Guo-Qiao Zha

Proximity-induced pseudogap in mesoscopic superconductor/normal-metal bilayers

The temperature evolution of the proximity effect in Au/La(2-x)Sr(x)CuO(4) and La(1.55)Sr(0.45)CuO(4)/La(2-x)Sr(x)CuO(4) bilayers was investigated using scanning tunneling microscopy. Proximity-induced gaps, centered at the chemical potential, were found to persist above the superconducting transition temperature, T(c), and up to nearly the pseudogap crossover temperature in both systems. Such independence of the spectra on the details of the normal-metal cap layer is incompatible with a density-wave order origin. However, our results can be accounted for by a penetration of incoherent Cooper pairs into the normal metal above T(c).

Checkerboard-pattern vortex with the long-range Coulomb interaction in underdoped high-temperature superconductors

Vortex structures in high-transition-temperature superconductors are studied by solving Bogoliubov–de Gennes equations based on a model Hamiltonian with competing antiferromagnetic (AF) and d-wave superconducting orderings in the presence of a long-range Coulomb interaction. We show that transition from a checkerboard pattern to stripe structure for spin density wave (SDW), charge density wave, and d-wave orderings may occur by enhancing the strength of the on-site repulsion U in the absence of the long-range Coulomb interaction. The long-range Coulomb interaction provides an intrinsic mechanism for electron depletion inside an AF-like vortex core, and the field-induced AF order was screened and confined onto the near core region. Consequently, two-dimensional modulations or the checkerboard patterns of vortex charge distribution and SDW order recovered for underdoped samples of large U when a reasonably large long-range Coulomb interaction Vc strength had been introduced into the model Hamiltonian.

Half-flux quanta and vortex-antivortex pairs in two-band superconductors

Vortex states of two-band superconductors are investigated based on the microscopic Ginzburg-Landau model. We show that the nonlocal interband coupling can lead to the breaking of translation symmetry and the axial rotation symmetry of the conventional Abrikosov vortex states, resulting in a novel vortex structure comprising a mixture of triangle and square-like vortex lattices that have no counterparts in the single-band superconductors. Half-quantized vortices and stable vortex-antivortex pairs are found. These results may relate to the experimental observations of anomalous vortex structures in the two-band magnesium diboride superconductor.

Geometric effect on the phase transition in mesoscopic loops threaded by an Aharonov-Bohm flux

The quantum phase transition in mesoscopic noncircular loops threaded by an Aharonov-Bohm flux is systematically investigated by numerically solving the Bogoliubov-de Gennes equations self-consistently. We focus on the magnetic flux dependence of the s-wave superconducting order parameter and current in symmetric and asymmetric samples. The influence of surface indentation or bulge defects positioned at the inner or outer edge of the sample on the periodic oscillation is also discussed. We find various hc/e-flux periodicity evolution patterns, and the periodic phase transitions between the superconducing state and the resistive/normal state are demonstrated besides the superconducing state transitions. Our investigation may shed new light on material engineering and provide important insights to designing superconducting quantum devices.

Anomalous spin-triplet current in Ho/Co/Ho-based Josephson junctions

The supercurrent through a holmium/cobalt/holmium-based Josephson junction is investigated by using the extended the Blonder-Tinkham-Klapwijk (BTK) approach. It is found that the spiral magnetism of the Ho layer can induce both spin-split and spin-flip scatterings, which control the conversion between the spin-singlet and equal spin-triplet pairing correlations, generating a tunable long-range spin-triplet pair ordering. Spin-triplet current sharp peaks variations with the Ho layer thickness as well as a slow decay of the current amplitude with the Co layer thickness are revealed. The theoretical results are in good agreement with the relevant experimental observations (Robinson J. W. A. et al., Science, 329 (2010) 59).

Vortex-antivortex dynamics in mesoscopic symmetric and asymmetric superconducting loops with an applied ac current

In the framework of the time-dependent Ginzburg-Landau formalism, we study the dynamics of vortex-antivortex (V-Av) pairs in mesoscopic symmetric and asymmetric superconducting loops under an applied ac current. In contrast to the case of a constant biasing dc current, the process of the V-Av collision and annihilation is strongly affected by the time-periodic ac signal. As the direction of the applied ac current is reversed, the existed V-Av pair moves backward and then collides with a new created Av-V pair in a symmetric loop. In the presence of an appropriate external magnetic field, a novel sinusoidal-like oscillatory mode of the magnetization curve is observed, and the periodic dynamical process of the V-Av annihilation occurs in both branches of the sample. Moreover, for the asymmetric sample with an off-centered hole the creation point of the V-Av pair shifts away from the center of the sample, and the creation and annihilation dynamics of V-Av pairs turns out to be very different from the symmetric case.

Spin-orbit coupling effect in high-temperature superconductors

The spin-orbit (SO) coupling effect is investigated by self-consistently diagonalizing a model Hamiltonian with competing antiferromagnetic (AF) and d-wave superconductivity (DSC) orders for high-temperature superconductors. SO-coupling–induced spin-split and -flip competition with AF order can lead to a double-peak spectrum of the vortex-core state, and vortex charge sign change. Scanning tunneling microscopy spectra on differential conductance and nuclear magnetic resonance experiments on vortex charges for YBa2Cu3O7−x are interpreted.

Impurity-induced bound states in chiral p-wave superconducting nanoloops

The impurity-induced bound states in symmetric and asymmetric chiral p-wave superconducting nanoloops are investigated by numerically solving the Bogoliubov-de Gennes equations self-consistently. When a magnetic impurity is involved, the impurity bound level crosses the Fermi level at some impurity strength for the square loop, accompanied by a current jump at the zero-energy point. The evolution of zero-energy states can be effectively tuned by the located site of a single magnetic impurity. For the asymmetric rectangular loop, the zero mode is easier to be generated when a magnetic impurity is placed at the edge of the short side. In addition, the zero-energy states can be modulated by the threaded magnetic flux. Particularly, for an appropriate strength of the midway magnetic impurity, the zero-energy crossing of the impurity bound level takes place twice with increasing flux.

The chiral edge state and Majorana fermion in hole-doped YBa2Cu3O7−δ mesoscopic strip

Edge states of nontrivial topology are investigated by diagonalizing the tight-binding Hubbard model Hamiltonian for the copper-oxide superconductor YBCO strip in the presence of the Rashba spin-orbit interaction and the Zeeman field. It is found that chiral edge states may develop under appropriate spin-orbit coupling and the exchange field strengths. By defining the quasi-particle creation (annihilation) operators in terms of the obtained particle and hole functions, the zero-energy chiral edge states are proved to be the Majorana zero-energy modes on the opposite edges of the superconductor strip. Manipulations on the Majorana modes are promising by applying an external magnetic field normal to the strip plane. It is also showed that the chiral edge state and the Majorana fermions are more feasible in the underdoped samples. Moreover, domains distinguishing the d -wave pairings from the s -wave pairings are found, implying the coexistence of the d -wave and s -wave gaps for the underdoped YBCO samples.

Tunable odd-frequency triplet pairing states and skyrmion modes in chiral p-wave superconductor

Bogliubov-de Gennes equations are solved self-consistently to investigate the properties of bound states in chiral p-wave superconductive disks. It shows that either an s-wave or the mixed d- and s-wave state with odd-frequency and spin-triplet symmetry is induced at the vortex core, depending both on the chirality of the pairing states and on the vortex topology. It is also found that the odd-frequency triplet even parity (OTE) bound state can be manipulated with a local non-magnetic potential. Interestingly, with an appropriate potential amplitude, the zero-energy OTE bound state can be stabilized at a distance from the vortex core and from the local potential. Possible existences of the Majorana fermion modes are expected if the particle-hole symmetry property is applied to the zero-energy OTE bound state. Moreover, skyrmion modes with an integer topological charge have been found to exist.