15,463 research outputs found
Hot Electron Effects in the 2D Superconductor-Insulator Transition
The parallel magnetic field tuned two-dimensional superconductor-insulator
transition has been investigated in ultrathin films of amorphous Bi. The
resistance is found to be independent of temperature on both sides of the
transition below approximately 120 mK. Several observations suggest that this
regime is not intrinsically "metallic" but results from the failure of the
films' electrons to cool. The onset of this temperature-independent regime can
be moved to higher temperatures by either increasing the measuring current or
the level of electromagnetic noise. Temperature scaling is successful above 120
mK. Electric field scaling can be mapped onto temperature scaling by relating
the electric fields to elevated electron temperatures. These results cast doubt
on the existence of an intrinsic metallic regime and on the independent
determination of the correlation length and dynamical critical exponents
obtained by combining the results of electric field and temperature scaling.Comment: 4 pages, 4 figure
A computer program for the calculation of laminar and turbulent boundary layer flows
The results are presented of a study to produce a computer program to calculate laminar and turbulent boundary layer flows. The program is capable of calculating the following types of flow: (1) incompressible or compressible, (2) two dimensional or axisymmetric, and (3) flows with significant transverse curvature. Also, the program can handle a large variety of boundary conditions, such as blowing or suction, arbitrary temperature distributions and arbitrary wall heat fluxes. The program has been specialized to the calculation of equilibrium air flows and all of the thermodynamic and transport properties used are for air. For the turbulent transport properties, the eddy viscosity approach has been used. Although the eddy viscosity models are semi-empirical, the model employed in the program has corrections for pressure gradients, suction and blowing and compressibility. The basic method of approach is to put the equations of motion into a finite difference form and then solve them by use of a digital computer. The program is written in FORTRAN 4 and requires small amounts of computer time on most scientific machines. For example, most laminar flows can be calculated in less than one minute of machine time, while turbulent flows usually require three or four minutes
Kodaira-Spencer formality of products of complex manifolds
We shall say that a complex manifold is emph{Kodaira-Spencer formal} if its Kodaira-Spencer differential graded Lie algebra
is formal; if this happen, then the deformation theory of
is completely determined by the graded Lie algebra and the base space of the semiuniversal deformation is a quadratic singularity..
Determine when a complex manifold is Kodaira-Spencer formal is generally difficult and
we actually know only a limited class of cases where this happen. Among such examples we have
Riemann surfaces, projective spaces, holomorphic Poisson manifolds with surjective anchor map
and every compact K"{a}hler manifold with trivial or torsion canonical
bundle.
In this short note we investigate the behavior of this property under finite products. Let be compact complex manifolds; we prove that whenever and are
K"{a}hler, then is Kodaira-Spencer formal if and only if the same
holds for and . A revisit of a classical example by Douady shows that the above result fails if the K"{a}hler assumption is droppe
Electrostatic Tuning of the Superconductor-Insulator Transition in Two Dimensions
Superconductivity has been induced in insulating ultra-thin films of
amorphous bismuth using the electric field effect. The screening of
electron-electron interaction was found to increase with electron concentration
in a manner correlated with the tendency towards superconductivity. This does
not preclude an increase in the density of states being important in the
development of superconductivity. The superconductor-insulator transition
appears to belong to the universality class of the three dimensional XY model.Comment: Four pages, three figures. Revised slightly to reflect referees'
comment
Introduction: Urban revolutions in the age of global urbanism
This special issue, papers presented at an Urban Studies Foundation-funded conference in Jakarta (March 2011), examines the current ‘urban century’ in terms of three revolutions. Revolutions from above index the logics and norms of mainstream global urbanism, particularly the form they have taken as policymakers work with municipal officials worldwide to organise urban development around neoliberal norms. Revolutions from below refer to the multifaceted contestations of global urbanism that take place in and around cities, ranging from urban street demonstrations and occupations (such as those riveting the world in early 2011 when these papers were written) to the quotidian actions of those pursuing politics and livelihoods that subvert the norms of mainstream global urbanism. It also highlights conceptual revolutions, referencing the ongoing challenge of reconceptualising urban theory from the South – not simply as a hemispheric location or geopolitical category but an epistemological stance, staged from many different locations but always fraught with the differentials of power and the weight of historical geographies. Drawing on the insights of scholars writing from, and not just about, such locations, a further iteration in this ‘southern’ turn of urban theorising is proposed. This spatio-temporal conjunctural approach emphasises how the specificity of cities – their existence as entities that are at once singular and universal – emerges from spatio-temporal dynamics, connectivities and horizontal and vertical relations. Practically, such scholarship entails taking the field seriously through collaborative work that is multi-sited, engages people along the spectrum of academics and activists, and is presented before and scrutinised by multiple publics
Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom. Application to 1H NMR reversion experiments in nematic liquid crystals
An experimental study of NMR spin decoherence in nematic liquid crystals (LC)
is presented. Decoherence dynamics can be put in evidence by means of
refocusing experiments of the dipolar interactions. The experimental technique
used in this work is based on the MREV8 pulse sequence. The aim of the work is
to detect the main features of the Irreversible Quantum Decoherence (IQD) in
LC, on the basis of the theory presented by the authors recently. The focus is
laid on experimentally probing the eigen-selection process in the intermediate
time scale, between quantum interference of a closed system and thermalization,
as a signature of the IQD of the open quantum system, as well as on quantifying
the effects of non-idealities as possible sources of signal decays which could
mask the intrinsic IQD. In order to contrast experiment and theory, the theory
was adapted to obtain the IQD function corresponding to the MREV8 reversion
experiments. Non-idealities of the experimental setting are analysed in detail
within this framework and their effects on the observed signal decay are
numerically estimated. It is found that, though these non-idealities could in
principle affect the evolution of the spin dynamics, their influence can be
mitigated and they do not present the characteristic behavior of the IQD. As
unique characteristic of the IQD, the experimental results clearly show the
occurrence of eigen-selectivity in the intermediate timescale, in complete
agreement with the theoretical predictions. We conclude that the
eigen-selection effect is the fingerprint of IQD associated with a quantum open
spin system in LC. Besides, these features of the results account for the
quasi-equilibrium states of the spin system, which were observed previously in
these mesophases, and lead to conclude that the quasi-equilibrium is a definite
stage of the spin dynamics during its evolution towards equilibriu
On the type of triangle groups
We prove a conjecture of R. Schwartz about the type of some complex
hyperbolic triangle groups.Comment: 10 pages, 3 figure
Charmed Mesons Have No Discernable Color-Coulomb Attraction
Starting with a confining linear Lorentz scalar potential V_s and a Lorentz
vector potential V_v which is also linear but has in addition a color-Coulomb
attraction piece, -alpha_s/r, we solve the Dirac equation for the ground-state
c- and u-quark wave functions. Then, convolving V_v with the u-quark density,
we find that the Coulomb attraction mostly disappears, making an essentially
linear barV_v for the c-quark. A similar convolution using the c-quark density
also leads to an essentially linear tildeV_v for the u-quark. For bound cbar-c
charmonia, where one must solve using a reduced mass for the c-quarks, we also
find an essentially linear widehatV_v. Thus, the relativistic quark model
describes how the charmed-meson mass spectrum avoids the need for a
color-Coulomb attraction.Comment: 9 pages, 5 PDF figure
- …