Tunable 2-dimensional/3-dimensional electron gases by submonolayer La doping of SrTiO_{3}

First-principles calculation was used to study the structural and electronic features of the low dimensional oxide structure, SrTiO_{3}/Sr_{1-x}La_{x}TiO_{3} (x=0.25) superlattices, constructed by submonolayer low dimensional La doping into SrTiO_{3}. We demonstrate a dimensionality crossover from three-dimensional to two-dimensional (3D \to 2D) electronic behavior in the system. Two types of carriers, one confined to 2D and the other extended, exhibit distinct tunable (3D \to 2D) transport characteristics that will enable the study of many properties (e.g., superconductivity) through this change in dimensionality

Modified Gravity and Cosmology

In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, Scalar-Tensor, Einstein-Aether, and Bimetric theories, as well as TeVeS, f(R), general higher-order theories, Horava-Lifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including Kaluza-Klein, Randall-Sundrum, DGP, and higher co-dimension braneworlds. We also review attempts to construct a Parameterised Post-Friedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a self-contained, comprehensive and up-to-date introduction to the subject as a whole.Comment: 312 pages, 15 figure

Equation of state and thermodynamic properties of BCC metals

The moment method in statistical dynamics is used to study the equation of state and thermodynamic properties of the bcc metals taking into account the anharmonicity effects of the lattice vibrations and hydrostatic pressures. The explicit expressions of the lattice constant, thermal expansion  oefficient, and the specific heats of the bcc metals are derived within the fourth order moment approximation. The termodynamic quantities of W, Nb, Fe,and Ta metals are calculated as a function of the pressure, and they are in good agreement with the corresponding results obtained from the first principles calculations and experimental results. The effective pair potentials work well for the calculations of bcc metals

Structural transition and dielectric response of an epitaxially strained BaTiO<SUB>3</SUB>/SrTiO<SUB>3</SUB> superlattice: a first-principles study

We determine the structure and static dielectric response of the BaTiO3 1 unit cell/SrTiO3 1 unit cell (BTO/STO) superlattice as a function of epitaxial strain using first-principles density functional theory (DFT) calculations based on pseudopotentials and a plane-wave basis. We find a structural transition from the tetragonal phase at compressive strains to the monoclinic phase at tensile strains, with a nonzero in-plane component of polarization in the monoclinic phase as its order parameter. For the stable structures determined as a function of in-plane strain, we obtained optical phonon frequencies, Born effective charges and static dielectric constants using DFT linear response. Our calculations predict a large zero-temperature dielectric response with a strong anisotropy, whose origin is traced to soft phonons of the superlattice

Direct Observation of Surface Induced Metal-Insulator Transition

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Polarization of strained BaTiO<SUB>3</SUB>/SrTiO<SUB>3</SUB> artificial superlattice: first-principles study

We performed first-principles calculation to investigate the effect of epitaxial strain on lattice instabilities and polarization behavior of BaTiO3/SrTiO3 artificial lattice with very short stacking period, i.e., BaTiO3 1 unit cell/SrTiO3 1 unit cell (BTO/STO). The structural analysis of BTO/STO artificial superlattice under in-plane compressive state showed enhanced stability of the tetragonal phase. On the other hand, the stability of monoclinic phase was enhanced when the BTO/STO was in the in-plane tensile state. The phase transition from tetragonal to the monoclinic phase occurs at the misfit strain of -0.25%. As the misfit strain of BTO/STO superlattice increases from -0.25% to -1.5% (in-plane compressive state), the tetragonal superlattice exhibits an increasing polarization along the [001] direction. In the monoclinic phase, the polarization of the superlattice rotates progressively toward [110] direction with increasing the misfit strain, and the magnitude of the polarization simultaneously increases with the rotation. The first-principles study shows that the phase stability and polarization vector is sensitively influenced by the lattice misfit strain

Dielectric properties of BaTiO<SUB>3</SUB>/SrTiO<SUB>3</SUB> ferroelectric thin film artificial lattice

Dielectric behavior on BaTiO<SUB>3</SUB>/SrTiO<SUB>3</SUB> artificial lattices has been investigated along with quantum mechanical simulation (first principles calculation). From the oxide artificial lattice approach, strain manipulation was performed to obtain a wide range of lattice deformation in the consisting BaTiO<SUB>3</SUB> and SrTiO<SUB>3</SUB> layers, which leads to two important consequences. First, we obtained enhanced dielectric constant and extremely large nonlinearity in the artificial lattices with very short stacking periods. Second, it is found that there exists a maximum dielectric constant in each BaTiO<SUB>3</SUB> lattice and SrTiO<SUB>3</SUB> lattice at a certain degree of lattice deformation. The first principles study successfully explains the dielectric behavior of strained BaTiO<SUB>3</SUB> and SrTiO<SUB>3</SUB> lattices, the existence of the maximum dielectric constant. The first principles study on BaTiO<SUB>3</SUB>/SrTiO<SUB>3</SUB> artificial lattices with very short stacking periods also reveals that the artificial lattice undergoes phase transition between the tetragonal and monoclinic phases with a misfit lattice strain and exhibits an anomalous dielectric behavior at the phase boundary. Optical phonon behavior of the BaTiO<SUB>3</SUB>/SrTiO<SUB>3</SUB> artificial lattice resembles that of strained SrTiO<SUB>3</SUB> lattice and optical phonon softening primarily derives the anomaly of the dielectric tensor at the phase boundary. The lattice deformation is a primary influencing factor to phonon and dielectric behaviors rather than interface layer effect in BaTiO<SUB>3</SUB>/SrTiO<SUB>3</SUB> artificial lattice with very short stacking periods

Metal-insulator transition in low dimensional La{sub 0.75}Sr{sub 0.25}VO{sub 3} thin films

We report on the metal-insulator transition that occurs as a function of film thickness in ultrathin La{sub 0.75}Sr{sub 0.25}VO{sub 3} films. The metal-insulator transition displays a critical thickness of 5 unit cell. Above the critical thickness, metallic films exhibit a temperature driven metal-insulator transition with weak localization behavior. With decreasing film thickness, oxygen octahedron rotation in the films increases, causing enhanced electron-electron correlation. The electronelectron correlations in ultrathin films induce the transition from metal to insulator in addition to Anderson localization