687 research outputs found
Magnetic Phase Diagram of the Hole-doped CaNaCuOCl Cuprate Superconductor
We report on the magnetic phase diagram of a hole-doped cuprate
CaNaCuOCl, which is free from buckling of CuO
planes, determined by muon spin rotation and relaxation. It is characterized by
a quasi-static spin glass-like phase over a range of sodium concentration
(), which is held between long range antiferromagnetic
(AF) phase () and superconducting phase where the system is
non-magnetic for . The obtained phase diagram qualitatively agrees
well with that commonly found for hole-doped high-\tc cuprates, strongly
suggesting that the incomplete suppression of the AF order for is an
essential feature of the hole-doped cuprates.Comment: 5 pages, submitted to Phys. Rev. Let
Visualizing the emergence of the pseudogap state and the evolution to superconductivity in a lightly hole-doped Mott insulator
Superconductivity emerges from the cuprate antiferromagnetic Mott state with
hole doping. The resulting electronic structure is not understood, although
changes in the state of oxygen atoms appear paramount. Hole doping first
destroys the Mott state yielding a weak insulator where electrons localize only
at low temperatures without a full energy gap. At higher doping, the
'pseudogap', a weakly conducting state with an anisotropic energy gap and
intra-unit-cell breaking of 90\degree-rotational (C4v) symmetry appears.
However, a direct visualization of the emergence of these phenomena with
increasing hole density has never been achieved. Here we report atomic-scale
imaging of electronic structure evolution from the weak-insulator through the
emergence of the pseudogap to the superconducting state in Ca2-xNaxCuO2Cl2. The
spectral signature of the pseudogap emerges at lowest doping from a weakly
insulating but C4v-symmetric matrix exhibiting a distinct spectral shape. At
slightly higher hole-density, nanoscale regions exhibiting pseudogap spectra
and 180\degree-rotational (C2v) symmetry form unidirectional clusters within
the C4v-symmetric matrix. Thus, hole-doping proceeds by the appearance of
nanoscale clusters of localized holes within which the broken-symmetry
pseudogap state is stabilized. A fundamentally two-component electronic
structure11 then exists in Ca2-xNaxCuO2Cl2 until the C2v-symmetric clusters
touch at higher doping, and the long-range superconductivity appears.Comment: See the Nature Physics website for the published version available at
http://dx.doi.org/10.1038/Nphys232
Evidence for time-reversal symmetry breaking of the superconducting state near twin-boundary interfaces in FeSe
Junctions and interfaces consisting of unconventional superconductors provide
an excellent experimental playground to study exotic phenomena related to the
phase of the order parameter. Not only the complex structure of unconventional
order parameters have an impact on the Josephson effects, but also may
profoundly alter the quasi-particle excitation spectrum near a junction. Here,
by using spectroscopic-imaging scanning tunneling microscopy, we visualize the
spatial evolution of the local density of states (LDOS) near twin boundaries
(TBs) of the nodal superconductor FeSe. The rotation of the
crystallographic orientation across the TB twists the structure of the
unconventional order parameter, which may, in principle, bring about a
zero-energy LDOS peak at the TB. The LDOS at the TB observed in our study, in
contrast, does not exhibit any signature of a zero-energy peak and an apparent
gap amplitude remains finite all the way across the TB. The low-energy
quasiparticle excitations associated with the gap nodes are affected by the TB
over a distance more than an order of magnitude larger than the coherence
length . The modification of the low-energy states is even more
prominent in the region between two neighboring TBs separated by a distance
. In this region the spectral weight near the Fermi level
(0.2~meV) due to the nodal quasiparticle spectrum is almost
completely removed. These behaviors suggest that the TB induces a fully-gapped
state, invoking a possible twist of the order parameter structure which breaks
time-reversal symmetry.Comment: 12 pages, 6 figure
Spectroscopic Fingerprint of Phase-Incoherent Superconductivity in the Cuprate Pseudogap State
A possible explanation for the existence of the cuprate "pseudogap" state is
that it is a d-wave superconductor without quantum phase rigidity. Transport
and thermodynamic studies provide compelling evidence that supports this
proposal, but few spectroscopic explorations of it have been made. One
spectroscopic signature of d-wave superconductivity is the particle-hole
symmetric "octet" of dispersive Bogoliubov quasiparticle interference
modulations. Here we report on this octet's evolution from low temperatures to
well into the underdoped pseudogap regime. No pronounced changes occur in the
octet phenomenology at the superconductor's critical temperature Tc, and it
survives up to at least temperature T ~ 1.5Tc. In the pseudogap regime, we
observe the detailed phenomenology that was theoretically predicted for
quasiparticle interference in a phase-incoherent d-wave superconductor. Thus,
our results not only provide spectroscopic evidence to confirm and extend the
transport and thermodynamics studies, but they also open the way for
spectroscopic explorations of phase fluctuation rates, their effects on the
Fermi arc, and the fundamental source of the phase fluctuations that suppress
superconductivity in underdoped cuprates.Comment: 27 pages, 12 figure
Quasi-particle interference and superconducting gap in a high-temperature superconductor Ca2-xNaxCuO2Cl2
High-transition-temperature (high-Tc) superconductivity is ubiquitous in the
cuprates containing CuO2 planes but each cuprate has its own character. The
study of the material dependence of the d-wave superconducting gap (SG) should
provide important insights into the mechanism of high-Tc. However, because of
the 'pseudogap' phenomenon, it is often unclear whether the energy gaps
observed by spectroscopic techniques really represent the SG. Here, we report
spectroscopic imaging scanning tunneling microscopy (SI-STM) studies of
nearly-optimally-doped Ca2-xNaxCuO2Cl2 (Na-CCOC) with Tc = 25 ~ 28 K. They
enable us to observe the quasi-particle interference (QPI) effect in this
material, through which unambiguous new information on the SG is obtained. The
analysis of QPI in Na-CCOC reveals that the SG dispersion near the gap node is
almost identical to that of Bi2Sr2CaCu2Oy (Bi2212) at the same doping level,
while Tc of Bi2212 is 3 times higher than that of Na-CCOC. We also find that SG
in Na-CCOC is confined in narrower energy and momentum ranges than Bi2212. This
explains at least in part the remarkable material dependence of TcComment: 13pages, 4fig
Fully Gapped Single-Particle Excitations in the Lightly Doped Cuprates
The low-energy excitations of the lightly doped cuprates were studied by
angle-resolved photoemission spectroscopy. A finite gap was measured over the
entire Brillouin zone, including along the d_{x^2 - y^2} nodal line. This
effect was observed to be generic to the normal states of numerous cuprates,
including hole-doped La_{2-x}Sr_{x}CuO_{4} and Ca_{2-x}Na_{x}CuO_{2}Cl_{2} and
electron-doped Nd_{2-x}Ce_{x}CuO_{4}. In all compounds, the gap appears to
close with increasing carrier doping. We consider various scenarios to explain
our results, including the possible effects of chemical disorder, electronic
inhomogeneity, and a competing phase.Comment: To appear in Phys. Rev.
Universality of pseudogap and emergent order in lightly doped Mott insulators
It is widely believed that high-temperature superconductivity in the cuprates
emerges from doped Mott insulators. The physics of the parent state seems
deceivingly simple: The hopping of the electrons from site to site is
prohibited because their on-site Coulomb repulsion U is larger than the kinetic
energy gain t. When doping these materials by inserting a small percentage of
extra carriers, the electrons become mobile but the strong correlations from
the Mott state are thought to survive; inhomogeneous electronic order, a
mysterious pseudogap and, eventually, superconductivity appear. How the
insertion of dopant atoms drives this evolution is not known, nor whether these
phenomena are mere distractions specific to hole-doped cuprates or represent
the genuine physics of doped Mott insulators. Here, we visualize the evolution
of the electronic states of (Sr1-xLax)2IrO4, which is an effective spin-1/2
Mott insulator like the cuprates, but is chemically radically different. Using
spectroscopic-imaging STM, we find that for doping concentration of x=5%, an
inhomogeneous, phase separated state emerges, with the nucleation of pseudogap
puddles around clusters of dopant atoms. Within these puddles, we observe the
same glassy electronic order that is so iconic for the underdoped cuprates.
Further, we illuminate the genesis of this state using the unique possibility
to localize dopant atoms on topographs in these samples. At low doping, we find
evidence for much deeper trapping of carriers compared to the cuprates. This
leads to fully gapped spectra with the chemical potential at mid-gap, which
abruptly collapse at a threshold of around 4%. Our results clarify the melting
of the Mott state, and establish phase separation and electronic order as
generic features of doped Mott insulators.Comment: This version contains the supplementary information and small updates
on figures and tex
Selfsimilar solutions in a sector for a quasilinear parabolic equation
We study a two-point free boundary problem in a sector for a quasilinear
parabolic equation. The boundary conditions are assumed to be spatially and
temporally "self-similar" in a special way. We prove the existence, uniqueness
and asymptotic stability of an expanding solution which is self-similar at
discrete times. We also study the existence and uniqueness of a shrinking
solution which is self-similar at discrete times.Comment: 23 page
Mean curvature flow with triple junctions in higher space dimensions
We consider mean curvature flow of n-dimensional surface clusters. At
(n-1)-dimensional triple junctions an angle condition is required which in the
symmetric case reduces to the well-known 120 degree angle condition. Using a
novel parametrization of evolving surface clusters and a new existence and
regularity approach for parabolic equations on surface clusters we show local
well-posedness by a contraction argument in parabolic Hoelder spaces.Comment: 31 pages, 2 figure
Functional dissociation in sweet taste receptor neurons between and within taste organs of Drosophila
Finding food sources is essential for survival. Insects detect nutrients with external taste receptor neurons. Drosophila possesses multiple taste organs that are distributed throughout its body. However, the role of different taste organs in feeding remains poorly understood. By blocking subsets of sweet taste receptor neurons, we show that receptor neurons in the legs are required for immediate sugar choice. Furthermore, we identify two anatomically distinct classes of sweet taste receptor neurons in the leg. The axonal projections of one class terminate in the thoracic ganglia, whereas the other projects directly to the brain. These two classes are functionally distinct: the brain-projecting neurons are involved in feeding initiation, whereas the thoracic ganglia-projecting neurons play a role in sugar-dependent suppression of locomotion. Distinct receptor neurons for the same taste quality may coordinate early appetitive responses, taking advantage of the legs as the first appendages to contact food
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