4,741 research outputs found
Theory of integer quantum Hall polaritons in graphene
We present a theory of the cavity quantum electrodynamics of the graphene
cyclotron resonance. By employing a canonical transformation, we derive an
effective Hamiltonian for the system comprised of two neighboring Landau levels
dressed by the cavity electromagnetic field (integer quantum Hall polaritons).
This generalized Dicke Hamiltonian, which contains terms that are quadratic in
the electromagnetic field and respects gauge invariance, is then used to
calculate thermodynamic properties of the quantum Hall polariton system.
Finally, we demonstrate that the generalized Dicke description fails when the
graphene sheet is heavily doped, i.e. when the Landau level spectrum of 2D
massless Dirac fermions is approximately harmonic. In this case we `integrate
out' the Landau levels in valence band and obtain an effective Hamiltonian for
the entire stack of Landau levels in conduction band, as dressed by strong
light-matter interactions.Comment: 20 pages, 7 figure
Nonequilibrium spin-dependent phenomena in mesoscopic superconductor-normal metal tunnel structures
We analyze the broad range of spin-dependent nonequilibrium transport
properties of hybrid systems composed of a normal region tunnel coupled to two
superconductors with exchange fields induced by the proximity to thin
ferromagnetic layers and highlight its functionalities. By calculating the
quasiparticle distribution functions in the normal region we find that they are
spin-dependent and strongly sensitive to the relative angle between exchange
fields in the two superconductors. The impact of inelastic collisions on their
properties is addressed. As a result, the electric current flowing through the
system is found to be strongly dependent on the relative angle between exchange
fields, giving rise to a huge value of magnetoresistance. Moreover, the current
presents a complete spin-polarization in a wide range of bias voltages, even in
the quasiequilibrium case. In the nonequilibrium limit we parametrize the
distributions with an ``effective`` temperature, which turns out to be strongly
spin-dependent, though quite sensitive to inelastic collisions. By tunnel
coupling the normal region to an additional superconducting electrode we show
that it is possible to implement a spin-polarized current source of both spin
species, depending on the bias voltages applied.Comment: Published version: 12 pages, 14 figures; new text added and one
figure modifie
Two-state wave packet for strong field-free molecular orientation
We demonstrate strong laser-field-free orientation of absolute-ground-state
carbonyl sulfide molecules. The molecules are oriented by the combination of a
485-ps-long non-resonant laser pulse and a weak static electric field. The
edges of the laser pulse create a coherent superposition of two rotational
states resulting in revivals of strong transient molecular orientation after
the laser pulse. The experimentally attained degree of orientation of 0.6
corresponds to the theoretical maximum for mixing of the two states. Switching
off the dc field would provide the same orientation completely field-free
Detecting phonon blockade with photons
Measuring the quantum dynamics of a mechanical system, when few phonons are
involved, remains a challenge. We show that a superconducting microwave
resonator linearly coupled to the mechanical mode constitutes a very powerful
probe for this scope. This new coupling can be much stronger than the usual
radiation pressure interaction by adjusting a gate voltage. We focus on the
detection of phonon blockade, showing that it can be observed by measuring the
statistics of the light in the cavity. The underlying reason is the formation
of an entangled state between the two resonators. Our scheme realizes a
phonotonic Josephson junction, giving rise to coherent oscillations between
phonons and photons as well as a self-trapping regime for a coupling smaller
than a critical value. The transition from the self-trapping to the oscillating
regime is also induced dynamically by dissipation.Comment: 6 pages, 5 figure
Entanglement Trapping in Structured Environments
The entanglement dynamics of two independent qubits each embedded in a
structured environment under conditions of inhibition of spontaneous emission
is analyzed, showing entanglement trapping. We demonstrate that entanglement
trapping can be used efficiently to prevent entanglement sudden death. For the
case of realistic photonic band-gap materials, we show that high values of
entanglement trapping can be achieved. This result is of both fundamental and
applicative interest since it provides a physical situation where the
entanglement can be preserved and manipulated, e.g. by Stark-shifting the qubit
transition frequency outside and inside the gap.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Lett. on Friday 16 May
200
Luther-Emery Phase and Atomic-Density Waves in a Trapped Fermion Gas
The Luther-Emery liquid is a state of matter that is predicted to occur in
one-dimensional systems of interacting fermions and is characterized by a
gapless charge spectrum and a gapped spin spectrum. In this Letter we discuss a
realization of the Luther-Emery phase in a trapped cold-atom gas. We study by
means of the density-matrix renormalization-group technique a two-component
atomic Fermi gas with attractive interactions subject to parabolic trapping
inside an optical lattice. We demonstrate how this system exhibits compound
phases characterized by the coexistence of spin pairing and atomic-density
waves. A smooth crossover occurs with increasing magnitude of the atom-atom
attraction to a state in which tightly bound spin-singlet dimers occupy the
center of the trap. The existence of atomic-density waves could be detected in
the elastic contribution to the light-scattering diffraction pattern.Comment: 10 pages, 3 figures, 1 Table, submitted to Phys. Rev. on July 25th
200
The PEP Survey: Infrared Properties of Radio-Selected AGN
By exploiting the VLA-COSMOS and the Herschel-PEP surveys, we investigate the
Far Infrared (FIR) properties of radio-selected AGN. To this purpose, from
VLA-COSMOS we considered the 1537, F[1.4 GHz]>0.06 mJy sources with a reliable
redshift estimate, and sub-divided them into star-forming galaxies and AGN
solely on the basis of their radio luminosity. The AGN sample is complete with
respect to radio selection at all z<~3.5. 832 radio sources have a counterpart
in the PEP catalogue. 175 are AGN. Their redshift distribution closely
resembles that of the total radio-selected AGN population, and exhibits two
marked peaks at z~0.9 and z~2.5. We find that the probability for a
radio-selected AGN to be detected at FIR wavelengths is both a function of
radio power and redshift, whereby powerful sources are more likely to be FIR
emitters at earlier epochs. This is due to two distinct effects: 1) at all
radio luminosities, FIR activity monotonically increases with look-back time
and 2) radio activity of AGN origin is increasingly less effective at
inhibiting FIR emission. Radio-selected AGN with FIR emission are
preferentially located in galaxies which are smaller than those hosting
FIR-inactive sources. Furthermore, at all z<~2, there seems to be a
preferential (stellar) mass scale M ~[10^{10}-10^{11}] Msun which maximizes the
chances for FIR emission. We find such FIR (and MIR) emission to be due to
processes indistinguishable from those which power star-forming galaxies. It
follows that radio emission in at least 35% of the entire AGN population is the
sum of two contributions: AGN accretion and star-forming processes within the
host galaxy.Comment: 13 pages, 14 figures, to appear in MNRA
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