92 research outputs found
Hall coefficient and Hc2 in underdoped LaFeAsO0.95F0.05
The electrical resistivity and Hall coefficient of LaFeAsO0.95F0.05
polycrystalline samples were measured in pulsed magnetic fields up to m0H = 60
T from room temperature to 1.5 K. The resistance of the normal state shows a
negative temperature coefficient (dr/dT < 0) below 70 K for this composition,
indicating insulating ground state in underdoped LaFeAsO system in contrast to
heavily doped compound. The charge carrier density obtained from Hall effect
can be described as constant plus a thermally activated term with an energy gap
DE = 630 K. Upper critical field, Hc2, estimated from resistivity measurements,
exceeds 75 T with zero-field Tc = 26.3 K, suggesting an unconventional nature
for superconductivity.Comment: 12 pages and 4 figure
Doping dependence of upper critical field and Hall resistivity in LaFeAsO1-xFx
The electrical resistivity (Rxx) and Hall resistivity (Rxy) of LaFeAsO1-xFx
have been measured over a wide fluorine doping range 0 =< x =< 0.14 using 60 T
pulsed magnets. While the superconducting phase diagram (Tc, x) displays the
classic dome-shaped structure, we find that the resistive upper critical field
(Hc2) increases monotonically with decreasing fluorine concentration, with the
largest Hc2 >= 75 T for x = 0.05. This is reminiscent of the composition
dependence in high-Tc cuprates and might correlate with opening of a pseudo-gap
in the underdoped region. Further, the temperature dependence of Hc2(T) for
superconducting samples can be understood in terms of multi-band
superconductivity. Rxy data for non-superconducting samples show non-linear
field dependence, which is also consistent with a multi-carrier scenario.Comment: 15 pages, 5 figures, Accepted by PR
The order parameter-entropy relation in some universal classes: experimental evidence
The asymptotic behaviour near phase transitions can be suitably characterized
by the scaling of with , where is
the excess entropy and is the order parameter. As is obtained by
integration of the experimental excess specific heat of the transition , it displays little experimental noise so that the curve versus is better constrained than, say,
versus . The behaviour of for different
universality classes is presented and compared. In all cases, it clearly
deviates from being a constant. The determination of this function can then be
an effective method to distinguish asymptotic critical behaviour. For
comparison, experimental data for three very different systems, Rb2CoF4,
Rb2ZnCl4 and SrTiO3, are analysed under this approach. In SrTiO3, the function
does not deviate within experimental resolution from a straight
line so that, although Q can be fitted with a non mean-field exponent, the data
can be explained by a classical Landau mean-field behaviour. In contrast, the
behaviour of for the antiferromagnetic transition in Rb2CoF4 and
the normal-incommensurate phase transition in Rb2ZCl4 is fully consistent with
the asymptotic critical behaviour of the universality class corresponding to
each case. This analysis supports, therefore, the claim that incommensurate
phase transitions in general, and the ABX compounds in particular, in
contrast with most structural phase transitions, have critical regions large
enough to be observable.Comment: 13 pp. 9 ff. 2 tab. RevTeX. Submitted to J. Phys.: Cond. Matte
Fast relaxation in a fragile liquid under pressure
The incoherent dynamic structure factor of ortho-terphenyl has been measured
by neutron time-of-flight and backscattering technique in the pressure range
from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K.
Tagged-particle correlations in the compressed liquid decay in two steps. The
alpha-relaxation lineshape is independent of pressure, and the relaxation time
proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of
beta relaxation. The beta-relaxation regime can be described by the
mode-coupling scaling function; amplitudes and time scales allow a consistent
determination of the critical pressure P_c(T). alpha and beta relaxation depend
in the same way on the thermodynamic state; close to the mode-coupling
cross-over, this dependence can be parametrised by an effective coupling Gamma
~ n*T**{-1/4}.Comment: 4 Pages of RevTeX, 4 figures (submitted to Physical Review Letters
Universal mechanism of discontinuity of commensurate-incommensurate transitions in three-dimensional solids: Strain dependence of soliton self-energy
We show that there exists a universal mechanism of long-range soliton
attraction in three-dimensional solids and, therefore, of discontinuity of any
commensurate-incommensurate (C-IC) phase transition. This mechanism is due to
the strain dependence of the soliton self-energy and specific features of the
solid-state elasticity. The role of this mechanism is studied in detail for a
class of C-IC transitions where the IC modulation is one-dimensional, the
anisotropy in the order parameter space is small, and the symmetry of the
systems allows the existence of the Lifshitz invariant. Two other mechanisms of
soliton attraction are operative here but the universal mechanism considered in
this paper is found to be the most important one in some cases. Comparison with
the most extensively studied C-IC transition in shows that the
experimentally observed thermal anomalies can be understood as a result of the
smearing of the theoretically predicted discontinuous transition.Comment: 8 pages (extended version, title changed
Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario
We discuss the nuclear magnetic relaxation rate and the superfluid density
with the use of the effective five-band model by Kuroki et al. [Phys. Rev.
Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a
fully-gapped anisotropic \pm s-wave superconductivity consistently explains
experimental observations. In our phenomenological model, the gaps are assumed
to be anisotropic on the electron-like \beta Fermi surfaces around the M point,
where the maximum of the anisotropic gap is about four times larger than the
minimum.Comment: 10 pages, 8 figures; Submitted versio
Polarization separated Zeeman spectra from magnetic dipole transitions in highly charged argon in the large helical device
Visible spectral emission lines from magnetic dipole transitions in ArX, ArXI, ArXIV, and ArXV are observed from plasmas heated with neutral-beam injection (NBI) in the Large Helical Device [O. Motojima et al., Phys. Plasmas6, 1843 (1999)]. Orthogonal linearly polarized components of the emission line profiles are observed with a polarization separation optical system and high-resolution spectrometer. Zeeman split profiles reveal polarization characteristics of magnetic dipole transitions. Ion temperatures and emission locations are estimated from the profiles with the magnetic field information on the lines of sight (LOS). The spatially resolved emissions are observed by the array of absolutely calibrated views. The time histories of line profiles and emission intensities at the poloidal view are presented. The observed line profiles and the intensity distribution at the poloidal view indicate the localization of these charge states in the edge region just inside the last closed flux surface. The emission line of ArX in the tangential observation indicates Doppler shifts of the Zeeman split profiles. The velocity components of ArX ion flow along the LOS at the tangential view are 7.7 and 2.0km/s at the outer and inner edge plasmas, respectively, in the opposite direction to the NBI
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