70,835 research outputs found
Analysis of broadband microwave conductivity and permittivity measurements of semiconducting materials
We perform broadband phase sensitive measurements of the reflection
coefficient from 45 MHz up to 20 GHz employing a vector network analyzer with a
2.4 mm coaxial sensor which is terminated by the sample under test. While the
material parameters (conductivity and permittivity) can be easily extracted
from the obtained impedance data if the sample is metallic, no direct solution
is possible if the material under investigation is an insulator. Focusing on
doped semiconductors with largely varying conductivity, here we present a
closed calibration and evaluation procedure for frequencies up to 5 GHz, based
on the rigorous solution for the electromagnetic field distribution inside the
sample combined with the variational principle; basically no limiting
assumptions are necessary. A simple static model based on the electric current
distribution proves to yield the same frequency dependence of the complex
conductivity up to 1 GHz. After a critical discussion we apply the developed
method to the hopping transport in Si:P at temperature down to 1 K.Comment: 9 pages, 10 figures, accepted for publication in the Journal of
Applied Physic
Engineering Time-Reversal Invariant Topological Insulators With Ultra-Cold Atoms
Topological insulators are a broad class of unconventional materials that are
insulating in the interior but conduct along the edges. This edge transport is
topologically protected and dissipationless. Until recently, all existing
topological insulators, known as quantum Hall states, violated time-reversal
symmetry. However, the discovery of the quantum spin Hall effect demonstrated
the existence of novel topological states not rooted in time-reversal
violations. Here, we lay out an experiment to realize time-reversal topological
insulators in ultra-cold atomic gases subjected to synthetic gauge fields in
the near-field of an atom-chip. In particular, we introduce a feasible scheme
to engineer sharp boundaries where the "edge states" are localized. Besides,
this multi-band system has a large parameter space exhibiting a variety of
quantum phase transitions between topological and normal insulating phases. Due
to their unprecedented controllability, cold-atom systems are ideally suited to
realize topological states of matter and drive the development of topological
quantum computing.Comment: 11 pages, 6 figure
Resonant Impurity States in the D-Density-Wave Phase
We study the electronic structure near impurities in the d-density-wave (DDW)
state, a possible candidate phase for the pseudo-gap region of the
high-temperature superconductors. We show that the local DOS near a
non-magnetic impurity in the DDW state is {\it qualitatively} different from
that in a superconductor with -symmetry. Since this result is a
robust feature of the DDW phase, it can help to identify the nature of the two
different phases recently observed by scanning tunneling microscopy experiments
in the superconducting state of underdoped Bi-2212 compounds
Detection of finite frequency current moments with a dissipative resonant circuit
We consider the measurement of higher current moments with a dissipative
resonant circuit, which is coupled inductively to a mesoscopic device in the
coherent regime. Information about the higher current moments is coded in the
histograms of the charge on the capacitor plates of the resonant circuit.
Dissipation is included via the Caldeira-Leggett model, and it is essential to
include it in order for the charge fluctuations (or the measured noise) to
remain finite. We identify which combination of current correlators enter the
measurement of the third moment. The latter remains stable for zero damping.
Results are illustrated briefly for a quantum point contact
Critical Lines and Massive Phases in Quantum Spin Ladders with Dimerization
We determine the existence of critical lines in dimerized quantum spin
ladders in their phase diagram of coupling constants using the finite-size DMRG
algorithm. We consider both staggered and columnar dimerization patterns, and
antiferromagnetic and ferromagnetic inter-leg couplings. The existence of
critical phases depends on the precise combination of these patterns. The
nature of the massive phases separating the critical lines are characterized
with generalized string order parameters that determine their valence bond
solid (VBS) content.Comment: 9 pages 10 figure
Non-abelian D=11 Supermembrane
We obtain a U(M) action for supermembranes with central charges in the Light
Cone Gauge (LCG). The theory realizes all of the symmetries and constraints of
the supermembrane together with the invariance under a U(M) gauge group with M
arbitrary. The worldvolume action has (LCG) N=8 supersymmetry and it
corresponds to M parallel supermembranes minimally immersed on the target M9xT2
(MIM2). In order to ensure the invariance under the symmetries and to close the
corresponding algebra, a star-product determined by the central charge
condition is introduced. It is constructed with a nonconstant symplectic
two-form where curvature terms are also present. The theory is in the strongly
coupled gauge-gravity regime. At low energies, the theory enters in a
decoupling limit and it is described by an ordinary N=8 SYM in the IR phase for
any number of M2-branes.Comment: Contribution to the Proceedings of the Dubna International SQS'09
Workshop ("Supersymmetries and Quantum Symmetries-2009", July 29 - August 3,
2009. 12pg, Late
Coulomb gauge Gribov copies and the confining potential
We study the approach, initiated by Marinari et al., to the static
inter-quark potential based on Polyakov lines of finite temporal extent,
evaluated in Coulomb gauge. We show that, at small spatial separations, the
potential can be understood as being between two separately gauge invariant
colour charges. At larger separations Gribov copies obstruct the
non-perturbative identification of individually gauge invariant colour states.
We demonstrate, for the first time, how gauge invariance can be maintained
quite generally by averaging over Gribov copies. This allows us to extend the
analysis of the Polyakov lines and the corresponding, gauge invariant
quark-antiquark state to all distance scales. Using large scale lattice
simulations, we show that this interpolating state possesses a good overlap
with the ground state in the quark-antiquark sector and yields the full static
inter-quark potential at all distances. A visual representation of the Gribov
copies on the lattice is also presented.Comment: 22 pages, 9 figures, v2: minor changes, references adde
- …