Current-Induced Nonequilibrium Phonons And Their Non-Hermitian Dynamics In Graphene And Graphene Nanoribbons

Heat conduction properties of solid-state materials play important roles in many fundamental processes of solids,and in maintaining the stability of the solid-state devices.It becomes even more important in low-dimensional materials.In insulators or semiconductors,the main heat carriers are collective motion of atoms,termed phonons.In real systems,the external forces that drive the system out of equilibrium could be of different types.The most common situation is heat current driven by temperature bias applied across the system in real space.In this thesis,we choose a rather different view,focusing on heat transport between electrons and phonons due to momentum and energy transfer in graphene and its nanoribbons under electrical current.Based on first-principles calculations at the density functional theory level,we obtained the electron,phonon band structures and electron-phonon coupling matrix.From these results,using semiclassical Boltzmann transport equation or Langevin molecular dynamics method,we study in details nonequilibrium phonon distribution and their non-Hermitian dynamics driving by electrical current.The main results are summarized as follows:(1)The electron system is in a nonequilibrium steady state under driving of external electric field.In the weak field limit,the electrons follow a shifted Fermi-Dirac distribution.Under this circumstance,we consider energy transport from electrons to phonons.We find quite different distributions and effective temperatures in different region of the Brillouin zone.Besides the familiar hot phonon effect,where phonons are heated by electrons,we also observe cold phonons in some region where the phonons are cooled by electrical current.Using graphene as a prototypical example,we analyzed the physical mechanism that leads to such situations.Moreover,from the nonequilibrium phonon distribution,we can calculate the change of lattice parameter,and the electron-phonon drag correction to the thermoelectric transport coefficient.(2)There are high-frequency dimer vibrations at the boundaries of hydrogenated armchair graphene nanoribbon,when the two hydrogen atoms are missed for a carbon dimer.Their frequency spills out of the bulk phonon band of graphene,due to short bond length of the low-coordinated carbon dimer.The resulting anharmonic coupling of these dimer modes with the rest phonons and among themselves are rather weak.We studied the indirect coupling of two dimer vibrations through flowing electrical current.The two dimers show typical non-Hermitian dynamics.We find that their dynamics can be effectively controlled by source-drain bias and gate voltage.The new eigen modes carry different angular momentum.The electrical current breaks the balance and lead to non-zero angular momentum of vibrations.(3)Moreover,we demonstrated that the coupled dimer vibrations can be tuned near to an exceptional point,where both their eigen values and eigen vectors coalesce.This paves the way of studying non-Hermitian dynamics and exceptional point physics down to atomic scale.