Electron-acoustic Phonon Research Articles

Graphene resistivity in diffusive limit due to scalar and vector potential electron-phonon scattering

We calculate the contribution of unscreened and screened scalar (DP) and vector potential(VP) electron-acoustic phonon coupling to resistivity in disordered graphene through Keldysh Green’s function method within the diffusive limit, kFl ≫ 1. We obtain analytical results in the asymptotic limits of clean and strong impurities for both DP and VP coupling, which is in observed to be in good agreement with the resistivity behavior in these limits. We find that the complete numerical results approach the analytical result in the extreme limits, but give different temperature dependencies in between these limits. The graphene resistivity has been investigated as functions of temperature, mean free path and carrier density. We also evaluated the screened behavior in the Thomas-Fermi and Random phase approximation dielectric function and obtained the temperature power exponents. We find that in the absence of screening when the electronic disorder (T < Tdr), the two coupling mechanisms are affected differently and the relaxation rate associated with the VP coupling is suppressed by disorder as compared to the DP coupling, and the DP coupling is enhanced by disorder.

Tuning the Structural and Optoelectronic Properties of Cs2 AgBiBr6 Double-Perovskite Single Crystals through Alkali-Metal Substitution.

Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs2 AgBiBr6 has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs2 AgBiBr6 and alkali-metal-substituted (Cs1- x Yx )2 AgBiBr6 (Y: Rb+ , K+ , Na+ ; x= 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron-acoustic phonon scattering is found compared to Cs2 AgBiBr6 . The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.

Study of the Quantum Magneto-Thermoelectric Effect in the Two-Dimensional Compositional Superlattice GaAs/AlGaAs under the influence of an Electromagnetic Wave by Using the Quantum Kinetic Equation

The quantum magneto-thermoelectric effect in a two-dimensional compositional superlattice under the influence of an electromagnetic wave (EMW) in two cases is investigated. Two cases of the electron scattering mechanism are considered: the electron-acoustic phonon scattering and electron-optical phonon scattering. Analytical expressions for the quantum Ettingshausen coefficient (EC), the thermopower tensor, the thermoelectric tensor and the kinetic tensor are obtained by using a quantum kinetic equation. These expressions are numerically solved for the two-dimensional compositional superlattice GaAs/AlGaAs and the results are discussed. The results show that in the case of electron-acoustic phonon scattering, the Shubnikov-de Haas oscillations appear when we examine the dependences of the quantum EC, the thermopower tensor and the thermoelectric tensor on the magnetic field. In the case of electron-optical phonon scattering, resonance peaks that satisfy the condition of the inter-subband magneto-phonon resonance appear. In the two cases, the superlattice period (a parameter specific to the material) strongly affects the quantum magneto-thermoelectric effect. When the superlattice period is small, quantum EC oscillations (in the case of electron-acoustic phonon interaction) and quantum EC resonance peaks (in the case of electron-optical phonon interaction) appear. However, when the superlattice period is large, these oscillations and resonance peaks are not observed. Especially, the influence of electromagnetic waves on the quantum magneto-thermoelectric effect is also clarified. The quantum theory of the magneto-thermoelectric effect has been studied from low temperature to high temperature. This overcomes the limitations of the Boltzmann kinetic equation which was studied at high temperature. The results are new and can serve as a basis for further development of the theory of quantum magneto-thermoelectric effects in low-dimensional semiconductor systems.

Phonon-induced giant linear-in- T resistivity in magic angle twisted bilayer graphene: Ordinary strangeness and exotic superconductivity

We study the effect of electron-acoustic phonon interactions in twisted bilayer graphene on resistivity in the high-temperature transport and superconductivity in the low-temperature phase diagram. We theoretically show that twisted bilayer graphene should have an enhanced and strongly twist-angle dependent linear-in-temperature resistivity in the metallic regime with the resistivity magnitude increasing as the twist angle approaches the magic angle. The slope of the resistivity versus temperature could approach one hundred ohms per kelvin with a strong angle dependence, but with a rather weak dependence on the carrier density. This higher-temperature density-independent linear-in-$T$ resistivity crosses over to a $T^4$ dependence at a low density-dependent characteristic temperature, becoming unimportant at low temperatures. This angle-tuned resistivity enhancement arises from the huge increase in the effective electron-acoustic phonon coupling in the system due to the suppression of graphene Fermi velocity induced by the flatband condition in the moir\'e superlattice system. Our calculated temperature dependence is reminiscent of the so-called `strange metal' transport behavior except that it is arising from the ordinary electron-phonon coupling in a rather unusual parameter space due to the generic moir\'e flatband structure of twisted bilayer graphene. We also show that the same enhanced electron-acoustic phonon coupling also mediates effective attractive interactions in $s$, $p$, $d$ and $f$ pairing channels with a theoretical superconducting transition temperature on the order of $\sim$5 K near magic angle. The fact that ordinary acoustic phonons can produce exotic non-$s$-wave superconducting pairing arises from the unusual symmetries of the system.

Linear-in- T resistivity in dilute metals: A Fermi liquid perspective

We consider a short-range deformation potential scattering model of electron-acoustic phonon interaction to calculate the resistivity of an ideal metal as a function of temperature (T) and electron density (n). We consider both 3D metals and 2D metals, and focus on the dilute limit, i.e., low effective metallic carrier density of the system. The main findings are: (1) a phonon scattering induced linear-in-T resistivity could persist to arbitrarily low T in the dilute limit independent of the Debye temperature ($T_D$) although eventually the low-T resistivity turns over to the expected Bloch-Gruneisen (BG) behavior with $T^5$ ($T^4$) dependence, in 3D (2D) respectively; (2) because of low values of n, the phonon-induced resistivity could be very high in the system; (3) the resistivity shows an intrinsic saturation effect at very high temperatures (for $T>T_D$), and in fact, decreases with increasing T above a high crossover temperature with this crossover being dependent on both $T_D$ and n in a non-universal manner. We also provide high-T linear-in-T resistivity results for 2D and 3D Dirac materials. Our work brings out the universal features of phonon-induced transport in dilute metals, and we comment on possible implications of our results for strange metals, emphasizing that the mere observation of a linear-in-T metallic resistivity at low temperatures or a very high metallic resistivity at high temperatures is not necessarily a reason to invoke an underlying quantum critical strange metal behavior. We discuss the temperature variation of the effective transport scattering rate showing that the scattering rate could be below or above $k_BT$, and in particular, purely coincidentally, the calculated scattering rate happens to be $k_BT$ in normal metals with no implications for the so-called Planckian behavior.

Multi-Field Coupling Behaviors on Phonon and Thermal/Electrical Properties in Semiconductor Nanostructures

Low-dimensional semiconductor structures such as nanofilms and nanowires have stimulated considerable interest due to their potential applications in nanoelectronic or nanomechanical devices. In this presentation, the effects of pre-stress field and surface stress on the phonon and thermal/electrical properties for semiconductor nanostructures are investigated theoretically. The continuum elastic model is employed to calculate the spatially confined phonon properties. The acoustoelastic effects and surface energy effects are taken into account in calculating the phonon properties of nanostructures. Since the thermal and electric properties are associated with phonon properties of semiconductors, the phonon thermal conductivity, electron-acoustic phonon scattering rate and the carrier mobility are obtained for naonfilms and nanowires under prestress fields and surface stress. The numerical results show that the stress fields have a significant influence on the phonon properties such as the phonon dispersion relation, density of state as well as the group velocity, leading to completely altering the thermal conductivity and the carrier mobility. It is also showed that the appearance of stress can adjust the sensitivity of phonon thermal conductivity and carrier mobility to temperature as well as to the physical size of the nanostructures. These results provide an approach of stress/strain engineering through tuning the phonon properties to control the thermal and electrical properties of nanostructured semiconductors in nanoelectronic devices.

The Mechanism for the Half‐Metal to Insulator Transition at the Fe3O4/ZnAl2O4 Interface

The mechanism of Verwey transition remains unsolved. In this work, the gap opening at the Fe3O4/ZnAl2O4 interface is discussed based on first‐principles calculations. Here, with constraint of the charge transfer, superlattices with different orbital ordering show different gaps corresponding to the different electron–acoustic phonon couplings. When electrons (near the Fermi level) are at the out‐of‐plane orbitals, the interface exhibits a wider band gap with stronger electron–acoustic phonon coupling compared to the superlattice with electrons at in‐plane orbitals. This finding unravels the link between the electron–acoustic phonon coupling and orbital ordering, which is significant to explain the mechanism for the Verwey transition.

Spin relaxation mediated by spin-orbit and acoustic phonon interactions in a single-electron two-dimensional quantum dot

The spin relaxation caused by both spin-orbit and electron-acoustic phonon interactions in a two-dimensional quantum dot embedded into nanosize semiconductor slabs in a perpendicular magnetic field is studied. The spin relaxation rate due to electron coupling to both dilatational and flexural acoustic phonon modes is analytically and numerically evaluated. It is shown that the magnetic field dependency of the spin relaxation rate has allowed and forbidden regions and it can change in a very large region depending on magnetic field. The obtained results show that by means of gate-voltage it is possible to realize the confinement, which makes the required transition rate between certain states available.

Electron Mobility Non-Damping Fluctuations in Semiconductors

Equilibrium state disturbance and restoration processes of an electron gas interacting with phonon system in an equilibrium semiconductor have been investigated. Damping peculiarities of small deviations (fluctuations) of electron system from equilibrium state due to electron-acoustic phonon random scattering have been analyzed. A second-order linear partial differential equation for symmetric component of fluctuation of electron distribution function was obtained via linearization of the Boltzmann equation. This equation describes the chaotic movement of electrons along the energy axis, which can be interpreted as diffusion in momentum space. Another equation which describes the time dependence of electron lattice mobility fluctuations was obtained. It was shown that in the Boltzmann equation linearization approximation, lattice mobility fluctuations do not decay over time. The time dependence of the mobility fluctuations is described by stochastic harmonic function with random amplitude and random initial phase.

Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers

Modern society relies on high charge mobility for efficient energy production and fast information technologies. The power factor of a material—the combination of electrical conductivity and Seebeck coefficient—measures its ability to extract electrical power from temperature differences. Recent advancements in thermoelectric materials have achieved enhanced Seebeck coefficient by manipulating the electronic band structure. However, this approach generally applies at relatively low conductivities, preventing the realization of exceptionally high-power factors. In contrast, half-Heusler semiconductors have been shown to break through that barrier in a way that could not be explained. Here, we show that symmetry-protected orbital interactions can steer electron–acoustic phonon interactions towards high mobility. This high-mobility regime enables large power factors in half-Heuslers, well above the maximum measured values. We anticipate that our understanding will spark new routes to search for better thermoelectric materials, and to discover high electron mobility semiconductors for electronic and photonic applications.

Effect of Ligand Exchange on the Photoluminescence Properties of Cu-Doped Zn-In-Se Quantum Dots

The surface-bound ligands of a semiconductor nanocrystal can affect its electron transition behavior. We investigate the photoluminescence (PL) properties of Cu-doped Zn-In-Se quantum dots (QDs) through the exchange of oleylamine with 6-mercaptohexanol (MCH). Fourier transform infrared and 1H nuclear magnetic resonance spectroscopies, and mass spectrometry reveal that the short-chain MCH molecules are bound to the QD surface. The emission peaks remain unchanged after ligand exchange, and the PL quantum yield is reduced from 49% to 38%. The effects of particle size and defect type on the change in PL behavior upon ligand substitution are excluded through high-resolution transmission electron microscopy, UV–Vis absorption, and PL spectroscopies. The origin of the decreased PL intensity is associated with increased ligand density and the stronger ligand electron-donating abilities of MCH-capped QDs that induce an increase in the nonradiative transition probability. A lower PL quenching transition temperature is observed for MCH-capped QDs and is associated with increasing electron-acoustic phonon coupling due to the lower melting temperature of MCH.

Transport Through a Quantum Dot with Electron-Phonon Interaction

We theoretically study the electrical transport properties of a single level quantum dot connected to two normal conducting leads, which is coupled to the lattice vibrations. We determine the current through the quantum dot in two different situations: time-independent and time-averaged. In all situations we consider three cases: when there is no electron-phonon interaction, when the dot electrons interact with optical phonons or when they interact with acoustic phonons. At finite temperatures we take into account the temperature dependence of the chemical potential. We treat the electron-phonon interaction by the canonical transformation method. In the case of electron-longitudinal optical phonon interaction the spectrum contains a subpeak. In the case of electron-acoustic phonon interaction the spectrum is continuous. In the time-averaged situation many parasite peaks appear in the spectrum, due to the external time-modulation.

Hot electrons in silicon oxide

One particular application of amorphous silicon oxide (SiO2), a material crucial for silicon device technology and design, is as a flash memory tunnel dielectric. The breakdown field of SiO2 exceeds 107 V cm−1. Strong electric fields in SiO2 give rise to phenomena that do not occur in crystalline semiconductors. In relatively weak electric fields (104–106 V cm−1), the electron distribution function is determined by the scattering of electrons on longitudinal optical phonons. In high fields (in excess of 106 V cm−1), the distribution function is determined by electron–acoustic phonon scattering.

Interface defect-assisted phonon scattering of hot carriers in graphene

The broadband and ultrafast photoresponse of graphene has been extensively studied in recent years, although the photoexcited carrier dynamics is still far from being completely understood. Different experimental approaches imply either one of two fundamentally different scattering mechanisms for hot electrons. One is high-energy optical phonons, while the other is disorder-driven supercollisions with acoustic phonons. However, the concurrent relaxation via both optical and acoustic phonons has not been considered so far, hindering the interpretation of different experiments within a unified framework. Here we expand the optical phonon-mediated cooling model, to include electron scattering with the acoustic phonons. By assuming the enhancement of electron-acoustic phonon supercollisions from the localized defect at the photothermoelectric current-generating interface, we provide a broader perspective to the ultrafast photoresponse of graphene, highlighting the previously overlooked effect of the interface for cooling dynamics. We show that the transient photothermoelectric response, which has been attributed exclusively to supercollisions, can be successfully explained without rejecting the established optical phonon relaxation pathway, demonstrating that the two cooling mechanisms are not mutually exclusive but complement each other.

A new relation between the hot electron power loss and acoustic phonon limited mobility in Bloch-Grüneisen regime

Expressions for the electron power loss F(T) and mobility μp due to acoustic phonon scattering are given in the Bloch-Grüneisen (BG) regime for three- and two-dimensional electron gas in semiconductors and Dirac-fermions. We obtain a simple relation F(T) μp = ηevs2, where η (~ 1) is a constant, e is the electron charge and vs is the acoustic phonon velocity. It is found to be independent of temperature and electron concentration. This relation is applied to GaAs heterojucntions and graphene, to obtain μp from the measured F(T). We propose that, using this relation, the measurements of F(T), in BG regime, which depends exclusively upon the electron - acoustic phonon coupling, could serve as a tool to determine the low temperature μp, which is otherwise difficult to measure due to the contributions from the lattice disorders.

Suppression of supercollision carrier cooling in high mobility graphene on SiC( 0001¯ )

Graphene, a two-dimensional monatomic layer of crystal carbon, has recently emerged as a potential material for next-generation optoelectronic devices owing to unique properties arising from its massless Dirac fermions. However, the intrinsic carrier dynamics of graphene has remained a mystery even for the simple case of carrier cooling after the photoexcitation. This is because the observed temporal variations of the nonequilibrium carriers in graphene have been thoroughly described in terms of a defect-induced extrinsic effect known as ``supercollision'' (SC). The SC process is based on defect-mediated electron-acoustic phonon scattering and theoretically has been predicted to reduce with the increase of mobility of a material. Here, the authors have prepared extremely high mobility graphene and traced the dynamics of photoexcited carriers in the Dirac bands directly by time- and angle-resolved photoemission spectroscopy. They successfully observed suppression of SC and extracted the intrinsic dynamical properties of graphene, such as anharmonic decay of the optical phonons and the bottleneck relaxation at the Dirac point. Breaking the limit of SC, their research also has technological significance in developing graphene-based optoelectronic devices.

Temperature dependent energy gap shifts of single color center in diamond based on modified Varshni equation

Although the diamond has the similar structure (diamond cubic) and symmetry (Oh) as many other conventional semiconductor materials, the extraordinary large Debye temperature and small lattice expansion coefficient make the temperature dependence of energy gaps in diamond to be totally different from those in other semiconductors. Here we propose a modified Varshni formula to describe the temperature dependent zero field splitting and zero phonon line of the nitrogen vacancy center in diamond. The shift of energy gaps shows a T4 scaling at low temperature and a T2 scaling at high temperature. The crossover between these two regimes is determined by two positive parameters in the modified formula. This empirical formula can give an excellent prediction of the temperature dependent energy gaps up to 700K. We experimentally verify this temperature dependent behavior with single negatively charged nitrogen vacancy centers in diamond. These observations demonstrate the leading role of electron-acoustic phonon interaction in the temperature dependent shift of energy gaps in diamond. This empirical equation can benefit the diamond-based applications such as quantum sensing and computing.

Temperature-dependent modulus of resilience in metallic solids – Calculated from strain-electron-phonon interactions

Abstract Modulus of resilience ( R ) in metallic solids is calculated based on Helmholtz energy analysis, free electron gas and acoustic phonon approximations: R = C + 2200 −1 (T - T D ) + 2200 −2 (T –T c ) 2 , where T, T D , T c is respectively the test, Debye, and Curie temperature, C is a material-specific constant. The model reproduces well experimental data in commercial titanium-, copper-, and four steels solids, and evaluates HfB 2 where experimental data are missing. It further suggests that only ∼0.1% of all the thermal free electrons/acoustic phonons are mechanically activated during the tensile test, which provides important insights on strain-electron-phonon coupling in solids.

Electron\u2013acoustic phonon coupling in single crystal CH3NH3PbI3 perovskites revealed by coherent acoustic phonons

Despite the great amount of attention CH3NH3PbI3 has received for its solar cell application, intrinsic properties of this material are still largely unknown. Mobility of charges is a quintessential property in this aspect; however, there is still no clear understanding of electron transport, as reported values span over three orders of magnitude. Here we develop a method to measure the electron and hole deformation potentials using coherent acoustic phonons generated by femtosecond laser pulses. We apply this method to characterize a CH3NH3PbI3 single crystal. We measure the acoustic phonon properties and characterize electron-acoustic phonon scattering. Then, using the deformation potential theory, we calculate the carrier intrinsic mobility and compare it to the reported experimental and theoretical values. Our results reveal high electron and hole mobilities of 2,800 and 9,400 cm2 V−1 s−1, respectively. Comparison with literature values of mobility demonstrates the potential role played by polarons in charge transport in CH3NH3PbI3.

Thermoelectric properties of γ-graphyne from first-principles calculations

The two-dimensional graphene-like carbon allotrope, graphyne, has been recently fabricated and exhibits many interesting electronic properties. In this work, we investigate the thermoelectric properties of γ-graphyne by performing first-principles calculations combined with Boltzmann transport theory for both electron and phonon. The carrier relaxation time is accurately evaluated from the ultra-dense electron-phonon coupling matrix elements calculated by adopting the density functional perturbation theory and Wannier interpolation, rather than the generally used deformation potential theory which only considers the electron-acoustic phonon scattering. It is found that the thermoelectric performance of γ-graphyne exhibits a strong dependence on the temperature and carrier type. At an intermediate temperature of 600 K, a maximum ZT value of 0.77 can be achieved for the n-type system and can be further enhanced to 1.4 by considering isotopic effect.