arXiv subjects

Antonio Cervellino

Publications and source records attributed to Antonio Cervellino.

18 recordsLinked to original sources

arXiv2025

Understanding the evolution of the magnetic ground state in Ba$_4$NaRu$_3$O$_{12}$

We report a comprehensive investigation of the quadruple perovskite Ba$_4$NaRu$_3$O$_{12}$, in which we discover a robust spin-lattice coupled ground state characterized by a long-range antiferromagnetic ordering at $T_N \sim$ 257 K. The system's unique structural motif of three symmetrically distinct magnetic ions, including Ru dimers separated by non-magnetic layers, is intimately correlated with its magnetic behavior, as evidenced by temperature-dependent diffraction measurements and specific heat data. The powder neutron diffraction patterns at 13 K showed that the spins within the dimers are antiparallel, leading to a net zero moment contribution and a staggered arrangement of the triangular layers formed by the Ru moments within the corner-shared octahedra along the $c$-axis. The low-temperature specific heat revealed an extra boson peak contribution from optical modes with a maximum vibrational energy of $\sim$55cm$^{-1}$. The charge transport exhibited variable-range hopping (VRH) behaviour below $T_N$, with a stronger energy-dependence than expected from the Efros-Shklovskii model, suggesting the presence of multiparticle correlation effects.

cond-mat.str-el↗

arXiv2023

Charge fluctuations in the intermediate-valence ground state of SmCoIn$_5$

The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn$_5$ and other Ce-based heavy fermion materials, depends strongly on the efficiency with which $f$ electrons are delocalized from the rare earth sites and participate in a Kondo lattice. Replacing Ce$^{3+}$ ($4f^1$, $J=5/2$) with Sm$^{3+}$ ($4f^5$, $J=5/2$), we show that a combination of crystal field and on-site Coulomb repulsion causes SmCoIn$_5$ to exhibit a $Γ_7$ ground state similar to CeCoIn$_5$ with multiple $f$ electrons. Remarkably, we also find that with this ground state, SmCoIn$_5$ exhibits a temperature-induced valence crossover consistent with a Kondo scenario, leading to increased delocalization of $f$ holes below a temperature scale set by the crystal field, $T_v$ $\approx$ 60 K. Our result provides evidence that in the case of many $f$ electrons, the crystal field remains the most important tuning knob in controlling the efficiency of delocalization near a heavy fermion quantum critical point, and additionally clarifies that charge fluctuations play a general role in the ground state of "115" materials.

cond-mat.str-el↗

arXiv2023

Structural Phase Transition and Superconductivity in 2H-BaGaGe with Buckled Honeycomb Layers

We report on the structural and superconducting properties of the intermetallic compound BaGaGe. We find that this material undergoes a structural second-order phase transition from the distorted AlB$_2$-type structure (1H, $a$ = 4.3254(2) Å, $c$ = 5.1078(3) Å, P6/mmm) into the CaIn$_2$-type structure (2H, $a$ = 4.3087(3) Å, $c$ = 10.2117(6) Å, P6$_3$/mmc) at a transition temperature of $T_{\rm S}$ = 253 K. We find that the structural phase-transition corresponds to a coherent buckling of the honeycomb layers, which we can interpret as a disorder-to-order transition of the atoms located within this layer. We show that the 2H-BaGaGe phase becomes superconducting at a critical temperature of $T_{\rm c}$ = 2.1 K. The bulk nature of the superconductivity in 2H-BaGaGe is confirmed by means of specific heat measurements, where we determine a value of $ΔC$/$γT_{\rm c}$ = 1.59, which is close to the expected BCS value in the weak coupling limit.

cond-mat.supr-con↗

arXiv2022

Diffraction from nanocrystal superlattices

Diffraction from a lattice of periodically spaced crystals is a topic of current interest because of the great development of self-organised superlattices (SL) of nanocrystals (NC). The self-organisation of NC into SL has theoretical interest, but especially a rich application prospect, as the coherent organisation has large effects on a wide range of material properties. Diffraction is a key method to understand the type and quality of SL ordering. Hereby the characteristic diffraction signature of a SL of NC - together with the characteristic types of disorder - are theoretically explored.

cond-mat.mtrl-sci↗

arXiv2022

Dynamic Magnetic Crossover at the Origin of the Hidden-Order in van der Waals Antiferromagnet CrSBr

The van der Waals material CrSBr stands out as a promising two-dimensional magnet. Especially, its high magnetic ordering temperature and versatile magneto-transport properties make CrSBr an important candidate for new devices in the emergent field of two-dimensional magnetic materials. To date, the magnetic and structural properties of CrSBr have not been fully elucidated. Here, we report on the detailed temperature-dependent magnetic and structural properties of this material, by comprehensively combining neutron scattering, muon spin relaxation spectroscopy, synchrotron X-ray diffraction, and magnetization measurements. We evidence that this material undergoes a transition to an A-type antiferromagnetic state below $T_{\rm N} \approx$ 140 K, with a pronounced two-dimensional character as deduced from the determined critical exponent of $β\approx $ 0.18. In our analysis of the field-induced metamagnetic transition, we find that the ferromagnetic correlations within the monolayers persist clearly above the Néel temperature in this material. Furthermore, we unravel the low-temperature (i.e. $T < T_{\rm N}$) magnetic hidden order within the long-range magnetically ordered state. We find that it is associated to a slowing down of the magnetic fluctuations, accompanied by a continuous reorientation of the internal magnetic field. These take place upon cooling below $T_s$ $\approx$ 100 K, until a spin freezing process occurs at $T$* $\approx$ 40 K. We argue this complex dynamic behavior to reflect a magnetic crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by the mixed-anion character of the material. Our findings indicate that the magnetic and structural properties of CrSBr widen its potential application as a component for spin-based electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Frustration, strain and phase co-existence in the mixed valent hexagonal iridate Ba$_{3}$NaIr$_{2}$O$_{9}$

Using detailed synchrotron diffraction, magnetization, thermodynamic and transport measurements, we investigate the relationship between the mixed valence of Ir, lattice strain and the resultant structural and magnetic ground states in the geometrically frustrated triple perovskite iridate Ba$_{3}$NaIr$_{2}$O$_{9}$. We observe a complex interplay between lattice strain and structural phase co-existence, which is in sharp contrast to what is typically observed in this family of compounds. The low temperature magnetic ground state is characterized by the absence of long range order, and points towards the condensation of a cluster glass state from an extended regime of short range magnetic correlations.

cond-mat.str-el↗

arXiv2021

Structural disorder and magnetic correlations driven by oxygen doping in Nd2NiO4+d (d ~ 0.11)

We investigated the influence of oxygen over-stoichiometry on apical oxygen disorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperature range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron single crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystalizes in a tetragonal commensurate structure with the P42/ncm space group with excess oxygen atoms occupy the 4b (3/4 1/4 1/4) interstitial sites, coordinated by four apical oxygen atoms. Large and anisotropic thermal displacement parameters are found for equatorial and apical oxygen atoms, which are strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase. Maximum Entropy analysis of the neutron single crystal diffraction data uncovered anharmonic contributions to the displacement parameters of the apical oxygen atoms, toward the nearest vacant 4b interstitial site, related to the phonon assisted oxygen diffusion mechanism. Macroscopic magnetization measurements and neutron powder diffraction studies reveal long-range antiferromagnetic ordering of the Ni-sublattice at TN ~ 53 K with a weak ferromagnetic component along the c-axis, while the long-range magnetic ordering of the Nd-sublattice occurs below 10 K. Temperature dependent neutron diffraction patterns show the appearance of a commensurate magnetic order at TN with the propagation vector k = (100) and the emergence of an additional incommensurate phase below 30 K, while both phases coexist at 2 K. The commensurate magnetic structure is best described by the P42/nc`m` Shubnikov space group. Refined magnetic moments of the Ni and Nd-sites at 2 K are 1.144(76) muB and 1.632(52) muB respectively. A possible origin of the incommensurate phase is discussed and a tentative magnetic phase diagram is proposed.

cond-mat.mtrl-sci↗

arXiv2020

Phase coexistence and negative thermal expansion in the triple perovskite iridate Ba$_{3}$CoIr$_{2}$O$_{9}$

The anomalous thermal expansion in a layered 3$d$-5$d$ based triple perovskite iridate Ba$_{3}$CoIr$_{2}$O$_{9}$ is investigated using high resolution synchrotron diffraction. Below the magneto-structural transition at 107\,K, the onset of antiferromagnetic order is associated with a monoclinic distortion of the hexagonal structure. Deeper within the magnetically ordered state, a part of the monoclinic phase distorts even further, and both these structural phases co-exist down to the lowest measured temperatures. We observe negative thermal expansion in this phase co-existence regime, which appears to be intimately connected to the temperature driven relative fractions of these monoclinic phases. The significant NTE observed in this system could be driven by magnetic exchange striction, and is of relevance to a number of systems with pronounced spin orbit interactions.

cond-mat.str-el↗

arXiv2020

Evolution of the structural, magnetic and electronic properties of the triple perovskite Ba$_{3}$CoIr$_{2}$O$_{9}$

We report a comprehensive investigation of the triple perovskite iridate Ba$_{3}$CoIr$_{2}$O$_{9}$. Stabilizing in the hexagonal $P6_{3}/mmc$ symmetry at room temperature, this system transforms to a monoclinic $C2/c$ symmetry at the magnetic phase transition. On further reduction in temperature, the system partially distorts to an even lower symmetry ($P2/c$), with both these structurally disparate phases coexisting down to the lowest measured temperatures. The magnetic structure as determined from neutron diffraction data indicates a weakly canted antiferromagnetic structure, which is also supported by first-principles calculations. Theory indicates that the Ir$^{5+}$ carries a finite magnetic moment, which is also consistent with the neutron data. This suggests that the putative $J=0$ state is avoided. Measurements of heat capacity, electrical resistance noise and dielectric susceptibility all point towards the stabilization of a highly correlated ground state in the Ba$_{3}$CoIr$_{2}$O$_{9}$ system.

cond-mat.str-el↗

arXiv2020

Tetragonal mixed system $Cs_2CuCl_{4-x}Br_x$ complemented by the tetragonal phase realisation of $Cs_2CuCl_4$

Realisation of the tetragonal phase of $Cs_2CuCl_4$ is possible using specific crystal growth conditions at a temperature below $281K$. This work deals with the comparison of the magnetic susceptibility and the magnetization of this new tetragonal compound with the magnetic behaviour of tetragonal $Cs_2CuCl_{2.9}Br_{1.1}$, $Cs_2CuCl_{2.5}Br_{1.5}$, $Cs_2CuCl_{2.2}Br_{1.8}$ and presents consistent results for such quasi $2-D$ antiferromagnets. Structural investigation at low temperature for $Cs_2CuCl_{2.2}Br_{1.8}$ shows no phase transition. The structure remains in the tetragonal symmetry $I4/mmm$. Furthermore, several magnetic reflections corresponding to the propagation vector $k = (0, 0, 0)$ are observed for this tetragonal compound through neutron diffraction experiments below the magnetic phase transition at $T_N = 11.3K$ confirming its antiferromagnetic nature.

cond-mat.mtrl-sci↗

arXiv2020

Texture corrections for total scattering functions

Many functional materials are today synthesised in form of nanoparticles displaying preferred orientation effects to some small or large extent. The analysis of diffraction data of such kind of systems is best performed in the framework of the total scattering approach that prescinds from translation symmetry assumptions. We therefore derived modified expressions for the most common total scattering functions, in particular the Debye Scattering Equation (DSE) that yields the texture-averaged differential cross section as a function of atomic coordinates and texture parameters. The modified DSE encodes higher-order even spherical Bessel functions which account for the texture effect. Selection rules arising from experimental geometries and symmetries are discussed. In addition the duality of the texture effect is introduced showing the effects of texture on both the I(Q) and G(r). The paper includes several definitions and appendices which are meant to be useful for those involved in the development of crystallographic computing.

physics.app-ph↗

arXiv2019

In-situ liquid SAXS studies on the early stage of calcium carbonate formation

Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and non-classical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different areas of science, little is known about the early stage of its formation. We, therefore, designed contactless devices capable to provide informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as Wide-Angle X-ray Scattering (WAXS) and Small-Angle X-ray Scattering (SAXS) techniques. In particular, SAXS was exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level was controlled by mixing four diluted solutions (i.e., NaOH, CaCl2, NaHCO3, H2O) at constant T and pH. The scattering data were collected on a liquid jet generated about 75 sec after the mixing point. The data were modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.

cond-mat.mtrl-sci↗

arXiv2018

A dilute gold nanoparticles suspension as SAXS standard for absolute scale using an extended Guinier approximation

In this article, a practical procedure for absolute intensity calibration for SAXS studies on liquid microjets is established, using a gold nanoparticle suspension as standard so that the intercept at $Q=0$ of the SAXS scattering curve would provide a scaling reference. In order to get the most precise extrapolation at $Q=0$, we used an extension to the Guinier approximation, with a second-order term in the fit that adapts to a larger $Q$-range.

cond-mat.mtrl-sci↗

arXiv2017

Coulomb spin liquid in anion-disordered pyrochlore Tb$_2$Hf$_2$O$_7$

The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. In the pyrochlore Tb$_2$Hf$_2$O$_7$, we demonstrate that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly-fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.

cond-mat.str-el↗

arXiv2016

The local structure of electrically stressed liquid water and implications for modelling of dielectric relaxation

In a floating water bridge the total radiation scattering of water stressed by a moderately strong electric field (1mV/nm) was compared to water without an applied electric field using X-ray and small angle neutron scattering. Structure refinement was carried out using the EPSR method and the TIP4P/2005 water model. These results did not reveal a significant difference in the local static structure of water however analysis of the simulation indicated that the modeled local potential energy surface reveals a departure between electrically stressed and unstressed water. The observed differences show that the local environment is changed by the applied electric field although weak relative to the intermolecular coulombic field. When discussing the results we show that the current methods used to simulate the pair potentials are still insufficient to treat such non-equilibrium systems and further simulation techniques have to be developed to properly reconstruct the microscopic dielectric relaxation process.

cond-mat.mes-hall↗

arXiv2009

Magnetoelastic coupling in triangular lattice antiferromagnet CuCrS2

CuCrS2 is a triangular lattice Heisenberg antiferromagnet with a rhombohedral crystal structure. We report on neutron and synchrotron powder diffraction results which reveal a monoclinic lattice distortion at the magnetic transition and verify a magnetoelastic coupling. CuCrS2 is therefore an interesting material to study the influence of magnetism on the relief of geometrical frustration.

cond-mat.str-el↗

arXiv2006

Generalization of a theorem of Carathéodory

Carathéodory showed that $n$ complex numbers $c_1,...,c_n$ can uniquely be written in the form $c_p=\sum_{j=1}^m ρ_j {ε_j}^p$ with $p=1,...,n$, where the $ε_j$s are different unimodular complex numbers, the $ρ_j$s are strictly positive numbers and integer $m$ never exceeds $n$. We give the conditions to be obeyed for the former property to hold true if the $ρ_j$s are simply required to be real and different from zero. It turns out that the number of the possible choices of the signs of the $ρ_j$s are {at most} equal to the number of the different eigenvalues of the Hermitian Toeplitz matrix whose $i,j$-th entry is $c_{j-i}$, where $c_{-p}$ is equal to the complex conjugate of $c_{p}$ and $c_{0}=0$. This generalization is relevant for neutron scattering. Its proof is made possible by a lemma - which is an interesting side result - that establishes a necessary and sufficient condition for the unimodularity of the roots of a polynomial based only on the polynomial coefficients. Keywords: Toeplitz matrix factorization, unimodular roots, neutron scattering, signal theory, inverse problems. PACS: 61.12.Bt, 02.30.Zz, 89.70.+c, 02.10.Yn, 02.50.Ga

math-ph↗

arXiv2006

Folding a 2-D powder diffraction image into a 1-D scan: a new procedure

A new procedure aiming at folding a powder diffraction 2-D into a 1-D scan is presented. The technique consists of three steps: tracking the beam centre by means of a Simulated Annealing (SA) of the diffraction rings along the same axis, detector tilt and rotation determination by a Hankel Lanczos Singular Value Decomposition (HLSVD) and intensity integration by an adaptive binning algorithm. The X-ray powder diffraction (XRPD) intensity profile of the standard NIST Si 640c sample is used to test the performances. Results show the robustness of the method and its capability of efficiently tagging the pixels in a 2-D readout system by matching the ideal geometry of the detector to the real beam-sample-detector frame. The whole technique turns out in a versatile and user-friendly tool for the $2\vartheta$ scanning of 2-D XRPD profiles.

cond-mat.mtrl-sci↗