322 research outputs found
Pressure Tuning of an Ionic Insulator into a Heavy Electron Metal: An Infrared Study of YbS
Optical conductivity [] of YbS has been measured under
pressure up to 20 GPa. Below 8 GPa, is low since YbS is an
insulator with an energy gap between fully occupied 4 state and unoccupied
conduction () band. Above 8 GPa, however, increases
dramatically, developing a Drude component due to heavy carriers and
characteristic infrared peaks. It is shown that increasing pressure has caused
an energy overlap and hybridization between the band and 4 state, thus
driving the initially ionic and insulating YbS into a correlated metal with
heavy carriers
A Closed Formula for the Barrier Transmission Coefficient in Quaternionic Quantum mechanics
In this paper, we analyze, by using a matrix approach, the dynamics of a
non-relativistic particle in presence of a quaternionic potential barrier. The
matrix method used to solve the quaternionic Schrodinger equation allows to
obtain a closed formula for the transmission coefficient. Up to now, in
quaternionic quantum mechanics, almost every discussion on the dynamics of
non-relativistic particle was motived by or evolved from numerical studies. A
closed formula for the transmission coefficient stimulates an analysis of
qualitative differences between complex and quaternionic quantum mechanics,
and, by using the stationary phase method, gives the possibility to discuss
transmission times.Comment: 10 pages, 2 figure
Exploring Large-scale Structure with Billions of Galaxies
We consider cosmological applications of galaxy number density correlations
to be inferred from future deep and wide multi-band optical surveys. We mostly
focus on very large scales as a probe of possible features in the primordial
power spectrum. We find the proposed survey of the Large Synoptic Survey
Telescope may be competitive with future all-sky CMB experiments over a broad
range of scales. On very large scales the inferred power spectrum is robust to
photometric redshift errors, and, given a sufficient number density of
galaxies, to angular variations in dust extinction and photometric calibration
errors. We also consider other applications, such as constraining dark energy
with the two CMB-calibrated standard rulers in the matter power spectrum, and
controlling the effect of photometric redshift errors to facilitate the
interpretation of cosmic shear data. We find that deep photometric surveys over
wide area can provide constraints that are competitive with spectroscopic
surveys in small volumes.Comment: 11 pages, 7 figures, ApJ accepted, references added, expanded
discussion in Sec. 3.
Optical study of superconducting Ga-rich layers in silicon
We performed phase-sensitive terahertz (0.12 - 1.2 THz) transmission
measurements of Ga-enriched layers in silicon. Below the superconducting
transition, T_{c} = 6.7 K, we find clear signatures of the formation of a
superconducting condensate and of the opening of an energy gap in the optical
spectra. The London penetration depth, \lambda(T), and the condensate density,
n_{s} = \lambda^{2} 0)/\lambda^{2}(T), as functions of temperature demonstrate
behavior, typical for conventional superconductors with \lambda(0) = 1.8 \mu m.
The terahertz spectra can be well described within the framework of Eliashberg
theory with strong electron-phonon coupling: the zero-temperature energy gap is
2\Delta(0) = 2.64 meV and 2\Delta(0)/k_{B}T_{c} = 4.6 \pm 0.1, consistent with
the amorphous state of Ga. At temperatures just above T_{c}, the optical
spectra demonstrate Drude behavior.Comment: 5 pages, 4 figure
Oxygen isotope effect and phase separation in the optical conductivity of (LaPr)CaMnO thin films
The optical conductivities of films of
(LaPr)CaMnO with different oxygen isotopes
(O and O) have been determined in the spectral range from 0.3 to
4.3 eV using a combination of transmission in the mid-infrared and ellipsometry
from the near-infrared to ultra-violet regions. We have found that the isotope
exchange strongly affects the optical response in the ferromagnetic phase in a
broad frequency range, in contrast to the almost isotope-independent optical
conductivity above . The substitution by O strongly suppresses the
Drude response and a mid-infrared peak while enhancing the conductivity peak at
1.5 eV. A qualitative explanation can be given in terms of the phase separation
present in these materials. Moreover, the optical response is similar to the
one extracted from measurements in polished samples and other thin films, which
signals to the importance of internal strain.Comment: 11 pages, 11 figures, to appear in PR
Far-infrared and submillimeter-wave conductivity in electron-doped cuprate La_{2-x}Ce_xCuO_4
We performed far-infrared and submillimeter-wave conductivity experiments in
the electron-doped cuprate La_{2-x}Ce_xCuO_4 with x = 0.081 (underdoped regime,
T_c = 25 K). The onset of the absorption in the superconducting state is
gradual in frequency and is inconsistent with the isotropic s-wave gap.
Instead, a narrow quasiparticle peak is observed at zero frequency and a second
peak at finite frequencies, clear fingerprints of the conductivity in a d-wave
superconductor. A far-infrared conductivity peak can be attributed to 4Delta_0,
or to 2Delta_0 + Delta_spin, where Delta_spin is the resonance frequency of the
spin-fluctuations. The infrared conductivity as well as the suppression of the
quasiparticle scattering rate below T_c are qualitatively similar to the
results in the hole-doped cuprates.Comment: 5 pages, 4 figures include
Metal nanofilm in strong ultrafast optical fields
We predict that a metal nanofilm subjected to an ultrashort (single
oscillation) optical pulse of a high field amplitude at
normal incidence undergoes an ultrafast (at subcycle times ) transition to a state resembling semimetal. Its reflectivity is
greatly reduced, while the transmissivity and the optical field inside the
metal are greatly increased. The temporal profiles of the optical fields are
predicted to exhibit pronounced subcycle oscillations, which are attributed to
the Bloch oscillations and formation of the Wannier-Stark ladder of electronic
states. The reflected, transmitted, and inside-the-metal pulses have non-zero
areas approaching half-cycle pulses. The effects predicted are promising for
applications to nanoplasmonic modulators and field-effect transistors with
petahertz bandwidth
String amplitudes in arbitrary dimensions
We calculate gravitational dressed tachyon correlators in non critcal
dimensions. The 2D gravity part of our theory is constrained to constant
curvature. Then scaling dimensions of gravitational dressed vertex operators
are equal to their bare conformal dimensions. Considering the model as d+2
dimensional critical string we calculate poles of generalized Shapiro-Virasoro
amplitudes.Comment: 14 page
An Inversion Method for Measuring Beta in Large Redshift Surveys
A precision method for determining the value of Beta= Omega_m^{0.6}/b, where
b is the galaxy bias parameter, is presented. In contrast to other existing
techniques that focus on estimating this quantity by measuring distortions in
the redshift space galaxy-galaxy correlation function or power spectrum, this
method removes the distortions by reconstructing the real space density field
and determining the value of Beta that results in a symmetric signal. To remove
the distortions, the method modifies the amplitudes of a Fourier plane-wave
expansion of the survey data parameterized by Beta. This technique is not
dependent on the small-angle/plane-parallel approximation and can make full use
of large redshift survey data. It has been tested using simulations with four
different cosmologies and returns the value of Beta to +/- 0.031, over a factor
of two improvement over existing techniques.Comment: 16 pages including 6 figures Submitted to The Astrophysical Journa
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