Browsing by Author "Rosenstein, Baruch"

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Chiral universality class of normal-superconducting and exciton condensation transitions on surface of topological insulator
(2015-06-01)
Li, Dingping
;
Rosenstein, Baruch
;
Shapiro, B. Ya.
;
Shapiro, I.
;
電子物理學系
New two-dimensional systems such as the surfaces of topological insulators (TIs) and graphene offer the possibility of experimentally investigating situations considered exotic just a decade ago. These situations include the quantum phase transition of the chiral type in electronic systems with a relativistic spectrum. Phonon-mediated (conventional) pairing in the Dirac semimetal appearing on the surface of a TI causes a transition into a chiral superconducting state, and exciton condensation in these gapless systems has long been envisioned in the physics of narrow-band semiconductors. Starting from the microscopic Dirac Hamiltonian with local attraction or repulsion, the Bardeen-Cooper-Schrieffer type of Gaussian approximation is developed in the framework of functional integrals. It is shown that owing to an ultrarelativistic dispersion relation, there is a quantum critical point governing the zero-temperature transition to a superconducting state or the exciton condensed state. Quantum transitions having critical exponents differ greatly from conventional ones and belong to the chiral universality class. We discuss the application of these results to recent experiments in which surface superconductivity was found in TIs and estimate the feasibility of phonon pairing.
Concurrent intersection point of magnetization and magnetoconductivity curves in strongly fluctuating superconductors
(2017-12-20)
Chi, Cheng
;
Jiang, Xujiang
;
Wang, Jiangfan
;
Li, Dingping
;
Rosenstein, Baruch
The thermal fluctuations contribution to magnetization and magnetoconductivity of type-II layered superconductors is calculated in the framework of the Lawrence-Doniach model. For numerous high-temperature cuprate superconductors, it was discovered that the magnetization dependence on temperature in a wide range of fields exhibits an intersection point at a temperature slightly below Tc. We notice a similar intersection point of the magnetoconductivity curves at the approximate same temperature. The phenomenon is explained by strong (non-Gaussian) thermal fluctuations with interactions treated using a self-consistent theory. All higher Landau levels should be included. Dimensionality of the fluctuations is defined, and the two- to three-dimensional crossover is the key for the existence of intersection points.
Covariant Bethe-Salpeter approximation in models of strongly correlated electron systems
(2020-02-20)
Fan, Zhenhao
;
Sun, Zhipeng
;
Li, Dingping
;
Berenstein, Itzhak
;
Leshem, Guy
Strongly correlated electron systems are generally described by tight-binding lattice Hamiltonians with strong local (onsite) interactions, the most popular being the Hubbard model. Although the half-filled Hubbard model can be simulated by Monte Carlo (MC), physically more interesting cases beyond half-filling are plagued by the sign problem. One therefore should resort to other methods. It was demonstrated recently that a systematic truncation of the set of Dyson-Schwinger equations for correlators of the Hubbard, supplemented by a "covariant" calculation of correlators leads to a convergent series of approximants. The covariance preserves all the Ward identities among correlators describing various condensed matter probes. While first-order (classical), second-order (Hartree-Fock or Gaussian), and third-order (Cubic) covariant approximation were worked out, the fourth-order (quartic) seems too complicated to be effectively calculable in fermionic systems. It turns out that the complexity of the quartic calculation in local interaction models,is manageable computationally. The quartic (Bethe-Salpeter-type) approximation is especially important in 1D and 2D models in which the symmetry-broken state does not exists (the Mermin-Wagner theorem), although strong fluctuations dominate the physics at strong coupling. Unlike the lower-order approximations, it respects the Mermin-Wagner theorem. The scheme is tested and exemplified on the single-band 1D and 2D Hubbard model.
Covariant cubic approximation for many-body electronic systems
(2018-10-15)
Rosenstein, Baruch
;
Li, Dingping
;
電子物理學系
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Department of Electrophysics
An approximation scheme determining the excitations of an electronic model on the lattice with pairwise interactions is proposed. A systematic truncation of the set of Dyson-Schwinger equations for correlators, supplemented by a "covariant" calculation of correlators lead to a converging series of approximates. The covariance preserves all the Ward identities among correlators describing various condensed matter probes. It is shown that the third-order approximant of this kind beyond classical and Gaussian (Hartree-Fock) is precise enough and due to several fortunate features the complexity of calculation is surprisingly low so that a realistic material computation might be feasible. Focus here is on the electron field correlator describing the electron (hole) excitations measured in photoemission and other probes. The scheme is tested on several solvable benchmark models.
Dynamical instability of the electric transport in superconductors
(2018-09-20)
Qiao, Lei
;
Li, Dingping
;
Postolova, Svetlana V.
;
Mironov, Alexey Yu.
;
Vinokur, Valerii
We develop a nonlinear theory of the electronic transport in superconductors in the framework of the time-dependent Ginzburg-Landau (TDGL) equation. We utilize self-consistent Gaussian approximation and reveal the conditions under which the current-voltage V(I) dependence (I-V characteristics) acquires an S-shape form leading to switching instabilities. We demonstrate that in two-dimensions the emergence of such an instability is a hallmark of the Berezinskii-Kosterlitz-Thou less (BKT) transition that we have detected by transport measurements of titanium nitride (TiN) films. Our theoretical findings compare favorably with our experimental results.
Dynamics of the vortex-glass transition
(2006)
Bel, Golan
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Rosenstein, Baruch
;
電子物理學系
;
Department of Electrophysics
The dynamic of moving vortex matter is considered in the framework of the time dependent Ginzburg-Landau equation beyond linear response. Both disorder and thermal fluctuations are included using the Martin-Siggia-Rose formalism within the lowest Landau level approximation. We determine the critical current as function of magnetic field and temperature. The surface in the J-B-T space defined by the function separates between the dissipative moving vortex matter regime (qualitatively appearing as either the vortex creep and flux flow) and dissipation less current state in which vortices are pinned creating an amorphous vortex "glass". Both the thermal depinning and the depinning by a driving force are taken into account. The static irreversibility line is compared to experiments and is consistent with the one obtained in the replica approach. The non-Ohmic I-V curve (in the depinned phase) is obtained and resistivity compared with experiments in layered superconductors and thin films.
Effect of the type-I to type-II Weyl semimetal topological transition on superconductivity
(2017-03-21)
Li, Dingping
;
Rosenstein, Baruch
;
Shapiro, B. Ya.
;
Shapiro, I.
;
電子物理學系
The influence of recently discovered topological transition between type-I and type-II Weyl semimetals on superconductivity is considered. A set of Gorkov equations for weak superconductivity inWeyl semimetal under topological phase transition is derived and solved. The critical temperature and superconducting gap both have spikes in the transition point as functions of the tilt parameter of the Dirac cone determined, in turn, by the material parameters like pressure. The spectrum of superconducting excitations is different in two phases: The sharp cone pinnacle is characteristic for type I, while two parallel almost flat bands, are formed in type II. Spectral density is calculated on both sides of transition to demonstrate the different weights of the bands. The superconductivity thus can be used as a clear indicator for the topological transformation. Results are discussed in the light of recent experiments.
Electrical conductivity beyond a linear response in layered superconductors under a magnetic field
(2010-06-29)
Tinh, Bui Duc
;
Li, Dingping
;
Rosenstein, Baruch
;
電子物理學系
;
Department of Electrophysics
The time-dependent Ginzburg-Landau approach is used to investigate nonlinear response of a strongly type-II superconductor. The dissipation takes a form of the flux flow which is quantitatively studied beyond linear response. Thermal fluctuations, represented by the Langevin white noise, are assumed to be strong enough to melt the Abrikosov vortex lattice created by the magnetic field into a moving vortex liquid and marginalize the effects of the vortex pinning by inhomogeneities. The layered structure of the superconductor is accounted for by means of the Lawrence-Doniach model. The nonlinear interaction term in dynamics is treated within self-consistent Gaussian approximation and we go beyond the often used lowest Landau level approximation to treat arbitrary magnetic fields. The I-V curve is calculated for arbitrary temperature and the results are compared to experimental data on high-T-c. superconductor YBa2Cu3O7-delta.
Ginzburg-Landau theory of type II superconductors in magnetic field
(2010-01-01)
Rosenstein, Baruch
;
Li, Dingping
;
電子物理學系
;
Department of Electrophysics
Thermodynamics of type II superconductors in electromagnetic field based on the Ginzburg-Landau theory is presented. The Abrikosov flux lattice solution is derived using an expansion in a parameter characterizing the "distance" to the superconductor-normal phase transition line. The expansion allows a systematic improvement of the solution. The phase diagram of the vortex matter in magnetic field is determined in detail. In the presence of significant thermal fluctuations on the mesoscopic scale (for example, in high T-c materials) the vortex crystal melts into a vortex liquid. A quantitative theory of thermal fluctuations using the lowest Landau level approximation is given. It allows one to determine the melting line and discontinuities at melt, as well as important characteristics of the vortex liquid state. In the presence of quenched disorder (pinning) the vortex matter acquires certain "glassy" properties. The irreversibility line and static properties of the vortex glass state are studied using the "replica" method. Most of the analytical methods are introduced and presented in some detail. Various quantitative and qualitative features are compared to experiments in type II superconductors, although the use of a rather universal Ginzburg-Landau theory is not restricted to superconductivity and can be applied with certain adjustments to other physical systems, for example, rotating Bose-Einstein condensate.
Glass transition in vortex matter
(2006)
Rosenstein, Baruch
;
Li, Dingping
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Vinokur, Valery M.
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電子物理學系
;
Department of Electrophysics
We develop a unifying approach bridging between the spin and vortex glasses, based, on one hand, on the application of the replica method to Ginzburg-Landau description of the system in terms of the superconductor order parameter near Tc and, on the other hand, on the notion of the extreme value statistics that govern the distribution of the energy barriers controlling the glassy slow dynamics of the vortices. Focusing on the vortex systems not very far from the upper critical field we demonstrate that the glassy behavior is associated with the continuous replica symmetry breaking, which is the inherent feature of the long range spin glasses. We apply our results to the description of vortex phase diagram and find the vortex glass transition line, which appears different and well separated from both the melting line and from the so-called second peak lines.
High-temperature superconductivity in single unit cell layer FeSe due to soft phonons in the interface layer of the SrTiO3 substrate
(2019-08-19)
Rosenstein, Baruch
;
Shapiro, B. Ya
;
電子物理學系
;
Department of Electrophysics
Using a microscopic model of lattice vibrations in the STO(001) substrate, an additional Omega(s) = 50 meV longitudinal optical (LO) interface mode is identified. The soft mode propagating mainly in the first TiO2 layer ("O chains") has stronger electron-phonon coupling to electron gas in FeSe than a well-known Omega(h) = 100 meV hard mode. The coupling constant, critical temperature, and replica band are calculated. Although there exists a forward in the electron-phonon scattering peak, it is clearly not as sharp as assumed in recent theories (delta-function-like). The critical temperature is obtained by solution of the gap equation and agrees with the observed one. The corresponding electron phonon coupling constant lambda = 0.23. The quasiparticle normal state "satellite" in spectral weight is broad and its peak appears at frequency much higher than Omega(s) consistent with observations usually associated with Omega(h). Possible relation of the transversal counterpart of the surface LO soft mode with known phonons is discussed.
Magnetic impurities make superconductivity in 3D Dirac semi-metal triplet
(2015-03-01)
Rosenstein, Baruch
;
Shapiro, B. Ya.
;
Li, Dingping
;
Shapiro, I.
;
電子物理學系
Conventional electron-phonon coupling induces either odd (triplet) or even (singlet) pairing states in a time reversal and inversion invariant Dirac semi-metal. It was shown that the energy of the singlet channel, always lower, prevails over the triplet. The role of both magnetic and non-magnetic spatial disorder was discovered. It is predicted that the magnetic part of scattering establishes a triplet ground state suppressing the singlet phase. On the other hand, the nonmagnetic impurities slightly affecting the singlet phase may significantly suppress the triplet state. Copyright (C) EPLA, 2015
Magnetic properties of type-I and type-II Weyl semimetals in the superconducting state
(2018-04-12)
Rosenstein, Baruch
;
Shapiro, B. Ya.
;
Li, Dingping
;
Shapiro, I.
;
電子物理學系
Superconductivity was observed in certain range of pressure and chemical composition in Weyl semimetals of both type I and type II (when the Dirac cone tilt parameter kappa > 1). Magnetic properties of these superconductors are studied on the basis of microscopic phonon-mediated pairing model. The Ginzburg-Landau effective theory for the order parameter is derived using the Gorkov approach and used to determine anisotropic coherence length, the penetration depth determining the Abrikosov parameter for a layered material and applied to recent extensive experiments on MoTe2. It is found that superconductivity is of second kind near the topological transition at kappa = 1. For a larger tilt parameter, superconductivity becomes first kind. For kappa < 1, the Abrikosov parameter also tends to be reduced, often crossing over to the first kind kappa For the superconductors of the second kind, the dependence of critical fields H-c2 and H-c1 on the tilt parameter. (governed by pressure) is compared with the experiments. Strength of thermal fluctuations is estimated and it is found that they are strong enough to cause Abrikosov vortex lattice melting near H-c2. The melting line is calculated and is consistent with experiments provided the fluctuations are three dimensional in the type-I phase (large pressure) and two dimensional in the type-II phase (small pressure).
Mean field theory of short-range order in strongly correlated low dimensional electronic systems
(2019-09-18)
Rosenstein, Baruch
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Li, Dingping
;
Ma, Tianxing
;
Kao, H. C.
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電子物理學系
Mean field approach, although a generally reliable tool that captures major short-range correlations, often fails in symmetric low dimensional strongly correlated electronic systems like those described by the Hubbard model. In these situations a symmetry is almost broken. The problem is linked to the restoration of the symmetry due to strong fluctuations (both quantum and thermal) on all scales. The restoration of symmetry in statistical models of scalar order parameter fields was treated recently successfully on the Gaussian approximation level by symmetrization of the correlators. Here the idea is extended to fermionic systems in which the order parameter is composite. Furthermore, the precision of the correlators can be improved perturbatively. Such a scheme (based on covariant Gaussian approximation) is demonstrated on the one dimensional (1D) and 2D one band Hubbard models by comparison of the correlator with exact diagonalization and MC simulations, respectively.
Quantum critical point in the superconducting transition on the surface of a topological insulator
(2014-08-25)
Li, Dingping
;
Rosenstein, Baruch
;
Shapiro, B. Ya
;
Shapiro, I.
;
電子物理學系
Pairing in the Weyl semimetal appearing on the surface of a topological insulator is considered. It is shown that due to an "ultrarelativistic" dispersion relation there is a quantum critical point governing the zero-temperature transition to a superconducting state. Starting from the microscopic Hamiltonian with local attraction, we calculated using the Gor'kov equations, the phase diagram of the superconducting transition at arbitrary chemical potential, and its magnetic properties and critical exponents close to the quantum critical point. The Ginzburg-Landau (GL) effective theory is derived for small chemical potential, allowing us to consider effects of spatial dependence of order parameters in a magnetic field. The GL equations are very different from the conventional ones reflecting the chiral universality class of the quantum phase transition. The order-parameter distribution of a single vortex is found to be different as well. The magnetization near the upper critical field is found to be quadratic, not linear as usual. We discuss the application of these results to recent experiments in which surface superconductivity was found for some three-dimensional topological insulators, and we estimate feasibility of the phonon pairing.
Strong magnetic field induces superconductivity in a Weyl semimetal
(2017-12-29)
Rosenstein, Baruch
;
Shapiro, B. Ya.
;
Li, Dingping
;
Shapiro, I.
;
電子物理學系
Microscopic theory of the normal-to-superconductor coexistence line of a multiband Weyl superconductor subjected to magnetic field is constructed. It is shown that the Weyl semimetal that is nonsuperconducting or having a small critical temperature T-c at zero field might become a superconductor at higher temperatures when the magnetic field is tuned to a series of quantized values H-n. The pairing occurs on Landau levels. It is argued that the phenomenon is detectable much easier in Weyl semimetals than in parabolic band metals since the quantum limit already has been approached in several Weyl materials. The effect of Zeeman coupling leading to splitting of the reentrant superconducting regions on the magnetic phase diagram is considered. An experimental signature of the superconductivity on Landau levels is the reduction of magnetoresistivity. This has been observed already in Cd3As2 and several other compounds. The novel kind of quantum oscillations of magnetoresistance detected in ZrTe5 is discussed along these lines.
Strong thermal fluctuations in cuprate superconductors in magnetic fields above T-c
(2014-02-18)
Jiang, Xujiang
;
Li, Dingping
;
Rosenstein, Baruch
;
電子物理學系
;
Department of Electrophysics
Recent measurements of fluctuation diamagnetism in high-temperature superconductors show distinct features above and below T-c, which can not be explained by simple Gaussian fluctuation theory. Self-consistent calculation of magnetization in layered high-temperature superconductors, based on the Ginzburg-Landau-Lawrence-Doniach model and including all Landau levels is presented. The results agree well with the experimental data in a wide region around T-c, including both the vortex liquid below T-c and the normal state above T-c. The Gaussian fluctuation theory significantly overestimates the diamagnetism for strong fluctuations. It is demonstrated that the intersection point of magnetization curves appears in the region where the lowest Landau level contribution dominates and magnetization just below T-c is nonmonotonic. Our calculation supports the phase disordering picture of fluctuations above T-c.
Superconductivity in the two-dimensional electron gas induced by high-energy optical phonon mode and large polarization of the SrTiO3 substrate
(2016-07-11)
Rosenstein, Baruch
;
Shapiro, B. Ya.
;
Shapiro, I.
;
Li, Dingping
;
電子物理學系
Pairing in one-atomic-layer-thick two-dimensional electron gas (2DEG) by a single flat band of high-energy longitudinal optical phonons is considered. The polar dielectric SrTiO3 (STO) exhibits such an energetic phonon mode and the 2DEG is created both when one unit cell FeSe layer is grown on its (100) surface and on the interface with another dielectric like LaAlO3 (LAO). We obtain a quantitative description of both systems solving the gap equation for T-c for arbitrary Fermi energy is an element of(F), electron-phonon coupling lambda, and the phonon frequency Omega, and direct (random-phase approximation) electron-electron repulsion strength alpha. The focus is on the intermediate region between the adiabatic, is an element of(F) >> Omega, and the nonadiabatic, is an element of(F) << Omega, regimes. The high-temperature superconductivity in single-unit-cell FeSe/STO is possible due to a combination of three factors: high-longitudinal-optical phonon frequency, large electron-phonon coupling lambda similar to 0.5, and huge dielectric constant of the substrate suppression the Coulomb repulsion. It is shown that very low density electron gas in the interfaces is still capable of generating superconductivity of the order of 0.1 K in LAO/STO.
The Vortex Core Excitation Spectrum in Gapped Topological d-wave Superconductors
(2013-12-01)
Rosenstein, Baruch
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Shapiro, Irina
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Shapiro, Boris Y.
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電子物理學系
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Department of Electrophysics
There are indications that some high temperature unconventional superconductors have a "complex" d-wave order parameter (with an admixture of s-wave) leading to nonzero energy gap. Since the coherence length is short and the Fermi energy is relatively small the quasiclassical approach is inapplicable and the more complicated Bogoliubov-deGennes equations should be used to investigate the excitation spectrum of such a material in a magneric field. It turns out that equations for the chiral d-wave superconductor simplify considerably and is the basis for any superconductor of that type with a sufficiently large gap. The spectrum of core excitations of the Abrikosov vortex in an anisotropic 3D sample exhibits several features. Unlike in conventional and gapless superconductors the core has a single excitation mode of order energetic gap for each value of momentum along the field. This has a large impact on thermal transport and vortex dynamics.
Theory of Nernst effect in high-T(c) superconductors
(2009-01-01)
Tinh, Bui Duc
;
Rosenstein, Baruch
;
電子物理學系
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Department of Electrophysics
We calculated, using the time-dependent Ginzburg-Landau equation with thermal noise, the transverse thermoelectric conductivity alpha(xy) and the Nernst signal e(N), describing the Nernst effect, in type-II superconductor in the vortex-liquid regime. The Gaussian method used is an elaboration of the Hartree-Fock utilized by Ullah and Dorsey [Phys. Rev. Lett. 65, 2066 (1990)]. An additional assumption often made in analytical calculations that only the lowest Landau level significantly contributes to physical quantities of interest in the high-field limit is lifted by including all the Landau levels. The resulting values in two dimensions are significantly lower than the numerical simulation data of the same model but are in reasonably good quantitative agreement with experimental data on La(2)SrCuO(4) above the irreversibility line (below the irreversibility line at which alpha(xy) diverges and theory should be modified by including pinning effects). The values of e(N) calculated in three dimensions are also in good quantitative agreement with experimental data for temperature close to T(c) on YBa(2)Cu(3)O(7). For each of the materials we consider, the melting and the irreversibility lines are also fitted with the same set of parameters using a recent quantitative Ginzburg-Landau theory.