Papers by Gonzalo Alvarez
<title>Complexity in transition metal oxides</title>
Strongly Correlated Electron Materials: Physics and Nanoengineering, 2005
A vast array of experimental and theoretical results in manganites has shown that the colossal ma... more A vast array of experimental and theoretical results in manganites has shown that the colossal magnetoresistance effect can be understood based on the competition of insulating and ferromagnetic phases. The general aspects of the theoretical description appear to be valid for any compound with intrinsic phase competition. In high temperature superconductors, recent experiments have shown the existence of intrinsic inhomogeneities in many materials, revealing a phenomenology quite similar to that of manganese oxides. We will review briefly the results for manganites in order to propose a similar scenario in the high Tc cuprates.
Physical Review Letters, 2010
The loss of coherence in quantum mechanical superposition states limits the time for which quantu... more The loss of coherence in quantum mechanical superposition states limits the time for which quantum information remains useful. Similarly, it limits the distance over which quantum information can be transmitted. Here, we investigate in a nuclear spin-based quantum simulator, the localization of the size of spin clusters that are generated by a Hamiltonian driving the transmission of information, while a variablestrength perturbation counteracts the spreading. We find that the system reaches a dynamic equilibrium size, which decreases with the square of the perturbation strength.
Physical Review Letters, 2008
Efficient simulations of quantum evolutions of spin-1/2 systems are relevant for ensemble quantum... more Efficient simulations of quantum evolutions of spin-1/2 systems are relevant for ensemble quantum computation as well as in typical NMR experiments. We propose an efficient method to calculate the dynamics of an observable provided that the initial excitation is "local". It resorts a single entangled pure initial state built as a superposition, with random phases, of the pure elements that compose the mixture. This ensures self-averaging of any observable, drastically reducing the calculation time. The procedure is tested for two representative systems: a spin star (cluster with random long range interactions) and a spin ladder.
Physical Review Letters, 2002
A lattice spin-fermion model for diluted magnetic semiconductors (DMS) is investigated numericall... more A lattice spin-fermion model for diluted magnetic semiconductors (DMS) is investigated numerically, improving on previously used mean-field (MF) approximations. Curie temperatures are obtained varying the Mn-spin x and hole n densities, and the impurity-hole exchange J in units of the hopping amplitude t. Optimal values are found in the subtle intermediate regime between itinerant and localized carriers. Our main result is the behavior of the Curie temperature at large J/t, where a "clustered" state is observed and ferromagnetism is suppressed. Formal analogies between DMS and manganites are also discussed. PACS numbers: 75.50.Pp,75.10.Lp,75.30.Vn Diluted magnetic semiconductors based on III-V compounds are attracting considerable attention due to their combination of magnetic and semiconducting properties, that may lead to spintronic applications [1, 2]. Ga 1−x Mn x As is the most studied of these compounds with a maximum Curie temperature T C ≈110 K at low doping x, and with a carrier concentration p=(n/x)<1 due to the presence of As antisite defects or Mn intersticials . It is widely believed that this ferromagnetism is carrier-induced, with holes introduced by doping mediating the interaction between S=5/2 Mn-spins. This Zener mechanism operates in other materials as well .
Physical Review Letters, 2010
Large scale Monte Carlo simulation results for the two-orbital model for manganites, including Ja... more Large scale Monte Carlo simulation results for the two-orbital model for manganites, including Jahn-Teller lattice distortions, are here presented. At hole density x = 1/4 and in the vicinity of the region of competition between the ferromagnetic metallic and spin-charge-orbital ordered insulating phases, the colossal magnetoresistance (CMR) phenomenon is observed with a magnetoresistance ratio ∼ 10, 000%. Our main result is that this CMR transition is found to be of first order in some portions of the phase diagram, in agreement with early results from neutron scattering, specific heat, and magnetization, thus solving a notorious discrepancy between experiments and previous theoretical studies. The first-order characteristics of the transition survive, and are actually enhanced, when weak quenched disorder is introduced.
Physical Review B, 2006
Using dynamical mean-field theory, we have evaluated the magnetic instabilities and T = 0 phase d... more Using dynamical mean-field theory, we have evaluated the magnetic instabilities and T = 0 phase diagram of the double-exchange model on a Bethe lattice in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, we also study a class of disordered phases with magnetic short-range order (SRO). In the weak-coupling limit, a SRO phase has a higher transition temperature than the AF phase for all fillings p below 1 and can even have a higher transition temperature than the FM phase. At T = 0 and for small Hund's coupling J H , a SRO state has lower energy than either the FM or AF phases for 0.26 ≤ p < 1. Phase separation is absent in the J H → 0 limit but appears for any non-zero value of J H .
Physical Review B, 2008
The modulation of charge density and spin order in ͑LaMnO 3 ͒ 2n / ͑SrMnO 3 ͒ n ͑n =1-4͒ superlat... more The modulation of charge density and spin order in ͑LaMnO 3 ͒ 2n / ͑SrMnO 3 ͒ n ͑n =1-4͒ superlattices is studied via Monte Carlo simulations of the double-exchange model. G-type antiferromagnetic barriers in the SrMnO 3 regions with low charge density are found to separate ferromagnetic LaMnO 3 layers with high charge density. A metal-insulator transition with increasing n is observed in the direction perpendicular to the interfaces. Our simulations provide insight into how disorder-induced localization may cause the metal-insulator transition occurring at n = 3 in experiments.
Physical Review A, 2013
Active protection of quantum states is an essential prerequisite for the implementation of quantu... more Active protection of quantum states is an essential prerequisite for the implementation of quantum computing. Dynamical decoupling (DD) is a promising approach that applies sequences of control pulses to the system in order to reduce the adverse effect of system-environment interactions. Since every hardware device has finite precision, the errors of the DD control pulses can themselves destroy the stored information rather than protect it. We experimentally compare the performance of different DD sequences in the presence of an environment that was chosen such that all relevant DD sequences can equally suppress its effect on the system. Under these conditions, the remaining decay of the qubits under DD allows us to compare very precisely the robustness of the different DD sequences with respect to imperfections of the control pulses. * mustafa.ahmed@tu-dortmund.de †
DCA++: A case for science driven application development for leadership computing platforms
Journal of Physics: Conference Series, 2009
ABSTRACT
Europhysics Letters (EPL), 1999
The Schwinger model is studied in a finite lattice by means of the P-representation. The vacuum e... more The Schwinger model is studied in a finite lattice by means of the P-representation. The vacuum energy, mass gap and chiral condensate are evaluated showing good agreement with the expected values in the continuum limit.
Quantum State Transfer and Network Engineering, 2013
This is a shortened and slightly edited version of a chapter [1] in the collection Quantum State ... more This is a shortened and slightly edited version of a chapter [1] in the collection Quantum State Transfer and Network Engineering, edited by G.M. Nikolopoulos and I. Jex [2], where we review our own research about the robustness of spin-chain state-transfer schemes along with other approaches to the topic. Since our own research is documented elsewhere to a large extent we here restrict ourselves to a review of other approaches which might be useful to other researchers in the field.
Studies in Computational Intelligence, 2011
The idea of using chaotic transformations in cryptography is explicit in the foundational papers ... more The idea of using chaotic transformations in cryptography is explicit in the foundational papers of Shannon on secrecy systems (e.g., ). Although the word "chaos" was not minted till the 1970s [71], Shannon clearly refers to this very concept when he proposes the construction of secure ciphers by means of measure-preserving, mixing maps which depend 'sensitively' on their parameters. The implementation of Shannon's intuitions had to wait till the development of Chaos Theory in the 1980s. Indeed, it was around 1990 when the first chaos-based ciphers were proposed (e.g., , ). Moreover, in 1990 chaos synchronization [91] entered the scene and shortly thereafter, the first applications to secure communications followed . The idea is remarkably simple: mask the message with a chaotic signal and use synchronization at the receiver to filter out the chaotic signal. The realization though had to overcome the desynchronization induced by the message itself. After this initial stage, the number of proposals which exploited the properties of chaotic maps for cryptographical purposes, grew in a spectacular way.
Physics Procedia, 2010
We investigate the one-orbital model for manganites with cooperative phonons and superexchange co... more We investigate the one-orbital model for manganites with cooperative phonons and superexchange coupling J AF via large-scale Monte Carlo (MC) simulations. Results for two-orbitals are also briefly discussed. Focusing on an the realistic electronic density n = 0.75, a regime of competition between ferromagnetic (FM) metallic and charge-ordered (CO) insulating states was identified in the finite temperature phase diagram. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects were observed. The CMR magnitude is much larger than recently reported when randomly distributed polarons form the competing insulator. The appearance of CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state. We present calculations of charge-charge correlations as well as typical Monte Carlo snapshots to support this view. Cengiz Şen et al. / Physics Procedia 00 (2010) 1-13 2
Quantum phase transition between orbital-selective Mott states in Hund's metals
Physical Review B, 2014
Physical Review Letters, 2005
Tennessee 37996 and Condensed Matter Sciences Division, Oak Ridge National Lab, TN 37381, USA
2009 6th International Multi-Conference on Systems, Signals and Devices, 2009
Chaos Solitons & Fractals, 2005
Recently, a new scheme was proposed for deniable authentication. Its main originality lied on app... more Recently, a new scheme was proposed for deniable authentication. Its main originality lied on applying a chaos-based encryption-hash parallel algorithm and the semi-group property of the Chebyshev chaotic map. Although original and practicable, its insecurity and inefficiency are shown in this paper, thus rendering it inadequate for adoption in e-commerce.
Physics Letters A, 2003
Recently a chaotic encryption system has been proposed by P. Garcı́a et al. It represents an impr... more Recently a chaotic encryption system has been proposed by P. Garcı́a et al. It represents an improvement over an algorithm previously presented by some of the same authors. In this Letter, several weaknesses of the new cryptosystem are pointed out and four successful cryptanalytic attacks are described.
Chaos Solitons & Fractals, 2004
This paper describes the security weakness of a recently proposed secure communication method bas... more This paper describes the security weakness of a recently proposed secure communication method based on parameter modulation of a chaotic system and adaptive observer-based synchronization scheme. We show that the security is compromised even without precise knowledge of the chaotic system used.
Computer Physics Communications, 2004
Recently a new chaotic encryption system has been proposed as a modified version of the chaotic c... more Recently a new chaotic encryption system has been proposed as a modified version of the chaotic cryptographic method based on iterating a logistic map. A fundamental weakness of this new cryptosystem is pointed out that allows for three successful cryptanalytic attacks.
Uploads
Papers by Gonzalo Alvarez