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Register a new userFirst-principles study of the effects of electron-phonon coupling on the thermoelectric properties: a case study of SiGe compound
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It is generally assumed in the thermoelectric community that the lattice thermal conductivity of a given material is independent of the electronic properties. This perspective is however questionable since the electron-phonon coupling could have certain effects on both the carrier and phonon transport, which in turn will affect the thermoelectric properties. Using SiGe compound as a prototypical example, we give an accurate prediction of the carrier relaxation time by considering scattering from all the phonon modes, as opposed to the simple deformation potential theory. It is found that the carrier relaxation time does not change much with the concentration, which is however not the case for the phonon transport where the lattice thermal conductivity can be significantly reduced by electron-phonon coupling at higher carrier concentration. As a consequence, the figure-of-merit of SiGe compound is obviously enhanced at optimized carrier concentration, and becomes more pronounced at elevated temperature.
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Using first-principles pseudopotential method and Boltzmann transport theory, we give a comprehensive understanding of the electronic and phonon transport properties of thermoelectric material BiCuSeO. By choosing proper hybrid functional for the exchange-correlation energy, we find that the system is semiconducting with a direct band gap of ~0.8 eV, which is quite different from those obtained previously using standard functionals. Detailed analysis of a three-dimensional energy band structure indicates that there is a valley degeneracy of eight around the valence band maximum, which leads to a sharp density of states and is responsible for a large p-type Seebeck coefficient. Moreover, we find that the density of states effective masses are much larger and results in very low hole mobility of BiCuSeO. On the other hand, we find larger atomic displacement parameters for the Cu atoms, which indicates that the stronger anharmonicity of BiCuSeO may originate from the rattling behavior of Cu instead of previously believed Bi atoms.
The electronic and phonon transport properties of quaternary tetradymite BiSbSeTe2 are investigated using first-principles approach and Boltzmann transport theory. Unlike the binary counterpart Bi2Te3, we obtain a pair of Rashba splitting bands induced by the absence of inversion center. Such unique characteristic could lead to a large Seebeck coefficient even at relatively higher carrier concentration. Besides, we find an ultralow lattice thermal conductivity of BiSbSeTe2, especially along the interlayer direction, which can be traced to the extremely small phonon relaxation time mainly induced by the mixed covalent bonds. As a consequence, a considerably large ZT value of ~2.0 can be obtained at 500 K, indicating that the unique lattice structure of BiSbSeTe2 caused by isoelectronic substitution could be an advantage to achieving high thermoelectric performance.
Strain engineering is a very effective method to continuously tune the electronic, topological, optical and thermoelectric properties of materials. In this work, strain-dependent phonon transport of recently-fabricated antimonene (Sb monolayer) under biaxial strain is investigated from a combination of first-principles calculations and the linearized phonon Boltzmann equation. It is found that the ZA dispersion of antimonene with strain less than -1% gives imaginary frequencies, which suggests that compressive strain can induce structural instability. Experimentally, it is possible to enhance structural stability by tensile strain. Calculated results show that lattice thermal conductivity increases with strain changing from -1% to 6%, and lattice thermal conductivity at 6% strain is 5.6 times larger than that at -1% strain at room temperature. It is interesting that lattice thermal conductivity is in inverse proportion to buckling parameter $h$ in considered strain range. Such a strain dependence of lattice thermal conductivity is attributed to enhanced phonon lifetimes caused by increased strain, while group velocities have a decreased effect on lattice thermal conductivity with increasing strain. It is found that acoustic branches dominate the lattice thermal conductivity over the full strain range. The cumulative room-temperature lattice thermal conductivity at -1% strain converges to maximum with phonon mean free path (MFP) at 50 nm, while one at 6% strain becomes as large as 44 $mathrm{mu m}$, which suggests that strain can give rise to very strong size effects on lattice thermal conductivity in antimonene. These results may provide guidance on fabrication techniques of antimonene, and offer perspectives on tuning lattice thermal conductivity by size and strain for applications of thermal management and thermoelectricity.
The extremely large magnetoresistance (XMR) material LaBi was reported to become superconducting under pressure accompanying with suppressed magnetoresistance. However, the underlying mechanism is unclear. By using first-principles electronic structure calculations in combination with a semiclassical model, we have studied the electron-phonon coupling and magnetoresistance of LaBi in the pressure range from 0 to 18 GPa. Our calculations show that LaBi undergoes a structural phase transition from a face-centered cubic lattice to a primitive tetragonal lattice at $sim$7 GPa, verifying previous experimental results. Meanwhile, LaBi remains topologically nontrivial across the structural transition. Under all pressures that we have studied, the phonon-mediated mechanism based on the weak electron-phonon coupling cannot account for the observed superconductivity in LaBi, and the calculated magnetoresistance for LaBi does not show a suppression. The distinct difference between our calculations and experimental observations suggests either the existence of extra Bi impurities in the real LaBi compound or the possibility of other unknown mechanism.
Recent experiments reported giant magnetoresistance at room temperature in LaOMnAs. Here a density functional theory calculation is performed to investigate magnetic properties of LaOMnAs. The ground state is found to be the G-type antiferromagnetic order within the $ab$ plane but coupled ferromagnetically between planes, in agreement with recent neutron investigations. The electronic band structures suggest an insulating state which is driven by the particular G-type magnetic order, while a metallic state accompanies the ferromagnetic order. This relation between magnetism and conductance may be helpful to qualitatively understand the giant magnetoresistance effects.
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