234 research outputs found
Analysis of longitudinal bunching inan FEL driven two-beam accelerator
Recent experiments [1] have explored the use of a free-electron laser (FEL)
as a buncher for a microwave two-beam accelerator, and the subsequent driving
of a standing-wave rf output cavity. Here we present a deeper analysis of the
longitudinal dynamics of the electron bunches as they are transported from the
end of the FEL and through the output cavity. In particular, we examine the
effect of the transport region and cavity aperture to filter the bunched
portion of the beam.
[1] T. Lefevre, et. al., Phys. Rev. Lett. 84 (2000), 1188.Comment: 3 pages, 8 figures. Submitted to XX Int'l LINAC Conferenc
Different effects of Ni and Co substitution on the transport properties of BaFe2As2
We report resistivity and Hall effect results on Ba(Fe1-xNix)2As2 and compare
them with those in Ba(Fe1-xCox)2As2. The Hall number RH is negative for all x
values from 0.01 to 0.14, which indicates that electron carriers dominate the
transport both in the magnetic and paramagnetic regime. We analyse the data in
the framework of a two-band model. Without any assumption on the number of
carriers, we show that the electron resistivity can be estimated with good
accuracy in the low temperature paramagnetic range. Although the phase diagrams
of the two families are very similar with respect to the extra electrons added
in the system, we find that the transport properties differ in several aspects.
First, we evidence that the contribution of holes to the transport is more
important for Ni doping than for Co doping. Secondly, Ni behaves as a stronger
scatterer for the electrons, as the increase of the residual electron
resistivity rho/x is about four times larger for Ni than for Co in the most
doped samples.Comment: 8 pages, 8 figure
Hole and Electron Contributions to the Transport Properties of Ba(Fe_(1-x)Ru_x)_2As_2 Single Crystals
We report a systematic study of structural and transport properties in single
crystals of Ba(Fe_(1-x)Ru_x)_2As_2 for x ranging from 0 to 0.5. The isovalent
substitution of Fe by Ru leads to an increase of the a parameter and a decrease
of the c parameter, resulting in a strong increase of the AsFeAs angle and a
decrease of the As height above the Fe planes. Upon Ru substitution, the
magnetic order is progressively suppressed and superconductivity emerges for x
> 0.15, with an optimal Tc ~ 20K at x = 0.35 and coexistence of magnetism and
superconductivity between these two Ru contents. Moreover, the Hall coefficient
RH which is always negative and decreases with temperature in BaFe2As2, is
found to increase here with decreasing T and even change sign for x > 0.15. For
x_Ru = 0.35, photo-emission studies have shown that the number of holes and
electrons are similar with n_e = n_h ~ 0.11, that is twice larger than found in
BaFe2As2 [1]. Using this estimate, we find that the transport properties of
Ba(Fe_0.65Ru_0.35)_2As_2 can be accounted for by the conventional multiband
description for a compensated semi-metal. In particular, our results show that
the mobility of holes is strongly enhanced upon Ru addition and overcomes that
of electrons at low temperature when x_Ru > 0.15.Comment: new version with minor correction
Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs
The resistivity, Hall effect and transverse magnetoresistance (MR) have been
measured in low residual resistivity single crystals of LiFeAs. A comparison
with angle resolved photoemission spectroscopy and quantum oscillation data
implies that four carrier bands unevenly contribute to the transport. However
the scattering rates of the carriers all display the T^2 behavior expected for
a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2
variation permit us to extract the superconducting fluctuation contribution to
the conductivity. Though below Tc the anisotropy of superconductivity is rather
small, the superconducting fluctuations display a quasi ideal two-dimensional
behavior which persists up to 1.4 Tc. These results call for a refined
theoretical understanding of the multiband behavior of superconductivity in
this pnictide.Comment: 8pages with supplementary material, 6 figure
High-field muSR studies of superconducting and magnetic correlations in cuprates above Tc
The advent of high transverse-field muon spin rotation (TF-muSR) has led to
recent muSR investigations of the magnetic-field response of cuprates above the
superconducting transition temperature T_c. Here the results of such
experiments on hole-doped cuprates are reviewed. Although these investigations
are currently ongoing, it is clear that the effects of high field on the
internal magnetic field distribution of these materials is dependent upon a
competition between superconductivity and magnetism. In La_{2-x}Sr_xCuO_4 the
response to the external field above Tc is dominated by heterogeneous spin
magnetism. However, the magnetism that dominates the observed inhomogeneous
line broadening below x ~ 0.19 is overwhelmed by the emergence of a completely
different kind of magnetism in the heavily overdoped regime. The origin of the
magnetism above x ~ 0.19 is currently unknown, but its presence hints at a
competition between superconductivity and magnetism that is reminiscent of the
underdoped regime. In contrast, the width of the internal field distribution of
underdoped YBa_2Cu_3O_y above Tc is observed to track Tc and the density of
superconducting carriers. This observation suggests that the magnetic response
above Tc is not dominated by electronic moments, but rather inhomogeneous
fluctuating superconductivity.Comment: 28 pages, 11 figures, 104 reference
Significant reduction of electronic correlations upon isovalent Ru substitution of BaFe2As2
We present a detailed investigation of Ba(Fe0.65Ru0.35)2As2 by transport
measurements and Angle Resolved photoemission spectroscopy. We observe that Fe
and Ru orbitals hybridize to form a coherent electronic structure and that Ru
does not induce doping. The number of holes and electrons, deduced from the
area of the Fermi Surface pockets, are both about twice larger than in
BaFe2As2. The contribution of both carriers to the transport is evidenced by a
change of sign of the Hall coefficient with decreasing temperature. Fermi
velocities increase significantly with respect to BaFe2As2, suggesting a
significant reduction of correlation effects. This may be a key to understand
the appearance of superconductivity at the expense of magnetism in undoped iron
pnictides
Effect of controlled disorder on quasiparticle thermal transport in BiSrCaCuO
Low temperature thermal conductivity, , of optimally-doped Bi2212 was
studied before and after the introduction of point defects by electron
irradiation. The amplitude of the linear component of remains
unchanged, confirming the universal nature of heat transport by zero-energy
quasiparticles. The induced decrease in the absolute value of at
finite temperatures allows us to resolve a nonuniversal term in due to
conduction by finite-energy quasiparticles. The magnitude of this term provides
an estimate of the quasiparticle lifetime at subkelvin temperatures.Comment: 5 pages including 2 .eps figuer
Hall effect and resistivity study of the magnetic transition, carrier content and Fermi liquid behavior in Ba(Fe(1-x) Cox)2As2
The negative Hall constant R_H measured all over the phase diagram of
Ba(Fe(1-x)Cox)2As2 allows us to show that electron carriers always dominate the
transport properties. The evolution of R_H with x at low doping (x<2%)
indicates that important band structure changes happen for x<2% prior to the
emergence of superconductivity. For higher x, an change of the electron
concentration with T is required to explain the low T variations of R_H, while
the electron scattering rate displays the T^2 law expected for a Fermi liquid.
The T=0 residual scattering is affected by Co disorder in the magnetic phase,
but is rather dominated by incipient disorder in the paramagnetic state.Comment: 5 pages, 4 figures, published versio
Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2
75As nuclear magnetic resonance (NMR) experiments were performed on
Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer
magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift
and is found similar to that of BaFe2As2, implying that Mn does not induce
charge doping. A satellite line associated with the Mn nearest neighbours
(n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries
a local magnetic moment. This is confirmed by the main line broadening typical
of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to
the localization of the additional Mn hole. These findings explain why Mn does
not induce superconductivity in the pnictides contrary to other dopants such as
Co, Ni, Ru or K.Comment: 6 pages, 7 figure
Reduction of Tc due to Impurities in Cuprate Superconductors
In order to explain how impurities affect the unconventional
superconductivity, we study non-magnetic impurity effect on the transition
temperature using on-site U Hubbard model within a fluctuation exchange (FLEX)
approximation. We find that in appearance, the reduction of Tc roughly
coincides with the well-known Abrikosov-Gor'kov formula. This coincidence
results from the cancellation between two effects; one is the reduction of
attractive force due to randomness, and another is the reduction of the damping
rate of quasi-particle arising from electron interaction. As another problem,
we also study impurity effect on underdoped cuprate as the system showing
pseudogap phenomena. To the aim, we adopt the pairing scenario for the
pseudogap and discuss how pseudogap phenomena affect the reduction of Tc by
impurities. We find that 'pseudogap breaking' by impurities plays the essential
role in underdoped cuprate and suppresses the Tc reduction due to the
superconducting (SC) fluctuation.Comment: 14 pages, 28 figures To be published in JPS
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