arXiv subjects

Roser Valentí

Publications and source records attributed to Roser Valentí.

At least 19 recordsLinked to original sources

arXiv2026

Quetzalcoatlite as a Disorder-Free Platform for Chiral Magnetism and Frustration

The natural mineral quetzalcoatlite Zn$_6$Cu$_3$(TeO$_6$)$_2$(OH)$_6$ $\cdot$ (Ag$_x$Pb$_y$Cl$_{x+2y}$) is a structurally ideal kagome magnet, providing a platform for exploring the interplay of geometric frustration, chirality, and tunability in a disorder-free framework. Here, we present the (first) comprehensive ab initio study of its electronic and magnetic properties. The electronic structure is dominated by localized half-filled Cu $d_{x^2-y^2}$ orbitals that become insulating through electronic correlations. Mapping the low-energy physics onto a Heisenberg model reveals that the magnetism is governed primarily by two exchange interactions: a nearest-neighbor intralayer kagome coupling and a next-nearest-neighbor interlayer coupling. Their competition stabilizes an unconventional three-dimensional chiral magnetic state. Each kagome layer hosts a $\sqrt{3}\times\sqrt{3}$ order, while adjacent layers are rotated by $60^\circ$, producing a right-handed spiral along the crystallographic $c$-axis. This intrinsic chiral order emerges naturally from the crystal structure and magnetic interactions, establishing quetzalcoatlite as a distinctive realization of chiral magnetism on a perfect kagome lattice. At the same time, the small energy scale of the exchange interactions places the material close to competing magnetic regimes, suggesting that moderate pressure, chemical substitution, or structural modifications may strongly enhance frustration, suppress long-range order, and potentially drive the system toward a quantum spin-liquid state.

cond-mat.str-el↗

arXiv2026

Finite-Temperature Flat-Band Ferromagnetism in the Kagome Hubbard Model

Kagome metals exhibit a rich interplay of topology, electronic correlations, and lattice dynamics. Recent discoveries of Kagome materials with a flat band near the Fermi level have revealed a variety of correlated electronic phases. However, elucidating their microscopic origin remains challenging, as realistic descriptions require accounting for multiple orbitals and competing interactions on an equal footing. To disentangle correlation effects from material-specific details and identify the essential physics of the flat-band regime, we study the single-orbital Kagome-Hubbard model at flat-band fillings using dynamical mean-field theory. We find strong signatures of flat-band ferromagnetism, consistent with exact and mean-field ground-state results. Moreover, we uncover an unconventional quasi-ordered phase in which a partially filled spin-polarized flat band pinned at the Fermi level gives rise to persistent local spin fluctuations down to zero temperature, in striking contrast to the classical behavior expected for a conventional ferromagnet. Our results demonstrate that these anomalous fluctuations are an intrinsic consequence of the flat-band degeneracy and establish a minimal framework for understanding correlation effects in flat-band Kagome systems.

cond-mat.str-el↗

arXiv2026

Visualizing impurity-driven scattering phase textures in EuCd2As2

Understanding how disorder modifies electronic states in magnetic semiconductors is important for controlling spin-dependent transport and topological responses. Here we use scanning tunneling microscopy to visualize scattering phase textures in EuCd2As2. By isolating a single surface wavevector we reconstruct spatial phase maps of the local density of states and identify phase dislocations characterized by 2pi winding around impurity sites. These phase singularities emerge systematically within charge puddles generated by Eu interstitials and their positions evolve with bias voltage. We show that their spatial structure is consistent with interference between multiple scattering channels, including contributions from spin-orbit coupling. We provide a model which reproduces phase dislocations and relates the decay of the phase gradient to the relative strength of spin-orbit and scalar scattering. Our results establish a route to access the phase of electronic scattering in real space and study the role of local disorder and spin-orbit interactions in shaping electronic states in quantum materials.

cond-mat.mtrl-sci↗

arXiv2026

Altermagnetism from a Cu-Fe Lieb Lattice in FeSe/Cuprate Heterostructures

Realizing altermagnetism in high-$T_c$ cuprate-based systems would provide a direct route for studying spin-split electronic bands in the absence of net magnetization and investigate their interplay with unconventional superconductivity. Here, we propose that FeSe/cuprate heterostructures offer such a platform, where a 45$^\circ$ twist of Cu and Fe layers creates an effective CuFe$_2$ Lieb lattice in which Fe magnetic order and Cu-Fe hybridization through the ligands induces altermagnetic $d$-wave spin splitting. A minimal tight-binding model shows that this mechanism is generic. Furthermore, a substrate-induced inequivalence of the two Se sites in FeSe provides a second route in which altermagnetism originates in the Fe layer and is transferred to the cuprate layer by proximity. Density functional theory calculations for FeSe/Bi$_2$Sr$_2$CuO$_6$ heterostructures confirm the viability of both mechanisms and reveal ways to enhance the spin splitting. These results establish superconducting cuprate/transition metal chalcogenide heterostructures as a promising setting for engineering altermagnetism and studying its coupling to unconventional superconductivity.

cond-mat.str-el↗

arXiv2026

Stacking-dependent anisotropic altermagnetism in V$_{1/3}$NbS$_2$

We report profound impacts of the stacking sequence of triangular lattices of magnetic transition metal ions intercalated between the layers of the van der Waals material NbS$_2$. Using single crystal x-ray and neutron diffraction, and transport and magnetization measurements, we show there are two distinct polytypes of $\rm V_{1/3}NbS_2$ with disparate easy axes of magnetization and different anomalous Hall responses. Self-consistent analysis of inelastic neutron scattering data provides evidence for oscillatory RKKY interactions that extend to 1 nm and stabilize quasi-collinear A-type altermagnetic orders in both polytypes though with perpendicular easy axes. The detailed stacking sequence of a bulk polytype crystal dramatically impact its macroscopic anomalous Hall response and magnetism, which suggests a new path to engineer the bulk properties of a layered three dimensional solid.

cond-mat.str-el↗

arXiv2026

Bobkingite, a new coupled sawtooth chain platform

We investigate the mineral bobkingite, \ce{Cu5(OH)8Cl2(H2O)2}, as a potential realization of the sawtooth chain. Using \textit{ab initio} methods, we estimate the magnetic exchange couplings and find that bobkingite hosts quasi-one-dimensional sawtooth chains, with residual three-dimensional interactions strongly suppressed by the crystal geometry. Examining the full exchange network, we find that the classical model exhibits an extensive manifold of nearly degenerate states with emergent two-dimensional character, which spin-wave theory shows to persist to leading order in quantum fluctuations as Ising degrees of freedom. Unlike other sawtooth candidates, bobkingite has negligible vertical interchain coupling, preserving a one-dimensional degeneracy even in the presence of ordering, suggesting that any long-range order is weak. Thermal fluctuations may thus stabilize a finite-temperature classical spin liquid regime, with a cascade of transitions upon cooling into successively lower-dimensional degenerate states, making bobkingite a compelling platform for exploring sawtooth chain physics.

cond-mat.str-el↗

arXiv2026

$α$-RuCl$_3$ intercalated into graphite: a new three-dimensional platform for exotic quantum phases

Multilayer graphene with different stacking sequences has emerged as a powerful setting for correlated and topological phases. In parallel, progress in graphene heterostructures with magnetic or correlated materials-most notably the Kitaev candidate $α$-RuCl$_3$-has demonstrated charge transfer, magnetic proximity effects, and interfacial reconstruction, creating new opportunities for engineered quantum systems. Motivated by these developments, we explore a three-dimensional analogue in which $α$-RuCl$_3$ layers are inserted directly into the van der Waals gaps of graphite, forming an intercalated system. Here, we report the successful synthesis and comprehensive characterization of graphite intercalated with $α$-RuCl$_3$. Using a combination of X-ray diffraction, quantum oscillation measurements, and first-principles electronic structure calculations, we study the structural and electronic properties of these intercalated crystals. Our results demonstrate that graphite intercalated with $α$-RuCl$_3$ offers a robust route to develop three-dimensional materials with access to novel correlated and topological states.

cond-mat.str-el↗

arXiv2026

Emergence of an antiferromagnetic topological Anderson insulator in the interacting Haldane model

We examine the emergence of topological Anderson insulating phases in the spinful Haldane model with Hubbard and next-neighbor density-density interactions, subject to Anderson disorder. Using finite-size exact diagonalization, we characterize the phases that arise from the interplay between topology, interactions, and disorder. In addition to standard $C=2$ topological Anderson phases, we observe an antiferromagnetic $C=1$ topological Anderson phase, consistent with the antiferromagnetic quantum anomalous Hall insulator previously identified in the clean model at finite staggered mass. We further analyze these phases using a neural network trained on the exact diagonalization data. Our results support the hypothesis that an explicit charge imbalance is required to induce the $C=1$ phase, generated by Anderson disorder rather than by a staggered mass.

cond-mat.mes-hall↗

arXiv2026

Competing incommensurability, electronic correlations, and superconductivity in a hybrid transition metal dichalcogenide

The engineering of superlattices in two-dimensional van der Waals materials has enabled the realization of rich phase diagrams hosting topological and strongly correlated phases. While incommensurability is widespread in three-dimensional systems, the role of moiré potentials in bulk materials remains largely unexplored. Here, using scanning tunneling microscopy, we demonstrate that a bulk transition-metal dichalcogenide polytype, 4Hb-TaS$_2$, hosts an emergent incommensurate potential between its alternating 1T and 1H layers. Interplay with a concomitant incommensurate charge-density wave suppresses the long-range order of this potential, leading to intricate coupling with electronic correlations in the doped 1T surface layer. Combining density functional theory with dynamical mean-field theory, we show that the lattice mismatch locally modulates the interlayer distance, thereby tuning both hybridization and charge transfer between the correlated 1T and metallic 1H layers. This redistribution of charge drives the system towards a doped Mott regime, in which the remaining local moments become self-screened, giving rise to a zero-bias resonance. We further find that bulk superconductivity competes with both the underlying landscape and the associated charge transfer. Our results establish incommensurate potentials as a previously overlooked ingredient in hybrid transition-metal dichalcogenides, highlighting their central role in the interplay between electronic correlations, charge-density-wave order, and unconventional superconductivity.

cond-mat.str-el↗

arXiv2026

Interplay between many-body correlations, strain and lattice relaxation in twisted bilayer graphene

In twisted bilayer graphene, a unified understanding of the mechanisms governing temperature-dependent electronic spectra and thermodynamic properties remains controversial despite extensive theoretical efforts. Here, we present a comprehensive theoretical framework that quantitatively accounts for scanning tunneling spectroscopy, quantum twisting microscopy, and thermodynamic properties of magic angle twisted bilayer graphene. We demonstrate that the observed behavior arises from the interplay between electron correlations and external symmetry-breaking induced by strain and lattice relaxation. These effects act cooperatively to shape the emergent electronic behavior, leaving characteristic signatures across spectroscopy, compressibility and entropy.

cond-mat.str-el↗

arXiv2026

Exact downfolding and its perturbative approximation

Solving the many-electron problem, even approximately, is one of the most challenging and simultaneously most important problems in contemporary condensed matter physics with various connections to other fields. The standard approach is to follow a divide and conquer strategy that combines various numerical and analytical techniques. A crucial step in this strategy is the derivation of an effective model for a subset of degrees of freedom by a procedure called downfolding, which often corresponds to integrating out energy scales far away from the Fermi level. In this work we present a rigorous formulation of this downfolding procedure, which complements the renormalization group picture put forward by Honerkamp [PRB 85, 195129 (2012)}]. We derive an exact effective model in an arbitrarily chosen target space (e.g. low-energy degrees of freedom) by explicitly integrating out the the rest space (e.g. high-energy degrees of freedom). Within this formalism we state conditions that justify a perturbative truncation of the downfolded effective interactions to just a few low-order terms. Furthermore, we utilize the exact formalism to formally derive the widely used constrained random phase approximation (cRPA), uncovering underlying approximations and highlighting relevant corrections in the process. Lastly, we detail different contributions in the material examples of fcc Nickel and the infinite-layer cuprate SrCuO$_2$. Our results open up a new pathway to obtain effective models in a controlled fashion and to judge whether a chosen target space is suitable.

cond-mat.str-el↗

arXiv2026

Oxygen-vacancy-induced Raman softening in the catalyst Fe$_2$(MoO$_4$)$_3$

Iron molybdate (Fe$_2$(MoO$_4$)$_3$) is a widely used commercial catalyst for oxidative dehydrogenation. Recently, the possibility that bulk oxygen atoms participate in catalytic reactions has been proposed based on the experimentally observed significant reduction in Raman intensity during the catalytic process, which implies the formation of oxygen defects. In this work, we performed density functional theory (DFT) calculations to elucidate the microscopic mechanism of the experimentally observed Raman intensity variation. Our phonon analysis reveals that oxygen-dominated vibrational modes, with a small contribution from Mo, occur near 782cm$^{-1}$-- the same frequency region where the Raman intensity reduction was measured. To make the calculations computationally feasible for this large system, we introduced an effective frozen-phonon approach to mimic defect effects into the Raman intensity. Our results suggest that oxygen vibrations are primarily responsible for the decrease in the calculated Raman intensity. Moreover, structural relaxation of Fe$_2$(MoO$_4$)$_3$ containing an oxygen vacancy indicates that oxygen diffusion from the bulk to the surface may occur very rapidly, such that the local symmetry remains effectively unchanged. This interpretation is in line with the absence of measurable peak shifts or broadening in the experimental Raman spectra.

cond-mat.mtrl-sci↗

arXiv2026

Revealing the microscopic origin of the magnetization plateau in Na$_3$Ni$_2$BiO$_6$

Recent experimental studies of the spin-1 honeycomb antiferromagnet Na$_3$Ni$_2$BiO$_6$ have revealed a pronounced one-third magnetization plateau under applied magnetic fields, highlighting the presence of strong magnetic frustration and anisotropy in this material. Such behavior has been attributed to substantial bond-dependent Kitaev interactions in combination with single-ion anisotropy, placing Na$_3$Ni$_2$BiO$_6$ among honeycomb compounds of interest for unconventional magnetic phases. Motivated by these observations, we present a first-principles-based analysis of the magnetic interactions in Na$_3$Ni$_2$BiO$_6$. By combining density-functional calculations with microscopic modeling, we extract the relevant exchange parameters and construct an effective spin model that quantitatively reproduces both the elastic neutron-scattering spectra and the magnetization curve. The model captures the experimentally observed zero-field zigzag magnetic order, and proposes a $\textit{double-zigzag}$ state at intermediate magnetic fields, realizing the 1/3-magnetization plateau in a simpler way than suggested in previous works. Crucially, we show that the one-third magnetization plateau does not require Kitaev interactions; instead, it arises from the interplay of strong out-of-plane single-ion anisotropy and competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic third-neighbor ($J_3$) Heisenberg couplings. These results establish a consistent microscopic description of Na$_3$Ni$_2$BiO$_6$ and clarify the origin of its field-induced plateau phase.

cond-mat.str-el↗

arXiv2026

Many-body Euler topology

Integer and fractional Chern insulators exhibit a nonzero quantized anomalous Hall conductivity due to a spontaneous breaking of time reversal symmetry. To identify nontrivial topology in their time-reversal symmetric many-body spectra, we introduce many-body Euler numbers as a counterpart to many-body Chern numbers. Exemplarily, we perform calculations in a topological Hubbard model that can realize Chern and fractional Chern insulating phases. Furthermore, we lay out a classification scheme to realize different topological phases in interacting systems using symmetry indicators in analogy to topological band theory.

cond-mat.str-el↗

arXiv2026

Pressure Tuning of Electronic Correlations and Flat Bands in CsCr$_3$Sb$_5$

CsCr$_3$Sb$_5$ is a newly identified strongly correlated kagome superconductor, characterized by non-Fermi-liquid behavior at elevated temperatures and intertwined charge- and spin-density-wave order below $T_{DW}\approx 54$K. Under external pressure, this order is suppressed and a superconducting phase emerges. This phase diagram, which closely resembles that of high-$T_c$ superconductors, together with a kagome flat band near the Fermi level and possible altermagnetic order, has motivated extensive theoretical and experimental investigations. To better understand how pressure influences the ordered states, we present a systematic study of the evolution of the electronic properties under applied pressure. Performing DFT+DMFT (density functional theory combined with dynamical mean field theory) calculations, we uncover a complex interplay between the redistribution of spectral weight in the flat bands and the strength of electronic correlations under pressure. Our results further strengthen the interpretation that pressure effectively weakens electronic correlations through enhanced orbital hybridization. This, in turn, strongly suggests that superconductivity emerges as a direct consequence of the suppression of the system's ordered phase.

cond-mat.str-el↗

arXiv2025

Obtaining the Spectral Function of Moiré Graphene Heavy-Fermions Using Iterative Perturbation Theory

The spectral functions of twisted bilayer graphene (TBG) in the absence of strain have recently been investigated in both the symmetric and symmetry-broken phases using dynamical mean-field theory (DMFT). The theoretically predicted Mott-Hubbard bands and gapless semimetallic state at half-filling have since been confirmed experimentally. Here, we develop several second-order perturbation theory approaches to the topological heavy-fermion (THF) model of TBG and twisted symmetric trilayer graphene (TSTG). In the symmetric phase, we adapt, implement, and benchmark an iterative perturbation theory (IPT) impurity solver within DMFT, enabling computationally efficient yet accurate spectral function calculations. We present momentum- and energy-resolved spectra over a broad range of temperatures and fillings for both symmetric and symmetry-broken states. In addition, we derive analytic expressions for the spectral function within the ``Hubbard-I'' approximation of the THF model and, as expected, find that while it provides a tractable description of Mott physics, it does not capture the low-energy Kondo peak or the finite lifetime broadening of the bands. Our methodology can be extended to include strain, lattice relaxation, and parameter variations, thereby allowing systematic predictions of TBG and TSTG spectral properties across a wide range of physical regimes. Because our perturbative approaches are far less computationally intensive than DMFT with numerically exact impurity solvers, they can be used to efficiently benchmark and scan extensive phase diagrams of the THF parameters, paving the way for full DMFT analyses of the TBG spectral function in the presence of strain and relaxation.

cond-mat.str-el↗

arXiv2025

Exploring d-Wave Magnetism in Cuprates from Oxygen Moments

The antiferromagnetic parent phase of high-T$_c$ cuprates has been established as a Néel state of copper moments, but early work pointed out the important role of ligand oxygen orbitals. Using the three-orbital Emery model, we explore how, and under which conditions, doping-induced antiferromagnetic ordering of weak magnetic moments on the oxygen sites can lead to unconventional d-wave magnetism with spin-split electronic bands. The mechanism for forming such altermagnetic (AM) states in cuprates does not rely on a lowering of the crystal symmetry but rather on interaction-induced formation of magnetic moments on directional oxygen orbitals within the crystallographic unit cell. Therefore, we obtain two different types of AM, namely a (0,0)-AM and a ($π$,$π$)-AM. We explore different regimes and challenges for realizing oxygen AM supported by Hartree-Fock calculations and complementary exact diagonalization of small clusters. While the region of interacting parameters needed to realize these states may be difficult to achieve in known high-T$_c$ cuprates, we propose a scenario to realize AM induced by oxygen magnetic moments in a cuprate-based candidate compound using density functional theory and discuss experimental implications.

cond-mat.str-el↗

arXiv2025

Symmetry-breaking-induced topology in FeSe

FeSe has been one of the most intensively studied iron-based superconductors over the past two decades, exhibiting a wide range of phenomena such as unconventional superconductivity, nematic order, magnetism, orbital-selective correlations, and structural phase transitions. While topologically non-trivial phases have been identified in certain cases -- such as Te-doped FeSe and monolayer FeSe -- topology in bulk FeSe has largely remained unexplored. In this work, we propose a new route to realize topological phases directly in bulk FeSe. We demonstrate that breaking the tetragonal $C_4$ rotational symmetry, thereby lowering the crystal symmetry, can drive FeSe into a strong topological insulating phase. To support this, we perform density functional theory calculations and analyze the band structure using Topological Quantum Chemistry and symmetry-based indicators. Our results show that both uniaxial strain and temperature-induced structural changes lead to non-trivial band topology. Moreover, incorporating electronic correlations through dynamical mean field theory reveals that the topological characteristics near the Fermi level remain robust, as the relevant bands experience only moderate renormalization. These findings highlight strain as a promising mechanism to induce topological phases in FeSe

cond-mat.mtrl-sci↗