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We discuss a recent resonant ultrasound spectroscopy (RUS) study of YBa2Cu3O6+Δ, which infers a line of phase transitions bounding the pseudogap phase and argue that this scenario is not supported by thermodynamic evidence. We show that the anomalies in RUS, heat capacity, and thermal expansion at the superconducting transition temperatures agree well, but there are large discrepancies between RUS and thermodynamic measurements at T× where the pseudogap phase transitions are purported to occur. Moreover, the frequency and temperature dependence of the RUS data for the crystal with Δ = 0.98, interpreted in terms of critical slowing down near an electronic phase transition, is five orders of magnitude smaller than what is expected. For this crystal the RUS data near T× are more consistent with nonequilibrium effects, such as oxygen relaxation. [ABSTRACT FROM AUTHOR]

Spin injection and transport in solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) are investigated using vertical CoPt/TIPS-pentacene/AlOx/Co spin valve architectures. The morphology and the molecular orientation of TIPS-pentacene spin-coated from a chloroform solution as grown in the spin valve devices are characterized by means of atomic force microscopy and x-ray diffraction, where the π-π stacking is found to be in planes oriented parallel to the substrate plane on CoPt (not L10-ordered). The magnetization hysteresis loops recorded with a superconducting quanmm interference device show an individual switching of the magnetization of the two ferromagnetic layers. The antiparallel state of the relative orientation of CoPt and Co is achieved due to their different coercive fields. A spin valve effect is observed at room temperature. The scaling of the magnetoresistance (MR) with the bulk mobility of TIPS-pentacene as a function of temperature together with the dependence of the MR on the interlayer thickness clearly indicates spin injection and transport in TIPS-pentacene. From organic semiconductor-spacer thickness-dependent MR measurements, we estimate a spin relaxation length of TIPS-pentacene of λs = (24 ± 6) nm and a spin relaxation time of approximately τs = 3.5 μs at r = 300 K, taking the measured bulk mobility of holes into account. [ABSTRACT FROM AUTHOR]

We studied the valency and spin behavior of M = Mn, Fe, Co, Li, and Al in the high-temperature superconducting compound Bi2.15 Sr1.85Ca(Cu1-zMz)2O8+δ (Bi-2212) for small values of z. Mn, Fe, aud Co retain their magnetic moments, and our thermopower and magnetic susceptibility data imply ionization states Mn3+, Fe-2+, and Co2+, while Li and Al are accommodated in the charge reservoir layers. Single-crystal studies show that the susceptibility of Co2+ ions in Bi-2212 is strongly anisotropic, with a weak anisotropy detected for Mn3+ and none for Fe2+. Fits to a pseudogap formula for a pure Bi-2212 crystal suggest that the spin susceptibility of the host compound is more anisotropic than previously realized. Data in the superconducting state allow us to compare the pair-breaking properties of the different impurities. Several aspects of the data, including the stronger suppression of the superconducting transition temperature Tc by Co compared with Fe for underdoped and optimally doped samples, show that the d-level structure of the magnetic ions and multiorbital effects are important. We also find that the temperatures of the magnetization crossing points are equal to the low-field Tc values to within 1% or 2%. This agrees with a 2D thermodynamic fluctuation argument given by Junod et al. [ABSTRACT FROM AUTHOR]

Using a high-resolution differential technique we have determined the electronic specific heat coefficient γ(T)=Cel/T of Ba1-xKxFe2As2 with x=0 to 1.0, at temperatures (T) from 2 K to 380 K and in magnetic fields H=0 to 13 T. In the normal state γn(x,T) increases strongly with x at low temperature, compatible with a mass renormalization ~12 at x=1, and decreases weakly with x at high temperature. A superconducting transition is seen in all samples from x=0.2 to 1, with transition temperatures and condensation energies peaking sharply at x=0.4. Superconducting coherence lengths ξab~20 Å and ξc~3 Å are estimated from an analysis of Gaussian fluctuations. For many dopings we see features in the H and T dependencies of γs(T,H) in the superconducting state that suggest superconducting gaps in three distinct bands. A broad "knee" and a sharp mean-field-like peak are typical of two coupled gaps. However, several samples show a shoulder above the sharp peak with an abrupt onset at Tc,s and a T dependence γs(T)∝√1-T/Tc,s. We provide strong evidence that the shoulder is not due to doping inhomogeneity and suggest it is a distinct gap with an unconventional T dependence Δs(T)∝(1-T/Tc,s)0.75 near Tc,s. We estimate band fractions and T=0 gaps from three-band α-model fits to our data and compare the x dependencies of the band fractions with spectroscopic studies of the Fermi surface. [ABSTRACT FROM AUTHOR]

Torque magnetization measurements on YBa2Cu3Oy (YBCO) at doping y=6.67 (p=0.12), in dc fields (B) up to 33 T and temperatures down to 4.5 K, show that weak diamagnetism persists above the extrapolated irreversibility field Hirr(T=0)≈24 T. The differential susceptibility dM/dB, however, is more rapidly suppressed for B≳16 T than expected from the properties of the low field superconducting state, and saturates at a low value for fields B≳24 T. In addition, torque measurements on a p=0.11 YBCO crystal in pulsed field up to 65 T and temperatures down to 8 K show similar behavior, with no additional features at higher fields. We offer two candidate scenarios to explain these observations: (a) superconductivity survives but is heavily suppressed at high field by competition with charge-density-wave (CDW) order; (b) static superconductivity disappears near 24 T and is followed by a region of fluctuating superconductivity, which causes dM/dB to saturate at high field. The diamagnetic signal observed above 50 T for the p=0.11 crystal at 40 K and below may be caused by changes in the normal state susceptibility rather than bulk or fluctuating superconductivity. There will be orbital (Landau) diamagnetism from electron pockets and possibly a reduction in spin susceptibility caused by the stronger three-dimensional ordered CDW. [ABSTRACT FROM AUTHOR]

Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of twoferromagnetic Co layers can be used to influence the superconducting state of Nb. We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the twoferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation. [ABSTRACT FROM AUTHOR]

We report a systematic and quantitative study of near-infrared intersubband absorption in strained AlGaN/GaN and lattice-matched AHnN/GaN superlattices grown by plasma-assisted molecular-beam epitaxy as a function of Si-doping profile with and without S doping. For AlGaN/GaN, we obtained good theoretical agreement with experimental measurements of transition energy, integrated absorbance and linewidth by considering many-body effects, interface roughness, and calculations of the transition lifetime that include dephasing. For the AHnN/GaN system, experimental measurements of the integrated absorbance due to the superlattice transitions produced values more than one order of magnitude lower than AlGaN/GaN heterostructures at similar doping levels. Furthermore, observed transition energies were roughly 150 meV higher than expected. The weak absorption and high transition energies measured in these structures is attributed to columnar alloy inhomogeneity in the AlInN barriers observed in high-angle annular dark-field scanning transmission electron microscopy. We simulated the effect of these inhomogeneities using three-dimensional band-structure calculations. The inhomogeneities were modeled as AlInN nanorods with radially varying In composition embedded in the barrier material of the superlattice. We show that inclusion of the nanorods leads to the depletion of the quantum wells (QWs) due to localization of charge carriers in high-in-containing regions. The higher energy of the intersubband transitions was attributed to the relatively uniform regions of the QWs surrounded by high Al (95%) composition barriers. The calculated transition energy assuming Al0.95ln0.05 N barriers was in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

The magnetization of three high-quality single crystals of YBa2Cu3O6+x, from slightly overdoped to heavily underdoped, has been measured using torque magnetometry. Striking effects in the angular dependence of the torque for the two underdoped crystals, a few degrees above the superconducting transition temperature (Tc), are described well by the theory of Gaussian superconducting fluctuations using a single adjustable parameter. The data at higher temperatures (T) are consistent with a strong cutoff in the fluctuations for T ≳ 1.1 Tc. Numerical estimates suggest that inelastic scattering could be responsible for this cutoff. [ABSTRACT FROM AUTHOR]

We present a detailed study of quantum oscillations in the antiferromagnetically ordered pnictide compound SrFe2As2 as the angle between the applied magnetic field and crystalline axes is varied. Our measurements were performed on high-quality single crystals in a superconducting magnet, and in pulsed magnetic fields up to 60 T allowing us to observe orbits from several small Fermi-surface pockets. We extract the cyclotron effective mass m* and frequency F for these orbits and track their values as the field is rotated away from the c axis. While a constant ratio of m*/F is expected for a parabolic band, we observe deviations from this behavior. We conclude that this observation points to orbits derived from a band with Dirac dispersion near the Fermi level. [ABSTRACT FROM AUTHOR]

We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as a function of thickness. The magnetic moment, measured by polarized neutron reflectometry, and the Curie temperature are found to decrease with reducing thickness. Our results indicate that these surface-induced effects are caused by the reduced number of nearest neighbors, band bending, and the partial depopulation of the 4f states of Eu. [ABSTRACT FROM AUTHOR]