961 research outputs found
Broadening effects due to alloy scattering in Quantum Cascade Lasers
We report on calculations of broadening effects in QCL due to alloy
scattering. The output of numerical calculations of alloy broadened Landau
levels compare favorably with calculations performed at the self-consistent
Born approximation. Results for Landau level width and optical absorption are
presented. A disorder activated forbidden transition becomes significant in the
vicinity of crossings of Landau levels which belong to different subbands. A
study of the time dependent survival probability in the lowest Landau level of
the excited subband is performed. It is shown that at resonance the population
relaxation occurs in a subpicosecond scale.Comment: 7 pages, 8 figure
Poisson algebra of 2d dimensionally reduced gravity
Using a Lax pair based on twisted affine Kac-Moody and Virasoro
algebras, we deduce a r-matrix formulation of two dimensional reduced vacuum
Einstein's equations. Whereas the fundamental Poisson brackets are
non-ultralocal, they lead to pure c-number modified Yang-Baxter equations. We
also describe how to obtain classical observables by imposing reasonable
boundaries conditions.Comment: 16 pages, minor corrections. To appear in JHE
Characterizing the many-body localization transition through the entanglement spectrum
We numerically explore the many body localization (MBL) transition through
the lens of the {\it entanglement spectrum}. While a direct transition from
localization to thermalization is believed to obtain in the thermodynamic limit
(the exact details of which remain an open problem), in finite system sizes
there exists an intermediate `quantum critical' regime. Previous numerical
investigations have explored the crossover from thermalization to criticality,
and have used this to place a numerical {\it lower} bound on the critical
disorder strength for MBL. A careful analysis of the {\it high energy} part of
the entanglement spectrum (which contains universal information about the
critical point) allows us to make the first ever observation in exact numerics
of the crossover from criticality to MBL and hence to place a numerical {\it
upper bound} on the critical disorder strength for MBL.Comment: 4 pages+appendi
Many body localization and thermalization: insights from the entanglement spectrum
We study the entanglement spectrum in the many body localizing and
thermalizing phases of one and two dimensional Hamiltonian systems, and
periodically driven `Floquet' systems. We focus on the level statistics of the
entanglement spectrum as obtained through numerical diagonalization, finding
structure beyond that revealed by more limited measures such as entanglement
entropy. In the thermalizing phase the entanglement spectrum obeys level
statistics governed by an appropriate random matrix ensemble. For Hamiltonian
systems this can be viewed as evidence in favor of a strong version of the
eigenstate thermalization hypothesis (ETH). Similar results are also obtained
for Floquet systems, where they constitute a result `beyond ETH', and show that
the corrections to ETH governing the Floquet entanglement spectrum have
statistical properties governed by a random matrix ensemble. The particular
random matrix ensemble governing the Floquet entanglement spectrum depends on
the symmetries of the Floquet drive, and therefore can depend on the choice of
origin of time. In the many body localized phase the entanglement spectrum is
also found to show level repulsion, following a semi-Poisson distribution (in
contrast to the energy spectrum, which follows a Poisson distribution). This
semi-Poisson distribution is found to come mainly from states at high
entanglement energies. The observed level repulsion only occurs for interacting
localized phases. We also demonstrate that equivalent results can be obtained
by calculating with a single typical eigenstate, or by averaging over a
microcanonical energy window - a surprising result in the localized phase. This
discovery of new structure in the pattern of entanglement of localized and
thermalizing phases may open up new lines of attack on many body localization,
thermalization, and the localization transition.Comment: 17 pages, 20 figure
Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet
We demonstrate that local modulations of magnetic couplings have a profound
effect on the temperature dependence of the relaxation rate of optical magnons
in a wide class of antiferromagnets in which gapped excitations coexist with
acoustic spin waves. In a two-dimensional collinear antiferromagnet with an
easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode,
Gamma_imp=Gamma_0+A(TlnT)^2, greatly exceeds the magnon-magnon damping,
Gamma_m-m=BT^5, negligible at low temperatures. We measure the lifetime of
gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a
high-resolution neutron-resonance spin-echo technique and find experimental
data in close accord with the theoretical prediction. Similarly strong effects
of disorder in the three-dimensional case and in noncollinear antiferromagnets
are discussed.Comment: 4.5 pages + 2.5 pages supplementary material, published versio
Emergent particle-hole symmetry in spinful bosonic quantum Hall systems
When a fermionic quantum Hall system is projected into the lowest Landau
level, there is an exact particle-hole symmetry between filling fractions
and . We investigate whether a similar symmetry can emerge in bosonic
quantum Hall states, where it would connect states at filling fractions
and . We begin by showing that the particle-hole conjugate to a
composite fermion `Jain state' is another Jain state, obtained by reverse flux
attachment. We show how information such as the shift and the edge theory can
be obtained for states which are particle-hole conjugates. Using the techniques
of exact diagonalization and infinite density matrix renormalization group, we
study a system of two-component (i.e., spinful) bosons, interacting via a
-function potential. We first obtain real-space entanglement spectra
for the bosonic integer quantum Hall effect at , which plays the role of
a filled Landau level for the bosonic system. We then show that at
the system is described by a Jain state which is the particle-hole conjugate of
the Halperin (221) state at . We show a similar relationship between
non-singlet states at and . We also study the case of
, providing unambiguous evidence that the ground state is a composite
Fermi liquid. Taken together our results demonstrate that there is indeed an
emergent particle-hole symmetry in bosonic quantum Hall systems.Comment: 10 pages, 8 figures, 4 appendice
Composite-fermionization of bosons in rapidly rotating atomic traps
The non-perturbative effect of interaction can sometimes make interacting
bosons behave as though they were free fermions. The system of neutral bosons
in a rapidly rotating atomic trap is equivalent to charged bosons coupled to a
magnetic field, which has opened up the possibility of fractional quantum Hall
effect for bosons interacting with a short range interaction. Motivated by the
composite fermion theory of the fractional Hall effect of electrons, we test
the idea that the interacting bosons map into non-interacting spinless fermions
carrying one vortex each, by comparing wave functions incorporating this
physics with exact wave functions available for systems containing up to 12
bosons. We study here the analogy between interacting bosons at filling factors
with non-interacting fermions at for the ground state
as well as the low-energy excited states and find that it provides a good
account of the behavior for small , but interactions between fermions become
increasingly important with . At , which is obtained in the limit
, the fermionization appears to overcompensate for the
repulsive interaction between bosons, producing an {\em attractive}
interactions between fermions, as evidenced by a pairing of fermions here.Comment: 8 pages, 3 figures. Submitted to Phys. Rev.
Tunneling in Fractional Quantum Hall line junctions
We study the tunneling current between two counterpropagating edge modes
described by chiral Luttinger liquids when the tunneling takes place along an
extended region. We compute this current perturbatively by using a tunnel
Hamiltonian. Our results apply to the case of a pair of different
two-dimensional electron gases in the fractional quantum Hall regime separated
by a barrier, e. g. electron tunneling. We also discuss the case of strong
interactions between the edges, leading to nonuniversal exponents even in the
case of integer quantum Hall edges. In addition to the expected nonlinearities
due to the Luttinger properties of the edges, there are additional interference
patterns due to the finite length of the barrier.Comment: 7 pages, RevTex, 12 figs, submitted to Phys Rev
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