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We analyze the magnetic and electronic properties of the quantum critical
heavy fermion superconductor beta-YbAlB4, calculating the Fermi surface and the
angular dependence of the extremal orbits relevant to the de Haas--van Alphen
measurements. Using a combination of the realistic materials modeling and
single-ion crystal field analysis, we are led to propose a layered Kondo
lattice model for this system, in which two dimensional boron layers are Kondo
coupled via interlayer Yb moments in a $J_{z}=\pm 5/2$ state. This model fits
the measured single ion magnetic susceptibility and predicts a substantial
change in the electronic anisotropy as the system is pressure-tuned through the
quantum critical point.
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In the presence of a magnetic impurity the spin-up and down band states are
modified differently by the impurity. If the multi-electron scalar product
(MESP) between the occupied spin-up and down states approaches zero then this
defines an orthogonality catastrophe. In the present paper the MESP is
investigated for the FAIR (Friedel Artificial Iserted Resonance) solution for a
Friedel-Anderson impurity. A basis of Wilson states is used. The MESP is
numerically determined for the (enforced) magnetic, the singlet, and the
triplet states as a function of the number N of Wilson states. The magnetic and
the triplet state show an exponentially decreasing MESP as a function of N.
Surprisingly it is not the number of states which causes this decrease. It is
instead the energy separation of the highest occupied state from the Fermi
energy which determines the reduction of the MESP. In the singlet state the
ground-state requires a finite MESP to optimize its energy. As a consequence
there is no orthogonality catastrophe. The MESP approaches a saturation value
as function of N.
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We study photon bunching phenomena associated with biexciton-exciton cascade
in single GaAs self-assembled quantum dots. Experiments carried out with a
pulsed excitation source show that significant bunching is only detectable at
very low excitation, where the typical intensity of photon streams is less than
the half of their saturation value. Our findings are qualitatively understood
with a model which accounts for Poissonian statistics in the number of
excitons, predicting the height of a bunching peak being determined by the
inverse of probability of finding more than one exciton.
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We first explain the pseudogap of high-temperature superconductivity based on
an approach of quantum optics. After introducing a damping factor for the
lifetime $\tau$ of quasiparticles, the superconducting dome is naturally
produced, and the pseudogap is the consequence of pairing with damped
coherence. We derive a new expression of Ginzburg-Landau free energy density,
in which a six-order term due to decoherence damping effect is included.
Without invoking any microscopic pairing mechanism, this approach provides a
simple universal equation of second-order phase transition, which can be
reduced to two well-known empirical scaling equations: the superconducting dome
Presland-Tallon equation, and the normal-state pseudogap crossover temperature
$T^{*}$ line.
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We study a system composed of a quantum dot in contact with ferromagnetic
leads, held at different temperatures. Spin analogues to the thermopower and
thermoelectric figures of merit are defined and studied as a function of
junction parameters. It is shown that in contrast to bulk ferromagnets, the
spin thermopower coefficient in a junction can be as large as the Seebeck
coefficient, resulting in a large spin figure of merit. In addition, it is
demonstrated that the junction can be tuned to supply only spin current but no
charge current. We also discuss experimental systems where our predictions can
be verified.
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The local lattice structure in newly discovered LaFeAsO1-xFx superconductors
is studied by extended x-ray absorption fine structure measurements. An
anomalous upturn of the mean-square relative displacement of the Fe-As bond is
detected below ~70 K as electron carriers are introduced,reflecting the
occurrence of Fe-As bond local lattice fluctuation. Similar to that in
cuprates, this lattice fluctuation exhibits an abrupt depression at the onset
superconducting transition temperature. The results indicate that strong
electron-lattice interaction is involved in the superconducting transition in
oxypcnictide superconductors, putting a strict limitation on possible
theoretical models.
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The structural properties of the SrFe2As2 and CaFe2As2 compounds have been
extensively analyzed by transmission electron microscopy (TEM) from room
temperature down to 20K. The experimental results demonstrate that the SrFe2As2
crystal, in consistence with previous x-ray data, has a tetragonal structure at
room temperature and undergoes a tetragonal (T)-orthorhombic (O) phase
transition at about 210K. Moreover, twinning lamella arising from T-O
transition evidently appears in the orthorhombic phase. On the other hand, TEM
observations of CaFe2As2 reveal the presence of a pseudo-periodic structural
modulation with the periodicity of around 40nm at room temperature. This
modulation is likely in connection with the local structural distortions within
the Ca layer. In-situ cooling TEM observations of CaFe2As2 reveal the presence
of complex domain structures in the low-temperature orthorhombic phase.
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The universal scaling function of the square lattice Ising model in a
magnetic field is obtained numerically via Baxter's variational corner transfer
matrix approach. The high precision numerical data is in perfect agreement with
the remarkable field theory results obtained by Fonseca and Zamolodchikov, as
well as with many previously known exact and numerical results for the 2D Ising
model. This includes excellent agreement with analytic results for the magnetic
susceptibility obtained by Orrick, Nickel, Guttmann and Perk. In general the
high precision of the numerical results underline the potential and full power
of the variational corner transfer matrix approach.
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Interlayer exchange couplings were examined for Co2FeAl0.5Si0.5(CFAS)/Cr/CFAS
trilayered films grown on MgO (001) single crystal and thermally oxidized Si
substrates. The films were (001) epitaxial on MgO and (110) textured
polycrystalline on SiO2. Strong exchange couplings were observed for the films
with the 1.5 nm thick Cr spacer layer. A 90 degree coupling is dominant in the
(001) epitaxial film. In contrast, an antiparallel coupling exists in the
polycrystalline one. The relationship of interlayer couplings with the
structure is discussed.
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A theory of self-propelled particles is developed in two dimensions assuming
that the particles can be deformed from a circular shape when the propagating
velocity is increased. A coupled set of equations in terms of the velocity and
a tensor variable to represent the deformation is introduced to show that there
is a bifurcation from a straight motion to a circular motion of a single
particle. Dynamics of assembly of the particles is studied numerically where
there is a global interaction such that the particles tend to cause an
orientational order.
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We introduce a lattice spin model that mimics a system of interacting
particle through a short range repulsive potential and a long range attractive
power law decaying potential. We performed a detailed analysis of the general
equilibrium phase diagram of the model at finite temperature, showing that the
only possible equilibrium pases are the ferromagnetic and the antiferromagnetic
ones. We then studied the non equilibrium behavior of the model after a quench
to subcritical temperatures, in the antiferromagnetic region of the phase
diagram region, where the pair interaction potential behaves in the same
qualitative way as in a Lennard-Jones gas. We found that, even in the absence
of quenched disorder or geometric frustration, the competition between
interactions gives rise to non--equilibrium disordered structures at low enough
temperatures that strongly slow down the relaxation of the system.
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We analyze a quantum spin Hall (QSH) device with a point contact connecting
two of its edges. The contact supports a net spin tunneling current that can be
probed experimentally via a two-terminal resistance measurement. We find that
the low-bias tunneling current and the differential conductance exhibit scaling
with voltage and temperature that depend nonlinearly on the strength of the
electron-electron interaction.
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We generalize the Poland-Scheraga (PS) model to the case of a circular DNA,
taking into account the twisting of the two strains around each other. Guided
by recent single-molecule experiments on DNA strands, we assume that the
torsional stress induced by denaturation enforces formation of supercoils whose
writhe absorbs the linking number expelled by the loops. We find that when the
the entropy parameter of a loop satisfies $c \le 2$, denaturation transition
does not take place. On the other hand for $c>2$ a first-order denaturation
transition takes place, as in the case with no supercoil. These results are in
contrast with other treatments of circular DNA melting where denaturation is
assumed to be accompanied by an increase in twist rather than writhe.
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For coherent electron spins, hyperfine coupling to nuclei in the host
material can either be a dominant source of unwanted spin decoherence or, if
controlled effectively, a resource allowing storage and retrieval of quantum
information. To investigate the effect of a controllable nuclear environment on
the evolution of confined electron spins, we have fabricated and measured
gate-defined double quantum dots with integrated charge sensors made from
single-walled carbon nanotubes with a variable concentration of 13C (nuclear
spin I=1/2) among the majority zero-nuclear-spin 12C atoms. Spin-sensitive
transport in double-dot devices grown using methane with the natural abundance
(~ 1%) of 13C is compared with similar devices grown using an enhanced (~99%)
concentration of 13C. We observe strong isotope effects in spin-blockaded
transport, and from the dependence on external magnetic field, estimate the
hyperfine coupling in 13C nanotubes to be on the order of 100 micro-eV, two
orders of magnitude larger than anticipated theoretically. 13C-enhanced
nanotubes are an interesting new system for spin-based quantum information
processing and memory, with nuclei that are strongly coupled to gate-controlled
electrons, differ from nuclei in the substrate, are naturally confined to one
dimension, lack quadrupolar coupling, and have a readily controllable
concentration from less than one to 10^5 per electron.
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We use charge sensing of Pauli blockade (including spin and isospin) in a
two-electron 13C nanotube double quantum dot to measure relaxation and
dephasing times. The relaxation time, T1, first decreases with parallel
magnetic field then goes through a minimum in a field of 1.4 T. We attribute
both results to the spin-orbit-modified electronic spectrum of carbon
nanotubes, which suppresses hyperfine mediated relaxation and enhances
relaxation due to soft phonons. The inhomogeneous dephasing time, T2*, is
consistent with previous data on hyperfine coupling strength in 13C nanotubes.
