57 research outputs found
Approximative treatment of 5f-systems with partial localization due to intra-atomic correlations
Increasing experimental and theoretical evidence points towards a dual nature
of the 5 electrons in actinide-based strongly correlated metallic compounds,
with some 5 electrons being localized and others delocalized. In a recent
paper (PRB xxx, 2004), we suggested the interplay of intra-atomic correlations
as described by Hund's rules and a weakly anisotropic hopping (hybridization)
as a possible mechanism. The purpose of the present work is to provide a first
step towards a microscopic description of partial localization in solids by
analyzing how well various approximation schemes perform when applied to small
clusters. It is found that many aspects of partial localization are described
appropriately both by a variational wavefunction of Gutzwiller type and by a
treatment which keeps only those interactions which are present in LDA+U
calculations. In contrast, the energies and phase diagram calculated within the
Hartree Fock approximation show little resemblence with the exact results.
Enhancement of hopping anisotropy by Hund's rule correlations are found in all
approximations.Comment: 9 pages, 9 figure
Multiple-charge transfer and trapping in DNA dimers
We investigate the charge transfer characteristics of one and two excess
charges in a DNA base-pair dimer using a model Hamiltonian approach. The
electron part comprises diagonal and off-diagonal Coulomb matrix elements such
a correlated hopping and the bond-bond interaction, which were recently
calculated by Starikov [E. B. Starikov, Phil. Mag. Lett. {\bf 83}, 699 (2003)]
for different DNA dimers. The electronic degrees of freedom are coupled to an
ohmic or a super-ohmic bath serving as dissipative environment. We employ the
numerical renormalization group method in the nuclear tunneling regime and
compare the results to Marcus theory for the thermal activation regime. For
realistic parameters, the rate that at least one charge is transferred from the
donor to the acceptor in the subspace of two excess electrons significantly
exceeds the rate in the single charge sector. Moreover, the dynamics is
strongly influenced by the Coulomb matrix elements. We find sequential and pair
transfer as well as a regime where both charges remain self-trapped. The
transfer rate reaches its maximum when the difference of the on-site and
inter-site Coulomb matrix element is equal to the reorganization energy which
is the case in a GC-GC dimer. Charge transfer is completely suppressed for two
excess electrons in AT-AT in an ohmic bath and replaced by damped coherent
electron-pair oscillations in a super-ohmic bath. A finite bond-bond
interaction alters the transfer rate: it increases as function of when
the effective Coulomb repulsion exceeds the reorganization energy (inverted
regime) and decreases for smaller Coulomb repulsion
Quantum Resistive Transition in Type II Superconductors under Magnetic Field
It is shown that, within a Ginzburg-Landau (GL) formalism, the
superconducting fluctuation is insulating at zero temperature even if the
fluctuation dynamics is metallic (dissipative). Based on this fact, the low
temperature behavior of the -line and the resistivity curves near a
zero temperature transition are discussed. In particular, it is pointed out
that the neglect of quantum fluctuations in data analysis of the dc resistivity
may lead to an under-estimation of the values near zero temperature.Comment: 7 page
Kondo Effect in a Metal with Correlated Conduction Electrons: Diagrammatic Approach
We study the low-temperature behavior of a magnetic impurity which is weakly
coupled to correlated conduction electrons. To account for conduction electron
interactions a diagrammatic approach in the frame of the 1/N expansion is
developed. The method allows us to study various consequences of the conduction
electron correlations for the ground state and the low-energy excitations. We
analyse the characteristic energy scale in the limit of weak conduction
electron interactions. Results are reported for static properties (impurity
valence, charge susceptibility, magnetic susceptibility, and specific heat) in
the low-temperature limit.Comment: 16 pages, 9 figure
Electronic structure of spinel-type LiV_2O_4
The band structure of the cubic spinel compound LiV_2O_4, which has been
reported recently to show heavy Fermion behavior, has been calculated within
the local-density approximation using a full-potential version of the linear
augmented-plane-wave method. The results show that partially-filled V 3d bands
are located about 1.9 eV above the O 2p bands and the V 3d bands are split into
a lower partially-filled t_{2g} complex and an upper unoccupied e_{g} manifold.
The fact that the conduction electrons originate solely from the t_{2g} bands
suggests that the mechanism for the mass enhancement in this system is
different from that in the 4f heavy Fermion systems, where these effects are
attributed to the hybridization between the localized 4f levels and itinerant
spd bands.Comment: 5 pages, revte
Conventional type-II superconductivity in locally non-centrosymmetric LaRhAs single crystals
We report on the observation of superconductivity in LaRhAs, which is
the analogue without -electrons of the heavy-fermion system with two
superconducting phases CeRhAs. A zero-resistivity transition, a
specific-heat jump and a drop in magnetic ac susceptibility consistently point
to a superconducting transition at a transition temperature of \,K.
The magnetic field-temperature superconducting phase diagrams determined from
field-dependent ac-susceptibility measurements reveal small upper critical
fields \,mT for and
\,mT for . The observed
is larger than the estimated thermodynamic critical field
derived from the heat-capacity data, suggesting that LaRhA is a
type-II superconductor with Ginzburg-Landau parameters and . The microscopic Eliashberg
theory indicates superconductivity to be in the weak-coupling regime with an
electron-phonon coupling constant . Despite a
similar and the same crystal structure as the Ce compound, LaRhAs
displays conventional superconductivity, corroborating the substantial role of
the 4 electrons for the extraordinary superconducting state in
CeRhAs.Comment: 11 pages, 8 figure
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]
Orbital-based Scenario for Magnetic Structure of Neptunium Compounds
In order to understand a crucial role of orbital degree of freedom in the
magnetic structure of recently synthesized neptunium compounds NpTGa_5 (T=Fe,
Co, and Ni), we propose to discuss the magnetic phase of an effective
two-orbital model, which has been constructed based on a j-j coupling scheme to
explain the magnetic structure of uranium compounds UTGa_5. By analyzing the
model with the use of numerical technique such as exact diagonalization, we
obtain the phase diagram including several kinds of magnetic states. An
orbital-based scenario is discussed to understand the change in the magnetic
structure among C-, A-, and G-type antiferromagnetic phases, experimentally
observed in NpFeGa_5, NpCoGa_5, and NpNiGa_5.Comment: 18 pages, 8 figures, to appear in New Journal of Physic
Probing the phase diagram of CeRu_2Ge_2 by thermopower at high pressure
The temperature dependence of the thermoelectric power, S(T), and the
electrical resistivity of the magnetically ordered CeRu_2Ge_2 (T_N=8.55 K and
T_C=7.40 K) were measured for pressures p < 16 GPa in the temperature range 1.2
K < T < 300 K. Long-range magnetic order is suppressed at a p_c of
approximately 6.4 GPa. Pressure drives S(T) through a sequence of temperature
dependences, ranging from a behaviour characteristic for magnetically ordered
heavy fermion compounds to a typical behaviour of intermediate-valent systems.
At intermediate pressures a large positive maximum develops above 10 K in S(T).
Its origin is attributed to the Kondo effect and its position is assumed to
reflect the Kondo temperature T_K. The pressure dependence of T_K is discussed
in a revised and extended (T,p) phase diagram of CeRu_2Ge_2.Comment: 7 pages, 6 figure
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