124 research outputs found
Magnetic and electronic structures of superconducting RuSrGdCuO
The coexistence of ferromagnetism and superconductivity in
RuSrGdCuO was reported both from experiments (by Tallon et. al.)
and first-principles calculations (by Pickett et. al.). Here we report that our
first-principles full-potential linearized augmented plane wave (FLAPW)
calculations, employing the precise crystal structure with structural
distortions (i.e., RuO rotations) determined by neutron diffraction,
demonstrate that antiferromagnetic ordering of the Ru moments is energetically
favored over the previously proposed ferromagnetic ordering. Our results are
consistent with recently performed magnetic neutron diffraction experiments
(Lynn et. al). Ru states, which are responsible for the magnetism,
have only a very small interaction with Cu states, which results in a
small exchange splitting of these states. The Fermi surface, characterized by
strongly hybridized orbitals, has nesting features similar to those
in the two-dimensional high cuprate superconductors.Comment: 6 pages,6 figures, accepted for publication in Phys. Rev.
Neutron Scattering Study of Crystal Field Energy Levels and Field Dependence of the Magnetic Order in Superconducting HoNi2B2C
Elastic and inelastic neutron scattering measurements have been carried out
to investigate the magnetic properties of superconducting (Tc~8K) HoNi2B2C. The
inelastic measurements reveal that the lowest two crystal field transitions out
of the ground state occurat 11.28(3) and 16.00(2) meV, while the transition of
4.70(9) meV between these two levels is observed at elevated temperatures. The
temperature dependence of the intensities of these transitions is consistent
with both the ground state and these higher levels being magnetic doublets. The
system becomes magnetically long range ordered below 8K, and since this
ordering energy kTN ~ 0.69meV << 11.28meV the magnetic properties in the
ordered phase are dominated by the ground-state spin dynamics only. The low
temperature structure, which coexists with superconductivity, consists of
ferromagnetic sheets of Ho{3+ moments in the a-b plane, with the sheets coupled
antiferromagnetically along the c-axis. The magnetic state that initially forms
on cooling, however, is dominated by an incommensurate spiral antiferromagnetic
state along the c-axis, with wave vector qc ~0.054 A-1, in which these
ferromagnetic sheets are canted from their low temperature antiparallel
configuration by ~17 deg. The intensity for this spiral state reaches a maximum
near the reentrant superconducting transition at ~5K; the spiral state then
collapses at lower temperature in favor of the commensurate antiferromagnetic
state. We have investigated the field dependence of the magnetic order at and
above this reentrant superconducting transition. Initially the field rotates
the powder particles to align the a-b plane along the field direction,
demonstrating that the moments strongly prefer to lie within this plane due to
the crystal field anisotropy. Upon subsequently increasing the field atComment: RevTex, 7 pages, 11 figures (available upon request); Physica
Superconductivity in a Ferromagnetic Layered Compound
We examine superconductivity in layered systems with large Fermi-surface
splitting due to coexisting ferromagnetic layers. In particular, the hybrid
ruthenate-cuprate compound RuSr_2GdCu_2O_8 is examined on the coexistence of
the superconductivity and the ferromagnetism, which has been observed recently.
We calculate critical fields of the superconductivity taking into account the
Fulde-Ferrell-Larkin-Ovchinnikov state in a model with Fermi-surfaces which
shapes are similar to those obtained by a band calculation. It is shown that
the critical field is enhanced remarkably due to a Fermi-surface effect, and
can be high enough to make the coexistence possible in a microscopic scale. We
also clarify the direction of the spatial oscillation of the order parameter,
which may be observed by scanning tunneling microscope experiments.Comment: 4 pages, 4 figures, (Latex, revtex.sty, epsf.sty
Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors
The phonon-mode decomposition of the electron-phonon coupling in the
MgB2-like system Li_{1-x}BC is explored using first principles calculations. It
is found that the high temperature superconductivity of such systems results
from extremely strong coupling to only ~2% of the phonon modes. Novel
characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and
greater compared to a mean of for other modes, (2) a precipitous
Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the
frequency itself, indicating impending breakdown of linear electron-phonon
theory. This behavior in borne out by recent inelastic x-ray scattering studies
of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4
changed. Comparison with new data include
Changes in Optical Conductivity due to Readjustments in Electronic Density of States
Within the model of elastic impurity scattering, we study how changes in the
energy dependence of the electronic density of states (EDOS)
around the Fermi energy are reflected in the frequency-dependent
optical conductivity . While conserving the total number of
states in we compute the induced changes in as a
function of and in the corresponding optical scattering rate
. These quantities mirror some aspects of the EDOS
changes but the relationship is not direct. Conservation of optical oscillator
strength is found not to hold, and there is no sum rule on the optical
scattering rate although one does hold for the quasiparticle scattering.
Temperature as well as increases in impurity scattering lead to additional
changes in optical properties not seen in the constant EDOS case. These effects
have their origin in an averaging of the EDOS around the Fermi energy
on an energy scale set by the impurity scattering.Comment: 13 pages, 7 figure
Josephson Plasma in RuSr2GdCu2O8
Josephson plasma in RuSrGdCuO,
RuSrGdCuO (x = 0.3), and
RuSrEuCeCuO (x = 0.5) compounds is
investigated by the sphere resonance method. The Josephson plasma is observed
in a low-frequency region (around 8.5 cm at T ) for
ferromagnetic RuSrGdCuO, while it increases to 35 cm
for non-ferromagnetic RuSrGdCuO (x = 0.3), which
represents a large reduction in the Josephson coupling at ferromagnetic
RuO block layers. The temperature dependence of the plasma does not shift
to zero frequency ({\it i.e.} = 0) at low temperatures, indicating that
there is no transition from the 0-phase to the -phase in these compounds.
The temperature dependence and the oscillator strength of the peak are
different from those of other non-magnetic cuprates, and the origins of these
anomalies are discussed.Comment: to appear in Phys. Rev.B Rapid Com
Influence of next-nearest-neighbor electron hopping on the static and dynamical properties of the 2D Hubbard model
Comparing experimental data for high temperature cuprate superconductors with
numerical results for electronic models, it is becoming apparent that a hopping
along the plaquette diagonals has to be included to obtain a quantitative
agreement. According to recent estimations the value of the diagonal hopping
appears to be material dependent. However, the values for discussed
in the literature were obtained comparing theoretical results in the weak
coupling limit with experimental photoemission data and band structure
calculations. The goal of this paper is to study how gets renormalized as
the interaction between electrons, , increases. For this purpose, the effect
of adding a bare diagonal hopping to the fully interacting two dimensional
Hubbard model Hamiltonian is investigated using numerical techniques. Positive
and negative values of are analyzed. Spin-spin correlations, ,
vs , and local magnetic moments are studied for values
of ranging from 0 to 6, and as a function of the electronic density. The
influence of the diagonal hopping in the spectral function
is also discussed, and the changes in the gap present in the density of states
at half-filling are studied. We introduce a new criterion to determine probable
locations of Fermi surfaces at zero temperature from data obtained
at finite temperature. It appears that hole pockets at
may be induced for negative while a positive produces similar
features at and . Comparisons with the standard 2D
Hubbard () model indicate that a negative hopping amplitude appears
to be dynamically generated. In general, we conclude that it is very dangerous
to extract a bare parameter of the Hamiltonian from PES data whereComment: 9 pages (RevTex 3.0), 12 figures (postscript), files packed with
uufile
Soft phonons and structural phase transition in superconducting Ba0.59K0.41BiO3
We have observed a softening of phonons and a structural phase transition in
a superconducting Ba0.59K0.41BiO3 (Tc = 31 K) single crystal using elastic and
inelastic neutron scattering measurements. The soft phonon occurs for the [111]
transverse acoustic mode at the zone boundary. The phonon energies in this
vicinity are found to continuously decrease with decreasing temperature from
above room temperature to 200 K, where a structural phase transition from cubic
to tetragonal symmetry occurs. The overall results are consistent with previous
data that reported phonon softening and a (0.5, 0.5, 0.5) type superstructure
in several Ba1-xKxBiO3 systems. However, we also find weak (0.5, 0.5, 0) type
superstructure peaks that reveal an additional component to the modulation. No
significant change related to the superconductivity was observed for the soft
phonon energies or linewidths.Comment: 15 page
Spin Susceptibility and Superexchange Interaction in the Antiferromagnet CuO
Evidence for the quasi one-dimensional (1D) antiferromagnetism of CuO is
presented in a framework of Heisenberg model. We have obtained an experimental
absolute value of the paramagnetic spin susceptibility of CuO by subtracting
the orbital susceptibility separately from the total susceptibility through the
Cu NMR shift measurement, and compared directly with the theoretical
predictions. The result is best described by a 1D antiferromagnetic
Heisenberg (AFH) model, supporting the speculation invoked by earlier authors.
We also present a semi-quantitative reason why CuO, seemingly of 3D structure,
is unexpectedly a quasi 1D antiferromagnet.Comment: 7 pages including 4 tables and 9 figure
Palaeontology, the biogeohistory of Victoria
The broad-scale distribution of fossils within Victoria is controlled by general global patterns in the biological evolution of life on Earth, the local development and environmental evolution of habitats, and the occurrence of geological processes conducive to the preservation of fossil floras and faunas. Early Palaeozoic fossils are mostly marine in origin because of the predominance of marine sedimentary rocks in Victoria and because life on land was not significant during most of this time interval. Middle Palaeozoic sequences have both terrestrial and marine fossil records. Within Victoria, marine rocks are only very minor components of strata deposited during the late Palaeozoic, so that few marine fossils are known from this time period. A similar situation existed during most of the Mesozoic except towards the end of this era when marine conditions began to prevail in the Bass Strait region. During long intervals in the Cainozoic, large areas of Victoria were flooded by shallow-marine seas, particularly in the southern basins of Bass Strait, as well as in the northwest of the State (Murray Basin). Cainozoic sediments contain an extraordinary range of animal and plant fossils. During the Quaternary, the landscape of Victoria became, and continues to be, dominated by continental environments including, at times, extensive freshwater lake systems. Fossil floras and faunas from sediments deposited in these lake systems and from other continental sediments, as well as from Quaternary sediments deposited in marginal marine environments, collectively record a history of rapid fluctuations in climate and sea level.<br /
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