184 research outputs found
Magnetism of PdNi alloys near the critical concentration for ferromagnetism
We report results of a muon spin rotation and relaxation (SR) study of
dilute PdNi alloys, with emphasis on Ni concentrations 0.0243
and 0.025. These are close to the critical value for the onset
of ferromagnetic long-range order (LRO), which is a candidate for a quantum
critical point. The 2.43 and 2.5 at.% Ni alloys exhibit similar SR
properties. Both samples are fully magnetic, with average muon local fields
2.0 and 3.8 mT and Curie temperatures
1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively, at . The
temperature dependence of suggests ordering of
Ni spin clusters rather than isolated spins. Just above a two-phase
region is found with separate volume fractions of quasistatic short-range order
(SRO) and paramagnetism. The SRO fraction decreases to zero with increasing
temperature a few kelvin above . This mixture of SRO and paramagnetism is
consistent with the notion of an inhomogeneous alloy with Ni clustering. The
measured values of extrapolate to = 0.0236 0.0027.
The dynamic muon spin relaxation in the vicinity of differs for the two
samples: a relaxation-rate maximum at is observed for = 0.0243,
reminiscent of critical slowing down, whereas for 0.025 no dynamic
relaxation is observed within the SR time window. The data suggest a
mean-field-like transition in this alloy.Comment: 15 pages, 15 figures, to be published in Phys. Rev.
Spin-driven Phase Transitions in ZnCrSe and ZnCrS Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction
The crystal and magnetic structures of the spinel compounds ZnCrS and
ZnCrSe were investigated by high resolution powder synchrotron and
neutron diffraction. ZnCrSe exhibits a first order phase transition at
K into an incommensurate helical magnetic structure. Magnetic
fluctuations above are coupled to the crystal lattice as manifested by
negative thermal expansion. Both, the complex magnetic structure and the
anomalous structural behavior can be related to magnetic frustration.
Application of an external magnetic field shifts the ordering temperature and
the regime of negative thermal expansion towards lower temperatures. Thereby,
the spin ordering changes into a conical structure. ZnCrS shows two
magnetic transitions at K and K that are accompanied by
structural phase transitions. The crystal structure transforms from the cubic
spinel-type (space group \={3}) at high temperatures in the paramagnetic
state, via a tetragonally distorted intermediate phase (space group /
) for into a low temperature orthorhombic phase
(space group ) for . The cooperative displacement of
sulfur ions by exchange striction is the origin of these structural phase
transitions. The low temperature structure of ZnCrS is identical to the
orthorhombic structure of magnetite below the Verwey transition. When applying
a magnetic field of 5 T the system shows an induced negative thermal expansion
in the intermediate magnetic phase as observed in ZnCrSe.Comment: 11 pages, 13 figures, to be published in PR
Optical Properties of Heavy Fermion Systems with SDW Order
The dynamical conductivity , reflectivity , and
tunneling density of states of strongly correlated systems (like
heavy fermions) with a spin-density wave (SDW) magnetic order are studied as a
function of impurity scattering rate and temperature. The theory is generalized
to include strong coupling effects in the SDW order. The results are discussed
in the light of optical experiments on heavy-fermion SDW materials. With some
modifications the proposed theory is applicable also to heavy fermions with
localized antiferromagnetic (LAF) order.Comment: 9 pages, 10 figure
Incorporating interactive 3-dimensional graphics in astronomy research papers
Most research data collections created or used by astronomers are
intrinsically multi-dimensional. In contrast, all visual representations of
data presented within research papers are exclusively 2-dimensional. We present
a resolution of this dichotomy that uses a novel technique for embedding
3-dimensional (3-d) visualisations of astronomy data sets in electronic-format
research papers. Our technique uses the latest Adobe Portable Document Format
extensions together with a new version of the S2PLOT programming library. The
3-d models can be easily rotated and explored by the reader and, in some cases,
modified. We demonstrate example applications of this technique including: 3-d
figures exhibiting subtle structure in redshift catalogues, colour-magnitude
diagrams and halo merger trees; 3-d isosurface and volume renderings of
cosmological simulations; and 3-d models of instructional diagrams and
instrument designs.Comment: 18 pages, 7 figures, submitted to New Astronomy. For paper with
3-dimensional embedded figures, see http://astronomy.swin.edu.au/s2plot/3dpd
The Nature of Heavy Quasiparticles in Magnetically Ordered Heavy Fermions
The optical conductivity of the heavy fermions UPd2Al3 and UPt3 has been
measured in the frequency range from 10 GHz to 1.2 THz (0.04 meV to 5 meV) at
temperatures 1 K < T < 300 K. In both compounds a well pronounced pseudogap of
less than a meV develops in the optical response at low temperatures; we relate
this to the antiferromagnetic ordering. From the energy dependence of the
effective electronic mass and scattering rate we derive the energies essential
for the heavy quasiparticle. We find that the enhancement of the mass mainly
occurs below the energy which is related to magnetic correlations between the
local magnetic moments and the itinerant electrons. This implies that the
magnetic order in these compounds is the pre-requisite to the formation of the
heavy quasiparticle and eventually of superconductivity.Comment: RevTeX, 4 pages, 3 figures, email:
[email protected]
Staggered Pairing Phenomenology for UPd_2Al_3 and UNi_2Al_3
We apply the staggered-pairing Ginzburg-Landau phenomenology to describe
superconductivity in UPd_2Al_3 and UNi_2Al_3. The phenomenology was applied
successfully to UPt_3 so it explains why these materials have qualitatively
different superconducting phase diagrams although they have the same
point-group symmetry. UPd_2Al_3 and UNi_2Al_3 have a two-component
superconducting order parameter transforming as an H-point irreducible
representation of the space group. Staggered superconductivity can induce
charge-density waves characterized by new Bragg peaks suggesting experimental
tests of the phenomenology.Comment: 4 pages, REVTeX, 2 Postscript figure
Incommensurate structure of the spin-Peierls compound TiOCl in zero and finite magnetic fields
Electronic and optical properties of LiBC
LiBC, a semiconducting ternary borocarbide constituted of the lightest
elements only, has been synthesized and characterized by x-ray powder
diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing
an infrared microscope the phonon spectrum has been investigated in single
crystals. The in-plane B-C stretching mode has been detected at 150 meV,
noticeably higher than in AlB2, a non-superconducting isostructural analog of
MgB2. It is this stretching mode, which reveals a strong electron-phonon
coupling in MgB2, driving it into a superconducting state below 40 K, and is
believed to mediate predicted high-temperature superconductivity in hole-doped
LiBC [H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88,
127001 (2002)].Comment: 4 pages, 4 figure
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