1,359 research outputs found
Magnetic Order Beyond RKKY in the Classical Kondo Lattice
We study the Kondo lattice model of band electrons coupled to classical
spins, in three dimensions, using a combination of variational calculation and
Monte Carlo. We use the weak coupling `RKKY' window and the strong coupling
regime as benchmarks, but focus on the physically relevant intermediate
coupling regime. Even for modest electron-spin coupling the phase boundaries
move away from the RKKY results, the non interacting Fermi surface no longer
dictates magnetic order, and weak coupling `spiral' phases give way to
collinear order. We use these results to revisit the classic problem of 4f
magnetism and demonstrate how both electronic structure and coupling effects
beyond RKKY control the magnetism in these materials.Comment: 6 pages, 4 figs. Improved figures, expanded captions. To appear in
Europhys. Let
Spin-dynamic field coupling in strongly THz driven semiconductors : local inversion symmetry breaking
We study theoretically the optics in undoped direct gap semiconductors which
are strongly driven in the THz regime. We calculate the optical sideband
generation due to nonlinear mixing of the THz field and the near infrared
probe. Starting with an inversion symmetric microscopic Hamiltonian we include
the THz field nonperturbatively using non-equilibrium Green function
techniques. We find that a self induced relativistic spin-THz field coupling
locally breaks the inversion symmetry, resulting in the formation of odd
sidebands which otherwise are absent.Comment: 8 pages, 6 figure
Effect of Al mole fraction on carrier diffusion lengths and lifetimes in AlxGa1âxAs
The ambipolar diffusion length and carrier lifetime are measured in AlxGa1âxAs for several mole fractions in the interval 0<x<0.38. These parameters are found to have significantly higher values in the higher mole fraction samples. These increases are attributed to occupation of states in the indirect valleys, and supporting calculations are presented
A double-lined spectroscopic orbit for the young star HD 34700
We report high-resolution spectroscopic observations of the young star HD
34700, which confirm it to be a double-lined spectroscopic binary. We derive an
accurate orbital solution with a period of 23.4877 +/- 0.0013 days and an
eccentricity of e = 0.2501 +/- 0.0068. The stars are found to be of similar
mass (M2/M1 = 0.987 +/- 0.014) and luminosity. We derive also the effective
temperatures (5900 K and 5800 K) and projected rotational velocities (28 km/s
and 22 km/s) of the components. These values of v sin i are much higher than
expected for main-sequence stars of similar spectral type (G0), and are not due
to tidal synchronization. We discuss also the indicators of youth available for
the object. Although there is considerable evidence that the system is young
--strong infrared excess, X-ray emission, Li I 6708 absorption (0.17 Angstroms
equivalent width), H alpha emission (0.6 Angstroms), rapid rotation-- the
precise age cannot yet be established because the distance is unknown.Comment: 17 pages, including 2 figures and 2 tables. Accepted for publication
in AJ, to appear in February 200
First Principles Study of Zn-Sb Thermoelectrics
We report first principles LDA calculations of the electronic structure and
thermoelectric properties of -ZnSb. The material is found
to be a low carrier density metal with a complex Fermi surface topology and
non-trivial dependence of Hall concentration on band filling. The band
structure is rather covalent, consistent with experimental observations of good
carrier mobility. Calculations of the variation with band filling are used to
extract the doping level (band filling) from the experimental Hall number. At
this band filling, which actually corresponds to 0.1 electrons per 22 atom unit
cell, the calculated thermopower and its temperature dependence are in good
agreement with experiment. The high Seebeck coefficient in a metallic material
is remarkable, and arises in part from the strong energy dependence of the
Fermiology near the experimental band filling. Improved thermoelectric
performance is predicted for lower doping levels which corresponds to higher Zn
concentrations.Comment: 5 pages, 6 figure
Quasienergy Spectroscopy of Excitons
We theoretically study nonlinear optics of excitons under intense THz
irradiation. In particular, the linear near infrared absorption and resonantly
enhanced nonlinear sideband generation are described. We predict a rich
structure in the spectra which can be interpreted in terms of the quasienergy
spectrum of the exciton, via a remarkably transparent expression for the
susceptibility, and show that the effects of strongly avoided quasienergy
crossings manifest themselves directly, both in the absorption and transmitted
sidebands.Comment: 4 pages RevTex, 3 eps figs included, as publishe
Dynamical Masses for Pre-Main Sequence Stars: A Preliminary Physical Orbit for V773 Tau A
We report on interferometric and radial-velocity observations of the
double-lined 51-d period binary (A) component of the quadruple pre-main
sequence (PMS) system V773 Tau. With these observations we have estimated
preliminary visual and physical orbits of the V773 Tau A subsystem. Among other
parameters, our orbit model includes an inclination of 66.0 2.4 deg, and
allows us to infer the component dynamical masses and system distance. In
particular we find component masses of 1.54 0.14 and 1.332 0.097
M_{\sun} for the Aa (primary) and Ab (secondary) components respectively.
Our modeling of the subsystem component spectral energy distributions finds
temperatures and luminosities consistent with previous studies, and coupled
with the component mass estimates allows for comparison with PMS stellar models
in the intermediate-mass range. We compare V773 Tau A component properties with
several popular solar-composition models for intermediate-mass PMS stars. All
models predict masses consistent to within 2-sigma of the dynamically
determined values, though some models predict values that are more consistent
than others.Comment: ApJ in press; 25 pages, 6 figures; data tables available in journal
versio
The Machine Learning Landscape of Top Taggers
Based on the established task of identifying boosted, hadronically decaying
top quarks, we compare a wide range of modern machine learning approaches.
Unlike most established methods they rely on low-level input, for instance
calorimeter output. While their network architectures are vastly different,
their performance is comparatively similar. In general, we find that these new
approaches are extremely powerful and great fun.Comment: Yet another tagger included
Exact boundary conditions in numerical relativity using multiple grids: scalar field tests
Cauchy-Characteristic Matching (CCM), the combination of a central 3+1 Cauchy
code with an exterior characteristic code connected across a time-like
interface, is a promising technique for the generation and extraction of
gravitational waves. While it provides a tool for the exact specification of
boundary conditions for the Cauchy evolution, it also allows to follow
gravitational radiation all the way to infinity, where it is unambiguously
defined.
We present a new fourth order accurate finite difference CCM scheme for a
first order reduction of the wave equation around a Schwarzschild black hole in
axisymmetry. The matching at the interface between the Cauchy and the
characteristic regions is done by transfering appropriate characteristic/null
variables. Numerical experiments indicate that the algorithm is fourth order
convergent. As an application we reproduce the expected late-time tail decay
for the scalar field.Comment: 14 pages, 5 figures. Included changes suggested by referee
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