3,064 research outputs found
Pseudo-Goldstone Modes in Isospin-Asymmetric Nuclear Matter
We analyze the chiral limit in dense isospin-asymmetric nuclear matter. It is
shown that the pseudo-Goldstone modes in this system are qualitatively
different from the case of isospin-symmetric matter.Comment: 15 pages, ReVTeX, no figure
Defect and equivalence of unitary matrices. The Fourier case
Consider the real space D_U of directions moving into which from a unitary N
x N matrix U we do not disturb its unitarity and the moduli of its entries in
the first order. dim( D_U ) is called the defect of U and denoted D(U). We give
an account of Alexander Karabegov's theory where D_U is parametrized by the
imaginary subspace of the eigenspace, associated with lambda = 1, of a certain
unitary operator I_U on the N x N complex matrices, and where D(U) is the
multiplicity of 1 in the spectrum of I_U. This characterization allows us to
establish dependence of D(U_1 x ... x U_r) - where x stands for the Kronecker
product - on D(U_k)'s, to derive formulas expressing D(F) for a Fourier matrix
F of the size being a power of a prime number, as well as to show the
multiplicativity of D(F) with respect to Kronecker factors of F if their sizes
are pairwise relatively prime. Also partly due to the role of symmetries of U
in the determination of the eigenspaces of I_U we study the 'permute and
enphase' symmetries and equivalence of Fourier matrices, associated with
arbitrary finite abelian groups.Comment: 92 pages. The paper has undergone hundreds of minor corrections and
improvements. Some paragraphs have been completely changed, a few addded.
Sections Introduction and Conclusions have been added, as well as a new
abstrac
Quantum Chaotic Environments, The Butterfly Effect, And Decoherence
We investigate the sensitivity of quantum systems that are chaotic in a
classical limit, to small perturbations of their equations of motion. This
sensitivity, originally studied in the context of defining quantum chaos, is
relevant to decoherence in situations when the environment has a chaotic
classical counterpart.Comment: 4 pages, 3 figure
Resilient Quantum Computation: Error Models and Thresholds
Recent research has demonstrated that quantum computers can solve certain
types of problems substantially faster than the known classical algorithms.
These problems include factoring integers and certain physics simulations.
Practical quantum computation requires overcoming the problems of environmental
noise and operational errors, problems which appear to be much more severe than
in classical computation due to the inherent fragility of quantum
superpositions involving many degrees of freedom. Here we show that arbitrarily
accurate quantum computations are possible provided that the error per
operation is below a threshold value. The result is obtained by combining
quantum error-correction, fault tolerant state recovery, fault tolerant
encoding of operations and concatenation. It holds under physically realistic
assumptions on the errors.Comment: 19 pages in RevTex, many figures, the paper is also avalaible at
http://qso.lanl.gov/qc
Spin-gap opening accompanied by a strong magnetoelastic response in the S=1 magnetic dimer system Ba3BiRu2O9
Neutron diffraction, magnetization, resistivity, and heat capacity
measurements on the 6H-perovskite Ba3BiRu2O9 reveal simultaneous magnetic and
structural dimerization driven by strong magnetoelastic coupling. An
isostructural but strongly displacive first-order transition on cooling through
T*=176 K is associated with a change in the nature of direct Ru-Ru bonds within
Ru2O9 face-sharing octahedra. Above T*, Ba3BiRu2O9 is an S=1 magnetic dimer
system with intradimer exchange interactions J0/kB=320 K and interdimer
exchange interactions J'/kB=-160 K. Below T*, a spin-gapped state emerges with
\Delta\approx220 K. Ab initio calculations confirm antiferromagnetic exchange
within dimers, but the transition is not accompanied by long range-magnetic
order.Comment: 5 pages, 5 figures, accepted by Physical Review
Size fluctuations of the initial source and the event-by-event transverse momentum fluctuations in relativistic heavy-ion collisions
We show that the event-by-event fluctuations of the transverse size of the
initial source, which follow directly from the Glauber treatment of the
earliest stage of relativistic heavy-ion collisions, cause, after hydrodynamic
evolution, fluctuations of the transverse flow velocity at hadronic freeze-out.
This in turn leads to event-by-event fluctuations of the average transverse
momentum, p_T. Simulations with GLISSANDO for the Glauber phase, followed by a
realistic hydrodynamic evolution and statistical hadronization carried out with
THERMINATOR, lead to agreement with the RHIC data. In particular, the magnitude
of the effect, its centrality dependence, and the weak dependence on the
incident energy are properly reproduced. Our results show that bulk of the
observed event-by-event p_T fluctuations may be explained by the fluctuations
of the size of the initial source.Comment: 5 pages, 4 figures, version accepted in PR
High intensity study of THz detectors based on field effect transistors
Terahertz power dependence of the photoresponse of field effect transistors,
operating at frequencies from 0.1 to 3 THz for incident radiation power density
up to 100 kW/cm^2 was studied for Si metal-oxide-semiconductor field-effect
transistors and InGaAs high electron mobility transistors. The photoresponse
increased linearly with increasing radiation power up to kW/cm^2 range. The
saturation of the photoresponse was observed for all investigated field effect
transistors for intensities above several kW/cm^2. The observed signal
saturation is explained by drain photocurrent saturation similar to saturation
in direct currents output characteristics. The theoretical model of terahertz
field effect transistor photoresponse at high intensity was developed. The
model explains quantitatively experimental data both in linear and nonlinear
(saturation) range. Our results show that dynamic range of field effect
transistors is very high and can extend over more than six orderd of magnitudes
of power densities (from 0.5 mW/cm^2 to 5 kW/cm^2)
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