Session F61: Electrons, Phonons, Electron-Phonon Scattering and Phononics III

Vibrational dynamics driven by structural symmetries and complexities

Vibrational excitations and their influence on material structure and function often govern the potential for applications, such as thermoelectrics, or set limitations for these, such as spin-phonon couplings in magnetic storage and processing devices. As such, developing deep insights into the rich physics of phonons and their quasiparticle interactions is critical to developing further fundamental insights toward design of improved or new material functionalities. Here, I will discuss recent advances in our understanding of vibrational behaviors of materials, particularly highlighting the role of structural complexities and symmetries in determining vibrational dynamics and transport. More specifically, I will discuss twist phase relations in materials with screw axes, translational phase relations between primitive and conventional geometries, how such relations manifest in measured spectra, and phonon scattering behaviors in layered structures. I will discuss this physics in the context of a variety of materials, including wide-band gap compounds and superlattices [(Al/Ga)N], magnetic semiconductors [Mn(Te/Sb/Bi)], elemental metals (Te), and cleavable ferromagnets (CrCl₃).

 

L.L. acknowledges support from the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.   

Quartic Anharmonic Lattice Thermal Conductivity in 2D InS Monolayer: Self-consistent Phonon Calculations

There are many reports on low lattice thermal conductivity in 2D van der Waal’s based materials for thermoelectric applications using Density Functional Theory (DFT) and Molecular Dynamics simulations. Wickramaratne [1] et. al. have extensively studied the excellent thermoelectric properties of 2D group III-VI materials such GaX and InX (X = S, Se) owing to the formation of ‘Mexican hat’ in the valence band of electronic structure. However, they have only focused on the electronic thermal conductivity and estimated the ZT using the upper-bound value of κl. This gap in research was motivation for several studies focusing on phonon transport and thermal properties of 2D InS monolayer. All the above reports are limited to the calculations of 1) phonon dispersion relations using the harmonic approximation and 2) lattice thermal conductivity using third order interatomic force constants (IFC’s) by solving BTE. However, a more reliable calculation of lattice transport properties and the effect of finite temperature on phonon spectra requires the inclusion of higher order anharmonicity. Self-consistent phonon theory based on quantum mechanical description of the phonons is an efficient non-perturbative way of including the anharmonicity. In this study, the above-mentioned approach is used to study the phonon transport properties and the thermodynamic parameters using quartic anharmonicity by including the quartic IFCs of InS monolayer to find their significant effect on the value of lattice thermal conductivity.

1. Wickramaratne, D., F. Zahid and R. K. Lake. "Electronic and thermoelectric properties of van der waals materials with ring-shaped valence bands." Journal of Applied Physics 118 (2015): 075101. 10.1063/1.4928559. https://doi.org/10.1063/1.4928559.

    

Zone-center chiral phonons from broken time reversal symmetry

In conventional ab initio methodologies, phonons are calculated by solving equations of motion involving static interatomic force constants and atomic masses. However, this approach does not fully account for the effects of broken time-reversal symmetry in systems with magnetic order. Recent attempts to rectify this involve the inclusion of the velocity dependence of the interatomic forces in the equations of motion, which is given by the nuclear Berry curvature. This can result in "chiral" phonon modes with non-zero angular momentum, even at the zone center. We show, via a novel density-functional theory methodology based on finite displacements, that the main contribution to this nuclear Berry curvature is from the spin rotations caused by the phonons. This requires spin and phonon degrees of freedom to be treated on the same footing. We propose a model involving Hessian matrices and Berry curvature tensors in terms of both spin and phonon degrees of freedom, and develop a first-principles methodology to calculate them. We present results on both ferromagnets and antiferromagnets, and the implication for splitting of chiral phonon modes is discussed.   

Chiral phonons throughout reciprocal space

The properties of chiral phonons (phonons with associated angular momentum and associated magnetic moments) have been studied mainly at the Γ point (where they can be created by circularly polarized excitations) and other high-symmetry points in the Brillouin zone. However, they can arise at arbitrary wavevectors in materials which lack a centre of symmetry. Thereby, phonon chirality can affect the bulk properties of solids, especially in cases where time reversal symmetry is broken, leading to, for example, the phonon thermal Hall effect. We have selected noncentrosymmetric materials in which chiral phononic effects are enhanced by structural and chemical factors, and calculated the distribution of chiral phonons throughout the Brillouin zone and their properties in order to predict chiral phononic responses to external perturbations (e.g. electromagnetic and thermal stimuli).

    

Entropy contributions to explain thermal expansion: thermodynamics of the Invar effect

The anomalously low thermal expansion observed in some metals, known as the Invar effect, has long been associated with magnetism. Its microscopic underpinnings, however, are still an open scientific question, long after C. E. Guillaume received the 1920 physics Nobel Prize for its discovery. In particular the role of phonons, their contribution to the thermal expansion, and their dependence on magnetism has not been previously quantified. We explore a new method for obtaining the thermal expansion from the pressure dependence of the entropy. By combining two nuclear x-ray scattering techniques, suitable for pressure experiments in diamond-anvil cells, we probe contributions from phonons and spins to the entropy of Fe-Ni Invar. We show that the Invar behavior stems from a competition between phonons and spins, resulting in a cancellation of their entropy contributions. Ab initio phonon calculations reveal a spin-lattice coupling at the root of the Invar anomaly. Such couplings of excitations from phonons and spins go beyond the case of Invar, and could be the cause of the thermal expansion behavior in other magnetic materials. 

    

Electronic density of states of body-centered-cubic Fe under phonon excitations, boron doping and amorphization

It has been shown that the intrinsic magnon damping in metallic ferromagnetic materials is proportional to the electronic density of states (EDOS) at Fermi level. This is theoretically described by the breathing Fermi surface model. Understanding how the EDOS at the Fermi level is affected by different factors has potential value for experimentalists to optimize low-damping magnetic materials for applications in spintronics and magnonics. To investigate this question, we consider a paradigmatic ferromagnetic material, the body-centered-cubic iron (Fe). We calculate the EDOS of Fe based on the density functional theory (DFT) using the Vienna Ab-Initio Simulation Package (VASP). Three different scenarios are considered: 1) Fe with thermal lattice disorder, 2) Fe doped by boron (B) and 3) Fe in an amorphous state. We have observed that, while the EDOS at Fermi level is not much affected by phonons up to the temperature of 500K, it is significantly reduced by B doping. Lowered EDOS at Fermi level can also be achieved in the amorphous Fe and FeB. 

Acknowledgements: this work was supported by the US Department of Energy, Office of Science, Materials Science and Engineering Division, under Contract No. DE-SC0022060. This work made use of the Illinois Campus Cluster, a computing resource that is operated by the Illinois Campus Cluster Program (ICCP) in conjunction with the National Center for Supercomputing Applications (NCSA) and which is supported by funds from the University of Illinois at Urbana-Champaign. 

 

 

    

Quantum nuclear vibrations and the electronic properties of molecular crystals

We present a study of the electronic properties of molecular crystals, with a focus on the effect of nuclear quantum motion and anharmonicity on their band gap. We consider a system composed of relatively rigid molecules, a diamondoid crystal, and one composed of floppier molecules, NAI-DMAC, a thermally activated delayed fluorescence compound. We compute the electronic properties at the DFT level of theory, by coupling first principles molecular dynamics with a nuclear quantum thermostat, following a protocol previously applied to diamond and amorphous carbon [1]. We use the Qbox code (http://qboxcode.org/) coupled with i-PI (http://ipi-code.org/). We find a sizable zero-point-renormalization (ZPR) of the band gaps, which is much larger in the case of diamondoids (~ 0.6 eV) than for NAI-DMAC (~ 0.22 eV). We show that when using the frozen phonon (FP) approximation, which neglects inter-molecular anharmonic effects, we introduce a large error (~ 50%) in the calculation of the band gap ZPR. Instead, when using a stochastic method [2], we obtain results in good agreement with those of our quantum simulations for the diamondoid crystal. However, the agreement is worse for NAI-DMAC where intra-molecular anharmonicities largely contribute to the ZPR.  Our results highlight the importance of accurately including nuclear and anharmonic quantum effects to predict the electronic properties of molecular crystals.

[1] A. Kundu et al, Phys. Rev. Mat. 2021 & PNAS 2022; [2] M. Zacharias, F. Giustino. Phys. Rev. B 2016.   

Impacts of Short-range Order on Thermal conductivity of Si-Ge-Sn Alloys

Si-Ge-Sn alloys are silicon-based, versatile materials for electronic, photonic, and topological quantum applications. Thermal management in such applications is an important issue but has not been well addressed. As our recent studies^1-3 predicted the complex short-range order behaviors in Si-Ge-Sn alloys, it remains to be understood how thermal transport is affected by the non-random distribution of alloying elements. Here, by computing lattice thermal conductivity from first-principles calculations, we find that the thermal conductivity of Si-Ge-Sn alloys shows strong variation with atomic ordering. The underlying mechanisms are explored and discussed. Our preliminary results suggest that short-range order may provide a new route for engineering the thermal properties of Group IV alloys.

1. ACS Appl Mater Interfaces 12, 57245 (2020)

2. Phys Rev Mater 5, 104606 (2021)

3. Communs Mater 3, 66 (2022)   

Interfacial thermal transport in Bi2Te3

Highly efficient, room-temperature thermoelectrics are key to the future of environmental energy harvesting and solid-state cooling, among many other applications. To advance beyond the current state-of-the-art room-temperature thermoelectrics it is critical to understand the impact of interfaces on thermal transport in currently utilised materials, such as Bi2Te3, at an atomistic level. The importance of this understanding is highlighted even further considering the prevalence of nanostructured (often polycrystalline) materials. In this work, we employ reverse non-equilibrium molecular dynamics simulations (rNEMD) with a classical two-body interatomic potential (IP) [1], to examine the effect of specific interfacial structures on thermal transport in Bi2Te3. The interfacial thermal resistance (Kapitza resistance) and interfacial structures are compared across a number of twin boundaries. In addition, we will examine finite-length effects in rNEMD simulations of interfaces; often these are overlooked. We will also discuss the limitations of existing IPs and consider the development of machine-learned IPs for Bi2Te3.

[1] B. Huang et al. J. Phys. D Appl. Phys. 52 (42), ?425303 (2019)   

Anomalous thermoelectric transport phenomena from interband electron-phonon scattering

The Seebeck coefficient and electrical conductivity are two central quantities to be optimized simultaneously in designing thermoelectric materials, and they are determined by the dynamics of carrier scattering. We uncover a new regime where the presence of multiple electron bands with different effective masses, crossing near the Fermi level, leads to strongly energy-dependent carrier lifetimes due to intrinsic electron-phonon scattering. In this anomalous regime, electrical conductivity decreases with carrier concentration, Seebeck coefficient reverses sign even at high doping, and power factor exhibits an unusual second peak. We explain the origin and magnitude of this effect using a general simplified model as well as first-principles Boltzmann transport calculations in recently discovered half-Heusler alloys. We identify general design rules for using this paradigm to engineer enhanced performance in thermoelectric materials.   

A Study Of The Heusler Alloy (Fe2V0.8W0.2Al)

Thermoelectric materials offer a great potential for directly converting heat into electricity and are essential for wide-scale renewable energy application. For practical thermoelectric generation applications, a large figure of merit (ZT) is beneficial. The goal of our research is to search for large figure of merit (ZT) thermoelectric materials. Because of their reasonably large ZT values Fe₂V_0.8W_0.2Al and related Heusler systems are potential thermoelectric materials. Here we describe synthesis, as well as experimental details and numerical results on thermal and electrical properties of bulk Fe₂V_0.8W_0.2Al. Briefly, we find that both electrical resistivity, r and thermal conductivity, k increases with temperature. For instance, around room temperature over 100K, both r and K increase by ~10% . Also, Seebeck coefficient is about 40 μV/K around room temperature and mildly increases with increasing temperature The implication of these results will be discussed.

Quasiharmonic approximation via irreducible derivatives: low symmetry crystals

The quasiharmonic approximation is a standard approach to include anharmonicity when evaluating temperature-dependent vibrational observables, yet computing arbitrary observables is nontrivial.  Here we execute the irreducible derivative approach to the quasiharmonic approximation, greatly reducing the computational cost and facilitating the study of lower symmetry crystals.  Specifically, we study PbTiO$_3$ using density functional theory within various exchange-correlation functionals, computing the thermal expansion and the full elastic constant tensor as a function of temperature.  The Born-Oppenheimer potential and the irreducible components of the dynamical matrix are parametrized by a Taylor series expansion in symmetrized strain, allowing for the systematic study of successively higher order truncations of the quasiharmonic potential.  Additionally, we explore the validity of the quasiharmonic approximation in metals, including ZrN.  Results are compared to existing experimental measurements.

    

First-Principles Simulation of Lattice Thermal Conductivity: Uncertainties from Different Flavors of Temperature-Dependent Force Constants

First-principles simulations of lattice thermal conductivity, κ_L, of highly anharmonic crystals have long been challenging in condensed matter physics. With recent theoretical advances, the calculation of κ_L has evolved into a sophisticated process requiring the consideration of high-order phonon-phonon scattering, anharmonic phonon renormalization, and the temperature-dependent potential energy surface. Interatomic force constants (IFCs), however, as a shared pillar of the above concepts, are ambiguously implemented in this process, resulting in nonnegligible discrepancies between different studies. Here, we revisit the ultralow κ_L of Tl₃VSe₄ and make a rigorous comparison using different flavors of IFCs. We find that the phonon spectrum obtained with 0 K IFCs is prone to get over stiffened after anharmonic renormalization, which leads to a doubled κ_L even with up to four-phonon scattering considered. Moreover, the different flavors of fitting 2nd-, 3rd-, and 4th-order IFCs at finite temperatures using force-displacement data leads to significantly dispersive κ_L. Our work illuminates how choosing different IFCs alters κ_L, which contributes to a better understanding of lattice dynamics simulation based on the perturbation theory.