276 research outputs found
Optical phonons in new ordered perovskite Sr2Cu(Re0.69Ca0.31) Oy system observed by infrared reflectance spectroscopy
We report infrared reflectivity spectra for a new correlated cupric oxide
system Sr2Cu(Re0.69Ca0.31)Oy with y ~ 0.6 at several temperatures ranging
between 8 and 380 K. The reflectivity spectrum at 300 K comprises of several
optical phonons. A couple of residual bands located around 315 and 653 cm-1
exhibit exceptionally large intensity as compared to the other ones. The
overall reflectivity spectrum lifts up slightly with increasing temperature.
The energy and damping factor of transverse-optical phonons are determined by
fitting the imaginary dielectric constant by Lorentz oscillator model and
discussed as a function of temperature in terms of lattice anharmonicity.Comment: 9 pages, 3 figures, presented at ISS2005, to appear in Physica
Field-induced breakdown of the quantum Hall effect
A numerical analysis is made of the breakdown of the quantum Hall effect
caused by the Hall electric field in competition with disorder. It turns out
that in the regime of dense impurities, in particular, the number of localized
states decreases exponentially with the Hall field, with its dependence on the
magnetic and electric field summarized in a simple scaling law. The physical
picture underlying the scaling law is clarified. This intra-subband process,
the competition of the Hall field with disorder, leads to critical breakdown
fields of magnitude of a few hundred V/cm, consistent with observations, and
accounts for their magnetic-field dependence \propto B^{3/2} observed
experimentally. Some testable consequences of the scaling law are discussed.Comment: 7 pages, Revtex, 3 figures, to appear in Phys. Rev.
Single-mode approximation and effective Chern-Simons theories for quantum Hall systems
A unified description of elementary and collective excitations in quantum
Hall systems is presented within the single-mode approximation (SMA) framework,
with emphasis on revealing an intimate link with Chern-Simons theories. It is
shown that for a wide class of quantum Hall systems the SMA in general yields,
as an effective theory, a variant of the bosonic Chern-Simons theory. For
single-layer systems the effective theory agrees with the standard Chern-Simons
theory at long wavelengths whereas substantial deviations arise for collective
excitations in bilayer systems. It is suggested, in particular, that Hall-drag
experiments would be a good place to detect out-of-phase collective excitations
inherent to bilayer systems. It is also shown that the intra-Landau-level modes
bear a similarity in structure (though not in scale) to the inter-Landau-level
modes, and its implications on the composite-fermion and composite-boson
theories are discussed.Comment: 9 pages, Revtex
Electromagnetic characteristics and effective gauge theory of double-layer quantum Hall systems
The electromagnetic characteristics of double-layer quantum Hall systems are
studied, with projection to the lowest Landau level taken into account and
intra-Landau-level collective excitations treated in the single-mode
approximation. It is pointed out that dipole-active excitations, both
elementary and collective, govern the long-wavelength features of quantum Hall
systems. In particular, the presence of the dipole-active interlayer
out-of-phase collective excitations, inherent to double-layer systems, modifies
the leading O(k) and O(k^{2}) long-wavelength characteristics (i.e., the
transport properties and characteristic scale) of the double-layer quantum Hall
states substantially. We apply bosonization techniques and construct from such
electromagnetic characteristics an effective theory, which consists of three
vector fields representing the three dipole-active modes, one interlayer
collective mode and two inter-Landau-level cyclotron modes. This effective
theory properly incorporates the spectrum of collective excitations on the
right scale of the Coulomb energy and, in addition, accommodates the favorable
transport properties of the standard Chern-Simons theories.Comment: 10 pages, Revtex, sec. II slightly shortened, to appear in Phys. Rev.
Electromagnetic characteristics of bilayer quantum Hall systems in the presence of interlayer coherence and tunneling
The electromagnetic characteristics of bilayer quantum Hall systems in the
presence of interlayer coherence and tunneling are studied by means of a
pseudospin-texture effective theory and an algebraic framework of the
single-mode approximation, with emphasis on clarifying the nature of the
low-lying neutral collective mode responsible for interlayer tunneling
phenomena. A long-wavelength effective theory, consisting of the collective
mode as well as the cyclotron modes, is constructed. It is seen explicitly from
the electromagnetic response that gauge invariance is kept exact, this
implying, in particular, the absence of the Meissner effect in bilayer systems.
Special emphasis is placed on exploring the advantage of looking into quantum
Hall systems through their response; in particular, subtleties inherent to the
standard Chern-Simons theories are critically examined.Comment: 9 pages, Revtex, to appear in Phys. Rev.
The DNA sequence of human chromosome 22
Knowledge of the complete genomic DNA sequence of an organism allows a systematic approach to defining its genetic components. The genomic sequence provides access to the complete structures of all genes, including those without known function, their control elements, and, by inference, the proteins they encode, as well as all other biologically important sequences. Furthermore, the sequence is a rich and permanent source of information for the design of further biological studies of the organism and for the study of evolution through cross-species sequence comparison. The power of this approach has been amply demonstrated by the determination of the sequences of a number of microbial and model organisms. The next step is to obtain the complete sequence of the entire human genome. Here we report the sequence of the euchromatic part of human chromosome 22. The sequence obtained consists of 12 contiguous segments spanning 33.4 megabases, contains at least 545 genes and 134 pseudogenes, and provides the first view of the complex chromosomal landscapes that will be found in the rest of the genome
Central charge and renormalization in supersymmetric theories with vortices
Some quantum features of vortices in supersymmetric theories in 1+2
dimensions are studied in a manifestly supersymmetric setting of the superfield
formalism. A close examination of the supercurrent that accommodates the
central charge and super-Poincare charges in a supermultiplet reveals that
there is no genuine quantum anomaly in the supertrace identity and in the
supercharge algebra, with the central-charge operator given by the bare
Fayet-Iliopoulos term alone. The central charge and the vortex spectrum undergo
renormalization on taking the expectation value of the central-charge operator.
It is shown that the vortex spectrum is exactly determined at one loop while
the spectrum of the elementary excitations receives higher-order corrections.Comment: 9 pages, revte
Predictable and robust performance of a Bi-2223 superconducting coil for compact isochronous cyclotrons
The development of ever smaller medical particle accelerators is motivated by a desire to make proton therapy accessible to more patients. Reducing the footprint of particle accelerators and subsequently proton therapy facilities allows for cheaper and broader usage of proton therapy. By employing superconducting technologies for field shaping, the size of particle accelerators can be reduced further below what is possible with saturated iron. This article discusses experiments on a first-of-its-kind double pancake (DP), and an assembly of six DP coils, designed to be used as a so-called âflutter coilâ for a compact isochronous cyclotron for proton therapy, fabricated from high-temperature superconducting (HTS) Bi 2 â x Pbx Sr2Ca2Cu3Oy (Bi-2223) tape. The coils were mounted under pre-stress within a stainless-steel structure to maintain mechanical stability during the experiments. The critical current as a function of the temperature of both coils was measured in a conduction-cooled setup. A model describing the coils, based on tape data, was created and revealed that the measurements were in excellent agreement with the predictions. Additional experiments were performed to study the quench and thermal runaway behaviour of the HTS coils, determining whether such coils can be protected against fault scenarios, using realistic quench-detection levels and discharge extraction-rates. These experiments demonstrate that the coils are very robust and can be well protected against quenches and thermal-runaway events using common quench-protection measures with realistic parameters.</p
Magnetic Response in Quantized Spin Hall Phase of Correlated Electrons
We investigate the magnetic response in the quantized spin Hall (SH) phase of
layered-honeycomb lattice system with intrinsic spin-orbit coupling lambda_SO
and on-site Hubbard U. The response is characterized by a parameter g= 4 U a^2
d / 3, where a and d are the lattice constant and interlayer distance,
respectively. When g< (sigma_{xy}^{s2} mu)^{-1}, where sigma_{xy}^{s} is the
quantized spin Hall conductivity and mu is the magnetic permeability, the
magnetic field inside the sample oscillates spatially. The oscillation vanishes
in the non-interacting limit U -> 0. When g > (sigma_{xy}^{s2} mu)^{-1}, the
system shows perfect diamagnetism, i.e., the Meissner effect occurs. We find
that superlattice structure with large lattice constant is favorable to see
these phenomena. We also point out that, as a result of Zeeman coupling, the
topologically-protected helical edge states shows weak diamagnetism which is
independent of the parameter g.Comment: 7 pages, the final version will be published in J. Phys. Soc. Jp
On Low-Energy Effective Actions in N = 2, 4 Superconformal Theories in Four Dimensions
We study some aspects of low-energy effective actions in 4-d superconformal
gauge theories on the Coulomb branch. We describe superconformal invariants
constructed in terms of N=2 abelian vector multiplet which play the role of
building blocks for the N=2,4 supersymmetric low-energy effective actions. We
compute the one-loop effective actions in constant N=2 field strength
background in N=4 SYM theory and in N=2 SU(2) SYM theory with four
hypermultiplets in fundamental representation. Using the classification of
superconformal invariants we then find the manifestly N=2 superconformal form
of these effective actions. While our explicit computations are done in the
one-loop approximation, our conclusions about the structure of the effective
actions in N=2 superconformal theories are general. We comment on some
applications to supergravity - gauge theory duality in the description of
D-brane interactions.Comment: 18 pages, latex, comments/reference adde
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