Magnetic topological insulators provide an important materials platform to explore emergent quantum phenomena such as the quantized anomalous Hall (QAH) effect, Majorana modes and the axion insulator state, etc. Recently, MnBi2Te4 was discovered to be the first material realization of a van der Waals (vdW) antiferromagnetic topological insulator (TI). In the two-dimensional (2D) limit, at a record high temperature of 4.5 K, MnBi2Te4 manifests the QAH effect in the forced ferromagnetic state above 12 T. To realize the QAH effect at lower fields, it is essential to search for magnetic TIs with lower saturation fields. By realizing a bulk vdW material MnBi4Te7 with alternating [MnBi2Te4]and [Bi2Te3] layers, we suggest that it is ferromagnetic in plane but antiferromagnetic along the c axis with a small out-of-plane saturation field of ~ 0.22 T at 2 K. Our angle-resolved photoemission spectroscopy and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Therefore, MnBi4Te7 provides a new material platform to investigate emergent topological phenomena associated with the QAH effect at much lower magnetic fields in its 2D limit.

High spatial resolution magnetic imaging has driven important developments in fields ranging from materials science to biology. However, to uncover finer details approaching the nanoscale with greater sensitivity requires the development of a radically new sensor technology. The nitrogenvacancy (NV) defect in diamond has emerged as a promising candidate for such a sensor based on its atomic size and quantum-limited sensing capabilities afforded by long spin coherence times. Although the NV center has been successfully implemented as a nanoscale scanning magnetic probe at room temperature, it has remained an outstanding challenge to extend this capability to cryogenic temperatures, where many solid-state systems exhibit non-trivial magnetic order. Here we present NV magnetic imaging down to 6 K with 6 nm spatial resolution and 3 μT/√Hz field sensitivity, first benchmarking the technique with a magnetic hard disk sample, then utilizing the technique to image vortices in the iron pnictide superconductor BaFe2(As0.7P0.3)2 with Tc = 30 K. The expansion of NVbased magnetic imaging to cryogenic temperatures represents an important advance in state-of-theart magnetometry, which will enable future studies of heretofore inaccessible nanoscale magnetism in condensed matter systems. and Lorentz transmission electron microscopy (TEM) 7 , and reciprocal space techniques including neutron scattering 8 have been successfully utilized to study magnetism in these systems. However, each of these techniques has limitations that must be considered. In MFM, a ferromagnetic tip must be placed in close proximity to a sample, which can perturb the magnetic order that is being probed.Scanning SQUIDs typically require a probe temperature of 10 K or lower, and generally offer micron-size spatial resolution, although recent studies have enhanced the resolution to submicron scales 9 . Lorentz TEM can provide images with high spatial resolution and magnetic contrast, but requires very thin samples, typically less than 100 nm thick. Neutron scattering requires the growth of large, high purity single-crystal samples, and is an ensemble-averaged measurement. There is therefore a significant opportunity to develop a real-space, non-invasive magnetic sensor capable of studying magnetic order at sub-10 nm spatial resolution and sub-T/Hz DC field sensitivities.The nitrogen vacancy (NV) defect center in diamond is an exceptionally versatile single spin system with unique quantum properties that have driven its application in diverse areas ranging from quantum information and photonics to quantum metrology [10][11][12][13][14][15][16][17][18][19] . Cryogenic scanning magnetometry stands out as potentially the most impactful application of NV centers, taking advantage of the exquisite magnetic field sensitivity and intrinsic atomic scale of the NV center for high resolution imaging 20 . The operation of an NV-based magnetic probe is dependent on a fundamentally different sensing principle than other imaging methods, namely the spin-dependent photolum...

Topological semimetals are characterized by protected crossings between conduction and valence bands. These materials have recently attracted significant interest because of the deep connections to high-energy physics, the novel topological surface states, and the unusual transport phenomena. While Dirac and Weyl semimetals have been extensively studied, the nodal-line semimetal remains largely unexplored due to the lack of an ideal material platform. In this paper, we report the magneto-transport properties of two nodal-line semimetal candidates CaAgAs and CaCdGe. First, our single crystalline CaAgAs supports the first "hydrogen atom" nodal-line semimetal, where only the topological nodal-line is present at the Fermi level. Second, our CaCdGe sample provides an ideal platform to perform comparative studies because it features the same topological nodal line but has a more complicated Fermiology with irrelevant Fermi pockets. As a result, the magnetoresistance of our CaCdGe sample is more than 100 times larger than that of CaAgAs. Through our systematic magneto-transport and first-principles band structure calculations, we show that our CaTX compounds can be used to study, isolate, and control the novel topological nodal-line physics in real materials.

We investigate systematically the bulk and surface electronic structure of the candidate nodal-line semimetal CaAgAs by angle-resolved photoemission spectroscopy and density functional calculations. We observed a metallic, linear, non-k z -dispersive surface band that coincides with the high-binding-energy part of the theoretical topological surface state, proving the topological nontriviality of the system. An overall downshift of the experimental Fermi level points to a rigid-band-like p doping of the samples, due possibly to Ag vacancies in the as-grown crystals. DOI: 10.1103/PhysRevB.96.161112 The discovery of topologically nontrivial systems has dominated the field of condensed matter physics over the past decade. Unique nontrivial topological properties in these fermionic systems relate concepts from high-energy physics to various quasiparticle excitations in condensed matter, causing the systems to resist small perturbations due to the protection by particular topological invariants [1,2]. In the so-called topological semimetals, such nontrivialities appear as the touching of valence and conduction bands at isolated points, closed lines, or planes in momentum space. Such materials exhibit chiral anomaly [3,4], topological surface Fermi arcs [5][6][7][8][9][10][11][12][13], and/or superconducting zero modes [14][15][16], whose quasiparticle excitations correspond directly to the Dirac, Weyl, and Majorana fermions. These quasiparticles differ from the actual entities in high-energy physics, but obey the same underlying principles in quantum field theory, offering the opportunity to investigate the fundamental physical laws that govern a large subset of quantum condensed matter as well as creating a new approach for developing a broad range of low-power, high-efficiency spintronic and quantum computing devices [1,2,17].Topological nodal-line semimetals (NLSMs) exhibit novel topological properties that are manifested by surface states in the form of a drumlike membrane living in the continuous toroidal isosurface gap in three-dimensional (3D) momentum space. With strong spin-orbit coupling (SOC), the nodal lines in these materials are either protected by reflection or mirror symmetry, or gapped out due to the lack of such symmetries [18][19][20][21][22] realizing a clean, "hydrogen-atom-like" NLSM is therefore of urgent need. Single-crystalline CaAgAs is theoretically predicted to be among the best candidate NLSMs since its theoretical Fermi surface contains no more than a circular nodal contour linked by the topological surface state [36], which gives rise to the ultralow magnetoresistance found by transport measurements [37]. In this Rapid Communication, we systematically investigate the NLSM state of CaAgAs which is solely protected by mirror reflection symmetry through a comparison between angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations. Our DFT calculations show that without SOC, a topological nodal ring enclosing appears in the first Bri...