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Superconducting aluminum thin films are integral to many astrophysics detector applications. Using x-ray absorption spectroscopy (XAS), we have studied the residues and adsorbates created during various standard lithography and etch steps, which are commonly used to pattern thin aluminum films into device structures. We have observed the formation of aluminum oxide as α-Al2O3 and aluminum fluoride as β-AlF3. We have observed correlations between these XAS signatures and the Al film's microwave loss due to two-level systems. This study, which guides the way for future device optimization, further explores the chemical impact of different process steps, including standard silicon substrate wafer cleaning processes, sulfur-hexa-fluoride plasma etching, passivation with a fluorocarbon, and exposure to photoresist adhesion promoters during the lithography process with the help of control samples. [ABSTRACT FROM AUTHOR]

Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient second-harmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac states.

Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the indirectly intersurface-coupled topological insulator Bi2-x Mn x Se3 films (12 nm thick) revealed a strong dependence of the rise-time and initial decay-time constants on photoexcited carrier density and Mn content. In undoped samples (x  =  0), these time constants are exclusively governed by electron-electron and electron-phonon scattering, respectively, whereas in films with x  =  0.013-0.27 ultrafast carrier dynamics are completely controlled by photoexcited electron trapping by ionized Mn(2+) acceptors and their dimers. The shortest decay-time (~0.75 ps) measured for the film with x  =  0.27 suggests a great potential of Mn-doped Bi2Se3 films for applications in high-speed optoelectronic devices. Using Raman spectroscopy exploiting similar laser photon energy (1.58 eV), we demonstrate that due to indirect intersurface coupling in the films, the photoexcited electron trapping in the bulk enhances the electron-phonon interaction strength in Dirac surface states.

Analysis of the results from our investigations of the temperature dependence of ac susceptibilities (χ' and χ") and magnetic viscosity in a 7.6 nm thin film of La0.7Sr0.3MnO3 grown on SrTiO3 (001) substrate is presented. The Curie temperature (TC) of this film is magnetic field (H) dependent, varying as [TC(H) -- TC(0)] ~ Hn with TC(0) = 270 K and n = 0.15. Temperature dependence of χ' and χ" measured at ac frequencies fm in the range of 0.1-10 kHz shows a broad peak near 230 K associated with the blocking temperature (TB) of spin clusters present in the 1.4 nm surface layer and a frequency dependent peak near 270 K associated with TC. The mean relaxation time τ determined from the Cole-Cole analysis of the temperature dependent χ' and χ" is shown to fit the Vogel-Fulcher law: τ = τ0exp[ΔE/kB(T -- T0)] with T0 = 245 K, ΔE/kB = 270 K = TC, and τ0 = 1.2 × 10-9 s. Magnetic viscosity S (measured in H = 0 for a sample cooled in H = 50 Oe) determined from the fit to the magnetization: M (t) =M (0) - S ln t, with time t up to 2 h, shows a peak at 230 K above which M (2 h) switches to negative values for temperatures up to TC(0). It is argued that this negative magnetization results from an interaction between the superparamagnetic spin clusters and the ferromagnetic phase, both being present in the temperature range of TB < T < TC. [ABSTRACT FROM AUTHOR]

The depth-dependent magnetization in thin film oxygen stoichiometric and oxygen-deficient La0.7Sr0.3MnO3 is investigated by using polarized neutron reflectivity and DC bulk magnetometry. The polarized neutron reflectivity results reveal that the stoichiometric sample shows enhanced interfacial magnetization relative to the rest of the film. The oxygen-deficient sample exhibits a reduced average magnetization from the optimized recipe. Both films show regions of suppressed magnetization at the surface regardless of the growth pressure. The oxygen stoichiometric film does not show an interfacial dead layer, whereas the oxygen-deficient film exhibits a dead layer whose thickness changes with temperature. At a low applied field, we observe striking differences in the depth dependence of the magnetic ordering, with the oxygen-deficient film exhibiting exchange spring behavior, while the stoichiometric film shows a constant magnetization direction across the film. These results suggest that the incorporation of oxygen vacancies during growth leads to an accumulation of vacancies at the interface, which is enhanced at higher temperature due to increased oxygen mobility, creating a region of reduced magnetism that couples to the rest of the film. These results offer insight into the complex behavior and role of oxygen vacancies in the magnetism of these systems. Additionally, the study reveals further details of the negative magnetization in La0.7Sr0.3MnO3 reported in previous studies, which are discussed here. [ABSTRACT FROM AUTHOR]

The effects of Mn doping on the structural properties of the topological insulator Bi2Se3 in thin film form were studied in samples grown via molecular beam epitaxy. Extended x-ray absorption fine structure measurements, supported by density functional theory calculations, indicate that preferential incorporation occurs substitutionally in Bi sites across the entire film volume. This finding is consistent with x-ray diffraction measurements which show that the out of plane lattice constant expands while the in plane lattice constant contracts as the Mn concentration is increased. X-ray photoelectron spectroscopy indicates that the Mn valency is 2+ and that the Mn bonding is similar to that in MnSe. The expansion along the out of plane direction is most likely due to weakening of the Van der Waals interactions between adjacent Se planes. Transport measurements are consistent with this Mn2+ substitution of Bi sites if additional structural defects induced by this substitution are taken into account. [ABSTRACT FROM AUTHOR]

Transient reflectivity traces measured for nanometer-sized films (6-40 nm) of the topological insulator Bi2Se3 revealed GHz-range oscillations driven within the relaxation of hot carriers photoexcited with ultrashort (∼100 fs) laser pulses of 1.51 eV photon energy. These oscillations have been suggested to result from acoustic phonon dynamics, including coherent longitudinal acoustic phonons in the form of standing acoustic waves. An increase of oscillation frequency from ∼35 to ∼70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the interplay between two different regimes employing traveling-acoustic-waves for films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR) modes for films thinner than 40 nm. The amplitude of oscillations decays rapidly for films below 15 nm thick when the indirect intersurface coupling in Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The frequency range of coherent longitudinal acoustic phonons is in good agreement with elastic properties of Bi2Se3. [ABSTRACT FROM AUTHOR]

Growth of pure phase delafossite CuFeO2 thin films on Al2O3 (00.1) substrates by pulsed laser deposition was systematically investigated as a function of growth temperature and oxygen pressure. X-ray diffraction, transmission electron microscopy, Raman scattering, and x-ray absorption spectroscopy confirmed the existence of the delafossite phase. Infrared reflectivity spectra determined a band edge at 1.15?eV, in agreement with the bulk delafossite data. Magnetization measurements on CuFeO2 films demonstrated a phase transition at TC ≈ 15 ± 1?K, which agrees with the first antiferromagnetic transition at 14?K in the bulk CuFeO2. Low temperature magnetic phase is best described by commensurate, weak ferromagnetic spin ordering along the c-axis. [ABSTRACT FROM AUTHOR]

Films of CuFeO2 and CuFe0.75Ga0.25O2 were grown over sapphire substrates in high vacuum using a pulsed laser deposition technique. The films grew with rhombohedral delafossite structure and highly epitaxial in the c-direction. Samples were characterized by X-ray diffraction, Raman spectroscopy and atomic force microscopy. Surface of the films were inspected with X-ray and UV photoelectron spectroscopy. Adsorption of CO2 and H2O was studied by a thermal program desorption technique. In both films Cu and Fe were exposed at the surface-gas interface. X-ray photoelectron data indicated that CO2 adsorbs preferentially at Cu sites forming a similar coordination to CuCO3. The energy for desorption of CO2 and H2O was estimated to be 30 kcal mol−1 (1.3 eV atom−1) for CuFeO2 and 36 kcal mol−1 (1.6 eV mol−1) for CuFe0.75Ga0.25O2. UV photoelectron spectroscopy showed that the valence band of the CuFeO2 delafossite oxides is modified with the substitution of Fe by Ga in the crystal lattice. The semiconductor band gap of CuFeO2 delafossite oxides also increased from 1.2 to 1.5 eV due to the substitution of Fe by Ga in the crystal lattice. [ABSTRACT FROM AUTHOR]

Transient reflectivity (TR) from thin films (6-40 nm thick) of the topological insulator Bi2Se3 revealed ultrafast carrier dynamics, which suggest the existence of both radiative and non-radiative recombination between electrons residing in the upper cone of initially unoccupied high energy Dirac surface states (SS) and holes residing in the lower cone of occupied low energy Dirac SS. The modeling of measured TR traces allowed us to conclude that recombination is induced by the depletion of bulk electrons in films below ~20 nm thick due to the charge captured on the surface defects. We predict that such recombination processes can be observed using time-resolved photoluminescence techniques. [ABSTRACT FROM AUTHOR]