Giant magneto-photoelectric effect in graphene

• Applicant: Dr. Friedemann Queisser
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 398912239

The efficiency of solar cells is bounded by the so-called Shockley-Queisser-limit. Within this research project, we want to propose an alternative setting which converts radiation into electric energy an might be able to overcome this limit. In particular, we want to study the current generation in a graphene fold (or a graphene flake with boundary) which is subject to a constant magnetic field: Electrons and holes move in opposite directions such that reflections at the graphene fold (graphene boundary) separate the charges and generate a current along the fold (boundary). Since the mean free path in graphene is on the micrometer scale whereas the typical cyclotron radius for an electron (for an energy in the meV range and a magnetic field of a few Tesla) is in the nanometer range, the charge separation is only slightly disturbed by scattering events.The band structure of graphene has no energy gap, therefore also long-wave photons can be absorbed.Due to these reasons, it might be feasible that the efficiency of the proposed setting exceeds the efficiency of commercially available solar cells. Indeed, a first experimental realization showed that 100 incoming photons generate up to 17 particle-hole pairs which is in contradiction to graphene's absorption probability of 2.3%. A possible origin of the observed giant-magnetophotoelectric effect is the generation of secondary particle-hole pairs at the graphene fold (edge) via Coulomb scattering.The first part of this proposal concerns the absorption probability of a graphene fold (a graphene boundary) within the single-particle picture.Using the continuum description of electrons in graphene we shall investigate its dependenceon the value of the magnetic field, the value of the chemical potential and external electric fields. Using these findings, we will investigate the secondary particle-hole pair creation. As a first step, we shall employ Fermi's golden rule to estimate qualitatively the probabilities of the scattering processes.Since the dimensionless parameter of the perturbation expansion(the effective fine-structure constant in graphene) is about 300 times larger than the fine-structure constant of quantum electrodynamics,we will apply two non-perturbative methods in order to obtain quantitative estimates.The first method is based on the continued fraction representation of solutions of the many-particle Lippmann-Schwinger-equation.Owing to the non-polynomial nature of this approximation scheme, we expect a better convergence compared to a polynomial perturbation theory.The second method is based on a hierarchy of correlation functions. In a further subproject, we shall investigate the impact of the boundary shape on the current generation.

• DFG Programme: Research Grants