2,251 research outputs found
Magnon Supersolid and Anomalous Hysteresis in Spin Dimers on a Triangular Lattice
We study the magnetic phase diagram and hysteresis behavior of weakly coupled
spin dimers on a triangular lattice using the cluster mean-field method with
cluster-size scaling. We find that the magnetization curve has plateaus at 1/3
and 2/3 of the total magnetization, in which local singlet and triplet states
form a superlattice pattern. Moreover, if increasing (decreasing) the magnetic
field from the 1/3 (2/3) plateau, the Bose-Einstein condensation (BEC) of
triplons occurs on the superlattice background, leading to the transition into
magnon supersolid phase. We also find that the first-order transition between
these solid states and the standard magnon BEC state exhibits an anomalous
hysteresis upon cycling the magnetic field; the transition can occur only from
solid to BEC, and the system cannot return to the initial solid state in the
reverse process.Comment: 6 pages, 6 figure
Collective excitation and stability of flow-induced gapless Fermi superfluids
We study the collective excitation and stability of superfluid Fermi gases
flowing with a constant velocity in three-dimensional free space. In
particular, we investigate a possible gapless superfluid state induced by the
superflow using the mean-field theory and the generalized random-phase
approximation (GRPA). For weak attractive interactions, we find that the
mean-field superfluid order parameter can take a nonzero value even after the
superflow velocity exceeds the threshold for the onset of Bogoliubov
quasiparticle excitations. Since the Cooper pairs are only partially broken by
the quasiparticle excitations, a gapless superfluid state can be formed over a
certain range of superflow velocity above the pair-breaking onset. In addition
to the usual quasiparticle-pair continuum and the Anderson-Bogoliubov
collective mode, the GRPA excitation spectrum of the gapless superfluid state
has a quasiparticle-quasihole continuum and a second collective mode. We find
that the long-wavelength excitations of the second collective mode eventually
cause dynamical instability of the system when the superflow velocity
increases. However, the gapless superfluid state still remains stable in a
narrow but finite range of superflow velocity.Comment: 12 pages, 8 figure
Quantum Phase Diagram of the Triangular-Lattice XXZ Model in a Magnetic Field
The triangular lattice of S=1/2 spins with XXZ anisotropy is a ubiquitous
model for various frustrated systems in different contexts. We determine the
quantum phase diagram of the model in the plane of the anisotropy parameter and
the magnetic field by means of a large-size cluster mean-field method with a
scaling scheme. We find that quantum fluctuations break up the nontrivial
continuous degeneracy into two first-order phase transitions. In between the
two transition boundaries, the degeneracy lifting results in the emergence of a
new coplanar phase not predicted in the classical counterpart of the model. We
suggest that the quantum phase transition to the nonclassical coplanar state
can be observed in triangular-lattice antiferromagnets with large easy-plane
anisotropy or in the corresponding optical-lattice systems.Comment: Published version (5 pages, 4 figures) + Supplementary Material (5
pages, 5 figures, 1 table). Erratum added at the end (1 page, 1 figure;
submitted to PRL). The correction is minor and does not affect the main
conclusion
Cubic-quintic nonlinearity in superfluid Bose-Bose mixtures in optical lattices: Heavy solitary waves, barrier-induced criticality, and current-phase relations
We study superfluid (SF) states of strongly interacting Bose-Bose mixtures
with equal mass and intra-component interaction in optical lattices both in the
presence and absence of a barrier potential (BP). We show that the SF order
parameters obey the two-component nonlinear Schroedinger equation (NLSE) with
not only cubic but also quintic nonlinearity in the vicinity of the first-order
transitions to the Mott insulators with even fillings. In the case of no BP, we
analyze solitary-wave (SW) solutions of the cubic-quintic NLSE. When the SF
state changes from a ground state to a metastable one, a standard dark SW turns
into a bubble-like dark SW, which has a non-vanishing density dip and no pi
phase kink even in the case of a standing SW. It is shown that the former and
latter SW are dynamically unstable against an out-of-phase fluctuation and an
in-phase fluctuation, respectively, and the dynamical instabilities are
weakened when one approaches the transition point. We find that the size and
the inertial mass of the SW diverge at the first-order transition point. We
suggest that the divergence of the inertial mass may be detected through
measurement of the relation between the velocity and the phase jump of the SW.
In the presence of BP, we reveal that when the barrier strength exceeds a
certain critical value, the SF state that was metastable without the barrier is
destabilized towards complete disjunction of the SF. The presence of the
critical BP strength indicates that the strong BP qualitatively changes the
criticality near the metastability limit of the SF state. We derive critical
behaviors of the density, the compressibility, and the critical current near
the metastability limit induced by the BP. It is also found that the relation
between the supercurrent and the phase jump across the BP exhibits a peculiar
behavior, owing to the non-topological nature of the bubble-like SW.Comment: 25 pages, 19 figure
Quantum tricriticality at the superfluid-insulator transition of binary Bose mixtures
Quantum criticality near a tricritical point (TCP) is studied in the
two-component Bose-Hubbard model on square lattices. The existence of quantum
TCP on a boundary of superfluid-insulator transition is confirmed by quantum
Monte Carlo simulations. Moreover, we analytically derive the quantum
tricritical behaviors on the basis of an effective field theory. We find two
significant features of the quantum tricriticality, that are the chemical
potential dependence of the superfluid transition temperature and a strong
density fluctuation. We suggest that these features are directly observable in
existing experimental setups of Bose-Bose mixtures in optical lattices.Comment: 5+10 pages, 5 figure
Quantum phases of hardcore bosons with long-range interactions on a square lattice
We study the ground-state phase diagrams of hardcore bosons with long-range
interactions on a square lattice using the linear spin-wave theory and a
cluster mean-field method. Specifically, we consider the two types of
long-range interaction: One consists only of the nearest- and
next-nearest-neighbor interactions, and the other is the dipole-dipole
interaction that decays with the interparticle distance as .
It is known from previous analyses by quantum Monte Carlo methods that a
checkerboard supersolid (CSS) is absent in the ground-state phase diagram of
the former case while it is present in the latter. In the former, we find that
quantum fluctuations around mean-field solutions are enhanced by the direct
competition between the checkerboard and striped solid orders and that they
destabilize the CSS phase. On the other hand, the emergence of the CSS phase in
the latter case can be attributed to the absence of such a competition with
other solid orders. We also show that the cluster mean-field method allows for
the determination of phase boundaries in a precise quantitative manner when
scaling with respect to the cluster size is taken into account. It is found
that the phase transition between the superfluid and the solid (or CSS) is of
the first order in the vicinity of the particle-hole symmetric line.Comment: 19 pages, 16 figures, 1 tabl
Microscopic Model Calculations for the Magnetization Process of Layered Triangular-Lattice Quantum Antiferromagnets
Magnetization processes of spin-1/2 layered triangular-lattice
antiferromagnets (TLAFs) under a magnetic field H are studied by means of a
numerical cluster mean-field method with a scaling scheme. We find that small
antiferromagnetic couplings between the layers give rise to several types of
extra quantum phase transitions among different high-field coplanar phases.
Especially, a field-induced first-order transition is found to occur at
H\approx 0.7 H_s, where H_s is the saturation field, as another common quantum
effect of ideal TLAFs in addition to the well-established one-third plateau.
Our microscopic model calculation with appropriate parameters show excellent
agreement with experiments on Ba_3CoSb_2O_9 [T. Susuki et al., Phys. Rev. Lett.
110, 267201 (2013)]. Given this fact, we suggest that the Co^{2+}-based
compounds may allow for quantum simulations of intriguing properties of this
simple frustrated model, such as quantum criticality and supersolid states.Comment: 5+2 pages, 5+4 figure
Magnetization process of spin-1/2 Heisenberg antiferromagnets on a layered triangular lattice
We study the magnetization process of the spin-1/2 antiferromagnetic
Heisenberg model on a layered triangular lattice by means of a numerical
cluster mean-field method with a scaling scheme (CMF+S). It has been known that
antiferromagnetic spins on a two-dimensional (2D) triangular lattice with
quantum fluctuations exhibit a one-third magnetization plateau in the
magnetization curve under magnetic field. We demonstrate that the CMF+S
quantitatively reproduces the magnetization curve including the stabilization
of the plateau. {We also discuss the effects of a finite interlayer coupling,
which is unavoidable in real quasi-2D materials. It has been recently argued
for a model of the layered-triangular-lattice compound Ba3CoSb2O9 that such
interlayer coupling can induce an additional first-order transition at a strong
field. We present the detailed CMF+S results for the magnetization and
susceptibility curves of the fundamental Heisenberg Hamiltonian in the presence
of magnetic field and weak antiferromagnetic interlayer coupling. The extra
first-order transition appears as a quite small jump in the magnetization curve
and a divergence in the susceptibility at a strong magnetic field ~ 0.712 of
the saturation field.Comment: 14 pages, 7 figure
Umbrella-coplanar transition in the triangular XXZ model with arbitrary spin
The quantum triangular XXZ model has recently enjoyed a wealth of new
theoretical results, especially in relation to the modeling of the
BaCoSbO compound. In particular, it has been understood that in a
longitudinal magnetic field the umbrella (cone) phase, classically stable in
all the easy-plane region of the ground-state phase diagram, is considerably
reduced by the effect of quantum fluctuations. We provide more quantitative
information for this phenomenon at arbitrary value of the site spin , by
employing the dilute Bose gas expansion, valid in the high-field regime; our
results improve the available estimates of the expansion. We quantify the
extent to which a higher spin suppresses the effect of quantum fluctuations.
Besides, we show how in three-dimensional layered systems a relatively small
antiferromagnetic interlayer coupling has a similar consequence of bringing
back the umbrella phase in some part of the phase diagram.Comment: 8 pages, 7 figures; v2 results unchanged, title and abstract slightly
modified, introduction expanded, presentation generally improved, references
added, matches published versio
Secure multiplex coding to attain the channel capacity in wiretap channels
It is known that a message can be transmitted safely against any wiretapper
via a noisy channel without a secret key if the coding rate is less than the
so-called secrecy capacity , which is usually smaller than the channel
capacity . In order to remove the loss , we propose a multiplex
coding scheme with plural independent messages. In this paper, it is shown that
the proposed multiplex coding scheme can attain the channel capacity as the
total rate of the plural messages and the perfect secrecy for each message. The
coding theorem is proved by extending Hayashi's proof, in which the coding of
the channel resolvability is applied to wiretap channels.Comment: 15 pages, submitted to the IEEE Transactions on Information Theor
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