37,242 research outputs found
The diamagnetism above the superconducting transition in underdoped La(1.9)Sr(0.1)CuO(4) revisited: Chemical disorder or phase incoherent superconductivity?
The interplay between superconducting fluctuations and inhomogeneities
presents a renewed interest due to recent works supporting an anomalous [beyond
the conventional Gaussian-Ginzburg-Landau (GGL) scenario] diamagnetism above Tc
in underdoped cuprates. This conclusion, mainly based in the observation of new
anomalies in the low-field isothermal magnetization curves, is in contradiction
with our earlier results in the underdoped La(1.9)Sr(0.1)CuO(4) [Phys. Rev.
Lett. 84, 3157 (2000)]. These seemingly intrinsic anomalies are being presented
in various influential works as a 'thermodynamic evidence' for phase incoherent
superconductivity in the pseudogap regime, this last being at present a central
and debated issue of the cuprate superconductors' physics. Here we have
extended our magnetization measurements in La(1.9)Sr(0.1)CuO(4) to two samples
with different chemical disorder, in one of them close to the one associated
with the random distribution of Sr ions. For this sample, the corresponding
Tc-distribution may be approximated as symmetric around the average Tc, while
in the most disordered sample is strongly asymmetric. The comparison between
the magnetization measured in both samples provides a crucial check of the
chemical disorder origin of the observed diamagnetism anomalies, which are
similar to those claimed as due to phase fluctuations by other authors. This
conclusion applies also to the sample affected only by the intrinsic-like
chemical disorder, providing then a further check that the intrinsic
diamagnetism above the superconducting transition of underdoped cuprates is not
affected by the opening of a pseudogap in the normal state. It is also shown
here that once these disorder effects are overcome, the remaining precursor
diamagnetism may be accounted at a quantitative level in terms of the GGL
approach under a total energy cutoff.Comment: 13 pages, 7 figures. Minor corrections include
A Method for Solving Distributed Service Allocation Problems
We present a method for solving service allocation problems in which a set of
services must be allocated to a set of agents so as to maximize a global
utility. The method is completely distributed so it can scale to any number of
services without degradation. We first formalize the service allocation problem
and then present a simple hill-climbing, a global hill-climbing, and a
bidding-protocol algorithm for solving it. We analyze the expected performance
of these algorithms as a function of various problem parameters such as the
branching factor and the number of agents. Finally, we use the sensor
allocation problem, an instance of a service allocation problem, to show the
bidding protocol at work. The simulations also show that phase transition on
the expected quality of the solution exists as the amount of communication
between agents increases
Fluctuation diamagnetism around the superconducting transition in a cuprate crystal with a reduced Meissner fraction
The magnetization around the superconducting transition was measured in a
TlPbSrCaCuO crystal affected by a considerable
reduction (55%) of its effective superconducting volume fraction but
still with a relatively sharp low-field Meissner transition, a behaviour that
may be attributed to the presence of structural inhomogeneities. By taking into
account these inhomogeneities just through the Meissner fraction, the observed
diamagnetism may still be explained, consistently above and below the
superconducting transition, in terms of the conventional Ginzburg-Landau
approach with fluctuations of Cooper pairs and vortices.Comment: 4 pages, 4 figure
Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"
By using high magnetic field data to estimate the background conductivity,
Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522
(2011)] experimental evidence that the in-plane paraconductivity in cuprates is
almost independent of doping. In this Comment we also show that, in contrast
with their claims, these useful data may be explained at a quantitative level
in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors,
extended by Carballeira and coworkers to high reduced-temperatures by
introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When
combined, these two conclusions further suggest that the paraconductivity in
cuprates is conventional, i.e., associated with fluctuating superconducting
pairs above the mean-field critical temperature.Comment: 9 pages, 1 figur
Diamagnetism around the Meissner transition in a homogeneous cuprate single crystal
The in-plane diamagnetism around the Meissner transition was measured in a
TlBaCaCuO single crystal of high chemical and structural
quality, which minimizes the inhomogeneity and disorder rounding effects on the
magnetization. When analyzed quantitatively and consistently above and below
the transition in terms of the Ginzburg-Landau (GL) approach with fluctuations
of Cooper pairs and vortices, these data provide a further confirmation that
the observed Meissner transition is a conventional GL superconducting
transition in a homogeneous layered superconductor.Comment: 5 pages, including 3 figure
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Exposure to “real life” professional experiences through a shadowing scheme that matches penultimate and final year students with employers
This paper reports on a learning development project undertaken during 2010 – 2011. The Work-Shadowing Scheme (WSS) was primarily developed in response to feedback following the National Student Survey (NSS). The NSS brought to light the apparent discrepancy between, the number of structured programmes City University London host that offer exposure within professional environments (for example, formal placement schemes) and the number of students looking for experience within a professional context. In an attempt to bridge this gap, the Career and Skills Development Service began thinking about how to implement a scheme that would provide City students with a unique opportunity to gain professional experience across a variety of industries. The WSS project was designed to provide students with the opportunity to shadow a guide from a sector of their choice. Typically, students requested to shadow guides in industries associated with their course, however an additional benefit of the WSS was the potential to shadow a guide within a sector that was not traditionally associated with their degree. On completion of a shadowing experience, students were given the option to shadow two more employers. This aspect of the scheme was further commended, and students reiterated the benefits of gaining „snapshots‟ of multiple sectors before making crucial career decisions
Pointwise universal consistency of nonparametric linear estimators
This paper presents sufficient conditions for pointwise universal consistency of nonparametric delta estimators. We show the applicability of these conditions for some classes of nonparametric estimators
A generalization of Bohr's Equivalence Theorem
Based on a generalization of Bohr's equivalence relation for general
Dirichlet series, in this paper we study the sets of values taken by certain
classes of equivalent almost periodic functions in their strips of almost
periodicity. In fact, the main result of this paper consists of a result like
Bohr's equivalence theorem extended to the case of these functions.Comment: Because of a mistake detected in one of the references, the previous
version of this paper has been modified by the authors to restrict the scope
of its application to the case of existence of an integral basi
A universal quantum circuit for two-qubit transformations with three CNOT gates
We consider the implementation of two-qubit unitary transformations by means
of CNOT gates and single-qubit unitary gates. We show, by means of an explicit
quantum circuit, that together with local gates three CNOT gates are necessary
and sufficient in order to implement an arbitrary unitary transformation of two
qubits. We also identify the subset of two-qubit gates that can be performed
with only two CNOT gates.Comment: 3 pages, 7 figures. One theorem, one author and references added.
Change of notational conventions. Minor correction in Theorem
Bohr's equivalence relation in the space of Besicovitch almost periodic functions
Based on Bohr's equivalence relation which was established for general
Dirichlet series, in this paper we introduce a new equivalence relation on the
space of almost periodic functions in the sense of Besicovitch,
, defined in terms of polynomial approximations. From
this, we show that in an important subspace , where Parseval's equality and Riesz-Fischer theorem
holds, its equivalence classes are sequentially compact and the family of
translates of a function belonging to this subspace is dense in its own class.Comment: Because of a mistake detected in one of the references, the
equivalence relation which is inspired by that of Bohr is revised to adapt
correctly the situation in the general case. arXiv admin note: text overlap
with arXiv:1801.0803
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