Spectrum of Andreev bound states in Josephson junctions with a ferromagnetic insulator

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–π$0–\pi$ 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 π(0)$\pi (0)$ state is always more stable than the 0 (π$\pi$) state if L is odd (even).     

Inhomogeneous magnetostriction states in uniaxial ferromagnetic films

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H _c, different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k=k _c≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate. Fiz. Tverd. Tela (St. Petersburg) 41, 665–671 (April 1999)     

Incomplete Andreev reflection in a clean SFS junction

We study the stationary Josephson effect in a ballistic superconductor/ferromagnet/superconductor junction for arbitrarily large spin polarizations. Due to the exchange interaction in the ferromagnet, the Andreev reflection is incomplete. We describe how this effect modifies the Josephson current in the crossover from a superconductor/normal metal/superconductor junction to a superconductor/half metal/superconductor junction.     

铁磁金属量子阱态的共振隧穿

非铁磁金属层中的量子阱态在磁输运过程中的重要性已被广泛认识．铁磁金属层中自旋极化的量子阱态以前并没有详尽的理论研究；实验上也没有清晰地观测到自旋极化量子阱态的隧穿．文章介绍了最近由卢仲毅、张晓光和Pantelides预言的Fe／MgO／FeO／Fe／Cr和其他铁磁量子阱隧道结中的共振隧穿，并解释铁、钴、铬的△1能带的对称性在这种共振隧穿中的作用．     

铁磁金属量子阱态的共振隧穿

非铁磁金属层中的量子阱态在磁输运过程中的重要性已被广泛认识.铁磁金属层中自旋极化的量子阱态以前并没有详尽的理论研究;实验上也没有清晰地观测到自旋极化量子阱态的隧穿.文章介绍了最近由卢仲毅、张晓光和Pantelides［1］预言的Fe/MgO/FeO/Fe/Cr和其他铁磁量子阱隧道结中的共振隧穿,并解释铁、钴、铬的Δ1能带的对称性在这种共振隧穿中的作用.     

Andreev bound states in high temperature superconductors

Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa₂Cu₃O (YBCO), Bi₂Sr₂CaCu₂O(BSCCO), and La_1.85Sr_0.15CuO₄ (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd_1.85Ce_0.15CuO_4-y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented. Received: 17 February 1998 / Revised: 27 April 1998 / Accepted: 23 June 1998     

Andreev states near short-ranged pairing potential impurities

We study Andreev states near atomic scale modulations in the pairing potential in both s- and d-wave superconductors with short coherence lengths. For a moderate reduction of the local gap, the states exist only close to the gap edge. If one allows for local sign changes of the order parameter, however, resonances can occur at energies close to the Fermi level. The local density of states (LDOS) around such pairing potential defects strongly resembles the patterns observed by tunneling measurements around Zn impurities in Bi2Sr2CaCu2O8+x (BSCCO). We discuss how this phase impurity model of the Zn LDOS pattern can be distinguished from other proposals experimentally.     

Andreev states and the Josephson effect in superconducting heterojunctions on thin YBa2Cu3Ox films

Conductance anomalies at low bias voltages and superconducting currents in Au/YBa₂Cu₃O_x and Nb/Au/YBa₂Cu₃O_x heterojunctions in which the c axis of the YBa₂Cu₃O_x (YBCO) epitaxial film is rotated in the (110) YBCO plane through 11° with respect to the normal to the substrate plane were studied experimentally. The films were prepared by laser deposition onto (7 2 10)-oriented NdGaO₃ substrates. The current-voltage characteristics of the heterojunctions exhibit conductance anomalies at low voltages. The behavior of these anomalies is studied at various temperatures and in various magnetic fields. The critical current and Shapiro steps observed in the current-voltage characteristics of Nb/Au/YBa₂Cu₃O_x were evidence of the Josephson effect in these heterojunctions. The experimental results are analyzed in terms of the model of the arising of bound states caused by Andreev reflection in superconductors with d-type symmetry of the superconducting order parameter.     

Tunneling states in ferromagnetic glasses

We study the interaction between structural defects, represented by two level systems, and spin waves in a ferromagnetic glass. The damping and energy shift of the spin wave due to this interaction have a “resonant” and a “relaxation” contribution in analogy with the case of phonons. Ferromagnetic resonance in amorphous ferromagnets can provide useful information on structural relaxation in these materials.     

Zero-energy Andreev surface bound states in the lattice model

The conditions for zero-energy Andreev surface bound states to exist are found for the lattice model of a d-wave superconductor with arbitrary surface orientation. Both nearest-neighbors and next-nearest-neighbors models are considered. It is shown that the results are very sensitive to the surface orientation. In particular, for a half-filled (hl0)-surface, zero-energy Andreev surface states only appear under the condition that h and l are odd simultaneously.     

Spin dependent Andreev reflection in ferromagnetic semiconductor/d-wave superconductor ballistic junction

The Andreev reflection (AR) probability and transmission of quasiparticles in ferromagnetic semiconductor/d-wave superconductor (FS/DS) ballistic junctions are studied based on an extended Blonder–Tinkham–Klapwijk (BTK) theory. It is shown that the dependence of AR probability and pair potential on the spin orientation of incident quasiparticles for the heavy holes is much different from that for light holes due to the different mismatches in the effective mass and Fermi velocity between FS and DS. The junction conductance is dominated by the quasiparticles which undergo AR processes with the largest probability, and this provides a method for measuring the spin polarization in FS.     

Andreev probe of persistent current states in superconducting quantum circuits

Using the extraordinary sensitivity of Andreev interferometers to the superconducting phase difference associated with currents, we measure the persistent current quantum states in superconducting loops interrupted by Josephson junctions. Straightforward electrical resistance measurements of the interferometers give a continuous readout of the states, allowing us to construct the energy spectrum of the quantum circuit. The probe is estimated to be more precise and faster than previous methods, and can measure the local phase difference in a wide range of superconducting circuits.     

Tunneling between ferromagnetic films

FeGeCo junctions conductance G(V) is studied when mean magnetizations of the two ferromagnetic film are parrallel or antiparallel. Conductance measurement, in these two cases, is related to the spin polarizations of the conduction electrons.     

Density of states in ferromagnetic NiCu alloys

We have calculated the concentration dependence of the density of states at Fermi level in ferromagnetic NiCu alloys using a two-band model Hamiltonian and the coherent potential theory. Good agreement with experimental data has been obtained.     

Andreev states and shot noise in bicrystal junctions of cuprate superconductors

Experimentally observed features of the electrical and noise characteristics of bicrystal junctions of cuprate superconductors, such as linearity of the critical current density versus square root of the junction transparency and increase in the spectral density of shot noise for small bias voltages (below the superconducting gap), indicate that the superconducting current in cuprate bicrystal junctions is determined by the passage of quasi-particles through a potential barrier at the superconductor boundaries. This process involves bound states appearing as a result of multiple Andreev reflections in superconductors with dominant wavefunction components of the d _x ² ? _y ² symmetry type. At the same time, interpretation of the experimental current-phase and current-magnetic field curves requires that the character of faceting at the bicrystal junctions would be also taken into account.     

Collective states of interacting ferromagnetic nanoparticles

Discontinuous magnetic multilayers [CoFe/Al₂O₃] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co₈₀Fe₂₀ nanoparticles are embedded in a diamagnetic insulating a-Al₂O₃ matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.     

Remanent states of small ferromagnetic cylinder

The internal magnetic structures of a cylinder are computed without prior assumption of axial symmetry. A three-dimensional magnetostatic solution is presented. A detailed distribution of the magnetization is obtained through numerical integration of the Landau-Lifshitz equation with the demagnetization field. Various entry conditions are used. Materials with small uniaxial anisotropy typically demonstrate vortex structures, while a higher anisotropy leads to the formation of concentric cylindrical magnetic bubbles inside the cylinder and a monodomain flower state.