431 research outputs found
Phase Diagram of Interacting Bosons on the Honeycomb Lattice
We study the ground state properties of repulsively interacting bosons on the
honeycomb lattice using large-scale quantum Monte Carlo simulations. In the
hard-core limit the half-filled system develops long ranged diagonal order for
sufficiently strong nearest-neighbor repulsion. This staggered solid melts at a
first order quantum phase transition into the superfluid phase, without the
presence of any intermediate supersolid phase. Within the superfluid phase,
both the superfluid density and the compressibility exhibit local minima near
particle- (hole-) density one quarter, while the density and the condensate
fraction show inflection points in this region. Relaxing the hard-core
constraint, supersolid phases emerge for soft-core bosons. The suppression of
the superfluid density is found to persist for sufficiently large, finite
on-site repulsion.Comment: 4 pages with 5 figure
Superfluid Suppression in d-Wave Superconductors due to Disordered Magnetism
The influence of static magnetic correlations on the temperature-dependent
superfluid density \rho_s(T) is calculated for d-wave superconductors. In
self-consistent calculations, itinerant holes form incommensurate spin density
waves (SDW) which coexist with superconductivity. In the clean limit, the
density of states is gapped, and \rho_s(T << T_c) is exponentially activated.
In inhomogeneously-doped cases, the SDW are disordered and both the density of
states and \rho_s(T) obtain forms indistinguishable from those in dirty but
pure d-wave superconductors, in accordance with experiments. We conclude that
the observed collapse of \rho_s at x\approx 0.35 in underdoped YBCO may
plausibly be attributed to the coexistence of SDW and superconductivity.Comment: 6 pages, 5 figures. Expanded discussio
Change Management in Large-Scale Enterprise Information Systems
Abstract. The information infrastructure in today’s businesses consists of many interoperating autonomous systems. Changes to a single system can therefore have an unexpected impact on other, dependent systems. In our Caro approach we try to cope with this problem by observing each system participating in the infrastructure and analyzing the impact of any change that occurs. The analysis process is driven by declaratively defined rules and works with a generic and ex-tensible graph model to represent the relevant metadata that is subject to changes. This makes Caro applicable to heterogeneous scenarios and customizable to spe-cial needs.
Phase separation in supersolids
We study quantum phase transitions in the ground state of the two dimensional
hard-core boson Hubbard Hamiltonian. Recent work on this and related models has
suggested ``supersolid'' phases with simultaneous diagonal and off-diagonal
long range order. We show numerically that, contrary to the generally held
belief, the most commonly discussed ``checkerboard'' supersolid is
thermodynamically unstable. Furthermore, this supersolid cannot be stabilized
by next near neighbour interaction. We obtain the correct phase diagram using
the Maxwell construction. We demonstrate the ``striped'' supersolid is
thermodynamically stable and is separated from the superfluid phase by a
continuous phase transition.Comment: 4 pages, 4 eps figures, include
Strong-coupling perturbation theory for the two-dimensional Bose-Hubbard model in a magnetic field
The Bose-Hubbard model in an external magnetic field is investigated with
strong-coupling perturbation theory. The lowest-order secular equation leads to
the problem of a charged particle moving on a lattice in the presence of a
magnetic field, which was first treated by Hofstadter. We present phase
diagrams for the two-dimensional square and triangular lattices, showing a
change in shape of the phase lobes away from the well-known power-law behavior
in zero magnetic field. Some qualitative agreement with experimental work on
Josephson-junction arrays is found for the insulating phase behavior at small
fields.Comment: 7 pages, 5 figures include
Investigating tiredness in Australian general practice. Do pathology tests help in diagnosis?
Melbourn
The Bose Metal: gauge field fluctuations and scaling for field tuned quantum phase transitions
In this paper, we extend our previous discussion of the Bose metal to the
field tuned case. We point out that the recent observation of the metallic
state as an intermediate phase between the superconductor and the insulator in
the field tuned experiments on MoGe films is in perfect consistency with the
Bose metal scenario. We establish a connection between general dissipation
models and gauge field fluctuations and apply this to a discussion of scaling
across the quantum phase boundaries of the Bose metallic state. Interestingly,
we find that the Bose metal scenario implies a possible {\em two} parameter
scaling for resistivity across the Bose metal-insulator transition, which is
remarkably consistent with the MoGe data. Scaling at the superconductor-metal
transition is also proposed, and a phenomenolgical model for the metallic state
is discussed. The effective action of the Bose metal state is described and its
low energy excitation spectrum is found to be .Comment: 15 pages, 1 figur
Anisotropy in the helicity modulus of a 3D XY-model: application to YBCO
We present a Monte Carlo study of the helicity moduli of an anisotropic
classical three-dimensional (3D) XY-model of YBCO in superconducting state. It
is found that both the ab-plane and the c-axis helicity moduli, which are
proportional to the inverse square of the corresponding magnetic field
penetration depth, vary linearly with temperature at low temperatures. The
result for the c-axis helicity modulus is in disagreement with the experiments
on high quality samples of YBCO. Thus we conclude that purely classical phase
fluctuations of the superconducting order parameter cannot account for the
observed c-axis electrodynamics of YBCO.Comment: 7 pages, 1 figur
Anisotropy in the helicity modulus of a quantum 3D XY-model: application to YBCO
We present a variational study of the helicity moduli of an anisotropic
quantum three-dimensional (3D) XY-model of YBCO in superconducting state. It is
found that both the ab-plane and the c-axis helicity moduli, which are
proportional to the inverse square of the corresponding magnetic field
penetration depth, vary with temperature T as T to the fourth power in the zero
temperature limit. Moreover, the c-axis helicity modulus drops with temperature
much faster than the ab-plane helicity modulus because of the weaker Josephson
couplings along the c-axis compared to those along the ab-plane. These findings
are in disagreement with the experiments on high quality samples of YBCO.Comment: 9 pages, 1 figur
Phase fluctuations in superconductors: from Galilean invariant to quantum XY models
We analyze the corrections to the superfluid density due to phase
fluctuations within both a continuum and a lattice model for - and d-wave
superconductors. We expand the phase-only action beyond the Gaussian level and
compare our results with the quantum XY model both in the quantum and in the
classical regime. We find new dynamic anharmonic vertices, absent in the
quantum XY model, which are responsible for the vanishing of the correction to
the superfluid density at zero temperature in a continuum (Galilean invariant)
model. Moreover the phase-fluctuation effects are reduced with respect to the
XY model by a factor at least of order .Comment: 4 pages; shorter version, accepted for publication on Phys. Rev. B
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