154 research outputs found
Structural and electronic properties of an azamacrocycle, C26H18N6
We compute the structure of an azamacrocycle, C26H18N6. Two approximatively
planar elliptical structures with C2 or CI symmetry are found to be nearly
degenerate. The roughly circular conformation observed in metal complexes turns
out to be ~ 0.6 eV higher in energy. We suggest that this difference is mainly
due to electrostatic interactions. We discuss the results on various levels of
theory (Hartree-Fock, local density and gradient corrected density functional
calculations).Comment: to appear in J Phys Chem
Fermi surface and heavy masses for UPdAl
We calculate the Fermi surface and the anisotropic heavy masses of UPd2Al3 by
keeping two of the 5f electrons as localized. Good agreement with experiments
is found. The theory contains essentially no adjustable parameter except for a
small shift of the position of the Fermi energy of the order of a few meV. A
discussion is given why localization of two f electrons is justified.Comment: 4 pages, 2 figure
Moment screening in the correlated Kondo lattice model
The magnetic correlations, local moments and the susceptibility in the
correlated 2D Kondo lattice model at half filling are investigated. We
calculate their systematic dependence on the control parameters J_K/t and U/t.
An unbiased and reliable exact diagonalization (ED) approach for ground state
properties as well as the finite temperature Lanczos method (FTLM) for specific
heat and the uniform susceptibility are employed for small tiles on the square
lattice. They lead to two major results: Firstly we show that the screened
local moment exhibits non-monotonic behavior as a function of U for weak Kondo
coupling J_K. Secondly the temperature dependence of the susceptibility
obtained from FTLM allows to extract the dependence of the characteristic Kondo
temperature scale T* on the correlation strength U. A monotonic increase of T*
for small U is found resolving the ambiguity from earlier investigations. In
the large U limit the model is equivalent to the 2D Kondo necklace model with
two types of localized spins. In this limit the numerical results can be
compared to those of the analytical bond operator method in mean field
treatment and excellent agreement for the total paramagnetic moment is found,
supporting the reliability of both methods.Comment: 19 pages, 9 figure
Superconductivity in heavy fermion compounds
We review the current state of experimental and theoretical investigations of
heavy fermion superconductors. We discuss most of the Ce-based compounds like
Ce122, Ce115, Ce218 and Ce131 classes and U-based superconductors like UBe_13
and UPd_2Al_3. In the former the emphasis is on the connection to quantum
critical phenomena and non-Fermi liquid behaviour. Recent neutron scattering
and hydrostatic pressure results on SDW/SC competition in the Ce122 system are
included. For the U-compounds we discuss the significance of dual models with
both localised and itinerant 5f electrons for mass enhancement and
superconducting pair formation. Itinerant spin fluctuation theories for
unconventional superconductivity are also reviewed.Comment: 74 pages, 29 figures. For a version of the manuscript including
higher-resolution figures, see http://www.cpfs.mpg.de/~thalm/SCMaterials.pd
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
Competition between Kondo screening and quantum Hall edge reconstruction
We report on a Kondo correlated quantum dot connected to two-dimensional
leads where we demonstrate the renormalization of the g-factor in the pure
Zeeman case i.e, for magnetic fields parallel to the plane of the quantum dot.
For the same system we study the influence of orbital effects by investigating
the quantum Hall regime i.e. a perpendicular magnetic field is applied. In this
case an unusual behaviour of the suppression of the Kondo effect and of the
split zero-bias anomaly is observed. The splitting decreases with magnetic
field and shows discontinuous changes which are attributed to the intricate
interplay between Kondo screening and the quantum Hall edge structure
originating from electrostatic screening. This edge structure made up of
compressible and incompressible stripes strongly affects the Kondo temperature
of the quantum dot and thereby influences the renormalized g-factor
Lifshitz transitions and quasiparticle de-renormalization in YbRhSi
We study the effect of magnetic fields up to 15 T on the heavy fermion state
of YbRhSi via Hall effect and magnetoresistance measurements down to 50
mK. Our data show anomalies at three different characteristic fields. We
compare our data to renormalized band structure calculations through which we
identify Lifshitz transitions associated with the heavy fermion bands. The Hall
measurements indicate that the de-renormalization of the quasiparticles, {\it
i.e} the destruction of the local Kondo singlets, occurs smoothly while the
Lifshitz transitions occur within rather confined regions of the magnetic
field.Comment: 7 pages, 5 figure
Similar temperature scale for valence changes in Kondo lattices with different Kondo temperatures
The Kondo model predicts that both the valence at low temperatures and its
temperature dependence scale with the characteristic energy T_K of the Kondo
interaction. Here, we study the evolution of the 4f occupancy with temperature
in a series of Yb Kondo lattices using resonant X-ray emission spectroscopy. In
agreement with simple theoretical models, we observe a scaling between the
valence at low temperature and T_K obtained from thermodynamic measurements. In
contrast, the temperature scale T_v at which the valence increases with
temperature is almost the same in all investigated materials while the Kondo
temperatures differ by almost four orders of magnitude. This observation is in
remarkable contradiction to both naive expectation and precise theoretical
predictions of the Kondo model, asking for further theoretical work in order to
explain our findings. Our data exclude the presence of a quantum critical
valence transition in YbRh2Si2
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