Strong electron–phonon coupling influences carrier transport and thermoelectric performances in group-IV/V elemental monolayers

The interactions between electrons and phonons play the key role in determining the carrier transport properties in semiconductors. In this work, comprehensive investigations on full electron–phonon (el–ph) couplings and their influences on carrier mobility and thermoelectric (TE) performances of 2D group IV and V elemental monolayers are performed, and we also analyze the selection rules on el–ph couplings using group theory. For shallow n/p-dopings in Si, Ge, and Sn, ZA/TA/LO phonon modes dominate the intervalley scatterings. Similarly strong intervalley scatterings via ZA/TO phonon modes can be identified for CBM electrons in P, As, and Sb, and for VBM holes, ZA/TA phonon modes dominate intervalley scatterings in P while LA phonons dominate intravalley scatterings in As and Sb. By considering full el–ph couplings, the TE performance for these two series of monolayers are predicted, which seriously downgrades the thermoelectric figures of merits compared with those predicted by the constant relaxation time approximation.

Beyond the relaxation time approximation

The relaxation time approximation (RTA) is a well known method of describing the time evolution of a statistical ensemble by linking distributions of the variables of interest at different stages of their temporal evolution. We show that if all the distributions occurring in the RTA have the same functional form of a quasi-power Tsallis distribution the time evolution of which depends on the time evolution of its control parameter, nonextensivity q(t), then it is more convenient to consider only the time evolution of this control parameter.

Approximate Models for the Lattice Thermal Conductivity of Alloy Thermoelectrics

Thermoelectric generators (TEGs) convert waste heat to electricity and are a leading contender for improving energy efficiency at a range of scales. Ideal TE materials show a large Seebeck effect, high electrical conductivity, and low thermal conductivity. Alloying is a widely-used approach to engineering the heat transport in TEs, but despite many successes the underlying mechanisms are poorly understood. In previous work, first-principles modelling has successfully been used to study the thermodynamics of alloy formation and to investigate its effect on the electronic structure and phonon spectrum. However, it has so far only been possible to examine qualitatively the impact of alloying on the lattice thermal conductivity. In this work, we develop and test two new approaches to addressing this. The constant relaxation-time approximation (CRTA) assumes the primary effect of alloying is on the phonon group velocities, and allows the thermal conductivity to be calculated assuming a suitable constant lifetime. Alternatively, setting the three-phonon interaction strengths to a constant further enables an assessment of how changes to the phonon frequency spectrum influence the lifetimes. We test both approaches for the <i>Pnma</i> Sn(S_1-<i>x</i>Se_<i>x</i>) alloy system and are able to account for the substantially-reduced thermal conductivity measured in experiments.

Convergence of hydrodynamic modes: insights from kinetic theory and holography

We study the mechanisms setting the radius of convergence of hydrodynamic dispersion relations in kinetic theory in the relaxation time approximation. This introduces a quali\-tatively new feature with respect to holography: a nonhydrodynamic sector represented by a branch cut in the retarded Green's function. In contrast with existing holographic examples, we find that the radius of convergence in the shear channel is set by a collision of the hydrodynamic pole with a branch point. In the sound channel it is set by a pole-pole collision on a non-principal sheet of the Green's function. More generally, we examine the consequences of the Implicit Function Theorem in hydrodynamics and give a prescription to determine a set of points that necessarily includes all complex singularities of the dispersion relation. This may be used as a practical tool to assist in determining the radius of convergence of hydrodynamic dispersion relations.

Efficient calculation of carrier scattering rates from first principles

The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH3NH3PbI3 hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment (rs = 0.93) improves significantly on results obtained using a constant relaxation time approximation (rs = 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power.

Phonons and Thermal Transport in Si/SiO2 Multishell Nanotubes: Atomistic Study

Thermal transport in the Si/SiO2 multishell nanotubes is investigated theoretically. The phonon energy spectra are obtained using the atomistic lattice dynamics approach. Thermal conductivity is calculated using the Boltzmann transport equation within the relaxation time approximation. Redistribution of the vibrational spectra in multishell nanotubes leads to a decrease of the phonon group velocity and the thermal conductivity as compared to homogeneous Si nanowires. Phonon scattering on the Si/SiO2 interfaces is another key factor of strong reduction of the thermal conductivity in these structures (down to 0.2 Wm−1K−1 at room temperature). We demonstrate that phonon thermal transport in Si/SiO2 nanotubes can be efficiently suppressed by a proper choice of nanotube geometrical parameters: lateral cross section, thickness and number of shells. We argue that such nanotubes have prospective applications in modern electronics, in cases when low heat conduction is required.