A magnetic topological semimetal Sr1-yMn1-zSb2 (y, z < 0.1)

Synthesis, Crystal and Electronic Structures, and Magnetic and Electrical Transport Properties of Bismuthides NdZn0.6Bi2 and (La0.5RE0.5)Zn0.6Bi2 (RE = Pr or Nd)

Bismuth is a good constituent element for many quantum materials due to its large atomic number, 6s26p3 orbitals, and strong spin-orbital coupling. In this work, three new bismuthides, NdZn0.6Bi2, (La0.5Pr0.5)Zn0.6Bi2, and (La0.5Nd0.5)Zn0.6Bi2, were grown by a metal flux method, and their crystal structures were accurately determined by single-crystal X-ray diffraction. These new bismuthides belong to the RE-T-Pn2 (RE = La-Lu, T = Mn, Fe, Co, Ni, or Zn, and Pn = P, As, Sb, or Bi) family, are isostructural, and crystallize in the HfCuSi2 structure type. The bismuth elements have two possible oxidation states, Bi3- and Bi-, which were studied by X-ray photoelectron spectroscopy (XPS). Two binding energy peaks of 155.91 and 161.23 eV were observed for Bi atoms within NdZn0.6Bi2, and similar binding energy peaks were detected in NdBi and LiBi. XPS also confirmed the trivalent nature of Nd, which was further verified by magnetic measurements. Additionally, magnetic measurements revealed that NdZn0.6Bi2 exhibits an antiferromagnetic transition around 3 K, while the mixed-cation compounds do not show any magnetic transition down to 2 K. Electronic transport measurements reveal weak magnetoresistance in all three compounds, with a maximum value of ∼25% at 2 K and 9 T for (La0.5Nd0.5)Zn0.6Bi2.

Extremely Large Anomalous Hall Conductivity and Unusual Axial Diamagnetism in a Quasi-1D Dirac Material La3MgBi5

Anomalous Hall effect (AHE), one of the most important electronic transport phenomena, generally appears in ferromagnetic materials but is rare in materials without magnetic elements. Here, a study of La3MgBi5 is presented, whose band structure carries multitype Dirac fermions. Although magnetic elements are absent in La3MgBi5, the signals of AHE can be observed. In particular, the anomalous Hall conductivity is extremely large, reaching 42,356 Ω-1 cm-1 with an anomalous Hall angle of 8.8%, the largest one that has been observed in the current AHE systems. The AHE is suggested to originate from the combination of skew scattering and Berry curvature. Another unique property discovered in La3MgBi5 is the axial diamagnetism. The diamagnetism is significantly enhanced and dominates the magnetization in the axial directions, which is the result of the restricted motion of the Dirac fermion at the Fermi level. These findings not only establish La3MgBi5 as a suitable platform to study AHE and quantum transport but also indicate the great potential of 315-type Bi-based materials for exploring novel physical properties.

Layer-resolved electronic behavior in a Kondo lattice system, CeAgAs2

We investigate the electronic structure of an antiferromagnetic Kondo lattice system CeAgAs₂employing hardx-ray photoemission spectroscopy. CeAgAs₂, an orthorhombic variant of HfCuSi₂structure, exhibits antiferromagnetic ground state, Kondo like resistivity upturn and compensation of magnetic moments at low temperatures. The photoemission spectra obtained at different photon energies suggest termination of the cleaved surface at cis-trans-As layers. The depth-resolved data show significant surface-bulk differences in the As and Ce core level spectra. The As 2pbulk spectrum shows distinct two peaks corresponding to two different As layers. The peak at higher binding energy correspond to cis-trans-As layers and is weakly hybridized with the adjacent Ce layers. The As layers between Ce and Ag-layers possess close to trivalent configuration due to strong hybridization with the neighboring atoms and the corresponding feature appear at lower binding energy. Ce 3dcore level spectra show multiple features reflecting strong Ce-As hybridization and strong correlation. Intensef₀peak is observed in the surface spectrum while it is insignificant in the bulk. In addition, we observe a features at binding energy lower than the well-screened feature indicating the presence of additional interactions. This feature becomes more intense in the bulk spectra suggesting it to be a bulk property. Increase in temperature leads to a spectral weight transfer to higher binding energies in the core level spectra and a depletion of spectral intensity at the Fermi level as expected in a Kondo material. These results reveal interesting surface-bulk differences, complex interplay of intra- and inter-layer covalency, and electron correlation in the electronic structure of this novel Kondo lattice system.

Strong room-temperature bulk nonlinear Hall effect in a spin-valley locked Dirac material

Nonlinear Hall effect (NLHE) is a new type of Hall effect with wide application prospects. Practical device applications require strong NLHE at room temperature (RT). However, previously reported NLHEs are all low-temperature phenomena except for the surface NLHE of TaIrTe4. Bulk RT NLHE is highly desired due to its ability to generate large photocurrent. Here, we show the spin-valley locked Dirac state in BaMnSb2 can generate a strong bulk NLHE at RT. In the microscale devices, we observe the typical signature of an intrinsic NLHE, i.e. the transverse Hall voltage quadratically scales with the longitudinal current as the current is applied to the Berry curvature dipole direction. Furthermore, we also demonstrate our nonlinear Hall device's functionality in wireless microwave detection and frequency doubling. These findings broaden the coupled spin and valley physics from 2D systems into a 3D system and lay a foundation for exploring bulk NLHE's applications.

Superconductivity and improved electrical conduction in anti-ThCr2Si2-type RE 2O2Sb and RE 2O2Bi with pnictogen square net

Layered compounds have shown rich physical properties such as high-temperature superconductivity. Recently, monatomic honeycomb lattice systems, such as graphene, have been studied extensively, whereas monatomic pnictogen square nets in layered compounds also exhibit interesting electronic properties owing to their unusual negative valence states. Among them, anti-ThCr2Si2-type RE 2O2 Pn (RE = rare earth, Pn = Sb, Bi) with monoatomic Pn square nets were recently found to exhibit interesting electronic properties such as superconductivity and high carrier mobility. In this article, we review recent studies on crystal structures, electronic properties, and thin-film growth of RE 2O2 Pn.

Hidden Local Symmetry Breaking in a Kagome-Lattice Magnetic Weyl Semimetal

Exploring the relationship between intriguing physical properties and structural complexity is a central topic in studying modern functional materials. Co₃Sn₂S₂, a newly discovered kagome-lattice magnetic Weyl semimetal, has triggered intense interest owing to the intimate coupling between topological semimetallic states and peculiar magnetic properties. However, the origins of the magnetic phase separation and spin glass state below T_C in this ordered compound are two unresolved yet important puzzles in understanding its magnetism. Here, we report the discovery of local symmetry breaking surprisingly co-emerges with the onset of ferromagnetic order in Co₃Sn₂S₂, by a combined use of neutron total scattering and half-polarized neutron diffraction. An anisotropic distortion of the cobalt kagome lattice at the atomic/nano level is also found, with distinct distortion directions among the two Co1 and four Co2 atoms. The mismatch of local and average symmetries occurs below T_C, indicating that Co₃Sn₂S₂ evolves to an intrinsically lattice disordered system when the ferromagnetic order is established. The local symmetry breaking with intrinsic lattice disorder provides new understanding of the puzzling magnetic properties. Our density functional theory (DFT) calculation indicates that the local symmetry breaking is expected to reorient local ferromagnetic moments, unveiling the existence of the ferromagnetic instability associated with the lattice instability. Furthermore, DFT calculation unveils that the local symmetry breaking could affect the Weyl property by breaking the mirror plane. Our findings highlight the fundamentally important role that the local symmetry breaking plays in advancing our understanding on the magnetic and topological properties in Co₃Sn₂S₂, which may draw attention to explore the overlooked local symmetry breaking in Co₃Sn₂S₂, its derivatives and more broadly in other topological Dirac/Weyl semimetals and kagome-lattice magnets.

Giant Negative Magnetoresistance beyond Chiral Anomaly in Topological Material YCuAs2

The large negative magnetoresistance (MR) effect, which usually emerges in various magnetic systems, is a technologically important property for spintronics. Recently, the so-called "chiral anomaly" in topological semimetals offers an alternative to generate a considerable negative MR effect without utilizing magnetism. However, it requires that the applied magnetic field must be strictly along the electric current direction, which sets a strong limit for practical applications. Here, a giant negative MR effect is discovered with a value of up to -40% in 9 T at 2 K in the nonmagnetic Dirac material YCuAs₂ , which is not restricted to the specific configuration for applied magnetic fields. Based on magnetic susceptibility and NMR measurements, the giant negative MR effect is tightly connected with the unexpected spin-dependent scattering from vacancy-induced local moments, which is also beyond the classical Kondo effect. The present work not only offers an alternative route for spintronics based on nonmagnetic topological materials, but also helps to further understand the negative MR effect in topological materials.

Single crystal growth and physical properties of layered compound SrCdBi2

We have grown the high quality single crystals of SrCdBi2successfully and investigated the physical properties systematically through measurements of magnetoresistance (MR), Hall effect, magnetic susceptibility, and specific heat measurements. The compound is a nonmagnetic 112-type pnictide with a Bi square net layer, which is potential for hosting Dirac fermions. We found that it exhibited metallic behavior with an anomaly appearing at around 210 K. MR study reveal that the electronic structure of SrCdBi2is quasi-two-dimensional. At low temperatures, we observed magnetic field induced metal-to-insulator-like transition and resistivity plateau, nonsaturating quasilinear MR, and high carrier mobility in magnetotransport measurements, which indicate the possible existence of nearly massless Dirac fermions in SrCdBi2. The anomaly at around 210 K can be observed in resistivity, Hall effect, and magnetic susceptibility, but cannot be detected in heat capacity. This implies the anomaly might be caused by domain formation or disorder. We found that the nonsaturating linear MR in SrCdBi2is likely caused by both of the quantum linear dispersion and the classical disorder. Our findings suggest that SrCdBi2is a natural experimental platform for realizing the topological properties of nonmagnetic 112-type pnictides.

Effect of Cu Doping on Structure and Physical Properties in the Antiferromagnetic Dirac Semimetal CaMnBi2

The newly discovered AMnBi₂ (A = Ca, Sr, Ba, Eu, and Yb) materials composed of two-dimensional Bi square nets provide an excellent platform to investigate the effect of magnetism on topological band structures. Effectively tuning the magnetic interaction in AMnBi₂ is of great importance to advance this issue. Here, we describe an effective route to tune the magnetism in Dirac semimetal CaMnBi₂ through Cu doping. Structural analysis on CaMn_1-xCu_xBi₂ single crystals indicates that Cu atoms occupy the Mn sites randomly, with the maximum doping level of 25%. After Cu doping, the Bi square net in charge of the Dirac band is still retained, but the Bi-Bi bond distance is markedly shortened. The antiferromagnetic interaction of CaMnBi₂ is strongly weakened in the Cu-doped crystals, with the transition temperature decreased from 260 to 85 K. On the contrary, the ferromagnetic component that originated from the canted AFM is enhanced, suggesting that the spin canting in this system is tunable. In addition, the magnetoresistance is decreased upon Cu doping, probably due to the disorder in structure. Our work suggests that the CaMn_1-xCu_xBi₂ (0 ≤ x ≤ 0.25) system can offer a suitable playground to address the interplay between magnetism and the topological state.

Giant anomalous Nernst signal in the antiferromagnet YbMnBi2

A large anomalous Nernst effect (ANE) is crucial for thermoelectric energy conversion applications because the associated unique transverse geometry facilitates module fabrication. Topological ferromagnets with large Berry curvatures show large ANEs; however, they face drawbacks such as strong magnetic disturbances and low mobility due to high magnetization. Herein, we demonstrate that YbMnBi2, a canted antiferromagnet, has a large ANE conductivity of ~10 A m-1 K-1 that surpasses large values observed in other ferromagnets (3-5 A m-1 K-1). The canted spin structure of Mn guarantees a non-zero Berry curvature, but generates only a weak magnetization three orders of magnitude lower than that of general ferromagnets. The heavy Bi with a large spin-orbit coupling enables a large ANE and low thermal conductivity, whereas its highly dispersive px/y orbitals ensure low resistivity. The high anomalous transverse thermoelectric performance and extremely small magnetization make YbMnBi2 an excellent candidate for transverse thermoelectrics.

The High Energy Density Scientific Instrument at the European XFEL

The European XFEL delivers up to 27000 intense (>1012 photons) pulses per second, of ultrashort (≤50 fs) and transversely coherent X-ray radiation, at a maximum repetition rate of 4.5 MHz. Its unique X-ray beam parameters enable groundbreaking experiments in matter at extreme conditions at the High Energy Density (HED) scientific instrument. The performance of the HED instrument during its first two years of operation, its scientific remit, as well as ongoing installations towards full operation are presented. Scientific goals of HED include the investigation of extreme states of matter created by intense laser pulses, diamond anvil cells, or pulsed magnets, and ultrafast X-ray methods that allow their diagnosis using self-amplified spontaneous emission between 5 and 25 keV, coupled with X-ray monochromators and optional seeded beam operation. The HED instrument provides two target chambers, X-ray spectrometers for emission and scattering, X-ray detectors, and a timing tool to correct for residual timing jitter between laser and X-ray pulses.

Thermoelectric Properties of Novel Semimetals: A Case Study of YbMnSb2

The emerging class of topological materials provides a platform to engineer exotic electronic structures for a variety of applications. As complex band structures and Fermi surfaces can directly benefit thermoelectric performance it is important to identify the role of featured topological bands in thermoelectrics particularly when there are coexisting classic regular bands. In this work, the contribution of Dirac bands to thermoelectric performance and their ability to concurrently achieve large thermopower and low resistivity in novel semimetals is investigated. By examining the YbMnSb₂ nodal line semimetal as an example, the Dirac bands appear to provide a low resistivity along the direction in which they are highly dispersive. Moreover, because of the regular-band-provided density of states, a large Seebeck coefficient over 160 µV K^-1 at 300 K is achieved in both directions, which is very high for a semimetal with high carrier concentration. The combined highly dispersive Dirac and regular bands lead to ten times increase in power factor, reaching a value of 2.1 mW m^-1 K^-2 at 300 K. The present work highlights the potential of such novel semimetals for unusual electronic transport properties and guides strategies towards high thermoelectric performance.

Enhanced Mobility and Large Linear Nonsaturating Magnetoresistance in the Magnetically Ordered States of TmNiC_{2}

We have studied the magnetic, magnetotransport, and galvanomagnetic properties of TmNiC_{2}. We find that the antiferromagnetic and field induced metamagnetic and ferromagnetic orderings do not suppress the charge density wave. The persistence of Fermi surface pockets, open as a result of imperfect nesting accompanying the Peierls transition, results in an electronic carriers mobility of the order of 4×10^{3}  cm^{2} V^{-1} s^{-1} in ferromagnetic state, without any signatures for a significant deterioration of nesting properties. This is independently evidenced by high, nonsaturating linear magnetoresistance reaching 440% at T=2  K and an analysis of the Hall conductivity. We thus demonstrate that, the coexistence of charge density wave and magnetism provides an alternative route to maintain high electronic mobility in the magnetically ordered state.

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.

Spin-Canting-Induced Band Reconstruction in the Dirac Material Ca_{1-x}Na_{x}MnBi_{2}

The ternary AMnBi_{2} (A is alkaline as well as rare-earth atom) materials provide an arena for investigating the interplay between low-dimensional magnetism of the antiferromagnetic MnBi layers and the electronic states in the intercalated Bi layers, which harbor relativistic fermions. Here, we report on a comprehensive study of the optical properties and magnetic torque response of Ca_{1-x}Na_{x}MnBi_{2}. Our findings give evidence for a spin canting occurring at T_{s}∼50-100  K. With the support of first-principles calculations we establish a direct link between the spin canting and the reconstruction of the electronic band structure, having immediate implications for the spectral weight reshuffling in the optical response, signaling a partial gapping of the Fermi surface, and the dc transport properties below T_{s}.

High mobility in a van der Waals layered antiferromagnetic metal

Van der Waals (vdW) materials with magnetic order have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far, they are mainly insulating or semiconducting, and none of them has a high electronic mobility-a property that is rare in layered vdW materials in general. The realization of a high-mobility vdW material that also exhibits magnetic order would open the possibility for novel magnetic twistronic or spintronic devices. Here, we report very high carrier mobility in the layered vdW antiferromagnet GdTe₃. The electron mobility is beyond 60,000 cm² V^-1 s^-1, which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe₃ is comparable to that of black phosphorus. By mechanical exfoliation, we further demonstrate that GdTe₃ can be exfoliated to ultrathin flakes of three monolayers.

Strong and Tunable Electrical Anisotropy in Type-II Weyl Semimetal Candidate WP2 with Broken Inversion Symmetry

A transition metal diphosphide, WP₂ , is a candidate for type-II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP₂ , spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP₂ electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle-resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first-principle studies of WP₂ are reported. The energies of the observed Raman-active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first-principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP₂ . Moreover, the measured ratio (R_c /R_a ) between the crystalline c-axis and a-axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in-plane electrical resistivity anisotropy among the reported type-II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first-principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the R_c /R_a of WP₂ at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so-far-strongest and magnetic-field-tunable electrical resistivity anisotropy found in WP₂ can serve as a degree of freedom for tuning the electrical properties of type-II WSMs, which paves the way for the development of novel electronic applications based on type-II WSMs.

Singular angular magnetoresistance in a magnetic nodal semimetal

Transport coefficients of correlated electron systems are often useful for mapping hidden phases with distinct symmetries. Here we report a transport signature of spontaneous symmetry breaking in the magnetic Weyl semimetal cerium-aluminum-germanium (CeAlGe) system in the form of singular angular magnetoresistance (SAMR). This angular response exceeding 1000% per radian is confined along the high-symmetry axes with a full width at half maximum reaching less than 1° and is tunable via isoelectronic partial substitution of silicon for germanium. The SAMR phenomena is explained theoretically as a consequence of controllable high-resistance domain walls, arising from the breaking of magnetic point group symmetry strongly coupled to a nearly nodal electronic structure. This study indicates ingredients for engineering magnetic materials with high angular sensitivity by lattice and site symmetries.

Large magnetoresistance and large magnetothermopower effect in the Dirac material EuMn0.8Sb2

In this article, the structure, transport and magnetic properties were studied in details for EuMn_0.8Sb₂ crystals with the orthorhombic structure and Mn deficiencies. It was found that the temperature dependence of the resistivity exhibits a metallic behavior in the whole measuring temperature range, different from that in the crystals without Mn deficiencies. A large positive magnetoresistance (MR) (∼127% at 2 K and  ∼25% at 300 K, in 9 T field) was observed, which can be ascribed to the combination of semiclassical MR and quantum limit MR of Dirac electrons. We also observed the high mobility of the carriers and large magnetothermopower effect at low temperatures, and two magnetic transitions emerging at  ∼24 K and  ∼10 K, respectively, corresponding to the antiferromagnetic ordering and canted arrangement of the Eu moments. Our findings shed new light on the intrinsic properties of EuMn_0.8Sb₂ and demonstrate the existence of Dirac fermions.

High Hole Mobility and Nonsaturating Giant Magnetoresistance in the New 2D Metal NaCu4Se4 Synthesized by a Unique Pathway

The new compound NaCu₄Se₄ forms by the reaction of CuO and Cu in a molten sodium polyselenide flux, with the existence of CuO being unexpectedly critical to its synthesis. It adopts a layered hexagonal structure (space group P6₃/ mmc with cell parameters a = 3.9931(6) Å and c = 25.167(5) Å), consisting of infinite two-dimensional [Cu₄Se₄]^- slabs separated by Na⁺ cations. X-ray photoelectron spectroscopy suggests that NaCu₄Se₄ is mixed-valent with the formula (Na⁺)(Cu⁺)₄(Se^2-)(Se^-)(Se₂)^2-. NaCu₄Se₄ is a p-type metal with a carrier density of ∼10²¹ cm^-3 and a high hole mobility of ∼808 cm² V^-1 s^-1 at 2 K based on electronic transport measurements. First-principles calculations suggest the density of states around the Fermi level are composed of Cu-d and Se-p orbitals. At 2 K, a very large transverse magnetoresistance of ∼1400% was observed, with a nonsaturating, linear dependence on field up to 9 T. Our results indicate that the use of metal oxide chemical precursors can open reaction paths to new low-dimensional compounds.

Chiral Landau levels in Weyl semimetal NbAs with multiple topological carriers

Recently, Weyl semimetals have been experimentally discovered in both inversion-symmetry-breaking and time-reversal-symmetry-breaking crystals. The non-trivial topology in Weyl semimetals can manifest itself with exotic phenomena, which have been extensively investigated by photoemission and transport measurements. Despite the numerous experimental efforts on Fermi arcs and chiral anomaly, the existence of unconventional zeroth Landau levels, as a unique hallmark of Weyl fermions, which is highly related to chiral anomaly, remains elusive owing to the stringent experimental requirements. Here, we report the magneto-optical study of Landau quantization in Weyl semimetal NbAs. High magnetic fields drive the system toward the quantum limit, which leads to the observation of zeroth chiral Landau levels in two inequivalent Weyl nodes. As compared to other Landau levels, the zeroth chiral Landau level exhibits a distinct linear dispersion in magnetic field direction and allows the optical transitions without the limitation of zero z momentum or [Formula: see text] magnetic field evolution. The magnetic field dependence of the zeroth Landau levels further verifies the predicted particle-hole asymmetry of the Weyl cones. Meanwhile, the optical transitions from the normal Landau levels exhibit the coexistence of multiple carriers including an unexpected massive Dirac fermion, pointing to a more complex topological nature in inversion-symmetry-breaking Weyl semimetals. Our results provide insights into the Landau quantization of Weyl fermions and demonstrate an effective tool for studying complex topological systems.

Casting a Wider Net: Rational Synthesis Design of Low-Dimensional Bulk Materials

The discovery of novel magnetic and electronic properties in low-dimensional materials has led to the pursuit of hierarchical materials with specific substructures. Low-dimensional solids are highly anisotropic by nature and show promise in new quantum materials leading to exotic physical properties not realized in three-dimensional materials. We have the opportunity to extend our synthetic strategy of the flux-growth method to designing single crystalline low-dimensional materials in bulk. The goal of this Account is to highlight the synthesis and physical properties of several low-dimensional intermetallic compounds containing specific structural motifs that are linked to desirable magnetic and electrical properties. We turned our efforts toward intermetallic compounds consisting of antimony nets because they are closely linked to properties such as high carrier mobility (the velocity of an electron moving through a material under a magnetic field) and large magnetoresistance (the change in resistivity with an applied magnetic field), both of which are desirable properties for technological applications. The SmSb₂ structure type is of particular interest because it is comprised of rectangular antimony nets and rare earth ions stacked between the antimony nets in a square antiprismatic environment. LnSb₂ (Ln = La-Nd, Sm) have been shown to be highly anisotropic with SmSb₂ exhibiting magnetoresistance of over 50000% for H∥c axis and ∼2400% for H∥ab. Using this structure type as an initial building block, we envision the insertion of transition metal substructures into the SmSb₂ structure type to produce ternary materials. We describe compounds adopting the HfCuSi₂ structure type as an insertion of a tetrahedral transition metal-antimony subunit into the LnSb₂ host structure. We studied LnNi_1-xSb₂ (Ln = Y, Gd-Er), where positive magnetoresistance reaching above 100% was found for the Y, Gd, and Ho analogues. We investigated the influence of the transition metal sublattice by substituting Ni into Ce(Cu_1-xNi_x)_ySb₂ (y < 0.8) and found that the material is highly anisotropic and metamagnetic transitions appear at ∼0.5 and 1 T in compounds with higher Ni concentration. Metamagnetism is characterized by a sharp increase in the magnetic response of a material with increasing applied magnetic field, which was also observed in LnSb₂ (Ln = Ce-Nd). We also endeavored to study materials that possess a transition metal sublattice with the potential for geometric frustration. An example is the La₂Fe₄Sb₅ structure type, which consists of antimony square nets and an iron-based network arranged in nearly equilateral triangles, a feature found in magnetically frustrated systems. We discovered spin glass behavior in Ln₂Fe₄Sb₅ (Ln = La-Nd, Sm) and evidence that the transition metal sublattice contributes to the magnetic interactions of Ln₂Fe₄Sb₅. We investigated the magnetic properties of Pr₂Fe_4-xCo_xSb₅ (x < 2.3) and found that as the Co concentration increases, a second magnetic transition leads from a localized to an itinerant system. The La₂Fe₄Sb₅ structure type is quite robust and allows for the incorporation of other transition metals, thereby making it an excellent candidate to study competing magnetic interactions in lanthanide-containing intermetallic compounds. In this manuscript, we aim to share our experiences of bulk intermetallic compounds to inspire the development of new low-dimensional materials.

Fe-Cluster Compounds of Chalcogenides: Candidates for Rare-Earth-Free Permanent Magnet and Magnetic Nodal-Line Topological Material

Fe-cluster-based crystal structures are predicted for chalcogenides Fe₃X₄ (X = S, Se, Te) using an adaptive genetic algorithm. Topologically different from the well-studied layered structures of iron chalcogenides, the newly predicted structures consist of Fe clusters that are either separated by the chalcogen atoms or connected via sharing of the vertex Fe atoms. Using first-principles calculations, we demonstrate that these structures have competitive or even lower formation energies than the experimentally synthesized Fe₃X₄ compounds and exhibit interesting magnetic and electronic properties. In particular, we show that Fe₃Te₄ can be a good candidate as a rare-earth-free permanent magnet and Fe₃S₄ can be a magnetic nodal-line topological material.

Unusual interlayer quantum transport behavior caused by the zeroth Landau level in YbMnBi2

Relativistic fermions in topological quantum materials are characterized by linear energy-momentum dispersion near band crossing points. Under magnetic fields, relativistic fermions acquire Berry phase of π in cyclotron motion, leading to a zeroth Landau level (LL) at the crossing point, a signature unique to relativistic fermions. Here we report the unusual interlayer quantum transport behavior resulting from the zeroth LL mode observed in the time reversal symmetry breaking type II Weyl semimetal YbMnBi2. The interlayer magnetoresistivity and Hall conductivity of this material are found to exhibit surprising angular dependences under high fields, which can be well fitted by a model, which considers the interlayer quantum tunneling transport of the zeroth LL's Weyl fermions. Our results shed light on the unusual role of zeroth LLl mode in transport.The transport behavior of the carriers residing in the lowest Landau level is hard to observe in most topological materials. Here, Liu et al. report a surprising angular dependence of the interlayer magnetoresistivity and Hall conductivity arising from the lowest Landau level under high magnetic field in type II Weyl semimetal YbMnBi2.

Nearly massless Dirac fermions hosted by Sb square net in BaMnSb2

Layered compounds AMnBi2 (A = Ca, Sr, Ba, or rare earth element) have been established as Dirac materials. Dirac electrons generated by the two-dimensional (2D) Bi square net in these materials are normally massive due to the presence of a spin-orbital coupling (SOC) induced gap at Dirac nodes. Here we report that the Sb square net in an isostructural compound BaMnSb2 can host nearly massless Dirac fermions. We observed strong Shubnikov-de Haas (SdH) oscillations in this material. From the analyses of the SdH oscillations, we find key signatures of Dirac fermions, including light effective mass (~0.052m0; m0, mass of free electron), high quantum mobility (1280 cm(2)V(-1)S(-1)) and a π Berry phase accumulated along cyclotron orbit. Compared with AMnBi2, BaMnSb2 also exhibits much more significant quasi two-dimensional (2D) electronic structure, with the out-of-plane transport showing nonmetallic conduction below 120 K and the ratio of the out-of-plane and in-plane resistivity reaching ~670. Additionally, BaMnSb2 also exhibits a G-type antiferromagnetic order below 283 K. The combination of nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes BaMnSb2 an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.