Excitation wavelength dependence of the anomalous circular photogalvanic effect in undoped InGaAs/AlGaAs quantum wells L. P. Zhu, Y. Liu, C. Y. Jiang, X. D. Qin, Y. Li, H. S. Gao, and Y. H. Chen Citation: Journal of Applied Physics 115, 083509 (2014); doi: 10.1063/1.4867039 View online: http://dx.doi.org/10.1063/1.4867039 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spin depolarization under low electric fields at low temperatures in undoped InGaAs/AlGaAs multiple quantum well Appl. Phys. Lett. 105, 152103 (2014); 10.1063/1.4898344 Electron spin relaxation time in (110) InGaAs/InAlAs quantum wells J. Appl. Phys. 116, 023507 (2014); 10.1063/1.4887803 Spectra of Rashba- and Dresselhaus-type circular photogalvanic effect at inter-band excitation in GaAs/AlGaAs quantum wells and their behaviors under external strain Appl. Phys. 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Box 912, Beijing 100083, People’s Republic of China (Received 7 December 2013; accepted 15 February 2014; published online 26 February 2014) The excitation wavelength dependence of the anomalous circular photogalvanic effect (ACPGE) current arising from the reciprocal spin Hall effect (RSHE) in undoped InGaAs/AlGaAs quantum wells is measured under normal incidence of circularly polarized light at room temperature. We found that the spot location with the maximum ACPGE current is wavelength independent. And the normalized ACPGE current decreases at smaller wavelengths, which can be attributed to the sharp decrease of the spin relaxation time (ss ) and the hot electron relaxation time (s1 ) at smaller wavelengths. The study of the excitation wavelength dependence of ACPGE current is a good supplement to the in-depth C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4867039] investigation of RSHE. V I. INTRODUCTION Much attention has been given to semiconductor spintronics for the promising applications in information technology.1,2 However, techniques for efficient generation and manipulation of spin currents are still the major issues. The spin Hall effect (SHE) which has been studied extensively opens avenues to convert a charge current into a spin current due to the spin-orbit interaction (SOI).2,3 As a reverse process, the SOI can also convert a spin current into a transverse charge current, which is known as the reciprocal spin Hall effect (RSHE).4–9 RSHE has so far been observed in bulk GaAs,4 Pt wire,5 AlGaN/GaN heterostructures,6 GaAs/AlGaAs heterostructures,7 InN films,8 and MgZnO/ZnO heterostructures.9 The ordinary circular photogalvanic effect (CPGE) has been applied widely to measure the Rashba and Dresselhaus spin-orbit coupling coefficient in semiconductors.10–12 According to CPGE, the oblique incidence of circularly polarized light induces a spin polarized charge current whose direction and magnitude depend on the polarization degree of the light.10 Different from the ordinary CPGE, a charge current based on RSHE is excited exactly under normal incidence, while the current induced by CPGE is zero on this occasion; thus, this effect can be named as anomalous circular photogalvanic effect (ACPGE).6,10 The radiation sources applied in the former studies for ACPGE were all single wavelength lasers. But in this letter, we focus on the wavelength dependence of the ACPGE in undoped InGaAs/AlGaAs quantum wells (QWs), which might reveal more information about RSHE. Usually the SOI is weak in GaAs system and consequently RSHE is too weak to be detected at room temperature; however, the undoped InGaAs/AlGaAs QWs studied here had demonstrated a strong Rashba spin-obit coupling.13 II. SAMPLE AND EXPERIMENT The sample studied here is an undoped In0.15Ga0.85As/ Al0.3Ga0.7As QWs grown by molecular beam epitaxy. A a) [email protected] 0021-8979/2014/115(8)/083509/5/$30.00 200 nm buffer layer is initially deposited on (001) SI-GaAs ˚ -In0.15Ga0.85As/ substrate, followed by ten periods of 100 A ˚ ˚ 100 A-Al0.3Ga0.7As QWs. Then, a 500 A Al0.3Ga0.7As layer ˚ GaAs cap layer are deposited. The sample is and 100 A cleaved into a narrow strip along the GaAs ½1 10 direction with a width of 4 mm and a length of 12 mm, respectively. The geometry has been shown in Fig. 2(b), where two ohmic electrodes with a distance of 3 mm were made along y direction by indium deposition and annealed at about 420 C in nitrogen atmosphere. The experimental setup is sketched in Fig. 1. A modelocked Ti:sapphire laser with a repetition rate of 80 MHz serves as the radiation source. As the full width at half maximum (FWHM) of the laser pulse is about 7 nm, the steplength of the spectral scanning should not be too small. In this experiment, the excitation wavelength is tuned from 865 nm to 1000 nm with a step-length of 5 nm, which covers both 1hh-1e (the first valence subband of heavy hole to the first conduction) transition and 1lh-1e (the first valence subband of light hole to the first conduction) transition.14 The incident light goes through a polarizer and a photoelastic modulator (PEM), of which the peak retardation is set to be k/4, to yield a modulated circularly polarized light with a fixed modulating frequency at 50 KHz. A chopper with a frequency at 220 Hz is applied to produce a polarization independent light, which is used to measure the common photoinduced current (JPC). The Gaussian profile light beam irradiates vertically on the sample with a diameter of about 2 mm at the central line x between two electrodes (see Fig. 2(b)). The photogalvanic current is collected by two lock-in amplifiers through the two circle electrodes. III. THEORY BACKGROUND For the undoped In0.15Ga0.85As/Al0.3Ga0.7As QWs with C2v symmetry, the CPGE current can be expressed as jy ¼ cyx e^x E20 Pcirc ; (1) where cyx is a second-rank pseudotensor proportional to the spin-orbit coupling parameter; x, y are two orthogonal 115, 083509-1 C 2014 AIP Publishing LLC V [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.228.14 On: Tue, 17 Mar 2015 01:23:55 083509-2 Zhu et al. J. Appl. Phys. 115, 083509 (2014) momentum of electron’s orbit motion. This transformation means a coupling of two axial vectors and should not be limited to the gyrotropic materials. The intensity of the light spot has a Gauss profile; thus an inhomogeneous spin density will be excited on the sample plane, as shown in Fig. 2(b). Under normal incidence, the gradient of the spin density will induce a diffused spin polarization current (SPC) FIG. 1. The schematic diagram of the experimental setup. directions in the plane of the QWs; E0 ; Pcirc ; e^ are the electric field amplitude of the light, the degree of circular polarization, and the unit vector pointing in the direction of light propagation, respectively.10 For normal incidence, e^ is parallel to [001] crystallographic orientation and hence the current vanishes as e^x ¼ 0.10 However, under normal incidence, a sizeable charge current induced by circularly polarized light is observed with the light spot moving away from the original point of the x direction. This current corresponds to the typical characteristics of CPGE which can be described by formula jc ¼ j0 sin 2u; (2) where j0 is the amplitude of current and u is the angle between the polarization directions of incident light and the optical axis of the quarter-wave plate (the PEM equals to a quarter-wave plate), so it can be named as ACPGE current. It has been believed that this current is derived from a swirly current depending on the spin orientation of photo-induced carriers.6 The swirling charge current can be regarded as a transformation of the photon angular momentum into the angular qzr ¼ Drr nz ðrÞsin 2u; (3) where D is the electron diffusion coefficient, nz(r) is the spin density along the z direction, and r is the radial direction in the x–y plane. Since the radius of light spot is about 2 mm which is much bigger than the spin diffusion length, the contribution of the SPC outside the light spot can be neglected. Thus, nz(r) is decided by the distribution of the light intensity, namely, nz ðrÞ ¼ N0z GðrÞ, where N0z is the average electron spin density with the maximal circular polarization degree and G(r) is the Gaussian distribution of the light spot. According to the RSHE, the SPC flowing in the plane of the sample will suffer from a spin transverse force.6,15 As a result, a transverse electric current (density) perpendicular to both the direction of the SPC and the direction of the spin polarization is produced, which can be expressed as j ¼ ceqzr ^z ; (4) where e, c are the elementary charge, the spin-orbit interaction coefficient based on RSHE, respectively. Similar to the SHE, RSHE has the extrinsic and intrinsic mechanisms. The former originates from spin-dependent scattering of defects and the latter is based on the spin band splitting. Since the FIG. 2. (a) The ACPGE currents as a function of the spot location corresponding to 1hh-1e and 1lh-1e. The dashed lines are for ease of viewing. (b) Geometry of the sample. And schemed mechanism of RSHE induced by Gauss-profile light spot. Illustration of the movement of spin polarized electrons under normal incidence of rþ polarized light. The red dashed arrows denote the spin diffusional direction and the black dashed arrows denote the spin polarized direction. The solid (black) arrows denote the SOC induced transverse electric current. The blue circles denote two electrodes, and the ACPGE current is collected by the two electrodes. (c) The two-dimensional colored figure denotes the ACPGE currents at different spot locations and different wavelengths (near the transitions of 1hh-1e and 1lh-1e). The crosses (the triangles) mark the maximum (the minimum) positions at certain wavelengths. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.228.14 On: Tue, 17 Mar 2015 01:23:55 083509-3 Zhu et al. J. Appl. Phys. 115, 083509 (2014) undoped QWs studied here has a sizeable Rashba effect,13,14 the extrinsic and intrinsic mechanisms probably coexist. As a result, a swirling electric current will be induced around the light spot, which will further generate an observed ACPGE current. IV. RESULTS AND DISCUSSION In the experiment, the ACPGE currents are measured as a function of the spot location at different wavelengths (near the transitions of 1hh-1e (905 nm) and 1lh-1e (955 nm)). It is worth noting that the current corresponding to 1lh-1e is actually contributed by not only the 1lh-1e transition but also the 1hh-1e transition. As shown in Fig. 2(a), the ACPGE currents reverse the sign from the left to right side, just like a sine curve. It suggests that there is not a directed current but a current swirling over the center of the light spot. The movement of the swirling current along x direction will change the amplitude of the ACPGE current. The sine-like curve can be understood as follows. When the light spot irradiates on the left side of the original point (see Fig. 2(b)), a pure spin current along þx direction will flow through the original point. Due to the RSHE, an electric current along y direction is collected by the two circle electrodes. On the contrary, when the light spot irradiates on the right side of the original point, the pure spin current will flow along x direction; therefore, the induced electric current is along þy direction. When the light spot irradiates just on the original point, the ACPGE current is zero because there is no pure spin current flow through the original point. When the light spot irradiates far away from the original point (more than 4.5 mm), the ACPGE current disappears for the smaller pure spin current. According to Eq. (3), Eq. (4) and Fig. 2, the ACPGE current can be expressed phenomenologically as JACPGE ¼ ceDSN0z wðx; rÞrr GðrÞ; (5) where S is the cross sectional area of the current and wðx; rÞ is a sine-like function depending on the location and the radius of the light spot. As shown in Fig. 2(c), the extreme positions are focused on the location of x ¼ 61.5 mm (the little bit of shift near 1lh-1e transition is probably due to the contribution from the 1hh-1e transition), which indicates that the extreme position of ACPGE current is wavelength independent. Before giving a theoretical explanation to this independence, we will deduce the specific expression of the average electron spin density N0z in Eq. (10). Under normal incidence, the circularly polarized light will induce spin polarized electrons. In particular, the left-handed circular polarized light ðrþ Þ will induce spin-up electrons, and a small part of spin-down electrons still exist due to the spin-flipping of spin-up electrons. In order to explain the experimental result, an LOP (longitudinal optical phonons)-assisted, spin-preserving relaxation time of the hot electrons which jump from the excited states to the bottom states of conduction band (inelastic process) is introduced.16–18 In the relaxation time approximation, one can get dn" n" n" n" n# n# ¼ g þ þ ; dt s0 s1 ss s1 ss (6) dn# n# n# n# n" n" ¼ þ þ ; dt s0 s1 ss s1 ss (7) where n" ; n# , g are the density of spin-up electrons, the density of spin-down electrons, and the generation rate of spinup electrons, respectively; s0 is the lifetime of photoinduced electrons, s1 the LOP-assisted, spin-preserving relaxation time of the hot electrons, and ss is the spin relaxation time. The reason why there is the spin relaxation time in the above two expressions rather than the momentum relaxation time will be discussed later. Under the condition of steady state, one can get further expressions as n" þ n# ¼ gs0 ; 1 1 2 2 : ¼ þ þ n" n# ¼ gs ; s s0 s1 ss (8) (9) In Eq. (8), n" þ n# is the total electron spin density, which is proportional to the PC current. And in Eq. (9), s is an effective relaxation time. We regard n" n# as the average electron spin density, namely, N0z ¼ gs . Then, the ACPGE current can be further expressed as JACPGE ¼ gs ceDSwðx; rÞrr GðrÞ: (10) When the wavelength is tuned from 905 nm to 965 nm, the changes of the size of the light spot can be neglected. In fact, the vary of wavelength only changes the generation rate g and the effective relaxation time s . Therefore, according to Eq. (10), the vary of wavelength only affects the amplitude of the ACPGE current but does not change the extreme position. Fig. 3(a) shows the ACPGE current as a function of wavelength when the light spot is fixed on the position of x ¼ þ1.5 mm. If the ACPGE current origins from RSHE, one FIG. 3. The light spot is fixed on the position of x ¼ þ1.5 mm. (a) The red empty circles denote the ACPGE current as a function of excitation wavelength. (b) The black empty circles denote the common photoinduced current as a function of excitation wavelength. The external electric field applied on the two circle electrodes is 100 V/cm. (c) The blue (yellow) empty circles denote the ACPGE (ALPGE) current normalized by the common photoinduced current. All of the solid lines are for ease of viewing. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.228.14 On: Tue, 17 Mar 2015 01:23:55 083509-4 Zhu et al. J. Appl. Phys. 115, 083509 (2014) should observe that the ACPGE current reverses the direction when the optical interband transition changes between 1lh-1e and 1hh-1e, as a result of sign change of electron spin involved. However, we did not observe obvious opposite in the sign, which is probably due to the fact that the signal corresponding to 1lh-1e is contributed from not only the 1lh-1e transition but also the 1hh-1e transition. In other words, signal corresponding to 1lh-1e is small and the current corresponding to the so-called 1lh-1e marked in Figs. 2 and 3 is actually the combined action of 1lh-1e and 1hh-1e. As a comparison, the common photoinduced current is also measured under the same conditions, as shown in Fig. 3(b). According to Eq. (8) and Ohm’s law, the common photoinduced current can be expressed as JPC ¼ gs0 elES; (11) where l is the mobility of electrons, E is the electric field applied on the two circle electrodes (in Fig. 3(b), E ¼ 100 V/cm), and S is the cross sectional area of the current. One can surprisingly find that the two curve shapes in Figs. 3(a) and 3(b) are quite different. To eliminate the difference of the nonequilibrium electron’s density at different wavelengths, we normalize the ACPGE current with the common photoinduced current, because the common photoinduced current is proportional to the density of photoinduced nonequilibrium electrons. And the normalized ACPGE current is shown in Fig. 3(c). What is more, the normalized anomalous linear photogalvanic effect (ALPGE) current is also illustrated in Fig. 3(c), which is measured by the lock-in amplifier with the reference frequency at 100 KHz. (The peak retardation of PEM still remains to be k/4.) For further details of ALPGE, please see Ref. 19. It is clear from Fig. 3(c) that the two curves are different in not only the shape but also the size of value. Since the ALPGE is derived from the momentum relaxation, the ACPGE will not come from the momentum relaxation, and we believe that the spin relaxation is dominant to the ACPGE. Thus, there is the spin relaxation time rather than momentum relaxation time in Eqs. (6) and (7). According to Eq. (10), Eq. (11), and Einstein’s relation (Dl ¼ kBeT ), the normalized JACPGE (see Fig. 3(c)) can be expressed by JACPGE s kB Tcwðx; rÞrr GðrÞ : ¼ JPC s0 eE (12) c, T, E are invariable at different excitation wavelengths. The little difference of wðx; rÞ or G(r) at different wavelengths can be ignored. Therefore, Eq. (12) is simplified as JACPGE s / : JPC s0 (13) The normalized ACPGE current that decreases at smaller wavelengths can be understood as follows. The current corresponding to the transition of 1lh-1e is so small that it can be ignored. And the current mainly comes from the transition of 1hh-1e. As the mixing of 1hh band and 1lh band, the current coming from the transition of 1hh-1e extends to a broad spectrum (see Fig. 3(a)). The splitting of the conduction band can be described as the result of action on the electron spin of an effective magnetic field. Spin precession around this field in the intervals between collisions gives rise to spin relaxation. Previously, Tackeuchi et al.20 have shown that the spin relaxation at room temperature in GaAs/AlGaAs quantum well is mainly governed by “Dyakonov-Perel” (DP) interaction. Since InGaAs has many physical similarities with GaAs, DP interaction seems to be the most plausible spin relaxation mechanism.21 Taking the spin splitting of the conduction band into consideration, the spin relaxation rate of nondegenerate carriers can be simplified by neglecting the energy dependence of sp as21,22 1 2kB TðaE1e =hÞ2 sp ; ¼ exp Eg ss kB T (14) where a, E1e, are the dimensionless numerical coefficient governing the spin splitting of the conduction band, the quantum confined energy denoting the bottom energy of the first conduction subband, and the kinetic energy of electrons in the first conduction subband, respectively. ss in Eq. (14), different from that in Ref. 21 where it is an average relaxation time, is a local variable. If we just consider the transitions near the bottom of conduction subband, especially for < kB T; ss will decrease rapidly with the increase of . Since is decided by the energy of exciting light, ss will decrease rapidly with the decrease of wavelength. What is more, with the decrease of excitation wavelength, the relaxation time s1 of hot electrons jumping from the excited states to the bottom states of conduction band decreases sharply for the scattering of longitudinal optical phonon.16,18,23,24 The lifetime s0 of photoinduced electrons is a constant at room temperature. To sum up, the effective relaxation time s in Eq. (13) will decrease, and then the normalized JACPGE (shown in Fig. 3(c)) will decrease rapidly at smaller excitation wavelengths. V. CONCLUSIONS In conclusion, the excitation wavelength dependence of the ACPGE current arising from RSHE in undoped InGaAs/ AlGaAs quantum wells is measured under normal incidence of circularly polarized light at room temperature. We found that the spot location with the maximum ACPGE current is wavelength independent. And the normalized ACPGE current decreases at smaller wavelengths, which can be attributed to the sharp decrease of spin relaxation time (ss ) and hot electron relaxation time (s1 ) at smaller wavelengths. The study of the excitation wavelength dependence of ACPGE current is a good supplement to the in-depth investigation of RSHE. This work shows promising applications of undoped InGaAs/ AlGaAs QWs in spintronics based on RSHE. ACKNOWLEDGMENTS The work was supported by the 973 program (2012CB921304, 2012CB619306) and the National Natural Science Foundation of China (No. 60990313). [This article is copyrighted as indicated in the article. 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