"Intrinsic" acceptor ground state splitting in silicon: an isotopic effect

One of the oldest open questions in semiconductor physics is the origin of the small splittings of the neutral acceptor ground state in silicon which lead to a distribution of doublet splittings rather than the fourfold-degenerate state of Gamma(8) symmetry expected in the absence of perturbations. Here we show that these acceptor ground state splittings are absent in the photoluminescence spectra of acceptor bound excitons in isotopically purified 28Si, demonstrating conclusively the surprising result that the splittings previously observed in natural Si result from the randomness of the Si isotopic composition.     

铁小檗碱化合物结构的密度泛函理论研究

本工作采用密度泛函理论(B3LYP)方法对铁小檗碱化合物的中性分子及离子进行了结构优化和频率计算.结果表明:无论是中性分子还是离子,铁小檗碱物化合物基态结构都是小檗碱基态结构中的与N原子相邻的C10原子上键合一个Fe原子形成的.其阳离子结构具有C_1对称性和~3A电子态;中性分子的基态结构具有C_1对称性和2A电子态;阴离子的基态结构具有C1对称性和5A电子态.当铁小檗碱物化合物的阳离子得到一个电子后,C(10)-Fe(26)及Fe(26)-H(44)的键长都有所变短;当中性分子得到一个电子变成阴离子后,C(10)-Fe(26)及Fe(26)-H(44)的键长都有所增加.     

铁小檗碱化合物结构的密度泛函理论研究

本工作采用密度泛函理论（B3LYP） 方法对铁小檗碱化合物的中性分子及离子进行了结构优化和频率计算。结果表明:无论是中性分子还是离子,铁小檗碱物化合物基态结构都是小檗碱基态结构中的与N原子相邻的C10原子上键合一个Fe原子形成的。其阳离子结构具有C1对称性和3A电子态;中性分子的基态结构具有C1对称性和2A电子态;阴离子的基态结构具有C1对称性和5A电子态。当铁小檗碱物化合物的阳离子得到一个电子后,C(10)-Fe(26)及Fe(26)-H(44)的键长都有所变短;当中性分子得到一个电子变成阴离子后,C(10)-Fe(26)及Fe(26)-H(44)的键长都有所增加。     

Effect of shear on phase-ordering dynamics with order-parameter-dependent mobility: the large-n limit

The effect of shear on the ordering kinetics of a conserved order-parameter system with O(n) symmetry and order-parameter-dependent mobility Gamma(straight phi-->) approximately (1-straight phi-->(2)/n)(alpha) is studied analytically within the large-n limit. In the late stage, the structure factor becomes anisotropic and exhibits multiscaling behavior with characteristic length scales (t(2alpha+5)/ln t)(1/2(alpha+2)) in the flow direction and (t/ln t)(1/2(alpha+2)) in directions perpendicular to the flow. As in the alpha=0 case, the structure factor in the shear-flow plane has two parallel ridges.     

Evidence for a new low-lying resonance state in 7He

Low-lying resonance states in 7He(6He+n), formed after fragmentation reactions of a 227 MeV/nucleon 8He beam on a carbon target, have been studied. Coincidences between 6He nuclei and neutrons, corresponding to the one-neutron knockout channel in 8He, were selected. The relative energy spectrum in the 6He+n system shows a structure, which is interpreted as the 7He ( Ipi = 3/2(-)) ground state, unbound with 0.43(2) MeV relative to the 6He+n system and a width of Gamma = 0.15(8) MeV overlapping with an excited ( Ipi = 1/2(-)) state observed at 1.0(1) MeV with a width of Gamma = 0.75(8) MeV.     

Electronic structure of MgB2 from angle-resolved photoemission spectroscopy

The first angle-resolved photoemission spectroscopy results from MgB2 single crystals are reported. Along the GammaK and GammaM directions, we observed three distinct dispersive features approaching the Fermi energy. These can be assigned to the theoretically predicted sigma (B 2p(x,y)) and pi (B 2p(z)) bands. In addition, a small parabolic-like band is detected around the Gamma point, which can be attributed to a surface-derived state. The overall agreement between our results and the band calculations suggests that the electronic structure of MgB2 is of a conventional nature, thus implying that electron correlations are weak and may be of little importance to superconductivity in this system.     

Magneto-Luminescence of excitons bound to neutral acceptor in cadmium telluride

The radiative recombination of excitons bound to neutral acceptor (A⁰, X) in high-purity CdTe has been investigated in magnetic fields up to 100 KG. A doublet structure observed in (A⁰, X) emission line is explained by considering the j-j coupling between two holes and an electron. Zeeman splittings in emission lines originate from the transition between J = 1/2 and 3/2 states in bound-exciton complex, and the acceptor-ground state. It is supposed that the acceptor which binds excitons is due to certain complex having C_3υ symmetry which is lower than the host lattice.     

Direct evidence for shallow acceptor states with nonspherical symmetry in GaAs

We investigate the energy and symmetry of Zn and Be dopant-induced acceptor states in GaAs using cross-sectional scanning tunnelling microscopy (STM) and spectroscopy at low temperatures. The ground and first excited states are found to have a nonspherical symmetry. In particular, the first excited acceptor state has a T(d) symmetry. Its major contribution to the STM empty-state images allows us to explain the puzzling triangular shaped contrast observed in the empty-state STM images of acceptor impurities in III-V semiconductors.     

Atomic and electronic structures of p-type dopants in 4H-SiC

Using hybrid density functional calculation,we study the atomic and electronic structures of p-type dopants,B,Al and Ga,in 4 H-SiC.For B,depending on the growth condition,it can occupy both Si and C sites.In contrast,Al and Ga on the C sites exhibit too high formation energy to exist in a significant amount.In 4 H-SiC,there exist two types of Si sites in wurtzite-like and zincblende-like local coordination,respectively.Our calculations suggest that the dopant atoms have negligible preference occupying the two sites.In neutral charge state,all the dopants exhibit significant distortions from the structure in the negatively charged state.For most cases,our calculations yield three distorted structures,in which the most stable one has the dopant atom displaced along its bond with one of the surrounding equatorial Si or C atoms,lowering the C_(3 v) symmetry to Cs symmetry(i.e.,a mirror symmetry only).Among the three dopant elements,Al on Si sites exhibits overall the lowest formation energy and the shallowest acceptor level.Nevertheless,it is not a hydrogenic dopant with the acceptor level 0.12 eV above the valence band maximum based on calculation using a 400-atom supercell.Its corresponding defect state exhibits apparent localization along the [0001] direction,but it is relatively delocalized in the(0001) plane.     

Triple-q(-->) octupolar ordering in NpO2

We report the results of resonant x-ray scattering experiments performed at the Np M(4,5) edges in NpO2. Below T(0)=25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q(-->) longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Gamma(5) symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole-magnetic moment.     

理论研究V2O6-的结构和电子性质

采用自旋非限制密度泛函(B3LYP)方法研究了V2O6-的结构和电子性质,得到的基态和前人的理论计算结果不同．V2O6-阴离子是具有C2v对称性的双重基态,而以前Vyboishchikov等人得到的结果为D2h对称性双重基态．V2O6中性分了是具有D2h对称性的开壳层单重基态,它的能量和三重态的非常接近,而Vyboishchikov等人和Calatayud等人分别得到D2h对称性的闭壳层单重基态和Cs对称性的三重基态．从该基态结果出发计算出的电子分离能和实验上的PES谱相符,相反前人得到的电子分离能和实验相差较大．     

Resonant tunneling in truly axial symmetry Mn12 single-molecule magnets: sharp crossover between thermally assisted and pure quantum tunneling

Magnetization measurements of a truly axial symmetry molecular nanomagnet with a spin ground state of S = 10 show resonant tunneling. This compound has the same magnetic anisotropy as but the molecules are better isolated and the crystals have less disorder and a higher symmetry. Hysteresis loop measurements at several temperatures reveal a well-resolved step fine structure which is due to level crossings of excited states. All step positions can be modeled by a simple spin Hamiltonian. The results establish a sharp crossover between thermally assisted and pure quantum tunneling, as had been previously predicted.     

Axial crystals – macroscopic symmetry and tensor properties

ABSTRACT

Axial crystals have axial symmetry which keeps invariant straight line with a fixed point. Axial symmetry groups include 27 non-cubic crystallographic point groups and 5 limit groups describing symmetry of textures and liquid crystals. We show that, except for four cases, each axial symmetry belongs to one of five axial types: polar, chiral, pseudopolar (three basic axial types), directional (possessing none of characteristic properties of basic types) or rotational (exhibiting characteristic properties of all basic types). Each basic type can appear in two structurally different variants with the same symmetry. These variants can coexist and form a mesoscopic structure (antiparallel ferroelectric or chiral domain structure, mixture of enantiomers). We examine macroscopic properties of axial types and variants, and experimental accessivity of their characteristic features.     

Strong valence-band offset bowing of ZnO1-xSx enhances p-type nitrogen doping of ZnO-like alloys

Photoelectron spectroscopy, optical characterization, and density functional calculations of ZnO1-xSx reveal that the valence-band (VB) offset E(v)(x) increases strongly for small S content, whereas the conduction-band edge E(c)(x) increases only weakly. This is explained as the formation of local ZnS-like bonds in the ZnO host, which mainly affects the VB edge and thereby narrows the energy gap: E(g)(x=0.28) approximately E(g)(ZnO)-0.6 eV. The low-energy absorption tail is a direct Gamma(v)-->Gamma(c) transition from ZnS-like VB. The VB bowing can be utilized to enhance p-type N(O) doping with lower formation energy DeltaH(f) and shallower acceptor state in the ZnO-like alloys.     

THE SYMMETRY OF STRATONS IN

In this paper,the color symmetry of stratons for a theoretical model proposed some times ago is investigated.This symmetry contains two kinds of transformations, the first is color gauge transformations which leave the color mumber of stratons conserved,the second is color permutations which indicate the equivaluence of dif- ferent color stratons.The structure of this symmetry group（S3α）is investigated.The irreducible representations（IR's）of S3α and the reduction of their direct product are given.The relation between IR's of S3α and those of color U（3）group is discussed. In a simplest physical model only two neutral vector gluons are needed and the super-strong ihteraetions among stratons exhibit saturation properties similar to but somewhat different from the case of the Han-Nambu model.Not only the color singlet, but also other color neutral states belonging to IR's 2 or 1 of S3α all belong to the lowest energy level of the super-strong interactions.If there were eight degenerate vector gluons belonging to IR's 2 and 6,the Han-Nambu model would be reproduced.     

Time-reversal symmetry breaking by a (d+id) density-wave state in underdoped cuprate superconductors

It was proposed that the id(x(2)-y(2)) density-wave state (DDW) may be responsible for the pseudogap behavior in the underdoped cuprates. Here we show that the admixture of a small d(xy) component to the DDW state breaks the symmetry between the counterpropagating orbital currents of the DDW state and, thus, violates the macroscopic time-reversal symmetry. This symmetry breaking results in a nonzero polar Kerr effect, which has recently been observed in the pseudogap phase.     

Focusing ground-state atoms with an electrostatic field

A general formula for the trajectory of atoms in an arbitrary potential with axial symmetry is derived. We apply this formula to show that an axially symmetric electrostatic imaging lens for ground-state neutral atoms is possible. Because of its simple construction and capability of focusing atoms of any species, such a lens system can be used in a variety of applications. PACS 03.75.Be; 39.25.+k; 32.80.Pj     

Rotational symmetry breaking in the ground state of sodium-doped cuprate superconductors

We use an extended t-J model to study a single hole bound to a Na+ acceptor in Ca2-xNaxCuO2Cl2. For parameters suitable to cuprates, the ground state has a twofold degeneracy, corresponding to even (odd) reflection symmetry around the x (y) axes. The conductance pattern of the broken symmetry state is anisotropic as the tip of a tunneling microscope scans above the Cu-O-Cu bonds along the x (y) axes. This anisotropy is pronounced at lower voltages but reduced at higher voltages. Our theory agrees qualitatively with recent data of scanning tunneling microscopy showing broken local rotational symmetry.     

Photoemission insight into heavy-fermion behavior in YbRh2Si2

As shown by angle-resolved photoemission (PE), hybridization of bulk Yb 4f(2+) states with a shallow-lying valence band of the same symmetry leads in YbRh2Si2 to dispersion of a 4f PE signal in the region of the Kondo resonance with a Fermi-energy crossing close to Gamma[over ]. Additionally, renormalization of the valence state results in the formation of a heavy band that disperses parallel to the 4f originating signal. The symmetry and character of the states are probed by circular dichroism and the photon-energy dependence of the PE cross sections.     

Resonant impurity states in the d-density-wave phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudogap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with dx(2)(-y(2)) symmetry. Thus, the electronic structure near impurities can provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.