Exciton Condensation in Molecular-Scale van der Waals Stacks

Abstract

Recent experiments have realized the Bose–Einstein condensation of excitons, known as exciton condensation, in extended systems such as bilayer graphene and van der Waals heterostructures. Here we computationally demonstrate the beginnings of exciton condensation in multilayer, molecular-scale van der Waals stacks composed of benzene subunits. The populations of excitons, which are computed from the largest eigenvalue of the particle-hole reduced density matrix (RDM) through advanced variational RDM calculations, are shown to increase with the length of the stack. The large eigenvalue indicates a nonclassical long-range ordering of the excitons that can support the frictionless flow of energy. Moreover, we use chemical substitutions and geometric modifications to tune the extent of the condensation. Results suggest exciton condensation in a potentially large family of molecular systems with applications to energy-efficient transport.

Simultaneous fermion and exciton condensations from a model Hamiltonian

“…(See the Appendix for more details on how the signature of exciton condensation, λ G was computed.) This computational signature has been utilized to study exciton condensation is possible in quantum and molecular systems [29,[38][39][40]64]. One model known to achieve a large λ G value and hence exhibit exciton condensate character in the limit of a large correlation is the Lipkin quasispin model [15][16][17][18][19][20][21].…”

Quantum Many-Body Theory from a Solution of the N -Representability Problem

“…More recently, the author [10] showed that the complete N -representability conditions are derivable from convex combinations of higher-particle, constraint matrices. In the past decade approximate conditions have been employed in variational calculations of the 2-RDM to solve significant problems such as the elucidation of the electronic properties of a glassy, highly conductive amorphous polymer [38] and the revelation of exciton condensation in molecular analogs of double-layer graphene [39][40][41][42] (refer to Refs. [43][44][45][46][47][48] for their use in one-particle RDM functional theories).…”

Beginnings of exciton condensation in coronene analog of graphene double layer

“…Exciton condensation-a Bose-Einstein condensation of particle-hole pairs into a single quantum statehas generated considerable experimental and theoretical interest [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] due to the resultant superfluidity [20][21][22][23] of the constituent excitons (particle-hole pairs) allowing for the dissipationless transport of energy 24,25 , which presents the possibility for uniquely energy efficient materials. Further, the greater binding energy and lesser mass of excitonic quasiparticles relative to particle-particle Cooper pairs indicates that exciton condensation should occur at higher temperatures 26 relative to the temperatures at which traditional superconductivity-i.e., the condensation of particle-particle pairs into a single quantum state [27][28][29][30] -occurs.…”

“…Specifically, van der Waal heterostructures 4,33,39,43 as well as graphene bilayers 17,31,44 demonstrate promise in the search for higher-temperature exciton condensate phases, with the tuneability of electronic states afforded by twisting graphene layers relative to each other being particularly of interest in recent literature 35,41 . a) Electronic mail: damazz@uchicago.edu Small, molecularly-scaled systems have also been revealed to support exciton condensation via theoretical explorations utilizing a signature of such condensation found in the modified particle-hole reduced density matrix (RDM) [8][9][10][11] . These molecular systems are able to be treated using theoretical approaches at lower computational costs and can be used as an analog for similar larger-scaled systems; moreover, molecular-scaled exciton condensation in and of itself may have potential applications in the design of more energy-efficient molecular-structures and devices.…”