Spectral Properties and Stability in the Two-Dimensional Lattice-Hubbard Model

The two-dimensional Hubbard model on the square lattice is studied in the presence of lattice distortions in the adiabatic approximation. The self energy is computed within perturbation theory up to second order, which provides a way for studying the quasiparticle dispersion. We compute numerically the second order contribution to the self-energy using a standard fast Fourier transform algorithm for finite sizes system. The stability of the lattice distortions is investigated and a schematic phase diagram is drawn. The Fermi surface is analyzed for densities close to half filling, the presence of lattice distortions changes some spectral properties of the model and gives an anisotropic interacting Fermi surface. The spectral function is calculated along several lines in momentum space and the renormalized quasiparticle dispersion is obtained. The behavior of the density of states is shown for different values of the intrasite repulsion U in the different phases.Instituto de Física La Plat

Spectral Properties and Stability in the Two-Dimensional Lattice-Hubbard Model

The two-dimensional Hubbard model on the square lattice is studied in the presence of lattice distortions in the adiabatic approximation. The self energy is computed within perturbation theory up to second order, which provides a way for studying the quasiparticle dispersion. We compute numerically the second order contribution to the self-energy using a standard fast Fourier transform algorithm for finite sizes system. The stability of the lattice distortions is investigated and a schematic phase diagram is drawn. The Fermi surface is analyzed for densities close to half filling, the presence of lattice distortions changes some spectral properties of the model and gives an anisotropic interacting Fermi surface. The spectral function is calculated along several lines in momentum space and the renormalized quasiparticle dispersion is obtained. The behavior of the density of states is shown for different values of the intrasite repulsion U in the different phases.Instituto de Física La Plat

Spectral Properties and Stability in the Two-Dimensional Lattice-Hubbard Model

The two-dimensional Hubbard model on the square lattice is studied in the presence of lattice distortions in the adiabatic approximation. The self energy is computed within perturbation theory up to second order, which provides a way for studying the quasiparticle dispersion. We compute numerically the second order contribution to the self-energy using a standard fast Fourier transform algorithm for finite sizes system. The stability of the lattice distortions is investigated and a schematic phase diagram is drawn. The Fermi surface is analyzed for densities close to half filling, the presence of lattice distortions changes some spectral properties of the model and gives an anisotropic interacting Fermi surface. The spectral function is calculated along several lines in momentum space and the renormalized quasiparticle dispersion is obtained. The behavior of the density of states is shown for different values of the intrasite repulsion U in the different phases.Instituto de Física La Plat

Teorías efectivas de sistemas fuertemente correlacionados en bajas dimensiones

Los sistemas altamente correlacionados presentan una amplia variedad de fascinantes propiedades y se están estudiando de forma intensa desde hace ya varios años. Particularmente, desde el descubrimiento de los cupratos superconductores en 1986, se han escrito una gran cantidad de trabajos concernientes al mecanismo que genera la superconductividad de alta temperatura. La famosa teoría BCS en la cual el apareamiento de los electrones se obtiene como consecuencia de una atracción inducida por la interacción con los fonones es consistente con un gran número de experimentos en superconductores convencionales. Sin embargo, esta teoría no explica las altas temperaturas críticas que presentan algunos superconductores no convencionales. Aunque a lo largo de los últimos años se ha trabajado intensamente para poder explicar la existencia de superconductividad no convencional, aun no se ha logrado consensuar con respecto a la teoría mas apropiada. Una de las teorías que parece tener los ingredientes necesarios para alcanzar este fin está basada esencialmente en el proceso de dopar un aislador de Mott y en que la superconductividad se genera directamente de la interacción repulsiva entre los electrones. Por este motivo entender el diagrama de fases en todo el régimen de dopajes se ha vuelto un problema de importancia en materia condensada. Desde el límite de grandes dopajes, donde los sistemas están descritos por la teoría de Landau, se debe entender el mecanismo por el cual esta descripción se vuelve inviable dando lugar a fases exóticas como los cristales líquidos o fases de tipo nemáticas. Por otro lado en el régimen de muy bajo dopaje el estado fundamental generalmente es un estado antiferromagnético. Al aumentar el dopaje este estado se vuelve inestable dando lugar a nuevas fases. En este límite el sistema suele estar descrito por un modelo de Heisenberg antiferromagnético como teoría efectiva de bajas energías. En esta tesis se investigaron estos dos límites en el ámbito de los sistemas altamente correlacionados. Por un lado, se estudiaron las condiciones por las cuales se producen rupturas en la descripción del líquido de Fermi y las simetrías de las posibles fases más allá de esta inestabilidad. Desarrollamos para esto un método que puede aplicarse en forma sistemática a un amplio espectro de modelos de electrones sobre redes bidimensionales. Por otro lado se estudió el límite opuesto donde se desea entender cuales son los factores que desestabilizan el orden antiferromagnético del sistema. En este régimen, los modelos antiferromagnéticos en dos dimensiones han cobrado un interés a causa de su conexión con los superconductores de alta temperatura. En particular, es necesario comprender los factores que hacen que fases ordenadas se desestabilicen dando lugar a fases exóticas como los llamados líquidos de espín. Esta tesis se divide en tres partes: en la primera se presentan las motivaciones y algunos de los modelos y técnicas que se usarán a lo largo del resto de la tesis. La segunda parte esta centrada en el estudio de las posibles inestabilidades del líquido de Fermi en redes bidimensionales. Dando primero una introducción simple a la teoría de Landau del líquido de Fermi, luego desarrollamos un método que permite detectar inestabilidades del mismo en una gran cantidad de modelos. Aunque se describen aplicaciones del mismo a diferentes modelos en la red, la presentación del método se hace independientemente del modelo en cuestión de manera de que pueda ser utilizado de forma sistemática en otros contextos. En la tercera parte de la tesis se presenta el estudio realizado sobre un antiferromagneto frustrado en dos dimensiones sobre la red hexagonal en 2 dimensiones (o panal de abejas). Usando una teoría de campo medio basada en la representación de los operadores de espín en términos de bosones de Schwinger, se estudia la estabilidad del estado de Néel a medida que se incrementa la frustración del sistema.Facultad de Ciencias Exacta

Combined analytical and numerical approach to study magnetization plateaux in doped quasi-one-dimensional antiferromagnets

We investigate the magnetic properties of quasi-one-dimensional quantum spin-S antiferromagnets. We use a combination of analytical and numerical techniques to study the presence of plateaux in the magnetization curve. The analytical technique consists in a path integral formulation in terms of coherent states. This technique can be extended to the presence of doping and has the advantage of a much better control for large spins than the usual bosonization technique. We discuss the appearance of doping-dependent plateaux in the magnetization curves for spin-S chains and ladders. The analytical results are complemented by a density matrix renormalization group (DMRG) study for a trimerized spin-1/2 and anisotropic spin-3/2 doped chains.Instituto de Física La Plat

Current jumps in flat-band ladders with Dzyaloshinskii-Moriya interactions

Localized magnons states, due to flat bands in the spectrum, is an intensely studied phenomenon and can be found in many frustrated magnets of different spatial dimensionality. The presence of Dzyaloshinskii-Moriya (DM) interactions may change radically the behavior in such systems. In this context, we study a paradigmatic example of a one-dimensional frustrated antiferromagnet, the sawtooth chain in the presence of DM interactions. Using both path integrals methods and numerical Density Matrix Renormalization Group, we revisit the physics of localized magnons and determine the consequences of the DM interaction on the ground state. We have studied the spin current behavior, finding three different regimes. First, a Luttinger-liquid regime where the spin current shows a step behavior as a function of parameter D, at a low magnetic field. Increasing the magnetic field, the system is in the Meissner phase at the m = 1/2 plateau, where the spin current is proportional to the DM parameter. Finally, further increasing the magnetic field and for finite D there is a small stiffness regime where the spin current shows, at fixed magnetization, a jump to large values at D = 0, a phenomenon also due to the flat band.Fil: Acevedo, Santiago Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Pujol, P.. Université Paul Sabatier; Francia. Université de Toulouse; FranciaFil: Lamas, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin

Comparison between disordered quantum spin-1/2 chains

We study the magnetic properties of two types of one-dimensional XX spin-1/2 chains. The first type has only nearest-neighbor interactions which can be either antiferromagnetic or ferromagnetic, and the second type has both nearest-neighbor and next-nearest-neighbor interactions, but only antiferromagnetic in character. We study these systems in the presence of low magnetic fields both analytically and numerically. Comparison of results shows a close relation between the two systems, which is in agreement with results previously found in Heisenberg chains by means of a numerical real-space renormalization-group procedure.Facultad de Ciencias Exacta

Dimerized ground states in spin-S frustrated systems

We study a family of frustrated antiferromagnetic spin-S systems with a fully dimerized ground state. Starting from the simplest case of the frustrated zigzag spin ladder, we generalize the family to more complex geometries like tetrahedral ladders and spin tubes. After presenting some numerical results about the phase diagram of these systems, we show that the ground state is robust against the inclusion of weak disorder in the couplings as well as several kinds of perturbations, allowing to study some other interesting models as a perturbative expansion of the exact one. A discussion on how to determine the dimerization region in terms of quantum information estimators is also presented. Finally, we explore the relation of these results with the case of a four-leg spin tube, which recently was proposed as a model for the description of the compound Cu₂Cl₄D₈C₄SO₂, delimiting the region of the parameter space where this model presents dimerization in its ground state.Quantum Information Research GroupInstituto de Física La Plat

Nematic quantum phases in the bilayer honeycomb antiferromagnet

The spin-1/2 Heisenberg antiferromagnet on the honeycomb bilayer lattice is shown to display a rich variety of semiclassical and genuinely quantum phases, controlled by the interplay between intralayer frustration and interlayer exchange. Employing a complementary set of techniques, comprising spin rotationally invariant Schwinger boson mean-field theory, bond operators, and series expansions, we unveil the quantum phase diagram, analyzing low-energy excitations and order parameters. By virtue of Schwinger bosons we scan the complete range of exchange parameters, covering both long-range-ordered and quantum disordered ground states, and reveal the existence of an extended, frustration-induced lattice nematic phase in a range of intermediate exchange unexplored so far.Fil: Zhang, Hao. Sun Yat-sen University; ChinaFil: Lamas, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Arlego, Marcelo José Fabián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Brenig, Wolfram. Technische Universität Braunschweig; Alemani

Magnon crystals and magnetic phases in a kagome-stripe antiferromagnet

In this paper we analyze the magnetization properties of an antiferromagnetic kagome-stripe lattice, motivated by the recent synthesis of materials exhibiting this structure. By employing a variety of techniques that include numerical methods such as density-matrix renormalization-group and Monte Carlo simulations, as well as analytical techniques such as perturbative low-energy effective models and exact solutions, we characterize the magnetization process and magnetic phase diagram of a kagome-stripe lattice. The model captures a variety of behaviors present in the two-dimensional kagome lattice, which are described here by analytical models and numerically corroborated. In addition to the characterization of semiclassical intermediate plateaus, it is worth noting the determination of an exact magnon crystal phase which breaks the underlying symmetry of the lattice. This magnon crystal phase generalizes previous findingsFil: Acevedo, Santiago Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Lamas, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Arlego, Marcelo José Fabián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Pujol, Pierre. Universitè de Toulouse; Franci