9,132 research outputs found
Magnetic Inhomogeneity and Magnetotransport in Electron-Doped Ca(1-x)La(x)MnO(3) (0<=x<=0.10)
The dc magnetization (M) and electrical resistivity (\rho) as functions of
magnetic field and temperature are reported for a series of lightly electron
dopedCa(1-x)La(x)MnO(3) (0<=x<=0.10) specimens for which magnetization [Phys.
Rev. B {\bf 61}, 14319 (2000)] and scattering studies [Phys. Rev. B {\bf 68},
134440 (2003)] indicate an inhomogeneous magnetic ground state composed of
ferromagnetic (FM) droplets embedded in a G-type antiferromagnetic matrix. A
change in the magnetic behavior near x=0.02 has been suggested to be the
signature of a crossover to a long-ranged spin-canted phase. The data reported
here provide further detail about this crossover in the magnetization, and
additional insight into the origin of this phenomenon through its manifestation
in the magnetotransport. In the paramagnetic phase (T>=125 K) we find a
magnetoresistance =-C(M/M_S)^2 (M_S is the low-T saturation magnetization), as
observed in many manganites in the ferromagnetic (FM), colossal
magnetoresistance (CMR) region of the phase diagram, but with a value of C that
is two orders of magnitude smaller than observed for CMR materials. The doping
behavior C(x) follows that of M_S(x), indicating that electronic inhomogeneity
associated with FM fluctuations occurs well above the magnetic ordering
transition.Comment: 7 pp., 10 Fig.s, submitted to PR
Giant Dielectric Permittivity of Electron-Doped Manganite Thin Films, Ca(1-x)La(x)MnO(3) ($0<= x<= 0.03)
A giant low-frequency, in-plane dielectric constant, eps~10^6, for epitaxial
thin films of Ca(1-x)La(x)MnO(3) (x<=0.03) was observed over a broad
temperature range, 4K <= T 300K. This phenomenon is attributed to an internal
barrier-layer capacitor (IBLC) structure, with insulating boundaries between
semiconducting grains. The room-temperature eps increases substantially with
electron (La) doping. The measured values of eps exceed those of conventional
two-phase IBLC materials based on (Ba,Sr)TiO(3) as well as recently discovered
CaCu(3)Ti(4)O(12) and (Li,Ti) doped NiO.Comment: 5 pages, 4 fig.s, J. Appl. Phys., Jan. 15, 2005 (in press
Anisotropic In-Plane Strain and Transport in Epitaxial Nd(0.2)Sr(0.8)MnO(3) Thin Films
The structure, morphology, and electrical properties of epitaxial a-axis
oriented thin films of Nd(0.2)Sr(0.8)MnO(3) are reported for thicknesses 10 nm
<= t <= 150 nm. Films were grown with both tensile and compressive strain on
various substrates. It is found that the elongated crystallographic c-axes of
the films remain fully strained to the substrates for all thicknesses in both
strain states. Relaxation of the a and b axes is observed for t>= 65 nm with
films grown under tensile strain developing uniaxial crack arrays (running
along the c axis) due to a highly anisotropic thermal expansion. For the latter
films, the room-temperature in-plane electrical resistivity anisotropy,
rho_b/rho_c, increases approximately exponentially with increasing film
thickness to values of ~1000 in the thickest films studied. Films under tension
have their Neel temperatures enhanced by ~25 K independent of thickness,
consistent with an enhancement of ferromagnetic exchange along their expanded c
axes.Comment: 14 pp., 9 Fig.
Extreme thermopower anisotropy and interchain transport in the quasi-one-dimensional metal Li(0.9)Mo(6)O(17)
Thermopower and electrical resistivity measurements transverse to the
conducting chains of the quasi-one-dimensional metal Li(0.9)Mo(6)O(17) are
reported in the temperature range 5 K = 400 K the
interchain transport is determined by thermal excitation of charge carriers
from a valence band ~ 0.14 eV below the Fermi level, giving rise to a large,
p-type thermopower that coincides with a small, n-type thermopower along the
chains. This dichotomy -- semiconductor-like in one direction and metallic in a
mutually perpendicular direction -- gives rise to substantial transverse
thermoelectric (TE) effects and a transverse TE figure of merit among the
largest known for a single compound.Comment: PRL in press, manuscript (5pp, 4 Fig.'s) and Supplementary Material
(3pp, 3 Fig.'s
Scalable Text and Link Analysis with Mixed-Topic Link Models
Many data sets contain rich information about objects, as well as pairwise
relations between them. For instance, in networks of websites, scientific
papers, and other documents, each node has content consisting of a collection
of words, as well as hyperlinks or citations to other nodes. In order to
perform inference on such data sets, and make predictions and recommendations,
it is useful to have models that are able to capture the processes which
generate the text at each node and the links between them. In this paper, we
combine classic ideas in topic modeling with a variant of the mixed-membership
block model recently developed in the statistical physics community. The
resulting model has the advantage that its parameters, including the mixture of
topics of each document and the resulting overlapping communities, can be
inferred with a simple and scalable expectation-maximization algorithm. We test
our model on three data sets, performing unsupervised topic classification and
link prediction. For both tasks, our model outperforms several existing
state-of-the-art methods, achieving higher accuracy with significantly less
computation, analyzing a data set with 1.3 million words and 44 thousand links
in a few minutes.Comment: 11 pages, 4 figure
Impurity Conduction and Magnetic Polarons in Antiferromagnetic Oxides
Low-temperature transport and magnetization measurements for the
antiferromagnets SrMnO(3) and CaMnO(3) identify an impurity band of mobile
states separated by energy E from electrons bound in Coulombic potentials. Very
weak electric fields are sufficient to excite bound electrons to the impurity
band, increasing the mobile carrier concentration by more than three orders of
magnitude. The data argue against the formation of self-trapped magnetic
polarons (MPs) predicted by theory, and rather imply that bound MPs become
stable only for kT<<E.Comment: 4 pp., 4 fig
Several new catalysts for reduction of oxygen in fuel cells
Test results prove nickel carbide or nitride, nickel-cobalt carbide, titanium carbide or nitride, and intermetallic compounds of the transition or noble metals to be efficient electrocatalysts for oxygen reduction in alkaline electrolytes in low temperature fuel cells
Hole Localization in Underdoped Superconducting Cuprates Near 1/8th Doping
Measurements of thermal conductivity versus temperature over a broad range of
doping in YBaCuO and HgBaCaCuO
(=1,2,3) suggest that small domains of localized holes develop for hole
concentrations near =1/8. The data imply a mechanism for localization that
is intrinsic to the CuO-planes and is enhanced via pinning associated with
oxygen-vacancy clusters.Comment: 4 pages, 4 eps fig.'s, to be published, Phys. Rev.
Stoichiometry, structure, and transport in the quasi-one-dimensional metal, Li(0.9)Mo(6)O(17)
A correlation between lattice parameters, oxygen composition, and the
thermoelectric and Hall coefficients is presented for single-crystal
Li(0.9)Mo(6)O(17), a quasi-one-dimensional (Q1D) metallic compound. The
possibility that this compound is a compensated metal is discussed in light of
a substantial variability observed in the literature for these transport
coefficients.Comment: 5 pages, 4 Figures; Phys. Rev. B (in press
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