arXiv subjects

Omar Chmaissem

Publications and source records attributed to Omar Chmaissem.

17 recordsLinked to original sources

arXiv2025

4D-QENS Analysis of Correlated Ionic Conduction in SrCl$_2$

Methods of elucidating the mechanisms of fast-ion conduction in solid-state materials are pivotal for advancements in energy technologies such as batteries, fuel cells, sensors, and supercapacitors. In this study, we examine the ionic conduction pathways in single crystal SrCl$_2$, which is a fast-ion conductor above 900~K, using four-dimensional Quasi-Elastic Neutron Scattering (4D-QENS). We explore both coherent and incoherent neutron scattering at temperatures above the transition temperature into the superionic phase to explore the correlated motion of hopping anions. Refinements of the incoherent QENS yield residence times and jump probabilities between lattice sites in good agreement with previous studies, confirming that ionic hopping along nearest-neighbor directions is the most probable conduction pathway. However, the coherent QENS reveals evidence of de Gennes narrowing, indicating the importance of ionic correlations in the conduction mechanism. This highlights the need for improvements both in the theory of ionic transport in fluorite compounds and the modeling of coherent 4D-QENS in single crystals.

cond-mat.mtrl-sci↗

arXiv2024

Structural and magnetic properties of CoTeMoO$_6$ revisited

We have conducted a comprehensive investigation into the magnetic properties of the chiral multiferroic material CoTeMoO$_6$. In contrast with the previous claim of canted antiferromagnetic order with ferromagnetic components, our investigation reveals an antiferromagnetic ground state with compensated moments, providing an interesting platform for exploring exotic material properties. Through careful measurements of magnetization under a series of applied field, we demonstrate that there exist two sequential field-induced magnetic transitions in CoTeMoO$_6$, with one occurring at $H_{c1}$=460 Oe along the a-axis, and the other at $H_{c2}$=1.16 T with the field along the b-axis. The values of $H_{c1}$ and $H_{c2}$ exhibit strong angular dependence and diverge with different rates as the applied field is rotated 90 degrees within the ab plane. This reflects the distinct nature of these transitions, which is further supported by the different critical behavior of $H_{c1}$ and $H_{c2}$, characterized by the values of $γ$,in the function of $H_c=H_0\times(1-\frac{T}{T_c})^n$. Furthermore, we have demonstrated that there exist structural and magnetic twin domains in CoTeMoO$_6$ that strongly affect the experimental measurement of their macroscopic properties. Intriguingly, these twin domains can be related to the orthorhombicity/chirality of the crystal structure with the space group $P2_1 2_1 2$. We further explored the magnetic and structural domains with uniaxial pressure and polarized light microscopy. Our results suggest that CoTeMoO$_6$ could be used as a unique platform for investigating the intriguing physics involving intertwined degrees of freedom. The tunability of the underlying domain distribution and its strong anisotropy could also be useful for developing functional devices and applications.

cond-mat.mtrl-sci↗

arXiv2024

Local atomic and magnetic structure of multiferroic (Sr,Ba)(Mn,Ti)O$_3$

We present a detailed study of the local atomic and magnetic structure of the type-I multiferroic perovskite system (Sr,Ba)(Mn,Ti)O$_3$ using x-ray and neutron pair distribution function (PDF) analysis, polarized neutron scattering, and muon spin relaxation ($μ$SR) techniques. The atomic PDF analysis reveals widespread nanoscale tetragonal distortions of the crystal structure even in the paraelectric phase with average cubic symmetry, corresponding to incipient ferroelectricity in the local structure. Magnetic PDF analysis, polarized neutron scattering, and $μ$SR likewise confirm the presence of short-range antiferromagnetic correlations in the paramagnetic state, which grow in magnitude as the temperature approaches the magnetic transition. We show that these short-range magnetic correlations coincide with a reduction of the tetragonal (i.e. ferroelectric) distortion in the average structure, suggesting that short-range magnetism can play an important role in magnetoelectric and/or magnetostructural phenomena even without genuine long-range magnetic order. The reduction of the tetragonal distortion scales linearly with the local magnetic order parameter, pointing to spontaneous linear magnetoelectric coupling in this system. These findings provide greater insight into the multiferroic properties of (Sr,Ba)(Mn,Ti)O$_3$ and demonstrate the importance of investigating the local atomic and magnetic structure to gain a deeper understanding of the intertwined degrees of freedom in multiferroics.

cond-mat.str-el↗

arXiv2023

Frustration-Mediated Crossover from Long-Range to Short-Range Magnetic Ordering in $\mathbf{Y_{1-x}Lu_xBaCo_4O_7} $

We present a comprehensive magnetic and structural phase diagram for $\mathrm{Y_{1-x}Lu_xBaCo_4O_7}$, established through neutron diffraction and magnetization measurements. Our results outline the evolution of various nuclear structures, encompassing both orthorhombic and monoclinic symmetries, in response to changes in composition and temperature. The phase transition temperature of the orthorhombic phase, $T_{s1}$, decreases with increasing Lu content from 310 K for x = 0.0 to 110 K for x = 1.0. In Lu-rich compositions (0.7 \le x \le 1.0), first-order structural transitions are observed with coexisting and competing orthorhombic Pbn$2_1$ and metastable monoclinic Cc phases. Composition- and temperature-dependent refinements of the magnetic structure reveal an antiferromagnetic arrangement of Co spin pairs, below the magnetic transition temperatures of the Y-rich compositions, in the ab plane within both the triangular and Kagomé layers. A gradual suppression of long-range magnetic order is observed with increasing the Lu content, accompanied by the development of short-range magnetic correlations present in all the samples.

cond-mat.str-el↗

arXiv2022

Spin and charge density waves in the quasi-one-dimensional KMn6Bi5

AMn6Bi5 materials (A = Na, K, Rb and Cs) consisting of unique Mn-cluster chains emerge as a new family of superconductors with the suppression of their antiferromagnetic (AFM) order under high pressures. Here, we report transverse incommensurate spin density waves (SDWs) for the Mn atoms with a propagating direction along the chain axes as a ground state for KMn6Bi5 by single crystal neutron diffraction. The SDWs have a refined amplitude of ~2.46 Bohr magnetons for the Mn atoms in the pentagons and ~0.29 Bohr magnetons with a large standard deviation for Mn atoms in the center between the pentagons. AFM dominate both the nearest-neighbor Mn-Mn interactions within the pentagon and next-nearest-neighbor Mn-Mn interactions out of the pentagon (along the propagating wave). The SDWs exhibit both local and itinerant characteristics probably formed by a cooperative interaction between local magnetic exchange and conduction electrons. A significant magnetoelastic effect during the AFM transition, especially along the chain direction, has been demonstrated by temperature-dependent x-ray powder diffraction. Single crystal x-ray diffraction below the AFM transition revealed satellite peaks originating from charge density waves along the chain direction with a q-vector twice as large as the SDW one, pointing to a strong real space coupling between them. Our work not only manifests a fascinating interplay among spin, charge, lattice and one dimensionality to trigger intertwined orders in KMn6Bi5 but also provides important piece of information for the magnetic structure of the parent compound to understand the mechanism of superconductivity in this new family.

cond-mat.str-el↗

arXiv2022

Order-Disorder Transitions in (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$

The classification of structural phase transitions as displacive or order-disorder in character is usually based on spectroscopic data above the transition. We use single crystal x-ray diffraction to investigate structural correlations in the quasi-skutterudites, (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$, which have a quantum phase transition at $x\sim0.9$. Three-dimensional pair distribution functions show that the amplitudes of local atomic displacements are temperature-independent below the transition and persist to well above the transition, a signature of order-disorder behavior. The implications for the associated electronic transitions are discussed.

cond-mat.str-el↗

arXiv2019

High pO2 Floating Zone Crystal Growth of the Perovskite Nickelate PrNiO3

Single crystals of PrNiO3 were grown under an oxygen pressure of 295 bar using a unique high-pressure optical-image floating zone furnace. The crystals, with volume in excess of 1 mm3, were characterized structurally using single crystal and powder X-ray diffraction. Resistivity, specific heat, and magnetic susceptibility were measured, all of which evidenced an abrupt, first order metal-insulator transition (MIT) at ~130 K, in agreement with previous literature reports on polycrystalline specimens. Temperature-dependent single crystal diffraction was performed to investigate changes through the MIT. Our study demonstrates the opportunity space for high fugacity, reactive environments for single crystal growth specifically of perovskite nickelates but more generally to correlated electron oxides.

cond-mat.mtrl-sci↗

arXiv2018

Widespread orthorhombic fluctuations in the (Sr,Na)Fe$_2$As$_2$ family of superconductors

We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr$_{1-x}$Na$_{x}$Fe$_2$As$_2$. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels ($x \gtrsim 0.48$, near optimal superconductivity) and high temperatures (above room temperature for $x = 0$, decreasing to 150 K for $x = 0.48$), with a typical length scale of 1-3 nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

cond-mat.supr-con↗

arXiv2017

Local orthorhombicity in the magnetic $C_4$ phase of the hole-doped iron-arsenide superconductor Sr$_{1-x}$Na$_{x}$Fe$_2$As$_2$

We report temperature-dependent pair distribution function measurements of Sr$_{1-x}$Na$_{x}$Fe$_2$As$_2$, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic $C_4$ phase. Quantitative refinements indicate that the instantaneous local structure in the $C_4$ phase is comprised of fluctuating orthorhombic regions with a length scale of $\sim$2 nm, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic $C_4$ phase and the neighboring $C_2$ and superconducting phases.

cond-mat.supr-con↗

arXiv2015

Tetragonal magnetic phase in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ from x-ray and neutron diffraction

Combined neutron and x-ray diffraction experiments demonstrate the formation of a low-temperature minority tetragonal phase in Ba$_{0.76}$K$_{0.24}$Fe$_2$As$_2$ in addition to the majority magnetic, orthorhombic phase. A coincident enhancement in the magnetic ($\frac{1}{2}$ $\frac{1}{2}$ 1) peaks shows that this minority phase is of the same type that was observed in Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ ($0.24 \leq x \leq 0.28$), in which the magnetic moments reorient along the $c$-axis. This is evidence that the tetragonal magnetic phase is a universal feature of the hole-doped iron-based superconductors.

cond-mat.supr-con↗

arXiv2015

Magnetically-Driven Suppression of Nematic Order in an Iron-Based Superconductor

A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d-electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. In this article, we report the observation by neutron powder diffraction of an additional four- fold-symmetric phase in Ba1-xNaxFe2As2 close to the suppression of SDW order, which is consistent with the predictions of magnetically-driven models of nematic order.

cond-mat.supr-con↗

arXiv2014

Coincident structural and magnetic order in BaFe$_2$(As$_{1-x}$)P$_x$)$_2$ revealed by high-resolution neutron diffraction

We present neutron diffraction analysis of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ over a wide temperature (10 to 300 K) and compositional ($0.11 \leq x \leq 0.79$) range, including the normal state, the magnetically ordered state, and the superconducting state. The paramagnetic to spin-density wave and orthorhombic to tetragonal transitions are first order and coincident within the sensitivity of our measurements ($\sim 0.5$ K). Extrapolation of the orthorhombic order parameter down to zero suggests that structural quantum criticality cannot exist at compositions higher than $x = 0.28$, which is much lower than values determined using other methods, but in good agreement with our observations of the actual phase stability range. The onset of spin-density wave order shows a stronger structural anomaly than the charge-doped system in the form of an enhancement of the $c/a$ ratio below the transition.

cond-mat.supr-con↗

arXiv2014

Neutron Scattering Studies of the Ferroelectric Distortion and Spin Dynamics in the Type-1 Multiferroic Perovskite Sr0.56Ba0.44MnO3

The magnetic order, spin dynamics, and crystal structure of the multiferroic Sr0.56Ba0.44MnO3 have been investigated using neutron and x-ray scattering. Ferroelectricity develops at TC=305 K with a polarization of 4.2 microC/cm2 associated with the displacements of the Mn ions, while the Mn4+ spins order below TN = 200 K into a simple G-type commensurate magnetic structure. Below TN the ferroelectric order decreases dramatically demonstrating that the two order parameters are strongly coupled. The ground state spin dynamics are characterized by a spin gap of 4.6(5) meV and the magnon density of states peaking at 43 meV. Detailed spin wave simulations with a gap and isotropic exchange of J=4.8(2) meV describe the excitation spectrum well. Above TN strong spin correlations coexist with robust ferroelectric order.

cond-mat.mtrl-sci↗

arXiv2013

Structural, Magnetic, and Superconducting Properties of Ba1-xNaxFe2As2

We report the results of a systematic investigation of the phase diagram of the iron-based superconductor system, Ba1-xNaxFe2As2, from x = 0.1 to x = 1.0 using high resolution neutron and x-ray diffraction and magnetization measurements. We find that the coincident structural and magnetic phase transition to an orthorhombic (O) structure with space group Fmmm and a striped antiferromagnet (AF) with space group F(C)mm'm' in Ba1-xNaxFe2As2 is of first order. A complete suppression of the magnetic phase is observed by x = 0.30, and bulk superconductivity occurs at a critical concentration near 0.15. We compare the new findings to the previously reported results of the hole-doped Ba1-xKxFe2As2 solid solution in order to resolve the differing effects of band filling and A-site cation size on the properties of the magnetic and superconducting ground states. The substantial size difference between Na and K causes various changes in the lattice trends, yet the overarching property phase diagram from the Ba1-xKxFe2As2 phase diagram carries over to the Ba1-xNaxFe2As2 solid solution. We note that the composition dependence of the c axis turns over from positive to negative around x = 0.35, unlike the K-substituted materials. We show that this can be understood by invoking steric effects; primarily the Fe2As2 layer shape is dictated mostly by the electronic filling, which secondarily induces an interlayer spacing adjusted to compensate for the given cation volume. This exemplifies the primacy of even subtle features in the Fe2As2 layer in controlling both the structure and properties in the uncollapsed 122 phases.

cond-mat.supr-con↗

arXiv2012

Phase Diagram of Ba1-xKxFe2As2

We report the results of a systematic investigation of the phase diagram of the iron-based superconductor, Ba1-xKxFe2As2, from x = 0 to x = 1.0 using high resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of \Deltax <~ 0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba1-xKxFe2As2 crystallizes in a tetragonal structure with the space group symmetry of I4/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with space group Fmmm and a striped antiferromagnet (AF) with space group Fcmm'm'. The transition temperature falls from a maximum of 139K in the undoped compound to 0K at x = 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of x = 0.130(3) and the superconducting transition temperature grows linearly with x until it crosses the AF/O phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum Tc of 38 K at x = 0.4 falling to 3 K at x = 1.0. We discuss reasons for the suppression of the spin-density-wave order and the electron-hole asymmetry in the phase diagram.

cond-mat.supr-con↗

arXiv2011

Magnetoelastic Coupling in the Phase Diagram of Ba1-xKxFe2As2

We report a high resolution neutron diffraction investigation of the coupling of structural and magnetic transitions in Ba1xKxFe2As2. The tetragonal-orthorhombic and antiferromagnetic transitions are suppressed with potassium-doping, falling to zero at x <~ 0.3. However, unlike Ba(Fe1xCox)2As2, the two transitions are first-order and coincident over the entire phase diagram, with a biquadratic coupling of the two order parameters. The phase diagram is refined showing that the onset of superconductivity is at x = 0.133 with all three phases coexisting until x >~ 0.24.

cond-mat.supr-con↗

arXiv2008

Universal doping dependence of the ground state staggered magnetization in cuprates

Using muon spin rotation we determine the zero temperature staggered antiferromagnetic order parameter M0 versus hole doping measured from optimum Dp_m, in the (Ca_{x}La_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_{3}O_{y} system. In this system the maximum Tc and the superexchange J vary by 30% between families (x). M0(x,Dp_m) is found to be x-independent. Using neutron diffraction we also determine the lattice parameters variations for all x and doping. The oxygen buckling angle is found to change with x, implying a change in the holes kinetic energy. We discuss the surprising insensitivity of M0(x,Dp_m) to the kinetic energy variations in the framework of the t-J model.

cond-mat.supr-con↗