Determination of gap solution and critical temperature in doped graphene superconductivity (original) (raw)

Theoretical investigation of superconductivity in graphene doped systems

2012

The compound of Ba(AlSn) from ternary superconductors exhibits the superconductivity behaviour below the temperature 2.9 K. We report the results of an ab initio study based on electronic, and detailed lattice dynamical properties as a function of pressure of superconducting material. The phonon dispersion curves along the high-symmetry directions and phonon frequencies parameters at the Brillouin zone center are computed by using density functional perturbation theory while the elastic constants are calculated in metric-tensor formulation. The Vickers hardness belonging to the compound is also evaluated clearly. The band structure, partial densities of states and Fermi surface topology are also discussed in detail. At the same time we describe the relationship between the properties determined and superconducting characteristic.

Kohn-Luttinger superconductivity in graphene

Physical Review B, 2008

We investigate the development of superconductivity in graphene when the Fermi level becomes close to one of the Van Hove singularities of the electron system. The origin of the pairing instability lies in the strong anisotropy of the e-e scattering at the Van Hove filling, which leads to a channel with attractive coupling when making the projection of the BCS vertex on the symmetry modes with nontrivial angular dependence along the Fermi line. We show that the scale of the superconducting instability may be pushed up to temperatures larger than 10 K, depending on the ability to tune the system to the proximity of the Van Hove singularity.

Theory of superconductivity for Dirac electrons in graphene

Journal of Experimental and Theoretical Physics, 2010

Phonon exchange induced superconducting pairing of effectively ultrarelativistic electrons in graphene is investigated. The Eliashberg equation obtained for describing pairing in the Cooper channel with allowance for delayed interaction are matrix equations with indices corresponding to the valence and con duction bands. The equations are solved in the high doping limit, in which pairing is effectively a single band process, and in the vicinity of a critical quantum point of underdoped graphene for a value of the coupling constant for which pairing is an essentially multiband process. For such cases, analytic estimates are obtained for the superconducting transition temperature of the system. It is shown that the inclusion of dynamic effects makes it possible to determine the superconducting transition temperature, as well as the critical coupling constant for underdoped graphene, more accurately than in the static approximation of the BCS type. Esti mates of the constants of electron interaction with the scalar optical phonon mode in graphene indicate that an appreciable superconducting transition temperature can be attained under a high chemical doping level of graphene.

Investigation of the two-gap superconductivity in a few-layer NbSe2 -graphene heterojunction

Physical Review B, 2018

We investigated the superconductivity in a few-layer NbSe 2-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in NbSe 2 were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in NbSe 2 followed the expected temperature behavior in the limit of weak interband scattering, with a gap to T c ratio suggesting a weak to moderately strong coupling in few-layer systems.

Enhancement of superconductivity upon reduction of carrier density in proximitized graphene

Physical review, 2022

The superconducting transition temperature (Tc) of a single layer graphene coupled to an Indium oxide (InO) film, a low carrier-density superconductor, is found to increase with decreasing carrier density and is largest close to the average charge neutrality point in graphene. Such an effect is very surprising in conventional BCS superconductors. We study this phenomenon both experimentally and theoretically. Our analysis suggests that the InO film induces random electron and hole-doped puddles in the graphene. The Josephson effect across these regions of opposite polarity enhances the Josephson coupling between the superconducting clusters in InO, along with the overall Tc of the bilayer heterostructure. This enhancement is most effective when the chemical potential of the system is tuned between the charge neutrality points of the electron and hole-doped regions.

Subgap states in two-dimensional spectroscopy of graphene-based superconducting hybrid junctions

Physical review, 2019

The two-dimensional nature of graphene makes it an ideal platform to explore proximity-induced unconventional planar superconductivity and the possibility of topological superconductivity. Using Green's functions techniques, we study the transport properties of a finite size ballistic graphene layer placed between a normal state electrode and a graphene lead with proximity-induced unconventional superconductivity. Our microscopic description of such a junction allows us to consider the effect of edge states in the graphene layer and the imperfect coupling to the electrodes. The tunnel conductance through the junction and the spectral density of states feature a rich interplay between graphene's edge states, interface bound states formed at the graphene-superconductor junction, Fabry-Pérot resonances originated from the finite size of the graphene layer, and the characteristic Andreev surface states of unconventional superconductors. Within our analytical formalism, we identify the separate contribution from each of these subgap states to the conductance and density of states. Our results show that graphene provides an advisable tool to determine experimentally the pairing symmetry of proximity-induced unconventional superconductivity.

Unconventional Superconductivity in Semiconductor Artificial Graphene

arXiv: Superconductivity, 2019

Unconventional superconductivity featuring large pairing energies has attracted immense interest, yet tractable microscopic theories have proven elusive. A major breakthrough has been the advent of twisted bilayer graphene (TBG), which serves as a simple model system to 'look under the hood' of unconventional superconductivity. We propose a new model, within current experimental reach, to investigate the microscopics of strong-binding superconductivity. Our proposed device is semiconductor artificial graphene (AG), a two dimensional electron gas overlaid with a periodic potential (superlattice). We demonstrate a new mechanism for superconductivity that originates solely from the repulsive Coulomb interaction. The superlattice promotes certain interactions, which are antiscreened, cause attractive ppp-wave pairing and - in contrast to graphene - can be strongly enhanced through device engineering. The strength of the pairing energy is similar to TBG, and we find within the ac...

Proximity-induced superconductivity in graphene

JETP Letters, 2008

We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, Tc, can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, T ≪ Tc, and zero magnetic field, the density of states is characterized by a small gap Eg ≤ Tc resulting from the collective proximity effect. Transverse magnetic field Hg(T) ∝ Eg is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field Hg2(T).

Pairing symmetry of superconducting graphene

The European Physical Journal B, 2010

The possibility of intrinsic superconductivity in alkali-coated graphene monolayers has been recently suggested theoretically. Here, we derive the possible pairing symmetries of a carbon honeycomb lattice and discuss their phase diagram. We also evaluate the superconducting local density of states (LDOS) around an isolated impurity. This is directly related to scanning tunneling microscopy experiments, and may evidence the occurrence of unconventional superconductivity in graphene.

High-temperature surface superconductivity in rhombohedral graphite

Physical Review B, 2013

Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high transition temperature. 74.20.Pq, 74.70.Wz A low critical temperature of conventional superconductors results from a constant density of states (DOS) due to a linear-in-momentum electronic spectrum near the Fermi energy. With a higher-order dispersion, the relation between the critical temperature and the coupling constant becomes stronger, boosting the superconductivity. The extreme case would be a completely dispersionless energy spectrum, a flat band, which has been predicted in many condensed matter systems, see e.g. Refs. 1-4. In some cases the flat bands are protected by topology in momentum space; they emerge in gapless topological matter . A singular DOS associated with the dispersionless spectrum was recently shown to essentially enhance the transition temperature opening a new route to room-temperature superconductivity.