31 research outputs found
Partial preservation of chiral symmetry and colossal magnetoresistance in adatom doped graphene
We analyze the electronic properties of adatom doped graphene in the low
impurity concentration regime. We focus on the Anderson localized regime and
calculate the localization length () as a function of the electron doping
and an external magnetic field. The impurity states hybridize with carbon's
states and form a partially filled band close to the Dirac point. Near
the impurity band center, the chiral symmetry of the system's effective
Hamiltonian is partially preserved which leads to a large enhancement of .
The sensitivity of transport properties, namely Mott's variable range hopping
scale , to an external magnetic field perpendicular to the graphene sheet
leads to a colossal magnetoresistance effect, as observed in recent
experiments.Comment: 5 pages, 4 figs. Few comments and references added. To appear in PR
On the nature of the Mott transition in multiorbital systems
We analyze the nature of Mott metal-insulator transition in multiorbital
systems using dynamical mean-field theory (DMFT). The auxiliary multiorbital
quantum impurity problem is solved using continuous time quantum Monte Carlo
(CTQMC) and the rotationally invariant slave-boson (RISB) mean field
approximation. We focus our analysis on the Kanamori Hamiltonian and find that
there are two markedly different regimes determined by the nature of the lowest
energy excitations of the atomic Hamiltonian. The RISB results at
suggest the following rule of thumb for the order of the transition at zero
temperature: a second order transition is to be expected if the lowest lying
excitations of the atomic Hamiltonian are charge excitations, while the
transition tends to be first order if the lowest lying excitations are in the
same charge sector as the atomic ground state. At finite temperatures the
transition is first order and its strength, as measured e.g. by the jump in the
quasiparticle weight at the transition, is stronger in the parameter regime
where the RISB method predicts a first order transition at zero temperature.
Interestingly, these results seem to apply to a wide variety of models and
parameter regimes.Comment: Accepted for publication in Physical Review
Tunable Charge and Spin Seebeck Effects in Magnetic Molecular Junctions
We study the charge and spin Seebeck effects in a spin-1 molecular junction
as a function of temperature (T), applied magnetic field (H), and magnetic
anisotropy (D) using Wilson's numerical renormalization group. A hard-axis
magnetic anisotropy produces a large enhancement of the charge Seebeck
coefficient Sc (\sim k_B/|e|) whose value only depends on the residual
interaction between quasiparticles in the low temperature Fermi-liquid regime.
In the underscreened spin-1 Kondo regime, the high sensitivity of the system to
magnetic fields makes it possible to observe a sizable value for the spin
Seebeck coefficient even for magnetic fields much smaller than the Kondo
temperature. Similar effects can be obtain in C60 junctions where the control
parameter is the gap between a singlet and a triplet molecular state.Comment: 5 pages, 4 figure
Dynamical magnetic anisotropy and quantum phase transitions in a vibrating spin-1 molecular junction
We study the electronic transport through a spin-1 molecule in which
mechanical stretching produces a magnetic anisotropy. In this type of device, a
vibron mode along the stretching axis will couple naturally to the molecular
spin. We consider a single molecular vibrational mode and find that the
electron-vibron interaction induces an effective correction to the magnetic
anisotropy that shifts the ground state of the device toward a non-Fermi liquid
phase. A transition into a Fermi liquid phase could then be achieved, by means
of mechanical stretching, passing through an underscreened spin-1 Kondo regime.
We present numerical renormalization group results for the differential
conductance, the spectral density, and the magnetic susceptibility across the
transition.Comment: 7 pages, 7 figure
Thermometry and signatures of strong correlations from Raman spectroscopy of fermionic atoms in optical lattices
We propose a method to directly measure the temperature of a gas of weakly
interacting fermionic atoms loaded into an optical lattice. This technique
relies on Raman spectroscopy and is applicable to experimentally relevant
temperature regimes. Additionally, we show that a similar spectroscopy scheme
can be used to obtain information on the quasiparticle properties and Hubbard
bands of the metallic and Mott-insulating states of interacting fermionic spin
mixtures. These two methods provide experimentalists with novel probes to
accurately characterize fermionic quantum gases confined to optical lattices.Comment: 13 pages, 22 figure
State-of-the-art techniques for calculating spectral functions in models for correlated materials
The dynamical mean field theory (DMFT) has become a standard technique for
the study of strongly correlated models and materials overcoming some of the
limitations of density functional approaches based on local approximations. An
important step in this method involves the calculation of response functions of
a multiorbital impurity problem which is related to the original model.
Recently there has been considerable progress in the development of techniques
based on the density matrix renormalization group (DMRG) and related matrix
product states (MPS) implying a substantial improvement to previous methods. In
this article we review some of the standard algorithms and compare them to the
newly developed techniques, showing examples for the particular case of the
half-filled two-band Hubbard model.Comment: 8 pages, 4 figures, to be published in EPL Perspective