147 research outputs found
Thermal Fluctuations in d-wave Layered Superconductors
We study the thermal fluctuations of anisotropic order parameters (OP) in
layered superconductors. In particular, for copper oxides and a d-wave OP, we
present some experimental consequences of fluctuations in the direction normal
to the layers. It is shown that the c-axis penetration depth can
have a "disorder-like" quadratic temperature dependence at low temperature.
The fluctuations are analyzed in the framework of a Lawrence-Doniach model
with an isotropic Fermi surface. Anisotropies pin the orientation of the OP to
the crystallographic axes of the lattice. Then we study an extended t-J model
that fits Fermi suface data of bilayers and . This leads to a
d-wave OP with two possible orientations and, including the thermal
fluctuations, yields the announced temperature dependence of .
Furthermore a reservoir layer is introduced. It implies a finite density of
states at the Fermi energy which is successfully compared to conductance and
specific heat measurements.Comment: 21 pages, Latex, 3 uuencoded figure
DLCQ Strings, Twist Fields and One-Loop Correlators on a Permutation Orbifold
We investigate some aspects of the relationship between matrix string theory
and light-cone string field theory by analysing the correspondence between the
two-loop thermal partition function of DLCQ strings in flat space and the
integrated two-point correlator of twist fields in a symmetric product orbifold
conformal field theory at one-loop order. This is carried out by deriving
combinatorial expressions for generic twist field correlation functions in
permutation orbifolds using the covering surface method, by deriving the
one-loop modification of the twist field interaction vertex, and by relating
the two-loop finite temperature DLCQ string theory to the theory of Prym
varieties for genus two covers of an elliptic curve. The case of bosonic Z(2)
orbifolds is worked out explicitly and precise agreement between both
amplitudes is found. We use these techniques to derive explicit expressions for
Z(2) orbifold spin twist field correlation functions in the Type II and
heterotic string theories.Comment: 48 pages, 1 figure; v2: typos correcte
Symmetry breaking and ascending in the magnetic kagome metal FeGe
Spontaneous symmetry breaking-the phenomenon where an infinitesimal
perturbation can cause the system to break the underlying symmetry-is a
cornerstone concept in the understanding of interacting solid-state systems. In
a typical series of temperature-driven phase transitions, higher temperature
phases are more symmetric due to the stabilizing effect of entropy that becomes
dominant as the temperature is increased. However, the opposite is rare but
possible when there are multiple degrees of freedom in the system. Here, we
present such an example of a symmetry-ascending phenomenon in a magnetic kagome
metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. In
the paramagnetic state at 460K, we confirm that the crystal structure is indeed
hexagonal kagome lattice. On cooling to TN, the crystal structure changes from
hexagonal to monoclinic with in-plane lattice distortions on the order of
10^(-4) and the associated splitting of the double degenerate phonon mode of
the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice
shows a small negative thermal expansion, and the crystal structure becomes
more symmetric gradually upon further cooling. Increasing the crystalline
symmetry upon cooling is unusual, it originates from an extremely weak
structural instability that coexists and competes with the CDW and magnetic
orders. These observations are against the expectations for a simple model with
a single order parameter, hence can only be explained by a Landau free energy
expansion that takes into account multiple lattice, charge, and spin degrees of
freedom. Thus, the determination of the crystalline lattice symmetry as well as
the unusual spin-lattice coupling is a first step towards understanding the
rich electronic and magnetic properties of the system and sheds new light on
intertwined orders where the lattice degree of freedom is no longer dominant
Non-anticommutative solitons
Certain supersymmetric sigma models in 2+1 dimensions feature multi-soliton
solutions, with and without scattering. We subject these systems to a
non-anticommutative deformation by replacing the Grassmann algebra of the odd
superspace coordinates with a Clifford algebra. Static CP^1 and scattering U(2)
solitons are constructed and carry an additional spin-1/2 degree of freedom due
to the deformation. Abelian BPS solutions exist as well but have infinite
action, in contrast to the Moyal case.Comment: 1+10 page
In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFeAs
A small in-plane external uniaxial pressure has been widely used as an
effective method to acquire single domain iron pnictide BaFeAs, which
exhibits twin-domains without uniaxial strain below the
tetragonal-to-orthorhombic structural (nematic) transition temperature .
Although it is generally assumed that such a pressure will not affect the
intrinsic electronic/magnetic properties of the system, it is known to enhance
the antiferromagnetic (AF) ordering temperature () and create
in-plane resistivity anisotropy above . Here we use neutron polarization
analysis to show that such a strain on BaFeAs also induces a static or
quasi-static out-of-plane (-axis) AF order and its associated critical spin
fluctuations near . Therefore, uniaxial pressure necessary to detwin
single crystals of BaFeAs actually rotates the easy axis of the
collinear AF order near , and such effect due to spin-orbit coupling
must be taken into account to unveil the intrinsic electronic/magnetic
properties of the system.Comment: 11 pages, 4 figures, Supplementary information is available upon
reques
Single particle tracking in systems showing anomalous diffusion: the role of weak ergodicity breaking
Anomalous diffusion has been widely observed by single particle tracking
microscopy in complex systems such as biological cells. The resulting time
series are usually evaluated in terms of time averages. Often anomalous
diffusion is connected with non-ergodic behaviour. In such cases the time
averages remain random variables and hence irreproducible. Here we present a
detailed analysis of the time averaged mean squared displacement for systems
governed by anomalous diffusion, considering both unconfined and restricted
(corralled) motion. We discuss the behaviour of the time averaged mean squared
displacement for two prominent stochastic processes, namely, continuous time
random walks and fractional Brownian motion. We also study the distribution of
the time averaged mean squared displacement around its ensemble mean, and show
that this distribution preserves typical process characteristic even for short
time series. Recently, velocity correlation functions were suggested to
distinguish between these processes. We here present analytucal expressions for
the velocity correlation functions. Knowledge of the results presented here are
expected to be relevant for the correct interpretation of single particle
trajectory data in complex systems.Comment: 15 pages, 15 figures; References adde
Data submission and curation for caArray, a standard based microarray data repository system
caArray is an open-source, open development, web and programmatically accessible array data management system developed at National Cancer Institute. It was developed to support the exchange of array data across the Cancer Biomedical Informatics Grid (caBIG™), a collaborative information network that connect scientists and practitioners through a shareable and interoperable infrastructure to share data and knowledge. caArray adopts a federated model of local installations, in which data deposited are shareable across caBIG™. 

Comprehensive in annotation yet easy to use has always been a challenge to any data repository system. To alleviate this difficulty, caArray accepts data upload using the MAGE-TAB, a spreadsheet-based format for annotating and communicating microarray data in a MIAME-compliant fashion ("http://www.mged.org/mage-tab":http://www.mged.org/mage-tab). MAGE-TAB is built on community standards – MAGE, MIAME, and Ontology. The components and work flow of MAGE-TAB files are organized in such a way which is already familiar to bench scientists and thus minimize the time and frustration of reorganizing their data before submission. The MAGE-TAB files are also structured to be machine readable so that they can be easily parsed into database. Users can control public access to experiment- and sample-level data and can create collaboration groups to support data exchange among a defined set of partners. 

All data submitted to caArray at NCI will go through strict curation by a group of scientists against these standards to make sure that the data are correctly annotated using proper controlled vocabulary terms and all required information are provided. Two of mostly used ontology sources are MGED ontology ("http://mged.sourceforge.net/ontologies/MGEDontology.php":http://mged.sourceforge.net/ontologies/MGEDontology.php) and NCI thesaurus ("http://nciterms.nci.nih.gov/NCIBrowser/Dictionary.do":http://nciterms.nci.nih.gov/NCIBrowser/Dictionary.do). The purpose of data curation is to ensure easy comparison of results from different labs and unambiguous report of results. 

Data will also undergo automatic validation process before parsed into database, in which minimum information requirement and data consistency with the array designs are checked. Files with error found during validation are flagged with error message. Curators will re-examine those files and make necessary corrections before re-load the files. The iteration repeats until files are validated successfully. Data are then imported into the system and ready for access through the portal or through API. Interested parties are encouraged to review the installation package, documentation, and source code available from "http://caarray.nci.nih.gov":http://caarray.nci.nih.gov
Identification of the bulk pairing symmetry in high-temperature superconductors: Evidence for an extended s-wave with eight line nodes
we identify the intrinsic bulk pairing symmetry for both electron and
hole-doped cuprates from the existing bulk- and nearly bulk-sensitive
experimental results such as magnetic penetration depth, Raman scattering,
single-particle tunneling, Andreev reflection, nonlinear Meissner effect,
neutron scattering, thermal conductivity, specific heat, and angle-resolved
photoemission spectroscopy. These experiments consistently show that the
dominant bulk pairing symmetry in hole-doped cuprates is of extended s-wave
with eight line nodes, and of anisotropic s-wave in electron-doped cuprates.
The proposed pairing symmetries do not contradict some surface- and
phase-sensitive experiments which show a predominant d-wave pairing symmetry at
the degraded surfaces. We also quantitatively explain the phase-sensitive
experiments along the c-axis for both Bi_{2}Sr_{2}CaCu_{2}O_{8+y} and
YBa_{2}Cu_{3}O_{7-y}.Comment: 11 pages, 9 figure
"Pair" Fermi contour and repulsion-induced superconductivity in cuprates
The pairing of charge carriers with large pair momentum is considered in
connection with high-temperature superconductivity of cuprate compounds. The
possibility of pairing arises due to some essential features of
quasi-two-dimensional electronic structure of cuprates: (i) The Fermi contour
with strong nesting features; (ii) The presence of extended saddle point near
the Fermi level; (iii) The existence of some ordered state (for example,
antiferromagnetic) close to the superconducting one as a reason for an
appearing of "pair" Fermi contour resulting from carrier redistribution in
momentum space. In an extended vicinity of the saddle point, momentum space has
hyperbolic (pseudoeuclidean) metrics, therefore, the principal values of
two-dimensional reciprocal reduced effective mass tensor have unlike signs.
Rearrangement of holes in momentum space results in a rise of "pair" Fermi
contour which may be defined as zero-energy line for relative motion of the
pair. The superconducting gap arises just on this line. Pair Fermi contour
formation inside the region of momentum space with hyperbolic metrics results
in not only superconducting pairing but in a rise of quasi-stationary state in
the relative motion of the pair. Such a state has rather small decay and may be
related to the pseudogap regime of underdoped cuprates. It is concluded that
the pairing in cuprates may be due to screened Coulomb repulsion. In this case,
the superconducting energy gap in hole-doped cuprates exists in the region of
hole concentration which is bounded both above and below. The superconducting
state with positive condensation energy exists in more narrow range of doping
level inside this region. Such hole concentration dependence correlates with
typical phase diagram of cuprates.Comment: 23 pages, 11 figures. Submitted to Phys. Rev.
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