arXiv subjects

Kazuhiro Kuboki

Publications and source records attributed to Kazuhiro Kuboki.

28 records · Page 2Linked to original sources

arXiv2005

Superconductivity on a Möbius strip: numerical studies on order parameter and quasiparticles

Superconducting states of an anisortopic s-wave superconductor on a Möbius strip are studied numerically based on the Ginzburg-Landau theory and the Bogoliubov-de Gennes theory. In both, the equations are solved numerically on discitized lattice and the nonlinearity and the self-consistency are fully taken into account. First, we study the superconducting states on the Möbius strip in the presence of the Aharonov-Bohm flux threading the ring by employing the Ginzburg-Landau theory, and confirm the phase diagram previously proposed by Hayashi and Ebisawa [J. Phys. Soc. Jpn. {\bf 70}, 3495 (2002)]. The metastable states as well as the equilibrium energy state are studied and the nonequiriblium processes when the magnetic field is varied at a fixed temperature are discussed. Next, we study the microscopic superconducting states on the Möbius strip based on the Bogoliubov-de Gennes theory, especially focusing on the state with a real-space node in the superconducting gap, which is expected to appear when the flux threading the ring is half the superconducting flux quantum. The local density of states in this {\it nodal state} is calculated in detail and the existence of the zero-energy bound states is shown.

cond-mat.supr-con↗

arXiv2004

Spontaneous spin current near the interface between unconventional superconductors and ferromagnets

We study theoretically the proximity effect between ferromagnets (F) and superconductors (S) with broken time-reversal symmetry (${\cal T}$). A chiral $(p_x\pm ip_y)$-wave, and a $d_{x^2-y^2}$-wave superconductor, the latter of which can form ${\cal T}$-breaking surface state, i.e., $(d_{x^2-y^2} \pm is)$-state, are considered for the S side. The spatial variations of the superconducting order parameters and the magnetization are determined by solving the Bogoliubov de Gennes equation. In the case of a chiral $(p_x\pm ip_y)$-wave superconductor, a spontaneous spin current flows along the interface, but not in the case of a $d_{x^2-y^2}$-wave superconductor. For F/S$(p_x+ip_y)$/F trilayer system, total spin current can be finite while total charge current vanishes, if the magnetization of two F layers are antiparallel.

cond-mat.supr-con↗

arXiv2002

Spontaneous spin current near the interface between ferromagnets and unconventional superconductors

Proximity effects between ferromagnets (F) and superconductors (S) with broken time-reversal symmetry (T) are studied theoretically. For the S side we consider a chiral (p_x \pm ip_y)-wave, and a d_{x^2-y^2}-wave superconductor, the latter of which can form T-breaking surface state, i.e., (d_{x^2-y^2} \pm is)-state. The Bogoliubov de Gennes equation which describes the spatial variations of the superconducting order parameter and the magnetization is derived and solved numerically. It is found that a spontaneous spin current flows along the interface between the (p_x \pm ip_y)-wave superconductor and the ferromagnet. On the contrary, in the case of a [110] interface of the d_{x^2-y^2}-wave SC, the surface state has a (d \pm p_x \pm p_y)-wave (or (d_{x^2-y^2} \pm is)-wave) symmetry, and thus no (only charge) spontaneous current arises.

cond-mat.str-el↗

arXiv2001

Effect of Band Structure on the Symmetry of Superconducting States

Effects of the band structure on the symmetry of superconducting (SC) states are studied. For a square lattice system with a nearest-neighbor attractive interaction, SC states with various symmetries are found by changing the band structure, or, the shape of the Fermi surface. The spin-triplet ($(p_x + ip_y)$-wave) and spin-singlet ($d$- or s-wave) SC states, and states with their coexistence ($d + ip_y$, $s + ip_y$) can be stabilized within the same type of interaction. The stability of interlayer-pairing states with line nodes is also examined, and its relation to the SC state of Sr$_2$RuO$_4$ is discussed.

cond-mat.supr-con↗

arXiv2000

Effect of spin-orbit impurity scattering in the superconducting state of t-J model

We study the effect of magnetic impurities in the d_{x^2-y^2}-wave superconducting (SC) state of the two dimensional t-J model.The spin-orbit and the spin-exchange interactions are examined by treating the impurity as a classical spin. The Bogoliubov de Gennes equation derived within a slave-boson mean-field approximation is solved numerically at T = 0. The spin-exchange scattering induces spin-triplet p-wave SC order parameters near the impurity, while a SC state with broken time-reversal symmetry and a spontaneous current appears in the presence of the spin-orbit interaction. When both interactions coexist, it turns out that a state which carries a spontaneous spin current occurs.

cond-mat.supr-con↗

arXiv1999

Proximity effects near the interface between d-wave superconductors and ferro/antiferromagnets

We study the proximity effects near the interface between a d-wave superconductor (S) and a ferromagnet (F) or an antiferromagnet (AF). The S-side (F and AF-side) is described by the attractive (repulsive) Hubbard model, and the Bogoliubov de Gennes equation derived within the Hartree-Fock approximation is solved numerically. The superconducting order parameter and the magnetization (m) can coexist near the interface, and the spatial variation of m induces the spin-triplet p-wave component in both F and AF cases. The local density of states is also calculated and discussed.

cond-mat.str-el↗

arXiv1998

Study on nonmagnetic impurities in the superconducting state of two-dimensional t-J model

We study the effect of nonmagnetic impurities in the superconducting state of the t-J model. The Bogoliubov de Gennes equation is derived using a slave-boson mean-field approximation, and we solve it numerically at T=0. The d_{x^2-y^2}-wave order parameter is suppressed near an impurity and its spatial variation induces the extended s-wave component. We also find the local charge density wave (CDW)-like feature due to the Friedel oscillations. This leads to the splitting of the expected zero-energy Andreev bound state. Therefore the system is unlikely introducing a localized state of other symmetry such as superconductivity with broken time reversal symmetry or antiferromagnetism.

cond-mat.supr-con↗

arXiv1998

Time-reversal symmetry breaking surface states in t-J model

Recently a phenomenological Ginzburg-Landau (GL) theory has been proposed to describe the occurrence of a locally time-reversal symmetry (T) breaking state near a Josephson junction between unconventional superconductors. In this paper we derive this type of GL free energy microscopically from the t-J model within a slave-boson mean-field approximation. The resulting GL free energy is shown to satisfy the conditions to have a T-violating surface state. The existence of this junction state may explain some of the recent experiments on High-Tc superconductors.

cond-mat.supr-con↗

arXiv1995

Energy Gap Structure in Bilayer Oxide Superconductors

We consider a model for bilayer superconductors where an interlayer pairing amplitude $Δ_\perp$ co-exists with intralayer pairing $Δ_\|$ of $d_{x^2 - y^2}$ symmetry. This model is motivated by a recent photoemission experiment reporting the splitting of the nodes of the energy gap. In addition to offering a natural explanation of this observation, the model has a number of new experimental consequences. We find that the new state is accompanied by a spontaneous breaking of the tetragonal symmetry. We also find that the out-of-phase oscillation of $Δ_\perp$ and $Δ_\|$ gives rise to a new Raman active mode. The phase of $Δ_\perp$ may also become imaginary, leading to a state which breaks time reversal symmetry, which may have important implications for tunnelling experiments.

cond-mat↗

arXiv1995

Proximity-induced time-reversal symmetry breaking at Josephson junctions between unconventional superconductors

We argue that a locally time-reversal symmetry breaking state can occur at Josephson junctions between unconventional superconductors. Order parameters induced by the proximity effect can combine with the bulk order parameter to form such a state. This property is specifically due to the intrinsic phase structure of the pairing wave function in unconventional superconductors. Experimental consequences of this effect in high-temperature superconductors are examined.

cond-mat↗