14,199 research outputs found
Asymmetric tunneling, Andreev reflection and dynamic conductance spectra in strongly correlated metals
Landau Fermi liquid theory predicts that the differential conductivity
between metallic point and metal is a symmetric function of voltage bias V.
This symmetry holds if the particle-hole symmetry is preserved. We show that
the situation can be different when one of the two metals is a strongly
correlated one whose electronic system can be represented by a heavy fermion
liquid. When the heavy fermion liquid undergoes fermion condensation quantum
phase transition, the particle-hole symmetry is violated making both the
differential tunneling conductivity and dynamic conductance asymmetric as a
function of applied voltage. This asymmetry can be observed when the strongly
correlated metal is either normal or superconducting. We show that at small
values of $V the asymmetric part of the dynamic conductance is a linear
function of V and inversely proportional to the maximum value of the gap and
does not depend on temperature provided that metal is superconducting, when it
becomes normal the asymmetric part diminishes at elevated temperatures.Comment: 8 pages, 7 figure
Pair Production Beyond the Schwinger Formula in Time-Dependent Electric Fields
We investigate electron-positron pair production in pulse-shaped electric
background fields using a non-Markovian quantum kinetic equation. We identify a
pulse-length range for subcritical fields still in the nonperturbative regime
where the number of produced pairs significantly exceeds that of a naive
expectation based on the Schwinger formula. From a conceptual viewpoint, we
find a remarkable quantitative agreement between the (real-time) quantum
kinetic approach and the (imaginary-time) effective action approach.Comment: 5 pages, 3 figures. Typos corrected and references added, PRD Versio
Band-aid for information loss from black holes
We summarize, simplify and extend recent work showing that small deviations
from exact thermality in Hawking radiation, first uncovered by Kraus and
Wilczek, have the capacity to carry off the maximum information content of a
black hole. This goes a considerable way toward resolving a long-standing
"information-loss paradox"
Finite-size fluctuations and photon statistics near the polariton condensation transition in a single-mode microcavity
We consider polariton condensation in a generalized Dicke model, describing a
single-mode cavity containing quantum dots, and extend our previous mean-field
theory to allow for finite-size fluctuations. Within the fluctuation-dominated
regime the correlation functions differ from their (trivial) mean-field values.
We argue that the low-energy physics of the model, which determines the photon
statistics in this fluctuation-dominated crossover regime, is that of the
(quantum) anharmonic oscillator. The photon statistics at the crossover are
different in the high- and low- temperature limits. When the temperature is
high enough for quantum effects to be neglected we recover behavior similar to
that of a conventional laser. At low enough temperatures, however, we find
qualitatively different behavior due to quantum effects.Comment: 12 pages, 5 figures. v2: Revised version with minor corrections
(typos, added reference, correction in argument following Eq. 25). v3:
further typos correcte
Virtual Processes and Superradiance in Spin-Boson Models
We consider spin-boson models composed by a single bosonic mode and an
ensemble of identical two-level atoms. The situation where the coupling
between the bosonic mode and the atoms generates real and virtual processes is
studied, where the whole system is in thermal equilibrium with a reservoir at
temperature . Phase transitions from ordinary fluorescence to
superradiant phase in three different models is investigated. First a model
where the coupling between the bosonic mode and the atom is via the
pseudo-spin operator is studied. Second, we investigate the
generalized Dicke model, introducing different coupling constants between the
single mode bosonic field and the environment, and for rotating
and counter-rotating terms, respectively. Finally it is considered a modified
version of the generalized Dicke model with intensity-dependent coupling in the
rotating terms. In the first model the zero mode contributes to render the
canonical entropy a negative quantity for low temperatures. The last two models
presents phase transitions, even when only Hamiltonian terms which generates
virtual processes are considered
Fermion condensation: a strange idea successfully explaining behavior of numerous objects in Nature
Strongly correlated Fermi systems are among the most intriguing, best
experimentally studied and fundamental systems in physics. These are, however,
in defiance of theoretical understanding. The ideas based on the concepts like
Kondo lattice and involving quantum and thermal fluctuations at a quantum
critical point have been used to explain the unusual physics. Alas, being
suggested to describe one property, these approaches fail to explain the
others. This means a real crisis in theory suggesting that there is a hidden
fundamental law of nature, which remains to be recognized. A theory of fermion
condensation quantum phase transition, preserving the extended quasiparticles
paradigm and intimately related to unlimited growth of the effective mass as a
function of temperature, magnetic field etc, is capable to resolve the problem.
We discuss the construction of the theory and show that it delivers theoretical
explanations of vast majority of experimental results in strongly correlated
systems such as heavy-fermion metals and quasi-two-dimensional Fermi systems.Comment: 12 pages, 14 figures, Invited talk at Bogolyubov Kyiv Conference,
Modern Problems of Theoretical and Mathematical Physics, 2009, Kyiv, Ukrain
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