arXiv subjects

Susumu Kurihara

Publications and source records attributed to Susumu Kurihara.

At least 19 recordsLinked to original sources

arXiv2016

Incommensurate spiral magnetic order on anisotropic triangular lattice: Dynamical mean field study in a spin-rotating frame

We study the ground-state magnetism of the half-filled Hubbard model on the anisotropic triangular lattice, where two out of three bonds have hopping $t$ and the third one has $t^\prime$ in a unit triangle. Working in a spin-rotating frame and using the density matrix renormalization group method as an impurity solver, we provide a proper description of incommensurate magnetizations at zero temperature in the framework of the dynamical mean-field theory (DMFT). It is shown that the incommensurate spiral magnetic order for $t^\prime/t\gtrsim 0.7$ survives the dynamical fluctuations of itinerant electrons in the Hubbard interaction range from the strong-coupling (localized-spin) limit down to the insulator-to-metal transition. We also find that the magnetic moment reduction from the localized-spin limit is pronounced in the vicinity of the transition between the commensurate Néel and incommensurate spiral phases at $t^\prime/t\sim 0.7$. When the anisotropy parameter $t^\prime/t$ increases from the Néel-to-spiral transition, the magnitude of the magnetic moment immediately reaches a maximum and then rapidly decreases in the range of larger $t^\prime/t$ including the isotropic triangular lattice point $t^\prime/t=1$. This work gives a solid foundation for further extension of the study including nonlocal correlation effects neglected at the standard DMFT level.

cond-mat.str-el↗

arXiv2014

Spontaneous loop-spin current with topological characters in the Hubbard model

We find a state characterized by a spontaneous loop-spin current and a single-particle gap in the Hubbard model within the variational cluster approach. This state exists for arbitrarily small interaction in a half-filled honeycomb lattice. Moreover, from the calculations of the topological invariants for the interacting system, it is shown that this gapped state has nontrivial topological characters; this state is the topological Mott insulating state. This result implies the ubiquity of topological Mott insulating phases.

cond-mat.str-el↗

arXiv2014

Tunable rotons in square lattice antiferromagnets under strong magnetic fields

Excitation spectra of square lattice Heisenberg antiferromagnets in magnetic fields are investigated by the spin-wave theory. It is pointed out that a rotonlike structure appears in a narrow range of magnetic fields, as a result of strong nonlinear effects. It is shown that the energy gap and the mass of the "roton" are quite sensitive to the magnetic field: the roton gap softens rapidly and eventually closes as a precursor of a quantum phase transition. The possibility of the experimental observation of the roton and a new ground state after its softening are discussed.

cond-mat.str-el↗

arXiv2014

A Phenomenological Theory of Loop-Current Phases

A phenomenological theory of the loop-current and loop-spin-current phases is proposed. In order to investigate the stability of these phases, a Ginzburg-Landau-Wilson type action is constructed as a functional of the orbital magnetization. From the analysis of this action based on the Landau theory and momentum-shell one-loop renormalization group theory, it is found that the loop-current and loop-spin-current phases are stable if a certain interaction between the orbital magnetizations is sufficiently large. Moreover, these phases are likely to be stable in systems with large orbital susceptibility, for example, Dirac electron systems.

cond-mat.str-el↗

arXiv2013

Spin Conductivity in Two-Dimensional Non-Collinear Antiferromagnets

We propose a method to derive the spin current operator for non-collinear Heisenberg antiferromagnets. We show that the spin conductivity calculated by the spectral representation with the spin current satisfies the f-sum rule. We also study the spin conductivity at T=0 within spin wave theory. We show how the spin conductivity depends on the external magnetic field with changing magnon spectrum. We also find that the spin Drude weight vanishes for any external magnetic field at T=0.

cond-mat.str-el↗

arXiv2010

Coherence effect in a two-band superconductor: Application to iron pnictides

From a theoretical point of view, we propose an experimental method to determine the pairing symmetry of iron pnictides. We focus on two kinds of pairing symmetries, $s_{+-}$ and $s_{++}$, which are strong candidates for the pairing symmetry of iron pnictides. For each of these two symmetries, we calculate both the density and spin response functions by using the two-band BCS model within the one-loop approximation. As a result, a clear difference is found between the $s_{+-}$- and $s_{++}$-wave states in the temperature dependence of the response functions at nesting vector $\bf{Q}$, which connects the hole and electron Fermi surfaces. We point out that this difference comes from the coherence effect in the two-band superconductor. We suggest that the pairing symmetry could be clarified by observing the temperature dependence of both the density and spin structure factors at the nesting vector $\bf{Q}$ in neutron scattering measurements.

cond-mat.supr-con↗

arXiv2008

Tomonaga-Luttinger Liquid Renormalized by a Single Impurity

Effects of a single impurity potential on bulk properties of a spinful Tomonaga-Luttinger (TL) liquid is studied. A boundary bosonization technique is developed to include the impurity potential of {\it arbitrary} strength $V$. Our new bosonization formula for fermion field $ψ$ smoothly connects the two existing expressions in the strong ($V = \infty$) and the weak ($V = 0$) impurity limits. With use of the formula, we found the TL parameters determined from the long-distance correlation functions are renormalized due to the partial transmission through the impurity potential.

cond-mat.mes-hall↗

arXiv2008

Fano effect in a Josephson junction with a quantum dot

We theoretically investigate the Fano effect in dc Josephson current at the absolute zero of temperature. The system under consideration is a double-path Josephson junction in which one path is through an insulating barrier and the other one is through a quantum dot (QD). Here the Kondo temperature is assumed to be much smaller than the superconducting gap, and the Coulomb interaction inside the QD is treated by the Hartree-Fock approximation. It is shown that the Josephson critical current exhibits an asymmetric resonance against the QD energy level. This behavior is caused by the interference between the two tunneling processes between the superconductors; the direct tunneling across the insulating barrier and the resonant one through the QD. Moreover, we find that the Josephson critical current changes its sign around the resonance when the Coulomb interaction is sufficiently strong. Our results suggest that 0-$π$ transition is induced by the cooperation of the Fano effect and the Coulomb interaction inside the QD.

cond-mat.mes-hall↗

arXiv2008

Green's function theory for spin-1/2 ferromagnets with an easy-plane exchange anisotropy

The many-body Green's function theory with the random-phase approximation is applied to the study of easy-plane spin-1/2 ferromagnets in an in-plane magnetic field. We demonstrate that the usual procedure, in which only the three Green's functions $<< S_i^μ;S_j^->>$ ($μ=+,-,z$) are used, yields unreasonable results in this case. Then the problem is discussed in more detail by considering all combinations of Green's functions. We can derive one more equation, which cannot be obtained by using only the set of the above three Green's functions, and point out that the two equations contradict each other if one demands that the identities of the spin operators are exactly satisfied. We discuss the cause of the contradiction and attempt to improve the method in a self consistent way. In our procedure, the effect of the anisotropy can be appropriately taken into account, and the results are in good agreement with the quantum Monte Carlo calculations.

cond-mat.str-el↗

arXiv2008

Band-Selective Filter in a Zigzag Graphene Nanoribbon

Electric transport of a zigzag graphene nanoribbon through a step-like potential or a barrier potential is investigated by using the recursive Green's function method. In the case of the step-like potential, we demonstrate numerically that scattering processes obey a selection rule for the band indices when the number of zigzag chains is even; the electrons belonging to the ``even'' (``odd'') bands are scattered only into the even (odd) bands so that the parity of wavefunctions is preserved. The so-called valley-valve effect can be explained by this selection rule. In the case of the barrier potential, by tuning the barrier height to be an appropriate value, we show that it can work as the ``band-selective filter'', which transmits electrons selectively with respect to the indices of the bands to which the incident electrons belong. Finally, we suggest that this selection rule can be observed in the conductance by applying two barrier potentials.

cond-mat.mes-hall↗

arXiv2007

AC Josephson current and supercurrent noise through one-dimensional correlated electron systems

AC Josephson effect in one-dimensional Tomonaga-Luttinger liquid (TLL) adiabatically connected to superconducting electrodes is theoretically investigated. It is found that density fluctuations due to repulsive electron-electron interactions in TLL inhibit Josephson oscillations, whereas they do not affect time-independent current part. We also show that the fluctuations reduce supercurrent noise caused by multiple Andreev reflections. This indicates that the quantum fluctuations in TLL disturb the superconducting phase coherence spreading across the junction.

cond-mat.mes-hall↗

arXiv2007

Collective Modes and Stability of Bose-Fermi Mixtures with a BCS-BEC Crossover

We investigate an ultracold Bose-Fermi mixture with a Feshbach resonance between two hyperfine states of fermions. Using a functional integral method, we calculate collective modes associated with fermion-pairs and bosons in a superfluid phase. We derive a stability condition of the mixtures, which is valid in the entire region of the BCS-BEC crossover. This stability condition, which smoothly connects the well-known results obtained in both the BCS and BEC limits, shows that sufficiently strong FB interactions destabilize the mixtures. In order to investigate the consequence of this instability, we study the ground state energy of both the uniformly mixed and the phase-separated states. We find that the FB repulsion induces a phase separation, whereas the FB attraction should cause a collapse of the mixture. The possibilities for the experimental observation of such instabilities are also discussed.

cond-mat.other↗

arXiv2006

Antiferromagnetism in two-dimensional t-J model: pseudospin representation

We discuss a pseudospin representation of the two-dimensional t-J model. We introduce pseudospins associated with empty sites, deriving a new representation of the t-J model that consists of local spins and spinless fermions. We show, within a mean-field approximation, that our representation of t-J model corresponds to the {\it isotropic} antiferromagnetic Heisenberg model in an effective magnetic field. The strength and the direction of the effective field are determined by the hole doping $δ$ and the orientation of pseudospins associated with empty sites, respectively. We find that the staggered magnetization in the standard representation corresponds to the component of magnetization perpendicular to the effective field in our pseudospin representation. Using a many-body Green's function method, we show that the staggered magnetization decreases with increasing hole doping $δ$ and disappears at ${δ\approx 0.06-0.15}$ for $t/J=2-5$. Our results are in good agreement with experiments and numerical calculations in contradistinction to usual mean-field methods.

cond-mat.str-el↗

arXiv2006

Spin accumulation caused by confining potential in a two dimensional electron system

We investigate spin accumulation caused by spin-orbit interactions (SOIs) in a two dimensional electron system confined in one direction. We calculate spin density caused by three kinds of SOIs; arising from edge potential, Rashba and Dresselhaus mechanisms. All SOIs are shown to generate out-of-plane spin accumulations at the lateral edges of the system. We also find that the Rashba and the Dresselhaus mechanisms are competitive. Especially, when the strength of both interactions are equal, their effects cancel out each other. In that case, only the edge potential mechanism becomes relevant, and analytical expression is obtained for the spin density. The edge potential mechanism is shown to induce spin accumulation similar to and consistent with the experiment [Sih {\it et al}., Nature Phys {\bf 1}, 31 (2005)]. We discuss the mechanism of the spin accumulation for each SOIs in some detail.

cond-mat.mes-hall↗

arXiv2006

Spin-charge mixing effects on resonant tunneling in a polarized Luttinger Liquid

We investigate spin-charge mixing effect on resonant tunneling in spin-polarized Tomonaga-Luttinger liquid with double impurities. The mixing arises from Fermi velocity difference between two spin species due to Zeeman effect. Zero bias conductance is calculated as a function of gate voltage $V_{\rm g}$, gate magnetic field $B_{\rm g}$, temperature and magnetic field applied to the system. Mixing effect is shown to cause rotation of the lattice pattern of the conductance peaks in $(V_{\rm g},B_{\rm g})$ plane, which can be observed in experiments. At low temperatures, the contour shapes are classified into three types, reflecting the fact that effective barrier potential is renormalized towards ``perfect reflection'', ``perfect transmission'' and magnetic field induced ``spin-filtering'', respectively.

cond-mat.str-el↗

arXiv2006

Phase Dependence of Phonon Tunneling in Bosonic Superfluid-Insulator-Superfluid Junctions

We consider the tunneling of phonon excitations across a potential barrier spatially separating two condensates with different macroscopic phases. We analyze the relation between the phase difference $ϕ$ of the two condensates and the transmission coefficient {\it T} by solving the Bogoliubov equations. It is found that {\it T} strongly depends on $ϕ$, and that the perfect transmission of low-energy excitations disappears when the phase difference reaches the critical value which gives the maximum supercurrent of the condensate. We also discuss the feasibility of observing the phase differences in experiments.

cond-mat.soft↗

arXiv2005

Collective Excitations of Bose-Einstein Condensates in a Double-Well Potential

We investigate collective excitations of Bose-Einstein condensates at absolute zero in a double-well trap. We solve the Bogoliubov equations with a double-well trap, and show that the crossover from the dipole mode to the Josephson plasma mode occurs in the lowest energy excitation. It is found that the anomalous tunneling property of low energy excitations is crucial to the crossover.

cond-mat.other↗

arXiv2005

Gutzwiller study of spin-1 bosons in an optical lattice under a magnetic field

We study spin-1 bosons in an optical lattice under a magnetic field by the Gutzwiller approximation. Our results thus obtained join the discontinuous phase boundary curves obtained by perturbative studies through a first-order transition. On the phase boundary curve, we also find a peculiar cusp structure originating from the degeneracy between different spin Mott states under a magnetic field. The magnetic field dependence of both fluctuation in the total number of bosons and the spin magnetization clarifies that the superfluid phase is divided into two regions reflecting the coexisting first- and second-order superfluid transitions.

cond-mat.other↗