Session P52: Magnetic Topological Materials 4: STM

Live

Tunneling into topological magnets: discovery of Chern and many-body quantum phenomena

The search for topological phases of matter is evolving towards magnetic systems, where exotic effects emerge from the quantum level interplay between geometry, correlation, and topology. Equipped with unprecedented spatial resolution, electronic detection, and magnetic tunability, scanning tunneling microscopy (STM) has become a powerful tool to probe and discover topological magnet. Via spectroscopic Landau quantization and edge state imaging, we discover the Chern phase in TbMn6Sn6 [1]. Through quasi-particle interferences based on vectorial magnetization, we discover giant and anisotropic spin-orbit tunability in Fe3Sn2 [2]. Using spin-polarized or magnetization polarized STM, we observe flat band orbital magnetism and spin-orbit quantum impurity in a Co-based magnet [3,4]. We also uncover many-body resonance in an Mn-based antiferromagnet [5] and many-body fermion-boson interplay in a Co-based paramagnet [6]. These research developments not only demonstrate STM's ability to discover interacting topological phenomena but also allow researchers to connect to the results obtained by other techniques including angle-resolved photoemission, magneto-transport, density functional and tight-binding modeling.

1. J-X Yin et al. Nature 583, 533 (2020).
2. J-X Yin et al. Nature 562, 91-95 (2018).
3. J-X Yin et al. Nature Physics 15, 443 (2019).
4. J-X Yin et al. Nat Commun 11, 4415 (2020).
5. S. S. Zhang et al. Phys. Rev. Lett. 125, 046401 (2020).
6. J-X Yin et al. Nat Commun 11, 4003 (2020).

These studies are in collaboration with M. Zahid Hasan, Songtian S. Zhang, Shuang Jia, Wenhong Wang, Hechang Lei, Satoru Nakatsuji, Ziqiang Wang, Titus Neupert, Hsin Lin, Guoqing Chang, Tay-Rong Chang, Biao Lian, Brian Møller Andersen, and Rui Wang.   

Live

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Although the mean mass gap can exceed 28 meV (∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using Landau level spectroscopic imaging, we compare the electronic structure of FMTI Cr_0.08(Bi_0.1Sb_0.9)_1.92Te₃ to that of its nonmagnetic parent (Bi_0.1Sb_0.9)₂Te₃ to find electrostatic and magnetic disorders conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits QAH in Sb₂Te₃-based FMTI materials to very low temperatures.   

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The ZrSiS family compounds host various exotic quantum phenomena due to the presence of both topological nonsymmorphic Dirac fermions and nodal line fermions. The inherent magnetism in LnSbTe (Ln= lanthanide rare earth) compounds offers opportunity to study possible new quantum states. Here, we report magnetic and transport properties of previously unexplored topological nodal line semimetal candidates NdSbTe and SmSbTe. These materials show antiferromagnetic ground state and strong electronic correlations unlike many other known LnSbTe compounds. All the findings suggest that these materials can offer a rare platform to explore exotic quantum phenomena arising from the interplay between magnetism, topology, and electronic correlations.   

Live

Magnetic Weyl semimetals—in which time reversal symmetry is broken—present an opportunity to control band topology with external parameters such as temperature and magnetic field. CeAlGe lacks inversion symmetry and becomes magnetically ordered below 4.5K, serving as an ideal material to explore the interactions of magnetism and electronic band topology. We report on anomalous electronic and thermal transport behavior in CeAlGe above the magnetic transition temperature. In particular, we find that the magnetoresistance (MR) is zero at 12.8K whereas the thermal conductivity is greatly suppressed by magnetic field at this temperature. The thermal transport and electronic transport phase diagrams suggest that this intermediate temperature region just above the magnetic transition temperature has distinct properties from the high temperature and magnetically ordered phases. We propose a mechanism for this new phase arising from the interaction between the Weyl fermions and fluctuating magnetic moments.   

Live

SrRuO₃, a 4d ferromagnetic perovskite, provides a promising opportunity to seek the existence of Weyl fermions in magnetic materials. However, such a quest has been hampered until our latest report [1] due to difficulties in preparing high-quality specimens [2]. Here, we demonstrate the quantum transport evidence of the Weyl fermions in an epitaxial SrRuO₃ film with the best crystalline quality ever reported. We grew 63-nm thick SrRuO₃ films on SrTiO₃ substrates by our recently developed machine-learning-assisted molecular beam epitaxy [3]. The residual resistivity ratio [ρ(300 K)/ρ(T→0 K)] of the film reached the world’s best value of 84, enabling direct observations of quantum transport evidence of Weyl fermions [1,4]: (i) unsaturated linear positive magnetoresistance (MR), (ii) chiral-anomaly-induced negative MR, (iii) π Berry phase accumulated along cyclotron orbits, (iv) light cyclotron masses, and (v) high quantum mobility of about 10000 cm²/Vs. Therefore, our results establish SrRuO₃ as a magnetic Weyl semimetal.

[1] K. Takiguchi, Y. K. Wakabayashi, et al., Nat. Commun. 11, 4969 (2020).
[2] G. Koster, et al., Rev. Mod. Phys. 84, 253 (2012).
[3] Y. K. Wakabayashi, et al., APL Mater. 7, 101114 (2019).
[4] X. Huang, et al., Phys. Rev. X 5 031023 (2015).   

Live

YMn₆Sn₆ is a member of the ternary Kagome magnets, known for their frustration-driven ground states and novel phenomena. YMn₆Sn₆ orders in an easy plane commensurate antiferromagnetic phase below T_N = 345K, transitioning to an incommensurate double helical (DH) structure upon further cooling. With field in the ab-plane, this DH structure evolves between multiple helical and fan-like phases. Of these, the transverse conical spiral, shows a large topological Hall effect where the scalar spin chirality is stabilized by thermal fluctuation, similar to the nematic susceptibility of Fe-based superconductors¹. Electronic band structure calculations show topological character near E_F, making YMn₆Sn₆ a candidate to investigate the interplay between the electronic topology and magnetic topological properties. Here we report signatures in the magnetoresistance indicating strong coupling between the magnetic structure and the electronic band structures. Regardless of field orientation, a decrease in resistivity above 6T indicates a strong effect on the electronic properties, independent of magnetization, implying alteration of the topological electronic bands via applied field.

¹Ghimire, N. J., et al. arXiv:2007.02399 (2020).   

Live

YMn₆Sn₆ exhibits topologically protected characteristics which are unusual given the underlying centrosymmetric crystal lattice (P6/mmm). In zero-applied magnetic field, polarized neutron diffraction measurements presented here show a preferential spin chirality in the double flat spiral spin state. At elevated temperatures and modest in-plane magnetic fields, a topological Hall effect emerges, [1,2] despite a null scalar spin chirality. The competing magnetic exchange interactions in this material are delicately balanced due to frustration and as a result, thermal fluctuations, temperature, and applied magnetic fields can drive the system into a number of different magnetic phases. Unpolarized neutron diffraction measurements were used to solve the structure of many of these phases and will also be presented. These measurements neatly explain the multiple features seen in ac susceptibility measurements, [1] and our corresponding theoretical studies align with the experimental evidence to provide a satisfyingly complete picture of the in-field magnetic phase diagram for YMn₆Sn₆.

[1] Ghimire, N. J., et al. arXiv preprint arXiv:2007.02399 (2020).
[2] Wang, Q., et al. arXiv preprint arXiv:1906.07986 (2019).   

Live

Kagome magnets have been proposed to host many exotic phenomena, such as spin liquid and Weyl semimetal phases. A hallmark signature of their band structure is a dispersionless "flat" band, accopanied by a Dirac dispersion near the Brillouin zone edge. We use low-temperature spectroscopic-imaging scanning tunneling microscopy (SI-STM) to investigate the surface structure and low energy electronic properties of a kagome magnet YMn₆Sn₆. We find a sharp spectral peak E_p close to the Fermi level, which can be linearly tunned by magnetic field closer to Fermi level. By further imaging scattering and interference of electrons on the kagome Mn surface, we are also able to visualize a Dirac like band dispersion, terminating near E_p. Remarkably, external magnetic field also shifts this Dirac dispersion to higher energy regardless of the relative direction of the magnetic field. This diamagnetic behavior cannot be explained by spin Zeeman coupling, and points towards the emergence of orbital magnetism in this system.   

Live

A Weyl semimetal arises when a bulk Dirac point is split into two Weyl nodes by breaking inversion or time-reversal symmetry (TRS). At low T and increasing B-field, the candidate Weyl material CeBi exhibits a cascade of TRS-breaking magnetic phases. Here we focus on the ferrimagnetic and fully-polarized phases of CeBi, where our density functional theory (DFT) calculations predict several Weyl nodes near the Fermi level (EF). We use spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy to image the surface magnetic order, and quasiparticle interference (QPI) measurements to quantify the band splitting. Strong suppression of the surface spin-polarization at EF, coincident with a Fano line shape in dI/dV, suggests the Bi p states partially Kondo screen the f magnetic moments, and this p-f mixing causes strong Fermi-level band renormalization. The QPI measurements support p band flattening and ~100 meV splitting, suggesting a strongly interacting magnetic Weyl semimetal with robustly spaced nodes.   

Live

Kagome-net magnets form a popular class of materials for topologically nontrivial magnetic and electronic structures. YMn₆Sn₆ is a prototype of such compounds. It forms an hexagonal P6/mmm structure consisting of kagome planes [Mn3Sn] separated by two inequivalent Sn3 and Sn2Y layers, thereby providing a platform for topological electronic features inherent to the kagome geometry (e.g. Dirac cones), and complex magnet structures arising from the parametric frustration. We have recently realized a series of competing magnetic phases together with a fluctuation-driven topological Hall effect in this compound [1]. Here we present the calculated electronic structure of YMn₆Sn₆ along with de Haas-van Alphen and Shubnikov-de Haas oscillation data in magnetic fields of up to 60 T and temperatures down to 0.5 K. A plethora of oscillations is observed, reflecting the intricate Fermi surface, whilst phase and effective mass analysis hints at the role of Dirac fermions.
[1] Ghimire et al., arXiv:2007.02399.   

Live

The kagome lattice has long been regarded as a conceptual lattice network that connects its characteristic atomic arrangement to unusual singularities in the electronic structure. The recent discovery of Dirac dispersions and flat bands in transition metal stannide T-Sn (T = Fe, Co) kagome compounds has brought the long-sought theoretical framework into realistic material systems [1]. Here we report the surface-sensitive planar tunneling spectroscopy of antiferromagnetic kagome metal FeSn across a Schottky heterointerface with an n-type degenerate semiconductor [2, 3]. We present measurements across a wide range of temperatures, barrier widths, and magnetic fields probing the underlying electronic structure. We also discuss the relevance of our results to potential applications for spintronic devices.
References:
[1] M. Kang, L. Ye et al., Nat. Mater. 19, 163 (2020)
[2] H. Inoue, M. Han et al., Appl. Phys. Lett. 115, 072403 (2019)
[3] M. Han, H. Inoue et al., in preparation (2020)   

Live

The kagome lattice, along with the honeycomb lattice, is among the simplest theoretical models anticipated to host Dirac fermions in their tight-binding band structures. In recent years, electronic Dirac fermions have been discovered in a series of transition element-based kagome metals, among which the ferromagnetic Fe₃Sn₂ stands as one of the initial examples. Spectroscopic, transport, and thermodynamic experiments have revealed the presence of quasi-two-dimensional massive Dirac fermions in Fe₃Sn₂ [1,2]. Here we present a comprehensive first-principles study of the electronic structure of Fe₃Sn₂ to clarify the origin of the observed Dirac fermions and the nature of the electronic topology in the system. Our findings suggest that the massless Dirac fermions in the non-relativistic limit in Fe₃Sn₂ are subject to a Kane-Mele type spin-orbit coupling. We will also discuss our study's implications on realizing low-dimensional lattice models using three-dimensional crystalline materials.

[1] L. Ye, M. Kang et al., Nature 555 638-642 (2018).
[2] L. Ye et al., Nat. Commun. 10, 4870 (2019).   

On Demand

The axion is a hypothetical but experimentally undetected particle. Recently, the antiferromagnetic topological insulator MnBi₂Te₄ has been predicted to host the axion insulator, but the experimental evidence remains elusive. Specifically, the axion insulator is believed to carry "half-quantized" chiral hinge currents running antiparallel on its top and bottom hinges. However, it is challenging to measure precisely the half-quantization. Here, we propose a nonlocal surface transport device, in which the axion insulator can be distinguished from normal insulators without a precise measurement of the half-quantization. More importantly, we show that the nonlocal surface transport, as a qualitative measurement, is robust in realistic situations when the gapless side surfaces and disorder come to play. Moreover, thick electrodes can be used in the device of MnBi₂Te₄ thick films, enhancing the feasibility of the surface measurements. This proposal will be insightful for the search of the axion insulator and axion in topological matter.

arXiv: 2005.14074