81 research outputs found
A momentum-dependent perspective on quasiparticle interference in Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}
Angle Resolved Photoemission Spectroscopy (ARPES) probes the momentum-space
electronic structure of materials, and provides invaluable information about
the high-temperature superconducting cuprates. Likewise, the cuprate
real-space, inhomogeneous electronic structure is elucidated by Scanning
Tunneling Spectroscopy (STS). Recently, STS has exploited quasiparticle
interference (QPI) - wave-like electrons scattering off impurities to produce
periodic interference patterns - to infer properties of the QP in
momentum-space. Surprisingly, some interference peaks in
Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} (Bi-2212) are absent beyond the
antiferromagnetic (AF) zone boundary, implying the dominance of particular
scattering process. Here, we show that ARPES sees no evidence of quasiparticle
(QP) extinction: QP-like peaks are measured everywhere on the Fermi surface,
evolving smoothly across the AF zone boundary. This apparent contradiction
stems from different natures of single-particle (ARPES) and two-particle (STS)
processes underlying these probes. Using a simple model, we demonstrate
extinction of QPI without implying the loss of QP beyond the AF zone boundary
Discovery of microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x
The parent compounds of the copper oxide high-Tc superconductors are unusual
insulators. Superconductivity arises when they are properly doped away from
stoichiometry1. In Bi2Sr2CaCu2O8+x, superconductivity results from doping with
excess oxygen atoms, which introduce positive charge carriers (holes) into the
CuO2 planes, where superconductivity is believed to originate. The role of
these oxygen dopants is not well understood, other than the fact that they
provide charge carriers. However, it is not even clear how these charges
distribute in the CuO2 planes. Accordingly, many models of high-Tc
superconductors simply assume that the charge carriers introduced by doping
distribute uniformly, leading to an electronically homogeneous system, as in
ordinary metals. Here we report the observation of an electronic inhomogeneity
in the high-Tc superconductor Bi2Sr2CaCu2O8+x using scanning tunnelling
microscopy/spectroscopy. This inhomogeneity is manifested as spatial variations
in both the local density of states spectrum and the superconducting energy
gap. These variations are correlated spatially and vary on a surprisingly short
length scale of ~ 14 Angs. Analysis suggests that the inhomogeneity observed is
a consequence of proximity to a Mott insulator resulting in poor screening of
the charge potentials associated with the oxygen ions left behind in the BiO
plane after doping. Hence this experiment is a direct probe of the local nature
of the superconducting state, which is not easily accessible by macroscopic
measurements.Comment: 6 pages, 4 figure
Effective Rheology of Bubbles Moving in a Capillary Tube
We calculate the average volumetric flux versus pressure drop of bubbles
moving in a single capillary tube with varying diameter, finding a square-root
relation from mapping the flow equations onto that of a driven overdamped
pendulum. The calculation is based on a derivation of the equation of motion of
a bubble train from considering the capillary forces and the entropy production
associated with the viscous flow. We also calculate the configurational
probability of the positions of the bubbles.Comment: 4 pages, 1 figur
Alignment of the ALICE Inner Tracking System with cosmic-ray tracks
37 pages, 15 figures, revised version, accepted by JINSTALICE (A Large Ion Collider Experiment) is the LHC (Large Hadron Collider) experiment devoted to investigating the strongly interacting matter created in nucleus-nucleus collisions at the LHC energies. The ALICE ITS, Inner Tracking System, consists of six cylindrical layers of silicon detectors with three different technologies; in the outward direction: two layers of pixel detectors, two layers each of drift, and strip detectors. The number of parameters to be determined in the spatial alignment of the 2198 sensor modules of the ITS is about 13,000. The target alignment precision is well below 10 micron in some cases (pixels). The sources of alignment information include survey measurements, and the reconstructed tracks from cosmic rays and from proton-proton collisions. The main track-based alignment method uses the Millepede global approach. An iterative local method was developed and used as well. We present the results obtained for the ITS alignment using about 10^5 charged tracks from cosmic rays that have been collected during summer 2008, with the ALICE solenoidal magnet switched off.Peer reviewe
An unusual isotope effect in a high-transition-temperature superconductor
In conventional superconductors, the electron pairing that allows
superconductivity is caused by exchange of virtual phonons, which are quanta of
lattice vibration. For high-transition-temperature (high-Tc) superconductors,
it is far from clear that phonons are involved in the pairing at all. For
example, the negligible change in Tc of optimally doped Bi2Sr2CaCu2O8 (Bi2212)
upon oxygen isotope substitution (16O to 18O leads to Tc decreasing from 92 to
91 K) has often been taken to mean that phonons play an insignificant role in
this material. Here we provide a detailed comparison of the electron dynamics
of Bi2212 samples containing different oxygen isotopes, using angle-resolved
photoemission spectroscopy. Our data show definite and strong isotope effects.
Surprisingly, the effects mainly appear in broad high-energy humps, commonly
referred to as "incoherent peaks". As a function of temperature and electron
momentum, the magnitude of the isotope effect closely correlates with the
superconducting gap - that is, the pair binding energy. We suggest that these
results can be explained in a dynamic spin-Peierls picture, where the singlet
pairing of electrons and the electron-lattice coupling mutually enhance each
other
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