1,721,445 research outputs found
Tight-binding description of impurity states in semiconductors
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The author thanks J. Munárriz and C. González-Santander for a critical reading of the manuscript. This work was supported by MICINN (Project MAT2010-17180).Introductory textbooks in solid state physics usually present the hydrogenic impurity model to calculate the energy of carriers bound to donors or acceptors in semiconductors. This model treats the pure semiconductor as a homogeneous medium and the impurity is represented as a fixed point charge. This approach is only valid for shallow impurities and it also departs from the conventional view in solid state physics, where carriers move in a crystal lattice. As an alternative description of impurities in semiconductors, we present a minimal one-dimensional lattice model within the tight-binding approximation. The lattice model is valid for deep and shallow impurities. In the latter case, the results are in agreement with the predictions of the hydrogenic impurity model. The underlying ideas are simple and knowledge of advanced quantum mechanics is not required. Thus, this alternative model could be suitable for introductory courses in solid state physics and materials science.MICINNDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
Landau level shift under the influence of short-range impurities in gapless graphene
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This work was supported by MICINN (projects Mosaico and MAT2010-17180). The authors thank A V Malyshev for helpful comments.We study theoretically the level shift of massless Dirac fermions in a graphene monolayer subjected to a quantizing perpendicular magnetic field under the influence of short-range impurities. A Green function method is used to obtain closed expressions for the Landau level shift for any sharply peaked impurity potential approaching a delta-shell potential.MICINNDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
Spectroscopy of the Dirac oscillator perturbed by a surface delta potential
Get PDF©2012 Elsevier B.V. All rights reserved.
Work in Madrid was supported by MICINN (project MAT2010- 17180). R.P.A. Lima would like to thank CAPES via project PPCPMercosul, CNPq, and FINEP (Brazilian Research Agencies) as well as FAPEAL (Alagoas State Research Agency) for partial financial support.We study theoretically the level shift of the Dirac oscillator perturbed by any sharply peaked potential approaching a surface delta potential. A Green function method is used to obtain closed expressions for all partial waves and parities.MICINNCAPES (Brazilian Research Agency)FINEP (Brazilian Research Agency)FAPEAL (Alagoas State Research Agency)CNPq (Brazilian Research Agency)Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
Super Bloch oscillations in the Peyrard–Bishop–Holstein model
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This work was supported by MICINN (projects MAT2010-17180 and MOSAICO). C. H. acknowledges financial support by MEC (Program Becas de Colaboración).Recently, polarons in the Peyrard-Bishop-Holstein model under DC electric fields were established to perform Bloch oscillations, provided the charge-lattice coupling is not large. In this work, we study this model when the charge is subjected to an applied field with both DC and AC components. Similarly to what happens in the rigid lattice, we find that the carrier undergoes a directed motion or coherent oscillations when the AC field is resonant or detuned with respect to the Bloch frequency, respectively. The electric density current and its Fourier spectrum are also studied to reveal the frequencies involved in the polaron dynamics.MICINNMECDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
Non-local separable solutions of two interacting particles in a harmonic trap
Get PDF©2010 Elsevier B.V. All rights reserved.
The authors thank J. M. R. Parrondo for discussions. This work was supported by MICINN (projects Mosaico and MAT2010-17180). C. G.-S. acknowledges financial support from Comunidad de Madrid and European Social Foundation.We calculate the energy levels of two particles trapped in a harmonic potential. The actual two-body potential, assumed to be spherically symmetric, is replaced by a projective operator (non-local separable potential) to determine the energy levels in a closed form. This approach overcomes the limitations of the regularized Fermi pseudopotential when the characteristic length of the two-body interaction potential is of the order of the size of the harmonic trap. In addition, we recover the results obtained with the Fermi pseudopotential when the length of the interaction is much smaller than the size of the trap.Ministerio de Ciencia e Innovación (MICINN)Comunidad de MadridEuropean Social FoundationDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
Graphene nanoring as a tunable source of polarized electrons
Get PDF© 2012 IOP Publishing Ltd Printed in the UK & the USA.
Work at Madrid was supported by MICINN (projects Mosaico and MAT2010-17180). PAO acknowledges support from FODNECYT (project 1100560).We propose a novel spin filter based on a graphene nanoring fabricated above a ferromagnetic strip. The exchange interaction between the magnetic moments of the ions in the ferromagnet and the electron spin splits the electronic states, and gives rise to spin polarization of the conductance and the total electric current. We demonstrate that both the current and its polarization can be controlled by a side-gate voltage. This opens the possibility to use the proposed device as a tunable source of polarized electrons.MICINNFODNECYTDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Heat transport and thermal rectification in molecular junctions: A minimal model approach
Get PDF©2011 American Physical Society.
This work was supported by DFG-Projekt CU 44/20-1 Project No. MAT2010-17180 and by the South Korea Ministry of Education, Science and Technology Program “World Class University” under Contract No. R31-2008-000-10100-0. E.D. acknowledges financial support by MEC and the program Flores Valles-UCM. We acknowledge fruitful discussions with H. Sevincli and S. Avdoshenko.Heat conduction properties are investigated in a molecular junction modeled as a two-strand ladder with strongly asymmetric thermal transport pathways. By confining anharmonic contributions to only one of the strands, it is shown that tuning of the interstrand coupling can lead to normal heat transport and to the emergence of a well-defined temperature gradient. More interestingly, thermal rectification is obtained around a critical value of the interstrand interaction and by appropriate asymmetries induced by the coupling to the thermal baths.DFGMECProgram Flores Valles-UCMDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
Scattering of massless Dirac particles by oscillating barriers in one dimension
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This work was supported by MICINN (projects MAT2010-17180 and FIS2009-07880), JCYL (project SA226U13) and USAL (project KBBB). C.G.-S. acknowledges financial upport from Comunidad de Madrid and European Social Foundation.We study the scattering of massless Dirac particles by oscillating barriers in one dimension. Using the Floquet theory, we find the exact scattering amplitudes for time-harmonic barriers of arbitrary shape. In all cases the scattering amplitudes are found to be independent of the energy of the incoming particle and the transmission coefficient is unity. This is a manifestation of the Klein tunneling in time-harmonic potentials. Remarkably, the transmission amplitudes for arbitrary sharply-peaked potehtials also become independent of the driving frequency. Conditions for which barriers of finite width can be replaced by sharply-peaked potentials are discussed.Ministerio de Ciencia e Innovación (MICINN)JCYLUSALComunidad de MadridEuropean Social FoundationDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu
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