398 research outputs found
Nonadiabatic effects of rattling phonons and 4f excitations in Pr(Os{1-x}Ru{x})4Sb12
In the skutterudite compounds the anharmonic 'rattling' oscillations of
4f-host ions in the surrounding Sb12 cages are found to have significant
influence on the low temperature properties. Recently specific heat analysis of
Pr(Os{1-x}Ru{x})4Sb12 has shown that the energy of crystalline electric field
(CEF) singlet-triplet excitations increases strongly with Ru-concentration x
and crosses the almost constant rattling mode frequency at about x ~
0.65. Due to magnetoelastic interactions this may entail prominent nonadiabatic
effects in inelastic neutron scattering (INS) intensity and quadrupolar
susceptibility. Furthermore the Ru- concentration dependence of the
superconducting Tc, notably the minimum at intermediate x is explained as a
crossover effect from pairforming aspherical Coulomb scattering to pairbreaking
exchange scattering.Comment: 12 pages, 5 figures; to appear in Phys. Rev.
CPA for charge ordering in the extended Hubbard model
We study charge ordering in the extended Hubbard model with both on-site and
nearest neighbor Coulomb repulsion (U and V, respectively) within the Coherent
potential approximation (CPA). The phase boundary between the homogeneous and
charge ordered phase for the square lattice is obtained for different values of
U. It is shown that at quarter filling for all values of U the charge ordering
exists only if the inter-site Coulomb repulsion V exceeds certain critical
value which is of the order of the kinetic energy t. At finite temperature a
reentrant transition is found in some region of V.Comment: 5 pages, 4 figure
Magnetic excitations in charge- ordered NaV2O5
An investigation of the spin excitation spectrum of charge ordered (CO)
NaV2O5 is presented. We discuss several different exchange models which may be
relevant for this compound, namely in- line and zig-zag chain models with weak
as well as strong inter- chain coupling and also a ladder model and a CO/MV
(mixed valent) model. We put special emphasis on the importance of large
additional exchange across the diagonals of V- ladders and the presence of
exchange anisotropies on the excitation spectrum. It is shown that the observed
splitting of transverse dispersion branches may both be interpreted as
anisotropy effect as well as acoustic- optic mode splitting in the weakly
coupled chain models. In addition we calculate the field dependence of
excitation modes in these models. Furthermore we show that for strong inter-
chain coupling, as suggested by recent LDA+U results, an additional high energy
optical excitation appears and the spin gap is determined by anisotropies. The
most promising CO/MV model predicts a spin wave dispersion perpendicular to the
chains which agrees very well with recent results obtained by inelastic neutron
scattering.Comment: Revtex, 22 pages, 7 postscript figure
Spin Exciton Formation inside the Hidden Order Phase of CeB6
The heavy fermion metal CeB6 exhibits hidden order of antiferroquadrupolar
(AFQ) type below T_Q=3.2K and subsequent antiferromagnetic (AFM) order at
T_N=2.3K. It was interpreted as ordering of the quadrupole and dipole moments
of a quartet of localised Ce electrons. This established
picture has been profoundly shaken by recent inelastic neutron scattering (G.
Friemel et al., arXiv:1111.4151) that found the evolution of a feedback spin
exciton resonance within the hidden order phase at the AFQ wave vector which is
stabilized by the AFM order. We develop an alternative theory based on a
fourfold degenerate Anderson lattice model, including both order parameters as
particle-hole condensates of itinerant heavy quasiparticles. This explains in a
natural way the appearance of the spin exciton resonance and the momentum
dependence of its spectral weight, in particular around the AFQ vector and its
rapid disappearance in the disordered phase. Analogies to the feedback effect
in unconventional heavy fermion superconductors are pointed out.Comment: 5 pages, 3 figure
Superconductivity in a magnetically ordered background
Borocarbide compounds with the formula RNi2B2C show interesting
superconducting and magnetic properties and the coexistence of the two
phenomena. BCS theory is extended to systems with underlying commensurate
magnetic order. In the case of helical phases the technique may be extended to
any Q-vector and there exists a well defined limit for incommensurate values.
The way magnetic order influences superconductivity depends crucially on the
details of both the magnetic structure and the electron bands, but some
qualitative criteria may be given. As an example we give a brief analysis of
the compound HoNi2B2C.Comment: 3 pages, 1 figure, proceedings to the conference "Anomalous Complex
Superconductors", Crete 199
Reentrant Kondo effect in Landau quantized graphene
We have studied the interplay of an Anderson impurity in Landau quantized
graphene, with special emphasis on the influence of the chemical potential.
Within the slave-boson mean-field theory, we found reentrant Kondo behaviour by
varying the chemical potential or gate voltage. Between Landau levels, the
density of states is suppressed, and by changing the graphene's Fermi energy,
we cross from metallic to semiconducting regions. Hence, the corresponding
Kondo behaviour is also influenced. The f-level spectral function reveals both
the presence of Landau levels in the conduction band and the Kondo resonance.Comment: 8 pages, 6 figure
Field induced quantum phase transition in the anisotropic Kondo necklace model
The anisotropic Kondo necklace model in 2D and 3D is treated as a genuine
model for magnetic to Kondo singlet quantum phase transitions in the heavy
fermion (HF) compounds. The variation of the quantum critical point (QCP) with
anisotropy parameters has been investigated previously in the zero field case
[1]. Here we extend the treatment to finite fields using a generalised bond
operator representation including all triplet states. The variation of critical
tc with external field H and the associated phase diagram is derived. The
influence of anisotropies and the different g-factors for localised and
itinerant spins on tc(H) is also investigated. It is found that three different
types of behaviour may appear: (i) Destruction of antiferromangetism and
appearance of a singlet state above a critical field. (ii) The inverese
behaviour, namely field induced antiferromagnetism out of the Kondo singlet
phase. (iii) Reentrance behaviour of the Kondo singlet phase as function of
field strength.Comment: 12 pages, 5 figure
Dynamic spin susceptibility of paramagnetic spinel LiV2O4
In an attempt to explain inelastic neutron scattering data for LiV2O4 the
dynamical spin susceptibility \chi(Q,w) at zero temperature is calculated.
Starting point is a weak coupling approach based on the LDA bandstructure for
that material. It is supplemented by a RPA treatment of local on-site
interactions and contains an adjustable parameter. Due to the geometrically
frustrated lattice structure the magnetic response is strongly enhanced in the
vicinity of a nearly spherical surface in Q-space. We compare these results
with recent low-temperature neutron scattering data. The measured spin
relaxation rate \Gamma is used to estimate the spin fluctuation contribution to
the specific heat.Comment: 26 pages, 6 figures, submitted to PR
Evidence for time-reversal symmetry breaking in superconducting PrPt4Ge12
Zero and longitudinal field muon spin rotation (muSR) experiments were
performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc
a spontaneous magnetization with a temperature variation resembling that of the
superfluid density appears. This observation implies time-reversal symmetry
(TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an
anomalous superconductor and the weak and gradual change of Tc and of the
related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12)
suggests that the TRS breaking is due to orbital degrees of freedom of the
Cooper pairs.Comment: To appear in Phys. Rev. B. 5 pages, 3 figure
Cluster approach study of intersite electron correlations in pyrochlore and checkerboard lattices
To treat effects of electron correlations in geometrically frustrated
pyrochlore and checkerboard lattices, an extended single-orbital Hubbard model
with nearest neighbor hopping and Coulomb repulsion is
applied. Infinite on-site repulsion, , is assumed, thus double
occupancies of sites are forbidden completely in the present study. A
variational Gutzwiller type approach is extended to examine correlations due to
short-range interaction and a cluster approximation is developed to
evaluate a variational ground state energy of the system. Obtained analytically
in a special case of quarter band filling appropriate to LiVO, the
resulting simple expression describes the ground state energy in the regime of
intermediate and strong coupling . Like in the Brinkman-Rice theory based on
the standard Gutzwiller approach to the Hubbard model, the mean value of the
kinetic energy is shown to be reduced strongly as the coupling approaches a
critical value . This finding may contribute to explaining the observed
heavy fermion behavior in LiVO
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