Epitaxy-stabilizedn-type superconducting cuprates

Abstract

We report the growth of n-type superconducting T′-(La,Ce)2CuO4 and infinite-layer (IL) (Sr,La)CuO2 thin films by means of molecular beam epitaxy (MBE). The bulk synthesis of T′-(La,Ce)2CuO4 and IL-(Sr,La)CuO2 requires complicated techniques: synthesis at low temperatures below 600 °C for the former and at high pressures above 3 GPa for the latter. This makes it difficult to grow bulk single crystals. We have found, however, that high-quality single-crystalline films of both compounds can be rather easily prepared by thin-film processes. Single-phase T′-(La,Ce)2CuO4 films can be obtained for a wide range of x (0.0 ≤ x ≤ ∼0.4). The best Tcend is over 30 K, which is the highest in the T′ family. For IL-(Sr,La)CuO2, by using KTaO3 substrates, high Tcend over 39 K and also metallic resistivity were achieved for the first time to our knowledge. We describe the key parameters in the growth and the properties of the resultant films.

Superconductivity in undoped T’-RE2CuO4 with Tc over 30K

“…Employing a thin film process, we also demonstrated that the superconducting range in T'-La 2-x Ce x CuO 4 is extended down to x = 0.045 [3], and also that the end-member T'-La 2 CuO 4 , which has been supposed to be a Mott insulator, shows metallic resistivity down to 150 K with ρ(300 K) as low as 2 mΩcm [4]. Furthermore, in the previous article, we reported the superconductivity with T c ~ 25 K in "undoped" T'-(La,RE) 2 CuO 4 , which was achieved by state-of-the-art molecular beam epitaxy (MBE) technique [5].…”

Superconductivity in bulk T′-(La,Sm)2CuO4 prepared via a molten alkaline hydroxide route

“…In La 2-x Ce x CuO 4 , which can be stabilized by thin film synthesis [7], the superconducting region extend to lower doping (0.45 < x < 0.22) as compared with Pr 2-x Ce x CuO 4 and Nd 2-x Ce x CuO 4 . Although superconductivity does not appear at x = 0, the end-member T'-La 2 CuO 4 shows low resistivity (2 mΩcm at 300 K) and metallic behavior down to 150 K [8].…”

Ce doping in T-La2CuO4 films: Broken electron–hole symmetry for high-Tc superconductivity

“…As seen in the figure, the T-phase films become more insulating with increasing x. The metallic behavior with a superconducting transition at about 30 K observed in the film with x = 0.09 is due to the impurity phase T'-La 2-x Ce x CuO 4 [6,7]. Figure 5 plots the resistivity at 300 K [ρ(300 K)] as a function of the Ce content.…”

“…The first concerns the inclusion of T'-La 2-x Ce x CuO 4 as an impurity phase. Because of the low synthesis temperature employed in MBE growth, the T' phase is easy to form and even predominates in preparation of La 2-x Ce x CuO 4 films for x even as small as 0.05 [6,7]. T'-La 2-x Ce x CuO 4 films are more conductive than T-La 2-x Ce x CuO 4 films by three to six orders of magnitude.…”

“…Figure 5 plots the resistivity at 300 K [ρ(300 K)] as a function of the Ce content. It also includes our data for T-La 2-x Sr x CuO 4 (hole doping into the T-structure) [12,13] and T'-La 2-x Ce x CuO 4 (electron doping into the T'-structure) [7,14] [14], indicating that the ionic size of Ce dopants is close to Ce 4+ (0.97 Å) rather than Ce 3+ (1.196 Å) in either compound [16]. Hence, these results suggest that effective electron doping in T-La 2 CuO 4 is achieved by tetravalent Ce doping.…”