Placing molecules with Bohr radius resolution using DNA origami.
Funke, Jonas J; Dietz, Hendrik
2016-01-01
Molecular self-assembly with nucleic acids can be used to fabricate discrete objects with defined sizes and arbitrary shapes. It relies on building blocks that are commensurate to those of biological macromolecular machines and should therefore be capable of delivering the atomic-scale placement accuracy known today only from natural and designed proteins. However, research in the field has predominantly focused on producing increasingly large and complex, but more coarsely defined, objects and placing them in an orderly manner on solid substrates. So far, few objects afford a design accuracy better than 5?nm, and the subnanometre scale has been reached only within the unit cells of designed DNA crystals. Here, we report a molecular positioning device made from a hinged DNA origami object in which the angle between the two structural units can be controlled with adjuster helices. To test the positioning capabilities of the device, we used photophysical and crosslinking assays that report the coordinate of interest directly with atomic resolution. Using this combination of placement and analysis, we rationally adjusted the average distance between fluorescent molecules and reactive groups from 1.5 to 9?nm in 123 discrete displacement steps. The smallest displacement step possible was 0.04?nm, which is slightly less than the Bohr radius. The fluctuation amplitudes in the distance coordinate were also small (±0.5?nm), and within a factor of two to three of the amplitudes found in protein structures. PMID:26479026
Placing molecules with Bohr radius resolution using DNA origami
NASA Astrophysics Data System (ADS)
Funke, Jonas J.; Dietz, Hendrik
2016-01-01
Molecular self-assembly with nucleic acids can be used to fabricate discrete objects with defined sizes and arbitrary shapes. It relies on building blocks that are commensurate to those of biological macromolecular machines and should therefore be capable of delivering the atomic-scale placement accuracy known today only from natural and designed proteins. However, research in the field has predominantly focused on producing increasingly large and complex, but more coarsely defined, objects and placing them in an orderly manner on solid substrates. So far, few objects afford a design accuracy better than 5â€…nm, and the subnanometre scale has been reached only within the unit cells of designed DNA crystals. Here, we report a molecular positioning device made from a hinged DNA origami object in which the angle between the two structural units can be controlled with adjuster helices. To test the positioning capabilities of the device, we used photophysical and crosslinking assays that report the coordinate of interest directly with atomic resolution. Using this combination of placement and analysis, we rationally adjusted the average distance between fluorescent molecules and reactive groups from 1.5 to 9â€…nm in 123 discrete displacement steps. The smallest displacement step possible was 0.04â€…nm, which is slightly less than the Bohr radius. The fluctuation amplitudes in the distance coordinate were also small (Â±0.5â€…nm), and within a factor of two to three of the amplitudes found in protein structures.
NASA Astrophysics Data System (ADS)
Heyrovska, R.; Narayan, S.
2005-10-01
Recently, the ground state Bohr radius (aB) of hydrogen was shown to be divided into two Golden sections, aB,p = aB/Ã¸2 and aB,e = aB/Ã¸ at the point of electrical neutrality, where Ã¸ = 1.618 is the Golden ratio (R. Heyrovska, Molecular Physics 103: 877-882, and the literature cited therein). The origin of the difference of two energy terms in the Rydberg equation was thus shown to be in the ground state energy itself, as shown below: EH = (1/2)e2/(ÎºaB) = (1/2)(e2/Îº) [(1/aB,p - (1/aB,e)] (1). This work brings some new results that 1) a unit charge in vacuum has a magnetic moment, 2) (e2/2Îº) in eq. (1) is an electromagnetic condenser constant, 3) the de Broglie wavelengths of the proton and electron correspond to the Golden arcs of a circle with the Bohr radius, 4) the fine structure constant (Î±) is the ratio of the Planck's constants without and with the interaction of light with matter, 5) the g-factors of the electron and proton, ge/2 and gp/2 divide the Bohr radius at the magnetic center and 6) the ``mysterious'' value (137.036) of Î± -1 = (360/Ã¸2) - (2/Ã¸3), where (2/Ã¸3) arises from the difference, (gp - ge).
Exciton binding energy in GaAs V-shaped quantum wires
NASA Astrophysics Data System (ADS)
Rinaldi, R.; Cingolani, R.; Lepore, M.; Ferrara, M.; Catalano, I. M.; Rossi, F.; Rota, L.; Molinari, E.; Lugli, P.; Marti, U.; Martin, D.; Morier-Gemoud, F.; Ruterana, P.; Reinhart, F. K.
1994-11-01
We have determined the main parameters of the quasi-one-dimensional excitons confined in GaAs V-shaped quantum wires, namely exciton Bohr radius and binding energy, by two-photon absorption and magnetoluminescence experiments. The experimental results are in excellent agreement with our calculations based on realistic wave functions for the actual wire geometry.
Exciton binding energy in semiconductor quantum dots
Pokutnii, S. I.
2010-04-15
In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass {mu} = {mu}(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy E{sub ex}(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii a{sub ex}, the exciton binding energy E{sub ex}(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.
Exciton effective mass enhancement in coupled quantum wells in electric and magnetic fields
NASA Astrophysics Data System (ADS)
Wilkes, J.; Muljarov, E. A.
2016-02-01
We present a calculation of exciton states in semiconductor coupled quantum wells in the presence of electric and magnetic fields applied perpendicular to the QW plane. The exciton SchrÃ¶dinger equation is solved in real space in three-dimensions to obtain the Landau levels of both direct and indirect excitons. Calculation of the exciton energy levels and oscillator strengths enables mapping of the electric and magnetic field dependence of the exciton absorption spectrum. For the ground state of the system, we evaluate the Bohr radius, optical lifetime, binding energy and dipole moment. The exciton mass renormalization due to the magnetic field is calculated using a perturbative approach. We predict a non-monotonous dependence of the exciton ground state effective mass on magnetic field. Such a trend is explained in a classical picture, in terms of the ground state tending from an indirect to a direct exciton with increasing magnetic field.
NASA Astrophysics Data System (ADS)
Narayan, Monishka Rita; Singh, Jai
2013-10-01
Primary steps of exciton formation in organic solar cells are presented here. The rates of absorption of photons to excite singlet and triplet excitons are derived using exciton-photon and exciton-spin-orbit-photon-interaction, respectively, as perturbation operators. In both singlet and triplet absorptions, the rates are found to depend on the absorption energy, excitonic Bohr radius, and the dielectric constant of the donor organic material. Incorporation of heavy metal atoms enhances the exciton-spin-orbit-photon interaction and hence the rate of excitation of triplet excitons because it depends on the square of the heaviest atomic number. The new exciton-spin-orbit-photon interaction operator flips the spin to a singlet form leading to faster dissociation into charge carriers and resulting in higher photon to electron-hole pair conversion efficiency in organic solar cells.
Exciton Mapping at Subwavelength Scales in Two-Dimensional Materials
NASA Astrophysics Data System (ADS)
Tizei, Luiz H. G.; Lin, Yung-Chang; Mukai, Masaki; Sawada, Hidetaka; Lu, Ang-Yu; Li, Lain-Jong; Kimoto, Koji; Suenaga, Kazu
2015-03-01
Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.
Two-dimensional excitons in three-dimensional hexagonal boron nitride
Cao, X. K.; Lin, J. Y. Jiang, H. X.; Clubine, B.; Edgar, J. H.
2013-11-04
The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8?Å and 740?meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54m{sub o}, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.
Scaling of exciton binding energy and virial theorem in semiconductor quantum wells and wires
NASA Astrophysics Data System (ADS)
Zhang, Yong; Mascarenhas, A.
1999-01-01
Recent numerical calculations [F. Rossi, G. Goldoni, and E. Molinari, Phys. Rev. Lett. 78, 3527 (1997)] have revealed a shape-independent hyperbolic scaling rule for the exciton binding energy versus the exciton Bohr radius in semiconductor quantum wires, and an enhancement in the exciton binding energy in a quantum wire with respect to a quantum well for a given exciton Bohr radius. These findings were attributed to the existence of a constant (shape- and/or size-independent) virial theorem value (potential- to kinetic-energy ratio), respectively, for the wires and wells, and its value was found to be larger (=4) for wires than (=2) for wells. In order to elucidate the physics underlying the above results, we reexamine this subject by calculating the exciton binding energy and the corresponding virial theorem value in quantum wells and wires with infinite confinement barriers. We find the following. (i) The virial theorem value is nonconstant but approaches 2 from above when reducing the finite extension of the electron and hole wave functions in the confined directions. This is because the origin of the virial theorem value of 2 lies in the inverse square Coulomb force being the only interaction seen by the electron and hole. (ii) The scaling rule is nonhyperbolic, because the virial theorem value is not a constant. (iii) The virial theorem value and the exciton binding energy are larger in a wire than in a well for a given exciton Bohr radius, because the wire exciton has a smaller kinetic energy in the nonconfined direction. (iv) The origin of the shape-independent scaling rule for wires lies in the close similarity of the effective Coulomb potentials for wires with different shapes and widths. The virial theorem value being or not being a constant is irrelevant to the scaling rule. (v) There exists a more fundamental and practically more useful shape-independent scaling rule.
NASA Astrophysics Data System (ADS)
Crease, Robert P.
2008-05-01
In his book Niels Bohr's Times, the physicist Abraham Pais captures a paradox in his subject's legacy by quoting three conflicting assessments. Pais cites Max Born, of the first generation of quantum physics, and Werner Heisenberg, of the second, as saying that Bohr had a greater influence on physics and physicists than any other scientist. Yet Pais also reports a distinguished younger colleague asking with puzzlement and scepticism "What did Bohr really do?".
NASA Astrophysics Data System (ADS)
Litvinenko, K. L.; Li, Juerong; Stavrias, N.; Meaney, A. J.; Christianen, P. C. M.; Engelkamp, H.; Homewood, K. P.; Pidgeon, C. R.; Murdin, B. N.
2016-04-01
We have measured the near-infrared photoluminescence spectrum of phosphorus doped silicon (Si:P) and extracted the donor-bound exciton (D0X) energy at magnetic fields up to 28 T. At high field the Zeeman effect is strongly nonlinear because of the diamagnetic shift, also known as the quadratic Zeeman effect (QZE). The magnitude of the QZE is determined by the spatial extent of the wave-function. High field data allows us to extract values for the radius of the neutral donor (D0) ground state, and the light and heavy hole D0X states, all with more than an order of magnitude better precision than previous work. Good agreement was found between the experimental state radius and an effective mass model for D0. The D0X results are much more surprising, and the radius of the m J = Â±3/2 heavy hole is found to be larger than that of the m J = Â±1/2 light hole.
ERIC Educational Resources Information Center
Willden, Jeff
2001-01-01
"Bohr's Atomic Model" is a small interactive multimedia program that introduces the viewer to a simplified model of the atom. This interactive simulation lets students build an atom using an atomic construction set. The underlying design methodology for "Bohr's Atomic Model" is model-centered instruction, which means the central model of the…
Bristow, Alan D; Zhang, Tianhao; Siemens, Mark E; Cundiff, Steven T; Mirin, R P
2011-05-12
Optical two-dimensional Fourier-transform spectroscopy is used to study the heavy- and light-hole excitonic resonances in weakly disordered GaAs quantum wells. Homogeneous and inhomogeneous broadening contribute differently to the two-dimensional resonance line shapes, allowing separation of homogeneous and inhomogeneous line widths. The heavy-hole exciton exhibits more inhomogeneous than homogeneous broadening, whereas the light-hole exciton shows the reverse. This situation occurs because of the interplay between the length scale of the disorder and the exciton Bohr radius, which affects the exciton localization and scattering. Utilizing this separation of line widths, excitation-density-dependent measurements reveal that many-body interactions alter the homogeneous dephasing, while disorder-induced dephasing is unchanged. PMID:21384940
NASA Astrophysics Data System (ADS)
Cundiff, Steven; Bristow, Alan; Zhang, Tianhao; Siemens, Mark; Mirin, Richard
2011-03-01
Optical two-dimensional Fourier-transform spectroscopy is used to study the heavy- and light-hole excitonic resonances in GaAs quantum wells with weak structural disorder. Homogeneous and inhomogeneous broadening contribute differently to the two-dimensional resonance line shapes, allowing separation of homogeneous and inhomogeneous line widths. The heavy-hole exciton exhibits more inhomogeneous than homogeneous broadening, whereas the light-hole exciton shows the opposite. This situation arises from the interplay between the length scale of the disorder and the exciton Bohr radius, which affects the exciton localization and scattering. Utilizing this separation of line widths, excitation-density-dependent measurements reveal that many-body interactions alter the homogeneous dephasing, while disorder-induced dephasing is unchanged.
Takagi, Hidetsugu; Kunugita, Hideyuki; Ema, Kazuhiro; Sato, Mikio; Takeoka, Yuko
2013-12-04
We have investigated experimentally excitonic properties in organic-inorganic hybrid multi quantum well crystals, (C{sub 4}H{sub 9}NH{sub 3}){sub 2}PbBr{sub 4} and (C{sub 6}H{sub 5}âˆ’C{sub 2}H{sub 4}NH{sub 3}){sub 2}PbBr{sub 4}, by measuring photoluminescence, reflectance, photoluminescence excitation spectra. In these materials, the excitonic binding energies are enhanced not only by quantum confinement effect (QCE) but also by image charge effect (ICE), since the dielectric constant of the barrier layers is much smaller than that of the well layers. By comparing the 1s-exciton and 2s-exciton energies, we have investigated the influence of ICE with regard to the difference of the Bohr radius.
NASA Astrophysics Data System (ADS)
Takagi, Hidetsugu; Sato, Mikio; Takeoka, Yuko; Kunugita, Hideyuki; Ema, Kazuhiro
2013-12-01
We have investigated experimentally excitonic properties in organic-inorganic hybrid multi quantum well crystals, (C4H9NH3)2PbBr4 and (C6H5-C2H4NH3)2PbBr4, by measuring photoluminescence, reflectance, photoluminescence excitation spectra. In these materials, the excitonic binding energies are enhanced not only by quantum confinement effect (QCE) but also by image charge effect (ICE), since the dielectric constant of the barrier layers is much smaller than that of the well layers. By comparing the 1s-exciton and 2s-exciton energies, we have investigated the influence of ICE with regard to the difference of the Bohr radius.
Nondispersing Bohr Wave Packets
Maeda, H.; Gurian, J. H.; Gallagher, T. F.
2009-03-13
Long-lived, nondispersing circular, or Bohr, wave packets are produced starting from Li Rydberg atoms by exposing them first to a linearly polarized microwave field at the orbital frequency, 17.6 GHz at principal quantum number n=72, which locks the electron's motion into an approximately linear orbit in which the electron oscillates in phase with the microwave field. The microwave polarization is changed to circular polarization slowly compared to the orbital frequency, and the electron's motion follows, resulting in a nondispersing Bohr wave packet.
Nondispersing Bohr Wave Packets
NASA Astrophysics Data System (ADS)
Maeda, H.; Gurian, J. H.; Gallagher, T. F.
2009-03-01
Long-lived, nondispersing circular, or Bohr, wave packets are produced starting from Li Rydberg atoms by exposing them first to a linearly polarized microwave field at the orbital frequency, 17.6 GHz at principal quantum number n=72, which locks the electronâ€™s motion into an approximately linear orbit in which the electron oscillates in phase with the microwave field. The microwave polarization is changed to circular polarization slowly compared to the orbital frequency, and the electronâ€™s motion follows, resulting in a nondispersing Bohr wave packet.
NASA Astrophysics Data System (ADS)
Bellac, Michel Le
2014-11-01
The final form of quantum physics, in the particular case of wave mechanics, was established in the years 1925-1927 by Heisenberg, Schrödinger, Born and others, but the synthesis was the work of Bohr who gave an epistemological interpretation of all the technicalities built up over those years; this interpretation will be examined briefly in Chapter 10. Although Einstein acknowledged the success of quantum mechanics in atomic, molecular and solid state physics, he disagreed deeply with Bohr's interpretation. For many years, he tried to find flaws in the formulation of quantum theory as it had been more or less accepted by a large majority of physicists, but his objections were brushed away by Bohr. However, in an article published in 1935 with Podolsky and Rosen, universally known under the acronym EPR, Einstein thought he had identified a difficulty in the by then standard interpretation. Bohr's obscure, and in part beyond the point, answer showed that Einstein had hit a sensitive target. Nevertheless, until 1964, the so-called Bohr-Einstein debate stayed uniquely on a philosophical level, and it was actually forgotten by most physicists, as the few of them aware of it thought it had no practical implication. In 1964, the Northern Irish physicist John Bell realized that the assumptions contained in the EPR article could be tested experimentally. These assumptions led to inequalities, the Bell inequalities, which were in contradiction with quantum mechanical predictions: as we shall see later on, it is extremely likely that the assumptions of the EPR article are not consistent with experiment, which, on the contrary, vindicates the predictions of quantum physics. In Section 3.2, the origin of Bell's inequalities will be explained with an intuitive example, then they will be compared with the predictions of quantum theory in Section 3.3, and finally their experimental status will be reviewed in Section 3.4. The debate between Bohr and Einstein goes much beyond a simple controversy, which is after all almost eighty years old and has been settled today. In fact, the concept introduced in this debate, that of entanglement, lies at the heart of many very important developments of modern quantum physics, in particular all those linked to quantum information (Chapter 8). Moreover, we shall see that the phenomenon of non-local correlations compels us to revise in depth our space-time representation of quantum processes. These are the two reasons why a whole chapter is devoted to this debate.
The methodological lesson of complementarity: Bohr’s naturalistic epistemology
NASA Astrophysics Data System (ADS)
Folse, H. J.
2014-12-01
Bohr’s intellectual journey began with the recognition that empirical phenomena implied the breakdown of classical mechanics in the atomic domain; this, in turn, led to his adoption of the ‘quantum postulate’ that justifies the ‘stationary states’ of his atomic model of 1913. His endeavor to develop a wider conceptual framework harmonizing both classical and quantum descriptions led to his proposal of the new methodological goals and standards of complementarity. Bohr’s claim that an empirical discovery can demand methodological revision justifies regarding his epistemological lesson as supporting a naturalistic epistemology.
NASA Astrophysics Data System (ADS)
Dotson, Allen
2013-07-01
Jon Cartwright's interesting and informative article on quantum philosophy ("The life of psi", May pp26â€“31) mischaracterizes Niels Bohr's stance as anti-realist by suggesting (in the illustration on p29) that Bohr believed that "quantum theory [does not] describe an objective reality, independent of the observer".
Exciton-exciton annihilation in organic polariton microcavities
Akselrod, G. M.; Tischler, Jonathan R.; Young, E. R.; Nocera, D.G.; Bulovic, Vladimir
2010-09-27
We investigate the incoherent diffusion of excitons in thin films (5.1±0.1 nm thick) of a highly absorbing J-aggregated cyanine dye material (10^{6} cm^{-1} absorption constant) as the excitonic component of a polariton microcavity. Under high-intensity pulsed laser excitation, the J-aggregated molecular films exhibit significant exciton-exciton annihilation, indicating a large exciton diffusion radius of more than 100 nm. When the material is strongly coupled to a cavity, the polaritonic structure also shows exciton-exciton annihilation, which is a competing process against the establishment of a threshold population of polaritons needed for polariton lasing. This study suggests that exciton-exciton annihilation is a loss process which can significantly increase the lasing threshold in polariton microcavities.
ERIC Educational Resources Information Center
Brunori, Maurizio
2012-01-01
Before the outbreak of World War II, Jeffries Wyman postulated that the "Bohr effect" in hemoglobin demanded the oxygen linked dissociation of the imidazole of two histidines of the polypeptide. This proposal emerged from a rigorous analysis of the acid-base titration curves of oxy- and deoxy-hemoglobin, at a time when the information on the…
A Simple Relativistic Bohr Atom
ERIC Educational Resources Information Center
Terzis, Andreas F.
2008-01-01
A simple concise relativistic modification of the standard Bohr model for hydrogen-like atoms with circular orbits is presented. As the derivation requires basic knowledge of classical and relativistic mechanics, it can be taught in standard courses in modern physics and introductory quantum mechanics. In addition, it can be shown in a class that…
A Simple Relativistic Bohr Atom
ERIC Educational Resources Information Center
Terzis, Andreas F.
2008-01-01
A simple concise relativistic modification of the standard Bohr model for hydrogen-like atoms with circular orbits is presented. As the derivation requires basic knowledge of classical and relativistic mechanics, it can be taught in standard courses in modern physics and introductory quantum mechanics. In addition, it can be shown in a class thatâ€¦
ERIC Educational Resources Information Center
Brunori, Maurizio
2012-01-01
Before the outbreak of World War II, Jeffries Wyman postulated that the "Bohr effect" in hemoglobin demanded the oxygen linked dissociation of the imidazole of two histidines of the polypeptide. This proposal emerged from a rigorous analysis of the acid-base titration curves of oxy- and deoxy-hemoglobin, at a time when the information on theâ€¦
Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films
NASA Astrophysics Data System (ADS)
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong-Wei; Cao, Linyou
2015-11-01
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5â€“7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5â€“7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.
Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; Van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong-Wei; Cao, Linyou
2015-01-01
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5â€“7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5â€“7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters. PMID:26598075
Exciton-dominated dielectric function of atomically thin MoS_{2} films
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; Van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong -Wei; Cao, Linyou
2015-11-24
We systematically measure the dielectric function of atomically thin MoS_{2} films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5â€“7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5â€“7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS_{2} films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. Lastly, the knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS_{2} films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.
Exciton-dominated dielectric function of atomically thin MoS2 films
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; Van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong -Wei; et al
2015-11-24
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5â€“7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5â€“7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function maymoreÂ Â» dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. Lastly, the knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.Â«Â less
Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films.
Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E; Van de Walle, Chris G; Nguyen, Nhan V; Zhang, Yong-Wei; Cao, Linyou
2015-01-01
We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters. PMID:26598075
Corda, Christian
2015-03-10
The idea that black holes (BHs) result in highly excited states representing both the â€œhydrogen atomâ€ and the â€œquasi-thermal emissionâ€ in quantum gravity is today an intuitive but general conviction. In this paper it will be shown that such an intuitive picture is more than a picture. In fact, we will discuss a model of quantum BH somewhat similar to the historical semi-classical model of the structure of a hydrogen atom introduced by Bohr in 1913. The model is completely consistent with existing results in the literature, starting from the celebrated result of Bekenstein on the area quantization.
The Bohr effect before Perutz.
Brunori, Maurizio
2012-01-01
Before the outbreak of World War II, Jeffries Wyman postulated that the Bohr effect in hemoglobin demanded the oxygen linked dissociation of the imidazole of two histidines of the polypeptide. This proposal emerged from a rigorous analysis of the acid-base titration curves of oxy- and deoxy-hemoglobin, at a time when the information on the chemistry and structure of the protein was essentially nil. The magnetochemical properties of hemoglobin led Linus Pauling to hypothesize that the (so called) Bohr histidines were coordinated to the heme iron in the fifth and sixth positions; and Wyman shared this opinion. However, this structural hypothesis was abandoned in 1951 when J. Wyman and D. W. Allen proposed the pK shift of the oxygen linked histidines to be the result of "...a change of configuration of the hemoglobin molecule as a whole accompanying oxygenation." This shift in paradigm, that was published well before the 3D structure of hemoglobin was solved by M.F. Perutz, paved the way to the concept of allostery. After 1960 the availability of the crystallographic structure opened new horizons to the interpretation of the allosteric properties of hemoglobin. PMID:22987550
What classicality? Decoherence and Bohr's classical concepts
NASA Astrophysics Data System (ADS)
Schlosshauer, Maximilian; Camilleri, Kristian
2011-03-01
Niels Bohr famously insisted on the indispensability of what he termed "classical concepts." In the context of the decoherence program, on the other hand, it has become fashionable to talk about the "dynamical emergence of classicality" from the quantum formalism alone. Does this mean that decoherence challenges Bohr's dictumâ€”for example, that classical concepts do not need to be assumed but can be derived? In this paper we'll try to shed some light down the murky waters where formalism and philosophy cohabitate. To begin, we'll clarify the notion of classicality in the decoherence description. We'll then discuss Bohr's and Heisenberg's take on the quantumâ€”classical problem and reflect on different meanings of the terms "classicality" and "classical concepts" in the writings of Bohr and his followers. This analysis will allow us to put forward some tentative suggestions for how we may better understand the relation between decoherence-induced classicality and Bohr's classical concepts.
Bohr model as an algebraic collective model
Rowe, D. J.; Welsh, T. A.; Caprio, M. A.
2009-05-15
Developments and applications are presented of an algebraic version of Bohr's collective model. Illustrative examples show that fully converged calculations can be performed quickly and easily for a large range of Hamiltonians. As a result, the Bohr model becomes an effective tool in the analysis of experimental data. The examples are chosen both to confirm the reliability of the algebraic collective model and to show the diversity of results that can be obtained by its use. The focus of the paper is to facilitate identification of the limitations of the Bohr model with a view to developing more realistic, computationally tractable models.
Excitonic effects in GeC hybrid: Many-body Green's function calculations
NASA Astrophysics Data System (ADS)
Drissi, L. B.; Ramadan, F. Z.
2015-11-01
Many-body effects on the electronic and optical absorption properties of a GeC sheet are studied by means of first principle many-body Green's function and Bethe-Salpeter equation formalism. The absence of soft modes in the phonon-spectrum indicates the stability of the system. The inclusion of quasiparticle corrections increases significantly the band gap. The local field effects induce significant change in the absorption spectra for the out-plane polarization rendering the GeC monolayer transparent below 7 eV. The excitonic effects are significant on the optical absorption properties. A detailed analysis of the spectrum shows a strong binding energy of 1.82 eV assigned to the lowest-energy bound excitons that is characterized by an effective mass of 1.68 m0 and a Bohr radius of 2 Ã…. The results of this study hold the promise for potential applications of the GeC hybrid in optoelectronics.
NASA Astrophysics Data System (ADS)
Young, Eliot F.; Young, L. A.; Buie, M.
2007-10-01
The size of Pluto has been difficult to measure. Stellar occultations by Pluto have not yet probed altitudes lower than 1198 km, assuming the clear atmosphere model of Elliot, Person and Qu (2003). Differential refraction by Pluto's atmosphere attenuates the light from an occulted star to a level that is indistinguishable from the zero-level baseline long before Pluto's solid surface is a factor. Since Charon has no detectable atmosphere, its radius was well determined from a stellar occultation in 2005 (Gulbis et al. 2006, Sicardy et al. 2006). Combined with the mutual event photometry (Charon transited Pluto every 6.38 days between 1986 through 1992) - for which differential refraction is a negligible effect - the well-known radius of Charon translates into a more accurate radius for Pluto's solid surface. Our preliminary solid radius estimate for Pluto is 1161 km. We will discuss error bars and the correlations of this determination with Pluto albedo maps. We will also discuss the implications for Pluto's thermal profile, surface temperature and pressure, and constraints on the presence of a haze layer. This work is funded by NASA's Planetary Astronomy program. References Elliot, J.L., Person, M.J., & Qu, S. 2003, "Analysis of Stellar Occultation Data. II. Inversion, with Application to Pluto and Triton." AJ, 126, 1041. Gulbis, A.A.S. et al. 2006, "Charon's radius and atmospheric constraints from observations of a stellar occultation." Nature, 49, 48. Sicardy, B. et al. 2006, "Charon's size and an upper limit on its atmosphere from a stellar occultation." Nature, 49, 52.
The Influence of Bohr on Delbruck
NASA Astrophysics Data System (ADS)
Holladay, Wendell
2000-11-01
The book by Robert Lagemann on the history of physics and astronomy at Vanderbilt University contains a chapter on Max Delbruck, a member of the Vanderbilt physics department from 1940 - 1947, where he did seminal work in establishing microbial genetics, for which he received the Nobel prize in physiology in 1969. Delbruck, a Ph.D. in physics for work with Max Born in Gottingen, had been inspired by Niels Bohr's suggestion of a complementary relation between biology and atomic physics to work in biology. We will explore exactly what Bohr said in this connection and argue that Delbruck's own work leads to a conclusion in opposition to Bohr's suggestion, namely that the existence of life is reducible to molecular physics, through the remarkable properties of DNA. The lesson for scientific methodology to be learned from this example is that science can lead to truth even if motivated by an ideology pushing in the opposite direction.
Quantum Monte Carlo treatment of elastic exciton-exciton scattering
NASA Astrophysics Data System (ADS)
Shumway, J.; Ceperley, D. M.
2001-04-01
We calculate cross sections for low energy, elastic, bulk, s-wave exciton-exciton scattering within the single-band effective mass approximation. Unlike previous theoretical approaches, we give a complete, nonperturbative treatment of the four-particle scattering problem. Diffusion Monte Carlo is used to calculate the essentially exact energies of scattering states, from which phase shifts are determined. For the case of equal-mass electrons and holes, which is equivalent to positronium-positronium scattering, we find as=2.1ax for scattering of singlet excitons and as=1.5ax for triplet excitons, where ax is the excitonic radius. The spin dependence of the cross sections arises from the spatial exchange symmetry of the scattering wave functions. A significant triplet-triplet to singlet-singlet scattering process is found, which is similar to the reported effects in recent experiments and theory for excitons in quantum wells. We also show that the scattering length can change sign and diverge for some values of the mass ratio mh/me, an effect not seen in previous perturbative treatments.
Energy Transfer of Excitons Between Quantum Wells Separated by a Wide Barrier
LYO,SUNGKWUN K.
1999-12-06
We present a microscopic theory of the excitonic Stokes and anti-Stokes energy transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Exciton transfer through dipolar coupling, photon-exchange coupling and over-barrier ionization of the excitons through exciton-exciton Auger processes are examined. The energy transfer rate is calculated as a function of T and the center-to-center distance d between the two wells. The rates depend sensitively on T for plane-wave excitons. For located excitons, the rates depend on T only through the T-dependence of the localization radius.
Bohr-Sommerfeld quantization of pseudospin Hamiltonians
Shankar, R.
1980-09-29
It is shown here how to map the problem with pseudospin J into an equivalent one in which 1/J plays the role of h and canonical variables exist at the classical level. Bohr-Sommerfeld quantization of the equivalent theory is found to produce a spectrum in very good agreement with the exact results for the Lipkin-Meshkov-Glick model at J=15 and 25. The method readily extends to the SU(n) case.
Niels Bohr and the Third Quantum Revolution
NASA Astrophysics Data System (ADS)
Scharff Goldhaber, Alfred
2013-04-01
In the history of science few developments can rival the discovery of quantum mechanics, with its series of abrupt leaps in unexpected directions stretching over a quarter century. The result was a new world, even more strange than any previously imagined subterranean (or in this case submicroscopic) kingdom. Niels Bohr made the third of these leaps (following Planck and Einstein) when he realized that still-new quantum ideas were essential to account for atomic structure: Rutherford had deduced, using entirely classical-physics principles, that the positive charge in an atom is contained in a very small kernel or nucleus. This made the atom an analogue to the solar system. Classical physics implied that negatively charged electrons losing energy to electromagnetic radiation would ``dive in'' to the nucleus in a very short time. The chemistry of such tiny atoms would be trivial, and the sizes of solids made from these atoms would be much too small. Bohr initially got out of this dilemma by postulating that the angular momentum of an electron orbiting about the nucleus is quantized in integer multiples of the reduced quantum constant = h/2Ï€. Solving for the energy of such an orbit in equilibrium immediately produces the famous Balmer formula for the frequencies of visible light radiated from hydrogen as an electron jumps from any particular orbit to another of lower energy. There remained mysteries requiring explanation or at least exploration, including two to be discussed here: 1. Rutherford used classical mechanics to compute the trajectory and hence the scattering angle of an Î± particle impinging on a small positively charged target. How could this be consistent with Bohr's quantization of particle orbits about the nucleus? 2. Bohr excluded for his integer multiples of the value 0. How can one justify this exclusion, necessary to bar tiny atoms of the type mentioned earlier?
Niels Bohr and the Third Quantum Revolution
NASA Astrophysics Data System (ADS)
Goldhaber, Alfred
2013-04-01
In the history of science few developments can rival the discovery of quantum mechanics, with its series of abrupt leaps in unexpected directions stretching over a quarter century. The result was a new world, even more strange than any previously imagined subterranean (or in this case submicroscopic) kingdom. Niels Bohr made the third of these leaps (following Planck and Einstein) when he realized that still-new quantum ideas were essential to account for atomic structure: Rutherford had deduced, using entirely classical-physics principles, that the positive charge in an atom is contained in a very small kernel or nucleus. This made the atom an analogue to the solar system. Classical physics implied that negatively charged electrons losing energy to electromagnetic radiation would ``dive in'' to the nucleus in a very short time. The chemistry of such tiny atoms would be trivial, and the sizes of solids made from these atoms would be much too small. Bohr initially got out of this dilemma by postulating that the angular momentum of an electron orbiting about the nucleus is quantized in integer multiples of the reduced quantum constant â„ = h/2 Ï€. Solving for the energy of such an orbit in equilibrium immediately produces the famous Balmer formula for the frequencies of visible light radiated from hydrogen as an electron jumps from any particular orbit to another of lower energy. There remained mysteries requiring explanation or at least exploration, including two to be discussed here: 1. Rutherford used classical mechanics to compute the trajectory and hence the scattering angle of an Î± particle impinging on a small positively charged target. How could this be consistent with Bohr's quantization of particle orbits about the nucleus? 2. Bohr excluded for his integer multiples of â„ the value 0. How can one justify this exclusion, necessary to bar tiny atoms of the type mentioned earlier?
Analytical method for determining quantum well exciton properties in a magnetic field
NASA Astrophysics Data System (ADS)
Stépnicki, Piotr; Piétka, Barbara; Morier-Genoud, François; Deveaud, Benoît; Matuszewski, Micha?
2015-05-01
We develop an analytical approximate method for determining the Bohr radii of Wannier-Mott excitons in thin quantum wells under the influence of magnetic field perpendicular to the quantum well plane. Our hybrid variational-perturbative method allows us to obtain simple closed formulas for exciton binding energies and optical transition rates. We confirm the reliability of our method through exciton-polariton experiments realized in a GaAs/AlAs microcavity with an 8 nm InxGa1 -xAs quantum well and magnetic field strengths as high as 14 T.
Realization of Localized Bohr-like Wavepackets
Mestayer, J. J.; Wyker, B.; Lancaster, J. C.; Dunning, F. B.; Reinhold, Carlos O; Yoshida, S.; Burgdorfer, J.
2008-01-01
We demonstrate a protocol to create localized wavepackets in very-high-n Rydberg states which travel in nearly circular orbits around the nucleus. Although these wavepackets slowly dephase and eventually lose their localization, their motion can be monitored over several orbital periods. These wavepackets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in circular classical orbit around the nucleus. Possible extension of the approach to create so far elusive "planetary atoms" in highly correlated stable multiply-excited states is discussed.
Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.
1980-01-01
The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)
Bohr's Creation of his Quantum Atom
NASA Astrophysics Data System (ADS)
Heilbron, John
2013-04-01
Fresh letters throw new light on the content and state of Bohr's mind before and during his creation of the quantum atom. His mental furniture then included the atomic models of the English school, the quantum puzzles of Continental theorists, and the results of his own studies of the electron theory of metals. It also included the poetry of Goethe, plays of Ibsen and Shakespeare, novels of Dickens, and rhapsodies of Kierkegaard and Carlyle. The mind that held these diverse ingredients together oscillated between enthusiasm and dejection during the year in which Bohr took up the problem of atomic structure. He spent most of that year in England, which separated him for extended periods from his close-knit family and friends. Correspondence with his fianc'ee, Margrethe Nørlund, soon to be published, reports his ups and downs as he adjusted to J.J. Thomson, Ernest Rutherford, the English language, and the uneven course of his work. In helping to smooth out his moods, Margrethe played an important and perhaps an enabling role in his creative process.
Electron-hole interaction in the presence of excitons
NASA Astrophysics Data System (ADS)
Collet, J. H.; Amand, T.
1984-10-01
We calculate the effective electron-hole interaction V re in the presence of an exciton gas, which reads in real space: V re(r)= {-e 2}/{r}{1+ limitâˆ‘i=14(-1) iC iexp (-Z i{r}/{a}} The parameters Ci and Zi are given explicitly for GaAs. For this material, we show the binding energy of the exciton is weakly modified so long as {8Ï€R 0Ï± exa 03}/{kTâ©½1}. ( R0, exciton Rydberg, a0 exciyon radius, Ï± ex exciton density, T temperature).
Davidson potential and SUSYQM in the Bohr Hamiltonian
Georgoudis, P. E.
2013-06-10
The Bohr Hamiltonian is modified through the Shape Invariance principle of SUper-SYmmetric Quantum Mechanics for the Davidson potential. The modification is equivalent to a conformal transformation of Bohr's metric, generating a different {beta}-dependence of the moments of inertia.
Frenkel-like Wannier-Mott excitons in few-layer Pb I2
NASA Astrophysics Data System (ADS)
Toulouse, Alexis S.; Isaacoff, Benjamin P.; Shi, Guangsha; Matuchová, Marie; Kioupakis, Emmanouil; Merlin, Roberto
2015-04-01
Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer Pb I2 indicate that the n =1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases in the gap and exciton binding energy with a decreasing number of layers and a transition of the fundamental gap, which becomes indirect for one and two monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.
Resisting the Bohr Atom: The Early British Opposition
NASA Astrophysics Data System (ADS)
Kragh, Helge
2011-03-01
When Niels Bohr's theory of atomic structure appeared in the summer and fall of 1913, it quickly attracted attention among British physicists. While some of the attention was supportive, others was critical. I consider the opposition to Bohr's theory from 1913 to about 1915, including attempts to construct atomic theories on a classical basis as alternatives to Bohr's. I give particular attention to the astrophysicist John W. Nicholson, who was Bohr's most formidable and persistent opponent in the early years. Although in the long run Nicholson's objections were inconsequential, for a short period of time his atomic theory was considered to be a serious rival to Bohr's. Moreover, Nicholson's theory is of interest in its own right.
Bohr effect of hemoglobins: Accounting for differences in magnitude.
Okonjo, Kehinde O
2015-09-01
The basis of the difference in the Bohr effect of various hemoglobins has remained enigmatic for decades. Fourteen amino acid residues, identical in pairs and located at specific 'Bohr group positions' in human hemoglobin, are implicated in the Bohr effect. All 14 are present in mouse, 11 in dog, eight in pigeon and 13 in guinea pig hemoglobin. The Bohr data for human and mouse hemoglobin are identical: the 14 Bohr groups appear at identical positions in both molecules. The dog data are different from the human because three Bohr group positions are occupied by non-ionizable groups in dog hemoglobin; the pigeon data are vastly different from the human because six Bohr group positions are occupied by non-ionizable groups in pigeon hemoglobin. The guinea pig data are quite complex. Quantitative analyses showed that only the pigeon data could be fitted with the Wyman equation for the Bohr effect. We demonstrate that, apart from guinea pig hemoglobin, the difference between the Bohr effect of each of the other hemoglobins and of pigeon hemoglobin can be accounted for quantitatively on the basis of the occupation of some of their Bohr group positions by non-ionizable groups in pigeon hemoglobin. We attribute the anomalous guinea pig result to a new salt-bridge formed in its R2 quaternary structure between the terminal NH3(+) group of one Î²-chain and the COO(-) terminal group of the partner Î²-chain in the same molecule. The pKas of this NH3(+) group are 6.33 in the R2 and 4.59 in the T state. PMID:26102019
Realization of Bohr-like circular wavepackets
NASA Astrophysics Data System (ADS)
Mestayer, Jeff; Wyker, B.; Dunning, F. B.; Reinhold, C.; Yoshida, S.; BurgdÃ¶rfer, J.
2008-05-01
We demonstrate a protocol to create localized wavepackets in very-high n Rydberg states that travel in near-circular orbits around the nucleus. Although these wavepackets slowly dephase and eventually lose their localization, their motion can be followed for several orbital periods. These wavepackets represent the closest analog yet achieved to the original Bohr model of the atom, i.e., an electron in circular classical orbit around the nucleus. The time evolution of the momentum and position of the wavepackets is monitored using, respectively, short half-cycle pulses and sudden field steps, which are applied after a variable time delay and along different directions. The present technique is explained with the aid of CTMC simulations, and possible extension to creation of elusive ``planetary atoms'' in highly correlated stable multiply-excited states is discussed. Research supported by the NSF, the Robert A. Welch Foundation, the OBES, U.S. DoE to ORNL, and by the FWF (Austria).
Wiseman, Howard M.
2013-11-15
By rigorously formalizing the Einstein–Podolsky–Rosen (EPR) argument, and Bohr’s reply, one can appreciate that both arguments were technically correct. Their opposed conclusions about the completeness of quantum mechanics hinged upon an explicit difference in their criteria for when a measurement on Alice’s system can be regarded as not disturbing Bob’s system. The EPR criteria allow their conclusion–incompleteness–to be reached by establishing the physical reality of just a single observable q (not of both q and its conjugate observable p), but I show that Bohr’s definition of disturbance prevents the EPR chain of reasoning from establishing even this. Moreover, I show that Bohr’s definition is intimately related to the asymmetric concept of quantum discord from quantum information theory: if and only if the joint state has no Alice-discord, she can measure any observable without disturbing (in Bohr’s sense) Bob’s system. Discord can be present even when systems are unentangled, and this has implications for our understanding of the historical development of notions of quantum nonlocality. -- Highlights: •Both the EPR argument, and Bohr’s reply, were technically correct. •Their opposed conclusions came from different criteria for disturbance. •Bohr’s criterion works against even the simplified (one-variable) EPR argument. •Bohr’s criterion for disturbance is intimately related to quantum discord. •This illuminates the historical development of notions of quantum nonlocality.
Excitons in nanoscale systems.
Scholes, Gregory D; Rumbles, Garry
2006-09-01
Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes. PMID:16946728
NASA Astrophysics Data System (ADS)
Scholes, Gregory D.; Rumbles, Garry
2006-09-01
Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.
NASA Astrophysics Data System (ADS)
Keldysh, L. V.
1992-10-01
A model which allows to consider self-consistently many-body exciton effects, including biexciton formation, is proposed for the theoretical description of an excitonic polarization induced by intense electromagnetic field. In the presence of a pump the normal electromagnetic wave spectrum in the vicinity of the nonlinear biexciton resonance is shown to be composed by four polariton branches, each being as a matter of fact the superposition of two photons and two excitons. In the self-consistent field approximation the structure of the interconnected strongly nonlinear exciton and biexciton resonances is studied, including optical Stark shift, multistable behaviour, etc. For a certain range of the exciton interaction parameters at strong enough pump wave the system of excitons and excitonic molecules transforms abruptly to a dense coherent electron-hole "liquid". Appearance and temporary evolution of an exciton-biexciton system after the switching on of a pump is also considered.
Paul Ehrenfest, Niels Bohr, and Albert Einstein: Colleagues and Friends
NASA Astrophysics Data System (ADS)
Klein, Martin J.
2010-09-01
In May 1918 Paul Ehrenfest received a monograph from Niels Bohr in which Bohr had used Ehrenfest's adiabatic principle as an essential assumption for understanding atomic structure. Ehrenfest responded by inviting Bohr, whom he had never met, to give a talk at a meeting in Leiden in late April 1919, which Bohr accepted; he lived with Ehrenfest, his mathematician wife Tatyana, and their young family for two weeks. Albert Einstein was unable to attend this meeting, but in October 1919 he visited his old friend Ehrenfest and his family in Leiden, where Ehrenfest told him how much he had enjoyed and profited from Bohr's visit. Einstein first met Bohr when Bohr gave a lecture in Berlin at the end of April 1920, and the two immediately proclaimed unbounded admiration for each other as physicists and as human beings. Ehrenfest hoped that he and they would meet at the Third Solvay Conference in Brussels in early April 1921, but his hope was unfulfilled. Einstein, the only physicist from Germany who was invited to it in this bitter postwar atmosphere, decided instead to accompany Chaim Weizmann on a trip to the United States to help raise money for the new Hebrew University in Jerusalem. Bohr became so overworked with the planning and construction of his new Institute for Theoretical Physics in Copenhagen that he could only draft the first part of his Solvay report and ask Ehrenfest to present it, which Ehrenfest agreed to do following the presentation of his own report. After recovering his strength, Bohr invited Ehrenfest to give a lecture in Copenhagen that fall, and Ehrenfest, battling his deep-seated self-doubts, spent three weeks in Copenhagen in December 1921 accompanied by his daughter Tanya and her future husband, the two Ehrenfests staying with the Bohrs in their apartment in Bohr's new Institute for Theoretical Physics. Immediately after leaving Copenhagen, Ehrenfest wrote to Einstein, telling him once again that Bohr was a prodigious physicist, and again expressing the hope that he soon would see both of them in Leiden.
Relativistic Corrections to the Bohr Model of the Atom
ERIC Educational Resources Information Center
Kraft, David W.
1974-01-01
Presents a simple means for extending the Bohr model to include relativistic corrections using a derivation similar to that for the non-relativistic case, except that the relativistic expressions for mass and kinetic energy are employed. (Author/GS)
Stacking in colloidal nanoplatelets: tuning excitonic properties.
Guzelturk, Burak; Erdem, Onur; Olutas, Murat; Kelestemur, Yusuf; Demir, Hilmi Volkan
2014-12-23
Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient Förster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Förster radius of ?13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening. PMID:25469555
Hemoglobin Bohr effects: atomic origin of the histidine residue contributions.
Zheng, Guishan; Schaefer, Michael; Karplus, Martin
2013-11-26
The Bohr effect in hemoglobin, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiological function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of His residues of the unliganded (deoxy) and liganded (carbonmonoxy) structures. Using recent high resolution structures, the residue pKa values corresponding to the two structures are calculated. The method is based on determining the electrostatic interactions between residues in the protein, relative to those of the residue in solution, by use of the linearized finite difference Poisson-Boltzmann equation and Monte Carlo sampling of protonation states. Given that good agreement is obtained with the available experimental values for the contribution of His residues in HbA to the Bohr effect, the calculated results are used to determine the atomic origin of the pKa shift between deoxy and carbonmonoxy HbA. The contributions to the pKa shift calculated by means of the linear response approximation show that the salt bridge involving His146 plays an important role in the alkaline Bohr effect, as suggested by Perutz but that other interactions are significant as well. A corresponding analysis is made for the contribution of His143 to the acid Bohr effect for which there is no proposed explanation. The method used is summarized and the program by which it is implemented is described in the Appendix . PMID:24224786
Gravitational Bohr's model with Newton's and Weber's potentials
NASA Astrophysics Data System (ADS)
Tiandho, Yuant; Triyanta
2015-09-01
Similarity between Newtonian gravitational force and Coulomb's force leads one to define gravitational electric and magnetic fields or gravito-electromagnetic fields. Also, one may define a gravitational Bohr's theory in analogy with the electromagnetic Bohr's theory for which the latter is quite successful in explaining some properties of hydrogen atom. Unlike to the Coulomb's force, the Newton`s force is believed to be an approximated theory of gravity as it is unable to explain the Mercury's orbit. This paper discusses the gravitational Bohr's theory by taking into account the Newton's potential and Weber's potential; the later is a modified Newton's gravitational potential that conforms, to some degrees, to the anomaly of the Mercury's orbit.
Radiative lifetimes of excitons and trions in monolayers of the metal dichalcogenide MoS2
NASA Astrophysics Data System (ADS)
Wang, Haining; Zhang, Changjian; Chan, Weimin; Manolatou, Christina; Tiwari, Sandip; Rana, Farhan
2016-01-01
We present results on the radiative lifetimes of excitons and trions in a monolayer of metal dichalcogenide MoS2. The small exciton radius and the large exciton optical oscillator strength result in radiative lifetimes in the 0.18-0.30 ps range for excitons that have small in-plane momenta and couple to radiation. Average lifetimes of thermally distributed excitons depend linearly on the exciton temperature and can be in the few picoseconds range at small temperatures and more than a nanosecond near room temperature. Localized excitons exhibit lifetimes in the same range and the lifetime increases as the localization length decreases. The radiative lifetimes of trions are in the hundreds of picosecond range and increase with the increase in the trion momentum. Average lifetimes of thermally distributed trions increase with the trion temperature as the trions acquire thermal energy and larger momenta. We expect our theoretical results to be applicable to most other 2D transition metal dichalcogenides.
Steering quantum states toward classical Bohr-like orbits
NASA Astrophysics Data System (ADS)
Dunning, F. B.; Reinhold, C. O.; Yoshida, S.; BurgdÃ¶rfer, J.
2010-08-01
We describe how, almost 100 years after the introduction of the Bohr model of the atom, it is now possible using pulsed electric fields to create localized wavepackets in high n (n â‰ˆ300) Rydberg atoms that travel in near-circular Bohr-like orbits mimicking the behavior of a classical electron. The protocols employed are explained with the aid of quantum and classical dynamics. Although many aspects of the underlying behavior can be described using classical arguments, purely quantum effects such as revivals can be seen even for n â‰ˆ300.
NASA Astrophysics Data System (ADS)
Zhu, Xiaoyang
2011-03-01
Solar photovoltaics based on molecular and nano materials commonly involve excitons. This results from strong Coulomb attraction between an electron and a hole due to the low dielectric constants of molecules or quantum confinement of nano materials. In this lecture, I will address the question of how excitons dissociate at donor/acceptor interfaces. The first example deals with charge separation in organic photovoltaics. Due to the low dielectric constant of organic materials, an electron-hole pair across an organic donor/acceptor interface is bound by the Coulomb potential. This gives rise to a set of H-atom like states called charge-transfer excitons, as observed experimentally. The lowest energy charge transfer exciton state has a binding energy much higher than kT at room temperature. This leads to the conclusion that hot charge transfer exciton states must be involved in charge separation in organic photovoltaics. The second example deals with hot exciton dissociation due to electron transfer from photo-excited semiconductor nanocrystals (PbSe) to an electron acceptor (TiO2), an issue of particular interest to hot carrier solar cells with theoretical solar conversion efficiency surpassing the Shockley-Queisser limit. We show that, with appropriate chemical treatment of the nanocrystal surface, ultrafast transfer of a hot electron can be competitive with hot exciton relaxation due to phonon scattering. The last example will show recent development on hot carrier scattering and multiple exciton generation (MEG) in semiconductor nanorystals.
NASA Technical Reports Server (NTRS)
Harris, Michael R.
1987-01-01
Resonator configurations permitting operation with large mode radius while maintaining good transverse mode discrimination are considered. Stable resonators incorporating an intracavity telescope and unstable resonator geometries utilizing an output coupler with a Gaussian reflectivity profile are shown to enable large radius single mode laser operation. Results of heterodyne studies of pulsed CO2 lasers with large (11mm e sup-2 radius) fundamental mode sizes are presented demonstrating minimal frequency sweeping in accordance with the theory of laser-induced medium perturbations.
Optically programmable excitonic traps
Alloing, Mathieu; LemaÃ®tre, Aristide; Galopin, Elisabeth; Dubin, FranÃ§ois
2013-01-01
With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits. PMID:23546532
Neutral and positively charged excitons in narrow quantum ring
NASA Astrophysics Data System (ADS)
Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D.
2014-05-01
We study theoretically quantized states of a neutral and a positively charged exciton (trion X+) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.
Neutral and positively charged excitons in narrow quantum ring
Porras Monroy, L. C.; RodrÃguez-Prada, F. A.; Mikhailov, I. D.
2014-05-15
We study theoretically quantized states of a neutral and a positively charged exciton (trion X{sup +}) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.
Cation influence on exciton localization in homologue scheelites
NASA Astrophysics Data System (ADS)
Laasner, R.; Nagirnyi, V.; Vielhauer, S.; Kirm, M.; Spassky, D.; Sirutkaitis, V.; Grigonis, R.; Vasil'ev, A. N.
2015-09-01
Homologue scheelite crystals CaWO4, SrWO4, and BaWO4 possess similar crystal and electronic structure, but their luminescence exhibits drastically different thermal stabilities. By measuring the temperature dependence of the decay time of the intrinsic luminescence and fitting it to a three level model, we have qualitatively shown the effective exciton radius to increase in the order CaWO4 \\to SrWO4 \\to BaWO4, which explains the differences in the thermal stability. The origin of the variation in the exciton radii is suggested to be related to differences in the excited state dynamics in these crystals. From the decay kinetics measured under conditions of high excitation density, the efficiency of dipole-dipole interaction between excitons is shown to grow with exciton delocalization.
Anomalous magnetization of a carbon nanotube as an excitonic insulator
NASA Astrophysics Data System (ADS)
Rontani, Massimo
2014-11-01
We show theoretically that an undoped carbon nanotube might be an excitonic insulator—the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone, and M. Bockrath, Science 323, 106 (2009), 10.1126/science.1165799]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.
Electronic properties of exciton and biexciton in a CdTe/ZnTe nano-heterostructure
NASA Astrophysics Data System (ADS)
Sujanah, P.; Peter, A. John
2015-06-01
Energy eigen values and the binding energies of exciton and biexciton in a CdTe/ZnTe quantum dot are studied with the geometrical confinement effect. The single exciton binding energy and the biexciton binding energy are calculated as a function of dot radius. Overlap integral of the exciton and the biexciton is found in the CdTe/ZnTe quantum dot. The electron and hole potentials are calculated from the Poisson equations. Self-consistent method is to compute the energy eigenvalues of the exciton and the biexciton. The Hartree potential is employed to obtain the Coulomb interaction energy.
Photovoltaics: Separating Multiple Excitons
Nozik, A. J.
2012-05-01
Scientists have demonstrated an efficient process for generating multiple excitons in adjacent silicon nanocrystals from a single high-energy photon. Their findings could prove useful for a wide range of photovoltaic applications.
Steering Quantum States Towards Classical Bohr-Like Orbits
Dunning, F. B.; Reinhold, Carlos O; Yoshida, S.; Burgdorfer, J.
2010-01-01
This article furnishes an introduction to the properties of time-dependent electronic wavefunctions in atoms and to physics at the interface between the quantum and classical worlds. We describe how, almost 100 years after the introduction of the Bohr model of the atom, it is now possible using pulsed electric fields to create in the laboratory localized wavepackets in high-n (n ~ 300) Rydberg atoms that travel in near-circular Bohr-like orbits mimicking the behavior of a classical electron. The control protocols employed are explained with the aid of quantum and classical dynamics. Remarkably, while many aspects of the underlying behavior can be described using classical arguments, even at n ~ 300 purely quantum effects such as revivals can be seen.
Analytical solutions of the Bohr Hamiltonian with the Morse potential
Boztosun, I.; Inci, I.; Bonatsos, D.
2008-04-15
Analytical solutions of the Bohr Hamiltonian are obtained in the {gamma}-unstable case, as well as in an exactly separable rotational case with {gamma}{approx_equal}0, called the exactly separable Morse (ES-M) solution. Closed expressions for the energy eigenvalues are obtained through the asymptotic iteration method (AIM), the effectiveness of which is demonstrated by solving the relevant Bohr equations for the Davidson and Kratzer potentials. All medium mass and heavy nuclei with known {beta}{sub 1} and {gamma}{sub 1} bandheads have been fitted by using the two-parameter {gamma}-unstable solution for transitional nuclei and the three-parameter ES-M for rotational ones. It is shown that bandheads and energy spacings within the bands are well reproduced for more than 50 nuclei in each case.
Adiabatic Bohr-Sommerfeld calculations for the hydrogenic Stark effect
Byrd, David; Mitchell, David
2004-12-01
A general prescription is presented for the semiclassical calculation of quantized energy levels of a class of parametric quantum systems. Starting with the Bohr-Sommerfeld quantization conditions of an exactly solvable system in parabolic coordinates, the calculation evolves the classical equations of motion while adiabatically introducing a nonzero parameter. We present accurate results for the quadratic Stark effect in addition to the linear Stark effect, an improvement over previous semiclassical studies.
Creation of nondispersive Bohr-like wave packets
NASA Astrophysics Data System (ADS)
Mestayer, J. J.; Wyker, B.; Dunning, F. B.; Yoshida, S.; Reinhold, C. O.; BurgdÃ¶rfer, J.
2009-03-01
We demonstrate the use of a periodic train of half-cycle pulses to maintain strongly-localized wave packets in very-high- n (Ã±300) Rydberg atoms that travel in near-circular orbits about the nucleus. This motion can be followed for hundreds of orbital periods and mimics the original Bohr model of the hydrogen atom which envisioned an electron in circular classical orbit about the nucleus.
Microscopic Uni-axial Bohr-Mottelson Rotational Model
Gulshani, P.
2010-08-04
A microscopic version of the phenomenological Bohr-Mottelson unified adiabatic rotational model is derived using only space-fixed particle coordinates, and without imposing any constraints on the particle coordinates or the intrinsic wavefunction. It is shown that this can done only for rigid flow. A collective-rotation velocity field is defined and is used to show that, although their Hamiltonians are closely related, the flows in a multi-fermion and single-particle system are inherently different.
Creation of Non-dispersive Bohr-like Wavepackets
Mestayer, J. J.; Wyker, B.; Dunning, F. B.; Yoshida, S.; Reinhold, Carlos O; Burgdorfer, J.
2009-01-01
We demonstrate the use of a periodic train of half-cycle pulses to maintain strongly-localized wavepackets in very-high-n (n~300) Rydberg atoms that travel in near circular orbits about the nucleus. This motion can be followed for hundreds of orbital periods and mimics the original Bohr model of the hydrogen atom which envisioned an electron in circular classical orbit about the nucleus.
Bohr-Sommerfeld Lagrangians of moduli spaces of Higgs bundles
NASA Astrophysics Data System (ADS)
Biswas, Indranil; Gammelgaard, Niels Leth; Logares, Marina
2015-08-01
Let X be a compact connected Riemann surface of genus at least two. Let MH(r, d) denote the moduli space of semistable Higgs bundles on X of rank r and degree d. We prove that the compact complex Bohr-Sommerfeld Lagrangians of MH(r, d) are precisely the irreducible components of the nilpotent cone in MH(r, d) . This generalizes to Higgs G-bundles and also to the parabolic Higgs bundles.
Jahan K, Luhluh Boda, Aalu; Chatterjee, Ashok
2015-05-15
The problem of an exciton trapped in a three dimensional Gaussian quantum dot is studied in the presence of an external magnetic field. A variational method is employed to obtain the ground state energy of the exciton as a function of the quantum dot size, the confinement strength and the magnetic field. It is also shown that the variation of the size of the exciton with the radius of the quantum dot.
Variable Radius Nacelle Studies
NASA Technical Reports Server (NTRS)
McGowan, David M.
2001-01-01
An overview of the active shape control for a variable radius nacelle leading edge program is presented. The current technical plan and schedule will be discussed. Results from the structural shape change of curved plates demonstration will be presented, as well as the NASA LaRC concept for a variable radius nacelle leading edge. Results of a Boeing systems integration study of this concept will be discussed briefly. The status of the sensors, actuators, and computational design tools tasks will also be presented.
Distal Radius Fracture (Broken Wrist)
.org Distal Radius Fracture (Broken Wrist) Page ( 1 ) The radius is the larger of the two bones of the forearm. The end toward the wrist is called the distal end. A fracture of the distal radius occurs when the area ...
De novo nonsense mutations in ASXL1 cause Bohring-Opitz syndrome.
Hoischen, Alexander; van Bon, Bregje W M; Rodríguez-Santiago, Benjamín; Gilissen, Christian; Vissers, Lisenka E L M; de Vries, Petra; Janssen, Irene; van Lier, Bart; Hastings, Rob; Smithson, Sarah F; Newbury-Ecob, Ruth; Kjaergaard, Susanne; Goodship, Judith; McGowan, Ruth; Bartholdi, Deborah; Rauch, Anita; Peippo, Maarit; Cobben, Jan M; Wieczorek, Dagmar; Gillessen-Kaesbach, Gabriele; Veltman, Joris A; Brunner, Han G; de Vries, Bert B B A
2011-08-01
Bohring-Opitz syndrome is characterized by severe intellectual disability, distinctive facial features and multiple congenital malformations. We sequenced the exomes of three individuals with Bohring-Opitz syndrome and in each identified heterozygous de novo nonsense mutations in ASXL1, which is required for maintenance of both activation and silencing of Hox genes. In total, 7 out of 13 subjects with a Bohring-Opitz phenotype had de novo ASXL1 mutations, suggesting that the syndrome is genetically heterogeneous. PMID:21706002
Engineering directed excitonic energy transfer
Perdomo, Alejandro; Vogt, Leslie; Najmaie, Ali; Aspuru-Guzik, Alan
2010-01-01
We provide an intuitive platform for engineering exciton transfer dynamics. We show that careful consideration of the spectral density, which describes the system-bath interaction, leads to opportunities to engineer exciton transfer. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dynamics. We carefully describe the validity of the model and use experimentally relevant material parameters to show counter-intuitive examples of directed exciton transfer in a linear chain of quantum dots.
NASA Astrophysics Data System (ADS)
Byrnes, Tim; Kim, Na Young; Yamamoto, Yoshihisa
2014-11-01
Recently a new type of system exhibiting spontaneous coherence has emerged--the exciton-polariton condensate. Exciton-polaritons (or polaritons for short) are bosonic quasiparticles that exist inside semiconductor microcavities, consisting of a superposition of an exciton and a cavity photon. Above a threshold density the polaritons macroscopically occupy the same quantum state, forming a condensate. The polaritons have a lifetime that is typically comparable to or shorter than thermalization times, giving them an inherently non-equilibrium nature. Nevertheless, they exhibit many of the features that would be expected of equilibrium Bose-Einstein condensates (BECs). The non-equilibrium nature of the system raises fundamental questions as to what it means for a system to be a BEC, and introduces new physics beyond that seen in other macroscopically coherent systems. In this review we focus on several physical phenomena exhibited by exciton-polariton condensates. In particular, we examine topics such as the difference between a polariton BEC, a polariton laser and a photon laser, as well as physical phenomena such as superfluidity, vortex formation, and Berezinskii-Kosterlitz-Thouless and Bardeen-Cooper-Schrieffer physics. We also discuss the physics and applications of engineered polariton structures.
Realization of localized Bohr-like wave packets.
Mestayer, J J; Wyker, B; Lancaster, J C; Dunning, F B; Reinhold, C O; Yoshida, S; BurgdÃ¶rfer, J
2008-06-20
We demonstrate a protocol to create localized wave packets in very-high-n Rydberg states which travel in nearly circular orbits around the nucleus. Although these wave packets slowly dephase and eventually lose their localization, their motion can be monitored over several orbital periods. These wave packets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in a circular classical orbit around the nucleus. The possible extension of the approach to create "planetary atoms" in highly correlated stable multiply excited states is discussed. PMID:18643582
Realization of Localized Bohr-Like Wave Packets
NASA Astrophysics Data System (ADS)
Mestayer, J. J.; Wyker, B.; Lancaster, J. C.; Dunning, F. B.; Reinhold, C. O.; Yoshida, S.; BurgdÃ¶rfer, J.
2008-06-01
We demonstrate a protocol to create localized wave packets in very-high-n Rydberg states which travel in nearly circular orbits around the nucleus. Although these wave packets slowly dephase and eventually lose their localization, their motion can be monitored over several orbital periods. These wave packets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in a circular classical orbit around the nucleus. The possible extension of the approach to create â€œplanetary atomsâ€ in highly correlated stable multiply excited states is discussed.
Revisiting Bohr's principle of complementarity with a quantum device
NASA Astrophysics Data System (ADS)
Tang, Jian-Shun; Li, Yu-Long; Li, Chuan-Feng; Guo, Guang-Can
2013-07-01
Bohr's principle of complementarity (BPC) is the cornerstone of quantum mechanics. According to this principle, the total wavelike and particlelike information of a particle is limited by the Englert-Greenberger (EG) duality relation. Here, by introducing a quantum detecting device into the experiment, we find that the limit of the EG duality relation is exceeded because of the interference between the wave and particle properties of the photon. A generalized EG duality relation is further developed. Our work provides a generalization of BPC and gives new insights into quantum mechanics.
Exciton Transport in Nanostructured Solids
NASA Astrophysics Data System (ADS)
Bulovic, Vladimir
2015-03-01
Transport of nanoscale energy in the form of excitons is at the core of operation of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. The talk will present recent advancements in directly visualizing exciton transport, with spatial, temporal and spectral evolution recorded for molecular crystals, disordered thin films, and colloidal quantum dot solids. Our measurements demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology and the design of nanoscale building blocks. In addition, the talk will show that the excitonic energy landscape can be directly manipulated in solid-state thin films using dipole -dipole interactions, which can be increased under mechanical pressure, or molecular doping with polar molecules, leading to dramatic shifts in the exciton energy structure.
Singlet exciton fission photovoltaics.
Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A
2013-06-18
Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses long- and short-wavelength donors and an acceptor and a simpler, two-layer combination of a singlet-fission donor and a long-wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared-absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption. PMID:23611026
Molecular Basis of the Bohr Effect in Arthropod Hemocyanin
Hirota, S.; Kawahara, T; Beltramini, M; Di Muro, P; Magliozzo, R; Peisach, J; Powers, L; Tanaka, N; Nagao, S; Bubacco, L
2008-01-01
Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, kon, rather than changes in koff. In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s ? 4pz transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs.
Experimental Observation of Bohr's Nonlinear Fluidic Surface Oscillation.
Moon, Songky; Shin, Younghoon; Kwak, Hojeong; Yang, Juhee; Lee, Sang-Bum; Kim, Soyun; An, Kyungwon
2016-01-01
Niels Bohr in the early stage of his career developed a nonlinear theory of fluidic surface oscillation in order to study surface tension of liquids. His theory includes the nonlinear interaction between multipolar surface oscillation modes, surpassing the linear theory of Rayleigh and Lamb. It predicts a specific normalized magnitude of 0.416Î·(2) for an octapolar component, nonlinearly induced by a quadrupolar one with a magnitude of Î· much less than unity. No experimental confirmation on this prediction has been reported. Nonetheless, accurate determination of multipolar components is important as in optical fiber spinning, film blowing and recently in optofluidic microcavities for ray and wave chaos studies and photonics applications. Here, we report experimental verification of his theory. By using optical forward diffraction, we measured the cross-sectional boundary profiles at extreme positions of a surface-oscillating liquid column ejected from a deformed microscopic orifice. We obtained a coefficient of 0.42â€‰Â±â€‰0.08 consistently under various experimental conditions. We also measured the resonance mode spectrum of a two-dimensional cavity formed by the cross-sectional segment of the liquid jet. The observed spectra agree well with wave calculations assuming a coefficient of 0.414â€‰Â±â€‰0.011. Our measurements establish the first experimental observation of Bohr's hydrodynamic theory. PMID:26803911
NASA Astrophysics Data System (ADS)
Ye, Yu; Wong, Zi Jing; Lu, Xiufang; Ni, Xingjie; Zhu, Hanyu; Chen, Xianhui; Wang, Yuan; Zhang, Xiang
2015-11-01
Two-dimensional van der Waals materials have opened a new paradigm for fundamental physics exploration and device applications because of their emerging physical properties. Unlike gapless graphene, monolayer transition-metal dichalcogenides (TMDCs) are two-dimensional semiconductors that undergo an indirect-to-direct bandgap transition, creating new optical functionalities for next-generation ultra-compact photonics and optoelectronics. Although the enhancement of spontaneous emission has been reported on TMDC monolayers integrated with photonic crystals and distributed Bragg reflector microcavities, coherent light emission from a TMDC monolayer has not been demonstrated. Here, we report the realization of a two-dimensional excitonic laser by embedding monolayer WS2 in a microdisk resonator. Using a whispering gallery mode with a high quality factor and optical confinement, we observe bright excitonic lasing at visible wavelengths. This demonstration of a two-dimensional excitonic laser marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications.
Shamirzaev, T. S. Toropov, A. I.; Bakarov, A. K.; Zhuravlev, K. S.; Kobitski, A. Yu.; Wagner, H. P.; Zahn, D. R. T.
2006-05-15
The stationary and time-resolved polariton radiation in ultrahigh quality AIGaAs layers have been studied. It has been found that elastic exciton-exciton collisions lead to the appearance of a low-energy line of polariton radiation. We show that the rate of exciton-to-polariton transitions caused by elastic exciton-exciton collisions is determined not only by the density of the excitonic gas, but also by its temperature; this is in accordance with existing theoretical predictions.
Measurement of solar radius changes
LaBonte, B.J.; Howard, R.
1981-11-20
Photoelectric solar radius measurements since 1974 at Mount Wilson show no change in the solar radius, with a limit of about 0.1 arc second (1 standard deviation), over the interval. The limit is set by residual systematic effects.
NASA Astrophysics Data System (ADS)
Ma, Xuedan; Roslyak, Oleskiy; Duque, Juan G.; Pang, Xiaoying; Doorn, Stephen K.; Piryatinski, Andrei; Dunlap, David H.; Htoon, Han
2015-07-01
Pump-dependent photoluminescence imaging and second-order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature. These studies enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these parameters is attributed to the effect of environmental disorder in setting the exciton mean free path and capture-limited Auger recombination at this length scale. A suppression of photon antibunching is attributed to the creation of multiple spatially nonoverlapping excitons in SWCNTs, whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of the exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization.
Ma, Xuedan; Roslyak, Oleskiy; Duque, Juan G; Pang, Xiaoying; Doorn, Stephen K; Piryatinski, Andrei; Dunlap, David H; Htoon, Han
2015-07-01
Pump-dependent photoluminescence imaging and second-order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature. These studies enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these parameters is attributed to the effect of environmental disorder in setting the exciton mean free path and capture-limited Auger recombination at this length scale. A suppression of photon antibunching is attributed to the creation of multiple spatially nonoverlapping excitons in SWCNTs, whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of the exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization. PMID:26182119
Turnaround radius in modified gravity
NASA Astrophysics Data System (ADS)
Faraoni, Valerio
2016-03-01
In an accelerating universe in General Relativity there is a maximum radius above which a shell of test particles cannot collapse, but is dispersed by the cosmic expansion. This radius could be used in conjunction with observations of large structures to constrain the equation of state of the universe. We extend the concept of turnaround radius to modified theories of gravity for which the gravitational slip is non-vanishing.
Momentum dependence of the excitons in pentacene
Roth, Friedrich; Schuster, Roman; Koenig, Andreas; Knupfer, Martin; Berger, Helmuth
2012-05-28
We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*, b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.
Complications of Distal Radius Fixation.
Lee, Dennis S; Weikert, Douglas R
2016-04-01
Complications following any form of distal radius fixation remain prevalent. With an armamentarium of fixation options available to practicing surgeons, familiarity with the risks of newer plate technology as it compares with other conventional methods is crucial to optimizing surgical outcome and managing patient expectations. This article presents an updated review on complications following various forms of distal radius fixation. PMID:26772950
Measurement of solar radius changes.
Labonte, B J; Howard, R
1981-11-20
Photoelectric solar radius measurements since 1974 at Mount Wilson show no change in the solar radius, with a limit of abouit 0.1 arc second (1 standard deviation), over the interval. The limit is set by residual systematic effects. PMID:17782444
Hyperspherical theory of anisotropic exciton
NASA Astrophysics Data System (ADS)
Muljarov, E. A.; Yablonskii, A. L.; Tikhodeev, S. G.; Bulatov, A. E.; Birman, Joseph L.
2000-09-01
A new approach to the theory of anisotropic exciton based on Fock transformation, i.e., on a stereographic projection of the momentum to the unit four-dimensional (4D) sphere, is developed. Hyperspherical functions are used as a basis of the perturbation theory. The binding energies, wave functions and oscillator strengths of elongated as well as flattened excitons are obtained numerically. It is shown that with an increase of the anisotropy degree the oscillator strengths are markedly redistributed between optically active and formerly inactive states, making the latter optically active. An approximate analytical solution of the anisotropic exciton problem taking into account the angular momentum conserving terms is obtained. This solution gives the binding energies of moderately anisotropic exciton with a good accuracy and provides a useful qualitative description of the energy level evolution.
Energy transfer of excitons between quantum wells separated by a wide barrier
Lyo, S. K.
2000-11-15
We present a microscopic theory of the excitonic Stokes and anti-Stokes energy-transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Several important intrinsic energy-transfer mechanisms have been examined, including dipolar coupling, real and virtual photon-exchange coupling, and over-barrier ionization of the excitons via exciton-exciton Auger processes. The transfer rate is calculated as a function of T and the center-to-center distance d between the wells. The rates depend sensitively on T for plane-wave excitons. For localized excitons, the rates depend on T only through the T dependence of the exciton localization radius. For Stokes energy transfer, the dominant energy transfer occurs through a photon-exchange interaction, which enables the excitons from the higher-energy wells to decay into free electrons and holes in the lower-energy wells. The rate has a slow dependence on d, yielding reasonable agreement with recent data from GaAs/Al{sub x}Ga{sub 1-x}As quantum wells. The dipolar rate is about an order of magnitude smaller for large d (e.g., d=175Aa) with a stronger range dependence proportional to d{sup -4}. However, the latter can be comparable to the radiative rate for small d (e.g., d{<=}80Aa). For anti-Stokes transfer through exchange-type (e.g., dipolar and photon-exchange) interactions, we show that thermal activation proportional to exp(-{Delta}/k{sub B}T) is essential for the transfer, contradicting a recent nonactivated result based on the Fo''rster-Dexter's spectral-overlap theory. Phonon-assisted transfer yields a negligibly small rate. On the other hand, energy transfer through over-barrier ionization of excitons via Auger processes yields a significantly larger nonactivated rate which is independent of d. The result is compared with recent data.
Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites
NASA Astrophysics Data System (ADS)
Bityurin, N.; Ermolaev, N.; Smirnov, A. A.; Afanasiev, A.; Agareva, N.; Koryukina, T.; Bredikhin, V.; Kamensky, V.; Pikulin, A.; Sapogova, N.
2016-03-01
UV irradiation of materials consisting of a polymer matrix that possesses precursors of different kinds can result in creation of nanoparticles within the irradiated domains. Such photoinduced nanocomposites are promising for photonic applications due to the strong alteration of their optical properties compared to initial non-irradiated materials. We report our results on the synthesis and investigation of plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites. Plasmonic nanocomposites contain metal nanoparticles of noble metals with a pronounced plasmon resonance. Excitonic nanocomposites possess semiconductor nanoclusters (quantum dots). We consider the CdS-Au pair because the luminescent band of CdS nanoparticles enters the plasmon resonance band of gold nanoparticles. The obtaining of such particles within the same composite materials is promising for the creation of media with exciton-plasmon resonance. We demonstrate that it is possible to choose appropriate precursor species to obtain the initially transparent poly(methyl methacrylate) (PMMA) films containing both types of these molecules either separately or together. Proper irradiation of these materials by a light-emitting diode operating at the wavelength of 365 nm provides material alteration demonstrating light-induced optical absorption and photoluminescent properties typical for the corresponding nanoparticles. Thus, an exciton-plasmonic photoinduced nanocomposite is obtained. It is important that here we use the precursors that are different from those usually employed.
Why has the bohr-sommerfeld model of the atom been ignoredby general chemistry textbooks?
Niaz, Mansoor; Cardellini, Liberato
2011-12-01
Bohr's model of the atom is considered to be important by general chemistry textbooks. A major shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. In order to increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study has the following objectives: 1) Formulation of criteria based on a history and philosophy of science framework; and 2) Evaluation of university-level general chemistry textbooks based on the criteria, published in Italy and U.S.A. Presentation of a textbook was considered to be "satisfactory" if it included a description of the Bohr-Sommerfeld model along with diagrams of the elliptical orbits. Of the 28 textbooks published in Italy that were analyzed, only five were classified as "satisfactory". Of the 46 textbooks published in U.S.A., only three were classified as "satisfactory". This study has the following educational implications: a) Sommerfeld's innovation (auxiliary hypothesis) by introducing elliptical orbits, helped to restore the viability of Bohr's model; b) Bohr-Sommerfeld's model went no further than the alkali metals, which led scientists to look for other models; c) This clearly shows that scientific models are tentative in nature; d) Textbook authors and chemistry teachers do not consider the tentative nature of scientific knowledge to be important; e) Inclusion of the Bohr-Sommerfeld model in textbooks can help our students to understand how science progresses. PMID:24061142
Bohr's Electron was Problematic for Einstein: String Theory Solved the Problem
NASA Astrophysics Data System (ADS)
Webb, William
2013-04-01
Neils Bohr's 1913 model of the hydrogen electron was problematic for Albert Einstein. Bohr's electron rotates with positive kinetic energies +K but has addition negative potential energies - 2K. The total net energy is thus always negative with value - K. Einstein's special relativity requires energies to be positive. There's a Bohr negative energy conflict with Einstein's positive energy requirement. The two men debated the problem. Both would have preferred a different electron model having only positive energies. Bohr and Einstein couldn't find such a model. But Murray Gell-Mann did! In the 1960's, Gell-Mann introduced his loop-shaped string-like electron. Now, analysis with string theory shows that the hydrogen electron is a loop of string-like material with a length equal to the circumference of the circular orbit it occupies. It rotates like a lariat around its centered proton. This loop-shape has no negative potential energies: only positive +K relativistic kinetic energies. Waves induced on loop-shaped electrons propagate their energy at a speed matching the tangential speed of rotation. With matching wave speed and only positive kinetic energies, this loop-shaped electron model is uniquely suited to be governed by the Einstein relativistic equation for total mass-energy. Its calculated photon emissions are all in excellent agreement with experimental data and, of course, in agreement with those -K calculations by Neils Bohr 100 years ago. Problem solved!
Exciton dispersion in molecular solids.
Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo
2015-03-25
The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators. PMID:25738755
Exciton dispersion in molecular solids
NASA Astrophysics Data System (ADS)
Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo
2015-03-01
The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.
Exciton dynamicsstudied via internal THz transitions
Kaindl, R.A.; Hagele, D.; Carnahan, M.A.; Lovenich, R.; Chemla,D.S.
2003-02-26
We employ a novel, ultrafast terahertz probe to investigatethe dynamical interplay of optically-induced excitons and unboundelectron-hole pairs in GaAs quantum wells. Resonant creation ofheavy-hole excitons induces a new low-energy oscillator linked totransitions between the internal exciton degrees of freedom. The timeresolved terahertz optical conductivity is found to be a probe wellsuited for studies of fundamental processes such as formation, relaxationand ionization of excitons.
Exciton coupling in molecular crystals
NASA Technical Reports Server (NTRS)
Ake, R. L.
1976-01-01
The implications of perfect exciton coupling and molecular vibrations were investigated, as well as the effect they have on the lifetime of singlet and triplet excitons coupled in a limiting geometry. Crystalline bibenzyl, Cl4Hl4, provided a situation in which these mechanisms involving exciton coupling can be studied in the limit of perfect coupling between units due to the crystal's geometry. This geometry leads to a coupling between the two halves of the molecule resulting in a splitting of the molecular excited states. The study reported involves an experimental spectroscopic approach and begins with the purification of the bibenzyl. The principal experimental apparatus was an emission spectrometer. A closed cycle cryogenic system was used to vary the temperature of the sample between 20 K and 300 K. The desired results are the temperature-dependent emission spectra of the bibenzyl; in addition, the lifetimes and quantum yields measured at each temperature reveal the effect of competing radiationless processes.
Crystallization of an exciton superfluid
Boening, J.; Filinov, A.; Bonitz, M.
2011-08-15
Indirect excitons--pairs of electrons and holes spatially separated in semiconductor bilayers or quantum wells--are known to undergo Bose-Einstein condensation and to form a quantum fluid. Here we show that this superfluid may crystallize upon compression. However, further compression results in quantum melting back to a superfluid. This unusual behavior is explained by the effective interaction potential between indirect excitons, which strongly deviates from a dipole potential at small distances due to many-particle and quantum effects. Based on first-principles path-integral Monte Carlo simulations, we compute the complete phase diagram of this system and predict the relevant parameters necessary to experimentally observe exciton crystallization in semiconductor quantum wells.
Exciton-photon correlations in bosonic condensates of exciton-polaritons.
Kavokin, Alexey V; Sheremet, Alexandra S; Shelykh, Ivan A; Lagoudakis, Pavlos G; Rubo, Yuri G
2015-01-01
Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979
Exciton-photon correlations in bosonic condensates of exciton-polaritons
Kavokin, Alexey V.; Sheremet, Alexandra S.; Shelykh, Ivan A.; Lagoudakis, Pavlos G.; Rubo, Yuri G.
2015-01-01
Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979
NASA Astrophysics Data System (ADS)
Iizuka, Hideyuki; Nakayama, Takashi
2016-02-01
Quantum processes of bound exciton dissociation at organic donor/acceptor interfaces were studied by the time-evolution simulation of a wave packet, focusing on the effects of interface roughness and hot excitons. It is shown that the dissociation is remarkably enhanced by the interface roughness that is comparable to the exciton diameter. Moreover, the dissociation probability increases for the hot excitons compared with the ground-state exciton owing to their small binding energies and large diameters.
Exciton Polarization in Carbon Nanotubes
NASA Astrophysics Data System (ADS)
Konobeeva, N. N.; Belonenko, M. B.
2015-09-01
The nonlinear process of propagation of optical pulses in the spectral region inside the polarization gap in carbon nanotubes has been investigated. A simultaneous solution of the Maxwell equation and the equation of motion for exciton polarization has been carried out. The dynamics of an electromagnetic pulse has been examined as a function of the parameters of the problem. It is shown that taking exciton polarization into account does not have a substantial effect on the propagation process, but alters the shape of the optical pulse.
Generation of Quasiclassical Bohr-Like Wave Packets Using Half-Cycle Pulses
Mestayer, J. J.; Wyker, B.; Dunning, F. B.; Reinhold, Carlos O; Yoshida, S.; Burgdorfer, J.
2008-08-01
We demonstrate the experimental realization of Bohr-like atoms by applying a pulsed unidirectional field, termed a half-cycle pulse (HCP), to atoms in quasi-two-dimensional near-circular states. This leads to creation of localized wave packets that travel in near-circular orbits and mimic the dynamics of an electron in the original Bohr model of the hydrogen atom. This motion can be followed for several orbital periods before the localization of the wave packet is lost due to dephasing. We show, however, that localization can be recovered by application of further HCPs.
Atomically thin spherical shell-shaped superscatterers based on a Bohr model
NASA Astrophysics Data System (ADS)
Li, Rujiang; Lin, Xiao; Lin, Shisheng; Liu, Xu; Chen, Hongsheng
2015-12-01
Graphene monolayers can be used for atomically thin three-dimensional shell-shaped superscatterer designs. Due to the excitation of the first-order resonance of transverse magnetic (TM) graphene plasmons, the scattering cross section of the bare subwavelength dielectric particle is enhanced significantly by five orders of magnitude. The superscattering phenomenon can be intuitively understood and interpreted with a Bohr model. In addition, based on the analysis of the Bohr model, it is shown that contrary to the TM case, superscattering is hard to achieve by exciting the resonance of transverse electric (TE) graphene plasmons due to their poor field confinements.
Atomically thin spherical shell-shaped superscatterers based on a Bohr model.
Li, Rujiang; Lin, Xiao; Lin, Shisheng; Liu, Xu; Chen, Hongsheng
2015-12-18
Graphene monolayers can be used for atomically thin three-dimensional shell-shaped superscatterer designs. Due to the excitation of the first-order resonance of transverse magnetic (TM) graphene plasmons, the scattering cross section of the bare subwavelength dielectric particle is enhanced significantly by five orders of magnitude. The superscattering phenomenon can be intuitively understood and interpreted with a Bohr model. In addition, based on the analysis of the Bohr model, it is shown that contrary to the TM case, superscattering is hard to achieve by exciting the resonance of transverse electric (TE) graphene plasmons due to their poor field confinements. PMID:26580845
Spectral dimension and Bohr's formula for Schrödinger operators on unbounded fractal spaces
NASA Astrophysics Data System (ADS)
Chen, Joe P.; Molchanov, Stanislav; Teplyaev, Alexander
2015-09-01
We establish an asymptotic formula for the eigenvalue counting function of the Schrödinger operator -{{? }}+V for some unbounded potentials V on several types of unbounded fractal spaces. We give sufficient conditions for Bohr’s formula to hold on metric measure spaces which admit a cellular decomposition, and then verify these conditions for fractafolds and fractal fields based on nested fractals. In particular, we partially answer a question of Fan, Khandker, and Strichartz regarding the spectral asymptotics of the harmonic oscillator potential on the infinite blow-up of a Sierpinski gasket.
Exactly separable Bohr Hamiltonian with the Killingbeck potential for triaxial nuclei
NASA Astrophysics Data System (ADS)
Neyazi, H.; Rajabi, A. A.; Hassanabadi, H.
2016-01-01
After pioneering work by Bohr, Mottelson and their numerous colleagues, the essential framework for understanding collective model is introduced. One of the applications of this framework is the study of shape phase transition, vibrational and rotational energy spectrum of nuclei. We consider the Bohr Hamiltonian and solve the beta and gamma part equation of it, by considering that reduced potential and wave function are exactly separable. In the beta part equation we consider the Killingbeck potential and derive the wave function and energy spectrum of it.
Measurement of exciton correlations using electrostatic lattices
NASA Astrophysics Data System (ADS)
Remeika, M.; Leonard, J. R.; Dorow, C. J.; Fogler, M. M.; Butov, L. V.; Hanson, M.; Gossard, A. C.
2015-09-01
We present a method for determining correlations in a gas of indirect excitons in a semiconductor quantum well structure. The method involves subjecting the excitons to a periodic electrostatic potential that causes modulations of the exciton density and photoluminescence (PL). Experimentally measured amplitudes of energy and intensity modulations of exciton PL serve as an input to a theoretical estimate of the exciton correlation parameter and temperature. We also present a proof-of-principle demonstration of the method for determining the correlation parameter and discuss how its accuracy can be improved.
Stirring potential for indirect excitons
NASA Astrophysics Data System (ADS)
Hasling, M. W.; Kuznetsova, Y. Y.; Andreakou, P.; Leonard, J. R.; Calman, E. V.; Dorow, C. J.; Butov, L. V.; Hanson, M.; Gossard, A. C.
2015-01-01
We demonstrate experimental proof of principle for a stirring potential for indirect excitons. The azimuthal wavelength of this stirring potential is set by the electrode periodicity, the amplitude is controlled by the applied AC voltage, and the angular velocity is controlled by the AC frequency.
Exciton size and quantum transport in nanoplatelets.
Pelzer, Kenley M; Darling, Seth B; Gray, Stephen K; Schaller, Richard D
2015-12-14
Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of FÃ¶rster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport. PMID:26671357
Exciton size and quantum transport in nanoplatelets
NASA Astrophysics Data System (ADS)
Pelzer, Kenley M.; Darling, Seth B.; Gray, Stephen K.; Schaller, Richard D.
2015-12-01
Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of FÃ¶rster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.
Subdiffusive exciton transport in quantum dot solids.
Akselrod, Gleb M; Prins, Ferry; Poulikakos, Lisa V; Lee, Elizabeth M Y; Weidman, Mark C; Mork, A Jolene; Willard, Adam P; Bulovi?, Vladimir; Tisdale, William A
2014-06-11
Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications. PMID:24807586
Quantum Monte Carlo Simulations of Exciton-Exciton Scattering in Quantum Wells
NASA Astrophysics Data System (ADS)
Shumway, John
2005-03-01
Exciton-exciton interactions are characterized by the scattering length, which is a property of excited states of a four-particle wavefunction at the zero energy limit. As is well-known in atomic physics, the scattering length can be notoriously hard to predict theoretically, since correlation and van der Waals forces can play a large role. We have developed a quantum Monte Carlo (QMC) approach that can accurately calculate the bulk exciton-exciton scattering length within the effective mass approximation (Shumway and Ceperley, PRB 63, 165209, 2001). As an added benefit of this technique, all bound biexciton states are also calculated, providing an additional test for the simulations. Now we have adapted this excited-state QMC technique to exciton-exciton interactions in quantum wells, where there is current interest in exciton or polariton condensates. We discuss predictions of our simulations, especially ways to modify exciton-exciton interaction strength with different well geometries and external fields.
Caprio, M. A.
2011-06-15
Detailed quantitative predictions are obtained for phonon and multiphonon excitations in well-deformed rotor nuclei within the geometric framework, by exact numerical diagonalization of the Bohr Hamiltonian in an SO(5) basis. Dynamical {gamma} deformation is found to significantly influence the predictions through its coupling to the rotational motion. Basic signatures for the onset of rigid triaxial deformation are also obtained.
The Thermal Casimir Effect for Conducting Plates and the Bohr-Van Leeuwen Theorem
NASA Astrophysics Data System (ADS)
Bimonte, Giuseppe
2010-04-01
We examine recent prescriptions for estimating the thermal Casimir force between two metallic plates from the point of view of their consistency with the Bohr-van Leeuwen theorem of classical statistical physics. We find that prescriptions including the effect of ohmic dissipation satisfy the theorem, while prescriptions that neglect ohmic dissipation do not.
Why We Should Teach the Bohr Model and How to Teach it Effectively
ERIC Educational Resources Information Center
McKagan, S. B.; Perkins, K. K.; Wieman, C. E.
2008-01-01
Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits students' ability to learn the true quantum nature of electrons in atoms. Although the evidence for this claim is weak, many have accepted it. This claim has implications for how to present atoms in classes ranging from elementary schoolâ€¦
What Can the Bohr-Sommerfeld Model Show Students of Chemistry in the 21st Century?
ERIC Educational Resources Information Center
Niaz, Mansoor; Cardellini, Liberato
2011-01-01
Bohr's model of the atom is considered to be important by general chemistry textbooks. A shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. To increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study aims to elaborate a frameworkâ€¦
EPR before EPR: A 1930 Einstein-Bohr thought Experiment Revisited
ERIC Educational Resources Information Center
Nikolic, Hrvoje
2012-01-01
In 1930, Einstein argued against the consistency of the time-energy uncertainty relation by discussing a thought experiment involving a measurement of the mass of the box which emitted a photon. Bohr seemingly prevailed over Einstein by arguing that Einstein's own general theory of relativity saves the consistency of quantum mechanics. We revisit…
Why We Should Teach the Bohr Model and How to Teach it Effectively
ERIC Educational Resources Information Center
McKagan, S. B.; Perkins, K. K.; Wieman, C. E.
2008-01-01
Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits students' ability to learn the true quantum nature of electrons in atoms. Although the evidence for this claim is weak, many have accepted it. This claim has implications for how to present atoms in classes ranging from elementary school…
What Can the Bohr-Sommerfeld Model Show Students of Chemistry in the 21st Century?
ERIC Educational Resources Information Center
Niaz, Mansoor; Cardellini, Liberato
2011-01-01
Bohr's model of the atom is considered to be important by general chemistry textbooks. A shortcoming of this model was that it could not explain the spectra of atoms containing more than one electron. To increase the explanatory power of the model, Sommerfeld hypothesized the existence of elliptical orbits. This study aims to elaborate a framework…
Quantum Explorers: Bohr, Jordan, and DelbrÃ¼ck Venturing into Biology
NASA Astrophysics Data System (ADS)
Joaquim, Leyla; Freire, Olival; El-Hani, Charbel N.
2015-09-01
This paper disentangles selected intertwined aspects of two great scientific developments: quantum mechanics and molecular biology. We look at the contributions of three physicists who in the 1930s were protagonists of the quantum revolution and explorers of the field of biology: Niels Bohr, Pascual Jordan, and Max DelbrÃ¼ck. Their common platform was the defense of the Copenhagen interpretation in physics and the adoption of the principle of complementarity as a way of looking at biology. Bohr addressed the problem of how far the results reached in physics might influence our views about life. Jordan and DelbrÃ¼ck were followers of Bohr's ideas in the context of quantum mechanics and also of his tendency to expand the implications of the Copenhagen interpretation to biology. We propose that Bohr's perspective on biology was related to his epistemological views, as Jordan's was to his political positions. DelbrÃ¼ck's propensity to migrate was related to his transformation into a key figure in the history of twentieth-century molecular biology.
EPR before EPR: A 1930 Einstein-Bohr thought Experiment Revisited
ERIC Educational Resources Information Center
Nikolic, Hrvoje
2012-01-01
In 1930, Einstein argued against the consistency of the time-energy uncertainty relation by discussing a thought experiment involving a measurement of the mass of the box which emitted a photon. Bohr seemingly prevailed over Einstein by arguing that Einstein's own general theory of relativity saves the consistency of quantum mechanics. We revisitâ€¦
Why we should teach the Bohr model and how to teach it effectively
NASA Astrophysics Data System (ADS)
McKagan, S. B.; Perkins, K. K.; Wieman, C. E.
2008-06-01
Some education researchers have claimed that we should not teach the Bohr model of the atom because it inhibits students’ ability to learn the true quantum nature of electrons in atoms. Although the evidence for this claim is weak, many have accepted it. This claim has implications for how to present atoms in classes ranging from elementary school to graduate school. We present results from a study designed to test this claim by developing a curriculum on models of the atom, including the Bohr and Schrödinger models. We examine student descriptions of atoms on final exams in transformed modern physics classes using various versions of this curriculum. We find that if the curriculum does not include sufficient connections between different models, many students still have a Bohr-like view of atoms rather than a more accurate Schrödinger model. However, with an improved curriculum designed to develop model-building skills and with better integration between different models, it is possible to get most students to describe atoms using the Schrödinger model. In comparing our results with previous research, we find that comparing and contrasting different models is a key feature of a curriculum that helps students move beyond the Bohr model and adopt Schrödinger’s view of the atom. We find that understanding the reasons for the development of models is much more difficult for students than understanding the features of the models. We also present interactive computer simulations designed to help students build models of the atom more effectively.
Electric arc radius and characteristics
Fang, T.M.
1980-09-30
The heat transfer equation of an arc discharge has been solved. The arc is assumed to be a cylinder with negligible axial variation and the dominant heat transfer process is conduction radially inside the column and radiation/convection at the outside edge. The symmetric consideration allows a simple one-dimensional formulation. By taking into account proper variation of the electrical conductivity as function of temperature, the heat balance equation has been solved analytically. The radius of the arc and its current-field characteristics have also been obtained. The conventional results that E approx. I/sup 0/ /sup 5385/ and R approx. I/sup 0/ /sup 7693/ with E being the applied field, I the current, and R the radius of the cylindrical arc, have been proved to be simply limiting cases of our more general characteristics. The results can be applied quite widely including, among others, the neutral beam injection project in nuclear fusion and MHD energy conversion.
Spatially indirect excitons in coupled quantum wells
Lai, Chih-Wei Eddy
2004-03-01
Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.
Contrastive analysis of multiple exciton generation theories
NASA Astrophysics Data System (ADS)
Tan, Hengyu; Chang, Qing
2015-10-01
Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.
Exciton Seebeck effect in molecular systems
Yan, Yun-An; Cai, Shaohong
2014-08-07
We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.
Exciton Seebeck effect in molecular systems.
Yan, Yun-An; Cai, Shaohong
2014-08-01
We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect. PMID:25106568
NASA Astrophysics Data System (ADS)
Noltemeyer, Martin; Bertram, Frank; Hempel, Thomas; Bastek, Barbara; Christen, Juergen; Brandt, Matthias; Lorenz, Michael; Grundmann, Marius
2012-02-01
The temperature dependence of diffusion length and lifetime or diffusivity of the free exciton is measured in a commercial ZnO-substrate and in an epitaxial ZnO quantum well using nm-spatially and ps-time resolved cathodoluminescence (CL) spectroscopy. The characteristic temperature dependence of the exciton mobility is a fingerprint of the underlying excitonic scattering processes. Since excitons are neutral particles scattering at ionized impurities should be not effective. With decreasing temperature diffusion lengths and lifetimes give rise to a monotonous increase of the excitonic mobility. Two different methods for determining the excitonic transport parameters will be presented. On the one hand we are able to perform completely pulsed excitation experiments and on the other hand a combination of cw- and pulsed excitation in two independent measurements are needed.
Exciton Binding Energy of Monolayer WS2
Zhu, Bairen; Chen, Xi; Cui, Xiaodong
2015-01-01
The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01?eV around K valley in the Brillouin zone. PMID:25783023
Localization of Surface-exciton Polaritons
NASA Astrophysics Data System (ADS)
Wang, Qing-Song; Cheng, Ze
2002-07-01
We develop a numerical model to investigate the localization of surface-exciton polaritons in the presence of random roughness and spatial dispersion. It is found that the localization occurs in a limited frequency range near the maximum frequency of surface-exciton polaritons. The decaying length in the extended region is much larger than that in the localized region. The localization of surface-exciton polaritons is due to the destructive interference between waves scattered from the rough surface.
Relaxation kinetics of excitons in cuprous oxide
NASA Astrophysics Data System (ADS)
O'Hara, Keith E.
Cuprous oxide is a thoroughly studied semiconductor with long-lived, mobile excitons. The kinetic energy distribution of the excitons is reproduced in phonon-assisted luminescence spectra. When non-equilibrium excitons are produced by a laser pulse, nanosecond-time-resolved luminescence reveals the relaxation of their kinetic energy distribution. It has been known for several years that, when the excitons are produced with the highest attainable densities, the time-sequence of exciton kinetic energy distributions closely resembles a sequence of Bose-Einstein distributions, with apparent temperatures greater than that of the lattice. A widely used hypothesis is that the excitons exchange energy through elastic collisions quickly enough to establish a quasi-equilibrium, with the exciton gas having a temperature distinct from that of the lattice. The transient kinetic energy distributions have thus been interpreted simply as quasi-equilibrium distributions, with the gas density near enough the quantum density that the Bose statistics of the excitons is noticeable. This work is an experimental and theoretical study of such luminescence spectra. The absolute brightness of the photoluminescence, compared with that of a known number of excitons, indicates that the density of excitons is at all times too low to reveal their Bose statistics---assuming they act as an ideal gas. The assumption of a quasi-equilibrium among the excitons is abandoned, and the relaxation problem is treated through a Boltzmann equation. Only relaxation processes which have been measured experimentally are included: acoustic phonon emission and absorption, interconversion between spin states, and non-radiative two-exciton decay. Numerical modeling reproduces most of the experimental observations, without invoking Bose statistics.
Rate limiting processes in the bohr shift in human red cells
Forster, R. E.; Steen, J. B.
1968-01-01
1. The rates of the Bohr shift of human red cells and some of its constituent reactions have been studied with a modified Hartridgeâ€”Roughton rapid reaction apparatus using an oxygen electrode to measure the progress of the reaction. 2. The rate of the Bohr shift was compatible with the hypothesis that the transfer of H+ across the membrane by means of CO2 exchange and reaction with buffers is generally the rate-limiting step. (a) When the Bohr off-reaction was produced by a marked increase in PCO2 around the cells, the half-time at 37Â° C was 0Â·12 sec. In this case CO2 was available initially to diffuse into the cells, the process being predominantly limited by the rate of intracellular CO2 hydration. (b) When the Bohr off-shift was produced by an increase of [H+] outside the cell, PCO2 being low and equal within and outside the cells, the half time became 0Â·31 sec. In this case, even at the start, the H2CO3 formed by the almost instantaneous neutralization reaction of H+ and HCO3- had to dehydrate to form CO2 and this in turn had to diffuse into and react within the red cell before the [HbO2] could change. When a carbonic anhydrase inhibitor was added to slow the CO2 reaction inside the cell, the half-time rose to 10 sec. (c) The Bohr off-shift in a haemolysed cell suspension produced by an increase in PCO2 appeared to be limited by the rate at which the CO2 could hydrate to form H+. 3. The Bohr off-shift has an average Q10 of 2Â·5 between 42Â·5 and 28Â° C with an activation energy of 8000 cal. 4. The pronounced importance of the CO2-bicarbonate system for rapid intracellular pH changes is discussed in connexion with some physiological situations. PMID:5664232
Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong
2015-01-15
The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. Moreover, it is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.
Ballistic spin transport in exciton gases
NASA Astrophysics Data System (ADS)
Kavokin, A. V.; Vladimirova, M.; Jouault, B.; Liew, T. C. H.; Leonard, J. R.; Butov, L. V.
2013-11-01
Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.
Nonlinear optical response of interacting excitons
NASA Astrophysics Data System (ADS)
Keldysh, Leonid V.
1995-02-01
In the vicinity of the direct intrinsic absorption threshold in semiconductors nonlinearity of the optical response is dominated by manybody interaction effects in the polarization cloud accompanying the propagation of the electromagnetic field. This polarization is essentially the system of electron-hole pairs. Its evolution with increasing density (radiation intensity) is analyzed in the framework of simplified model substituting effective short range pseudopotential instead of real Coulomb potential. At moderate densities excitonic resonance is essentially modified by the formation of excitonic molecules. Multistable behavior arise. Transient processes are discussed including gradual formation of bound state-excitons and excitonic molecules. The latter manifest themselves in anomalously large four-wave mixing amplitude.
Cross-polarized excitons in carbon nanotubes
Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K.; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R.
2008-01-01
Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293
Quantum Humor: The Playful Side of Physics at Bohr's Institute for Theoretical Physics
NASA Astrophysics Data System (ADS)
Halpern, Paul
2012-09-01
From the 1930s to the 1950s, a period of pivotal developments in quantum, nuclear, and particle physics, physicists at Niels Bohr's Institute for Theoretical Physics in Copenhagen took time off from their research to write humorous articles, letters, and other works. Best known is the Blegdamsvej Faust, performed in April 1932 at the close of one of the Institute's annual conferences. I also focus on the Journal of Jocular Physics, a humorous tribute to Bohr published on the occasions of his 50th, 60th, and 70th birthdays in 1935, 1945, and 1955. Contributors included LÃ©on Rosenfeld, Victor Weisskopf, George Gamow, Oskar Klein, and Hendrik Casimir. I examine their contributions along with letters and other writings to show that they offer a window into some issues in physics at the time, such as the interpretation of complementarity and the nature of the neutrino, as well as the politics of the period.
On Quasi-Normal Modes, Area Quantization and Bohr Correspondence Principle
NASA Astrophysics Data System (ADS)
Corda, Christian
2015-10-01
In (Int. Journ. Mod. Phys. D 14, 181 2005), the author Khriplovich verbatim claims that "the correspondence principle does not dictate any relation between the asymptotics of quasinormal modes and the spectrum of quantized black holes" and that "this belief is in conflict with simple physical arguments". In this paper we analyze Khriplovich's criticisms and realize that they work only for the original proposal by Hod, while they do not work for the improvements suggested by Maggiore and recently finalized by the author and collaborators through a connection between Hawking radiation and black hole (BH) quasi-normal modes (QNMs). This is a model of quantum BH somewhat similar to the historical semi-classical model of the structure of a hydrogen atom introduced by Bohr in 1913. Thus, QNMs can be really interpreted as BH quantum levels (the "electrons" of the "Bohr-like BH model").Our results have also important implications on the BH information puzzle.
Josephson effects in condensates of excitons and exciton polaritons
Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.
2008-07-15
We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.
Radius of curvature controlled mirror
Neil, George R.; Rathke, John Wickham; Schultheiss, Thomas John; Shinn, Michelle D.; Dillon-Townes, Lawrence A.
2006-01-17
A controlled radius of curvature mirror assembly comprising: a distortable mirror having a reflective surface and a rear surface; and in descending order from the rear surface; a counter-distortion plate; a flow diverter having a flow diverter aperture at the center thereof; a flow return plate having a flow return aperture at the center thereof; a thermal isolation plate having a thermal isolation plate aperture at the center thereof and a flexible heater having a rear surface and a flexible heater aperture at the center thereof; a double walled tube defining a coolant feed chamber and a coolant return chamber; said coolant feed chamber extending to and through the flow diverter aperture and terminating at the counter-distortion plate and the coolant return chamber extending to and through the thermal isolation backplate and terminating at the flow diverter; and a coolant feed and a coolant return exit at the rear of said flexible heater.
Observation of a Rydberg Series in H{sup +}H{sup -}: A Heavy Bohr Atom
Vieitez, M. O.; Ivanov, T. I.; Lange, C. A. de; Ubachs, W.; Reinhold, E.
2008-10-17
We report on the realization of a heavy 'Bohr atom', through the spectroscopic observation of a Rydberg series of bound quantum states at principal quantum numbers n=140 to 230. The system is made heavy by replacing an electron inside a hydrogen atom by a composite H{sup -} particle, thus forming a H{sup +}H{sup -} Coulombically bound system obeying the physical laws of a generalized atom with appropriate mass scaling.
Bohr-Sommerfeld quantization condition for Dirac states derived from an Ermakov-type invariant
Thylwe, Karl-Erik; McCabe, Patrick
2013-05-15
It is shown that solutions of the second-order decoupled radial Dirac equations satisfy Ermakov-type invariants. These invariants lead to amplitude-phase-type representations of the radial spinor solutions, with exact relations between their amplitudes and phases. Implications leading to a Bohr-Sommerfeld quantization condition for bound states, and a few particular atomic/ionic and nuclear/hadronic bound-state situations are discussed.
How Sommerfeld extended Bohr's model of the atom (1913-1916)
NASA Astrophysics Data System (ADS)
Eckert, Michael
2014-04-01
Sommerfeld's extension of Bohr's atomic model was motivated by the quest for a theory of the Zeeman and Stark effects. The crucial idea was that a spectral line is made up of coinciding frequencies which are decomposed in an applied field. In October 1914 Johannes Stark had published the results of his experimental investigation on the splitting of spectral lines in hydrogen (Balmer lines) in electric fields, which showed that the frequency of each Balmer line becomes decomposed into a multiplet of frequencies. The number of lines in such a decomposition grows with the index of the line in the Balmer series. Sommerfeld concluded from this observation that the quantization in Bohr's model had to be altered in order to allow for such decompositions. He outlined this idea in a lecture in winter 1914/15, but did not publish it. The First World War further delayed its elaboration. When Bohr published new results in autumn 1915, Sommerfeld finally developed his theory in a provisional form in two memoirs which he presented in December 1915 and January 1916 to the Bavarian Academy of Science. In July 1916 he published the refined version in the Annalen der Physik. The focus here is on the preliminary Academy memoirs whose rudimentary form is better suited for a historical approach to Sommerfeld's atomic theory than the finished Annalen-paper. This introductory essay reconstructs the historical context (mainly based on Sommerfeld's correspondence). It will become clear that the extension of Bohr's model did not emerge in a singular stroke of genius but resulted from an evolving process.
Discreteness of the volume of space from Bohr-Sommerfeld quantization.
Bianchi, Eugenio; Haggard, Hal M
2011-07-01
A major challenge for any theory of quantum gravity is to quantize general relativity while retaining some part of its geometrical character. We present new evidence for the idea that this can be achieved by directly quantizing space itself. We compute the Bohr-Sommerfeld volume spectrum of a tetrahedron and show that it reproduces the quantization of a grain of space found in loop gravity. PMID:21797533
Electric quadrupole transitions of the Bohr Hamiltonian with the Morse potential
Inci, I.; Bonatsos, D.; Boztosun, I.
2011-08-15
Eigenfunctions of the collective Bohr Hamiltonian with the Morse potential have been obtained by using the asymptotic iteration method (AIM) for both {gamma}-unstable and rotational structures. B(E2) transition rates have been calculated and compared to experimental data. Overall good agreement is obtained for transitions within the ground-state band, while some interband transitions appear to be systematically underpredicted in {gamma}-unstable nuclei and overpredicted in rotational nuclei.
Closed analytical solutions of Bohr Hamiltonian with Manning-Rosen potential model
NASA Astrophysics Data System (ADS)
Chabab, M.; Lahbas, A.; Oulne, M.
2015-11-01
In the present paper, we have obtained closed analytical expressions for eigenvalues and eigenfunctions of the Bohr Hamiltonian with the Manning-Rosen potential for Î³â€‘unstable nuclei as well as exactly separable rotational ones with Î³ â‰ˆ 0. Some heavy nuclei with known Î² and Î³ bandheads have been fitted by using two parameters in the Î³â€‘unstable case and three parameters in the axially symmetric prolate deformed one. A good agreement with experimental data has been achieved.
Bohr Hamiltonian with a deformation-dependent mass term: physical meaning of the free parameter
NASA Astrophysics Data System (ADS)
Bonatsos, Dennis; Minkov, N.; Petrellis, D.
2015-09-01
Embedding the five-dimensional (5D) space of the Bohr Hamiltonian with a deformation-dependent mass (DDM) into a six-dimensional (6D) space shows that the free parameter in the dependence of the mass on the deformation is connected to the curvature of the 5D space, with the special case of constant mass corresponding to a flat 5D space. Comparison of the DDM Bohr Hamiltonian to the 5D classical limit of Hamiltonians of the 6D interacting boson model (IBM), shows that the DDM parameter is proportional to the strength of the pairing interaction in the U(5) (vibrational) symmetry limit, while it is proportional to the quadrupole-quadrupole interaction in the SU(3) (rotational) symmetry limit, and to the difference of the pairing interactions among s, d bosons and d bosons alone in the O(6) (?-soft) limit. The presence of these interactions leads to a curved 5D space in the classical limit of IBM, in contrast to the flat 5D space of the original Bohr Hamiltonian, which is made curved by the introduction of the DDM.
The theory of the Bohr-Weisskopf effect in the hyperfine structure
NASA Astrophysics Data System (ADS)
Karpeshin, F. F.; Trzhaskovskaya, M. B.
2015-09-01
Description of the Bohr-Wesskopf effect in the hyperfine structure of few-electron heavy ions is a challenging problem, which can be used as a test of both QED and atomic calculations. However, for twenty years the research has actually been going in a wrong direction, aimed at fighting the Bohr-Weisskopf effect through its cancellation in specific differences. Alternatively, we propose the constructive model-independent way, which enables the nuclear radii and their momenta to be retrieved from the hyper-fine splitting (HFS). The way is based on analogy of HFS to internal conversion coefficients, and the Bohr-Weisskopf effect - to the anomalies in the internal conversion coefficients. It is shown that the parameters which can be extracted from the data are the even nuclear momenta of the magnetization distribution. The radii R2 and - for the first time - R4 are obtained in this way by analysis of the experimental HFS values for the H- and Li-like ions of 209Bi. The critical prediction concerning the HFS for the 2p1/2 state is made. The present analysis shows high sensitivity of the method to the QED effects, which offers a way of precision test of QED. Experimental recommendations are given, which are aimed at retrieving data on the HFS values for a set of a few-electron configurations of each atom.
Baranov, P. G.; Poluektov, O. G.; Schmidt, J.
2010-01-01
Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4? complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron–nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn–Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra. PMID:21151483
Optical properties of anisotropic exciton: Hyperspherical theory
NASA Astrophysics Data System (ADS)
Muljarov, E. A.; Yablonskii, A. L.; Tikhodeev, S. G.; Bulatov, A. E.; Birman, Joseph L.
1999-02-01
A perturbation Brillouin-Wigner approach to anisotropic exciton problem, based on a hyperspherical formalism, is developed. The binding energies and oscillator strengths of elongated as well as flattened excitons are calculated numerically. It is shown that with an increase of the anisotropy degree the oscillator strengths are markedly redistributing between optically active and formerly inactive states, making the latter optically active.
DNA-mediated excitonic upconversion FRET switching
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-17
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via FÃ¶rster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.
DNA-mediated excitonic upconversion FRET switching
NASA Astrophysics Data System (ADS)
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-01
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via FÃ¶rster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy upconversion via upconversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based upconversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an upconversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy upconversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy upconversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.
DNA-mediated excitonic upconversion FRET switching
Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.
2015-11-17
Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via FÃ¶rster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffersmoreÂ Â» from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.Â«Â less
NASA Astrophysics Data System (ADS)
Winbow, Alexander Graham
Indirect excitons in GaAs coupled quantum well nanostructures are a versatile system for fundamental study of cold neutral bosonic gases and demonstration of novel optoelectronic devices based on excitons --- a bound electron--hole pair --- rather than electrons. Indirect exciton lifetimes range from nanoseconds to microseconds and cool rapidly after photoexcitation to the lattice temperature. Lithographically-patterned electrodes enable design of potential energy landscapes, and both energy and lifetime can be controlled in situ, rapidly, on timescales much shorter than the exciton lifetime. Such intrinsically optoelectronic devices can operate at speeds relevant to optical networks, and later be fabricated in other semiconductors for higher-temperature operation. Two different kinds of devices are demonstrated: Photon storage --- an optical memory --- with 250 ps rise time of the readout optical signal and storage time reaching microseconds was implemented with indirect excitons in CQW. The storage and release of photons was controlled by the gate voltage pulse, and the transient processes in the CQW studied by measuring the kinetics of the exciton emission spectra. This control of excitons on timescales much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps. The Exciton Conveyer is a laterally moving electrostatic lattice potential for actively transporting excitons. Generated by laterally modulated electrodes, the potential velocity and depth are controlled in situ by frequency and voltage. We observed exciton transport characterized by average exciton cloud spatial extension over several tens of microns, and observed dynamical localization--delocalization transitions for the excitons in the conveyer: In the localization regime of deeper potentials and moderate exciton density, excitons are moved by the conveyer; in the delocalized regime of shallower lattice potential or high exciton density, excitons do not follow the conveyer motion. We explore conveyer velocities both slower and faster than phonon velocities. Realizing subnanosecond manipulations of exciton energy and lifetime required versatile control of pulsed and multiple AC RF electrical signals in optical, liquid helium cryogenic systems. Considerable detail is presented of design, construction, and test of flexible experimental apparatus.
Thermodynamic efficiency limit of excitonic solar cells
Giebink, Noel C.; Wiederrecht, Gary P.; Wasielewski, Michael R.; Forrest, Stephen R.
2011-01-01
Excitonic solar cells, comprised of materials such as organic semiconductors, inorganic colloidal quantum dots, and carbon nanotubes, are fundamentally different than crystalline, inorganic solar cells in that photogeneration of free charge occurs through intermediate, bound exciton states. Here, we show that the Second Law of Thermodynamics limits the maximum efficiency of excitonic solar cells below the maximum of 31% established by Shockley and Queisser [J. Appl. Phys. 32, 510 (1961)] for inorganic solar cells (whose exciton-binding energy is small). In the case of ideal heterojunction excitonic cells, the free energy for charge transfer at the interface, ?G, places an additional constraint on the limiting efficiency due to a fundamental increase in the recombination rate, with typical -?G in the range 0.3 to 0.5 eV decreasing the maximum efficiency to 27% and 22%, respectively.
The structure and dynamics of molecular excitons.
Bardeen, Christopher J
2014-01-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet â†” triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion. PMID:24313684
The Structure and Dynamics of Molecular Excitons
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-04-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet -- triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion.
NASA Astrophysics Data System (ADS)
Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel
2015-03-01
Monolayer transition-metal dichalcogenides form a direct bandgap predicted in the visible regime making them attractive host materials for various electronic and optoelectronic applications. Due to a weak dielectric screening in these materials, strongly bound electron-hole pairs or excitons have binding energies up to at least several hundred meV's. While the conventional wisdom is to think of excitons as hydrogen-like quasi-particles, we show that the hydrogen model breaks down for these experimentally observed strongly bound, room-temperature excitons. To capture these non-hydrogen-like photo-excitations, we introduce an atomistic model for excitons that predicts both bright excitons and dark excitons, and their broken degeneracy in these two-dimensional materials. For strongly bound exciton states, the lattice potential significantly distorts the envelope wave functions, which affects predicted exciton peak energies. The combination of large binding energies and non-degeneracy of exciton states in monolayer transition metal dichalogendies may furthermore be exploited in room temperature applications where prolonged exciton lifetimes are necessary. This work has been funded by the Office of Naval Research (ONR), directly and through the Naval Research Laboratory (NRL). F.T and E.S acknowledge support from NRL through the NRC Research Associateship Program and ONR Summer Faculty Program, respectively.
Can Disorder Enhance Incoherent Exciton Diffusion?
Lee, Elizabeth M Y; Tisdale, William A; Willard, Adam P
2015-07-30
Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we introduce a general model, based upon Förster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates, which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific hopping rates is broadened in a manner that results in a decrease in average exciton diffusivity relative to that in a perfectly ordered film. Second, since excitons prefer to make transitions that are downhill in energy, the steady state distribution of exciton energies is biased toward low-energy molecular subunits, those that exhibit reduced diffusivity relative to a perfectly ordered film. These effects combine to reduce the net diffusivity in a manner that is time dependent and grows more pronounced as disorder is increased. Notably, however, we demonstrate that the presence of energetic disorder can give rise to a population of molecular subunits with exciton transfer rates exceeding those of subunits in an energetically uniform material. Such enhancements may play an important role in processes that are sensitive to molecular-scale fluctuations in exciton density field. PMID:26106811
Mirror with thermally controlled radius of curvature
Neil, George R.; Shinn, Michelle D.
2010-06-22
A radius of curvature controlled mirror for controlling precisely the focal point of a laser beam or other light beam. The radius of curvature controlled mirror provides nearly spherical distortion of the mirror in response to differential expansion between the front and rear surfaces of the mirror. The radius of curvature controlled mirror compensates for changes in other optical components due to heating or other physical changes. The radius of curvature controlled mirror includes an arrangement for adjusting the temperature of the front surface and separately adjusting the temperature of the rear surface to control the radius of curvature. The temperature adjustment arrangements can include cooling channels within the mirror body or convection of a gas upon the surface of the mirror. A control system controls the differential expansion between the front and rear surfaces to achieve the desired radius of curvature.
Superfluorescence spectra of excitons in quantum wells
NASA Astrophysics Data System (ADS)
GrÃ¼nwald, P.; Burau, G. K. G.; Stolz, H.; Vogel, W.
2013-11-01
We study the fluorescence light emitted from GaAs excitons in semiconductor quantum wells. The excitons are modeled as interacting bosons. By combining quantum optical methods for the excitonic emission spectrum with many particle descriptions of the transmission through the medium, we can evaluate the spectra outside the well. Comparing with experimental spectra, we get a very good agreement. The method helps to explain the main features of the observed spectra. It is demonstrated that the observed spectra show clear evidence of superfluorescent emission.
Exciton dephasing in quantum dot molecules.
Borri, P; Langbein, W; Woggon, U; Schwab, M; Bayer, M; Fafard, S; Wasilewski, Z; Hawrylak, P
2003-12-31
We have measured the exciton dephasing time in InAs/GaAs quantum dot molecules having different interdot barrier thicknesses in the temperature range from 5 to 60 K, using a highly sensitive four-wave mixing heterodyne technique. At 5 K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed. These results show how the quantum-mechanical coupling of the electronic wave functions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction. PMID:14754087
Ultrahigh exciton diffusion in intrinsic diamond
NASA Astrophysics Data System (ADS)
Morimoto, Hikaru; Hazama, Yuji; Tanaka, Koichiro; Naka, Nobuko
2015-11-01
We observe exceptionally high diffusion of excitons in diamond using time-resolved photoluminescence imaging. The diffusivity is found to increase with decreasing temperature more rapidly than well-established predictions. The highest diffusion constant, 9.2 ×103cm2/s , measured for thermalized excitons is 150 times that recorded previously in diamond. We elucidate the momentum relaxation mechanisms determining transport: intraband and interband scattering by acoustic phonons in the exciton fine-structure levels. The enhanced diffusivity above 100 K is explained by a free-carrier contribution.
Excitons in the Fractional Quantum Hall Effect
DOE R&D Accomplishments Database
Laughlin, R. B.
1984-09-01
Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.
Excitation energy dependence of the exciton inner ring
NASA Astrophysics Data System (ADS)
Kuznetsova, Y. Y.; Leonard, J. R.; Butov, L. V.; Wilkes, J.; Muljarov, E. A.; Campman, K. L.; Gossard, A. C.
2012-04-01
We report on the excitation energy dependence of the inner ring in the exciton emission pattern. The contrast of the inner ring is found to decrease with lowering excitation energy. Excitation by light tuned to the direct exciton resonance is found to effectively suppress excitation-induced heating of indirect excitons and facilitate the realization of a cold and dense exciton gas. The excitation energy dependence of the inner ring is explained in terms of exciton transport and cooling.
Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong
2015-05-13
The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. It is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system. PMID:25641931
Multiple Exciton Generation Solar Cells
Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.
2012-01-01
Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.
Singlet exciton fission in solution.
Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight. PMID:24256865
Singlet exciton fission in solution
NASA Astrophysics Data System (ADS)
Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.
2013-12-01
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.
Bohr effect and temperature sensitivity of hemoglobins from highland and lowland deer mice.
Jensen, Birgitte; Storz, Jay F; Fago, Angela
2016-05-01
An important means of physiological adaptation to environmental hypoxia is an increased oxygen (O2) affinity of the hemoglobin (Hb) that can help secure high O2 saturation of arterial blood. However, the trade-off associated with a high Hb-O2 affinity is that it can compromise O2 unloading in the systemic capillaries. High-altitude deer mice (Peromyscus maniculatus) have evolved an increased Hb-O2 affinity relative to lowland conspecifics, but it is not known whether they have also evolved compensatory mechanisms to facilitate O2 unloading to respiring tissues. Here we investigate the effects of pH (Bohr effect) and temperature on the O2-affinity of high- and low-altitude deer mouse Hb variants, as these properties can potentially facilitate O2 unloading to metabolizing tissues. Our experiments revealed that Bohr factors for the high- and low-altitude Hb variants are very similar in spite of the differences in O2-affinity. The Bohr factors of deer mouse Hbs are also comparable to those of other mammalian Hbs. In contrast, the high- and low-altitude variants of deer mouse Hb exhibited similarly low temperature sensitivities that were independent of red blood cell anionic cofactors, suggesting an appreciable endothermic allosteric transition upon oxygenation. In conclusion, high-altitude deer mice have evolved an adaptive increase in Hb-O2 affinity, but this is not associated with compensatory changes in sensitivity to changes in pH or temperature. Instead, it appears that the elevated Hb-O2 affinity in high-altitude deer mice is compensated by an associated increase in the tissue diffusion capacity of O2 (via increased muscle capillarization), which promotes O2 unloading. PMID:26808972
Optical nutation in the exciton range of spectrum
Khadzhi, P. I.; Vasiliev, V. V.
2013-08-15
Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.
Tailoring Quantum Dot Assemblies to Extend Exciton Coherence Times and Improve Exciton Transport
NASA Astrophysics Data System (ADS)
Seward, Kenton; Lin, Zhibin; Lusk, Mark
2012-02-01
The motion of excitons through nanostructured assemblies plays a central role in a wide range of physical phenomena including quantum computing, molecular electronics, photosynthetic processes, excitonic transistors and light emitting diodes. All of these technologies are severely handicapped, though, by quasi-particle lifetimes on the order of a nanosecond. The movement of excitons must therefore be as efficient as possible in order to move excitons meaningful distances. This is problematic for assemblies of small Si quantum dots (QDs), where excitons quickly localize and entangle with dot phonon modes. Ensuing exciton transport is then characterized by a classical random walk reduced to very short distances because of efficient recombination. We use a combination of master equation (Haken-Strobl) formalism and density functional theory to estimate the rate of decoherence in Si QD assemblies and its impact on exciton mobility. Exciton-phonon coupling and Coulomb interactions are calculated as a function of dot size, spacing and termination to minimize the rate of intra-dot phonon entanglement. This extends the time over which more efficient exciton transport, characterized by partial coherence, can be maintained.
Intrinsic frame inverse mass tensor as a function of {beta} and {gamma} in the Bohr Hamiltonian
Jolos, R. V.; Brentano, P. von
2012-04-15
Analytical expressions are derived for the components of the intrinsic frame inverse mass tensor of the Bohr Hamiltonian. These expressions contain parameters which are determined by the experimental data on the B(E2)'s and the excitation energies of the low-lying collective states. It is shown that the nondiagonal component of the intrinsic frame mass tensor has a small effect on the collective motion. It is shown also that the values of the B{sub {beta}{beta}}, B{sub {gamma}{gamma}} and the rotational mass coefficientB{sub 1} differ in the well-deformed nuclei by factor 3 or more.
AGU's historical records move to the Niels Bohr Library and Archives
NASA Astrophysics Data System (ADS)
Harper, Kristine C.
2012-11-01
As scientists, AGU members understand the important role data play in finding the answers to their research questions: no dataâ€”no answers. The same holds true for the historians posing research questions concerning the development of the geophysical sciences, but their data are found in archival collections comprising the personal papers of geophysicists and scientific organizations. Now historians of geophysicsâ€”due to the efforts of the AGU History of Geophysics Committee, the American Institute of Physics (AIP), and the archivists of the Niels Bohr Library and Archives at AIPâ€”have an extensive new data source: the AGU manuscript collection.
Exactly separable Bohr Hamiltonian with the Morse potential for triaxial nuclei
NASA Astrophysics Data System (ADS)
Inci, I.
2014-08-01
In this paper, the Morse potential is used in the ?-part of the collective Bohr Hamiltonian for triaxial nuclei. Energy eigenvalues and eigenfunctions are obtained in a closed form through exactly separating the Hamiltonian into its variables by using an appropriate form of the potential. The results are applied to generate the nuclear spectrum of 192Pt, 194Pt and 196Pt isotopes which are known to be the best candidate exhibiting triaxiality. Electric quadrupole transition ratios are calculated and then compared with the experimental data and the Z(5) model results.
Bohr Hamiltonian with Hulthén plus ring-shaped potential for triaxial nuclei
NASA Astrophysics Data System (ADS)
Chabab, M.; Lahbas, A.; Oulne, M.
2015-10-01
In this paper, we solve the eigenvalues and eigenvectors problem with the Bohr collective Hamiltonian for triaxial nuclei. The ?-part of the collective potential is taken to be equal to the Hulthén potential while the ?-part is defined by a new generalized potential obtained from a ring-shaped one. Analytical expressions for spectra and wave functions are derived by means of a recent version of the asymptotic iteration method and the usual approximations. The calculated energies and B( E2) transition rates are compared with experimental data and the available theoretical results in the literature.
Cavity-Enhanced Transport of Excitons
NASA Astrophysics Data System (ADS)
Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido
2015-05-01
We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions.
Cavity-enhanced transport of excitons.
Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido
2015-05-15
We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions. PMID:26024186
Triplet excitons: Bringing dark states to light
NASA Astrophysics Data System (ADS)
Bardeen, Christopher J.
2014-11-01
Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.
Exciton properties of selected aromatic hydrocarbon systems
NASA Astrophysics Data System (ADS)
Roth, Friedrich; Mahns, Benjamin; Hampel, Silke; Nohr, Markus; Berger, Helmuth; Büchner, Bernd; Knupfer, Martin
2013-02-01
We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.
Exciton-polariton wakefields in semiconductor microcavities
NASA Astrophysics Data System (ADS)
TerÃ§as, H.; MendonÃ§a, J. T.
2016-02-01
We consider the excitation of polariton wakefields due to a propagating light pulse in a semiconductor microcavity. We show that two kinds of wakes are possible, depending on the constituents fraction (either exciton or photon) of the polariton wavefunction. The nature of the wakefields (pure excitonic or polaritonic) can be controlled by changing the speed of propagation of the external pump. This process could be used as a diagnostic for the internal parameters of the microcavity.
Anisotropic exciton Stark shift in black phosphorus
NASA Astrophysics Data System (ADS)
Chaves, A.; Low, Tony; Avouris, P.; ?ak?r, D.; Peeters, F. M.
2015-04-01
We calculate the excitonic spectrum of few-layer black phosphorus by direct diagonalization of the effective mass Hamiltonian in the presence of an applied in-plane electric field. The strong attractive interaction between electrons and holes in this system allows one to investigate the Stark effect up to very high ionizing fields, including also the excited states. Our results show that the band anisotropy in black phosphorus becomes evident in the direction-dependent field-induced polarizability of the exciton.
Diffusion of a Plasmon-Exciton Polaron
NASA Astrophysics Data System (ADS)
Cherqui, Charles; Dunlap, David; Piryatinski, Andrei
2012-02-01
We consider the motion of an exciton constrained to a quasi-one-dimensional geometry in the vicinity of a metal interface. For weak coupling, the metal causes damping of the center of mass motion, leading to a decrease in the exciton diffusion constant. This can be modeled as non-contact dielectric friction between an oscillating dipole and a substrate, where the frictional force is related to the response of the metal through the fluctuation dissipation theorem [1]. When the exciton frequency is in the neighborhood of the plasma resonance, the interaction can no longer be described by linear response theory, for the exciton and plasmon form a quasiparticle, an exciton-plasmon polaron. We calculate the transmission and reflection coefficients for the exciton-plasmon polaron in the neighborhood of a metal interface, as well as the diffusion rate and radiative lifetime versus coupling strength.[4pt] [1] Seppe Kuehn, John A. Marohn, and Roger F. Loring, 110(30) J. Phys. Chem. B, (2006) 1425
Exciton quasicondensation in one-dimensional systems
NASA Astrophysics Data System (ADS)
Werman, Yochai; Berg, Erez
2015-06-01
Two Luttinger liquids, with an equal density and opposite sign of charge carriers, may exhibit enhanced excitonic correlations. We term such a system an exciton quasicondensate, with a possible realization being two parallel oppositely doped quantum wires, coupled by repulsive Coulomb interactions. We show that this quasiexciton condensate can be stabilized in an extended range of parameters, in both spinless and spinful systems. We calculate the interwire tunneling current-voltage characteristic, and find that a negative differential conductance is a signature of excitonic correlations. For spinful electrons, the excitonic regime is shown to be distinct from the usual quasi-long-range ordered Wigner crystal phase characterized by power-law density wave correlations. The two phases can be clearly distinguished through their interwire tunneling current-voltage characteristics. In the quasiexciton condensate regime the tunneling conductivity diverges at low temperatures and voltages, whereas in the Wigner crystal it is strongly suppressed. Both the Wigner crystal and the excitonic regime are characterized by a divergent Coulomb drag at low temperature. Finally, metallic carbon nanotubes are considered as a special case of such a one-dimensional setup, and it is shown that exciton condensation is favorable due to the additional valley degree of freedom.
Exciton Resonances in Novel Silicon Carbide Polymers
NASA Astrophysics Data System (ADS)
Burggraf, Larry; Duan, Xiaofeng
2015-05-01
A revolutionary technology transformation from electronics to excitionics for faster signal processing and computing will be advantaged by coherent exciton transfer at room temperature. The key feature required of exciton components for this technology is efficient and coherent transfer of long-lived excitons. We report theoretical investigations of optical properties of SiC materials having potential for high-temperature excitonics. Using Car-Parinello simulated annealing and DFT we identified low-energy SiC molecular structures. The closo-Si12C12 isomer, the most stable 12-12 isomer below 1100 C, has potential to make self-assembled chains and 2-D nanostructures to construct exciton components. Using TDDFT, we calculated the optical properties of the isomer as well as oligomers and 2-D crystal formed from the isomer as the monomer unit. This molecule has large optical oscillator strength in the visible. Its high-energy and low-energy transitions (1.15 eV and 2.56 eV) are nearly pure one-electron silicon-to-carbon transitions, while an intermediate energy transition (1.28 eV) is a nearly pure carbon-to-silicon one-electron charge transfer. These results are useful to describe resonant, coherent transfer of dark excitons in the nanostructures. Research supported by the Air Force Office of Scientific Research.
Russell, Bianca; Johnston, Jennifer J; Biesecker, Leslie G; Kramer, Nancy; Pickart, Angela; Rhead, William; Tan, Wen-Hann; Brownstein, Catherine A; Kate Clarkson, L; Dobson, Amy; Rosenberg, Avi Z; Vergano, Samantha A Schrier; Helm, Benjamin M; Harrison, Rachel E; Graham, John M
2015-09-01
Bohring-Opitz syndrome is a rare genetic condition characterized by distinctive facial features, variable microcephaly, hypertrichosis, nevus flammeus, severe myopia, unusual posture (flexion at the elbows with ulnar deviation, and flexion of the wrists and metacarpophalangeal joints), severe intellectual disability, and feeding issues. Nine patients with Bohring-Opitz syndrome have been identified as having a mutation in ASXL1. We report on eight previously unpublished patients with Bohring-Opitz syndrome caused by an apparent or confirmed de novo mutation in ASXL1. Of note, two patients developed bilateral Wilms tumors. Somatic mutations in ASXL1 are associated with myeloid malignancies, and these reports emphasize the need for Wilms tumor screening in patients with ASXL1 mutations. We discuss clinical management with a focus on their feeding issues, cyclic vomiting, respiratory infections, insomnia, and tumor predisposition. Many patients are noted to have distinctive personalities (interactive, happy, and curious) and rapid hair growth; features not previously reported. PMID:25921057
Exciton formation and diffusion in OLEDs (Presentation Recording)
NASA Astrophysics Data System (ADS)
Ingram, Grayson L.; Lu, Zheng-Hong
2015-10-01
This talk will discuss recent experiments designed to study the formation of excitons and their subsequent diffusions in OLEDs. These experimental results suggest that contrary to conventional wisdom, host singlet exciton diffusion can occur over long distances, while host triplet excitons are confined close to the exciton formation region for the archetype host and hole transport layer CBP. The exciton formation mechanism is studied and we show that the ratio of excitons formed on the host to excitons formed on the dopant varies strongly with the applied voltage. Refinements to models of efficiency roll off are discussed in light of the improved understanding of exciton formation and we suggest design guidelines to improve efficiency by engineering exciton formation.
Exciton hopping probed by picosecond time-resolved cathodoluminescence
NASA Astrophysics Data System (ADS)
Shahmohammadi, Mehran; Jacopin, Gwénolé; Fu, Xuewen; Ganière, Jean-Daniel; Yu, Dapeng; Deveaud, Benoît
2015-10-01
The exciton transport is studied in high quality ZnO microwires using time resolved cathodoluminescence. Owing to the available picosecond temporal and nanometer spatial resolution, a direct estimation of the exciton average speed has been measured. When raising the temperature, a strong decrease of the effective exciton mobility (hopping speed of donor-bound excitons) has been observed in the absence of any remarkable change in the effective lifetime of excitons. Additionally, the exciton hopping speed was observed to be independent of the strain gradient value, revealing the hopping nature of exciton movement. These experimental results are in good agreement with the behavior predicted for impurity-bound excitons in our previously published theoretical model based on Monte-Carlo simulations, suggesting the hopping process as the main transport mechanism of impurity-bound excitons at low temperatures.
A Bohr-type model of a composite particle using gravity as the attractive force
NASA Astrophysics Data System (ADS)
Vayenas, C. G.; Souentie, S.; Fokas, A.
2014-07-01
We formulate a Bohr-type rotating particle model for three light particles of rest mass mo each, forming a bound rotational state under the influence of their gravitational attraction, in the same way that electrostatic attraction leads to the formation of a bound proton-electron state in the classical Bohr model of the H atom. By using special relativity, the equivalence principle and the de Broglie wavelength equation, we find that when each of the three rotating particles has the same rest mass as the rest mass of a neutrino or an antineutrino (˜0.05 eV/c2) then surprisingly the composite rotating state has the rest mass of the stable baryons, i.e. of the proton and the neutron (˜1 GeV/c2). This rest mass is due almost exclusively to the kinetic energy of the rotating particles. The results are found to be consistent with the theory of general relativity. The model contains no unknown parameters, describes both asymptotic freedom and confinement and also provides good agreement with QCD regarding the QCD condensation temperature. Predictions for the thermodynamic and other physical properties of these bound rotational states are compared with experimental values.
The boundary conditions for Bohr's law: when is reacting faster than acting?
Pinto, Yaïr; Otten, Marte; Cohen, Michael A; Wolfe, Jeremy M; Horowitz, Todd S
2011-02-01
In gunfights in Western movies, the hero typically wins, even though the villain draws first. Niels Bohr (Gamow, The great physicists from Galileo to Einstein. Chapter: The law of quantum, 1988) suggested that this reflected a psychophysical law, rather than a dramatic conceit. He hypothesized that reacting is faster than acting. Welchman, Stanley, Schomers, Miall, and Bülthoff (Proceedings of the Royal Society of London B: Biological Sciences, 277, 1667-1674, 2010) provided empirical evidence supporting "Bohr's law," showing that the time to complete simple manual actions was shorter when reacting than when initiating an action. Here we probe the limits of this effect. In three experiments, participants performed a simple manual action, which could either be self-initiated or executed following an external visual trigger. Inter-button time was reliably faster when the action was externally triggered. However, the effect disappeared for the second step in a two-step action. Furthermore, the effect reversed when a choice between two actions had to be made. Reacting is faster than acting, but only for simple, ballistic actions. PMID:21264708
Einstein-Bohr recoiling double-slit gedanken experiment performed at the molecular level
NASA Astrophysics Data System (ADS)
Liu, Xiao-Jing; Miao, Quan; Gel'Mukhanov, Faris; Patanen, Minna; Travnikova, Oksana; Nicolas, Christophe; Ågren, Hans; Ueda, Kiyoshi; Miron, Catalin
2015-02-01
Double-slit experiments illustrate the quintessential proof for wave-particle complementarity. If information is missing about which slit the particle has traversed, the particle, behaving as a wave, passes simultaneously through both slits. This wave-like behaviour and corresponding interference is absent if ‘which-slit’ information exists. The essence of Einstein-Bohr's debate about wave-particle duality was whether the momentum transfer between a particle and a recoiling slit could mark the path, thus destroying the interference. To measure the recoil of a slit, the slits should move independently. We showcase a materialization of this recoiling double-slit gedanken experiment by resonant X-ray photoemission from molecular oxygen for geometries near equilibrium (coupled slits) and in a dissociative state far away from equilibrium (decoupled slits). Interference is observed in the former case, while the electron momentum transfer quenches the interference in the latter case owing to Doppler labelling of the counter-propagating atomic slits, in full agreement with Bohr's complementarity.
Microscopic theories of excitons and their dynamics
NASA Astrophysics Data System (ADS)
Berkelbach, Timothy C.
This thesis describes the development and application of microscopically-defined theories of excitons in a wide range of semiconducting materials. In Part I, I consider the topic of singlet exciton fission, an organic photophysical process which generates two spin-triplet excitons from one photoexcited spin-singlet exciton. I construct a theoretical framework that couples a realistic treatment of the static electronic structure with finite-temperature quantum relaxation techniques. This framework is applied separately, but consistently, to the problems of singlet fission in pentacene dimers, crystalline pentacene, and crystalline hexacene. Through this program, I am able to rationalize observed behaviors and make non-trivial predictions, some of which have been confirmed by experiment. In Part II, I present theoretical developments on the properties of neutral excitons and charged excitons (trions) in atomically thin transition metal dichalcogenides. This work includes an examination of material trends in exciton binding energies via an effective mass approach. I also present an experimental and theoretical collaboration, which links the unconventional disposition of excitons in the Rydberg series to the peculiar screening properties of atomically thin materials. The light-matter coupling in these materials is examined within low-energy models and is shown to give rise to bright and dark exciton states, which can be qualitatively labeled in analogy with the hydrogen series. In Part III, I explore theories of relaxation dynamics in condensed phase environments, with a focus on methodology development. This work is aimed towards biological processes, including resonant energy transfer in chromophore complexes and electron transfer in donor-bridge-acceptor systems. Specifically, I present a collaborative development of a numerically efficient but highly accurate hybrid approach to reduced dynamics, which exploits a partitioning of environmental degrees of freedom into those that evolve "fast" and "slow," as compared to the internal system dynamics. This method is tested and applied to the spin-boson model, a two-site Frenkel exciton model, and the seven-site Fenna-Matthews-Olson complex. I conclude with a collaborative analysis of a recently developed polaron-transformed quantum master equation, which is shown to accurately interpolate between the well-known Redfield and Forster theories, even in challenging donor-bridge-acceptor arrangements.
Yan, Yun-An
2016-01-14
The quantum interference is an intrinsic phenomenon in quantum physics for photon and massive quantum particles. In principle, the quantum interference may also occur with quasi-particles, such as the exciton. In this study, we show how the exciton quantum interference can be significant in aggregates through theoretical simulations with hierarchical equations of motion. The systems under investigation are generalized donor-bridge-acceptor model aggregates with the donor consisting of six homogeneous sites assuming the nearest neighbor coupling. For the models with single-path bridge, the exciton transfer time only shows a weak excitation energy dependence. But models with double-path bridge have a new short transfer time scale and the excitation energy dependence of the exciton transfer time assumes clear peak structure which is detectable with today's nonlinear spectroscopy. This abnormality is attributed to the exciton quantum interference and the condition for a clear observation in experiment is also explored. PMID:26772569
NASA Astrophysics Data System (ADS)
Yan, Yun-An
2016-01-01
The quantum interference is an intrinsic phenomenon in quantum physics for photon and massive quantum particles. In principle, the quantum interference may also occur with quasi-particles, such as the exciton. In this study, we show how the exciton quantum interference can be significant in aggregates through theoretical simulations with hierarchical equations of motion. The systems under investigation are generalized donor-bridge-acceptor model aggregates with the donor consisting of six homogeneous sites assuming the nearest neighbor coupling. For the models with single-path bridge, the exciton transfer time only shows a weak excitation energy dependence. But models with double-path bridge have a new short transfer time scale and the excitation energy dependence of the exciton transfer time assumes clear peak structure which is detectable with today's nonlinear spectroscopy. This abnormality is attributed to the exciton quantum interference and the condition for a clear observation in experiment is also explored.
Radius stabilization in a supersymmetric warped compactification
Eto, Minoru; Sakai, Norisuke; Maru, Nobuhito
2004-10-15
A supersymmetric (SUSY) model of radius stabilization is constructed for the S{sup 1}/Z{sub 2} warped compactifications with a hypermultiplet in five dimensions. Requiring the continuity of scalar field across the boundaries, we obtain radius stabilization preserving SUSY, realizing the SUSY extension of the Goldberger-Wise mechanism. Even if we allow discontinuity of the Z{sub 2} odd field across the boundary, we always obtain SUSY preservation but obtain the radius stabilization only when the discontinuity is fixed by other mechanisms.
Exciton transport and dissociation at organic interfaces
NASA Astrophysics Data System (ADS)
Beljonne, David
2011-03-01
This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length LD in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger LD values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules (> 50nm) to values actually measured in CPs (<10nm). 2) A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intra-layer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature and the presence of energetic disorder. 3) The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how these interactions in turn affect the energy landscape around the interface and therefore also the energy barrier for exciton dissociation into free carriers.
Large-Larmor-radius interchange instability
Ripin, B.H.; McLean, E.A.; Manka, C.K.; Pawley, C.; Stamper, J.A.; Peyser, T.A.; Mostovych, A.N.; Grun, J.; Hassam, A.B.; Huba, J.
1987-11-16
We observe linear and nonlinear features of a strong plasma/magnetic field interchange Rayleigh-Taylor instability in the limit of large ion Larmor radius. The instability undergoes rapid linear growth culminating in free-streaming flute tips.
Laser differential confocal paraboloidal vertex radius measurement.
Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Shen, Yang; Jiang, Hongwei
2014-02-15
This Letter proposes a laser differential confocal paraboloidal vertex radius measurement (DCPRM) method for the high-accuracy measurement of the paraboloidal vertex radius of curvature. DCPRM constructs an autocollimation vertex radius measurement light path for the paraboloid by placing a reflector in the incidence light path. This technique is based on the principle that a paraboloid can aim a parallel beam at its focus without aberration and uses differential confocal positioning technology to identify the paraboloid focus and vertex accurately. Measurement of the precise distance between these two positions is achieved to determine the paraboloid vertex radius. Preliminary experimental results indicate that DCPRM has a relative expanded uncertainty of less than 0.001%. PMID:24562218
Mass-radius relationships in icy satellites
NASA Technical Reports Server (NTRS)
Lupo, M. J.; Lewis, J. S.
1979-01-01
Using published laboratory data for H2O ice, a modeling technique was developed by which the bulk density, density and temperature profile, rotational moment of inertia, central pressure, and location of the rock-ice interface can all be obtained as a function of the radius, the heliocentric distance, and the silicate composition. Models of the interiors of Callisto, Ganymede, Europa, Rhea, and Titan are given, consistent with present mass and radius data. The radius and mass of spheres of ice under self-gravitation for two different temperature classes are given (103 and 77 deg K). Measurements of mass, radius and I/MR2 by spacecraft can be interpreted by this model to yield substantial information about the internal structure and the ice/rock ratio of the icy satellites of Jupiter and Saturn.
Photoexcitation and Exciton Transport in Molecular Crystals
NASA Astrophysics Data System (ADS)
Irkhin, Pavel
This work is dedicated to the investigation of exciton transport processes in organic molecular crystals, and to the optical characterization of an important organic semiconductor, rubrene single crystal. Rubrene is a high quality molecular crystal that has been found to have exceptional functionalities in electronic applications such as field effect transistors. I present the intrinsic absorption and photoluminescence spectra of rubrene single crystals, deriving them from a series of experiments performed in different experimental geometries. I describe the absorption spectra for all three principal light polarizations in the crystal, and discuss how the strongly anisotropic absorption and emission properties affect the spectral characteristics of observed photoluminescence spectra. I identify vibronic progressions both in absorption and emission and discuss their parameters and the main vibrational modes that are responsible for them. Through careful analysis of the data, I arrive at a conclusion that absorption and emission of rubrene that is not polarized along a specific crystallographic and molecular direction (c-axis of the crystal, corresponding to the M-axis of the molecule) is not due to an electronic dipole matrix element that has components in that direction. Instead, it is caused by vibronically-induced depolarization of the electronic HOMO-LUMO transition that is described by a dipole matrix element that has components along the c-axis. Further, this work developed and demonstrated a direct imaging technique that allowed to directly observe the diffusion of excitons in rubrene single crystals. This simple and intuitive technique uses localized photoexcitation and spatially resolved detection of the excitonic luminescence to visualize the spatial distribution of excitons. The technique was then used to directly measure the diffusion length of triplet excitons in rubrene. To do this, I exploited the fact that in rubrene photoexcited singlet excitons undergo efficient fission into triplet excitons, and the latter can interact with each other to create photoluminescing singlet excitons again. I show that the exciton mobility in rubrene is strongly anisotropic, with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, the same direction for which a large charge carrier mobility has been observed in experiments with field effect transistors.
Exciton-polaritonic quasicrystalline and aperiodic structures
NASA Astrophysics Data System (ADS)
Poddubny, A. N.; Pilozzi, L.; Voronov, M. M.; Ivchenko, E. L.
2009-09-01
We have theoretically studied propagation of exciton-polaritons in deterministic aperiodic multiple-quantum-well structures, particularly, in the Fibonacci and Thue-Morse chains. The attention is concentrated on the structures tuned to the resonant Bragg condition with two-dimensional quantum-well exciton. Depending on the number of wells, the super-radiant either photonic-quasicrystal regimes are realized in these aperiodic structures. For moderate values of the exciton nonradiative damping rate ? , the developed theory based on the two-wave approximation allows one to perceive and describe analytically the exact transfer-matrix computations for transmittance and reflectance spectra in the whole frequency range except for a narrow region near the exciton resonance ?0 . In this region the optical spectra and the exciton-polariton dispersion demonstrate scaling invariance and self-similarity which can be interpreted in terms of the “band-edge” cycle of the trace map, in the case of Fibonacci structures, and in terms of zero reflection frequencies, in the case of Thue-Morse structures. With decreasing ? , in the whole allowed polariton band the two-wave approximation stops to be valid, and a transition occurs from Bloch-like to localized states, with modes closer to ?0 becoming localized first.
Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides
NASA Astrophysics Data System (ADS)
Hao, Kai; Moody, Galan; Dass, Chandriker; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Bergäuser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin
2015-03-01
Excitons in monolayer transition metal dichalcogenides (TMDCs) exhibit exceptionally large binding energy, strong optical absorption, and spin valley coupling. These characteristics make TMDCs a promising system for optoelectronics and valleytronics. An important yet unknown property of excitons in TMDCs is the intrinsic homogeneous linewidth, which reflect radiative recombination and irreversible dissipative decay. Here, we use optical coherent two-dimensional spectroscopy to reveal the exciton homogeneous linewidth in monolayer CVD grown Tungsten Diselenide (WSe2). With excitation density and temperature dependent measurements, exciton-exciton interaction and exciton-phonon interactions are quantitatively evaluated. Extrapolating to zero density and temperature, we obtain a residual homogeneous linewidth of ~ 1.5 meV, which places a lower bound of 0.2 ps on the exciton radiative lifetime. This result is consistent with microscopic calculations, which suggest that fast radiative decay of delocalized excitons arises from their large oscillator strength. We acknowledge AFOSR and NSF for funding.
ERIC Educational Resources Information Center
Gjedde, Albert
2010-01-01
The year 2010 is the centennial of the publication of the "Seven Little Devils" in the predecessor of "Acta Physiologica". In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activityâ€¦
Inci, I.; Boztosun, I.; Bonatsos, D.
2008-11-11
Analytical solutions of the collective Bohr Hamiltonian with the Morse potential have been obtained for the U(5)-O(6) and U(5)-SU(3) transition regions through the Asymptotic Iteration Method (AIM). The obtained energy eigenvalue equations have been used to get the experimental excitation energy spectrum of Xe and Yb isotopes. The results are in good agreement with experimental data.
ERIC Educational Resources Information Center
Gjedde, Albert
2010-01-01
The year 2010 is the centennial of the publication of the "Seven Little Devils" in the predecessor of "Acta Physiologica". In these seven papers, August and Marie Krogh sought to refute Christian Bohr's theory that oxygen diffusion from the lungs to the circulation is not entirely passive but rather facilitated by a specific cellular activity…
NASA Astrophysics Data System (ADS)
Fennel, Franziska; Lochbrunner, Stefan
2015-10-01
Exciton annihilation dynamics in a disordered organic model system is investigated by ultrafast absorption spectroscopy. We show that the temporal evolution of the exciton density can be quantitatively understood by applying Förster energy transfer theory to describe the diffusion of the excitons as well as the annihilation step itself. To this end, previous formulations of Förster theory are extended to account for the inhomogeneous distribution of the S0-S1 transition energies resulting in an effective exciton diffusion constant. Two annihilation pathways are considered, the direct transfer of an exciton between two excited molecules and diffusive motion by multiple transfer steps towards a second exciton preceding the annihilation event. One pathway can be emphasized with respect to the other by tuning the exciton diffusion constant via the chromophore concentration. The investigated system allows one to extract all relevant parameters for the description and provides in this way a proof that the annihilation dynamics can be entirely understood and modeled by Förster energy transfer.
Exciton-Exciton Annihilation in Copper-Phthalocyanine Single-Crystal Nanowires
Ma, Yingzhong; Xiao, Kai; Shaw, Robert W
2012-01-01
Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a bi-exponential decay function with lifetimes of 22 and 204 ps, and corresponding relative amplitudes depending on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. While this phenomenon has been found previously in crystalline metallophthalocyanine (MPc) polymorphs such as colloidal particles and thin films, the results obtained using the CuPc nanowires are markedly distinct, namely, much longer decay times and a linear intensity dependence of the initial peak amplitudes. Despite these differences, detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intra-chain exciton diffusion along 1D molecular stacks. The significantly long-lived excitons of CuPc nanowires in comparison to those of other crystalline polymorphs make them particularly suitable for photovoltaic applications.
Bohr Hamiltonian with a deformation-dependent mass term for the Davidson potential
Bonatsos, Dennis; Georgoudis, P. E.; Lenis, D.; Minkov, N.; Quesne, C.
2011-04-15
Analytical expressions for spectra and wave functions are derived for a Bohr Hamiltonian, describing the collective motion of deformed nuclei, in which the mass is allowed to depend on the nuclear deformation. Solutions are obtained for separable potentials consisting of a Davidson potential in the {beta} variable, in the cases of {gamma}-unstable nuclei, axially symmetric prolate deformed nuclei, and triaxial nuclei, implementing the usual approximations in each case. The solution, called the deformation-dependent mass (DDM) Davidson model, is achieved by using techniques of supersymmetric quantum mechanics (SUSYQM), involving a deformed shape invariance condition. Spectra and B(E2) transition rates are compared to experimental data. The dependence of the mass on the deformation, dictated by SUSYQM for the potential used, reduces the rate of increase of the moment of inertia with deformation, removing a main drawback of the model.
Wo die Quantentheorie Wurzeln schlug: Die Schulen von Sommerfeld, Bohr und Born
NASA Astrophysics Data System (ADS)
Eckert, Michael
Die Geburtsstunde der Quantentheorie liegt hundert Jahre zurÃ¼ck, aber es dauerte ein Vierteljahrhundert, bis die neue Physik Wurzeln schlug und daraus die Quantenmechanik hervorging. Auf dem Weg dahin spielten die frÃ¼hen Schulen der theoretischen Physik eine wichtige Rolle. Das erste dieser Zentren war die von Arnold Sommerfeld an der MÃ¼nchner UniversitÃ¤t gegrÃ¼ndete PflanzstÃ¤tte fÃ¼r theoretische Physik. In Niels Bohrs 1921 erÃ¶ffneten Institut fÃ¼r theoretische Physik gaben sich die hoffnungsvollen Nachwuchswissenschaftler die Klinke in die Hand, und in Deutschland war nach dem Ersten Weltkrieg die UniversitÃ¤t GÃ¶ttingen eine der ersten Adressen fÃ¼r moderne Physik. Der dortige Mentor war Max Born.
NASA Astrophysics Data System (ADS)
Chabab, M.; Lahbas, A.; Oulne, M.
2015-06-01
In a recent work [Phys. Rev. C 84, 044321 (2011), 10.1103/PhysRevC.84.044321] M. J. Ermamatov and P. R. Fraser have studied rotational and vibrational excited states of axially symmetric nuclei within the Bohr Hamiltonian with different mass parameters. However, the energy formula that the authors have used contains some inaccuracies. So the numerical results they obtained seem to be controversial. In this paper, we revisit all calculations related to this problem and determine the appropriate formula for the energy spectrum. Moreover, in order to improve such calculations, we reconsider this problem within the framework of the deformation-dependent mass formalism. Also, unlike the work of Bonatsos et al. [Phys. Rev. C 83, 044321 (2011), 10.1103/PhysRevC.83.044321], in which the mass parameter has not been considered, we will show the importance of this parameter and its effect on numerical predictions.
Inspirations from the theories of Bohr and Mottelson: a Canadian perspective
NASA Astrophysics Data System (ADS)
Ward, D.; Waddington, J. C.; Svensson, C. E.
2016-03-01
The theories developed by Bohr and Mottelson have inspired much of the world-wide experimental investigation into the structure of the atomic nucleus. On the occasion of the 40th anniversary of the awarding of their Nobel prize, we reflect on some of the experimental developments made in understanding the structure of nuclei. We have chosen to focus on experiments performed in Canada, or having strong ties to Canada, and the work included here spans virtually the whole of the second half of the 20th century. The 8Ï€ Spectrometer, which figures prominently in this story, was a novel departure for funding science in Canada that involved an intimate collaboration between a Crown Corporation (Atomic Energy of Canada Ltd) and University research, and enabled many of the insights discussed here.
Bohr-van Leeuwen theorem and the thermal Casimir effect for conductors
NASA Astrophysics Data System (ADS)
Bimonte, Giuseppe
2009-04-01
The problem of estimating the thermal corrections to the Casimir and Casimir-Polder interactions in systems involving conducting plates has attracted considerable attention in the recent literature on dispersion forces. Alternative theoretical models, based on distinct low-frequency extrapolations of the plate’s reflection coefficient for transverse electric (TE) modes, provide widely different predictions for the magnitude of this correction. In this paper we examine the most widely used prescriptions for this reflection coefficient from the point of view of their consistency with the Bohr-van Leeuwen theorem of classical statistical physics, stating that at thermal equilibrium transverse electromagnetic fields decouple from matter in the classical limit. We find that the theorem is satisfied if and only if the TE reflection coefficient vanishes at zero frequency in the classical limit. This criterion appears to rule out some of the models that have been considered recently for describing the thermal correction to the Casimir pressure with nonmagnetic metallic plates.
Analytical solutions for the Bohr Hamiltonian with the Woods-Saxon potential
NASA Astrophysics Data System (ADS)
Çapak, M.; Petrellis, D.; Gönül, B.; Bonatsos, Dennis
2015-09-01
Approximate analytical solutions in closed form are obtained for the five-dimensional Bohr Hamiltonian with the Woods-Saxon potential, taking advantage of the Pekeris approximation and the exactly solvable one-dimensional extended Woods-Saxon potential with a dip near its surface. Comparison with the data for several ?-unstable and prolate deformed nuclei indicates that the potential can describe well the ground state and {? }1 bands of many prolate deformed nuclei corresponding to a large enough ‘well size’ and diffuseness, while it fails in describing the {? }1 bands, due to its lack of a hard core, as well as in describing ?-unstable nuclei, because of the small ‘well size’ and diffuseness they exhibit.
The ``graviton picture'': a Bohr model for gravitation on galactic scales?
NASA Astrophysics Data System (ADS)
Trippe, Sascha
2015-02-01
Modified Newtonian Dynamics (MOND) provides a successful description of stellar and galactic dynamics on almost all astronomical scales. A key feature of MOND is the transition function from Newtonian to modified dynamics which corresponds to the empirical mass discrepancy--acceleration (MDA) relation. However, the functional form of the MDA relation does not follow from theory in a straightforward manner; in general, empirical MDA relations are inserted ad hoc into analyses of stellar dynamics. I revisit the possibility of gravity being mediated by massive virtual particles, gravitons. Under certain reasonable assumptions, the resulting "graviton picture" implies a MDA relation that is equivalent to the - empirical - "simple mu" function of MOND which is in very good agreement with observations. I conclude that the "graviton picture" offers a simple description of gravitation on galactic scales, potentially playing a role for gravitation analogous to the role played by Bohr's model for atomic physics.
Mass tensor in the Bohr Hamiltonian from the nondiagonal energy weighted sum rules
Jolos, R. V.; Brentano, P. von
2009-04-15
Relations are derived in the framework of the Bohr Hamiltonian that express the matrix elements of the deformation-dependent components of the mass tensor through the experimental data on the energies and the E2 transitions relating the low-lying collective states. These relations extend the previously obtained results for the intrinsic mass coefficients of the well-deformed axially symmetric nuclei on nuclei of arbitrary shape. The expression for the mass tensor is suggested, which is sufficient to satisfy the existing experimental data on the energy weighted sum rules for the E2 transitions for the low-lying collective quadrupole excitations. The mass tensor is determined for {sup 106,108}Pd, {sup 108-112}Cd, {sup 134}Ba, {sup 150}Nd, {sup 150-154}Sm, {sup 154-160}Gd, {sup 164}Dy, {sup 172}Yb, {sup 178}Hf, {sup 188-192}Os, and {sup 194-196}Pt.
Excitons in Time-Dependent Density-Functional Theory.
Ullrich, Carsten A; Yang, Zeng-hui
2016-01-01
This chapter gives an overview of the description of the optical and dielectric properties of bulk insulators and semiconductors in time-dependent density-functional theory (TDDFT), with an emphasis on excitons. We review the linear-response formalism for periodic solids, discuss excitonic exchange-correlation kernels, calculate exciton binding energies for various materials, and compare the treatment of excitons with TDDFT and with the Bethe-Salpeter equation. PMID:25805143
Disorder-enhanced exciton delocalization in an extended dendrimer
NASA Astrophysics Data System (ADS)
Pouthier, Vincent
2014-08-01
The exciton dynamics in a disordered extended dendrimer is investigated numerically. Because a homogeneous dendrimer exhibits few highly degenerate energy levels, a dynamical localization arises when the exciton is initially located on the periphery. However, it is shown that the disorder lifts the degeneracy and favors a delocalization-relocalization transition. Weak disorder enhances the delocalized nature of the exciton and improves any quantum communication, whereas strong disorder prevents the exciton from propagating in accordance with the well-known Anderson theory.
Image excitons and plasmon-exciton strong coupling in two-dimensional perovskite semiconductors
NASA Astrophysics Data System (ADS)
Niu, Wendy; Ibbotson, Lindsey A.; Leipold, David; Runge, Erich; Prakash, G. Vijaya; Baumberg, Jeremy J.
2015-04-01
We present evidence for "image biexcitons" within organic-inorganic perovskite-coated silver gratings. These composite quasiparticles are formed by the interaction between an exciton and its image in the metal mirror below, with binding energy 100 meV at room temperature. By changing the polar and azimuthal angles of the incident light, we observe strong coupling between excitons and surface plasmon polaritons on the grating, with Rabi splittings of 150 and 125 meV for the exciton and biexciton, respectively. A detailed analysis of the field polarizations and dipole orientations shows how these Rabi couplings arise from the strongly compressed field volume.
Observation of charged excitons in V-groove quantum wires
NASA Astrophysics Data System (ADS)
Otterburg, T.; Oberli, D. Y.; Dupertuis, M.-A.; Dwir, B.; Pelucchi, E.; Kapon, E.
2004-02-01
We report on the observation of negatively and positively charged excitons in the photoluminescence spectra of V-groove quantum wires. The charged exciton binding energy increases with the strength of the quantum confinement. We demonstrate that the charged excitons are localized by the fluctuations of the confinement potential and estimate a minimal value of the localization length.
The dynamics of one-dimensional excitons in liquids
NASA Astrophysics Data System (ADS)
van Burgel, Mirjam; Wiersma, Douwe A.; Duppen, Koos
1995-01-01
The properties of excitons in one-dimensional molecular aggregates, dissolved at room temperature in a liquid, were studied by means of femtosecond nonlinear optical experiments. Both the one-exciton band (i.e., Frenkel-excitons) and multiexciton bands contribute to the observed nonlinear optical response. The rapid motions in the liquid lead to ultrafast perturbations of the molecular energy levels. This localizes the excitons on limited sections of the chains of aggregated molecules. Ultrafast frequency-resolved pump-probe spectroscopy on the lowest two exciton bands was employed to determine the delocalization length of the optical excitations. The kinetics of the exciton populations was measured by ultrafast grating scattering experiments and time-resolved single photon counting. A model is described in which the multiexciton bands act as doorway states in the exciton-exciton annihilation process. These bands thereby determine the population decay of the Frenkel excitons at high excitation densities. Room temperature photon echo experiments show that stochastic perturbations of the exciton transition frequencies occur on two distinct time scales. In particular the slow components of the fluctuations are affected by motional narrowing, associated with the exciton delocalization length. It is therefore argued that the optical dephasing of excitons is directly related to the spatial extent of the excitation on the aggregate chain.
Device design for optimal exciton harvesting
NASA Astrophysics Data System (ADS)
Ingram, G. L.; Lu, Z. H.
2014-10-01
Organic light emitting diodes (OLEDs) show potential as the next generation solid state lighting technology. A major barrier to widespread adoption at this point is the efficiency droop that occurs for OLEDs at practical brightness (~ 5000 cd/m2) levels necessary for general lighting. We highlight recent progress in highly efficient OLEDs at high brightness, where improvements are made by managing excitons in these devices through rational device design. General design principles for monochrome OLEDs are discussed based on recent device architectures that have been successfully implemented. We expect that an improved understanding of exciton dynamics in OLEDs in combination with innovative device design will drive future development.
Control of exciton transport using quantum interference
NASA Astrophysics Data System (ADS)
Lusk, Mark T.; Stafford, Charles A.; Zimmerman, Jeramy D.; Carr, Lincoln D.
2015-12-01
It is shown that quantum interference can be employed to create an exciton transistor. An applied potential gates the quasiparticle motion and also discriminates between quasiparticles of differing binding energy. When implemented within nanoscale assemblies, such control elements could mediate the flow of energy and information. Quantum interference can also be used to dissociate excitons as an alternative to using heterojunctions. A finite molecular setting is employed to exhibit the underlying discrete, two-particle, mesoscopic analog to Fano antiresonance. Selected entanglement measures are shown to distinguish regimes of behavior which cannot be resolved from population dynamics alone.
Exciton dynamics in perturbed vibronic molecular aggregates.
BrÃ¼ning, C; Wehner, J; Hausner, J; Wenzel, M; Engel, V
2016-07-01
A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840
Exciton dynamics in perturbed vibronic molecular aggregates
BrÃ¼ning, C.; Wehner, J.; Hausner, J.; Wenzel, M.; Engel, V.
2015-01-01
A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840
Binding energy of exciton in a nanowire superlattice in magnetic and electric fields
NASA Astrophysics Data System (ADS)
Galván-Moya, J. E.; Gutiérrez, W.; Moscoso, C.
2010-02-01
We study the binding energy of excitons in a cylindrical GaAs/Ga1-xAlxAs nanowire superlattice, embedded in Ga1-yAlyAs matrix, in the presence of magnetic and electric fields applied parallel to the growth direction. We express the exciton trial function as a product of one-particle wave functions of the electron and the hole with variationally determined envelope function, which describes the exciton intrinsic properties and depends only on the electron-hole separation. By using a functional derivative technique, we derive a differential equation for this envelope function, which we solve numerically. By varying the wire radius, interwell barrier width and well sizes we obtain binding energies ranging in character from one for strongly coupled superlattice to that for a system of stack of isolated disks. The behaviour of the binding energies and the charge distributions as functions of the interwell coupling, well sizes, and the external fields is consistently described with our simple formalism.
Ligth-hole exciton in cylindrical microtube with two quantum wells
NASA Astrophysics Data System (ADS)
González, J. D.; Gonzalez, J. E.; Barba-Ortega, J.
2014-03-01
We report on microtube with double quantum well and large radius of curvature. Method for calculating the ground-state energy of light hole exciton and density of states confined in a square potential model that consist of a narrow well, which is produced by a symmetrical structure. The exciton trial function is taken as a product of the ground state wave functions of both the unbound electron and hole in the heterostructure, with an arbitrary correlation function that depends only on electron-hole separation. A renormalized Schrödinger equation for the correlation function is derived and coincides with the corresponding equation for a hydrogen atom in an effective and space-isotropic homogeneous. The binding energy of the ground state to an exciton in this heterostructure, the contribution to the energy given by the sublevels and the density of states is determined as a function of the width of the well, the aluminum concentration and confinement potential profile is obtained by solving the equation calculated by the variational model proposed.
NASA Astrophysics Data System (ADS)
Singh, Jai; Koblov, Alexander
2013-02-01
Using the derived expression for the light yield in a scintillator, the influence of linear radiative and non-radiative (quenching) rates on the nonproportionality in light yield is studied. It is found that if the excitation created within the electron track initiated by a ?-photon incident on a scintillator remains mainly excitonic, then nonproportionality can be minimised by inventing a scintillator material with linear radiative rate >107 s-1, linear quenching rate <106 s-1 and track radius ?70 nm along with maintaining the rates of other nonlinear processes as discovered earlier. If one can increase the linear radiative rate to 109 s-1, then the nonproportionality can be eliminated at a track radius >20 nm.
NASA Astrophysics Data System (ADS)
Nakayama, M.; Hirao, T.; Hasegawa, T.
2009-06-01
We have investigated photoluminescence (PL) properties of GaAs (15 nm)/AlAs (15 nm) and GaAs (20 nm)/AlAs (20 nm) multiple quantum wells at 10 K under high density excitation conditions at excitation energies in the region of the fundamental excitons. It has been found that the PL due to exciton-exciton scattering, the so-called P emission, is observed with a threshold nature in addition to the appearance of the biexciton PL. The energy spacing between the P-PL band and the heavy-hole exciton depends on the layer thickness, which reflects the change of the exciton binding energy by the quantum size effect. The intensity of the biexciton-PL band is saturated by the appearance of the P-PL band. Both the exciton-exciton scattering process and the biexciton formation process require the collision of two excitons. Thus, the exciton-exciton scattering process prevents the formation of biexcitons, which leads to the saturation behavior of the biexciton-PL intensity. Furthermore, we have confirmed the existence of optical gain leading to stimulated emission due to the exciton-exciton scattering process with use of a variable-stripe-length method.
Bright and dark excitons in semiconductor carbon nanotubes
Tretiak, Sergei
2008-01-01
We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.
A Maximum Radius for Habitable Planets.
Alibert, Yann
2015-09-01
We compute the maximum radius a planet can have in order to fulfill two constraints that are likely necessary conditions for habitability: 1- surface temperature and pressure compatible with the existence of liquid water, and 2- no ice layer at the bottom of a putative global ocean, that would prevent the operation of the geologic carbon cycle to operate. We demonstrate that, above a given radius, these two constraints cannot be met: in the Super-Earth mass range (1-12 Mearth), the overall maximum that a planet can have varies between 1.8 and 2.3 Rearth. This radius is reduced when considering planets with higher Fe/Si ratios, and taking into account irradiation effects on the structure of the gas envelope. PMID:26159097
Mass and radius of cosmic balloons
NASA Technical Reports Server (NTRS)
Wang, Yun
1994-01-01
Cosmic balloons are spherical domain walls with relativistic particles trapped inside. We derive the exact mass and radius relations for a static cosmic balloon using Gauss-Codazzi equations. The cosmic balloon mass as a function of its radius, M(R), is found to have a functional form similar to that of fermion soliton stars, with a fixed point at 2GM(R)/R approximately or equal to 0.486 which corresponds to the limit of infinite central density. We derive a simple analytical approximation for the mass density of a spherically symmetric relativistic gas star. When applied to the computation of the mass and radius of a cosmic balloon, the analytical approximation yields fairly good agreement with the exact numerical solutions.
A Maximum Radius for Habitable Planets
NASA Astrophysics Data System (ADS)
Alibert, Yann
2015-09-01
We compute the maximum radius a planet can have in order to fulfill two constraints that are likely necessary conditions for habitability: 1- surface temperature and pressure compatible with the existence of liquid water, and 2- no ice layer at the bottom of a putative global ocean, that would prevent the operation of the geologic carbon cycle to operate. We demonstrate that, above a given radius, these two constraints cannot be met: in the Super-Earth mass range (1-12 Mearth), the overall maximum that a planet can have varies between 1.8 and 2.3 Rearth. This radius is reduced when considering planets with higher Fe/Si ratios, and taking into account irradiation effects on the structure of the gas envelope.
Solar Radius Variations: An Inquisitive Wavelength Dependence
NASA Astrophysics Data System (ADS)
Rozelot, Jean Pierre; Kosovichev, Alexander; Kilcik, Ali
2015-10-01
Recent solar radius determinations from space observations of Mercury and Venus transits have been made by different teams in 2003, 2006, 2012, and 2014. Seemingly the results are not consistent: the authors interpreted the discrepancies as caused by the different methods of analysis. However, looking at the wavelength dependence and adding other available observations from X-EUV up to radio, a typical wavelength dependence can be found, reflecting the different heights at which the lines are formed. Measurements obtained during different periods of time would, in principle, allow us to detect a signature of radius temporal dependence. However, the available data are not sufficiently numerous to detect a significant dependence, at least at the level of the uncertainty at which the observations were made. Lastly, no unique theoretical model is available today to reproduce the strong wavelength dependence of the solar radius, which shows an unexpected minimum at around (6.6 ± 1.9) ?m, after a parabolic fit.
Exciton-exciton annihilation and relaxation pathways in semiconducting carbon nanotubes
NASA Astrophysics Data System (ADS)
Chmeliov, Jevgenij; Narkeliunas, Jonas; Graham, Matt W.; Fleming, Graham R.; Valkunas, Leonas
2016-01-01
We present a thorough analysis of one- and two-color transient absorption measurements performed on single- and double-walled semiconducting carbon nanotubes. By combining the currently existing models describing exciton-exciton annihilation--the coherent and the diffusion-limited ones--we are able to simultaneously reproduce excitation kinetics following both E11 and E22 pump conditions. Our simulations revealed the fundamental photophysical behavior of one-dimensional coherent excitons and non-trivial excitation relaxation pathways. In particular, we found that after non-linear annihilation a doubly-excited exciton relaxes directly to its E11 state bypassing the intermediate E22 manifold, so that after excitation resonant with the E11 transition, the E22 state remains unpopulated. A quantitative explanation for the observed much faster excitation kinetics probed at E22 manifold, comparing to those probed at the E11 band, is also provided.
Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.
Rao, Akshay; Wilson, Mark W B; Hodgkiss, Justin M; Albert-Seifried, Sebastian; BÃ¤ssler, Heinz; Friend, Richard H
2010-09-15
Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs. PMID:20735067
Disorder-Induced Relaxation of Frenkel Excitons in Molecular Aggregates
NASA Astrophysics Data System (ADS)
Bednarz, M.; Lemaistre, J. P.
2001-08-01
A model, based on the intraband scattering of excitons in one-dimensional J-aggregates is proposed to describe the lengthening of the experimentally observed radiative lifetime with temperature. According to this mechanism, the exciton-phonon scattering transfers the oscillator strength from the lowest k ? 0 optically allowed state to the other states within the excitonic band. A Pauli master equation, in which the hopping rates are calculated, is used to describe the thermalization of the excitonic band. Assuming a fast relaxation mechanism, the temperature dependence of the exciton radiative lifetime is simulated for various chain lengths.
Disorder-Induced Relaxation of Frenkel Excitons in Molecular Aggregates
NASA Astrophysics Data System (ADS)
Bednarz, M.; Lemaistre, J. P.
A model, based on the intraband scattering of excitons in one-dimensional J-aggregates is proposed to describe the lengthening of the experimentally observed radiative lifetime with temperature. According to this mechanism, the exciton-phonon scattering transfers the oscillator strength fromt he lowest k?0 optically allowed state to the other states within the excitonic band. A Pauli master equation, in which the hopping rates are calculated, is used to describe the thermalization of the excitonic band. Assuming a fast relaxation mechanism, the temperature dependence of the exciton radiative lifetime is simulated for various chain lengths.
Muonic Hydrogen and the Proton Radius Puzzle
NASA Astrophysics Data System (ADS)
Pohl, Randolf; Gilman, Ronald; Miller, Gerald A.; Pachucki, Krzysztof
2013-10-01
The extremely precise extraction of the proton radius obtained by Pohl et al. from the measured energy difference between the 2P and 2S states of muonic hydrogen disagrees significantly with that extracted from electronic hydrogen or elastic electron-proton scattering. This discrepancy is the proton radius puzzle. In this review, we explain the origins of the puzzle and the reasons for believing it to be very significant. We identify various possible solutions of the puzzle and discuss future research needed to resolve the puzzle.
Tailoring quantum dot assemblies to extend exciton coherence times and improve exciton transport
NASA Astrophysics Data System (ADS)
Seward, Kenton T.
Electron energy transfer (EET) through nanostructured assemblies plays a crucial role in a wide range of emerging technologies such as quantum dot solar cells, quantum computing, molecular electronics, excitonic transistors, and light emitting diodes. These technologies are very dependent on excitonic lifetimes which are short on the order of a nanosecond. In order to efficiently use this short time scale, EET needs to be as fast as possible. This leads to an interest in the application of coherent exciton transfer. To examine the possibility of coherent transfer, we ask a simple question: How rapidly do coherent superpositions of excitonic states dephase between quantum dots?. We assume that the major source of decoherence at room temperature is from the internal phonon modes of silicon quantum dots. The question is then addressed using a combination of ab initio calculations and a master equation formulation for the evolution of the electronic density operator for a dimer of interacting two-level systems coupled to a shared bath of harmonic oscillators. A combination of density functional theory (DFT) and frozen phonon method (FPM) analysis was used to obtain exciton-phonon coupling in various sizes of silicon quantum dots. As expected, coherent EET is faster in comparison to incoherent EET in assemblies of identical nanostructures. In cases of non-identical assemblies of nanostructures, the low energy regions act as trap states for the exciton. In this case, a combination of coherent and incoherent transport leads to the fastest transport rate.
Intracavity optical pumping of J-aggregate microcavity exciton polaritons
Bradley, M. Scott; Bulovic, Vladimir
2010-07-29
We demonstrate intracavity optical pumping of J-aggregate microcavity exciton polaritons. The use of ultrathin layer-by-layer J-aggregate thin films as the strongly coupled exciton medium allows for inclusion of a thermally evaporated luminescent cavity spacer layer, through which the lower-branch exciton-polariton states are resonantly pumped. We present a measurement of the lower-branch exciton-polariton occupation in room-temperature microcavity devices containing J-aggregated molecular thin films under low-density steady-state excitation. The observed exciton-polariton occupation shows a Maxwell-Boltzmann distribution at T=300 K , indicating thermalization of exciton polaritons in the lower energy branch. This device design enables us to propose a new type of “polariton laser” architecture for microcavity exciton polaritons.
Dynamics of the excitonic coupling in organic crystals.
AragÃ³, Juan; Troisi, Alessandro
2015-01-16
We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene. PMID:25635554
Dynamics of the Excitonic Coupling in Organic Crystals
NASA Astrophysics Data System (ADS)
AragÃ³, Juan; Troisi, Alessandro
2015-01-01
We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene.
Spatiotemporal dynamics of excitons in monolayer and bulk WS2
NASA Astrophysics Data System (ADS)
He, Jiaqi; He, Dawei; Wang, Yongsheng; Cui, Qiannan; Ceballos, Frank; Zhao, Hui
2015-05-01
Spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time-delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22 +/- 1 and 110 +/- 10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi-exciton recombination processes. Exciton diffusion coefficients of 60 +/- 20 and 3.5 +/- 0.5 cm2 s-1 are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications.
NASA Astrophysics Data System (ADS)
Kowalski, Piotr; Machnikowski, Pawe?
2015-12-01
We study the coherent mixing between two-particle (single exciton) and four-particle (biexciton) states of a semiconductor nanocrystal resulting from the Coulomb coupling between states with different numbers of electron-hole pairs. Using a simple model of the nanocrystal wave functions and an envelope function approach, we estimate the efficiency of the multiple exciton generation (MEG) process resulting from such coherent admixture mechanism, including all the relevant states in a very broad energy interval. We show that in a typical ensemble of nanocrystals with an average radius of 3nm, the onset of the MEG process appears about 1 eV above the lower edge of the biexciton density of states. This is due to the angular momentum conservation that imposes selection rules and limits the available MEG pathways, thus taking over the role of momentum conservation that hinders this process in bulk. The efficiency of the MEG process reaches 50% for photon energies around 5 eV. The MEG onset shifts to lower energies and therefore the efficiency increases in a certain energy range as the radius grows. The energy dependence of the MEG efficiency differs considerably between ensembles with small and large inhomogeneity of nanocrystal sizes.
Excitons in T-shaped quantum wires
NASA Astrophysics Data System (ADS)
Szymanska, M. H.; Littlewood, P. B.; Needs, R. J.
2001-05-01
We calculate energies, oscillator strengths for radiative recombination, and two-particle wave functions for the ground-state exciton and around 100 excited states in a T-shaped quantum wire. We include the single-particle potential and the Coulomb interaction between the electron and hole on an equal footing, and perform exact diagonalization of the two-particle problem within a finite-basis set. We calculate spectra for all of the experimentally studied cases of T-shaped wires including symmetric and asymmetric GaAs/AlxGa1-xAs and InyGa1-yAs/AlxGa1-xAs structures. We study in detail the shape of the wave functions to gain insight into the nature of the various states for selected symmetric and asymmetric wires in which laser emission has been experimentally observed. We also calculate the binding energy of the ground-state exciton and the confinement energy of the one-dimensional (1D) quantum-wire-exciton state with respect to the 2D quantum-well exciton for a wide range of structures, varying the well width and the Al molar fraction x. We find that the largest binding energy of any wire constructed to date is 16.5 meV. We also notice that in asymmetric structures, the confinement energy is enhanced with respect to the symmetric forms with comparable parameters but the binding energy of the exciton is then lower than in the symmetric structures. For GaAs/AlxGa1-xAs wires we obtain an upper limit for the binding energy of around 25 meV in a 10-Å -wide GaAs/AlAs structure that suggests that other materials must be explored in order to achieve room-temperature applications. There are some indications that InyGa1-yAs/AlxGa1-xAs might be a good candidate.
Baym, Gordon; Ozawa, Tomoki
2009-01-01
We analyze Niels Bohr's proposed two-slit interference experiment with highly charged particles which argues that the consistency of elementary quantum mechanics requires that the electromagnetic field must be quantized. In the experiment a particle's path through the slits is determined by measuring the Coulomb field that it produces at large distances; under these conditions the interference pattern must be suppressed. The key is that, as the particle's trajectory is bent in diffraction by the slits, it must radiate and the radiation must carry away phase information. Thus, the radiation field must be a quantized dynamical degree of freedom. However, if one similarly tries to determine the path of a massive particle through an inferometer by measuring the Newtonian gravitational potential the particle produces, the interference pattern would have to be finer than the Planck length and thus indiscernible. Unlike for the electromagnetic field, Bohr's argument does not imply that the gravitational field must be quantized. PMID:19218440
Femtosecond correlation spectroscopy of excitons in molecular aggregates
NASA Astrophysics Data System (ADS)
Zhang, Wei Min
2000-08-01
This thesis focuses on the development of theoretical approaches toward the understanding of the complex nature of molecular aggregates. Applications are made to four- wave-mixing spectroseopies of the B820 Chlorophyll dimers and the B850 LH2 antenna in purple bacteria. Two types of theories are developed. The first type rigorously partitions the third-order polarization into a coherent and a sequential contribution. The latter is given by a sum of an exciton-hopping and a ground state (bleaching) terms, both expressed using the doorway-window representation. The theory includes effects of two- exciton states, static disorder, and coupling to a phonon bath with an arbitrary spectral density. It accounts for strong energetic disorder and applies when the disorder- induced exciton localization length is shorter than the phonon-induced exciton self-trapping length (polaron size) in the absence of disorder. Exciton transport is described by a master equation. Two-exciton states are expressed using Bethe ansatz, hence, the final expressions apply for a cyclic Frenkel-exciton model with the nearest-neighbor intermolecular interactions. The second type of theory describes the combined effects of two-exciton resonances and exciton transport. It is derived using the nonlinear exciton equations (NEE), which follow explicitly the complete set of one-, two-, and three-point dynamical exciton variables relevant for the third-order response. The two-exciton state is calculated through an exciton-exciton scattering matrix. Effects of nuclear motions are incorporated through relaxation superoperators calculated perturbatively in exciton-phonon coupling. Thus the approach is limited to weak exciton-phonon coupling and is not sensitive to the detailed features of the spectral densities. A Green function expression for the third order response is obtained by solving the NEE using a truncation scheme based on factorizing the three-point relaxation kernels. These results set the stage for designing multidimensional spectroscopies of excitons and analyzing them using coherence-transfer pathways. New two- dimensional femtosecond resonant spectroscopic techniques which have the capacity to probe directly the excitonic couplings among chromophores are proposed. Model calculations illustrate that 2D techniques could fully characterize the system by providing information on intermolecular coupling constants.
On the Bohr-van Leeuwen theorem, the non-existence of classical magnetism in thermal equilibrium
NASA Astrophysics Data System (ADS)
Kaplan, T. A.; Mahanti, S. D.
2009-07-01
We point out that the classical Langevin equation for a charged particle moving on a ring in a constant magnetic field perpendicular to the ring leads at long times to thermal equilibrium and to zero magnetic moment, consistent with the Bohr-van Leeuwen theorem. This fact is a counter-example to the recent claim that the Langevin equation leads to a non-zero diamagnetic moment in thermal equilibrium when the particle moves in a finite but unbounded space.
One-dimensional exciton diffusion in perylene bisimide aggregates.
Marciniak, Henning; Li, Xue-Qing; Würthner, Frank; Lochbrunner, Stefan
2011-02-10
The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Fo?rster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems. PMID:21192672
MASS-RADIUS RELATIONSHIPS FOR EXOPLANETS
Swift, D. C.; Eggert, J. H.; Hicks, D. G.; Hamel, S.; Caspersen, K.; Schwegler, E.; Collins, G. W.; Nettelmann, N.; Ackland, G. J.
2012-01-01
For planets other than Earth, particularly exoplanets, interpretation of the composition and structure depends largely on comparing the mass and radius with the composition expected given their distance from the parent star. The composition implies a mass-radius relation which relies heavily on equations of state calculated from electronic structure theory and measured experimentally on Earth. We lay out a method for deriving and testing equations of state, and deduce mass-radius and mass-pressure relations for key, relevant materials whose equation of state (EOS) is reasonably well established, and for differentiated Fe/rock. We find that variations in the EOS, such as may arise when extrapolating from low-pressure data, can have significant effects on predicted mass-radius relations and on planetary pressure profiles. The relations are compared with the observed masses and radii of planets and exoplanets, broadly supporting recent inferences about exoplanet structures. Kepler-10b is apparently 'Earth-like', likely with a proportionately larger core than Earth's, nominally 2/3 of the mass of the planet. CoRoT-7b is consistent with a rocky mantle over an Fe-based core which is likely to be proportionately smaller than Earth's. GJ 1214b lies between the mass-radius curves for H{sub 2}O and CH{sub 4}, suggesting an 'icy' composition with a relatively large core or a relatively large proportion of H{sub 2}O. CoRoT-2b is less dense than the hydrogen relation, which could be explained by an anomalously high degree of heating or by higher than assumed atmospheric opacity. HAT-P-2b is slightly denser than the mass-radius relation for hydrogen, suggesting the presence of a significant amount of matter of higher atomic number. CoRoT-3b lies close to the hydrogen relation. The pressure at the center of Kepler-10b is 1.5{sup +1.2}{sub -1.0} TPa. The central pressure in CoRoT-7b is probably close to 0.8 TPa, though may be up to 2 TPa. These pressures are accessible by planar shock and ramp-loading experiments at large laser facilities. The center of HAT-P-2b is probably around 210 TPa, in the range of planned National Ignition Facility experiments, and that of CoRoT-3b around 1900 TPa.
An investigation of the nature of Bohr, Root, and Haldane effects in Octopus dofleini hemocyanin.
Miller, K I; Mangum, C P
1988-01-01
1. The pH dependence of Octopus dofleini hemocyanin oxygenation is so great that below pH 7.0 the molecule does not become fully oxygenated, even in pure O2 at 1 atm pressure. However, the curves describing percent oxygenation as a function of PO2 appear to be gradually increasing in oxygen saturation, rather than leveling out at less than full saturation. Hill plots indicate that at pH 6.6 and below the molecule is stabilized in its low affinity conformation. Thus, the low saturation of this hemocyanin in air is due to the very large Bohr shift, and not to the disabling of one or more functionally distinct O2 binding sites on the native molecule. 2. Experiments in which pH was monitored continuously while oxygenation was manipulated in the presence of CO2 provide no evidence of O2 linked binding of CO2. While CO2 does influence O2 affinity independently of pH, its effect may be due to high levels of HCO3- and CO3-, rather than molecular CO2, and it may entail a lowering of the activities of the allosteric effectors Mg2+ and Ca2+. PMID:3150406
What is complementarity?: Niels Bohr and the architecture of quantum theory
NASA Astrophysics Data System (ADS)
Plotnitsky, Arkady
2014-12-01
This article explores Bohrâ€™s argument, advanced under the heading of â€˜complementarity,â€™ concerning quantum phenomena and quantum mechanics, and its physical and philosophical implications. In Bohr, the term complementarity designates both a particular concept and an overall interpretation of quantum phenomena and quantum mechanics, in part grounded in this concept. While the argument of this article is primarily philosophical, it will also address, historically, the development and transformations of Bohrâ€™s thinking, under the impact of the development of quantum theory and Bohrâ€™s confrontation with Einstein, especially their exchange concerning the EPR experiment, proposed by Einstein, Podolsky and Rosen in 1935. Bohrâ€™s interpretation was progressively characterized by a more radical epistemology, in its ultimate form, which was developed in the 1930s and with which I shall be especially concerned here, defined by his new concepts of phenomenon and atomicity. According to this epistemology, quantum objects are seen as indescribable and possibly even as inconceivable, and as manifesting their existence only in the effects of their interactions with measuring instruments upon those instruments, effects that define phenomena in Bohrâ€™s sense. The absence of causality is an automatic consequence of this epistemology. I shall also consider how probability and statistics work under these epistemological conditions.
Transporting near-circular Bohr-like wave packets using chirped pulse trains
NASA Astrophysics Data System (ADS)
Yoshida, S.; BurgdÃ¶rfer, J.; Reinhold, C. O.; Wyker, B.; Ye, S.; Dunning, F. B.
2011-05-01
Protocols for driving localized high- n (n 300) wave packets in near-circular Bohr-like orbits to higher n states using chirped sine waves are described. While Rydberg wave packets involving several n levels are known to be dispersive, circular wave packets can be stabilized by the application of a circularly- or linearly-polarized resonant sine wave. A similar stabilization mechanism is known for the formation of the Trojan asteroids in celestial mechanics. We demonstrate that by slowly chirping the drive frequency parent high- n wave packets can be transported to a narrow distribution of much higher n states because the motion of the wave packet remains locked to the sine wave during the chirping. Use of a chirped HCP train instead of a sine wave allows similar transport but also provides some control of the orbit eccentricity. Research supported by the NSF, the Robert A. Welch Foundation, the OBES US DoE to ORNL, and by the FWF (Austria)
Generation of localized ``Bohr-like'' wavepackets in near-circular orbit about the nucleus
NASA Astrophysics Data System (ADS)
Dunning, F. Barry
2009-05-01
Atoms in high-lying (n Ëœ 300) Rydberg states provide a valuable laboratory in which to explore the engineering of electronic wavefunctions using carefully-tailored sequences of short electric field pulses whose characteristic times (duration and/or rise/fall times) are less than the classical electron orbital period. The level of control that can be exercised is illustrated with reference to the generation of localized wavepackets in ``Bohr-like'' near circular orbits. While such wavepackets slowly dephase and undergo dispersion, their localization can be maintained for extended periods (many hundreds of orbits) through external driving using a periodic train of pulses. The wavepackets can be further manipulated by slowly varying, or ``chirping,'' the pulse repetition frequency. The physics underlying these control protocols is explained using classical trajectory Monte Carlo simulations. Even in the absence of external driving, however, wavepacket relocalization is expected at late times due to quantum revivals. The observation of such relocalization is described and demonstrates that quantum phenomena can be seen even in mesoscopic very-high-n atoms. Research undertaken in collaboration with J. J. Mestayer, B. Wyker, C. O. Reinhold, S. Yoshida and J. BurgdÃ¶rfer.
A rigorous proof of the Bohr-van Leeuwen theorem in the semiclassical limit
NASA Astrophysics Data System (ADS)
Savoie, Baptiste
2015-10-01
The original formulation of the Bohr-van Leeuwen (BvL) theorem states that, in a uniform magnetic field and in thermal equilibrium, the magnetization of an electron gas in the classical Drude-Lorentz model vanishes identically. This stems from classical statistics which assign the canonical momenta all values ranging from -? to ? that makes the free energy density magnetic-field-independent. When considering a classical (Maxwell-Boltzmann) interacting electron gas, it is usually admitted that the BvL theorem holds upon condition that the potentials modeling the interactions are particle-velocities-independent and do not cause the system to rotate after turning on the magnetic field. From a rigorous viewpoint, when treating large macroscopic systems, one expects the BvL theorem to hold provided the thermodynamic limit of the free energy density exists (and the equivalence of ensemble holds). This requires suitable assumptions on the many-body interactions potential and on the possible external potentials to prevent the system from collapsing or flying apart. Starting from quantum statistical mechanics, the purpose of this paper is to give, within the linear-response theory, a proof of the BvL theorem in the semiclassical limit when considering a dilute electron gas in the canonical conditions subjected to a class of translational invariant external potentials.
Taming excitons in II-VI semiconductor nanowires and nanobelts
NASA Astrophysics Data System (ADS)
Xu, Xinlong; Zhang, Qing; Zhang, Jun; Zhou, Yixuan; Xiong, Qihua
2014-10-01
Excitons are one of the most important fundamental quasi-particles, and are involved in a variety of processes forming the basis of a wide range of opto-electronic and photonic devices based on II-VI semiconductor nanowires and nanobelts, such as light-emitting diodes, photovoltaic cells, photodetectors and nanolasers. A clear understanding of their properties and unveiling the potential engineering for excitons is of particular importance for the design and optimization of nanoscale opto-electronic and photonic devices. Herein, we present a comprehensive review on discussing the fundamental behaviours of the excitons in one-dimensional (1D) II-VI semiconductor nanomaterials (nanowires and nanobelts). We will start with a focus on the unique properties (origin, generation, etc) and dynamics of excitons and exciton complexes in the II-VI semiconductor nanowires and nanobelts. Then we move to the recent progress on the excitonic response in 1D nanomaterials and focus on the tailoring and engineering of excitonic properties through rational controlling of the physical parameters and conditions, intrinsically and extrinsically. These include (1) exciton-exciton interaction, which is important for 1D nanomaterial nanolasing; (2) exciton-phonon interaction, which has interesting applications for laser cooling; and (3) exciton-plasmon interaction, which is the cornerstone towards the realization of plasmonic lasers. The potential of electric field, morphology and size control for excitonic properties is also discussed. Unveiling and controlling excitonic properties in II-VI semiconductor nanowires and nanobelts would promote the development of 1D nanoscience and nanotechnology.
Okonjo, Kehinde O; Olatunde, Abimbola M; Fodeke, Adedayo A; Babalola, J Oyebamiji
2014-06-01
We have measured the affinity of the CysF9[93]Î² sulfhydryl group of human deoxyhemoglobin and oxyhemoglobin for 5,5'-dithiobis(2-nitrobenzoate), DTNB, between pH â‰ˆ5.6 and 9 in order to understand the basis of the reported reduction of the Bohr effect induced by chemical modification of the sulfhydryl. We analyzed the results quantitatively on the basis of published data indicating that the sulfhydryl exists in two conformations that are coupled to the transition between two tertiary structures of hemoglobin in dynamic equilibrium. Our analyses show that the ionizable groups linked to the DTNB reaction have lower pKas of ionization in deoxyhemoglobin compared to oxyhemoglobin. So these ionizable groups should make negative contributions to the Bohr effect. We identify these groups as HisNA2[2]Î², HisEF1[77]Î² and HisH21[143]Î². We provide explanations for the finding that hemoglobin, chemically modified at CysF9[93]Î², has a lower Bohr effect and a higher oxygen affinity than unmodified hemoglobin. PMID:24824171
Radius measurement by laser confocal technology.
Yang, Jiamiao; Qiu, Lirong; Zhao, Weiqian; Zhang, Xin; Wang, Xu
2014-05-01
A laser confocal radius measurement (LCRM) method is proposed for high-accuracy measurement of the radius of curvature (ROC). The LCRM uses the peak points of confocal response curves to identify the cat eye and confocal positions precisely. It then accurately measures the distance between these two positions to determine the ROC. The LCRM also uses conic fitting, which significantly enhances measurement accuracy by restraining the influences of environmental disturbance and system noise on the measurement results. The experimental results indicate that LCRM has a relative expanded uncertainty of less than 10 ppm for both convex and concave spheres. Thus, LCRM is a feasible method for ROC measurements with high accuracy and concise structures. PMID:24921872
Ulnar Shortening Osteotomy for Distal Radius Malunion
Kamal, Robin N.; Leversedge, Fraser J.
2014-01-01
Backgroundâ€ƒMalunion is a common complication of distal radius fractures. Ulnar shortening osteotomy (USO) may be an effective treatment for distal radius malunion when appropriate indications are observed. Methodsâ€ƒThe use of USO for treatment of distal radius fracture malunion is described for older patients (typically patients >50 years) with dorsal or volar tilt less than 20 degrees and no carpal malalignment or intercarpal or distal radioulnar joint (DRUJ) arthritis. Description of Techniqueâ€ƒPreoperative radiographs are examined to ensure there are no contraindications to ulnar shortening osteotomy. The neutral posteroanterior (PA) radiograph is used to measure ulnar variance and to estimate the amount of ulnar shortening required. An ulnar, mid-sagittal incision is used and the dorsal sensory branch of the ulnar nerve is preserved. An USO-specific plating system with cutting jig is used to create parallel oblique osteotomies to facilitate shortening. Intraoperative fluoroscopy and clinical range of motion are checked to ensure adequate shortening and congruous reduction of the ulnar head within the sigmoid notch. Resultsâ€ƒPrevious outcomes evaluation of USO has demonstrated improvement in functional activities, including average flexion-extension and pronosupination motions, and patient reported outcomes. Conclusionâ€ƒThe concept and technique of USO are reviewed for the treatment of distal radius malunion when specific indications are observed. Careful attention to detail related to surgical indications and to surgical technique typically will improve range of motion, pain scores, and patient-reported outcomes and will reduce the inherent risks of the procedure, such as ulnar nonunion or the symptoms related to unrecognized joint arthritis. Level of Evidence:â€ƒLevel IV PMID:25097811
Fractures of Distal Radius: An Overview
Meena, Sanjay; Sharma, Pankaj; Sambharia, Abhishek Kumar; Dawar, Ashok
2014-01-01
Fractures of distal radius account for up to 20% of all fractures treated in emergency department. Initial assessment includes a history of mechanism of injury, associated injury and appropriate radiological evaluation. Treatment options include conservative management, internal fixation with pins, bridging and non-bridging external fixation, dorsal or volar plating with/without arthroscopy assistance. However, many questions regarding these fractures remain unanswered and good prospective randomized trials are needed. PMID:25657938
DVR plating of distal radius fractures.
Vanhaecke, J; Fernandez, D L
2015-11-01
Volar plating has become the standard of care for most distal radius fractures. When done for the right indication and with adequate mastering of the technique complication ratio is low. The concept of subchondral support is key in this technique. Osteoporotic patients will especially benefit from this type of fixation which allows early immobilization, quick return to activities of daily living and early good outcome. PMID:26319206
Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes
Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, B.; Wassner, T. A.; Eickhoff, M.; Feneberg, M.; Goldhahn, R.
2011-07-01
The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg{sub x}Zn{sub 1-x}O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.
QED confronts the radius of the proton
NASA Astrophysics Data System (ADS)
De Rújula, A.
2011-02-01
Recent results on muonic hydrogen (Pohl et al., 2010) [1] and the ones compiled by CODATA on ordinary hydrogen and ep-scattering (Mohr et al., 2008) [2] are 5? away from each other. Two reasons justify a further look at this subject: (1) One of the approximations used in Pohl et al. (2010) [1] is not valid for muonic hydrogen. This amounts to a shift of the proton's radius by ?3 of the standard deviations of Pohl et al. (2010) [1], in the “right” direction of data-reconciliation. In field-theory terms, the error is a mismatch of renormalization scales. Once corrected, the proton radius “runs”, much as the QCD coupling “constant” does. (2) The result of Pohl et al. (2010) [1] requires a choice of the “third Zemach moment”. Its published independent determination is based on an analysis with a p-value - the probability of obtaining data with equal or lesser agreement with the adopted (fit form-factor) hypothesis - of 3.92×10. In this sense, this quantity is not empirically known. Its value would regulate the level of “tension” between muonic- and ordinary-hydrogen results, currently at most?4?. There is no tension between the results of Pohl et al. (2010) [1] and the proton radius determined with help of the analyticity of its form-factors.
Excitonic gap formation in neutral bilayer structures
NASA Astrophysics Data System (ADS)
Apinyan, V.; KopeÄ‡, T. K.
2015-08-01
We consider the pairing between conduction band electrons, and the valence band holes in the neutral bilayer-type structures. By employing the bilayer Hubbard model, we show the possibility of the inter-plane exciton formation in the system without applied external field. The in-plane and inter-plane Coulomb interaction effects on the pairing mechanism are considered, and the role of the in-plane particle hopping asymmetry on the gap behavior is analyzed in the paper. We show that both Frenkel-type pairing channel and Wannier-Mott-type excitonic pairings are present in the considered system. We analyze also the structure of the chemical potential in the bilayer system. The temperature effects, and the tunable inter-plane electron hopping effects are discussed. For the Frenkel channel, we have shown a particular behavior of the chemical potential at very low temperatures, which is related to the degenerated Frenkel-gap.
Subdiffusive exciton motion in systems with heavy-tailed disorder
NASA Astrophysics Data System (ADS)
Vlaming, S. M.; Malyshev, V. A.; Eisfeld, A.; Knoester, J.
2013-06-01
We study the transport of collective excitations (Frenkel excitons) in systems with static disorder in the transition energies, not limiting ourselves to Gaussian transition energy distributions. Instead, we generalize this model to the wider class of Lévy stable distributions, characterized by heavy tails. Phonon-assisted scattering of excitons, localized by the disorder, leads to thermally activated exciton motion. The time evolution of the second moment of the exciton distribution is shown to be sublinear, thus indicating that the exciton dynamics in such systems is not diffusive, but rather subdiffusive instead. The heavier the tail in the transition energy distribution is, the larger are the deviations from the diffusive regime. This from fluctuations of site energies larger than the exciton band width (outliers). We show that the occurrence of subdiffusive transport for heavy-tailed disorder distributions can be understood from the scattering rate distributions, which possess a (second) peak at zero scattering rate.
Evidence for a Bose-Einstein condensate of excitons
NASA Astrophysics Data System (ADS)
Alloing, Mathieu; Beian, Mussie; Lewenstein, Maciej; Fuster, David; González, Yolanda; González, Luisa; Combescot, Roland; Combescot, Monique; Dubin, François
2014-07-01
We report compelling evidence for a “gray” condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas (\\sim 10^{10}\\ \\text{cm}^{-2}) that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the “gray” condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.
Observation of High Angular Momentum Excitons in Cuprous Oxide
NASA Astrophysics Data System (ADS)
Thewes, J.; Heckötter, J.; Kazimierczuk, T.; Aßmann, M.; Fröhlich, D.; Bayer, M.; Semina, M. A.; Glazov, M. M.
2015-07-01
The recent observation of dipole-allowed P excitons up to principal quantum numbers of n =25 in cuprous oxide has given insight into exciton states with unprecedented spectral resolution. While so far the exciton description as a hydrogenlike complex has been fully adequate for cubic crystals, we demonstrate here distinct deviations: The breaking of rotational symmetry leads to mixing of high angular momentum F and H excitons with the P excitons so that they can be observed in absorption. The F excitons show a threefold splitting that depends systematically on n , in agreement with theoretical considerations. From detailed comparison of experiment and theory we determine the cubic anisotropy parameter of the Cu2O valence band.
Excitonic properties of ZnSe/ZnSeS superlattices
NASA Astrophysics Data System (ADS)
Cingolani, R.; Lomascolo, M.; Lovergine, N.; Dabbicco, M.; Ferrara, M.; Suemune, I.
1994-05-01
We report an optical investigation of the excitons in ZnSe/ZnSeS superlattices of well widths ranging between 2 and 15 nm. An almost constant exciton binding energy is found. The exciton confinement is found to be dominated by the hole quantization, consistent with the expectation of negligible conduction-band discontinuity in these heterostructures. The effect of strain has been included to properly reproduce the well width dependence of the light-hole energies. Finally strong evidence of hot exciton photogeneration is obtained from the oscillatory behavior of the photoluminescence excitation spectra. This is consistent with the strong exciton-phonon coupling deduced from the temperature dependence of the excitonic linewidth measured by transmission experiments.
Ultrafast dynamics of excitons in tetracene single crystals
NASA Astrophysics Data System (ADS)
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-01
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states Sn on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Ultrafast dynamics of excitons in tetracene single crystals.
Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S(n) on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale. PMID:24655187
Singlet Exciton Fission in Nanostructured Organic Solar Cells
Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc
2011-04-13
Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C{sub 6}0. Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C{sub 60}. We measure a singlet fission efficiency of (71 Â± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
Ultrafast dynamics of excitons in tetracene single crystals
Birech, Zephania; Schwoerer, Heinrich; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens
2014-03-21
Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S{sub n} on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S{sub 1} on a 40 ps timescale. The high energy Davydov component of the S{sub 1} exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Optics of excitonic molecules in semiconductors and semiconductor microstructures
NASA Astrophysics Data System (ADS)
Ivanov, A. L.; Haug, H.; Keldysh, L. V.
1998-03-01
A bipolariton concept of an excitonic molecule in direct-band-gap semiconductors is reviewed. The bipolariton model interprets a molecule as two quasi-bound polaritons provided that both the exciton-exciton Coulombic interaction and the exciton-photon coupling (polariton effect) are treated simultaneously and beyond a low-order perturbation theory. The molecule-mediated optics is reformulated within the bipolariton concept for bulk semiconductors (e.g., CuCl and CdS) and semiconductor microstructures (e.g., GaAs quantum wells and wires). We discuss such phenomena as two-photon absorption and four-wave-mixing due to excitonic molecules, molecule-mediated optical Stark effect and other coherent processes which effectively involve excitonic molecules, and polariton solitons coupled through the molecule state.
Haines, D.E.; Watson, D.D.; Verow, A.F. )
1990-07-01
Myocardial heating by transcatheter delivery of radiofrequency (RF) energy has been proposed as an effective means of arrhythmia ablation. A thermodynamic model describing the radial temperature gradient at steady state during RF-induced heating is proposed. If one assumes that RF power output is adjusted to maintain a constant electrode-tissue interface temperature at all times, then this thermodynamic model predicts that the radius of the RF-induced lesion will be directly proportional to the electrode radius. A total of 76 RF-induced lesions were created in a model of isolated canine right ventricular free wall perfused and superfused with oxygenated Krebs-Henseleit buffer. Electrode radius was varied between 0.75 and 2.25 mm. RF energy (500 kHz) was delivered for 90 seconds, and the power output was adjusted to maintain a constant electrode-tissue interface temperature of 60 degrees C. A strong linear correlation was observed between electrode radius and lesion radius in two dimensions: transverse (p = 0.0001, r = 0.85) and transmural (p = 0.0001, r = 0.89). With these data, the temperature correlation with irreversible myocardial injury in this model was calculated at 46.6-48.8 degrees C. Therefore, the proposed thermodynamic model closely predicts the observed relation between electrode radius and lesion size during RF myocardial heating.
Microcavity controlled coupling of excitonic qubits
NASA Astrophysics Data System (ADS)
Albert, F.; Sivalertporn, K.; Kasprzak, J.; Strauß, M.; Schneider, C.; Höfling, S.; Kamp, M.; Forchel, A.; Reitzenstein, S.; Muljarov, E. A.; Langbein, W.
2013-04-01
Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.
Fractional Solitons in Excitonic Josephson Junctions
NASA Astrophysics Data System (ADS)
Hsu, Ya-Fen; Su, Jung-Jung
2015-10-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ?0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q?=??0/2? is the lowest energy state starting from zero ?0 – until ?0?>?? – then the alternative group of solitons with Q?=??0/2????1 takes place and switches the polarity of CPR.
Microcavity controlled coupling of excitonic qubits.
Albert, F; Sivalertporn, K; Kasprzak, J; Strauß, M; Schneider, C; Höfling, S; Kamp, M; Forchel, A; Reitzenstein, S; Muljarov, E A; Langbein, W
2013-01-01
Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits--like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots--is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample's coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. PMID:23612288
Excitonic effects in oxyhalide scintillating host compounds
Shwetha, G.; Kanchana, V.; Valsakumar, M. C.
2014-10-07
Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.
Topological Excitonic Superfluids in Three Dimensions
NASA Astrophysics Data System (ADS)
Gilbert, Matthew; Hankiewicz, Ewelina; Kim, Youngseok
2013-03-01
We study the equilibrium and non-equilibrium properties of topological dipolar intersurface exciton condensates within time-reversal invariant topological insulators in three spatial dimensions without a magnetic field. We elucidate that, in order to correctly identify the proper pairing symmetry within the condensate order parameter, the full three-dimensional Hamiltonian must be considered. As a corollary, we demonstrate that only particles with similar chirality play a significant role in condensate formation. Furthermore, we find that the intersurface exciton condensation is not suppressed by the interconnection of surfaces in three-dimensional topological insulators as the intersurface polarizability vanishes in the condensed phase. This eliminates the surface current flow leaving only intersurface current flow through the bulk. We conclude by illustrating how the excitonic superfluidity may be identified through an examination of the terminal currents above and below the condensate critical current. Army Research Office (ARO) under contract number W911NF-09-1-0347, the Office of Naval Research (ONR) under contract number N0014-11-1-0728, and the Air Force Office of Scientific Research (AFOSR) under contract number FA9550-10-1-0459, DFG Grant HA 5893
Fractional Solitons in Excitonic Josephson Junctions
Hsu, Ya-Fen; Su, Jung-Jung
2015-01-01
The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ?0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q?=??0/2? is the lowest energy state starting from zero ?0 – until ?0?>?? – then the alternative group of solitons with Q?=??0/2????1 takes place and switches the polarity of CPR. PMID:26511770
Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.
Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; MÃ¼llen, Klaus; Cerullo, Giulio
2016-01-01
Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of â‰ˆ250â€‰meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics. PMID:26984281
Ultrafast exciton dynamics in ZnO: Excitonic versus electron-hole plasma lasing
NASA Astrophysics Data System (ADS)
Shih, T.; Mazur, E.; Richters, J.-P.; Gutowski, J.; Voss, T.
2011-02-01
The use of ZnO bulk and especially nanolayer and nanowire structures for novel device applications has led to a renewal of interest in high-electron-density processes in ZnO, such as those occurring during lasing in ZnO. Using a pump-probe reflectometry technique, we investigate the ultrafast exciton dynamics of bulk ZnO under femtosecond laser excitation close to lasing conditions. Under intense excitation by 266-nm femtosecond (fs) pump pulses, the exciton resonance becomes highly damped and does not recover for several picoseconds. This slow recovery indicates a significant screening of the Coulomb interaction. Even below the lasing thresholds typically found for ZnO nanolayers and nanowires, we observe damping of the exciton resonance for several picoseconds, which indicates that the primary mechanism for lasing in ZnO induced by femtosecond laser pumping is electron-hole plasma recombination.
One-dimensional excitons in V-shaped quantum wires
NASA Astrophysics Data System (ADS)
Rinaldi, R.; Cingolani, R.; Lepore, M.; Ferrara, M.; Catalano, I. M.; Marti, U.; Martin, D.; Morier-Gemoud, U.; Ruterana, P.; Reinhart, F. K.
We report a detailed study of one-dimensional excitons in a planar array of single V-shaped GaAs quantum wires. Two-photon absorption, magnetoluminescence and linear photoluminescence spectroscopy have been used to measure the exciton binding energy, the excited 2 p states of the excitons, the higher index transitions and the extension of the confined wavefunctions in the V-shaped region of the quantum wires.
Dielectrically confined excitons in natural superlattices: perovskite lead iodide semiconductors
NASA Astrophysics Data System (ADS)
Muljarov, E.; Tikhodeev, Sergei G.; Ishihara, Terry
1993-11-01
A large class of new layered semiconductors -- perovskite lead iodide compounds -- is of great interest because of possible optoelectronic applications due to pronounced excitonic effects. It has been predicted earlier that excitons in these structures should be enhanced due to the so called `dielectric confinement' arising from large differences of dielectric constants of adjoining layers. In the present work we calculate the energies, the wave functions, and the diamagnetic coefficient of excitons in these naturally grown superlattices with allowance for the image potential and the superlattice structure of the compounds. The fitting of our theory to the experiments makes it possible to evaluate the reduced mass of excitons in these compounds.
Charged excitons in modulation-doped quantum wires
NASA Astrophysics Data System (ADS)
Otterburg, T.; Oberli, D. Y.; Dupertuis, M.-A.; Moret, N.; Malko, A.; Pelucchi, E.; Dwir, B.; Kapon, E.
2005-06-01
We report on the observation of negatively- and positively-charged excitons in the photoluminescence spectra of V-groove quantum wires. The charged exciton binding energy increases with the strength of the quantum confinement. We demonstrate that fluctuations of the confinement potential cause the localization of the exciton and of the charged exitons on the same location. We discover that a large fraction of the enhancement of the charged exciton "binding energies" has a kinetic origin associated with the recoil energy transferred to the remaining carrier during the emission process.
Minimal model for charge transfer excitons at the dielectric interface
NASA Astrophysics Data System (ADS)
Ono, Shota; Ohno, Kaoru
2016-03-01
A theoretical description of the charge transfer (CT) exciton across the donor-acceptor interface without the use of a completely localized hole (or electron) is a challenge in the field of organic solar cells. We calculate the total wave function of the CT exciton by solving an effective two-particle SchrÃ¶dinger equation for the inhomogeneous dielectric interface. We formulate the magnitude of the CT and construct a minimal model of the CT exciton under the breakdown of inversion symmetry. We demonstrate that both a light hole mass and a hole localization along the normal to the dielectric interface are crucial to yield the CT exciton.
Polarization-dependent exciton dynamics in tetracene single crystals.
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-28
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors. PMID:25554147
Localization length scales of triplet excitons in singlet fission materials
NASA Astrophysics Data System (ADS)
Bayliss, Sam L.; Thorley, Karl J.; Anthony, John E.; Bouchiat, HÃ©lÃ¨ne; Greenham, Neil C.; Chepelianskii, Alexei D.
2015-09-01
We measure the dielectric confinement length scales of triplet excitons in organic semiconductors by jointly measuring their microwave-domain electric and magnetic susceptibilities. We apply this technique to characterize triplet excitons in two singlet fission materials with distinct solid-state packing and correlate the extracted localization length scales with the role of the excitonic environment. By using the magnetic susceptibility simultaneously determined through our experiments, we compare the independently extracted dielectric and spin-spin localization length scales, highlighting the role of local anisotropy on the properties of excitonic triplet states.
Polarization-dependent exciton dynamics in tetracene single crystals
Zhang, Bo; Zhang, Chunfeng Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-28
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Triplet diffusion in singlet exciton fission sensitized pentacene solar cells
NASA Astrophysics Data System (ADS)
Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.
2013-10-01
Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 Â± 15%.
Exciton dynamics in organic molecular crystals and nanostructures
NASA Astrophysics Data System (ADS)
Bardeen, Chris
2014-03-01
The photophysical behavior of organic semiconductors is governed by their excitonic states. In this talk, we classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge-transfer) typically encountered in organic semiconductors. The availability of several different exciton bands provides the possibility of interband processes. One such process is singlet fission, where an initially excited singlet exciton can spontaneously split into a pair of spin-entangled triplet excitons. We analyze this phenomenon in detail, emphasizing the role of spin state coherence and magnetic fields in studying singlet <-- --> triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge-transfer) interact to generate unique nonlinear excitonic processes in molecular systems. These processes may be useful for applications like solar energy conversion, where the generation of two excitons per absorbed photon could lead to significant enhancements in the efficiency of single junction photovoltaic cells. Finally, we will briefly describe how excitons can also be used to initiate photochemical reactions in molecular crystal nanostructures, resulting in large shape changes and deformations.
Polarization-dependent exciton dynamics in tetracene single crystals
NASA Astrophysics Data System (ADS)
Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min
2014-12-01
We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.
Nonthermally activated exciton transport in crystalline organic semiconductor thin films
NASA Astrophysics Data System (ADS)
Topczak, Anna K.; Roller, Tobias; Engels, Bernd; Brütting, Wolfgang; Pflaum, Jens
2014-05-01
The temperature dependent exciton transport in the prototypical organic semiconductor di-indeno-perylene (DIP) is investigated by photoluminescence quenching. Analysis by an advanced diffusion model including interference and morphological aspects reveals an exciton diffusion length of about 60 nm at room temperature, which relates to the long-range order induced by the DIP molecular shape. Above 80 K, singlet exciton transport is thermally activated with an energy of 10 to 20 meV. Below 80 K, exciton motion becomes temperature independent and is supported by the crystalline structure of the transport layer in combination with the reduced phonon interaction.
Coherence Length of Excitons in a Semiconductor Quantum Well
NASA Astrophysics Data System (ADS)
Zhao, Hui; Moehl, Sebastian; Kalt, Heinz
2002-08-01
We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300-400nm, about 25-30times the exciton deBroglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by considering only the acoustic-phonon scattering suggests an important influence of static disorder.
Coherence length of excitons in a semiconductor quantum well.
Zhao, Hui; Moehl, Sebastian; Kalt, Heinz
2002-08-26
We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300-400 nm, about 25-30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by considering only the acoustic-phonon scattering suggests an important influence of static disorder. PMID:12190436
Abramavicius, Darius; Nemeth, Alexandra; Milota, Franz; Sperling, Jaroslaw; Mukamel, Shaul; Kauffmann, Harald F.
2013-01-01
The two-exciton manifold of a double-wall cylindrical molecular aggregate is studied using a coherent third order optical technique. Experiments reveal the anharmonic character of the exciton bands. Atomistic simulations of the exciton-exciton scattering show that the excitons can be treated as weakly coupled hard-core bosons. The weak coupling stems from the extended exciton delocalization made possible by the nanotube geometry. PMID:22401120
Photoluminescence properties of exciton-exciton scattering in a GaAs/AlAs multiple quantum well
NASA Astrophysics Data System (ADS)
Nakayama, M.; Hirao, T.; Hasegawa, T.
2010-09-01
We report on the photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well under high-density-excitation conditions at excitation energies near the fundamental exciton energies. The biexciton-PL band is dominant in a relatively low-excitation-power region. The PL originating from exciton-exciton scattering, the so-called P emission, suddenly appears with an increase in excitation power. The excitation-energy dependence of the intensity of the P-PL band indicates that the excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical (LO) phonon is the most efficient for the P PL. This suggests that the LO-phonon scattering plays an important role in the relaxation process of excitons leading to the P PL. The appearance of the P-PL band remarkably suppresses the intensity of the biexciton-PL band; namely, the exciton-exciton scattering process prevents the formation of biexcitons. Furthermore, we have confirmed the existence of optical gain due to the exciton-exciton scattering process with use of a variable-stripe-length method.
Nuclear Charge Radius of Lithium-11
Sanchez, Rodolfo; Nortershauser, W.; Dax, A. ..; Ewald, G.; Gotte, S.; Kirchner, R. G.; Kluge, H. J.; Kuhl, T. H.; Wojtaszek, A.; Bushaw, Bruce A.; Drake, Gordon W. F.; Yan, Z. C.; Zimmermann, Claus; Albers, Daniel; Behr, John; Bricault, Pierre; Dilling, Jens; Dombsky, Marik; Lassen, J.; Levy, C.D. P.; Pearson, Matthew; Prime, Erika; Ryjkov, Vladimir L.
2006-07-01
We have determined the nuclear charge radius of 11Li by high-precision laser spectroscopy. The experiment was performed at the TRIUMF-ISAC facility where the 7Li-11Li isotope shift was measured in the 2s to 3s electronic transition using Doppler-free two-photon spectroscopy with a relative accuracy better than 10 5. The accuracy reached in previous experiments on the other lithium isotopes was improved. Most of the isotope shifts measured in the experiment are due to difference in the mass of the nuclei but small contributions are produced by the change in proton distribution, QED and relativistic effects have to be taken into account as well. By comparing the experimental results with sophisticated atomic calculations of the mass dependent effect the nuclear charge radii of the lithium isotopes are found to decrease monotonically from 6Li to 9Li while the nuclear charge radius of 11Li is about 11% larger than that of 9Li.
NASA Astrophysics Data System (ADS)
Schröter, M.; Ivanov, S. D.; Schulze, J.; Polyutov, S. P.; Yan, Y.; Pullerits, T.; Kühn, O.
2015-03-01
The influence of exciton-vibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these protein-pigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of exciton-vibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of exciton-vibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of exciton-vibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the exciton-vibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real system-bath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be given in terms of a stochastic decoupling ansatz. This method has become the standard in exciton-vibrational theory and illustrative examples will be presented as well as a comparison with ML-MCTDH. Applications will be shown for generic model systems as well as for small aggregates mimicking those formed by perylene bisimide dyes. Further, photosynthetic antenna complexes will be discussed, including spectral densities and the role of exciton-vibrational coupling in two-dimensional electronic spectroscopy.
Visualization of Exciton Transport in Molecular and Quantum Dot Solids
NASA Astrophysics Data System (ADS)
Akselrod, Gleb; Deotare, Parag; Prins, Ferry; Thompson, Nicholas; Poulikakos, Lisa; Lee, Elizabeth; Weidman, Mark; Mork, Jolene; Lee, Jiye; Willard, Adam; Baldo, Marc; Menon, Vinod; Tisdale, William; Bulovic, Vladimir
2015-03-01
Transport of nanoscale energy in the form excitons is at the core of the operation of a wide range of nanostructured optoelectronic devices such as solar cells, light emitting diodes and excitonic transistors. Particularly important is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal, and spectral visualization of exciton transport in molecular crystals and quantum dot solids. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of tetracene, we show that the transition to subdiffusive transport occurs at earlier times as disorder is increased. In colloidal quantum dot films, we show that diffusion does not occur by a random-walk process; instead, energetic disorder causes the exciton diffusivity to decrease over time. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology and disorder.
Effect of exciton dragging by a surface acoustic wave
NASA Astrophysics Data System (ADS)
Kovalev, V. M.; Chaplik, A. V.
2015-02-01
We present the theory of the effect of exciton dragging by a Rayleigh surface acoustic wave at temperatures above the condensation temperature of the exciton gas and at zero temperature, where the effects of the Bose-Einstein condensation of the exciton gas are most pronounced. The magnitude of the acoustic drag flux in the exciton gas at high temperatures has been calculated taking into account the exciton-exciton interaction. It has been shown that the drag flux at typical experimental parameters (at a given intensity of the surface acoustic wave (SAW)) is independent of the frequency of the acoustic wave, whereas the interaction between excitons leads to screening of the SAW-induced perturbation, which results in an exponentially fast decrease in the drag flux with an increase in the exciton density. At low temperatures, in the presence of a condensate, the drag flux of condensate particles exhibits a resonance character when the velocity of Bogoliubov excitations approaches the velocity of the acoustic wave and the magnitude of the flux is linear in the SAW frequency. The drag flux of the above-condensate particles has a threshold character: the above-condensate particles are dragged by the wave at a velocity of the acoustic wave higher than the bogolon velocity. The magnitude of the above-condensate flux is inversely proportional to the SAW frequency.
Architectures for enhanced exciton collection in organic photovoltaic cells
NASA Astrophysics Data System (ADS)
Holmes, Russell
2010-03-01
Organic semiconductors have received considerable attention for application in a variety of optoelectronic systems including light-emitting devices, lasers, and photovoltaic cells. Due to their compatibility with lightweight flexible substrates and high throughput processing techniques, organic photovoltaic cells (OPVs) represent an intriguing renewable energy option. In these materials, photogenerated excitons must be dissociated in order to generate a photocurrent. Exciton dissociation is typically realized using a donor-acceptor (D-A) heterojunction, where the energy level offset exceeds the exciton binding energy. Mobile excitons diffuse to the D-A heterojunction and are dissociated into their component charge carriers. In most organic materials, the exciton diffusion length is much shorter than the optical absorption length. Consequently, not all of the photogenerated excitons reach the D-A interface, limiting cell efficiency. For small molecule active materials, routes around this bottleneck have centered on the use of mixed D-A film morphologies to increase the area of the dissociating interface. This work instead focuses on the use of OPVs with continuously graded film composition and morphology as a means to simultaneously optimize the exciton diffusion and charge collection efficiencies. In these graded heterojunction OPVs, the power conversion efficiency is noted to exceed that of comparable devices containing a planar or mixed heterojunction. Overall, this approach provides the ability to tune the exciton diffusion and charge collection efficiencies based on the composition profile, permitting greater control over device performance.
Directing energy transport in organic photovoltaic cells using interfacial exciton gates.
Menke, S Matthew; Mullenbach, Tyler K; Holmes, Russell J
2015-04-28
Exciton transport in organic semiconductors is a critical, mediating process in many optoelectronic devices. Often, the diffusive and subdiffusive nature of excitons in these systems can limit device performance, motivating the development of strategies to direct exciton transport. In this work, directed exciton transport is achieved with the incorporation of exciton permeable interfaces. These interfaces introduce a symmetry-breaking imbalance in exciton energy transfer, leading to directed motion. Despite their obvious utility for enhanced exciton harvesting in organic photovoltaic cells (OPVs), the emergent properties of these interfaces are as yet uncharacterized. Here, directed exciton transport is conclusively demonstrated in both dilute donor and energy-cascade OPVs where judicious optimization of the interface allows exciton transport to the donor-acceptor heterojunction to occur considerably faster than when relying on simple diffusion. Generalized systems incorporating multiple exciton permeable interfaces are also explored, demonstrating the ability to further harness this phenomenon and expeditiously direct exciton motion, overcoming the diffusive limit. PMID:25798712
Optical diode based on exciton-polaritons
NASA Astrophysics Data System (ADS)
Espinosa-Ortega, T.; Liew, T. C. H.; Shelykh, I. A.
2013-11-01
We propose theoretically an optical diode based on exciton-polaritons in semiconductor microcavities. A flow of polaritons in the bistable regime is used to send signals through an asymmetric fixed potential that favours the bridging of particles in one direction. Through dynamic modelling of the coherent polariton field, we demonstrate the characteristics of an ideal diode, namely, that the forward signal is fully transmitted while the transmission in the reverse direction tends to zero, without any additional external control. Moreover, the system proves to be robust to the presence of disorder, intrinsic to microcavities, and can function at gigahertz repetition rates.
Entangled exciton states in quantum dot molecules
NASA Astrophysics Data System (ADS)
Bayer, Manfred
2002-03-01
Currently there is strong interest in quantum information processing(See, for example, The Physics of Quantum Information, eds. D. Bouwmeester, A. Ekert and A. Zeilinger (Springer, Berlin, 2000).) in a solid state environment. Many approaches mimic atomic physics concepts in which semiconductor quantum dots are implemented as artificial atoms. An essential building block of a quantum processor is a gate which entangles the states of two quantum bits. Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate(P. Hawrylak, S. Fafard, and Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).)(M. Bayer, P. Hawrylak, K. Hinzer, S. Fafard, M. Korkusinski, Z.R. Wasilewski, O. Stern, and A. Forchel, Science 291, 451 (2001).): The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the isospin and leads to superposition of these states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. Here we present spectrocsopic studies of single self-assembled InAs/GaAs quantum dot molecules that support the feasibility of this proposal. The evolution of the excitonic recombination spectrum with varying separation between the dots allows us to demonstrate coherent tunneling of carriers across the separating barrier and the formation of entangled exciton states: Due to the coupling between the dots the exciton states show a splitting that increases with decreasing barrier width. For barrier widths below 5 nm it exceeds the thermal energy at room temperature. For a given barrier width, we find only small variations of the tunneling induced splitting demonstrating a good homogeneity within a molecule ensemble. The entanglement may be controlled by application of electromagnetic field. For example, using an electric field along the molecule axis we can break the entanglement. Tunneling of carriers is prevented then and emission from intra-dot and inter-dot excitons in which electron and hole are located on the same and on opposite dots, respectively, is observed. The voltage required for the entanglement breaking increases with decreasing barrier width reflecting the increasing 'robustness' of the entanglement for narrow barriers.
Exciton fine structure in coupled quantum dots
NASA Astrophysics Data System (ADS)
Lyanda-Geller, Y. B.; Reinecke, T. L.; Bayer, M.
2004-04-01
We have developed a theoretical treatment of exciton fine structure effects in coupled quantum dots. We find that the dominant contributions giving mixing of optically bright and dark states and anticrossings between them are (i) an anisotropic Zeeman interaction for the hole and (ii) the electron spin-orbit interaction at nonzero magnetic field. Detailed calculations are given for vertically coupled InAs/GaAs dots that give a framework for the interpretation of recent experimental results. We find that anticrossings arise from deviations from axial symmetry in these systems.
Hastings, Rob; Cobben, Jan-Maarten; Gillessen-Kaesbach, Gabriele; Goodship, Judith; Hove, Hanne; Kjaergaard, Susanne; Kemp, Helena; Kingston, Helen; Lunt, Peter; Mansour, Sahar; McGowan, Ruth; Metcalfe, Kay; Murdoch-Davis, Catherine; Ray, Mary; Rio, Marlène; Smithson, Sarah; Tolmie, John; Turnpenny, Peter; van Bon, Bregje; Wieczorek, Dagmar; Newbury-Ecob, Ruth
2011-01-01
Bohring–Opitz syndrome (BOS) is a rare congenital disorder of unknown etiology diagnosed on the basis of distinctive clinical features. We suggest diagnostic criteria for this condition, describe ten previously unreported patients, and update the natural history of four previously reported patients. This is the largest series reported to date, providing a unique opportunity to document the key clinical features and course through childhood. Investigations undertaken to try and elucidate the underlying pathogenesis of BOS using array comparative genomic hybridization and tandem mass spectrometry of cholesterol precursors did not show any pathogenic changes responsible. PMID:21368916
Charge-transfer excitons at organic semiconductor surfaces and interfaces.
Zhu, X-Y; Yang, Q; Muntwiler, M
2009-11-17
When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D/A interface can be higher than that to CT(1s) as expected from energy resonance arguments. We suggest a design principle in organic heterojunction solar cells: there must be strong electronic coupling between molecular excitons in the donor and hot CT excitons across the D/A interface. PMID:19378979
Nonlinear photoluminescence spectroscopy of carbon nanotubes with localized exciton states.
Iwamura, Munechiyo; Akizuki, Naoto; Miyauchi, Yuhei; Mouri, Shinichiro; Shaver, Jonah; Gao, Zhenghong; Cognet, Laurent; Lounis, Brahim; Matsuda, Kazunari
2014-11-25
We report distinctive nonlinear behavior of photoluminescence (PL) intensities from localized exciton states embedded in single-walled carbon nanotubes (SWNTs) at room temperature. We found that PL from the local states exhibits strong nonlinear behavior with increasing continuous-wave excitation power density, whereas free exciton PL shows only weak sublinear behavior. The strong nonlinear behavior was observed regardless of the origin of the local states and found to be nearly independent of the local state density. These results indicate that the strong PL nonlinearity arises from a universal mechanism to SWNTs with sparse local states. The significant nonlinear PL is attributed to rapid ground-state depletion of the local states caused by an efficient accumulation of photogenerated free excitons into the sparse local states through one-dimensional diffusional migration of excitons along the nanotube axis; this mechanism is verified by Monte Carlo simulations of exciton diffusion dynamics. PMID:25331628
Modeling excitonic line shapes in weakly disordered semiconductor nanostructures
NASA Astrophysics Data System (ADS)
Kuznetsova, I.; G?gh, N.; Förstner, J.; Meier, T.; Cundiff, S. T.; Varga, I.; Thomas, P.
2010-02-01
Excitonic spectra of weakly disordered semiconductor heterostructures are simulated on the basis of a one-dimensional tight-binding model. The influence of the length scale of weak disorder in quantum wells on the redshift of the excitonic peak and its linewidth is studied. By calculating two-dimensional Fourier-transform spectra we are able to determine the contribution of disorder to inhomogeneous and also to homogeneous broadenings separately. This disorder-induced dephasing is related to a Fano-type coupling and leads to contributions to the homogeneous linewidth that depends on energy within the inhomogeneously broadened line. The model includes heavy- and light-hole excitons and yields smaller inhomogeneous broadening for the light-hole exciton if compared to the heavy-hole exciton, which agrees qualitatively with the experiment.
Dynamics of a single exciton in strongly correlated bilayers
NASA Astrophysics Data System (ADS)
Rademaker, Louk; Wu, Kai; Zaanen, Jan
2012-08-01
We formulated an effective theory for a single interlayer exciton in a bilayer quantum antiferromagnet, in the limit when the holon and doublon are strongly bound onto one interlayer rung by the Coulomb force. Upon using a rung linear spin-wave approximation of the bilayer Heisenberg model, we calculated the spectral function of the exciton for a wide range of the interlayer Heisenberg coupling ? = J?/Jz. In the disordered phase at large ?, a coherent quasi-particle peak appears, representing free motion of the exciton in a spin singlet background. In the Néel phase, which applies to more realistic model parameters, a ladder spectrum arises due to Ising confinement of the exciton. The exciton spectrum is visible in measurements of the dielectric function, such as c-axis optical conductivity measurements.
Excitonic states of an impurity in a Fermi gas
NASA Astrophysics Data System (ADS)
Lan, Zhihao; Lobo, Carlos
2015-11-01
We study excitonic states of an atomic impurity in a Fermi gas, i.e., bound states consisting of the impurity and a hole. Previous studies considered bound states of the impurity with particles from the Fermi sea where the holes formed only part of the particle-hole dressing. Within a two-channel model, we find that, for a wide range of parameters, excitonic states are not ground but metastable states. We further calculate the decay rates of the excitonic states to polaronic and dimeronic states and find they are long-lived, scaling as ?PolExc?(?? ) 5.5 and ?DimExc?(?? ) 4 . We also find that a new continuum of exciton-particle states should be considered alongside the previously known dimeron-hole continuum in spectroscopic measurements. Excitons must therefore be considered a new ingredient in the study of metastable physics currently being explored experimentally.
Excitonic correlation in the Mott crossover regime in Ge
NASA Astrophysics Data System (ADS)
Sekiguchi, Fumiya; Shimano, Ryo
2015-04-01
Exciton Mott transition (EMT) in Ge was investigated by using optical-pump and terahertz-probe spectroscopy. From the quantitative analysis of optical conductivity and dielectric function, we evaluated the densities of unbound electron-hole pairs and excitons after the photoexcitation, from which we determined the ionization ratio of excitons Î±. The Mott crossover density region in Ge was elucidated from the density dependence of Î± in the temperature range above the critical temperature of electron-hole droplets. The 1 s -2 p excitonic transition energy hardly shifted with increasing density toward the EMT. Combined with the similar results recently observed in bulk Si, we suggest that the robustness of excitonic correlation against the Coulomb screening is a universal feature in bulk semiconductors in the Mott crossover regime.
High efficiency organic multilayer photodetectors based on singlet exciton fission
NASA Astrophysics Data System (ADS)
Lee, J.; Jadhav, P.; Baldo, M. A.
2009-07-01
We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45Â±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7Â±0.2)% under an applied magnetic field of H =0.4 T, while the C60 photocurrent is relatively unchanged.
Calculating exciton downconversion rates in Coulombically coupled chromophores
NASA Astrophysics Data System (ADS)
Chapman, Craig; Schatz, George
2015-03-01
Exciton downconversion is a second order energy transfer process that splits a high energy exciton in a donor chromophore into multiple lower energy excitons in acceptor chromophores. Downconversion has been seen in a variety of materials including rare-earth doped glassy matrices, organic crystals, and semiconductor nanocrystals, and has the potential to efficiently convert a single high energy photon into a broad distribution of lower energy excitons. A comprehensive mechanistic understanding of the energy conversion process will allow for the rational engineering of materials that can control the flow of energy in a guided fashion. To this end we formulate and implement a method for calculating multi-chromophore FÃ¶rster-like exciton transfer rates using transition charges obtained from time-dependent density functional theory.
Multiphonon excitonic absorption in semiconductors and quantum wells
Bardyszewski, Witold; Prywata, Miroslaw; Yevick, David
2001-07-01
In this article, we present a straightforward procedure based on the cumulant expansion for computing the contribution of exciton{endash}phonon coupling to the exciton line-shape function and thus absorption in bulk and quantum well semiconductors. Using the functional integral technique we derive the effective equation of motion for the two-particle Green{close_quote}s function in the presence of interaction with phonons. We show that our many body formulation in the limit of linear coupling of carriers with polar longitudinal optical phonons in bulk semiconductors is equivalent to the model exciton{endash}phonon Hamiltonian approach. Our calculations for the hydrogenic model of excitons in three- and two-dimensional systems reproduce not only the correct polaron induced shift (renormalization) of the exciton energy but also the observed asymmetric spectral line shape and exponential Urbach absorption tail below the fundamental absorption edge. {copyright} 2001 American Institute of Physics.
Bose-Einstein Condensation of Excitons and Biexcitons
NASA Astrophysics Data System (ADS)
Moskalenko, S. A.; Snoke, D. W.
2000-02-01
Bose-Einstein condensation of excitons is a unique effect in which the electronic states of a solid can self-organize to acquire quantum phase coherence. The phenomenon is closely linked to Bose-Einstein condensation in other systems such as liquid helium and laser-cooled atomic gases. Covering theoretical aspects as well as recent experimental work, the book provides a comprehensive survey of the field. After introducing the relevant basic physics of excitons, the authors discuss exciton-phonon interactions as well as the behavior of biexcitons. They also cover exciton phase-transitions and give particular attention to nonlinear optical effects including the optical Stark effect and chaos in excitonic systems. The thermodynamics of equilibrium, quasiequilibrium, and nonequilibrium systems are examined in detail. Throughout, the authors interweave theoretical and experimental results. The book will be of great interest to graduate students and researchers in semiconductor and superconductor physics, quantum optics, and atomic physics.
Understanding the mass-radius relation for sub-Neptunes: radius as a proxy for composition
Lopez, Eric D.; Fortney, Jonathan J.
2014-09-01
Transiting planet surveys like Kepler have provided a wealth of information on the distribution of planetary radii, particularly for the new populations of super-Earth- and sub-Neptune-sized planets. In order to aid in the physical interpretation of these radii, we compute model radii for low-mass rocky planets with hydrogen-helium envelopes. We provide model radii for planets 1-20 M {sub âŠ•}, with envelope fractions 0.01%-20%, levels of irradiation 0.1-1000 times Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide simple analytic fits that summarize how radius depends on each of these parameters. Most importantly, we show that at fixed H/He envelope fraction, radii show little dependence on mass for planets with more than âˆ¼1% of their mass in their envelope. Consequently, planetary radius is to a first order a proxy for planetary composition, i.e., H/He envelope fraction, for Neptune- and sub-Neptune-sized planets. We recast the observed mass-radius relationship as a mass-composition relationship and discuss it in light of traditional core accretion theory. We discuss the transition from rocky super-Earths to sub-Neptune planets with large volatile envelopes. We suggest âˆ¼1.75 R {sub âŠ•} as a physically motivated dividing line between these two populations of planets. Finally, we discuss these results in light of the observed radius occurrence distribution found by Kepler.
Lebedev, Vladimir S; Medvedev, A S
2012-08-31
This paper examines plasmon - exciton coupling effects in light absorption and scattering by hybrid nanoparticles consisting of a metallic core and organic dye J-aggregate shell. The spectroscopic characteristics of such particles are calculated using generalised Mie theory for two concentric spheres in a wide spectral range for various geometric parameters of the system, core materials (Ag, Au, Cu and Al) and cyanine dyes (TC, OC and PIC). We determine the eigenfrequencies of hybrid modes in the system and photoabsorption peak heights as functions of the oscillator strength of the transition in the J-band of the dye, core radius and shell thickness, and demonstrate that the interactions of a Frenkel exciton with dipole and multipole plasmons have radically different effects on the optical properties of the composite nanoparticles. Varying the particle size and the optical constants of the core and shell materials influences the number of peaks in the spectra of the particles and leads to a significant redistribution of peak heights. We identify regions where the extinction spectrum of the particles is dominated by light absorption or scattering processes. (nanophotonics)
Trofymchuk, Kateryna; Prodi, Luca; Reisch, Andreas; Mély, Yves; Altenhöner, Kai; Mattay, Jochen; Klymchenko, Andrey S
2015-06-18
Photoswitching of bright fluorescent nanoparticles opens new possibilities for bioimaging with superior temporal and spatial resolution. However, efficient photoswitching of nanoparticles is hard to achieve using Förster resonance energy transfer (FRET) to a photochromic dye, because the particle size is usually larger than the Förster radius. Here, we propose to exploit the exciton diffusion within the FRET donor dyes to boost photoswitching efficiency in dye-doped polymer nanoparticles. To this end, we utilized bulky hydrophobic counterions that prevent self-quenching and favor communication of octadecyl rhodamine B dyes inside a polymer matrix of poly(D,L-lactide-co-glycolide). Among tested counterions, only perfluorinated tetraphenylborate that favors the exciton diffusion enables high photoswitching efficiency (on/off ratio ?20). The switching improves with donor dye loading and requires only 0.1-0.3 wt % of a diphenylethene photochromic dye. Our nanoparticles were validated both in solution and at the single-particle level. The proposed concept paves the way to new efficient photoswitchable nanomaterials. PMID:26266601
NASA Astrophysics Data System (ADS)
Szczytko, J.; Kappei, L.; Berney, J.; Morier-Genoud, F.; Portella-Oberli, M. T.; Deveaud, B.
2005-05-01
We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum well: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We demonstrate, by comparing the temperature dependence of 1s and 2s excitonic transitions, that excitons provide the dominant contribution to the luminescence signal. Furthermore, our analysis gives evidence that the Coulomb correlated plasma contribution to the luminescence signal might be neglected for densities, temperatures, and time scales actually used in time-resolved experiments.
Exciton states and interband absorption of cylindrical quantum dot with Morse confining potential
NASA Astrophysics Data System (ADS)
Hayrapetyan, D. B.; Kazaryan, E. M.; Kotanjyan, T. V.; Tevosyan, H. Kh.
2015-02-01
In this paper the exciton and electron sates in cylindrical quantum dot with Morse potential made of GaAs are studied. For the regime of strong size quantization, energy spectrum with the parabolic approximation case are compared. For strong and weak size quantization regimes analytic expressions for the particle energy spectrum, absorption coefficient and dependencies of effective threshold frequencies of absorption on the geometrical parameters quantum dot are obtained. For the intermediate size quantization regime the problem solved in the framework of variation method. The selection rules corresponding to different transitions between quantum levels are found. The size dispersion distribution of growing quantum dots by the radius and height by two experimentally realizing distribution functions have been taken into account. Distribution functions of Gauss, Lifshits-Slezov have been considered.
21 CFR 886.1450 - Corneal radius measuring device.
Code of Federal Regulations, 2014 CFR
2014-04-01
... 21 Food and Drugs 8 2014-04-01 2014-04-01 false Corneal radius measuring device. 886.1450 Section 886.1450 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES OPHTHALMIC DEVICES Diagnostic Devices Â§ 886.1450 Corneal radius measuring device. (a) Identification. A corneal radius...
Robust excitons inhabit soft supramolecular nanotubes
Eisele, Dörthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A.; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G.
2014-01-01
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions—prerequisites for efficient energy transport—are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature’s high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials. PMID:25092336
Exciton Dynamics in Semiconducting Carbon Nanotubes
Graham, Matt; Chmeliov, Javgenij; Ma, Yingzhong; Shinohara, Nori; Green, Alexander A.; Hersam, Mark C.; Valkunas, Leonas; Fleming, Graham
2010-01-01
We report femtosecond transient absorption spectroscopic study on the (6, 5) single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled carbon nanotube species. We found that the dynamics of exciton relaxation probed at the first transition-allowed state (E11) of a given tube type exhibits a markedly slower decay when the second transition-allowed state (E22) is excited than that measured by exciting its first transition-allowed state (E11). A linear intensity dependence of the maximal amplitude of the transient absorption signal is found for the E22 excitation, whereas the corresponding amplitude scales linearly with the square root of the E11 excitation intensity. Theoretical modeling of these experimental findings was performed by developing a continuum model and a stochastic model with explicit consideration of the annihilation of coherent excitons. Our detailed numerical simulations show that both models can reproduce reasonably well the initial portion of decay kinetics measured upon the E22 and E11 excitation of the chosen tube species, but the stochastic model gives qualitatively better agreement with the intensity dependence observed experimentally than those obtained with the continuum model.
Photoluminescence from exciton-exciton scattering in a GaAs1-xNx thin film
NASA Astrophysics Data System (ADS)
Hashimoto, J.; Maeda, Y.; Nakayama, M.
2010-02-01
We have investigated photoluminescence (PL) properties under high-density-excitation conditions at 10 K in a GaAs1-xNx thin film (x =0.008) with a narrow band-gap energy of 1.34 eV. A PL band was observed with a threshold-like nature, and its intensity was found to exhibit quadratic dependence on the excitation power. At the threshold excitation power, the PL-peak energy is lower than the energy of the fundamental exciton by the magnitude of the exciton binding energy that is ˜4 meV. The results described above indicate that the PL band originates from exciton-exciton scattering, the so-called P emission, which is typically observed in wide-gap semiconductors with large exciton binding energies. Furthermore, we have confirmed the existence of optical gain in the energy region of the P band using a variable-stripe-length method.
Theory of exciton transfer and diffusion in conjugated polymers.
Barford, William; Tozer, Oliver Robert
2014-10-28
We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ~10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations. PMID:25362268
Theory of exciton transfer and diffusion in conjugated polymers
Barford, William; Tozer, Oliver Robert
2014-10-28
We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ?10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.
Self-Trapped Excitons in Semiconductor Quantum Wires Inside a Polar Dielectric Matrix
NASA Astrophysics Data System (ADS)
Muljarov, E. A.; Tikhodeev, S. G.
1997-11-01
We analyze theoretically the conditions of the exciton self-trapping in a thin semiconductor wire, imbedded into a polar dielectric matrix. We show that in such systems the exciton self-trapping can be strongly enhanced due to (i) increased exciton-phonon coupling and (ii) possibility of spatial charge separation in the exciton. We use the standard adiabatic polaron approach to evaluate the effective exciton Hamiltonian, which takes into account the exciton-optical phonon interaction as well as the electric field renormalization due to image charges, and calculate variationally the self-trapped exciton binding energy and the Stokes shift between the peak energies in the optical absorption and emission.
Nondiffusive excitonic transport in GaAs and the effects of momentum scattering
NASA Astrophysics Data System (ADS)
Schaefer, A. C.; Erland, J.; Steel, D. G.
1996-10-01
Measurements of exciton transport using frequency-domain nearly degenerate four-wave mixing (NDFWM) show a failure of Fick's law and near-ballistic excitonic motion for scale lengths less than 10 ?m. A solution of the Boltzmann transport equation assuming a simple momentum-changing collision model is in agreement with the observed NDFWM line shapes and apparent subquadratic dependence on inverse scale length. The measurements show the surprising result that exciton-exciton interactions lead to a large increase in exciton linewidth with little corresponding change in exciton center-of-mass momentum, unlike exciton interactions with phonons and impurities.
NASA Astrophysics Data System (ADS)
Ben-Amots, N.
2015-05-01
We discuss exploding stars exceeding 8 MSun. Supernova processes start with implosion before the explosion. The processes causing implosion are relatively well understood. Not so the processes that reverse the implosion into explosion. We propose and discuss three possible processes that can reverse implosion of a star into an explosion, causing supernova: a) energy accumulation in sub-Bohr electron orbitals populated by an electron moving at high relativistic velocities [1], b) Franklin's relativistic rotation of quarks [2], [3] and c) Einstein's gravitational field bounceback[4]. The relativistic sub-Bohr orbitals are derived by using an analogy to exponential gravitation (recent refs [1], [4], [5], [6], [7] and [8]).
Broadband THz response of high-density excitons
NASA Astrophysics Data System (ADS)
Huber, R.
2005-03-01
Many-body interactions in electron-hole (e-h) gases determine their nature as a conductive unbound e-h plasma or insulating exciton gas. THz spectroscopy, unlike bandgap luminescence or absorption, probes transitions between internal exciton states. In this way, exciton formation in a dilute, optically-generated e-h gas was recently investigated in GaAs quantum wells (R. Kaindl, et al. Nature 423, 734, 2003). Here, we report THz studies of a high-density exciton gas. For dilute, insulating excitons, the THz conductivity peaks around 7 meV due to 1s-2p transitions. With increasing density, the peak shows a striking red-shift and broadening, and finally evolves into a Drude shape. Quantitative analysis reveals a broadening that gradually exceeds the level spacing and is larger than expected for 1s excitons. This agrees with enhanced scattering from p-like states. In contrast to optical studies, which are limited due to counteracting bandgap renormalization and reduced e-h attraction, THz radiation provides a direct way to measure the impact of phase-space filling and screening on exciton levels at high densities.
Excitonic properties of ZnS quantum wells
NASA Astrophysics Data System (ADS)
Urbaszek, B.; Townsley, C. M.; Tang, X.; Morhain, C.; Balocchi, A.; Prior, K. A.; Nicholas, R. J.; Cavenett, B. C.
2001-10-01
The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by molecular-beam epitaxy on GaP substrates has allowed the observation of heavy- and light-hole exciton transitions with values for the full width at half maximum as narrow as 5 meV. The 2s state of the heavy-hole exciton is identified and exciton binding energies of as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings of the order of 10 meV for both the light- and heavy-hole exciton transitions appear in magnetoreflectivity spectra in magnetic fields up to 54 T. Large light-hole exciton g values of the order of 4 for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile-strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.
Dynamical fermion mass generation and exciton spectra in graphene
Zhang Chunxu; Liu Guozhu; Huang Mingqiu
2011-03-15
The Coulomb interaction between massless Dirac fermions may induce dynamical chiral symmetry breaking by forming excitonic pairs in clean graphene, leading to semimetal-insulator transition. If the Dirac fermions have zero bare mass, an exact continuous chiral symmetry is dynamically broken and thus there are massless Goldstone excitons. If the Dirac fermions have a small bare mass, an approximate continuous chiral symmetry is dynamically broken and the resultant Goldstone-type excitons become massive, which is analogous to what happens in QCD. In this paper, after solving the Dyson-Schwinger gap equation in the presence of a small bare fermion mass, we found a remarkable reduction of the critical Coulomb interaction strength for excitonic pair formation and a strong enhancement of dynamical fermion mass. We then calculate the masses of Goldstone-type excitons using the Shifman-Vainshtein-Zakharov sum-rule method and operator product expansion technique developed in QCD and find that the exciton masses are much larger than bare fermion mass but smaller than the width of dynamical fermion mass gap. We also study the spin susceptibilities and estimate the masses of non-Goldstone-type excitons using the same tools.
Experimental study of finite Larmor radius effects
Struve, K.W.
1980-08-01
Linear Z-pinches in Ar, Kr, Xe, N/sub 2/, and He are experimentally studied in regimes where strong finite Larmor radius effects could provide a significant stabilizing effect. Scaling arguments show that for deuterium such a pinch has an electron line density of order 2 x 10/sup 15//cm. For higher Z plasmas a higher line density is allowed, the exact value of which depends on the average ion charge. The pinch is formed by puffing gas axially through the cathode towards the anode of an evacuated pinch chamber. When the gas reaches the anode, the pinch bank is fired. The pinch current rises in 2 to 3 ..mu..sec to a maximum of 100 to 200 kA. The pinch bank capacitance is 900 ..mu..F, and the external inductance is 100 nH. Additionally, the bank is fused to increase dI/dt. The primary diagnostics are a framing camera, a spatially resolved Mach-Zehnder interferometer, and X-ray absorption.
PHOTOSPHERIC RADIUS EXPANSION DURING MAGNETAR BURSTS
Watts, Anna L.; Van der Klis, Michiel; Wijers, Ralph A. M. J.; Kouveliotou, Chryssa; Van der Horst, Alexander J.; Goegues, Ersin; Kaneko, Yuki; Harding, Alice K.; Baring, Matthew G.
2010-08-10
On 2008 August 24 the new magnetar SGR 0501+4516 (discovered by Swift) emitted a bright burst with a pronounced double-peaked structure in hard X-rays, reminiscent of the double-peaked temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However, for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24 burst.
New Physics and the Proton Radius Problem
Carl E. Carlson, Benjamin C. Rislow
2012-08-01
Background: The recent disagreement between the proton charge radius extracted from Lamb shift measurements of muonic and electronic hydrogen invites speculation that new physics may be to blame. Several proposals have been made for new particles that account for both the Lamb shift and the muon anomalous moment discrepancies. Purpose: We explore the possibility that new particles' couplings to the muon can be fine-tuned to account for all experimental constraints. Method: We consider two fine-tuned models, the first involving new particles with scalar and pseudoscalar couplings, and the second involving new particles with vector and axial couplings. The couplings are constrained by the Lamb shift and muon magnetic moments measurements while mass constraints are obtained by kaon decay rate data. Results: For the scalar-pseudoscalar model, masses between 100 to 200 MeV are not allowed. For the vector model, masses below about 200 MeV are not allowed. The strength of the couplings for both models approach that of electrodynamics for particle masses of about 2 GeV. Conclusions: New physics with fine tuned couplings may be entertained as a possible explanation for the Lamb shift discrepancy.
Compensation of dipolar-exciton spin splitting in magnetic field
NASA Astrophysics Data System (ADS)
Gorbunov, A. V.; Timofeev, V. B.
2013-03-01
Magnetoluminescence of spatially indirect dipolar excitons in 25 nm GaAs/AlGaAs single quantum well collected within a lateral potential trap has been studied in Faraday geometry. The paramagnetic spin splitting of the luminescence line of the heavy-hole excitons in the trap centre is completely compensated at magnetic field below critical value ?2 ?. The effect of spin splitting compensation is caused by the exchange interaction in dense exciton Bose gas which is in qualitative agreement with the existing theoretical concepts.
Majorana edge modes of topological exciton condensate with superconductors
NASA Astrophysics Data System (ADS)
Seradjeh, Babak
2012-09-01
I study the edge states of the topological exciton condensate formed by a Coulomb interaction between two parallel surfaces of a strong topological insulator. When the condensate is contacted by superconductors with a ? phase shift across the two surfaces, a pair of counterpropagating Majorana modes close the gap at the boundary. I propose a nanostructured system of topological insulators and superconductors that hosts unpaired Majorana fermions when and only when the exciton condensate forms. Therefore, measuring the Majorana signal in this structure provides a way of detecting the topological exciton condensate that is uniquely related to its topological nature. The relevant experimental signatures as well as implications for related systems are discussed.
Spin-excitons in heavy-fermion semimetals
NASA Astrophysics Data System (ADS)
Riseborough, Peter S.; Magalhaes, S. G.
2016-02-01
Spin-excitons are sharp and dispersive magnetic fluctuations in paramagnetic semiconductors where the dispersion relation lies within the semiconducting gap. Spin-excitons are found in the vicinity of magnetic quantum critical points in semiconductors, much the same as antiparamagnons are precursor fluctuations for quantum critical points in metals. Here we show that this concept of spin-exciton excitations can be extended to heavy-fermion semimetals and provides a natural explanation of the magnetic modes found by inelastic neutron scattering experiments on paramagnetic CeFe2Al10.
Nonresonant radiative exciton transfer by near field between quantum wells
Aleshkin, V. Ya.; Gavrilenko, L. V. Gaponova, D. M.; Kadykov, A. M.; Lysenko, V. G.; Krasil’nik, Z. F.
2013-11-15
We experimentally observed an increase in the intensity of photoluminescence from a wider quantum well (QW) when an exciton transition was induced in the neighboring narrower QW separated from the former one by a tunneling-nontransparent AlGaAs barrier. The dependence of the efficiency of the near-field radiative transfer of excitons on the distance between QWs was studied in heterostructures without coincidence of exciton resonances in the adjacent QWs. Theoretical results were qualitatively consistent with the available experimental data.
Confocal shift interferometry of coherent emission from trapped dipolar excitons
Repp, J.; Schinner, G. J.; Schubert, E.; Rai, A. K.; Wieck, A. D.; Reuter, D.; Wurstbauer, U.; Holleitner, A. W.; and others
2014-12-15
We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.
Topological polaritons and excitons in garden-variety systems
NASA Astrophysics Data System (ADS)
Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Timothy C. H.
2015-04-01
We present a practical scheme for creating topological polaritons in garden-variety systems based, for example, on zinc-blende semiconductor quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as an appealing alternative for topological states in exciton-based systems. Topological polaritons and indirect excitons open a new frontier for topological states in solid-state systems, which can be directly probed and manipulated while offering a system with nonlinear interactions.
Charged two-exciton emission from a single semiconductor nanocrystal
NASA Astrophysics Data System (ADS)
Hu, Fengrui; Zhang, Qiang; Zhang, Chunfeng; Wang, Xiaoyong; Xiao, Min
2015-03-01
Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the "on" and "off" levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate "grey" level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state.
Photocurrent, photoluminescence and exciton dynamics in rubrene molecular single crystals
NASA Astrophysics Data System (ADS)
Lyu, ByungGook
This work discusses the photocurrent and photoluminescence that can be induced by short-pulse illumination in rubrene single crystals. The pulsed illumination excites a rubrene molecule from the ground state to its first optically accessible excited state, resulting in a singlet exciton state. In rubrene, a singlet exciton can transform into two triplet excitons - which together have a spin of zero - by an efficient spin-conserved fission process. On the other hand, two triplet excitons can interact to again form a singlet exciton by a fusion process. Quantitative modeling of the transformation of singlet excitons into triplet excitons and vice-versa shows that both photoconductivity dynamics and photocurrent dynamics after pulsed excitations can be understood within the same framework. The photoluminescence observed after pulsed excitation is only emitted upon radiative recombination of singlet excitons. A simple model of fission and fusion based on rate equations leads to a qualitatively different photoluminescence dynamics depending on the time scale. In particular, it predicts a fast exponential decay corresponding to the initial fission process, later a power-law (quadratic) decay corresponding to a regime when triplet-triplet interaction is dominant, and a final exponential decay with a time-constant which is half the triplet exciton lifetime. This last exponential decay corresponds to the case when only a lower density of triplet excitons is left. The same model can be used to predict the photocurrent dynamics after pulsed excitation. Experimental observations after pulsed illumination show that, for low excitation pulse energies, a large photocurrent grows exponentially with a time constant of the order of 100 microseconds. This photocurrent build-up time then becomes shorter at higher excitation energies, with the peak photocurrent also saturating. One finds that the observed photocurrent dynamics can be reproduced with the same model based on exciton fission and fusion that successfully explained photoluminescence dynamics. The only additional assumption that is required to do so is that triplet excitons be able dissociate and release free holes by direct interaction with a defect state. The 100 microsecond build-up time of the impulsively induced photocurrent then corresponds to the triplet lifetime.
Kinetics of exciton photoluminescence in type-II semiconductor superlattices
NASA Astrophysics Data System (ADS)
Braginsky, L. S.; Zaharov, M. Yu.; Gilinsky, A. M.; Preobrazhenskii, V. V.; Putyato, M. A.; Zhuravlev, K. S.
2001-05-01
The exciton decay rate at a rough interface in type-II semiconductor superlattices is investigated. It is shown that the possibility of recombination of the indirect excitons at a plane interface essentially affects the kinetics of the exciton photoluminescence at a rough interface. This is the result of the quantum interference of electrons scattered from the plane interface and at the roughnesses. Expressions that relate the parameters of the luminescence kinetics with the statistical characteristics of the rough interface are obtained. The mean height and length of the roughnesses in GaAs/AlAs superlattices are estimated from the experimental data.
Enhanced resonant backscattering of excitons in disordered quantum wells.
Langbein, W; Runge, E; Savona, V; Zimmermann, R
2002-10-01
A clear signature of enhanced backscattering of excitons is observed in the directional resonant Rayleigh scattering of light from localized two-dimensional excitons in disordered quantum wells. Its spectral dependence and time dynamics are measured and theoretically predicted in a quantitative way. The intensity enhancement has a large momentum span extending beyond the external light emission cone. This is a consequence of the small localization length of the exciton as a massive particle probed close to the band bottom. The localization length can be controlled by the photon kinetic energy. This constitutes a qualitative difference to backscattering phenomena in other branches of physics. PMID:12366019
Enhanced Resonant Backscattering of Excitons in Disordered Quantum Wells
NASA Astrophysics Data System (ADS)
Langbein, W.; Runge, E.; Savona, V.; Zimmermann, R.
2002-09-01
A clear signature of enhanced backscattering of excitons is observed in the directional resonant Rayleigh scattering of light from localized two-dimensional excitons in disordered quantum wells. Its spectral dependence and time dynamics are measured and theoretically predicted in a quantitative way. The intensity enhancement has a large momentum span extending beyond the external light emission cone. This is a consequence of the small localization length of the exciton as a massive particle probed close to the band bottom. The localization length can be controlled by the photon kinetic energy. This constitutes a qualitative difference to backscattering phenomena in other branches of physics.
Role of phonons in Josephson oscillations of excitonic and polaritonic condensates
Magnusson, E. B.; Flayac, H.; Malpuech, G.; Shelykh, I. A.
2010-11-15
We analyze theoretically the role of the exciton-phonon interactions in phenomena related to the Josephson effect between two spatially separated exciton and exciton-polariton condensates. We consider the role of the dephasing introduced by phonons in such phenomena as Josephson tunneling, self-trapping and spontaneous polarization separation. In the regime of cw pumping we find a remarkable bistability effect arising from exciton-exciton interactions as well as regimes of self-sustained regular and chaotic oscillations.
Control of Exciton Photon Coupling in Nano-structures
NASA Astrophysics Data System (ADS)
Liu, Xiaoze
In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley polarization has also been discussed in this MoS2 microcavity for the possible applications in spin switches and logic gates. The direct dipole-dipole type excitonic interactions have also been studied in inorganic-organic nanocomposites, where ZnO nanowire is taken as the inorganic constituent and NTCDA thin films as the organic constituent. The excitonic interactions can be classified into weak coupling regime and strong coupling regime. Forster Resonant Energy Transfer (FRET), which is in the weak coupling regime, has been observed in this hybrid system. The optimized optical nonlinearity has also been determined in the hybrid system via Z-scan measurements. Exciton-plasmon polariton, another example of strongly coupled state which results from the interaction between excitons and plasmons when they are in resonance, has also been investigated in this thesis. Two rhodamine dyes spincoated on the silver thin films have separately been observed to be strongly coupled to the surface plasmon modes. With observed new polariton states, energy transfer mechanism has been discussed for nonlinear optical applications.
Exciton dynamics in emergent Rydberg lattices
NASA Astrophysics Data System (ADS)
Bettelli, S.; Maxwell, D.; Fernholz, T.; Adams, C. S.; Lesanovsky, I.; Ates, C.
2013-10-01
The dynamics of excitons in a one-dimensional ensemble with partial spatial order are studied. During optical excitation, cold Rydberg atoms spontaneously organize into regular spatial arrangements due to their mutual interactions. This emergent lattice is used as the starting point to study resonant energy transfer triggered by driving a nS to n'P transition using a microwave field. The dynamics are probed by detecting the survival probability of atoms in the nS Rydberg state. Experimental data qualitatively agree with our theoretical predictions including the mapping onto the XXZ spin model in the strong-driving limit. Our results suggest that emergent Rydberg lattices provide an ideal platform to study coherent energy transfer in structured media without the need for externally imposed potentials.
Plasmon-exciton self-induced transparency
Zabolotskii, A. A.
2011-04-15
The possibility of forming stable bound plasmon-polariton states in an extended metallic cylinder surrounded by a two-level medium has been investigated. The dynamics of plasmons is described in the hydrodynamic approximation. It has been shown that the equations of motion of charge-density bunches and the Bloch equations for the two-level medium are reduced in certain approximations to integrable equations for both transverse and longitudinal plasmons. In the former case, the initial system of equations after the application of the slow-envelope approximation is reduced to equations equivalent to the Maxwell-Bloch equations. In the latter case, the equations describe wave dynamics beyond the slow-envelope approximation. In the approximation of unidirectional wave propagation, the initial system of equations is reduced to equations related to the reduced Maxwell-Bloch equations. Soliton and breather-like solutions of the derived equations describe plasmon-exciton self-induced transparency.
Optical properties of Rydberg excitons and polaritons
NASA Astrophysics Data System (ADS)
ZieliÅ„ska-RaczyÅ„ska, Sylwia; Czajkowski, Gerard; Ziemkiewicz, David
2016-02-01
We show how to compute the optical functions when Rydberg excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. We use the real density matrix approach (RDMA), which, combined with the Green's function method, enables one to derive analytical expressions for the optical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a Cu20 crystal, being a semiconductor with an indirect gap. The effect of the coherence is displayed in the line shape. We also examine in detail and explain the dependence of the oscillator strength and the resonance placement on the state number. We report good agreement with recently published experimental data. We also show that the presented method can be applied to semiconductors with a direct gap.
Hot exciton dissociation in polymer solar cells
NASA Astrophysics Data System (ADS)
Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G.
2013-01-01
The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC60BM system. Exciton splitting occurs within the first 50?fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200?fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.
Hot exciton dissociation in polymer solar cells.
Grancini, G; Maiuri, M; Fazzi, D; Petrozza, A; Egelhaaf, H-J; Brida, D; Cerullo, G; Lanzani, G
2013-01-01
The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC(60)BM system. Exciton splitting occurs within the first 50?fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200?fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield. PMID:23223127
Tidal radius estimates for three open clusters
NASA Astrophysics Data System (ADS)
Danilov, V. M.; Loktin, A. V.
2015-10-01
A new method is developed for estimating tidal radii and masses of open star clusters (OCL) based on the sky-plane coordinates and proper motions and/or radial velocities of cluster member stars. To this end, we perform the correlation and spectral analysis of oscillations of absolute values of stellar velocity components relative to the cluster mass center along three coordinate planes and along each coordinate axis in five OCL models. Mutual correlation functions for fluctuations of absolute values of velocity field components are computed. The spatial Fourier transform of the mutual correlation functions in the case of zero time offset is used to compute wavenumber spectra of oscillations of absolute values of stellar velocity components. The oscillation spectra of these quantities contain series of local maxima at equidistant wavenumber k values. The ratio of the tidal radius of the cluster to the wavenumber difference ? k of adjacent local maxima in the oscillation spectra of absolute values of velocity field components is found to be the same for all five OCL models. This ratio is used to estimate the tidal radii and masses of the Pleiades, Praesepe, and M67 based on the proper motions and sky-plane coordinates of the member stars of these clusters. The radial dependences of the absolute values of the tangential and radial projections of cluster star velocities computed using the proper motions relative to the cluster center are determined, along with the corresponding autocorrelation functions and wavenumber spectra of oscillations of absolute values of velocity field components. The Pleiades virial mass is estimated assuming that the cluster is either isolated or non-isolated. Also derived are the estimates of the Pleiades dynamical mass assuming that it is non-stationary and non-isolated. The inferred Pleiades tidal radii corresponding to these masses are reported.
Lee, Chi-Cheng; Chen, Xiaoqian M; Gan, Yu; Yeh, Chen-Lin; Hsueh, H C; Abbamonte, Peter; Ku, Wei
2013-10-11
We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures. PMID:24160627
Min, Kyung-Gu; Jho, Young-Dahl; Yee, Ki-Ju; Stanton, C. J.; Song, Jin-Dong
2013-12-04
We have studied the characteristics of longitudinal-optical-phonon--plasmon coupled (LOPC) mode as a function of thickness in InAs epilayers, ranging from 10 to 900 nm. The absence of LOPC modes in a scale less than exciton Bohr radius manifests the role of electron diffusion rather than the carrier screening via drift motion in surface depletion region.
Synthesis and Characterization of Quantum Dots: A Case Study Using PbS
ERIC Educational Resources Information Center
Pan, Yi; Li, Yue Ru; Zhao, Yu; Akins, Daniel L.
2015-01-01
A research project for senior undergraduates of chemistry has been developed to introduce syntheses of a series of monodispersed semiconductor PbS quantum dots (QDs) and their characterization methodologies. In this paper, we report the preparation of monodispersed semiconductor PbS QDs with sizes smaller than the exciton Bohr radius using a…
Synthesis and Characterization of Quantum Dots: A Case Study Using PbS
ERIC Educational Resources Information Center
Pan, Yi; Li, Yue Ru; Zhao, Yu; Akins, Daniel L.
2015-01-01
A research project for senior undergraduates of chemistry has been developed to introduce syntheses of a series of monodispersed semiconductor PbS quantum dots (QDs) and their characterization methodologies. In this paper, we report the preparation of monodispersed semiconductor PbS QDs with sizes smaller than the exciton Bohr radius using aâ€¦
Excitonic Josephson effect in double-layer graphene junctions
NASA Astrophysics Data System (ADS)
Zenker, B.; Fehske, H.; Beck, H.
2015-08-01
We show that double-layer graphene (DLG), where an external potential induces a charge imbalance between n - and p -type layers, is a promising candidate to realize an exciton condensate in equilibrium. To prove this phenomenon experimentally, we suggest coupling two DLG systems, separated by a thin insulating barrier, and measuring the excitonic Josephson effect. For this purpose we calculate the ac and dc Josephson currents induced by tunneling excitons and show that the former only occurs when the gate potentials of the DLG systems differ, irrespective of the phase relationship of their excitonic order parameters. A dc Josephson current develops if a finite order-parameter phase difference exists between two coupled DLG systems with identical gate potentials.
Electrical Tuning of Exciton Binding Energies in Monolayer WS2
NASA Astrophysics Data System (ADS)
Chernikov, Alexey; van der Zande, Arend M.; Hill, Heather M.; Rigosi, Albert F.; Velauthapillai, Ajanth; Hone, James; Heinz, Tony F.
2015-09-01
We demonstrate continuous tuning of the exciton binding energy in monolayer WS2 by means of an externally applied voltage in a field-effect transistor device. Using optical spectroscopy, we monitor the ground and excited excitonic states as a function of gate voltage and track the evolution of the quasiparticle band gap. The observed decrease of the exciton binding energy over the range of about 100 meV, accompanied by the renormalization of the quasiparticle band gap, is associated with screening of the Coulomb interaction by the electrically injected free charge carriers at densities up to 8 ×1012 cm-2 . Complete ionization of the excitons due to the electrical doping is estimated to occur at a carrier density of several 1013 cm-2 .
Excitonic condensation in spatially separated one-dimensional systems
Abergel, D. S. L.
2015-05-25
We show theoretically that excitons can form from spatially separated one-dimensional ground state populations of electrons and holes, and that the resulting excitons can form a quasicondensate. We describe a mean-field Bardeen-Cooper-Schrieffer theory in the low carrier density regime and then focus on the core-shell nanowire giving estimates of the size of the excitonic gap for InAs/GaSb wires and as a function of all the experimentally relevant parameters. We find that optimal conditions for pairing include small overlap of the electron and hole bands, large effective mass of the carriers, and low dielectric constant of the surrounding media. Therefore, one-dimensional systems provide an attractive platform for the experimental detection of excitonic quasicondensation in zero magnetic field.
How bilayer excitons can greatly enhance thermoelectric efficiency
NASA Astrophysics Data System (ADS)
Wu, Kai; Rademaker, Louk; Zaanen, Jan
2015-03-01
Presently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer exciton thermocouple. Based on current experimental parameters, a bilayer exciton heterostructures of p- and n-doped Bi2Te3 can enhance the figure of merit an order of magnitude compared to bulk Bi2Te3. Another material suggestion is to make a bilayer out of electron-doped SrTiO3 and hole-doped Ca3Co4O9.
Effect of correlation of local fluctuations on exciton coherence
NASA Astrophysics Data System (ADS)
Chen, Xin; Silbey, Robert J.
2010-05-01
Recent experimental studies have shown both oscillations of exciton populations and long lasting coherence in multichromophoric systems such as photosynthetic light harvesting systems and conjugated polymers. It has been suggested that this quantum effect is due to correlations of the fluctuations of site energies among the closely packed chromophores in the protein environment. In addition to these, there is the strong possibility of correlations between site energies and transfer matrix elements. In order to understand the role of such correlations we generalize the Haken-Strobl-Reineker (HSR) model to include the energetic correlations and the site diagonal-off-diagonal correlations in a systematic way. The extended HSR model in the exciton basis is also constructed and allows us to study the dynamics of the exciton populations and coherences. With the extended model, we can provide insight into how these correlations affect the evolution of the populations and coherences of excitons by comparing to the original HSR model with uncorrelated fluctuating environments.
Effect of correlation of local fluctuations on exciton coherence
Chen, Xin; Silbey, Robert J.
2010-01-01
Recent experimental studies have shown both oscillations of exciton populations and long lasting coherence in multichromophoric systems such as photosynthetic light harvesting systems and conjugated polymers. It has been suggested that this quantum effect is due to correlations of the fluctuations of site energies among the closely packed chromophores in the protein environment. In addition to these, there is the strong possibility of correlations between site energies and transfer matrix elements. In order to understand the role of such correlations we generalize the Hakenâ€“Stroblâ€“Reineker (HSR) model to include the energetic correlations and the site diagonal-off-diagonal correlations in a systematic way. The extended HSR model in the exciton basis is also constructed and allows us to study the dynamics of the exciton populations and coherences. With the extended model, we can provide insight into how these correlations affect the evolution of the populations and coherences of excitons by comparing to the original HSR model with uncorrelated fluctuating environments.
Electrical Tuning of Exciton Binding Energies in Monolayer WS_{2}.
Chernikov, Alexey; van der Zande, Arend M; Hill, Heather M; Rigosi, Albert F; Velauthapillai, Ajanth; Hone, James; Heinz, Tony F
2015-09-18
We demonstrate continuous tuning of the exciton binding energy in monolayer WS_{2} by means of an externally applied voltage in a field-effect transistor device. Using optical spectroscopy, we monitor the ground and excited excitonic states as a function of gate voltage and track the evolution of the quasiparticle band gap. The observed decrease of the exciton binding energy over the range of about 100 meV, accompanied by the renormalization of the quasiparticle band gap, is associated with screening of the Coulomb interaction by the electrically injected free charge carriers at densities up to 8×10^{12} cm^{-2}. Complete ionization of the excitons due to the electrical doping is estimated to occur at a carrier density of several 10^{13} cm^{-2}. PMID:26431003
Topological Polaritons and Excitons in Garden Variety Systems
NASA Astrophysics Data System (ADS)
Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Tim
2015-03-01
Topological polaritons (aka topolaritons) present a new frontier for topological behavior in solid-state systems. They combine light and matter, which allows to probe and manipulate them in a variety of ways. They can also be made strongly interacting, due to their excitonic component. Here we present a scheme which allows to realize topolaritons in garden variety zinc-blende quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as a particularly appealing alternative for topological states in exciton-based systems. Indirect excitons are robust and long lived (with lifetimes up to milliseconds), and, therefore, provide a flexible platform for the realization, probing, and utilization of topological coupled light-matter states. Funded by: Institute for Quantum Information and Matter, Swiss National Science Foundation, Packard Foundation, NSF.
Excitonic condensation in systems of strongly correlated electrons
NASA Astrophysics Data System (ADS)
KuneÅ¡, Jan
2015-08-01
The idea of exciton condensation in solids was introduced in the 1960s with the analogy of superconductivity in mind. While exciton supercurrents have been realised only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g. cold atoms physics. One of our goals is to provide a â€˜dictionaryâ€™ that would allow the reader to efficiently combine results obtained in these different fields.
Excitons in ultrathin organic-inorganic perovskite crystals
NASA Astrophysics Data System (ADS)
Yaffe, Omer; Chernikov, Alexey; Norman, Zachariah M.; Zhong, Yu; Velauthapillai, Ajanthkrishna; van der Zande, Arend; Owen, Jonathan S.; Heinz, Tony F.
2015-07-01
We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals.
Singlet exciton fission-sensitized infrared quantum dot solar cells.
Ehrler, Bruno; Wilson, Mark W B; Rao, Akshay; Friend, Richard H; Greenham, Neil C
2012-02-01
We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells. PMID:22257168
Exciton separation at the interface between pentacene and dipolar chromophore
NASA Astrophysics Data System (ADS)
Park, Byoungnam; Park, Jonghoo
2013-11-01
We demonstrate photo-induced charge transfer between pentacene and dipolar chromophore, disperse red 19 (DR19), which can be applied in fabricating optoelectronic devices with abundant flexibility due to the functionality of the dipolar chromophore at the interface. Photo-induced charge transfer phenomenon between pentacene and DR19 is explored through pentacene thickness-dependent threshold voltage measurements using pentacene/DR19 bilayer field effect transistors under illumination. Threshold voltage increases as pentacene thickness increases up to a certain thickness followed by a decrease, resulting in a peak threshold voltage. The presence of the peak explains competition between optical absorption in the pentacene layer and exciton diffusion followed by exciton separation at the pentacene/DR19 interface. The exciton diffusion length in pentacene is estimated using an exciton diffusion-dissociation model.
An iterative algorithm for calculating stylus radius unambiguously
NASA Astrophysics Data System (ADS)
Vorburger, T. V.; Zheng, A.; Renegar, T. B.; Song, J.-F.; Ma, L.
2011-08-01
The stylus radius is an important specification for stylus instruments and is commonly provided by instrument manufacturers. However, it is difficult to measure the stylus radius unambiguously. Accurate profiles of the stylus tip may be obtained by profiling over an object sharper than itself, such as a razor blade. However, the stylus profile thus obtained is a partial arc, and unless the shape of the stylus tip is a perfect sphere or circle, the effective value of the radius depends on the length of the tip profile over which the radius is determined. We have developed an iterative, least squares algorithm aimed to determine the effective least squares stylus radius unambiguously. So far, the algorithm converges to reasonable results for the least squares stylus radius. We suggest that the algorithm be considered for adoption in documentary standards describing the properties of stylus instruments.
Mechanism for radiative recombination in ZnCdO alloys
Buyanova, I. A.; Bergman, J. P.; Pozina, G.; Chen, W. M.; Rawal, S.; Norton, D. P.; Pearton, S. J.; Osinsky, A.; Dong, J. W.
2007-06-25
Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content.
Nonlinear optical response and exciton dephasing in quantum dots
NASA Astrophysics Data System (ADS)
Muljarov, E. A.; Zimmermann, R.
2006-08-01
The full time-dependent four-wave mixing polarization in quantum dots is microscopically calculated, taking into account acoustic phonon-assisted transitions between different exciton states of the dot. It is shown that quite different dephasing times of higher exciton states in pancake anisotropic InGaAs quantum dots are responsible for the experimentally observed [1] double-exponential decay in the photon echo signal.
Excitonic coupling in van der waals complexes: The anisole dimers
NASA Astrophysics Data System (ADS)
Pasquini, Massimiliano; Pietraperzia, Giangaetano; Piani, Giovanni; Becucci, Maurizio
2011-05-01
We report on the determination of the excitonic coupling occurring between the two anisole molecular units bound to form the anisole dimer in the gas phase. The experimental data are summarized and then analyzed in terms of time-independent perturbation theory. The effective perturbation term for the molecular hamiltonian giving rise to the excitonic coupling is determined. The relative relevance of different coupling terms in the hamiltonian of the system is discussed as well.
Observation of two-dimensional exciton-phonon quasibound states
NASA Astrophysics Data System (ADS)
Pelekanos, N. T.; Haas, H.; Magnea, N.; Belitsky, V. I.; Cantarero, A.
1997-10-01
We demonstrate the existence of robust exciton-phonon quasibound states (EPQBS) in a two-dimensional semiconductor system, resulting from the binding of the e1h1 and e1h2 heavy-hole quantum-well excitons with an LO phonon. We show that increasing quantum confinement drastically weakens these two-dimensional EPQBS. A theoretical model including phonon confinement accounts qualitatively for our results.
Quantum confinement of excitons in wurtzite InP nanowires
Pemasiri, K.; Jackson, H. E.; Smith, L. M.; Wong, B. M.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.
2015-05-21
Exciton resonances are observed in photocurrent spectra of 80â€‰nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30â€‰nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands.
Kekule-distortion-induced exciton instability in graphene
Dillenschneider, Raoul
2008-09-15
Effects of a Kekule distortion on exciton instability in single-layer graphene are discussed. In the framework of quantum electrodynamics the mass of the electron generated dynamically is worked out using a Schwinger-Dyson equation. For homogeneous lattice distortion it is shown that the generated mass is independent of the amplitude of the lattice distortion at the one-loop approximation. Formation of excitons induced by the homogeneous Kekule distortion could appear only through direct dependence of the lattice distortion.
Anterior corneal asphericity calculated by the tangential radius of curvature
NASA Astrophysics Data System (ADS)
Ying, Jinglu; Wang, Bo; Shi, Mingguang
2012-07-01
We propose a method of calculating the corneal asphericity (Q) and analyze the characteristics of the anterior corneal shape using the tangential radius. Fifty-eight right eyes of 58 subjects were evaluated using the Orbscan II corneal topographer. The Q-values of the flat principal semi-meridians calculated by the sagittal radius were compared to those by the tangential radius. Variation in the Q-value with semi-meridian in the nasal and temporal cornea calculated by the tangential radius was analyzed. There were significant differences in Q-values (P<0.001) between the two methods. The mean Q-values of the flat principal semi-meridians calculated by tangential radius with -0.33±0.10 in the nasal and -0.22±0.12 in the temporal showed more negative than the corresponding Q-values calculated by the sagittal radius. The Q-values calculated by tangential radius became less negative gradually from horizontal semi-meridians to oblique semi-meridians in both nasal and temporal cornea. Variation in Q-value with semi-meridian was more obvious in the nasal cornea. The method of calculating corneal Q using the tangential radius could provide more reasonable and complete Q-value than that by the sagittal radius. The model of a whole anterior corneal surface could be reconstructed on the basis of the above method.
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.
Excitonic effects in the luminescence of quantum wells
NASA Astrophysics Data System (ADS)
Deveaud, B.; Kappei, L.; Berney, J.; Morier-Genoud, F.; Portella-Oberli, M. T.; Szczytko, J.; Piermarocchi, C.
2005-11-01
We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high-quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum wells: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We are also able to give the boundaries of the Mott transition in our system, and to show the absence of observable renormalization of the gap below the onset of this transition. We detail the characteristics of the trion formation and evidence the possible formation of both positive and negative trions in the absence of any resident free carrier populations.
Femtosecond THz Studies of Intra-Excitonic Transitions
Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.
2007-10-02
Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.
Probing the origin of excitonic states in monolayer WSe2
Huang, Jiani; Hoang, Thang B.; Mikkelsen, Maiken H.
2016-01-01
Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60â€‰K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65â€‰K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198â€‰meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications. PMID:26940069
Exciton Dynamics in Hexagonal InP Nanowires
NASA Astrophysics Data System (ADS)
Kaveh-Baghbadorani, Masoud; Langbein, Wolfgang; Gao, Qiang; Jagadish, Chennupati; Wagner, Hans-Peter
2013-03-01
We study the exciton dynamics in InP nanowire ensembles by intensity- and temperature-dependent photoluminescence (PL) measurements, time-correlated-single-photon-counting (TCSPC) and heterodyne detected four-wave-mixing experiments (HFWM). The InP nanowires were grown on fused silica substrate by 50 nm gold catalyst metal-organic-vapor-phase-epitaxy at a temperature of 450 °C resulting in nearly wurtzite type nanowires. The PL measurements at 15 K show a strong emission band at 837 nm and two weak side bands at nearly 820 and 860 nm. The bands are tentatively attributed to trapped, free and zinc-blende related exciton transitions, respectively. With increasing temperature the free-exciton band gains importance relative to the dominating trapped exciton band while the low energy band vanishes. TCSPC measurements show an increasing PL decay rate of all emission bands with increasing temperature most pronounced for the low energy band. The result agrees with the exciton population dynamics obtained from three-beam HFWM measurements. Photon echo experiments at 80 K reveal an ultrafast exciton dephasing time of less than 100 fs which is attributed to scattering with a high carrier background in these nanowires. The support of the Australian Research Council is kindly acknowledged.
Singlet fission in pentacene through multi-exciton quantum states.
Zimmerman, Paul M; Zhang, Zhiyong; Musgrave, Charles B
2010-08-01
Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here. PMID:20651727
Molecular packing determines singlet exciton fission in organic semiconductors.
Kolata, Kolja; Breuer, Tobias; Witte, Gregor; Chatterjee, Sangam
2014-07-22
Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockleyâ€“Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission. PMID:24957197
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 abÂ 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. PMID:26919006
Exciton energy-momentum map of hexagonal boron nitride
NASA Astrophysics Data System (ADS)
Fugallo, Giorgia; Aramini, Matteo; Koskelo, Jaakko; Watanabe, Kenji; Taniguchi, Takashi; Hakala, Mikko; Huotari, Simo; Gatti, Matteo; Sottile, Francesco
2015-10-01
Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe-Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum q , as previously reported, but also at large q . We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials.
Exciton-dominant electroluminescence from a diode of monolayer MoS{sub 2}
Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Wang, Yuan; Zhao, Mervin; Yin, Xiaobo; Zhang, Xiang
2014-05-12
In two-dimensional monolayer MoS{sub 2}, excitons dominate the absorption and emission properties. However, the low electroluminescent efficiency and signal-to-noise ratio limit our understanding of the excitonic behavior of electroluminescence. Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS{sub 2} fabricated on a heavily p-type doped silicon substrate. Direct and bound-exciton related recombination processes are identified from the electroluminescence. At a high electron-hole pair injection rate, Auger recombination of the exciton-exciton annihilation of the bound exciton emission is observed at room temperature. Moreover, the efficient electrical injection demonstrated here allows for the observation of a higher energy exciton peak of 2.255â€‰eV in the monolayer MoS{sub 2} diode, attributed to the excited exciton state of a direct-exciton transition.
Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai
2015-10-01
Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a FÃ¶rster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion. PMID:26391077
Exciton scattering mechanism in a single semiconducting MgZnO nanorod.
Yoo, Jinkyoung; Dang, Le Si; Chon, Bonghwan; Joo, Taiha; Yi, Gyu-Chul
2012-02-01
Excitonic phenomena, such as excitonic absorption and emission, have been used in many photonic and optoelectronic semiconductor device applications. As the sizes of these nanoscale materials have approached to exciton diffusion lengths in semiconductors, a fundamental understanding of exciton transport in semiconductors has become imperative. We present exciton transport in a single MgZnO nanorod in the spatiotemporal regime with several nanometer-scale spatial resolution and several tens of picosecond temporal resolution. This study was performed using temperature-dependent cathodoluminescence and time-resolved photoluminescence spectroscopies. The exciton diffusion length in the MgZnO nanorod decreased from 100 to 70 nm with increasing temperature in the range of 5 and 80 K. The results obtained for the temperature dependence of exciton diffusion length and luminescence lifetime revealed that the dominant exciton scattering mechanism in MgZnO nanorod is exciton-phonon assisted piezoelectric field scattering. PMID:22214177
NASA Astrophysics Data System (ADS)
Das, Subhasis; Sharma, Akant Sagar; Das, T. D.; Dhar, S.
2015-10-01
The ground state binding energy of heavy hole excitons confined in GaAs1-xBix/GaAs spherical quantum dots is calculated as a function of dot radius and the Bi content using a Variational method based on 1-s hydrogenic wave functions with effective mass approximation. The parameter shows strong dependence on the Bi mole fraction x, particularly at smaller values of the dot radii. The strain associated with the quantum dot is found to decrease exponentially with increase in dot radius and shows a linear increase with Bi composition.
Midgett, Aaron G; Luther, Joseph M; Stewart, John T; Smith, Danielle K; Padilha, Lazaro A; Klimov, Victor I; Nozik, Arthur J; Beard, Matthew C
2013-07-10
Using ultrafast transient absorption and time-resolved photoluminescence spectroscopies, we studied multiple exciton generation (MEG) in quantum dots (QDs) consisting of either PbSe, PbS, or a PbSxSe1-x alloy for various QD diameters with corresponding bandgaps (Eg) ranging from 0.6 to 1 eV. For each QD sample, we determine the MEG efficiency, ?MEG, defined in terms of the electron-hole pair creation energy (?eh) such that ?MEG = Eg/?eh. In previous reports, we found that ?MEG is about two times greater in PbSe QDs compared to bulk PbSe, however, little could be said about the QD-size dependence of MEG. In this study, we find for both PbS and PbSxSe1-x alloyed QDs that ?MEG decreases lineally with increasing QD diameter within the strong confinement regime. When the QD radius is normalized by a material-dependent characteristic radius, defined as the radius at which the electron-hole Coulomb and confinement energies are equivalent, PbSe, PbS, and PbSxSe1-x exhibit similar MEG behaviors. Our results suggest that MEG increases with quantum confinement, and we discuss the interplay between a size-dependent MEG rate versus hot exciton cooling. PMID:23750998
Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy
Huber, Rupert; Kaindl, Robert A.; Schmid, Ben A.; Chemla, Daniel S.
2005-06-25
We report the density-dependent crossover of a resonantly photoexcited exciton gas from insulating to conducting phases. Broadband terahertz spectroscopy gives direct access to the exciton binding energy via intra-excitonic 1s-2p transitions. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with decreasing inter-particle distance. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite free-carrier and exciton terahertz conductivity. Comparison with near-infrared absorption changes illustrates a significantly enhanced energy shift and broadening of the intra-excitonic resonance.
Studying the proton 'radius' puzzle with ?p elastic scattering
Gilman, R.
2013-11-07
The disagreement between the proton radius determined from muonic hydrogen and from electronic measurements is called the proton radius puzzle. The resolution of the puzzle remains unclear and appears to require new experimental results. An experiment to measure muon-proton elastic scattering is presented here.
The Origin of the Ionic-Radius Ratio Rules
ERIC Educational Resources Information Center
Jensen, William B.
2010-01-01
In response to a reader query, this article traces the origins of the ionic-radius ratio rules and their incorrect attribution to Linus Pauling in the chemical literature and to Victor Goldschmidt in the geochemical literature. In actual fact, the ionic-radius ratio rules were first proposed within the context of the coordination chemistry…
21 CFR 886.1450 - Corneal radius measuring device.
Code of Federal Regulations, 2012 CFR
2012-04-01
... 21 Food and Drugs 8 2012-04-01 2012-04-01 false Corneal radius measuring device. 886.1450 Section 886.1450 Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES OPHTHALMIC DEVICES Diagnostic Devices Â§ 886.1450 Corneal radius measuring...
The Origin of the Ionic-Radius Ratio Rules
ERIC Educational Resources Information Center
Jensen, William B.
2010-01-01
In response to a reader query, this article traces the origins of the ionic-radius ratio rules and their incorrect attribution to Linus Pauling in the chemical literature and to Victor Goldschmidt in the geochemical literature. In actual fact, the ionic-radius ratio rules were first proposed within the context of the coordination chemistryâ€¦
Fragment-Specific Fixation for Fractures of the Distal Radius.
Geissler, William B; Clark, Sonja M
2016-03-01
This article summarizes the management of distal fractures utilizing Acumed fragment-specific family of plates. No single plate option can address every fracture pattern of the distal radius. These fragment-specific plates are usually adjuncts to allow the surgeon to expand the armamentarium in the management of complex volar and dorsal comminuted distal radius fracture patterns. PMID:26855832
Benchmarking Calculations of Excitonic Couplings between Bacteriochlorophylls.
Kenny, Elise P; Kassal, Ivan
2016-01-14
Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. Understanding these uncertainties can guard against striving for unrealistic precision; at the same time, detailed benchmarks can allow important qualitative questions-which do not depend on the precise values of the simulation parameters-to be addressed with greater confidence about the conclusions. PMID:26651217
Exciton condensation and perfect Coulomb drag.
Nandi, D; Finck, A D K; Eisenstein, J P; Pfeiffer, L N; West, K W
2012-08-23
Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, double quantum well systems can support exciton condensates, which consist of electrons in one well tightly bound to holes in the other. 'Perfect' drag is therefore expected; a steady transport current of electrons driven through one quantum well should be accompanied by an equal current of holes in the other. Here we demonstrate this effect, taking care to ensure that the electron-hole pairs dominate the transport and that tunnelling of charge between the quantum wells, which can readily compromise drag measurements, is negligible. We note that, from an electrical engineering perspective, perfect Coulomb drag is analogous to an electrical transformer that functions at zero frequency. PMID:22914164
NASA Astrophysics Data System (ADS)
Tanaka, I.; Nakayama, M.
2002-10-01
We have investigated high-excitation photoluminescence properties of CuI thin films with a thickness of 100 nm grown on a NaCl substrate by vacuum deposition. The energies of the heavy-hole and light-hole excitons in the CuI thin film are split at low temperatures by the thermal strain resulting from the difference in the thermal expansion coefficients of CuI and NaCl: the heavy-hole exciton is lower in energy than the light-hole exciton. Under resonant excitation of the heavy-hole exciton, a new photoluminescence band appears on the low-energy side of the heavy-hole exciton. The energy spacing between the new photoluminescence band and the heavy-hole exciton is almost equal to the splitting energy of the heavy-hole and light-hole excitons. The photoluminescence intensity exhibits an almost quadratic dependence on the excitation power. These results indicate that the new photoluminescence band originates from an inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state. We have confirmed the existence of optical gain in the energy region of the new photoluminescence band by using a variable-stripe-length method. This fact demonstrates that the inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state may lead to stimulated emission.
Singlet fission of hot excitons in Ï€-conjugated polymers.
Zhai, Yaxin; Sheng, Chuanxiang; Vardeny, Z Valy
2015-06-28
We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(Ï‰)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent Ï€-conjugated polymers. From the high energy step in the triplet EXPA(Ï‰) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at Eâ‰ˆ2ET (=2.8â€‰eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8â€‰eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2ET, which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(Ï‰) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=Eg (â‰ˆ2.25â€‰eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2Ag exciton, dominates the triplet photogeneration in PDA polymer as Eg>2ET. The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in Ï€-conjugated polymers is a much more general process than thought previously. PMID:25987576
Chaudhary, Indradeo Prasad; Ranjan, Ram Bilas; Kumar, Prashant
2015-01-01
Thrombocytopenia - absent radii (TAR) syndrome is an autosomal recessive genetic rare disorder with hypomegakaryocytic thrombocytopenia and bilateral absent radius that may have additional anomalies. This disorder is characterized by thrombocytopenia resulting in potentially severe bleeding episodes primarily during infancy. We report the case of a 7-day-old term appropriate for gestational age (AGA) male baby, product of non consanguineous marriage presented with bloody loose stool, right sided upper limb deformity and paleness of the body, was diagnosed as TAR syndrome with some atypical presentation. Such type of atypical presentation has not been previously reported in a case with TAR Syndrome.Patient was managed in our hospital with packed cell transfusion and two units platelets concentrates transfusion, Intra-venous antimicrobials, and other supportive treatment. He gradually improved and was discharged after seven days of hospital stay with advice to consult orthopedic surgeon for opinion regarding limb reconstruction. PMID:25954675
Intermolecular exciton-exciton annihilation in phospholipid vesicles doped with [Ru(bpy)2dppz]2+
NASA Astrophysics Data System (ADS)
De la Cadena, Alejandro; Pascher, TorbjÃ¶rn; Davydova, Dar'ya; Akimov, Denis; Herrmann, Felix; Presselt, Martin; WÃ¤chtler, Maria; Dietzek, Benjamin
2016-01-01
The ultrafast photophysics of [Ru(bpy)2dppz]2+ (dppz = dipyrido[3,2-a:2â€§,3â€§-c]-phenazine) embedded into the walls of phospholipid vesicles has been studied by femtosecond time-resolved pump-probe spectroscopy. While [Ru(bpy)2dppz]2+ has been studied intensively with respect to its intramolecular charge transfer processes, which are associated with the well known light-switch effect, this study focuses on intermolecular energy transfer processes taking place upon dense packing of the complexes into a phospholipid membrane composed of dipalmitoyl-L-Î±-phosphatidylglycerol, which can be thought of as a simplistic model of a cellular membrane. The data indicate additional quenching of excited [Ru(bpy)2dppz]2+ upon increasing the pump-pulse intensity. Hence, the observed photophysics, which is assigned to the presence of intermolecular exciton-exciton annihilation at high pump-intensities, might be related to the ultrafast photophysics of [Ru(bpy)2dppz]2+ when used as a chromophore to stain cells, an effect that may be taken into account during the employment of novel cellular markers based on Ru polypyridine complexes.
Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation.
Scholes, Gregory D
2015-12-24
Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet-triplet annihilation to upconvert excitation, have been extensively studied. Comparatively little work has sought to examine the properties of the intermediate state in both of these processes-the bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported. The lowest-energy states obtained from the branching diagram method are the triplet pairs with overall singlet spin |X1âŸ© â‰ˆ (1)[TT] and quintet spin |QâŸ© â‰ˆ (5)[TT]. It is shown that triplet pair states can be separated by a triplet-triplet energy-transfer mechanism to give a separated, yet entangled triplet pair (1)[TÂ·Â·Â·T]. Independent triplets are produced by decoherence of the separated triplet pair. Recombination of independent triplets by exciton-exciton annihilation to form the correlated triplet pair (i.e., nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the separated correlated triplet pair (geminate recombination). PMID:26595530
How to Draw Energy Level Diagrams in Excitonic Solar Cells.
Zhu, X-Y
2014-07-01
Emerging photovoltaic devices based on molecular and nanomaterials are mostly excitonic in nature. The initial absorption of a photon in these materials creates an exciton that can subsequently dissociate in each material or at their interfaces to give charge carriers. Any attempt at mechanistic understanding of excitonic solar cells must start with drawing energy level diagrams. This seemingly elementary exercise, which is described in textbooks for inorganic solar cells, has turned out to be a difficult subject in the literature. The problem stems from conceptual confusion of single-particle energy with quasi-particle energy and the misleading practice of mixing the two on the same energy level diagram. Here, I discuss how to draw physically accurate energy diagrams in excitonic solar cells using only single-particle energies (ionization potentials and electron affinities) of both ground and optically excited states. I will briefly discuss current understanding on the electronic energy landscape responsible for efficient charge separation in excitonic solar cells. PMID:26279547
Quantum dots with light-hole exciton ground state
NASA Astrophysics Data System (ADS)
Witek, Barbara; Akopian, Nika; Huo, Yongheng; Kumar, Santosh; Cardenas, Ricardo; Bester, Gabriel; Rastelli, Armando; Schmidt, Oliver; Zwiller, Val
2013-03-01
A light-hole exciton is a quasiparticle formed from a single electron and a single light-hole (LH). This is a fundamental excitation in a semiconductor quantum dot (QD), which could potentially lead to new and simpler schemes in quantum information science and technology, However, it has not been explored so far because the ground state of a hole in a QD has dominant heavy-hole character. Here we develop a novel type of a QD system that allows us to engineer GaAs/ AlGaAs QDs with a light-hole (LH) ground state by embedding them in tensile strained membranes. We fully characterize LH exciton states in polarization resolved ?-photoluminesce in the external magnetic field. LH exciton manifests itself in three orthogonally-polarized bright transitions and a large fine-structure. Further, we determine LH g-factor and observe different diamagnetic coefficients for LH px,y and pz orbitals. Finally, we provide a comprehensive theoretical description of all the observed LH exciton properties: fine structure, polarization, oscillator strength and g-factors. Our work paves the way to explore the fundamental properties and potential relevance of LH-excitons in QD for quantum information technologies.
Density functional study of tetraphenylporphyrin long-range exciton coupling.
Moore, Barry; Autschbach, Jochen
2012-08-01
The performance of time-dependent density functional theory (TDDFT) for calculations of long-range exciton circular dichroism (CD) is investigated. Tetraphenylporphyrin (TPP) is used as a representative of a class of strongly absorbing chromophores for which exciton CD with chromophore separations of 50 Å and even beyond has been observed experimentally. A dimer model for TPP is set up to reproduce long-range exciton CD previously observed for a brevetoxin derivative. The calculated CD intensity is consistent with TPP separations of over 40 Å. It is found that a hybrid functional with fully long-range corrected range-separated exchange performs best for full TDDFT calculations of the dimer. The range-separation parameter is optimally tuned for TPP, resulting in a good quality TPP absorption spectrum and small DFT delocalization error (measured by the curvature of the energy calculated as a function of fractional electron numbers). Calculated TDDFT data for the absorption spectra of TPP are also used as input for a 'matrix method' (MM) model of the exciton CD. For long-range exciton CD, comparison of MM spectra with full TDDFT CD spectra for the dimer shows that the matrix method is capable of producing very accurate results. A MM spectrum obtained from TPP absorption data calculated with the nonhybrid Becke88-Perdew86 (BP) functional is shown to match the experimental brevetoxin spectrum 'best', but for the wrong reasons. PMID:24551508
Density Functional Study of Tetraphenylporphyrin Long-Range Exciton Coupling
Moore, Barry; Autschbach, Jochen
2012-01-01
The performance of time-dependent density functional theory (TDDFT) for calculations of long-range exciton circular dichroism (CD) is investigated. Tetraphenylporphyrin (TPP) is used as a representative of a class of strongly absorbing chromophores for which exciton CD with chromophore separations of 50 Å and even beyond has been observed experimentally. A dimer model for TPP is set up to reproduce long-range exciton CD previously observed for a brevetoxin derivative. The calculated CD intensity is consistent with TPP separations of over 40 Å. It is found that a hybrid functional with fully long-range corrected range-separated exchange performs best for full TDDFT calculations of the dimer. The range-separation parameter is optimally tuned for TPP, resulting in a good quality TPP absorption spectrum and small DFT delocalization error (measured by the curvature of the energy calculated as a function of fractional electron numbers). Calculated TDDFT data for the absorption spectra of TPP are also used as input for a ‘matrix method’ (MM) model of the exciton CD. For long-range exciton CD, comparison of MM spectra with full TDDFT CD spectra for the dimer shows that the matrix method is capable of producing very accurate results. A MM spectrum obtained from TPP absorption data calculated with the nonhybrid Becke88–Perdew86 (BP) functional is shown to match the experimental brevetoxin spectrum ‘best’, but for the wrong reasons. PMID:24551508
Nonmonotonic energy harvesting efficiency in biased exciton chains.
Vlaming, S M; Malyshev, V A; Knoester, J
2007-10-21
We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average time for the exciton to be trapped after the initial excitation. The exciton transport is treated as the intraband energy relaxation over the states obtained by numerically diagonalizing the Frenkel Hamiltonian that corresponds to the biased chain. The relevant intraband scattering rates are obtained from a linear exciton-phonon interaction. Numerical solution of the Pauli master equation that describes the relaxation and trapping processes reveals a complicated interplay of factors that determine the overall harvesting efficiency. Specifically, if the trapping step is slower than or comparable to the intraband relaxation, this efficiency shows a nonmonotonic dependence on the bias: it first increases when introducing a bias, reaches a maximum at an optimal bias value, and then decreases again because of dynamic (Bloch) localization of the exciton states. Effects of on-site (diagonal) disorder, leading to Anderson localization, are addressed as well. PMID:17949203
Dynamics of exciton formation and relaxation in photoexcited semiconductors
NASA Astrophysics Data System (ADS)
Jankovi?, Veljko; Vukmirovi?, Nenad
2015-12-01
We investigate the dynamics of the exciton formation and relaxation on a picosecond time scale following a pulsed photoexcitation of a semiconductor. The study is conducted in the framework of the density matrix theory complemented with the dynamics controlled truncation scheme. We truncate the phonon branch of the resulting hierarchy of equations and propose the form of coupling among single-phonon-assisted and higher-order phonon-assisted density matrices so as to ensure the energy and particle-number conservation in a closed system. Time scales relevant for the exciton formation and relaxation processes are determined from numerical investigations performed on a one-dimensional model for the values of model parameters representative of a typical organic and inorganic semiconductor. The exciton dynamics is examined for different values of central frequency of the exciting field, temperature, and microscopic model parameters, such as the strengths of carrier-carrier and carrier-phonon couplings. We find that for typical organic semiconductor parameters, formation of bound excitons occurs on a several-hundred-femtosecond time scale, while their subsequent relaxation and equilibration take at least several picoseconds. These time scales are consistent with recent experimental studies of the exciton formation and relaxation in conjugated polymer-based materials.
Incoherent exciton trapping in self-similar aperiodic lattices
Dominguez-Adame, F.; Macia, E. ); Sanchez, A. Escuela Politecnica Superior, Universidad Carlos III de Madrid, C./Butarque 15, E-28911 Leganes, Madrid )
1995-01-01
Incoherent exciton dynamics in one-dimensional perfect lattices with traps at sites arranged according to aperiodic deterministic sequences is studied. We focus our attention on Thue-Morse and Fibonacci systems as canonical examples of self-similar aperiodic systems. Solving numerically the corresponding master equation we evaluate the survival probability and the mean-square displacement of an exciton initially created at a single site. Results are compared to systems of the same size with the same concentration of traps randomly as well as periodically distributed over the whole lattice. Excitons progressively extend over the lattice on increasing time and, in this sense, they act as a probe of the particular arrangements of traps in each system considered. The analysis of the characteristic features of their time decay indicates that exciton dynamics in self-similar aperiodic arrangements of traps is quite close to that observed in periodic ones, but differs significantly from that corresponding to random lattices. We also report on characteristic features of exciton motion suggesting that Fibonacci and Thue-Morse orderings might be clearly observed by appropriate experimental measurements. In the conclusions we comment on the implications of our work on the way towards a unified theory of the ordering of matter.
Excitons in time-dependent density-functional theory
NASA Astrophysics Data System (ADS)
Ullrich, Carsten
2014-03-01
Excitons are the dominant feature in the optical spectra of insulators and semiconductors close to the absorption edge. They are collective excitations of the many-body system, but can often be discussed in a simplified picture as bound electron-hole pairs. To describe excitons in bulk materials with time-dependent density-functional theory (TDDFT), exchange-correlation functionals with a proper long-range behavior are required. The first part of this talk will present a TDDFT approach for directly calculating singlet and triplet exciton binding energies, which is based on an adaptation of the Casida formalism for periodic solids. Several exchange-correlation kernels have been tested for a variety of semiconductors and large-gap insulators. The second part of this talk will discuss a method to visualize exciton dynamics in large organic molecules in real time, based on the time-dependent transition density matrix. The method is applied to study the optical properties of intramolecular charge-transfer excitons in photoexcited molecular donor-acceptor systems that are of interest in organic photovoltaics. This work has been supported by NSF Grant DMR-1005651.
ON THE VARIATION OF SOLAR RADIUS IN ROTATION CYCLES
Qu, Z. N.; Kong, D. F.; Xiang, N. B.; Feng, W.
2015-01-10
The Date Compensated Discrete Fourier Transform and CLEANest algorithm are used to study the temporal variations of the solar radius observed at Rio de Janeiro Observatory from 1998 March 2 to 2009 November 6. The CLEANest spectra show several significant periodicities around 400, 312, 93.5, 86.2, 79.4, 70.9, 53.2, and 26.3 days. Then, combining the data on the daily solar radius measured at Calern Observatory and Rio de Janeiro Observatory and the corresponding daily sunspot areas, we study the short-term periodicity of the solar radius and the role of magnetic field in the variation of the solar radius. The rotation period of the daily solar radius is determined to be statistically significant. Moreover, its temporal evolution is anti-phase with that of sunspot activity, and it is found anti-phase with solar activity. Generally, the stronger solar activity is, the more obvious is the anti-phase relation of radius with solar activity. This indicates that strong magnetic fields have a greater inhibitive effect than weak magnetic fields on the variation of the radius.
Schweitzer-Stenner, Reinhard; Bosenbeck, Michael; Dreybrodt, Wolfgang
1993-01-01
The depolarization ratios of heme protein Raman lines arising from vibrations of the heme group exhibit significant dependence on the excitation wavelength. From the analysis of this depolarization ratio dispersion, one obtains information about symmetry-lowering distortions ?Q? of the heme group that can be classified in terms of the symmetry races ? = A1g, B1g, B2g, and A2g in D4h symmetry. The heme-protein interaction can be changed by the protonation of distinct amino acid side chains (i.e., for instance the Bohr groups in hemoglobin derivates), which gives rise to specific static heme distortions for each protonation state. From the Raman dispersion data, it is possible to obtain parameters by fitting to a theoretical expression of the Raman tensor, which provide information on these static distortions and also about the pK values of the involved titrable side chains. We have applied this method to the ?4 (1,355 cm-1) and ?10 (1,620 cm-1) lines of deoxygenated hemoglobin of the fourth component of trout and have measured their depolarization ratio dispersion as a function of pH between 6 and 9. From the pH dependence of the thus derived parameters, we obtain pK values identical to those of the Bohr groups, which were earlier derived from the corresponding O2-binding isotherms. These are pK?1 = pK?2 = 8.5 for the ? and pK?1 = 7.5, pK?2 = 7.4 for the ? chains. We also obtain the specific distortion parameters for each protonation state. As shown in earlier studies, the ?4 mode mainly probes distortions from interactions between the proximal histidine and atoms of the heme core (i.e., the nitrogens and the C? atoms of the pyrroles). Group theoretical argumentation allows us to relate specific changes of the imidazole geometry as determined by its tilt and azimuthal angle and the iron-out-of-plane displacement to distinct variations of the normal distortions ?Q? derived from the Raman dispersion data. Thus, we found that the pH dependence of the heme distortions ?QA1g (totally symmetric) and ?QB1g (asymmetric) is caused by variations of the azimuthal rather than the tilt angle of the Fe-His (F8) bond. In contrast to this, the ?10 line mainly monitors changes resulting from the interaction between peripheral substituents of the porphyrin macrocycle (vinyl). From the pH dependence of the parameters, it is possible to separately identify distortions ?Q? affecting the hemes in the ? and ? chains, respectively. From this, we find that in the ? subunit structural changes induced on protonation of the corresponding Bohr groups are mainly transferred via the Fe—N? bond and give rise to changes in the azimuthal angle. In the ? subunit, however, in addition, structural changes of the heme pocket arise, which most probably result from protonation of the imidazole of the COOH-terminal His (HC3 ?). This rearranges the net of H bonds between His HC3 ?, Ser (F9 ?), and Glu (F7 ?). PMID:19431886
An Asian Perspective on the Management of Distal Radius Fractures
Sebastin, Sandeep J.; Chung, Kevin C.
2012-01-01
Synopsis There is little data with regards to the epidemiology, pathology, or management of distal radius fractures from centers in Asia. Asia includes five advanced economies, namely Hong Kong SAR, Japan, Korea, Singapore, and Taiwan and a number of emerging economies prominent among which are China, India, Malaysia, Philippines, and Thailand. This article examines the available epidemiological data from Asia, and compares the management of distal radius fractures in the advanced and emerging Asian economies and how they match up to the current management in the west. It concludes by offering solutions for improving outcomes of distal radius fractures in both the advanced and emerging economies of Asia. PMID:22554658
An Asian perspective on the management of distal radius fractures.
Sebastin, Sandeep J; Chung, Kevin C
2012-05-01
There is limited data regarding the epidemiology, pathology, and management of distal radius fractures from centers in Asia. The advanced economies in Asia include Hong Kong, Japan, Korea, Singapore, and Taiwan, whereas the prominent emerging economies are China, India, Malaysia, Philippines, and Thailand. This article examines the available epidemiological data from Asia, compares the management of distal radius fractures in the advanced and emerging Asian economies and how they compare with the current management in the west. It concludes by offering solutions for improving outcomes of distal radius fractures in Asia. PMID:22554658
Finite Larmor radius modification of the Mercier criterion
Connor, J.W.; Tang, W.M.; Allen, L.
1984-04-01
The finite Larmor radius modification of the Suydam criterion involves a competition between stabilizing finite Larmor radius effects and destabilizing curvature. In the case of the toroidal calculation, corresponding to the Mercier criterion, ballooning effects from regions of unfavorable curvature must be taken into account. In the case of a model equilibrium, valid near the magnetic axis, a complete solution is obtained. Results indicate that the amount of finite Larmor radius stabilization needed to overcome the effects of unfavorable average curvature increases as a function of the toroidal ballooning parameter.
Parity violation, the neutron radius of lead, and neutron stars
NASA Astrophysics Data System (ADS)
Piekarewicz, J.
2007-06-01
The neutron radius of a heavy nucleus is a fundamental nuclear-structure observable that remains elusive. Progress in this arena has been limited by the exclusive use of hadronic probes that are hindered by large and controversial uncertainties in the reaction mechanism. The parity radius experiment at the Jefferson Laboratory offers an attractive electro-weak alternative to the hadronic program and promises to measure the neutron radius of 208Pb accurately and model independently via parity-violating electron scattering. In this contribution we examine the far-reaching implications that such a determination will have in areas as diverse as nuclear structure, atomic parity violation, and astrophysics.
Percolation and cluster structure parameters: The radius of gyration
NASA Astrophysics Data System (ADS)
Hoshen, Joseph
1997-12-01
The squared radius of gyration of percolation clusters are determined in terms of the clusters' second spatial moments using the enhanced Hoshen - Kopelman algorithm for square and triangular lattices containing 0305-4470/30/24/011/img1 sites. Correlation length exponents and related exponents as well as their corresponding amplitudes are calculated in terms of the squared radius of gyration above and below the percolation threshold. A coefficient for cluster compactness that is based on the squared radius of gyration of a cluster is introduced. That coefficient is compared with a similar coefficient of compactness that is based on the cluster cyclomatic number.
Primary nonunion of the distal radius fractures in healthy children.
Song, Kwang Soon; Lee, Si Wook; Bae, Ki Cheor; Yeon, Chang Jin; Naik, Premal
2016-03-01
There are no published case series of nonunion of distal radius fractures in healthy children because of the rarity of its occurrence. We searched for all reported cases of this condition in Pubmed, Google scholar, and SCOPUS. We found three series, which included one previously reported by our group. The aim of the present study was to define the predisposing factors leading to nonunion after treatment of distal radius fractures in healthy children. We also aimed to emphasize that nonunion should be included in the list of complications of distal radius fractures in children and be mentioned in the textbook of pediatric trauma. PMID:26583931
Novel Quantum Condensates in Excitonic Matter
Littlewood, P. B.; Keeling, J. M. J.; Simons, B. D.; Eastham, P. R.; Marchetti, F. M.; Szymanska, M. H.
2009-08-20
These lectures interleave discussion of a novel physical problem of a new kind of condensate with teaching of the fundamental theoretical tools of quantum condensed matter field theory. Polaritons and excitons are light mass composite bosons that can be made inside solids in a number of different ways. As bosonic particles, they are liable to make a phase coherent ground state - generically called a Bose-Einstein condensate (BEC) - and these lectures present some models to describe that problem, as well as general approaches to the theory. The focus is very much to explain how mean-field-like approximations that are often presented heuristically can be derived in a systematic fashion by path integral methods. Going beyond the mean field theory then produces a systematic approach to calculation of the excitation energies, and the derivation of effective low energy theories that can be generalised to more complex dynamical and spatial situations than is practicable for the full theory, as well as to study statistical properties beyond the semi-classical regime. in particular, for the polariton problem, it allows one to connect the regimes of equilibrium BEC and non-equilibrium laser. The lectures are self-sufficient, but not highly detailed. The methodological aspects are covered in standard quantum field theory texts and the presentation here is deliberately cursory: the approach will be closest to the book of Altland and Simons. Since these lectures concern a particular type of condensate, reference should also be made to texts on BEC, for example by Pitaevskii and Stringari. A recent theoretically focussed review of polariton systems covers many of the technical issues associated with the polariton problem in greater depth and provides many further references.
Exciton transport and exchange self-energy in semiconducting carbon nanotubes
NASA Astrophysics Data System (ADS)
Crochet, Jared; Sau, Jay; Cohen, Marvin; Duque, Juan; Cognet, Laurent; Doorn, Stephen
2012-02-01
We present direct measurements of S1 exciton transport in (6,5) carbon nanotubes. Exciton diffusion lengths associated with end quenching, photoluminescence lifetimes, and homogeneous emission linewidths provide a basis for determining an intrinsic diffusion constant of 5 cm^2s-1 within the dispersion of light. Exciton diffusion is modeled in terms of an anomalous dispersion within a marginal Fermi liquid description of the exciton exchange self-energy and acoustic phonon scattering.
Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics
NASA Astrophysics Data System (ADS)
Scully, Shawn Ryan
Organic photovoltaics have attracted significant interest over the last decade due to their promise as clean low-cost alternatives to large-scale electric power generation such as coal-fired power, natural gas, and nuclear power. Many believe power conversion efficiency targets of 10-15% must be reached before commercialization is possible. Consequently, understanding the loss mechanisms which currently limit efficiencies to 4-5% is crucial to identify paths to reach higher efficiencies. In this work, we investigate the dominant loss mechanisms in some of the leading organic photovoltaic architectures. In the first class of architectures, which include planar heterojunctions and bulk heterojunctions with large domains, efficiencies are primarily limited by the distance photogenerated excitations (excitons) can be transported (termed the exciton diffusion length) to a heterojunction where the excitons may dissociate. We will discuss how to properly measure the exciton diffusion length focusing on the effects of optical interference and of energy transfer when using fullerenes as quenching layers and show how this explains the variety of diffusion lengths reported for the same material. After understanding that disorder and defects limit exciton diffusion lengths, we suggest some approaches to overcome this. We then extensively investigate the use of long-range resonant energy transfer to increase exciton harvesting. Using simulations and experiments as support, we discuss how energy transfer can be engineered into architectures to increase the distance excitons can be harvested. In an experimental model system, DOW Red/PTPTB, we will show how the distance excitons are harvested can be increased by almost an order of magnitude up to 27 nm from a heterojunction and give design rules and extensions of this concept for future architectures. After understanding exciton harvesting limitations we will look at other losses that are present in planar heterojunctions. One of the primary losses that puts stringent requirements on the charge carrier mobilities in these cells is the recombination losses due to space charge build up at the heterojunction. Because electrons are confined to the acceptor and holes to the donor, net charge density always exists even when mobilities are matched, in contrast to bulk heterojunctions wherein matched mobilities lead to zero net charge. This net charge creates an electric field which opposes the built-in field and limits the current that can be carried away from this heterojunction. Using simulations we show that for relevant current densities charge carrier mobilities must be higher than 10-4 cm2/V.s to avoid significant losses due to space charge formation. In the last part of this work, we will focus on the second class of architectures in which exciton harvesting is efficient. We will present a systematic analysis of one of the leading polymer:fullerene bulk heterojunction cells to show that losses in this architecture are due to charge recombination. Using optical measurements and simulations, exciton harvesting measurements, and device characteristics we will show that the dominant loss is likely due to field-dependent geminate recombination of the electron and hole pair created immediately following exciton dissociation. No losses in this system are seen due to bimolecular recombination or space charge which provides information on charge-carrier mobility targets necessary for the future design of high efficiency organic photovoltaics.
Long-range exciton dissociation in organic solar cells.
Caruso, Domenico; Troisi, Alessandro
2012-08-21
It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the interface and generate partially separated electrons and holes, which can more easily overcome their coulombic attraction and form free charges. We first establish under what conditions long-range exciton dissociation is likely (using a kinetic model and a microscopic model for the calculation of the long-range electron transfer rate). Then, defining a rather general model Hamiltonian for the donor material, we show that the phenomenon is extremely common in the majority of polymer:fullerene bulk heterojunction solar cells. PMID:22869702
Effects of fermion exchange on the polarization of exciton condensates.
Combescot, Monique; Combescot, Roland; Alloing, Mathieu; Dubin, François
2015-03-01
Exchange interaction is responsible for the stability of elementary boson condensates with respect to momentum fragmentation. This remains true for composite bosons when single fermion exchanges are included but spin degrees of freedom are ignored. Here, we show that their inclusion can produce a spin fragmentation of the dark exciton condensate, i.e., an unpolarized condensate with an equal amount of spin (+2) and (-2) excitons not coupled to light. The composite boson many-body formalism allows us to predict that, for spatially indirect excitons, the condensate polarization switches from unpolarized to fully polarized when the distance between the layers confining electrons and holes increases. Importantly, the threshold distance for this switch lies in a regime fully accessible to experiments. PMID:25793784
Binding energies of excitons in ionic quantum well structures
NASA Astrophysics Data System (ADS)
Antonelli, A.; Cen, J.; Bajaj, K. K.
1996-01-01
We have calculated the binding energies of excitons in quantum well structures based on ionic semiconductors by including the electron - hole interactions with the longitudinal optical phonon field. We have taken into account these interactions by using different effective interaction potentials between the electron and the hole as derived by Haken, by Aldrich and Bajaj, and by Pollman and BÃ¼ttner. We have calculated the binding energies of excitons in several ionic quantum well structures as functions of well width using these effective potentials by following a variational approach. We find that the values of the exciton binding energies calculated using these potentials are always larger than those obtained using a Coulomb potential screened by a static dielectric constant. We compare our results with those of some recent calculations.
Microscopic theory of singlet exciton fission. I. General formulation.
Berkelbach, Timothy C; Hybertsen, Mark S; Reichman, David R
2013-03-21
Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach. PMID:23534622
Quantum confinement-induced tunable exciton states in graphene oxide
Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M.; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin
2013-01-01
Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology. PMID:23872608
Strain Effects on Excitonic Transitions in GaN
NASA Astrophysics Data System (ADS)
Shan, W.; Fischer, A. J.; Hauenstein, R. J.; Song, J. J.; Perry, W. G.; Bremser, M. D.; Davis, R. F.; Goldenberg, B.
1997-03-01
We present the results of experimental studies of strain effects on the excitonic transitions in wurtzite GaN epitaxial layers on sapphire and SiC substrates, with the emphasis on the determination of deformation potentials. Photoluminescence and reflectance spectroscopies were performed to measure the energy positions of exciton transitions, and high-precision X-ray diffraction measurements were conducted to examine the lattice parameters of GaN epitaxial layers grown on different substrates. Residual strain induced by the mismatch of lattice constants and thermal-expansion between GaN epitaxial layers and substrates was found to have a strong influence in determining the energies of excitonic transitions. The overall effects of the strain generated in GaN are compressive for GaN grown on sapphire and tensile for GaN on SiC substrate. The uniaxial and hydrostatic deformation potentials of wurtzite GaN were derived from the experimental results.
Charged two-exciton emission from a single semiconductor nanocrystal
Hu, Fengrui; Zhang, Qiang; Zhang, Chunfeng; Wang, Xiaoyong; Xiao, Min
2015-03-30
Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the â€œonâ€ and â€œoffâ€ levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate â€œgreyâ€ level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state.
Coulomb Mediated Hybridization of Excitons in Coupled Quantum Dots
NASA Astrophysics Data System (ADS)
Ardelt, P.-L.; Gawarecki, K.; MÃ¼ller, K.; Waeber, A. M.; Bechtold, A.; Oberhofer, K.; Daniels, J. M.; Klotz, F.; Bichler, M.; Kuhn, T.; Krenner, H. J.; Machnikowski, P.; Finley, J. J.
2016-02-01
We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantum dot molecules. By probing the optical response of an individual quantum dot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k .p calculations taking into account a realistic quantum dot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule.
Optical control of charged exciton states in tungsten disulfide
Currie, M.; Hanbicki, A. T.; Jonker, B. T.; Kioseoglou, G.
2015-05-18
A method is presented for optically preparing WS{sub 2} monolayers to luminescence from only the charged exciton (trion) state–completely suppressing the neutral exciton. When isolating the trion state, we observed changes in the Raman A{sub 1g} intensity and an enhanced feature on the low energy side of the E{sup 1}{sub 2g} peak. Photoluminescence and optical reflectivity measurements confirm the existence of the prepared trion state. This technique also prepares intermediate regimes with controlled luminescence amplitudes of the neutral and charged exciton. This effect is reversible by exposing the sample to air, indicating the change is mitigated by surface interactions with the ambient environment. This method provides a tool to modify optical emission energy and to isolate physical processes in this and other two-dimensional materials.
Microscopic theory of singlet exciton fission. I. General formulation
NASA Astrophysics Data System (ADS)
Berkelbach, Timothy C.; Hybertsen, Mark S.; Reichman, David R.
2013-03-01
Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized exciton bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach.
Excitonic positronium emission from n-Si(111)
NASA Astrophysics Data System (ADS)
Cassidy, D. B.; Hisakado, T. H.; Tom, H. W. K.; Mills, A. P., Jr.
2012-10-01
Recently it was found that energetic positronium (Ps) emission from both n- and p-type Si(100) occurs primarily via an exciton-like surface state PsX [D. B. Cassidy, T. H. Hisakado, H. W. K. Tom, and A. P. Mills, Jr., Phys. Rev. B1098-012110.1103/PhysRevB.84.195312 84, 195312 (2011)]. The underlying Ps production mechanism was deduced from the observed Ps emission yield and kinetic energy, using laser and thermally generated electrons to populate the PsX state, but not all of the observed effects have been explained. In this brief report we show that excitonic Ps emission also occurs from n-Si(111), but with characteristics that more closely resemble p-Si(100) than n-Si(100). Considering both (100) and (111) crystal orientations may help in the formulation of a more complete theoretical description of the excitonic Ps production mechanism.
Coulomb Mediated Hybridization of Excitons in Coupled Quantum Dots.
Ardelt, P-L; Gawarecki, K; Müller, K; Waeber, A M; Bechtold, A; Oberhofer, K; Daniels, J M; Klotz, F; Bichler, M; Kuhn, T; Krenner, H J; Machnikowski, P; Finley, J J
2016-02-19
We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantum dot molecules. By probing the optical response of an individual quantum dot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k·p calculations taking into account a realistic quantum dot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule. PMID:26943557
Surface plasmon mediated energy transfer of electrically-pumped excitons.
An, Kwang Hyup; Shtein, Max; Pipe, Kevin P
2010-03-01
We report strong surface plasmon polariton mediated transfer of energy between molecular excitons across the metallic cathode of an electrically-pumped organic heterostructure. The donor molecular excitons at the organic heterojunction resonantly excite surface plasmon modes on both sides of the optically thick metal electrode, which evanescently couple to dye molecules near the electrode's exterior surface. Dye fluorescence in the capping layer on the exterior of the device shows a 6.5-fold increase in intensity due to this effect, far exceeding any enhancement attributable to Purcell or optical microcavity effects. Demonstration of this energy transfer mechanism for electrically-pumped excitons suggests new sensing and imaging applications with high signal to noise ratio and new routes for performance improvement in energy harvesting devices, plasmonic devices, and organic LEDs (including white light emission). PMID:20389418
Split radius-form blocks for tube benders
NASA Technical Reports Server (NTRS)
Lange, D. R.; Seiple, C. W.
1970-01-01
Two-piece, radius-form block permits accurate forming and removing of parts with more than a 180 degree bend. Tube bender can shape flexible metal tubing in applications dealing with plumbing, heating, and pressure transmission lines.
Excitonic splittings in jet-cooled molecular dimers.
Ottiger, Philipp; Leutwyler, Samuel
2011-01-01
In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S0-->S1 electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S0-S1 and S0-->S2 transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as 12C/13C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless O0(0) bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)2 and (2-aminopyridine)2 that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-12C- and 13C- isotopomer spectra yield excitonic splittings of delta(exp) = 43.5 and 10.5 cm(-1), respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced ('quenched') by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced. PMID:21678767
Estimating the cutting force when skiving with a radius cutter
NASA Astrophysics Data System (ADS)
Filippov, A. V.; Filippova, E. O.
2015-09-01
The paper considers the method of determining the components of the cutting force under cutting completed with a radius cutter. The authors provide the design of the experimental study. The authors provide the data on the cutting force change in the process of turning with a radius cutter according to the cutting edge inclination, cutting depth and supply obtained experimentally and calculated analytically. The paper also provides the results of experimental work approximation and theoretical error checking related to experimental data.
Phonon-Induced Exciton Dephasing in Quantum Dot Molecules
NASA Astrophysics Data System (ADS)
Muljarov, E. A.; Takagahara, T.; Zimmermann, R.
2005-10-01
A new microscopic approach to the optical transitions in quantum dots and quantum dot molecules, which accounts for both diagonal and nondiagonal exciton-phonon interaction, is developed. The cumulant expansion of the linear polarization is generalized to a multilevel system and is applied to calculation of the full time dependence of the polarization and the absorption spectrum. In particular, the broadening of zero-phonon lines is evaluated directly and discussed in terms of real and virtual phonon-assisted transitions. The influence of Coulomb interaction, tunneling, and structural asymmetry on the exciton dephasing in quantum dot molecules is analyzed.
Crossed excitons in a semiconductor nanostructure of mixed dimensionality
Owschimikow, Nina Kolarczik, Mirco; Kaptan, YÃ¼cel I.; Grosse, Nicolai B.; Woggon, Ulrike
2014-09-08
Semiconductor systems of reduced dimensionality, e.g., quantum dots or quantum wells, display a characteristic spectrum of confined excitons. Combining several of these systems may lead to the formation of â€œcrossedâ€ excitons, and thus new equilibrium states and scattering channels. We derive gain excitation spectra from two-color pump-probe experiments on an In(Ga)As based quantum dot semiconductor optical amplifier by analyzing the amplitudes of the traces. This grants access to the quantum dot response, even in the presence of strong absorption by the surroundings at the excitation energy. The gain excitation spectra yield evidence of crossed quantum dot-bulk states.
Resolving multi-exciton generation by attosecond spectroscopy.
Neukirch, A J; Neumark, D M; Kling, M F; Prezhdo, O V
2014-10-20
We propose an experimentally viable attosecond transient absorption spectroscopy scheme to resolve controversies regarding multiexciton (ME) generation in nanoscale systems. Absence of oscillations indicates that light excites single excitons, and MEs are created by incoherent impact ionization. An oscillation indicates the coherent mechanism, involving excitation of superpositions of single and MEs. The oscillation decay, ranging from 5 fs at ambient temperature to 20 fs at 100 K, gives the elastic exciton-phonon scattering time. The signal is best observed with multiple-cycle pump pulses. PMID:25401661
Exciton autoionization in ion-induced electron emission.
Bajales, N; Cristina, L; Mendoza, S; Baragiola, R A; Goldberg, E C; Ferrón, J
2008-06-01
We report on measurements of electron emission spectra from surfaces of highly oriented pyrolytic graphite (HOPG) excited by 1-5 keV He+ and Li+ which, for He+, exhibit a previously unreported high-energy structure. Through a full quantum dynamic description that allows for the calculation of neutralization and electron-hole pair excitation, we show that these high-energy electrons can arise from autoionization of excitons formed by electron promotion to conduction band states close to the vacuum level. The same calculation explains the observed absence of high-energy excitons for Li+ on HOPG. PMID:18643460
Enhanced energy transport in genetically engineered excitonic networks.
Park, Heechul; Heldman, Nimrod; Rebentrost, Patrick; Abbondanza, Luigi; Iagatti, Alessandro; Alessi, Andrea; Patrizi, Barbara; Salvalaggio, Mario; Bussotti, Laura; Mohseni, Masoud; Caruso, Filippo; Johnsen, Hannah C; Fusco, Roberto; Foggi, Paolo; Scudo, Petra F; Lloyd, Seth; Belcher, Angela M
2016-02-01
One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime. PMID:26461447
Crossed excitons in a semiconductor nanostructure of mixed dimensionality
NASA Astrophysics Data System (ADS)
Owschimikow, Nina; Kolarczik, Mirco; Kaptan, Yücel I.; Grosse, Nicolai B.; Woggon, Ulrike
2014-09-01
Semiconductor systems of reduced dimensionality, e.g., quantum dots or quantum wells, display a characteristic spectrum of confined excitons. Combining several of these systems may lead to the formation of "crossed" excitons, and thus new equilibrium states and scattering channels. We derive gain excitation spectra from two-color pump-probe experiments on an In(Ga)As based quantum dot semiconductor optical amplifier by analyzing the amplitudes of the traces. This grants access to the quantum dot response, even in the presence of strong absorption by the surroundings at the excitation energy. The gain excitation spectra yield evidence of crossed quantum dot-bulk states.
Frequency Combs with Weakly Lasing Exciton-Polariton Condensates
NASA Astrophysics Data System (ADS)
Rayanov, K.; Altshuler, B. L.; Rubo, Y. G.; Flach, S.
2015-05-01
We predict the spontaneous modulated emission from a pair of exciton-polariton condensates due to coherent (Josephson) and dissipative coupling. We show that strong polariton-polariton interaction generates complex dynamics in the weak-lasing domain way beyond Hopf bifurcations. As a result, the exciton-polariton condensates exhibit self-induced oscillations and emit an equidistant frequency comb light spectrum. A plethora of possible emission spectra with asymmetric peak distributions appears due to spontaneously broken time-reversal symmetry. The lasing dynamics is affected by the shot noise arising from the influx of polaritons. That results in a complex inhomogeneous line broadening.
Enhanced energy transport in genetically engineered excitonic networks
NASA Astrophysics Data System (ADS)
Park, Heechul; Heldman, Nimrod; Rebentrost, Patrick; Abbondanza, Luigi; Iagatti, Alessandro; Alessi, Andrea; Patrizi, Barbara; Salvalaggio, Mario; Bussotti, Laura; Mohseni, Masoud; Caruso, Filippo; Johnsen, Hannah C.; Fusco, Roberto; Foggi, Paolo; Scudo, Petra F.; Lloyd, Seth; Belcher, Angela M.
2016-02-01
One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime.
Plasmon and exciton superconductivity mechanisms in layered structures
NASA Technical Reports Server (NTRS)
Gabovich, A. M.; Pashitskiy, E. A.; Uvarova, S. K.
1977-01-01
Plasmon and exciton superconductivity mechanisms are discussed. Superconductivity in a three layer metal semiconductor metal and insulator semimetal insulator sandwich structure was described in terms of the temperature dependent Green function of the longitudinal (Coulomb) field. The dependences of the superconducting transition temperature on structure parameters were obtained. In a semiconducting film, as a result of interactions of degenerate free carriers with excitons, superconductivity exists only in a certain range of parameter values, and the corresponding critical temperature is much lower than in the plasmon mechanism of superconductivity.
An exciton-polariton mediated all-optical router
NASA Astrophysics Data System (ADS)
Flayac, H.; Savenko, I. G.
2013-11-01
We propose an all-optical nonlinear router based on a double barrier gate connected to periodically modulated guides. A semiconductor microcavity is driven nonresonantly in-between the barriers to form an exciton-polariton condensate on a discrete state that is subject to the exciton blueshift. The subsequent coherent optical signal is allowed to propagate through a guide provided that the condensate energy is resonant with a miniband or is blocked if it faces a gap. While a symmetric sample operates as an optical switch, its asymmetric counterpart embodies a router turned to be polarization selective under applied magnetic field.
Frequency combs with weakly lasing exciton-polariton condensates.
Rayanov, K; Altshuler, B L; Rubo, Y G; Flach, S
2015-05-15
We predict the spontaneous modulated emission from a pair of exciton-polariton condensates due to coherent (Josephson) and dissipative coupling. We show that strong polariton-polariton interaction generates complex dynamics in the weak-lasing domain way beyond Hopf bifurcations. As a result, the exciton-polariton condensates exhibit self-induced oscillations and emit an equidistant frequency comb light spectrum. A plethora of possible emission spectra with asymmetric peak distributions appears due to spontaneously broken time-reversal symmetry. The lasing dynamics is affected by the shot noise arising from the influx of polaritons. That results in a complex inhomogeneous line broadening. PMID:26024173
Realization of an all optical exciton-polariton router
NASA Astrophysics Data System (ADS)
Marsault, FÃ©lix; Nguyen, Hai Son; Tanese, Dimitrii; LemaÃ®tre, Aristide; Galopin, Elisabeth; Sagnes, Isabelle; Amo, Alberto; Bloch, Jacqueline
2015-11-01
We report on the experimental realization of an all optical router for exciton-polaritons. This device is based on the design proposed by Flayac and Savenko [Appl. Phys. Lett. 103, 201105 (2013)], in which a zero-dimensional island is connected through tunnel barriers to two periodically modulated wires of different periods. Selective transmission of polaritons injected in the island, into either of the two wires, is achieved by tuning the energy of the island state across the band structure of the modulated wires. We demonstrate routing of ps polariton pulses using an optical control beam which controls the energy of the island quantum states, thanks to polariton-exciton interactions.
Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.
Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H
2013-06-18
Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (âˆ¼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley-Queisser limit. PMID:23656886
Exciton localization-delocalization transition in an extended dendrimer
NASA Astrophysics Data System (ADS)
Pouthier, Vincent
2013-12-01
Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number Gc ? 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.
Exciton localization-delocalization transition in an extended dendrimer
Pouthier, Vincent
2013-12-21
Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number G{sub c} â‰ˆ 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.
LONG-TERM PERIODICITY VARIATIONS OF THE SOLAR RADIUS
Qu, Z. N.; Xie, J. L.
2013-01-01
In order to study the long-term periodicity variations of the solar radius, daily solar radius data from 1978 February to 2000 September at the Calern Observatory are used. Continuous observations of the solar radius are difficult due to the weather, seasonal effects, and instrument characteristics. Thus, to analyze these data, we first use the Dixon criterion to reject suspect values, then we measure the cross-correlation between the solar radius and sunspot numbers. The result indicates that the solar radius is in complete antiphase with the sunspot numbers and shows lead times of 74 months relative to the sunspot numbers. The Lomb-Scargle and date compensated discrete Fourier transform methods are also used to investigate the periodicity of the solar radius. Both methods yield similar significance periodicities around {approx}1 yr, {approx}2.6 yr, {approx}3.6 yr, and {approx}11 yr. Possible mechanisms for these periods are discussed. The possible physical cause of the {approx}11 yr period is the cyclic variation of the magnetic pressure of the concentrated flux tubes at the bottom of the solar convection zone.
NASA Astrophysics Data System (ADS)
Chen, R.; Bajaj, K. K.
1997-02-01
We have derived an expression for the effective interaction potential between the constituents of an exciton, namely an electron and a hole in an ionic quantum well structure by taking into account the electron-hole interactions with the confined optical, interface and half-space longitudinal optical phonon modes using a variational method. We follow an approach similar to that used by Haken to derive an effective potential between an electron and a hole in bulk ionic crystals. Using our interaction potential we have calculated the binding energies of excitons in several ionic quantum well structures. We find that the values of the exciton binding energies calculated using this effective interaction potential are always larger than those obtained using a Coulomb potential screened by the static dielectric constant. We compare our results with those of some recent calculations.
NASA Astrophysics Data System (ADS)
González, J. D.; Joya, Miryam R.; Barba-Ortega, J.
2014-01-01
In this paper, we propose a theoretical model to calculate the ground-state energy of light-hole exciton and its density of states in a microtube. This heterostructure has been fabricated and studied experimentally, and contains a double quantum well of AlGaAs/GaAs and large radius of curvature. The exciton trial function is taken as a product of the ground state wave functions of both the unbound electron and hole in the heterostructure, with an arbitrary correlation function that depends only on electron-hole separation. A renormalized Schrödinger equation for the correlation function is derived and coincides with the corresponding equation for a hydrogen atom in an effective isotropic and nonhomogeneous space. The binding energy of the ground state for an exciton in this heterostructure is calculated by the variational model proposed. The contribution to the energy given by the sublevels and the density of states is determined as a function of the width of the well and the aluminum concentration. The obtained results show good agreement with recent experimental observations.
Nogues, Gilles Den Hertog, Martien; Auzelle, Thomas; Gayral, Bruno; Daudin, Bruno
2014-03-10
We perform correlated studies of individual GaN nanowires in scanning electron microscopy combined to low temperature cathodoluminescence, microphotoluminescence, and scanning transmission electron microscopy. We show that some nanowires exhibit well localized regions emitting light at the energy of a stacking fault bound exciton (3.42?eV) and are able to observe the presence of a single stacking fault in these regions. Precise measurements of the cathodoluminescence signal in the vicinity of the stacking fault give access to the exciton diffusion length near this location.
NASA Astrophysics Data System (ADS)
Ping, Er-Xuan; Dalal, Vikram
1993-11-01
A model which includes the transfer of excitons among different growth islands with one atomic layer difference in well thickness is developed and used to study the combined effects of the exciton thermal migration and the inherent interface fluctuation on the exciton photoluminescence (PL) of a quantum well (QW). Results show that the exciton migration to lower-energy islands or large well thickness islands eventuates in a red shifting of the exciton PL and a change of the PL linewidth. Analytical expressions are obtained for low and high temperature limits.
Plasmon-exciton coupling in neighboring metal nanoparticles and a semiconductor quantum well: Theory
NASA Astrophysics Data System (ADS)
Kosobukin, V. A.
2016-02-01
Spectroscopic effects of plasmon-exciton coupling are studied theoretically for metal nanoparticles and a semiconductor quantum-well located nearby. Resonant polarizability is self-consistently calculated for dipole plasmons supported by nanoparticles and influenced by image dipoles due to exciton polarization of the quantum well. Spectra of elastic scattering and reflection of light are analyzed in conformity with high-quality plasmons of silver nanoparticles and quasi-two-dimensional excitons of AlGaAs/GaAs quantum well. Essential two-peak resonant structure associated with plasmon-exciton Rabi splitting is found. The related spectral dip is interpreted in terms of excitonic response enhancement by plasmons of nanoparticles.
All-optical depletion of dark excitons from a semiconductor quantum dot
Schmidgall, E. R.; Schwartz, I.; Cogan, D.; Gershoni, D.; Gantz, L.; Heindel, T.; Reitzenstein, S.
2015-05-11
Semiconductor quantum dots are considered to be the leading venue for fabricating on-demand sources of single photons. However, the generation of long-lived dark excitons imposes significant limits on the efficiency of these sources. We demonstrate a technique that optically pumps the dark exciton population and converts it to a bright exciton population, using intermediate excited biexciton states. We show experimentally that our method considerably reduces the dark exciton population while doubling the triggered bright exciton emission, approaching thereby near-unit fidelity of quantum dot depletion.
Yamada, Yasuhiro; Yamaji, Youhei; Imada, Masatoshi
2015-11-01
Energy dissipation and decoherence are at first glance harmful to acquiring the long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental, as suggested in photosynthesis. By simulating the quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of a carbon nanotube, the exciton lifetime increases by nearly 2 orders of magnitude when the crossover triggers a stable high population in the dark excitons. PMID:26588415
NASA Astrophysics Data System (ADS)
Yamada, Yasuhiro; Yamaji, Youhei; Imada, Masatoshi
2015-11-01
Energy dissipation and decoherence are at first glance harmful to acquiring the long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental, as suggested in photosynthesis. By simulating the quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of a carbon nanotube, the exciton lifetime increases by nearly 2 orders of magnitude when the crossover triggers a stable high population in the dark excitons.
Light-hole exciton spin relaxation in quantum dots
NASA Astrophysics Data System (ADS)
Tsitsishvili, E.
2015-04-01
The phonon-induced flip of the exciton spin in single flat semiconductor quantum dots with a light-hole exciton ground state is studied. The corresponding quartet, split by the exchange interaction, consists of three bright states and a dark state located energetically below the bright exciton. The two in-plane polarized bright states contribute to single-phonon transitions to the dark state and also to the upper bright state polarized in the z growth direction of the dot. For these processes, the presented analytical results are calculated for the relaxation driven by the spin-orbit interaction in the conduction and the light-hole valence subbands. The estimated spin-relaxation times at low temperature are (at least) one order of magnitude lower than the bright exciton lifetime. Two other possible transitions, within the in-plane polarized doublet and between the z -polarized bright and dark states as well, proceed via intermediate states with a contribution from two acoustic phonons. These processes are strongly suppressed at low temperature, whereas they appear to be of the same intensity as single-phonon transitions at high enough temperatures.
Exciton condensation in microcavities under three-dimensional quantization conditions
Kochereshko, V. P. Platonov, A. V.; Savvidis, P.; Kavokin, A. V.; Bleuse, J.; Mariette, H.
2013-11-15
The dependence of the spectra of the polarized photoluminescence of excitons in microcavities under conditions of three-dimensional quantization on the optical-excitation intensity is investigated. The cascade relaxation of polaritons between quantized states of a polariton Bose condensate is observed.
Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus
NASA Astrophysics Data System (ADS)
Wang, Xiaomu; Jones, Aaron M.; Seyler, Kyle L.; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian
2015-03-01
Recently, black phosphorus emerged as a promising new 2D material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form, and its unique properties at the truly 2D quantum confinement have yet to be demonstrated. Here, we reveal highly anisotropic and tightly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centers around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. In addition, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of around 0.9 eV, consistent with theoretical results based on first-principles. The experimental observation of highly anisotropic, bright excitons with exceedingly large binding energy not only opens avenues for the future explorations of many-electron effects in this unusual 2D material, but also suggests a promising future in optoelectronic devices such as on-chip infrared light sources.
Highly anisotropic and robust excitons in monolayer black phosphorus
NASA Astrophysics Data System (ADS)
Wang, Xiaomu; Jones, Aaron M.; Seyler, Kyle L.; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian
2015-06-01
Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3?eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2?eV, from which we estimate an exciton binding energy of ?0.9?eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices.
Organic photosensitive optoelectronic device having a phenanthroline exciton blocking layer
Thompson, Mark E. (Anaheim Hills, CA); Li, Jian (Los Angeles, CA); Forrest, Stephen (Princeton, NJ); Rand, Barry (Princeton, NJ)
2011-02-22
An organic photosensitive optoelectronic device, having an anode, a cathode, and an organic blocking layer between the anode and the cathode is described, wherein the blocking layer comprises a phenanthroline derivative, and at least partially blocks at least one of excitons, electrons, and holes.
National solar technology roadmap: Multiple-exciton-generation PV
Ellingson, Randy
2007-06-01
This roadmap addresses the development of solar cells based on inorganic semiconductor nanocrystals (NCs)â€”such as spherical quantum dots (QDs), quantum rods (QRs), or quantum wires (QWs)â€”focusing on their potential to improve upon bulk semiconductor cell efficiencies by efficient multiple-exciton generation (MEG
Highly anisotropic and robust excitons in monolayer black phosphorus.
Wang, Xiaomu; Jones, Aaron M; Seyler, Kyle L; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian
2015-06-01
Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3?eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2?eV, from which we estimate an exciton binding energy of ?0.9?eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices. PMID:25915195
Effect of correlation of local fluctuations on exciton coherence.
Chen, Xin; Silbey, Robert J
2010-05-28
Recent experimental studies have shown both oscillations of exciton populations and long lasting coherence in multichromophoric systems such as photosynthetic light harvesting systems and conjugated polymers. It has been suggested that this quantum effect is due to correlations of the fluctuations of site energies among the closely packed chromophores in the protein environment. In addition to these, there is the strong possibility of correlations between site energies and transfer matrix elements. In order to understand the role of such correlations we generalize the Haken-Strobl-Reineker (HSR) model to include the energetic correlations and the site diagonal-off-diagonal correlations in a systematic way. The extended HSR model in the exciton basis is also constructed and allows us to study the dynamics of the exciton populations and coherences. With the extended model, we can provide insight into how these correlations affect the evolution of the populations and coherences of excitons by comparing to the original HSR model with uncorrelated fluctuating environments. PMID:20515096
Magnetic control of Coulomb scattering and terahertz transitions among excitons
NASA Astrophysics Data System (ADS)
Bhattacharyya, J.; Zybell, S.; Eßer, F.; Helm, M.; Schneider, H.; Schneebeli, L.; Böttge, C. N.; Breddermann, B.; Kira, M.; Koch, S. W.; Andrews, A. M.; Strasser, G.
2014-03-01
Time-resolved terahertz quenching studies of the magnetoexcitonic photoluminescence from GaAs/AlGaAs quantum wells are performed. A microscopic theory is developed to analyze the experiments. Detailed experiment-theory comparisons reveal a remarkable magnetic-field controllability of the Coulomb and terahertz interactions in the excitonic system.
Bimolecular quenching of excitons and fluorescence in the photosynthetic unit.
Swenberg, C E; Geacintov, N E; Pope, M
1976-01-01
The recent results of Campillo et al. and Mauzerall on the quenching of the fluorescence of chlorophyll a in Chlorella pyrenoidosa as a function of the intensity of the laser excitation pulses are rationalized by applying a model invoking singlet-singlet exciton annihilation. PMID:990396
Singlet fission in pentacene through multiple exciton quantum states
NASA Astrophysics Data System (ADS)
Zhang, Zhiyong; Zimmerman, Paul; Musgrave, Charles
2010-03-01
Multi-exciton generation (MEG) has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission is the molecular analogue of MEG and has been observed in various systems, including tetracene and pentacene, however, no fundamental mechanism for singlet fission has yet been described, although it may govern MEG processes in a variety of materials. Because photoexcited states have single-exciton character, singlet fission to produce a pair of triplet excitons must involve an intermediate state that: (1) exhibits multi-exciton (ME) character, (2) is accessible from S1 and satisfies the fission energy requirement, and (3) efficiently dissociates into multiple electron-hole pairs. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through a dark state (D) of ME character that lies just below S1, satisfies the fission energy requirement (ED>2ET0), and splits into two triplets (2xT0). In tetracene, D lies just above S1, consistent with the observation that singlet fission is thermally activated in tetracene. Rational design of photovoltaic systems that exploit singlet fission will require ab initio analysis of ME states such as D.
Ultrafast exciton dissociation at donor/acceptor interfaces
NASA Astrophysics Data System (ADS)
Grancini, G.; Fazzi, D.; Binda, M.; Maiuri, M.; Petrozza, A.; Criante, L.; Perissinotto, S.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G.
2013-09-01
Charge generation at donor/acceptor interface is a highly debated topic in the organic photovoltaics (OPV) community. The primary photoexcited state evolution happens in few femtosecond timescale, thus making very intriguing their full understanding. In particular charge generation is believed to occur in < 200 fs, but no clear picture emerged so far. In this work we reveal for the first time the actual charge generation mechanism following in real time the exciton dissociation mechanism by means of sub-22 fs pump-probe spectroscopy. We study a low-band-gap polymer: fullerene interface as an ideal system for OPV. We demonstrate that excitons dissociation leads, on a timescale of 20-50 fs, to two byproducts: bound interfacial charge transfer states (CTS) and free charges. The branching ratio of their formation depends on the excess photon energy provided. When high energy singlet polymer states are excited, well above the optical band gap, an ultrafast hot electron transfer happens between the polymer singlet state and the interfacial hot CTS* due to the high electronic coupling between them. Hot exciton dissociation prevails then on internal energy dissipation that occurs within few hundreds of fs. By measuring the internal quantum efficiency of a prototypical device a rising trend with energy is observed, thus indicating that hot exciton dissociation effectively leads to a higher fraction of free charges.
Singlet Fission and Multi-Exciton Generation in Organic Systems
NASA Astrophysics Data System (ADS)
Musgrave, Charles
2012-02-01
Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited in solar-cells to dramatically increase efficiency. In tetracene and pentacene MEG has been attributed to singlet fission (SF), however a fundamental mechanism for SF has not been previously described. Here, we use sophisticated ab initio calculations to show that MEG in pentacene proceeds by transition of the lowest optically allowed excited state S1 to a dark state (D) of multi-exciton character, which subsequently undergoes SF to generate two triplets (2xT0). D satisfies the energy requirement for SF (ED>2ET0) and lies just below S1 in pentacene, but above S1 in tetracene, consistent with the observed thermally activated SF process in tetracene, but no thermal activation in pentacene. While S1 exhibits single exciton character, D shows multi-exciton character comprising two separated electron-hole pairs. Dimer simulations predict S1 excimer formation and that fission of D into triplets proceeds through the excimer. The predicted energetics, wavefunctions and excimer interaction support the proposed mechanism, which accounts for the observed rapid, unactivated SF in pentacene. Results for SF in polyacenes, grapheme nanoribbons, rubrene and carbon nanotubes will be presented.
Heavy ion collisions and the pre-equilibrium exciton model
Betak, E.
2012-10-20
We present a feasible way to apply the pre-equilibrium exciton model in its masterequation formulation to heavy-ion induced reactions including spin variables. Emission of nucleons, {gamma}'s and also light clusters is included in our model.
Polaritons in semiconductor/insulator superlattices with nonlocal excitonic response
NASA Astrophysics Data System (ADS)
Gippius, N. A.; Tikhodeev, S. G.; Keldysh, L. V.
1994-06-01
We report a polariton mode structure calculations based on nonlocal model for semiconductor/insulator superlattices. The reflection spectra and the spatial distributions of intrinsic electric fields near polariton resonance are presented. We show that near the polariton resonance the electric component of polariton mode changes drastically and depends strongly upon the quantum confined exciton wavefunctions.
Coherent control of multipartite excitonic entanglement in quantum dot arrays
NASA Astrophysics Data System (ADS)
Rolon, Juan E.; Drut, Joaquin E.
2015-03-01
We propose a coherent control scheme for multipartite entanglement of exciton states in optically driven quantum dot arrays (QDAs) coupled by charge tunneling and resonant energy transfer (RET) processes. An adiabatic manipulation of the entanglement dynamics is devised by pulse shaping and time-dependent electric field sweeps. By varying the inter-dot distance and number of quantum dots (QDs) comprising the QDA, the excitonic qubit manifolds are obtained by a Feshbach projection over the resulting multilevel exciton configurations. We identify regimes in which the dynamics is confined to decoherence-free excitonic qubit manifolds taking into account spontaneous recombination and non-Markovian effects introduced by a phonon bath. We present results for entanglement monotones and measures such as the entanglement of formation and entanglement entropy for different QDA geometries and carrier injection conditions. Our results indicate that in spite of the effects of phonon-assisted relaxation, entanglement can be optimized and transferred between QDs by the controlled interplay of system geometry, pulse shaping, RET and carrier tunneling.
Effect of magnetic confinement on manybody nonlinearities of Lorentzian excitons
Kner, P.; Bar-Ad, S.; Marquezini, M.V.; Chemla, D.S.
1995-11-01
100-fs time- and frequency-resolved four-wave mixing in GaAs at T=1.6K under magnetic field, 0{<=}|{rvec B}|{<=}12T, is used for investigating how magnetic confinement modifies the processes that govern the manybody nonlinearities of the Lorentzian excitons.
Resonant Rayleigh scattering dynamics of excitons in single quantum wells
NASA Astrophysics Data System (ADS)
Kocherscheidt, G.; Langbein, W.; Woggon, U.; Savona, V.; Zimmermann, R.; Reuter, D.; Wieck, A. D.
2003-08-01
The resonant Rayleigh scattering dynamics of excitons in single GaAs quantum wells is investigated. The deviation of the measured intensity from the ideal ensemble average is analyzed as a function of the speckle ensemble size. The influence of the amplitude and correlation length of the exciton disorder potential is traced using a series of samples with varying inhomogeneous broadening and interface island sizes. The experimental data are compared with theoretical predictions using exciton states of spatially uncorrelated energies, as well as full calculations of the exciton polarization dynamics using a more realistic disorder potential including the formation of monolayer islands. Deviations from the dynamics of uncorrelated states are found at early times after excitation. They are reproduced by the calculations possessing a state correlation due to quantum mechanical level repulsion and a finite correlation length for the disorder potential. Additionally, the presence of a long-range disorder potential in the micrometer range is suggested. Changing the disorder potential by varying the island size influences the observed dynamics systematically according to the different disorder correlation lengths.