arXiv subjects

V. Pardo

Publications and source records attributed to V. Pardo.

At least 19 recordsLinked to original sources

arXiv2026

Symmetry-Selective Stabilization of Charge-Density Wave in ScV$_6$Sn$_6$

Charge-density-wave (CDW) order in kagome metals is highly sensitive to external tuning parameters such as chemical substitution and hydrostatic pressure, which generally suppress long-range order. Here, using high-resolution X-ray diffraction under controlled uniaxial strain, we show that anisotropic lattice deformation instead stabilizes and enhances the CDW state in ScV$_6$Sn$_6$. Compression along the [H00] and [HH0] directions lowers the crystal symmetry from hexagonal to orthorhombic, lifts the degeneracy between symmetry-equivalent in-plane CDW domains, and promotes long-range order while preserving the underlying trimer instability. Phonon calculations indicate only a moderate stabilization of the imaginary flat phonon mode, demonstrating that the increase in T$_\mathrm{CDW}$ is primarily driven by the in-plane ordering of the Sn$^\mathrm{T}$--Sc--Sn$^\mathrm{T}$ \textit{rattling} chains within the frustrated kagome lattice. A phenomenological model incorporating strain-dependent Ising couplings within a three-state Potts framework successfully reproduces the evolution of T$_\mathrm{CDW}$ under compression and captures the continuous nature of the transition. Our results establish uniaxial strain as a powerful symmetry-selective tuning parameter for order-disorder transformations in frustrated lattices.

cond-mat.str-el↗

arXiv2025

First-order phase transition driven by competing charge-order fluctuations in 1T'-TaTe$_{2}$

First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector $\mathbf{q}^* = (0, \frac{1}{4}+\delta, \frac{1}{2})$ in the 1T' phase of TaTe$_2$, which competes with fluctuating high-temperature Ta trimer bonding states at $\mathbf{q}_\mathrm{CO} =(0, \frac{1}{3}, 0)$. The precursor state follows the temperature dependence of the hidden incommensurability of the $\textit{quasi}$-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at $\mathbf{q}_\mathrm{CO}$, characterized by a charge disproportionation of the inequivalent Ta sites, appears to be driven by local chemical bonding. Dynamical lattice calculations identify an imaginary optical mode at $\mathbf{q}^*$, involving an in-plane vibration of the Ta atoms forming a chain-like structure that renormalizes below $T_\mathrm{CO}$. Our experimental and theoretical observations suggest that the controversial first-order phase transition, as captured by phenomenological Ginzburg-Landau theory, results from the competition between two order parameters: one involving Fermi surface nesting and the other involving local chemical bonding.

cond-mat.str-el↗

arXiv2025

Magnetoelastic coupling in intercalated transition metal dichalcogenides

The large van der Waals gap in transition metal dichalcogenides (TMDs) offers an avenue to host external metal atoms that modify the ground state of these 2D materials. Here, we experimentally and theoretically address the charge correlations in a family of intercalated TMDs. While short-range charge fluctuations develop in Co$_{1/3}$TaS$_{2}$ and Fe$_{1/3}$TaS$_{2}$, long-range charge order switches-on in Fe$_{1/3}$NbS$_{2}$ driven by the interplay of magnetic order and lattice degrees of freedom. The magnetoelastic coupling is demonstrated in Fe$_{1/3}$NbS$_{2}$ by the enhancement of the charge modulations upon magnetic field below T$_\mathrm{N}$, although Density Functional Perturbation Theory (DFPT) calculations predict negligible electron(spin)-phonon coupling. Furthermore, we show that Co-intercalated TaS$_2$ displays a kagome-like Fermi surface, hence opening the path to engineer electronic band structures and study the entanglement of spin, charge, and spin-phonon mechanisms in the large family of intercalated TMDs.

cond-mat.str-el↗

arXiv2024

Self-stacked 1$\mathrm{T}$-1$\mathrm{H}$ layers in 6$\mathrm{R}$-NbSeTe and the emergence of charge and magnetic correlations due to ligand disorder

The emergence of correlated phenomena arising from the combination of 1$\mathrm{T}$ and 1$\mathrm{H}$ van der Waals layers is the focus of intense research. Here, we synthesize a novel self-stacked 6$\mathrm{R}$ phase in NbSeTe, showing a perfect alternating 1T and 1H layers that grow coherently along the c-direction, as revealed by scanning transmission electron microscopy. Angle resolved photoemission spectroscopy shows a mixed contribution of the trigonal and octahedral Nb bands to the Fermi level. Diffuse scattering reveals temperature-independent short-range charge fluctuations with propagation vector $\mathrm{q_{CO}}$=(0.25,0), derived from the condensation of a longitudinal mode in the 1T layer. We observe that ligand disorder quenches the formation of a charge density wave. Magnetization measurements suggest the presence of an inhomogeneous, short-range magnetic order, further supported by the absence of a clear phase transition in the specific heat. These experimental analyses in combination with \textit{ab initio} calculations indicate that the ground state of 6$\mathrm{R}$-NbSeTe is described by a statistical distribution of short-range charge-modulated and spin-correlated regions driven by ligand disorder. Our results devise a route to synthesize 1$\mathrm{T}$-1$\mathrm{H}$ self-stacked bulk heterostructures to study emergent phases of matter.

cond-mat.mtrl-sci↗

arXiv2023

Electronic structure and lattice dynamics of 1T-VSe$_2$: origin of the 3D-CDW

In order to characterize in detail the charge density wave (CDW) transition of 1$T$-VSe$_2$, its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering (IXS), and state-of-the-art first principles density functional theory calculations. Resonant elastic x-ray scattering (REXS) does not show any resonant enhancement at either V or Se K-edges, indicating that the CDW peak describes a purely structural modulation of the electronic ordering. ARPES identifies (i) a pseudogap at T$>$T$_{CDW}$, which leads to a depletion of the density of states in the $ML-M'L'$ plane at T$<$T$_{CDW}$, and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon ($\omega_1$) with propagation vector (0.25 0 -0.3) r.l.u. We show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependence EPI with a small contribution from nesting. The pressure dependence of the $\omega_1$ soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic $C2/m$ phase. The wide set of experimental data are well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals (vdW) corrections in the exchange-correlation functional. The description of the electronics and dynamics of VSe$_2$ reported here adds important pieces of information to the understanding of the electronic modulations of TMDs.

cond-mat.str-el↗

arXiv2022

Anharmonicity reveals the tunability of the charge density wave orders in monolayer VSe$_2$

VSe$_2$ is a layered compound that has attracted great attention due to its proximity to a ferromagnetic state that is quenched by the presence of a charge density wave (CDW) phase. In the monolayer limit, unrelated experiments have reported different CDW orders with transition temperatures in the range of 130 to 220 K, making this monolayer very controversial. Here we perform first-principles non-perturbative anharmonic phonon calculations in monolayer VSe$_2$ in order to estimate the CDW order and the corresponding transition temperature. Our analysis solves previous experimental contradictions as we reveal that monolayer VSe$_2$ develops two independent charge density wave orders associated to $\sqrt{3} \times \sqrt{7}$ and $4 \times 4$ modulations that compete as a function of strain. In fact, tiny variations of only 1.5% in the lattice parameter are enough to stabilize one order or the other, which underlines that the CDW order becomes substrate-dependent. The predicted CDW temperature is strain-dependent and has a value of around 220 K, in good agreement with experiments. Moreover, we analyze the impact of external Lennard-Jones interactions on the CDW. We show that these can act together with the anharmonicity to suppress the CDW orders. In the particular case of monolayer VSe$_2$, this may give rise to the emergence of a ferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2021

Tuning Coherent-Phonon Heat Transport in LaCoO$_3$/SrTiO$_3$ Superlattices

Accessing the regime of coherent phonon propagation in nanostructures opens enormous possibilities to control the thermal conductivity in energy harvesting devices, phononic circuits, etc. In this paper we show that coherent phonons contribute substantially to the thermal conductivity of LaCoO3/SrTiO3 oxide superlattices, up to room temperature. We show that their contribution can be tuned through small variations of the superlattice periodicity, without changing the total superlattice thickness. Using this strategy, we tuned the thermal conductivity by 20% at room temperature. We also discuss the role of interface mixing and epitaxial relaxation as an extrinsic, material dependent key parameter for understanding the thermal conductivity of oxide superlattices.

cond-mat.mtrl-sci↗

arXiv2020

Synthesis, Engineering, and Theory of 2D van der Waals Magnets

Spontaneous magnetic order is a routine instance in three-dimensional (3D) materials but for a long time, it remained elusive in the 2D world. Recently, the first examples of (stand-alone) 2D van der Waals (vdW) crystals with magnetic order, either antiferromagnetic or ferromagnetic, have been reported. In this review, we describe the state of the art of the nascent field of magnetic 2D materials focusing on synthesis, engineering, and theory aspects. We also discuss challenges and some of the many different promising directions for future work.

cond-mat.mtrl-sci↗

arXiv2017

Mimicking cuprates: large orbital polarization in a metallic square-planar nickelate

High temperature cuprate superconductivity remains a defining problem in condensed matter physics. Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics. None of these analogs has been superconducting, and few are even metallic. Here, we report that the low-valent, quasi-two-dimensional trilayer compound, Pr4Ni3O8 avoids a charge-stripe ordered phase previously reported for La4Ni3O8, leading to a metallic ground state. By combining x-ray absorption spectroscopy and density functional theory calculations, we further find that metallic Pr4Ni3O8 exhibits a low-spin configuration and significant orbital polarization of the unoccupied eg states with pronounced dx2-y2 character near the Fermi energy, both hallmarks of the cuprate superconductors. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus represents one of the closest analogies to cuprates yet reported and a singularly promising candidate for high-Tc superconductivity if appropriately doped.

cond-mat.str-el↗

arXiv2016

Quantum spin Hall effect in rutile-based oxide multilayers

Dirac points in two-dimensional electronic structures are a source for topological electronic states due to the $\pm π$ Berry phase that they sustain. Here we show that two rutile multilayers (namely (WO$_2$)$_2$/(ZrO$_2$)$_n$ and (PtO$_2$)$_2$/(ZrO$_2$)$_n$, where an active bilayer is sandwiched by a thick enough (n=6 is sufficient) band insulating substrate, show semi-metallic Dirac dispersions with a total of four Dirac cones along the $Γ-M$ direction. These become gapped upon the introduction of spin-orbit coupling, giving rise to an insulating ground state comprising four edge states. We discuss the origin of the lack of topological protection in terms of the valley spin-Chern numbers and the multiplicity of Dirac points. We show with a model Hamiltonian that mirror-symmetry breaking would be capable of creating a quantum phase transition to a strong topological insulator, with a single Kramers pair per edge.

cond-mat.mes-hall↗

arXiv2016

Charge ordering in Ni$^{1+}$/Ni$^{2+}$ nickelates: La$_4$Ni$_3$O$_8$ and La$_3$Ni$_2$O$_6$

Ab initio calculations have been performed to unravel the origin of the recently found superlattice peaks in the trilayer nickelate La$_4$Ni$_3$O$_8$. These peaks arise from static charge ordering of Ni$^{2+}$/ Ni$^{1+}$ stripes oriented at 45$^{\circ}$ to the Ni-O bonds. An insulating state originates from a combination of structural distortions and magnetic order, with the gap being formed solely within the d$_{x^2-y^2}$ manifold of states. When doped, electrons or holes would go into these states, in a similar fashion to what occurs in the cuprates. Analogous calculations suggest that checkerboard charge order should occur in the bilayer nickelate La$_3$Ni$_2$O$_6$. These results reveal a close connection between La$_4$Ni$_3$O$_8$ and La$_3$Ni$_2$O$_6$ with La$_{2-x}$Sr$_x$NiO$_4$ for x=1/3 and x=1/2, respectively.

cond-mat.str-el↗

arXiv2016

Ab initio study of the strain dependence of thermopower in electron-doped SrTiO$_3$

In this paper we explore the different mechanisms that affect the thermopower of a band insulating perovskite (in this case, SrTiO$_3$) when subject to strain (both compressive or tensile). We analyze the high temperature, entropy dominated limit and the lower temperature, energy-transport regime. We observe that the effect of strain in the high-temperature Seebeck coefficient is small at the concentration levels of interest for thermoelectric applications. However, the effective mass changes substantially with strain, which produces an opposite effect to that of the degeneracy-breakups produced by strain. In particular, we find that the thermopower can be enhanced by applying tensile strain in the adequate regime. We conclude that the detrimental effect of strain in thermopower due to band splitting is a minor effect that will not hamper the optimization of the thermoelectric properties of oxides with t$_{2g}$-active bands by applying strain.

cond-mat.mtrl-sci↗

arXiv2016

Dirac topological insulator in the d$_{z^2}$ manifold of a honeycomb oxide

We show by means of ab initio calculations and tight-binding modeling that an oxide system based on a honeycomb lattice can sustain topologically non-trivial states if a single orbital dominates the spectrum close to the Fermi level. In such situation, the low energy spectra is described by two Dirac equations that become non-trivially gapped when spin-orbit coupling (SOC) is switched on. We provide one specific example for this but the recipe is general. We discuss a realization of this starting from a conventional spin-a-half honeycomb antiferromagnet whose states close to the Fermi energy are d$_{z^2}$ orbitals. Switching off magnetism by atomic substitution and ensuring that the electronic structure becomes two-dimensional is sufficient for topologicality to arise in such a system. We show that the gap in such model scales linearly with SOC, opposed to other oxide-based topological insulators, where smaller gaps tend to appear by construction of the lattice. We also provide a study of the quantum Hall effect in such system, showing the close connections with the physics of graphene but in a d-electron system.

cond-mat.mes-hall↗

arXiv2015

Non-collinear vs collinear description of the Ir-based one-$t_{2g}$ -hole perovskite-related compounds: SrIrO$_3$ and Sr$_2$IrO$_4$

We present an analysis of the electronic structure of perovskite-related iridates, 5d electron compounds where a subtle interplay between spin-orbit coupling, tetragonal distortions and electron correlations determines the electronic structure properties. We suggest via electronic structure calculations that a non-collinear calculation is required to obtain solutions close to the usually quoted $j_{eff}$ = 1/2 state to describe the $t_{2g}$ hole in the $Ir^{4+} :d^5$ cation, while a collinear calculation yields a different solution, the hole is in a simpler xz/yz complex combination with a smaller $L_z /S_z$ ratio. We describe what the implications of this are in terms of the electronic structure; surprisingly, both solutions barely differ in terms of their band structure, and are similar to the one obtained by a tight binding model involving $t_{2g}$ orbitals with mean field interactions. We also analyze how the electronic structure and magnetism evolve with strain, with the spin-orbit coupling strength and with the on-site Coulomb repulsion, suggest the way the band structure gets modified and draw some comparisons with available experimental observations.

cond-mat.str-el↗

arXiv2015

Electron degeneracy and intrinsic magnetic properties of epitaxial Nb:SrTiO$_3$ thin-films controlled by defects

We report thermoelectric power experiments in e-doped thin films of SrTiO$_3$ (STO) which demonstrate that the electronic band degeneracy can be lifted through defect management during growth. We show that even small amounts of cationic vacancies, combined with epitaxial stress, produce a homogeneous tetragonal distortion of the films, resulting in a Kondo-like resistance upturn at low temperature, large anisotropic magnetoresistance, and non-linear Hall effect. Ab-initio calculations confirm a different occupation of each band depending on the degree of tetragonal distortion. The phenomenology reported in this paper for tetragonally distorted e-doped STO thin films, is similarto that observed in LaAlO$_3$/STO interfaces and magnetic STO quantum wells.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic Field Induced Transition in Vanadium Spinels

We study vanadium spinels $A$V$_2$O$_4$ ($A$ = Cd, Mg) in pulsed magnetic fields up to 65 T. A jump in magnetization at $μ_0 H \approx$ 40 T is observed in the single-crystal MgV$_2$O$_4$, indicating a field induced quantum phase transition between two distinct magnetic orders. In the multiferroic CdV$_2$O$_4$, the field-induced transition is accompanied by a suppression of the electric polarization. By modeling the magnetic properties in the presence of strong spin-orbit coupling characteristic of vanadium spinels, we show that both features of the field-induced transition can be successfully explained by including the effects of the local trigonal crystal field.

cond-mat.str-el↗

arXiv2013

Charge States of Ions, and Mechanisms of Charge-Ordering Transitions

To gain insight into the mechanism of charge-ordering transitions, which conventionally are pictured as a disproportionation of an ion M as 2M$^{n+}$ $\rightarrow$ M$^{(n+1)+}$ + M$^{(n-1)+}$, we (1) review and reconsider the charge state (or oxidation number) picture itself, (2) introduce new results for the putative charge ordering compound AgNiO$_2$ and the dual charge state insulator AgO, and (3) analyze cationic occupations of actual (not formal) charge, and work to reconcile the conundrums that arise. We establish that several of the clearest cases of charge ordering transitions involve no disproportion (no charge transfer between the cations, hence no charge transfer), and that the experimental data used to support charge ordering can be accounted for within density functional based calculations that contain no charge transfer between cations. We propose that the charge state picture retains meaning and importance, at least inn many cases, if one focuses on Wannier functions rather than atomic orbitals. The challenge of modeling charge ordering transitions with model Hamiltonians is discussed.

cond-mat.str-el↗

arXiv2013

Ab initio study of $Z_2$ topological phases in perovskite (111) $(\text{SrTiO}_3)_7/(\text{SrIrO}_3)_2$ and $(\text{KTaO}_3)_7/(\text{KPtO}_3)_2$ multilayers

Honeycomb structures formed by the growth of perovskite 5d transition metal oxide heteroestructures along the (111) direction in $t_{2g}^5$ configuration can give rise to topological ground states characterized by a topological index $ν$=1. Using a combination of a tight binding model and ab initio calculations we study the multilayers $(\text{SrTiO}_3)_7/(\text{SrIrO}_3)_2$ and $(\text{KTaO}_3)_7/(\text{KPtO}_3)_2$ as a function of parity asymmetry, on-site interaction and uniaxial strain and determine the nature and evolution of the gap. $(\text{SrTiO}_3)_7/(\text{SrIrO}_3)_2$ is found to be a topological semimetal. $(\text{KTaO}_3)_7/(\text{KPtO}_3)_2$ is a topological Mott insulator that can be driven to a trivial insulating phase by an external electric field.

cond-mat.str-el↗