Synthesis and Magnetic Properties of the Multiferroic [C(NH2)3]Cr(HCOO)3 Metal-Organic Framework: The Role of Spin-Orbit Coupling and Jahn-Teller Distortions.

We report for the first time the synthesis of [C(NH2)3]Cr(HCOO)3 stabilizing Cr2+ in formate perovskite, which adopts a polar structure and orders magnetically below 8 K. We discuss in detail the magnetic properties and their coupling to the crystal structure based on first-principles calculations, symmetry, and model Hamiltonian analysis. We establish a general model for the orbital magnetic moment of [C(NH2)3]M(HCOO)3 (M = Cr, Cu) based on perturbation theory, revealing the key role of the Jahn-Teller distortions. We also analyze their spin and orbital textures in k-space, which show unique characteristics.

Growth of bilayer MoTe2 single crystals with strong non-linear Hall effect.

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T' MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer Td phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer Td MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.

Electronic structure and magnetic properties of transition metal kagome metal-organic frameworks.

Kagome metal-organic frameworks (MOFs) are considered a new class of materials that can host two-dimensional (2D) magnetism and correlated electron phenomena such as superconductivity and quantum anomalous Hall effect. Despite its potential for spintronics applications and others, the systematic understanding between the electronic structure and magnetic properties of kagome MOFs is still missing. This work determines the crystal structure, magnetic ground states, and anisotropy of a series of transition metal atoms and ligands from first-principles calculations. We reveal that the coexistence of covalent and ionic bonding characters of 3d orbitals is a distinctive feature of the 2D kagome MOFs. Furthermore, we demonstrate that the occupancies of active bands near the Fermi level are responsible for different superexchange mechanisms: the partially filled bands with empty for V and Co MOFs lead to antiferromagnetic ordering, and the partially filled bands with full for Mn, Fe, and Co MOFs lead to ferromagnetic ordering between transition metal ions. It is pointed out that the bands are formed through dpπ-hybridization between the transition metal dyz, dzx and ligand pz orbitals in the square planar coordination of metal atoms. This mechanism provides valuable insights into understanding magnetism in 2D kagome MOFs.

Chern insulator with a nearly flat band in the metal-organic-framework-based Kagome lattice.

Based on first-principles density-functional theory (DFT) calculations, we report that the transition-metal bis-dithiolene, M3C12S12 (M = Mn and Fe), complexes can be a two-dimensional (2D) ferromagnetic insulator with nontrivial Chern number. Among various synthetic pathways leading to metal bis-dithiolenes, the simplest choice of ligand, Benzene-hexathiol, connecting metal cations to form a Kagome lattice is studied following the experimental report of time-reversal symmetric isostructural compound Ni3C12S12. We show sulfur and carbon-based ligands play the key role in making the complexes topologically nontrivial. An unusual topological quantum phase transition induced by the on-site Coulomb interaction brings a nearly flat band with a nonzero Chern number as the highest occupied band. With this analysis we explain the electronic structure of the class M3C12S12 and predict the existence of nearly flat band with nonzero Chern number and it can be a fractional Chern insulator candidate with carrier doping.

A Room-Temperature Ferroelectric Ferromagnet in a 1D Tetrahedral Chain Network.

Ferroelectricity occurs in crystals with broken spatial inversion symmetry. In conventional perovskite oxides, concerted ionic displacements within a 3D network of transition-metal-oxygen polyhedra (MOx ) manifest spontaneous polarization. Meanwhile, some 2D networks of MOx foster geometric ferroelectricity with magnetism, owing to the distortion of the polyhedra. Because of the fundamentally different mechanism of ferroelectricity in a 2D network, one can further challenge an uncharted mechanism of ferroelectricity in a 1D channel of MOx and estimate its feasibility. Here, ferroelectricity and coupled ferromagnetism in a 1D FeO4 tetrahedral chain network of a brownmillerite SrFeO2.5 epitaxial thin film are presented. The result provides a new paradigm for designing low-dimensional MOx networks, which is expected to benefit the realization of macroscopic ferro-ordering materials including ferroelectric ferromagnets.

Graphene analogue in (111)-oriented BaBiO3 bilayer heterostructures for topological electronics.

Topological electronics is a new field that uses topological charges as current-carrying degrees of freedom. For topological electronics applications, systems should host topologically distinct phases to control the topological domain boundary through which the topological charges can flow. Due to their multiple Dirac cones and the π-Berry phase of each Dirac cone, graphene-like electronic structures constitute an ideal platform for topological electronics; graphene can provide various topological phases when incorporated with large spin-orbit coupling and mass-gap tunability via symmetry-breaking. Here, we propose that a (111)-oriented BaBiO3 bilayer (BBL) sandwiched between large-gap perovskite oxides is a promising candidate for topological electronics by realizing a gap-tunable, and consequently a topology-tunable, graphene analogue. Depending on how neighboring perovskite spacers are chosen, the inversion symmetry of the BBL heterostructure can be either conserved or broken, leading to the quantum spin Hall (QSH) and quantum valley Hall (QVH) phases, respectively. BBL sandwiched by ferroelectric compounds enables switching of the QSH and QVH phases and generates the topological domain boundary. Given the abundant order parameters of the sandwiching oxides, the BBL can serve as versatile topological building blocks in oxide heterostructures.

Graphene as a flexible template for controlling magnetic interactions between metal atoms.

Metal-doped graphene produces magnetic moments that have potential application in spintronics. Here we use density function theory computational methods to show how the magnetic interaction between metal atoms doped in graphene can be controlled by the degree of flexure in a graphene membrane. Bending graphene by flexing causes the distance between two substitutional Fe atoms covalently bonded in graphene to gradually increase and these results in the magnetic moment disappearing at a critical strain value. At the critical strain, a carbon atom can enter between the two Fe atoms and blocks the interaction between relevant orbitals of Fe atoms to quench the magnetic moment. The control of interactions between doped atoms by exploiting the mechanical flexibility of graphene is a unique approach to manipulating the magnetic properties and opens up new opportunities for mechanical-magnetic 2D device systems.

Charge and magnetic states of rutile TiO2 doped with Cr ions.

We observe that the electronic and magnetic properties of Cr-doped rutile TiO2 single crystals are highly dependent on growth conditions. The ferromagnetic component of magnetic susceptibility is observed to be enhanced for samples grown under oxygen-rich conditions. To understand the charge state of Cr dopants and their role in response to an external magnetic field, we carry out density functional theory calculations for Cr-doped rutile TiO2. Using the results of formation energy calculations in the presence of oxygen vacancies and Cr atom substitution at the Ti sites, we demonstrate that the Cr3+ state is a source of Curie-Weiss-type magnetic response, whereas the Cr4+ defect states contribute to the ferromagnetic component. We also provide the electronic structures of various defect configurations and attempt to explain the optical and electronic properties of the Cr-doped system.

Large in-plane deformation of RuO6 octahedron and ferromagnetism of bulk SrRuO3.

SrRuO3 is a ferromagnetic metal with several unusual physical properties such as zero thermal expansion below Tc, so-called Invar behavior. Another anomalous feature is that the a-axis lattice constant is larger than the b-axis lattice constant, a clear deviation from the predictions of the Glazer structural description with rigid RuO6 octahedron motion. Using high resolution neutron diffraction techniques, we show how these two structural anomalies arise from the irregular in-plane deformation, i.e. plastic behavior of the RuO6 octahedron, a weak band Jahn-Teller distortion. We further demonstrate that the ferromagnetic instability of SrRuO3 is related to the temperature-induced localization of Ru 4d bands.

Modulation of electron carrier density at the n-type LaAlO3/SrTiO3 interface by water adsorption.

We investigate the energetic stability and dissociation dynamics of water adsorption at the LaAlO3 surface of the n-type LaAlO3/SrTiO3 (LAO/STO) interface and its effect on the electronic properties of the interface by carrying out first-principles electronic structure calculations. In an ambient atmosphere at room temperature the configuration of 1 monolayer (ML) of water molecules including 3/4 ML of dissociated water molecules adsorbed at the surface is found to be most stable, whereas the configuration of 1 ML of dissociated water molecules is metastable. Water molecule dissociation induces an up-shift of the valence band maximum (VBM) of the LAO surface, reducing the gap between the VBM of the LAO surface and the conduction band minimum of the STO. For the LAO/STO interface with three LAO unit-cell layers, once the coverage of dissociated water molecules reaches 1/2 ML the gap is closed, the interface becomes metallic and the carrier density at the LAO/STO interface increases with increasing coverage of dissociated water molecules. Our findings suggest two ways to control the conductivity at the LAO/STO interface: (I) an insulator-metal transition by adsorbing an amount of water at the bare surface; (II) a carrier density change by the transition between the most stable and the metastable adsorption configurations for 1 ML coverage in an ambient atmosphere at room temperature.

Effective control of the charge and magnetic states of transition-metal atoms on single-layer boron nitride.

Developing approaches to effectively control the charge and magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here we suggest that the magnetic and charge states of transition-metal (TM) doped single-layer boron-nitride (SLBN) systems can be easily controlled by the (internal) defect engineering and (external) electric fields (Eext). The relative positions and symmetries of the in-gap levels induced by defect engineering and the TM d-orbital energy levels effectively determine the charge states and magnetic properties of the TM/SLBN system. Remarkably, the application of an Eext can easily control the size of the crystal field splitting of the TM d orbitals and thus, leading to the spin crossover in TM/SLBN, which could be used as Eext-driven nonvolatile memory devices. Our conclusion obtained from TM/SLBN is valid generally in other TM adsorbed layered semiconductors.

Topological quantum phase transition in 5d transition metal oxide Na2IrO3.

We predict a quantum phase transition from normal to topological insulators in the 5d transition metal oxide Na2IrO3, where the transition can be driven by the change of the long-range hopping and trigonal crystal field terms. From the first-principles-derived tight-binding Hamiltonian, we determine the phase boundary through the parity analysis. In addition, our first-principles calculations for Na2IrO3 model structures show that the interlayer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. Na2IrO3 is suggested to be a candidate material which can have both a nontrivial topology of bands and strong electron correlations.

X-ray absorption spectroscopy studies of spin-orbit coupling in 5d transition metal oxides.

In order to examine the effects of strong valence band spin-orbit coupling (SOC) in 5d transition metal oxides (TMOs), we have investigated the L(2) and L(3) edge white-line intensities of the x-ray absorption spectra of several 5d TMOs. The white-line intensities at both edges are found to decrease monotonously with increasing 5d electron occupancy, while their ratio showed anomalous behavior for late 5d TMOs (IrO(2), PtO(2), and Au(2)O(3)), deviating significantly from the theoretical value of 2 expected for the case of weak SOC. This observation serves as a clear experimental indication of strong SOC effects in 5d TMOs. We also discussed how the 5d TMOs can have charge transfer effects different from their counterpart 5d elemental metals by making comparative studies. Our works demonstrate the importance of j quantum states due to strong SOC in the 5d system.

Orbital-angular-momentum based origin of Rashba-type surface band splitting.

We propose that the existence of local orbital angular momentum (OAM) on the surfaces of high-Z materials plays a crucial role in the formation of Rashba-type surface band splitting. Local OAM state in a Bloch wave function produces an asymmetric charge distribution (electric dipole). The surface-normal electric field then aligns the electric dipole and results in chiral OAM states and the relevant Rashba-type splitting. Therefore, the band splitting originates from electric dipole interaction, not from the relativistic Zeeman splitting as proposed in the original Rashba picture. The characteristic spin chiral structure of Rashba states is formed through the spin-orbit coupling and thus is a secondary effect to the chiral OAM. Results from first-principles calculations on a single Bi layer under an external electric field verify the key predictions of the new model.

First-principles study of ultrathin (2 x 2) Gd nanowires encapsulated in carbon nanotubes.

Using density-functional calculations, we investigate the structural and magnetic properties of ultrathin Gd and Gd-carbide nanowires (NWs) encapsulated in narrow carbon nanotubes (CNTs). The equilibrium geometry of an encapsulated (2 x 2) Gd-NW is markedly different from that of bulk Gd crystals. The charge-density analysis shows pronounced spin-dependent electron transfer in the encapsulated Gd-NW in comparison with that of Gd-carbide NWs. We conclude that Gd-CNT hybridization is primarily responsible for both the structural difference and electron transfer in the encapsulated Gd-NW.

Mapping atomic contact between pentacene and a Au surface using scanning tunneling spectroscopy.

We mapped spatially varying intramolecular electronic structures on a pentacene-gold interface using scanning tunneling spectroscopy. Along with ab initio calculations based on density functional theory, we found that the directional nature of the d orbitals of Au atoms plays an important role in the interaction at the pentacene-gold contact. The gold-induced interface states are broadened and shifted by various pentacene-gold distances determined by the various registries of a pentacene molecule on a gold substrate.

Competition between structural distortion and magnetic moment formation in fullerene C20.

We investigate the effect of on-site Coulomb interactions on the structural and magnetic ground state of C(20) fullerene based on density-functional-theory calculations within the local density approximation (LDA) plus on-site Coulomb corrections (LDA+U). The total energies of the high symmetry (I(h)) and the distorted (D(3d)) structures of C(20) are calculated for different spin configurations. The ground state configurations are found to depend on the forms of exchange-correlation potentials and the on-site Coulomb interaction parameter U, reflecting the subtle nature of the competition between Jahn-Teller distortion and magnetic instability in the C(20) fullerene. While the nonmagnetic state of the distorted D(3d) structure is robust for small U, a magnetic ground state of the undistorted I(h) structure emerges for U larger than 4 eV when the LDA exchange-correlation potential is employed.

Novel Jeff=1/2 Mott state induced by relativistic spin-orbit coupling in Sr2IrO4.

We investigated the electronic structure of 5d transition-metal oxide Sr2IrO4 using angle-resolved photoemission, optical conductivity, x-ray absorption measurements, and first-principles band calculations. The system was found to be well described by novel effective total angular momentum Jeff states, in which the relativistic spin-orbit coupling is fully taken into account under a large crystal field. Despite delocalized Ir 5d states, the Jeff states form such narrow bands that even a small correlation energy leads to the Jeff=1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of Jeff quantum spin driven correlated-electron phenomena.

Photonic crystal alloys: a new twist in controlling photonic band structure properties.

We identified new photonic structures and phenomenon that are analogous to alloy crystals and the associated electronic bandgap engineering. From a set of diamond-lattice microwave photonic crystals of randomly mixed silica and alumina spheres but with a well defined mixing composition, we observed that both bandedges of the L-point bandgap monotonically shifted with very little bowing as the composition was varied. The observed results were in excellent agreement with the virtual crystal approximation theory originally developed for electronic properties of alloy crystals. This result signifies the similarity and correspondence between photonics and electronics.

Enhanced charge gap in the bilayer manganite La2-2xSr1+2xMn2O7 near x = 0.4.

We have investigated the optical conductivity spectra of La2-2xSr1+2xMn2O7 (0.3