Superconductivity In Bilayers Research Articles

A twisty trilayer

Superconductivity The discovery of superconductivity in bilayers consisting of graphene sheets twisted with respect to each other by just the right “magic” angle has inspired enormous interest in twisted materials. Hao et al. constructed twisted trilayer graphene, in which the middle layer is twisted with respect to the bottom layer and the top layer is roughly parallel to the bottom layer (see the Perspective by Yazdani). Such trilayers exhibited superconductivity that was tunable by an external electric field and consistent with having an unconventional nature. Science , this issue p. [1133][1]; see also p. [1098][2] [1]: /lookup/doi/10.1126/science.abg0399 [2]: /lookup/doi/10.1126/science.abg5641

Electric field-tunable superconductivity in alternating-twist magic-angle trilayer graphene.

Engineering moiré superlattices by twisting layers in van der Waals (vdW) heterostructures has uncovered a wide array of quantum phenomena. We constructed a vdW heterostructure that consists of three graphene layers stacked with alternating twist angles ±θ. At the average twist angle θ ~ 1.56°, a theoretically predicted "magic angle" for the formation of flat electron bands, we observed displacement field-tunable superconductivity with a maximum critical temperature of 2.1 kelvin. By tuning the doping level and displacement field, we found that superconducting regimes occur in conjunction with flavor polarization of moiré bands and are bounded by a van Hove singularity (vHS) at high displacement fields. Our findings display inconsistencies with a weak coupling description, suggesting that the observed moiré superconductivity has an unconventional nature.

High-temperature interface superconductivity in bilayer copper oxide films by pulsed laser deposition

In a seminal work, Gozar et al. reported on the high-temperature interface superconductivity in bilayers of insulating La2CuO4 and metallic La2−xSrxCuO4 (x=0.45). An interesting question to address is how general and robust this interface superconductivity is. In the past, the cuprate bilayers were grown in a unique atomic-layer molecular beam epitaxy system, with a Sr doping range of x≤0.47, and the atomically flat interface was thought to be indispensable. Here, we have fabricated bilayers of La2CuO4 and La2−xSrxCuO4 by pulsed laser deposition. We have tried to extend the nominal doping range of Sr from the previous maximum of 0.47 to the present 1.70 (the nominal Sr content in the targets). X-ray diffraction result indicates that our La2−xSrxCuO4 films with x≤0.60 have very high crystalline quality; but the film crystalline structure degrades gradually with further increasing x, and finally the structure is fully lost when x reaches 1.40 and higher. Although the film quality scatters dramatically, our experiments show that there exists superconductivity for bilayers in nearly the entire over-doped Sr range, except for a non-superconducting region at x∼0.80. These observations demonstrate that the interface superconductivity in copper oxides is very general and robust.

Superconductivity of bilayer titanium/indium thin film grown on SiO2/Si (001)

Bilayer superconducting films with tunable transition temperature (Tc) are a critical ingredient to the fabrication of high-performance transition edge sensors. Commonly chosen materials include Mo/Au, Mo/Cu, Ti/Au, and Ti/Al systems. Here in this work, titanium/indium (Ti/In) bilayer superconducting films are successfully fabricated on SiO2/Si (001) substrates by molecular beam epitaxy (MBE). The success in the epitaxial growth of indium on titanium is achieved by lowering the substrate temperature to −150 °C during indium evaporation. We measure the critical temperature under a bias current of 10 μA, and obtain different superconducting transition temperatures ranging from 645 mK to 2.7 K by adjusting the thickness ratio of Ti/In. Our results demonstrate that the transition temperature decreases as the thickness ratio of Ti/In increases.

Observation of two-dimensional superconductivity in bilayers of BaBiO3 and BaPbO3

Besides the cuprate superconductors lead-doped barium bismuthate still attracts intense interest concerning its relatively high critical temperature and the nature of its superconductivity in vicinity to a charge density wave ordered topological insulator. Here we present bilayers of barium bismuthate and barium leadate and find two-dimensional superconductivity at their interfaces well described by current-voltage characteristics compatible with a Berezinskii-Kosterlitz-Thouless transition. Magneto-transport studies independently prove the two-dimensional character of the superconducting properties in these bilayers. The particular dependence of the superconducting transition temperature on the thickness of the barium leadate top layer suggests the formation of the superconducting state to be strain-induced through the interface.

Effect of a magnetic field on the critical temperature of a ferromagnet-superconductor transition in layered heterostructures

Effect of an external magnetic field H on the superconductivity of bilayer (FS) and trilayer (FSF) heterostructures consisting of layers of ferromagnetic (F) and superconducting (S) metals has been studied. The case has been considered where the scattering in both metals is sufficiently strong (the so-called “dirty limit”), which makes it possible to use the Usadel equations to describe the proximity effect. For the Gor’kov function, a boundary problem is formulated that is free of restrictions on the magnitudes of the transparency of the FS (SF) interface and takes into account both the wave and diffusional types of motion of quasi-particles. Special attention is given to the case of a trilayer system, for which a detailed analysis of two states with parallel (P) and antiparallel (AP) directions of the magnetizations in the F layers has been performed. Based on the results obtained, the possibility of a technical application of the FSF system is considered.

High-temperature interface superconductivity between metallic and insulating copper oxides

The realization of high-transition-temperature (high-T(c)) superconductivity confined to nanometre-sized interfaces has been a long-standing goal because of potential applications and the opportunity to study quantum phenomena in reduced dimensions. This has been, however, a challenging target: in conventional metals, the high electron density restricts interface effects (such as carrier depletion or accumulation) to a region much narrower than the coherence length, which is the scale necessary for superconductivity to occur. By contrast, in copper oxides the carrier density is low whereas T(c) is high and the coherence length very short, which provides an opportunity-but at a price: the interface must be atomically perfect. Here we report superconductivity in bilayers consisting of an insulator (La(2)CuO(4)) and a metal (La(1.55)Sr(0.45)CuO(4)), neither of which is superconducting in isolation. In these bilayers, T(c) is either approximately 15 K or approximately 30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm of the interface. If such a bilayer is exposed to ozone, T(c) exceeds 50 K, and this enhanced superconductivity is also shown to originate from an interface layer about 1-2 unit cells thick. Enhancement of T(c) in bilayer systems was observed previously but the essential role of the interface was not recognized at the time.