223 research outputs found

### Localized Flux Lines and the Bose Glass

Columnar defects provide effective pinning centers for magnetic flux lines in
high--$T_{\rm c}$ superconductors. Utilizing a mapping of the statistical
mechanics of directed lines to the quantum mechanics of two--dimensional
bosons, one expects an entangled flux liquid phase at high temperatures,
separated by a second--order localization transition from a low--temperature
``Bose glass'' phase with infinite tilt modulus. Recent decoration experiments
have demonstrated that below the matching field the repulsive forces between
the vortices may be sufficiently large to produce strong spatial correlations
in the Bose glass. This is confirmed by numerical simulations, and a remarkably
wide soft ``Coulomb gap'' at the chemical potential is found in the
distribution of pinning energies. At low currents, the dominant transport
mechanism in the Bose glass phase proceeds via the formation of double kinks
between not necessarily adjacent columnar pins, similar to variable--range
hopping in disordered semiconductors. The strong correlation effects
originating in the long--range vortex interactions drastically reduce
variable--range hopping transport.Comment: 10 pages, latex ("lamuphys.sty" file included), 6 figures can be
obtained from the author ([email protected]); to appear in Proc. XIV
Sitges conference on "Complex Behaviour of Glassy Systems" (Springer--Verlag

### On the Coexistence of Diagonal and off-Diagonal Long-Range Order, a Monte Carlo Study

The zero temperature properties of interacting 2 dimensional lattice bosons
are investigated. We present Monte Carlo data for soft-core bosons that
demonstrate the existence of a phase in which crystalline long-range order and
off-diagonal long-range order (superfluidity) coexist. We comment on the
difference between hard and soft-core bosons and compare our data to mean-field
results that predict a larger coexistence region. Furthermore, we determine the
critical exponents for the various phase transitions.Comment: 7 pages and 8 figures appended in postscript, KA-TFP-93-0

### Crossover and scaling in a two-dimensional field-tuned superconductor

Using an analysis similar to that of Imry and Wortis, it is shown that the
apparent first order superconductor to metal transition, which has been claimed
to exist at low values of the magnetic field in a two-dimensional field-tuned
system at zero temperature,can be consistentlyinterpreted as a sharp crossover
from a strong superconductor to an inhomogeneous state, which is a weak
superconductor. The true zero-temperature superconductor to insulator
transition within the inhomogenous state is conjectured to be that of randomly
diluted XY model. An explaination of the observed finite temperature
approximate scaling of resistivity close to the critical point is speculated
within this model.Comment: 5 pages, 2 figures, corrected and modified according to referee
Report

### Quantum vortex fluctuations in cuprate superconductors

We study the effects of quantum vortex fluctuations in two-dimensional
superconductors using a dual theory of vortices, and investigate the relevance
to underdoped cuprates where the superconductor-insulator transition (SIT) is
possibly driven by quantum vortex proliferation. We find that a broad enough
phase fluctuation regime may exist for experimental observation of the quantum
vortex fluctuations near SIT in underdoped cuprates. We propose that this
scenario can be tested via pair-tunneling experiments which measure the
characteristic resonances in the zero-temperature pair-field susceptibility in
the vortex-proliferated insulating phase.Comment: RevTex 5 pages, 2 eps figures; expanded; to appear in Phys. Rev.

### Quantum Phase Transitions and Conserved Charges

The constraints on the scaling properties of conserved charge densities in
the vicinity of a zero temperature ($T$), second-order quantum phase transition
are studied. We introduce a generalized Wilson ratio, characterizing the
non-linear response to an external field, $H$, coupling to any conserved
charge, and argue that it is a completely universal function of $H/T$: this is
illustrated by computations on model systems. We also note implications for
transitions where the order parameter is a conserved charge (as in a $T=0$
ferromagnet-paramagnet transition).Comment: 19 pages, REVTEX 3.0, 8 uuencoded Postscript figues appended,
YCTP-xxx

### Superconductor-Insulator Transition in a Capacitively Coupled Dissipative Environment

We present results on disordered amorphous films which are expected to
undergo a field-tuned Superconductor-Insulator Transition.The addition of a
parallel ground plane in proximity to the film changes the character of the
transition.Although the screening effects expected from "dirty-boson" theories
are not evident,there is evidence that the ground plane couples a certain type
of dissipation into the system,causing a dissipation-induced phase
transition.The dissipation due to the phase transition couples similarly into
quantum phase transition systems such as superconductor-insulator transitions
and Josephson junction arrays.Comment: 4 pages, 4 figure

### Off-Diagonal Long Range Order and Scaling in a Disordered Quantum Hall System

We have numerically studied the bosonic off-diagonal long range order,
introduced by Read to describe the ordering in ideal quantum Hall states, for
noninteracting electrons in random potentials confined to the lowest Landau
level. We find that it also describes the ordering in disordered quantum Hall
states: the proposed order parameter vanishes in the disordered
($\sigma_{xy}=0$) phase and increases continuously from zero in the ordered
($\sigma_{xy}=e^2/h$) phase. We study the scaling of the order parameter and
find that it is consistent with that of the one-electron Green's function.Comment: 10 pages and 4 figures, Revtex v3.0, UIUC preprint P-94-03-02

### Critical Exponents for Three-Dimensional Superfluid--Bose-Glass Phase Transition

The critical phenomenon of the zero temperature superfluid--Bose-glass phase
transition for hard-core bosons on a three-dimensional disordered lattice is
studied using a quantum real-space renormalization-group method. The
correlation-length exponent $\nu$ and the dynamic exponent z are computed. The
critical exponent z is found to be 2.5 for compressible states and 1.3 for
incompressible states. The exponent $\nu$ is shown to be insensitive to z as
that in the two-dimensional case, and has value roughly equal to 1.Comment: 11 pages, REVTE

### Ring exchange, the Bose metal, and bosonization in two dimensions

Motivated by the high-T_c cuprates, we consider a model of bosonic Cooper
pairs moving on a square lattice via ring exchange. We show that this model
offers a natural middle ground between a conventional antiferromagnetic Mott
insulator and the fully deconfined fractionalized phase which underlies the
spin-charge separation scenario for high-T_c superconductivity. We show that
such ring models sustain a stable critical phase in two dimensions, the *Bose
metal*. The Bose metal is a compressible state, with gapless but uncondensed
boson and ``vortex'' excitations, power-law superconducting and charge-ordering
correlations, and broad spectral functions. We characterize the Bose metal with
the aid of an exact plaquette duality transformation, which motivates a
universal low energy description of the Bose metal. This description is in
terms of a pair of dual bosonic phase fields, and is a direct analog of the
well-known one-dimensional bosonization approach. We verify the validity of the
low energy description by numerical simulations of the ring model in its exact
dual form. The relevance to the high-T_c superconductors and a variety of
extensions to other systems are discussed, including the bosonization of a two
dimensional fermionic ring model

### Soluble `Supersymmetric' Quantum XY Model

We present a `supersymmetric' modification of the $d$-dimensional quantum
rotor model whose ground state is exactly soluble. The model undergoes a
vortex-binding transition from insulator to metal as the rotor coupling is
varied. The Hamiltonian contains three-site terms which are relevant: they
change the universality class of the transition from that of the ($d+1$)--- to
the $d$-dimensional classical XY model. The metallic phase has algebraic ODLRO
but the superfluid density is identically zero. Variational wave functions for
single-particle and collective excitations are presented.Comment: 12 pages, REVTEX 3.0, IUCM93-00

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