60 research outputs found
Theoretical investigation of magnetic order in ReOFeAs, Re = Ce, Pr
Density functional theory (DFT) calculations are carried out on ReOFeAs, Re =
Ce, Pr, the parent compounds of the high-T superconductors
ReOFFeAs, in order to determine the magnetic order of the ground
state. It is found that the magnetic moments on the Fe sites adopt a collinear
antiferromagnetic order, similar to the case of LaOFeAs. Within the generalized
gradient approximation along with Coulomb onsite repulsion (GGA+U), we show
that the Re magnetic moments also adopt an antiferromagnetic order for which,
within the ReO layer, same spin Re sites lie along a zigzag line perpendicular
to the Fe spin stripes. While within GGA the Re 4f band crosses the Fermi
level, upon inclusion of onsite Coulomb interaction the 4f band splits and
moves away from the Fermi level, making ReOFeAs a Mott insulator.Comment: 5 pages, 4 figure
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
Physics of Polymorphic Transitions in CeRuSn
We report a detailed study of the polymorphic transitions in ternary stannide
CeRuSn on high quality single crystals through a combination of X-ray
diffraction experiments conducted at 300, 275 and 120 K, and measurements of
the thermal expansion, magnetization, and resistivity, along main
crystallographic axes. In addition, the transition was followed as a function
of pressure up to 0.8 GPa. The present X-ray diffraction data show that the
room temperature polymorph consists of the lattice doubled along the c axis
with respect to the CeCoAl-type structure consistent with previous reports.
Upon cooling, the compound undergoes two successive transitions, first to a
quintuple (290 K) and than to a triple CeCoAl superstructure at 225 K. The
transitions are accompanied by a tremendous volume change due to a strong
shrinking of the lattice along the c axis, which is clearly observed in thermal
expansion. We advance arguments that the volume collapse originates from an
increasing number of crystallographically inequivalent Ce sites and the change
of ratio between the short and long Ce-Ru bonds. The observed properties of the
polymorphic transition in CeRuSn are reminiscent of the transition in
elementary Cerium, suggesting that similar physics, i.e., a Kondo influenced
transition and strong lattice vibrations might be the driving forces
First order valence transition in YbInCu_4 in the (B,T) - plane
The puzzling properties of the first order phase transition in YbInCu and
its alloys in the wide range of magnetic fields and temperatures are perfectly
described in terms of a simple entropy transition for free Yb ions. In
particular, it turns out that the transition line in the -plane is very
close to the elliptic shape, as it has been observed experimentally. Similar
calculations are done, and the experiments are proposed for the
phase transition in Ce in Megagauss fields. We speculate,
that in case of YbInCu the first order transition is a Mott transition
between a higher temperature phase in which localized moments are stabilized by
the entropy terms in the free energy, and a band-like non-magnetic ground state
of the -electrons.Comment: RevTeX, 5 pages, 2 figure
Low-lying energy spectrum of the cerium dimer
The electronic structure of Ce2 is studied in a valence bond model with two
4f electrons localized at two cerium sites. It is shown that the low-lying
energy spectrum of the simplest cerium chemical bond is determined by
peculiarities of the occupied 4f states. The model allows for an analytical
solution which is discussed along with the numerical analysis. The energy
spectrum is a result of the interplay between the 4f valence bond exchange, the
4f Coulomb repulsion and the spin-orbit coupling. The calculated ground state
is the even \Omega=\Lambda=\Sigma=0 level, the lowest excitations situated at
30 K are the odd \Omega=\Lambda=\Sigma=0 state and the doublet
(\Omega=\pm5,\, \Lambda=\pm6,\, \Sigma=\mp1). The calculated magnetic
susceptibility displays different behavior at high and low temperatures. In the
absence of the spin-orbit coupling the ground state is the
triplet. The results are compared with other many-electron calculations and
experimental data.Comment: 10 pages, 3 figures, 5 Tables, 18 Reference
A robust but disordered collapsed-volume phase in a cerium alloy under the application of pulsed magnetic fields
We report synchrotron x-ray powder diffraction measurements of
Ce0.8La0.1Th0.1 subject to pulsed magnetic fields as high as 28 Tesla. This
alloy is known to exhibit a continuous volume collapse on cooling at ambient
pressure, which is a modification of the gamma -> alpha transition in elemental
cerium. Recently, it has been suggested on the basis of field-cooled
resistivity and pulsed field magnetization measurements that the volume
collapse in this alloy can be suppressed by the application of magnetic fields.
Conversely, our direct diffraction measurements show a robust collapsed phase,
which persists in magnetic fields as high as 28 Tesla. We also observe
nanoscale disorder in the collapsed phase, which increasingly contaminates the
high temperature phase on thermal cycling.Comment: 6 pages, 6 figures, submitted to PR
Superconductivity emerging near quantum critical point of valence transition
The nature of the quantum valence transition is studied in the
one-dimensional periodic Anderson model with Coulomb repulsion between f and
conduction electrons by the density-matrix renormalization group method. It is
found that the first-order valence transition emerges with the quantum critical
point and the crossover from the Kondo to the mixed-valence states is strongly
stabilized by quantum fluctuation and electron correlation. It is found that
the superconducting correlation is developed in the Kondo regime near the sharp
valence increase. The origin of the superconductivity is ascribed to the
development of the coherent motion of electrons with enhanced valence
fluctuation, which results in the enhancement of the charge velocity, but not
of the charge compressibility. Statements on the valence transition in
connection with Ce metal and Ce compounds are given.Comment: 9 pages, 4 figure
The Structure of Barium in the hcp Phase Under High Pressure
Recent experimental results on two hcp phases of barium under high pressure
show interesting variation of the lattice parameters. They are here interpreted
in terms of electronic structure calculation by using the LMTO method and
generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II
the dramatic drop in c/a is an instability analogous to that in the group II
metals but with the transfer of s to d electrons playing a crucial role in Ba.
Meanwhile in phase V, the instability decrease a lot due to the core repulsion
at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx,
71.15LaComment: 29 pages, 8 figure
Tricritical Phenomena at the Cerium Transition
The isostructural transition in the
CeLaTh system is measured as a function of La alloying
using specific heat, magnetic susceptibility, resistivity, thermal
expansivity/striction measurements. A line of discontinuous transitions, as
indicated by the change in volume, decreases exponentially from 118 K to close
to zero with increasing La doping and the transition changes from being
first-order to continuous at a critical concentration . At the tricritical point, the coefficient of the linear term in the
specific heat and the magnetic susceptibility start to increase
rapidly near = 0.14 and gradually approaches large values at =0.35
signifying that a heavy Fermi-liquid state evolves at large doping. Near ,
the Wilson ratio, , has a value of 3.0, signifying the presence of
magnetic fluctuations. Also, the low-temperature resistivity shows that the
character of the low-temperature Fermi-liquid is changing
High field x-ray diffraction study on a magnetic-field-induced valence transition in YbInCu4
We report the first high-field x-ray diffraction experiment using synchrotron
x-rays and pulsed magnetic fields exceeding 30 T. Lattice deformation due to a
magnetic-field-induced valence transition in YbInCu4 is studied. It has been
found that the Bragg reflection profile at 32 K changes significantly at around
27 T due to the structural transition. In the vicinity of the transition field
the low-field and the high-field phases are observed simultaneously as the two
distinct Bragg reflection peaks: This is a direct evidence of the fact that the
field-induced valence state transition is the first order phase transition. The
field-dependence of the low-field-phase Bragg peak intensity is found to be
scaled with the magnetization.Comment: 5 pages, 6 figures, submitted to J. Phys. Soc. Jp
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