arXiv subjects

Mohammed ElMassalami

Publications and source records attributed to Mohammed ElMassalami.

5 recordsLinked to original sources

arXiv2021

Structural metastability and Fermi surface Topology of SrAl2Si2

SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, αphase or a superconducting, BaZn2P2-type, βphase. We explore possible α--Pc;Tc--> βtransformations by employing pressure- and temperature-dependent free-energy calculations, vibrational spectra calculations, and room-temperature synchrotron X-ray powder diffraction (XRPD) measurements up to 14 GPa using diamond anvil cell. Our theoretical and empirical analyses together with all baric and thermal reported events on both phases allow us to construct a preliminary P-T diagram of transformations. Our calculations show a relatively low critical pressure for the αto βtransition (4.9 GPa at 0 K, 5.0 GPa at 300 K and 5.3 GPa at 900 K); nevertheless, our nonequilibrium analysis indicates that the low-pressure-low-temperature αphase is separated from metastable βphase by a relatively high activation barrier. This analysis is supported by our XRPD data at ambient temperature and P < 14 GPa which shows an absence of βphase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl2Si2-dimorphs or total chemical substitution of Ca by Sr in isomorphous αCaAl2Si2; empirically, manifestation of such topology modification is evident when comparing the evolution of (magneto-)transport properties of members of SrAl2Si2-dimorphs and αisomorphs.

cond-mat.mtrl-sci↗

arXiv2020

Enhancement of giant refrigerant capacity in Ho$_{1-x}$Gd$_{x}$B$_{2}$ alloys (0.1 $\leq$ x $\leq$ 0.4)

Intending to optimize the giant magnetocaloric properties of HoB$_{2}$, we synthesized and magnetocalorically characterized Ho$_{1-x}$Gd$_{x}$B$_{2}$(0.1 $\leq$ x $\leq$ 0.4) alloys. We found out that Gd enters stoichiometrically and randomly into the Ho site, leading to a Vegard-type structural change. The addition of spherical S$^{7/2}$ Gd$^{3+}$ moments prompts an enhancement in Curie temperature, a reduction in peak value of the magnetic entropy change while still being relatively high, and a broadening of the magnetic entropy change curves. The overall influence is a relatively high refrigerant capacity and relative cooling power, and an extension of the thermal working range to higher temperatures; thus, electing Ho$_{1-x}$Gd$_{x}$B$_{2}$(as potential candidates for cryogenic refrigeration applications.

cond-mat.mtrl-sci↗

arXiv2017

Phase-separation control of K$_x$Fe$_{2-y}$Se$_2$ superconductor through rapid-quenching process

K$_x$Fe$_{2-y}$Se$_2$ exhibits an iron-vacancy ordering at $T_{\rm s} {\sim}270°$C and separates into two phases: a minor superconducting (iron-vacancy-disordered) phase and a major non-superconducting (iron-vacancy-ordered) phase. The microstructural and superconducting properties of this intermixture can be tuned by an appropriate control of the quenching process through $T_{\rm s}$. A faster quenching rate leads to a finer microstructure and a suppression of formation of the non-superconducting phase by up to 50%. Nevertheless, such a faster cooling rate does induce a monotonic reduction in the superconducting transition temperature (from 30.7 K down to 26.0 K) and, simultaneously, a decrease in the iron content within the superconducting phase such that the compositional ratio changed from K$_{0.35}$Fe$_{1.83}$Se$_2$ to K$_{0.58}$Fe$_{1.71}$Se$_2$.

cond-mat.supr-con↗

arXiv2011

Linear magnetoresistivity in the ternary AM2B2 and A3Rh8B6 phases (A = Ca, Sr; M = Rh, Ir)

We studied the magnetoresistivity of the AM2B2 and A3Rh8B6 (A = Ca, Sr; M = Rh, Ir) compounds within the ranges 1.8<=T<=300 K and 0<=H<=50 kOe. The zero-field resistivity ρ0(T) is metallic and follows closely the Bloch-Grüneisen description. A positive, nonsaturating, and dominantly linear-in-H magnetoresistivity was observed in all samples, including the ones with a superconducting ground state. Such ΔρT(H)/ρT(0), reaching 1200% in favorable cases, was found to be much stronger for the AM2B2 compounds and to decrease with temperature as well as when Ca is replaced by Sr, or Rh is replaced by Ir. Finally, the general features of the observed magnetoresistivity will be discussed in terms of the Abrikosov model for the linear magnetoresistivity in inhomogeneous materials.

cond-mat.supr-con↗

arXiv2011

A New Noncentrosymmetric Superconducting Phase in the Li-Rh-B System

Superconductivity, at 2-3 K, was observed in a novel phase of the ternary Li-Rh-B system. The structural phase exhibits a large noncentrosymmetric cubic unit cell with the a-parameter being within 1.208 \leq a \leq 1.215 nm. This phase is stable over a wider compositional range of LixRhBy (0.6 < x < 2, 1 < y < 2). The superconductivity, as well as the unit cell volume, is sensitive to the Li/B content but it is manifested with Tc \geq 1.8 K over a wider compositional range: the highest Tc \approx 3 K occurs for x : y \approx 0.9:1.5 with a \approx 1.209 nm. The superconducting shielding fraction of most samples is almost 80% of that of Sn. The lower critical field, Hc1(0), is ~65 Oe while the upper one, Hc2(0) is determined from extrapolation to be higher than 14 kOe. We discuss the influence of pressure on Tc and also the influence of the lack of inversion symmetry on the superconducting properties.

cond-mat.supr-con↗