arXiv subjects

H. Tsunetsugu

Publications and source records attributed to H. Tsunetsugu.

14 recordsLinked to original sources

arXiv2025

Orbital moiré and quadrupolar triple-q physics in a triangular lattice

We numerically study orders of planer type $(xy,x^2-y^2)$ quadrupoles on a triangular lattice with nearest-neighbor isotropic $J$ and anisotropic $K$ interactions. This type of quadrupoles possesses unique single-ion anisotropy proportional to a third order of the quadrupole moments. This provides an unconventional mechanism of triple-$q$ orders which does not exist for the degrees of freedom with odd parity under time-reversal operation such as magnetic dipoles. In addition to several single-$q$ orders, we find various orders including incommensurate triple-$q$ quasi-long-range orders with orbital moiré and a four-sublattice triple-$q$ partial order. Our Monte-Carlo simulations demonstrate that the phase transition to the latter triple-$q$ state belongs to the universality class of the critical line of the Ashkin-Teller model in two dimensions close to the four-state Potts class. These results indicate a possibility of realizing unique quadrupole textures in simple triangular systems.

cond-mat.str-el↗

arXiv2013

p-wave superconductivity near a transverse saturation field

We investigate reentrant superconductivity in an Ising ferromagnetic superconductor URhGe under a transverse magnetic field h_x. The superconducting transition temperature for p-wave order parameters T_{sc} is calculated and shows two domes as a function of h_x. We find strong enhancement of T_{sc} in the high-field dome near a saturation field h_s where the spins align in the transverse direction. Soft magnons generate strong attractive interactions there. Spin components of the pairing show a significant change between h_x h_s. We also discuss the appearance of superconductivity with zero-spin pair due to cancellation between external and exchange fields.

cond-mat.supr-con↗

arXiv2012

Non-Fermi liquid, unscreened scalar chirality and parafermions in a frustrated tetrahedron Anderson model

We investigate a four-impurity Anderson model where localized orbitals are located at vertices of a regular tetrahedron and find a novel fixed point in addition to the ordinary Fermi liquid phase. That is characterized by unscreened scalar chirality of a tetrahedron. In this phase, parafermions emerges in the excitation spectrum and quasiparticle mass diverges as 1/|T log^3 T| at low temperatures (T). The diverging effective mass is a manifestation of singular Fermi liquid states as in the underscreened Kondo problem. Between the two phases, our Monte Carlo results show the existence of a non Fermi liquid critical point where the Kondo effects and the intersite antiferromagnetic interactions are valanced. Singular behaviors are prominent in the dynamics and we find that the frequency (omega) dependence of the self-energy (Sigma) is the marginal Fermi liquid like, -Im Sigma \sim |omega|.

cond-mat.str-el↗

arXiv2011

Conventional and charge six superfluids from melting hexagonal Fulde-Ferrell-Larkin-Ovchinnikov phases in two dimensions

We consider defect mediated melting of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and pair density wave (PDW) phases in two dimensions. Examining mean-field ground states in which the spatial oscillations of the FFLO/PDW superfluid order parameter exhibit hexagonal lattice symmetry, we find that thermal melting leads to a variety of novel phases. We find that a spatially homogeneous charge six superfluid can arise from melting a hexagonal vortex-anitvortex lattice FFLO/PDW phase. The charge six superfluid has an order parameter corresponding to a bound state of six fermions. We further find that a hexagonal vortex-free FFLO/PDW phase can melt to yield a conventional (charge two) homogeneous superfluid. A key role is played by topological defects that combine fractional vortices of the superfluid order and fractional dislocations of the lattice order.

cond-mat.supr-con↗

arXiv2009

Order parameter and vortices in the superconducting Q-phase of CeCoIn5

Recently, it has been reported that the low-temperature high-magnetic field phase in CeCoIn5 (Q-phase), has spin-density wave (SDW) order that only exists within this phase. This indicates that the SDW order is the result of the development of pair density wave (PDW) order in the superconducting phase that coexists with d-wave superconductivity. Here we develop a phenomenological theory for these coexisting orders. This provides selection rules for the PDW order and further shows that the detailed structure of this order is highly constrained. We then apply our theory to the the vortex phase. This reveals vortex phases in which the d-wave vortex cores exhibit charge density wave (CDW) order and further reveals that the SDW order provides a unique probe of the vortex phase.

cond-mat.str-el↗

arXiv2009

Dislocations and vortices in pair density wave superconductors

With the ground breaking work of the Fulde, Ferell, Larkin, and Ovchinnikov (FFLO), it was realized that superconducting order can also break translational invariance; leading to a phase in which the Cooper pairs develop a coherent periodic spatially oscillating structure. Such pair density wave (PDW) superconductivity has become relevant in a diverse range of systems, including cuprates, organic superconductors, heavy fermion superconductors, cold atoms, and high density quark matter. Here we show that, in addition to charge density wave (CDW) order, there are PDW ground states that induce spin density wave (SDW) order when there is no applied magnetic field. Furthermore, we show that PDW phases support topological defects that combine dislocations in the induced CDW/SDW order with a fractional vortex in the usual superconducting order. These defects provide a mechanism for fluctuation driven non-superconducting CDW/SDW phases and conventional vortices with CDW/SDW order in the core.

cond-mat.str-el↗

arXiv2007

Lattice gas description of pyrochlore and checkerboard antiferromagnets in a strong magnetic field

Quantum Heisenberg antiferromagnets on pyrochlore and checkerboard lattices in a strong external magnetic field are mapped onto hard-core lattice gases with an extended exclusion region. The effective models are studied by the exchange Monte Carlo simulations and by the transfer matrix method. The transition point and the critical exponents are obtained numerically for a square-lattice gas of particles with the second-neighbor exclusion, which describes a checkerboard antiferromagnet. The exact structure of the magnon crystal state is determined for a pyrochlore antiferromagnet.

cond-mat.str-el↗

arXiv2005

High field properties of geometrically frustrated magnets

Above the saturation field, geometrically frustrated quantum antiferromagnets have dispersionless low-energy branches of excitations corresponding to localized spin-flip modes. Transition into a partially magnetized state occurs via condensation of an infinite number of degrees of freedom. The ground state below the phase transition is a magnon crystal, which breaks only translational symmetry and preserves spin-rotations about the field direction. We give a detailed review of recent works on physics of such phase transitions and present further theoretical developments. Specifically, the low-energy degrees of freedom of a spin-1/2 kagomé antiferromagnet are mapped to a hard hexagon gas on a triangular lattice. Such a mapping allows to obtain a quantitative description of the magnetothermodynamics of a quantum kagomé antiferromagnet from the exact solution for a hard hexagon gas. In particular, we find the exact critical behavior at the transition into a magnon crystal state, the universal value of the entropy at the saturation field, and the position of peaks in temperature- and field-dependence of the specific heat. Analogous mapping is presented for the sawtooth chain, which is mapped onto a model of classical hard dimers on a chain. The finite macroscopic entropies of geometrically frustrated magnets at the saturation field lead to a large magnetocaloric effect.

cond-mat.str-el↗

arXiv1999

Dynamic correlations in doped 1D Kondo insulator: Finite-T DMRG study

The finite-T DMRG method is applied to the one-dimensional Kondo lattice model to calculate dynamic correlation functions. Dynamic spin and charge correlations, S_f(omega), S_c(omega), and N_c(omega), and quasiparticle density of states rho(omega) are calculated in the paramagnetic metallic phase for various temperatures and hole densities. Near half filling, it is shown that a pseudogap grows in these dynamic correlation functions below the crossover temperature characterized by the spin gap at half filling. A sharp peak at omega=0 evolves at low temperatures in S_f(omega) and N_c(omega). This may be an evidence of the formation of the collective excitations, and this confirms that the metallic phase is a Tomonaga-Luttinger liquid in the low temperature limit.

cond-mat.str-el↗

arXiv1998

Thermodynamics of doped Kondo insulator in one dimension: Finite Temperature DMRG Study

The finite-temperature density-matrix renormalization-group method is applied to the one-dimensional Kondo lattice model near half filling to study its thermodynamics. The spin and charge susceptibilities and entropy are calculated down to T=0.03t. We find two crossover temperatures near half filling. The higher crossover temperature continuously connects to the spin gap at half filling, and the susceptibilities are suppressed around this temperature. At low temperatures, the susceptibilities increase again with decreasing temperature when doping is finite. We confirm that they finally approach to the values obtained in the Tomonaga-Luttinger (TL) liquid ground state for several parameters. The crossover temperature to the TL liquid is a new energy scale determined by gapless excitations of the TL liquid. The transition from the metallic phase to the insulating phase is accompanied by the vanishing of the lower crossover temperature.

cond-mat.str-el↗

arXiv1994

Strong Coupling Expansions for Antiferromagnetic Heisenberg S=1/2 Ladders

The properties of antiferromagnetic Heisenberg $S=\frac{1}{2}$ ladders with 2, 3, and 4 chains are expanded in the ratio of the intra- and interchain coupling constants. A simple mapping procedure is introduced to relate the 4 and 2-chain ladders which holds down to moderate values of the expansion parameters. A second order calculation of the spin gap to the lowest triplet excitation in the 2- and 4-chain ladders is found to be quite accurate even at the isotropic point where the couplings are equal. Similar expansions and mapping procedures are presented for the 3-chain ladders which are in the same universality class as single chains.

cond-mat↗

arXiv1994

Spin Gaps in Coupled t-J Ladders

Spin gaps in coupled $t$-$J$ ladders are investigated by exact diagonalization of small clusters up to 4$\times$8 sites. At half-filling, the numerical results for the triplet excitation spectrum are in very good agreement with a second order perturbation expansion in term of small inter-ladder and intra-ladder exchange couplings between rungs ($J/J^\prime$$<$$0.25$). The band of local triplet excitations moving coherently along the ladder (with momenta close to $π$) is split by the inter-ladder coupling. For intermediate couplings finite size scaling is used to estimate the spin gap. In the isotropic infinite 4-chain system (two coupled ladders) we find a spin gap of $0.245 J$, roughly half of the single ladder spin gap. When the system is hole doped, bonding and anti-bonding bound pairs of holes can propagate coherently along the chains and the spin gap remains finite.

cond-mat↗

arXiv1993

Evidence for Spin-Charge Separation in the Two-Dimensional T-J Model

We have calculated high temperature expansions for the momentum distribution function $n_{\scriptscriptstyle\bf k}$ and the equal time spin and density correlation functions $S({\bf q})$ and $N({\bf q})$ of the two-dimensional $t$-$J$ model. On extrapolation to low temperatures we find that $n_{\scriptscriptstyle\bf k}$ has a step-like feature at ${\bf k}_{\scriptscriptstyle F}$ and $S({\bf q})$ has $2{\bf k}_{\scriptscriptstyle F}$ as a characteristic wavevector, whereas $N({\bf q})$ has $2{\bf k}_{\scriptscriptstyle F}^{\scriptscriptstyle SF}$ as a characteristic wavevector. Here ${\bf k}_{\scriptscriptstyle F}$ and ${\bf k}_{\scriptscriptstyle F}^{\scriptscriptstyle SF}$ are the Fermi wavevectors of the nearest-neighbor square lattice tight-binding and spinless fermion models, respectively. By comparison to the known results for one dimension this suggests spin-charge separation in the two-dimensional $t$-$J$ model.

cond-mat↗

arXiv1992

Spin Gap and Superconductivity in the One-Dimensional t-J Model with Coulomb Repulsion

The one-dimensional t-J model with density-density repulsive interactions is investigated using exact diagonalization and quantum Monte Carlo methods. A short-range repulsion pushes phase separation to larger values of J/t, and leads to a widened precursor region in which a spin gap and strengthened superconducting correlations appear. The correlation exponent is calculated. On the contrary, a long-range repulsion of $1/r$-form suppresses superconductivity in the precursor region.

cond-mat↗