4,526 research outputs found
Charge order, metallic behavior and superconductivity in La_{2-x}Ba_xCuO_4 with x=1/8
The ab-plane optical properties of a cleaved single crystal of
La_{2-x}Ba_xCuO_4 for x=1/8 (T_c ~ 2.4 K) have been measured over a wide
frequency and temperature range. The low-frequency conductivity is Drude-like
and shows a metallic response with decreasing temperature. However, below ~ 60
K, corresponding to the onset of charge-stripe order, there is a rapid loss of
spectral weight below about 40 meV. The gapping of single-particle excitations
looks surprisingly similar to that observed in superconducting
La_{2-x}Sr_{x}CuO_4, including the presence of a residual Drude peak with
reduced weight; the main difference is that the lost spectral weight moves to
high, rather than zero, frequency, reflecting the absence of a bulk
superconducting condensate.Comment: 4 pages, with 1 table and 3 figure
Google Earth® Models with COLLADA and WxAzygy® Transparent Interface: An example from Grotto Creek, Front Ranges, Canadian Cordillera
Virtual globes represent a paradigm shift for geoscience education. It is now possible to explore real world experiences across the entire Earth, the Moon, and Mars, and also to combine multiple 2-D images into one 3-D image with topography. Models viewed in Google Earth® are more intuitive for visualizing 3-D geological structures than traditional paper maps and cross-sections. Here a student-constructed geological map and cross-sections from an introductory field school are used to illustrate the creation of a draped geological map over topography. A custom vertical slider elevates the cross-sections above topography and a horizontal slider restores thrust faulting. Models located in situ in topography are made queryable via a ‘cut-away’ using the WxAzygy® transparent interface.SOMMAIRE La notion de « globes virtuels » constitue un changement de paradigme dans le domaine de l’éducation en géoscience. Il est maintenant possible de traiter de la réalité de tous les recoins de la Terre, de la lune et de Mars, et aussi de combiner de multiples images 2-D en une image 3-D affichant la topographie. Les modèles de Google Earth® permettent une visualisation 3-D plus intuitive des structures géologiques que ne le permettent les cartes papiers usuelles et les coupes. Dans la présente, une carte géologique et une coupe réalisées par un étudiant d’un cours d’introduction au travail de terrain sont utilisées pour illustrer la confection d’une carte géologique appliquée sur la topographie correspondante. Un curseur vertical personnalisé dessine les coupes au-dessus de la topographie, et un curseur horizontal permet de restaurer les failles de chevauchement. Ces modèles ancrés au droit de la topographie peuvent être exploiter au moyen d’un écorché produit par l’interface transparent WxAzygy®
Anomalous Metal-Insulator Transition in Filled Skutterudite CeOsSb
Anomalous metal-insulator transition observed in filled skutterudite
CeOsSb is investigated by constructing the effective tight-binding
model with the Coulomb repulsion between f electrons. By using the mean field
approximation, magnetic susceptibilities are calculated and the phase diagram
is obtained. When the band structure has a semimetallic character with small
electron and hole pockets at and H points, a spin density wave
transition with the ordering vector occurs due to the
nesting property of the Fermi surfaces. Magnetic field enhances this phase in
accord with the experiments.Comment: 4 pages, 4 figure
Improving knowledge of Funiculina quadrangularis and vulnerable marine ecosystems in the south Adriatic
The Adriatic Sea is one of the most exploited areas in the Mediterranean; however, a large part of the South Adriatic Sea remains largely unexplored. Unfortunately, direct and indirect anthropogenic impacts are increasing exponentially, causing a loss of flagship species and threatening ecosystem functioning. This has led to several international conventions demanding better protection and management of sensitive species and areas worldwide. Soft mud facies bearing the sea pen Funiculina quadrangularis are indicators of vulnerable marine ecosystems (VMEs). This study aimed a) to contribute to the knowledge on the distributions of F. quadrangularis and VMEs, and b) to assess the main threats, thus enabling better identification of areas that need protection and ecosystem base management. The results show that F. quadrangularis are distributed from 160 m to 400 m depth, with the highest population density of 0.83 colony/m(2) reported in the Adriatic Sea at a 162 m depth. However, using video ground truthing, we registered stresses on deep-sea habitats originating from fishery and marine litter. Further research is needed to ascertain other potentially vulnerable areas. Moreover, international discussion is needed to determine future steps for the protection and sustainable use of resources beyond national jurisdictions
Optical conductivity of filled skutterudites
A simple tight-binding model is constructed for the description of the
electronic structure of some Ce-based filled skutterudite compounds showing an
energy gap or pseudogap behavior. Assuming band-diagonal electron interactions
on this tight-binding model, the optical conductivity spectrum is calculated by
applying the second-order self-consistent perturbation theory to treat the
electron correlation. The correlation effect is found to be of great importance
on the description of the temperature dependence of the optical conductivity.
The rapid disappearance of an optical gap with increasing temperature is
obtained as observed in the optical experiment for Ce-based filled-skutterudite
compounds.Comment: 6 pages, 7 figures, use jpsj2.cls, to appear in J. Phys. Soc. Jpn.
Vol.73, No.10 (2004
Electrodynamics of the Nodal Metal in Weakly Doped High- Cuprates
We report on the detailed analysis of the infrared (IR) conductivity of two
prototypical high- systems YBaCuO and
LaSrCuO throughout the complex phase diagram of these
compounds. Our focus in this work is to thoroughly document the electromagnetic
response of the nodal metal state which is initiated with only few holes doped
in parent antiferromagnetic systems and extends up to the pseudogap boundary in
the phase diagram. The key signature of the nodal metal is the two-component
conductivity: the Drude mode at low energies followed by a resonance in mid-IR.
The Drude component can be attributed to the response of coherent
quasiparticles residing on the Fermi arcs detected in photoemission
experiments. The microscopic origin of the mid-IR band is yet to be understood.
A combination of transport and IR data uncovers fingerprints of the Fermi
liquid behavior in the response of the nodal metal. The comprehensive nature of
the data sets presented in this work allows us to critically re-evaluate common
approaches to the interpretation of the optical data. Specifically we
re-examine the role of magnetic excitations in generating electronic self
energy effects through the analysis of the IR data in high magnetic field.Comment: 14 pages, 11 figure
Calculation of Optical Conductivity, Resistivity and Thermopower of Filled Skutterudite CeRuSb based on a Realistic Tight-binding Model with Strong Correlation
The filled-skutterudite compound CeRuSb shows a pseudo-gap
structure in the optical conductivity spectra similar to the Kondo insulators,
but metallic behavior below 80 K. The resistivity shows a large peak at 80 K,
and the Seebeck coefficient is positive and also shows a large peak at nearly
the same temperature. In order to explain all these features, a simplified
tight-binding model, which captures the essential features of the band
calculation, is proposed. Using this model and introducing the correlation
effect within the framework of the dynamical mean field approximation and the
iterative perturbation theory, the temperature dependences of the optical
conductivity, resistivity and the Seebeck coefficient are calculated, which can
explain the experiments.Comment: 4 pages, 6 figure
Scaling of the superfluid density in high-temperature superconductors
A scaling relation \rho_s \simeq 35\sigma_{dc}T_c has been observed in the
copper-oxide superconductors, where \rho_s is the strength of the
superconducting condensate, T_c is the critical temperature, and \sigma_{dc} is
the normal-state dc conductivity close to T_c. This scaling relation is
examined within the context of a clean and dirty-limit BCS superconductor.
These limits are well established for an isotropic BCS gap 2\Delta and a
normal-state scattering rate 1/\tau; in the clean limit 1/\tau \ll 2\Delta, and
in the dirty limit 1/\tau > 2\Delta. The dirty limit may also be defined
operationally as the regime where \rho_s varies with 1/\tau. It is shown that
the scaling relation \rho_s \propto \sigma_{dc}T_c is the hallmark of a BCS
system in the dirty-limit. While the gap in the copper-oxide superconductors is
considered to be d-wave with nodes and a gap maximum \Delta_0, if 1/\tau >
2\Delta_0 then the dirty-limit case is preserved. The scaling relation implies
that the copper-oxide superconductors are likely to be in the dirty limit, and
that as a result the energy scale associated with the formation of the
condensate is scaling linearly with T_c. The a-b planes and the c axis also
follow the same scaling relation. It is observed that the scaling behavior for
the dirty limit and the Josephson effect (assuming a BCS formalism) are
essentially identical, suggesting that in some regime these two effects may be
viewed as equivalent. This raises the possibility that electronic
inhomogeneities in the copper-oxygen planes may play an important role in the
nature of the superconductivity in the copper-oxide materials.Comment: 8 pages with 5 figures and 1 tabl
Optical Conductivity and Electronic Structure of CeRu4Sb12 under High Pressure
Optical conductivity [s(w)] of Ce-filled skutterudite CeRu4Sb12 has been
measured at high pressure to 8 GPa and at low temperature, to probe the
pressure evolution of its electronic structures. At ambient pressure, a
mid-infrared peak at 0.1 eV was formed in s(w) at low temperature, and the
spectral weight below 0.1 eV was strongly suppressed, due to a hybridization of
the f electron and conduction electron states. With increasing external
pressure, the mid-infrared peak shifts to higher energy, and the spectral
weight below the peak was further depleted. The obtained spectral data are
analyzed in comparison with band calculation result and other reported physical
properties. It is shown that the electronic structure of CeRu4Sb12 becomes
similar to that of a narrow-gap semiconductor under external pressure.Comment: 8 pages, 9 figure
Universal scaling relation in high-temperature superconductors
Scaling laws express a systematic and universal simplicity among complex
systems in nature. For example, such laws are of enormous significance in
biology. Scaling relations are also important in the physical sciences. The
seminal 1986 discovery of high transition-temperature (high-T_c)
superconductivity in cuprate materials has sparked an intensive investigation
of these and related complex oxides, yet the mechanism for superconductivity is
still not agreed upon. In addition, no universal scaling law involving such
fundamental properties as T_c and the superfluid density \rho_s, a quantity
indicative of the number of charge carriers in the superconducting state, has
been discovered. Here we demonstrate that the scaling relation \rho_s \propto
\sigma_{dc} T_c, where the conductivity \sigma_{dc} characterizes the
unidirectional, constant flow of electric charge carriers just above T_c,
universally holds for a wide variety of materials and doping levels. This
surprising unifying observation is likely to have important consequences for
theories of high-T_c superconductivity.Comment: 11 pages, 2 figures, 2 table
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