4,983 research outputs found
Fiber Optic Tactical Local Network (FOTLAN)
A 100 Mbit/s FDDI (fiber distributed data interface) network interface unit is described that supports real-time data, voice and video. Its high-speed interrupt-driven hardware architecture efficiently manages stream and packet data transfer to the FDDI network. Other enhancements include modular single-mode laser-diode fiber optic links to maximize node spacing, optic bypass switches for increased fault tolerance, and a hardware performance monitor to gather real-time network diagnostics
Metal-insulator transition from combined disorder and interaction effects in Hubbard-like electronic lattice models with random hopping
We uncover a disorder-driven instability in the diffusive Fermi liquid phase
of a class of many-fermion systems, indicative of a metal-insulator transition
of first order type, which arises solely from the competition between quenched
disorder and interparticle interactions. Our result is expected to be relevant
for sufficiently strong disorder in d = 3 spatial dimensions. Specifically, we
study a class of half-filled, Hubbard-like models for spinless fermions with
(complex) random hopping and short-ranged interactions on bipartite lattices,
in d > 1. In a given realization, the hopping disorder breaks time reversal
invariance, but preserves the special ``nesting'' symmetry responsible for the
charge density wave instability of the ballistic Fermi liquid. This disorder
may arise, e.g., from the application of a random magnetic field to the
otherwise clean model. We derive a low energy effective field theory
description for this class of disordered, interacting fermion systems, which
takes the form of a Finkel'stein non-linear sigma model [A. M. Finkel'stein,
Zh. Eksp. Teor. Fiz. 84, 168 (1983), Sov. Phys. JETP 57, 97 (1983)]. We analyze
the Finkel'stein sigma model using a perturbative, one-loop renormalization
group analysis controlled via an epsilon-expansion in d = 2 + epsilon
dimensions. We find that, in d = 2 dimensions, the interactions destabilize the
conducting phase known to exist in the disordered, non-interacting system. The
metal-insulator transition that we identify in d > 2 dimensions occurs for
disorder strengths of order epsilon, and is therefore perturbatively accessible
for epsilon << 1. We emphasize that the disordered system has no localized
phase in the absence of interactions, so that a localized phase, and the
transition into it, can only appear due to the presence of the interactions.Comment: 47 pages, 25 figures; submitted to Phys. Rev. B. Long version of
arXiv:cond-mat/060757
Mott-Kondo Insulator Behavior in the Iron Oxychalcogenides
We perform a combined experimental-theoretical study of the
Fe-oxychalcogenides (FeO\emph{Ch}) series
LaOFeO\emph{M} (\emph{M}=S, Se), which is the latest
among the Fe-based materials with the potential \ to show unconventional
high-T superconductivity (HTSC). A combination of incoherent Hubbard
features in X-ray absorption (XAS) and resonant inelastic X-ray scattering
(RIXS) spectra, as well as resitivity data, reveal that the parent
FeO\emph{Ch} are correlation-driven insulators. To uncover microscopics
underlying these findings, we perform local density
approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that
unravel a Mott-Kondo insulating state. Based upon good agreement between theory
and a range of data, we propose that FeO\emph{Ch} may constitute a new, ideal
testing ground to explore HTSC arising from a strange metal proximate to a
novel selective-Mott quantum criticality
Selective interlayer ferromagnetic coupling between the Cu spins in YBa Cu O grown on top of La Ca MnO
Studies to date on ferromagnet/d-wave superconductor heterostructures focus
mainly on the effects at or near the interfaces while the response of bulk
properties to heterostructuring is overlooked. Here we use resonant soft x-ray
scattering spectroscopy to reveal a novel c-axis ferromagnetic coupling between
the in-plane Cu spins in YBa Cu O (YBCO) superconductor when it
is grown on top of ferromagnetic La Ca MnO (LCMO) manganite
layer. This coupling, present in both normal and superconducting states of
YBCO, is sensitive to the interfacial termination such that it is only observed
in bilayers with MnO_2but not with La Ca interfacial
termination. Such contrasting behaviors, we propose, are due to distinct
energetic of CuO chain and CuO plane at the La Ca and
MnO terminated interfaces respectively, therefore influencing the transfer
of spin-polarized electrons from manganite to cuprate differently. Our findings
suggest that the superconducting/ferromagnetic bilayers with proper interfacial
engineering can be good candidates for searching the theorized
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state in cuprates and studying the
competing quantum orders in highly correlated electron systems.Comment: Please note the change of the title. Text might be slightly different
from the published versio
Disorder-induced Majorana metal in interacting non-Abelian anyon systems
We demonstrate that a thermal metal of Majorana fermions forms in a
two-dimensional system of interacting non-Abelian (Ising) anyons in the
presence of moderate disorder. This bulk metallic phase arises in the
quantum Hall state when disorder pins the anyonic quasiparticles. More
generally, it naturally occurs for various proposed systems supporting Majorana
fermion zero modes when disorder induces the random pinning of a finite density
of vortices. This includes all two-dimensional topological superconductors in
so-called symmetry class D. A distinct experimental signature of the thermal
metal phase is the presence of bulk heat transport down to zero temperature.Comment: 4 pages, 6 figure
Topological Surface States and Dirac point tuning in ternary Bi2Te2Se class of topological insulators
Using angle-resolved photoemission spectroscopy, we report electronic
structure for representative members of ternary topological insulators. We show
that several members of this family, such as Bi2Se2Te, Bi2Te2Se, and GeBi2Te4,
exhibit a singly degenerate Dirac-like surface state, while Bi2Se2S is a fully
gapped insulator with no measurable surface state. One of these compounds,
Bi2Se2Te, shows tunable surface state dispersion upon its electronic alloying
with Sb (SbxBi2-xSe2Te series). Other members of the ternary family such as
GeBi2Te4 and BiTe1.5S1.5 show an in-gap surface Dirac point, the former of
which has been predicted to show nonzero weak topological invariants such as
(1;111); thus belonging to a different topological class than BiTe1.5S1.5. The
measured band structure presented here will be a valuable guide for
interpreting transport, thermoelectric, and thermopower measurements on these
compounds. The unique surface band topology observed in these compounds
contributes towards identifying designer materials with desired flexibility
needed for thermoelectric and spintronic device fabrication.Comment: 9 pages, 6 figures; Related results at
http://online.kitp.ucsb.edu/online/topomat11/hasan
Surface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPES
The Kondo insulator SmB6 has long been known to exhibit low temperature (T <
10K) transport anomaly and has recently attracted attention as a new
topological insulator candidate. By combining low-temperature and high
energy-momentum resolution of the laser-based ARPES technique, for the first
time, we probe the surface electronic structure of the anomalous conductivity
regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and
identify in-gap low-lying states within a 4 meV window of the Fermi level on
the (001)-surface of this material. The low-lying states are found to form
electron-like Fermi surface pockets that enclose the X and the Gamma points of
the surface Brillouin zone. These states disappear as temperature is raised
above 15K in correspondence with the complete disappearance of the 2D
conductivity channels in SmB6. While the topological nature of the in-gap
metallic states cannot be ascertained without spin (spin-texture) measurements
our bulk and surface measurements carried out in the
transport-anomaly-temperature regime (T < 10K) are consistent with the
first-principle predicted Fermi surface behavior of a topological Kondo
insulator phase in this material.Comment: 4 Figures, 6 Page
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