323 research outputs found
Many-particle effects in adsorbed magnetic atoms with easy-axis anisotropy: the case of Fe on CuN/Cu(100) surface
We study the effects of the exchange interaction between an adsorbed magnetic
atom with easy-axis magnetic anisotropy and the conduction-band electrons from
the substrate. We model the system using an anisotropic Kondo model and we
compute the impurity spectral function which is related to the differential
conductance (dI/dV) spectra measured using a scanning tunneling microscope. To
make contact with the known experimental results for iron atoms on the
CuN/Cu(100) surface [Hirjibehedin et al., Science {\bf 317}, 1199 (2007)], we
calculated the spectral functions in the presence of an external magnetic field
of varying strength applied along all three spatial directions. It is possible
to establish an upper bound on the coupling constant J: in the range of the
magnetic fields for which the experimental results are currently known (up to
7T), the low-energy features in the calculated spectra agree well with the
measured dI/dV spectra if the exchange coupling constant J is at most half as
large as that for cobalt atoms on the same surface. We show that for even
higher magnetic field (between 8 and 9T) applied along the ``hollow
direction'', the impurity energy states cross, giving rise to a Kondo effect
which takes the form of a zero-bias resonance. The paper introduces an approach
for calculating the expectation values of global spin operators and all
components of the impurity magnetic susceptibility tensor in numerical
renormalization group (NRG) calculations with no spin symmetry. An appendix
contains a density-functional-theory (DFT) study of the Cu and Fe adsorbates on
CuN/Cu(100) surface: we compare magnetic moments, as well as orbital energies,
occupancies, centers, and spreads by calculating the maximally localized
Wannier orbitals of the adsorbates.Comment: 18 pages, 7 figure
priročnik regionalne zgodovine Istre z navedbami o mestu Reka. Fiume, Unione italiana - Trieste, Universita popolare, Rovigno, Centro di Ricerche Storiche, 2011, 735 str.
Kondo resonance line-shape of magnetic adatoms on decoupling layers
The zero-bias resonance in the dI/dV tunneling spectrum recorded using a
scanning tunneling microscope above a spin-1/2 magnetic adatom (such as Ti)
adsorbed on a decoupling layer on metal surface can be accurately fitted using
the universal spectral function of the Kondo impurity model both at zero field
and at finite external magnetic field. Excellent agreement is found both for
the asymptotic low-energy part and for the high-energy logarithmic tails of the
Kondo resonance. For finite magnetic field, the nonlinear fitting procedure
consists in repeatedly solving the impurity model for different Zeeman energies
in order to obtain accurate spectral functions which are compared with the
experimental dI/dV curves. The experimental results at zero field are
sufficiently restraining to enable an unprecedented reliability in the
determination of the Kondo temperature, while at finite fields the results are
more ambiguous and two different interpretations are proposed
Van Hove singularities in the paramagnetic phase of the Hubbard model: a DMFT study
Using the dynamical mean-field theory (DMFT) we study the paramagnetic phase
of the Hubbard model with the density of states (DOS) corresponding to the
three-dimensional cubic lattice and the two-dimensional square lattice, as well
as a DOS with inverse square root singularity. We show that the electron
correlations rapidly smooth out the square-root van Hove singularities (kinks)
in the spectral function for the 3D lattice and that the Mott metal-insulator
transition (MIT) as well as the magnetic-field-induced MIT differ only little
from the well-known results for the Bethe lattice. The consequences of the
logarithmic singularity in the DOS for the 2D lattice are more dramatic. At
half filling, the divergence pinned at the Fermi level is not washed out, only
its integrated weight decreases as the interaction is increased. While the Mott
transition is still of the usual kind, the magnetic-field-induced MIT falls
into a different universality class as there is no field-induced localization
of quasiparticles. In the case of a power-law singularity in the DOS at the
Fermi level, the power-law singularity persists in the presence of interaction,
albeit with a different exponent, and the effective impurity model in the DMFT
turns out to be a pseudo-gap Anderson impurity model with a hybridization
function which vanishes at the Fermi level. The system is then a generalized
Fermi liquid. At finite doping, regular Fermi liquid behavior is recovered.Comment: 7 pages, 9 figure
Magnetic anisotropy effects on quantum impurities in superconducting host
We study the magnetic anisotropy effects on the localized sub-gap excitations
induced by quantum impurities coupled to a superconducting host. We establish
the ground-state phase diagrams for single-channel and two-channel high-spin
Kondo impurities; they unveil surprising complexity that results from the
(multi-stage) Kondo screening in competition with the superconducting
correlations and the magnetic anisotropy splitting of the spin multiplets. We
discuss the possibility of detecting the Zeeman splitting of the sub-gap
states, which would provide an interesting spectroscopic tool for studying the
magnetism on the single-atom level. We also study the problem of two impurities
coupled by the Heisenberg exchange interaction, and we follow the evolution of
the sub-gap states for both antiferromagnetic and ferromagnetic coupling. For
sufficiently strong antiferromagnetic coupling, the impurities bind into a
singlet state that is non-magnetic, thus the sub-gap states move to the edge of
the gap and can no longer be discerned. For ferromagnetic coupling, some
excited states remain present inside the gap.Comment: 14 pages, 16 figures (significantly extended version, includes a
section on the two-impurity effects
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