光学极化子性质对耦合强度和磁场的依赖性

应用线性组合算符和幺正变换方法,研究磁场和耦合强度对光学极化子性质的影响。数值计算表明:当电子接近晶体表面时,光学极化子的振动频率、基态能量和第一激发能仅与磁场有关,且随磁场强度的增强而增大;当电子远离晶体表面时,基态能量和第一激发能与磁场强度和耦合参数均有关,且随磁场强度和耦合参数的增加而增加。     

Probing magnetic fields on crystals of the nanomagnet Mn12-acetate by electron paramagnetic resonance

We report on electron paramagnetic resonance (EPR) probing of magnetic fields and magnetic field gradients near the surface of a single crystal of the nanomagnet [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-Ac). As the EPR probe, we utilized a 0.7 mm x 30 microm x 30 microm fibrous needle of the organic conductor N-methylphenazinium-tetracyanoquinodimethane (NMP-TCNQ), which yields an exceptionally sharp peak, with a 0.2 G (approximately 20 microT) width. In the presence of Mn12-Ac, the probe's peak exhibits splitting on temperature lowering, which depends on the orientation of the Zeeman field relative to the axis of easy magnetization of the employed Mn12-Ac crystal. The shifted peaks yield the magnitude of the magnetic field from Mn12-Ac crystal to which the various fibers of the probe are subjected. In conjunction with electron microscopy, the shifts yield the field gradient at the crystal surface and its change with temperature. For Mn12-Ac at 10 K, the surface magnetic field was measured to be in the mT range and its gradient on the order of 50 T/m.     

STRONG-COUPLING SURFACE OPTICAL POLARON IN A MAGNETIC FIELD

A variational scheme of the Pekar type is employed to study the strong-coupling optical polaron on the outer surface of the polar crystal under the influence of a magnetic field. The ground state energy and the effective mass of the polaron are obtained as functions of the magnetic field strength and the electron-surface optical phonon coupling constant. It is shown that the effective mass of the surface polaron appears in nonisotropic nature due to the coupling between the electron and the surface optical phonon.     

On the crystallization of two-dimensional electron system in strong magnetic field

The possibility of the two-dimensional electron crystallization in the strong magnetic field is discussed by comparison between electron free energies in liquid and crystal states. The electron free energy in the liquid state is determined in Hartree-Fock approximation, while the correlation phenomena are proved to be negligible. The electron free energy in the crystal state is calculated in the harmonic approximation. The magnetic field is shown to “nonmonotonously” affect the crystal formation. From one hand, an electron crystal may exist in an interval of the strong magnetic field H at rather high density, where that is impossible at H=0. From the other hand, at any density the sufficiently strong field forces electron lattice to melt at finite temperature.     

表面磁极化子的回旋共振性质

在磁场中有不少的极性晶体,电子和体纵光学声子的耦合弱,而与表面光学声子的耦合强。本文讨论电子和体纵光学声子耦合弱,与表面光学声子耦合强时对表面磁极化子的性质的影响。采用改进了的线性组合算符法导出了磁场中表面极化子的回旋共振频率和回旋共振质量,对AgCl晶体进行了数值计算。结果表明,磁场中表面极化子的回旋共振频率和回旋共振质量随磁场的增加而增加。     

Magnetoresistance effect obtained from the band structure of a crystalline solid

This paper demonstrates that the Lorentz equation combined with the equation for electron drift in an external magnetic field gives a definite value for the relaxation time of the electron motion in a crystalline solid in the aforementioned field. The main result is that the product of the relaxation time and the strength of the magnetic field remain constant and are dependent only on the structure of the energy band of the solid. Free electrons, or nearly free electrons, result in the diagonal components of the tensor for magnetoresistance tending to zero for all electron states. For the electrons in a crystal lattice, the relaxation time considered along the cross-section line in the reciprocal space of a plane normal to the magnetic field and the surface of constant energy become highly anisotropic quantities.     

Magnetic field dependence of vibrational excitations in a two-dimensional triangular Wigner crystal

It is shown that a static magnetic field applied perpendicular to a quasi twodimensional Wigner crystal lifts the degeneracy, e.g., of transverse and longitudinal vibrational frequencies and modifies their dispersion. The dependence of the effect on the magnitude of the applied field, on the electron surface concentration, and its relevance for observing a two-dimensional Wigner crystal are investigated quantitatively for a triangular lattice of a three-layer structure in harmonic approximation.     

自旋和声子之间相互作用对TlBr晶体中表经极化子性质的影响

在考虑电子自旋和声子之间相互作用的同时,采用线性组合算符和微扰法研究半无限Ｔ１Ｂｒ晶体内表面磁极化子处于基态时的振动频率和诱生势与磁场Ｂ和距晶体表面距离（坐标）ｚ之间的依赖关系。     

磁场中极性晶体板内的强耦合极化子

采用推广的强耦合变分波函数方法，研究了在板面垂直方向施加磁场的极性晶体板内电子与体纵光学声子和表面光学声子强相互作用系统的能量和有效质量，得出了作为厚度和磁场强度函数的极化子的能量和有效质量表达式．对Cu₂O材料作了数值计算，并讨论极化子的基态能、电子与声子相互作用能以及有效质量随板厚及磁场强度变化的规律 关键词：     

Photoemission electron microscopy of neodymium-iron-boron (Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B)

The magnetic domain structure of a neodymium-iron-boron single crystal (Nd₂Fe₁₄B) was investigated in a photoemission electron microscope equipped with an aperture for partial restriction of the electron beam. As a result of the influence of magnetic microfields, electron trajectories are deflected in such a way that some of them are stopped by the aperture in the electron optical path. As a result, the contrast caused by the stray fields of the magnetic domains is significantly enhanced. The distribution of the local magnetic fields at the surface is reconstructed from the image by means of the proposed theory on the contrast mechanism. The size of the stray field close to the sample surface under study was 0.5–0.7 T. PACS 68.37.Xy; 75.50.Bb; 75.70-i; 75.70.Kw     

强磁场下Sm-Fe薄膜不同晶态组织演化及磁性能调控

针对Sm-Fe薄膜的不同晶态组织演化和磁性能调控问题,采用分子束气相沉积方法制备Sm-Fe薄膜时,通过改变Sm含量、膜厚和强磁场来调节薄膜的晶态和磁性能.结果表明,Sm含量可以调节Sm-Fe薄膜的晶态组织演化,而晶态组织的演化和强磁场对磁性能有显著影响.Sm-Fe薄膜在Sm原子比为5.8%时是体心立方晶态组织,在Sm含量为33.0%时为非晶态组织,而膜厚和强磁场不会影响薄膜的晶态组织.非晶态薄膜的表面粗糙度和表面颗粒尺寸都比晶态薄膜的小,施加6 T强磁场会使表面颗粒尺寸增大,而表面粗糙度降低.非晶态薄膜的饱和磁化强度M_s比晶态薄膜的M_s(1466 emu/cm~3,1 emu/cm~3=4π×10-10T)低约47.6%,施加6 T强磁场使非晶态和晶态薄膜的M_s均降低约50%.Sm-Fe薄膜的矫顽力H_c在6—130 Oe(1 Oe=103/(4π)A/m)之间,其中,非晶态薄膜的H_c比晶态薄膜的H_c大.施加6 T强磁场使晶态薄膜的H_c增大,而使非晶态薄膜的H_c减小,最高可以减少95%.结果表明含量和强磁场可以用于调控Sm-Fe薄膜的晶态和磁性能.     

Detection of vortex excitations in a Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10</Subscript> crystal at temperatures above the critical temperature by EPR of the surface layer

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi₂Sr₂Ca₂Cu₃O₁₀ single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ T _c. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.     

氢负离子在磁场和金属面附近电子通量分布的研究

利用半经典开轨道理论, 研究了磁场和金属面附近氢负离子的剥离电子通量分布, 并揭示了电子通量分布中的振荡结构与经典轨道之间的关系.固定离子到金属面的距离, 研究了不同的磁场强度对电子通量分布的影响. 结果表明, 由于与电子通量分布相联系的剥离电子的经典轨迹增加, 随着磁场强度的增加, 通量分布变得复杂. 此外发现剥离电子的能量变化也会影响电子通量分布. 因此可以通过改变磁场强度大小和剥离电子的能量来调控剥离电子通量分布和干涉图样. 研究结果对于理解负离子在外场、表面附近的电子流通量和剥离电子干涉图样问题以及将来实验研究负离子的光剥离显微问题都可以提供一定的参考. 关键词： 开轨道理论 电子通量 金属面 磁场     

Interface Polaron Within Parallel Electric and Magnetic Fields

A combinative method of variational wavefunction and harmonic oscillator operator algebra is used to treat the interface polaron in a semi-infinite polar crystal within parallel electric and magnetic fields perpendicular to the interface. Both the bulk longitudinal optical phonon and the interface optical phonon together with the anisotropic mass of the electron are included. The energy level correction up to the second-order perturbation, cyclotron-resonance frequency and cyclotron mass are expressed as functions of the electric and magnetic fields and a parameter characterizing the mean distance of the polaron from the interface. This theory is used to calculate numerically the single heterostructure AlAs/GaAs, when the electron is at the X high-symmetry point of the conduction band of AlAs. The results show that the magnetic field greatly enhances the polaronic correction of the electron energy levels while the electric field only increases the correction of their surface optical phonon part but obviously decreases that of their bulk optical mode part and thus the total energy correction decreases as the electric field increases. The change of red shift due to the electron-phonon interaction with electric and magnetic fields is also obf ained.     

块状TiO2气凝胶的溶胶-凝胶过程及结构

对块状TiO2气凝胶的溶胶-凝胶过程及结构进行了实验研究,结果发现:增加催化剂的量,凝胶化时间缩短,湿凝胶的透明度降低,强度提高;增加前驱体的量,凝胶化时间缩短,湿凝胶的透明度变化不大,强度提高;增加水量,凝胶化时间先缩短后增加,湿凝胶透明度先减小后增加,强度先增加后减小。利用扫描电镜对超临界干燥法制备的不同催化剂量和密度的块状TiO2气凝胶的结构进行了表征,并对结构与溶胶-凝胶过程之间的联系进行了分析。结果表明:增加催化剂量,由于缩聚反应进行的程度提高,气凝胶粒子粒径较小且总的孔径较大。减小前驱体量,气凝胶粒子粒径增大且结构逐渐疏松。     

Theory of the three-dimensional quantum Hall effect in graphite

We predict the existence of a three-dimensional quantum Hall effect plateau in a graphite crystal subject to a magnetic field. The plateau has a Hall conductivity quantized at 4e2/variant Planck's over 2pi 1/c0 with c0 the c-axis lattice constant. We analyze the three-dimensional Hofstadter problem of a realistic tight-binding Hamiltonian for graphite, find the gaps in the spectrum, and estimate the critical value of the magnetic field above which the Hall plateau appears. When the Fermi level is in the bulk Landau gap, Hall transport occurs through the appearance of chiral surface states. We estimate the magnetic field necessary for the appearance of the effect to be 15.4 T for electron carriers and 7.0 T for holes.     

Study of Cyclotron Resonance of Interface Polarons

This paper presents a study of the cyclotron resonance of the interface polarons in a semiinfinite polar crystal. In the resonant magnetic field region the Landau-level corrections due to the resonant coupling of the electron with the bulk longitudinal-optical (LO) phonon and the surface optical (SO) phonon are calculated. At resonance the n-th Landau level (n > 0) splits into a triplet. The cyclotron-resonance mass of the electron is evaluated for an arbitrary magnetic-field strength. For the interface system the magneto-optical anomalies display three branches of the cyclotron-resonance mass spectra. The numerical results show that in the interface system the polaron effect is weakened with respect to the ideal twodimensional system. It is found that in the very weak magnetic fields the polaron effect is contributed mainly by the SO phonons, whereas in the large magnetic fields the LO-phonon contribution to the polaron.effect becomes predominant.     

磁场和耦合强度对极化子有效质量和平均声子数的影响

应用线性组合算符和幺正变换方法,研究磁场和耦合强度对极化子有效质量和平均声子数的影响.数值计算表明:极化子的有效质量随耦合强度的增加而增加,这是由于耦合强度增加时,电子与晶格振动之间的相互作用增加所致;而磁场强度增加时,有效质量是先增加,达到一个极大值后,再逐渐减少,出现共振现象.平均声子数随耦合强度的增加而增加,当磁场强度大于共振时的磁场强度时,随磁场强度的增加而减少,反之,结论相反.     

极性晶体中强耦合表面磁极化子特性

研究极性晶体中表面磁极化子的特性。采用Huybrechts线性组合算符和幺正变换方法,导出了晶体中强耦合表面磁极化子处于基态的振动频率和有效哈密顿量,讨论了坐标z的两种极限情况,对RbCl晶体进行了数值计算。结果表明:振动频率和平行有效哈密顿量随磁场的增加而增大。     

两种外磁场形式对介质面次级电子倍增的抑制

利用自编1D3V PIC程序,数值研究了不同外加磁场方式对次级电子倍增抑制的物理过程,给出了次级电子数目、平均能量、密度、运动轨迹、渡越时间、介质表面静电场及沉积功率等物理量时空分布关系。模拟结果表明:不同方向外加磁场抑制次级电子倍增的机理有所不同。轴向外加磁场利用电子回旋运动干扰微波电场对电子加速过程,使其碰壁能量降低以达到抑制二次电子倍增的效果;横向外加磁场利用电子回旋漂移过程中,电子半个周期被推离介质表面（不发生次级电子倍增）,半个周期被推回介质表面（降低电子碰撞能量）的作用机理,达到抑制二次电子倍增的效果。讨论了横向磁场在回旋共振下,电子回旋同步加速导致回旋半径增大,电子能量持续增加的特殊过程。两种外加磁场方式都可以通过增加磁场达到进一步抑制次级电子倍增的目的。轴向外加磁场加载容易,但对磁场要求较高;横向外加磁场需要磁场较低,但加载较为困难。