Superconductivity In Graphene Research Articles

Andreev reflection in hybrid [formula omitted] lattices junction

We explore local Andreev reflection and crossed Andreev reflection in a normal metal (N)–superconductor (S)–normal metal (N) hybrid junction based on the α−T3 lattice. Initially, both the left and right sections feature dice lattice structures (α=1). We compute probabilities for electron reflection, local Andreev reflection, electron transmission, and crossed Andreev reflection, as well as local conductance, electron transmission conductance, and crossed Andreev reflection conductance. Subsequently, we analyze structures with the left side featuring dice lattice and the right side graphene (α=0), as well as the reverse configuration. Our findings reveal that crossed Andreev reflections are more prevalent in the graphene–superconductor–dice lattice (GSD) junction, particularly with moderate superconducting region lengths, larger electron incidence angles, and lower electron incidence energy.

Shaping Graphene Superconductivity with Nanometer Precision.

Graphene holds great potential for superconductivity due to its pure 2D nature, the ability to tune its carrier density through electrostatic gating, and its unique, relativistic-like electronic properties. At present, still far from controlling and understanding graphene superconductivity, mainly because the selective introduction of superconducting properties to graphene is experimentally very challenging. Here, a method is developed that enables shaping at will graphene superconductivity through a precise control of graphene-superconductor junctions. The method combines the proximity effect with scanning tunnelling microscope (STM) manipulation capabilities. Pb nano-islands are first grown that locally induce superconductivity in graphene. Using a STM, Pb nano-islands can be selectively displaced, over different types of graphene surfaces, with nanometre scale precision, in any direction, over distances of hundreds of nanometres. This opens an exciting playground where a large number of predefined graphene-superconductor hybrid structures can be investigated with atomic scale precision. To illustrate the potential, a series of experiments are performed, rationalized by the quasi-classical theory of superconductivity, going from the fundamental understanding of superconductor-graphene-superconductor heterostructures to the construction of superconductor nanocorrals, further used as "portable" experimental probes of local magnetic moments in graphene.

Andreev and normal reflections in gapped bilayer graphene–superconductor junctions

Andreev and normal reflections in gapped bilayer graphene–superconductor junctions

Effect of Andreev processes on the Goos–Hänchen (GH) shift in the Graphene–Superconductor–Graphene (GSG) junctions

In this article, we study the transport properties of Graphene–Superconductor–Graphene (GSG) heterojunction where the superconducting region is created in the middle of a graphene sheet, as contrasted to widely studied transport properties through a Superconductor–Graphene–Superconductor (SGS) type of Josephson junction. We particularly analyse in detail the Goos–Hänchen shift of the electron and the hole at the GS interface in such a junction, due to normal as well as Andreev reflection, using a transfer matrix-based approach. Additionally, we evaluate the normalised differential conductance as a function of bias voltage that characterises the transport through such junction and point out how they are influenced by Andreev and normal reflection. In the subsequent parts of the article we demonstrate how the GH shift for both electron and hole changes with the width of the superconducting region. The behaviour of the differential conductance in such junctions as a function of the bias voltage in the region, dominated by Andreev and normal reflection, is also presented and analysed.

Predicting of tunneling resistivity between adjacent nanosheets in graphene–polymer systems

In this work, the tunneling resistivity between neighboring nanosheets in grapheme–polymer nanocomposites is expressed by a simple equation as a function of the characteristics of graphene and tunnels. This expression is obtained by connecting two advanced models for the conductivity of graphene-filled materials reflecting tunneling role and interphase area. The predictions of the applied models are linked to the tested data of several samples. The impressions of all factors on the tunneling resistivity are evaluated and interpreted using the suggested equation. The calculations of tunneling resistivity for the studied examples by the model and suggested equation demonstrate the same levels, which confirm the presented methodology. The results indicate that the tunneling resistivity decreases by super-conductive graphene, small tunneling width, numerous contacts among nanosheets and short tunneling length.

Andreev reflection between aluminum and graphene across van der Waals barriers

We present planar aluminum superconductor–graphene junctions whose hybrid interface is engineered for couplings ranging from tunneling to the strongly coupled regime by employing an atomically thin van der Waals tunneling barrier. Without the vdW barrier, we find Al makes strongly coupled contacts with the fully proximities graphene channel underneath. Using a large band gap hexagonal boron nitride (hBN) barrier, we find the junctions always remain in the weak coupling regime, exhibiting tunneling characteristics. Using monolayer semi-conducting transition metal dichalcogenides (TMDs) such as MoS2, we realize intermediate coupling with enhanced junction conductance due to the Andreev process. In this intermediate regime, we find that junction resistance changes in discrete steps when sweeping a perpendicular magnetic field. The period of the resistance steps in the magnetic field is inversely proportional to the junction area, suggesting the physical origin of our observations is due to magnetic-field-induced vortex formation in the planar junction.

Stabilizing Fluctuating Spin-Triplet Superconductivity in Graphene via Induced Spin-Orbit Coupling.

A recent experiment showed that a proximity-induced Ising spin-orbit coupling enhances the spin-triplet superconductivity in Bernal bilayer graphene. Here, we show that, due to the nearly perfect spin rotation symmetry of graphene, the fluctuations of the spin orientation of the triplet order parameter suppress the superconducting transition to nearly zero temperature. Our analysis shows that both an Ising spin-orbit coupling and an in-plane magnetic field can eliminate these low-lying fluctuations and can greatly enhance the transition temperature, consistent with the recent experiment. Our model also suggests the possible existence of a phase at small anisotropy and magnetic field which exhibits quasilong-range ordered spin-singlet charge 4e superconductivity, even while the triplet 2e superconducting order only exhibits short-ranged correlations. Finally, we discuss relevant experimental signatures.

Scalability of non-adiabatic effects in lithium-decorated graphene superconductor

The analysis is conducted to unveil how the non-adiabatic effects scale within the superconducting phase of lithium-decorated graphene . Based on the Eliashberg formalism it is shown that the non-adiabatic effects notably reduce essential superconducting parameters in and arise as a significant oppressor of the discussed phase. Moreover, non-adiabaticity is found to scale with the strength of superconductivity, proportionally to the phonon energy scale and inversely with respect to the electron-phonon coupling. These findings are partially in contrast to other theoretical studies and show that superconductivity in is more peculiar than previously anticipated. In this context, the guidelines for enhancing superconducting phase in and sibling materials are also proposed.

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.

Critical Field and Specific Heat in Electron- and Hole-Doped Graphene Superconductors

Doping is one of the most prominent techniques to alter properties of a given material. Herein, the influence of the electron- and hole-doping on the selected superconducting properties of graphene are considered. In details, the Migdal-Eliashberg formalism is employed to analyze the specific heat and the critical magnetic field in the representative case of graphene doped with nitrogen or boron, respectively. It is found that the electron doping is much more favorable in terms of enhancing the aforementioned properties than its hole counterpart. These findings are appropriately summarized by the means of the dimensionless thermodynamic ratios, familiar in the Bardeen-Cooper-Schrieffer theory. To this end, the perspectives for future research on superconductivity in graphene are drawn.

Single magnetic impurity effects in graphene based superconductors

The magnetic impurity effects and the existence of bound states (i.e. Yu-Shiba-Rusinov states) in superconductors have been a topic of great interest. Recently, the existence of Yu-Shiba-Rusinov states in graphene-based superconducting materials has been successfully observed in the laboratory. In this work, an effective Hamiltonian in real space is established to describe the superconducting state of graphene materials by considering a single magnetic impurity. Thus the Bogoliubov-de Gennes (BdG) equation is constructed and the self-consistency calculations of the superconducting order parameter are conducted. On this basis, the effects of magnetic impurities on graphene-like superconductors are investigated theoretically. The numerical results show that the Yu-Shiba-Rusinov state can only appear in the symmetry of the superconducting pair of the traditional <i>s</i>-wave coupling. The position and strength of the bound state are related to the magnetic moment of the impurity, showing a notable electron-hole asymmetry. There are no bound states in the energy gap for other pairing symmetries. This theoretical calculation not only provides a reasonable explanation for experimental phenomena, but also demonstrates that the heterojunction system composed of graphene and traditional superconductors has an <i>s</i>-wave superconducting pairing induced by the proximity effect in the graphene layer.

The study of contact properties in edge-contacted graphene–aluminum Josephson junctions

Transparent contact interfaces in superconductor–graphene hybrid systems are critical for realizing superconducting quantum applications. Here, we examine the effect of the edge contact fabrication process on the transparency of the superconducting aluminum–graphene junction. We show significant improvement in the transparency of our superconductor–graphene junctions by promoting the chemical component of the edge contact etch process. Our results compare favorably with state-of-the-art graphene Josephson junctions. The findings of our study contribute to advancing the fabrication knowledge of edge-contacted superconductor–graphene junctions.

Acoustic-phonon-mediated superconductivity in moiréless graphene multilayers

We investigate the competition between acoustic-phonon-mediated superconductivity and the long-range Coulomb interaction in moir\'eless graphene multilayers, specifically, Bernal bilayer graphene, rhombohedral trilayer graphene, and ABCA-stacked tetralayer graphene. In these graphene multilayers, the acoustic phonons can realize, through electron-phonon coupling, both spin-singlet and spin-triplet pairings, and the intra-sublattice pairings ($s$-wave spin-singlet and $f$-wave spin-triplet) are the dominant channels. Our theory naturally explains the distinct recent experimental findings in Bernal bilayer graphene and rhombohedral trilayer graphene, and we further predict existence of superconductivity in ABCA tetralayer graphene arising from electron-phonon interactions. In particular, we demonstrate that the acoustic-phonon-mediated superconductivity prevails over a wide range of doping in rhombohedral trilayer graphene and ABCA tetralayer graphene while superconductivity exists only in a narrow range of doping near the Van Hove singularity in Bernal bilayer graphene. Key features of our theory are the inclusion of realistic band structures with the appropriate Van Hove singularities and Coulomb repulsion effects (the so-called "$\mu^*$ effect") opposing the phonon-induced superconducting pairing. We also discuss how intervalley scatterings can suppress the spin-triplet spin-polarized superconductivity. Our work provides detailed prediction based on electron-acoustic-phonon-interaction-induced graphene superconductivity, which should be investigated in future experiments.

A Moir\xe9 interference pattern formation of magnetic nanoparticles by rotational magnetic field controlled interfacial self-assembly

‘Magic-angled’ structure currently attracts increasing attention from condensed matter physics due to its amazing performance in superconductivity of graphene. The similar pattern formation of nanoparticles will bring about novel physics in nanoscale that causes the next generation nanodevices. However, it remains a challenging task to acquire such patterns by self-assembly process. In this article, we reported a Moiré interference pattern formation of magnetic nanoparticles regulated by a rotational magnetic field. The colloidal concentration, the strength, and the frequency of magnetic field were found to affect the pattern formation. The mechanism lied in a two-step process. First, the magnetic nanoparticles formed bilayers of superlattice driven by hydrophobic interaction of interfacial self-assembly. Then, the bilayers of superlattice yielded asynchronous rotational displacement resulting from the magnetic force so that the Moiré interference pattern of nanoparticles formed. This fabrication methodology based on the field-controlled self-assembly provided a novel and simple means to get ‘magic angled’ structures in nanoscale that will enrich the physical understanding of nanotechnology.

Study of differential shot noise in ferromagnet-insulator-superconductor graphene junction with TRSB

In this paper, we study differential shot noise in ferromagnet-insulator- superconductor graphene junction with time-reversal symmetry broken (TRSB) where we consider two different Cooper pairing states: one unconventional TRSB D ​+ ​iS-wave and an unconventional TRSB D ​+ ​iDxy-wave. We consider two regimes: the first one is the equal Fermi energies on both sides of the junction and the second regime is the Fermi energies that are far from each other. In the first regime, for D ​+ ​iS when the main part i. e. D-wave is dominant, we have obtained the shot noise peaks at the edge of the superconducting gap. The height of peaks reduces with the increase of the ferromagnetic exchange field. For D ​+ ​iS, when the s-wave part plays the main role, the peaks split in high exchange fields. For D ​+ ​iDxy-wave, the peaks split as well, but the distance between the peaks for D ​+ ​iDxy is smaller than D ​+ ​iS. Also, differential shot noise at zero-bias shows an oscillating behavior with respect to the barrier potential of the insulator such that its absolute value decreases when the exchange field enhances.In the second regime (Fermi energies are selected far from each other in the both sides of the junction), zero-bias peaks are formed when the main part i. e. D-wave is dominant while the peaks are shifted to non-zero voltages when the sub-dominant s-wave part is significant compared with the main part. Also, differential shot noise at zero-bias does not show any oscillating behavior versus the strength of barrier potential while its absolute value enhances with the increase of the exchange fields.Therefore, differential shot noise may serve as a valuable tool to understand more about various candidates of pair symmetries including TRSB states.

Two-Dimensional Superconductivity of Ca-Intercalated Graphene on SiC: Vital Role of the Interface between Monolayer Graphene and the Substrate.

Ca-intercalation has enabled superconductivity in graphene on SiC. However, the atomic and electronic structures that are critical for superconductivity are still under discussion. We find an essential role of the interface between monolayer graphene and the SiC substrate for superconductivity. In the Ca-intercalation process, at the interface a carbon layer terminating SiC changes to graphene by Ca-termination of SiC (monolayer graphene becomes a bilayer), inducing more electrons than a free-standing model. Then, Ca is intercalated in between the graphene layers, which shows superconductivity with the updated critical temperature (TC) of up to 5.7 K. In addition, the relation between TC and the normal-state conductivity is unusual, "dome-shaped". These findings are beyond the simple C6CaC6 model in which s-wave BCS superconductivity is theoretically predicted. This work proposes a general picture of the intercalation-induced superconductivity in graphene on SiC and indicates the potential for superconductivity induced by other intercalants.

Surprising superconductivity of graphene.

An ordinary graphene bilayer exhibits extraordinary superconductivity.

Progressing of Kovacs model for conductivity of graphene-filled products by total contact resistance and actual filler amount

In this work, the simple model recommended by Kovacs is progressed for tunneling conductivity of graphene-based samples assuming the total contact resistance and interphase pieces. The amount, dimensions and conduction of graphene nanosheets as well as the tunneling resistivity and contact size express the total contact resistance. Moreover, the actual filler content and percolation beginning take into account the interphase pieces. The forecasts of the novel model are linked to the measured results. In addition, the stimuli of factors on the contact resistance and conductivity of system are studied. The model’s productions acceptably follow the experimented data of several examples. Thinner graphene nanosheets simultaneously increase the contact resistance and the conductivity of system. Also, larger graphene nanosheets and shorter contact size definitely diminish the contact resistance and improve the conductivity. Nonetheless, the graphene’s conductivity does not affect the contact resistance and nanocomposite’s conductivity, because super-conductive graphene causes too poorer resistance in the contact pieces compared to insulated polymer layer.

Active Terahertz Modulator and Slow Light Metamaterial Devices with Hybrid Graphene-Superconductor Photonic Integrated Circuits.

Metamaterial photonic integrated circuits with arrays of hybrid graphene–superconductor coupled split-ring resonators (SRR) capable of modulating and slowing down terahertz (THz) light are introduced and proposed. The hybrid device’s optical responses, such as electromagnetic-induced transparency (EIT) and group delay, can be modulated in several ways. First, it is modulated electrically by changing the conductivity and carrier concentrations in graphene. Alternatively, the optical response can be modified by acting on the device temperature sensitivity by switching Nb from a lossy normal phase to a low-loss quantum mechanical phase below the transition temperature (Tc) of Nb. Maximum modulation depths of 57.3% and 97.61% are achieved for EIT and group delay at the THz transmission window, respectively. A comparison is carried out between the Nb-graphene-Nb coupled SRR-based devices with those of Au-graphene-Au SRRs, and significant enhancements of the THz transmission, group delay, and EIT responses are observed when Nb is in the quantum mechanical phase. Such hybrid devices with their reasonably large and tunable slow light bandwidth pave the way for the realization of active optoelectronic modulators, filters, phase shifters, and slow light devices for applications in chip-scale future communication and computation systems.

Graphene Superconductivity at Room‐Temperature of a Wide Range and Standard Atmosphere, Based on Vacuum Channels and White‐Light Interferometry

AbstractThe thickness of a monolayer graphene is extremely thin, ≈0.34 nm, so exact measurement of the optical path length and optical path length difference of graphene is challenging. This paper uses white‐light interferometry and a horizontal grating to obtain the optical characteristic curve of graphene. Then, the real existence of vacuum channels (VCs) in monolayer graphene is discovered. These VCs naturally eliminate the collision phenomena, and provide zero electrical resistance and superconductivity (SC) states. Multiple SC states (SCSs), SC persistence, magnetic field response, VCs transmitting a SC current with high efficiency, screen characteristics of VCs, etc. are discussed. Experiments show that only when a graphene ribbon is located at the exact position of the VC, does it transmit the SC current and provide a SCS. The duration of the persistence tests is 2 months. All experiments are conducted in a completely open space of a laboratory. Graphene SC at room‐temperatures of a wide range and standard atmosphere is achieved. This paper uses the optical technology to realize graphene SC, points out the basic reason of realizing it, and provides the experimental evidences.