Rashba自旋轨道耦合下square-octagon晶格的拓扑相变

本文研究了各向同性square-octagon晶格在内禀自旋轨道耦合、Rashba自旋轨道耦合和交换场作用下的拓扑相变,同时引入陈数和自旋陈数对系统进行拓扑分类.系统在自旋轨道耦合和交换场的影响下会出现许多拓扑非平庸态,包括时间反演对称破缺的量子自旋霍尔态和量子反常霍尔态.特别的是,在时间反演对称破缺的量子自旋霍尔效应中,无能隙螺旋边缘态依然能够完好存在.调节交换场或者填充因子的大小会导致系统发生从时间反演对称破缺的量子自旋霍尔态到自旋过滤的量子反常霍尔态的拓扑相变.边缘态能谱和自旋谱的性质与陈数和自旋陈数的拓扑刻画完全一致.这些研究成果为自旋量子操控提供了一个有趣的途径.     

Quantum spin chains with quantum group symmetry

We consider actions of quantum groups on lattice spin systems. We show that if an action of a quantum group respects the local structure of a lattice system, it has to be an ordinary group. Even allowing weakly delocalized (quasi-local) tails of the action, we find that there are no actions of a properly quantum group commuting with lattice translations. The non-locality arises from the ordering of factors in the quantum groupC ^*-algebra, and can be made one-sided, thus allowing semi-local actions on a half chain. Under such actions, localized quantum group invariant elements remain localized. Hence the notion of interactions invariant under the quantum group and also under translations, recently studied by many authors, makes sense even though there is no global action of the quantum group. We consider a class of such quantum group invariant interactions with the property that there is a unique translation invariant ground state. Under weak locality assumptions, its GNS representation carries no unitary representation of the quantum group.Supported in part by NSF Grant # PHY90-19433 A02Copyright © 1995 by the authors. Faithful reproduction of this article by any means is permitted for non-commercial purposes.     

带有Dzyaloshinski-Mariya相互作用的两比特纠缠量子Otto热机和量子Stirling热机

研究了以带有Dzyaloshinski-Mariya(DM)相互作用的两比特自旋体系为工质的量子纠缠Otto热机和量子Stirling热机.两种不同热机在各自的循环过程中,通过保持其他参量不变,只有DM相互作用发生改变,从而分析热机循环中DM相互作用与热传递、做功以及效率等热力学量之间的关系.研究结果表明:DM相互作用对两种热机的基本量子热力学量都具有重要的影响,但量子Stirling热机由于回热器的使用,其循环效率会大于量子Otto纠缠热机的效率,甚至会超过Carnot效率;得到了量子Otto纠缠热机和量子Stirling热机做正功的条件.因此,在这两个纠缠体系中,热力学第二定律都依然成立.     

Infrared bounds and the peierls argument in two dimensions

We propose a definition of contours for spin systems which leads to improved estimates on the region of parameters where several phases coexist. We discuss as examples anisotropic rotators and a ⁴ lattice field theory. Our contours are estimated using infrared bounds and they are related to those of Euclidean Field Theory.Part of this work was done while Jean Bricmont was at the Mathematics Department of Princeton University and Jean-Raymond Fontaine was at the Mathematics Department of Rutgers University. This work was supported by NSF Grant No. MCS 78-01885 at Princeton University and by NSF Grant No. PHY 78-15920 at Rutgers University     

A general relation between kink-exchange and kink-rotation

The normal correlation between spin and statistics is shown to be valid for arbitrary kinks, among them theSU(n) Skyrmions forn3. It is assumed in the proof that no gauge-ambiguity attaches to the values of the underlying scalar field, and that conversely each configuration of this field is represented quantum mechanically by a Hilbert subspace of dimension precisely one.Supported in part by NSF Grant No. PHY 83-18350     

Raman磁共振谱中强单量子信号的压抑

通常单量子信号的强度在Raman磁共振谱中要比其它多量子信号的强度大得多,因而易引起接收机饱和,使信号发生变形,不利于多量子信号的检测,因此压抑强的单量子峰是必要的.积算符理论分析表明,异核体系Raman磁共振谱的单,多量子峰的共振频率及其信号强度关于频偏满足对称关系:当频偏由正变为负时,单量子信号的强度亦随之改变了符号.因此,对正负频偏(数值相等)的两次Raman磁共振实验的FID信号进行累加,就可压抑强的单量子峰,而对于多量子峰则在大的频偏条件下不会相互抵消.因此,这样的实验累加可以保持多量子信号的强度而压抑掉强的单量子峰.作为实验验证,我们给出了异核AX_n体系(CHCl₃,CH₂Cl₂,CH₃OH)的按上述正负频偏循环累加采样方法得到Raman磁共振¹³C实验谱.实验结果同理论预言完全一致.     

Pulse sequences for realizing the quantum Fourier transform on multilevel systems

Sequences of selective pulses of an RF magnetic field for realizing the quantum Fourier transform by NMR methods on systems with four, six, and eight nonequidistant levels are found using the virtual spin formalism. The results can be applied to other quantum systems when laser pulses are used.     

Spin Splitting in Different Semiconductor Quantum Wells

We theoretically investigate the spin splitting in four undoped asymmetric quantum wells in the absence of external electric field and magnetic field. The quantum well geometry dependence of spin splitting is studied with the Rashba and the Dresselhaus spin-orbit coupling included. The results show that the structure of quantum well plays an important role in spin splitting. The Rashba and the Dresselhaus spin splitting in four asymmetric quantum wells are quite different. The origin of the distinction is discussed in this work.     

Highly spin-polarized field emissions induced by quantum size effects in ultrathin films of Fe on W(001)

Spin-polarized field emissions from Fe pseudomorphic ultrathin films on W(001) surfaces are studied by density functional calculations. We found that nearly completely spin-polarized field emission currents can be realized in two and four Fe layers on W(001) and that these systems have the additional advantages of thermal stability and low work functions. The unusually high spin polarizations of the field emission current is traced to the Fe film's quantum size effects leading to spin-polarized quantum well states and surface resonance states.     

Protected two-qubit entangling gate with mechanical driven continuous dynamical decoupling

In this work, we propose a high-fidelity phonon-mediated entangling gate in a hybrid mechanical system based on two silicon-vacancy color centers in diamond. In order to suppress the influence of the spin decoherence on the entangling gate, we use a continuous dynamical decoupling approach to create new dressed spin states, which are less sensitive to environmental fluctuations and exhibit an extended ${T}_{2}^{* }$ spin dephasing time. The effective spin–spin Hamiltonian modified by the mechanical driving field and the corresponding master equation are derived in the dispersive regime. We show that in the presence of the mechanical driving field, the effective spin–spin coupling can be highly controlled. By calculating the entangling gate fidelity in the dressed basis, we find that once the mechanical field is turned on, the gate fidelity can be significantly improved. In particular, under an optimized spin-phonon detuning and a stronger Rabi frequency of the mechanical driving field, the two-qubit gate is capable of reaching fidelity exceeding 0.99. Moreover, by employing appropriate driving modulation, we show that a high-fidelity full quantum gate can be also realized, in which the initial and final spin states are on a bare basis. Our work provides a promising scheme for realizing high-fidelity quantum information processing.     

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.     

Spin of the ground quantum state of electrons from first principles in the representation of Feynman path integrals

A method for calculating the spin of the ground quantum state of nonrelativistic electrons and distance between energy levels of quantum states differing in the spin magnitude from first principles is proposed. The approach developed is free from the one-electron approximation and applicable in multielectron systems with allowance for all spatial correlations. The possibilities of the method are demonstrated by the example of calculating the energy gap between spin states in model ellipsoidal quantum dots with a harmonic confining field. The results of computations by the Monte Carlo method point to high sensitivity of the energy gap to the break of spherical symmetry of the quantum dot. For three electrons, the phenomenon of inversion has been revealed for levels corresponding to high and low values of the spin. The calculations demonstrate the practical possibility to obtain spin states with arbitrarily close energies by varying the shape of the quantum dot, which is a key condition for development prospects in technologies of storage systems based on spin qubits.     

Algebras of local observables on a manifold

We propose a generalization of the Haag-Kastler axioms for local observables to Lorentzian manifolds. The framework is intended to resolve ambiguities in the construction of quantum field theories on manifolds. As an example we study linear scalar fields for globally hyperbolic manifolds.Supported by National Science Foundation PHY 77-21740.On leave from Department of Mathematics, SUNY at Buffalo, Buffalo, NY 14214, USA     

An estimate on the large N behavior of Z N spin systems in two dimensions

We consider spin systems in two dimensions having the discrete symmetry group Z _N . We give a bound on the spontaneous magnetization for such systems which reduces to the classical Mermin-Wager argument as N goes to infinity. Research supported in part by NSF PHY 78-23952.     

Ward identities for non-commutative geometry

We interpret the cocycle condition in quantum field theory as a set of integrated Ward identities for non-commutative geometry.Dedicated to Res Jost and Arthur WightmanSupported in part by the National Science Foundation under Grants DMS/PHY 88-16214 and INT 87-22044     

Quantum Correlation Properties in Composite Parity-Conserved Matrix Product States

We give a new thought for constructing long-range quantum correlation in quantum many-body systems. Our proposed composite parity-conserved matrix product state has long-range quantum correlation only for two spin blocks where their spin-block length larger than 1 compared to any subsystem only having short-range quantum correlation, and we investigate quantum correlation properties of two spin blocks varying with environment parameter and spacing spin number. We also find that the geometry quantum discords of two nearest-neighbor spin blocks and two next-nearest-neighbor spin blocks become smaller and for other conditions the geometry quantum discord becomes larger than that in any subcomponent, i.e., the increase or the production of the long-range quantum correlation is at the cost of reducing the short-range quantum correlation compared to the corresponding classical correlation and total correlation having no any characteristic of regulation. For nearest-neighbor and next-nearest-neighbor all the correlations take their maximal values at the same points, while for other conditions no whether for spacing same spin number or for different spacing spin numbers all the correlations taking their maximal values are respectively at different points which are very close. We believe that our work is helpful to comprehensively and deeply understand the organization and structure of quantum correlation especially for long-range quantum correlation of quantum many-body systems; and further helpful for the classification, the depiction and the measure of quantum correlation of quantum many-body systems.     

Correlation inequalities and contour estimates

We give a simple estimate on the probability of contours in classical ferromagnetic spin systems, based on Griffiths' or Ginibre's correlation inequalities. This includes quite general one- and two-component spin models. Some extension also holds for alln-component anisotropic or isotropic rotators.Supported by NSF grant No. MCS78-01885.On leave from: Institut de Physique Théorique, Université de Louvain, Belgium.Supported by NSF grant No. PHY78-15920.     

A note on cluster expansions,tree graph identities,extra 1/N! factors!!!

We draw attention to a new tree graph identity which substantially improves on the usual tree graph method of proving convergence of cluster expansions in statistical mechanics and quantum field theory. We can control expansions that could not be controlled before.This work was supported in part by the National Science Foundation under Grants No. PHY01011 and MCS 83 01116.     

A new class of ideal clocks

We have found a new class of ideal clocks within general relativity. They are self-gravitating systems such as rotating stars, rotating black holes, and binary star systems. The gravitational red shift of the observed period of rotation of such clocks in a given, weak external gravitational field is shown to be the same as that of an ideal atomic clock. Because the clocks have structure and dynamics determined by gravitational interactions, the full nonlinear machinery of general relativity must be used. This result is important for the binary pulsar PSR 1913+16, where the gravitational red shift of the pulsar's frequency caused by the companion's gravitational field is an observable effect.Supported in part by the National Science Foundation Grant Nos. PHY81-15800 and PHY83-13545.This essay received the third award from the Gravity Research Foundation for 1984-Ed.