Electron-phonon Scatterings Research Articles

Giant Enhancement of Hole Mobility for 4H-Silicon Carbide through Suppressing Interband Electron-Phonon Scattering.

4H-silicon carbide (4H-SiC) possesses a high Baliga figure of merit, making it a promising material for power electronics. However, its applications are limited by low hole mobility. Herein, we found that the hole mobility of 4H-SiC is mainly limited by the strong interband electron-phonon scattering using mode-level first-principles calculations. Our research indicates that applying compressive strain can reverse the sign of crystal-field splitting and change the ordering of electron bands close to the valence band maximum. Therefore, the interband electron-phonon scattering is severely suppressed and the electron group velocity is significantly increased. The out-of-plane hole mobility of 4H-SiC can be greatly enhanced by ∼200% with 2% uniaxial compressive strain applied. This work provides new insights into the electron transport mechanisms in semiconductors and suggests a strategy to improve hole mobility that could be applied to other semiconductors with hexagonal crystalline geometries.

Effect of single and dual doping on the thermoelectric properties of FeVSb0.95Sn0.05 and Fe0.95Co0.05VSb0.90Sn0.10 half-Heusler alloys

FeVSb in addition to a set of novel FeVSb0.95Sn0.05 and Fe0.95Co0.05VSb0.90Sn0.10 half-Heusler alloys were fabricated by arc melting followed by induction melting simple methods. Impacts of single Sn-doping and double Co- and Sn-doping on the structural and thermoelectric properties of FeVSb were investigated in this article. Structural and thermoelectric properties of FeVSb, FeVSb0.95Sn0.05 and Fe0.95Co0.05VSb0.90Sn0.10 compounds have been extensively studied in this work. N-type conduction was confirmed for the FeVSb system and for the doped FeVSb0.95Sn0.05 and Fe0.95Co0.05VSb0.90Sn0.10 alloys. The absolute value of Seebeck coefficient for the single Sn-doped sample increased from 127 μV/K to 155 μV/K, with the temperature increasing. Similarly, the Seebeck coefficient of the double Co- and Sn-doped alloy also increased from 40 μV/K to 60 μV/K. The thermoelectric power factor was notably enhanced for the dual doped Fe0.95Co0.05VSb0.90Sn0.10 alloy reaching 14 μW/cm.K2. Lattice thermal conductivity of the single Sn-doped FeVSb0.95Sn0.05 alloy decreased considerably over the whole temperature range. A maximum zT of 0.084 was achieved for FeVSb0.95Sn0.05 alloy at 575 K. The improved figure of merit due to doping is attributed to the phonon scattering at point defects. Sn-doping has significantly reduced thermal conductivity due to the enhanced point-defect and electron–phonon scatterings which contributed to improved thermoelectric figure of merit. Our findings showed also that the room-temperature thermal conductivity is seriously reduced via Sn and Co dual doping leading to higher ZT value for Fe0.95Co0.05VSb0.90Sn0.10 alloy. Promising candidates for further thermoelectric applications could be achieved by Sn-doping and double Co- and Sn-doping.

A Delicate Balance Between Spin-Wave Mediated Weak Localization and Electron-Phonon Scattering in the Design of Zero Temperature Coefficient of Resistivity.

Zero thermal coefficients of resistivity (ZTCR) materials exhibit minimal changes in resistance with temperature variations, making them essential in modern advanced technologies. The current ZTCR materials, which are based on the resistivity saturation effect of heavy metals, tend to function at elevated temperatures because the mean free path approaches the lower limit of the semiclassical Boltzmann theory when the temperature is sufficiently high. ZTCR materials working at low-temperatures are difficult to achieve due to electron-phonon scattering, which results in increased resistivity according to Bloch's theory. In this work, the ZTCR behavior at low-temperatures is realized in pre-microstrained Mn3NiN. The delicate balance between the resistivity contribution from electron-phonon scattering and spin-wave mediated weak localization is well revealed. A remarkable temperature coefficient of resistivity (TCR) value as low as 1.9ppmK-1 (50K≤T≤200K) is obtained, which is significantly superior to the threshold value of ZTCR behavior and the application standard of commercial ZTCR materials. The demonstration provides a unique paradigm in the design of ZTCR materials through the contraction effects of two opposite conductance mechanisms with positive and negative thermal coefficients of resistivity.

Ab Initio Predictions of Spin Relaxation, Dephasing, and Diffusion in Solids.

Spin relaxation, dephasing, and diffusion are at the heart of spin-based information technology. Accurate theoretical approaches to simulate spin lifetimes (τs), determining how fast the spin polarization and phase information will be lost, are important to the understanding of the underlying mechanism of these spin processes, and invaluable in searching for promising candidates of spintronic materials. Recently, we develop a first-principles real-time density-matrix (FPDM) approach to simulate spin dynamics for general solid-state systems. Through the complete first-principles descriptions of light-matter interaction and scattering processes including electron-phonon, electron-impurity, and electron-electron scatterings with self-consistent spin-orbit coupling, as well as ab initio Landé g-factor, our method can predict τs at various conditions as a function of carrier density and temperature, under electric and magnetic fields. By employing this method, we successfully reproduce experimental results of disparate materials and identify the key factors affecting spin relaxation, dephasing, and diffusion in different materials. Specifically, we predict that germanene has long τs (∼100 ns at 50 K), a giant spin lifetime anisotropy, and spin-valley locking effect under electric fields, making it advantageous for spin-valleytronic applications. Based on our theoretical derivations and ab initio simulations, we propose a new useful electronic quantity, named spin-flip angle θ↑↓, for the understanding of spin relaxation through intervalley spin-flip scattering processes. Our method can be further applied to other emerging materials and extended to simulate exciton spin dynamics and steady-state photocurrents due to photogalvanic effect.

Intrinsic Berry curvature driven anomalous Nernst thermopower in the semimetallic Heusler alloy CoFeVSb

Understanding of spin-heat coupling mechanisms and magnetothermoelectric phenomena, including the anomalous Nernst effect (ANE), in emergent quaternary Heusler alloys is of practical importance for applications in thermal management and energy harvesting. Here, we demonstrate an intrinsic Berry curvature mediated anomalous Nernst thermopower in CoFeVSb, which orders magnetically at high temperature (${T}_{C}\ensuremath{\approx}850\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) with a large saturation magnetization of $\ensuremath{\approx}2.2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ at room temperature. We show that the electron-electron elastic and electron-magnon inelastic scattering dominate longitudinal electrical transport at low temperatures ($T\ensuremath{\le}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$), whereas the electron-phonon and electron-magnon scatterings govern it at higher $\phantom{\rule{0.28em}{0ex}}T$. The longitudinal thermopower is resulted mainly from the diffusive contribution with a very large longitudinal Seebeck coefficient ($42\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 395 K). The value of the anomalous Nernst coefficient (${S}_{\mathrm{ANE}}$) for CoFeVSb at room temperature is $0.039\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ which is higher than the compressively strained $\mathrm{SrRu}{\mathrm{O}}_{3}$ film ($0.03\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) as well as the spin gapless semiconductor CoFeCrGa ($0.018\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$). On lowering $T$, both the ordinary Nernst coefficient and carrier mobility increase but an opposite trend is found for ${S}_{\mathrm{ANE}}$. Our ab initio simulations reveal the topological semimetallic nature of CoFeVSb with a pair of Weyl points. These Weyl crossings result in a significant contribution to the Berry curvature, leading to an intrinsic anomalous Hall conductivity (${\ensuremath{\sigma}}_{xy}^{\mathrm{AHE}}$) of $\ensuremath{\approx}85$ S/cm, which matches well with experiment (77 S/cm at 2 K). Our experimental findings and ab initio calculations support the dominance of the intrinsic Berry curvature in the observed ANE. The ratio of ${\ensuremath{\sigma}}_{xy}^{\mathrm{AHE}}$ to the transverse anomalous thermoelectric conductivity (${\ensuremath{\alpha}}_{xy}^{\mathrm{ANE}}$) shows an increasing trend with $T$ attaining a sizable fraction of $\phantom{\rule{0.28em}{0ex}}\frac{{k}_{B}}{e}\phantom{\rule{0.28em}{0ex}}(\ensuremath{\approx}0.35\frac{{k}_{B}}{e})$ at room temperature.

Dynamical renormalization of electron-phonon coupling in conventional superconductors

The adiabatic Born-Oppenheimer approximation is considered to be a robust approach that very rarely breaks down. Consequently, it is predominantly utilized to address various electron-phonon properties in condensed matter physics. By combining many-body perturbation and density functional theories we demonstrate the importance of dynamical (nonadiabatic) effects in estimating superconducting properties in various bulk and two-dimensional materials. Apart from the expected long-wavelength nonadiabatic effects, we found sizable nonadiabatic Kohn anomalies away from the Brillouin zone center for materials with strong intervalley electron-phonon scatterings. Compared to the adiabatic result, these dynamical phonon anomalies can significantly modify electron-phonon coupling strength $\ensuremath{\lambda}$ and superconducting transition temperature ${T}_{c}$. Further, the dynamically induced modifications of $\ensuremath{\lambda}$ have a strong impact on transport properties, where probably the most interesting is the rescaling of the low-temperature and low-frequency regime of the scattering time $1/\ensuremath{\tau}$ from about ${T}^{3}$ to about ${T}^{2}$, resembling the Fermi liquid result for electron-electron scattering. Our goal is to point out the potential implications of these nonadiabatic effects and reestablish their pivotal role in computational estimations of electron-phonon properties.

The Impact of Electron Phonon Scattering, Finite Size and Lateral Electric Field on Transport Properties of Topological Insulators: A First Principles Quantum Transport Study.

We study, using non-equilibrium Green's function simulations combined with first-principles density functional theory, the edge-state transport in two-dimensional topological insulators. We explore the impact of electron-phonon coupling on carrier transport through the protected states of two widely known topological insulators with different bulk gaps, namely stanene and bismuthene. We observe that the transport in a topological insulator with a small bulk gap (such as stanene) can be heavily affected by electron-phonon scattering, as the bulk states broaden into the bulk gap. In bismuthene with a larger bulk gap, however, a significantly higher immunity to electron-phonon scattering is observed. To mitigate the negative effects of a small bulk gap, finite-size effects are studied in stanene ribbons. The bulk gap increases in ultra-narrow stanene ribbons, but the transport results revealed no improvement in the dissipative case, as the states in the enlarged bulk gaps aren't sufficiently localized. To investigate an application, we also used topological insulator ribbons as a material for field-effect transistors with side gates imposing a lateral electric field. Our results demonstrate that the lateral electric field could offer another avenue to manipulate the edge states and even open a gap in stanene ribbons, leading to an ION/IOFF of 28 in the ballistic case. These results shed light on the opportunities and challenges in the design of topological insulator field-effect transistors.

Revealing the decisive factors of the lattice thermal conductivity reduction by electron-phonon interactions in half-Heusler semiconductors

The role of electron-phonon (EP) scattering in lattice thermal conductivity (κL) of thermoelectric material has not been fully studied until now, especially for the decisive factors that influence the reduction of κL. To address this issue, we report the effect of EP interactions on κL at a series of temperatures and carrier concentrations for 18 half-Heusler compounds. Among all the compounds investigated, the hole-doped TiCoSb and the electron-doped ZrIrSb have the largest κL reductions (32% & 20%) by EP interactions at 300 K, under the carrier concentration of 1021 cm−3. Detailed analyses reveal that the system with strong EP coupling strength and high electronic density of states at the Fermi level (N (EF)) favor the κL reduction, because these two factors are beneficial for EP scattering rates. And a high N (EF) can be caused by a high carrier concentration and/or a large effective mass. Temperature is another factor that affects the reduction of κL by EP interactions due to its imbalance influence on EP and phonon-phonon (PP) scatterings. Furthermore, after considering the influences from the aliovalent doping and grain boundary, the EP interactions still play a non-negligible role on κL reduction, especially at low temperatures and high carrier concentrations. Our work provides a complete picture for understanding the mechanism of EP interactions in material's thermal transport.

Spin-Dependent Momentum Conservation of Electron–Phonon Scattering in Chirality-Induced Spin Selectivity

The elucidation of the mechanisms underpinning chirality-induced spin selectivity remains an outstanding scientific challenge. Here we consider the role of delocalized phonon modes in electron transport in chiral structures and demonstrate that spin selectivity can originate from spin-dependent energy and momentum conservation in electron-phonon scattering events. While this mechanism is robust to the specific nature of the vibrational modes, the degree of spin polarization depends on environmental factors, such as the specific temperature and phonon relaxation rates, as well as the presence of external driving fields. This parametric dependence is used to present experimentally testable predictions of our model.

Strain engineering for enhanced hot-carrier photodetection

Hot-carrier devices in metal–semiconductor junctions have attracted considerable attention but still with quantum efficiencies far from expectations. Introducing the lattice strain to the material can effectively modulate the electronic structure, providing a way to control the hot-carrier dynamics. Here, we study how this strain affects the generation, transport, and injection of hot carriers in gold (Au) by using first-principles calculations and evaluate the overall responses of Au-based hot-carrier devices by Monte Carlo simulation. We find that the compressive strain can significantly increase the hot-electron generation from direct transition at E > 1.1 eV for Au. The compressive strain delocalizes the band structure and decreases the electron density of state, which, in turn, reduce electron–electron and electron–phonon scatterings to improve the transport of hot carriers. Taking the Au/TiO2 device as an example, we find that the compressive strain (−6%) can enable a 1.5- to 3-fold enhancement of quantum efficiency and responsivity at a photon energy between 1.2 and 3 eV.

Hot-Carrier Physics of Transition-Metal Carbides and Nitrides: Insight from Electronic Structure

Hot-carrier photodetectors have been extensively studied in recent years, but still exhibit particularly low efficiencies. The physics behind this is closely related to the intrinsic electronic structures of hot-carrier materials. Transition-metal carbides and nitrides (TMCNs) show intriguing potential for high-performance hot-carrier applications; however, a systematic study from the electronic structure perspective remains elusive. Herein, we study the underlying physics, including the electronic structures, hot-carrier energy distribution, electron-electron and electron-phonon scatterings, and hot-carrier injection for TMCNs by first-principles calculation and Monte Carlo simulations. We investigate the complete hot-carrier behavior from the microscopic dynamics (e.g., generation, transport, and injection) of hot carriers to the macroscopic responses (e.g., device responsivity) of the photodetectors. It is found that a well-manipulated electronic structure can greatly improve the performance of hot-carrier photodetectors. Our systematic study shows that hot-carrier systems based on $\mathrm{Hf}\mathrm{C}$, $\mathrm{Zr}\mathrm{N}$, or $\mathrm{Ta}\mathrm{C}$ display significantly higher efficiencies, benefitting from their better electronic structures; in particular, the unbiased responsivity of the $\mathrm{Zr}\mathrm{N}/{\mathrm{Ti}\mathrm{O}}_{2}$ system can be up to 20 mA/W at a wavelength of 1033 nm, which is about 70 times higher than that of the conventional $\mathrm{Au}$ system, enabling the realization of high-performance hot-carrier photodetectors by using non-noble metals.

2D MXenes for Hot‐Carrier Photodetection

AbstractFor 2D MXenes (Mo2C and Ti3C2), the fundamental hot‐carrier‐related processes are studied by using the first‐principles calculation, including the material electronic structures, indirect/direct transitions for hot‐carrier generation, electron–electron and electron–phonon scatterings during hot‐carrier transport, and electronic modulation on material to evaluate the potentials of 2D MXenes in hot‐carrier photodetection. It is found that Mo2C and Ti3C2 show higher hot‐carrier generation efficiencies than Au in energy over 1 eV, especially for Mo2C, which shows better transport performance than Ti3C2. Biaxial strain to adjust the electronic structure of Mo2C is further used to optimize the hot‐carrier transport properties. A Mo2C/MoGeSiN4 hot‐electron photodetector is proposed, which shows that the compressive strain (−4%) on Mo2C material enables a twofold enhancement of the quantum efficiency and responsivity. The Monte Carlo device simulation predicts an extremely high responsivity up ≈176 mA W−1 at the communication wavelength of 1550 nm, which is one order of magnitude over the existing hot‐carrier devices and can be competitive with the existing semiconductor‐based photodetectors.

Anomalous Thermoelectric Transport Phenomena from First‐Principles Computations of Interband Electron–Phonon Scattering

AbstractThe 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. Here a new regime is uncovered where the presence of multiple electron bands with different effective masses, crossing near the Fermi level, leads to strong 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. The origin and magnitude of this effect is explained using a general simplified model as well as first‐principles Boltzmann transport calculations in recently discovered half‐Heusler alloys. General design rules for using this paradigm to engineer enhanced performance in thermoelectric materials are identified.

Plasmon Mediated Electron Transfer and Temperature Dependent Electron-Phonon Scattering in Gold Nanoparticles Embedded in Dielectric Films.

Excitation of localized surface plasmon resonance in metal nanoparticles (NPs) embedded in a glassy matrix generates hot electrons, which can be extracted for different optoelectronic applications. The insights of plasmon relaxation dynamics with varying surrounding dielectric environments and temperature dependence electron-phonon scattering process in gold (Au) NPs are still not very clear. Here, we have employed ultrafast transient absorption (TA) spectroscopy to explore the hot plasmon mediated electron transfer (PMET) and electron-phonon dynamics of photo-excited Au NPs in glassy film matrix with variable SiO2 /TiO2 compositions at cryogenic (5 K) to room temperature (300 K). Herein, we have chosen two pump excitation wavelengths (400 and 700 nm). The 400 nm excitation (d→sp) generates hot electron and the 700 nm excitation (sp→sp) provide information of direct plasmon relaxation. Drastic reduction of the transient signal of Au NPs in the high TiO2 content film as compared to pure SiO2 confirm hot electron transfer (HET) from Au plasmon to TiO2 . Electron-phonon scattering time constant (τe-ph ) of Au NPs in the glassy film is found to be faster in presence of TiO2 due to facile electron transfer/injection. Temperature dependent TA studies suggest that electron-phonon scattering time decreases with temperature. These findings would assist to develop more advanced photo-voltaic, opto-electronic and quantum optic-based devices using the plasmonic metal NPs.

On the importance of electron–electron and electron–phonon scatterings and energy renormalizations during carrier relaxation in monolayer transition-metal dichalcogenides

An ab initio based fully microscopic many-body approach is used to study the carrier relaxation dynamics in monolayer transition-metal dichalcogenides. Bandstructures and wavefunctions as well as phonon energies and coupling matrix elements are calculated using density functional theory. The resulting dipole and Coulomb matrix elements are implemented in the Dirac–Bloch equations to calculate carrier–carrier and carrier–phonon scatterings throughout the whole Brillouin zone (BZ). It is shown that carrier scatterings lead to a relaxation into hot quasi-Fermi distributions on a single femtosecond timescale. Carrier cool down and inter-valley transitions are mediated by phonon scatterings on a picosecond timescale. Strong, density-dependent energy renormalizations are shown to be valley-dependent. For MoTe2, MoSe2 and MoS2 the change of energies with occupation is found to be about 50% stronger in the Σ and Λ side valleys than in the K and K′ valleys. However, for realistic carrier densities, the materials always maintain their direct bandgap at the K points of the BZ.

Ab initio ultrafast spin dynamics in solids

Spin relaxation and decoherence is at the heart of spintronics and spin-based quantum information science. Currently, theoretical approaches that can accurately predict spin relaxation of general solids including necessary scattering pathways and capable for ns to ms simulation time are urgently needed. We present a first-principles real-time density-matrix approach based on Lindblad dynamics to simulate ultrafast spin dynamics for general solid-state systems. Through the complete first-principles descriptions of pump, probe and scattering processes including electron-phonon, electron-impurity and electron-electron scatterings with self-consistent electronic spin-orbit couplings, our method can directly simulate the ultrafast pump-probe measurements for coupled spin and electron dynamics over ns at any temperatures and doping levels. We first apply this method to a prototypical system GaAs and obtain excellent agreement with experiments. We find that the relative contributions of different scattering mechanisms and phonon modes differ considerably between spin and carrier relaxation processes. In sharp contrast to previous work based on model Hamiltonians, we point out that the electron-electron scattering is negligible at room temperature but becomes dominant at low temperatures for spin relaxation in n-type GaAs. We further examine ultrafast dynamics in novel spin-valleytronic materials - monolayer and bilayer WSe2 with realistic defects. We find that spin relaxation is highly sensitive to local symmetry and chemical bonds around defects. Our work provides a predictive computational platform for spin dynamics in solids, which has unprecedented potentials for designing new materials ideal for spintronics and quantum information technology.

Giant Spin Lifetime Anisotropy and Spin-Valley Locking in Silicene and Germanene from First-Principles Density-Matrix Dynamics.

Through first-principles real-time density-matrix (FPDM) dynamics simulations, we investigated spin relaxation due to electron-phonon and electron-impurity scatterings with spin-orbit coupling (SOC) in two-dimensional Dirac materials silicene and germanene at finite temperatures. We discussed the applicability of conventional descriptions of spin relaxation mechanisms by Elliott-Yafet (EY) and D'yakonov-Perel' (DP) compared to the FPDM method, which is determined by a complex interplay of intrinsic SOC, external fields, and scattering strength. For example, the electric field dependence of the spin lifetime by FPDM is close to the DP mechanism for silicene at room temperature but similar to the EY mechanism for germanene. Because of its stronger SOC strength and buckled structure in contrast to graphene, germanene has a giant spin lifetime anisotropy and spin-valley locking effect under nonzero Ez and low temperatures. More importantly, germanene has a long spin lifetime (∼100 ns at 50 K) and an ultrahigh carrier mobility, making it advantageous for spin-valleytronic applications.

(Invited) Enhanced Electron Phonon Scattering in Si Nanowires Covered By Oxide

In order to maintain gate controllability of electric current in nanoscale transistors, nanosheet or nanowire channels are required for future nodes. In Si covered by oxides, deformation potential greater than that in bulk has been used to reproduce electron mobility. Increase of the deformation potential results in decrease of phonon-limited mobility. However, physical origin of the increase of phonon scattering in nanostructures was not completely understood. Although atomistic calculations with H-terminated Si nanosheets partially explained the increase of phonon scattering, the amount of the increase of phonon scattering was not sufficient to reproduce experimental mobility or empirical values of deformation potential [1]. In this work, we calculated mobility in oxide covered semiconductor nanowires by Molecular dynamics (MD)-Landauer approach [2]. Based on MD simulations followed by transport calculations of tight-binding nonequilibrium Green’s function model, effects of the oxide layers on phonon scattering in the semiconductor nanowires was considered.The derived phonon-limited mobility of the free-standing Si nanowire (205 cm2V-1s-1) was almost the same as previously reported value in H-terminated Si nanowire [2], confirming the validity of our calculations. In the SiO2-covered Si nanowires, the mobility was decreased by half due to the SiO2 (95 cm2V-1s-1). Corresponding effective deformation potential was extracted by comparing the mobility calculated from MD-Landauer approach with the mobility calculated from Boltzmann transport equation. The corresponding deformation potential in SiO2-covered Si nanowires (D ac=22 eV) roughly agrees with the empirical deformation potentials in SOI films of the same thickness [3]. From the comparison between the free-standing and the SiO2-covered nanowires, we found the atomic displacements in the SiO2-covered Si nanowires were abnormally increased at the surface, which would result in the enhance of phonon scattering. Therefore, in order to precisely model the transport properties in future node’s transistors, atomistic descriptions of the interface between semiconductor and oxide were indispensable.[1] J. Li, JAP, 120, 174301, 2016.[2] T. Markussen, PRB, 95, 245210, 2017.[3] T. Ohashi, IEEE JEDS, 4, 278, 2016. Figure 1

Observation of a giant mass enhancement in the ultrafast electron dynamics of a topological semimetal

Topological phases of matter offer exciting possibilities to realize lossless charge and spin information transport on ultrafast time scales. However, this requires detailed knowledge of their nonequilibrium properties. Here, we employ time-, spin- and angle-resolved photoemission to investigate the ultrafast response of the Sb(111) spin-polarized surface state to femtosecond-laser excitation. The surface state exhibits a giant mass enhancement which is observed as a kink structure in its energy-momentum dispersion above the Fermi level. The kink structure, originating from the direct coupling of the surface state to the bulk continuum, is characterized by an abrupt change in the group velocity by ~70%, in agreement with our GW-based band structure calculations. Our observation of this connectivity in the transiently occupied band structure enables the unambiguous experimental verification of the topological nature of the surface state. The influence of bulk-surface coupling is further confirmed by our measurements of the electron dynamics, which show that bulk and surface states behave as a single thermalizing electronic population with distinct contributions from low-k electron-electron and high-k electron-phonon scatterings. These findings are important for future applications of topological semimetals and their excitations in ultrafast spintronics.

Electron-Phonon Coupling and Electron-Phonon Scattering in SrVO3.

Understanding the physics of strongly correlated electronic systems has been a central issue in condensed matter physics for decades. In transition metal oxides, strong correlations characteristic of narrow d bands are at the origin of remarkable properties such as the opening of Mott gap, enhanced effective mass, and anomalous vibronic coupling, to mention a few. SrVO3 with V4+ in a 3d1 electronic configuration is the simplest example of a 3D correlated metallic electronic system. Here, the authors' focus on the observation of a (roughly) quadratic temperature dependence of the inverse electron mobility of this seemingly simple system, which is an intriguing property shared by other metallic oxides. The systematic analysis of electronic transport in SrVO3 thin films discloses the limitations of the simplest picture of e–e correlations in a Fermi liquid (FL); instead, it is shown show that the quasi‐2D topology of the Fermi surface (FS) and a strong electron–phonon coupling, contributing to dress carriers with a phonon cloud, play a pivotal role on the reported electron spectroscopic, optical, thermodynamic, and transport data. The picture that emerges is not restricted to SrVO3 but can be shared with other 3d and 4d metallic oxides.