Half-metallic ferromagnetism in I2-VI compounds with non-magnetic dopants

First-principles calculations based on the tight-binding linear muffin-tin orbital (TB-LMTO) method were performed to investigate the occurrence of spin polarization in the alkali metal oxides (M₂O) [M: Li, Na, K, Rb] in antifluorite (anti- CaF₂-type) structure with non-magnetic (N, P, As, Sb and Bi) dopants. The calculations reveal that non-magnetic substitutional doping at anion site can induce stable half-metallic ferromagnetic ground state in I₂-VI compounds. Total energy calculations show that the antifluorite ferromagnetic state is energetically more stable than the antifluorite non-magnetic state at equilibrium volume. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The calculated magnetic moment is found to be 1.00 μ_B per dopant atom. The magnetic moment is mainly contributed by p orbitals of dopant atom.     

Excitonic-vibronic coupled dimers: A dynamic approach

The dynamical properties of exciton transfer coupled to polarization vibrations in a two site system are investigated in detail. A fixed point analysis of the full system of Bloch-oscillator equations representing the coupled excitonic-vibronic flow is performed. For overcritical polarization a bifurcation converting the stable bonding ground state to a hyperbolic unstable state which is basic to the dynamical properties of the model is obtained. The phase space of the system is generally of a mixed type: Above bifurcation chaos develops starting from the region of the hyperbolic state and spreading with increasing energy over the Bloch sphere leaving only islands of regular dynamics. The behaviour of the polarization oscillator accordingly changes from regular to chaotic.     

Anisotropic property in interacting quantum wires embedded in (110) plane

We investigate the theoretically combined effect of spin-orbit interactions and Coulomb interaction on the ground state and transport property of a quantum wire oriented along different crystallographic directions in the (110) plane. We find that the electron’s ground state exhibits phase transition among spin density wave, charge density wave, singlet superconductivity and metamagnetism, which can be controlled by changing the crystallographic orientation, the strengths of the spin-orbit interactions and the Coulomb interaction. The ac conductance exhibits a significant anisotropic behavior and a out-of-plane spin polarization which can be tuned by an in-plane electric field.     

Carrier-envelope phase measurement by all-optical poling with a polar side-chain polymer

Dependence of all-optical poling efficiency on carrier-envelope phase (CEP) could be measured using photoisomerization of dye molecules which are covalently bound to a polymer main chain and have large difference in static dipole moment between the ground state and excited state. Increased chromophore density leads to an order of magnitude reduction in signal-detection time from a dye doped polymer. Analysis of all-optical poling experiments with CEP changes clearly showed the presence of polarization restoring force to zero polarization. This enables resetting of SH activity in the all-optical poling process to be used for fast response loop of CEP stabilization. Phenomenological model could explain well the difference in the growth-and-decay dynamics of poling between sample of dye doped in polymer studied previously and that grafted to a polymer main chain used in the present paper.     

Multiphoton resonant transitions in atomic Ba in the presence of additional strong off-resonant radiation

The multiphoton transitions in atomic Ba are experimentally studied in the presence of strong off-resonant radiation that generates additional atomic polarization in the ground state. It is demonstrated that the additionally induced polarization of the Ba atoms in the ground state leads to a higher probability of the multiphoton transitions from this state.     

Dynamics of optical switching phenomena in dense media

The ultrafast optical switching phenomena in a dense medium of two-level atoms induced by arbitrary varying pulses are explained in terms of the adiabatic cancellation of the pulse by the induced polarization. The final population inversion of the medium after the passage of the pulse is found to depend on the number of oscillations the inversion exhibits during the time interval when the normalized pulse amplitude exceeds the maximum allowed value of the atomic polarization. If the inversion undergoes an integer number of oscillations in this region, then the final state of the system returns to the ground state. On the other hand, if the inversion undergoes a half integer number of oscillations in this region, the final state of the system is fully inverted. This behavior is explored analytically and illustrated numerically for the constant, sine and secant pulse shapes.     

On the possibility of polarizing atoms and nuclei by pulsed electromagnetic fields without using ultra-low temperatures

A method is proposed for fast and deep polarization of the system of hyperfine sublevels of the ground state of an atom having an optical excited state by means of two-component microwave pulses. The pulse of the bichromatic optical field that induces the transitions between the ground state and excited state of the atom is supposed to provide coherence among the hyperfine sublevels of the atomic ground state via the effect of coherent population trapping. The subsequent resonance microwave pulses create the polarization of equally populated ground state sublevels of the atom. The proposed polarization technique may be used for designing the new schemes of quantum computers, for the pulse transformation in optical experiments when light passes through a resonant medium containing rear-earth ions, as well as for producing polarized nuclear targets.     

Polarization phenomena under fast electron scattering by deuterons

The polarization of nucleons, which arises in deuteron electrodisintegration as a function of the angle between nucleon trajectories is considered. In electron scattering by large angles the polarization is affected by the interaction in the final state as well as by the presence of a D-wave component in the deuteron ground state. It is shown that the polarization degree of producing nucleons essentially depends on the choice of radial parts of the deuteron wave-function and the D-wave contribution in the deuteron ground state.     

Spin polarized current in the ground state of superconductor-ferromagnet-insulator trilayers

We study the ground state properties of a superconductor-ferromagnet-insulator trilayer on the basis of a Hubbard Model featuring exchange splitting in the ferromagnet and electron-electron attraction in the superconductor. We solve the spin-polarized Hartree-Fock-Gorkov equations together with the Maxwell's equation (Ampere's law) fully self-consistently with respect to the order parameter and the current. For certain values of the exchange splitting we find that a spontaneous spin polarized current is generated in the ground state and is intimately related to Andreev bound states at the Fermi level. Moreover, the polarization of the current strongly depends on the band filling. Received 23 September 2002 / Received in final form 13 December 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: m.a.krawiec@bristol.ac.uk     

Half-polarized quantum hall states

We report a theoretical analysis of the half-polarized quantum Hall states observed in a recent experiment. Our numerical results indicate that the ground state energy of the quantum Hall nu = 2 / 3 and nu = 2 / 5 states versus spin polarization has a downward cusp at half the maximal spin polarization. We map the two-component fermion system onto a system of excitons and describe the ground state as a liquid state of excitons with nonzero values of exciton angular momentum.     

Control of spin state in CdSe quantum dots by coupling with magnetic semiconductor quantum dots

We studied spin states of CdSe quantum dots (QDs) coupled with CdMnSe QDs by probing circular polarization of photoluminescence spectrum under external magnetic fields. The bandgap energies of CdSe and CdMnSe QDs are close to each other and photoluminescence mainly originates from CdSe QDs due to relatively low radiation efficiency of CdMnSe QDs. The photoluminescence lifetime as well as its intensity was decreased with increasing magnetic field, which was ascribed to the increase in the ground state wavefunctions in CdMnSe QDs. The decrease was more pronounced for spin down electrons, which was explained by the difference in spin up and down wave functions under magnetic fields. Our results show that the spin state of CdSe QDs can be manipulated by coupling with CdMnSe QDs.     

Polarized nonequilibrium Bose-Einstein condensates of spinor exciton polaritons in a magnetic field

The effect of a magnetic field on a spinor exciton-polariton condensate has been investigated. A quenching of a polariton Zeeman splitting and an elliptical polarization of the condensate have been observed at low magnetic fields B<2 T. The effects are attributed to a competition between the magnetic field induced circular polarization buildup and the spin-anisotropic polariton-polariton interaction which favors a linear polarization. The sign of the circular polarization of the condensate emission at B<3 T is negative, suggesting that a dynamic condensation in the excited spin state rather than the ground spin state takes place in this magnetic field range. From about 2T on, the Zeeman splitting opens and from then on the slope of the circular polarization degree changes its sign. For magnetic fields larger than the 3 T, the upper spin state occupation is energetically suppressed and circularly polarized condensation takes place in the ground state.     

Metal-free spin channels in graphitic boron-nitrogen nanostructures

Evidence is given for the existence of metal-free spin channels in an insulating medium. First-principles calculations indicate the presence of an unpaired spin, in a ground state boron-nitrogen nanostructure with a carbon zig-zag chain generated by the inclusion of a disclination with either negative or positive Gaussian curvature. The spin-polarized states are delocalized on the carbon chain suggesting possible spintronics applications.     

Tilted foil polarization of radioactive beam nuclei

Tilted foil polarization has up to now been mostly applied to nuclear reaction products recoiling out of a target traversed by a primary particle beam. Being a universal phenomenon it can be applied equally well to beams of particles, primary or secondary, radioactive or other. There are however some technical considerations arising from the nature of the beam particles. Radioactive beams are associated with ground state nuclei. They usually have low nuclear spin and as a consequence-as will be shown later-low polarization. Secondary beams are usually low in intensity and do not impose any constraints on the foils they traverse; unlike intense primary heavy ion beams which, if they traverse the foils, essentially limit the foil material to carbon. We review here briefly the tilted foil polarization process and then discuss an experiment with an isomer beam. Finally we review experiments with radioactive beams, past, present and planned for the future.     

Ground state structures in the polonium based II-VI compounds

We have performed ab-initio self-consistent calculations using the full-potential linear augmented plane-wave method to investigate the structural and the electronic properties of the less known II-VI compounds: ZnPo, CdPo, and HgPo. Total energy calculations of the cubic zinc-blende, wurtzite, rocksalt, cesuim chloride, orthorhombic Cmcm, and tetragonal PbO phases are investigated. Ground state parameters are computed, and compared with available theoretical and experimental works. The zinc-blende structure is found to be the ground state phase of ZnPo and CdPo, while HgPo prefers the tetragonal PbO structure. The calculated band structure of II-Po shows features that differ considerably from those of typical II-VI semiconductors. In particular we found an inverted band gap, reflecting a semi-metallic character for these compounds.     

Elliptic dichroism in hydrogen ionization by a coherent superposition of two harmonics

The two-photon ionization of the hydrogen atom from its ground state by a two-colour electromagnetic field consisting of two odd harmonics of the same IR laser is analyzed. The influence of the state of polarization of the bichromatic field on the azimuthal angular distribution and the dependence of the elliptic dichroism on the photon frequencies are reported. Received 11 December 2002 Published online 29 April 2003 RID="a" ID="a"e-mail: fritz.ehlotzky@uibk.ac.at     

From zigzag to armchair: the energetic stability, electronic and magnetic properties of chiral graphene nanoribbons with hydrogen-terminated edges

The energetic stability, electronic and magnetic properties of chiral graphene nanoribbons (GNRs) with hydrogen-terminated edges are investigated using density functional theory. Our calculations show that the percentage of carbon atoms at the zigzag sites (P(z)) is the key factor determining the electronic and magnetic properties of chiral GNRs. Within the local spin density approximation, chiral GNRs with P(z) ≥ 50% have a semiconducting antiferromagnetic ground state. Otherwise, chiral GNRs are spin degenerate semiconductors. Thus, the critical chiral angle for the occurrence of spin polarization is determined to be 13.9°. In contrast to the antiferromagnetic state that is independent of the width of GNRs investigated, size effects occur for the ferromagnetic metastable state. These findings are helpful for the design of GNR-based spintronic devices.     

Production of a highly polarized sodium atomic beam

Two experimental techniques for preparing atoms in selected states were combined in order to obtain a highly polarized sodium beam: Firstly state selection by an inhomogeneous magnetic field and secondly optical pumping with laser light. This results in a dominating population of the 3s ² S _1/2 ground state levelF=2M _F=+2 (or alternativelyF=2M _F=−2) corresponding to high electron as well as nuclear spin polarization. Polarization values of 0.85±0.05 were easily obtained. The sign of the polarization can be reversed by changing the light polarization. The method can also be applied to other atoms. In addition, it is demonstrated that the optical pumping process allows a determination of the spin-selectivity of hexapole magnets.     

用脉冲半导体激光观察铷原子基态塞曼能级的相干共振

用半导体激光脉冲序列泵浦汽泡铷原子,观察其基态能级的△m=1和△m=2的塞曼(Zeeman)相干.利用直接调制激光二极管注入电流的增益开关技术,在波长780nm处产生约200ps的激光脉冲.对△m=1,用偏振光谱学方法检测;而对△m=2,则直接使用前向散射光谱学方法检测.     

Magnetic and electronic structures of zinc-blende FeX (X=P,As, Sb) by first principles calculations

Magnetic and electronic structure calculations are carried out for hypothetical zinc-blende (zb) phase of FeX (X=P, As, Sb) by using the full-potential linearized augmented plane wave (FLAPW) method. For zb FeSb, the total energy has been calculated as a function of lattice constant in ferromagnetic (FM) and antiferromagnetic (AFM) states. We found that the ground state of zb FeSb is very stable with respect to compression and expansion of the unit cell. The magnetic moment of zb FeSb in the AFM state is increasing with the lattice constant. The magnetic and electronic structures calculations of FeAs (FeP) are carried out for the lattice constants of GaAs (GaP), InAs (InP), and Si. Our finding shows that AFM is the ground state for all of our calculated zb FeX compounds and do not belong to the class of zb half metallic ferromagnets.