arXiv subjects

Yasmine Sassa

Publications and source records attributed to Yasmine Sassa.

At least 19 recordsLinked to original sources

arXiv2026

Persistent Uncorrelated Magnetic Domains in Fe/Si Multilayers and their suppression by incorporating 11B4C

This study investigates magnetic domains in Fe/Si and Fe/Si + B4C multilayers using spin flip off-specular polarized neutron reflectometry. The results show that Fe/Si multilayers exhibit pronounced spin flip off-specular scattering originating from magnetic domains that are uncorrelated out of plane. With increasing external magnetic field the domains progressively coalesce and their magnetization rotates toward alignment with the applied field, approaching a homogeneous magnetic state at higher fields. In contrast, Fe/Si + B4C multilayers exhibit no detectable spin flip off-specular scattering already at low fields, indicating that the multilayer reaches magnetic saturation at significantly lower applied fields. The scattering patterns are interpreted using distorted wave Born approximation simulations in BornAgain, enabled by our added code for simulating magnetic domains and magnetic ordering. To further probe the magnetic behavior, low-energy mu+SR measurements were performed, representing the first mu+SR investigation of polarizing neutron optics multilayers. Together with comparison to previously reported VSM data, these measurements provide insight into the magnetic behavior across short range, medium range, and long range length scales. The results show that incorporating approximately 15 vol.% B4C makes the magnetic configuration highly responsive to external magnetic fields, with clear sensitivity to both in-plane and out-of-plane field geometries. These results show that B4C suppresses magnetic domains and spin flip off-specular scattering, improving Fe/Si coatings for neutron polarization optics in regards to off-specular scattering, and other applications requiring easy magnetic manipulation.

cond-mat.mtrl-sci↗

arXiv2025

Topological Metal-Insulator Transition within the Ferromagnetic state

A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions (MITs) are central to bridging these two domains while simultaneously serving as 'on-off' switches for electronic states. Here, we demonstrate how the prototypical material of K2Cr8O16 undergoes a ferromagnetic MIT accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we demonstrate that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological MIT within the ferromagnetic phase (topological-FM-MIT) with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. This work pioneers the discovery of a topological-FM-MIT and represents a fundamentally new class of topological phase transitions, revealing a unique pathway through which magnetism, topology, and electronic correlations interact.

cond-mat.str-el↗

arXiv2025

Characterization of Nuclear and Magnetic Structures of Wolframite-Type MgReO4 and ZnReO4

We utilized high-pressure methods to synthesize the oxides AReO$4$ (A=Zn, Mg) and characterized their crystal structures as monoclinic wolframite-type. By combining muon spin spectroscopy ($μ^+$SR) with DFT calculations for muon stopping sites, we identify two possible magnetic spin structures for both compounds: $Γ_3$ with the propagation vector $\mathbf{k} = (0,1/2,0)$ and $Γ_4$ with $\mathbf{k} = (0,0,0)$. In both cases, the magnetic moments are canted from the principal axes within the $ac$-plane. The ordered moment of the proposed structures is $\mathbf{0.29(5)~μ\mathrm{B}}$ for $Γ_3$ and $\mathbf{0.25(8)~μ_\mathrm{B}}$ for $Γ_4$. The low moment is consistent with the absence of a magnetic contribution to the neutron powder diffraction (NPD) spectra. Bond valence sum (BVS) analysis supports the oxidation state of Re being Re$^{6+}$ in the compounds, and we suggest that a combination of $t_\mathrm{2g}$ orbital splitting due to spin-orbit coupling (SOC) and $d$-$p$ orbital hybridization is responsible for the strongly suppressed ordered magnetic moment.

cond-mat.mtrl-sci↗

arXiv2025

Concurrent Multifractality and Anomalous Hall Response in the Nodal Line Semimetal Fe$_3$GeTe$_2$ Near Localization

Topological states of matter exhibit unique protection against scattering by disorder. Different topological classes exhibit distinct forms and degrees of protection. Here, we investigate the response of the ferromagnetic nodal line semimetal Fe$_3$GeTe$_2$ to disorder and electronic interactions. By combining global magneto-transport with atomic-scale scanning tunneling spectroscopy we find a simultaneous onset of diverse phenomena below a common temperature scale of about 15 K: A crossover from metallic to insulating temperature dependence of the longitudinal resistivity, saturation of the anomalous Hall conductivity to its maximal value, formation of a sharp zero-bias dip in the tunneling density of state, and emergence of multi-fractal structure of the electronic wavefunction peaking at the Fermi energy. These concurrent observations reflect the emergence of a novel energy scale possibly related to the opening of a gap in the nodal line band of Fe$_3$GeTe$_2$. Our study provides overarching insight into the role of disorder, electronic interactions and Berry curvature in setting the micro- and macro-scale responses of topological semimetals.

cond-mat.str-el↗

arXiv2025

Anatomy of anomalous Hall effect due to magnetic fluctuations

The anomalous Hall {\color{black} e}ffect (AHE) has emerged as a key indicator of time-reversal symmetry breaking (TRSB) and topological features in electronic band structures. Absent of a magnetic field, the AHE requires spontaneous TRSB but has proven hard to probe due to averaging over domains. The anomalous component of the Hall effect is thus frequently derived from extrapolating the magnetic field dependence of the Hall response. We show that discerning whether the AHE is an intrinsic property of the field free system becomes intricate in the presence of strong magnetic fluctuations. {\color{black}As a study case,} we use the Weyl semimetal PrAlGe, where TRSB can be toggled via a ferromagnetic transition, providing a transparent view of the AHE's topological origin. Through a combination of thermodynamic, transport and muon spin relaxation measurements, we contrast the behaviour below the ferromagnetic transition temperature to that of strong magnetic fluctuations above. Our results {\color{black}on PrAlGe provide general insights into the} interpretation of anomalous Hall signals in systems where TRSB is debated, such as families of Kagome metals or certain transition metal dichalcogenides.

cond-mat.str-el↗

arXiv2024

Tuning of Charge Order by Uniaxial Stress in a Cuprate Superconductor

Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct 'islands'. Using X-ray diffraction, we investigate the competition between charge order and superconductivity in the archetypal cuprate La(2-x)BaxCuO4, around the x = 1/8-doping, where uniaxial stress restores optimal 3D superconductivity at approximately 0.06 GPa. We find that the charge order peaks and the correlation length along the stripe are strongly reduced up to the critical stress, above which they stay constant. Simultaneously, the charge order onset temperature only shows a modest decrease. Our findings suggest that optimal 3D superconductivity is not linked to the absence of charge stripes but instead requires their arrangement into smaller 'islands'. Our results provide insight into the length scales over which the interplay between superconductivity and charge order takes place.

cond-mat.supr-con↗

arXiv2023

Nesting-driven antiferromagnetic order in Kondo lattice CePd5Al2

We investigated the electronic structure of the antiferromagnetic Kondo lattice CePd5Al2 using high-resolution angle-resolved photoemission spectroscopy. The experimentally determined band structure of the conduction electrons is predominated by the Pd 4d character. It contains multiple hole and electron Fermi pockets, in good agreement with density functional theory calculations. The Fermi surface is folded over Q0 = (0, 0, 1), manifested by Fermi surface reconstruction and band folding. Our results suggest that Fermi surface nesting drives the formation of antiferromagnetic order in CePd5Al2.

cond-mat.str-el↗

arXiv2023

Non-Conventional Critical Behavior and Q-dependent Electron-Phonon Coupling Induced Phonon Softening in the CDW Superconductor LaPt2Si2

This paper reports the first experimental observation of phonons and their softening on single crystalline LaPt$_2$Si$_2$ via inelastic neutron scattering. From the temperature dependence of the phonon frequency in close proximity to the charge-density wave (CDW) $q$-vector, we obtain a CDW transition temperature of T$_{CDW}$ = 230 K and a critical exponent $β$ = 0.28 $\pm$ 0.03. This value is suggestive of a non-conventional critical behavior for the CDW phase transition in LaPt$_2$Si$_2$, compatible with a scenario of CDW discommensuration (DC). The DC would be caused by the existence of two CDWs in this material, propagating separately in the non equivalent (Si1-Pt2-Si1) and (Pt1-Si2-Pt1) layers respectively, with transition temperatures T$_{CDW-1}$ = 230 K and T$_{CDW-2}$ = 110 K. A strong $q$-dependence of the electron-phonon coupling has been identified as the driving mechanism for the CDW transition at T$_{CDW-1}$ = 230 K while a CDW with 3-dimensional character, and Fermi surface quasi-nesting as a driving mechanism, is suggested for the transition at T$_{CDW-2}$ = 110 K. Our results clarify some aspects of the CDW transition in LaPt$_2$Si$_2$, which have been so far misinterpreted by both theoretical predictions and experimental observations, and give direct insight into its actual temperature dependence.

cond-mat.supr-con↗

arXiv2022

Confirming the high pressure phase diagram of the Shastry-Sutherland model

A Muon Spin Rotation ($μ$+SR) study was conducted to investigate the magnetic properties of SrCu2(BO3)2 (SCBO) as a function of temperature/pressure. Measurements in zero field and transverse field confirm the absence of long range magnetic order at high pressures and low temperatures. These measurements suggest changes in the Cu spin fluctuations characteristics above 21 kbar, consistent with the formation of a plaquette phase as previously suggested by inelastic neutron scattering measurements. SCBO is the only known realisation of the Shatry-Sutherland model, thus the ground state mediating the dimer and antiferromagnetic phase is likekly to be a plaquette state.

cond-mat.str-el↗

arXiv2022

Magnetic Properties of Multifunctional $^7$LiFePO$_4$ under Hydrostatic Pressure

LiFePO$_4$ (LFPO) is an archetypical and well-known cathode material for rechargeable Li-ion batteries. However, its quasi-one-dimensional (Q1D) structure along with the Fe ions, LFPO also displays interesting low-temperature magnetic properties. Our team has previously utilized the muon spin rotation ($μ^+$SR) technique to investigate both magnetic spin order as well as Li-ion diffusion in LFPO. In this initial study we extend our investigation and make use of high-pressure $μ^+$SR to investigate effects on the low-$T$ magnetic order. Contrary to theoretical predictions we find that the magnetic ordering temperature as well as the ordered magnetic moment increase at high pressure (compressive strain).

cond-mat.str-el↗

arXiv2022

Engineering a pure Dirac regime in ZrTe$_5$

Real-world topological semimetals typically exhibit Dirac and Weyl nodes that coexist with trivial Fermi pockets. This tends to mask the physics of the relativistic quasiparticles. Using the example of ZrTe5, we show that strain provides a powerful tool for in-situ tuning of the band structure such that all trivial pockets are pushed far away from the Fermi energy, but only for a certain range of Van der Waals gaps. Our results naturally reconcile contradicting reports on the presence or absence of additional pockets in ZrTe$_5$, and provide a clear map of where to find a pure three-dimensional Dirac semimetallic phase in the structural parameter space of the material.

cond-mat.mtrl-sci↗

arXiv2022

Structural Evolution and Onset of the Density Wave Transition in the CDW Superconductor LaPt$_2$Si$_2$ Clarified with Synchrotron XRD

The quasi-2D Pt-based rare earth intermetallic material LaPt$_2$Si$_2$ has attracted attention as it exhibits strong interplay between charge density wave (CDW) and and superconductivity (SC). However, the most of the results reported on this material come from theoretical calculations, preliminary bulk investigations and powder samples, which makes it difficult to uniquely determine the temperature evolution of its crystal structure and, consequently, of its CDW transition. Therefore, the published literature around LaPt$_2$Si$_2$ is often controversial. In this paper, we clarify the complex evolution of the crystal structure, and the temperature dependence of the development of density wave transitions, in good quality LaPt$_2$Si$_2$ single crystals, with high resolution synchrotron X-ray diffraction data. According to our findings, on cooling from room temperature LaPt$_2$Si$_2$ undergoes a series of subtle structural transitions which can be summarised as follows: second order commensurate tetragonal ($P4/nmm$)-to-incommensurate structure followed by a first order incommensurate-to-commensurate orthorhombic ($Pmmn$) transition and then a first order commensurate orthorhombic ($Pmmn$)-to-commensurate tetragonal ($P4/nmm$). The structural transitions are accompanied by both incommensurate and commensurate superstructural distortions of the lattice. The observed behavior is compatible with discommensuration of the CDW in this material.

cond-mat.str-el↗

arXiv2022

Revised Magnetic Structure and Tricritical Behavior of the CMR Compound NaCr$_2$O$_4$ Investigated with High Resolution Neutron Diffraction and $μ^+$SR

The mixed valence Cr compound NaCr$_2$O$_4$, synthesized using a high-pressure technique, offers a unique playground for investigating unconventional physical properties in condensed matter. In the present study, muon spin rotation/relaxation ($μ^+$SR) and high-resolution neutron powder diffraction (NPD) measurements were carried out to clarify the true magnetic ground state of this interesting compound. Our detailed study brings new insight, allowing us to confirm the existence of a commensurate antiferromagnetic order (C-AFM) and to extract its ordered Cr moment $μ^{\rm C}_{\rm Cr}=(4.30\pm0.01)μ_B$. Such a value of the ordered moment is in fact compatible with the existence of high-spin Cr sites. Further, the value of the canting angle of the Cr spin axial vector is refined as $θ_{\rm c}=(8.8\pm0.5)^{\circ}$. Employing high-quality samples in combination with time-of-flight NPD, a novel magnetic supercell was also revealed. Such supercell display an incommensurate (IC)-AFM propagation vector (0~0~${\textstyle \frac{1}{2}-}δ$), having an ordered moment $μ^{\rm IC}_{\rm Cr}=(2.20\pm0.03)μ_B$. It is suggested that the C-AFM and IC-AFM modulations are due to itinerant and localized contributions to the magnetic moment, respectively. Finally, the direct measurement of the magnetic order parameter provided a value of the critical exponent $β= 0.245 \approx \frac{1}{4}$, suggesting a non conventional critical behavior for the magnetic phase transition in NaCr$_2$O$_4$.

cond-mat.str-el↗

arXiv2022

Na-ion Dynamics in the Solid Solution Na$_{\rm x}$Ca$_{1- \rm x}$Cr$_2$O$_4$ Studied by Muon Spin Rotation and Neutron Diffraction

In this work we present systematic set of measurements carried out by muon spin rotation/relaxation ($μ^+$SR) and neutron powder diffraction (NPD) on the solid solution Na$_{\rm x}$Ca$_{1- \rm x}$Cr$_2$O$_4$. This study investigates Na-ion dynamics in the quasi-1D (Q1D) diffusion channels created by the honeycomb-like arrangement of CrO$_6$ octahedra, in the presence of defects introduced by Ca doping. With increasing Ca content, the size of the diffusion channels is enlarged, however, this effect does not enhance the Na ion mobility. Instead the overall diffusivity is hampered by the local defects and the Na hopping probability is lowered. The diffusion mechanism in Na$_{\rm x}$Ca$_{1- \rm x}$Cr$_2$O$_4$ was found to be interstitial and the activation energy as well as diffusion coefficient were determined for all the members of the solid solution.

cond-mat.mtrl-sci↗

arXiv2022

Gradual emergence of superconductivity in underdoped LSCO

We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.05$, $0.06$ and $0.07$, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings $x=0.06$ and $x=0.07$, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO$_2$ planes is detectable for $x=0.05$. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2022

Refined magnetic structure of VI$_3$

The van der Waals ferromagnet (FM), VI$_3$, was studied by muon spin relaxation ($μ^+$SR) and first principle calculations based on density functional theory (DFT). Temperature dependent zero field muon spin relaxation ($μ^+$SR) measurements confirm the onset of long range FM order and the time spectra exhibits clear muon spin precession frequencies for $T<T_{\rm C}=50.03(1)$~K. The calculated internal magnetic fields at the predicted muon sites, based on the established magnetic structure from neutron diffraction, is inconsistent with the measured one. This inconsistency is because of strong incoherent neutron scattering and absorption originating from the elements V and I. Instead, a new and a more accurate magnetic structure is derived based on a combined study using $μ^+$SR and DFT. These results suggest strong contritions from orbital angular momentum, providing experimental evidence for the existence of unquenched orbital angular momentum of V$^{3+}$ in VI$_3$. Finally, an unusual form of a short range ordering is present above $T_{\rm C}$. Its temperature dependence is unlike previously reported cases in other layered compounds and its microscopic origin is discussed.

cond-mat.str-el↗

arXiv2022

TRIM Simulations Tool for $μ^+$ Stopping Fraction in Hydrostatic Pressure Cells

For quantum systems or materials, a common procedure for probing their behaviour is to tune electronic/magnetic properties using external parameters, e.g. temperature, magnetic field or pressure. Pressure application as an external stimuli is a widely used tool, where the sample in question is inserted into a pressure cell providing a hydrostatic pressure condition. Such device causes some practical problems when using in Muon Spin Rotation/Relaxation ($μ^+$SR) experiments as a large proportion of the muons will be implanted in the pressure cell rather than in the sample, resulting in a higher background signal. This issue gets further amplified when the temperature dependent response from the sample is much smaller than that of the pressure cell,which may cause the sample response to be lost in the background and cause difficulties in aligning the sample within the beam. To tackle this issue, we have used pySRIM to construct a practical and helpful simulation tool for calculating muon stopping fractions, specifically for the pressure cell setup at the $μ$E1 beamline using the GPD spectrometer at the Paul Scherrer Institute, with the use of TRIM simulations. The program is used to estimate the number of muon stopping in both the sample and the pressure cell at a given momentum. The simulation tool is programmed into a GUI, making it accessible to user to approximate prior to their experiments at GPD what fractions will belong to the sample and the pressure cell in their fitting procedure.

physics.comp-ph↗

arXiv2022

Magnetic nature of wolframite MgReO$_4$

Rhenium oxides belonging to the family $A$ReO$_4$ where $A$ is a metal cation, exhibit interesting electronic and magnetic properties. In this study we have utilized the muon spin rotation/relaxation ($μ^+$SR) technique to study the magnetic properties of the MgReO$_4$ compound. To the best of our knowledge, this is the first investigation reported on this interesting material, that is stabilized in a wolframite crystal structure using a special high-pressure synthesis technique. Bulk magnetic studies show the onset of an antiferromagnetic (AF) long range order, or a possible singlet spin state at $T_{\rm C1}\approx90$~K, with a subtle second high-temperature transition at $T_{\rm C2}\approx280$~K. Both transitions are also confirmed by heat capacity ($C_p$) measurements. From our $μ^+$SR measurements, it is clear that the sample enters an AF order below $T_{\rm C1}=T_{\rm N}\approx85$~K. We find no evidence of magnetic signal above $T_{\rm N}$, which indicates that $T_{\rm C2}$ is likely linked to a structural transition. Further, via sensitive zero field (ZF) $μ^+$SR measurements we find evidence of a spin reorientation at $T_{\rm Cant}\approx65$~K. This points towards a transition from a collinear AF into a canted AF order at low temperature, which is proposed to be driven by competing magnetic interactions.

cond-mat.str-el↗