Kazuhiro Kuboki

Magnetic properties of t-J model

The dynamical spin susceptibility χ( q , ω), of the t-J model in two-dimensional square lattice have been studied based on the mean field approximation to the slave-boson formalism. The static spin susceptibility, χ( q , 0), and Im χ( q , ω) at various energy, ω, have been calculated. We found; 1) χ(0, 0) has only weak temperature dependence 2) T 1 T ∝( T +θ) for \(T\gtrsim0.1{J}\) with θ linearly dependent on the doping rate, δ 3) T 1 T has a minimum at T =(0.1∼0.2) J 4) Im χ( q , ω) at q = Q ≡(π, π) has a gap like structure as a function of ω if the temperature is low. All these are consistent with experimental results.

Spin-Peierls Transition with Competing Interactions

The one-dimensional spin-1/2 Heisenberg antiferromagnet which has next-nearest-, as well as nearest-neighbor exchange interaction and couples to the lattice distortion is investigated. The next-nearest-neighbor interaction is shown to result from the non-adiabaticity of the lattice distortion or the itineracy of electrons. The groundstate of this system is studied by the renormalization group method. In the absence of the coupling to the lattice distortion, the phase diagram consists of three states, i.e., the spin-fluid, the dimer and the Neel state, in essential agreement with Haldane, while in its presence, there are two states, the spin-Peierls and the Neel state. The energy gain due to the lattice distortion is estimated by the self-consistent harmonic approximation.

Effects of Superconducting Fluctuations on NMR Relaxation Rate

Effects of superconducting fluctuations on the relaxation rate of NMR are evaluated in the dirty limit for the s -wave and d -wave states. In the s -wave state, there is a divergent contribution from the Maki process, but its magnitude is sensitive to the degree of three-dimensionality, whereas the contribution of the same process is vanishingly small in the d -wave state.

Effect of Band Structure on the Symmetry of Superconducting States

Effects of band structure on the symmetry of superconducting (SC) states are studied. We show that various symmetries of SC states are possible within the same type of interaction when the shape of the Fermi surface is changed. As a simple model we consider a square lattice system with a nearest-neighbor attractive interaction, and find that the spin-triplet (( p x + i p y )-wave) and spin-singlet ( d - or s -wave) SC states, and states with their coexistence ( d + i p y , s + i p y ) can be stabilized if the electron density (and thus the Fermi surface) is changed. By taking into account the interlayer attractive interaction in this model, the stability of interlayer-pairing states with line nodes is examined, and its relation to the SC state of Sr 2 RuO 4 is discussed.

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.

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

Anyons in Commensurate Flux Phase

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Effect of gauge fields on nuclear magnetic relaxation rate and spin susceptibility

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Microscopic Derivation of Ginzburg–Landau Equations for Coexistent States of Superconductivity and Magnetism

({p}_{x}\ifmmode\pm\else\textpm\fi{}i{p}_{y})

Study on Nonmagnetic Impurities in the Superconducting State of Two-Dimensional t-J Model

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