1,206 research outputs found
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
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
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
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
Renormalization group and Ward identities in quantum liquid phases and in unconventional critical phenomena
By reviewing the application of the renormalization group to different
theoretical problems, we emphasize the role played by the general symmetry
properties in identifying the relevant running variables describing the
behavior of a given physical system. In particular, we show how the constraints
due to the Ward identities, which implement the conservation laws associated
with the various symmetries, help to minimize the number of independent running
variables. This use of the Ward identities is examined both in the case of a
stable phase and of a critical phenomenon. In the first case we consider the
problems of interacting fermions and bosons. In one dimension general and
specific Ward identities are sufficient to show the non-Fermi-liquid character
of the interacting fermion system, and also allow to describe the crossover to
a Fermi liquid above one dimension. This crossover is examined both in the
absence and presence of singular interaction. On the other hand, in the case of
interacting bosons in the superfluid phase, the implementation of the Ward
identities provides the asymptotically exact description of the acoustic
low-energy excitation spectrum, and clarifies the subtle mechanism of how this
is realized below and above three dimensions. As a critical phenomenon, we
discuss the disorder-driven metal-insulator transition in a disordered
interacting Fermi system. In this case, through the use of Ward identities, one
is able to associate all the disorder effects to renormalizations of the Landau
parameters. As a consequence, the occurrence of a metal-insulator transition is
described as a critical breakdown of a Fermi liquid.Comment: 47 pages, 11 figure
Gap and pseudogap evolution within the charge-ordering scenario for superconducting cuprates
We describe the spectral properties of underdoped cuprates as resulting from
a momentum-dependent 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-T_c superconductors, we reproduce also
the T_c-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.Comment: 13 pages (EPY style), 7 enclosed figures, to appear on Eur. Phys. J.
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