Spin relaxation of two-dimensional electrons in a hall ferromagnet

Electron spin resonance in a system of two-dimensional electrons with a high electron mobility has been investigated and the position, width, intensity, and line shape of the resonance microwave absorption have been studied as functions of the filling factor and temperature. It has been shown that the ESR linewidth in high-electron-mobility GaAs/AlGaAs quantum wells may reach 30 MHz, which corresponds to a spin relaxation time of the two-dimensional electrons of 10 ns. The experimental data on the linewidth of the spin resonance at a filling factor of 1 are compared with the theoretical results for various spin relaxation mechanisms. It has been shown that the dominant mechanism of spin relaxation at a filling factor of 1 and a temperature of 1.5–4 K is the mutual scattering of spin excitons.     

Spin relaxation and decoherence of holes in quantum dots

We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is 2 times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g factor.     

量子混沌系统中的自旋压缩性质

量子信息是21世纪的一门新兴交叉学科,现已经成为世界关注的热门研究领域.近年来,量子计算机的研究正成为大家十分感兴趣的课题.在寻找量子计算的实现方案过程中,量子混沌引起了研究人员的极大关注,因为在量子计算机执行一些量子运算法则的过程中可能产生量子混沌,并可能破坏量子计算机的运算操作条件.近期有关量子纠缠与量子混沌之间的关系已经有所报道,而自旋压缩作为另外一种典型的纯量子效应,是否也与量子混沌之间存在一定关系呢?讨论了量子混沌研究中一个非常典型的QKT模型,研究了量子混沌系统中自旋压缩的性质.通过数值模拟计算,给出了两种不同定义的自旋压缩系数与混沌系数κ之间的变化关系,结果发现在经典相空间中,如果在规则区域占优势的情况下,当初始自旋相干态波包位于椭圆形中心时,随着时间的演化,系统压缩行为表现得非常强;而对于经典相空间中混沌区域占优势的情况下,初始自旋相干态波包同样位于椭圆形中心,则系统的压缩行为表现得非常弱,说明自旋压缩对相应的经典混沌非常敏感.通过比较还发现,采用Wineland等定义的自旋压缩系数比采用Kitagawa和Ueda等定义的自旋压缩系数对经典混沌更敏感一些,从而得出用自旋压缩可以刻画量子混沌的结论.     

量子自旋液体候选材料的缪子自旋弛豫研究

量子自旋液体是一种新奇的磁性物态。由于极强的量子涨落，直至零温都不会出现长程序。量子自旋液体的基态不能用序参量描述，并且缺少对称性破缺，因此该物态的实现打破朗道理论的范式。对于量子自旋液体的研究有助于理解高温超导的机理，并且可以被应用在量子计算和量子信息中。目前，尽管理论上有了长足的发展，但仍旧没有任何一个材料被证实为量子自旋液体。因此，探测和确认一个真正的量子自旋液体材料是当前的研究重点。缪子自旋弛豫是一个对磁场极为敏感的实验技术，被广泛应用于量子自旋液体候选材料的研究中。该技术可以观测基态中是否存在磁有序，测量系统中的涨落频率，这两点都是表征量子自旋液体的重要性质。本文简要介绍了量子自旋液体态和缪子自旋弛豫技术，回顾了近期在不同体系的量子自旋液体候选材料中的实验结果，特别是缪子自旋弛豫的成果。这些体系包括一维反铁磁海森堡链（苯甲酸铜），三角格子（YbMgGaO4，NaYbO2 和TbInO3），笼目格[ZnCu3(OH)6Cl2 和 m3Sb3Zn2O14]，蜂窝状格子（Na2IrO3 和 α-RuCl3），以及烧绿石结构（Tb2Ti2O7，Pr2Ir2O7 和Ce2Zr2O7）。     

Weak localization of Dirac fermions in HgTe quantum wells

Weak localization in a system of gapless two-dimensional Dirac fermions in HgTe quantum wells with thickness d = 6.6 nm, which corresponds to the transition from a normal to an inverted spectrum, has been investigated experimentally. A negative logarithmic correction to the conductivity of the system has been observed both at the Dirac point and in the vicinity of this point. The anomalous magnetoresistance of two-dimensional Dirac fermions is positive. This indicates that weak localization in the system of two-dimensional Dirac fermions occurs owing to localization and interaction effects in the presence of rapid spin relaxation.     

Spin dephasing of a two-dimensional electron gas in a GaAs quantum well near odd filling factors

The coherent spin dynamics of a two-dimensional electron gas in a GaAs/AlGaAs quantum well is experimentally studied near the filling factors ν = 3 and 1. The nonmonotonic character of the dependence of the spin dephasing time of a Goldstone spin exciton on the filling factor is found experimentally. The observed effect can be due to the formation of a new spin relaxation channel, when the main state of the two-dimensional electron system is a spin-textured liquid.     

Quantum competitions between the effects of the interlayer exchange couplings and the magnetically structural symmetry in a four-layer ferrimagnetic superlattice

The magnon energy spectra, the sublayer magnetization and the quantum fluctuations in a ferrimagnetic superlattice consisting of four different magnetic sublayers are studied by employing the linear spin-wave approach and Green's function technique. The effects of the interlayer exchange couplings and the spin quantum numbers on the sublayer magnetization and the quantum fluctuations of the systems are discussed for three different spin configurations. The roles of quantum competitions among the interlayer exchange couplings and the symmetry of the different spin configurations have been understood. The magnetizations of some sublayers increase monotonously, while those of others can exhibit their maximum, and the quantum fluctuations of the whole superlattice system can show a minimum when one of the antiferromagnetic interlayer exchange couplings increases. This is due to the quantum competition/transmission of effects of the interlayer exchange couplings. When the spin quantum number of sublayers varies, the system goes through from a quantum region of small spin numbers to a classical region of large spin numbers. The quantum fluctuations of the system exhibit a maximum as a function of the spin quantum number of a sublayer, which is related with higher symmetry of the system. It belongs to the type III Shubnikov group of magnetic groups. This magnetically structural symmetry consists of not only the symmetry of space group, but also the symmetry of the direction and strength of spins.     

Zeeman energy and spin relaxation in a one-electron quantum dot

We have measured the relaxation time, T1, of the spin of a single electron confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic field, applied parallel to the two-dimensional electron gas in which the quantum dot is defined, Zeeman splitting of the orbital states is directly observed by measurements of electron transport through the dot. By applying short voltage pulses, we can populate the excited spin state with one electron and monitor relaxation of the spin. We find a lower bound on T1 of 50 micros at 7.5 T, only limited by our signal-to-noise ratio. A continuous measurement of the charge on the dot has no observable effect on the spin relaxation.     

Semi-Classical Dynamics in Quantum Spin Systems

We consider two limiting regimes, the large-spin and the mean-field limit, for the dynamical evolution of quantum spin systems. We prove that, in these limits, the time evolution of a class of quantum spin systems is determined by a corresponding Hamiltonian dynamics of classical spins. This result can be viewed as a Egorov-type theorem. We extend our results to the thermodynamic limit of lattice spin systems and continuum domains of infinite size, and we study the time evolution of coherent spin states in these limiting regimes.     

The robustness of the quantum spin Hall effect to the thickness fluctuation in HgTe quantum wells

The quantum spin Hall effect(QSHE) was first realized in HgTe quantum wells(QWs),which remain the only known two-dimensional topological insulator so far.In this paper,we have systematically studied the effect of the thickness fluctuation of HgTe QWs on the QSHE.We start with the case of constant mass with random distributions,and reveal that the disordered system can be well described by a virtual uniform QW with an effective mass when the number of components is small.When the number is infinite and corresponds to the real fluctuation,we find that the QSHE is not only robust,but also can be generated by relatively strong fluctuation.Our results imply that the thickness fluctuation does not cause backscattering,and the QSHE is robust to it.     

Quantum percolation in two-dimensional antiferromagnets

The interplay of geometric randomness and strong quantum fluctuations is an exciting topic in quantum many-body physics, leading to the emergence of novel quantum phases in strongly correlated electron systems. Recent investigations have focused on the case of homogeneous site and bond dilution in the quantum antiferromagnet on the square lattice, reporting a classical geometric percolation transition between magnetic order and disorder. In this study we show how inhomogeneous bond dilution leads to percolative quantum phase transitions, which we have studied extensively by quantum Monte Carlo simulations. Quantum percolation introduces a new class of two-dimensional spin liquids, characterized by an infinite percolating network with vanishing antiferromagnetic order parameter.     

Spin dynamics and magnetic correlation length in two-dimensional quantum heisenberg antiferromagnets

The correlated spin dynamics and temperature dependence of the correlation length xi(T) in two-dimensional quantum (S = 1/2) Heisenberg antiferromagnets (2DQHAF) on a square lattice are discussed in light of experimental results of proton spin lattice relaxation in copper formiate tetradeuterate. In this compound the exchange constant is much smaller than the one in recently studied 2DQHAF, such as La2CuO4 and Sr2CuO2Cl2. Thus the spin dynamics can be probed in detail over a wider temperature range. The NMR relaxation rates turn out to be in excellent agreement with a theoretical mode-coupling calculation. The deduced temperature behavior of xi(T) is in agreement with high-temperature expansions, quantum Monte Carlo simulations, and the pure quantum self-consistent harmonic approximation. Contrary to the predictions of the theories based on the nonlinear sigma model, no evidence of crossover between different quantum regimes is observed.     

Goldstone-mode relaxation in a quantized Hall ferromagnet

We report on a study of the spin relaxation of a strongly correlated two-dimensional electron gas in the nu=2kappa+1 quantum Hall regime. As the initial state we consider a coherent deviation of the spin system from the B direction and investigate a breakdown of this Goldstone-mode (GM) state due to the spin-orbit coupling and smooth disorder. The relaxation is considered in terms of annihilation processes in the system of spin waves. The problem is solved at an arbitrary value of the deviation. We predict that the GM relaxation occurs nonexponentially with time.     

SPIN POLARIZATION IN A MODEL QUANTUM WIRE WITH A CROSS-SECTION

Numerical investigation of the electronic structure of a small closed quantum system fabricated on the two-dimensional electron gas forming on the interference between GaAs and Al_xGa_1-xAs is reported. The Kohn-Sham spin-density-fun ctional theory is applied. Self-consistent results show that the quantum system assumes unequal numbers of electrons for spin up and spin down because of the existence of bound states. The profiles of effective potential energies for spin up and spin down electrons are then found to be very different, which will result in different transmission coefficients. It is notable that this spin polarization is caused by the geometry of the nanostructure.     

Nuclear spin conversion in diatomic molecules

A mechanism of the internal interaction in dimers that mixes different nuclear spin modifications has been proposed. It has been shown that the intramolecular current associated with transitions between electronic terms of different parities can generate different magnetic fields on nuclei, leading to transitions between spin modifications and to the corresponding changes in rotational states. In the framework of the known quantum relaxation process, this interaction initiates irreversible conversion of nuclear spin modifications. The estimated conversion rate for nitrogen at atmospheric pressure is quite high (10^?3–10^?5 s^?1).     

Effect of electron-electron interaction on spin relaxation of charge carriers in semiconductors

An analysis of spin dynamics is presented for semiconductor systems without inversion symmetry that exhibit spin splitting. It is shown that electron-electron interaction reduces the rate of the Dyakonov-Perel (precession) mechanism of spin relaxation both via spin mixing in the momentum space and via the Hartree-Fock exchange interaction in spin-polarized electron gas. The change in the Hartree-Fock contribution with increasing nonequilibrium spin polarization is analyzed. Theoretical predictions are compared with experimental results on spin dynamics in GaAs/AlGaAs-based quantum-well structures. The effect of electron-electron collisions is examined not only for two-dimensional electron gas in a quantum well, but also for electron gas in a bulk semiconductor and a quantum wire.     

半导体核磁共振显微压力的质子全自旋量子门的实现

以实现质子全自旋量子门、观察半导体核子自旋态和量子计算为目的, 依据样品的自旋-晶格弛豫时间和自旋-自旋弛豫时间,采用脉冲调制序列控制磁共振的条件和翻转旋转框架,计算了共振显微压力. 结果表明,质子全自旋量子门具有高灵敏度和高Q操控性,通过扫描片段和激光干涉可以得到磁共振压力. 共振压力兼具MRI和AFM优点,是一种强有力的通过核自旋实现量子计算获得量子信息的有效方法. 关键词： 空间分辨率 共振显微 半导体光刻 电子束刻印     

Evidence for skyrmion crystallization from NMR relaxation experiments

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate (1/T(1)) was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of (75)As is found to relax much more efficiently with T --> 0 and when a well developed quantum Hall state with R(xx) approximately 0 occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e., a Skyrmion crystal.     

Electric dipole spin resonance for heavy holes in quantum dots

We propose and analyze a new method for manipulation of a heavy-hole spin in a quantum dot. Because of spin-orbit coupling between states with different orbital momenta and opposite spin orientations, an applied rf electric field induces transitions between spin-up and spin-down states. This scheme can be used for detection of heavy-hole spin resonance signals, for the control of the spin dynamics in two-dimensional systems, and for determining important parameters of heavy holes such as the effective g factor, mass, spin-orbit coupling constants, spin relaxation, and decoherence times.     

Parametric excitation of a magnetic nanocontact by a microwave field

We demonstrate that magnetic oscillations of a current-biased magnetic nanocontact can be parametrically excited by a microwave field applied at twice the resonant frequency of the oscillation. The threshold microwave amplitude for the onset of the oscillation decreases with increasing bias current, and vanishes at the transition to the auto-oscillation regime. Theoretical analysis shows that measurements of parametric excitation provide quantitative information about the relaxation rate, the spin transfer efficiency, and the nonlinearity of the nanomagnetic system.