3,691 research outputs found
N-particle sector of quantum field theory as a quantum open system
We give an exposition of a technique, based on the Zwanzig projection
formalism, to construct the evolution equation for the reduced density matrix
corresponding to the n-particle sector of a field theory. We consider the case
of a scalar field with a interaction as an example and construct the
master equation at the lowest non-zero order in perturbation theory.Comment: 12 pages, Late
The temperature dependent bandstructure of a ferromagnetic semiconductor film
The electronic quasiparticle spectrum of a ferromagnetic film is investigated
within the framework of the s-f model. Starting from the exact solvable case of
a single electron in an otherwise empty conduction band being exchange coupled
to a ferromagnetically saturated localized spin system we extend the theory to
finite temperatures. Our approach is a moment-conserving decoupling procedure
for suitable defined Green functions. The theory for finite temperatures
evolves continuously from the exact limiting case. The restriction to zero
conduction band occupation may be regarded as a proper model description for
ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a
simple cubic film cut parallel to the (100) crystal plane, we find some marked
correlation effects which depend on the spin of the test electron, on the
exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure
Bond Orientational Order, Molecular Motion and Free Energy of High Density DNA Mesophases
By equilibrating condensed DNA arrays against reservoirs of known osmotic
stress and examining them with several structural probes, it has been possible
to achieve a detailed thermodynamic and structural characterization of the
change between two distinct regions on the liquid crystalline phase digram: a
higher-density hexagonally packed region with long-range bond orientational
order in the plane perpendicular to the average molecular direction; and a
lower-density cholesteric region with fluid-like positional order. X-rays
scattering on highly ordered DNA arrays at high density and with the helical
axis oriented parallel to the incoming beam showed a six-fold azimuthal
modulation of the first order diffraction peak that reflects the macroscopic
bond-orientational order. Transition to the less-dense cholesteric phase
through osmotically controlled swelling shows the loss of this bond
orientational order that had been expected from the change in optical
birefringence patterns and that is consistent with a rapid onset of molecular
positional disorder. This change in motion was previously inferred from
intermolecular force measurements and is now confirmed by NMR.
Controlled reversible swelling and compaction under osmotic stress, spanning a
range of densities between mg/ml to mg/ml, allows
measurement of the free energy changes throughout each phase and at the phase
transition, essential information for theories of liquid-crystalline states.Comment: 14 pages, 3 figures in gif format available at
http://abulafia.mgsl.dcrt.nih.gov/pics.html E-mail: [email protected]
Efimov states and their Fano resonances in a neutron-rich nucleus
Asymmetric resonances in elastic n+C scattering are attributed to
Efimov states of such neutron-rich nuclei, that is, three-body bound states of
the n+n+C system when none of the pairs is bound or some of them only
weakly bound. By fitting to the general resonance shape described by Fano, we
extract resonance position, width, and the "Fano profile index". While Efimov
states have been discussed extensively in many areas of physics, there is only
one very recent experimental observation in trimers of cesium atoms. The
conjunction that we present of the Efimov and Fano phenomena may lead to
experimental realization in nuclei.Comment: 4 double-column pages, 3 figure
Unusual magnetic properties of the low-dimensional quantum magnet Na2V3O7
We report the results of low-temperature measurements of the specific heat
Cp(T), ac susceptibility chi(T) and 23Na nuclear magnetic resonance NMR of
Na2V3O7. At liquid He temperatures Cp(T)/T exhibits broad field-dependent
maxima, which shift to higher temperatures upon increasing the applied magnetic
field H. Below 1.5 K the ac magnetic susceptibility chi(T) follows a
Curie-Weiss law and exhibits a cusp at 0.086 mK which indicates a phase
transition at very low temperatures. These results support the previous
conjecture that Na2V3O7 is close to a quantum critical point (QCP) at mu_{0}H =
0 T. The entire data set, including results of measurements of the NMR
spin-lattice relaxation 1/T1(T), reveals a complex magnetic behavior at low
temperatures. We argue that it is due to a distribution of singlet-triplet
energy gaps of dimerized V moments. The dimerization process evolves over a
rather broad temperature range around and below 100 K. At the lowest
temperatures the magnetic properties are dominated by the response of only a
minor fraction of the V moments.Comment: 10.5 pages, 15 figures. Submitted to Phys. Rev.
Feed-Forward Chains of Recurrent Attractor Neural Networks Near Saturation
We perform a stationary state replica analysis for a layered network of Ising
spin neurons, with recurrent Hebbian interactions within each layer, in
combination with strictly feed-forward Hebbian interactions between successive
layers. This model interpolates between the fully recurrent and symmetric
attractor network studied by Amit el al, and the strictly feed-forward
attractor network studied by Domany et al. Due to the absence of detailed
balance, it is as yet solvable only in the zero temperature limit. The built-in
competition between two qualitatively different modes of operation,
feed-forward (ergodic within layers) versus recurrent (non- ergodic within
layers), is found to induce interesting phase transitions.Comment: 14 pages LaTex with 4 postscript figures submitted to J. Phys.
Are multiphase competition & order-by-disorder the keys to understanding Yb2Ti2O7?
If magnetic frustration is most commonly known for undermining long-range
order, as famously illustrated by spin liquids, the ability of matter to
develop new collective mechanisms in order to fight frustration is no less
fascinating, providing an avenue for the exploration and discovery of
unconventional properties of matter. Here we study an ideal minimal model of
such mechanisms which, incidentally, pertains to the perplexing quantum spin
ice candidate Yb2Ti2O7. Specifically, we explain how thermal and quantum
fluctuations, optimized by order-by-disorder selection, conspire to expand the
stability region of an accidentally degenerate continuous symmetry U(1)
manifold against the classical splayed ferromagnetic ground state that is
displayed by the sister compound Yb2Sn2O7. The resulting competition gives rise
to multiple phase transitions, in striking similitude with recent experiments
on Yb2Ti2O7 [Lhotel et al., Phys. Rev. B 89 224419 (2014)]. Considering the
effective Hamiltonian determined for Yb2Ti2O7, we provide, by combining a gamut
of numerical techniques, compelling evidence that such multiphase competition
is the long-sought missing key to understanding the intrinsic properties of
this material. As a corollary, our work offers a pertinent illustration of the
influence of chemical pressure in rare-earth pyrochlores.Comment: 9 page
Rovibrationally resolved photodissociation of HeH+
Accurate photodissociation cross sections have been obtained for the A-X
electronic transition of HeH+ using ab initio potential curves and dipole
transition moments. Partial cross sections have been evaluated for all
rotational transitions from the vibrational levels v"=0-11 and over the entire
accessible wavelength range 100-1129 Angstrom. Assuming a Boltzmann
distribution of the rovibrational levels of the X state, photodissociation
cross sections are presented for temperatures between 500 and 12,000 K. A
similar set of calculations was performed for the pure rovibrational
photodissociation in the X-X electronic ground state, but covering photon
wavelengths into the far infrared. Applications of the cross sections to the
destruction of HeH+in the early Universe and in UV-irradiated environments such
as primordial halos and protoplanetary disks are briefly discussed
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