6,316 research outputs found
Possible indicators for low dimensional superconductivity in the quasi-1D carbide Sc3CoC4
The transition metal carbide Sc3CoC4 consists of a quasi-one-dimensional (1D)
structure with [CoC4]_{\inft} polyanionic chains embedded in a scandium
matrix. At ambient temperatures Sc3CoC4 displays metallic behavior. At lower
temperatures, however, charge density wave formation has been observed around
143K which is followed by a structural phase transition at 72K. Below T^onset_c
= 4.5K the polycrystalline sample becomes superconductive. From Hc1(0) and
Hc2(0) values we could estimate the London penetration depth ({\lambda}_L ~=
9750 Angstroem) and the Ginsburg-Landau (GL) coherence length ({\xi}_GL ~= 187
Angstroem). The resulting GL-parameter ({\kappa} ~= 52) classifies Sc3CoC4 as a
type II superconductor. Here we compare the puzzling superconducting features
of Sc3CoC4, such as the unusual temperature dependence i) of the specific heat
anomaly and ii) of the upper critical field H_c2(T) at T_c, and iii) the
magnetic hysteresis curve, with various related low dimensional
superconductors: e.g., the quasi-1D superconductor (SN)_x or the 2D
transition-metal dichalcogenides. Our results identify Sc3CoC4 as a new
candidate for a quasi-1D superconductor.Comment: 4 pages, 5 figure
Competing magnetic interactions in CeNi9-xCoxGe4
CeNi9Ge4 exhibits outstanding heavy fermion features with remarkable
non-Fermi- liquid behavior which is mainly driven by single-ion effects. The
substitution of Ni by Cu causes a reduction of both, the RKKY coupling and
Kondo interaction, coming along with a dramatic change of the crystal field
(CF) splitting. Thereby a quasi-quartet ground state observed in CeNi9Ge4
reduces to a two-fold degenerate one in CeNi8CuGe4. This leads to a
modiffcation of the effective spin degeneracy of the Kondo lattice ground state
and to the appearance of antiferromagnetic (AFM) order. To obtain a better
understanding of consequences resulting from a reduction of the effective spin
degeneracy, we stepwise replaced Ni by Co. Thereby an increase of the Kondo and
RKKY interactions through the reduction of the effective d-electron count is
expected. Accordingly, a paramagnetic Fermi liquid ground state should arise.
Our experimental studies, however, reveal AFM order already for small Co
concentrations, which becomes even more pronounced with increasing Co content
x. Thereby the modiffcation of the effective spin degeneracy seems to play a
crucial role in this system
Antiferromagnetic behavior in CeCoGe
We investigate the novel intermetallic ternary compounds
\emph{R}CoGe with \emph{R} = La and Ce by means of -ray
diffraction, susceptibility and specific heat measurements. CeCoGe
crystallizes in the space group 4/ and is characterized by the
coexistence of two different magnetic sublattices. The Ce-based sublattice,
with an effective moment close to the expected value for a Ce-ion,
exhibits a magnetically ordered ground state with K. The
Co-based sublattice, however, exhibits magnetic moments due to itinerant 3
electrons. The magnetic specific heat contribution of the Ce-sublattice is
discussed in terms of a resonance-level model implying the interplay between an
antiferromagnetic phase transition and the Kondo-effect and an underlying
Schottky-anomaly indicating a crystal field level scheme splitting into three
twofold degenerated micro states ( K, K).Comment: 4 pages, 3 figures, conference SCES0
Strong Coupling Theory of Two Level Atoms in Periodic Fields
We present a new convergent strong coupling expansion for two-level atoms in
external periodic fields, free of secular terms. As a first application, we
show that the coherent destruction of tunnelling is a third-order effect. We
also present an exact treatment of the high-frequency region, and compare it
with the theory of averaging. The qualitative frequency spectrum of the
transition probability amplitude contains an effective Rabi frequency.Comment: 4 pages with 3 figure
Crossover from Single-Ion to Coherent Non-Fermi Liquid Behavior in CeLaNiGe
We report specific heat and magneto-resistance studies on the compound
CeLaNiGe for various concentrations over the entire
stoichiometric range. Our data reveal single-ion scaling with Ce-concentration
between and 0.95. Furthermore, CeNiGe turns out to have
the largest ever recorded value of the electronic specific heat 5.5 J at K which was found in Cerium
f-electron lattice systems. In the doped samples increases
logarithmically in the temperature range between 3 K and 50 mK typical for
non-Fermi liquid (nFl) behavior, while exhibits a Kondo-like minimum
around 30 K, followed by a single-ion local nFl behavior. In contrast to this,
CeNiGe flattens out in below 300 mK and displays a
pronounced maximum in the resistivity curve at 1.5 K indicating a coherent
heavy fermion groundstate. These properties render the compound
CeLaNiGe a unique system on the borderline between
Fermi liquid and nFl physics.Comment: 2 pages, 3 figures, SCES0
Quantitative effects of abrupt deceleration on pulmonary diffusion in man Technical report,
Quantitative effects of abrupt deceleration on pulmonary diffusion in ma
Spontaneous breaking of spatial and spin symmetry in spinor condensates
Parametric amplification of quantum fluctuations constitutes a fundamental
mechanism for spontaneous symmetry breaking. In our experiments, a spinor
condensate acts as a parametric amplifier of spin modes, resulting in a twofold
spontaneous breaking of spatial and spin symmetry in the amplified clouds. Our
experiments permit a precise analysis of the amplification in specific spatial
Bessel-like modes, allowing for the detailed understanding of the double
symmetry breaking. On resonances that create vortex-antivortex superpositions,
we show that the cylindrical spatial symmetry is spontaneously broken, but
phase squeezing prevents spin-symmetry breaking. If, however, nondegenerate
spin modes contribute to the amplification, quantum interferences lead to
spin-dependent density profiles and hence spontaneously-formed patterns in the
longitudinal magnetization.Comment: 5 pages, 4 figure
Gauge symmetric delta(1232) couplings and the radiative muon capture in hydrogen
Using the difference between the gauge symmetric and standard pi-N-delta
couplings, a contact pi-pi-N-N term, quadratic in the pi-N-delta coupling, is
explicitly constructed. Besides, a contribution from the delta excitation
mechanism to the photon spectrum for the radiative muon capture in hydrogen is
derived from the gauge symmetric pi-N-delta and gamma-N-delta couplings. It is
shown for the photon spectrum, studied recently experimentally, that the new
spectrum is for the photon momentums k > 60 MeV by 4-10 % smaller than the one
obtained from standardly used couplings with the on-shell deltas.Comment: 9 pages, 3 figure
Radio frequency association of heteronuclear Feshbach molecules
We present a detailed analysis of the production efficiency of weakly bound
heteronuclear KRb-Feshbach molecules using radio frequency association in a
harmonic trap. The efficiency was measured in a wide range of temperatures,
binding energies and radio frequencies. A comprehensive analytical model is
presented, explaining the observed asymmetric spectra and achieving good
quantitative agreement with the measured production rates. This model provides
a deep understanding of the molecule association process and paves the way for
future experiments which rely on Feshbach molecules e.g. for the production of
deeply bound molecules.Comment: 5 pages, 4 figure
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