11,732 research outputs found
Terahertz magneto-spectroscopy of transient plasmas in semiconductors
Using synchronized near-infrared (NIR) and terahertz (THz) lasers, we have
performed picosecond time-resolved THz spectroscopy of transient carriers in
semiconductors. Specifically, we measured the temporal evolution of THz
transmission and reflectivity after NIR excitation. We systematically
investigated transient carrier relaxation in GaAs and InSb with varying NIR
intensities and magnetic fields. Using this information, we were able to
determine the evolution of the THz absorption to study the dynamics of
photocreated carriers. We developed a theory based on a Drude conductivity with
time-dependent density and density-dependent scattering lifetime, which
successfully reproduced the observed plasma dynamics. Detailed comparison
between experimental and theoretical results revealed a linear dependence of
the scattering frequency on density, which suggests that electron-electron
scattering is the dominant scattering mechanism for determining the scattering
time. In InSb, plasma dynamics was dramatically modified by the application of
a magnetic field, showing rich magneto-reflection spectra, while GaAs did not
show any significant magnetic field dependence. We attribute this to the small
effective masses of the carriers in InSb compared to GaAs, which made the
plasma, cyclotron, and photon energies all comparable in the density, magnetic
field, and wavelength ranges of the current study.Comment: 8 pages, 9 figures, submitted to Phys. Rev.
Non-Markovian finite-temperature two-time correlation functions of system operators: beyond the quantum regression theorem
An extremely useful evolution equation that allows systematically calculating
the two-time correlation functions (CF's) of system operators for non-Markovian
open (dissipative) quantum systems is derived. The derivation is based on
perturbative quantum master equation approach, so non-Markovian open quantum
system models that are not exactly solvable can use our derived evolution
equation to easily obtain their two-time CF's of system operators, valid to
second order in the system-environment interaction. Since the form and nature
of the Hamiltonian are not specified in our derived evolution equation, our
evolution equation is applicable for bosonic and/or fermionic environments and
can be applied to a wide range of system-environment models with any factorized
(separable) system-environment initial states (pure or mixed). When applied to
a general model of a system coupled to a finite-temperature bosonic environment
with a system coupling operator L in the system-environment interaction
Hamiltonian, the resultant evolution equation is valid for both L = L^+ and L
\neq L^+ cases, in contrast to those evolution equations valid only for L = L^+
case in the literature. The derived equation that generalizes the quantum
regression theorem (QRT) to the non-Markovian case will have broad applications
in many different branches of physics. We then give conditions on which the QRT
holds in the weak system-environment coupling case, and apply the derived
evolution equation to a problem of a two-level system (atom) coupled to a
finite-temperature bosonic environment (electromagnetic fields) with L \neq
L^+.Comment: To appear in the Journal of Chemical Physics (12 pages, 1 figure
Observation of Feshbach-like resonances in collisions between ultracold molecules
We observe magnetically tuned collision resonances for ultracold Cs2
molecules stored in a CO2-laser trap. By magnetically levitating the molecules
against gravity, we precisely measure their magnetic moment. We find an avoided
level crossing which allows us to transfer the molecules into another state. In
the new state, two Feshbach-like collision resonances show up as strong
inelastic loss features. We interpret these resonances as being induced by Cs4
bound states near the molecular scattering continuum. The tunability of the
interactions between molecules opens up novel applications such as controlled
chemical reactions and synthesis of ultracold complex molecules
On the Analytic Structure of the Quark Self-Energy in Nambu-Jona- Lasinio Models
The self-energy of quarks is investigated for various models which are
inspired by the Nambu--Jona-Lasinio (NJL) model. Including, beyond the
Hartree-Fock approximation, terms up to second-order in the quark interaction,
the real and imaginary parts of scalar and vector components of the self-energy
are discussed. The second-order contributions depend on the energy and momentum
of the quark under consideration. This leads to solutions of the Dirac equation
which are significantly different from those of a free quark or a quark with
constant effective mass, as obtained in the Hartree-Fock approximation.Comment: 15 pages LaTeX, 6 figures can be obtained from author
The Complete Characterization of Fourth-Order Symplectic Integrators with Extended-Linear Coefficients
The structure of symplectic integrators up to fourth-order can be completely
and analytical understood when the factorization (split) coefficents are
related linearly but with a uniform nonlinear proportional factor. The analytic
form of these {\it extended-linear} symplectic integrators greatly simplified
proofs of their general properties and allowed easy construction of both
forward and non-forward fourth-order algorithms with arbitrary number of
operators. Most fourth-order forward integrators can now be derived
analytically from this extended-linear formulation without the use of symbolic
algebra.Comment: 12 pages, 2 figures, submitted to Phys. Rev. E, corrected typo
Design and Construction of Compaction Grouting for Foundation Soil Improvements
This paper presents the design and construction of compaction grouting work completed for a tank replacement project in Portland, Oregon. The project site is located along the west bank of the Willamette River. The subsurface soils at the project site were determined to be highly susceptible to soil liquefaction and lateral spreading under a design earthquake event per the building code. Compaction grouting was designed and constructed to strengthen the foundation soils supporting the new steel tank that is 115 feet in diameter and 40 feet in height. The design of the compaction grouting was completed using the design guidelines outlined in ASCE/G-I Standard 53-10. Detailed quality assurance/quality control processes were implemented during grouting operations to account for the variability in soil conditions being grouted. Real time monitoring was also completed to evaluate the ground movement induced by the grouting process and its impact to adjacent structures and critical utilities. Pre- and post-grouting CPTs were completed to verify that the intended ground densification was achieved. A hydrostatic test was also completed with the tank filled with water. The tank foundation settlement under the hydrostatic test was found to range between ÂĽ to Âľ inches and met the acceptance criteria per API-650 and API-653 Standards
A new broken U(1)-symmetry in extreme type-II superconductors
A phase transition within the molten phase of the Abrikosov vortex system
without disorder in extreme type-II superconductors is found via large-scale
Monte-Carlo simulations. It involves breaking a U(1)-symmetry, and has a
zero-field counterpart, unlike vortex lattice melting. Its hallmark is the loss
of number-conservation of connected vortex paths threading the entire system
{\it in any direction}, driving the vortex line tension to zero. This tension
plays the role of a generalized ``stiffness'' of the vortex liquid, and serves
as a probe of the loss of order at the transition, where a weak specific heat
anomaly is found.Comment: 5 pages, 3 figure
Non-Markovian dynamics of a nanomechanical resonator measured by a quantum point contact
We study the dynamics of a nanomechanical resonator (NMR) subject to a
measurement by a low transparency quantum point contact (QPC) or tunnel
junction in the non-Markovian domain. We derive the non-Markovian
number-resolved (conditional) and unconditional master equations valid to
second order in the tunneling Hamiltonian without making the rotating-wave
approximation and the Markovian approximation, generally made for systems in
quantum optics. Our non-Markovian master equation reduces, in appropriate
limits, to various Markovian versions of master equations in the literature. We
find considerable difference in dynamics between the non-Markovian cases and
its Markovian counterparts. We also calculate the time-dependent transport
current through the QPC which contains information about the measured NMR
system. We find an extra transient current term proportional to the expectation
value of the symmetrized product of the position and momentum operators of the
NMR. This extra current term, with a coefficient coming from the combination of
the imaginary parts of the QPC reservoir correlation functions, has a
substantial contribution to the total transient current in the non-Markovian
case, but was generally ignored in the studies of the same problem in the
literature. Considering the contribution of this extra term, we show that a
significantly qualitative and quantitative difference in the total transient
current between the non-Markovian and the Markovian wide-band-limit cases can
be observed. Thus, it may serve as a witness or signature of the non-Markovian
features in the coupled NMR-QPC system.Comment: Accepted for publication in Physical Review B (20 pages, 13 figures
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