Interfaces between materials have led to revolutionary discoveries. When precisely crafted at the atomic scale, proximity effects, reduced dimensionality and symmetry breaking compete at interfaces to bring out quantum phenomena and unforeseen functionality. In this talk I will discuss examples from my work where heteroepitaxial interfaces play a pivotal role in the aggregate response of a system. These include: 1. Dielectric superlattices consisting of precisely integer molecular layers of titanate phases - CaTiO3, BaTiO3 and SrTiO3, where the symmetry of the superlattice and hence its electronic response was controlled by the sequence of interfaces built into the structure. 2. Mixed-valent manganites, where ordered pseudo alloys constructed through digital superlattices to have the same average doping as their random alloy counterparts albeit with removed structural disorder, resulted in modified electronic and magnetic properties. 3. Extreme heteroepitaxy between SrTiO3 and (001) Si avoiding the formation of an amorphous interfacial SiO2 layer where a commensurately strained SrTiO3 film and thus a strain induced ferroelectric directly on silicon was obtained. Through these examples, I will show how such nanoscale sample design, when carefully combined with other techniques, can be used to probe basic and applied physics.

The microscopic mechanism behind the superconductivity is the formation of bound pairs of electrons (Cooper pairing). So far the complete disappearance of the electrical resistance was the major known consequence of Cooper pairing. I will present novel findings, demonstrating that Cooper pairing is responsible not only for the zero-resistive superconducting state but, paradoxically, also for the zero-conducting superinsulating state. The latter is dual to the superconducting state: It appears at a finite temperature and has all the attributes complementary to the superconducting state, namely, the magnetic field dependent critical temperature and the threshold voltage, which plays the same role as critical current, breaking down the zero-resistance state of superconducting films.

This talk will present my recent work at the intersection of photonics, nanofabrication, and materials design. I will start with an overview of the light emission properties of semiconductor nanowires, including single-nanowire light-emitting diodes, and optically pumped ultraviolet nanowire lasers. Then, I will introduce nanoskiving as a technique for engineering the optical response of metallic nanostructures. Finally, I will discuss two novel approaches for the design of new nanomaterials: point-defect engineered silicon for silicon photonics and rotationally twinned nanowires as a new type of superlattice.

The study of quantum phase transitions in the presence of disorder is at the forefront of research in the field of correlated electron systems, yet there have been relatively few experimental model materials. We have succeeded in the growth of large single crystals of the randomly-diluted spin-1/2 square-lattice Heisenberg antiferromagnet La2(Cu,Zn,Mg)O4 up to high dilution concentrations. Our neutron scattering measurements of the instantaneous antiferromagnetic (AF) spin correlations, complemented by numerical experiments, demonstrate that this compound is an excellent system for the study of site percolation in the quantum spin-1/2 limit 1. High transition-temperature (Tc) superconductivity develops near AF phases, and it is possible that magnetic excitations contribute to the superconducting (SC) pairing mechanism. In order to assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional AF spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the AF phase extends much further with doping and it appears to overlap with the SC phase: the archetypical compound Nd(2-x)CexCuO{4\pm\delta} shows bulk superconductivity above , while evidence for AF order has been found up to . However, our new inelastic magnetic neutron scattering measurements point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not co-exist. Our measurements furthermore demonstrate that the pseudogap phenomenon in the electron-doped materials arises from a build-up of spin correlations 2.

1 O.P. Vajk et al., Science 296, 1691 (2002).
2 E.M. Motoyama et al., Nature 455, 186 (2007).

We predict that d-wave (or other exotic)superconductors demonstrate (at least) two sequential transitions as a function of increasing disorder: a d-wave to s-wave, and then an s-wave to metal transition.