1,099 research outputs found
Effects of dissipation on quantum phase transitions
We discuss the effect of dissipation on quantum phase transitions. In
particular we concentrate on the Superconductor to Insulator and Quantum-Hall
to Insulator transitions. By invoking a phenomenological parameter to
describe the coupling of the system to a continuum of degrees of freedom
representing the dissipative bath, we obtain new phase diagrams for the quantum
Hall and superconductor-insulator problems. Our main result is that, in
two-dimensions, the metallic phases observed in finite magnetic fields
(possibly also strictly zero field) are adiabatically deformable from one to
the other. This is plausible, as there is no broken symmetry which
differentiates them.Comment: 13 pages, 4 figure
Transmission lines and resonators based on quantum Hall plasmonics: electromagnetic field, attenuation and coupling to qubits
Quantum Hall edge states have some characteristic features that can prove
useful to measure and control solid state qubits. For example, their high
voltage to current ratio and their dissipationless nature can be exploited to
manufacture low-loss microwave transmission lines and resonators with a
characteristic impedance of the order of the quantum of resistance . The high value of the impedance guarantees that the
voltage per photon is high and for this reason high impedance resonators can be
exploited to obtain larger values of coupling to systems with a small charge
dipole, e.g. spin qubits. In this paper, we provide a microscopic analysis of
the physics of quantum Hall effect devices capacitively coupled to external
electrodes. The electrical current in these devices is carried by edge
magnetoplasmonic excitations and by using a semiclassical model, valid for a
wide range of quantum Hall materials, we discuss the spatial profile of the
electromagnetic field in a variety of situations of interest. Also, we perform
a numerical analysis to estimate the lifetime of these excitations and, from
the numerics, we extrapolate a simple fitting formula which quantifies the
factor in quantum Hall resonators. We then explore the possibility of reaching
the strong photon-qubit coupling regime, where the strength of the interaction
is higher than the losses in the system. We compute the Coulomb coupling
strength between the edge magnetoplasmons and singlet-triplet qubits, and we
obtain values of the coupling parameter of the order ;
comparing these values to the estimated attenuation in the resonator, we find
that for realistic qubit designs the coupling can indeed be strong
The Quantum Hall Effect of Field Induced Spin Density Wave Phases: the Physics of the Ultra Quantum Crystal
The Quantum Hall Effect of Field Induced Spin Density Wave Phases is
accounted for within a weak coupling theory which assumes that in the relevant
low temperature part of the phase diagram the quasi one dimensional conductor
is well described by Fermi liquid theory. Recent experimental results show that
sign inversion of the Hall Plateaux takes place all the way down from the
instability line of the normal state. The Quantum Nesting model, when it takes
into account small perturbations away from perfect nesting, describes well not
only the usual sequence of Hall plateaux, but also the anomalies connected with
sign inversion of the Hall Effect. Experimental observation of de-doubling of
sub-phase to sub-phase transition lines suggests that superposition of SDW
order parameters occurs in some parts of the phase diagram. The collective
excitations of the Ultra Quantum Crystal have a specific magneto-roton
structure. The SDW case exhibits,apart from the usual spin waves, topological
excitations which are either skyrmions or half skyrmions. It is suggested that
magneto-rotons may have been observed some years ago in specific heat
experiments.Comment: 17 pages, latex, 6 figures, available on request at
[email protected]
Transport equation for 2D electron liquid under microwave radiation plus magnetic field and the Zero Resistance State
A general transport equation for the center of mass motion is constructed to
study transports of electronic system under uniform magnetic field and
microwave radiation. The equation is applied to study 2D electron system in the
limit of weak disorder where negative resistance instability is observed when
the radiation field is strong enough. A solution of the transport equation with
spontaneous AC current is proposed to explain the experimentally observed
Radiation-Induced Zero Resistance State.Comment: 9 pages, 1 figur
Quantum Hall effect in exfoliated graphene affected by charged impurities: metrological measurements
Metrological investigations of the quantum Hall effect (QHE) completed by
transport measurements at low magnetic field are carried out in
a-few--wide Hall bars made of monolayer (ML) or bilayer (BL)
exfoliated graphene transferred on substrate. From the
charge carrier density dependence of the conductivity and from the measurement
of the quantum corrections at low magnetic field, we deduce that transport
properties in these devices are mainly governed by the Coulomb interaction of
carriers with a large concentration of charged impurities. In the QHE regime,
at high magnetic field and low temperature (), the Hall
resistance is measured by comparison with a GaAs based quantum resistance
standard using a cryogenic current comparator. In the low dissipation limit, it
is found quantized within 5 parts in (one standard deviation, ) at the expected rational fractions of the von Klitzing constant,
respectively and in the ML and BL
devices. These results constitute the most accurate QHE quantization tests to
date in monolayer and bilayer exfoliated graphene. It turns out that a main
limitation to the quantization accuracy, which is found well above the
accuracy usually achieved in GaAs, is the low value of the QHE
breakdown current being no more than . The current dependence
of the longitudinal conductivity investigated in the BL Hall bar shows that
dissipation occurs through quasi-elastic inter-Landau level scattering,
assisted by large local electric fields. We propose that charged impurities are
responsible for an enhancement of such inter-Landau level transition rate and
cause small breakdown currents.Comment: 14 pages, 9 figure
Compendium for precise ac measurements of the quantum Hall resistance
In view of the progress achieved in the field of the ac quantum Hall effect,
the Working Group of the Comite Consultatif d'Electricite et Magnetisme (CCEM)
on the AC Quantum Hall Effect asked the authors of this paper to write a
compendium which integrates their experiences with ac measurements of the
quantum Hall resistance. In addition to the important early work performed at
the Bureau International des Poids et Mesures and the National Physical
Laboratory, UK, further experience has been gained during a collaboration of
the authors' institutes NRC, METAS, and PTB, and excellent agreement between
the results of different national metrology institutes has been achieved. This
compendium summarizes the present state of the authors' knowledge and reviews
the experiences, tests and precautions that the authors have employed to
achieve accurate measurements of the ac quantum Hall effect. This work shows
how the ac quantum Hall effect can be reliably used as a quantum standard of ac
resistance having a relative uncertainty of a few parts in 10^8.Comment: 26 pages, 8 figure
Quantum Anomalous Hall Effect in HgMnTe Quantum Wells
The quantum Hall effect is usually observed when the two-dimensional electron
gas is subjected to an external magnetic field, so that their quantum states
form Landau levels. In this work we predict that a new phenomenon, the quantum
anomalous Hall effect, can be realized in HgMnTe quantum wells,
without the external magnetic field and the associated Landau levels. This
effect arises purely from the spin polarization of the atoms, and the
quantized Hall conductance is predicted for a range of quantum well thickness
and the concentration of the atoms. This effect enables dissipationless
charge current in spintronics devices.Comment: 5 pages, 3 figures. For high resolution figures see final published
version when availabl
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