Ga空位对GaN:Gd体系磁性影响的第一性原理研究

采用基于密度泛函理论的第一性原理结合投影缀加平面波的方法,研究了GaN中Ga被稀土元素Gd替代以及与邻近N或Ga空位组成的缺陷复合体的晶格常数、磁矩、形成能以及电子结构等性质.结果发现,Gd掺杂GaN后禁带宽度变窄,由直接带隙半导体转为间接带隙半导体;单个Gd原子掺杂给体系引入大约7μB的磁矩;在Gd与Ga或N空位形成的缺陷复合体系中,N空位对引入磁矩贡献很小,大约0.1μB,Ga空位能引入约2μB的磁矩.随着Ga空位的增多,体系总磁矩增加,但增加量与Ga空位的位置分布密切相关.当Ga空位分布较为稀疏时,Gd单原子磁矩受影响较小,但当Ga空位距离较近且倾向于形成团簇时,Gd单原子磁矩明显增加,而且这种情况下空位形成能也最小.     

Electron range-energy relationships for calculating backscattering coefficients in elemental and compound semiconductors

Backscattering coefficients for electrons normally impinging on Si, Ge, GaN, GaAs and InSb targets have been calculated by using the Vicanek and Urbassek theory [M. Vicanek, H.M. Urbassek, Phys. Rev. B 44 (1991) 7234] for incident energies ≤5 keV. Electron range has been calculated from various semi-empirical analytical expressions. The cross-sections used to describe the electron transport are determined via the appropriate analytical expression given by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470] whose new improved version has been recently reported by Rouabah et al. [Z. Rouabah, N. Bouarissa, C. Champion, N. Bouaouadja, Appl. Surf. Sci. 255 (2009) 6217]. The results may be seen as the first predictions for low-energy electron backscattering coefficients impinging on GaN, GaAs and InSb semiconductors. The models used in the calculation of the electron range affect both the accuracy and behaviour of the electron backscattering coefficients.     

Optical pumping of the Zeeman components in the rubidium vapor

We investigated the optical pumping of the Zeeman components of rubidium atoms, in the presence of the external magnetic field ranging from the geomagnetic up to 130 Gauss. Using the saturated absorption spectroscopy with linearly polarized pump and probe laser beams, the rubidium Doppler-free spectra at different magnetic field strengths were measured. The dips (negative intensity signals) in the saturated absorption spectra of the ⁸⁷Rb hyperfine transition lines were observed. They come as a result of the alignment process induced by the incoherent population transfer due to the hyperfine optical pumping. By inspection of the dips for different magnetic field strengths we were able to conclude about the dynamics of the alignment process in the external magnetic field. Present work is a part of the investigations concerning the influence of the magnetic field on the velocity selective optical pumping of the rubidium atoms induced by femtosecond frequency comb [D. Aumiler, T. Ban, H. Skenderovi?, G. Pichler, Phys. Rev. Lett. 95 (2005) 233001; T. Ban, D. Aumiler, H. Skenderovi?, G. Pichler, Phys. Rev. A 73 (2006) 043407].     

Twining effects in the magnetism of small Pd clusters

We report a theoretical study of the magnetic behavior of symmetrical twined Pd_N (N≤220) clusters. The twined Pd_N particles were built from two equal Pd_M seed-clusters with fcc-like structure for M=38, 55, 79 and 116. The optimized geometrical structures of Pd_N (with N<2M) were obtained from an uniform relaxation of the fcc-like twined configurations using the embedded atom method (EAM). The spin-polarized electronic structure and related magnetic properties of those optimized geometries were calculated by solving self-consistently a spd tight-binding Hamiltonian. We observe that, in some cases the twining process may induce and/or enhance the magnetic moment of the clusters even in the case when the seed-clusters are non-magnetic. Our results also suggest a strong dependence on the twining orientation, providing further support to the influence of symmetry effects on the magnetic properties of finite transition-metal systems. We discuss our results in comparison with some recent experimental observations for Pd nanoparticles [Shinohara et al., Phys. Rev. Lett. 91 (2003) 197201. [14]; Sampedro et al., Phys. Rev. Lett. 91 (2003) 237203. [13]].     

General Properties of Thermal Entanglement in an Arbitrary-Length Heisenberg Spin Chain

We investigate general properties of thermal entanglement in arbitrary-length 1D Helsenberg spin-1/2 chain based on classifications of its eigenstates. The influences of magnetic field and temperature on entanglement are qualitatively discussed and three features are presented. The conclusions hold for both bipartite and multipartite entanglement, and are in agreement with the results numerically proven by Arnesen et al. [Phys. Rev. Lett. 50 （2001） 017901].     

Electronic structure and magnetic properties of metastable SmCo_7 compound

1 Introduction The rare-earth transition-metal intermetallic compounds have been widely investi-gated for many years, among them the Sm-Co series compounds with 1:5 and 2:17 crys-tal structures. These compounds have been used as sintered and bonded permanent magnets since the 1960s[1,2]. Interest recently has been focused on the TbCu7-type struc-ture Sm-Co intermetallic compounds with a strong uniaxial magnetocrytalline anisot-ropy and a low temperature coefficient (β = ?0.11%)[3―6] due t…     

Efficient Multiparty-to-Multiparty Quantum Secret Sharing via Continuous Variable Operations

The one-to-multiparty quantum secret sharing scheme [Phys. Rev. A 71 （2005） 044301] proposed recently is extended to a multiparty-to-multiparty case. Furthermore, the continuous variable operations are employed in the extended scheme to replace the specific discrete unitary operations used in the original scheme. The complete randomicity of the continuous variable characterizing the unitary operations can ensure the security of secret sharing. Moreover, the present scheme is compared with the recent similar scheme [Phys. Rev. A 72 （2005） 012304]. It is found that the efficiency of the present scheme is n times of that of the previous one.     

Spin gap and Luttinger liquid description of the NMR relaxation in carbon nanotubes

Recent NMR experiments by Singer et al. [Singer, Phys. Rev. Lett. 95, 236403 (2005).] showed a deviation from Fermi-liquid behavior in carbon nanotubes with an energy gap evident at low temperatures. Here, a comprehensive theory for the magnetic field and temperature dependent NMR 13C spin-lattice relaxation is given in the framework of the Tomonaga-Luttinger liquid. The low temperature properties are governed by a gapped relaxation due to a spin gap ( approximately 30 K), which crosses over smoothly to the Luttinger liquid behavior with increasing temperature.     

First Principles Calculation of Universal Conductance Fluctuation in Monatomic Metal Chains

Combining the non-equilibrium Green＇s function method and density functional theory, we provide a first-principle scheme to calculate the universal conductance fluctuation （UCF） in quasi one-dimensional monatomic chains subject to a magnetic field. Our results show that for these monatomic chains, the amplitude of the UCF is much smaller than the previous theoretical prediction for mesoscopic conductors by Lee et al. [Phys. Rev. Lett. 55 （1985） 1622; Phys. Rev. B 36 （1987） 1039] The reason is that the ergodic hypothesis fails in these nanowires due to the confinement of geometry. We ascribe the phenomenon to the flux-dependent density of states fluctuation.     

Evidences of defect contribution in magnetically ordered Sm-implanted GaN

Samarium (Sm) ions of 200 keV in energy were implanted into highly-resistive molecular-beam-epitaxy grown GaN thin films with a focused-ion-beam implanter at room temperature. The implantation doses range between 10¹⁴ and 10¹⁶ cm⁻². X-ray diffraction revealed Sm incorporation into GaN matrix without secondary phase. Raman-scattering spectroscopy identified impurity-independent defect-related oscillation modes. Slight decrease in band gap and significant reduction in transmittance were observed by optical transmission spectroscopy. Photoluminescence spectra showed emission peaks related to background p-type impurity. Ferromagnetic hysteresis loops were recorded from GaN implanted with highest Sm dose, and magnetic ordering was observed from Sm-implanted GaN with dose of and above 10¹⁵ cm⁻². The long-range magnetic ordering can be attributed to interaction of Sm ions through the implantation-induced Ga vacancy.     

Magnetic field dependence on transition temperature Tc in cuprate superconductors

We investigate the magnetic field dependence on T_c in the high transition temperature superconductors. It is shown that phonon-enhanced spin fluctuations drive this superconductivity once more suggested by us [Phys. Rev. B 61 (2001) 4289]. We know magnetic field dependence on our transition temperature is in good correspondence with experimental data. It is elucidated that the external field is closely related to the local internal field in order to influence spin fluctuations.     

Colossal magnetic moment of Gd in GaN

We investigate the magnetic properties of epitaxial GaN:Gd layers as a function of the external magnetic field and temperature. An unprecedented magnetic moment is observed in this diluted magnetic semiconductor. The average value of the moment per Gd atom is found to be as high as 4000 micro(B) as compared to its atomic moment of 8 micro(B). The long-range spin polarization of the GaN matrix by Gd is also reflected in the circular polarization of magnetophotoluminescence measurements. Moreover, the materials system is found to be ferromagnetic above room temperature in the entire concentration range under investigation (7 x 10(15) to 2 x 10(19) cm(-3)). We propose a phenomenological model to understand the macroscopic magnetic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd.     

Influence of the magnetic field strength and excitation intensity on the shape of microphotoluminescence spectra of quantum-well structures based on GaN/InGaN doped with Sm and Eu + Sm

This paper presents the results of complex measurements of the microphotoluminescence spectra of quantum-well structures based on InGaN/GaN〈Sm〉 and the determination of the concentration and charge state of the Sm dopant. It has been shown that an increase in the magnetic field strength and the excitation intensity of the microphotoluminescence spectra leads to an increase in the luminescence intensity and a shift in the position of the maximum of the emission wavelength toward the short-wavelength region of the spectrum. Measurements of the microphotoluminescence spectra with variations in the external magnetic field strength, as well as with the introduction of paramagnetic and magnetic impurities, provide additional information on the mechanisms of formation of luminescence spectra in the quantum-well structures InGaN/GaN〈Sm〉,〈Eu + Sm〉. In the long-wavelength region, the influence of the magnetic field on the shape of the microphotoluminescence spectra of the InGaN/GaN structures doped with Sm and Sm + Eu is less pronounced than that in the short-wavelength region.     

EDM of the muon, deuteron, and proton in storage rings

I discuss the progression of ideas over the last decade that has led to extremely sensitive dedicated electric dipole moment (edm) storage ring designs. These ideas grew out of our experience in BNL E821: a precision measurement of the anomalous magnetic moment of the muon (Bennett et al. Phys Rev D73:072003, 2006).     

Determining exchange splitting in a magnetic semiconductor by spin-filter tunneling

A large exchange splitting of the conduction band in ultrathin films of the ferromagnetic semiconductor EuO was determined quantitatively, by using EuO as a tunnel barrier and fitting the current-voltage characteristics and temperature dependence to tunneling theory. This exchange splitting leads to different tunnel barrier heights for spin-up and spin-down electrons and is large enough to produce a near-fully spin-polarized current. Moreover, the magnetic properties of these ultrathin films (<6 nm) show a reduction in Curie temperature with decreasing thickness, in agreement with theoretical calculation [R. Schiller, Phys. Rev. Lett. 86, 3847 (2001)10.1103/Phys. Rev. Lett.86.3847].     

Comment on breakup densities of hot nuclei

In [V.E. Viola et al., Phys. Rev. Lett. 93 (2004) 132701, D.S. Bracken et al., Phys. Rev. C 69 (2004) 034612] the observed decrease in spectral peak energies of IMFs emitted from hot nuclei was interpreted in terms of a breakup density that decreased with increasing excitation energy. Subsequently, Raduta et al. [Ad. Raduta et al., Phys. Lett. B 623 (2005) 43] performed MMM simulations that showed decreasing spectral peaks could be obtained at constant density. In this Letter we point out that this apparent inconsistency is due to a selective comparison of theory and data that overlooks the evolution of the fragment multiplicities as a function of excitation energy.     

Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce–Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin–orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.     

Anomalous Hall effect for the phonon heat conductivity in paramagnetic dielectrics

The theory of the anomalous Hall effect for the heat transfer in a parmagnetic dielectric, discovered by Strohm, Rikken, and Wyder [Phys. Rev. Lett. 95, 155901 (2005)]10.1103/PhysRevLett.95.155901, is developed. The appearance of the phonon heat flux normal to both the temperature gradient and the magnetic field is connected with the interaction of magnetic ions with the crystal field oscillations. In crystals with an arbitrary phonon spectrum this interaction creates the elliptical polarization of phonons. The kinetics related to phonon scattering induced by the spin-phonon interaction determines an origin of the off-diagonal phonon density matrix. The combination of both factors is decisive for the phenomenon under consideration.     

Anomalous Hall effect in the phonon thermal conductivity of paramagnetic dielectrics

A theory of the phonon Hall effect during heat transfer in a paramagnetic dielectric discovered by Strohm et al. [Phys. Rev. 95, 155901 (2005)] is developed. The heat flux emerging in the direction perpendicular to the magnetic field and to the temperature gradient is associated with the interaction of magnetic ions with the oscillating crystal field. In crystals with an arbitrary phonon spectrum, this interaction induces elliptic polarization of phonons. On the other hand, for any type of scattering, the temperature gradient forms part of the phonon density matrix, which is nondiagonal in modes. The combined action of these factors leads to the anomalous Hall effect.     

Commensurability effects in superconducting Nb films with quasiperiodic pinning arrays

We study experimentally the critical depinning current I(c) versus applied magnetic field B in Nb thin films which contain 2D arrays of circular antidots placed on the nodes of quasiperiodic (QP) fivefold Penrose lattices. Close to the transition temperature T(c) we observe matching of the vortex lattice with the QP pinning array, confirming essential features in the I(c)(B) patterns as predicted by Misko et al. [Phys. Rev. Lett. 95, 177007 (2005)]. We find a significant enhancement in I(c)(B) for QP pinning arrays in comparison to I(c) in samples with randomly distributed antidots or no antidots.