89 research outputs found
Multi-band Superconductivity in the Chevrel Phases SnMo6S8 and PbMo6S8
Sub-Kelvin scanning tunnelling spectroscopy in the Chevrel Phases SnMo6S8 and
PbMo6S8 reveals two distinct superconducting gaps with Delta_1 = 3 meV, Delta_2
~ 1.0 meV and Delta_1 = 3.1 meV, Delta_2 ~ 1.4 meV respectively. The gap
distribution is strongly anisotropic, with Delta_2 predominantly seen when
scanning across unit-cell steps on the (001) sample surface. The spectra are
well-fitted by an anisotropic two-band BCS s-wave gap function. Our
spectroscopic data are confirmed by electronic heat capacity measurements which
also provide evidence for a twin-gap scenario.Comment: 5 pages, 4 figure
A motivation to move: Juxtaposing the embodied practices of Pina Bausch and Ingemar Lindh
This is an Accepted Manuscript of an article published by Taylor & Francis in Theatre, Dance and Performance Training on 26/3/2015, available online: http://wwww.tandfonline.com/10.1080/19443927.2014.986286In their summer newsletter of 1996, the Centre for Performance Research (CPR) announced a workshop retreat to be led by Swedish theatre practitioner Ingemar Lindh at Druidstone in West Wales. The workshop, which was supposed to run in July of 1997, did not happen due to Lindh’s untimely death in Malta a few days before. The announcement described Lindh’s work as ‘oscillating between sensuality, even eroticism, on the one hand, and a kind of choreography of everyday life, similar sometimes to the work of Pina Bausch, on the other’ (CPR 1996, p. 9). Taking the CPR comparison as its cue, this article investigates an overlapping concern between the tanztheater practice of Bausch and the laboratory theatre work of Lindh: that whether called ‘movement’ or ‘action’, a performer’s work needs to be motivated by one’s personal input (memories, thoughts, images, and other mental processes) rather than executed as an estranged and dictated vocabulary of movement. This premise was largely a result of two major influential figures in Bausch’s and Lindh’s careers: Rudolph von Laban and Étienne Decroux. The article starts with a concise contextualisation of a reaction to rigid methodology in both tanztheater and laboratory theatre, i.e. Bausch’s and Lindh’s backgrounds respectively. It then juxtaposes Laban’s and Decroux’s reflections on embodied practice, leading the way to a discussion of the matter in the practices of Bausch and Lindh. To achieve broader understanding, the juxtaposition is supported by a close reading of Rick Kemp’s (2012) and Erika Fischer-Lichte’s (2008) accounts of ‘embodied mind’
Parton energy loss at strong coupling and the universal bound
The apparent universality of jet quenching observed in heavy ion collisions
at RHIC for light and heavy quarks, as well as for quarks and gluons, is very
puzzling and calls for a theoretical explanation. Recently it has been proposed
that the synchrotron--like radiation at strong coupling gives rise to a
universal bound on the energy of a parton escaping from the medium. Since this
bound appears quite low, almost all of the observed particles at high
transverse momentum have to originate from the surface of the hot fireball.
Here I make a first attempt of checking this scenario against the RHIC data and
formulate a "Universal Bound Model" of jet quenching that can be further tested
at RHIC and LHC.Comment: 8 pages, 2 figures, invited plenary talk given at "Hard Probes 2008"
Conference, 8-14 June 2008, Illa da Toxa, Galicia, Spai
Current-induced highly dissipative domains in high Tc thin films
We have investigated the resistive response of high Tc thin films submitted
to a high density of current. For this purpose, current pulses were applied
into bridges made of Nd(1.15)Ba(1.85)Cu3O7 and Bi2Sr2CaCu2O8. By recording the
time dependent voltage, we observe that at a certain critical current j*, a
highly dissipative domain develops somewhere along the bridge. The successive
formation of these domains produces stepped I-V characteristics. We present
evidences that these domains are not regions with a temperature above Tc, as
for hot spots. In fact this phenomenon appears to be analog to the nucleation
of phase-slip centers observed in conventional superconductors near Tc, but
here in contrast they appear in a wide temperature range. Under some
conditions, these domains will propagate and destroy the superconductivity
within the whole sample. We have measured the temperature dependence of j* and
found a similar behavior in the two investigated compounds. This temperature
dependence is just the one expected for the depairing current, but the
amplitude is about 100 times smaller.Comment: 9 pages, 9 figures, Revtex, to appear in Phys. Rev.
Studies of YBCO Strip Lines under Voltage Pulses: Optimisation of the Design of Fault Current Limiters
We present experimental results on the behaviour of a superconducting YBCO/Au
meander of length L submitted to short circuit tests with constant voltage
pulses. The meander, at the beginning of the short-circuit, is divided in two
regions; one, with a length L1 proportional to the applied voltage, which first
switches into a highly dissipative state (HDS) while the rest remains
superconducting. Then the rest of the meander will progressively switch into
the normal state due to the propagation of this HDS (few m/s) from both ends.
The part L1 has to initially support a power density proportional to r.Jp^2 (r
is the resistivity of the bilayer and Jp the peak current density). To avoid
local excessive dissipation of power and over heating on one part of the wafer
in the initial period, we have developed a novel design in order to distribute
the dissipating section of the meander into many separated small dissipative
zones. Furthermore the apparent propagation velocity of these dissipative zones
is increased by the number of propagation fronts. We will show results obtained
on 3kW (300V, 10A) FCL on a 2" wafer which confirm the benefits of this new
design.Comment: 4 pages, 6 figures; presented at the Applied Superconductivity
Conference in Houston, TX (August 2002); to be published in IEEE Trans. On
Appl. Supercon
Hall-effect studies in YBa2Cu3O7/PrBa2Cu3O7 superlattices
We measured the resistivity and the Hall coefficient RH in a series of YBa2Cu3O7/PrBa2Cu3O7 multilayers. We found no systematic change of the transport properties with decreasing layer thicknesses down to one unit cell. The resistivity and RH evaluated for the YBa2Cu3O7 layers are slightly higher than in bulk material, suggesting a small decrease of the carrier density in the multilayers. However, we observe no change in the Hall number 1/eRH when the thickness of the YBa1Cu3O7 layers decreases, so that the lowering of Tc observed in the superlattices cannot simply be related to a change in the carrier density. Furthermore, we find that the temperature dependence of RH is very similar to that of bulk materials
Node-like excitations in superconducting PbMo6S8 probed by scanning tunneling spectroscopy
We present the first scanning tunneling spectroscopy study on the Chevrel
phase PbMo6S8, an extreme type II superconductor with a coherence length only
slightly larger than in high-Tc cuprates. Tunneling spectra measured on
atomically flat terraces are spatially homogeneous and show well-defined
coherence peaks. The low-energy spectral weight, the zero bias conductance and
the temperature dependence of the gap are incompatible with a conventional
isotropic s-wave interpretation, revealing the presence of low-energy
excitations in the superconducting state. We show that our data are consistent
with the presence of nodes in the superconducting gap.Comment: To appear in PRB; 5 pages, 4 figure
Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process
Manufacturing superconducting circuits out of ultrathin films is a
challenging task when it comes to patterning complex compounds, which are
likely to be deteriorated by the patterning process. With the purpose of
developing high-T superconducting photon detectors, we designed a novel
route to pattern ultrathin YBCO films down to the nanometric scale. We believe
that our method, based on a specific use of a focused-ion beam, consists in
locally implanting Ga^{3+} ions and/or defects instead of etching the film.
This protocol could be of interest to engineer high-T superconducting
devices (SQUIDS, SIS/SIN junctions and Josephson junctions), as well as to
treat other sensitive compounds.Comment: 13 pages, 7 figure
Electronic structure properties and BCS superconductivity in beta-pyrochlore oxides: KOs_2O_6
We report a first-principles density-functional calculation of the electronic
structure and properties of the recently discovered superconducting
beta-pyrochlore oxide KOs_2O_6. We find that the electronic structure near the
Fermi energy E_F is dominated by strongly hybridized Os-5d and O-2p states. A
van Hove singularity very close to E_F leads to a relatively large density of
states at E_F, and the Fermi surface exhibits strong nesting along several
directions. These features could provide the scattering processes leading to
the observed anomalous temperature dependence of the resistivity and to the
rather large specific heat mass enhancement we obtain from the calculated
density of states and the observed specific heat coefficient. An estimate of
T_c within the framework of the BCS theory of superconductivity taking into
account the possible effects of spin fluctuations arising from nesting yields
the experimental value.Comment: 5 pages, 4 figures; submitted for publicatio
Phonon Mode Spectroscopy, Electron-Phonon Coupling and the Metal-Insulator Transition in Quasi-One-Dimensional M2Mo6Se6
We present electronic structure calculations, electrical resistivity data and
the first specific heat measurements in the normal and superconducting states
of quasi-one-dimensional M2Mo6Se6 (M = Tl, In, Rb). Rb2Mo6Se6 undergoes a
metal-insulator transition at ~170K: electronic structure calculations indicate
that this is likely to be driven by the formation of a dynamical charge density
wave. However, Tl2Mo6Se6 and In2Mo6Se6 remain metallic down to low temperature,
with superconducting transitions at Tc = 4.2K and 2.85K respectively. The
absence of any metal-insulator transition in these materials is due to a larger
in-plane bandwidth, leading to increased inter-chain hopping which suppresses
the density wave instability. Electronic heat capacity data for the
superconducting compounds reveal an exceptionally low density of states DEF =
0.055 states eV^-1 atom^-1, with BCS fits showing 2Delta/kBTc >= 5 for
Tl2Mo6Se6 and 3.5 for In2Mo6Se6. Modelling the lattice specific heat with a set
of Einstein modes, we obtain the approximate phonon density of states F(w).
Deconvolving the resistivity for the two superconductors then yields their
electron-phonon transport coupling function a^2F(w). In Tl2Mo6Se6 and
In2Mo6Se6, F(w) is dominated by an optical "guest ion" mode at ~5meV and a set
of acoustic modes from ~10-30meV. Rb2Mo6Se6 exhibits a similar spectrum;
however, the optical phonon has a lower intensity and is shifted to ~8meV.
Electrons in Tl2Mo6Se6 couple strongly to both sets of modes, whereas In2Mo6Se6
only displays significant coupling in the 10-18meV range. Although pairing is
clearly not mediated by the guest ion phonon, we believe it has a beneficial
effect on superconductivity in Tl2Mo6Se6, given its extraordinarily large
coupling strength and higher Tc compared to In2Mo6Se6.Comment: 16 pages, 13 figure
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