Session Q02: Magnetic Topological Semimetals I

Coupling of magnetism and Dirac fermions in layered topological semimetals

Dirac semimetals remain at the forefront of research on topological materials because of the fascinating quantum electronic phenomena they exhibit and of their potential technological applications. The family of 112 ternary pnictogens with the general formula A/RMnX2 (A = Ca, Sr; R = Yb, Eu; X = Bi, Sb) have attracted particular attention due to the combination of highly anisotropic Dirac dispersion in quasi-two-dimensional (2D) square nets of X atoms and strongly correlated magnetism of Mn. Both the interlayer charge transport and the magnetic correlations between the Mn layers require that Dirac carriers are coupled to strongly correlated Mn electrons. This work reveals effects of such coupling on antiferromagnetic spin waves of Mn moments and establishes the trend connecting the strength of spin-fermion coupling and the ensuing spin-wave damping with the magnetic interaction parameters and the strength of spin-orbit interaction. These results establish the systematics of spin-fermion interactions in layered magnetic Dirac materials, guiding both theoretical and technological development.   

Isotropic Field Driven Insulator-to-Metal transition in GdPS.

We synthesized single crystals for GdPS and studied its electronic properties. GdPS crystalizes in an orthorhombically distorted ZrSiS-type structure. Unlike ZrSiS which possess topological semimetal phase with excellent metallicity, GdPS exhibit non-metallic transport that is consistent with the existence of a bulk band gap found in electronic band calculations. Furthermore, in contrast to the extremely large positive magnetoresistance in ZrSiS, GdPS display a colossal negative magnetoresistance reaching nearly 100% (normalized to zero field resistivity) at 2 K, which effectively leads to a field driven insulator-to-metal transition. Furthermore, such colossal negative magnetoresistance is insensitive to the orientation of the applied magnetic field, which is distinct from most of the previously established negative magnetoresistance material systems. Through combined experimental and theoretical investigation, we found that such unusual behavior is caused by the isotropic d-f magnetic exchange splitting for Gd, which drives an insulator-to-metal transition that is isotropic with respect to magnetic field. Our findings may establish a new direction towards antiferromagnetic materials with isotropic magneto transport properties which creates a rare platform for spintronics.   

Evidence of Hidden Magnetic Order in non-centrosymmetric Weyl semimetal GdAlSi

In recent decades, spintronics has emerged as a compelling alternative to traditional electronics. The discovery of topological phases of matter characterized by safeguarded spin-polarized states has introduced exciting possibilities. Concurrently, there has been a recent revelation of intriguing non-relativistic spin splitting in collinear antiferromagnetic materials possessing specific symmetries.

This study unveils the coexistence of these two remarkable phases within a single material, GdAlSi. GdAlSi adopts a body-centered tetragonal structure with a non-centrosymmetric space group, denoted as I4₁md (109). Magnetization data reveals antiferromagnetic ordering with a critical temperature (T_N) of 32 K.

Ab-initio calculations establish GdAlSi as a collinear antiferromagnetic Weyl semimetal, featuring an unconventional, momentum-dependent spin splitting, often referred to as "altermagnet." Angle-resolved photoemission spectroscopy measurements conducted on GdAlSi single crystals subsequently affirm the presence of Fermi arcs-like feature, a distinctive hallmark of Weyl semimetals. Electric and magnetic multipole analysis deepens our understanding of the symmetry-mediated, momentum-dependent spin splitting, which possesses a strictly non-relativistic origin.

To the best of our knowledge, the coexistence of unconventional antiferromagnetic order and non-trivial topology in a single material, as observed in GdAlSi, represents a groundbreaking discovery unprecedented in any material. This uniqueness positions GdAlSi as a promising material for topotronic applications.   

Novel surface state in GdBi and DyBi induced by long-range magnetic order

Several rare-earth monopnictides, including GdBi, were predicted to host topologically nontrivial states. Earlier ARPES studies of GdBi in the paramagnetic phase confirmed the presence of Dirac states in the corner of the Brillouin zone. Also, several recent ARPES studies reported the development of unconventional surface states in several rare-earth monopnictides (NdBi, NdSb, CeBi) in the antiferromagnetic phase, which form spin textured Fermi arcs and unusual magnetic splitting. Here, we investigate the evolution of the electronic structure of GdBi and DyBi upon antiferromagnetic transition using Angle-resolved photoemission spectroscopy. These materials are known to be magnetically ordered along (111) direction, which is different from the direction of magnetic ordering in NdBi, NdSb, CeBi. We observed the emergence of additional states in the electronic structure of GdBi and DyBi in the antiferromagnetic phase, including Dirac-like dispersions. These new states are different from those previously observed in other rare-earth monopnictides.   

Colossal anomalous Hall conductivity in half-metallic material Fe-doped CoS2

While it is well-established that the intrinsic anomalous Hall conductivity (AHC) arises from Berry curvature (BC) induced by spin-orbit gapped band anti-crossing, reported AHC values typically fall within the range of 100 to 1000 Ω^-1cm^-1. The primary reason for the often modest AHC lies in the cancellation of BC sources in the momentum space. This study delves into the AHC of cobalt disulfides (CoS₂), revealing resonant behavior as the chemical potential approaches the middle of the gap through varying Fe-doping levels. Co_0.95Fe_0.05S₂, in particular, exhibits a resonant peak of 2507 Ω^-1cm^-1, more than four times larger than that of CoS₂. Our density functional theory and tight-binding analyses trace this substantial AHC to four spin-polarized massive Dirac dispersions in the k_z = 0 plane of the Brillouin zone, positioned just below the Fermi level. The observed colossal AHC stems from the four BC sources, a consequence of d-wave-like spin-orbit coupling among spin-polarized e_g orbitals with the same sign, preventing cancellation. This result unveils the mechanism behind the significantly tunable AHC in CoS₂ and provides insights into a strategy for identifying similar materials.   

Novel Antiferromagnetic Topological Insulator NdBi Studied by Micro-ARPES

Antiferromagnetic topological insulators (AF TI) are known to host a massive Dirac cone (DC) surface state despite the absence of net magnetic moment. Moreover, a massless feature of DC shows a significant dependence on the orientation of the Néel vector. In order to clarify such DC characteristics associated with the AF-TI nature, we performed angle-resolved photoemission spectroscopy (ARPES) to directly visualize the DC states of AF TI.

Here, we focus on the rare-earth monopnictide NdBi, which shows semimetallic properties, topologically non-trivial nature, and AF transition at low temperature. To overcome the experimental difficulty in resolving ARPES signal from multiple AF domains at the cleaved surface, we have developed a micro-ARPES system with a spot size of 12 × 10 µm² at Photon Factory, KEK, and performed domain selective ARPES measurements on NdBi. As a result, we have succeeded in directly observing massive and massless DC states on the cleaved surface. Furthermore, we have identified the relationship between the AF domains and the DC states by clarifying the rotational symmetry of the surface at each AF domain. We discuss the origin of this DC behavior in terms of the observed symmetry pattern in ARPES spectra and crystal symmetry in the AF state.   

Giant Anomalous Nernst Effect Coefficient and Thermal Hall Angle in the Direction of Spin Canting in Antiferromagnet YbMnBi2

A large anomalous Nernst effect (ANE) has been sought after for thermoelectric applications in transverse geometry, as it contributes to greatly simplified thermoelectric device fabrication. YbMnBi₂, a topological antiferromagnet (AFM), was demonstrated to have a large ANE thermopower of ~ 6 µV K^-1,with a magnetic field B applied along [100], temperature gradient∇T applied along [010] and Nernst voltage V_N measured along [001], which was claimed to outshine all the other ANE thermopowers that had ever been reported in antiferromagnets¹. In this study, we present that in a different configuration in which B is applied along canted antiferromagnetic moment direction [110],∇T along [1-10] and V_N along [001], a remarkably large ANE coefficient of at least 30 µV K^-1 was observed and reproduced in different samples, which is probably ranked as the largest ANE thermopower reported in AFMs so far. The thermal Hall angle is also anomalously large (∇_yT/∇_xT  > 0.05) across the experiment range of 40 K ~ 300 K and shows a step in field at certain temperatures, suggesting the existence of an Anomalous Thermal Hall Effect (ATHE). The relatively low carrier density, which puts the chemical potential near the Weyl points, and high carrier mobility of these samples are key to these observations. The carrier polarity flip at high fields points at the topological origin of these anomalous transport phenomena.

1. Pan, Y., Le, C., He, B. et al. Giant anomalous Nernst signal in the antiferromagnet YbMnBi₂. Nat. Mater. 21, 203–209 (2022).   

Electronic structure and physical properties of antiferromagnetic Weyl semimetal GdAlSi

We have studied the electronic structure and physical properties of an antiferromagnetic Weyl semimetal GdAlSi. This compound crystallizes in a LaPtSi-type tetragonal structure with the non-centrosymmetric space group I4₁md. An antiferromagnetic transition is observed at T_N= 32 K due to the ordering of Gd³⁺ moments. In-plane and out of plane magnetic susceptibility measurements confirm a very weak magneto-crystalline anisotropy in this compound. Moreover, in-plane isothermal magnetization shows an unusual hysteresis behavior below T_N, suggesting a possible spiral magnetic order. We observe an exceptionally large anomalous Hall conductivity (AHC) of ∼ 1310 Ω⁻¹cm⁻¹ at 2 K. Interestingly, the anomalous Hall effect persists up to room temperature with a significant value of AHC (∼ 155 Ω⁻¹cm⁻¹). First-principles calculations reveal that GdAlSi hosts multiple Weyl points near the Fermi energy, which is further supported by angle-resolved photoemission spectroscopy (ARPES) measurements. The calculated AHC from Berry curvature associated with the Weyl nodes is close to the experimental value of AHC.   

Revealing spin-valve-like and exchange bias effect in Co3Sn1.9In0.1S2 crystal

In our scientific investigation, we present findings pertaining to a distinct magnetic phenomenon known as “sharp spin-valve-like magnetoresistance (MR)” occurring at temperatures below the ferromagnetic ordering temperature (T_C). It is worth noting that this particular MR behavior has not previously been explored in either the pristine crystalline material or in crystals subjected to doping. Furthermore, we have observed a characteristic manifestation of the “exchange bias (EB)” effect, which is indicated by systematic shifts of the loop observed in MR driven by the cooling field. The EB effect appears below a characteristic temperature (T_A), where prior theoretical considerations have suggested the potential coexistence of both ferromagnetic and antiferromagnetic phases within the material. The incorporation of Indium (In) leads to the emergence of sharp spin-valve-like MR and EB effects across a broad temperature range. Importantly, these effects persist even at significantly higher temperatures than the liquid nitrogen temperature. These compelling observations point towards the prospective suitability of this system for applications in the field of topological spintronics.   

Characterization of the Anisotropic Anomalous Nernst Effect in Antiferromagnetic YbMnBi2 and YbMnSb2

This work characterizes thermoelectric transport properties of antiferromagnetic topological semimetals YbMnSb₂ and YbMnBi₂, which exhibit large anomalous effects with simultaneously low magnetization and high mobility. Polycrystalline samples of YbMnBi₂ are synthesized for the first time, and the anomalous Nernst and magneto-Seebeck effects are characterized as a function of temperature and magnetic field. Previous work in single-crystalline YbMnBi₂ [1] indicated anisotropy in the Nernst effect, and emphasis is placed here on its removal in polycrystalline samples. Similarly, the anomalous Nernst and magneto-Seebeck effects are characterized in single crystalline YbMnSb₂ as a function of temperature, magnetic field, and crystallographic orientation such that the relationship between the orientation of the net Berry curvature and fluxes/fields associated with thermomagnetic transport can be determined.

[1] Y. Pan et al. Nat. Mater 21, 203-209 (2022).   

Multiple magnetic transitions, metamagnetism, and large magnetoresistance in GdAuGe single crystals

We have synthesized the single crystals of GdAuGe using the Bi flux and investigated their physical properties using magnetic, heat capacity, and magnetotransport measurements as well as electronic band calculations. The temperature-dependent magnetic susceptibility data reveal an antiferromagnetic ground state with Neel temperature T_N = 17.2 K, which is further confirmed through electrical resistivity and heat capacity measurements. Further, we observed two field-induced metamagnetic (MM) transitions in magnetization measurements for H ∥ c in the magnetically ordered state. The transverse magnetoresistance is positive and large near the critical fields of MM transitions (MR ~169 % at 2 K and 9 T). We also observed a very large anomalous Hall conductivity ~ 1270 Ω^-1 cm^-1 at 2 K. The field temperature phase diagram of GdAuGe has multiple magnetic phase transitions, including a collinear antiferromagnetic ground state, two successive spin-flop transitions, and a polarized paramagnetic state with two field-induced anomalies. Our first-principles calculations suggest that GdAuGe is a Dirac nodal line semimetal.   

Oral: Anomalous angular magnetoresistance of a proposed type-II Weyl semimetal CeAlGe

CeAlGe, a proposed type-II Weyl semimetal, orders antiferromagnetically below 5 K in a zero magnetic field. In the magnetically ordered state, the magnetic moment lies in the tetragonal ab plane, and the M(H) data show a four-fold symmetry along the principal directions in the ab plane. However, anomalously robust and complex two-fold symmetry is observed in the angular dependence of resistivity and magnetic torque data in the magnetically ordered state if the field is swept in the ab plane. This two-fold symmetry is independent of temperature and field-hystereses. We will show that this anomalous behavior is the result of complex magnetic structures and can be tuned by an Al deficiency.