Session S48: Electrons, Phonons, Electron-Phonon Scattering, and Phononics V

First-principles calculation of thermomagnetic transport effects

As the need for energy-efficient devices has become more pressing, so has interest in finding ways to enhance the thermoelectric performance of materials. While the Seebeck effect has been intensely studied, thermomagnetic transport has rarely seen computational investigation. Recent results, such as those showing enhanced thermopower in compensated semimetals via the Nernst effect, suggest that it also has potential to provide enhancements in thermoelectric performance in the right circumstances. Additionally, established results such as those for bismuth and graphite, which show low-temperature peaks in the Nernst coefficient, could be better understood or further engineered for potential applications. However, these interesting and possibly useful behaviors can be difficult to interpret from experimental results alone. Using first-principles solutions of the Boltzmann transport equation as implemented in our new open-source code package Phoebe [1], we calculate the Seebeck and Nernst coefficients to better understand the physics underlying these effects.

[1] https://mir-group.github.io/phoebe/   

Thermodynamic explanation of the Invar effect by computation and experiments

Phonons, electrons, spins and their interactions give the entropy and free energy of magnetic materials. A Maxwell relation implies that the pressure dependence of these entropy contributions must somehow sum to zero, to give Invar materials their known zero thermal expansion. With experiments and computations, we evaluated these thermodynamic contributions from lattice vibrations and spin disorder versus pressure for Fe-36%Ni Invar. From nuclear resonant inelastic X-ray scattering and Mössbauer spectroscopy at varying temperatures and pressures, we measured the phonon density of states and the magnetization, and determined their individual contributions to the entropy of Invar. Calculations of the phonon modes performed with an ab initio effective potential method that includes the magnetic disorder are able to capture the anomalies in the thermal expansion, and are in good agreement with experiments. The low thermal expansion of Invar is explained by the cancellation of entropy effects from lattice vibrations and spin disordering.   

Froehlich condensate of Phonons and heat control

Different from Bose-Einstein Condensate (BEC) that occurs at equilibrium, phonon condensate happens in the condition of out of equilibrium[1]. In this work, we will present that the Fröhlich condensate of phonons can be realized in optomechanical systems [2] as well as in a self-feedback classical mechanical system [3]. Our optomechanical system consists of a one-dimensional array of membranes coupled to the cavity field via a quadratic interaction, and the cavity is pumped by an external laser. Analytical and numerical results demonstrate that the high phonon occupancy of the lowest or highest mechanical mode is achievable depending on the detuning of the driving laser, the optomechanical strength, and the temperature. The feasibility of experimental implementation is discussed. Our results shed light on energy conversion and transfer, heat control, and multimode cooling using optomechanical systems.   

Coherent state representation of lattice vibrations and its application to electron-phonon interaction

Since the introduction of the deformation potential model to describe electron-phonon interaction, the mechanism of the scattering of the electron by the lattice vibrations was described by phonon creation and annihilation using second quantization for the lattice vibrations. Furthermore, an incoherent concatenation of first-order scattering events using Boltzmann transport theory was used to calculate resistivity.

However, as the lattice vibrations can be treated as a classical field, it is natural to choose a coherent state, instead of a phonon Fock state, to describe lattice vibrations. In this picture, the electron scattering by the lattice vibrations can be explained without phonon creation and annihilation like an impurity scattering. It turned out that the calculated resistivity in this way gives the same result to the conventional theory up to a factor of order unity. Also, Anderson localization of an electron by the deformation potential was observed. Using the coherent state representation helps understanding electron coherence effects such as Anderson localization and Shubnikov-de Haas effect, which was not naturally described in the incoherent Boltzmann transport theory.   

Identifying thermal insulators via ab initio Green Kubo simulations guided by anharmonicity

We present a systematic first-principles search for thermal insulators in material space, covering hundreds of compounds, five lattice types and seven space groups, including simple rocksalt and zinc blende structures up to complex perovskites with 20 atoms per unit cell. Using the high-throughput framework FHI-vibes [1,2] and a recently developed measure for the strength of anharmonicity [3], we identify 120 candidate materials with potential for ultra-low thermal conductivity <2 W/mK at room temperature, i.e., comparable to those of thermoelectrics such as SnSe or MgSb. We perform non-perturbative ab initio Green-Kubo simulations [4] for the 60 most promising candidates, thereby including all anharmonic effects responsible for low thermal conductivity.

With this approach, we identify seven new compounds with calculated ultra-low thermal conductivity. We analyze the underlying dynamical mechanisms, in particular the importance of strong anharmonic effects not accessible in perturbative phonon formalisms, e.g., short-lived metastable configurations and precursors of structural phase transitions. Eventually, we discuss the relevance of ab initio Green-Kubo methods for the discovery of novel, anharmonic, and increasingly complex materials.

[1] F. Knoop et al., J. Open Source Softw. 5, 2671 (2020)

[2] V. Blum et al., Comput. Phys. Commun. 180, 2175 (2009)

[3] F. Knoop, et al., Phys. Rev. Mater. 4, 083809 (2020)

[4] C. Carbogno, R. Ramprasad, and M. Scheffler, Phys. Rev. Lett. 118, 175901 (2017)   

Symmetry-based lattice dynamics and thermal transport in magnetic CrCl3 and RuCl3

The primitive Wigner-Seitz cell, and corresponding first Brillouin zone (FBZ), is typically used for calculations of lattice vibrational properties as it contains the smallest number of degrees of freedom and thus has cheapest computational cost. However, in complex materials, the FBZ can take on irregular shapes where lattice symmetries are not apparent. Thus conventional cells (with more atoms and regular shape) are often used to describe materials, though dynamical calculations are more expensive. Here we discuss mapping of conventional cell dynamics to primitive cell dynamics based on translational symmetries and phases. This leads to phase interference conditions that act like conserved quantum numbers that enable a conventional cell description of vibrations and transport at nearly the same computational cost as those done in the primitive cell. We demonstrate this dynamics using first principles calculations to simulate phonons in two technologically-relevant magnetic systems: the cleavable antiferromagnet CrCl₃ and the quantum spin liquid candidate a-RuCl₃. Additionally, stacking fault limited, low temperature phonon transport in a-RuCl₃ is examined with regards to measured data, thus providing physical insights into thermal Hall behaviors of Kitaev spin-liquid materials.   

Theory of dynamical structure factors in complex twisted materials

Structural complexity underlies a variety of novel quasiparticle behaviors that can have profound impacts on material functionalities. Here we intimately examine the theory of lattice dynamical spectra of ‘twisted’ materials with combined rotational and translational symmetries. We advance a dynamical theory that incorporates the twist symmetry directly into the description of the phonon frequencies and eigenvectors, which naturally elucidates the underlying angular momenta of phonon bands. We apply this theory to build insights into dynamical structure factors and spectral observations from inelastic neutron and x-ray scattering experiments.   

The quasiharmonic approximation via space group irreducible derivatives

The quasiharmonic approximation (QHA) is the simplest nontrivial approximation for interacting phonons under constant pressure, bringing the effects of anharmonicity into temperature dependent observables.  Nonetheless, the most general version of the QHA is often implemented with additional approximations due to the complexity of computing phonons under arbitrary strains.  Here we circumvent the aforementioned complexity by employing irreducible second order displacement derivatives of the Born-Oppenheimer potential and their strain dependence, which are efficiently and precisely computed using the lone irreducible derivative approach.  We execute two complementary strain parametrizations: a discretized strain grid interpolation and a Taylor series expansion in symmetrized strain.  We illustrate our approach by evaluating the thermal expansion and temperature dependence of the elastic constant tensor in thoria and lead titanate using density functional theory, and compare to experimental measurements.  Our irreducible derivative approach to the QHA will facilitate reproducible, high throughput applications.   

Topological Invariant of Acoustic Phonons in 2D materials

2D materials that are allowed to flex out-of-plane display an unusual acoustic phonon mode – the flexural mode. This mode disperses quadratically away from the center of the Brillouin zone, as opposed to the linear dispersion of the in-plane modes. This leads to an unusual triple degeneracy at the zone center, with two linear and one quadratic band touching. This band touching is unusual because it is enforced by Goldstone’s theorem rather than symmetry, as will be discussed. I will also discuss the topological invariant associated with this crossing and how this invariant affects the physics. The invariant is a priori of quaternionic type, but it reduces to a Z2 invariant under rather general assumptions. This turns out to have important implications for the band splitting that occurs when a 2D material is grown on a substrate. This splitting will be discussed for the specific case of graphene, where the Z2 invariant associated with the acoustic bands turns out to be non-trivial.   

Extremely high thermoelectric performance of SnSe at optimal carrier densities

High-efficiency thermoelectric materials are desirable for their capability of functioning as all solid-state modules for electric power generation from waste heat or distributed spot-size refrigeration. Bulk tin selenide (SnSe) has received much attention since its excellent thermoelectric figure of merit (zT) was reported. Using extensive first-principles calculations of carrier-impurity and carrier-phonon scattering processes, we investigate the optimal carrier densities to maximize zT in p- and n-doped SnSe over a broad range of temperatures. We predict an ultrahigh zT > 4 for in-plane p-doped and out-of-plane n-doped SnSe at high temperatures, which is robust against variations in the concentration of ionized impurities. An extensive analysis of the transport properties as a function of the carrier density and temperature indicates that the Lorenz number reaches a minimum at the optimal conditions. Our results are motivation for improving the efficiency of doping in SnSe in order to attain optimal carrier densities.   

Self-consistent extended Hubbard functional study on electronic and structural properties of BaBiO3 and BaSbO3

Ba_1-xK_xBiO₃ (BKBO) has long been a subject of great interest due to its intertwined electronic and structural properties. Recently, comparative material Ba_1-xK_xSbO₃ (BKSO) is also successfully synthesized and exhibits a similar phase diagram with BKBO. It is well established that the simple DFT fails to describe several physical properties of BKBO compound while the hybrid functional or GW methods are better in many aspects although comprehensive studies for its entire phase diagram are limited due to their expensive computational cost. Here, based on a newly developed ab initio method counting self-consistent on- and inter-site Coulomb interactions, we examine the electronic, structural, and phonon properties of BKBO and BKSO with various potassium doping levels. Our new method can describe the bandgaps and phonon dispersions of semiconductors precisely in a similar level to the GW method, but at the simple DFT computational cost. We found that the new method gives much improved results compared to the simple DFT calculations. A comprehensive comparative study on the electronic and structural properties of BKBO and BKSO will be discussed.   

Thermoelectric properties and carrier dynamics in two-dimensional chalcogenides

In recent years, 2D layered metal chalcogenides and transition metal dichalcogenides (TMDs) have received tremendous attention due to their unique electronic, thermal and optoelectronic properties leading to their applications in optoelectronics and thermoelectrics (TE).[1,2] Understanding of electron and phonon scattering mechanisms and carrier dynamics is critical for optimization of the transport properties in new devices. TMDs like MoS₂, MoSe₂ and WS₂ are being investigated because of the electronic, optical and spin properties exhibited by them as they go from bulk to monolayer. The TE properties of few layers and monolayer TMDs are of great interest due to low thermal conductivity and increased power factor. In this context, we use the Boltzmann transport formalism to assess the potential of monolayer and bilayer TMDs and monolayer metal chalcogenide[3] as TE material by employing different electron and phonon scattering mechanisms. Carrier dynamics is also studied to understand the device efficiency.

References:

1. Q. H. Wang et al., Nat. Nanotechnol. 7,699(2012)

2. R. Gupta et al., J. Phys. Chem. C 124,17476(2020)

3. R. Gupta et al., J. Appl. Phys. 130,054301(2021)