77 research outputs found
Conductance of a molecule with a center of mass motion
We calculate the zero temperature conductance and characteristic correlation
functions of a molecule with a center of mass (CM) motion which modulates
couplings to the leads. In the first model studied, the CM vibrational mode is
simultaneously coupled to the electron density on the molecule. The conductance
is suppressed in regimes corresponding to non-integer occupancy of the
molecule. In the second model, where the CM mode is not directly coupled to the
electron density, the suppression of conductance is related to the dynamic
breaking of the inversion symmetry.Comment: to appear in Phys. Rev.
Electronic correlations, magnetism and Hund's rule coupling in the ruthenium perovskites SrRuO and CaRuO
A comparative density functional plus dynamical mean field theory study of
the pseudocubic ruthenate materials CaRuO and SrRuO is presented. Phase
diagrams are determined for both materials as a function of Hubbard repulsion
and Hund's rule coupling . Metallic and insulating phases are found, as
are ferromagnetic and paramagnetic states. The locations of the relevant phase
boundaries are determined. Based on the computed phase diagrams, Mott-dominated
and Hund's dominated regimes of strong correlation are distinguished.
Comparison of calculated properties to experiments indicates that the actual
materials are in the Hund's coupling dominated region of the phase diagram so
can be characterized as Hund's metals, in common with other members of the
ruthenate family. Comparison of the phase diagrams for the two materials
reveals the role played by rotational and tilt (GdFeO-type) distortions of
the ideal perovskite structure. The presence of magnetism in SrRuO and its
absence in CaRuO despite the larger mass and larger tilt/rotational
distortion amplitude of CaRuO can be understood in terms of density of
states effects in the presence of strong Hund's coupling. Comparison of the
calculated low- properties of CaRuO to those of SrRuO provides
insight into the effects of magnetic order on the properties of a Hund's metal.
The study provides a simultaneous description of magnetism and correlations and
explicates the roles played by band theory and Hubbard and Hund's interactions
Spin qubits in double quantum dots - entanglement versus the Kondo effect
We investigate the competition between pair entanglement of two spin qubits
in double quantum dots attached to leads with various topologies and the
separate entanglement of each spin with nearby electrodes. Universal behavior
of entanglement is demonstrated in dependence on the mutual interactions
between the spin qubits, the coupling to their environment, temperature and
magnetic field. As a consequence of quantum phase transition an abrupt switch
between fully entangled and unentangled states takes place when the dots are
coupled in parallel.Comment: 3 figure
Kondo effect and channel mixing in oscillating molecules
We investigate the electronic transport through a molecule in the Kondo
regime. The tunneling between the electrode and the molecule is asymmetrically
modulated by the oscillations of the molecule, i.e., if the molecule gets
closer to one of the electrodes the tunneling to that electrode will increase
while for the other electrode it will decrease. The system is described by a
two-channel Anderson model with phonon-assisted hybridization, which is solved
with the Wilson numerical renormalization group method. The results for several
functional forms of tunneling modulation are presented. For a linearized
modulation the Kondo screening of the molecular spin is caused by the even or
odd conduction channel. At the critical value of the electron-phonon coupling
an unstable two-channel Kondo fixed point is found. For a realistic modulation
the spin at the molecular orbital is Kondo screened by the even conduction
channel even in the regime of strong coupling. A universal consequence of the
electron-phonon coupling is the softening of the phonon mode and the related
instability to perturbations that break the left-right symmetry. When the
frequency of oscillations decreases below the magnitude of such perturbation,
the molecule is abruptly attracted to one of the electrodes. In this regime,
the Kondo temperature is enhanced and, simultaneously, the conductance through
the molecule is suppressed.Comment: published versio
Conductance of deformable molecules with interaction
Zero temperature linear response conductance of molecules with Coulomb
interaction and with various types of phonon modes is analysed together with
local occupation, local moment, charge fluctuations and fluctuations of
molecular deformation. Deformation fluctuations are quantitatively related to
charge fluctuations which exhibit similarity also to static charge
susceptibility.Comment: 4 pages, color figure
Impact of electronic correlations on the equation of state and transport in -Fe
We have obtained the equilibrium volumes, bulk moduli, equations of state of
the ferromagnetic cubic and paramagnetic hexagonal phases
of iron in close agreement with experiment using an ab initio dynamical
mean-field theory approach. The local dynamical correlations are shown to be
crucial for a successful description of the ground-state properties of
paramagnetic -Fe. Moreover, they enhance the effective mass of the
quasiparticles and reduce their lifetimes across the
transition leading to a step-wise increase of the resistivity, as observed in
experiment. The calculated magnitude of the jump is significantly
underestimated, which points to non-local correlations. The implications of our
results for the superconductivity and non-Fermi-liquid behavior of
-Fe are discussed.Comment: 6 pages, 3 figure
Entanglement and the Kondo effect in double quantum dots
We investigate entanglement between electrons in serially coupled double
quantum dots attached to non interacting leads. In addition to local repulsion
we consider the influence of capacitive inter-dot interaction. We show how the
competition between extended Kondo and local singlet phases determines the
ground state and thereby the entanglement.Comment: EPJ Special Topics 200
Optical Response of SrRuO Reveals Universal Fermi-liquid Scaling and Quasiparticles Beyond Landau Theory
We report optical measurements demonstrating that the low-energy relaxation
rate () of the conduction electrons in SrRuO obeys scaling
relations for its frequency () and temperature () dependence in
accordance with Fermi-liquid theory. In the thermal relaxation regime,
1/\tau\propto (\hbar\omega)^2 + (p\pi\kB T)^2 with , and
scaling applies. Many-body electronic structure calculations using dynamical
mean-field theory confirm the low-energy Fermi-liquid scaling, and provide
quantitative understanding of the deviations from Fermi-liquid behavior at
higher energy and temperature. The excess optical spectral weight in this
regime provides evidence for strongly dispersing "resilient" quasiparticle
excitations above the Fermi energy
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