Tunable coupling of two-dimensional superconductors in bilayer SrTiO3heterostructures

Abstract

Interlayer coupling effects between high mobility two-dimensional superconductors are studied in bilayer δ-doped SrTiO3 heterostructures. By tuning the undoped SrTiO3 spacer layer between the dopant planes, clear tunable coupling is demonstrated in the variation of the sheet carrier density, Hall mobility, superconducting transition temperature, and the temperature-and angle-dependences of the superconducting upper critical field. Systematic variation is found between one effective (merged) two-dimensional superconductor to two decoupled two-dimensional superconductors. In the intermediate coupled regime, a crossover is observed due to coupling arising from inter-sub-band interactions.Interlayer coupling between two-dimensional (2D) planes in layered materials is a key parameter which dramatically influences their physics and functionality. Paradigmatic examples in the field of superconductivity include the high temperature superconducting cuprates [1, 2], and the iron pnictides [3], as well as artificially created superlattices, in which the Josephson coupling between the layers can be tunable continuously using thin film control [4,5]. There have also been extensive studies of coupled bilayer 2D systems, where exotic phases arise from the interlayer interactions [6][7][8][9][10][11][12].Among the various 2D systems currently under investigation, SrTiO 3 (STO) is of particular interest since it is a rare example of a high mobility doped semiconductor that is simultaneously superconducting. By confining electrons in STO two-dimensionally using field effect gating [13], heterointerfaces with LaTiO 3 [14], or LaAlO 3 [15,16], and δ-doping [17], the interplay between subband quantization and superconductivity can be investigated. Here we exploit interlayer coupling in a δ-doped bilayer system to spatially and flexibly control the subbands. By systematically tuning the interlayer coupling by varying the spacer layer thickness, we can examine the effects of the sub-band quantization on superconductivity.The bilayer samples consist of two Nb:STO δ-doped layers, with an undoped STO spacer of thickness d inter , as well as 100 nm thick undoped STO cap and buffer layers. All samples were fabricated using pulsed laser deposition using growth conditions as reported elsewhere [18]. The thickness of each of the two δ-layers was fixed at a constant value of d doped = 5.4 ± 1 nm, as calibrated from the total thickness and the laser pulse count. The interlayer undoped STO thickness d inter was varied in the range 3.7 ≤ d inter ≤ 272 nm. The dopant concentration of the δ-doped layers was 1 at.%. A schematic of the bilayer samples is shown in Fig.

Odd-parity superconductivity by competing spin-orbit coupling and orbital effect in artificial heterostructures

“…Furthermore, the multilayer structure has been artificially controlled in CeCoIn 5 /YbCoIn 5 29 and δ-doped SrTiO 3 . 41 Thus, we expect that odd-parity topological superconductivity will be created in these systems by tuning the multilayer structure and the magnetic field.…”

Dynamics of the metal-insulator transition of donor-dopedSrTiO3

“…Note that electron depletion is not observed in delta-doped structures in which the n-doped STO layer is covered by an additional undoped top layer before a postannealing process in oxidizing conditions is performed [91,92]. In this case, Sr vacancy formation in the buried n-STO layer would require significant diffusion of Sr towards the surface and is, therefore, sluggish and can be suppressed by limiting the annealing time.…”

Topological Crystalline Superconductivity in Locally Noncentrosymmetric Multilayer Superconductors

“…Considering the spatially modulating order parameter in the trilayer, we call it the "pair-density wave (PDW) state" 24 . Multilayer structures of this kind are not only theoretical constructs, but have indeed been produced recently, for example, in the artificially grown superlattices CeCoIn 5 /YbCoIn 5 [25][26][27] and in transition-metaloxide interfaces 28 . The PDW state is stabilized when the three conditions, (a) Pauli-limited SC, (b) quasi-twodimensional structure, and (c) large spin-orbit coupling, are satisfied.…”

“…Nearest-neighbor layers are coupled by the hopping matrix element t ⊥ . We focus on the intra-layer Cooper pairing which is relevant for 2D SCs, as realized in CeCoIn 5 /YbCoIn 5 superlattices [25][26][27] and δ-doped SrTiO 3 28 , although an inter-layer Cooper pairing has been considered for Cu x Bi 2 Se 3 13,16 .…”

“…It is straightforward to extend our analysis to more than three layers and we find that the PDW state is a topological SC independent of parameters, if the number of layers is odd. Thus, the design of the topological crystalline SC is feasible for artificially grown multilayers using the available technology [25][26][27][28] . The superlattice CeCoIn 5 /YbCoIn 5 25 is considered to be a D+p-wave SC, and will be similarly a topological crystalline SC as will be discussed elsewhere 45 .…”