Extrinsic and intrinsic charge trapping at the graphene/ferroelectric interface.

The interface between graphene and the ferroelectric superlattice PbTiO3/SrTiO3 (PTO/STO) is studied. Tuning the transition temperature through the PTO/STO volume fraction minimizes the adorbates at the graphene/ferroelectric interface, allowing robust ferroelectric hysteresis to be demonstrated. "Intrinsic" charge traps from the ferroelectric surface defects can adversely affect the graphene channel hysteresis and can be controlled by careful sample processing, enabling systematic study of the charge trapping mechanism.

Ferroelectric PbTiO3/SrRuO3 superlattices with broken inversion symmetry.

We have fabricated PbTiO3/SrRuO3 superlattices with ultrathin SrRuO3 layers. Because of the superlattice geometry, the samples show a large anisotropy in their electrical resistivity, which can be controlled by changing the thickness of the PbTiO3 layers. Therefore, along the ferroelectric direction, SrRuO3 layers can act as dielectric, rather than metallic, elements. We show that, by reducing the concentration of PbTiO3, an increasingly important effect of polarization asymmetry due to compositional inversion symmetry breaking occurs. The results are significant as they represent a new class of ferroelectric superlattices, with a rich and complex phase diagram. By expanding our set of materials we are able to introduce new behaviors that can only occur when one of the materials is not a perovskite titanate. Here, compositional inversion symmetry breaking in bicolor superlattices, due to the combined variation of A and B site ions within the superlattice, is demonstrated using a combination of experimental measurements and first principles density functional theory.

Engineering polarization rotation in a ferroelectric superlattice.

A key property that drives research in ferroelectric perovskite oxides is their strong piezoelectric response in which an electric field is induced by an applied strain, and vice versa for the converse piezoelectric effect. We have achieved an experimental enhancement of the piezoelectric response and dielectric tunability in artificially layered epitaxial PbTiO(3)/CaTiO(3) superlattices through an engineered rotation of the polarization direction. As the relative layer thicknesses within the superlattice were changed from sample to sample we found evidence for polarization rotation in multiple x-ray diffraction measurements. Associated changes in functional properties were seen in electrical measurements and piezoforce microscopy. The results demonstrate a new approach to inducing polarization rotation under ambient conditions in an artificially layered thin film.

New phenomena at the interfaces of very thin ferroelectric oxides.

We present a brief review of the role of interfacial physics in ferroelectric oxides, with an emphasis on the importance of boundary conditions that determine the properties of very thin ferroelectric films and superlattices. As well as discussing the screening problem, and the role of strain and electrostatics in ferroelectrics, we highlight some of the possibilities in fine period superlattices where the high density of interfaces can lead to new and potentially useful phenomena.

Cat sleep: a unique first night effect.

Longitudinal sleep studies in chronically implanted cats revealed a unique first night effect. Unlike other species, during the first 24 hr of sleep recording, the electroencephalogram and reticular formation multiple unit activity revealed more rapid eye movement sleep occurred than during subsequent 24 hr recording sessions. In addition, higher rapid eye movement to slow wave sleep ratios were observed for the first 24 hr as compared to the following days. An increase in wakefulness and decrease in slow wave sleep occurred on the third recording day as compared to the initial 24 hr period. These data have important implications for studies of variables on the sleep pattern of animals.

A new anorexigen assay: stress-induced hyperphagia in rats.

Humans often respond to stress by overeating. A rat model of hyperphagia induced by a stressful tail-pinch has been described. Consumption of sweetened condensed milk by vaive rats was measured durin the stress. This milk drinking was prevented by pretreatment with d-amphetamine, chlorphentermine, diethylpropion, fenfluramine, methamphetamine, morphine, chlorpromazine and haloperidol. The benzodiazepines chlordiazepoxide and diazepam increased the hyperphagia. The relevance of dopaminergic agonistic and antagonist activities to anorexigenic activity are questioned.

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices.

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.