arXiv subjects

Ivana Vobornik

Publications and source records attributed to Ivana Vobornik.

At least 19 recordsLinked to original sources

arXiv2026

Type-I superconductivity in a quasi-2D topologically nontrivial YbBi$_2$

Intrinsic superconductivity in stoichiometric materials with nontrivial electronic topology remains uncommon, limiting opportunities to investigate how these two phenomena coexist within a single electronic system. Here, we report bulk type-I superconductivity below $T_c$ $\sim$ 0.9~K in YbBi$_2$, a layered rare-earth compound with a nonsymmorphic crystal structure and a quasi-two-dimensional Fermi surface. Thermodynamic and transport measurements establish the superconducting ground state, while quantum oscillations reveal exceptionally light carriers, with a cyclotron mass as low as 0.07 $m_e$, and a nonzero Berry phase of approximately 0.82 $π$. The latter closely matches the calculated value of the corresponding Wilson phase 0.99 $π$ for the corresponding orbit near a symmetry-protected band degeneracy. ARPES measurements show good agreement with key features of the calculated electronic structure, providing complementary experimental constraints on the normal-state band structure. The combination of intrinsic type-I superconductivity, light quasi-two-dimensional carriers, and signatures of nontrivial electronic topology identifies YbBi$_2$ as a distinct stoichiometric platform for investigating superconductivity in a topologically nontrivial electronic environment.

cond-mat.supr-con↗

arXiv2026

Correlation-driven origin of shallow electron pocket in Co$_{1/3}$TaS$_2$ revealed by ARPES and cluster perturbation theory

We investigate the electronic structure and Fermi surface of Co$_{1/3}$TaS$_2$ using angle-resolved photoemission spectroscopy (ARPES) combined with theoretical modeling beyond standard density functional theory (DFT+U). A shallow electron pocket, the so-called $β$ feature, is observed at the Fermi level near the corner of the superlattice Brillouin zone, representing the first experimental observation of this feature in an intercalated TaS$_2$ compound. Similar pockets have been reported in $X_{1/3}$NbS$_2$ ($X$ = Co, Cr, Ni), where their surface versus bulk origin remains actively debated. Because conventional DFT+U does not capture this feature, we employ cluster perturbation theory (CPT) to incorporate an explicit treatment of strong electron correlations ($U$) on the Co sites. CPT successfully reproduces the $β$ feature, demonstrating its origin from correlation-driven bulk states rather than surface effects. To further substantiate this conclusion, we studied a reduced Co-content sample, Co$_{0.22}$TaS$_2$, where the reduced charge transfer modifies the Co-derived states near the Fermi level. Its electronic structure remains largely similar to that of pristine 2H-TaS$_2$, showing only a minor overall energy shift and lacking the $β$ feature, consistent with disrupted long-range Co ordering and modified orbital character near the Fermi level. We demonstrate that the $β$ feature arises from strong local correlations on the Co sites and requires long-range crystallographic order among intercalated Co atoms to maintain coherence. These results highlight the importance of strong electronic correlations in magnetically intercalated transition-metal dichalcogenides and provide a microscopic understanding of features not captured by conventional DFT+U.

cond-mat.str-el↗

arXiv2026

Orbital mixing and strong Hund's coupling stabilize spin order in van der Waals ferromagnet CrI3

Recent years have seen a vast increase in research into van der Waals magnetic materials. In many of these systems, magnetism is introduced via light 3d-transition metal elements, combined with chalcogenides or halogens. Despite the high technological promise in the field of spintronics, the connection between the d-orbital configuration and the occurrence of low-dimensional magnetic order is currently unclear. Here we address the prototypical two-dimensional ferromagnet CrI3, via complementary spectroscopies and density functional theory calculations. We reveal the electronic structure and orbital character of bulk CrI3 in the paramagnetic and ferromagnetic phases, describing the couplings underpinning its energy diagram, and providing a robust experimental demonstration that the stabilization of ferromagnetism is attributable to orbital mixing between I p and Cr eg states, and to the presence of strong Hund's coupling. These findings reveal the microscopic connection between orbital and spin degrees of freedom, providing fundamental insights into the behavior of low-dimensional magnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Generation of an anomalous linearly dispersing spin-polarized band in Bi-based topological insulators

We report the generation of an anomalous linearly dispersing, spin-polarized band in Bi-based topological insulator (TI) thin films, induced by soft Ar-ion bombardment followed by annealing. This extra band -- which we call the anomalous linearly dispersing state (ALS) -- is superimposed on the regular band structure including the topological surface state (TSS), spans an unusually large energetic range of up to ${\sim}\,\SI{650}{\milli\electronvolt}$ at the $Γ$-point, and appears near the Fermi energy. Spin-resolved measurements indicate spin-momentum locking with a helicity \emph{opposite} to that of the regular TSS. The Fermi velocity of the ALS, $v_\mathrm{F} = (5.1\pm 0.4)\times 10^{5}\,\frac m s$, is indistinguishable from that of the regular TSS, $(5.3\pm 0.5)\times 10^{5}\,\frac m s$. The observation is reproducible across samples of varying thickness and was confirmed at two independent synchrotron radiation facilities. We discuss different mechanisms for the physical origin of the observed ALS including sputtering-induced TSS relocation, bi-layer formation by,e.g., chalcogen removal, and high-index surface relocation.

cond-mat.mtrl-sci↗

arXiv2026

Rashba engineering at van der Waals interfaces

Two-dimensional transition metal dichalcogenide (TMD) interfaces offer a versatile platform for studying emergent quantum phenomena and enabling novel device functionalities. When distinct TMD monolayers are stacked vertically or laterally stitched, their interfaces can exhibit unique electronic band alignments, giving rise to long-lived interlayer excitons, charge transfer effects, and moiré superlattices with correlated states. Here, we demonstrate that the interface between a large variety of two different epitaxially grown TMD monolayers controls the intensity and sign of the Rashba spin splitting, which is probed using THz spintronic emission. Optimized TMD heterobilayers, such as HfSe$_2$/PtSe$_2$, show enhanced THz emission that surpass the spin-to-charge conversion efficiency of bulk TMDs, confirming the presence of Rashba states with large spin splitting at the interface. By combining spin- and angle-resolved photoemission spectroscopy with density functional theory, we reveal that the electronic hybridization between the two different TMD monolayers gives rise to extended in-gap states with strong Rashba spin-orbit coupling. The choice of TMD layers enables to engineer the sign and strength of spin-to-charge conversion in van der Waals heterobilayers opening up perspectives to build efficient and tunable THz spintronic emitters.

cond-mat.mes-hall↗

arXiv2026

Emergent surface resonance from charge density wave symmetry breaking in TiSe2

Surface confined electronic states provide a fertile ground for discovering emergent phenomena that have no counterpart in the bulk, offering new routes to manipulate correlations, symmetry breaking, and dimensionality at the atomic scale. Here, we show that charge density wave (CDW) symmetry breaking can yield a surface states in 1T-TiSe2. Micro angle resolved photoemission spectroscopy resolves a sharp, two dimensional surface resonant state (SRS) that emerges within the CDW reconstructed low energy spectrum. The SRS exhibits notable temperature dependence and its spectral weight collapses around 160 K, while CDW transition temperature TCDW is commonly reported as 202 K. Slab DFT+U calculations reproduce a surface localized resonance when CDW folding brings valence and conduction states into near degeneracy, suggesting a correlation tuned, surface selective origin. These results point to a form of correlation-tuned surface resonance in a layered CDW compound and suggest a framework for engineering low dimensional quantum states in van der Waals materials via symmetry breaking and electronic structure tuning.

cond-mat.mtrl-sci↗

arXiv2026

Interplay of Kondo Physics with Incommensurate Charge Density Waves in CeTe$_3$

CeTe$_3$ is a 2--dimensional (2D) Van der Waals (VdW) material with incommensurate charge density waves (CDW), extremely high transition temperature ($T_{CDW}$) and a large momentum--dependent CDW gap that leaves a significant portion of the Fermi surface intact. It is also considered to be a weak Kondo system, a property unexpected for a material with incommensurate CDW, where each atomic site is slightly different. Here, we study the properties of the CDW state in several RTe$_3$ (R is rare earth) materials and examine the hybridization of itinerant states with the localized Ce $4f$ multiplet in CeTe$_3$ by using angle resolved photoemission spectroscopy (ARPES). We find that the renormalization of the itinerant states originating from the hybridization with the deeper localized $4f$ states at $-260$ meV is $k-$dependent and extends to the Fermi level. As these localized states are far from the Fermi level, the observed hybridization affects the effective masses only marginally and does not lead to heavy fermions. However, since the same renormalizing mechanism normally leads to the heavy fermion physics when the localized $4f$ states are near the Fermi level, our observation of its strong $k-$dependence suggests that this could be the reason for discrepancy between the heavy masses in specific heat and light ones in Shubnikov de Haas oscillations, often observed in heavy fermions.

cond-mat.str-el↗

arXiv2026

Robust spin splitting and fermiology in a layered altermagnet

Altermagnetism defies conventional classifications of collinear magnetic phases, standing apart from ferromagnetism and antiferromagnetism with its unique combination of spin-dependent symmetries, net-zero magnetization, and anomalous Hall transport. Although altermagnetic states have been realized experimentally, their integration into functional devices has been hindered by the structural rigidity and poor tunability of existing materials. First, through cobalt intercalation of the superconducting 2H-NbSe$_2$ polymorph, we induce and stabilize a robust altermagnetic phase and using both theory and experiment, we directly observe the lifting of Kramers degeneracy. Additionally, we present spectroscopic insight into a previously hinted low-temperature phase, and provide evidence of its electronic origin. While shedding light on overlooked aspects of altermagnetism, these findings open pathways to spin-based technologies and lay a foundation for advancing the emerging field of altertronics.

cond-mat.str-el↗

arXiv2025

Strongly Entangled Kondo and Kagome Lattices and the Emergent Magnetic Ground State in Heavy-Fermion Kagome Metal YbV$_6$Sn$_6$

Applying angle-resolved photoemission spectroscopy and density functional theory calculations, we present compelling spectroscopic evidence demonstrating the intertwining and mutual interaction between the Kondo and kagome sublattices in heavy-fermion intermetallic compound YbV$_6$Sn$_6$. We reveal the Yb 4$f$-derived states near the Fermi level, along with the presence of bulk kagome bands and topological surface states. We unveil strong interactions between the 4$f$ and itinerant electrons, where the kagome bands hosting the Dirac fermions and van Hove singularities predominate. Such findings are well described using a $c$-$f$ hybridization model. On the other hand, our systematic characterization of magnetic properties demonstrates an unusually enhanced antiferromagnetic ordering, where the kagome-derived van Hove singularities near $E_F$ play a vital role in determining the unconventional nature of the Ruderman-Kittel-Kasuya-Yosida interaction and Kondo coupling. These unique kagome-state-mediated exchange interactions have never been reported before and could lead to a novel phase diagram and various quantum critical behaviors in YbV$_6$Sn$_6$ and its siblings. Our results not only expand the family of exotic quantum phases entangled with kagome structure to the strongly correlated regime, but also establish YbV$_6$Sn$_6$ as an unprecedented platform to explore unconventional many-body physics beyond the standard Kondo picture.

cond-mat.str-el↗

arXiv2025

Mixed Triplet-Singlet Order Parameter in Decoupled Superconducting 1H Monolayers of Transition-Metal Dichalcogenides

Understanding the emergence of unconventional superconductivity, where the order parameter deviates from simple isotropic s-wave pairing, is a central puzzle in condensed matter physics. Transition-metal dichalcogenides (TMDCs), though generally regarded as conventional superconductors, display signatures of this unusual behavior and thus provide a particularly intriguing platform to explore how exotic states arise. Here we investigate the misfit compound (SnS)$_{1.15}$(TaS$_2$), a heterostructure composed of alternating SnS and 1H-TaS$_2$ layers. Using transport, photoemission, and scanning tunneling spectroscopy, we demonstrate that the SnS layers effectively decouple the TaS$_2$ into electronically isolated 1H sheets. In this limit, the tunneling density of states reveals a clear two-gap superconducting spectrum with T$_c \sim$ 3.1 K. A theoretical model based on lack of inversion symmetry and finite-range attraction reproduces the observed multi-gap structure as a mixed singlet-triplet state. These results establish misfit compounds as a powerful platform for studying unconventional superconductivity in isolated 1H layers and for realizing multiple uncoupled superconductors within a single crystal.

cond-mat.supr-con↗

arXiv2025

Anomalous spin-optical helical effect in Ti-based kagome metal

The kagome lattice stands as a rich platform for hosting a wide array of correlated quantum phenomena, ranging from charge density waves and superconductivity to electron nematicity and loop current states. Direct detection of loop currents in kagome systems has remained a formidable challenge due to their intricate spatial arrangements and the weak magnetic field signatures they produce. This has left their existence and underlying mechanisms a topic of intense debate. In this work, we uncover a hallmark reconcilable with loop currents: spin handedness-selective signals that surpass conventional dichroic, spin, and spin-dichroic responses. We observe this phenomenon in the kagome metal CsTi$_3$Bi$_5$ and we call it the anomalous spin-optical helical effect. This effect arises from the coupling of light' s helicity with spin-orbital electron correlations, providing a groundbreaking method to visualize loop currents in quantum materials. Our discovery not only enriches the debate surrounding loop currents but also paves the way for new strategies to exploit the electronic phases of quantum materials via light-matter interaction.

cond-mat.str-el↗

arXiv2025

Three-dimensional mapping of the altermagnetic spin splitting in CrSb

Altermagnetism, a kind of collinear magnetism that is characterized by a momentum-dependent band and spin splitting without net magnetization, has recently attracted considerable interest. Finding altermagnetic materials with large splitting near the Fermi level necessarily requires three-dimensional k-space mapping. While this is crucial for spintronic applications and emergent phenomena, it remains challenging. Here, using synchrotron-based angle-resolved photoemission spectroscopy (ARPES), spin-resolved ARPES and model calculations, we uncover a large altermagnetic splitting, up to ~1.0 eV, near the Fermi level in CrSb. We verify its bulk-type g-wave altermagnetism through systematic three-dimensional k-space mapping, which unambiguously reveals the altermagnetic symmetry and associated nodal planes. Spin-resolved ARPES measurements further verify the spin polarizations of the split bands near Fermi level. Tight-binding model analysis indicates that the large altermagnetic splitting arises from strong third-nearest-neighbor hopping mediated by Sb ions. The large band/spin splitting near Fermi level in metallic CrSb, together with its high TN (up to 705 K) and simple spin configuration, paves the way for exploring emergent phenomena and spintronic applications based on altermagnets.

cond-mat.mtrl-sci↗

arXiv2024

Measurements of the quantum geometric tensor in solids

Understanding the geometric properties of quantum states and their implications in fundamental physical phenomena is at the core of modern physics. The Quantum Geometric Tensor (QGT) is a central physical object in this regard, encoding complete information about the geometry of the quantum state. The imaginary part of the QGT is the well-known Berry curvature, which plays a fundamental role in the topological magnetoelectric and optoelectronic phenomena. The real part of the QGT is the quantum metric, whose importance has come to prominence very recently, giving rise to a new set of quantum geometric phenomena, such as anomalous Landau levels, flat band superfluidity, excitonic Lamb shifts, and nonlinear Hall effect. Despite the central importance of the QGT, its experimental measurements have been restricted only to artificial two-level systems. In this work, we develop a framework to measure the QGT (both quantum metric and Berry curvature) in crystalline solids using polarization-, spin-, and angle-resolved photoemission spectroscopy. Using this framework, we demonstrate the effective reconstruction of the QGT in solids in the archetype kagome metal CoSn, which hosts topological flat bands. The key idea is to introduce another geometrical tensor, the quasi-QGT, whose components, the band Drude weight and orbital angular momentum, are experimentally accessible and can be used for extracting the QGT. Establishing such a momentum- and energy-resolved spectroscopic probe of the QGT is poised to significantly advance our understanding of quantum geometric responses in a wide range of crystalline systems.

cond-mat.mtrl-sci↗

arXiv2024

A topological material in the III-V family: heteroepitaxial InBi on InAs

InBi(001) is formed epitaxially on InAs(111)-A by depositing Bi on to an In-rich surface. Angle-resolved photoemission measurements reveal topological electronic surface states, close to the M bar high symmetry point. This demonstrates a heteroepitaxial system entirely in the III-V family with topological electronic properties. InBi shows coexistence of Bi and In surface terminations, in contradiction with other III-V materials. For the Bi termination, the study gives a consistent physical picture of the topological surface electronic structure of InBi(001) terminated by a Bi bilayer rather than a surface formed by splitting to a Bi monolayer termination. Theoretical calculations based on relativistic density functional theory and the one-step model of photoemission clarify the relationship between the InBi(001) surface termination and the topological surface states, supporting a predominant role of the Bi bilayer termination. Furthermore, a tight-binding model based on this Bi bilayer termination with only Bi-Bi hopping terms, and no Bi-In interaction, gives a deeper insight into the spin texture.

cond-mat.str-el↗

arXiv2024

Uncovering the lowest thickness limit for room-temperature ferromagnetism of Cr$_{1.6}$Te$_{2}$

Metallic ferromagnetic transition metal dichalcogenides have emerged as important building blocks for scalable magnonics and memory applications. Downscaling such systems to the ultra-thin limit is critical to integrate them into technology. Here, we achieved layer-by-layer control over the transition metal dichalcogenide Cr$_{1.6}$Te$_{2}$ by using pulsed laser deposition, and we uncovered the minimum critical thickness above which room temperature magnetic order is maintained. The electronic and magnetic structure is explored experimentally and theoretically and it is shown that the films exhibit strong in-plane magnetic anisotropy as a consequence of large spin-orbit effects. Our study elucidates both magnetic and electronic properties of Cr$_{1.6}$Te$_{2}$, and corroborates the importance of intercalation to tune the magnetic properties of nanoscale materials architectures.

cond-mat.mtrl-sci↗

arXiv2024

Charge transfer and Spin-Valley locking in 4Hb-TaS$_{2}$

4Hb-TaS$_2$ is a superconductor that exhibits unique characteristics such as time-reversal symmetry breaking, hidden magnetic memory, and topological edge modes. It is a naturally occurring heterostructure comprising of alternating layers of 1H-TaS$_2$ and 1T-TaS$_2$. The former is a well-known superconductor, while the latter is a correlated insulator with a possible non-trivial magnetic ground state. In this study, we use angle resolved photoemission spectroscopy to investigate the normal state electronic structure of this unconventional superconductor. Our findings reveal that the band structure of 4H-TaS$_2$ fundamentally differs from that of its constituent materials. Specifically, we observe a significant charge transfer from the 1T layers to the 1H layers that drives the 1T layers away from half-filling. In addition, we find a substantial reduction in inter-layer coupling in 4Hb-TaS$_2$ compared to the coupling in 2H-TaS$_2$ that results in a pronounced spin-valley locking within 4Hb-TaS$_2$

cond-mat.supr-con↗

arXiv2024

Strain-induced enhancement of the charge-density-wave in the kagome metal ScV$_6$Sn$_6$

The kagome geometry is an example of frustrated configuration in which rich physics takes place, including the emergence of superconductivity and charge density wave (CDW). Among the kagome metals, ScV$_6$Sn$_6$ hosts an unconventional CDW, with its electronic order showing a different periodicity than that of the phonon which generates it. In this material, a CDW-softened flat phonon band has a second-order collapse at the same time that the first order transition occurs. This phonon band originates from the out-of-plane vibrations of the Sc and Sn atoms, and it is at the base of the electron-phonon-coupling driven CDW phase of ScV$_6$Sn$_6$. Here, we use uniaxial strain to tune the frequency of the flat phonon band, tracking the strain evolution via time-resolved optical spectroscopy and first-principles calculations. Our findings emphasize the capability to induce an enhancement of the unconventional CDW properties in ScV$_6$Sn$_6$ kagome metal through control of strain.

cond-mat.str-el↗

arXiv2024

Signatures of a surface spin-orbital chiral metal

The relation between crystal symmetries, electron correlations, and electronic structure steers the formation of a large array of unconventional phases of matter, including magneto-electric loop currents and chiral magnetism. Detection of such hidden orders is a major goal in condensed matter physics. However, to date, nonstandard forms of magnetism with chiral electronic ordering have been experimentally elusive. Here, we develop a theory for symmetry-broken chiral ground states and propose a methodology based on circularly polarized spin-selective angular-resolved photoelectron spectroscopy to probe them. We exploit the archetypal quantum material Sr2RuO4 and reveal spectroscopic signatures which, even though subtle, may be reconciled with the formation of spin-orbital chiral currents at the material surface. As we shed light on these chiral regimes, our findings pave the way for a deeper understanding of ordering phenomena and unconventional magnetism.

cond-mat.str-el↗