arXiv subjects

Anna Isaeva

Publications and source records attributed to Anna Isaeva.

27 records · Page 2Linked to original sources

arXiv2018

Possible Experimental Realization of a Basic Z2 Topological Semimetal

We report experimental and theoretical evidence that GaGeTe is a basic $Z_2$ topological semimetal with three types of charge carriers: bulk-originated electrons and holes as well as surface state electrons. This electronic situation is qualitatively similar to the primer 3D topological insulator Bi2Se3, but important differences account for an unprecedented transport scenario in GaGeTe. High-resolution angle-resolved photoemission spectroscopy combined with advanced band structure calculations show a small indirect energy gap caused by a peculiar band inversion in the \textit{T}-point of the Brillouin zone in GaGeTe. An energy overlap of the valence and conduction bands brings both electron- and hole-like carriers to the Fermi level, while the momentum gap between the corresponding dispersions remains finite. We argue that peculiarities of the electronic spectrum of GaGeTe have a fundamental importance for the physics of topological matter and may boost the material's application potential.

cond-mat.mtrl-sci↗

arXiv2018

Coexisting surface states in the weak and crystalline topological insulator Bi_2TeI

The established diversity of electronic topology classes lends the opportunity to pair them into dual topological complexes. Bulk-surface correspondence then ensures the coexistence of a combination of boundary states that cannot be realized but only at the various surfaces of such a dual topological material. We show that the layered compound Bi_2TeI realizes a dual topological insulator. It exhibits band inversions at two time reversal symmetry points of the bulk band which classify it as a weak topological insulator with metallic states on its (010) 'side' surfaces. Additional mirror symmetry of the crystal structure concurrently classifies it as a topological crystalline insulator. Bi2TeI is therefore predicted to host a pair of Dirac cones protected by time reversal symmetry on its 'side' surfaces and three pairs of Dirac cones protected by mirror symmetry on its 'top' and 'bottom' (001) surfaces. We spectroscopically map the top cleaved surface of Bi_2TeI, and crystallographic step edges therein. We show the existence of both two dimensional surface states which are susceptible to mirror symmetry breaking, as well as one dimensional channels that reside along the step edges. Their mutual coexistence on the step edge where both facets join is facilitated by momentum and energy segregation. Our observations of a dual topological insulator make way to additional pairing of other dual topology classes with distinct surface manifestations coexisting at their boundaries.

cond-mat.mes-hall↗

arXiv2018

Large Thermal Hall Effect in $α$-RuCl$_3$: Evidence for Heat Transport by Kitaev-Heisenberg Paramagnons

The honeycomb Kitaev model in a magnetic field is a source of a topological quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. We present experimental results for the thermal Hall effect of the material $α$-RuCl$_{3}$ which recently emerged as a prime candidate for realizing such physics. At temperatures above long-range magnetic ordering $T\gtrsim T_N\approx8$ K, we observe with an applied magnetic field $B$ perpendicular to the honeycomb layers a sizeable positive transversal heat conductivity $κ_{xy}$ which increases linearly with $B$. Upon raising the temperature, $κ_{xy}(T)$ increases strongly, exhibits a broad maximum at around 30 K, and eventually becomes negligible at $T\gtrsim 125$ K. Remarkably, the longitudinal heat conductivity $κ_{xx}(T)$ exhibits a sizeable positive thermal magnetoresistance effect. Thus, our findings provide clear-cut evidence for longitudinal and transverse magnetic heat transport and underpin the unconventional nature of the quasiparticles in the paramagnetic phase of $α$-RuCl$_{3}$.

cond-mat.str-el↗

arXiv2017

Low temperature enhancement of ferromagnetic Kitaev correlations in α-RuCl3

Kitaev-type interactions between neighbouring magnetic moments emerge in the honeycomb material $α$-RuCl3. It is debated however whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a d4 triplet in the final state and therefore is sensitive to nearest-neighbor spin-spin correlations. At low temperature the spectral weight of these triplets is strongly enhanced, in accordance with optical data. We show that the magnetic correlation function that determines this EELS spectral weight is directly related to a Kitaev-type spin-spin correlator and that the temperature dependence agrees very well with the results of a microscopic magnetic Hamiltonian for $α$-RuCl3 with ferromagnetic Kitaev coupling.

cond-mat.str-el↗

arXiv2017

Designing 3D topological insulators by 2D-Xene (X = Ge, Sn) sheet functionalization in the GaGeTe-type structures

State-of-the-art theoretical studies anticipate a 2D Dirac system in the "heavy" analogues of graphene, free-standing buckled honeycomb-like Xenes (X = Si, Ge, Sn, Pb, etc.). Herewith a structurally and electronically resembling 2D sheet, which can be regarded as Xene functionalized by covalent interactions within a 3D periodic structure, is predicted to constitute a 3D strong topological insulator with Z2 = 1;(111) (primitive cell, rhombohedral setting) in the structural family of layered AXTe (A = Ga, In; X = Ge, Sn) bulk materials. The host structure GaGeTe is a long-known bulk semiconductor; the "heavy", isostructural analogues InSnTe and GaSnTe are predicted to be dynamically stable. Spin-orbit interaction in InSnTe opens a small topological band gap with inverted gap edges that are mainly composed of the In-5s and Te-5p states. Our simulations classify GaSnTe as a semimetal with topological properties, whereas the verdict for GaGeTe is not conclusive and urges further experimental verification. AXTe family structures can be regarded as stacks of 2D layered cut-outs from a zincblende-type lattice and are composed by elements that are broadly used in modern semiconductor devices; hence they represent an accessible, attractive alternative for applications in spintronics. The layered nature of AXTe should facilitate exfoliation of its hextuple layers and manufacture of heterostuctures.

cond-mat.mtrl-sci↗

arXiv2017

Large field-induced gap of Kitaev-Heisenberg paramagnons in $α$-RuCl$_{3}$

The honeycomb Kitaev-Heisenberg model is a source of a quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. In the quest of finding a pertinent material, $α$-RuCl$_{3}$ recently emerged as a prime candidate. Here we unveil highly unusual low-temperature heat conductivity $κ$ of $α$-RuCl$_{3}$: beyond a magnetic field of $B_c\approx$ 7.5 T, $κ$ increases by about one order of magnitude, resulting in a large magnetic field dependent peak at about 7 K, both for in-plane as well as out-of-plane transport. This clarifies the unusual magnetic field dependence unambiguously to be the result of severe scattering of phonons off putative Kitaev-Heisenberg excitations in combination with a drastic field-induced change of the magnetic excitation spectrum. In particular, an unexpectedly large energy gap arises, which increases approximately linearly with the magnetic field and reaches a remarkably large $\hbarω_0/k_B\approx $ 50 K at 18 T.

cond-mat.str-el↗

arXiv2016

A Novel Quasi-One-Dimensional Topological Insulator in Bismuth Iodide $β$-Bi$_4$I$_4$

Recent progress in the field of topological states of matter(1,2) has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs)(3-6), followed by closely related ternary compounds(7-16) and predictions of several weak TIs(17-19). However, both the conceptual richness of Z$_2$ classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z$_2$ topological insulator is theoretically predicted and experimentally confirmed in the $β$-phase of quasi-one-dimensional bismuth iodide Bi$_4$I$_4$. The electronic structure of $β$-Bi$_4$I$_4$, characterized by Z$_2$ invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.

cond-mat.mtrl-sci↗

arXiv2015

Correlation between topological band character and chemical bonding in a $\mathbf{Bi_{14}Rh_{3}I_{9}}$-based family of insulators

Recently the presence of topologically protected edge-states in Bi$_{14}$Rh$_3$I$_9$ was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi$_{14}$Rh$_3$I$_9$ parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi$_{14}$Rh$_3$I$_9$ and graphene in terms of $p_z$-molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt- and Pd-based systems, whereas the Os- and Ru-systems remain trivial. Furthermore, the energy position of the metal $d$-band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The $d$-band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.

cond-mat.mtrl-sci↗

arXiv2013

Stacked topological insulator built from bismuth-based graphene sheet analogues

Commonly materials are classified as either electrical conductors or insulators. The theoretical discovery of topological insulators (TIs) in 2005 has fundamentally challenged this dichotomy. In a TI, spin-orbit interaction generates a non-trivial topology of the electronic band-structure dictating that its bulk is perfectly insulating, while its surface is fully conducting. The first TI candidate material put forward -graphene- is of limited practical use since its weak spin-orbit interactions produce a band-gap of ~0.01K. Recent reinvestigation of Bi2Se3 and Bi2Te3, however, have firmly categorized these materials as strong three-dimensional TI's. We have synthesized the first bulk material belonging to an entirely different, weak, topological class, built from stacks of two-dimensional TI's: Bi14Rh3I9. Its Bi-Rh sheets are graphene analogs, but with a honeycomb net composed of RhBi8-cubes rather than carbon atoms. The strong bismuth-related spin-orbit interaction renders each graphene-like layer a TI with a 2400K band-gap.

cond-mat.mtrl-sci↗