1,274,691 research outputs found
Structural, magnetic, and transport properties of CoFeSi Heusler films
We report the deposition of thin CoFeSi films by RF magnetron sputtering.
Epitaxial (100)-oriented and L2 ordered growth is observed for films grown
on MgO(100) substrates. (110)-oriented films on AlO(110) show several
epitaxial domains in the film plane. Investigation of the magnetic properties
reveals a saturation magnetization of 5.0 at low temperatures. The
temperature dependence of the resistivity exhibits a crossover
from a T^3.5 law at T<50K to a T^1.65 behaviour at elevated temperatures.
shows a small anisotropic magnetoresistive effect. A weak
dependence of the normal Hall effect on the external magnetic field indicates
the compensation of electron and hole like contributions at the Fermi surface.Comment: 10 pages, 9 figures to be published in J. Phys. D: Appl. Phy
c-axis Raman Scattering in MgB2: Observation of a Dirty-Limit Gap in the pi-bands
Raman scattering spectra from the ac-face of thick MgB2 single crystals were
measured in zz, xz and xx polarisations. In zz and xz polarisations a threshold
at around 29 cm^{-1} forms in the below Tc continuum but no pair-breaking peak
is seen, in contrast to the sharp pair-breaking peak at around 100 cm^{-1} seen
in xx polarisation. The zz and xz spectra are consistent with Raman scattering
from a dirty superconductor while the sharp peak in the xx spectra argues for a
clean system. Analysis of the spectra resolves this contradiction, placing the
larger and smaller gap magnitudes in the sigma and pi bands, and indicating
that relatively strong impurity scattering is restricted to the pi bands.Comment: Revised manuscript accepted for publication in Physical Review
Letter
High Frequency Conductivity in the Quantum Hall Regime
We have measured the complex conductivity of a two-dimensional
electron system in the quantum Hall regime up to frequencies of 6 GHz at
electron temperatures below 100 mK. Using both its imaginary and real part we
show that can be scaled to a single function for different
frequencies and for all investigated transitions between plateaus in the
quantum Hall effect. Additionally, the conductivity in the variable-range
hopping regime is used for a direct evaluation of the localization length
. Even for large filing factor distances from the critical
point we find with a scaling exponent
Discovery of the spectroscopic binary nature of the Cepheids X Puppis and XX Sagittarii
We present the analysis of photometric and spectroscopic data of two bright
Galactic Cepheids, X Puppis and XX Sagittarii. Based on the available data in
the literature as well as own observations spanning 75 years, we conclude that
both Cepheids belong to spectroscopic binary systems. However, the data are not
sufficient to determine the orbital periods nor other elements for the orbit.
This discovery corroborates the statement on the high frequency of occurrence
of binaries among the classical Cepheids, a fact to be taken into account when
calibrating the period-luminosity relationship for Cepheids. The photometric
data revealed that the pulsation period of X Pup is continuously increasing
with Delta P=0.007559 d/century likely caused by stellar evolution. The
pulsation period of XX Sgr turned out to be very stable in the last ~100 years.Comment: 6 pages, 5 figure
Thermoelectric Hall conductivity and figure of merit in Dirac/Weyl materials
We calculate the thermoelectric response coefficients of three-dimensional
Dirac or Weyl semimetals as a function of magnetic field, temperature, and
Fermi energy. We focus in particular on the thermoelectric Hall coefficient
and the Seebeck coefficient , which are well-defined even
in the dissipationless limit. We contrast the behaviors of and
with those of traditional Schr\"{o}dinger particle systems, such as
doped semiconductors. Strikingly, we find that for Dirac materials
acquires a constant, quantized value at sufficiently large
magnetic field, which is independent of the magnetic field or the Fermi energy,
and this leads to unprecedented growth in the thermopower and the
thermoelectric figure of merit. We further show that even relatively small
fields, such that (where is the cyclotron
frequency and is the scattering time), are sufficient to produce a more
than increase in the figure of merit.Comment: 10 pages, 5 figure
Status Report on the Low-Energy Demonstration Accelerator (LEDA)
The 75-keV injector and 6.7-MeV RFQ that comprise the first portion of the
cw, 100-mA proton linac for the accelerator production of tritium (APT) project
have been built and operated. The LEDA RFQ has been extensively tested for
pulsed and cw output-beam currents <=100 mA. Up to 2.2 MW of cw rf power from
the 350-MHz rf system is coupled into the RFQ, including 670 kW for the cw
proton beam. The emittance for a 93-mA pulsed RFQ output beam, as determined
from quadrupole-magnet-scan measurements, is ex x ey = 0.25 x 0.31 (pi mm
mrad)2 [rms normalized]. A follow-on experiment, to intentionally introduce and
measure beam halo on the RFQ output beam, is now being installed.Comment: Paper TUD14 for the XX International Linac Conference (LINAC2000), 3
pages, 10 figure
Itinerant Ferromagnetism in the electronic localization limit
We present Hall effect, , and magnetoresistance, ,
measurements of ultrathin films of Ni, Co and Fe with thicknesses varying
between 0.2-8 nm and resistances between 1 M - 100 Both
measurements show that films having resistance above a critical value, ,
(thickness below a critical value, ) show no signs for ferromagnetism.
Ferromagnetism appears only for films with , where is material
dependent. We raise the possibility that the reason for the absence of
spontaneous magnetization is suppression of itinerant ferromagnetism by
electronic disorder in the strong localization regime.Comment: 4 pages, 4 figure
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