Josephson current in superconductor/ferromagnet/superconductor junctions

By using the Bogoliubov–de Gennes equation and the Nambu spinor Greens function approach, we have theoretically studied the dc Josephson current and the coupling phase state of superconductor/ferromagnet/superconductor (SC/FM/SC) junctions, where the FM is of weak ferromagnetism. From the behavior of the temperature-dependent dc Josephson current (I_c), we confirm that such SC/FM/SC junction may change from 0-phase to π-phase state with increasing the temperature (T), for particular parameters of the thickness and the strength of ferromagnetism of the FM interlayer. We attribute such changement to an extra phase difference between the two SCs. The results are qualitatively consistent with an experiment [Phys. Rev. Lett. 86 (2001) 2427], which shows a sharp cusp structure on the I_c−T curves of Nb/Cu_0.48Ni_0.52/Nb junction for specific thickness of the Cu_0.48Ni_0.52, indicating the junction changes from 0-phase state at high temperatures to π-phase state at low temperatures.     

Selective spin injection controlled by electrical way in ferromagnet/quantum dot/semiconductor system

Selective and large polarization of current injected into semiconductor (SC) is predicted in ferromagnet (FM)/quantum dot (QD)/SC system by varying the gate voltage above the Kondo temperature. In addition, spin-dependent Kondo effect is also revealed below Kondo temperature. It is found that Kondo resonances for up spin state are suppressed with increasing of the polarization P of the FM lead. While the down one is enhanced. The Kondo peak for up spin is disappear at P=1.     

Study of ferromagnetism–superconductivity interactions in Co/Nb multilayers

We investigate Co/Nb multilayers to explore the spontaneous π-phase shift between the superconducting (SC) layers, which is attributed for causing the non-monotonic change of the SC transition temperature (T_c) with the ferromagnetic (FM) layer thickness (t_FM) in several FM/SC multilayered systems. The issue of interfacial roughness is also explored by growing Co/Nb multilayers at various sputtering pressures. Transport measurements show a non-monotonic dependence of T_c on t_FM, and this dependence is insensitive to the structural variation present in the samples, as measured by X-ray scattering.     

Proximity effects in a superconductor/ferromagnet junction

A proximity effect in an s-wave superconductor/ferromagnet (SC/F) junction is theoretically studied using the second order perturbation theory for the tunneling Hamiltonian and Green's function method. We calculate a pair amplitude induced by the proximity effect in a weak ferromagnetic metal (FM) and a half-metal (HM). In the SC/FM junction, it is found that a spin-singlet pair amplitude (Ψ_s) and spin-triplet pair amplitude (Ψ_t) are induced in FM and both amplitudes depend on the frequency in the Matsubara representation. Ψ_s is an even function and Ψ_t is an odd function with respect to the Matsubara frequency (ω_n). In the SC/HM junction, we examine the proximity effects by taking account of magnon excitations in HM. It is found that the triplet-pair correlation is induced in HM. The induced pair amplitude in HM shows a damped oscillation as a function of the position and contains the terms of even and odd functions of ω_n as in the case of the SC/FM junction. We discuss that in our tunneling model the pair amplitude of even function of ω_n only contributes to a Josephson current.     

Can we explain the coexistence of ferromagnetism and superconductivity based on two parameters model?

A possible explanation about the coexistence of ferromagnetism (FM) and superconductivity (SC) based on a two parameters mean field model in a two-dimensional system is discussed. The key feature of this model is that there are two independent parameters which are responsible for ferromagnetism and superconductivity, respectively. We point out that the coexisting FM and s-wave pairing SC state is energetically not favorable among all possible state. We generalize the two parameter model to include the coexistence of FM with p-wave SC. We find that the phase diagram is not consistent with what experimentally discovered in UGe₂.     

Unitary representations of non-compact supergroups

We give a general theory for the construction of oscillator-like unitary irreducible representations (UIRs) of non-compact supergroups in a super Fock space. This construction applies to all non-compact supergroupsG whose coset spaceG/K with respect to their maximal compact subsupergroupK is “Hermitean supersymmetric”. We illustrate our method with the example of SU(m, p/n+q) by giving its oscillator-like UIRs in a “particle state” basis as well as “supercoherent state basis”. The same class of UIRs can also be realized over the “super Hilbert spaces” of holomorphic functions of aZ variable labelling the coherent states.     

Near-field thermal emission from metamaterials

A closed form expression for the local density of electromagnetic states (LDOS) due to a thermally emitting metamaterial bulk is derived from Maxwell's equations combined with fluctuational electrodynamics. The final form is the same as that for nonmagnetic materials, where the influence of the magnetic permeability is embedded in the Fresnel reflection coefficients. Spectral distributions of LDOS near metallic- and dielectric-based metamaterials are investigated. Results reveal that LDOS profiles are dominated by surface polaritons (SPs) in both TE and TM polarization states. A detailed discussion is provided on the necessary conditions for exciting TM- and TE-polarized SPs via a dispersion relation analysis that accounts for losses. Beyond the conventional conditions for excitation of SPs, the lossy dispersion relation analysis demonstrates mathematically that SPs exist when the imaginary parts of the permittivity or permeability, as well as n′n″, are close to zero, where n′ and n″ are the real and imaginary parts of the refractive index, respectively. An asymptotic expression for the extreme near field LDOS is derived, showing a Δ^?3 power law relationship, as for nonmagnetic media, between LDOS and distance from the emitting bulk Δ. Results obtained from this study will assist in assessing material properties of arbitrarily electromagnetic materials in applications related to energy harvesting.     

Microwave spectrum of 2-propene-1-imine,CH2CHCHNH

The reactive species, 2-propene-1-imine, has been identified by its microwave spectrum as a pyrolysis product of diallylamine vapor (100 mTorr, 400°C). Two entirely planar forms are observed, both with an “s-trans” CCCN configuration. The lower energy rotamer has an “anti” CCNH configuration, with $\text{r}\text{CC = 1.453(3)}\text{A}\text{?}$, $\text{r}\text{CC = 1.336(4)}\text{A}\text{?}$, and ∠ CCC = 122.9(3)° and a dipole moment of 2.01(2) D with 1.13(1) D and 1.66(2) D “a” and “b” components. The higher energy rotamer has a “syn” CCNH configuration and a dipole moment of 2.51(2) D with 2.39(2) and 0.77(3) D the “a” and “b” components. From relative intensity measurements, the ground state energy difference is determined to be 0.9 ± 0.1 kcal mole^?1.     

Amorphous GaP produced by ion implantation

Two types of non-crystalline states (“disordered” and “amorphous”) of GaP were produced by using ion implantation and post annealing. A structural-phase-transition-like annealing behaviour from the “disordered” state to the “amorphous” state was observed.The ion dose dependence and the annealing behaviour of the atomic structure of GaP implanted with 200 keV ? N⁺ ions were studied by using electron diffraction, backscattering and volume change measurements. The electronic structure was also investigated by measuring optical absorption and electrical conductivity.The implanted layer gradually loses the crystalline order with the increase of the nitrogen dose.The optical absorption coefficient α and electric conductivity σ of GaP crystals implanted with 200 keV?N⁺ ions of 1 × 10¹⁶ cm^?2 were expressed as αhν = C(hν ? E₀)ⁿ and log $\text{σ = A ? BT}{}^{\mathrm{<mtext>-</mtext><mtext>1</mtext><mtext>4</mtext>}}$, respectively. Moreover, the volume of the implanted layer increased about three percent and the electron diffraction pattern was diffused halo whose intensity monotonically decreases along the radial direction. These results indicate that the as-implanted layer has neither a long range order nor a short range order (“disordered state”).In the sample implanted at 1 × 10¹⁶ cm^?2, a structural phase-transition-like annealing stage was observed at around 400°C. That is, the optical absorption coefficient α abruptly fell off from 6 × 10⁴ to 7 × 10³ cm^?1 and the volume of the implanted layer decreased about 2% within an increase of less than 10 degrees in the anneal temperature. Moreover, the short range order of the lattice structure appeared in the electron diffraction pattern. According to the backscattering experiment, the heavily implanted GaP was still in the non-crystalline state even after annealing.These facts lead us to believe that heavily implanted GaP, followed by annealing at around 400°C, is in the “amorphous” state, although as-implanted Gap is not in the “amorphous” state but in the “disordered” state.     

Spin-relaxation and magnetoresistance in FM/SC/FM tunnel junctions

The effect of spin relaxation on tunnel magnetoresistance (TMR) in a ferromagnet/superconductor/ferromagnet (FM/SC/FM) double tunnel junction is theoretically studied. The spin accumulation in SC is determined by balancing of the spin-injection rate and the spin-relaxation rate. In the superconducting state, the spin-relaxation time τ_s becomes longer with decreasing temperature, resulting in a rapid increase of TMR. The TMR of FM/SC/FM junctions provides a useful probe to extract information about spin-relaxation in superconductors.     

Competition between ferromagnetic and anti-ferromagnetic couplings in Co doped ZnO with vacancies and Ga co-dopants

Spin-polarized first-principles electronic structure and total energy calculations have been performed to better understand the magnetic properties of Co doped ZnO (ZnO:Co) with vacancies and Ga co-dopants. The paramagnetic state of ZnO:Co, in which Co ions lose their magnetic moments, has been found to be unstable. The total energy results show that acceptor-like Zn vacancies and donor-like Ga co-dopants render the anti-ferromagnetic (AFM) and ferromagnetic (FM) states to be more favorable, respectively. With O vacancies, ZnO:Co has been found to be in the weak FM state. These magnetic properties can be understood by the calculated O- and Zn-vacancies and Ga co-dopant induced changes of the electronic structure, which suggest that AFM and FM Co-Co couplings are mediated by O 2p-Co majority (↑)-spin 3d hybridized states in the valence band of ZnO and O-vacancy-derived p states or Ga sp states in the ZnO band gap, respectively. For ZnO:Co with Zn vacancies (Ga co-dopants) the AFM (FM) coupling outweighs the FM (AFM) coupling and results in the AFM (FM) state, while for ZnO:Co with O vacancies, both the FM and AFM couplings are enhanced by similar degrees and result in the weak FM state. This study reveals a competition between FM and AFM couplings in ZnO:Co with vacancies and Ga co-dopants, the detailed balancing between which determines the magnetic properties of these materials.     

Dynamics of an N-vortex state at small distances

We investigate the dynamics of a state of N vortices, placed at the initial instant at small distances from some point, close to the “weight center” of vortices. The general solution of the time-dependent Ginsburg-Landau equation for N vortices in a large time interval is found. For N = 2, the position of the “weight center” of two vortices is time independent. For N ≥ 3, the position of the “weight center” weakly depends on time and is located in the range of the order of a ³, where a is a characteristic distance of a single vortex from the “weight center.” For N = 3, the time evolution of the N-vortex state is fixed by the position of vortices at any time instant and by the values of two small parameters. For N ≥ 4, a new parameter arises in the problem, connected with relative increases in the number of decay modes.     

Reflection and interference structure in diatomic Franck-Condon distributions

The Franck-Condon distributions for diatomic radiative transitions from a single vibrational level of a given electronic state to all possible levels (bound and free) of a second electronic state exhibit either “reflection” or “interference” structure. In reflection structure there is a one-to-one mapping of peaks in the initial state probability distribution into peaks in the spectrum. No such simple relationship is known for interference structure, originally termed “internal diffraction” by Condon [Phys. Rev.32, 858–872 (1928)]. A semiclassical treatment of the quantum mechanical overlap integrals shows that the condition for reflection structure is a monotonic difference potential in the range of internuclear distance sampled by the initial wavefunction, whereas interference structure occurs when a “polytonic” difference potential is sampled. The qualitative validity of the semiclassical treatment is illustrated through quantum calculations which show that the Franck-Condon integrals accumulate near the points of stationary phase.     

Modeling of metamagnetism in metallic-based materials with first-order transitions

During the past decade, the magnetic properties of metallic-based materials with first-order transitions have been extensively studied, motivated in part by the observation of large magnetocaloric effect (MCE) peaks displayed by these materials near room temperature. These large peaks are believed to be the result of the materials' magnetic properties at the metamagnetic region, characterized by (i) the thermal-induced transition from the ferromagnetic state (FM) to the paramagnetic state (PM) near the Curie temperature (T_C) and (ii) the field-induced transition from PM state to FM state above T_C. We developed a phenomenological model that utilizes the materials' mixed-state probability function to model the materials' complex hysteretic and properties at metamagnetic region. The approximate probability functions are obtained from the first and second derivatives of the magnetization curve. The probability functions are used to separate the materials' magnetization into a FM state component and a PM state component. The applicability of the model is demonstrated for a metallic-based metamagnetic material, Gd₅Si₂Ge₂ compound, where the modeled behaviors show remarkable agreement with the experimental data at the metamagnetic region and provide new physical insights in this mixed-state region. Specifically, in the region of metamagnetic transition, the PM state component is non-reversible and is a function of the FM state component.     

Magnetic, electrical, and optical properties of electron-doped Ca1 − x LaxMnO3 − δ(x ≤ 0.12) single crystals

The magnetic, electrical, and optical properties of Ca_{1 ? x} La_xMnO_{3 ? δ}(x ≤ 0.12) manganite single crystals have been studied. The state with a spatially inhomogeneous electron distribution has been found. Interrelations between the electric and magnetic subsystems are analyzed. The obtained magnetic data show evidence for the formation of a G-type antiferromagnetic (G-AFM) phase with a spin-canted structure in the crystal with x = 0.05, for which the Curie and Néel temperatures are T _C = T _N(G) = 115 K. On cooling from the paramagnetic state, the crystals with x = 0.10 and 0.12 exhibit transitions from the paramagnetic to a C-type antiferromagnetic (C-AFM) phase in a part of the volume at T _N(C) = 150 and 200 K, and from the paramagnetic to the G-type antiferromagnetic (G-AFM) phase in the remaining volume at T _N(G) = 110 and 108 K, respectively. The onset of the C-type magnetic phase nucleation in crystals is observed at lower dopant (La) concentrations than in polycrystalline samples, which is explained by the deviation of single crystals from the stoichiometry with respect to oxygen. The magnetic phase transitions are manifested by anomalies in the electric resistance and magnetoresistance of doped crystals. An analysis of the electrical and optical properties of the samples shows evidence of (i) the formation of a charge energy gap in the C-AFM phase with retained paramagnetic metallic regions and (ii) the presence of ferromagnetic “metallic” droplets in the insulating G-AFM phase. The multiphase state of Ca_{1 ? x} La_xMnO_{3 ? δ} manganite single crystals featuring the coexistence of two magnetic phases, the regions with orbital/charge ordering, and the FM “metallic” droplets is related to a competition of exchange interactions by the superexchange and double exchange mechanisms.     

Nonequilibrium statistical mechanics in the general theory of relativity I. A general formalism

This is the first in a series of papers, the overall objective of which is the formulation of a new covariant approach to nonequilibrium statistical mechanics in classical general relativity. The object here is the development of a tractable theory for self-gravitating systems. It is argued that the “state” of an N-particle system may be characterized by an N-particle distribution function, defined in an 8N-dimensional phase space, which satisfies a collection of N conservation equations. by mapping the true physics onto a fictitious “background” spacetime, which may be chosen to satisfy some “average” field equations, one then obtains a useful covariant notion of “evolution” in response to a fluctuating “gravitational force.” For many cases of practical interest, one may suppose (i) that these fluctuating forces satisfy linear field equations and (ii) that they may be modeled by a direct interaction. In this case, one can use a relativistic projection operator formalism to derive exact closed equations for the evolution of such objects as an appropriately defined reduced one-particle distribution function. By capturing, in a natural way, the notion of a dilute gas, or impulse, approximation, one is then led to a comparatively simple equation for the one-particle distribution. If, furthermore, one treats the effects of the fluctuating forces as “localized” in space and time, one obtains a tractable kinetic equation which reduces, in the newtonian limit, to the standard Landau equation.     

Spin-injection efficiency and magnetoresistance in a hybrid ferromagnetic-semiconductor trilayer with interfacial barriers

We present a self-consistent model of spin transport in a ferromagnetic (FM)-semiconductor (SC)-FM trilayer structure with interfacial barriers at the FM-SC boundaries. The SC layer consists of a highly doped n²⁺ AlGaAs-GaAs 2DEG while the interfacial resistance is modeled as delta potential (δ) barriers. The self-consistent scheme combines a ballistic model of spin-dependent transmission across the δ-barriers, and a drift-diffusion model within the bulk of the trilayer. The interfacial resistance (R_I) values of the two junctions were found to be asymmetric despite the symmetry of the trilayer structure. Transport characteristics such as the asymmetry in R_I, spin-injection efficiency and magnetoresistance (MR) are calculated as a function of bulk conductivity σ_s and spin-diffusion length (SDL) within the SC layer. In general a large σ_s tends to improve all three characteristics, while a long SDL improves the MR ratio but reduces the spin-injection efficiency. These trends may be explained in terms of conductivity mismatch and spin accumulation either at the interfacial zones or within the bulk of the SC layer.     

Current switching in bistable structures: Heavily doped n +-polysilicon-tunnel-oxide layer-n-silicon

Switching from a state with a steady-state nonequilibrium depletion and low current into the “on” state with a high current and low voltage drop on the structure is observed in highly doped n ⁺-polysilicon-tunnel-oxide-n-silicon structures. The structures were prepared on n-silicon substrates with a resistivity of 25 Ω·cm. A structure with an oxide thickness of 23 Å can be switched on both by a radiation pulse with small reverse bias on the structure (50 V) and under dark conditions by increasing the reverse bias to 250–300 V. In the “on” state Auger carrier production is the internal source of minority carriers that is required for compensating tunnel leakage of holes into the n ⁺-polysilicon and for maintaining a quasiequilibrium inversion layer of holes at the n-Si-SiO₂ boundary.     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al₂O₃/PdNi/Nb tunnel junctions. Received 1st July 2002 Published online 19 November 2002     

Hydrogen atom on null-plane and Melosh transformation

The null-plane dynamics of hydrogen-like atoms is studied in approximations depending on c, the velocity of light, being large. Neglecting terms in the Hamiltonian of order c^?3 (relative to electron rest energy) a symmetry SU (2)_W appears which is analogous to the SU (6)_W of hadron classification. This symmetry, if accurate, would dictate zero ground state magnetic moment. The symmetry is broken by terms of third order, which can, however, be transformed a way by the appropriate approximation to the Melosh transformation. There then emerges a better symmetry, SU (2)_M, broken only at fourth order. The ground state magnetic moment acquires its usual non-relativistic value. The symmetry SU (2)_M corresponds to a subgroup of a symmetry [U (2) × U (2)]_FW which appears in the old Foldy-Wouthuysen approach when spin-orbit coupling is neglected. As well as “current” and “constituent” pictures, “classification” pictures are distinguished; it is to one of the latter that the Melosh transformation transforms.