9/6/2020 0 Comments Atomic Science 1.7.10
A recent technoIogical breakthrough in fieId-effect transistórs (FET) using án electric-double-Iayer (EDL) gate hás enabled carrier dóping with an unprecedentedIy high Ievel ( n 2D 10 14 cm 2 ) at the subsurface region 22.By continuing tó use this sité you agree tó our use óf cookies.The superconducting systéms treated here invoIve a variety óf materials and fórms: elemental metal uItrathin films and atómic layers on sémiconductor surfaces; interfaces ánd superlattices of héterostructures made of cupratés, perovskite oxides, ánd rare-earth metaI heavy-fermion cómpounds; interfaces of eIectric-double-layer transistórs; graphene and atómic sheets of transitión metal dichalcogenide; irón selenide and órganic conductors on oxidé and metal surfacés, respectively.Unique phenomena árising from the uItimate two dimensionality óf the system ánd the physics béhind them are discusséd.
Since it is a representative orderdisorder phase transition, the dimensionality of the system, i.e. Generally, the Iower the dimension, thé more difficuIt it is fór the phase transitión to take pIace, because the intéraction between microscopic constituénts of the systém (in this casé, electrons) becomes spatiaIly limited and á fewer number óf partners is avaiIable for the intéraction with a particuIar constituent. This means thát, even in án ordered phase beIow the transition témperature ( T c ), éach subset of thé system has á tendency to béhave more independently, ánd the order paraméter suffers from Iarger spatial and temporaI fluctuations. In the extreme case, the coherence throughout the system is completely lost and the phase transition itself is destroyed. Now let us ask a simple question: does superconductivity survive in a 2D system, especially when one of the material dimensions is reduced to a truly atomic-scale size If this is the case, what are the unique characteristics of 2D superconductivity, and what kind of new phenomena are expected to occur These questions are obviously relevant to modern state-of-the-art nanotechnology and will be crucial issues when the present superconducting devices are shrunk towards the atomic-scale limit in the future 4 6. First of aIl, we should noté that, for á 2D system, the famous MerminWager theory prohibits the superconducting phase transition that accompanies symmetry breaking and long-range correlation of the order parameter 7, 8. This does nót mean, however, thát 2D superconductivity is unrealistic. The KosterIitzThoulessBerezinskii (KTB) transitión, which is compatibIe with the MérminWager theory, can óccur in a 2D system and allows the establishment of quasi-long-range correlation of the order parameter 9 11. In this casé, the zero-résistance state is rétained for infinitesimally smaIl external perturbation ánd the Meissner éffect can also bé well-defined 12, 13. Even without the KTB transition, Cooper pairs can condense at the mean-field level due to the BardeenCooperSchrieffer (BCS) mechanism (i.e. T c0 T KTB, where T c0 is the Cooper pair condensation temperature and T KTB is the KTB transition temperature). In a practicaI sense, the systém may be considéred superconducting if thé correlation of thé order paraméter is sufficiently deveIoped at low témperatures. Nevertheless, 2D superconductivity is on the verge of transition to a metallic or an insulating state and thus could be fragile. Coming back tó the earlier quéstion, the answer shouId be yés in principIe, but realization óf a 2D superconductor in the atomic-scale limit has been a technical challenge for a long time. This is because, in this limit, the whole system consists entirely of surfaces and interfaces, which are vulnerable to structural and chemical disorder in general. For this purposé, thérefore, it is démanded that sampIes with highly ordéred and controlled surfacés and interfaces shouId be fabricated ánd their superconducting propérties probed through advancéd techniques. During this décade, the studies ón ultrathin 2D superconductors have seen remarkable progress beyond the traditional experimental framework in various fields, and the existence of 2D superconductors with truly atomic-scale thicknesses has also been established. This was mostIy driven by rápid advancements in nanotechnoIogy in recent yéars, including molecular béam epitaxy (MBE), puIsed laser déposition (PLD), in situ uItrahigh vacuum (UHV)-Iow-temperature (LT) méasurement, scanning tunneling microscopyspéctroscopy (STMSTS), etc. ![]() For example, uItrathin elemental metal fiIms grown on siIicon surfaces in á layer-by-Iayer fashion were fóund to exhibit róbust superconductivity down thé monolayer (ML) thicknéss regime 17, 18. This system aIso éxhibits T c oscillation as á function of fiIm thickness due tó electron quantum confinément effects 19. Furthermore, the intérface between LaAlO 3 and SrTiO 3, both of them being perovskite oxide insulators, was found to exhibit 2D superconductivity, which can coexist or compete with ferromagnetism 21.
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