Origin of Negative Photoconductivity at the Interface of Ba0.8Sr0.2TiO3/LaMnO3/Ba0.8Sr0.2TiO3 Heterostructures.

The study of Ba0.8Sr0.2TiO3/LaMnO3/Ba0.8Sr0.2TiO3 heterostructures on a MgO substrate with Ba0.8Sr0.2TiO3 ferroelectric films revealed the occurrence of a metallic character of the temperature behavior of the resistance at a temperature less than 175 K. This behavior is associated with an increased charge concentration at the interface due to a discontinuity in the ferroelectric polarization at the interface between the films. At these temperatures, the effect of negative photoconductivity is observed under uniform illumination with the light of a selected spectral composition event on the surface of the ferroelectric film. The combined exposure to green and infrared light led to an addition of the effects. As a result, a cumulative effect was observed. The effect of metallic conductivity is due to the discontinuity of ferroelectric polarization. Therefore, we explain that the partial screening of the ferroelectric polarization by photogenerated charge carriers causes a reduction in the carrier concentration at the interface. Measurements in the Kelvin mode of atomic force microscopy showed that illumination influences the surface charge concentration in a similar way; this observation confirms our hypothesis.

Fabrication of High-Temperature Quasi-Two-Dimensional Superconductors at the Interface of a Ferroelectric Ba_{0.8}Sr_{0.2}TiO_{3} Film and an Insulating Parent Compound of La_{2}CuO_{4}.

We report the first observation of superconductivity in a heterostructure consisting of an insulating ferroelectric film (Ba_{0.8}Sr_{0.2}TiO_{3}) grown on an insulating parent compound of La_{2}CuO_{4} with [001] orientation. The heterostructure was prepared by magnetron sputtering on a nonatomically flat surface with inhomogeneities of the order of 1-2 nm. The measured superconducting transition temperature T_{c} is about 30 K. We have shown that superconductivity is confined near the interface region. Application of a weak magnetic field perpendicular to the interface leads to the appearance of the finite resistance. That confirms the quasi-two-dimensional nature of the superconductive state. The proposed concept promises ferroelectrically controlled interface superconductivity which offers the possibility of novel design of electronic devices.

Nonequilibrium optical control of dynamical states in superconducting nanowire circuits.

Optical control of states exhibiting macroscopic phase coherence in condensed matter systems opens intriguing possibilities for materials and device engineering, including optically controlled qubits and photoinduced superconductivity. Metastable states, which in bulk materials are often associated with the formation of topological defects, are of more practical interest. Scaling to nanosize leads to reduced dimensionality, fundamentally changing the system's properties. In one-dimensional superconducting nanowires, vortices that are present in three-dimensional systems are replaced by fluctuating topological defects of the phase. These drastically change the dynamical behavior of the superconductor and introduce dynamical periodic long-range ordered states when the current is driven through the wire. We report the control and manipulation of transitions between different dynamically stable states in superconducting δ3-MoN nanowire circuits by ultrashort laser pulses. Not only can the transitions between different dynamically stable states be precisely controlled by light, but we also discovered new photoinduced hidden states that cannot be reached under near-equilibrium conditions, created while laser photoexcited quasi-particles are outside the equilibrium condition. The observed switching behavior can be understood in terms of dynamical stabilization of various spatiotemporal periodic trajectories of the order parameter in the superconductor nanowire, providing means for the optical control of the superconducting phase with subpicosecond control of timing.

Hole interactions with molecular vibrations on DNA.

We report on a study of the interactions between holes and molecular vibrations on dry DNA using photoinduced infrared absorption spectroscopy. Laser photoexcited holes are found to have a room-temperature lifetime in excess of tau > 1 ms, clearly indicating the presence of localization. However, from a quantitative model analysis of the frequency shifts of vibrational modes caused by the holes, we find the hole-vibrational coupling constant to be relatively small, lambda approximately 0.2. This interaction leads to a change in the conformational energy of DeltaE0 approximately 0.015 eV, which is too small to cause self-trapping at room temperature. We conclude that, at least in the dry (A) form, DNA is best understood in terms of a double chain of coupled quantum dots arising from the pseudorandom chain sequence of base pairs, in which Anderson localization prevents the formation of a metallic state.