6 research outputs found
Crystallographic Phase Transition and High-Tc Superconductivity in LaFeAsO:F
Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor,
exhibits a sharp decrease in the temperature-dependent resistivity at ~160 K.
The anomaly can be suppressed by F doping and the superconductivity appears
correspondingly, suggesting a close associate of the anomaly with the
superconductivity. We examined the crystal structures, magnetic properties and
superconductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO (Tc
= 20 K) by synchrotron X-ray diffraction, DC magnetic measurements, and ab
initio calculations to demonstrate that the anomaly is associated with a phase
transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160 K as
well as an antiferromagnetic transition at ~140 K. These transitions can be
explained by spin configuration-dependent potential energy surfaces derived
from the ab initio calculations. The suppression of the transitions is ascribed
to interrelated effects of geometric and electronic structural changes due to
doping by F- ions.Comment: 22 pages, 8 figures, 2 tables, Supplementary information is included
at the end of the document, accepted for publication in Supercond. Sci.
Techno
The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs
We report the temperature dependent x-ray powder diffraction of the
quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 K
and 95 K. We have detected the structural phase transition from the tetragonal
phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with
Cmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150
K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structural
phase transition is strongly reduced, by about ~30K, by increasing the internal
chemical pressure going from LaOFeAs to NdOFeAs. In contrast the
superconducting critical temperature increases from 27 K to 51 K going from
LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped
orthorhombic Cmma phase competes with the superconducting tetragonal phase.
Therefore by controlling the internal chemical pressure in new materials it
should be possible to push toward zero the critical temperature T0 of the
structural phase transition, giving the striped phase, in order to get
superconductors with higher Tc.Comment: 9 pages, 3 figure
Nernst effect of the new iron-based superconductor LaOFFeAs
We report the first Nernst effect measurement on the new iron-based
superconductor LaOFFeAs . In the normal state, the
Nernst signal is negative and very small. Below a large positive peak
caused by vortex motion is observed. The flux flowing regime is quite large
compared to conventional type-II superconductors. However, a clear deviation of
the Nernst signal from normal state background and an anomalous depression of
off-diagonal thermoelectric current in the normal state between and 50
K are observed. We propose that this anomaly in the normal state Nernst effect
could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde
Feshbach resonances and mesoscopic phase separation near a quantum critical point in multiband FeAs-based superconductors
High Tc superconductivity in FeAs-based multilayers (pnictides), evading
temperature decoherence effects in a quantum condensate, is assigned to a
Feshbach resonance (called also shape resonance) in the exchange-like interband
pairing. The resonance is switched on by tuning the chemical potential at an
electronic topological transition (ETT) near a band edge, where the Fermi
surface topology of one of the subbands changes from 1D to 2D topology. We show
that the tuning is realized by changing i) the misfit strain between the
superconducting planes and the spacers ii) the charge density and iii) the
disorder. The system is at the verge of a catastrophe i.e. near a structural
and magnetic phase transition associated with the stripes (analogous to the 1/8
stripe phase in cuprates) order to disorder phase transition. Fine tuning of
both the chemical potential and the disorder pushes the critical temperature Ts
of this phase transition to zero giving a quantum critical point. Here the
quantum lattice and magnetic fluctuations promote the Feshbach resonance of the
exchange-like anisotropic pairing. This superconducting phase that resists to
the attacks of temperature is shown to be controlled by the interplay of the
hopping energy between stripes and the quantum fluctuations. The
superconducting gaps in the multiple Fermi surface spots reported by the recent
ARPES experiment of D. V. Evtushinsky et al. arXiv:0809.4455 are shown to
support the Feshbach scenario.Comment: 31 pages, 7 figure