Session T65: Topology in Correlated and Magnetic Phases

Magnetic-direction tunable Weyl line in A-A stacking kagome magnet

Kagome magnets provide a fascinating platform for realizing a plethora of exotic topological orders, such as Chern insulator, magnetic Weyl and nodal line semimetals, which depend on the interplay between hopping strength, stacking order and magnetic ground state. Here, utilizing angle-resolved photoelectron spectroscopy, we demonstrate the existence of magnetic nodal lines near Fermi level in an A-A stacking Kagome metal GdMn₆Sn₆, via imaging the conical in-plane dispersion at Brillouin zone boundary and strong out-of-plane dispersion, associated with the interlayer coupling of kagome sheets. More importantly, we directly observe the spin-orbit coupling effect in the out-of-plane ferrimagnetic phase of Tb doped GdMn₆Sn₆, which is absent in the undoped compound with in-plane magnetic order.  Our results not only provide the evidence of three-dimensional topological magnetic nodal lines in GdMn₆Sn₆ , but also experimentally illustrate the magnetic direction as the efficient tuning knob of the topological band structure in kagome materials.   

Observation of multiple nodal-lines in SmSbTe

Within the family of ZrSiS-type 111 materials, the  LnSbTe (Ln = lanthanide elements)-type materials are attracting research interests in recent times because of the possible correlation effects and magnetism arising from the 4f electrons of the lanthanides, which can provide an important platform  to study the linking between topology, magnetism, and correlation. In this talk, we present the systematic report of the electronic structure of SmSbTe by utilizing angle-resolved photoemission spectroscopy in conjunction with first-principles calculations, transport, and magnetic measurements. Our experimental results identify multiple Dirac nodes forming the nodal-lines along the Γ – M and Z – R directions in the bulk Brillouin zone (BZ) as predicted by our theoretical calculations. A surface Dirac-like state that arises from the square net plane of the Sb atoms is also observed at the X point of the surface BZ. Our study highlights SmSbTe as a promising candidate to understand the topological electronic structure of LnSbTe materials.   

Electronic structure through the Curie temperature in magnetic Weyl semimetal Co3Sn2S2

Co₃Sn₂S₂ is a magnetic Weyl semimetal below its Curie temperature (T_c) of 177K. We will discuss our temperature-dependent angle-resolved photoemission spectroscopy (ARPES) studies in this system.  Across T_c, we observe spectral changes in bulk bands which are inconsistent with a simple lifting of exchange interactions, suggesting enhanced electronic correlations in the non-magnetic regime.  We will also discuss spatial-dependent microARPES data which quantify the characteristic differences between Sn- and S- terminated surfaces.   

Evidence of multiple topological band structures in EuGa2Al2

The intermetallic compounds of the typical ThCr₂Si₂-structure type have uncovered a large family of material base exhibiting rich interplay of the lattice, magnetic, orbital and charge degrees of freedom, leading to a rich variety of states including antiferromagnetism, high temperature superconductivity, heavy fermion behavior, charge density wave (CDW), etc. EuGa₂Al₂ is such a rare-earth-based ThCr₂Si₂-structure type intermetallic with multiple magnetic transitions and CDW order. In this talk, a comprehensive electronic structure of EuGa₂Al₂ measured by angle-resolved photoemission spectroscopy will be presented. The existence of multiple topological bands will be elucidated with the combination of first-principles calculations. We will also discuss the effect of magnetism and CDW order on the electronic structures.   

Anisotropically large anomalous and topological Hall effect in a kagome magnet

Recently, kagome materials have caught massive attention due to the interplay among symmetry, magnetism, topology, and electron correlation. The latest works on RMn₆Sn₆ (R = rare-earth metal) compounds have illustrated that this family could be intriguing to investigate various physical phenomena due to large spin-orbit coupling and strong magnetic ordering. However, combined transport and spectroscopic studies in RMn₆Sn₆ materials are still limited. In this talk, we discuss magnetic, magneto-transport, and angle-resolved photoemission spectroscopy measurements of a kagome magnet ErMn₆Sn₆ that undergoes antiferromagnetic (T_N = 345 K) to ferrimagnetic (T_C = 68 K) phase transitions in the presence of the field. We observe large anomalous and topological Hall effects serving as transport signatures of the nontrivial Berry curvature. The isothermal magnetization exhibits strong anisotropic nature, and the topological Hall effect of the compound depends on the critical field of metamagnetic transition. Our spectroscopic results complemented by theoretical calculations show the multi-orbital kagome fermiology. This work provides new insight into the tunability and interplay of topology and magnetism in a kagome magnet.   

Unusual magnetic and transport properties in HoMn6Sn6 kagome magnet

With intricate lattice structures, kagome materials are an excellent platform to study various fascinating topological quantum states. In particular, kagome materials, revealing large responses to external stimuli such as pressure or magnetic field, are subject to special investigation. Here, we study the kagome-net HoMn₆Sn₆ magnet that undergoes paramagnetic to ferrimagnetic transition (below 376 K) and spin-reorientation transition (below 200 K). In this compound, we observe the topological Hall effect and substantial contribution of anomalous Hall effect above 100 K. We unveil the pressure effects on magnetic ordering at a low magnetic field from the pressure tunable magnetization measurement. By utilizing high-resolution angle-resolved photoemission spectroscopy, Dirac-like dispersion at the high-symmetry point K is revealed in the vicinity of the Fermi level, which is well supported by the first-principles calculations, suggesting a possible Chern-gapped Dirac cone in this compound. Our investigation will pave the way to understand the magneto-transport and electronic properties of various rare-earth-based kagome magnets.   

Electronic Structure of Topological Kagome Magnet HoMn6Sn6 studied by ARPES

Recent discoveries in magnetic RMn₆Sn₆ compounds (R=rare earth element) have opened a new path toward topological state engineering [1,2]. Topological states found in these compounds’ electronic structure are the result of both lattice geometry and electronic correlations. Interactions between Mn 3d electrons arranged in a kagome lattice, and 4f electrons from R atoms act as a source of a Chern gap and massive Dirac fermions. The size of the Chern gap and magnetic order in RMn₆Sn₆ can be manipulated by changing the R atom type. Different types of magnetic anisotropy are observed: easy ab-plane for Dy, easy c-axis for Tb and a conical magnetic structure for Ho and Dy.

Here, we present studies of the electronic structure of HoMn₆Sn₆ by means of angle resolved photoelectron spectroscopy (ARPES) at low temperature (~7 K). Our results show that the electronic structure of this system is three-dimensional, with significant dispersion along c-axis. We have observed a band structure typical of a topological kagome metal:  Dirac cones at K points and flat bands at M points. We find that the Dirac points are located very close to the Fermi level. Additionally, we resolved the electron band around the Γ point, which shows an enhanced effective mass. We will discuss the influence of electronic correlations, originating mostly from Ho atoms, on the topological states.

[1] J.-X. Yin et al., Nature 583, 533 (2020).

[2] W. Ma et al., Phys. Rev. Lett. 126, 246602 (2021).   

Edge current generation in 2d topological insulators through exciton dissociation

Exciton dissociation in semiconductor junctions constitute one of the main mechanisms for photovoltaic current generation. The quest for more efficient solar panels has caused lots of efforts to be put into research of materials with enhanced exciton dissociation rates, and alternative photocurrent mechanisms. On the other hand, topological insulators are a promising platform for spintronics, due to the presence of helical edge states which are robust to disorder and have a spin-locked propagation direction. As insulating materials, they are subject to exciton formation, so we conjecture that if an exciton is formed in bulk, it could dissociate on the edges leading to some effective current. In this work, by considering a ribbon of Bi(111) which is known to be a 2d time-reversal topological insulator, we compute the exciton spectrum within tight-binding theory using the Tamm-Dancoff approximation, and show the effect of topology in both the spectrum and the bulk excitonic wavefunction, as well as its consequences over the exciton transition rate to edge states. By performing energy band engineering, modifying the edge bands appropiately through different substractes at each edge, we conclude that it is possible to obtain a non-zero net current from exciton dissociation formed at the bulk.   

Electronic structure of Cr1/3NbSe2 epitaxial thin films studied by angle-resolved photoemission spectroscopy

In transition metal dichalcogenide (TMD), intercalation of 3d-transition metal atoms has been widely studied due to the presence of magnetic phases with varied spin-ordering structures. Recently, the role of intercalation has been revisited in the lens of topological physics, as the ferromagnetic phase of 3d-transition metal intercalated Nb and Ta dichalcogenide (space group: P6₃22) have been predicted to host topological band crossing [1]. Within the intercalated TMD, Cr-intercalated niobium diselenide (Cr_xNbSe2) is one such example, which for x=1/3 the material develops a ferromagnetic ground-state [2]. In this work, we employed molecular-beam epitaxy (MBE) to obtain highly ordered Cr_1/3NbSe₂ epitaxial thin-films. We focused on clarifying the electronic structure of the Cr_1/3NbSe₂ epitaxial thin-films via angle-resolved photoemission spectroscopy (ARPES). By comparing the result with the first principles band calculation, the role of Cr intercalation to the electronic structure and topological features of the material is investigated.

 

References

1. T. Inoshita et al. Phys. Rev. B 100, 121112 (2019).

2. N. M. Toporova et al. J. Alloys Compd. 848, 156534 (2020).   

Realizing kagome band structure in two-dimensional kagome surface states of RV6Sn6 (R=Gd, Ho)

We report angle resolved photoemission experiments on a newly discovered family of kagome metals RV₆Sn₆ (R=Gd, Ho). Intrinsic bulk states and surface states of the vanadium kagome layer are differentiated from those of other atomic sublattices by the real-space resolution of the measurements with a small beam spot. Characteristic Dirac cone, saddle point and flat bands of the kagome lattice are observed. Our results establish the two-dimensional (2D) kagome surface states as a new platform to investigate the intrinsic kagome physics.   

Behavior of Dirac Fermions in a non-centrosymmetric superconductor, BiPd

Topological materials having Dirac fermions exhibiting superconductivity is believed to be the candidate material to realize exotic physics as well as advanced technology. We studied the electronic structure of a complex topological material BiPd, which has non-centrosymmetric crystal structure and exhibit superconductivity below 3.7 K. Employing high resolution photoemission spectroscopy in a wide photon energy range (ultraviolet to hard x-ray), we show that the surface electronic structure of this material is confined only within the top few layers and there is an enhancement in orbital moment contribution leading to a deviation from atomic description of the core level spectroscopy. Our high-resolution ARPES study at varied conditions helped to identify the location of the Dirac node accurately and show the two dimensional behavior of the Dirac states. These states have significantly high binding energy and hence may not be responsible for the superconductivity in this system. The dispersion of the Dirac bands show unusual anisotropy - isotropic close to the Dirac point and becomes anisotropic at higher momenta. We have constructed a model Hamiltonian considering higher-order spin-orbit terms and demonstrate that this model provides an excellent description of the observed anisotropy.   

Study of the Fermi surface in Kagome Superconductor AV$_3$Sb$_5$(A = K,Rb,Cs) by de Haas-van Alphen and Shubnikov-de Haas effect

The recent discovery of novel charge density wave (CDW) Kagome superconductor AV$_3$Sb$_5$(A = K,Rb,Cs) have attracted great interest. We measured the de Haas-van Alphen effect of AV$_3$Sb$_5$ by using torque magnetometry and Shubnikov-de Haas (SdH) effect by using proximity detector oscillator (PDO) circuits under intense magnetic fields. Combining both dHvA and SdH effect, we provide a comprehensive study of the Fermi surface with the CDW gap considered.   

Thermal transport study in kagome superconductor CsV3Sb5

Thermal transport measurement has been an extremely powerful tool to explore the rich phases and hidden orders in various quantum materials. Recently, the discovery of the Kagome superconducting compound CsV₃Sb₅ provides a new platform to investigate the interplay between superconductivity, nematic phase, Dirac dispersion, and charge density wave (CDW) order. To understand the nature of these intertwined orders, we present a comprehensive study of the thermal conductivity as well as the thermal hall effect in the CsV₃Sb₅ single crystals. With the help of our self-developed differential amplifier and careful calibration, we measured the longitudinal and transverse thermal conductivity with high resolution in the precursor region of the superconducting state. The experimental methods, verification, and possible explanations will be discussed.