Yang Hai-Feng

Tuning the Electronic Structure of Sr2IrO4 Thin Films by Bulk Electronic Doping Using Molecular Beam Epitaxy

M. Y. Li,1 Z. T. Liu,1 H. F. Yang,1 J. L. Zhao,1 Q. Yao,1, 2 C. C. Fan,1 J. S. Liu,1 B. Gao,1 D. W. Shen,1, ∗ and X. M. Xie1 1State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences, Shanghai 200050, China 2State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, ChinaBy means of oxide molecular beam epitaxy with shutter-growth mode, we fabricate a series of electron-doped (Sr1−xLax)2IrO4 (001) (x=0, 0.05, 0.1 and 0.15) single crystalline thin films and then investigate the doping dependence of the electronic structure utilizing in-situ angle-resolved photoemission spectroscopy. It is found that with the increasing doping content, the Fermi levels of samples progressively shift upward. Prominently, an extra electron pocket crossing the Fermi level around the M point is evidently observed in the 15% nominal doping sample. Moreover, bulk-sensitive transport measurements confirm that the doping effectively suppresses the insulating state with respect to the as-grown Sr2IrO4, though the doped samples still remain insulating at low temperatures due to the localization effect possibly stemming from disorders including oxygen deficiencies. Our work provides another feasible doping method to tune electronic structure of Sr2IrO4.

Spectral decomposition at a complex laser polarization configuration

We study the role of laser polarization in the diamagnetic spectrum for the transition from the ground state to the highly excited Rydberg states through a single photon absorption. For simplicity, one usually polarizes the irradiation laser to the selected main quantum axis, which is along the applied external electric or magnetic field. The transition selection rule is simply expressed as Delta m = 0, which corresponds to the pi transition. When the polarization is circularly polarized around the main axis, the s + or s transition occurs, corresponding to the selection rule of Delta m = 1 or Delta m = -1, respectively. A slightly more complex case is that the laser is linearly polarized perpendicular to the main axis. The numerical calculation shows that we can decompose the transition into the sum of sigma(+) and sigma(-) transitions, it is noted as the sigma transition. For the more complex case in which the laser is linearly polarized with an arbitrary angle with respect to the main axis, we have to decompose the polarization into one along the main axis and the other one perpendicular to the main axis. They correspond to p and s transitions, respectively. We demonstrate that these transitions in the diamagnetic spectrum and the above spectral decomposition well explain the experimentally observed spectra.

Stark spectrum of barium in highly excited Rydberg states

We present observations of Stark spectra of barium in highly excited Rydberg states in the energy region around n = 35. The one-photon excitation concerns the pi transition. The observed Stark spectra at electric fields ranging from 0 to 60 V.cm(-1) are well explained by the diagonalization of the Hamiltonian incorporating the core effects. From the Stark maps, the anti-crossings between energy levels are identified experimentally and theoretically. The time of flight spectra at the specified Stark states are recorded, where the deceleration and acceleration of barium atoms are observed. This is very consistent with the prediction derived from the Stark maps from the point of view of energy conservation.

Lifetime Measurement for 6snp Rydberg States of Barium

We present a simple and efficient method for measuring the atomic lifetimes in order of tens of microseconds and demonstrate it in the lifetime determination of barium Rydberg states. This method extracts the lifetime information from the time-of-flight spectrum directly, which is much more efficient than other methods such as the time-delayed field ionization and the traditional laser induced fluorescence. The lifetimes determined with our method for barium Rydberg 6snp (n = 37-59) series are well coincident with the values deduced from the absolute oscillator strengths of barium which were given in the literature [J. Phys. B 14 ( 1981) 4489, 29 (1996) 655] on experiments.

Photodetachment of H(-) near an elastic surface in a magnetic field

This paper investigates the photodetachment of the negative hydrogen ion H- near an elastic wall in a magnetic field. The magnetic field confines the perpendicular motion of the electron, which results in a real three-dimensional well for the detached electron. The analytical formulas for the cross section of the photodetachment in the three-dimensional quantum well are derived based on both the quantum approach and closed-orbit theory. The magnetic field and the elastic surface lead to two completely different modulations to the cross section of the photodetachment. The oscillation amplitude depends on the strength of the magnetic field, the ion-wall distance and the photon polarization as well. Specially, for the circularly polarized photon-induced photodetachment, the cross sections display a suppressed (E - E-th)(1/2) threshold law with energy E in the vicinity above Landau energy E-th, contrasting with the (E - Eth)(-1/2) threshold law in the presence of only the magnetic field. The semiclassical calculation fits the quantum result quite well, although there are still small deviations. The difference is attributed to the failure of semiclassical mechanics.

Spatial variation of moisture content in shallow aeolian sandy soil in southern marginal zone of Gurbantonggut Desert.

In May 2008,the vegetation,terrain and landforms were investigated at a plot of 200 m x 50 m in a typical region of longitudinal sand dunes in the southern marginal zone of the Gurbantonggut Desert.At the plot 180 quadrats were selected for the observation,measurement and soil sampling.These soil samples were sampled from the surface layer(0～10 cm)and shallow layer(10～30 cm),and their moisture content values were measured. Under the support of geostatistical software,such as GS~+ and geostatistical model in ArcGis,this paper analyzes the spatial varying pattern of soil moisture content at the different positions of two longitudinal sand dunes in the study area.The results show that the semi-variance functional values of soil moisture content in the two layers can be well fitted as an exponential model.The variation ranges of surface layer and shallow layer of aeolian sandy soil are 37.8 m and 21.3 m,and the ratios of partial sill and sill[C/(C_0+C)]are 75.1%and 75.6%,respectively.These reveal that there is a high spatial autocorrelation in soil moisture between the surface layer and the shallow layer of aeolian sandy soil,and the spatial variation of the soil moisture content comes mainly from a structural difference. The results can objectively reflect the spatial structure pattern of soil moisture in the plot.The figure of Kriging interpolation shows that soil moisture content is correspondently increased with the height increase of terrain.The analyzed results show that the values of soil moisture content are in an order of crestwindward slopeleeward slopeinter-dune land in the surface layer(0～10 cm)of aeolian sandy soil,and an order of windward slopecrestleeward slopeinter-dune land in the shallow layer(1030 cm).

Scaled-energy spectroscopy of a ｜M｜= 1 Rydberg barium atom in an electric field

We observe strong energy-dependent quantum defects in the scaled-energy Stark spectra for |M| = 1 Rydberg states of barium atoms at three scaled energies: e = –2.000, e = –2.500 and e = –3.000. In an attempt to explain the observations, theoretical calculations of closed orbit theory based on a model potential including core effect are performed for non-hydrogenic atoms. While such a potential has been uniformly successful for alkali atoms with a single valence electron, it fails to match experimental results for barium atoms in the 6snp Rydberg states with two valence electrons. Our study points out that this discrepancy is due to the strong perturbation from the 5d8p state, which voids the simple approximation for constant quantum defects of principle quantum number n.

Tunneling between double wells of atom in crossed electromagnetic fields

The tunneling between double wells of atom in crossed electromagnetic fields is investigated by a one-dimensional Hamiltonian model. The crossed fields induced outer well is apart from the nuclear origin and it is very difficult to access by means of spectroscopy but it will be possible if there exists the tunneling of the electron between the outer well and the Coulomb potential predominated well at the nuclear origin. A one-dimensional quantum calculation with B-spline basis has been performed for hydrogen atom in crossed fields accessible in our laboratory, at B = 0.8 T and F = -220 V.cm(-1). The calculation shows that the wavefunctions of some excited states close to the Stark saddle point in the outer well extend over to the Coulomb potential well, making it possible to penetrate the quantum information of the outer well. However, the tunneling rate is very small and the spectral measurement of the transitions from the ground state should be of a high resolution and high sensitivity.

Non-linear spectral splitting of Rydberg sodium in external fields

We have studied highly excited sodium in various electric fields, parallel electric and magnetic fields, with one sigma and pi photon irradiation, and even in a magnetic field with a complex laser polarization configuration. The sigma spectra shows a simple linear Stark effect with the applied electric field, while the pi spectra exhibits a strong non-linear dependence on the electric field. The pi transitions in parallel fields show a similar behavior to that in a pure electric field but the spectra get more smooth due to the magnetic field. The diamagnetic spectrum with laser polarization angles between 0 and pi/2 proves that it can be reproduced by simple linear combination of pi and sigma components, indicating there is no interference between the pi and sigma channels. A full quantum calculation considering the quantum defects accounts for all the observations. The quantum defects, especially for the channel n pi, play an important role in the spectral profile.

Nonlinear spectroscopy of barium in parallel electric and magnetic fields

We report on the experimental spectral observations of barium in parallel electric and magnetic fields. The laser pulse is linearly polarized along or perpendicular to the fields, leading to the states m = 0 and the states m = +/- 1 populated, respectively, by one photon excitation. By sweeping the electric field, we observe the linear and nonlinear splitting of the diamagnetic spectrum as the electric field increases. The spectral anticrossing is induced by the atomic core effect. The Stark spectrum also shows an obvious nonlinear quadratic behavior when the applied magnetic field varies strongly. All spectra are well explained by the full quantum calculation after taking the quantum defect effects of the channel ns up to nf into account.