Fishtail effect and vortex dynamics in LiFeAs single crystals

We investigate the fishtail effect, critical current density ($J_c$) 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 $J_c$, 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 Sr$_2$RuO$_4$. 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 $T_c\simeq 18$ 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 Ca0.32_{0.32}Na0.68_{0.68}Fe2_2As2_2 single crystals: unconventional s±_\pm 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 Ca$_{0.32}$Na$_{0.68}$Fe$_2$As$_2$. This compound exhibits bulk superconductivity with a transition temperature $T_c \approx 34$\,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 $\gamma \approx 53$ mJ/mol K$^{2}$. 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$_\pm$ pairing symmetry and gap magnitudes $\Delta_i$ 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 $T_c$. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-$T_c$ 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