arXiv subjects

M. Grilli

Publications and source records attributed to M. Grilli.

At least 91 records · Page 5Linked to original sources

arXiv2002

First-Order Pairing Transition and Single-Particle Spectral Function in the Attractive Hubbard Model

A Dynamical Mean Field Theory analysis of the attractive Hubbard model is carried out. We focus on the normal state upon restricting to solutions where superconducting order is not allowed. Nevertheless a clear first-order pairing transition as a function of the coupling takes place at all the electron densities out of half-filling. The transition occurs between a Fermi liquid, stable for $U < U_c$, and an insulating bound pairs phase for $U > U_c$. The spectral function in the Fermi liquid phase is constituted by a low energy structure around the Fermi level (similar to the Kondo resonance of the repulsive half-filled model), which disappears discontinuously at $U=U_c$, and two high energy features (lower and upper Hubbard bands), which persist in the insulating phase.

cond-mat.str-el↗

arXiv2001

Strong correlation, electron-phonon interaction and critical fluctuations: Isotope effect, pseudogap formation, and phase diagram of the cuprates

Within the Hubbard-Holstein model with long-range Coulomb forces, we revisit the charge-ordering scenario for the superconducting cuprates and account for the presence or the absence of a formed stripe phase in different classes of cuprates. We also evaluate the mean-field and the fluctuation-corrected critical lines for charge ordering and we relate them with the various pseudogap crossover lines occurring in the cuprates and we discuss a mechanism for their peculiar isotopic dependence. Considering the dynamical nature of the charge-ordering transition, we explain the spread of T* and of its isotopic shift, obtained with experimental probes with different characteristic time scales.

cond-mat.supr-con↗

arXiv2001

Single-particle spectra near a stripe instability

We analyze the single-particle spectra of a bi-layered electron system near a stripe instability and compare the results with ARPES experiments on the Bi2212 cuprate superconductor near optimum doping, addressing also the issue of the puzzling absence of bonding-antibonding splitting.

cond-mat.supr-con↗

arXiv2001

Anomalous optical absorption in overdoped cuprates near the charge-ordering instability

We propose an interpretation for the hump observed in the optical conductivity at or below a few hundreds of cm$^{-1}$, in overdoped cuprates like the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\gtrsim 0.15 and the hole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on the direct excitation of charge collective modes, which become nearly critical in the proximity to a charge-ordering instability. The nearly critical character of these excitations entails a peculiar temperature dependence and a pseudo-scaling form of the lineshapes, which are in agreement with the experimental data.

cond-mat.supr-con↗

arXiv2001

Anomalous isotopic effect near the charge-ordering quantum criticality

Within the Hubbard-Holstein model, we evaluate the various crossover lines marking the opening of pseudogaps in the cuprates, which, in our scenario, are ruled by the proximity to a charge-ordering quantum criticality (stripe formation). We provide also an analysis of their isotopic dependencies, as produced by critical fluctuations. We find no isotopic shift of the temperature $T^0$ marked as a reduction of the quasiparticle density of states in various experiments, and a substantial positive shift of the pseudogap-formation temperature $T^*$. We infer that the superconducting critical temperature $T_c$ has almost no shift in the optimally- and overdoped regimes while it has a small negative isotopic shift in the underdoped, which increses upon underdoping. We account also for the possible dynamical nature of the charge-ordering transition, and explain in this way the spread of the values of $T^*$ and its of isotopic shift, obtained with experimental probes having different characteristic timescales.

cond-mat.supr-con↗

arXiv2001

Charge-ordering quantum criticality in the phase diagram of the cuprates

We discuss how the quantum-critical-point scenario for high-T_c superconductors with density-driven charge order or stripe formation finds support from recent EXAFS experiments. Our phase diagram has the interesting feature, which is also suggested by the EXAFS experiments, that some cuprates may not show well-formed stripe phase. We also consider the extensions of the scenario to include the interesting possibility of first-order transitions which call for further experimental and theoretical investigation.

cond-mat.supr-con↗

arXiv2000

Influence of incommensurate dynamic charge-density wave scattering on the line shape of high-T$_c$ cuprates

We show that the spectral lineshape of superconducting La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) can be well described by the coupling of the charge carriers to collective incommensurate charge-density wave (CDW) excitations. Our results imply that besides antiferromagnetic (AF) fluctuations also low-energy CDW modes can contribute to the observed dip-hump structure in the Bi2212 photoemission spectra. In case of underdoped LSCO we propose a possible interpretation of ARPES data in terms of a grid pattern of fluctuating stripes where the charge and spin scattering directions deviate by $α=π/4$. Within this scenario we find that the spectral intensity along $(0,0) \to (π,π)$ is strongly suppressed consistent with recent photoemission experiments. In addition the incommensurate charge-density wave scattering leads to a significant broadening of the quasiparticle-peak around $(π,0)$.

cond-mat.supr-con↗

arXiv2000

Jahn-Teller, Charge and Magnetic Ordering in half-doped Manganese Oxides

The phase diagram of half-doped manganite systems of formula A_{0.5}A'_{0.5}MnO_3 is investigated within a single-orbital model incorporating magnetic double-exchange and superexchange, together with intersite Coulomb and electron-lattice interactions. Strong Jahn-Teller and breathing mode deformations compete together and result in shear lattice deformations. The latters stabilize the charge-ordered CE-type phase, which undergo first-order transitions with temperature or magnetic field to either Ferromagnetic metallic or Paramagnetic insulating phases. An essential feature is the self-consistent screening of Coulomb and electron-phonon interactions in the ferromagnetic phase.

cond-mat.str-el↗

arXiv2000

Two-gap model for underdoped cuprate superconductors

Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface respectively. We find that a gap forms near the M points resulting into incoherent pairing due to strong fluctuations. Instead the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).

cond-mat.supr-con↗

arXiv2000

Stabilization of A-type layered antiferromagnetic phase in LaMnO_3 by cooperative Jahn-Teller deformations

It is shown that the layered antiferromagnetic order in stoechiometric LaMnO_3 cannot be understood purely from electronic interactions. On the contrary, it mainly results from strong cooperative Jahn-Teller deformations. Those involve a compression of the Mn-O octahedron along the c-axis (mode Q_3 < 0), while alternate Jahn-Teller deformations occur in the ab-plane (mode Q_2). These deformations stabilize a certain type of orbital ordering. The resulting superexchange couplings are calculated by exact diagonalization, taking into account both e_g and t_{2g} orbitals. The main result is that antiferromagnetic (ferromagnetic) coupling along the c-direction (ab-planes) can be understood only if the Jahn-Teller energy is much larger than the superexchange couplings, which is consistent with experiments. This mechanism contrasts with that based on weak Jahn-Teller coupling which instead predicts elongation along the c-axis (Q_3 > 0). The crucial role of the deformation anisotropy Q_2/Q_3 is also emphasized.

cond-mat.str-el↗

arXiv2000

The physics of the stripe quantum critical point in the superconducting cuprates

We elaborate on several observable consequences of the Quantum-Critical-Point scenario. In particular we show that the strong k-dependent scattering of the quasiparticles with the quasi-critical charge and spin fluctuations reproduces the main features of the low-energy spectral weights and of the observed Fermi surfaces. In the underdoped cuprates the attractive k-dependent charge scattering drives the formation of the pseudogap at the M points below the crossover temperature T^*. In this context we discuss models for pseudogap formation with relevant scattering in the particle-particle and particle-hole channels. The experimental consequences for the pair-fluctuation and for the pseudogap behavior are investigated.

cond-mat.supr-con↗

arXiv2000

Stripe ordering and two-gap model for underdoped cuprates

The evidence of edge-gaps around the M-points in the metallic state of underdoped cuprates has triggered a very active debate on their origin. We first consider the possibility that this spectroscopic feature results from a quasi-static charge ordering taking place in the underdoped regime. It comes out that to explain the coexistence of gaps and arcs on the Fermi surface the charge modulation should be in an eggbox form. In the lack of evidences for that, we then investigate the local pairing induced by charge-stripe fluctuations. A proper description of the strong anisotropy of both the interactions and the Fermi velocities requires a two-gap model for pairing. We find that a gap due to incoherent pairing forms near the M-points, while coherence is established by the stiffness of the pairing near the nodal points. The model allows for a continuos evolution from a pure BCS pairing (over- and optimally doped regime) to a mixed boson-fermion model (heavily underdoped regime).

cond-mat.supr-con↗

arXiv1999

Stripe formation in electron-doped cuprates

We investigate the formation of charge domain walls in an electron-doped extended Hubbard model for the superconducting cuprates. Within an unrestricted Hartree-Fock approach, extended by the introduction of slave-bosons to obtain a more proper treatment of strong correlations, we demonstrate the occurrence of stripes in the (1,1) and (1,-1) directions having one doped electron per stripe site. The different filling, direction and width of these electron-doped stripes with respect to those obtained in the hole-doped systems have interesting observable consequences, which are discussed.

cond-mat.supr-con↗

arXiv1999

Charge and spin inhomogeneity as a key to the physics of the high Tc cuprates

We present a coherent scenario for the physics of cuprate superconductors, which is based on a charge-driven inhomogeneity, i.e. the ``stripe phase''. We show that spin and charge critical fluctuations near the stripe instability of strongly correlated electron systems provide an effective interaction between the quasiparticles, which is strongly momentum, frequency, temperature and doping dependent. This accounts for the various phenomena occurring in the overdoped, optimally and underdoped regimes both for the normal and the superconductive phase.

cond-mat.supr-con↗

arXiv1999

Kosterlitz-Thouless vs Ginzburg-Landau description of 2D superconducting fluctuations

We evaluate the charge and spin susceptibilities of the 2D attractive Hubbard model and we compare our results with Montecarlo simulations on the same model. We discuss the possibility to include topological Kosterlitz-Thouless superconducting fluctuations in a standard perturbative approach substituting in the fluctuation propagator the Ginzburg-Landau correlation length with the Kosterlitz-Thouless correlation length.

cond-mat.supr-con↗

arXiv1999

Spectral properties of incommensurate charge-density wave systems

The concept of frustrated phase separation is applied to investigate its consequences for the electronic structure of the high T_c cuprates. The resulting incommensurate charge density wave (CDW) scattering is most effective in creating local gaps in k-space when the scattering vector connects states with equal energy. Starting from an open Fermi surface we find that the resulting CDW is oriented along the (10)- and (or) (01)-direction which allows for a purely one-dimensional or a two-dimensional ``eggbox type'' charge modulation. In both cases the van Hove singularities are substantially enhanced, and the spectral weight of Fermi surface states near the M-points, tends to be suppressed. Remarkably, a leading edge gap arises near these points, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless. We discuss our results with repect to possible consequences for photoemission experiments.

cond-mat.supr-con↗

arXiv1998

On localization effects in underdoped cuprates

We comment on transport experiments in underdoped LaSrCuO in the non-superconducting phase. The temperature dependence of the resistance strongly resembles what is expected from standard localization theory. However this theory fails, when comparing with experiments in more detail.

cond-mat.str-el↗

arXiv1998

The Stripe-Phase Quantum-Critical-Point Scenario for High-Tc Superconductors

A summary is given of the main outcomes of the quantum-critical-point scenario for high-$T_c$ superconductors, developed in the last few years by the Rome group. Phase separation, which commonly occurs in strongly correlated electronic systems, turns into a stripe instability when Coulomb interaction is taken into account. The stripe phase continuously connects the high doping regime, dominated by charge degrees of freedom to the low doping regime where spin degrees of freedom are most relevant. Dynamical stripe fluctuations enslave antiferromagnetic fluctuations at high doping. Critical fluctuations near the stripe instability mediate a singular interaction between quasiparticles, which is responsible for the non-Fermi liquid behavior in the metallic phase and for the Cooper pairing with d-wave symmetry in the superconducting phase.

cond-mat.supr-con↗