Troy C. Messina

A student’s guide to searching the literature using online databases

A method is described to empower students to efficiently perform general and specific literature searches using online resources. The method was tested on undergraduate and graduate students with varying backgrounds in scientific literature. Students involved in this study showed marked improvement in their awareness of how and where to find accurate scientific information.

External field effects on the resonant frequency of magnetically capped oscillators for magnetic resonance force microscopy

We study the resonant frequency shift of CoPt-capped single-crystal-silicon micro-oscillators when a magnetic field is applied perpendicular to the magnetic film, as required for application to nuclear magnetic resonance force microscopy. The oscillator resonant frequencies show two distinct regimes of behavior. At low fields, when the magnetic moment is nearly perpendicular to the external field, the frequency decreases sharply with field, while at high fields, when the moment and field are nearly aligned, the frequency increases. We present models that accurately describe both behaviors. The transition point between these two regimes scales with the volume of the micromagnets.

Single-crystal silicon triple-torsional micro-oscillators for use in magnetic resonance force microscopy

Single-crystal silicon triple-torsional micro-oscillators have been fabricated, characterized, and modeled primarily for use in a magnetic resonance force microscope. These structures exploit a high-Q triple-torsional mode of oscillation while providing added stability. Fabrication involves lithography, reactive ion etch, and a final KOH wet-etch, with the final oscillator material being single-crystal boron-doped silicon. Typical oscillators were 250 nm thick and 10 - 200 microns in lateral dimensions. Finite element modeling provided the sequence and structure of the ten lowest-frequency modes and indicated that the upper torsional mode best isolates the motion from losses to the base. The oscillators were excited piezoelectrically and the resulting frequency-dependent motion was detected with fiber-optic interferometry, with a 0.002 nm/Hz1/2 resolution. Phase-sensitive motion detection at various points on the oscillator facilitated the assignment of the principle modes. Magnetic excitation was also investigated in order to best excite the torsional resonances. Cobalt micromagnets with moments below 10-12 J/T were electron-beam deposited onto oscillators, and the magnetic forces were measured. MRFM, the primary intended application of these novel structures, is discussed; in particular, an overview is given of an experiment which uses a double-torsional micro-oscillator for the force detection of nuclear magnetic resonance. All topics discussed in this work are being combined in order to achieve a NMRFM single-sweep sensitivity as low as 10-16 N/Hz1/2 at room temperature.

Steric quenching of the switchable mirror effect

Scandium was substituted for yttrium to observe the effect of unit cell size on the optical metal-to-insulator (MIT) transition in the Y_(1-z)Sc_(z)H_(x) alloy system. The optical transmittance decreases significantly for z>0.10. Simultaneous electrical resistivity measurements confirm the transition from trihydride to dihydride behavior with increasing z. These observations imply a quenching of the MIT when the unit cell volume falls below a critical level that is consistent with the boundary between trihydride and non-trihydride forming rare-earth elements. A combinatoric model reveals this formation boundary corresponds to two or more Sc per unit cell.

Observation of step-flow growth in laser-ablated thin films of the T'-phase compound Pr2CuO4

We report the results of laser ablation studies of T′-phase copper-oxide compounds. Variations in film morphology as a function of deposition parameters are discussed. Highly-oriented films were obtained, and a true step-flow growth mode accompanied by a coherent tilt boundary was observed.

NMR of 89Y in the Copper-Oxide Spin-Chain Compound Ca2+xY2−xCu5O10

We report NMR lineshape, spin-lattice relaxation time T1, and spin-spin relaxation time T2 data at 17 MHz (8.07 T) for 89Y in the copper-oxide spin-chain compound Ca2+xY2−xCu5O10. For x=0, a broad, asymmetric line with width Δν≈90 kHz is observed for T=250–300 K. The spectra exhibit an appreciable average shift (ΔH/H≈+0.7%) and sharpen at lower temperature, possibly due to increasing intrachain ferromagnetic correlations. T1 and T2 decrease with decreasing temperature. The Tl data imply a short correlation-time limit, with τe=3–5×10−11 s. The T2 data apparently include a contribution from dipolar interactions with copper nuclei. Relaxation time data for a doped (x=0.5) compound surprisingly show more rapid relaxation.

Studies of Infinite-Layer, T’-Phase, and 1-D-Ladder Copper-Oxide Compounds

Recent experimental studies of electron-doped infinite-layer compounds (e.g., Sr1-xLaxCuO2), electron-doped T’-phase compounds (e.g., Nd2-xCexCuO4), and spin-1/2 quasi-1-D chain/ladder compounds (e.g., Sr14Cu24O41) are overviewed, including new structural and magnetic measurements. Studies of steric effects indicate that superconductivity disappears in both electron-doped systems for values of the in-plane lattice constant a below a critical value, acr ≈ 3.92 A. Apparently, delocalized holes cannot be introduced into the ambient-pressure T’-phase materials (Ln2CuO4; Ln = Nd-Gd); some studies of low-temperature-synthesis rare-earth-free T’-phase La2-xYxCuO4 are discussed. The infinite-layer parent compound series Ca1-xSrxCuO2 makes accessible a large range of Cu-O bond lengths; we discuss high pressure synthesis and properties of Ca1-x-ySrxMyCuO2 for both electron (M = La, etc.) and hole (M = Na, K) doping. For the quasi-1-D chain/ladder materials, substitutions in the ambient-pressure parent compound Sr14Cu24O41 are studied (La is soluble in Sr14-xLaxCu24O41 for 0 ≤x ≤5); changes in susceptibility with such decreases in hole doping are discussed. Collaborative studies of these systems using µSR, NMR, and other spectroscopies are overviewed.

89Y NMR of the Spin‐Chain System Ca2Y2Cu5O10

We report measurements of the 89Y NMR lineshape, spin‐lattice relaxation time T1, and spin‐spin relaxation time T2 for the copper‐oxide spin‐chain compound Ca2Y2Cu5O10. The NMR data presented here, obtained at 17 MHz (8.1 T), show broad, asymmetric, two‐peaked lineshapes with width Δν ≈ 90 kHz. The appreciable shift of the line, ΔH/H ≈ 0.7%, decreases with increasing temperature. T2 increases with increasing temperature at lower temperatures, but plateaus at T2 = 13 ± 1 ms for temperatures above 325 K. For the spin‐lattice relaxation, we observe a T1 minimum around 275 K that is 100 times more shallow than expected for random 89Y−Cu2+ dipolar interactions. Such behavior indicates unusually strong spin correlations, and is surprising for the apparently small intrachain exchange coupling, Jnn/kB ≈ 40 K. The long Cu2+ electronic timescale (τe ∼ 10 −8 s) evident at temperatures below the T1 minimum indicates a slowing down of spin correlations far above TN.

Extreme smallness of the transverse force on moving vortices in Bi2Sr2CaCu2O8

Abstract We report the results of direct force measurements of the longitudinal (pinning and viscous) forces and the transverse (“Magnus”) force on vortices along the c -direction in Bi 2 Sr 2 CaCu 2 O 8 single crystals. The magnetic flux is applied locally to the center of a crystal using a micro-electromagnet, thus avoiding edge pinning and geometrical-pinning effects. The pinning and viscous forces measured by two different high- Q mechanical oscillator techniques are in good agreement, and agree with theoretical predictions. The measured transverse force on moving vortices is extremely small, in sharp contrast to recent “universal” theories, which predict the full hydrodynamic value. The data indicate that the transverse force on a moving vortex is smaller than the hydrodynamic value by a factor α ⩽0.015.

Steric effects on the metal–insulator (mirror–transparent) transition in YHx

Abstract In order to further study the role of structure on the metal–insulator (mirror–transparent) transition in yttrium-hydride (YH x ) films [Nature 380 (1996) 231], we have substituted scandium (Sc) into the Y lattice. This substitution shows the effect of reduction of unit cell size on this transition. Electron-beam evaporation was used to deposit 100-nm-thick films of Y 1− z Sc z alloys for 0≤ z ≤1. The films are capped with a 10 nm overlayer of palladium (Pd) to prevent oxidation and to catalyze hydrogen absorption. Fully hydrogenated alloys with z ≤0.10 exhibit optical transmittance of up to 0.14 in agreement with the literature [J. Alloys. Comp. 253–254 (1997) 41]. This transmittance decreases by a factor of 3–12 for scandium concentrations of 20% and greater. The cell volume for z ≥0.20 is consistent with rare-earth elements that do not form trihydrides. Details of sample synthesis, lattice structure, and optical transmission spectra are discussed.