We developed the uniaxial strain method to compress a crystalline sample along any direction without involving Poisson’s effect. The uniaxial strain is realized by inserting a composite of either the sample embedded in epoxy or the sample in frozen oil into a cylinder much harder than the sample composite followed by the application of external forces to a piston put on the sample composite. We verified, by using a strain gauge embedded in the epoxy or the frozen oil, that the strain thus created has the uniaxial nature. Resistance measurements on two kinds of organic conductors, α-(BEDT–TTF)2KHg(SCN)4 and α-(BEDT–TTF)2I3 under the uniaxial strain showed novel electric properties that have never been found under hydrostatic pressures. Their properties were found to be largely dominated by the direction of the uniaxial strain even when it is applied along directions in the conducting plane having a two-dimensional nature.

We have performed angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles electronic structure calculations on the electron-doped high-Tc superconductors Ln1.85Ce0.15CuO4 (Ln = Nd, Sm, and Eu). The observed Fermi surface and band dispersion show such changes that with decreasing ionic size of Ln 3+ , the curvature of the Fermi surface or −t ′ /t decreases, where t and t ′ are transfer integrals between the nearest-neighbor and next-nearestneighbor Cu sites, respectively. The increase of t with chemical pressure is found to be significant, which may explain the apparently inconsistent behavior seen in the hole-doped La2−xSrxCuO4 under epitaxial strain [M. Abrecht et al., Phys. Rev. Lett. 91, 057002 (2003)]. A gap due to the antiferromagnetism opens even in the nodal region for the Sm and Eu compounds, and the gap size increases in going from Ln = Sm to Eu.

Systematic uniaxial strain studies of ␣-͑BEDT-TTF͒ 2 M Hg͑SCN͒ 4 (M ϭK,NH 4 ͒ have been carried out. By controlling the in-plane lattice parameters a and c independently, we find that both compounds have essentially the same phase diagram in electronic properties, although, under the hydrostatic pressures, they show quite different phase diagrams. The K salt shows the superconductivity like NH 4 salt under the c-axis uniaxial strain, and the latter undergoes a transition to the density-wave state like the former under the a-axis uniaxial strain. These changes in electronic structures are verified by measuring the angle-dependent magnetoresistance. The electronic properties of these compounds can be systematically understood as a function of c/a, the ratio of the in-plane lattice parameters.

Ru and RuO2 thin films are considered to be new electrode materials for dynamic random access memories (DRAMs) and ferroelectric nonvolatile memories because of their low resistivity and good thermal and chemical stabilities. In this study these thin films were pepared by reactively sputtering a Ru metal target (99.9% purity) in an argon and oxygen atmosphere. XPS spectra were collected with a PHI 1600 spectrometer equipped with a monochromatic Al Kα x-ray source and a multichannel detector. This report includes XPS spectra of Ru 3d and O 1s core regions for these samples. The binding energy of Ru 3d5/2 is determined as 280.0 and 280.8 eV for Ru and RuO2 films, respectively. The presence of a small amount of Ru with higher oxidation states, such as Ru6+ and Ru8+, is shown at the surface of the RuO2 thin film.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of Nd1.85Ce0.15CuO4 (NCCO) in order to elucidate the origin of the high-energy kink (HEK) observed in the high-Tc superconductors (HTSCs). The energy scale of the HEK in NCCO is large compared with that in hole-doped HTSCs, consistent with previous ARPES studies. From measurement in a wide momentum region, we have demonstrated that between the hole-and electron-doped HTSCs the energy position of the HEK is shifted approximately by the amount of the chemical potential difference. Also, we have found that around (π, 0) the HEK nearly coincides with the band bottom while around the node the band reaches the incoherent region and the HEK appears at the boundary between the coherent and incoherent regions.

Momentum-resolved inelastic resonant x-ray scattering is used to map the doping evolution of bulk electronic modes in the doped Mott insulator class Nd 2−x Ce x CuO 4 . As the doping induced antiferromagnet/ superconductor ͑AFM/SC͒ transition is approached, we observe an anisotropic redistribution of the spectral weight of collective excitations over a large energy scale along the ⌫ → ͑ , ͒ direction, whereas the modes exhibit broadening ͑ϳ1 eV͒ with relatively little softening along ⌫ → ͑ ,0͒ with respect to the parent Mott state ͑x =0͒. Our study reveals a closing of the charge gap in the vicinity of the zone center even though the mode softening and spectral redistribution involve an unusually large energy scale over the full Brillouin zone. The collective behavior of modes in the vicinity of the AFM/SC critical transition is demonstrated. DOI: 10.1103/PhysRevB.78.073104 PACS number͑s͒: 78.70.Ck, 74.20.Mn, 74.25.Jb, 74.72.Ϫh The evolution of a strongly correlated material with doping-from a Mott insulator to a conducting metal-is one of the most intensively studied issues in modern condensed matter physics. This fascinating evolution has proven to be full of surprises, such as the appearance of high-T c superconductivity, non-Fermi liquid behavior, and nanoscale phase separation. 1 Mott insulators often exhibit phase transitions upon doping, which are signaled or hallmarked by the softening or redistribution of the spectral weight of collective charge or spin modes. The behavior of spin modes has been extensively investigated via neutron scattering.2 Although charge excitations near the Brillouin zone ͑BZ͒ center can be accessed by optical techniques, 3 their behavior with momentum over the full BZ remains largely unexplored. Here, as demonstrated in recent experimental 4,5 and theoretical studies, 6,7 inelastic x-ray scattering provides such a unique opportunity. While previous studies have focused largely on either undoped one-dimensional 8 or two-dimensional 4,5 insulators, or hole-doped superconductors, 9 Mott insulators can be doped with electrons as well. In fact, it appears that with electron doping, cuprate bands evolve in a much more straightforward and systematic manner 10-14 than with hole doping. A limited previous work 15 that focused on a superconductor with a fit to the one-band theory does not provide insights into the way the collective modes of a nonsuperconducting Mott insulator evolve into a superconductor wherein the high-energy excitations of the Mott insulator are intimately connected to the lower-energy physics of the superconductor. Here, we report a high-resolution study of the evolution of the collective charge excitations of the Mott insulating state ͑x =0͒ with electron doping in approaching the critical antiferromagnet/superconductor ͑AFM/SC͒ transition. Our finding, which was made possible by studying the doped insulating states, is that as the electron-doping induced AFM/SC transition is approached from the x = 0 Mott side, the system exhibits anisotropic softening of the e...

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C-axis-oriented Ru thin films were deposited on glass substrates by sputtering a Ru target in Ar and O 2 gas mixture with O 2 flow ratios which were lower than that required for RuO 2 formation. A minimum value of 3.5 • was obtained for the full-width at half maximum (FWHM) of the rocking curve of a Ru (002) peak for the Ru film deposited at a substrate temperature of 500 • C and O 2 flow ratio of 4%. The c-axis-oriented Ru films were observed to be formed from the initial stage of crystal growth and became continuous even at a film thickness of 3 nm. Two-dimensional crystal growth of the single-axis-oriented Ru films was suggested.