arXiv subjects

E. Arrigoni

Publications and source records attributed to E. Arrigoni.

At least 55 records · Page 3Linked to original sources

arXiv2003

Scaling properties of the projected SO(5) model in three dimensions

We study the scaling properties of the quantum ``projected'' SO(5) model in three dimensions by means of a highly accurate Quantum-Monte-Carlo analysis. Within the parameter regime studied (temperature and system size), we show that the scaling behavior is consistent with a SO(5)-symmetric critical behavior in a relative extendent transient regime. This holds both when the symmetry breaking is caused by quantum fluctuations only as well as when also the static (mean-field) symmetry is moderately broken. We argue that possible departure away from the SO(5)-symmetric scaling occurs only in an extremely narrow parameter regime, which is inaccessible both experimentally and numerically.

cond-mat.str-el↗

arXiv2003

Phase-fluctuation induced reduction of the kinetic energy at the superconducting transition

Recent reflectivity measurements provide evidence for a "violation" of the in-plane optical integral in the underdoped high-T_c compound Bi_2Sr_2CaCu_2O_{8+δ} up to frequencies much higher than expected by standard BCS theory. The sum rule violation may be related to a loss of in-plane kinetic energy at the superconducting transition. Here, we show that a model based on phase fluctuations of the superconducting order parameter can account for this change of in-plane kinetic energy at T_c. The change is due to a transition from a phase-incoherent Cooper-pair motion in the pseudogap regime above T_c to a phase-coherent motion at T_c.

cond-mat.supr-con↗

arXiv2002

The optimal inhomogeneity for superconductivity

We study the effect of nonuniform transverse couplings on a quasi-one dimensional superconductor. We show that inhomogeneous couplings quite generally increase the superconducting (pairing) gap relative to the uniform system, but that beyond an ``optimal'' degree of inhomogeneity, they lead to a suppression of the tendency to phase coherence. The optimal conditions for superconductivity are derived. We also show that a {\it delocalized}, spin-gapped phase is stable against weak disorder in a four-leg-ladder with moderate repulsive interactions.

cond-mat.str-el↗

arXiv2002

Pair Phase Fluctuations and the Pseudogap

The single-particle density of states and the tunneling conductance are studied for a two-dimensional BCS-like Hamiltonian with a d_{x^2-y^2}-gap and phase fluctuations. The latter are treated by a classical Monte Carlo simulation of an XY model. Comparison of our results with recent scanning tunneling spectra of Bi-based high-T_c cuprates supports the idea that the pseudogap behavior observed in these experiments can be understood as arising from phase fluctuations of a d_{x^2-y^2} pairing gap whose amplitude forms on an energy scale set by T_c^{MF} well above the actual superconducting transition.

cond-mat.supr-con↗

arXiv2002

Interrelation between antiferromagnetic and superconducting gaps in high-Tc materials

We propose a phenomenological model, comprising a microscopic \sof model plus the on-site Hubbard interaction $U$ (``projected \sof model'') to understand the interrelation between the d-wave-gap modulation observed by recent angle-resolved photoemission experiments in the insulating antiferromagnet Ca$_2$CuO$_2$Cl$_2$ and the d-wave gap of high-Tc superconducting materials. The on-site interaction $U$ is important in order to produce a Mott gap of the correct order of magnitude, which would be absent in an exact \sof theory. The projected \sof-model explains the gap characteristics, namely both the symmetry and the different order of magnitude of the gap modulations between the \af and the \sc phases. Furthermore, it is shown that the projected \sof theory can provide an explanation for a recent observation [E. Pavarini et al., Phys. Rev. Lett. 87, 47003 (2001)], i. e. that the maximum Tc observed in a large variety of high-Tc cuprates scales with the next-nearest-neighbor hopping matrix element $t'$.

cond-mat.supr-con↗

arXiv2002

Phase diagram and dynamics of the projected SO(5)-symmetric model of high-$T_c$ superconductivity

We present numerical studies of a quantum ``projected'' SO(5) model which aims at a unifying description of antiferromagnetism and superconductivity in the high-T$_c$ cuprates, while properly taking into account the Mott insulating gap. Our numerical results, obtained by the Quantum Monte Carlo technique of Stochastic Series Expansion, show that this model can give a realistic description of the global phase diagram of the high-$T_c$ superconductors and accounts for many of their physical properties. Moreover, we address the question of dynamic restoring of the SO(5) symmetry at the critical point.

cond-mat.supr-con↗

arXiv2001

Spectral Properties of High-T$_c$ Cuprates via a Cluster-Perturbation Approach

Angular-resolved photoemission data on half-filled doped cuprate materials are compared with an exact-diagonalization analysis of the three-band Hubbard model, which is extended to the infinite lattice by means of a perturbation in the intercluster hopping (cluster perturbation theory). A study of the band dispersion and spectral weight of the insulating cuprate Sr$_2$CuO$_2$Cl$_2$ allows us to fix a consistent parameter set, which turns out to be appropriate at finite dopings as well. In the overdoped regime, our results for the spectral weight and for the Fermi surface give a good description of the experimental data on Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. In particular, the Fermi surface is hole-like and centered around $k=(π,π)$. Finally, we introduce a hopping between two layers and address the issue of bilayer splitting.

cond-mat.str-el↗

arXiv2001

Stripes and superconducting pairing in the t-J model with Coulomb interactions

We study the competition between long- and short-range interactions among charge carriers in strongly-correlated electronic systems employing a new method which combines the density-matrix renormalization-group technique with a self-consistent treatment of the long-range interactions. We apply the method to an extended t-J model which exhibits ``stripe'' order. The Coulomb interactions, while not destroying stripes, induce large transverse stripe fluctuations with associated charge delocalization. This leads to a substantial Coulomb-repulsion-induced {\it enhancement} of long-range superconducting pair-field correlations.

cond-mat.str-el↗

arXiv2000

Spin-wave spectrum of a two-dimensional itinerant electron system: Analytic results for the incommensurate spiral phase in the strong-coupling limit

We study the zero-temperature spin fluctuations of a two-dimensional itinerant-electron system with an incommensurate magnetic ground state described by a single-band Hubbard Hamiltonian. We introduce the (broken-symmetry) magnetic phase at the mean-field (Hartree-Fock) level through a \emph{spiral spin configuration} with characteristic wave vector $\gmathbf{Q}$ different in general from the antiferromagnetic wave vector $\gmathbf{Q_{AF}}$, and consider spin fluctuations over and above it within the electronic random-phase (RPA) approximation. We obtain a \emph{closed} system of equations for the generalized wave vector and frequency dependent susceptibilities, which are equivalent to the ones reported recently by Brenig. We obtain, in addition, analytic results for the spin-wave dispersion relation in the strong-coupling limit of the Hubbard Hamiltonian and find that at finite doping the spin-wave dispersion relation has a \emph{hybrid form} between that associated with the (localized) Heisenberg model and that associated with the (long-range) RKKY exchange interaction. We also find an instability of the spin-wave spectrum in a finite region about the center of the Brillouin zone, which signals a physical instability toward a different spin- or, possibly, charge-ordered phase, as, for example, the stripe structures observed in the high-Tc materials. We expect, however, on physical grounds that for wave vectors external to this region the spin-wave spectrum that we have determined should survive consideration of more sophisticated mean-field solutions.

cond-mat.str-el↗

arXiv2000

Critical properties of projected SO(5) models at finite temperatures

We consider the projected SO(5) bosonic model introduced in order to connect the SO(5) theory of high-T$_c$ superconductivity with the physics of the Mott-insulating gap, and derive the corresponding effective functional describing low-energy degrees of freedom. At the antiferromagnetic-superconducting transition, SO(5) symmetry-breaking effects due to the gap are purely quantum mechanical and become irrelevant in the neighborhood of a possible finite-temperature multicritical point separating the normal from the antiferromagnetic and the superconducting phases. A difference in the magnon and hole-pair mobility always takes the system away from the SO(5)-symmetric fixed point towards a region of instability, and the phase transition between the normal and the two ordered phases becomes first order before merging into the antiferromagnetic-superconducting line. Quantum fluctuations at intermediate temperatures, while introducing symmetry-breaking terms in the case of equal mobilities, tend to cancel the symmetry-breaking effects in the case of different mobilities.

cond-mat.str-el↗

arXiv1999

Crossover to Fermi-liquid behavior for weakly-coupled Luttinger liquids in the anisotropic large-dimension limit

We study the problem of the crossover from one- to higher-dimensional metals by considering an array of Luttinger liquids (one-dimensional chains) coupled by a weak interchain hopping {\tp.} We evaluate the exact asymptotic low-energy behavior of the self-energy in the anisotropic infinite-dimension limit. This limit extends the dinamical mean field concept to the case of a chain embedded in a self-consistent medium. The system flows to a Fermi-liquid fixed point for energies below the dimensional crossover temperature, and the anomalous exponent $\al$ renormalizes to zero, in the case of equal spin and charge velocities. In particular, the single-particle spectral function shows sharp quasiparticle peaks with nonvanishing weight along the whole Fermi surface, in contrast to the lowest-order result. Our result is obtained by carring out a resummation of all diagrams of the expansion in \tp contributing to the anisotropic $D\to\infty$ limit. This is done by solving, in an almost completely analytic way, an asymptotically exact recursive equation for the renormalized vertices, within a skeleton expansion. Our outcome shows that perturbation expansions in \tp restricted to lowest orders are unreliable below the crossover temperature. The extension to finite dimensions is discussed. This work extends our recent Letter [Phys. Rev. Lett. {\bf 83}, 128 (1999)], and includes all mathematical details.

cond-mat.str-el↗

arXiv1999

Renormalized SO(5) symmetry in ladders with next-nearest-neighbor hopping

We study the occurrence of SO(5) symmetry in the low-energy sector of two-chain Hubbard-like systems by analyzing the flow of the running couplings ($g$-ology) under renormalization group in the weak-interaction limit. It is shown that SO(5) is asymptotically restored for low energies for rather general parameters of the bare Hamiltonian. This holds also with inclusion of a next-nearest-neighbor hopping which explicitly breaks particle-hole symmetry provided one accounts for a different single-particle weight for the quasiparticles of the two bands of the system. The physical significance of this renormalized SO(5) symmetry is discussed.

cond-mat.str-el↗

arXiv1998

Magnetic Properties of YBa_2Cu_3O_{7-δ} in a self-consistent approach: Comparison with Quantum-Monte-Carlo Simulations and Experiments

We analyze single-particle electronic and two-particle magnetic properties of the Hubbard model in the underdoped and optimally-doped regime of \YBCO by means of a modified version of the fluctuation-exchange approximation, which only includes particle-hole fluctuations. Comparison of our results with Quantum-Monte Carlo (QMC) calculations at relatively high temperatures ($T\sim 1000 K$) suggests to introduce a temperature renormalization in order to improve the agreement between the two methods at intermediate and large values of the interaction $U$. We evaluate the temperature dependence of the spin-lattice relaxation time $T_1$ and of the spin-echo decay time $T_{2G}$ and compare it with the results of NMR measurements on an underdoped and an optimally doped \YBCO sample. For $U/t=4.5$ it is possible to consistently adjust the parameters of the Hubbard model in order to have a good {\it semi-quantitative} description of this temperature dependence for temperatures larger than the spin gap as obtained from NMR measurements. We also discuss the case $U/t\sim 8$, which is more appropriate to describe magnetic and single-particle properties close to half-filling. However, for this larger value of $U/t$ the agreement with QMC as well as with experiments at finite doping is less satisfactory.

cond-mat.str-el↗

arXiv1998

Interplane magnetic coupling effects in the multilattice compound Y_2Ba_4Cu_7O_{15}

We investigate the interplane magnetic coupling of the multilattice compound Y_2Ba_4Cu_7O_{15} by means of a bilayer Hubbard model with inequivalent planes. We evaluate the spin response, effective interaction and the intra- and interplane spin-spin relaxation times within the fluctuation exchange approximation. We show that strong in-plane antiferromagnetic fluctuations are responsible for a magnetic coupling between the planes, which in turns leads to a tendency of the fluctuation in the two planes to equalize. This equalization effect grows whit increasing in-plane antiferromagnetic fluctuations, i. e., with decreasing temperature and decreasing doping, while it is completely absent when the in-layer correlation length becomes of the order of one lattice spacing. Our results provide a good qualitative description of NMR and NQR experiments in Y_2Ba_4Cu_7O_{15}.

cond-mat.str-el↗

arXiv1998

Magnetic fluctuations in coupled inequivalent Hubbard layers as a model for Y2Ba4Cu7O15

We investigate, within the fluctuation-exchange approximation, a correlated-electron model for Y2Ba4Cu7O15 represented by two inequivalent Hubbard layers coupled by an interlayer hopping $t_\perp$. An energy offset $δ$ is introduced in order to produce a different charge carrier concentration in the two layers. We compare several single-particle and magnetic excitations, namely, the single particle scattering rate, the spectral function and the spin lattice as well as spin-spin relaxation times in the two layers as a function of $δ$. We show that the induced interlayer magnetic coupling produces a tendency to ``equalization'' of the magnetic properties in the two layers whereby antiferromagnetic fluctuations are suppressed in the less doped layer and enhanced in the heavily doped one. The strong antiferromagnetic bilayer coupling causes the charge carriers in the plane with larger doping concentration to behave similar to those of the underdoped layer, they are coupled to. This effect grows for decreasing temperature. For high temperatures or if both layers are optimally or overdoped, i.e. when the antiferromagnetic correlation length becomes of the order or smaller than one lattice site the charge carrier and magnetic dynamics of the two layers is disconnected and the equalization effect disappears. These results are in good agreement with NMR experiments on Y2Ba4Cu7O15 by Stern et al. [Phys. Rev B 51, 15478 (1995)]. We also compare the results with calculations on bilayer systems with equivalent layers as models for the constituent compounds YBa2Cu3O7 and YBa2Cu4O8.

cond-mat↗

arXiv1998

SO(5) Symmetry in t-J and Hubbard Models

Numerical and analytical results are reviewed, which support SO(5) symmetry as a concept unifying superconductivity and antiferromagnetism in the high-temperature superconductors. Exact cluster diagonalizations verify that the low-energy states of the two-dimensional t-J and Hubbard models, widely used microscopic models for the high-Tc cuprates, form SO(5) symmetry multiplets. Apart from a small standard deviation ~J/10, these multiplets become degenerate at a critical chemical potential (transition into doped system). As a consequence, the d-wave superconducting states away from half-filling are obtained from the higher spin states at half-filling through SO(5) rotations. Between one and two dimensions, using weak-coupling renormalization, a rather general ladder Hamiltonian including next-nearest-neighbor hopping can be shown to flow to an SO(5) symmetric point. Experimental tests and consequences such as the existence of a pi-Goldstone mode both in the insulator and superconductor and, in particular, the relationship between the photoemission spectra of the insulator and superconductor, are emphasized.

cond-mat.str-el↗

arXiv1997

Systematic numerical study of spin-charge separation in one dimension

The problem of spin-charge separation is analyzed numerically in the metallic phase of the one-band Hubbard model in one dimension by studying the behavior of the single-particle Green's function and of the spin and charge susceptibilities. We first analyze the Quantum-Monte Carlo data for the imaginary-time Green's function within the Maximum Entropy method in order to obtain the spectral function at real frequencies. For some values of the momentum sufficiently away from the Fermi surface two separate peaks are found, which can be identified as charge and spin excitations. In order to improve our accuracy and to be able to extend our study to a larger portion of the Brillouin zone, we also fit our data with the imaginary-time Green's function obtained from the Luttinger-model solution with two different velocities as fitting parameters. The excitation energies associated with these velocities turn out to agree, in a broad range of momenta, with the ones calculated from the charge and spin susceptibilities. This allows us to identify these single-particle excitations as due to a separation of spin and charge. Remarkably, the range of momenta where spin-charge separation is seen extends well beyond the region of linear dispersion about the Fermi surface. We finally discuss a possible extension of our method to detect spin-charge separation numerically in two dimensions.

cond-mat.str-el↗

arXiv1997

Universal scaling behavior of coupled chains of interacting fermions

The single-particle hopping between two chains is investigated by exact-diagonalizations techniques supplemented by finite-size scaling analysis. In the case of two coupled strongly-correlated chains of spinless fermions, the Taylor expansion of the expectation value of the single-particle interchain hopping operator of an electron at momentum k_F in powers of the interchain hopping t_perp is shown to become unstable in the thermodynamic limit. In the regime alpha alpha_{tp}, the scaling law is proven to change its functional form, thus signaling, for the first time numerically, the onset of coherent transverse two-particle hopping.

cond-mat.str-el↗