John T. Markert

Dielectron production in Ar+KCl collisions at 1.76A GeV

We present results on dielectron production in 40 Ar+KCl collisions at 1.76A GeV. For the first time ω mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The ω multiplicity has been extracted and compared to the yields of other particles, in particular of the φ meson. At intermediate e + e invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions suggesting the onset of non-trivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for ]

Superfluid and normal fluid density in high-Tc superconductors

The electronic properties of the cuprate superconductors have been studied by measuring the reflectance over the frequency range from the far-infrared to the near-ultraviolet(roughly, 10 meV-5 eV). There is an interesting behavior in both the normal state and the superconducting state. In the normal state, there is the well-known non-Drude distribution of the doping-induced spectral weight. In the superconducting state, the spectral weight of the superconducting condensate correlates with Tc in a variety of materials. Remarkably, in optimally doped superconductors, only about 20% of the doping-induced carriers joins the superfluid; the rest of the spectral weight remains at finite frequencies.

Surface structure of YBa2Cu3O7−x probed by reversed‐bias scanning tunneling microscopy

We report the results of scanning tunneling microscopy studies on high‐quality single crystals of YBa2Cu3O7−x (YBCO) which were cleaved along a basal plane in situ at 20 K prior to measurement. The initial results of a systematic study of the YBCO surface are presented. Reversed‐bias images of the CuO chain layer reveal modulations 3.3±0.3‖b‖=1.3±0.1 nm in wavelength which change phase by 180° under bias polarity reversal along solid chains, and are invariant under bias polarity reversal at a point defect. Regions of sharp unit‐cell‐sized square corrugations with disordered islands are also observed. We interpret these new results in terms of our previous model [H. L. Edwards, J. T. Markert, and A. L. de Lozanne, Phys. Rev. Lett. 69, 2967 (1992)] of the cleaved YBCO surface.

DESIGN AND CONSTRUCTION OF A SENSITIVE NUCLEAR MAGNETIC RESONANCE FORCE MICROSCOPE

We report our progress in the design and construction of a sensitive and versatile nuclear magnetic resonance force microscope. Improvements over previous designs include the use of higher Q(Q≈105–107), single-crystal double-torsional mechanical oscillators for force detection and the development of extremely convenient positioning and approach capabilities. We describe both a demonstration experiment using large (∼1 cm) torsional oscillators and the micromachining of the small (∼100 μm) torsional oscillators.We report our progress in the design and construction of a sensitive and versatile nuclear magnetic resonance force microscope. Improvements over previous designs include the use of higher Q(Q≈105–107), single-crystal double-torsional mechanical oscillators for force detection and the development of extremely convenient positioning and approach capabilities. We describe both a demonstration experiment using large (∼1 cm) torsional oscillators and the micromachining of the small (∼100 μm) torsional oscillators.

Superconductivity and spin fluctuations in the electron-doped infinitely-layered high Tc superconductor Sr0.9La0.1CuO2 (Tc = 42 K)

We report 63 Cu NMR studies on uniaxially aligned and unaligned powder samples of an electron-doped infinitely-layered high T c cuprate superconductor Sr 0.9 La 0.1 CuO 2 (T c = 42 K). We measured the nuclear spin-lattice relaxation rate 1/T 1 up to 800 K. We found from the temperature dependence of 1/T 1 T that the wave-vector-averaged spin susceptibility is highly enhanced by spin-fluctuations in the normal state, resulting in a Curie-Weiss behavior. Below T c , we did not observe a Hebel-Slichter coherence peak of 1/T 1 , suggesting an unconventional nature of,the symmetry of the superconducting order parameter. These results are quite similar to those observed for some hole-doped high T c cuprates.

Chemical Signature of the Superconducting Phase in the Nd-Ce-Cu-O System

The electron-doped material Nd2–xCexCuO4 becomes superconducting with a Ce4+ composition around 0.16, but only after removal of a minuscule amount of extraneous oxygen. This enigmatic behavior was addressed here. A small fraction of copper in the CuO2 planes of Nd2–xCexCuO4 was substituted by cobalt-57, which serves as a microprobe of the chemical environment. Deoxygenation brought about little change in the M�ssbauer spectra both above and below the optimal superconducting concentration; however, for x = 0.16 a change was observed. In the latter, a major fraction of the magnetically split, five-coordinate species showed itself as a paramagnetically relaxed doublet upon deoxygenation. The abundance of the paramagnetically relaxed species corresponds closely to the diamagnetic volume fraction and thus provides a microscopic signature of the superconducting phase.

Inclusive dielectron production in proton-proton collisions at 2.2 GeV beam energy

Data on inclusive dielectron production are presented for the reaction p+p at 2.2 GeV measured with the High Acceptance DiElectron Spectrometer (HADES). Our results supplement data obtained earlier in this bombarding energy regime by DLS and HADES. The comparison with the 2.09 GeV DLS data is discussed. The reconstructed e+e- distributions are confronted with simulated pair cocktails, revealing an excess yield at invariant masses around 0.5 GeV/c2. Inclusive cross sections of neutral pion and eta production are obtained.

Medium effects in proton-induced

We present the analysis of the inclusive K 0 production in p + p and p + Nb collisions measured with the HADES detector (GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt) at a beam kinetic energy of 3.5 GeV. Data are compared to the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) transport model. The data suggest the presence of a repulsive momentum-dependent kaon potential as predicted by the chiral perturbation theory (ChPT). For the kaon at rest and at normal nuclear density, the ChPT potential amounts to ≈ 35 MeV. A detailed tuning of the kaon production cross sections implemented in the model has been carried out to reproduce the experimental data measured in p + p collisions. The uncertainties in the parameters of the model were examined with respect to the sensitivity of the experimental results from p + Nb collisions to the in-medium kaon potential.

K^{0}

Controlling the morphology of inorganic nanocrystals is important because many of their electronic attributes are highly sensitive to shape and aspect ratio. FePt nanocrystals have potential as advanced magnetic materials for ultrahigh-density memory. This is due to their high shape and/or magnetocrystalline anisotropy, which allows bits as small as 3 nm to be thermally stable over typical data storage periods of 10 years. Herein, nanocrystals were simply fabricated by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in properly chosen conditions of solvent/surfactant proportions and temperature for rational design of their shape and magnetic properties. This work has combined magnetometry measurements and micromagnetic simulations to illustrate the role of the external shape on the rotation of the magnetization vector for colloidal assemblies.

production at 3.5 GeV

Micro-oscillators of different designs and dimensions have been fabricated for use in a nuclear magnetic resonance force microscope. The various designs include double and triple torsional oscillators which have high Qs at room temperature (approximately equals 10,000) when operating at the upper cantilever and upper torsional resonances. Depending on design and dimensions, the resonance frequencies vary from tens to hundreds of kHz. Typical dimensions of the designs are (200 X 150) micrometers 2 X 200 nm thick. To fabricate these devices, microelectric fabrication techniques were employed. Si (100) wafers were patterned, etched, and boron-implanted at a dose of 4.2 X 1016 cm-2 and an energy of 134 keV. A post-implant anneal was then performed at 1000 degree(s)C, followed by a KOH wet-etch which leaves the free-standing boron-doped oscillators. Depending on the doping level, anneal, and etch parameters, the thickness of the oscillators varies from 100 - 400 nm. In order to optimize the design and fabrication process, resonance frequencies and Qs have been characterized using fiber-optic interferometry. For example, the upper cantilever resonance of one design has been found to have a minimum detectable force of 1.5 X 10-16 N/(root)Hz at room temperature.