54 research outputs found
The crossover between lasing and polariton condensation in optical microcavities
We study a model of a photon mode dipole-coupled to a medium of two-level
oscillators in a microcavity in the presence of dephasing processes introduced
by coupling to external baths. Decoherence processes can be classified as
pair-breaking or non-pair-breaking in analogy with magnetic or non-magnetic
impurities in superconductors. In the absence of dephasing, the ground state of
the model is a polariton condensate with a gap in the excitation spectrum.
Increase of the pair-breaking parameter reduces the gap, which becomes
zero at a critical value ; for large , the conventional
laser regime is obtained in a way that demonstrates its close analogy to a
gapless superconductor. In contrast, weak non-pair-breaking processes have no
qualitative effect on the condensate or the existence of a gap, although they
lead to inhomogeneous broadening of the excitations
Condensation and Lasing of Microcavity Polaritons: Comparison between two Models
Condensation of microcavity polaritons and the substantial influence of
pair-breaking disorder and decoherence leading to a laser regime has been
recently considered using two different models: a model for direct two band
excitons in a disordered quantum well coupled to light and a model where the
cavity mode couples instead to a medium of localised excitons, represented by
two-level oscillators in the presence of dephasing processes. Even if
complementary from the point of view of assumptions, the models share most of
the main conclusions and show similar phase diagrams. The issue whether
excitons are propagating or localised seems secondary for the polariton
condensation and the way in which pair-breaking disorder and decoherence
processes influence the condensation and drive the microcavity into a lasing
regime is, within the approximations used in each model, generic. The reasons
for the similarities between the two physical situations are analysed and
explained.Comment: Proceeding of the First International Conference on Spontaneous
Coherence in Excitonic Systems (ICSCE'04); 7 pages, 2 eps figure
Branching Transport Model of Alkali-Halide Scintillators
We measure the time dependence of the scintillator light-emission pulses in
NaI(Tl) crystals at different temperatures, after activation by gamma rays. We
confirm that there are two main nonexponential components to the time decay and
find that their amplitude ratio shows Arrhenius temperature dependence. We
explain these nonexponential components as arising from two competing
mechanisms of carrier transport to the Tl activation levels. The total light
output of the NaI(Tl) detectors shows a linear temperature dependence explained
by our model
Bose condensation in a model microcavity
We study the equilibrium properties of a system of dipole-active excitons
coupled to a single photon mode at fixed total excitation. Treating the
presence or absence of a trapped exciton as a two-level system produces a model
that is exactly soluble. It gives a simple description of the physics of
polariton condensation in optical cavities beyond the low-density bosonic
regime.Comment: 5 pages, 3 figures, uses RevTeX and psfig. Revised version:
(1)Corrects an error in our treatment of the constraint, leading to a
rescaled transition temperature, and (2)Extends our discussion of the
relevance of the model to real system
Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors
We calculate the c-axis infrared conductivity in
underdoped cuprate superconductors for spinfluctuation exchange scattering
within the CuO-planes including a phenomenological d-wave pseudogap of
amplitude . For temperatures decreasing below a temperature , a gap for develops in in the
incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting'
behavior, i.e. it increases for low temperatures above the constant behavior
for . We find that the pseudogap structure in the in-plane optical
conductivity is about twice as big as in the interplane conductivity
, in qualitative agreement with experiment. This is a
consequence of the fact that the spinfluctuation exchange interaction is
suppressed at low frequencies as a result of the opening of the pseudogap.
While the c-axis conductivity in the underdoped regime is described best by
incoherent transmission, in the overdoped regime coherent conductance gives a
better description.Comment: to be published in Phys. Rev. B (November 1, 1999
Thermal Hall conductivity of marginal Fermi liquids subject to out-of plane impurities in high- cuprates
The effect of out-of-plane impurities on the thermal Hall conductivity
of in-plane marginal-Fermi-liquid (MFL) quasiparticles in
high- cuprates is examined by following the work on electrical Hall
conductivity by Varma and Abraham [Phys. Rev. Lett. 86, 4652
(2001)]. It is shown that the effective Lorentz force exerted by these
impurities is a weak function of energies of the MFL quasiparticles, resulting
in nearly the same temperature dependence of and ,
indicative of obedience of the Wiedemann-Franz law. The inconsistency of the
theoretical result with the experimental one is speculated to be the
consequence of the different amounts of out-of-plane impurities in the two
YBaCuO samples used for the and measurements.Comment: 5 pages, 2 eps figures; final versio
Phase locking in quantum and classical oscillators: polariton condensates, lasers, and arrays of Josephson junctions
We connect three phenomena in which a coherent electromagnetic field could be
generated: polariton condensation, phase-locking in arrays of underdamped
Josephson junctions, and lasing. All these phenomena have been described using
Dicke-type models of spins coupled to a single photon mode. These descriptions
may be distinguished by whether the spins are quantum or classical, and whether
they are strongly or weakly damped.Comment: 6 pages, RevTex. To appear in a special edition of Solid State
Communications on "Quantum Phases at the Nanoscale
Mass-renormalized electronic excitations at (, 0) in the superconducting state of
Using high-resolution angle-resolved photoemission spectroscopy on
, we have made the first observation of a
mass renormalization or "kink" in the E vs. dispersion relation
localized near . Compared to the kink observed along the nodal
direction, this new effect is clearly stronger, appears at a lower energy near
40 meV, and is only present in the superconducting state. The kink energy scale
defines a cutoff below which well-defined quasiparticle excitations occur. This
effect is likely due to coupling to a bosonic excitation, with the most
plausible candidate being the magnetic resonance mode observed in inelastic
neutron scattering
Optical absorption in the strong coupling limit of Eliashberg theory
We calculate the optical conductivity of superconductors in the
strong-coupling limit. In this anomalous limit the typical energy scale is set
by the coupling energy, and other energy scales such as the energy of the
bosons mediating the attraction are negligibly small. We find a universal
frequency dependence of the optical absorption which is dominated by bound
states and differs significantly from the weak coupling results. A comparison
with absorption spectra of superconductors with enhanced electron-phonon
coupling shows that typical features of the strong-coupling limit are already
present at intermediate coupling.Comment: 10 pages, revtex, 4 uuencoded figure
Low-frequency incommensurate magnetic response in strongly correlated systems
It is shown that in the t-J model of Cu-O planes at low frequencies the
dynamic spin structure factor is peaked at incommensurate wave vectors
(1/2+-delta,1/2)$, (1/2,1/2+-delta). The incommensurability is connected with
the momentum dependencies of the magnon frequency and damping near the
antiferromagnetic wave vector. The behavior of the incommensurate peaks is
similar to that observed in La_{2-x}(Ba,Sr)_xCuO_{4+y} and YBa_2Cu_3O_{7-y}:
for hole concentrations 0.02<x<=0.12 we find that delta is nearly proportional
to x, while for x>0.12 it tends to saturation. The incommensurability
disappears with increasing temperature. Generally the incommensurate magnetic
response is not accompanied by an inhomogeneity of the carrier density.Comment: 4 pages, 4 figure
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