Intervalley Transitions Research Articles

THz generation by AlGaAs/GaAs heterostructured p-i-n diode

The generation of terahertz radiation by heterostructure p-i-n AlxGa1−xAs/GaAs diodes excited by femtosecond optical pulses was studied experimentally and using the Monte Carlo method. It is shown that when the reverse bias varies, the terahertz generation mechanism changes. With a positive bias on the p-i-n diode, the THz generation mechanism is due to the reflection of the photoexcited electrons from the interface. With a large internal electric field, THz generation in the p-i-n diode occurs due to the acceleration of electrons at the ballistic stage of their movement in the electric field to velocities significantly exceeding the steady state velocity (“velocity overshoot”). The subsequent sharp decrease in velocity of electrons is associated with their inter-valley transitions from the Γ-valley to the L-valley of the conduction band. At electric fields less than 22 kV/cm, the effect of electric field screening by photoexcited carriers has a significant impact on the formation of photocurrent and, accordingly, on the THz generation mechanism. As the reverse bias decreases, this effect leads to a shift in the maximum of the THz pulse toward shorter times and it begins to dominate at electric fields less than 10 kV/cm.

2D‐Material‐Fused High‐Emittance Plasmo–Photonic Metasurfaces

AbstractTransition metal dichalcogenides (TMDC) are a family of atomic‐layer 2D materials (2dMs). Many researches have attempted to reinforce the unique properties by incorporating artificially designed nanostructures. Here, a highly photoluminescence (PL) enhancing platform is reported that is formed by combining high‐emittance plasmo–photonic metasurfaces with a large‐area 2dM, which is a continuous monolayer of TMDC of cm2 dimension. It is experimentally demonstrated that the 2dM‐PL intensity at room temperature is enhanced by more than 1030‐fold in comparison with that observed on the flat gold film regions. The enhancement is associated with spectral transformation indicating a resonant effect of the metasurfaces. It is moreover revealed that the confocal PL image originates from a coherent single‐mode emission that forms a photon number distribution obeying the Poisson probability distribution. Thus, the 2dM‐fused metasurfaces function as efficient coherent quantum light sources. PL decay‐time analysis also reveals the coexistence of two components in the enhanced PL. The fast component exhibits a lifetime of 18 ps whereas the slow component decays at 4.4 ns. Both components are most likely to undergo radiative processes, which indicates that metal‐induced PL quenching is suppressed on these 2dM‐fused metasurfaces. The long lifetime component is ascribed to an excitonic intervalley transition.

Valley filters using graphene blister defects from first principles

Valleytronics, which makes use of the two valleys in graphenes, attracts considerable attention and a valley filter is expected to be the central component in valleytronics. We propose the application of the graphene valley filter using blister defects to the investigation of the valley-dependent transport properties of the Stone–Wales and blister defects of graphenes by density functional theory calculations. It is found that the intervalley transition from the K valley to the valleys is completely suppressed in some defects. Using a large bipartite honeycomb cell (BHC) including several carbon atoms in a cell and replacing atomic orbitals with molecular orbitals in the tight-binding model, we demonstrate analytically and numerically that the symmetry between the A and B sites of the BHC contributes to the suppression of the intervalley transition. In addition, the universal rule for the atomic structures of the blisters suppressing the intervalley transition is derived. Furthermore, by introducing additional carbon atoms to graphenes to form blister defects, we can split the energies of the states at which resonant scattering occurs on the and channel electrons. Because of this split, the fully valley-polarized current will be achieved by the local application of a gate voltage.

First-principles analysis of intravalley and intervalley electron-phonon scattering in thermoelectric materials

Intervalley collisions, which scatter electrons from one valley or band to another, can be detrimental to thermoelectric performance in materials with multiple valleys/bands. In this study, density functional theory is used to investigate the electron-phonon scattering characteristics of three lead chalcogenides (PbS, PbSe, PbTe) and three half-Heuslers (ScNiBi, ScPdSb, ZrNiSn), which all possess multiple equivalent conduction valleys, in order to characterize and analyze their intravalley/intervalley components. To elucidate what controls the degree of intravalley and intervalley transitions, the scattering rates are decomposed into the product of the phase space (a measure of how much scattering is possible) and the average electron-phonon coupling. To help guide the search for improved thermoelectric and high-conductivity materials, simple and approximate approaches are demonstrated that can be adopted to identify materials with reduced intervalley scattering, which circumvent the need for computationally demanding electron-phonon scattering calculations. In addition, the benefits of selecting materials with large-energy zone-edge phonons are explored in the limit $\ensuremath{\hbar}\ensuremath{\omega}$ $\ensuremath{\gg}$ ${k}_{B}T$, and found to potentially suppress intervalley processes by up to an order of magnitude, leading to a 70% and 100% increase in conductivity and power factor, respectively.

Interlayer RKKY interaction in ferromagnet/tilted Weyl semimetal/ferromagnet trilayer system

We have investigated the interlayer RKKY interaction in ferromagnet/Weyl semimetal/ferromagnet (FM/WSM/FM) trilayer system in the presence of the tilting effect of Weyl cones. Our investigation focuses on the influence of the unique features of WSMs: anisotropy dispersion, tilt effect, intervalley transitions, and Fermi arc states. Due to the separation of Weyl nodes, the RKKY behaviors, such as spin model, decaying rate, and oscillation period, heavily depend on the stacked direction of FMs, exhibiting strong magnetic anisotropy. Tilt of Weyl cones can increase the RKKY amplitude, change spin model, switch the antiferromagnetism and ferromagnetism, and more importantly generate the noncollinear Dzyaloshinskii-Moriya or spin-frustrated terms. Intervalley transition generates extra oscillation with period determined by separated distance between Weyl nodes, which together with Fermi-energy-induced oscillation leads to an interesting battering pattern. These signatures are significantly different from the interlayer RKKY interaction with single-node topological insulators or impurity RKKY case reported in previous literatures. More importantly, we find that the interlayer RKKY interaction decays more slowly with the spacer thickness than one reported extensively in WSMs doped with magnetic impurities, which is explained by providing a general formula. In addition, we discuss the effect from Weyl Fermi arcs. Compared to the bulk contributions, a more slowly decaying rate is found at the edges of ferromagnetic layer if the edges are parallel to the line connecting two Weyl nodes. Our research suggests that trilayer FM/WSM/FM is a more powerful platform to understand the WSM magnetic property and to extract the parameters of WSM materials by measuring the RKKY interaction.

Electron velocity distribution on the abrupt change in source–drain current of GaN devices

An analytical model of the channel electron energy distribution in an on-state GaN transistor has been proposed based on the assumption that drift velocities of channel electrons obey the two-dimensional Maxwell–Boltzmann distribution. The validity of such an assumption was confirmed by Monte Carlo simulation. It was found that there could be a larger number of high-energy channel electrons whose energy is higher than the intervalley energy between [Formula: see text] and [Formula: see text] valleys in a GaN transistor with a high electron temperature. The fraction of hot electrons with its energy higher than the intervalley energy between [Formula: see text] and [Formula: see text] valleys to the total channel electrons can easily reach 50% when the electron temperature is higher than 3000 K. Such an electron temperature in a GaN transistor had been determined in experiments. Thus, hot electrons in the [Formula: see text] valley can transit into [Formula: see text] valleys. It suggests that intervalley transitions could be one possible physical origin of the abrupt change in the source−drain current in GaN devices. The proposed model can well explain how an abrupt change in the source–drain current in GaN transistor experiments depends on the voltage-dependent gate, the trap, etc.

Unidirectional valley-contrasting photocurrent in strained transition metal dichalcogenide monolayers

We examine the full static non-linear optical response of uniaxially strained transition metal dichalcogenide monolayers doped with a finite carrier density in the conduction band, in the presence of disorder. We find that the customary shift current is suppressed, yet we identify a strong, valley-dependent non-reciprocal response, which we term a \textit{unidirectional valley-contrasting photo-current} (UVCP). This DC current originates from the combined effect of strain and Kramers symmetry breaking by trigonal warping, while the contributions due to individual valleys can be separated by introducing an energy offset between them by means of a magnetization. This latter fact enables one to monitor inter-valley transitions. The UVCP is proportional to the mobility and is enhanced by the excitonic Coulomb interaction and inter-valley scattering, as well as by a top gate bias. We discuss detection strategies in state-of-the-art experiments.

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.

Electrostatic control of photoluminescence from A and B excitons in monolayer molybdenum disulfide.

Tailoring excitonic photoluminescence (PL) in molybdenum disulfide (MoS2) is critical for its various applications. Although significant efforts have been devoted to enhancing the PL intensity of monolayer MoS2, simultaneous tailoring of emission from both A excitons and B excitons remains largely unexplored. Here, we demonstrate that both A-excitonic and B-excitonic PL of chemical vapor deposition (CVD)-grown monolayer MoS2 can be tuned by electrostatic doping in air. Our results indicate that the B-excitonic PL changed in the opposite direction compared to A-excitonic PL when a gate voltage (Vg) was applied, both in S-rich and Mo-rich monolayer MoS2. Through the combination of gas adsorption and electrostatic doping, a 12-fold enhancement of the PL intensity for A excitons in Mo-rich monolayer MoS2 was achieved at Vg = −40 V, and a 26-fold enhancement for the ratio of B/A excitonic PL was observed at Vg = +40 V. Our results demonstrate not only the control of the conversion between A0 and A−, but also the modulation of intravalley and intervalley conversion between A excitons and B excitons. With electrostatic electron doping, the population of B excitons can be promoted due to the enhanced intravalley and intervalley transition process through electron–phonon coupling. The electrostatic control of excitonic PL has potential applications in exciton physics and valleytronics involving the B excitons.

Uniaxial strain-induced electronic property alterations of MoS2 monolayer

Molybdenum disulfide (MoS2) has attracted interests owing to its strain-tuned electronic and optical properties, making it a promising candidate for applications in strain engineering devices. In this study, we investigate the effect of uniaxial strain on the electronic properties of MoS2 monolayer using first-principles calculations. Results show that a crossover of the K–K direct to Γ–K indirect bandgap transitions occurs at a strain of 1.743%. Moreover, a strong correlation is observed between the modified bandgap and the density of states (DOS) of the Mo–4d and S-3p orbitals at the valence band maximum and conduction band minimum. The uniaxial strain–tuned interatomic distance along the a-crystallographic axis not only alters the bandgap at different rates but also affects the DOS of the Mo–4d orbital and possible electronic transitions. This study clarifies the mechanism of the electronic structural modification of two-dimensional MoS2 monolayer, which may affect intervalley transitions.

Effect of Transition Mechanism of Photon-Induced Carriers on Time Jitter of GaAs Photoconductive Semiconductor Switches

To investigate the effect of photon-induced carrier transition mechanism on switch stability, the time jitters of 2-mm-gap gallium arsenide photoconductive semiconductor switches (GaAs PCSSs) are compared when triggered by lasers of two different wavelengths, i.e., 1064 and 532 nm. The time jitter model is proposed to clarify the synergy of the multiple adjustable macroparameters of trigger conditions. The carrier valley occupation rates with respect to bias electric field are simulated, where the influence of the microprocedures, i.e., carrier generation, carrier intervalley transition, and carrier transportation, is considered. Our experimental results illustrate that the time jitter of the GaAs PCSS exhibits a nonmonotonous behavior and obtains a local minimum at ~3 kV/cm for 1064 nm; however, it shows a monotonous decrease to a constant value at about 3 kV/cm for 532 nm. Our analysis indicates that the discrepancy in GaAs PCSS time jitter is attributed to the difference in the relative fluctuation of the carrier velocity affected by the carrier valley occupation rate. This article is of great significance in optimizing the stability of GaAs PCSS and pulse power system by adjusting the macroparameters in the time jitter model.

Междолинные процессы релаксации состояний мелких доноров в германии

The role of the intervalley processes of electron-phonon interaction in the relaxation of excited shallow arsenic donors in germanium is analyzed. The rates of intracenter inter-valley transitions with emission of TA phonons in ger-manium are calculated in dependence on the uniaxial compression stress along {111} crystallographic direction. It is shown that inter-valley transitions to the ground state of the donor with emission of phonons can play a significant role in the relaxation of excited impurities only upon uniaxial stress of the crystal, since at zero stress, there are no exact resonances between impurity transitions and intervalley phonons. There are also transitions from highly excited states lying in a narrow band of energies (~ 0.5 meV) under very bottom of the conduction band to the first excited state 1s(3)(Г5) (in stressed germanium crystal to 1s(3)(Г3) state). The average rate of these transitions is estimated at 0.3×109 s-1.

Multipath Optical Recombination of Intervalley Dark Excitons and Trions in Monolayer WSe_{2}.

Excitons and trions (or exciton polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe_{2}. We observe the intervalley dark excitons, which can recombine by the assistance of defect scattering or chiral-phonon emission. We also reveal that a trion can decay in two distinct paths-through intravalley or intervalley electron-hole recombination-into two different final valley states. Although these two paths are energy degenerate, we can distinguish them by lifting the valley degeneracy under a magnetic field. In addition, the intra- and inter-valley trion transitions are coupled to zone-center and zone-corner chiral phonons, respectively, to produce distinct phonon replicas. The observed multipath optical decays of dark excitons and trions provide insight into the internal quantum structure of trions and the complex excitonic interactions with defects and chiral phonons in monolayer valley semiconductors.

Electronic and optical conductivity of Kekulé-patterned graphene: Intravalley and intervalley transport

A Kubo formalism is used to calculate the electronic and optical conductivity of a graphene superlattice with Y-shaped kekul\'e bond texture, similar to that visualized in recent experiments. We show that new conduction channels between the valleys in graphene are opened by the kekul\'e distortion. This intervalley contribution to the electronic transport is not present in pristine graphene and here appears due to the folding of the Dirac cones $K$, $K'$ on top of each other. The contribution of intervalley transport to the conductivity of graphene, as well as the modification of the intravalley transport, are analyzed in detail for different frequency, temperature, chemical potential and scattering rate limits. We obtain analytical expressions for the conductivity that reproduce previous expressions used to fit experimental measurements in graphene and compare with direct numerical evaluations of the Kubo formula, finding great agreement. The optical absorption arising from intervalley transitions displays a maximum at a special frequency that can be tuned by doping. Our results show how the single parameter describing the valley coupling in this system could be obtained by measuring graphene's optical absorbance in the region where interband transitions are blocked by the impurities. Finally, we use Fermi's golden rule to independently verify some of the previous results.

Spin-valley system in a gated MoS2-monolayer quantum dot

The aim of presented research is to design a nanodevice based on a gate-defined quantum dot within a MoS2 monolayer in which we confine a single electron. By applying control voltages to the device gates we modulate the confinement potential and force intervalley transitions. The present Rashba spin–orbit coupling additionally allows for spin operations. Moreover, both effects enable the spin-valley SWAP. The device structure is modeled realistically, taking into account feasible dot-forming potential and electric field that controls the Rasha coupling. Therefore, by performing reliable numerical simulations, we show how by electrically controlling the state of the electron in the device, we can obtain single- and two-qubit gates in a spin-valley two-qubit system. Through simulations we investigate possibility of implementation of two qubits locally, based on single electron, with an intriguing feature that two-qubit gates are easier to realize than single ones.

Spin and valley waves in Dirac semimetals with population imbalance

We find an intervalley wave collective mode in two- and three-dimensional Dirac semimetals in the presence of a valley population imbalance. The dispersion relation of this mode is gapless, proportional to the square of the wave vector at small frequencies, and inversely proportional to the electron-electron exchange interaction energy. The valley wave serves as an energy gain source for the external field, that generates the intervalley transitions. The spin wave analog is discussed for the case of a semimetal with nonequilibrium spin orientation.

A Quasi-Two-Dimensional Physics-Based Model of HEMTs without Smoothing Functions for Joining Linear and Saturation Regions of I-V Characteristics

A quasi-two-dimensional physics-based model of HEMT transistor without using any smoothing functions for joining the linear and saturation regions of current-voltage (I-V) characteristics was developed. Considering the intervalley transitions of electrons and the presence of holes in the channel of transistor, we calculated the nonuniform spatial distributions of the electrical field, electron temperature, and electron mobility within the channel. The model is in a good agreement with experimental data over the linear and saturation regions of operation. The model provides precise simulating of HEMT transistors and can be utilized as a tool for analysis and prediction of influence of the material parameters on device and circuit characteristics.

Research on Time Jitter of GaAs Photoconductive Semiconductor Switches in the Negative Differential Mobility Region

Time jitter of GaAs photoconductive semiconductor switches (PCSS) is investigated at an optical excitation of 1053-nm wavelength and 500-ps pulse duration. The experimental results indicate that the time jitter of the GaAs PCSS exhibits a nonmonotonic variation in the negative differential mobility (NDM) region. All of these time jitters are lower than 4% of the rise time of the switching waveform. The optimum time jitter of ~15 ps is achieved at the onset of the NDM region. The theoretical relationship between the optical excitation parameters, the bias electric field, and the time jitter of the GaAs PCSS is built up. The nonmonotonic behavior of the time jitter with electric field is attributed to the instability of the relative fluctuation of drift velocity caused by inter-valley transition of carriers in GaAs.

Two Topics of Optical Excitation Dynamics, Newly Unveiled by the Time- and Momentum-Resolved Photo-Electron Emission from the Conduction Band of GaAs: A Theoretical Review

We review the recent two topics of optical excitation and relaxation dynamics, newly unveiled by the time- and momentum-resolved photo-electron emission from the conduction band of GaAs. One is the real-time collective relaxation dynamics, resulting in the Fermi degeneracy formation in the Γ valley. We show that it takes almost infinite time to realize the exact Fermi degeneracy, due to a restricted selection rule for the intravalley transition of the photo-excited electrons. The other is the spontaneous and instantaneous intervalley transition from the Γ valley to the L one. By considering the electron-phonon coupling before the photo-excitation, such spontaneous intervalley transition is realized within the framework of the Franck–Condon principle of the photo-excitation.

Simultaneously high electron and hole mobilities in cubic boron-V compounds: BP, BAs, and BSb

Through first-principles calculations, the phonon-limited transport properties of cubic boron-V compounds (BP, BAs and BSb) are studied. We find that the high optical phonon frequency in these compounds leads to the substantial suppression of polar scattering and the reduction of inter-valley transition mediated by large-wavevector optical phonons, both of which significantly facilitate charge transport. We also discover that BAs simultaneously has a high hole mobility (2110 cm2/V-s) and electron mobility (1400 cm2/V-s) at room temperature, which is rare in semiconductors. Our findings present a new insight in searching high mobility polar semiconductors, and point to BAs as a promising material for electronic and photovoltaic devices in addition to its predicted high thermal conductivity.