32 research outputs found
Critical property of spin-glass transition in a bond-disordered classical antiferromagnetic Heisenberg model with a biquadratic interaction
Motivated by puzzling spin-glass behaviors observed in many pyrochlore-based
magnets, effects of magnetoelastic coupling to local lattice distortions were
recently studied by the authors for a bond-disordered antiferromagnet on a
pyrochlore lattice [Phys. Rev. Lett. 107, 047204 (2011)]. Here, we extend the
analyses with focusing on the critical property of the spin-glass transition
which occurs concomitantly with a nematic transition. Finite-size scaling
analyses are performed up to a larger system size with 8192 spins to estimate
the transition temperature and critical exponents. The exponents are compared
with those in the absence of the magnetoelastic coupling and with those for the
canonical spin-glass systems. We also discuss the temperature dependence of the
specific heat in comparison with that in canonical spin-glass systems as well
as an experimental result.Comment: 4 pages, 2 figures, proceedings for LT2
Loop algorithm for classical Heisenberg models with spin-ice type degeneracy
In many frustrated Ising models, a single-spin flip dynamics is frozen out at
low temperatures compared to the dominant interaction energy scale because of
the discrete "multiple valley" structure of degenerate ground-state manifold.
This makes it difficult to study low-temperature physics of these frustrated
systems by using Monte Carlo simulation with the standard single-spin flip
algorithm. A typical example is the so-called spin ice model, frustrated
ferromagnets on the pyrochlore lattice. The difficulty can be avoided by a
global-flip algorithm, the loop algorithm, that enables to sample over the
entire discrete manifold and to investigate low-temperature properties. We
extend the loop algorithm to Heisenberg spin systems with strong easy-axis
anisotropy in which the ground-state manifold is continuous but still retains
the spin-ice type degeneracy. We examine different ways of loop flips and
compare their efficiency. The extended loop algorithm is applied to the
following two models, a Heisenberg antiferromagnet with easy-axis anisotropy
along the z axis, and a Heisenberg spin ice model with the local
easy-axis anisotropy. For both models, we demonstrate high efficiency of our
loop algorithm by revealing the low-temperature properties which were hard to
access by the standard single-spin flip algorithm. For the former model, we
examine the possibility of order-from-disorder and critically check its
absence. For the latter model, we elucidate a gas-liquid-solid transition,
namely, crossover or phase transition among paramagnet, spin-ice liquid, and
ferromagnetically-ordered ice-rule state.Comment: 12 pages, 11 figures, accepted for publication in Phys. Rev.
Spin-glass transition in bond-disordered Heisenberg antiferromagnets coupled with local lattice distortions on a pyrochlore lattice
Motivated by puzzling characteristics of spin-glass transitions widely
observed in pyrochlore-based frustrated materials, we investigate effects of
coupling to local lattice distortions in a bond-disordered antiferromagnet on
the pyrochlore lattice by extensive Monte Carlo simulations. We show that the
spin-glass transition temperature \TSG is largely enhanced by the
spin-lattice coupling, and furthermore, becomes almost independent of
in a wide range of the disorder strength . The critical property of the
spin glass transition is indistinguishable from that of the canonical
Heisenberg spin glass in the entire range of . These peculiar behaviors
are ascribed to a modification of the degenerate manifold from continuous to
semidiscrete one by the spin-lattice coupling.Comment: 4 pages, 3 figures, major revisions, accepted for publication in PR
Hidden covalent insulator and spin excitations in SrRuO
The density functional plus dynamical mean-field theory is used to study the
spin excitation spectra of SrRuO. A good quantitative agreement with
experimental spin excitation spectra is found. Depending on the size of the
Hund's coupling the systems chooses either Mott insulator or covalent
insulator state when magnetic ordering is not allowed. We find that the nature
of the paramagnetic state has negligible influence on the charge and spin
excitation spectra. We find that antiferromagnetic correlations hide the
covalent insulator state for realistic choices of the interaction parameters.Comment: 8 pages, 7 figure
Electronic and magnetic properties of metallic phases under coexisting short-range interaction and diagonal disorder
We study a three-dimensional Anderson-Hubbard model under the coexistence of
short-range interaction and diagonal disorder within the Hartree-Fock
approximation. We show that the density of states at the Fermi energy is
suppressed in the metallic phases near the metal-insulator transition as a
proximity effect of the soft Hubbard gap in the insulating phases. The
transition to the insulator is characterized by a vanishing DOS in contrast to
formation of a quasiparticle peak at the Fermi energy obtained by the dynamical
mean field theory in pure systems. Furthermore, we show that there exist frozen
spin moments in the paramagnetic metal.Comment: 4 pages, 2 figures, published versio
Orbital degeneracy and Mott transition in Mo pyrochlore oxides
We present our theoretical results on an effective two-band double-exchange
model on a pyrochlore lattice for understanding intricate phase competition in
Mo pyrochlore oxides. The model includes the twofold degeneracy of
orbitals under trigonal field splitting, the interorbital Coulomb repulsion,
the Hund's-rule coupling between itinerant electrons and localized
spins, and the superexchange antiferromagnetic interaction between the
spins. By Monte Carlo simulation with treating the Coulomb repulsion
at an unrestricted-type mean-field level, we obtain the low-temperature phase
diagram as functions of the Coulomb repulsion and the superexchange
interaction. The results include four dominant phases with characteristic spin
and orbital orders and the metal-insulator transitions among them. The
insulating region is characterized by a `ferro'-type orbital ordering of the
orbitals along the local axis, irrespective of the spin
ordering.Comment: 6 pages, proceedings for ICFC
Single-particle excitations under coexisting electron correlation and disorder: a numerical study of the Anderson-Hubbard model
Interplay of electron correlation and randomness is studied by using the
Anderson-Hubbard model within the Hartree-Fock approximation. Under the
coexistence of short-range interaction and diagonal disorder, we obtain the
ground-state phase diagram in three dimensions, which includes an
antiferromagnetic insulator, an antiferromagnetic metal, a paramagnetic
insulator (Anderson-localized insulator) and a paramagnetic metal. Although
only the short-range interaction is present in this model, we find
unconventional soft gaps in the insulating phases irrespective of electron
filling, spatial dimensions and long-range order, where the single-particle
density of states (DOS) vanishes with a power-law scaling in one dimension (1D)
or even faster in two dimensions (2D) and three dimensions (3D) toward the
Fermi energy. We call it soft Hubbard gap. Moreover, exact-diagonalization
results in 1D support the formation of the soft Hubbard gap beyond the
mean-field level. The formation of the soft Hubbard gap cannot be attributed to
a conventional theory by Efros and Shklovskii (ES) owing the emergence of soft
gaps to the long-range Coulomb interaction. Indeed, based on a picture of
multivalley energy landscape, we propose a phenomenological scaling theory,
which predicts a scaling of the DOS in perfect agreement with the numerical
results. We further discuss a correction of the scaling of the DOS by the
long-range part of the Coulomb interaction, which modifies the scaling of Efros
and Shklovskii. Furthermore, explicit formulae for the temperature dependence
of the DC resistivity via variable-range hopping under the influence of the
soft gaps are derived. Finally, we compare the present theory with experimental
results of SrRu_{1-x}Ti_xO_3.Comment: 22 pages, 19 figure
Efficient implementation of the continuous-time interaction-expansion quantum Monte Carlo method
We describe an open-source implementation of the continuous-time interaction-expansion quantum Monte Carlo method for cluster-type impurity models with onsite Coulomb interactions and complex Weiss functions. The code is based on the ALPS libraries
Continuous-time hybridization expansion quantum impurity solver for multi-orbital systems with complex hybridizations
We describe an open-source implementation of the continuous-time hybridization-expansion quantum Monte Carlo method for impurity models with general instantaneous two-body interactions and complex hybridization functions. The code is built on an updated version of the core libraries of ALPS (Applications and Libraries for Physics Simulations) [ALPSCore libraries]
Efficient implementation of the continuous-time interaction-expansion quantum Monte Carlo method
We describe an open-source implementation of the continuous-time interaction-expansion quantum Monte Carlo method for cluster-type impurity models with onsite Coulomb interactions and complex Weiss functions. The code is based on the ALPS libraries