The dependence of superlattice thermal conductivity on period length is investigated by molecular dynamics simulation. For perfectly lattice matched superlattices, a minimum is observed when the period length is of the order of the effective phonon mean free path. As temperature decreases and interatomic potential strength increases, the position of the minimum shifts to larger period lengths. The depth of the minimum is strongly enhanced as mass and interatomic potential ratios of the constituent materials increase. The simulation results are consistent with phonon transmission coefficient calculations, which indicate increased stop bandwidth and thus strongly enhanced Bragg scattering for the same conditions under which strong reductions in thermal conductivity are found. When nonideal interfaces are created by introducing a 4% lattice mismatch, the minimum disappears and thermal conductivity increases monotonically with period length. This result may explain why minimum thermal conductivity has not been observed in a large number of experimental studies. DOI: http://dx.doi.org/10.1103/PhysRevB.72.174302 © 2005 The American Physical Society

Effective control of heat transfer is vital for energy saving and carbon emission reduction. In contrast to achievements in electrical conduction, active control of heat transfer is much more challenging. Ferroelectrics are promising candidates for thermal switching as a result of their tunable domain structures. However, switching ratios in ferroelectrics are low (2.2), fast-speed (10 ⁷ ) thermal switching under a small voltage (<10 V). In situ reciprocal space mapping and atomistic modelings reveal that the field-driven antiferroelectric-ferroelectric phase transition induces a substantial change of primitive cell size, which modulates phonon-phonon scattering phase space drastically and results in high switching ratio. These results advance the concept of thermal transport control in ferroic materials.

Two-dimensional (2D) intrinsic magnetic materials with room temperature ferromagnetism and complete spin-polarization are highly desirable for realizing advanced spintronic devices. However owing to the weak d–p–d super-exchange interaction, many recently realized 2D ferromagnetic (FM) materials present a very low Curie temperature (T_C). Here, based on first principles calculations, we report a series of Cr₃X₄ (X = S, Se, Te) monolayers, in which the coexistence of two oxidation states of Cr atoms results in double-exchange interaction and thereby enhances FM coupling greatly and improves T_C up to 370 K for Cr₃Se₄ and 460 K for Cr₃Te₄, respectively. Spin-polarized calculations further demonstrate that Cr₃Se₄ and Cr₃Te₄ monolayers are FM half-metals, which ideally could achieve 100% spin-polarized currents. Meanwhile, excellent dynamical and thermal stabilities are identified, and a possible synthetic strategy is proposed. Our work not only provides two competitive ferromagnetic candidate materials for nanoscale spintronic applications, but also shows that double-exchange interaction may be a way to realize 2D room temperature FM half-metals.

Low-dimensional materials such as graphene provide an ideal platform to probe the correlation between thermal transport and lattice defects, which could be engineered at the molecular level. In this work, we perform molecular dynamics simulations and non-contact optothermal Raman measurements to study this correlation. We find that oxygen plasma treatment could reduce the thermal conductivity of graphene significantly even at extremely low defect concentration (∼83% reduction for ∼0.1% defects), which could be attributed mainly to the creation of carbonyl pair defects. Other types of defects such as hydroxyl, epoxy groups and nano-holes demonstrate much weaker effects on the reduction where the sp² nature of graphene is better preserved. With the capability of selectively functionalizing graphene, we propose an asymmetric junction between graphene and defective graphene with a high thermal rectification ratio of ∼46%, as demonstrated by our molecular dynamics simulation results. Our findings provide fundamental insights into the physics of thermal transport in defective graphene and two-dimensional materials in general, which could help on the future design of functional applications such as optothermal and electrothermal devices.