3,217 research outputs found
Vortex glass transitions in disordered three-dimensional XY models: Simulations for several different sets of parameters
The anisotropic frustrated 3D XY model with strong disorder in the coupling
constants is studied as a model of a disordered superconductor in an applied
magnetic field. Simulations with the exchange Monte Carlo method are performed
for frustrations f=1/5 and f=1/4, corresponding to two different values of the
magnetic field along the z direction. The anisotropy is also varied. The
determination of the helicity modulus from twist histograms is discussed in
some detail and the helicity modulus is used in finite size scaling analyses of
the vortex glass transition. The general picture is that the behavior in [Phys.
Rev. Lett. 91, 077002 (2003)] is confirmed. For strong (e.g. isotropic)
coupling in the z direction the helicity modulus fails to scale and it is
argued that this is due to a too small effective randomness of such systems for
the accessible system sizes
How much entanglement is needed to reduce the energy variance?
We explore the relation between the entanglement of a pure state and its
energy variance for a local one dimensional Hamiltonian, as the system size
increases. In particular, we introduce a construction which creates a matrix
product state of arbitrarily small energy variance for spins,
with bond dimension scaling as , where is a
constant. This implies that a polynomially increasing bond dimension is enough
to construct states with energy variance that vanishes with the inverse of the
logarithm of the system size. We run numerical simulations to probe the
construction on two different models, and compare the local reduced density
matrices of the resulting states to the corresponding thermal equilibrium. Our
results suggest that the spatially homogeneous states with logarithmically
decreasing variance, which can be constructed efficiently, do converge to the
thermal equilibrium in the thermodynamic limit, while the same is not true if
the variance remains constant.Comment: small changes to fix typos and bibliographic reference
Ground State of the Kagome Lattice Heisenberg Antiferromagnet
Using series expansions around the dimer limit, we show that the ground state
of the Heisenberg Antiferromagnet on the Kagome Lattice appears to be a Valence
Bond Crystal (VBC) with a 36-site unit cell, and an energy per site of
. It is a honeycomb lattice of `perfect hexagons' as
discussed by Nikolic and Senthil. The energy difference between the ground
state and other ordered states with the maximum number of `perfect hexagons',
such as a stripe-ordered state, is of order . The energy of the
36-site system with periodic boundary conditions is further lowered by an
amount of , consistent with Exact Diagonalization. Every unit
cell of the VBC has two singlet states whose degeneracy is not lifted to
order in the expansion. We estimate this energy difference to be smaller than
. Two leading orders of perturbation theory find the lowest-energy
triplet excitations to be dispersionless and confined to the `perfect
hexagons'
Trimers, molecules and polarons in imbalanced atomic Fermi gases
We consider the ground state of a single "spin-down" impurity atom
interacting attractively with a "spin-up" atomic Fermi gas. By constructing
variational wave functions for polarons, molecules and trimers, we perform a
detailed study of the transitions between each of these dressed bound states as
a function of mass ratio and interaction strength.
We find that the presence of a Fermi sea enhances the stability of the -wave
trimer, which can be viewed as a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)
molecule that has bound an additional majority atom. For sufficiently large
, we find that the transitions lie outside the region of phase separation in
imbalanced Fermi gases and should thus be observable in experiment, unlike the
well-studied equal-mass case.Comment: 5 pages, 2 figure
Enlarging and cooling the N\'eel state in an optical lattice
We propose an experimental scheme to favor both the realization and the
detection of the N\'eel state in a two-component gas of ultracold fermions in a
three-dimensional simple-cubic optical lattice. By adding three compensating
Gaussian laser beams to the standard three pairs of retroreflected lattice
beams, and adjusting the relative waists and intensities of the beams, one can
significantly enhance the size of the N\'eel state in the trap, thus increasing
the signal of optical Bragg scattering. Furthermore, the additional beams
provide for adjustment of the local chemical potential and the possibility to
evaporatively cool the gas while in the lattice. Our proposals are relevant to
other attempts to realize many-body quantum phases in optical lattices.Comment: 8 pages, 10 figures (significantly revised text and figures
Universality and Crossover of Directed Polymers and Growing Surfaces
We study KPZ surfaces on Euclidean lattices and directed polymers on
hierarchical lattices subject to different distributions of disorder, showing
that universality holds, at odds with recent results on Euclidean lattices.
Moreover, we find the presence of a slow (power-law) crossover toward the
universal values of the exponents and verify that the exponent governing such
crossover is universal too. In the limit of a 1+epsilon dimensional system we
obtain both numerically and analytically that the crossover exponent is 1/2.Comment: LateX file + 5 .eps figures; to appear on Phys. Rev. Let
VELO Module Production - Module Assembly
This note describes in detail the procedures used in the gluing of sensors to hybrid and hybrid to pedestal for the LHCb VELO detector module assembly
Competing density-wave orders in a one-dimensional hard-boson model
We describe the zero-temperature phase diagram of a model of bosons,
occupying sites of a linear chain, which obey a hard-exclusion constraint: any
two nearest-neighbor sites may have at most one boson. A special case of our
model was recently proposed as a description of a ``tilted'' Mott insulator of
atoms trapped in an optical lattice. Our quantum Hamiltonian is shown to
generate the transfer matrix of Baxter's hard-square model. Aided by exact
solutions of a number of special cases, and by numerical studies, we obtain a
phase diagram containing states with long-range density-wave order with period
2 and period 3, and also a floating incommensurate phase. Critical theories for
the various quantum phase transitions are presented. As a byproduct, we show
how to compute the Luttinger parameter in integrable theories with
hard-exclusion constraints.Comment: 16 page
Do the surface Fermi arcs in Weyl semimetals survive disorder?
We theoretically study the topological robustness of the surface physics
induced by Weyl Fermi-arc surface states in the presence of short-ranged
quenched disorder and surface-bulk hybridization. This is investigated with
numerically exact calculations on a lattice model exhibiting Weyl Fermi-arcs.
We find that the Fermi-arc surface states, in addition to having a finite
lifetime from disorder broadening, hybridize with nonperturbative bulk rare
states making them no longer bound to the surface (i.e. they lose their purely
surface spectral character). Thus, we provide strong numerical evidence that
the Weyl Fermi-arcs are not topologically protected from disorder. Nonetheless,
the surface chiral velocity is robust and survives in the presence of strong
disorder, persisting all the way to the Anderson-localized phase by forming
localized current loops that live within the localization length of the
surface. Thus, the Weyl semimetal is not topologically robust to the presence
of disorder, but the surface chiral velocity is.Comment: Single column; 24 pages, 12 figure
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