396 research outputs found
Quantum critical fluctuations in disordered d-wave superconductors
Quasiparticles in the cuprates appear to be subject to anomalously strong
inelastic damping mechanisms. To explain the phenomenon, Sachdev and
collaborators recently proposed to couple the system to a critically
fluctuating order parameter mode of either id_{xy}- or is-symmetry. Motivated
by the observation that the energies relevant for the dynamics of this mode are
comparable to the scattering rate induced by even moderate impurity
concentrations, we here generalize the approach to the presence of static
disorder. In the id-case, we find that the coupling to disorder renders the
order parameter dynamics diffusive but otherwise leaves much of the
phenomenology observed in the clean case intact. In contrast, the interplay of
impurity scattering and order parameter fluctuations of is-symmetry entails the
formation of a secondary superconductor transition, with a critical temperature
exponentially sensitive to the disorder concentration.Comment: 4 pages, 2 figures include
Universal low-temperature behavior of the CePd_{1-x}Rh_x ferromagnet
The heavy-fermion metal CePd_{1-x}Rh_x evolves from ferromagnetism at x=0 to
a non-magnetic state at some critical concentration x_c. Utilizing the
quasiparticle picture and the concept of fermion condensation quantum phase
transition (FCQPT), we address the question about non-Fermi liquid (NFL)
behavior of ferromagnet CePd_{1-x}Rh_x and show that it coincides with that of
both antiferromagnet YbRh_2(Si_{0.95}Ge_{0.05})_2 and paramagnet CeRu_2Si_2 and
CeNi_2Ge_2. We conclude that the NFL behavior being independent of the
peculiarities of specific alloy, is universal, while numerous quantum critical
points assumed to be responsible for the NFL behavior of different HF metals
can be well reduced to the only quantum critical point related to FCQPT.Comment: 6 pages, 7 figure
Superconducting charge-ordered states in cuprates
Motivated by recent neutron scattering and scanning tunneling microscopy
(STM) experiments on cuprate superconductors, we discuss charge-ordered states,
in particular with two-dimensional charge modulation patterns, co-existing with
superconductivity. We extend previous studies of a large-N mean-field
formulation of the t-J model. In addition to bond-centered superconducting
stripe states at low doping, we find checkerboard-modulated superconducting
states which are favorable in an intermediate doping interval. We also analyze
the energy dependence of the Fourier component of the local density of states
at the ordering wavevector for several possible modulation patterns, and
compare with STM results.Comment: 5 pages, 4 figs; (v2) extended discussion; final version as publishe
Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment
In this paper, we review theoretical and experimental research on rare region
effects at quantum phase transitions in disordered itinerant electron systems.
After summarizing a few basic concepts about phase transitions in the presence
of quenched randomness, we introduce the idea of rare regions and discuss their
importance. We then analyze in detail the different phenomena that can arise at
magnetic quantum phase transitions in disordered metals, including quantum
Griffiths singularities, smeared phase transitions, and cluster-glass
formation. For each scenario, we discuss the resulting phase diagram and
summarize the behavior of various observables. We then review several recent
experiments that provide examples of these rare region phenomena. We conclude
by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative
Ising chain fixed, references added, v3: final version as publishe
Indications of Spin-Charge Separation at Short Distance and Stripe Formation in the Extended t-J Model on Ladders and Planes
The recently discussed tendency of holes to generate nontrivial spin
environments in the extended two-dimensional t-J model (G. Martins, R. Eder,
and E. Dagotto, Phys. Rev. B{\bf 60}, R3716 (1999)) is here investigated using
computational techniques applied to ladders with several number of legs. This
tendency is studied also with the help of analytic spin-polaron approaches
directly in two dimensions. Our main result is that the presence of robust
antiferromagnetic correlations between spins located at both sides of a hole
either along the x or y axis, observed before numerically on square clusters,
is also found using ladders, as well as applying techniques based on a
string-basis expansion. This so-called "across-the-hole" nontrivial structure
exists even in the two-leg spin-gapped ladder system, and leads to an effective
reduction in dimensionality and spin-charge separation at short-distances, with
a concomitant drastic reduction in the quasiparticle (QP) weight Z. In general,
it appears that holes tend to induce one-dimensional-like spin arrangements to
improve their mobility. Using ladders it is also shown that the very small
J/t0.1 regime of the standard t-J model may be more realistic than
anticipated in previous investigations, since such regime shares several
properties with those found in the extended model at realistic couplings.
Another goal of the present article is to provide additional information on the
recently discussed tendencies to stripe formation and spin incommensurability
reported for the extended t-J model.Comment: 14 pages, 21 figures, LateX, submited to Phys. Rev.
Spin orthogonality catastrophe in two-dimensional antiferromagnets and superconductors
We compute the spectral function of a spin S hole injected into a
two-dimensional antiferromagnet or superconductor in the vicinity of a magnetic
quantum critical point. We show that, near van Hove singularities, the problem
maps onto that of a static vacancy carrying excess spin S. The hole creation
operator is characterized by a new boundary anomalous dimension and a vanishing
quasiparticle residue at the critical point. We discuss possible relevance to
photoemission spectra of cuprate superconductors near the anti-nodal points.Comment: (v1) 4 pages, 2 figures; field theory afficionados - see also
cond-mat/0011233; (v2) added figure of Monte Carlo data; (v3) corrected typo
Spin ordering quantum transitions of superconductors in a magnetic field
We argue that recent neutron scattering measurements by Lake et. al. (Science
291, 1759 (2001)) of the spin excitation spectrum of LSCO in a magnetic field
can be understood in terms of proximity to a phase with co-existing
superconductivity and spin density wave order. We present a general theory for
such quantum transitions, and argue that their low energy spin fluctuations are
controlled by a singular correction from the superflow kinetic energy, acting
in the region outside the vortex cores. We propose numerous experimental tests
of our theory.Comment: 4 pages, 3 eps fig
Anomalous criticality near semimetal-to-superfluid quantum phase transition in a two-dimensional Dirac cone model
We analyze the scaling behavior at and near a quantum critical point
separating a semimetallic from a superfluid phase. To this end we compute the
renormalization group flow for a model of attractively interacting electrons
with a linear dispersion around a single Dirac point. We study both ground
state and finite temperature properties. In two dimensions, the electrons and
the order parameter fluctuations exhibit power-law scaling with anomalous
scaling dimensions. The quasi-particle weight and the Fermi velocity vanish at
the quantum critical point. The order parameter correlation length turns out to
be infinite everywhere in the semimetallic ground state.Comment: 8 pages, 2 figures, typos correcte
Local versus Nonlocal Order Parameter Field Theories for Quantum Phase Transitions
General conditions are formulated that allow to determine which quantum phase
transitions in itinerant electron systems can be described by a local
Landau-Ginzburg-Wilson or LGW theory solely in terms of the order parameter. A
crucial question is the degree to which the order parameter fluctuations couple
to other soft modes. Three general classes of zero-wavenumber order parameters,
in the particle-hole spin-singlet and spin-triplet channels, and in the
particle-particle channel, respectively, are considered. It is shown that the
particle-hole spin-singlet class does allow for a local LGW theory, while the
other two classes do not. The implications of this result for the critical
behavior at various quantum phase transitions are discussed, as is the
connection with nonanalyticities in the wavenumber dependence of order
parameter susceptibilities in the disordered phase.Comment: 9 pp., LaTeX, no figs, final version as publishe
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