405 research outputs found
Superconducting phase formation in random neck syntheses: a study of the Y-Ba-Cu-O system by magneto-optics and magnetometry
Magneto-optical imaging and magnetization measurements were applied to
investigate local formation of superconducting phase effected by a random neck
synthesis in Y-Ba-Cu-O system. Polished pellets of strongly inhomogeneous
ceramic samples show clearly the appearance of superconducting material in the
intergrain zones of binary primary particles reacted under different
conditions. Susceptibility measurements allows evaluation of superconducting
critical temperature, which turned out to be close to that of optimally doped
YBCO.Comment: 6 pages, 11 figure
Detailed magnetization study of superconducting properties of YBCO ceramic spheres
We present a magnetization study of low density YBCO ceramics carried out in
magnetic fields 0.5 Oe < H < 50 kOe. It was demonstrated that superconducting
links between grains may be completely suppressed either by a magnetic field of
the order of 100 Oe (at low temperatures) or by an increase of temperature
above 70 K. This property of present samples allowed to evaluate the ratio
between an average grain size and the magnetic field penetration depth lambda.
Furthermore, at temperatures T > 85 K, using low-field magnetization
measurements, we could evaluate the temperature dependence of lambda, which
turned out to be very close to predictions of the conventional Ginzburg-Landau
theory. Although present samples consisted of randomly oriented grains,
specifics of magnetization measurements allowed for evaluation of lambda_ab(T).
Good agreement between our estimation of the grain size with the real sample
structure provides evidence for the validity of this analysis of magnetization
data. Measurements of equilibrium magnetization in high magnetic fields were
used for evaluation of Hc2(T). At temperatures close to T_c, the Hc2(T)
dependence turned out to be linear in agreement with the Ginzburg-Landau
theory. The value of temperature, at which Hc2 vanishes, coincides with the
superconducting critical temperature evaluated from low-field measurements.Comment: 10 pages, 12 figure
Czochralski growth of Yb3+ and Pr3+ doped Ca-fluoroapatite
Pure, Pr3+, and Yb3+ doped Ca-fluoroapatite (Ca-FAP) crystals were grown by the Czochralski method. The effective distribution coefficient keff for Yb3+ was 0.5. For Pr3+ a very high keff of 1.4 was obtained. Values for keff are discussed in terms of an elastic model accounting for the strain energy originating from the difference in the size of Ln3+ ions. The Ln3+ concentrations were measured by absorbance spectroscopy, by inductively coupled plasma optical emission spectroscopy, and by electron microprobe analyse
Change of Electronic Structure Induced by Magnetic Transitions in CeBi
The temperature dependence of the electronic structure of CeBi arising from
two types of antiferromagnetic transitions based on optical conductivity
() was observed. The spectrum continuously and
discontinuously changes at 25 and 11 K, respectively. Between these
temperatures, two peaks in the spectrum rapidly shift to the opposite energy
sides as the temperature changes. Through a comparison with the band
calculation as well as with the theoretical spectrum, this
peak shift was explained by the energy shift of the Bi band due to the
mixing effect between the Ce and Bi states. The single-layer
antiferromagnetic () transition from the paramagnetic state was concluded
to be of the second order. The marked changes in the spectrum
at 11 K, however, indicated the change in the electronic structure was due to a
first-order-like magnetic transition from a single-layer to a double-layer
() antiferromagnetic phase.Comment: 4 pages, to be published in J. Phys. Soc. Jpn. 73 Aug. (2004
Strongly Correlated Cerium Systems: Non-Kondo Mechanism for Moment Collapse
We present an ab initio based method which gives clear insight into the
interplay between the hybridization, the coulomb exchange, and the
crystal-field interactions, as the degree of 4f localization is varied across a
series of strongly correlated cerium systems. The results for the ordered
magnetic moments, magnetic structure, and ordering temperatures are in
excellent agreement with experiment, including the occurence of a moment
collapse of non-Kondo origin. In contrast, standard ab initio density
functional calculations fail to predict, even qualitatively, the trend of the
unusual magentic properties.Comment: A shorter version of this has been submitted to PR
The structures and thermoelectric properties of the infinitely adaptive series (Bi2)m(Bi2Te3)n
The structures and thermoelectric properties of the (Bi2)m(Bi2Te3)n
homologous series, derived from stacking hexagonal Bi2 and Bi2Te3 blocks, are
reported. The end-members of this series are metallic Bi and semiconducting
Bi2Te3; nine members of the series have been studied. The structures form an
infinitely adaptive series and a unified structural description based on a
modulated structure approach is presented. The as-synthesized samples have
thermopowers (S) that vary from n-type for Bi2Te3 to p-type for phases rich in
Bi2 blocks but with some Bi2Te3 blocks present, to n-type again for Bi metal.
The thermoelectric power factor (S2/rho) is highest for Bi metal (43 muW/K2 cm
at 130 K), followed by Bi2Te3 (20 muW/K2 cm at 270 K), while Bi2Te (m:n = 5:2)
and Bi7Te3 (m:n = 15:6) have 9 muW/K2 cm (at 240 K) and 11 muW/K2 (at 270 K),
respectively. The results of doping studies with Sb and Se into Bi2Te are
reported.Comment: accepted for publication in PR
Near-Zero Moment Ferromagnetism in the Semiconductor SmN
The magnetic behaviour of SmN has been investigated in stoichiometric
polycrystalline films. All samples show ferromagnetic order with Curie
temperature (T_c) of 27 +/- 3 K, evidenced by the occurrence of hysteresis
below T_c. The ferromagnetic state is characterised by a very small moment and
a large coercive field, exceeding even the maximum applied field of 6 T below
about 15 K. The residual magnetisation at 2 K, measured after cooling in the
maximum field, is 0.035 mu_B per Sm. Such a remarkably small moment results
from a near cancellation of the spin and orbital contributions for Sm3+ in SmN.
Coupling to an applied field is therefore weak, explaining the huge coercive
field . The susceptibility in the paramagnetic phase shows
temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck
contribution is in quantitative agreement with the field-induced admixture of
the J=7/2 excited state and the 5/2 ground state. The Curie-Weiss contribution
returns a Curie temperature that agrees with the onset of ferromagnetic
hysteresis, and a conventional paramagnetic moment with an effective moment of
0.4 mu_B per Sm ion, in agreement with expectations for the crystal-field
modified effective moment on the Sm3+ ions.Comment: 5 pages, 3 figure
Effect of finite temperature and uniaxial anisotropy on the Casimir effect with three-dimensional topological insulators
In this work we study the Casimir effect with three-dimensional topological
insulators including the effects of temperature and uniaxial anisotropy.
Although precise experimental values for the optical properties of these
materials are yet to be established, qualitative analysis is still possible. We
find qualitatively that the reported repulsive behavior and the equilibrium
point are robust features of the system, and are favored by low temperatures
and the enhancement of the optical response parallel to the optical axis. The
dependence of the equilibrium point with temperature and with the topological
magnetoelectric polarizability characteristic of three-dimensional topological
insulators is also discussed.Comment: 17 pages, 7 figures. Published versio
Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates
The two-dimensional layer of molybdenum disulfide (MoS2) has recently
attracted much interest due to its direct-gap property and potential
applications in optoelectronics and energy harvesting. However, the synthetic
approach to obtain high quality and large-area MoS2 atomic thin layers is still
rare. Here we report that the high temperature annealing of a thermally
decomposed ammonium thiomolybdate layer in the presence of sulfur can produce
large-area MoS2 thin layers with superior electrical performance on insulating
substrates. Spectroscopic and microscopic results reveal that the synthesized
MoS2 sheets are highly crystalline. The electron mobility of the bottom-gate
transistor devices made of the synthesized MoS2 layer is comparable with those
of the micromechanically exfoliated thin sheets from MoS2 crystals. This
synthetic approach is simple, scalable and applicable to other transition metal
dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to
arbitrary substrates, providing great opportunities to make layered composites
by stacking various atomically thin layers.Comment: manuscript submitted on 11-Dec-2011, revision submitted on
16-Feb-201
Low-temperature properties of the heavy-fermion system U Cd
We present electrical-resistivity, magnetic-susceptibility, specific-heat, and thermal-expansion data for UCd11. The low-temperature specific heat indicates that the electronic subsystem has a highly enhanced specific heat which is partially removed by a phase transition at 5.0 K. © 1984 The American Physical Society
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