226 research outputs found
Pseudogap in the Optical Spectra of UPd_2Al_3
The in-plane optical conductivity of UPd_2Al_3 was measured at temperatures
K in the spectral range from 1 cm^{-1} to 40 cm^{-1} (0.14
meV to 5 meV). As the temperature decreases below 25 K a well pronounced
pseudogap of 0.2 meV develops in the optical response. In addition we observe a
narrow conductivity peak at zero frequency which at 2 K is less than 1 cm^{-1}
wide but which contains only a fraction of the delocalized carriers. The gap in
the electronic excitations might be an inherent feature of the heavy fermioin
ground state.Comment: 4 pages, 4 figures (submitted to Phys. Rev. Lett.
On the magnetism of Ln{2/3}Cu{3}Ti{4}O{12} (Ln = lanthanide)
The magnetic and thermodynamic properties of the complete
LnCuTiO series were investigated. Here stands for
the lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. %Most
of the compounds were prepared as single phase polycrystalline powder %without
any traces of impurities. Marginal amounts of %impurities were
detected Gd, Er, and Tm. %Significant amounts of impurity phases were
found for Ce and Yb. All the samples investigated crystallize in the
space group with lattice constants that follow the lanthanide
contraction. The lattice constant of the Ce compound reveals the presence of
Ce leading to the composition CeCuTiO. From
magnetic susceptibility and electron-spin resonance experiments it can be
concluded that the copper ions always carry a spin and order
antiferromagnetically close to 25\,K. The Curie-Weiss temperatures can
approximately be calculated assuming a two-sublattice model corresponding to
the copper and lanthanide ions, respectively. It seems that the magnetic
moments of the heavy rare earths are weakly coupled to the copper spins, while
for the light lanthanides no such coupling was found. The moments remain
paramagnetic down to the lowest temperatures, with the exception of the Tm
compound, which indicates enhanced Van-Vleck magnetism due to a non-magnetic
singlet ground state of the crystal-field split manifold. From
specific-heat measurements we accurately determined the antiferromagnetic
ordering temperature and obtained information on the crystal-field states of
the rare-earth ions. The heat-capacity results also revealed the presence of a
small fraction of Ce in a magnetic state.Comment: 10 pages, 10 figure
X-Ray Scattering at FeCo(001) Surfaces and the Crossover between Ordinary and Normal Transitions
In a recent experiment by Krimmel et al. [PRL 78, 3880 (1997)], the critical
behavior of FeCo near a (001) surface was studied by x-ray scattering. Here the
experimental data are reanalyzed, taking into account recent theoretical
results on order-parameter profiles in the crossover regime between ordinary
and normal transitions. Excellent agreement between theoretical expectations
and the experimental results is found.Comment: 9 pages, Latex, 1 PostScript figure, to be published in Phys.Rev.
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
Disorder-to-order transition in the magnetic and electronic properties of URh_2Ge_2
We present a study of annealing effects on the physical properties of
tetragonal single--crystalline URh_2Ge_2. This system, which in as-grown form
was recently established as the first metallic 3D random-bond heavy-fermion
spin glass, is transformed by an annealing treatment into a long-range
antiferromagnetically (AFM) ordered heavy-fermion compound. The transport
properties, which in the as-grown material were dominated by the structural
disorder, exhibit in the annealed material signs of typical metallic behavior
along the crystallographic a axis. From our study URh_2Ge_2 emerges as
exemplary material highlighting the role and relevance of structural disorder
for the properties of strongly correlated electron systems. We discuss the link
between the magnetic and electronic behavior and how they are affected by the
structural disorder.Comment: Phys. Rev. B, in print (scheduled 1 Mar 2000
Spin Frustration and Magnetic Exchange in Cobalt Aluminum Oxide Spinels
We report on x-ray diffraction, magnetic susceptibility, electron- spin
resonance and heat- capacity studies of Co[Al_1-xCo_x]_2O_4 for Co
concentrations 0<x<1. In this spinel system only the A-site Co^2+ cation is
magnetic, while the non-magnetic B-site Al^3+ is substituted by the low-spin
non-magnetic Co^3+, and it is possible to investigate the complete phase
diagram from Co^2+Al^3+_2O_4 to Co^2+Co^3+_2O_4. All samples reveal large
negative Curie-Weiss temperatures Theta_CW of the order of -110 K independent
of concentration, which is attributed to a high multiplicity of the
superexchange interactions between the A-site Co^2+ cations. A pure
antiferromagnetic state is found for x = 1.0 and 0.9 with Neel temperatures T_N
= 29.5 K and 21.2 K, respectively, as evidenced by lambda-like anomalies in the
specific heat. Compositions with 0.3<x<0.75 show smeared out strongly reduced
magnetic ordering temperatures. At low temperatures, a T^2.5 dependence of the
specific heat is indicative of a spin-liquid state. For x < 0.2 a T^2
dependence of the specific heat and a spin-glass like behavior of the
susceptibility below T_f = 4.7 K are observed. The high value of the
frustration parameter f = |Theta_CW|/T_f > 10 indicates the presence of strong
spin frustration at least for x < 0.6. The frustration mechanism is attributed
to competing nearest neighbor and next-nearest neighbor superexchange
interactions between the A-site Co^2+ ions.Comment: 19 pages, 9 figures, 46 reference
Spin-fluctuations in the quarter-filled Hubbard ring : significances to LiVO
Using the quantum Monte Carlo method, we investigate the spin dynamics of
itinerant electrons in the one-dimensional Hubbard system. Based on the model
calculation, we have studied the spin-fluctuations in the quarter-filled
metallic Hubbard ring, which is aimed at the vanadium ring or chain defined
along corner-sharing tetrahedra of LiVO, and found the dramatic changes
of magnetic responses and spin-fluctuation characteristics with the
temperature. Such results can explain the central findings in the recent
neutron scattering experiment for LiVO.Comment: 5 pages, 3 figure
Spiral spin-liquid and the emergence of a vortex-like state in MnScS
Spirals and helices are common motifs of long-range order in magnetic solids,
and they may also be organized into more complex emergent structures such as
magnetic skyrmions and vortices. A new type of spiral state, the spiral
spin-liquid, in which spins fluctuate collectively as spirals, has recently
been predicted to exist. Here, using neutron scattering techniques, we
experimentally prove the existence of a spiral spin-liquid in MnScS by
directly observing the 'spiral surface' - a continuous surface of spiral
propagation vectors in reciprocal space. We elucidate the multi-step ordering
behavior of the spiral spin-liquid, and discover a vortex-like triple-q phase
on application of a magnetic field. Our results prove the effectiveness of the
- Hamiltonian on the diamond lattice as a model for the spiral
spin-liquid state in MnScS, and also demonstrate a new way to realize a
magnetic vortex lattice.Comment: 10 pages, 11 figure
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