First-principles prediction of doped graphane as a high-temperature electron-phonon superconductor.

We predict by first-principles calculations that p-doped graphane is an electron-phonon superconductor with a critical temperature above the boiling point of liquid nitrogen. The unique strength of the chemical bonds between carbon atoms and the large density of electronic states at the Fermi energy arising from the reduced dimensionality give rise to a giant Kohn anomaly in the optical phonon dispersions and push the superconducting critical temperature above 90 K. As evidence of graphane was recently reported, and doping of related materials such as graphene, diamond, and carbon nanostructures is well established, superconducting graphane may be feasible.

A Stochastic Model for Electron Multiplication Charge-Coupled Devices - From Theory to Practice

Electron multiplication charge-coupled devices (EMCCD) are widely used for photon counting experiments and measurements of low intensity light sources, and are extensively employed in biological fluorescence imaging applications. These devices have a complex statistical behaviour that is often not fully considered in the analysis of EMCCD data. Robust and optimal analysis of EMCCD images requires an understanding of their noise properties, in particular to exploit fully the advantages of Bayesian and maximum-likelihood analysis techniques, whose value is increasingly recognised in biological imaging for obtaining robust quantitative measurements from challenging data. To improve our own EMCCD analysis and as an effort to aid that of the wider bioimaging community, we present, explain and discuss a detailed physical model for EMCCD noise properties, giving a likelihood function for image counts in each pixel for a given incident intensity, and we explain how to measure the parameters for this model from various calibration images. © 2013 Hirsch et al.

Nitrogen defects and ferromagnetism in Cr-doped dilute magnetic semiconductor AlN from first principles

High Curie temperature of 900 K has been reported in Cr-doped AlN diluted magnetic semiconductors prepared by various methods, which is exciting for spintronic applications. It is believed that N defects play important roles in achieving the high-temperature ferromagnetism in good samples. Motivated by these experimental advances, we use a full-potential density-functional-theory method and supercell approach to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies (V-N). We investigate the structural and electronic properties of V-N, single Cr atom, Cr-Cr atom pairs, Cr-V-N pairs, and so on. In each case, the most stable structure is obtained by comparing different atomic configurations optimized in terms of the total energy and the force on every atom, and then it is used to calculate the defect formation energy and study the electronic structures. Our total-energy calculations show that the nearest substitutional Cr-Cr pair with the two spins in parallel is the most favorable and the nearest Cr-V-N pair makes a stable complex. Our formation energies indicate that V-N regions can be formed spontaneously under N-poor condition because the minimal V-N formation energy equals -0.23 eV or Cr-doped regions with high enough concentrations can be formed under N-rich condition because the Cr formation energy equals 0.04 eV, and hence real Cr-doped AlN samples are formed by forming some Cr-doped regions and separated V-N regions and through subsequent atomic relaxation during annealing. Both of the single Cr atom and the N vacancy create filled electronic states in the semiconductor gap of AlN. N vacancies enhance the ferromagnetism by adding mu(B) to the Cr moment each but reduce the ferromagnetic exchange constants between the spins in the nearest Cr-Cr pairs. These calculated results are in agreement with experimental observations and facts of real Cr-doped AlN samples and their synthesis. Our first-principles results are useful to elucidate the mechanism for the ferromagnetism and to explore high-performance Cr-doped AlN diluted magnetic semiconductors.

One-dimensional quantum waveguide theory of Rashba electrons

The ballistic spin transport in one-dimensional waveguides with the Rashba effect is studied. Due to the Rashba effect, there are two electron states with different wave vectors for the same energy. The wave functions of two Rashba electron states are derived, and it is found that their phase depend on the direction of the circuit and the spin directions of two states are perpendicular to the circuit, with the +pi/2 and -pi/2 angles, respectively. The boundary conditions of the wave functions and their derivatives at the intersection of circuits are given, which can be used to investigate the waveguide transport properties of Rashba spin electron in circuits of any shape and structure. The eigenstates of the closed circular and square loops are studied by using the transfer matrix method. The transfer matrix M(E) of a circular arc is obtained by dividing the circular arc into N segments and multiplying the transfer matrix of each straight segment. The energies of eigenstates in the closed loop are obtained by solving the equation det[M(E)-I]=0. For the circular ring, the eigenenergies obtained with this method are in agreement with those obtained by solving the Schrodinger equation. For the square loop, the analytic formula of the eigenenergies is obtained first The transport properties of the AB ring and AB square loop and double square loop are studied using the boundary conditions and the transfer matrix method In the case of no magnetic field, the zero points of the reflection coefficients are just the energies of eigenstates in closed loops. In the case of magnetic field, the transmission and reflection coefficients all oscillate with the magnetic field; the oscillating period is Phi(m)=hc/e, independent of the shape of the loop, and Phi(m) is the magnetic flux through the loop. For the double loop the oscillating period is Phi(m)=hc/2e, in agreement with the experimental result. At last, we compared our method with Koga's experiment. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3253752]

Spin dynamics of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system

We report on time-resolved Kerr rotation measurements of spin coherence of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While the transverse spin lifetime (T-2(*)) of electrons decreases monotonically with increasing magnetic field, it has a nonmonotonic dependence on the temperature and reaches a peak value of 596 ps at 36 K, indicating the effect of intersubband electron-electron scattering on the electron-spin relaxation.

Effective-mass theory for hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots

The electronic structures in the hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels and optical transition energies are calculated. In our calculation, the effect of finite offset, valence-band mixing, the effects due to the different effective masses of electrons and holes in different regions, and the real quantum dot structures are all taken into account. The results show that (1) electronic energy levels decrease monotonically, and the energy difference between the energy levels increases as the GaAs quantum dot (QD) height increases; (2) strong state mixing is found between the different energy levels as the GaAs QD width changes; (3) the hole energy levels decrease more quickly than those of the electrons as the GaAs QD size increases; (4) in excited states, the hole energy levels are closer to each other than the electron ones; (5) the first heavy- and light-hole transition energies are very close. Our theoretical results agree well with the available experimental data. Our calculated results are useful for the application of the hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots to photoelectric devices.

Atomic configurations of dislocation core and twin boundaries in 3C-SiC studied by high-resolution electron microscopy

The defects in 3C-SiC film grown on (001) plane of Si substrate were studied using a 200 kV high-resolution electron microscope with point resolution of 0.2 nm. A posterior image processing technique, the image deconvolution, was utilized in combination with the image contrast analysis to distinguish atoms of Si from C distant from each other by 0.109 nm in the [110] projected image. The principle of the image processing technique utilized and the related image contrast theory is briefly presented. The procedures of transforming an experimental image that does not reflect the crystal structure intuitively into the structure map and of identifying Si and C atoms from the map are described. The atomic configurations for a 30 degrees partial dislocation and a microtwin have been derived at atomic level. It has been determined that the 30 degrees partial dislocation terminates in C atom and the segment of microtwin is sandwiched between two 180 degrees rotation twins. The corresponding stacking sequences are derived and atomic models are constructed according to the restored structure maps for both the 30 degrees partial dislocation and microtwin. Images were simulated based on the two models to affirm the above-mentioned results.

Theory of ultrafast optical manipulation of electron spins in quantum wells

Based on a multiparticle-state stimulated Raman adiabatic passage approach, a comprehensive theoretical study of the ultrafast optical manipulation of electron spins in quantum wells is presented. In addition to corroborating experimental findings [Gupta , Science 292, 2458 (2001)], we improve the expression for the optical-pulse-induced effective magnetic field, in comparison with the one obtained via the conventional single-particle ac Stark shift. Further study of the effect of hole-spin relaxation reveals that, while the coherent optical manipulation of electron spin in undoped quantum wells would deteriorate in the presence of relatively fast hole-spin relaxation, the coherent control in doped systems can be quite robust against decoherence. The implications of the present results on quantum dots will also be discussed. (c) 2005 American Institute of Physics.

Theory of terahertz-photocurrent resonances in miniband superlattices

The currents of de and ac components and their phase-angle cosines for a superlattice under a direct bias and alternating field are calculated with the balance equations. It is found that the de currents as functions of the direct field show resonance peaks at the fields corresponding to the Bloch frequency equal to n omega. With increasing alternating field intensity the resonance peaks of higher harmonic increase, and simultaneously the first peak caused by the de field decreases. The results are in good agreement with the experimental results, indicating that this resonance can be understood in terms of electron acceleration within the miniband, i.e., it is a bulk superlattice effect, rather than caused by the electric-field localization mechanism (Wannier Stark ladder). The phase-angle cosine for the first harmonic cos phi(1) becomes negative when the Bloch frequency increases to be larger than the frequency of the ac field omega, and it also shows resonance peaks at the resonance frequencies n omega. The negative cos phi(1) may cause the energy transferred to the alternating field, i.e., oscillation of the system.

On detection wavelength and electron-hole wave function overlap of type II InAs/In-x Ga1-x Sb superlattice infrared photodetector

In this paper, the detection wavelength and the electron-hole wave function overlap of InAs/IrxGa1-xSb type II superlattice photodetectors are numerically calculated by using the envelope function and the transfer matrix methods. The band offset is dealt with by employing the model solid theory, which already takes into account the lattice mismatch between InAs and InxGa1-xSb layers. Firstly, the detection wavelength and the wave function overlap are investigated in dependence on the InAs and InxGa1-xSb layer thicknesses, the In mole fraction, and the periodic number. The results indicate that the detection wavelength increases with increasing In mole fraction, InAs and InxGa1-xSb layer thicknesses, respectively. When increasing the periodic number, the detection wavelength first increases distinctly for small periodic numbers then increases very slightly for large period numbers. Secondly, the wave function overlap diminishes with increasing InAs and InxGa1-xSb layer thicknesses, while it enhances with increasing In mole fraction. The dependence of the wave function overlap on the periodic number shows the same trend as that of the detection wavelength on the periodic number. Moreover, for a constant detection wavelength, the wave function overlap becomes greater when the thickness ratio of the InAs over InxGa1-xSb is larger.

1ST PRINCIPLE CALCULATIONS OF QUASI-PARTICLE ENERGIES FOR BAND STRUCTURES OF SI AND GAAS

We have applied the Green function theory in GW approximation to calculate the quasiparticle energies for semiconductors Si and GaAs. Good agreements of the calculated excitation energies and fundamental energy gaps with the experimental band structures were achieved. We obtained the calculated fundamental gaps of Si and GaAs to be 1.22 and 1.42 eV in comparison to the experimental values of 1.17 and 1.52 eV, respectively. Ab initio pseudopotential method has been used to generate basis wavefunctions and charge densities for calculating dielectric matrix elements and electron self-energies.

Advanced superconducting electron cyclotron resonance ion source SECRAL: Design, construction, and the first test result

Superconducting electron cyclotron resonance (ECR) ion source with advanced design in Lanzhou (SECRAL) is a next generation ECR ion source and aims for developing a very compact superconducting ECR ion source with a structure and high performances for highly charged ion-beam production. The ion source was designed to be operated at 18 GHz at initial operation and finally will be extended to 28 GHz. The superconducting magnet confinement configuration of the ion source consists of three axial solenoid coils and six sextupole coils with a cold iron structure as field booster and clamping. At full excitation, this magnet assembly can produce peak mirror fields on the axis of 3.6 T at injection, 2.2 T at extraction, and a radial sextupole field of 2.0 T at plasma chamber wall. What is different from the traditional design, such as LBNL VENUS and LNS SERSE, is that the three axial solenoid coils are located inside of the sextupole bore in order to reduce the interaction forces between the sextupole coils and the solenoid coils. SECRAL may open the way for building a compact and high-performance 18-28 GHz superconducting ECR ion source. Very preliminary commissioning results are promising. Detailed design, construction issues and very preliminary test results of the ion source at 18 GHz are presented.