An introduction to heat transfer

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-020259.pdf

Gate-tunable phase transitions in thin flakes of 1T-TaS$_{2}$

An Anisotropically High Thermal Conductive Boron Nitride/Epoxy Composite Based on Nacre-Mimetic 3D Network

We report a new type of phononic crystals with topologically nontrivial band gaps for both longitudinal and transverse polarizations, resulting in protected one-way elastic edge waves. In our design, gyroscopic inertial effects are used to break the time-reversal symmetry and realize the phononic analogue of the electronic quantum (anomalous) Hall effect. We investigate the response of both hexagonal and square gyroscopic lattices and observe bulk Chern numbers of 1 and 2, indicating that these structures support single and multimode edge elastic waves immune to backscattering. These robust one-way phononic waveguides could potentially lead to the design of a novel class of surface wave devices that are widely used in electronics, telecommunication, and acoustic imaging.

Topological Phononic Crystals with One-Way Elastic Edge Waves

Two-Dimensional Materials for Thermal Management Applications

We investigated thermal properties of the epoxy-based composites with the high loading fraction-up to f ≈ 45 vol %-of the randomly oriented electrically conductive graphene fillers and electrically insulating boron nitride fillers. It was found that both types of the composites revealed a distinctive thermal percolation threshold at the loading fraction fT > 20 vol %. The graphene loading required for achieving thermal percolation, fT, was substantially higher than the loading, fE, for electrical percolation. Graphene fillers outperformed boron nitride fillers in the thermal conductivity enhancement. It was established that thermal transport in composites with high filler loadings, f ≥ fT, is dominated by heat conduction via the network of percolating fillers. Unexpectedly, we determined that the thermal transport properties of the high loading composites were influenced strongly by the cross-plane thermal conductivity of the quasi-two-dimensional fillers. The obtained results shed light on the debated mechanism of the thermal percolation, and facilitate the development of the next generation of the efficient thermal interface materials for electronic applications.

/pdf/thermal-percolation-threshold-and-thermal-properties-of-4r6kxff3hu.pdf

Thermal Percolation Threshold and Thermal Properties of Composites with High Loading of Graphene and Boron Nitride Fillers

Author(s): Kargar, Fariborz; Barani, Zahra; Balinskiy, Michael; Magana, Andres Sanchez; Lewis, Jacob S.; Balandin, Alexander A.

/pdf/dual-functional-graphene-composites-for-electromagnetic-3nt0d6depq.pdf

Dual-Functional Graphene Composites for Electromagnetic Shielding and Thermal Management

Thermal Properties of the Binary‐Filler Hybrid Composites with Graphene and Copper Nanoparticles

Magnetically-functionalized self-aligning graphene fillers for high-efficiency thermal management applications

We report the results of ultraviolet Raman spectroscopy of NiO, which allowed us to determine the spin-phonon coupling coefficients in this important antiferromagnetic material. The use of the second-order phonon scattering and ultraviolet laser excitation (λ = 325 nm) was essential for overcoming the problem of the optical selection rules and dominance of the two-magnon band in the visible Raman spectrum of NiO. We established that the spins of Ni atoms interact more strongly with the longitudinal than transverse optical phonons and produce opposite effects on the phonon energies. The peculiarities of the spin-phonon coupling are consistent with the trends given by density functional theory. The obtained results shed light on the nature of the spin-phonon coupling in antiferromagnetic insulators and can help in developing spintronic devices.

/pdf/spin-phonon-coupling-in-antiferromagnetic-nickel-oxide-19v1fkn7nq.pdf

Fariborz Kargar

Papers

Thermal Percolation Threshold and Thermal Properties of Composites with High Loading of Graphene and Boron Nitride Fillers

Dual-Functional Graphene Composites for Electromagnetic Shielding and Thermal Management

Thermal Properties of the Binary‐Filler Hybrid Composites with Graphene and Copper Nanoparticles

Magnetically-functionalized self-aligning graphene fillers for high-efficiency thermal management applications

Spin-phonon coupling in antiferromagnetic nickel oxide