Zeeman splitting of interacting two-dimensional electrons with two effective masses

We have realized an AlAs two-dimensional electron system in which electrons occupy conduction-band valleys with different Fermi contours and effective masses. In the quantum Hall regime, we observe both resistivity spikes and persistent gaps at crossings between the Landau levels originating from these two valleys. From the positions of the spikes in tilted magnetic fields and measurements of the energy gaps away from the crossings, we find that, after occupation of the minority valley, the spin susceptibility drops rapidly, and the electrons possess a single interaction-enhanced g-factor, despite the dissimilarity of the two occupied valleys.     

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.     

Lipkin-Meshkov-Glick模型中的能级劈裂与宇称振荡研究

Lipkin-Meshkov-Glick (LMG)模型原本描述的是核物理系统,然而近年来,人们发现它广泛存在于凝聚态物理、量子信息、量子光学中,因此对其研究兴趣正在升温.本文采用精确对角化的方法以及量子微扰理论计算和分析了LMG模型在费米子数量为有限N时的能谱结构.在U(1)极限下给出它的能级精确解,发现其相互交错成渔网结构.而离开U(1)极限,系统的能级总是奇偶宇称成对地分组,形成束缚态,并且宇称会发生振荡,给出了宇称交叉点的临界塞曼场的位置.而达到Z2极限,系统能级则在零塞曼场附近形成劈裂,解析地计算了这些能隙与塞曼场之间关系,并发现对于奇数和偶数的N,各能态宇称的行为有所差别,具体而言,奇数N系统各态在零塞曼场处会发生宇称改变,而偶数N不会.     

Features of the fine structure of the fundamental absorption edge in low-doped semiconductor quantum wells with allowance for the Coulomb gap

Influence of the Coulomb gap on the fine structure of the fundamental absorption edge in quantum wells is studied. Using the interpolation formula, which adequately describes the effect of the Coulomb gap in quantum wells under conditions of low doping regardless of the compensation degree, an explicit expression is obtained for the absorption coefficient related to transitions from the valence level of size quantization to the impurity band. The dependence of the absorption coefficient of a quantum well on the levels of doping and compensation is also studied.     

Surface electron doping induced double gap opening in Td-WTe2

By using scanning tunneling microscopy, we investigated the electronic evolution of T_d-WTe₂ via in-situ surface alkali K atoms deposition. The T_d-WTe₂ surface is electron doped upon K deposition, and as the K coverage increases, two gaps are sequentially opened near Fermi energy, which probably indicates that two phase transitions concomitantly occur during electron doping. The two gaps both show a dome-like dependence on the K coverage. While the bigger gap shows no prominent dependence on the magnetic field, the smaller one can be well suppressed and thus possibly corresponds to the superconducting transition. This work indicates that T_d-WTe₂ exhibits rich quantum states closely related to the carrier concentration.     

利用不变量本征算符求解二维耦合量子谐振子的能级间隔

目前不变量本征算符方法已成功地解决了某些量子系统哈密顿量能级问题.对于二维耦合量子谐振子,利用这一方法可以非常简捷有效地给出其能级信息,而不需要使其哈密顿量对角化.计算结果表明,不同耦合形式的二维耦合量子谐振子的能级间隔是不同的.     

Dynamic tunneling polarization as a quantum rotor analogue of dynamic nuclear polarization and the NMR solid effect

The populations of the tunneling states of CH(3) are manipulated by rf irradiation of weakly allowed sideband transitions within the manifold of tunneling-magnetic levels. Substantial positive and negative CH(3) tunneling polarizations are observed, providing a quantum rotor analogue of dynamic nuclear polarization and the solid effect in NMR. The field-cycling NMR technique used in the experiments employs level crossings between tunneling and Zeeman systems to report on the tunneling polarization. The tunneling lifetimes are measured and the field dependence investigated.     

Electron emission from diamondoids: a diffusion quantum Monte Carlo study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.     

Spectrum of the graphene-based planar quantum dot

The dependence of energy levels of the planar narrow-gap semiconductor quantum dot on its radius, energy gap, and potential barrier height was calculated in the two-band approximation. Based on the results obtained, the energy spectrum of the graphene-based planar quantum dot was studied in detail.     

Interacting dirac fermions on a topological insulator in a magnetic field

We study the fractional quantum Hall states on the surface of a topological insulator thin film in an external magnetic field, where the Dirac fermion nature of the charge carriers have been experimentally established only recently. Our studies indicate that the fractional quantum Hall states should indeed be observable in the surface Landau levels of a topological insulator. The strength of the effect will however be different, compared to that in graphene, due to the finite thickness of the topological insulator film and due to the admixture of Landau levels of the two surfaces of the film. At a small film thickness, that mixture results in a strongly nonmonotonic dependence of the excitation gap on the film thickness. At a large enough thickness of the film, the excitation gap in the lowest two Landau levels are comparable in strength.     

Excitation and entanglement transfer near quantum critical points

Recently, there has been growing interest in employing condensed matter systems such as quantum spin or harmonic chains as quantum channels for short distance communication. Many properties of such chains are determined by the spectral gap between their ground and excited states. In particular this gap vanishes at critical points of quantum phase transitions. In this article we study the relation between the transfer speed and quality of such a system and the size of its spectral gap. We find that the transfer is almost perfect but slow for large spectral gaps and fast but rather inefficient for small gaps. The text was submitted by the authors in English.     

The impact of dense plasma environments on the 1s3l fine structure levels of He-like ions

Finite-temperature dense plasma relativistic atomic structure calculations are employed to study the energy eigenvalues and transition probabilities related to the 1s3l fine structure levels of He-like ions. We discovered a particular response of the level structure that is distinctly different from simple nuclear charge screening effects that exists also for atoms and ions in vacuum as Z decreases. It is demonstrated that characteristic level crossings are driven by the free electron screening due to a particular dependence on the quantum numbers of each level of the 1s3l-configuration. Levels with large l shift stronger to the continuum than levels with small l whereas spin dependent dense plasma effects are relatively small. As a result, the known energy level fine structure in vacuum is subject to characteristic changes (level crossings, level order). Numerical calculations performed for a wide range of density, temperature and different chemical elements indicate that induced level crossings and change in level order are general characteristics of dense plasma effects.     

Electronic properties of a quantum ring perturbed with a quantum well in the presence of perpendicular magnetic flux

Here, we study the effects of the number of sites, quantum ring radius and potential well depth on the energy levels, persistent current, magnetic susceptibility and density of states (DOS) of a quantum ring with a quantum well within its circumstance in a magnetic flux perpendicular to its plane. We show that, for small radius quantum ring systems, there are periodic local gaps along the magnetic flux axis in the DOS plots and along the axis ‘energy’. For large radius quantum ring systems, a uniform gap along the energy axis exists and along the phi axis nothing changes. In quantum rings with a quantum well in their circumstance, by using the large confining potential, we can create uniform gaps in the Energy–phi plane. The energy eigenvalues, persistent current and magnetic susceptibility decrease by increasing the confining potential. A quantum ring even with a very small confining potential in its circumstance can sensibly decrease the persistent current and magnetic susceptibility, although it may do not change the energy eigenvalues and DOS maximum considerably. Thus, by using the abovementioned parameters, we are able to tune the DOS, persistent current, magnetic susceptibility and energy levels, desirably.     

Intervalley gap anomaly of two-dimensional electrons in silicon

We report here a systematic study of the energy gaps at the odd-integer quantum Hall states nu = 3 and 5 under tilted magnetic (B) fields in a high quality Si two-dimensional electron system. Out of the coincidence region, the valley splitting is independent of the in-plane fields. However, the nu = 3 valley gap differs by about a factor of 3 (Deltav approximately 0.4 vs 1.2 K) on different sides of the coincidence. More surprisingly, instead of reducing to zero, the energy gaps at nu = 3 and 5 rise rapidly when approaching the coincidence angles. We believe that such an anomaly is related to strong couplings of the nearly degenerate Landau levels.     

Does adiabatic quantum optimization fail for NP-complete problems?

It has been recently argued that adiabatic quantum optimization would fail in solving NP-complete problems because of the occurrence of exponentially small gaps due to crossing of local minima of the final Hamiltonian with its global minimum near the end of the adiabatic evolution. Using perturbation expansion, we analytically show that for the NP-hard problem known as maximum independent set, there always exist adiabatic paths along which no such crossings occur. Therefore, in order to prove that adiabatic quantum optimization fails for any NP-complete problem, one must prove that it is impossible to find any such path in polynomial time.     

Structural stability and optical properties of nanomaterials with reconstructed surfaces

We present density functional and quantum Monte Carlo calculations of the stability and optical properties of semiconductor nanomaterials with reconstructed surfaces. We predict the relative stability of silicon nanostructures with reconstructed and unreconstructed surfaces, and we show that surface step geometries unique to highly curved surfaces dramatically reduce the optical gaps and decrease excitonic lifetimes. These predictions provide an explanation of both the variations in the photoluminescence spectra of colloidally synthesized nanoparticles and observed deep gap levels in porous silicon.     

Ordering of Ground State Energy Levels of Two-Electron Quantum Dot in a Magnetic Field

The energy levels structure of two interacting electrons in a parabolic quantum dot under an external magnetic field of arbitrary strength is studied via the asymptotic iteration method. The method gives accurate results over the full range of quantum dot parameters. A crossings between spin-singlet and spin-triplet ground states energies as a function of the magnetic field is predicted.     

Chern semimetal and the quantized anomalous Hall effect in HgCr2Se4

In 3D momentum space, a topological phase boundary separating the Chern insulating layers from normal insulating layers may exist, where the gap must be closed, resulting in a "Chern semimetal" state with topologically unavoidable band crossings at the Fermi level. This state is a condensed-matter realization of Weyl fermions in (3+1)D, and should exhibit remarkable features, such as magnetic monopoles and Fermi arcs. Here we predict, based on first principles calculations, that such a novel quantum state can be realized in a known ferromagnetic compound HgCr2Se4, with a single pair of Weyl fermions separated in momentum space. The quantum Hall effect without an external magnetic field can be achieved in its quantum-well structure.     

Fractional quantum Hall States of Dirac electrons in graphene

We have investigated the fractional quantum Hall states of Dirac electrons in a graphene layer in different Landau levels. The relativistic nature of the energy dispersion relation of electrons in graphene significantly modifies the interelectron interactions. This results in a specific dependence of the ground state energy and the energy gaps for electrons on the Landau-level index. For the valley-polarized states, i.e., at nu=1/m, m being an odd integer, the energy gaps have the largest values in the n=1 Landau level. For the valley-unpolarized states, e.g., for the 2/3 state, the energy gaps are suppressed for n=1 as compared to those at n=0. For both n=1 and n=0, the ground state of the 2/3 system is fully valley-unpolarized.     

硅量子点的形状及其弯曲表面效应

硅量子点的弯曲表面引起系统的对称性破缺, 致使某些表面键合在能带的带隙中形成局域电子态.计算结果表明:硅量子点的表面曲率不同形成的表面键合结合能和电子态分布明显不同. 例如, Si–O–Si桥键在曲率较大的表面键合能够在带隙中形成局域能级, 而在硅量子点曲率较小的近平台表面上键合不会形成任何局域态, 但此时的键合结合能较低. 用弯曲表面效应(CS)可以解释较小硅量子点的光致荧光光谱的红移现象. CS效应揭示了纳米物理中又一奇妙的特性. 实验证实, CS效应在带隙中形成的局域能级可以激活硅量子点发光. 关键词： 硅量子点 弯曲表面效应 表面键合 局域能级