arXiv subjects

M. ElMassalami

Publications and source records attributed to M. ElMassalami.

14 recordsLinked to original sources

arXiv2025

Pressure-induced hole delocalization in the strongly correlated quasicubic charge-transfer perovskite $LaBa_2Fe_3O_{8+\delta}$d

Analysis of the thermal and baric evolution of resistance in $LaBa_2Fe_3O_{8+\delta}$ enabled the construction of its pressure-temperature (P-T) phase diagram, which prominently displays a critical boundary, $P^{MIT}_c(T)$, marking the transition from localized to hole-type extended states. The relatively low critical pressures [$P^{MIT}_c(T) \approx 3$-8 GPa] suggest that, as $P \rightarrow P_c$ in this narrow-gap, strongly correlated charge-transfer system, both the hybridization strength and the charge-transfer character are progressively enhanced - ultimately leading to the emergence of metallicity. Emphasizing the electronic nature of this transition, pressure-dependent structural analyses at room temperature reveal no associated structural phase transition at $P^{MIT}_c(T)$; the system retains a (weakly tetragonally distorted) quasicubic perovskite structure with Murnaghan-type compressibility up to 30\,GPa. The emergence of hole delocalization and metallic conduction, coupled with suppressed antiferromagnetism, suggests proximity to quantum criticality.

cond-mat.str-el↗

arXiv2021

Quantum-Critical, Spin-Fluctuation-driven Residual Resistivity and Emergent Universal Correlations in the Fermi-Liquid Regime of Heavy-Fermion Superconductors

We investigate correlations within the unconventional Fermi-liquid (FL) regime of quantum-critical (QC) heavy-fermion superconductors (HFSs) by tracking the pressure dependence of three quantities: the temperature-independent, SF-driven residual resistivity, $\rho^{ sf}_{0}(P)$; the FL scattering coefficient, $A(P)$; and the superconducting transition temperature, $T_c(P)$. The first two define the spin-fluctuation contribution to the resistivity, $\rho(T)=\rho^{sf}_0+AT^2$. Using experimental data from archetypal heavy-fermion systems, we identify three robust empirical correlations: $\ln(\frac{T_c}{\theta}) \propto A^{-1/2}$, $A \propto (\rho^{sf}_0)^2$, and $\ln(\frac{T_c}{\theta}) \propto \big(\rho^{sf}_0\big)^{-1}$ ($\theta$ is a characteristic temperature scale). Absent in conventional FL superconductors, these relationships indicate that QC fluctuations not only mediate inelastic scattering and Cooper pairing, but also generate an effective elastic channel responsible for $\rho^{sf}_0$. We explicitly calculate $\rho^{sf}_0$ on the high-pressure side of the quantum critical point (QCP) and introduce a characteristic length scale, $\ell \sim \big(\rho^{sf}_0\big)^{-1}$, that captures the spatial extent of fluctuation-induced scattering. Within this regime, and within the Migdal--Eliashberg framework combined with Boltzmann transport theory, we derive analytic expressions for $T_c(\ell)$ and $A(\ell)$, together with their interrelations, which are consistent with the observed empirical trends. These findings highlight the quantum-critical FL regime in HFSs as an intrinsically correlated phase, governed by fluctuations and marked by unconventional transport and pairing mechanisms.

cond-mat.str-el↗

arXiv2020

Defect-engineered, universal kinematic correlations between superconductivity and Fermi liquid transport

Identifying universal scaling relations between two or more variables in a complex system plays a pivotal role in understanding various phenomena in different branches of science. Examples include the allometric scaling among food webs in biology, the scaling relationship between fluid flow and fracture stiffness in geophysics, and the gap-to-$T_c$ ratio, between energy gap, $Δ$, and critical transition temperature, $T_c$, hallmarks of superconductivity. Kinematics, in turn, is the branch of physics that governs the motion of bodies by imposing constraints correlating their masses, momenta, and energy; it is an essential ingredient for the analysis of high-energy quarkonium production, galaxy formation, as well as the $ρ_\circ+AT^2$ contribution to the normal state resistivity in a Fermi liquid (FL), $ρ_\circ$ being a measure of disorder and $A$ the hallmark of FL. Here, we report on the identification of a novel, universal kinematic scaling relation between $T_c(ρ_\circ)$ and $A(ρ_\circ)$ found in a plethora of defect-bearing conventional and non-conventional superconductors within their FL regime. We traced back this relation to the triggering and stabilization of an electron-electron scattering channel within a very specific, yet common, type of amorphized regions, ubiquitous in all such superconductors. Our theoretical treatment consisted of, first, analyzing the construct of a distorted lattice as a mimic of the kinematic aftermath of the formation of such amorphized regions. Then we applied standard many body techniques to derive expressions for $T_c(ρ_\circ)$, $A(ρ_\circ)$, and their correlations. Our results are in agreement with experiments and provide a solid theoretical foundation for reconciling superconductivity with FL transport in these systems.

cond-mat.supr-con↗

arXiv2018

A mean-field approach to Kondo-attractive-Hubbard model

With the purpose of investigating coexistence between magnetic order and superconductivity, we consider a model in which conduction electrons interact with each other, via an attractive Hubbard on-site coupling $U$, and with local moments on every site, via a Kondo-like coupling, $J$. The model is solved on a simple cubic lattice through a Hartree-Fock approximation, within a `semi-classical' framework which allows spiral magnetic modes to be stabilized. For a fixed electronic density, $n_c$, the small $J$ region of the ground state ($T=0$) phase diagram displays spiral antiferromagnetic (SAFM) states for small $U$. Upon increasing $U$, a state with coexistence between superconductivity (SC) and SAFM sets in; further increase in $U$ turns the spiral mode into a Néel antiferromagnet. The large $J$ region is a (singlet) Kondo phase. At finite temperatures, and in the region of coexistence, thermal fluctuations suppress the different ordered phases in succession: the SAFM phase at lower temperatures and SC at higher temperatures; also, reentrant behaviour is found to be induced by temperature. Our results provide a qualitative description of the competition between local moment magnetism and superconductivity in the borocarbides family.

cond-mat.supr-con↗

arXiv2017

Enhanced superconductivity, Kondo behavior and negative-curvature resistivity of oxygen-irradiated thin films of aluminium

We followed the evolution of the normal and superconducting properties of Al thin films after each session of various successive oxygen irradiations at ambient temperature. Such irradiated films, similar to the granular ones, exhibit enhanced superconductivity, Kondo behavior and negative-curvature resistivity. Two distinct roles of oxygen are identified: as a damage-causing projectile and as an implanted oxidizing agent. The former gives rise to the processes involved in the conventional recovery stages. The latter, considered within the context of the Cabrera-Mott model, gives rise to a multistep process which involves charges transfer and creation of stabilized vacancies and charged defects. Based on the outcome of this multistep process, we consider (i) the negative curvature resistivity as a manifestation of a thermally-assisted liberation of trapped electric charges, (ii) the Kondo contribution as a spin-flip scattering from paramagnetic, color-center-type defects, and (iii) the enhancement of T_{c} as being due to a lattice softening facilitated by the stabilized defects and vacancies. The similarity in the phase diagrams of granular and irradiated films as well as the aging effects are discussed along the same line of reasoning.

cond-mat.supr-con↗

arXiv2014

On the superconductivity of the Li$_{x}$RhB$_{y}$ compositions

We observed superconductivity ($T_{c}$ $\simeq$2-3 K) in Li$_{x}$RhB$_{y}$ intermetallics wherein $x$ and $y$ vary over a wide compositional range. The crystal structure consists of cubic unit-cell ($a$ $\simeq$ 12.1 Å) with centro-symmetric space group $Pn\bar{3}n$. A weak but positive pressure-induced increase of $T_{c}$ was observed. The correlations between the composition and each of the followings were followed over a wide range of $x$ and $y$: the unit-cell dimensions, $T_{c}$ , Sommerfeld coefficient $γ$, Debye temperature $θ_{D}$, and critical fields $H$$_{c1}$ and $H$$_{c2}$. The thermal evolution of the electronic specific heat within the superconducting phase was observed to follow a quadratic-in-$T$ behavior. In addition, a paramagnetic Meissner Effect (PME) is manifested during a low-field-cooled magnetization cycle. This manifestation of quadratic-in-$T$ behavior and PME feature will be discussed.

cond-mat.supr-con↗

arXiv2014

Pressure-dependent magnetization and magnetoresistivity studies on the tetragonal FeS (mackinawite): revealing its intrinsic metallic character

The transport and magnetic properties of the tetragonal Fe$_{1+δ}$S were investigated using magnetoresistivity and magnetization within 2$\leq T\leq $300 K, $H\leq$70 kOe and $P\leq$ 3.0 GPa. In addition, room-temperature X-ray diffraction and photoelectron spectroscopy were also applied. In contrast to previously reported nonmetallic character, Fe$_{1+δ}$S is intrinsically metallic but due to a presence of a weak localization such metallic character is not exhibited below room temperature. An applied pressure reduces strongly this additional resistive contribution and as such enhances the temperature range of the metallic character which, for $\sim$3 GPa, is evident down to 75 K. The absence of superconductivity as well as the mechanism behind the weak localization will be discussed.

cond-mat.str-el↗

arXiv2014

On the bad metallicity and phase diagrams of Fe$_{1+δ}X$ ($X$ =Te, Se, S, solid solutions): an electrical resistivity study

Based on a systematic analysis of the thermal evolution of the resistivities of Fe-based chalcogenides Fe$_{1+δ}$Te$_{1-x}X_{x}$ ($X$= Se, S), it is inferred that their often observed nonmetallic resistivities are related to a presence of two resistive channels: one is a high-temperature thermally-activated process while the other is a low-temperature log-in-$T$ process. On lowering temperature, there are often two metal-to-nonmetall crossover events: one from the high-$T$ thermally-activated nonmetallic regime into a metal-like phase and the other from the log-in-$T$ regime into a second metal-like phase. Based on these events, together with the magnetic and superconducting transitions, a phase diagram is constructed for each series. We discuss the origin of both processes as well as the associated crossover events. We also discuss how these resistive processes are being influenced by pressure, intercalation, disorder, doping, or sample condition and, in turn, how these modifications are shaping the associated phase diagrams.

cond-mat.supr-con↗

arXiv2012

Evolution of the magnetism of Tb(Co_{x}Ni_{1-x})_{2}B_{2}C

The magnetic properties of polycrystalline Tb(Co_{x}Ni_{1-x})_{2}B_{2}C (x=0.2,0.4,0.6,0.8) samples were probed by magnetization, specific heat, ac susceptibility, and resistivity techniques. For x{\neq}0.4, the obtained curves are consistent with the features expected for the corresponding magnetic modes, namely k_{1}=(0.55,0,0) at x=0; k_{2}=([nicefrac]\nicefrac{1}{2} ,0,[nicefrac] \nicefrac{1}{2}) at x= 0.2; k_{3}=(0,0,[nicefrac]\nicefrac{1}{3}) at x= 0.6, and k_{4}=(0,0,0) at x= 0.8 and 1. For x=0.4, even though the neutron diffraction indicates a k_{2} mode, but with a reduced magnetic moment, the magnetization, the ac susceptibility, and resistivity indicate two magnetic events; furthermore, deviation from Curie-Weiss behavior is observed below 150 K for this sample. These features, together with the evolution of both magnetic moment and critical temperature, are attributed to an interplay between competing magnetic couplings; for the particular x=0.4 case, additional factors such as crystalline electric field effects may be in operation.

cond-mat.str-el↗

arXiv2012

Tuning in magnetic modes in Tb(Co_{x}Ni_{1-x})_{2}B_{2}C: from longitudinal spin-density waves to simple ferromagnetism

Neutron diffraction and thermodynamics techniques were used to probe the evolution of the magnetic properties of Tb(Co_{x}Ni_{1-x})_{2}B_{2}C. A succession of magnetic modes was observed as x is varied: the longitudinal modulated k=(0.55,0,0) state at x=0 is transformed into a collinear k=([nicefrac] \nicefrac{1}{2} ,0,[nicefrac] \nicefrac{1}{2} ) antiferromagnetic state at x= 0.2, 0.4; then into a transverse c-axis modulated k=(0,0,[nicefrac] \nicefrac{1}{3} ) mode at x= 0.6, and finally into a simple ferromagnetic structure at x= 0.8 and 1. Concomitantly, the low-temperature orthorhombic distortion of the tetragonal unit cell at x=0 is reduced smoothly such that for x >= 0.4 only a tetragonal unit cell is manifested. Though predicted theoretically earlier, this is the first observation of the k=(0,0,[nicefrac] \nicefrac{1}{3} ) mode in borocarbides; our findings of a succession of magnetic modes upon increasing x also find support from a recently proposed theoretical model. The implication of these findings and their interpretation on the magnetic structure of the RM_{2}B_{2}C series are also discussed.

cond-mat.str-el↗

arXiv2011

On the anomalous thermal evolution of the low-temperature, normal-state specific heat of various nonmagnetic intermetallic compounds

The low-temperature normal-state specific heat and resistivity curves of various nonmagnetic intermetallic compounds manifest an anomalous thermal evolution. Such an anomaly is exhibited as a break in the slope of the linearized C/T versus T^2 curve and as a drop in the R versus T curve, both at the same T_βγ. It is related, not to a thermodynamic phase transition, but to an anomaly in the density of states curves of the phonon or electron subsystems. On representing these two anomalies as additional Dirac-type delta functions, situated respectively at kB.θ_L (for lattice) and kB.θ_E (for electrons), an analytical expression for the total specific heat can be obtained. A least-square fit of this expression to experimental specific heat curves of various compounds reproduced satisfactorily all the features of the anomalous thermal evolution. The obtained fit parameters (in particular the Sommerfeld constant, γ_{0}, and Debye temperatures, θ_D, compare favorably with the reported values. Furthermore, the analysis shows that (i) (T_βγ) / (θ_D) = 0.2(1\pm1/\surd6) and (ii) γ_{0} {\propto} (θ_D)^2; both relations are in a reasonable agreement with the experimental results. Finally, this analysis (based on the above arguments) justifies the often-used procedure that treats the above anomaly in terms of either a thermal variation of θ_D or an additional Einstein mode.

cond-mat.mtrl-sci↗

arXiv2008

Magnetic structures of quaternary intermetallic borocarbides RCo2B2C (R=Dy, Ho, Er)

The magnetic structures of the title compounds have been studied by neutron diffraction. In contrast to the isomorphous RNi2B2C compounds wherein a variety of exotic incommensurate modulated structures has been observed, the magnetic structure of ErCo2B2C is found to be collinear antiferromagnet with k=((1/2),0,(1/2)) while that of HoCo2B2C and DyCo2B2C are observed to be simple ferromagnets. For all studied compounds, the moments are found to be confined within the basal plane and their magnitudes are in good agreement with the values obtained from the low-temperature isothermal magnetization measurements. The absence of modulated magnetic structures in the RCo2B2C series (for ErCo2B2C, verified down to 50 mK) is attributed to the quenching of the Fermi surface nesting features.

cond-mat.str-el↗

arXiv2008

On the ferromagnetic structure of the intermetallic borocarbide TbCo2B2C

Based on magnetization, specific heat, magnetostriction, and neutron diffraction studies on single-crystal TbCo2B2C, it is found out that the paramagnetic properties, down to liquid nitrogen temperatures, are well described by a Curie-Weiss behavior of the Tb+3 moments. Furthermore, below Tc= 6.3 K, the Tb-sublattice undergoes a ferromagnetic (FM) phase transition with the easy axis being along the (100) direction and, concomitantly, the unit cell undergoes a tetragonal-to-orthorhhombic distortion. For fields up to 90 kOe, no field-induced splitting of the Co 3d orbitals was observed; as such the internal field must be well below the critical value needed to polarize the Co 3d subsystem. The manifestation of a FM state in TbCo2B2C is unique among all other isomorphous borocarbides, in particular TbNi2B2C (Tn=15 K, incommensurate modulated magnetic state) even though the Tb-ions in both isomorphs have almost the same crystalline electric field properties. The difference in the magnetic modes of these Tb-based isomorphs is attributed to a difference in their exchange couplings caused by a variation in their lattice parameters and in the position of their Fermi levels.

cond-mat.str-el↗

arXiv2008

Synthesis and magnetic characterization of TmCo2B2C

A new quaternary intermetallic borocarbide TmCo2B2C has been synthesized via a rapid-quench of an arc-melted ingot. Elemental and powder-diffraction analyses established its correct stoichiometry and single-phase character. The crystal structure is isomorphous to that of TmNi2B2C(I4/mmm) and is stable over the studied temperature range. Above 7 K, the paramagnetic state follows the modified Curie-Weiss behavior (X=C/(T-theta)+X0 wherein X0=0.008(1) emu/mole and the temperature-dependent term reflecting the paramagnetism of the Tm subsystem: ueff=7.6(2) uB [in agreement with the expected value for a free Tm3+ ion] and theta = -4.5(3) K. Long range ferromagnetic order of the Tm sublattice is observed to develop around ~1 K. No superconductivity is detected in TmCo2B2C down to 20 mK, a feature which is consistent with the general trend in the RCo2B2C series. Finally, the influence of the rapid-quench process on the magnetism (and superconductivity) of TmNi2B2C will be discussed and compared to that of TmCo2B2C.

cond-mat.str-el↗