arXiv subjects
Publications and source records attributed to J. Lorenzana.
We study phase separation frustrated by the long-range Coulomb interaction in two dimensional electronic systems with emphasys in the case of a metallic and an insulating phase. We find that two-dimensional systems are more prone to mesoscopic frustrated phase separation than the three dimensional ones.
We investigate the interplay between the electron-electron and the electron-phonon interaction in the Hubbard-Holstein model. We implement the flow-equation method to investigate within this model the effect of correlation on the electron-phonon effective coupling and, conversely, the effect of phonons in the effective electron-electron interaction. Using this technique we obtain analytical momentum-dependent expressions for the effective couplings and we study their behavior for different physical regimes. In agreement with other works on this subject, we find that the electron-electron attraction mediated by phonons in the presence of Hubbard repulsion is peaked at low transferred momenta. The role of the characteristic energies involved is also analyzed.
We present a non-linear elastic model of a coherent transition with discontinuous volume change in an isotropic solid. The model reproduces the anomalous thermodynamics typical of coherent equilibrium including intrinsic hysteresis (for a pressure driven experiment) and a negative bulk modulus. The novelty of the model is that the statistical mechanics solution can be easily worked out. We find that coherency leads to an infinite-range density--density interaction, which drives classical critical behavior. The pressure width of the hysteresis loop shrinks with increasing temperature, ending at a critical point at a temperature related to the shear modulus. The bulk modulus softens with a 1/2 exponent at the transition even far from the critical point. Many well known features of the phase diagram of Ce and related systems are explained by the model.
We have studied the magnetization of a series of spin-charge ordered La(2-x)Sr(x)NiO(4+delta) single crystals with 0 < x < 0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature T(F1) ~ 50 K and a smaller irreversibility that persists up to near the charge ordering temperature. We observed a novel memory effect in the thermo-remnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed, and that there is a pronounced increase in the thermo-remnant magnetization when the system is warmed through a spin reorientation transition.
We present estimates in the Hubbard and Heisenberg models for the spectral weight in magnetic neutron scattering experiments on the cuprates. With the aid of spin-wave theory and the time dependent Gutzwiller approximation we discuss how the spectral weight is distributed among the different channels and between high and low energies. In addition to the well known total moment sum rule we discuss sum rules for each component of the dynamical structure factor tensor which are peculiar for spin 1/2 systems. The various factors that reduce the spectral weight at the relevant energies are singled out and analyzed like: shielding factors, weight at electronic energies, multimagnon process etc. Although about 10% ~ 15% of the naively expected weight is detected in experiments after consideration of these factors the missing weight is within the experimental uncertainties. A large fraction of the spectral weight is hard to detect with present experimental conditions.
Within the time-dependent Gutzwiller approximation for the Hubbard model we compute the magnetic fluctuations of vertical metallic stripes with parameters appropriate for La$_{1.875}$Ba$_{0.125}$CuO$_4$ (LBCO). For bond- and site-centered stripes the excitation spectra are similar, consisting of a low-energy incommensurate acoustic branch which merges into a ``resonance peak'' at the antiferromagnetic wave vector and several high-energy optical branches. The acoustic branch is similar to the result of theories assuming localized spins whereas the optical branches are significantly different. Results are in good agreement with a recent inelastic neutron study of LBCO.
We carry out a theoretical analysis of the momentum dependence of the Fourier-transformed local density of states (LDOS) in the superconducting cuprates within a model considering the interference of quasiparticles scattering on quenched impurities. The impurities introduce an external scattering potential, which is either nearly local in space or it can acquire a substantial momentum dependence due to a possible strong momentum dependence of the electronic screening near a charge modulation instability. The key new effect that we introduce is an additional mesoscopic disorder aiming to reproduce the inhomogeneities experimentally observed in scanning tunnelling microscopy. The crucial effect of this mesoscopic disorder is to give rise to point-like spectroscopic features, to be contrasted with the curve-like shape of the spectra previously calculated within the interfering-quasiparticle schemes. It is also found that stripe-like charge modulations play a relevant role to correctly reproduce all the spectral features of the experiments.
We present a first principle computation of vertical stripes in $La_{15/8}Sr_{1/8}CuO_4$ within the LDA+U method. We find that Cu centered stripes are unstable toward O centered stripes. The metallic core of the stripe is quite wide and shows reduced magnetic moments with suppressed antiferromagnetic (AF) interactions. The system can be pictured as alternating metallic and AF two-leg ladders the latter with strong AF interaction and a large spin gap. The Fermi surface shows warping due to interstripe hybridization. The periodicity and amplitude of the warping is in good agreement with angle resolved photoemission experiment. We discuss the connection with low-energy theories of the cuprates.
We use a spin-rotational invariant Gutzwiller energy functional to compute random-phase-approximation-like (RPA) fluctuations on top of the Gutzwiller approximation (GA). The method can be viewed as an extension of the previously developed GA+RPA approach for the charge sector [G. Seibold and J. Lorenzana, Phys. Rev. Lett. {\bf 86}, 2605 (2001)] with respect to the inclusion of the magnetic excitations. Unlike the charge case, no assumptions about the time evolution of the double occupancy are needed in this case. Interestingly, in a spin-rotational invariant system, we find the correct degeneracy between triplet excitations, showing the consistency of both computations. Since no restrictions are imposed on the symmetry of the underlying saddle-point solution, our approach is suitable for the evaluation of the magnetic susceptibility and dynamical structure factor in strongly correlated inhomogeneous systems. We present a detailed study of the quality of our approach by comparing with exact diagonalization results and show its much higher accuracy compared to the conventional Hartree-Fock+RPA theory. In infinite dimensions, where the GA becomes exact for the Gutzwiller variational energy, we evaluate ferromagnetic and antiferromagnetic instabilities from the transverse magnetic susceptibility. The resulting phase diagram is in complete agreement with previous variational computations.
Based on an unrestricted Gutzwiller approximation (GA) we investigate the stripe orientation and periodicity in an extended one-band Hubbard model. A negative ratio between next-nearest and nearest neighbor hopping t'/t, as appropriate for cuprates, favors partially filled (metallic) stripes for both vertical and diagonal configurations. At around optimal doping diagonal stripes, site centered (SC) and bond centered (BC) vertical stripes become degenerate suggesting strong lateral and orientational fluctuations. We find that within the GA the resulting phase diagram is in agreement with experiment whereas it is not in the Hartree-Fock approximation due to a strong overestimation of the stripe filling. Results are in agreement with previous calculations within the three-band Hubbard model but with the role of SC and BC stripes interchanged.
We present an exactly solvable non-linear elastic model of a volume collapse transition in an isotropic solid. Integrity of the lattice through the transition leads to an infinite-range density-density interaction, which drives classical critical behavior. Nucleation is forbidden within a pressure window leading to intrinsic hysteresis and an unavoidable discontinuity of the thermodynamic potential (zeroth order transition). The window shrinks with increasing temperature ending at a critical point at a temperature related to the shear modulus. Mixed phases behave non-extensively and show negative compressibility. We discuss the implications for Ce, SmS, and related systems.
We study the dynamics of metallic vertical stripes in cuprates within the three-band Hubbard model based on a recently developed time dependent Gutzwiller approximation. As doping increases the optical conductivity shows transfer of spectral weight from the charge transfer band towards i) an incoherent band centered at 1.3eV, {ii} a Drude peak, mainly due to motion along the stripe, {iii} a low energy collective mode which softens with doping and merges with ii} at optimum doping in good agreement with experiment. The softening is related to the quasidegeneracy between Cu centered and O centered mean-field stripe solutions close to optimal doping.
We present a detailed study of the time-dependent Gutzwiller approximation for the Hubbard model. The formalism, labelled GA+RPA, allows us to compute random-phase approximation-like (RPA) fluctuations on top of the Gutzwiller approximation (GA). No restrictions are imposed on the charge and spin configurations which makes the method suitable for the calculation of linear excitations around symmetry-broken solutions. Well-behaved sum rules are obeyed as in the Hartree-Fock (HF) plus RPA approach. Analytical results for a two-site model and numerical results for charge-charge and current-current dynamical correlation functions in one and two dimensions are compared with exact and HF+RPA results, supporting the much better performance of GA+RPA with respect to conventional HF+RPA theory.
We perform a systematic slave-boson mean-field analysis of the three-band model for cuprates with first-principle parameters. Contrary to widespread believe based on earlier mean-field computations low doping stripes have a linear density close to 1/2 added hole per lattice constant. We find a dimensional crossover from 1D to 2D at doping $\sim 0.1$ followed by a breaking of particle-hole symmetry around doping 1/8 as doping increases. Our results explain in a simple way the behavior of the chemical potential, the magnetic incommensurability, and transport experiments as a function of doping. Bond centered and site-centered stripes become degenerate for small overdoping.
We analyze the combined effect of the long range Coulomb (LRC) interaction and of surface energy on first order density-driven phase transitions in the presence of a compensating rigid background. We study mixed states formed by regions of one phase surrounded by the other in the case in which the scale of the inhomogeneities is much larger than the interparticle distance. Two geometries are studied in detail: spherical drops of one phase into the other and a layered structure of one phase alternating with the other. We find the optimum density profile in an approximation in which the free energy is a functional of the local density (LDA). It is shown that an approximation in which the density is assumed to be uniform (UDA) within each phase region gives results very similar to those of the more involved LDA approach. Within the UDA we derive the general equations for the chemical potential and the pressures of each phase which generalize the Maxwell construction to this situation. The equations are valid for a rather arbitrary geometry. We find that the transition to the mixed state is quite abrupt i.e. inhomogeneities of the first phase appear with a finite value of the radius and of the phase volume fraction. The maximum size of the inhomogeneities is found to be on the scale of a few electric field screening lengths. Contrary to the ordinary Maxwell construction, the inverse specific volume of each phase depends here on the global density in the coexistence region and can decrease as the global density increases. The range of densities in which coexistence is observed shrinks as the LRC interaction increases until it reduces to a singular point. We argue that close to this singular point the system undergoes a lattice instability as long as the inverse lattice compressibility is finite.
The theory of first order density-driven phase transitions with frustration due to the long range Coulomb (LRC) interaction develop on paper I of this series is applied to the following physical systems: i) the low density electron gas ii) electronic phase separation in the low density three dimensional $t-J$ model iii) in the manganites near the charge ordered phase. We work in the approximation that the density within each phase is uniform and we assume that the system separates in spherical drops of one phase hosted by the other phase with the distance between drops and the drop radius much larger than the interparticle distance. For i) we study a well known apparent instability related to a negative compressibility at low densities. We show that this does not lead to macroscopic drop formation as one could expect naively and the system is stable from this point of view. For ii) we find that the LRC interaction significantly modifies the phase diagram favoring uniform phases and mixed states of antiferromagnetic (AF) regions surrounded by metallic regions over AF regions surrounded by empty space. For iii) we show that the dependence of local densities of the phases on the overall density found in paper I gives a non-monotonous behavior of the Curie temperature on doping in agreement with experiments.
We develop a time-dependent Gutzwiller approximation (GA) for the Hubbard model analogous to the time-dependent Hartree-Fock (HF) method. The formalism incorporates ground state correlations of the random phase approximation (RPA) type beyond the GA. Static quantities like ground state energy and double occupancy are in excellent agreement with exact results in one dimension up to moderate coupling and in two dimensions for all couplings. We find a substantial improvement over traditional GA and HF+RPA treatments. Dynamical correlation functions can be easily computed and are also substantially better than HF+RPA ones and obey well behaved sum rules.
We use a high-statistic quantum Monte Carlo and Maximum Entropy regularization method to compute the dynamical energy correlation function (DECF) of the one-dimensional (1D) $S=1/2$ antiferromagnetic Heisenberg model at finite temperatures. We also present a finite temperature analytical ansatz for the DECF which is in very good agreement with the numerical data in all the considered temperature range. From these results, and from a finite temperature generalisation of the mechanism proposed by Lorenzana and Sawatsky [Phys. Rev. Lett. {\bf 74}, 1867 (1995)], we compute the line shape for the optical absorption spectra of multimagnon excitations assisted by phonons for quasi 1D compounds. The line shape has two contributions analogous to the Stokes and anti-Stokes process of Raman scattering. Our low temperature data is in good agreement with optical absorption experiments of CuO$_3$ chains in Sr$_2$CuO$_3$. Our finite temperature results provide a non trivial prediction on the dynamics of the Heisenberg model at finite temperatures that is easy to verify experimentally.