12,982 research outputs found
Difference of optical conductivity between one- and two-dimensional doped nickelates
We study the optical conductivity in doped nickelates, and find the dramatic
difference of the spectrum in the gap (\alt4 eV) between one- (1D)
and two-dimensional (2D) nickelates. The difference is shown to be caused by
the dependence of hopping integral on dimensionality. The theoretical results
explain consistently the experimental data in 1D and
2D nickelates, YCaBaNiO and LaSrNiO,
respectively. The relation between the spectrum in the X-ray aborption
experiments and the optical conductivity in LaSrNiO is
discussed.Comment: RevTeX, 4 pages, 4 figure
A Variational Principle Based Study of KPP Minimal Front Speeds in Random Shears
Variational principle for Kolmogorov-Petrovsky-Piskunov (KPP) minimal front
speeds provides an efficient tool for statistical speed analysis, as well as a
fast and accurate method for speed computation. A variational principle based
analysis is carried out on the ensemble of KPP speeds through spatially
stationary random shear flows inside infinite channel domains. In the regime of
small root mean square (rms) shear amplitude, the enhancement of the ensemble
averaged KPP front speeds is proved to obey the quadratic law under certain
shear moment conditions. Similarly, in the large rms amplitude regime, the
enhancement follows the linear law. In particular, both laws hold for the
Ornstein-Uhlenbeck process in case of two dimensional channels. An asymptotic
ensemble averaged speed formula is derived in the small rms regime and is
explicit in case of the Ornstein-Uhlenbeck process of the shear. Variational
principle based computation agrees with these analytical findings, and allows
further study on the speed enhancement distributions as well as the dependence
of enhancement on the shear covariance. Direct simulations in the small rms
regime suggest quadratic speed enhancement law for non-KPP nonlinearities.Comment: 28 pages, 14 figures update: fixed typos, refined estimates in
section
Interplay between Superconductivity and Antiferromagnetism in a Multi-layered System
Based on a microscopic model, we study the interplay between
superconductivity and antiferromagnetism in a multi-layered system, where two
superconductors are separated by an antiferromagnetic region. Within a
self-consistent mean-field theory, this system is solved numerically. We find
that the antiferromagnetism in the middle layers profoundly affects the
supercurrent flowing across the junction, while the phase difference across the
junction influences the development of antiferromagnetism in the middle layers.
This study may not only shed new light on the mechanism for high-
superconductors, but also bring important insights to building
Josephson-junction-based quantum devices, such as SQUID and superconducting
qubit.Comment: 4+ pages, 5 figures, Accepted for publication in Phys. Rev.
Quantum refrigerator driven by current noise
We proposed a scheme to implement a self-contained quantum refrigerator
system composed of three rf-SQUID qubits, or rather, flux-biased phase qubits.
The three qubits play the roles of the target, the refrigerator and the heat
engine respectively. We provide different effective temperatures for the three
qubits, by imposing external current noises of different strengths. The
differences of effective temperatures give rise to the flow of free energy and
that drives the refrigerator system to cool down the target. We also show that
the efficiency of the system approaches the Carnot efficiency.Comment: 5 pages, 1 figur
Statistical Analysis of a Semilinear Hyperbolic System Advected by a White in Time Random Velocity Field
We study a system of semilinear hyperbolic equations passively advected by
smooth white noise in time random velocity fields. Such a system arises in
modeling non-premixed isothermal turbulent flames under single-step kinetics of
fuel and oxidizer. We derive closed equations for one-point and multi-point
probability distribution functions (PDFs) and closed form analytical formulas
for the one point PDF function, as well as the two-point PDF function under
homogeneity and isotropy. Exact solution formulas allows us to analyze the
ensemble averaged fuel/oxidizer concentrations and the motion of their level
curves. We recover the empirical formulas of combustion in the thin reaction
zone limit and show that these approximate formulas can either underestimate or
overestimate average concentrations when reaction zone is not tending to zero.
We show that the averaged reaction rate slows down locally in space due to
random advection induced diffusion; and that the level curves of ensemble
averaged concentration undergo diffusion about mean locations.Comment: 18 page
Observations of Feedback from Radio-Quiet Quasars: I. Extents and Morphologies of Ionized Gas Nebulae
Black hole feedback -- the strong interaction between the energy output of
supermassive black holes and their surrounding environments -- is routinely
invoked to explain the absence of overly luminous galaxies, the black hole vs.
bulge correlations and the similarity of black hole accretion and star
formation histories. Yet direct probes of this process in action are scarce and
limited to small samples of active nuclei. We present Gemini IFU observations
of the distribution of ionized gas around luminous, obscured, radio-quiet (RQ)
quasars at z~0.5. We detect extended ionized gas nebulae via [O III]5007
emission in every case, with a mean diameter of 28 kpc. These nebulae are
nearly perfectly round. The regular morphologies of nebulae around RQ quasars
are in striking contrast with lumpy or elongated nebulae seen around radio
galaxies at low and high redshifts. We present the uniformly measured
size-luminosity relationship of [O III] nebulae around Seyfert 2 galaxies and
type 2 quasars spanning 6 orders of magnitude in luminosity and confirm the
flat slope of the correlation (R ~ L^{0.25+/-0.02}). We find a universal
behavior of the [O III]/H-beta ratio in our entire RQ quasar sample: it
persists at a constant value (~10) in the central regions, until reaching a
"break" isophotal radius ranging from 4 to 11 kpc where it starts to decrease.
We propose a model of clumpy nebulae in which clouds that produce line emission
transition from being ionization-bounded at small distances from the quasar to
being matter-bounded in the outer parts of the nebula, which qualitatively
explains the observed line ratio and surface brightness profiles. It is
striking that we see such smooth and round large-scale gas nebulosities in this
sample, which are inconsistent with illuminated merger debris and which we
suggest may be the signature of accretion energy from the nucleus reaching gas
at large scales.Comment: 44 pages, 11 figures, 3 tables. Accepted for publication in MNRA
Observations of Feedback from Radio-Quiet Quasars - II. Kinematics of Ionized Gas Nebulae
The prevalence and energetics of quasar feedback is a major unresolved
problem in galaxy formation theory. In this paper, we present Gemini Integral
Field Unit observations of ionized gas around eleven luminous, obscured,
radio-quiet quasars at z~0.5 out to ~15 kpc from the quasar; specifically, we
measure the kinematics and morphology of [O III]5007 emission. The round
morphologies of the nebulae and the large line-of-sight velocity widths (with
velocities containing 80% of the emission as high as 1000 km/s combined with
relatively small velocity difference across them (from 90 to 520 km/s) point
toward wide-angle quasi-spherical outflows. We use the observed velocity widths
to estimate a median outflow velocity of 760 km/s, similar to or above the
escape velocities from the host galaxies. The line-of-sight velocity dispersion
declines slightly toward outer parts of the nebulae (by 3% per kpc on average).
The majority of nebulae show blueshifted excesses in their line profiles across
most of their extents, signifying gas outflows. For the median outflow
velocity, we find a kinetic energy flow between 4x10^{44} and 3x10^{45} erg/s
and mass outflow rate between 2000 and 20000 Msun/yr. These values are large
enough for the observed quasar winds to have a significant impact on their host
galaxies. The median rate of converting bolometric luminosity to kinetic energy
of ionized gas clouds is ~2%. We report four new candidates for "super-bubbles"
-- outflows that may have broken out of the denser regions of the host galaxy.Comment: 23 pages, 10 figures, 2 tables, accepted for publication in MNRA
Decoherence and the retrieval of lost information
We found that in contrast with the common premise, a measurement on the
environment of an open quantum system can {\em reduce} its decoherence rate. We
demonstrate it by studying an example of indirect qubit's measurement, where
the information on its state is hidden in the environment. This information is
extracted by a distant device, coupled with the environment. We also show that
the reduction of decoherence generated by this device, is accompanied with
diminution of the environmental noise in a vicinity of the qubit. An
interpretation of these results in terms of quantum interference on large
scales is presented.Comment: 9 pages, 8 figures, additional explanations added, Phys. Rev. B, in
pres
Energy-efficiency improvements for optical access
This article discusses novel approaches to improve energy efficiency of different optical access technologies, including time division multiplexing passive optical network (TDM-PON), time and wavelength division multiplexing PON (TWDM-PON), point-to-point (PTP) access network, wavelength division multiplexing PON (WDM-PON), and orthogonal frequency division multiple access PON (OFDMA-PON). These approaches include cyclic sleep mode, energy-efficient bit interleaving protocol, power reduction at component level, or frequency band selection. Depending on the target optical access technology, one or a combination of different approaches can be applied
- …