328 research outputs found
The behavioural and neurophysiological modulation of microsaccades in monkeys
Systematic modulations of microsaccades have been observed in humans during covert orienting. We show here that monkeys are a suitable model for studying the neurophysiology governing these modulations of microsaccades. Using various cue-target saccade tasks, we observed the effects of visual and auditory cues on microsaccades in monkeys. As in human studies, following visual cues there was an early bias in cue-congruent microsaccades followed by a later bias in cue-incongruent microsaccades. Following auditory cues there was a cue-incongruent bias in left cues only. In a separate experiment, we observed that brainstem omnipause neurons, which gate all saccades, also paused during microsaccade generation. Thus, we provide evidence that at least part of the same neurocircuitry governs both large saccades and microsaccades
New Phase Induced by Pressure in the Iron-Arsenide Superconductor K-Ba122
The electrical resistivity rho of the iron-arsenide superconductor
Ba1-xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four
underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic
ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases
linearly with pressure. At pressures above around 1.0 GPa, a second sharp
anomaly is detected at a lower temperature T_0, which rises with pressure. We
attribute this second anomaly to the onset of a phase that causes a
reconstruction of the Fermi surface. This new phase expands with increasing x
and it competes with superconductivity. We discuss the possibility that a
second spin-density wave orders at T_0, with a Q vector distinct from that of
the spin-density wave that sets in at T_N.Comment: Two higher K concentrations were added, revealing a steady expansion
of the new phase in the T-P phase diagra
Universal heat conduction in the iron-arsenide superconductor KFe2As2 : Evidence of a d-wave state
The thermal conductivity of the iron-arsenide superconductor KFe2As2 was
measured down to 50 mK for a heat current parallel and perpendicular to the
tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both
current directions, confirming the presence of nodes in the superconducting
gap. Our value of the RLT in the plane is equal to that reported by Dong et al.
[Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity
was ten times larger. This independence of the RLT on impurity scattering is
the signature of universal heat transport, a property of superconducting states
with symmetry-imposed line nodes. This argues against an s-wave state with
accidental nodes. It favors instead a d-wave state, an assignment consistent
with five additional properties: the magnitude of the critical scattering rate
for suppressing Tc to zero; the magnitude of the RLT, and its dependence on
current direction and on magnetic field; the temperature dependence of the
thermal conductivity.Comment: To appear in Physical Review Letter
From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2
The nature of the pairing state in iron-based superconductors is the subject
of much debate. Here we argue that in one material, the stoichiometric iron
pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state,
characterized by symmetry-imposed vertical line nodes in the superconducting
gap. This evidence is reviewed, with a focus on thermal conductivity and the
strong impact of impurity scattering on the critical temperature Tc. We then
compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the
pairing symmetry is s-wave and the Tc is ten times higher. The transition from
d-wave to s-wave within the same crystal structure provides a rare opportunity
to investigate the connection between band structure and pairing mechanism. We
also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which
the pairing symmetry is probably not d-wave, but more likely s-wave with
accidental line nodes
Doping Evolution of the Superconducting Gap Structure in the Underdoped Iron Arsenide BaâââKâFeâAsâ Revealed by Thermal Conductivity
The thermal conductivity Îș of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping (x = 0.34, Tc = 39 K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity (x = 0.16, Tc = 7 K). In zero field, there is no residual linear term in Îș(T) as Tâ0 at any doping, whether for in-plane or interplane transport. This shows that there are no nodes in the superconducting gap. However, as x decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing x. We propose that the minima in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is reconstructed by the antiferromagnetic order
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