Current driven tri-stable resistance states in magnetic point contacts.

Point contacts between normal and ferromagnetic metals are investigated using magnetoresistance and transport spectroscopy measurements combined with micromagnetic simulations. Pronounced hysteresis in the point contact resistance versus both bias current and external magnetic field are observed. It is found that such hysteretic resistance can exhibit, in addition to bi-stable resistance states found in ordinary spin valves, tri-stable resistance states with a middle resistance level.

Surface spin-valve effect.

We report an observation of spin-valve-like hysteresis within a few atomic layers at a ferromagnetic interface. We use phonon spectroscopy of nanometer-sized point contacts as an in situ probe to study the mechanism of the effect. Distinctive energy phonon peaks for contacts with dissimilar nonmagnetic outer electrodes allow localizing the observed spin switching to the top or bottom interfaces for nanometer thin ferromagnetic layers.

Atomic-size oscillations in conductance histograms for gold nanowires and the influence of work hardening.

Nanowires of different natures have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for Au wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials, and we discover a new set of periodic structures, which we interpret as due to the atomic discreteness of the contact size for the three principal crystal orientations.

Spectroscopy of phonons and spin torques in magnetic point contacts.

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts.

Search for E(2g) phonon modes in MgB2 single crystals by point-contact spectroscopy.

The electron-phonon interaction in magnesium diboride MgB2 single crystals is investigated by point-contact (PC) spectroscopy. For the first time the electron coupling with E(2g) phonon modes is resolved in the PC spectra. The correlation between intensity of the extremely broad E(2g) modes in the PC spectra and value of the superconducting gap is established.

Crossover from electronic to atomic shell structure in alkali metal nanowires.

After making a cold weld by pressing two clean metal surfaces together, upon gradually separating the two pieces a metallic nanowire is formed, which progressively thins down to a single atom before contact is lost. In previous experiments we have observed that the stability of such nanowires is influenced by electronic shell filling effects, in analogy to shell effects in metal clusters. For sodium and potassium at larger diameters there is a crossover to crystalline wires with shell closings corresponding to the completion of additional atomic layers.

Supershell structure in alkali metal nanowires.

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes.

Mass-spectrometric investigations on hydration of nucleic acid components in vacuum.

Association reactions between water and alkylated uracils. occurring under field-ionization conditions in a mass spectrometer at the tungsten point emitter surface, were studied at several temperatures. The origin of peaks observed in the mass spectra at m/e ratios corresponding to M+H and M+H-H2O were attributed to M-H2O and M-(H2O)2 hydrates, respectively, hydrogen-bonded via carbonyl groups of the diketopyrimidines (M) investigated.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. IV. Enthalpies of hydration of 5-alkyluracils.

Enthalpies of sublimation, DeltaH degrees (subl) and of solution in water, DeltaH degrees (sol) were determined for a series of crystalline 1,3-dimethyl-uracil derivatives substituted at the C5-ring carbon atom with alkyl groups (-C(n)H(2n+1), n = 2-4) and some of their C(5.6)-cyclooligomethylene analogues (-(CH2)(n)-, n = 3-5). From these data.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. III. Enthalpies of hydration of uracil, thymine and their derivatives.

Enthalpies of sublimation DeltaH(0)(subl) crystalline uracil, thymine and their methylated derivatives as well as of N,N-diethylthymine were determinated by the quartz-resonator method and mass spectrometry. Enthalpies of solution at infinite dilution DeltaH(0)(sol) in water of aBcylated compounds were obtained calorimetrically. Hence the calculated enthalpies of hydration: DeltaH(0)(hydrsubal) = DeltaH(0)(sol) - DeltaH(0)(subl), were corrected for energies of cavity formation in pure liquid water to yield enthalpies of interaction DeltaH(0)(sint) of the solutes with their hydration shells.