Growth characteristics of sputter-deposited thin films

Abstract

Thin films of the heavy-fermion superconductor UPt 3 were deposited on various substrate materials in various orientations by means of a quasi-multilayer sputter process. Strongly (0001)-textured growth of the hexagonal compound was found for a uranium content in the range of 23% to 28% on sapphire (10 12), LaAlO 3 (111) and SrTiO 3 (111) with perfect in-plane order on the latter substrate material. Atomic force microscopy and scanning electron microscopy revealed a Vollmer-Weber-like growth mode resulting in the development of large compressive strain in films on SrTiO 3 (111). As a result the electronic transport properties-in particular the temperature dependence of the resistivity-were strongly renormalized. Strong deviations from the typical heavy-fermion characteristics known from UPt 3 bulk samples were found in films on SrTiO 3 with residual resistance ratios ρ(300 K)/ρ 0 up to 930 and a significantly reduced superconducting transition temperature of 130 mK.

Selection rules for Cooper pairing in two-dimensional interfaces and sheets

“…We note in this context that superconducting (001)-oriented films of UPt 3 have been reported in ref. 12 From a symmetry point of view, there are ten possible superconducting phases-four associated …”

“…Due to C ⊥ 2 ∈ C 6v , the triplet vector which is largely aligned along the z axis in the bulk condensate [36] must rotate to be parallel to the xy plane. Note that superconductivity has been reported in (001)-oriented films of UPt 3 [10]. From a symmetry point of view, there are 10 possible superconducting phases -four associated with the four 1D IRs of C 6v and 3 with each of the two 2D IRs E 1 and E 2 .…”

“…Let us first focus on the upper signs in Eq. (12). Suppose that the point group contains a twofold rotation C ?…”

“…This proves statement (II) and shows that it results from the observation that a crystalline point group has a multidimensional (or complex) irreducible representation which is not forced to be odd under C Spinless fermions Similar reasoning can be applied to the case of spinless fermions, i.e., for the lower signs in Eq. (12). For instance, one can show that, as opposed to (II), the spin-0 TRS must be necessarily broken at the superconducting transition if the point group contains a proper or improper fourfold rotation symmetry.…”

“…Particularly interesting examples are given by LaAlO 3 /SrTiO 3 heterostructures, that show very rich electronic behavior [8], and single-layer FeSe on [001] SrTiO 3 with significantly enhanced transition temperatures compared to its bulk value [3]. This also motivates closer inspection of superconducting thin films of other correlated materials such as Sr 2 RuO 4 [9] and UPt 3 [10] which, in addition, promises to offer insights into the electronic properties of the bulk material.…”