arXiv subjects

Cameron Rudderham

Publications and source records attributed to Cameron Rudderham.

3 recordsLinked to original sources

arXiv2023

The influence of film thickness and uniaxial anisotropy on in-plane skyrmions: Numerical investigations of the phase space of chiral magnets

The equilibrium phase space of magnetic textures in thin-films of cubic chiral ferromagnets, including skyrmions, is explored as a function of in-plane magnetic field strength, film thickness and uniaxial anisotropy. The interplay between these system parameters is found to give rise to a phase space with a rich structure, distinct from that of the nano-stripes that have been previously studied. For certain values of the anisotropy, the range of thicknesses supporting in-plane skyrmions and/or helicoids with an out-of-plane propagation vector is found to be disconnected, suggesting a possible direction for future experiments. We explain how this interesting phase space topology arises due to the geometric confinement of the thin-film system, and identify the optimal parameter ranges for future explorations of novel magnetic textures such as the oblique spiral phase.

cond-mat.mtrl-sci↗

arXiv2020

Thermoelectric calculations of ring-shaped bands in two-dimensional Bi$_2$Te$_3$, Bi$_2$Se$_3$ and Sb$_2$Te$_3$: a comparison of simple scattering approximations

Materials with ring-shaped electronic bands are promising thermoelectric candidates, since their unusual dispersion shape is predicted to give large power factors. While previous calculations of these materials have relied on the assumption of a constant mean-free-path or relaxation time, recent first-principles modeling of electron-phonon scattering suggests that the scattering rates may be better approximated by the electron density-of-states (so-called DOS scattering model). In this work, we use density functional theory to investigate single and double quintuple-layer Bi$_2$Te$_3$, Bi$_2$Se$_3$ and Sb$_2$Te$_3$, with a focus on understanding how the three aforementioned scattering approximations impact thermoelectric performance -- emphasis is placed on the DOS scattering model. The single quintuple-layer materials possess two ring-shaped valence band maxima that provide an abrupt increase in conducting channels, which benefits the power factor. Additionally, below the band edge a ring-shaped minimum, located between the two maxima, is found to further enhance the thermoelectric performance but only with the DOS scattering model. This comes from a sharp drop in the DOS, and thus scattering, just below the ring-shaped minimum. An analytic octic dispersion model is introduced and shown to qualitatively capture the observed features. The double quintuple-layer materials display notably worse thermoelectric properties, since their dispersions are significantly modified compared to the single quintuple-layer case. The benefits of ring-shaped bands are sensitive to the alignment of the two ring maxima and to the degree of ring anisotropy. Overall, single quintuple-layer Bi$_2$Te$_3$ and Bi$_2$Se$_3$ are most promising, with the DOS scattering model giving the highest power factors.

cond-mat.mes-hall↗

arXiv2020

Analysis of simple scattering models on the thermoelectric performance of analytical electron dispersions

Recent first-principles electron-phonon scattering calculations of heavily-doped semiconductors suggest that a simple DOS scattering model, wherein the electronic scattering rates are assumed to be proportional to the density-of-states, better approximates the rigorous scattering characteristics compared to the commonly used constant relaxation-time and constant mean-free-path approximations. This work investigates how the thermoelectric properties predicted with the DOS model compare to the other two scattering models, using three analytical electron dispersions (parabolic band in 3D/2D/1D, Kane band in 3D/2D/1D, and ring-shaped quartic band in 2D). Our findings show that the scattering models can lead to significant differences, and can disagree about whether certain band structures can provide benefits. A constant relaxation-time is found to always be optimistic compared to a constant mean-free-path, while the DOS scattering model shows no such clear trend. Notably, the 1D parabolic band and 2D quartic band exhibit the highest power factors with the DOS model, resulting from a rapid decrease in density-of-states, and thus scattering - suggesting a possible strategy for improved thermoelectrics based on engineering band structures with sharp/discontinuous drops in density-of-states. The DOS scattering approximation also suggests that searches for materials with a delta function-like DOS (as a proxy to the transport distribution) or converged bands may yield limited benefits, due to the increase in scattering. This work highlights the importance of simple and accurate scattering models when rigorous ab-initio scattering calculations are not feasible.

cond-mat.mtrl-sci↗