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.     

Spin gauge fields: From Berry phase to topological spin transport and Hall effects

The paper examines the emergence of gauge fields during the evolution of a particle with a spin that is described by a matrix Hamiltonian with n different eigenvalues. It is shown that by introducing a spin gauge field a particle with a spin can be described as a spin multiplet of scalar particles situated in a non-Abelian pure gauge (forceless) field U (n). As the result, one can create a theory of particle evolution that is gauge-invariant with regards to the group Uⁿ (1). Due to this, in the adiabatic (Abelian) approximation the spin gauge field is an analogue of n electromagnetic fields U (1) on the extended phase space of the particle. These fields are force ones, and the forces of their action enter the particle motion equations that are derived in the paper in the general form. The motion equations describe the topological spin transport, pumping, and splitting. The Berry phase is represented in this theory analogously to the Dirac phase of a particle in an electromagnetic field. Due to the analogy with the electromagnetic field, the theory becomes natural in the four-dimensional form. Besides the general theory, the article considers a number of important particular examples, both known and new.     

Response of spin-accumulated ferromagnet/superconductor/ferromagnet double tunnel junction to applied magnetic field

Tunneling current in a ferromagnet/superconductor/ferromagnet double tunnel junction induces a nonequilibrium spin accumulation in the superconductor. We study theoretically the response of such a system to applied magnetic field. We show that the interplay between the magnetic field and the spin accumulation could lead to novel bias voltage dependence and magnetic field dependence of the superconducting gap function, and bring in anomalous asymmetry in the spin-dependent transport. Our study also indicates a possible application of the spin injection.     

Magnetic properties,electronic structure,and optical properties of the filled skutterudite BaFe4Sb12

The magnetic properties, electronic structure, and optical properties of the filled skutterudite BaFe₄Sb₁₂ are calculated by the first-principles full-potential linearized augmented plane wave (FPLAPW) plus local orbital method. It is found that the local spin density approximation (LSDA) method appears more accurate than the generalized gradient approximation (GGA) method in calculating the electronic structures and optical properties of this compound. Furthermore, our calculated lattice constant and spin magnetic moments with the LSDA method are in overall better agreement with experiment. In contrast with recent experiment, our calculations are in good agreement with experimental reflectivity spectra and optical conductivity spectrum.     

CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction as a spin current diode predicted by first-principles calculations

We report on first-principles calculations of spin-dependent quantum transport in a CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction and predict a strong diode effect of charge and spin current. The minority spin current is absolutely inhibited when the bias voltage is applied to the terminals of both CrAs and AlAs. The majority spin current is inhibited when the bias voltage is applied to the terminal of CrAs and “relaxed” when the bias voltage is applied to the terminal of AlAs. The charge and spin current diode are promising for reprogrammable logic applications in the field of spintronics.     

Magnetic structure of geometrically frustrated compound Co2Cl(OH)3 determined by proton NMR

The magnetic structure of a geometrically frustrated system Co₂Cl(OH)₃ is determined by comparing the observed proton NMR spectrum with numerical calculations based on various magnetic models. The best fit is obtained with a model that the magnetic moments of Co²⁺ ions in the triangular plane are parallel to the principal axis of local crystal field and those of Co²⁺ ions in the kagome lattice plane are randomly disordered in the a-b plane, which nearly bisects the angle between the principal axis of the local field and a line pointing towards the body center of the tetrahedron. The coexistence of the ferromagnetic order in the triangular plane and the random disorder in the kagome plane is consistent with the results of measurements by Zheng et al. However, the magnetic moments of Co²⁺ ions are not directed towards the body center of the tetrahedron as characteristic in the “spin ice” magnetic structure. Furthermore, the Co²⁺ ions in the triangular plane have a smaller magnitude of magnetic moment than those in the kagome plane. Thus, our result suggests that the transition metal compound Co₂Cl(OH)₃ is different from the “spin ice” in magnetic structure, although it is similar to rare-earth pyrochlores in crystal structure.     

Stern-Gerlach spin filter using surface acoustic waves

We propose the ambipolar carrier transport by surface acoustic waves (SAWs) in a semiconductor quantum well (QW) for the realization of the Stern-Gerlach (SG) experiment in the solid phase. The well-defined and very low carrier velocity in the moving SAW field leads to a large deflection angle and thus to efficient spin separation, even for the weak field gradients and short (μm-long) interaction lengths that can be produced by micromagnets. The feasibility of a SG spin filter is discussed for different QW materials.     

On the contribution of nearly critical spin and charge collective modes to the Raman spectra of high-Tc cuprates

We discuss how Raman spectra are affected by nearly critical spin and charge collective modes, which are coupled to charge carriers near a stripe quantum critical point. We show that specific fingerprints of nearly critical collective modes can indeed be observed in Raman spectra and that the selectivity of Raman spectroscopy in momentum space may also be exploited to distinguish the spin and charge contribution. We apply our results to discuss the spectra of high-T_c superconducting cuprates finding that the collective modes should have masses with substantial temperature dependence in agreement with their nearly critical character. Moreover, spin modes should be more diffusive than charge modes indicating that in stripes the charge is nearly ordered, while spin modes are strongly overdamped and fluctuate with high frequency.     

Spin polarization of the injected carriers in C-doped BN nanotubes

We investigated BN nanotubes with two carbon substitutions for one boron atom and one nitrogen atom based on density functional theory (DFT) with local spin density approximation (LSDA). When the two carbon dopants are separated without C-C bond formation, we found that the injected carriers are spin polarized, although there is no net spin in the neutral systems. Here we call the material as ‘spin polarizer’ which can polarize the carriers passing through.     

Schottky-like anomaly in the low-temperature specific heat of single-crystal NdMnO3

The specific heat of single-crystal NdMnO₃ was investigated from 2 to 20 K under different magnetic fields up to 8 T. All the specific heat data show a Schottky-like anomaly, which becomes more indistinctive as increasing magnetic field. The experiment data were successfully fitted by taking into account factors such as crystal-field splitting, the two-level Schottky anomaly, the lattice vibration, and type-A antiferromagnetic (A-AF) spin waves. It was found that the splitting of the ground state doublet of Nd³⁺ ion increases linearly with magnetic field. The above phenomena can be interpreted in terms of the model of unchanged effective molecular field at Nd³⁺ site caused by the ferromagnetic component of A-AF structure of Mn spins. This ferromagnetic component is likely caused by the GdFeO₃-type octahedron rotation. In addition, it was also found that the magnetic field increases the spin-wave stiffness coefficient, but reduces the Debye temperature.     

Interchain impurity substitution effects on the magnetoelastic coupling in CuGeO3

We report the polarized far-infrared transmittance of Si-doped CuGeO₃ single crystals as a function of impurity concentration and applied magnetic field at low temperature. We use the behavior of the 44 cm⁻¹ spin gap excitation and the 98 cm⁻¹ zone-folding mode structure to investigate the interaction between the magnetic system and the lattice distortion. We find that interchain impurity substitution collapses the spin gap before it suppresses the lattice dimerization, a result that is understood in terms of the relative length scales for lattice distortion and spin singlet formation as well as criteria for magnetoelastic coupling in chains.     

Spin glass theory: Numerical and experimental results in three-dimensional systems

Here I will review the theoretical results that have been obtained for spin glasses. I will concentrate my attention on the predictions of the mean field approach in three dimensional systems and on its numerical and experimental verifications.     

Coulomb drag, magnetoresistance, and spin-current injection in magnetic multilayers

We study the effect of spin Coulomb drag on the magnetoresistance and the spin-current injection efficiency of a layered structure consisting of a nonmagnetic semiconductor sandwiched between two ferromagnetic electrodes of spin polarization p. The calculations are done within the framework of the drift-diffusion theory, which we generalize to include the spin trans-conductivity σ_↑↓. We find that for p close to 100% the spin drag enhances the magnetoresistance, while for smaller values of p it reduces it. A new approach to the measurement of σ_↑↓ is suggested.     

Spin injection-driven switching in a magnetic junction

We investigate a perpendicular electric current passing through a “ferromagnetic nanojunction”, that is through some layered nanosized structure of spin-valve type, containing two ferromagnetic metallic layers. Spacer may be used between the metallic layers to prevent the rotation of the moving spin phases. Such an arrangement is typical for spin valves: one of the metallic layers has strongly pinned magnetic lattice and the other one has free magnetic lattice and free mobile spins. Further the conditions are derived to provide a very high nonequilibrium spin injection level. It appears that the so-called spin resistances of the constitutive layers should be in definite relations to each other. These relations lead to the situation where the spin injection becomes dominant and significantly suppresses the “ordinary” spin-transfer torque. As a result, the threshold current becomes lowered down to 2-3 and even more orders of magnitude.     

Half-metallicity characteristic at zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1)

Electronic and magnetic properties of the zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1) semiconductors are studied within the framework of the density-functional theory using the FPLAPW+lo approach. We found that the Cr-terminated surfaces retain the half-metallic character, while the half-metallicity is destroyed for the Sb-terminated surfaces due to surface states, which originate from p electrons. The phase diagram obtained through the ab-initio atomistic thermodynamics shows that at phase transition has occurred. Also the half-metallicity character is preserved at both CrSb/GaSb and CrSb/InAs interfaces. The conduction band minimum (CBM) of CrSb in the minority spin case lies about 0.63 eV above that of InAs, suggesting that the majority spin can be injected into InAs without being flipped to the conduction bands of the minority spin. On the other hand the CrSb/GaSb interface has a greater valence band offset (VBO) compared with the CrSb/InAs interface and the minority electrons have lower contribution in the injected currents and hence more efficient spin injection into the GaSb semiconductor. Thus the CrSb/GaSb and CrSb/InAs heterojunctions can be useful in the field of spintronics.     

Theory of spin precession monitored by laser pulse

We first predict the splitting of a spin degenerate impurity level when this impurity is irradiated by a circularly polarized laser beam tuned in the transparency region of a semiconductor. This splitting, which comes from different exchange processes between the impurity electron and the virtual pairs coupled to the pump beam, induces a spin precession around the laser beam axis, which lasts as long as the pump pulse. It can thus be used for ultrafast spin manipulation. This effect, which has similarities with the exciton optical Stark effect we studied long ago, is here derived using the concepts we developed very recently to treat many-body interactions between composite excitons and which make the physics of this type of effects quite transparent. They, in particular, allow to easily extend this work to other experimental situations in which a spin rotates under laser irradiation.     

Transferred hyperfine field of Rb2CoCl4 single crystals in the ferroelectric-incommensurate-normal phase by Rb NMR

The molecular susceptibility and paramagnetic shift of Rb₂CoCl₄ single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co²⁺ ions to Rb⁺ ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of H_hf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co²⁺ ions to the Rb(I) and Rb(II) ions are large above T_i. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in T_c1 and T_i, and are not affected by the ferroelectric-incommensurate-normal phase transitions.     

Huge exchange-coupling field observed in FeMn-pinned spin valve with ultra-thin pinned NiFe layer

A conventional Ta/NiFe/Cu/NiFe/FeMn/Ta spin valve multilayer was prepared to investigate the exchange bias variations of the pinned NiFe layer. An exchange bias field of 560 Oe has been found in a valve multilayer with ultra-thin pinned NiFe layers (1 nm), in which a large constant magnetic field of 700 Oe was applied during film deposition procession. The observed results are attributed to the large applied magnetic field, which produced more net spins of the antiferromagnet at the interface. These interfacial uncompensated spins provide the net spin moments required for exchange coupling and bias.     

EPR spectra of Fe centers in layered TlGaSe2 single crystal

A single-crystal TlGaSe₂ doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet (S=5/2, L=0) of Fe³⁺ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe³⁺ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe³⁺ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe₄ tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes.     

Transport through a quantum dot subject to spin and charge bias

Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron-phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.