Hiroki Tsuchiura

Crossover of superconducting properties and kinetic-energy gain in two-dimensional Hubbard model

Superconductivity in the Hubbard model on a square lattice near half filling is studied using an optimization (or correlated) variational Monte Carlo method. Second-order processes of a strong-coupling expansion are considered in wave functions beyond the Gutzwiller projection. Superconductivity of d x 2 - y 2 -wave symmetry is widely stable, and exhibits a crossover around U = U co ∼12 t from a BCS type to a new type. For (Ugtrsim U_{text{co}}) ((Ulesssim U_{text{co}})), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and ∝exp (- t / J ) [tiny] on the strong [weak] coupling side of U co . With reference to experiments on optical conductivity, cuprates belong to the strong-coupling regime.

Electronic states around a vortex core in high-Tc superconductors based on the t-J model

Electronic states around vortex cores in high-Tc superconductors are studied using the twodimensional t-J model in order to treat the d-wave superconductivity with short coherence length and the antiferromagnetic (AF) instability within the same framework. We focus on the disappearance of the large zero-energy peak in the local density of states observed in high-Tc superconductors. When the system is near the optimum doping, we find that the local AF correlation develops inside the vortex cores. However, the detailed doping dependence calculations confirm that the experimentally observed reduction of the zero-energy peak is more reasonably attributed to the smallness of the core size rather than to the AF correlation developed inside the core. The correlation between the spatial dependence of the core states and the core radius is discussed. There are several experimental results in high-Tc superconductors which have not been explained in the conventional BCS dx2−y2-wave superconductivity. The electronic structure inside the vortex core is one of the most interesting issues among them. The conventional theory for d-wave vortices based on Bogoliubovde Gennes (BdG) mean-field theory predicts a large and broad peak at the Fermi energy in the local density of states (LDOS), so-called zero-energy peak (ZEP), at the vortex core[1]. However, scanning tunneling spectroscopy (STS) spectrum in one of the high-Tc materials, BSCCO, giving directly the LDOS around the vortex core, shows only a small-double peak structure at energies ±7 meV[2]. A similar situation was also observed in YBCO compounds[3].

Quasiparticle properties around a nonmagnetic impurity in the superconducting state of the two-dimensional t-J model

The superconducting state around a well-isolated nonmagnetic impurity in the high- T c superconductors is studied using the two-dimensional t - J model. The spatial dependence of the order parameter and the local density of states are obtained from the numerical diagonalization of the Bogoliubov-de Gennes equation derived using the Gutzwiller approximation. We find a zero-energy peak in the local density of states around the impurity. Different from the previous results on a vortex or surfaces in the t - J model, the splitting of the zero-energy peak is not found. The zero-energy states corresponding to the zero-energy peak is approximately localized around the impurity.

Influence of Magnetic Field on Tunneling Conductance in Normal Metal/ dx2-y2-Wave Superconductor Junctions

Influences of magnetic field on tunneling conductance spectra in normal metal/ d x 2 - y 2 -wave superconductor junctions are studied on the basis of the quasiclassical Greens function method where the spatial dependence of the pair potential is determined self-consistently. Focuses are set on the field response of the zero-bias conductance peaks (ZBCP). For the detection of the splitting of the ZBCP due to broken time-reversal symmetry states, we found the presence of a critical value for the magnetic field strength in contrast to previous theories. The critical field H C is almost proportional to the width of ZBCP in the absence of field and equivalently to the transparency of the junctions. The present result may explain the origin for the inconsistency of the magnetic field responses in several experiments.

A model calculation for photo-stimulated desorption

A model calculation is presented for photo-stimulated desorption of adsorbates from a metal surface. In the model, the electron system of the material is described by a two-band tight-binding Hamiltonian. Interband electronic excitation is induced by laser pulse irradiation and causes a change in adsorbate trajectory. Close investigations are made of the dependence of the adsorbate trajectory on the photon energy and the time-width of laser pulse within one-photon process. For that purpose, an effective excited-state potential for adsorbate motion is introduced tentatively. When the electronic excitation is spatially localized in the neighborhood of the surface, substantial deviation of shape from the ground state potential can be seen in the effective excited-state potential, and gives rise to adsorbate desorption. It can be expected from the obtained results that the desorption probability increases as the photon energy is increased and as the time-width of laser pulse is decreased.

Josephson effect in d-wave superconductor junctions in a lattice model

Josephson current between two d -wave superconductors is calculated by using a lattice model. Here we consider two types of junctions, i.e., the parallel junction and the mirror-type junction. The maximum Josephson current ( J c ) shows a wide variety of temperature ( T ) dependence depending on the misorientation angles and the types of junctions. When the misorientation angles are not zero, the Josephson current has anomalous temperature dependencies because of a zero energy state (ZES) at the interfaces. In the case of mirror-type junctions, J c has a non monotonic temperature dependence. These results are consistent with previous results based on the quasiclassical theory [Y. Tanaka and S. Kashiwaya: Phys. Rev. B 56 (1997) 892]. On the other hand, we find that the ZES disappears in several junctions because of the Friedel oscillations of the wave function, which is peculiar to the lattice model. In such junctions, the temperature dependence of J c is close to the Ambegaokar–Baratoffs relation.

Tight-binding formulation of the electrical transport in mesoscopic system with unconventional superconductors

We formulate the electrical conductance of normal metal/insulator/d-wave superconductor (N/I/D) junctions using the linear response theory and real-space Greens functions so that the realistic electronic structure and the randomness can be treated. We demonstrate the conductance calculation for N/I/D junctions using realistic band parameters obtained for d-wave superconductor.

Theory of the spin-dependent transport in superconducting quantum point contacts with a ferromagnetic metal

We calculate the tunneling conductance numerically in ferromagnetic metal/superconducting quantum wire/superconductor junctions. It is shown that a wide variety of the tunneling conductance is obtained depending on the exchange interaction in the ferromagnet and the geometry of the quantum wire.

Josephson Effect in Quasi One-dimensional Unconventional Superconductors

Josephson effect in junctions of quasi one-dimensional superconductors at the quarter-filling electron density is discussed by using the tight-binding model on a two-dimensional square lattice. To consider the triangular lattice structures in real materials such as (TMTSF) 2 X (X=PF 6 , ClO 4 , etc), the asymmetric hopping integral is introduced among second nearest lattice sites. A combination of these characters in superconductors (quasi one-dimensionality, quarter-filling electron density and weak triangular lattice structures) seriously affects the formation of a zero-energy state at a junction interface. As a consequence, calculated results of the Josephson current show anomalous dependences on temperatures.

Quasiparticle states around vortex cores in the t–J model

Abstract Electronic states around vortex cores in high- T c cuprates are studied using the t – J model based on the conventional Gutzwiller approximation and Bogoliubov–de Gennes (BdG) theory. When the system is near the optimum doping, local density of states at the center of the core does not have the zero-energy peak (ZEP). We have clarified that the absence of ZEP is attributed to the existence of the staggered flux state inside the core which is a peculiar feature to small size of vortex core.