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Start date: 2020 × DDC: 530 × Type: Text × Publisher: College Park, MD : APS × search.filters.applied.f.series: Physical Review Research 5 (2023), Nr. 4 ×

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Now showing 1 - 4 of 4
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    Phononic-magnetic dichotomy of the thermal Hall effect in the Kitaev material Na2 Co2 TeO6
    (College Park, MD : APS, 2023) Gillig, Matthias; Hong, Xiaochen; Wellm, Christoph; Kataev, Vladislav; Yao, Weiliang; Li, Yuan; Büchner, Bernd; Hess, Christian
    The quest for a half-quantized thermal Hall effect of a Kitaev system represents an important tool to probe topological edge currents of emergent Majorana fermions. Pertinent experimental findings for α-RuCl3 are, however, strongly debated, and it has been argued that the thermal Hall signal stems from phonons or magnons rather than from Majorana fermions. Here, we investigate the thermal Hall effect of the Kitaev candidate material Na2Co2TeO6, and we show that the measured signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the longitudinal and transversal heat conductivities share clear phononic signatures, while the transversal signal changes sign upon entering the low-temperature, magnetically ordered phase. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in Kitaev topological quantum spin liquids such as α-RuCl3 and Na2Co2TeO6 requires disentangling phonon vs magnetic contributions, including potentially fractionalized excitations such as the expected Majorana fermions.
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    Fate of density waves in the presence of a higher-order van Hove singularity
    (College Park, MD : APS, 2023) Zervou, Alkistis; Efremov, Dmitry V.; Betouras, Joseph J.
    Topological transitions in electronic band structures, resulting in van Hove singularities in the density of states, can considerably affect various types of orderings in quantum materials. Regular topological transitions (of neck formation or collapse) lead to a logarithmic divergence of the electronic density of states (DOS) as a function of energy in two dimensions. In addition to the regular van Hove singularities, there are higher-order van Hove singularities (HOVHS) with a power-law divergence in DOS. By employing renormalization group techniques, we study the fate of a spin-density wave phase formed by nested parts of the Fermi surface, when a HOVHS appears in parallel. We find that the phase formation can be boosted by the presence of the singularity, with the critical temperature increasing by orders of magnitude, under certain conditions. We discuss possible applications of our findings to a range of quantum materials such as Sr3Ru2O7, Sr2RuO4, and transition metal dichalcogenides.
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    Correlation induced magnetic topological phases in the mixed-valence compound SmB6
    (College Park, MD : APS, 2023) Liu, Huimei; Hirschmann, Moritz M.; Sawatzky, George A.; Khaliullin, Giniyat; Schnyder, Andreas P.
    SmB6 is a mixed-valence compound with flat f-electron bands that have a propensity to magnetism. Here, using a realistic Γ8 quartet model, we investigate the dynamical spin susceptibility and describe the in-gap collective mode observed in neutron scattering experiments. We show that as the Sm valence increases with pressure, the magnetic correlations enhance and SmB6 undergoes a first-order phase transition into a metallic antiferromagnetic state, whose symmetry depends on the model parameters. The magnetic orderings give rise to distinct band topologies: while the A-type order leads to an overlap between valence and conduction bands in the form of Dirac nodal lines, the G-type order has a negative indirect gap with weak Z2 indices. We also consider the spin polarized phase under a strong magnetic field, and find that it exhibits Weyl points as well as nodal lines close to the Fermi level. The magnetic phases show markedly different surface states and tunable bulk transport properties, with important implications for experiments. Our theory predicts that a magnetic order can be stabilized also by lifting the Γ8 cubic symmetry, thus explaining the surface magnetism reported in SmB6.
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    Suppression of nematicity by tensile strain in multilayer FeSe/SrTiO3 films
    (College Park, MD : APS, 2023) Lou, Rui; Suvorov, Oleksandr; Grafe, Hans-Joachim; Kuibarov, Andrii; Krivenkov, Maxim; Rader, Oliver; Büchner, Bernd; Borisenko, Sergey; Fedorov, Alexander
    The nematicity in multilayer FeSe/SrTiO3 films has been previously suggested to be enhanced with decreasing film thickness. Motivated by this, there have been many discussions about the competing relation between nematicity and superconductivity. However, the criterion for determining the nematicity strength in FeSe remains highly debated. The understanding of nematicity as well as its relation to superconductivity in FeSe films is therefore still controversial. Here, we fabricate multilayer FeSe/SrTiO3 films using molecular beam epitaxy and study the nematic properties by combining angle-resolved photoemission spectroscopy, Se77 nuclear magnetic resonance, and scanning tunneling microscopy experiments. We unambiguously demonstrate that, near the interface, the nematic order is suppressed by the SrTiO3-induced tensile strain; in the bulk region further away from the interface, the strength of nematicity recovers to the bulk value. Our results not only solve the recent controversy about the nematicity in multilayer FeSe films, but also offer valuable insights into the relationship between nematicity and superconductivity.