In this work, we study the near-field radiative heat transfer between two suspended sheets of anisotropic 2D materials. It is found that the radiative heat transfer can be enhanced with orders-of-magnitude over the blackbody limit for nanoscale separation. The enhancement is attributed to the excitation of anisotropic and hyperbolic plasmonic modes. Meanwhile, a large thermal modulation effect, depending on the twisted angle of principal axes between the upper and bottom sheets of anisotropic 2D materials, is revealed. The near-field radiative heat transfer for different concentrations of electron is demonstrated and the role of hyperbolic plasmonic modes is analyzed. Our finding of radiative heat transfer between anisotropic 2D materials may find promising applications in thermal nano-devices, such as non-contact thermal modulators, thermal lithography, thermosphotovoltaics, etc.Thermal radiation is an important physical phenomenon. Any object with temperature T>0K emits electromagnetic (EM) waves due to the fluctuating current generated from thermal motion of charge carriers. According to the Stefan-Boltzmann law, the radiative heat flux between two separated black bodies is given as 2 4 4 4 1 2 3 2 ( ) 60 B bb k S T T c π = −  , where k B is the Boltzmann constant,  is the reduced Planck's constant, c is the speed of light in vacuum, T 1 and T 2 is the high and low temperatures,respectively. For the Stefan-Boltzmann law, the separation distance d is much larger than the thermal wavelength λ th =c/k B T and only the propagation modes are taken into account in the process of radiative heat transfer. However, an additional contribution, i.e., evanescent waves, is dominant and should be considered when the separation distance d is much smaller than λ th (about 10 µm at room temperature). 1-2 The evanescent waves near the surface can be surface plasmon polaritions (SPPs), 3 surface phonon polaritions (SPhPs), 3,4 or even frustrated modes from hyperbolic materials. 5-6 Due to large density of states of evanescent waves, near-field radiative heat transfer (NFRHT) can exceed the blackbody limit by several orders of _____________________________ a) Author to whom correspondence should be addressed. Electronic

Near-field radiative heat transfer (NFRHT) is strongly related with many applications such as near-field imaging, thermos-photovoltaics and thermal circuit devices. The active control of NFRHT is of great interest since it provides a degree of tunability by external means. In this work, a magnetically tunable multi-band NFRHT is revealed in a system of two suspended graphene sheets at room temperature. It is found that the single-band spectra for B=0 split into multi-band spectra under an external magnetic field. Dual-band spectra can be realized for a modest magnetic field (e.g., B=4 T). One band is determined by intra-band transitions in the classical regime, which undergoes a blue shift as the chemical potential increases. Meanwhile, the other band is contributed by inter-Landau-level transitions in the quantum regime, which is robust against the change of chemical potentials. For a strong magnetic field (e.g., B=15 T), there is an additional band with the resonant peak appearing at near-zero frequency (microwave regime), stemming from the magneto-plasmon zero modes. The great enhancement of NFRHT at such low frequency has not been found in any previous systems yet. This work may pave a way for multi-band thermal information transfer based on atomically thin graphene sheets.

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a b s t r a c tTotal energy calculations of germanium nitride, done at three different phases under extreme conditions in the b, wII and g structures using the plane-wave pseudo-potential method plus GGA-PBE in the framework of quantum mechanical density functional theory. Bulk properties such as the equilibrium lattice parameters, elastic constants and bulk moduli are predicted and compared to available theoretical and experimental data. g-Ge 3 N 4 could not resistant to thermal shocks due to its brittleness. The ductility of b-Ge 3 N 4 is larger than that of wII-Ge 3 N 4 . The b / wII / g phase transitions are also successfully predicted.The phase boundary of b / wII transition can be described as P ¼ 10.80087 À 8.58508 Â 10 À4 T þ 5.00991 Â 10 À6 T 2 À 1.84732 Â 10 À9 T 3 . Pressure (0e58 GPa) and temperature (0e1300 K) dependent thermal quantities including the bulk modulus, coefficient of thermal expansion, entropy, heat capacity and Debye temperature are obtained and analyzed through the quasi-harmonic approximation, in which the lattice vibrations and phonon effects are taken into account. Some interesting features can be observed in the temperature range of 300~1200 K. It is worthy of note that most of the investigated properties are not reported by previous literatures. Our calculations need to be verified by the future experiments.

The recently discovered tetragonal, monoclinic and orthorhombic polymorphs of M3N4 (M=C, Si, Sn) are investigated by using first-principles calculations. A set of anisotropic elastic quantities, i.e., the bulk and shear moduli, Young's modulus, Poisson ratio, B/G ratio and Vickers hardness of M3N4 (M=C, Si, Sn) are predicted. The quasi-harmonic Debye model, assuming that the solids are isotopic, may lead to large errors for the non-cubic crystals. The thermal effects are obtained by the traditional quasi-harmonic approach. The dependences of heat capacity, thermal expansion coefficient and Debye temperature on temperature and pressure are systematically discussed in the pressure range of 0–10 GPa and in the temperature range of 0–1100 K. More importantly, o-C3N4 is a negative thermal expansion material. Our results may have important consequences in shaping the understanding of the fundamental properties of these binary nitrides.