[Show abstract][Hide abstract] ABSTRACT: Using the time-dependent density-matrix renormalization group (tDMRG), we
study the time evolution of electron wave packets in one-dimensional (1D)
metal-superconductor heterostructures. The results show Andreev reflection at
the interface, as expected. By combining these results with the well-known
single-spin-species electron-hole transformation in the Hubbard model, we
predict an analogous spin Andreev reflection in metal-Mott insulator
heterostructures. This effect is numerically confirmed using 1D tDMRG, but it
is expected to be present also in higher dimensions, as well as in more general
Hamiltonians. We present an intuitive picture of the spin reflection, analogous
to that of Andreev reflection at metal-superconductors interfaces. This allows
us to discuss a novel antiferromagnetic proximity effect. Possible experimental
realizations are discussed.
Preview · Article · Feb 2015 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: We report a quantum phase transition between orbital-selective Mott states,
with different localized orbitals, in a Hund's metals model. Using the density
matrix renormalization group, the phase diagram is constructed varying the
electronic density and Hubbard $U$, at robust Hund's coupling. We demonstrate
that this transition is preempted by charge fluctuations and the emergence of
free spinless fermions, as opposed to the magnetically-driven Mott transition.
The Luttinger correlation exponent is shown to have a universal value in the
strong-coupling phase, whereas it is interaction dependent at intermediate
couplings. At weak coupling we find a second transition from a normal metal to
the intermediate-coupling phase.
[Show abstract][Hide abstract] ABSTRACT: We investigate the real-time dynamics of photoexcited electronic
instabilities in a charge-transfer system model, using the time-dependent
density matrix renormalization group method. The model of choice was the
quarter-filled one-dimensional extended Peierls-Hubbard Hamiltonian interacting
with classical few-cycle electromagnetic radiation. The results show that only
one electronic instability drives the main features of the photogenerated
time-dependent behavior. Indeed, the photoresponse of the system shows a large
enhancement of the $4k_F$ (bond and charge) instability whereas the $2k_F$
state remains largely unaffected. This conclusion holds regardless of the
nature of the optical excitations and whether the system is perturbed
resonantly or not. Our results suggest potential applications of
charge-transfer systems with slow phononic dynamics as optoelectronic switching
[Show abstract][Hide abstract] ABSTRACT: The orbital-selective Mott phase of multiorbital Hubbard models has been extensively analyzed before using static and dynamical mean-field approximations. In parallel, the properties of block states (antiferromagnetically coupled ferromagnetic spin clusters) in Fe-based superconductors have also been much discussed. The present effort uses numerically exact techniques in one-dimensional systems to report the observation of block states within the orbital-selective Mott phase regime, connecting two seemingly independent areas of research, and providing analogies with the physics of double-exchange models.
Preview · Article · Mar 2014 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: Using time-dependent density-matrix renormalization group, we study the time
evolution of electronic wave packets in the one-dimensional extended Hubbard
model with on-site and nearest neighbor repulsion, U and V, respectively. As
expected, the wave packets separate into spin-only and charge-only excitations
(spin-charge separation). Charge and spin velocities exhibit non-monotonic
dependence on V. For small and intermediate values of V, both velocities
increase with V. However, the charge velocity exhibits a stronger dependence
than that of the spin, leading to a more pronounced spin-charge separation.
Charge fractionalization, on the other hand, is weakly affected by V. The
results are explained in terms of Luttinger liquid theory in the weak-coupling
limit, and an effective model in the strong-coupling regime.
[Show abstract][Hide abstract] ABSTRACT: For a mobile spin-1/2 impurity, coupled antiferromagnetically to a
one-dimensional gas of fermions, perturbative ideas have been used to argue in
favor of two-channel Kondo behavior of the impurity spin. Here we combine
general considerations and extensive numerical simulations to show that the
problem displays a novel quantum phase transition between two-channel and
one-channel Kondo screening upon increasing the Kondo coupling. We construct a
ground-state phase diagram and discuss the various non-trivial crossovers as
well as possible experimental realizations.
Full-text · Article · Jun 2013 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Neutron scattering experiments have unveiled a dominant spin arrangement
in the two-leg ladder selenide compound BaFe2Se3,
involving ferromagnetically ordered 2x2 iron-superblocks, that are
antiferromagnetically coupled among them (the ``block-AFM'' state). Our
numerical study of the electronic five-orbital Hubbard model, within the
Hartree-Fock approximation and using first principles techniques for the
hopping amplitudes, has shown that the exotic block-AFM state is indeed
stable at realistic electronic densities n ˜6.0. Another state
with wavevector (,) becomes stable in other portions of the phase
diagrams, including n ˜5.5, as found experimentally in
KFe2Se3. In addition, our study unveils several
competing magnetic phases that could be experimentally stabilized
varying either n chemically or the electronic bandwidth by pressure.
Similar results were obtained using two-orbital models, studied here via
Lanczos and DMRG techniques .  Qinlong Luo, et al, arXiv:
1205.3239, and references therein.
[Show abstract][Hide abstract] ABSTRACT: Interfaces alter the subtle balance among different degrees of freedom
responsible for exotic phenomena in complex oxides, such as
cuprate-manganite interfaces. We study these interfaces by means of
scanning transmission electron microscopy and theoretical calculations.
Microscopy and EEL spectroscopy indicate that the interfaces are sharp,
and the chemical profile is symmetric with two equivalent interfaces.
Spectroscopy also allows us to establish an oxidation state profile with
sub-nanometer resolution. We find an anomalous charge redistribution: a
non-monotonic behavior of the occupancy of d orbitals in the manganite
layers as a function of distance to the interface. Relying on model
calculations, we establish that this profile is a result of the
competition between standard charge transfer tendencies involving
materials with different chemical potentials and strong bonding effects
across the interface. The competition can be tuned by different factors
(temperature, doping, magnetic fields...). As examples, we report
different charge distributions as a function of doping of the manganite
layers. ACKNOWLEDGEMENTS ORNL:U.S. DOE-BES, Material Sciences and
Engineering Division & ORNL's ShaRE. UCM:Juan de la Cierva, Ramon y
Cajal, & ERC Starting Investigator Award programs.
Full-text · Article · Mar 2013 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: We study a mobile spin-1/2 impurity, coupled antiferromagnetically to a
one-dimensional gas of fermions. Combining perturbative ideas and
extensive density matrix renormalization group calculations, we study
the interplay between the screening of the impurity by the electrons and
the kinetic and magnetic properties of the impurity. We show that this
problem displays a quantum phase transition between one- and two-channel
Kondo physics. Using finite-size scaling, we construct a ground-state
phase diagram and discuss various non-trivial regimes.
[Show abstract][Hide abstract] ABSTRACT: We consider the problem of a dispersive magnetic impurity interacting
antiferromagnetically with a one dimensional fermionic gas. By combining
general considerations and extensive numerical simulations we show that
the problem displays a quantum phase transition between two-channel and
one-channel Kondo behaviour upon increasing the Kondo coupling and
construct a phase diagramme. We also discuss possible experimental
[Show abstract][Hide abstract] ABSTRACT: Many of the most intriguing quantum effects are observed or could be measured in transport experiments through nanoscopic systems such as quantum dots, wires and rings formed by large molecules or arrays of quantum dots. In particular, the separation of charge and spin degrees of freedom and interference effects have important consequences in the conductivity through these systems. Charge-spin separation was predicted theoretically in one-dimensional strongly inter-acting systems (Luttinger liquids) and, although observed indirectly in several materials formed by chains of correlated electrons, it still lacks direct observation. We present results on transport properties through Aharonov-Bohmrings (pierced by a magnetic flux) with one or more channels represented by paradigmatic strongly-correlated models. For a wide range of parameters we observe characteristic dips in the conductance as a function of magnetic flux which are a signature of spin and charge separation. Interference effects could also be controlled in certain molecules and interesting properties could be observed. We analyze transport properties of conjugated molecules, benzene in particular, and find that the conductance depends on the lead configuration. In molecules with translational symmetry, the conductance can be controlled by breaking or restoring this symmetry, e.g. by the application of a local external potential. These results open the possibility of observing these peculiar physical properties in anisotropic ladder systems and in real nanoscopic and molecular devices.
[Show abstract][Hide abstract] ABSTRACT: Several of the most interesting quantum effects can or could be observed in nanoscopic systems. For example, the effect of strong correlations between electrons and of quantum interference can be measured in transport experiments through quantum dots, wires, individual molecules and rings formed by large molecules or arrays of quantum dots. In addition, quantum coherence and entanglement can be clearly observed in quantum corrals. In this paper we present calculations of transport properties through Aharonov–Bohm strongly correlated rings where the characteristic phenomenon of charge–spin separation is clearly observed. Additionally quantum interference effects show up in transport through π-conjugated annulene molecules producing important effects on the conductance for different source–drain configurations, leading to the possibility of an interesting switching effect. Finally, elliptic quantum corrals offer an ideal system to study quantum entanglement due to their focalizing properties. Because of an enhanced interaction between impurities localized at the foci, these systems also show interesting quantum dynamical behaviour and offer a challenging scenario for quantum information experiments.
No preview · Article · Nov 2010 · Journal of Statistical Mechanics Theory and Experiment
[Show abstract][Hide abstract] ABSTRACT: Coherent electronic transport through individual molecules is crucially sensitive to quantum interference. We investigate the zero-bias and zero-temperature conductance through pi-conjugated annulene molecules weakly coupled to two leads for different source-drain configurations, finding an important reduction for certain transmission channels and for particular geometries as a consequence of destructive quantum interference between states with definite momenta. When translational symmetry is broken by an external perturbation we find an abrupt increase of the conductance through those channels. Previous studies concentrated on the effect at the Fermi energy, where this effect is very small. By analyzing the effect of symmetry breaking on the main transmission channels we find a much larger response thus leading to the possibility of a larger switching of the conductance through single molecules.
Preview · Article · Dec 2009 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: We calculate the conductance through Aharonov–Bohm chain and ladder rings pierced by a magnetic flux which couples with the charge degrees of freedom. The system is weakly coupled to two leads and contains strongly interacting electrons modeled by the prototypical t-J and Hubbard models. For a wide range of parameters we observe characteristic dips in the conductance as a function of magnetic flux which are a signature of spin and charge separation. We also show how the dips evolve when the parameters of the models depart from the ideal case of total spin–charge separation. The ladder ring can be mapped onto an effective model for large anisotropy which can be easily analyzed. These results open the possibility of observing this peculiar many-body phenomenon in anisotropic ladder systems and in real nanoscopic devices.
No preview · Article · Oct 2009 · Physica B Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: A distributed-memory parallelization strategy for the density matrix renormalization group is proposed for cases where correlation functions are required. This new strategy has substantial improvements with respect to previous works. A scalability analysis shows an overall serial fraction of 9.4% and an efficiency of around 60% considering up to eight nodes. Sources of possible parallel slowdown are pointed out and solutions to circumvent these issues are brought forward in order to achieve a better performance. Comment: 8 pages, 4 figures; version published in Computer Physics Communications
[Show abstract][Hide abstract] ABSTRACT: Using non-equilibrium renormalized perturbation theory, we calculate the conductance G as a function of temperature T and bias voltage V for an Anderson model, suitable for describing transport properties through a quantum dot. For renormalized parameters that correspond to the extreme Kondo limit, we do not find a simple scaling formula beyond a quadratic dependence in T and V. However, if valence fluctuations are allowed, we find agreement with recent experiments. Comment: Eq.(10) corrected
[Show abstract][Hide abstract] ABSTRACT: We study the conductance through finite Aharonov-Bohm rings of interacting electrons weakly coupled to non-interacting leads at two arbitrary sites. This model can describe an array of quantum dots with a large charging energy compared to the interdot overlap. As a consequence of the spin-charge separation, which occurs in these highly correlated systems, the transmittance is shown to present pronounced dips for particular values of the magnetic flux piercing the ring. We analyze this effect by numerical and analytical means and show that the zero-temperature equilibrium conductance in fact presents these striking features which could be observed experimentally. Comment: 4 pages, 3 figures. FCM 2008 proceedings
Full-text · Article · Jan 2009 · Physica B Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: We calculate the conductance through rings with few sites $L$ described by the $t-J$ model, threaded by a magnetic flux $\Phi$ and weakly coupled to conducting leads at two arbitrary sites. The model can describe a circular array of quantum dots with large charging energy $U$ in comparison with the nearest-neighbor hopping $t$. We determine analytically the particular values of $\Phi$ for which a depression of the transmittance is expected as a consequence of spin-charge separation. We show numerically that the equilibrium conductance at zero temperature is depressed at those particular values of $\Phi $ for most systems, in particular at half filling, which might be easier to realize experimentally. Comment: 8 pages, 7 figures