453 research outputs found
The change of Fermi surface topology in Bi2Sr2CaCu2O8 with doping
We report the observation of a change in Fermi surface topology of
Bi2Sr2CaCu2O8 with doping. By collecting high statistics ARPES data from
moderately and highly overdoped samples and dividing the data by the Fermi
function, we answer a long standing question about the Fermi surface shape of
Bi2Sr2CaCu2O8 close to the (pi,0) point. For moderately overdoped samples
(Tc=80K) we find that both the bonding and antibonding sheets of the Fermi
surface are hole-like. However for a doping level corresponding to Tc=55K we
find that the antibonding sheet becomes electron-like. This change does not
directly affect the critical temperature and therefore the superconductivity.
However, since similar observations of the change of the topology of the Fermi
surface were observed in LSCO and Bi2Sr2Cu2O6, it appears to be a generic
feature of hole-doped superconductors. Because of bilayer splitting, though,
this doping value is considerably lower than that for the single layer
materials, which again argues that it is unrelated to Tc
Identifying the Background Signal in ARPES of High Temperature Superconductors
One of the interesting features of the photoemission spectra of the high
temperature cuprate superconductors is the presence of a large signal (referred
to as the "background'') in the unoccupied region of the Brillouin zone. Here
we present data indicating that the origin of this signal is extrinsic and is
most likely due to strong scattering of the photoelectrons. We also present an
analytical method that can be used to subtract the background signal
Dual character of the electronic structure in YBa2Cu4O8: conduction bands of CuO2 planes and CuO chains
We use microprobe Angle-Resolved Photoemission Spectroscopy (muARPES) to
separately investigate the electronic properties of CuO2 planes and CuO chains
in the high temperature superconductor, YBa2Cu4O8. In the CuO2 planes, a two
dimensional (2D) electronic structure with nearly momentum independent bilayer
splitting is observed. The splitting energy is 150 meV at (pi,0), almost 50%
larger than in Bi2Sr2CaCu2O(8+d) and the electron scattering at the Fermi level
in the bonding band is about 1.5 times stronger than in the antibonding band.
The CuO chains have a quasi one dimensional (1D) electronic structure. We
observe two 1D bands separated by ~ 550meV: a conducting band and an insulating
band with an energy gap of ~ 240meV. We find that the conduction electrons are
well confined within the planes and chains with a non-trivial hybridization.Comment: 4 pages, 4 figure
Momentum anisotropy of the scattering rate in cuprate superconductors
We examine the momentum and energy dependence of the scattering rate of the
high temperature cuprate superconductors using angle resolved photoemission
spectroscopy. The scattering rate is of the form a + b*w. The inelastic
coefficient b is found to be isotropic. The elastic term, a, however, is found
to be highly anisotropic in the pseudogap phase of optimal doped samples, with
an anisotropy which correlates with that of the pseudogap. This can be
contrasted with heavily overdoped samples, which show an isotropic scattering
rate in the normal state
Non-dispersive Fermi arcs and absence of charge ordering in the pseudogap phase of Bi2Sr2CaCu2O8+d
The autocorrelation of angle resolved photoemission data from the high
temperature superconductor Bi2Sr2CaCu2O8+d shows distinct peaks in momentum
space which disperse with binding energy in the superconducting state, but not
in the pseudogap phase. Although it is tempting to attribute a non-dispersive
behavior in momentum space to some ordering phenomenon, a de-construction of
the autocorrelation reveals that the non-dispersive peaks arise not from
ordering, but rather from the tips of the Fermi arcs, which themselves do not
change with binding energy.Comment: 4 pages, 3 figure
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
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The Tabby cat locus maps to feline chromosome B1.
The Tabby markings of the domestic cat are unique coat patterns for which no causative candidate gene has been inferred from other mammals. In this study, a genome scan was performed on a large pedigree of cats that segregated for Tabby coat markings, specifically for the Abyssinian (Ta-) and blotched (tbtb) phenotypes. There was linkage between the Tabby locus and eight markers on cat chromosome B1. The most significant linkage was between marker FCA700 and Tabby (Z = 7.56, theta = 0.03). Two additional markers in the region supported linkage, although not with significant LOD scores. Pairwise analysis of the markers supported the published genetic map of the cat, although additional meioses are required to refine the region. The linked markers cover a 17-cM region and flank an evolutionary breakpoint, suggesting that the Tabby gene has a homologue on either human chromosome 4 or 8. Alternatively, Tabby could be a unique locus in cats
Electronic Spectra and Their Relation to the (pi,pi) Collective Mode in High-Tc Superconductors
Photoemission spectra of Bi2Sr2CaCu2O8 reveal that the high energy feature
near (pi,0), the "hump", scales with the superconducting gap and persists above
Tc in the pseudogap phase. As the doping decreases, the dispersion of the hump
increasingly reflects the wavevector (pi,pi) characteristic of the undoped
insulator, despite the presence of a large Fermi surface. This can be
understood from the interaction of the electrons with a collective mode,
supported by our observation that the doping dependence of the resonance
observed by neutron scattering is the same as that inferred from our data.Comment: 4 pages (revtex), 5 figures (eps
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