16O + 16O molecular structures of positive- and negative-parity superdeformed bands in 34S
NASA Astrophysics Data System (ADS)
Taniguchi, Yasutaka
2016-05-01
The structures of excited states in 34S are investigated using the antisymmetrized molecular dynamics and generator coordinate method(GCM). The GCM basis wave functions are calculated via energy variation with a constraint on the quadrupole deformation parameter β. By applying the GCM after parity and angular momentum projections, the coexistence of two positive- and one negative-parity super de formed(SD) bands are predicted, and low-lying states and other deformed bands are obtained. The SD bands have structures of 16O + 16O + two valence neutrons in molecular orbitals around the two 16O cores in a cluster picture. The configurations of the two valence neutrons are δ2 and π2 for the positive-parity SD bands and π1δ1 for the negative parity SD band.
NASA Astrophysics Data System (ADS)
Tyc, Michał H.; Salejda, Włodzimierz; Klauzer-Kruszyna, Agnieszka; Tarnowski, Karol
2007-05-01
The dispersion relation for polarized light transmitting through a one-dimensional superlattice composed of aperiodically arranged layers made of ordinary dielectric and negative refraction metamaterials is calculated with finite element method. Generalized Fibonacci, generalized Thue-Morse, double-periodic and Rudin-Shapiro superlattices are investigated, using their periodic approximants. Strong dispersion of metamaterials is taken into account. Group velocities and effective refraction indices in the structures are calculated. The self-similar structure of the transmission spectra is observed.
Negative parity bands of {sup 115}Pd and band structures in {sup 113,115,117}Pd
Fong, D.; Hwang, J.K.; Ramayya, A.V.; Hamilton, J.H.; Gore, P.M.; Jones, E.F.; Luo, Y.X.; Walters, W.B.; Rasmussen, J.O.; Lee, I.Y.; Macchiavelli, A.O.; Wu, S.C.; Stoyer, M.A.; Zhu, S.J.; Daniel, A.V.; Ter-Akopian, G.M.; Oganessian, Yu.Ts.; Cole, J.D.; Donangelo, R.; Ma, W.C.
2005-07-01
Level structures of {sup 113,115,117}Pd have been studied using the Gammasphere and a spontaneous fission source of {sup 252}Cf. A new 85.1-keV transition was identified in {sup 113}Pd. This indicates that the spin and parity of the isomeric state is 9/2{sup -} rather than the previously assigned 11/2{sup -}. New low-energy transitions are confirmed in {sup 115,117}Pd. In {sup 115}Pd, the 39.0.- and 49.0-keV transitions are shown to be in prompt coincidence. This coincidence relationship indicates a spin and parity assignment of 1/2{sup +} for the ground state rather than the previously assigned 3/2{sup +}.
NASA Astrophysics Data System (ADS)
Abujetas, D. R.; Paniagua-Domínguez, R.; Nieto-Vesperinas, M.; Sánchez-Gil, J. A.
2015-12-01
We investigate theoretically and numerically the photonic band structure in the optical domain of an array of core-shell metal-semiconductor nanowires. Corresponding negative-index photonic bands are calculated, showing isotropic equifrequency surfaces. The effective (negative) electric permittivity and magnetic permeability, retrieved from S-parameters, are used to compare the performance of such nanowire arrays with homogeneous media in canonical examples, such as refraction through a prism and flat-lens focusing. Very good agreement is found, confirming the effective medium behavior of the nanowire array as a low-loss, isotropic (2D) and bulk, optical negative index metamaterial. Indeed, disorder is introduced to further stress its robustness.
NASA Astrophysics Data System (ADS)
Singh, V.; Sihotra, S.; Bhat, G. H.; Sheikh, J. A.; Kaur, M.; Kumar, S.; Singh, K.; Goswamy, J.; Saha, S.; Sethi, J.; Palit, R.; Malik, S. S.; Singh, N.; Garg, U.; Mehta, D.
2017-06-01
Excited states in the 101Pd nucleus were investigated through the 75As(31P,2 p 3 n ) fusion-evaporation reaction at Elab=125 MeV by using the Indian National Gamma Array spectrometer equipped with 21 clover Ge detectors. The level scheme is considerably extended for medium spin values. New positive-parity band structures in 101Pd have been studied within the framework of the projected shell model and are found to undergo transition from single quasiparticle to high-K three quasiparticle configuration after band crossing, i.e., from principal-axis rotation to tilted-axis rotation. The negative-parity band structures are discussed in the framework of the hybrid version of tilted-axis cranking shell model calculations. The observed alignment gain in the lowest excited ν h11 /2 negative-parity band results from successive (νg7 /2) 2 and (πg9 /2) 2 pair alignments. The higher excited negative-parity bands are reproduced for the ν [h11 /2(g7/2/d5 /2) 2] and (πg9 /2) -2⊗ν h11 /2 configurations.
NASA Astrophysics Data System (ADS)
Cho, Sungjin; Kim, Boseung; Min, Dongki; Park, Junhong
2015-10-01
This paper presents a two-dimensional heat-exhaust and sound-proof acoustic meta-structure exhibiting tunable multi-band negative effective mass density. The meta-structure was composed of periodic funnel-shaped units in a square lattice. Each unit cell operates simultaneously as a Helmholtz resonator (HR) and an extended pipe chamber resonator (EPCR), leading to a negative effective mass density creating bandgaps for incident sound energy dissipation without transmission. This structure allowed large heat-flow through the cross-sectional area of the extended pipe since the resonance was generated by acoustic elements without using solid membranes. The pipes were horizontally directed to a flow source to enable small flow resistance for cooling. Measurements of the sound transmission were performed using a two-load, four-microphone method for a unit cell and small reverberation chamber for two-dimensional panel to characterize the acoustic performance. The effective mass density showed significant frequency dependent variation exhibiting negative values at the specific bandgaps, while the effective bulk modulus was not affected by the resonator. Theoretical models incorporating local resonances in the multiple resonator units were proposed to analyze the noise reduction mechanism. The acoustic meta-structure parameters to create broader frequency bandgaps were investigated using the theoretical model. The negative effective mass density was calculated to investigate the creation of the bandgaps. The effects of design parameters such as length, cross-sectional area, and volume of the HR; length and cross-sectional area of the EPCR were analyzed. To maximize the frequency band gap, the suggested acoustic meta-structure panel, small neck length, and cross-sectional area of the HR, large EPCR length was advantageous. The bandgaps became broader when the two resonant frequencies were similar.
Negative pressure in shear thickening band of a dilatant fluid
NASA Astrophysics Data System (ADS)
Nagahiro, Shin-ichiro; Nakanishi, Hiizu
2016-12-01
We perform experiments and numerical simulations to investigate spatial distribution of pressure in a sheared dilatant fluid of the Taylor-Couette flow under a constant external shear stress. In a certain range of shear stress, the flow undergoes the shear thickening oscillation around 20 Hz. We find that, during the oscillation, a localized thickened band rotates around the axis with the flow. Based upon experiments and numerical simulations, we show that a major part of the thickened band is under negative pressure even in the case of discontinuous shear thickening, which indicates that the thickening is caused by Reynolds dilatancy; the dilatancy causes the negative pressure in interstitial fluid, which generates contact structure in the granular medium, then frictional resistance hinders rearrangement of the structure and solidifies the medium.
Yablonovitch, E.
1993-05-01
We learned how to create 3-dimensionally periodic dielectric structures which are to photon waves, as semiconductor crystals are to electron waves. That is, these photonic crystals have a photonic bandgap, a band of frequencies in which electromagnetic waves are forbidden, irrespective of propagation direction in space. Photonic bandgaps provide for spontaneous emission inhibition and allow for a new class of electromagnetic micro-cavities. If the perfect 3-dimensional periodicity is broken by a local defect, then local electromagnetic modes can occur within the forbidden bandgap. The addition of extra dielectric material locally, inside the photonic crystal, produces {open_quotes}donor{close_quotes} modes. Conversely, the local removal of dielectric material from the photonic crystal produces {open_quotes}acceptor{close_quotes} modes. Therefore, it will now be possible to make high-Q electromagnetic cavities of volume {approx_lt}1 cubic wavelength, for short wavelengths at which metallic cavities are useless. These new dielectric micro-resonators can cover the range all the way from millimeter waves, down to ultraviolet wavelengths.
Band Structures of Plasmonic Polarons
NASA Astrophysics Data System (ADS)
Caruso, Fabio; Lambert, Henry; Giustino, Feliciano
2015-03-01
In angle-resolved photoemission spectroscopy (ARPES), the acceleration of a photo-electron upon photon absorption may trigger shake-up excitations in the sample, leading to the emission of phonons, electron-hole pairs, and plasmons, the latter being collective charge-density fluctuations. Using state-of-the-art many-body calculations based on the `GW plus cumulant' approach, we show that electron-plasmon interactions induce plasmonic polaron bands in group IV transition metal dichalcogenide monolayers (MoS2, MoSe2, WS2, WSe2). We find that the energy vs. momentum dispersion relations of these plasmonic structures closely follow the standard valence bands, although they appear broadened and blueshifted by the plasmon energy. Based on our results we identify general criteria for observing plasmonic polaron bands in the angle-resolved photoelectron spectra of solids.
NASA Astrophysics Data System (ADS)
Banerjee, P.; Ganguly, S.; Pradhan, M. K.; Sharma, H. P.; Muralithar, S.; Singh, R. P.; Bhowmik, R. K.
2016-07-01
The structure of collective bands in 113Sn, populated in the reaction 100Mo(19F,p 5 n ) at a beam energy of 105 MeV, has been studied. A new positive-parity sequence of eight states extending up to 7764.9 keV and spin (39 /2+) has been observed. The band is explained as arising from the coupling of the odd valence neutron in the g7 /2 or the d5 /2 orbital to the deformed 2p-2h proton configuration of the neighboring even-A Sn isotope. Lifetimes of six states up to an excitation energy of 9934.9 keV and spin 47 /2-belonging to a Δ I =2 intruder band have been measured for the first time, including an upper limit for the last state, from Doppler-shift-attenuation data. A moderate average quadrupole deformation β2=0.22 ±0.02 is deduced from these results for the five states up to spin 43 /2- . The transition quadrupole moments decrease with increase in rotational frequency, indicating a reduction of collectivity with spin, a feature common for terminating bands. The behavior of the kinematic and dynamic moments of inertia as a function of rotational frequency has been studied and total Routhian surface calculations have been performed in an attempt to obtain an insight into the nature of the states near termination.
A study of oligoclonal band negative multiple sclerosis.
Zeman, A Z; Kidd, D; McLean, B N; Kelly, M A; Francis, D A; Miller, D H; Kendall, B E; Rudge, P; Thompson, E J; McDonald, W I
1996-01-01
OBJECTIVES--To determine whether oligoclonal band (OCB) negative multiple sclerosis is a reliable diagnosis and, if so, whether it has a distinctive prognosis. METHODS--Retrospective and matched prospective comparison of the clinical and laboratory features of patients with clinical definite multiple sclerosis with and without intrathecal synthesis of oligoclonal IgG. RESULTS--Thirty four patients were identified with apparent OCB negative clinically definite multiple sclerosis. The results of oligoclonal banding proved to have been equivocal in 14 of 34; the clinical diagnosis of multiple sclerosis was questionable in 8 of 34. The remaining 12 patients with "true" OCB negative multiple sclerosis were significantly less disabled than matched OCB positive controls. Re-examination of CSF-serum pairs from six OCB negative patients showed that three remained OCB negative while three showed evidence of intrathecal synthesis of OCBs. CONCLUSIONS--OCB negative clinically definite multiple sclerosis is rare and should be diagnosed with caution; in unequivocal cases it seems to have a relatively benign prognosis. PMID:8558146
A new miniaturized negative-index meta-atom for tri-band applications
NASA Astrophysics Data System (ADS)
Hossain, Mohammad Jakir; Faruque, Mohammad Rashed Iqbal; Abdullah, Sharifah Mastura Syed; Roslan, Muhamad Roszaini; Islam, Mohammad Tariqul
2017-07-01
In this paper, a miniature negative index meta-atom was designed; simulated, fabricated and measured based on parallel incidence of electromagnetic wave that can maintain a tri-band applications in microwave spectra. Compare to the other multi-band conventional metamaterial, the proposed meta-atom structure allows miniaturization factor and follows better effective medium ratio (EMR). Finite-integration technique (FIT) based computer simulation technology (CST) electromagnetic simulator was adopted to examine the design of the meta-atom. It exhibits tri-band response in conjunction with backward wave property over a certain frequency band in the microwave regime. Furthermore, the effective medium ratio is considerably improved compared to previously reported metamaterial. Moreover, few parametric analyses were done with the meta-atom. The size, scattering parameters and effective medium parameters of the proposed negative index miniaturized meta-atom is appropriate for tri-band applications.
NASA Astrophysics Data System (ADS)
Alqadami, Abdulrahman Shueai Mohsen; Jamlos, Mohd Faizal; Soh, Ping Jack; Rahim, Sharul Kamal Abdul; Vandenbosch, Guy A. E.; Narbudowicz, Adam
2017-01-01
A miniaturized dual-band antenna array using a negative index metamaterial is presented for WiMAX, LTE, and WLAN applications. This left-handed metamaterial plane is located behind the antenna array, and its unit cell is a combination of split-ring resonator, square electric ring resonator, and rectangular electrical coupled resonator. This enables the achievement of a metamaterial structure exhibiting both negative permittivity and permeability, which results in antenna size miniaturization, efficiency, and gain enhancement. Moreover, the proposed metamaterial antenna has realized dual-band operating frequencies compared to a single frequency for normal antenna. The measured reflection coefficient (S11) shows a 50.25% bandwidth in the lower band (from 2.119 to 3.058 GHz) and 4.27% in the upper band (from 5.058 to 5.276 GHz). Radiation efficiency obtained in the lower and upper band are >95 and 80%, respectively.
Photonic band gap structure simulator
Chen, Chiping; Shapiro, Michael A.; Smirnova, Evgenya I.; Temkin, Richard J.; Sirigiri, Jagadishwar R.
2006-10-03
A system and method for designing photonic band gap structures. The system and method provide a user with the capability to produce a model of a two-dimensional array of conductors corresponding to a unit cell. The model involves a linear equation. Boundary conditions representative of conditions at the boundary of the unit cell are applied to a solution of the Helmholtz equation defined for the unit cell. The linear equation can be approximated by a Hermitian matrix. An eigenvalue of the Helmholtz equation is calculated. One computation approach involves calculating finite differences. The model can include a symmetry element, such as a center of inversion, a rotation axis, and a mirror plane. A graphical user interface is provided for the user's convenience. A display is provided to display to a user the calculated eigenvalue, corresponding to a photonic energy level in the Brilloin zone of the unit cell.
Structures with negative index of refraction
Soukoulis, Costas M [Ames, IA; Zhou, Jiangfeng [Ames, IA; Koschny, Thomas [Ames, IA; Zhang, Lei [Ames, IA; Tuttle, Gary [Ames, IA
2011-11-08
The invention provides simplified negative index materials (NIMs) using wire-pair structures, 4-gap single ring split-ring resonator (SRR), fishnet structures and overleaf capacitor SRR. In the wire-pair arrangement, a pair of short parallel wires and continuous wires are used. In the 4-gap single-ring SRR, the SRRs are centered on the faces of a cubic unit cell combined with a continuous wire type resonator. Combining both elements creates a frequency band where the metamaterial is transparent with simultaneously negative .di-elect cons. and .mu.. In the fishnet structure, a metallic mesh on both sides of the dielectric spacer is used. The overleaf capacitor SRR changes the gap capacities to small plate capacitors by making the sections of the SRR ring overlap at the gaps separated by a thin dielectric film. This technique is applicable to conventional SRR gaps but it best deploys for the 4-gap single-ring structures.
Triple negative permeability band in plasmon-hybridized cut-wire-pair metamaterials
NASA Astrophysics Data System (ADS)
Thuy, V. T. T.; Viet, D. T.; Hieu, N. V.; Lee, Y. P.; Lam, V. D.; Tung, N. T.
2010-11-01
We expand the picture of plasmon hybridization in metamagnetic structure via numerically studying the electromagnetic coupling in the metallic cut-wire-pair super cells. It is shown that a triple negative permeability band can be achieved by systematically controlling the plasmon hybridization in such the structure. The corresponding transmission properties as well as the electromagnetic responses of the plasmon-hybridized structures were presented by using the finite integration technique simulations. Our results would reveal a promising design to obtain the multiple negative refractions based on the combination of hybridized cut-wire-pairs and continuous wires.
Effective band structure of random alloys.
Popescu, Voicu; Zunger, Alex
2010-06-11
Random substitutional A(x)B(1-x) alloys lack formal translational symmetry and thus cannot be described by the language of band-structure dispersion E(k(→)). Yet, many alloy experiments are interpreted phenomenologically precisely by constructs derived from wave vector k(→), e.g., effective masses or van Hove singularities. Here we use large supercells with randomly distributed A and B atoms, whereby many different local environments are allowed to coexist, and transform the eigenstates into an effective band structure (EBS) in the primitive cell using a spectral decomposition. The resulting EBS reveals the extent to which band characteristics are preserved or lost at different compositions, band indices, and k(→) points, showing in (In,Ga)N the rapid disintegration of the valence band Bloch character and in Ga(N,P) the appearance of a pinned impurity band.
Dynamically variable negative stiffness structures
Churchill, Christopher B.; Shahan, David W.; Smith, Sloan P.; Keefe, Andrew C.; McKnight, Geoffrey P.
2016-01-01
Variable stiffness structures that enable a wide range of efficient load-bearing and dexterous activity are ubiquitous in mammalian musculoskeletal systems but are rare in engineered systems because of their complexity, power, and cost. We present a new negative stiffness–based load-bearing structure with dynamically tunable stiffness. Negative stiffness, traditionally used to achieve novel response from passive structures, is a powerful tool to achieve dynamic stiffness changes when configured with an active component. Using relatively simple hardware and low-power, low-frequency actuation, we show an assembly capable of fast (<10 ms) and useful (>100×) dynamic stiffness control. This approach mitigates limitations of conventional tunable stiffness structures that exhibit either small (<30%) stiffness change, high friction, poor load/torque transmission at low stiffness, or high power active control at the frequencies of interest. We experimentally demonstrate actively tunable vibration isolation and stiffness tuning independent of supported loads, enhancing applications such as humanoid robotic limbs and lightweight adaptive vibration isolators. PMID:26989771
Dynamically variable negative stiffness structures.
Churchill, Christopher B; Shahan, David W; Smith, Sloan P; Keefe, Andrew C; McKnight, Geoffrey P
2016-02-01
Variable stiffness structures that enable a wide range of efficient load-bearing and dexterous activity are ubiquitous in mammalian musculoskeletal systems but are rare in engineered systems because of their complexity, power, and cost. We present a new negative stiffness-based load-bearing structure with dynamically tunable stiffness. Negative stiffness, traditionally used to achieve novel response from passive structures, is a powerful tool to achieve dynamic stiffness changes when configured with an active component. Using relatively simple hardware and low-power, low-frequency actuation, we show an assembly capable of fast (<10 ms) and useful (>100×) dynamic stiffness control. This approach mitigates limitations of conventional tunable stiffness structures that exhibit either small (<30%) stiffness change, high friction, poor load/torque transmission at low stiffness, or high power active control at the frequencies of interest. We experimentally demonstrate actively tunable vibration isolation and stiffness tuning independent of supported loads, enhancing applications such as humanoid robotic limbs and lightweight adaptive vibration isolators.
Low-loss negative index metamaterials for X, Ku, and K microwave bands
Lee, David A.; Vedral, L. James; Smith, David A.; Pinchuk, Anatoliy O.; Musselman, Randall L.
2015-04-15
Low-loss, negative-index of refraction metamaterials were designed and tested for X, Ku, and K microwave frequency bands. An S-shaped, split-ring resonator was used as a unit cell to design homogeneous slabs of negative-index metamaterials. Then, the slabs of metamaterials were cut unto prisms to measure experimentally the negative index of refraction of a plane electromagnetic wave. Theoretical simulations using High-Frequency Structural Simulator, a finite element equation solver, were in good agreement with experimental measurements. The negative index of refraction was retrieved from the angle- and frequency-dependence of the transmitted intensity of the microwave beam through the metamaterial prism and compared well to simulations; in addition, near-field electromagnetic intensity mapping was conducted with an infrared camera, and there was also a good match with the simulations for expected frequency ranges for the negative index of refraction.
Negative capacitance switching via VO{sub 2} band gap engineering driven by electric field
He, Xinfeng; Xu, Jing; Xu, Xiaofeng Gu, Congcong; Chen, Fei; Wu, Binhe Wang, Chunrui Xing, Huaizhong; Chen, Xiaoshuang; Chu, Junhao
2015-03-02
We report the negative capacitance behavior of an energy band gap modulation quantum well with a sandwich VO{sub 2} layer structure. The phase transition is probed by measuring its capacitance. With the help of theoretical calculations, it shows that the negative capacitance changes of the quantum well device come from VO{sub 2} band gap by continuously tuning the temperature or voltage. Experiments reveal that as the current remains small enough, joule heating can be ignored, and the insulator-metal transition of VO{sub 2} can be induced by the electric field. Our results open up possibilities for functional devices with phase transitions induced by external electric fields other than the heating or electricity-heat transition.
Negative capacitance switching via VO2 band gap engineering driven by electric field
NASA Astrophysics Data System (ADS)
He, Xinfeng; Xu, Jing; Xu, Xiaofeng; Gu, Congcong; Chen, Fei; Wu, Binhe; Wang, Chunrui; Xing, Huaizhong; Chen, Xiaoshuang; Chu, Junhao
2015-03-01
We report the negative capacitance behavior of an energy band gap modulation quantum well with a sandwich VO2 layer structure. The phase transition is probed by measuring its capacitance. With the help of theoretical calculations, it shows that the negative capacitance changes of the quantum well device come from VO2 band gap by continuously tuning the temperature or voltage. Experiments reveal that as the current remains small enough, joule heating can be ignored, and the insulator-metal transition of VO2 can be induced by the electric field. Our results open up possibilities for functional devices with phase transitions induced by external electric fields other than the heating or electricity-heat transition.
New design of multi-band negative-index metamaterial and absorber at visible frequencies
NASA Astrophysics Data System (ADS)
Gong, Boyi; Guo, Fan; Zou, Wenkang; Chen, Lin; Song, Kun; Zhao, Xiaopeng
2017-08-01
A new negative-index metamaterial (NIM) structure is proposed by designing the metallic holes of traditional double-fishnet (DF) structures from uniform sizes to several different sizes. Numerical results demonstrate that the new metamaterial, as an improved variant of the DF structure, achieved a multi-band negative refractive index across a wide range of visible frequencies from 470 THz to 540 THz, which covers the red, orange, yellow, and green regions of the visible spectra. Meanwhile, a low-profile nanostrctured absorber was obtained when one side of the perforated metal layer of this multi-band NIM was substituted with a continuous metal film with the same thickness. The absorber showed the high absorption of more than 95% at multiple frequencies of 511, 520, 523, 525, and 527 THz. The behavior of multi-frequency response effectively broadened the working bandwidth. Finally, the physical mechanism of the multi-band operating characteristics of NIM and absorber was analyzed with the distributions of current intensity at different resonant frequencies.
Creating wide-band negative-index-of-refraction metamaterials with fractal-based geometry
NASA Astrophysics Data System (ADS)
Penney, Keith
2009-11-01
A burgeoning topic of modern research in electrodynamics and antenna design is the design and fabrication of ``left-handed'' metamaterials. This ``left-handedness'' is often created through use of an array of conductive structures with geometry appropriate for coupling on the wavelength scale with incident radiation to produce a phase-shifted reflected wave that cancels out incoming radiation and prevents transmission. This property has been demonstrated in several papers published in the last decade. In every instance, though the ``left-handed'' response is only exhibited in a small bandwidth centered about a specific frequency (bandwidth typically less that 0.1 GHz). I will show that through use of tessellated, fractal-based structures, one can create a repeatable geometry that exhibits a negative index of refraction (NIR) for multiple frequency bands, limited only by fabrication precision, with the ultimate goal being a wide-band absorptive response.
Nonreciprocal microwave band-gap structures.
Belov, P A; Tretyakov, S A; Viitanen, A J
2002-07-01
An electrically controlled nonreciprocal electromagnetic band-gap material is proposed and studied. The new material is a periodic three-dimensional regular lattice of small magnetized ferrite spheres. In this paper, we consider plane electromagnetic waves in this medium and design an analytical model for the material parameters. An analytical solution for plane-wave reflection from a planar interface is also presented. In the proposed material, a new electrically controlled stop band appears for one of the two circularly polarized eigenwaves in a frequency band around the ferrimagnetic resonance frequency. This frequency can be well below the usual lattice band gap, which allows the realization of rather compact structures. The main properties of the material are outlined.
NASA Astrophysics Data System (ADS)
Goswami, R.; Sethi, B.; Sarkar, M. S.; Sen, S.
1995-12-01
The band structures of121, 123I nuclei have been studied using a version of the particle-rotor-model in which the experimental excitation energies of the neighbouring (A-1) cores can be fed directly as input parameters. The calculations have been carried out with axially symmetric Nilsson potential with both prolate and oblate deformations. The parameters of the model have been chosen from earlier theoretical work and experimental odd-even mass differences. Only the Coriolis attenuation factor has been treated as adjustable parameter. The theoretical band structures are in very good agreement with the available experimental data.
Structurally Integrated X-Band Array Development
2006-10-01
work in progress towards the development of a very large structural x-band electronically scanned array (ESA). A building block approach that...significant structural testing from coupon through large scale structural validation have been complete and is reported. The active array testing will...mechanical performance, so trades could be used to develop the most promising configuration. This involved evaluating coupons to understand the
NASA Astrophysics Data System (ADS)
Singh, Bipin K.; Dikshit, Ashutosh K.; Thapa, Khem B.; Pandey, Praveen C.
2016-05-01
We have investigated the photonic band gaps (PBG) and omnidirectional band gaps in one-dimensional photonic crystals made up of alternate layer of exponential graded index material and negative index material. We have considered the influence of material properties, geometrical parameters and material composition on the PBG and omnidirectional band gap. Results show that the parameters of exponential graded index material and negative index material can change the photonic and omnidirectional band structures remarkably. Number and bandwidth of PBG increases with increasing the negative index material layer thicknesses while thicknesses of graded index layer only have an effect on the bandwidth of PBGs. The bandwidth of PBG also depends on grading profile parameter of exponential graded index layers and bandwidth can be tuned with increase the value of grading profile parameter. This work can facilitate the design of filters and reflectors, and provide the basic understanding of the influence of graded index materials and metamaterials on the PBG properties.
Dual-band double-negative-index fishnet metamaterial at millimeter-waves.
Navarro-Cía, Miguel; García-Meca, Carlos; Beruete, Miguel; Martínez, Alejandro; Sorolla, Mario
2011-11-01
An effective negative refractive index (NRI) is demonstrated and experimentally verified for the first two propagation bands of a fishnet-like metamaterial at millimeter-wave frequencies. The dual-band NRI behavior is achieved by engineering the diffraction order (±1, ±1) associated with the internal mode supported between holey layers to correspond with the second propagation band. In addition to the experimental interferometric technique that accounts for the handedness of the propagation, numerical results are given to predict the dual-band effective NRI and to confirm dual-band negative refraction for a prism composed of the proposed metamaterial.
Midfrequency band dynamics of large space structures
NASA Astrophysics Data System (ADS)
Coppolino, Robert N.; Adams, Douglas S.; Levine, Marie B.
2004-09-01
High and low intensity dynamic environments experienced by a spacecraft during launch and on-orbit operations, respectively, induce structural loads and motions, which are difficult to reliably predict. Structural dynamics in low- and mid-frequency bands are sensitive to component interface uncertainty and non-linearity as evidenced in laboratory testing and flight operations. Analytical tools for prediction of linear system response are not necessarily adequate for reliable prediction of mid-frequency band dynamics and analysis of measured laboratory and flight data. A new MATLAB toolbox, designed to address the key challenges of mid-frequency band dynamics, is introduced in this paper. Finite-element models of major subassemblies are defined following rational frequency-wavelength guidelines. For computational efficiency, these subassemblies are described as linear, component mode models. The complete structural system model is composed of component mode subassemblies and linear or non-linear joint descriptions. Computation and display of structural dynamic responses are accomplished employing well-established, stable numerical methods, modern signal processing procedures and descriptive graphical tools. Parametric sensitivity and Monte-Carlo based system identification tools are used to reconcile models with experimental data and investigate the effects of uncertainties. Models and dynamic responses are exported for employment in applications, such as detailed structural integrity and mechanical-optical-control performance analyses.
Tunable dual-band ferrite-based metamaterials with dual negative refractions
NASA Astrophysics Data System (ADS)
Huang, Y. J.; Wen, G. J.; Yang, Y. J.; Xie, K.
2012-01-01
We report on three types of tunable dual-band metamaterial with dual negative refraction in this paper. The three types of metamaterial are composed of ferrite slabs and three different metallic resonators, including split-ring resonators (SRR), Ω-like resonators, and short wire pairs. The ferrite slabs under an applied magnetic bias provide one magnetic resonance frequency band and the three metallic resonators provide another magnetic resonance frequency band, respectively. The continuous wires within the metamaterials provide the negative permittivity in a wide frequency band covering the two magnetic resonance bands. We give the design, analysis and numerical demonstrations of three such types of metamaterial in detail. The effective electromagnetic parameters obtained from the simulated S-parameters indicate that the three types of metamaterial indeed exhibit two negative refraction passbands and the two passbands can also be shifted by changing the magnetic bias. Our results open the way to fabricate tunable dual-band metamaterial cloaks, absorbers, and antennas.
Pichard, Hélène; Richoux, Olivier; Groby, Jean-Philippe
2012-10-01
The propagation of audible acoustic waves in two-dimensional square lattice tunable sonic crystals (SC) made of square cross-section infinitely rigid rods embedded in air is investigated experimentally. The band structure is calculated with the plane wave expansion (PWE) method and compared with experimental measurements carried out on a finite extend structure of 200 cm width, 70 cm depth and 15 cm height. The structure is made of square inclusions of 5 cm side with a periodicity of L = 7.5 cm placed inbetween two rigid plates. The existence of tunable complete band gaps in the audible frequency range is demonstrated experimentally by rotating the scatterers around their vertical axis. Negative refraction is then analyzed by use of the anisotropy of the equi-frequency surface (EFS) in the first band and of a finite difference time domain (FDTD) method. Experimental results finally show negative refraction in the audible frequency range.
A new wideband negative refractive index metamaterial for dual-band operation
NASA Astrophysics Data System (ADS)
Islam, S. S.; Faruque, M. R. I.; Islam, M. T.; Ali, M. T.
2017-04-01
A new wideband negative refractive index (NRI) metamaterial for dual-band operation is introduced in this study. Initially, a bare-H-shaped resonator was designed over the FR-4 substrate material. The refractive index curve for the unit cell, displays more than 3-GHz negative real peak from C-band to some portion of X-band. The proposed design also displays NRI property in the same frequency bands with wider bandwidth, when the Rogers RT 3010 substrate material was employed instead of FR-4 substrate material.
Q-band tunable negative refractive index metamaterial using Sc-doped BaM hexaferrite
NASA Astrophysics Data System (ADS)
He, P.; Gao, J.; Chen, Y.; Parimi, P. V.; Vittoria, C.; Harris, V. G.
2009-08-01
A simple structured tunable negative refractive index (n) metamaterial (TNIM) has been designed, fabricated and tested in a Q-band rectangular waveguide. The structure consists of one slab of single crystalline scandium-doped barium hexaferrite (Sc-BaM), aligned parallel to two rows of periodic copper wires. The magnetic field tunable passband is measured indicating the occurrence of negative n. The centre frequency of the 5 GHz wide passband, having a transmission peak of -13 dB, is shifted linearly from 40.9 to 43.9 GHz by varying the bias field (H) from 4.0 to 7.0 kOe. The impact of ferrite volume factor (FVF) of the Sc-BaM slab upon the performance of the TNIM composite has been studied qualitatively. A tradeoff effect is illustrated in which the desirable negative permeability (μ) of the ferrite is offset by the detrimental impact of its dielectric property in suppressing the negative permittivity (ɛ) of the nearby plasmonic wires.
Electronic band structure of defect chalcopyrites
NASA Astrophysics Data System (ADS)
Jiang, Xiaoshu; Lambrecht, Walter R. L.
2001-03-01
The defect chalcopyrites of chemical composition II-III-VI4 in which II, III and VI mean group-II elements such as Cd or Hg, group-III elements such as Al and Ga and group-VI elements such as S, Se, Te, form an interesting family of semiconductor compounds with potential nonlinear optical applications. They can be thought of as derived from the regular I-III-VI2 chalcopyrites by doubling the formula unit and replacing the group I element, for example, Ag by the group-II element and a vacancy in an ordered manner. The chalcopyrites themselves are derived from II-VI compounds by replacing the group-II by a group I and a group-III element. In this contribution we present electronic band structure calculations of some of these compounds, calculated using the linear muffin-tin orbital method combined with the local density functional approximation. We discuss the relation of the band structures of the corresponding zincblende, chalcopyrite and defect chalcopyrite compounds. In particular, the role of the group I or group II d-band energy will be shown to be important. The trends with chemical substutions and the effects of structural distortions c/a and internal parameters accompanying the chemical distortion will be discussed.
The band-gap enhanced photovoltaic structure
Tessler, Nir
2016-05-02
We critically examine the recently suggested structure that was postulated to potentially add 50% to the photo-conversion efficiency of organic solar cells. We find that the structure could be realized using stepwise increase in the gap as long as the steps are not above 0.1 eV. We also show that the charge extraction is not compromised due to an interplay between the contact's space charge and the energy level modification, which result in a flat energy band at the extracting contact.
Topological Insulators: Electronic Band Structure and Spectroscopy
NASA Astrophysics Data System (ADS)
Palaz, S.; Koc, H.; Mamedov, A. M.; Ozbay, E.
2017-02-01
In this study, we present the results of our ab initio calculation of the elastic constants, density of states, charge density, and Born effective charge tensors for ferroelectric (rhombohedral) and paraelectric phases (cubic) of the narrow band ferroelectrics (GeTe, SnTe) pseudopotentials. The related quantities such as bulk modulus and shear modulus using obtained elastic constants have also been estimated in the present work. The total and partial densities of states corresponding to the band structure of Sn(Ge)Te(S,Se) were calculated. We also calculated the Born effective charge tensor of an atom (for instance, Ge, Sn, Te, etc.), which is defined as the induced polarization of the solid along the main direction by a unit displacement in the perpendicular direction of the sublattice of an atom at the vanishing electric field.
Band structure of doubly-odd nuclei around mass 130
Higashiyama, Koji; Yoshinaga, Naotaka
2011-05-06
Nuclear structure of the doublet bands in the doubly-odd nuclei with mass A{approx}130 is studied in terms of a pair-truncated shell model. The model reproduces quite well the energy levels of the doublet bands and the electromagnetic transitions. The analysis of the electromagnetic transitions reveals new band structure of the doublet bands.
5 CFR 9701.321 - Structure of bands.
Code of Federal Regulations, 2013 CFR
2013-01-01
... 5 Administrative Personnel 3 2013-01-01 2013-01-01 false Structure of bands. 9701.321 Section 9701... Structure of bands. (a) DHS may, after coordination with OPM, establish ranges of basic pay for bands, with... rates. (b) For each band within an occupational cluster, DHS will establish a common rate range...
Photonic band gaps in quasiperiodic photonic crystals with negative refractive index
NASA Astrophysics Data System (ADS)
Vasconcelos, M. S.; Mauriz, P. W.; de Medeiros, F. F.; Albuquerque, E. L.
2007-10-01
We investigate the photonic band gaps in quasiperiodic photonic crystals made up of both positive (SiO2) and negative refractive index materials using a theoretical model based on a transfer matrix treatment. The quasiperiodic structures are characterized by the nature of their Fourier spectrum, which can be dense pure point (Fibonacci sequences) or singular continuous (Thue-Morse and double-period sequences). These substitutional sequences are described in terms of a series of generations that obey peculiar recursion relations. We discussed the photonic band gap spectra for both the ideal cases, where the negative refractive index material can be approximated as a constant in the frequency range considered, as well as the more realistic case, taking into account the frequency-dependent electric permittivity γ and magnetic permeability μ . We also present a quantitative analysis of the results, pointing out the distribution of the allowed photonic bandwidths for high generations, which gives a good insight about their localization and power laws.
Numerical study of tunable band-pass filter made of one dimensional composite periodic structure
NASA Astrophysics Data System (ADS)
Anusha N., P.; Sharan, Alok
2017-06-01
Band-pass filters are used to allow transmission of certain range of frequencies and attenuate the rest. In this report we propose a 1-D periodic structure made of alternate layer of single negative index medium and zero index medium which works as band-pass filter. We have numerically simulated the band-pass filter using Finite Difference Time Domain (FDTD) method in Matlab. Entire range of frequency is reflected back when the light is incident on a single negative index medium slab. We were able to create a tunable narrow band-pass filter using the proposed periodic structure by choosing the appropriate values of permittivity and permeability.
Band structure of absorptive two-dimensional photonic crystals
NASA Astrophysics Data System (ADS)
van der Lem, Han; Tip, Adriaan; Moroz, Alexander
2003-06-01
The band structure for an absorptive two-dimensional photonic crystal made from cylinders consisting of a Drude material is calculated. Absorption causes the spectrum to become complex and form islands in the negative complex half-plane. The boundaries of these islands are not always formed by the eigenvalues calculated for Bloch vectors on the characteristic path, and we find a hole in the spectrum. For realistic parameter values, the real part of the spectrum is hardly influenced by absorption, typically less than 0.25%. The employed method uses a Korringa-Kohn-Rostoker procedure together with analytical continuation. This results in an efficient approach that allows these band-structure calculations to be done on a Pentium III personal computer.
Band-structure parameters by genetic algorithm
Starrost, F.; Bornholdt, S.; Solterbeck, C.; Schattke, W.
1996-05-01
A genetic algorithm has been used to solve a complex multidimensional parameter-fitting problem. We will focus on the parameters of an empirical tight-binding Hamiltonian. The method is used to approximate the electronic energy band structure if energy values are known for a few wave vectors of high symmetry. Compared to the usual manual procedure this method is more accurate and automatic. This approach, based on the extended H{umlt u}ckel theory (EHT), has provided a list of EHT parameters for IV-IV and III-V semiconductors with zinc-blende structure and helped us to find a symmetry in the EHT. {copyright} {ital 1996 The American Physical Society.}
Tunable Negative Refractive Index Metamaterials and Applications at X and Q-bands
2008-03-02
The goal of the current DARPA-ARO funded project is to design, fabricate, and test a tunable negative index metamaterial (TNIM) using ferrites as...Mater., 313 (2007) 187-191. Y. He, P. He, V. G. Harris, and C. Vittoria, “Role of ferrites in negative index metamaterials ”, IEEE Trans. Magnetics...2 3 Final Progress Report Tunable Negative Refractive Index Metamaterials and Applications at X and Q- bands Peng He, Jinsheng Gao, P
Elucidating the stop bands of structurally colored systems through recursion
NASA Astrophysics Data System (ADS)
Amir, Ariel; Vukusic, Peter
2013-04-01
Interference is the source of some of the spectacular colors of animals and plants in nature. In some of these systems, the physical structure consists of an ordered array of layers with alternating high and low refractive indices. This periodicity leads to an optical band structure that is analogous to the electronic band structure encountered in semiconductor physics: specific bands of wavelengths (the stop bands) are perfectly reflected. Here, we present a minimal model for optical band structure in a periodic multilayer structure and solve it using recursion relations. The stop bands emerge in the limit of an infinite number of layers by finding the fixed point of the recursion. We compare to experimental data for various beetles, whose optical structure resembles the proposed model. Thus, using only the phenomenon of interference and the idea of recursion, we are able to elucidate the concept of band structure in the context of the experimentally observed high reflectance and iridescent appearance of structurally colored beetles.
Rotational band structure in 32Mg
NASA Astrophysics Data System (ADS)
Crawford, H. L.; Fallon, P.; Macchiavelli, A. O.; Poves, A.; Bader, V. M.; Bazin, D.; Bowry, M.; Campbell, C. M.; Carpenter, M. P.; Clark, R. M.; Cromaz, M.; Gade, A.; Ideguchi, E.; Iwasaki, H.; Langer, C.; Lee, I. Y.; Loelius, C.; Lunderberg, E.; Morse, C.; Richard, A. L.; Rissanen, J.; Smalley, D.; Stroberg, S. R.; Weisshaar, D.; Whitmore, K.; Wiens, A.; Williams, S. J.; Wimmer, K.; Yamamato, T.
2016-03-01
There is significant evidence supporting the existence of deformed ground states within the neutron-rich N ≈20 neon, sodium, and magnesium isotopes that make up what is commonly called the "island of inversion." However, the rotational band structures, which are a characteristic fingerprint of a rigid nonspherical shape, have yet to be observed. In this work, we report on a measurement and analysis of the yrast (lowest lying) rotational band in 32Mg up to spin I =6+ produced in a two-step projectile fragmentation reaction and observed using the state-of-the-art γ -ray tracking detector array, GRETINA (γ -ray energy tracking in-beam nuclear array). Large-scale shell-model calculations using the SDPF-U-MIX effective interaction show excellent agreement with the new data. Moreover, a theoretical analysis of the spectrum of rotational states as a function of the pairing gap, together with cranked-shell-model calculations, provides intriguing evidence for a reduction in pairing correlations with increased angular momentum, also in line with the shell-model results.
Rotational Band Structure in 32Mg
NASA Astrophysics Data System (ADS)
Crawford, Heather; NSCL E11029 Collaboration Team
2016-03-01
There is significant evidence supporting the existence of deformed ground states within the neutron-rich N =20 neon, sodium, and magnesium isotopes that make up what is commonly called the ``Island of Inversion''. However, rotational band structures, a characteristic fingerprint of a rigid non-spherical shape, have yet to be observed. We report on a measurement and analysis of the yrast (lowest lying) rotational band in 32Mg up to spin I = 6+, produced in a two-step projectile fragmentation reaction and observed using the state-of-the-art γ-ray tracking detector array, GRETINA. Large-scale shell model calculations using the SDPF-U-MIX effective interaction show excellent agreement with the new data. Moreover, a theoretical analysis of the spectrum of rotational states as a function of the pairing gap, together with cranked shell model calculations, provides intriguing evidence for a reduction in pairing correlations with increased angular momentum, also in line with the shell-model results. This material is based upon work supported by the U.S. DOE, Office of Science, NP Office under Contract No. DE-AC02-05CH11231 (LBNL). GRETINA was funded by the U.S. DOE Office of Science. Operation of the array at NSCL was supported by NSF.
NASA Astrophysics Data System (ADS)
Yang, Chuanhui; Wu, Jiu Hui; Cao, Songhua; Jing, Li
2016-08-01
This paper studies a novel kind of low-frequency broadband acoustic metamaterials with small size based on the mechanisms of negative mass density and multi-cavity coupling. The structure consists of a closed resonant cavity and an open resonant cavity, which can be equivalent to a homogeneous medium with effective negative mass density in a certain frequency range by using the parameter inversion method. The negative mass density makes the anti-resonance area increased, which results in broadened band gaps greatly. Owing to the multi-cavity coupling mechanism, the local resonances of the lower frequency mainly occur in the closed cavity, while the local resonances of the higher frequency mainly in the open cavity. Upon the interaction between the negative mass density and the multi-cavity coupling, there exists two broad band gaps in the range of 0-1800 Hz, i.e. the first-order band gap from 195 Hz to 660 Hz with the bandwidth of 465 Hz and the second-order band gap from 1157 Hz to 1663 Hz with the bandwidth of 506 Hz. The acoustic metamaterials with small size presented in this paper could provide a new approach to reduce the low-frequency broadband noises.
Segmental structure in banded mongoose calls.
Fitch, W Tecumseh
2012-12-03
In complex animal vocalizations, such as bird or whale song, a great variety of songs can be produced via rearrangements of a smaller set of 'syllables', known as 'phonological syntax' or 'phonocoding' However, food or alarm calls, which function as referential signals, were previously thought to lack such combinatorial structure. A new study of calls in the banded mongoose Mungos mungo provides the first evidence of phonocoding at the level of single calls. The first portion of the call provides cues to the identity of the caller, and the second part encodes its current activity. This provides the first example known in animals of something akin to the consonants and vowels of human speech.
Segmental structure in banded mongoose calls
2012-01-01
In complex animal vocalizations, such as bird or whale song, a great variety of songs can be produced via rearrangements of a smaller set of 'syllables', known as 'phonological syntax' or 'phonocoding' However, food or alarm calls, which function as referential signals, were previously thought to lack such combinatorial structure. A new study of calls in the banded mongoose Mungos mungo provides the first evidence of phonocoding at the level of single calls. The first portion of the call provides cues to the identity of the caller, and the second part encodes its current activity. This provides the first example known in animals of something akin to the consonants and vowels of human speech. See research article http://www.biomedcentral.com/1741-7007/10/97 PMID:23206277
Thunderstorm Charge Structures Producing Negative Gigantic Jets
NASA Astrophysics Data System (ADS)
Boggs, L.; Liu, N.; Riousset, J. A.; Shi, F.; Rassoul, H.
2016-12-01
Here we present observational and modeling results that provide insight into thunderstorm charge structures that produce gigantic jet discharges. The observational results include data from four different thunderstorms producing 9 negative gigantic jets from 2010 to 2014. We used radar, very high frequency (VHF) and low frequency (LF) lightning data to analyze the storm characteristics, charge structures, and lightning activity when the gigantic jets emerged from the parent thunderstorms. A detailed investigation of the evolution of one of the charge structures by analyzing the VHF data is also presented. The newly found charge structure obtained from the observations was analyzed with fractal modeling and compared with previous fractal modeling studies [Krehbiel et al., Nat. Geosci., 1, 233-237, 2008; Riousset et al., JGR, 115, A00E10, 2010] of gigantic jet discharges. Our work finds that for normal polarity thunderstorms, gigantic jet charge structures feature a narrow upper positive charge region over a wide middle negative charge region. There also likely exists a `ring' of negative screening charge located around the perimeter of the upper positive charge. This is different from previously thought charge structures of the storms producing gigantic jets, which had a very wide upper positive charge region over a wide middle negative charge region, with a very small negative screening layer covering the cloud top. The newly found charge structure results in leader discharge trees in the fractal simulations that closely match the parent flashes of gigantic jets inside and outside the thundercloud. The previously used charge structures, while vital to the understanding of gigantic jet initiation and the role of charge imbalances inside the cloud, do not produce leader discharge trees that agree with observed gigantic jet discharges.Finally, the newly discovered gigantic jet charge structures are formed near the end of a convective pulse [Meyer et al., JGR, 118
5 CFR 9701.321 - Structure of bands.
Code of Federal Regulations, 2014 CFR
2014-01-01
... 5 Administrative Personnel 3 2014-01-01 2014-01-01 false Structure of bands. 9701.321 Section 9701.321 Administrative Personnel DEPARTMENT OF HOMELAND SECURITY HUMAN RESOURCES MANAGEMENT SYSTEM... Structure of bands. (a) DHS may, after coordination with OPM, establish ranges of basic pay for bands, with...
A Theoretical Structure of High School Concert Band Performance
ERIC Educational Resources Information Center
Bergee, Martin J.
2015-01-01
This study used exploratory (EFA) and confirmatory factor analysis (CFA) to verify a theoretical structure for high school concert band performance and to test that structure for viability, generality, and invariance. A total of 101 university students enrolled in two different bands rated two high school band performances (a "first"…
A Theoretical Structure of High School Concert Band Performance
ERIC Educational Resources Information Center
Bergee, Martin J.
2015-01-01
This study used exploratory (EFA) and confirmatory factor analysis (CFA) to verify a theoretical structure for high school concert band performance and to test that structure for viability, generality, and invariance. A total of 101 university students enrolled in two different bands rated two high school band performances (a "first"…
Achieving Higher Energies via Passively Driven X-band Structures
NASA Astrophysics Data System (ADS)
Sipahi, Taylan; Sipahi, Nihan; Milton, Stephen; Biedron, Sandra
2014-03-01
Due to their higher intrinsic shunt impedance X-band accelerating structures significant gradients with relatively modest input powers, and this can lead to more compact particle accelerators. At the Colorado State University Accelerator Laboratory (CSUAL) we would like to adapt this technology to our 1.3 GHz L-band accelerator system using a passively driven 11.7 GHz traveling wave X-band configuration that capitalizes on the high shunt impedances achievable in X-band accelerating structures in order to increase our overall beam energy in a manner that does not require investment in an expensive, custom, high-power X-band klystron system. Here we provide the design details of the X-band structures that will allow us to achieve our goal of reaching the maximum practical net potential across the X-band accelerating structure while driven solely by the beam from the L-band system.
NASA Technical Reports Server (NTRS)
Solomon, S. C.; Head, J. W.
1984-01-01
Two classes of tectonic models for the formation of band structure on Venus mountain ranges are quantitatively evaluated: folding models and extensional models. The characteristics of banded terrain on Ishtar Terra are reviewed, particularly the evidence for a tectonic origin and such important details of band geometry as the spacing between adjacent bands. The tectonic models are tested against these observed features, and it it shown that both classes of models are possible explanations of banded terrain as long as the outer elastic-brittle layer of the Venus crust is at most a few kilometers thick. A general assessment is given of hypotheses for the origin of banded terrain and of the relationship between mechanical models for band formation and the thermal structure of the Venus lithosphere.
Birefringence and band structure of CdP2 crystals
NASA Astrophysics Data System (ADS)
Beril, S. I.; Stamov, I. G.; Syrbu, N. N.; Zalamai, V. V.
2013-08-01
The spatial dispersion in CdP2 crystals was investigated. The dispersion is positive (nk||с>nk||у) at λ>λ0 and negative (nk||с
Negative radiation pressure on gain medium structures.
Mizrahi, Amit; Fainman, Yeshaiahu
2010-10-15
We demonstrate negative radiation pressure on gain medium structures, such that light amplification may cause a nanoscale body to be pulled toward a light source. Optically large gain medium structures, such as slabs and spheres, as well as deep subwavelength bodies, may experience this phenomenon. The threshold gain for radiation pressure reversal is obtained analytically for Rayleigh spheres, thin cylinders, and thin slabs. This threshold vanishes when the gain medium structure is surrounded by a medium with a matched refractive index, thus eliminating the positive scattering forces.
Theoretical study of band structure of odd-mass 115,117I isotopes
NASA Astrophysics Data System (ADS)
Singh, Dhanvir; Kumar, Amit; Sharma, Chetan; Singh, Suram; Bharti, Arun
2016-05-01
By using the microscopic approach of Projected Shell Model (PSM), negative-parity band structures of odd mass neutron-rich 115,117I nuclei have been studied with the deformed single-particle states generated by the standard Nilsson potential. For these isotopes, the band structures have been analyzed in terms of quasi-particles configurations. The phenomenon of back bending in moment of inertia is also studied in the present work.
Theoretical study of band structure of odd-mass {sup 115,117}I isotopes
Singh, Dhanvir Kumar, Amit Sharma, Chetan; Singh, Suram; Bharti, Arun
2016-05-06
By using the microscopic approach of Projected Shell Model (PSM), negative-parity band structures of odd mass neutron-rich {sup 115,117}I nuclei have been studied with the deformed single-particle states generated by the standard Nilsson potential. For these isotopes, the band structures have been analyzed in terms of quasi-particles configurations. The phenomenon of back bending in moment of inertia is also studied in the present work.
The first-negative ( B2Σ +→ X2Σ +) band system in 13C16O+: further investigations
NASA Astrophysics Data System (ADS)
Kępa, R.; Malak, Z.; Szajna, W.; Zachwieja, M.
2003-07-01
Ten new 0-4, 1-0, 1-1, 3-3, 3-5, 3-6, 3-7, 4-6, 4-7, and 5-8 bands of the first-negative ( B2Σ +→ X2Σ +) system of 13C16O+ were observed and their rotational structure was analyzed for the first time. These bands were recorded by means of classical spectrographical methods under high resolution. This allowed one to observe a spin splitting in most lines of the recorded bands. The effective Hamiltonian of Brown with the matrix elements for the 2Σ+ states taken from a work by Amiot et al. [J. Mol. Spectrosc. 87 (1981) 196-218] has been used to obtain the rovibronic structure parameters of the individual bands from the observed wavenumbers of lines. The molecular constants obtained in this way were then merged with those derived by us earlier. For this purpose the least-squares merge procedure was used. This made it possible to obtain a wide set of precise constants of the rotational structure in 15 vibrational levels, for the X 2Σ+ (v=0-8) ground state and for the second B2Σ +( v=0-5) excited state in 13C16O+. On the basis of these final constants it was possible to calculate the equilibrium molecular constants, and the RKR potential curves for the X2Σ + and B2Σ + electronic states as well as Franck-Condon factors and r-centroids for the first-negative bands in 13C16O+.
Simultaneous description of low-lying positive and negative parity bands in heavy even-even nuclei
NASA Astrophysics Data System (ADS)
Ganev, H. G.
2014-05-01
The low-lying spectra including the first few excited positive and negative parity bands of some heavy even-even nuclei from the rare earth and actinide mass regions are investigated within the framework of the symplectic interacting vector boson model with the Sp(12,R) dynamical symmetry group. Symplectic dynamical symmetries allow the change of the number of excitation quanta or phonons building the collective states, providing for larger representation spaces and richer subalgebraic structures to incorporate more complex nuclear spectra. The theoretical predictions for the energy levels and the electromagnetic transitions between the collective states of the ground-state band and Kπ=0- band are compared with experiment and some other collective models incorporating octupole and/or dipole degrees of freedom. The energy staggering, which is a sensitive indicator of the octupole correlations in even-even nuclei, is also calculated and compared with experiment. The results obtained for the energy levels, energy staggering, and transition strengths reveal the relevance of the dynamical symmetry used in the model to simultaneously describe both positive and negative parity low-lying collective bands.
Half-filled energy bands induced negative differential resistance in nitrogen-doped graphene.
Li, Xiao-Fei; Lian, Ke-Yan; Qiu, Qi; Luo, Yi
2015-03-07
Nitrogen-doping brings novel properties and promising applications into graphene, but the underlying mechanism is still in debate. To determine the key factor in motivating the negative differential resistance (NDR) behaviour of nitrogen-doped graphene, the electronic structure and transport properties of an 11-dimer wide nitrogen-doped armchair graphene nanoribbon (N-AGNR) were systematically studied by first principles calculations. Both the effect of interaction between N-dopants and the effect of doping-sublattice on the NDR were examined for the first time. Taking into account the two effects, N-AGNR becomes metallic or semiconducting depending on the doping configuration, and its Fermi level varies in a large range. NDR was firmly verified not to be intrinsic for N-AGNRs. However, it is totally determined by whether nitrogen-doping induces half-filled energy bands (HFEBs) because it is HFEBs that cross the Fermi level and determine the transport properties of N-AGNR under low biases. With the bias increasing, the transmission spectrum near the Fermi level showed a flag shape, and therefore, the corresponding transport channel is totally suppressed at a certain bias, resulting in the NDR behaviour with a configuration-dependent peak-to-valley current ratio (PVCR) up to 10(4). Our findings give new insights into the microscopic mechanism of chemical doping induced NDR behaviour and will be useful in building NDR-based nanodevices in the future.
Analysis of the N(+/2) first negative band system in the Earth's upper atmosphere dayglow
NASA Astrophysics Data System (ADS)
Stone, Thomas Coleman
The First Negative (1N) band emission of the molecular nitrogen ion, N+2 , is one of the most prominent features of the terrestrial dayglow spectrum. However, past N+2 studies have encountered problems in validating the intensity of this emission. Also, some anomalous characteristics of the dayglow 1N spectrum remain unexplained, such as a highly developed rotational and vibrational structure. These anomalies appear to be due to the charge exchange reaction: O+ + N2 --> N+2 + O, which dominates N+2 ion production at high altitudes. This thesis examines dayglow 1N spectra acquired by the Arizona Airglow Experiment (GLO) flown on the space shuttle mission STS- 74. In the analysis the emission is separated into two components. First is the emission from ions produced by photoionization and electron bombardment. Second is emission from ions produced by the charge exchange reaction, which cannot be modeled. The first source is evaluated and subtracted from the observed spectrum. The remaining emission is then used to derive empirical parameters related to the charge exchange reaction. These parameters can be used to estimate the 1N emission rate expected from the thermosphere, based on model atmosphere predictions. This emission rate can be used to determine the dayside 0+ concentration using the GLO observations.
The energy band structure of Si and Ge nanolayers
NASA Astrophysics Data System (ADS)
Wu, Xueke; Huang, Weiqi; Huang, Zhongmei; Qin, Chaojie; Tang, Yanlin
2016-12-01
First-principles calculation based on density functional theory (DFT) with the generalized gradient approximation (GGA) were carried out to investigate the energy band gap structure of Si and Ge nanofilms. Calculation results show that the band gaps of Si(111) and Ge(110) nanofilms are indirect structures and independent of film thickness, the band gaps of Si(110) and Ge(100) nanofilms could be transfered into the direct structure for nanofilm thickness of less than a certain value, and the band gaps of Si(100) and Ge(111) nanofilms are the direct structures in the present model thickness range (about 7 nm). Moreover, the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects. It will be a good way to obtain direct band gap emission in Si and Ge materials, and to develop Si and Ge laser on Si chip.
Engineering the Electronic Band Structure for Multiband Solar Cells
Lopez, N.; Reichertz, L.A.; Yu, K.M.; Campman, K.; Walukiewicz, W.
2010-07-12
Using the unique features of the electronic band structure of GaNxAs1-x alloys, we have designed, fabricated and tested a multiband photovoltaic device. The device demonstrates an optical activity of three energy bands that absorb, and convert into electrical current, the crucial part of the solar spectrum. The performance of the device and measurements of electroluminescence, quantum efficiency and photomodulated reflectivity are analyzed in terms of the Band Anticrossing model of the electronic structure of highly mismatched alloys. The results demonstrate the feasibility of using highly mismatched alloys to engineer the semiconductor energy band structure for specific device applications.
Engineering the electronic band structure for multiband solar cells.
López, N; Reichertz, L A; Yu, K M; Campman, K; Walukiewicz, W
2011-01-14
Using the unique features of the electronic band structure of GaN(x)As(1-x) alloys, we have designed, fabricated and tested a multiband photovoltaic device. The device demonstrates an optical activity of three energy bands that absorb, and convert into electrical current, the crucial part of the solar spectrum. The performance of the device and measurements of electroluminescence, quantum efficiency and photomodulated reflectivity are analyzed in terms of the band anticrossing model of the electronic structure of highly mismatched alloys. The results demonstrate the feasibility of using highly mismatched alloys to engineer the semiconductor energy band structure for specific device applications.
Experimental Studies of Band-Structure Properties in Bloch Transistors
NASA Astrophysics Data System (ADS)
Flees, Daniel J.
1998-03-01
One of the most striking features in small SIS tunnel junctions is the energy-band structure produced by Josephson coupling and charging effects. These energy bands are analogous to Bloch bands in crystalline solids. The superconducting single-electron (Bloch) transistor is the simplest system in which the energy bands can be readily studied. It consists of a superconducting island coupled to a source and drain through two small tunnel junctions. The elastic tunneling of Cooper-Pairs onto the island mixes the discrete charge states of the island. The shapes of the resulting energy bands can be modified by changing the electrostatic energies of these charge states with a voltage applied to a capacitively coupled gate. The maximum zero-voltage current (supercurrent) of each band depends upon the shape of the band and so the gate modulates the supercurrent. Each band has a different characteristic supercurrent modulation, with excited bands generally having lower currents. Thus! we can use the reduction in super current associated with a transition to an excited band to begin probing band- structure properties such as the band-gap.(Daniel J. Flees, Siyuan Han, and J.E. Lukens, Phys. Rev. Lett. 78), 4817 (1997).
Vorticity structuring and velocity rolls triggered by gradient shear bands.
Fielding, Suzanne M
2007-07-01
We suggest a mechanism by which vorticity structuring and velocity rolls can form in complex fluids, triggered by the linear instability of one-dimensional gradient shear banded flow. We support this with a numerical study of the diffusive Johnson-Segalman model. In the steady vorticity structured state, the thickness of the interface between the bands remains finite in the limit of zero stress diffusivity, presenting a possible challenge to the accepted theory of shear banding.
Universality of Mallmann correlations for nuclear band structures
NASA Astrophysics Data System (ADS)
Bucurescu, D.; Zamfir, N. V.; Cǎta-Danil, G.; Ivaşcu, M.; Mǎrginean, N.
2008-11-01
It is shown that the Mallmann's energy ratio correlations, first time proposed 50 years ago for the ground state bands of the even-even nuclei, are universal: all band structures in collective nuclei obey the same systematics. Based on a second order anharmonic vibrator description, parameter-free recurrence relations are proposed for Mallmann-type energy ratios, which can be used to extrapolate band structures to higher spin.
Bi-directional evolutionary optimization for photonic band gap structures
Meng, Fei; Huang, Xiaodong; Jia, Baohua
2015-12-01
Toward an efficient and easy-implement optimization for photonic band gap structures, this paper extends the bi-directional evolutionary structural optimization (BESO) method for maximizing photonic band gaps. Photonic crystals are assumed to be periodically composed of two dielectric materials with the different permittivity. Based on the finite element analysis and sensitivity analysis, BESO starts from a simple initial design without any band gap and gradually re-distributes dielectric materials within the unit cell so that the resulting photonic crystal possesses a maximum band gap between two specified adjacent bands. Numerical examples demonstrated the proposed optimization algorithm can successfully obtain the band gaps from the first to the tenth band for both transverse magnetic and electric polarizations. Some optimized photonic crystals exhibit novel patterns markedly different from traditional designs of photonic crystals.
Weak morphology dependent valence band structure of boron nitride
NASA Astrophysics Data System (ADS)
Zhi, Chunyi; Ueda, Shigenori; Zeng, Haibo; Wang, Xuebin; Tian, Wei; Wang, Xi; Bando, Yoshio; Golberg, Dmitri
2013-08-01
We report a hard X-ray photoelectron spectroscopy (HX-PES) investigation on valence band structure of Boron Nitrides (BN) having different morphologies, including nanosheets, nanotubes, and micro-sized particles. Very weak morphology/valence band structure dependence was observed. For each case, the B-N π-band overlapping with σ-band between 0 to -12.5 eV and the s-band below -15 eV were identified. No obvious morphology-induced band shifts and intensity variations were observed. First-principles calculations based on density functional theory were performed and the results were compared with the experimental data. This theoretical analysis well explains the weak morphology dependent valence band spectra of BN nanomaterials obtained during HX-PES measurements.
Topological structure of the inter-band phase difference soliton in two-band superconductivity
NASA Astrophysics Data System (ADS)
Tanaka, Y.; Iyo, A.; Tokiwa, K.; Watanabe, T.; Crisan, A.; Sundaresan, A.; Terada, N.
2010-11-01
Two-component superconductivity based on the two-band superconductor has a functional topology such as an inter-band phase difference soliton (i-soliton) to realize topological electronics (topolonics). Many gauge field theories are applied to investigate the topology of two-band superconductivity. To ease experimental and electronics applications, these theories should be refined. Weinberg-Salam theory and SU(2) (two-dimensional special unitary symmetry) gauge field theory are proper starting points. An effective extra force field because of the crystal structure and inter-band Josephson interaction, rather than spontaneous symmetry breaking, simplifies the conventional gauge field theory.
NASA Astrophysics Data System (ADS)
Ryan, Colan Graeme Matthew
Focused on the quad-band generalized negative-refractive-index transmission line (G-NRI-TL), this thesis presents a variety of novel printed G-NRI-TL multi-band microwave device and antenna prototypes. A dual-band coupled-line coupler, an all-pass G-NRI-TL bridged-T circuit, a dual-band metamaterial leaky-wave antenna, and a multi-band G-NRI-TL resonant antenna are all new developments resulting from this research. In addition, to continue the theme of multi-band components, negative-refractive-index transmission lines are used to create a dual-band circularly polarized transparent patch antenna and a two-element wideband decoupled meander antenna system. High coupling over two independently-specified frequency bands is the hallmark of the G-NRI-TL coupler: it is 0.35lambda0 long but achieves approximately -3 dB coupling over both bands with a maximum insertion loss of 1 dB. This represents greater design flexibility than conventional coupled-line couplers and less loss than subsequent G-NRI-TL couplers. The single-ended bridged-T G-NRI-TL offers a metamaterial unit cell with an all-pass magnitude response up to 8 GHz, while still preserving the quad-band phase response of the original circuit. It is shown how the all-pass response leads to wider bandwidths and improved matching in quad-band inverters, power dividers, and hybrid couplers. The dual-band metamaterial leaky-wave antenna presented here was the first to be reported in the literature, and it allows broadside radiation at both 2 GHz and 6 GHz without experiencing the broadside stopband common to conventional periodic antennas. Likewise, the G-NRI-TL resonant antenna is the first reported instance of such a device, achieving quad-band operation between 2.5 GHz and 5.6 GHz, with a minimum radiation efficiency of 80%. Negative-refractive-index transmission line loading is applied to two devices: an NRI-TL meander antenna achieves a measured 52% impedance bandwidth, while a square patch antenna incorporates
Band structure controlled by chiral imprinting
NASA Astrophysics Data System (ADS)
Castro-Garay, P.; Adrian Reyes, J.; Ramos-Garcia, R.
2007-09-01
Using the configuration of an imprinted cholesteric elastomer immersed in a racemic solvent, the authors find the solution of the boundary-value problem for the reflection and transmission of incident optical waves due to the elastomer. They show a significant width reduction of the reflection band for certain values of nematic penetration depth, which depends on the volume fraction of molecules from the solvent, whose handedness is preferably absorbed. The appearance of nested band gaps of both handednesses during the sorting mixed chiral process is also obtained. This suggests the design of chemically controlled optical filters and optically monitored chiral pumps.
Calculation of the band structure of polyguanilic acid in the presence of water and Na+ ions
NASA Astrophysics Data System (ADS)
Ladik, J.; Bende, A.; Bogar, F.
2007-08-01
Using the Hartree-Fock crystal orbital method with a combined symmetry (helix) operation, the band structure of polyguanilic acid was calculated in the presence of water and Na+ ions. The water structure was optimized with the help of molecular mechanics.The obtained band structure shows that both the valence and conduction bands are purely guanine type. The three impurity bands in the 10.66eV large gap are close to the conduction band and therefore cannot play any role in the assumed hole conduction of the system. Namely, according to detailed x-ray diffraction investigations of the nucleosomes in chromatin, there are possibilities of charge transfer from the negative sites of DNA to the positive ones in histones. Therefore most probably there is a hole conduction in DNA and an electronic one in the histone proteins.
Tunable dual-band negative refractive index in ferrite-based metamaterials.
Bi, Ke; Zhou, Ji; Zhao, Hongjie; Liu, Xiaoming; Lan, Chuwen
2013-05-06
A tunable dual-band ferrite-based metamaterial has been investigated by experiments and simulations. The negative permeability is realized around the ferromagnetic resonance (FMR) frequency which can be influenced by the dimension of the ferrites. Due to having two negative permeability frequency regions around the two FMR frequencies, the metamaterials consisting of metallic wires and ferrite rods with different sizes possess two passbands in the transmission spectra. The microwave transmission properties of the ferrite-based metamaterials can be not only tuned by the applied magnetic field, but also adjusted by the dimension of the ferrite rods. A good agreement between experimental and simulated results is demonstrated, which confirms that the tunable dual-band ferrite-based metamaterials can be used for cloaks, antennas and absorbers.
Band warping, band non-parabolicity, and Dirac points in electronic and lattice structures
NASA Astrophysics Data System (ADS)
Resca, Lorenzo; Mecholsky, Nicholas A.; Pegg, Ian L.
2017-10-01
We illustrate at a fundamental level the physical and mathematical origins of band warping and band non-parabolicity in electronic and vibrational structures. We point out a robust presence of pairs of topologically induced Dirac points in a primitive-rectangular lattice using a p-type tight-binding approximation. We analyze two-dimensional primitive-rectangular and square Bravais lattices with implications that are expected to generalize to more complex structures. Band warping is shown to arise at the onset of a singular transition to a crystal lattice with a larger symmetry group, which allows the possibility of irreducible representations of higher dimensions, hence band degeneracy, at special symmetry points in reciprocal space. Band warping is incompatible with a multi-dimensional Taylor series expansion, whereas band non-parabolicities are associated with multi-dimensional Taylor series expansions to all orders. Still band non-parabolicities may merge into band warping at the onset of a larger symmetry group. Remarkably, while still maintaining a clear connection with that merging, band non-parabolicities may produce pairs of conical intersections at relatively low-symmetry points. Apparently, such conical intersections are robustly maintained by global topology requirements, rather than any local symmetry protection. For two p-type tight-binding bands, we find such pairs of conical intersections drifting along the edges of restricted Brillouin zones of primitive-rectangular Bravais lattices as lattice constants vary relatively to each other, until these conical intersections merge into degenerate warped bands at high-symmetry points at the onset of a square lattice. The conical intersections that we found appear to have similar topological characteristics as Dirac points extensively studied in graphene and other topological insulators, even though our conical intersections have none of the symmetry complexity and protection afforded by the latter more
Gao, Li; Shigeta, Kazuki; Vazquez-Guardado, Abraham; Progler, Christopher J; Bogart, Gregory R; Rogers, John A; Chanda, Debashis
2014-06-24
We report advances in materials, designs, and fabrication schemes for large-area negative index metamaterials (NIMs) in multilayer "fishnet" layouts that offer negative index behavior at wavelengths into the visible regime. A simple nanoimprinting scheme capable of implementation using standard, widely available tools followed by a subtractive, physical liftoff step provides an enabling route for the fabrication. Computational analysis of reflection and transmission measurements suggests that the resulting structures offer negative index of refraction that spans both the visible wavelength range (529-720 nm) and the telecommunication band (1.35-1.6 μm). The data reveal that these large (>75 cm(2)) imprinted NIMs have predictable behaviors, good spatial uniformity in properties, and figures of merit as high as 4.3 in the visible range.
Strain-driven electronic band structure modulation of si nanowires.
Hong, Ki-Ha; Kim, Jongseob; Lee, Sung-Hoon; Shin, Jai Kwang
2008-05-01
One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.
Transition probabilities and Franck-Condon factors for the second negative band system of O2(+)
NASA Technical Reports Server (NTRS)
Fox, J. L.; Dalgarno, A.
1990-01-01
Transition probabilities for the second negative band system of O2(+) are computed using the dipole transition moment presented by Wetmore et al. (1984). Vibrational levels v double prime = 0 - 54 of the X2Pi(g) ground state and v prime = - 33 of the excited A2Pi(u) state are included. Franck-Condon factors for ionization-excitation of O2 to O2(+) are also presented.
Formation of Deformation Band Structures Normal to the Shear Plane
NASA Astrophysics Data System (ADS)
Balasko, C. M.; Schultz, R. A.
2001-12-01
Recent fieldwork has shown that many of the geometric relationships previously observed in cataclastic deformation bands are based on a two dimensional view of a three dimensional structure. When the bands are viewed in the shear direction lenses and inosculating bands are seen, as first noted by Aydin [1978] and later by many others. When viewed normal to the shear direction "ladder structures" (linked mode II echelon stepovers or duplexes) and conjugate structures are seen [Davis, 1999]. Davis [1999] grouped these two geometries as different classes of deformation bands and many have interpreted the duplexes as strain localization in Riedel shear zones. Here we demonstrate an alternative explanation. When viewed normal to the shear direction, deformation bands display either mutually crosscutting ("conjugate") sets or contractional stepovers ("duplexes") between interacting mode-II bands. Our field observations reveal that the principal difference between the two is the relative distance separating the parallel echelon bands. Bands that are sufficiently close interact mechanically to promote linking bands within the stepovers. These linking bands are younger than the bounding echelon bands. Because the orientations of bands within the stepovers are the same as that of "conjugate" bands that are more widely spaced, we infer that the magnitude of stress within the stepovers was increased somewhat over background values, but that stress rotations within the stepovers were negligible. To accomplish this, the echelon deformation bands must be strong relative to the host rock and accommodate only small offsets, leading to only minor perturbations of the local stress state in their vicinity. Ladder structure, "radiator rock" [Davis, 1999], and linked echelon stepovers demonstrate a progression from distributed to more localized strain of the sandstone, with the scale dependent on the size and offset magnitude of individual bands. Occurrence of linked echelon stepovers
Band structures in the nematic elastomers phononic crystals
NASA Astrophysics Data System (ADS)
Yang, Shuai; Liu, Ying; Liang, Tianshu
2017-02-01
As one kind of new intelligent materials, nematic elastomers (NEs) represent an exciting physical system that combines the local orientational symmetry breaking and the entropic rubber elasticity, producing a number of unique physical phenomena. In this paper, the potential application of NEs in the band tuning is explored. The band structures in two kinds of NE phononic crystals (PCs) are investigated. Through changing NE intrinsic parameters, the influence of the porosity, director rotation and relaxation on the band structures in NE PCs are analyzed. This work is a meaningful try for application of NEs in acoustic field and proposes a new intelligent strategy in band turning.
New Band Structures in Aapprox110 Neutron-Rich Nuclei
Zhu, S. J.; Wang, J. G.; Ding, H. B.; Gu, L.; Xu, Q.; Yeoh, E. Y.; Xiao, Z. G.; Hamilton, J. H.; Ramayya, A. V.; Hwang, J. K.; Liu, S. H.; Li, K.; Luo, Y. X.; Rasmussen, J. O.; Lee, I. Y.; Qi, B.; Meng, J.
2010-05-12
The high spin states of neutron-rich nuclei in Aapprox110 region have been carefully investigated by measuring prompt gamma-gamma-gamma coincident measurements populated in the spontaneous fission of {sup 252}Cf with the Gammasphere detector array. Many new collective bands have been discovered. In this proceeding paper, we introduce some interesting new band structures recently observed by our cooperative groups, that is, the one-phonon- and two-phonon gamma-vibrational bands in odd-A {sup 103}Nb, {sup 105}Mo and {sup 107}Tc, the chiral doublet bands in even-even {sup 106}Mo, {sup 110}Ru and {sup 112}Ru, and the pseudospin partner bands with in {sup 108}Tc. The characteristics of these band structures have been discussed.
Universality of Mallmann correlations for nuclear band structures
NASA Astrophysics Data System (ADS)
Bucurescu, D.; Zamfir, N. V.; Căta-Danil, G.; Ivaşcu, M.; Mărginean, N.
2008-10-01
It is shown that the Mallmann's energy ratio correlations, for the first time observed for the ground state band of the even-even nuclei, are universal: various band structures in all collective nuclei obey the same systematics, and consequently the same spin dependence. Based on a second order anharmonic vibrator description, parameter-free recurrence relations between Mallmann-type energy ratios are deduced, which can be used to extrapolate bands to higher spin.
Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon
Zhu, Zhen; Shao, Hezhu; Dong, Xiao; Li, Ning; Ning, Bo-Yuan; Ning, Xi-Jing; Zhao, Li; Zhuang, Jun
2015-01-01
We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing. PMID:26012369
Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon.
Zhu, Zhen; Shao, Hezhu; Dong, Xiao; Li, Ning; Ning, Bo-Yuan; Ning, Xi-Jing; Zhao, Li; Zhuang, Jun
2015-05-27
We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.
Chaos in the band structure of a soft Sinai lattice
NASA Astrophysics Data System (ADS)
Porter, Max D.; Barr, Aaron; Barr, Ariel; Reichl, L. E.
2017-05-01
We study the effect of broken spatial and dynamical symmetries on the band structure of two lattices with unit cells that are soft versions of the classic Sinai billiard. We find significant signatures of chaos in the band structure of these lattices, in energy regimes where the underlying classical unit cell undergoes a transition to chaos. Broken dynamical symmetries and the presence of chaos can diminish the feasibility of changing and controlling band structure in a wide variety of two-dimensional lattice-based devices, including two-dimensional solids, optical lattices, and photonic crystals.
Shell model description of band structure in 48Cr
Vargas, Carlos E.; Velazquez, Victor M.
2007-02-12
The band structure for normal and abnormal parity bands in 48Cr are described using the m-scheme shell model. In addition to full fp-shell, two particles in the 1d3/2 orbital are allowed in order to describe intruder states. The interaction includes fp-, sd- and mixed matrix elements.
NASA Astrophysics Data System (ADS)
Ababil, M. H.; Saimoom, M. F.; Chowdhury, S. M. H.; Mahdy, M. R. C.; Matin, M. A.
Based on the idea of additional modified mode(s) (Mahdy et al., IEEE Antennas Wirel Propag Lett 10:869-872, 2011), metamaterial-loaded triple-band rectangular patch antennas have been reported in (Mahdy et al., Prog Electromagn Res Lett 21:99-107, 2011). But the idea of additional modified mode(s) to achieve multiband performance in circular patch antenna has not been reported so far. Recently we have reported the idea of "additional modified mode(s)" in circular shaped patch antennas loaded with metamaterials to achieve multiband performance (Ferdous et al., IET Microw Antennas Propag J 7:768-776, 2013). On the basis of the design algorithm reported in (Ferdous et al., IET Microw Antennas Propag J 7:768-776, 2013), in this chapter, a triple-band circular patch antenna loaded with ENG metamaterial has been proposed. The proposed antenna not only provides good resonance, but also ensures satisfactory radiation performances (directivity, radiation efficiency, and gain) for all the three bands. Achieving a triple-band performance has been possible by modifying TM δ10(1 < δ < 2) mode (using ɛ negative metamaterial) along with TM210 mode modification (due to symmetrical slotting). It is expected that this sort of antenna will be really effective in multiband highly directive applications, especially in satellite communication.
Transport in bilayer and trilayer graphene: band gap engineering and band structure tuning
NASA Astrophysics Data System (ADS)
Zhu, Jun
2014-03-01
Controlling the stacking order of atomically thin 2D materials offers a powerful tool to control their properties. Linearly dispersed bands become hyperbolic in Bernal (AB) stacked bilayer graphene (BLG). Both Bernal (ABA) and rhombohedral (ABC) stacking occur in trilayer graphene (TLG), producing distinct band structures and electronic properties. A symmetry-breaking electric field perpendicular to the sample plane can further modify the band structures of BLG and TLG. In this talk, I will describe our experimental effort in these directions using dual-gated devices. Using thin HfO2 film deposited by ALD as gate dielectric, we are able to apply large displacement fields D > 6 V/nm and observe the opening and saturation of the field-induced band gap Eg in bilayer and ABC-stacked trilayer graphene, where the conduction in the mid gap changes by more than six decades. Its field and temperature dependence highlights the crucial role played by Coulomb disorder in facilitating hopping conduction and suppressing the effect of Eg in the tens of meV regime. In contrast, mid-gap conduction decreases with increasing D much more rapidly in clean h-BN dual-gated devices. Our studies also show the evolution of the band structure in ABA-stacked TLG, in particular the splitting of the Dirac-like bands in large D field and the signatures of two-band transport at high carrier densities. Comparison to theory reveals the need for more sophisticated treatment of electronic screening beyond self-consistent Hartree calculations to accurately predict the band structures of trilayer graphene and graphenic materials in general.
Unfolding the band structure of non-crystalline photonic band gap materials
NASA Astrophysics Data System (ADS)
Tsitrin, Samuel; Williamson, Eric Paul; Amoah, Timothy; Nahal, Geev; Chan, Ho Leung; Florescu, Marian; Man, Weining
2015-08-01
Non-crystalline photonic band gap (PBG) materials have received increasing attention, and sizeable PBGs have been reported in quasi-crystalline structures and, more recently, in disordered structures. Band structure calculations for periodic structures produce accurate dispersion relations, which determine group velocities, dispersion, density of states and iso-frequency surfaces, and are used to predict a wide-range of optical phenomena including light propagation, excited-state decay rates, temporal broadening or compression of ultrashort pulses and complex refraction phenomena. However, band calculations for non-periodic structures employ large super-cells of hundreds to thousands building blocks, and provide little useful information other than the PBG central frequency and width. Using stereolithography, we construct cm-scale disordered PBG materials and perform microwave transmission measurements, as well as finite-difference time-domain (FDTD) simulations. The photonic dispersion relations are reconstructed from the measured and simulated phase data. Our results demonstrate the existence of sizeable PBGs in these disordered structures and provide detailed information of the effective band diagrams, dispersion relation, iso-frequency contours, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study introduces a powerful tool to investigate photonic properties of non-crystalline structures and provides important effective dispersion information, otherwise difficult to obtain.
Unfolding the band structure of non-crystalline photonic band gap materials.
Tsitrin, Samuel; Williamson, Eric Paul; Amoah, Timothy; Nahal, Geev; Chan, Ho Leung; Florescu, Marian; Man, Weining
2015-08-20
Non-crystalline photonic band gap (PBG) materials have received increasing attention, and sizeable PBGs have been reported in quasi-crystalline structures and, more recently, in disordered structures. Band structure calculations for periodic structures produce accurate dispersion relations, which determine group velocities, dispersion, density of states and iso-frequency surfaces, and are used to predict a wide-range of optical phenomena including light propagation, excited-state decay rates, temporal broadening or compression of ultrashort pulses and complex refraction phenomena. However, band calculations for non-periodic structures employ large super-cells of hundreds to thousands building blocks, and provide little useful information other than the PBG central frequency and width. Using stereolithography, we construct cm-scale disordered PBG materials and perform microwave transmission measurements, as well as finite-difference time-domain (FDTD) simulations. The photonic dispersion relations are reconstructed from the measured and simulated phase data. Our results demonstrate the existence of sizeable PBGs in these disordered structures and provide detailed information of the effective band diagrams, dispersion relation, iso-frequency contours, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study introduces a powerful tool to investigate photonic properties of non-crystalline structures and provides important effective dispersion information, otherwise difficult to obtain.
Unfolding the band structure of non-crystalline photonic band gap materials
Tsitrin, Samuel; Williamson, Eric Paul; Amoah, Timothy; Nahal, Geev; Chan, Ho Leung; Florescu, Marian; Man, Weining
2015-01-01
Non-crystalline photonic band gap (PBG) materials have received increasing attention, and sizeable PBGs have been reported in quasi-crystalline structures and, more recently, in disordered structures. Band structure calculations for periodic structures produce accurate dispersion relations, which determine group velocities, dispersion, density of states and iso-frequency surfaces, and are used to predict a wide-range of optical phenomena including light propagation, excited-state decay rates, temporal broadening or compression of ultrashort pulses and complex refraction phenomena. However, band calculations for non-periodic structures employ large super-cells of hundreds to thousands building blocks, and provide little useful information other than the PBG central frequency and width. Using stereolithography, we construct cm-scale disordered PBG materials and perform microwave transmission measurements, as well as finite-difference time-domain (FDTD) simulations. The photonic dispersion relations are reconstructed from the measured and simulated phase data. Our results demonstrate the existence of sizeable PBGs in these disordered structures and provide detailed information of the effective band diagrams, dispersion relation, iso-frequency contours, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study introduces a powerful tool to investigate photonic properties of non-crystalline structures and provides important effective dispersion information, otherwise difficult to obtain. PMID:26289434
Band structure in Yang-Mills theories
NASA Astrophysics Data System (ADS)
Bachas, Constantin; Tomaras, Theodore
2016-05-01
We show how Yang-Mills theory on S3 × ℝ can exhibit a spectrum with continuous bands if coupled either to a topological 3-form gauge field, or to a dynamical axion with heavy Peccei-Quinn scale. The basic mechanism consists in associating winding histories to a bosonic zero mode whose role is to convert a circle in configuration space into a helix. The zero mode is, respectively, the holonomy of the 3-form field or the axion momentum. In these models different θ sectors coexist and are only mixed by (non-local) volume operators. Our analysis sheds light on, and extends Seiberg's proposal for modifying the topological sums in quantum field theories. It refutes a recent claim that B + L violation at LHC is unsuppressed.
Band structure peculiarities of magnetic photonic crystals
NASA Astrophysics Data System (ADS)
Gevorgyan, A. H.; Golik, S. S.
2017-10-01
In this work we studied light diffraction in magneto-photonic crystals (MPC) having large magneto-optical activity and modulation large depth. The case of arbitrary angles between the direction of the external static magnetic field and the normal to the border of the MPC layer is considered. The problem is solved by Ambartsumian's modified layer addition method. It is found that there is a new type of non-reciprocity, namely, the relation R (α) ≠ R (- α) takes place, where R is the reflection coefficient, and α is the incidence angle. It is shown the formation of new photonic band gap (PBG) at oblique incidence of light, which is not selective for the polarization of the incident light, in the case when the external magnetic field is directed along the medium axis. Such a system can be used as: a tunable polarization filter, polarization mirror, circular (elliptical) polarizer, tunable optical diode, etc.
Unfolding the band structure of GaAsBi
NASA Astrophysics Data System (ADS)
Maspero, R.; Sweeney, S. J.; Florescu, Marian
2017-02-01
Typical supercell approaches used to investigate the electronic properties of GaAs(1-x)Bi(x) produce highly accurate, but folded, band structures. Using a highly optimized algorithm, we unfold the band structure to an approximate E≤ft(\\mathbf{k}\\right) relation associated with an effective Brillouin zone. The dispersion relations we generate correlate strongly with experimental results, confirming that a regime of band gap energy greater than the spin-orbit-splitting energy is reached at around 10% bismuth fraction. We also demonstrate the effectiveness of the unfolding algorithm throughout the Brillouin zone (BZ), which is key to enabling transition rate calculations, such as Auger recombination rates. Finally, we show the effect of disorder on the effective masses and identify approximate values for the effective mass of the conduction band and valence bands for bismuth concentrations from 0-12%.
Locally resonant periodic structures with low-frequency band gaps
NASA Astrophysics Data System (ADS)
Cheng, Zhibao; Shi, Zhifei; Mo, Y. L.; Xiang, Hongjun
2013-07-01
Presented in this paper are study results of dispersion relationships of periodic structures composited of concrete and rubber, from which the frequency band gap can be found. Two models with fixed or free boundary conditions are proposed to approximate the bound frequencies of the first band gap. Studies are conducted to investigate the low-frequency and directional frequency band gaps for their application to engineering. The study finds that civil engineering structures can be designed to block harmful waves, such as earthquake disturbance.
Calculation of 2D electronic band structure using matrix mechanics
NASA Astrophysics Data System (ADS)
Pavelich, R. L.; Marsiglio, F.
2016-12-01
We extend previous work, applying elementary matrix mechanics to one-dimensional periodic arrays (to generate energy bands), to two-dimensional arrays. We generate band structures for the square-lattice "2D Kronig-Penney model" (square wells), the "muffin-tin" potential (circular wells), and Gaussian wells. We then apply the method to periodic arrays of more than one atomic site in a unit cell, specifically to the case of materials with hexagonal lattices like graphene. These straightforward extensions of undergraduate-level calculations allow students to readily determine band structures of current research interest.
Active Narrow-Band Vibration Isolation of Large Engineering Structures
NASA Technical Reports Server (NTRS)
Rahman, Zahidul; Spanos, John
1994-01-01
We present a narrow-band tracking control method using a variant of the Least Mean Squares (LMS) algorithm to isolate slowly changing periodic disturbances from engineering structures. The advantage of the algorithm is that it has a simple architecture and is relatively easy to implement while it can isolate disturbances on the order of 40-50 dB over decades of frequency band. We also present the results of an experiment conducted on a flexible truss structure. The average disturbance rejection achieved is over 40 dB over the frequency band of 5 Hz to 50 Hz.
Active Narrow-Band Vibration Isolation of Large Engineering Structures
NASA Technical Reports Server (NTRS)
Rahman, Zahidul; Spanos, John
1994-01-01
We present a narrow-band tracking control method using a variant of the Least Mean Squares (LMS) algorithm to isolate slowly changing periodic disturbances from engineering structures. The advantage of the algorithm is that it has a simple architecture and is relatively easy to implement while it can isolate disturbances on the order of 40-50 dB over decades of frequency band. We also present the results of an experiment conducted on a flexible truss structure. The average disturbance rejection achieved is over 40 dB over the frequency band of 5 Hz to 50 Hz.
Banded Electron Structure Formation in the Inner Magnetosphere
NASA Technical Reports Server (NTRS)
Liemohn, M. W.; Khazanov, G. V.
1997-01-01
Banded electron structures in energy-time spectrograms have been observed in the inner magnetosphere concurrent with a sudden relaxation of geomagnetic activity. In this study, the formation of these banded structures is considered with a global, bounce-averaged model of electron transport, and it is concluded that this structure is a natural occurrence when plasma sheet electrons are captured on closed drift paths near the Earth. These bands do not appear unless there is capture of plasma sheet electrons; convection along open drift paths making open pass around the Earth do not have time to develop this feature. The separation of high-energy bands from the injection population due to the preferential advection of the gradient-curvature drift creates spikes in the energy distribution, which overlap to form a series of bands in the energy spectrograms. The lowest band is the bulk of the injected population in the sub-key energy range. Using the Kp history for an observed banded structure event, a cloud of plasma sheet electrons is captured and the development of their distribution function is examined and discussed.
Automated effective band structures for defective and mismatched supercells
NASA Astrophysics Data System (ADS)
Brommer, Peter; Quigley, David
2014-12-01
In plane-wave density functional theory codes, defects and incommensurate structures are usually represented in supercells. However, interpretation of E versus k band structures is most effective within the primitive cell, where comparison to ideal structures and spectroscopy experiments are most natural. Popescu and Zunger recently described a method to derive effective band structures (EBS) from supercell calculations in the context of random alloys. In this paper, we present bs_sc2pc, an implementation of this method in the CASTEP code, which generates an EBS using the structural data of the supercell and the underlying primitive cell with symmetry considerations handled automatically. We demonstrate the functionality of our implementation in three test cases illustrating the efficacy of this scheme for capturing the effect of vacancies, substitutions and lattice mismatch on effective primitive cell band structures.
Automated effective band structures for defective and mismatched supercells.
Brommer, Peter; Quigley, David
2014-12-03
In plane-wave density functional theory codes, defects and incommensurate structures are usually represented in supercells. However, interpretation of E versus k band structures is most effective within the primitive cell, where comparison to ideal structures and spectroscopy experiments are most natural. Popescu and Zunger recently described a method to derive effective band structures (EBS) from supercell calculations in the context of random alloys. In this paper, we present bs_sc2pc, an implementation of this method in the CASTEP code, which generates an EBS using the structural data of the supercell and the underlying primitive cell with symmetry considerations handled automatically. We demonstrate the functionality of our implementation in three test cases illustrating the efficacy of this scheme for capturing the effect of vacancies, substitutions and lattice mismatch on effective primitive cell band structures.
Deformed band structures at high spin in 200Tl
NASA Astrophysics Data System (ADS)
Bhattacharya, Soumik; Bhattacharyya, S.; Das Gupta, S.; Pai, H.; Mukherjee, G.; Palit, R.; Xu, F. R.; Wu, Q.; Shrivastava, A.; Asgar, Md. A.; Banik, R.; Bhattacharjee, T.; Chanda, S.; Chatterjee, A.; Goswami, A.; Nanal, V.; Pandit, S. K.; Saha, S.; Sethi, J.; Roy, T.; Thakur, S.
2017-01-01
High-spin band structures of 200Tl have been studied by γ -ray spectroscopic methods using the 198Pt(7Li,5 n )200Tl reaction at 45 MeV of beam energy. The level scheme of 200Tl has been extended significantly and several new band structures have been established with the observation of 60 new transitions. The π h9 /2⊗ν i13 /2 oblate band has been extended beyond the particle alignment frequencies. The band structures and the other excited states have been compared with the neighboring odd-odd Tl isotopes. Total Routhian surface calculations have been performed to study the deformation and shape changes as a function of spin in this nucleus. These calculations could reproduce the particle alignment frequency and suggest that the neutron pair alignment in ν i13 /2 orbital induces γ softness in 200Tl.
Electronic band structure of surface-doped black phosphorus
NASA Astrophysics Data System (ADS)
Kim, Jimin; Ryu, Sae Hee; Sohn, Yeongsup; Kim, Keun Su
2015-03-01
There are rapidly growing interests in the study of few-layer black phosphorus owing to its promising device characteristics that may impact our future electronics technology. The low-energy band structure of black phosphorus has been widely predicted to be controllable by external perturbations, such as strain and doping. In this work, we attempt to control the electronic band structure of black phosphorous by in-situ surface deposition of alkali-metal atoms. We found that surface doping induces steep band bending towards the bulk, leading to the emergence of new 2D electronic states that are confined within only few phosphorene layers of black phosphorus. Using angle-resolved photoemission spectroscopy, we directly measured the electronic band structure and its evolution as a function of dopant density. Supported by IBS.
Band-structure engineering in conjugated 2D polymers.
Gutzler, Rico
2016-10-26
Conjugated polymers find widespread application in (opto)electronic devices, sensing, and as catalysts. Their common one-dimensional structure can be extended into the second dimension to create conjugated planar sheets of covalently linked molecules. Extending π-conjugation into the second dimension unlocks a new class of semiconductive polymers which as a consequence of their unique electronic properties can find usability in numerous applications. In this article the theoretical band structures of a set of conjugated 2D polymers are compared and information on the important characteristics band gap and valence/conduction band dispersion is extracted. The great variance in these characteristics within the investigated set suggests 2D polymers as exciting materials in which band-structure engineering can be used to tailor sheet-like organic materials with desired electronic properties.
Development of X-Band Dielectric-Loaded Accelerating Structures
Gold, S. H.; Jing, C.; Kanareykin, A.; Gai, W.; Konecny, R.; Power, J. G.; Kinkead, A. K.
2010-11-04
This paper presents a progress report on the development and testing of X-band dielectric-loaded accelerating structures. Recent tests on several quartz DLA structures with different inner diameters are reported. Designs for gap-free DLA structures are presented. Also, planned new experiments are discussed, including higher gradient traveling-wave and standing-wave structures and special grooved structures for multipactor suppression.
Negative derivative feedback for vibration control of flexible structures
NASA Astrophysics Data System (ADS)
Cazzulani, G.; Resta, F.; Ripamonti, F.; Zanzi, R.
2012-07-01
In this paper a resonant control technique, called negative derivative feedback (NDF), for structural vibration control is presented. Resonant control is a class of control logics, based on the modal approach, which calculates the control action through a dynamic compensator in order to achieve a damping increase on a certain number of system modes. The NDF compensator is designed to work as a band-pass filter, cutting off the control action far from the natural frequencies associated with the controlled modes and reducing the so-called spillover effect. In the paper the proposed control logic is compared both theoretically and experimentally with the most common state-of-the-art resonant control techniques.
Outstanding problems in the band structures of 152Sm
NASA Astrophysics Data System (ADS)
Gupta, J. B.; Hamilton, J. H.
2017-09-01
The recent data on B (E 2 ) values, deduced from the multi-Coulex excitation of the low spin states in the decay of 152Sm, and other experimental findings in the last two decades are compared with the predictions from the microscopic dynamic pairing plus quadrupole model of Kumar and Baranger. The 1292.8 keV 2+ state is assigned to the 03 + band, and the K =2 assignment of the 1769 keV 2+ state is confirmed. The anomaly of the shape coexistence of the assumed spherical β band versus the deformed ground band is resolved. The values from the critical point symmetry X(5) support the collective character of the β band. The problem with the two-term interacting boson model Hamiltonian in predicting β and γ bands in 152Sm leads to interesting consequences. The collective features of the second excited Kπ=03 + band are preferred over the "pairing isomer" view. Also the multiphonon nature of the higher lying Kπ=22 +β γ band and Kπ=4+ band are illustrated vis-à-vis the new data and the nuclear structure theory.
Effective band structure of random III-V alloys
NASA Astrophysics Data System (ADS)
Popescu, Voicu; Zunger, Alex
2010-03-01
Random substitutional alloys have no long range order (LRO) or translational symmetry so rigorously speaking they have no E(k) band structure or manifestations thereof. Yet, many experiments on alloys are interpreted using the language of band theory, e.g. inferring Van Hove singularities, band dispersion and effective masses. Many standard alloy theories (VCA- or CPA-based) have the LRO imposed on the alloy Hamiltonian, assuming only on-site disorder, so they can not be used to judge the extent of LRO that really exists. We adopt the opposite way, by using large (thousand atom) randomly generated supercells in which chemically identical alloy atoms are allowed to have different local environments (a polymorphous representation). This then drives site-dependent atomic relaxation as well as potential fluctuations. The eigenstates from such supercells are then mapped onto the Brillouin zone (BZ) of the primitive cell, producing effective band dispersion. Results for (In,Ga)X show band-like behaviour only near the centre and faces of the BZ but rapidly lose such characteristics away from γ or for higher bands. We further analyse the effects of stoichiometry variation, internal relaxation, and short-range order on the alloy band structure.
Valence band structure in crystalline pentacene thin films
NASA Astrophysics Data System (ADS)
Hatch, Richard; Huber, David; Höchst, Hartmut
2009-03-01
Organic semiconductors, such as pentacene (Pn), are beginning to show promise as a low-cost substitute for conventional semiconductors for a variety of electronic devices. The overlap of π-orbitals in the Pn crystal leads to molecular orbital-derived bands. We used angle-resolved photoemission spectroscopy (ARPES) to reveal the Pn in-plane band structure of the two highest occupied molecular orbital-derived bands in crystalline thin film Pn (grown on a Bi substrate) for various temperatures between 75 K and 300 K. We mapped these two bands in several crystallographic directions with special attention given to the region near the top of the valence band and show, within the limits of our experimental resolution, that temperature does not change the dispersions of these bands. We fit the band structure to a tight binding model and compared our results with recent theoretical predictions[1-2]. We also calculated the in-plane reciprocal effective mass for the M point and compared it with the measured mobility. [1] H. Yoshida et. al. Phys. Rev. B 77, 235205 (2008). [2] G. A. de Wijs et. al. Synth. Met. 139, 109 (2003).
Band structures and localization properties of aperiodic layered phononic crystals
NASA Astrophysics Data System (ADS)
Yan, Zhi-Zhong; Zhang, Chuanzeng
2012-03-01
The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.
Li, Chao; Zhou, Yun-Song; Wang, Huai-Yu
2012-03-26
In this paper, we applied the band structure theory to investigate the plasmonic band (PB) structures and optical properties of subwavelength metal/dielectric/metal Bragg waveguides in the near infrared range with either dielectric or geometric modulation. The Bloch wave vector, density of states, slowdown factor, propagation length and transmittance are calculated and analyzed. Both the modulations are in favor of manipulating surface-plasmon-polariton (SPP) waves. For the dielectric modulation, the PB structure is mainly formed by SPP modes and possesses a "regular pattern" in which the bands and gaps have a relatively even distribution. For the geometric modulation, due to the strong transverse scattering, the contributions of higher modes have to be considered and the gap widths have a significant increase compared to the dielectric modulation. A larger slowdown factor may emerge at the band edge; especially for the geometric modulation, the group velocity can be reduced to 1/100 of light, and negative group velocity is observed as well. While inside the bands, the slowdown factor is smaller and the bands are flat. The contribution of each eigenmode to the PB structure is analyzed.
Transition probabilities in negative parity bands of the 119I nucleus
NASA Astrophysics Data System (ADS)
Srebrny, J.; Droste, Ch.; Morek, T.; Starosta, K.; Wasilewski, A. A.; Pasternak, A. A.; Podsvirova, E. O.; Lobach, Yu. N.; Hagemann, G. H.; Juutinen, S.; Piiparinen, M.; Törmänen, S.; Virtanen, A.
2001-02-01
Lifetimes in four negative-parity bands of 119I were measured using DSAM and RDM. 119I nuclei were produced in the 109Ag( 13C,3n) reaction, γγ coincidences were collected using the NORDBALL array. RDDSA — a new method of RDM analysis — is described. This method allowed for the self-calibration of stopping power. From 31 measured lifetimes, 39 values of B(E2) were established. Calculations in the frame of the Core Quasi Particle Coupling (CQPC) model were focused on the problem of susceptibility of the nucleus to γ-deformation. It was established that nonaxial quadrupole deformation of 119I plays on important role. The Wilets-Jean model of a γ-soft nucleus describes the 119I nucleus in a more consistent way then the Davydov-Filippov model of a γ-rigid nucleus.
Fang, Yun-Tuan; Ni, Zhi-Yao; Zhu, Na; Zhou, Jun
2016-01-13
We propose a new mechanism to achieve light localization and slow light. Through the study on the coupling of two magnetic surface modes, we find a special convex band that takes on a negative refraction effect. The negative refraction results in an energy flow concellation effect from two degenerated modes on the convex band. The energy flow concellation effect leads to forming of the self-trapped and slow light bands. In the self-trapped band light is localized around the source without reflection wall in the waveguide direction, whereas in the slow light band, light becomes the standing-waves and moving standing-waves at the center and the two sides of the waveguide, respectively.
Novel band structures in germanene on aluminium nitride substrate
NASA Astrophysics Data System (ADS)
Ren, Miaojuan; Li, Mingming
2017-09-01
Germanene is difficult to grow epitaxially on conventional semiconductor substrates, and to open a sizable band gap is also a hot topic. Using first-principles calculations, we investigate the structural and electronic properties of germanene/aluminium nitride heterobilayers (Ge/AlN HBLs). The results indicate that the Dirac cone with the nearly linear band dispersion of germanene is almost maintained in the band gap of the substrate. Very low effective masses and high carrier mobilities are achieved, and the band gap of germanene can be effectively tuned from 0.004 to 0.46 eV. These results suggest a possible method to design effective field effect transistors that can operate at room temperature.
Calculation of RIXS spectra of cuprates using band structure parameters
NASA Astrophysics Data System (ADS)
Shi, Yifei; Klich, Israel; Benjamin, David; Demler, Eugene
2015-03-01
We explore the quasi particle theory to study the Resonant Inelastic X-ray Scattering(RIXS) by using the band structure parameters. We use both the determinant method(D. Benjamin, I. Klich and E. Demler, Phys. Rev. Lett. 112, 247002(2014)) and the expansion in the core-hole potential. The methods are applied to the (CaxLa1?x)(Ba1 . 75 - xLa0 . 25 + x)Cu3Oy, (x= 0 . 1 and 0 . 4). We find that by using the band structure alone we can obtain quantitative agreement with the experimental data, especially for the position of the peak.
Structure of dipole bands in doubly odd 102Ag
NASA Astrophysics Data System (ADS)
Singh, V.; Sihotra, S.; Malik, S. S.; Bhat, G. H.; Palit, R.; Sheikh, J. A.; Kumar, S.; Singh, N.; Singh, K.; Goswamy, J.; Sethi, J.; Saha, S.; Trivedi, T.; Mehta, D.
2016-10-01
Excited states in the transitional doubly odd 102Ag nucleus were populated in the 75As(31P,p 3 n ) fusion-evaporation reaction using the 125 MeV incident 31P beam. The subsequent deexcitations were investigated through in-beam γ -ray spectroscopic techniques using the Indian National Gamma Array spectrometer equipped with 21 clover Ge detectors. The level scheme in 102Ag has been established up to excitation energy ˜6.5 MeV and angular momentum 19 ℏ . The earlier reported level scheme is considerably extended and modified to result in a pair of nearly degenerate negative-parity dipole bands. Lifetime measurements for the states of these two dipole bands have been performed by using the Doppler-shift attenuation method. The two nearly degenerate bands exhibit different features with regard to kinetic moment of inertia, and the reduced transition probabilities B (M 1 ) and B (E 2 ) , which do not favor these to be chiral partners. These bands are discussed in the framework of the hybrid version of tilted-axis cranking (tac) model calculations and assigned the π g9 /2⊗ν h11 /2 and π g9 /2⊗ν h11 /2(d5/2/g7 /2) 2 configurations. The tac model calculations are extended to the nearly degenerate bands observed in the heavier doubly odd Ag-108104 isotopes.
Tunable band structure and effective mass of disordered chalcopyrite
NASA Astrophysics Data System (ADS)
Wang, Ze-Lian; Xie, Wen-Hui; Zhao, Yong-Hong
2017-02-01
The band structure and effective mass of disordered chalcopyrite photovoltaic materials Cu1- x Ag x Ga X 2 ( X = S, Se) are investigated by density functional theory. Special quasirandom structures are used to mimic local atomic disorders at Cu/Ag sites. A local density plus correction method is adopted to obtain correct semiconductor band gaps for all compounds. The bandgap anomaly can be seen for both sulfides and selenides, where the gap values of Ag compounds are larger than those of Cu compounds. Band gaps can be modulated from 1.63 to 1.78 eV for Cu1- x Ag x Ga Se 2, and from 2.33 to 2.64 eV for Cu1- x Ag x Ga S 2. The band gap minima and maxima occur at around x = 0:5 and x = 1, respectively, for both sulfides and selenides. In order to show the transport properties of Cu1- x Ag x Ga X 2, the effective mass is shown as a function of disordered Ag concentration. Finally, detailed band structures are shown to clarify the phonon momentum needed by the fundamental indirect-gap transitions. These results should be helpful in designing high-efficiency photovoltaic devices, with both better absorption and high mobility, by Ag-doping in CuGa X 2.
Band structure engineering at heterojunction interfaces via the piezotronic effect.
Shi, Jian; Starr, Matthew B; Wang, Xudong
2012-09-04
Engineering the electronic band structure using the piezopotential is an important aspect of piezotronics, which describes the coupling between the piezoelectric property and semiconducting behavior and functionalities. The time-independent band structure change under short-circuit condition is believed to be due to the remnant piezopotential present at the interface, a result of the finite charge-screening depth at the interface. A series of materials, including metals, semiconductors and electrolytes, are selected to investigate the interfacial band structure engineered by remnant piezopotential when they are in contact with a strained piezoelectric semiconductor. The remnant piezopotential at the interface can switch the junction between Ohmic and Schottky characters, enhance charge combination/separation, regulate barrier height, and modulate reaction kinetics. The difference between the regular time-dependent, pulse-type piezopotential and constant remnant piezopotential is also discussed in detail using a ZnO-based photoelectrochemical anode as an example. The piezotronic effect offers a new pathway for engineering the interface band structure without altering the interface structure or chemical composition, which is promising for improving the performance of many electronics, optoelectronics, and photovoltaic devices.
Electronic band structure and phonons in V2O5
NASA Astrophysics Data System (ADS)
Bhandari, Churna; Lambrecht, Walter R. L.
2013-03-01
Among the vanadium oxides, V2O5 presents special interest as a layered material. As for other layered materials, it is of interest to search for changes in its electronic structure and phonon spectrum in the monolayer modification of this material. For example, reduced screening may modify phonon modes affected by long-range Coulomb interactions. As a preliminary we here present a first-principles study of the bulk electronic band structure and the phonons at the Γ-point. Density functional calculations in the local density approximation were carried out for the electronic band structure and the density functional perturbation method was used for the phonon calculations. We used LDA and norm-conserving pseudopotentials in the abinit code. A group theoretical analysis is used to label the phonon modes. Non-analyticity is included for the LO modes. The band structures are in good agreement with previous work and yield an indirect band gap. Relaxed structural properties are also in good agreement with experiment. Simulated infrared and Raman spectra will be presented. Our results will be compared with experimental and previous theoretical work.
Reconfigurable wave band structure of an artificial square ice
lacocca, Ezio; Gliga, Sebastian; Stamps, Robert L.; Heinonen, Olle
2016-04-18
Artificial square ices are structures composed of magnetic nanoelements arranged on the sites of a twodimensional square lattice, such that there are four interacting magnetic elements at each vertex, leading to geometrical frustration. Using a semianalytical approach, we show that square ices exhibit a rich spin-wave band structure that is tunable both by external magnetic fields and the magnetization configuration of individual elements. Internal degrees of freedom can give rise to equilibrium states with bent magnetization at the element edges leading to characteristic excitations; in the presence of magnetostatic interactions these form separate bands analogous to impurity bands in semiconductors. Full-scale micromagnetic simulations corroborate our semianalytical approach. Our results show that artificial square ices can be viewed as reconfigurable and tunable magnonic crystals that can be used as metamaterials for spin-wave-based applications at the nanoscale.
Band structure mapping of bilayer graphene via quasiparticle scattering
NASA Astrophysics Data System (ADS)
Yankowitz, Matthew; Wang, Joel I.-Jan; Li, Suchun; Birdwell, A. Glen; Chen, Yu-An; Watanabe, Kenji; Taniguchi, Takashi; Quek, Su Ying; Jarillo-Herrero, Pablo; LeRoy, Brian J.
2014-09-01
A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as γ0 = 3.1 eV, γ1 = 0.39 eV, and γ4 = 0.22 eV.
Reconfigurable wave band structure of an artificial square ice
lacocca, Ezio; Gliga, Sebastian; Stamps, Robert L.; ...
2016-04-18
Artificial square ices are structures composed of magnetic nanoelements arranged on the sites of a twodimensional square lattice, such that there are four interacting magnetic elements at each vertex, leading to geometrical frustration. Using a semianalytical approach, we show that square ices exhibit a rich spin-wave band structure that is tunable both by external magnetic fields and the magnetization configuration of individual elements. Internal degrees of freedom can give rise to equilibrium states with bent magnetization at the element edges leading to characteristic excitations; in the presence of magnetostatic interactions these form separate bands analogous to impurity bands in semiconductors.more » Full-scale micromagnetic simulations corroborate our semianalytical approach. Our results show that artificial square ices can be viewed as reconfigurable and tunable magnonic crystals that can be used as metamaterials for spin-wave-based applications at the nanoscale.« less
Reconfigurable wave band structure of an artificial square ice
lacocca, Ezio; Gliga, Sebastian; Stamps, Robert L.; Heinonen, Olle
2016-04-18
Artificial square ices are structures composed of magnetic nanoelements arranged on the sites of a twodimensional square lattice, such that there are four interacting magnetic elements at each vertex, leading to geometrical frustration. Using a semianalytical approach, we show that square ices exhibit a rich spin-wave band structure that is tunable both by external magnetic fields and the magnetization configuration of individual elements. Internal degrees of freedom can give rise to equilibrium states with bent magnetization at the element edges leading to characteristic excitations; in the presence of magnetostatic interactions these form separate bands analogous to impurity bands in semiconductors. Full-scale micromagnetic simulations corroborate our semianalytical approach. Our results show that artificial square ices can be viewed as reconfigurable and tunable magnonic crystals that can be used as metamaterials for spin-wave-based applications at the nanoscale.
Negative refraction of acoustic waves using a foam-like metallic structure
NASA Astrophysics Data System (ADS)
Hladky-Hennion, A.-C.; Vasseur, J. O.; Haw, G.; Croënne, C.; Haumesser, L.; Norris, A. N.
2013-04-01
A phononic crystal (PC) slab made of a single metallic phase is shown, theoretically and experimentally, to display perfect negative index matching and focusing capability when surrounded with water. The proposed PC slab is a centimeter scale hollow metallic foam-like structure in which acoustic energy is mediated via the metal lattice. The negative index property arises from an isolated branch of the dispersion curves corresponding to a mode that can be coupled to incident acoustic waves in surrounding water. This band also intercepts the water sound line at a frequency in the ultrasonic range. The metallic structure is consequently a candidate for the negative refraction of incident longitudinal waves.
Band-structure loops and multistability in cavity QED
NASA Astrophysics Data System (ADS)
Prasanna Venkatesh, B.; Larson, J.; O'Dell, D. H. J.
2011-06-01
We calculate the band structure of ultracold atoms located inside a laser-driven optical cavity. For parameters where the atom-cavity system exhibits bistability, the atomic band structure develops loop structures akin to the ones predicted for Bose-Einstein condensates in ordinary (noncavity) optical lattices. However, in our case the nonlinearity derives from the cavity back-action rather than from direct interatomic interactions. We find both bi- and tristable regimes associated with the lowest band, and show that the multistability we observe can be analyzed in terms of swallowtail catastrophes. Dynamic and energetic stability of the mean-field solutions is also discussed, and we show that the bistable solutions have, as expected, one unstable and two stable branches. The presence of loops in the atomic band structure has important implications for proposals concerning Bloch oscillations of atoms inside optical cavities [Peden , Phys. Rev. APLRAAN1050-294710.1103/PhysRevA.80.043803 80, 043803 (2009); Prasanna Venkatesh , Phys. Rev. APLRAAN1050-294710.1103/PhysRevA.80.063834 80, 063834 (2009)].
Band structure of W and Mo by empirical pseudopotential method
NASA Technical Reports Server (NTRS)
Sridhar, C. G.; Whiting, E. E.
1977-01-01
The empirical pseudopotential method (EPM) is used to calculate the band structure of tungsten and molybdenum. Agreement between the calculated reflectivity, density of states, density of states at the Fermi surface and location of the Fermi surface from this study and experimental measurements and previous calculations is good. Also the charge distribution shows the proper topological distribution of charge for a bcc crystal.
X-Band Photonic Band-Gap Accelerator Structure Breakdown Experiment
Marsh, Roark A.; Shapiro, Michael A.; Temkin, Richard J.; Dolgashev, Valery A.; Laurent, Lisa L.; Lewandowski, James R.; Yeremian, A.Dian; Tantawi, Sami G.; /SLAC
2012-06-11
In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at X band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of 65 MV/m yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above 110 MV/m was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of 100 MV/m and a surface magnetic field of 890 kA/m. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electric field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about 14 MV/m, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.
Photonic crystal digital alloys and their band structure properties.
Lee, Jeongkug; Kim, Dong-Uk; Jeon, Heonsu
2011-09-26
We investigated semi-disordered photonic crystals (PCs), digital alloys, and made thorough comparisons with their counterparts, random alloys. A set of diamond lattice PC digital alloys operating in a microwave regime were prepared by alternately stacking two kinds of sub-PC systems composed of alumina and silica spheres of the same size. Measured transmission spectra as well as calculated band structures revealed that when the digital alloy period is short, band-gaps of the digital alloys are practically the same as those of the random alloys. This study indicates that the concept of digital alloys holds for photons in PCs as well.
Electronic band structure of magnetic bilayer graphene superlattices
Pham, C. Huy; Nguyen, T. Thuong
2014-09-28
Electronic band structure of the bilayer graphene superlattices with δ-function magnetic barriers and zero average magnetic flux is studied within the four-band continuum model, using the transfer matrix method. The periodic magnetic potential effects on the zero-energy touching point between the lowest conduction and the highest valence minibands of pristine bilayer graphene are exactly analyzed. Magnetic potential is shown also to generate the finite-energy touching points between higher minibands at the edges of Brillouin zone. The positions of these points and the related dispersions are determined in the case of symmetric potentials.
Temperature Effects in the Band Structure of Topological Insulators.
Monserrat, Bartomeu; Vanderbilt, David
2016-11-25
We study the effects of temperature on the band structure of the Bi_{2}Se_{3} family of topological insulators using first-principles methods. Increasing temperature drives these materials towards the normal state, with similar contributions from thermal expansion and from electron-phonon coupling. The band gap changes with temperature reach 0.3 eV at 600 K, of similar size to the changes caused by electron correlation. Our results suggest that temperature-induced topological phase transitions should be observable near critical points of other external parameters.
Inter-band optoelectronic properties in quantum dot structure of low band gap III-V semiconductors
Dey, Anup; Maiti, Biswajit; Chanda, Debasree
2014-04-14
A generalized theory is developed to study inter-band optical absorption coefficient (IOAC) and material gain (MG) in quantum dot structures of narrow gap III-V compound semiconductor considering the wave-vector (k{sup →}) dependence of the optical transition matrix element. The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted for analysis of the IOAC and MG taking InAs, InSb, Hg{sub 1−x}Cd{sub x}Te, and In{sub 1−x}Ga{sub x}As{sub y}P{sub 1−y} lattice matched to InP, as example of III–V compound semiconductors, having varied split-off energy band compared to their bulk band gap energy. It has been found that magnitude of the IOAC for quantum dots increases with increasing incident photon energy and the lines of absorption are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of energy band parameters. The results show a significant deviation to the MG spectrum of narrow-gap materials having band nonparabolicity compared to the parabolic band model approximations. The results reflect the important role of valence band split-off energies in these narrow gap semiconductors.
Structurally tunable resonant absorption bands in ultrathin broadband plasmonic absorbers.
Butun, Serkan; Aydin, Koray
2014-08-11
Light absorption is a fundamental optical process playing significantly important role in wide variety of applications ranging from photovoltaics to photothermal therapy. Semiconductors have well-defined absorption bands with low-energy edge dictated by the band gap energy, therefore it is rather challenging to tune the absorption bandwidth of semiconductors. However, resonant absorbers based on plasmonic nanostructures and optical metamaterials emerged as alternative light absorbers due to spectrally selective absorption bands resulting from optical resonances. Recently, a broadband plasmonic absorber design was introduced by Aydin et al. with a reasonably high broadband absorption. Based on that design, here, structurally tunable, broadband absorbers with improved performance are demonstrated. This broadband absorber has a total thickness of 190 nm with 80% average measured absorption (90% simulated absorption) over the entire visible spectrum (400 - 700 nm). Moreover, the effect of the metal and the oxide thicknesses on the absorption spectra are investigated and results indicate that the shorter and the longer band-edge of broadband absorption can be structurally tuned with the metal and the oxide thicknesses, as well as with the resonator size. Detailed numerical simulations shed light on the type of optical resonances that contribute to the broadband absorption response and provide a design guideline for realizing plasmonic absorbers with structurally tunable bandwidths.
Gochitashvili, Malkhaz R.; Lomsadze, Ramaz A.; Kezerashvili, Roman Ya.
2010-08-15
The absolute cross sections for the e-N{sub 2} and p-N{sub 2} collisions for the first negative B{sup 2{Sigma}}{sub u}{sup +}-X{sup 2{Sigma}}{sub g}{sup +} and Meinel A{sup 2{Pi}}{sub u}-X{sup 2{Sigma}}{sub g}{sup +} bands have been measured in the energy region of 400-1500 eV for electrons and 0.4-10 keV for protons, respectively. Measurements are performed in the visible spectral region of 400-800 nm by an optical spectroscopy method. The ratio of the cross sections of the Meinel band system to the cross section of the first negative band system (0,0) does not depend on the incident electron energy. The populations of vibrational levels corresponding to A{sup 2{Pi}}{sub u} states are consistent with the Franck-Condon principle. The ratios of the cross sections of (4,1) to (3,0) bands and (5,2) to (3,0) bands exhibit slight dependence on the proton energy. A theoretical estimation within the quasimolecular approximation provides a reasonable description of the total cross section for the first negative band.
Polarimetric and Structural Properties of a Boreal Forest at P-Band and L-Band
NASA Astrophysics Data System (ADS)
Tebaldini, S.; Rocca, F.
2010-12-01
With this paper we investigate the structural and polarimetric of the boreal forest within the Krycklan river catchment, Northern Sweden, basing on multi-polarimetric and multi-baseline SAR surveys at P-Band and L-Band collected in the framework of the ESA campaign BioSAR 2008. The analysis has been carried out by applying the Algebraic Synthesis (AS) technique, recently introduced in literature, which provides a theoretical framework for the decomposition of the backscattered signal into ground-only and volume-only contributions, basing on both baseline and polarization diversity. The availability of multiple baselines allows the formation of a synthetic aperture not only along the azimuth direction but also in elevation. Accordingly, the backscattered echoes can be focused not only in the slant range, azimuth plane, but in the whole 3D space. This is the rationale of the SAR Tomography (T-SAR) concept, which has been widely considered in the literature of the last years. It follows that, as long as the penetration in the scattering volume is guaranteed, the vertical profile of the vegetation layer is retrieved by separating backscatter contributions along the vertical direction, which is the main reason for the exploitation of Tomographic techniques at longer wavelengths. Still, the capabilities of T-SAR are limited to imaging the global vertical structure of the electromagnetic scattering in a certain polarization. It then becomes important to develop methodologies for the investigation of the vertical structure of different Scattering Mechanisms (SMs), such as ground and volume scattering, in such a way as to derive information that can be delivered also outside the field of Radar processing. This is an issue that may become relevant at longer wavelengths, such as P-Band, where the presence of multiple scattering arising from the interaction with terrain could hinder the correct reconstruction of the forest structure. The availability of multiple polarizations
Measurement of valence band structure in arbitrary dielectric films
Uhm, Han S.; Choi, Eun H.
2012-10-15
A new way of measuring the band structure of various dielectric materials using the secondary electron emission from Auger neutralization of ions is introduced. The first example of this measurement scheme is the magnesium oxide (MgO) films with respect to the application of the films in the display industries. The density of state in the valence bands of MgO film and MgO film with a functional layer (FL) deposited over a dielectric surface reveals that the density peak of film with a FL is considerably less than that of film, thereby indicating a better performance of MgO film with functional layer in display devices. The second example of the measurement is the boron-zinc oxide (BZO) films with respect to the application of the films to the development of solar cells. The measurement of density of state in BZO film suggests that a high concentration of boron impurity in BZO films may enhance the transition of electrons and holes through the band gap from the valence to the conduction band in zinc oxide crystals; thereby improving the conductivity of the film. Secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials.
From lattice Hamiltonians to tunable band structures by lithographic design
NASA Astrophysics Data System (ADS)
Tadjine, Athmane; Allan, Guy; Delerue, Christophe
2016-08-01
Recently, new materials exhibiting exotic band structures characterized by Dirac cones, nontrivial flat bands, and band crossing points have been proposed on the basis of effective two-dimensional lattice Hamiltonians. Here, we show using atomistic tight-binding calculations that these theoretical predictions could be experimentally realized in the conduction band of superlattices nanolithographed in III-V and II-VI semiconductor ultrathin films. The lithographed patterns consist of periodic lattices of etched cylindrical holes that form potential barriers for the electrons in the quantum well. In the case of honeycomb lattices, the conduction minibands of the resulting artificial graphene host several Dirac cones and nontrivial flat bands. Similar features, but organized in different ways, in energy or in k -space are found in kagome, distorted honeycomb, and Lieb superlattices. Dirac cones extending over tens of meV could be obtained in superlattices with reasonable sizes of the lithographic patterns, for instance in InAs/AlSb heterostructures. Bilayer artificial graphene could be also realized by lithography of a double quantum-well heterostructure. These new materials should be interesting for the experimental exploration of Dirac-based quantum systems, for both fundamental and applied physics.
Negative-group-delay and non-foster electromagnetic structures
NASA Astrophysics Data System (ADS)
Mirzaei, Hassan
of the feedline. The active approach employs unilateral reactive non-Foster elements, synthesized by NGD networks, in the feedline. Chapters 7 presents the properties of positive-index/negative-index coupled-line structures, where simplified relations describing these structures in the complex-mode band are derived. Based on these relations, the properties of these structures are discussed. By employing these properties, two applications, namely a NGD structure and a novel traveling-wave resonator, are demonstrated. Although this dissertation focuses on non-Foster and negative-group-delay structures, extra work is described in parts of the thesis to explore relevant structures and to propose novel applications. In this regard, in Chapters 4, before presenting the antenna with an embedded matching network and in a relevant application, a frequency-reconfigurable antenna is presented. Moreover, in Chapters 5, passive approaches for squint-free beamforming in series-fed arrays are explored and implemented. Further, in Chapters 6, the applications of positive-index/negative-index guides in constructing a novel type of resonator with multiple resonant frequencies, in a certain bandwidth, are investigated.
Zhang, Hai-Feng E-mail: lsb@nuaa.edu.cn; Liu, Shao-Bin E-mail: lsb@nuaa.edu.cn; Jiang, Yu-Chi
2014-09-15
In this paper, the tunable all-angle negative refraction and photonic band gaps (PBGs) in two types of two-dimensional (2D) plasma photonic crystals (PPCs) composed of homogeneous plasma and dielectric (GaAs) with square-like Archimedean lattices (ladybug and bathroom lattices) for TM wave are theoretically investigated based on a modified plane wave expansion method. The type-1 structure is dielectric rods immersed in the plasma background, and the complementary structure is named as type-2 PPCs. Theoretical simulations demonstrate that the both types of PPCs with square-like Archimedean lattices have some advantages in obtaining the higher cut-off frequency, the larger PBGs, more number of PBGs, and the relative bandwidths compared to the conventional square lattices as the filling factor or radius of inserted rods is same. The influences of plasma frequency and radius of inserted rod on the properties of PBGs for both types of PPCs also are discussed in detail. The calculated results show that PBGs can be manipulated by the parameters as mentioned above. The possibilities of all-angle negative refraction in such two types of PPCs at low bands also are discussed. Our calculations reveal that the all-angle negative phenomena can be observed in the first two TM bands, and the frequency range of all-angle negative refraction can be tuned by changing plasma frequency. Those properties can be used to design the optical switching and sensor.
NASA Astrophysics Data System (ADS)
Zhang, Hai-Feng; Liu, Shao-Bin; Jiang, Yu-Chi
2014-09-01
In this paper, the tunable all-angle negative refraction and photonic band gaps (PBGs) in two types of two-dimensional (2D) plasma photonic crystals (PPCs) composed of homogeneous plasma and dielectric (GaAs) with square-like Archimedean lattices (ladybug and bathroom lattices) for TM wave are theoretically investigated based on a modified plane wave expansion method. The type-1 structure is dielectric rods immersed in the plasma background, and the complementary structure is named as type-2 PPCs. Theoretical simulations demonstrate that the both types of PPCs with square-like Archimedean lattices have some advantages in obtaining the higher cut-off frequency, the larger PBGs, more number of PBGs, and the relative bandwidths compared to the conventional square lattices as the filling factor or radius of inserted rods is same. The influences of plasma frequency and radius of inserted rod on the properties of PBGs for both types of PPCs also are discussed in detail. The calculated results show that PBGs can be manipulated by the parameters as mentioned above. The possibilities of all-angle negative refraction in such two types of PPCs at low bands also are discussed. Our calculations reveal that the all-angle negative phenomena can be observed in the first two TM bands, and the frequency range of all-angle negative refraction can be tuned by changing plasma frequency. Those properties can be used to design the optical switching and sensor.
Elevated noise power in gamma band related to negative symptoms and memory deficit in schizophrenia.
Suazo, Vanessa; Díez, Álvaro; Martín, Carmen; Ballesteros, Alejandro; Casado, Pilar; Martín-Loeches, Manuel; Molina, Vicente
2012-08-07
There is an increasing consideration for a disorganized cerebral activity in schizophrenia, perhaps relating to a synaptic inhibitory deficit in the illness. Noise power (scalp-recorded electroencephalographic activity unlocked to stimuli) may offer a non-invasive window to assess this possibility. 29 minimally-treated patients with schizophrenia (of which 17 were first episodes) and 27 healthy controls underwent clinical and cognitive assessments and an electroencephalographic recording during a P300 paradigm to calculate signal-to-noise ratio and noise power magnitudes in the theta and gamma bands. In comparison to controls, a significantly higher gamma noise power was common to minimally-treated and first episode patients over P3, P4, T5 and Fz electrode sites. Those high values were directly correlated to negative symptom severity and inversely correlated to verbal memory scores in the patients. There were no differences in signal-to-noise ratio magnitudes among the groups. Gamma noise power at Fz discriminated significantly between patients and controls. No significant differences were found in theta noise power or in gamma noise power over the other electrode sites between the groups of patients and controls. We have not assessed phase-locked and non-phase locked power changes, a complementary approach that may yield useful information. Gamma noise power may represent a useful and non-invasive tool for studying brain dysfunction in psychotic illness. These results suggest an inefficient activation pattern in schizophrenia. Copyright © 2012 Elsevier Inc. All rights reserved.
NASA Astrophysics Data System (ADS)
Xu, Xiaohu; Li, Yuan; Miao, Xiangyang
2016-11-01
We experimentally study the guided wave along the interface between ε-negative (ENG) material and μ-negative (MNG) material when the interface is distorted. It is demonstrated that the guided mode is robust against various interface perturbations, owing to the band inversion between ENG and MNG materials. For comparison, the guided modes in double-positive material sandwiched by two MNG materials are also investigated under the same interface perturbations. Without band inversion, the guided modes in the sandwich waveguide are strongly affected by the interface distortions. Numerical simulations agree well with the experimental results.
Negative thermal expansion and broad band photoluminescence in a novel material of ZrScMo2VO12
Ge, Xianghong; Mao, Yanchao; Liu, Xiansheng; Cheng, Yongguang; Yuan, Baohe; Chao, Mingju; Liang, Erjun
2016-01-01
In this paper, we present a novel material with the formula of ZrScMo2VO12 for the first time. It was demonstrated that this material exhibits not only excellent negative thermal expansion (NTE) property over a wide temperature range (at least from 150 to 823 K), but also very intense photoluminescence covering the entire visible region. Structure analysis shows that ZrScMo2VO12 has an orthorhombic structure with the space group Pbcn (No. 60) at room temperature. A phase transition from monoclinic to orthorhombic structure between 70 and 90 K is also revealed. The intense white light emission is tentatively attributed to the n- and p-type like co-doping effect which creates not only the donor- and acceptor-like states in the band gap, but also donor-acceptor pairs and even bound exciton complexes. The excellent NTE property integrated with the intense white-light emission implies a potential application of this material in light emitting diode and other photoelectric devices. PMID:27098924
Negative thermal expansion and broad band photoluminescence in a novel material of ZrScMo2VO12.
Ge, Xianghong; Mao, Yanchao; Liu, Xiansheng; Cheng, Yongguang; Yuan, Baohe; Chao, Mingju; Liang, Erjun
2016-04-21
In this paper, we present a novel material with the formula of ZrScMo2VO12 for the first time. It was demonstrated that this material exhibits not only excellent negative thermal expansion (NTE) property over a wide temperature range (at least from 150 to 823 K), but also very intense photoluminescence covering the entire visible region. Structure analysis shows that ZrScMo2VO12 has an orthorhombic structure with the space group Pbcn (No. 60) at room temperature. A phase transition from monoclinic to orthorhombic structure between 70 and 90 K is also revealed. The intense white light emission is tentatively attributed to the n- and p-type like co-doping effect which creates not only the donor- and acceptor-like states in the band gap, but also donor-acceptor pairs and even bound exciton complexes. The excellent NTE property integrated with the intense white-light emission implies a potential application of this material in light emitting diode and other photoelectric devices.
Negative thermal expansion and broad band photoluminescence in a novel material of ZrScMo2VO12
NASA Astrophysics Data System (ADS)
Ge, Xianghong; Mao, Yanchao; Liu, Xiansheng; Cheng, Yongguang; Yuan, Baohe; Chao, Mingju; Liang, Erjun
2016-04-01
In this paper, we present a novel material with the formula of ZrScMo2VO12 for the first time. It was demonstrated that this material exhibits not only excellent negative thermal expansion (NTE) property over a wide temperature range (at least from 150 to 823 K), but also very intense photoluminescence covering the entire visible region. Structure analysis shows that ZrScMo2VO12 has an orthorhombic structure with the space group Pbcn (No. 60) at room temperature. A phase transition from monoclinic to orthorhombic structure between 70 and 90 K is also revealed. The intense white light emission is tentatively attributed to the n- and p-type like co-doping effect which creates not only the donor- and acceptor-like states in the band gap, but also donor-acceptor pairs and even bound exciton complexes. The excellent NTE property integrated with the intense white-light emission implies a potential application of this material in light emitting diode and other photoelectric devices.
Development of X-band accelerating structures for high gradients
NASA Astrophysics Data System (ADS)
Bini, S.; Chimenti, V.; Marcelli, A.; Palumbo, L.; Spataro, B.; A. Dolgashev, V.; Tantawi, S.; D. Yeremian, A.; Higashi, Y.; G. Grimaldi, M.; Romano, L.; Ruffino, F.; Parodi, R.
2012-07-01
Short copper standing wave (SW) structures operating at an X-band frequency have been recently designed and manufactured at the Laboratori Nazionali di Frascati of the Istituto Nazionale di Fisica Nucleare (INFN) using the vacuum brazing technique. High power tests of the structures have been performed at the SLAC National Accelerator Laboratory. In this manuscript we report the results of these tests and the activity in progress to enhance the high gradient performance of the next generation of structures, particularly the technological characterization of high performance coatings obtained via molybdenum sputtering.
Effect of pressure on band structure properties of Zinc Chalcogenides
NASA Astrophysics Data System (ADS)
Singh, Dharmbir
2013-06-01
The first principal calculations have been carried out to study the effect of pressure on band structure of Zinc Chalcogenides. The tight-binding linear muffin-tin orbital method (TB-LMTO) within local density approximation (LDA) has been used to study the band structure properties at abmbient and high pressure. The phase stability is determined from the total energy calculations within the atomic-sphere approximation (ASA). The purely theoretical calculations show that in these materials (i) at ambient pressure, zinc blende type (B3) phase is more stable than rock salt type (B1) phase; and (ii) it exhibits a phase transition from zinc blende (B3) type to NaCl type (B1) type structure at high pressure. At further ultrahigh pressure there is phase transition from NaCl type (B1) phase to CsCl type (B2) phase. Although the calculated lattice parameter, transition pressure, volume of collapse is found to be little less than the experimentally observed value, but this has been explained from the fact that the calculation has been carried out at 0 K while the experiments have been performed at room temperature. Ambient & high pressure band structural results are compared with earlier obtained similar results and explained in details. The Author would like to thank to Director, PIET for all the motivation and support.
NASA Astrophysics Data System (ADS)
Guo, Yuzheng; Robertson, John
2017-09-01
We present a detailed study of the electronic structure of the layered semiconductor InSe. We calculate the band structure of the monolayer and bulk material using density functional theory, hybrid functionals, and G W . The band gap of the monolayer InSe is calculated to be 2.4 eV in screened exchange hybrid functional, close to the experimental photoluminescence gap. The electron affinities and band offsets are calculated for vertical stacked-layer heterostructures, and are found to be suitable for tunnel field effect transistors (TFETs) in combination with WS e2 or similar. The valence-band edge of InSe is calculated to lie 5.2 eV below the vacuum level, similar to that for the closed shell systems HfS e2 or SnS e2 . Hence InSe would be suitable to act as a p -type drain in the TFET. The intrinsic defects are calculated. For Se-rich layers, the Se adatom (interstitial) is found to be the most stable defect, whereas for In-rich layers, the Se vacancy is the most stable for the neutral state. Antisites tend to have energies just above those of vacancies. The Se antisite distorts towards a bond-breaking distortion as in the EL2 center of GaAs. Both substitutional donors and acceptors are calculated to be shallow, and effective dopants. They do not reconstruct to form nondoping configurations as occurs in black phosphorus. Finally, the Schottky barriers of metals on InSe are found to be strongly pinned by metal induced gap states (MIGS) at ˜0.5 eV above the valence-band edge. Any interfacial defects would lead to a stronger pinning at a similar energy. Overall, InSe is an effective semiconductor combining the good features of 2D (lack of dangling bonds, etc.) with the good features of 3D (effective doping), which few others achieve.
Energetic stabilization of the Mizoguchi structure for magnetite by band-structure effects
NASA Astrophysics Data System (ADS)
Mishra, S. K.; Satpathy, S.
1993-03-01
We show that the Mizoguchi structure is energetically stabilized over the Verwey structure for magnetite by electron hopping on the B sublattice. We use the one-band Cullen-Callen model Hamiltonian for the electronic band structure taking the nearest-neighbor and the second-neighbor Coulomb interactions, U1 and U2, into account. There is a competition between the Coulomb and the band-structure energies. The Coulomb energy tends to favor the Verwey structure while the band-structure energy tends to favor the Mizoguchi structure. We find that for U1<~0.25 eV, the kinetic-energy (band-structure energy) term dominates making the Mizoguchi structure energetically favorable over the Verwey structure. For a larger value of U1, the band-structure effect alone is insufficient, making it necessary to invoke other mechanisms such as the electron-phonon coupling earlier proposed by other authors, to stabilize the Mizoguchi structure. The energy of a single ``reversed-ring'' excitation in the Mizoguchi structure is calculated to be of the order of a few meV. The small energy is consistent with Cullen's explanation of the absence of cell doubling in the Ca plane as observed in diffraction experiments. The Mizoguchi order is unstable with respect to the formation of reversed-ring excitations if only U1 is present, but is stabilized by a small value of U2.
Exciton band structure in two-dimensional materials
NASA Astrophysics Data System (ADS)
Cudazzo, Pier Luigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo
In low-dimensional materials the screening of the Coulomb interaction is strongly reduced. As a consequence, the binding energy of both Wannier and Frenkel excitons in the optical spectra is large and comparable in size. Therefore, contrarily to bulk materials, it cannot serve as a criterion to distinguish different kinds of excitons. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the hopping terms related to the electronic band structure.
Band structure engineered layered metals for low-loss plasmonics
Gjerding, Morten N.; Pandey, Mohnish; Thygesen, Kristian S.
2017-01-01
Plasmonics currently faces the problem of seemingly inevitable optical losses occurring in the metallic components that challenges the implementation of essentially any application. In this work, we show that Ohmic losses are reduced in certain layered metals, such as the transition metal dichalcogenide TaS2, due to an extraordinarily small density of states for scattering in the near-IR originating from their special electronic band structure. On the basis of this observation, we propose a new class of band structure engineered van der Waals layered metals composed of hexagonal transition metal chalcogenide-halide layers with greatly suppressed intrinsic losses. Using first-principles calculations, we show that the suppression of optical losses lead to improved performance for thin-film waveguiding and transformation optics. PMID:28436432
Band structure engineered layered metals for low-loss plasmonics.
Gjerding, Morten N; Pandey, Mohnish; Thygesen, Kristian S
2017-04-24
Plasmonics currently faces the problem of seemingly inevitable optical losses occurring in the metallic components that challenges the implementation of essentially any application. In this work, we show that Ohmic losses are reduced in certain layered metals, such as the transition metal dichalcogenide TaS2, due to an extraordinarily small density of states for scattering in the near-IR originating from their special electronic band structure. On the basis of this observation, we propose a new class of band structure engineered van der Waals layered metals composed of hexagonal transition metal chalcogenide-halide layers with greatly suppressed intrinsic losses. Using first-principles calculations, we show that the suppression of optical losses lead to improved performance for thin-film waveguiding and transformation optics.
Talebzadeh, Robabeh; Namdar, Abdolrahman
2012-09-20
We study the electromagnetic beam reflection from layered structures that include the so-called ε-negative and the μ-negative materials, also called single negative materials. We predict that such structures can demonstrate a giant lateral Goos-Hänchen shift of the resonant excitation of surface waves at the interface between the conventional and single negative materials, as well as due to the excitation of leaky modes in the layered structures. Then we replace the conventional layer with a left-handed layer (a material with both ε<0 and μ<0). We show that the Goos-Hänchen shift can be positive and negative depending on the type of this layer (conventional or LH material), which can support TE or TM surface waves.
Semiempirical band structure calculations on skutterudite-type compounds
NASA Astrophysics Data System (ADS)
Partik, M.; Lutz, H. D.
Semiempirical band structure calculations were performed on several skutterudite-type compounds by using the extended Hückel method. Starting with the molecular orbital calculations on isolated P4 and As4 rings, the reason for the band dispersions of the skutterudites was found to be the interactions between the nonmetal atoms. Both the intermolecular and the intramolecular interactions between the phosphorus atoms are stronger than those between the arsenic atoms. Hence, the dispersion of the bands in CoP3 is larger than that in CoAs3. The COOP (crystal orbital overlap population) integrals of the intramolecular P-P bonds reveal the relation between the valence electron count and the observed bond lengths. The P-P bonds in the skutterudite-type compounds like TP3 (T=Co, Rh, Ir) become stronger by reduction as in NiP3 and weaker by oxidation as in RT4X12 (X=P, As, Sb; R=alkaline earth or rare earth metals) because the bands near the Fermi level are bonding. The electronic reason for the geometric distortion of the Ge2Y2 (Y=S, Se) units of mixed skutterudites TGe1.5Y1.5 is caused by an electron pair gap on germanium, which corresponds to low electron density perpendicular to the ring plane on the germanium atoms.
Majumdar, Kausik
2014-05-07
In this paper, we use a tight binding Hamiltonian with spin orbit coupling to study the real and complex band structures of relaxed and strained GaAs. A simple d orbital on-site energy shift coupled with appropriate scaling of the off-diagonal terms is found to correctly reproduce the band-edge shifts with strain. Four different 〈100〉 strain combinations, namely, uniaxial compressive, uniaxial tensile, biaxial compressive, and biaxial tensile strain are studied, revealing rich valence band structure and strong relative orientation dependent tunneling. It is found that complex bands are unable to provide unambiguous tunneling paths away from the Brillouin zone center. Tunneling current density distribution over the Brillouin zone is computed using non-equilibrium Green's function approach elucidating a physical picture of band to band tunneling.
A Study of Higher-Band Dipole Wakefields in X-Band Accelerating Structures for the G/NLC
Jones, R
2004-09-02
The X-band linacs for the G/NLC (Global/Next Linear Collider) have evolved from the DDS (Damped Detuned Structure) series. The present accelerating structures are 60 cm in length and incorporate damping and detuning of the dipole modes which comprise the wakefield. In order to adequately damp the wakefield, frequencies of adjacent structures are interleaved. Limited analysis has been done previously on the higher order dipole bands. Here, we calculate the contribution of higher order bands of interleaved structures to the wakefield. Beam dynamics issues are also studied.
Band gaps in grid structure with periodic local resonator subsystems
NASA Astrophysics Data System (ADS)
Zhou, Xiaoqin; Wang, Jun; Wang, Rongqi; Lin, Jieqiong
2017-09-01
The grid structure is widely used in architectural and mechanical field for its high strength and saving material. This paper will present a study on an acoustic metamaterial beam (AMB) based on the normal square grid structure with local resonators owning both flexible band gaps and high static stiffness, which have high application potential in vibration control. Firstly, the AMB with variable cross-section frame is analytically modeled by the beam-spring-mass model that is provided by using the extended Hamilton’s principle and Bloch’s theorem. The above model is used for computing the dispersion relation of the designed AMB in terms of the design parameters, and the influences of relevant parameters on band gaps are discussed. Then a two-dimensional finite element model of the AMB is built and analyzed in COMSOL Multiphysics, both the dispersion properties of unit cell and the wave attenuation in a finite AMB have fine agreement with the derived model. The effects of design parameters of the two-dimensional model in band gaps are further examined, and the obtained results can well verify the analytical model. Finally, the wave attenuation performances in three-dimensional AMBs with equal and unequal thickness are presented and discussed.
Band structure engineering in topological insulator based heterostructures.
Menshchikova, T V; Otrokov, M M; Tsirkin, S S; Samorokov, D A; Bebneva, V V; Ernst, A; Kuznetsov, V M; Chulkov, E V
2013-01-01
The ability to engineer an electronic band structure of topological insulators would allow the production of topological materials with tailor-made properties. Using ab initio calculations, we show a promising way to control the conducting surface state in topological insulator based heterostructures representing an insulator ultrathin films on the topological insulator substrates. Because of a specific relation between work functions and band gaps of the topological insulator substrate and the insulator ultrathin film overlayer, a sizable shift of the Dirac point occurs resulting in a significant increase in the number of the topological surface state charge carriers as compared to that of the substrate itself. Such an effect can also be realized by applying the external electric field that allows a gradual tuning of the topological surface state. A simultaneous use of both approaches makes it possible to obtain a topological insulator based heterostructure with a highly tunable topological surface state.
Miniaturized Band Stop FSS Using Convoluted Swastika Structure
NASA Astrophysics Data System (ADS)
Bilvam, Sridhar; Sivasamy, Ramprabhu; Kanagasabai, Malathi; Alsath M, Gulam Nabi; Baisakhiya, Sanjay
2017-01-01
This paper presents a miniaturized frequency selective surface (FSS) with stop band characteristics at the resonant frequency of 5.12 GHz. The unit cell size of the proposed FSS design is in the order of 0.095 λ×0.095 λ. The proposed unit cell is obtained by convoluting the arms of the basic swastika structure. The design provides fractional bandwidth of 9.0 % at the center frequency of 5.12 GHz in the 20 dB reference level of insertion loss. The symmetrical aspect of the design delivers identical response for both transverse electric (TE) and transverse magnetic (TM) modes thereby exhibiting polarization independent operation. The miniaturized design provides good angular independency for various incident angles. The dispersion analysis is done to substantiate the band stop operation of the convoluted swastika FSS. The proposed FSS is fabricated and its working is validated through measurements.
Fabrication of x-band accelerating structures at Fermilab
Tug T Arkan et al.
2004-07-20
The RF Technology Development group at Fermilab is working together with the NLC and GLC groups at SLAC and KEK on developing technology for room temperature X-band accelerating structures for a future linear collider. We built six 60-cm long, high phase advance, detuned structures (HDS or FXB series). These structures have 150 degrees phase advance per cell, and are intended for high gradient tests. The structures were brazed in a vacuum furnace with a partial pressure of argon, rather than in a hydrogen atmosphere. We have also begun to build 60-cm long, damped and detuned structures (HDDS or FXC/FXD series). We have built 5 FXC and 1 FXD structures. Our goal was to build six structures for the 8-pack test at SLAC by the end of March 2004, as part of the GLC/NLC effort to demonstrate the readiness of room temperature RF technology for a linear collider. This paper describes the RF structure factory infrastructure (clean rooms, vacuum furnaces, vacuum equipment, RF equipment etc.), and the fabrication techniques utilized (the machining of copper cells/couplers, quality control, etching, vacuum brazing, cleanliness requirements etc.) for the production of FXB and FXC/FXD structures.
Negative index meta-materials based on two-dimensional metallic structures
NASA Astrophysics Data System (ADS)
Shvets, Gennady; Urzhumov, Yaroslav A.
2006-04-01
The electromagnetic properties of two-dimensional metallic nanostructures in the optical frequency range are studied. One example of such a structure is a periodic array of thin metallic strip pairs. The magnetic response of these structures is studied, as is the closely related emergence of the negative index of refraction propagation bands. The presence of such bands is found to critically depend on the proximity of electric and magnetic dipole resonances. It is demonstrated that the frequencies of those resonances are strongly dependent on the ratio of the structure thickness and the plasmonic skin depth. Electromagnetic structures that are much thicker than the plasmonic skin depth are shown to exhibit standard broad antenna resonances at the wavelength roughly twice the strip length. As the structures are scaled down to resonate in the visible/mid-infrared, electrostatic resonances determine the electromagnetic properties of such materials.
Engineering band structure in nanoscale quantum-dot supercrystals.
Baimuratov, Anvar S; Rukhlenko, Ivan D; Fedorov, Anatoly V
2013-07-01
Supercrystals made of periodically arranged semiconductor quantum dots (QDs) are promising structures for nanophotonics applications due to almost unlimited degrees of freedom enabling fine tuning of their optical responses. Here we demonstrate broad engineering opportunities associated with the possibility of tailoring the energy bands of excitons in two-dimensional quantum-dot supercrystals through the alteration in the QD arrangement. These opportunities offer an unprecedented control over the optical properties of the supercrystals, which may be used as a versatile material base for advanced photonics devices on the nanoscale.
Photofield emission spectroscopy of the tungsten <112> band structure
NASA Astrophysics Data System (ADS)
Radoń, T.; Jaskółka, S.
1991-05-01
Optical transitions in photofield emission (PFE) characteristics from the (112) plane of tungsten have been observed at five values of photon energy in the visible range. To measure very small PFE currents modulated laser radiation and phase-sensitive detection have been used. Shoulders in the characteristics obtained with s-polarized light correspond to transitions in the bulk band structure near the Fermi level in accordance with the theoretical results of Christensen and Feuerbacher [1]. Using p-polarized light, peaks of the surface density of states, lying below the Fermi level, were observed in a good agreement with both the field and photofield emission distributions.
Anomalous quasiparticle lifetime in graphite: band structure effects.
Spataru, C D; Cazalilla, M A; Rubio, A; Benedict, L X; Echenique, P M; Louie, S G
2001-12-10
We report ab initio calculations of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW approximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.
Band Structure Asymmetry of Bilayer Graphene Revealed by Infrared Spectroscopy
Li, Z.Q.; Henriksen, E.A.; Jiang, Z.; Hao, Zhao; Martin, Michael C.; Kim, P.; Stormer, H.L.; Basov, Dimitri N.
2008-12-10
We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observe a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the next-nearest-neighbor interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined.
X-BAND TRAVELING WAVE RF DEFLECTOR STRUCTURES
Wang, J.W.; Tantawi, S.; /SLAC
2008-12-18
Design studies on the X-Band transverse RF deflectors operating at HEM{sub ll} mode have been made for two different applications. One is for beam measurement of time-sliced emittance and slice energy spread for the upgraded LCLS project, its optimization in RF efficiency and system design are carefully considered. Another is to design an ultra-fast RF kicker in order to pick up single bunches from the bunch-train of the B-factory storage ring. The challenges are to obtain very short structure filling time with high RF group velocity and good RF efficiency with reasonable transverse shunt impedance. Its RF system will be discussed.
Band structure and optical properties of silicon carbide
NASA Astrophysics Data System (ADS)
Gavrilenko, Vladimir I.; Frolov, Sergey I.
1991-03-01
Silicon carbide is an interesting high-temperature large band gap semiconquctor. it ispromising as a basical material for optoelectronic devices . The optical properties of SiC have been studied by several authors. The absrption coefficient of SiC 6H3 has been measured by Choyke and Patrick up to 4.9 eV and by Makarov to 5.8 eV. Reflection spectra of 6H, 15R, and 3C SiC in the range 3.0 to 13 eV have been stidied in . The optical constants of SiC 6H have been measured by reflectivity in the range 4 to 25 eV, The energies of direct optical transitions between subbands in the conduction band, resulting from confinement in a one dimensional superI,at,,tice, have been measured in8sveral polytypes of SiC by absorption ' and electroreflection (ER) ' The electron energy band structure (85) of SiC of1he1 halerite structure (3C SiC) has been calculated by several 1tu4r2 ' . BS of wurtzite modification of SiC have been calculated 1 in ' ' ' for 2H iC. BS of 4H and 6H SiC has been calculated by the semiempirical pseudopotential method at high-symmetry points of the Brillouin zone (BZ). Tight binding calculations of 2H SiC show valence bands which agree with experiment, but unrealistic conductive bands due to the restriction to nearest neighbours in the Hamiltonian matrix In this work we report the electroreflectance (ER) spectra of hexagonal (4H and 6H) and cubic SiC measured in the range 1.0 to 5.6 eV. Values of direct optical gps1ave been obtained from the ER spectra using a multiple oscillator model ' . BS of SiC has been calculated by the first-principles self-consistent linear muffin-tin orbital (LMTO-ASA) method (2H, 4H, and 6H SiC) and by the semiempirical pseudopotential method (3C SiC). Calculated BS parameters have been compared with experimental data measured in this work and those available in the literature.
Exciton Band Structure in Two-Dimensional Materials
NASA Astrophysics Data System (ADS)
Cudazzo, Pierluigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo
2016-02-01
Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene.
Exciton Band Structure in Two-Dimensional Materials.
Cudazzo, Pierluigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo
2016-02-12
Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene.
Multi-band circular polarizer using planar spiral metamaterial structure.
Ma, Xiaoliang; Huang, Cheng; Pu, Mingbo; Hu, Chenggang; Feng, Qin; Luo, Xiangang
2012-07-02
A multi-band circular polarizer is proposed by using multi layered planar spiral metamaterial structure in analogy with classic spiral antenna. At three distinct resonant frequencies, the incident linearly polarized wave with electric field paralleling to one specific direction is transformed into left/right-handed circularly polarized waves through electric field coupling. Measured and simulated results show that right-handed circularly polarized wave is produced at 13.33 GHz and 16.75 GHz while left-handed circularly polarized wave is obtained at 15.56 GHz. The surface current distributions are studied to investigate the transformation behavior for both circular polarizations. The relationship between the resonant positions and the structure parameters is discussed as well.
Levels of Structural Integration and Facial Expressions of Negative Emotions.
Bock, Astrid; Huber, Eva; Benecke, Cord
2016-09-01
For a clinically relevant understanding of facial displays of patients with mental disorders it is crucial to go beyond merely counting frequencies of facial expressions, but include the contextual information of the expression. We assume that patients with different levels of structural integration differ in the contextual embedding of their negative facial expressions of emotions. Facial affective behaviour of 80 female participants during an OPD interview was analysed using FACS (Facial Action Coding System) and the RFE coding system (Referencesof- Facial-Expression coding system; Bock et al. 2015).Using the RFE coding system, 2192 negative facial expressions of emotions were attributed to different references (e.g., interactive, self-related, object-related) by relying on contextual variables. Pure frequency of negative facial affect was not related to level of structural integration. Negative facial expressions of emotions directed towards the interviewer (interactive reference), as well as negative facial expressions directed towards the displayer's whole self were associated with lower levels of structural integration. In contrast, negative facial affects directed to single aspects of the self, to single aspects of objects, or to external situations were associated with higher levels of structural integration. The differentiation of references of facial affective behavior allows a deeper understanding of the connection between facial displays and structural levels of psychic integration.
Santos-Ortiz, Reinaldo; Volkov, Vyacheslav; Schmid, Stefan; Kuo, Fang-Ling; Kisslinger, Kim; Nag, Soumya; Banerjee, Rajarshi; Zhu, Yimei; Shepherd, Nigel D
2013-04-10
Battery electrodes in thin-film form are free of the binders used with traditional powder electrodes and present an ideal platform to obtain basic insight to the evolution of the electrode-electrolyte interface passivation layer, the formation of secondary phases, and the structural underpinnings of reversibility. This is particularly relevant to the not yet fully understood conversion electrode materials, which possess enormous potential for providing transformative capacity improvements in next-generation lithium-ion batteries. However, this necessitates an understanding of the electronic charge transport properties and band structure of the thin films. This work presents an investigation of the electron transport properties of iron fluoride (FeF2) thin-film electrodes for Li-ion batteries. FeF2 thin films were prepared by pulsed-laser deposition, and their phase purity was characterized by electron microscopy and diffraction. The grown materials are polycrystalline FeF2 with a P42/mnm crystallographic symmetry. Room-temperature Hall measurements reveal that as-deposited FeF2 is n-type: the Hall coefficients were negative, electron mobility was 0.33 cm2/(V s) and resistivity was 0.255 Ω cm. The electronic band diagram of FeF2 was obtained using a combination of ultraviolet photoelectron spectroscopy, photoluminescence, photoluminescence excitation and optical absorption, which revealed that FeF2 is a direct bandgap, n-type semiconductor whose band structure is characterized by a 3.4 eV bandgap, a workfunction of ∼4.51 eV, and an effective Fermi level that resides approximately 0.22 eV below the conduction band edge. We propose that the shallow donor levels at 0.22 eV are responsible for the measured n-type conductivity. The band diagram was used to understand electron transport in FeF2 thin film and FeF2-C composite electrodes.
Structural Evolution of a Warm Frontal Precipitation Band During GCPEx
NASA Technical Reports Server (NTRS)
Colle, Brian A.; Naeger, Aaron; Molthan, Andrew; Nesbitt, Stephen
2015-01-01
A warm frontal precipitation band developed over a few hours 50-100 km to the north of a surface warm front. The 3-km WRF was able to realistically simulate band development, although the model is somewhat too weak. Band genesis was associated with weak frontogenesis (deformation) in the presence of weak potential and conditional instability feeding into the band region, while it was closer to moist neutral within the band. As the band matured, frontogenesis increased, while the stability gradually increased in the banding region. Cloud top generating cells were prevalent, but not in WRF (too stable). The band decayed as the stability increased upstream and the frontogenesis (deformation) with the warm front weakened. The WRF may have been too weak and short-lived with the band because too stable and forcing too weak (some micro issues as well).
Low porosity metallic periodic structures with negative Poisson's ratio.
Taylor, Michael; Francesconi, Luca; Gerendás, Miklós; Shanian, Ali; Carson, Carl; Bertoldi, Katia
2014-04-16
Auxetic behavior in low porosity metallic structures is demonstrated via a simple system of orthogonal elliptical voids. In this minimal 2D system, the Poisson's ratio can be effectively controlled by changing the aspect ratio of the voids. In this way, large negative values of Poisson's ratio can be achieved, indicating an effective strategy for designing auxetic structures with desired porosity.
NASA Astrophysics Data System (ADS)
Hsueh, W. J.; Chen, R. F.; Tang, K. Y.
2008-11-01
We present a divergence-free method to determine the characteristics of band structures and projected band structures of transverse acoustic phonons in Fibonacci superlattices. A set of bandedge equations is formulated to solve the band structures for the phonon instead of using the traditional dispersion relation. Numerical calculations show band structures calculated by the present method for the Fibonacci superlattice without numerical instability, which may occur in traditional methods. Based on the present formalism, the band structure for the acoustic phonons has been characterized by closure points and the projected bandgaps of the forbidden bands. The projected bandgaps are determined by the projected band structure, which is characterized by the cross points of the projected bandedges. We observed that the band structure and projected band structure and their characteristics were quite different for different generation orders and the basic layers for the Fibonacci superlattice. In this study, concise rules to determine these characteristics of the band structure and the projected band structure, including the number and the location of closure points of forbidden bands and those of projected bandgaps, in Fibonacci superlattices with arbitrary generation order and basic layers are proposed.
Band structure of ABC-trilayer graphene superlattice
NASA Astrophysics Data System (ADS)
Uddin, Salah; Chan, K. S.
2014-11-01
We investigate the effect of one-dimensional periodic potentials on the low energy band structure of ABC trilayer graphene first by assuming that all the three layers have the same potential. Extra Dirac points having the same electron hole crossing energy as that of the original Dirac point are generated by superlattice potentials with equal well and barrier widths. When the potential height is increased, the numbers of extra Dirac points are increased. The dispersions around the Dirac points are not isotropic. It is noted that the dispersion along the ky direction for kx = 0 oscillates between a non-linear dispersion and a linear dispersion when the potential height is increased. When the well and barrier widths are not identical, the symmetry of the conduction and valence bands is broken. The extra Dirac points are shifted either upward or downward depending on the barrier and well widths from the zero energy, while the position of the central Dirac point oscillates with the superlattice potential height. By considering different potentials for different layers, extra Dirac points are generated not from the original Dirac points but from the valleys formed in the energy spectrum. Two extra Dirac points appear from each pair of touched valleys, so four Dirac points appeared in the spectrum at particular barrier height. By increasing the barrier height of superlattice potential two Dirac points merge into the original Dirac point. This emerging and merging of extra Dirac points is different from the equal potential case.
Negative index fishnet structures with nanopillars formed by nanoimprint lithography
NASA Astrophysics Data System (ADS)
Sharp, Graham J.; Khan, Saima I.; Khokhar, Ali Z.; De La Rue, Richard M.; Johnson, Nigel P.
2014-05-01
Since their inception, metamaterial fishnet structures have frequently been used to exhibit a negative refractive index. Their shape and structure make it possible to independently produce both a negative permeability (μ) and a negative permittivity (ɛ). Fishnets that display this characteristic can be referred to as a double negative metamaterial. Although other techniques have been demonstrated, fishnets are commonly fabricated using electron-beam lithography (EBL) or focused ion-beam (FIB) milling. In this paper we demonstrate the fabrication of fishnets using nano-imprint lithography (NIL). Advantages associated with NIL include a shorter fabrication time, a larger feasible pattern area and reduced costs. In addition to these advantages, the quality of the fabricated structures is excellent. We imprint a stamp directly into a metal-dielectric-metal stack which creates the fishnet and, as an artifact of the technique, a periodic array of nanopillars. Two different designs of the fishnet and nanopillar structure have been fabricated and optical measurements have been taken from both. In addition to the experimental measurements the structures have also been extensively simulated, suggesting a negative refractive index with a real part as large in magnitude as five can be achieved.
Polar semiconductor heterojunction structure energy band diagram considerations
Lin, Shuxun; Wen, Cheng P. Wang, Maojun; Hao, Yilong
2016-03-28
The unique nature of built-in electric field induced positive/negative charge pairs of polar semiconductor heterojunction structure has led to a more realistic device model for hexagonal III-nitride HEMT. In this modeling approach, the distribution of charge carriers is dictated by the electrostatic potential profile instead of Femi statistics. The proposed device model is found suitable to explain peculiar properties of GaN HEMT structures, including: (1) Discrepancy in measured conventional linear transmission line model (LTLM) sheet resistance and contactless sheet resistance of GaN HEMT with thin barrier layer. (2) Below bandgap radiation from forward biased Nickel Schottky barrier diode on GaN HEMT structure. (3) GaN HEMT barrier layer doping has negligible effect on transistor channel sheet charge density.
The structure of band-tail states in amorphous silicon
NASA Astrophysics Data System (ADS)
Drabold, D. A.; Dong, Jianjun
1997-03-01
We compute and analyze the electronic eigenstates of the realistic and large (4096) atom model of Djordjevic, Thorpe and Wooten(B. R. Djordjevic, M. F. Thorpe and F. Wooten, Phys. Rev. B 52) 5685 (1995) in the vicinity of the gap. We discuss the structure, localization, and conductivity (from the Kubo formula) for states from midgap, well into the interior of the valence band. Tight-binding and ab initio (LDA) approximations for the states are discussed; comparisons are made to total yield photoelectron spectroscopy measurements(S. Aljisji, J. D. Cohen, S. Jin and L. Ley, Phys. Rev. Lett. 64), 2811 (1990). This is an extension of our earlier work on amorphous diamond( J. Dong and D. A. Drabold, Phys. Rev. B 54) 10284 (1996) to a-Si.
Band Structure for a Lattice with a Single Resonance
NASA Astrophysics Data System (ADS)
Monsivais, G.; Moshinsky, M.
1998-03-01
We study the band structure of a chain of scatterers that in general cannot be described by means of a potential. In order to describe these kind of systems we have followed the ideas of Wigner who stressed though that an interaction should be described by a R matrix. In particular, we have considered an infinite sequence of scatterers, each one described by means of a R matrix with a single resonance. This study is an extension of a recent paper ( M. Moshinsky and G. Monsivais, J. Phys. G: Nucl. Part. Phys. 23), 573-588, (1997) where we have studied the delay time for a single scatterer using a R matrix. We compare our results with those than appear in the description of some superlattices.
Dynamical theory of shear bands in structural glasses
NASA Astrophysics Data System (ADS)
Wisitsorasak, Apiwat; Wolynes, Peter G.
2017-02-01
The heterogeneous elastoplastic deformation of structural glasses is explored using the framework of the random first-order transition theory of the glass transition along with an extended mode-coupling theory that includes activated events. The theory involves coupling the continuum elastic theory of strain transport with mobility generation and transport as described in the theory of glass aging and rejuvenation. Fluctuations that arise from the generation and transport of mobility, fictive temperature, and stress are treated explicitly. We examine the nonlinear flow of a glass under deformation at finite strain rate. The interplay among the fluctuating fields leads to the spatially heterogeneous dislocation of the particles in the glass, i.e., the appearance of shear bands of the type observed in metallic glasses deforming under mechanical stress.
Dynamical theory of shear bands in structural glasses.
Wisitsorasak, Apiwat; Wolynes, Peter G
2017-02-07
The heterogeneous elastoplastic deformation of structural glasses is explored using the framework of the random first-order transition theory of the glass transition along with an extended mode-coupling theory that includes activated events. The theory involves coupling the continuum elastic theory of strain transport with mobility generation and transport as described in the theory of glass aging and rejuvenation. Fluctuations that arise from the generation and transport of mobility, fictive temperature, and stress are treated explicitly. We examine the nonlinear flow of a glass under deformation at finite strain rate. The interplay among the fluctuating fields leads to the spatially heterogeneous dislocation of the particles in the glass, i.e., the appearance of shear bands of the type observed in metallic glasses deforming under mechanical stress.
Negative Stains Containing Trehalose: Application to Tubular and Filamentous Structures
NASA Astrophysics Data System (ADS)
Harris, J. Robin; Gerber, Max; Gebauer, Wolfgang; Wernicke, Wolfgang; Markl, Jürgen
1996-02-01
Several examples are presented that show the successful application of uranyl acetate and ammonium molybdate negative staining in the presence of trehalose for TEM studies of filamentous and tubular structures. The principal benefit to be gained from the inclusion of trehalose stems from the considerably reduced flattening of the large tubular structures and the greater orientational freedom of single molecules due to an increased depth of the negative stain in the presence of trehalose. Trehalose is likely to provide considerable protection to protein molecules and their assemblies during the drying of negatively stained specimens. Some reduction in the excessive density imparted by uranyl acetate around large assemblies is also achieved. Nevertheless, in the presence of 1% (w/v) trehalose, it is desirable to increase the concentration of negative stain to 5% (w/v) for ammonium molybdate and to 4% for uranyl acetate to produce satisfactory image contrast. In general, the ammonium molybdate-trehalose negative stain is more satisfactory than the uranyl acetate-trehalose combination, because of the greater electron beam sensitivity of the uranyl negative stain. Reassembled taxol-stabilized pig brain microtubules, together with collagen fibrils, sperm tails, helical filaments, and reassociated hemocyanin (KLH2), all from the giant keyhole limpet Megathura crenulata, have been studied by negative staining in the presence of trehalose. In all cases satisfactory TEM imaging conditions were readily obtained on the specimens, as long as regions of excessively deep stain were avoided.
NASA Astrophysics Data System (ADS)
Piyadasa, Adimali; Wang, Sibo; Gao, Pu-Xian
2017-07-01
The electronic band structure of a solid state semiconductor determines many of its physical and chemical characteristics such as electrical, optical, physicochemical, and catalytic activity. Alteration or modification of the band structure could lead to significant changes in these physical and chemical characteristics, therefore we introduce new mechanisms of creating novel solid state materials with interesting properties. Over the past three decades, research on band structure engineering has allowed development of various methods to modify the band structure of engineered materials. Compared to bulk counterparts, nanostructures generally exhibit higher band structure modulation capabilities due to the quantum confinement effect, prominent surface effect, and higher strain limit. In this review we will discuss various band structure engineering strategies in semiconductor nanowires and other related nanostructures, mostly focusing on metal oxide systems. Several important strategies of band structure modulation are discussed in detail, such as doping, alloying, straining, interface and core-shell nanostructuring.
Band Structure Engineering for Ultra-Low Threshold Laser Diodes
1988-01-01
is lifted above the heavy hole band in the k-vector directions parellel to the applied strain. Since the heavy hole is five times heavier than the...band above the I Permanent Address : Columbia University , Dept. of Electrical Engi- neering, New York, N.Y.10027. 89 2 2 013 2 heavy hole band by strain
Band structure systematics and symmetries in even-even nuclei
NASA Astrophysics Data System (ADS)
Bucurescu, D.; Cata-Danil, Gh.; Ivascu, M.; Ur, C. A.
1993-07-01
It is shown that the experimental in-band energy ratios for the even-even nuclei obey universal systematics similar to those observed by Mallmann for the quasiground band. Systematic correlations between energy ratios belonging to different bands are also found in certain cases. Finally, correlations between mixed energy ratios are shown to be useful in characterizing the evolution of the nulcear collectivity.
Band structure systematics and symmetries in even-even nuclei
Bucurescu, D.; Cata-Danil, G.; Ivascu, M.; Ur, C.A. )
1993-07-01
It is shown that the experimental in-band energy ratios for the even-even nuclei obey universal systematics similar to those observed by Mallmann for the quasiground band. Systematic correlations between energy ratios belonging to different bands are also found in certain cases. Finally, correlations between mixed energy ratios are shown to be useful in characterizing the evolution of the nulcear collectivity.
Excitation of guided waves in layered structures with negative refraction.
Shadrivov, Ilya; Ziolkowski, Richard; Zharov, Alexander; Kivshar, Yuri
2005-01-24
We study the electromagnetic beam reflection from layered structures that include the so-called double-negative metamaterials, also called left-handed metamaterials. We predict that such structures can demonstrate a giant lateral Goos-Hänchen shift of the scattered beam accompanied by a splitting of the reflected and transmitted beams due to the resonant excitation of surface waves at the interfaces between the conventional and double-negative materials as well as due to the excitation of leaky modes in the layered structures. The beam shift can be either positive or negative, depending on the type of the guided waves excited by the incoming beam. We also perform finite-difference time-domain simulations and confirm the major effects predicted analytically.
Harding, T.P.
1985-04-01
Negative and positive flower structures and positive inverted structures imply specific modes of formation, and their distinctive characteristics make them important criteria for the identification of certain structural styles. A negative flower structure from the Andaman Sea consists of a shallow synform bounded by upward-spreading strands of a wrench fault that have mostly normal separations. Paralleling monoclines and oblique, en echelon normal faults flank the divergent wrench fault. A positive flower structure from the Ardmore basin, Oklahoma, consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations. En echelon folds are present on either side of this convergent wrench fault. Positive structural inversion at the Rambutan oil field, South Sumatra basin, has formed a shallow anticlinorium and has partly uplifted the underlying graben. Deeper fault segments bounding the graben have retained their normal fault profiles, but at shallow levels some of these faults have reverse separations.
Investigations of the Band Structure and Morphology of Nanostructured Surfaces
NASA Astrophysics Data System (ADS)
Knox, Kevin R.
2011-12-01
In this dissertation, I examine the electronic structure of two very different types of two-dimensional systems: valence band electrons in single layer graphene and electronic states created at the vacuum interface of single crystal copper surfaces. The characteristics of both electronic systems depend intimately on the morphology of the surfaces they inhabit. Thus, in addition to discussing the respective band structures of these systems, a significant portion of this dissertation will be devoted to measurements of the surface morphology of these systems. Free-standing exfoliated monolayer graphene is an ultra-thin flexible membrane and, as such, is known to exhibit large out-of-plane deformation due to substrate and adsorbate interaction as well as thermal vibrations and, possibly, intrinsic buckling. Such crystal deformation is known to limit mobility and increase local chemical reactivity. Additionally, deformations present a measurement challenge to researchers wishing to determine the band structure by angle-resolved photoemission since they limit electron coherence in such measurements. In this dissertation, I present low energy electron microscopy and micro probe diffraction measurements, which are used to image and characterize corrugation in SiO2-supported and suspended exfoliated graphene at nanometer length scales. Diffraction line-shape analysis reveals quantitative differences in surface roughness on length scales below 20 nm which depend on film thickness and interaction with the substrate. Corrugation decreases with increasing film thickness, reflecting the increased stiffness of multilayer films. Specifically, single-layer graphene shows a markedly larger short range roughness than multilayer graphene. Due to the absence of interactions with the substrate, suspended graphene displays a smoother morphology and texture than supported graphene. A specific feature of suspended single-layer films is the dependence of corrugation on both adsorbate load
Banded Structure and Domain Arrangements in PbTiO3 Single Crystals
NASA Astrophysics Data System (ADS)
Chou, Chen-Chia; Chen, Cheng-Sao
1998-09-01
In the present work we report the ferroelectric domain arrangements and characteristics of banded structures observed in flux-grown PbTiO3 single crystals. Investigations of etched crystals indicate that most of the specimens show banded structures which may correspond to surface relief characteristics of as-grown crystals. Banded structures, which are different from domain structures, possess structural characteristics similar to those of martensite variants in various alloys and ceramics. The stresses produced during transformation were relieved by the formation of the banded structures and the 90° domain structures, suggesting that the band structures and 90° domains are the products of self-accommodation in as-grown PbTiO3 crystals during the cubic/tetragonal (C/T) transformation. Image characteristics imply that banded-structure-induced domain intersections may store high strain energy in lead titanate crystals and therefore hinder polarization switching of the crystals.
X-band rf structure with integrated alignment monitors
NASA Astrophysics Data System (ADS)
Dehler, M.; Raguin, J.-Y.; Citterio, A.; Falone, A.; Wuensch, W.; Riddone, G.; Grudiev, A.; Zennaro, R.
2009-06-01
We present the electrical design for an X-band traveling wave accelerator structure with integrated alignment monitors to measure the transverse wake, which will be used as part of the PSI-XFEL project and in the CLIC structure testing program. At PSI, it will compensate nonlinearities in the longitudinal phase space at the injector prototype of the PSI-XFEL. At CLIC it will be tested for breakdown limits and rates in the high gradient regime. The prolonged operation of such a structure in the PSI-XFEL injector, albeit not for the CLIC parameter regime, will constitute a good quality test of the manufacturing procedures employed. The operation in the PSI-XFEL injector will be at a relatively modest beam energy of 250 MeV, at which transverse wakes can easily destroy the beam emittance. For this reason, the layout chosen employs a large iris, 5π/6 phase advance geometry, which minimizes transverse wakefield effects while still retaining a good efficiency. As a second important feature, the design includes two wakefield monitors coupling to the transverse higher order modes, which allow steering the beam to the structure axis, potentially facilitating a higher precision than mechanical alignment strategies. Of special interest is the time domain envelope of these monitor signals. Local offsets due to bends or tilts have individual signatures in the frequency spectrum, which in turn are correlated with different delays in the signal envelope. By taking advantage of this combined with the single bunch mode at the PSI-XFEL, the use of a relatively simple detector-type rf front end should be possible, which will not only show beam offsets but also higher order misalignments such as tilts in the structure. The resolution of these monitors is determined by the tolerance of the random cell-to-cell misalignment leading to a spurious signal in the monitors.
Phononic Band Gaps in 2D Quadratic and 3D Cubic Cellular Structures
Warmuth, Franziska; Körner, Carolin
2015-01-01
The static and dynamic mechanical behaviour of cellular materials can be designed by the architecture of the underlying unit cell. In this paper, the phononic band structure of 2D and 3D cellular structures is investigated. It is shown how the geometry of the unit cell influences the band structure and eventually leads to full band gaps. The mechanism leading to full band gaps is elucidated. Based on this knowledge, a 3D cellular structure with a broad full band gap is identified. Furthermore, the dependence of the width of the gap on the geometry parameters of the unit cell is presented. PMID:28793713
Electronic structure of graphene: (Nearly) free electron bands versus tight-binding bands
NASA Astrophysics Data System (ADS)
Kogan, E.; Silkin, V. M.
2017-09-01
In our previous paper (Phys. Rev. B {\\bf 89}, 165430 (2014)) we have found that in graphene, in distinction to the four occupied bands, which can be described by the simple tight-binding model (TBM) with four atomic orbitals per atom, the two lowest lying at the $\\Gamma$-point unoccupied bands (one of them of a $\\sigma$ type and the other of a $\\pi$ type) can not be described by such model. In the present work we suggest a minimalistic model for these two bands, based on (nearly) free electrons model (FEM), which correctly describes the symmetry of these bands, their dispersion law and their localization with respect to the graphene plane.
Gupta, Sandhya; Tuttle, Gary L.; Sigalas, Mihail; McCalmont, Jonathan S.; Ho, Kai-Ming
2001-08-14
A method of manufacturing a flexible metallic photonic band gap structure operable in the infrared region, comprises the steps of spinning on a first layer of dielectric on a GaAs substrate, imidizing this first layer of dielectric, forming a first metal pattern on this first layer of dielectric, spinning on and imidizing a second layer of dielectric, and then removing the GaAs substrate. This method results in a flexible metallic photonic band gap structure operable with various filter characteristics in the infrared region. This method may be used to construct multi-layer flexible metallic photonic band gap structures. Metal grid defects and dielectric separation layer thicknesses are adjusted to control filter parameters.
Band-structure analysis from photoreflectance spectroscopy in (Ga,Mn)As
Yastrubchak, Oksana; Gluba, Lukasz; Zuk, Jerzy; Wosinski, Tadeusz; Andrearczyk, Tomasz; Domagala, Jaroslaw Z.; Sadowski, Janusz
2013-12-04
Modulation photoreflectance spectroscopy has been applied to study the band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn content. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap-transition energy in the (Ga,Mn)As layers with increasing Mn content are interpreted in terms of a disordered valence band, extended within the band gap, formed, in highly Mn-doped (Ga,Mn)As, as a result of merging the Mn-related impurity band with the host GaAs valence band.
NASA Astrophysics Data System (ADS)
Medeiros, Paulo V. C.; Stafström, Sven; Björk, Jonas
2014-01-01
We use a band unfolding technique to recover an effective primitive cell picture of the band structure of graphene under the influence of different types of perturbations. This involves intrinsic perturbations, such as structural defects, and external ones, comprising nitrogen substitutions and the inclusion of graphene in adsorbed systems. In such cases, the band unfolding provides a reliable and efficient tool for quantitatively analyzing the effect of doping and defects on the electronic structure of graphene. We envision that this approach will become a standard method in the computational analysis of graphene's electronic structure in related systems.
Collective Band Structures in the Neutron-Rich 107,109Ru Nuclei
NASA Astrophysics Data System (ADS)
Zhu, Sheng-jiang; Gan, Cui-yun; J, Hamilton H.; A, Ramayya V.; B, Babu R. S.; M, Sakhaee; W, Ma C.; Long, Gui-lu; Deng, Jing-kang; Zhu, Ling-yan; Li, Ming; Yang, Li-ming; J, Komicki; J, Cole D.; R, Aryaeinejad; Y, Dardenne K.; M, Drigert W.; J, Rasmussen O.; M, Stoyer A.; S, Chu Y.; K, Gregorich E.; M, Mohar F.; S, Prussin G.; I, Lee Y.; N, Johnson R.; F, McGowan K.
1998-11-01
The levels in neutron-rich odd-A 107,109Ru nuclei have been investigated by using γ-γ- and γ-γ-γ-coincidence studies of the prompt γ-rays from the spontaneous fission of 252Cf. The ground state bands and the negative parity bands are identified and expanded in both nuclei. Triaxial rotor plus particle model calculations indicate the ground state bands originate from ν(d5/2 + g7/2) quasiparticle configurations and the negative parity bands are from νh11/2 orbital.
Band structure of ABC-trilayer graphene superlattice
Uddin, Salah Chan, K. S.
2014-11-28
We investigate the effect of one-dimensional periodic potentials on the low energy band structure of ABC trilayer graphene first by assuming that all the three layers have the same potential. Extra Dirac points having the same electron hole crossing energy as that of the original Dirac point are generated by superlattice potentials with equal well and barrier widths. When the potential height is increased, the numbers of extra Dirac points are increased. The dispersions around the Dirac points are not isotropic. It is noted that the dispersion along the k{sub y} direction for k{sub x} = 0 oscillates between a non-linear dispersion and a linear dispersion when the potential height is increased. When the well and barrier widths are not identical, the symmetry of the conduction and valence bands is broken. The extra Dirac points are shifted either upward or downward depending on the barrier and well widths from the zero energy, while the position of the central Dirac point oscillates with the superlattice potential height. By considering different potentials for different layers, extra Dirac points are generated not from the original Dirac points but from the valleys formed in the energy spectrum. Two extra Dirac points appear from each pair of touched valleys, so four Dirac points appeared in the spectrum at particular barrier height. By increasing the barrier height of superlattice potential two Dirac points merge into the original Dirac point. This emerging and merging of extra Dirac points is different from the equal potential case.
Band-Gap Design of Quaternary (In,Ga) (As,Sb) Semiconductors via the Inverse-Band-Structure Approach
Piquini, P.; Graf, P. A.; Zunger. A.
2008-01-01
Quaternary systems illustrated by (Ga,In)(As,Sb) manifest a huge configurational space, offering in principle the possibility of designing structures that are lattice matched to a given substrate and have given electronic properties (e.g., band gap) at more than one composition. Such specific configurations were however, hitherto, unidentified. We show here that using a genetic-algorithm search with a pseudopotential Inverse-band-structure (IBS) approach it is possible to identify those configurations that are naturally lattice matching (to GaSb) and have a specific band gap (310 meV) at more than one composition. This is done by deviating from randomness, allowing the IBS to find a partial atomic ordering. This illustrates multitarget design of the electronic structure of multinary systems.
Pattern reconfigurable antenna using electromagnetic band gap structure
NASA Astrophysics Data System (ADS)
Ismail, M. F.; Rahim, M. K. A.; Majid, H. A.; Hamid, M. R.; Yusoff, M. F. M.; Dewan, R.
2017-01-01
In this paper, a single rectangular patch antenna incorporated with an array of electromagnetic band gap (EBG) structures is proposed. The proposed antenna features radiation pattern agility by means of connecting the shorting pin vias to the EBG unit cells. The proposed design consists of 32 mm × 35.5 mm rectangular patch antenna and 10.4-mm-square mushroom-like EBG unit cells. The EBGs are placed at both sides of the antenna radiating patch and located on the thicker substrate of thickness, h. The copper tape which represents the PIN diode is used to control the connection between the EBG's via and the ground plane as reconfigurable mechanism of the antenna. The simulated result shows by switching the ON and OFF EBG structures in either sides or both, the directional radiation pattern can be tilted from 0 to +14°. The proposed antenna exhibits 7.2 dB realized gain at 2.42 GHz. The parametric study on EBG and antenna is also discussed.
Deformation mechanisms in negative Poisson's ratio materials - Structural aspects
NASA Technical Reports Server (NTRS)
Lakes, R.
1991-01-01
Poisson's ratio in materials is governed by the following aspects of the microstructure: the presence of rotational degrees of freedom, non-affine deformation kinematics, or anisotropic structure. Several structural models are examined. The non-affine kinematics are seen to be essential for the production of negative Poisson's ratios for isotropic materials containing central force linkages of positive stiffness. Non-central forces combined with pre-load can also give rise to a negative Poisson's ratio in isotropic materials. A chiral microstructure with non-central force interaction or non-affine deformation can also exhibit a negative Poisson's ratio. Toughness and damage resistance in these materials may be affected by the Poisson's ratio itself, as well as by generalized continuum aspects associated with the microstructure.
Sakai, Osamu; Sakaguchi, Takui; Tachibana, Kunihide
2007-04-01
Two theoretical approaches appropriate for two-dimensional plasma photonic crystals reveal dispersions of propagating waves including photonic (electromagnetic) band gaps and multiflatbands. A modified plane-wave expansion method yields dispersions of collisional periodical plasmas, and the complex-value solution of a wave equation by a finite difference method enables us to obtain dispersions with structure effects in an individual microplasma. Periodical plasma arrays form band gaps as well as normal photonic crystals, and multiflatbands are present below the electron plasma frequency in the transverse electric field mode. Electron elastic collisions lower the top frequency of the multiflatbands but have little effect on band gap properties. The spatial gradient of the local dielectric constant resulting from an electron density profile widens the frequency region of the multiflatbands, as demonstrated by the change of surface wave distributions. Propagation properties described in dispersions including band gaps and flatbands agree with experimental observations of microplasma arrays.
NASA Astrophysics Data System (ADS)
Pomogaeva, Anna; Springborg, Michael; Kirtman, Bernard; Gu, Feng Long; Aoki, Yuriko
2012-12-01
A method developed for building band structure of polymer from elongation oligomer calculations has been applied for studying electronic structures of nanoscaled periodic systems. Band structures of single-walled (4,4) carbon and boron nitrogen nanotubes have been built at different levels of approximations. This method also has been applied for band structure construction of molecular systems where periodicity is disturbed by presence of local defects.
NASA Astrophysics Data System (ADS)
Walrath, Jenna Cherie
Low-dimensional semiconductor structures are important for a wide variety of applications, and recent advances in nanoscale fabrication are paving the way for increasingly precise nano-engineering of a wide range of materials. It is therefore essential that the physics of materials at the nanoscale are thoroughly understood to unleash the full potential of nanotechnology, requiring the development of increasingly sophisticated instrumentation and modeling. Of particular interest is the relationship between the local density of states (LDOS) of low-dimensional structures and the band structure and local electronic properties. This dissertation presents the investigation of the band structure, LDOS, and local electronic properties of nanostructures ranging from zero-dimensional (0D) quantum dots (QDs) to two-dimensional (2D) thin films, synthesizing computational and experimental approaches including Poisson-Schrodinger band structure calculations, scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and scanning thermoelectric microscopy (SThEM). A method is presented for quantifying the local Seebeck coefficient (S) with SThEM, using a quasi-3D conversion matrix approach to directly convert temperature gradient-induced voltages S. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces. The local carrier concentration, n, is profiled across epitaxial InAs/GaAs QDs, where SThEM is used to profile the temperature gradient-induced voltage, which is converted to a profile of the local S and finally to an n profile. The S profile is converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. The combined computational-experimental approach suggests a reduced n in the QD center in
Projected shell model study of band structure of 90Nb
NASA Astrophysics Data System (ADS)
Kumar, Amit; Singh, Dhanvir; Gupta, Anuradha; Singh, Suram; Bharti, Arun
2016-05-01
A systematic study of two-quasiparticle bands of the odd-odd 90Nb nucleus is performed using the projected shell model approach. Yrast band with some other bands have been obtained and back-bending in moment of inertia has also been calculated and compared with the available experimental. On comparing the available experimental data, it is found that the treatment with PSM provides a satisfactory explanation of the available data.
Influence of banded structure on the mechanical properties of a high-strength maraging steel
Ahmed, M.; Salam, I.; Hashmi, F.H.; Khan, A.Q.
1997-04-01
Chemical inhomogeneity results in the formation of banded structure in high-strength maraging steels. Segregation of titanium and molybdenum was found to be the primary cause of banded structure formation. When the concentrations of these elements increased beyond certain critical levels, bands comprising different grain sizes formed. The inclusions existed preferentially along the interface of the bands. A high-temperature homogenization treatment substantially reduced or eliminated the banded structure. The large grain size resulting from the homogenization treatment was subsequently reduced by a grain refinement treatment. The mechanical properties of the steel substantially improved following homogenization and grain refinement.
Design of an X-band accelerating structure using a newly developed structural optimization procedure
NASA Astrophysics Data System (ADS)
Huang, Xiaoxia; Fang, Wencheng; Gu, Qiang; Zhao, Zhentang
2017-05-01
An X-band high gradient accelerating structure is a challenging technology for implementation in advanced electron linear accelerator facilities. The present work discusses the design of an X-band accelerating structure for dedicated application to a compact hard X-ray free electron laser facility at the Shanghai Institute of Applied Physics, and numerous design optimizations are conducted with consideration for radio frequency (RF) breakdown, RF efficiency, short-range wakefields, and dipole/quadrupole field modes, to ensure good beam quality and a high accelerating gradient. The designed X-band accelerating structure is a constant gradient structure with a 4π/5 operating mode and input and output dual-feed couplers in a racetrack shape. The design process employs a newly developed effective optimization procedure for optimization of the X-band accelerating structure. In addition, the specific design of couplers providing high beam quality by eliminating dipole field components and reducing quadrupole field components is discussed in detail.
Huttner, Hagen B; Schellinger, Peter D; Struffert, Tobias; Richter, Gregor; Engelhorn, Tobias; Bassemir, Teresa; Mäurer, Mathias; Garcia, Meritxell; Schwab, Stefan; Köhrmann, Martin; Doerfler, Arnd
2009-07-01
In multiple sclerosis (MS) more than 95% of the patients have positive oligoclonal bands (OCB) in the cerebrospinal fluid (CSF). Previous studies have reported differences between patients with and without OCB mainly with regard to clinical parameters such as age, gender, disease duration, and clinical severity. However, several MRI characteristics have also been hypothesized to be distinct, and a varying lesion load in OCB-negative and -positive patients is proposed. In this study, we aimed to evaluate whether Barkhof's diagnostic MRI criteria are unequally frequently fulfilled in OCB-negative and -positive MS patients. We screened our database for all OCB-negative MS patients who had (1) been treated with the diagnosis of a clinical definite relapsing-remitting MS in our institution as well as (2) undergone CSF analysis and MR brain imaging during hospital stay between January 2004 and December 2007. Eleven OCB-negative patients were identified who fulfilled these criteria. In a second step, we carefully matched each of them to two OCB-positive controls according to age, gender, EDSS, and disease duration. The separate analysis of the several parameters of Barkhof's criteria revealed a less frequent prevalence of infratentorial (3/11 vs. 18/22; P = 0.005) and a more frequent occurrence of juxtacortical lesions (10/11 vs. 10/22; P = 0.022) in OCB-negative as compared to OCB-positive patients. The overall fulfillment of the Barkhof criteria did not differ in OCB-negative and -positive patients (7/11 vs. 16/22; P = 0.696). Further analyses of MRI findings between OCB-negative and -positive MS patients might contribute to a better pathophysiological understanding of the genesis and evidence of OCB in the CSF of MS patients.
Band structures in {sup 106}Ag and systematics of shears mechanism in the Aapprox110 mass region
He, C. Y.; Wu, X. G.; Wen, S. X.; Li, G. S.; Liu, Y.; Wang, Z. M.; Li, X. Q.; Ma, R. G.; Yang, C. X.; Zhu, L. H.; Cui, X. Z.; Sun, H. B.
2010-05-15
High-spin states of {sup 106}Ag were populated via the fusion-evaporation reaction {sup 100}Mo ({sup 11}B,5n){sup 106}Ag at a beam energy of 60 MeV. A rich level scheme with six band structures has been established. A high-K negative-parity band has been identified and assigned to the pig{sub 9/2} x nu[h{sub 11/2}(g{sub 7/2}/d{sub 5/2}){sup 2}] configuration. The positive parity DELTAI=1 band, built on the 12{sup +} state, is discussed in terms of the magnetic rotation model. The systematic study of similar bands in the Aapprox110 mass region indicates that the magnetic rotation phenomenon has, as lower boundary, the Ag isotopes at Z=47.
NASA Astrophysics Data System (ADS)
Eichhubl, P.; O'Brien, C. M.; Elliott, S. J.
2016-12-01
Mechanisms of brittle deformation of sediments and sedimentary rock change with burial because of increasing confining stress, change in pore fluid chemical and temperature conditions, and diagenetic state. In the field, these changes are observed in a transition from early non-cataclastic to later cataclastic deformation bands and to joint-based structures. Jurassic eolian sandstones in the San Rafael monocline and adjacent San Rafael Desert region, Utah, allow comparison of deformation band structures and their diagenetic attributes in contractional and extensional tectonic settings in close proximity. In the Entrada and Navajo Sandstones, we observe up to six generations of deformation bands, with earliest non-cataclastic bands having diffuse boundaries to host rock, and short and irregular traces. Later bands are cataclastic, more sharply defined, with long and straight traces. Cataclastic bands in the San Rafael monocline are interpreted to form as reverse faults during progressive rotation of the steeply dipping fold limb, resulting in an array of bands of varying dip. Bands in the San Rafael Desert form as normal faults with a narrower dip range. Although structural characteristics of bands differ in extensional and contractional tectonic regimes, cataclastic bands in either regime have comparable amount of porosity loss and quartz cementation indicating that tectonic regime does not influence band diagenesis. Abundance of quartz cement in bands, determined by point counting of SEM images, increases from earlier to later generations of bands and, within a single generation, with increasing slip along the band, reaching up to 24% of band volume. This trend is attributed to an increase in cataclasis with increasing host rock cementation and confining stress during burial, and, within the same generation, with increasing slip. Porosity loss by cementation tends to dominate over porosity loss by mechanical compaction. These findings demonstrate that quartz
Structure of a positive-parity band in 130Pr
NASA Astrophysics Data System (ADS)
Ma, K. Y.; Lu, J. B.; Xu, X.; Liu, Y. M.; Zhang, Z.; Li, X. Y.; Yang, D.; Liu, Y. Z.; Wu, X. G.; He, C. Y.; Zheng, Y.; Li, C. B.
2017-01-01
High-spin states of 130Pr have been populated using the 99Ru(35Cl, 2n2p)130Pr reaction at a beam energy of 151 MeV. A new positive-parity side band has been identified and possible interpretations on the origin of the side band have been discussed.
Multilayer structures as negative refractive and left-handed materials.
Chui, S T; Chan, C T; Lin, Z F
2006-02-15
We examine multilayer structures as negative refractive index and left-handed materials, and find that for one polarization there is a wide range (≈90°) of incident angle within which negative refraction will occur. This comes about because the group velocity and the Poynting vector have a large component parallel to the layers, no matter what the angle of incidence of the incoming radiation is. This behaviour in turn comes from the large anisotropy of the phase velocities. If one of the components is a ferromagnetic metal, the system can be a left-handed material above the ferromagnetic resonance frequency.
2015-03-20
This is a spin polarized state. ii) The graphene system with two monovacancie gives rise to an effective in direct band gap (pseudo gap ) of ~ 1eV...graphene with single vacancy [Figs. 4a and 4b], we can observe the separation of the tips of the Dirac cones at the “K” point. The presence of topological
Graphene band structure and its 2D Raman mode
NASA Astrophysics Data System (ADS)
Narula, Rohit; Reich, Stephanie
2014-08-01
High-precision simulations are used to generate the 2D Raman mode of graphene under a range of screening conditions and laser energies EL. We reproduce the decreasing trend of the 2D mode FWHM vs EL and the nearly linearly increasing dispersion ∂ω2D/∂EL seen experimentally in freestanding (unscreened) graphene, and propose relations between these experimentally accessible quantities and the local, two-dimensional gradients |∇ | of the electronic and TO phonon bands. In light of state-of-the-art electronic structure calculations that acutely treat the long-range e-e interactions of isolated graphene and its experimentally observed 2D Raman mode, our calculations determine a 40% greater slope of the TO phonons about K than given by explicit phonon measurements performed in graphite or GW phonon calculations in graphene. We also deduce the variation of the broadening energy γ [EL] for freestanding graphene and find a nominal value γ ˜140 meV, showing a gradually increasing trend for the range of frequencies available experimentally.
Global Kinetic Modeling of Banded Electron Structures in the Plasmasphere
NASA Technical Reports Server (NTRS)
Liemohn, M. W.; Khazanov, G. V.
1997-01-01
Significant fluxes of 10 eV to 30 keV electrons have been detected in the plasmasphere, appearing as banded structures in energy with broad spatial extents and slowly evolving over several days. It is thought that these populations are decaying plasma sheet electrons injected into the corotating region of near-Earth space. This capture can occur when the convective electric field drops rapidly and the Alfven boundary suddenly outward, trapping the inner edge of the plasma sheet along closed drift paths. Our bounce-averaged kinetic model of superthermal electron transport is able to simulate this capture and the subsequent drift, diffusion, and decay of the plasma cloud. Results of this simulation will be shown and discussed, from the initial injection during the elevated convection to the final loss of the particles. It is thought that not only Coulomb collisions but also wave-particle interactions play a significant role in altering the plasma cloud. Quasilinear diffusion is currently being incorporated into the model and the importance of this mechanism will be examined. Also, the high anisotropy of the trapped population could be unstable and generate plasma waves. These and other processes will be investigated to determine the final fate of the cloud and to quantify where, how, and when the energy of the plasma cloud is deposited. Comparisons with CRRES observations of these events are shown to verify the model and explain the data.
Halbgebauer, Steffen; Huss, André; Buttmann, Mathias; Steinacker, Petra; Oeckl, Patrick; Brecht, Isabel; Weishaupt, Andreas; Tumani, Hayrettin; Otto, Markus
2016-05-01
Oligoclonal immunoglobulin G bands (OCBs) restricted to the cerebrospinal fluid indicate intrathecal inflammation. Using isoelectric focusing and immunoblotting, they are detected in about 95 % of patients with clinically definite multiple sclerosis (MS). To elucidate whether in the remaining 5 % OCBs are truly absent or alternatively missed due to insufficient sensitivity of the routine measurement, we employed a new, highly sensitive nanoscale method for OCB detection. Capillary isoelectric focusing followed by immunological detection served to analyze OCBs in 33 well-characterized OCB-negative and 10 OCB-positive MS patients as well as in 100 OCB-negative control patients with non-inflammatory neurological diseases and 30 OCB-positive control patients with inflammatory neurological diseases. We detected intrathecal immunoglobulin G production in 10 out of 33 MS patients (30 %), initially diagnosed as being OCB-negative, and in all 10 OCB-positive MS patients, but in only 3 out of 100 non-inflammatory neurological controls (3 %) and in 29 of 30 inflammatory neurological controls (97 %). At least about one-third of MS patients without intrathecal immunoglobulin G synthesis according to standard methods are OCB-positive. Advanced methods for OCB detection may increase the analytical sensitivity for detecting OCB in patients with MS who are OCB-negative according to current routine methods.
Negative-parity high-spin states and a possible magnetic rotation band in 76 59 135Pr
NASA Astrophysics Data System (ADS)
Garg, Ritika; Kumar, S.; Saxena, Mansi; Goyal, Savi; Siwal, Davinder; Kalkal, Sunil; Verma, S.; Singh, R.; Pancholi, S. C.; Palit, R.; Choudhury, Deepika; Ghugre, S. S.; Mukherjee, G.; Kumar, R.; Singh, R. P.; Muralithar, S.; Bhowmik, R. K.; Mandal, S.
2015-11-01
Excited states in 135Pr have been investigated using the reaction 123Sb(16O,4 n )135Pr at an incident beam energy of 82 MeV. The partial level scheme has been established for negative-parity states with addition of new γ -ray transitions. The directional correlation and polarization measurements have been performed to assign spin parity for most of the reported γ -ray transitions. At high spin, a negative-parity dipole band (Δ I =1 ) has been reported along with the observation of new crossover E 2 transitions. Tilted Axis Cranking (TAC) calculations have been performed by considering a three-quasiparticle (3qp) configuration π (h11/2) 1⊗ν (h11/2) -2 and a five-quasiparticle (5qp) configuration π (h11/2) 1(g7/2) 2⊗ν (h11/2) -2 for the lower and upper parts of the band, respectively. The observed results are compared with the results of the theoretical (TAC) calculations.
Valence band energy spectrum of HgTe quantum wells with an inverted band structure
NASA Astrophysics Data System (ADS)
Minkov, G. M.; Aleshkin, V. Ya.; Rut, O. E.; Sherstobitov, A. A.; Germanenko, A. V.; Dvoretski, S. A.; Mikhailov, N. N.
2017-07-01
The energy spectrum of the valence band in HgTe /CdxHg1 -xTe quantum wells of a width (8 -20 ) nm has been studied experimentally by magnetotransport effects and theoretically in the framework of a four-band k P method. Comparison of the Hall density with the density found from a period of the Shubnikov-de Haas (SdH) oscillations clearly shows that the degeneracy of states of the top of the valence band is equal to 2 at the hole density p <5.5 ×1011cm-2 . Such degeneracy does not agree with the calculations of the spectrum performed within the framework of the four-band k P method for symmetric quantum wells. These calculations show that the top of the valence band consists of four spin-degenerate extremes located at k ≠0 (valleys) which gives the total degeneracy K =8 . It is shown that taking into account the "mixing of states" at the interfaces leads to the removal of the spin degeneracy that reduces the degeneracy to K =4 . Accounting for any additional asymmetry, for example, due to the difference in the mixing parameters at the interfaces, the different broadening of the boundaries of the well, etc., leads to reduction of the valleys degeneracy, making K =2 . It is noteworthy that for our case twofold degeneracy occurs due to degeneracy of two single-spin valleys. The hole effective mass (mh) determined from analysis of the temperature dependence of the amplitude of the SdH oscillations shows that mh is equal to (0.25 ±0.02 ) m0 and weakly increases with the hole density. Such a value of mh and its dependence on the hole density are in a good agreement with the calculated effective mass.
A unified perspective of complex band structure: interpretations, formulations, and applications.
Reuter, Matthew G
2017-02-08
Complex band structure generalizes conventional band structure by also considering wavevectors with complex components. In this way, complex band structure describes both the bulk-propagating states from conventional band structure and the evanescent states that grow or decay from one unit cell to the next. Even though these latter states are excluded by translational symmetry, they become important when translational symmetry is broken via, for example, a surface or impurity. Many studies over the last 80 years have directly or indirectly developed complex band structure for an impressive range of applications, but very few discuss its fundamentals or compare its various results. In this work we build upon these previous efforts to expose the physical foundation of complex band structure, which mathematically implies its existence. We find that a material's static and dynamic electronic structure are both completely described by complex band structure. Furthermore, we show that complex band structure reflects the minimal, intrinsic information contained in the material's Hamiltonian. These realizations then provide a context for comparing and unifying the different formulations and applications of complex band structure that have been reported over the years. Ultimately, this discussion introduces the idea of examining the amount of information contained in a material's Hamiltonian so that we can find and exploit the minimal information necessary for understanding a material's properties.
A unified perspective of complex band structure: interpretations, formulations, and applications
NASA Astrophysics Data System (ADS)
Reuter, Matthew G.
2017-02-01
Complex band structure generalizes conventional band structure by also considering wavevectors with complex components. In this way, complex band structure describes both the bulk-propagating states from conventional band structure and the evanescent states that grow or decay from one unit cell to the next. Even though these latter states are excluded by translational symmetry, they become important when translational symmetry is broken via, for example, a surface or impurity. Many studies over the last 80 years have directly or indirectly developed complex band structure for an impressive range of applications, but very few discuss its fundamentals or compare its various results. In this work we build upon these previous efforts to expose the physical foundation of complex band structure, which mathematically implies its existence. We find that a material’s static and dynamic electronic structure are both completely described by complex band structure. Furthermore, we show that complex band structure reflects the minimal, intrinsic information contained in the material’s Hamiltonian. These realizations then provide a context for comparing and unifying the different formulations and applications of complex band structure that have been reported over the years. Ultimately, this discussion introduces the idea of examining the amount of information contained in a material’s Hamiltonian so that we can find and exploit the minimal information necessary for understanding a material’s properties.
Negation, questions, and structure building in a homesign system
Franklin, Amy; Giannakidou, Anastasia; Goldin-Meadow, Susan
2013-01-01
Deaf children whose hearing losses are so severe that they cannot acquire spoken language, and whose hearing parents have not exposed them to sign language, use gestures called homesigns to communicate. Homesigns have been shown to contain many of the properties of natural languages. Here we ask whether homesign has structure building devices for negation and questions. We identify two meanings (negation, question) that correspond semantically to propositional functions, that is, to functions that apply to a sentence (whose semantic value is a proposition, φ) and yield another proposition that is more complex (¬φ for negation; ?φ for question). Combining φ with¬ or ? thus involves sentence modification. We propose that these negative and question functions are structure building operators, and we support this claim with data from an American homesigner. We show that: (a) each meaning is marked by a particular form in the child’s gesture system (side-to-side headshake for negation, manual flip for question); (b) the two markers occupy systematic, and different, positions at the periphery of the gesture sentences (headshake at the beginning, flip at the end); and (c) the flip is extended from questions to other uses associated with the wh-form (exclamatives, referential expressions of location) and thus functions like a category in natural languages. If what we see in homesign is a language creation process (Goldin-Meadow, 2003), and if negation and question formation involve sentential modification, then our analysis implies that homesign has at least this minimal sentential syntax. Our findings thus contribute to ongoing debates about properties that are fundamental to language and language learning. PMID:23630971
Photonic band gaps structure properties of two-dimensional function photonic crystals
NASA Astrophysics Data System (ADS)
Ma, Ji; Wang, Zhi-Guo; Liu, Xiao-Jing; Zhang, Si-Qi; Liang, Yu; Wu, Xiang-Yao
2017-05-01
The tunable two-dimensional photonic crystals band gap, absolute photonic band gap and semi-Dirac point are beneficial to designing the novel optical devices. In this paper, tunable photonic band gaps structure was realized by a new type two-dimensional function photonic crystals, which dielectric constants of medium columns are functions of space coordinates. However for the two-dimensional conventional photonic crystals the dielectric constant does not change with space coordinates. As the parameter adjustment, we found that the photonic band gaps structures are dielectric constant function coefficient, medium columns radius, dielectric constant function form period number and pump light intensity dependent, namely, the photonic band gaps position and width can be tuned. we also obtained absolute photonic band gaps and semi-Dirac point in the photonic band gaps structures of two-dimensional function photonic crystals. These results provide an important theoretical foundation for design novel optical devices.
First principle study of band structure of SrMO3 perovskites
NASA Astrophysics Data System (ADS)
Daga, Avinash; Sharma, Smita
2016-05-01
First principle study of band structure calculations in the local density approximations (LDA) as well as in the generalized gradient approximations (GGA) have been used to determine the electronic structure of SrMO3 where M stands for Ti, Zr and Mo. Occurrence of band gap proves SrTiO3 and SrZrO3 to be insulating. A small band gap is observed in SrMoO3 perovskite signifies it to be metallic. Band structures are found to compare well with the available data in the literature showing the relevance of this approach. ABINIT computer code has been used to carry out all the calculations.
Simulation Analysis of a Strip Dipole Excited Electromagnetic Band-Gap (EBG) Structure
2015-07-01
ARL-TR-7337 ● JULY 2015 US Army Research Laboratory Simulation Analysis of a Strip Dipole Excited Electromagnetic Band-Gap (EBG...Simulation Analysis of a Strip Dipole Excited Electromagnetic Band-Gap (EBG) Structure by Seth A McCormick and William O Coburn Sensors and...4/2015 4. TITLE AND SUBTITLE Simulation Analysis of a Strip Dipole Excited Electromagnetic Band-Gap (EBG) Structure 5a. CONTRACT NUMBER 5b
Semi-decoupled band and other high-spin band structures in 188Pt
NASA Astrophysics Data System (ADS)
Mukhopadhyay, S.; Biswas, D. C.; Tandel, S. K.; Frauendorf, S.; Danu, L. S.; Prashanth, P. N.; Joshi, B. N.; Prajapati, G. K.; John, B. V.; Nag, Somnath; Trivedi, T.; Saha, S.; Sethi, J.; Palit, R.; Joshi, P. K.
2017-07-01
High-spin states in the 188Pt nucleus have been investigated employing the 174Yb(18O,4 n )188Pt reaction. The level scheme has been significantly extended with the addition of several new γ rays. The Indian National Gamma Array (INGA) spectrometer consisting of eighteen clover Ge detectors facilitated angular correlation and linear polarization measurements for spin-parity assignment of levels. The Iπ=5- semi-decoupled band has been investigated in detail, and the long-standing ambiguity regarding the correct ordering of associated γ rays is now removed with the new observation of transitions linking the even-spin levels to the odd-spin ones. A weak, Δ I =2 sequence of γ rays with alignment and moment of inertia similar to the yrast, even-spin, rotation-aligned neutron i13 /2 band has been newly observed. Cranking calculations suggest that this new odd-spin sequence arises from the excitation of a valence quasineutron to the next i13 /2 level.
The band gap variation of a two dimensional binary locally resonant structure in thermal environment
NASA Astrophysics Data System (ADS)
Li, Zhen; Wang, Xian; Li, Yue-ming
2017-01-01
In this study, the numerical investigation of thermal effect on band gap dynamical characteristic for a two-dimensional binary structure composed of aluminum plate periodically filled with nitrile rubber cylinder is presented. Initially, the band gap of the binary structure variation trend with increasing temperature is studied by taking the softening effect of thermal stress into account. A breakthrough is made which found the band gap being narrower and shifting to lower frequency in thermal environment. The complete band gap which in higher frequency is more sensitive to temperature that it disappears with temperature increasing. Then some new transformed models are created by changing the height of nitrile rubber cylinder from 1mm to 7mm. Simulations show that transformed model can produce a wider band gap (either flexure or complete band gap). A proper forbidden gap of elastic wave can be utilized in thermal environment although both flexure and complete band gaps become narrower with temperature. Besides that, there is a zero-frequency flat band appearing in the first flexure band, and it becomes broader with temperature increasing. The band gap width decreases trend in thermal environment, as well as the wider band gap induced by the transformed model with higher nitrile rubber cylinder is useful for the design and application of phononic crystal structures in thermal environment.
Band structures in silicene on monolayer gallium phosphide substrate
NASA Astrophysics Data System (ADS)
Ren, Miaojuan; Li, Mingming; Zhang, Changwen; Yuan, Min; Li, Ping; Li, Feng; Ji, Weixiao; Chen, Xinlian
2016-07-01
Opening a sizable band gap in the zero-gap silicene is a key issue for its application in nanoelectronics. We design new 2D silicene and GaP heterobilayer (Si/GaP HBL) composed of silicene and monolayer (ML) GaP. Based on first-principles calculations, we find that the interaction energies are in the range of -295.5 to -297.5 meV per unit cell, indicating a weak interaction between silicene and gallium phosphide (GaP) monolayer. The band gap changes ranging from 0.06 to 0.44 eV in hybrid HBLs. An unexpected indirect-direct band gap crossover is also observed in HBLs, dependent on the stacking pattern. These provide a possible way to design effective FETs out of silicene on GaP monolayer.
Vibrational structure of defect luminescence bands in GaN from electronic structure calculations
NASA Astrophysics Data System (ADS)
Alkauskas, Audrius; van de Walle, Chris G.
2012-02-01
Optical methods are among the most powerful to characterize defects in materials. The study of optical signatures based on state-of-the-art electronic structure methods is therefore very important. In this work we investigate the vibrational structure of luminescence bands pertaining to deep defect levels in GaN. Since luminescence lineshapes depend sensitively on defect geometries and vibrational frequencies, these should be described accurately. The latter is achieved through the use of hybrid density functionals. Both quasi-localized and bulk phonons are included in our description. In the case of transitions accompanied by very large lattice relaxations, anharmonic effects become sizeable, and these are also accounted for. For the defects studied a very good agreement with available experimental data is achieved. For instance, in the case of wide luminescence bands the resulting line widths are within 0.05 eV of the experimental values. This work was supported by the Swiss NSF and by NSF.
Band structure and many body effects in graphene
NASA Astrophysics Data System (ADS)
Bostwick, A.; Ohta, T.; McChesney, J. L.; Seyller, T.; Horn, K.; Rotenberg, E.
2007-09-01
We have determined the electronic bandstructure of clean and potassium-doped single layer graphene, and fitted the graphene π bands to a one- and three-near-neighbor tight binding model. We characterized the quasiparticle dynamics using angle resolved photoemission spectroscopy. The dynamics reflect the interaction between holes and collective excitations, namely plasmons, phonons, and electron-hole pairs. Taking the topology of the bands around the Dirac energy for n-doped graphene into account, we compute the contribution to the scattering lifetimes due to electron-plasmon and electron phonon coupling.
The sheath structure around a negatively charged rocket payload
NASA Technical Reports Server (NTRS)
Neubert, T.; Gilchrist, B. E.; Banks, P. M.; Mandell, M. J.; Sasaki, S.
1990-01-01
The sheath structure around a rocket payload charged up to 460 V negative relative to the ambient ionospheric plasma is investigated experimentally and by computer simulations. The experimental results come from the Charge 2 sounding rocket experiment in which the payload was split into two separate sections (mother and daughter) connected with a conducting, insulated tether. In one of the experimental modes, the voltage between the payloads was increased linearly from 0 to 460 V in 2.5 s. A floating probe array was mounted on the mother with probes located 25, 50, 75, and 100 cm from the rocket surface. The internal impedance of the array was smaller than the probe/plasma impedance, which influenced the potential measurements. The measurements contain signatures, resulting from the outward expansion of the ion sheath with increasing negative mother potential. This conclusion is substantiated by computer simulations of space charge limited flow.
The Band Structure of Polymers: Its Calculation and Interpretation. Part 3. Interpretation.
ERIC Educational Resources Information Center
Duke, B. J.; O'Leary, Brian
1988-01-01
In this article, the third part of a series, the results of ab initio polymer calculations presented in part 2 are discussed. The electronic structure of polymers, symmetry properties of band structure, and generalizations are presented. (CW)
Narrow-band anisotropic electronic structure of ReS2
NASA Astrophysics Data System (ADS)
Biswas, D.; Ganose, Alex M.; Yano, R.; Riley, J. M.; Bawden, L.; Clark, O. J.; Feng, J.; Collins-Mcintyre, L.; Sajjad, M. T.; Meevasana, W.; Kim, T. K.; Hoesch, M.; Rault, J. E.; Sasagawa, T.; Scanlon, David O.; King, P. D. C.
2017-08-01
We have used angle-resolved photoemission spectroscopy to investigate the band structure of ReS2, a transition-metal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of structural distortion and spin-orbit coupling. We further show how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zigzag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone center along kz. These experiments are in good agreement with our density-functional theory calculations, shedding light on the bulk electronic structure of ReS2, and how it can be expected to evolve when thinned to a single layer.
Kim, Min Su; Seo, Changwon; Kim, Hyun; Lee, Jubok; Luong, Dinh Hoa; Park, Ji-Hoon; Han, Gang Hee; Kim, Jeongyong
2016-06-28
Heterostacking of layered transition-metal dichalcogenide (LTMD) monolayers (1Ls) offers a convenient way of designing two-dimensional exciton systems. Here we demonstrate the simultaneous hosting of positive trions and negative trions in heterobilayers made by vertically stacking 1L MoSe2 and 1L MoS2. The charge transfer occurring between the 1Ls of MoSe2 and MoS2 converted the polarity of trions in 1L MoSe2 from negative to positive, resulting in the presence of positive trions in the 1L MoSe2 and negative trions in the 1L MoS2 of the same heterostacked bilayer. Significantly enhanced MoSe2 photoluminescence (PL) in the heterostacked bilayers compared to the PL of 1L MoSe2 alone suggests that, unlike other previously reported heterostacked bilayers, direct band transition of 1L MoSe2 in heterobilayer was enhanced after the vertical heterostacking. Moreover, by inserting hexagonal BN monolayers between 1L MoSe2 and 1L MoS2, we were able to adjust the charge transfer to maximize the MoSe2 PL of the heteromultilayers and have achieved a 9-fold increase of the PL emission. The enhanced optical properties of our heterostacked LTMDs suggest the exciting possibility of designing LTMD structures that exploit the superior optical properties of 1L LTMDs.
Electronic transitions in GdN band structure
Vidyasagar, R. Kita, T.; Sakurai, T.; Ohta, H.
2014-05-28
Using the near-infrared (NIR) absorbance spectroscopy, electronic transitions and spin polarization of the GdN epitaxial film have been investigated; and the GdN epitaxial film was grown by a reactive rf sputtering technique. The GdN film exhibited three broad bands in the NIR frequency regimes; and those bands are attributable primarily to the minority and majority spin transitions at the X-point and an indirect transition along the Γ-X symmetric direction of GdN Brillouin zone. We experimentally observe a pronounced red-shift of the indirect band gap when cooling down below the Curie temperature which is ascribed to the orbital-dependent coulomb interactions of Gd-5dxy electrons, which tend to push-up the N-2p bands. On the other hand, we have evaluated the spin polarization of 0.17 (±0.005), which indicates that the GdN epitaxial film has almost 100% spin-polarized carriers. Furthermore, the experimental result of GdN electronic transitions are consistent with the previous reports and are thus well-reproduced. The Arrott plots evidenced that the Curie temperature of GdN film is 36 K and the large spin moment is explained by the nitrogen vacancies and the intra-atomic exchange interaction.
NASA Astrophysics Data System (ADS)
Dahal, Dipendra; Gumbs, Godfrey
2017-01-01
A remarkable property of intrinsic graphene is that upon doping, electrons and holes travel through the monolayer thick material with constant velocity which does not depend on energy up to about 0.3 eV (Dirac fermions), as though the electrons and holes are massless particles and antiparticles which move at the Fermi velocity vF. Consequently, there is Klein tunneling at a p-n junction, in which there is no backscattering at normal incidence of massless Dirac fermions. However, this process yielding perfect transmission at normal incidence is expected to be affected when the group velocity of the charge carriers is energy dependent and there is non-zero effective mass for the target particle. We investigate how away from normal incidence the combined effect of incident electron energy ɛ and band gap parameter Δ can determine whether a p-n junction would allow focusing of an electron beam by behaving like a Veselago lens with negative refractive index. We demonstrate that there is a specific region in ɛ - Δ space where the index of refraction is negative, i.e., where monolayer graphene behaves as a metamaterial. Outside this region, the refractive index may be positive or there may be no refraction at all. We compute the ballistic conductance across a p-n junction as a function of Δ and ɛ and compare our results with those for a single electrostatic potential barrier and multiple barriers.
Human chromosomal bands: nested structure, high-definition map and molecular basis.
Costantini, Maria; Clay, Oliver; Federico, Concetta; Saccone, Salvatore; Auletta, Fabio; Bernardi, Giorgio
2007-02-01
In this paper, we report investigations on the nested structure, the high-definition mapping, and the molecular basis of the classical Giemsa and Reverse bands in human chromosomes. We found the rules according to which the approximately 3,200 isochores of the human genome are assembled in high (850-band) resolution bands, and the latter in low (400-band) resolution bands, so forming the nested mosaic structure of chromosomes. Moreover, we identified the borders of both sets of chromosomal bands at the DNA sequence level on the basis of our recent map of isochores, which represent the highest-resolution, ultimate bands. Indeed, beyond the 100-kb resolution of the isochore map, the guanine and cytosine (GC) profile of DNA becomes turbulent owing to the contribution of specific sequences such as exons, introns, interspersed repeats, CpG islands, etc. The isochore-based level of definition (100 kb) of chromosomal bands is much higher than the cytogenetic definition level (2-3 Mb). The major conclusions of this work concern the high degree of order found in the structure of chromosomal bands, their mapping at a high definition, and the solution of the long-standing problem of the molecular basis of chromosomal bands, as these could be defined on the basis of compositional DNA properties alone.
Ding Xueyong; Li Hongfan; Lv Zhensu
2012-09-15
Based on the mode-coupling method, numerical analysis is presented to demonstrate the influence of ripple taper on band-gap overlap in a coaxial Bragg structure operating at terahertz frequency. Results show that the interval between the band-gaps of the competing mode and the desired working mode is narrowed by use of positive-taper ripples, but is expanded if negative-taper ripples are employed, and the influence of the negative-taper ripples is obviously more advantageous than the positive-taper ripples; the band-gap overlap of modes can be efficiently separated by use of negative-taper ripples. The residual side-lobes of the frequency response in a coaxial Bragg structure with ripple taper also can be effectively suppressed by employing the windowing-function technique. These peculiarities provide potential advantage in constructing a coaxial Bragg cavity with high quality factor for single higher-order-mode operation of a high-power free-electron maser in the terahertz frequency range.
Quantum structure of negation and conjunction in human thought.
Aerts, Diederik; Sozzo, Sandro; Veloz, Tomas
2015-01-01
We analyze in this paper the data collected in a set of experiments investigating how people combine natural concepts. We study the mutual influence of conceptual conjunction and negation by measuring the membership weights of a list of exemplars with respect to two concepts, e.g., Fruits and Vegetables, and their conjunction Fruits And Vegetables, but also their conjunction when one or both concepts are negated, namely, Fruits And Not Vegetables, Not Fruits And Vegetables, and Not Fruits And Not Vegetables. Our findings sharpen and advance existing analysis on conceptual combinations, revealing systematic deviations from classical (fuzzy set) logic and probability theory. And, more important, our results give further considerable evidence to the validity of our quantum-theoretic framework for the combination of two concepts. Indeed, the representation of conceptual negation naturally arises from the general assumptions of our two-sector Fock space model, and this representation faithfully agrees with the collected data. In addition, we find a new significant and a priori unexpected deviation from classicality, which can exactly be explained by assuming that human reasoning is the superposition of an "emergent reasoning" and a "logical reasoning," and that these two processes are represented in a Fock space algebraic structure.
Quantum structure of negation and conjunction in human thought
Aerts, Diederik; Sozzo, Sandro; Veloz, Tomas
2015-01-01
We analyze in this paper the data collected in a set of experiments investigating how people combine natural concepts. We study the mutual influence of conceptual conjunction and negation by measuring the membership weights of a list of exemplars with respect to two concepts, e.g., Fruits and Vegetables, and their conjunction Fruits And Vegetables, but also their conjunction when one or both concepts are negated, namely, Fruits And Not Vegetables, Not Fruits And Vegetables, and Not Fruits And Not Vegetables. Our findings sharpen and advance existing analysis on conceptual combinations, revealing systematic deviations from classical (fuzzy set) logic and probability theory. And, more important, our results give further considerable evidence to the validity of our quantum-theoretic framework for the combination of two concepts. Indeed, the representation of conceptual negation naturally arises from the general assumptions of our two-sector Fock space model, and this representation faithfully agrees with the collected data. In addition, we find a new significant and a priori unexpected deviation from classicality, which can exactly be explained by assuming that human reasoning is the superposition of an “emergent reasoning” and a “logical reasoning,” and that these two processes are represented in a Fock space algebraic structure. PMID:26483715
High-Pressure Crystal Structure, Lattice Vibrations, and Band Structure of BiSbO4.
Errandonea, Daniel; Muñoz, Alfonso; Rodríguez-Hernández, Placida; Gomis, Oscar; Achary, S Nagabhusan; Popescu, Catalin; Patwe, Sadeque J; Tyagi, Avesh K
2016-05-16
The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ∼33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations.
Effect of Uniaxial Strain on Band Structure of Multi-layer WS2
NASA Astrophysics Data System (ADS)
Troha, Conrad; Le, Duy; Rahman, Talat
The ability to tailor band structure of a multi-layer transition metal dichalcogenide is of interest because it opens up utilizations of the material for various applications. Strain is considered a robust way to alter the electronic structure of a material. We performed calculations, using density functional theory, of band structure of multi-layer WS2 under the effects of uniaxial strain. We show that the position of the bottom of conduction band (BCB) at ∑ moves to higher, and at K to lower, energy levels under the effects of uniaxial tensile strain, making multi-layer WS2 closer to a direct band gap material. Our results suggest that uniaxial tensile strain can be used to alter band structure of multi-layer WS2 to achieve higher yield photo luminescence. This work is supported in part by U.S. Department of Energy (DOE DE-FG02-07ER15842).
NASA Astrophysics Data System (ADS)
Rubio-Ponce, A.; Olguín, D.
2015-01-01
The structural and electronic properties of CdTe(001), CdSe(001), and ZnSe(001) C(2 x 2) reconstructed surfaces have been investigated through the use of first-principles calculations. To simulate the surface, we employed the slab model. Using the experimentally determined lattice parameters as inputs, we relaxed the internal atomic positions of the outer atomic layers. We demonstrate that our model appropriately reproduces both the surface structural parameters and the known electronic properties found for these semiconductor compounds in bulk. Finally, we discuss our results of the projected bulk bands and the surface and resonance states found for these surfaces.
The sheath structure around a negatively charged rocket payload
Neubert, T.; Gilchrist, B.E.; Banks, P.M.; Williamson, P.R. ); Mandell, M.J.; Katz, I. ); Sasaki, S.; Oyama, K.I. ); Raitt, W.J.; Meyers, N.B. )
1990-05-01
The sheath structure around a rocket payload charged up to 460 V negative relative to the ambient ionospheric plasma is investigated experimentally and by computer simulations. In one of the experimental modes, the voltage between the payloads was increased linearly from 0 to 460 V in 2.5 s. In this case the tethered mother/daughter functioned as a double probe, the negative probe (mother) reaching large negative potentials, while the positive probe (daughter) stayed close to the ambient plasma potential. A floating probe array was mounted on the mother with probes located, 25, 50, 75, and 100 cm from the rocket surface. The internal impedance of the array was smaller than the probe/plasma impedance, which influenced the potential measurements. However, the measurements contain signatures, which the authors interpret as resulting from the outward expansion of the ion sheath with increasing negative mother potential. This conclusion is substantiated by NASCAP/LEO computer simulations of space charge limited flow. At high potentials, the observed ion current flowing to the mother increased more strongly with bias potential than found from the simulations. It is suggested that the enhancement of the current is generated by secondary electrons emitted by the ions bombarding the payload skin. The effects of the motion of the mother (540-580 m/s) and of the ambient magnetic field have been assessed by the code. It was estimated that the ion current to the mother was increased by 20% relative to a stationary payload, while the incorporation of a magnetic field had no practical influence on the simulation results.
Valence Band Structure of Highly Efficient p-type Thermoelectric PbTe-PbS Alloys
Jaworski, C. M.; Nielsen, Mechele; Wang, Hsin; Girard, Steven N.; Cai, Wei; Porter, Wallace D; Kanatzidis, Mercouri G.; Heremans, J. P.
2013-01-01
New experimental evidence is given relevant to the temperature-dependence of valence band structure of PbTe and PbTe1-xSx alloys (0.04 x 0.12), and its effect on the thermoelectric figure of merit zT. The x = 0.08 sample has zT ~ 1.55 at 773K. The magnetic field dependence of the high-temperature Hall resistivity of heavily p-type (> 1019 cm-3) Na-doped PbTe1-xSx reveals the presence of high-mobility electrons. This put in question prior analyses of the Hall coefficient and the conclusion that PbTe would be an indirect gap semiconductor at temperatures where its zT is optimal. Possible origins for these electrons are discussed: they can be induced by photoconductivity, or by the topology of the Fermi surface when the L and -bands merge. Negative values for the low-temperature thermopower are also observed. Our data show that PbTe continues to be a direct gap semiconductor at temperatures where the zT and S2 of p-type PbTe are optimal e.g. 700-900K. The previously suggested temperature induced rapid rise in energy of the heavy hole LVB relative to the light hole UVB is not supported by the experimental data.
Fine structure of the amide i band in acetanilide
NASA Astrophysics Data System (ADS)
Careri, G.; Gratton, E.; Shyamsunder, E.
1988-05-01
Their absorption spectrum of both single crystals and powdered samples of acetanilide (a model system for proteins) has been studied in the amide i region, where a narrow band has been identified as a highly trapped soliton state. The powder-sample spectra have been decomposed using four Lorentzian bands. A strong temperature dependence has been found for the intensity of two of the subbands, which also show a complementary behavior. Polarization studies performed on thin crystals have shown that the subbands have the same polarization. Low-temperature spectra of partially deuterated samples show the presence of the subbands at the same absorption frequencies found using the fitting procedure in the spectra of nondeuterated samples. The soliton model currently proposed to explain the origin of the anomalous amide i component at 1650 cm-1 still holds, but some modification of the model is required to account for the new features revealed by this study.
Band structure of germanium carbides for direct bandgap silicon photonics
Stephenson, C. A. Stillwell, R. A.; Wistey, M. A.; O'Brien, W. A.; Penninger, M. W.; Schneider, W. F.; Gillett-Kunnath, M.; Zajicek, J.; Yu, K. M.; Kudrawiec, R.
2016-08-07
Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge{sub 1−x}C{sub x} (x = 0.78%) using density functional theory with HSE06 hybrid functionals predicts a splitting of the conduction band at Γ and a strongly direct bandgap, consistent with band anticrossing. Photoreflectance of Ge{sub 0.998}C{sub 0.002} shows a bandgap reduction supporting these results. Growth of Ge{sub 0.998}C{sub 0.002} using tetrakis(germyl)methane as the C source shows no signs of C-C bonds, C clusters, or extended defects, suggesting highly substitutional incorporation of C. Optical gain and modulation are predicted to rival III–V materials due to a larger electron population in the direct valley, reduced intervalley scattering, suppressed Auger recombination, and increased overlap integral for a stronger fundamental optical transition.
Electronic band structures of graphene nanoribbons with self-passivating edge reconstructions
NASA Astrophysics Data System (ADS)
Nguyen, L. Tung; Pham, C. Huy; Nguyen, V. Lien
2011-07-01
Using the nearest-neighbor tight-binding approach we study the electronic band structures of graphene nanoribbons with self-passivating edge reconstructions. For zigzag ribbons the edge reconstruction moves both the Fermi energy and the flat band down by several hundred meV, and the flat band is always found to be below the Fermi energy. The states featured by the flat band are shown to be mainly localized at the atoms belonging to several lattice lines closest to the edges. For armchair ribbons the edge reconstruction strongly modifies the band structure in the region close to the Fermi energy, leading to the appearance of a band gap even for ribbons which were predicted to be metallic in the model of standard armchair edges. The gap widths are, however, strongly different in magnitude and behave in different ways regarding the ribbon width.
Quasiparticle band structures and optical properties of magnesium fluoride.
Yi, Zhijun; Jia, Ran
2012-02-29
The quasiparticle and optical properties of magnesium fluoride (MgF(2)) are computed within the GW approximation based on many-body perturbation theory (MBPT). The many-body effects appearing in self-energy and electron-hole interactions have an important influence on the electronic and optical properties. The DFT-LDA calculation shows a 6.78 eV band gap. Two methods are employed to evaluate the self-energy within the GW approximation in the present work. The generalized plasmon pole model (GPP) provides a band gap of 12.17 eV, which agrees well with the experimental value of 12.4 eV (Thomas et al 1973 Phys. Status Solidi b 56 163). Another band gap value of 11.30 eV is obtained by using a full frequency-dependent self-energy, which is also not far from the experimental value and is much better than the result from the LDA calculation. The calculated optical spectrum within DFT is significantly different from the experiment. Although the calculated optical absorption threshold within the GW method is close to the experiment, the overall shape of the spectrum is still similar to the case of DFT. However, the overall shape of the spectrum via the Bethe-Salpeter equation (BSE) method agrees well with the experiment.
Liu, Wei-Sheng; Chu, Ting-Fu; Huang, Tien-Hao
2014-12-15
This study presents an band-alignment tailoring of a vertically aligned InAs/GaAs(Sb) quantum dot (QD) structure and the extension of the carrier lifetime therein by rapid thermal annealing (RTA). Arrhenius analysis indicates a larger activation energy and thermal stability that results from the suppression of In-Ga intermixing and preservation of the QD heterostructure in an annealed vertically aligned InAs/GaAsSb QD structure. Power-dependent and time-resolved photoluminescence were utilized to demonstrate the extended carrier lifetime from 4.7 to 9.4 ns and elucidate the mechanisms of the antimony aggregation resulting in a band-alignment tailoring from straddling to staggered gap after the RTA process. The significant extension in the carrier lifetime of the columnar InAs/GaAsSb dot structure make the great potential in improving QD intermediate-band solar cell application.
Wormser, Maximilian; Wein, Fabian; Stingl, Michael; Körner, Carolin
2017-09-22
We present a novel approach for gradient based maximization of phononic band gaps. The approach is a geometry projection method combining parametric shape optimization with density based topology optimization. By this approach, we obtain, in a two dimension setting, cellular structures exhibiting relative and normalized band gaps of more than 8 and 1.6, respectively. The controlling parameter is the minimal strut size, which also corresponds with the obtained stiffness of the structure. The resulting design principle is manually interpreted into a three dimensional structure from which cellular metal samples are fabricated by selective electron beam melting. Frequency response diagrams experimentally verify the numerically determined phononic band gaps of the structures. The resulting structures have band gaps down to the audible frequency range, qualifying the structures for an application in noise isolation.
Three-dimensional structure of the Z band in a normal mammalian skeletal muscle
1996-01-01
The three-dimensional structure of the vertebrate skeletal muscle Z band reflects its function as the muscle component essential for tension transmission between successive sarcomeres. We have investigated this structure as well as that of the nearby I band in a normal, unstimulated mammalian skeletal muscle by tomographic three- dimensional reconstruction from electron micrograph tilt series of sectioned tissue. The three-dimensional Z band structure consists of interdigitating axial filaments from opposite sarcomeres connected every 18 +/- 12 nm (mean +/- SD) to one to four cross-connecting Z- filaments are observed to meet the axial filaments in a fourfold symmetric arrangement. The substantial variation in the spacing between cross-connecting Z-filament to axial filament connection points suggests that the structure of the Z band is not determined solely by the arrangement of alpha-actinin to actin-binding sites along the axial filament. The cross-connecting filaments bind to or form a "relaxed interconnecting body" halfway between the axial filaments. This filamentous body is parallel to the Z band axial filaments and is observed to play an essential role in generating the small square lattice pattern seen in electron micrographs of unstimulated muscle cross sections. This structure is absent in cross section of the Z band from muscles fixed in rigor or in tetanus, suggesting that the Z band lattice must undergo dynamic rearrangement concomitant with crossbridge binding in the A band. PMID:8636232
Chaos and band structure in a three-dimensional optical lattice.
Boretz, Yingyue; Reichl, L E
2015-04-01
Classical chaos is known to affect wave propagation because it signifies the presence of broken symmetries. The effect of chaos has been observed experimentally for matter waves, electromagnetic waves, and acoustic waves. When these three types of waves propagate through a spatially periodic medium, the allowed propagation energies form bands. For energies in the band gaps, no wave propagation is possible. We show that optical lattices provide a well-defined system that allows a study of the effect of chaos on band structure. We have determined the band structure of a body-centered-cubic optical lattice for all theoretically possible couplings, and we find that the band structure for those lattices realizable in the laboratory differs significantly from that expected for the bands in an "empty" body-centered-cubic crystal. However, as coupling is increased, the lattice becomes increasingly chaotic and it becomes possible to produce band structure that has behavior qualitatively similar to the "empty" body-centered-cubic band structure, although with fewer degeneracies.
NASA Astrophysics Data System (ADS)
Appalakondaiah, S.; Vaitheeswaran, G.; Lebègue, S.
2013-05-01
We report a detailed theoretical study of the structural and vibrational properties of solid nitromethane using first principles density functional calculations. The ground state properties were calculated using a plane wave pseudopotential code with either the local density approximation, the generalized gradient approximation, or with a correction to include van der Waals interactions. Our calculated equilibrium lattice parameters and volume using a dispersion correction are found to be in reasonable agreement with the experimental results. Also, our calculations reproduce the experimental trends in the structural properties at high pressure. We found a discontinuity in the bond length, bond angles, and also a weakening of hydrogen bond strength in the pressure range from 10 to 12 GPa, picturing the structural transition from phase I to phase II. Moreover, we predict the elastic constants of solid nitromethane and find that the corresponding bulk modulus is in good agreement with experiments. The calculated elastic constants show an order of C11> C22 > C33, indicating that the material is more compressible along the c-axis. We also calculated the zone center vibrational frequencies and discuss the internal and external modes of this material under pressure. From this, we found the softening of lattice modes around 8-11 GPa. We have also attempted the quasiparticle band structure of solid nitromethane with the G0W0 approximation and found that nitromethane is an indirect band gap insulator with a value of the band gap of about 7.8 eV with G0W0 approximation. Finally, the optical properties of this material, namely the absorptive and dispersive part of the dielectric function, and the refractive index and absorption spectra are calculated and the contribution of different transition peaks of the absorption spectra are analyzed. The static dielectric constant and refractive indices along the three inequivalent crystallographic directions indicate that this material
Electron momentum density, band structure, and structural properties of SrS
Sharma, G.; Munjal, N.; Vyas, V.; Kumar, R.; Sharma, B. K.; Joshi, K. B.
2013-10-15
The electron momentum density, the electronic band structure, and the structural properties of SrS are presented in this paper. The isotropic Compton profile, anisotropies in the directional Compton profiles, the electronic band structure and density of states are calculated using the ab initio periodic linear combination of atomic orbitals method with the CRYSTAL06 code. Structural parameters of SrS-lattice constants and bulk moduli in the B1 and B2 phases-are computed together with the transition pressure. The computed parameters are well in agreement with earlier investigations. To compare the calculated isotropic Compton profile, measurement on polycrystalline SrS is performed using 5Ci-{sup 241}Am Compton spectrometer. Additionally, charge transfer is studied by means of the Compton profiles computed from the ionic model. The nature of bonding in the isovalent SrS and SrO compounds is compared on the basis of equal-valenceelectron-density profiles and the bonding in SrS is found to be more covalent than in SrO.
Three-dimensional band structure of LaSb and CeSb: Absence of band inversion
NASA Astrophysics Data System (ADS)
Oinuma, H.; Souma, S.; Takane, D.; Nakamura, T.; Nakayama, K.; Mitsuhashi, T.; Horiba, K.; Kumigashira, H.; Yoshida, M.; Ochiai, A.; Takahashi, T.; Sato, T.
2017-07-01
We have performed angle-resolved photoemission spectroscopy (ARPES) of LaSb and CeSb, a candidate of topological insulators. Using soft-x-ray photons, we have accurately determined the three-dimensional bulk band structure and revealed that the band inversion at the Brillouin-zone corner, a prerequisite for realizing the topological-insulator phase, is absent in both LaSb and CeSb. Moreover, unlike the ARPES data obtained with soft-x-ray photons, those with VUV photons were found to suffer significant kz broadening. These results suggest that LaSb and CeSb are topologically trivial semimetals, and unusual Dirac-cone-like states observed with VUV photons are not of the topological origin.
NASA Astrophysics Data System (ADS)
Antonova, Irina V.; Kurkina, Irina I.; Nebogatikova, Nadezhda A.; Komonov, Alexander I.; Smagulova, Svetlana A.
2017-02-01
The band structure and electric properties of films created from a partially fluorinated graphene suspension are analyzed in this paper. As may be inferred from the structural study, graphene islands (quantum dots) are formed in these films. Various types of negative differential resistance (NDR) and a step-like increase in the current are found for films created from the fluorinated graphene suspension. NDR resulting from the formation of the potential barrier system in the film and corresponding to the theoretical prediction is observed for a relatively low fluorination degree. The origin of the NDR varies with an increase in the fluorination degree of the suspension. The observation of NDR in the fluorinated films widens the range of application of such films, including as active device layers fabricated using 2D printed technologies on rigid and flexible substrates.
Band structure of InAsSb strained-layer superlattices
NASA Astrophysics Data System (ADS)
Liu, Lifeng; Lee, G. S.; Marshak, A. H.
1992-02-01
Band structures of relaxed InSb/InAs1-xSbx strained-layer superlattices (SLSs) were calculated based on Herman-Skillman neutral atom ionization energies, effective dipole theory, and strain shift of band edges. The conduction-band minimum and the valence-band maximum of InAs1-xSbx are lower in energy than those of InSb, respectively. The band structures of InSb/InAs1-xSbx are type II SLSs and an extreme type II superlattice is predicted for x<0.82. This extreme type II superlattice with a proper structure factor can have favorable properties for infrared detectors covering the 8-12 μm wavelength range. The requirement for layer thickness of InSb/InAs1-xSbx SLSs with different absorption mechanisms was discussed.
One-dimensional electromagnetic band gap structures formed by discharge plasmas in a waveguide
Arkhipenko, V. I.; Simonchik, L. V. Usachonak, M. S.; Callegari, Th.; Sokoloff, J.
2014-09-28
We demonstrate the ability to develop one-dimensional electromagnetic band gap structure in X-band waveguide solely by using the positive columns of glow discharges in neon at the middle pressure. Plasma inhomogeneities are distributed uniformly along a typical X-band waveguide with cross section of 23×10 mm². It is shown that electron densities larger than 10¹⁴ cm ⁻³ are needed in order to create an effective one-dimensional electromagnetic band gap structure. Some applications for using the one-dimensional electromagnetic band gap structure in waveguide as a control of microwave (broadband filter and device for variation of pulse duration) are demonstrated.
Tuning the band gap in hybrid tin iodide perovskite semiconductors using structural templating.
Knutson, Jeremy L; Martin, James D; Mitzi, David B
2005-06-27
Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.
NASA Astrophysics Data System (ADS)
Sedghi, Aliasghar; Valiaghaie, Soma; Soufiani, Ahad Rounaghi
2014-10-01
By virtue of the efficiency of the Dirichlet-to-Neumann map method, we have calculated, for H-polarization (TE mode), the band structure of 2D photonic crystals with a square lattice composed of metallic rods embedded in an air background. The rod in the unit cell is chosen to be circular in shape. Here, from a practical point of view, in order to obtain maximum band gaps, we have studied the band structure as a function of the size of the rods. We have also studied the flat bands appearing in the band structures and have shown that for frequencies around the surface plasmon frequency, the modes are highly localized at the interface between the metallic rods and the air background.
NASA Astrophysics Data System (ADS)
Gripp, J. A. B.; Góes, L. C. S.; Heuss, O.; Scinocca, F.
2015-12-01
Piezoelectric shunt damping is a well-known technique to damp mechanical vibrations of a structure, using a piezoelectric transducer to convert mechanical vibration energy into electrical energy, which is dissipated in an electrical resistance. Resonant shunts consisting of a resistance and an inductance connected to a piezoelectric transducer are used to damp structural vibrations in narrow frequency bands, but their performance is very sensitive to variations in structural modal frequencies and transducer capacitance. In order to overcome this drawback, a piezoelectric shunt damping technique with improved performance and robustness is presented in this paper. The design of the adaptive circuit considers the variation of the host structure’s natural frequency as a project parameter. This paper describes an adaptive resonant piezoelectric vibration absorber enhanced by a synthetic negative capacitance applied to a shell structure. The resonant shunt circuit autonomously adapts its inductance value by comparing the phase difference of the vibration velocity and the current flowing through the shunt circuit. Moreover, a synthetic negative capacitance is added to the shunt circuit to enhance the vibration attenuation provided by the piezoelectric absorber. The circuitry is implemented using analog components. Validation of the proposed method is done by bonding the piezoelectric absorber on a free-formed metallic shell.
First direct observation of a nearly ideal graphene band structure
Sprinkle, M.; Siegel, D.; Hu, Y.; Hicks, J.; Tejeda, A.; Taleb-Ibrahimi, A.; Le Fèvre, P.; Bertran, F.; Vizzini, S.; Enriquez, H.; Chiang, S.; Soukiassian, P.; Berger, C.; de Heer, W.A.; Lanzara, A.; Conrad, E.H.
2009-12-10
Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(000{bar 1}) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.
Influence of spin correlations on band structure of magnetic semiconductors
NASA Astrophysics Data System (ADS)
Sinkkonen, J.
1981-06-01
A perturbation treatment of the s-f interaction in ferromagnetic semiconductors is presented. The many-spin correlation functions are expressed in terms of connected correlation functions which are constructed by the meanfield theory. For the self-energy an integral equation is obtained which includes correlation effects. The method of calculation is closely connected with the coherent-potential approximation. As an application the density of states is shown in various cases by allowing the bandwidth to vary from broad- to narrow-band regime. The calculation is limited to the paramagnetic phase. Correlation effects are seen as temperature-dependent changes in the density of states.
Compact electromagnetic bandgap structures for notch band in ultra-wideband applications.
Rotaru, Mihai; Sykulski, Jan
2010-01-01
This paper introduces a novel approach to create notch band filters in the front-end of ultra-wideband (UWB) communication systems based on electromagnetic bandgap (EBG) structures. The concept presented here can be implemented in any structure that has a microstrip in its configuration. The EBG structure is first analyzed using a full wave electromagnetic solver and then optimized to work at WLAN band (5.15-5.825 GHz). Two UWB passband filters are used to demonstrate the applicability and effectiveness of the novel EBG notch band feature. Simulation results are provided for two cases studied.
Compact Electromagnetic Bandgap Structures for Notch Band in Ultra-Wideband Applications
Rotaru, Mihai; Sykulski, Jan
2010-01-01
This paper introduces a novel approach to create notch band filters in the front-end of ultra-wideband (UWB) communication systems based on electromagnetic bandgap (EBG) structures. The concept presented here can be implemented in any structure that has a microstrip in its configuration. The EBG structure is first analyzed using a full wave electromagnetic solver and then optimized to work at WLAN band (5.15–5.825 GHz). Two UWB passband filters are used to demonstrate the applicability and effectiveness of the novel EBG notch band feature. Simulation results are provided for two cases studied. PMID:22163430
Photonic band structure of dielectric membranes periodically textured in two dimensions
NASA Astrophysics Data System (ADS)
Pacradouni, V.; Mandeville, W. J.; Cowan, A. R.; Paddon, P.; Young, Jeff F.; Johnson, S. R.
2000-08-01
The real and imaginary photonic band structure of modes attached to two-dimensionally textured semiconductor membranes is determined experimentally and theoretically. These porous waveguides exhibit large (1000 cm-1 at 9500 cm-1) second-order optical gaps, highly dispersive lifetimes, and bands with well-defined polarization along directions of high symmetry.
Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3
Ho, Ching-Hwa; Chen, Hsin-Hung
2014-01-01
The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga2S3. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga2S3. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga2S3 crystal is revealed. PMID:25142550
Triaxial superdeformed and normal-deformed high-spin band structures in {sup 170}Hf
Neusser-Neffgen, A.; Huebel, H.; Bringel, P.; Domscheit, J.; Mergel, E.; Nenoff, N.; Singh, A.K.; Hagemann, G.B.; Jensen, D.R.; Bhattacharya, S.; Curien, D.; Dorvaux, O.; Hannachi, F.; Lopez-Martens, A.
2006-03-15
The high-spin structure of {sup 170}Hf was investigated using the EUROBALL spectrometer. The previously known level scheme was extended in the low-spin region as well as to higher spins, and several new bands were discovered. In particular, two bands were identified which show the characteristics of triaxial superdeformation. One of these bands is strongly populated, and its excitation energy and spins are established. Configuration assignments are made to the normal-deformed bands based on comparisons of their properties with cranked shell model calculations. The results for the very high spin states provide important input for such calculations.
Features of the band structure for semiconducting iron, ruthenium, and osmium monosilicides
Shaposhnikov, V. L. Migas, D. B.; Borisenko, V. E.; Dorozhkin, N. N.
2009-02-15
The pseudopotential method has been used to optimize the crystal lattice and calculate the energy band spectra for iron, ruthenium and, osmium monosilicides. It is found that all these compounds are indirect-gap semiconductors with band gaps of 0.17, 0.22, and 0.50 eV (FeSi, RuSi, and OsSi, respectively). A distinctive feature of their band structure is the 'loop of extrema' both in the valence and conduction bands near the center of the cubic Brillouin zone.
Electronic band structure effects in monolayer, bilayer, and hybrid graphene structures
NASA Astrophysics Data System (ADS)
Puls, Conor
Since its discovery in 2005, graphene has been the focus of intense theoretical and experimental study owing to its unique two-dimensional band structure and related electronic properties. In this thesis, we explore the electronic properties of graphene structures from several perspectives including the magnetoelectrical transport properties of monolayer graphene, gap engineering and measurements in bilayer graphene, and anomalous quantum oscillation in the monolayer-bilayer graphene hybrids. We also explored the device implications of our findings, and the application of some experimental techniques developed for the graphene work to the study of a complex oxide, Ca3Ru2O7, exhibiting properties of strongly correlated electrons. Graphene's high mobility and ballistic transport over device length scales, make it suitable for numerous applications. However, two big challenges remain in the way: maintaining high mobility in fabricated devices, and engineering a band gap to make graphene compatible with logical electronics and various optical devices. We address the first challenge by experimentally evaluating mobilities in scalable monolayer graphene-based field effect transistors (FETs) and dielectric-covered Hall bars. We find that the mobility is limited in these devices, and is roughly inversely proportional to doping. By considering interaction of graphene's Dirac fermions with local charged impurities at the interface between graphene and the top-gate dielectric, we find that Coulomb scattering is responsible for degraded mobility. Even in the cleanest devices, a band gap is still desirable for electronic applications of graphene. We address this challenge by probing the band structure of bilayer graphene, in which a field-tunable energy band gap has been theoretically proposed. We use planar tunneling spectroscopy of exfoliated bilayer graphene flakes demonstrate both measurement and control of the energy band gap. We find that both the Fermi level and
NASA Astrophysics Data System (ADS)
Wang, Zefang; Zhao, Liang; Mak, Kin Fai; Shan, Jie
2017-02-01
We study the electronic band structure in the K/K' valleys of the Brillouin zone of monolayer WSe2 and MoSe2 by optical reflection and photoluminescence spectroscopy on dual-gated field-effect devices. Our experiment reveals the distinct spin polarization in the conduction bands of these compounds by a systematic study of the doping dependence of the A and B excitonic resonances. Electrons in the highest-energy valence band and the lowest-energy conduction band have antiparallel spins in monolayer WSe2, and parallel spins in monolayer MoSe2. The spin splitting is determined to be hundreds of meV for the valence bands and tens of meV for the conduction bands, which are in good agreement with first principles calculations. These values also suggest that both n- and p-type WSe2 and MoSe2 can be relevant for spin- and valley-based applications
Wang, Zefang; Zhao, Liang; Mak, Kin Fai; Shan, Jie
2017-02-08
We study the electronic band structure in the K/K' valleys of the Brillouin zone of monolayer WSe2 and MoSe2 by optical reflection and photoluminescence spectroscopy on dual-gated field-effect devices. Our experiment reveals the distinct spin polarization in the conduction bands of these compounds by a systematic study of the doping dependence of the A and B excitonic resonances. Electrons in the highest-energy valence band and the lowest-energy conduction band have antiparallel spins in monolayer WSe2 and parallel spins in monolayer MoSe2. The spin splitting is determined to be hundreds of meV for the valence bands and tens of meV for the conduction bands, which are in good agreement with first-principles calculations. These values also suggest that both n- and p-type WSe2 and MoSe2 can be relevant for spin- and valley-based applications.
Quantitative study of band structure in BaTiO3 particles with vacant ionic sites
NASA Astrophysics Data System (ADS)
Oshime, Norihiro; Kano, Jun; Ikeda, Naoshi; Teranishi, Takashi; Fujii, Tatsuo; Ueda, Takeji; Ohkubo, Tomoko
2016-10-01
Levels of the conduction band minimum and the valence band maximum in ion-deficient BaTiO3 particles were investigated with optical band gap and ionization energy measurements. Though it is known that the quantification of the band structure in an insulator is difficult, due to the poor electrical conductivity of BaTiO3, systematic variation in the band energy levels was found that correlated with the introduction of vacancies. Photoelectron yield spectroscopy provided direct observation of the occupancy level of electrons, which is altered by the presence of oxygen and barium vacancies. In addition, the conduction band deviation from the vacuum level was determined by optical reflectance spectroscopy. Our results show that: (1) Introduction of oxygen vacancies forms a donor level below the conduction band. (2) The conduction band is shifted to a lower level by a larger number of oxygen vacancies, while the valence band also shifts to a lower level, due to the reduction in the density of O 2p orbitals. (3) Introduction of barium vacancies widens the band gap. Since barium vacancies can induce a small number of oxygen vacancies with accompanying charge compensation, this behavior suppresses any large formation of donor levels in the gap states, indicating that cation vacancies can control the number of both donor and acceptor levels.
Multi-large low-frequency band gaps in a periodic hybrid structure
NASA Astrophysics Data System (ADS)
Wang, T.; Sheng, M. P.; Guo, H. B.
2016-03-01
A hybrid structure composed of a local resonance mass and an external oscillator is proposed in this paper for restraining the elastic longitudinal wave propagation. Theoretical model has been established to investigate the dispersion relation and band gaps of the structure. The results show that the hybrid structure can produce multi-band gaps wider than the multi-resonator acoustic metamaterials. It is much easier for the hybrid structure to yield wide and low band gaps by adjusting the mass and stiffness of the external oscillator. Small series spring constant ratio results in low-frequency band gaps, in which the external oscillator acts as a resonator and replaces the original local resonator to hold the band gaps in low frequency range. Compared with the one-dimensional phononic crystal (PC) lattice, a new band gap emerges in lower frequency range in the hybrid structure because of the added local resonance, which will be a significant assistance in low-frequency vibration and noise reduction. Further, harmonic response analysis using finite element method (FEM) has been performed, and results show that elastic longitudinal waves are efficiently forbidden within the band gaps.
Latent structure of negative valence measures in childhood.
Lee, Minyoung; Aggen, Steven H; Carney, Dever M; Hahn, Shannon; Moroney, Elizabeth; Machlin, Laura; Brotman, Melissa A; Towbin, Kenneth E; Leibenluft, Ellen; Pine, Daniel S; Roberson-Nay, Roxann; Hettema, John M
2017-08-01
Internalizing disorders (IDs), consisting of the syndromes of anxiety and depression, are common, debilitating conditions often having onsets in adolescence. Scientists have developed dimensional self-report instruments that assess putative negative valence system (NVS) trait-like constructs as complimentary phenotypes to clinical symptoms. These include various measures that index temperamental predispositions to IDs and correlate with neural substrates of fear, anxiety, and affective regulation. This study sought to elucidate the overarching structure of putative NVS traits and their relationship to early manifestations of ID symptomatology. The sample consisted of 768 juvenile twin subjects ages 9-13. Together with ID symptoms, extant validated instruments were chosen to assess a broad spectrum of NVS traits: anxiety sensitivity, irritability, fearfulness, behavioral activation and inhibition, and neuroticism and extraversion. Exploratory and confirmatory factor analyses (EFA/CFA) were used to investigate the latent structure of the associations among these different constructs and ID symptoms. Bifactor modeling in addition to standard correlated-factor analytic approaches were applied. Factor analyses produced a primary tripartite solution comprising anxiety/fear, dysphoria, and positive affect among all these measures. Competing DSM-like correlated factors and an RDoC-like NVS bifactor structure provided similar fit to these data. Our findings support the conceptual organization of a tripartite latent internalizing domain in developing children. This structure includes both clinical symptoms and a variety of self-report dimensional traits currently in use by investigators. These various constructs are, therefore, most informatively investigated using an inclusive, integrated approach. © 2017 Wiley Periodicals, Inc.
Valence and conduction band structure of the quasi-two-dimensional semiconductor Sn S2
NASA Astrophysics Data System (ADS)
Racke, David A.; Neupane, Mahesh R.; Monti, Oliver L. A.
2016-02-01
We present the momentum-resolved photoemission spectroscopy of both the valence and the conduction band region in the quasi-two-dimensional van der Waals-layered indirect band gap semiconductor Sn S2 . Using a combination of angle-resolved ultraviolet photoemission and angle-resolved two-photon photoemission (AR-2PPE) spectroscopy, we characterize the band structure of bulk Sn S2 . Comparison with density functional theory calculations shows excellent quantitative agreement in the valence band region and reveals several localized bands that likely originate from defects such as sulfur vacancies. Evidence for a moderate density of defects is also observed by AR-2PPE in the conduction band region, leading to localized bands not present in the computational results. The energetic structure and dispersion of the conduction bands is captured well by the computational treatment, with some quantitative discrepancies remaining. Our results provide a broader understanding of the electronic structure of Sn S2 in particular and van der Waals-layered semiconductors in general.
Disorder enabled band structure engineering of a topological insulator surface
Xu, Yishuai; Chiu, Janet; Miao, Lin; He, Haowei; Alpichshev, Zhanybek; Kapitulnik, A.; Biswas, Rudro R.; Wray, L. Andrew
2017-01-01
Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport. PMID:28155858
Disorder enabled band structure engineering of a topological insulator surface
NASA Astrophysics Data System (ADS)
Xu, Yishuai; Chiu, Janet; Miao, Lin; He, Haowei; Alpichshev, Zhanybek; Kapitulnik, A.; Biswas, Rudro R.; Wray, L. Andrew
2017-02-01
Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.
Disorder enabled band structure engineering of a topological insulator surface
Xu, Yishuai; Chiu, Janet; Miao, Lin; ...
2017-02-03
Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localizedmore » regime usually associated with impurity bands. Lastly, at native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.« less
Broadband locally resonant band gaps in periodic beam structures with embedded acoustic black holes
NASA Astrophysics Data System (ADS)
Tang, Liling; Cheng, Li
2017-05-01
The Acoustic Black Hole (ABH) effect can be used to effectively reduce structural vibrations by trapping flexural waves in a thin-walled structure with a power-law thickness variation. In the present study, we used a wavelet-decomposed energy method to investigate an Euler-Bernoulli beam embedded with multiple ABHs. Broadband transmission attenuation bands at relatively low frequencies are observed in a beam containing only a few ABH elements. To explain the underlying phenomena, an infinite structure with periodic ABH elements is analyzed. Numerical results show that the periodic boundary conditions in terms of displacement and rotational slope of a unit cell, based on the finite model, are sufficient to describe the band structures, without requiring full treatment of the entire infinite structure. This provides an efficient and flexible means to predict, and eventually optimize, the band structure based on a single element. Meanwhile, the ABH-induced locally resonant band gaps coincide with the attenuation bands observed in the finite beams. Because of the unique ABH feature, the proposed beam requires only a small number of elements to obtain broad attenuation bands, which offers great potential for vibrational isolation applications and wave filter designs in beam structures.
NASA Astrophysics Data System (ADS)
Wei, Xiaojun; Tanaka, Takeshi; Yomogida, Yohei; Sato, Naomichi; Saito, Riichiro; Kataura, Hiromichi
2016-10-01
Experimental band structure analyses of single-walled carbon nanotubes have not yet been reported, to the best of our knowledge, except for a limited number of reports using scanning tunnelling spectroscopy. Here we demonstrate the experimental determination of the excitonic band structures of single-chirality single-walled carbon nanotubes using their circular dichroism spectra. In this analysis, we use gel column chromatography combining overloading selective adsorption with stepwise elution to separate 12 different single-chirality enantiomers. Our samples show higher circular dichroism intensities than the highest values reported in previous works, indicating their high enantiomeric purity. Excitonic band structure analysis is performed by assigning all observed Eii and Eij optical transitions in the circular dichroism spectra. The results reproduce the asymmetric structures of the valence and conduction bands predicted by density functional theory. Finally, we demonstrate that an extended empirical formula can estimate Eij optical transition energies for any (n,m) species.
Band structure engineering through orbital interaction for enhanced thermoelectric power factor
Zhu, Hong; Sun, Wenhao; Ceder, Gerbrand; Armiento, Rickard; Lazic, Predrag
2014-02-24
Band structure engineering for specific electronic or optical properties is essential for the further development of many important technologies including thermoelectrics, optoelectronics, and microelectronics. In this work, we report orbital interaction as a powerful tool to finetune the band structure and the transport properties of charge carriers in bulk crystalline semiconductors. The proposed mechanism of orbital interaction on band structure is demonstrated for IV-VI thermoelectric semiconductors. For IV-VI materials, we find that the convergence of multiple carrier pockets not only displays a strong correlation with the s-p and spin-orbit coupling but also coincides with the enhancement of power factor. Our results suggest a useful path to engineer the band structure and an enticing solid-solution design principle to enhance thermoelectric performance.
Wei, Xiaojun; Tanaka, Takeshi; Yomogida, Yohei; Sato, Naomichi; Saito, Riichiro; Kataura, Hiromichi
2016-10-05
Experimental band structure analyses of single-walled carbon nanotubes have not yet been reported, to the best of our knowledge, except for a limited number of reports using scanning tunnelling spectroscopy. Here we demonstrate the experimental determination of the excitonic band structures of single-chirality single-walled carbon nanotubes using their circular dichroism spectra. In this analysis, we use gel column chromatography combining overloading selective adsorption with stepwise elution to separate 12 different single-chirality enantiomers. Our samples show higher circular dichroism intensities than the highest values reported in previous works, indicating their high enantiomeric purity. Excitonic band structure analysis is performed by assigning all observed Eii and Eij optical transitions in the circular dichroism spectra. The results reproduce the asymmetric structures of the valence and conduction bands predicted by density functional theory. Finally, we demonstrate that an extended empirical formula can estimate Eij optical transition energies for any (n,m) species.
Wei, Xiaojun; Tanaka, Takeshi; Yomogida, Yohei; Sato, Naomichi; Saito, Riichiro; Kataura, Hiromichi
2016-01-01
Experimental band structure analyses of single-walled carbon nanotubes have not yet been reported, to the best of our knowledge, except for a limited number of reports using scanning tunnelling spectroscopy. Here we demonstrate the experimental determination of the excitonic band structures of single-chirality single-walled carbon nanotubes using their circular dichroism spectra. In this analysis, we use gel column chromatography combining overloading selective adsorption with stepwise elution to separate 12 different single-chirality enantiomers. Our samples show higher circular dichroism intensities than the highest values reported in previous works, indicating their high enantiomeric purity. Excitonic band structure analysis is performed by assigning all observed Eii and Eij optical transitions in the circular dichroism spectra. The results reproduce the asymmetric structures of the valence and conduction bands predicted by density functional theory. Finally, we demonstrate that an extended empirical formula can estimate Eij optical transition energies for any (n,m) species. PMID:27703139
S-band accelerating structures for the PAL-XFEL
NASA Astrophysics Data System (ADS)
Lee, Heung-Soo; Park, Young Jung; Joo, Young-Do; Heo, Hoon; Heo, Jinyul; Kim, Sang-Hee; Park, Soung-Soo; Hwang, Woon Ha; Kang, Heung-Sik; Kim, Kwang-woo; Ko, In-Soo; Oh, Kyoung-Min; Noh, Sung-Joo; Bak, Yong Hwan; Matsumoto, Hiroshi
2015-02-01
One hundred seventy-two accelerating structures are required for the Pohang Accelerator Laboratory X-ray free-electron laser's (PAL-XFEL's) 10-GeV main linear accelerator. So far, we have purchased 80 structures from Mitsubishi Heavy Industry (MHI), which have quasi-symmetric couplers in the accelerating structure to reduce the quadruple and the sextuple components of the electric field in the coupling cavity. High-power tests have been conducted for the first structure of the MHI structure, and Research Instruments (RI) has developed a 3-m long accelerating structure that has an operating frequency of 2856 MHz and in/out couplers of quasi-symmetric racetrack shape for the PAL-XFEL linear accelerator. This structure also has been tested by PAL and RI in the Pohang accelerator laboratory (PAL) to check the maximum available electric field gradient. We will describe the test results of these structures and the current status for the fabrication of the other accelerating structures in this paper.
NASA Astrophysics Data System (ADS)
Jiang, Tao; Wang, Yanyan; Li, Yingsong
2017-07-01
In this paper, a triple stop-band filter with a ratioed periodical defected microstrip structure is proposed for wireless communication applications. The proposed ratioed periodical defected microstrip structures are spiral slots, which are embedded into a 50 Ω microstrip line to obtain multiple stop-bands. The performance of the proposed triple stop-band filter is investigated numerically and experimentally. Moreover, the equivalent circuit model of the proposed filter is also established and discussed. The results are given to verify that the proposed triple stop-band filter has three stop bands at 3.3 GHz, 5.2 GHz, 6.8 GHz to reject the unwanted signals, which is promising for integrating into UWB communication systems to efficiently prevent the potential interferences from unexpected narrowband signals such as WiMAX, WLAN and RFID communication systems.
Magnon band structure and magnon density in one-dimensional magnonic crystals
NASA Astrophysics Data System (ADS)
Qiu, Rong-ke; Huang, Te; Zhang, Zhi-dong
2014-11-01
By using Callen's Green's function method and the Tyablikov and Anderson-Callen decoupling approximations, we systematically study the magnon band structure and magnon density perpendicular to the superlattice plane of one-dimensional magnonic crystals, with a superlattice consisting of two magnetic layers with ferromagnetic (FM) or antiferromagnetic (AFM) interlayer exchange coupling. The effects of temperature, interlayer coupling, anisotropy and external magnetic field on the magnon-energy band and magnon density in the Kx-direction are investigated in three situations: a) the magnon band of magnetic superlattices with FM interlayer coupling, b) separate and c) overlapping magnon bands of magnetic superlattices with AFM interlayer coupling. In the present work, a quantum approach is developed to study the magnon band structure and magnon density of magnonic crystals and the results are beneficial for the design of magnonic-crystal waveguides or gigahertz-range spin-wave filters.
Electronic energy band structure of the double perovskite Ba2MnWO6.
Fujioka, Yukari; Frantti, Johannes; Nieminen, Risto M
2008-06-05
The electronic and magnetic structures of the double perovskite oxide Ba 2MnWO6 (BMW) were determined by employing the density functional theory within the generalized gradient approximation (GGA) + U approach. BMW is considered a prototype double perovskite due to its high degree of B-site ordering and is a good case study for making a comparison between computations and experiments. By adjusting the U-parameter, the electronic energy band structure and magnetic properties, which were consistent with the experimental results, were obtained. These computations revealed that the valence bands are mainly formed from Mn 3d and O 2p states, while the conduction bands are derived from W 5d and O 2p states. The localized bands composed from Mn 3d states are located in the bandgap. The results imply that the formation of polarons in the conduction band initiate the resonance Raman modes observed as a series of equidistant peaks.
Double stop-band structure near half-integer tunes in high-intensity rings
NASA Astrophysics Data System (ADS)
Moriya, K.; Ota, M.; Fukushima, K.; Yamaguchi, M.; Ito, K.; Okamoto, H.
2016-11-01
This paper addresses a detailed experimental study of collective instability bands generated near every half-integer tune per lattice period by coherent dipole and quadrupole resonances. Both instabilities appear side by side or overlap each other but are mostly separable because the dipole resonance often creates a narrower stop band accompanied by more severe particle losses. The separation of these low-order resonance bands becomes greater as the beam intensity increases. In principle, the double stop-band structure can be formed even without machine imperfections when the beam's initial phase-space profile is deviated from the ideal stationary distribution. The tabletop ion-trap system called "S-POD" is employed to experimentally demonstrate the parameter dependence of the double stop-band structure. Numerical simulations are also performed for comparison with experimental observations.
First-principle study of energy band structure of armchair graphene nanoribbons
NASA Astrophysics Data System (ADS)
Ma, Fei; Guo, Zhankui; Xu, Kewei; Chu, Paul K.
2012-07-01
First-principle calculation is carried out to study the energy band structure of armchair graphene nanoribbons (AGNRs). Hydrogen passivation is found to be crucial to convert the indirect band gaps into direct ones as a result of enhanced interactions between electrons and nuclei at the edge boundaries, as evidenced from the shortened bond length as well as the increased differential charge density. Ribbon width usually leads to the oscillatory variation of band gaps due to quantum confinement no matter hydrogen passivated or not. Mechanical strain may change the crystal symmetry, reduce the overlapping integral of C-C atoms, and hence modify the band gap further, which depends on the specific ribbon width sensitively. In practical applications, those effects will be hybridized to determine the energy band structure and subsequently the electronic properties of graphene. The results can provide insights into the design of carbon-based devices.
X-band dielectric loaded traveling-wave acceleration structure
NASA Astrophysics Data System (ADS)
Zou, P.; Gai, W.; Konecny, R.; Sun, X.; Wong, T.
2001-05-01
We report on the construction, numerical modeling and experimental testing of a traveling-wave acceleration structure based on a dielectric-lined circular waveguide. This type of structure has similar acceleration properties to disk-loaded metal slow wave structures but with some distinct advantages in terms of simplicity of fabrication, suppression of parasitic wakefield effects, and having no dark current. Efficient coupling of external RF power to the cylindrical dielectric waveguide is a technical challenge, particularly to structures loaded with very high dielectric constant materials. We have designed and constructed an 11.4 GHz structure loaded with ceramic with dielectric constant of 20, to be powered by an external RF power source. High efficiency RF coupling has been achieved using a combination of a tapered dielectric end section and a carefully adjusted coupling slot. Bench tests using a network analyzer have demonstrated a power coupling efficiency in excess of 95% with bandwidth of 30 MHz, and vacuum tests have also shown that this dielectric loaded structure can be operated in an ultra-high vacuum environment. Thus, this work provides a necessary basis for construction of an accelerator using this kind of structure. We have also simulated the parameters of this structure using MAFIA. Within the limits of the approximations used, the results are in agreement with the bench measurements.
Observation of interface band structure by ballistic-electron-emission microscopy
NASA Technical Reports Server (NTRS)
Bell, L. D.; Kaiser, W. J.
1988-01-01
The paper reports an advanced ballistic electron spectroscopy technique that was used to directly measure semiconductor band structure properties at a subsurface interface. Two interface systems having contrasting band structures were investigated by this method: Au-Si and Au-GaAs. It is concluded that the proposed method, based on scanning tunneling microscopy, enables the spatially resolved carrier-transport spectroscopy of interfaces.
Observation of interface band structure by ballistic-electron-emission microscopy
NASA Technical Reports Server (NTRS)
Bell, L. D.; Kaiser, W. J.
1988-01-01
The paper reports an advanced ballistic electron spectroscopy technique that was used to directly measure semiconductor band structure properties at a subsurface interface. Two interface systems having contrasting band structures were investigated by this method: Au-Si and Au-GaAs. It is concluded that the proposed method, based on scanning tunneling microscopy, enables the spatially resolved carrier-transport spectroscopy of interfaces.
B3LYP, BLYP and PBE DFT band structures of the nucleotide base stacks
NASA Astrophysics Data System (ADS)
Szekeres, Zs; Bogár, F.; Ladik, J.
DFT crystal orbital (band structure) calculations have been performed for the nucleotide base stacks of cytosine, thymine, adenine, and guanine arranged in DNA B geometry. The band structures obtained with PBE, BLYP, and B3LYP functionals are presented and compared to other related experimental and theoretical results. The influence of the quality of the basis set on the fundamental gap values was also investigated using Clementi's double ζ, 6-31G and 6-31G* basis sets.
Zhang Haifeng; Liu Shaobin; Kong Xiangkun; Bian Borui; Dai Yi
2012-11-15
In this paper, an omnidirectional photonic band gap realized by one-dimensional ternary unmagnetized plasma photonic crystals based on a new Fibonacci quasiperiodic structure, which is composed of homogeneous unmagnetized plasma and two kinds of isotropic dielectric, is theoretically studied by the transfer matrix method. It has been shown that such an omnidirectional photonic band gap originates from Bragg gap in contrast to zero-n gap or single negative (negative permittivity or negative permeability) gap, and it is insensitive to the incidence angle and the polarization of electromagnetic wave. From the numerical results, the frequency range and central frequency of omnidirectional photonic band gap can be tuned by the thickness and density of the plasma but cease to change with increasing Fibonacci order. The bandwidth of omnidirectional photonic band gap can be notably enlarged. Moreover, the plasma collision frequency has no effect on the bandwidth of omnidirectional photonic band gap. It is shown that such new structure Fibonacci quasiperiodic one-dimensional ternary plasma photonic crystals have a superior feature in the enhancement of frequency range of omnidirectional photonic band gap compared with the conventional ternary and conventional Fibonacci quasiperiodic ternary plasma photonic crystals.
Plasmonic nanochannel structure for narrow-band selective thermal emitter
NASA Astrophysics Data System (ADS)
Wang, Zhiyu; Clark, J. Kenji; Huang, Li-Chung; Ho, Ya-Lun; Delaunay, Jean-Jacques
2017-06-01
A plasmonic structure consisting of a periodic arrangement of vertical silicon nanochannels connected by U-shaped gold layers is demonstrated as a spectrally selective thermal emitter. The plasmonic nanochannel structure sustains a coupled mode between a surface plasmon polariton and a stationary surface plasmon resonance, which induces a strong and sharp resonance observed in the form of a reflectance dip in the far field. Upon heating the structure, a strong and narrow-bandwidth thermal emittance peak is observed with a maximum emittance value of 0.72 and a full-width-at-half-maximum of 248 nm at a wavelength of 5.66 μm, which corresponds to the reflectance dip wavelength. Moreover, we demonstrate the control of the emission peak wavelength by varying the period of the structure. The plasmonic nanochannel structure realizes a small-size and selective infrared thermal emitter, which is expected to be applicable as an infrared light source.
Multi-cavity locally resonant structure with the low frequency and broad band-gaps
NASA Astrophysics Data System (ADS)
Jiang, Jiulong; Yao, Hong; Du, Jun; Zhao, Jinbo
2016-11-01
A multi-cavity periodic structure with the characteristic of local resonance was proposed in the paper. The low frequency band-gap structure was comparatively analyzed by the finite element method (FEM) and electric circuit analogy (ECA). Low frequency band-gap can be opened through the dual influence of the coupling's resonance in the cavity and the interaction among the couplings between structures. Finally, the influence of the structural factors on the band-gap was analyzed. The results show that the structure, which is divided into three parts equally, has a broader effective band-gap below the frequency of 200 Hz. It is also proved that reducing the interval between unit structures can increase the intensity of the couplings among the structures. And in this way, the width of band-gap would be expanded significantly. Through the parameters adjustment, the structure enjoys a satisfied sound insulation effect below the frequency of 500Hz. In the area of low frequency noise reduction, the structure has a lot of potential applications.
RF Breakdown in High Vacuum Multimegawatt X-Band Structures
Dolgashev, V
2004-06-15
Increasing the power handling capabilities of rf components is an important issue for the design of rf accelerators and rf sources. RF breakdown is a phenomena that limit the high power performance. A major concern is the damage that can occur in rf components from breakdown. To better understand this damage, we have studied rf breakdown in a rectangular waveguide experimentally and theoretically. The breakdown process in a waveguide is both easier to measure and simulate than breakdown in a complex geometry such as an accelerating structure. We used a particle tracking code and a Particle-In-Cell code to model the breakdown behavior. Models developed for the waveguide were applied to the breakdown in accelerating structures. RF breakdown in traveling wave and standing wave accelerating structures was simulated. We compare the experimental data with results of the simulations for the accelerating structures.
Quashie, Edwin E.; Saha, Bidhan C.; Correa, Alfredo A.
2016-10-05
Here, we present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities v = 0.02–10a.u. where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results aremore » in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of ~1.5) in the velocity range v = 0.07–0.3a.u., which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping v → 0 is not as simple as phenomenological models suggest and it is plagued by band-structure effects.« less
Quashie, Edwin E.; Saha, Bidhan C.; Correa, Alfredo A.
2016-10-05
Here, we present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities v = 0.02–10a.u. where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results are in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of ~1.5) in the velocity range v = 0.07–0.3a.u., which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping v → 0 is not as simple as phenomenological models suggest and it is plagued by band-structure effects.
Quashie, Edwin E.; Saha, Bidhan C.; Correa, Alfredo A.
2016-10-05
Here, we present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities v = 0.02–10a.u. where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results are in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of ~1.5) in the velocity range v = 0.07–0.3a.u., which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping v → 0 is not as simple as phenomenological models suggest and it is plagued by band-structure effects.
Low-frequency photonic band structures in graphene-like triangular metallic lattice
NASA Astrophysics Data System (ADS)
Wang, Kang
2016-11-01
We study the low frequency photonic band structures in triangular metallic lattice, displaying Dirac points in the frequency spectrum, and constructed upon the lowest order regular polygonal tiles. We show that, in spite of the unfavourable geometrical conditions intrinsic to the structure symmetry, the lowest frequency photonic bands are formed by resonance modes sustained by local structure patterns, with the corresponding electric fields following a triangular distribution at low structure filling rate and a honeycomb distribution at high filling rate. For both cases, the lowest photonic bands, and thus the plasma gap, can be described in the framework of a tight binding model, and analysed in terms of local resonance modes and their mutual correlations. At high filling rate, the Dirac points and their movement following the structure deformation are described in the same framework, in relation with local structure patterns and their variations, as well as the particularity of the metallic lattice that enhances the topological anisotropy.
Control over band structure and tunneling in bilayer graphene induced by velocity engineering.
Cheraghchi, Hosein; Adinehvand, Fatemeh
2014-01-08
The band structure and transport properties of massive Dirac fermions in bilayer graphene with velocity modulation in space are investigated in the presence of a previously created band gap. It is pointed out that velocity engineering may be considered as a factor to control the band gap of symmetry-broken bilayer graphene. The band gap is direct and independent of velocity value if the velocity modulated in two layers is set up equally. Otherwise, in the case of interlayer asymmetric velocity, not only is the band gap indirect, but also the electron-hole symmetry fails. This band gap is controllable by the ratio of the velocity modulated in the upper layer to the velocity modulated in the lower layer. In more detail, the shift of momentum from the conduction band edge to the valence band edge can be engineered by the gate bias and velocity ratio. A transfer matrix method is also elaborated to calculate the four-band coherent conductance through a velocity barrier possibly subjected to a gate bias. Electronic transport depends on the ratio of velocity modulated inside the barrier to that for surrounding regions. As a result, a quantum version of total internal reflection is observed for thick enough velocity barriers. Moreover, a transport gap originating from the applied gate bias is engineered by modulating the velocities of the carriers in the upper and lower layers.
High-spin band structure of 192Tl
NASA Astrophysics Data System (ADS)
Kreiner, A. J.; Filevich, A.; García Bermúdez, G.; Mariscotti, M. A. J.; Baktash, C.; der Mateosian, E.; Thieberger, P.
1980-03-01
High-spin states in 192Tl, excited through the 181Ta(18O,7n) and 181Ta(16O,5n) reactions, were studied using in-beam γ-ray spectroscopic techniques. Excitation functions, activity spectra, γ-ray angular distributions, and multidimensional coincidences were measured. The strongly Coríolis-distorted π~h92×ν~i132 two-quasiparticle band already known in the heavier 194,196,198Tl isotopes has also been found in this case based on an Iπ=8- isomeric state at 250.6 keV above the known long-lived 7+ level. Trends already noted in the other Tl isotopes and also predicted by two-quasiparticle plus-rotor model calculations are confirmed thus reinforcing such a theoretical description. NUCLEAR REACTIONS 181Ta(18O,xnγ), E=105-125 MeV; 181Ta(16O,xnγ), E=95-105 MeV; measured Eγ, Iγ, σ(E, Eγ, θγ), γ-γ coin.; 192Tl levels deduced, J, π, T12. Natural target. Ge(Li) detectors.
Fine structure of the red luminescence band in undoped GaN
Reshchikov, M. A.; Usikov, A.; Helava, H.; Makarov, Yu.
2014-01-20
Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RL band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band.
NASA Astrophysics Data System (ADS)
Seyidov, MirHasan Yu.; Suleymanov, Rauf A.
2010-09-01
We conducted comparison of the original experimental data of the temperature dependences of thermal expansion in crystals with layered crystalline structure. It is shown that in most crystals with layered structure (graphite, boron nitride, GaSe, GaS, and InSe) the effect of negative thermal expansion can be explained by the specific character of the phonon spectra. It was shown, that in contrast to other crystals with layered structure, negative thermal expansion in the layers' plane of TlGaSe2 is the result of negative area compressibility. We demonstrate that the thermal expansion of TlGaSe2 crystals can be controlled by illumination, external electric field, and thermal annealing. The nature of observed effects and a special mechanism of the negative area compressibility in TlGaSe2 crystals are discussed.
First principle study of band structure of SrMO{sub 3} perovskites
Daga, Avinash; Sharma, Smita
2016-05-06
First principle study of band structure calculations in the local density approximations (LDA) as well as in the generalized gradient approximations (GGA) have been used to determine the electronic structure of SrMO{sub 3} where M stands for Ti, Zr and Mo. Occurrence of band gap proves SrTiO{sub 3} and SrZrO{sub 3} to be insulating. A small band gap is observed in SrMoO{sub 3} perovskite signifies it to be metallic. Band structures are found to compare well with the available data in the literature showing the relevance of this approach. ABINIT computer code has been used to carry out all the calculations.
Energy loss of ions at metal surfaces: Band-structure effects
Alducin, M.; Silkin, V.M.; Juaristi, J.I.; Chulkov, E.V.
2003-03-01
We study band-structure effects on the energy loss of protons scattered off the Cu (111) surface. The distance dependent stopping power for a projectile traveling parallel to the surface is calculated within the linear response theory. The self-consistent electronic response of the system is evaluated within the random-phase approximation. In order to characterize the surface band structure, the electronic single-particle wave functions and energies are obtained by solving the Schroedinger equation with a realistic one-dimensional model potential. This potential reproduces the main features of the Cu (111) surface: the energy band gap for electron motion along the surface normal, as well as the binding energy of the occupied surface state and the first image state. Comparison of our results with those obtained within the jellium model allows us to characterize the band-structure effects in the energy loss of protons interacting with the Cu (111) surface.
Shape dependence of band-edge exciton fine structure in CdSe nanocrystals.
Zhao, Qingzhong; Graf, Peter A; Jones, Wesley B; Franceschetti, Alberto; Li, Jingbo; Wang, Lin-Wang; Kim, Kwiseon
2007-11-01
The band-edge exciton fine structure of wurtzite CdSe nanocrystals is investigated by a plane-wave pseudopotential method that includes spin-orbit coupling, screened electron-hole Coulomb interactions, and exchange interactions. Large-scale, systematic simulations have been carried out on quantum dots, nanorods, nanowires, and nanodisks. The size and shape dependence of the exciton fine structure is explored over the whole diameter-length configuration space and is explained by the interplay of quantum confinement, intrinsic crystal-field splitting, and electron-hole exchange interactions. Our results show that the band-edge exciton fine structure of CdSe nanocrystals is determined by the origin of their valence-band single-particle wave functions. Nanocrystals where the valence-band maximum originates from the bulk A band have a "dark" ground-state exciton. Nanocrystals where the valence-band maximum is derived from the bulk B band have a "quasi-bright" ground-state exciton. Thus, the diameter-length configuration map can be divided into two regions, corresponding to dark and quasi-bright ground-state excitons. We find that the dark/quasi-bright ground-state exciton crossover is not only diameter-dependent but also length-dependent, and it is characterized by a curve in the two-parameter space of diameter and length.
Shank, Joshua C.; Tellekamp, M. Brooks; Doolittle, W. Alan
2015-01-21
The theoretically suggested band structure of the novel p-type semiconductor lithium niobite (LiNbO{sub 2}), the direct coupling of photons to ion motion, and optically induced band structure modifications are investigated by temperature dependent photoluminescence. LiNbO{sub 2} has previously been used as a memristor material but is shown here to be useful as a sensor owing to the electrical, optical, and chemical ease of lithium removal and insertion. Despite the high concentration of vacancies present in lithium niobite due to the intentional removal of lithium atoms, strong photoluminescence spectra are observed even at room temperature that experimentally confirm the suggested band structure implying transitions from a flat conduction band to a degenerate valence band. Removal of small amounts of lithium significantly modifies the photoluminescence spectra including additional larger than stoichiometric-band gap features. Sufficient removal of lithium results in the elimination of the photoluminescence response supporting the predicted transition from a direct to indirect band gap semiconductor. In addition, non-thermal coupling between the incident laser and lithium ions is observed and results in modulation of the electrical impedance.
Electronic band structures and excitonic properties of delafossites: A GW-BSE study
NASA Astrophysics Data System (ADS)
Wang, Xiaoming; Meng, Weiwei; Yan, Yanfa
2017-08-01
We report the band structures and excitonic properties of delafossites CuMO2 (M=Al, Ga, In, Sc, Y, Cr) calculated using the state-of-the-art GW-BSE approach. We evaluate different levels of self-consistency of the GW approximations, namely G0W0, GW0, GW, and QSGW, on the band structures and find that GW0, in general, predicts the band gaps in better agreement with experiments considering the electron-hole effect. For CuCrO2, the HSE wave function is used as the starting point for the perturbative GW0 calculations, since it corrects the band orders wrongly predicted by PBE. The discrepancy about the valence band characters of CuCrO2 is classified based on both HSE and QSGW calculations. The PBE wave functions, already good, are used for other delafossites. All the delafossites are shown to be indirect band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV, in consistent with experimental findings. The excitation mechanisms are explained by examining the exciton amplitude projections on the band structures. Discrepancies compared with experiments are also addressed. The lowest and strongest exciton, mainly contributed from either Cu 3d → Cu 3p (Al, Ga, In) or Cu 3d → M 3d (M = Sc, Y, Cr) transitions, is always located at the L point of the rhombohedral Brillouin zone.
An open-structure sound insulator against low-frequency and wide-band acoustic waves
NASA Astrophysics Data System (ADS)
Chen, Zhe; Fan, Li; Zhang, Shu-yi; Zhang, Hui; Li, Xiao-juan; Ding, Jin
2015-10-01
To block sound, i.e., the vibration of air, most insulators are based on sealed structures and prevent the flow of the air. In this research, an acoustic metamaterial adopting side structures, loops, and labyrinths, arranged along a main tube, is presented. By combining the accurately designed side structures, an extremely wide forbidden band with a low cut-off frequency of 80 Hz is produced, which demonstrates a powerful low-frequency and wide-band sound insulation ability. Moreover, by virtue of the bypass arrangement, the metamaterial is based on an open structure, and thus air flow is allowed while acoustic waves can be insulated.
The band structure of birefractive CdGa2S4 crystals
NASA Astrophysics Data System (ADS)
Stamov, I. G.; Syrbu, N. N.; Parvan, V. I.; Zalamai, V. V.; Tiginyanu, I. M.
2013-11-01
In this paper, we report on the spectral dependence of Δn=no-ne for CdGa2S4 single crystals for shorter and longer wavelengths than the isotropic wavelength λ0=485.7 nm (300 K). It was established that Δn is positive at λ>λ0 and it is negative in the spectral range λ<λ0. The isotropic wavelength λ0 exhibits blue spectral shift with temperature decreasing. The ground and excited states of three excitonic series A, B and C with binding energies of 53 meV, 52 meV and 46 meV, respectively, were found out at 10 K. The effective masses of electrons for k=0 were derived from the calculation of excitonic spectra: mc∥(Е∥с)=0.21m0 and mc⊥(Е⊥с)=0.19m0. The holes masses are equal to 0.59m0 and 0.71m0 for Е∥с and Е⊥с, respectively. The value of valence bands splitting, V1-V2, by crystalline field equals 24 meV, and V2-V3 splitting due to the spin-orbital interaction equals to 130 meV. The optical functions n, k, ε1 and ε2 for Е⊥с and Е∥с polarizations were calculated by means of Kramers-Kronig analyses in the energy interval 3-6 eV. The evidenced features are discussed taking into account the results of new theoretical calculations of CdGa2S4 band structure.
Berry phase and band structure analysis of the Weyl semimetal NbP
Sergelius, Philip; Gooth, Johannes; Bäßler, Svenja; Zierold, Robert; Wiegand, Christoph; Niemann, Anna; Reith, Heiko; Shekhar, Chandra; Felser, Claudia; Yan, Binghai; Nielsch, Kornelius
2016-01-01
Weyl semimetals are often considered the 3D-analogon of graphene or topological insulators. The evaluation of quantum oscillations in these systems remains challenging because there are often multiple conduction bands. We observe de Haas-van Alphen oscillations with several frequencies in a single crystal of the Weyl semimetal niobium phosphide. For each fundamental crystal axis, we can fit the raw data to a superposition of sinusoidal functions, which enables us to calculate the characteristic parameters of all individual bulk conduction bands using Fourier transform with an analysis of the temperature and magnetic field-dependent oscillation amplitude decay. Our experimental results indicate that the band structure consists of Dirac bands with low cyclotron mass, a non-trivial Berry phase and parabolic bands with a higher effective mass and trivial Berry phase. PMID:27667203
Reducing support loss in micromechanical ring resonators using phononic band-gap structures
NASA Astrophysics Data System (ADS)
Hsu, Feng-Chia; Hsu, Jin-Chen; Huang, Tsun-Che; Wang, Chin-Hung; Chang, Pin
2011-09-01
In micromechanical resonators, energy loss via supports into the substrates may lead to a low quality factor. To eliminate the support loss, in this paper a phononic band-gap structure is employed. We demonstrate a design of phononic-crystal (PC) strips used to support extensional wine-glass mode ring resonators to increase the quality factor. The PC strips are introduced to stop elastic-wave propagation by the band-gap and deaf-band effects. Analyses of resonant characteristics of the ring resonators and the dispersion relations, eigenmodes, and transmission properties of the PC strips are presented. With the proposed resonator architecture, the finite-element simulations show that the leaky power is effectively reduced and the stored energy inside the resonators is enhanced simultaneously as the operating frequencies of the resonators are within the band gap or deaf bands. Realization of a high quality factor micromechanical ring resonator with minimized support loss is expected.
NASA Astrophysics Data System (ADS)
Ozkaya, Efe; Yilmaz, Cetin
2017-02-01
The effect of eddy current damping on a novel locally resonant periodic structure is investigated. The frequency response characteristics are obtained by using a lumped parameter and a finite element model. In order to obtain wide band gaps at low frequencies, the periodic structure is optimized according to certain constraints, such as mass distribution in the unit cell, lower limit of the band gap, stiffness between the components in the unit cell, the size of magnets used for eddy current damping, and the number of unit cells in the periodic structure. Then, the locally resonant periodic structure with eddy current damping is manufactured and its experimental frequency response is obtained. The frequency response results obtained analytically, numerically and experimentally match quite well. The inclusion of eddy current damping to the periodic structure decreases amplitudes of resonance peaks without disturbing stop band width.
Promoting Photochemical Water Oxidation with Metallic Band Structures.
Liu, Hongfei; Moré, René; Grundmann, Henrik; Cui, Chunhua; Erni, Rolf; Patzke, Greta R
2016-02-10
The development of economic water oxidation catalysts is a key step toward large-scale water splitting. However, their current exploration remains empirical to a large extent. Elucidating the correlations between electronic properties and catalytic activity is crucial for deriving general and straightforward catalyst design principles. Herein, strongly correlated electronic systems with abundant and easily tunable electronic properties, namely La(1-x)Sr(x)BO3 perovskites and La(2-x)Sr(x)BO4 layered perovskites (B = Fe, Co, Ni, or Mn), were employed as model systems to identify favorable electronic structures for water oxidation. We established a direct correlation between the enhancement of catalytic activity and the insulator to metal transition through tuning the electronic properties of the target perovskite families via the La(3+)/Sr(2+) ratio. Their improved photochemical water oxidation performance was clearly linked to the increasingly metallic character. These electronic structure-activity relations provide a promising guideline for constructing efficient water oxidation catalysts.
Kimura, Kenichi; Langford, S. C.; Dickinson, J. T.
2007-12-01
Many wide band gap materials yield charged and neutral emissions when exposed to sub-band-gap laser radiation at power densities below the threshold for optical breakdown and plume formation. In this work, we report the observation of negative alkali ions from several alkali halides under comparable conditions. We observe no evidence for negative halogen ions, in spite of the high electron affinities of the halogens. Significantly, the positive and negative alkali ions show a high degree of spatial and temporal overlap. A detailed study of all the relevant particle emissions from potassium chloride (KCl) suggests that K{sup -} is formed by the sequential attachment of two electrons to K{sup +}.
Band structures of 4f and 5f materials studied by angle-resolved photoelectron spectroscopy.
Fujimori, Shin-ichi
2016-04-20
Recent remarkable progress in angle-resolved photoelectron spectroscopy (ARPES) has enabled the direct observation of the band structures of 4f and 5f materials. In particular, ARPES with various light sources such as lasers (hν ~ 7 eV) or high-energy synchrotron radiations (hν >/~ 400 eV) has shed light on the bulk band structures of strongly correlated materials with energy scales of a few millielectronvolts to several electronvolts. The purpose of this paper is to summarize the behaviors of 4f and 5f band structures of various rare-earth and actinide materials observed by modern ARPES techniques, and understand how they can be described using various theoretical frameworks. For 4f-electron materials, ARPES studies of CeMIn5(M = Rh, Ir, and Co) and YbRh2Si2 with various incident photon energies are summarized. We demonstrate that their 4f electronic structures are essentially described within the framework of the periodic Anderson model, and that the band-structure calculation based on the local density approximation cannot explain their low-energy electronic structures. Meanwhile, electronic structures of 5f materials exhibit wide varieties ranging from itinerant to localized states. For itinerant U5f compounds such as UFeGa5, their electronic structures can be well-described by the band-structure calculation assuming that all U5f electrons are itinerant. In contrast, the band structures of localized U5f compounds such as UPd3 and UO2 are essentially explained by the localized model that treats U5f electrons as localized core states. In regards to heavy fermion U-based compounds such as the hidden-order compound URu2Si2, their electronic structures exhibit complex behaviors. Their overall band structures are generally well-explained by the band-structure calculation, whereas the states in the vicinity of EF show some deviations due to electron correlation effects. Furthermore, the electronic structures of URu2Si2 in the paramagnetic and hidden-order phases are
Band structures of 4f and 5f materials studied by angle-resolved photoelectron spectroscopy
NASA Astrophysics Data System (ADS)
Fujimori, Shin-ichi
2016-04-01
Recent remarkable progress in angle-resolved photoelectron spectroscopy (ARPES) has enabled the direct observation of the band structures of 4f and 5f materials. In particular, ARPES with various light sources such as lasers (hν ∼ 7~\\text{eV} ) or high-energy synchrotron radiations (hν ≳ 400~\\text{eV} ) has shed light on the bulk band structures of strongly correlated materials with energy scales of a few millielectronvolts to several electronvolts. The purpose of this paper is to summarize the behaviors of 4f and 5f band structures of various rare-earth and actinide materials observed by modern ARPES techniques, and understand how they can be described using various theoretical frameworks. For 4f-electron materials, ARPES studies of \\text{Ce}M\\text{I}{{\\text{n}}5} (M=\\text{Rh} , \\text{Ir} , and \\text{Co} ) and \\text{YbR}{{\\text{h}}2}\\text{S}{{\\text{i}}2} with various incident photon energies are summarized. We demonstrate that their 4f electronic structures are essentially described within the framework of the periodic Anderson model, and that the band-structure calculation based on the local density approximation cannot explain their low-energy electronic structures. Meanwhile, electronic structures of 5f materials exhibit wide varieties ranging from itinerant to localized states. For itinerant \\text{U}~5f compounds such as \\text{UFeG}{{\\text{a}}5} , their electronic structures can be well-described by the band-structure calculation assuming that all \\text{U}~5f electrons are itinerant. In contrast, the band structures of localized \\text{U}~5f compounds such as \\text{UP}{{\\text{d}}3} and \\text{U}{{\\text{O}}2} are essentially explained by the localized model that treats \\text{U}~5f electrons as localized core states. In regards to heavy fermion \\text{U} -based compounds such as the hidden-order compound \\text{UR}{{\\text{u}}2}\\text{S}{{\\text{i}}2} , their electronic structures exhibit complex behaviors. Their overall band structures
Electronic band structure of TiN/MgO nanostructures
NASA Astrophysics Data System (ADS)
Kobayashi, Kazuaki; Takaki, Hirokazu; Shimono, Masato; Kobayashi, Nobuhiko; Hirose, Kenji
2017-04-01
Various nanostructured TiN(001)/MgO(001) superlattices based on a repeated slab model with a vacuum region have been investigated by the total energy pseudopotential method. They are rectangular and rectangular parallelepiped TiN(001) dot structures on MgO(001)-2×2 and 3×3 substrates. A rectangular TiN(001) structure on a MgO(001)-2×1 substrate has also been calculated. Their detailed electronic and internal lattice properties were investigated systematically. The internal atomic coordinates in a unit cell were fully relaxed. The rectangular TiN(001) structure on the MgO(001)-2×1 superlattice, which is not a dot owing to its periodicity, corresponds to metallicity. The electronic states of relaxed rectangular TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices are semiconducting. All relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices correspond to metallicity. The electronic properties depend on the shape of the TiN dot and the size of the MgO substrate.
Structural Properties of the Anterolateral Capsule and Iliotibial Band of the Knee.
Rahnemai-Azar, Ata A; Miller, R Matthew; Guenther, Daniel; Fu, Freddie H; Lesniak, Bryson P; Musahl, Volker; Debski, Richard E
2016-04-01
The role of the anterolateral capsule in knee stability has recently been advocated by studies reporting that a distinct ligament exists in this area. Defining the structural properties of the anterolateral capsule can provide insight into its contribution to joint stability. The structural properties of the iliotibial band also need to be determined, as it is a common graft used for extra-articular tenodesis. The purpose of this study was to determine the structural properties of the anterolateral capsule and iliotibial band. The hypothesis was that the iliotibial band will have comparable structural properties to the anterolateral capsule because it is generally an accepted graft for extra-articular reconstruction surgeries. Controlled laboratory study. Nine human cadaveric knees (average age, 57 ± 10 years) were dissected to assess the presence of a discrete capsular thickness originating from the lateral femoral epicondyle to the lateral tibial plateau between the Gerdy tubercle and the fibular head. For each knee, 2 constructs were prepared: (1) a bone-anterolateral capsule-bone specimen and (2) a strip of iliotibial band attached to the Gerdy tubercle. Structural properties, including ultimate load, ultimate elongation, and stiffness, were determined for the anterolateral capsule and the iliotibial band. After tensile testing, plain radiographs were obtained for evaluation of the Segond fracture. A paired t test was used to compare the structural properties of the anterolateral capsule with the iliotibial band. Significance was set at P < .05. Two of the 9 specimens were found to have a discrete thickening of the anterolateral capsule. The iliotibial band had almost 50% higher ultimate load and nearly 3 times higher stiffness (487.9 ± 156.9 N and 73.2 ± 24.1 N/mm, respectively) compared with the anterolateral capsule (319.7 ± 212.6 N and 26.0 ± 11.5 N/mm, respectively) (P < .05 for both). The anterolateral capsule had about double the ultimate elongation
Triaxial-band structures, chirality, and magnetic rotation in La133
Petrache, C. M.; Chen, Q. B.; Guo, S.; ...
2016-12-05
The structure of 133La has been investigated using the 116Cd(22Ne,4pn) reaction and the Gammasphere array. Three new bands of quadrupole transitions and one band of dipole transitions are identified and the previously reported level scheme is revised and extended to higher spins. The observed structures are discussed using the cranked Nilsson-Strutinsky formalism, covariant density functional theory, and the particle-rotor model. Triaxial configurations are assigned to all observed bands. For the high-spin bands it is found that rotations around different axes can occur, depending on the configuration. The orientation of the angular momenta of the core and of the activemore » particles is investigated, suggesting chiral rotation for two nearly degenerate dipole bands and magnetic rotation for one dipole band. As a result, it is shown that the h11/2 neutron holes present in the configuration of the nearly degenerate dipole bands have significant angular momentum components not only along the long axis but also along the short axis, contributing to the balance of the angular momentum components along the short and long axes and thus giving rise to a chiral geometry.« less
Triaxial-band structures, chirality, and magnetic rotation in 133La
NASA Astrophysics Data System (ADS)
Petrache, C. M.; Chen, Q. B.; Guo, S.; Ayangeakaa, A. D.; Garg, U.; Matta, J. T.; Nayak, B. K.; Patel, D.; Meng, J.; Carpenter, M. P.; Chiara, C. J.; Janssens, R. V. F.; Kondev, F. G.; Lauritsen, T.; Seweryniak, D.; Zhu, S.; Ghugre, S. S.; Palit, R.
2016-12-01
The structure of 133La has been investigated using the (22Ne 116Cd,4 p n ) reaction and the Gammasphere array. Three new bands of quadrupole transitions and one band of dipole transitions are identified and the previously reported level scheme is revised and extended to higher spins. The observed structures are discussed using the cranked Nilsson-Strutinsky formalism, covariant density functional theory, and the particle-rotor model. Triaxial configurations are assigned to all observed bands. For the high-spin bands it is found that rotations around different axes can occur, depending on the configuration. The orientation of the angular momenta of the core and of the active particles is investigated, suggesting chiral rotation for two nearly degenerate dipole bands and magnetic rotation for one dipole band. It is shown that the h11 /2 neutron holes present in the configuration of the nearly degenerate dipole bands have significant angular momentum components not only along the long axis but also along the short axis, contributing to the balance of the angular momentum components along the short and long axes and thus giving rise to a chiral geometry.
Electronic- and band-structure evolution in low-doped (Ga,Mn)As
Yastrubchak, O.; Gluba, L.; Żuk, J.; Sadowski, J.; Krzyżanowska, H.; Domagala, J. Z.; Andrearczyk, T.; Wosinski, T.
2013-08-07
Modulation photoreflectance spectroscopy and Raman spectroscopy have been applied to study the electronic- and band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn doping in the range of low Mn content, up to 1.2%. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap-transition energy with increasing Mn content in very low-doped (Ga,Mn)As layers with n-type conductivity are interpreted as a result of merging the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy with increasing Mn content in (Ga,Mn)As layers with higher Mn content and p-type conductivity indicates the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in the (Ga,Mn)As diluted ferromagnetic semiconductor.
NASA Astrophysics Data System (ADS)
Zhang, Xiaochuan; Wang, Yong; Yang, Jia; Qiao, Zhixia; Ren, Chunhua; Chen, Cheng
2016-10-01
The ferrite/pearlite banded structure causes the anisotropic behavior of steel. In this paper, digital image correlation (DIC) was used to analyze the micro deformation of this microstructure under uniaxial tension. The reliability of DIC for this application was verified by a zero-deformation experiment. The results show that the performance of DIC can satisfy the requirements of the tensile deformation measurement. Then, two uniaxial tensile tests in different directions (longitudinal direction and transverse direction) were carried out and DIC was used to measure the micro deformation of the ferrite/pearlite banded structure. The measured results show that the ferrite bands undergo the main deformation in the transverse tension, which results in the relatively weaker tensile properties in the transverse direction than in the longitudinal direction. This work is useful to guide the modification of the bands morphology and extend the application scope of DIC.
The LDA+U calculation of electronic band structure of GaAs
Bahuguna, B. P. Sharma, R. O.; Saini, L. K.
2016-05-06
We present the electronic band structure of bulk gallium arsenide (GaAs) using first principle approach. A series of calculations has been performed by applying norm-conserving pseudopotentials and ultrasoft non-norm-conserving pseudopotentials within the density functional theory. These calculations yield too small band gap as compare to experiment. Thus, we use semiemperical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U), which is quite effective in order to describe the band gap of GaAs.
Two-zone heterogeneous structure within shear bands of a bulk metallic glass
Shao, Yang; Yao, Kefu; Liu, Xue; Li, Mo
2013-10-21
Shear bands, the main plastic strain carrier in metallic glasses, are severely deformed regions often considered as disordered and featureless. Here we report the observations of a sandwich-like heterogeneous structure inside shear bands in Pd{sub 40.5}Ni{sub 40.5}P{sub 19} metallic glass sample after plastic deformation by high-resolution transmission electron microscopy. The experimental results suggest a two-step plastic deformation mechanism with corresponding microstructure evolution at atomic scale, which may intimately connected to the stability of the shear band propagation and the overall plastic deformability.
NASA Astrophysics Data System (ADS)
Chegel, Raad
2016-06-01
By using the third nearest neighbor modified tight binding (3NN-TB) method, the electronic structure and band gap of BNNRs under transverse electric fields are explored. The band gap of the BNNRs has a decreasing with increasing the intensity of the applied electric field, independent on the ribbon edge types. Furthermore, an analytic model for the dependence of the band gap in armchair and zigzag BNNRs on the electric field is proposed. The reduction of E g is similar for some N a armchair and N z zigzag BNNRs independent of their edges.
NASA Technical Reports Server (NTRS)
Asnin, V. M.; Krainsky, I. L.
1998-01-01
A fine structure was discovered in the low-energy peak of the secondary electron emission spectra of the diamond surface with negative electron affinity. We studied this structure for the (100) surface of the natural type-IIb diamond crystal. We have found that the low-energy peak consists of a total of four maxima. The relative energy positions of three of them could be related to the electron energy minima near the bottom of the conduction band. The fourth peak, having the lowest energy, was attributed to the breakup of the bulk exciton at the surface during the process of secondary electron emission.
Study on band gap structure of Fibonacci quantum superlattices by using the transfer matrix method
NASA Astrophysics Data System (ADS)
Ferrando, V.; Castro-Palacio, J. C.; Marí, B.; Monsoriu, J. A.
2014-02-01
The scattering properties of particles in a one-dimensional Fibonacci sequence based potential have been analyzed by means of the Transfer Matrix Method. The electronic band gaps are examined comparatively with those obtained using the corresponding periodic potentials. The reflection coefficient shows self-similar properties for the Fibonacci superlattices. Moreover, by using the generalized Bragg's condition, the band gaps positions are derived from the golden mean involved in the design of the superlattice structure.
NASA Astrophysics Data System (ADS)
Nazrul Rosli, Ahmad; Fatimah Wahab, Izzati; Zabidi, Noriza Ahmad; Abu Kassim, Hasan
2015-06-01
Sodium intercalation in graphite (GIC-Na) was investigated by the first principle calculation. The structure of GIC-Na was calculated using density functional theory (DFT) with the aid of CASTEP module of Material Studio. The exchange correlation functional has been treat by local density approximation (LDA) and generalized gradient approximation (GGA). It was shown that, unlike potassium GIC and lithium GIC, the band gap of GIC-Na was not induced and has same value of band gap with bulk graphite.
Chen, Hsieh; Alexander-Katz, Alfredo
2014-03-01
Using hydrodynamic simulations, we demonstrate that confined colloidal suspensions can greatly enhance the unfolding of collapsed single polymers in flow. When colloids come in direct contact with the polymers due to the flow, the collapsed chains become flattened or elongated on the surface of the colloids, increasing the probability of forming large chain protrusions that the flow can pull out to unfold the polymers. This phenomenon may be suppressed if the colloid size is commensurate with the confining channels, where the colloids form well-defined banding structures. Here, we analyze the colloid banding structures in detail and their relation to the chain unfolding. We find that for colloid volume fractions up to 30%, the confined colloids form simple cubic (sc), hexagonal (hex), or a mixture of sc + hex structures. By directly changing the heights of the confining channels, we show that the collapsed polymers unfold the most in the mixed sc + hex structures. The diffuse (not well-defined) bands in the mixed sc + hex structures provide the highest collision probability for the colloids and the polymers, thus enhancing unfolding the most. Without colloidal suspensions, we show that the confining channels alone do not have an observable effect on the unfolding of collapsed polymers. The well-defined colloid bands also suppress the unfolding of noncollapsed polymers. In fact, the average size for noncollapsed chains is even smaller in the well-defined bands than in a channel without any colloids. The appearance of well-defined bands in this case also indicates that lift forces experienced by the polymers in confinement are negligible compared to those exerted by the colloidal band structures. Our results may be important for understanding the dynamics of mixed colloid polymer solutions.
Dual-Band Perfect Absorption by Breaking the Symmetry of Metamaterial Structure
NASA Astrophysics Data System (ADS)
Hai, Le Dinh; Qui, Vu Dinh; Dinh, Tiep Hong; Hai, Pham; Giang, Trinh Thị; Cuong, Tran Manh; Tung, Bui Son; Lam, Vu Dinh
2017-02-01
Since the first proposal of Landy et al. (Phys Rev Lett 100:207402, 2008), the metamaterial perfect absorber (MPA) has rapidly become one of the most crucial research trends. Recently, dual-band, multi-band and broadband MPA have been highly desirable in electronic applications. In this paper, we demonstrate and evaluate a MPA structure which can generate dual-band absorption operating at the microwave frequency by breaking the symmetry of structure. There is an agreement between simulation and experimental results. The results can be explained by using the equivalent LC circuit and the electric field distribution of this structure. In addition, various structures with different symmetry configurations were studied to gain greater insight into the absorption.
Observation of Nonlinear Looped Band Structure of Bose-Einstein condensates in an optical lattice
NASA Astrophysics Data System (ADS)
Goldschmidt, Elizabeth; Koller, Silvio; Brown, Roger; Wyllie, Robert; Wilson, Ryan; Porto, Trey
2016-05-01
We study experimentally the stability of excited, interacting states of bosons in a double-well optical lattice in regimes where the nonlinear interactions are expected to induce ``swallow-tail'' looped band structure. By carefully preparing different initial coherent states and observing their subsequent decay, we observe distinct decay rates, which provide direct evidence for multi-valued band structure. The double well lattice both stabilizes the looped band structure and allows for dynamic preparation of different initial states, including states within the loop structure. We confirm our state preparation procedure with dynamic Gross-Pitaevskii calculations. The excited loop states are found to be more stable than dynamically unstable ground states, but decay faster than expected based on a mean-field stability calculation, indicating the importance of correlations beyond a mean-field description. Now at Georgia Tech Research Institute.
Paavilainen, Sami; Ropo, Matti; Nieminen, Jouko; Akola, Jaakko; Räsänen, Esa
2016-06-08
We uncover the electronic structure of molecular graphene produced by adsorbed CO molecules on a copper (111) surface by means of first-principles calculations. Our results show that the band structure is fundamentally different from that of conventional graphene, and the unique features of the electronic states arise from coexisting honeycomb and Kagome symmetries. Furthermore, the Dirac cone does not appear at the K-point but at the Γ-point in the reciprocal space and is accompanied by a third, almost flat band. Calculations of the surface structure with Kekulé distortion show a gap opening at the Dirac point in agreement with experiments. Simple tight-binding models are used to support the first-principles results and to explain the physical characteristics behind the electronic band structures.
Band structure and phonon properties of lithium fluoride at high pressure
Panchal, J. M.; Joshi, Mitesh; Gajjar, P. N.
2016-05-23
High pressure structural and electronic properties of Lithium Fluoride (LiF) have been studied by employing an ab-initio pseudopotential method and a linear response scheme within the density functional theory (DFT) in conjunction with quasi harmonic Debye model. The band structure and electronic density of states conforms that the LiF is stable and is having insulator behavior at ambient as well as at high pressure up to 1 Mbar. Conclusions based on Band structure, phonon dispersion and phonon density of states are outlined.
Lousse, V; Vigneron, J P
2001-02-01
The theory of photonic crystals is extended to include the optical Kerr effect taking place in weak third-order, nonlinear materials present in the unit cell. The influence on the dispersion relations of the illumination caused by a single Bloch mode transiting through the crystal structure is examined. Special attention is given to the modification of the photonic gap width and position. Assuming an instantaneous change of refractive index with illumination, the nonlinear band structure problem is solved as a sequence of ordinary, linear band structure calculations, carried out in a plane-wave field representation.
Polarization-dependent diffraction in all-dielectric, twisted-band structures
Kardaś, Tomasz M.; Jagodnicka, Anna; Wasylczyk, Piotr
2015-11-23
We propose a concept for light polarization management: polarization-dependent diffraction in all-dielectric microstructures. Numerical simulations of light propagation show that with an appropriately configured array of twisted bands, such structures may exhibit zero birefringence and at the same time diffract two circular polarizations with different efficiencies. Non-birefringent structures as thin as 3 μm have a significant difference in diffraction efficiency for left- and right-hand circular polarizations. We identify the structural parameters of such twisted-band matrices for optimum performance as circular polarizers.
Effect of pressure on the band structure of BC{sub 3}
Manju, M.S.; Harikrishnan, G.; Ajith, K.M.; Valsakumar, M.C.
2016-05-23
Density functional theory (DFT) calculations were carried out to study the effect of pressure on the band structure of two dimensional BC{sub 3} sheet. BC{sub 3} is a semiconductor at ambient conditions having a band gap of ~0.3 eV. Electronic structure calculations are carried out on BC{sub 3} at pressures of 5, 20, 50 and 100 GPa. The system shows a semiconductor – metal transition by the application of pressure without any structural transition.
NASA Astrophysics Data System (ADS)
Yoon, Gil Ho; Mo, Jun Su
2017-05-01
A new structure with a bidirectional negative stiffness (BNS) value utilizing buckling phenomena (often called bi-stable or snap-through) and a mechanical diode are presented with regard to mechanical metamaterial applications. The need for cost and mass efficient vibration isolation parts within the modern aerospace, automotive, and civil industries has been the subject of many interesting studies in the last several decades. With conventional materials in nature, many innovative approaches have been proposed, but with some limitations for vibration suppression. To cope with these difficulties, it was possible to employ a man-made material with a negative density or negative stiffness, not existing in nature. Recently metamaterials, i.e., man-made repeating structures with negative densities or/and stiffness values, have been proposed and their applications have been reported. This research presents a new man-made structure with a BNS, using the help of bi-stable mechanisms and a mechanical diode. With regard to the metamaterial application of this phenomenon, the dispersion nature of a 1D bi-stable mechanism has been studied and a new novel structure having BNS with a mechanical diode is presented. The effects of states of snap-through were highlighted and quantified experimentally.
The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation
NASA Astrophysics Data System (ADS)
Dolgos, Michelle R.; Paraskos, Alexandra M.; Stoltzfus, Matthew W.; Yarnell, Samantha C.; Woodward, Patrick M.
2009-07-01
The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba 3(VO 4) 2, Pb 3(VO 4) 2, YVO 4, BiVO 4, CeVO 4 and Ag 3VO 4 were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba 3(VO 4) 2 and YVO 4 have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb 3(VO 4) 2 and BiVO 4 the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6 s orbitals with nonbonding O 2 p states at the top of the valence band, and (b) overlap of empty 6 p orbitals with antibonding V 3 d-O 2 p states at the bottom of the conduction band. In Ag 3VO 4 mixing between filled Ag 4 d and O 2 p states destabilizes states at the top of the valence band leading to a large decrease in the band gap ( Eg=2.2 eV). In CeVO 4 excitations from partially filled 4 f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce 1-xBi xVO 4 (0≤ x≤0.5) and Ce 1-xY xVO 4 ( x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi 3+ or Y 3+ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4 f orbitals.
Low band gap frequencies and multiplexing properties in 1D and 2D mass spring structures
NASA Astrophysics Data System (ADS)
Aly, Arafa H.; Mehaney, Ahmed
2016-11-01
This study reports on the propagation of elastic waves in 1D and 2D mass spring structures. An analytical and computation model is presented for the 1D and 2D mass spring systems with different examples. An enhancement in the band gap values was obtained by modeling the structures to obtain low frequency band gaps at small dimensions. Additionally, the evolution of the band gap as a function of mass value is discussed. Special attention is devoted to the local resonance property in frequency ranges within the gaps in the band structure for the corresponding infinite periodic lattice in the 1D and 2D mass spring system. A linear defect formed of a row of specific masses produces an elastic waveguide that transmits at the narrow pass band frequency. The frequency of the waveguides can be selected by adjusting the mass and stiffness coefficients of the materials constituting the waveguide. Moreover, we pay more attention to analyze the wave multiplexer and DE-multiplexer in the 2D mass spring system. We show that two of these tunable waveguides with alternating materials can be employed to filter and separate specific frequencies from a broad band input signal. The presented simulation data is validated through comparison with the published research, and can be extended in the development of resonators and MEMS verification.
Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt
Eich, Steffen; Plötzing, Moritz; Rollinger, Markus; Emmerich, Sebastian; Adam, Roman; Chen, Cong; Kapteyn, Henry Cornelius; Murnane, Margaret M.; Plucinski, Lukasz; Steil, Daniel; Stadtmüller, Benjamin; Cinchetti, Mirko; Aeschlimann, Martin; Schneider, Claus M.; Mathias, Stefan
2017-01-01
The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization. PMID:28378016
A novel banded structure ceramic phosphor for high-power white LEDs.
Zhang, Ji-Yun; Luo, Zhao-Hua; Jiang, Hao-Chuan; Jiang, Jun; Chen, Chun-Hua; Zhang, Jing-Xian; Gui, Zhen-Zhen
2017-06-20
A novel banded structure ceramic phosphor has been fabricated in this research. This structure provides a convenient and effective method for regulating the full spectrum. It has the advantage of being able to adjust the intensity of all three primary colors independently, regardless of the mutual absorption among different active ions.
International X-Band Linear Collider Accelerator Structure R&D
Wang, J.W.; /SLAC
2009-03-04
For more than fifteen years before the International Technology Recommendation Panel (ITRP) decision in August, 2004, there were intensive R&D activities and broad international collaboration among the groups at SLAC, KEK, FNAL, LLNL and other labs for the room temperature X-Band accelerator structures. The goal was to provide an optimized design of the main linac structure for the NLC (Next Linear Collider) or GLC (Global Linear Collider). There have been two major challenges in developing X-band accelerator structures for the linear colliders. The first is to demonstrate stable, long-term operation at the high gradient (65 MV/m) that is required to optimize the machine cost. The second is to strongly suppress the beam induced long-range wakefields, which is required to achieve high luminosity. More than thirty X-band accelerator structures with various RF parameters, cavity shapes and coupler types have been fabricated and tested since 1989. A summary of the main achievements and experiences are presented in this talk including the structure design, manufacturing techniques, high power performance, and other structure related issues. Also, the new progress in collaborating with the CLIC, high gradient structures and X-Band structure applications for RF deflectors and others are briefly introduced.
NASA Astrophysics Data System (ADS)
Usui, Hidetomo; Kuroki, Kazuhiko
2017-04-01
We study the relationship between the shape of the electronic band structure and the thermoelectric properties. In order to study the band shape dependence of the thermoelectric properties generally, we first adopt models with band structures having the dispersion E ( k ) ˜ | k | n with n = 2, 4, and 6. We consider one-, two-, and three-dimensional systems and calculate the thermoelectric properties using the Boltzmann equation approach within the constant quasi-particle lifetime approximation. n = 2 corresponds to the usual parabolic band structure, while the band shape for n = 4 and 6 has a flat portion at the band edge, so that the density of states diverges at the bottom of the band. We call this kind of band structure the "pudding mold type band". n ≥ 4 belong to the pudding mold type band, but since the density of states diverges even for n = 2 in the one dimensional system, this is also categorized as the pudding mold type. Due to the large density of states and the rapid change of the group velocity around the band edge, the spectral conductivity of the pudding mold type band structures becomes larger than that of the usual parabolic band structures. It is found that the pudding mold type band has a coexistence of a large Seebeck coefficient and a large electric conductivity and a small Lorenz number in the Wiedemann-Franz law due to the specific band shape. We also find that the low dimensionality of the band structure can contribute to large electronic conductivity and hence a small Lorenz number. We conclude that the pudding mold type band, especially in low dimensional systems, can enhance not only the power factor but also the dimensionless figure of merit due to stronger reduction of the Lorenz number.
NASA Astrophysics Data System (ADS)
Rahimi, H.; Rezaei, M.
2013-08-01
Transfer matrix method is used to investigate the electromagnetic transmission spectra of a one-dimensional photonic fractal multilayer structure composed of single-negative metamaterial when common positive dielectric defect layers of are introduced. It is found that the frequency of the resonance defect modes can be tuned independently by varying the defect layer thicknesses. It is also found that by increasing the value of refractive index of defect layers and the number of periods, the full width half maximum of defect modes will be narrowed and shifted to lower frequencies. Also, our investigations show that for both TE and TM polarizations moving away from normal incidence to oblique incidence shows that the defect modes shift to upper frequencies. In other words, the defect modes inside the band gap depend on the incident angle and polarization. More interesting, for angles of incidence greater than 55° the defect modes for TE polarization (unlike to TM polarization) are eliminated. Moreover, the electromagnetic fields in the defect layers are strongly localized, and they can be excite independently. We believe that the proposed fractal structures can be useful in designing tunable independently high-Q filters with specific channels by adjusting their structural parameters.
UWB Band-notched Adjustable Antenna Using Concentric Split-ring Slots Structure
NASA Astrophysics Data System (ADS)
Yin, Y.; Hong, J. S.
2014-09-01
In this paper, a kind of concentric split-ring slots structure is utilized to design a novel triple-band-notched UWB antenna. Firstly, a concentric split-ring slots structure that has a higher VSWR than that of a single slot at notch frequency is presented. What's more, the structure is very simple and feasible to obtain notched-band at different frequency by adjustment of the length of slot. Secondly, a triple-band-notched antenna, whose notched bands are at 3.52-3.81 GHz for WiMAX and 5.03-5.42 GHz and 5.73-56.17 GHz for WLAN, is designed by using this structure. At last, a compact size of 24 × 30 mm2 of the proposed antenna has been fabricated and measured and it is shown that the proposed antenna has a broadband matched impedance (3.05-14 GHz, VSWR < 2), relatively stable gain and good omnidirectional radiation patterns at low bands.
Electron-Phonon Renormalization of Electronic Band Structures of C Allotropes and BN Polymorphs
NASA Astrophysics Data System (ADS)
Tutchton, Roxanne M.; Marchbanks, Christopher; Wu, Zhigang
The effect of lattice vibration on electronic band structures has been mostly neglected in first-principles calculations because the electron-phonon (e-ph) renormalization of quasi-particle energies is often small (< 100 meV). However, in certain materials, such as diamond, the electron-phonon coupling reduces the band gap by nearly 0.5 eV, which is comparable to the many-body corrections of the electronic band structures calculated using the density functional theory (DFT). In this work, we compared two implementations of the Allen-Heine-Cardona theory in the EPW code and the ABINIT package respectively. Our computations of Si and diamond demonstrate that the ABINIT implementation converges much faster. Using this method, the e-ph renormalizations of electronic structures of three C allotropes (diamond, graphite, graphene) and four BN polymorphs (zincblend, wurtzite, mono-layer, and layered-hexagonal) were calculated. Our results suggest that (1) all of the zero-point renormalizations of band gaps in these materials, except for graphene, are larger than 100 meV, and (2) there are large variations in e-ph renormalization of band gaps due to differences in crystal structure. This work was supported by a U.S. DOE Early Career Award (Grant No. DE-SC0006433). Computations were carried out at the Golden Energy Computing Organization at CSM and the National Energy Research Scientific Computing Center (NERSC).
Observation of 'Band' Structures in Spacecraft Observations of Inner Magnetosphere Plasma Electrons
NASA Astrophysics Data System (ADS)
Mohan Narasimhan, Kirthika; Fazakerley, Andrew; Milhaljcic, Branislav; Grimald, Sandrine; Dandouras, Iannis; Owen, Chris
2013-04-01
In previous studies, several authors have reported inner magnetosphere observations of proton distributions confined to narrow energy bands in the range of 1-25 keV. These structures have been known as "nose structures", with reference to their appearance in energy-time spectrograms and are known as "bands" if they are observed for extended periods of time. These proton structures have been studied quite extensively with multiple mechanisms proposed for their formation, not all of which apply for electrons. We examine Double-Star TC1 PEACE electron data recorded in the inner magnetosphere (L<15) near the equatorial plane to see if these features are also observed in the electron energy spectra. These "bands" also appear in electron spectrograms, spanning an energy range of 0.2-30 keV, and are shown to occur predominantly towards the dayside and dusk sectors. We also see multiple bands in some instances. We investigate the properties of these multi-banded structures and carry out a statistical survey analysing them as a function of geomagnetic activity, looking at both the Kp and Auroral Indices, in an attempt to explain their presence.
Yamaguchi, Hisato; Ogawa, Shuichi; Watanabe, Daiki; Hozumi, Hideaki; Gao, Yongqian; Eda, Goki; Mattevi, Cecilia; Fujita, Takeshi; Yoshigoe, Akitaka; Ishizuka, Shinji; Adamska, Lyudmyla; Yamada, Takatoshi; Dattelbaum, Andrew M.; Gupta, Gautam; Doorn, Stephen K.; Velizhanin, Kirill A.; Teraoka, Yuden; Chen, Mingwei; Htoon, Han; Chhowalla, Manish; Mohite, Aditya D.; Takakuwa, Yuji
2016-09-01
We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 °C. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 °C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart.
Yamaguchi, Hisato; Ogawa, Shuichi; Watanabe, Daiki; ...
2016-09-01
We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 °C. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a setmore » of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 °C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart.« less
Quasienergy band structure of the harmonically driven δ-function chain
NASA Astrophysics Data System (ADS)
Martinez, D. F.; Reichl, L. E.; Luna-Acosta, Germán A.
2002-11-01
We study the quasienergy band structure of a potential consisting of a periodic array of harmonically oscillating δ functions. The perturbative and non-perturbative regimes are investigated using Floquet-Bloch states and the Floquet translation matrix whose eigenvalues and eigenvectors are given in terms of continued fractions. We study the structure of these eigenstates and relate it to the structure of the quasibound state of a single δ-function potential. We also study the dynamics of the bands as a function of the strength of the oscillating potential and find that the collapse of one of the quasienergy bands is related to the quenching of the transmission through a single δ-function potential.
Band structures of nonmagnetic transition-metal oxides: PdO and PtO
Hass, K.C. ); Carlsson, A.E. )
1992-08-15
The electronic band structures of PdO and PtO are calculated using the augmented-spherical-wave method and the local-density approximation. Our results are consistent with the widely held view of these materials as conventional band insulators with the crystal-field splitting of metal {ital d} states primarily responsible for gap formation. A significant role for correlation effects as well cannot be ruled out. The predicted valence-band structure for PdO agrees well with published photoemission data. The electronic structure of PtO is qualitatively similar. In both cases the calculated gap is direct and occurs at the {ital M} point of the Brillouin zone. The magnitude of the gap is found to be larger in PtO, which we attribute to the more relativistic nature of Pt compared to Pd.
Yamaguchi, Hisato; Ogawa, Shuichi; Watanabe, Daiki; Hozumi, Hideaki; Gao, Yongqian; Eda, Goki; Mattevi, Cecilia; Fujita, Takeshi; Yoshigoe, Akitaka; Ishizuka, Shinji; Adamska, Lyudmyla; Yamada, Takatoshi; Dattelbaum, Andrew M.; Gupta, Gautam; Doorn, Stephen K.; Velizhanin, Kirill A.; Teraoka, Yuden; Chen, Mingwei; Htoon, Han; Chhowalla, Manish; Mohite, Aditya D.; Takakuwa, Yuji
2016-09-01
We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 °C. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 °C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart.
Pan, Xiaoyang; Yi, Zhiguo
2015-12-16
A facile, one-step hydrothermal method has been developed to fabricate tin oxide-reduced graphene oxide (Sn-RGO) nanocomposites with tunable composition, morphology, and energy band structure by utilizing graphene oxide (GO) as a multifunctional two-dimensional scaffold. By adjusting the GO concentration during synthesis, a variety of tin oxide nanomaterials with diverse composition and morphology are obtained. Simultaneously, the varying of GO concentration can also narrow the bandgap and tune the band edge positions of the Sn-RGO nanocomposites. As a result, the Sn-RGO nanocomposites with controllable composition, morphology, and energy band structure are obtained, which exhibit efficient photoactivities toward methyl orange (MO) degradation under visible-light irradiation. It is expected that our work would point to the new possibility of using GO for directing synthesis of semiconductor nanomaterials with tailored structure and physicochemical properties.
NASA Astrophysics Data System (ADS)
Cui, Ning; Liang, Renrong; Wang, Jing; Xu, Jun
2012-06-01
Choosing novel materials and structures is important for enhancing the on-state current in tunnel field-effect transistors (TFETs). In this paper, we reveal that the on-state performance of TFETs is mainly determined by the energy band profile of the channel. According to this interpretation, we present a new concept of energy band profile modulation (BPM) achieved with gate structure engineering. It is believed that this approach can be used to suppress the ambipolar effect. Based on this method, a Si TFET device with a symmetrical tri-material-gate (TMG) structure is proposed. Two-dimensional numerical simulations demonstrated that the special band profile in this device can boost on-state performance, and it also suppresses the off-state current induced by the ambipolar effect. These unique advantages are maintained over a wide range of gate lengths and supply voltages. The BPM concept can serve as a guideline for improving the performance of nanoscale TFET devices.
NASA Astrophysics Data System (ADS)
Winther, Kirsten T.; Thygesen, Kristian S.
2017-06-01
The idea of combining different two-dimensional (2D) crystals in van der Waals heterostructures (vdWHs) has led to a new paradigm for band structure engineering with atomic precision. Due to the weak interlayer couplings, the band structures of the individual 2D crystals are largely preserved upon formation of the heterostructure. However, regardless of the details of the interlayer hybridisation, the size of the 2D crystal band gaps are always reduced due to the enhanced dielectric screening provided by the surrounding layers. The effect can be significant (on the order of electron volts) but its precise magnitude is non-trivial to predict because of the non-local nature of the screening in quasi-2D crystals. Moreover, the effect is not captured by effective single-particle methods such as density functional theory. Here we present an efficient and general method for calculating the band gap renormalization of a 2D material embedded in an arbitrary vdWH. The method evaluates the change in the GW self-energy of the 2D material from the change in the screened Coulomb interaction. The latter is obtained using the quantum-electrostatic heterostructure (QEH) model. We benchmark the GΔW method against full first-principles GW calculations and use it to unravel the importance of screening-induced band structure renormalisation in various vdWHs. A main result is the observation that the size of the band gap reduction of a given 2D material when inserted into a heterostructure scales inversely with the polarisability of the 2D material. Our work demonstrates that dielectric engineering via van der Waals heterostructuring represents a promising strategy for tailoring the band structure of 2D materials.
Electronic band structure and optical properties of the cubic, Sc, Y and La hydride systems
Peterman, D.J.
1980-01-01
Electronic band structure calculations are used to interpret the optical spectra of the cubic Sc, Y and La hydride systems. Self-consistent band calculations of ScH/sub 2/ and YH/sub 2/ were carried out. The respective joint densities of states are computed and compared to the dielectric functions determined from the optical measurements. Additional calculations were performed in which the Fermi level or band gap energies are rigidly shifted by a small energy increment. These calculations are then used to simulate the derivative structure in thermomodulation spectra and relate the origin of experimental interband features to the calculated energy bands. While good systematic agreement is obtained for several spectral features, the origin of low-energy interband transitions in YH/sub 2/ cannot be explained by these calculated bands. A lattice-size-dependent premature occupation of octahedral sites by hydrogen atoms in the fcc metal lattice is suggested to account for this discrepancy. Various non-self-consistent calculations are used to examine the effect of such a premature occupation. Measurements of the optical absorptivity of LaH/sub x/ with 1.6 < x < 2.9 are presented which, as expected, indicate a more premature occupation of the octahedral sites in the larger LaH/sub 2/ lattice. These experimental results also suggest that, in contrast to recent calculations, LaH/sub 3/ is a small-band-gap semiconductor.
Band structures in two-dimensional phononic crystals with periodic Jerusalem cross slot
NASA Astrophysics Data System (ADS)
Li, Yinggang; Chen, Tianning; Wang, Xiaopeng; Yu, Kunpeng; Song, Ruifang
2015-01-01
In this paper, a novel two-dimensional phononic crystal composed of periodic Jerusalem cross slot in air matrix with a square lattice is presented. The dispersion relations and the transmission coefficient spectra are calculated by using the finite element method based on the Bloch theorem. The formation mechanisms of the band gaps are analyzed based on the acoustic mode analysis. Numerical results show that the proposed phononic crystal structure can yield large band gaps in the low-frequency range. The formation mechanism of opening the acoustic band gaps is mainly attributed to the resonance modes of the cavities inside the Jerusalem cross slot structure. Furthermore, the effects of the geometrical parameters on the band gaps are further explored numerically. Results show that the band gaps can be modulated in an extremely large frequency range by the geometry parameters such as the slot length and width. These properties of acoustic waves in the proposed phononic crystals can potentially be applied to optimize band gaps and generate low-frequency filters and waveguides.
Studies of Breakdown in High Gradient X-Band Accelerator Structures Using Acoustic Emission
Frisch, Josef C
2002-08-23
X-band accelerator structures meeting the Next Linear Collider (NLC) design requirements have been found to suffer damage due to RF breakdown when processed to high gradients. Improved understanding of these breakdown events is desirable for the development of structure designs, fabrication procedures, and processing techniques that minimize structure damage [1]. Acoustic emission sensors attached to an accelerator structure can detect both nominal and breakdown RF pulses [2]. Using an array of acoustic sensors, we have been able to pinpoint both the cell and azimuth location of individual breakdown events. This allows studies of breakdown time and position sequences so that underlying causes can be determined. The technique provided a significant advance in studies of breakdown in the structure input coupler. In this paper we present acoustic emission sensor data and analysis from the breakdown studies in several x-band accelerator structures.
Cherenkov oscillator operating at the second band gap of leakage waveguide structures
NASA Astrophysics Data System (ADS)
Jang, Kyu-Ha; Park, Seong Hee; Lee, Kitae; Jeong, Young Uk
2016-10-01
An electromagnetic wave source operating around second band gaps of metallic grating structures is presented. The considered metallic grating structures are not perfect periodic but inhomogeneously structured within a period to have a second band gap where the wavelength is equal to the period of the structures. The radiation mechanism by an electron beam in the structures is different from the well-known Smith-Purcell radiation occurring in perfect periodic grating structures. That is, the radiating wave has a single frequency and the radiation is unidirectional. When the energy of the electron beam is synchronized at the standing wave point in the dispersion curves, strong interaction happens and coherent radiation perpendicular to the grating surface is generated with relatively lower starting oscillation current.
NASA Astrophysics Data System (ADS)
Taha, M. M.
2015-11-01
The anomalous negative-parity bands of odd-mass nuclei W/Os/Pt for N = 103 isotones are studied within the framework of particle rotor model (PRM). The phenomenon of Δ I = 1 staggering or signature splitting in energies occurs as one plots the gamma transitional energy over spin (EGOS) versus spin for the 1/2-[521] band originating from N = 5 single particle orbital. The rotational band with K = 1/2 separates into two signature partners. The levels with I = 1/2, 5/2, 9/2,… are displaced relatively to the levels with I = 3/2,7/2,11/2,…. The deviations of the level energies from the rigid rotor values is described by Coriolis coupling.
NASA Astrophysics Data System (ADS)
Nguyen-Minh, T.; Sidor, J. J.; Petrov, R. H.; Kestens, L. A. I.
2015-04-01
Due to progressive deformation, the dislocation densities in crystals are accumulated and the resistance of grains to further deformation increases. Homogeneous deformation becomes energetically less favorable, which may result for some orientations in strain localization. In-grain shear banding, a typical kind of localized deformation in metals with BCC crystal structure, has been accounted for by the geometric softening of crystals. In this study, the occurrence of shear bands in rotated Goss ({110}<110>) orientations of Fe-Si steel is predicted by crystal plasticity simulations and validated by EBSD measurements. It was observed and confirmed by crystal plasticity modeling that such shear bands exhibit stable cube orientations The orientation evolution of crystals in shear bands and its impact on annealing texture of materials are also described.
TUNABLE Band Structures of 2d Multi-Atom Archimedean-Like Phononic Crystals
NASA Astrophysics Data System (ADS)
Xu, Y. L.; Chen, C. Q.; Tian, X. G.
2012-06-01
Two dimensional multi-atom Archimedean-like phononic crystals (MAPCs) can be obtained by adding "atoms" at suitable positions in primitive cells of traditional simple lattices. Band structures of solid-solid and solid-air MAPCs are computed by the finite element method in conjunction with the Bloch theory. For the solid-solid system, our results show that the MAPCs can be suitably designed to split and shift band gaps of the corresponding traditional simple phononic crystal (i.e., with only one scatterer inside a primitive cell). For the solid-air system, the MAPCs have more and wider band gaps than the corresponding traditional simple phononic crystal. Numerical calculations for both solid-solid and solid-air MAPCs show that the band gap of traditional simple phononic crystal can be tuned by appropriately adding "atoms" into its primitive cell.
Transient band structures in the ultrafast demagnetization of ferromagnetic gadolinium and terbium
NASA Astrophysics Data System (ADS)
Teichmann, Martin; Frietsch, Björn; Döbrich, Kristian; Carley, Robert; Weinelt, Martin
2015-01-01
We compare the laser-driven demagnetization dynamics of the rare earths gadolinium and terbium by mapping their transient valance band structures with time- and angle-resolved photoelectron spectroscopy. In both metals, the minority and majority spin valence bands evolve independently with different time constants after optical excitation. The ultrafast shift of the partially unoccupied minority spin bulk band to higher binding energy and of the majority spin surface state to lower binding energy suggests spin transport between surface and bulk. The slower response of the fully occupied majority spin band follows the lattice temperature and is attributed to Elliott-Yafet type spin-flip scattering. Terbium shows a stronger and faster decay of the exchange splitting, pointing to ultrafast magnon emission via 4 f spin-to-lattice coupling.
NASA Astrophysics Data System (ADS)
Tamura, Ryo; Tsukada, Masaru
1999-03-01
The band structures of the periodic nanotube junctions are investigated by the effective mass theory and the tight binding model. The periodic junctions are constructed by introducing pairs of a pentagonal defect and a heptagonal defect periodically in the carbon nanotube. We treat the periodic junctions composed by two kinds of metallic nanotubes with almost same radii, the ratio of which is between 0.7 and 1. The discussed energy region is near the undoped Fermi level. The energy bands are expressed with closed analytical forms by the effective mass theory. They are similar to the dispersion relation of Kronig-Penny model and coincide well with the numerical results by the tight binding model. The width of the gap and the band are in inverse proportion to the length of the unit cell. The degeneracy and repulsion between the two bands are determined only from symmetries.
Full band structure calculation of two-photon indirect absorption in bulk silicon
Cheng, J. L.; Rioux, J.; Sipe, J. E.
2011-03-28
Degenerate two-photon indirect absorption in silicon is an important limiting effect on the use of silicon structures for all-optical information processing at telecommunication wavelengths. We perform a full band structure calculation to investigate two-photon indirect absorption in bulk silicon, using a pseudopotential description of the energy bands and an adiabatic bond charge model to describe phonon dispersion and polarization. Our results agree well with some recent experimental results. The transverse acoustic/optical phonon-assisted processes dominate.
Enlarged band gap and electron switch in graphene-based step-barrier structure
Lu, Wei-Tao Ye, Cheng-Zhi; Li, Wen
2013-11-04
We study the transmission through a step-barrier in gapped graphene and propose a method to enlarge the band gap. The step-barrier structure consists of two or more barriers with different strengths. It is found that the band gap could be effectively enlarged and controlled by adjusting the barrier strengths in the light of the mass term. Klein tunneling at oblique incidence is suppressed due to the asymmetry of step-barrier, contrary to the cases in single-barrier and superlattices. Furthermore, a tunable conductance channel could be opened up in the conductance gap, suggesting an application of the structure as an electron switch.
Effective parameters in beam acoustic metamaterials based on energy band structures
NASA Astrophysics Data System (ADS)
Jing, Li; Wu, Jiu Hui; Guan, Dong; Hou, Mingming; Kuan, Lu; Shen, Li
2016-07-01
We present a method to calculate the effective material parameters of beam acoustic metamaterials. The effective material parameters of a periodic beam are calculated as an example. The dispersion relations and energy band structures of this beam are calculated. Subsequently, the effective material parameters of the beam are investigated by using the energy band structures. Then, the modal analysis and transmission properties of the beams with finite cells are simulated in order to confirm the correctness of effective approximation. The results show that the periodic beam can be equivalent to the homogeneous beam with dynamic effective material parameters in passband.
On the origin of a band gap in compounds of diamond-like structures.
Köhler, Jürgen; Deng, Shuiquan; Lee, Changhoon; Whangbo, Myung-Hwan
2007-03-19
Electronic structure calculations were performed to examine the origin of a band gap present in most 18-electron half-Heusler compounds and its absence in NaTl. In these compounds of diamond-like structures, the presence or absence of a band gap is controlled by the sigma antibonding between the valence s orbitals, and the bonding characteristics of the late-main-group elements depend on the extent of their ns/np hybridization. Implications of these observations on the formal oxidation state and the covalent bonding of the transition-metal atoms in 18-electron half-Heusler and related compounds were discussed.
Structural studies and band gap tuning of Cr doped ZnO nanoparticles
Srinet, Gunjan Kumar, Ravindra Sajal, Vivek
2014-04-24
Structural and optical properties of Cr doped ZnO nanoparticles prepared by the thermal decomposition method are presented. X-ray diffraction studies confirmed the substitution of Cr on Zn sites without changing the wurtzite structure of ZnO. Modified form of W-H equations was used to calculate various physical parameters and their variation with Cr doping is discussed. Significant red shift was observed in band gap, i.e., a band gap tuning is achieved by Cr doping which could eventually be useful for optoelectronic applications.
NASA Astrophysics Data System (ADS)
Kuroki, Kazuhiko; Aoki, Hideo
1992-12-01
The occurrence of superconductivity due to nonretarded attraction is demonstrated in a class of repulsively interacting electron systems that consists of a carrier band interacting with an insulating band. The superconductivity, which can be mapped to that in the attractive Hubbard model, has been confirmed from both the canonical transformation of the Hamiltonian and quantum Monte Carlo results. We indicate essential differences of the present models from existing models, including the d-p sub sigma model, and also discuss the relevance of our models to high TC materials.
Understanding the electronic band structure of Pt-alloys for surface reactivity
NASA Astrophysics Data System (ADS)
Jung, Jongkeun; Kim, Beomyoung; Hong, Ji Sook; Jin, Tae Won; Shim, Ji Hoon; Nemsak, Slavomir; Denlinger, Jonathan D.; Masashi, Arita; Kenya, Shimada; Kim, Changyoung; Mun, Bongjin Simon
In polymer exchange membrane fuel cell (PEMFC), the oxygen reduction reaction (ORR) at cathode side has been continuously investigated due to its critical importance in performance of fuel cell. So far, even with best industrial catalyst made with Pt, the performance of ORR is too far below from the commercial purpose. In 2007, Stamenkovic et al. showed that Pt alloys with 3- dtransition metal exhibited significantly improved ORR performance and pointed out the altered electronic structure of surface as the major contributing factor for enhanced ORR. Since 1990, with the advance of DFT calculation, the trend of surface chemical reactivity is explained with the analysis of d-band structures, known as d-band model. While d-band provides valid insight on surface chemical reactivity based on the valence band DOS, the relation between surface work function and DOS has not been well understood. The element-specific local electronic band structure of Pt alloys are identified by ARPES measurement, and the correlation between surface work function and local charge density is investigated.
Band structure engineering and thermoelectric properties of charge-compensated filled skutterudites
Shi, Xiaoya; Yang, Jiong; Wu, Lijun; ...
2015-10-12
Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co4Sb12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content,more » we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively.« less
Zhang, Ning; Chen, Chen; Mei, Zongwei; Liu, Xiaohe; Qu, Xiaolei; Li, Yunxiang; Li, Siqi; Qi, Weihong; Zhang, Yuanjian; Ye, Jinhua; Roy, Vellaisamy A L; Ma, Renzhi
2016-04-27
Exploring surface-exposed highly active crystal facets for photocatalytic oxidations is promising in utilizing monoclinic WO3 semiconductor. However, the previously reported highly active facets for monoclinic WO3 were mainly toward enhancing photocatalytic reductions. Here we report that the WO3 with {100} facet orientation and tuned surface electronic band structure can effectively enhance photocatalytic oxidation properties. The {100} faceted WO3 single crystals are synthesized via a facile hydrothermal method. The UV-visible diffuse reflectance, X-ray photoelectron spectroscopy valence band spectra, and photoelectrochemical measurements suggest that the {100} faceted WO3 has a much higher energy level of valence band maximum compared with the normal WO3 crystals without preferred orientation of the crystal face. The density functional theory calculations reveal that the shift of O 2p and W 5d states in {100} face induce a unique band structure. In comparison with the normal WO3, the {100} faceted WO3 exhibits an O2 evolution rate about 5.1 times in water splitting, and also shows an acetone evolution rate of 4.2 times as well as CO2 evolution rate of 3.8 times in gaseous degradation of 2-propanol. This study demonstrates an efficient crystal face engineering route to tune the surface electronic band structure for enhanced photocatalytic oxidations.
Band structure engineering and thermoelectric properties of charge-compensated filled skutterudites
Shi, Xiaoya; Yang, Jiong; Wu, Lijun; Salvador, James R.; Zhang, Cheng; Villaire, William L.; Haddad, Daad; Yang, Jihui; Zhu, Yimei; Li, Qiang
2015-10-12
Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co_{4}Sb_{12} substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively.
Band Structure Engineering and Thermoelectric Properties of Charge-Compensated Filled Skutterudites
NASA Astrophysics Data System (ADS)
Shi, Xiaoya; Yang, Jiong; Wu, Lijun; Salvador, James R.; Zhang, Cheng; Villaire, William L.; Haddad, Daad; Yang, Jihui; Zhu, Yimei; Li, Qiang
2015-10-01
Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co4Sb12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively.
Band Structure Engineering and Thermoelectric Properties of Charge-Compensated Filled Skutterudites
Shi, Xiaoya; Yang, Jiong; Wu, Lijun; Salvador, James R.; Zhang, Cheng; Villaire, William L.; Haddad, Daad; Yang, Jihui; Zhu, Yimei; Li, Qiang
2015-01-01
Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co4Sb12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively. PMID:26456013
The Calculation of the Band Structure in 3D Phononic Crystal with Hexagonal Lattice
NASA Astrophysics Data System (ADS)
Aryadoust, Mahrokh; Salehi, H.
2015-12-01
In this article, the propagation of acoustic waves in the phononic crystals (PCs) of three dimensions with the hexagonal (HEX) lattice is studied theoretically. The PCs are constituted of nickel (Ni) spheres embedded in epoxy. The calculations of the band structure and the density of states are performed using the plane wave expansion (PWE) method in the irreducible part of the Brillouin zone (BZ). In this study, we analyse the dependence of the band structures inside (the complete band gap width) on c/a and filling fraction in the irreducible part of the first BZ. Also, we have analysed the band structure of the ALHA and MLHKM planes. The results show that the maximum width of absolute elastic band gap (AEBG) (0.045) in the irreducible part of the BZ of HEX lattice is formed for c/a=6 and filling fraction equal to 0.01. In addition, the maximum of the first and second AEBG widths are 0.0884 and 0.0474, respectively, in the MLHKM plane, and the maximum of the first and second AEBG widths are 0.0851 and 0.0431, respectively, in the ALHA plane.
Electron band structure of the high pressure cubic phase of AlH3
NASA Astrophysics Data System (ADS)
Shi, Hongliang; Zarifi, Niliffar; Yim, Wai-Leung; Tse, J. S.
2012-07-01
The electronic band structure of the cubic Pm3n phase of AlH3 stable above 100 GPa is examined with semi-local, Tran-Blaha modified Becke-Johnson local density approximation (TB-mBJLDA), screened hybrid density functionals and GW methods. The shift of the conduction band to higher energy with increasing pressure is predicted by all methods. However, there are significant differences in detail band structure. In the pressure range from 90 to160 GPa, semi-local, hybrid functional and TB-mBJLDA calculations predicted that AlH3 is a poor metal. In comparison, GW calculations show a gap opening at 160 GPa and AlH3 becomes a small gap semi-conductor. From the trends of the calculated band shifts, it can be concluded that the favourable conditions leading to the nesting of Fermi surfaces predicted by semi-local calculation have disappeared if the exchange term is included. The results highlight the importance of the correction to the exchange energy on the band structure of hydrogen dominant dense metal hydrides at high pressure hydrides and may help to rationalize the absence of superconductivity in AlH3 from experimental measurements.
Band structure of the solar system: an objective test of the grouping of planets and satellites.
Arrhenius, G; Arrhenius, S
1988-01-01
Alfvén in his early work on the origin of the solar system (1942-1946) noted a pronounced band structure in the gravitational potential distribution of secondary bodies, and suggested this feature to be directly related to the formation process. When the critical velocity phenomenon was later discovered, a close agreement was found between the planet-satellite bands on one hand, and the critical velocity limits of the major compound elements in the interstellar medium on the other, suggesting a specific emplacement mechanism for the dusty plasma which presumably constituted the solar nebula. Since the originally perceived band structure was outlined in a qualitative fashion, an attempt is made here to analyze the distribution by a statistical technique, testing the significance of clustering of the observational data in the bands. The results show that, with proper scaling of the parameters, such a band structure indeed appears, with features closely similar to those originally conceived. Some deviations are indicated by the cluster analysis, however; their significance is discussed in terms of process involved in the formation of the solar system.
NASA Astrophysics Data System (ADS)
Yi, Seho; Choi, Jin-Ho; Lee, Kimoon; Kim, Sung Wng; Park, Chul Hong; Cho, Jun-Hyung
2016-12-01
The electronic band structure of crystals is generally influenced by the periodic arrangement of their constituent atoms. Specifically, the emerging two-dimensional (2D) layered structures have shown different band structures with respect to their stacking configurations. Here, based on first-principles density-functional theory calculations, we demonstrate that the band structure of the recently synthesized 2D Ca2N electride changes little for the stacking sequence as well as the lateral interlayer shift. This intriguing invariance of band structure with respect to geometrical variations can be attributed to a complete screening of [Ca2N ] + cationic layers by anionic excess electrons delocalized between the cationic layers. The resulting weak interactions between 2D dressed cationic layers give rise to not only a shallow potential barrier for bilayer sliding but also an electron-doping-facilitated shear exfoliation. Our findings open a route for exploration of the peculiar geometry-insensitive electronic properties in 2D electride materials, which will be useful for future thermally stable electronic applications.
Complete multipactor suppression in an X-band dielectric-loaded accelerating structure
Jing, C.; Gold, S. H.; Fischer, Richard; Gai, W.
2016-05-09
Multipactor is a major issue limiting the gradient of rf-driven Dielectric-Loaded Accelerating (DLA) structures. Theoretical models have predicted that an axial magnetic field applied to DLA structures may completely block the multipactor discharge. However, previous attempts to demonstrate this magnetic field effect in an X-band traveling-wave DLA structure were inconclusive, due to the axial variation of the applied magnetic field, and showed only partial suppression of the multipactor loading [Jing et al., Appl. Phys. Lett. 103, 213503 (2013)]. The present experiment has been performed under improved conditions with a uniform axial magnetic field extending along the length of an X-band standing-wave DLA structure. Multipactor loading began to be continuously reduced starting from 3.5 kG applied magnetic field and was completely suppressed at 8 kG. Dependence of multipactor suppression on the rf gradient inside the DLA structure was also measured.
The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique
Sutherland, Kevin Jerome
2001-01-01
Photonic band gap (PBG) crystals are periodic dielectric structures that manipulate electromagnetic radiation in a manner similar to semiconductor devices manipulating electrons. Whereas a semiconductor material exhibits an electronic band gap in which electrons cannot exist, similarly, a photonic crystal containing a photonic band gap does not allow the propagation of specific frequencies of electromagnetic radiation. This phenomenon results from the destructive Bragg diffraction interference that a wave propagating at a specific frequency will experience because of the periodic change in dielectric permitivity. This gives rise to a variety of optical applications for improving the efficiency and effectiveness of opto-electronic devices. These applications are reviewed later. Several methods are currently used to fabricate photonic crystals, which are also discussed in detail. This research involves a layer-by-layer micro-transfer molding ({mu}TM) and stacking method to create three-dimensional FCC structures of epoxy or titania. The structures, once reduced significantly in size can be infiltrated with an organic gain media and stacked on a semiconductor to improve the efficiency of an electronically pumped light-emitting diode. Photonic band gap structures have been proven to effectively create a band gap for certain frequencies of electro-magnetic radiation in the microwave and near-infrared ranges. The objective of this research project was originally two-fold: to fabricate a three dimensional (3-D) structure of a size scaled to prohibit electromagnetic propagation within the visible wavelength range, and then to characterize that structure using laser dye emission spectra. As a master mold has not yet been developed for the micro transfer molding technique in the visible range, the research was limited to scaling down the length scale as much as possible with the current available technology and characterizing these structures with other methods.
Reconstruction of band structure induced by electronic nematicity in an FeSe superconductor.
Nakayama, K; Miyata, Y; Phan, G N; Sato, T; Tanabe, Y; Urata, T; Tanigaki, K; Takahashi, T
2014-12-05
We have performed high-resolution angle-resolved photoemission spectroscopy on an FeSe superconductor (T_{c}∼8 K), which exhibits a tetragonal-to-orthorhombic structural transition at T_{s}∼90 K. At low temperature, we found splitting of the energy bands as large as 50 meV at the M point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to T∼110 K, slightly above T_{s}, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives rise to a van Hove singularity within 5 meV of the Fermi energy. The present result strongly suggests that this unusual electronic state is responsible for the unconventional superconductivity in FeSe.
The influence of line shape and band structure on temperatures in planetary atmospheres
NASA Technical Reports Server (NTRS)
Arking, A.; Grossman, K.
1972-01-01
Numerical experiments are performed to examine the effects of line shape and band structure on the radiative equilibrium temperature profile in planetary atmospheres. In order to accurately determine these effects, a method for calculating radiative terms is developed which avoids the usual approximations. It differs from the more commonly used methods in that it allows arbitrary dependence of the absorption coefficient on wave number, without requiring tedious line by line integration and without the constraints of band models. The present formulation is restricted to homogeneous atmospheres but the concept can be extended to the more general case. The numerical experiments reveal that the line shape and band structure of the absorbing gases have a large effect on temperatures in the higher layers of the atmosphere (corresponding to the stratosphere and mesosphere). The more nongrey the spectrum (that is, the higher the peaks and the deeper the troughs in the spectrum), the lower the temperature.
Damping Effect Studies for X-band Normal Conducting High Gradient Standing Wave Structures
Pei, S.; Li, Z.; Tantawi, S.G.; Dolgashev, V.A.; Wang, J.; /SLAC
2009-08-03
The Multi-TeV colliders should have the capability to accelerate low emittance beam with high rf efficiency, X-band normal conducting high gradient accelerating structure is one of the promising candidate. However, the long range transverse wake field which can cause beam emittance dilution is one of the critical issues. We examined effectiveness of dipole mode damping in three kinds of X-band, {pi}-mode standing wave structures at 11.424GHz with no detuning considered. They represent three damping schemes: damping with cylindrical iris slot, damping with choke cavity and damping with waveguide coupler. We try to reduce external Q factor below 20 in the first two dipole bands, which usually have very high (R{sub T}/Q){sub T}. The effect of damping on the acceleration mode is also discussed.
Band structures of cylindrical AlN/GaN quantum dots with fully coupled piezoelectric models
NASA Astrophysics Data System (ADS)
Prabhakar, Sanjay; Melnik, Roderick
2010-08-01
We study the coupled electro-mechanical effects in the band structure calculations of low dimensional semiconductor nanostructures (LDSNs) such as AlN/GaN quantum dots. Some effects in these systems are essentially nonlinear. Strain, piezoelectric effects, eigenvalues and wave functions of a quantum dot have been used as tuning parameters for the optical response of LDSNs in photonics, band gap engineering and other applications. However, with a few noticeable exceptions, the influence of piezoelectric effects in the electron wave functions in Quantum Dots (QDs) studied with fully coupled models has been largely neglected in the literature. In this paper, by using the fully coupled model of electroelasticity, we analyze the piezoelectric effects into the band structure of cylindrical quantum dots. Results are reported for III-V type semiconductors with a major focus given to AlN/GaN based QD systems.
Photoelectron spectroscopic study of band alignment of polymer/ZnO photovoltaic device structure
Nagata, T.; Chikyow, T.; Oh, S.; Wakayama, Y.; Yamashita, Y.; Yoshikawa, H.; Kobayashi, K.; Ikeno, N.
2013-01-28
Using x-ray photoelectron spectroscopy, we investigated the band alignment of a Ag/poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO photovoltaic structure. At the P3HT/ZnO interface, a band bending of P3HT and a short surface depletion layer of ZnO were observed. The offset between the highest occupied molecular orbital of P3HT and the conduction band minimum of ZnO at the interface contributed to the open circuit voltage (Voc) was estimated to be approximately 1.5 {+-} 0.1 eV, which was bigger than that of the electrically measured effective Voc of P3HT/ZnO photovoltaic devices, meaning that the P3HT/ZnO photovoltaic structure has the potential to provide improved photovoltaic properties.
Tan, Chih-Shan; Huang, Michael H
2017-09-04
Density functional theory calculations have been performed on Si (100), (110), (111), and (112) planes with tunable number of planes for evaluation of their band structures and density of states profiles. The purpose is to see whether silicon can exhibit facet-dependent properties derived from the presence of a thin surface layer having different band structures. No changes have been observed for single to multiple layers of Si (100) and (110) planes with a consistent band gap between the valence band and the conduction band. However, for 1, 2, 4, and 5 Si (111) and (112) planes, metal-like band structures were obtained with continuous density of states going from the valence band to the conduction band. For 3, 6, and more Si (111) planes, as well as 3 and 6 Si (112) planes, the same band structure as that seen for Si (100) and (110) planes has been obtained. Thus, beyond a layer thickness of five Si (111) planes at ≈1.6 nm, normal semiconductor behavior can be expected. The emergence of metal-like band structures for the Si (111) and (112) planes are related to variation in Si-Si bond length and bond distortion plus 3s and 3p orbital electron contributions in the band structure. This work predicts possession of facet-dependent electrical properties of silicon with consequences in FinFET transistor design. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Electronic structure descriptor for the discovery of narrow-band red-emitting phosphors
Wang, Zhenbin; Chu, Iek -Heng; Zhou, Fei; ...
2016-05-09
Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu2+-activated emission identified through a comparison of the electronic structures of known narrow-band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu2+ 4f7 bands. By incorporating this descriptor in a high-throughput first-principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu2+-activated red-emitting phosphors that are predicted to exhibit goodmore » chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Lastly, our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth.« less
Electronic structure descriptor for the discovery of narrow-band red-emitting phosphors
Wang, Zhenbin; Chu, Iek -Heng; Zhou, Fei; Ong, Shyue Ping
2016-05-09
Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu^{2+}-activated emission identified through a comparison of the electronic structures of known narrow-band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu^{2+} 4f^{7} bands. By incorporating this descriptor in a high-throughput first-principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu^{2+}-activated red-emitting phosphors that are predicted to exhibit good chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Lastly, our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth.
NASA Astrophysics Data System (ADS)
Kanayama, Kaoru; Nagashio, Kosuke; Nishimura, Tomonori; Toriumi, Akira
2014-03-01
Although upper conduction and valence sub-bands in bilayer graphene are known to be asymmetric, a detailed analysis based on the electrical measurements is very limited due to the infirm quality of gate insulator. In this study, the electrical quality of the top-gate Y2O3 insulator is drastically improved by the high-pressure O2 post-deposition annealing at 100 atm and the carrier density of ~8*1013 cm-2 is achieved. In quantum capacitance measurements, the drastic increase of the density of states is observed in addition to the van Hove singularity, suggesting that the Fermi energy reaches upper sub-band. At the same carrier density, the sudden reduction of the conductivity is observed, indicating that the inter-band scattering occurs. The estimated carrier density required to fill the upper sub-bands is different between electron and hole sides, i.e., asymmetric band structure between upper conduction and valence bands is revealed by the electrical measurements.
NASA Astrophysics Data System (ADS)
Kishi, Ayaka; Oda, Masato; Shinozuka, Yuzo
2016-05-01
This paper reports on the electronic states of compound semiconductor alloys of wurtzite structure calculated by the recently proposed interacting quasi-band (IQB) theory combined with empirical sp3 tight-binding models. Solving derived quasi-Hamiltonian 24 × 24 matrix that is characterized by the crystal parameters of the constituents facilitates the calculation of the conduction and valence bands of wurtzite alloys for arbitrary concentrations under a unified scheme. The theory is applied to III-V and II-VI wurtzite alloys: cation-substituted Al1- x Ga x N and Ga1- x In x N and anion-substituted CdS1- x Se x and ZnO1- x S x . The obtained results agree well with the experimental data, and are discussed in terms of mutual mixing between the quasi-localized states (QLS) and quasi-average bands (QAB): the latter bands are approximately given by the virtual crystal approximation (VCA). The changes in the valence and conduction bands, and the origin of the band gap bowing are discussed on the basis of mixing character.
Electronic structure descriptor for the discovery of narrow-band red-emitting phosphors
Wang, Zhenbin; Chu, Iek -Heng; Zhou, Fei; Ong, Shyue Ping
2016-05-09
Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu^{2+}-activated emission identified through a comparison of the electronic structures of known narrow-band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu^{2+} 4f^{7} bands. By incorporating this descriptor in a high-throughput first-principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu^{2+}-activated red-emitting phosphors that are predicted to exhibit good chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Lastly, our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth.
Band structure, cohesive properties, and Compton profile of γ- and α-cerium
NASA Astrophysics Data System (ADS)
Podloucky, R.; Glötzel, D.
1983-03-01
Recent Compton scattering experiments on the high-volume (γ) and low-volume (α) phases of fcc cerium and their interpretation in terms of the renormalized-free-atom model cast severe doubts on the promotional model of Pauling and Zachariasen for the γ-α transition. Stimulated by these results, we have extended a previous self-consistent local-density band-structure investigation to study the Compton profiles of γ- and α-cerium. For the band structure, Bloch functions, and their Fourier transforms we use the linear muffin-tin orbital method in the atomic-sphere approximation. We analyze the calculated Compton profiles in terms of band structure and local angular momentum character of the wave functions. The change in band structure and wave functions under compression (with approximately one electron per atom in the 4f band of both phases) accounts well for the observed change in the Compton profile. This provides further evidence against the promotional model in agreement with the analysis of Kornstädt et al. In addition, we study the cohesive energy of fcc cerium as a function of volume in the local-density approximation. For α-cerium in the 4f1(5d 6s)3 configuration we find a cohesive energy of 5.4 eV/atom in good agreement with experiment, whereas the "promotional" 4f0(5d 6s)4 state yields a binding energy of 0.6 eV/atom only. Therefore the fourth valence electron has to be a 4f electron, and α-cerium has to be regarded as an f-band metal.
Burgoyne, Thomas; Morris, Edward P; Luther, Pradeep K
2015-11-06
The Z-band in vertebrate striated muscle crosslinks actin filaments of opposite polarity from adjoining sarcomeres and transmits tension along myofibrils during muscular contraction. It is also the location of a number of proteins involved in signalling and myofibrillogenesis; mutations in these proteins lead to myopathies. Understanding the high-resolution structure of the Z-band will help us understand its role in muscle contraction and the role of these proteins in the function of muscle. The appearance of the Z-band in transverse-section electron micrographs typically resembles a small-square lattice or a basketweave appearance. In longitudinal sections, the Z-band width varies more with muscle type than species: slow skeletal and cardiac muscles have wider Z-bands than fast skeletal muscles. As the Z-band is periodic, Fourier methods have previously been used for three-dimensional structural analysis. To cope with variations in the periodic structure of the Z-band, we have used subtomogram averaging of tomograms of rat cardiac muscle in which subtomograms are extracted and compared and similar ones are averaged. We show that the Z-band comprises four to six layers of links, presumably α-actinin, linking antiparallel overlapping ends of the actin filaments from the adjoining sarcomeres. The reconstruction shows that the terminal 5-7nm of the actin filaments within the Z-band is devoid of any α-actinin links and is likely to be the location of capping protein CapZ. Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.
NASA Astrophysics Data System (ADS)
Ye, Zhen-Yu; Deng, Hui-Xiong; Wu, Hui-Zhen; Li, Shu-Shen; Wei, Su-Huai; Luo, Jun-Wei
2015-11-01
Group-IV tellurides have exhibited exotic band structures. Specifically, despite the fact that Sn sits between Ge and Pb in the same column of the periodic table, cubic SnTe is a topological crystalline insulator with band inversion, but both isovalent GeTe and PbTe are trivial semiconductors with normal band order. By performing first-principles band structure calculations, we unravel the origin of this abnormal behaviour by using symmetry analysis and the atomic orbital energy levels and atomic sizes of these elements. In group-IV tellurides, the s lone pair band of the group-IV element is allowed by symmetry to couple with the anion valence p band at the L-point, and such s-p coupling leads to the occurrence of bandgap at the L-point. We find that such s-p coupling is so strong in SnTe that it inverts the band order near the bandgap; however, it is not strong enough in both GeTe and PbTe, so they remain normal semiconductors. The reason for this is the incomplete screening of the core of the relatively tight-binding Ge 4s orbital by its 3d orbitals and the large atomic size and strong relativistic effect in Pb, respectively. Interestingly, we also find that the rhombohedral distortion removes the inversion symmetry and the reduced s-p coupling transforms the α-SnTe back to a normal semiconductor. Our study demonstrates that, in addition to spin-orbital coupling, strain and interface dipole fields, inter-orbital coupling is another effective way to engineer the topological insulators.
Observation of 'Band' Structures in Spacecraft Observations of Inner Magnetosphere Plasma Ions
NASA Astrophysics Data System (ADS)
Mohan, Kirthika; Fazakerley, Andrew; Grimald, Sandrine; Dandouras, Iannis; Owen, Chris
2014-05-01
In previous studies, several authors have reported inner magnetosphere observations of proton distributions confined to narrow energy bands in the range of 1-25 keV (Smith and Hoffman (1974), etc). These structures have been described as "nose structures", with reference to their appearance in energy-time spectrograms and are also known as "bands" if they occur for extended periods of time. Statistical surveys (Buzulukova et al. (2003); Vallat et al. (2007)) of these features in Interball and Cluster data highlight the presence of single nose in nightside sectors and multi-nose strutures in the dayside sectors. We examine Double-Star TC1 HIA data mainly recorded in the equatorial plane of the inner magnetosphere (L<15) to see how observations of "multi-banded structures" compare to the observations from more inclined orbits of Cluster and Interball. We investigate the properties of these multi-banded structures and carry out a statistical survey analysing them as a function of geomagnetic activity. This is a comparison study to a similar study conducted using DoubleStar TC-1 PEACE electron data.
k.p Parameters with Accuracy Control from Preexistent First-Principles Band Structure Calculations
NASA Astrophysics Data System (ADS)
Sipahi, Guilherme; Bastos, Carlos M. O.; Sabino, Fernando P.; Faria Junior, Paulo E.; de Campos, Tiago; da Silva, Juarez L. F.
The k.p method is a successful approach to obtain band structure, optical and transport properties of semiconductors. It overtakes the ab initio methods in confined systems due to its low computational cost since it is a continuum method that does not require all the atoms' orbital information. From an effective one-electron Hamiltonian, the k.p matrix representation can be calculated using perturbation theory and the parameters identified by symmetry arguments. The parameters determination, however, needs a complementary approach. In this paper, we developed a general method to extract the k.p parameters from preexistent band structures of bulk materials that is not limited by the crystal symmetry or by the model. To demonstrate our approach, we applied it to zinc blende GaAs band structure calculated by hybrid density functional theory within the Heyd-Scuseria-Ernzerhof functional (DFT-HSE), for the usual 8 ×8 k.p Hamiltonian. Our parameters reproduced the DFT-HSE band structure with great accuracy up to 20% of the first Brillouin zone (FBZ). Furthermore, for fitting regions ranging from 7-20% of FBZ, the parameters lie inside the range of values reported by the most reliable studies in the literature. The authors acknowledge financial support from the Brazilian agencies CNPq (Grant #246549/2012-2) and FAPESP (Grants #2011/19333-4, #2012/05618-0 and #2013/23393-8).
Electric polarization and the photogalvanic effect in solids with a topological band structure
NASA Astrophysics Data System (ADS)
Fregoso, Benjamin M.
2015-03-01
It is known that solids without inversion symmetry can exhibit photogalvanic effects and intrinsic electric polarization, e.g., ferroelectrics. Understanding the relation between the strength of the induced current and the electric polarization has proven challenging. We report on model calculations with topologically non-trivial band structure aimed at quantifying these contributions. Partial support form Conacyt.
Band structure of one-dimensional plasma photonic crystals using the Fresnel coefficients method
NASA Astrophysics Data System (ADS)
Jafari, A.; Rahmat, A.
2017-04-01
The current study has examined the band structures of two types of photonic crystals (PCs). The first is a one-dimensional metamaterial photonic crystal (1DMMPC) composed of double-layered units for which both layers of each unit are dielectric. The second type is a very similar one-dimensional plasma photonic crystal (1DPPC) also composed of double-layered units in which the first layer is a dielectric material but the second is a plasma layer. This study compares the band structures of the 1DMMPC with specific optical characteristics of the 1DPPC using the Fresnel coefficients method and also compares the results of this method with the results of the transfer matrix method. It is concluded that the dependency of the electric permittivity of the plasma layer on the incident field frequency causes differences in the band structures in 1DMMPC and 1DPPC for both TE and TM polarizations and their gaps reside in different frequencies. The band structures of the 1DMMPC and 1DPPC are confirmed by the results of the transfer matrix method.
Band structure of one-dimensional plasma photonic crystals using the Fresnel coefficients method
NASA Astrophysics Data System (ADS)
Jafari, A.; Rahmat, A.
2016-11-01
The current study has examined the band structures of two types of photonic crystals (PCs). The first is a one-dimensional metamaterial photonic crystal (1DMMPC) composed of double-layered units for which both layers of each unit are dielectric. The second type is a very similar one-dimensional plasma photonic crystal (1DPPC) also composed of double-layered units in which the first layer is a dielectric material but the second is a plasma layer. This study compares the band structures of the 1DMMPC with specific optical characteristics of the 1DPPC using the Fresnel coefficients method and also compares the results of this method with the results of the transfer matrix method. It is concluded that the dependency of the electric permittivity of the plasma layer on the incident field frequency causes differences in the band structures in 1DMMPC and 1DPPC for both TE and TM polarizations and their gaps reside in different frequencies. The band structures of the 1DMMPC and 1DPPC are confirmed by the results of the transfer matrix method.
Doping and strain dependence of the electronic band structure in Ge and GeSn alloys
NASA Astrophysics Data System (ADS)
Xu, Chi; Gallagher, James; Senaratne, Charutha; Brown, Christopher; Fernando, Nalin; Zollner, Stefan; Kouvetakis, John; Menendez, Jose
2015-03-01
A systematic study of the effect of dopants and strain on the electronic structure of Ge and GeSn alloys is presented. Samples were grown by UHV-CVD on Ge-buffered Si using Ge3H8 and SnD4 as the sources of Ge and Sn, and B2H6/P(GeH3)3 as dopants. High-energy critical points in the joint-density of electronic states were studied using spectroscopic ellipsometry, which yields detailed information on the strain and doping dependence of the so-called E1, E1 +Δ1 , E0' and E2 transitions. The corresponding dependencies of the lowest direct band gap E0 and the fundamental indirect band gap Eindwere studied via room-T photoluminescence spectroscopy. Of particular interest for this work were the determination of deformation potentials, band gap renormalization effects, Burstein-Moss shifts due to the presence of carriers at band minima, and the dependence of other critical point parameters, such as amplitudes and phase angles, on the doping concentration. The selective blocking of transitions due to high doping makes it possible to investigate the precise k-space location of critical points. These studies are complemented with detailed band-structure calculations within a full-zone k-dot- p approach. Supported by AFOSR under DOD AFOSR FA9550-12-1-0208 and DOD AFOSR FA9550-13-1-0022.
Electronic band structure, doping, and defects in the semiconducting Half Heusler compound CoTiSb
NASA Astrophysics Data System (ADS)
Kawasaki, Jason; Johansson, Linda; Hjort, Martin; Timm, Rainer; Schultz, Brian; Balasubramanian, Thiagarajan; Mikkelsen, Anders; Palmstrom, Chris
2013-03-01
We report transport and electronic band structure measurements on epitaxial films of the Half Heusler compound CoTiSb. CoTiSb belongs to the family of Half Heuslers with 18 valence electrons per formula unit that are predicted to be semiconducting despite being composed of all metallic components. Here the CoTiSb films were grown by molecular beam epitaxy on a lattice matched InAlAs buffer. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction and X-ray diffraction. Scanning tunnelling spectroscopy and temperature-dependent transport measurements reveal that the films are semiconducting, with unintentionally doped carrier concentrations comparable to that of highly doped conventional compound semiconductors. These carrier concentrations can be modulated by doping with Sn. The band structure of the films was measured by angle resolved photoemission spectroscopy at the MAX-Lab Synchrotron facility. The bulk bands are in general agreement with density functional theory calculations, with a valence band maximum at Γ and surface states within the bulk band gap. The effects of defects are explored in order to explain the ARPES results. This work was supported by the ARO, AFOSR, ONR, and NSF.
Liu, Hao; Xu, Ziqiang
2013-01-01
A modified electromagnetic-bandgap (M-EBG) structure and its application to planar monopole ultra-wideband (UWB) antenna are presented. The proposed M-EBG which comprises two strip patch and an edge-located via can perform dual notched bands. By properly designing and placing strip patch near the feedline, the proposed M-EBG not only possesses a simple structure and compact size but also exhibits good band rejection. Moreover, it is easy to tune the dual notched bands by altering the dimensions of the M-EBG. A demonstration antenna with dual band-notched characteristics is designed and fabricated to validate the proposed method. The results show that the proposed antenna can satisfy the requirements of VSWR < 2 over UWB 3.1-10.6 GHz, except for the rejected bands of the world interoperability for microwave access (WiMAX) and the wireless local area network (WLAN) at 3.5 GHz and 5.5 GHz, respectively.
Xu, Ziqiang
2013-01-01
A modified electromagnetic-bandgap (M-EBG) structure and its application to planar monopole ultra-wideband (UWB) antenna are presented. The proposed M-EBG which comprises two strip patch and an edge-located via can perform dual notched bands. By properly designing and placing strip patch near the feedline, the proposed M-EBG not only possesses a simple structure and compact size but also exhibits good band rejection. Moreover, it is easy to tune the dual notched bands by altering the dimensions of the M-EBG. A demonstration antenna with dual band-notched characteristics is designed and fabricated to validate the proposed method. The results show that the proposed antenna can satisfy the requirements of VSWR < 2 over UWB 3.1–10.6 GHz, except for the rejected bands of the world interoperability for microwave access (WiMAX) and the wireless local area network (WLAN) at 3.5 GHz and 5.5 GHz, respectively. PMID:24170984
Valley-dependent band structure and valley polarization in periodically modulated graphene
NASA Astrophysics Data System (ADS)
Lu, Wei-Tao
2016-08-01
The valley-dependent energy band and transport property of graphene under a periodic magnetic-strained field are studied, where the time-reversal symmetry is broken and the valley degeneracy is lifted. The considered superlattice is composed of two different barriers, providing more degrees of freedom for engineering the electronic structure. The electrons near the K and K' valleys are dominated by different effective superlattices. It is found that the energy bands for both valleys are symmetric with respect to ky=-(AM+ξ AS) /4 under the symmetric superlattices. More finite-energy Dirac points, more prominent collimation behavior, and new crossing points are found for K' valley. The degenerate miniband near the K valley splits into two subminibands and produces a new band gap under the asymmetric superlattices. The velocity for the K' valley is greatly renormalized compared with the K valley, and so we can achieve a finite velocity for the K valley while the velocity for the K' valley is zero. Especially, the miniband and band gap could be manipulated independently, leading to an increase of the conductance. The characteristics of the band structure are reflected in the transmission spectra. The Dirac points and the crossing points appear as pronounced peaks in transmission. A remarkable valley polarization is obtained which is robust to the disorder and can be controlled by the strain, the period, and the voltage.
Results from the CLIC X-Band Structure Test Program at NLCTA
Adolphsen, Chris; Bowden, Gordon; Dolgashev, Valery; Laurent, Lisa; Tantawi, Sami; Wang, Faya; Wang, Juwen W.; Doebert, Steffen; Grudiev, Alexej; Riddone, Germana; Wuensch, Walter; Zennaro, Riccardo; Higashi, Yasuo; Higo, Toshiyasu; /KEK, Tsukuba
2009-07-06
As part of a SLAC-CERN-KEK collaboration on high gradient X-band structure research, several prototype structures for the CLIC linear collider study have been tested using two of the high power (300 MW) X-band rf stations in the NLCTA facility at SLAC. These structures differ in terms of their fabrication (brazed disks and clamped quadrants), gradient profile (amount by which the gradient increases along the structure, which optimizes efficiency and maximizes sustainable gradient) and HOM damping (use of slots or waveguides to rapidly dissipate dipole mode energy). The CLIC goal in the next few years is to demonstrate the feasibility of a CLIC-ready baseline design and to investigate alternatives that could increase efficiency. This paper summarizes the high gradient test results from NLCTA in support of this effort.
Crystal structure of the anion exchanger domain of human erythrocyte band 3.
Arakawa, Takatoshi; Kobayashi-Yurugi, Takami; Alguel, Yilmaz; Iwanari, Hiroko; Hatae, Hinako; Iwata, Momi; Abe, Yoshito; Hino, Tomoya; Ikeda-Suno, Chiyo; Kuma, Hiroyuki; Kang, Dongchon; Murata, Takeshi; Hamakubo, Takao; Cameron, Alexander D; Kobayashi, Takuya; Hamasaki, Naotaka; Iwata, So
2015-11-06
Anion exchanger 1 (AE1), also known as band 3 or SLC4A1, plays a key role in the removal of carbon dioxide from tissues by facilitating the exchange of chloride and bicarbonate across the plasma membrane of erythrocytes. An isoform of AE1 is also present in the kidney. Specific mutations in human AE1 cause several types of hereditary hemolytic anemias and/or distal renal tubular acidosis. Here we report the crystal structure of the band 3 anion exchanger domain (AE1(CTD)) at 3.5 angstroms. The structure is locked in an outward-facing open conformation by an inhibitor. Comparing this structure with a substrate-bound structure of the uracil transporter UraA in an inward-facing conformation allowed us to identify the anion-binding position in the AE1(CTD), and to propose a possible transport mechanism that could explain why selected mutations lead to disease.
Ferromagnetism and the electronic band structure in (Ga,Mn)(Bi,As) epitaxial layers
Yastrubchak, O.; Sadowski, J.; Domagala, J. Z.; Andrearczyk, T.; Wosinski, T.
2014-08-18
Impact of Bi incorporation into (Ga,Mn)As layers on their electronic- and band-structures as well as their magnetic and structural properties has been studied. Homogenous (Ga,Mn)(Bi,As) layers of high structural perfection have been grown by the low-temperature molecular-beam epitaxy technique. Post-growth annealing treatment of the layers results in an improvement of their structural and magnetic properties and an increase in the hole concentration in the layers. The modulation photoreflectance spectroscopy results are consistent with the valence-band model of hole-mediated ferromagnetism in the layers. This material combines the properties of (Ga,Mn)As and Ga(Bi,As) ternary compounds and offers the possibility of tuning its electrical and magnetic properties by controlling the alloy composition.
The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation
Dolgos, Michelle R.; Paraskos, Alexandra M.; Stoltzfus, Matthew W.; Yarnell, Samantha C.; Woodward, Patrick M.
2009-07-15
The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba{sub 3}(VO{sub 4}){sub 2}, Pb{sub 3}(VO{sub 4}){sub 2}, YVO{sub 4}, BiVO{sub 4}, CeVO{sub 4} and Ag{sub 3}VO{sub 4} were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba{sub 3}(VO{sub 4}){sub 2} and YVO{sub 4} have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb{sub 3}(VO{sub 4}){sub 2} and BiVO{sub 4} the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6s orbitals with nonbonding O 2p states at the top of the valence band, and (b) overlap of empty 6p orbitals with antibonding V 3d-O 2p states at the bottom of the conduction band. In Ag{sub 3}VO{sub 4} mixing between filled Ag 4d and O 2p states destabilizes states at the top of the valence band leading to a large decrease in the band gap (E{sub g}=2.2 eV). In CeVO{sub 4} excitations from partially filled 4f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce{sub 1-x}Bi{sub x}VO{sub 4} (0<=x<=0.5) and Ce{sub 1-x}Y{sub x}VO{sub 4} (x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi{sup 3+} or Y{sup 3+} are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4f orbitals. - Graphical abstract: The electronic structures of six vanadate salts, Ba{sub 3}(VO{sub 4}){sub 2}, Pb{sub 3}(VO{sub 4}){sub 2}, YVO{sub 4}, BiVO{sub 4}, Ag{sub 3}VO{sub 4} and CeVO{sub 4}, are studied. The results show that the oxygen to vanadium charge transfer, which is largely responsible for the
Electromagnetic wave band structure due to surface plasmon resonances in a complex plasma
NASA Astrophysics Data System (ADS)
Vladimirov, S. V.; Ishihara, O.
2016-07-01
The dielectric properties of complex plasma containing either metal or dielectric spherical inclusions (macroparticles, dust) are investigated. We focus on surface plasmon resonances on the macroparticle surfaces and their effect on electromagnetic wave propagation. It is demonstrated that the presence of surface plasmon oscillations can significantly modify plasma electromagnetic properties by resonances and cutoffs in the effective permittivity. This leads to related branches of electromagnetic waves and to the wave band gaps. The conditions necessary to observe the band-gap structure in laboratory dusty plasma and/or space (cosmic) dusty plasmas are discussed.
Investigation of band structure of 103,105Rh using microscopic computational technique
NASA Astrophysics Data System (ADS)
Kumar, Amit; Singh, Suram; Bharti, Arun
2015-08-01
The high-spin structure in 61Cu nucleus is studied in terms of effective two body interaction. In order to take into account the deformed BCS basis, the basis states are expanded in terms of the core eigenfunctions. Yrast band with some other bands havew been obtained and back-bending in moment of inertia has also been calculated and compared with the available experimental data for 61Cu nucleus. On comparing the available experimental as well as other theoretical data, it is found that the treatment with PSM provides a satisfactory explanation of the available data.
NASA Astrophysics Data System (ADS)
Narimani, M.; Nourbakhsh, Z.
2016-05-01
In this paper, the structural, electronic and optical properties of LuPdBi and ScPdBi compounds are investigated using the density functional theory by WIEN2K package within the generalized gradient approximation, local density approximation, Engel-Vosco generalized gradient approximations and modified Becke-Johnson potential approaches. The topological phases and band orders of these compounds are studied. The effect of pressure on band inversion strength, electron density of states and the linear coefficient of the electronic specific heat of these compounds is investigated. Furthermore, the effect of pressure on real and imaginary parts of dielectric function, absorption and reflectivity coefficients of these compounds is studied.
Investigation of band structure of {sup 103,105}Rh using microscopic computational technique
Kumar, Amit; Singh, Suram; Bharti, Arun
2015-08-28
The high-spin structure in {sup 61}Cu nucleus is studied in terms of effective two body interaction. In order to take into account the deformed BCS basis, the basis states are expanded in terms of the core eigenfunctions. Yrast band with some other bands havew been obtained and back-bending in moment of inertia has also been calculated and compared with the available experimental data for {sup 61}Cu nucleus. On comparing the available experimental as well as other theoretical data, it is found that the treatment with PSM provides a satisfactory explanation of the available data.
Crystal Structure and Band Gap Engineering in Polyoxometalate-Based Inorganic-Organic Hybrids.
Roy, Soumyabrata; Sarkar, Sumanta; Pan, Jaysree; Waghmare, Umesh V; Dhanya, R; Narayana, Chandrabhas; Peter, Sebastian C
2016-04-04
We have demonstrated engineering of the electronic band gap of the hybrid materials based on POMs (polyoxometalates), by controlling its structural complexity through variation in the conditions of synthesis. The pH- and temperature-dependent studies give a clear insight into how these experimental factors affect the overall hybrid structure and its properties. Our structural manipulations have been successful in effectively tuning the optical band gap and electronic band structure of this kind of hybrids, which can find many applications in the field of photovoltaic and semiconducting devices. We have also addressed a common crystallographic disorder observed in Keggin-ion (one type of heteropolyoxometalate [POMs])-based hybrid materials. Through a combination of crystallographic, spectroscopic, and theoretical analysis of four new POM-based hybrids synthesized with tactically varied reaction conditions, we trace the origin and nature of the disorder associated with it and the subtle local structural coordination involved in its core picture. While the crystallography yields a centrosymmetric structure with planar coordination of Si, our analysis with XPS, IR, and Raman spectroscopy reveals a tetrahedral coordination with broken inversion symmetry, corroborated by first-principles calculations.
Electronic band structure effects in the stopping of protons in copper
NASA Astrophysics Data System (ADS)
Quashie, Edwin E.; Saha, Bidhan C.; Correa, Alfredo A.
2016-10-01
We present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities v =0.02 -10 a .u . where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results are in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of ˜1.5 ) in the velocity range v =0.07 -0.3 a .u . , which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping v →0 is not as simple as phenomenological models suggest and it is plagued by band-structure effects.
Spin-resolved electronic band structure of fct-cobalt (100) (abstract)
NASA Astrophysics Data System (ADS)
Schneider, C. M.; Schuster, P.; Hammond, M.; Ebert, H.; Noffke, J.; Kirschner, J.
1991-04-01
The spin-polarized electronic structure of the fcc high-temperature modification of cobalt has not been investigated yet, because of the experimental difficulties of photoemission at high temperature. We stabilized fcc Co by molecular-beam epitaxy on a Cu(100) substrate with large, atomically flat terasses (0.5-1 μm wide) as revealed by STM. The structure of the layers was studied by LEED and MEED, showing a tetragonal distortion of the fcc lattice perpendicular to the (100) surface plane. The dispersion of the exchange split bands perpendicular to the surface was determined for a 5-monolayer-thick sample (tetragonal distortion on average 4%-5%) by spin- and momentum-resolved photoemission. The results are compared to two relativistic spin-polarized band-structure calculations for fcc cobalt. Somewhat surprisingly, even a 5-ML-thick sample shows three-dimensional dispersion in good agreement with the calculations, as far as the average exchange splitting (1.2±0.2 eV), and the symmetry character of the bands is concerned. There are, however, some systematic deviations of minority bands near the Fermi energy which are attributed to the tetragonal compression.
NASA Astrophysics Data System (ADS)
Zhang, Hai-Feng; Liu, Shao-Bin; Li, Bing-Xiang
2014-08-01
In this paper, the properties of anisotropic photonic band gaps (PBGs) for three-dimensional (3D) photonic crystals (PCs) composed of the anisotropic positive-index materials (the uniaxial materials) and the epsilon-negative (ENG) materials with body-centered-cubic (bcc) lattices are theoretically studied by a modified plane wave expansion (PWE) method, which are the uniaxial materials spheres inserted in the epsilon-negative materials background. The anisotropic photonic band gaps (PBGs) and one flatbands region can be achieved in first irreducible Brillouin zone. The influences of the ordinary-refractive index, extraordinary-refractive index, filling factor, the electronic plasma frequency, the dielectric constant of ENG materials and the damping factor on the properties of anisotropic PBGs for such 3D PCs are studied in detail, respectively, and some corresponding physical explanations are also given. The numerical results show that the anisotropy can open partial band gaps in such 3D PCs with bcc lattices composed of the ENG materials and uniaxial materials, and the complete PBGs can be obtained compared to the conventional 3D PCs containing the isotropic materials. The calculated results also show that the anisotropic PBGs can be manipulated by the parameters as mentioned above except for the damping factor. Introducing the uniaxial materials into 3D PCs containing the ENG materials can obtain the larger complete PBGs as such 3D PCs with high symmetry, and also provides a way to design the tunable devices.
NASA Astrophysics Data System (ADS)
Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.
2017-04-01
We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.
Disorder effects on the band structure of ZnGeN2: Role of exchange defects
NASA Astrophysics Data System (ADS)
Skachkov, Dmitry; Quayle, Paul C.; Kash, Kathleen; Lambrecht, Walter R. L.
2016-11-01
The role of exchange defects on the band structure of ZnGeN2 is investigated. Exchange defects are defined through the exchange of cations Zn and Ge starting from the ideal P n a 21 crystal structure, which obeys the local octet rule. Each such exchange creates several nitrogen-centered tetrahedra which violate the local octet rule, although overall charge neutrality is preserved. We study several distributions of exchange defects, some with all antisites making up the exchange defect close to each other and with increasing numbers of exchange defects, and others where the two types of antisites ZnGe and GeZn are kept separated from each other. We also compare the results for these models with a fully random distribution of Zn and Ge on the cation sites. We show that for a single-nearest-neighbor exchange defect, the band gap is narrowed by about 0.5 eV due to two effects: (1) the ZnGe antisites form filled acceptor states just above and merging with the valence-band maximum (VBM) of perfect crystal ZnGeN2 and (2) the GeZn antisites form a resonance in the conduction band which lowers the conduction-band minimum (CBM). When more exchange defects are created, these acceptor states broaden into bands which can lower the gap further. When tetrahedra occur surrounded completely by four Zn atoms, states even deeper in the gap are found localized all near these tetrahedra, forming a separate intermediate band. Finally, for phase-segregated ZnGe and GeZn, the gap is significantly more reduced, but no separate band is found to occur. The ZnGe acceptorlike states now form a percolating defect band which is significantly wider and hence reaches deeper into the gap. In all cases, the wave functions near the top of the new VBM remain, to some extent, localized near the ZnGe sites. For a fully random case, the gap is even more severely reduced by almost 3 eV. The total energy of the system increases with the number of octet-rule-violating tetrahedra and the energy cost per
Spin-dependent band structure of the ferromagnetic semimetal EuB6
NASA Astrophysics Data System (ADS)
Xiong, Peng; Zhang, X.; von Molnar, S.; Fisk, Z.
2008-03-01
The spin polarization of EuB6 crystals has been measured using Andreev reflection spectroscopy. The conductance spectra of the EuB6/Pb junctions are well-described by the spin-polarized BTK model, which yields a spin polarization of about 56%. The results demonstrate that ferromagnetic EuB6 is not half-metallic. Further analyses of the Hall effect and magnetoresistivity indicate a semi-metallic band structure with complete spin polarization for the hole band only. The values and the spread of the measured spin polarization are quantitatively consistent with Fermi surface determined by quantum oscillation measurements^1 and carrier densities obtained from standard two-band model fits to the low temperature magnetoresistivity and Hall resistivity. This work was supported by a FSU Research Foundation PEG, NSF DMR 0710492 and 0503360 grants. ^1R. Goodrich et al., PRB 58, 14896 (1998); M. Aronson et al., PRB 59, 4720 (1999).
Band structures of two dimensional solid/air hierarchical phononic crystals
NASA Astrophysics Data System (ADS)
Xu, Y. L.; Tian, X. G.; Chen, C. Q.
2012-06-01
The hierarchical phononic crystals to be considered show a two-order “hierarchical” feature, which consists of square array arranged macroscopic periodic unit cells with each unit cell itself including four sub-units. Propagation of acoustic wave in such two dimensional solid/air phononic crystals is investigated by the finite element method (FEM) with the Bloch theory. Their band structure, wave filtering property, and the physical mechanism responsible for the broadened band gap are explored. The corresponding ordinary phononic crystal without hierarchical feature is used for comparison. Obtained results show that the solid/air hierarchical phononic crystals possess tunable outstanding band gap features, which are favorable for applications such as sound insulation and vibration attenuation.
Pathway to oxide photovoltaics via band-structure engineering of SnO
Peng, Haowei; Bikowski, Andre; Zakutayev, Andriy; ...
2016-10-04
All-oxide photovoltaics could open rapidly scalable manufacturing routes, if only oxide materials with suitable electronic and optical properties were developed. SnO has exceptional doping and transport properties among oxides, but suffers from a strongly indirect band gap. Here, we address this shortcoming by band-structure engineering through isovalent but heterostructural alloying with divalent cations (Mg, Ca, Sr, and Zn). Furthermore, using first-principles calculations, we show that suitable band gaps and optical properties close to that of direct semiconductors are achievable, while the comparatively small effective masses are preserved in the alloys. Initial thin film synthesis and characterization support the feasibility ofmore » the approach.« less
Ab initio electronic band structure study of III-VI layered semiconductors
NASA Astrophysics Data System (ADS)
Olguín, Daniel; Rubio-Ponce, Alberto; Cantarero, Andrés
2013-08-01
We present a total energy study of the electronic properties of the rhombohedral γ-InSe, hexagonal ɛ-GaSe, and monoclinic GaTe layered compounds. The calculations have been done using the full potential linear augmented plane wave method, including spin-orbit interaction. The calculated valence bands of the three compounds compare well with angle resolved photoemission measurements and a discussion of the small discrepancies found has been given. The present calculations are also compared with recent and previous band structure calculations available in the literature for the three compounds. Finally, in order to improve the calculated band gap value we have used the recently proposed modified Becke-Johnson correction for the exchange-correlation potential.
Täuber, Daniela; Dobrovolsky, Alexander; Camacho, Rafael; Scheblykin, Ivan G
2016-08-10
Understanding electronic processes in organometal halide perovskites, flourishing photovoltaic, and emitting materials requires unraveling the origin of their electronic transitions. Light polarization studies can provide important information regarding transition dipole moment orientations. Investigating individual methylammonium lead triiodide perovskite nanocrystals enabled us to detect the polarization of photoluminescence intensity and photoluminescence excitation, hidden in bulk samples by ensemble averaging. Polarization properties of the crystals were correlated with their photoluminescence spectra and electron microscopy images. We propose that distortion of PbI6 octahedra leads to peculiarities of the electronic band structure close to the band-edge. Namely, the lowest band transition possesses a transition dipole moment along the apical Pb-I-Pb bond resulting in polarized photoluminescence. Excitation of photoluminescence above the bandgap is unpolarized because it involves molecular orbitals delocalized both in the apical and equatorial directions of the perovskite octahedron. Trap-assisted emission at 77 K, rather surprisingly, was polarized similar to the bandgap emission.
Band gap engineering in nano structured graphane by applying elastic strain
NASA Astrophysics Data System (ADS)
Kumar, Naveen; Sharma, Jyoti Dhar; Kumar, Ashok; Ahluwalia, P. K.
2013-02-01
The first principle calculations have been performed to investigate the influence of elastic strain namely: uniaxial compression (-a), symmetrical biaxial compression (-a-b) and asymmetric biaxial (-a+b) stain, on the electronic structure of graphane in chair conformation. It is found that the band gap can be tuned by elastic strain and depends on the type of applied strain. The nature of the graphane has been found to change from wide band gap semiconductor to metal at three types of strain i. e. 19% uniaxial compression, 16% biaxial compression and 16.5% asymmetric biaxial strain. Tunable band gap of graphane with elastic strain can make it suitable for the applications of electromechanical devices and for the fabrication of strain sensors.
Band like Electronic Structures in Square Hollow Quantum Dots by 3D-MHFKS Calculation
NASA Astrophysics Data System (ADS)
Takizawa, Tokihiro; Okada, Hoshihito; Matsuse, Takehiro
To find novel aspects of the electronic structures in quantum dots (QD) from a view point of spatial broken symmetry, 3-dimensional-mesh Hartree-Fock-Kohn-Sham (3D-MHFKS) calculations1 are applied to the interacting electron system of electron number N in a symmetry broken hollow QD. For the case of a square hollow quantum dot confined in square hard wall (HW) potential (SSHQD), the magnetic (B) field dependence of the obtained single particle energy levels and chemical potentials in B-N diagram are shown to have a band like electronic structures over the wide B-field range up to 20T. To clarify the origin of the band like electronic structures in SSHQD, 3D-MHFKS calculations are also applied for the mixed symmetry QD's with a circular hollow in square HW potential (SCHQD) and with a square hollow in circular HW potential (CSHQD).
Simulation of pulsed optical logic gates based on photonic band gap structures
NASA Astrophysics Data System (ADS)
Nefedov, Igor; Morozov, Yurii; Gusyatnikov, Viktor; Zheltikov, Aleksei
2001-04-01
The change in the refractive index of GaAs due to the light-induced generation of nonequilibrium charge carriers is shown to substantially change the transmission of a one-dimensional GaAs/GaAlAs photonic band-gap structure, allowing low-threshold photonic-crystal optical logic gates to be created. Elementary logic operations of the Boolean algebra performed with nonlinear PBG structures, including the biconditional implication, modulo-two addition, Peirce's arrow, and disjunction, are demonstrated. It is demonstrated that different results of optical switching can be achieved and, consequently, different logic operations can be performed with the same photonic band-gap structure at different delay times by varying the energy of the control light pulse.
Setyawan, Wahyu; Gaume, Romain M; Lam, Stephanie; Feigelson, Robert S; Curtarolo, Stefano
2011-07-11
For the purpose of creating a database of electronic structures of all the known inorganic compounds, we have developed a computational framework based on high-throughput ab initio calculations (AFLOW) and an online repository (www.aflowlib.org). In this article, we report the first step of this task: the calculation of band structures for 7439 compounds intended for the research of scintillator materials for γ-ray radiation detection. Data-mining is performed to select the candidates from 193,456 compounds compiled in the Inorganic Crystal Structure Database. Light yield and scintillation nonproportionality are predicted based on semiempirical band gaps and effective masses. We present a list of materials, potentially bright and proportional, and focus on those exhibiting small effective masses and effective mass ratios.
Multi-band polarization insensitive metamaterial absorber with dual cross-wires structure
NASA Astrophysics Data System (ADS)
Yao, Li-fang; Li, Min-hua; Zhai, Xiao-min; Wang, Hui-bo; Dong, Jian-feng
2015-11-01
A five-band metamaterial absorber (MMA) based on a simple planar structure is proposed. It utilizes different areas of a single unit cell to match impedance, and produces different absorptive frequencies. Numerical calculation shows that the MMA has five different absorption peaks at 3.78 GHz, 7.66 GHz, 10.9 GHz, 14.5 GHz and 16.7 GHz, and their absorption rates reach 95.5%, 98.6%, 95.7%, 96.6% and 99.8%, respectively. The proposed structure is polarization insensitive for transverse electric (TE) and transverse magnetic (TM) incident waves. Also, the absorptive characteristics over large incident angles are examined. In addition, we analyze the absorption mechanism by the surface current density and power flow density distributions. This simple structure provides a way to design multi-band MMA, and also saves the cost of fabrication.
Electronic band structure and Fermi surface of ferromagnetic Tb: Experiment and theory
NASA Astrophysics Data System (ADS)
Döbrich, K. M.; Bihlmayer, G.; Starke, K.; Prieto, J. E.; Rossnagel, K.; Koh, H.; Rotenberg, E.; Blügel, S.; Kaindl, G.
2007-07-01
We have investigated the bulk valence-band structure of Tb metal in the ferromagnetic phase by angle-resolved photoelectron spectroscopy and full-potential-linearized-augmented-plane-wave calculations. The experiments were performed at undulator beamline 7.0.1 of the Advanced Light Source using a three-axis rotatable low-temperature goniometer and a display-type photoelectron spectrometer that give access to a large region of momentum space. The results of our calculations, which make use of recent progress in the theoretical description of the magnetic properties of 4f metals, are in remarkably good agreement with experiment. This can be best seen from a comparison of the electronic structure in high-symmetry directions, at critical points, on Fermi contours, and at band crossings with the Fermi level. To our knowledge, the present work represents the most detailed combined experimental and theoretical study of the electronic structure of a magnetic lanthanide metal to date.
Spatially resolved methane band photometry of Saturn. II - Cloud structure models at four latitudes
NASA Technical Reports Server (NTRS)
West, R. A.
1983-01-01
Saturn's cloud vertical structures in the Equatorial Zone, South Equatorial Belt, and North and South Temperate Regions near + or - 30 deg latitudes are determined by means of an analysis of spatially resolved reflectivity measurements in the 6190, 7250, and 8996 A methane bands. Radiative transfer models are computed for a structure whose parameters are the methane column abundance in an aerosol-free layer at the top of the atmosphere, and the specific abundance of methane in a semiinfinite homogeneous gas-and-cloud mixture deep in the atmosphere. The structure for the South Equatorial Belt resembles that for the North Temperate Region. The level where unit cloud optical depth occurs in the South Temperate Region is deeper than the corresponding level at other latitudes. The differences between model parameters derived by means of different absorption bands are discussed.
Ground-based testing of the dynamics of flexible space structures using band mechanisms
NASA Technical Reports Server (NTRS)
Yang, L. F.; Chew, Meng-Sang
1991-01-01
A suspension system based on a band mechanism is studied to provide the free-free conditions for ground based validation testing of flexible space structures. The band mechanism consists of a noncircular disk with a convex profile, preloaded by torsional springs at its center of rotation so that static equilibrium of the test structure is maintained at any vertical location; the gravitational force will be directly counteracted during dynamic testing of the space structure. This noncircular disk within the suspension system can be configured to remain unchanged for test articles with the different weights as long as the torsional spring is replaced to maintain the originally designed frequency ratio of W/k sub s. Simulations of test articles which are modeled as lumped parameter as well as continuous parameter systems, are also presented.
Engineered band structure for an enhanced performance on quantum dot-sensitized solar cells
Jin, Bin Bin; Wang, Ye Feng; Wei, Dong; Chen, Yu; Zeng, Jing Hui; Cui, Bin
2016-06-20
A photon-to-current efficiency of 2.93% is received for the Mn-doped CdS (MCdS)-quantum dot sensitized solar cells (QDSSCs) using Mn:ZnO (MZnO) nanowire as photoanode. Hydrothermal synthesized MZnO are spin-coated on fluorine doped tin oxide (FTO) glass with P25 paste to serve as photoanode after calcinations. MCdS was deposited on the MZnO film by the successive ionic layer adsorption and reaction method. The long lived excitation energy state of Mn{sup 2+} is located inside the conduction band in the wide bandgap ZnO and under the conduction band of CdS, which increases the energetic overlap of donor and acceptor states, reducing the “loss-in-potential,” inhibiting charge recombination, and accelerating electron injection. The engineered band structure is well reflected by the electrochemical band detected using cyclic voltammetry. Cell performances are evidenced by current density-voltage (J-V) traces, diffuse reflectance spectra, transient PL spectroscopy, and incident photon to current conversion efficiency characterizations. Further coating of CdSe on MZnO/MCdS electrode expands the light absorption band of the sensitizer, an efficiency of 4.94% is received for QDSSCs.
Engineered band structure for an enhanced performance on quantum dot-sensitized solar cells
NASA Astrophysics Data System (ADS)
Jin, Bin Bin; Wang, Ye Feng; Wei, Dong; Cui, Bin; Chen, Yu; Zeng, Jing Hui
2016-06-01
A photon-to-current efficiency of 2.93% is received for the Mn-doped CdS (MCdS)-quantum dot sensitized solar cells (QDSSCs) using Mn:ZnO (MZnO) nanowire as photoanode. Hydrothermal synthesized MZnO are spin-coated on fluorine doped tin oxide (FTO) glass with P25 paste to serve as photoanode after calcinations. MCdS was deposited on the MZnO film by the successive ionic layer adsorption and reaction method. The long lived excitation energy state of Mn2+ is located inside the conduction band in the wide bandgap ZnO and under the conduction band of CdS, which increases the energetic overlap of donor and acceptor states, reducing the "loss-in-potential," inhibiting charge recombination, and accelerating electron injection. The engineered band structure is well reflected by the electrochemical band detected using cyclic voltammetry. Cell performances are evidenced by current density-voltage (J-V) traces, diffuse reflectance spectra, transient PL spectroscopy, and incident photon to current conversion efficiency characterizations. Further coating of CdSe on MZnO/MCdS electrode expands the light absorption band of the sensitizer, an efficiency of 4.94% is received for QDSSCs.
2011-02-28
2,3,4 Prashanth C. Upadhya, 1 Rohit P. Prasankumar, 1 Antoinette J. Taylor, 1 and S. R. J. Brueck 2,* 1Center for Integrated Nanotechnologies...2009). 23. A. Mary , S. G. Rodrigo, F. J. Garcia-Vidal, and L. Martin-Moreno, “Theory of negative-refractive-index response of double-fishnet
NASA Technical Reports Server (NTRS)
Tripathi, A. K.; Singhal, R. P.; Khazanov, G. V.; Avanov, L. A.
2016-01-01
Electron pitch angle (D (alpha)) and momentum (D(pp)) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L = 4.6 and 6.8 for electron energies 10 keV. Landau (n = 0) resonance and cyclotron harmonic resonances n = +/-1, +/-2,...+/-5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (alpha) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n = +1 and n = +2. A major contribution to momentum diffusion coefficients appears from n = +2. However, the banded structures in D alpha and Dpp coefficients appear only in the profile of diffusion coefficients for n = +2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The Dpp diffusion coefficient for ECH waves is one to two orders smaller than D alpha coefficients. For chorus waves, Dpp coefficients are about an order of magnitude smaller than D alpha coefficients for the case n does not = 0. In case of Landau resonance, the values of Dpp coefficient are generally larger than the values of D alpha coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89 deg and harmonic resonances n = +1, +2, and +3, whereas for whistler mode waves, the frequencies have been calculated for angle
NASA Technical Reports Server (NTRS)
Tripathi, A. K.; Singhal, R. P.; Khazanov, G. V.; Avanov, L. A.
2016-01-01
Electron pitch angle (D(sub (alpha alpha))) and momentum (D(sub pp)) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L=4.6 and 6.8 for electron energies less than or equal to 10 keV. Landau (n=0) resonance and cyclotron harmonic resonances n= +/- 1, +/-2, ... +/-5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (alpha) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n=+1 and n=+2. A major contribution to momentum diffusion coefficients appears from n=+2. However, the banded structures in D(sub alpha alpha) and D(sub pp) coefficients appear only in the profile of diffusion coefficients for n=+2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The D(sub pp) diffusion coefficient for ECH waves is one to two orders smaller than D(sub alpha alpha) coefficients. For chorus waves, D(sub pp) coefficients are about an order of magnitude smaller than D(sub alpha alpha) coefficients for the case n does not equal 0. In case of Landau resonance, the values of D(sub pp) coefficient are generally larger than the values of D(sub alpha alpha) coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89 deg and harmonic resonances
NASA Technical Reports Server (NTRS)
Tripathi, A. K.; Singhal, R. P.; Khazanov, G. V.; Avanov, L. A.
2016-01-01
Electron pitch angle (D (alpha)) and momentum (D(pp)) diffusion coefficients have been calculated due to resonant interactions with electrostatic electron cyclotron harmonic (ECH) and whistler mode chorus waves. Calculations have been performed at two spatial locations L = 4.6 and 6.8 for electron energies 10 keV. Landau (n = 0) resonance and cyclotron harmonic resonances n = +/-1, +/-2,...+/-5 have been included in the calculations. It is found that diffusion coefficient versus pitch angle (alpha) profiles show large dips and oscillations or banded structures. The structures are more pronounced for ECH and lower band chorus (LBC) and particularly at location 4.6. Calculations of diffusion coefficients have also been performed for individual resonances. It is noticed that the main contribution of ECH waves in pitch angle diffusion coefficient is due to resonances n = +1 and n = +2. A major contribution to momentum diffusion coefficients appears from n = +2. However, the banded structures in D alpha and Dpp coefficients appear only in the profile of diffusion coefficients for n = +2. The contribution of other resonances to diffusion coefficients is found to be, in general, quite small or even negligible. For LBC and upper band chorus waves, the banded structures appear only in Landau resonance. The Dpp diffusion coefficient for ECH waves is one to two orders smaller than D alpha coefficients. For chorus waves, Dpp coefficients are about an order of magnitude smaller than D alpha coefficients for the case n does not = 0. In case of Landau resonance, the values of Dpp coefficient are generally larger than the values of D alpha coefficients particularly at lower energies. As an aid to the interpretation of results, we have also determined the resonant frequencies. For ECH waves, resonant frequencies have been estimated for wave normal angle 89 deg and harmonic resonances n = +1, +2, and +3, whereas for whistler mode waves, the frequencies have been calculated for angle
Ab initio electronic structure of a small band gap polymer: Poly-aminosquaraine
NASA Astrophysics Data System (ADS)
Brocks, G.
1995-02-01
Poly-aminosquaraine is the prototype of a class of organic polymers which recently has been shown to provide a route towards small band gap materials. We predict that poly-aminosquaraine has a small band gap of ˜0.5 eV. Our prediction is based upon a detailed analysis of first-principles calculations of the geometrical and the electronic structure, using the Car-Parrinello technique of simultaneous optimization. We analyze the bands around the Fermi level in terms of a simple tight-binding model based upon the highest occupied and lowest occupied (HOMO/LUMO) states of the individual squaraine molecules. The small band gap of the polymer is shown to be the result of the small splitting between the occupied and the unoccupied states of the squaraine molecule combined with a favorable hybridization in the polymer. It should be possible to analyze the electronic structure of a wide class of squaraine based polymers in the same way.
NASA Astrophysics Data System (ADS)
Szczęśniak, Dominik; Ennaoui, Ahmed; Ahzi, Saïd
2016-09-01
Recently, the transition metal dichalcogenides have attracted renewed attention due to the potential use of their low-dimensional forms in both nano- and opto-electronics. In such applications, the electronic and transport properties of monolayer transition metal dichalcogenides play a pivotal role. The present paper provides a new insight into these essential properties by studying the complex band structures of popular transition metal dichalcogenide monolayers (MX 2, where M = Mo, W; X = S, Se, Te) while including spin-orbit coupling effects. The conducted symmetry-based tight-binding calculations show that the analytical continuation from the real band structures to the complex momentum space leads to nonlinear generalized eigenvalue problems. Herein an efficient method for solving such a class of nonlinear problems is presented and yields a complete set of physically relevant eigenvalues. Solutions obtained by this method are characterized and classified into propagating and evanescent states, where the latter states manifest not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gap of MX 2 monolayers, where electrons can directly tunnel between the band gap edges. To describe these tunneling currents, decay behavior of electronic states in the forbidden energy region is elucidated and their importance within the ballistic transport regime is briefly discussed.
Szczęśniak, Dominik; Ennaoui, Ahmed; Ahzi, Saïd
2016-09-07
Recently, the transition metal dichalcogenides have attracted renewed attention due to the potential use of their low-dimensional forms in both nano- and opto-electronics. In such applications, the electronic and transport properties of monolayer transition metal dichalcogenides play a pivotal role. The present paper provides a new insight into these essential properties by studying the complex band structures of popular transition metal dichalcogenide monolayers (MX 2, where M = Mo, W; X = S, Se, Te) while including spin-orbit coupling effects. The conducted symmetry-based tight-binding calculations show that the analytical continuation from the real band structures to the complex momentum space leads to nonlinear generalized eigenvalue problems. Herein an efficient method for solving such a class of nonlinear problems is presented and yields a complete set of physically relevant eigenvalues. Solutions obtained by this method are characterized and classified into propagating and evanescent states, where the latter states manifest not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gap of MX 2 monolayers, where electrons can directly tunnel between the band gap edges. To describe these tunneling currents, decay behavior of electronic states in the forbidden energy region is elucidated and their importance within the ballistic transport regime is briefly discussed.
Hediger, R; Ansari, H A; Stranzinger, G F
1991-01-01
By using three gene probes, one derived from the porcine major histocompatibility complex (MHC) and two from bovine cytokeratin genes, type I (KRTA) and type II (KRTB), the hypothesis of conservation of genome structure in two members of the family Bovidae was examined. Gene mapping data revealed the MHC to be in chromosome region 23q15----q23 in cattle (BOLA) and 20q15----q23 in sheep (OLA). KRTA was localized to chromosome region 19q25----q29 in cattle and 11q25----q29 in sheep and KRTB to 5q14----q22 in cattle and 3q14----q22 in sheep. The banding patterns of the chromosome arms to which the loci were assigned were identical in both species. Moreover, the resemblances of GTG- or QFQ-banding patterns between the cattle and sheep karyotypes illustrated further chromosome homologies. These studies, based on gene mapping comparisons and comparative cytogenetics, document that within bovid chromosomes, homology of banding patterns corresponds to a homologous genetic structure. Hence, we propose that gene assignments on identified chromosomal segments in one species of the Bovidae can be extrapolated, in general, to other bovid species based on the banding homologies presented here.
NASA Astrophysics Data System (ADS)
Islam, M. Fhokrul; Bohr, Henrik G.; Malik, F. B.
2009-12-01
Beyond the second row of elements in the Mendeleev periodic table, the consideration of the relativistic effect is important in determining proper configurations of atoms and ions, in many cases. Many important quantities of interest in determining physical and chemical properties of matter, such as the effective charge, root mean square radii, and higher moments of radii used in many calculations, e.g. in the determinations of legend stabilization bond energies depend on whether the treatment is relativistic or not. In general, these quantities for a given l-orbital having two different j-values, e.g. d2/3 and d5/2, differ from each other, hence, making it necessary to treat them as separate orbitals. This also necessitates characterizing bands with their j-values in many instants and not l-values, particularly for "d" and f-orbitals. For example, in Au, 5d3/2 and 5d5/2 are to be dealt with as two distinct bands. The observed enhancement of laser induced field emission in W, which is not understood in terms of non-relativistic band-structures, can be explained in terms of the expected relativistic band structure. Spin-orbit coupling, which is the manifestation of the relativistic effect, is a prime factor in facilitating intersystem crossing in bio-molecules.
NASA Astrophysics Data System (ADS)
Fhokrul Islam, M.; Bohr, Henrik G.; Malik, F. B.
Beyond the second row of elements in the Mendeleev periodic table, the consideration of the relativistic effect is important in determining proper configurations of atoms and ions, in many cases. Many important quantities of interest in determining physical and chemical properties of matter, such as the effective charge, root mean square radii, and higher moments of radii used in many calculations, e.g. in the determinations of legend stabilization bond energies depend on whether the treatment is relativistic or not. In general, these quantities for a given l-orbital having two different j-values, e.g. d3/2 and d5/2, differ from each other, hence, making it necessary to treat them as separate orbitals. This also necessitates characterizing bands with their j-values in many instants and not l-values, particularly for "d" and f-orbitals. For example, in Au, 5d3/2 and 5d5/2 are to be dealt with as two distinct bands. The observed enhancement of laser induced field emission in W, which is not understood in terms of non-relativistic band-structures, can be explained in terms of the expected relativistic band structure. Spin-orbit coupling, which is the manifestation of the relativistic effect, is a prime factor in facilitating intersystem crossing in bio-molecules.
NASA Astrophysics Data System (ADS)
Izuani Che Rosid, N. A.; Ahmadi, M. T.; Ismail, Razali
2016-09-01
The effect of tensile uniaxial strain on the non-parabolic electronic band structure of armchair graphene nanoribbon (AGNR) is investigated. In addition, the density of states and the carrier statistic based on the tight-binding Hamiltonian are modeled analytically. It is found that the property of AGNR in the non-parabolic band region is varied by the strain. The tunable energy band gap in AGNR upon strain at the minimum energy is described for each of n-AGNR families in the non-parabolic approximation. The behavior of AGNR in the presence of strain is attributed to the breakable AGNR electronic band structure, which varies the physical properties from its normality. The linear relation between the energy gap and the electrical properties is featured to further explain the characteristic of the deformed AGNR upon strain. Project supported by the Ministry of Higher Education (MOHE), Malaysia under the Fundamental Research Grant Scheme (FRGS) (Grant No.Q.J130000.7823.4F477). We also thank the Research Management Center (RMC) of Universiti Teknologi Malaysia (UTM) for providing an excellent research environment.
The structure of the stable negative ion of calcium
Pegg, D.J.; Thompson, J.S.; Compton, R.N.; Alton, G.D.
1988-01-01
The structure of the Ca/sup /minus// ion has been determined using a crossed laser-ion beams apparatus. The photoelectron detachment spectrum shows that, contrary to earlier expectations, the Ca/sup /minus// ion is stably bound in the (4s/sup 2/4p)/sup 2/p state. The electron affinity of Ca was measured to be 0.043 /sup + -/ 0.007 eV.
Demonstration of molecular beam epitaxy and a semiconducting band structure for I-Mn-V compounds
Jungwirth, T.; Novak, V.; Cukr, M.; Zemek, J.; Marti, X.; Horodyska, P.; Nemec, P.; Holy, V.; Maca, F.; Shick, A. B.; Masek, J.; Kuzel, P.; Nemec, I.; Gallagher, B. L.; Campion, R. P.; Foxon, C. T.; Wunderlich, J.
2011-01-15
Our ab initio theory calculations predict a semiconducting band structure of I-Mn-V compounds. We demonstrate on LiMnAs that high-quality materials with group-I alkali metals in the crystal structure can be grown by molecular beam epitaxy. Optical measurements on the LiMnAs epilayers are consistent with the theoretical electronic structure. Our calculations also reproduce earlier reports of high antiferromagnetic ordering temperature and predict large, spin-orbit-coupling-induced magnetic anisotropy effects. We propose a strategy for employing antiferromagnetic semiconductors in high-temperature semiconductor spintronics.
Evidence for a diamondlike electronic band structure of Si multilayers on Ag(111)
NASA Astrophysics Data System (ADS)
Mahatha, S. K.; Moras, P.; Sheverdyaeva, P. M.; Flammini, R.; Horn, K.; Carbone, C.
2015-12-01
Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced reconstruction of Si(111) demonstrate, from the close match in the valence level band structures, that the films exhibit a s p3 diamondlike structure. This refutes the interpretation of silicon multilayers on Ag(111) as silicene, a conclusion that is strengthened by the observation from core level photoemission that significant silver segregation occurs to the surface of these layers.
Alternative structure of TiO2 with higher energy valence band edge
NASA Astrophysics Data System (ADS)
Coh, Sinisa; Yu, Peter Y.; Aoki, Yuta; Saito, Susumu; Louie, Steven G.; Cohen, Marvin L.
2017-02-01
We propose an alternative structure of TiO2 anatase that has a higher energy oxygen p -like valence band maximum than pristine TiO2 anatase and thus has a much better alignment with the water splitting levels. This alternative structure is unique when considering a large subspace of possible structural distortions of TiO2 anatase. We propose two routes towards this state and argue that one of them might have been realized in the recently discovered so-called black TiO2.
Autopsy on an RF-Processed X-band Travelling Wave Structure
Le Pimpec, Frederic
2002-09-25
In an effort to locate the cause(s) of high electric-field breakdown in x-band accelerating structures, we have cleanly-autopsied (no debris added by post-operation structure disassembly) an RF-processed structure. Macroscopic localization provided operationally by RF reflected wave analysis and acoustic sensor pickup was used to connect breakdowns to autopsied crater damage areas. Surprisingly, the microscopic analyses showed breakdown craters in areas of low electric field. High currents induced by the magnetic field on sharp corners of the input coupler appears responsible for the extreme breakdown damage observed.
SLAC High Gradient Testing of a KEK X-Band Accelerator Structure
Loewen, Rod
2000-03-30
The high accelerating gradients required for future linear colliders demands a better study of field emission and RF breakdown in accelerator structures. Changes in structure geometry, vacuum pumping, fabrication methods, and surface finish can all potentially impact the conditioning process, dark current emission, and peak RF power handling capability. Recent tests at SLAC of KEK's ``M2'' travelling wave x-band accelerator section provides an opportunity to investigate some of these effects by comparing its performance to previously high power tested structures at SLAC. In addition to studying ultimate power limitations, this test also demonstrates the use of computer automated conditioning to reach practical, achievable gradients.
Experimental Studies of W-Band Accelerator Structures at High Field
Hill, Marc E
2001-02-09
A high-gradient electron accelerator is desired for high-energy physics research, where frequency scalings of breakdown and trapping of itinerant beamline particles dictates operation of the accelerator at short wavelengths. The first results of design and test of a high-gradient mm-wave linac with an operating frequency at 91.392 GHz (W-band) are presented. A novel approach to particle acceleration is presented employing a planar, dielectric lined waveguide used for particle acceleration. The traveling wave fields in the planar dielectric accelerator (PDA) are analyzed for an idealized structure, along with a circuit equivalent model used for understanding the structure as a microwave circuit. Along with the W-band accelerator structures, other components designed and tested are high power rf windows, high power attenuators, and a high power squeeze-type phase shifter. The design of the accelerator and its components where eased with the aide of numerical simulations using a finite-difference electromagnetic field solver. Manufacturing considerations of the small, delicate mm-wave components and the steps taken to reach a robust fabrication process are detailed. These devices were characterized under low power using a two-port vector network analyzer to verify tune and match, including measurements of the structures' fields using a bead-pull. The measurements are compared with theory throughout. Addition studies of the W-band structures were performed under high power utilizing a 11.424 GHz electron linac as a current source. Test results include W-band power levels of 200 kW, corresponding to fields in the PDA of over 20 MV/m, a higher gradient than any collider. Planar accelerator devices naturally have an rf quadrupole component of the accelerating field. Presented for the first time are the measurements of this effect.
NASA Astrophysics Data System (ADS)
Fu, Xiao-Long; Wu, Guo-Cheng; Bai, Wei-Xiong; Wang, Guang-Ming; Liang, Jian-Gang
2017-01-01
In this paper, an ultra-compact single negative (SNG) electric waveguided metamaterial (WG-MTM) is first investigated and used to reduce the mutual coupling in E & H planes of a dual-band microstrip antenna array. The proposed SNG electric WG-MTM unit cell is designed by etching two different symmetrical spiral lines on the ground, and has two stopbands operating at 1.86 GHz and 2.40 GHz. The circuit size is very compact, which is only {λ }0/33.6× {λ }0/15.1 (where λ 0 is the wavelength at 1.86 GHz in free space). Taking advantage of the dual-stopband property of the proposed SNG electric WG-MTM, a dual-band microstrip antenna array operating at 1.86 GHz and 2.40 GHz with very low mutual coupling is designed by embedding a cross shaped array of the proposed SNG electric WG-MTM. The measured and simulated results of the designed dual-band antenna array are in good agreement with each other, indicating that the mutual coupling of the fabricated dual-band antenna array realizes 9.8/11.1 dB reductions in the H plane, 8.5/7.9 dB reductions in the E plane at 1.86 GHz and 2.40 GHz, respectively. Besides, the distance of the antenna elements in the array is only 0.35λ 0 (where λ 0 is the wavelength at 1.86 GHz in free space). The proposed strategy is used for the first time to reduce the mutual coupling in E & H planes of the dual-band microstrip antenna array by using ultra-compact SNG electric WG-MTM. Project supported by the National Natural Science Foundation of China (Grant No. 61372034).
Fang, A. F.; Xu, G.; Dong, T.; Zheng, P.; Wang, N. L.
2013-01-01
Ir1−xPtxTe2 is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe2 in an effort to elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the energy levels of Te (px, py) and Te pz bands. PMID:23362455
NASA Astrophysics Data System (ADS)
Tran, Manh Cuong; Nguyen, Thi Thuy; Ho, Tuan Hung; Do, Hoang Tung
2017-01-01
We present a simple method to achieve a multiband perfect metamaterial absorber for use in the K band by applying defects to the absorber structure. Open boundary conditions with an excitation port are used for simulation of the whole considered structure. A defect was then introduced into the structure to obtain multiband absorption. Two perfect absorption peaks were observed at 19.8 GHz and 23.1 GHz for the structure with a defect of 2 × 2 unit cells. The multiple resonance frequencies could be tuned by varying the defect dimensions. In addition, it was found that the absorber structure is insensitive to the polarization angle of the incident electromagnetic wave over a wide range due to the symmetry of the configuration. This represents a simpler method to create a multifrequency absorber compared with previous works. To the best of our knowledge, this is the first study considering the influence of structural defects on the absorption frequencies of a metamaterial absorber.
Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids
NASA Astrophysics Data System (ADS)
Tancogne-Dejean, Nicolas; Mücke, Oliver D.; Kärtner, Franz X.; Rubio, Angel
2017-02-01
An accurate analytic model describing the microscopic mechanism of high-harmonic generation (HHG) in solids is derived. Extensive first-principles simulations within a time-dependent density-functional framework corroborate the conclusions of the model. Our results reveal that (i) the emitted HHG spectra are highly anisotropic and laser-polarization dependent even for cubic crystals; (ii) the harmonic emission is enhanced by the inhomogeneity of the electron-nuclei potential; the yield is increased for heavier atoms; and (iii) the cutoff photon energy is driver-wavelength independent. Moreover, we show that it is possible to predict the laser polarization for optimal HHG in bulk crystals solely from the knowledge of their electronic band structure. Our results pave the way to better control and optimize HHG in solids by engineering their band structure.
Observations of LHR noise with banded structure by the sounding rocket S29 barium-GEOS
NASA Technical Reports Server (NTRS)
Koskinen, H. E. J.; Holmgren, G.; Kintner, P. M.
1983-01-01
The measurement of electrostatic noise near the lower hybrid frequency made by the sounding rocket S29 barium-GEOS is reported. The noise is related to the spin of the rocket and reaches well below the local lower hybrid resonance frequency. Above the altitude of 300 km the noise shows banded structure roughly organized by the hydrogen cyclotron frequency. Simultaneously with the banded structure a signal near the hydrogen cyclotron frequency is detected. This signal is also spin modulated. The character of the noise strongly suggests that it is locally generated by the rocket payload disturbing the plasma. If this interpretation is correct, plasma wave experiments on other spacecrafts are expected to observe similar phenomena.
Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids.
Tancogne-Dejean, Nicolas; Mücke, Oliver D; Kärtner, Franz X; Rubio, Angel
2017-02-24
An accurate analytic model describing the microscopic mechanism of high-harmonic generation (HHG) in solids is derived. Extensive first-principles simulations within a time-dependent density-functional framework corroborate the conclusions of the model. Our results reveal that (i) the emitted HHG spectra are highly anisotropic and laser-polarization dependent even for cubic crystals; (ii) the harmonic emission is enhanced by the inhomogeneity of the electron-nuclei potential; the yield is increased for heavier atoms; and (iii) the cutoff photon energy is driver-wavelength independent. Moreover, we show that it is possible to predict the laser polarization for optimal HHG in bulk crystals solely from the knowledge of their electronic band structure. Our results pave the way to better control and optimize HHG in solids by engineering their band structure.
Band structure of topological insulators from noise measurements in tunnel junctions
NASA Astrophysics Data System (ADS)
Cascales, Juan Pedro; Martínez, Isidoro; Katmis, Ferhat; Chang, Cui-Zu; Guerrero, Rubén; Moodera, Jagadeesh S.; Aliev, Farkhad G.
2015-12-01
The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al2O3/Co tunnel junctions with bottom TI electrodes of either Bi2Te3 or Bi2Se3. We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems that can further lead to the manipulation of their spin-polarized properties for technological purposes.
Band structure of topological insulators from noise measurements in tunnel junctions
NASA Astrophysics Data System (ADS)
Cascales Sandoval, Juan Pedro; Martinez, Isidoro; Guerrero, Ruben; Chang, Cui-Zu; Katmis, Ferhat; Moodera, Jagadeesh; Aliev, Farkhad
The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunnelling and noise spectroscopy utilizing TI/Al2O3/Co tunnel junctions with bottom TI electrodes of either Bi2Te3 or Bi2Se3. We demonstrate that features related to the band structure of the TI materials show up in the tunnelling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems that can further lead to the manipulation of their spin-polarized properties for technological purposes.
Band gap and chemically ordered domain structure of a graphene analogue BCN
NASA Astrophysics Data System (ADS)
Venu, K.; Kanuri, S.; Raidongia, K.; Hembram, K. P. S. S.; Waghmare, U. V.; Datta, R.
2010-12-01
Chemically synthesized few layer graphene analogues of B xC yN z are characterized by aberration corrected transmission electron microscopy and high resolution electron energy loss spectroscopy (HREELS) to determine the local phase, electronic structure and band gap. HREELS band gap studies of a B xC yN z composition reveal absorption edges at 2.08, 3.43 and 6.01 eV, indicating that the B xC yN z structure may consist of domains of different compositions. The K-absorption edge energy position of the individual elements in B xC yN z is determined and compared with h-BN and graphite. An understanding of these experimental findings is developed with complementary first-principles based calculations of the various ordered configurations of B xC yN z.
Ab initio Green-function formulation of the transfer matrix: Application to complex band structures
NASA Astrophysics Data System (ADS)
Wortmann, D.; Ishida, H.; Blügel, S.
2002-04-01
A method for the first-principles calculation of the transfer matrix is presented. The method is based on a Green-function formulation and allows one to relate the wave functions and their derivatives on boundaries at opposite sides of a film or junction of finite thickness. Both the underlying theory and an actual implementation in the full-potential linearized augmented plane wave method are described. Currently the embedding method is used to evaluate the Green-function matrix elements and in turn we show that the transfer matrix can be used to construct the embedding potential. Some possible applications of the transfer-matrix method such as the calculations of the complex band structure or the calculation of the transmission and reflection coefficients for ballistic transport are discussed. As a first example, complex band structures of Cu, Fe, and Si are presented.
Efficient quasiparticle band-structure calculations for cubic and noncubic crystals
Wenzien, B.; Cappellini, G.; Bechstedt, F.
1995-05-15
An efficient method developed for the calculation of quasiparticle corrections to density-functional-theory--local-density-approximation (DFT-LDA) band structures of diamond and zinc-blende materials is generalized for crystals with other cubic, hexagonal, tetragonal, and orthorhombic Bravais lattices. Local-field effects are considered in the framework of a LDA-like approximation. The dynamical screening is treated by expanding the self-energy linearly in energy. The anisotropy of the inverse dielectric matrix is taken into account. The singularity of the Coulomb potential in the screened-exchange part of the electronic self-energy is treated using auxiliary functions of the appropriate symmetry. An application to the electronic quasiparticle band structure of wurtzite 2{ital H}-SiC is presented within the approach of norm-conserving, nonlocal, fully separable pseudopotentials and a plane-wave expansion of the wave functions for the underlying DFT-LDA.
Measurements of higher order modes in a 30 cm long X-band structure
NASA Astrophysics Data System (ADS)
Xiao, L.; Liang, Y.; Tong, D.; Zhang, H.
2001-05-01
The use of a cage of metallic wires as a bead is proposed to measure the higher order modes (HOMs) in an X-band accelerating structure. These long thin wires can isolate the longitudinal electric field component from other field components and produce sufficient frequency shift in bead-pull measurements. In the setup described in this paper, the bead is made by sputtering silver film onto a thin nylon line through a specially designed fixture. The cage has a size of approximately 0.5 mm in diameter, 2 mm in length and more than six metallic wires of less than 0.1 mm in width. The fabrication and calibration of the cage are described. The longitudinal electric fields of the lowest passband dipole mode TM 1 1 0 in a 30 cm long X-band structure are measured by bead-pull measurements. Results are compared with the calculated ones obtained from URMELT-code.
Li, Qiang; Zhu, He; Zheng, Lirong; Fan, Longlong; Ren, Yang; Chen, Jun; Deng, Jinxia; Xing, Xianran
2016-11-01
The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.
NASA Astrophysics Data System (ADS)
Nakamura, H.; Hayashi, N.; Nakai, N.; Okumura, M.; Machida, M.
2009-10-01
screening is unusual, e.g., an overscreening occurs, the estimation of U may not simply give a positive value. In this paper, we explain why the negative U correction successfully works on these iron-based compounds. We would like to suggest that a situation, in which the on-site intra-band U becomes smaller than the inter-band U‧, emerges in these compounds. Such a situation is peculiar to multi-band cases and leads to an effective intra-band attraction. Moreover, the situation can create a full-gap Cooper pair, which is consistent with several observations. The contents of this paper are as follows. Section 2 briefly explains the calculation framework and gives U dependences of the magnetic moment for two cases, i.e., the mother and doped compounds. In addition, we compare the Fermi-surfaces between U=0 and U=-1. In Section 3, we discuss unique features of the electronic structure in iron-based superconductors and explain which type of situation requires the negative U correction.
Hattori, Haroldo T; Schneider, Vitor M; Cazo, Rogério M; Barbosa, Carmem L
2005-05-20
Recently, photonic crystal band-edge structures have been analyzed in the literature. However, most devices that have been presented so far emit light in different directions. We present a modal analysis (no gain included) of a few schemes to improve the directionality of these devices, i.e., in such a way that light that exits from them will travel mainly in a certain direction, eventually coupling its energy to a wide waveguide.
Electronic band structure and optical properties of antimony selenide under pressure
Abhijit, B.K.; Jayaraman, Aditya; Molli, Muralikrishna
2016-05-23
In this work we present the optical properties of Antimony Selenide (Sb{sub 2}Se{sub 3}) under ambient conditions and under pressure of 9.2 GPa obtained using first principles calculations. We investigated the electronic band structure using the FP-LAPW method within the sphere of the density functional theory. Optical properties like refractive index, absorption coefficient and optical conductivity are calculated using the WIEN2k code.
NASA Astrophysics Data System (ADS)
Viñas, Florinda; Xu, H. Q.; Leijnse, Martin
2017-03-01
Nanowires with a GaSb core and an InAs shell (and the inverted structure) are interesting for studies of electron-hole hybridization and interaction effects due to the bulk broken band-gap alignment at the material interface. We have used eight-band k .p theory together with the envelope function approximation to calculate the band structure of such nanowires. For a fixed core radius, as a function of shell thickness the band structure changes from metallic (for a thick shell) to semiconducting (for a thin shell) with a gap induced by quantum confinement. For intermediate shell thickness, a different gapped band structure can appear, where the gap is induced by hybridization between the valence band in GaSb and the conduction band in InAs. To establish a relationship between the nanowire band structures and signatures in thermoelectrical measurements, we use the calculated energy dispersions as input to the Boltzmann equation and to ballistic transport equations to study the diffusive limit and the ballistic limit, respectively. Our theoretical results provide a guide for experiments, showing how thermoelectric measurements in a gated setup can be used to distinguish between different types of band gaps, or tune the system into a regime with few electrons and few holes, which can be of interest for studies of exciton physics.
Structural Biomass Estimation from L-band Interferometric SAR and Lidar
NASA Astrophysics Data System (ADS)
Treuhaft, R. N.; Chapman, B. D.; Goncalves, F.; Hensley, S.; dos Santos, J. R.; Graca, P. A.; Dutra, L.
2011-12-01
After a review of biomass estimation from interferometric SAR (InSAR) at all bands over the last 15 years, and a brief review of lidar biomass estimation, this paper discusses structure and biomass estimation from simultaneously acquired (not repeat-track) InSAR at L-band. We will briefly discuss the history of regression of biomass to InSAR raw observations (coherence and phase) and structural parameters (height, standard deviation, Fourier component). Lidar biomass estimation from functions of the waveform will be discussed. We review our structural and biomass estimation results for C-band InSAR at vertical polarization for 12-14 baselines in La Selva Biological Station, Costa Rica. C-band vertical scales were between 12 and 100 m for structure estimation, but only between 50 and 100 m for biomass estimation, due to phase calibration problems at the shorter vertical wavelengths (larger baselines). Most of the talk will be spent on L-band, simultaneously acquired multibaseline InSAR, also at La Selva, acquired at vertical polarization. Because the vertical interferometric scale is proportional to the radar altitude times the wavelength over the baseline length, the AirSAR aircraft had to be flown very low (1.2 km) to realize vertical scales at L-band of 60 m and higher. Our lidar biomass estimation suggests that vertical scales of 14 m-100 m are optimal for biomass estimation. We will try three different approaches to biomass estimation with the limited high vertical scales we have available: 1) We will regress biomass to Fourier transforms as in the C-band and lidar study, but with 60 m - 100+ m vertical scales we do not expect accuracies to be as high as for the lidar demonstration (58 Mg/ha RMS scatter of estimated about field biomass for biomasses up to 450 Mg/ha), which used Fourier vertical wavelengths of 15 m-20 m. In addition to using Fourier components, 2) we will report the use of the derivative of the InSAR complex coherence with respect to Fourier
NASA Astrophysics Data System (ADS)
Zhong, C.; Zhang, H.; Cao, Q. P.; Wang, X. D.; Zhang, D. X.; Ramamurty, U.; Jiang, J. Z.
2016-08-01
Molecular dynamics simulations were employed to investigate the plastic deformation within the shear bands in three different metallic glasses (MGs). To mimic shear bands, MG specimens were first deformed until flow localization occurs, and then the volume of the material within the localized regions was extracted and replicated. Homogeneous deformation that is independent of the size of the specimen was observed in specimens with shear band like structure, even at a temperature that is far below the glass transition temperature. Structural relaxation and rapid cooling were employed to examine the effect of free volume content on the deformation behavior. This was followed by detailed atomic structure analyses, employing the concepts of Voronoi polyhedra and “liquid-like” regions that contain high fraction of sub-atomic size open volumes. Results suggest that the total fraction of atoms in liquid-like regions is a key parameter that controls the plastic deformation in MGs. These are discussed in the context of reported experimental results and possible strategies for synthesizing monolithic amorphous materials that can accommodate large tensile plasticity are suggested.
Resonant inelastic x-ray scattering as a probe of band structure effects in cuprates
NASA Astrophysics Data System (ADS)
Kanász-Nagy, M.; Shi, Y.; Klich, I.; Demler, E. A.
2016-10-01
We analyze within quasiparticle theory a recent resonant inelastic x-ray scattering (RIXS) experiment on YBa2Cu3O6+x with the incoming photon energy detuned at several values from the resonance maximum [Minola et al., Phys. Rev. Lett. 114, 217003 (2015), 10.1103/PhysRevLett.114.217003]. Surprisingly, the data show a much weaker dependence on detuning than expected from recent measurements on a different cuprate superconductor, Bi2Sr2CuO6+x [Guarise et al., Nat. Commun. 5, 5760 (2014), 10.1038/ncomms6760]. We demonstrate here that this discrepancy, originally attributed to collective magnetic excitations, can be understood in terms of the differences between the band structures of these materials. We find good agreement between theory and experiment over a large range of dopings, both in the underdoped and overdoped regimes. Moreover, we demonstrate that the RIXS signal depends sensitively on excitations at energies well above the Fermi surface that are inaccessible to traditionally used band structure probes, such as angle-resolved photoemission spectroscopy. This makes RIXS a powerful probe of band structure, not suffering from surface preparation problems and small sample sizes, making it potentially applicable to a number of cuprate materials.
Effective mass and Fermi surface complexity factor from ab initio band structure calculations
NASA Astrophysics Data System (ADS)
Gibbs, Zachary M.; Ricci, Francesco; Li, Guodong; Zhu, Hong; Persson, Kristin; Ceder, Gerbrand; Hautier, Geoffroy; Jain, Anubhav; Snyder, G. Jeffrey
2017-02-01
The effective mass is a convenient descriptor of the electronic band structure used to characterize the density of states and electron transport based on a free electron model. While effective mass is an excellent first-order descriptor in real systems, the exact value can have several definitions, each of which describe a different aspect of electron transport. Here we use Boltzmann transport calculations applied to ab initio band structures to extract a density-of-states effective mass from the Seebeck Coefficient and an inertial mass from the electrical conductivity to characterize the band structure irrespective of the exact scattering mechanism. We identify a Fermi Surface Complexity Factor:
Band structure, Fermi surface, superconductivity, and resistivity of actinium under high pressure
Dakshinamoorthy, M.; Iyakutti, K.
1984-12-15
The electronic band structures of fcc actinium (Ac) have been calculated for a wide range of pressures by reducing the unit-cell volume from 1.0V/sub 0/ to 0.5V/sub 0/ with use of the relativistic augmented-plane-wave method. The density of states and Fermi-surface cross sections corresponding to various volumes are obtained. Calculations for the band-structure-related quantities such as electron-phonon mass enhancement factor lambda, superconducting transition temperature T/sub c/, and resistivity rho corresponding to different volumes are performed. It is seen that T/sub c/ increases with pressure, i.e., with decreasing volume. A new empirical relation for the volume dependence of T/sub c/ is proposed and its validity is checked using the T/sub c/ values obtained from the above band-structure results. The resistivity rho first increases with increasing pressure (i.e., with decreasing volume) and then decreases for higher pressures (i.e., for smaller volumes).
Zhong, C.; Zhang, H.; Cao, Q. P.; Wang, X. D.; Zhang, D. X.; Ramamurty, U.; Jiang, J. Z.
2016-01-01
Molecular dynamics simulations were employed to investigate the plastic deformation within the shear bands in three different metallic glasses (MGs). To mimic shear bands, MG specimens were first deformed until flow localization occurs, and then the volume of the material within the localized regions was extracted and replicated. Homogeneous deformation that is independent of the size of the specimen was observed in specimens with shear band like structure, even at a temperature that is far below the glass transition temperature. Structural relaxation and rapid cooling were employed to examine the effect of free volume content on the deformation behavior. This was followed by detailed atomic structure analyses, employing the concepts of Voronoi polyhedra and “liquid-like” regions that contain high fraction of sub-atomic size open volumes. Results suggest that the total fraction of atoms in liquid-like regions is a key parameter that controls the plastic deformation in MGs. These are discussed in the context of reported experimental results and possible strategies for synthesizing monolithic amorphous materials that can accommodate large tensile plasticity are suggested. PMID:27480496
STM investigation of FeSe/SrTiO3 band structure
NASA Astrophysics Data System (ADS)
Webb, Tatiana A.; Huang, Dennis; Song, Can-Li; Chang, Cui-Zu; Moodera, Jagadeesh; Hoffman, Jennifer E.
2015-03-01
Growing a single unit cell of FeSe on a SrTiO3 substrate (1 u.c. FeSe/STO) enhances the superconducting transition temperature (Tc) by an order of magnitude. While the dramatic effect of the interface is evident, a mechanism is not. ARPES studies have revealed that the band structure differs significantly from bulk FeSe and the majority of other Fe-based superconductors, most notably in lacking a hole pocket at the Fermi level. ARPES, however, is limited to probing the filled electron states. STM/STS has access to the band structure, both above and below the Fermi level, with spatial resolution, and the data encodes electronic properties including orbital character and interactions. We present an STM/STS study of 1 u.c. FeSe/STO grown by molecular beam epitaxy (MBE), focusing on the empty-state band structure of this new high Tc superconductor. Work supported by the US National Science Foundation grant DMR-0847433, and STC Center for Integrated Quantum Materials under NSF grant DMR-1231319.
Robust topology optimization of three-dimensional photonic-crystal band-gap structures.
Men, H; Lee, K Y K; Freund, R M; Peraire, J; Johnson, S G
2014-09-22
We perform full 3D topology optimization (in which "every voxel" of the unit cell is a degree of freedom) of photonic-crystal structures in order to find optimal omnidirectional band gaps for various symmetry groups, including fcc (including diamond), bcc, and simple-cubic lattices. Even without imposing the constraints of any fabrication process, the resulting optimal gaps are only slightly larger than previous hand designs, suggesting that current photonic crystals are nearly optimal in this respect. However, optimization can discover new structures, e.g. a new fcc structure with the same symmetry but slightly larger gap than the well known inverse opal, which may offer new degrees of freedom to future fabrication technologies. Furthermore, our band-gap optimization is an illustration of a computational approach to 3D dispersion engineering which is applicable to many other problems in optics, based on a novel semidefinite-program formulation for nonconvex eigenvalue optimization combined with other techniques such as a simple approach to impose symmetry constraints. We also demonstrate a technique for robust topology optimization, in which some uncertainty is included in each voxel and we optimize the worst-case gap, and we show that the resulting band gaps have increased robustness to systematic fabrication errors.
Resonant inelastic x-ray scattering as a band structure probe of high-temperature superconductors
NASA Astrophysics Data System (ADS)
Kanasz-Nagy, Marton; Shi, Yifei; Klich, Israel; Demler, Eugene
I will analyze recent resonant inelastic x-ray scattering (RIXS) experimental data on YBa2Cu3O6 + x [Minola et al., Phys. Rev. Lett. 114, 217003 (2015)] within quasi-particle theory. This measurement has been performed with the incoming photon energy detuned at several values from the resonance maximum, and, surprisingly, the data shows much weaker dependence on detuning than expected from recent measurements on a different cuprate superconductor, Bi2Sr2CuO6 + x [Guarise et al., Nat. Commun. 5, 5760 (2014)]. I will demonstrate, that this discrepancy, originally attributed to collective magnetic excitations, can be understood in terms of the differences between the band structures of these materials. We found good agreement between theory and experiment over a large range of dopings [M. Kanasz-Nagy et al., arXiv:1508.06639]. Moreover, I will demonstrate that the RIXS signal depends sensitively on excitations at energies well above the Fermi surface, that are inaccessible to traditionally used band structure probes, such as angle-resolved photoemission spectroscopy. This makes RIXS a powerful probe of band structure, not suffering from surface preparation problems and small sample sizes, making it potentially applicable to a wide range of materials. The work of M. K.-N. was supported by the Harvard-MIT CUA, NSF Grant No. DMR-1308435, AFOSR Quantum Simulation MURI, the ARO-MURI on Atomtronics, and ARO MURI Quism program.
Zhong, C; Zhang, H; Cao, Q P; Wang, X D; Zhang, D X; Ramamurty, U; Jiang, J Z
2016-08-02
Molecular dynamics simulations were employed to investigate the plastic deformation within the shear bands in three different metallic glasses (MGs). To mimic shear bands, MG specimens were first deformed until flow localization occurs, and then the volume of the material within the localized regions was extracted and replicated. Homogeneous deformation that is independent of the size of the specimen was observed in specimens with shear band like structure, even at a temperature that is far below the glass transition temperature. Structural relaxation and rapid cooling were employed to examine the effect of free volume content on the deformation behavior. This was followed by detailed atomic structure analyses, employing the concepts of Voronoi polyhedra and "liquid-like" regions that contain high fraction of sub-atomic size open volumes. Results suggest that the total fraction of atoms in liquid-like regions is a key parameter that controls the plastic deformation in MGs. These are discussed in the context of reported experimental results and possible strategies for synthesizing monolithic amorphous materials that can accommodate large tensile plasticity are suggested.
A High-Power Test of An X-Band Molybdenum Iris Structure
Grudiev, A.
2005-02-18
In order to achieve accelerating gradients above 150 MV/m, alternative materials to copper are being investigated by the CLIC study. The potential of refractory metals has already been demonstrated in tests in which a tungsten-iris and a molybdenum-iris structure reached 150 and 193 MV/m respectively (30 GHz and a pulse length of 15 ns). In order to extend the investigation to the pulse lengths required for a linear collider, a molybdenum-iris structure scaled to X-band was tested at the Next Linear Collider Test Accelerator (NLCTA). The structure conditioned to only 65 MV/m (100 ns pulse length) in the available testing time and much more slowly than is typical of a copper structure. However the structure showed no sign of saturation and a microscopic inspection of the rf surfaces corroborated that the structure was still at an early stage of conditioning. The X-band and 30 GHz results are compared and what has been learned about material quality, surface preparation and conditioning strategy is discussed.
Peculiarities of the band structure of multi-component photonic crystals with different dimensions.
Samusev, A K; Samusev, K B; Rybin, M V; Limonov, M F
2010-03-24
In this work we offer a simple analytical method which allows us to determine and study the effects of the selective switching of photonic stop-bands in multi-component photonic crystals (Mc-PhCs) of any dimensionality. The calculations for Mc-PhCs with low dielectric contrast have been performed in the framework of the model based on the scattering form factor analysis. It has been shown that the effects of selective switching of photonic stop-bands predicted theoretically and found experimentally before in three-dimensional (3D) Mc-PhC have a general character and may be observed also in one-dimensional (1D) and two-dimensional (2D) Mc-PhCs. It is found that 1D, 2D and 3D Mc-PhCs demonstrate unexpectedly similar quasi-periodic behaviour of photonic stop-bands as a function of the reciprocal lattice vector. A proper choice of the structural and dielectric parameters can create a resonance photonic stop-band determining the Bragg wavelengths, to which a photonic crystal can never be transparent.
A Ku-band magnetically insulated transmission line oscillator with overmoded slow-wave-structure
NASA Astrophysics Data System (ADS)
Jiang, Tao; He, Jun-Tao; Zhang, Jian-De; Li, Zhi-Qiang; Ling, Jun-Pu
2016-12-01
In order to enhance the power capacity, an improved Ku-band magnetically insulated transmission line oscillator (MILO) with overmoded slow-wave-structure (SWS) is proposed and investigated numerically and experimentally. The analysis of the dispersion relationship and the resonant curve of the cold test indicate that the device can operate at the near π mode of the TM01 mode, which is useful for mode selection and control. In the particle simulation, the improved Ku-band MILO generates a microwave with a power of 1.5 GW and a frequency of 12.3 GHz under an input voltage of 480 kV and input current of 42 kA. Finally, experimental investigation of the improved Ku-band MILO is carried out. A high-power microwave (HPM) with an average power of 800 MW, a frequency of 12.35 GHz, and pulse width of 35 ns is generated under a diode voltage of 500 kV and beam current of 43 kA. The consistency between the experimental and simulated far-field radiation pattern confirms that the operating mode of the improved Ku-band MILO is well controlled in π mode of the TM01 mode. Project supported partly by the National Natural Science Foundation of China (Grant No. 61171021).
Quasiparticle band structures and thermoelectric transport properties of p-type SnSe
Shi, Guangsha; Kioupakis, Emmanouil
2015-02-14
We used density functional and many-body perturbation theory to calculate the quasiparticle band structures and electronic transport parameters of p-type SnSe both for the low-temperature Pnma and high-temperature Cmcm phases. The Pnma phase has an indirect band gap of 0.829 eV, while the Cmcm has a direct band gap of 0.464 eV. Both phases exhibit multiple local band extrema within an energy range comparable to the thermal energy of carriers from the global extrema. We calculated the electronic transport coefficients as a function of doping concentration and temperature for single-crystal and polycrystalline materials to understand the previous experimental measurements. The electronic transport coefficients are highly anisotropic and are strongly affected by bipolar transport effects at high temperature. Our results indicate that SnSe exhibits optimal thermoelectric performance at high temperature when doped in the 10{sup 19}–10{sup 20 }cm{sup −3} range.
NASA Astrophysics Data System (ADS)
Guillot-Deudon, Catherine; Harel, Sylvie; Mokrani, Arezki; Lafond, Alain; Barreau, Nicolas; Fernandez, Vincent; Kessler, John
2008-12-01
The aim of the present work is to complete a preliminary study concerning the electronic band structure investigations of NaxCu1-xIn5S8 compounds with 0≤x≤1 , which are expected to be formed at the Cu(In,Ga)Se2/In2S3 interface. The band structure calculations demonstrate that for the compounds containing both Na and Cu, as the Cu content increases the band gap tends to decrease, and x-ray photoemission spectroscopy measurements show that this variation is mainly due to valence-band-maximum shift along the solid solution. The band gap strongly depends on the nature of the monovalent cation, and the band structure calculations demonstrate that the d electrons of copper are responsible for the shift of the valence band. In addition, it is worth noting that the Cu-containing compounds have indirect gaps.
NASA Astrophysics Data System (ADS)
Mosallaei, Hossein
The main objective of this dissertation is to characterize and create insight into the electromagnetic performances of two classes of composite structures, namely, complex multi-layered media and periodic Electromagnetic Band-Gap (EBG) structures. The advanced and diversified computational techniques are applied to obtain their unique propagation characteristics and integrate the results into some novel applications. In the first part of this dissertation, the vector wave solution of Maxwell's equations is integrated with the Genetic Algorithm (GA) optimization method to provide a powerful technique for characterizing multi-layered materials, and obtaining their optimal designs. The developed method is successfully applied to determine the optimal composite coatings for Radar Cross Section (RCS) reduction of canonical structures. Both monostatic and bistatic scatterings are explored. A GA with hybrid planar/curved surface implementation is also introduced to efficiently obtain the optimal absorbing materials for curved structures. Furthermore, design optimization of the non-uniform Luneburg and 2-shell spherical lens antennas utilizing modal solution/GA-adaptive-cost function is presented. The lens antennas are effectively optimized for both high gain and suppressed grating lobes. The second part demonstrates the development of an advanced computational engine, which accurately computes the broadband characteristics of challenging periodic electromagnetic band-gap structures. This method utilizes the Finite Difference Time Domain (FDTD) technique with Periodic Boundary Condition/Perfectly Matched Layer (PBC/PML), which is efficiently integrated with the Prony scheme. The computational technique is successfully applied to characterize and present the unique propagation performances of different classes of periodic structures such as Frequency Selective Surfaces (FSS), Photonic Band-Gap (PBG) materials, and Left-Handed (LH) composite media. The results are
Transparent, metallo-dielectric, one-dimensional, photonic band-gap structures
NASA Astrophysics Data System (ADS)
Scalora, M.; Bloemer, M. J.; Pethel, A. S.; Dowling, J. P.; Bowden, C. M.; Manka, A. S.
1998-03-01
We investigate numerically the properties of metallo-dielectric, one-dimensional, photonic band-gap structures. Our theory predicts that interference effects give rise to a new transparent metallic structure that permits the transmission of light over a tunable range of frequencies, for example, the ultraviolet, the visible, or the infrared wavelength range. The structure can be designed to block ultraviolet light, transmit in the visible range, and reflect all other electromagnetic waves of lower frequencies, from infrared to microwaves and beyond. The transparent metallic structure is composed of a stack of alternating layers of a metal and a dielectric material, such that the complex index of refraction alternates between a high and a low value. The structure remains transparent even if the total amount of metal is increased to hundreds of skin depths in net thickness.
RF Processing of X-Band Accelerator Structures at the NLCTA
Adolphsen, Chris
2000-08-24
During the initial phase of operation, the linacs of the Next Linear Collider (NLC) will contain roughly 5,000 X-Band accelerator structures that will accelerate beams of electrons and positrons to 250 GeV. These structures will nominally operate at an unloaded gradient of 72 MV/m. As part of the NLC R and D program, several prototype structures have been built and operated at the Next Linear Collider Test Accelerator (NLCTA) at SLAC. Here, the effect of high gradient operation on the structure performance has been studied. Significant progress was made during the past year after the NLCTA power sources were upgraded to reliably produce the required NLC power levels and beyond. This paper describes the structures, the processing methodology and the observed effects of high gradient operation.
Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure
Simakov, Evgenya I.; Arsenyev, Sergey A.; Buechler, Cynthia E.; Edwards, Randall L.; Romero, William P.; Conde, Manoel; Ha, Gwanghui; Power, John G.; Wisniewski, Eric E.; Jing, Chunguang
2016-02-10
We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic band gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. We conducted an experiment at the Argonne Wakefield Accelerator (AWA) test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Lastly, excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.
Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure.
Simakov, Evgenya I; Arsenyev, Sergey A; Buechler, Cynthia E; Edwards, Randall L; Romero, William P; Conde, Manoel; Ha, Gwanghui; Power, John G; Wisniewski, Eric E; Jing, Chunguang
2016-02-12
We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic-band-gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have the potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. We conducted an experiment at the Argonne Wakefield Accelerator test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.
Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure
NASA Astrophysics Data System (ADS)
Simakov, Evgenya I.; Arsenyev, Sergey A.; Buechler, Cynthia E.; Edwards, Randall L.; Romero, William P.; Conde, Manoel; Ha, Gwanghui; Power, John G.; Wisniewski, Eric E.; Jing, Chunguang
2016-02-01
We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic-band-gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have the potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. We conducted an experiment at the Argonne Wakefield Accelerator test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.
Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure
Simakov, Evgenya I.; Arsenyev, Sergey A.; Buechler, Cynthia E.; ...
2016-02-10
We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic band gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. Wemore » conducted an experiment at the Argonne Wakefield Accelerator (AWA) test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Lastly, excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.« less
Band structure calculation of GaSe-based nanostructures using empirical pseudopotential method
NASA Astrophysics Data System (ADS)
Osadchy, A. V.; Volotovskiy, S. G.; Obraztsova, E. D.; Savin, V. V.; Golovashkin, D. L.
2016-08-01
In this paper we present the results of band structure computer simulation of GaSe- based nanostructures using the empirical pseudopotential method. Calculations were performed using a specially developed software that allows performing simulations using cluster computing. Application of this method significantly reduces the demands on computing resources compared to traditional approaches based on ab-initio techniques and provides receiving the adequate comparable results. The use of cluster computing allows to obtain information for structures that require an explicit account of a significant number of atoms, such as quantum dots and quantum pillars.