1,535 research outputs found
2PI Effective Action and Evolution Equations of N = 4 super Yang-Mills
We employ nPI effective action techniques to study N = 4 super Yang-Mills,
and write down the 2PI effective action of the theory. We also supply the
evolution equations of two-point correlators within the theory.Comment: 16 pages, 6 figures. Figure 2 replaced, approximation scheme
clarified, references adde
Controlling the interfacial conductance in LaAlO₃/SrTiO₃ in 90∘ off-axis sputter deposition
We report on the fabrication of conducting interfaces between
LaAlO
3
and
SrTiO
3
by
90
∘
off-axis sputtering in an Ar atmosphere. At a growth pressure of 0.04 mbar the interface is metallic, with a carrier density of the order of
1
×
10
13
cm
−
2
at 3 K. By increasing the growth pressure, we observe an increase of the out-of-plane lattice constants of the
LaAlO
3
films while the in-plane lattice constants do not change. Also, the low-temperature sheet resistance increases with increasing growth pressure, leading to an insulating interface when the growth pressure reaches 0.10 mbar. We attribute the structural variations to an increase of the La/Al ratio, which also explains the transition from metallic behavior to insulating behavior of the interfaces. Our research shows that the control which is furnished by the Ar pressure makes sputtering as versatile a process as pulsed laser deposition, and emphasizes the key role of the cation stoichiometry of
LaAlO
3
in the formation of the conducting interface
Optimality Theory as a Framework for Lexical Acquisition
This paper re-investigates a lexical acquisition system initially developed
for French.We show that, interestingly, the architecture of the system
reproduces and implements the main components of Optimality Theory. However, we
formulate the hypothesis that some of its limitations are mainly due to a poor
representation of the constraints used. Finally, we show how a better
representation of the constraints used would yield better results
Inhomogeneous superconductivity and quasilinear magnetoresistance at amorphous LaTiO3/SrTiO3 interfaces
We have studied the transport properties of LaTiO3/SrTiO3 (LTO/STO)
heterostructures. In spite of 2D growth observed in reflection high energy
electron diffraction, Transmission Electron Microscopy images revealed that the
samples tend to amorphize. Still, we observe that the structures are
conducting, and some of them exhibit high conductance and/or superconductivity.
We established that conductivity arises mainly on the STO side of the
interface, and shows all the signs of the 2-dimensional electron gas usually
observed at interfaces between SrTiO3 and LaTiO3 or LaAlO3, including the
presence of two electron bands and tunability with a gate voltage. Analysis of
magnetoresistance (MR) and superconductivity indicates presence of a spatial
fluctuations of the electronic properties in our samples. That can explain the
observed quasilinear out-of-plane MR, as well as various features of the
in-plane MR and the observed superconductivity.Comment: 10 pages, 6 figures, plus Supplementary Informatio
Waiting and Residence Times of Brownian Interface Fluctuations
We report on the residence times of capillary waves above a given height
and on the typical waiting time in between such fluctuations. The measurements
were made on phase separated colloid-polymer systems by laser scanning confocal
microscopy. Due to the Brownian character of the process, the stochastics vary
with the chosen measurement interval . In experiments, the discrete
scanning times are a practical cutoff and we are able to measure the waiting
time as a function of this cutoff. The measurement interval dependence of the
observed waiting and residence times turns out to be solely determined by the
time dependent height-height correlation function . We find excellent
agreement with the theory presented here along with the experiments.Comment: 5 figure
Degenerate distributions in complex Langevin dynamics: one-dimensional QCD at finite chemical potential
We demonstrate analytically that complex Langevin dynamics can solve the sign
problem in one-dimensional QCD in the thermodynamic limit. In particular, it is
shown that the contributions from the complex and highly oscillating spectral
density of the Dirac operator to the chiral condensate are taken into account
correctly. We find an infinite number of classical fixed points of the Langevin
flow in the thermodynamic limit. The correct solution originates from a
continuum of degenerate distributions in the complexified space.Comment: 20 pages, several eps figures, minor comments added, to appear in
JHE
Enhancement of the superconducting transition temperature in Nb/Permalloy bilayers by controlling the domain state of the ferromagnet
In (S/F) hybrids the suppression of superconductivity by the exchange field
h_ex of the ferromagnet can be partially lifted when different directions of
h_ex are sampled simultaneously by the Cooper pair. In F/S/F trilayer
geometries where the magnetization directions of the two F-layers can be
controlled separately, this leads to the so-called spin switch. Here we show
that domain walls in a single F-layer yield a similar effect. We study the
transport properties of Ni_0.8Fe_0.2/Nb bilayers structured in strips of
different sizes. For large samples a clear enhancement of superconductivity
takes place in the resistive transition, in the very narrow field range (order
of 0.5 mT) where the magnetization of the Py layer switches and many domains
are present. This effect is absent in microstructured samples. Comparison of
domain wall width \delta_w to the temperature dependent superconductor
coherence length \xi_S(T) shows that \delta_w ~ \xi_S(T), which means that the
Cooper pairs sample a large range of different magnetization directions.Comment: 4 pages, 5 figure
Complex Langevin dynamics in the SU(3) spin model at nonzero chemical potential revisited
The three-dimensional SU(3) spin model is an effective Polyakov loop model
for QCD at nonzero temperature and density. It suffers from a sign problem at
nonzero chemical potential. We revisit this model using complex Langevin
dynamics and assess in particular the justification of this approach, using
analyticity at small mu^2 and the criteria for correctness developed recently.
Finite-stepsize effects are discussed in some detail and a higher-order
algorithm is employed to eliminate leading stepsize corrections. Our results
strongly indicate that complex Langevin dynamics is reliable in this theory in
both phases, including the critical region. This is in sharp contrast to the
case of the XY model, where correct results were obtained in only part of the
phase diagram.Comment: 23 pages, several figures, minor typos corrected, to appear in JHE
Spin Screening and Antiscreening in a Ferromagnet/Superconductor Heterojunction
We present a theoretical study of spin screening effects in a
ferromagnet/superconductor (F/S) heterojunction. It is shown that the magnetic
moment of the ferromagnet is screened or antiscreened, depending on the
polarization of the electrons at the Fermi level. If the polarization is
determined by the electrons of the majority (minority) spin band then the
magnetic moment of the ferromagnet is screened (antiscreened) by the electrons
in the superconductor. We propose experiments that may confirm our theory: for
ferromagnetic alloys with certain concentration of Fe or Ni ions there will be
screening or antiscreening respectively. Different configurations for the
density of states are also discussed.Comment: 5 pages; 4 figures. to be published in Phys. Rev,
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