We describe the spectral properties of underdoped cuprates as resulting from a momentumdependent ... more We describe the spectral properties of underdoped cuprates as resulting from a momentumdependent pseudogap in the normal state spectrum. Such a model accounts, within a BCS approach, for the doping dependence of the critical temperature and for the two-parameter leading-edge shift observed in the cuprates. By introducing a phenomenological temperature dependence of the pseudogap, which finds a natural interpretation within the stripe quantum-critical-point scenario for high-Tc superconductors, we reproduce also the Tc − T * bifurcation near optimum doping. Finally, we briefly discuss the different role of the gap and the pseudogap in determining the spectral and thermodynamical properties of the model at low temperatures.
We have analyzed the band structure of a CuOz plane including two copper orbitals {d 2 2 and X d»... more We have analyzed the band structure of a CuOz plane including two copper orbitals {d 2 2 and X d») and two oxygen orbitals {p", p~) in a tight-binding approximation. A Hubbard interaction between copper d holes has been considered with use of a slave-boson technique, while oxygen and copper holes interact via a nearest-neighbors Coulombic repulsion treated in the Hartree-Fock approximation. The resulting band structure opens the way to a comparison with the experiments and to some theoretical implications on the pairing mechanism of high-T, superconductors. The electronic structure of the high-temperature superconductors has been a controversial issue ever since their discovery by Bednorz and Muller. ' There is a general agreement in the scientific community in assigning a major role to the Cu02 layers in determining the normal and superconducting properties of these materials. At zero doping the Cu02 system has one hole per unit cell residing on Cu sites with d, 2(b,) symmetry. The additionx al holes introduced by doping in the superconducting samples reside on oxygen sites because of the large Hubbard repulsion on Cu sites Ud (of the order of 8-10 eV).
The strange metal behavior, usually characterized by a linear-in-temperature (T) resistivity, is ... more The strange metal behavior, usually characterized by a linear-in-temperature (T) resistivity, is a still unsolved mystery in solid-state physics. It is often associated with the proximity to a quantum critical point (a second order transition at temperature T=0, leading to a broken symmetry phase) focusing on the related divergent order parameter correlation length. Here, we propose a paradigmatic shift, focusing on a divergent characteristic time scale due to a divergent dissipation acting on the fluctuating critical modes while their correlation length stays finite. To achieve a divergent dissipation, we propose a mechanism based on the coupling between a local order parameter fluctuation and electron density diffusive modes that accounts both for the linear-in-T resistivity and for the logarithmic specific heat versus temperature ratio CV/T∼log(1/T), down to low temperatures.
Anomalous metallic properties are often observed in the proximity of quantum critical points, wit... more Anomalous metallic properties are often observed in the proximity of quantum critical points, with violation of the Fermi Liquid paradigm. We propose a scenario where, near the quantum critical point, dynamical fluctuations of the order parameter with finite correlation length mediate a nearly isotropic scattering among the quasiparticles over the entire Fermi surface. This scattering produces an anomalous metallic behavior, which is extended to the lowest temperatures by an increase of the damping of the fluctuations. We phenomenologically identify one single parameter ruling this increasing damping when the temperature decreases, accounting for both the linear-in-temperature resistivity and the seemingly divergent specific heat observed, e.g., in high-temperature superconducting cuprates and some heavy-fermion metals
I present here a short memory of my scientific contacts with K.A. Müller starting from the Interl... more I present here a short memory of my scientific contacts with K.A. Müller starting from the Interlaken Conference (1988), Erice (1992 and 1993), and Cottbus (1994) on the initial studies on phase separation (PS) and charge inhomogeneity in cuprates carried out against the view of the majority of the scientific community at that time. Going over the years and passing through the charge density wave (CDW) instability of the correlated Fermi liquid (FL) and to the consequences of charge density fluctuations (CDFs), I end with a presentation of my current research activity on CDWs and the related two-dimensional charge density fluctuations (2D-CDFs). A scenario follows of the physics of cuprates, which includes the solution of the decades-long problem of the strange metal (SM) state.
A scenario is presented, in which the presence of a quantum critical point due to formation of in... more A scenario is presented, in which the presence of a quantum critical point due to formation of incommensurate charge density waves accounts for the basic features of the high temperature superconducting cuprates, both in the normal and in the superconducting states. Specifically, the singular interaction arising close to this charge-driven quantum critical point gives rise to the non-Fermi liquid behavior universally found at optimal doping. This interaction is also responsible for d-wave Cooper pair formation with a superconducting critical temperature strongly dependent on doping in the overdoped region and with a plateau in the optimally doped region. In the underdoped region a temperature dependent pairing potential favors local pair formation without superconducting coherence, with a peculiar temperature dependence of the pseudogap and a nontrivial relation between the pairing temperature and the gap itself. This last property is in good qualitative agreement with so far unexplained features of the experiments.
We investigate the formation of partially filled domain walls in the two-dimensional Hubbard mode... more We investigate the formation of partially filled domain walls in the two-dimensional Hubbard model in the presence of long-range interaction. Using an unrestricted Gutzwiller variational approach we show that: i) the strong local interaction favors charge segregation in stripe domain walls; ii) The long-range interaction favors the formation of half-filled vertical stripes with a period doubling due to the charge and a period quadrupling due to the spins along the wall. Our results show that, besides the underlying lattice structure, also the electronic interactions can contribute to determine the different domain wall textures in Nd doped copper oxides and nickel oxides.
The recent solution by Finkel shtein of a model of two-dimensional interacting disordered electro... more The recent solution by Finkel shtein of a model of two-dimensional interacting disordered electrons is reconsidered. The scaling equation requires a small modification, with the result that the conductivity remains finite down to zero temperature instead of going to infinity as envisaged by Finkel'shtein. Strong divergence in the spin susceptibility and a vanishing of the spin-diffusion constant are found at low temperatures. We also discuss the possibility that the spin fluctuation may lead to crossover to the high magnetic field or singlet-only universality classes, restoring the metal-insulator transition.
We analyze interacting lattice fermions with anisotropic hopping amplitudes. Calculating the line... more We analyze interacting lattice fermions with anisotropic hopping amplitudes. Calculating the linear response of the momentum distribution function, we find that one-dimensional Luttinger liquid phases are unstable with respect to arbitrarily small transverse hopping, even if the interaction is strong. This instability is expected to signal the formation of states with broken symmetry in the strongly anisotropic regime.
We study a four-band model for the iron oxypnictides, in which the superconducting properties are... more We study a four-band model for the iron oxypnictides, in which the superconducting properties are assumed to be determined by the interband coupling between hole-like and electron-like Fermi sheets. We show that reasonable parameters can account for the angle-resolved photoemission spectra showing multiple gaps in Ba1−xKxFe2As2, and for the temperature dependence of the superfluid density. At the same time, the zero-temperature value of the superfluid density shows a conventional scaling with the number of carriers.
ABSTRACT Phase separation (PS) is a generic feature of electron systems in the presence of strong... more ABSTRACT Phase separation (PS) is a generic feature of electron systems in the presence of strong interaction: When the local electron-electron (e-e) repulsion drastically reduces the electron kinetic energy, the addition of short-range interactions drives the system to a PS instability in the absence of long-range Coulomb repulsion. When this latter is present, incommensurate charge-density-waves replace the PS region. The phenomenon of PS has been shown to occur irrespective to the nature of the short range interaction and it is present in models with magnetic, nearest-neighbor Coulombic or electron-lattice interactions. Moreover, Cooper instabilities take place close to the PS regions, as a result of the attractive effective interaction, which eventually lead to PS. Superconductivity (SC) also acts to (partially) stabilize the system reducing the regions of the phase-diagram where PS is present.
We describe the spectral properties of underdoped cuprates as resulting from a momentumdependent ... more We describe the spectral properties of underdoped cuprates as resulting from a momentumdependent pseudogap in the normal state spectrum. Such a model accounts, within a BCS approach, for the doping dependence of the critical temperature and for the two-parameter leading-edge shift observed in the cuprates. By introducing a phenomenological temperature dependence of the pseudogap, which finds a natural interpretation within the stripe quantum-critical-point scenario for high-Tc superconductors, we reproduce also the Tc − T * bifurcation near optimum doping. Finally, we briefly discuss the different role of the gap and the pseudogap in determining the spectral and thermodynamical properties of the model at low temperatures.
We have analyzed the band structure of a CuOz plane including two copper orbitals {d 2 2 and X d»... more We have analyzed the band structure of a CuOz plane including two copper orbitals {d 2 2 and X d») and two oxygen orbitals {p", p~) in a tight-binding approximation. A Hubbard interaction between copper d holes has been considered with use of a slave-boson technique, while oxygen and copper holes interact via a nearest-neighbors Coulombic repulsion treated in the Hartree-Fock approximation. The resulting band structure opens the way to a comparison with the experiments and to some theoretical implications on the pairing mechanism of high-T, superconductors. The electronic structure of the high-temperature superconductors has been a controversial issue ever since their discovery by Bednorz and Muller. ' There is a general agreement in the scientific community in assigning a major role to the Cu02 layers in determining the normal and superconducting properties of these materials. At zero doping the Cu02 system has one hole per unit cell residing on Cu sites with d, 2(b,) symmetry. The additionx al holes introduced by doping in the superconducting samples reside on oxygen sites because of the large Hubbard repulsion on Cu sites Ud (of the order of 8-10 eV).
The strange metal behavior, usually characterized by a linear-in-temperature (T) resistivity, is ... more The strange metal behavior, usually characterized by a linear-in-temperature (T) resistivity, is a still unsolved mystery in solid-state physics. It is often associated with the proximity to a quantum critical point (a second order transition at temperature T=0, leading to a broken symmetry phase) focusing on the related divergent order parameter correlation length. Here, we propose a paradigmatic shift, focusing on a divergent characteristic time scale due to a divergent dissipation acting on the fluctuating critical modes while their correlation length stays finite. To achieve a divergent dissipation, we propose a mechanism based on the coupling between a local order parameter fluctuation and electron density diffusive modes that accounts both for the linear-in-T resistivity and for the logarithmic specific heat versus temperature ratio CV/T∼log(1/T), down to low temperatures.
Anomalous metallic properties are often observed in the proximity of quantum critical points, wit... more Anomalous metallic properties are often observed in the proximity of quantum critical points, with violation of the Fermi Liquid paradigm. We propose a scenario where, near the quantum critical point, dynamical fluctuations of the order parameter with finite correlation length mediate a nearly isotropic scattering among the quasiparticles over the entire Fermi surface. This scattering produces an anomalous metallic behavior, which is extended to the lowest temperatures by an increase of the damping of the fluctuations. We phenomenologically identify one single parameter ruling this increasing damping when the temperature decreases, accounting for both the linear-in-temperature resistivity and the seemingly divergent specific heat observed, e.g., in high-temperature superconducting cuprates and some heavy-fermion metals
I present here a short memory of my scientific contacts with K.A. Müller starting from the Interl... more I present here a short memory of my scientific contacts with K.A. Müller starting from the Interlaken Conference (1988), Erice (1992 and 1993), and Cottbus (1994) on the initial studies on phase separation (PS) and charge inhomogeneity in cuprates carried out against the view of the majority of the scientific community at that time. Going over the years and passing through the charge density wave (CDW) instability of the correlated Fermi liquid (FL) and to the consequences of charge density fluctuations (CDFs), I end with a presentation of my current research activity on CDWs and the related two-dimensional charge density fluctuations (2D-CDFs). A scenario follows of the physics of cuprates, which includes the solution of the decades-long problem of the strange metal (SM) state.
A scenario is presented, in which the presence of a quantum critical point due to formation of in... more A scenario is presented, in which the presence of a quantum critical point due to formation of incommensurate charge density waves accounts for the basic features of the high temperature superconducting cuprates, both in the normal and in the superconducting states. Specifically, the singular interaction arising close to this charge-driven quantum critical point gives rise to the non-Fermi liquid behavior universally found at optimal doping. This interaction is also responsible for d-wave Cooper pair formation with a superconducting critical temperature strongly dependent on doping in the overdoped region and with a plateau in the optimally doped region. In the underdoped region a temperature dependent pairing potential favors local pair formation without superconducting coherence, with a peculiar temperature dependence of the pseudogap and a nontrivial relation between the pairing temperature and the gap itself. This last property is in good qualitative agreement with so far unexplained features of the experiments.
We investigate the formation of partially filled domain walls in the two-dimensional Hubbard mode... more We investigate the formation of partially filled domain walls in the two-dimensional Hubbard model in the presence of long-range interaction. Using an unrestricted Gutzwiller variational approach we show that: i) the strong local interaction favors charge segregation in stripe domain walls; ii) The long-range interaction favors the formation of half-filled vertical stripes with a period doubling due to the charge and a period quadrupling due to the spins along the wall. Our results show that, besides the underlying lattice structure, also the electronic interactions can contribute to determine the different domain wall textures in Nd doped copper oxides and nickel oxides.
The recent solution by Finkel shtein of a model of two-dimensional interacting disordered electro... more The recent solution by Finkel shtein of a model of two-dimensional interacting disordered electrons is reconsidered. The scaling equation requires a small modification, with the result that the conductivity remains finite down to zero temperature instead of going to infinity as envisaged by Finkel'shtein. Strong divergence in the spin susceptibility and a vanishing of the spin-diffusion constant are found at low temperatures. We also discuss the possibility that the spin fluctuation may lead to crossover to the high magnetic field or singlet-only universality classes, restoring the metal-insulator transition.
We analyze interacting lattice fermions with anisotropic hopping amplitudes. Calculating the line... more We analyze interacting lattice fermions with anisotropic hopping amplitudes. Calculating the linear response of the momentum distribution function, we find that one-dimensional Luttinger liquid phases are unstable with respect to arbitrarily small transverse hopping, even if the interaction is strong. This instability is expected to signal the formation of states with broken symmetry in the strongly anisotropic regime.
We study a four-band model for the iron oxypnictides, in which the superconducting properties are... more We study a four-band model for the iron oxypnictides, in which the superconducting properties are assumed to be determined by the interband coupling between hole-like and electron-like Fermi sheets. We show that reasonable parameters can account for the angle-resolved photoemission spectra showing multiple gaps in Ba1−xKxFe2As2, and for the temperature dependence of the superfluid density. At the same time, the zero-temperature value of the superfluid density shows a conventional scaling with the number of carriers.
ABSTRACT Phase separation (PS) is a generic feature of electron systems in the presence of strong... more ABSTRACT Phase separation (PS) is a generic feature of electron systems in the presence of strong interaction: When the local electron-electron (e-e) repulsion drastically reduces the electron kinetic energy, the addition of short-range interactions drives the system to a PS instability in the absence of long-range Coulomb repulsion. When this latter is present, incommensurate charge-density-waves replace the PS region. The phenomenon of PS has been shown to occur irrespective to the nature of the short range interaction and it is present in models with magnetic, nearest-neighbor Coulombic or electron-lattice interactions. Moreover, Cooper instabilities take place close to the PS regions, as a result of the attractive effective interaction, which eventually lead to PS. Superconductivity (SC) also acts to (partially) stabilize the system reducing the regions of the phase-diagram where PS is present.
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Papers by C. Di Castro