Probing Magnetic Exchange Interactions with Helium.

Controlling and sensing spin polarization of electrons forms the basis of spintronics. Here, we report a study of the effect of helium on the spin polarization of the tunneling current and magnetic contrast in spin-polarized scanning tunneling microscopy (SP STM). We show that the magnetic contrast in SP STM images recorded in the presence of helium depends sensitively on the tunneling conditions. From tunneling spectra and their variation across the atomic lattice we establish that the helium can be reversibly ejected from the tunneling junction by the tunneling electrons. The energy of the tunneling electrons required to eject the helium depends on the relative spin polarization of the tip and sample, making the microscope sensitive to the magnetic exchange interactions. We show that the time-averaged spin polarization of the tunneling current is suppressed in the presence of helium and thereby demonstrate voltage control of the spin polarization of the tunneling current across the tip-sample junction.

Cryogenic STM in 3D vector magnetic fields realized through a rotatable insert.

Spin-polarized scanning tunneling microscopy (SP-STM) performed in vector magnetic fields promises atomic scale imaging of magnetic structure, providing complete information on the local spin texture of a sample in three dimensions. Here, we have designed and constructed a turntable system for a low temperature STM which in combination with a 2D vector magnet provides magnetic fields of up to 5 T in any direction relative to the tip-sample geometry. This enables STM imaging and spectroscopy to be performed at the same atomic-scale location and field-of-view on the sample, and most importantly, without experiencing any change on the tip apex before and after field switching. Combined with a ferromagnetic tip, this enables us to study the magnetization of complex magnetic orders in all three spatial directions.

Structure of a model TiO2 photocatalytic interface.

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

Engineering Polarons at a Metal Oxide Surface.

Polarons in metal oxides are important in processes such as catalysis, high temperature superconductivity, and dielectric breakdown in nanoscale electronics. Here, we study the behavior of electron small polarons associated with oxygen vacancies at rutile TiO_{2}(110), using a combination of low temperature scanning tunneling microscopy (STM), density functional theory, and classical molecular dynamics calculations. We find that the electrons are symmetrically distributed around isolated vacancies at 78 K, but as the temperature is reduced, their distributions become increasingly asymmetric, confirming their polaronic nature. By manipulating isolated vacancies with the STM tip, we show that particular configurations of polarons are preferred for given locations of the vacancies, which we ascribe to small residual electric fields in the surface. We also form a series of vacancy complexes and manipulate the Ti ions surrounding them, both of which change the associated electronic distributions. Thus, we demonstrate that the configurations of polarons can be engineered, paving the way for the construction of conductive pathways relevant to resistive switching devices.

Oxygen vacancy origin of the surface band-gap state of TiO2(110).

Scanning tunneling microscopy and photoemission spectroscopy have been used to determine the origin of the band-gap state in rutile TiO2(110). This state has long been attributed to oxygen vacancies (O{b} vac). However, recently an alternative origin has been suggested, namely, subsurface interstitial Ti species. Here, we use electron bombardment to vary the O{b} vac density while monitoring the band-gap state with photoemission spectroscopy. Our results show that O{b} vac make the dominant contribution to the photoemission peak and that its magnitude is directly proportional to the O{b} vac density.

Investigations of photoreceptor synaptic transmission and light adaptation in the zebrafish visual mutant nrc.

PURPOSE: To characterize the retinal physiology of the zebrafish visual mutant no optokinetic response c (nrc) and to identify the genetic map position of the nrc mutation. METHODS: Electroretinograms were recorded from wild-type and nrc zebrafish larvae between 5 to 6 days postfertilization. Responses to flash stimuli, On and Off responses to prolonged light stimuli, and responses to flash stimuli with constant background illumination were characterized. The glutamate agonist, 2-amino-4-phosphonobutyric acid (APB) was used to examine the photoreceptor specific a-wave component of the electroretinogram. Amplified fragment length polymorphism methodology was used to place the nrc mutation on the zebrafish genomic map. RESULTS: nrc and wild-type zebrafish larvae 5 to 6 days postfertilization have similar threshold responses to light, but the b-wave of the nrc electroretinogram is significantly delayed and reduced in amplitude. On and Off responses of nrc larvae to prolonged light have multiple oscillations that do not occur in normal zebrafish larvae after 5 days postfertilization. Analysis of the b-wave demonstrated a light adaptation defect in nrc that causes saturation at background light levels approximately 1 order of magnitude less than those with wild-type larvae. Application of the glutamate analog, APB, uncovered the photoreceptor component of the electroretinogram and revealed a light adaptation defect in nrc photoreceptors. The nrc mutation was placed approximately 0.2 cM from sequence length polymorphism marker Z7504 on linkage group 10. CONCLUSIONS: The zebrafish mutant nrc is a possible model for human retinal disease. nrc has defects in photoreceptor synaptic transmission and light adaptation. The nrc mutant phenotype shows striking similarities with phenotypes of dystrophin glycoprotein complex mutants, including patients with Duchenne/Becker muscular dystrophy. Localization of the nrc mutation now makes it possible to evaluate candidate genes and clone the nrc gene.