Session S29: Two-dimensional Topological Insulators: Growth, Structure and Electronic Properties

Quantum Anomalous Hall Effect in Low-buckled Honeycomb Lattice with In-plane Magnetization

With out-of-plane magnetization, the quantum anomalous Hall effect has been extensively studied in quantum wells and two-dimensional atomic crystal layers [1]. Here, we investigate the possibility of realizing quantum anomalous Hall effect (QAHE) in honeycomb lattices with in-plane magnetization. We show that the QAHE can only occur in low-buckled honeycomb lattice where both intrinsic and intrinsic Rashba spin-orbit coupling appear spontaneously. The extrinsic Rashba spin-orbit coupling is detrimental to this phase. In contrast to the out-of-plane magnetization induced QAHE, the QAHE from in-plane magnetization is achieved in the vicinity of the time reversal symmetric momenta at $M$ points rather than Dirac points. In monolayer case, the QAHE can be characterized by Chern number $\mathcal{C}=\pm 1$ whereas additional phases with Chern number $\mathcal{C}=\pm 2$ appear in chiral stacked bilayer system. The Chern number strongly depends on the orientation of the magnetization. The bilayer system also provides additional tunability via out-of-plane electric field, which can reduce the critical magnetization strength required to induce QAHE. It can also lead to topological phase transitions from $\mathcal{C}=\pm 2$ to $\pm1$ and finally to $0$. [1] Review article: arXiv:1509.09016   

The theoretical studies of topology electronic states in HgTe Hall Bar and Quantum Dot

In recent years, there is an extensive attention on the new properties of topology materials and their potential applications. Our interest is on the physics in the quantum confined systems based on topology materials. To consider two such systems, i.e., quantum dot and Hall bar constructed on the HgTe quantum well, we study the electronic properties and their dependence on various material parameters with and without an in-plane electric field. For both systems, we find that 1) the exotic edge states appear in bulk energy gap, resulting from the non-trivial topological property of quantum well system. 2) by the magnetic doping, there are tunable phase transitions, e.g., transition from trivial insulating phase to topological insulating phase or anomalous quantum Hall insulating phase. 3) the in-plane electric field can introduce effective control on the electronic states.   

A new structure of two-dimensional allotropes of group V elements

The elemental two-dimensional (2D) materials such as graphene, silicene, germanene, and black phosphorus have attracted considerable attention due to their fascinating physical properties. Structurally they possess the honeycomb or distorted honeycomb lattices, which are composed of six-atom rings. Here we find a new structure of 2D allotropes of group V elements composed of eight-atom rings, which we name as the octagonal tiling (OT) structure. First-principles calculations indicate that these allotropes are dynamically stable and are also thermally stable at temperatures up to 600 K. These allotropes are semiconductors with band gaps ranging from 0.3 to 2.0 eV, thus they are potentially useful in near- and mid-infrared optoelectronic devices. OT-Bi is also a 2D topological insulator (TI) with a band gap of 0.33 eV, which is the largest among the reported elemental 2D TIs, and this gap can be increased further by applying compressive strains.   

Topological phase transition in layered transition metal dichalcogenides

Despite considerable interests in transition metal dichalcogenides (TMDs), such as MX$_{\mathrm{2}}$ with M $=$ (Mo, W) and X $=$ (S, Se, Te), the physical origin of their topological nature is still in its infancy. The conventional view of topological phase transition (TPT) in TMDs is that the band inversion occurs between the metal d and chalcogen p orbital bands. More precisely, the former is pulled down below the latter. Here we introduce an explicit scheme for analyzing TPT in topological materials and find that the TPT in TMDs is different from the conventional speculation. When the 1T phase undergoes a structural transformation to the 1T' phase in monolayer MX$_{\mathrm{2}}$, the band topology changes from trivial to non-trivial, leading to the TPT. We discuss the exact role of the metal d and chalcogen p orbital bands during the TPT. Our finding would provide clear guidelines for understanding the topological nature not only in TMDs but also in other topological materials yet to be explored.   

Effects of edge terminations on monolayer topological crystalline insulators via group theory and DFT

Topological crystalline insulators (TCIs) are a counterpart of usual topological insulators in materials where the topological edge (or surface) states are protected by the underlying crystal symmetries. Here we discuss the band structure of IV-VI monolayer materials, where the edge states are protected by mirror symmetry, considering ribbons cutted in different orientations and distinct edge terminations. We show that although the Chern numbers and the topological classification remain a bulk property, the nature of edge terminations and orientation play a significant role in TCIs. For each ribbon an effective Hamiltonian is derived by group theory and proper boundary conditions are presented. These show good agreement with DFT calculations and illustrate the effects of the distinct reduced symmetry of each ribbon type.   

Antiperovskite Sr3PbO thin films grown by molecular beam epitaxy

Several antiperovskite compounds have recently been predicted to host bulk three dimensional Dirac dispersion as well as surface states protected by crystal symmetry. Here, we present fabrication of cubic antiperovskite Sr$_3$PbO films epitaxially grown on LaAlO$_3$ by molecular beam epitaxy. Fabricated films were capped with polymer without breaking vacuum to facilitate ex-situ transport characterization. All of the films showed metallic temperature dependence. The Hall effect measurement suggests that the carrier type is hole, whose density is around $5 \times 10^{19}$ cm$^{-3}$. Details of magnetotransport at low temperature is also described.   

Spin-Phonon coupling in a candidate 2D atomic crystal magnetic semiconductor

Cr$_{\mathrm{2}}$Ge$_{\mathrm{2}}$Te$_{\mathrm{6}}$ is a particularly interesting material since it is in the very rare class of ferromagnetic semiconductors and possesses a layered, nearly two dimensional structure due to van Der Waals bonds. The van Der Waals bonds make it a candidate two dimensional atomic crystal, which is predicted as a platform to study 2D semiconducting ferromagnets and for single layered spintronics devices. Spin-phonon coupling can be a key factor for the spin relaxation in a spintronics devices. We use polarized temperature dependent Raman scattering to study Cr$_{\mathrm{2}}$Ge$_{\mathrm{2}}$Te$_{\mathrm{6}}$. The spin-phonon coupling has been confirmed in three ways: below T$_{\mathrm{C}}$ we observe a split of two phonon modes due to the breaking of time reversal symmetry; an anomalous hardening of an additional three modes; and dramatic decrease of the phonon lifetimes upon warming into the paramagnetic phase. Our results also suggest the possibility of probing the magneto-elastic coupling using Raman spectroscopy, opening a door for the further study of exfoliated 2D Cr$_{\mathrm{2}}$Ge$_{\mathrm{2}}$Te$_{\mathrm{6}}$. We gratefully acknowledge support from the National Science Foundation (Grant No. DMR-1410846)   

Magnetism at grain boundary interfacesin the colossal permittivity dielectric material; In+Nb Co-Doped Rutile

With the emphasis in recent years on understanding novel materials with potential technological applications this work seeks to understand magnetic ordering within the colossal-permittivity material, In+Nb co-doped rutile (TiO$_2$). Evidence for a spin-freezing transition was reported from a step like feature in the dielectic data below 50 K but this was largly glossed over. Within this work we show that below 300 K there is a slowing down of magnetic fluctuations associated with the electronic magnetism due to the defect-dipoles created by the co-doping, but the muon spectroscopy results are strongly suggestive of the behaviour being localised to the edges/interfaces of particles/grains. The $T_{\rm C}$ is strongly dependent on the doping level of the samples that presents novel way to control the magnetism and ultimately magneto-electric coupling within a dielectric material.   

Photoconductivity of transparent perovskite oxide semiconductors BaSnO$_{\mathrm{3}}$ and SrTiO$_{\mathrm{3}}$ epitaxial thin films

We have measured the photoconductivity (PC) of epitaxial thin films of transparent semiconductor BaSnO$_{\mathrm{3}}$ (BSO) and SrTiO$_{\mathrm{3}}$ (STO) at room temperature. The epitaxial thin films of BSO and STO were grown by pulsed laser ablation technique on the MgO substrates to exclude any conductance from the substrate owing to its large bandgap (\textasciitilde 7.8 eV).~ Despite the same crystalline structure and similar band gap sizes (\textasciitilde 3.2 eV), the PC of BSO behaved very differently. The slowly varying component in the PC of BSO is much larger than that of STO; the PC of BSO increased slowly, reached higher magnitude after the same duration of illumination, and persisted longer than many hours after the light was turned off, whereas the PC of STO showed little persistent conductivity. The spectral responses of the PC of BSO and STO showed their highest peaks below 400 nm when measured by a UV monochromator system, suggesting that the electron-hole pair generation is the main mechanism of the PC for both materials. ~The higher mobility of BSO~should be partially responsible for the higher PC. ~The large persistent PC of BSO seems related to the dislocations that trap electrons easily.   

Change In The Electronic Structure And Optical Absorption Of Cuprate Delafossites Via B-site Alloying

Delafossite oxides are a family of materials with the form $ABO_2$, where the A-site is a monovalent cation ($Cu, Ag, Au$) and the B-site is a trivalent cation ($Ga, Al, In$). Delafossites typically have a wide optical band gap, this band gap may be tuned by adding a second B-site element forming an $AB^1_{(1-x)}B^2_{(x)}O_2$ alloy. We investigate changes in the electronic structure of $CuAlO_2$, $CuGaO_2$, and $CuInO_2$ when alloyed with $CuFeO_2$. Using the FIREBALL program to optimize the atomic structure, calculate the total and partial density of states, calculate the valence band edge for each alloy level, and investigate the clustering factor of the second B-site atom, it is found that alloying with $Fe$ creates midgap states caused by $Fe-O$ interactions. From the partial density of state, each type of atoms contribution to the change in the valence band edge can be seen. Observed changes to the materials include increased optical absorption in the visible range, and symmetry breaking because of the deformation in the crystal structure. The $CuFeO_2$ alloying percentages range from 0-5\%. We are synthesizing these alloys to experimentally verify the changes in the optical absorption spectra.   

Growth, structural, dielectric and magnetic properties of epitaxial multiferroic NaMnF$_{\mathrm{3}}$ thin films

Epitaxial NaMnF$_{\mathrm{3}}$ thin films were grown on SrTiO$_{\mathrm{3\thinspace }}$(100) single crystal substrates via molecular beam epitaxy (MBE). The orthorhombically distorted perovskite fluoride NaMnF$_{\mathrm{3}}$ (\textit{Pnma} space group) has been predicted to have a polar instability at low temperatures due to MnF$_{\mathrm{6}}$ octahedral tilts. Structural, magnetic and dielectric properties were studied. Thin film structural quality as a function of the substrate temperature and film thickness was investigated using X-ray diffraction (XRD), in-situ reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The best films were smooth and single phase grown with four different twin domains. Magnetic characterization was performed using superconducting quantum interference device (SQUID) magnetometry. In-plane magnetization measurements revealed antiferromagnetic ordering with a Neel temperature $T_{N}_{\mathrm{\thinspace }}=$ 66 K. For the dielectric studies, NaMnF$_{\mathrm{3}}$ films were grown on top of SrRuO$_{\mathrm{3}}$ (100) buffer layers grown via pulsed laser deposition that were used as bottom electrodes. Dielectric spectroscopy was performed at different temperatures between 11K and room temperature in a frequency range 100 Hz to 100 kHz. Significant temperature dependent dielectric properties were observed.   

Molecular Beam Epitaxial (MBE) Growth and Characterization of Thin Films of Semiconductor Tin

Recent theoretical predictions that a two-dimensional monolayer of semiconductor tin is a two-dimensional topological insulator and experimental evidence of three-dimensional topological insulator behavior in strained ultrathin films of semiconductor tin grown by MBE on InSb has generated intense research interest. This research is primarily focused on the MBE growth and topological characteristics of ultrathin films of semiconductor tin. In this talk we present results of a study on the MBE growth and the transport, structural and optical characterization of thin films of semiconductor tin on several different substrates.   

Phase dependent structural and electronic properties of Lanthanum Orthophosphate (LaPO$_{\mathrm{4}})$

Lanthanum orthophosphate (LaPO$_{\mathrm{4}})$ belongs to the family of rare-earth (RE) orthophosphates. The La-ion lacks valence 4f-electron, so for it to exhibit f-electron dependent physics, it must be doped with additional RE elements. In the bulk form, LaPO$_{\mathrm{4}}$ exist in both a stable monoclinic and a metastable hexagonal phase, which both possess indirect energy transition characteristics. Though the overall optoelectronic properties of the RE-doped LaPO$_{\mathrm{4}}$ depend on the accuracy of the observed bulk energy gap, the reported experimental and theoretical energy gaps varies between \textasciitilde 8$^{\mathrm{1,2}}$ and \textasciitilde 5$^{\mathrm{3}}$ eV, respectively. Through this theoretical study, we attempt to establish a correlation between electronic properties of bulk LaPO$_{\mathrm{4}}$ and various levels of first principle theories. Compared to experimental data, the PBE0 functional over-predicts energy gaps and the energy differences between the indirect-to-direct transition energies by 25{\%}. The HSE06 gives a good description of electronic properties and predicts the energy gaps to be 7.68 (monoclinic) and 7.29 eV (hexagonal). Analysis on the structural stability also reveals that the total energy difference between the two phases is 6meV, consistent with the experimentally observed instantaneous pressure and temperature dependent phase transition. [1] J. Lumin. 72--74, 255, 1997, [2] J. Lumin. 15-18, 255, 1977, [3] App. Surface Science, 268, 458-463, 2013.   

Landau Theory of Trifluoride Negative Thermal Expansion Materials

Negative thermal expansion (NTE) is a desirable property in designing materials that are dimensionally stable and resistant to thermal shocks. Transition metal trifluorides (MF$_3$, M=Al, Cr, Fe, Ga, In, Ti, V) are a class of materials with ReO$_3$ structure that exhibit large, isotropic, and tunable NTE over a wide temperature range, which makes them attractive material candidates. They exhibit large coefficients of thermal expansion near their cubic-to-rhombohedral structural phase change, which can be thermally or pressure induced. Though they have recently been the subject of intense experimental research, little work has been done on the theory side and it has almost exclusively focused on zero temperature properties. In this talk, we construct a simple Landau theory of trifluorides and use it to calculate the temperature dependence of the elastic constants, soft phonon frequencies, and volume expansion near their structural transition. We compare our results to existing experimental data on trifluorides.   

Type-II Dirac cones as unified topological origin of the exotic electronic properties of WTe$_2$

WTe$_2$ is a recently discovered layered material with remarkable electronic properties. Transport measurements show an extremely large non-saturating magnetoresistance (MR) with  mobilities as high as 167~000~cm$^2$/Vs at 2~K.  Furthermore, recent photoemission experiments discovered circular dichroism in the bulk band structure. We propose a unified explanation for these exotic observations by relating key properties of the bulk electronic structure to that of to that of the mono- and bi-layer material. In particular, we demonstrate that the monolayer is a novel type-II Dirac semimetal in absence of spin-orbit coupling, with Dirac cones that are sufficiently anisotropic to simultaneously harbor electron and hole pockets. The band structure can be characterized by a new $\mathbb{Z}_2 \times \mathbb{Z}_2$ topological invariant defined through non-Abelian Wilson loops. We develop a tight-binding model for the mono- and bilayer of WTe$_2$ based on Wannier functions from {\it ab-inito} calculations and extend our findings to the iso-structural compounds MoTe$_2$ and ZrI$_2$.