Dynamics of nuclear polarization in InGaAs quantum dots in a transverse magnetic field

The time-resolved Hanle effect is examined for negatively charged InGaAs/GaAs quantum dots. Experimental data are analyzed by using an original approach to separate behavior of the longitudinal and transverse components of nuclear polarization. This made it possible to determine the rise and decay times of each component of nuclear polarization and their dependence on transverse magnetic field strength. The rise and decay times of the longitudinal component of nuclear polarization (parallel to the applied field) were found to be almost equal (approximately 5 ms). An analysis of the transverse component of nuclear polarization shows that the corresponding rise and decay times differ widely and strongly depend on magnetic field strength, increasing from a few to tens of milliseconds with an applied field between 20 and 100 mT. Current phenomenological models fail to explain the observed behavior of nuclear polarization. To find an explanation, an adequate theory of spin dynamics should be developed for the nuclear spin system of a quantum dot under conditions of strong quadrupole splitting.     

Electron–Nuclear Spin Dynamics in Semiconductor QDs

This work presents an overview of investigations of the nuclear spin dynamics in nanostructures with negatively charged InGaAs/GaAs quantum dots characterized by strong quadrupole splitting of nuclear spin sublevels. The main method of the investigations is the experimental measurements and the theoretical analysis of the photoluminescence polarization as a function of the transverse magnetic field (effect Hanle). The dependence of the Hanle curve profile on the temporal protocol of optical excitation is examined. Experimental data are analyzed using an original approach based on separate consideration of behavior of the longitudinal and transverse components of the nuclear polarization. The rise and decay times of each component of the nuclear polarization and their dependence on transverse magnetic field strength are determined. To study the role of the Knight field in the dynamic of nuclear polarization, a weak additional magnetic field parallel to the optical axis is used. We have found that, only taking into account the nuclear spin fluctuations, we can accurately describe the measured Hanle curves and evaluate the parameters of the electron–nuclear spin system in the studied quantum dots. A new effect of the resonant optical pumping of nuclear spin polarization in an ensemble of the singly charged (In,Ga)As/GaAs quantum dots subjected to a transverse magnetic field is discussed. Nuclear spin resonances for all isotopes in the quantum dots are detected in that way. In particular, transitions between the states split off from the ±1/2 doublets by the nuclear quadrupole interaction are identified.     

Quantum cellular automaton theory of light

We present a quantum theory of light based on the recent derivation of Weyl and Dirac quantum fields from general principles ruling the interactions of a countable set of abstract quantum systems, without using space–time and mechanics (D’Ariano and Perinotti, 2014). In a Planckian interpretation of the discreteness, the usual quantum field theory corresponds to the so-called relativistic regime of small wave-vectors. Within the present framework the photon is a composite particle made of an entangled pair of free Weyl Fermions, and the usual Bosonic statistics is recovered in the low photon density limit, whereas the Maxwell equations describe the relativistic regime. We derive the main phenomenological features of the theory in the ultra-relativistic regime, consisting in a dispersive propagation in vacuum, and in the occurrence of a small longitudinal polarization, along with a saturation effect originated by the Fermionic nature of the photon. We then discuss whether all these effects can be experimentally tested, and observe that only the dispersive effects are accessible to the current technology via observations of gamma-ray bursts.     

Trimodal random-field spin- Ising systems in a transverse field

The trimodal random-field spin- Ising system in a transverse field is investigated by combining the pair approximation with the discretized path-integral representation by introducing a parameter p to simulate the fractions of the spins not exposed to the external longitudinal magnetic field. The variation of the critical reduced transverse field and longitudinal magnetic field with the parameter p is studied for different coordination numbers and it is found that the system does not exhibit any tricritical points for p>0.22. The phase diagrams with respect to the external longitudinal random-field and the second-order phase transition temperature are obtained for given values of the transverse field, coordination numbers and the parameter p. It is found that for appropriate values of the system parameters the system does present tricritical points and reentrant phase transitions, which may be caused by the competition between the quantum effects and randomness.     

Phase and polarization singularities of nonparaxial Gaussian vortex beams

The analytical expression for nonparaxial Gaussian vortex beams propagating in free space is derived, which enables us to study phase and polarization singularities in nonparaxial vector wavefields. Differing from the polarization singularities formed by two transverse electric-field components in the paraxial regime, the polarization singularities can be formed by the transverse and longitudinal electric-field components of nonparaxial beams, and there exist C-points and L-lines. The variation of the beam parameters and propagation distance will result in a shift of phase and polarization singularities, but their position relation remains unchanged and the topological relationship holds true.     

A study of the ultrafast optical response of magnetized GaAs/AlGaAs quantum wire structures

We have studied analytically the ultrafast optical response of GaAs/AlGaAs quantum wire subjected to a moderately strong transverse magnetic field. The energy dispersion relations have been numerically calculated and show a significant deviation from parabolic behaviour as the magnetic field is increased. The effective semiconductor Bloch equation technique is used to calculate the induced polarization and differential transmission spectra in the quantum wire. The calculated induced polarization is used to study the optical coherent transient phenomenon of optical nutation. The analysis demonstrates that the magnetic field effectively alters the optical response of the semiconductor quantum wire nanostructures. It is observed that the nutating signal frequency enhances with an increasing magnetic field. The results are useful to explain magnetic field effects on the transient optical properties of semiconductor nanostructures.     

Multiphoton analysis of the free electron laser

A model of the free electron laser is proposed which is based on the classical current of the electron in the wiggler field interacting with a quantized radiation field. To calculate the gain, the quantum recoil for the processes of n-photon emission and absorption must be put in by hand from kinematical considerations. Apart from a spontaneous emission term the gain agrees essentially with the usual small signal expression, for the radiation field being in either an eigenstate of photon number or in a coherent state. The distribution of the electrons after the interaction is, however, essentially quantum mechanical.     

Vortex dynamics in superfluids and the Berry phase

There is a close analogy between the dynamics of electrons in a strong magnetic field and the dynamics of quantized vortices in superfluids and superconductors. In both systems an important part is played by a term in the Lagrangian linear in velocity that corresponds to a Berry phase in the quantum theory. This Berry phase can be calculated from the usual trial wave function for a vortex. This has important consequences for quantum tunneling of vortices, and leads unambiguously to the form of the Magnus force in a superconductor.     

From magnetically doped topological insulator to the quantum anomalous Hall effect

Quantum Hall effect （QHE）, as a class of quantum phenomena that occur in macroscopic scale, is one of the most important topics in condensed matter physics. It has long been expected that QHE may occur without Landau levels so that neither external magnetic field nor high sample mobility is required for its study and application, Such a QHE free of Landau levels, can appear in topological insulators （TIs） with ferromagnetism as the quantized version of the anomalous Hall effect, i.e., quantum anomalous Hall （QAH） effect. Here we review our recent work on experimental realization of the QAH effect in magnetically doped TIs. With molecular beam epitaxy, we prepare thin films of Cr-doped （Bi,Sb）2Te3 TIs with well- controlled chemical potential and long-range ferromagnetic order that can survive the insulating phase. In such thin films, we eventually observed the quantization of the Hall resistance at h/e2 at zero field, accompanied by a considerable drop in the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes, whereas the Hall resistance remains at the quantized value. The realization of the QAH effect provides a foundation for many other novel quantum phenomena predicted in TIs, and opens a route to practical applications of quantum Hall physics in low-power-consumption electronics.     

光子的态矢量函数

从广义Schr?dinger方程出发，在三个充分必要的量子化条件规范下，得到了一个新颖的光子一维态矢量函数，除了光子的能量和动量特征以外，它还包含有光子的角动量属性，完整地描述了光子作为量子力学中相对论自由粒子的行为.对一维光子态矢量函数的分析不仅定义了描述光子行为的微观参量——概率幅和相位，而且将这些微观参量与光束的宏观偏振联系了起来，具体剖析了一个人们一直感到困惑的偏振问题，得到了圆满的解释. 关键词： 光子态矢量函数 概率幅 相位 偏振     

Recoil-proton polarization in high-energy deuteron photodisintegration with circularly polarized photons

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.     

Mechanism of exchange switching of spin valves by an inverse current

The exchange switching of spin valves by an inverse current can be explained by the interaction of the charge carriers with the spin-injection effective magnetic field. Such an interaction gives rise to transverse spin components, which are transferred to the magnetic lattice and cause its instability and switching. The spin-injection field is produced by longitudinal spin components, but it opens up a channel for the transverse spin transfer to the lattice. The spin transfer to the lattice and the switching occur in the free layer of the spin valve.     

Diamagnetism of microcavity polaritons induced by spin-dependent polariton–polariton interactions

Diamagnetism of condensed microcavity polaritons in a vertically applied magnetic field is theoretically studied by using the density of free energy of polaritons. The magnetic dependence of polariton–polariton interactions and spin polarization degree of polaritons are derived, and are used to show the diamagnetic behavior of the polariton spin polarization, which is discussed for GaAs-based microcavities. We show that for strong magnetic field the spin polarization of the polaritons is paramagnetic as usual, while around positive exciton–photon detuning and special Rabi splitting, the spin polarization of the polaritons could be diamagnetic. In addition, weak magnetic field and high polariton density are beneficial to observe the polariton diamagnetism.     

Electromagnetic Propagation in a Relativistic Electron Gas at Finite Temperatures

The electromagnetic propagation in a relativistic electron gas at finite temperatures and carrier densities is described. Using quantum electrodynamics at finite temperatures, electric and magnetic responses and general constitutive relations are obtained. Rewriting the propagator for the electromagnetic field in terms of the electric and magnetic responses, the modes that propagate in the gas are identified. As expected, the usual collective excitations are obtained, i.e., a longitudinal electric and two transverse magnetic plasmonic modes. In addition, a purely photonic mode is found, which satisfies the wave equation in vacuum, for which the electron gas is transparent. Dispersion relations for the plasmon modes at zero and finite temperatures are presented and the intervals of frequency and wavelength where both electric and magnetic responses are simultaneously negative are identified, a behavior previously thought not to occur in natural systems. The investigation of the electromagnetic responses of a relativistic electron gas shows that, apart from the usual longitudinal electric plasmon mode and the two transverse magnetic plasmon modes, there is also a pure photonic mode that propagates with the speed of light, as if the medium were transparent. Furthermore, there is a region of frequencies and wavenumbers of the external fields where both the longitudinal electric permittivity and magnetic permeability are simultaneously negative, a property found in artificially constructed metamaterials.     

Continuous quantum phase transitions in the one-dimensional spin-1/2 axial next-nearest-neighbour Ising model in two orthogonal magnetic fields

We have investigated the one-dimensional spin-1/2 axial next-nearest-neighbour Ising (ANNNI) model in two orthogonal magnetic fields at zero temperature. There are four different possible ground state configurations for the ANNNI model in a longitudinal field, in the thermodynamic limit. The inclusion of a transverse field introduces quantum fluctuations which destroy the existing spin order along certain critical lines. The effects of the fluctuations in three of the four ordered regions were investigated using the finite-size scaling technique. The phase boundaries of the ANNNI model in two orthogonal magnetic fields were thus determined numerically. For certain limits of the Hamiltonian we compared the obtained results with the existing literature and our results were in good agreement with the results in the existing literature.     

The influence of nuclear spins on the transport properties of molecular gases and application to HD

Coupling between the angular momentum J and the nuclear spins of a molecule modifies those contributions to the transport properties of a molecular gas which arise from the macroscopic orientational polarization. This (low pressure) effect is studied in detailfor molecular HD at low temperature (J = 1 states). Because the rotational quantum numbers are low (J = 1), a quantum treatment is essential and moreover be classical motion of J differs significantly from simple precession: the “nutation” of J being also important. This nutation gives rise to significant phase randomization of the angular-momentum polarizations at zero field, and to a field effect on the axial polarizations which are in addition to the usual field effects on nonaxial polarizations.The complicated dependence of the microscopic transport coefficients on the magnetic field and pressure reflects not only the various intramolecular couplings involved, but also the discreteness of the energy-level structure. In fact, it is possible to resolve effects of individual energy off-diagonalities in the density matrix, which persist at energy level crossings. These contribute adsorption and dispersion peaks respectively to the field dependence of the even and odd transport coefficients. The collisionally uncoupled model used in the calculation leads to good agreement both with the experimental transverse thermal-conductivity coefficient, and with the experimental thermomagnetic torque.     

Quantized four-terminal resistances in a ferromagnetic graphene p-n junction

The quantum Hall and longitudinal resistances in four-terminal ferromagnetic graphene p-n junctions under a perpendicular magnetic field are investigated. In the Hall measurement, the transverse contacts are assumed to be located at the p-n interface to avoid the mixing of edge states at the interface and the resulting quantized resistances are then topologically protected. According to the charge carrier type, the resistances in a four-terminal p-n junction can be naturally divided into nine different regimes. The symmetric Hall and longitudinal resistances are observed, with many new robust quantum plateaus revealed due to the competition between spin splitting and local potentials.     

Effect of magnetic field on the stimulated photon echo in ytterbium vapors

Polarization of the stimulated photon echo (SPE) at the 0 ? 1 transition in ytterbium vapors in the presence of a longitudinal magnetic field of 0–40 G is experimentally and theoretically studied. The SPE is generated using three light pulses with identical linear polarizations, so that the SPE polarization is the same at zero magnetic field. In the presence of a weak magnetic field, the SPE polarization vector rotates around the magnetic field vector and the depolarization of the SPE signal takes place. Each of the SPE polarization components exhibits biharmonic oscillations depending on the magnetic field. In the presence of a strong magnetic field, these oscillations vanish and the SPE becomes depolarized. The experimental data are in qualitative agreement with the results of the numerical calculations performed with the method of the evolution operator for the finite-duration excitation pulses. The application of the results for the processing of optical data is discussed.     

Addition to the Solution of the Problem on the Motion of a Free Nonrelativistic Electron in a Magnetic Field

In addition to the solution of the quantum mechanical problem on the motion of a free electron in a magnetic field, given by L. D. Landau in his pioneering work, this quantum mechanical problem is solved in view of the fact that the sum of the components of the free kinetic energy of an electron along two axes is a periodic quantity varying identically within the boundaries of every Landau energy level. This periodicity is a consequence of the quantized motion of an electron in a plane normal to the direction of the external magnetic field.     

超导与规范对称性,希格斯机制与希格斯粒子——电磁学与电动力学中的超导Ⅱ

本文是文献[1]和[2]联合的后继文章,在文中我们依据电磁学和电动力学中的麦克斯韦方程组建立了有质量光子导致导体中的超导现象这一事实的规范不变描写,文献[1]的结果是目前理论选取洛伦兹规范的特殊情形.我们发现在这种规范不变的理论中存在一个零质量的标量场,它可以和规范势的纵向分量相互转化.这正是文献[2]所介绍的2013年诺贝尔物理学奖中著名的希格斯机制,即规范粒子吃掉Goldstone玻色子而产生纵向分量,因而获得质量.这个新引进的零质量标量场对应量子场论中激发Goldstone玻色子的标量场,它可以被看成是一个更一般的两分量复标量场的相角分量.而此推广的复标量场的常数模分量可以被看成是另一个动力学场——希格斯场的真空期望值.希格斯场的激发是希格斯粒子,即所谓上帝的粒子;而光子的质量则起源于希格斯场的真空期望值.