285 research outputs found
First direct observation of the Van Hove singularity in the tunneling spectra of cuprates
In two-dimensional lattices the electronic levels are unevenly spaced, and
the density of states (DOS) displays a logarithmic divergence known as the Van
Hove singularity (VHS). This is the case in particular for the layered cuprate
superconductors. The scanning tunneling microscope (STM) probes the DOS, and is
therefore the ideal tool to observe the VHS. No STM study of cuprate
superconductors has reported such an observation so far giving rise to a debate
about the possibility of observing directly the normal state DOS in the
tunneling spectra. In this study, we show for the first time that the VHS is
unambiguously observed in STM measurements performed on the cuprate Bi-2201.
Beside closing the debate, our analysis proves the presence of the pseudogap in
the overdoped side of the phase diagram of Bi-2201 and discredits the scenario
of the pseudogap phase crossing the superconducting dome.Comment: 4 pages, 4 figure
Disentangling Cooper-pair formation above Tc from the pseudogap state in the cuprates
The discovery of the pseudogap in the cuprates created significant excitement
amongst physicists as it was believed to be a signature of pairing, in some
cases well above the room temperature. In this "pre-formed pairs" scenario, the
formation of pairs without quantum phase rigidity occurs below T*. These pairs
condense and develop phase coherence only below Tc. In contrast, several recent
experiments reported that the pseudogap and superconducting states are
characterized by two different energy scales, pointing to a scenario, where the
two compete. However a number of transport, magnetic, thermodynamic and
tunneling spectroscopy experiments consistently detect a signature of
phase-fluctuating superconductivity above leaving open the question of whether
the pseudogap is caused by pair formation or not. Here we report the discovery
of a spectroscopic signature of pair formation and demonstrate that in a region
of the phase diagram commonly referred to as the "pseudogap", two distinct
states coexist: one that persists to an intermediate temperature Tpair and a
second that extends up to T*. The first state is characterized by a doping
independent scaling behavior and is due to pairing above Tc, but significantly
below T*. The second state is the "proper" pseudogap - characterized by a
"checker board" pattern in STM images, the absence of pair formation, and is
likely linked to Mott physics of pristine CuO2 planes. Tpair has a universal
value around 130-150K even for materials with very different Tc, likely setting
limit on highest, attainable Tc in cuprates. The observed universal scaling
behavior with respect to Tpair indicates a breakdown of the classical picture
of phase fluctuations in the cuprates.Comment: 9 pages, 4 figure
(pi,pi)-electronic order in iron arsenide superconductors
The distribution of valence electrons in metals usually follows the symmetry
of an ionic lattice. Modulations of this distribution often occur when those
electrons are not stable with respect to a new electronic order, such as spin
or charge density waves. Electron density waves have been observed in many
families of superconductors[1-3], and are often considered to be essential for
superconductivity to exist[4]. Recent measurements[5-9] seem to show that the
properties of the iron pnictides[10, 11] are in good agreement with band
structure calculations that do not include additional ordering, implying no
relation between density waves and superconductivity in those materials[12-15].
Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp
disagreement with those band structure calculations[12-15], instead revealing a
reconstruction characterized by a (pi,pi) wave vector. This electronic order
coexists with superconductivity and persists up to room temperature
STM imaging of symmetry-breaking structural distortion in the Bi-based cuprate superconductors
A complicating factor in unraveling the theory of high-temperature (high-Tc)
superconductivity is the presence of a "pseudogap" in the density of states,
whose origin has been debated since its discovery [1]. Some believe the
pseudogap is a broken symmetry state distinct from superconductivity [2-4],
while others believe it arises from short-range correlations without symmetry
breaking [5,6]. A number of broken symmetries have been imaged and identified
with the pseudogap state [7,8], but it remains crucial to disentangle any
electronic symmetry breaking from pre-existing structural symmetry of the
crystal. We use scanning tunneling microscopy (STM) to observe an orthorhombic
structural distortion across the cuprate superconducting Bi2Sr2Can-1CunO2n+4+x
(BSCCO) family tree, which breaks two-dimensional inversion symmetry in the
surface BiO layer. Although this inversion symmetry breaking structure can
impact electronic measurements, we show from its insensitivity to temperature,
magnetic field, and doping, that it cannot be the long-sought pseudogap state.
To detect this picometer-scale variation in lattice structure, we have
implemented a new algorithm which will serve as a powerful tool in the search
for broken symmetry electronic states in cuprates, as well as in other
materials.Comment: 4 figure
Appearance of fluctuating stripes at the onset of the pseudogap in the high-Tc Superconductor Bi2Sr2CaCu2O8+x
Doped Mott insulators have been shown to have a strong propensity to form
patterns of holes and spins often referred to as stripes. In copper-oxides,
doping also gives rise to the pseudogap state, which transforms into a high
temperature superconductor with sufficient doping or by reducing the
temperature. A long standing question has been the interplay between pseudogap,
which is generic to all hole-doped cuprates, and stripes, whose static form
occurs in only one family of cuprates over a narrow range of the phase diagram.
Here we examine the spatial reorganization of electronic states with the onset
of the pseudogap state at T* in the high-temperature superconductor
Bi2Sr2CaCu2O8+x using spectroscopic mapping with the scanning tunneling
microscope (STM). We find that the onset of the pseudogap phase coincides with
the appearance of electronic patterns that have the predicted characteristics
of fluctuating stripes. As expected, the stripe patterns are strongest when the
hole concentration in the CuO2 planes is close to 1/8 (per Cu). While
demonstrating that the fluctuating stripes emerge with the onset of the
pseudogap state and occur over a large part of the cuprate phase diagram, our
experiments indicate that they are a consequence of pseudogap behavior rather
than its cause.Comment: preprint version, 25 pages including supplementary informatio
Particle-Hole Symmetry Breaking in the Pseudogap State of Bi2201
In conventional superconductors, a gap exists in the energy absorption
spectrum only below the transition temperature (Tc), corresponding to the
energy price to pay for breaking a Cooper pair of electrons. In high-Tc cuprate
superconductors above Tc, an energy gap called the pseudogap exists, and is
controversially attributed either to pre-formed superconducting pairs, which
would exhibit particle-hole symmetry, or to competing phases which would
typically break it. Scanning tunnelling microscopy (STM) studies suggest that
the pseudogap stems from lattice translational symmetry breaking and is
associated with a different characteristic spectrum for adding or removing
electrons (particle-hole asymmetry). However, no signature of either spatial or
energy symmetry breaking of the pseudogap has previously been observed by
angle-resolved photoemission spectroscopy (ARPES). Here we report ARPES data
from Bi2201 which reveals both particle-hole symmetry breaking and dramatic
spectral broadening indicative of spatial symmetry breaking without long range
order, upon crossing through T* into the pseudogap state. This symmetry
breaking is found in the dominant region of the momentum space for the
pseudogap, around the so-called anti-node near the Brillouin zone boundary. Our
finding supports the STM conclusion that the pseudogap state is a
broken-symmetry state that is distinct from homogeneous superconductivity.Comment: Nature Physics advance online publication, 04/04/2010
(doi:10.1038/nphys1632) Author's version of the paper
Giant phonon anomalies and central peak due to charge density wave formation in YBaCuO
The electron-phonon interaction is a major factor influencing the competition
between collective instabilities in correlated-electron materials, but its role
in driving high-temperature superconductivity in the cuprates remains poorly
understood. We have used high-resolution inelastic x-ray scattering to monitor
low-energy phonons in YBaCuO (superconducting
K), which is close to a charge density wave (CDW) instability. Phonons in a
narrow range of momentum space around the CDW ordering vector exhibit extremely
large superconductivity-induced lineshape renormalizations. These results imply
that the electron-phonon interaction has sufficient strength to generate
various anomalies in electronic spectra, but does not contribute significantly
to Cooper pairing. In addition, a quasi-elastic "central peak" due to CDW
nanodomains is observed in a wide temperature range above and below ,
suggesting that the gradual onset of a spatially inhomogeneous CDW domain state
with decreasing temperature is a generic feature of the underdoped cuprates
Error correction in bimanual coordination benefits from bilateral muscle activity: evidence from kinesthetic tracking
Although previous studies indicated that the stability properties of interlimb coordination largely result from the integrated timing of efferent signals to both limbs, they also depend on afference-based interactions. In the present study, we examined contributions of afference-based error corrections to rhythmic bimanual coordination using a kinesthetic tracking task. Furthermore, since we found in previous research that subjects activated their muscles in the tracked (motor-driven) arm, we examined the functional significance of this activation to gain more insight into the processes underlying this phenomenon. To these aims, twelve subjects coordinated active movements of the right hand with motor-driven oscillatory movements of the left hand in two coordinative patterns: in-phase (relative phase 0°) and antiphase (relative phase 180°). They were either instructed to activate the muscles in the motor-driven arm as if moving along with the motor (active condition), or to keep these muscles as relaxed as possible (relaxed condition). We found that error corrections were more effective in in-phase than in antiphase coordination, resulting in more adequate adjustments of cycle durations to compensate for timing errors detected at the start of each cycle. In addition, error corrections were generally more pronounced in the active than in the relaxed condition. This activity-related difference was attributed to the associated bilateral neural control signals (as estimated using electromyography), which provided an additional reference (in terms of expected sensory consequences) for afference-based error corrections. An intimate relation was revealed between the (integrated) motor commands to both limbs and the processing of afferent feedback
Direct evidence for a competition between the pseudogap and high temperature superconductivity in the cuprates
A pairing gap and coherence are the two hallmarks of superconductivity. In a
classical BCS superconductor they are established simultaneously at Tc. In the
cuprates, however, an energy gap (pseudogap) extends above Tc. The origin of
this gap is one of the central issues in high temperature superconductivity.
Recent experimental evidence demonstrates that the pseudogap and the
superconducting gap are associated with different energy scales. It is however
not clear whether they coexist independently or compete. In order to understand
the physics of cuprates and improve their superconducting properties it is
vital to determine whether the pseudogap is friend or foe of high temperature
supercondctivity. Here we report evidence from angle resolved photoemission
spectroscopy (ARPES) that the pseudogap and high temperature superconductivity
represent two competing orders. We find that there is a direct correlation
between a loss in the low energy spectral weight due to the pseudogap and a
decrease of the coherent fraction of paired electrons. Therefore, the pseudogap
competes with the superconductivity by depleting the spectral weight available
for pairing in the region of momentum space where the superconducting gap is
largest. This leads to a very unusual state in the underdoped cuprates, where
only part of the Fermi surface develops coherence.Comment: Improved version was published in Natur
Exclusive Leptoproduction of rho^0 Mesons from Hydrogen at Intermediate Virtual Photon Energies
Measurements of the cross section for exclusive virtual-photoproduction of
rho^0 mesons from hydrogen are reported. The data were collected by the HERMES
experiment using 27.5 GeV positrons incident on a hydrogen gas target in the
HERA storage ring. The invariant mass W of the photon-nucleon system ranges
from 4.0 to 6.0 GeV, while the negative squared four-momentum Q^2 of the
virtual photon varies from 0.7 to 5.0 GeV^2. The present data together with
most of the previous data at W > 4 GeV are well described by a model that
infers the W-dependence of the cross section from the dependence on the Bjorken
scaling variable x of the unpolarized structure function for deep-inelastic
scattering. In addition, a model calculation based on Off-Forward Parton
Distributions gives a fairly good account of the longitudinal component of the
rho^0 production cross section for Q^2 > 2 GeV^2.Comment: 10 pages, 6 embedded figures, LaTeX for SVJour(epj) document class.
Revisions: curves added to Fig. 1, several clarifications added to tex
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