G.A. Farnan

Growth of YBa2Cu3O7-delta thin films in pulsed laser deposition: influence of target, substrate and deposition rate

The growth of smooth, high quality, c -axis thin films of optimally doped YBa2 Cu3 O7- on (001) MgO substrates has been studied using pulsed laser deposition. The films were characterized with Auger spectroscopy, x-ray diffraction, self-inductance and four-probe resistance techniques. We have found an interesting interaction between deposition rate, substrate topography and target oxygenation in the determination of film quality. The best films are produced with dense underoxygenated targets at low laser repetition rates (1 Hz) on very smooth substrates; the optimal deposition parameters for these are given. Atomic force microscopy shows spiral growth formations with a unit-cell edge height. Oriented outgrowths observed on some films are attributed to heat-modified substrate surface morphology. a -axis films also have been grown using PrBa2 Cu3 Ox template layers.

Doping and characterization of YBa2Cu3O7-δ thin films prepared by pulsed laser deposition

High-quality, smooth, optimally doped, c-axis thin films of YBa2Cu3O7-δ were grown on (001) MgO substrates by pulsed laser deposition and subsequent oxygen annealing. They were characterized structurally by θ-2θ and four-axis x-ray diffraction and, electrically, by a self-inductance measurement. Through subsequent annealing in reduced pressure oxygen environments, underdoped films were produced and again characterized using the same techniques. The doping is reversible and reproducible. A monotonic decrease of Tc and an increase of the c-axis length were found with a decrease of the oxygen partial pressure during the annealing. From these data corresponding values of δ were determined. The width of the rocking curve of the (005) peak was found to be substantially unchanged, at 0.28-0.29°, between the optimally doped and the underdoped films. The relative peak intensities showed a systematic variation with δ in agreement with model calculations by Ye and Nakamura. The grain alignment is exclusively along the MgO high symmetry axes throughout the film. Resistance-temperature characteristics allowed the pseudogap onset temperature, T*, to be determined as a function of doping.

Temperature dependence of the mid-infrared dielectric function of YBCO in the a–b plane: a re-evaluation

Abstract The a–b plane dielectric function (e) of c-axis YBa2Cu3O7−δ thin films with Tc>85 K was measured at λ=3.392 μm in the temperature range 85–300 K, using an attenuated total reflectance (ATR) technique based on the excitation of surface plasmons. The results show that |er| decreases quasi-linearly with increasing temperature, while ei is invariant to temperature within experimental uncertainties. Typical values are eab=−23+16.5i at ∼295 K and eab=−27+15.5i at ∼90 K. A generalised Drude analysis yields effective scattering rates (1/τ*) that increase with temperature from ∼1500 to ∼1900 cm−1. The temperature dependent rates best fit an equation of the form 1/τ*=a+bTα with α=1.46±0.40. The effective plasma frequencies of ωp*∼18,500 cm−1 are almost independent of temperature. The uniquely detailed temperature dependence of the results confirm and consolidate data obtained by other groups using normal reflectance methods, but contradict our previously published ATR measurements. Technical shortcomings in the earlier work are identified as the source of the discrepancy.

Fabrication and characterisation of underdoped YBa2Cu3O7-δ thin films

Abstract Thin c-axis films of YBa2Cu3O7-δ have been grown on single crystal (001) MgO by pulsed laser deposition. The films have been deoxygenated by sintering and annealing in a reduced pressure of O2. The process is reproducible, controllable and reversible. The reduction in Tc is accompanied by a systematic elongation in c-axis, facilitating an inference of δ. Resistance measurements show a departure from linearity at the pseudogap formation temperature T∗, which has been determined as a function of δ.

Dielectric function of YBCO in the mid-infrared: temperature and doping dependence at 3.392μm

Abstract The ab-plane dielectric function at 3392nm of thin films of YBa2Cu3O7−δ with a wide range of doping has been measured using attenuated total reflectance. While e2, and therefore σ1, are temperature independent, |e1| decreases by ∼25% with increasing temperature. A stronger dependence on doping is observed: the magnitudes of e1 and e2 are both strongly suppressed as δ increases. A generalised Drude analysis shows that the plasma frequency is temperature-independent, but scales with doping. The scattering rate increases with temperature, and also increases with decreasing doping, consistent with stronger coupling in the underdoped regime.

Femtosecond mid-infrared study of YBa2Cu3O7−δ

Abstract Femtosecond near-infrared excitation of optimally-doped and underloped YBa 2 Cu 3 O 7− δ leads to an ultrafast fill-in of the much-discussed (ab)-plane mid-infrared conductivity gap. Transients taken from the underdoped material reveal two gap constituents with distinctly different recovery dynamics and saturation behaviour.

Femtosecond mid-infrared study of the high-Tc superconductor YBa2Cu3O7-d

Ultrafast nearinfrared excitation of optimally-doped and underdoped YBa2Cu3O7-−δ leads to an ultrafast fill-in of the (ab)-plane mid-infrared conductivity gap. Underdoped material shows two gap constituents in the transients with different recovery dynamics and saturation behaviour.