129 research outputs found
Strong enhancement of drag and dissipation at the weak- to strong- coupling phase transition in a bi-layer system at a total Landau level filling nu=1
We consider a bi-layer electronic system at a total Landau level filling
factor nu =1, and focus on the transition from the regime of weak inter-layer
coupling to that of the strongly coupled (1,1,1) phase (or ''quantum Hall
ferromagnet''). Making the assumption that in the transition region the system
is made of puddles of the (1,1,1) phase embedded in a bulk of the weakly
coupled state, we show that the transition is accompanied by a strong increase
in longitudinal Coulomb drag, that reaches a maximum of approximately
. In that regime the longitudinal drag is increased with decreasing
temperature.Comment: four pages, one included figur
Coulomb Drag at the Onset of Anderson Insulators
It is shown that the Coulomb drag between two identical layers in the
Anderson insulting state indicates a striking difference between the Mott and
Efros-Shklovskii (ES) insulators. In the former, the trans-resistance
is monotonically increasing with the localization length ; in the latter,
the presence of a Coulomb gap leads to an opposite result: is enhanced
with a decreasing , with the same exponential factor as the single layer
resistivity. This distinction reflects the relatively pronounced role of
excited density fluctuations in the ES state, implied by the enhancement in the
rate of hopping processes at low frequencies. The magnitude of drag is
estimated for typical experimental parameters in the different cases. It is
concluded that a measurement of drag can be used to distinguish between
interacting and non-interacting insulating state.Comment: 15 pages, revte
Coulomb drag of Luttinger liquids and quantum-Hall edges
We study the transconductance for two coupled one-dimensional wires or edge
states described by Luttinger liquid models. The wires are assumed to interact
over a finite segment. We find for the interaction parameter that the
drag rate is finite at zero temperature, which cannot occur in a Fermi-liquid
system. The zero temperature drag is, however, cut off at low temperature due
to the finite length of the wires. We also consider edge states in the
fractional quantum Hall regime, and we suggest that the low temperature
enhancement of the drag effect might be seen in the fractional quantum Hall
regime.Comment: 5 pages, 2 figures; to appear in Phys. Rev. Let
Effects of proximity to an electronic topological transition on normal state transport properties of the high-Tc superconductors
Within the time dependent Ginzburg-Landau theory, the effects of the
superconducting fluctuations on the transport properties above the critical
temperature are characterized by a non-zero imaginary part of the relaxation
rate gamma of the order parameter. Here, we evaluate Im gamma for an
anisotropic dispersion relation typical of the high-Tc cuprate superconductors
(HTS), characterized by a proximity to an electronic topological transition
(ETT). We find that Im gamma abruptly changes sign at the ETT as a function of
doping, in agreement with the universal behavior of the HTS. We also find that
an increase of the in-plane anisotropy, as is given by a non-zero value of the
next-nearest to nearest hopping ratio r=t'/t, increases the value of | Im gamma
| close to the ETT, as well as its singular behavior at low temperature,
therefore enhancing the effect of superconducting fluctuations. Such a result
is in qualitative agreement with the available data for the excess Hall
conductivity for several cuprates and cuprate superlattices.Comment: to appear in Phys. Rev.
Missing 2k_F Response for Composite Fermions in Phonon Drag
The response of composite Fermions to large wavevector scattering has been
studied through phonon drag measurements. While the response retains
qualitative features of the electron system at zero magnetic field, notable
discrepancies develop as the system is varied from a half-filled Landau level
by changing density or field. These deviations, which appear to be inconsistent
with the current picture of composite Fermions, are absent if half-filling is
maintained while changing density. There remains, however, a clear deviation
from the temperature dependence anticipated for 2k_F scattering.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. Let
From Kondo Effect to Fermi Liquid
The Kondo effect has been playing an important role in strongly correlated
electon systems. The important point is that the magnetic impurity in metals is
a typical example of the Fermi liquid. In the system the local spin is
conserved in the ground state and continuity with respect to Coulomb repulsion
is satisfied. This nature is satisfied also in the periodic systems as far
as the systems remain as the Fermi liquid. This property of the Fermi liquid is
essential to understand the cuprate high-Tc superconductors (HTSC). On the
basis of the Fermi liquid theory we develop the transport theory such as the
resistivity and the Hall coefficient in strongly correlated electron systems,
such as HTSC, organic metals and heavy Fermion systems. The significant role of
the vertex corrections for total charge- and heat-currents on the transport
phenomena is explained. By taking the effect of the current vertex corrections
into account, various typical non-Fermi-liquid-like transport phenomena in
systems with strong magnetic and/or superconducting flucutations are explained
within the Fermi liquid theory.Comment: 14 pages, an article for the special edition of JPSJ "Kondo Effect --
40 Years after the Discovery
How to detect fluctuating order in the high-temperature superconductors
We discuss fluctuating order in a quantum disordered phase proximate to a
quantum critical point, with particular emphasis on fluctuating stripe order.
Optimal strategies for extracting information concerning such local order from
experiments are derived with emphasis on neutron scattering and scanning
tunneling microscopy. These ideas are tested by application to two model
systems - the exactly solvable one dimensional electron gas with an impurity,
and a weakly-interacting 2D electron gas. We extensively review experiments on
the cuprate high-temperature superconductors which can be analyzed using these
strategies. We adduce evidence that stripe correlations are widespread in the
cuprates. Finally, we compare and contrast the advantages of two limiting
perspectives on the high-temperature superconductor: weak coupling, in which
correlation effects are treated as a perturbation on an underlying metallic
(although renormalized) Fermi liquid state, and strong coupling, in which the
magnetism is associated with well defined localized spins, and stripes are
viewed as a form of micro-phase separation. We present quantitative indicators
that the latter view better accounts for the observed stripe phenomena in the
cuprates.Comment: 43 pages, 11 figures, submitted to RMP; extensively revised and
greatly improved text; one new figure, one new section, two new appendices
and more reference
Colossal thermomagnetic response in the exotic superconductor URu2Si2
When a superconductor is heated above its critical temperature ,
macroscopic coherence vanishes, leaving behind droplets of thermally
fluctuating Cooper pair. This superconducting fluctuation effect above
has been investigated for many decades and its influence on the transport,
thermoelectric and thermodynamic quantities in most superconductors is well
understood by the standard Gaussian fluctuation theories. The transverse
thermoelectric (Nernst) effect is particularly sensitive to the fluctuations,
and the large Nernst signal found in the pseudogap regime of the underdoped
high- cuprates has raised much debate on its connection to the origin of
superconductivity. Here we report on the observation of a colossal Nernst
signal due to the superconducting fluctuations in the heavy-fermion
superconductor URuSi. The Nernst coefficient is enhanced by as large as
one million times over the theoretically expected value within the standard
framework of superconducting fluctuations. This, for the first time in any
known material, results in a sizeable thermomagnetic figure of merit
approaching unity. Moreover, contrary to the conventional wisdom, the
enhancement in the Nernst signal is more significant with the reduction of the
impurity scattering rate. This anomalous Nernst effect intimately reflects the
highly unusual superconducting state embedded in the so-called hidden-order
phase of URuSi. The results invoke possible chiral or Berry-phase
fluctuations originated from the topological aspect of this superconductor,
which are associated with the effective magnetic field intrinsically induced by
broken time-reversal symmetry of the superconducting order parameter.Comment: Original version. Accepted for publication in Nature Physic
Decrease of upper critical field with underdoping in cuprate superconductors
The transition temperature Tc of cuprate superconductors falls when the
doping p is reduced below a certain optimal value. It is unclear whether this
fall is due to strong phase fluctuations or to a decrease in the pairing gap.
Different interpretations of photoemission data disagree on the evolution of
the pairing gap and different estimates of the upper critical field Hc2 are in
sharp contradiction. Here we resolve this contradiction by showing that
superconducting fluctuations in the underdoped cuprate Eu-LSCO, measured via
the Nernst effect, have a characteristic field scale that falls with
underdoping. The critical field Hc2 dips at p = 0.11, showing that
superconductivity is weak where stripe order is strong. In the archetypal
cuprate superconductor YBCO, Hc2 extracted from other measurements has the same
doping dependence, also with a minimum at p = 0.11, again where stripe order is
present. We conclude that competing states such as stripe order weaken
superconductivity and this, rather than phase fluctuations, causes Tc to fall
as cuprates become underdoped.Comment: Supplementary Information file available upon request; Nature Physics
(2012
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