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The generation mechanisms of electron spin polarization (ESP) of charge carriers (electrons and holes, called "doublets") in doublet-doublet (D-D) recombination and triplet-doublet (T-D) quenching in disordered organic semiconduct...
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The generation mechanisms of electron spin polarization (ESP) of charge carriers (electrons and holes, called "doublets") in doublet-doublet (D-D) recombination and triplet-doublet (T-D) quenching in disordered organic semiconductors are analyzed in detail. The ESP is assumed to result from nonadiabatic transitions between the states of the spin Hamiltonian of D-D or T-D pairs. The transitions are induced by the hyperfine and anisotropic Zeeman interactions (for D-D process), or zero field splitting interaction (for T-D process). The proposed mechanism of ESP generation is essentially different from those based on polarized spin injection (using ferromagnetic electrodes) and optical pumping. In this mechanism, the ESP magnitude appears to depend on specific features of relative motion of particles. In our work, we have considered the cage and free diffusion models of this motion. The effect of possible attractive spin-independent interactions between particles is also analyzed. Estimations with obtained formulas show that the proposed mechanisms can lead to a fairly strong ESP much larger than the thermal one (at room temperatures).
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We use spin density functional theory ab initio calculations to theoretically explore the possibility of achieving useful gate control over exchange coupling between cobalt clusters placed on a graphene sheet. By applying an elect...
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We use spin density functional theory ab initio calculations to theoretically explore the possibility of achieving useful gate control over exchange coupling between cobalt clusters placed on a graphene sheet. By applying an electric field across supercells, we demonstrate that the exchange interaction is strongly dependent on gate voltage, and find that it is also sensitive to the relative sublattice registration of the cobalt clusters. We use our results to discuss strategies for achieving strong and reproducible magnetoelectric effects in graphene/transition-metal hybrid systems.
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Magnetic linear dichroism and birefringence in (Ga,Mn)As epitaxial layers is investigated by measuring the polarization plane rotation of reflected linearly polarized light when magnetization lies in the plane of the sample. We re...
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Magnetic linear dichroism and birefringence in (Ga,Mn)As epitaxial layers is investigated by measuring the polarization plane rotation of reflected linearly polarized light when magnetization lies in the plane of the sample. We report on the spectral dependence of the rotation and ellipticity angles in a broad energy range of 0.12-2.7 eV for a series of optimized samples covering a wide range on Mn dopings and Curie temperatures and find a clear blueshift of the dominant peak at energy exceeding the host material band gap. These results are discussed within the framework of the κ·ρ and mean-field kinetic-exchange model of the (Ga,Mn)As band structure. We infer that disorder-induced nondirect transitions significantly influence magneto-optical properties of (Ga,Mn)As.
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We study the electrical and thermoelectric transport properties of the surface state of a topological insulator and graphene in the presence of randomly distributed impurities. For finite impurity strength, the dependence of the t...
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We study the electrical and thermoelectric transport properties of the surface state of a topological insulator and graphene in the presence of randomly distributed impurities. For finite impurity strength, the dependence of the transport coefficients as a function of the gate voltage, magnetic field, and impurity potential are obtained numerically. In the limit of zero impurities (clean limit), analytic results for the peak values of the magneto-oscillations in thermopower are derived. Analogous with the conventional two-dimensional electron gas, the peak values are universal in the clean limit. Unlike graphene, in topological insulators the coupling of the electron spin to its momentum leads to a dependence of the transport coefficients on the gyromagnetic ratio (g). We compare our results with data on graphene and identify unique signatures expected in topological insulators due to the magnetoelectric coupling.
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We discuss the influence of a uniform current j on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy ε(q) has a current-induced contribution proportional to q · J, where J is the spin current, a...
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We discuss the influence of a uniform current j on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy ε(q) has a current-induced contribution proportional to q · J, where J is the spin current, and predict that collective dynamics will be more strongly damped at finite j. We obtain similar results for models with and without local moment participation in the magnetic order. For transition metal ferromag-nets, we estimate that the uniform magnetic state will be destabilized for j approx >10~9 A cm~(-2). We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems.
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The in-plane resistivity anisotropy has been measured for detwinned single crystals of Ba(Fe_(1-x)T_x)2As_2 and Ba(Fe_(1-x)Cu_x)2As_2 The data reveal a nonmonotonic doping dependence, similar to previous observations for Ba(Fe_(1-...
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The in-plane resistivity anisotropy has been measured for detwinned single crystals of Ba(Fe_(1-x)T_x)2As_2 and Ba(Fe_(1-x)Cu_x)2As_2 The data reveal a nonmonotonic doping dependence, similar to previous observations for Ba(Fe_(1-x)Co_x)2As_2 Magnetotransport measurements of the parent compound reveal a nonlinear Hall coefficient and a large linear term in the transverse magnetoresistance. Both effects are rapidly suppressed with chemical substitution over a similar compositional range as the onset of the large in-plane resistivity anisotropy. This suggests that the relatively small in-plane anisotropy of the parent compound in the spin-density wave state is due to the presence of an isotropic, high mobility pocket of the reconstructed Fermi surface. Progressive suppression of the contribution to the conductivity arising from this isotropic pocket with chemical substitution eventually reveals the underlying in-plane anisotropy associated with the remaining Fermi surface pockets.
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We report a study of the Ce doping effect on the thermal conductivity (κ) of Nd_(2-x)Ce_xCuO_4 (NCCO) at low temperatures down to 0.3 K and in magnetic fields up to 14 T. It is found that with Ce doping, the electronic thermal co...
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We report a study of the Ce doping effect on the thermal conductivity (κ) of Nd_(2-x)Ce_xCuO_4 (NCCO) at low temperatures down to 0.3 K and in magnetic fields up to 14 T. It is found that with Ce doping, the electronic thermal conductivity increases; at the same time, the a-axis field-induced changes in κ(H), associated with the spin flop and spin polarization of the Nd~(3+) sublattice, and the spin flop of the Cu~(2+) sublattice, gradually disappear. These are clearly due to the electron doping and the destruction of the antiferromagnetic orders. In the superconducting NCCO with x = 0.14 and 0.18, although the electronic thermal conductivity shows sizable field dependencies with H||c, the paramagnetic scattering of phonons is still playing the dominant role in the heat transport, which is different from many other cuprates. In the lightly doped samples (x = 0.03 and 0.06), the low-T κ(H) isotherms with H||c show a steplike anomaly and are likely related to the spin/charge stripes.
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We consider a two-dimensional disordered composite medium made of three constituents: a normal conductor, a perfect conductor, and an insulator, and we examine its macroscopic electrical response in cases where it is subject to a ...
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We consider a two-dimensional disordered composite medium made of three constituents: a normal conductor, a perfect conductor, and an insulator, and we examine its macroscopic electrical response in cases where it is subject to a strong magnetic field applied perpendicular to its plane. To this end, we exploit a discrete network model and apply a Monte Carlo procedure for sampling ensembles of finite-size three-constituent networks of this kind. The simulations indicate that when the perfectly conducting and insulating constituents are below the percolation threshold, such that the material has a finite, nonvanishing conductivity tensor, two distinct behaviors of the macroscopic magnetoresistance appear, according to whether the normal conductor by itself is below or above the percolation threshold. When the area fraction of the normal conductor is below that threshold, the macroscopic induced magnetoresistance is found to keep increasing with the magnetic field, without any saturation, whereas when that area fraction is above the percolation threshold, the magnetoresistance is found to saturate. Thus, the percolation threshold of the normal conductor is identified as a critical point. This critical phenomenon is associated with both a geometrical percolation and the presence of a large Hall effect. Its origin can be qualitatively understood by noticing the surprising fact that, in the strong-field limit, a perfectly conducting inclusion surrounded by a normally conducting neighborhood tends to expel currents almost like an insulating inclusion. The simulations also provide insights into difficulties that arise when simulating finite-size conducting networks at strong magnetic fields.
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A previous work [Shen, Ma, Xie, and Zhang, Phys. Rev. Lett. 92, 256603 (2004)] on two-dimensional quantum wells with Rashba type spin-orbit interaction under a strong perpendicular magnetic field is generalized to include the Dres...
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A previous work [Shen, Ma, Xie, and Zhang, Phys. Rev. Lett. 92, 256603 (2004)] on two-dimensional quantum wells with Rashba type spin-orbit interaction under a strong perpendicular magnetic field is generalized to include the Dresselhaus coupling. The Rashba coupling and the Dresselhaus coupling interplay with the Zeeman effect in opposing ways. The former tends to produce a resonant spin Hall effect at certain magnetic fields while the latter suppresses it. Due to the resonant spin Hall effect, the spin Hall current is highly nonohmic at low temperatures. The condition for the resonant spin Hall conductance in the presence of both Rashba and Dresselhaus couplings is derived using a perturbation method. In the presence of disorder, we argue that the resonant spin Hall conductance occurs when the two Zeeman split extended states near the Fermi level become degenerate due to the Rashba coupling, and that the quantized charge Hall conductance changes by 2e~2/h instead of e~2/h as the magnetic field changes through the resonant field.
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Self-interaction corrected local spin-density approximation calculations were performed for La_(1-x)Sr_xMnO_3 (LSMO) (0.0展开
Self-interaction corrected local spin-density approximation calculations were performed for La_(1-x)Sr_xMnO_3 (LSMO) (0.0收起
