Quantum Theory of Advanced Materials

Quasiparticle band structures for the cubic trihydrides YH3 and LaH3 have been calculated by evaluating the self-energy in the GW approximation using ab initio pseudopotentials and plane waves. These are the prototype metal hydrides that exhibit switchable optical properties. For both materials, the local-density approximation (LDA) yields semimetallic energy bands with a direct overlap of about 1 eV. We find the self-energy correction to the LDA energies opens a gap at Gamma of 0.8-0.9 eV for LaH3 and 0.2-0.3 eV for YH3, where the latter is in sharp contrast to a previous study using linear-muffin-tin orbitals. The quasiparticle band gaps are analyzed as a function of an initial shift in the LDA bands used to evaluate the random-phase approximation screening in constructing the self-energy. We also make a comparison of results obtained by using two different pseudopotentials, each designed to better approximate exchange and correlation between the semicore states and valence states of Y and La.

We report on a first-principles study of the conductance through graphene suspended between Al contacts as a function of junction length, width, and orientation. The charge transfer at the leads and into the freestanding section gives rise to an electron-hole asymmetry in the conductance and in sufficiently long junctions induces two conductance minima at the energies of the Dirac points for suspended and clamped regions, respectively. We obtain the potential profile along a junction caused by doping and provide parameters for effective model calculations of the junction conductance with weakly interacting metallic leads.

Graphene is believed to be an excellent candidate material for next-generation electronic devices. However, one needs to take into account the nontrivial effect of metal contacts in order to precisely control the charge injection and extraction processes. We have performed transport calculations for graphene junctions with wetting metal leads (metal leads that bind covalently to graphene) using nonequilibrium Green's functions and density functional theory. Quantitative information is provided on the increased resistance with respect to ideal contacts and on the statistics of current fluctuations. We find that charge transport through the studied two-terminal graphene junction with Ti contacts is pseudo-diffusive up to surprisingly high energies.

We present a detailed study of the exchange-correlation hole and exchange-correlation energy per particle in the Si crystal as calculated by the variational Monte Carlo method and predicted by various density-functional models. Nonlocal density-averaging methods prove to be successful in correcting severe errors in the local-density approximation (LDA) at low densities where the density changes dramatically over the correlation length of the LDA hole. but fail to provide systematic improvements at higher densities where the effects of density inhomogeneity are more subtle. Exchange and correlation considered separately show a sensitivity to the nonlocal semiconductor-crystal environment, particularly within the Si bond. which is not predicted by the nonlocal approaches based on density averaging. The exchange hole is well described by a bonding-orbital picture, while the correlation hole has a significant component due to the polarization of the nearby bonds, which partially screens out the anisotropy in the exchange hole.

The atomic geometry, electronic structure, and magnetic moment of 4d transition-metal clusters with 13 atoms are studied by pseudopotential density-functional calculations. We find a new buckled biplanar structure with a C-2v symmetry stabilized by enhanced s-d hybridization. It has a lower energy than the close-packed icosahedral or cuboctahedral structure for elements with more than half-filled d shells. The magnetic moments of this buckled biplanar structure are found to be smaller than those of the icosahedral structure and closer to available experimental results.

Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 × 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 ± 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two-layer and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.

{Probing and controlling the valley degree of freedom in graphene systems by transport measurements has been a major challenge to fully exploit the unique properties of this two-dimensional material. In this theoretical work, we show that this goal can be achieved by a quantum-wire geometry made of gapped graphene that acts as a valley filter with the following favorable features: (i) all electrical gate control, (ii) electrically switchable valley polarity, (iii) robustness against configuration fluctuation, and (iv) potential for room temperature operation. This valley filtering is accomplished by a combination of gap opening in either bilayer graphene with a vertical electrical field or single layer graphene on h-BN, valley splitting with a horizontal electric field, and intervalley mixing by defect scattering. In addition to functioning as a building block for valleytronics, the proposed configuration makes it possible to convert signals between electrical and valleytronic forms, thus allowing for the integration of electronic and valleytronic components for the realization of electro-valleytronics.}

We show that the continuous feedback approach is highly effective for controlling chaotic systems. The control design for the Lorenz system is presented as an example to demonstrate the strength of this approach. The proposed control is able to eliminate chaos and bring the system toward any of the three steady states. Two different control input locations are considered. Only one system variable is used in the feedback. The control scheme can tolerate both measurement noise and modeling uncertainty as long as they are bounded.

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Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum-spin-Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe2 single layer with the 1T′ structure that does not exist in the bulk form of WSe2. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observe a gap of 129 meV in the 1T′ layer and an in-gap edge state located near the layer boundary. The system′s 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator–semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.

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One third of a monolayer of Sn adsorbed on Ge(111) undergoes a broad phase transition upon cooling from a (root3 x root3)R30degrees normal phase at room temperature to a (3 x 3) phase at low temperatures. Since band-structure calculations for the ideal (root3 x root3)R30degrees phase show no Fermi-surface nesting, the underlying mechanism for this transition has been a subject of much debate. Evidently, defects formed by Ge substitution for Sn in the adlayer, at a concentration of just a few percent, play a key role in this complex phase transition. Surface areas near these defects are pinned to form (3 x 3) patches above the transition temperature. Angle-resolved photoemission is employed to examine the temperature-dependent band structure, and the results show an extended gap forming in k-space as a result of band splitting at low temperatures. On account of the fact that the room temperature phase is actually a mixture of (root3 x root3)R30degrees areas and defect-pinned (3 x 3) areas, the band structure for the pure (root3 x root3)R30degrees phase is extracted by a difference-spectrum method. The results are in excellent agreement with band calculations. The mechanism for the (3 x 3) transition is discussed in terms of a response function and a tight-binding cluster calculation. A narrow bandwidth and a small group velocity near the Fermi surface render the system highly sensitive to surface perturbations, and formation of the (3 x 3) phase is shown to involve a Peierls-like lattice distortion mediated by defect doping. Included in the discussion, where appropriate, are dynamic effects and many-body effects that have been previously proposed as possible mechanisms for the phase transition.