摘要 :
A counting formula for computing the number of (Majorana) zero modes bound to topological point defects is evaluated in a gradient expansion for systems with charge-conjugation symmetry. This semi-classical counting of zero modes ...
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A counting formula for computing the number of (Majorana) zero modes bound to topological point defects is evaluated in a gradient expansion for systems with charge-conjugation symmetry. This semi-classical counting of zero modes is applied to some examples that include graphene and a chiral p-wave superconductor in two-dimensional space. In all cases, we explicitly relate the counting of zero modes to Chern numbers.
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We obtain multisoliton solutions of the time-dependent Bogoliubov-de Gennes equations or, equivalently, Gorkov equations that describe the dynamics of a fermionic condensate in the dissipationless regime. There are two kinds of so...
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We obtain multisoliton solutions of the time-dependent Bogoliubov-de Gennes equations or, equivalently, Gorkov equations that describe the dynamics of a fermionic condensate in the dissipationless regime. There are two kinds of solitons-normal and anomalous. At large times, normal multisolitons asymptote to unstable stationary states of the BCS Hamiltonian with zero order parameter (normal states), while the anomalous ones tend to eigenstates characterized by a nonzero anomalous average. Under certain circumstances, multisoliton solutions break up into sums of single solitons. In the linear analysis near the stationary states, solitons correspond to unstable modes. Generally, they are nonlinear extensions of these modes, so that a stationary state with k unstable modes gives rise to a k-soliton solution. We relate parameters of the multisolitons to those of the asymptotic stationary state, which determines the conditions necessary for exciting solitons. We further argue that the dynamics in many physical situations is multisoliton.
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Aliovalent rare-earth substitution into the alkaline-earth site of CaFe_2As_2 single crystals is used to fine tune structural, magnetic, and electronic properties of this iron-based superconducting system. Neutron and single-cryst...
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Aliovalent rare-earth substitution into the alkaline-earth site of CaFe_2As_2 single crystals is used to fine tune structural, magnetic, and electronic properties of this iron-based superconducting system. Neutron and single-crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by the choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of 180 × 10~(-6) K~(-1). Electrical transport and magnetic susceptibility measurements reveal abrupt changes in the physical properties through the collapse as a function of temperature, including a reconstruction of the electronic structure. Superconductivity with onset transition temperatures as high as 47 K is stabilized by the suppression of antiferromagnetic order via chemical pressure, electron doping, or a combination of both. Extensive investigations are performed to understand the observations of partial volume-fraction diamagnetic screening, ruling out extrinsic sources such as strain mechanisms, surface states, or foreign phases as the cause of this superconducting phase that appears to be stable in both collapsed and uncollapsed structures.
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We report the effect of applied pressures on magnetic and superconducting order in single crystals of the aliovalent La-doped iron pnictide material Ca_(1-x)La_xFe_2As_2. Using electrical transport, elastic neutron scattering, and...
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We report the effect of applied pressures on magnetic and superconducting order in single crystals of the aliovalent La-doped iron pnictide material Ca_(1-x)La_xFe_2As_2. Using electrical transport, elastic neutron scattering, and resonant tunnel diode oscillator measurements on samples under both quasihydrostatic and hydrostatic pressure conditions, we report a series of phase diagrams spanning the range of substitution concentrations for both antiferromagnetic and superconducting ground states that include pressure-tuning through the antiferromagnetic (AFM) superconducting critical point. Our results indicate that the observed superconducting phase with a maximum transition temperature of T_c = 47 K is intrinsic to these materials, appearing only upon suppression of magnetic order by pressure-tuning through the AFM critical point. Thus, the superconducting phase appears to exist exclusively in juxtaposition to the antiferromagnetic phase in a manner similar to the oxygen- and fluorine-based iron-pnictide superconductors with the highest transition temperatures reported to date. Unlike the lower- T_c systems, in which superconductivity and magnetism usually coexist, the tendency for the highest-T_c systems to show noncoexistence provides an important insight into the distinct transition temperature limits in different members of the iron-based superconductor family.
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Neutron and x-ray powder and single crystal synchrotron diffraction of Cs_y,Fe_(2-x)Se_2 show the presence of superstructure reflections with propagation vector k = [2/5,1/5,1] with respect to the average crystal structure I4/mmm ...
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Neutron and x-ray powder and single crystal synchrotron diffraction of Cs_y,Fe_(2-x)Se_2 show the presence of superstructure reflections with propagation vector k = [2/5,1/5,1] with respect to the average crystal structure I4/mmm {a ~ 4, c ~ 15 A). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation A = 2a + b, B = —a + 2b, C = c. A solution for the atomic structure is found in the space group 14/m with an ordered pattern of iron vacancies corresponding to the iron deficiency x = 0.29 and Cs stoichiometry y = 0.83. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out all possible symmetry adapted magnetic configurations and found that the presence of antiferromagnetic ordering with the ordered magnetic moment of Fe with approx=2μ_b does not contradict the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in (ab) plane or along c axis. The superstructure is destroyed above T_s approx= 500 K by a first-order-like transition.
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We present detailed magnetometry and muon-spin rotation data on polycrystalline samples of overdoped, nonsuperconducting LiFe_(1-X)Ni_xAs (x=0.1,0.2) and Li(1-y),Fe_(1+y)As (0≤ y≤0.04) as well as superconducting LiFeAs. While Li...
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We present detailed magnetometry and muon-spin rotation data on polycrystalline samples of overdoped, nonsuperconducting LiFe_(1-X)Ni_xAs (x=0.1,0.2) and Li(1-y),Fe_(1+y)As (0≤ y≤0.04) as well as superconducting LiFeAs. While LiFe_(1-X)Ni_x As exhibits weak antiferromagnetic fluctuations down to 1.5 K, Li_(1-y),Fe_(1+y) As samples, which have a much smaller deviation from the 1 : 1 : 1 stoichiometry, show a crossover from ferromagnetic to antiferromagnetic fluctuations on cooling and a freezing of dynamically fluctuating moments at low temperatures. We do not find any signatures of time-reversal symmetry breaking in stoichiometric LiFeAs that would support recent predictions of triplet pairing.
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We study the quantum anomalous thermal Hall effect in a topological superconductor, which possesses an integer bulk topological number and supports Majorana excitations on the surface. To realize the quantum thermal Hall effect, a...
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We study the quantum anomalous thermal Hall effect in a topological superconductor, which possesses an integer bulk topological number and supports Majorana excitations on the surface. To realize the quantum thermal Hall effect, a finite gap at the surface is induced by applying an external magnetic field or by the proximity effects with magnetic materials or s-wave superconductors with complex pair potentials. Basing on the lattice model Hamiltonian for superconducting states in Cu-doped Bi_2Se_3, we compute the thermal Hall conductivity as a function of various parameters such as the chemical potential, the pair potential, and the spin-orbit coupling-induced band gap. It is argued that the bulk topological invariant corresponds to the quantization rule of the thermal Hall conductivity induced by complex s-wave pair potentials.
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摘要 :
The relation between bulk topological invariants and experimentally observable physical quantities is a fundamental property of topological insulators and superconductors. In the case of chiral symmetric systems in odd spatial dim...
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The relation between bulk topological invariants and experimentally observable physical quantities is a fundamental property of topological insulators and superconductors. In the case of chiral symmetric systems in odd spatial dimensions such as time-reversal invariant topological superconductors and topological insulators with sublattice symmetry, this relation has not been well understood. We clarify that the winding number which characterizes the bulk Z nontriviality of these systems can appear in electromagnetic and thermal responses in a certain class of heterostructure systems. It is also found that the Z nontriviality can be detected in the bulk "chiral polarization," which is induced by magneto-electric effects.
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We study nontrivial responses of topological superconductors and superfluids to the temperature gradient and rotation of the system. In two-dimensional gapped systems, the Streda formula for the electric Hall conductivity is gener...
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We study nontrivial responses of topological superconductors and superfluids to the temperature gradient and rotation of the system. In two-dimensional gapped systems, the Streda formula for the electric Hall conductivity is generalized to the thermal Hall conductivity. Applying this formula to the Majorana surface states of three-dimensional topological superconductors predicts cross-correlated responses between the orbital angular momentum and thermal polarization (entropy polarization). These results can be naturally related to the gravitoelectromagnetism description of three-dimensional topological superconductors and superfluids, analogous to the topological magnetoelectric effect in Z_2 topological insulators.
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The longitudinal and Hall resistances have recently been measured for quantum Hall bilayers at total filling v= 1 in the superfluid state with interlayer pairing, both for currents flowing parallel to one another and for "counterf...
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The longitudinal and Hall resistances have recently been measured for quantum Hall bilayers at total filling v= 1 in the superfluid state with interlayer pairing, both for currents flowing parallel to one another and for "counterflowing" currents in the two layers. Here I examine the contribution to these resistances from the motion of unpaired vortices in these systems, developing some possible explanations of various qualitative features of these data.
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