A quadruple quantum-dot (QQD) cell is proposed as a spin filter. The transport properties of the QQD cell were studied in linear response regime on the basis of the equations of motion for retarded Green's functions. The developed approach allowed us to take into account the influence of both intra- and interdot Coulomb interactions on charge carriers' spin polarization. It was shown that the presence of the insulating bands in the conductance due to the Coulomb correlations results in the emergence of spin-polarized windows (SPWs) in magnetic field leading to the high spin polarization. We demonstrated that the SPWs can be effectively manipulated by gate fields and considering the hopping between central dots in both isotropic and anisotropic regimes.
Ferromagnetic materials with exchange fields E_ex smaller or of the order of the superconducting gap Delta are important for applications of corresponding (s-wave) superconductor/ ferromagnet/ superconductor (SFS) junctions. Presently such materials are not known but there are several proposals how to create them. Small exchange fields are in principle difficult to detect. Based on our results we propose reliable detection methods of such small E_ex. For exchange fields smaller than the superconducting gap the subgap differential conductance of the normal metal - ferromagnet - insulator - superconductor (NFIS) junction shows a peak at the voltage bias equal to the exchange field of the ferromagnetic layer, eV=E_ex. Thus measuring the subgap conductance one can reliably determine small E_ex < Delta. In the opposite case E_ex > Delta one can determine the exchange field in scanning tunneling microscopy (STM) experiment. The density of states of the FS bilayer measured at the outer border of the ferromagnet shows a peak at the energy equal to the exchange field, E=E_ex. This peak can be only visible for small enough exchange fields of the order of few Delta.
We investigate electron cooling based on a clean normal-metal/spin-filter/superconductor junction. Due to the suppression of the Andreev reflection by the spin-filter effect, the cooling power of the system is found to be extremely higher than that for conventional normal-metal/nonmagnetic-insulator/superconductor coolers. Therefore we can extract large amount of heat from normal metals. Our results strongly indicate the practical usefulness of the spin-filter effect for cooling detectors, sensors, and quantum bits.
Structural, magnetic and magnetotransport properties of (Bi1-xEux)2Se3 thin films have been studied experimentally as a function of Eu content. The films were synthesized by MBE. It is demonstrated that Eu distribution is not uniform, it enter quint-layers forming inside them plain (pancake-like) areas containing Eu atoms, which sizes and concentration increase with the growth of Eu content. Positive magnetoresistance related to the weak antilocalization was observed up to 15 K. The antilocalization was not followed by weak localization as theory predicts for nontrivial topological states. Surprisingly, the features of antilocalization were seen even at Eu content x = 0.21. With the increase of Eu content the transition to ferromagnetic state occurs at x about 0.1 and with the Curie temperature 8 K, that rises up to 64 K for x = 0.21. At temperatures above 1–2 K, the dephasing length is proportional to T^-1/2 indicating the dominant contribution of inelastic e-e scattering into electron phase breaking. However, at low temperatures, the dephasing length saturates, that could be due to the scattering on magnetic ions.
A set of thin film MnxSi1-x alloy samples with different manganese concentration x≈0.44−0.63 grown by the pulsed laser deposition (PLD) method onto the Al2O3(0001) substrate was investigated in the temperature range 4–300 K using ferromagnetic resonance (FMR) measurements in the wide range of frequencies () and magnetic fields (). For samples with x≈0.52−0.55, FMR data show clear evidence of ferromagnetism with high Curie temperatures . These samples demonstrate a complex magnetic anisotropy described phenomenologically as a combination of the essential second order easy plane anisotropy contribution and the additional fourth order uniaxial anisotropy contribution with easy direction normal to the film plane. The observed anisotropy is attributed to a polycrystalline (mosaic) structure of the films caused by the film-substrate lattice mismatch. The existence of extra strains at the crystallite boundaries initiates a random distribution of local in-plane anisotropy axes in the samples. As a result, the symmetry of the net magnetic anisotropy is axial with the symmetry axis normal to the film plane. The principal features of the observed anisotropy are explained qualitatively within the proposed microscopic model.
We present the magnetic properties of (Y1-xNdx) 2BaNiO5 (x=1, 0.15) investigated by means of specific heat, magnetic susceptibility, and spectroscopic measurements. Magnetic ordering occurs at 47 K and 13 K in the compounds with x=1 and x=0.15, respectively. We estimate the magnetic contribution of the neodymium subsystem to magnetization and specific heat using temperature dependences of the splitting of ground Kramers doublet of Nd3+ ion obtained from spectroscopic experiment. We show that both Nd-Ni and Nd-Nd interactions should be taken into account. An origin of the observed spin-glass state below ∼5 K and a contribution of the Ni chain breaks to the magnetization and specific heat are discussed. © 2012 Elsevier B.V. All rights reserved.
We investigate the critical temperature Tc of F2/F1/S trilayers (Fi is a ferromagnetic metal and S is a singlet superconductor), where the long-range triplet superconducting component is generated at noncollinear magnetizations of the F layers. In this paper we demonstrate a possibility of the spin-valve effect mode selection (standard switching effect, the triplet spin-valve effect or reentrant Tc(α)Tc(α) dependence) by the variation of the F2/F1 interface transparency.
Quasi-one-dimensional superconducting channels host sound-like plasma modes propagating along the which are associated with fluctuations of the phase of the superconducting order parameter (Mooiji and 1985) . Interaction between these electromagnetic excitations and charge carriers affects the electron density of states (DOS). I-V characteristics of tunnel S1-I-S2 junctions, where superconducting S2 electrode nanowire in the regime of quantum fluctuations have been studied. The observed broadening of the I-V dependencies at the gap edge is interpreted as the renormalization of DOS. The results are in reasonable agreement with the model, taking into consideration plasma modes in quasi-one-dimensional superconductors.
Ferromagnetic-insulator (FI) based Josephson junctions are promising candidates for a coherent superconducting quantum bit as well as a classical superconducting logic circuit. Recently the appearance of an intriguing atomic-scale 0-pi transition has been theoretically predicted. In order to uncover the mechanism of this phenomena, we numerically calculate the spectrum of Andreev bound states in a FI barrier by diagonalizing the Bogoliubov-de Gennes equation. We show that Andreev spectrum drastically depends on the parity of the FI-layer number L and accordingly the pi (0) state is always more stable than the 0 (pi) state if L is odd (even).
Collective plasmon excitations in a helical electron liquid on the surface of strong three-dimensional topological insulator are considered. The properties and internal structure of these excitations are studied. Due to spin-momentum locking in helical liquid on a surface of topological insulator, the collective excitations should manifest themselves as coupled charge- and spin-density waves.
It is known that Dirac nodes can be present at high-symmetry points of Brillouin zone only for certain space groups. For these cases, the eect of strain is treated by symmetry considerations. The dependence of strain-induced potentials on the strain tensor is found. In all but two cases, the pseudomagnetic eld potential is present. It can be used to control valley currents.
The non-stationary response of local magnetic moment to abrupt switching ‘‘on” and ‘‘off” of external magnetic ﬁeld was studied for a single-level quantum dot (QD) coupled to a reservoir. We found that transient processes look different for the shallow and deep localized energy level. It was demonstrated that for deep energy level the relaxation rates of the local magnetic moment strongly differ in the case of magnetic ﬁeld switching ‘‘on” or ‘‘off”. Obtained results can be applied in the area of dynamic memory devices stabilization in the presence of magnetic ﬁeld
A review of our recent results on the spin valve effect is presented. We have used a theoretically proposed spin switch design F1/F2/S comprising a ferromagnetic bilayer (F1/F2) as a ferromagnetic component, and an ordinary superconductor (S) as the second interface component. Based on it we have prepared and studied in detail a set of multilayers CoOx/Fe1/Cu/Fe2/S (S=In or Pb). In these heterostructures we have realized for the first time a full spin switch effect for the superconducting current, have observed its sign-changing oscillating behavior as a function of the Fe2-layer thickness and finally have obtained direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the magnetizations of the Fe1 and Fe2 layers.