Dimensionality-Driven Metal-Insulator Transition in Spin-Orbit-Coupled SrIrO_{3}.

Upon reduction of the film thickness we observe a metal-insulator transition in epitaxially stabilized, spin-orbit-coupled SrIrO_{3} ultrathin films. By comparison of the experimental electronic dispersions with density functional theory at various levels of complexity we identify the leading microscopic mechanisms, i.e.

Nonlocal Parity Order in the Two-Dimensional Mott Insulator.

The Mott insulator is characterized by having small deviations around the (integer) average particle density n, with pairs with n-1 and n+1 particles forming bound states. In one dimension, the effect is captured by a nonzero value of a nonlocal "string" of parities, which instead vanishes in the superfluid phase where density fluctuations are large. Here, we investigate the interaction induced transition from the superfluid to the Mott insulator, in the paradigmatic Bose Hubbard model at n=1.

The strength of electron electron correlation in Cs3C60.

Cs3C60 is an antiferromagnetic insulator that under pressure (P) becomes metallic and superconducting below Tc = 38 K. The superconducting dome present in the T - P phase diagram close to a magnetic state reminds what found in superconducting cuprates and pnictides, strongly suggesting that superconductivity is not of the conventional Bardeen-Cooper-Schrieffer (BCS) type We investigate the insulator to metal transition induced by pressure in Cs3C60 by means of infrared spectroscopy supplemented by Dynamical Mean-Field Theory calculations. The insulating compound is driven towards a metallic-like behaviour, while strong correlations survive in the investigated pressure range.

Design of a Mott Multiferroic from a Nonmagnetic Polar Metal.

We examine the electronic properties of the newly discovered "ferroelectric metal" LiOsO3 combining density-functional and dynamical mean-field theories. We show that the material is close to a Mott transition and that electronic correlations can be tuned to engineer a Mott multiferroic state in the 1/1 superlattice of LiOsO3 and LiNbO3. We use electronic structure calculations to predict that the (LiOsO3)1/(LiNbO3)1 superlattice exhibits strong coupling between magnetic and ferroelectric degrees of freedom with a ferroelectric polarization of 41.

Phase diagram of the triangular extended Hubbard model.

We study the extended Hubbard model on the triangular lattice as a function of filling and interaction strength. The complex interplay of kinetic frustration and strong interactions on the triangular lattice leads to exotic phases where long-range charge order, antiferromagnetic order, and metallic conductivity can coexist. Variational Monte Carlo simulations show that three kinds of ordered metallic states are stable as a function of nearest neighbor interaction and filling.

Bottom-up approach for the low-cost synthesis of graphene-alumina nanosheet interfaces using bimetallic alloys.

The production of high-quality graphene-oxide interfaces is normally achieved by graphene growth via chemical vapour deposition on a metallic surface, followed by transfer of the C layer onto the oxide, by atomic layer and physical vapour deposition of the oxide on graphene or by carbon deposition on top of oxide surfaces. These methods, however, come with a series of issues: they are complex, costly and can easily result in damage to the carbon network, with detrimental effects on the carrier mobility. Here we show that the growth of a graphene layer on a bimetallic Ni3Al alloy and its subsequent exposure to oxygen at 520 K result in the formation of a 1.

DFT and TB study of the geometry of hydrogen adsorbed on graphynes.

Using density-functional calculations (DFT) and a tight-binding model, we investigate the origin of distinct favorable geometries which depend on the type of graphyne used. The change in the H geometry is described in terms of the tuning of the hopping between sp(2)-bonded C atoms and sp-bonded C atoms hybridized with the H atoms. We find that the different preferred geometry for each type of graphyne is associated with the electronic effects due to different symmetries rather than a steric effect minimizing the repulsive interaction between the H atoms.

Selective Mott physics as a key to iron superconductors.

We show that electron- and hole-doped BaFe(2)As(2) are strongly influenced by a Mott insulator that would be realized for half-filled conduction bands. Experiments show that weakly and strongly correlated conduction electrons coexist in much of the phase diagram, a differentiation which increases with hole doping. This selective Mottness is caused by the Hund's coupling effect of decoupling the charge excitations in different orbitals.

Electronic correlations stabilize the antiferromagnetic Mott state in Cs3C60.

Cs(3)C(60) in the A15 structure is an antiferromagnet at ambient pressure in contrast with other superconducting trivalent fullerides. Superconductivity is recovered under pressure and reaches the highest critical temperature of the family. Comparing density-functional calculations with generalized gradient approximation to the hybrid functional of Heyd, Scuseria, and Ernzerhof, which includes a suitable component of exchange, we establish that the antiferromagnetic state of Cs(3)C(60) is not due to a Slater mechanism, and it is stabilized by electron correlation.

Microscopic origin of large negative magnetoelectric coupling in Sr(1/2)Ba(1/2)MnO3.

With a combined ab initio density functional and model Hamiltonian approach we establish that in the recently discovered multiferroic phase of the manganite Sr(1/2)Ba(1/2)MnO3 the polar distortion of Mn and O ions is stabilized via enhanced in-plane Mn-O hybridizations. The magnetic superexchange interaction is very sensitive to the polar bond-bending distortion, and we find that this dependence directly causes a strong magnetoelectric coupling. This novel mechanism for multiferroicity is consistent with the experimentally observed reduced ferroelectric polarization upon the onset of magnetic ordering.

A spectroscopic and ab initio study of the formation of graphite and carbon nanotubes from thermal decomposition of silicon carbide.

We report an experimental and first-principles study of the thermal decomposition of 6H-SiC wafers, yielding graphite on the Si-terminated face and carbon nanotubes on the C-terminated face. The asymmetry of the carbon structure formation mechanisms is rationalized in terms of the different termination geometries of the opposite SiC faces. First-principles modeling reveals that horizontal, xr-delocalized carbon structures form on the Si-terminated face.

Analysis of methane-to-methanol conversion on clean and defective Rh surfaces.

We investigate by density-functional theory simulations several elementary reactions associated to direct methane-to-methanol conversion on clean Rh(111) surfaces and on Rh adatoms on Rh(111). Energy barriers and reaction paths have been determined by the nudged elastic band method. The rate-limiting step in the process, C-O bond formation, has higher activation energy than the one for complete methane dehydrogenation.

Resonating valence bond wave function for the two-dimensional fractional spin liquid.

The unconventional low-lying spin excitations, recently observed in neutron scattering experiments on Cs2CuCl4, are explained with a spin liquid wave function. The dispersion relation as well as the wave vector of the incommensurate spin correlations are well reproduced within a projected BCS wave function with gapless and fractionalized spin-1/2 excitations around the nodes of the BCS gap function. The proposed wave function is shown to be very accurate for one-dimensional spin-1/2 systems and remains similarly accurate in the two-dimensional model corresponding to Cs2CuCl4, thus representing a good ansatz for describing spin fractionalization in two dimensions.

Chiral spin liquid wave function and the Lieb-Schultz-Mattis theorem.

We study a chiral spin liquid wave function defined as a Gutzwiller projected BCS state with a complex pairing function. After projection, spontaneous dimerization is found for any odd but finite number of chains, thus satisfying the Lieb-Schultz-Mattis theorem, whereas for an even number of chains there is no dimerization. The two-dimensional thermodynamic limit is consistently reached for a large number of chains since the dimer order parameter vanishes in this limit.

Tetramerization of a frustrated spin-1/2 chain.

We investigate a model of a frustrated spin-1/2 Heisenberg chain coupled to adiabatic phonons with a general form of magnetoelastic coupling. For large enough frustration and lattice coupling, a new tetramerized phase with three different bond lengths is found. We argue that the zigzag spin-1/2 chain LiV2O5 might be a good candidate to observe such a phase.