Model of an exotic chiral superconducting phase in a graphene bilayer

We theoretically demonstrate the formation of a new type of unconventional superconductivity in graphene materials, which exhibits a gapless property. The studied superconductivity is based on an interlayer pairing of chiral electrons in bilayer graphene, which results in an exotic s-wave spin-triplet condensate order with anomalous thermodynamic properties. These include the possibility of a temperature-induced condensation causing an increase of the pairing gap with increasing temperature and an entropy of the stable superconducting state which can be higher than its value in the normal state. Our study reveals the analogy of the interlayer superconductivity in graphene materials to the color superconductivity in dense quark matter and the gapless pairing states in nuclear matter and ultracold atomic gases.     

On Superconductivity State in Pure Graphene

We study theoretically the possibility of superconductivity state in pure graphene within the extended attractive Hubbard model. In the absence of disorder, when we use the local attractive interaction potential, U≌5t, where t is hopping term, pure graphene can be in superconductivity state.     

Graphite,Graphene, and the Flat Band Superconductivity

Superconductivity has been observed in bilayer graphene [1, 2]. The main factor that determines the mechanism of the formation of this superconductivity is the “magic angle” of twist of two graphene layers, at which the electronic band structure becomes nearly flat. The specific role played by twist and by the band flattening has been earlier suggested for explanations of the signatures of room-temperature superconductivity observed in the highly oriented pyrolytic graphite (HOPG), when the quasi two-dimensional interfaces between the twisted domains are present. The interface contains the periodic array of misfit dislocations (analogs of the boundaries of the unit cell of the Moiré superlattice in bilayer graphene), which provide the possible source of the flat band. This demonstrates that it is high time for combination of the theoretical and experimental efforts in order to reach the reproducible room-temperature superconductivity in graphite or in similar real or artificial materials.     

Electronic structure of superconducting KC8 and nonsuperconducting LiC6 graphite intercalation compounds: evidence for a graphene-sheet-driven superconducting state

We have performed photoemission studies of the electronic structure in LiC(6) and KC(8), a nonsuperconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC(8) than in LiC(6), opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and superconductivity monotonically increase with the filling of graphene π(*) states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.     

Superconducting states of pure and doped graphene

We study the superconducting phases of the two-dimensional honeycomb lattice of graphene. We find two spin singlet pairing states; s wave and an exotic p+ip that is possible because of the special structure of the honeycomb lattice. At half filling, the p+ip phase is gapless and superconductivity is a hidden order. We discuss the possibility of a superconducting state in metal coated graphene.     

二维硼碳基纳米结构上的吸附及其性质

综述金属原子与非金属原子和分子在石墨烯、BC₃平面等二维硼碳基纳米结构上的吸附所表现出的各种物理性质及可能的应用.纯净的石墨烯为零带隙的半金属、无磁且自旋轨道耦合效应非常弱,BC₃平面为间接带隙半导体,但金属原子与非金属原子和分子的吸附可能使石墨烯体系在Dirac点处打开带隙、具有强自旋轨道耦合效应,可能使石墨烯体系与二维BC₃体系具有磁有序、超导电性及应用在氢存储上.另外石墨烯表现出非常好的分子探测性能.     

Doped graphene as a superconductor

We study s-wave superconductivity state in doped graphene within the extended attractive Hubbard model and BCS theory. We use the Green's function approach and coherent potential approximation. We obtain critical temperature of graphene, Tc, as a function of the impurity concentration, c, as well as impurity strength, δ. The results show that when c and δ, are increased, Tc remains finite and doped graphene can be a superconductor.     

Surface superconductivity in multilayered rhombohedral graphene: Supercurrent

The supercurrent for the surface superconductivity of a flat-band multilayered rhombohedral graphene is calculated. Despite the absence of dispersion of the excitation spectrum, the supercurrent is finite. The critical current is proportional to the zero-temperature superconducting gap, i.e., to the superconducting critical temperature and to the size of the flat band in the momentum space.     

Stanene: A Promising Material for New Electronic and Spintronic Applications

The attractive mechanical and electronic properties of freestanding graphene has led to the exploration of two‐dimensional (2D) materials which can be integrated with contemporary electronics. As a 2D analog of graphene, stanene has become a hopeful candidate for 2D films due to its excellent quantum effects, superconductivity, and thermoelectric properties. Focusing on the promising 2D elemental material stanene, the fundamental electronic properties and experimental preparation of this material are reviewed. The prospects of utilizing the ability to manipulate the electronic properties of stanene for nanoelectronic and optoelectronic applications are determined.     

双层石墨烯材料中无序导致超导-绝缘体相变的数值研究

本文利用一种新的数值方法研究了在较大的双层石墨烯样品中杂质的无序 效应对超导态特性的影响. 采用核多项式方法 (Kernel Polynomial Method) 来自洽求解双层石墨烯系统的Bogoliubov-de-Gennes (BdG) 方程, 从而得到了由无序效应所引起的超导序参量的空间涨落精确解. 进一步, 计算了系统处于超导态时的态密度、光电导和广义逆参与率 (inverse participation ratio) 等物理量, 并发现随着无序强度的不断增大态密度中的能隙被 完全抑制, 同时光电导的Drude权重也迅速减小并最终降为零, 这表明双层石墨烯中的低能电子态发生了安德森局域化, 系统因而发生了由无序效应诱导的超导-绝缘体相变. 关键词： 双层石墨烯 安德森局域化 超导-绝缘体相变 核多项式方法     

Triplet superconducting p+ip pairs existing in a graphene sandwiched by superconductor and ferromagnet

Since the even-parity (s- or d-wave) superconducting pairs are incompatible with the ferromagnetism, a proper parity for these superconducting pairs existing in the interfaces of superconductors (SCs) and ferromagnets (FMs) should be odd-parity (p-wave). In this study a sandwiched structure constructed by even-parity superconductor/graphene/ferromagnet (ESC/GH/FM) is theoretically proposed to play a role to accommodate stable triplet superconducting pairs in the graphene (GH). The interactions in the graphene induced from the heavy magnetic atoms in FM including Kondo interaction, Kane–Mele and Rashba spin–orbit interactions (SOI) are considered. The calculations reveal a fact that p+ip state in the graphene is stable than pure p state and the former interaction suppress the triplet superconductivity but enhance it by later two SOIs.     

Formation and lithium doping of graphene on the surface of cobalt silicide

The intercalation of silicon under graphene on the Co(0001) surface, which is accompanied by the formation of a silicon solid solution in cobalt and by the formation of a surface crystalline Co₂Si phase, has been investigated using photoelectron spectroscopy. It has been shown that the formation of cobalt silicide leads to a substantial weakening of the hybridization of electronic states of graphene and cobalt and to the recovery of the Dirac spectrum of electronic states of graphene near the Fermi level. This has made it possible to investigate the electron doping of graphene on the cobalt silicide substrate upon deposition of lithium on its surface. It has been found that doping with lithium leads to a significant charge transfer onto graphene, and the electron concentration reaches 3.1 × 10¹⁴ cm^?2. Moreover, the specific form of the Fermi surface creates favorable conditions for the enhancement of the electron-phonon coupling. As a result, the formed system can be considered as a candidate for the creation of superconductivity in single-layer graphene.     

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.     

Five-fold way to new high <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors

Discovery of high T _c superconductivity in La_2?x Ba _x CuO₄ by Bednorz and Muller in 1986 was a breakthrough in the 75-year long search for new superconductors. Since then new high T _c superconductors, not involving copper, have also been discovered. Superconductivity in cuprates also inspired resonating valence bond (RVB) mechanism of superconductivity. In turn, RVB theory provided a new hope for finding new superconductors through a novel electronic mechanism. This article first reviews an electron correlation-based RVB mechanism and our own application of these ideas to some new noncuprate superconducting families. In the process we abstract, using available phenomenology and RVB theory, that there are five directions to search for new high T _c superconductors. We call them five-fold way. As the paths are reasonably exclusive and well-defined, they provide more guided opportunities, than before, for discovering new superconductors. The five-fold ways are (i) copper route, (ii) pressure route, (iii) diamond route, (iv) graphene route and (v) double RVB route. Copper route is the doped spin-½ Mott insulator route. In this route one synthesizes new spin-½ Mott insulators and dopes them chemically. In pressure route, doping is not external, but internal, a (chemical or external) pressure-induced self-doping suggested by organic ET-salts. In the diamond route we are inspired by superconductivity in boron-doped diamond and our theory. Here one creates impurity band Mott insulators in a band insulator template that enables superconductivity. Graphene route follows from our recent suggestion of superconductivity in doped graphene, a two-dimensional broadband metal with moderate electron correlations, compared to cuprates. Double RVB route follows from our recent theory of doped spin-1 Mott insulator for superconductivity in iron pnictide family.     

Superconductivity in octagraphene

This article reviews the basic theoretical aspects of octagraphene, an one-atom-thick allotrope of carbon, with unusual two-dimensional(2 D) Fermi nesting, hoping to contribute to the new family of quantum materials. Octagraphene has an almost strongest sp²hybrid bond similar to graphene, and has the similar electronic band structure as iron-based superconductors, which makes it possible to realize high-temperature superconductivity. We have compared various possible mechanisms of superconductivity, including the unconventional s;superconductivity driven by spin fluctuation and conventional superconductivity based on electron–phonon coupling. Theoretical studies have shown that octagraphene has relatively high structural stability. Although many 2 D carbon materials with C;carbon ring and C;carbon ring structures have been reported, it is still challenging to realize the octagraphene with pure square-octagon structure experimentally.This material holds hope to realize new 2 D high-temperature superconductivity.     

Graphene/Mo_2C heterostructure directly grown by chemical vapor deposition

Graphene-based heterostructure is one of the most attractive topics in physics and material sciences due to its intriguing properties and applications. We report the one-step fabrication of a novel graphene/Mo_2C heterostructure by using chemical vapor deposition(CVD). The composition and structure of the heterostructure are characterized through energydispersive spectrometer, transmission electron microscope, and Raman spectrum. The growth rule analysis of the results shows the flow rate of methane is a main factor in preparing the graphene/Mo_2C heterostructure. A schematic diagram of the growth process is also established. Transport measurements are performed to study the superconductivity of the heterostructure which has potential applications in superconducting devices.     

Superconductivity in hydrogenated carbon nanostructures

We present an application of density functional theory for superconductors to superconductivity in hydrogenated carbon nanotubes and fullerane (hydrogenated fullerene). We show that these systems are chemically similar to graphane (hydrogenated graphene) and like graphane, upon hole doping, develop a strong electron phonon coupling. This could lead to superconducting states with critical temperatures approaching 100 K, however this possibility depends crucially on if and how metallization is achieved.     

Specular Andreev reflection in graphene

By combining the Dirac equation of relativistic quantum mechanics with the Bogoliubov-de Gennes equation of superconductivity we investigate the electron-hole conversion at a normal-metal-superconductor interface in graphene. We find that the Andreev reflection of Dirac fermions has several unusual features: (1) the electron and hole occupy different valleys of the band structure; (2) at normal incidence the electron-hole conversion happens with unit efficiency in spite of the large mismatch in Fermi wavelengths at the two sides of the interface; and, most fundamentally: (3) away from normal incidence the reflection angle may be the same as the angle of incidence (retroreflection) or it may be inverted (specular reflection). Specular Andreev reflection dominates in weakly doped graphene, when the Fermi wavelength in the normal region is large compared to the superconducting coherence length.     

Unconventional superconductivity in low density electron systems and conventional superconductivity in hydrogen metallic alloys

In this short review, we first discuss the results, which are mainly devoted to the generalizations of the famous Kohn–Luttinger mechanism of superconductivity in purely repulsive fermion systems at low electron densities. In the context of repulsive-U Hubbard model and Shubin–Vonsovsky model we consider briefly the superconducting phase diagrams and the symmetries of the order parameter in novel strongly correlated electron systems including idealized monolayer and bilayer graphene. We stress that purely repulsive fermion systems are mainly the subject of unconventional low-temperature superconductivity. To get the high temperature superconductivity in cuprates (with T_C of the order of 100 K) we should proceed to the t–J model with the van der Waals interaction potential and the competition between short-range repulsion and long-range attraction. Finally we note that to describe superconductivity in metallic hydrogen alloys under pressure (with T_C of the order of 200 K) it is reasonable to reexamine more conventional mechanisms connected with electron–phonon interaction. These mechanisms arise in the attractive-U Hubbard model with static onsite or intersite attractive potential or in more realistic theories (which include retardation effects) such as Migdal–Eliashberg strong coupling theory or even Fermi–Bose mixture theory of Ranninger et al. and its generalizations.