909 research outputs found
Shear viscosity of hot scalar field theory in the real-time formalism
Within the closed time path formalism a general nonperturbative expression is
derived which resums through the Bethe-Salpter equation all leading order
contributions to the shear viscosity in hot scalar field theory. Using a
previously derived generalized fluctuation-dissipation theorem for nonlinear
response functions in the real-time formalism, it is shown that the
Bethe-Salpeter equation decouples in the so-called (r,a) basis. The general
result is applied to scalar field theory with pure lambda*phi**4 and mixed
g*phi**3+lambda*phi**4 interactions. In both cases our calculation confirms the
leading order expression for the shear viscosity previously obtained in the
imaginary time formalism.Comment: Expanded introduction and conclusions. Several references and a
footnote added. Fig.5 and its discussion in the text modified to avoid double
counting. Signs in Eqs. (45) and (53) correcte
Beam divergence measurements of InGaN/GaN micro-array light-emitting diodes using confocal microscopy
The recent development of high-density, two-dimensional arrays of micrometer-sized InGaN/GaN light-emitting diodes (micro-LEDs) with potential applications from scientific instrumentation to microdisplays has created an urgent need for controlled manipulation of the light output from these devices. With directed light output these devices can be used in situations where collimated beams or light focused onto several thousand matrix points is desired. In order to do this effectively, the emission characteristics of the devices must be fully understood and characterized. Here we utilize confocal microscopy to directly determine the emission characteristics and angular beam divergences from the individual micro-LED elements. The technique is applied to both top (into air) and bottom (through substrate) emission in arrays of green (540 nm), blue (470 nm), and UV (370 nm) micro-LED devices, at distances of up to 50 ”m from the emission plane. The results are consistent with simple optical modeling of the expected beam profiles
Transport coefficients from the 2PI effective action
We show that the lowest nontrivial truncation of the two-particle irreducible
(2PI) effective action correctly determines transport coefficients in a weak
coupling or 1/N expansion at leading (logarithmic) order in several
relativistic field theories. In particular, we consider a single real scalar
field with cubic and quartic interactions in the loop expansion, the O(N) model
in the 2PI-1/N expansion, and QED with a single and many fermion fields.
Therefore, these truncations will provide a correct description, to leading
(logarithmic) order, of the long time behavior of these systems, i.e. the
approach to equilibrium. This supports the promising results obtained for the
dynamics of quantum fields out of equilibrium using 2PI effective action
techniques.Comment: 5 pages, explanation in introduction expanded, summary added; to
appear in PR
Boundary Effects on Dynamic Behavior of Josephson-Junction Arrays
The boundary effects on the current-voltage characteristics in
two-dimensional arrays of resistively shunted Josephson junctions are examined.
In particular, we consider both the conventional boundary conditions (CBC) and
the fluctuating twist boundary conditions (FTBC), and make comparison of the
obtained results. It is observed that the CBC, which have been widely adopted
in existing simulations, may give a problem in scaling, arising from rather
large boundary effects; the FTBC in general turn out to be effective in
reducing the finite-size effects, yielding results with good scaling behavior.
To resolve the discrepancy between the two boundary conditions, we propose that
the proper scaling in the CBC should be performed with the boundary data
discarded: This is shown to give results which indeed scale well and are the
same as those from the FTBC.Comment: RevTex, Final version to appear in Phys. Rev.
Benefits of active middle ear implants over hearing aids in patients with sloping high tone hearing loss: comparison with hearing aids
In questo studio retrospettivo, abbiamo confrontato i benefici oggettivi e soggettivi degli impianti attivi dellâorecchio medio (AMEI) rispetto alle tradizionali protesi acustiche (HA) nei pazienti con perdita dellâudito per le frequenze acute. Trentaquattro pazienti con ipoacusia neurosensoriale sono stati trattati con lâimpianto di AMEI. Tra questi, sei avevano un audiogramma âin discesaâ con perdita dellâudito per le frequenze acute, ed avevano usato per piĂč di sei mesi HA. Ă stata quindi eseguita una valutazione oggettiva, tramite lâaudiometria tonale e il test di riconoscimento delle parole, una versione coreana del âHearing in Noise Testâ (K-HINT), ed una valutazione soggettiva tramite il seguente questionario: Abbreviated Profile of Hearing Aid Benefit (APHAB). I pazienti sono stati sottoposti ai suddetti test in tre occasioni distinte: 1) prima della chirurgia, senza protesi; 2) prima della chirurgia, con HA; 3) tre mesi dopo lâimpianto di AMEI. Il guadagno medio per le alte frequenze (â„ 2 kHz) si Ăš rivelato migliore con AMEI che con HA. Sebbene il risultato non ha raggiunto un livello di significativitĂ statistica, gli impianti attivi dellâorecchio medio hanno mostrato un punteggio di riconoscimento delle parole superiore rispetto a HA. Ad ogni modo, il livello di comoda udibilitĂ al quale il punteggio di riconoscimento delle parole Ăš stato testato si Ăš rivelato significativamente piĂč basso con AMEI rispetto ad HA. Al K-HINT i pazienti con AMEI hanno mostrato un migliore riconoscimento rispetto ai risultati ottenuti con HA, sia in condizione di quiete sia di rumore. Gli score APAHB hanno rivelato che i pazienti erano piĂč soddisfatti con AMEI. Lâuso degli impianti attivi dellâorecchio medio in pazienti con perdita dellâudito per le frequenze acute ha permesso di ottenere risultati migliori rispetto allâutilizzo delle protesi tradizionali. Basandoci su questi dati, gli AMEI hanno offerto risultati oggettivi e soggettivi migliori, e pertanto, potrebbero rappresentare una valida alternativa per il trattamento delle ipoacusie con audiogramma in discesa
Towards a Nonequilibrium Quantum Field Theory Approach to Electroweak Baryogenesis
We propose a general method to compute -violating observables from
extensions of the standard model in the context of electroweak baryogenesis. It
is alternative to the one recently developed by Huet and Nelson and relies on a
nonequilibrium quantum field theory approach. The method is valid for all
shapes and sizes of the bubble wall expanding in the thermal bath during a
first-order electroweak phase transition. The quantum physics of -violation
and its suppression coming from the incoherent nature of thermal processes are
also made explicit.Comment: 19 pages, 1 figure available upon e-mail reques
Quantum phase transitions in superconducting arrays under external magnetic fields
We study the zero-temperature phase transitions of two-dimensional
superconducting arrays with both the self- and the junction capacitances in the
presence of external magnetic fields. We consider two kinds of excitations from
the Mott insulating phase: charge-dipole excitations and single-charge
excitations, and apply the second-order perturbation theory to find their
energies. The resulting phase boundaries are found to depend strongly on the
magnetic frustration, which measures the commensurate-incommensurate effects.
Comparison of the obtained values with those in recent experiment suggests the
possibility that the superconductor-insulator transition observed in experiment
may not be of the Berezinskii-Kosterlitz-Thouless type. The system is also
transformed to a classical three-dimensional XY model with the magnetic field
in the time-direction; this allows the analogy to bulk superconductors,
revealing the nature of the phase transitions.Comment: 9 pages including 7 figures, to appear in Phys. Rev.
XY model in small-world networks
The phase transition in the XY model on one-dimensional small-world networks
is investigated by means of Monte-Carlo simulations. It is found that
long-range order is present at finite temperatures, even for very small values
of the rewiring probability, suggesting a finite-temperature transition for any
nonzero rewiring probability. Nature of the phase transition is discussed in
comparison with the globally-coupled XY model.Comment: 5 pages, accepted in PR
Critical behavior of the frustrated antiferromagnetic six-state clock model on a triangular lattice
We study the anti-ferromagnetic six-state clock model with nearest neighbor
interactions on a triangular lattice with extensive Monte-Carlo simulations. We
find clear indications of two phase transitions at two different temperatures:
Below a chirality order sets in and by a thorough finite size scaling
analysis of the specific heat and the chirality correlation length we show that
this transition is in the Ising universality class (with a non-vanishing
chirality order parameter below ). At the spin-spin
correlation length as well as the spin susceptibility diverges according to a
Kosterlitz-Thouless (KT) form and spin correlations decay algebraically below
. We compare our results to recent x-ray diffraction experiments on the
orientational ordering of CFBr monolayers physisorbed on graphite. We argue
that the six-state clock model describes the universal feature of the phase
transition in the experimental system and that the orientational ordering
belongs to the KT universality class.Comment: 8 pages, 9 figure
Shear Viscosity in the O(N) Model
We compute the shear viscosity in the O(N) model at first nontrivial order in
the large N expansion. The calculation is organized using the 1/N expansion of
the 2PI effective action (2PI-1/N expansion) to next-to-leading order, which
leads to an integral equation summing ladder and bubble diagrams. We also
consider the weakly coupled theory for arbitrary N, using the three-loop
expansion of the 2PI effective action. In the limit of weak coupling and
vanishing mass, we find an approximate analytical solution of the integral
equation. For general coupling and mass, the integral equation is solved
numerically using a variational approach. The shear viscosity turns out to be
close to the result obtained in the weak-coupling analysis.Comment: 37 pages, few typos corrected; to appear in JHE
- âŠ