The nanostructuring of thermoelectric materials is a well-established method of suppressing lattice thermal conductivity. However, our understanding of the interfaces that form as a result of nanostructure engineering is still limited. In this work, we utilise a simple two-body pair potential to calculate the thermal boundary resistance of basal plane twin boundaries in Bi2Te3 at 300 K using reverse non-equilibrium molecular dynamics simulations. The considered interatomic potential gives an excellent description of the twin boundary formation energies and the lattice thermal conductivity of bulk Bi2Te3. Using this potential, we find that the twin boundary located at the Bi layer is not thermally stable (unlike those located at the Te layers), and undergoes a phase transition into two distinct structures. We compare the thermal boundary resistance across these different twin boundaries and link the observed trends to overall geometry, van der Waals gap sizes and degree of structural disorder in atomic layers near the boundary.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Colm O'Sullivan, Stephen Fahy; Teaching about Magnetic Materials—A Pedagogical Dilemma. Phys. Teach. 1 March 2024; 62 (3): 188–190. https://doi.org/10.1119/5.0120835 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAmerican Association of Physics TeachersThe Physics Teacher Search Advanced Search |Citation Search

The band-anticrossing (BAC) model provides the basis for the self-consistent Green’s function method that we have previously developed to calculate the density of states of GaNxAs1−x dilute nitride alloys. In this paper, we extend this Green’s function method to include the complex energy states and to find the poles of the Green’s function, thereby allowing one to calculate the dispersion relation, group velocity, and the carrier decay rate in disordered dilute nitride alloys. Two different models of the N states have been studied to investigate the band structure of these materials: (1) the conventional two-band BAC model, which assumes that all N states are located at the same energy, and (2) a model which includes N states distributed over a range of energies, as expected in actual dilute nitride samples. Our results for the second model show a much shorter carrier mean-free path, and lower carrier mobility for GaNxAs1−x, with the magnitude of the calculated mobility in good agreement with the experimental data.

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi_{2}Te_{3} and Bi_{2}Se_{3}, following the excitation of coherent A_{1g} optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A_{1g}-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES. We find the coupling in Bi_{2}Te_{3} and Bi_{2}Se_{3} to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling.

We investigate experimentally and using first-principles theory the generation of phonons and the relaxation of carriers on picosecond timescales across the Brillouin zone of photo-excited Ge by inter-valley electron–phonon scattering. The phonons generated are typical of those generated in semiconductor devices, contributing to the accumulation of heat within the material. We simulate the time-evolution of phonon populations, based on first-principles band structure and electron–phonon and phonon–phonon matrix elements, and compare them to data from time-resolved x-ray diffuse scattering experiments, performed at the Linac Coherent Light Source x-ray free-electron laser facility, following photo-excitation by a 50 fs near-infrared optical pulse. We show that the intensity of the non-thermal x-ray diffuse scattering signal, which is observed to grow substantially near the L-point of the Brillouin zone over 3–5 ps, is due to phonons generated by scattering of carriers between the Δ and L valleys. These phonons have low group velocities, resulting in a heat bottleneck. With the inclusion of phonon decay through 3-phonon processes, the simulations also account for other non-thermal features observed in the x-ray diffuse scattering intensity, which are due to anharmonic phonon–phonon scattering of the phonons initially generated by electron–phonon scattering.

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi2Te3 and Bi2Se3, following the excitation of coherent A(1g) optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A(1g)-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES. We find the coupling in Bi2Te3 and Bi2Se3 to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling.

We investigate coupled electron-lattice dynamics in the topological insulator Bi2Te3 with time-resolved photoemission and time-resolved x-ray diffraction. It is well established that coherent phonons can be launched by optical excitation, but selection rules generally restrict these modes to zone-center wavevectors and Raman-active branches. We find that the topological surface state couples to additional modes, including a continuum of surface-projected bulk modes from both Raman- and infrared-branches, with possible contributions from surface-localized modes when they exist. Our calculations show that this surface vibrational spectrum occurs naturally as a consequence of the translational and inversion symmetries broken at the surface, without requiring the splitting-off of surface-localized phonon modes. The generality of this result suggests that coherent phonon spectra are useful by providing unique fingerprints for identifying surface states in more controversial materials. These effects may also expand the phase space for tailoring surface state wavefunctions via ultrafast optical excitation.

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi2Te3 and Bi2Se3, following the excitation of coherent A1g optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A1g-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES. We find the coupling in Bi2Te3 and Bi2Se3 to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling.5 MoreReceived 22 July 2023Revised 8 October 2023Accepted 15 November 2023DOI:https://doi.org/10.1103/PhysRevX.13.041050Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectron-phonon couplingPhononsSurface statesUltrafast phenomenaPhysical SystemsTopological materialsTechniquesDensity functional theoryPhotoemission spectroscopyUltrafast femtosecond pump probeX-ray diffractionCondensed Matter, Materials & Applied Physics

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi_2Te_3 and Bi_2Se_3, following the excitation of coherent A_1g optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A_1g-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES.We find the coupling in Bi_2Te_3 and Bi_2Se_3 to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling.

Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, topological superconductivity, and potentially how to manipulate the surface state using a structural degree of freedom. We perform time-resolved x-ray diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on Bi$_2$Te$_3$ and Bi$_2$Se$_3$, following the excitation of coherent A$_{1g}$ optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A$_{1g}$-like displacements in these two materials using the experimentally determined atomic displacements from XRD and electron band shifts from ARPES.We find the coupling in Bi$_2$Te$_3$ and Bi$_2$Se$_3$ to be similar and in general in agreement with expectations from density functional theory. We establish a methodology that quantifies the mode-specific electron-phonon coupling experimentally, allowing detailed comparison to theory. Our results shed light on fundamental processes in topological insulators involving electron-phonon coupling.

Achieving high valley degeneracy (i.e."band convergence") in a material usually results in considerably enhanced thermoelectric properties. However, it is still unclear why this strategy of designing efficient thermoelectric materials is so successful, since the benefit of increased density of states may be severely degraded by intervalley scattering. Using first-principles calculations, we investigate these effects in $p$-type PbTe, where temperature induces alignment of the $L$ and $\Sigma$ valleys at $\sim$~620~K. We explicitly show that the thermoelectric power factor and figure of merit peak near the band convergence temperature. The figure of merit maximum is larger than those of the individual $L$ and $\Sigma$ valleys. Surprisingly, intervalley scattering does not considerably affect the figure of merit near the band convergence temperature and optimal doping conditions, although it reduces the power factor by almost a factor of 2. Our results suggest that band convergence will significantly increase the figure of merit if intervalley scattering is roughly proportional to the density of states and the lattice thermal conductivity is considerably lower than the electronic thermal conductivity, even if intervalley scattering is strong.

Experimental investigations of the phase transition in GeTe provide contradictory conclusions regarding the nature of the phase transition. Considering growing interest in technological applications of GeTe, settling these disputes is of great importance. To that end, we present a molecular dynamics study of the structural phase transition in GeTe using a machine-learned interatomic potential with ab-initio accuracy. First, we calculate the asymmetric shape of the radial distribution function of the nearest-neighbor bonds above the critical temperature, in agreement with previous studies. However, we show that this effect is not necessarily linked with the order-disorder phase transition and can occur as a result of large anharmonicity. Next, we study in detail the static and dynamic properties of the order parameter in the vicinity of the phase transition and find fingerprints of both order-disorder and displacive phase transition.

We report the observation of photo-induced plasmon–phonon coupled modes in the group IV–VI semiconductor PbTe using ultrafast x-ray diffuse scattering at the Linac Coherent Light Source. We measure the near-zone-center excited-state dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following above bandgap photoexcitation. We suggest that upon photoexcitation, the LO phonon-plasmon coupled (LOPC) modes themselves become coupled to longitudinal acoustic modes that drive electron band shifts via acoustic deformation potentials and possibly to low-energy single-particle excitations within the plasma and that these couplings give rise to displacement-correlations that oscillate in time with a period given effectively by the heavily screened LOPC frequency.

IV-VI materials are some of the most efficient bulk thermoelectric materials due to their proximity to soft-mode phase transitions, which leads to low lattice thermal conductivity. However, the effect of soft phonon modes on their electronic thermoelectric properties remains poorly understood. Using first principles calculations, we show that the soft zone center transverse optical phonons do not deteriorate the electronic thermoelectric properties of PbTe driven closer to the phase transition via lattice expansion due to external stress. Scattering between electrons and soft phonons remains weak even very near the phase transition as a result of symmetry restrictions. In contrast, phonon softening considerably reduces the lattice thermal conductivity of PbTe with the lattice constants $\sim 1.5$\% larger than that at equilibrium and doubles the thermoelectric figure of merit. Our results indicate that the thermoelectric figure of merit of materials with soft phonon modes that interact strongly with other phonons, but weakly with electronic states relevant for transport, could be significantly increased by bringing these materials closer to the soft-mode phase transition.

We demonstrate an experimental approach to determining the excited-state interatomic forces using femtosecond x-ray pulses from an x-ray free-electron laser. We determine experimentally the excited-state interatomic forces that connect photoexcited carriers to the nonequilibrium lattice dynamics in the prototypical Peierls-distorted material, bismuth. The forces are obtained by a constrained least-squares fit of a pairwise interatomic force model to the excited-state phonon dispersion relation as measured by the time- and momentum-resolved x-ray diffuse scattering. We find that photoexcited carriers weaken predominantly the nearest-neighbor forces, which drives the measured softening of the transverse acoustic modes throughout the Brillouin zone as well as the zone-center A(1g) optical mode. This demonstrates a bond-selective approach to measuring electron-phonon coupling relevant to a broad range of photoinduced phase transitions and transient light-driven states in quantum materials.

The passivation of thin Bi(1 1 1) films with hydrogen and oxide capping layers is investigated from first principles. Considering termination-related changes of the crystal structure, we show how the bands and density of states are affected. In the context of the much-discussed semimetal-to-semiconductor transition and the band topology of the bulk material, we consider the effects of confinement in the whole Brillouin zone and go beyond standard density functional theory by including many-body interactions via the G(0) W-0 approximation. The conductivity of unterminated films is calculated via the Boltzmann transport equation using the simple constant relaxation-time approximation and compared to experimental observations that have suggested a two-channel model.

We report the observation of photo-induced plasmon-phonon coupled modes in the group IV-VI semiconductor PbTe using Fourier-transform inelastic X-ray scattering at the Linac Coherent Light Source (LCLS). We measure the near-zone-center dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following above band gap photoexcitation.