Session N43: Magnetic Topological Semimetals

In-situ fabrication of pyrochlore epitaxial thin film and heterostructures

We report on the successful in-situ fabrication of a variety (111) oriented thin films with pyrochlore structure R2Ir2O7 (R is rare-earth) by a hybrid approach utilizing solid phase epitaxy and layer-by-layer pulsed laser deposition. Specifically, we report on developing an entirely in-situ annealing protocol, which is superior to the conventional ex-situ routine that requires multi-hour annealing to stabilize the proper pyrochlore structure. In addition, the excellent morphological quality of the films was verified by x-ray diffraction, reflectivity, reciprocal space mapping, XPS, and TEM, demonstrating their high crystallinity with a pure pyrochlore phase, smooth surface, proper electronic structure, and an expected epitaxial relation to the substrate. Our findings open a new solution to the long-standing challenge of the growth of pyrochlore single-crystalline thin films and heterostructures based on them. Furthermore, this all-in-situ approach provides new opportunities for applying surface-sensitive probes.   

Magneto-transport properties of (111)-oriented thin films of pyrochlore iridates

Magnetic topological materials have attracted significant interest due to their emergent quantum properties stemming from the interplay between magnetism and non-trivial band topology. However, high-quality compounds with coexisting magnetic and topological properties at elevated temperatures are still rare. Pyrochlore iridates family with the general formula, R2Ir2O7 (R = Y, rare-earth elements), is among one of them; they are predicted to harbor a magnetic Weyl semimetallic (WSM) phase accompanied by the all-in-all-out (AIAO) antiferromagnetic (AFM) configuration. Notably, in (111)-oriented thin films of pyrochlore iridates, the theory also predicts the emergence of additional topological phases generally unattainable in their bulk-crystal counterpart. Here, we report on unusual magneto-transport properties arising from the interplay between the WSM and AFM phases and the presence of surface/interface in the thin films of iridates. The results indicate that (111)-oriented thin films of pyrochlores present itself as a promising system for zero-dissipation spintronics devices.   

Tuning the topological Hall effect of CeAlGe by external pressure

 A topological Hall effect (THE) may arise from non-trivial topological spin textures, such as chiral domain walls, which have potential applications in spintronics [1]. The interplay of Weyl fermions and domain walls yields a topological Hall torque, which can efficiently manipulate the magnetization [2].

  At ambient pressure, CeAlGe is a Weyl semimetal hosting a topological magnetic phase [3], making it an excellent candidate to study non-trivial topologies in real and momentum space. Our results reveal that, similar to its magnetism [4], the THE in CeAlGe is sensitive to slight stoichiometric variations. The application of external pressure modifies the domain walls landscape leading to a pressure-induced evolution of a single THE region to two distinct regions in magnetic fields, in agreement with other results [5]. In addition, a THE was induced by pressure even in samples, where it was absent at low pressures. Our findings showing the high tunability of CeAlGe are a promising result on the way to induce THE in devices with external stimuli in a controlled way.

 

[1] S.-H. Yang, et al., Nat. Rev. Phys. 3, 328 (2021).

[2] M. Yamanouchi et al., Science advances 8 15 eabl6192 (2022).

[3] P. Puphal, et al., Phys. Rev. Lett. 124 017202 (2020).

[4] P. Puphal, et al., Phys. Rev. Mat. 3 024204 (2019).

[5] X. He, et al., arXiv:2207.08442   

Understanding transport and topology in EuCd2X2 through electronic structure: X = P vs. As

The symmetry breaking required to host a single pair of Weyl points seems to be realized in EuCd₂As₂, and importantly, can be tuned by energetically close magnetic ground states. Another tuning parameter is to change out the pnictogen for phosphorus instead of arsenic. The cousin compound, EuCd₂P₂ exhibits a surprisingly enormous collossal magneto-resistance (CMR), much larger than EuCd₂As₂ and even outperforming the traditional manganese oxides while not possessing any of the properties that explain their CMR. Therefore, this series of compounds provides a rich intersection where physics of strong correlation lie in close proximity to nontrivial topology. We systematically investigate the electronic structure of EuCd₂X₂ with X= {P, As} using both angle-resolved photo-emission spectroscopy (ARPES) and density functional theory (DFT). With remarkable agreement between experiment and theory, we provide a solid foundation to elucidate the rich transport behavior, spectroscopic signal related to magneto-resistance, and topological classification for these compounds.   

Negligible Hall resistance in Dirac semimetal EuCu2Sb2

Magnetic Dirac semimetal is of great interest among researchers these days. Many of these Dirac semimetals are protected by the combination of crystal symmetries and a special antiferromagnetic time-reversal symmetry ^[1]. There are limited systems that show magnetic Dirac state. Recently, the EuCd₂As₂ compound has been identified as magnetic Weyl semimetal ^[2]. A single crystal of EuCu₂Sb₂ was synthesized by flux growth method using Cu-Sb flux. The crystal structure of EuCu2Sb2 contains a subsequent magnetic Eu²⁺ and non-magnetic [Cu₂Sb₂]^2- layers perpendicular to c- axis, and Eu moments align in antiferromagnetically between two adjacent layers. Electrical resistivity and specific heat data show antiferromagnetic transition at 5.1 K. Interestingly, it shows negligible Hall resistance with the applied magnetic field 0 – 14 T. In Angle-resolved photoemission spectroscopy measurement, the linear band crossing near the Fermi level suggests Dirac like band dispersion which results in unconventional Hall resistance.

[1] Young, Steve M. and Wieder, Benjamin J., Phys. Rev. Lett. 118, 186401 (2017).

[2] Soh, J.-R. et al., Phys. Rev. B 100, 201102 (2019).

    

Ultrafast dynamics of SrCd2Sb2-xAsx single crystals studied by optical pump-probe spectroscopy

Topological materials exhibiting symmetry-protected surface states such as topological insulators have recently gained attention due to its unique physical properties and potential applications. In this context, the present study utilizes AB₂X₂ type SrCd₂Sb_2-xAs_x single crystals to study the ultrafast dynamics of carriers and phonons to gain insight into critical changes in crystal structure through chemical substitution. Here, we use optical pump-probe (OP-OP) spectroscopy on the as-synthesized SrCd₂Sb₂, SrCd₂As₂, and As (x=0.1) doped SrCd₂As_0.1Sb_1.9 single crystals. The transient reflectivity (ΔR/R) curve shows similar fitting behavior and paramters for SrCd₂Sb₂ and SrCd₂As_0.1Sb_1.9 which is in contrast to SrCd₂As₂. The significant observations from the OP-OP measurements include, (i) absence of sub-ps decay component in SrCd₂Sb₂, (ii) appearance of ∼10 ps decay term (τ₃) for SrCd₂Sb₂, and (iii) the change of sign for the amplitude (A₂) of several ps τ₂ decay term. The results revealed contrast ultrafast dynamics between SrCd₂As₂ and SrCd₂Sb₂ thereby implying that SrCd₂As₂ and SrCd₂Sb₂ have distinct electronic structures near the band edge. Further, we will carry out OP-OP measurements for the whole doping range (x=0.1 to 2.0) in SrCd₂Sb_2-xAs_x to elucidate the evolving electronic band structure and to observe the desired transition in ultrafast spectroscopy around x = 1.0.   

Rare-earth monopnictides: Family of antiferromagnets hosting magnetic Fermi arcs

Since the discovery of topological insulators a great deal of research effort has been devoted to magnetic topological materials, in which nontrivial spin properties can be controlled by magnetic fields, culminating in a wealth of fundamental phenomena and possible applications. The main focus was on ferromagnetic materials that can host Weyl fermions and therefore spin-textured Fermi arcs. The recent discovery of Fermi arcs and new magnetic band splitting in the antiferromagnet (AFM) NdBi has opened up new avenues for exploration. We show that these uncharted effects are not restricted to this specific compound, but also emerge in CeBi and NdSb when they undergo paramagnetic to AFM transition. Also, the relative intensity of the new bands and splitting of these bands scale with the magnetic moments of the rare-earth elements. Our data show that the Fermi arcs in NdSb have twofold symmetry, leading to strong anisotropy that may enhance effects of spin textures on transport properties.   

Unusual transport properties of GdPS.

 

We report the growth and property study of GdPS, a new member of the ZrSiS-family materials. ZrSiS materials are characterized by the coexistence of nodal-line and non-symmorphic Dirac states, which lead to exotic properties such as electronic correlation enhancement. Introduction of magnetism by rare earth elements further enrich the properties. We found that the magnetic GdPS exhibit very different properties from non-magnetic ZrSiS, which is manifested by unusual magnetoresistance and anisotropy. This discovery offers a new platform to engineer topological states for fundamental physics study and device applications.   

Spin motive force by the momentum-space Berry phase in magnetic Weyl semimetals

Electrons with topologically non-trivial band structures show unique phenomena reflecting their topological nature. For instance, in recent years, studies on the transport and optical properties of Weyl semimetal discovered the photovoltaic effect reflecting the Berry phase and magnetoresistance related to chiral anomaly. In addition, the experimental discovery of magnetic Weyl semimetals opened a route to study novel phenomena arising from the interplay of Weyl electrons and magnetism. Drawing inspiration from the adiabatic charge pump, we explore how the dynamics of the magnetic moment and the Berry phase arising from the dynamics affect the material properties. We show that the precession of ferromagnetic moment in a Weyl semimetal induces a Berry curvature with the time derivative, similar to the adiabatic pump. This phenomenon resembles Faraday's law of electromagnetism, in which the circular electric current induces a magnetic field; the circular motion of the Weyl node, a magnetic charge in the momentum space, induces a momentum-space electric field that appears in the adiabatic pump. The Berry phase effect results in a macroscopic current in a setup similar to the ferromagnetic resonance, in which the current appears as a spin motive force. Unlike the conventional spin motive force, this phenomenon produces a finite current without a magnetic gradient. The result shed light on a novel material property arising from the Berry phase effect and its potential for spintronics application.   

Observation of Large Wiedemann-Franz Law Violation in Ferromagnetic, Heusler, Weyl Semimetal Co2MnAl

The Wiedemann-Franz (WF) law states that in metals the ratio between the electronic component of thermal conductivity and the electrical conductivity is proportional to the temperature because both are mediated by charge carriers. This proportionality constant is called the Lorenz number. This relation has been found to hold for many metals over a wide temperature range, however a number of topological materials have been found to violate the WF law[1,2]. Co2MnAl is a Heusler, ferromagnetic, Weyl semimetal whose band structure is dependent on applied magnetic field. Under no applied field, Co2MnAl’s band structure has 4 Weyl nodal rings that form a Hopf-like chain protected by mirror symmetries. When a magnetic field is applied, these rings can be gapped out creating Weyl nodes and changing the Berry curvature[3,4]. In this talk, we report thermal conductivity and electrical resistivity measurements and analysis that demonstrates a substantial violation of the WF law in Co2MnAl. Further, we observe nonclassical temperature dependence of the Lorenz number. Comparison between our results and those of other WF law violating materials indicates that there is no one effect that can fully explain the observed violation and temperature dependence. Rather, our observations are likely due to a number of effects, related to Co2MnAl’s exotic band structure, working in tandem.   

Complex intrinsic surface states of pyrite ferromagnetic topological semimetal CoS2

The ferromagnet CoS₂, a typical pyrite structure, was recently discovered to have multi-nodal structures. In this work, we observe many complex intrinsic surface states in CoS₂ and find the different origins of these various surface states. We find that CoS₂ not only exhibits a variety of topological nodal structures but also has an obstructed-atomic-limit feature. Thus, Fermi arc, drumhead, and obstructed atomic-center-induced surface states (OASs) are accessible in this system. Interestingly, OASs exhibit a clear Dirac-like crossing sitting at Fermi level protected by glide mirror symmetry and some surface states spreading the whole Brillouin zone. Furthermore, combining first-principle calculation and experimental results, we observe that some additional spin-polarized surface states cannot be explained by the multi-nodal structures and OASs. These unique surface states spreading throughout the whole Brillouin zone are dubbed crystal field-enforced surface states (CFSs), i.e., this is because the S octahedron surrounding each Co is broken at the surface. In addition, we find that due to the bulk nature of the CoS₂ crystal structure, at least one of the OASs and CFSs always appears regardless of the choice of surface termination. These diverse origins of intrinsic surface states are beneficial to plenty of potential applications and explain the novel physical properties recently discovered in experiments, such as high-performance electrocatalytic activity.   

Strong correlations in a magnetic chiral topological semimetal close to a correlated metal-insulator transition

Chiral topological semimetals are a new class of quantum materials that host multifold fermions with large topological charges and long Fermi-arc surface states. We have recently identified a family of non-magnetic intermetallic compounds crystallizing in the cubic B20 as chiral topological semimetals and visualized their multifold fermions and long fermi-arcs with angle-resolved photoemission (ARPES) [1,2]. In this talk, I will present our latest ARPES study of a magnetic chiral topological semimetal. The spectral function of the majority spin-channel is strongly broadened and almost completely smeared out in the vicinity of the Gamma point where a multifold fermion has been predicted by ab-initio calculations, whilst the minority spin channel shows long-lived quasiparticles up to 500 meV binding energy.  Such spin-dependent electronic correlations have previously been observed in elemental ferromagnets, but to the best of my knowledge, such a drastic loss of quasiparticle weight has never been observed in a topological metal and may indicate a correlated topological metal-insulator transition. 

[1] N. B. M. Schröter et al., Nat. Phys. 15, 759–765 (2019).

[2] N. B. M. Schröter et al., Science 369, 179 (2020).

    

Observation of Band-Splitting by Paramagnetic Spin-Fluctuations in EuZn2Sb2*

The simultaneous presence of time-reversal symmetry and spatial inversion symmetry imposes a two-fold degeneracy of the bands structure of a solid. Typically, the lifting of this degeneracy in centrosymmetric materials is indicative of long-range magnetization. However, correlated alignment of spins, even in a paramagnetic sample, is also capable of lifting the spin-degeneracy of the electronic states. We report the observation of parallelly dispersing split bands in Eu-ternary pnictide EuZn2Sb2 through the use of high-resolution ARPES. Our DFT calculations account for the observed splitting by including the effects of long-range ferromagnetic correlation of the spins. These results indicate the possibility of observing further time-reversal violating physics in non-magnetic materials.