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Search Results: 1 - 10 of 404025 matches for " J. V. Alvarez "
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Nonperturbative approach to the Hubbard model in C60 cluster
J. Gonzalez,J. V. Alvarez
Physics , 1995, DOI: 10.1103/PhysRevB.53.11729
Abstract: We propose a computational scheme for the Hubbard model in the C60 cluster in which the interaction with the Fermi sea of charges added to the neutral molecule is switched on sequentially. This is applied to the calculation of the balance of charging energies, within a low-energy truncation of the space of states which produces moderate errors for an intermediate range of the interaction strength.
Strong coupling phases of the $t-t'$ Hubbard model
J. Gonzalez,J. V. Alvarez
Physics , 1996, DOI: 10.1103/PhysRevB.56.367
Abstract: We study the strong coupling regime of the $t-t'$ Hubbard model,filled up to the level of the van Hove singularities, by means of an exact diagonalization approach. We characterize the different phases of the model by the different sectors of the Hilbert space with given quantum numbers. By looking for the ground state of the system, we find essentially the competition between a state with incipient ferromagnetism and other mimicking a $d$-wave condensate, which has the lowest energy in a large region of the phase space.
Insulating, superconducting and large-compressibility phases in nanotube ropes
J. V. Alvarez,J. Gonzalez
Physics , 2003, DOI: 10.1103/PhysRevLett.91.076401
Abstract: The superconducting properties of carbon nanotube ropes are studied using a new computational framework that incorporates the renormalization of intratube interactions and the effect of intertube Coulomb screening. This method allows to study both the limits of thin and thick ropes ranging from purely one-dimensional physics to the setting of three-dimensional Cooper-pair coherence, providing good estimates of the critical temperature as a function of the rope physical parameters. We discuss the connection of our results with recent experiments.
Charge fluctuations in the unconventional metallic state of Li0.9Mo6O17
J. Merino,J. V. Alvarez
Physics , 2014, DOI: 10.1103/PhysRevB.91.035135
Abstract: Charge fluctuations in the quasi-one-dimensional material Li0.9Mo6O17 are analyzed based on a multi orbital extended Hubbard model. A charge ordering transition induced by Coulomb repulsion is found with a charge ordering pattern different from a conventional charge density wave driven by Fermi surface nesting. The metallic state displays a characteristic charge collective mode which softens signalling the proximity to the transition. We argue that the strong scattering between electrons generated by these charge order fluctuations can lead to the unconventional metallic state observed above the superconducting transition temperature in Li0.9Mo6O17.
Relaxational behavior of the infinite-range Ising spin-glass in a transverse field
J. V. Alvarez,Felix Ritort
Physics , 1996, DOI: 10.1088/0305-4470/29/23/006
Abstract: We study the zero-temperature behavior of the infinite-ranged Ising spin glass in a transverse field. Using spin summation techniques and Monte Carlo methods we characterize the zero-temperature quantum transition. Our results are well compatible with a value $\nu=\frac{1}{4}$ for the correlation length exponent, $z=4$ for the dynamical exponent and an algebraic decay $t^{-1}$ for the imaginary-time correlation function. The zero-temperature relaxation of the energy in the presence of the transverse field shows that the system monotonically reaches the ground state energy due to tunneling processes and displays strong glassy effects.
The antiferromagnetic/paramagnetic transition in mixed-spin compounds R2BaNiO5
J. V. Alvarez,Roser Valenti
Physics , 2005, DOI: 10.1140/epjb/e2005-00143-2
Abstract: We present an extensive Quantum Monte Carlo study of the magnetic properties of the mixed-spin quantum systems R2BaNiO5 (R= magnetic rare earth) which show coexistence of 3-dimensional magnetic long-range order with 1-dimensional quantum gap excitations. We discuss the validity of the performed simulations in the critical region and show the excellent agreement with experimental results. We emphasize the importance of quantum fluctuations contained in our study which is absent in previous mean-field-like treatments.
Disordered and Ordered States in a Frustrated Anisotropic Heisenberg Hamiltonian
S. Moukouri,J. V. Alvarez
Physics , 2004,
Abstract: We use a recently proposed perturbative numerical renormalization group algorithm to investigate ground-state properties of a frustrated three dimensional Heisenberg model on an anisotropic lattice. We analyze the ground state energy, the finite size spin gap and the static magnetic structure factor. We find in two dimensions a frustration-induced gapless spin liquid state which separates two magnetically ordered phases. In the spin liquid state, the magnetic structure factor shows evidence that this state is made of nearly disconnected chains. This spin liquid state is unstable against unfrustrated interplane couplings.
Anomalous thermal conductivity of frustrated Heisenberg spin-chains and ladders
J. V. Alvarez,Claudius Gros
Physics , 2002, DOI: 10.1103/PhysRevLett.89.156603
Abstract: We study the thermal transport properties of several quantum spin chains and ladders. We find indications for a diverging thermal conductivity at finite temperatures for the models examined. The temperature at which the non-diverging prefactor \kappa^{(th)}(T) peaks is in general substantially lower than the temperature at which the corresponding specific heat c_V(T) is maximal. We show that this result of the microscopic approach leads to a substantial reduction for estimates of the magnetic mean-free path \lambda extracted by analyzing recent experiments, as compared to similar analyses by phenomenological theories.
Low-temperature transport in Heisenberg chains
J. V. Alvarez,Claudius Gros
Physics , 2001, DOI: 10.1103/PhysRevLett.88.077203
Abstract: A technique to determine accurately transport properties of integrable and non-integrable quantum-spin chains at finite temperatures by Quantum Monte-Carlo is presented. The reduction of the Drude weight by interactions in the integrable gapless regime is evaluated. Evidence for the absence of a Drude weight in the gapless regime of a non-integrable system with longer-ranged interactions is presented. We estimate the effect of the non-integrability on the transport properties and compare with recent experiments on one-dimensional quantum-spin chains.
Renormalization group method for weakly coupled quantum chains: comparison with exact diagonalization
J. V. Alvarez,S. Moukouri
Physics , 2004,
Abstract: We show that numerical quasi-one-dimensional renormalization group allows accurate study of weakly coupled chains with modest computational effort. We perform a systematic comparison with exact diagonalization results in two and three-leg spin ladders with a transverse Hamiltonian that can involve frustration. Due to the variational nature of the algorithm, the accuracy can be arbitrarily improved enlarging the basis of eigenstates of the density matrix defined in the transverse direction. We observe that the precision of the algorithm is directly correlated to the binding of the chains. We also show that the method performs especially well in frustrated systems.
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