1,980 research outputs found
Phonon Dispersion Effects and the Thermal Conductivity Reduction in GaAs/AlAs Superlattices
The experimentally observed order-of-magnitude reduction in the thermal
conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n)
superlattices is investigated theoretically for (2x2), (3x3) and (6x6)
structures using an accurate model of the lattice dynamics. The modification of
the phonon dispersion relation due to the superlattice geometry leads to
flattening of the phonon branches and hence to lower phonon velocities. This
effect is shown to account for a factor-of-three reduction in the thermal
conductivity with respect to bulk GaAs along the growth direction; the
remainder is attributable to a reduction in the phonon lifetime. The
dispersion-related reduction is relatively insensitive to temperature (100 < T
< 300K) and n. The phonon lifetime reduction is largest for the (2x2)
structures and consistent with greater interface scattering. The thermal
conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs
force constant differences than to those associated with molecular masses.Comment: 5 figure
Predictions for Impurity-Induced Tc Suppression in the High-Temperature Superconductors
We address the question of whether anisotropic superconductivity is
compatible with the evidently weak sensitivity of the critical temperature Tc
to sample quality in the high-Tc copper oxides. We examine this issue
quantitatively by solving the strong-coupling Eliashberg equations numerically
as well as analytically for s-wave impurity scattering within the second Born
approximation. For pairing interactions with a characteristically low energy
scale, we find an approximately universal dependence of the d-wave
superconducting transition temperature on the planar residual resistivity which
is independent of the details of the microscopic pairing. These results, in
conjunction with future systematic experiments, should help elucidate the
symmetry of the order parameter in the cuprates.Comment: 13 pages, 4 figures upon request, revtex version
Origin of Intrinsic Josephson Coupling in the Cuprates and Its Relation to Order Parameter Symmetry: An Incoherent Hopping Model
Experiments on the cuprate superconductors demonstrate that these materials
may be viewed as a stack of Josephson junctions along the c-direction. In this
paper, we present a model which describes this intrinsic Josephson coupling in
terms of incoherent quasiparticle hopping along the c-axis arising from
wave-function overlap, impurity-assisted hopping, and boson-assisted hopping.
We use this model to compute the magnitude and temperature T dependence of the
resulting Josephson critical current j_c (T) for s- and d-wave superconductors.
Contrary to other approaches, d-wave pairing in this model is compatible with
an intrinsic Josephson effect at all hole concentrations and leads to j_c (T)
\propto T at low T. By parameterizing our theory with c-axis resistivity data
from YBCO, we estimate j_c (T) for optimally doped and underdoped members of
this family. Our estimates suggest that further experiments on this compound
would be of great help in elucidating the validity of our model in general and
the pairing symmetry in particular. We also discuss the implications of our
model for LSCO and BSCCO.Comment: 28 pages, REVTEX, 5 compressed PostScript figures. Substantially
expanded and revised from the earlier version. To appear in Physica
Counterproductive Work Behaviors and Work Climate:The Role of an Ethically Focused Management Control System and Peers’ Self-Focused Behavior
The importance of curtailing undesirable behaviors and, ultimately, self-focused work climates in organizations is undeniable. This study examines how management control systems (MCSs), as a crucial part of a firm’s formal ethical infrastructure, can contribute to this objective. We conceptualize an ethically focused MCS as one that communicates ethical values and motivates employees to act accordingly. Our study is based on data from a sample of 120 department managers from 120 different firms. We show that department managers’ perceptions of the extent to which the MCS imposed on them is ethically focused are associated with a reduction in their counterproductive work behaviors (CWBs). We also examine department managers’ perceptions of peer managers’ self-focused behaviors, as a core part of a firm’s informal ethical infrastructure and find that peers’ behaviors are not associated with an increase in CWBs of the department manager. However, we find some evidence that the negative association between an ethically focused MCS and managers’ CWBs is limited when peers act in ways that are more self-focused. Finally, we find that CWBs of department managers are not only relevant in and of themselves, but they translate into more self-focused behaviors of department employees (as manifested in their work climates). Overall, this study suggests that, while including and emphasizing ethical content in the MCS is associated with less CWB and, in turn, with a work climate less focused on self, peer managers’ behaviors are also seemingly important
Active motion assisted by correlated stochastic torques
The stochastic dynamics of an active particle undergoing a constant speed and
additionally driven by an overall fluctuating torque is investigated. The
random torque forces are expressed by a stochastic differential equation for
the angular dynamics of the particle determining the orientation of motion. In
addition to a constant torque, the particle is supplemented by random torques
which are modeled as an Ornstein-Uhlenbeck process with given correlation time
. These nonvanishing correlations cause a persistence of the particles'
trajectories and a change of the effective spatial diffusion coefficient. We
discuss the mean square displacement as a function of the correlation time and
the noise intensity and detect a nonmonotonic dependence of the effective
diffusion coefficient with respect to both correlation time and noise strength.
A maximal diffusion behavior is obtained if the correlated angular noise
straightens the curved trajectories, interrupted by small pirouettes, whereby
the correlated noise amplifies a straightening of the curved trajectories
caused by the constant torque.Comment: 8 pages, 9 figure
Polarization correlations in the two--photon decay of hydrogen--like ions
Polarization properties of the photons emitted in the two-photon decay of
hydrogen-like ions are studied within the framework of the density matrix and
second-order perturbation theory. In particular, we derive the polarization
correlation function that gives the probability of the (two-photon) coincidence
measurement performed by polarization-sensitive detectors. Detailed
calculations of this function are performed for the
transition in neutral hydrogen as well as Xe and U ions. The
obtained results allow us to understand the influence of relativistic and
non-dipole effects on the polarization correlations in the bound-bound
two-photon transitions in heavy ions
Stochastic dynamics of resistive switching: fluctuations lead to optimal particle number
Resistive switching (RS) is one of the foremost candidates for building novel
types of non-volatile random access memories. Any practical implementation of
such a memory cell calls for a strong miniaturization, at which point
fluctuations start playing a role that cannot be neglected. A detailed
understanding of switching mechanisms and reliability is essential. For this
reason, we formulate a particle model based on the stochastic motion of oxygen
vacancies. It allows us to investigate fluctuations in the resistance states
of a switch with two active zones. The vacancies' dynamics are governed by a
master equation. Upon the application of a voltage pulse, the vacancies travel
collectively through the switch. By deriving a generalized Burgers equation we
can interpret this collective motion as nonlinear traveling waves, and
numerically verify this result. Further, we define binary logical states by
means of the underlying vacancy distributions, and establish a framework of
writing and reading such memory element with voltage pulses. Considerations
about the discriminability of these operations under fluctuations together
with the markedness of the RS effect itself lead to the conclusion, that an
intermediate vacancy number is optimal for performance
Identification of epidermal Pdx1 expression discloses different roles of Notch1 and Notch2 in murine KrasG12D-induced skin carcinogenesis in vivo
Background
The Ras and Notch signaling pathways are frequently activated during development to control many diverse cellular processes and are often dysregulated during tumorigenesis. To study the role of Notch and oncogenic Kras signaling in a progenitor cell population, Pdx1-Cre mice were utilized to generate conditional oncogenic KrasG12D mice with ablation of Notch1 and/or Notch2.
Methodology/Principal Findings
Surprisingly, mice with activated KrasG12D and Notch1 but not Notch2 ablation developed skin papillomas progressing to squamous cell carcinoma providing evidence for Pdx1 expression in the skin. Immunostaining and lineage tracing experiments indicate that PDX1 is present predominantly in the suprabasal layers of the epidermis and rarely in the basal layer. Further analysis of keratinocytes in vitro revealed differentiation-dependent expression of PDX1 in terminally differentiated keratinocytes. PDX1 expression was also increased during wound healing. Further analysis revealed that loss of Notch1 but not Notch2 is critical for skin tumor development. Reasons for this include distinct Notch expression with Notch1 in all layers and Notch2 in the suprabasal layer as well as distinctive p21 and β-catenin signaling inhibition capabilities.
Conclusions/Significance
Our results provide strong evidence for epidermal expression of Pdx1 as of yet not identified function. In addition, this finding may be relevant for research using Pdx1-Cre transgenic strains. Additionally, our study confirms distinctive expression and functions of Notch1 and Notch2 in the skin supporting the importance of careful dissection of the contribution of individual Notch receptors
Exchange Instabilities in Semiconductor Double Quantum Well Systems
We consider various exchange-driven electronic instabilities in semiconductor
double-layer systems in the absence of any external magnetic field. We
establish that there is no exchange-driven bilayer to monolayer charge transfer
instability in the double-layer systems. We show that, within the unrestricted
Hartree-Fock approximation, the low density stable phase (even in the absence
of any interlayer tunneling) is a quantum ``pseudospin rotated'' spontaneous
interlayer phase coherent spin-polarized symmetric state rather than the
classical Ising-like charge-transfer phase. The U(1) symmetry of the double
quantum well system is broken spontaneously at this low density quantum phase
transition, and the layer density develops quantum fluctuations even in the
absence of any interlayer tunneling. The phase diagram for the double quantum
well system is calculated in the carrier density--layer separation space, and
the possibility of experimentally observing various quantum phases is
discussed. The situation in the presence of an external electric field is
investigated in some detail using the
spin-polarized-local-density-approximation-based self-consistent technique and
good agreement with existing experimental results is obtained.Comment: 24 pages, figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng/preprint/ct.uu/ . Revised final
version to appear in PR
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