arXiv subjects

Sara A. Lopez-Paz

Publications and source records attributed to Sara A. Lopez-Paz.

4 recordsLinked to original sources

arXiv2026

Disentangling pre-transitional fluctuations in metallic VO2

VO2 features concomitant structural and metal-insulator transitions. This poses a challenge for understanding the underlying mechanism: is the transition triggered by a structural or by an electronic instability? The two scenarios are expected to produce very different pre-transitional fluctuations above TC. By combining magnetic susceptibility, IR reflectivity and X-ray diffuse scattering measurements, we observe that metallic VO2 features strong electronic and structural fluctuations towards the insulating monoclinic phase. By measuring resonant diffuse X-ray scattering across the Vanadium K-edge, we search for a potential decoupling between electronic and structural ordering in these fluctuations, finding no evidence of it. While our results do not completely rule out pure electronic fluctuations, they constrain them, favoring the interpretation that the VO2 metal-insulator transition is triggered by a structural instability. Our work offers a novel approach to solve similar problems in other strongly correlated systems.

cond-mat.str-el↗

arXiv2024

Controlling the Magnetic Properties of the van der Waals Multiferroic Crystals Co$_{1-x}$Ni$_{x}$I$_2$

The structurally related compounds NiI$_2$ and CoI$_2$ are multiferroic van der Waals materials, in which helimagnetic orders exist simultaneously with electric polarization. Here, we report on the evolution of the crystal structure and of the magnetic properties across the solid solution Co$_{1-x}$Ni$_{x}$I$_2$. We have successfully grown crystals of the whole range of the solid solution, i.e. $x = 0-1$, by employing the self-selecting vapor growth (SSVG) technique and by carefully tuning the synthesis conditions according to the chemical composition. Our structural investigations show that the crystal symmetry changes from $P\bar{3}m1$ to $R\bar{3}m$ when Ni substitutes for Co beyond $x = 0.2$. Both the lattice parameters and magnetic properties evolve continuously and smoothly from one end member to the other, showing that they can be finely tuned by the chemical composition. We also observe that the Ni substitution degree in the solid solution affects the metamagnetic transition typical for CoI$_2$ at high magnetic fields. In particular, we find the existence of the metamagnetic transition similar to that for CoI$_2$ in the NiI$_2$ structure. Based on magnetic measurements we construct the phase diagram of the Co$_{1-x}$Ni$_{x}$I$_2$ system. Controlling the magnetic properties by the chemical composition may open new pathways for the fabrication of electronic devices made of two-dimensional (2D) flakes of multiferroic van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2023

Anomalous T-dependence of phonon lifetimes in metallic VO2

We investigate phonon lifetimes in VO2 single crystals. We do so in the metallic state above the metal-insulator transition (MIT), where strong structural fluctuations are known to take place. By combining inelastic X-ray scattering and Raman spectroscopy, we track the temperature dependence of several acoustic and optical phonon modes up to 1000 K. Contrary to what is commonly observed, we find that phonon lifetimes decrease with decreasing temperature. Our results show that pre-transitional fluctuations in the metallic state give rise to strong electron-phonon scattering that onsets hundreds of degrees above the transition and increases as the MIT is approached. Notably, this effect is not limited to specific points of reciprocal space that could be associated with the structural transition.

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↗