Two-dimensional electron gas with universal subbands at the surface of SrTiO(3).

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO(3)) is the foundation of the emerging field of oxide electronics. SrTiO(3) is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal-insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO(3) (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ∼0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO(3)-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO(3) lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO(3)-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

First-order insulator-to-metal Mott transition in the paramagnetic 3D system GaTa4Se8.

The nature of the Mott transition in the absence of any symmetry breaking remains a matter of debate. We study the correlation-driven insulator-to-metal transition in the prototypical 3D Mott system GaTa(4)Se(8), as a function of temperature and applied pressure. We report novel experiments on single crystals, which demonstrate that the transition is of first order and follows from the coexistence of two states, one insulating and one metallic, that we toggle with a small bias current. We provide support for our findings by contrasting the experimental data with calculations that combine local density approximation with dynamical mean-field theory, which are in very good agreement.

Electronic subband reconfiguration in a d0-perovskite induced by strain-driven structural transformations.

It is well known that transport in lightly n-doped SrTiO(3) involves light and heavy electron bands. We have found that upon application of moderate quasi-isotropic pressures, the relative positions of these subbands are changed by a few meV and, eventually, a band inversion occurs at ~1 kbar. Such effects are, however, suppressed in the closely related KTaO(3) perovskite. We show that the extremely subtle electronic reconfiguration in SrTiO(3) is triggered by strain-induced structural transformations that are accompanied by remarkable mobility enhancements up to about Δµ/µ≈300%. Our results provide a microscopic rationale for the recently discovered transport enhancement under strain and underscore the role of the internal structural degrees of freedom in the modulation of the perovskite electronic properties.

Electronic fine structure in the electron-hole plasma in SrB6

Electron-hole mixing-induced fine structure in alkaline earth hexaborides leads to lower energy (temperature) scales, and thus a stronger tendency toward an excitonic instability than in their doped counterparts (viz. Ca1-xLaxB6, x approximately 0.005), which are high-Curie-temperature, small-moment ferromagnets. Comparison of Fermi surfaces and spectral distributions with de Haas-van Alphen, optical, transport, and tunneling data indicates that SrB6 remains a fermionic semimetal down to (at least) 5 K, rather than forming an excitonic condensate. For the doped system the Curie temperature is higher than the degeneracy temperature.

New superhard phases for three-dimensional C60-based fullerites

We have explored new possible phases of 3D C60-based fullerites using semiempirical potentials and ab initio density functional methods. We have found three closely related structures-two body-centered orthorhombic and one body-centered cubic-having 52, 56, and 60 tetracoordinated atoms per molecule. These 3D polymers result in semiconductors with bulk moduli near 300 GPa, and shear moduli around 240 GPa, which make them good candidates for new low density superhard materials.

Colossal magnetic anisotropy of monatomic free and deposited platinum nanowires.

Whenever a nanosystem such as an adatom, a cluster or a nanowire spontaneously magnetizes, a crucial parameter is its magnetic anisotropy, the intrinsic preference of magnetization to lie along an easy axis. Anisotropy is important in nanosystems because it helps reduce the magnitude of thermal (superparamagnetic) fluctuations, it can modify the flow of current, and it can induce new phenomena, such as the quantum tunnelling of magnetization. We discuss here, on the basis of density functional calculations, the novel and unconventional feature of colossal magnetic anisotropy--the strict impossibility of magnetization to rotate from the parallel to the orthogonal direction--which, owing to a quantum mechanical selection rule, the recently predicted Pt nanowire magnetism should exhibit. Model calculations suggest that the colossal magnetic anisotropy of a Pt chain should persist after weak adsorption on an inert substrate or surface step.

Magnetism in atomic-size palladium contacts and nanowires.

We have investigated Pd nanowires theoretically, and found that, unlike either metallic or free atomic Pd, they exhibit Hund's rule magnetism. In long, monostrand nanowires, we find a spin moment of 0.7 mu(B) per atom, whereas for short, monostrand nanowires between bulk leads, the predicted moment is about 0.3 mu(B) per nanowire atom. In contrast, a coaxial (6,1) nanowire was found to be nonmagnetic. The origin of the nanowire magnetism is analyzed.

Superconductivity near ferromagnetism in MgCNi3.

An unusual quasi-two-dimensional heavy band mass van Hove singularity (vHs) lies very near the Fermi energy in MgCNi3, recently reported to superconduct at 8.5 K. This compound is strongly exchange enhanced and unstable to ferromagnetism upon hole doping with approximately 12% Mg-->Na or Li (i.e., 0.04 hole/Ni). We identify an essentially infinite mass along the M-Gamma line, which accounts for the two dimensionality of this vHs. This compound provides new opportunities to probe the ferromagnetic critical point as well as introducing the novelties of 2D behavior into a 3D system.