arXiv subjects

Iñigo Robredo

Publications and source records attributed to Iñigo Robredo.

At least 19 recordsLinked to original sources

arXiv2026

Emergent Surface Kondo Flat Band Driven by Competing Interactions in a Topological Ferromagnet

A central goal of modern condensed matter physics is to uncover new quantum states of matter arising from the intertwined effects of strong electron correlations, magnetism, and band topology. Heavy-fermion phases, generated by Kondo interactions, represent one of the most remarkable manifestations of electronic correlations, and topological heavy-fermion states have been identified in several non-magnetic materials. Yet, the consequences of their competition with magnetic order have remained largely unexplored. Here, we reveal a new phenomenon: the spontaneous spatial separation of correlated quantum phases. By showing that magnetism can drive distinct strongly correlated electronic states to coexist in different regions of a single material, our work establishes a previously unknown mechanism for organizing quantum matter and opens a new direction in the study of correlated topological systems. Using \emph{bulk-sensitive} probes, we show that UAsS crystals are, in the bulk, metallic ferromagnets with only moderate correlation-driven band renormalizations. First-principles calculations reveal a topological electronic structure hosting both nodal lines and Weyl points, pointing to a rich underlying topology. Angle-resolved photoemission spectroscopy (ARPES) measurements are consistent with these predictions, resolving the nodal lines and Weyl crossings. In striking contrast, \emph{surface-sensitive} ARPES and scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal a pronounced flat band pinned at the Fermi level, accompanied by a sharp resonance -- hallmarks of an emergent, strongly correlated Kondo state not captured by first-principles calculations.

cond-mat.str-el↗

arXiv2026

Quantifying quasiparticle chirality in a chiral topological semimetal

Recently, the projection of the electron's spin on its crystal momentum has been proposed as a metric to quantify electronic chirality of Bloch states in crystals, which is expected to affect a wide range of physical properties, such as magnetoelectric and optical responses. However, a direct experimental quantification of this chirality metric over an entire iso-energy surface has remained elusive. Here, we have used spin- and angle-resolved photoemission spectroscopy to directly probe the electronic chirality by measuring the bulk spin texture of Kramers-Weyl and Weyl cones in RhSi, a chiral topological semimetal with strong spin-orbit coupling (SOC). After quantifying the SOC splitting of Weyl cones, we determine their spin direction along different azimuthal angles to extract energy dependent the deviations (up to ~40°) from perfect parallel spin-momentum locking. From these deviations we define an energy-dependent normalized electron chirality density (NECD), a directly accessible metric of bulk electronic chirality. In RhSi, the NECD decreases from 1 at the Kramers-Weyl point to ~0.8 at ~200 meV below it. Finally, we show that this experimentally grounded NECD provides predictive power for magneto-optical and transport responses of chiral materials, exemplified by the longitudinal Edelstein effect.

cond-mat.mes-hall↗

arXiv2026

Control of the MoTe$_2$ Fermi Surface by Nb Doping

Ab initio calculations and angle-resolved photoemission experiments show that the bulk and surface electronic structure of Weyl semimetal candidate MoTe$_2$ changes significantly by tuning the chemical potential by less than 0.4 eV. Calculations show that several Lifshitz transitions can occur among multiple electron and hole Fermi pockets of differing orbital character. Experiments show that 18% Nb-Mo substitution reduces the occupation of bulk and (001) surface bands, effectively producing a chemical potential shift of $\approx 0.3$ eV. Orbital character and dimensionality of the bulk bands is examined by soft X-ray angle resolved photoemission with control of the excitation light polarization. The band filling at the surface is shown to increase upon deposition of alkali atoms. The results indicate that multiple regimes of electronic properties can be easily accessed in this versatile, layered material.

cond-mat.mtrl-sci↗

arXiv2025

Emergent heavy-fermion physics in a new family of topological insulators RAsS (R = Y, La, and Sm)

Realizing topological phases in strongly correlated materials has become a major impetus in condensed matter physics. Although many compounds are now classified as topological insulators, $f$-electron systems (with their strong electron correlations) provide an especially fertile platform for emergent heavy-fermion phenomena driven by the interplay of topology and many-body effects. In this study, we examine the crystalline topology of a new RAsS series (R = Y, La, Sm), revealing a structural variant from previous reports. We demonstrate that YAsS and SmAsS host hourglass fermions protected by glide symmetry. SmAsS notably exhibits a strong effective-mass enhancement, placing it alongside SmB${}_6$ and YbB${}_{12}$ as a material that couples topological surface states with emergent Kondo physics, yet distinguished by its crystalline symmetry constraints and $f$-$p$ orbital hybridization. To capture these features, we construct a minimal model incorporating $f$-electron degrees of freedom, which reproduces the observed topological properties and predicts that the surface states survive in the correlated regime, albeit shifted in energy. Our work thus introduces a new family of correlated topological materials and forecasts the robustness of their surface states under Kondo correlations.

cond-mat.mtrl-sci↗

arXiv2025

Spin-to-charge conversion in orthorhombic RhSi topological semimetal crystalline thin films

The rise of non-magnetic topological semimetals, which provide a promising platform for observing and controlling various spin-orbit effects, has led to significant advancements in the field of topological spintronics. RhSi exists in two distinct polymorphs: cubic and orthorhombic crystal structures. The noncentrosymmetric B20 cubic structure has been extensively studied for hosting unconventional multifold fermions. In contrast, the orthorhombic structure, which crystallizes in the Pnma space group (No. 62), remains less explored and belongs to the family of topological Dirac semimetals. In this work, we investigate the structural, magnetic, and electrical properties of RhSi textured-epitaxial films grown on Si(111) substrates, which crystallize in the orthorhombic structure. We investigate the efficiency of pure spin current transport across RhSi/permalloy interfaces and the subsequent spin-to-charge current conversion via inverse spin Hall effect measurements. The xperimentally determined spin Hall conductivity in orthorhombic RhSi reaches a maximum value of 126 ($\hbar$/e)($Ω$.cm)$^{-1}$ at 10 K, which aligns reasonably well with first-principles calculations that attribute the spin Hall effect in RhSi to the spin Berry curvature mechanism. Additionally, we demonstrate the ability to achieve a sizable spin-mixing conductance (34.7 nm$^{-2}$) and an exceptionally high interfacial spin transparency of 88$%$ in this heterostructure, underlining its potential for spin-orbit torque switching applications. Overall, this study broadens the scope of topological spintronics, emphasizing the controlled interfacial spin-transport processes and subsequent spin-to-charge conversion in a previously unexplored topological Dirac semimetal RhSi/ferromagnet heterostructure.

cond-mat.mtrl-sci↗

arXiv2025

Concurrent Multifractality and Anomalous Hall Response in the Nodal Line Semimetal Fe$_3$GeTe$_2$ Near Localization

Topological states of matter exhibit unique protection against scattering by disorder. Different topological classes exhibit distinct forms and degrees of protection. Here, we investigate the response of the ferromagnetic nodal line semimetal Fe$_3$GeTe$_2$ to disorder and electronic interactions. By combining global magneto-transport with atomic-scale scanning tunneling spectroscopy we find a simultaneous onset of diverse phenomena below a common temperature scale of about 15 K: A crossover from metallic to insulating temperature dependence of the longitudinal resistivity, saturation of the anomalous Hall conductivity to its maximal value, formation of a sharp zero-bias dip in the tunneling density of state, and emergence of multi-fractal structure of the electronic wavefunction peaking at the Fermi energy. These concurrent observations reflect the emergence of a novel energy scale possibly related to the opening of a gap in the nodal line band of Fe$_3$GeTe$_2$. Our study provides overarching insight into the role of disorder, electronic interactions and Berry curvature in setting the micro- and macro-scale responses of topological semimetals.

cond-mat.str-el↗

arXiv2025

Origins of the anomalous Hall conductivity in the symmetry enforced Fe3GeTe2 nodal-line ferromagnet

Fe$_3$GeTe$_2$ has gained attention in the condensed matter community for its potential to be exfoliated into thin films with ferromagnetic (FM) order, thanks to its van der Waals layered structure and significant intrinsic anomalous Hall conductivity (AHC). In this work, we analyze the electronic structure and show that, contrary to prior claims, the bulk of the AHC cannot arise from gapped nodal lines. By studying the material's symmetry properties, both with and without spin-orbit coupling (SOC) and across paramagnetic and FM phases, we find that Fe$_3$GeTe$_2$ hosts mirror-symmetry-protected nodal lines, which support surface drumhead states. Additionally, we identify three key sources of AHC: nodal lines in the paramagnetic phase gapped by the FM order, Weyl points within specific energy ranges, and gaps between spin-up and spin-down bands caused by SOC. Finally, our calculations suggest that electron doping could increase the AHC up to four times compared to its value at the computed Fermi level.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic Weyl-Kondo semimetals induced by quantum fluctuations

Weyl-Kondo semimetals are strongly correlated topological semimetals that develop through the cooperation of the Kondo effect with space group symmetries. The Kondo effect, capturing quantum fluctuations associated with strong correlations, is usually suppressed by magnetic order. Here we develop the theory of magnetic Weyl-Kondo semimetal. The key of the proposed mechanism is that the magnetic order comes from conduction $d$ electrons, such that the local $f$ moments can still fluctuate. We illustrate the extreme case where the magnetic space group symmetries prevent any spontaneous magnetization on the sites with the $f$-orbitals. In this case, topological degeneracies, including hourglass Weyl-Kondo nodal lines, appear when the magnetic space group symmetry constrains the Kondo-driven low-energy excitations; they lead to a third-order nonlinear anomalous Hall response. Based on the proposed mechanism, we explore the interplay between strong correlations and symmetries with database search leading to several candidate materials. The most prominent candidates are antiferromagnetic $\rm UNiGa$ and $\rm UNiAl$, with a third-order anomalous Hall response, as well as ferromagnetic $\rm USbTe$ and $\rm CeCoPO$, with a first-order one. Our findings pave the way for future experimental and theoretical investigations that promise to further advance the overarching theme of strongly correlated topology.

cond-mat.str-el↗

arXiv2024

New magnetic topological materials from high-throughput search

We conducted a high-throughput search for topological magnetic materials on 522 new, experimentally reported commensurate magnetic structures from MAGNDATA, doubling the number of available materials on the Topological Magnetic Materials database. This brings up to date the previous studies which had become incomplete due to the discovery of new materials. For each material, we performed first-principle electronic calculations and diagnosed the topology as a function of the Hubbard U parameter. Our high-throughput calculation led us to the prediction of 250 experimentally relevant topologically non-trivial materials, which represent 47.89% of the newly analyzed materials. We present five remarkable examples of these materials, each showcasing a different topological phase: Mn${}_2$AlB${}_2$ (BCSID 1.508), which exhibits a nodal line semimetal to topological insulator transition as a function of SOC, CaMnSi (BCSID 0.599), a narrow gap axion insulator, UAsS (BCSID 0.594) a 5f-orbital Weyl semimetal, CsMnF${}_4$ (BCSID 0.327), a material presenting a new type of quasi-symmetry protected closed nodal surface and FeCr${}_2$S${}_4$ (BCSID 0.613), a symmetry-enforced semimetal with double Weyls and spin-polarised surface states.

cond-mat.mtrl-sci↗

arXiv2024

Multifold topological semimetals

The discovery of topological semimetals with multifold band crossings has opened up a new and exciting frontier in the field of topological physics. These materials exhibit large Chern numbers, leading to long double Fermi arcs on their surfaces, which are protected by either crystal symmetries or topological order. The impact of these multifold crossings extends beyond surface science, as they are not constrained by the Poincaré classification of quasiparticles and only need to respect the crystal symmetry of one of the 1651 magnetic space groups. Consequently, we observe the emergence of free fermionic excitations in solid-state systems that have no high-energy counterparts, protected by non-symmorphic symmetries. In this work, we review the recent theoretical and experimental progress made in the field of multifold topological semimetals. We begin with the theoretical prediction of the so-called multifold fermions and discuss the subsequent discoveries of chiral and magnetic topological semimetals. Several experiments that have realized chiral semimetals in spectroscopic measurements are described, and we discuss the future prospects of this field. These exciting developments have the potential to deepen our understanding of the fundamental properties of quantum matter and inspire new technological applications in the future.

cond-mat.mtrl-sci↗

arXiv2023

Controllable orbital angular momentum monopoles in chiral topological semimetals

The emerging field of orbitronics aims at generating and controlling currents of electronic orbital angular momentum (OAM) for information processing. Structurally chiral topological crystals could be particularly suitable orbitronic materials because they have been predicted to host topological band degeneracies in reciprocal space that are monopoles of OAM. Around such a monopole, the OAM is locked isotopically parallel or antiparallel to the direction of the electron's momentum, which could be used to generate large and controllable OAM currents. However, OAM monopoles have not yet been directly observed in chiral crystals, and no handle to control their polarity has been discovered. Here, we use circular dichroism in angle-resolved photoelectron spectroscopy (CD-ARPES) to image OAM monopoles in the chiral topological semimetals PtGa and PdGa. Moreover, we also demonstrate that the polarity of the monopole can be controlled via the structural handedness of the host crystal by imaging OAM monopoles and anti-monopoles in the two enantiomers of PdGa, respectively. For most photon energies used in our study, we observe a sign change in the CD-ARPES spectrum when comparing positive and negative momenta along the light direction near the topological degeneracy. This is consistent with the conventional view that CD-ARPES measures the projection of the OAM monopole along the photon momentum. For some photon energies, however, this sign change disappears, which can be understood from our numerical simulations as the interference of polar atomic OAM contributions, consistent with the presence of OAM monopoles. Our results highlight the potential of chiral crystals for orbitronic device applications, and our methodology could enable the discovery of even more complicated nodal OAM textures that could be exploited for orbitronics.

cond-mat.str-el↗

arXiv2023

Topology of SmB6 revisited by means of topological quantum chemistry

The mixed-valence compound SmB6 with partially filled samarium 4f flat bands hybridizing with 5d conduction bands is a paramount example of a correlated topological heavy-fermion system. In this study we revisit the topology of SmB6 with the band theory paradigm and uncover previously overlooked aspects resulting from the formation of multiple topological gaps in the electronic structure. By invoking topological quantum chemistry (TQC) we provide a detailed classification of the strong and crystalline topological features that derive from the existence of such topological gaps. To corroborate this classification, we calculate Wilson loops and simulate the surface electronic structure using a minimal tight-binding model, allowing us to describe its surface states and confirm the crystalline topology. We finally discuss its implications for experiments.

cond-mat.str-el↗

arXiv2023

Monopole-like orbital-momentum locking and the induced orbital transport in topological chiral semimetals

The interplay between chirality and topology nurtures many exotic electronic properties. For instance, topological chiral semimetals display multifold chiral fermions that manifest nontrivial topological charge and spin texture. They are an ideal playground for exploring chirality-driven exotic physical phenomena. In this work, we reveal a monopole-like orbital-momentum locking texture on the three-dimensional Fermi surfaces of topological chiral semimetals with B20 structures (e.g., RhSi and PdGa). This orbital texture enables a large orbital Hall effect (OHE) and a giant orbital magnetoelectric (OME) effect in the presence of current flow. Different enantiomers exhibit the same OHE which can be converted to the spin Hall effect by spin-orbit coupling in materials. In contrast, the OME effect is chirality-dependent and much larger than its spin counterpart. Our work reveals the crucial role of orbital texture for understanding OHE and OME effects in topological chiral semimetals and paves the path for applications in orbitronics, spintronics, and enantiomer recognition.

cond-mat.mes-hall↗

arXiv2023

Axion Topology in Photonic Crystal Domain Walls

Axion insulators are 3D magnetic higher-order topological insulators protected by inversion-symmetry that exhibit hinge-localized chiral channels and induce quantized topological magnetoelectric effects. Recent research has suggested that axion insulators may be capable of detecting dark-matter axion-like particles by coupling to their axionic excitations. Beyond its fundamental theoretical interest, designing a photonic AXI offers the potential to enable the development of magnetically-tunable photonic switch devices through the manipulation of the axionic modes and their chiral propagation using external magnetic fields. Motivated by these facts, in this work, we propose a novel approach to induce axionic band topology in gyrotropic 3D Weyl photonic crystals gapped by supercell modulation. To quantize an axionic angle, we create domain-walls across inversion-symmetric photonic crystals, incorporating a phase-obstruction in the supercell modulation of their dielectric elements. This allows us to bind chiral channels on inversion-related hinges, ultimately leading to the realization of an axionic chiral channel of light. Moreover, by controlling the material gyrotropic response, we demonstrate a physically accessible way of manipulating the axionic modes through a small external magnetic bias, which provides an effective topological switch between different 1D chiral photonic fiber configurations. Remarkably, the unidirectional axionic hinge states supported by the photonic axion insulator are buried in a fully connected 3D dielectric structure, thereby being protected from radiation through the electromagnetic continuum. As a result, they are highly suitable for applications in guided-light communication, where the preservation and non-reciprocal propagation of photonic signals are of paramount importance.

physics.optics↗

arXiv2022

Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire

Internal interfaces in Weyl semimetals (WSMs) are predicted to host distinct topological features that are different from the commonly studied external interfaces (crystal-to-vacuum boundaries). However, the lack of atomically sharp and crystallographically oriented internal interfaces in WSMs makes it difficult to experimentally investigate hidden topological states buried inside the material. Here, we study a unique internal interface known as merohedral twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi, a chiral WSM of space group P213 (No. 198). High resolution scanning transmission electron microscopy reveals that this internal interface is (001) twin plane and connects two enantiomeric counterparts at an atomically sharp interface with inversion twinning. Ab-initio calculations show localized internal Fermi arcs at the (001) twin boundary that can be clearly distinguished from both external Fermi arcs and bulk states. These merohedrally twinned CoSi NWs provide an ideal material system to probe unexplored topological properties associated with internal interfaces in WSMs.

cond-mat.mtrl-sci↗

arXiv2022

Large anomalous Hall, Nernst effect and topological phases in the 3d-4d/5d based oxide double perovskites

Magnetic topological quantum materials are attracting considerable attention owing to their potential technological applications. However, only a small number of these materials have been experimentally realized, thereby giving rise to the need for new stable magnetic topological quantum materials. Magnetism and spin-orbit coupling, two essential ingredients of the oxide materials, lead to various topological transport phenomena such as the anomalous Hall and anomalous Nernst effects, which can be significantly enhanced by designing an electronic structure with a large Berry curvature. In that respect, double perovskites with the general formula A$_2$BB'O$_6$ with an alternating ordered arrangement of two transition metal sites, B(3d) and B'(4d/5d), present attractive possibilities as they are robustly stable against oxidation under ambient conditions and versatile. These double perovskites also offer a high energy scale for magnetism as well as strong spin-orbit coupling with a high magnetic ordering temperature. Here, using first-principles density functional theory calculations, we present a comprehensive study of the intrinsic anomalous transport for 3d-4d/5d based cubic and tetragonal stable double perovskite (DP) compounds. A few of the DPs exhibit a very large anomalous Hall effect with a distinct topological band crossing in the vicinity of the Fermi energy. Our results show the importance of symmetries, particularly the mirror planes, as well as the clean topological band crossing near the Fermi energy, which is primarily contributed by the 5d-t$_{2g}$ for large anomalous Hall and Nernst effects.

cond-mat.mtrl-sci↗

arXiv2022

Unconventional charge-to-spin conversions in graphene/MoTe2 van der Waals heterostructures

Spin-charge interconversion (SCI) is a central phenomenon to the development of spintronic devices from materials with strong spin-orbit coupling (SOC). In the case of materials with high crystal symmetry, the only allowed SCI processes are those where the spin current, charge current and spin polarization directions are orthogonal to each other. Consequently, standard SCI experiments are designed to maximize the signals arising from the SCI processes with conventional mutually orthogonal geometry. However, in low-symmetry materials, certain non-orthogonal SCI processes are also allowed. Since the standard SCI experiment is limited to charge current flowing only in one direction in the SOC material, certain allowed SCI configurations remain unexplored. In this work, we performed a thorough SCI study in a graphene-based lateral spin valve combined with low-symmetry MoTe$_2$. Due to a very low contact resistance between the two materials, we could detect SCI signals using both a standard configuration, where the charge current is applied along the MoTe$_2$, and a recently introduced (3D-current) configuration, where the charge current flow can be controlled in three directions within the heterostructure. As a result, we observed three different SCI components, one orthogonal and two non-orthogonal, giving new insight into the SCI processes in low-symmetry materials. The large SCI signals obtained at room temperature, along with the versatility of the 3D-current configuration, provide feasibility and flexibility to the design of the next generation of spin-based devices.

cond-mat.mes-hall↗

arXiv2022

Berry curvature induced anomalous Hall conductivity in magnetic topological oxide double perovskite Sr2FeMoO6

Oxide materials exhibit several novel structural, magnetic, and electronic properties. Their stability under ambient conditions, easy synthesis, and high transition temperatures provide such systems with an ideal ground for realizing topological properties and real-life technological applications. However, experimental evidence of topological states in oxide materials is rare. In this study, we have synthesized single crystals of oxide double perovskite Sr2FeMoO6 and revealed its topological nature by investigating its structural, magnetic, and electronic properties. We observed that the system crystallized in the cubic space group Fm-3m, which is a half-metallic ferromagnet. Transport measurements show an anomalous Hall effect, and it is evident that the Hall contribution originates from the Berry curvature. Assuming a shift of the Fermi energy towards the conduction band, the contribution of the anomalous Hall effect is enhanced owing to the presence of a gaped nodal line. This study can be used to explore and realize the topological properties of bulk oxide systems.

cond-mat.mtrl-sci↗