Spin fluctuation induced Weyl semimetal state in the paramagnetic phase of EuCd2As2

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.

Recent advances in understanding and manipulating magnetic and electronic properties of EuM2X2(M= Zn, Cd;X= P, As)

Over the past five years, significant progress has been made in understanding the magnetism and electronic properties of CaAl2Si2-type EuM2X2(M= Zn, Cd;X= P, As) compounds. Prior theoretical work and experimental studies suggested that EuCd2As2had the potential to host rich topological phases, particularly an ideal magnetic Weyl semimetal state when the spins are polarized along thecaxis. However, this perspective is challenged by recent experiments utilizing samples featuring ultra-low carrier densities, as well as meticulous calculations employing various approaches. Nonetheless, the EuM2X2family still exhibit numerous novel properties that remain to be satisfactorily explained, such as the giant nonlinear anomalous Hall effect and the colossal magnetoresistance effect. Moreover, EuM2X2compounds can be transformed from semiconducting antiferromagnets to metallic ferromagnets by introducing a small number of carriers or applying external pressure, and a further increase in the ferromagnetic transition temperature can be achieved by reducing the unit cell volume. These features make the EuM2X2family a fertile platform for studying the interplay between magnetism and charge transport, and an excellent candidate for applications in spintronics. This paper presents a comprehensive review of the magnetic and transport behaviors of EuM2X2compounds with varying carrier densities, as well as the current insights into these characteristics. An outlook for future research opportunities is also provided.

Experimental progress in Eu(Al,Ga)4topological antiferromagnets

The non-trivial magnetic and electronic phases occurring in topological magnets are often entangled, thus leading to a variety of exotic physical properties. Recently, the BaAl4-type compounds have been extensively investigated to elucidate the topological features appearing in their real- and momentum spaces. In particular, the topological Hall effect and the spin textures, typical of the centrosymmetric Eu(Al,Ga)4family, have stimulated extensive experimental and theoretical research. In this topical review, we discuss the latest findings on the Eu(Al,Ga)4topological antiferromagnets and related materials, arising from a wide range of experimental techniques. We show that Eu(Al,Ga)4represents a suitable platform to explore the interplay between lattice-, charge-, and spin degrees of freedom, and associated emergent phenomena. Finally, we address some key questions open to future investigation.

Colossal anomalous Hall effect in the layered antiferromagnetic EuAl2Si2 compound

The anomalous Hall effect (AHE), significantly enhanced by the extrinsic mechanism, has attracted attention for its almost unlimited Hall response, which exceeds the upper limit of the Berry curvature mechanism. However, due to the high conductivity in the clean regime and weak skew scattering, it is a great challenge to obtain large anomalous Hall conductivities and large anomalous Hall angles at the same time. Here, we unveil a new magnetic metal system, EuAl2Si2, which hosts both colossal anomalous Hall conductivity (σAxy ≥ 104 Ω-1 cm-1) and large anomalous Hall angle (AHA >10%). The scaling relation suggests that the skew scattering mechanism is dominant in the colossal anomalous Hall response and gives rise to a large skew scattering constant. The large effective SOC and large magnetic moment may account for this anomaly. Our results indicate that EuAl2Si2 is a good platform to study the extrinsic AHE mechanism.

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.

Tunable positions of Weyl nodes via magnetism and pressure in the ferromagnetic Weyl semimetal CeAlSi

The noncentrosymmetric ferromagnetic Weyl semimetal CeAlSi with simultaneous space-inversion and time-reversal symmetry breaking provides a unique platform for exploring novel topological states. Here, by employing multiple experimental techniques, we demonstrate that ferromagnetism and pressure can serve as efficient parameters to tune the positions of Weyl nodes in CeAlSi. At ambient pressure, a magnetism-facilitated anomalous Hall/Nernst effect (AHE/ANE) is uncovered. Angle-resolved photoemission spectroscopy (ARPES) measurements demonstrated that the Weyl nodes with opposite chirality are moving away from each other upon entering the ferromagnetic phase. Under pressure, by tracing the pressure evolution of AHE and band structure, we demonstrate that pressure could also serve as a pivotal knob to tune the positions of Weyl nodes. Moreover, multiple pressure-induced phase transitions are also revealed. These findings indicate that CeAlSi provides a unique and tunable platform for exploring exotic topological physics and electron correlations, as well as catering to potential applications, such as spintronics.

Emergence of Weyl fermions by ferrimagnetism in a noncentrosymmetric magnetic Weyl semimetal

Condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics. Here, using angle-resolved photoemission spectroscopy, we provide an example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the emergence of new Weyl fermions in the ferrimagnetic state. Our results establish NdAlSi as a magnetic Weyl semimetal and provide an experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.

EuCd_{2}As_{2}: A Magnetic Semiconductor

EuCd_{2}As_{2} is now widely accepted as a topological semimetal in which a Weyl phase is induced by an external magnetic field. We challenge this view through firm experimental evidence using a combination of electronic transport, optical spectroscopy, and excited-state photoemission spectroscopy. We show that the EuCd_{2}As_{2} is in fact a semiconductor with a gap of 0.77 eV. We show that the externally applied magnetic field has a profound impact on the electronic band structure of this system. This is manifested by a huge decrease of the observed band gap, as large as 125 meV at 2 T, and, consequently, by a giant redshift of the interband absorption edge. However, the semiconductor nature of the material remains preserved. EuCd_{2}As_{2} is therefore a magnetic semiconductor rather than a Dirac or Weyl semimetal, as suggested by ab initio computations carried out within the local spin-density approximation.

Topology stabilized fluctuations in a magnetic nodal semimetal

The interplay between magnetism and electronic band topology enriches topological phases and has promising applications. However, the role of topology in magnetic fluctuations has been elusive. Here, we report evidence for topology stabilized magnetism above the magnetic transition temperature in magnetic Weyl semimetal candidate CeAlGe. Electrical transport, thermal transport, resonant elastic X-ray scattering, and dilatometry consistently indicate the presence of locally correlated magnetism within a narrow temperature window well above the thermodynamic magnetic transition temperature. The wavevector of this short-range order is consistent with the nesting condition of topological Weyl nodes, suggesting that it arises from the interaction between magnetic fluctuations and the emergent Weyl fermions. Effective field theory shows that this topology stabilized order is wavevector dependent and can be stabilized when the interband Weyl fermion scattering is dominant. Our work highlights the role of electronic band topology in stabilizing magnetic order even in the classically disordered regime.

Intrinsic exchange bias state in silicene and germanene materials EuX2

2D magnets have recently emerged as a host for unconventional phases and related phenomena. The prominence of 2D magnetism stems from its high amenability to external stimuli and structural variations. The low dimensionality facilitates competition between magnetic orders which may give rise to exchange bias, in particular in magnetic heterostructures. Here, we propose a strategy for the search of exchange bias state in 2D individual compounds. We track the evolution of magnetic orders driven by the number of monolayers in a system exhibiting antiferromagnetism in the multilayer and ferromagnetism in the monolayer limit. The material, EuSi₂, has the structure of multilayer silicene intercalated by Eu. A strong intrinsic exchange bias effect accompanies the dimensional crossover. Comparison with silicene-based GdSi₂ and germanene-based EuGe₂ suggests the competition between magnetic orders to be a common property of this class of materials that may be useful in spintronic applications.

Magnetic Weyl Semimetal in BaCrSe2 with Long-Distance Distribution of Weyl Points

Weyl semimetals (WSMs) have attracted great attentions that provide intriguing platforms for exploring fundamental physical phenomena and future topotronics applications. Despite the fact that numerous WSMs are achieved, WSMs with long-distance distribution of Weyl points (WPs) in given material candidates remain elusive. Here, the emergence of intrinsic ferromagnetic WSMs in BaCrSe₂ with the nontrivial nature explicitly confirmed by the Chern number and Fermi arc surface states analysis is theoretically demonstrated. Remarkably, unlike previous WSMs for which opposite chirality WPs are located very close to each other, the WPs of BaCrSe₂ host a long-distance distribution, as much as half of the reciprocal space vector, suggesting that the WPs are highly robust and difficult to be annihilated by perturbations. The presented results not only advance the general understanding of magnetic WSMs but also put forward potential applications in topotronics.

Anomalous Hall Conductivity and Nernst Effect of the Ideal Weyl Semimetallic Ferromagnet EuCd2 As2

Weyl semimetal is a unique topological phase with topologically protected band crossings in the bulk and robust surface states called Fermi arcs. Weyl nodes always appear in pairs with opposite chiralities, and they need to have either time-reversal or inversion symmetry broken. When the time-reversal symmetry is broken the minimum number of Weyl points (WPs) is two. If these WPs are located at the Fermi level, they form an ideal Weyl semimetal (WSM). In this study, intrinsic ferromagnetic (FM) EuCd2 As2 are grown, predicted to be an ideal WSM and studied its electronic structure by angle-resolved photoemission spectroscopy, and scanning tunneling microscopy which agrees closely with the first principles calculations. Moreover, anomalous Hall conductivity and Nernst effect are observed, resulting from the non-zero Berry curvature, and the topological Hall effect arising from changes in the band structure caused by spin canting produced by magnetic fields. These findings can help realize several exotic quantum phenomena in inorganic topological materials that are otherwise difficult to assess because of the presence of multiple pairs of Weyl nodes.

Canted antiferromagnetic order in EuZn2As2 single crystals

Compounds containing Eu show a vast range of unique physical properties due to the interplay of electronic and magnetic properties, which can lead to a nontrivial electronic topology combined with magnetic order. We report on the growth of trigonal (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$P\overline{3 }m1$$\end{document}P3¯m1 space group) EuZn₂As₂ single crystals and on the studies of their structural, electronic and magnetic properties. A range of experimental techniques was applied including X-ray diffraction, electron microscopy, magnetic susceptibility, magnetization, heat capacity and Mössbauer spectroscopy in the study. We found that Eu has solely a 2+ valence state and its magnetic moments below T_N = 19.2 K form a canted antiferromagnetic structure, tilted from the basal plane.

Anomalous Electromagnetic Field Penetration in a Weyl or Dirac Semimetal

The current response to an electromagnetic field in a Weyl or Dirac semimetal becomes nonlocal due to the chiral anomaly activated by an applied static magnetic field. The nonlocality develops under the conditions of the normal skin effect and is related to the valley charge imbalance generated by the joint effect of the electric field of the impinging wave and the static magnetic field. We elucidate the signatures of this nonlocality in the transmission of electromagnetic waves. The signatures include enhancement of the transmission amplitude and its specific dependence on the wave's frequency and the static magnetic field strength.

Comparative Raman spectroscopy of magnetic topological material EuCd2X2(XP, As)

EuCd₂X₂(X = P, As) is a new class of magnetic topological materials discovered recently. The electronic structure and the band topology are intimately coupled with its magnetism, giving rise to interesting properties such as spin fluctuation and colossal magnetoresistance. Phonon excitation can contribute to the quasi-particle response of the topological matters through spin-lattice and electron-phonon coupling. However, the phonon properties of this material family remain unexplored. Here we report a comparative study of Raman-active vibration modes in EuCd₂X₂(X = P, As) by means of angle-resolved, temperature-resolved, and magnetic-field-resolved Raman spectroscopy together with the first-principle calculations and Raman tensor analysis. The phonon properties can be tuned by chemical potential and temperature within the material family. All the phonon modes are softened with increased chemical pressure by replacing P with As. Angle-resolved polarized Raman spectroscopy reveals the configuration-sensitive Raman activity and the isotropic intensity response. In addition, the magneto-Raman spectrum indicates the stability of Raman-active vibration modes against the magnetic field at room temperature. Our work sheds light on the phonon dynamics of magnetic topological matters, which are potentially coupled with the topological charge and spin excitation.

The Zintl phases compound AEIn2As2 (AE=Ca, Sr, Ba): topological phase transition under pressure

AEIn2As2 (AE=Ca, Sr, Ba), as a new crucial nonmagnetic thermoelectric candidate, is desired to be understood in terms of its potential physical properties and controversial structural phases in both experimental...

Visualizing band selective enhancement of quasiparticle lifetime in a metallic ferromagnet

Electrons navigate more easily in a background of ordered magnetic moments than around randomly oriented ones. This fundamental quantum mechanical principle is due to their Bloch wave nature and also underlies ballistic electronic motion in a perfect crystal. As a result, a paramagnetic metal that develops ferromagnetic order often experiences a sharp drop in the resistivity. Despite the universality of this phenomenon, a direct observation of the impact of ferromagnetic order on the electronic quasiparticles in a magnetic metal is still lacking. Here we demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Our detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications.

Magnetism-induced topological transition in EuAs3

The nature of the interaction between magnetism and topology in magnetic topological semimetals remains mysterious, but may be expected to lead to a variety of novel physics. We systematically studied the magnetic semimetal EuAs3, demonstrating a magnetism-induced topological transition from a topological nodal-line semimetal in the paramagnetic or the spin-polarized state to a topological massive Dirac metal in the antiferromagnetic ground state at low temperature. The topological nature in the antiferromagnetic state and the spin-polarized state has been verified by electrical transport measurements. An unsaturated and extremely large magnetoresistance of ~2 × 105% at 1.8 K and 28.3 T is observed. In the paramagnetic states, the topological nodal-line structure at the Y point is proven by angle-resolved photoemission spectroscopy. Moreover, a temperature-induced Lifshitz transition accompanied by the emergence of a new band below 3 K is revealed. These results indicate that magnetic EuAs3 provides a rich platform to explore exotic physics arising from the interaction of magnetism with topology.

High Carrier Mobility in a Layered Antiferromagnet Integrated with Silicon

Coupling various functional properties in one material is always a challenge, more so if the material should be nanostructured for practical applications. Magnetism and high carrier mobility are key components for spintronic applications but rather difficult to bundle together. Here, we establish EuAl₂Si₂ as a layered antiferromagnet supporting high carrier mobility. Its topotactic synthesis via a sacrificial two-dimensional template results in epitaxial nanoscale films on silicon. Their outstanding structural quality and atomically sharp interfaces are demonstrated by diffraction and microscopy techniques. EuAl₂Si₂ films exhibit extreme magnetoresistance and a carrier mobility of above 10,000 cm² V^-1 s^-1. The marriage of these properties and magnetism makes EuAl₂Si₂ a promising spintronic material. Importantly, the seamless integration of EuAl₂Si₂ with silicon technology is particularly appealing for applications.

Electric Field Control of the Magnetic Weyl Fermion in an Epitaxial SrRuO3 (111) Thin Film

The magnetic Weyl fermion originates from the time reversal symmetry (TRS)-breaking in magnetic crystalline structures, where the topology and magnetism entangle with each other. Therefore, the magnetic Weyl fermion is expected to be effectively tuned by the magnetic field and electrical field, which holds promise for future topologically protected electronics. However, the electrical field control of the magnetic Weyl fermion has rarely been reported, which is prevented by the limited number of identified magnetic Weyl solids. Here, the electric field control of the magnetic Weyl fermion is demonstrated in an epitaxial SrRuO₃ (111) thin film. The magnetic Weyl fermion in the SrRuO₃ films is indicated by the chiral anomaly induced magnetotransport, and is verified by the observed Weyl nodes in the electronic structures characterized by the angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Through the ionic-liquid gating experiment, the effective manipulation of the Weyl fermion by electric field is demonstrated, in terms of the sign-change of the ordinary Hall effect, the nonmonotonic tuning of the anomalous Hall effect, and the observation of the linear magnetoresistance under proper gating voltages. The work may stimulate the searching and tuning of Weyl fermions in other magnetic materials, which are promising in energy-efficient electronics.

Observation of a singular Weyl point surrounded by charged nodal walls in PtGa

Constrained by the Nielsen-Ninomiya no-go theorem, in all so-far experimentally determined Weyl semimetals (WSMs) the Weyl points (WPs) always appear in pairs in the momentum space with no exception. As a consequence, Fermi arcs occur on surfaces which connect the projections of the WPs with opposite chiral charges. However, this situation can be circumvented in the case of unpaired WP, without relevant surface Fermi arc connecting its surface projection, appearing singularly, while its Berry curvature field is absorbed by nontrivial charged nodal walls. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, we show experimentally that a singular Weyl point emerges in PtGa at the center of the Brillouin zone (BZ), which is surrounded by closed Weyl nodal walls located at the BZ boundaries and there is no Fermi arc connecting its surface projection. Our results reveal that nontrivial band crossings of different dimensionalities can emerge concomitantly in condensed matter, while their coexistence ensures the net topological charge of different dimensional topological objects to be zero. Our observation extends the applicable range of the original Nielsen-Ninomiya no-go theorem which was derived from zero dimensional paired WPs with opposite chirality.

Electronic State Unfolding for Plane Waves: Energy Bands, Fermi Surfaces, and Spectral Functions

Present day computing facilities allow for first-principles density functional theory studies of complex physical and chemical phenomena. Often such calculations are linked to large supercells to adequately model the desired property. However, supercells are associated with small Brillouin zones in the reciprocal space, leading to folded electronic eigenstates that make the analysis and interpretation extremely challenging. Various techniques have been proposed and developed to reconstruct the electronic band structures of super cells unfolded into the reciprocal space of an ideal primitive cell. Here we propose an unfolding scheme embedded directly in the Vienna Ab initio Simulation Package (VASP) that requires modest computational resources and allows for an automatized mapping from the reciprocal space of the supercell to the primitive cell Brillouin zone. This algorithm can compute band structures, Fermi surfaces, and spectral functions by using an integrated postprocessing tool (bands4vasp). Here the method is applied to a selected variety of complex physical situations: the effect of doping on the band dispersion in the BaFe_2(1-x)Ru_2x As₂ superconductor, the interaction between adsorbates and polaronic states on the TiO₂(110) surface, and the band splitting induced by noncollinear spin fluctuations in EuCd₂As₂.

Realization of an ideal Weyl semimetal band in a quantum gas with 3D spin-orbit coupling

Weyl semimetals are three-dimensional (3D) gapless topological phases with Weyl cones in the bulk band. According to lattice theory, Weyl cones must come in pairs, with the minimum number of cones being two. A semimetal with only two Weyl cones is an ideal Weyl semimetal (IWSM). Here we report the experimental realization of an IWSM band by engineering 3D spin-orbit coupling for ultracold atoms. The topological Weyl points are clearly measured via the virtual slicing imaging technique in equilibrium and are further resolved in the quench dynamics. The realization of an IWSM band opens an avenue to investigate various exotic phenomena that are difficult to access in solids.

Colossal Magnetoresistance without Mixed Valence in a Layered Phosphide Crystal

Materials with strong magnetoresistive responses are the backbone of spintronic technology, magnetic sensors, and hard drives. Among them, manganese oxides with a mixed valence and a cubic perovskite structure stand out due to their colossal magnetoresistance (CMR). A double exchange interaction underlies the CMR in manganates, whereby charge transport is enhanced when the spins on neighboring Mn³⁺ and Mn⁴⁺ ions are parallel. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here an enormous CMR at low temperatures in EuCd₂ P₂ without manganese, oxygen, mixed valence, or cubic perovskite structure is shown. EuCd₂ P₂ has a layered trigonal lattice and exhibits antiferromagnetic ordering at 11 K. The magnitude of CMR (10⁴ %) in as-grown crystals of EuCd₂ P₂ rivals the magnitude in optimized thin films of manganates. The magnetization, transport, and synchrotron X-ray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics.

A magnetic topological insulator in two-dimensional EuCd2Bi2: giant gap with robust topology against magnetic transitions

Magnetic topological states open up exciting opportunities for exploring fundamental topological quantum physics and innovative design of topological spintronics devices. However, the nontrivial topologies, for most known magnetic topological states, are usually associated with and may be heavily deformed by fragile magnetism. Here, using a tight-binding model and first-principles calculations, we demonstrate that a highly robust magnetic topological insulator phase, which remains intact under both ferromagnetic and antiferromagnetic configurations, can emerge in two-dimensional EuCd₂Bi₂ quintuple layers. Because of spin-orbital coupling, an inverted gap with intrinsic band inversions occuring simultaneously for up and down spin channels is obtained, accompanied by a nonzero spin Chern number and a pair of gapless edge states, and remarkably the magnitude of the nontrivial band gap for EuCd₂Bi₂ reaches as much as 750 meV. Moreover, the robustness of the magnetic TI phase is further confirmed by rotating the magnetization directions, indicating that EuCd₂Bi₂ represents a promising material for understanding and utilizing the topological insulating states in two-dimensional spin-orbit magnets.

Unconventional Transverse Transport above and below the Magnetic Transition Temperature in Weyl Semimetal EuCd_{2}As_{2}

As exemplified by the growing interest in the quantum anomalous Hall effect, the research on topology as an organizing principle of quantum matter is greatly enriched from the interplay with magnetism. In this vein, we present a combined electrical and thermoelectrical transport study on the magnetic Weyl semimetal EuCd_{2}As_{2}. Unconventional contribution to the anomalous Hall and anomalous Nernst effects were observed both above and below the magnetic transition temperature of EuCd_{2}As_{2}, indicating the existence of significant Berry curvature. EuCd_{2}As_{2} represents a rare case in which this unconventional transverse transport emerges both above and below the magnetic transition temperature in the same material. The transport properties evolve with temperature and field in the antiferromagnetic phase in a different manner than in the paramagnetic phase, suggesting different mechanisms to their origin. Our results indicate EuCd_{2}As_{2} is a fertile playground for investigating the interplay between magnetism and topology, and potentially a plethora of topologically nontrivial phases rooted in this interplay.

The Zintl phases AIn2As2 (A = Ca, Sr, Ba): new topological insulators and thermoelectric material candidates

Recently, there has been a lot of interest in topological insulators (TIs), being electronic materials, which are insulating in their bulk but with the gapless exotic metallic state on their surface. The surface states observed in such materials behave as a perfect conductor thereby making them more suited for several cutting-edge technological applications such as spintronic devices. Here, we report the synthesis and structural characterization of the Zintl phases AIn2As2 (A = Ca, Sr, Ba), which could become a new class of TIs. Crystal structure elucidation by single-crystal X-ray diffraction reveals that CaIn2As2 and SrIn2As2 are isostructural and crystallize in the EuIn2P2 structure type (space group P63/mmc, no. 194, Z = 2) with unit cell parameters a = 4.1482(6) Å, c = 17.726(4) Å; and a = 4.2222(6) Å, c = 18.110(3) Å, respectively. Their hexagonal structure is made up of alternating [In2As2]2- layers separated by slabs of A2+ cations. BaIn2As2 on the other hand crystallizes in the monoclinic EuGa2P2 structure type (space group P2/m, no. 10, Z = 4) with unit cell parameters a = 10.2746(11) Å, b = 4.3005(5) Å, c = 13.3317(14) Å and β = 95.569(2)°. This structure is also layered, and it is made up of different type of polyanionic [In2As2]2- units and Ba2+ cations. The valence electron count for all three compounds adheres to the Zintl-Klemm formalism, and all elements achieve closed-shell electronic configurations. Bulk electronic structure calculations indicate the opening of a bandgap Eg ∼ 0.03 eV (CaIn2As2 and Sr2In2As2), and Eg ∼0.21 eV (BaIn2As2) in the absence of strain and spin-orbit coupling (SOC). These results argue in favor of the realization of a nontrivial topological insulator state under the influence of tensile strain and SOC. Preliminary transport properties on BaIn2As2 are suggestive of a degenerate p-type semiconductor-a behavior which is sought after in thermoelectric (TE) materials. Since both TIs and excellent TE materials are known to favor the same material properties such as narrow bandgap, heavy elements, and strong SOC, these three Zintl phases are also projected as candidates TE materials.

Persistent surface states with diminishing gap in MnBi2Te4/Bi2Te3 superlattice antiferromagnetic topological insulator

Magnetic topological quantum materials (TQMs) provide a fertile ground for the emergence of fascinating topological magneto-electric effects. Recently, the discovery of intrinsic antiferromagnetic (AFM) topological insulator MnBi₂Te₄ that could realize quantized anomalous Hall effect and axion insulator phase ignited intensive study on this family of TQM compounds. Here, we investigated the AFM compound MnBi₄Te₇ where Bi₂Te₃ and MnBi₂Te₄ layers alternate to form a superlattice. Using spatial- and angle-resolved photoemission spectroscopy, we identified ubiquitous (albeit termination dependent) topological electronic structures from both Bi₂Te₃ and MnBi₂Te₄ terminations. Unexpectedly, while the bulk bands show strong temperature dependence correlated with the AFM transition, the topological surface states with a diminishing gap show negligible temperature dependence across the AFM transition. Together with the results of its sister compound MnBi₂Te₄, we illustrate important aspects of electronic structures and the effect of magnetic ordering in this family of magnetic TQMs.

Chiral-Anomaly-Driven Casimir-Lifshitz Torque between Weyl Semimetals

We propose a new mechanism to generate the Casimir-Lifshitz torque between Weyl semimetals arising from the chiral anomaly. For short distances ranging from a nanometer to a few tens of nanometers, chiral anomaly is manifested via a Casimir-Lifshitz torque ∼sin(θ) with θ being the twisting angle. As the distance between Weyl semimetals increases from a submicrometer to a few micrometers, chiral-anomaly-driven Casimir-Lifshitz torque between Weyl semimetals is remarkably large, which is comparable with that of conventional birefringent materials.

Emergence of Nontrivial Low-Energy Dirac Fermions in Antiferromagnetic EuCd2 As2

Parity-time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd₂ As₂ . As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time-reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c-axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system.

Acoustogalvanic Effect in Dirac and Weyl Semimetals

The acoustogalvanic effect is proposed as a nonlinear mechanism to generate a direct electric current by passing acoustic waves in Dirac and Weyl semimetals. Unlike the standard acoustoelectric effect, which relies on the sound-induced deformation potential and the corresponding electric field, the acoustogalvanic one originates from the pseudoelectromagnetic fields, which are not subject to screening. The longitudinal acoustogalvanic current scales at least quadratically with the relaxation time, which is in contrast to the photogalvanic current where the scaling is linear. Because of the interplay of pseudoelectric and pseudomagnetic fields, the current could show a nontrivial dependence on the direction of sound wave propagation. Being within the experimental reach, the effect can be utilized to probe dynamical deformations and corresponding pseudoelectromagnetic fields, which are yet to be experimentally observed in Weyl and Dirac semimetals.