886 research outputs found
Quantum tunneling induced Kondo effect in single molecular magnets
We consider transport through a single-molecule magnet strongly coupled to
metallic electrodes. We demonstrate that for half-integer spin of the molecule
electron- and spin-tunneling \emph{cooperate} to produce both quantum tunneling
of the magnetic moment and a Kondo effect in the linear conductance. The Kondo
temperature depends sensitively on the ratio of the transverse and easy-axis
anisotropies in a non-monotonic way. The magnetic symmetry of the transverse
anisotropy imposes a selection rule on the total spin for the occurrence of the
Kondo effect which deviates from the usual even-odd alternation.Comment: 4 pages, 4 figure
Kondo-transport spectroscopy of single molecule magnets
We demonstrate that in a single molecule magnet (SMM) strongly coupled to
electrodes the Kondo effect involves all magnetic excitations. This Kondo
effect is induced by the quantum tunneling of the magnetic moment (QTM).
Importantly, the Kondo temperature can be much larger than the magnetic
splittings. We find a strong modulation of the Kondo effect as function of the
transverse anisotropy parameter or a longitudinal magnetic field. For both
integer and half-integer spin this can be used for an accurate transport
spectroscopy of the magnetic states in low magnetic fields on the order of the
easy-axis anisotropy parameter. We set up a relationship between the Kondo
effects for successive integer and half-integer spins.Comment: 5 pages, 3 figure
Magnetism and domain formation in SU(3)-symmetric multi-species Fermi mixtures
We study the phase diagram of an SU(3)-symmetric mixture of three-component
ultracold fermions with attractive interactions in an optical lattice,
including the additional effect on the mixture of an effective three-body
constraint induced by three-body losses. We address the properties of the
system in by using dynamical mean-field theory and variational Monte
Carlo techniques. The phase diagram of the model shows a strong interplay
between magnetism and superfluidity. In the absence of the three-body
constraint (no losses), the system undergoes a phase transition from a color
superfluid phase to a trionic phase, which shows additional particle density
modulations at half-filling. Away from the particle-hole symmetric point the
color superfluid phase is always spontaneously magnetized, leading to the
formation of different color superfluid domains in systems where the total
number of particles of each species is conserved. This can be seen as the SU(3)
symmetric realization of a more general tendency to phase-separation in
three-component Fermi mixtures. The three-body constraint strongly disfavors
the trionic phase, stabilizing a (fully magnetized) color superfluid also at
strong coupling. With increasing temperature we observe a transition to a
non-magnetized SU(3) Fermi liquid phase.Comment: 36 pages, 17 figures; Corrected typo
Magnetism and d-wave superconductivity on the half-filled square lattice with frustration
The role of frustration and interaction strength on the half-filled Hubbard
model is studied on the square lattice with nearest and next-nearest neighbour
hoppings t and t' using the Variational Cluster Approximation (VCA). At
half-filling, we find two phases with long-range antiferromagnetic (AF) order:
the usual Neel phase, stable at small frustration t'/t, and the so-called
collinear (or super-antiferromagnet) phase with ordering wave-vector
or , stable for large frustration. These are separated by a phase with
no detectable long-range magnetic order. We also find the d-wave
superconducting (SC) phase (), which is favoured by frustration if
it is not too large. Intriguingly, there is a broad region of coexistence where
both AF and SC order parameters have non-zero values. In addition, the physics
of the metal-insulator transition in the normal state is analyzed. The results
obtained with the help of the VCA method are compared with the large-U
expansion of the Hubbard model and known results for the frustrated J1-J2
Heisenberg model. These results are relevant for pressure studies of undoped
parents of the high-temperature superconductors: we predict that an insulator
to d-wave SC transition may appear under pressure.Comment: 12 pages, 10 figure
Unified description of correlations in double quantum dots
The two-level model for a double quantum dot coupled to two leads, which is
ubiquitously used to describe charge oscillations, transmission-phase lapses
and correlation-induced resonances, is considered in its general form. The
model features arbitrary tunnelling matrix elements among the two levels and
the leads and between the levels themselves (including the effect of
Aharonov-Bohm fluxes), as well as inter-level repulsive interactions. We show
that this model is exactly mapped onto a generalized Anderson model of a single
impurity, where the electrons acquire a pseudo-spin degree of freedom, which is
conserved by the tunnelling but not within the dot. Focusing on the
local-moment regime where the dot is singly occupied, we show that the
effective low-energy Hamiltonian is that of the anisotropic Kondo model in the
presence of a tilted magnetic field. For moderate values of the (renormalized)
field, the Bethe ansatz solution of the isotropic Kondo model allows us to
derive accurate expressions for the dot occupation numbers, and henceforth its
zero-temperature transmission. Our results are in excellent agreement with
those obtained from the Bethe ansatz for the isotropic Anderson model, and with
the functional and numerical renormalization-group calculations of Meden and
Marquardt [Phys. Rev. Lett. 96, 146801 (2006)], which are valid for the general
anisotropic case. In addition we present highly accurate estimates for the
validity of the Schrieffer-Wolff transformation (which maps the Anderson
Hamiltonian onto the low-energy Kondo model) at both the high- and low-magnetic
field limits. Perhaps most importantly, we provide a single coherent picture
for the host of phenomena to which this model has been applied.Comment: 23 pages, 7 figure
Partial Wave Analysis of Scattering with Nonlocal Aharonov-Bohm Effect and Anomalous Cross Section induced by Quantum Interference
Partial wave theory of a three dmensional scattering problem for an arbitray
short range potential and a nonlocal Aharonov-Bohm magnetic flux is
established. The scattering process of a ``hard shere'' like potential and the
magnetic flux is examined. An anomalous total cross section is revealed at the
specific quantized magnetic flux at low energy which helps explain the
composite fermion and boson model in the fractional quantum Hall effect. Since
the nonlocal quantum interference of magnetic flux on the charged particles is
universal, the nonlocal effect is expected to appear in quite general potential
system and will be useful in understanding some other phenomena in mesoscopic
phyiscs.Comment: 6 figure
Relation between Energy Level Statistics and Phase Transition and its Application to the Anderson Model
A general method to describe a second-order phase transition is discussed. It
starts from the energy level statistics and uses of finite-size scaling. It is
applied to the metal-insulator transition (MIT) in the Anderson model of
localization, evaluating the cumulative level-spacing distribution as well as
the Dyson-Metha statistics. The critical disorder and the critical
exponent are computed.Comment: 9 pages, Latex, 6 PostScript figures in uuencoded compressed tar file
are appende
Interference and interaction effects in multi-level quantum dots
Using renormalization group techniques, we study spectral and transport
properties of a spinless interacting quantum dot consisting of two levels
coupled to metallic reservoirs. For strong Coulomb repulsion and an applied
Aharonov-Bohm phase , we find a large direct tunnel splitting
between the levels of
the order of the level broadening . As a consequence we discover a
many-body resonance in the spectral density that can be measured via the
absorption power. Furthermore, for , we show that the system can be
tuned into an effective Anderson model with spin-dependent tunneling.Comment: 5 pages, 4 figures included, typos correcte
Zero-Bias Conductance Through Side-Coupled Double Quantum Dots
Low temperature zero-bias conductance through two side-coupled quantum dots
is investigated using Wilson's numerical renormalization group technique. A
low-temperature phase diagram is computed. Near the particle-hole symmetric
point localized electrons form a spin-singlet associated with weak conductance.
For weak inter-dot coupling we find enhanced conductance due to the two-stage
Kondo effect when two electrons occupy quantum dots. When quantum dots are
populated with a single electron, the system enters Kondo regime with enhanced
conductance. Analytical expressions for the width of the Kondo regime and the
Kondo temperature in this regime are given.Comment: to be published in the Proceedings of the NATO Advanced Research
Workshop on "Electron Correlations in New Materials and Nanosystems" held in
Yalta, Ukraine, 19 - 23 September 2005 (NATO Science Series II, Springer
2006
- …