Browsing by Author "Shapiro, B. Ya."

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3D DIRAC SEMI-METAL IN THE TRIPLET SUPERCONDUCTING STATE
(2015-01-01)
Rosenstein, B.
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Shapiro, B. Ya.
;
Shapiro, I.
;
交大名義發表
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National Chiao Tung University
It was recently shown that conventional phonon-electron interactions may induce a triplet pairing state in time-reversal invariant 3D Dirac semi - metals. Starting from the microscopic model of the isotropic Dirac semi-metal, the Ginzburg-Landau equations for the vector order parameter is derived using the Gorkov technique. The time dependent Ginzburg - Landau model in the presence of external fields is used to investigate some optical and magnetic properties of such superconductors. The AC conductivity of a clean sample depends on the orientation of the order parameter. It is demonstrated that the difference between the penetration depths results in rotation of the polarization vector of microwave passing a slab made of this material. The upper critical magnetic field H-c2 was found. It turns out that at fields close to H-c2 the order parameter orients itself perpendicular to the field direction. In certain range of parameters the triplet superconducting phase persists at arbitrarily high magnetic field like in some p wave superconductors.
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.
Collective modes, ac response, and magnetic properties of the three-dimensional Dirac semimetal in the triplet superconducting state
(2015-08-03)
Rosenstein, B.
;
Shapiro, B. Ya.
;
Shapiro, I.
;
電子物理學系
;
Department of Electrophysics
It was recently shown that conventional phonon-electron interactions may induce a triplet pairing state in time-reversal invariant three-dimensional Dirac semimetals. Starting from the microscopic model of the isotropic Dirac semimetal, the Ginzburg-Landau equations for the vector order parameter is derived using the Gor'kov technique. The collective modes including gapless Goldstone modes and gapped Higgs modes of various polarizations are identified. They are somewhat analogous to the modes in the B phase of He-3, although in the present case quantitatively there is a pronounced difference between longitudinal and transverse components. The difference is caused by the vector nature of the order parameter leading to two different coherence lengths or penetration depths. The system is predicted to be highly dissipative due to the Goldstone modes. The time-dependent Ginzburg-Landau model in the presence of external fields is used to investigate some optical and magnetic properties of such superconductors. The ac conductivity of a clean sample depends on the orientation of the order parameter. It is demonstrated that the difference between the penetration depths results in rotation of the polarization vector of microwave passing a slab made of this material. The upper critical magnetic field H-c2 was found. It turns out that at fields close to H-c2 the order parameter orients itself perpendicular to the field direction. In certain range of parameters the triplet superconducting phase persists at arbitrarily high magnetic field like in some p-wave superconductors.
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.
Extended object tunneling: Current-carrying states of Abrikosov vortices in a superconductor with artificial nanobarriers
(2012-02-24)
Rosenstein, B.
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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.
Influence of quenched disorder on the square-to-rhombohedral structural transformation of the vortex lattice of type-II superconductors
(2006-11-01)
Li, D. P.
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Lin, P. -J.
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Rosenstein, B.
;
Shapiro, B. Ya.
;
Shapiro, I.
A theory of structural transformations of the vortex lattice in a disordered fourfold symmetric type-II superconductor is constructed using two complementary descriptions of the vortex matter. At low temperatures we use a nonlocal London approach, while near H-c2(T) the lowest Landau level Ginzburg-Landau model with an asymmetric high derivative term is utilized. The quenched disorder influences the location of the square-to-rhombohedral structural transition line, the simplest transformation of that kind. In the clean case the slope of the line in the T-B plane is generally negative, as thermal fluctuations favor a more symmetric square lattice. We calculated the transition line in the presence of pinning and find that the disorder, which plays an important role in low-T-c materials, makes the slope positive, as has been observed in recent experiments.
Interaction between Josephson and pancake vortices investigated by the induced microwave dissipation by ac magnetic field technique
(2008-06-01)
Shaltiel, D.
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von Nidda, H. -A. Krug
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Shapiro, B. Ya.
;
Rosenstein, B.
;
Loidl, A.
In the "induced microwave dissipation by ac magnetic field" (IMDACMF) technique, only the motion of Josephson vortices (JV) but not of pancake vortices (PV) induces microwave dissipation. This unique property is used to investigate the interaction between JV and PV in the high anisotropy superconductor Bi2212 as a function of magnetic field and tilting angle away from the a-b plane. In general, pinning of JV by PV inhibits the motion of the JV and, thus, reduces the microwave dissipation. Almost no pinning of JV is obtained at low dc magnetic fields even at high-tilting angles. Strong pinning of JV due to their interaction with PV is observed at higher magnetic fields and at tilting angles between 50 degrees and 90 degrees. The technique enables to observe transitions from the JV state to the crossing lattice state, without and with interaction between JV and PV, and to the one-dimensional vortex chains. The results obtained by the present technique are compared to results of surface imaging techniques, which investigate the interaction between PV and JV probing the PV.
Interaction of AC magnetic field with Josephson vortices in high anisotropy superconductors Bi2212 and Bi2223
(2007-09-01)
Shaltiel, D.
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von Nidda, H.-A. Krug
;
Shapiro, B. Ya.
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Shapiro, I.
;
Loidl, A.
We show that in both high anisotropy superconductors Bi2212 (T-c = 90 K), and Bi2223 (T-c = 110 K), an AC magnetic field applied collinear to the DC field and parallel to the Cu layers induces strong microwave dissipation. It is theoretically predicted that the Josephson solitons are responsible for this absorption. Good qualitative agreement of experimental data with theoretical predictions is found. Differences observed in the experimental results between these two compounds are discussed. (c) 2007 Elsevier B.V. All rights reserved.
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).
Majorana states in a p-wave superconducting ring
(2013-06-01)
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 excitations of the chiral superconducting ring with internal and external radii R-i, R-e (comparable with coherence length xi) trapping a unit flux Phi(0) is calculated. We find within the Bogoliubov-de Gennes approach that there exists a pair of precisely zero-energy states when 2k(perpendicular to)(R-e - R-i)/pi is integer (here k(perpendicular to) is the momentum component in the disk plane while k(perpendicular to) xi > 1). They are not protected by topology, but are stable under certain deformations of the system. We discuss the ways to tune the system so that it grows into such a "Majorana disk". This condition has a character of a resonance phenomenon. Copyright (C) EPLA, 2013
Maximal persistent current in a type-II superconductor with an artificial pinning array at the matching magnetic field
(2010-02-01)
Rosenstein, B.
;
Shapiro, I.
;
Shapiro, B. Ya.
;
電子物理學系
;
Department of Electrophysics
The current carrying steady state of the pinned flux line lattice created by magnetic field is described. We calculate analytically the critical current for the case of the matching field (when the number of vortices is equal to that of the pinning centers) using a simple variational method in the framework of Ginzburg-Landau equations. The vortex cores are deformed and displaced in the current carrying state. Displacement of the centers of the vortices with respect to pinning centers and structure of these states are determined.
Microwave absorption in the cores of Abrikosov vortices pinned by artificial insulator inclusion
(2011-10-17)
Rosenstein, B.
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Shapiro, I.
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Deutch, E.
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Shapiro, B. Ya.
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電子物理學系
The spectrum of core excitations of the Abrikosov vortex pinned by a dielectric inclusion or nanoholes the size of the coherence length is considered using the Bogoliubov-deGennes equations beyond the semiclassical approximation. While the lowest excitation, minigap, in the unpinned (or pinned by a metallic defect) vortex is of the order of Delta(2)/E(F), it becomes of the order of the superconducting gap Delta (E(F) is Fermi energy). The reconstruction of the quasiparticle excitations' spectrum has a significant impact on optical properties and on the tunneling density of states. We calculate the absorption amplitude and point out that, while in scanning tunneling microscopy the energy gap Lambda(DOS) is between a quasiparticle state with angular momentum mu(e) = mu(0) > 1/2 and quasihole with mu(h) = -mu(0), the microwave absorption gap, Delta(dir), is between the states with mu(e) = mu(h) +/- 1. It is shown that Delta(dir) > Delta(DOS). The large minigap might play a role in magneto-transport phenomena broadly associated with the "superconductor-insulator" transition in quasi-two-dimensional systems in which small insulating inclusions are generally present.
Microwave absorption in the cores of the Abrikosov vortices pinned by artificial insulator inclusion
(2012-09-01)
Rosenstein, B.
;
Shapiro, I.
;
Shapiro, B. Ya.
;
Deutch, E.
;
電子物理學系
The spectrum of core excitations of the Abrikosov vortex pinned by a dielectric inclusion or nanoholes of the size of the coherence length is considered using the Bogoliubov-deGennes equations beyond the semiclassical approximation. While the lowest excitation, mini-gap, in the unpinned (or pinned by a metallic defect) vortex is of the order of Delta(2)/epsilon(F), it becomes of the order of the superconducting gap Delta. The reconstruction of the quasiparticle excitations' spectrum has a significant impact on optical properties and on the tunneling density of states. We calculate the absorption amplitude and point out that, while in STM the energy gap Delta(DOS) is between a quasiparticles state with angular momentum mu(e) = mu(s) > 1/2 and quasi-hole with mu(h) = mu(s) the microwave absorption gap, Delta(dir) is between states with mu(e) = mu(h) + 1. It is shown that Delta(dir) > Delta(DOS). The large mini-gap might play a role in magneto-transport phenomena broadly associated with the "superconductor-insulator" transition in quasi 2D systems in which small insulating inclusions are generally present. (C) 2012 Elsevier B. V. All rights reserved.
On investigation of high T(c) superconductors with Electron Paramagnetic Resonance spectrometers
(2010-11-15)
Shaltiel, D.
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von Nidda, H. -A. Krug
;
Shapiro, B. Ya.
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Bogoslavsky, B.
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Rosenstein, B.
Following the discovery of high T(c) superconductors, many researchers have applied EPR spectrometers to investigate their properties using the regular instructions to operate the spectrometer. As no substantial information was obtained it was soon practically abandoned. Investigations performed recently with the EPR technique, using a different operating method than the conventional one and the introduction of a detailed theoretical study related to the interaction of the JV with the microwave magnetic field lead to new and important results related to properties of high anisotropy superconductors indicating that the EPR spectrometer can be used to investigate properties of high anisotropy superconductors. The failures when using conventional operating method to obtain meaningful results and the method used in the present work to carry out the measurements in a proper manner are presented. Some important results obtained in investigating high anisotropy superconductors by the EPR technique are presented. (C) 2010 Elsevier B.V. All rights reserved.
On investigation of high T-c superconductors with Electron Paramagnetic Resonance spectrometers
(2010-11-15)
Shaltiel, D.
;
von Nidda, H. -A. Krug
;
Shapiro, B. Ya.
;
Bogoslavsky, B.
;
Rosenstein, B.
Following the discovery of high T-c superconductors, many researchers have applied EPR spectrometers to investigate their properties using the regular instructions to operate the spectrometer. As no substantial information was obtained it was soon practically abandoned. Investigations performed recently with the EPR technique, using a different operating method than the conventional one and the introduction of a detailed theoretical study related to the interaction of the JV with the microwave magnetic field lead to new and important results related to properties of high anisotropy superconductors indicating that the EPR spectrometer can be used to investigate properties of high anisotropy superconductors. The failures when using conventional operating method to obtain meaningful results and the method used in the present work to carry out the measurements in a proper manner are presented. Some important results obtained in investigating high anisotropy superconductors by the EPR technique are presented. (C) 2010 Elsevier B.V. All rights reserved.
Peak effect and the rhomb to square structural transition in the vortex lattice
(2006)
Bruckental, Y.
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Rosenstein, B.
;
Shapiro, B. Ya.
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Shapiro, I.
;
Shaulov, A.
The theory of structural transformation of the vortex lattice in a fourfold symmetric type II superconductor in the presence of both thermal fluctuations and quenched disorder is constructed. We show that in the absence of pinning, the slope of the square to rhomb transition in the T-H plane is generally negative: thermal fluctuations favor a more symmetric square lattice. Disorder\'s influence on the slope is just the opposite- it favors a less symmetric rhombic phase in which the fourfold symmetry is spontaneously broken. The second magnetization peak line in LaSCO in a wide range of doping is well described as a result of the transition.
Possibility of vortex lattice structural phase transition in the superconducting pnictide Ba(Fe(0.925)Co(0.075))(2)As(2)
(2010-03-01)
Kopeliansky, R.
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Shaulov, A.
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Shapiro, B. Ya.
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Yeshurun, Y.
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Rosenstein, B.
We present magnetic measurements in a single crystal of the newly discovered superconducting ironpnictide Ba(Fe(0.925)Co(0.075))(2)As(2). The magnetization loops exhibit a second magnetization peak (SMP) similar to that observed in most high-temperature superconductors (HTSs). Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, S = d ln M/d ln t, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in Ba(Fe(0.925)Co(0.075))(2)As(2) the onset and peak fields, as well as the minimum point in S, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in Ba(Fe(0.925)Co(0.075))(2)As(2) is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of S. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.
Possibility of vortex lattice structural phase transition in the superconducting pnictide Ba(Fe0.925Co0.075)(2)As-2
(2010-03-01)
Kopeliansky, R.
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Shaulov, A.
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Shapiro, B. Ya.
;
Yeshurun, Y.
;
Rosenstein, B.
We present magnetic measurements in a single crystal of the newly discovered superconducting ironpnictide Ba(Fe0.925Co0.075)(2)As-2. The magnetization loops exhibit a second magnetization peak (SMP) similar to that observed in most high-temperature superconductors (HTSs). Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, S = d ln M/d ln t, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in Ba(Fe0.925Co0.075)(2)As-2 the onset and peak fields, as well as the minimum point in S, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in Ba(Fe0.925Co0.075)(2)As-2 is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of S. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.
Shock waves and avalanches in type-II superconductors
(2006)
Shapiro, B. Ya.
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Rosenstein, B.
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Shapiro, I.
;
電子物理學系
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Department of Electrophysics
Rapid penetration of magnetic flux into a Meissner phase of the type-II superconductor is studied analytically and numerically. A sharp shock wave front of the magnetic induction is formed due to the non-analyticity of the resistivity at the transition from the mixed to the normal state. The shock wave magnetic induction and the temperature profiles move with constant velocity determined by the Joule heat produced by the electric current in the normal domain at the flux front. If the normal state resistance of the sample material is temperature dependent (like in HTSC), then the straight front shows instability. For a sufficiently small thermal diffusion constant a finger shaped avalanches are formed.