124 research outputs found
Fishtail effect and vortex dynamics in LiFeAs single crystals
We investigate the fishtail effect, critical current density () and
vortex dynamics in LiFeAs single crystals. The sample exhibits a second peak
(SP) in the magnetization loop only with the field c-axis. We calculate a
reasonably high , however, values are lower than in 'Ba-122' and
'1111'-type FeAs-compounds. Magnetic relaxation data imply a strong pinning
which appears not to be due to conventional defects. Instead, its behavior is
similar to that of the triplet superconductor SrRuO. Our data suggest
that the origin of the SP may be related to a vortex lattice phase transition.
We have constructed the vortex phase diagram for LiFeAs on the
field-temperature plane.Comment: 5 pages, 5 figure
Resistivity and Hall effect of LiFeAs: Evidence for electron-electron scattering
LiFeAs is unique among the broad family of FeAs-based superconductors,
because it is superconducting with a rather large K under
ambient conditions although it is a stoichiometric compound. We studied the
electrical transport on a high-quality single crystal. The resistivity shows
quadratic temperature dependence at low temperature giving evidence for strong
electron-electron scattering and a tendency towards saturation around room
temperature. The Hall constant is negative and changes with temperature, what
most probably arises from a van Hove singularity close to the Fermi energy in
one of the hole-like bands. Using band structure calculations based on angular
resolved photoemission spectra we are able to reproduce all the basic features
of both the resistivity as well as the Hall effect data.Comment: 6 pages, 3 figures included; V2 has been considerably revised and
contain a more detailed analysis of the Hall effect dat
Epitaxial Bi2FeCrO6 Multiferroic Thin Films
We present here experimental results obtained on Bi2FeCrO6 (BFCO) epitaxial
films deposited by laser ablation directly on SrTiO3 substrates. It has been
theoretically predicted, by Baettig and Spaldin, using first-principles density
functional theory that BFCO is ferrimagnetic (with a magnetic moment of 2 Bohr
magneton per formula unit) and ferroelectric (with a polarization of ~80
microC/cm2 at 0K). The crystal structure has been investigated using X-ray
diffraction which shows that the films are epitaxial with a high crystallinity
and have a degree of orientation depending of the deposition conditions and
that is determined by the substrate crystal structure. Chemical analysis
carried out by X-ray Microanalysis and X-ray Photoelectron Spectroscopy (XPS)
indicates the correct cationic stoichiometry in the BFCO layer, namely
(Bi:Fe:Cr = 2:1:1). XPS depth profiling revealed that the oxidation state of Fe
and Cr ions in the film remains 3+ throughout the film thickness and that both
Fe and Cr ions are homogeneously distributed throughout the depth.
Cross-section high-resolution transmission electron microscopy images together
with selected area electron diffraction confirm the crystalline quality of the
epitaxial BFCO films with no identifiable foreign phase or inclusion. The
multiferroic character of BFCO is proven by ferroelectric and magnetic
measurements showing that the films exhibit ferroelectric and magnetic
hysteresis at room temperature. In addition, local piezoelectric measurements
carried out using piezoresponse force microscopy (PFM) show the presence of
ferroelectric domains and their switching at the sub-micron scale.Comment: Accepted for publication in Philosophical Magazine Letter
Specific heat of CaNaFeAs single crystals: unconventional s multi-band superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings
We report a low-temperature specific heat study of high-quality single
crystals of the heavily hole doped superconductor
CaNaFeAs. This compound exhibits bulk
superconductivity with a transition temperature \,K, which is
evident from the magnetization, transport, and specific heat measurements. The
zero field data manifests a significant electronic specific heat in the normal
state with a Sommerfeld coefficient mJ/mol K. Using a
multi-band Eliashberg analysis, we demonstrate that the dependence of the zero
field specific heat in the superconducting state is well described by a
three-band model with an unconventional s pairing symmetry and gap
magnitudes of approximately 2.35, 7.48, and -7.50 meV. Our analysis
indicates a non-negligible attractive intraband coupling,which contributes
significantly to the relatively high value of . The Fermi surface averaged
repulsive and attractive coupling strengths are of comparable size and outside
the strong coupling limit frequently adopted for describing high- iron
pnictide superconductors. We further infer a total mass renormalization of the
order of five, including the effects of correlations and electron-boson
interactions.Comment: 8 Figures, Submitted to PR
Growth, Structure and Properties of BiFeO3-BiCrO3 Films obtained by Dual Cross Beam PLD
The properties of epitaxial Bi2FeCrO6 thin films, recently synthesized by
pulsed laser deposition, have partially confirmed the theoretical predictions
(i.e. a magnetic moment of 2 muB per formula unit and a polarization of ~80
microC/cm2 at 0K). The existence of magnetic ordering at room temperature for
this material is an unexpected but very promising result that needs to be
further investigated. Since magnetism is assumed to arise from the exchange
interaction between the Fe and Cr cations, the magnetic behaviour is strongly
dependent on both their ordering and the distance between them. We present here
the successful synthesis of epitaxial Bi2FexCryO6 (BFCO x/y) films grown on
SrTiO3 substrates using dual crossed beam pulsed laser deposition. The crystal
structure of the films has different types of (111)-oriented superstructures
depending on the deposition conditions. The multiferroic character of BFCO
(x/y) films is proven by the presence of both ferroelectric and magnetic
hysteresis at room temperature. The oxidation state of Fe and Cr ions in the
films is shown to be 3+ only and the difference in macroscopic magnetization
with Fe/Cr ratio composition could only be due to ordering of the Cr3+ and Fe3+
cations therefore to the modification of the exchange interaction between them.Comment: Manuscript accepted for publication in IEEE-UFF
Switching properties of self-assembled ferroelectric memory cells
In this letter, we report on the switching properties of an ordered system of Bi4Ti3O12 ferroelectric memory cells of an average lateral size of 0.18 μm formed via a self-assembling process. The ferroelectricity of these cells has been measured microscopically and it has been demonstrated that an individual cell of 0.18 μm size is switching. Switching of single nanoelectrode cells was achieved via scanning force microscopy working in piezoresponse mode
Raman Evidence for Superconducting Gap and Spin-Phonon Coupling in Superconductor Ca(Fe0.95Co0.05)2As2
Inelastic light scattering studies on single crystal of electron-doped
Ca(Fe0.95Co0.05)2As2 superconductor, covering the tetragonal to orthorhombic
structural transition as well as magnetic transition at TSM ~ 140 K and
superconducting transition temperature Tc ~ 23 K, reveal evidence for
superconductivity-induced phonon renormalization; in particular the phonon mode
near 260 cm-1 shows hardening below Tc, signaling its coupling with the
superconducting gap. All the three Raman active phonon modes show anomalous
temperature dependence between room temperature and Tc i.e phonon frequency
decreases with lowering temperature. Further, frequency of one of the modes
shows a sudden change in temperature dependence at TSM. Using first-principles
density functional theory-based calculations, we show that the low temperature
phase (Tc < T < TSM) exhibits short-ranged stripe anti-ferromagnetic ordering,
and estimate the spin-phonon couplings that are responsible for these phonon
anomalies
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