3,069 research outputs found
Doping evolution of spin and charge excitations in the Hubbard model
To shed light on how electronic correlations vary across the phase diagram of
the cuprate superconductors, we examine the doping evolution of spin and charge
excitations in the single-band Hubbard model using determinant quantum Monte
Carlo (DQMC). In the single-particle response, we observe that the effects of
correlations weaken rapidly with doping, such that one may expect the random
phase approximation (RPA) to provide an adequate description of the
two-particle response. In contrast, when compared to RPA, we find that
significant residual correlations in the two-particle excitations persist up to
hole and electron doping (the range of dopings achieved in the
cuprates). These fundamental differences between the doping evolution of
single- and multi-particle renormalizations show that conclusions drawn from
single-particle processes cannot necessarily be applied to multi-particle
excitations. Eventually, the system smoothly transitions via a
momentum-dependent crossover into a weakly correlated metallic state where the
spin and charge excitation spectra exhibit similar behavior and where RPA
provides an adequate description.Comment: 5 pages, 4 figures, plus supplementary materia
Topological Constraints at the Theta Point: Closed Loops at Two Loops
We map the problem of self-avoiding random walks in a Theta solvent with a
chemical potential for writhe to the three-dimensional symmetric
U(N)-Chern-Simons theory as N goes to 0. We find a new scaling regime of
topologically constrained polymers, with critical exponents that depend on the
chemical potential for writhe, which gives way to a fluctuation-induced
first-order transition.Comment: 5 pages, RevTeX, typo
Gravity as Backreaction
Quadratic theory of gravity is a complicated constraint system. We
investigate some consequences of treating quadratic terms perturbatively
(higher derivative version of backreaction effects). This approach is shown to
overcome some well known problems associated with higher derivative theories,
i.e., the physical gravitational degree of freedom remains unchanged from those
of Einstein gravity.
Using such an interpretation of gravity, we investigate a
classical and Wheeler DeWitt evolution of gravity for a
particular sign of , corresponding to non- tachyon case. Matter is
described by a phenomenological . It is concluded that
both the Friedmann potential () and the
Wheeler DeWitt potential () develop repulsive barriers near for
(i.e., ). The interpretations is clear. Repulsive
barrier in implies that a contracting FRW universe ()
will bounce to an expansion phase without a total gravitational collapse.
Repulsive barrier in means that is a classically forbidden
region. Therefore, probability of finding a universe with the big bang
singularity ( ) is exponentially suppressed.Comment: Accepted for publication in Phy. Rev. D.,18 pages, 6 figures, Latex
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Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor
Systematic ac susceptibility measurements have been performed on a MgB
bulk sample. We demonstrate that the flux creep activation energy is a
nonlinear function of the current density , indicating a
nonlogarithmic relaxation of the current density in this material. The
dependence of the activation energy on the magnetic field is determined to be a
power law , showing a steep decline in the activation
energy with the magnetic field, which accounts for the steep drop in the
critical current density with magnetic field that is observed in MgB. The
irreversibility field is also found to be rather low, therefore, the pinning
properties of this new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma
Probing the inter-layer exciton physics in a MoS/MoSe/MoS van der Waals heterostructure
Stacking atomic monolayers of semiconducting transition metal dichalcogenides
(TMDs) has emerged as an effective way to engineer their properties. In
principle, the staggered band alignment of TMD heterostructures should result
in the formation of inter-layer excitons with long lifetimes and robust valley
polarization. However, these features have been observed simultaneously only in
MoSe/WSe heterostructures. Here we report on the observation of long
lived inter-layer exciton emission in a MoS/MoSe/MoS trilayer van
der Waals heterostructure. The inter-layer nature of the observed transition is
confirmed by photoluminescence spectroscopy, as well as by analyzing the
temporal, excitation power and temperature dependence of the inter-layer
emission peak. The observed complex photoluminescence dynamics suggests the
presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We
show that circularly polarized optical pumping results in long lived valley
polarization of inter-layer exciton. Intriguingly, the inter-layer exciton
photoluminescence has helicity opposite to the excitation. Our results show
that through a careful choice of the TMDs forming the van der Waals
heterostructure it is possible to control the circular polarization of the
inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184
Can Modal Skepticism Defeat Humean Skepticism?
My topic is moderate modal skepticism in the spirit of Peter van Inwagen. Here understood, this is a conservative version of modal empiricism that severely limits the extent to which an ordinary agent can reasonably believe “exotic” possibility claims. I offer a novel argument in support of this brand of skepticism: modal skepticism grounds an attractive (and novel) reply to Humean skepticism. Thus, I propose that modal skepticism be accepted on the basis of its theoretical utility as a tool for dissolving philosophical paradox
Vibrational Spectra of a Mechanosensitive Channel
We report the simulated vibrational spectra of a mechanosensitive membrane channel in different gating states. Our results show that while linear absorption is insensitive to structural differences, linear dichroism and sum-frequency generation spectroscopies are sensitive to the orientation of the transmembrane helices, which is changing during the opening process. Linear dichroism cannot distinguish an intermediate structure from the closed structure, but sum-frequency generation can. In addition, we find that two-dimensional infrared spectroscopy can be used to distinguish all three investigated gating states of the mechanosensitive membrane channel.
Infrared Hall effect in high Tc superconductors: Evidence for non-Fermi liquid Hall scattering
Infrared (20-120 cm-1 and 900-1100 cm-1) Faraday rotation and circular
dichroism are measured in high Tc superconductors using sensitive polarization
modulation techniques. Optimally doped YBCO thin films are studied at
temperatures down to 15 K and magnetic fields up to 8 T. At 1000 cm-1 the Hall
conductivity varies strongly with temperature in contrast to the longitudinal
conductivity which is nearly independent of temperature. The Hall scattering
rate has a T^2 temperature dependence but, unlike a Fermi liquid, depends only
weakly on frequency. The experiment puts severe constraints on theories of
transport in the normal state of high Tc superconductors.Comment: 8 pages, 3 figure
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