Publications (23)79.9 Total impact
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ABSTRACT: Using the timedependent densitymatrix renormalization group (tDMRG), we study the time evolution of electron wave packets in onedimensional (1D) metalsuperconductor heterostructures. The results show Andreev reflection at the interface, as expected. By combining these results with the wellknown singlespinspecies electronhole transformation in the Hubbard model, we predict an analogous spin Andreev reflection in metalMott 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 metalsuperconductors interfaces. This allows us to discuss a novel antiferromagnetic proximity effect. Possible experimental realizations are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: We report a quantum phase transition between orbitalselective 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 magneticallydriven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strongcoupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediatecoupling phase. 
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ABSTRACT: We investigate the realtime dynamics of photoexcited electronic instabilities in a chargetransfer system model, using the timedependent density matrix renormalization group method. The model of choice was the quarterfilled onedimensional extended PeierlsHubbard Hamiltonian interacting with classical fewcycle electromagnetic radiation. The results show that only one electronic instability drives the main features of the photogenerated timedependent 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 chargetransfer systems with slow phononic dynamics as optoelectronic switching devices.  [Show abstract] [Hide abstract]
ABSTRACT: The orbitalselective Mott phase of multiorbital Hubbard models has been extensively analyzed before using static and dynamical meanfield approximations. In parallel, the properties of block states (antiferromagnetically coupled ferromagnetic spin clusters) in Febased superconductors have also been much discussed. The present effort uses numerically exact techniques in onedimensional systems to report the observation of block states within the orbitalselective Mott phase regime, connecting two seemingly independent areas of research, and providing analogies with the physics of doubleexchange models.  [Show abstract] [Hide abstract]
ABSTRACT: Using timedependent densitymatrix renormalization group, we study the time evolution of electronic wave packets in the onedimensional extended Hubbard model with onsite and nearest neighbor repulsion, U and V, respectively. As expected, the wave packets separate into spinonly and chargeonly excitations (spincharge separation). Charge and spin velocities exhibit nonmonotonic 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 spincharge separation. Charge fractionalization, on the other hand, is weakly affected by V. The results are explained in terms of Luttinger liquid theory in the weakcoupling limit, and an effective model in the strongcoupling regime.  [Show abstract] [Hide abstract]
ABSTRACT: For a mobile spin1/2 impurity, coupled antiferromagnetically to a onedimensional gas of fermions, perturbative ideas have been used to argue in favor of twochannel 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 twochannel and onechannel Kondo screening upon increasing the Kondo coupling. We construct a groundstate phase diagram and discuss the various nontrivial crossovers as well as possible experimental realizations.  [Show abstract] [Hide abstract]
ABSTRACT: Neutron scattering experiments have unveiled a dominant spin arrangement in the twoleg ladder selenide compound BaFe2Se3, involving ferromagnetically ordered 2x2 ironsuperblocks, that are antiferromagnetically coupled among them (the ``blockAFM'' state). Our numerical study of the electronic fiveorbital Hubbard model, within the HartreeFock approximation and using first principles techniques for the hopping amplitudes, has shown that the exotic blockAFM 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 twoorbital models, studied here via Lanczos and DMRG techniques [1]. [1] 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 cupratemanganite 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 subnanometer resolution. We find an anomalous charge redistribution: a nonmonotonic 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. DOEBES, Material Sciences and Engineering Division & ORNL's ShaRE. UCM:Juan de la Cierva, Ramon y Cajal, & ERC Starting Investigator Award programs.  [Show abstract] [Hide abstract]
ABSTRACT: We study a mobile spin1/2 impurity, coupled antiferromagnetically to a onedimensional 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 twochannel Kondo physics. Using finitesize scaling, we construct a groundstate phase diagram and discuss various nontrivial regimes. 
Article: Dispersive Impurities in onedimensional Fermi Gases: From one to two Channel Kondo Polarons
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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 twochannel and onechannel Kondo behaviour upon increasing the Kondo coupling and construct a phase diagramme. We also discuss possible experimental realisations. 
Conference Paper: Quantum properties in transport through nanoscopic rings: Chargespin separation and interference effects
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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. Chargespin separation was predicted theoretically in onedimensional strongly interacting 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 AharonovBohmrings (pierced by a magnetic flux) with one or more channels represented by paradigmatic stronglycorrelated 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 πconjugated 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. 
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ABSTRACT: Coherent electronic transport through individual molecules is crucially sensitive to quantum interference. We investigate the zerobias and zerotemperature conductance through piconjugated annulene molecules weakly coupled to two leads for different sourcedrain 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. 
Article: Effect of charge–spin separation on the conductance through interacting lowdimensional rings
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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 tJ 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 manybody phenomenon in anisotropic ladder systems and in real nanoscopic devices.  [Show abstract] [Hide abstract]
ABSTRACT: A distributedmemory 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 nonequilibrium 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 AharonovBohm rings of interacting electrons weakly coupled to noninteracting 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 spincharge 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 zerotemperature equilibrium conductance in fact presents these striking features which could be observed experimentally. Comment: 4 pages, 3 figures. FCM 2008 proceedings  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the conductance through rings with few sites $L$ described by the $tJ$ 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 nearestneighbor hopping $t$. We determine analytically the particular values of $\Phi$ for which a depression of the transmittance is expected as a consequence of spincharge 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
Publication Stats
119  Citations  
79.90  Total Impact Points  
Top Journals
Institutions

20132015

Oak Ridge National Laboratory
 Materials Science and Technology Division
Oak Ridge, Florida, United States


2011

Instituto Balseiro
Сан Карлос де Барилоче, Rio Negro, Argentina


20092010

Comisión Nacional de Energía Atómica
 Departamento de Física
Buenos Aires, Buenos Aires F.D., Argentina 
Centro Atómico Bariloche
Сан Карлос де Барилоче, Rio Negro, Argentina


20082009

Universidad Nacional de Río Negro
Viedma, Rio Negro, Argentina
