Room-temperature ferromagnetism in epitaxial bilayer FeSb/SrTiO3(001) terminated with a Kagome lattice

Two-dimensional (2D) magnets exhibit unique physical properties for potential applications in spintronics. To date, most 2D ferromagnets are obtained by mechanical exfoliation of bulk materials with van der Waals interlayer interactions, and the synthesis of single or few-layer 2D ferromagnets with strong interlayer coupling remains experimentally challenging. Here, we report the epitaxial growth of 2D non-van der Waals ferromagnetic bilayer FeSb on SrTiO3(001) substrates stabilized by strong coupling to the substrate, which exhibits in-plane magnetic anisotropy and a Curie temperature above 300 K. In-situ low-temperature scanning tunneling microscopy/spectroscopy and density-functional theory calculations further reveal that a Fe Kagome layer terminates the bilayer FeSb. Our results open a new avenue for further exploring emergent quantum phenomena from the interplay of ferromagnetism and topology for application in spintronics

Magnetocapacitance effect and magnetoelectric coupling in type-II multiferroic HoFeWO6

We have investigated the multiferroicity and magnetoelectric (ME) coupling in HoFeWO6. With a noncentrosymmetric polar structure (space group Pna21) at room temperature, this compound shows an onset of electric polarization with an antiferromagnetic ordering at the Néel temperature (TN ) of 17.8 K. The magnetic properties of the polycrystalline samples were studied by DC and AC magnetization and heat capacity measurements. The metamagnetic behavior at low temperatures was found to be directly related to the dielectric properties of the compound. In particular, field-dependent measurements of capacitance show a magnetocapacitance (MC) effect with double-hysteresis loop behavior in direct correspondence with the magnetization. Our x-ray diffraction results show the Pna21 structure down to 8 K and suggest the absence of a structural phase transition across TN . Soft x-ray absorption spectroscopy and soft x-ray magnetic circular dichroism (XMCD) measurements at the Fe L2,3 and Ho M4,5 edges revealed the oxidation state of Fe and Ho cations to be 3+. Fe L2,3 XMCD further shows that Fe3+ cations are antiferromagnetically ordered in a noncollinear fashion with spins arranged 90◦ with respect to each other. Our findings show that HoFeWO6 is a type-II multiferroic exhibiting a MC effect. The observed MC effect and the change in polarization by the magnetic field, as well as their direct correspondence with magnetization, further support the strong ME coupling in this compound.The work at University of Houston (UH) is supported by U. S. Air Force Office of Scientific Research Grants FA9550-15-1-0236 and FA9550-20-1-0068, the T. L. L. Temple Foundation, the John J. and Rebecca Moores Endowment, and the State of Texas through the Texas Center for Superconductivity at the University of Houston. The XRD patterns were collected at the National Synchrotron Radiation Research Center at Taiwan. The synchrotron XAS/XMCD experiments were performed at the BOREAS beamline of the ALBA Synchrotron Light Facility in collaboration with ALBA staff. Computational resources were provided by the Extreme Science and Engineering Discovery Environment (XSEDE) [55] supported by the National Science Foundation (ACI-1548562) and the National Energy Research Scientific Computing (NERSC) Center, a DOE Office of Science User Facility supported by the Office of Science, U. S. Department of Energy, under Contract No. DE-AC02-05CH11231. Additional support for this work was provided through resources of the uHPC cluster managed by UH and acquired through NSF Award 1531814. The authors acknowledge the use of the Maxwell/Opuntia/Sabine Cluster and the advanced support from the Research Computing Data Core at UH. The work at National Sun Yat-Sen University was partially supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST 109-2112-M-110-019.Peer reviewe

Scanning Tunneling Microscopy of Thin Films Grown by Molecular Beam Epitaxy to Study Topological Surface States in Silver Chalcogenides and Atomic and Electronic Properties of Intermetallic CaBi2

Silver chalcogenides have attracted significant interest as promising candidates for novel topological properties due to their highly anisotropic Dirac cones. We used our combined molecular beam epitaxy (MBE) and low temperature-scanning tunneling microscopy/spectroscopy (LT-STM/S) combined system to explore these binary topological insulators. Using LT-STM/S, we have characterized the atomic structure and electronic properties of Ag2Se thin films grown on SrTiO3 (STO)(001) substrates by MBE for the first time. Based on atomic-resolved STM images, a monoclinic structure is proposed for the MBE-grown Ag2Se films, which has not been clearly discussed in this system previously. Three different types of Ag2Se atomic terminations were observed on the surface stemming from different growth directions. STS analysis of these atomic terminations uncovered different features near the Fermi level, indicating constituent- and direction-dependent electronic properties. We also obtained epitaxial monoclinic Ag2Te(001) thin films grown on STO(001) substrates using MBE, which strongly supports the growth of new monoclinic Ag2Se(001) thin films on the STO(001) substrate. To test the unique features of topological surface states (TSSs), we conducted a LT-STM/S study on the surface states of Ag2Se thin films. On the selenium (Se)-terminated surfaces with different types of defect densities, evidence for TSSs was observed in the quasiparticle interference (QPI) patterns. By studying the voltage-dependent standing wave patterns, we determined the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. To further confirm the topological nature of monoclinic Ag2Se, density functional theory (DFT) calculations were also performed and compared to the experimental results. The existence of standing waves strongly supports the topological nature of surface states. Our findings provide a convenient method to produce the monoclinic Ag2Se thin film and lead to a deeper understanding of the topological nature of this compound. Separately, we systematically characterized the atomic structure and electronic properties of epitaxial CaBi2(010) thin films grown on (STO)(001) substrates by MBE. Intermetallic bismuth-based compounds have attracted great interest as promising candidates for novel topological superconductivity. Our findings on epitaxial CaBi2(010) thin films provide insight for deeper understanding of the physical properties of this two-dimensional layered material compound