Induced superconductivity in graphene grown on rhenium

Proximity-Induced Superconductivity in Atomically Precise Nanographene on Ag/Nb(110)

Obtaining a robust superconducting state in atomically precise nanographene (NG) structures by proximity to a superconductor could foster the discovery of topological superconductivity in graphene. On-surface synthesis of such NGs has been achieved on noble metals and metal oxides; however, it is still absent on superconductors. Here, we present a synthetic method to induce superconductivity of polymeric chains and NGs adsorbed on the superconducting Nb(110) substrate covered by thin Ag films. Using atomic force microscopy at low temperature, we characterize the chemical structure of each subproduct formed on the superconducting Ag layer. Scanning tunneling spectroscopy further allows us to elucidate the electronic properties of these nanostructures, which consistently show a superconducting gap.

Resonant Scattering in Proximity-Coupled Graphene/Superconducting Mo2 C Heterostructures

The realization of high-quality heterostructures or hybrids of graphene and superconductor is crucial for exploring various novel quantum phenomena and devices engineering. Here, the electronic transport on directly grown high-quality graphene/Mo₂ C vertical heterostructures with clean and sharp interface is comprehensively investigated. Owing to the strong interface coupling, the graphene layer feels an effective confinement potential well imposed by two-dimensional (2D) Mo₂ C crystal. Employing cross junction device geometry, a series of resonance-like magnetoresistance peaks are observed at low temperatures. The temperature and gate voltage dependences of the observed resonance peaks give evidence for geometric resonance of electron cyclotron orbits with the formed potential well. Moreover, it is found that both the amplitude of resonance peaks and conductance fluctuation exhibit different temperature-dependent behaviors below the superconducting transition temperature of 2D Mo₂ C, indicating the correlation of quantum fluctuations and superconductivity. This study offers a promising route toward integrating graphene with 2D superconducting materials, and establishes a new way to investigate the interplay of massless Dirac fermion and superconductivity based on graphene/2D superconductor vertical heterostructures.

Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

Two-dimensional heterostructures composed of layers with slightly different lattice vectors exhibit new periodic structure known as moiré lattices, which, in turn, can support novel correlated and topological phenomena. Moreover, moiré superstructures can emerge from multiple misaligned moiré lattices or inhomogeneous strain distributions, offering additional degrees of freedom in tailoring electronic structure. High-resolution imaging of the moiré lattices and superstructures is critical for understanding the emerging physics. Here, we report the imaging of moiré lattices and superstructures in graphene-based samples under ambient conditions using an ultrahigh-resolution implementation of scanning microwave impedance microscopy. Although the probe tip has a gross radius of ~100 nm, spatial resolution better than 5 nm is achieved, which allows direct visualization of the structural details in moiré lattices and the composite super-moiré. We also demonstrate artificial synthesis of novel superstructures, including the Kagome moiré arising from the interplay between different layers.

Atmospheric Pressure Catalytic Vapor Deposition of Graphene on Liquid Sn and Cu-Sn Alloy Substrates

The chemical vapor deposition (CVD) of graphene on liquid substrates produces high quality graphene films due to the defect-free and atomically flat surfaces of the liquids. Through the detailed study of graphene growth on liquid Sn using atmospheric pressure CVD (APCVD), the quality of graphene has been found to have a close relationship with hydrogen flow rate that reflects on hydrogen partial pressure inside the reactor (P_H2) and hydrogen solubility of the growth substrates. The role of P_H2 was found to be crucial, with a low defect density monolayer graphene being obtained in low P_H2 (90.4 mbar), while partial graphene coverage occurred at high P_H2 (137.3 mbar). To further understand the role of substrate’s composition, binary alloy with compositions of 20, 30, 50, 60 and 80 wt.% tin in copper were made by arc-melting. Graphene quality was found to decrease with increasing the content of copper in the Cu–Sn alloys when grown using the conditions optimised for Sn substrates and this was related to the change in hydrogen solubility and the high catalytic activity of Cu compared to Sn. This shall provide a tool to help optimising CVD conditions for graphene growth based on the properties of the used catalytic substrate.

Spin Relaxation in s-Wave Superconductors in the Presence of Resonant Spin-Flip Scatterers

Employing analytical methods and quantum transport simulations we investigate the relaxation of quasiparticle spins in graphene proximitized by an s-wave superconductor in the presence of resonant magnetic and spin-orbit active impurities. Off resonance, the relaxation increases with decreasing temperature when electrons scatter off magnetic impurities-the Hebel-Slichter effect-and decreases when impurities have spin-orbit coupling. This distinct temperature dependence (not present in the normal state) uniquely discriminates between the two scattering mechanisms. However, we show that the Hebel-Slichter picture breaks down at resonances. The emergence of Yu-Shiba-Rusinov bound states within the superconducting gap redistributes the spectral weight away from magnetic resonances. The result is opposite to the Hebel-Slichter expectation: the spin relaxation decreases with decreasing temperature. Our findings hold for generic s-wave superconductors with resonant magnetic impurities, but also, as we show, for resonant magnetic Josephson junctions.

In-gap states of magnetic impurity in quantum spin Hall insulator proximitized to a superconductor

We study in-gap states of a single magnetic impurity embedded in a honeycomb monolayer which is deposited on superconducting substrate. The intrinsic spin-orbit coupling induces the quantum spin Hall insulating (QSHI) phase gapped around the Fermi energy. Under such circumstances we consider the emergence of Shiba-like bound states driven by the superconducting proximity effect. We investigate their topography, spin-polarization and signatures of the quantum phase transition manifested by reversal of the local currents circulating around the magnetic impurity. These phenomena might be important for more exotic in-gap quasiparticles in such complex nanostructures as magnetic nanowires or islands, where the spin-orbit interaction along with the proximity induced electron pairing give rise to topological phases hosting the protected boundary modes.

Proximity-Induced Superconductivity in Monolayer MoS2

Proximity effects in superconducting normal (SN) material heterostructures with metals and semiconductors have long been observed and theoretically described in terms of Cooper pair wave functions and Andreev reflections. Whereas the semiconducting N-layer materials in the proximity experiments to date have been doped and tens of nanometers thick, we present here a proximity tunneling study involving a pristine single-layer transition-metal dichalcogenide film of MoS₂ placed on top of a Pb thin film. Scanning tunneling microscopy and spectroscopy experiments together with parallel theoretical analysis based on electronic structure calculations and Green's function modeling allow us to unveil a two-step process in which MoS₂ first becomes metallic and then is induced into becoming a conventional s-wave Bardeen-Cooper-Schrieffer-type superconductor. The lattice mismatch between the MoS₂ overlayer and the Pb substrate is found to give rise to a topographic moiré pattern. Even though the induced gap appears uniform in location, the coherence peak height of the tunneling spectra is modulated spatially into a moiré pattern that is similar to but shifted with respect to the moiré pattern observed in topography. The aforementioned modulation is shown to originate from the atomic-scale structure of the SN interface and the nature of local atomic orbitals that are involved in generating the local pairing potential. Our study indicates that the local modulation of induced superconductivity in MoS₂ could be controlled via geometrical tuning, and it thus shows promise toward the integration of monolayer superconductors into next-generation functional electronic devices by exploiting proximity-effect control of quantum phases.

Size-Selective Carbon Clusters as Obstacles to Graphene Growth on a Metal

Chemical vapor deposition (CVD) on metals is so far the best suited method to produce high-quality, large-area graphene. We discovered an unprecedentedly large family of small size-selective carbon clusters that form together with graphene during CVD. Using scanning tunneling microscopy (STM) and density functional theory (DFT), we unambiguously determine their atomic structure. For that purpose, we use grids based on a graphene moiré and a dilute atomic lattice that unambiguously reveal the binding geometry of the clusters. We find that the observed clusters bind in metastable configurations on the substrate, while the thermodynamically stable configurations are not observed. We argue that the clusters are formed under kinetic control and establish that the evolution of the smallest clusters is blocked. They are hence products of surface reactions in competition with graphene growth, rather than intermediary species to the formation of extended graphene, as often assumed in the literature. We expect such obstacles to the synthesis of perfect graphene to be ubiquitous on a variety of metallic surfaces.

Origin of the moiré superlattice scale lateral force modulation of graphene on a transition metal substrate

The moiré superlattice formed between graphene and a transition metal substrate is capable of tuning the frictional properties of graphene. For instance, a moiré superlattice scale modulation on the lateral force will be experienced by the tip of an atomic force microscope (AFM). However, the origin of this long-range force modulation still needs to be clarified. In this study, density functional theory (DFT) calculations have been carried out to investigate the indentation processes of a one-Ar-atom tip and a 10-atom Ir tip, sliding on graphene/Re(0001) and graphene/Pt(111) moiré superlattices, respectively. The calculation results indicate that the interfacial interaction between graphene and a transition metal substrate determines the morphological corrugation of graphene and the characteristics of the lateral force modulation. Moreover, when the tip-graphene interaction is strong enough, it will influence the evolutions of the adsorption energy Ead and tip sliding trajectory. Thus, the moiré superlattice scale lateral force modulation of graphene on a transition metal substrate originates from the joint effects of the graphene-substrate interfacial interaction and tip-graphene interaction.

Large local lattice expansion in graphene adlayers grown on copper

Variations of the lattice parameter can significantly change the properties of a material, and, in particular, its electronic behaviour. In the case of graphene, however, variations of the lattice constant with respect to graphite have been limited to less than 2.5% due to its well-established high in-plane stiffness. Here, through systematic electronic and lattice structure studies, we report regions where the lattice constant of graphene monolayers grown on copper by chemical vapour deposition increases up to ~7.5% of its relaxed value. Density functional theory calculations confirm that this expanded phase is energetically metastable and driven by the enhanced interaction between the substrate and the graphene adlayer. We also prove that this phase possesses distinctive chemical and electronic properties. The inherent phase complexity of graphene grown on copper foils revealed in this study may inspire the investigation of possible metastable phases in other seemingly simple heterostructure systems.

Design and performance of an ultra-high vacuum spin-polarized scanning tunneling microscope operating at 30 mK and in a vector magnetic field

We describe the design and performance of a scanning tunneling microscope (STM) that operates at a base temperature of 30 mK in a vector magnetic field. The cryogenics is based on an ultra-high vacuum (UHV) top-loading wet dilution refrigerator that contains a vector magnet allowing for fields up to 9 T perpendicular and 4 T parallel to the sample. The STM is placed in a multi-chamber UHV system, which allows in situ preparation and exchange of samples and tips. The entire system rests on a 150-ton concrete block suspended by pneumatic isolators, which is housed in an acoustically isolated and electromagnetically shielded laboratory optimized for extremely low noise scanning probe measurements. We demonstrate the overall performance by illustrating atomic resolution and quasiparticle interference imaging and detail the vibrational noise of both the laboratory and microscope. We also determine the electron temperature via measurement of the superconducting gap of Re(0001) and illustrate magnetic field-dependent measurements of the spin excitations of individual Fe atoms on Pt(111). Finally, we demonstrate spin resolution by imaging the magnetic structure of the Fe double layer on W(110).

Tune the chemical activity of graphene via the transition metal substrate

To achieve the chemical modification of graphene efficiently is desirable and essential to promote the technological applications of graphene. In this study, the density functional theory (DFT) calculations have been carried out to investigate the hydrogenation and fluorination activities of graphene on Ni(111), Re(0001) and Pt(111). The calculation results indicate that the chemical activity of graphene is related to both the characteristics of the graphene–substrate interfacial interaction and the local atomic stacking, namely the chemical activity of graphene is position-dependent. The strong covalently interacting substrates Ni(111) and Re(0001) will remarkably enhance the chemical activity of graphene, while the modulation effects from the weak van der Waals interacting substrate Pt(111) is trivial. Electronic structure studies reveal that the intensive graphene–substrate interfacial interaction can gain in chemical energy to offset the strain energy caused by the C atom sp²–sp³ transition and stabilize the absorbing state.

The chemical activity of graphene can be tuned by the transition metal substrate.

Strong Adlayer-Substrate Interactions "Break" the Patching Growth of h-BN onto Graphene on Re(0001)

Hetero-epitaxial growth of hexagonal boron nitride (h-BN) from the edges of graphene domains or vice versa has been widely observed during synthesis of in-plane heterostructures of h-BN-G on Rh(111), Ir(111), and even Cu foil. We report that on a strongly coupled Re(0001) substrate via a similar two-step sequential growth strategy, h-BN preferably nucleated on the edges of Re(0001) steps rather than on the edges of existing graphene domains. Statistically, one-third of the domain boundaries of graphene and h-BN were patched seamlessly, and the others were characterized by obvious "defect lines" when the total coverage approached a full monolayer. This imperfect merging behavior can be explained by translational misalignment and lattice mismatch of the resulting separated component domains. According to density functional theory calculations, this coexisting patching and non-patching growth behavior was radically mediated by the strong adlayer-substrate (A-S) interactions, as well as the disparate formation energies of the attachment of B-N pairs or B-N lines along the edges of the Re(0001) steps versus the graphene domains. This work will be of fundamental significance for the controllable synthesis of in-plane heterostructures constructed from two-dimensional layered materials with consideration of A-S interactions.

p-wave triggered superconductivity in single-layer graphene on an electron-doped oxide superconductor

Electron pairing in the vast majority of superconductors follows the Bardeen-Cooper-Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.

Influence of interface interaction on the moiré superstructures of graphene on transition-metal substrates

The formation of moiré superstructures between graphene and its underlying substrate has attracted significant attention because it significantly influences the morphology and properties of graphene.

Effects of Pb Intercalation on the Structural and Electronic Properties of Epitaxial Graphene on SiC

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of PbSi chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

Universal classification of twisted, strained and sheared graphene moiré superlattices

Moiré superlattices in graphene supported on various substrates have opened a new avenue to engineer graphene's electronic properties. Yet, the exact crystallographic structure on which their band structure depends remains highly debated. In this scanning tunneling microscopy and density functional theory study, we have analysed graphene samples grown on multilayer graphene prepared onto SiC and on the close-packed surfaces of Re and Ir with ultra-high precision. We resolve small-angle twists and shears in graphene, and identify large unit cells comprising more than 1,000 carbon atoms and exhibiting non-trivial nanopatterns for moiré superlattices, which are commensurate to the graphene lattice. Finally, a general formalism applicable to any hexagonal moiré is presented to classify all reported structures.

Strain engineering of graphene: a review

Graphene has intrigued the science community by many unique properties not found in conventional materials. In particular, it is the strongest two-dimensional material ever measured, being able to sustain reversible tensile elastic strain larger than 20%, which yields an interesting possibility to tune the properties of graphene by strain and thus opens a new field called "straintronics". In this article, the current progress in the strain engineering of graphene is reviewed. We first summarize the strain effects on the electronic structure and Raman spectra of graphene. We then highlight the electron-phonon coupling greatly enhanced by the biaxial strain and the strong pseudomagnetic field induced by the non-uniform strain with specific distribution. Finally, the potential application of strain-engineering in the self-assembly of foreign atoms on the graphene surface is also discussed. Given the short history of graphene straintronics research, the current progress has been notable, and many further advances in this field are expected.

Scanning Tunneling Spectroscopy of Proximity Superconductivity in Epitaxial Multilayer Graphene

We report on spatial measurements of the superconducting proximity effect in epitaxial graphene induced by a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial multilayer graphene on SiC. The aluminum films were discontinuous with networks of trenches in the film morphology reaching down to exposed graphene terraces. Scanning tunneling spectra measured on the graphene terraces show a clear decay of the superconducting energy gap with increasing separation from the graphene-aluminum edges. The spectra were well described by Bardeen-Cooper-Schrieffer (BCS) theory. The decay length for the superconducting energy gap in graphene was determined to be greater than 400 nm. Deviations in the exponentially decaying energy gap were also observed on a much smaller length scale of tens of nanometers.