251 research outputs found
A Mott Glass to Superfluid Transition for Random Bosons in Two Dimensions
We study the zero temperature superfluid-insulator transition for a
two-dimensional model of interacting, lattice bosons in the presence of
quenched disorder and particle-hole symmetry. We follow the approach of a
recent series of papers by Altman, Kafri, Polkovnikov, and Refael, in which the
strong disorder renormalization group is used to study disordered bosons in one
dimension. Adapting this method to two dimensions, we study several different
species of disorder and uncover universal features of the superfluid-insulator
transition. In particular, we locate an unstable finite disorder fixed point
that governs the transition between the superfluid and a gapless, glassy
insulator. We present numerical evidence that this glassy phase is the
incompressible Mott glass and that the transition from this phase to the
superfluid is driven by percolation-type process. Finally, we provide estimates
of the critical exponents governing this transition.Comment: (24 pages + 7 page appendix, 28 figures) This version has been
accepted to PRB. We have acquired new data that resolves the contradiction
between two estimates of the critical exponents in the earlier version of the
pape
Local superfluid densities probed via current-induced superconducting phase gradients
We have developed a superconducting phase gradiometer consisting of two
parallel DNA-templated nanowires connecting two thin-film leads. We have ramped
the cross current flowing perpendicular to the nanowires, and observed
oscillations in the lead-to-lead resistance due to cross-current-induced phase
differences. By using this gradiometer we have measured the temperature and
magnetic field dependence of the superfluid density and observed an
amplification of phase gradients caused by elastic vortex displacements. We
examine our data in light of Miller-Bardeen theory of dirty superconductors and
a microscale version of Campbell's model of field penetration.Comment: 5 pages, 6 figure
Suppression of 2\pi\ phase-slip due to hidden zero modes in one dimensional topological superconductors
We study phase slips in one-dimensional topological superconducting wires.
These wires have been proposed as building blocks for topologically protected
qubits in which the quantum information is distributed over the length of the
device and thus is immune to local sources of decoherence. However, phase-slips
are non-local events that can result in decoherence. Phase slips in topological
superconductors are peculiar for the reason that they occur in multiples of
4\pi\ (instead of 2\pi\ in conventional superconductors). We re-establish this
fact via a beautiful analogy to the particle physics concept of dynamic
symmetry breaking by explicitly finding a "hidden" zero mode in the fermion
spectrum computed in the background of a 2\pi\ phase-slip. Armed with the
understanding of phase-slips in topological superconductors, we propose a
simple experimental setup with which the predictions can be tested by
monitoring tunneling rate of a superconducting flux quantum through a
topological superconducting wire.Comment: 18 pages,14 figures, Updated referenc
Three-dimensional electronic instabilities in polymerized solid A1C60
The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of
polymerized C60 chains, with magnetic properties that suggest a non-metallic
ground state. We study the paramagnetic state of this phase using
first-principles electronic-structure methods, and examine the magnetic
fluctuations around this state using a model Hamiltonian. The electronic and
magnetic properties of even this polymerized phase remain strongly three
dimensional, and the magnetic fluctuations favor an unusual three-dimensional
antiferromagnetically ordered structure with a semi-metallic electronic
spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from
[email protected]
Alfvén Eigenmodes in shear reversed plasmas
Experiments on JT-60U and JET have shown that plasma configurations with shear reversal are prone to the excitation of unusual Alfvén eigenmodes by energetic particles. These modes emerge outside the TAE frequency gap, where one might expect them to be strongly damped. The modes often appear in bunches and they exhibit a quasi-periodic pattern of predominantly upward frequency sweeping (Alfvén Cascades) as the safety factor q changes in time. This work presents a theory that explains the key features of the observed unusual modes including their connection to TAE’s as well as the modifications of TAE’s themselves near the shear reversal point. The developed theory has been incorporated into a reduced numerical model and verified with full geometry codes. JET experimental data on Alfvén spectroscopy have been simulated to infer the mode numbers and the evolution of qmin in the discharge. This analysis confirms the values of q that characterize the internal transport barrier triggering in reversed shear plasmas
Structure and properties of the stable two-dimensional conducting polymer Mg5C60
We present a study on the structural, spectroscopic, conducting,
and
magnetic properties of Mg5C60, which is a two-dimensional (2D)
fulleride polymer. The polymer phase is stable up to the
exceptionally
high temperature of 823 K. The infrared and Raman studies
suggest the
formation of single bonds between the fulleride ions and
possibly
Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient
temperature, as
shown by electron spin resonance and microwave conductivity
measurements. The smooth transition from a metallic to a
paramagnetic
insulator state below 200 K is attributed to Anderson
localization
driven by structural disorder
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