19 research outputs found
Strong-coupling limit of depleted Kondo- and Anderson-lattice models
Fourth-order strong-coupling degenerate perturbation theory is used to derive
an effective low-energy Hamiltonian for the Kondo-lattice model with a depleted
system of localized spins. In the strong-J limit, completely local Kondo
singlets are formed at the spinful sites which bind a fraction of conduction
electrons. The low-energy theory describes the scattering of the excess
conduction electrons at the Kondo singlets as well as their effective
interactions generated by virtual excitations of the singlets. Besides the
Hubbard term, already discussed by Nozieres, we find a ferromagnetic Heisenberg
interaction, an antiferromagnetic isospin interaction, a correlated hopping
and, in more than one dimensions, three- and four-site interactions. The
interaction term can be cast into highly symmetric and formally simple
spin-only form using the spin of the bonding orbital symmetrically centered
around the Kondo singlet. This spin is non-local. We show that, depending on
the geometry of the depleted lattice, spatial overlap of the non-local spins
around different Kondo singlets may cause ferromagnetic order. This is
sustained by a rigorous argument, applicable to the half-filled model, by a
variational analysis of the stability of the fully polarized Fermi sea of
excess conduction electrons as well as by exact diagonalization of the
effective model. A similar fourth-order perturbative analysis is performed for
the depleted Anderson lattice in the limit of strong hybridization. Even in a
parameter regime where the Schrieffer-Wolff transformation does not apply, this
yields the same effective theory albeit with a different coupling constant.Comment: 14 pages, 5 figure
Competition between Kondo screening and indirect magnetic exchange in a quantum box
Nanoscale systems of metal atoms antiferromagnetically exchange coupled to
several magnetic impurities are shown to exhibit an unconventional re-entrant
competition between Kondo screening and indirect magnetic exchange interaction.
Depending on the atomic positions of the magnetic moments, the total
ground-state spin deviates from predictions of standard
Ruderman-Kittel-Kasuya-Yosida perturbation theory. The effect shows up on an
energy scale larger than the level width induced by the coupling to the
environment and is experimentally verifiable by studying magnetic field
dependencies.Comment: 5 pages, 2 figures, v3 with minor change
a retrospective study
Introduction Emergency treatment of major sub-/total traumatic amputations
continue to represent a clinical challenge due to high infection rates and
serious handicaps. Effective treatment is based on two columns: surgery and
antimicrobial therapy. Detailed identification of pathogen spectrum and
epidemiology associated with these injuries is of tremendous importance as it
guides the initial empiric antibiotic regimen and prevents adverse septic
effents. Methods In this retrospective study 51 patients with major traumatic
amputations (n = 16) and subtotal amputations (n = 35) treated from 2001 to
2010 in our trauma center were investigated. All patients received emergency
surgery, debridement with microbiological testing within 6 h after admission
and empircic antimicrobial therapy. Additionally to baseline patient
characteristics, the incidence of positive standardized microbiologic testing
combined with clinical signs of infection, pathogen spectrum, administered
antimicrobial agents and clinical complications were analyzed. Results 70.6%
of the patients (n = 36) acquired wound infection. In 39% wounds were
contaminated on day 1, whereas the mean length of duration until first
pathogen detection was 9.1 ± 13.4 days after injury. In 37% polymicrobial
colonization and 28% Pseudomonas were responsible for wound infections during
hospitalization. In 45% the empirc antimicrobial therapy focussed on Gram
positive strains did not cover the detected bacteria, according antimicrobial
resistogram. It was significantly more often found in infections associated
with Pseudomonas (p 0.02) or polymicrobial wound infections. Conclusions This
epidemiologic study reveals a pathogen shift from Gram-positive to Gram-
negative strains with high incidence of Pseudomonas and polymicrobial
infections in sub-/total major traumatic amputations. Therefore, empiric
antimicrobial treatment historically focussing on Gram-positive strains must
be adjusted. We recommend the use of Piperacillin/Tazobactam for these
injuries. As soon as possible antimicrobial treatment should be changed from
empiric to goal directed therapy according to the microbiological tests and
resistogram results
Impact of high prevalence of pseudomonas and polymicrobial gram-negative infections in major sub-/total traumatic amputations on empiric antimicrobial therapy: a retrospective study
INTRODUCTION: Emergency treatment of major sub-/total traumatic amputations continue to represent a clinical challenge due to high infection rates and serious handicaps. Effective treatment is based on two columns: surgery and antimicrobial therapy. Detailed identification of pathogen spectrum and epidemiology associated with these injuries is of tremendous importance as it guides the initial empiric antibiotic regimen and prevents adverse septic effents. METHODS: In this retrospective study 51 patients with major traumatic amputations (n = 16) and subtotal amputations (n = 35) treated from 2001 to 2010 in our trauma center were investigated. All patients received emergency surgery, debridement with microbiological testing within 6 h after admission and empircic antimicrobial therapy. Additionally to baseline patient characteristics, the incidence of positive standardized microbiologic testing combined with clinical signs of infection, pathogen spectrum, administered antimicrobial agents and clinical complications were analyzed. RESULTS: 70.6% of the patients (n = 36) acquired wound infection. In 39% wounds were contaminated on day 1, whereas the mean length of duration until first pathogen detection was 9.1 ± 13.4 days after injury. In 37% polymicrobial colonization and 28% Pseudomonas were responsible for wound infections during hospitalization. In 45% the empirc antimicrobial therapy focussed on Gram positive strains did not cover the detected bacteria, according antimicrobial resistogram. It was significantly more often found in infections associated with Pseudomonas (p 0.02) or polymicrobial wound infections. CONCLUSIONS: This epidemiologic study reveals a pathogen shift from Gram-positive to Gram-negative strains with high incidence of Pseudomonas and polymicrobial infections in sub-/total major traumatic amputations. Therefore, empiric antimicrobial treatment historically focussing on Gram-positive strains must be adjusted. We recommend the use of Piperacillin/Tazobactam for these injuries. As soon as possible antimicrobial treatment should be changed from empiric to goal directed therapy according to the microbiological tests and resistogram results
Dynamical mean-field theory of indirect magnetic exchange
To analyze the physical properties arising from indirect magnetic exchange
between several magnetic adatoms and between complex magnetic nanostructures on
metallic surfaces, the real-space extension of dynamical mean-field theory
(R-DMFT) appears attractive as it can be applied to systems of almost arbitrary
geometry and complexity. While R-DMFT describes the Kondo effect of a single
adatom exactly, indirect magnetic (RKKY) exchange is taken into account on an
approximate level only. Here, we consider a simplified model system consisting
of two magnetic Hubbard sites ("adatoms") hybridizing with a non-interacting
tight-binding chain ("substrate surface"). This two-impurity Anderson model
incorporates the competition between the Kondo effect and indirect exchange but
is amenable to an exact numerical solution via the density-matrix
renormalization group (DMRG). The particle-hole symmetric model at half-filling
and zero temperature is used to benchmark R-DMFT results for the magnetic
coupling between the two adatoms and for the magnetic properties induced in the
substrate. In particular, the dependence of the local adatom and the nonlocal
adatom-adatom static susceptibilities as well as the magnetic response of the
substrate on the distance between the adatoms and on the strength of their
coupling with the substrate is studied. We find both, excellent agreement with
the DMRG data even on subtle details of the competition between RKKY exchange
and the Kondo effect but also complete failure of the R-DMFT, depending on the
parameter regime considered. R-DMFT calculations are performed using the
Lanczos method as impurity solver. With the real-space extension of the
two-site DMFT, we also benchmark a simplified R-DMFT variant.Comment: 14 pages, 8 figure