380 research outputs found
Solid-state time-to-pulse-height converter developed
Solid-state circuit produces an output pulse with an amplitude directly proportional to the time interval between two input pulses. It uses selected circuit options to achieve variable mode operation and a tunnel diode controls the charging time of a capacitor in proportion to the time interval being measured
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
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
One-dimensional Josephson arrays as superlattices for single Cooper pairs
We investigate uniform one-dimensional arrays of small Josephson junctions
(, ) with a realistic Coulomb interaction (here is the screening length
in units of the lattice constant of the array). At low energies this system can
be described in terms of interacting Bose particles (extra single Cooper pairs)
on the lattice. With increasing concentration of extra Cooper pairs, a
crossover from the Bose gas phase to the Wigner crystal phase and then to the
superlattice regime occurs. The phase diagram in the superlattice regime
consists of commensurable insulating phases with ( is integer)
separated by superconducting regions where the current is carried by
excitations with {\em fractional} electric charge . The Josephson
current through a ring-shaped array pierced by magnetic flux is calculated for
all of the phases.Comment: 4 pages (LATEX), 2 figure
Classical Phase Fluctuations in High Temperature Superconductors
Phase fluctuations of the superconducting order parameter play a larger role
in the cuprates than in conventional BCS superconductors because of the low
superfluid density of a doped insulator. In this paper, we analyze an XY model
of classical phase fluctuations in the high temperature superconductors using a
low-temperature expansion and Monte Carlo simulations. In agreement with
experiment, the value of the superfluid density at temperature T=0 is a quite
robust predictor of Tc, and the evolution of the superfluid density with T,
including its T-linear behavior at low temperature, is insensitive to
microscopic details.Comment: 4 pages, 1 figur
Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations
Using a single coil mutual inductance technique, we measure the low
temperature dependence of the magnetic penetration depth in superconducting NbN
films prepared with similar critical temperatures around 16 K but with
different microstructures. Only (100) epitaxial and weakly granular (100)
textured films display the characteristic exponential dependence of
conventional BCS s-wave superconductors. More granular (111) textured films
exhibit a linear dependence, indicating a gapless state in spite of the s-wave
gap. This result is quantitatively explained by a model of thermal phase
fluctuations favored by the granular structure.Comment: 10 pages, 4 figures, to appear in Phys. Rev.
Quantum-Phase Transitions of Interacting Bosons and the Supersolid Phase
We investigate the properties of strongly interacting bosons in two
dimensions at zero temperature using mean-field theory, a variational Ansatz
for the ground state wave function, and Monte Carlo methods. With on-site and
short-range interactions a rich phase diagram is obtained. Apart from the
homogeneous superfluid and Mott-insulating phases, inhomogeneous charge-density
wave phases appear, that are stabilized by the finite-range interaction.
Furthermore, our analysis demonstrates the existence of a supersolid phase, in
which both long-range order (related to the charge-density wave) and
off-diagonal long-range order coexist. We also obtain the critical exponents
for the various phase transitions.Comment: RevTex, 20 pages, 10 PostScript figures include
Monte Carlo study of the superfluid weight in doped antiferromagnets
The phase fluctuations of the condensate in doped antiferromagnets, described
by a t-t'-J model and a suitable 1/N expansion, provide a mechanism for a
Kosterlitz-Thouless (KT) type of transition to a superconducting state below
T_{c}. In this paper, we present a Monte Carlo study of the corresponding
superfluid weight D_{s}(T) in the classical (large-N) limit, as a function of
temperature and doping. Consistent with generic experimental trends, D_{s}(T)
exhibits a T-linear decrease at low temperatures, with the magnitude of the
slope D_{s}'(0) increasing upon doping. Finite-size scaling in the underdoped
regime predicts values for the dimensionless ratio A=k_{B}T_{c}/D_{s}(0) of
order unity, with A=0.4435(5) in the half-filled-band limit, thus confirming
D_{s}(0) as the fundamental energy scale determining T_{c}. Our Monte Carlo
results for D_{s}(T)/D_{s}(0) vs k_{B}T/D_{s}(0), at 10% hole doping, are found
to be in reasonable agreement with recent measurements on
La_{2-x}Sr_{x}CuO_{4}, with x=0.10, throughout the temperature range below the
theoretical KT transition temperature T_{c}.Comment: 9 pages, REVTEX file (4 Postscript figures). To appear in Phys. Rev.
Mean Field Theory of Josephson Junction Arrays with Charge Frustration
Using the path integral approach, we provide an explicit derivation of the
equation for the phase boundary for quantum Josephson junction arrays with
offset charges and non-diagonal capacitance matrix. For the model with nearest
neighbor capacitance matrix and uniform offset charge , we determine,
in the low critical temperature expansion, the most relevant contributions to
the equation for the phase boundary. We explicitly construct the charge
distributions on the lattice corresponding to the lowest energies. We find a
reentrant behavior even with a short ranged interaction. A merit of the path
integral approach is that it allows to provide an elegant derivation of the
Ginzburg-Landau free energy for a general model with charge frustration and
non-diagonal capacitance matrix. The partition function factorizes as a product
of a topological term, depending only on a set of integers, and a
non-topological one, which is explicitly evaluated.Comment: LaTex, 24 pages, 8 figure
Simultaneous Diagonal and Off Diagonal Order in the Bose--Hubbard Hamiltonian
The Bose-Hubbard model exhibits a rich phase diagram consisting both of
insulating regimes where diagonal long range (solid) order dominates as well as
conducting regimes where off diagonal long range order (superfluidity) is
present. In this paper we describe the results of Quantum Monte Carlo
calculations of the phase diagram, both for the hard and soft core cases, with
a particular focus on the possibility of simultaneous superfluid and solid
order. We also discuss the appearance of phase separation in the model. The
simulations are compared with analytic calculations of the phase diagram and
spin wave dispersion.Comment: 28 pages plus 24 figures, uuencoded Revtex+postscript file
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