Hole concentration and phonon renormalization in Ca-doped YBa_2Cu_3O_y (6.76 < y < 7.00)

In order to access the overdoped regime of the YBa_2Cu_3O_y phase diagram, 2% Ca is substituted for Y in YBa_2Cu_3O_y (y = 7.00,6.93,6.88,6.76). Raman scattering studies have been carried out on these four single crystals. Measurements of the superconductivity-induced renormalization in frequency (Delta \omega) and linewidth (\Delta 2\gamma) of the 340 cm^{-1} B_{1g} phonon demonstrate that the magnitude of the renormalization is directly related to the hole concentration (p), and not simply the oxygen content. The changes in \Delta \omega with p imply that the superconducting gap (\Delta_{max}) decreases monotonically with increasing hole concentration in the overdoped regime, and \Delta \omega falls to zero in the underdoped regime. The linewidth renormalization \Delta 2\gamma is negative in the underdoped regime, crossing over at optimal doping to a positive value in the overdoped state.Comment: 18 pages; 5 figures; submitted to Phys. Rev. B Oct. 24, 2002 (BX8292

Interfacing topological insulators and ferrimagnets: Bi2_2Te3_3 and Fe3_3O4_4 heterostructures grown by molecular beam epitaxy

Relying on the magnetism induced by the proximity effect in heterostructures of topological insulators and magnetic insulators is one of the promising routes to achieve the quantum anomalous Hall effect. Here we investigate heterostructures of Bi$_2$Te$_3$ and Fe$_3$O$_4$. By growing two different types of heterostructures by molecular beam epitaxy, Fe$_3$O$_4$ on Bi$_2$Te$_3$ and Bi$_2$Te$_3$ on Fe$_3$O$_4$, we explore differences in chemical stability, crystalline quality, electronic structure, and transport properties. We find the heterostructure Bi$_2$Te$_3$ on Fe$_3$O$_4$ to be a more viable approach, with transport signatures in agreement with a gap opening in the topological surface states.Comment: accepted for publication in APL Material

Nernst Effect and Superconducting Fluctuations in Zn-doped YBa2_{2}Cu3_{3}O7−δ_{7-\delta}

We report the measurements of in-plane resistivity, Hall effect, and Nernst effect in Zn doped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ epitaxial thin films grown by pulsed laser deposition technique. The pseudogap temperature, $T^*$, determined from the temperature dependence of resistivity, does not change significantly with Zn doping. Meanwhile the onset temperature ($T^{\nu}$) of anomalous Nernst signal above $T_{c0}$, which is interpreted as evidence for vortex-like excitations, decreases sharply as the superconducting transition temperature $T_{c0}$ does. A significant decrease in the maximum of vortex Nernst signal in mixed state is also observed, which is consistent with the scenario that Zn impurities cause a decrease in the superfluid density and therefore suppress the superconductivity. The phase diagram of $T^*$, $T^{\nu}$, and $T_{c0}$ versus Zn content is presented and discussed.Comment: 6 pages, 5 figures, Latex; v2: to be published in PR

Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation

We have succeeded in growing epitaxial and highly stoichiometric films of EuO on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation process during the molecular beam epitaxy assisted growth enables the consistent achievement of stoichiometry. We have also succeeded in growing the films in a layer-by-layer fashion by fine tuning the Eu vs. oxygen deposition rates. The initial stages of growth involve the limited supply of oxygen from the YSZ substrate, but the EuO stoichiometry can still be well maintained. The films grown were sufficiently smooth so that the capping with a thin layer of aluminum was leak tight and enabled ex situ experiments free from trivalent Eu species. The findings were used to obtain recipes for better epitaxial growth of EuO on MgO (001).Comment: 10 pages, 15 figure

Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)

We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm the absence of Eu3+ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%.Comment: 8 pages, 9 figure

Electronic signature of the vacancy ordering in NbO (Nb3O3)

We investigated the electronic structure of the vacancy-ordered 4d-transition metal monoxide NbO (Nb3O3) using angle-integrated soft- and hard-x-ray photoelectron spectroscopy as well as ultra-violet angle-resolved photoelectron spectroscopy. We found that density-functional-based band structure calculations can describe the spectral features accurately provided that self-interaction effects are taken into account. In the angle-resolved spectra we were able to identify the so-called vacancy band that characterizes the ordering of the vacancies. This together with the band structure results indicates the important role of the very large inter-Nb-4d hybridization for the formation of the ordered vacancies and the high thermal stability of the ordered structure of niobium monoxide