Browsing by Author "Li, Dingping"

Now showing 1 - 20 of 28

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
;
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.
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.
Exchange Interaction Makes Superconductivity in 3D Dirac Semi-metal Triplet
(2015-04-01)
Rosenstein, B.
;
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. In a certain range of the chemical potential and parameters characterizing the pairing attraction (effective electron-electron coupling constant and the Debye energy ) the energy of the singlet although always lower, prevails by a very slim margin over the triplet. This means that interactions that are small but discriminate between the spin singlet and the spin triplet determine the nature of the superconducting order there. It is shown that in materials close enough to the Dirac point ( ) a moderate exchange constant (below Stoner instability to ferromagnetism) stabilizes the odd pairing superconducting state. The 3D quantum critical point at of transition to the triplet superconductivity governs the physics of the superconductor.
Extended object tunneling: Current-carrying states of Abrikosov vortices in a superconductor with artificial nanobarriers
(2012-02-24)
Rosenstein, B.
;
Shapiro, I.
;
Shapiro, B. Ya.
;
Li, Dingping
;
Frydman, A.
We consider the structure and dynamics of two-dimensional fluxons created by a magnetic field in a type-II superconductor film with critical temperature T-1 in the presence of nanostripes of material with a higher critical temperature T-2. The width of the stripes is of the order of the coherence length xi. Such a stripe plays the role of a potential barrier for vortices. When subjected to the current J parallel to the stripe, vortices move toward the barrier. Below the critical current J(c), the flux flow is effectively halted by the barrier, while above J(c), flux penetrates the stripe and passes the barrier. The mechanism of the barrier penetration ("soliton tunneling" occurs even at zero temperature) is rather unusual: the vortices adjust their shape, and upon passing the barrier, they shrink their cores. The critical current and the I-V curves are calculated numerically and analytically using variational approach.
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 and the replica symmetry breaking in vortex matter: MC study
(2011-01-01)
Li, Dingping
;
Rosenstein, B.
;
Chen, Shou
;
Lin, H. H.
;
Berco, D.
We investigate effects of disorder and thermal fluctuations on the Abrikosov state of type II superconductors applying the Monte Carlo method to Ginzburg-Landau theory to confirm earlier replica calculation The vortex phase diagram has two transition lines the melting line and the vortex glass transition lines in both crystalline and homogeneous states The glass line is always a continuous transition while the melting line is the first order (C) 2010 Elsevier BV All rights reserved
Glass transition in vortex matter
(2006)
Rosenstein, Baruch
;
Li, Dingping
;
Vinokur, Valery M.
;
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.
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
;
Li, Dingping
;
Ma, Tianxing
;
Kao, H. C.
;
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.
Superconducting critical temperature and upper magnetic field in Weyl semimetal
(2018-01-01)
Li, Dingping
;
Rosenstein, B.
;
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 semi-metals on superconductivity is considered. A set of Gorkov equations for weak superconductivity in Weyl semi-metal under topological phase transition is derived and solved. The critical temperature has spike in the point the transition point as function of the tilt parameter of the Dirac cone determined in turn by the material parameters like pressure. The second critical magnetic field is calculated for phase I. It shows narrow domains of reentrant superconductivity associated with Landau quantization relativistic 2D electron gas.
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.