arXiv subjects

Andrei Varykhalov

Publications and source records attributed to Andrei Varykhalov.

18 recordsLinked to original sources

arXiv2026

Large Jahn-Teller shifts and splittings observed in halide perovskite CsPbBr3

Despite the fundamental role of Jahn-Teller effects of first and second order in shaping structural and electronic properties, there is hardly any observation in angle-resolved photoemission of solids. In oxide and halide perovskites, band structure replicas have occasionally been reported as fingerprints of Jahn-Teller effects, but no accompanying energy shifts or splittings that would allow a conclusion about their origin. In CsPbBr3, we uncover both key signatures: Upon cooling, orthorhombic replica bands emerge which had eluded earlier studies. This includes an extra valence band maximum at Γ which can even be distinguished at room temperature. Most importantly, band narrowing along Γ-M, a splitting at Γ, and the lifting of degeneracy between nonequivalent M points, all of several 100 meV, become apparent. Temperature-dependent x-ray diffraction, used as input for band structure calculations, links these effects directly to tilts and rotations of the PbBr6 octahedra. Our results uncover a strong electron-lattice interaction which is at the heart of so-far unresolved questions concerning polaronic transport, dynamic disorder, and exciton trapping in halide perovskites.

cond-mat.mtrl-sci↗

arXiv2026

Light-induced trion-exciton competition revealed by ultrafast photoemission

Strong Coulomb interactions in low-dimensional quantum materials give rise to emergent bound states such as excitons and trions. Trions are conventionally secondary excitations, requiring both optical excitation and charge doping. In quasi-one-dimensional Ta$_2$NiS$_5$, however, an exceptionally large binding energy exceeding the single-particle band gap stabilizes an equilibrium trion gas under surface doping alone. Here, using time- and angle-resolved photoemission spectroscopy, we show that trions can also be generated purely optically, without external charge. Following photoexcitation of pristine Ta$_2$NiS$_5$ we observe a bright, momentum-localized, in-gap feature with a slow, fluence-dependent relaxation. Rate-equation modeling identifies it as a mixed population of trions and excitons, with the trions formed via an unconventional single-particle pathway that requires no photoexcited hole. The trion-exciton composition is controlled by pump fluence, with trions dominating the late-time relaxation. In surface doped samples, the same model with unchanged rates reproduces the full dynamics, linking the light-induced trions to their equilibrium counterparts through pump-induced dissociation and recapture. These results establish trARPES as a direct probe of charged quasiparticles far from equilibrium and open routes to optical control of neutral and charged excitations in correlated materials.

cond-mat.str-el↗

arXiv2026

Trion gas on the surface of a failed excitonic insulator

Trions, three-body bound states composed of an exciton and an additional charge, are typically fragile and require external excitation to form. Here, we report the spontaneous emergence of a stable trion gas at the surface of the layered semiconductor Ta2NiS5, revealed through angle-resolved photoemission spectroscopy. We observe a sharp, highly localized in-gap feature that cannot be explained by conventional band-theory. Instead, we argue that it arises from the formation of negative trions, stabilized by surface-induced band bending and the material's quasi-one-dimensional geometry. Unlike excitons, these trions form without optical pumping and persist at equilibrium, marking a rare example of an interaction-driven surface state in a nominally conventional semiconductor. Our findings establish Ta2NiS5 as a unique platform for exploring many-body physics at surfaces and open new avenues for studying and controlling collective excitations in low-dimensional systems.

cond-mat.str-el↗

arXiv2025

Chiral charge density wave in 4Hb- and 1T-TaS$_2$: The Role of interlayer coupling

We use micro-angle-resolved photoemission spectroscopy (micro-ARPES) to investigate chiral charge density waves (CDWs) in 4Hb-TaS$_2$ with micron-scale spatial resolution. In the 1T layers of 4Hb-TaS$_2$, we uncover coexisting left- and right-handed CDW domains and resolve four distinct spectral patterns arising from the interplay of chirality and rotational stacking. In contrast, bulk 1T-TaS$_2$ exhibits a uniform chirality. In addition, 4Hb-TaS$_2$ shows negligible out-of-plane dispersion of the 1T-derived bands, in contrast to the pronounced interlayer coupling observed in bulk 1T-TaS$_2$. Density functional theory (DFT) calculations corroborate this picture, revealing that the interlayer interaction of the chiral order in 4Hb-TaS$_2$ is nearly two orders of magnitude weaker than in the 1T polytype. Our findings establish 4Hb-TaS$_2$ as a quasi-two-dimensional platform for exploring tunable chiral CDW phenomena.

cond-mat.str-el↗

arXiv2025

Halide Perovskites as Spin-1 Dirac Materials

Halide perovskites are a promising class of materials for optoelectronic and photovoltaic applications, exhibiting high power conversion efficiency due to strong light absorption and long carrier diffusion lengths. While various aspects of their crystal and electronic structure have been studied, we identify a fundamental property previously overlooked that may significantly impact their efficiency. We demonstrate that halide perovskites realize a three-dimensional (3D) Lieb lattice, giving rise to a gapped 3D Dirac cone of spin-1 fermions. This leads to a fivefold reduction in effective mass compared to a conventional cubic structure and suppressed carrier backscattering due to Klein tunneling. Our conclusions are supported by band structure calculations and angle-resolved photoemission spectroscopy from CsPbBr$_3$ and CsSnBr$_3$. In particular, we reveal the transformation of the flat band of the Lieb lattice and the emergence of a dark corridor effect in photoemission from the Dirac cone, which increases as the band gap is decreased from CsPbBr$_3$ to CsSnBr$_3$.

cond-mat.mtrl-sci↗

arXiv2023

One-dimensional electronic structure of phosphorene chains

Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing because of its semiconducting properties and narrow band gap. Further reduction of the phosphorene dimensionality may spawn exotic properties of its electronic structure, including lateral quantum confinement and topological edge states. Phosphorene atomic chains self-assembled on Ag(111) have recently been characterized structurally but were found by angle-resolved photoemission (ARPES) to be electronically 2D. We show that these chains, although aligned equiprobably to three <$1 \bar{1} 0$> directions of the Ag(111) surface, can be characterized by ARPES because the three rotational variants are separated in the angular domain. The dispersion of the phosphorus band measured along and perpendicular to the chains reveals pronounced electronic confinement resulting in a 1D band, flat and dispersionless perpendicular to the chain direction in momentum space. Our density functional theory calculations reproduce the 1D band for the experimentally determined structure of P/Ag(111). We predict a semiconductor-to-metal phase transition upon increasing the density of the chain array so that a 2D structure would be metallic.

cond-mat.mtrl-sci↗

arXiv2022

Colossal negative magnetoresistance in the complex charge density wave regime of an antiferromagnetic Dirac semimetal

Colossal magnetoresistance (MR) is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. Though topological semimetals also show large MR, its origin and nature are completely different. Here, we show that in the highly electron doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb$_{0.11}$Te$_{1.90}$ hosts multiple charge density wave (CDW) modulation-vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor-metal-like transition, which results in a colossal negative MR. Moreover, signatures of the coupling between the CDW and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

cond-mat.mtrl-sci↗

arXiv2022

Is there a polaron signature in angle-resolved photoemission of CsPbBr3?

The formation of large polarons has been proposed as reason for the high defect tolerance, low mobility, low charge carrier trapping and low nonradiative recombination rates of lead halide perovskites. Recently, direct evidence for large-polaron formation has been reported from a 50% effective mass enhancement in angle-resolved photoemission of CsPbBr3 over theory for the orthorhombic structure. We present in-depth band dispersion measurements of CsPbBr3 and GW calculations which lead to almost identical effective masses at the valence band maximum of 0.203+/-0.016 m0 in experiment and 0.226 m0 in orthorhombic theory. We argue that the effective mass can be explained solely on the basis of electron-electron correlation and large-polaron formation cannot be concluded from photoemission data.

cond-mat.mtrl-sci↗

arXiv2021

Band Engineering of Dirac Semimetals using Charge Density Waves

New developments in the field of topological matter are often driven by materials discovery, including novel topological insulators, Dirac semimetals and Weyl semimetals. In the last few years, large efforts have been performed to classify all known inorganic materials with respect to their topology. Unfortunately, a large number of topological materials suffer from non-ideal band structures. For example, topological bands are frequently convoluted with trivial ones, and band structure features of interest can appear far below the Fermi level. This leaves just a handful of materials that are intensively studied. Finding strategies to design new topological materials is a solution. Here we introduce a new mechanism that is based on charge density waves and non-symmorphic symmetry to design an idealized Dirac semimetal. We then show experimentally that the antiferromagnetic compound GdSb$_{0.46}$Te$_{1.48}$ is a nearly ideal Dirac semimetal based on the proposed mechanism, meaning that most interfering bands at the Fermi level are suppressed. Its highly unusual transport behavior points to a thus far unknown regime, in which Dirac carriers with Fermi energy very close to the node seem to gradually localize in the presence of lattice and magnetic disorder.

cond-mat.mtrl-sci↗

arXiv2021

Signatures of dephasing by mirror-symmetry breaking in weak-antilocalization magnetoresistance across the topological transition in Pb$_{1-x}$Sn$_{x}$Se

Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities or two-level systems specific to non-crystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) Pb$_{1-x}$Sn$_x$Se thin film with Sn contents $x$ corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.

cond-mat.mes-hall↗

arXiv2019

Modular Arithmetic with Nodal Lines: Drumhead Surface States in ZrSiTe

We study the electronic structure of the nodal line semimetal ZrSiTe both experimentally and theoretically. We find two different surface states in ZrSiTe - topological drumhead surface states and trivial floating band surface states. Using the spectra of Wilson loops, we show that a non-trivial Berry phase that exists in a confined region within the Brillouin Zone gives rise to the topological drumhead-type surface states. The $\mathbb{Z}_2$ structure of the Berry phase induces a $\mathbb{Z}_2$ 'modular arithmetic' of the surface states, allowing surface states deriving from different nodal lines to hybridize and gap out, which can be probed by a set of Wilson loops. Our findings are confirmed by \textit{ab-initio} calculations and angle-resolved photoemission experiments, which are in excellent agreement with each other and the topological analysis. This is the first complete characterization of topological surface states in the family of square-net based nodal line semimetals and thus fundamentally increases the understanding of the topological nature of this growing class of topological semimetals.

cond-mat.mes-hall↗

arXiv2017

Surface floating 2D bands in layered nonsymmorphic semimetals: ZrSiS and related compounds

In this work, we present a model of the surface states of nonsymmorphic semimetals. These are derived from surface mass terms that lift the high degeneracy imposed in the band structure by the nonsymmorphic bulk symmetries. Reflecting the reduced symmetry at the surface, the bulk bands are strongly modified. This leads to the creation of two-dimensional floating bands, which are distinct from Shockley states, quantum well states or topologically protected surface states. We focus on the layered semimetal ZrSiS to clarify the origin of its surface states. We demonstrate an excellent agreement between DFT calculations and ARPES measurements and present an effective four-band model in which similar surface bands appear. Finally, we emphasize the role of the surface chemical potential by comparing the surface density of states in samples with and without potassium coating. Our findings can be extended to related compounds and generalized to other crystals with nonsymmorphic symmetries.

cond-mat.mtrl-sci↗

arXiv2017

Tunable Weyl and Dirac states in the nonsymmorphic compound $\rm\mathbf{CeSbTe}$

Recent interest in topological semimetals has lead to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry and the introduction of magnetic order can influence these states by breaking time reversal symmetry. Here we show that we can realize a rich variety of different topological semimetal states in a single material, $\rm CeSbTe$. This compound can exhibit different types of magnetic order that can be accessed easily by applying a small field. It allows, therefore, for tuning the electronic structure and can drive it through a manifold of topologically distinct phases, such as the first nonsymmorphic magnetic topological material with an eight-fold band crossing at a high symmetry point. Our experimental results are backed by a full magnetic group theory analysis and ab initio calculations. This discovery introduces a realistic and promising platform for studying the interplay of magnetism and topology.

cond-mat.mtrl-sci↗

arXiv2016

Non-symmorphic band degeneracy at the Fermi level in ZrSiTe

Non-symmorphic materials have recently been predicted to exhibit many different exotic features in their electronic structures. These originate from forced band degeneracies caused by the non-symmorphic symmetry, which not only creates the possibility to realize Dirac semimetals, but also recently resulted in the prediction of novel quasiparticles beyond the usual Dirac, Weyl or Majorana fermions, which can only exist in the solid state. Experimental realization of non-symmorphic materials that have the Fermi level located at the degenerate point is difficult, however, due to the requirement of an odd band filling. In order to investigate the effect of forced band degeneracies on the transport behavior, a material that has such a degeneracy at or close to the Fermi level is desired. Here, we show with angular resolved photoemission experiments supported by density functional calculations, that ZrSiTe hosts several fourfold degenerate Dirac crossings at the X point, resulting from non-symmorphic symmetry. These crossings form a Dirac line node along XR, which is located almost directly at the Fermi level and shows almost no dispersion in energy. ZrSiTe is thus the first real material that allows for transport measurements investigating Dirac fermions that originate from non-symmorphic symmetry.

cond-mat.mtrl-sci↗

arXiv2015

Dirac states with knobs on: interplay of external parameters and the surface electronic properties of 3D topological insulators

Topological insulators are a novel materials platform with high applications potential in fields ranging from spintronics to quantum computation. In the ongoing scientific effort to demonstrate controlled manipulation of their electronic structure by external means, stoichiometric variation and surface decoration are two effective approaches that have been followed. In ARPES experiments, both approaches are seen to lead to electronic band structure changes. Such approaches result in variations of the energy position of bulk and surface-related features and the creation of two-dimensional electron gases.The data presented here demonstrate that a third manipulation handle is accessible by utilizing the amount of illumination a topological insulator surface has been exposed to under typical experimental ARPES conditions. Our results show that this new, third, knob acts on an equal footing with stoichiometry and surface decoration as a modifier of the electronic band structure, and that it is in continuous competition with the latter. The data clearly point towards surface photovoltage and photo-induced desorption as the physical phenomena behind modifications of the electronic band structure under exposure to high-flux photons. We show that the interplay of these phenomena can minimize and even eliminate the adsorbate-related surface band bending on typical binary, ternary and quaternary Bi-based topological insulators. Including the influence of the sample temperature, these data set up a framework for the external control of the electronic band structure in topological insulator compounds in an ARPES setting. Four external knobs are available: bulk stoichiometry, surface decoration, temperature and photon exposure. These knobs can be used in conjunction to tune the band energies near the surface and consequently influence the topological properties of the relevant electronic states.

cond-mat.mtrl-sci↗

arXiv2014

ARPES insights on the metallic states of YbB6(001): E(k) dispersion, temporal changes and spatial variation

We report high resolution Angle Resolved PhotoElectron Spectroscopy (ARPES) results on the (001) cleavage surface of YbB$_{6}$, a rare-earth compound which has been recently predicted to host surface electronic states with topological character. We observe two types of well-resolved metallic states, whose Fermi contours encircle the time-reversal invariant momenta of the YbB$_{6}$(001) surface Brillouin zone, and whose full (E,$k$)-dispersion relation can be measured wholly unmasked by states from the rest of the electronic structure. Although the two-dimensional character of these metallic states is confirmed by their lack of out-of-plane dispersion, two new aspects are revealed in these experiments. Firstly, these states do not resemble two branches of opposite, linear velocity that cross at a Dirac point, but rather straightforward parabolas which terminate to high binding energy with a clear band bottom. Secondly, these states are sensitive to time-dependent changes of the YbB$_{6}$ surface under ultrahigh vacuum conditions. Adding the fact that these data from cleaved YbB$_{6}$ surfaces also display spatial variations in the electronic structure, it appears there is little in common between the theoretical expectations for an idealized YbB$_{6}$(001) crystal truncation on the one hand, and these ARPES data from real cleavage surfaces on the other.

cond-mat.str-el↗

arXiv2014

Observation of Quantum-Tunneling Modulated Spin Texture in Ultrathin Topological Insulator Bi2Se3 Films

Understanding the spin-texture behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. Here by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe strongly binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultra-thin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunneling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model which captures this delicate relationship between quantum tunneling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nano-device.

cond-mat.mes-hall↗

arXiv2013

Anisotropic two-dimensional electron gas at SrTiO3(110) protected by its native overlayer

Two dimensional electron gases (2DEGs) at a oxide heterostructures are attracting considerable attention, as these might substitute conventional semiconductors for novel electronic devices [1]. Here we present a minimal set-up for such a 2DEG -the SrTiO3(110)-(4 x 1) surface, natively terminated with one monolayer of chemically-inert titania. Oxygen vacancies induced by synchrotron radiation migrate under- neath this overlayer, this leads to a confining potential and electron doping such that a 2DEG develops. Our angular resolved photoemission spectroscopy (ARPES) and theoretical results show that confinement along (110) is strikingly different from a (001) crystal orientation. In particular the quantized subbands show a surprising "semi-heavy" band, in contrast to the analogue in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system.

cond-mat.mtrl-sci↗