Anomalous Hall effect in Weyl semimetal half-Heusler compounds RPtBi (R = Gd and Nd)

Tuning of anomalous magnetotransport properties in half-Heusler topological semimetal GdPtBi

Half-Heusler compounds from the REPtBi family exemplify Weyl semimetals in which an external magnetic field induces Weyl nodes. These materials exceptionally host topologically non-trivial states near the Fermi level and their manifestation can be clearly seen in the magnetotransport properties. In this study, we tune the Fermi level of the archetypal half-Heusler Weyl semimetal GdPtBi through high-energy electron irradiation, moving it away from the Weyl nodes to investigate the resilience of the contribution of topologically non-trivial states to magnetotransport properties. Remarkably, we observe that the negative longitudinal magnetoresistance, which is a definitive indicator of the chiral magnetic anomaly occurring in topological semimetals, persists even when the Fermi level is shifted by 100 meV from its original position in the pristine sample. Additionally, the anomalous Hall effect shows complex variations as the Fermi level is altered, attributed to the energy-dependent nature of the Berry curvature, which arises from avoided band crossing. Our findings show the robust influence of Weyl nodes on the magneto-transport properties of GdPtBi, irrespective of the Fermi level position, a behaviour likely applicable to many half-Heusler Weyl semimetals.

Tuning anomalous Hall conductivity via antiferromagnetic configurations in GdPtBi

The magnetic, electronic, and topological properties of GdPtBi were systematically investigated using first-principles density functional theory (DFT) calculations. Various magnetic configurations were examined, including ferromagnetic (FM) and antiferromagnetic (AFM) states, with particular focus on AFM states where the Gd magnetic moments align either parallel (AFM‖) or perpendicular (AFM⊥) to the [111] crystal direction. For AFM⊥, the in-plane angles ϕ were varied at ϕ = 0°, 15°, and 30° (denoted as AFM⊥,ϕ=0°, AFM⊥,ϕ=15°, and AFM⊥,ϕ=30°, respectively). The ground-state magnetic structure of GdPtBi was validated through dipolar magnetic field calculations at the muon sites, corroborating the internal magnetic fields observed in muon spin relaxation (μSR) experiments. The results indicate that the AFM⊥,ϕ=30° configuration aligns with the μSR-measured internal field. The DFT-calculated band structure and Berry curvature reveal that AFM⊥ belongs to the triple-point semimetals (TPSMs) where the triple-point nodes positioned along the Z-Γ-Z and F-Γ-F paths have energies that shift as the spin-orbit coupling strength varies with ϕ. Notably, this shift in triple-point energy corresponds to a significant change in the anomalous Hall conductivity (AHC, σxy), with a difference of 75.06 Ω-1 cm-1 at the Fermi energy between AFM⊥,ϕ=0° and AFM⊥,ϕ=30°. These findings highlight the potential for controlling the AHC through precise manipulation of the AFM structure.

Giant Topological Hall Effect and Colossal Magnetoresistance in Heusler Ferromagnet near Room Temperature

Colossal magnetoresistance (CMR) is an exotic phenomenon that allows for the efficient magnetic control of electrical resistivity and has attracted significant attention in condensed matter due to its potential for memory and spintronic applications. Heusler alloys are the subject of considerable interest in this context due to the electronic properties that result from the nontrivial band topology. Here, the observation of CMR near room temperature is reported in the shape memory Heusler alloy Ni2Mn1.4In0.6, which is attributed to the combined effects of magnetic field-induced martensite twin variant reorientation (MFIR) and magnetic field-induced structural phase transformation (MFIPT). This compound undergoes a structural phase transition from a cubic (austenite-L21) ferromagnetic (FM) to a monoclinic (martensite) antiferromagnetic (AFM), which leads to an effective increase in the size of the Fermi surface and consequently in CMR. Additionally, it exhibits significant anomalous Hall conductivity in both antiferromagnetic and ferromagnetic phases. Furthermore, it demonstrates a giant topological Hall resistivity (THR)ρ yx T $\rho _{{\mathrm{yx}}}^{\mathrm{T}}$ ≈6 µΩ.cm in the vicinity of martensite transition due to the enhanced spin chirality resulting from the formation of magnetic domains with Bloch-type domain walls. The findings contribute to the understanding of the magnetotransport of Ni-Mn-In Heusler alloys, which are prospective candidates for room-temperature spintronic applications.

Giant Domain Wall Anomalous Hall Effect in a Layered Antiferromagnet EuAl_{2}Si_{2}

Generally, the dissipationless Hall effect in solids requires time-reversal symmetry breaking (TRSB), where TRSB induced by external magnetic field results in the ordinary Hall effect, while TRSB caused by spontaneous magnetization gives rise to the anomalous Hall effect (AHE) which scales with the net magnetization. The AHE is therefore not expected in antiferromagnets with vanishing small magnetization. However, large AHE was recently observed in certain antiferromagnets with noncollinear spin structure and nonvanishing Berry curvature. Here, we report another origin of AHE in a layered antiferromagnet EuAl_{2}Si_{2}, namely, the domain wall (DW) skew scattering with Weyl points near the Fermi level, in experiments for the first time. Interestingly, the DWs form a unique periodic stripe structure with controllable periodicity by external magnetic field, which decreases nearly monotonically from 975 nm at 0 T to 232 nm at 4 T. Electrons incident on DW with topological bound states experience strong asymmetric scattering, leading to a giant AHE, with the DW Hall conductivity (DWHC) at 2 K and 1.2 T reaching a record value of ∼1.51×10^{4}  Scm^{-1} among bulk systems and being 2 orders of magnitude larger than the intrinsic anomalous Hall conductivity. The observation not only sets a new paradigm for exploration of large anomalous Hall effect, but also provides potential applications in spintronic devices.

Emergence of Weyl points and large anomalous Hall conductivity in layered Bi2TeMnI2

In recent years, narrow band gap layered materials were reported as an interesting candidate for energy efficient devices. Here, we chose BiTeI, a layered material that has significant Rashba spin splitting, for charge modification with the purpose of exploring the electronic, magnetic and topological properties. Chemical doping with an Mn atom is done to the Te site in BiTeI. On the basis of density functional theory calculations, we found that the parent material BiTeI is a semiconductor with an indirect band gap of ∼0.46 eV within full-relativistic mode. The orbital contributions around the Fermi level are found to be mainly from the Bi-6p, I-5p and Te-5p states in the electronic structure. Upon chemical doping by Mn to Bi, Te and I separately, doping to the Te site is energetically favorable with a ferromagnetic ground state and a semimetallic behaviour. The doped material, i.e., Bi2TeMnI2, is found to be a magnetic Weyl semimetal with six Weyl points close to the Fermi level (around 100 meV in the conduction region). Our calculations suggest Bi2TeMnI2 as a probable candidate of a Weyl semimetal. The emergence of Weyl points gives rise to a large intrinsic anomalous Hall conductivity of up to ∼750 Ω-1 cm-1. The calculated negative value of formation energy (-0.233 eV) and the positive phonon frequency suggests Bi2TeMnI2 to be thermodynamically favorable and dynamically stable. This work deserves a transport experiment to confirm our claim, which might provide insights towards discovering new quantum materials suitable for high-speed electronics, spintronics and quantum computing.

Nonmonotonic Hall Effect of Weyl Semimetals under a Magnetic Field

The Hall effect of topological quantum materials often reveals essential new physics and possesses potential for application. The magnetic Weyl semimetal is one especially interesting example that hosts an interplay between the spontaneous time-reversal symmetry-breaking topology and the external magnetic field. However, it is less known beyond the anomalous Hall effect thereof, which is unable to account for plenty of magnetotransport measurements. We propose a new Hall effect characteristically nonmonotonic with respect to the external field, intrinsic to the three-dimensional Weyl topology, and free from chemical potential fine-tuning. Two related mechanisms from the Landau level bending and chiral Landau level shifting are found, together with their relation to the Shubnikov-de Hass effect. This field-dependent Hall response, universal to thin films and bulk samples, provides a concrete physical picture for existing measurements and is promising to guide future experiments.

Extended Berry Curvature Tail in Ferromagnetic Weyl Semimetals NiMnSb and PtMnSb

Heusler compounds belong to a large family of materials and exhibit numerous physical phenomena with promising applications, particularly ferromagnetic Weyl semimetals for their use in spintronics and memory devices. Here, anomalous Hall transport is reported in the room-temperature ferromagnets NiMnSb (half-metal with a Curie temperature (TC) of 660 K) and PtMnSb (pseudo half-metal with a TC of 560 K). They exhibit 4 µB/f.u. magnetic moments and non-trivial topological states. Moreover, NiMnSb and PtMnSb are the first half-Heusler ferromagnets to be reported as Weyl semimetals, and they exhibit anomalous Hall conductivity (AHC) due to the extended tail of the Berry curvature in these systems. The experimentally measured AHC values at 2 K are 1.8 × 102 Ω-1 cm-1 for NiMnSb and 2.2 × 103 Ω-1 cm-1 for PtMnSb. The comparatively large value between them can be explained in terms of the spin-orbit coupling strength. The combined approach of using ab initio calculations and a simple model shows that the Weyl nodes located far from the Fermi energy act as the driving mechanism for the intrinsic AHC. This contribution of topological features at higher energies can be generalized.

The discovery of three-dimensional Van Hove singularity

Arising from the extreme/saddle point in electronic bands, Van Hove singularity (VHS) manifests divergent density of states (DOS) and induces various new states of matter such as unconventional superconductivity. VHS is believed to exist in one and two dimensions, but rarely found in three dimension (3D). Here, we report the discovery of 3D VHS in a topological magnet EuCd2As2 by magneto-infrared spectroscopy. External magnetic fields effectively control the exchange interaction in EuCd2As2, and shift 3D Weyl bands continuously, leading to the modification of Fermi velocity and energy dispersion. Above the critical field, the 3D VHS forms and is evidenced by the abrupt emergence of inter-band transitions, which can be quantitatively described by the minimal model of Weyl semimetals. Three additional optical transitions are further predicted theoretically and verified in magneto-near-infrared spectra. Our results pave the way to exploring VHS in 3D systems and uncovering the coordination between electronic correlation and the topological phase.

Non-trivial topological phases in transition metal rich half-Heusler oxides

Topological insulators with gapless surface states and insulating bulk in non-centrosymmetric cubic systems have been extensively explored following the discovery of two-dimensional quantum spin hall effect in zincblende HgTe. In such systems the negative band inversion strength EBIS(= EΓ6-EΓ8<0) governs the robustness of the non-trivial topological states at ambient conditions. Hence, realizing large negative values of EBIShas been a guiding motivation of several investigations reported in literature. Here, we present a material design approach which can be employed to realize large negative values of EBISin cubic materials such as half-Heusler (HH) oxides with 18 valence electron configurations. We explore 27 HH oxides of the form ABO (A = Li, K, Rb; B = Cu, Ag, Au) inα-,β-, andγ-phase (by placing transition metal atom at different Wyckoff positions) for their non-trivial topological phase. Off these three phases, we found that, theα-phase of nine HH oxides (wherein the transition metal atoms occupy 4a Wyckoff positions in the crystal structure) is the most promising with non-trivial topological phase which is governed by the mass-Darwin fully-relativistic effects enhancing EBIS. Whereas the other phases were found to be either trivial semiconductors or semimetals or metals and most of them being dynamically unstable. We focus on RbAuO inα-phase with EBISof -1.29 eV and the effect of strain fields on the topological surface states of this compound. We conclude that theα-phase of HH oxide presented here can be synthesized experimentally for diverse room temperature applications in spintronics and nanoelectronics.

Colossal negative magnetoresistance in field-induced Weyl semimetal of magnetic half-Heusler compound

The discovery of topological insulators and semimetals triggered enormous interest in exploring emergent electromagnetic responses in solids. Particular attention has been focused on ternary half-Heusler compounds, whose electronic structure bears analogy to the topological zinc-blende compounds while also including magnetic rare-earth ions coupled to conduction electrons. However, most of the research in this system has been in band-inverted zero-gap semiconductors such as GdPtBi, which still does not fully exhaust the large potential of this material class. Here, we report a less-studied member of half-Heusler compounds, HoAuSn, which we show is a trivial semimetal or narrow-gap semiconductor at zero magnetic field but undergoes a field-induced transition to a Weyl semimetal, with a negative magnetoresistance exceeding four orders of magnitude at low temperatures. The combined study of Shubnikov-de Haas oscillations and first-principles calculation suggests that the exchange field from Ho 4f moments reconstructs the band structure to induce Weyl points which play a key role in the strong suppression of large-angle carrier scattering. Our findings demonstrate the unique mechanism of colossal negative magnetoresistance and provide pathways towards realizing topological electronic states in a large class of magnetic half-Heusler compounds.

Topological Superconductors from a Materials Perspective

Topological superconductors (TSCs) have garnered significant research and industry attention in the past two decades. By hosting Majorana bound states which can be used as qubits that are robust against local perturbations, TSCs offer a promising platform toward (nonuniversal) topological quantum computation. However, there has been a scarcity of TSC candidates, and the experimental signatures that identify a TSC are often elusive. In this Perspective, after a short review of the TSC basics and theories, we provide an overview of the TSC materials candidates, including natural compounds and synthetic material systems. We further introduce various experimental techniques to probe TSCs, focusing on how a system is identified as a TSC candidate and why a conclusive answer is often challenging to draw. We conclude by calling for new experimental signatures and stronger computational support to accelerate the search for new TSC candidates.

Semimetallic, Half-Metallic, Semiconducting, and Metallic States in Gd-Sb Compounds

The electronic and band structures of the Gd- and Sb-based intermetallic materials have been explored using the theoretical ab initio approach, accounting for strong electron correlations of the Gd-4f electrons. Some of these compounds are being actively investigated because of topological features in these quantum materials. Five compounds were investigated theoretically in this work to demonstrate the variety of electronic properties in the Gd-Sb-based family: GdSb, GdNiSb, Gd₄Sb₃, GdSbS₂O, and GdSb₂. The GdSb compound is a semimetal with the topological nonsymmetric electron pocket along the high-symmetry points Γ-X-W, and hole pockets along the L-Γ-X path. Our calculations show that the addition of nickel to the system results in the energy gap, and we obtained a semiconductor with indirect gap of 0.38 eV for the GdNiSb intermetallic compound. However, a quite different electronic structure has been found in the chemical composition Gd₄Sb₃; this compound is a half-metal with the energy gap of 0.67 eV only in the minority spin projection. The molecular GdSbS₂O compound with S and O in it is found to be a semiconductor with a small indirect gap. The GdSb₂ intermetallic compound is found to have a metallic state in the electronic structure; remarkably, the band structure of GdSb₂ has a Dirac-cone-like feature near the Fermi energy between high-symmetry points Г and S, and these two Dirac cones are split by spin-orbit coupling. Thus, studying the electronic and band structure of several reported and new Gd-Sb compounds revealed a variety of the semimetallic, half-metallic, semiconducting, or metallic states, as well topological features in some of them. The latter can lead to outstanding transport and magnetic properties, such as a large magnetoresistance, which makes Gd-Sb-based materials very promising for applications.

Spin and current transport in the robust half-metallic magnetc-CoFeGe

Spintronics is an emerging form of electronics based on the electrons' spin degree of freedom for which materials with robust half-metallic ferromagnet character are very attractive. Here we determine the structural stability, electronic, magnetic, and mechanical properties of the half-Heusler (hH) compound CoFeGe, in particular also in its cubic form. The first-principles calculations suggest that the electronic structure is robust with 100% spin polarization at the Fermi level under hydrostatic pressure and uni-axial strain. Both the longitudinal and Hall current polarization are calculated and the longitudinal current polarization (PL) is found to be>99%and extremely robust under uniform pressure and uni-axial strain. The anomalous Hall conductivity and spin Hall conductivity of hH cubic CoFeGe (c-CoFeGe) are found to be∼-100S cm^-1and∼39 ℏ/eS cm^-1, respectively. Moreover, the Curie temperature of the alloy is calculated to be ∼524 K with a 3μBmagnetic moment. Lastly, the calculated mechanical properties indicate thatc-CoFeGe is ductile and mechanically stable with a bulk modulus of ≈154 GPa. Overall, this analysis reveals that cubic CoFeGe is a robust half-metallic ferromagnet and an interesting material for spintronic applications.

Topology and Symmetry in Quantum Materials

Interest in topological materials continues to grow unabated in view of their conceptual novelties as well as their potential as platforms for transformational new technologies. Electronic states in a topological material are robust against perturbations and support unconventional electromagnetic responses. The first-principles band-theory paradigm has been a key player in the field by providing successful prediction of many new classes of topological materials. This perspective presents a cross section through the recent work on understanding the role of geometry and topology in generating topological states and their responses to external stimuli, and as a basis for connecting theory and experiment within the band theory framework. In this work, effective strategies for topological materials discovery and impactful directions for future topological materials research are also commented.

Revealing the Potential of Ternary Medium-Entropy Alloys as Exceptional Electrocatalysts toward Nitrogen Reduction: An Example of Heusler Alloys

With less energy consumption and environmental pollution, electrochemical ammonia synthesis is regarded as the most promising way to replace the industrial Haber-Bosch process, which greatly contributes to global energy consumption and CO₂ emission. At present, the best metal electrocatalyst for N₂ fixation is ruthenium although its performance still suffers from a low Faradaic efficiency and a high overpotential. Alloy engineering is a promising way to discover more metal-based electrocatalysts for dinitrogen reduction reaction (N2RR), and almost all reported alloy catalysts so far are binary alloys. In this work, we proposed a large group of ternary alloy electrocatalysts (Heusler alloys) for N2RR and demonstrated their superior catalytic performance. As an example, alloying Ru with Mn and Si led to a reduced Ru-Ru distance on the surface, which facilitates an uncommon horizontal adsorption mode of N₂ and results in effective activation of N₂ molecules. The theoretical overpotential of N2RR on Ru₂MnSi(100-Ru) is only around 0.28 V, which ranks among the best reported results, and the usage of precious Ru is greatly reduced. Meanwhile, the adsorption of N₂ on Ru₂MnSi(100-Ru) was much stronger than that of protons, and it also took less energy to drive N2RR than the hydrogen evolution reaction (HER), making HER less competitive on this catalyst. Considering the successful synthesis of numerous Heusler alloys including the six members mentioned here, our work provided a wider range of practical and excellent N2RR electrocatalysts in terms of both catalytic performance and economical cost.

Progress and prospects in magnetic topological materials

Magnetic topological materials represent a class of compounds with properties that are strongly influenced by the topology of their electronic wavefunctions coupled with the magnetic spin configuration. Such materials can support chiral electronic channels of perfect conduction, and can be used for an array of applications, from information storage and control to dissipationless spin and charge transport. Here we review the theoretical and experimental progress achieved in the field of magnetic topological materials, beginning with the theoretical prediction of the quantum anomalous Hall effect without Landau levels, and leading to the recent discoveries of magnetic Weyl semimetals and antiferromagnetic topological insulators. We outline recent theoretical progress that has resulted in the tabulation of, for the first time, all magnetic symmetry group representations and topology. We describe several experiments realizing Chern insulators, Weyl and Dirac magnetic semimetals, and an array of axionic and higher-order topological phases of matter, and we survey future perspectives.

Anomalous magnetoresistance and magneto-thermal properties of the half-Heuslers,RPdSi (RY, Gd-Er)

Here, we present a detailed study on the magnetic, magneto-transport, and magneto-thermal properties of the equiatomic half-Heusler compounds with the general formula,RPdSi (R= Y and rare-earth, Gd-Er). These materials crystallize in two different superstructures of the TiNiSi-type orthorhombic unit cell with the space groupsPnmaandPmmn. Our magnetic and heat capacity measurements reveal the onset of an antiferromagnetic (AFM) ordering in the temperature range 3-16 K for all the local moments bearingRPdSi compounds, while the non-magnetic analog, YPdSi exhibits a Pauli-paramagnetic behaviour. The AFM state of these compounds can be tuned by magnetic field and temperature as demonstrated by the magnetic measurements below the Neel temperature (T_N). Most importantly, this tuning of the magnetic structure is well documented in the complex temperature and field dependence of magnetoresistance (MR) and magnetocaloric effect (MCE). Our study establishes a striking correlation of the commensurate/incommensurate AFM structure with that of positive/negative MR and MCE in this series of compounds. We emphasize that such a framework applies to a large number of AFM intermetallic systems.

Strain-tunable triple point Fermions in diamagnetic rare-earth half-Heusler alloys

Topologically non-trivial electronic structure is a feature of many rare-earth half-Heusler alloys, which host atoms with high spin-orbit coupling bringing in the non-triviality. In this article, using the first-principles simulations, rare-earth half-Heusler YPdBi, ScPdBi, LaPdBi, LuPdBi, YPtBi and LuPtBi alloys are studied under strain to reveal multiple band inversions associated with topological phase transitions. From our simulations we find that, as a result of first band-inversion, the Brillouin zone of the diamagnetic half-Heusler alloys hosts eight triple points whereas, the second band inversion causes the emergence of sixteen more triple points. These band-inversions are observed to be independent of the spin-orbit coupling and are the reason behind increasing occupation of bismuth 7s orbitals as volume of the unit cell increases. The surface electronic transport in different triple point semi-metallic phases is found to evolve under strain, as the number of Fermi arcs change due to multiple band inversions. Once the second band inversion occurs, further application of tensile strain does not increase the number of triple points and Fermi arcs. However, increasing tensile strain (or decreasing compressive strain) pushes the triple point crossing to higher momenta, making them more effective as source of highly mobile electrons. These observations make a pathway to tune the bulk as well as surface transport through these semi-metals by application of tensile or compressive strain depending on the unstrained relative band-inversion strength of the material.

Epitaxy, exfoliation, and strain-induced magnetism in rippled Heusler membranes

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al2O3 substrates. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. The weak Van der Waals interactions of graphene enable mechanical exfoliation to yield free-standing GdPtSb membranes, which form ripples when transferred to a flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic, measurements on rippled membranes show a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd. First-principles calculations show that the coupling to homogeneous strain is too small to induce ferromagnetism, suggesting a dominant role for strain gradients. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetism and magnetostriction) and strain gradients (flexomagnetism).

Giant Anomalous Hall Effect due to Double-Degenerate Quasiflat Bands

We propose a novel approach to achieve a giant anomalous Hall effect (AHE) in materials with flat bands (FBs). FBs are accompanied by small electronic bandwidths, which consequently increases the momentum separation (K) within pair of Weyl points and, thus, the integrated Berry curvature. Starting from a simple model with a single pair of Weyl nodes, we demonstrated the increase of K and the AHE by decreasing the bandwidth. It is further expanded to a realistic pyrochlore lattice model with characteristic double-degenerated FBs, where we discovered a giant AHE while maximizing the K with nearly vanishing band dispersion of FBs. We identify that such a model system can be realized and modulated through strain engineering in both pyrochlore and spinel compounds based on first-principles calculations, validating our theoretical model and providing a feasible platform for experimental exploration.

The topology of electronic band structures

The study of topology as it relates to physical systems has rapidly accelerated during the past decade. Critical to the realization of new topological phases is an understanding of the materials that exhibit them and precise control of the materials chemistry. The convergence of new theoretical methods using symmetry indicators to identify topological material candidates and the synthesis of high-quality single crystals plays a key role, warranting discussion and context at an accessible level. This Perspective provides a broad introduction to topological phases, their known properties, and material realizations. We focus on recent work in topological Weyl and Dirac semimetals, with a particular emphasis on magnetic Weyl semimetals and emergent fermions in chiral crystals and their extreme responses to excitations, and we highlight areas where the field can continue to make remarkable discoveries. We further examine open questions and directions for the topological materials science community to pursue, including exploration of non-equilibrium properties of Weyl semimetals and cavity-dressed topological materials.

Interface-induced sign reversal of the anomalous Hall effect in magnetic topological insulator heterostructures

The Berry phase picture provides important insights into the electronic properties of condensed matter systems. The intrinsic anomalous Hall (AH) effect can be understood as the consequence of non-zero Berry curvature in momentum space. Here, we fabricate TI/magnetic TI heterostructures and find that the sign of the AH effect in the magnetic TI layer can be changed from being positive to negative with increasing the thickness of the top TI layer. Our first-principles calculations show that the built-in electric fields at the TI/magnetic TI interface influence the band structure of the magnetic TI layer, and thus lead to a reconstruction of the Berry curvature in the heterostructure samples. Based on the interface-induced AH effect with a negative sign in TI/V-doped TI bilayer structures, we create an artificial “topological Hall effect”-like feature in the Hall trace of the V-doped TI/TI/Cr-doped TI sandwich heterostructures. Our study provides a new route to create the Berry curvature change in magnetic topological materials that may lead to potential technological applications.

Berry curvature connects to exotic electronic phases hence it provides important insights to understand quantum materials. Here, the authors report sign change of the anomalous Hall effect resulted from Berry curvature change at the interface of a topological insulator/magnetic topological insulator heterostructure.

Breakdown of Ohm's law and nontrivial Berry phase in magnetic Weyl semimetal Co3Sn2S2

The concept of realization of Weyl points close to the Fermi level in materials with broken time-reversal symmetry has significant theoretical and technological ramifications. Here, we review the investigation of magneto-transport measurements in single crystals of magnetic Weyl semimetal Co3Sn2S2. We see a turn-on like behaviour followed by saturation in resistivity under magnetic field in the low temperature region which is allocated to the topological surface states. A non-saturating magnetoresistance, linear at high fields, is observed at low temperatures where applied magnetic field is transverse to the current direction. The linear negative magnetoresistance at low magnetic fields (B< 0.1 T) provides evidence for time reversal symmetry breaking in Co3Sn2S2. Chiral anomaly in Weyl metallic state in Co3Sn2S2is confirmed from the breakdown of Ohm's law in the electronic transport. Shubnikov de Haas (SdH) oscillation measurement has unveiled the multiple sub-bands on the Fermi surface that corresponds to a non-trivial Berry phase. The non-linear behaviour in Hall resistivity validates the existence of two type of charge carriers with equal electron and hole densities. Strong temperature dependence of carrier mobilities reflects the systematic violation of Kohler's rule in Co3Sn2S2. Our findings open avenues to study kagome-lattice based magnetic Weyl semimetals that unfurl the basic topological aspects leading to significant ramification for spintronics.

Finite-temperature violation of the anomalous transverse Wiedemann-Franz law

The Wiedemann-Franz (WF) law has been tested in numerous solids, but the extent of its relevance to the anomalous transverse transport and the topological nature of the wave function, remains an open question. Here, we present a study of anomalous transverse response in the noncollinear antiferromagnet Mn₃Ge extended from room temperature down to sub-kelvin temperature and find that the anomalous Lorenz ratio remains close to the Sommerfeld value up to 100 K but not above. The finite-temperature violation of the WF correlation is caused by a mismatch between the thermal and electrical summations of the Berry curvature and not by inelastic scattering. This interpretation is backed by our theoretical calculations, which reveals a competition between the temperature and the Berry curvature distribution. The data accuracy is supported by verifying the anomalous Bridgman relation. The anomalous Lorenz ratio is thus an extremely sensitive probe of the Berry spectrum of a solid.

Signatures of Sixfold Degenerate Exotic Fermions in a Superconducting Metal PdSb2

Multifold degenerate points in the electronic structure of metals lead to exotic behaviors. These range from twofold and fourfold degenerate Weyl and Dirac points, respectively, to sixfold and eightfold degenerate points that are predicted to give rise, under modest magnetic fields or strain, to topological semimetallic behaviors. The present study shows that the nonsymmorphic compound PdSb₂ hosts six-component fermions or sextuplets. Using angle-resolved photoemission spectroscopy, crossing points formed by three twofold degenerate parabolic bands are directly observed at the corner of the Brillouin zone. The group theory analysis proves that under weak spin-orbit interaction, a band inversion occurs.

Intrinsic Anomalous Hall Effect in Ni-Substituted Magnetic Weyl Semimetal Co3Sn2S2

Topological Weyl semimetals have recently attracted considerable attention among materials scientists as their properties are predicted to be protected against perturbations such as lattice distortion and chemical substitution. However, any experimental proof of such robustness is still lacking. In this study, we experimentally demonstrate that the topological properties of the ferromagnetic kagomé compound Co3Sn2S2 are preserved upon Ni substitution. We systematically vary the Ni content in Co3Sn2S2 single crystals and study their magnetic and anomalous transport properties. For the intermediate Ni substitution, we observe a remarkable increase in the coercive field while still maintaining significant anomalous Hall conductivity. The large anomalous Hall conductivity of these compounds is intrinsic, consistent with first-principles calculations, which proves its topological origin. Our results can guide further studies on the chemical tuning of topological materials for better understanding.

Magnetism of 3d and 4d doped Mn0.7T0.3NiGe (T = Fe, Co, Ru and Rh): bulk magnetization and ab initio calculations

We compare the magnetic properties of 3d (Fe and Co) and 4d (Ru and Rh) transition metals doped MnNiGe using the combined results of magnetization and ab initio calculations. The alloys crystallize in austenite Ni₂In-type hexagonal phase (space group: P6₃/mmc) with insignificant difference in the lattice parameters. Mn_0.7Fe_0.3NiGe and Mn_0.7Co_0.3NiGe exhibit spin-glass behavior, resulting from the competing ferro- and antiferromagnetic interactions. These alloys exhibit spontaneous exchange bias field of about [Formula: see text] Oe and 323 Oe, respectively. From the 4d-metal doped alloys, Mn_0.7Ru_0.3NiGe shows glassy behavior while long-range ferromagnetic order is confirmed in Mn_0.7Rh_0.3NiGe. In Mn_0.7Rh_0.3NiGe, in agreement with experiment and the theoretical calculations, the ground state is confirmed to be ferromagnetic because of the FM exchange interactions of the Mn magnetic moments. But in Mn_1-x (Fe,Co,Ru) _x NiGe alloys the calculations revealed the competing and comparable FM and AFM exchange interaction parameters, resulting in the formation of spin-glassy characteristics.

Resolving the topological classification of bismuth with topological defects

The growing diversity of topological classes leads to ambiguity between classes that share similar boundary phenomenology. This is the status of bulk bismuth. Recent studies have classified it as either a strong or a higher-order topological insulator, both of which host helical modes on their boundaries. We resolve the topological classification of bismuth by spectroscopically mapping the response of its boundary modes to a screw-dislocation. We find that the one-dimensional mode, on step-edges, extends over a wide energy range and does not open a gap near the screw-dislocations. This signifies that this mode binds to the screw-dislocation, as expected for a material with nonzero weak indices. We argue that the small energy gap, at the time reversal invariant momentum L, positions bismuth within the critical region of a topological phase transition between a higher-order topological insulator and a strong topological insulator with nonzero weak indices.

Extremely high conductivity observed in the triple point topological metal MoP

Weyl and Dirac fermions have created much attention in condensed matter physics and materials science. Recently, several additional distinct types of fermions have been predicted. Here, we report ultra-high electrical conductivity in MoP at low temperature, which has recently been established as a triple point fermion material. We show that the electrical resistivity is 6 nΩ cm at 2 K with a large mean free path of 11 microns. de Haas-van Alphen oscillations reveal spin splitting of the Fermi surfaces. In contrast to noble metals with similar conductivity and number of carriers, the magnetoresistance in MoP does not saturate up to 9 T at 2 K. Interestingly, the momentum relaxing time of the electrons is found to be more than 15 times larger than the quantum coherence time. This difference between the scattering scales shows that momentum conserving scattering dominates in MoP at low temperatures.