29,763 research outputs found
Non-Fermi-Liquid Behavior from the Fermi-Liquid Approach
Non-Fermi liquid behavior of strongly correlated Fermi systems is derived
within the Landau approach. We attribute this behavior to a phase transition
associated with a rearrangement of the Landau state that leads to flattening of
a portion of the single-particle spectrum in the vicinity
of the Fermi surface. We demonstrate that the quasiparticle subsystem
responsible for the flat spectrum possesses the same thermodynamic properties
as a gas of localized spins. Theoretical results compare favorably with
available experimental data. While departing radically from prevalent views on
the origin of non-Fermi-liquid behavior, the theory advanced here is
nevertheless a conservative one of in continuing to operate within the general
framework of Landau theory.Comment: 8 pages, 4 figures, corrected list of author
The Akulov-Volkov Lagrangian, Symmetry Currents and Spontaneously Broken Extended Supersymmetry
A generalization of the Akulov-Volkov effective Lagrangian governing the self
interactions of the Nambu-Goldstone fermions associated with spontaneously
broken extended supersymmetry as well as their coupling to matter is presented
and scrutinized. The resulting currents associated with R-symmetry,
supersymmetry and space-time translations are constructed and seen to form a
supermultiplet structure.Comment: 17 pages, LaTeX; Title, abstract and introduction changes, references
adde
Calculations of Energy Losses due to Atomic Processes in Tokamaks with Applications to the ITER Divertor
Reduction of the peak heat loads on the plasma facing components is essential
for the success of the next generation of high fusion power tokamaks such as
the International Thermonuclear Experimental Reactor (ITER) 1 . Many present
concepts for accomplishing this involve the use of atomic processes to transfer
the heat from the plasma to the main chamber and divertor chamber walls and
much of the experimental and theoretical physics research in the fusion program
is directed toward this issue. The results of these experiments and
calculations are the result of a complex interplay of many processes. In order
to identify the key features of these experiments and calculations and the
relative role of the primary atomic processes, simple quasi-analytic models and
the latest atomic physics rate coefficients and cross sections have been used
to assess the relative roles of central radiation losses through
bremsstrahlung, impurity radiation losses from the plasma edge, charge exchange
and hydrogen radiation losses from the scrape-off layer and divertor plasma and
impurity radiation losses from the divertor plasma. This anaysis indicates that
bremsstrahlung from the plasma center and impurity radiation from the plasma
edge and divertor plasma can each play a significant role in reducing the power
to the divertor plates, and identifies many of the factors which determine the
relative role of each process. For instance, for radiation losses in the
divertor to be large enough to radiate the power in the divertor for high power
experiments, a neutral fraction of 10-3 to 10-2 and an impurity recycling rate
of netrecycle of ~ 10^16 s m^-3 will be required in the divertor.Comment: Preprint for the 1994 APSDPP meeting, uuencoded and gzipped
postscript with 22 figures, 40 pages
On Properties of the Isoscalar Giant Dipole Resonance
Main properties (strength function, energy-dependent transition density,
branching ratios for direct nucleon decay) of the isoscalar giant dipole
resonance in several medium-heavy mass spherical nuclei are described within a
continuum-RPA approach, taking into account the smearing effect. All model
parameters used in the calculations are taken from independent data.
Calculation results are compared with available experimental data.Comment: 12 pages, 2 figure
Asymmetric optical nuclear spin pumping in a single uncharged quantum dot
A highly asymmetric dynamic nuclear spin pumping is observed in a single self
assembled InGaAs quantum dot subject to resonant optical pumping of the neutral
exciton transition leading to a large maximum polarization of 54%. This dynamic
nuclear polarization is found to be much stronger following pumping of the
higher energy Zeeman state. Time-resolved measurements allow us to directly
monitor the buildup of the nuclear spin polarization in real time and to
quantitatively study the dynamics of the process. A strong dependence of the
observed dynamic nuclear polarization on the applied magnetic field is found,
with resonances in the pumping efficiency being observed for particular
magnetic fields. We develop a model that fully accounts for the observed
behaviour, where the pumping of the nuclear spin system is due to
hyperfine-mediated spin flip transitions between the states of the neutral
exciton manifold.Comment: published version; 4+ pages, 3 figures (eps
The Formation and Fragmentation of Disks around Primordial Protostars
The very first stars to form in the Universe heralded an end to the cosmic
dark ages and introduced new physical processes that shaped early cosmic
evolution. Until now, it was thought that these stars lived short, solitary
lives, with only one extremely massive star, or possibly a very wide binary
system, forming in each dark matter minihalo. Here we describe numerical
simulations that show that these stars were, to the contrary, often members of
tight multiple systems. Our results show that the disks that formed around the
first young stars were unstable to gravitational fragmentation, possibly
producing small binary and higher-order systems that had separations as small
as the distance between the Earth and the Sun.Comment: This manuscript has been accepted for publication in Science. This
version has not undergone final editing. Please refer to the complete version
of record at http://www.sciencemag.org
Superconducting Analogues of Quantum Optical Phenomena: Macroscopic Quantum Superpositions and Squeezing in a SQUID Ring
In this paper we explore the quantum behaviour of a SQUID ring which has a
significant Josephson coupling energy. We show that that the eigenfunctions of
the Hamiltonian for the ring can be used to create macroscopic quantum
superposition states of the ring. We also show that the ring potential may be
utilised to squeeze coherent states. With the SQUID ring as a strong contender
as a device for manipulating quantum information, such properties may be of
great utility in the future. However, as with all candidate systems for quantum
technologies, decoherence is a fundamental problem. In this paper we apply an
open systems approach to model the effect of coupling a quantum mechanical
SQUID ring to a thermal bath. We use this model to demonstrate the manner in
which decoherence affects the quantum states of the ring.Comment: 9 pages, 10 figures, To be submitted to Phys. Rev. A. (changes for
referee's and editior's comments - replaced to try to get PDF working
The stochastic dynamics of micron and nanoscale elastic cantilevers in fluid: fluctuations from dissipation
The stochastic dynamics of micron and nanoscale cantilevers immersed in a
viscous fluid are quantified. Analytical results are presented for long slender
cantilevers driven by Brownian noise. The spectral density of the noise force
is not assumed to be white and the frequency dependence is determined from the
fluctuation-dissipation theorem. The analytical results are shown to be useful
for the micron scale cantilevers that are commonly used in atomic force
microscopy. A general thermodynamic approach is developed that is valid for
cantilevers of arbitrary geometry as well as for arrays of multiple cantilevers
whose stochastic motion is coupled through the fluid. It is shown that the
fluctuation-dissipation theorem permits the calculation of stochastic
quantities via straightforward deterministic methods. The thermodynamic
approach is used with deterministic finite element numerical simulations to
quantify the autocorrelation and noise spectrum of cantilever fluctuations for
a single micron scale cantilever and the cross-correlations and noise spectra
of fluctuations for an array of two experimentally motivated nanoscale
cantilevers as a function of cantilever separation. The results are used to
quantify the noise reduction possible using correlated measurements with two
closely spaced nanoscale cantilevers.Comment: Submitted to Nanotechnology April 26, 200
Re-entrance and entanglement in the one-dimensional Bose-Hubbard model
Re-entrance is a novel feature where the phase boundaries of a system exhibit
a succession of transitions between two phases A and B, like A-B-A-B, when just
one parameter is varied monotonically. This type of re-entrance is displayed by
the 1D Bose Hubbard model between its Mott insulator (MI) and superfluid phase
as the hopping amplitude is increased from zero. Here we analyse this
counter-intuitive phenomenon directly in the thermodynamic limit by utilizing
the infinite time-evolving block decimation algorithm to variationally minimize
an infinite matrix product state (MPS) parameterized by a matrix size chi.
Exploiting the direct restriction on the half-chain entanglement imposed by
fixing chi, we determined that re-entrance in the MI lobes only emerges in this
approximate when chi >= 8. This entanglement threshold is found to be
coincident with the ability an infinite MPS to be simultaneously
particle-number symmetric and capture the kinetic energy carried by
particle-hole excitations above the MI. Focussing on the tip of the MI lobe we
then applied, for the first time, a general finite-entanglement scaling
analysis of the infinite order Kosterlitz-Thouless critical point located
there. By analysing chi's up to a very moderate chi = 70 we obtained an
estimate of the KT transition as t_KT = 0.30 +/- 0.01, demonstrating the how a
finite-entanglement approach can provide not only qualitative insight but also
quantitatively accurate predictions.Comment: 12 pages, 8 figure
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