869 research outputs found
Spontaneous time reversal symmetry breaking in the pseudogap state of high-Tc superconductors
When matter undergoes a phase transition from one state to another, usually a
change in symmetry is observed, as some of the symmetries exhibited are said to
be spontaneously broken. The superconducting phase transition in the underdoped
high-Tc superconductors is rather unusual, in that it is not a mean-field
transition as other superconducting transitions are. Instead, it is observed
that a pseudo-gap in the electronic excitation spectrum appears at temperatures
T* higher than Tc, while phase coherence, and superconductivity, are
established at Tc (Refs. 1, 2). One would then wish to understand if T* is just
a crossover, controlled by fluctuations in order which will set in at the lower
Tc (Refs. 3, 4), or whether some symmetry is spontaneously broken at T* (Refs.
5-10). Here, using angle-resolved photoemission with circularly polarized
light, we find that, in the pseudogap state, left-circularly polarized photons
give a different photocurrent than right-circularly polarized photons, and
therefore the state below T* is rather unusual, in that it breaks time reversal
symmetry11. This observation of a phase transition at T* provides the answer to
a major mystery of the phase diagram of the cuprates. The appearance of the
anomalies below T* must be related to the order parameter that sets in at this
characteristic temperature .Comment: 11 pages, 4 figure
Locally critical quantum phase transitions in strongly correlated metals
When a metal undergoes a continuous quantum phase transition, non-Fermi
liquid behaviour arises near the critical point. It is standard to assume that
all low-energy degrees of freedom induced by quantum criticality are spatially
extended, corresponding to long-wavelength fluctuations of the order parameter.
However, this picture has been contradicted by recent experiments on a
prototype system: heavy fermion metals at a zero-temperature magnetic
transition. In particular, neutron scattering from CeCuAu has
revealed anomalous dynamics at atomic length scales, leading to much debate as
to the fate of the local moments in the quantum-critical regime. Here we report
our theoretical finding of a locally critical quantum phase transition in a
model of heavy fermions. The dynamics at the critical point are in agreement
with experiment. We also argue that local criticality is a phenomenon of
general relevance to strongly correlated metals, including doped Mott
insulators.Comment: 20 pages, 3 figures; extended version, to appear in Natur
Accurate theoretical fits to laser ARPES EDCs in the normal phase of cuprate superconductors
Anderson has recently proposed a theory of the strange metal state above Tc
in the high Tc superconductors. [arXiv:cond-mat/0512471] It is based on the
idea that the unusual transport properties and spectral functions are caused by
the strong Mott- Hubbard interactions and can be computed by using the formal
apparatus of Gutzwiller projection. In ref. 1 Anderson computed only the
tunneling spectrum and the power-law exponent of the infrared conductivity. He
had calculated the energy distribution curves (EDCs) in angle resolved
photoemission spectroscopy (ARPES) but was discouraged when these differed
radically from the best ARPES measurements available at the time, and did not
include them. In this letter we compare the spectral functions computed within
this model to the novel laser-ARPES data of the Dessau group.These are found to
capture the shape of the experimental EDCs with unprecedented accuracy and in
principle have only one free parameter
Linear-T resistivity and change in Fermi surface at the pseudogap critical point of a high-Tc superconductor
A fundamental question of high-temperature superconductors is the nature of
the pseudogap phase which lies between the Mott insulator at zero doping and
the Fermi liquid at high doping p. Here we report on the behaviour of charge
carriers near the zero-temperature onset of that phase, namely at the critical
doping p* where the pseudogap temperature T* goes to zero, accessed by
investigating a material in which superconductivity can be fully suppressed by
a steady magnetic field. Just below p*, the normal-state resistivity and Hall
coefficient of La1.6-xNd0.4SrxCuO4 are found to rise simultaneously as the
temperature drops below T*, revealing a change in the Fermi surface with a
large associated drop in conductivity. At p*, the resistivity shows a linear
temperature dependence as T goes to zero, a typical signature of a quantum
critical point. These findings impose new constraints on the mechanisms
responsible for inelastic scattering and Fermi surface transformation in
theories of the pseudogap phase.Comment: 24 pages, 6 figures. Published in Nature Physics. Online at
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1109.htm
High-transition-temperature superconductivity in the absence of the magnetic-resonance mode
The fundamental mechanism that gives rise to high-transition-temperature
(high-Tc) superconductivity in the copper oxide materials has been debated
since the discovery of the phenomenon. Recent work has focussed on a sharp
'kink' in the kinetic energy spectra of the electrons as a possible signature
of the force that creates the superconducting state. The kink has been related
to a magnetic resonance and also to phonons. Here we report that infrared
spectra of Bi2Sr2CaCu2O(8+d), (Bi-2212) show that this sharp feature can be
separated from a broad background and, interestingly, weakens with doping
before disappearing completely at a critical doping level of 0.23 holes per
copper atom. Superconductivity is still strong in terms of the transition
temperature (Tc approx 55 K), so our results rule out both the magnetic
resonance peak and phonons as the principal cause of high-Tc superconductivity.
The broad background, on the other hand, is a universal property of the copper
oxygen plane and a good candidate for the 'glue' that binds the electrons.Comment: 4 pages, 3 figure
Field-induced quantum fluctuations in the heavy fermion superconductor CeCu2Ge2
Quantum-mechanical fluctuations in strongly correlated electron systems cause
unconventional phenomena such as non-Fermi liquid behavior, and arguably high
temperature superconductivity. Here we report the discovery of a field-tuned
quantum critical phenomenon in stoichiometric CeCu2Ge2, a spin density wave
ordered heavy fermion metal that exhibits unconventional superconductivity
under ~ 10 GPa of applied pressure. Our finding of the associated quantum
critical spin fluctuations of the antiferromagnetic spin density wave order,
dominating the local fluctuations due to single-site Kondo effect, provide new
information about the underlying mechanism that can be important in
understanding superconductivity in this novel compound.Comment: Heavy Fermion, Quantum Critical Phenomeno
Bounding the pseudogap with a line of phase transitions in YBCO cuprate superconductors
Close to optimal doping, the copper oxide superconductors show 'strange
metal' behavior, suggestive of strong fluctuations associated with a quantum
critical point. Such a critical point requires a line of classical phase
transitions terminating at zero temperature near optimal doping inside the
superconducting 'dome'. The underdoped region of the temperature-doping phase
diagram from which superconductivity emerges is referred to as the 'pseudogap'
because evidence exists for partial gapping of the conduction electrons, but so
far there is no compelling thermodynamic evidence as to whether the pseudogap
is a distinct phase or a continuous evolution of physical properties on
cooling. Here we report that the pseudogap in YBCO cuprate superconductors is a
distinct phase, bounded by a line of phase transitions. The doping dependence
of this line is such that it terminates at zero temperature inside the
superconducting dome. From this we conclude that quantum criticality drives the
strange metallic behavior and therefore superconductivity in the cuprates
An explanation for a universality of transition temperatures in families of copper oxide superconductors
A remarkable mystery of the copper oxide high-transition-temperature (Tc)
superconductors is the dependence of Tc on the number of CuO2 layers, n, in the
unit cell of a crystal. In a given family of these superconductors, Tc rises
with the number of layers, reaching a peak at n=3, and then declines: the
result is a bell-shaped curve. Despite the ubiquity of this phenomenon, it is
still poorly understood and attention has instead been mainly focused on the
properties of a single CuO2 plane. Here we show that the quantum tunnelling of
Cooper pairs between the layers simply and naturally explains the experimental
results, when combined with the recently quantified charge imbalance of the
layers and the latest notion of a competing order nucleated by this charge
imbalance that suppresses superconductivity. We calculate the bell-shaped curve
and show that, if materials can be engineered so as to minimize the charge
imbalance as n increases, Tc can be raised further.Comment: 15 pages, 3 figures. The version published in Natur
Powerlaw optical conductivity with a constant phase angle in high Tc superconductors
In certain materials with strong electron correlations a quantum phase
transition (QPT) at zero temperature can occur, in the proximity of which a
quantum critical state of matter has been anticipated. This possibility has
recently attracted much attention because the response of such a state of
matter is expected to follow universal patterns defined by the quantum
mechanical nature of the fluctuations. Forementioned universality manifests
itself through power-law behaviours of the response functions. Candidates are
found both in heavy fermion systems and in the cuprate high Tc superconductors.
Although there are indications for quantum criticality in the cuprate
superconductors, the reality and the physical nature of such a QPT are still
under debate. Here we identify a universal behaviour of the phase angle of the
frequency dependent conductivity that is characteristic of the quantum critical
region. We demonstrate that the experimentally measured phase angle agrees
precisely with the exponent of the optical conductivity. This points towards a
QPT in the cuprates close to optimal doping, although of an unconventional
kind.Comment: pdf format, 9 pages, 4 color figures include
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