Merrill W. Shafer

Bio: Merrill W. Shafer is an academic researcher from IBM. The author has contributed to research in topics: Superconductivity & Oxide. The author has an hindex of 16, co-authored 52 publications receiving 1074 citations.

Correlation of Tc with hole concentration in La2-xSrx

Abstract: The electron deficiency (hole concentration) in the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ system is determined by a chemical method. Our results show a direct correlation between ${T}_{c}$ and hole concentration. The hole concentration equals the Sr concentration to about $x=0.15$. For $xg0.15$, the hole concentration decreases and oxygen vacancies are formed. These results strongly support an all-electronic mechanism for superconductivity in this system.

Hole concentrations, Hall number, and Tc relationships in substituted YBa2Cu3Oy.

Abstract: One of the fundamental questions involving the high-temperature copper oxide containing superconductors is to understand the relationship between hole concentration and the superconducting transition temperature, ${T}_{c}$. We help resolve this question by a study of Nd-, La-, Ga-, Ca-, and Zn-substituted $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ with controlled oxygen stoichiometry. The total hole concentration is determined by a chemical method and trends in the mobile hole concentration by Hall measurements. By considering ${T}_{c}$ to be a function of the mobile sheet holes only, there is a maximum in ${T}_{c}$ (92 K) at a sheet-hole concentration of about 0.20-0.25 per Cu site.

Orientation of the O 2p holes in Bi2Sr2Ca1Cu2O8.

Abstract: Unoccupied O 2p states are observed in Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8/ near the Fermi level. They show up as a spike in the O 1s absorption edge at threshold (h..nu.. = 528.2 eV). The orientation of these O 2p states is determined via optical selection rules from the polarization dependence of the absorption using single crystals. The O 2p holes are found to have at least 95% p/sub x//sub ,//sub y/ character with x,y in the superconducting a-b plane.

Phase Equilibria in the System La2O3—Iron Oxide in Air

Abstract: Phase equilibria data are presented for compositions in the system La2O3-iron oxide in air. Liquidus and solidus curves were obtained by the quenching method in the iron-rich portion of the system. The remainder of the diagram was determined using a strip-furnace technique. Two compounds have been found, the ortho-rhombic perovskite LaFeO3 and a compound with the magnetoplumbite structure corresponding to a composition LaFe12O19. LaFeO3 was determined to melt congruently at about 1890°C. whereas LaFe12O19 has both a stability minimum and maximum at 1380° and 1421°C., respectively. The iron-rich portion of the system is essentially ternary whereas the remainder can be considered to be a simple binary. Liquid compositions have been determined and are plotted in terms of the ternary system La2O3-Fe2O3-FeO.

Infrared study of anisotropy in single-crystal La 2 − x Sr x Cu O 4

Abstract: We have made infrared reflectivity measurements on single crystals of ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ and ${\mathrm{La}}_{1.92}$${\mathrm{Sr}}_{0.08}$Cu${\mathrm{O}}_{4}$ with the infrared radiation polarized parallel and perpendicular to the copper-oxygen planes. In ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ we observe nearly insulating spectra, from which we easily identify the dominant in-plane and out-of-plane infrared-active phonons. When ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ is doped with Sr, we observe a dramatic increase in the electronic contribution to the $a\ensuremath{-}b$ plane conductivity, which tends to screen the in-plane phonons. In contrast, the out-of-plane response is essentially unchanged by doping. This leads to a conductivity anisotropy which we estimate to be at least 50 to 1 for our ${\mathrm{La}}_{1.92}$${\mathrm{Sr}}_{0.08}$Cu${\mathrm{O}}_{4}$ crystal. Our observations on single crystals resolve some of the controversies which have arisen over the interpretation of reflectivity measurements on polycrystalline samples.

Electronic structure of the high-temperature oxide superconductors

Abstract: Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Scanning tunneling microscopy and atomic force microscopy: application to biology and technology

Abstract: The scanning tunneling microscope (STM) and the atomic force microscope (AFM) are scanning probe microscopes capable of resolving surface detail down to the atomic level. The potential of these microscopes for revealing subtle details of structure is illustrated by atomic resolution images including graphite, an organic conductor, an insulating layered compound, and individual adsorbed oxygen atoms on a semiconductor. Application of the STM for imaging biological materials directly has been hampered by the poor electron conductivity of most biological samples. The use of thin conductive metal coatings and replicas has made it possible to image some biological samples, as indicated by recently obtained images of a recA-DNA complex, a phospholipid bilayer, and an enzyme crystal. The potential of the AFM, which does not require a conductive sample, is shown with molecular resolution images of a nonconducting organic monolayer and an amino acid crystal that reveals individual methyl groups on the ends of the amino acids. Applications of these new microscopes to technology are demonstrated with images of an optical disk stamper, a diffraction grating, a thin-film magnetic recording head, and a diamond cutting tool. The STM has even been used to improve the quality of diffraction gratings and magnetic recording heads.

Electrodynamics of high- T c superconductors

Abstract: Recent studies of the electromagnetic response of high-Tc superconductors using terahertz, infrared, and optical spectroscopies are reviewed. In combination these experimental techniques provide a comprehensive picture of the low-energy excitations and charge dynamics in this class of materials. These results are discussed with an emphasis on conceptual issues, including evolution of the electronic spectral weight in doped Mott-Hubbard insulators, the d-wave superconducting energy gap and the normal-state pseudogap, anisotropic superfluid response, electronic phase segregation, emergence of coherent electronic state as a function of both temperature and doping, the vortex state, and the energetics of the superconducting transition. Because the theoretical understanding of these issues is still evolving the review is focused on the analysis of the universal trends that are emerging out of a large body of work carried on by many research teams. Where possible data generated by infrared/optical techniques are compared with the data from other spectroscopic and transport methods.

Perovskite‐Type Oxides—the New Approach to High—Tc Superconductivity

Abstract: The review dwells on the problems of high-temperature superconductivity. Ideas encouraging the search for high-temperature superconductivity are elucidated. The way from cubical alloys, containing niobium, to laminated copper-containing oxides with the perovskite-type structure is shown. Properties of new laminated oxide superconductors are described.

Microwave ferrites, part 1: fundamental properties

Abstract: Ferrimagnets having low RF loss are used in passive microwave components such as isolators, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or by passing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe12O19 and SrFe12O19, which have high effective internal magnetic anisotropy that also contributes to the permanent bias. Such a self-biased material remains magnetized even after removing the external applied magnetic field, and thus, may not even require an external permanent magnet. In garnet and spinel ferrites, such as Y3Fe5O12 (YIG) and MgFe2O4, however, the uniaxial anisotropy is much smaller, and one would need to apply huge magnetic fields to achieve such high frequencies. In Part 1 of this review of microwave ferrites a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing, are presented. Operational principles of microwave passive components and electrical tuning of magnetization using magnetoelectric coupling are discussed in Part 2.