arXiv subjects

Yasuyuki Kato

Publications and source records attributed to Yasuyuki Kato.

88 records · Page 5Linked to original sources

arXiv2013

Efimov effect in quantum magnets

Physics is said to be universal when it emerges regardless of the underlying microscopic details. A prominent example is the Efimov effect, which predicts the emergence of an infinite tower of three-body bound states obeying discrete scale invariance when the particles interact resonantly. Because of its universality and peculiarity, the Efimov effect has been the subject of extensive research in chemical, atomic, nuclear and particle physics for decades. Here we employ an anisotropic Heisenberg model to show that collective excitations in quantum magnets (magnons) also exhibit the Efimov effect. We locate anisotropy-induced two-magnon resonances, compute binding energies of three magnons and find that they fit into the universal scaling law. We propose several approaches to experimentally realize the Efimov effect in quantum magnets, where the emergent Efimov states of magnons can be observed with commonly used spectroscopic measurements. Our study thus opens up new avenues for universal few-body physics in condensed matter systems.

cond-mat.str-el↗

arXiv2013

Efficient Langevin Simulation of Coupled Classical Fields and Fermions

We introduce an efficient Langevin method to study bilinear Fermionic Hamiltonians interacting with classical fields. Our method is suitable for very large systems and offers high accuracy. To demonstrate the method, we study complex non-coplanar chiral spin textures on the triangular Kondo lattice model. We also explore non-equilibrium mesoscale physics such as chiral domain coarsening and Z2 vortex annihilation.

cond-mat.stat-mech↗

arXiv2013

Generalized spin-wave theory: application to the bilinear-biquadratic model

We present a generalized spin-wave theory (GSWT) for treating spin Hamiltonians of arbitrary spin $S$. The generalization consists of an extension of the traditional spin-wave theory from SU(2) to SU($N$). Low energy excitations are waves of the local order parameter that fluctuates in the SU($N$) space of unitary transformations of the local spin states, instead of the SU(2) space of local spin rotations. Since the generators of the SU($N$) group can be represented as bilinear forms in $N$-flavored bosons, the low-energy modes of the GSWT are described with $N-1$ different bosons. The generalization allows treating quantum spin systems whose ground state exhibit multipolar ordering as well as detecting instabilities of magnetically ordered states (dipolar ordering) towards higher multipolar orderings. We illustrate these advantages by applying the GSWT to a bilinear-biquadratic model of arbitrary spin $S$ on hypercubic lattices.

cond-mat.str-el↗

arXiv2013

Multi-discontinuity algorithm for world-line Monte Carlo simulations

We introduce a novel multi-discontinuity algorithm for efficient global update of world-line configurations in Monte Carlo simulations of interacting quantum systems. This new algorithm is a generalization of the two-discontinuity algorithms introduced in Refs. [N. Prokof'ev, B. Svistunov, and I. Tupitsyn, Phys. Lett. A {\bf 238}, 253 (1998)] and [O. Syljuåsen and A. Sandvik, Phys. Rev. E {\bf 66}, 046701 (2002)] . This generalization is particularly effective for studying Bose-Einstein condensates (BEC) of composite particles. In particular, we demonstrate the utility of the generalized algorithm by simulating a Hamiltonian for an S=1 anti-ferromagnet with strong uniaxial single-ion anisotropy. The multi-discontinuity algorithm not only solves the freezing problem that arises in this limit, but also allows for efficiently computing the off-diagonal correlator that characterizes a BEC of composite particles.

cond-mat.stat-mech↗

arXiv2013

Phase Diagram and Magnetic Excitations of Anisotropic Spin-One Magnets

We use a generalized spin wave approach and large scale quantum Monte Carlo (QMC) simulations to study the quantum phase diagram and quasiparticle excitations of the S=1 Heisenberg model with an easy-plane single-ion anisotropy in dimensions d=2 and 3. We consider two alternative approximations for describing the quantum paramagnetic state: the standard Holstein-Primakoff approximation and a modified treatment in which the local constraint (finite dimension of the local Hilbert space) is enforced by introducing a Lagrange multiplier. While both approximations produce qualitatively similar results, the latter approach is the only one that is in good quantitative agreement with the quantum phase diagram and the quasiparticle dispersions obtained with QMC. This result is very important for low-temperature studies of quantum paramagnets in magnetic fields because it shows that a simple modification of the standard analytical approach should produce much better quantitative agreement between theory and experiment.

cond-mat.str-el↗

arXiv2012

Magnetic and nematic orderings in spin-1 antiferromagnets with single-ion anisotropy

We study a spin-one Heisenberg model with exchange interaction, J, uniaxial single-ion exchange anisotropy, D, and Zeeman coupling to a magnetic field, B, parallel to the symmetry axis. We com- pute the (D/J,B/J) quantum phase diagram for square and simple cubic lattices by combining an- alytical and Quantum Monte Carlo approaches, and find a transition between XY-antiferromagnetic and ferronematic phases that spontaneously break the U(1) symmetry of the model. In the language of bosonic gases, this is a transition between a Bose-Einstein condensate (BEC) of single bosons and a BEC of pairs. Our work opens up new avenues for measuring this transition in real magnets.

cond-mat.str-el↗

arXiv2012

Controllable chirality-induced geometrical Hall effect in a frustrated highly-correlated metal

A current of electrons traversing a landscape of localized spins possessing non-coplanar magnetic order gains a geometrical (Berry) phase which can lead to a Hall voltage independent of the spin-orbit coupling within the material--a geometrical Hall effect. We show that the highly-correlated metal UCu5 possesses an unusually large controllable geometrical Hall effect at T<1.2K due to its frustration-induced magnetic order. The magnitude of the Hall response exceeds 20% of the ν=1 quantum Hall effect per atomic layer, which translates into an effective magnetic field of several hundred Tesla acting on the electrons. The existence of such a large geometric Hall response in UCu5 opens a new field of inquiry into the importance of the role of frustration in highly-correlated electron materials.

cond-mat.str-el↗

arXiv2012

Structure of magnetic order in Pauli limited unconventional superconductors

We analyse the magnetic structure of the antiferromagnetic order induced in Pauli limited d-wave superconductors by Zeeman coupling to magnetic field. We determine the phase diagram in the H-T plane, and find that the magnetic phase, which is stabilized at low temperatures and just below the upper critical field, can have two realizations depending primarily on the shape of the underlying Fermi surface. The double-Q magnetic ordering may persist over the entire coexistence range. Alternatively, there may exist a weak first order transition from a double-Q structure at lower fields to a single-Q modulation at higher fields. Together with the calculations of the NMR line-shape these results suggest the second scenario as a serious candidate for describing the superconducting state of CeCoIn5.

cond-mat.supr-con↗

arXiv2012

Orbital disorder induced by charge fluctuations in vanadium spinels

Motivated by recent experiments on vanadium spinels, $A$V$_2$O$_4$, that show an increasing degree of electronic delocalization for smaller cation sizes, we study the evolution of orbital ordering (OO) between the strong and intermediate-coupling regimes of a multi-orbital Hubbard Hamiltonian. The underlying magnetic ordering of the Mott insulating state leads to a rapid suppression of OO due to enhanced charge fluctuations along ferromagnetic bonds. Orbital double-occupancy is rather low at the transition point indicating that the system is in the crossover region between strong and intermediate-coupling regimes when the orbital degrees of freedom become disordered.

cond-mat.str-el↗

arXiv2012

Quantum Monte-Carlo study of magnetic ordering in ZnV2O4

We study the magnetic ordering of Vanadium spinels by Quantum Monte Carlo simulations of a three-band Hubbard model. Vanadium spinels, AV$_2$O$_4$, exhibit a unique "up-up-down-down" spin ordering at low temperatures. While this magnetic ordering was originally measured in 1973, its origin has remained unclear for many years due to the lack of unbiased approaches for solving the relevant model. A three-band Hubbard model on the spinel lattice (corner sharing tetrahedra) is a minimal Hamiltonian for describing the $t_{2g}$ electrons of the V$^{2+}$ ions. One of the main difficulties is that this family of compounds belongs to the elusive intermediate-coupling regime ($U \gtrsim t$) for which there is no small parameter that can justify a perturbative expansion. We present a controlled Quantum Monte-Carlo approach to the three-band Hubbard model relevant for this materials that reproduces the up-up-down-down spin ordering. The method is free of the sign problem that is usually the main limiting factor for simulating fermionic systems in dimension higher than one.

cond-mat.str-el↗

arXiv2011

Novel Polaron State for Single Impurity in a Bosonic Mott Insulator

We show that a single impurity embedded in a cold atom bosonic Mott insulator leads to a novel polaron that exhibits correlated motion with an effective mass and a linear size that nearly diverge at critical value of the on-site impurity-boson interaction strength. Cold atom technology can tune the polaron's properties and break up the composite particle into a deconfined impurity-hole and boson particle state at finite, controllable polaron momentum.

cond-mat.quant-gas↗

arXiv2010

Reponse to Comment on "Direct mapping of the finite temperature phase diagram of strongly correlated quantum models"

Pollet et al have questioned our claim staked in Phys. Rev. Lett. 103, 085701 (2009) that from the location of sharp features in the compressibility in the trap, it is possible to map out the phase boundary between two phases within reasonable accuracy. We stand by it and use this reply to sharpen the conditions under which "sharp" features in compressibility reflect the effects of critical fluctuations.

cond-mat.quant-gas↗

arXiv2010

Stability of spontaneous quantum Hall state in the Triangular Kondo-lattice model

We study the behavior of the quarter-filled Kondo lattice model on a triangular lattice by combining a zero-temperature variational approach and finite-temperature Monte-Carlo simulations. For intermediate coupling between itinerant electrons and classical moments ${\bf S}_j$, we find a thermodynamic phase transition into an exotic spin ordering with uniform scalar spin chirality and $< {\bf S}_j >=0$. The state exhibits spontaneous quantum Hall effect. We also study how its properties are affected by application of an external magnetic field.

cond-mat.str-el↗

arXiv2009

Finite-Size Scaling for Quantum Criticality above the Upper Critical Dimension: Superfluid-Mott-Insulator Transition in Three Dimensions

Validity of modified finite-size scaling above the upper critical dimension is demonstrated for the quantum phase transition whose dynamical critical exponent is $z=2$. We consider the $N$-component Bose-Hubbard model, which is exactly solvable and exhibits mean-field type critical phenomena in the large-$N$ limit. The modified finite-size scaling holds exactly in that limit. However, the usual procedure, taking the large system-size limit with fixed temperature, does not lead to the expected (and correct) mean-field critical behavior due to the limited range of applicability of the finite-size scaling form. By quantum Monte Carlo simulation, it is shown that the same holds in the case of N=1.

cond-mat.stat-mech↗

arXiv2009

Strong-coupling expansion for the momentum distribution of the Bose Hubbard model with benchmarking against exact numerical results

A strong-coupling expansion for the Green's functions, self-energies and correlation functions of the Bose Hubbard model is developed. We illustrate the general formalism, which includes all possible inhomogeneous effects in the formalism, such as disorder, or a trap potential, as well as effects of thermal excitations. The expansion is then employed to calculate the momentum distribution of the bosons in the Mott phase for an infinite homogeneous periodic system at zero temperature through third-order in the hopping. By using scaling theory for the critical behavior at zero momentum and at the critical value of the hopping for the Mott insulator to superfluid transition along with a generalization of the RPA-like form for the momentum distribution, we are able to extrapolate the series to infinite order and produce very accurate quantitative results for the momentum distribution in a simple functional form for one, two, and three dimensions; the accuracy is better in higher dimensions and is on the order of a few percent relative error everywhere except close to the critical value of the hopping divided by the on-site repulsion. In addition, we find simple phenomenological expressions for the Mott phase lobes in two and three dimensions which are much more accurate than the truncated strong-coupling expansions and any other analytic approximation we are aware of. The strong-coupling expansions and scaling theory results are benchmarked against numerically exact QMC simulations in two and three dimensions and against DMRG calculations in one dimension. These analytic expressions will be useful for quick comparison of experimental results to theory and in many cases can bypass the need for expensive numerical simulations.

cond-mat.other↗

arXiv2009

Direct mapping of the finite temperature phase diagram of strongly correlated quantum models

Optical lattice experiments, with the unique potential of tuning interactions and density, have emerged as emulators of nontrivial theoretical models that are directly relevant for strongly correlated materials. However, so far the finite temperature phase diagram has not been mapped out for any strongly correlated quantum model. We propose a remarkable method for obtaining such a phase diagram for the first time directly from experiments using only the density profile in the trap as the input. We illustrate the procedure explicitly for the Bose Hubbard model, a textbook example of a quantum phase transition from a superfluid to a Mott insulator. Using "exact" quantum Monte Carlo simulations in a trap with up to $10^6$ bosons, we show that kinks in the local compressibility, arising from critical fluctuations, demarcate the boundaries between superfluid and normal phases in the trap. The temperature of the bosons in the optical lattice is determined from the density profile at the edge. Our method can be applied to other phase transitions even when reliable numerical results are not available.

cond-mat.supr-con↗