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The stacking of intrinsically magnetic van der Waals materials provides a fertile platform to explore tunable transport effects of magnons, presenting significant prospects for spintronic applications. The possibility of having topologically nontrivial magnons in these systems can further expand the scope of exploration. In this work, we consider a bilayer system with intralayer ferromagnetic exchange and a weak interlayer antiferromagnetic exchange and study the topological magnon-polaron excitations induced by magnetoelastic couplings. Under an applied magnetic field, the system features a metamagnetic transition, where the magnetic ground state changes from antiparallel layers to parallel. We show that the metamagnetic transition is accompanied by a transition of the topological structure of the magnon polarons, which results in discernible changes in the topology induced transport effects. The magnetic-field dependence of the thermal Hall conductivity and spin Nernst coefficient is analyzed with linear response theories. [ABSTRACT FROM AUTHOR]

We analyzed the temperature dependencies of the DC resistivity and Hall coefficient in thin films of samarium nickelate SmNiO3. A dominating hopping conductivity was revealed in the insulating phase below 400 K, which defines an exponential drop in the Hall coefficient with temperature. The estimated dependencies of the hopping activation energy, Hall mobility, and charge carrier density suggest that small polarons are responsible for hopping. The hopping transport explains the observed sign anomaly of the Hall coefficient in rare-earth nickelates. [ABSTRACT FROM AUTHOR]

The generation and manipulation of spin-polarized electrons are the basis of spintronic applications. Spin injection from a ferromagnetic surface or heterojunction to a semiconductor adjacent layer is often used to create spin-polarized carriers; however, for halide perovskites, due to the mismatch of lattice parameters with most ferromagnets at room temperature, this spin injection is challenging. In this study, we synthesize all-inorganic perovskite CsPbBr3 surrounded by Fe3O4 magnetic nanoparticles. It is found that this nanocomposite shows both magnetization and magnetic field-induced circularly polarized photoluminescence at room temperature. Specifically, with the attachment of Fe3O4 nanoparticles closely on the surface of CsPbBr3 nanocrystals, both the degree of circular polarization and g-factor enhanced by 3.5 times compared with that of pure CsPbBr3. The phenomenon should be due to the formation of exciton magnetic polaron through the coupling of the magnetic aligned nanoparticles with the excitonic state of the host semiconductor in the external magnetic field. The effective spin injection provides a method of controlling the excitonic spin polarization of all-inorganic halide perovskites for spintronic applications. [ABSTRACT FROM AUTHOR]

Oxygen vacancies are equilibrium defects in the vanadium pentoxide system that give rise to polaronic hopping transport via V4+ charge compensating defect. In this paper, we report the tunability of polaron formation, the hopping process, and their magnetic signature by substitution of isovalent (5+) phosphorus ions in the V5+ site. The powder x-ray diffraction data show a monotonous shift in lattice parameters with progressive P-doping, confirming the presence of a substitutional dopant. The polaron hopping energy reduced from 0.307 to 0.290 eV depicting a lower defect concentration in P-doping in V2O5. At low temperatures, it is found to obey the Efros–Shklovskii variable range hopping mechanism. The estimated hopping range increased to 1.6 ± 0.1 nm in doped V2O5 in contrast to ∼1.3 nm in the undoped one. The electron spin resonance measurements show a diminishing broad ferromagnetic signal and rising paramagnetic signal (g = 1.97) with progressive P-doping depicting predominant isolated electronic spins in the doped sample. The same is corroborated in room temperature M–H with a distinct hysteresis that diminishes with P-doping and a rise of a paramagnetic slope. Moreover, the reduced oxygen defects and lower V4+ relative occupancy together with fermi level fall toward intrinsic position are substantiated by photoelectron emission studies. [ABSTRACT FROM AUTHOR]

Achieving ferromagnetic ordering in two-dimensional carbon semiconductors like graphdiyne remains a challenge in spintronics. We synthesized Ni-doped graphdiyne (Ni-GDY) using an electrochemical method and found that adjusting the Ni atom concentration allows for a transition from paramagnetism to ferromagnetism, with a high Curie temperature of 175 K. Our density functional theory calculations revealed that the magnetic moment in Ni-GDY arises from Ni quantum dots. At low concentrations, the distant quantum dots result in paramagnetism, while at high concentrations, the formation of bound polarons and long-range exchange coupling through carbon p orbitals leads to ferromagnetism. This study clarifies the contradiction in magnetism observed in various transition metal-doped graphdiyne materials and highlights the potential applications of Ni-doped graphdiyne in electronic devices. [ABSTRACT FROM AUTHOR]

In ABO3 perovskite oxide ferroelectrics, electrical conductivity ranges from insulator- to superconductor-type and is virtually critical for all applications of these materials. Compared to bulk ceramics and crystals, ferroelectric thin films can enable advanced control of the conductivity. Here, small-polaron hopping conductivity was evidenced and examined in various pulsed-laser-deposited films of ferroelectric BaTiO3 and reference films of SrTiO3. For this, AC conductivity was studied in a broad range of temperatures and frequencies for films sandwiched between the bottom and top electrodes. In the BaTiO3 films, with increasing temperature, a significant increase in activation energy for small-polaron hopping was found and ascribed to strong electron–phonon coupling and complex lattice oscillations therein. Plain relations of the activation energy to microstructure, composition, or phase transitions were lacking, which corroborated the critical role of phonons. Additionally, a phonon-less transport was detected. It was anticipated that owing to strong electron–phonon coupling, rich phonon ensembles, and coexistence of phonon-stimulated and phonon-less processes, the small-polaron conductivity can heavily vary in ferroelectric films that necessitates further studies. [ABSTRACT FROM AUTHOR]

We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred to be about 33 meV. This small value of the bandgap renormalization rules out the formation of small polarons and points to weak electron–phonon coupling. The experiments are performed on the nanosheets of the archetypal halide perovskite, CsPbBr3. [ABSTRACT FROM AUTHOR]

Tin sulfide (SnS) is one of the promising materials for the applications of optoelectronics and photovoltaics. This study determines the nematic dynamics of photoexcited electrons and phonons in SnS single crystals using polarization-dependent pump–probe spectroscopy at various temperatures. As well as the fast (0.21–1.38 ps) and slow (>5 ps) relaxation processes, a 36–41 GHz coherent acoustic phonon with a sound velocity of 4883 m/s that is generated by the thermoelastic effect is also observed in the transient reflectivity change (ΔR/R) spectra. Electrons and coherent acoustic phonons show significant in-plane anisotropy from 330 to 430 K due to strong electron–phonon coupling. However, this in-plane anisotropy weakens dramatically in the low-temperature (<330 K) and high-temperature (>430 K) phases. These results add to the knowledge about the anisotropy of electrons and coherent acoustic phonons that give SnS applications in photovoltaic or optoelectronic devices. [ABSTRACT FROM AUTHOR]

A recent experimental work has reported an excess of the non-linear conductivity in the charge density wave (CDW) sliding mode of the quasi one-dimensional compound (TaSe4)2I, when a magnetic field is applied co-linearly to the electric field [Gooth et al., Nature 575, 315 (2019)]. This result has opened a conceptual approach, where the CDW gap in (TaSe4)2I is opened between Weyl fermions of opposite chirality with the assumption that this compound is a Weyl semi-metal in its undistorted high temperature phase. We report measurements in the sliding state of (TaSe4)2I performed in similar conditions. We have found no increase in the magnetoconductivity. In our attempts for understanding this unsettling discrepancy, we stress the specific nature of the Peierls transition in (TaSe4)2I and the strong electron-phonon coupling present in this compound. Given the lack of further evidence, we think that it is premature to assert that (TaSe4)2I is an axionic insulator. [ABSTRACT FROM AUTHOR]

Enhancement of electrical conductivity in fundamentally insulating ABO3 perovskite oxide ferroelectrics is crucial for innovative applications in resistive switching, photovoltaics, and catalysis. One of the methods to raise conductivity in bulk crystals or ceramics relies on the possibility to remove and/or substitute oxygen atoms. Here, we explored this approach for thin films of the representative perovskite oxide SrTiO3. Small-signal AC conductivity was investigated in epitaxial and polycrystalline films, where oxygen vacancies (VO), nitrogen (N), or hydrogen (H) were introduced in situ during film growth. Hopping mechanism of conductivity was evidenced by the observed strong growth of AC conductivity with temperature, frequency, and AC voltage in all films. Small polarons were identified as charge carriers. Oxygen vacancies/substitutions were suggested to facilitate hopping probability by generating sites for carrier localization. Important ferroelectric devices were proposed to benefit from the revealed hopping conductivity owing to its unique increase with an electric field. [ABSTRACT FROM AUTHOR]

A desirable driving force or dissociation energy greater than 0.300 eV is thought to be the prerequisite for an efficient dissociation of exciton in the organic bulk heterojunction photovoltaic system. This criterion has confronted the challenge in some nonfullerene acceptors (NFAs) based solar cells. Factors that govern the dissociation are still under debate. In this work, it is demonstrated that a large photocurrent can be produced by a NFA organic blend with a negligible driving force (0.070 eV) and a small dielectric constant (ε = 4). By evaluating the magnetic field dependent photocurrent density and photoluminescence, we postulate that the spin-dependent polaron pair dissociation at charge transfer states due to a significant singlet generation may prove critical for the photocurrent production. The driving force that originates from the energy offset may play a negligible role in the exciton dissociation. [ABSTRACT FROM AUTHOR]

We present coherent phonon-mediated dynamics for addressable motion transfer in a scalable mechanical transducer consisting of three nearest-neighboring coupled micro-cantilevers. To connect two distant cantilevers without direct interaction, two parametric pumps are applied to couple the cantilevers to an intermediate mechanical mode simultaneously. We demonstrate that the distant cantilevers can be coherently coupled even when the intermediate mode is effectively decoupled as a result of two-pump on-resonance coupling. The phonon-mediated coupling is controlled for long-range transferring of both iterating and intra-mode motions while avoid unwanted exciting of the intermediate mode. [ABSTRACT FROM AUTHOR]

The thermal conductivity of IIa diamond from low temperature (80 K) to room temperature was studied by laser-excited Raman spectroscopy. The laser has two complementary effects, one performing the Raman spectrum and the other resulting in the local heating on the surface of diamond. With the increase in measurement temperature on the surface of IIa diamond, the Raman peak shifts to the low energy side together with intensity quenching due to the strong electron–phonon coupling and lattice expansion, and the thermal conductivity of IIa diamond decreases as T−1, which is attributed to the creation and the annihilation of phonons. [ABSTRACT FROM AUTHOR]

Twisted bilayer and multilayer two-dimensional materials linked by van der Waals interactions exhibit various unique physical properties. The phonon properties of such systems are of great importance, but have not been explored in detail. In this work, we use a hybrid neural-network potential to systematically investigate the evolution of the phonon structure of twisted multilayer graphene under out-of-plane strain. With increasing out-of-plane strain, the evolution of the phonon structure of the moiré superlattice exhibits different behavior from that of AA and AB stacked multilayer graphene. Meanwhile, with twisting of the interlayer, a higher Grüneisen parameter and a lower phonon group velocity can be obtained. A possible method is revealed by which phonon anharmonic scattering in stacked multilayer graphene could be enhanced by varying the twist angle in combination with out-of-plane strain. Our work shows that the application of out-of-plane strain can serve as an effective way to amplify the effect of twist angle on phonon structures of twisted multilayer two-dimensional systems, with potential application to thermoelectric and thermal logical devices. [ABSTRACT FROM AUTHOR]

Observing the phonon blockade in a nanomechanical oscillator is clear evidence of its quantum nature. However, it is still a severe challenge to measure the strong phonon blockade in an optomechanical system with effective nonlinear coupling. In this paper, we put forward a theoretical proposal for predicting the phonon blockade effect in a quadratically coupled optomechanical system by exploiting supervised machine learning. The detected optical signals are injected into the neural network as the input, while the output is the mechanical equal-time second-order correlation. Our results show that our scheme has great advantages in predicting phonon blockade. Specifically, it is effective and feasible for nonlinear coupling systems; it shows a high precision for predicting strong phonon blockade; it is robust against the slight disturbance of systemic parameters. The trained neural network is convenient for measuring phonon blockade directly in the experiment. Our work provides a promising way to predict phonon blockade in nonlinear coupled quantum systems. [ABSTRACT FROM AUTHOR]

We report on the experimental determination of electron–electron conductance at Au/TiOx interfacial regions and electron–phonon coupling of thin TiOx layers for x = 0–2.62. Our study demonstrates that the electronic energy transport mechanisms at metal/metal oxide interfaces are enhanced through metallic defects that lead to electronic band alignment between the metal and metal oxide (in our case, Au and TiOx). Electronic heat transport processes are interrogated via a pump/probe technique, utilizing sub-picosecond laser pulses to monitor the ultrafast thermoreflectance responses of Au/TiOx systems, which were analyzed using a two-temperature model to extract electron–electron conductances at Au/TiOx interfaces and the electron–phonon coupling in TiOx layers. We find that TiOx stoichiometries near TiO2 have ultrahigh electron–phonon coupling factors similar to that of pure Ti and that electronic energy transmission from Au to TiOx layers is comparable to that of Au to Ti due to the presence of Ti0 defects. For x = 2.62 in TiOx, electron–phonon coupling is reduced by more than a factor of 5. Our experimental data are corroborated by real-time time-dependent density functional theory calculations, which show that excited electrons in Au do not participate in the TiOx phonon relaxation process, resulting in lower electron–electron energy transmission from Au and electron–phonon coupling due to the difference in the Fermi energy of Au relative to the conduction band minimum of TiOx when x >2. [ABSTRACT FROM AUTHOR]

We investigate the basic charge and heat transport properties of charge neutral epigraphene at sub-kelvin temperatures, demonstrating a nearly logarithmic dependence of electrical conductivity over more than two decades in temperature. Using graphene's sheet conductance as an in situ thermometer, we present a measurement of electron-phonon heat transport at mK temperatures and show that it obeys the T4 dependence characteristic for a clean two-dimensional conductor. Based on our measurement, we predict the noise-equivalent power of ∼ 10 − 22 W / Hz of the epigraphene bolometer at the low end of achievable temperatures. [ABSTRACT FROM AUTHOR]

We report electron transport measurements in dual-gated monolayer WS2 encapsulated in hexagonal boron-nitride. Using gated Ohmic contacts that operate from room temperature down to 1.5 K, we measure the intrinsic conductivity and carrier density as a function of temperature and gate bias. Intrinsic electron mobilities of 100 cm2/(V s) at room temperature and 2000 cm2/(V s) at 1.5 K are achieved. The mobility shows a strong temperature dependence at high temperatures, consistent with phonon scattering dominated carrier transport. At low temperature, the mobility saturates due to impurity and long-range Coulomb scattering. First-principles calculations of phonon scattering in monolayer WS2 are in good agreement with the experimental results, showing we approach the intrinsic limit of transport in these two-dimensional layers. [ABSTRACT FROM AUTHOR]

Trigonal α − G a 2 O 3 is an ultrawideband semiconductor with potential applications in power electronics and ultraviolet opto-electronic devices. In this Letter, we calculate the low field electron mobility in α − G a 2 O 3 from first principles calculations. The effect of all the 30 phonon modes is taken into account for the transport calculation. The phonon dispersion and the Raman and Infrared spectra are calculated under the density functional perturbation theory formalism and compared with experiments. The electron–phonon interaction (EPI) elements on a dense reciprocal space grid are obtained using the Wannier function interpolation. The full energy dispersion of the phonons is included in both the polar and nonpolar EPI calculations. The electron mobility is then evaluated incorporating the effects of the polar, nonpolar, and ionized impurity scattering using Rode's iterative method. At room temperature, the low field isotropic average electron mobility is estimated to be ∼220 c m 2 / V s predominantly limited by the polar optical phonon scattering at a doping density of 1.0 × 1 0 15 c m − 3 . The anisotropy in the mobility arising from the phonon scattering is also evaluated. Temperature and dopant concentration variation of mobility is also studied, which can help in optimization of the growth for transport measurements. [ABSTRACT FROM AUTHOR]

Graphitic carbon nitride (C2N and C3N) with various π electron distributions on layers have been studied under pressure through a combined theoretical and experimental approach and a comparison with graphite. It is found that as these materials transform into low compressibility phases in the pressure range from 15 to 45 GPa, strong electrostatic repulsion between π electrons and in-plane sp2 electrons may distort and soften the sp2 bonds, leading to anomalous pressure evolutions of the intralayer phonon vibrations, such as a plateau-like behavior of E2g mode (G-band) in C2N and C3N. This also causes a slow increase in the resistivity/resistance of C2N and C3N as pressure increases, and the gradual interlayer bonding leads to an abrupt increase in resistance of the materials but with different pressure responses due to their different π electron distributions. Moreover, the intensity enhancement of the G band in both CN materials may be related to their electronic structure changes. The results deepen our understanding of the effects of π electron distribution on the structural transition of graphitic materials and may explain some unexplained in previous studies. [ABSTRACT FROM AUTHOR]

Tuning the optical, electronic, and long-wavelength properties of group-III nitride alloys can be achieved by alloying AlN with ScN. We report here on the infrared dielectric functions determined from spectroscopic ellipsometry of (0001) wurtzite ScxAl1−xN with compositions of 0 ≤ x ≤ 0.20 grown by molecular beam epitaxy on c-plane sapphire substrates. We also report the optical phonons and their parameters determined in our analysis and compare with those in the previous literature. We find that all phonons shift to a lower wavenumber as a function of scandium incorporation, and we also see evidence of a decrease in crystal quality. Further, we report the high frequency and static dielectric constants and the Born effective charge as well as their evolution with the scandium content. [ABSTRACT FROM AUTHOR]

The weakest mode of the infrared active folded phonons in 4H–SiC, folded transverse acoustic (FTA) mode, has been experimentally studied. The lifetime of the FTA phonons has been determined. The lifetime is 58 ± 4 ps at room temperature and increases to 140 ± 7 ps when the temperature is decreased to 5 K. These results were obtained by the experimental study of far-infrared transmission of the SiC crystal and its theoretical simulation using an oscillator model for the dielectric function around the phonon resonance. The FTA phonon lifetime was a fitting parameter of the model when analyzing the irregular interference pattern in the transmission spectrum measured with a high resolution (0.2 cm−1). Oscillator strength of the FTA phonon resonance has also been determined. It is equal to (6.0 ± 0.5) × 10 − 4 and does not depend on temperature. [ABSTRACT FROM AUTHOR]

Rutile germanium dioxide (r-GeO2) is a recently predicted ultrawide-bandgap semiconductor with potential applications in high-power electronic devices, for which the carrier mobility is an important material parameter that controls the device efficiency. We apply first-principles calculations based on density functional and density functional perturbation theory to investigate carrier-phonon coupling in r-GeO2 and predict its phonon-limited electron and hole mobilities as a function of temperature and crystallographic orientation. The calculated carrier mobilities at 300 K are μ elec , ⊥ c → = 244 cm2 V−1 s−1, μ elec , ∥ c → = 377 cm2 V−1 s−1, μ hole , ⊥ c → = 27 cm2 V−1 s−1, and μ hole , ∥ c → = 29 cm2 V−1 s−1. At room temperature, carrier scattering is dominated by the low-frequency polar-optical phonon modes. The predicted Baliga figure of merit of n-type r-GeO2 surpasses several incumbent semiconductors such as Si, SiC, GaN, and β-Ga2O3, demonstrating its superior performance in high-power electronic devices. [ABSTRACT FROM AUTHOR]

We demonstrate the generation of coherent phonons in a quartz bulk acoustic wave (BAW) resonator through the photoelastic properties of the crystal, via coupling to a microwave cavity enhanced by a photonic lambda scheme. This is achieved by imbedding a single crystal BAW resonator between the post and the adjacent wall of a microwave re-entrant cavity resonator. This 3D photonic lumped LC resonator at the same time acts as the electrode of a BAW phonon resonator and allows the direct readout of coherent phonons via the linear piezoelectric response of the quartz. A microwave pump, ωp, is tuned to the cavity resonance ω0, while a probe frequency, ωprobe, is detuned and varied around the red and blue detuned values with respect to the BAW phonon frequency, Ωm. The pump and probe power dependence of the generated phonons unequivocally determines the process to be electrostrictive, with the phonons produced at the difference frequency between the pump and the probe, with no back action effects involved. Thus, the phonons are created without threshold and can be considered analogous to a passive coherent population trapped maser scheme. [ABSTRACT FROM AUTHOR]

This Letter reports measured cryogenic temperature trends for over 300 longitudinal phonon modes spanning >10 GHz in an epitaxial GaN/NbN/SiC high overtone bulk acoustic resonator (epi-HBAR). We present temperature profiles from 7.2 K to 200 K for the mode frequency (f), the quality factor (Q), the figure of merit (f × Q), and the phonon loss or attenuation coefficient (α). We show that for all m phonon modes, f m T follows an identical parabolic trend, with a zero-slope turnover temperature of 35 K. Thus, the epi-HBAR comb spectrum can be considered an ensemble of modes with the same temperature dependencies, potentially enabling the design of precise multi-mode temperature-stable RF oscillators and clocks operating at GHz frequencies. Using temperature trends for (f × Q) m and α m , we provide strong evidence that the epi-HBARs are fundamentally limited by anharmonic phonon scattering in the materials that make up the epi-HBAR. Crucially, we unambiguously demonstrate the evolution of this anharmonic phonon scattering from the low frequency Akhiezer scattering regime α m ∝ T 1 to the high frequency Landau–Rumer scattering regime α m ∝ T 4 , using hundreds of phonon modes in the same device. Finally, we show that at extremely low temperatures, other emergent loss mechanisms overshadow anharmonic phonon scattering. This finding motivates further investigation into the root causes of these limiting mechanisms for precision RF signal processing, quantum acoustodynamics, and other applications that require extremely low loss micromechanical devices. [ABSTRACT FROM AUTHOR]

Two-dimensional ultrawide bandgap materials have compelling potential advantages in nano high-power semiconductors, deep-ultraviolet optoelectronics, and so on. Recently, two-dimensional few-layer h-BeO predicted as an ultrawide bandgap material has been synthesized in the experiment. In the present work, the first-principles calculations show that monolayer h-BeO has an indirect bandgap of 7.05 eV with the HSE functional. The ultrawide bandgap results from the atomic electronegativity difference in the polar h-BeO. The electronic transport properties are also systematically investigated by using the Boltzmann transport theory. The polar LO phonons generate the macroscopic polarization field and strongly couple to electrons by the Fröhlich interaction. Limited by the electron-phonon scattering, monolayer h-BeO has a high mobility of 452 cm2 V−1 s−1 at room temperature. Further studies indicate that the biaxial tensile strain can reduce the electron effective mass and enhance the electron-phonon coupling strength. A suitable strain promotes the mobility to ∼1000 cm2 V−1 s−1 at room temperature. [ABSTRACT FROM AUTHOR]

Vibrational properties and ultrafast photocarrier dynamics of 2H–TaS2 under pressures are studied by the experiments of Raman scattering and femtosecond time resolved spectroscopy. With increasing pressure, the linewidth of two-phonon Raman mode broadens, especially features with a sudden rapid increase above 10 GPa. Meanwhile, the ultrafast dynamics show that the fast decay through optical phonon scattering is significantly suppressed in amplitude and prolonged in lifetime; however, the weight of the slow decay component originating from acoustic phonon scattering is enhanced at high pressures. Furthermore, the electron-phonon (e-ph) coupling constant, λq, and the density of states (DOS) near the Fermi level, N(0), are evaluated qualitatively, which reveals a drastic decrease in e-ph coupling strength and a sudden increase in the DOS beyond 10 GPa. This turning pressure is consistent with the critical point where T c of superconductivity peaks while the charge density wave (CDW) vanishes, indicating the direct correlations between λq, N(0), superconductivity, and the CDW under pressure. The anomaly of the DOS may imply the occurrence of a topological transition of the electronic structure. Our work not only provides valuable information for the understanding of competition between the CDW and superconductivity in 2H–TaS2 but also may open up a routine to unveil the pressure-induced electronic topological transition. [ABSTRACT FROM AUTHOR]

We report the temperature-dependent phonon modes and interband electronic transitions of InSe films on SiO2/Si substrates prepared by pulsed laser deposition. The microstructure results proved the ε phase structure and monochalcogenide phase composition with well-defined hexagonal InSe sheets. The temperature effects on lattice vibrations were discovered by Raman spectra from 123 K to 423 K. The frequency and full width at half maximum of the A 2 g 1 (LO) mode show a strong nonlinearity with the temperature. The energy band structure and electron–phonon interaction were studied by temperature-dependent spectroscopic ellipsometry with the aid of the Tauc–Lorentz model. It was found that five electronic transitions around 1.33, 1.61, 2.53, 3.73, and 4.64 eV generally show a redshift trend with the temperature. The present results can provide a valuable reference for future optoelectronic applications of InSe films. [ABSTRACT FROM AUTHOR]

We investigated the thermoelectric properties of a metal/semiconductor multilayer by using a simple parabolic band model and two-temperature model. The multilayer enables not only reducing thermal conductivity but also enhancing the power factor by using a nonequilibrium state between electrons and phonons created by the interface. Our results revealed that combining a metal with low lattice thermal conductivity and a semiconductor with high lattice thermal conductivity can compensate for their weak points each other in the thermoelectric properties, resulting in the remarkable enhancement of the figure of merit. [ABSTRACT FROM AUTHOR]

In organic field-effect transistors, conductivity is achieved by electronically injected charges that form high-density accumulation layers. We report self-consistent calculations of Poisson's equation, carrier statistics, and the Drude permittivity of the carrier gas at the interface between semiconductors and insulators. The results show that the injected carriers efficiently screen local potentials. Additionally, the AC permittivity of the carriers reduces electrical fields particularly at frequencies of several THz. This dynamic screening may affect the formation of large polarons and the transient localization of carriers. [ABSTRACT FROM AUTHOR]

We explore the optomechanical coupling in an on-chip lithium niobate (LN) photonic crystal nanocavity. A mechanical frequency shift and nonlinear mechanical oscillations are observed with the intracavity power increasing. A 14th harmonic oscillation is generated at the intracavity power of 430 μW in the LN photonic crystal, which potentially can function as a mechanical frequency comb. The photonic crystal structure also shows the positive temperature coefficient of frequency, while the LN crystal has a negative intrinsic temperature coefficient. These characters of optomechanics in LN may play important roles in photon-phonon coupling or detector systems such as sensitive displacement and the mass and force detection. [ABSTRACT FROM AUTHOR]

In this work, we study the electron mobility of near surface metal organic vapor phase epitaxy-grown InGaAs quantum wells. We utilize Hall mobility measurements in conjunction with simulations to quantify the surface charge defect density. Buried quantum wells are limited by polar optical phonon scattering at room temperature. In contrast, the quantum wells directly at the surface are limited by remote charge impurity scattering from defects situated at the III–V/oxide interface or inside the oxide, showing a mobility of 1500 cm2/V s. Passivating the InGaAs surface by depositing an oxide reduces the amount of defects at the interface, increasing the mobility to 2600 cm2/V s. [ABSTRACT FROM AUTHOR]

We study the electrical and optical properties of degenerate ZnGa2O4 films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire and semi-insulating films grown by pulsed laser deposition (PLD) on fused silica. After a forming-gas anneal at 700 °C, the MOCVD film is highly conducting, with a room-temperature carrier concentration of 2 × 1020 cm−3, a mobility of 20 cm2/V s, and direct bandgap absorptions at 3.65 eV and 4.60 eV. Under the same annealing conditions, the PLD film is semi-insulating, with a direct bandgap absorption at 5.25 eV. The phonon structure, important for electrical and thermal conduction as well as superconductivity and other quantum phenomena, is very complicated due to the large number of atoms (and, thus, phonon branches) in the unit cell. However, we show that the phonon contributions to electron mobility (μph) can be directly measured by quantum-based magnetoconductivity over the temperature span T = 10–200 K. From an approximate analytical formula, μph = function (Tph, T), we calculate an effective phonon energy kTph(T) that takes account of all phonon contributions at temperature T. For T = 10–200 K, the value of kTph ranges from about 10 to 90 meV, consistent with the energy range of the ZnGa2O4 phonon density of states (at 0 K) calculated by density functional theory. The total measured mobility can then be modeled by μtot−1 = μii−1 + μph−1, where μii is the mobility due to ionized-impurity scattering. With a high bandgap, controllable conductivity, high breakdown voltage, and bulk-growth capability, ZnGa2O4 offers opportunities for high-power electronics and UV detectors. [ABSTRACT FROM AUTHOR]

Recent efforts have targeted manipulation of nanomaterial vibrational modes in applications such as chemical/mass sensing, optical switching, and phonon-driven photochemistry. While impulsive photoexcitation can generate coherent phonons, multiple excitation pulses offer the prospect of control and manipulation of coherent phonon modes for functions of optical memory and logic. Here, we use such an approach to inject an arbitrary coherent phonon phase into a colloidal ensemble of highly monodisperse gold bipyramids. We then demonstrate that this technique can be applied to a system that exhibits plasmon-exciton coupling to further manipulate the hybridization of the system. This ability to manipulate acoustic phonons and hybridization can enable optical logic applications of acoustic phonons in addition to optical memory. [ABSTRACT FROM AUTHOR]

Temperature-dependent dielectric spectroscopy was used to observe multiple dielectric responses involving four sets of dielectric relaxations (DRs) in molecular beam epitaxy-deposited ultrathin (001) SrMnO3/Nb:SrTiO3 heterojunctions. Two sets of oxygen-vacancy (OV)-related DRs, which were sensitive to oxygen annealing, were attributed to short-range OV hopping and localized Jahn–Teller (JT)-polaron hopping-induced dipolar-type relaxations. Arising from electron-phonon coupling between eg electrons and JT-distortion in Mn3+ ions, the appearance of JT-polarons was closely related to the presence of oxygen vacancies, which leads to transitions from Mn4+ to Mn3+. [ABSTRACT FROM AUTHOR]

We show that the charge transport properties of single crystals of rubrene can be well described without using polaron theories along the two high-mobility axes. The charge carriers can be considered as holes with coupling to the lattice but not to the degree, which requires the use of polaron theories. It is possible to use the Boltzmann Transport Equation (BTE) in the relaxation time approximation (RTA) to evaluate mobilities due to various scattering mechanisms after introducing a transport reduction factor ( P TRF ). P TRF takes into account the fraction of charge carriers, which have path lengths that are larger than the lattice constant, and permits the use of the BTE in the RTA even when the magnitude of the overall mobility is lower than the value typically required for the use of the BTE. We are then able to calculate mobilities due to various scattering mechanisms. We calculate the effective electron–phonon coupling constant from the published values for various phonon modes. The values of the effective mass from calculations and measurements reported in the literature vary slightly; we assume an intermediate value for the effective mass. With no fitting parameters needed for calculating temperature-dependent mobilities for trap-free crystals, we are able to get excellent agreement with the measured values along the two high-mobility crystallographic directions. In samples, with some trapping, a small density of exponentially distributed trap states is assumed and gives a very good fit to the measured data. Our work provides strong evidence that it is not necessary to invoke polaronic effects to understand charge transport in rubrene crystals. [ABSTRACT FROM AUTHOR]

Coherent acoustic phonons detected by time-domain Brillouin spectroscopy (TDBS) in a liquid medium are strongly attenuated when silica nanoparticles sediment in the liquid. The decrease in the optical signal is due to the strong acoustic scattering by silica nanoparticles whose size matches the wavelength of phonons at the frequency of the Brillouin shift. The control of the acoustic wave-front with nanoparticles thus strongly reduces the undesired Brillouin response of the liquid medium. This is demonstrated with a thin transparent film in water, mimicking a biological sample in a nutritive serum. While strong Brillouin scattering in the particle-free liquid hampers the measurement in the film, we show that TDBS can however be performed in a film of a thickness less than the optical wavelength when scatterers sediment in the liquid. [ABSTRACT FROM AUTHOR]

The phonon-limited mobility in 4H-silicon carbide (SiC) inversion channels was precisely evaluated by employing ultralow net doping concentrations. The measured mobility in the inversion channels of these samples was comparable to the electron mobility in bulk 4H-SiC, and the temperature dependence indicated that the mobility can be ascribed to phonon-scattering-limited mobility. The strong dependence of the mobility on the net doping concentration cannot be explained by Coulomb scattering by dopant impurities. This indicates the existence of scattering origins at the SiO2/SiC interface. Comparison of dry oxidized samples and samples subjected to postoxidation annealing in nitric oxide revealed that the scattering origins were not attributable to trapped electrons at the SiO2/SiC interface states, although the nature of the scattering origins remains unclear. [ABSTRACT FROM AUTHOR]

Coherent vibrations detected in optical experiments can offer insights into material properties and electronic interactions, but also yield complex time-dependent optical signatures, especially in hybridized systems. Here, we adapt techniques from studies on vibrational wave packets in molecules to analyze the optical signatures of coherent acoustic phonons in nanoparticles. This strategy enables us to better understand the implications of energetic changes induced by coherent phonons. We then apply this approach to systems that target coherent acoustic phonons as a route to modulate plasmon-exciton coupling and compare the results to theoretical calculations. Taken together, the described approach provides an intuitive, simple means of analyzing future systems and facilitates attempts to utilize, rather than simply observe, nanomaterial phonon modes. [ABSTRACT FROM AUTHOR]

Surface-enhanced Raman scattering (SERS) spectroscopy with unparalleled sensitivity for fingerprint detection of aromatic dyes is demonstrated by loading highly dense Au@Ag core/shell nanoparticles (NPs) on carbon nanotubes (CNTs). The Raman spectrum of crystal violet molecules adsorbed on the optimal CNT/Au@Ag NPs (8.4% Au and 8.6% Ag) can be distinguished as low as 10−14 M, achieving ultralow SERS detection. Besides electromagnetic enhancement originating from the Au@Ag core-shell, the strong electron-phonon coupling effect in CNTs is highlighted by the formation of more defects via doping bimetallic NPs, further improving SERS activity. Thus, this finding will offer a strategy for boosting SERS performance in widespread applications. [ABSTRACT FROM AUTHOR]

While it is well-known that electron-phonon scattering often determines the electron mobility, its impact on lattice thermal conductivity is less clear. Dominant phonon scattering mechanisms that determine the lattice thermal conductivity have been attributed to phonon-phonon and phonon-defect interactions. However, recent studies in silicon have shown that strong electron-phonon interaction can also lead to significant phonon scatterings at high carrier concentrations. Here, we use first-principles simulations to study thermal transport in SiGe alloys and show that the effect of electron-phonon interaction on thermal transport is even more significant than that in Si because mass disorder scattering leaves long mean free path phonons behind, which are more strongly scattered by electrons. At the carrier concentration of 1 × 1020 cm−3, the room temperature lattice thermal conductivity of the Si0.9Ge0.1 alloy including electron-phonon interaction is only 40% of the value without this interaction. The results show that thermal transport in alloys at a high doping level can be significantly impacted by the free carriers, providing important insights into heat conduction mechanisms in thermoelectric materials which are mostly based on heavily doped alloys. [ABSTRACT FROM AUTHOR]

We report on the effects of the oxygen vacancy (VO) regarding the microstructure, conductivity, and charge transport mechanisms of epitaxial Ba0.6La0.4TiO3−δ (BLTO) films. The VO concentration can be largely regulated from 21.5% to 37.8% by varying the oxygen pressure (PO2) during film deposition. Resistivity-temperature and Hall effect measurements demonstrate that the BLTO films can be tuned remarkably from an insulator to a semiconductor, and even to a metallic conductor by regulating the VO concentration. The role of VO concentration in the charge transport mechanism is clarified. For films with low VO concentration, the charge transport is dominated by variable range hopping (VRH) at low temperatures, and it shows small polaron (SP) hopping at high temperatures. For films with high VO concentration, the carrier transport remains VRH at low temperatures, while it changes to SP hopping at moderate temperatures, and is dominated by thermal phonon scattering at high temperatures. Furthermore, the lower starting temperature of SP hopping for films with higher VO concentrations indicates that VO favors electron-phonon coupling. Different charge transport mechanisms are assumed to be due to different VO-induced defect energy levels in the BLTO films, which has been verified by their soft x-ray absorption spectroscopy results. [ABSTRACT FROM AUTHOR]

Polyaniline (PANI) based memristive devices have emerged as promising candidates for hardware implementation of artificial synapses (the key components of neuromorphic systems) due to their high flexibility, low cost, solution processability, three-dimensional stacking capability, and biocompatibility. Here, we report on a way of the significant improvement of the switching rate and endurance of PANI-based memristive devices. The reduction of the PANI active channel dimension leads to the increase in the resistive switching rate by hundreds of times in comparison with the conventional one. The miniaturized memristive device was shown to be stable within at least 104 cyclic switching events between high- and low-conductive states with a retention time of at least 103 s. The obtained results make PANI-based memristive devices potentially widely applicable in neuromorphic systems. [ABSTRACT FROM AUTHOR]

Organic light-emitting diode (OLED) displays are lighter and more flexible, have a wider color gamut, and consume less power than conventional displays. Stable materials and the structural design of the device are important for OLED longevity. Control of charge transport and accumulation in the device is particularly important because the interaction of excitons and polarons results in material degradation. This research investigated the charge dynamics of OLEDs experimentally and by drift diffusion modeling. Parallel capacitance–voltage measurements of devices provided knowledge of charge behavior at different driving voltages. A comparison of exciplex-forming co-host and single host structures established that the mobility balance in the emitting layers determined the amount of accumulated polarons in those layers. Consequently, an exciplex-forming co-host provides a superior structure in terms of device lifetime and efficiency because of its well-balanced mobility. Minimizing polaron accumulation is key to achieving long OLED device lifetimes. This is a crucial aspect of device physics that must be considered in the device design structure. [ABSTRACT FROM AUTHOR]

The colossal magnetoresistance effect endows La0.7A0.3MnO3 manganites distinctive fascination. Both theoretical and experimental studies demonstrated that the interplay among polarons could significantly influence magnetoresistance. However, the underlying microscopic mechanism of the influence remains elusive due to the lack of experimental evidences. Utilizing ultrafast optical spectroscopy to track the polaron dynamics around Curie temperatures, we observed a diverse two-step recovery process in three sibling manganite thin films with various magnetoresistance effects and Curie temperatures, while the slow step was proposed to be the formation evolution of correlated polarons through the polaron-polaron interaction. Polarons in La0.7A0.3MnO3 equilibrate much faster than those in La0.7(Ca0.58Sr0.42)0.3MnO3 and La0.7A0.3MnO3, indicating a comparatively tighter interaction between polarons and subsequently a stronger magnetoresistance effect. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) is used to directly observe and characterize neutral Mg acceptors (Mg0Ga) in aβ-Ga2O3 crystal. These acceptors, best considered as small polarons, are produced when the Mg-doped crystal is irradiated at or near 77K with x rays. During the irradiation, neutral acceptors are formed when holes are trapped at singly ionized Mg acceptors (Mg-Ga). Unintentionally present Fe3+ (3d5) and Cr3+ (3d3) transition-metal ions serve as the corresponding electron traps. The hole is localized in a nonbonding p orbital on a threefold-coordinated oxygen ion adjacent to an Mg ion at a sixfold-coordinated Ga site. These Mg0Ga acceptors (S=1/2) have a slightly anisotropic g matrix (principal values are 2.0038, 2.0153, and 2.0371). There is also partially resolved 69Ga and 71Ga hyperfine structure resulting from unequal interactions with the two Ga ions adjacent to the hole. With the magnetic field along the a direction, hyperfine parameters are 2.61 and 1.18 mT for the 69Ga nuclei at the two inequivalent neighboring Ga sites. The Mg0Ga acceptors thermally convert back to their nonparamagnetic Mg-Ga charge state when the temperature of the crystal is raised above approximately 250 K. [ABSTRACT FROM AUTHOR]

Measurements of the real and imaginary parts of the dielectric permittivity of reduced Ti-doped lithium niobate reveal a low-frequency dielectric relaxation as well as a strong increase in the conductivity compared to undoped samples. The low-frequency permittivity is increased by about an order of magnitude due to the relaxation. The analysis of the temperature-dependence of the relaxation frequency reveals a thermally activated behavior with an activation energy of Ea=0.65 eV, which is in agreement with a transition between NbLi4+ small bound polarons and NbNb4+ free polarons. Both low-field ac conductivity and high-field dc conductivity indicate a Poole-Frenkel-type electron hopping mechanism between NbLi4+ sites at low temperature, turning into similar hopping between NbNb4+ at high temperatures. [ABSTRACT FROM AUTHOR]

Environmental friendly, non-toxic double perovskite BaxSr2-xTiCoO6 compositions with 0≤x≤0.2 were synthesized using solid-state reaction route for high temperature thermoelectric (TE) applications. XRD and SEM studies confirmed the presence of single-phase solid solution with highly dense microstructure for all the oxide compositions. Temperature dependent electrical conductivity measurement showed semiconductor to metal (M-S) transition in these double perovskites. Incorporation of barium in Sr2TiCoO6 pushed M-S transition to higher temperature making it a potential candidate for high temperature TE applications. Conductivity behaviors of these oxides were explained by small polaron model. Furthermore, these oxides exhibit a glass like behavior resulting in low thermal conductivity. Low temperature dielectric measurement revealed relaxor ferroelectric behavior in these oxides below room temperature. Transition of these relaxors into a glassy state beyond Burns temperature (TD) was found responsible for having low thermal conductivity in these oxides. Maximum dimensionless TE figure-of-merit ZT=0.29 at 1223K was achieved for BaxSr2-xTiCoO6 composition with x=0.2. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

We determine both the lifetime and the mobility of photoexcited charge carriers in Sn2P2S6 using holographic time of flight in diffusion mode. From the build-up dynamics of space charge gratings created by pulsed interband illumination at 532 nm, we find a charge carrier lifetime of 2.6±0.5 ns and a charge carrier mobility along the x-axis of 2.4±0.6 cm2V-1s-1 at room temperature. This mobility is assigned to holes as more mobile carriers in tin hypothiodiphosphate (SPS), and it is found to decrease for decreasing temperature, a behavior consistent with small-polaron hopping. [ABSTRACT FROM AUTHOR]

We determine both the lifetime and the mobility of photoexcited charge carriers in Sn2P2S6 using holographic time of flight in diffusion mode. From the build-up dynamics of space charge gratings created by pulsed interband illumination at 532 nm, we find a charge carrier lifetime of 2.6±0.5 ns and a charge carrier mobility along the x-axis of 2.4±0.6 cm2V-1s-1 at room temperature. This mobility is assigned to holes as more mobile carriers in tin hypothiodiphosphate (SPS), and it is found to decrease for decreasing temperature, a behavior consistent with small-polaron hopping. [ABSTRACT FROM AUTHOR]