738 research outputs found
Electron Quasiparticles Drive the Superconductor-to-Insulator Transition in Homogeneously Disordered Thin Films
Transport data on Bi, MoGe, and PbBi/Ge homogeneously-disordered thin films
demonstrate that the critical resistivity, , at the nominal
insulator-superconductor transition is linearly proportional to the normal
sheet resistance, . In addition, the critical magnetic field scales
linearly with the superconducting energy gap and is well-approximated by
. Because is determined at high temperatures and is the
pair-breaking field, the two immediate consequences are: 1)
electron-quasiparticles populate the insulating side of the transition and 2)
standard phase-only models are incapable of describing the destruction of the
superconducting state. As gapless electronic excitations populate the
insulating state, the universality class is no longer the 3D XY model. The lack
of a unique critical resistance in homogeneously disordered films can be
understood in this context. In light of the recent experiments which observe an
intervening metallic state separating the insulator from the superconductor in
homogeneously disordered MoGe thin films, we argue that the two transitions
that accompany the destruction of superconductivity are 1) superconductor to
Bose metal in which phase coherence is lost and 2) Bose metal to localized
electron insulator via pair-breaking.Comment: This article is included in the Festschrift for Prof. Michael Pollak
on occasion of his 75th birthda
Developmental and age differences in visuomotor adaptation across the lifespan
Health and self-regulatio
Ferromagnetic Quantum Critical Point in CePdP with Pd Ni Substitution
An investigation of the structural, thermodynamic, and electronic transport
properties of the isoelectronic chemical substitution series
Ce(PdNi)P is reported, where a possible ferromagnetic
quantum critical point is uncovered in the temperature - concentration ()
phase diagram. This behavior results from the simultaneous contraction of the
unit cell volume, which tunes the relative strengths of the Kondo and RKKY
interactions, and the introduction of disorder through alloying. Near the
critical region at 0.7, the rate of contraction of the
unit cell volume strengthens, indicating that the cerium -valence crosses
over from trivalent to a non-integer value. Consistent with this picture, x-ray
absorption spectroscopy measurements reveal that while CePdP has a
purely trivalent cerium -state, CeNiP has a small ( 10 \%)
tetravalent contribution. In a broad region around , there is a
breakdown of Fermi liquid temperature dependences, signaling the influence of
quantum critical fluctuations and disorder effects. Measurements of clean
CePdP furthermore show that applied pressure has a similar initial
effect to alloying on the ferromagnetic order. From these results,
CePdP emerges as a keystone system to test theories such as the
Belitz-Kirkpatrick-Vojta model for ferromagnetic quantum criticality, where
distinct behaviors are expected in the dirty and clean limits.Comment: 9 pages, 8 figure
The shape of equality: discourses around the Section 28 repeal in Scotland
This article focuses on conceptualizations of equality in the discourses deployed in the campaign to repeal Section 28 in Scotland. I use the parliamentary debates and two newspapers: the Daily Record, which supported the campaign to Keep the Clause, and The Guardian, which supported repeal, to exemplify the different discursive articulations around equality and citizenship. I suggest that the Scottish example provides further evidence of the ways in which liberalism naturalizes heterosexuality as the standard for citizenship and thus bequeaths a hierarchy of 'equality' and citizenship in the realm of sexuality, wherein lesbian and gay citizenship is either rendered invalid or characterized as 'special rights'. However, within the narrow confines of the parliamentary debates, more expansive and differentiated notions of citizenship and equality are evident. Whilst I conclude that the 'shape' of equality achieved through the repeal has been moulded to support institutionalized heterosexuality - with Section 28 replaced by statutory guidelines on sex education which advocate marriage - I also suggest equality is contested, both through the recognition of transformations in heterosexual family forms and the appeal to non-discrimination as a democratic principle. It is possible, therefore, that current destabilizations of the heterosexual social order simultaneously destabilize the precepts of liberal democracy
Approximating the coefficients in semilinear stochastic partial differential equations
We investigate, in the setting of UMD Banach spaces E, the continuous
dependence on the data A, F, G and X_0 of mild solutions of semilinear
stochastic evolution equations with multiplicative noise of the form dX(t) =
[AX(t) + F(t,X(t))]dt + G(t,X(t))dW_H(t), X(0)=X_0, where W_H is a cylindrical
Brownian motion on a Hilbert space H. We prove continuous dependence of the
compensated solutions X(t)-e^{tA}X_0 in the norms
L^p(\Omega;C^\lambda([0,T];E)) assuming that the approximating operators A_n
are uniformly sectorial and converge to A in the strong resolvent sense, and
that the approximating nonlinearities F_n and G_n are uniformly Lipschitz
continuous in suitable norms and converge to F and G pointwise. Our results are
applied to a class of semilinear parabolic SPDEs with finite-dimensional
multiplicative noise.Comment: Referee's comments have been incorporate
Maximal -regularity for stochastic evolution equations
We prove maximal -regularity for the stochastic evolution equation
\{{aligned} dU(t) + A U(t)\, dt& = F(t,U(t))\,dt + B(t,U(t))\,dW_H(t),
\qquad t\in [0,T],
U(0) & = u_0, {aligned}. under the assumption that is a sectorial
operator with a bounded -calculus of angle less than on
a space . The driving process is a cylindrical
Brownian motion in an abstract Hilbert space . For and
and initial conditions in the real interpolation space
\XAp we prove existence of unique strong solution with trajectories in
L^p(0,T;\Dom(A))\cap C([0,T];\XAp), provided the non-linearities
F:[0,T]\times \Dom(A)\to L^q(\mathcal{O},\mu) and B:[0,T]\times \Dom(A) \to
\g(H,\Dom(A^{\frac12})) are of linear growth and Lipschitz continuous in their
second variables with small enough Lipschitz constants. Extensions to the case
where is an adapted operator-valued process are considered as well.
Various applications to stochastic partial differential equations are worked
out in detail. These include higher-order and time-dependent parabolic
equations and the Navier-Stokes equation on a smooth bounded domain
\OO\subseteq \R^d with . For the latter, the existence of a unique
strong local solution with values in (H^{1,q}(\OO))^d is shown.Comment: Accepted for publication in SIAM Journal on Mathematical Analysi
'Man up!': discursive constructions of non-drinkers among UK undergraduates
This study adopted a discursive approach to explore how not drinking alcohol (non-drinking) is construed in relation to masculine identity among 12 undergraduate interviewees. Three prominent discourses were revealed. First, non-drinking was constructed as something strange requiring explanation. Second, contradictory discourses constructed non-drinking as, simultaneously, unsociable yet reflective of greater sociability. Third, non-drinking was constructed as something which has greater negative social consequences for men than for women. Opportunities for challenging traditional gender role expectations are considered
Inertial Mass of a Vortex in Cuprate Superconductors
We present here a calculation of the inertial mass of a moving vortex in
cuprate superconductors. This is a poorly known basic quantity of obvious
interest in vortex dynamics. The motion of a vortex causes a dipolar density
distortion and an associated electric field which is screened. The energy cost
of the density distortion as well as the related screened electric field
contribute to the vortex mass, which is small because of efficient screening.
As a preliminary, we present a discussion and calculation of the vortex mass
using a microscopically derivable phase-only action functional for the far
region which shows that the contribution from the far region is negligible, and
that most of it arises from the (small) core region of the vortex. A
calculation based on a phenomenological Ginzburg-Landau functional is performed
in the core region. Unfortunately such a calculation is unreliable, the reasons
for it are discussed. A credible calculation of the vortex mass thus requires a
fully microscopic, non-coarse grained theory. This is developed, and results
are presented for a s-wave BCS like gap, with parameters appropriate to the
cuprates. The mass, about 0.5 per layer, for magnetic field along the
axis, arises from deformation of quasiparticle states bound in the core, and
screening effects mentioned above. We discuss earlier results, possible
extensions to d-wave symmetry, and observability of effects dependent on the
inertial mass.Comment: 27 pages, Latex, 3 figures available on request, to appear in
Physical Review
Neural correlates of multi-day learning and savings in sensorimotor adaptation
Abstract In the present study we evaluated changes in neural activation that occur over the time course of multiple days of sensorimotor adaptation, and identified individual neural predictors of adaptation and savings magnitude. We collected functional MRI data while participants performed a manual adaptation task during four separate test sessions over a three-month period. This allowed us to examine changes in activation and associations with adaptation and savings at subsequent sessions. Participants exhibited reliable savings of adaptation across the four sessions. Brain activity associated with early adaptation increased across the sessions in a variety of frontal, parietal, cingulate, and temporal cortical areas, as well as various subcortical areas. We found that savings was positively associated with activation in several striatal, parietal, and cingulate cortical areas including the putamen, precuneus, angular gyrus, dorsal anterior cingulate cortex (dACC), and cingulate motor area. These findings suggest that participants may learn how to better engage cognitive processes across days, potentially reflecting improvements in action selection. We propose that such improvements may rely on action-value assignments, which previously have been linked to the dACC and striatum. As correct movements are assigned a higher value than incorrect movements, the former are more likely to be performed again
Focal Gray Matter Plasticity as a Function of Long Duration Head Down Tilted Bed Rest: Preliminary Results
Long duration spaceflight (i.e., 22 days or longer) has been associated with changes in sensorimotor systems, resulting in difficulties that astronauts experience with posture control, locomotion, and manual control. The microgravity environment is an important causal factor for spaceflight induced sensorimotor changes. Whether these sensorimotor changes are solely related to peripheral changes from reduced vestibular stimulation, body unloading, body fluid shifts or that they may be related to structural and functional brain changes is yet unknown. However, a recent study reported associations between microgravity and flattening of the posterior eye globe and protrusion of the optic nerve [1] possibly as the result of increased intracranial pressure due to microgravity induced bodily fluid shifts [3]. Moreover, elevated intracranial pressure has been related to white matter microstructural damage [2]. Thus, it is possible that spaceflight may affect brain structure and thereby cognitive functioning. Long duration head down tilt bed rest has been suggested as an exclusionary analog to study microgravity effects on the sensorimotor system [4]. Bed rest mimics microgravity in body unloading and bodily fluid shifts. In consideration of the health and performance of crewmembers both in- and post-flight, we are conducting a prospective longitudinal 70-day bed rest study as an analog to investigate the effects of microgravity on brain structure [5]. Here we present results of the first six subjects. Six subjects were assessed at 12 and 7 days before-, at 7, 30, and ~70 days in-, and at 8 and 12 days post 70 days of bed rest at the NASA bed rest facility in UTMB, Galveston, TX, USA. At each time point structural MRI scans (i.e., high resolution T1-weighted imaging and Diffusion Tensor Imaging (DTI)) were obtained using a 3T Siemens scanner. Focal changes over time in gray matter density were assessed using the voxel based morphometry 8 (VBM8) toolbox under SPM. Longitudinal processing in VBM8 includes linear registration of each scan to the mean of the subject and subsequently transforming all scans in to MNI space by applying the warp from the mean subject to MNI to the individual gray matter segmentations. Modulation was applied so that all images represented the volume of the original structure in native space. Voxel wise analysis was carried out on the gray matter images after smoothing, using a flexible factorial design with family wise error correction. Focal changes in white matter microstructural integrity were assessed using tract based spatial statistics (TBSS) as part of FMRIB software library (FSL). TBSS registers all DTI scans to standard space. It subsequently creates a study specific white matter skeleton of the major white matter tracts. For each subject, for each DTI metric (i.e. fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD)), the maximum value in a line perpendicular to the skeleton tract is projected to the skeleton. Non-parametric permutation based t-tests and ANOVA's were used for voxel-wise comparison of the skeletons. For both VBM and TBSS, comparison of pre bed rest measurements did not show significant differences. VBM analysis revealed decreased gray matter density in bilateral areas including the frontal medial cortex, the insular cortex and the caudate (see Figure) from 'pre to in bed rest'. Over the same time period, there was an increase in gray matter density in the cerebellum, occipital-, and parietal cortex, including the precuneus (see Figure). The majority of these changes did not recover from 'during to post bed rest'. TBSS analysis did not reveal significant changes in white matter microstructural integrity after correction for multiple comparisons. Uncorrected analyses (p<.015) revealed an increase in RD in the cerebellum and brainstem from pre bed rest to the first week in bed rest that did not recover post bed rest. Extended bed rest, which is an analog for microgravity, can result in gray matter changes and potentially in microstructural white matter changes in areas that are important for neuro motor behavior and cognition. These changes did not recover at two weeks post bed rest. Whether the effects of bed rest wear off at longer times post bed rest, and if they are associated with behavior are important questions that warrant further research
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