35 research outputs found
Low energy scales of Kondo lattices: mean-field perspective
A review of the low temperature properties of Kondo lattice systems is
presented within the mean-field approximation, focusing on the different
characteristic energy scales. The Kondo temperature, T_K, and the Fermi liquid
coherence energy, T_0, are analyzed as functions of the electronic filling, the
shape of the non-interacting density of states, and the concentration of
magnetic moments. These two scales can vanish, corresponding to a breakdown of
the Kondo effect when an external magnetic field is applied. The Kondo
breakdown can also be reached by adding a superexchange term to the Kondo
lattice model, which mimics the intersite magnetic correlations neglected at
the mean-field level.Comment: 17 pages, 9 figures. Proceedings of the NATO Advanced Research
Workshop of Hvar, Croatia (2008
Lifshitz transition in Kondo alloys
We study the low energy states of Kondo alloys as function of the magnetic
impurity concentration per site, x, and the conduction electron average site
occupation, nc. Using two complementary approaches, the mean-field coherent
potential approximation and the strong coupling limit, we identify and
characterize two different Fermi liquid regimes. We propose that both regimes
are separated by a Lifshitz transition at x = nc. Indeed, we predict a
discontinuity of the number of quasiparticles which are enclosed in the Fermi
surface. This feature could provide a scenario for the non-Fermi liquid
properties that were recently observed in Kondo alloy systems around x = nc.Comment: 5 pages, 2 figure
Kondo screening by the surface modes of a strong topological insulator
We consider a magnetic impurity deposited on the surface of a strong
topological insulator and interacting with the surface modes by a Kondo
exchange interaction. Taking into account the warping of the Fermi line of the
surface modes, we derive a mapping to an effective one dimensional model and
show that the impurity is fully screened by the surface electrons except when
the Fermi level lies exactly at the Dirac point. Using an Abrikosov fermion
mean-field theory, we calculate the shape of the electronic density Friedel
oscillation resulting from the presence of the Kondo screening cloud. We
analyze quantitatively the observability of a six-fold symmetry in the Friedel
oscillations for two prototype compounds: BiSe and BiTe.Comment: 22 pages, 6 figure
An analysis of B_{d,s} mixing angles in presence of New Physics and an update of Bs -> K0* anti-K0*
We discuss a simple approach to measure the weak mixing angles phi_s and
phi_d of the Bs and Bd systems in the presence of New Physics. We present a new
expression that allows one to measure directly the New Physics mixing angles if
New Physics contributes significantly to the mixing only. We apply the method
to specific penguin-mediated B->PP, B->PV and B ->VV modes. We provide a very
stringent and simple bound on the direct CP asymmetries of all these modes, the
violation of which is a signal of New Physics in decay. Within the same
theoretical framework, an updated prediction for the branching ratio of Bs->K0*
anti-K0* is presented, which can be compared with a recent LHCb analysis.Comment: 11 pages, 3 figure
Mesoscopic Anderson Box: Connecting Weak to Strong Coupling
Both the weakly coupled and strong coupling Anderson impurity problems are
characterized by a Fermi-liquid theory with weakly interacting quasiparticles.
In an Anderson box, mesoscopic fluctuations of the effective single particle
properties will be large. We study how the statistical fluctuations at low
temperature in these two problems are connected, using random matrix theory and
the slave boson mean field approximation (SBMFA). First, for a resonant level
model such as results from the SBMFA, we find the joint distribution of energy
levels with and without the resonant level present. Second, if only energy
levels within the Kondo resonance are considered, the distributions of
perturbed levels collapse to universal forms for both orthogonal and unitary
ensembles for all values of the coupling. These universal curves are described
well by a simple Wigner-surmise type toy model. Third, we study the
fluctuations of the mean field parameters in the SBMFA, finding that they are
small. Finally, the change in the intensity of an eigenfunction at an arbitrary
point is studied, such as is relevant in conductance measurements: we find that
the introduction of the strongly-coupled impurity considerably changes the wave
function but that a substantial correlation remains.Comment: 17 pages, 7 figure
3D Modulated Spin Liquid model applied to URuSi
We have developed a 3D version for the Modulated Spin Liquid in a
body-centered tetragonal lattice structure to describe the hidden order
observed in URuSi at K. This second order transition
is well described by our model confirming our earlier hypothesis. The symmetry
of the modulation is minimized for . We assume a linear
variation of the interaction parameters with the lattice spacing and our
results show good agreement with uniaxial and pressure experiments.Comment: 5 pages, 4 figure
Metamagnetic transition in the two orbitals Kondo lattice model
In this work, we study the effects of a transverse magnetic field in a Kondo
lattice model with two orbitals interacting with the conduction electrons.
The electrons that are present on the same site interact through Hund's
coupling, while on neighboring sites they interact through intersite exchange.
We consider here that part of electrons are localized (orbital 1) while
another part (orbital 2) are delocalized, as it is frequent in uranium systems.
Then, only electrons in the localized orbital 1 interact through exchange
interaction with the neighboring ones, while electrons in orbital 2 are coupled
with conduction electrons through a Kondo interaction. We obtain a solution
where ferromagnetism and Kondo effect coexist for small values of an applied
transverse magnetic field for . Increasing the transverse field,
two situations can be obtained when Kondo coupling vanishes: first, a
metamagnetic transition occurs just before or at the same time of the fully
polarized state, and second, a metamagnetic transition occurs when the spins
are already pointing out along the magnetic field.Comment: 15 pages, 7 figure
From Weak- to Strong-Coupling Mesoscopic Fermi Liquids
We study mesoscopic fluctuations in a system in which there is a continuous
connection between two distinct Fermi liquids, asking whether the mesoscopic
variation in the two limits is correlated. The particular system studied is an
Anderson impurity coupled to a finite mesoscopic reservoir described by random
matrix theory, a structure which can be realized using quantum dots. We use the
slave boson mean field approach to connect the levels of the uncoupled system
to those of the strong coupling Nozi\`eres Fermi liquid. We find strong but not
complete correlation between the mesoscopic properties in the two limits and
several universal features.Comment: 6 pages, 3 figure
Cold atoms in space: community workshop summary and proposed road-map
We summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to exploit the full potential of cold atoms in space. Finally, we present a first draft of a possible road-map for achieving these goals, that we propose for discussion by the interested cold atom, Earth Observation, fundamental physics and other prospective scientific user communities, together with the European Space Agency (ESA) and national space and research funding agencies
Cold atoms in space: community workshop summary and proposed road-map
We summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to exploit the full potential of cold atoms in space. Finally, we present a first draft of a possible road-map for achieving these goals, that we propose for discussion by the interested cold atom, Earth Observation, fundamental physics and other prospective scientific user communities, together with the European Space Agency (ESA) and national space and research funding agencies.publishedVersio