46,175 research outputs found
Correlations between pressure and bandwidth effects in metal-insulator transitions in manganites
The effect of pressure on the metal-insulator transition in manganites with a
broad range of bandwidths is investigated. A critical pressure is found at
which the metal-insulator transition temperature, T, reaches a maximum
value in every sample studied. The origin of this universal pressure and the
relation between the pressure effect and the bandwidth on the metal-insulator
transition are discussed
Pressure Induced Reentrant Electronic and Magnetic State in Pr0.7Ca0.3MnO3 Manganite
In PrCaMnO, pressure induces reentrant magnetic and
electronic state changes in the range 1 atm to 6 GPa. The
metal-insulator and magnetic transition temperatures coincide from 1 to 5
GPa, decouple outside of this range and do not change monotonically with
pressure. The effects may be explained by pressure tuned competition between
double exchange and super exchange. The insulating state induced by pressure
above 5 GPa is possibly ferromagnetic, different from the ferromagnetic
and antiferromagnetic phase-separated insulating state below 0.8 GPa
Amplifying ultraweak transitions in collective systems via quantum interference
We investigate laser-induced quantum interference phenomena in superradiance
processes and in an ensemble of initially excited type closely packed
three-level emitters. The lower doublet levels are pumped with a coherent laser
field. Due to constructive quantum interference effects, the superradiance
occurs on a much weaker atomic transition which is not the case in the absence
of the coherent driving. This result may be of visible relevance for enhancing
ultraweak transitions in atomic or atomic-like systems, respectively, or for
high-frequency lasing effects.Comment: 12 pages, 3 figure
Carrier-envelope phase dependence in single-cycle laser pulse propagation with the inclusion of counter-rotating terms
We focus on the propagation properties of a single-cycle laser pulse through
a two-level medium by numerically solving the full-wave Maxwell-Bloch
equations. The counter-rotating terms in the spontaneous emission damping are
included such that the equations of motion are slightly different from the
conventional Bloch equations. The counter-rotating terms can considerably
suppress the broadening of the pulse envelope and the decrease of the group
velocity rooted from dispersion. Furthermore, for incident single-cycle pulses
with envelope area 4, the time-delay of the generated soliton pulse from
the main pulse depends crucially on the carrier-envelope phase of the incident
pulse. This can be utilized to determine the carrier-envelope phase of the
single-cycle laser pulse.Comment: 6 pages, 5 figure
Non-Thermal Dark Matter from Cosmic Strings
Cosmic strings can be created in the early universe during symmetry-breaking
phase transitions, such as might arise if the gauge structure of the standard
model is extended by additional U(1) factors at high energies. Cosmic strings
present in the early universe form a network of long horizon-length segments,
as well as a population of closed string loops. The closed loops are unstable
against decay, and can be a source of non-thermal particle production. In this
work we compute the density of WIMP dark matter formed by the decay of gauge
theory cosmic string loops derived from a network of long strings in the
scaling regime or under the influence of frictional forces. We find that for
symmetry breaking scales larger than 10^10 GeV, this mechanism has the
potential to account for the observed relic density of dark matter. For
symmetry breaking scales lower than this, the density of dark matter created by
loop decays from a scaling string network lies below the observed value. In
particular, the cosmic strings originating from a U(1) gauge symmetry broken
near the electroweak scale, that could lead to a massive Z' gauge boson
observable at the LHC, produce a negligibly small dark matter relic density by
this mechanism.Comment: 22 pages, 4 figures, added discussion about boosted decay products
from loop cusp
Stability of a Fully Magnetized Ferromagnetic state in Repulsively Interacting Ultracold Fermi Gases
We construct a variational wave function to study whether a fully polarized
Fermi sea is energetically stable against a single spin flip. Our variational
wave function contains sufficient short-range correlation at least to the same
level as Gutzwiller's projected wave function. For Hubbard lattice model and
continuum model with pure repulsive interaction, we show a fully polarized
Fermi sea is generally unstable even when the repulsive strength becomes
infinite. While for a resonance model, ferromagnetic state is possible if the
s-wave scattering length is positive and sufficiently large, and the system is
prepared in scattering state orthogonal to molecular bound state. However, we
can not rule out the possibility that more exotic correlation can destabilize
the ferromagnetic state.Comment: 4 pages, 3 figure
Maximum likelihood based estimation of frequency and phase offset in DCT OFDM systems under non-circular transmissions: algorithms, analysis and comparisons
Recently, the advantages of the discrete cosine transform (DCT) based orthogonal frequency-division multiplexing (OFDM) have come to the light. We thus consider DCT- OFDM with non-circular transmission (our results cover circular transmission as well) and present two blind joint maximum- likelihood frequency offset and phase offset estimators. Both our theoretical analysis and numerical comparisons reveal new advantages of DCT-OFDM over the traditional discrete Fourier transform (DFT) based OFDM. These advantages, as well as those already uncovered in the early works on DCT-OFDM, support the belief that DCT-OFDM is a promising multi-carrier modulation scheme
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