Session Y46: Electrons, Phonons, Electron-Phonon Scattering, and Phononics VII

Thermal conductivity switching through reversible crystallographic transformations in CaFeOx and SrFeOx thin films.

The development of functional materials whose thermal conductivity can be switched between different states plays a key role in several technologies, such as nanoelectronics or thermoelectric energy conversion.

Three-state reversible transformations between perovskite (PV) AFeO_3-x, brownmillerite (BM) AFeO_2.5, and infinite layer (IL) AFeO₂ (A=Ca²⁺, Sr²⁺) oxides can be induced by topotactic oxygen exchange at moderate temperatures under reducing/oxidizing conditions. Here we report the reversible change of thermal conductivity associated to such structural changes in epitaxial (Ca/Sr)FeO_2.5 thin films. The PV to BM can be induced, locally, by applying local electric fields through a voltage-biased AFM tip. Using Frequency Domain Thermoreflectance (FDTR), we were able to map the thermal conductivity associated to each of these phases with micrometer spatial resolution. The time-dependent thermal conductivity allowed us to characterize the oxygen diffusion in these films.

The reversibility of the structural transformations, along with the large thermal conductivity contrast, suggest that (Ca/Sr)FeO_x ferrites are suitable candidates for solid-state thermal switching devices.   

Effects of disorder on THz conductivity of PdCoO2 delafossite thin films

In this work, the development of high-quality thin films of PdCoO2 has enabled a thorough study of the conductivity as a function of film thickness using both dc transport and time-domain THz spectroscopy. By increasing the film thickness from 15nm to 100nm, the residual resistivity is reduced and we observe an apparent large deviation from Matthiessen's rule (DMR) in the dc resistivity. We also find that the complex THz conductivity demonstrates excellent consistency to a single Drude term, and fit the data to extract the spectral weight and scattering rate simultaneously. The temperature dependence of the Drude scattering rate is found to be largely independent of the residual resistivity; however, the spectral weight appears to increase nearly by a factor of two between the most disordered and least disordered samples. This suggests that the DMR observed in dc resistivity is caused by large changes in the spectral weight with varying levels of disorder. In PdCoO2 thin films we find there exists disorder-enhanced electron-phonon scattering that can be systematically tuned by film thickness.   

Strain Effects on Vibrational Modes of 2D Transition Metal Dichalcogenides in the 2H Structure

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are semiconducting materials that have applications in electronics, optoelectronics, spintronics, and catalysis. Strain can tune their optical and electronic properties, affecting device functionality, and also shifts phonon frequencies, an effect which can be used to characterize strain via Raman spectroscopy. To explore these issues, vibrational properties of the 2H bulk phase of six TMDs (MX₂ where M=Mo, W; X=S, Se, Te) were studied through density functional perturbation theory. We calculated vibrational modes under uniaxial strain, in plane and out of plane, and determined frequency shifts, splittings of degenerate modes, mode Grüneisen parameters, and changes in Raman and IR intensities. We compared the behavior between the six TMDs and analyzed the influence of the M and X atoms. This study helps understand anharmonicity in 2D TMDs, and provides the calibration data needed to infer strain from frequency shifts in experimental measurements such as micro-Raman or nano-infrared spectroscopy.   

Enhancing phonon moments through framework flexibility

When circularly polarized light excites two degenerate optic phonons, the induced excitation can possess a magnetic moment and pseudoangular momentum due to the circular motions of ions [1]. These moments are generally fairly small, and are difficult to detect in normal materials, let alone use in applications. Nonetheless, flexible framework materials (e.g. metal organic frameworks) possess qualities which potentially allow for large phonon moments, as they can sustain large amplitude atomic displacements without loss of stability. To take an initial step towards the rational design of these materials with enhanced anomalous phononic properties, theoretical maximum phonon moments in flexible framework materials will be discussed.

[1] D. M. Juraschek and N. A. Spaldin, Orbital magnetic moments of phonons, Phys. Rev. Materials 3, 064405.

Rattling and the Lattice Thermal Conductivity of Half-Heusler Thermoelectrics

The factors that affect the thermal conductivity of semiconductors is a topic of great scientific interest, especially in relation to thermoelectrics. Key developments have been the concept of the phonon-glass-electroncrystal (PGEC) and the related idea of rattling to achieve this. We use first-principles phonon and thermal conductivity calculations to explore the concept of rattling for stoichiometric-ordered half-Heusler compounds. These compounds can be regarded as filled zinc blende materials, and the filling atom could be viewed as a rattler if it is weakly bound. We use two simple metrics, one related to the frequency and the other to bond frustration and anharmonicity. We find that both measures correlate with thermal conductivity. This suggests that both may be useful in screening materials for low thermal conductivity.   

SpaRTaNS: Spatially Resolved Transport of Non-equilibrium Species

At steady state, non-equilibrium transport of carriers, such as electrons and phonons, is given by the semi-classical Boltzmann transport equation (BTE). In general, the BTE is a six-dimensional non-linear integro-differential equation which is non-trivial to solve. Typically, one proceeds by linearizing the BTE and approximating the collision operator using simple functional forms which relax towards a local equilibrium. Recently, there have been considerable efforts in retaining the full state-resolution of the collision operator, applied in computing bulk transport properties, effectively neglecting spatial variations in the resulting carrier distribution function.

Here, we present SpaRTaNS (Spatially Resolved Transport of Non-equilibrium Species): a recursive solver to the BTE which retains both state- and space-resolution. We demonstrate SpaRTaNS' utility by solving a recently-proposed non-equilibrium transport problem with distinct spatial signatures, namely electron "hydrodynamics". Specifically, we investigate the role of microscopic interactions in electron hydrodynamics by using crystal symmetries and conservation laws to randomly generate physically-allowed scattering matrices and quantify their variability on macroscopic observables such as current density curvature.   

Asymmetric damping of plasmons by electron-phonon instability

Graphene plasmon polaritons (GPP) under a direct current (dc) can be dragged in momentum [1][2] causing a break of optical reciprocity by current flow. We find that under substantial current (~1 mA/um), in addition to the Fizeau drag, there is a drastic change in quality factor of GPP which cannot be explained by regular thermal effects. A noticeable difference in propagation length of GPP is observed under different signs of current flow, revealing a directional perturbance of GPP due to the interplays among electrons, phonons and plasmons. The interdependence of electron-plasmon drag, electron-phonon instability [3] and phonon emission caused plasmon damping are demonstrated by nanoimaging and nearfield photocurrent maps on a dc biased graphene ribbon.    

Understanding the thermal transport at metal-dielectric interfaces in the presence of ultrathin metallic interlayers

Efficient thermal management due to nanoscaling of devices is becoming more critical in numerous technologies such as thermoelectrics, plasmonic devices, etc. In this work, we analyze the role of vibrational properties and electron-phonon coupling constant (EPC) in modifying the thermal conductance at metal-dielectric interfaces by inserting ultrathin metal interlayers. Our results show that for substrates with highly dissimilar debye temperatures, the measured thermal boundary conductance (TBC) between metal/interlayer/substrate remains the same. We suggest that comparing the maximum phonon frequency of the low-energy phonons is a better parameter than the debye temperature to predict the change in the thermal conductance at metal/interlayer/dielectric interfaces. Inserting metallic layers with EPC strength higher than the top layer has been reported to increase the TBC by dragging electrons and phonons into equilibrium quickly. Our results also show that the Ta interlayer with the highest EPC and poorest phonon frequency overlap with the substrate has the lowest measured TBC compared to other interlayers Ni and Cr. Hence, we suggest that the TBC depends on an interplay between the phonon vibrational properties and metal EPC strength in non-trivial ways.     

Comparing the expense and accuracy of methods to simulate atomic vibrations in rubrene.

Atomic vibrations can inform about materials properties from hole transport in organic semiconductors to correlated disorder in metal-organic frameworks. Currently, there are several methods for predicting these vibrations using simulations, but the accuracy-efficiency tradeoffs have not been examined in depth. In this study, rubrene was used as a model system to predict atomic vibrational properties using six different simulation methods: Density Functional Theory, Density Functional Tight Binding, a Machine Learning model based on a trained Neural Net, the pre-trained ANI-1 Machine Learning method, a Molecular Dynamics-based method, and Density Functional Tight Binding with a Chebyshev polynomial-based correction. The accuracy of each method is evaluated by comparison to the experimental inelastic neutron scattering spectrum. All methods discussed here show some accuracy across a wide energy region, though the Chebyshev-corrected tight binding method showed the optimal combination of high accuracy with low expense. We then offer broad simulation guidelines to yield efficient, accurate results for inelastic neutron scattering spectrum prediction.   

A new approach based on anisotropic relaxation time evaluation for size-dependent electrical conductivity

 As the device size becomes smaller in modern CMOS technology,  its resistivity increases too rapidly, so copper would not be appropriate for future interconnect applications. One of the most significant challenges is to search for other materials to replace copper for interconnect in future single-digit nanosized CMOS technology. For this purpose, we investigate the electrical transport properties of not only various single element metals but also binary and ternary metal alloys to find candidate materials. We use first-principles calculations to compute their electronic structures and phonon dispersions and to evaluate the electron-phonon interaction yielding the momentum- and energy-dependent electron relaxation time or mean free path. We then solve the Boltzmann transport equation to predict their conductivity tensors. We further develop a method to evaluate the size dependence of the momentum- and energy-dependent electronic mean free path to include the surface scattering effect on the resistivity. It turns out that our method produces the results in good agreement with available experimental data. We present a few materials with strongly anisotropic Fermi surfaces, the resistivity of which becomes comparable to or even lower than copper as their size becomes smaller.   

Route to increased hole mobility GaN induced by epitaxial uniaxial strain on II-IV nitrides

GaN is a prominent semiconductor candidate for highly efficient power electronics applications. The low hole mobility in GaN, < 40 cm²/Vs, poses challenges to realizing p-type GaN devices. Applying strain to GaN has shown promise to increase hole mobility. In this first principles study, we investigate the effect of epitaxial in-plane compressive strain of GaN to the lattice constants of II-IV nitrides ZnGeN₂ and MgSiN₂ on phonon-limitted carrier mobility. Carrier mobilities are calculated by the iterative Boltzmann transport equation in the EPW code. The effective compressive strain induces inversion of the heavy hole and crystal field bands, leading to low effective hole mass dispersions in the compressive direction. We find that strain matching GaN to ZnGeN₂ or MgSiN₂ results in a 46% or 258% increase in phonon-limited hole mobility at room temperature in the compressive strain direction, respectively. While the extreme strain state induced by strain matching GaN to MgSiN₂ severely limits thin film critical thickness, we predict that epitaxial strain matching GaN to an ordered alloy Zn_0.75Mg_0.25Ge_0.75Si_0.25N₂ could still promote a mobility increase of 164% while maintaining a film critical thickness of over 8 nm, opening a pathway to high hole mobility thin film GaN devices.    

Controlling sound propagation and scattering via spinning media

The propagation of waves in time-varying and/or moving media is expected to lead to enhanced control of optical or acoustical waves and a plethora of intriguing applications. In this realm, we will discuss in this talk our recent advances on physics of sound propagation in spinning fluids (air or water). For instance, a spinning cylindrical column of fluid in the same medium will be shown to possess intrinsic spin angular momentum. We will study the torque and force it experiences in evanescent acoustic fields, and we will show that the resulting discontinuity can scatter sound in unusual ways, e.g., a negative radiation force, although it has no imaginary part in its parameters [M. Farhat, et al. PRB(L) 104, L060104 (2021)].

Similarly, scattering cancellation technique (SCT) has proved to be an effective way to conceal static objects from acoustic waves. Yet, spinning objects carry peculiar scattering features that cannot be canceled by regular cloaks. Here, we will propose a generalized SCT theory to cloak spinning objects, and hide them from static observers, based on rotating shells with different angular velocity. Our work extends the realm of SCT and brings it one step closer to its practical realization that involves moving objects [M. Farhat, et al. PRB 101, 174111(2020)].