Study On The Coherent Phonon Effect And Thermal Manipulation In Two-Dimensional Materials

With the increasing integration of electronic devices,the heat dissipation generated by devices is increasing,and a large amount of electric energy is converted into waste heat.Energy utilization efficiency is a critical problem in today’s society.At the macro level,it has become an important research topic to look for materials with high thermal conductivity and construct thermal metamaterials with composite structures to regulate heat transfer.In the micro scale,heat is transferred in the form of lattice vibration,and the carrier of the lattice vibration energy is phonon.Up to now,most of the studies have focused on the properties related to the particle nature of phonon such as ballistic and scattering transport,but less on the properties related to the wave picture of phonon.As the quantization of lattice vibration energy,phonons also have significant wave-like properties.Similar with photons in optics,phonons also have coherence and can interfere with each other.Through constructive/constructive interference,the transport of phonons with a specific frequency will be enhanced or weakened significantly when passing through the interface.Finally,depending on the phonon coherence,heat transport can be regulated by controlling the transmission of phonons in a certain frequency band at the material interfaces.Due to the nonlinearity of solitons,they can defend disturbance more powerfully than coherent phonons,and can propagate stably without losing their internal information.Exploring the way of stable soliton emission plays a positive role in its related research.In this study,firstly,the transport of coherent phonons is indirectly verified and observed by using the artificial microstructure.Through simulation,it is concluded that the specific impact of the interaction of coherent phonons on the thermal conductivities is reflected in two aspects: one is that the thermal conductivity dominated by coherent phonons increases with the decreasing period length in ordered structures,and the other is that in disordered structures,the localization of coherent phonons caused by boundary scattering will cause a significant decrease in thermal conductivity.In the following two chapters,we have respectively studied and verified the application of the phonon localization caused by boundary scattering in controlling heat flow(thermal rectification),and then through wave packet simulation method,we also visually describe the coherence effects of phonon transport in ordered super-lattice structures with the conditions supposed to be satisfied.The above studies are all harmonic effects caused by the interference of coherent phonons themselves.Considering that the wave packet itself can be both a soliton and the carrier of lattice harmonic vibration(phonon),coherent phonons can also bring anharmonic effects through the form of wave packets.Based on this concept,we find that a harmonic coherent phonon wave packet can excite nonlinear soliton waves in real materials through simulation.The specific contents of this paper are summarized as follows:1)Through the non-equilibrium molecular dynamics simulation of graphene phononic crystal(GPnC)structures,we find that the thermal conductivity of GPnC shows a non-monotonic trend with the varying period lengths,which gives the numerical evidence that the coherent phonons participate in the heat transport in GPnC.With the decrease of the period length in GPnC,we can clearly observe the transition of phonon transport mode from incoherent to coherent: in the incoherent transport region,diffusion transport of incoherent phonons dominates in heat transfer,and the smaller the period length is,the lower the thermal conductivity will be;In the coherent transport region,the phonons mainly behave in wave picture,and the thermal conductivity will anomalously increase with the decrease of the period length.Next,when we introduce the random perturbation to the hole positions in GPnC,the phonon wave packet simulations reveal that the coherent phonons also have a significant Anderson localization effect in the disordered phononic crystals,resulting in a sharp decrease in the overall thermal conductivity,which is no longer dependent on the system length.The phonon Anderson localization effect originates from phonon’s wave nature and is expected to provide a new scheme to minimize the thermal conductivity.2)Considering that the coherent phonon localization effect can significantly affect the phonon transport and regulate the heat flow,we further propose a scheme to use the phonon localization effect for thermal rectification: by designing and adjusting the contact length between the heat reservoirs and the material at both ends,the ratio between local and non-local phonons in the heat reservoir can be manipulated,so as to control the properties and numbers of phonons emitted from the heat source and finally the heat flow.Using this method,significant thermal rectification effect can be achieved in pristine graphene without asymmetry.Our molecular dynamics simulations show that significant thermal rectification can be achieved when the length of the heat reservoir at one end is greater than the phonon localization length and the length at the other end is less than the phonon localization length;When the two heat reservoirs are both larger or smaller than the phonon localization length at the same time,the thermal rectification effect will disappear.Our simulation results show that a giant thermal rectification efficiency of 920% can be obtained in short samples with a total length of 200 nm,and an appreciable thermal rectification efficiency of about 110% can also be maintained even in micron samples,which is significantly higher than the results reported in previous literatures.The analysis of phonon mode participation ratio further verifies that the strong localization effect of low-frequency phonons at the boundary is the fundamental reason for the thermal rectification effect in this study.3)The control of heat flow in phononic crystals is mainly manifested in the coherent phonon transport,different from the diffusion transport of incoherent phonons,which has aroused widespread interest.However,in most studies,the coherent nature of phonons can only be reflected indirectly through the macro thermal properties(such as thermal conductivity),and there is not enough direct evidence.In this study,we directly observed the interference effect of a single phonon mode in graphene superlattice at the micro scale by using phonon wave packet simulation.After the wave packet enters the super-lattice structure,multiple scattering occurs between the interfaces.According to the Bragg scattering principle,the constructive and destructive interference between the reflected and incident phonons cause the minimum and maximum values of the phonon transmission coefficients respectively,resulting in the periodic oscillation of the phonon transmission function with varying super-lattice period lengths.More importantly,we also realize the total transmission and total reflection of a single phonon mode at the interface.Finally,the physical conditions for realizing the phonon interference are proposed and proved by the independent wave packet simulation results.4)Nonlinear evolution equations are widely used to describe some novel phenomena,such as solitons,in the fields of hydrodynamics,plasma physics,optical fiber,solid state physics and so on.Due to the effect of nonlinearity,solitons can keep their width,amplitude and shape unchanged for a long time in the propagation process.Although coherent phonons can also maintain their phase information,the conditions to be met are more stringent than solitons.Solitons have more advantages in information transmission.However,it is still a difficult problem to build a stable soliton source like soliton laser in practical materials.In this work,we obtain the soliton wave packet through the excitation from the phonon wave packet,and there is no need to solve the nonlinear differential equation analytically.Finally,the exact expression of the soliton wave packet is obtained by fitting the hyperbolic tangent function.In this way,we can avoid directly solving the nonlinear equations of solitons to generate soliton wave packets,provide a more convenient method for studying solitons,and expand the application of coherent phonons.Finally,we also summarize and discuss the shortcomings of the above researches,and look forward to the next step of research in the future.