124,541 research outputs found
Phonon anomalies in pure and underdoped R{1-x}K{x}Fe{2}As{2} (R = Ba, Sr) investigated by Raman light scattering
We present a detailed temperature dependent Raman light scattering study of
optical phonons in Ba{1-x}K{x}Fe{2}As{2} (x ~ 0.28, superconducting Tc ~ 29 K),
Sr{1-x}K{x}Fe{2}As{2} (x ~ 0.15, Tc ~ 29 K) and non-superconducting
BaFe{2}As{2} single crystals. In all samples we observe a strong continuous
narrowing of the Raman-active Fe and As vibrations upon cooling below the
spin-density-wave transition Ts. We attribute this effect to the opening of the
spin-density-wave gap. The electron-phonon linewidths inferred from these data
greatly exceed the predictions of ab-initio density functional calculations
without spin polarization, which may imply that local magnetic moments survive
well above Ts. A first-order structural transition accompanying the
spin-density-wave transition induces discontinuous jumps in the phonon
frequencies. These anomalies are increasingly suppressed for higher potassium
concentrations. We also observe subtle phonon anomalies at the superconducting
transition temperature Tc, with a behavior qualitatively similar to that in the
cuprate superconductors.Comment: 5 pages, 6 figures, accepted versio
Detection of a single-charge defect in a metal-oxide-semiconductor structure using vertically coupled Al and Si single-electron transistors
An Al-AlO_x-Al single-electron transistor (SET) acting as the gate of a
narrow (~ 100 nm) metal-oxide-semiconductor field-effect transistor (MOSFET)
can induce a vertically aligned Si SET at the Si/SiO_2 interface near the
MOSFET channel conductance threshold. By using such a vertically coupled Al and
Si SET system, we have detected a single-charge defect which is tunnel-coupled
to the Si SET. By solving a simple electrostatic model, the fractions of each
coupling capacitance associated with the defect are extracted. The results
reveal that the defect is not a large puddle or metal island, but its size is
rather small, corresponding to a sphere with a radius less than 1 nm. The small
size of the defect suggests it is most likely a single-charge trap at the
Si/SiO_2 interface. Based on the ratios of the coupling capacitances, the
interface trap is estimated to be about 20 nm away from the Si SET.Comment: 5 pages and 5 figure
Proximity and anomalous field-effect characteristics in double-wall carbon nanotubes
Proximity effect on field-effect characteristic (FEC) in double-wall carbon
nanotubes (DWCNTs) is investigated. In a semiconductor-metal (S-M) DWCNT, the
penetration of electron wavefunctions in the metallic shell to the
semiconducting shell turns the original semiconducting tube into a metal with a
non-zero local density of states at the Fermi level. By using a two-band
tight-binding model on a ladder of two legs, it is demonstrated that anomalous
FEC observed in so-called S-M type DWCNTs can be fully understood by the
proximity effect of metallic phases.Comment: 4 pages, 4 figure
Entanglement between two fermionic atoms inside a cylindrical harmonic trap
We investigate quantum entanglement between two (spin-1/2) fermions inside a
cylindrical harmonic trap, making use of the von Neumann entropy for the
reduced single particle density matrix as the pure state entanglement measure.
We explore the dependence of pair entanglement on the geometry and strength of
the trap and on the strength of the pairing interaction over the complete range
of the effective BCS to BEC crossover. Our result elucidates an interesting
connection between our model system of two fermions and that of two interacting
bosons.Comment: to appear in PR
High-Order Adiabatic Approximation for Non-Hermitian Quantum System and Complexization of Berry's Phase
In this paper the evolution of a quantum system drived by a non-Hermitian
Hamiltonian depending on slowly-changing parameters is studied by building an
universal high-order adiabatic approximation(HOAA) method with Berry's phase
,which is valid for either the Hermitian or the non-Hermitian cases. This
method can be regarded as a non-trivial generalization of the HOAA method for
closed quantum system presented by this author before. In a general situation,
the probabilities of adiabatic decay and non-adiabatic transitions are
explicitly obtained for the evolution of the non-Hermitian quantum system. It
is also shown that the non-Hermitian analog of the Berry's phase factor for the
non-Hermitian case just enjoys the holonomy structure of the dual linear bundle
over the parameter manifold. The non-Hermitian evolution of the generalized
forced harmonic oscillator is discussed as an illustrative examples.Comment: ITP.SB-93-22,17 page
Fourier transform and rigidity of certain distributions
Let be a finite dimensional vector space over a local field, and be
its dual. For a closed subset of , and of , consider the space
of tempered distributions on whose support are contained
in and support of whose Fourier transform are contained in . We show
that possesses a certain rigidity property, for ,
which are some finite unions of affine subspaces.Comment: 10 page
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