20 results
Search Results
Now showing 1 - 10 of 20
- ItemPhononic-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, ChristianThe 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.
- ItemStrong surface termination dependence of the electronic structure of polar superconductor LaFeAsO revealed by nano-ARPES([London] : IOP, 2022) Jung, Sung Won; Rhodes, Luke C; Watson, Matthew D; Evtushinsky, Daniil V; Cacho, Cephise; Aswartham, Saicharan; Kappenberger, Rhea; Wurmehl, Sabine; Büchner, Bernd; Kim, Timur KThe electronic structures of the iron-based superconductors have been intensively studied by using angle-resolved photoemission spectroscopy (ARPES). A considerable amount of research has been focused on the LaFeAsO family, showing the highest transition temperatures, where previous ARPES studies have found much larger Fermi surfaces than bulk theoretical calculations would predict. The discrepancy has been attributed to the presence of termination-dependent surface states. Here, using photoemission spectroscopy with a sub-micron focused beam spot (nano-ARPES) we have successfully measured the electronic structures of both the LaO and FeAs terminations in LaFeAsO. Our data reveal very different band dispersions and core-level spectra for different surface terminations, showing that previous macro-focus ARPES measurements were incomplete. Our results give direct evidence for the surface-driven electronic structure reconstruction in LaFeAsO, including formation of the termination-dependent surface states at the Fermi level. This experimental technique, which we have shown to be very powerful when applied to this prototypical compound, can now be used to study various materials with different surface terminations.
- ItemPhonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na2Co2TeO6([London] : Nature Publishing Group, 2024) Hong, Xiaochen; Gillig, Matthias; Yao, Weiliang; Janssen, Lukas; Kocsis, Vilmos; Gass, Sebastian; Li, Yuan; Wolter, Anja U. B.; Büchner, Bernd; Hess, ChristianThe report of a half-quantized thermal Hall effect and oscillatory structures in the magnetothermal conductivity in the Kitaev material α-RuCl3 have sparked a strong debate on whether it is generated by Majorana fermion edge currents, spinon Fermi surface, or whether other more conventional mechanisms are at its origin. Here, we report low temperature thermal conductivity (κ) of another candidate Kitaev material, Na2Co2TeO6. The application of a magnetic field (B) along different principal axes of the crystal reveals a strong directional-dependent B impact on κ, while no evidence for mobile quasiparticles except phonons can be concluded at any field. Instead, severely scattered phonon transport prevails across the B−T phase diagram, revealing cascades of phase transitions for all B directions. Our results thus cast doubt on recent proposals for significant itinerant magnetic excitations in Na2Co2TeO6, and emphasize the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials.
- ItemSixfold fermion near the Fermi level in cubic PtBi2(Amsterdam : SciPost Foundation, 2021) Thirupathaiah, Setti; Kushnirenko, Yevhen; Koepernik, Klaus; Piening, Boy Roman; Büchner, Bernd; Aswartham, Saicharan; van den Brink, Jeroen; Borisenko, Sergey; Fulga, Ion CosmaWe show that the cubic compound PtBi2, is a topological semimetal hosting a sixfold band touching point in close proximity to the Fermi level. Using angle-resolved photoemission spectroscopy, we map the bandstructure of the system, which is in good agreement with results from density functional theory. Further, by employing a low energy effective Hamiltonian valid close to the crossing point, we study the effect of a magnetic field on the sixfold fermion. The latter splits into a total of twenty Weyl cones for a Zeeman field oriented in the diagonal, [111] direction. Our results mark cubic PtBi2, as an ideal candidate to study the transport properties of gapless topological systems beyond Dirac and Weyl semimetals.
- ItemUnusual spin pseudogap behavior in the spin web lattice Cu3TeO6 probed by 125Te nuclear magnetic resonance(College Park, MD : APS, 2021) Baek, Seung-Ho; Yeo, Hyeon Woo; Park, Jena; Choi, Kwang-Yong; Büchner, BerndWe present a 125Te nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice Cu3TeO6 which harbors topological magnons. The 125Te NMR spectra and the Knight-shift K as a function of temperature show a drastic change at TS∼40K much lower than the Néel ordering temperature TN∼61K, providing evidence for the first-order structural phase transition within the magnetically ordered state. Most remarkably, the temperature dependence of the spin-lattice relaxation rate T−11 unravels spin-gap-like magnetic excitations, which sharply sets in at T∗∼75K, the temperature well above TN. The spin-gap behavior may be understood by weakly dispersive optical magnon branches of high-energy spin excitations originating from the unique corner-sharing Cu hexagon spin-1/2 network with low coordination number.
- ItemCharge transfer characteristics of F6TCNNQ–gold interface(Chichester [u.a.] : Wiley, 2020) Kuhrt, Robert; Hantusch, Martin; Knupfer, Martin; Büchner, BerndThe metal–organic interface between polycrystalline gold and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) was investigated by photoelectron spectroscopy with the focus on the charge transfer characteristics from the metal to the molecule. The valence levels, as well as the core levels of the heterojunction, indicate a full electron transfer and a change in the chemical environment. The changes are observed in the first F6TCNNQ layers, whereas for further film growth, only neutral F6TCNNQ molecules could be detected. New occupied states below the Fermi level were observed in the valence levels, indicating a lowest unoccupied molecular orbital (LUMO) occupation due to the charge transfer. A fitting of the spectra reveals the presence of a neutral and a charged F6TCNNQ molecules, but no further species were present.
- ItemElastoresistivity of Heavily Hole-Doped 122 Iron Pnictide Superconductors(Lausanne : Frontiers Media, 2022) Hong, Xiaochen; Sykora, Steffen; Caglieris, Federico; Behnami, Mahdi; Morozov, Igor; Aswartham, Saicharan; Grinenko, Vadim; Kihou, Kunihiro; Lee, Chul-Ho; Büchner, Bernd; Hess, ChristianNematicity in heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from magnetic instability were declared in RbFe2As2 and its sister compounds. Here, we report a systematic elastoresistance study of a series of isovalent- and electron-doped KFe2As2 crystals. We found divergent elastoresistance on cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven toward a Lifshitz transition. However, we conclude that none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism for how elastoresistivity is enhanced at a Lifshitz transition.
- ItemAxion Mie theory of electron energy loss spectroscopy in topological insulators(Amsterdam : SciPost Foundation, 2021) Schultz, Johannes; Nogueira, Flavio S.; Büchner, Bernd; van den Brink, Jeroen; Lubk, AxelElectronic topological states of matter exhibit novel types of responses to electromagnetic fields. The response of strong topological insulators, for instance, is characterized by a so-called axion term in the electromagnetic Lagrangian which is ultimately due to the presence of topological surface states. Here we develop the axion Mie theory for the electromagnetic response of spherical particles including arbitrary sources of fields, i.e., charge and current distributions. We derive an axion induced mixing of transverse magnetic and transverse electric modes which are experimentally detectable through small induced rotations of the field vectors. Our results extend upon previous analyses of the problem. Our main focus is on the experimentally relevant problem of electron energy loss spectroscopy in topological insulators, a technique that has so far not yet been used to detect the axion electromagnetic response in these materials.
- ItemInterplay of charge density waves, disorder, and superconductivity in 2H-TaSe2 elucidated by NMR([London] : IOP, 2022) Baek, Seung-Ho; Sur, Yeahan; Kim, Kee Hoon; Vojta, Matthias; Büchner, BerndSingle crystals of pristine and 6% Pd-intercalated 2H‐TaSe2 have been studied by means of 77Se nuclear magnetic resonance. The temperature dependence of the 77Se spectrum, with an unexpected line narrowing upon Pd intercalation, unravels the presence of correlated local lattice distortions far above the transition temperature of the charge density wave (CDW) order, thereby supporting a strong-coupling CDW mechanism in 2H‐TaSe2. While, the Knight shift data suggest that the incommensurate CDW transition involves a partial Fermi surface gap opening. As for spin dynamics, the 77Se spin-lattice relaxation rate T1-1 as a function of temperature shows that a pseudogap behavior dominates the low-energy spin excitations even within the CDW phase, and gets stronger along with superconductivity in the Pd-6% sample. We discuss that CDW fluctuations may be responsible for the pseudogap as well as superconductivity, although the two phenomena are unlikely to be directly linked each other.
- ItemEvidence for a percolative Mott insulator-metal transition in doped Sr2IrO4(College Park, MD : APS, 2021) Sun, Zhixiang; Guevara, Jose M.; Sykora, Steffen; Pärschke, Ekaterina M.; Manna, Kaustuv; Maljuk, Andrey; Wurmehl, Sabine; van den Brink, Jeroen; Büchner, Bernd; Hess, ChristianDespite many efforts to rationalize the strongly correlated electronic ground states in doped Mott insulators, the nature of the doping-induced insulator-to-metal transition is still a subject under intensive investigation. Here, we probe the nanoscale electronic structure of the Mott insulator Sr2IrO4−δ with low-temperature scanning tunneling microscopy and find an enhanced local density of states (LDOS) inside the Mott gap at the location of individual defects which we interpret as defects at apical oxygen sites. A chiral behavior in the topography for those defects has been observed. We also visualize the local enhanced conductance arising from the overlapping of defect states which induces finite LDOS inside of the Mott gap. By combining these findings with the typical spatial extension of isolated defects of about 2 nm, our results indicate that the insulator-to-metal transition in Sr2IrO4−δ could be percolative in nature.