Valley polarization in Si(100) at zero magnetic field

The valley splitting, which lifts the degeneracy of the lowest two valley states in a SiO(2)/Si(100)/SiO(2) quantum well, is examined through transport measurements. We demonstrate that the valley splitting can be observed directly as a step in the conductance defining a boundary between valley-unpolarized and -polarized regions. This persists to well above liquid helium temperature and shows no dependence on magnetic field, indicating that single-particle valley splitting and valley polarization exist in (100) silicon even at zero magnetic field.     

Influence of intervalley transitions on the photoconductivity in n-type Si 〈100〉 inversion layers

Transitions between the different sub-band systems corresponding to the valleys with low and high effective mass in the direction of surface quantization involve large momentum transfer. As a consequence the probabilities for impurity induced transitions between the respective sub-band systems are low. The corresponding transitions by phonon emission are energetically forbidden if the sub-band splitting does not exceed the energy of acoustic phonons near the K-point. Negative photoconductivity due to carrier transfer into the sub-band system with lower mobility and extremely long response times, as observed experimentally,¹ results if the sub-band splitting is less than this energy. Photoconductivity effects in 〈100〉 inversion layers will be weaker and response times will be much shorter for sub-band splitting larger than the critical value. For the corresponding case in 〈100〉 inversion layers, in contrast, strongly enhanced photoresponse is expected and the long response time should persist.     

Determination of valley splitting in (100) Si inversion layers

An experimental method is presented to determine the valley splitting in n-channel Si inversion layers. The method is an extension of the “tilted field” experiment under conditions, where the valley splitting is resolved at all tilt angles. In the framework of Ohkawa and Uemura's theory, the bare splitting as well as the effective exchange interaction are obtained directly from the angles of coincidence. The bare valley splitting in the samples investigated is 0.15 meV/10¹² cm^-2 ·n_s. Using experimentally determined parameters we have calculated the oscillatory conductivity σ_xx in a simplified version of Ohkawa and Uemura's model. The calculated curves reproduce the detailed structure of the experimental data for the various tilt angles of the experiment.     

Enhancement of valley splitting in (100) Si MOSFETs at high magnetic fields

We report the density and magnetic field dependence of the valley splitting of two-dimensional electrons in (100) Si metal–oxide–semiconductor field-effect transistors, as determined via activation measurements in the quantum Hall regime. We find that the valley activation gap can be greatly enhanced at high magnetic fields as compared to the bare valley splitting. The observation of strong dependence of the valley activation gap on orbital Landau level occupancy and similar behavior of nearby spin gaps suggest that electron–electron interactions play a large role in the observed enhancement.     

Spontaneous parity breaking of graphene in the Quantum Hall regime

We propose that the inversion symmetry of the graphene honeycomb lattice is spontaneously broken via a magnetic-field-dependent Peierls distortion. This leads to valley splitting of the n=0 Landau level but not of the other Landau levels. Compared to Quantum Hall valley ferromagnetism recently discussed in the literature, lattice distortion provides an alternative explanation to all of the currently observed Quantum Hall plateaus in graphene.     

Surface quantum oscillations in (100) inversion layers under uniaxial stress

Surface quantum oscillations have been measured with uniaxially stressed (100) n-type inversion layers. A relation between mechanical stress and cyclotron mass m_c has been observed. In the quantum limit the two-fold valley degeneracy is lifted by about 1 meV with compression.     

Magnetic field dependence of the valley splitting in n-type inverted silicon mosfet surfaces

The influence of a magnetic field dependence of the valley splitting in n-type inverted (100) silicon MOSFET surfaces is observed for the first time from an abrupt phase shift for the position of the Shubnikov - de Haas extrema in the osciallatory magnetoresistance at constant gate voltages. The results allow a quantitative determination of the valley splitting.     

Quantum oscillations in p-type inversion layers of (111) and (100) silicon field effect transistors

For the first time Shubnikov-de Haas oscillations have been observed in p-type inversion layers of (111) and (100) silicon field effect transistors. For both orientations a single electric subband was found. The effective mass of the surface carriers was determined from the temperature dependence of the oscillations, for (111) surfaces as a function of the surface electric field. At low gate voltages spin splitting of the Landau levels was resolved.     

Possibility of a spin-density-wave or a valley-density-wave in the ground state of a two-dimensional electron fluid

It is suggested that the two-dimensional electron fluid in the Si[100] inversion layers may spontaneously distort to form a spin density wave (SDW) or valley density wave (VDW) at densities ? 8 × 10¹¹ cm^-2. The relative stability of the SDW to VDW states is determined by the sign of the total intervalley interactions.     

Janus monolayer TaNF: A new ferrovalley material with large valley splitting and tunable magnetic properties

Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin−orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and large magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only −3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.     

Charge density waves in silicon inversion layers: Effect of four valley interaction terms

Kelly and Falicov (KF) have demonstrated that phonon mediated intervalley electronic exchange interactions can lead to a charge density wave type ground state in silicon (111) inversion layers. In their model only two valley interaction terms are included. There are four valley interaction terms which, in principle, are as large in magnitude as the two valley terms. We have investigated the effect of including the four-valley interaction terms on a number of Hartree-Fock solutions.     

Theoretical analysis of the oscillatory gate capacitance inn-channel inversion layers on ternary chalcopyrite semiconductors under magnetic quantization

An attempt is made to investigate theoretically the gate capacitance inn-channel inversion layers on ternary chalcopyrite semiconductors under both weak and strong electric field limits in the presence of a quantizing magnetic field, takingn-channel inversion layers on p-type Cd GaAs₂ as examples. It is found, on the basis of the newly derived 2D electron spectra in inversion layers on the above class of semiconductors, for both weak and strong electric field limits, that the gate capacitances oscillate with the quantizing magnetic field and the crystal field splitting parameter effectively enhances the oscillatory spikes. It has also been observed that the oscillatory behaviour is in qualitative agreement with experimental observation as reported elsewhere for MOS structure of Hg_1–xCd_xTe. In addition, the corresponding results for inversion layers on parabolic semi-conductors are also obtained from the expressions derived.     

Applied stresses,cyclotron masses and charge-density-waves in silicon inversion layers

The behaviour of cyclotron masses for n-type Si(100)/SiO₂ inversion layers under stress is explained by means of a charge-density-wave model. The model yields: (1) the observed occupied valley degeneracy of two; (2) a cyclotron mass which varies as a function of stress or as a function of electron number density between 0.19 m_e and 0.42 m_e; (3) a charge-density-wave solution only for a restricted range of stresses, with a different paramagnetic solution at each boundary of this range. Of the two tramsitions one is first order and the other second order both as a function of stress and carrier density. The approximations in the calculation are discussed.     

融雪期积雪深度高光谱反演方法研究

积雪在干旱区的水分平衡中发挥着极为重要的作用,积雪深度的监测主要依靠地面站点观测和遥感反演等技术,高光谱遥感为快速、大面积监测积雪的物理特性提供了可能。通过对融雪期不同厚度积雪表面的反射光谱以及积雪深度数据的观测,进而对二者进行相关性分析;采用相关性较高同时也是特征吸收谷的波段数据建立单波段雪深回归模型;采用呈显著相关的波段进行逐步回归,选用贡献率最高的波段作为神经网络模型的输入变量进行积雪深度的反演研究。结果表明：在天山北坡中段的军塘湖流域地区,1 022,1 241和1 492 nm附近是积雪的特征吸收谷;相比单波段反演雪深模型的估算精度(R2=0.53),BP神经网络模型具有更高的雪深反演水平,当隐含层节点数为4时,R2为0.86,RMSE为0.67,表明神经网络模型可以显著提高高光谱数据反演积雪深度的能力。     

Electron-phonon interaction and cyclotron resonance in two-dimensional electron gas

Results for the influence of electron-phonon interaction on the cyclotron effective mass and the resonance linewidth in a two-dimensional electron gas are presented. The temperature and magnetic field dependence is studied and the existence of quantum oscillations is demonstrated. It is shown that the relevant phonon frequency in typical MOS inversion layers is very small so that magneto-transport properties are temperature dependent even at a few degrees Kelvin. Results are consistent with the observed temperature, magnetic field and frequency dependence in Si(100) inversion layers.     

On the lowering of the electronic energy in model insulators due to surface reconstruction

Shubnikov-de Haas oscillations, piezoresistance, Hall mobility, and transverse “Hall” field due to mobility anisotropy have been studied on n-channel (111) Si inversion layers. The valley degeneracy was found to be 2 between 1.7 and 300 K. Under uniaxial mechanical stress the initially isotropic conductivity became strongly anisotropic. All results can be described by the existence of domains in the inversion layer.     

Anomalous resistance ridges along filling factor nu=4i

We report anomalous structure in the magnetoresistance of SiO(2)/Si(100)/SiO(2) quantum wells. When Landau levels of opposite valleys are driven through coincidence at the Fermi level, the longitudinal resistance displays elevations at filling factors that are integer multiples of 4 (nu=4i) accompanied by suppression on either side of nu=4i. This persists when either the magnetic field or the valley splitting is swept, leading to resistance ridges running along nu=4i. The range of field over which they are observed points to the role of spin degeneracy, which is directly confirmed by their disappearance with the addition of an in-plane magnetic field. The data suggest a new type of many-body effect arising from the combined degeneracy due to the valley and the spin degrees of freedom.     

Valley splitting and valley degeneracy in n-type silicon (110) inversion layers

The magneto-quantum transport phenomena of silicon (110) n-type inversion layers directly reveal the presence of two types of electrons in i = O and O' electric subband states from their respective quantum oscillations. While the higher i = O' states are occupied by only about 6% of the total electron concentration in our samples, the i = O electrons populate two valley pairs each, the level ladders of which are shifted in energy by less than the Landau splitting in the range where quantum oscillations are observed. We propose this effect to originate from a slight misorientation (Δθ < 0.5°) of the surfaces investigated with respect to the (110) axis. Hence, increasing the gate voltage in a quantizing magnetic field all four i = O valleys are alternatingly occupied in pairs with increasing Landau quantum number in our (110) surfaces contrary to former conclusions on samples with comparable properties. The effect of the magnitude of Δθ on the resulting magneto-quantum oscillations is unimportant for 0 < Δθ ? 0.5° as a result of energy minimization for the total electron system.     

Optical absorption in surface space-charge layers of anisotropic and tilted valley systems

In n-channel accumulation and inversion layers on the (110) and (111) surface of Si, inter-subband optical transitions can be induced by infrared light incident normal to the surface. The resonance screening and exciton-like effect depend strongly on the nature of the valley degeneracy in this configuration. Examples of numerical calculation of the absorption spectrum are presented.     

Controlling spin-orbit coupling strength of bulk transition metal dichalcogenide semiconductors

Transition metal dichalcogenide (TMD) semiconductors are attracting much attention in research regarding device physics based on their unique properties that can be utilized in spintronics and valleytronics. Although current studies concentrate on the monolayer form due to the explicitly broken inversion symmetry and the direct band gap, bulk materials also hold the capability of carrying spin and valley current. In this study, we report the methodology to continuously control the spin-orbit coupling (SOC) strength of bulk TMDs Mo_1-xW_xSe₂ by changing the atomic ratio between Mo and W. The results show the size of band splitting at the K valley the measure of the coupling strength is linearly proportional to the atomic ratio of Mo and W. Our results thus demonstrate how to precisely tune the SOC coupling strength, and the collected information of which can serve as a reference for future applications of bulk TMDs.