In this article, we review the recent progress in surface atomic-layer superconductors on semiconductor substrates with Rashba/Zeeman-type spin-orbit coupling (SOC). After introduction of some of the ...basics of Rashba/Zeeman-type SOC and its effects on superconductivity, representative surface structures with relevant features are described in terms of their crystalline and electronic properties. This is followed by recent experimental studies that have revealed anomalous superconducting phenomena, which can be attributed to the effects of Rashba/Zeeman-type SOC. Future prospects, likely to be driven by instrumentational developments, are given as a concluding remark.
Surface atomic-layer (SAL) superconductors consisting of epitaxially grown metal adatoms on a clean semiconductor surface have been recently established. Compared to conventional metal thin films, ...they have two important features: (i) space-inversion symmetry-breaking throughout the system and (ii) high sensitivity to surface adsorption of foreign species. These potentially lead to manifestation of the Rashba effect and a Zeeman field exerted by adsorbed magnetic organic molecules. After introduction of the archetypical SAL superconductor Si(111)-( 7 × 3)-In, we describe how these features are utilized to engineer a topological superconductor with Majorana fermions and discuss its promises and expected challenges.
Recent progress in superconducting metal-induced surface reconstructions on silicon is reviewed, mainly focusing on the results of the author s group. After a brief introduction of an ...ultrahigh-vacuum (UHV)-low-temperature (LT)-compatible electron transport measurement system, direct observation of the zero resistance state for the Si(111)-()-In surface is described, which demonstrates the existence of a superconducting transition in this class of two-dimensional (2D) materials. The measurement and analysis of the temperature dependence of the critical current density indicate that a surface atomic step works as a Josephson junction. This identification is further confirmed by LT-scanning tunneling microscopy (STM) observation of Josephson vortices trapped at atomic steps on the Si(111)-()-In surface. These experiments reveal unique features of metal-induced surface reconstructions on silicon that may be utilized to explore novel superconductivity.
Spin-momentum locking is essential to the spin-split Fermi surfaces of inversion-symmetry broken materials, which are caused by either Rashba-type or Zeeman-type spin-orbit coupling (SOC). While the ...effect of Zeeman-type SOC on superconductivity has experimentally been shown recently, that of Rashba-type SOC remains elusive. Here we report on convincing evidence for the critical role of the spin-momentum locking on crystalline atomic-layer superconductors on surfaces, for which the presence of the Rashba-type SOC is demonstrated. In-situ electron transport measurements reveal that in-plane upper critical magnetic field is anomalously enhanced, reaching approximately three times the Pauli limit at T = 0. Our quantitative analysis clarifies that dynamic spin-momentum locking, a mechanism where spin is forced to flip at every elastic electron scattering, suppresses the Cooper pair-breaking parameter by orders of magnitude and thereby protects superconductivity. The present result provides a new insight into how superconductivity can survive the detrimental effects of strong magnetic fields and exchange interactions.
Recent progress in two-dimensional superconductors with atomic-scale thickness is reviewed mainly from the experimental point of view. The superconducting systems treated here involve a variety of ...materials and forms: elemental metal ultrathin films and atomic layers on semiconductor surfaces; interfaces and superlattices of heterostructures made of cuprates, perovskite oxides, and rare-earth metal heavy-fermion compounds; interfaces of electric-double-layer transistors; graphene and atomic sheets of transition metal dichalcogenide; iron selenide and organic conductors on oxide and metal surfaces, respectively. Unique phenomena arising from the ultimate two dimensionality of the system and the physics behind them are discussed.
•Organic molecules induced doping in atomic layer materials formed on solid surfaces•Controlling the charge states by the coverage of organic molecules•Manipulating the Tc of atomic layer ...superconductors with organic molecules
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In/Si(111)-(7×3) is an atomic layer superconductor formed by an In double layer on Si(111). The size of its circular Fermi surface becomes smaller by the adsorption of copper-phthalocyanine (CuPc) molecules till a coverage of 1.0 monolayer, and then larger at higher coverage. This means that it is possible to control the amount of hole doping, and therefore to tune the energy of the band by CuPc adsorption. By comparing the change in transition temperature (Tc) reported in “Nano Lett. 17 (2017) 2287”, whose reports the manipulation of Tc by CuPc adsorption, and the behavior of the Fermi surface observed in the present study, we conclude that both the increase of Tc till a CuPc coverage of 1.0 ML and its decrease from 1.0 to 2.0 ML have strong correlations with the amount of hole doping from CuPc to In/Si(111)-(7×3).
Surface superconductors such as Si(111)-(√7 × √3)-In (referred to as (√7 × √3)-In, hereafter) can be sensitively doped by surface dopants to tune their transition temperature (T c) even via a weak ...interaction induced by molecular overlayers. Here, we examined the effect of Cu-phthalocyanine (CuPc) and Cu-hexadecafluorophthalocyanine (F16CuPc) overlayers on the electronic structure on (√7 × √3)-In. While both molecules formed well-ordered monolayers, molecular adsorption geometries were found to be different; CuPc formed an ordered monolayer of face-on molecules, while F16CuPc tended to adsorb in a slightly tilted geometry. The normal-state electronic structures of (√7 × √3)-In covered with these molecules were found to be similar. Photoelectron spectroscopy have suggested that the charge transfer to the molecules was negligible and that the interaction between the molecules and (√7 × √3)-In were weak. Nevertheless, CuPc enhanced the superconducting transition temperature (T c) of (√7 × √3)-In by approximately 0.2 K, while F16CuPc suppressed it by approximately 1.0 K. The density functional theory (DFT) calculations combined with work function measurements showed that the surface electrons were accumulated in the region between the surface In atoms and the molecules, while molecules remain nearly uncharged. The increase in T c in the case of CuPc overlayer is explained by a hole doping into (√7 × √3)-In due to the extraction of surface electrons. In contrast, the decrease in T c for F16CuPc overlayer is attributable to the tilting adsorption geometry, possibly inducing an enhanced exchange interaction between spin magnetic moments of the molecular orbitals of inclined F16CuPc and the conduction electrons of (√7 × √3)-In. The high sensitivity of T c to the small difference in the adsorption geometry of weakly interacting molecules suggests the possibilities of modifying the superconducting properties of various surface materials.
Diamond is a wide bandgap semiconductor that can work at high temperatures and resist very high electric fields. It endures harsh environments through its physical stability and conducts heat very ...well. These properties make diamond suitable for the fabrication of unique electronic devices. In particular, diamond field effect transistors (FETs) have promising applications, including high-power converters for trains and electric vehicles and high-power high-frequency amplifiers for telecommunications and radar. Although high mobility is desirable for these applications, it has been difficult to achieve in diamond FETs particularly when the carrier density is high. The low mobility is most probably due to fixed and trapped charges in the non-ideal amorphous gate dielectric and at the dielectric/diamond interface. Here, we report on diamond FETs with monocrystalline hexagonal boron nitride (h-BN) as a gate dielectric. Thanks to the low density of charged impurities in monocrystalline h-BN, we obtained unprecedentedly high mobilities (>300 cm2 V−1 s−1) for moderately high carrier densities (>5 × 1012 cm−2). The resulting minimum sheet resistance was exceptionally low (<3 kΩ). Our results show that a heterostructure consisting of monocrystalline h-BN and diamond is an excellent platform with which to manufacture high-performance electronic devices.
Self-assembled organic molecules can potentially be an excellent source of charge and spin for two-dimensional (2D) atomic-layer superconductors. Here we investigate 2D heterostructures based on In ...atomic layers epitaxially grown on Si and highly ordered metal-phthalocyanine (MPc, M = Mn, Cu) through a variety of techniques: scanning tunneling microscopy, electron transport measurements, angle-resolved photoemission spectroscopy, X-ray magnetic circular dichroism, and ab initio calculations. We demonstrate that the superconducting transition temperature (T c) of the heterostructures can be modified in a controllable manner. Particularly, the substitution of the coordinated metal atoms from Mn to Cu is found to reverse the T c shift from negative to positive directions. This distinctive behavior is attributed to a competition of charge and spin effects, the latter of which is governed by the directionality of the relevant d-orbitals. The present study shows the effectiveness of molecule-induced surface doping and the significance of microscopic understanding of the molecular states in these 2D heterostructures.
We investigate Ag(100) thin films epitaxially grown on a Fe(100) substrate using a low-temperature scanning tunneling microscope. Fabrication of a wedge structure by evaporating Ag through a shadow ...mask allows us to observe a systematic evolution of quantum well (QW) states for layer thicknesses varying from 3 to 16 monolayers (ML). Close inspection of differential conductance spectra and images reveals significant modulations of QW states in the lateral directions, presumably due to the local defects at the Ag/Fe interface. The area where QW states are modulated extends over ~5nm. In clear contrast, near a surface atomic step, QW states exhibit negligible changes at least up to 1nm away from the step, leaving unmixed the two sets of neighboring QW states belonging to different thicknesses. The results are discussed in terms of a simple electron wave diffraction model.
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