Laplacian versus adjacency matrix in quantum walk search
NASA Astrophysics Data System (ADS)
Wong, Thomas G.; Tarrataca, Luís; Nahimov, Nikolay
2016-06-01
A quantum particle evolving by Schrödinger's equation contains, from the kinetic energy of the particle, a term in its Hamiltonian proportional to Laplace's operator. In discrete space, this is replaced by the discrete or graph Laplacian, which gives rise to a continuous-time quantum walk. Besides this natural definition, some quantum walk algorithms instead use the adjacency matrix to effect the walk. While this is equivalent to the Laplacian for regular graphs, it is different for non-regular graphs and is thus an inequivalent quantum walk. We algorithmically explore this distinction by analyzing search on the complete bipartite graph with multiple marked vertices, using both the Laplacian and adjacency matrix. The two walks differ qualitatively and quantitatively in their required jumping rate, runtime, sampling of marked vertices, and in what constitutes a natural initial state. Thus the choice of the Laplacian or adjacency matrix to effect the walk has important algorithmic consequences.
49 CFR 236.404 - Signals at adjacent control points.
Code of Federal Regulations, 2010 CFR
2010-10-01
... 49 Transportation 4 2010-10-01 2010-10-01 false Signals at adjacent control points. 236.404 Section 236.404 Transportation Other Regulations Relating to Transportation (Continued) FEDERAL RAILROAD ADMINISTRATION, DEPARTMENT OF TRANSPORTATION RULES, STANDARDS, AND INSTRUCTIONS GOVERNING THE INSTALLATION, INSPECTION, MAINTENANCE, AND REPAIR...
Freely suspended quantum point contacts
NASA Astrophysics Data System (ADS)
Rössler, C.; Herz, M.; Bichler, M.; Ludwig, S.
2010-05-01
We present a versatile design of freely suspended quantum point contacts with particular large one-dimensional subband quantization energies of up to Δɛ≈10 meV. The nanoscale bridges embedding a two-dimensional electron system are fabricated from AlGaAs/GaAs heterostructures by electron-beam lithography and etching techniques. Narrow constrictions define quantum point contacts that are capacitively controlled via local in-plane side gates. Employing transport spectroscopy, we investigate the transition from electrostatic subbands to Landau quantization in a perpendicular magnetic field. The large subband quantization energies allow us to utilize a wide magnetic field range and thereby observe a large exchange split spin-gap of the two lowest Landau-levels.
Controlling superconductivity by tunable quantum critical points.
Seo, S; Park, E; Bauer, E D; Ronning, F; Kim, J N; Shim, J-H; Thompson, J D; Park, Tuson
2015-01-01
The heavy fermion compound CeRhIn5 is a rare example where a quantum critical point, hidden by a dome of superconductivity, has been explicitly revealed and found to have a local nature. The lack of additional examples of local types of quantum critical points associated with superconductivity, however, has made it difficult to unravel the role of quantum fluctuations in forming Cooper pairs. Here, we show the precise control of superconductivity by tunable quantum critical points in CeRhIn5. Slight tin-substitution for indium in CeRhIn5 shifts its antiferromagnetic quantum critical point from 2.3 GPa to 1.3 GPa and induces a residual impurity scattering 300 times larger than that of pure CeRhIn5, which should be sufficient to preclude superconductivity. Nevertheless, superconductivity occurs at the quantum critical point of the tin-doped metal. These results underline that fluctuations from the antiferromagnetic quantum criticality promote unconventional superconductivity in CeRhIn5. PMID:25737108
Code of Federal Regulations, 2012 CFR
2012-07-01
... REGULATIONS § 334.412 Albemarle Sound, Pamlico Sound, Harvey Point and adjacent waters, NC; restricted area. (a) The area. Beginning on the north shore of Albemarle Sound and the easternmost tip of Harvey Point..., Harvey Point and adjacent waters, NC; restricted area. 334.412 Section 334.412 Navigation and...
Code of Federal Regulations, 2010 CFR
2010-07-01
... REGULATIONS § 334.412 Albemarle Sound, Pamlico Sound, Harvey Point and adjacent waters, NC; restricted area. (a) The area. Beginning on the north shore of Albemarle Sound and the easternmost tip of Harvey Point..., Harvey Point and adjacent waters, NC; restricted area. 334.412 Section 334.412 Navigation and...
Code of Federal Regulations, 2014 CFR
2014-07-01
... REGULATIONS § 334.412 Albemarle Sound, Pamlico Sound, Harvey Point and adjacent waters, NC; restricted area. (a) The area. Beginning on the north shore of Albemarle Sound and the easternmost tip of Harvey Point..., Harvey Point and adjacent waters, NC; restricted area. 334.412 Section 334.412 Navigation and...
Code of Federal Regulations, 2013 CFR
2013-07-01
... REGULATIONS § 334.412 Albemarle Sound, Pamlico Sound, Harvey Point and adjacent waters, NC; restricted area. (a) The area. Beginning on the north shore of Albemarle Sound and the easternmost tip of Harvey Point..., Harvey Point and adjacent waters, NC; restricted area. 334.412 Section 334.412 Navigation and...
Code of Federal Regulations, 2011 CFR
2011-07-01
... REGULATIONS § 334.412 Albemarle Sound, Pamlico Sound, Harvey Point and adjacent waters, NC; restricted area. (a) The area. Beginning on the north shore of Albemarle Sound and the easternmost tip of Harvey Point..., Harvey Point and adjacent waters, NC; restricted area. 334.412 Section 334.412 Navigation and...
Spotlighting quantum critical points via quantum correlations at finite temperatures
Werlang, T.; Ribeiro, G. A. P.; Rigolin, Gustavo
2011-06-15
We extend the program initiated by T. Werlang et al. [Phys. Rev. Lett. 105, 095702 (2010)] in several directions. Firstly, we investigate how useful quantum correlations, such as entanglement and quantum discord, are in the detection of critical points of quantum phase transitions when the system is at finite temperatures. For that purpose we study several thermalized spin models in the thermodynamic limit, namely, the XXZ model, the XY model, and the Ising model, all of which with an external magnetic field. We compare the ability of quantum discord, entanglement, and some thermodynamic quantities to spotlight the quantum critical points for several different temperatures. Secondly, for some models we go beyond nearest neighbors and also study the behavior of entanglement and quantum discord for second nearest neighbors around the critical point at finite temperature. Finally, we furnish a more quantitative description of how good all these quantities are in spotlighting critical points of quantum phase transitions at finite T, bridging the gap between experimental data and those theoretical descriptions solely based on the unattainable absolute zero assumption.
Detecting quantum critical points using bipartite fluctuations.
Rachel, Stephan; Laflorencie, Nicolas; Song, H Francis; Le Hur, Karyn
2012-03-16
We show that the concept of bipartite fluctuations F provides a very efficient tool to detect quantum phase transitions in strongly correlated systems. Using state-of-the-art numerical techniques complemented with analytical arguments, we investigate paradigmatic examples for both quantum spins and bosons. As compared to the von Neumann entanglement entropy, we observe that F allows us to find quantum critical points with much better accuracy in one dimension. We further demonstrate that F can be successfully applied to the detection of quantum criticality in higher dimensions with no prior knowledge of the universality class of the transition. Promising approaches to experimentally access fluctuations are discussed for quantum antiferromagnets and cold gases. PMID:22540493
Quantum-to-classical crossover near quantum critical point
Vasin, M.; Ryzhov, V.; Vinokur, V. M.
2015-12-21
A quantum phase transition (QPT) is an inherently dynamic phenomenon. However, while non-dissipative quantum dynamics is described in detail, the question, that is not thoroughly understood is how the omnipresent dissipative processes enter the critical dynamics near a quantum critical point (QCP). Here we report a general approach enabling inclusion of both adiabatic and dissipative processes into the critical dynamics on the same footing. We reveal three distinct critical modes, the adiabatic quantum mode (AQM), the dissipative classical mode [classical critical dynamics mode (CCDM)], and the dissipative quantum critical mode (DQCM). We find that as a result of the transitionmore » from the regime dominated by thermal fluctuations to that governed by the quantum ones, the system acquires effective dimension d+zΛ(T), where z is the dynamical exponent, and temperature-depending parameter Λ(T)ε[0, 1] decreases with the temperature such that Λ(T=0) = 1 and Λ(T →∞) = 0. Lastly, our findings lead to a unified picture of quantum critical phenomena including both dissipation- and dissipationless quantum dynamic effects and offer a quantitative description of the quantum-to-classical crossover.« less
Quantum-to-classical crossover near quantum critical point
NASA Astrophysics Data System (ADS)
Vasin, M.; Ryzhov, V.; Vinokur, V. M.
2015-12-01
A quantum phase transition (QPT) is an inherently dynamic phenomenon. However, while non-dissipative quantum dynamics is described in detail, the question, that is not thoroughly understood is how the omnipresent dissipative processes enter the critical dynamics near a quantum critical point (QCP). Here we report a general approach enabling inclusion of both adiabatic and dissipative processes into the critical dynamics on the same footing. We reveal three distinct critical modes, the adiabatic quantum mode (AQM), the dissipative classical mode [classical critical dynamics mode (CCDM)], and the dissipative quantum critical mode (DQCM). We find that as a result of the transition from the regime dominated by thermal fluctuations to that governed by the quantum ones, the system acquires effective dimension d + zΛ(T), where z is the dynamical exponent, and temperature-depending parameter Λ(T) ∈ [0, 1] decreases with the temperature such that Λ(T = 0) = 1 and Λ(T → ∞) = 0. Our findings lead to a unified picture of quantum critical phenomena including both dissipation- and dissipationless quantum dynamic effects and offer a quantitative description of the quantum-to-classical crossover.
Quantum-to-classical crossover near quantum critical point
Vasin, M.; Ryzhov, V.; Vinokur, V. M.
2015-01-01
A quantum phase transition (QPT) is an inherently dynamic phenomenon. However, while non-dissipative quantum dynamics is described in detail, the question, that is not thoroughly understood is how the omnipresent dissipative processes enter the critical dynamics near a quantum critical point (QCP). Here we report a general approach enabling inclusion of both adiabatic and dissipative processes into the critical dynamics on the same footing. We reveal three distinct critical modes, the adiabatic quantum mode (AQM), the dissipative classical mode [classical critical dynamics mode (CCDM)], and the dissipative quantum critical mode (DQCM). We find that as a result of the transition from the regime dominated by thermal fluctuations to that governed by the quantum ones, the system acquires effective dimension d + zΛ(T), where z is the dynamical exponent, and temperature-depending parameter Λ(T) ∈ [0, 1] decreases with the temperature such that Λ(T = 0) = 1 and Λ(T → ∞) = 0. Our findings lead to a unified picture of quantum critical phenomena including both dissipation- and dissipationless quantum dynamic effects and offer a quantitative description of the quantum-to-classical crossover. PMID:26688102
Quantum-to-classical crossover near quantum critical point
Vasin, M.; Ryzhov, V.; Vinokur, V. M.
2015-12-21
A quantum phase transition (QPT) is an inherently dynamic phenomenon. However, while non-dissipative quantum dynamics is described in detail, the question, that is not thoroughly understood is how the omnipresent dissipative processes enter the critical dynamics near a quantum critical point (QCP). Here we report a general approach enabling inclusion of both adiabatic and dissipative processes into the critical dynamics on the same footing. We reveal three distinct critical modes, the adiabatic quantum mode (AQM), the dissipative classical mode [classical critical dynamics mode (CCDM)], and the dissipative quantum critical mode (DQCM). We find that as a result of the transition from the regime dominated by thermal fluctuations to that governed by the quantum ones, the system acquires effective dimension d+zΛ(T), where z is the dynamical exponent, and temperature-depending parameter Λ(T)ε[0, 1] decreases with the temperature such that Λ(T=0) = 1 and Λ(T →∞) = 0. Lastly, our findings lead to a unified picture of quantum critical phenomena including both dissipation- and dissipationless quantum dynamic effects and offer a quantitative description of the quantum-to-classical crossover.
Characterizations of fixed points of quantum operations
Li Yuan
2011-05-15
Let {phi}{sub A} be a general quantum operation. An operator B is said to be a fixed point of {phi}{sub A}, if {phi}{sub A}(B)=B. In this note, we shall show conditions under which B, a fixed point {phi}{sub A}, implies that B is compatible with the operation element of {phi}{sub A}. In particular, we offer an extension of the generalized Lueders theorem.
Quantum point contacts as heat engines
NASA Astrophysics Data System (ADS)
Pilgram, Sebastian; Sánchez, David; López, Rosa
2015-11-01
The efficiency of macroscopic heat engines is restricted by the second law of thermodynamics. They can reach at most the efficiency of a Carnot engine. In contrast, heat currents in mesoscopic heat engines show fluctuations. Thus, there is a small probability that a mesoscopic heat engine exceeds Carnot's maximum value during a short measurement time. We illustrate this effect using a quantum point contact as a heat engine. When a temperature difference is applied to a quantum point contact, the system may be utilized as a source of electrical power under steady state conditions. We first discuss the optimal working point of such a heat engine that maximizes the generated electrical power and subsequently calculate the statistics for deviations of the efficiency from its most likely value. We find that deviations surpassing the Carnot limit are possible, but unlikely.
Spontaneous Spin Polarization in Quantum Point Contacts
NASA Astrophysics Data System (ADS)
Rokhinson, Leonid
2007-03-01
Mesoscopic systems exhibit a range of non-trivial spin-related phenomena in the low density regime, where inter-particle Coulomb interactions become comparable to their kinetic energy. In zero-dimensional systems spontaneous polarization of a few-electron quantum dot leads to a spin blockade, a remarkable effect where mismatch of a single spin blocks macroscopic current flow. In two-dimensional hole gases there is an experimental evidence of a finite spin polarization even in the absence of a magnetic field. In one-dimensional systems quantum wires and quantum point contacts - a puzzling so-called ``0.7 structure'' has been observed below the first quantization plateau. Experiments suggest that an extra plateau in the conductance vs gate voltage characteristic at 0.7x2e^2/h is spin related, however, the origin of the phenomenon is not yet understood and is highly debated. We report direct measurements of finite polarization of holes in a quantum point contact (QPC) at conductances G < 2e^2/h [1]. We incorporated QPC into a magnetic focusing device so that polarization can be measured directly using a recently developed spatial spin separation technique [2]. Devices are fabricated from p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in low-density regime, when conductance of a point contact is tuned to < 2e^2/h. We found that polarization is stronger in samples with well defined ``0.7 structure''. [1] L.P. Rokhinson, L.N. Pfeiffer and K.W. West,``Spontaneous spin polarization in quantum point contacts,'' Physical Review Letters 96, 156602 (2006) [2] L.P. Rokhinson, V. Larkina, Y.B. Lyanda-Geller, L.N. Pfeiffer and K.W. West, ``Spin separation in cyclotron motion,'' Physicsl Review Letters 93, 146601 (2004)
Navigated Pin-Point Approach to Osteoid Osteoma Adjacent to the Facet Joint of Spine
Neo, Masashi; Takemoto, Mitsuru; Nishizawa, Kazuya; Imai, Shinji
2016-01-01
Osteoid osteoma (OO) is a benign osteoblastic tumor. Its curative treatment is complete removal of the nidus, where intraoperative localization of the nidus governs clinical results. However, treatment can be difficult since the lesion is often invisible over the bony surface. Accordingly, establishment of an ideal less invasive surgical strategy for spinal OO remains yet unsettled. We illustrate the efficacy of a computed tomography (CT)-based navigation system in excising OO located adjacent to the facet joint of spine. In our 2 cases, complete and pin-point removal of the nidus located close to the facet joint was successfully achieved, without excessive removal of the bone potentially leading to spinal instability and possible damage of nearby neurovascular structures. We advocate a less invasive approach to spinal OO, particularly in an environment with an available CT-based navigation system. PMID:26949472
Dynamic trapping near a quantum critical point
NASA Astrophysics Data System (ADS)
Kolodrubetz, Michael; Katz, Emanuel; Polkovnikov, Anatoli
2015-02-01
The study of dynamics in closed quantum systems has been revitalized by the emergence of experimental systems that are well-isolated from their environment. In this paper, we consider the closed-system dynamics of an archetypal model: spins driven across a second-order quantum critical point, which are traditionally described by the Kibble-Zurek mechanism. Imbuing the driving field with Newtonian dynamics, we find that the full closed system exhibits a robust new phenomenon—dynamic critical trapping—in which the system is self-trapped near the critical point due to efficient absorption of field kinetic energy by heating the quantum spins. We quantify limits in which this phenomenon can be observed and generalize these results by developing a Kibble-Zurek scaling theory that incorporates the dynamic field. Our findings can potentially be interesting in the context of early universe physics, where the role of the driving field is played by the inflaton or a modulus field.
NASA Astrophysics Data System (ADS)
Jafarizadeh, M. A.; Salimi, S.
2007-05-01
Using the spectral distribution associated with the adjacency matrix of graphs, we introduce a new method of calculation of amplitudes of continuous-time quantum walk on some rather important graphs, such as line, cycle graph Cn, complete graph Kn, graph Gn, finite path and some other finite and infinite graphs, where all are connected with orthogonal polynomials such as Hermite, Laguerre, Tchebichef, and other orthogonal polynomials. It is shown that using the spectral distribution, one can obtain the infinite time asymptotic behavior of amplitudes simply by using the method of stationary phase approximation (WKB approximation), where as an example, the method is applied to star, two-dimensional comb lattices, infinite Hermite and Laguerre graphs. Also by using the Gauss quadrature formula one can approximate the infinite graphs with finite ones and vice versa, in order to derive large time asymptotic behavior by WKB method. Likewise, using this method, some new graphs are introduced, where their amplitudes are proportional to the product of amplitudes of some elementary graphs, even though the graphs themselves are not the same as the Cartesian product of their elementary graphs. Finally, by calculating the mean end to end distance of some infinite graphs at large enough times, it is shown that continuous-time quantum walk at different infinite graphs belong to different universality classes which are also different from those of the corresponding classical ones.
Spin polarization in quantum point contact structures
NASA Astrophysics Data System (ADS)
Ngo, Anh; Ulloa, Sergio
2008-03-01
One of the important goals in the field of spintronics is to produce spin-polarized currents in semiconductors [1]. The Rashba spin-orbit interaction is useful in this regard, because its strength is controllable by applying an electric field. In this work we study ballistic transport through semiconductor quantum point contact systems under different confinement geometries and applied fields. In particular, we investigate how the lateral spin-orbit coupling, as induced by the lateral confinement potential, plays a non-trivial role on the spin polarization of the current, even in the absence of magnetic field. We find that high spin polarization can be obtained by controlling the asymmetric shape of the confinement potential, and contrast our results with previous work in the literature [2]. This behavior suggests a novel scheme to implement spin-filters without external magnetic fields, and we present its dependence on structural parameters. [1] S. A. Wolf, et al., Spintronics: a spin based electronic vision of the future, Science 294, 1488-1495 (2001). [2] M. Eto, et al., Spin polarization at semiconductor point contacts in absence of magnetic field, J. Phys. Soc. Jpn. 74, 1934 (2005).
Fermion-induced quantum critical points: beyond Landau criterion
NASA Astrophysics Data System (ADS)
Yao, Hong; Li, Zi-Xiang; Jiang, Yi-Fan; Jian, Shao-Kai
According to Landau criterion, phase transitions must be first-order when cubic terms of order parameters in the Landau-Ginzburg free energy are allowed by symmetry. Here, from both renormalization group analysis and sign-problem-free quantum Monte Carlo simulations, we show that second-order quantum phase transitions can occur at such putatively-first-order quantum phase transitions in strongly-interacting Dirac semimetals in two spatial dimensions. Such type of Landau-criterion-violating quantum critical points are induced by massless fermionic modes at the quantum phase transitions. We call them ``fermion-induced quantum critical points''. From Majorana-quantum-Monte-Carlo simulations and renormalization analysis, we find that the critical exponentials at the kekule valence-bond-solid transition of the Dirac fermions on the honeycomb lattice are highly-nonclassical. We also discuss experimental signatures of the kekule quantum critical point which may be realized in graphene-like systems.
Graphene-based superconducting quantum point contacts
NASA Astrophysics Data System (ADS)
Moghaddam, A. G.; Zareyan, M.
2007-11-01
We investigate the Josephson effect in the graphene nanoribbons of length L smaller than the superconducting coherence length and an arbitrary width W. We find that in contrast to an ordinary superconducting quantum point contact (SQPC), the critical supercurrent Ic is not quantized for the nanoribbons with smooth and armchair edges. For a low concentration of the carriers, Ic decreases monotonically with lowering W/L and tends to a constant minimum for a narrow nanoribbon with Wlesssim{}L. The minimum Ic is zero for the smooth edges but eΔ0/hbar for the armchair edges. At higher concentrations of the carriers this monotonic variation acquires a series of peaks. Further analysis of the current-phase relation and the Josephson coupling strength IcRN in terms of W/L and the concentration of carriers revels significant differences with those of an ordinary SQPC. On the other hand for a zigzag nanoribbon, we find that, similar to an ordinary SQPC, Ic is quantized but to the half-integer values (n+1/2)4eΔ0/hbar.
Cavity-assisted dynamical quantum phase transition at bifurcation points
NASA Astrophysics Data System (ADS)
Tian, Lin
2016-04-01
Coupling a quantum many-body system to a cavity can create bifurcation points in its phase diagram, where the ground state makes sudden switchings between different phases. Here we study the dynamical quantum phase transition of a transverse field Ising model coupled to a cavity. We show that an infinitesimal quench of the cavity driving at the bifurcation points induces gradual evolution of the Ising model to pass across the quantum critical point and excites quasiparticles. Meanwhile, when the driving is slowly ramped through the bifurcation points, the adiabaticity of the evolution and the number of quasiparticle excitations are strongly affected by cavity-induced nonlinearity. Introducing and manipulating cavity-induced nonlinearity hence provide an effective approach to control the dynamics and the adiabaticity of adiabatic quantum processes. This model can be implemented with superconducting quantum circuits.
Telegraph noise in coupled quantum dot circuits induced by a quantum point contact.
Taubert, D; Pioro-Ladrière, M; Schröer, D; Harbusch, D; Sachrajda, A S; Ludwig, S
2008-05-01
Charge detection utilizing a highly biased quantum point contact has become the most effective probe for studying few electron quantum dot circuits. Measurements on double and triple quantum dot circuits is performed to clarify a back action role of charge sensing on the confined electrons. The quantum point contact triggers inelastic transitions, which occur quite generally. Under specific device and measurement conditions these transitions manifest themselves as bounded regimes of telegraph noise within a stability diagram. A nonequilibrium transition from artificial atomic to molecular behavior is identified. Consequences for quantum information applications are discussed. PMID:18518321
Gravity from entanglement close to a quantum critical point
NASA Astrophysics Data System (ADS)
Faulkner, Thomas
2015-04-01
Entanglement entropy (EE) in quantum many-body systems reveal interesting non-local aspects of the state or phase of the system. For example, topological order in gapped phases may be characterized in this way. We present calculations of entanglement close to a quantum critical point with relativistic invariance that reveal the existence of an emergent gravitational theory in one higher dimension. The gravitational theory encodes the entanglement of the quantum system in an efficient way. In this way calculations of EE, a usually notoriously difficult quantity to calculate, are reduced to a simple computation in classical gravity. The answer we find is in the spirit of the AdS/CFT duality but goes beyond it since our results apply to any relativistic quantum critical point and not just the known theories with classical gravity duals.
Coulomb Interaction between InAs/GaAs Quantum Dots and Adjacent Impurities
Engstroem, O.; Kaniewska, M.; Kaczmarczyk, M.
2011-12-23
Defects positioned close to a plane of quantum dots (QDs) are shown to be influenced by coulomb interaction effect when the quantum dots are charged by electrons. Signals from deep level transient spectroscopy (DLTS) measurement give rise to a mirror effect in the spectrum depending on movement of the defect energy level in relation to the Fermi-level as a result of the electron traffic at the QDs.
Theory of microwave-assisted supercurrent in quantum point contacts.
Bergeret, F S; Virtanen, P; Heikkilä, T T; Cuevas, J C
2010-09-10
We present a microscopic theory of the effect of a microwave field on the supercurrent through a quantum point contact of arbitrary transmission. Our theory predicts that (i) for low temperatures and weak fields, the supercurrent is suppressed at certain values of the superconducting phase, (ii) at strong fields, the current-phase relation is strongly modified and the current can even reverse its sign, and (iii) at finite temperatures, the microwave field can enhance the critical current of the junction. Apart from their fundamental interest, our findings are also important for the description of experiments that aim at the manipulation of the quantum state of atomic point contacts. PMID:20867598
Thermal conductivity at a disordered quantum critical point
NASA Astrophysics Data System (ADS)
Hartnoll, Sean A.; Ramirez, David M.; Santos, Jorge E.
2016-04-01
Strongly disordered and strongly interacting quantum critical points are difficult to access with conventional field theoretic methods. They are, however, both experimentally important and theoretically interesting. In particular, they are expected to realize universal incoherent transport. Such disordered quantum critical theories have recently been constructed holographically by deforming a CFT by marginally relevant disorder. In this paper we find additional disordered fixed points via relevant disordered deformations of a holographic CFT. Using recently developed methods in holographic transport, we characterize the thermal conductivity in both sets of theories in 1+1 dimensions. The thermal conductivity is found to tend to a constant at low temperatures in one class of fixed points, and to scale as T 0.3 in the other. Furthermore, in all cases the thermal conductivity exhibits discrete scale invariance, with logarithmic in temperature oscillations superimposed on the low temperature scaling behavior. At no point do we use the replica trick.
Parallel quantum-point-contacts as high-frequency-mixers
NASA Astrophysics Data System (ADS)
Haubrich, A. G. C.; Wharam, D. A.; Kriegelstein, H.; Manus, S.; Lorke, A.; Kotthaus, J. P.; Gossard, A. C.
1997-06-01
The results of high-frequency mixing experiments performed upon parallel quantum point contacts defined in the two-dimensional electron gas of an AlxGa1-xAs/GaAs heterostructure are presented. The parallel geometry, fabricated using a novel double-resist technology, enables the point-contact device to be impedance matched over a wide frequency range and, in addition, increases the power levels of the mixing signal while simultaneously reducing the parasitic source-drain capacitance. Here, we consider two parallel quantum point-contact devices with 155 and 110 point contacts, respectively; both devices operated successfully at liquid helium and liquid nitrogen temperatures with a minimal conversion loss of 13 dB.
Reprint of : Quantum point contacts as heat engines
NASA Astrophysics Data System (ADS)
Pilgram, Sebastian; Sánchez, David; López, Rosa
2016-08-01
The efficiency of macroscopic heat engines is restricted by the second law of thermodynamics. They can reach at most the efficiency of a Carnot engine. In contrast, heat currents in mesoscopic heat engines show fluctuations. Thus, there is a small probability that a mesoscopic heat engine exceeds Carnot's maximum value during a short measurement time. We illustrate this effect using a quantum point contact as a heat engine. When a temperature difference is applied to a quantum point contact, the system may be utilized as a source of electrical power under steady state conditions. We first discuss the optimal working point of such a heat engine that maximizes the generated electrical power and subsequently calculate the statistics for deviations of the efficiency from its most likely value. We find that deviations surpassing the Carnot limit are possible, but unlikely.
Stochastic Approximation of Dynamical Exponent at Quantum Critical Point
NASA Astrophysics Data System (ADS)
Suwa, Hidemaro; Yasuda, Shinya; Todo, Synge
We have developed a unified finite-size scaling method for quantum phase transitions that requires no prior knowledge of the dynamical exponent z. During a quantum Monte Carlo simulation, the temperature is automatically tuned by the Robbins-Monro stochastic approximation method, being proportional to the lowest gap of the finite-size system. The dynamical exponent is estimated in a straightforward way from the system-size dependence of the temperature. As a demonstration of our novel method, the two-dimensional S = 1 / 2 quantum XY model, or equivalently the hard-core boson system, in uniform and staggered magnetic fields is investigated in the combination of the world-line quantum Monte Carlo worm algorithm. In the absence of a uniform magnetic field, we obtain the fully consistent result with the Lorentz invariance at the quantum critical point, z = 1 . Under a finite uniform magnetic field, on the other hand, the dynamical exponent becomes two, and the mean-field universality with effective dimension (2+2) governs the quantum phase transition. We will discuss also the system with random magnetic fields, or the dirty boson system, bearing a non-trivial dynamical exponent.Reference: S. Yasuda, H. Suwa, and S. Todo Phys. Rev. B 92, 104411 (2015); arXiv:1506.04837
Stochastic approximation of dynamical exponent at quantum critical point
NASA Astrophysics Data System (ADS)
Yasuda, Shinya; Suwa, Hidemaro; Todo, Synge
2015-09-01
We have developed a unified finite-size scaling method for quantum phase transitions that requires no prior knowledge of the dynamical exponent z . During a quantum Monte Carlo simulation, the temperature is automatically tuned by the Robbins-Monro stochastic approximation method, being proportional to the lowest gap of the finite-size system. The dynamical exponent is estimated in a straightforward way from the system-size dependence of the temperature. As a demonstration of our novel method, the two-dimensional S =1 /2 quantum X Y model in uniform and staggered magnetic fields is investigated in the combination of the world-line quantum Monte Carlo worm algorithm. In the absence of a uniform magnetic field, we obtain the fully consistent result with the Lorentz invariance at the quantum critical point, z =1 , i.e., the three-dimensional classical X Y universality class. Under a finite uniform magnetic field, on the other hand, the dynamical exponent becomes two, and the mean-field universality with effective dimension (2 +2 ) governs the quantum phase transition.
Local dynamic nuclear polarization using quantum point contacts
Wald, K.R.; Kouwenhoven, L.P.; McEuen, P.L. ); van der Vaart, N.C. ); Foxon, C.T. )
1994-08-15
We have used quantum point contacts (QPCs) to locally create and probe dynamic nuclear polarization (DNP) in GaAs heterostructures in the quantum Hall regime. DNP is created via scattering between spin-polarized Landau level electrons and the Ga and As nuclear spins, and it leads to hysteresis in the dc transport characteristics. The nuclear origin of this hysteresis is demonstrated by nuclear magnetic resonance (NMR). Our results show that QPCs can be used to create and probe local nuclear spin populations, opening up new possibilities for mesoscopic NMR experiments.
Experimental consequences of quantum critical points at high temperatures
NASA Astrophysics Data System (ADS)
Freitas, D. C.; Rodière, P.; Núñez, M.; Garbarino, G.; Sulpice, A.; Marcus, J.; Gay, F.; Continentino, M. A.; Núñez-Regueiro, M.
2015-11-01
We study the C r1 -xR ex phase diagram finding that its phase transition temperature towards an antiferromagnetic order TN follows a quantum [(xc-x ) /xc ] ψ law, with ψ =1 /2 , from the quantum critical point (QCP) at xc=0.25 up to TN≈600 K . We compare this system to others in order to understand why this elemental material is affected by the QCP up to such unusually high temperatures. We determine a general criterion for the crossover, as a function of an external parameter such as concentration, from the region controlled solely by thermal fluctuations to that where quantum effects become observable. The properties of materials with low coherence lengths will thus be altered far away from the QCP.
NASA Astrophysics Data System (ADS)
Kim, Song-Hyok; Kang, Chol-Jin; Kim, Yon-Il; Kim, Kwang-Hyon
2015-05-01
We consider a triple quantum dot system in a triangular geometry with one of the dots connected to metallic leads. We investigate quantum phase transition between local moment phase and Kondo screened strong coupling phase in triple quantum dots where energy levels of dots are deviated from the particle-hole symmetric point. The effect of on-site energy of dots on quantum phase transition between local moment phase and Kondo screened strong coupling phase in triple quantum dots is studied based on the analytical arguments and the numerical renormalization group method. The results show that the critical value of tunnel coupling between side dots decreases when the energy level of embedded dot rises up from the symmetric point to the Fermi level and the critical value increases when the energy levels of two side dots rise up. The study of the influence of on-site-energy changes on the quantum phase transitions in triple quantum dots has the importance for clarifying the mechanism of Kondo screening in triple quantum dots where energy levels of dots are deviated from the particle-hole symmetric point.
Intact quasiparticles at an unconventional quantum critical point
NASA Astrophysics Data System (ADS)
Sutherland, M. L.; O'Farrell, E. C. T.; Toews, W. H.; Dunn, J.; Kuga, K.; Nakatsuji, S.; Machida, Y.; Izawa, K.; Hill, R. W.
2015-07-01
We report measurements of in-plane electrical and thermal transport properties in the limit T →0 near the unconventional quantum critical point in the heavy-fermion metal β -YbAlB4 . The high Kondo temperature TK≃200 K in this material allows us to probe transport extremely close to the critical point, at unusually small values of T /TK<5 ×10-4 . Here we find that the Wiedemann-Franz law is obeyed at the lowest temperatures, implying that the Landau quasiparticles remain intact in the critical region. At finite temperatures we observe a non-Fermi-liquid T -linear dependence of inelastic-scattering processes to energies lower than those previously accessed. These processes have a weaker temperature dependence than in comparable heavy fermion quantum critical systems, revealing a temperature scale of T ˜0.3 K which signals a sudden change in the character of the inelastic scattering.
Fixed point structure of quenched, planar quantum electrodynamics
Love, S.T.
1986-07-01
Gauge theories exhibiting a hierarchy of fermion mass scales may contain a pseudo-Nambu-Boldstone boson of spontaneously broken scale invariance. The relation between scale and chiral symmetry breaking is studied analytically in quenched, planar quantum electrodynamics in four dimensions. The model possesses a novel nonperturbative ultraviolet fixed point governing its strong coupling phase which requires the mixing of four fermion operators. 12 refs.
Mode Specific Backscattering in a Quantum Point Contact.
Kozikov, A A; Steinacher, R; Rössler, C; Ihn, T; Ensslin, K; Reichl, C; Wegscheider, W
2015-12-01
We demonstrate a scanning gate grid measurement technique consisting in measuring the conductance of a quantum point contact (QPC) as a function of gate voltage at each tip position. Unlike conventional scanning gate experiments, it allows investigating QPC conductance plateaus affected by the tip at these positions. We compensate the capacitive coupling of the tip to the QPC and discover that interference fringes coexist with distorted QPC plateaus. We spatially resolve the mode structure for each plateau. PMID:26569040
Komnik, A; Saleur, H
2011-09-01
We verify the validity of the Cohen-Gallavotti fluctuation theorem for the strongly correlated problem of charge transfer through an impurity in a chiral Luttinger liquid, which is realizable experimentally as a quantum point contact in a fractional quantum Hall edge state device. This is accomplished via the development of an analytical method to calculate the full counting statistics of the problem in all the parameter regimes involving the temperature, the Hall voltage, and the gate voltage. PMID:21981487
Gate-Defined Graphene Quantum Point Contact in the Quantum Hall Regime
NASA Astrophysics Data System (ADS)
Nakaharai, S.; Williams, J. R.; Marcus, C. M.
2011-07-01
We investigate transport in a gate-defined graphene quantum point contact in the quantum Hall regime. Edge states confined to the interface of p and n regions in the graphene sheet are controllably brought together from opposite sides of the sample and allowed to mix in this split-gate geometry. Among the expected quantum Hall features, an unexpected additional plateau at 0.5h/e2 is observed. We propose that chaotic mixing of edge channels gives rise to the extra plateau.
Quantum percolation and transition point of a directed discrete-time quantum walk
Chandrashekar, C. M.; Busch, Th.
2014-01-01
Quantum percolation describes the problem of a quantum particle moving through a disordered system. While certain similarities to classical percolation exist, the quantum case has additional complexity due to the possibility of Anderson localisation. Here, we consider a directed discrete-time quantum walk as a model to study quantum percolation of a two-state particle on a two-dimensional lattice. Using numerical analysis we determine the fraction of connected edges required (transition point) in the lattice for the two-state particle to percolate with finite (non-zero) probability for three fundamental lattice geometries, finite square lattice, honeycomb lattice, and nanotube structure and show that it tends towards unity for increasing lattice sizes. To support the numerical results we also use a continuum approximation to analytically derive the expression for the percolation probability for the case of the square lattice and show that it agrees with the numerically obtained results for the discrete case. Beyond the fundamental interest to understand the dynamics of a two-state particle on a lattice (network) with disconnected vertices, our study has the potential to shed light on the transport dynamics in various quantum condensed matter systems and the construction of quantum information processing and communication protocols. PMID:25301394
Quantum percolation and transition point of a directed discrete-time quantum walk.
Chandrashekar, C M; Busch, Th
2014-01-01
Quantum percolation describes the problem of a quantum particle moving through a disordered system. While certain similarities to classical percolation exist, the quantum case has additional complexity due to the possibility of Anderson localisation. Here, we consider a directed discrete-time quantum walk as a model to study quantum percolation of a two-state particle on a two-dimensional lattice. Using numerical analysis we determine the fraction of connected edges required (transition point) in the lattice for the two-state particle to percolate with finite (non-zero) probability for three fundamental lattice geometries, finite square lattice, honeycomb lattice, and nanotube structure and show that it tends towards unity for increasing lattice sizes. To support the numerical results we also use a continuum approximation to analytically derive the expression for the percolation probability for the case of the square lattice and show that it agrees with the numerically obtained results for the discrete case. Beyond the fundamental interest to understand the dynamics of a two-state particle on a lattice (network) with disconnected vertices, our study has the potential to shed light on the transport dynamics in various quantum condensed matter systems and the construction of quantum information processing and communication protocols. PMID:25301394
Point form relativistic quantum mechanics and relativistic SU(6)
NASA Technical Reports Server (NTRS)
Klink, W. H.
1993-01-01
The point form is used as a framework for formulating a relativistic quantum mechanics, with the mass operator carrying the interactions of underlying constituents. A symplectic Lie algebra of mass operators is introduced from which a relativistic harmonic oscillator mass operator is formed. Mass splittings within the degenerate harmonic oscillator levels arise from relativistically invariant spin-spin, spin-orbit, and tensor mass operators. Internal flavor (and color) symmetries are introduced which make it possible to formulate a relativistic SU(6) model of baryons (and mesons). Careful attention is paid to the permutation symmetry properties of the hadronic wave functions, which are written as polynomials in Bargmann spaces.
Effects of dissipation on a quantum critical point with disorder.
Hoyos, José A; Kotabage, Chetan; Vojta, Thomas
2007-12-01
We study the effects of dissipation on a disordered quantum phase transition with O(N) order-parameter symmetry by applying a strong-disorder renormalization group to the Landau-Ginzburg-Wilson field theory of the problem. We find that Ohmic dissipation results in a nonperturbative infinite-randomness critical point with unconventional activated dynamical scaling while super-Ohmic damping leads to conventional behavior. We discuss applications to the superconductor-metal transition in nanowires and to the Hertz theory of the itinerant antiferromagnetic transition. PMID:18233349
Instability of the Quantum-Critical Point of Itinerant Ferromagnets
NASA Astrophysics Data System (ADS)
Chubukov, Andrey V.; Pépin, Catherine; Rech, Jerome
2004-04-01
We study the stability of the quantum-critical point for itinerant ferromagnets commonly described by the Hertz-Millis-Moriya (HMM) theory. We argue that in D≤3 long-range spatial correlations associated with the Landau damping of the order parameter field generate a universal negative, nonanalytic |q|(D+1)/2 contribution to the static magnetic susceptibility χs(q,0), which makes HMM theory unstable. We argue that the actual transition is either towards incommensurate ordering, or first order. We also show that singular corrections are specific to the spin problem, while charge susceptibility remains analytic at criticality.
Detection of Majorana Kramers Pairs Using a Quantum Point Contact.
Li, Jian; Pan, Wei; Bernevig, B Andrei; Lutchyn, Roman M
2016-07-22
We propose a setup that integrates a quantum point contact (QPC) and a Josephson junction on a quantum spin Hall sample, experimentally realizable in InAs/GaSb quantum wells. The confinement due to both the QPC and the superconductor results in a Kramers pair of Majorana zero-energy bound states when the superconducting phases in the two arms differ by an odd multiple of π across the Josephson junction. We investigate the detection of these Majorana pairs with the integrated QPC, and find a robust switching from normal to Andreev scattering across the edges due to the presence of Majorana Kramers pairs. Such a switching of the current represents a qualitative signature where multiterminal differential conductances oscillate with alternating signs when the external magnetic field is tuned. We show that this qualitative signature is also present in current cross-correlations. Thus, the change of the backscattering current nature affects both conductance and shot noise, the measurement of which offers a significant advantage over quantitative signatures such as conductance quantization in realistic measurements. PMID:27494493
Detection of Majorana Kramers Pairs Using a Quantum Point Contact
NASA Astrophysics Data System (ADS)
Li, Jian; Pan, Wei; Bernevig, B. Andrei; Lutchyn, Roman M.
2016-07-01
We propose a setup that integrates a quantum point contact (QPC) and a Josephson junction on a quantum spin Hall sample, experimentally realizable in InAs/GaSb quantum wells. The confinement due to both the QPC and the superconductor results in a Kramers pair of Majorana zero-energy bound states when the superconducting phases in the two arms differ by an odd multiple of π across the Josephson junction. We investigate the detection of these Majorana pairs with the integrated QPC, and find a robust switching from normal to Andreev scattering across the edges due to the presence of Majorana Kramers pairs. Such a switching of the current represents a qualitative signature where multiterminal differential conductances oscillate with alternating signs when the external magnetic field is tuned. We show that this qualitative signature is also present in current cross-correlations. Thus, the change of the backscattering current nature affects both conductance and shot noise, the measurement of which offers a significant advantage over quantitative signatures such as conductance quantization in realistic measurements.
Coherent tunnelling across a quantum point contact in the quantum Hall regime.
Martins, F; Faniel, S; Rosenow, B; Sellier, H; Huant, S; Pala, M G; Desplanque, L; Wallart, X; Bayot, V; Hackens, B
2013-01-01
The unique properties of quantum hall devices arise from the ideal one-dimensional edge states that form in a two-dimensional electron system at high magnetic field. Tunnelling between edge states across a quantum point contact (QPC) has already revealed rich physics, like fractionally charged excitations, or chiral Luttinger liquid. Thanks to scanning gate microscopy, we show that a single QPC can turn into an interferometer for specific potential landscapes. Spectroscopy, magnetic field and temperature dependences of electron transport reveal a quantitatively consistent interferometric behavior of the studied QPC. To explain this unexpected behavior, we put forward a new model which relies on the presence of a quantum Hall island at the centre of the constriction as well as on different tunnelling paths surrounding the island, thereby creating a new type of interferometer. This work sets the ground for new device concepts based on coherent tunnelling. PMID:23475303
Coherent tunnelling across a quantum point contact in the quantum Hall regime
Martins, F.; Faniel, S.; Rosenow, B.; Sellier, H.; Huant, S.; Pala, M. G.; Desplanque, L.; Wallart, X.; Bayot, V.; Hackens, B.
2013-01-01
The unique properties of quantum hall devices arise from the ideal one-dimensional edge states that form in a two-dimensional electron system at high magnetic field. Tunnelling between edge states across a quantum point contact (QPC) has already revealed rich physics, like fractionally charged excitations, or chiral Luttinger liquid. Thanks to scanning gate microscopy, we show that a single QPC can turn into an interferometer for specific potential landscapes. Spectroscopy, magnetic field and temperature dependences of electron transport reveal a quantitatively consistent interferometric behavior of the studied QPC. To explain this unexpected behavior, we put forward a new model which relies on the presence of a quantum Hall island at the centre of the constriction as well as on different tunnelling paths surrounding the island, thereby creating a new type of interferometer. This work sets the ground for new device concepts based on coherent tunnelling. PMID:23475303
Scanning gate spectroscopy of a quantum Hall island near a quantum point contact
NASA Astrophysics Data System (ADS)
Hackens, Benoit; Martins, Frederico; Faniel, Sebastien; Bayot, Vincent; Rosenow, Bernd; Desplanque, Ludovic; Wallart, Xavier; Pala, Marco; Sellier, Hermann; Huant, Serge
2013-03-01
We report on low temperature (100 mK) scanning gate experiments performed at high magnetic field (around 10 T) on a mesoscopic device patterned in an InGaAs/InAlAs heterostructure. Magnetotransport measurements yield signatures of ultra-small Quantum Hall Islands (QHI) formed by closed quantum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to locate and probe a single QHI near a quantum point contact. The presence of Coulomb diamonds in the local spectroscopy confirms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias across the device, we uncover the QHI discrete energy spectrum arising from electronic confinement and we extract estimates of the gradient of the confining potential and of the edge state velocity.
NASA Astrophysics Data System (ADS)
Bhole, Gaurav; Anjusha, V. S.; Mahesh, T. S.
2016-04-01
A robust control over quantum dynamics is of paramount importance for quantum technologies. Many of the existing control techniques are based on smooth Hamiltonian modulations involving repeated calculations of basic unitaries resulting in time complexities scaling rapidly with the length of the control sequence. Here we show that bang-bang controls need one-time calculation of basic unitaries and hence scale much more efficiently. By employing a global optimization routine such as the genetic algorithm, it is possible to synthesize not only highly intricate unitaries, but also certain nonunitary operations. We demonstrate the unitary control through the implementation of the optimal fixed-point quantum search algorithm in a three-qubit nuclear magnetic resonance (NMR) system. Moreover, by combining the bang-bang pulses with the crusher gradients, we also demonstrate nonunitary transformations of thermal equilibrium states into effective pure states in three- as well as five-qubit NMR systems.
Entanglement entropy of 2D conformal quantum critical points: hearing the shape of a quantum drum.
Fradkin, Eduardo; Moore, Joel E
2006-08-01
The entanglement entropy of a pure quantum state of a bipartite system A union or logical sumB is defined as the von Neumann entropy of the reduced density matrix obtained by tracing over one of the two parts. In one dimension, the entanglement of critical ground states diverges logarithmically in the subsystem size, with a universal coefficient that for conformally invariant critical points is related to the central charge of the conformal field theory. We find that the entanglement entropy of a standard class of z=2 conformal quantum critical points in two spatial dimensions, in addition to a nonuniversal "area law" contribution linear in the size of the AB boundary, generically has a universal logarithmically divergent correction, which is completely determined by the geometry of the partition and by the central charge of the field theory that describes the critical wave function. PMID:17026083
Quantum Density of Probability at the Classical Peculiar Point
NASA Astrophysics Data System (ADS)
Buonanno, L.; Renna, M.; Pavlotsky, I. P.
The so-called no-interaction theorem of D.G. Currie, T.F. Jordan, E.C. Sudarshan, H. Leutwyler, G. Marmo and N. Mukunda makes it possible to construct relativistic quasi-classical particle dynamics in the post-Galilean approximation only.1-4 In this approximation the Lagrangians are singular on some surfaces of the phase space. The dynamical properties are essentially peculiar on the singular surfaces.5-8 In the particular case of the rectilinear motion of two electrons the peculiar point appears when the distance between the particles r=r0, where r0=e2/mc2 (the so-called “radius of an electron”). Here m and e are respectively the mass and the charge of the electron, c is the speed of light. In this paper it is shown that in the simple case of a one-dimensional system of two electrons with the symmetrical initial condition v1=-v2 (v1 and v2 are the velocities of the particles), the density of probability tends to zero when the distance between electrons tends to r0. In other words, the point of the classical phase-space, which cannot be crossed by the trajectory of the system, reflects at the point where the corresponding quantum system has the vanishing probability.
Quantum Hall Effect near the charge neutrality point in graphene
NASA Astrophysics Data System (ADS)
Leon, Jorge; Gusev, Guennadii; Plentz, Flavio
2013-03-01
The Quantum Hall effect (QHE) of a two-dimensional (2D) electron gas in a strong magnetic field is one of the most fascinating quantum phenomena discovered in condensed matter physics. In this work we propose to study the transport properties of the single layer and bilayer of graphene at the charge neutrality point (CNP) and compare it with random magnetic model developed in theoretical papers in which we argue that at CNP graphene layer is still inhomogeneous, very likely due to random potential of impurities. The random potential fluctuations induce smooth fluctuations in the local filling factor around ν = 0. In this case the transport is determined by special class of trajectories, ``the snake states'', propagating along contour ν = 0. The situation is very similar to the transport of a two-dimensional particles moving in a spatially modulated random magnetic field with zero mean value. We especially emphasize that our results may be equally relevant to the composite fermions description of the half-filled Landau level. The authors thank to CNPq and FAPESP for financial support for this work.
Observation of conductance doubling in an Andreev quantum point contact
NASA Astrophysics Data System (ADS)
Kjaergaard, M.; Nichele, F.; Suominen, H.; Nowak, M.; Wimmer, M.; Akhmerov, A.; Folk, J.; Flensberg, K.; Shabani, J.; Palmstrom, C.; Marcus, C.
One route to study the non-Abelian nature of excitations in topological superconductors is to realise gateable two dimensional (2D) semiconducting systems, with spin-orbit coupling in proximity to an s-wave superconductor. Previous work on coupling 2D electron gases (2DEG) with superconductors has been hindered by a non-ideal interface and unstable gateability. We report measurements on a gateable 2DEG coupled to superconductors through a pristine interface, and use aluminum grown in situ epitaxially on an InGaAs/InAs electron gas. We demonstrate quantization in units of 4e2 / h in a quantum point contact (QPC) in such hybrid systems. Operating the QPC as a tunnel probe, we observe a hard superconducting gap, overcoming the soft-gap problem in 2D superconductor/semiconductor systems. Our work paves way for a new and highly scalable system in which to pursue topological quantum information processing. Research supported by Microsoft Project Q and the Danish National Research Foundation.
Oscillating fidelity susceptibility near a quantum multicritical point
Mukherjee, Victor; Dutta, Amit; Polkovnikov, Anatoli
2011-02-15
We study scaling behavior of the geometric tensor {chi}{sub {alpha},{beta}}({lambda}{sub 1},{lambda}{sub 2}) and the fidelity susceptibility {chi}{sub F} in the vicinity of a quantum multicritical point (MCP) using the example of a transverse XY model. We show that the behavior of the geometric tensor (and thus of {chi}{sub F}) is drastically different from that seen near a critical point. In particular, we find that it is a highly nonmonotonic function of {lambda} along the generic direction {lambda}{sub 1}{approx}{lambda}{sub 2}={lambda} when the system size L is bounded by the shorter and longer correlation lengths characterizing the MCP: 1/|{lambda}|{sup {nu}{sub 1}}<
Practical Point-to-Point Free-Space Quantum Key Distribution over 1/2 KM
Buttler, W.T.; Hughes, R.J.; Kwiat, P.G.; Lamoreaux, S.K.; Morgan, G.L.; Peterson, C.G.
1999-02-01
We have demonstrated point-to-point single-photon quantum key distribution (QKD) over a free-space optical path of {approximately}475 m under daylight conditions. This represents an increase of >1,000 times farther than any reported point-to-point demonstration, and >6 times farther than the previous folded path daylight demonstration. We expect to extend the daylight range to 2 km or more within the next few months. A brief description of the system is given here. The QKD transmitter, a.k.a. ''Alice'' (Fig. 1), consists of three thermoelectrically cooled diode lasers, a single interference filter (IF), two optical attenuators, two linear polarizers, two non-polarization beam-splitters (BSs), and a 27x beam expander. The two data-lasers' (dim-lasers') wavelengths are temperature controlled and constrained by the IF to {approximately}773 {+-} 0.5 nm, while the transmitted wavelength of the bright-laser (timing-laser) is {approximately}768 nm; the data-lasers are configured to emit a weak pulse of approximately 1 ns duration. The transmitter incorporates no active polarization switching--a first in QKD.
Petkewich, M.D.; Vroblesky, D.A.; Robertson, J.F.; Bradley, P.M.
1997-01-01
A 9-year scientific investigation to determine the potential for biore-mediation of ground-water contamination and to monitor the effectiveness of an engineered bioremediation system located at the Defense Fuel Supply Point and adjacent properties in Hanahan, S.C., has culminated in the collection of abundant water-quality and water-level data.This report presents the analytical results of the study that monitored the changes in surface- and ground-water quality and water-table elevations in the study area from December 1990 to January 1996. This report also presents analytical results of lake-bottom sediments collected in the study area.
Graphene Quantum Point Contact Transistor for DNA Sensing
NASA Astrophysics Data System (ADS)
Girdhar, Anuj
2014-03-01
Over the past few years the need has grown for low-cost, high-speed, and accurate biomolecule sensing technology. Graphene is a promising choice for use in such sensing applications, as its single-atom thickness and unique electronic structure is suitable for probing biomolecules like DNA at a very high resolution. We propose the design of a transistor containing a graphene nanoribbon sensing layer with a nanopore for the simultaneous detection and control of a translocating DNA molecule. Through the combination of molecular dynamics simulations, a self-consistent Poisson equation solver, and electronic transport theory, we show that the motion of a DNA molecule through a nanopore can be observed by measuring conductance modulations in the graphene nanoribbon. We also demonstrate that the sensitivity of the graphene sheet conductance to external charges can be enhanced by modulating its carrier concentration as well as by choosing a quantum point contact geometry for the graphene nanoribbon. In addition, we propose the use of extra gates to control both the lateral and translocating motion of a DNA molecule inside the nanopore. I would like to acknowledge Oxford Nanopore Technology as well as the Beckman Institute for Advanced Science for their support.
Probing dopants in wide semiconductor quantum point contacts.
Yakimenko, I I; Berggren, K-F
2016-03-16
Effects of randomly distributed impurities on conductance, spin polarization and electron localization in realistic gated semiconductor quantum point contacts (QPCs) have been simulated numerically. To this end density functional theory in the local spin-density approximation has been used. In the case when the donor layer is embedded far from the two-dimensional electron gas (2DEG) the electrostatic confinement potential exhibits the conventional parabolic form, and thus the usual ballistic transport phenomena take place both in the devices with split gates alone and with an additional metallic gate on the top. In the opposite case, i.e. when the randomly distributed donors are placed not far away from the 2DEG layer, there are drastic changes like the localization of electrons in the vicinity of confinement potential minima which give rise to fluctuations in conductance and resonances. The conductance as a function of the voltage applied to the top gate for asymmetrically charged split gates has been calculated. In this case resonances in conductance caused by randomly distributed donors are shifted and decrease in amplitude while the anomalies caused by interaction effects remain unmodified. It has been also shown that for a wide QPC the polarization can appear in the form of stripes. The importance of partial ionization of the random donors and the possibility of short range order among the ionized donors are emphasized. The motivation for this work is to critically evaluate the nature of impurities and how to guide the design of high-mobility devices. PMID:26885626
Avoided ferromagnetic quantum critical point in CeRuPO
NASA Astrophysics Data System (ADS)
Lengyel, E.; Macovei, M. E.; Jesche, A.; Krellner, C.; Geibel, C.; Nicklas, M.
2015-01-01
CeRuPO is a rare example of a ferromagnetic (FM) Kondo-lattice system. External pressure suppresses the ordering temperature to zero at about pc≈3 GPa. Our ac-susceptibility and electrical-resistivity investigations evidence that the type of magnetic ordering changes from FM to antiferromagnetic (AFM) at about p*≈0.87 GPa . Studies in applied magnetic fields suggest that ferromagnetic and antiferromagnetic correlations compete for the ground state at p >p* , but finally the AFM correlations win. The change in the magnetic ground-state properties is closely related to the pressure evolution of the crystalline-electric-field level scheme and the magnetic Ruderman-Kittel-Kasuya-Yosida exchange interaction. The Néel temperature disappears abruptly in a first-order-like fashion at pc, hinting at the absence of a quantum critical point. This is consistent with the low-temperature transport properties exhibiting Landau-Fermi-liquid behavior in the whole investigated pressure range up to 7.5 GPa.
Kim, Y H; Kaur, N; Atkins, B M; Dalal, N S; Takano, Y
2009-12-11
At a quantum critical point (QCP)--a zero-temperature singularity in which a line of continuous phase transition terminates--quantum fluctuations diverge in space and time, leading to exotic phenomena that can be observed at nonzero temperatures. Using a quantum antiferromagnet, we present calorimetric evidence that nuclear spins frozen in a high-temperature nonequilibrium state by temperature quenching are annealed by quantum fluctuations near the QCP. This phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, serves as an excellent marker of a quantum critical region around the QCP and provides a probe of the dynamics of the divergent quantum fluctuations. PMID:20366226
Quantum Hall effect in a quantum point contact at Landau filling fraction ν=52
NASA Astrophysics Data System (ADS)
Miller, Jeffrey; Radu, Iuliana; Zumbühl, Dominik; Levenson-Falk, Eli; Kastner, Marc; Marcus, Charles; Pfeiffer, Loren; West, Ken
2007-03-01
We study the transport properties of quantum point contacts (QPC) fabricated on a GaAs/AlGaAs two dimensional electron gas that exhibits excellent bulk fractional quantum Hall effect, including a strong plateau in the Hall resistance at Landau level filling fraction ν= 52. We find that the ν=52 plateau is identifiable in point contacts with lithographic separations as small as 0.8 microns, but is not present in a 0.5 micron QPC. We study the temperature and dc-current-bias dependence of the ν=52 plateau---as well as neighboring fractional and integer plateaus---in the QPC. We also discuss our method to study the QPC at one filling fraction while the bulk remains at a higher filling fraction. Research supported in part by Microsoft Corporation, Project Q, and HCRP at Harvard University, and ARO (W911NF-05-1-0062), the NSEC program of the NSF (PHY-0117795) and NSF (DMR-0353209) at MIT.
Universal Entanglement Entropy in 2D Conformal Quantum Critical Points
Hsu, Benjamin; Mulligan, Michael; Fradkin, Eduardo; Kim, Eun-Ah
2008-12-05
We study the scaling behavior of the entanglement entropy of two dimensional conformal quantum critical systems, i.e. systems with scale invariant wave functions. They include two-dimensional generalized quantum dimer models on bipartite lattices and quantum loop models, as well as the quantum Lifshitz model and related gauge theories. We show that, under quite general conditions, the entanglement entropy of a large and simply connected sub-system of an infinite system with a smooth boundary has a universal finite contribution, as well as scale-invariant terms for special geometries. The universal finite contribution to the entanglement entropy is computable in terms of the properties of the conformal structure of the wave function of these quantum critical systems. The calculation of the universal term reduces to a problem in boundary conformal field theory.
Quantum-ring spin interference device tuned by quantum point contacts
Diago-Cisneros, Leo; Mireles, Francisco
2013-11-21
We introduce a spin-interference device that comprises a quantum ring (QR) with three embedded quantum point contacts (QPCs) and study theoretically its spin transport properties in the presence of Rashba spin-orbit interaction. Two of the QPCs conform the lead-to-ring junctions while a third one is placed symmetrically in the upper arm of the QR. Using an appropriate scattering model for the QPCs and the S-matrix scattering approach, we analyze the role of the QPCs on the Aharonov-Bohm (AB) and Aharonov-Casher (AC) conductance oscillations of the QR-device. Exact formulas are obtained for the spin-resolved conductances of the QR-device as a function of the confinement of the QPCs and the AB/AC phases. Conditions for the appearance of resonances and anti-resonances in the spin-conductance are derived and discussed. We predict very distinctive variations of the QR-conductance oscillations not seen in previous QR proposals. In particular, we find that the interference pattern in the QR can be manipulated to a large extend by varying electrically the lead-to-ring topological parameters. The latter can be used to modulate the AB and AC phases by applying gate voltage only. We have shown also that the conductance oscillations exhibits a crossover to well-defined resonances as the lateral QPC confinement strength is increased, mapping the eigenenergies of the QR. In addition, unique features of the conductance arise by varying the aperture of the upper-arm QPC and the Rashba spin-orbit coupling. Our results may be of relevance for promising spin-orbitronics devices based on quantum interference mechanisms.
Fermionic quantum critical point of spinless fermions on a honeycomb lattice
NASA Astrophysics Data System (ADS)
Wang, Lei; Corboz, Philippe; Troyer, Matthias
2014-10-01
Spinless fermions on a honeycomb lattice provide a minimal realization of lattice Dirac fermions. Repulsive interactions between nearest neighbors drive a quantum phase transition from a Dirac semimetal to a charge-density-wave state through a fermionic quantum critical point, where the coupling of the Ising order parameter to the Dirac fermions at low energy drastically affects the quantum critical behavior. Encouraged by a recent discovery (Huffman and Chandrasekharan 2014 Phys. Rev. B 89 111101) of the absence of the fermion sign problem in this model, we study the fermionic quantum critical point using the continuous-time quantum Monte Carlo method with a worm-sampling technique. We estimate the transition point V/t=1.356(1) with the critical exponents ν =0.80(3) and η =0.302(7). Compatible results for the transition point are also obtained with infinite projected entangled-pair states.
Nonlinear I-V Curve at a Quantum Impurity Quantum Critical Point
NASA Astrophysics Data System (ADS)
Baranger, Harold; Chung, Chung-Hou; Lin, Chao-Yun; Zhang, Gu; Ke, Chung-Ting; Finkelstein, Gleb
The nonlinear I-V curve at an interacting quantum critical point (QCP) is typically out of reach theoretically. Here, however, we provide a striking example of an analytical calculation of the full nonlinear I-V curve at the QCP. The system that we consider is a quantum dot coupled to resistive leads - a spinless resonant level interacting with an ohmic EM environment in which a QCP similar to the two-channel Kondo QCP occurs. Recent experiments studied this criticality via transport measurements: the transmission approaches unity at low temperature and applied bias when tuned exactly to the QCP (on resonance and symmetric tunnel barriers) and approaches zero in all other cases. To obtain the current at finite temperature and arbitrary bias, we write the problem as a one-dimensional field theory and transform from electrons in the left/right leads to right-going and left-going channels between which there is weak two-body backscattering. Drawing on dynamical Coulomb blockade theory, we thus obtain an analytical expression for the full I-V curve. The agreement with the experimental result is remarkable.
Quantum Monte Carlo calculations for point defects in semiconductors
NASA Astrophysics Data System (ADS)
Hennig, Richard
2010-03-01
Point defects in silicon have been studied extensively for many years. Nevertheless the mechanism for self diffusion in Si is still debated. Direct experimental measurements of the selfdiffusion in silicon are complicated by the lack of suitable isotopes. Formation energies are either obtained from theory or indirectly through the analysis of dopant and metal diffusion experiments. Density functional calculations predict formation energies ranging from 3 to 5 eV depending on the approximations used for the exchange-correlation functional [1]. Analysis of dopant and metal diffusion experiments result in similar broad range of diffusion activation energies of 4.95 [2], 4.68 [3], 2.4 eV [4]. Assuming a migration energy barrier of 0.1-0.3 eV [5], the resulting experimental interstitial formation energies range from 2.1 - 4.9 eV. To answer the question of the formation energy of Si interstitials we resort to a many-body description of the wave functions using quantum Monte Carlo (QMC) techniques. Previous QMC calculations resulted in formation energies for the interstitials of around 5 eV [1,6]. We present a careful analysis of all the controlled and uncontrolled approximations that affect the defect formation energies in variational and diffusion Monte Carlo calculations. We find that more accurate trial wave functions for QMC using improved Jastrow expansions and most importantly a backflow transformation for the electron coordinates significantly improve the wave functions. Using zero-variance extrapolation, we predict interstitial formation energies in good agreement with hybrid DFT functionals [1] and recent GW calculations [7]. [4pt] [1] E. R. Batista, J. Heyd, R. G. Hennig, B. P. Uberuaga, R. L. Martin, G. E. Scuseria, C. J. Umrigar, and J. W. Wilkins. Phys. Rev. B 74, 121102(R) (2006).[0pt] [2] H. Bracht, E. E. Haller, and R. Clark-Phelps, Phys. Rev. Lett. 81, 393 (1998). [0pt] [3] A. Ural, P. B. Griffin, and J. D. Plummer, Phys. Rev. Lett. 83, 3454 (1999). [0pt
Thermal Ising transitions in the vicinity of two-dimensional quantum critical points
NASA Astrophysics Data System (ADS)
Hesselmann, S.; Wessel, S.
2016-04-01
The scaling of the transition temperature into an ordered phase close to a quantum critical point as well as the order parameter fluctuations inside the quantum critical region provide valuable information about universal properties of the underlying quantum critical point. Here, we employ quantum Monte Carlo simulations to examine these relations in detail for two-dimensional quantum systems that exhibit a finite-temperature Ising-transition line in the vicinity of a quantum critical point that belongs to the universality class of either (i) the three-dimensional Ising model for the case of the quantum Ising model in a transverse magnetic field on the square lattice or (ii) the chiral Ising transition for the case of a half-filled system of spinless fermions on the honeycomb lattice with nearest-neighbor repulsion. While the first case allows large-scale simulations to assess the scaling predictions to a high precision in terms of the known values for the critical exponents at the quantum critical point, for the later case, we extract values of the critical exponents ν and η , related to the order parameter fluctuations, which we discuss in relation to other recent estimates from ground-state quantum Monte Carlo calculations as well as analytical approaches.
Quantum Theory from Observer's Mathematics Point of View
Khots, Dmitriy; Khots, Boris
2010-05-04
This work considers the linear (time-dependent) Schrodinger equation, quantum theory of two-slit interference, wave-particle duality for single photons, and the uncertainty principle in a setting of arithmetic, algebra, and topology provided by Observer's Mathematics, see [1]. Certain theoretical results and communications pertaining to these theorems are also provided.
Tuning inter-dot tunnel coupling of an etched graphene double quantum dot by adjacent metal gates
Wei, Da; Li, Hai-Ou; Cao, Gang; Luo, Gang; Zheng, Zhi-Xiong; Tu, Tao; Xiao, Ming; Guo, Guang-Can; Jiang, Hong-Wen; Guo, Guo-Ping
2013-01-01
Graphene double quantum dots (DQDs) open to use charge or spin degrees of freedom for storing and manipulating quantum information in this new electronic material. However, impurities and edge disorders in etched graphene nano-structures hinder the ability to control the inter-dot tunnel coupling, tC, the most important property of the artificial molecule. Here we report measurements of tC in an all-metal-side-gated graphene DQD. We find that tC can be controlled continuously about a factor of four by employing a single gate. Furthermore, tC, can be changed monotonically about another factor of four as electrons are gate-pumped into the dot one by one. The results suggest that the strength of tunnel coupling in etched graphene DQDs can be varied in a rather broad range and in a controllable manner, which improves the outlook to use graphene as a base material for qubit applications. PMID:24213723
Experimental observation of saddle points over the quantum control landscape of a two-spin system
NASA Astrophysics Data System (ADS)
Sun, Qiuyang; Pelczer, István; Riviello, Gregory; Wu, Re-Bing; Rabitz, Herschel
2015-04-01
The growing successes in performing quantum control experiments motivated the development of control landscape analysis as a basis to explain these findings. When a quantum system is controlled by an electromagnetic field, the observable as a functional of the control field forms a landscape. Theoretical analyses have predicted the existence of critical points over the landscapes, including saddle points with indefinite Hessians. This paper presents a systematic experimental study of quantum control landscape saddle points. Nuclear magnetic resonance control experiments are performed on a coupled two-spin system in a 13C-labeled chloroform (13CHCl3) sample. We address the saddles with a combined theoretical and experimental approach, measure the Hessian at each identified saddle point, and study how their presence can influence the search effort utilizing a gradient algorithm to seek an optimal control outcome. The results have significance beyond spin systems, as landscape saddles are expected to be present for the control of broad classes of quantum systems.
The features of ballistic electron transport in a suspended quantum point contact
Shevyrin, A. A. Budantsev, M. V.; Bakarov, A. K.; Toropov, A. I.; Pogosov, A. G.; Ishutkin, S. V.; Shesterikov, E. V.
2014-05-19
A suspended quantum point contact and the effects of the suspension are investigated by performing identical electrical measurements on the same experimental sample before and after the suspension. In both cases, the sample demonstrates conductance quantization. However, the suspended quantum point contact shows certain features not observed before the suspension, namely, plateaus at the conductance values being non-integer multiples of the conductance quantum, including the “0.7-anomaly.” These features can be attributed to the strengthening of electron-electron interaction because of the electric field confinement within the suspended membrane. Thus, the suspended quantum point contact represents a one-dimensional system with strong electron-electron interaction.
Quantum Gravity from the Point of View of Locally Covariant Quantum Field Theory
NASA Astrophysics Data System (ADS)
Brunetti, Romeo; Fredenhagen, Klaus; Rejzner, Katarzyna
2016-08-01
We construct perturbative quantum gravity in a generally covariant way. In particular our construction is background independent. It is based on the locally covariant approach to quantum field theory and the renormalized Batalin-Vilkovisky formalism. We do not touch the problem of nonrenormalizability and interpret the theory as an effective theory at large length scales.
Area law for fixed points of rapidly mixing dissipative quantum systems
Brandão, Fernando G. S. L.; Cubitt, Toby S.; Lucia, Angelo; Michalakis, Spyridon; Perez-Garcia, David
2015-10-15
We prove an area law with a logarithmic correction for the mutual information for fixed points of local dissipative quantum system satisfying a rapid mixing condition, under either of the following assumptions: the fixed point is pure or the system is frustration free.
Athermal domain-wall creep near a ferroelectric quantum critical point
Kagawa, Fumitaka; Minami, Nao; Horiuchi, Sachio; Tokura, Yoshinori
2016-01-01
Ferroelectric domain walls are typically stationary because of the presence of a pinning potential. Nevertheless, thermally activated, irreversible creep motion can occur under a moderate electric field, thereby underlying rewritable and non-volatile memory applications. Conversely, as the temperature decreases, the occurrence of creep motion becomes less likely and eventually impossible under realistic electric-field magnitudes. Here we show that such frozen ferroelectric domain walls recover their mobility under the influence of quantum fluctuations. Nonlinear permittivity and polarization-retention measurements of an organic charge-transfer complex reveal that ferroelectric domain-wall creep occurs via an athermal process when the system is tuned close to a pressure-driven ferroelectric quantum critical point. Despite the heavy masses of material building blocks such as molecules, the estimated effective mass of the domain wall is comparable to the proton mass, indicating the realization of a ferroelectric domain wall with a quantum-particle nature near the quantum critical point. PMID:26880041
Athermal domain-wall creep near a ferroelectric quantum critical point.
Kagawa, Fumitaka; Minami, Nao; Horiuchi, Sachio; Tokura, Yoshinori
2016-01-01
Ferroelectric domain walls are typically stationary because of the presence of a pinning potential. Nevertheless, thermally activated, irreversible creep motion can occur under a moderate electric field, thereby underlying rewritable and non-volatile memory applications. Conversely, as the temperature decreases, the occurrence of creep motion becomes less likely and eventually impossible under realistic electric-field magnitudes. Here we show that such frozen ferroelectric domain walls recover their mobility under the influence of quantum fluctuations. Nonlinear permittivity and polarization-retention measurements of an organic charge-transfer complex reveal that ferroelectric domain-wall creep occurs via an athermal process when the system is tuned close to a pressure-driven ferroelectric quantum critical point. Despite the heavy masses of material building blocks such as molecules, the estimated effective mass of the domain wall is comparable to the proton mass, indicating the realization of a ferroelectric domain wall with a quantum-particle nature near the quantum critical point. PMID:26880041
Athermal domain-wall creep near a ferroelectric quantum critical point
NASA Astrophysics Data System (ADS)
Kagawa, Fumitaka; Minami, Nao; Horiuchi, Sachio; Tokura, Yoshinori
2016-02-01
Ferroelectric domain walls are typically stationary because of the presence of a pinning potential. Nevertheless, thermally activated, irreversible creep motion can occur under a moderate electric field, thereby underlying rewritable and non-volatile memory applications. Conversely, as the temperature decreases, the occurrence of creep motion becomes less likely and eventually impossible under realistic electric-field magnitudes. Here we show that such frozen ferroelectric domain walls recover their mobility under the influence of quantum fluctuations. Nonlinear permittivity and polarization-retention measurements of an organic charge-transfer complex reveal that ferroelectric domain-wall creep occurs via an athermal process when the system is tuned close to a pressure-driven ferroelectric quantum critical point. Despite the heavy masses of material building blocks such as molecules, the estimated effective mass of the domain wall is comparable to the proton mass, indicating the realization of a ferroelectric domain wall with a quantum-particle nature near the quantum critical point.
Quantum Transport of Disordered Weyl Semimetals at the Nodal Point
NASA Astrophysics Data System (ADS)
Sbierski, Björn; Pohl, Gregor; Bergholtz, Emil J.; Brouwer, Piet W.
2014-07-01
Weyl semimetals are paradigmatic topological gapless phases in three dimensions. We here address the effect of disorder on charge transport in Weyl semimetals. For a single Weyl node with energy at the degeneracy point and without interactions, theory predicts the existence of a critical disorder strength beyond which the density of states takes on a nonzero value. Predictions for the conductivity are divergent, however. In this work, we present a numerical study of transport properties for a disordered Weyl cone at zero energy. For weak disorder, our results are consistent with a renormalization group flow towards an attractive pseudoballistic fixed point with zero conductivity and a scale-independent conductance; for stronger disorder, diffusive behavior is reached. We identify the Fano factor as a signature that discriminates between these two regimes.
Quantum transport of disordered Weyl semimetals at the nodal point.
Sbierski, Björn; Pohl, Gregor; Bergholtz, Emil J; Brouwer, Piet W
2014-07-11
Weyl semimetals are paradigmatic topological gapless phases in three dimensions. We here address the effect of disorder on charge transport in Weyl semimetals. For a single Weyl node with energy at the degeneracy point and without interactions, theory predicts the existence of a critical disorder strength beyond which the density of states takes on a nonzero value. Predictions for the conductivity are divergent, however. In this work, we present a numerical study of transport properties for a disordered Weyl cone at zero energy. For weak disorder, our results are consistent with a renormalization group flow towards an attractive pseudoballistic fixed point with zero conductivity and a scale-independent conductance; for stronger disorder, diffusive behavior is reached. We identify the Fano factor as a signature that discriminates between these two regimes. PMID:25062216
Electron Phase Shift at the Zero-Bias Anomaly of Quantum Point Contacts
NASA Astrophysics Data System (ADS)
Brun, B.; Martins, F.; Faniel, S.; Hackens, B.; Cavanna, A.; Ulysse, C.; Ouerghi, A.; Gennser, U.; Mailly, D.; Simon, P.; Huant, S.; Bayot, V.; Sanquer, M.; Sellier, H.
2016-04-01
The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts.
Electron Phase Shift at the Zero-Bias Anomaly of Quantum Point Contacts.
Brun, B; Martins, F; Faniel, S; Hackens, B; Cavanna, A; Ulysse, C; Ouerghi, A; Gennser, U; Mailly, D; Simon, P; Huant, S; Bayot, V; Sanquer, M; Sellier, H
2016-04-01
The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts. PMID:27081995
Kondo phase shift at the zero-bias anomaly of quantum point contacts
NASA Astrophysics Data System (ADS)
Brun, Boris; Martins, Frederico; Faniel, Sébastien; Hackens, Benoit; Cavanna, Antonella; Ulysse, Christian; Ouerghi, Albdelkarim; Gennser, Ulf; Mailly, Dominique; Simon, Pascal; Huant, Serge; Bayot, Vincent; Sanquer, Marc; Sellier, Hermann
The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts.
Hybrid Quantum Point Contact-Superconductor Devices Using InSb Nanowires
NASA Astrophysics Data System (ADS)
Gill, Stephen; Damasco, John Jeffrey; Car, Diana; Bakkers, Erik; Mason, Nadya
Recent experiments using hybrid nanowire (NW)-superconductor (SC) devices have provided evidence for Majorana quasiparticles in tunneling experiments. However, these tunneling experiments are marked by a soft superconducting gap, which likely originates from disorder at the NW-SC interface. Hence, clean NW-SC interfaces are important for future Majorana studies. By carefully processing the NW-SC interface, we have realized quantized conductance steps in quantum point contacts fabricated from InSb NWs and superconducting contacts. We study the length dependence of ballistic behavior and the induced superconductivity in InSb NWs by quantum point contact spectroscopy. Additionally, we discuss how the transport in InSb NW-SC quantum point contacts evolves in magnetic field.
NASA Astrophysics Data System (ADS)
Liu, Bao; Zhang, Feng-Yang; Song, Jie; Song, He-Shan
2015-06-01
We propose a direct measurement scheme to read out the geometric phase of a coupled double quantum dot system via a quantum point contact(QPC) device. An effective expression of the geometric phase has been derived, which relates the geometric phase of the double quantum dot qubit to the current through QPC device. All the parameters in our expression are measurable or tunable in experiment. Moreover, since the measurement process affects the state of the qubit slightly, the geometric phase can be protected. The feasibility of the scheme has been analyzed. Further, as an example, we simulate the geometrical phase of a qubit when the QPC device is replaced by a single electron transistor(SET).
Doping-Induced Quantum Critical Point in an Itinerant Antiferromagnet TiAu
NASA Astrophysics Data System (ADS)
Santiago, Jessica; Svanidze, Eteri; Besara, Tiglet; Siegrist, Theo; Morosan, Emilia
The recently discovered itinerant magnet TiAu is the first antiferromagnet composed of non-magnetic constituents. The spin density wave ground state develops below TN ~36 K, about an order of magnitude smaller than in Cr. Achieving a quantum critical point in this material would provide a better understanding of weak itinerant antiferromagnets, while giving long sought-after insights into the effects of spin fluctuations in itinerant electron systems. While the application of pressure increases the ordering temperature TN, partial substitution of Ti provides an alternative avenue towards achieving a quantum critical point. The non-Fermi liquid behavior accompanies the quantum phase transition, as evidenced by the divergent specific heat coefficient and linear temperature dependence of the resistivity. The transition is accompanied by enhanced electron-electron correlations as well as strong spin-fluctuations, providing an experimental avenue for the verification of the self-consistent theory of spin fluctuations.
Spin current source based on a quantum point contact with local spin-orbit interaction
Nowak, M. P.; Szafran, B.
2013-11-11
Proposal for construction of a source of spin-polarized current based on quantum point contact (QPC) with local spin-orbit interaction is presented. We show that spin-orbit interaction present within the narrowing acts like a spin filter. The spin polarization of the current is discussed as a function of the Fermi energy and the width of the QPC.
Conductance oscillations in quantum point contacts of InAs/GaSb heterostructures
NASA Astrophysics Data System (ADS)
Papaj, Michał; Cywiński, Łukasz; Wróbel, Jerzy; Dietl, Tomasz
2016-05-01
We study quantum point contacts in two-dimensional topological insulators by means of quantum transport simulations for InAs/GaSb heterostructures and HgTe/(Hg,Cd)Te quantum wells. In InAs/GaSb, the density of edge states shows an oscillatory decay as a function of the distance to the edge. This is in contrast to the behavior of the edge states in HgTe quantum wells, which decay into the bulk in a simple exponential manner. The difference between the two materials is brought about by spatial separation of electrons and holes in InAs/GaSb, which affects the magnitudes of the parameters describing the particle-hole asymmetry and the strength of intersubband coupling within the Bernevig-Hughes-Zhang model. We show that the character of the wave-function decay impacts directly the dependence of the point contact conductance on the constriction width and the Fermi energy, which can be verified experimentally and serves to accurately determine the values of the relevant parameters. In the case of InAs/GaSb heterostructures, the conductance magnitude oscillates as a function of the constriction width following the oscillations of the edge state penetration, whereas in HgTe/(Hg,Cd)Te quantum wells a single switching from transmitting to reflecting contact is predicted.
Zero-point term and quantum effects in the Johnson noise of resistors: a critical appraisal
NASA Astrophysics Data System (ADS)
Kish, Laszlo B.; Niklasson, Gunnar A.; Granqvist, Claes G.
2016-05-01
There is a longstanding debate about the zero-point term in the Johnson noise voltage of a resistor. This term originates from a quantum-theoretical treatment of the fluctuation-dissipation theorem (FDT). Is the zero-point term really there, or is it only an experimental artifact, due to the uncertainty principle, for phase-sensitive amplifiers? Could it be removed by renormalization of theories? We discuss some historical measurement schemes that do not lead to the effect predicted by the FDT, and we analyse new features that emerge when the consequences of the zero-point term are measured via the mean energy and force in a capacitor shunting the resistor. If these measurements verify the existence of a zero-point term in the noise, then two types of perpetual motion machines can be constructed. Further investigation with the same approach shows that, in the quantum limit, the Johnson–Nyquist formula is also invalid under general conditions even though it is valid for a resistor-antenna system. Therefore we conclude that in a satisfactory quantum theory of the Johnson noise, the FDT must, as a minimum, include also the measurement system used to evaluate the observed quantities. Issues concerning the zero-point term may also have implications for phenomena in advanced nanotechnology.
Reprint of : Dynamics of a quantum wave emitted by a decaying and evanescent point source
NASA Astrophysics Data System (ADS)
Delgado, F.; Muga, J. G.
2016-08-01
We put forward a model that describes a decaying and evanescent point source of non-interacting quantum waves in 1D. This point-source assumption allows for a simple description that captures the essential aspects of the dynamics of a wave traveling through a classically forbidden region without the need to specify the details of the inner region. The dynamics of the resulting wave is examined and several characteristic times are identified. One of them generalizes the tunneling time-scale introduced by Büttiker and Landauer and it characterizes the arrival of the maximum of the wave function. Diffraction in time and deviations from exponential decay are also studied. Here we show that there exists an optimal injection frequency and detection point for the observation of these two quantum phenomena.
The Unicellular State as a Point Source in a Quantum Biological System
Torday, John S.; Miller, William B.
2016-01-01
A point source is the central and most important point or place for any group of cohering phenomena. Evolutionary development presumes that biological processes are sequentially linked, but neither directed from, nor centralized within, any specific biologic structure or stage. However, such an epigenomic entity exists and its transforming effects can be understood through the obligatory recapitulation of all eukaryotic lifeforms through a zygotic unicellular phase. This requisite biological conjunction can now be properly assessed as the focal point of reconciliation between biology and quantum phenomena, illustrated by deconvoluting complex physiologic traits back to their unicellular origins. PMID:27240413
The Unicellular State as a Point Source in a Quantum Biological System.
Torday, John S; Miller, William B
2016-01-01
A point source is the central and most important point or place for any group of cohering phenomena. Evolutionary development presumes that biological processes are sequentially linked, but neither directed from, nor centralized within, any specific biologic structure or stage. However, such an epigenomic entity exists and its transforming effects can be understood through the obligatory recapitulation of all eukaryotic lifeforms through a zygotic unicellular phase. This requisite biological conjunction can now be properly assessed as the focal point of reconciliation between biology and quantum phenomena, illustrated by deconvoluting complex physiologic traits back to their unicellular origins. PMID:27240413
Exact conductance through point contacts in the {nu}=1/3 fractional quantum Hall Effect
Fendley, P.; Ludwig, A.W.W.; Saleur, H. |
1995-04-10
The conductance for tunneling through an impurity in a Luttinger liquid is described by a universal scaling function. We compute this scaling function exactly, by using the thermodynamic Bethe ansatz and a kinetic (Boltzmann) equation. This model has been proposed to describe resonant tunneling through a point contact between two {nu}=1/3 quantum Hall edges. Recent experiments on quantum Hall devices agree well with our exact results. We also derive the exact conductance and {ital I}({ital V}) curve, out of equilibrium, in this fully interacting system.
Fluctuation theorem for a double quantum dot coupled to a point-contact electrometer
Golubev, D.; Utsumi, Y.; Marthaler, M.; Schön, G.
2013-12-04
Motivated by recent experiments on the real-time single-electron counting through a semiconductor GaAs double quantum dot (DQD) by a nearby quantum point contact (QPC), we develop the full-counting statistics of coupled DQD and QPC system. By utilizing the time-scale separation between the dynamics of DQD and QPC, we derive the modified master equation with tunneling rates depending on the counting fields, which fulfill the detailed fluctuation theorem. Furthermore, we derive universal relations between the non-linear corrections to the current and noise, which can be verified in experiments.
Ising nematic quantum critical point in a metal: a Monte Carlo study
NASA Astrophysics Data System (ADS)
Lederer, Samuel
The Ising nematic quantum critical point (QCP) associated with the zero temperature transition from a symmetric to a nematic metal is an exemplar of metallic quantum criticality. We have carried out a minus sign-free quantum Monte Carlo study of this QCP for a two dimensional lattice model with sizes up to 24 × 24 sites. The system remains non-superconducting down to the lowest accessible temperatures. The results exhibit critical scaling behavior over the accessible ranges of temperature, (imaginary) time, and distance. This scaling behavior has remarkable similarities with recently measured properties of the Fe-based superconductors proximate to their putative nematic QCP. With Yoni Schattner, Steven A. Kivelson, and Erez Berg.
Heat capacity and magnetization of CoNb2O6 near quantum critical point
NASA Astrophysics Data System (ADS)
Liang, Tian; Koohpayeh, Seyed; Krizan, Jason; Dutton, Sian; McQueen, Tyrel; Cava, Robert; Phuan Ong, N.
2012-02-01
CoNb2O6 is a quasi-1D quantum magnet in which magnetic Co^2+ ions are ferromagnetically arranged into nearly isolated chains along the c axis with the magnetic moment confined in the ac-plane. By applying transverse magnetic field along b-axis, quantum phase transition from magnetically ordered phase to paramagnetic phase occurs. Evidence for emergent E8 symmetry was recently obtained by neutron scattering near the quantum critical point (QCP) in an applied transverse magnetic field of 5.5 T We will report on experiments to investigate the behavior of the heat capacity and torque magnetization in the vicinity of the QCP and discuss their implications.
Quantum effects on Lagrangian points and displaced periodic orbits in the Earth-Moon system
NASA Astrophysics Data System (ADS)
Battista, Emmanuele; Dell'Agnello, Simone; Esposito, Giampiero; Simo, Jules
2015-04-01
Recent work in the literature has shown that the one-loop long distance quantum corrections to the Newtonian potential imply tiny but observable effects in the restricted three-body problem of celestial mechanics; i.e., at the Lagrangian libration points of stable equilibrium, the planetoid is not exactly at an equal distance from the two bodies of large mass, but the Newtonian values of its coordinates are changed by a few millimeters in the Earth-Moon system. First, we assess such a theoretical calculation by exploiting the full theory of the quintic equation, i.e., its reduction to Bring-Jerrard form and the resulting expression of roots in terms of generalized hypergeometric functions. By performing the numerical analysis of the exact formulas for the roots, we confirm and slightly improve the theoretical evaluation of quantum corrected coordinates of Lagrangian libration points of stable equilibrium. Second, we prove in detail that for collinear Lagrangian points the quantum corrections are also of the same order of magnitude in the Earth-Moon system. Third, we discuss the prospects of measuring, with the help of laser ranging, the above departure from the equilateral triangle picture, which is a challenging task. On the other hand, a modern version of the planetoid is the solar sail, and much progress has been made, in recent years, on the displaced periodic orbits of solar sails at all libration points, both stable and unstable. Therefore, the present paper investigates, eventually, a restricted three-body problem involving Earth, the Moon, and a solar sail. By taking into account the one-loop quantum corrections to the Newtonian potential, displaced periodic orbits of the solar sail at libration points are again found to exist.
Theory of the nematic quantum critical point in a nodal superconductor
NASA Astrophysics Data System (ADS)
Kim, Eun-Ah
2008-03-01
In the last several years, experimental evidence has accumulated in a variety of highly correlated electronic systems of new quantum phases which (for purely electronic reasons) spontaneously break the rotational (point group) symmetry of the underlying crystal. Such electron ``nematic'' phases have been seen in quantum Hall systems[1], in the metamagnetic metal Sr3Ru2O7[2], and more recently in magnetic neutron scattering studies of the high temperature superconductor, YBCO[3]. In the case of a high Tc superconductor, the quantum dynamics of nematic order parameter naturally couples strongly to quasiparticle (qp) excitations. In this talk, I will discuss our recent results on the effects of the coupling between quantum critical nematic fluctuations and the nodal qp's of a d-wave superconductor in the vicinity of a putative quantum critical point inside the superconducting phase. We solve a model system with N flavors of quasiparticles in the large N limit[4]. To leading order in 1/N, quantum fluctuations enhance the dispersion anisotropy of the nodal excitations, and cause strong scattering which critically broadens the quasiparticle peaks in the spectral function, except in the vicinity of ``the tips of the banana,'' where the qp's remain sharp. We will discuss the possible implications of our results to ARPES and STM experiments. [1] M.P. Lilly, K.B. Cooper, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, PRL 83, 824 (1999). [2] R. A. Borzi and S. A. Grigera and J. Farrell and R. S. Perry and S. J. S. Lister and S. L. Lee and D. A. Tennant and Y. Maeno and A. P. Mackenzie, Science 315, 214 (2007). [3] V. Hinkov, D. Haug, B. Fauqu'e, P. Bourges, Y. Sidis, A. Ivanov, C. Bernhard, C. T. Lin, B. Keimer, unpublished. [4] E.-A. Kim, M. Lawler, P. Oreto, E. Fradkin, S. Kivelson, cond-mat/0705.4099.
Thermodynamics in the vicinity of a relativistic quantum critical point in 2+1 dimensions.
Rançon, A; Kodio, O; Dupuis, N; Lecheminant, P
2013-07-01
We study the thermodynamics of the relativistic quantum O(N) model in two space dimensions. In the vicinity of the zero-temperature quantum critical point (QCP), the pressure can be written in the scaling form P(T)=P(0)+N(T(3)/c(2))F(N)(Δ/T), where c is the velocity of the excitations at the QCP and |Δ| a characteristic zero-temperature energy scale. Using both a large-N approach to leading order and the nonperturbative renormalization group, we compute the universal scaling function F(N). For small values of N (Nquantum critical regime (|x|quantum disordered (x>/~1) regimes, but fails to describe the nonmonotonic behavior of F(N) in the quantum critical regime. We discuss the renormalization-group flows in the various regimes near the QCP and make the connection with the quantum nonlinear sigma model in the renormalized classical regime. We compute the Berezinskii-Kosterlitz-Thouless transition temperature in the quantum O(2) model and find that in the vicinity of the QCP the universal ratio T(BKT)/ρ(s)(0) is very close to π/2, implying that the stiffness ρ(s)(T(BKT)(-)) at the transition is only slightly reduced with respect to the zero-temperature stiffness ρ(s)(0). Finally, we briefly discuss the experimental determination of the universal function F(2) from the pressure of a Bose gas in an optical lattice near the superfluid-Mott-insulator transition. PMID:23944420
Universal Scaling in the Fan of an Unconventional Quantum Critical Point
Melko, Roger G; Kaul, Ribhu
2008-01-01
We present the results of extensive finite-temperature Quantum Monte Carlo simulati ons on a SU(2) symmetric, $S=1/2$ quantum antiferromagnet with a frustrating four-s pin interaction -- the so-called 'JQ' model~[Sandvik, Phys. Rev. Lett. {\\bf 98}, 22 7202 (2007)]. Our simulations, which are unbiased, free of the sign-problem and car ried out on lattice sizes containing in excess of $1.6\\times 10^4$ spins, indicate that N\\'eel order is destroyed through a continuous quantum transition at a critica l value of the frustrating interaction. At larger values of this coupling the param agnetic state obtained has valence-bond solid order. The scaling behavior in the 'q uantum critical fan' above the putative critical point confirms a $z=1$ quantum pha se transition that is not in the conventional $O(3)$ universality class. Our result s are consistent with the predictions of the 'deconfined quantum criticality' scena rio.
Non-linear superflow of a unitary Fermi gas through a quantum point contact
NASA Astrophysics Data System (ADS)
Lebrat, Martin; Husmann, Dominik; Uchino, Shun; Krinner, Sebastian; Häusler, Samuel; Brantut, Jean-Philippe; Giamarchi, Thierry; Esslinger, Tilman
2016-05-01
Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors, or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However, for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. We studied resonantly interacting Fermi gases of 6 Li atoms connected by a tunable, ballistic quantum point contact, finding a nonlinear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.
Adiabatic Edge Channel Transport in a Nanowire Quantum Point Contact Register.
Heedt, S; Manolescu, A; Nemnes, G A; Prost, W; Schubert, J; Grützmacher, D; Schäpers, Th
2016-07-13
We report on a prototype device geometry where a number of quantum point contacts are connected in series in a single quasi-ballistic InAs nanowire. At finite magnetic field the backscattering length is increased up to the micron-scale and the quantum point contacts are connected adiabatically. Hence, several input gates can control the outcome of a ballistic logic operation. The absence of backscattering is explained in terms of selective population of spatially separated edge channels. Evidence is provided by regular Aharonov-Bohm-type conductance oscillations in transverse magnetic fields, in agreement with magnetoconductance calculations. The observation of the Shubnikov-de Haas effect at large magnetic fields corroborates the existence of spatially separated edge channels and provides a new means for nanowire characterization. PMID:27347816
Quantum point contact displacement transducer for a mechanical resonator at sub-Kelvin temperatures
Okazaki, Yuma; Mahboob, Imran; Onomitsu, Koji; Sasaki, Satoshi; Yamaguchi, Hiroshi
2013-11-04
Highly sensitive displacement transduction of a 1.67 MHz mechanical resonator with a quantum point contact (QPC) formed in a GaAs heterostructure is demonstrated. By positioning the QPC at the point of maximum mechanical strain on the resonator and operating at 80 mK, a displacement responsivity of 3.81 A/m is measured, which represents a two order of magnitude improvement on the previous QPC based devices. By further analyzing the QPC transport characteristics, a sub-Poisson-noise-limited displacement sensitivity of 25 fm/Hz{sup 1/2} is determined which corresponds to a position resolution that is 23 times the standard quantum limit.
On the existence of point spectrum for branching strips quantum graph
Popov, I. Yu. Skorynina, A. N.; Blinova, I. V.
2014-03-15
The quantum graph having the form of branching strips with hexagonal (honeycomb) structure is considered. The Hamiltonian is determined as free 1D Schrödinger operator on each edge and some “boundary” conditions at each vertex. We obtain the conditions ensuring the point spectrum's existence for the Schrödinger operator of the system and relations that give us the eigenvalues.
Chiral symmetry breaking in three-dimensional quantum electrodynamics as fixed point annihilation
NASA Astrophysics Data System (ADS)
Herbut, Igor F.
2016-07-01
Spontaneous chiral symmetry breaking in three-dimensional (d =3 ) quantum electrodynamics is understood as annihilation of an infrared-stable fixed point that describes the large-N conformal phase by another unstable fixed point at a critical number of fermions N =Nc. We discuss the root of universality of Nc in this picture, together with some features of the phase boundary in the (d ,N ) plane. In particular, it is shown that as d →4 , Nc→0 with a constant slope, our best estimate of which suggests that Nc=2.89 in d =3 .
Scaling near the Quantum-Critical Point in the SO(5) Theory of the High-T{sub c} Superconductivity
Kopec, T. K.; Zaleski, T. A.
2001-08-27
We study the quantum-critical point scenario within the unified theory of superconductivity and antiferromagnetism based on the SO(5) symmetry. Closed-form expression for the quantum-critical scaling function for the dynamic spin susceptibility is obtained from the lattice SO(5) quantum nonlinear {sigma} -model in three dimensions, revealing that in the quantum-critical region the frequency scale for spin excitations is simply set by the absolute temperature. Implications for the non-Fermi liquid behavior of the normal-state resistivity due to spin fluctuations in the quantum-critical region are also presented.
A Novel Quantum Dots–Based Point of Care Test for Syphilis
2010-01-01
One-step lateral flow test is recommended as the first line screening of syphilis for primary healthcare settings in developing countries. However, it generally shows low sensitivity. We describe here the development of a novel fluorescent POC (Point Of Care) test method to be used for screening for syphilis. The method was designed to combine the rapidness of lateral flow test and sensitiveness of fluorescent method. 50 syphilis-positive specimens and 50 healthy specimens conformed by Treponema pallidum particle agglutination (TPPA) were tested with Quantum Dot-labeled and colloidal gold-labeled lateral flow test strips, respectively. The results showed that both sensitivity and specificity of the quantum dots–based method reached up to 100% (95% confidence interval [CI], 91–100%), while those of the colloidal gold-based method were 82% (95% CI, 68–91%) and 100% (95% CI, 91–100%), respectively. In addition, the naked-eye detection limit of quantum dot–based method could achieve 2 ng/ml of anti-TP47 polyclonal antibodies purified by affinity chromatography with TP47 antigen, which was tenfold higher than that of colloidal gold–based method. In conclusion, the quantum dots were found to be suitable for labels of lateral flow test strip. Its ease of use, sensitiveness and low cost make it well-suited for population-based on-the-site syphilis screening. PMID:20672123
A novel quantum dots-based point of care test for syphilis.
Yang, Hao; Li, Ding; He, Rong; Guo, Qin; Wang, Kan; Zhang, Xueqing; Huang, Peng; Cui, Daxiang
2010-01-01
One-step lateral flow test is recommended as the first line screening of syphilis for primary healthcare settings in developing countries. However, it generally shows low sensitivity. We describe here the development of a novel fluorescent POC (Point Of Care) test method to be used for screening for syphilis. The method was designed to combine the rapidness of lateral flow test and sensitiveness of fluorescent method. 50 syphilis-positive specimens and 50 healthy specimens conformed by Treponema pallidum particle agglutination (TPPA) were tested with Quantum Dot-labeled and colloidal gold-labeled lateral flow test strips, respectively. The results showed that both sensitivity and specificity of the quantum dots-based method reached up to 100% (95% confidence interval [CI], 91-100%), while those of the colloidal gold-based method were 82% (95% CI, 68-91%) and 100% (95% CI, 91-100%), respectively. In addition, the naked-eye detection limit of quantum dot-based method could achieve 2 ng/ml of anti-TP47 polyclonal antibodies purified by affinity chromatography with TP47 antigen, which was tenfold higher than that of colloidal gold-based method. In conclusion, the quantum dots were found to be suitable for labels of lateral flow test strip. Its ease of use, sensitiveness and low cost make it well-suited for population-based on-the-site syphilis screening. PMID:20672123
A Novel Quantum Dots-Based Point of Care Test for Syphilis
NASA Astrophysics Data System (ADS)
Yang, Hao; Li, Ding; He, Rong; Guo, Qin; Wang, Kan; Zhang, Xueqing; Huang, Peng; Cui, Daxiang
2010-05-01
One-step lateral flow test is recommended as the first line screening of syphilis for primary healthcare settings in developing countries. However, it generally shows low sensitivity. We describe here the development of a novel fluorescent POC (Point Of Care) test method to be used for screening for syphilis. The method was designed to combine the rapidness of lateral flow test and sensitiveness of fluorescent method. 50 syphilis-positive specimens and 50 healthy specimens conformed by Treponema pallidum particle agglutination (TPPA) were tested with Quantum Dot-labeled and colloidal gold-labeled lateral flow test strips, respectively. The results showed that both sensitivity and specificity of the quantum dots-based method reached up to 100% (95% confidence interval [CI], 91-100%), while those of the colloidal gold-based method were 82% (95% CI, 68-91%) and 100% (95% CI, 91-100%), respectively. In addition, the naked-eye detection limit of quantum dot-based method could achieve 2 ng/ml of anti-TP47 polyclonal antibodies purified by affinity chromatography with TP47 antigen, which was tenfold higher than that of colloidal gold-based method. In conclusion, the quantum dots were found to be suitable for labels of lateral flow test strip. Its ease of use, sensitiveness and low cost make it well-suited for population-based on-the-site syphilis screening.
Song, Yunke; Zhang, Yi; Wang, Tza-Huei
2014-01-01
Gene point mutations present important biomarkers for genetic diseases. However, existing point mutation detection methods suffer from low sensitivity, specificity, and tedious assay processes. In this report, we propose an assay technology which combines the outstanding specificity of gap ligase chain reaction (Gap-LCR), the high sensitivity of single molecule coincidence detection and superior optical properties of quantum dots (QDs) for multiplexed detection of point mutations in genomic DNA. Mutant-specific ligation products are generated by Gap-LCR and subsequently captured by QDs to form DNA-QD nanocomplexes that are detected by single molecule spectroscopy (SMS) through multi-color fluorescence burst coincidence analysis, allowing for multiplexed mutation detection in a separation-free format. The proposed assay is capable of detecting zeptomoles of KRAS codon 12 mutation variants with near 100% specificity. Its high sensitivity allows direct detection of KRAS mutation in crude genomic DNA without PCR pre-amplification. PMID:23239594
Resolution of point sources of light as analyzed by quantum detection theory.
NASA Technical Reports Server (NTRS)
Helstrom, C. W.
1973-01-01
The resolvability of point sources of incoherent thermal light is analyzed by quantum detection theory in terms of two hypothesis-testing problems. In the first, the observer must decide whether there are two sources of equal radiant power at given locations, or whether there is only one source of twice the power located midway between them. In the second problem, either one, but not both, of two point sources is radiating, and the observer must decide which it is. The decisions are based on optimum processing of the electromagnetic field at the aperture of an optical instrument. In both problems the density operators of the field under the two hypotheses do not commute. The error probabilities, determined as functions of the separation of the points and the mean number of received photons, characterize the ultimate resolvability of the sources.
The resolution of point sources of light as analyzed by quantum detection theory
NASA Technical Reports Server (NTRS)
Helstrom, C. W.
1972-01-01
The resolvability of point sources of incoherent light is analyzed by quantum detection theory in terms of two hypothesis-testing problems. In the first, the observer must decide whether there are two sources of equal radiant power at given locations, or whether there is only one source of twice the power located midway between them. In the second problem, either one, but not both, of two point sources is radiating, and the observer must decide which it is. The decisions are based on optimum processing of the electromagnetic field at the aperture of an optical instrument. In both problems the density operators of the field under the two hypotheses do not commute. The error probabilities, determined as functions of the separation of the points and the mean number of received photons, characterize the ultimate resolvability of the sources.
Magnetocaloric effect and magnetic cooling near a field-induced quantum-critical point
Wolf, Bernd; Tsui, Yeekin; Jaiswal-Nagar, Deepshikha; Tutsch, Ulrich; Honecker, Andreas; Remović-Langer, Katarina; Hofmann, Georg; Prokofiev, Andrey; Assmus, Wolf; Donath, Guido; Lang, Michael
2011-01-01
The presence of a quantum-critical point (QCP) can significantly affect the thermodynamic properties of a material at finite temperatures T. This is reflected, e.g., in the entropy landscape S(T,r) in the vicinity of a QCP, yielding particularly strong variations for varying the tuning parameter r such as pressure or magnetic field B. Here we report on the determination of the critical enhancement of ∂S/∂B near a B-induced QCP via absolute measurements of the magnetocaloric effect (MCE), (∂T/∂B)S and demonstrate that the accumulation of entropy around the QCP can be used for efficient low-temperature magnetic cooling. Our proof of principle is based on measurements and theoretical calculations of the MCE and the cooling performance for a Cu2+-containing coordination polymer, which is a very good realization of a spin-½ antiferromagnetic Heisenberg chain—one of the simplest quantum-critical systems.
Weak phase stiffness and nature of the quantum critical point in underdoped cuprates
Yildirim, Yucel; Ku, Wei
2015-11-02
We demonstrate that the zero-temperature superconducting phase diagram of underdoped cuprates can be quantitatively understood in the strong binding limit, using only the experimental spectral function of the “normal” pseudogap phase without any free parameter. In the prototypical (La_{1–x}Sr_{x})_{2}CuO_{4}, a kinetics-driven d-wave superconductivity is obtained above the critical doping δ_{c} ~ 5.2%, below which complete loss of superfluidity results from local quantum fluctuation involving local p-wave pairs. Near the critical doping, an enormous mass enhancement of the local pairs is found responsible for the observed rapid decrease of phase stiffness. Lastly, a striking mass divergence is predicted at δ_{c} that dictates the occurrence of the observed quantum critical point and the abrupt suppression of the Nernst effects in the nearby region.
Weak phase stiffness and nature of the quantum critical point in underdoped cuprates
Yildirim, Yucel; Ku, Wei
2015-11-02
We demonstrate that the zero-temperature superconducting phase diagram of underdoped cuprates can be quantitatively understood in the strong binding limit, using only the experimental spectral function of the “normal” pseudogap phase without any free parameter. In the prototypical (La1–xSrx)2CuO4, a kinetics-driven d-wave superconductivity is obtained above the critical doping δc ~ 5.2%, below which complete loss of superfluidity results from local quantum fluctuation involving local p-wave pairs. Near the critical doping, an enormous mass enhancement of the local pairs is found responsible for the observed rapid decrease of phase stiffness. Lastly, a striking mass divergence is predicted at δc thatmore » dictates the occurrence of the observed quantum critical point and the abrupt suppression of the Nernst effects in the nearby region.« less
Single-point position and transition defects in continuous time quantum walks
Li, Z. J.; Wang, J. B.
2015-01-01
We present a detailed analysis of continuous time quantum walks (CTQW) with both position and transition defects defined at a single point in the line. Analytical solutions of both traveling waves and bound states are obtained, which provide valuable insight into the dynamics of CTQW. The number of bound states is found to be critically dependent on the defect parameters, and the localized probability peaks can be readily obtained by projecting the state vector of CTQW on to these bound states. The interference between two bound states are also observed in the case of a transition defect. The spreading of CTQW probability over the line can be finely tuned by varying the position and transition defect parameters, offering the possibility of precision quantum control of the system. PMID:26323855
Phase reconstruction near to the two-dimensional ferromagnetic quantum critical point
NASA Astrophysics Data System (ADS)
Pedder, Chris; Karahasanovic, Una; Kruger, Frank; Green, Andrew
2012-02-01
We study the formation of new phases in two dimensions near to the putative quantum critical point of the itinerant ferromagnet to paramagnet phase transition. In addition to the first order and helimagnetic behaviour found in non-analytic extensions to Hertz-Millis theory [1] and in the quantum order-by-disorder approach [2], we find a small region of spin nematic order. Our approach also admits a concurrent formation of superconducting order. We further study the effect of small deformations from quadratic electron dispersion -- as previously found in three dimensions, these enlarge the region of spin nematic order at the expense of spiral order.[4pt] [1] D. Belitz, T.R. Kirkpatrick and T. Vojta, Rev. Mod. Phys. 77, 579 (2005),. V. Efremov, J.J. Betouras, A.V. Chubukov Phys. Rev. B 77, 220401(R), (2008)[0pt] [2] G. J. Conduit Phys. Rev. A 82, 043604 (2010)
NASA Astrophysics Data System (ADS)
Muro, Tatsuya; Nishihara, Yoshitaka; Norimoto, Shota; Ferrier, Meydi; Arakawa, Tomonori; Kobayashi, Kensuke; Ihn, Thomas; Rössler, Clemens; Ensslin, Klaus; Reichl, Christian; Wegscheider, Werner
2016-05-01
We report a precise experimental study on the shot noise of a quantum point contact (QPC) fabricated in a GaAs/AlGaAs based high-mobility two-dimensional electron gas (2DEG). The combination of unprecedented cleanliness and very high measurement accuracy has enabled us to discuss the Fano factor to characterize the shot noise with a precision of 0.01. We observed that the shot noise at zero magnetic field exhibits a slight enhancement exceeding the single particle theoretical prediction, and that it gradually decreases as a perpendicular magnetic field is applied. We also confirmed that this additional noise completely vanishes in the quantum Hall regime. These phenomena can be explained by the electron heating effect near the QPC, which is suppressed with increasing magnetic field.
Metal-insulator quantum critical point beneath the high Tc superconducting dome
Sebastian, Suchitra E.; Harrison, N.; Altarawneh, M. M.; Mielke, C. H.; Liang, Ruixing; Bonn, D. A.; Lonzarich, G. G.; Hardy, W. N.
2010-01-01
An enduring question in correlated systems concerns whether superconductivity is favored at a quantum critical point (QCP) characterized by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high Tc cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultrastrong magnetic fields to measure quantum oscillations in underdoped YBa2Cu3O6+x, revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in Tc in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centered at a critical Fermi surface instability. PMID:20304800
Formation of a protected sub-band for conduction in quantum point contacts under extreme biasing
NASA Astrophysics Data System (ADS)
Lee, J.; Han, J. E.; Xiao, S.; Song, J.; Reno, J. L.; Bird, J. P.
2014-02-01
Managing energy dissipation is critical to the scaling of current microelectronics and to the development of novel devices that use quantum coherence to achieve enhanced functionality. To this end, strategies are needed to tailor the electron-phonon interaction, which is the dominant mechanism for cooling non-equilibrium (`hot') carriers. In experiments aimed at controlling the quantum state, this interaction causes decoherence that fundamentally disrupts device operation. Here, we show a contrasting behaviour, in which strong electron-phonon scattering can instead be used to generate a robust mode for electrical conduction in GaAs quantum point contacts, driven into extreme non-equilibrium by nanosecond voltage pulses. When the amplitude of these pulses is much larger than all other relevant energy scales, strong electron-phonon scattering induces an attraction between electrons in the quantum-point-contact channel, which leads to the spontaneous formation of a narrow current filament and to a renormalization of the electronic states responsible for transport. The lowest of these states coalesce to form a sub-band separated from all others by an energy gap larger than the source voltage. Evidence for this renormalization is provided by a suppression of heating-related signatures in the transient conductance, which becomes pinned near 2e2/h (e, electron charge; h, Planck constant) for a broad range of source and gate voltages. This collective non-equilibrium mode is observed over a wide range of temperature (4.2-300 K) and may provide an effective means to manage electron-phonon scattering in nanoscale devices.
Entropy excess in strongly correlated Fermi systems near a quantum critical point
Clark, J.W.; Zverev, M.V.; Khodel, V.A.
2012-12-15
A system of interacting, identical fermions described by standard Landau Fermi-liquid (FL) theory can experience a rearrangement of its Fermi surface if the correlations grow sufficiently strong, as occurs at a quantum critical point where the effective mass diverges. As yet, this phenomenon defies full understanding, but salient aspects of the non-Fermi-liquid (NFL) behavior observed beyond the quantum critical point are still accessible within the general framework of the Landau quasiparticle picture. Self-consistent solutions of the coupled Landau equations for the quasiparticle momentum distribution n(p) and quasiparticle energy spectrum {epsilon}(p) are shown to exist in two distinct classes, depending on coupling strength and on whether the quasiparticle interaction is regular or singular at zero momentum transfer. One class of solutions maintains the idempotency condition n{sup 2}(p)=n(p) of standard FL theory at zero temperature T while adding pockets to the Fermi surface. The other solutions are characterized by a swelling of the Fermi surface and a flattening of the spectrum {epsilon}(p) over a range of momenta in which the quasiparticle occupancies lie between 0 and 1 even at T=0. The latter, non-idempotent solution is revealed by analysis of a Poincare mapping associated with the fundamental Landau equation connecting n(p) and {epsilon}(p) and validated by solution of a variational condition that yields the symmetry-preserving ground state. Significantly, this extraordinary solution carries the burden of a large temperature-dependent excess entropy down to very low temperatures, threatening violation of the Nernst Theorem. It is argued that certain low-temperature phase transitions, notably those involving Cooper-pair formation, offer effective mechanisms for shedding the entropy excess. Available measurements in heavy-fermion compounds provide concrete support for such a scenario. - Highlights: Black-Right-Pointing-Pointer Extension of Landau
Feedback cooling of cantilever motion using a quantum point contact transducer
Montinaro, M.; Mehlin, A.; Solanki, H. S.; Peddibhotla, P.; Poggio, M.; Mack, S.; Awschalom, D. D.
2012-09-24
We use a quantum point contact (QPC) as a displacement transducer to measure and control the low-temperature thermal motion of a nearby micromechanical cantilever. The QPC is included in an active feedback loop designed to cool the cantilever's fundamental mechanical mode, achieving a squashing of the QPC noise at high gain. The minimum achieved effective mode temperature of 0.2 K and the displacement resolution of 10{sup -11} m/{radical}(Hz) are limited by the performance of the QPC as a one-dimensional conductor and by the cantilever-QPC capacitive coupling.
Influence of super-ohmic dissipation on a disordered quantum critical point.
Vojta, Thomas; Hoyos, José A; Mohan, Priyanka; Narayanan, Rajesh
2011-03-01
We investigate the combined influence of quenched randomness and dissipation on a quantum critical point with O(N) order-parameter symmetry. Utilizing a strong-disorder renormalization group, we determine the critical behavior in one space dimension exactly. For super-ohmic dissipation, we find a Kosterlitz-Thouless type transition with conventional (power-law) dynamical scaling. The dynamical critical exponent depends on the spectral density of the dissipative baths. We also discuss the Griffiths singularities, and we determine observables. PMID:21339559
Infinite randomness fixed point of the superconductor-metal quantum phase transition.
Del Maestro, Adrian; Rosenow, Bernd; Müller, Markus; Sachdev, Subir
2008-07-18
We examine the influence of quenched disorder on the superconductor-metal transition, as described by a theory of overdamped Cooper pairs which repel each other. The self-consistent pairing eigenmodes of a quasi-one-dimensional wire are determined numerically. Our results support the recent proposal by Hoyos et al. [Phys. Rev. Lett. 99, 230601 (2007)10.1103/PhysRevLett.99.230601] that the transition is characterized by the same strong-disorder fixed point describing the onset of ferromagnetism in the random quantum Ising chain in a transverse field. PMID:18764263
Tunable strength saddle-point contacts impact on quantum rings transmission
NASA Astrophysics Data System (ADS)
González, J. J.; Diago-Cisneros, L.
2016-09-01
A particular subject of investigation is the role of several sadle-point contact (QPC) parameters on the scattering properties of an Aharonov-Bohm-Aharonov-Casher quantum ring (QR) under Rashba-type spin orbit interaction. We discuss the interplay of the conductance with the confinement strengths and height of the QPC, which yields new and tunable harmonic and non-harmonics patterns, while one manipulates these constriction parameters. This phenomenology may be of utility to implement a novel way to modulate spin interference effects in semiconducting QRs, providing an appealing test-platform for spintronics applications.
Infinite Randomness Fixed Point of the Superconductor-Metal Quantum Phase Transition
NASA Astrophysics Data System (ADS)
Del Maestro, Adrian; Rosenow, Bernd; Müller, Markus; Sachdev, Subir
2008-07-01
We examine the influence of quenched disorder on the superconductor-metal transition, as described by a theory of overdamped Cooper pairs which repel each other. The self-consistent pairing eigenmodes of a quasi-one-dimensional wire are determined numerically. Our results support the recent proposal by Hoyos et al. [Phys. Rev. Lett. 99, 230601 (2007)PRLTAO0031-900710.1103/PhysRevLett.99.230601] that the transition is characterized by the same strong-disorder fixed point describing the onset of ferromagnetism in the random quantum Ising chain in a transverse field.
Evolution of hyperfine parameters across a quantum critical point in CeRhIn5
NASA Astrophysics Data System (ADS)
Lin, C. H.; Shirer, K. R.; Crocker, J.; Dioguardi, A. P.; Lawson, M. M.; Bush, B. T.; Klavins, P.; Curro, N. J.
2015-10-01
We report nuclear magnetic resonance (NMR) data for both the In(1) and In(2) sites in the heavy-fermion material CeRhIn5 under hydrostatic pressure. The Knight shift data reveal a suppression of the hyperfine coupling to the In(1) site as a function of pressure, and the electric field gradient να α at the In(2) site exhibits a change of slope d να α/d P at Pc 1=1.75 GPa. These changes to the coupling constants reflect alterations to the electronic structure at the quantum critical point.
Modeling A.C. Electronic Transport through a Two-Dimensional Quantum Point Contact
Aronov, I.E.; Beletskii, N.N.; Berman, G.P.; Campbell, D.K.; Doolen, G.D.; Dudiy, S.V.
1998-12-07
We present the results on the a.c. transport of electrons moving through a two-dimensional (2D) semiconductor quantum point contact (QPC). We concentrate our attention on the characteristic properties of the high frequency admittance ({omega}{approximately}0 - 50 GHz), and on the oscillations of the admittance in the vicinity of the separatrix (when a channel opens or closes), in presence of the relaxation effects. The experimental verification of such oscillations in the admittance would be a strong confirmation of the semi-classical approach to the a.c. transport in a QPC, in the separatrix region.
Nuclear Quantum Effects in Water at the Triple Point: Using Theory as a Link Between Experiments.
Cheng, Bingqing; Behler, Jörg; Ceriotti, Michele
2016-06-16
One of the most prominent consequences of the quantum nature of light atomic nuclei is that their kinetic energy does not follow a Maxwell-Boltzmann distribution. Deep inelastic neutron scattering (DINS) experiments can measure this effect. Thus, the nuclear quantum kinetic energy can be probed directly in both ordered and disordered samples. However, the relation between the quantum kinetic energy and the atomic environment is a very indirect one, and cross-validation with theoretical modeling is therefore urgently needed. Here, we use state of the art path integral molecular dynamics techniques to compute the kinetic energy of hydrogen and oxygen nuclei in liquid, solid, and gas-phase water close to the triple point, comparing three different interatomic potentials and validating our results against equilibrium isotope fractionation measurements. We will then show how accurate simulations can draw a link between extremely precise fractionation experiments and DINS, therefore establishing a reliable benchmark for future measurements and providing key insights to increase further the accuracy of interatomic potentials for water. PMID:27203358
Singularity of the London Penetration Depth at Quantum Critical Points in Superconductors
NASA Astrophysics Data System (ADS)
Chowdhury, Debanjan; Swingle, Brian; Berg, Erez; Sachdev, Subir
2013-10-01
We present a general theory of the singularity in the London penetration depth at symmetry-breaking and topological quantum critical points within a superconducting phase. While the critical exponents and ratios of amplitudes on the two sides of the transition are universal, an overall sign depends upon the interplay between the critical theory and the underlying Fermi surface. We determine these features for critical points to spin density wave and nematic ordering, and for a topological transition between a superconductor with Z2 fractionalization and a conventional superconductor. We note implications for recent measurements of the London penetration depth in BaFe2(As1-xPx)2 [K. Hashimoto , Science 336, 1554 (2012)SCIEAS0036-807510.1126/science.1219821].
Magnetic-field control of quantum critical points of valence transition.
Watanabe, Shinji; Tsuruta, Atsushi; Miyake, Kazumasa; Flouquet, Jacques
2008-06-13
We study the mechanism of how critical end points of first-order valence transitions are controlled by a magnetic field. We show that the critical temperature is suppressed to be a quantum critical point (QCP) by a magnetic field, and unexpectedly, the QCP exhibits nonmonotonic field dependence in the ground-state phase diagram, giving rise to the emergence of metamagnetism even in the intermediate valence-crossover regime. The driving force of the field-induced QCP is clarified to be cooperative phenomena of the Zeeman and Kondo effects, which create a distinct energy scale from the Kondo temperature. This mechanism explains the peculiar magnetic response in CeIrIn(5) and the metamagnetic transition in YbXCu(4) for X=In as well as the sharp contrast between X=Ag and Cd. PMID:18643524
Terahertz magnetospectroscopy of a point contact based on CdTe/CdMgTe quantum well
NASA Astrophysics Data System (ADS)
Grigelionis, I.; Bialek, M.; Grynberg, M.; Czapkiewicz, M.; Kolkovski, V.; Wiater, M.; Wojciechowski, M.; Wróbel, J.; Wojtowicz, T.; Diakonova, N.; Knap, W.; Łusakowski, J.
2014-09-01
THz response of a number of samples based on CdTe/CdMgTe quantum wells grown by a molecular beam epitaxy was investigated at low temperatures and high magnetic fields. The experiments involved magnetotransport, photocurrent, and transmission measurements carried out with a monochromatic THz sources or a Fourier spectrometer. Samples of different geometry, with and without a gate metallization were used. We observed excitations of a two-dimensional plasma in the form of optically-induced Shubnikov-de Haas oscillations, cyclotron resonance transitions and magnetoplasmon resonances. A polaron effect was observed at magnetic fields higher than 10 T. A point contact device processed with an electron beam lithography was investigated as a detector of THz radiation. It was shown that the main mechanism responsible for a THz performance of the point contact was excitation of magnetoplasmons with a wave vector defined by geometrical constrictions of the device mesa.
Multiscale modeling of point defects in Si-Ge(001) quantum wells
Yang, B.; Tewary, V. K.
2007-04-01
A computationally efficient hybrid Green's function (GF) technique is developed for multiscale modeling of point defects in a trilayer lattice system that links seamlessly the length scales from lattice (subnanometers) to continuum (bulk). The model accounts for the discrete structure of the lattice including nonlinear effects at the atomistic level and full elastic anisotropy at the continuum level. The model is applied to calculate the discrete core structure of point defects (vacancies and substitutional impurities) in Si-Ge(001) quantum wells (QWs) that are of contemporary technological interest. Numerical results are presented for the short range and long range lattice distortions and strains in the lattice caused by the defects and their formation energy and Kanzaki forces that are basic characteristics of the defects. The continuum and the lattice GFs of the material system are used to link the different length scales, which enables us to model the point defects and extended defects such as the quantum well in a unified formalism. Nonlinear effects in the core of the point defects are taken into account by using an iterative scheme. The Tersoff potential is used to set up the lattice structure, compute the unrelaxed forces and force constants in the lattice, and derive the elastic constants required for the continuum GF. It is found that the overall elastic properties of the material and the properties of defects vary considerably when the material is strained from the bulk to the QW state. This change in the defect properties is very significant and can provide a characteristic signature of the defect. For example, in the case of a single vacancy in Ge, the strain reverses the sign of the relaxation volume. It is also found that the defect properties, such as the defect core structures, change abruptly across a Ge/Si interface. The transition occurs over a region extending from two to four lattice constants, depending upon the defect species.
Exotic quantum critical point on the surface of three-dimensional topological insulator
NASA Astrophysics Data System (ADS)
Bi, Zhen; You, Yi-Zhuang; Xu, Cenke
2016-07-01
In the last few years a lot of exotic and anomalous topological phases were constructed by proliferating the vortexlike topological defects on the surface of the 3 d topological insulator (TI) [Fidkowski et al., Phys. Rev. X 3, 041016 (2013), 10.1103/PhysRevX.3.041016; Chen et al., Phys. Rev. B 89, 165132 (2014), 10.1103/PhysRevB.89.165132; Bonderson et al., J. Stat. Mech. (2013) P09016, 10.1088/1742-5468/2013/09/P09016; Wang et al., Phys. Rev. B 88, 115137 (2013), 10.1103/PhysRevB.88.115137; Metlitski et al., Phys. Rev. B 92, 125111 (2015), 10.1103/PhysRevB.92.125111]. In this work, rather than considering topological phases at the boundary, we will study quantum critical points driven by vortexlike topological defects. In general, we will discuss a (2 +1 )d quantum phase transition described by the following field theory: L =ψ ¯γμ(∂μ-i aμ) ψ +| (∂μ-i k aμ) ϕ| 2+r|ϕ | 2+g |ϕ| 4 , with tuning parameter r , arbitrary integer k , Dirac fermion ψ , and complex scalar bosonic field ϕ , which both couple to the same (2 +1 )d dynamical noncompact U(1) gauge field aμ. The physical meaning of these quantities/fields will be explained in the text. Making use of the new duality formalism developed in [Metlitski et al., Phys. Rev. B 93, 245151 (2016), 10.1103/PhysRevB.93.245151; Wang et al., Phys. Rev. X 5, 041031 (2015), 10.1103/PhysRevX.5.041031; Wang et al., Phys. Rev. B 93, 085110 (2016), 10.1103/PhysRevB.93.085110; D. T. Son, Phys. Rev. X 5, 031027 (2015), 10.1103/PhysRevX.5.031027], we demonstrate that this quantum critical point has a quasi-self-dual nature. And at this quantum critical point, various universal quantities such as the electrical conductivity and scaling dimension of gauge-invariant operators, can be calculated systematically through a 1 /k2 expansion, based on the observation that the limit k →+∞ corresponds to an ordinary 3 d X Y transition.
Mapping out spin and particle conductances in a quantum point contact.
Krinner, Sebastian; Lebrat, Martin; Husmann, Dominik; Grenier, Charles; Brantut, Jean-Philippe; Esslinger, Tilman
2016-07-19
We study particle and spin transport in a single-mode quantum point contact, using a charge neutral, quantum degenerate Fermi gas with tunable, attractive interactions. This yields the spin and particle conductance of the point contact as a function of chemical potential or confinement. The measurements cover a regime from weak attraction, where quantized conductance is observed, to the resonantly interacting superfluid. Spin conductance exhibits a broad maximum when varying the chemical potential at moderate interactions, which signals the emergence of Cooper pairing. In contrast, the particle conductance is unexpectedly enhanced even before the gas is expected to turn into a superfluid, continuously rising from the plateau at [Formula: see text] for weak interactions to plateau-like features at nonuniversal values as high as [Formula: see text] for intermediate interactions. For strong interactions, the particle conductance plateaus disappear and the spin conductance gets suppressed, confirming the spin-insulating character of a superfluid. Our observations document the breakdown of universal conductance quantization as many-body correlations appear. The observed anomalous quantization challenges a Fermi liquid description of the normal phase, shedding new light on the nature of the strongly attractive Fermi gas. PMID:27357668
Spin dynamics in a quantum point contact showing the 0.7-anomaly
NASA Astrophysics Data System (ADS)
von Delft, Jan; Bauer, Florian; Heyder, Jan; Schimmel, Dennis; CeNS/ASC Team
2015-03-01
The 0.7-anomaly in the first conductance step of a quantum point contact is believed to arise from an interplay of geometry, spin dynamics and interaction effects. Various scenarios have been proposed to explain it, each evoking a different concept, including spontaneous spin polarization, or a quasi-localized state, or ferromagnetic spin fluctuations, or a van Hove ridge (a geometry-induced maximum in the density-of states). Though these scenarios differ substantially regarding numerous details, they all imply anomalous dynamics for the spins in the vicinity of the QPC. We have performed a detailed study of this spin dynamics in the central region of a parabolic quantum point contact, by using the functional renormalization group to calculate the dynamical spin-spin correlation function χ (x ,x' , ω) =∫0∞
Spin-dependent masses and field-induced quantum critical points
NASA Astrophysics Data System (ADS)
McCollam, A.; Daou, R.; Julian, S. R.; Bergemann, C.; Flouquet, J.; Aoki, D.
2005-04-01
We discuss spin-dependent mass enhancements associated with field-induced quantum critical points in heavy-fermion systems. We have recently observed this phenomenon on a branch of the Fermi surface of CeRu2Si2 above its metamagnetic transition, complementing earlier work. In CeCoIn5, at high fields above a field-induced quantum critical point, we see a strong spin-dependence of the effective mass on the thermodynamically dominant sheets of the Fermi surface. These observations reinforce the suggestion that ‘missing mass’ in some cerium-based heavy-fermion systems will be found on heavy spin-polarised branches of the Fermi surface. In all cases where this phenomenon is observed the linear coefficient of specific heat is field dependent; however, CeCoIn5 seems to be the first such heavy-fermion system in which the f-electrons are definitely contributing to the Fermi volume, which puts it beyond the existing theory intended for metamagnetic systems.
Quantum critical point and spin fluctuations in lower-mantle ferropericlase
Lyubutin, Igor S.; Struzhkin, Viktor V.; Mironovich, A. A.; Gavriliuk, Alexander G.; Naumov, Pavel G.; Lin, Jung-Fu; Ovchinnikov, Sergey G.; Sinogeikin, Stanislav; Chow, Paul; Xiao, Yuming; Hemley, Russell J.
2013-01-01
Ferropericlase [(Mg,Fe)O] is one of the most abundant minerals of the earth’s lower mantle. The high-spin (HS) to low-spin (LS) transition in the Fe2+ ions may dramatically alter the physical and chemical properties of (Mg,Fe)O in the deep mantle. To understand the effects of compression on the ground electronic state of iron, electronic and magnetic states of Fe2+ in (Mg0.75Fe0.25)O have been investigated using transmission and synchrotron Mössbauer spectroscopy at high pressures and low temperatures (down to 5 K). Our results show that the ground electronic state of Fe2+ at the critical pressure Pc of the spin transition close to T = 0 is governed by a quantum critical point (T = 0, P = Pc) at which the energy required for the fluctuation between HS and LS states is zero. Analysis of the data gives Pc = 55 GPa. Thermal excitation within the HS or LS states (T > 0 K) is expected to strongly influence the magnetic as well as physical properties of ferropericlase. Multielectron theoretical calculations show that the existence of the quantum critical point at temperatures approaching zero affects not only physical properties of ferropericlase at low temperatures but also its properties at P-T of the earth’s lower mantle. PMID:23589892
Electron-photon interaction in a quantum point contact coupled to a microwave resonator
NASA Astrophysics Data System (ADS)
Mendes, Udson C.; Mora, Christophe
2016-06-01
We study a single-mode cavity weakly coupled to a voltage-biased quantum point contact. In a perturbative analysis, the lowest order predicts a thermal state for the cavity photons, driven by the emission noise of the conductor. The cavity is thus emptied as all transmission probabilities of the quantum point contact approach one or zero. Two-photon processes are identified at higher coupling, and pair absorption dominates over pair emission for all bias voltages. As a result, the number of cavity photons, the cavity damping rate, and the second-order coherence g(2 ) are all reduced and exhibit less bunching than the thermal state. These results are obtained with a Keldysh path-integral formulation and reproduced with rate equations. They can be seen as a backaction of the cavity measuring the electronic noise. Extending the standard P (E ) theory to a steady-state situation, we compute the modified noise properties of the conductor and find quantitative agreement with the perturbative calculation.
Mapping out spin and particle conductances in a quantum point contact
Krinner, Sebastian; Lebrat, Martin; Husmann, Dominik; Grenier, Charles; Brantut, Jean-Philippe; Esslinger, Tilman
2016-01-01
We study particle and spin transport in a single-mode quantum point contact, using a charge neutral, quantum degenerate Fermi gas with tunable, attractive interactions. This yields the spin and particle conductance of the point contact as a function of chemical potential or confinement. The measurements cover a regime from weak attraction, where quantized conductance is observed, to the resonantly interacting superfluid. Spin conductance exhibits a broad maximum when varying the chemical potential at moderate interactions, which signals the emergence of Cooper pairing. In contrast, the particle conductance is unexpectedly enhanced even before the gas is expected to turn into a superfluid, continuously rising from the plateau at 1/h for weak interactions to plateau-like features at nonuniversal values as high as 4/h for intermediate interactions. For strong interactions, the particle conductance plateaus disappear and the spin conductance gets suppressed, confirming the spin-insulating character of a superfluid. Our observations document the breakdown of universal conductance quantization as many-body correlations appear. The observed anomalous quantization challenges a Fermi liquid description of the normal phase, shedding new light on the nature of the strongly attractive Fermi gas. PMID:27357668
Unconventional Superconductivity in the Vicinity of the Local Quantum Critical Point
NASA Astrophysics Data System (ADS)
Si, Qimiao; Pixley, Jedediah; Deng, Lili; Ingersent, Kevin
2015-03-01
Unconventional superconductivity and its relationship with quantum criticality remains a central question in strongly correlated electron systems. In the case of heavy fermion metals, the existence of antiferromagnetic quantum critical points (QCPs) is well established. Theoretical work has identified the existence of a local QCP where the Kondo effect is driven critical concomitant with the vanishing of the magnetic order parameter. Experiments on the heavy fermion compound CeRhIn5 and other materials have provided strong evidence that such a QCP drives unconventional superconductivity. With this in mind we solve the periodic Anderson model using a cluster extended dynamical mean field theory. We show that the Kondo energy scale is continuously suppressed at the antiferromagnetic QCP, and we determine the scaling form of the order parameter susceptibility and find remarkable agreement with well-established experiments in the related heavy fermion system CeCu6-xAux. Most importantly, we find that the singlet pairing susceptibility is strongly enhanced at the QCP, which points towards a new pairing mechanism associated with both magnetic and local critical fluctuations.
Realization of an all-electric spin transistor using quantum point contacts
NASA Astrophysics Data System (ADS)
Chen, Tse-Ming; Chuang, Pojen; Ho, Sheng-Chin; Smith, Luke; Sfigakis, Francois; Pepper, Michael; Chen, Chin-Hung; Fan, Ju-Chun; Griffiths, Jonathan; Farrer, Ian; Beere, Harvey; Jones, Geb; Ritchie, Dave
The spin field effect transistor envisioned by Datta and Das opens a gateway to spin information processing. Although the coherent manipulation of electron spins in semiconductors is now possible, the realization of a functional spin field effect transistor for information processing has yet to be achieved, owing to several fundamental challenges such as the low spin-injection efficiency due to resistance mismatch, spin relaxation, and the spread of spin precession angles. Alternative spin transistor designs have therefore been proposed, but these differ from the field effect transistor concept and require the use of optical or magnetic elements, which pose difficulties for the incorporation into integrated circuits. Here, we present an all-electric all-semiconductor spin field effect transistor, in which these obstacles are overcome by employing two quantum point contacts as spin injectors and detectors. Distinct engineering architectures of spin-orbit coupling are exploited for the quantum point contacts and the central semiconductor channel to achieve complete control of the electron spins--spin injection, manipulation, and detection--in a purely electrical manner. Such a device is compatible with large-scale integration and hold promise for future spintronic devices for information processing. Ref: P. Chuang et al., Nat. Nanotechnol. 10, 35 (2015).
NASA Astrophysics Data System (ADS)
Byers, L. C.; Stearns, L. A.; Finnegan, D. C.; LeWinter, A. L.; Gadomski, P. J.; Hamilton, G. S.
2014-12-01
Flow near the termini of tidewater glaciers varies over short time-scales due to mechanisms that are poorly understood. Repeat observations with high temporal and spatial resolution, recorded around the terminus, are required to better understand the processes that control flow variability. Progress in light detection and ranging (LiDAR) technology permit such observations of the near-terminus and the pro-glacial ice mélange, though standard workflows for quantifying deformation from point clouds currently do not exist. Here, we test and develop methods for processing displacements from LiDAR data of complexly deforming bodies. We use data collected at 30-minute intervals over three-days in August 2013 at Helheim Glacier, Greenland by a long-range (6-10 km), 1064 nm wavelength Terrestrial LiDAR Scanner (TLS). The total area of coverage was ~25 km2. Distributed shear in glaciers prevents a simple transformation for aligning repeat point clouds, but within small regions (~100 m2) strain is assumed to be minor between scans. Registering a large number of these individual regions, subset from the full point-cloud, results in reduced alignment errors. By subsetting in a regular grid, rasters of velocities between scans are created. However, using data-dependent properties such as point density causes the generation of unevenly spaced velocity estimations, which can locally improve resolution or decrease registration errors. The choice of subsets therefore controls the output product's resolution and accuracy. We test how the spatial segmentation scheme affects the displacement results and alignment errors, finding that displacements can be quantified with limited assumption of the true value of displacement for the subset, barring great morphological changes. By identifying areas that do not deform over the temporal domain of the dataset, and using these as the subsets to align, it should be possible to deduce which structures are accommodating strain. This allows for
Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy.
Brun, B; Martins, F; Faniel, S; Hackens, B; Bachelier, G; Cavanna, A; Ulysse, C; Ouerghi, A; Gennser, U; Mailly, D; Huant, S; Bayot, V; Sanquer, M; Sellier, H
2014-01-01
Quantum point contacts exhibit mysterious conductance anomalies in addition to well-known conductance plateaus at multiples of 2e(2)/h. These 0.7 and zero-bias anomalies have been intensively studied, but their microscopic origin in terms of many-body effects is still highly debated. Here we use the charged tip of a scanning gate microscope to tune in situ the electrostatic potential of the point contact. While sweeping the tip distance, we observe repetitive splittings of the zero-bias anomaly, correlated with simultaneous appearances of the 0.7 anomaly. We interpret this behaviour in terms of alternating equilibrium and non-equilibrium Kondo screenings of different spin states localized in the channel. These alternating Kondo effects point towards the presence of a Wigner crystal containing several charges with different parities. Indeed, simulations show that the electron density in the channel is low enough to reach one-dimensional Wigner crystallization over a size controlled by the tip position. PMID:24978440
Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy
NASA Astrophysics Data System (ADS)
Brun, B.; Martins, F.; Faniel, S.; Hackens, B.; Bachelier, G.; Cavanna, A.; Ulysse, C.; Ouerghi, A.; Gennser, U.; Mailly, D.; Huant, S.; Bayot, V.; Sanquer, M.; Sellier, H.
2014-06-01
Quantum point contacts exhibit mysterious conductance anomalies in addition to well-known conductance plateaus at multiples of 2e2/h. These 0.7 and zero-bias anomalies have been intensively studied, but their microscopic origin in terms of many-body effects is still highly debated. Here we use the charged tip of a scanning gate microscope to tune in situ the electrostatic potential of the point contact. While sweeping the tip distance, we observe repetitive splittings of the zero-bias anomaly, correlated with simultaneous appearances of the 0.7 anomaly. We interpret this behaviour in terms of alternating equilibrium and non-equilibrium Kondo screenings of different spin states localized in the channel. These alternating Kondo effects point towards the presence of a Wigner crystal containing several charges with different parities. Indeed, simulations show that the electron density in the channel is low enough to reach one-dimensional Wigner crystallization over a size controlled by the tip position.
Non-linear conductance in quantum point contacts of noble metals
NASA Astrophysics Data System (ADS)
Yoshida, Makoto; Takayanagi, Kunio
2004-03-01
We studied the non-linear property of the electronic conductance of the noble metal nanocontact. Specimens were cleaned by Ar ion sputtering in UHV(`2 ˜10|7[Pa]) at room temperature. Current vs voltage curves (I-V curves) were obtained, while the metal contact was stretched by STM. The bias voltage at the contact was changed within 2V (using the triangle wave voltage 3`5kHz). Au, Pt, Ag and Cu quantum point contacts showed non-linear I-V curves. These metallic contacts presented the quantized conductance of the quantum unit G0(=2e2/h). I-V curves are fitted to a cubic function ( IaV+cV3 ). The value of c/a does not depend on the zero-bias conductance value, a. However, c/a values depend on metals (c/a ; Au=0.58 0.02, Ag=0.33 0.02, Cu= 0.40 0.03). The present result indicates that metals of lower resistance (higher mobility) give lower values of c/a.
NASA Astrophysics Data System (ADS)
Shahbazi, Maryam; Bourbonnais, Claude
2015-03-01
The electrical and thermal transport properties of the normal state of quasi-1D superconductors like Bechgaard salts are investigated by combining the linearised Boltzmann equation and the renormalisation group (RG) method. The collision integral operator is calculated using the Umklapp scattering amplitudes obtained by the RG method yielding the electrical resistivity(ρ) and Seebeck coefficient(S). The power law dependence, ρ (T) ~Tα , for resistivity is obtained by changing the antinesting parameter t⊥' simulating the pressure distance from the quantum critical point (QCP) between spin-density-wave (SDW) and d-wave SC (SCd) in the phase diagram. The resistivity evolves from a linear component (α ~= 1) at the QCP towards a Fermi liquid component (α ~= 2) with increasing t⊥', which confirms an extended region of quantum criticality as a result of interference between SCd and SDW causing an anomalous growth of Umklapp scattering. Its anisotropy is also tied to the k⊥-dependence of hot/cold scattering regions along the Fermi surface. Similar calculations for the Seebeck coefficient show deviations from the usual linear temperature dependence and also a change of sign near a SDW instability.
Quantum point contacts on two-dimensional electron gases with a strong spin-orbit coupling
NASA Astrophysics Data System (ADS)
Lee, Joon Sue; Pendaharkar, Mihir; Shojaei, Borzoyeh; McFadden, Anthony P.; Palmstrøm, Chris
Studies of electrical transport in one-dimensional semiconductors in a presence of a strong spin-orbit interaction are crucial not only for exploring the emergent phenomena, such as topological superconductivity, but also for potential spintronic applications by controlling of the electron spins. We investigate the electrical transport properties of one-dimensional confinement defined by electrostatic potentials on large area two-dimensional electron gases of InAs and InSb, which have a strong spin-orbit coupling. The high-quality InAs and InSb quantum wells are grown on antimonide buffers by molecular beam epitaxy, and the gate-tunable regions are created using Al2O3 or HfO2 gate dielectrics by atomic layer deposition. We will discuss the modulation of spin-orbit coupling in the two-dimensional electron gases and the spin-orbit-induced spin splitting by the split-gate quantum point contacts. This work was supported by Microsoft Research.
NASA Astrophysics Data System (ADS)
Kharkov, Yaroslav; Oleg P Sushkov Team
We consider two spin 1 / 2 fermions in a two-dimensional magnetic system that is close to the O (3) magnetic quantum critical point (QCP) which separates magnetically ordered and disordered phases. Focusing on the disordered phase in the vicinity of the QCP, we demonstrate that the criticality results in a strong long range attraction between the fermions, with potential V (r) ~ - 1 /rα , α ~ 0 . 75 , where r is separation between the fermions. The mechanism of the enhanced attraction is similar to Casimir effect and corresponds to multi-magnon exchange processes between the fermions. While we consider a model system, the problem is originally motivated by recent experimental establishment of magnetic QCP in hole doped cuprates under the superconducting dome at doping of about 10%. We suggest the mechanism of magnetic critical enhancement of pairing in cuprates.
Casten, R. F.; Bonatsos, Dennis; McCutchan, E. A.
2009-01-28
Recently, a new signature for quantum phase transitional regions has been discussed. This signature, based on degeneracies of yrast and intrinsic excitations, can distinguish first and second order phase transitions, and is valid not only at or near the analytic critical points described by X(5) and E(5), but along the phase transitional line connecting them as well. In addition, a study of a number of recent analytic solutions to the Bohr Hamiltonian and of the dynamical symmetries of the IBA Hamiltonian has revealed a set of extremely simple and general analytic formulas that describe the energies of 0{sup +} states. For the case of flat-bottomed geometrical potentials, the formula depends solely on the number of relevant dimensions. For the IBA (large boson number limit) a single formula describes all three dynamical symmetries.
Lian, Xiaojuan Cartoixà, Xavier; Miranda, Enrique; Suñé, Jordi; Perniola, Luca; Rurali, Riccardo; Long, Shibing; Liu, Ming
2014-06-28
We depart from first-principle simulations of electron transport along paths of oxygen vacancies in HfO{sub 2} to reformulate the Quantum Point Contact (QPC) model in terms of a bundle of such vacancy paths. By doing this, the number of model parameters is reduced and a much clearer link between the microscopic structure of the conductive filament (CF) and its electrical properties can be provided. The new multi-scale QPC model is applied to two different HfO{sub 2}-based devices operated in the unipolar and bipolar resistive switching (RS) modes. Extraction of the QPC model parameters from a statistically significant number of CFs allows revealing significant structural differences in the CF of these two types of devices and RS modes.
Ballistic Transport and Exchange Interaction in InAs Nanowire Quantum Point Contacts.
Heedt, S; Prost, W; Schubert, J; Grützmacher, D; Schäpers, Th
2016-05-11
One-dimensional ballistic transport is demonstrated for a high-mobility InAs nanowire device. Unlike conventional quantum point contacts (QPCs) created in a two-dimensional electron gas, the nanowire QPCs represent one-dimensional constrictions formed inside a quasi-one-dimensional conductor. For each QPC, the local subband occupation can be controlled individually between zero and up to six degenerate modes. At large out-of-plane magnetic fields Landau quantization and Zeeman splitting emerge and comprehensive voltage bias spectroscopy is performed. Confinement-induced quenching of the orbital motion gives rise to significantly modified subband-dependent Landé g factors. A pronounced g factor enhancement related to Coulomb exchange interaction is reported. Many-body effects of that kind also manifest in the observation of the 0.7·2e(2)/h conductance anomaly, commonly found in planar devices. PMID:27104768
Tunable graphene quantum point contact transistor for DNA detection and characterization
Girdhar, Anuj; Sathe, Chaitanya; Schulten, Klaus; Leburton, Jean-Pierre
2015-01-01
A graphene membrane conductor containing a nanopore in a quantum point contact (QPC) geometry is a promising candidate to sense, and potentially sequence, DNA molecules translocating through the nanopore. Within this geometry, the shape, size, and position of the nanopore as well as the edge configuration influences the membrane conductance caused by the electrostatic interaction between the DNA nucleotides and the nanopore edge. It is shown that the graphene conductance variations resulting from DNA translocation can be enhanced by choosing a particular geometry as well as by modulating the graphene Fermi energy, which demonstrates the ability to detect conformational transformations of a double-stranded DNA, as well as the passage of individual base pairs of a single-stranded DNA molecule through the nanopore. PMID:25765702
Electronic Magnetization of a Quantum Point Contact Measured by Nuclear Magnetic Resonance
NASA Astrophysics Data System (ADS)
Kawamura, Minoru; Ono, Keiji; Stano, Peter; Kono, Kimitoshi; Aono, Tomosuke
2015-07-01
We report an electronic magnetization measurement of a quantum point contact (QPC) based on nuclear magnetic resonance (NMR) spectroscopy. We find that NMR signals can be detected by measuring the QPC conductance under in-plane magnetic fields. This makes it possible to measure, from Knight shifts of the NMR spectra, the electronic magnetization of a QPC containing only a few electron spins. The magnetization changes smoothly with the QPC potential barrier height and peaks at the conductance plateau of 0.5 ×2 e2/h . The observed features are well captured by a model calculation assuming a smooth potential barrier, supporting a no bound state origin of the 0.7 structure.
Mapping the current-current correlation function near a quantum critical point
NASA Astrophysics Data System (ADS)
Prodan, Emil; Bellissard, Jean
2016-05-01
The current-current correlation function is a useful concept in the theory of electron transport in homogeneous solids. The finite-temperature conductivity tensor as well as Anderson's localization length can be computed entirely from this correlation function. Based on the critical behavior of these two physical quantities near the plateau-insulator or plateau-plateau transitions in the integer quantum Hall effect, we derive an asymptotic formula for the current-current correlation function, which enables us to make several theoretical predictions about its generic behavior. For the disordered Hofstadter model, we employ numerical simulations to map the current-current correlation function, obtain its asymptotic form near a critical point and confirm the theoretical predictions.
Multi-Valued Logic Gates based on Ballistic Transport in Quantum Point Contacts
Seo, M.; Hong, C.; Lee, S.-Y.; Choi, H. K.; Kim, N.; Chung, Y.; Umansky, V.; Mahalu, D.
2014-01-01
Multi-valued logic gates, which can handle quaternary numbers as inputs, are developed by exploiting the ballistic transport properties of quantum point contacts in series. The principle of a logic gate that finds the minimum of two quaternary number inputs is demonstrated. The device is scalable to allow multiple inputs, which makes it possible to find the minimum of multiple inputs in a single gate operation. Also, the principle of a half-adder for quaternary number inputs is demonstrated. First, an adder that adds up two quaternary numbers and outputs the sum of inputs is demonstrated. Second, a device to express the sum of the adder into two quaternary digits [Carry (first digit) and Sum (second digit)] is demonstrated. All the logic gates presented in this paper can in principle be extended to allow decimal number inputs with high quality QPCs. PMID:24448272
NASA Technical Reports Server (NTRS)
Rueda, A.
1985-01-01
That particles may be accelerated by vacuum effects in quantum field theory has been repeatedly proposed in the last few years. A natural upshot of this is a mechanism for cosmic rays (CR) primaries acceleration. A mechanism for acceleration by the zero-point field (ZPE) when the ZPE is taken in a realistic sense (in opposition to a virtual field) was considered. Originally the idea was developed within a semiclassical context. The classical Einstein-Hopf model (EHM) was used to show that free isolated electromagnrtically interacting particles performed a random walk in phase space and more importantly in momentum space when submitted to the perennial action of the so called classical electromagnrtic ZPE.
Fate of the Wiedemann-Franz Law near Quantum Critical Points of Electron Systems in Solids
NASA Astrophysics Data System (ADS)
Khodel, V. A.; Clark, J. W.; Shaginyan, V. R.; Zverev, M. V.
2015-12-01
We introduce and analyze two different scenarios for violation of the Wiedemann-Franz law in strongly correlated electron systems of solids, close to a topological quantum critical point (TQCP) where the density of states N(0) diverges. The first, applicable to the Fermi-liquid (FL) side of the TQCP, involves a transverse zero-sound collective mode that opens a new channel for the thermal conductivity, thereby enhancing the Lorenz number L(0) relative to the value L0 =π2 k B 2/3 e 2 dictated by conventional FL theory. The second mechanism for violation of the WF law, relevant to the non-Fermi-liquid (NFL) side of the TQCP, involves the formation of a flat band and leads instead to a reduction of the Lorenz number.
Equation of state for a trapped quantum gas: remnant of zero-point energy effects
NASA Astrophysics Data System (ADS)
Castilho, P. C. M.; Poveda-Cuevas, F. J.; Seman, J. A.; Shiozaki, R. F.; Roati, G.; Muniz, S. R.; Magalhães, D. V.; Bagnato, V. S.
2016-02-01
The study of the thermodynamic properties of trapped gases has attracted great attention during the last few years and can be used as a tool to characterize such clouds in the presence of other phenomena. Here, we obtain an equation of state for a harmonically trapped Bose-Einstein condensate taking the limit of T\\to 0 by means of global themodynamic variables. These variables allow us to explore limits in which the standard thermodynamics are not defined. Our results are taken in the high density limit, and the extrapolation for N\\to 1 is done later. Even in this situation, we qualitatively observe the well known existence of a zero-point energy for harmonic potentials in which the determination of conjugated variables is limited by the quantum nature of the system.
Entropy excess in strongly correlated Fermi systems near a quantum critical point
NASA Astrophysics Data System (ADS)
Clark, J. W.; Zverev, M. V.; Khodel, V. A.
2012-12-01
A system of interacting, identical fermions described by standard Landau Fermi-liquid (FL) theory can experience a rearrangement of its Fermi surface if the correlations grow sufficiently strong, as occurs at a quantum critical point where the effective mass diverges. As yet, this phenomenon defies full understanding, but salient aspects of the non-Fermi-liquid (NFL) behavior observed beyond the quantum critical point are still accessible within the general framework of the Landau quasiparticle picture. Self-consistent solutions of the coupled Landau equations for the quasiparticle momentum distribution n(p) and quasiparticle energy spectrum ɛ(p) are shown to exist in two distinct classes, depending on coupling strength and on whether the quasiparticle interaction is regular or singular at zero momentum transfer. One class of solutions maintains the idempotency condition n2(p)=n(p) of standard FL theory at zero temperature T while adding pockets to the Fermi surface. The other solutions are characterized by a swelling of the Fermi surface and a flattening of the spectrum ɛ(p) over a range of momenta in which the quasiparticle occupancies lie between 0 and 1 even at T=0. The latter, non-idempotent solution is revealed by analysis of a Poincaré mapping associated with the fundamental Landau equation connecting n(p) and ɛ(p) and validated by solution of a variational condition that yields the symmetry-preserving ground state. Significantly, this extraordinary solution carries the burden of a large temperature-dependent excess entropy down to very low temperatures, threatening violation of the Nernst Theorem. It is argued that certain low-temperature phase transitions, notably those involving Cooper-pair formation, offer effective mechanisms for shedding the entropy excess. Available measurements in heavy-fermion compounds provide concrete support for such a scenario.
Det-Det correlations for quantum maps: Dual pair and saddle-point analyses
NASA Astrophysics Data System (ADS)
Nonnenmacher, S.; Zirnbauer, M. R.
2002-05-01
An attempt is made to clarify the ballistic nonlinear sigma model formalism recently proposed for quantum chaotic systems, by looking at the spectral determinant Z(s)=Det(1-sU) for quantized maps U∈U(N), and studying the correlator ωU(s)=∫dθ|Z(eiθs)|2. By identifying U(N) as one member of a dual pair acting in the spinor representation of Spin(4N), the expansion of ωU(s) in powers of s2 is shown to be a decomposition into irreducible characters of U(N). In close analogy with the ballistic nonlinear sigma model, a coherent-state integral representation of ωU(s) is developed. For generic U this integral has (N2N) saddle points and the leading-order saddle-point approximation turns out to reproduce ωU(s) exactly, up to a constant factor. This miracle is explained by interpreting ωU(s) as a character of U(2N), and arguing that the leading-order saddle-point result corresponds to the Weyl character formula. Unfortunately, the Weyl decomposition behaves nonsmoothly in the semiclassical limit N→∞, and to make further progress some additional averaging needs to be introduced. Several schemes are investigated, including averaging over basis states and an "isotropic" average. The saddle-point approximation applied in conjunction with these schemes is demonstrated to give incorrect results in general, one notable exception being a semiclassical averaging scheme, for which all loop corrections vanish identically. As a side product of the dual pair decomposition with isotropic averaging, the crossover between the Poisson and CUE limits is obtained.
Magnification of signatures of a topological phase transition by quantum zero point motion
NASA Astrophysics Data System (ADS)
Lopes, Pedro L. e. S.; Ghaemi, Pouyan
2015-08-01
We show that the zero point motion of a vortex in superconducting doped topological insulators leads to significant changes in the electronic spectrum at the topological phase transition in this system. This topological phase transition is tuned by the doping level, and the corresponding effects are manifest in the density of states at energies which are on the order of the vortex fluctuation frequency. Although the electronic energy gap in the spectrum generated by a stationary vortex is but a small fraction of the bulk superconducting gap, the vortex fluctuation frequency may be much larger. As a result, this quantum zero point motion can induce a discontinuous change in the spectral features of the system at the topological vortex phase transition to energies which are well within the resolution of scanning tunneling microscopy. This discontinuous change is exclusive to superconducting systems in which we have a topological phase transition. Moreover, the phenomena studied in this paper present effects of Magnus forces on the vortex spectrum which are not present in the ordinary s -wave superconductors. Finally, we demonstrate explicitly that the vortex in this system is equivalent to a Kitaev chain. This allows for the mapping of the vortex fluctuating scenario in three dimensions into similar one-dimensional situations in which one may search for other novel signatures of topological phase transitions.
NASA Astrophysics Data System (ADS)
Roszak, K.; Cywiński, Ł.
2015-10-01
We study quantum teleportation via Bell-diagonal mixed states of two qubits in the context of the intrinsic properties of the quantum discord. We show that when the quantum-correlated state of the two qubits is used for quantum teleportation, the character of the teleportation efficiency changes substantially depending on the Bell-diagonal-state parameters, which can be seen when the worst-case-scenario or best-case-scenario fidelity is studied. Depending on the parameter range, one of two types of single-qubit states is hardest/easiest to teleport. The transition between these two parameter ranges coincides exactly with the transition between the range of classical correlation decay and quantum correlation decay characteristic for the evolution of the quantum discord. The correspondence provides a physical interpretation for the prominent feature of the decay of the quantum discord.
Measuring the distance from saddle points and driving to locate them over quantum control landscapes
NASA Astrophysics Data System (ADS)
Sun, Qiuyang; Riviello, Gregory; Wu, Re-Bing; Rabitz, Herschel
2015-11-01
Optimal control of quantum phenomena involves the introduction of a cost functional J to characterize the degree of achieving a physical objective by a chosen shaped electromagnetic field. The cost functional dependence upon the control forms a control landscape. Two theoretically important canonical cases are the landscapes associated with seeking to achieve either a physical observable or a unitary transformation. Upon satisfaction of particular assumptions, both landscapes are analytically known to be trap-free, yet possess saddle points at precise suboptimal J values. The presence of saddles on the landscapes can influence the effort needed to find an optimal field. As a foundation to future algorithm development and analyzes, we define metrics that identify the ‘distance’ from a given saddle based on the sufficient and necessary conditions for the existence of the saddles. Algorithms are introduced utilizing the metrics to find a control such that the dynamics arrive at a targeted saddle. The saddle distance metric and saddle-seeking methodology is tested numerically in several model systems.
Nematic quantum critical point without magnetism in FeSe1-xSx superconductors.
Hosoi, Suguru; Matsuura, Kohei; Ishida, Kousuke; Wang, Hao; Mizukami, Yuta; Watashige, Tatsuya; Kasahara, Shigeru; Matsuda, Yuji; Shibauchi, Takasada
2016-07-19
In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1-xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near [Formula: see text], the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1-xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity. PMID:27382157
Spatiotemporal evolution of topological order upon quantum quench across the critical point
NASA Astrophysics Data System (ADS)
Lee, Minchul; Han, Seungju; Choi, Mahn-Soo
2016-06-01
We consider a topological superconducting wire and use the string order parameter to investigate the spatiotemporal evolution of the topological order upon a quantum quench across the critical point. We also analyze the propagation of the initially localized Majorana bound states after the quench, in order to examine the connection between the topological order and the unpaired Majorana states, which has been well established at equilibrium but remains illusive in dynamical situations. It is found that after the quench the string order parameters decay over a finite time and that the decaying behavior is universal, independent of the wire length and the final value of the chemical potential (the quenching parameter). It is also found that the topological order is revived repeatedly although the amplitude gradually decreases. Further, the topological order can propagate into the region which was initially in the nontopological state. It is observed that all these behaviors are in parallel and consistent with the propagation and dispersion of the Majorana wave functions. Finally, we propose local probing methods which can measure the nonlocal topological order.
Spin splitting generated in a Y-shaped semiconductor nanostructure with a quantum point contact
Wójcik, P. Adamowski, J. Wołoszyn, M.; Spisak, B. J.
2015-07-07
We have studied the spin splitting of the current in the Y-shaped semiconductor nanostructure with a quantum point contact (QPC) in a perpendicular magnetic field. Our calculations show that the appropriate tuning of the QPC potential and the external magnetic field leads to an almost perfect separation of the spin-polarized currents: electrons with opposite spins flow out through different output branches. The spin splitting results from the joint effect of the QPC, the spin Zeeman splitting, and the electron transport through the edge states formed in the nanowire at the sufficiently high magnetic field. The Y-shaped nanostructure can be used to split the unpolarized current into two spin currents with opposite spins as well as to detect the flow of the spin current. We have found that the separation of the spin currents is only slightly affected by the Rashba spin-orbit coupling. The spin-splitter device is an analogue of the optical device—the birefractive crystal that splits the unpolarized light into two beams with perpendicular polarizations. In the magnetic-field range, in which the current is carried through the edges states, the spin splitting is robust against the spin-independent scattering. This feature opens up a possibility of the application of the Y-shaped nanostructure as a non-ballistic spin-splitter device in spintronics.
The Occurrence of Anomalous Conductance Plateaus and Spin Textures in Quantum Point Contacts
NASA Astrophysics Data System (ADS)
Wan, J.; Cahay, M.; Debray, P.; Newrock, R.
2010-03-01
Recently, we used a NEGF formalism [1] to provide a theoretical explanation for the experimentally observed 0.5G0 (G0=2e^2/h) plateau in the conductance of side-gated quantum point contacts (QPCs) in the presence of lateral spin-orbit coupling (LSOC) [2]. We showed that the 0.5G0 plateau appears in the QPCs without any external magnetic field as a result of three ingredients: an asymmetric lateral confinement, a LSOC, and a strong electron-electron (e-e) interaction. In this report, we present the results of simulations for a wide range of QPC dimensions and biasing parameters showing that the same physics predicts the appearance of other anomalous plateaus at non-integer values of G0, including the well-known 0.7G0 anomaly. These features are related to a plethora of spin textures in the QPC that depend sensitively on material, device, biasing parameters, temperature, and the strength of the e-e interaction. [1] J. Wan, M. Cahay, P. Debray, and R.S. Newrock, Phys. Rev. B 80, 155440 (2009). [2] P. Debray, S.M. Rahman, J. Wan, R.S. Newrock, M. Cahay, A.T. Ngo, S.E. Ulloa, S.T. Herbert, M. Muhammad, and M. Johnson, Nature Nanotech. 4, 759 (2009).
Second wind of the Dulong-Petit law at a quantum critical point
NASA Astrophysics Data System (ADS)
Khodel, V. A.; Clark, J. W.; Shaginyan, V. R.; Zverev, M. V.
2010-10-01
Renewed interest in 3He physics has been stimulated by experimental observation of non-Fermi-liquid behavior of dense 3He films at low temperatures. Abnormal behavior of the specific heat C( T) of two-dimensional liquid 3He is demonstrated in the occurrence of a T-independent term in C( T). To uncover the origin of this phenomenon, we have considered the group velocity of transverse zero sound propagating in a strongly correlated Fermi liquid. For the first time, it is shown that if two-dimensional liquid 3He is located in the vicinity of the quantum critical point associated with a divergent quasiparticle effective mass, the group velocity depends strongly on temperature and vanishes as T is lowered toward zero. The predicted vigorous dependence of the group velocity can be detected in experimental measurements on liquid 3He films. We have demonstrated that the contribution to the specific heat coming from the boson part of the free energy due to the transverse zero-sound mode follows the Dulong-Petit Law. In the case of two-dimensional liquid 3He, the specific heat becomes independent of temperature at some characteristic temperature of a few millikelvins.
NASA Astrophysics Data System (ADS)
Sahoo, Sharmistha; Stoudenmire, E. Miles; Stéphan, Jean-Marie; Devakul, Trithep; Singh, Rajiv R. P.; Melko, Roger G.
2016-02-01
At a quantum critical point, bipartite entanglement entropies have universal quantities which are subleading to the ubiquitous area law. For Renyi entropies, these terms are known to be similar to the von Neumann entropy, while being much more amenable to numerical and even experimental measurement. We show here that when calculating universal properties of Renyi entropies, it is important to account for unusual corrections to scaling that arise from relevant local operators present at the conical singularity in the multisheeted Riemann surface. These corrections grow in importance with increasing Renyi index. We present studies of Renyi correlation functions in the 1 +1 transverse-field Ising model (TFIM) using conformal field theory, mapping to free fermions, and series expansions, and the logarithmic entropy singularity at a corner in 2 +1 for both free bosonic field theory and the TFIM, using numerical linked cluster expansions. In all numerical studies, accurate results are only obtained when unusual corrections to scaling are taken into account. In the worst case, an analysis ignoring these corrections can get qualitatively incorrect answers, such as predicting a decrease in critical exponents with the Renyi index, when they are actually increasing. We discuss a two-step extrapolation procedure that can be used to account for the unusual corrections to scaling.
Interference features in scanning gate conductance maps of quantum point contacts with disorder
NASA Astrophysics Data System (ADS)
Kolasiński, K.; Szafran, B.; Brun, B.; Sellier, H.
2016-08-01
We consider quantum point contact (QPC) defined within a disordered two-dimensional electron gas as studied by scanning gate microscopy. We evaluate the conductance maps in the Landauer approach with a wave-function picture of electron transport for samples with both low and high electron mobility at finite temperatures. We discuss the spatial distribution of the impurities in the context of the branched electron flow. We reproduce the surprising temperature stability of the experimental interference fringes far from the QPC. Next, we discuss funnel-shaped features that accompany splitting of the branches visible in previous experiments. Finally, we study elliptical interference fringes formed by an interplay of scattering by the pointlike impurities and by the scanning probe. We discuss the details of the elliptical features as functions of the tip voltage and the temperature, showing that the first interference fringe is very robust against the thermal widening of the Fermi level. We present a simple analytical model that allows for extraction of the impurity positions and the electron-gas depletion radius induced by the negatively charged tip of the atomic force microscope, and apply this model on experimental scanning gate images showing such elliptical fringes.
Spin splitting generated in a Y-shaped semiconductor nanostructure with a quantum point contact
NASA Astrophysics Data System (ADS)
Wójcik, P.; Adamowski, J.; Wołoszyn, M.; Spisak, B. J.
2015-07-01
We have studied the spin splitting of the current in the Y-shaped semiconductor nanostructure with a quantum point contact (QPC) in a perpendicular magnetic field. Our calculations show that the appropriate tuning of the QPC potential and the external magnetic field leads to an almost perfect separation of the spin-polarized currents: electrons with opposite spins flow out through different output branches. The spin splitting results from the joint effect of the QPC, the spin Zeeman splitting, and the electron transport through the edge states formed in the nanowire at the sufficiently high magnetic field. The Y-shaped nanostructure can be used to split the unpolarized current into two spin currents with opposite spins as well as to detect the flow of the spin current. We have found that the separation of the spin currents is only slightly affected by the Rashba spin-orbit coupling. The spin-splitter device is an analogue of the optical device—the birefractive crystal that splits the unpolarized light into two beams with perpendicular polarizations. In the magnetic-field range, in which the current is carried through the edges states, the spin splitting is robust against the spin-independent scattering. This feature opens up a possibility of the application of the Y-shaped nanostructure as a non-ballistic spin-splitter device in spintronics.
Quench dynamics near a quantum critical point: Application to the sine-Gordon model
NASA Astrophysics Data System (ADS)
de Grandi, C.; Gritsev, V.; Polkovnikov, A.
2010-06-01
We discuss the quench dynamics near a quantum critical point focusing on the sine-Gordon model as a primary example. We suggest a unified approach to sudden and slow quenches, where the tuning parameter λ(t) changes in time as λ(t)˜υtr , based on the adiabatic expansion of the excitation probability in powers of υ . We show that the universal scaling of the excitation probability can be understood through the singularity of the generalized adiabatic susceptibility χ2r+2(λ) , which for sudden quenches (r=0) reduces to the fidelity susceptibility. In turn this class of susceptibilities is expressed through the moments of the connected correlation function of the quench operator. We analyze the excitations created after a sudden quench of the cosine potential using a combined approach of form-factors expansion and conformal perturbation theory for the low-energy and high-energy sector, respectively. We find the general scaling laws for the probability of exciting the system, the density of excited quasiparticles, the entropy and the heat generated after the quench. In the two limits where the sine-Gordon model maps to hard-core bosons and free massive fermions we provide the exact solutions for the quench dynamics and discuss the finite temperature generalizations.
Terahertz time domain interferometry of a SIS tunnel junction and a quantum point contact
Karadi, C
1995-09-01
The author has applied the Terahertz Time Domain Interferometric (THz-TDI) technique to probe the ultrafast dynamic response of a Superconducting-Insulating-Superconducting (SIS) tunnel junction and a Quantum Point Contact (QPC). The THz-TDI technique involves monitoring changes in the dc current induced by interfering two picosecond electrical pulses on the junction as a function of time delay between them. Measurements of the response of the Nb/AlO{sub x}/Nb SIS tunnel junction from 75--200 GHz are in full agreement with the linear theory for photon-assisted tunneling. Likewise, measurements of the induced current in a QPC as a function of source-drain voltage, gate voltage, frequency, and magnetic field also show strong evidence for photon-assisted transport. These experiments together demonstrate the general applicability of the THz-TDI technique to the characterization of the dynamic response of any micron or nanometer scale device that exhibits a non-linear I-V characteristic. 133 refs., 49 figs.
Is U3Ni3Sn4 best described as near a quantum critical point?
Booth, C.H.; Shlyk, L.; Nenkov, K.; Huber, J.G.; De Long, L.E.
2003-04-08
Although most known non-Fermi liquid (NFL) materials are structurally or chemically disordered, the role of this disorder remains unclear. In particular, very few systems have been discovered that may be stoichiometric and well ordered. To test whether U{sub 3}Ni{sub 3}Sn{sub 4} belongs in this latter class, we present measurements of the x-ray absorption fine structure (XAFS) of polycrystalline and single-crystal U{sub 3}Ni{sub 3}Sn{sub 4} samples that are consistent with no measurable local atomic disorder. We also present temperature-dependent specific heat data in applied magnetic fields as high as 8 T that show features that are inconsistent with the antiferromagnetic Griffiths' phase model, but do support the conclusion that a Fermi liquid/NFL crossover temperature increases with applied field. These results are inconsistent with theoretical explanations that require strong disorder effects, but do support the view that U{sub 3}Ni{sub 3}Sn{sub 4} is a stoichoiometric, ordered material that exhibits NFL behavior, and is best described as being near an antiferromagnetic quantum critical point.
Emergent Lorentz symmetry near fermionic quantum critical points in two and three dimensions
NASA Astrophysics Data System (ADS)
Roy, Bitan; Juričić, Vladimir; Herbut, Igor F.
2016-04-01
We study the renormalization group flow of the velocities in the field theory describing the coupling of the massless quasi-relativistic fermions to the bosons through the Yukawa coupling, as well as with both bosons and fermions coupled to a fluctuating U(1) gauge field in two and three spatial dimensions. Different versions of this theory describe quantum critical behavior of interacting Dirac fermions in various condensed-matter systems. We perform an analysis using one-loop ɛ-expansion about three spatial dimensions, which is the upper critical dimension in the problem. In two dimensions, we find that velocities of both charged fermions and bosons ultimately flow to the velocity of light, independently of the initial conditions, the number of fermionic and bosonic flavors, and the value of the couplings at the critical point. In three dimensions, due to the analyticity of the gauge field propagator, both the U(1) charge and the velocity of light flow, which leads to a richer behavior than in two dimensions. We show that all three velocities ultimately flow to a common terminal velocity, which is non-universal and different from the original velocity of light. Therefore, emergence of the Lorentz symmetry in the ultimate infrared regime seems to be a rather universal feature of this class of theories in both two and three dimensions.
Schroeder, Almut; Ubaid-Kassis, Sara; Vojta, Thomas
2011-03-01
We report magnetization measurements close to the ferromagnetic quantum phase transition of the d-metal alloy Ni(1 - x)V(x) at a vanadium concentration of x(c)≈11.4%. In the diluted regime (x > x(c)), the temperature (T) and magnetic field (H) dependences of the magnetization are characterized by nonuniversal power laws and display H/T scaling in a wide temperature and field range. The exponents vary strongly with x and follow the predictions of a quantum Griffiths phase. We also discuss the deviations and limits of the quantum Griffiths phase as well as the phase boundaries due to bulk and cluster physics. PMID:21339558
Virk, Sohrab S; Niedermeier, Steven; Yu, Elizabeth; Khan, Safdar N
2014-08-01
EDUCATIONAL OBJECTIVES As a result of reading this article, physicians should be able to: 1. Understand the forces that predispose adjacent cervical segments to degeneration. 2. Understand the challenges of radiographic evaluation in the diagnosis of cervical and lumbar adjacent segment disease. 3. Describe the changes in biomechanical forces applied to adjacent segments of lumbar vertebrae with fusion. 4. Know the risk factors for adjacent segment disease in spinal fusion. Adjacent segment disease (ASD) is a broad term encompassing many complications of spinal fusion, including listhesis, instability, herniated nucleus pulposus, stenosis, hypertrophic facet arthritis, scoliosis, and vertebral compression fracture. The area of the cervical spine where most fusions occur (C3-C7) is adjacent to a highly mobile upper cervical region, and this contributes to the biomechanical stress put on the adjacent cervical segments postfusion. Studies have shown that after fusion surgery, there is increased load on adjacent segments. Definitive treatment of ASD is a topic of continuing research, but in general, treatment choices are dictated by patient age and degree of debilitation. Investigators have also studied the risk factors associated with spinal fusion that may predispose certain patients to ASD postfusion, and these data are invaluable for properly counseling patients considering spinal fusion surgery. Biomechanical studies have confirmed the added stress on adjacent segments in the cervical and lumbar spine. The diagnosis of cervical ASD is complicated given the imprecise correlation of radiographic and clinical findings. Although radiological and clinical diagnoses do not always correlate, radiographs and clinical examination dictate how a patient with prolonged pain is treated. Options for both cervical and lumbar spine ASD include fusion and/or decompression. Current studies are encouraging regarding the adoption of arthroplasty in spinal surgery, but more long
Lyo, S.K.
1999-01-04
We show that the low-temperature conductance (G) of a quantum point contact consisting of ballistic tunnel-coupled double-layer quantum well wires is modulated by an in-layer magnetic field B{sub {parallel}} perpendicular to the wires due to the anticrossing. In a system with a small g factor, B{sub {parallel}} creates a V-shaped quantum staircase for G, causing it to decrease in steps of 2e{sup 2}/{Dirac_h} to a minimum and then increase to a maximum value, where G may saturate or decrease again at higher B{sub {parallel}}'s. The effect of B{sub {parallel}}-induced mass enhancement and spin splitting is studied. The relevance of the results to recent data is discussed.
NASA Astrophysics Data System (ADS)
Pakmehr, M.; Whiteside, V. R.; Bhandari, N.; Newrock, R.; Cahay, M.; McCombe, B. D.
2013-08-01
We have studied the THz magneto-photoresponse of a 2DEG in an InAs quantum well with an embedded Quantum Point Contact in the frequency/field region where electron cyclotron resonance (CR) dominates the response. The photoresponse near CR is manifested as an envelope of the amplitude of the Shubnikov-de Haas oscillations of the 2DEG with a peak near the CR field. Clear spin-splitting of the quantum oscillations is observed for B > 4 T. Data were simulated by a model of resonant carrier heating, and from the simulations the carrier density, the CR effective mass, scattering times and the g-factor were obtained. We find a significantly enhanced g-factor apparently due to exchange interaction.
Spin-orbit splitting of valence and conduction bands in HgTe quantum wells near the Dirac point
NASA Astrophysics Data System (ADS)
Minkov, G. M.; Germanenko, A. V.; Rut, O. E.; Sherstobitov, A. A.; Nestoklon, M. O.; Dvoretski, S. A.; Mikhailov, N. N.
2016-04-01
Energy spectra both of the conduction and valence bands of the HgTe quantum wells with a width close to the Dirac point were studied experimentally. Simultaneous analysis of the Shubnikov-de Haas oscillations and the Hall effect over a wide range of electron and hole densities yields surprising results: the top of the valence band is strongly split by spin-orbit interaction while the splitting of the conduction band is absent, within experimental accuracy. This holds true for the structures with normal and inverted band ordering. The results obtained are inconsistent with the results of kP calculations, in which the smooth electric field across the quantum well is only reckoned in. It is shown that taking into account the asymmetry of the quantum-well interfaces within a tight-binding method gives reasonable agreement with the experimental data.
Quantization and anomalous structures in the conductance of Si/SiGe quantum point contacts
NASA Astrophysics Data System (ADS)
von Pock, J. F.; Salloch, D.; Qiao, G.; Wieser, U.; Hackbarth, T.; Kunze, U.
2016-04-01
Quantum point contacts (QPCs) are fabricated on modulation-doped Si/SiGe heterostructures and ballistic transport is studied at low temperatures. We observe quantized conductance with subband separations up to 4 meV and anomalies in the first conductance plateau at 4e2/h. At a temperature of T = 22 mK in the linear transport regime, a weak anomalous kink structure arises close to 0.5(4e2/h), which develops into a distinct plateau-like structure as temperature is raised up to T = 4 K. Under magnetic field parallel to the wire up to B = 14 T, the anomaly evolves into the Zeeman spin-split level at 0.5(4e2/h), resembling the "0.7 anomaly" in GaAs/AlGaAs QPCs. Additionally, a zero-bias anomaly (ZBA) is observed in nonlinear transport spectroscopy. At T = 22 mK, a parallel magnetic field splits the ZBA peak up into two peaks. At B = 0, elevated temperatures lead to similar splitting, which differs from the behavior of ZBAs in GaAs/AlGaAs QPCs. Under finite dc bias, the differential resistance exhibits additional plateaus approximately at 0.8(4e2/h) and 0.2(4e2/h) known as "0.85 anomaly" and "0.25 anomaly" in GaAs/AlGaAs QPCs. Unlike the first regular plateau at 4e2/h, the 0.2(4e2/h) plateau is insensitive to dc bias voltage up to at least VDS = 80 mV, in-plane magnetic fields up to B = 15 T, and to elevated temperatures up to T = 25 K. We interpret this effect as due to pinching off one of the reservoirs close to the QPC. We do not see any indication of lifting of the valley degeneracy in our samples.
The Casimir Effect from the Point of View of Algebraic Quantum Field Theory
NASA Astrophysics Data System (ADS)
Dappiaggi, Claudio; Nosari, Gabriele; Pinamonti, Nicola
2016-06-01
We consider a region of Minkowski spacetime bounded either by one or by two parallel, infinitely extended plates orthogonal to a spatial direction and a real Klein-Gordon field satisfying Dirichlet boundary conditions. We quantize these two systems within the algebraic approach to quantum field theory using the so-called functional formalism. As a first step we construct a suitable unital ∗-algebra of observables whose generating functionals are characterized by a labelling space which is at the same time optimal and separating and fulfils the F-locality property. Subsequently we give a definition for these systems of Hadamard states and we investigate explicit examples. In the case of a single plate, it turns out that one can build algebraic states via a pull-back of those on the whole Minkowski spacetime, moreover inheriting from them the Hadamard property. When we consider instead two plates, algebraic states can be put in correspondence with those on flat spacetime via the so-called method of images, which we translate to the algebraic setting. For a massless scalar field we show that this procedure works perfectly for a large class of quasi-free states including the Poincaré vacuum and KMS states. Eventually Wick polynomials are introduced. Contrary to the Minkowski case, the extended algebras, built in globally hyperbolic subregions can be collected in a global counterpart only after a suitable deformation which is expressed locally in terms of a *-isomorphism. As a last step, we construct explicitly the two-point function and the regularized energy density, showing, moreover, that the outcome is consistent with the standard results of the Casimir effect.
Second-order coherence of microwave photons emitted by a quantum point contact
NASA Astrophysics Data System (ADS)
Hassler, Fabian; Otten, Daniel
2015-11-01
Shot noise of electrons that are transmitted with probability T through a quantum point contact (biased at a voltage V0) leads to a fluctuating current that in turn emits radiation in the microwave regime. By calculating the Fano factor F for the case where only a single channel contributes to the transport, it has been shown that the radiation produced at finite frequency ω0 close to e V0/ℏ and at low temperatures is nonclassical with sub-Poissonian statistics (F <1 ). The origin of this effect is the fermionic nature of the electrons producing the radiation, which reduces the probability of simultaneous emission of two or more photons. However, the Fano factor, being a time-averaged quantity, offers only limited information about the system. Here, we calculate the second-order coherence g(2 )(τ ) for this source of radiation. We show that due to the interference of two contributions, two photon processes (leading to bunching) are completely absent at zero temperature for T =50 % . At low temperatures, we find a competition of the contribution due to Gaussian current-current fluctuations (leading to bunching) with the one due to non-Gaussian fluctuations (leading to antibunching). At slightly elevated temperatures, the non-Gaussian contribution becomes suppressed, whereas the Gaussian contributions remain largely independent of temperature. We show that the competition of the two contributions leads to a nonmonotonic behavior of the second-order coherence as a function of time. As a result, g(2 )(τ ) obtains a minimal value for times τ*≃ω0-1 . Close to this time, the second-order coherence remains below 1 at temperatures where the Fano factor is already above 1. We identify realistic experimental parameters that can be used to test the sub-Poissonian nature of the radiation.
All-electrical nonlinear fano resonance in coupled quantum point contacts
NASA Astrophysics Data System (ADS)
Xiao, Shiran
This thesis is motivated by recent interest in the Fano resonance (FR). As a wave-interference phenomenon, this resonance is of increasing importance in optics, plasmon-ics, and metamaterials, where its ability to cause rapid signal modulations under variation of some suitable parameter makes it desirable for a variety of applications. In this thesis, I focus on a novel manifestation of this resonance in systems of coupled quantum point contacts (QPCs). The major finding of this work is that the FR in this system may be ma-nipulated by applying a nonlinear DC bias to the system. Under such conditions, we are able to induce significant distortions of resonance lineshape, providing a pathway to all-electrical manipulation of the FR. To interpret this behavior we apply a recently-developed model for a three-path FR, involving an additional "intruder" continuum. We have previously used this model to account for the magnetic-field induced distortions of the FR observed in coupled QPCs, and show here that this model also provides a frame-work for understanding the observed nonlinear behavior. Our work therefore reveals a new manifestation of the FR that can be sensitively tailored by external control, a finding that may eventually allow the application of this feature to nanoelectronics. Since the in-terference scheme involves in this thesis is a completely general one, it should be broadly applicable across a variety of different wave-based systems, including those in both pho-tonics and electronics and opening up the possibility of new applications in areas such as chemical and biological sensing and secure communications.
Zhang, Ying-Ying; An, Sheng-Bai; Song, Yuan-Hong Wang, You-Nian; Kang, Naijing; Mišković, Z. L.
2014-10-15
We study the wake effect in the induced potential and the stopping power due to plasmon excitation in a metal slab by a point charge moving inside the slab. Nonlocal effects in the response of the electron gas in the metal are described by a quantum hydrodynamic model, where the equation of electronic motion contains both a quantum pressure term and a gradient correction from the Bohm quantum potential, resulting in a fourth-order differential equation for the perturbed electron density. Thus, besides using the condition that the normal component of the electron velocity should vanish at the impenetrable boundary of the metal, a consistent inclusion of the gradient correction is shown to introduce two possibilities for an additional boundary condition for the perturbed electron density. We show that using two different sets of boundary conditions only gives rise to differences in the wake potential at large distances behind the charged particle. On the other hand, the gradient correction in the quantum hydrodynamic model is seen to cause a reduction in the depth of the potential well closest to the particle, and a reduction of its stopping power. Even for a particle moving in the center of the slab, we observe nonlocal effects in the induced potential and the stopping power due to reduction of the slab thickness, which arise from the gradient correction in the quantum hydrodynamic model.
NASA Astrophysics Data System (ADS)
Shah, Nayana; Lopatin, Andrei
2007-09-01
A microscopic analysis of the superconducting quantum critical point realized via a pair-breaking quantum phase transition is presented. Finite-temperature crossovers are derived for the electrical conductivity, which is a key probe of superconducting fluctuations. By using the diagrammatic formalism for disordered systems, we are able to incorporate the interplay between fluctuating Cooper pairs and electrons, that is outside the scope of a time-dependent Ginzburg-Landau or effective bosonic action formalism. It is essential to go beyond the standard approximation in order to capture the zero-temperature correction which results purely from the (dynamic) quantum fluctuations and dictates the behavior of the conductivity in an entire low-temperature quantum regime. All dynamic contributions are of the same order and conspire to add up to a negative total, thereby inhibiting the conductivity as a result of superconducting fluctuations. On the contrary, the classical and the intermediate regimes are dominated by the positive bosonic channel. Our theory is applicable in one, two, and three dimensions and is relevant for experiments on superconducting nanowires, doubly connected cylinders, thin films, and bulk in the presence of magnetic impurities, magnetic field, or other pair breakers. A window of nonmonotonic behavior is predicted to exist as either the temperature or the pair-breaking parameter is swept.
Nodal Fermi surface pocket approaching an optimal quantum critical point in YBCO
NASA Astrophysics Data System (ADS)
Sebastian, Suchitra; Tan, Beng; Lonzarich, Gilbert; Ramshaw, Brad; Harrison, Neil; Balakirev, Fedor; Mielke, Chuck; Sabok, S.; Dabrowski, B.; Liang, Ruixing; Bonn, Doug; Hardy, Walter
2014-03-01
I present new quantum oscillation measurements over the entire underdoped regime in YBa2Cu3O6+x and YBa2Cu4O8 using ultra-high magnetic fields to destroy superconductivity and access the normal ground state. A robust small nodal Fermi surface created by charge order is found to extend over the entire underdoped range, exhibiting quantum critical signatures approaching optimal doping.
Terahertz magneto-spectroscopy of a point contact based on CdTe/CdMgTe quantum well
NASA Astrophysics Data System (ADS)
Grigelionis, Ignas; Białek, Marcin; Grynberg, Marian; Czapkiewicz, Magdalena; Kolkovskiy, Valery; Wiater, Maciej; Wojciechowski, Tomasz; Wróbel, Jerzy; Wojtowicz, Tomasz; Diakonova, Nina; Knap, Wojciech; Łusakowski, Jerzy
2015-01-01
To understand a terahertz (THz) response of a point contact device, a number of samples based on CdTe/CdMgTe quantum wells grown by a molecular beam epitaxy were investigated at low temperatures and high magnetic fields. The experiments involved magneto-transport, photocurrent, and transmission measurements carried out with monochromatic THz sources or a Fourier spectrometer. Samples of different geometry with and without gate metallization were used. We observed excitations of a two-dimensional electron plasma in the form of optically induced Shubnikov-de Haas oscillations, cyclotron resonance transitions, and magneto-plasmon resonances. A polaron effect was observed at magnetic fields higher than 10 T. A point contact device processed with an electron beam lithography was investigated as a detector of THz radiation. It was shown that the main mechanism responsible for a THz performance of the point contact was excitation of magneto-plasmons with a wavevector defined by geometrical constrictions of the device mesa.
Zaletel, Michael P; Bardarson, Jens H; Moore, Joel E
2011-07-01
Universal logarithmic terms in the entanglement entropy appear at quantum critical points (QCPs) in one dimension (1D) and have been predicted in 2D at QCPs described by 2D conformal field theories. The entanglement entropy in a strip geometry at such QCPs can be obtained via the "Shannon entropy" of a 1D spin chain with open boundary conditions. The Shannon entropy of the XXZ chain is found to have a logarithmic term that implies, for the QCP of the square-lattice quantum dimer model, a logarithm with universal coefficient ±0.25. However, the logarithm in the Shannon entropy of the transverse-field Ising model, which corresponds to entanglement in the 2D Ising conformal QCP, is found to have a singular dependence on the replica or Rényi index resulting from flows to different boundary conditions at the entanglement cut. PMID:21797582
NASA Astrophysics Data System (ADS)
Zaletel, Michael P.; Bardarson, Jens H.; Moore, Joel E.
2011-07-01
Universal logarithmic terms in the entanglement entropy appear at quantum critical points (QCPs) in one dimension (1D) and have been predicted in 2D at QCPs described by 2D conformal field theories. The entanglement entropy in a strip geometry at such QCPs can be obtained via the “Shannon entropy” of a 1D spin chain with open boundary conditions. The Shannon entropy of the XXZ chain is found to have a logarithmic term that implies, for the QCP of the square-lattice quantum dimer model, a logarithm with universal coefficient ±0.25. However, the logarithm in the Shannon entropy of the transverse-field Ising model, which corresponds to entanglement in the 2D Ising conformal QCP, is found to have a singular dependence on the replica or Rényi index resulting from flows to different boundary conditions at the entanglement cut.
Murakami, Yuta; Werner, Philipp; Tsuji, Naoto; Aoki, Hideo
2014-12-31
We reveal that electron-phonon systems described by the Holstein model on a bipartite lattice exhibit, away from half filling, a supersolid (SS) phase characterized by coexisting charge order (CO) and superconductivity (SC), and an accompanying quantum critical point (QCP). The SS phase, demonstrated by the dynamical mean-field theory with a quantum Monte Carlo impurity solver, emerges in the intermediate-coupling regime, where the peak of the Tc dome is located and the metal-insulator crossover occurs. On the other hand, in the weak- and strong-coupling regimes the CO-SC boundary is of first order with no intervening SS phases. The QCP is associated with the continuous transition from SS to SC and characterized by a reentrant behavior of the SS around it. We further show that the SS-SC transition is hallmarked by diverging charge fluctuations and a kink (peak) in the superfluid density. PMID:25615362
NASA Astrophysics Data System (ADS)
Murakami, Yuta; Werner, Philipp; Tsuji, Naoto; Aoki, Hideo
2014-12-01
We reveal that electron-phonon systems described by the Holstein model on a bipartite lattice exhibit, away from half filling, a supersolid (SS) phase characterized by coexisting charge order (CO) and superconductivity (SC), and an accompanying quantum critical point (QCP). The SS phase, demonstrated by the dynamical mean-field theory with a quantum Monte Carlo impurity solver, emerges in the intermediate-coupling regime, where the peak of the Tc dome is located and the metal-insulator crossover occurs. On the other hand, in the weak- and strong-coupling regimes the CO-SC boundary is of first order with no intervening SS phases. The QCP is associated with the continuous transition from SS to SC and characterized by a reentrant behavior of the SS around it. We further show that the SS-SC transition is hallmarked by diverging charge fluctuations and a kink (peak) in the superfluid density.
Finite-temperature scaling at the quantum critical point of the Ising chain in a transverse field
NASA Astrophysics Data System (ADS)
Haelg, Manuel; Huvonen, Dan; Guidi, Tatiana; Quintero-Castro, Diana Lucia; Boehm, Martin; Regnault, Louis-Pierre; Zheludev, Andrey
2015-03-01
Inelastic neutron scattering is used to study the finite-temperature scaling behavior of spin correlations at the quantum critical point in an experimental realization of the one-dimensional Ising model in a transverse field. The target compound is the well-characterized, anisotropic and bond-alternating Heisenberg spin-1 chain material NTENP. The validity and the limitations of the dynamic structure factor scaling are tested, discussed and compared to theoretical predictions. For this purpose neutron data have been collected on the three-axes spectrometers IN14 at ILL and FLEXX at HZB as well as on the time of flight multi-chopper spectrometer LET at ISIS. In addition to the general statement about quantum criticality and universality, present study also reveals new insight into the properties of the spin chain compound NTENP in particular.
Pairing interaction near a nematic quantum critical point of a three-band CuO2 model
Maier, Thomas A.; Scalapino, Douglas J.
2014-11-21
In this paper, we calculate the pairing interaction and the k dependence of the gap function associated with the nematic charge fluctuations of a CuO2 model.We find that the nematic pairing interaction is attractive for small momentum transfer and that it gives rise to d-wave pairing. Finally, as the doping p approaches a quantum critical point, the strength of this pairing increases and higher d-wave harmonics contribute to the k dependence of the superconducting gap function, reflecting the longer range nature of the nematic fluctuations.
Pairing interaction near a nematic quantum critical point of a three-band CuO_{2} model
Maier, Thomas A.; Scalapino, Douglas J.
2014-11-21
In this paper, we calculate the pairing interaction and the k dependence of the gap function associated with the nematic charge fluctuations of a CuO_{2} model.We find that the nematic pairing interaction is attractive for small momentum transfer and that it gives rise to d-wave pairing. Finally, as the doping p approaches a quantum critical point, the strength of this pairing increases and higher d-wave harmonics contribute to the k dependence of the superconducting gap function, reflecting the longer range nature of the nematic fluctuations.
Thermal Conductivity through the Quantum Critical Point in YbRh2Si2 at Very Low Temperature
NASA Astrophysics Data System (ADS)
Taupin, M.; Knebel, G.; Matsuda, T. D.; Lapertot, G.; Machida, Y.; Izawa, K.; Brison, J.-P.; Flouquet, J.
2015-07-01
The thermal conductivity of YbRh2Si2 has been measured down to very low temperatures under field in the basal plane. An additional channel for heat transport appears below 30 mK, both in the antiferromagnetic and paramagnetic states, respectively, below and above the critical field suppressing the magnetic order. This excludes antiferromagnetic magnons as the origin of this additional contribution to thermal conductivity. Moreover, this low temperature contribution prevails a definite conclusion on the validity or violation of the Wiedemann-Franz law at the field-induced quantum critical point.
Quantum quenches in the sinh-Gordon model: steady state and one-point correlation functions
NASA Astrophysics Data System (ADS)
Bertini, Bruno; Piroli, Lorenzo; Calabrese, Pasquale
2016-06-01
We consider quantum quenches to the sinh-Gordon integrable quantum field theory from a particular class of initial states. Our analysis includes the case of mass and interaction quenches starting from a non-interacting theory. By means of the recently developed quench action method, we fully characterize the stationary state reached at long times after the quench in terms of the corresponding rapidity distribution. We also provide exact results for the expectation values of arbitrary vertex operators in the post-quench stationary state by proposing a formula based on the analogy with the standard thermodynamic Bethe ansatz. Finally, we comment on the behavior of the post-quench stationary state under the mapping between the sinh-Gordon field theory and the one-dimensional Lieb-Liniger model.
NASA Astrophysics Data System (ADS)
Khots, Boris; Khots, Dmitriy
2014-12-01
Certain results that have been predicted by Quantum Mechanics (QM) theory are not always supported by experiments. This defines a deep crisis in contemporary physics and, in particular, quantum mechanics. We believe that, in fact, the mathematical apparatus employed within today's physics is a possible reason. In particular, we consider the concept of infinity that exists in today's mathematics as the root cause of this problem. We have created Observer's Mathematics that offers an alternative to contemporary mathematics. This paper is an attempt to relay how Observer's Mathematics may explain some of the contradictions in QM theory results. We consider the Hamiltonian Mechanics, Newton equation, Schrodinger equation, two slit interference, wave-particle duality for single photons, uncertainty principle, Dirac equations for free electron in a setting of arithmetic, algebra, and topology provided by Observer's Mathematics (see www.mathrelativity.com). Certain results and communications pertaining to solution of these problems are provided.
Khots, Boris; Khots, Dmitriy
2014-12-10
Certain results that have been predicted by Quantum Mechanics (QM) theory are not always supported by experiments. This defines a deep crisis in contemporary physics and, in particular, quantum mechanics. We believe that, in fact, the mathematical apparatus employed within today's physics is a possible reason. In particular, we consider the concept of infinity that exists in today's mathematics as the root cause of this problem. We have created Observer's Mathematics that offers an alternative to contemporary mathematics. This paper is an attempt to relay how Observer's Mathematics may explain some of the contradictions in QM theory results. We consider the Hamiltonian Mechanics, Newton equation, Schrodinger equation, two slit interference, wave-particle duality for single photons, uncertainty principle, Dirac equations for free electron in a setting of arithmetic, algebra, and topology provided by Observer's Mathematics (see www.mathrelativity.com). Certain results and communications pertaining to solution of these problems are provided.
NASA Astrophysics Data System (ADS)
Isobe, Hiroki; Fu, Liang
2016-06-01
We study the effect of the long-range Coulomb interaction in j =3 /2 Dirac electrons in cubic crystals with the Oh symmetry, which serves as an effective model for antiperovskite topological crystalline insulators. The renormalization group analysis reveals three fixed points that are Lorentz invariant, rotationally invariant, and Oh invariant. Among them, the Lorentz- and Oh-invariant fixed points are stable in the low-energy limit, while the rotationally invariant fixed point is unstable. The existence of a stable Oh-invariant fixed point of Dirac fermions with finite velocity anisotropy presents an interesting counterexample to emergent Lorentz invariance in solids.
One-Dimensional Three-State Quantum Walk with Single-Point Phase Defects
NASA Astrophysics Data System (ADS)
Xu, Yong-Zhen; Guo, Gong-De; Lin, Song
2016-09-01
In this paper, we study a three-state quantum walk with a phase defect at a designated position. The coin operator is a parametrization of the eigenvectors of the Grover matrix. We numerically investigate the properties of the proposed model via the position probability distribution, the position standard deviation, and the time-averaged probability at the designated position. It is shown that the localization effect can be governed by the phase defect's position and strength, coin parameter and initial state.
Andreani, Carla; Romanelli, Giovanni; Senesi, Roberto
2016-06-16
This study presents the first direct and quantitative measurement of the nuclear momentum distribution anisotropy and the quantum kinetic energy tensor in stable and metastable (supercooled) water near its triple point, using deep inelastic neutron scattering (DINS). From the experimental spectra, accurate line shapes of the hydrogen momentum distributions are derived using an anisotropic Gaussian and a model-independent framework. The experimental results, benchmarked with those obtained for the solid phase, provide the state of the art directional values of the hydrogen mean kinetic energy in metastable water. The determinations of the direction kinetic energies in the supercooled phase, provide accurate and quantitative measurements of these dynamical observables in metastable and stable phases, that is, key insight in the physical mechanisms of the hydrogen quantum state in both disordered and polycrystalline systems. The remarkable findings of this study establish novel insight into further expand the capacity and accuracy of DINS investigations of the nuclear quantum effects in water and represent reference experimental values for theoretical investigations. PMID:27214268
NASA Astrophysics Data System (ADS)
Pakmehr, Mehdi; Whiteside, Vincent; Bhandari, Nikhil; Cahay, Marc; Newrock, Richard; McCombe, Bruce
2013-03-01
We have studied the THz magneto-photoresponse of a 2DEG in an InAs quantum well with an embedded Quantum Point Contact in the frequency/field region where electron cyclotron resonance (CR) dominates the response suing several lines from an optically pumped THz laser. The photoresponse near CR is manifested as an envelope of the amplitude of the Shubnikov-de Haas oscillations of the 2DEG with a peak near the CR field. Clear spin-splitting of the quantum oscillations is observed for B > 4, while the SdH oscillations do not show resolved spin-splitting up to 10 T. Data were simulated by a model of resonant carrier heating (due to CR), and from the simulations the carrier density, the CR effective mass, scattering times and the g-factor were obtained. We find a significantly enhanced g-factor, apparently due to many-electron exchange interaction effects. The g-factor determined from fitting spin-split Landau level peaks increases with magnetic field. Work at UB was supported by NSF DMR 1008138 and the Office of the Provost; work at the University of Cincinnati was supported by NSF ECCE 1028483.
Quantum Point Contacts and Valley Filters on a 6-fold Degenerate Hydrogen-terminated Si(111) Surface
NASA Astrophysics Data System (ADS)
Robertson, Luke D.; Hu, Binhui; Kane, B. E.
Hydrogen-terminated Si(111) surfaces preserve the 6-fold valley degeneracy and anisotropic electron mass predicted in bulk Si, providing a unique environment for 2-D electron systems (2DESs). Our group has demonstrated high mobility as well as the fractional quantum Hall effect for electrons confined on the Si(111) surfaces, establishing evidence that they are ideal platforms for 2DESs and lower dimensional systems. Recently, we have succeeded in fabricating high mobility ambipolar devices and have found that heavily p-doped regions can be used as lateral depletion gates for confinement of 2DESs induced by a top gate. Here, we describe our efforts to extend this technology to the nanoscale and in particular towards the fabrication of quantum point contacts (QPCs). QPCs realized in materials with anisotropic electron mass may exhibit valley filter phenomena leading to extreme sensitivity to single donor occupancy, and thus are of interest to measurement schemes for donor-based quantum information processing. Preliminary measurements and fabrication techniques will be discussed
Automatic synthesis of quantum circuits for point addition on ordinary binary elliptic curves
NASA Astrophysics Data System (ADS)
Budhathoki, Parshuram; Steinwandt, Rainer
2015-01-01
When designing quantum circuits for Shor's algorithm to solve the discrete logarithm problem, implementing the group arithmetic is a cost-critical task. We introduce a software tool for the automatic generation of addition circuits for ordinary binary elliptic curves, a prominent platform group for digital signatures. The resulting circuits reduce the number of -gates by a factor compared to the best previous construction, without increasing the number of qubits or -depth. The software also optimizes the (CNOT) depth for -linear operations by means of suitable graph colorings.
NASA Astrophysics Data System (ADS)
Kolasiński, K.; Mreńca-Kolasińska, A.; Szafran, B.
2016-01-01
We analyze the effective Landé factor g* and its dependence on the orientation of the external magnetic field for a quantum point contact defined in the two-dimensional electron gas. The paper simulates the experimental procedure for evaluation of the effective Landé factors from the transconductance of a biased device in an external magnetic field. The contributions of the orbital effects of the magnetic field, the electron-electron interaction, and spin-orbit (SO) coupling are studied in low-temperature conditions (0.5 K). The anisotropy of the g* factors for the in-plane magnetic field orientation, which seems counterintuitive from the perspective of the effective SO magnetic field, is explained in an analytical model of the constriction as due to the SO-induced subband mixing. The asymmetry of the transconductance as a function of the gate voltage is obtained in agreement with the experimental data and the results are explained as due to depletion of the electron gas within the quantum point contact constriction and the related reduction of the screening as described within the DFT approach. The results for transconductance and the g* factors obtained are in a good agreement with the experimental data [Martin et al., Phys. Rev. B 81, 041303 (2010), 10.1103/PhysRevB.81.041303].
NASA Astrophysics Data System (ADS)
Zuo, Zheng-Wei; Kang, Da-wei; Wang, Zhao-Wu; Li, Liben
2016-08-01
The tunneling junction between one-dimensional topological superconductor and integer (fractional) topological insulator (TI), realized via point contact, is investigated theoretically with bosonization technology and renormalization group methods. For the integer TI case, in a finite range of edge interaction parameter, there is a non-trivial stable fixed point which corresponds to the physical picture that the edge of TI breaks up into two sections at the junction, with one side coupling strongly to the Majorana fermion and exhibiting perfect Andreev reflection, while the other side decouples, exhibiting perfect normal reflection at low energies. This fixed point can be used as a signature of the Majorana fermion and tested by nowadays experiment techniques. For the fractional TI case, the universal low-energy transport properties are described by perfect normal reflection, perfect Andreev reflection, or perfect insulating fixed points dependent on the filling fraction and edge interaction parameter of fractional TI.
Stability of a spin-triplet nematic state near to a quantum critical point
NASA Astrophysics Data System (ADS)
Hannappel, G.; Pedder, C. J.; Krüger, F.; Green, A. G.
2016-06-01
We analyze a model of itinerant electrons interacting through a quadrupole density-density repulsion in three dimensions. At the mean-field level, the interaction drives a continuous Pomeranchuk instability towards d -wave, spin-triplet nematic order, which simultaneously breaks the SU(2) spin-rotation and spatial-rotation symmetries. This order is characterized by spin-antisymmetric, elliptical deformations of the Fermi surfaces of up and down spins. We show that the effects of quantum fluctuations are similar to those in metallic ferromagnets, rendering the nematic transition first order at low temperatures. Using the fermionic quantum order-by-disorder approach to self-consistently calculate fluctuations around possible modulated states, we show that the first-order transition is preempted by the formation of a helical spin-triplet d -density wave. Such a state is closely related to d -wave bond density wave order in square-lattice systems. Moreover, we show that it may coexist with a modulated, p -wave superconducting state.
NASA Astrophysics Data System (ADS)
Watanabe, Shinji; Tsuruta, Atsushi; Miyake, Kazumasa; Flouquet, Jacques
2009-03-01
Valence instability and its critical fluctuations have attracted much attention recently in the heavy-electron systems. Valence fluctuations are essentially charge fluctuations, and it is highly non-trivial how the quantum critical point (QCP) as well as the critical end point is controlled by the magnetic field. To clarify this fundamental issue, we have studied the mechanism of how the critical points of the first-order valence transitions are controlled by the magnetic field [1]. We show that the critical temperature is suppressed to be the QCP by the magnetic field and unexpectedly the QCP exhibits nonmonotonic field dependence in the ground-state phase diagram, giving rise to emergence of metamagnetism even in the intermediate valence-crossover regime. The driving force of the field-induced QCP is clarified to be a cooperative phenomenon of Zeeman effect and Kondo effect, which creates a distinct energy scale from the Kondo temperature. This mechanism explains a peculiar magnetic response in CeIrIn5 and metamagnetic transition in YbXCu4 for X=In as well as a sharp contrast between X=Ag and Cd. We present the novel phenomena under the magnetic field to discuss significance of the proximity of the critical points of the first-order valence transition. [1] S. Watanabe et al. PRL100, (2008) 236401.
Spectral dimension of the universe in quantum gravity at a lifshitz point.
Horava, Petr
2009-04-24
We extend the definition of "spectral dimension" d_{s} (usually defined for fractal and lattice geometries) to theories in spacetimes with anisotropic scaling. We show that in gravity with dynamical critical exponent z in D+1 dimensions, the spectral dimension of spacetime is d_{s}=1+D/z. In the case of gravity in 3+1 dimensions with z=3 in the UV which flows to z=1 in the IR, the spectral dimension changes from d_{s}=4 at large scales to d_{s}=2 at short distances. Remarkably, this is the behavior found numerically by Ambjørn et al. in their causal dynamical triangulations approach to quantum gravity. PMID:19518693
Fermi points and topological quantum phase transitions in a multi-band superconductor.
Puel, T O; Sacramento, P D; Continentino, M A
2015-10-28
The importance of models with an exact solution for the study of materials with non-trivial topological properties has been extensively demonstrated. The Kitaev model plays a guiding role in the search for Majorana modes in condensed matter systems. Also, the sp-chain with an anti-symmetric mixing among the s and p bands is a paradigmatic example of a topological insulator with well understood properties. Interestingly, these models share the same universality class for their topological quantum phase transitions. In this work we study a two-band model of spinless fermions with attractive inter-band interactions. We obtain its zero temperature phase diagram, which presents a rich variety of phases including a Weyl superconductor and a topological insulator. The transition from the topological to the trivial superconducting phase has critical exponents different from those of Kitaev's model. PMID:26440940
NASA Astrophysics Data System (ADS)
Mishra, Utkarsh; Rakshit, Debraj; Prabhu, R.
2016-04-01
The time dynamics of quantum correlations in the quantum transverse anisotropic X Y spin chain of infinite length is studied at zero and finite temperatures. The evolution occurs due to the instantaneous quenching of the coupling constant between the nearest-neighbor spins of the model, which is performed either within the same phase or across the quantum phase-transition point connecting the order-disorder phases of the model. We characterize the time-evolved quantum correlations, viz., entanglement and quantum discord, which exhibit varying behavior depending on the initial state and the quenching scheme. We show that the system is endowed with enhanced nearest-neighbor bipartite quantum correlations compared to that of the initial state, when quenched from the ordered to the deep disordered phase. However, nearest-neighbor quantum correlations are almost washed out when the system is quenched from the disordered to the ordered phase with the initial state being at the zero temperature. We also identify the condition for the occurrence of enhanced bipartite correlations when the system is quenched within the same phase. Moreover, we investigate the bipartite quantum correlations when the initial state is a thermal equilibrium state with finite temperature, which reveals the effects of thermal fluctuation on the phenomena observed at zero temperature. Finally, an analogous analysis is carried out for zero-temperature next-nearest-neighbor quantum correlations.
Temperature-dependent quantum electron transport in 2D point contacts.
Krishtop, T V; Nagaev, K E
2013-02-01
We consider the transmission of electrons through a two-dimensional ballistic point contact in the low-conductance regime near the pinch-off region. The scattering of electrons by Friedel oscillations of charge density results in a contribution to the conductance proportional to the temperature. The sign of this linear term depends on the range of the electron-electron interaction and appears to be negative for the relevant experimental parameters. PMID:23288558
Aramburu, José Antonio; García-Fernández, Pablo; García-Lastra, Juan María; Moreno, Miguel
2016-07-18
First-principle calculations together with analysis of the experimental data found for 3d(9) and 3d(7) ions in cubic oxides proved that the center found in irradiated CaO:Ni(2+) corresponds to Ni(+) under a static Jahn-Teller effect displaying a compressed equilibrium geometry. It was also shown that the anomalous positive g∥ shift (g∥ -g0 =0.065) measured at T=20 K obeys the superposition of the |3 z(2) -r(2) ⟩ and |x(2) -y(2) ⟩ states driven by quantum effects associated with the zero-point motion, a mechanism first put forward by O'Brien for static Jahn-Teller systems and later extended by Ham to the dynamic Jahn-Teller case. To our knowledge, this is the first genuine Jahn-Teller system (i.e. in which exact degeneracy exists at the high-symmetry configuration) exhibiting a compressed equilibrium geometry for which large quantum effects allow experimental observation of the effect predicted by O'Brien. Analysis of the calculated energy barriers for different Jahn-Teller systems allowed us to explain the origin of the compressed geometry observed for CaO:Ni(+) . PMID:27028895
Heat capacity peak at the quantum critical point of the transverse Ising magnet CoNb2O6
Liang, Tian; Koohpayeh, S. M.; Krizan, J. W.; McQueen, T. M.; Cava, R. J.; Ong, N. P.
2015-01-01
The transverse Ising magnet Hamiltonian describing the Ising chain in a transverse magnetic field is the archetypal example of a system that undergoes a transition at a quantum critical point (QCP). The columbite CoNb2O6 is the closest realization of the transverse Ising magnet found to date. At low temperatures, neutron diffraction has observed a set of discrete collective spin modes near the QCP. Here, we ask if there are low-lying spin excitations distinct from these relatively high-energy modes. Using the heat capacity, we show that a significant band of gapless spin excitations exists. At the QCP, their spin entropy rises to a prominent peak that accounts for 30% of the total spin degrees of freedom. In a narrow field interval below the QCP, the gapless excitations display a fermion-like, temperature-linear heat capacity below 1 K. These novel gapless modes are the main spin excitations participating in, and affected by, the quantum transition. PMID:26146018
NASA Astrophysics Data System (ADS)
Tremblay, A.-M. S.; Hébert, Charles-David; Sémon, Patrick
Layered organic superconductors of the BEDT family are model systems for understanding the interplay of the Mott transition with superconductivity, magnetic order and frustration. Recent experimental studies on a hole-doped compound reveal an enhancement of superconductivity and a rapid crossover between two different conducting phases above the superconducting dome. Using plaquette cellular dynamical mean field theory with state of the art continuous-time quantum Monte Carlo calculations, we study this problem with the two-dimensional Hubbard model on the anisotropic triangular lattice. Phase diagrams are in broad agreement with experiment. As in the case of the cuprates, we find, at finite doping in the unstable normal state, a first-order transition between a pseudogap and a correlated metal. We make several experimental predictions. This work also clearly shows that the superconducting dome in organic superconductors is tied to the Mott transition and its continuation as a transition separating pseudogap phase from correlated metal in doped compounds, as in the cuprates. Contrary to heavy fermions for example, the maximum Tc is definitely not attached to an antiferromagnetic quantum critical point. That can also be verified experimentally. Supported by NSERC, CIFAR and the Tier I Canada Research Chair Program.
Heat capacity peak at the quantum critical point of the transverse Ising magnet CoNb2O6
NASA Astrophysics Data System (ADS)
Liang, Tian; Koohpayeh, S. M.; Krizan, J. W.; McQueen, T. M.; Cava, R. J.; Ong, N. P.
2015-07-01
The transverse Ising magnet Hamiltonian describing the Ising chain in a transverse magnetic field is the archetypal example of a system that undergoes a transition at a quantum critical point (QCP). The columbite CoNb2O6 is the closest realization of the transverse Ising magnet found to date. At low temperatures, neutron diffraction has observed a set of discrete collective spin modes near the QCP. Here, we ask if there are low-lying spin excitations distinct from these relatively high-energy modes. Using the heat capacity, we show that a significant band of gapless spin excitations exists. At the QCP, their spin entropy rises to a prominent peak that accounts for 30% of the total spin degrees of freedom. In a narrow field interval below the QCP, the gapless excitations display a fermion-like, temperature-linear heat capacity below 1 K. These novel gapless modes are the main spin excitations participating in, and affected by, the quantum transition.
Heat capacity peak at the quantum critical point of the transverse Ising magnet CoNb2O6.
Liang, Tian; Koohpayeh, S M; Krizan, J W; McQueen, T M; Cava, R J; Ong, N P
2015-01-01
The transverse Ising magnet Hamiltonian describing the Ising chain in a transverse magnetic field is the archetypal example of a system that undergoes a transition at a quantum critical point (QCP). The columbite CoNb2O6 is the closest realization of the transverse Ising magnet found to date. At low temperatures, neutron diffraction has observed a set of discrete collective spin modes near the QCP. Here, we ask if there are low-lying spin excitations distinct from these relatively high-energy modes. Using the heat capacity, we show that a significant band of gapless spin excitations exists. At the QCP, their spin entropy rises to a prominent peak that accounts for 30% of the total spin degrees of freedom. In a narrow field interval below the QCP, the gapless excitations display a fermion-like, temperature-linear heat capacity below 1 K. These novel gapless modes are the main spin excitations participating in, and affected by, the quantum transition. PMID:26146018
Electrical control of the sign of the g factor in a GaAs hole quantum point contact
NASA Astrophysics Data System (ADS)
Srinivasan, A.; Hudson, K. L.; Miserev, D.; Yeoh, L. A.; Klochan, O.; Muraki, K.; Hirayama, Y.; Sushkov, O. P.; Hamilton, A. R.
2016-07-01
Zeeman splitting of one-dimensional hole subbands is investigated in quantum point contacts fabricated on a (311)-oriented GaAs-AlGaAs heterostructure. Transport measurements can determine the magnitude of the g factor, but cannot usually determine the sign. Here we use a combination of tilted fields and a unique off-diagonal element in the hole g tensor to directly detect the sign of g*. We are able to tune not only the magnitude, but also the sign of the g factor by electrical means, which is of interest for spintronics applications. Furthermore, we show theoretically that the resulting behavior of g* can be explained by the momentum dependence of the spin-orbit interaction.
NASA Astrophysics Data System (ADS)
Khatua, Pradip; Bansal, Bhavtosh; Shahar, Dan
2014-01-01
In a "thought experiment," now a classic in physics pedagogy, Feynman visualizes Young's double-slit interference experiment with electrons in magnetic field. He shows that the addition of an Aharonov-Bohm phase is equivalent to shifting the zero-field wave interference pattern by an angle expected from the Lorentz force calculation for classical particles. We have performed this experiment with one slit, instead of two, where ballistic electrons within two-dimensional electron gas diffract through a small orifice formed by a quantum point contact (QPC). As the QPC width is comparable to the electron wavelength, the observed intensity profile is further modulated by the transverse waveguide modes present at the injector QPC. Our experiments open the way to realizing diffraction-based ideas in mesoscopic physics.
Observation of a 0.5 conductance plateau in asymmetrically biased GaAs quantum point contact
NASA Astrophysics Data System (ADS)
Bhandari, N.; Das, P. P.; Cahay, M.; Newrock, R. S.; Herbert, S. T.
2012-09-01
We report the observation of a robust anomalous conductance plateau near G = 0.5 G0 (G0 = 2e2/h) in asymmetrically biased AlGaAs/GaAs quantum point contacts (QPCs), with in-plane side gates in the presence of lateral spin-orbit coupling. This is interpreted as evidence of spin polarization in the narrow portion of the QPC. The appearance and evolution of the conductance anomaly has been studied at T = 4.2 K as a function of the potential asymmetry between the side gates. Because GaAs is a material with established processing techniques, high mobility, and a relatively high spin coherence length, the observation of spontaneous spin polarization in a side-gated GaAs QPC could eventually lead to the realization of an all-electric spin-valve at tens of degrees Kelvin.
Khatua, Pradip; Bansal, Bhavtosh; Shahar, Dan
2014-01-10
In a "thought experiment," now a classic in physics pedagogy, Feynman visualizes Young's double-slit interference experiment with electrons in magnetic field. He shows that the addition of an Aharonov-Bohm phase is equivalent to shifting the zero-field wave interference pattern by an angle expected from the Lorentz force calculation for classical particles. We have performed this experiment with one slit, instead of two, where ballistic electrons within two-dimensional electron gas diffract through a small orifice formed by a quantum point contact (QPC). As the QPC width is comparable to the electron wavelength, the observed intensity profile is further modulated by the transverse waveguide modes present at the injector QPC. Our experiments open the way to realizing diffraction-based ideas in mesoscopic physics. PMID:24483873
Peng, Juan Duan, Yifeng; Chen, PeiJian; Peng, Yan
2015-03-15
Analysis of the electronic properties of a two-dimensional (2D) deformed honeycomb structure arrayed by semiconductor quantum dots (QDs) is conducted theoretically by using tight-binding method in the present paper. Through the compressive or tensile deformation of the honeycomb lattice, the variation of energy spectrum has been explored. We show that, the massless Dirac fermions are generated in this adjustable system and the positions of the Dirac cones as well as slope of the linear dispersions could be manipulated. Furthermore, a clear linear correspondence between the distance of movement d (the distance from the Dirac points to the Brillouin zone corners) and the tunable bond angle α of the lattice are found in this artificial planar QD structure. These results provide the theoretical basis for manipulating Dirac fermions and should be very helpful for the fabrication and application of high-mobility semiconductor QD devices.
Spin-orbital liquid and quantum critical point in Y1 -xLaxTiO3
NASA Astrophysics Data System (ADS)
Zhao, Z. Y.; Khosravani, O.; Lee, M.; Balicas, L.; Sun, X. F.; Cheng, J. G.; Brooks, J.; Zhou, H. D.; Choi, E. S.
2015-04-01
The specific heat, the susceptibility under pressure, and the dielectric constant were measured for single crystals Y1 -xLaxTiO3 . The observed T2-dependent specific heat at low temperatures for 0.17 ≤x ≤0.3 samples shows a spin-orbital liquid state between the ferromagnetic/orbital ordering (x <0.17 ) and antiferromagnetic/possible orbital liquid phase (x >0.3 ) . The nonmonotonous pressure dependence of TC and the glassy behavior of the dielectric loss for the x =0.23 sample suggest that it is approaching a possible quantum critical point. All these properties result from the coupling between the strong spin and orbital fluctuations while approaching the phase boundary.
NASA Astrophysics Data System (ADS)
Heyder, Jan; Bauer, Florian; Schubert, Enrico; Borowsky, David; Schuh, Dieter; Wegscheider, Werner; von Delft, Jan; Ludwig, Stefan
2015-11-01
Quantum point contacts (QPCs) and quantum dots (QDs), two elementary building blocks of semiconducting nanodevices, both exhibit famously anomalous conductance features: the 0.7 anomaly in the former case, the Kondo effect in the latter. For both the 0.7 anomaly and the Kondo effect, the conductance shows a remarkably similar low-energy dependence on temperature T , source-drain voltage Vsd, and magnetic field B . In a recent publication [F. Bauer et al., Nature (London) 501, 73 (2013), 10.1038/nature12421], we argued that the reason for these similarities is that both a QPC and a Kondo QD (KQD) feature spin fluctuations that are induced by the sample geometry, confined in a small spatial regime, and enhanced by interactions. Here, we further explore this notion experimentally and theoretically by studying the geometric crossover between a QD and a QPC, focusing on the B -field dependence of the conductance. We introduce a one-dimensional model with local interactions that reproduces the essential features of the experiments, including a smooth transition between a KQD and a QPC with 0.7 anomaly. We find that in both cases the anomalously strong negative magnetoconductance goes hand in hand with strongly enhanced local spin fluctuations. Our experimental observations include, in addition to the Kondo effect in a QD and the 0.7 anomaly in a QPC, Fano interference effects in a regime of coexistence between QD and QPC physics, and Fabry-Perot-type resonances on the conductance plateaus of a clean QPC. We argue that Fabry-Perot-type resonances occur generically if the electrostatic potential of the QPC generates a flatter-than-parabolic barrier top.
Goh, S K; Tompsett, D A; Saines, P J; Chang, H C; Matsumoto, T; Imai, M; Yoshimura, K; Grosche, F M
2015-03-01
The quasiskutterudite superconductor Sr_{3}Rh_{4}Sn_{13} features a pronounced anomaly in electrical resistivity at T^{*}∼138 K. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centered around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of T^{*} as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., x_{c}=0.9). This establishes the (Ca_{x}Sr_{1-x})_{3}Rh_{4}Sn_{13} series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures. PMID:25793843
Interior building details of Building A, dungeon cell adjacent to ...
Interior building details of Building A, dungeon cell adjacent to northwest cell: granite and brick threshold, poured concrete floors, plastered finished walls, vaulted veiling; northwesterly view - San Quentin State Prison, Building 22, Point San Quentin, San Quentin, Marin County, CA
NASA Astrophysics Data System (ADS)
Viel, Alexandra; Coutinho-Neto, Maurício D.; Manthe, Uwe
2007-01-01
Quantum dynamics calculations of the ground state tunneling splitting and of the zero point energy of malonaldehyde on the full dimensional potential energy surface proposed by Yagi et al. [J. Chem. Phys. 1154, 10647 (2001)] are reported. The exact diffusion Monte Carlo and the projection operator imaginary time spectral evolution methods are used to compute accurate benchmark results for this 21-dimensional ab initio potential energy surface. A tunneling splitting of 25.7±0.3cm-1 is obtained, and the vibrational ground state energy is found to be 15122±4cm-1. Isotopic substitution of the tunneling hydrogen modifies the tunneling splitting down to 3.21±0.09cm-1 and the vibrational ground state energy to 14385±2cm-1. The computed tunneling splittings are slightly higher than the experimental values as expected from the potential energy surface which slightly underestimates the barrier height, and they are slightly lower than the results from the instanton theory obtained using the same potential energy surface.
Viel, Alexandra; Coutinho-Neto, Maurício D; Manthe, Uwe
2007-01-14
Quantum dynamics calculations of the ground state tunneling splitting and of the zero point energy of malonaldehyde on the full dimensional potential energy surface proposed by Yagi et al. [J. Chem. Phys. 1154, 10647 (2001)] are reported. The exact diffusion Monte Carlo and the projection operator imaginary time spectral evolution methods are used to compute accurate benchmark results for this 21-dimensional ab initio potential energy surface. A tunneling splitting of 25.7+/-0.3 cm-1 is obtained, and the vibrational ground state energy is found to be 15 122+/-4 cm-1. Isotopic substitution of the tunneling hydrogen modifies the tunneling splitting down to 3.21+/-0.09 cm-1 and the vibrational ground state energy to 14 385+/-2 cm-1. The computed tunneling splittings are slightly higher than the experimental values as expected from the potential energy surface which slightly underestimates the barrier height, and they are slightly lower than the results from the instanton theory obtained using the same potential energy surface. PMID:17228955
NASA Astrophysics Data System (ADS)
Isobe, Hiroki; Yang, Bohm-Jung; Chubukov, Andrey; Schmalian, Jörg; Nagaosa, Naoto
2016-02-01
We study the effects of Coulomb interaction between 2D Weyl fermions with anisotropic dispersion which displays relativistic dynamics along one direction and nonrelativistic dynamics along the other. Such a dispersion can be realized in phosphorene under electric field or strain, in TiO2 /VO2 superlattices, and, more generally, at the quantum critical point between a nodal semimetal and an insulator in systems with a chiral symmetry. Using the one-loop renormalization group approach in combination with the large-N expansion, we find that the system displays interaction-driven non-Fermi liquid behavior in a wide range of intermediate frequencies and marginal Fermi liquid behavior at the smallest frequencies. In the non-Fermi liquid regime, the quasiparticle residue Z at energy E scales as Z ∝Ea with a >0 , and the parameters of the fermionic dispersion acquire anomalous dimensions. In the marginal Fermi-liquid regime, Z ∝(|log E |)-b with universal b =3 /2 .
NASA Astrophysics Data System (ADS)
Kormos, Márton; Wu, Jianda; Si, Qimiao
2014-03-01
When the transverse-field Ising chain at its quantum critical point is subjected to a small longitudinal field, the perturbed conformal field theory led to a field theory with an exotic E8 symmetry. Recent neutron scattering experiments have provided evidence for the lightest two particles in this E8 model in the quasi-1D Ising ferromagnet CoNb2O6. While the zero temperature dynamic of the model is well known, its finite-temperature counterpart has not yet been systematically studied. We study the low-frequency dynamical spin structure factor at finite temperatures using the form-factor method. We show that the dominant contribution to the spin dynamics comes from the channel between two lightest particles, and demonstrate how the spin dynamics differ from a diffusion form. Using these results, we determine the temperature dependence of the NMR relaxation rate. We suggest that, for CoNb2O6, measurements of the NMR relaxation rate provide a means to further test the applicability of the E8 model.
NASA Astrophysics Data System (ADS)
Oliver, Sean; Fairfield, Jessamyn; Lee, Sunghun; Bellew, Allen; Stone, Iris; Ruppalt, Laura; Boland, John; Vora, Patrick
Resistive switching is ideal for use in non-volatile memory where information is stored in a metallic or insulating state. Nanowire junctions formed at the intersection of two Ni/NiO core/shell nanowires have emerged as a leading candidate structure where resistive switching occurs due to the formation and destruction of conducting filaments. However, significant knowledge gaps remain regarding the conduction mechanisms as measurements are typically only performed at room temperature. Here, we combine temperature-dependent current-voltage (IV) measurements from 15 - 300 K with magnetoresistance studies and achieve new insight into the nature of the conducting filaments. We identify a novel semiconducting state that behaves as a quantum point contact and find evidence for a possible electric-field driven phase transition. The insulating state exhibits unexpectedly complex IV characteristics that highlight the disordered nature of the ruptured filament while we find clear signs of anisotropic magnetoresistance in the metallic state. Our results expose previously unobserved behaviors in nanowire resistive switching devices and pave the way for future applications where both electrical and magnetic switching can be achieved in a single device. This work was supported by ONR Grant N-00014-15-1-2357.
Strong enhancement of s -wave superconductivity near a quantum critical point of Ca3Ir4Sn13
Biswas, P. K.; Guguchia, Z.; Khasanov, R.; Chinotti, M.; Li, L.; Wang, Kefeng; Petrovic, C.; Morenzoni, E.
2015-11-11
We repormore » t microscopic studies by muon spin rotation/relaxation as a function of pressure of the Ca3Ir4Sn13 and Sr3Ir4Sn13 system displaying superconductivity and a structural phase transition associated with the formation of a charge density wave (CDW). Our findings show a strong enhancement of the superfluid density and a dramatic increase of the pairing strength above a pressure of ≈ 1.6 GPa giving direct evidence of the presence of a quantum critical point separating a superconducting phase coexisting with CDW from a pure superconducting phase. The superconducting order parameter in both phases has the same s-wave symmetry. In spite of the conventional phonon-mediated BCS character of the weakly correlated (Ca1-xSrx)3Ir4Sn13 system the dependence of the effective superfluid density on the critical temperature puts this compound in the “Uemura” plot close to unconventional superconductors. This system exemplifies that conventional BCS superconductors in the presence of competing orders or multi-band structure can also display characteristics of unconventional superconductors.« less
Isobe, Hiroki; Yang, Bohm-Jung; Chubukov, Andrey; Schmalian, Jörg; Nagaosa, Naoto
2016-02-19
We study the effects of Coulomb interaction between 2D Weyl fermions with anisotropic dispersion which displays relativistic dynamics along one direction and nonrelativistic dynamics along the other. Such a dispersion can be realized in phosphorene under electric field or strain, in TiO_{2}/VO_{2} superlattices, and, more generally, at the quantum critical point between a nodal semimetal and an insulator in systems with a chiral symmetry. Using the one-loop renormalization group approach in combination with the large-N expansion, we find that the system displays interaction-driven non-Fermi liquid behavior in a wide range of intermediate frequencies and marginal Fermi liquid behavior at the smallest frequencies. In the non-Fermi liquid regime, the quasiparticle residue Z at energy E scales as Z∝E^{a} with a>0, and the parameters of the fermionic dispersion acquire anomalous dimensions. In the marginal Fermi-liquid regime, Z∝(|logE|)^{-b} with universal b=3/2. PMID:26943551
NASA Astrophysics Data System (ADS)
Kinross, A. W.; Fu, M.; Munsie, T. J.; Dabkowska, H. A.; Luke, G. M.; Sachdev, Subir; Imai, T.
2014-07-01
The transverse field Ising chain model is ideally suited for testing the fundamental ideas of quantum phase transitions because its well-known T=0 ground state can be extrapolated to finite temperatures. Nonetheless, the lack of appropriate model materials hindered the past effort to test the theoretical predictions. Here, we map the evolution of quantum fluctuations in the transverse field Ising chain based on nuclear magnetic resonance measurements of CoNb2O6, and we demonstrate the finite-temperature effects on quantum criticality for the first time. From the temperature dependence of the Nb93 longitudinal relaxation rate 1/T1, we identify the renormalized classical, quantum critical, and quantum disordered scaling regimes in the temperature (T) vs transverse magnetic field (h ⊥) phase diagram. Precisely at the critical field h⊥c=5.25±0.15 T, we observe a power-law behavior, 1/T1˜T-3/4, as predicted by quantum critical scaling. Our parameter-free comparison between the data and theory reveals that quantum fluctuations persist up to as high as T ˜0.4J, where the intrachain exchange interaction J is the only energy scale of the problem.
NASA Astrophysics Data System (ADS)
Czachor, Marek; Wrzask, Klaudia
2009-09-01
Electromagnetic fields are quantized in a manifestly covariant way by means of a class of reducible “center-of-mass N-representations” of the algebra of canonical commutation relations (CCR). The four-potential A a ( x) transforms in these representations as a Hermitian four-vector field in Minkowski four-position space (without change of gauge), but in momentum space it splits into spin-1 massless photons and two massless scalars. What we call quantum optics is the spin-1 sector of the theory. The scalar fields have physical status similar to that of dark matter (spin-1 and spin-0 particle numbers are separately conserved). There are no negative-norm or zero-norm states. Unitary dynamics is given by the point-form interaction picture, with minimal-coupling Hamiltonian constructed from fields that are free on the null-cone boundary of the Milne universe. SL(2,C) transformations as well as the dynamics are represented unitarily in the Hilbert space corresponding to N four-dimensional oscillators. Vacuum is a Bose-Einstein condensate of the N-oscillator gas and is given by any N-oscillator product state annihilated by all annihilation operators. The form of A a ( x) is determined by an analogue of the twistor equation. The same equation guarantees that the set of vacuum states is Poincaré invariant. The formalism is tested on quantum fields produced by pointlike classical sources. Photon statistics is well defined even for pointlike charges, with ultraviolet and infrared regularizations occurring automatically as a consequence of the formalism. The probabilities are not Poissonian but of a Rényi type with α=1-1/ N; the Shannon limit N→∞ is an ultraviolet/infrared-regularized Poisson distribution. The average number of photons occurring in Bremsstrahlung splits into two parts: The one due to acceleration, and the one that remains nonvanishing even for inertially moving charges. Classical Maxwell electrodynamics is reconstructed from coherent-state averaged
NASA Astrophysics Data System (ADS)
Straßel, Dominik; Kopietz, Peter; Eggert, Sebastian
2015-04-01
Spin-dimer systems are a versatile playground for the detailed study of quantum phase transitions. Using the magnetic field as the tuning parameter, it is possible to observe a crossover from the characteristic scaling near critical points to the behavior of a finite-temperature phase transition. In this work we study two-dimensional coupled spin-dimer systems by comparing numerical quantum Monte Carlo simulations with analytical calculations of the susceptibility, the magnetocaloric effect, and the helicity modulus. The magnetocaloric behavior of the magnetization with temperature can be used to determine the critical fields with high accuracy, but the critical scaling does not show the expected logarithmic corrections. The zeros of the cooling rate are an excellent indicator of the competition between quantum criticality and vortex physics, but they are not directly associated with the quantum phase transition or the finite-temperature Berezinsky-Kosterlitz-Thouless transition. The results give a unified picture of the full quantum and finite-temperature phase diagram.
Reid, J.-Ph.; Tanatar, Makariy; Daou, R.; Hu, Rongwei; Petrovic, C.; Taillefer, Louis
2014-01-23
The in-plane thermal conductivity kappa and electrical resistivity rho of the heavy-fermion metal YbRh2Si2 were measured down to 50 mK for magnetic fields H parallel and perpendicular to the tetragonal c axis, through the field-tuned quantum critical point H-c, at which antiferromagnetic order ends. The thermal and electrical resistivities, w L0T/kappa and rho, show a linear temperature dependence below 1 K, typical of the non-Fermi-liquid behavior found near antiferromagnetic quantum critical points, but this dependence does not persist down to T = 0. Below a characteristic temperature T-star similar or equal to 0.35 K, which depends weakly on H, w(T) and rho(T) both deviate downward and converge as T -> 0. We propose that T-star marks the onset of short-range magnetic correlations, persisting beyond H-c. By comparing samples of different purity, we conclude that the Wiedemann-Franz law holds in YbRh2Si2, even at H-c, implying that no fundamental breakdown of quasiparticle behavior occurs in this material. The overall phenomenology of heat and charge transport in YbRh2Si2 is similar to that observed in the heavy-fermion metal CeCoIn5, near its own field-tuned quantum critical point.
NASA Astrophysics Data System (ADS)
Yu, Wing Chi; Cheung, Yiu Wing; Saines, Paul J.; Imai, Masaki; Matsumoto, Takuya; Michioka, Chishiro; Yoshimura, Kazuyoshi; Goh, Swee K.
The family of the superconducting quasiskutterudites (CaxSr1-x)3Rh4Sn13 features a structural quantum critical point at xc = 0 . 9 , around which a dome-shaped variation of the superconducting transition temperature Tc is found. In this talk, we present the specific heat data for the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching xc, which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2 Δ /kBTc and ΔC / γTc beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. Reference: Wing Chi Yu et al. Phys. Rev. Lett. (in press, 2015) This work was supported by the CUHK (Startup Grant, Direct Grant No. 4053071), UGC Hong Kong (ECS/24300214), Grants-in-Aid from MEXT (22350029 and 23550152), and Glasstone Bequest, Oxford.
NASA Astrophysics Data System (ADS)
McCarthy, Kimberly Ann
1990-01-01
Divisions in definitions of creativity have centered primarily on the working definition of discontinuity and the inclusion of intrinsic features such as unconscious processing and intrinsic motivation and reinforcement. These differences generally result from Cohen's two world views underlying theories of creativity: Organismic, oriented toward holism; or mechanistic, oriented toward cause-effect reductionism. The quantum world view is proposed which theoretically and empirically unifies organismic and mechanistic elements of creativity. Based on Goswami's Idealistic Interpretation of quantum physics, the quantum view postulates the mind -brain as consisting of both classical and quantum structures and functions. The quantum domain accesses the transcendent order through coherent superpositions (a state of potentialities), while the classical domain performs the function of measuring apparatus through amplifying and recording the result of the collapse of the pure mental state. A theoretical experiment, based on the 1980 Marcel study of conscious and unconscious word-sense disambiguation, is conducted which compares the predictions of the quantum model with those of the 1975 Posner and Snyder Facilitation and Inhibition model. Each model agrees that while conscious access to information is limited, unconscious access is unlimited. However, each model differently defines the connection between these states: The Posner model postulates a central processing mechanism while the quantum model postulates a self-referential consciousness. Consequently, the two models predict differently. The strength of the quantum model lies in its ability to distinguish between classical and quantum definitions of discontinuity, as well as clarifying the function of consciousness, without added assumptions or ad-hoc analysis: Consciousness is an essential, valid feature of quantum mechanisms independent of the field of cognitive psychology. According to the quantum model, through a
Terahertz excitations near the quantum critical point in the 1D Ising chain quantum magnet CoNb2O6
NASA Astrophysics Data System (ADS)
Morris, Christopher M.; Valdés Aguilar, R.; Koopayeh, S.; McQueen, T. M.; Armitage, N. P.
2013-03-01
The one-dimensional magnet CoNb2O6 was recently demonstrated to be an excellent realization of a one-dimensional quantum Ising spin chain. It has been shown to undergo a quantum phase transition in a magnetic field oriented transverse to its ferromagnetically aligned spin chains. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.[2] The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate these optically active spin flip excitations in CoNb2O6 in an external magnetic field. For static magnetic fields oriented transverse to the spin chains, the terahertz excitations show evidence of the phase transitions that occur near the quantum critical magnetic field. Additional spin flip excitations are also observed for longitudinally oriented magnetic fields. Work supported by The Institute of Quantum Matter under DOE grant DE-FG02-08ER46544 and by the Gordon and Betty Moore Foundation.
Wong, Kin-Yiu; Gao, Jiali
2009-01-01
In this paper, we describe an automated integration-free path-integral (AIF-PI) method, based on Kleinert’s variational perturbation (KP) theory, to treat internuclear quantum-statistical effects in molecular systems. We have developed an analytical method to obtain the centroid potential as a function of the variational parameter in the KP theory, which avoids numerical difficulties in path-integral Monte Carlo or molecular dynamics simulations, especially at the limit of zero-temperature. Consequently, the variational calculations using the KP theory can be efficiently carried out beyond the first order, i.e., the Giachetti-Tognetti-Feynman-Kleinert variational approach, for realistic chemical applications. By making use of the approximation of independent instantaneous normal modes (INM), the AIF-PI method can readily be applied to many-body systems. Previously, we have shown that in the INM approximation, the AIF-PI method is accurate for computing the quantum partition function of a water molecule (3 degrees of freedom) and the quantum correction factor for the collinear H3 reaction rate (2 degrees of freedom). In this work, the accuracy and properties of the KP theory are further investigated by using the first three order perturbations on an asymmetric double-well potential, the bond vibrations of H2, HF, and HCl represented by the Morse potential, and a proton-transfer barrier modeled by the Eckart potential. The zero-point energy, quantum partition function, and tunneling factor for these systems have been determined and are found to be in excellent agreement with the exact quantum results. Using our new analytical results at the zero-temperature limit, we show that the minimum value of the computed centroid potential in the KP theory is in excellent agreement with the ground state energy (zero-point energy) and the position of the centroid potential minimum is the expectation value of particle position in wave mechanics. The fast convergent property of
Taufour, Valentin; Kaluarachchi, Udhara S; Khasanov, Rustem; Nguyen, Manh Cuong; Guguchia, Zurab; Biswas, Pabitra Kumar; Bonfà, Pietro; De Renzi, Roberto; Lin, Xiao; Kim, Stella K; Mun, Eun Deok; Kim, Hyunsoo; Furukawa, Yuji; Wang, Cai-Zhuang; Ho, Kai-Ming; Bud'ko, Sergey L; Canfield, Paul C
2016-07-15
The temperature-pressure phase diagram of the ferromagnet LaCrGe_{3} is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFM_{Q}. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q=0 to finite Q, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe_{3} is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets. PMID:27472137
Taufour, Valentin; Kaluarachchi, Udhara S.; Khasanov, Rustem; Nguyen, Manh Cuong; Guguchia, Zurab; Biswas, Pabitra Kumar; Bonfa, Pietro; De Renzi, Roberto; Lin, Xiao; Kim, Stella K.; et al
2016-07-13
Here, the temperature-pressure phase diagram of the ferromagnet LaCrGe3 is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFMQ. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q=0 to finite Q, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe3 ismore » a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.« less
NASA Astrophysics Data System (ADS)
Taufour, Valentin; Kaluarachchi, Udhara S.; Khasanov, Rustem; Nguyen, Manh Cuong; Guguchia, Zurab; Biswas, Pabitra Kumar; Bonfà, Pietro; De Renzi, Roberto; Lin, Xiao; Kim, Stella K.; Mun, Eun Deok; Kim, Hyunsoo; Furukawa, Yuji; Wang, Cai-Zhuang; Ho, Kai-Ming; Bud'ko, Sergey L.; Canfield, Paul C.
2016-07-01
The temperature-pressure phase diagram of the ferromagnet LaCrGe3 is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFMQ . Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q =0 to finite Q , as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe3 is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.
Kim, M. G.; Wang, M.; Tucker, G. S.; Valdivia, P. N.; Abernathy, D. L.; Chi, Songxue; Christianson, A. D.; Aczel, A. A.; Hong, T.; Heitmann, T. W.; et al
2015-12-02
We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting Ba(Fe0.957Cu0.043)2As2, a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound BaFe2As2 and superconducting Ba(Fe1–xNix)2As2 compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe0.957Cu0.043)2As2, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. In addition, we show that themore » spin-spin correlation length ξ(T) increases rapidly as the temperature is lowered and find ω/T scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.« less
NASA Astrophysics Data System (ADS)
Kim, M. G.; Wang, M.; Tucker, G. S.; Valdivia, P. N.; Abernathy, D. L.; Chi, Songxue; Christianson, A. D.; Aczel, A. A.; Hong, T.; Heitmann, T. W.; Ran, S.; Canfield, P. C.; Bourret-Courchesne, E. D.; Kreyssig, A.; Lee, D. H.; Goldman, A. I.; McQueeney, R. J.; Birgeneau, R. J.
2015-12-01
We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting Ba (Fe 0.957Cu 0.043) 2As 2 , a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound BaFe2As2 and superconducting Ba (Fe1-xNix) 2As2 compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba (Fe0.957Cu0.043)2As2 , which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spin-spin correlation length ξ (T ) increases rapidly as the temperature is lowered and find ω /T scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.