量子阱中电子自旋注入及弛豫的飞秒光谱研究

采用飞秒脉冲的饱和吸收光谱方法研究了GaAs/AlGaAs多量子阱中电子自旋的注入和 弛豫特性，测得电子自旋极化弛豫时间为80±10ps.说明了电子自旋 轨道耦合相互作用引 起局域磁场的随机化，是导致电子的自旋极化弛豫的主要机理. 关键词： 自旋电子学 半导体量子阱 飞秒激光光谱 自旋 轨道耦合     

InGaN薄膜中电子自旋偏振弛豫的时间分辨吸收光谱研究

采用飞秒时间分辨圆偏振光抽运-探测光谱对In_01Ga_09N薄膜的电子自旋注入和弛豫进行了研究.获得初始自旋偏振度约为02，此结果支持在圆偏振光激发下，重、轻空穴带的跃迁强度比为3∶1，而不支持1∶1或1∶094的观点同时获得自旋偏振弛豫时间为490±70ps，定性分析了自旋弛豫机理，认为BAP机理是电子自旋弛豫的主要机理. 关键词： 电子自旋 InGaN 自旋极化 自旋弛豫     

Cd0.96Zn0.04Te光致载流子动力学特性的太赫兹光谱研究

采用光抽运-太赫兹探测技术研究Cd_0.96Zn_0.04Te的载流子弛豫和瞬:态电导率特性.在中心波长800 nm的飞秒抽运光激发下,Cd_0.96Zn_0.04Te的载流子弛豫过程用单指数函数进行了拟合,其载流子弛豫时间长达几个纳秒,且在一定光激发载流子浓度范围内随光激发载流子浓度的增大而减小,这与电子-空穴对的辐射复合有关.在低.光激发载流子浓度(4.51×10¹⁶—1.81×10¹⁷ cm^-3)下,Cd_0.96Zn_0.04Te的太赫兹(terahertz,THz)瞬态透射变化率不随光激发载流子浓度增大而变化,主要是由于陷阱填充效应造成的载流子损失与光激发新增的载流子数量近似.随着光激发载流子浓度继续增大(1.81×10¹⁷—1.44×10¹⁸ cm^-3),THz瞬态透射变化率随光激发载流子浓度的增大而线性增大,是由于缺陷逐渐被...     

室温下(Ga,Mn)As中载流子的自旋弛豫特性

运用飞秒时间分辨抽运-探测克尔光谱技术，研究了室温下退火及未退火(Ga,Mn)As的载流子自旋弛豫的激发能量密度依赖性，发现电子自旋弛豫时间随激发能量密度增加而增大，而在同一激发能量密度下，退火样品比未退火样品具有更短的载流子复合时间、电子自旋弛豫时间和更大的克尔转角，显示DP机理是室温下(Ga,Mn)As的电子自旋弛豫的主导机理.退火(Ga,Mn)As的超快克尔增强效应显示其在超高速全光自旋开关方面的潜在应用价值，也为(Ga,Mn)As铁磁性起源的p-d交换机理提供了证据. 关键词： (Ga Mn)As稀磁半导体 时间分辨克尔光谱 电子自旋弛豫 DP机理     

GaAs中带填充效应与带隙重整化效应的竞争

采用时间分辨线偏振光抽运-探测光谱研究常温下本征GaAs中载流子弛豫动力学,观察到饱和吸收和吸收增强现象.发现载流子浓度为2×10¹⁷ cm^-3, 探测光子能量小于1.549 eV时,饱和吸收现象比较明显,反之,有明显的吸收增强现象出现.载流子浓度大于7×10¹⁶ cm^-3的范围内,吸收增强信号随时间增大没有减弱的趋势,反而有继续增强的趋势.理论上,考虑带填充效应和带隙重整化效应的竞争,模拟得到与实验谱线相符合的结果. 关键词： 飞秒抽运-探测光谱 带填充效应 带隙重整化效应 载流子寿命     

飞秒时间分辨拉曼光谱用于研究β-胡萝卜素单重激发态内转换和振动弛豫过程

搭建了飞秒时间分辨受激拉曼光谱(FSRS)装置，并用于研究全反式β-胡萝卜素单重电子激发态超快内转换和振动弛豫过程.基于三脉冲“抽运-探测”方案搭建的时间分辨受激拉曼光谱装置同时实现了150fs的时间分辨率和23.7cm^-1的光谱分辨率，光谱检测范围为300—4000cm^-1.对全反式β-胡萝卜素电子激发态的飞秒时间分辨拉曼光谱研究表明，β-胡萝卜素被激发到S₂态后，经由寿命约为0.3ps的中间态S_X态实 关键词： 飞秒时间分辨拉曼光谱 β-胡萝卜素 激发态内转换 振动弛豫     

(110)-GaAs量子阱中光生载流子对电子自旋弛豫的影响

我们实验研究了(110)-GaAs量子阱中光生载流子对电子自旋弛豫的影响。通过测量量子阱的荧光寿命和光学吸收计算,我们能得到不同泵浦光功率下的带间吸收所产生的空穴浓度;相对应地,通过双色磁光科尔旋转技术,我们测量了该GaAs量子阱中电子自旋的动力学过程。结合两者,我们得到了电子自旋弛豫速率与空穴浓度的关系。实验结果表明电子自旋弛豫速率与空穴浓度呈线性依赖关系,验证了BirAronov-Pikus机制主导该体系的电子自旋弛豫。     

大分子中磁偶极弛豫的分离

提出采用仅与偶极弛豫有关的¹³C核同¹H核之间的核交叉弛豫速率来考察分子内部运动状况。给出了实际用于考察分子运动状况的关系式,从理论上阐明了本文方法比传统上直接采用常规测定的¹³C核纵向弛豫时间T₁、演向弛豫时间T₂及NOE增强因子η来研究分子运动状况的方法要合理。建立了既适用于小分子又适用于大分子自旋系统¹³C核总自旋弛豫中偶极弛豫分离的公式.     

n型Bi2Te3基化合物的类施主效应和热电性能

晶粒细化是提高Bi₂Te₃基热电材料力学性能的重要方法,但晶粒细化过程中伴随的类施主效应严重劣化了材料的热电性能,并且一旦产生类施主效应,就很难通过简单的热处理等工艺消除.本文系统研究n型Bi₂Te₃基化合物烧结前粉体颗粒尺寸对材料类施主效应和热电性能的影响规律.随着颗粒尺寸减小,氧诱导的类施主效应明显增强,载流子浓度从10 M烧结样品的3.36×10¹⁹ cm^-3急剧增加到120 M烧结样品的7.33×10¹⁹ cm^-3,严重偏离最佳载流子浓度2.51×10¹⁹ cm^-3,热电性能严重劣化.当粉体颗粒尺寸为1—2 mm时,烧结样品的Seebeck系数为–195 μV/K,载流子浓度为3.36×10¹⁹ cm^-3,与区熔样品沿着ab面方向的Seebeck系数为–203 μV/K和载流子浓度为2.51×10¹⁹     

电子自旋偏振度及其弛豫过程的飞秒激光吸收光谱研究

基于二能级体系的速率方程，获得了非完全初始自旋偏振极化条件下的自旋偏振向上和向下 载流子布居弛豫的解析解. 基于小信号近似，给出了左、右旋圆偏振探测光的饱和吸收变化 的表达式. 此表达式中含有电子布居的初始自旋偏振度参数，因而用此表达式拟合实验数据 能够直接获取电子布居的初始自旋偏振度，而电子布居的初始自旋偏振度在自旋偏振输运研 究中是一个非常重要的关键参数. 实验获得了GaAs/AlGaAs多量子阱结构中光注入电子布居 的初始自旋偏振度及其弛豫时间常数. 关键词： 圆偏振抽运_探测技术 电子自旋偏振度 自旋偏振弛豫 GaAs量子阱     

Temperature relaxation and energy loss of hot carriers in graphene

The temperature relaxation and energy loss of hot Dirac fermions are investigated theoretically in graphene with carrier–optical phonon scattering. The time evolutions of temperature and energy loss for hot Dirac fermions in graphene are calculated self-consistently. It shows that the carrier–optical phonon coupling results in the energy relaxation of hot carriers excited by an electric field, and the relaxation time for temperature is about 0.5–1 ps and the corresponding energy loss is about 10–25 nW per carrier for typically doped graphene samples with a carrier density range of 1–5×10¹² cm^?2. Moreover, we analyze the dependence of temperature and energy relaxation on initial hot carrier temperature, lattice temperature and carrier density in detail.     

Anomalous spin-orbit effects in a strained InGaAs/InP quantum well structure

There is currently a large effort to explore spin-orbit effects in semiconductor structures with the ultimate goal of manipulating electron spins with gates. A search for materials with large spin-orbit coupling is therefore important. We report results of a study of spin-orbit effects in a strained InGaAs/InP quantum well. The spin-orbit relaxation time, determined from the weak antilocalization effect, was found to depend nonmonotonically on gate voltage. The spin-orbit scattering rate had a maximum value of 5×10¹⁰ s^?1 at an electron density of n=3×10¹⁵ m^?2. The scattering rate decreased from this for both increasing and decreasing densities. The smallest measured value was approximately 10⁹ s^?1 at an electron concentration of n=6×10¹⁵ m^?2. This behavior could not be explained by either the Rashba or the bulk Dresselhaus mechanisms but is attributed to asymmetry or strain effects at dissimilar quantum well interfaces.     

The optical conductivity of free carriers in GaAs

The optical conductivity of free electrons in polar semiconducting compounds has recently been calculated by use of a generalized Boltzmann equation derived from the equation of motion of the quantum density matrix. This reduces to the quasi-classical Boltzmann transport equation in the low frequency limit: the optical conductivity thus obtained spans a spectral range from around 30cm^?1 to 1.2 × 10⁴cm^?1 in GaAs. In this paper, the optical conductivity is calculated for GaAs as a function of carrier concentration in terms of a frequency dependent relaxation time which reduces to the usual relaxation time in the limit of low frequencies and an elastic scattering mechanism. The low frequency limit of the relaxation time is used to estimate the mobility as a function of carrier concentration. The frequency dependent relaxation time is given for GaAs at 298 K over the spectral region from 45 cm^?1 to 2.3 × 10³cm^?1 for carrier concentrations from 3.4 × 10¹⁵cm^?3 to 8.7 × 10¹⁸cm^?3.     

Nuclear Quadrupole spin-lattice relaxation in Rhombohedral Arsenic

The temperature dependence of the spin-lattice relaxation time T₁ in rhombohedral arsenic has been measured by nuclear quadrupole resonance. The relaxation time is inversely proportional to the temperature and of a magnitude which indicates that the relaxation results from the Fermi contact interaction of the conduction electrons and holes and the arsenic nuclei. The density of electrons and holes at the site of the nucleus, averaged over the Fermi surface is approximately 2.6 × 10²¹ carriers cm^?3.     

Hole spin relaxation in Ge quantum dots

Hole spin relaxation in an isolated Ge quantum dot due to interaction with phonons is investigated. Spin relaxation in this case occurs through the mechanism of the modulation of the spin-orbit interaction by lattice vibrations. According to the calculations performed, the spin relaxation time due to direct single-phonon processes for the hole ground state equals 1.4 ms in the magnetic field H = 1 T at the temperature T = 4 K. The dependence of the relaxation time on the magnetic field is described by the power function H^?5. At higher temperatures, a substantial contribution to spin relaxation is made by two-phonon (Raman) processes. Because of this, the spin relaxation time decreases to nanoseconds as the temperature is raised to T = 20 K. Analysis of transition probabilities shows that the third and twelfth excited hole states, which are intermediate in two-step relaxation processes, play the main part in Raman processes.     

Proton spin relaxation in hexagonal ice

Measurements of the rotating frame proton spin relaxation timeT _1p in hexagonal ice single crystals as a function of temperature ? for various rotating magnetic field strengths reveal the expectedT _1p minimum at the lowest practicable field values. This allows a very precise determination of the proton correlation (? molecular jump) time τ_c and the related activation energy ΔE by means of the theoretical reasoning of relaxation spectroscopy. We find the Arrhenius-law temperature dependenceτ _c=1.99×10^?17exp(0.603/8.61×10^?5 ?)sec, which is in good agreement with our earlier indirect derivation.     

Spin dynamics and spin noise in the presence of randomly varying spin-orbit interaction in a semiconductor quantum wire

Using ensemble Monte Carlo simulation, we have studied hot carrier spin dynamics and spin noise in a multi-subband GaAs quantum wire in the presence of a randomly varying Rashba spin-orbit interaction. The random variation reduces the carrier ensemble's spin dephasing time due to the D'yakonov-Perel' mechanism, but otherwise makes no qualitative difference to the temporal spin relaxation characteristics. However, it makes a qualitative difference to the spatial spin relaxation characteristics which change from monotonic and smooth to non-monotonic and chaotic because of a complex interplay between carriers in different subbands. As far as spin fluctuation and spin noise are concerned, the random variation has no major effect except that the low-frequency noise power spectral density increases slightly when the magnitude of the Rashba spin-orbit interaction field is varied randomly while holding the direction constant.     

The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×10¹³ to 4.1× 10¹⁸ cm^?3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl⁰ acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ T ^q (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 10¹⁸ cm^?3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |A _hf (Al)/2π| ～ 2.5×10⁶s^?1.     

Spin relaxation under conditions of the quantum Hall effect with odd filling

The relaxation of spins in a completely polarized state such that the initial direction of polarization is different from the direction of the external magnetic field is studied. The relaxation mechanism is governed by the spin-orbit and electron-phonon interactions and involves the excitation of nonzero spin excitons. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 1, 43–48 (10 January 1996)