515 research outputs found
Enhanced self-field critical current density of nano-composite YBa(2)Cu(3)O(7) thin films grown by pulsed-laser deposition
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ EPLA, 2008.Enhanced self-field critical current density Jc of novel, high-temperature superconducting thin films is reported. Layers are deposited on (001) MgO substrates by laser ablation of YBa2Cu3O7âÎŽ(Y-123) ceramics containing Y2Ba4CuMOx (M-2411, M=Ag, Nb, Ru, Zr) nano-particles. The Jc of films depends on the secondary-phase content of the ceramic targets, which was varied between 0 and 15âmol%. Composite layers (2âmol% of Ag-2411 and Nb-2411) exhibit Jc values at 77âK of up to 5.1âMA/cm2, which is 3 to 4 times higher than those observed in films deposited from phase pure Y-123 ceramics. Nb-2411 grows epitaxially in the composite layers and the estimated crystallite size is ~10ânm.The Austrian Science Fund, the Austrian Federal Ministry of Economics and Labour, the European Science Foundation and the Higher Education Commission of Pakistan
Magnetic dephasing in mesoscopic spin glasses
We have measured Universal Conductance Fluctuations in the metallic spin
glass Ag:Mn as a function of temperature and magnetic field. From this
measurement, we can access the phase coherence time of the electrons in the
spin glass. We show that this phase coherence time increases with both the
inverse of the temperature and the magnetic field. From this we deduce that
decoherence mechanisms are still active even deep in the spin glass phase
Remotely pumped GHz anti-bunched emission from single exciton states
Quantum communication networks require on-chip transfer and manipulation of
single particles as well as their interconversion to single photons for
long-range information exchange. Flying excitons propelled by GHz surface
acoustic waves (SAWs) are outstanding messengers to fulfill these requirements.
Here, we demonstrate the acoustic manipulation of two-level states consisting
of individual excitons bound to shallow impurities centers embedded in a
semiconductor quantum well. Time-resolved photoluminescence studies show that
the emission intensity and energy from these centers oscillate at the SAW
frequency of 3.5 GHz. Furthermore, these centers can be remotely pumped via
acoustic transport along a quantum well channel over several micron. Time
correlation studies reveal that the centers emit anti-bunched light, thus
acting as single-photon sources operating at GHz frequencies. Our results pave
the way for the exciton-based on-demand manipulation and on-chip transfer of
single excitons at microwave frequencies with a natural photonic interface.Comment: 10 pages, 7 figure
Are there static texture?
We consider harmonic maps from Minkowski space into the three sphere. We are
especially interested in solutions which are asymptotically constant, i.e.
converge to the same value in all directions of spatial infinity. Physical
3-space can then be compactified and can be identified topologically (but not
metrically!) with a three sphere. Therefore, at fixed time, the winding of the
map is defined. We investigate whether static solutions with non-trivial
winding number exist. The answer which we can proof here is only partial: We
show that within a certain family of maps no static solutions with non-zero
winding number exist. We discuss the existing static solutions in our family of
maps. An extension to other maps or a proof that our family of maps is
sufficiently general remains an open problem.Comment: 12 page Latex file, 1 postscript figure, submitted to PR
Electronic structure and excited state dynamics in a dicyanovinyl-substituted oligothiophene on Au(111)
Dicyanovinyl (DCV)-substituted oligothiophenes are promising donor materials
in vacuum-processed small-molecule organic solar cells. Here, we studied the
structural and the electronic properties of DCV-dimethyl-pentathiophene
(DCV5T-Me2) adsorbed on Au(111) from submonolayer to multilayer coverages.
Using a multi-technique experimental approach (low-temperature scanning
tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM),
and two-photon photoemission (2PPE) spectroscopy), we determined the energetic
position of several affinity levels as well as ionization potentials
originating from the lowest unoccupied molecular orbitals (LUMO) and the
highest occupied molecular orbitals (HOMO), evidencing a transport gap of 1.4
eV. Proof of an excitonic state was found to be a spectroscopic feature
located at 0.6 eV below the LUMO affinity level. With increasing coverage
photoemission from excitonic states gains importance. We were able to track
the dynamics of several electronically excited states of multilayers by means
of femtosecond time-resolved 2PPE. We resolved an intriguing relaxation
dynamics involving four processes, ranging from sub-picosecond (ps) to several
hundred ps time spans. These show a tendency to increase with increasing
coverage. The present study provides important parameters such as energetic
positions of transport levels as well as lifetimes of electronically excited
states, which are essential for designing organic-molecule-based
optoelectronic devices
Angle-dependence of the Hall effect in HgBa2CaCu2O6 thin films
Superconducting compounds of the family Hg-Ba-Ca-Cu-O have been the subject
of intense study since the current record-holder for the highest critical
temperature of a superconductor belongs to this class of materials. Thin films
of the compound with two adjacent copper-oxide layers and a critical
temperature of about 120 K were prepared by a two-step process that consists of
the pulsed-laser deposition of precursor films and the subsequent annealing in
mercury-vapor atmosphere. Like some other high-temperature superconductors,
Hg-Ba-Ca-Cu-O exhibits a specific anomaly of the Hall effect, a double-sign
change of the Hall coefficient close to the superconducting transition. We have
investigated this phenomenon by measurements of the Hall effect at different
angles between the magnetic field direction and the crystallographic c-axis.
The results concerning the upper part of the transition, where the first sign
change occurs, are discussed in terms of the renormalized fluctuation model for
the Hall conductivity, adapted through the field rescaling procedure in order
to take into account the arbitrary orientation of the magnetic field.Comment: to be published in Phys. Rev.
Two Scenarios of the Quantum Critical Point
Two different scenarios of the quantum critical point (QCP), a
zero-temperature instability of the Landau state, related to the divergence of
the effective mass, are investigated. Flaws of the standard scenario of the
QCP, where this divergence is attributed to the occurrence of some second-order
phase transition, are demonstrated. Salient features of a different {\it
topological} scenario of the QCP, associated with the emergence of bifurcation
points in equation that ordinarily determines the Fermi
momentum, are analyzed. The topological scenario of the QCP is applied to
three-dimensional (3D) Fermi liquids with an attractive current-current
interaction.Comment: 6 pages, added new discussion and 2 figure
Scaling of the low temperature dephasing rate in Kondo systems
We present phase coherence time measurements in quasi-one-dimensional Ag
wires doped with Fe Kondo impurities of different concentrations . Due to
the relatively high Kondo temperature of this system, we
are able to explore a temperature range from above down to below . We show that the magnetic contribution to the dephasing rate
per impurity is described by a single, universal curve when plotted as a
function of . For , the dephasing rate is remarkably well
described by recent numerical results for spin impurities. At lower
temperature, we observe deviations from this theory. Based on a comparison with
theoretical calculations for , we discuss possible explanations for the
observed deviations.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Transport through side-coupled double quantum dots: from weak to strong interdot coupling
We report low-temperature transport measurements through a double quantum dot
device in a configuration where one of the quantum dots is coupled directly to
the source and drain electrodes, and a second (side-coupled) quantum dot
interacts electrostatically and via tunneling to the first one. As the interdot
coupling increases, a crossover from weak to strong interdot tunneling is
observed in the charge stability diagrams that present a complex pattern with
mergings and apparent crossings of Coulomb blockade peaks. While the weak
coupling regime can be understood by considering a single level on each dot, in
the intermediate and strong coupling regimes, the multi-level nature of the
quantum dots needs to be taken into account. Surprisingly, both in the strong
and weak coupling regimes, the double quantum dot states are mainly localized
on each dot for most values of the parameters. Only in an intermediate coupling
regime the device presents a single dot-like molecular behavior as the
molecular wavefunctions weight is evenly distributed between the quantum dots.
At temperatures larger than the interdot coupling energy scale, a loss of
coherence of the molecular states is observed.Comment: 9 pages, 5 figure
``Cosmological'' scenario for A-B phase transition in superfluid 3He
At a very rapid superfluid transition in He, follows after a reaction
with single neutron, the creation of topological defects (vortices) has
recently been demonstrated in accordance with the Kibble-Zurek scenario for the
cosmological analogue. We discuss here the extension of the Kibble-Zurek
scenario to the case when alternative symmetries may be broken and different
states nucleated independently. We have calculated the nucleation probability
of the various states of superfluid He during a superfluid transition. Our
results can explain the transition from supercooled phase to the phase,
triggered by nuclear reaction. The new scenario is an alternative to the
well-known ``baked Alaska'' scenario.Comment: RevTex file, 4 pages, 3 figures, submitted to Phys. Rev. Let
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