arXiv subjects

C. Di Castro

Publications and source records attributed to C. Di Castro.

62 records · Page 4Linked to original sources

arXiv1997

Comment on ``Insulator-to-Metal Crossover in the Normal State of $La_{2-x}Sr_xCuO_4$ Near Optimum Doping''

We underline and comment some features of the data obtained in the beautiful experiment by G. S. Boebinger, Y. Ando, A. Passner, T. Kimura, M. Okuya, J. Shimoyama, K. Kishio, K. Tamasaku, N. Ichikawa, and S. Uchida, Phys. Rev. Lett. 77, 5417 (1996). While in our general interpretation we agree with the authors that ``It is possible that charge ordering could underlie the unusual insulating behavior'' observed ``in all underdoped LSCO samples'', we further notice that some specific features of the data are most naturally interpreted in the light of our proposal for an incommensurate charge-density-wave quantum-critical-point scenario.

cond-mat.supr-con↗

arXiv1997

Infrared Behavior of Interacting Bosons at Zero Temperature

We exploit the symmetries associated with the stability of the superfluid phase to solve the long-standing problem of interacting bosons in the presence of a condensate at zero temperature. Implementation of these symmetries poses strong conditions on the renormalizations that heal the singularities of perturbation theory. The renormalized theory gives: For d>3 the Bogoliubov quasiparticles as an exact result; for 1<d<=3 a nontrivial solution with the exact exponent for the singular longitudinal correlation function, with phonons again as low-lying excitations.

cond-mat↗

arXiv1997

Non-Fermi-liquid behavior and d-wave superconductivity near the charge-density-wave quantum critical point

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 non-trivial 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.

cond-mat.supr-con↗

arXiv1997

Fermi Systems with Strong Forward Scattering

We review the theory of interacting Fermi systems whose low-energy physics is governed by forward scattering, i.e. scattering processes generated by effective interactions with small momentum transfers. These systems include Fermi liquids as well as several important non-Fermi liquid phases: one-dimensional Luttinger liquids, systems with long-range interactions, and fermions coupled to a gauge field. We report results for the critical dimensions separating different "universality classes", and discuss the behavior of physical quantities as the momentum distribution function, the single-particle propagator and low-energy response functions in each class. The renormalization group for Fermi systems will be reviewed and applied as a link between microscopic models and effective low-energy theories. Particular attention is payed to conservation laws, which constrain any effective low-energy theory of interacting Fermi systems. In scattering processes with small momentum transfers the velocity of each scattering particle is (almost) conserved. This asymptotic conservation law leads to non-trivial cancellations of Feynman diagrams and other simplifications, making thus possible a non-perturbative treatment of forward scattering via Ward identities or bosonization techniques.

cond-mat↗

arXiv1996

d-wave superconductivity near charge instabilities

We investigate the symmetry of the superconducting order parameter in the proximity of a phase-separation or of an incommensurate charge-density-wave instability. The attractive effective interaction at small or intermediate transferred momenta is singular near the instability. This strongly $q$-dependent interaction, together with a residual local repulsion between the quasiparticles and an enhanced density of states for band structures appropriate for the high temperature superconducting oxides, strongly favors the formation of $d$-wave superconductivity. The relative stability with respect to superconductivity in the $s$-wave channel is discussed in detail, finding this latter hardly realized in the above conditions. The superconducting temperature is mostly determined by the closeness to the quantum critical point associated to the charge instability and displays a stronger dependence on doping with respect to the simple proximity to a Van Hove singularity. The relevance of this scenario and the generic agreement of the resulting phase diagram with the properties displayed by high temperature superconducting oxides is discussed.

cond-mat↗

arXiv1996

Charge-density-waves and superconductivity as an alternative to phase separation in the infinite-U Hubbard-Holstein model

We investigate the density instabilities present in the infinite-U Hubbard-Holstein model both at zero and finite momenta as well as the occurrence of Cooper instabilities with a specific emphasis on the role of long-range Coulomb forces. In carrying out this analysis a special attention is devoted to the effects of the strong local $e$-$e$ interaction on the $e$-$ph$ coupling and particularly to both the static and dynamic screening processes dressing this coupling. We also clarify under which conditions in strongly correlated electron systems a weak additional interaction, e.g. a phonon-mediated attraction, can give rise to a charge instability. In the presence of long-range Coulomb forces, the frustrated phase separation leads to the formation of incommensurate charge density waves. These instabilities, in turn, lead to strong residual scattering processes between quasiparticles and to superconductivity, thus providing an interesting clue to the interpretation of the physics of the copper oxides.

cond-mat↗

arXiv1995

Singular quasiparticle scattering in the proximity of charge instabilities

We analyze the behavior of the dynamic scattering amplitude between Fermi liquid quasiparticles at the Fermi surface in the proximity of a charge instability, which may occur in the high temperature superconducting cuprates. Within the infinite-U Hubbard-Holstein model in the slave-boson large-N technique we find that, in the absence of long-range Coulomb forces the scattering amplitude is strongly singular at zero momentum transfer close to the phase separation instability and it has the same form provided by gauge-field theories. In the presence of long-range Coulomb forces the charge instability occurs at finite wavevectors and concomitantly the scattering is still singular but anisotropic. Nevertheless it remains strong over extended regions of the momentum space. In both cases we show how normal state properties are largely affected by this scattering.

cond-mat↗

arXiv1995

Electron-phonon coupling close to a metal-insulator transition in one dimension

We consider a one-dimensional system of electrons interacting via a short-range repulsion and coupled to phonons close to the metal-insulator transition at half filling. We argue that the metal-insulator transition can be described as a standard one dimensional incommensurate to commensurate transition, even if the electronic system is coupled to the lattice distortion. By making use of known results for this transition, we prove that low-momentum phonons do not play any relevant role close to half-filling, unless their coupling to the electrons is large in comparison with the other energy scales present in the problem. In other words the effective strength of the low-momentum transferred electron-phonon coupling does not increase close to the metal-insulator transition, even though the effective velocity of the mobile carriers is strongly diminished.

cond-mat↗