4,328 research outputs found
Spectroscopic evidences of quantum critical charge fluctuations in cuprates
We calculate the optical conductivity in a clean system of quasiparticles
coupled to charge-ordering collective modes. The absorption induced by these
modes may produce an anomalous frequency and temperature dependence of
low-energy optical absorption in some cuprates. However, the coupling with
lattice degrees of freedom introduces a non-universal energy scale leading to
scaling violation in low-temperature optical conductivity.Comment: Proceedings of M2S 2006. To appear in Physica
Antiferromagnetic integer-spin chains in a staggered magnetic field: approaching the thermodynamic limit through the infinite-size DMRG
We investigate the behavior of antiferromagnetic integer-spin chains in a
staggered magnetic field, by means of the density-matrix renormalization group,
carefully addressing the role of finite-size effects within the Haldane phase
at small fields. In the case of spin S=2, we determine the dependence of the
groundstate energy and magnetization on the external field, in the
thermodynamic limit, and show how the peculiar finite-size behavior can be
connected with the crossover in the groundstate from a spin liquid to a
polarized N\'eel state.Comment: 7 pages, 5 figure
Law without law or "just" limit theorems?
About 35 years ago Wheeler introduced the motto `law without law' to
highlight the possibility that (at least a part of) Physics may be understood
only following {\em regularity principles} and few relevant facts, rather than
relying on a treatment in terms of fundamental theories. Such a proposal can be
seen as part of a more general attempt (including the maximum entropy approach)
summarized by the slogan `it from bit', which privileges the information as the
basic ingredient. Apparently it seems that it is possible to obtain, without
the use of physical laws, some important results in an easy way, for instance,
the probability distribution of the canonical ensemble. In this paper we will
present a general discussion on those ideas of Wheeler's that originated the
motto `law without law'. In particular we will show how the claimed simplicity
is only apparent and it is rather easy to produce wrong results. We will show
that it is possible to obtain some of the results treated by Wheeler in the
realm of the statistical mechanics, using precise assumptions and nontrivial
results of probability theory, mainly concerning ergodicity and limit theorems.Comment: 9 pages, 3 figure
Luttinger liquid, singular interaction and quantum criticality in cuprate materials
With particular reference to the role of the renormalization group approach
and Ward identities, we start by recalling some old features of the
one-dimensional Luttinger liquid as the prototype of non-Fermi-liquid behavior.
Its dimensional crossover to the Landau normal Fermi liquid implies that a
non-Fermi liquid, as, e.g., the normal phase of the cuprate high temperature
superconductors, can be maintained in d>1, only in the presence of a
sufficiently singular effective interaction among the charge carriers. This is
the case when, nearby an instability, the interaction is mediated by
fluctuations. We are then led to introduce the specific case of
superconductivity in cuprates as an example of avoided quantum criticality. We
will disentangle the fluctuations which act as mediators of singular
electron-electron interaction, enlightening the possible order competing with
superconductivity and a mechanism for the non-Fermi-liquid behavior of the
metallic phase. This paper is not meant to be a comprehensive review. Many
important contributions will not be considered. We will also avoid using
extensive technicalities and making full calculations for which we refer to the
original papers and to the many good available reviews. We will here only
follow one line of reasoning which guided our research activity in this field.Comment: 23 pages, 10 figure
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.Comment: 4 pages, 2 enclosed figures, Proceedings of M2S-HTS
Intrinsic instability of electronic interfaces with strong Rashba coupling
We consider a model for the two-dimensional electron gas formed at the
interface of oxide heterostructures, which includes a Rashba spin-orbit
coupling proportional to the electric field perpendicular to the interface.
Based on the standard mechanism of polarity catastrophe, we assume that the
electric field is proportional to the electron density. Under these simple and
general assumptions, we show that a phase separation instability occurs for
realistic values of the spin-orbit coupling and of the band parameters. This
could provide an intrinsic mechanism for the recently observed inhomogeneous
phases at the LaAlO_3/SrTiO_3 or LaTiO_3/SrTiO_3 interfaces.Comment: 5 pages, 4 figure
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