arXiv subjects

Gaston Garbarino

Publications and source records attributed to Gaston Garbarino.

At least 37 records · Page 2Linked to original sources

arXiv2025

Absence of higher than 6-fold coordination in glassy $GeO_{2}$ up to 158 GPa revealed by X-ray absorption spectroscopy

Simple binary oxide glasses can exhibit a compression behavior distinct from that of their crystalline counterparts. In this study, we employed high-pressure X-ray absorption spectroscopy coupled to the diamond anvil cell to investigate in detail local structural changes around Ge in glassy $GeO_{2}$ up to 158 GPa. We conducted four independent runs, both with and without pressure-transmitting media. Up to 30 GPa, we observed no significant influence of the pressure medium on the pressure dependence of the $Ge-O$ bond length ($ $). Between 10 and 30 GPa, the evolution of $ $ shows substantial variability across our experiments and previous works. The measured values lie close to those reported for crystalline polymorphs, including the rutile- and $CaCl_{2}$-type phase of $GeO_{2}$. This finding suggests that the amorphous structure possesses considerable flexibility to transition among different atomic configurations. From 30 GPa to 158 GPa, our results for both $ $ and the non-bonded cation-cation distance $ $ demonstrate that edge-sharing octahedra remain the main structural motives in glassy $GeO_{2}$. Up to 100 GPa, compaction proceeds primarily via distortions of octahedral $O-Ge-O$ bond angles accompanied by octahedral bond shortening. Above 100 GPa, octahedral distortion becomes the prevailing mechanism. Compared to its crystalline analogues ($α-PbO_{2}$ and pyrite-like phase), glassy $GeO_{2}$ exhibits a slightly less efficient compaction mechanism, likely due to kinetic constraints that inhibit reconstructive lattice rearrangements.

cond-mat.mtrl-sci↗

arXiv2025

Hydrogen-rich hydrate at high pressures up to 104 GPa

Gas hydrates are considered fundamental building blocks of giant icy planets like Neptune and similar exoplanets. The existence of these materials in the interiors of giant icy planets, which are subject to high pressures and temperatures, depends on their stability relative to their constituent components. In this study, we reexamine the structural stability and hydrogen content of hydrogen hydrates, (H2O)(H2)n, up to 104 GPa, focusing on hydrogen-rich materials. Using synchrotron single-crystal X-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations, we find that the C2-filled ice phase undergoes a transformation to C3-filled ice phase over a broad pressure range of 47 - 104 GPa at room temperature. The C3 phase contains twice as much molecular H2 as the C2 phase. Heating the C2-filled ice above approximately 1500 K induces the transition to the C3 phase at pressures as low as 47 GPa. Upon decompression, this phase remains metastable down to 40 GPa. These findings establish new stability limits for hydrates, with implications for hydrogen storage and the interiors of planetary bodies.

cond-mat.mtrl-sci↗

arXiv2025

Multilayer Crystal Field states from locally broken centrosymmetry

Local charge, spin, or orbital degrees of freedom with intersite interactions are oftentimes sufficient to construct most quantum orders. This is conventionally true for f-electron systems, where the extent of the f-electrons and their associated crystal-electric-field (CEF) states are strongly localized. Here, polarized Raman spectroscopy measurements of a locally non-centrosymmetric compound, CeCoSi, unveil more CEF excitations than expected in the local model. We interpret this as experimental evidence for the entanglement of CEF states between cerium layers. This composite sublattice, spin, and orbital degree of freedom provides an unconsidered means to form novel orders, not only in this system, but in any system exhibiting globally preserved yet locally broken centrosymmetry.

cond-mat.str-el↗

arXiv2025

Charge-density-wave quantum critical point under pressure in 2$H$-TaSe$_2$

Suppressing of an ordered state that competes with superconductivity is one route to enhance superconducting transition temperatures. Whereas the effect of suppressing magnetic states is still not fully understood, materials featuring charge-density waves and superconductivity offer a clearer scenario as both states can be associated with electron-phonon coupling. Metallic transition-metal dichalcogenides are prime examples for such intertwined electron-phonon-driven phases, yet, various compounds do not show the expected interrelation or feature additional mechanisms which makes an unambiguous interpretation difficult. Here, we report high-pressure X-ray diffraction and inelastic X-ray scattering measurements of the prototypical transition-metal dichalcogenide 2$H$-TaSe$_2$ and determine the evolution of the charge-density-wave state and its lattice dynamics up to and beyond its suppression at the critical pressure $p_c = 19.9(1)\,\rm{GPa}$ and at low temperatures. The high quality of our data allows the full refinement of the commensurate charge-density-wave superstructure at low pressure and we find the quantum critical point of the charge-density-wave to be in close vicinity to the reported maximum superconducting transition temperature $T_{sc} = 8.2\,\rm{K}$. $Ab-initio$ calculations corroborate that 2$H$-TaSe$_2$ is a reference example of order-suppressed enhanced superconductivity and can serve as a textbook case to investigate superconductivity near a charge-density-wave quantum critical point.

cond-mat.str-el↗

arXiv2024

Local electronic properties of La3Ni2O7 under pressure

The recent discovery of superconductivity in $\rm La_3Ni_2O_7$ has attracted significant attention due to its high critical temperature and analogy to cuprate oxides. The oxidation and spin states of Ni ions are among the most important local properties in this compound, extensively discussed in the context of its superconductivity. Despite their direct link to the electron filling configurations of the relevant $\rm 3d_{x^2-y^2}$ and $\rm 3d_{z^2}$ orbitals, these local electronic properties of $\rm La_3Ni_2O_7$ yet to be systematically investigated. In this work, we address this issue using x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) measurements under pressure. Comparison of Ni \textit{K}-edge XAS and $\rm Kβ$ XES with the reference spectra of $\rm NiO$ and $\rm LaNiO_3$ shows that Ni ions, with an average valence of $\sim 2.53+$, are in a low-spin ($\rm S = 1/2$) ground state under ambient conditions. High pressure XAS and XES data clearly show that the oxidation ($\sim 2.5+$) and spin ($\rm S = 1/2$) states of Ni ions remain stable across the investigated pressure (up to 30 GPa) and temperature (down to 10 K) ranges, ruling out previously proposed spin transition scenarios.

cond-mat.supr-con↗

arXiv2024

Polymorphism of monatomic iodine

We applied synchrotron single-crystal X-ray diffraction in a diamond anvil cell at 48-51 GPa and first-principles theoretical calculations to study the crystal structure of solid atomic iodine at high pressure. We report the synthesis of two phases of atomic iodine at 48-51 GPa via laser heating of I-N2 mixtures. Unlike the familiar monatomic I4/mmm structure, which consists of crystallographically equivalent atoms, a new Pm-3n structure is of inclusion type, featuring two distinct kinds of atoms: a central detached one and peripheral ones forming the linear chains. Moreover, we observe crystallization of the familiar high-pressure face centered cubic (fcc) structure, albeit at much lower pressures compared to cold compressed iodine. The discovery of Pm-3n structure in iodine marks an important step in understanding of the pressure induced phase transition sequence in halogens.

cond-mat.mtrl-sci↗

arXiv2024

Narrowing band gap chemically and physically: Conductive dense hydrocarbon

Band gap energy of an organic molecule can be reduced by intermolecular interaction enhancement, and thus, certain polycyclic aromatic hydrocarbons (PAHs), which are insulators with wide band gaps, are expected to undergo insulator-metal transitions by simple compression. Such a pressure-induced electronic transition can be exploited to transform non-metallic organic materials into states featuring intriguing electronic characteristics such as high-temperature superconductivity. Numerous attempts have been made to metalize various small PAHs, but so far only pressure-induced amorphization well below the megabar region was observed. The wide band gap energy of the small PAHs and low chemical stability under simple compression are the bottlenecks. We have investigated the band gap energy evolution and the crystal structural compression of the large PAH molecules, where the band gap energy is significantly reduced by increasing the number of π-electrons and improved chemical stability with fully benzenoid molecular structure. Herein, we present a pressure-induced transition in dicoronylene, C48H20, an insulator at ambient conditions that transforms into a semi-metallic state above 23.0 GPa with a three-order-of-magnitude reduction in resistivity. In-situ UV-visible absorption, transport property measurement, Raman spectroscopy, X-ray diffraction and density functional theory calculations were performed to provide tentative explanations to the alterations in its electronic structure at high pressure. The discovery of an electronic transition at pressures well below the megabar is a promising step towards realization of a single component purely hydrocarbon molecular metal in the near future.

cond-mat.mtrl-sci↗

arXiv2024

Anomalous phonon Grüneisen parameters in semiconductor Ta$_2$NiS$_5$

Strain tuning is a powerful experimental method in probing correlated electron systems. Here we study the strain response of the lattice dynamics and electronic structure in semiconductor Ta$_2$NiS$_5$ by polarization-resolved Raman spectroscopy. We observe an increase of the size of the direct semiconducting band gap. Although the majority of the optical phonons show only marginal dependence to applied strain, the frequency of the two B$_{2g}$ phonon modes, which have quadrupolar symmetry and already anomalously soften on cooling under zero strain, increases significantly with tensile strain along the $a$ axis. The corresponding Grüneisen parameters are unusually large in magnitude and negative in sign. These effects are well captured by first-principles density functional theory calculations and indicate close proximity of Ta$_2$NiS$_5$ to a structural instability, similar to that encountered in excitonic insulator candidate Ta$_2$NiSe$_5$.

cond-mat.str-el↗

arXiv2024

Magnetic versus nonmagnetic polymorphs of RuBr$_3$ under pressure

Pressure evolution of the crystal structure and magnetism of the honeycomb $α$-RuBr$_3$ is studied using high-pressure x-ray diffraction, magnetometry, and density-functional band-structure calculations. Hydrostatic compression transforms antiferromagnetic $α$-RuBr$_3$ ($R\bar 3$) into paramagnetic $α'$-RuBr$_3$ ($P\bar 1$) where short Ru-Ru bonds cause magnetism collapse above 1.3 GPa at 0 K and 2.5 GPa at 295 K. Below this critical pressure, the Néel temperature of $α$-RuBr$_3$ increases with the slope of 1.8 K/GPa. Pressure tunes $α$-RuBr$_3$ away from the Kitaev limit, whereas increased third-neighbor in-plane coupling and interlayer coupling lead to a further stabilization of the collinear zigzag state. Both $α$- and $α'$-RuBr$_3$ are metastable at ambient pressure, but their transformation into the thermodynamically stable $β$-polymorph is kinetically hindered at room temperature.

cond-mat.str-el↗

arXiv2024

Using strain to uncover the interplay between two- and three-dimensional charge density waves in high-temperature superconducting YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$

Uniaxial pressure provides an efficient approach to control the competition between charge density waves (CDWs) and superconductivity in underdoped YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$. It can enhance the correlation volume of ubiquitous short-range 2D CDW correlations, and induces a long-range 3D CDW otherwise only accessible at large magnetic fields. Here, we use x-ray diffraction to study the strain and doping evolution of these CDWs. No signatures of discommensurations nor pair density waves are observed in the investigated strain-temperature parameter space, but direct evidence for a form of competition between 2D and 3D CDWs is uncovered. We show that the interplay between the 3D CDW, the 2D CDWs and superconductivity is qualitatively well described by including strain effects in simulations of a nonlinear sigma model of competing superconducting and CDW orders. From a broader perspective, our results underscore the potential of strain tuning as a powerful tool for probing and manipulating competing orders in quantum materials.

cond-mat.supr-con↗

arXiv2024

Robustness of the pyrochlore structure in rare-earth A2Ir2O7 iridates and pressure-induced structural transformation in IrO2

A comprehensive study of the structural properties of the heavily investigated rare-earth A2Ir2O7 series under extreme conditions is presented. The series is covered by studying iridates with A = Pr, Sm, Dy-Lu. Temperature- and pressure-dependent synchrotron X-ray powder diffraction experiments reveal robustness of the pyrochlore structure throughout the rare-earth series, down to 4 K and up to 20 GPa. The thermal expansivity and pressure compressibility are determined, including Debye temperature, bulk modulus and Grüneisen parameter. Temperature and pressure evolution of the fractional coordinate of oxygen at 48f Wyckoff position, the sole free atomic position parameter, is investigated and discussed regarding the antiferromagnetic ordering of the Ir magnetic moments. In addition, the pressure evolution of the crystal structure of an IrO2 minority phase is followed. The tetragonal rutile-type structure is orthorhombically distorted at 15 GPa, and the orthorhombic structure is not fully stabilised up to 20 GPa.

cond-mat.mtrl-sci↗

arXiv2024

The high-pressure phase diagram of BaNi$_2$As$_2$: unconventional charge-density-waves and structural phase transitions

Structural phase transitions accompanied by incommensurate and commensurate charge density wave (CDW) modulations of unconventional nature have been reported in BaNi$_2$As$_2$, a nonmagnetic cousin of the parent compound of Fe-based superconductors, BaFe$_2$As$_2$. The strong dependence of the structural and CDW transitions of BaNi$_2$As$_2$ on isoelectronic substitutions alongside original dynamical lattice effects suggests strong tunability of the electronic phase of the system through structural effects. Here, we present a comprehensive synchrotron x-ray diffraction and first-principles calculation study of the evolution of the crystal structure and lattice instabilities of BaNi$_2$As$_2$ as a function of temperature and hydrostatic pressure (up to 12 GPa). We report a cascade of pressure-induced structural phase transitions and electronic instabilities up to 10 GPa, above which all CDW superstructures disappear. We reveal that the stable high-pressure phase consists of planar Ni zigzag chains, from which the surrounding As atoms have been pushed away. This yields a strong reduction of the interlayer As-As distance (along the original c axis), akin to what is observed in the collapsed tetragonal structure of other pnictides, albeit here with a monoclinic structure. The discovery of polymorphs in the pressure-temperature phase diagram of BaNi$_2$As$_2$ emphasizes the importance of the relative Ni-Ni and Ni-As bond lengths in controlling the electronic ground state of this compound and increases our understanding of viable electronic phases under extreme conditions.

cond-mat.supr-con↗

arXiv2023

Pressure-induced formation of cubic lutetium hydrides derived from trigonal LuH$_3$

In recent years, there has been a fervent search for room-temperature superconductivity within the binary hydrides. However, as the number of untested compounds dwindled, it became natural to begin searching within the ternary hydrides. This led to the controversial discovery of room-temperature superconductivity at only 1GPa in nitrogen-doped lutetium hydride [Dasenbrock-Gammon et al., Nature 615, 244 (2023)] and consequently provided much impetus for the synthesis of nitrogen-based ternary hydrides. Here, we report the synthesis of stable trigonal LuH$_3$ by hydrogenating pure lutetium which was subsequently pressurised to $\sim$2GPa in a dilute-N$_2$/He-rich pressure medium. Raman spectroscopy and x-ray diffraction were used to characterise the structures throughout. After depressurising, energy-dispersive and wavelength-dispersive X-ray spectroscopies characterised the final compound. Though our compound under pressure exhibits similar structural behaviour to the Dasenbrock-Gammon et al. sample, we do not observe any nitrogen within the structure of the recovered sample at ambient pressure. We observe two cubic structures under pressure that simultaneously explain the X-ray diffraction and Raman spectra observed: the first corresponds well to $Fm\overline{3}m$ LuH$_{2+x}$, whilst the latter is an $Ia\overline{3}$-type structure.

cond-mat.supr-con↗

arXiv2023

Melting curve of black phosphorus: evidence for a solid-liquid-liquid triple point

Black phosphorus (bP) is a crystalline material that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. The knowledge of its pressure-temperature (P-T) phase diagram, and in particular, the slope and location of its melting curve is fundamental for better understanding the synthesis and stability conditions of this important material. Despite several experimental studies, important uncertainties remain in the determination of this melting curve. Here we report accurate melting points measurements, using in situ high-temperature and high-pressure high-resolution synchrotron x-ray diffraction. In particular, we have employed an original and accurate pressure and temperature metrology based on the unique anisotropic P-T response of bP, that we used as sensor for the simultaneous determination of pressure and temperature up to 5 GPa and 1700 K. We confirmed the existence of and located a solid-liquid-liquid triple point at the intersection of the low- and high-pressure melting curves. Finally, we have characterized the irreversibility of the transformation in the low-pressure regime below 1 GPa, as the low-density liquid does not crystallize back to bP but into red phosphorus on temperature quenching.

cond-mat.mtrl-sci↗

arXiv2023

Polytypism of Incommensurately Modulated Structures of Crystalline Bromine upon Molecular Dissociation under High Pressure

Polytypism of incommensurately modulated structures was hitherto unobserved. Here, we found the phenomenon in simple halogen systems of bromine and iodine upon molecular dissociation in the solids under pressure. Single-crystal synchrotron X-ray diffraction in laser heated diamond anvil cells pressurised up to 112 GPa revealed a number of allotropes of bromine and iodine including polytypes of Br-IIIγ (Fmmm(00γ)s00) with γ varying within 0.18 to 0.3.

cond-mat.mtrl-sci↗

arXiv2023

Anisotropic thermo-mechanical response of layered hexagonal boron nitride and black phosphorus: application as a simultaneous pressure and temperature sensor

Hexagonal boron nitride (hBN) and black phosphorus (bP) are crystalline materials that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. The knowledge of the thermal equations of state of hBN and bP is of great interest in the field of 2D materials for a better understanding of the anisotropic thermo-mechanical properties and exfoliation mechanism of these materials. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the thermoelastic parameters of hBN and bP. Here, we report accurate thermal expansion and compressibility measurements along the individual crystallographic axes, using in situ high-temperature and high-pressure high-resolution synchrotron X-ray diffraction. In particular, we have quantitatively determined the subtle variations of the in-plane thermo-mechanical parameters by subjecting these materials to hydrostatic pressure conditions and collecting a large number of data points in small pressure and temperature increments. Based on the anisotropic behavior of bP, we propose the use of this material as sensor for the simultaneous determination of pressure and temperature in the range 0-5 GPa, 298-1700 K.

cond-mat.mtrl-sci↗

arXiv2023

Synthesis and physical properties of LaNiO$_2$ crystals

Infinite-layer (IL) nickelates are an emerging family of superconductors whose similarities and differences to cuprate superconductors are under intense debate. To date, the IL phase of nickelates can only be reached via topotactic oxygen reduction of the perovskite phase, using H$_2$ gas or reducing agents such as CaH$_2$. While the topotactic reduction method has been widely employed on thin film and polycrystalline powder samples, the reduction of La$_{1-x}$Ca$_x$NiO$_3$ single-crystals with lateral dimensions up to 150 $μ$m was achieved only recently, using an indirect contact method with CaH$_2$. Here we report the topotactic transformation of much larger LaNiO$_3$ crystals with lateral dimensions of more than one millimeter, via direct contact with CaH$_2$. We characterize the crystalline, magnetic, and electronic properties of the obtained IL LaNiO$_{2}$ crystals by powder and single-crystal x-ray diffraction (XRD), magnetometry, electrical transport, and x-ray photoelectron spectroscopy (XPS) measurements. The amount of incorporated topotactic hydrogen due to the reduction process is determined by a gas extraction method. In addition, we investigate the evolution of the lattice parameters under hydrostatic pressure up to 12 GPa, using high-resolution synchrotron XRD. Furthermore, we provide a direct comparison of several physical properties of the LaNiO$_{2}$ crystals to their powder and thin film counterparts.

cond-mat.supr-con↗

arXiv2022

Pressure-tuning of $α$-RuCl$_3$ towards the ideal Kitaev-limit

We report the discovery of an intriguing pressure-driven phase transformation in the layered Kitaev-material $α$-RuCl$_3$. By analyzing both the Bragg scattering as well as the diffuse scattering of high-quality single crystals, we reveal a collective reorganization of the layer stacking throughout the crystal. Importantly, this transformation also effects the structure of the RuCl$_3$ honeycomb layers, which acquire a high trigonal symmetry with a single Ru--Ru distance of 3.41Å and a single Ru--Cl--Ru bond angle of 92.8°. Hydrostatic pressure therefore allows to tune the structure of $α$-RuCl$_3$ much closer to the ideal Kitaev-limit. The high-symmetry phase can also be stabilized by biaxial stress, which can explain conflicting results reported earlier and, more importantly, makes the high-symmetry phase accessible to a variety of experiments.

cond-mat.str-el↗