Thermal rectification is of importance not only for fundamental physics, butalso for potential applications in thermal manipulations and thermalmanagement. However, thermal rectification effect usually decays rapidly withsystem size. Here, we show that a mass-graded system, with two diffusive leadsseparated by a ballistic spacer, can exhibit large thermal rectificationeffect, with the rectification factor independent of system size. Theunderlying mechanism is explained in terms of the effective size-independentthermal gradient and the match/mismatch of the phonon bands. We also show therobustness of the thermal diode upon variation of the model's parameters. Ourfinding suggests a promising way for designing realistic efficient thermaldiodes.
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