arXiv subjects

M. L. Amigó

Publications and source records attributed to M. L. Amigó.

6 recordsLinked to original sources

arXiv2021

The role of the nominal iron content in the structural, compositional and physical properties of BaFe$_{2+δ}$S$_3$

BaFe$_2$S$_3$ is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature, $T_N$, as well as the presence of superconductivity have been found to be sample dependent. It has been argued that the Fe content may play a decisive role, with the use of 5%mol excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether an Fe off-stoichiometry is actually present in the samples, and how it affects the structural, magnetic and transport properties. Here, we present a systematic study of compositional, structural and physical properties of BaFe$_{2+δ}$S$_3$ as a function of the nominal Fe excess $δ$. As $δ$ increases, we observe the presence of an increasing fraction of secondary phases but no systematic change in the composition or crystal structure of the main phase. Magnetic susceptibility curves are influenced by the presence of magnetic secondary phases. The previously reported maximum of $T_N$ at $δ$=0.1 was not confirmed. Samples with nominal $δ$=0 present the lowest $T_N$ and the resistivity anomaly at the highest temperature $T^*$ while, for $δ\geq 0.05$, both quantities and the transport gap are seemingly $δ$-independent. Finally, we show that crystals free of ferromagnetic spurious phases can be obtained by remelting samples with nominal $δ$=0.05 in a Bridgman process.

cond-mat.supr-con↗

arXiv2019

Microstructural control of the transport properties of $β$-FeSe films grown by sputtering

We have investigated the correlation between structural and transport properties in sputtered $β$-FeSe films grown onto SrTiO$_3$ (100). The growth parameters, such as substrate temperature and thickness, have been varied in order to explore different regimes. In the limit of textured thick films, we found promising features like an enhanced $T_{\rm c}\sim12\,$K, a relatively high $H_{\rm c2}$ and a low anisotropy. By performing magnetoresistance and Hall coefficient measurements, we investigate the influence of the disorder associated with the textured morphology on some features attributed to subtle details of the multi-band electronic structure of $β$-FeSe. Regarding the superconductor-insulator transition (SIT) induced by reducing the thickness, we found a non-trivial evolution of the structural properties and morphology associated with a strained initial growth and the coalescence of grains. Finally, we discuss the origin of the insulating behavior in high-quality stressed epitaxial thin films. We found that a lattice distortion, described by the Poisson's coefficient associated with the lattice parameters \textit{a} and \textit{c}, may play a key role.

cond-mat.supr-con↗

arXiv2017

Vortex dynamics in $β$-FeSe single crystals: effects of proton irradiation and small inhomogeneous stress

We report on the critical current density Jc and the vortex dynamics of pristine and 3 MeV proton irradiated (cumulative dose equal to 2x10^16 cm^-2) $β$-FeSe single crystals. We also analyze a remarkable dependence of the superconducting critical temperature Tc, Jc and the flux creep rate S on the sample mounting method. Free-standing crystals present Tc =8.4(1)K, which increases to 10.5(1)K when they are fixed to the sample holder by embedding them with GE-7031 varnish. On the other hand, the irradiation has a marginal effect on Tc. The pinning scenario can be ascribed to twin boundaries and random point defects. We find that the main effect of irradiation is to increase the density of random point defects, while the embedding mainly reduces the density of twin boundaries. Pristine and irradiated crystals present two outstanding features in the temperature dependence of the flux creep rate: S(T) presents large values at low temperatures, which can be attributed to small pinning energies, and a plateau at intermediate temperatures, which can be associated with glassy relaxation. From Maley analysis, we observe that the characteristic glassy exponent μ changes from ~ 1.7 to 1.35-1.4 after proton irradiation.

cond-mat.supr-con↗

arXiv2017

Intrinsic pinning by naturally occurring correlated defects in FeSe$_\text{1-x}$Te$_\text{x}$ superconductors

We study the angular dependence of the dissipation in the superconducting state of FeSe and Fe(Se$_\text{1-x}$Te$_\text{x}$) through electrical transport measurements, using crystalline intergrown materials. We reveal the key role of the inclusions of the non superconducting magnetic phase Fe$_\text{1-y}$(Se$_\text{1-x}$Te$_\text{x}$), growing into the Fe(Se$_\text{1-x}$Te$_\text{x}$) pure $β$-phase, in the development of a correlated defect structure. The matching of both atomic structures defines the growth habit of the crystalline material as well as the correlated planar defects orientation.

cond-mat.supr-con↗

arXiv2016

Normal state magnetotransport properties of $β$-FeSe superconductors

We present $β$-FeSe magnetotransport data, and describe them theoretically. Using a simplified microscopic model with two correlated effective orbitals, we determined the normal state electrical conductivity and Hall coefficient, using Kubo formalism. With model parameters relevant for Fe-chalcogenides, we describe the observed effect of the structural transition on the ab-plane electrical resistivity, as well as on the magnetoresistance. Temperature-dependent Hall coefficient data were measured at 16 Tesla, and their theoretical description improves upon inclusion of moderate electron correlations. We confirm the effect of the structural transition on the electronic structure, finding deformation-induced band splittings comparable to those reported in angle-resolved photoemission.

cond-mat.supr-con↗

arXiv2014

Multiband character of $β$-FeSe: Angular dependence of the magnetoresistance and upper critical field

We studied $ab$-plane transport properties in single crystals of the superconductor $β$-FeSe up to 16 T. In the normal state, below 90 K, the crystals present a strongly anisotropic positive magnetoresistance that becomes negligible above that temperature. In the superconducting state (T$_c$=8.87(5) K) the upper critical field anisotropy $H$$_{c2}$$\parallel$$ab$ / $H$$_{c2}$$\parallel$$c$ changes with temperature and the angular dependence of the dissipation for fixed temperatures and fields reflects a strongly anisotropic behavior. Our results make evident that multiband effects are needed to describe the measured transport properties. We model the magnetoresistance and upper critical field behavior with a two-band model showing that the diffusivities ratio parameter remains unchanged going from the normal to the superconducting state.

cond-mat.supr-con↗