Search Results (7 total)

Recent initial experiments in magnetic resonance force microscopy (MRFM) have detected the magnetic force exerted by electrons and nuclei in microscopic samples. The experiments generate a force signal by modulating the sample magnetization with standard magnetic resonance techniques. Sample sizes of a few nanograms generate readily detected force signals of order 10^-14 to 10^-16 Newtons. This article describes the present status of MRFM technology, with particular attention to the feasibility of detecting single-electron magnetic moments, and the possible applications of MRFM in biological imaging.

The elastic response (Young’s modulus and internal friction) of solid C₆₀ has been measured in single-crystal specimens as a function of temperature. We determine a Debye temperature of ∼100 K and a room-temperature Young’s modulus of 2.0±0.5×10¹¹ dyn/cm². The elastic properties are strongly anharmonic and indicate phase transitions at ∼150 and ∼269 K.

The sample length of the charge-density-wave (CDW) conductor TaS₃ is found to be a sensitive function of the applied dc bias electric field E and time t. The length reflects transitions through metastable CDW configurations for both the pinned and sliding CDW in the presence of random impurities. Although a simple theoretical model can account for some qualitative features of the experimental findings, our experimental results are inconsistent with present CDW elasticity theory.

We have applied pressure (i.e., stress) to single-crystal YBa₂Cu₃O₇ along the c axis to reduce the separation between the copper-oxygen planes. For the normal state, increasing pressures up to ∼1 kbar causes the c-axis resistivity to decrease dramatically and tend toward a metalliclike temperature dependence. No change is observed in the ab-plane resistivity. The superconducting T_c increases with decreasing interplane separation with dT_c/dP≊0.1 K/kbar. Some implications of these findings for normal-state conduction mechanisms and models of high-T_c superconductivity for YBa₂Cu₃O₇ are discussed.

The magnitude of the oxygen isotope effect in the high-T_c superconductor YBa₂Cu₃O₇ has been determined by substitution of ¹⁸O for ¹⁶O. A series of cross exchanges was performed on highquality polycrystalline specimens to eliminate uncertainties due to sample heat treatments and sample inhomogeneities. Magnetic measurements suggest a relative isotope shift for the bulk material of -0.17±0.03 K at 80% ¹⁸O substitution, yielding α_bulk=0.019±0.004 where T_c∼M^-α and M is the oxygen mass. Resistivity measurements in freshly prepared specimens reveal filamentary superconductivity 1 or 2 K above the bulk superconducting transition temperature. The isotope shift associated with the filamentary superconductivity is similar to but slightly larger than the bulk shift: α_fil=0.028±0.003. The filamentary superconductivity is time dependent and disappears several months after sample preparation. We show that the above values of α are inconsistent with the standard three-dimensional phonon-mediated Bardeen-Cooper-Schrieffer theory, and discuss implications for alternative possibilities.

The elastic response (Young's modulus and internal friction) of the high-T_c superconductor YBa₂Cu₃O₇ has been measured in single-crystal and polycrystalline specimens. For the first time, we resolve the small expected lattice softening associated with the superconducting phase transition. The anomalous lattice stiffening below T_c in polycrystalline samples is present also in single crystals.

An oxygen isotope shift is observed in superconducting La_1.85Sr_0.15CuO₄ when ¹⁸O is substituted partially for ¹⁶O; the superconducting transition temperature T_c is lowered by 0.3 to 1.0 K in different samples. We examine these results using conventioanl phonon-mediated BCS theory and conclude that, for La_1.85Sr_0.15CuO₄, phonons play an important role in the pairing mechanism.