2,386 research outputs found
Thermodynamics of an incommensurate quantum crystal
We present a simple theory of the thermodynamics of an incommensurate quantum
solid. The ground state of the solid is assumed to be an incommensurate
crystal, with quantum zero-point vacancies and interstitials and thus a
non-integer number of atoms per unit cell. We show that the low temperature
variation of the net vacancy concentration should be as , and that the
first correction to the specific heat due to this varies as ; these are
quite consistent with experiments on solid He. We also make some
observations about the recent experimental reports of ``supersolidity'' in
solid He that motivate a renewed interest in quantum crystals.Comment: revised, new title, somewhat expande
Preferential adsorption of high density lipoprotein (HDL) in blood plasma/polymer interaction
A few studies on the adsorption of plasma proteins to polymeric surfaces show that major plasma proteins: albumin (Alb), fibrinogen (Fb) and immunoglobulin (IgG) are adsorbed in much smaller quantities from plasma than from protein solutions (1,2). Present results show that this difference in adsorption is due to the preferential adsorption of high density lipoprotein from plasma onto the material surfaces studied (PVC and PS)
Non-perturbative renormalization group analysis of nonlinear spiking networks
The critical brain hypothesis posits that neural circuits may operate close
to critical points of a phase transition, which has been argued to have
functional benefits for neural computation. Theoretical and computational
studies arguing for or against criticality in neural dynamics largely rely on
establishing power laws or scaling functions of statistical quantities, while a
proper understanding of critical phenomena requires a renormalization group
(RG) analysis. However, neural activity is typically non-Gaussian, nonlinear,
and non-local, rendering models that capture all of these features difficult to
study using standard statistical physics techniques. Here, we overcome these
issues by adapting the non-perturbative renormalization group (NPRG) to work on
(symmetric) network models of stochastic spiking neurons. By deriving a pair of
Ward-Takahashi identities and making a ``local potential approximation,'' we
are able to calculate non-universal quantities such as the effective firing
rate nonlinearity of the network, allowing improved quantitative estimates of
network statistics. We also derive the dimensionless flow equation that admits
universal critical points in the renormalization group flow of the model, and
identify two important types of critical points: in networks with an absorbing
state there is Directed Percolation (DP) fixed point corresponding to a
non-equilibrium phase transition between sustained activity and extinction of
activity, and in spontaneously active networks there is a \emph{complex valued}
critical point, corresponding to a spinodal transition observed, e.g., in the
Lee-Yang model of Ising magnets with explicitly broken symmetry. Our
Ward-Takahashi identities imply trivial dynamical exponents in
both cases, rendering it unclear whether these critical points fall into the
known DP or Ising universality classes
Temperature Dependence of Interlayer Magnetoresistance in Anisotropic Layered Metals
Studies of interlayer transport in layered metals have generally made use of
zero temperature conductivity expressions to analyze angle-dependent
magnetoresistance oscillations (AMRO). However, recent high temperature AMRO
experiments have been performed in a regime where the inclusion of finite
temperature effects may be required for a quantitative description of the
resistivity. We calculate the interlayer conductivity in a layered metal with
anisotropic Fermi surface properties allowing for finite temperature effects.
We find that resistance maxima are modified by thermal effects much more
strongly than resistance minima. We also use our expressions to calculate the
interlayer resistivity appropriate to recent AMRO experiments in an overdoped
cuprate which led to the conclusion that there is an anisotropic, linear in
temperature contribution to the scattering rate and find that this conclusion
is robust.Comment: 8 pages, 4 figure
Gate-tunable band structure of the LaAlO-SrTiO interface
The 2-dimensional electron system at the interface between LaAlO and
SrTiO has several unique properties that can be tuned by an externally
applied gate voltage. In this work, we show that this gate-tunability extends
to the effective band structure of the system. We combine a magnetotransport
study on top-gated Hall bars with self-consistent Schr\"odinger-Poisson
calculations and observe a Lifshitz transition at a density of
cm. Above the transition, the carrier density of one
of the conducting bands decreases with increasing gate voltage. This surprising
decrease is accurately reproduced in the calculations if electronic
correlations are included. These results provide a clear, intuitive picture of
the physics governing the electronic structure at complex oxide interfaces.Comment: 14 pages, 4 figure
Spin-independent origin of the strongly enhanced effective mass in a dilute 2D electron system
We have accurately measured the effective mass in a dilute two-dimensional
electron system in silicon by analyzing temperature dependence of the
Shubnikov-de Haas oscillations in the low-temperature limit. A sharp increase
of the effective mass with decreasing electron density has been observed. Using
tilted magnetic fields, we have found that the enhanced effective mass is
independent of the degree of spin polarization, which points to a
spin-independent origin of the mass enhancement and is in contradiction with
existing theories
The Impact of Phorate on the Genetic Diversity of Wetland Aquatic Invertebraes
Impacts of the insecticide phorate on the genetic diversity of wetland invertebrates were investigated using field and laboratory studies in 1991. Electrophoretic methods were evaluated for revealing the impact of insecticides. Objectives were to determine the ability of electrophoresis to reveal the impact of phorate on invertebrates and to determine the influence of phorate on the genetic diversity in two common invertebrates. Amphipods, Hyallela azteca and mayflies, Callibaetis ferrugineus (Walsh) were placed in constructed mesocosms in wetlands and were exposed to varying amounts of phorate. Survivors and individuals from the parent population were genetically tested using cellulose acetate electrophoresis techniques. Allele frequencies were calculated for invertebrates in treatments and invertebrates from populations not exposed to phorate. Mortality of test invertebrates was significantly greater in phorate treatments than in controls (F = 5.97, P = 0.019). Chi-square analysis revealed differences in allele frequencies between the untreated populations and individuals of both species treated with phorate cx2 \u3e 8.5; df = 1,2; p \u3c 0.05). In addition, phorate appeared to eliminate, or reduce the frequency of certain genotypes in both species. Results indicate phorate selected against sensitive individuals and electrophoresis was effective at detecting differences between untreated populations and invertebrates that survived treatments. Genetic techniques should enable wetland scientists to detect the effects of pollution on invertebrate populations by monitoring genetic composition
Analytical calculation of the Green's function and Drude weight for a correlated fermion-boson system
In classical Drude theory the conductivity is determined by the mass of the
propagating particles and the mean free path between two scattering events. For
a quantum particle this simple picture of diffusive transport loses relevance
if strong correlations dominate the particle motion. We study a situation where
the propagation of a fermionic particle is possible only through creation and
annihilation of local bosonic excitations. This correlated quantum transport
process is outside the Drude picture, since one cannot distinguish between free
propagation and intermittent scattering. The characterization of transport is
possible using the Drude weight obtained from the f-sum rule, although its
interpretation in terms of free mass and mean free path breaks down. For the
situation studied we calculate the Green's function and Drude weight using a
Green's functions expansion technique, and discuss their physical meaning.Comment: final version, minor correction
Active Microrheology of Networks Composed of Semiflexible Polymers. II. Theory and comparison with simulations
Building on the results of our computer simulation (ArXiv cond-mat/0503573)we
develop a theoretical description of the motion of a bead, embedded in a
network of semiflexible polymers, and responding to an applied force. The
theory reveals the existence of an osmotic restoring force, generated by the
piling up of filaments in front of the moving bead and first deduced through
computer simulations. The theory predicts that the bead displacement scales
like x ~ t^alfa with time, with alfa=0.5 in an intermediate- and alfa=1 in a
long-time regime. It also predicts that the compliance varies with
concentration like c^(-4/3) in agreement with experiment.Comment: 18 pages and 2 figure
Correlated enhancement of Hc2 and Jc in carbon nanotube-doped MgB2
The use of MgB2 in superconducting applications still awaits for the
development of a MgB2-based material where both current-carrying performance
and critical magnetic field are optimized simultaneously. We achieved this by
doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in
polycrystalline samples. The optimum nominal DWCNT content for increasing the
critical current density, Jc is in the range 2.5-10%at depending on field and
temperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with
extrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT
content. The measured Hc2 vs T in all samples are successfully described using
a theoretical model for a two-gap superconductor in the dirty limit first
proposed by Gurevich et al.Comment: 12 pages, 3 figure
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