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.     

Spin polarization in the relaxed excited state of the F center in case of saturated optical pumping

The dependence of the F center MCP on the polarization of pumping light is explained in terms of the spin polarization in the RES (relaxed excited state) which is derived by examining the spin-mixing processes; the spin-mixing parameter in the RES and that associated with the relaxation from the URES (unrelaxed excited state) to RES are evaluated. A new ESR method in the RES is proposed.     

Optical spin orientation of excitons in GaSe under longitudinal magnetic field

The degree of circular polarization of the free-exciton luminescence line has been measured in GaSe excited by circularly polarized light at 4.2 K under longitudinal magnetic field. The result shows that the spin relaxation time of exciton is field-dependent but the spin memory before reaching the exciton ground state is almost unaffected by the applied longitudinal magnetic field.     

Alignment and orientation of charge exchange excitation with polarized projectiles

The alignment and orientation of hydrogenlike excited atoms produced by electron capture in collisions of spin polarized bare atomic nuclei with atomic targets is described in terms of Stokes parameters. TheH(2p)^* excited state formed by spin polarized protons making charge exchange collisions on atomic targets is dealt with extensively for the purpose of illustration. The linear polarization and circular polarization of resulting Lyman-α when polarized protons make electron capture collision on atomic hydrogen target are plotted to compare observable aspects of different theories of electron capture.     

Electron spin resonance in the relaxed excited state of F-centres detected via saturated optical pumping

A fairly simple and effective method is described to detect spin resonances in the relaxed excited state (RES) of F-and similar color centres in alkali halides. It has been applied to F-centers in CsCl and CsBr, yielding g-factors of 1.77(4) and 1.54(4), respectively, for the RES.     

Spin Chemistry Investigation of Peculiarities of Photoinduced Electron Transfer in Donor–Acceptor Linked System

Photoinduced intramolecular electron transfer in linked systems, (R,S)- and (S,S)-naproxen-N-methylpyrrolidine dyads, has been studied by means of spin chemistry methods [magnetic field effect and chemically induced dynamic nuclear polarization (CIDNP)]. The relative yield of the triplet state of the dyads in different magnetic field has been measured, and dependences of the high-field CIDNP of the N-methylpyrrolidine fragment on solvent polarity have been investigated. However, both (S,S)- and (R,S)-enantiomers demonstrate almost identical CIDNP effects for the entire range of polarity. It has been demonstrated that the main peculiarities of photoprocesses in this linked system are connected with the participation of singlet exciplex alongside with photoinduced intramolecular electron transfer in chromophore excited state quenching.     

Optically detected magnetic resonance of a relaxed excited state of atomic hydrogen centres in KCl and RbCl doped with I-

The ESR spectrum of relaxed excited state (RES) of inerstitial hydrogen centres in the “mixed” configuration with 3 Cl^- and 1 I^- ions next could be measured via the magnetic circular dichroism of the UV absorption bands. The g factor and I superhyperfine interactions of the RES and the ground state are practically the same, whereas the proton hyperfine interaction in the RES is reduced by ～ $\text{1}\text{3}$. The RES is a spin quartet charge transfer state with a very large zero field splitting.     

Level-anticrossing effect on the magnetoluminescence of the triplet indirect bound exciton in GaSe

The presence of level-anticrossing in the triplet state of the indirect impurity-bound exciton in GaSe has been found for the first time in the magnetic field of 1.5 kOe applied along the c-axis. Measurements on the magnetic circular dichroism (MCD) of the photoluminescence and on the spin dependent formation rate (SDFR) show that the |S_z| = 1 level lies 0.016 meV higher than the S_z = 0 level in the absence of the magnetic field. The resonance-like behavior in these two types of polarization correlation measurements may be explained in terms of weak perturbation between |S_z| = 1 and S_z = 0 levels.     

Beta decay and nuclear structure: Rb86

It was aimed to gain information on the nuclear structure of Rb⁸⁶ by investigation of the 2^?→2⁺ β transition. For this purpose the energy dependence of theβγ angular correlation and the angle dependence of theβγ circular polarization correlation have been measured. A novel experimental set-up has been used for the angular correlation measurement allowing a simultaneous determination of the anisotropy coefficientsA ₂ andA ₄ under considerable reduction of systematical and statistical errors. For the polarization correlation measurement an unusual experimental arrangement has been applied providing the possibility of simultaneous observation under four different angles. Employing additional data on shape factor measurements and energy dependent circular polarization correlations from other authors the nuclear structure of the 2^? state in Rb⁸⁶ and the 2⁺ first excited state in Sr⁸⁶ have been evaluated. For the latter purpose the unified model with weak coupling has been chosen.     

Magnetic field effects on the fluorescence of Cr3+ in GdAlO3

The fluorescence spectrum of Cr³⁺ in GdAlO₃ has been examined at 4.2 K as a function of magnetic field up to 60 kG. The resulting splitting of the ²E → ⁴A₂ emission lines are explained in terms of a modified molecular field approximation, which incorporates the effect of the spin fluctuations. The exchange constant in the relaxed excited state is found to be 1.2 cm^-1, which differs from that reported from absorption data. It is suggested that the difference may be related to the Frank-Condon effect.     

Transverse energy flows in vectorial fields of paraxial beams with singularities

A general study of transverse energy flows (TEF) as physically meaningful and informative characteristics of paraxial light beams’ spatial structure is presented. The total TEF can be decomposed into the spin and orbital contributions giving rise to the spin and orbital angular momentums, correspondingly. Definitions and properties of these constituents are discussed in relation with the optical field representation through linear and circular orthogonal polarization bases. With the help of model examples, the results are applied to investigation of TEF singularities in connection with the usual polarization morphology characteristics of paraxial optical fields. An analysis of TEFs near singular points has been carried out; in particular, the behavior of TEF and its partial contributions near polarization singularities (C-points) has demonstrated the special role of a boundary flow in the origin of the spin angular momentum. The analytical and experimental applicability of the introduced concepts are discussed.     

Spin-dependent recombination in GaAsN solid solutions

Room-temperature spin-dependent recombination in a series of GaAs_1?xN_x solid solutions (x = 2.1, 2.7, 3.4%) has been observed as manifested by a more than threefold decrease in intensity of the edge photoluminescence upon switching from circular to linear polarization of the exciting light or upon the application of a transverse magnetic field (～300 G). The interband absorption of the circularly polarized light is accompanied by the spin polarization of conduction electrons, which reaches 35% with an increase in the pumping level. The observed effects are explained in terms of the dynamic polarization of deep paramagnetic centers and the spin-dependent trapping of conduction electrons on these centers. The electron spin relaxation time, as estimated from the dependence of the edge photoluminescence depolarization in the transverse magnetic field (the Hanle effect) on the pumping intensity, is on the order of 1 ns. According to the adopted theory, the electron spin relaxation time in the presence of spin-dependent recombination is determined by a slow spin relaxation of localized electrons. The sign (positive) of the g factor of localized electrons has been experimentally determined from the direction of the magnetic-field-induced rotation of their average spin observed in the three GaAsN crystals studied.     

The role of spin-orbit coupling and hyperfine coupling in optical pumping ofF-centres

Optical pumping experiments onF-centres in potassium halides are described. The ground state polarization of the electronic spins achieved by optical pumping of isolatedF-centres is strongly dependent on the magnetic field and the wave-length of the pumping light. Experimentally it does not show the simple relationship to the magnetic circular dichroism (MCD) that has generally been assumed. A closer theoretical analysis shows indeed that the phenomena depend critically on the kind of spin mixing that prevails in the pumping cycle.If spin orbit coupling in the absorption band is the dominant spin mixing mechanism the sign of the pumping effect will be the same everywhere in the absorption band, if some other mechanism like spin mixing by radiationless transitions or hyperfine coupling to the surrounding nuclear spins prevails, the sign will follow the MCD. In strong magnetic fields experiments argue in favour of the first alternative, in low fields spin mixing by hyperfine coupling becomes important. On this basis the mechanism of optical pumping ofF-centres is discussed, and rate equations are given.     

The effects of electron–phonon interaction on anisotropic RKKY interaction in graphene nanoribbon

We study the Ruderman–Kittle–Kasuya–Yosida (RKKY) interaction in doped armchair graphene nanoribbon. The effects of both external magnetic field and electron-Holstein phonon on RKKY interaction have been addressed. RKKY interaction as a function of distance between localized moments has been analyzed. It has been shown that a magnetic field along the z-axis mediates an anisotropic interaction which corresponds to a XXZ model interaction between two magnetic moments. In order to calculate the exchange interaction along arbitrary direction between two magnetic moments, we should obtain both transverse and longitudinal static spin susceptibilities of armchair graphene nanoribbon in the presence of electron-phonon coupling and magnetic field. The spin susceptibility components are calculated using the spin dependent Green’s function approach for Holstein model Hamiltonian. The effects of spin polarization on the dependence of exchange interaction on distance between moments are investigated via calculating correlation function of spin density operators. Our results show the influences of magnetic field on the spatial behavior of in-plane and longitudinal RKKY interactions are different in the presence of magnetic field.     

On Electron Spin Polarization Created in the Excited Triplet State of Accessory Chlorophyll via Photoinduced Charge-Recombination of the Photosystem II Reaction Center

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl_accD1) which reside near the P_D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet–triplet conversion of the light-induced charge-separated state composed of P_D1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet–triplet energy transfer processes. It has been clearly shown that the ESP created in the ³Chl_accD1* is dependent on the rate constant (k _TT) of the triplet–triplet energy transfer from the intermediate triplet state created by the CR. Also we show that the relative orientation of the principal axes of the spin dipolar interaction in the intermediate triplet state (³P_D1*, as an example) may play a role in the ESP pattern, when the k _TT is smaller than the angular frequency of the Zeeman energy. We have theoretically shown that the TREPR measurement of the ESP is very powerful to investigate the primary chemical process and to characterize the intermediate as a signature of the stepwise ESP transfer.     

Atomic beam preparation for hydrogen maser operation with unpolarized atoms

The conventional atomic beam preparation technique for hydrogen maser operation consists of a six pole magnetic field focussing in combination with adiabatically changing magnetic fields in the transition region between focusser and maser cavity, which results in a state selected, polarized ensemble of hydrogen atoms. Hydrogen maser operation with state selected, but unpolarized atoms has the advantage to reduce considerably the sensitivity of hydrogen maser frequency on coherently excited, low frequency,Δm _F =±1 Zeeman transitions. The influence of the magnetic field pattern in the transition region on the energy level population probabilities for the focussed hydrogen beam has been analysed. Atomic beam preparation techniques using non-adiabatically changing magnetic field configurations in the transition region are described. The dependence of hydrogen maser oscillation on the energy level population within the atomic beam is theoretically investigated and the results are used for the analysis of the experimentally achieved atomic beam populations. It has been shown that a pulsed magnetic spin guidance field in the transition region, which switches between adiabatically changing magnetic field configurations and a sudden magnetic field reversal, results in an ensemble of hydrogen atoms with vanishing time averaged polarization, which does not deteriorate the conventionally achieved maser oscillation amplitude.     

Magnetooptik und elektronische Struktur der magnetisch ordnenden Europiumchalkogenide

The absorption coefficient and the interband Faraday rotation of EuS, EuSe and EuTe thin films have been measured as function of the photon energy (1–6 eV), the temperature (2.7–300 K) and the applied magnetic field (0–11.5 kOe). In addition a magnetic field modulation technique has been developed, with a resolution of 2 ? 10^?4 deg. This allows the measurement of the Faraday rotation in fields of only 100 Oe, which is important for metamagnetic samples with low critical fields. A Kramers-Kronig transformation of the Faraday rotation leads to the circular dichroism and from these two quantities and the optical constants the off-diagonal elements of the conductivity tensor have been computed. From a comparison of this experimental result with values obtained from a modified atomic model, we deduce the character of the involved transitions and the spin polarization of the occupied ground states (4f ⁷,p(anion)). In addition the ratio of exchange splitting to band width of the empty 5d final state can be evaluated. The fine structure of the first main peak is discussed in terms of Kasuya's coupling scheme between the 4f ⁶ multiplett and the excited 5d electron. In the antiferromagnetic EuTe the temperature dependence of the Faraday rotation does not follow the net magnetization of the sample for all photon energies, but some transitions show a “ferromagnetic” behavior. This is interpreted in Slater's model of the magnetic Brillouin zone.     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.     

Vertex correction of the anisotropic magnetic impurities to the spin polarization of 2D electron gas

In terms of Kubo's formula and Green's function method, for the two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling (SOC), we study the spin polarization due to the effect from magnetic impurities with anisotropic spin dependent delta type coupling to electrons when an external dc electric field in plane is applied. The vertex correction of impurities in ladder approximation is carried out in the limit of EF?1/τ, Δ. We find that the strength of spin polarization can be significantly modified by vertex correction and the spin polarization is relevant to the anisotropy coefficient γ, but the direction of net spin polarization cannot be changed.