5,738 research outputs found
Magnetotransport in the low carrier density ferromagnet EuB_6
We present a magnetotransport study of the low--carrier density ferromagnet
EuB_6. This semimetallic compound, which undergoes two ferromagnetic
transitions at T_l = 15.3 K and T_c = 12.5 K, exhibits close to T_l a colossal
magnetoresistivity (CMR). We quantitatively compare our data to recent
theoretical work, which however fails to explain our observations. We attribute
this disagreement with theory to the unique type of magnetic polaron formation
in EuB_6.Comment: Conference contribution MMM'99, San Jos
Crystal-field effects in the first-order valence transition in YbInCu4 induced by an external magnetic field
As it was shown earlier [Dzero, Gor'kov, and Zvezdin, J. Phys.:Condens.
Matter 12, L711 (2000)] the properties of the first-order valence phase
transition in YbInCu4 in the wide range of magnetic fields and temperatures are
perfectly described in terms of a simple entropy transition for free Yb ions.
Within this approach, the crystal field effects have been taken into account
and we show that the phase diagram in the plane acquires some anisotropy
with respect to the direction of an external magnetic field.Comment: 4 pages, 3 eps figures; minor changes; to be piblished in J. of
Physics: Cond. Ma
Tuning Low Temperature Physical Properties of CeNiGe by Magnetic Field
We have studied the thermal, magnetic, and electrical properties of the
ternary intermetallic system CeNiGe by means of specific heat,
magnetization, and resistivity measurements. The specific heat data, together
with the anisotropic magnetic susceptibility, was analyzed on the basis of the
point charge model of crystalline electric field. The \,=\,5/2 multiplet of
the Ce is split by the crystalline electric field (CEF) into three
Kramers doublets, where the second and third doublet are separated from the
first (ground state) doublet by 100\,K and
170\,K, respectively. In zero field CeNiGe exhibits an
antiferromangeic order below = 5.0\,K. For
\textbf{H}\,\,\textbf{a} two metamagnetic transitions are clearly
evidenced between 2\,\,4\,K from the magnetization isotherm and extended
down to 0.4\,K from the magnetoresistance measurements. For
\textbf{H}\,\,\textbf{a}, shifts to lower temperature as
magnetic field increases, and ultimately disappears at
32.5\,kOe. For , the electrical resistivity shows the quadratic
temperature dependence (). For , an
unconventional -dependence of with emerges, the
exponent becomes larger as magnetic field increases. Although the
antiferromagnetic phase transition temperature in CeNiGe can be
continuously suppressed to zero, it provides an example of field tuning that
does not match current simple models of Quantum criticality.Comment: accepted PR
Quantum Oscillations in EuFe2As2 single crystals
Quantum oscillation measurements can provide important information about the
Fermi surface (FS) properties of strongly correlated metals. Here, we report a
Shubnikov-de Haas (SdH) effect study on the pnictide parent compounds
EuFeAs (Eu122) and BaFeAs (Ba122) grown by In-flux.
Although both members are isovalent compounds with approximately the same
density of states at the Fermi level, our results reveal subtle changes in
their fermiology. Eu122 displays a complex pattern in the Fourier spectrum,
with band splitting, magnetic breakdown orbits, and effective masses
sistematically larger when compared to Ba122, indicating that the former is a
more correlated metal. Moreover, the observed pockets in Eu122 are more
isotropic and 3D-like, suggesting an equal contribution from the Fe
orbitals to the FS. We speculate that these FS changes may be responsible for
the higher spin-density wave ordering temperature in Eu122.Comment: 5 pages, 4 figure
Electronic Structure of LuRh2Si2: "Small" Fermi Surface Reference to YbRh2Si2
We present band structure calculations and quantum oscillation measurements
on LuRh2Si2, which is an ideal reference to the intensively studied quantum
critical heavy-fermion system YbRh2Si2. Our band structure calculations show a
strong sensitivity of the Fermi surface on the position of the silicon atoms
zSi within the unit cell. Single crystal structure refinement and comparison of
predicted and observed quantum oscillation frequencies and masses yield zSi =
0.379c in good agreement with numerical lattice relaxation. This value of zSi
is suggested for future band structure calculations on LuRh2Si2 and YbRh2Si2.
LuRh2Si2 with a full f electron shell represents the "small" Fermi surface
configuration of YbRh2Si2. Our experimentally and ab initio derived quantum
oscillation frequencies of LuRh2Si2 show strong differences with earlier
measurements on YbRh2Si2. Consequently, our results confirm the contribution of
the f electrons to the Fermi surface of YbRh2Si2 at high magnetic fields. Yet
the limited agreement with refined fully itinerant local density approximation
calculations highlights the need for more elaborated models to describe the
Fermi surface of YbRh2Si2.Comment: 12 pages 10 figure
A precursor state to unconventional superconductivity in CeIrIn
We present sensitive measurements of the Hall effect and magnetoresistance in
CeIrIn down to temperatures of 50 mK and magnetic fields up to 15 T. The
presence of a low temperature coherent Kondo state is established. Deviations
from Kohler's rule and a quadratic temperature dependence of the cotangent of
the Hall angle are reminiscent of properties observed in the high temperature
superconducting cuprates. The most striking observation pertains to the
presence of a \textit{precursor} state--characterized by a change in the Hall
mobility--that appears to precede the superconductivity in this material, in
similarity to the pseudogap in the cuprate high superconductors.Comment: 4 figure
Analysis of suprathermal tails using hourly-averaged proton velocity distributions at 1 AU
We obtain hourly values of tail densities and of power law indices, γ, of suprathermal (speeds above 2.48 times the solar wind speed) protons from power law fits to hourly velocity distribution functions in the solar-wind-frame. ACE/SWICS and ULEIS data, which often include very low counting statistics, are used to derive hourly proton phase space densities. We find that during part of the recent deep solar minimum (first 82 days in 2009): (a) the spectrum averaged over the entire 82 day period reveals the bulk and the halo solar wind components, interstellar pickup protons (seldom seen at 1 AU), and the common Fisk and Gloeckler (F&G) suprathermal tail (v^(−5) in velocity v with an exponential rollover at some higher speed); (b) hourly values of the tail densities range from ~1•10^(−6) to ~3•10^(−3) cm^(−3) and vary by a factor of ~2-10 over periods of hours as well as in a quasi-periodic manner by factors of 20 to 50 over 4 to 10 days; (c) about 95% of the nearly 2000 hourly spectra have complex shapes and that are not power laws; (d) about half of the ~5% of the hourly spectra that are monotonically decreasing with increasing speed (e.g. exponentials or Max-wellians, or F&G) are observed at times of high tail densities (>−5•10^(−5) cm^(−3)) where the spectra have the common F&G shapes; (e) each of the six sharp (few day long), large (tail density > 5•10^(−4) cm^(−3)) increases observed during this time period is associated with solar wind compression regions; (f) the eight shocks recorded locally that were not contained in compression regions did not produce signif-icant increases in the tail densities. We conclude that during times of low solar activity the higher energy portions of locally accelerated suprathermal tail spectra are often obscured by significant contributions from remotely accelerated particles whose spectra below (1-3)•10^8 cm/s are modified (modulated) by propagation from this remote acceleration region. In those instances where strong acceleration occurs locally, the observed tail spectra have the common F&G spectral shapes
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