Valley Separation Research Articles

Optical valley separation in two-dimensional semimetals with tilted Dirac cones

Quasiparticles emerging in crystalline materials can possess a binary flavor known as the valley quantum number which can be used as a basis to encode information in an emerging class of valleytronic devices. Here we show that two-dimensional semimetals with tilted Dirac cones in the electronic band structure exhibit spatial separation of carriers belonging to different valleys under illumination. In stark contrast to gapped Dirac materials this optovalleytronic phenomenon occurs in systems with intact inversion and time-reversal symmetry that host gapless Dirac cones in the band structure, thereby retaining the exceptional graphene-like transport properties. We thus demonstrate that optical valley separation is possible at arbitrarily low photon frequencies including the deep infrared and terahertz regimes with full gate tunability via Pauli blocking. As a specific example of our theory, we predict tunable valley separation in the proposed two-dimensional tilted Dirac cone semimetal 8-Pmmn borophene for incident infrared photons at room temperature. This work highlights the potential of two-dimensional tilted Dirac cone materials as a platform for tunable broadband optovalleytronic applications.

Electron-Induced Chirality-Selective Routing of Valley Photons via Metallic Nanostructure.

Valleytronics in 2Dtransition metal dichalcogenides has raised a great impact in nanophotonic information processing and transport as it provides the pseudospin degree of freedom for carrier control. The imbalance of carrier occupation in inequivalent valleys can be achieved by external stimulations such as helical light and electric field. With metasurfaces, it is feasible to separate the valley exciton in real space and momentum space, which is significant for logical nanophotonic circuits. However, the control of valley-separated far-field emission by a single nanostructure is rarely reported, despite the fact that it is crucial for subwavelength research of valley-dependent directional emission. Here, it is demonstrated that the electron beam permits the chirality-selective routing of valley photons in a monolayer WS2 with Au nanostructures. The electron beam can locally excite valley excitons and regulate the coupling between excitons and nanostructures, hence controlling the interference effect of multipolar electric modes in nanostructures. Therefore, the separation degree can be modified by steering the electron beam, exhibiting the capability of subwavelength control of valley separation. This work provides a novel method to create and resolve the variation of valley emission distribution in momentum space, paving the way for the design of future nanophotonic integrated devices.

Г-L intervalley separation and electron mobility in GaAsSb grown on InP: Transport comparison with the GaInAs and GaInAsSb alloys

GaAs0.51Sb0.49 is lattice-matched to InP and finds electron transport applications in base or absorber layers in high-speed heterostructure bipolar transistors or photodiodes, because its staggered (“type-II”) band alignment with InP favors electron injection across abrupt heterojunctions. Little remains known about electron transport properties and band structure details of GaAsxSb1−x near x = 0.5. Particularly, based on the Γ-L intervalley separation in binary constituents (ΔΓL = 84 meV in GaSb and 290 meV in GaAs at 300 K), interpolation suggests a low Γ-L separation in GaAs0.51Sb0.49 before considering energy gap bowing effects. To gain insight into electron transport in GaAs0.51Sb0.49, we characterized experimental Hall electron mobilities vs carrier concentration at 300 and 77 K in n-type GaAs0.51Sb0.49, Ga0.47In0.53As, and Ga0.76In0.24As0.67Sb0.33 alloys nearly matched to InP. In marked contrast to the other two alloys, GaAs0.51Sb0.49 exhibits a sharp rise in 77 K electron mobility, which evidences L-valley de-population for lower electron concentrations. A two-band transport analysis reveals a Γ-L valley separation ΔΓL = 91 meV at 77 K, significantly lower than values recommended in the literature. Based on the reported temperature variations of ΔΓL in GaAs and GaSb, 84 < ΔΓL < 95 meV is expected at 300 K. The corresponding GaAsxSb1−x L-valley bowing parameter is cL = 1.63 eV, significantly higher than the 1.1–1.2 eV recommended in the literature. In contrast to GaAsxSb1−x, GaInAsSb grown on InP displays a strong alloy scattering, which limits its low-temperature electron mobility.

Understanding high-field electron transport properties and strain effects of monolayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides (MX2) are promising candidates for future electronics. Although the transport properties (e.g. mobility) at low electric field have been widely studied, there are limited studies on high-field properties, which are important for many applications. Particularly, there is lack of understanding of the physical origins underlying the property differences across different MX2. Here by combining first-principles calculations with Monte Carlo simulations, we study the high-field electron transport in defects-free unstrained and tensilely strained MX2 (M=Mo, W and X=S, Se). We find that WS2 has the highest peak velocity (due to its smallest effective mass) that can be reached at the lowest electric field (owing to its highest mobility). Strain can increase the peak velocity by increasing the scattering energy. After reaching the peak velocity, most MX2 demonstrates negative differential mobility (NDM). WS2 shows the largest NDM among unstrained MX2 due to the strongest effect of electron transfer from the low-energy small-mass valley to the high-energy large-mass valley. The tensile strain increases the valley separation, which on one hand suppresses the electron transfer in WS2, on the other hand allows the electrons to access the non-parabolic band region of the low-energy valley. The latter effect leads to an NDM for electrons in the low-energy valley, which can significantly increase the overall NDM at moderate strain. The valley-separation induced NDM in the low-energy valley is found to be a general phenomenon. Our work unveils the physical factors underlying the differences in high-field transport properties of different MX2, and also identifies the most promising candidate as well as effective approach for further improvement.

Photonic Rashba effect from quantum emitters mediated by a Berry-phase defective photonic crystal.

Heterostructures combining a thin layer of quantum emitters and planar nanostructures enable custom-tailored photoluminescence in an integrated fashion. Here, we demonstrate a photonic Rashba effect from valley excitons in a WSe2 monolayer, which is incorporated into a photonic crystal slab with geometric phase defects, that is, into a Berry-phase defective photonic crystal. This phenomenon of spin-split dispersion in momentum space arises from a coherent geometric phase pickup assisted by the Berry-phase defect mode. The valley excitons effectively interact with the defects for site-controlled excitation, photoluminescence enhancement and spin-dependent manipulation. Specifically, the spin-dependent branches of photoluminescence in momentum space originate from valley excitons with opposite helicities and evidence the valley separation at room temperature. To further demonstrate the versatility of the Berry-phase defective photonic crystals, we use this concept to separate opposite spin states of quantum dot emission. This spin-enabled manipulation of quantum emitters may enable highly efficient metasurfaces for customized planar sources with spin-polarized directional emission.

Chiral Discrimination of DL-Amino Acids by Trapped Ion Mobility Spectrometry after Derivatization with (+)-1-(9-Fluorenyl)ethyl Chloroformate.

A novel analytical method based on hybrid trapped ion mobility spectrometry-time-of-flight mass spectrometry (TIMS-TOFMS) has been developed to achieve fast enantiomeric separation of amino acids (AAs). Resolution of chiral AAs was achieved by forming diastereomers through derivatization with the chiral agent (+)-1-(9-fluorenyl)ethyl chloroformate (FLEC), avoiding the use of reference compounds. Electrospray ionization (ESI) in positive mode yielded sodiated FLEC-AAs ions of which the diastereomers could be separated by TIMS. The effect of other alkali metal ions (such as Li and K) on the enantioselectivity was studied, but chiral discrimination was only observed for Na. TIMS conditions, including voltage ramp, ramp time, and accumulation time were optimized for each AA, and collision cross sections (CCSs) were determined for all diastereomers. The migration order of the DL enantiomers was found to be dependent on the structure of the AA. The resulting TIMS resolution (K0/ΔK0) for the FLEC-AA diastereomers on average was 115, requiring a mobility (K0) difference of about 0.009 cm2/(V s) to achieve 50%-valley separation. From the 21 AAs studied, enantiomer separation was achieved for 17 AAs with mobility differences ranging from 0.009 for lysine up to 0.061 cm2/(V s) for asparagine. Moreover, the presented methodology provided mutual separation of various AAs, allowing chiral analysis of multiple AAs simultaneously which may be challenging with previous enantioselective IMS approaches. It appeared possible to fully resolve all studied DL-AAs using three distinct TIMS methods, resulting in a total MS run time of about 3 min (1 min per method) and a total analysis time (including derivatization) of less than 15 min. The method demonstrated capable to determine enantiomeric ratios down to 2.5% with detection limits for the D enantiomers in the nanomolar range. This new TIMS-based methodology opens up possibilities for easy and fast analysis of AA enantiomers.

Strain-controlled valley and spin separation in silicene heterojunctions

We adopt the tight-binding mode-matching method to study the strain effect on silicene heterojunctions. It is found that valley- and spin-dependent separation of electrons cannot be achieved by the electric field only. When a strain and an electric field are simultaneously applied to the central scattering region, not only are the electrons of valleys K and K' separated into two distinct transmission lobes in opposite transverse directions, but the up-spin and down-spin electrons will also move in the two opposite transverse directions. Therefore, one can realize an effective modulation of valley- and spin-dependent transport by changing the amplitude and the stretch direction of the strain. The phenomenon of the strain-induced valley and spin deflection can be exploited for silicene-based valleytronics devices.

Strained-graphene-based highly efficient quantum heat engine operating at maximum power.

A strained graphene monolayer is shown to operate as a highly efficient quantum heat engine delivering maximum power. The efficiency and power of the proposed device exceeds that of recent proposals. The reason for these excellent characteristics is that strain enables complete valley separation in transmittance through the device, implying that increasing strain leads to very high Seebeck coefficient as well as lower conductance. In addition, since time-reversal symmetry is unbroken in our system, the proposed strained graphene quantum heat engine can also act as a high-performance refrigerator.

Spatial valley separation in strained graphene pn junction

Valleytronics in analogy to spintronics aims to use the electron valley degree of freedom to carry and manipulate information, and one of urgent tasks in this field is to generate valley-polarized electrons. In this work, we propose using the electron focusing effect in a strained graphene pn junction to separate valleys spatially through a beam of valley-unpolarized electrons, since the strain-induced pseudo-gauge potentials are opposite for K and valleys and severely affect the trajectories of K and electron propagation. We numerically simulate this valley-separated Veselago lens effect in a lattice model and demonstrate that pseudo-gauge potentials can efficiently control valley separation patterns.

Homochiral metal-organic framework used as a stationary phase for high-performance liquid chromatography.

Metal-organic frameworks are promising porous materials. Chiral metal-organic frameworks have attracted considerable attention in controlling enantioselectivity. In this study, a homochiral metal-organic framework [Co(2) (D-cam)(2) (TMDPy)] (D-cam = D-camphorates, TMDPy = 4,4'-trimethylenedipyridine) with a non-interpenetrating primitive cubic net has been used as a chiral stationary phase in high-performance liquid chromatography. It has allowed the successful separation of six positional isomers and six chiral compounds. The good selectivity and baseline separation, or at least 60% valley separation, confirmed its excellent molecular recognition characteristics. The relative standard deviations for the retention time of run-to-run and column-to-column were less than 1.8 and 3.1%, respectively. These results demonstrate that [Co(2) (D-cam)(2) (TMDPy)] may represent a promising chiral stationary phase for use in high-performance liquid chromatography.

Competition of optical transitions between direct and indirect bandgaps in Ge1−xSnx

Temperature-dependent photoluminescence (PL) study has been conducted in Ge1−xSnx films with Sn compositions of 0.9%, 3.2%, and 6.0% grown on Si. The competing between the direct and indirect bandgap transitions was clearly observed. The relative peak intensity of direct transition with respect to the indirect transition increases with an increase in temperature, indicating the direct transition dominates the PL at high temperature. Furthermore, as Sn composition increases, a progressive enhancement of direct transition was observed due to the reduction of direct-indirect valley separation, which experimentally confirms that the Ge1−xSnx could become the group IV-based direct bandgap material grown on Si by increasing the Sn content.

A plateau–valley separation method for textured surfaces with a deterministic pattern

The effective characterization of textured surfaces presenting a deterministic pattern of lubricant reservoirs is an issue with which many researchers are nowadays struggling. Existing standards are not suitable for the characterization of such surfaces, providing at times values without physical meaning. A new method based on the separation between the plateau and valley regions is hereby presented allowing independent functional analyses of the detected features. The determination of a proper threshold between plateaus and valleys is the first step of a procedure resulting in an efficient division of the two regions, which can be studied separately according to their specific function. The case of a turned multifunctional profile is presented depicting the lacks in efficacy of standardized methods and therefore studied with this new methodology. Limitations of the method are eventually presented, in particular its dependence on a proper leveling of the profile beforehand.

Spin-valley lifetimes in a silicon quantum dot with tunable valley splitting

Although silicon is a promising material for quantum computation, the degeneracy of the conduction band minima (valleys) must be lifted with a splitting sufficient to ensure the formation of well-defined and long-lived spin qubits. Here we demonstrate that valley separation can be accurately tuned via electrostatic gate control in a metal-oxide-semiconductor quantum dot, providing splittings spanning 0.3-0.8 meV. The splitting varies linearly with applied electric field, with a ratio in agreement with atomistic tight-binding predictions. We demonstrate single-shot spin read-out and measure the spin relaxation for different valley configurations and dot occupancies, finding one-electron lifetimes exceeding 2 s. Spin relaxation occurs via phonon emission due to spin-orbit coupling between the valley states, a process not previously anticipated for silicon quantum dots. An analytical theory describes the magnetic field dependence of the relaxation rate, including the presence of a dramatic rate enhancement (or hot-spot) when Zeeman and valley splittings coincide.

Steady-state and transient electron transport in bulk GaN employing an analytic bandstructure

We have studied the steady-state and transient electron transport in bulk gallium nitride using an ensemble two valley Monte-Carlo approach. We employ an analytic form for the bandstructure which is non-parabolic and includes the inflection point predicted by empirical pseudopotential methods. It has been suggested that the inflection point could give rise to a negative effective mass which would, in turn, give rise to a negative-differential conductivity (NDC). We examine the character of the NDC by varying the energy separation to the upper valleys. We find that for steady-state velocity field characteristics, the transferred electron effect dominates over the negative mass effect. However, the transient characteristics show a small temporary dominance of the negative mass effect when the valley separation is 1.9 eV.

Comparative study on nano‐scale III‐V MOSFETs with various channel materials using quantum‐corrected Monte Carlo simulation

AbstractWe investigate the ability of the nano‐scale III‐V MOSFETs with InGaAs, GaAs or InP channels by using the quantum‐corrected Monte Carlo (MC) simulation. The InGaAs channel shows the largest average electron velocity at the bottleneck, vs, because of the smallest electron effective mass, m * in the Γ valley. However, it is degraded by the electron rebound from the drain through the accumulation in the L valleys, when the gate voltage, Vgs, is highly biased. The electron rebound is more pronounced in the GaAs channel, because of the narrow Γ ‐ L valley separation. Meanwhile, the InP channel shows the smallest vs because of the largest m * in the Γ valley. However, the electron rebound is less pronounced, because of the smaller accumulation in the L valleys. On the other hand, the electron density at the potential bottleneck, nb, is largest in the InP channel, because of the largest gate capacitance owing to the highest density of states, DOS. Eventually, the InP channel shows the largest drain current, Ids (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Valley separation in graphene by polarized light

We show that the optical excitation of graphene with polarized light leads to the pure valley current where carriers in the valleys counterflow. The current in each valley originates from asymmetry of optical transitions and electron scattering by impurities owing to the warping of electron energy spectrum. The valley current has strong polarization dependence, its direction is opposite for normally incident beams of orthogonal linear polarizations. In undoped graphene on a substrate with high susceptibility, electron-electron scattering leads to an additional contribution to the valley current that can dominate.

Photoluminescence from heavily doped GeSn:P materials grown on Si(100)

Photoluminescence has been observed at room temperature in phosphorus-dopedGe1−ySny/Si(100) alloys containing carrier densities in the 1-6 × 1019 cm−3 range. The emission intensity is one order of magnitude stronger than observed in similar undoped films, and the enhancement is consistent with theoretical predictions for doped-Ge like materials. The ratio Idir/Iind of direct over indirect gap emission is found to increase for high-Sn concentrations as a result of the reduced Γ-L valley separation in Ge1−ySny alloys. These results confirm that alloying with Sn is a viable alternative to tensile strain as a tool to enhance direct-gap emission in Ge-like semiconductors.

Numerical simulation of a Gaussian beam profile passing through a Kerr medium

The circular Gaussian beam shape has been simulated by numerical evaluation on beam shape based on z-scan method through beam radius. This method is useful in many studies such as z-scan experiments. The validity of the relation between peak and valley separation in close aperture data and the diffraction length of the beam has been investigated by this method for some real cases. The results are in good agreement with the experimental studies.

Moss–Burstein and plasma reflection characteristics of heavily doped n-type InxGa1−xAs and InPyAs1−y

Degenerately doped (>1019 cm−3) n-type InxGa1−xAs (x∼0.67) and InPyAs1−y (y∼0.65) possess a number of intriguing electrical and optical properties relevant to electro-optic devices and thermophotovoltaic devices in particular. Due to the low electron effective mass of these materials (m*<0.2) and the demonstrated ability to incorporate n-type dopants into the high 1019 cm−3 range, both the Moss–Burstein band gap shift and plasma reflection characteristics are particularly dramatic. For InGaAs films with a nominal undoped band gap of 0.6 eV and N=5×1019 cm−3, the fundamental absorption edge increased to 1.27 eV. InPAs films exhibit a shorter plasma wavelength (λp∼5 μm) in comparison to InGaAs films (λp∼6 μm) with similar doping concentrations. The behavior of the plasma wavelength and the fundamental absorption edge are investigated in terms of conduction band nonparabolicity and Γ-L valley separation using detailed band structure measurements and calculations.

Performance improvements for a miniature quadrupole with a micromachined mass filter

Conventional quadrupole mass filters are usually constructed from metallic rods of length 50–225 mm and diameters 5–15 mm. In this study the conventional arrangement has been replaced with a micromachined mass filter made from silicon with Au metallised specially drawn glass fibres of length 30 mm and diameter 0.5 mm. This assembly was mounted on a vacuum flange with a VG ANAVAC ion source and a Faraday plate collector. Conventional electronics were adapted to run at 6 MHz and mass spectra in the range 0–50 amu were obtained. The results indicate a linear mass scale with 5–10% valley separation between O 2/N 2 peaks and a best resolution at 10% peak height of 2.7 amu at mass 40. It is believed that the recent improvements in performance are due to optimisation of the electronics coupled with refinements in the micromachining technique employed.