48 research outputs found
Absence of Phase Stiffness in the Quantum Rotor Phase Glass
We analyze here the consequence of local rotational-symmetry breaking in the
quantum spin (or phase) glass state of the quantum random rotor model. By
coupling the spin glass order parameter directly to a vector potential, we are
able to compute whether the system is resilient (that is, possesses a phase
stiffness) to a uniform rotation in the presence of random anisotropy. We show
explicitly that the O(2) vector spin glass has no electromagnetic response
indicative of a superconductor at mean-field and beyond, suggesting the absence
of phase stiffness. This result confirms our earlier finding (PRL, {\bf 89},
27001 (2002)) that the phase glass is metallic, due to the main contribution to
the conductivity arising from fluctuations of the superconducting order
parameter. In addition, our finding that the spin stiffness vanishes in the
quantum rotor glass is consistent with the absence of a transverse stiffness in
the Heisenberg spin glass found by Feigelman and Tsvelik (Sov. Phys. JETP, {\bf
50}, 1222 (1979).Comment: 8 pages, revised version with added references on the vanishing of
the stiffness constant in the Heisenberg spin glas
Hall Conductivity near the z=2 Superconductor-Insulator Transition in 2D
We analyze here the behavior of the Hall conductivity near a
insulator-superconductor quantum critical point in a perpendicular
magnetic field. We show that the form of the conductivity is sensitive to the
presence of dissipation , and depends non-monotonically on once
is weak enough. passes through a maximum at in the quantum critical regime, suggesting that the limits and
do not commute.Comment: 4 pages, 1 .eps figure, to appear in Phys. Rev.
A Phase Glass is a Bose Metal: New Conducting State in 2D
In the quantum rotor model with random exchange interactions having a
non-zero mean, three phases, a 1) phase (Bose) glass, 2) superfluid, and 3)
Mott insulator, meet at a bi-critical point. We demonstrate that proximity to
the bi-critical point and the coupling between the energy landscape and the
dissipative degrees of freedom of the phase glass lead to a metallic state at
T=0. Consequently, the phase glass is unique in that it represents a concrete
example of a metallic state that is mediated by disorder, even in 2D. We
propose that the experimentally observed metallic phase which intervenes
between the insulator and the superconductor in a wide range of thin films is
in actuality a phase glass.Comment: 4 pages, 1 .eps figure, final version to appear in Phys. Rev. Let
Scratching the Bose surface
This is a `News and Views' article discussing recent proposals for ground
states of many boson systems which are neither superfluids nor Mott insulators.Comment: 4 pages, 1 figur
Electron Quasiparticles Drive the Superconductor-to-Insulator Transition in Homogeneously Disordered Thin Films
Transport data on Bi, MoGe, and PbBi/Ge homogeneously-disordered thin films
demonstrate that the critical resistivity, , at the nominal
insulator-superconductor transition is linearly proportional to the normal
sheet resistance, . In addition, the critical magnetic field scales
linearly with the superconducting energy gap and is well-approximated by
. Because is determined at high temperatures and is the
pair-breaking field, the two immediate consequences are: 1)
electron-quasiparticles populate the insulating side of the transition and 2)
standard phase-only models are incapable of describing the destruction of the
superconducting state. As gapless electronic excitations populate the
insulating state, the universality class is no longer the 3D XY model. The lack
of a unique critical resistance in homogeneously disordered films can be
understood in this context. In light of the recent experiments which observe an
intervening metallic state separating the insulator from the superconductor in
homogeneously disordered MoGe thin films, we argue that the two transitions
that accompany the destruction of superconductivity are 1) superconductor to
Bose metal in which phase coherence is lost and 2) Bose metal to localized
electron insulator via pair-breaking.Comment: This article is included in the Festschrift for Prof. Michael Pollak
on occasion of his 75th birthda
Can Short-Range Interactions Mediate a Bose Metal Phase in 2D?
We show here based on a 1-loop scaling analysis that short-range interactions
are strongly irrelevant perturbations near the insulator-superconductor (IST)
quantum critical point. The lack of any proof that short-range interactions
mediate physics which is present only in strong coupling leads us to conclude
that short-range interactions are strictly irrelevant near the IST quantum
critical point. Hence, we argue that no new physics, such as the formation of a
uniform Bose metal phase can arise from an interplay between on-site and
nearest-neighbour interactions.Comment: 3 pages, 1 .eps file. SUbmitted to Phys. Rev.
Bose Hubbard model in the presence of Ohmic dissipation
We study the zero temperature mean-field phase diagram of the Bose-Hubbard
model in the presence of local coupling between the bosons and an external
bath. We consider a coupling that conserves the on-site occupation number,
preserving the robustness of the Mott and superfluid phases. We show that the
coupling to the bath renormalizes the chemical potential and the interaction
between the bosons and reduces the size of the superfluid regions between the
insulating lobes. For strong enough coupling, a finite value of hopping is
required to obtain superfluidity around the degeneracy points where Mott phases
with different occupation numbers coexist. We discuss the role that such a bath
coupling may play in experiments that probe the formation of the
insulator-superfluid shell structure in systems of trapped atoms.Comment: 5 pages, 2 figures. Error found in v1, now corrected, leads to
qualitative changes in result
Anomalous Quantum Diffusion at the Superfluid-Insulator Transition
We consider the problem of the superconductor-insulator transition in the
presence of disorder, assuming that the fermionic degrees of freedom can be
ignored so that the problem reduces to one of Cooper pair localization. Weak
disorder drives the critical behavior away from the pure critical point,
initially towards a diffusive fixed point. We consider the effects of Coulomb
interactions and quantum interference at this diffusive fixed point. Coulomb
interactions enhance the conductivity, in contrast to the situation for
fermions, essentially because the exchange interaction is opposite in sign. The
interaction-driven enhancement of the conductivity is larger than the
weak-localization suppression, so the system scales to a perfect conductor.
Thus, it is a consistent possibility for the critical resistivity at the
superconductor-insulator transition to be zero, but this value is only
approached logarithmically. We determine the values of the critical exponents
and comment on possible implications for the interpretation of
experiments
Transport Properties near the z=2 Insulator-Superconductor Transition
We consider here the fluctuation conductivity near the point of the
insulator-superconductor transition in a system of regular Josephson junction
arrays in the presence of particle-hole asymmetry or equivalently homogeneous
charge frustration. The transition is characterised by the dynamic critical
exponent , opening the possibility of the perturbative
renormalization-group (RG) treatment. The quartic interaction in the
Ginzburg-Landau action and the coupling to the Ohmic heat bath, giving the
finite quasiparticle life-time, lead to the non-monotonic behavior of the dc
conductivity as a function of temperature in the leading logarithmic
approximation.Comment: Revised version for publication. To appear in PR
Melting transition of an Ising glass driven by magnetic field
The quantum critical behavior of the Ising glass in a magnetic field is
investigated. We focus on the spin glass to paramagnet transition of the
transverse degrees of freedom in the presence of finite longitudinal field. We
use two complementary techniques, the Landau theory close to the T=0 transition
and the exact diagonalization method for finite systems. This allows us to
estimate the size of the critical region and characterize various crossover
regimes. An unexpectedly small energy scale on the disordered side of the
critical line is found, and its possible relevance to experiments on metallic
glasses is briefly discussed.Comment: 4 pages, 3 figure