Browsing by Author "Shapiro, B. Ya"

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Current carrying vortex crystals
(2012)
Rosenstein, B.
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Shapiro, I.
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Berco, D.
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Shapiro, B. Ya
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電子物理學系
Abrikosov vortices in a type-II superconducting film subjected to strong magnetic field B with periodic array of nanoholes of the density n(pin) form sometimes a vortex crystal, even when they are driven by a transport current. It is shown numerically that the crystal melting and transition to the resistive state occurs as a coherent depinning of the single vortex dislocations. For a system with interstitial vortices, f = B/Phi(0)n(pin) > 1, the mechanism of depinning depends on the current direction with respect to the pinning array. It was found that slightly above the critical current trajectories of moving vortices are not straight, but rather acquire a snake - like shape enveloping the system of pins. In contrast to the matching field case, f = 1, the transition to a resistive state is not coherent and is developing through formation of the "snake - like" vortex trajectories. It is pointed out that the depinning is closely associated with the appearance of a strongly varying electric field. We calculated the electric fields accompanying vortex crystal melting and found the voltage-current characteristics. When the pinning array is made random, the critical current is reduced.
Effect of nanoholes on the vortex core fermion spectrum and heat transport in p-wave superconductors
(2013-02-20)
Rosenstein, B.
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Shapiro, I.
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Shapiro, B. Ya
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電子物理學系
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Department of Electrophysics
The spectrum of core excitations of the Abrikosov vortex pinned by a nanohole of the size of the coherence length is considered. While the neutral zero energy Majorana core state remains intact due to its topological origin, the energy of charged excitations is significantly enhanced compared to that in the unpinned vortex. As a consequence of the pinning the minigap separating the Majorana state from the charged levels increases from Delta(2)/E-F (E-F is the Fermi energy and Delta is the bulk p-wave superconducting gap) to a significant fraction of Delta. Suppression of the thermodynamic and kinetic effects of the charged excitations allows us to isolate the Majorana state so it can be used for quantum computation. It is proposed that thermal conductivity along the vortex cores is a sensitive method to demonstrate the minigap. Using the Butticker-Landauer-Kopnin formula, we calculate the thermal conductance beyond the linear response as a function of the hole radius.
Exchange Interaction Makes Superconductivity in 3D Dirac Semi-metal Triplet
(2015-04-01)
Rosenstein, B.
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Shapiro, B. Ya
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Li, Dingping
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Shapiro, I.
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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.
Field cooling memory effect in Bi2212 and Bi2223 single crystals
(2010-07-01)
Shaltiel, D.
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von Nidda, H-A Krug
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Rosenstein, B.
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Shapiro, B. Ya
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Golosovsky, M.
A memory effect in the Josephson vortex system created by a magnetic field in the highly anisotropic superconductors Bi2212 and Bi2223 is demonstrated using microwave power absorption. This surprising effect appears despite a very low viscosity of Josephson vortices compared to Abrikosov vortices. The superconductor is field-cooled in a DC magnetic field H(m) oriented parallel to the CuO planes through the critical temperature T(c) down to 4 K, with subsequent reduction of the field to zero and again above H(m). A large microwave power absorption signal is observed at a magnetic field just above the cooling field, clearly indicating a memory effect. The dependence of the signal on a deviation of the magnetic field from H(m) is the same for a wide range of H(m) from 1500 to 17 000 G.
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
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Shapiro, B. Ya
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電子物理學系
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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.
Quantum critical point in the superconducting transition on the surface of a topological insulator
(2014-08-25)
Li, Dingping
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Rosenstein, Baruch
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Shapiro, B. Ya
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Shapiro, I.
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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.
Triplet superconductivity in 3D Dirac semi-metal due to exchange interaction
(2014-01-21)
Rosenstein, Baruch
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Shapiro, B. Ya
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Li, Dingping
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Shapiro, I.
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電子物理學系
Conventional phonon-electron interaction induces either triplet or one of two (degenerate) singlet pairing states in time reversal and inversion invariant 3D Dirac semi-metal. Investigation of the order parameters and energies of these states at zero temperature in a wide range of values of chemical potential mu, the effective electron-electron coupling constant. and Debye energy T-D demonstrates that when the exchange interaction is neglected the singlet always prevails, however, in significant portions of the (mu, lambda, T-D) parameter space the energy difference is very small. This means that interactions that are small, but discriminate between the spin singlet and the spin triplet, are important in order to determine the nature of the superconducting order there. The best candidate for such an interaction in the materials under consideration is the exchange (the Stoner term) characterized by constant lambda(ex). We show that at values of lambda(ex), much smaller than ones creating Stoner instability to ferromagnetism lambda(ex) similar to 1, the triplet pairing becomes energetically favored over the singlet ones. The 3D quantum critical point at mu = 0 is considered in detail. This can be realized experimentally in optically trapped cold atom systems.
Triplet superconductivity in 3D Dirac semi-metal due to exchange interaction
(2015-01-21)
Rosenstein, Baruch
;
Shapiro, B. Ya
;
Li, Dingping
;
Shapiro, I.
;
電子物理學系
Conventional phonon-electron interaction induces either triplet or one of two (degenerate) singlet pairing states in time reversal and inversion invariant 3D Dirac semi-metal. Investigation of the order parameters and energies of these states at zero temperature in a wide range of values of chemical potential mu, the effective electron-electron coupling constant. and Debye energy T-D demonstrates that when the exchange interaction is neglected the singlet always prevails, however, in significant portions of the (mu, lambda, T-D) parameter space the energy difference is very small. This means that interactions that are small, but discriminate between the spin singlet and the spin triplet, are important in order to determine the nature of the superconducting order there. The best candidate for such an interaction in the materials under consideration is the exchange (the Stoner term) characterized by constant lambda(ex). We show that at values of lambda(ex), much smaller than ones creating Stoner instability to ferromagnetism lambda(ex) similar to 1, the triplet pairing becomes energetically favored over the singlet ones. The 3D quantum critical point at mu = 0 is considered in detail. This can be realized experimentally in optically trapped cold atom systems.
Triplet superconductivity in 3D Dirac semi-metal due to exchange interaction (vol 27, 025701, 2015)
(2015-02-25)
Rosenstein, Baruch
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Shapiro, B. Ya
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Li, Dingping
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Shapiro, I.
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電子物理學系
Type I superconductivity in Dirac materials
(2018-08-22)
Shapiro, B. Ya
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Shapiro, I.
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Li, Dingping
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Rosenstein, Baruch
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電子物理學系
Superconductivity of the second kind was observed in many 3D Weyl and Dirac semi-metals while in the PdTe2, superconductivity is clearly of the first kind. This is very rare in Dirac semi-metals, but is expected in clean conventional metallic superconductors with 3D parabolic dispersion relation. The conduction bands in this material exhibit the linear (Dirac) dispersion only along two directions, while in the third direction the dispersion is parabolic. Therefore the 'hybrid' Dirac-parabolic material is intermediate between the two extremes. A microscopic pairing theory is derived for arbitrary tilt parameter of the 2D cone and used to determine anisotropic coherence lengths, the penetration depths and applied to recent extensive experiments. Magnetic properties of these superconductors are then studied in the parallel to the layers magnetic field on the basis of microscopically derived Ginzburg-Landau effective theory for the order parameter.
Upper critical magnetic field in superconducting Dirac semimetal
(2018-10-01)
Rosenstein, Baruch
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Shapiro, B. Ya
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Li, Dingping
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Shapiro, I
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電子物理學系
Temperature dependence of the upper critical field H-c2 of the Dirac semimetal (DSM) with phonon-mediated pairing is considered within the semiclassical approximation. The low-temperature dependence deviates from conventional BCS superconductor with parabolic dispersion relation (WERTHAMER N. R., HELFAND E. and HOHENBERG P. C., Phys. Rev., 147 (1966) 295; HELFAND E. and WERTHAMER N. R., Phys. Rev. Lett., 13 (1964) 686) even for large adiabaticity parameter, gamma = mu/(sic)Omega), where mu is the chemical potential and Omega the Debye frequency. In particular the "reduced field", ratio of zero-temperature H (c2) to the derivative at critical temperature, h* = H (c2)(0)/(-T dH(c2)/dT)vertical bar T-c, depends on gamma and can be extended beyond the adiabatic limit. The reduced magnetic field ratio is universal (independent of the chemical potential, interaction strength, etc.) and smaller than the Werthamer ratio for clean superconductors: h* = 0.55 for DSM, h* (0) = 0.69 for parabolic band. The results are in good agreement compared with recent experiments on TaP. Copyright (C) EPLA, 2018