Temperature-dependent resonant tunneling in disordered quasi-one-dimensional <Emphasis Type="Italic">N</Emphasis>–<Emphasis Type="Italic">I</Emphasis>–<Emphasis Type="Italic">N</Emphasis> junctions

Formulas are obtained for the current–voltage characteristics and conductance of a quasi-one-dimensional N–I–N junction (where N is an ordinary metal and I is an insulator) with quantum resonance percolation trajectories (QRPTs) in a disordered I-layer at temperatures T > 0 K and the energy of local single-impurity electron level being equal to the Fermi energy ε₀ = ε_F. Under these conditions, the impact QRPTs have on the current through the junctions weakens as the temperature grows, and the conductance drops; this is in contrast to the rise in conductance of an empty junction (with no impurities in the I-layer).     

Superconducting-insulator transition in disordered Josephson junctions networks

The superconducting-insulator transition is simulated in disordered networks of Josephson junctions with thermally activated Arrhenius-like resistive shunt. By solving the conductance matrix of the network, the transition is reproduced in different experimental conditions by tuning thickness, charge density and disorder degree. In particular, on increasing fluctuations of the parameters entering the Josephson coupling and the Coulomb energy of the junctions, the transition occurs for decreasing values of the critical temperature T _c and increasing values of the activation temperature T _o . The results of the simulation compare well with recent experiments where the mesoscopic fluctuations of the phase have been suggested as the mechanism underlying the phenomenon of emergent granularityin otherwise homogeneous films. The proposed approach is compared with the results obtained on TiN films and nanopatterned arrays of weak-links, where the superconductor-insulator transition is directly stimulated.     

Spin-precession-induced currents through ferromagnet/nanomagnet/superconductor junctions

The spin-precession-induced current through ferromagnet/nanomagnet/superconductorjunctions is investigated by using the nonequilibrium Green’s function method. It is foundthat the charge current I _c for the spinprecession frequency ω less than the energy gap Δ onlyarises from the equal-spin Andreev reflection, which is independent of the spinpolarization p of the ferromagnetic lead, while that forω > Δ mainly originates from the quasiparticle’scontribution. Both equal-spin AR and quasiparticle scattering processes contribute to thespin current I _s and the quasiparticlescattering process plays a dominant role. WhileI _c forω < Δ can be enhanced by the spin polarizationp, I _s decreases withp. These features may be of interest for ongoing experiments in thefield of molecular spintronics.     

Effective decrease in the exchange energy in <Emphasis Type="Italic">S</Emphasis>-(<Emphasis Type="Italic">FN</Emphasis>)-<Emphasis Type="Italic">S</Emphasis> Josephson structures

The critical current I _c of S-(FN)-S Josephson structures has been calculated as a function of the distance L between superconducting (S) electrodes using the Usadel quasiclassical equations for the case of specifying the supercurrent in the direction parallel to the interface between the ferromagnetic (F) and normal (N) films of the composite weak-link region. It has been shown that, owing to the interaction between F and N films, both the typical decrease scale I _c(L) and the period of the critical current oscillations can be much larger than the respective quantities for the SFS junctions. The conditions have been determined under which these lengths are on the order of the effective depth ζ_N of superconductivity penetration to a normal metal.     

Effect of incommensurate potential on the resonant tunneling through Majorana bound states on the topological superconductor chains

We study the transport through the Kitaev chain with incommensurate potentials coupled to two normal leads by the numerical operator method. We find a quantized linear conductance of e ² / h, which is independent to the disorder strength and the gate voltage in a wide range, signaling the Majorana bound states. While the incommensurate potential suppresses the current at finite voltage bias, and then narrows the linear response regime of the I-V curve which exhibits two plateaus corresponding to the superconducting gap and the band edge, respectively. The linear conductance abruptly drops to zero as the disorder strength reaches the critical value 2g _s + 2Δ with Δ the p-wave pairing amplitude and g _s the hopping between neighbor sites, corresponding to the transition from the topological superconducting phase to the Anderson localized phase. Changing the gate voltage also causes an abrupt drop of the linear conductance by driving the chain into the topologically trivial superconducting phase, whose I-V curve exhibits an exponential shape.     

Current-phase relation of double-barrier Josephson junctions with a two-gap superconductor as intermediate electrode

The current-phase (I-?) relation of a double-barrier Josephson junction with a two-gap superconductor as intermediate electrode is derived by means of a simplified version of Ohta’s model. As in conventional double-barrier Josephson junctions, a marked skewness in the I-? curves is present. Moreover, as in heterotic Josephson devices, a reduction of the maximum Josephson current is predicted. An appropriate experiment to verify the rich behavior of this type of Josephson device is suggested.     

Niederenergetische Gammalinien vom Neutroneneinfang in Lu 175 und Lu 176

The low energy gamma-rays from neutron-capture in Lu 175 and Lu 176 have been investigated by means of the bent crystal-spectrometer at the DR-3-reactor at Risø. From the transitions in Lu 177 3 rotational bands have been determined. The levels of the (404)K=7/2⁺ groundstate rotational band are: 121,62 keV (I=9/2), 268,79keV (I=11/2), 440,66 keV (I=13/2), 636,22 keV (I=15/2), 854,34 keV (I=17/2). The level-sequence of the (514)K=9/2^?-band is: 150,39 keV (I=9/2), 288,99 keV (I=11/2), 451,49 keV (I=13/2), 637,05 keV (I=15/2) and 844,88 keV (I=17/2). At 457,92 keV is the basis for the (402)K=5/2⁺-band the higher levels of which are 552,05 keV (I=7/2), 671,89 keV (I=9/2), 816,63 keV (I=11/2), 985,23 keV (I=13/2), 1176,73keV (I=15/2) and probably 1389,5 keV (I=17/2). The energies of the levels apart from the 1389 keV-level have an accuracy of 7×10^?5. The energy differences between the 3 bands agree very well with the values expected from the Bohr-Mottelson-formulaE=A·I(I+1)+B·I ²(I+1)². The calculated branching-ratios within the 3 bands are in fairly good agreement with the experimental values. Theg _K -factors have been determined for 2 bands: It was found that for the (514)-bandg _K =1,16±0,04 and for the (402)-bandg _K =1,33±0,07.     

Threshold perturbations in current-carrying superconducting bridges with a finite length near the critical temperature

Near the critical temperature of a superconducting transition, the energy of the threshold perturbation δF_thr that transfers a superconducting bridge to a resistive state at a current below the critical current I_c has been determined. It has been shown that δF_thr increases with a decrease in the length of a bridge for short bridges with lengths L < ξ (where ξ is the coherence length) and is saturated for long bridges with L ? ξ. At certain geometrical parameters of banks and bridge, the function δF_thr(L) at the current I → 0 has a minimum at L ~ (2–3)ξ. These results indicate that the effect of fluctuations on Josephson junctions made in the form of short superconducting bridges is reduced and that the effect of fluctuations on bridges with lengths ~(2–3)ξ is enhanced.     

Critical current in S-FNF-S Josephson structures with the noncollinear magnetization vectors of ferromagnetic films

The critical current I _C of S-FNF-S Josephson junctions, which are ferromagnet (F)-normal metal (N)-ferromagnet multilayer structures whose ends are in contact with the superconducting (S) electrodes, has been calculated. It has been shown that both the magnitude and sign of I _C depends significantly on the misorientation angle α of the magnetization vectors M _{1, 2} of the ferromagnetic films and the distance L between the superconducting electrodes. The effect of the triplet superconducting component ～〈ψ↑ψ↑〉 ～〈ψ↓ψ↓〉 appearing in the structure on I _C(α) has been analyzed. It has been proven that a new type of the π junction exists, appearing due to the superposition of two contributions to I _C that decrease monotonically with L and are damped at lengths about the coherence length of the normal metal. It has been shown that the effective control over the magnitude and sign of I _C of the structure is achieved at a small deflection of the vectors M _{1, 2} from the antiferromagnetic (M ₁ ↑↓ M ₂) configuration.     

Transitions between À and 0 states in superconductor-ferromagnet-superconductor junctions

Experimental and theoretical study of superconductor-ferromagnet-superconductor junctions (SFS junctions) showed that, in a certain range of parameters (e.g., the length of the ferromagnet d_F, the exchange field E_ex), the ground state of the SFS junction corresponds to a superconducting phase difference π or 0. The phase diagram of an SFS junction with respect to π and 0 states is investigated in this letter in E_ex, d_F, T space. It is shown that the phase diagram is very sensitive to the geometry of the system, in particular, to the amount of disorder in the junction.     

Dependence of the magnitude and direction of the persistent current on the magnetic flux in superconducting rings

The obtained periodic magnetic-field dependences I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences I _c?(Φ/Φ₀) = I _c+(Φ/Φ₀ + Δ?) of the critical current measured in opposite directions. This shift means that the position of I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude I _{c, an}(Φ/Φ₀) = I _c+(Φ/Φ₀) ? I _c?(Φ/Φ₀) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature.     

Statistical mechanics of rotating systems

A microcanonical distribution function depending on the total energyE and thez-componentM of the total angular momentum of a rotating system is examined. ForM=0 the generalized microcanonical ensemble is found to give the same entropy as the usual microcanonical ensemble. The moment of inertia of a rotating gas is calculated, and the kinetic energy of rotation is given as a power series in the small parameterM ²/2I ₀E_int, whereI ₀ is the moment of inertia of the gas at rest andE _int the internal energy.     

The analysis of the densities of <Emphasis Type="Italic">s</Emphasis>-wave neutron resonances separated with respect to spin

The density ratio of s-wave neutron resonances z=ρ(J₁)/ρ(J₂) was analyzed on the basis of the experimental data for 22 atomic nuclei and the Gilbert-Cameron formula for ρ(J). Here, J₁=I_x—1/2 and J₂=I_x+1/2, where I_x denotes the spin of the target nucleus in the ground state. Our aim was to verify whether the factor η(I_x), as a multiplier, can be applied in the expression describing ρ(J₁), with the assumption that ρ(J₂) values remain unchanged, or whether the factor 1η(I_x) can be applied, as a multiplier with ρ(J₂), while the ρ(J₁) values remain unchanged. The final conclusions, e.g., the confirmation or the negation of the fact that it may be necessary to apply the η(I_x) factor, depend on the values of “real” errors Δz of the z variable, which can be calculated if the optimal values of Δρ(J₁) and Δρ(J₂) are known.     

Magnetoresistance hysteresis in granular HTSCs as a manifestation of the magnetic flux trapped by superconducting grains in YBCO + CuO composites

Hysterestic behavior of the magnetoresistance of granular HTSCs and its interaction with the magnetic hysteresis are studied by measuring magnetoresistance R(H) and critical current I _c(H) of composites formed by HTSC Y_0.75Lu_0.25Ba₂Cu₃O₇ and CuO. A network of Josephson junctions is formed in such composites, in which the nonsuperconducting component plays the role of barriers between HTSC grains. Hysteretic dependences R(H) of magnetoresistance are studied in a wide range of transport current density j and are analyzed in the framework of the two-level model of a granular superconductor, in which dissipation takes place in the Josephson medium and the magnetic flux can be pinned both in grains and in the Josephson medium. The interrelation between the hysteresis of critical current I _c(H) and the evolution of the hysterestic dependence R(H) of the magnetoresistance upon transport current variation is demonstrated experimentally. The effect of the magnetic past history on the hysteretic behavior of R(H) and the emergence of a segment with a negative magnetoresistance are analyzed. It is shown for the first time that the R(H) dependences are characterized by a parameter that is independent of the transport current, viz., the width of the R(H) hysteresis loop.     

Josephson effects in unconventional heavy fermion superconductors

We discuss the Josephson effect for pairing states which break crystal symmetries in addition to gauge symmetry. We consider theE _1g andE _2u models for the low-temperature phase ofUPt ₃, with order parameters Δ(E _1g )～p _z (p _x +ip _y ) and Δ(E _2u )～p _z (p _x +ip _y )². We report calculations of Josephson critical currents, taking into account the effects of depairing at the interface. For singlet-triplet junctions the critical current is non-zero only for spin-orbit, spin-flip tunneling, and is found to be much smaller than the Ambegaokar-Baratoff value even when the spin-orbit tunneling amplitude is comparable to the spin-independent amplitude.     

Superconducting properties of superlattices containing ferromagnetic layers

Using the microscopic theory formulated by de Gennes and extended by Takahashi and Tachiki, we calculate the transition temperatureT _c and the pair functionF for the superlattices consisting of superconducting and ferromagnetic layers. Superconducting layers. (s) and ferromagnetic layers (f) are modeled byV _s ≠0 andI _m,s =0 andV _f =0 andI _m,f ≠0, whereV _s .(V _f ) is the BCS coupling constant andI _m,s (I _m,f ) is the molecular field fors (f) layers.     

Electrical instability of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

The temperature behavior of I-U curves and the field and temperature dependences of the electrical resistivity and dielectric permittivity of crystals of the LiCu₂O₂ phase have been studied. It was established that the crystals belong to p-type semiconductors and that their static resistivity in the range 80–260 K follows the Mott law ρ=Aexp(T₀/T)^1/4 describing variable-range hopping over localized states. At comparatively low electric fields, the crystals exhibit threshold switching and characteristic S-shaped I-U curves containing a region of negative differential resistivity. In the critical voltage region, jumps in the conductivity and dielectric permittivity are observed. Possible mechanisms of the disorder and electrical instability in these crystals are discussed.     

Vergleich der Davydov-Theorie mit dem Modell des achsialsymmetrischen Kreisels mit Rotation-Vibrations-Wechselwirkung für deformiertegg-Kerne

For deformed even-mass nuclei the model of the axial-symmetric vibrating top with rotation-vibration-interaction is compared to the Davydov-model without and with aβ-band. TheE2-transition-probabilities are calculated. The coefficientsB _ze andC fromE=A I(I+1) ?B _ze I ²(I+1)²+C I ³(I+1)³ are obtained. For the transition-probabilities from the states of the γ-band to the states of the ground state band as well as forB andC the Davydov-model yields values only about half of those obtained from the RV-theory. A comparision to the present availuable experimental data is made. In the RV-model the agreement is better than in the Davydov-theory.     

Ab initio study of I<Subscript>2</Subscript> and T<Subscript>2</Subscript> stacking faults in C14 Laves phase MgZn<Subscript>2</Subscript>

Based on the synchroshear mechanism, the formation of intrinsic stacking fault I ₂ and twin-like stacking fault T ₂ in C14 Laves phases has been modeled in detail and the generalised stacking fault energy curve of I ₂ and T ₂ for C14 Laves phase MgZn₂ has been calculated from first-principles. The results demonstrate that the unstable stacking fault energy of I ₂ by synchroshear is still very large, and the stable stacking fault energy of I ₂ is higher in comparison with pure Mg implying that the formation of I ₂ stacking fault in MgZn₂ is difficult. Starting with the I ₂ configuration, the T ₂ stacking fault can be formed by an additional synchroshear. The unstable and stable stacking fault energies of T ₂ are only slightly larger than those of I ₂, implying that the formation of T ₂ may be essentially similar to that of I ₂. From the obtained generalised stacking fault energy, the relevant deformation mechanism of MgZn₂ is also discussed. Finally, the electronic structure during synchroshear process is further studied.     

Optical 4<Emphasis Type="Italic">f</Emphasis>-4<Emphasis Type="Italic">f</Emphasis> transitions in multiferroic HoMnO<Subscript>3</Subscript>

In the absorption spectra of the hexagonal single-crystal manganite HoMnO₃ in the paramagnetic ferroelectric state, lines near 1.1 and 2.0 μm were observed associated with the transitions ⁵ I ₈ → ⁵ I ₆ and ⁵ I ₈ → ⁵ I ₇, respectively, within the 4f ¹⁰ configuration of the Ho³⁺ ion. At T = 80 K, to the ⁵ I ₈ → ⁵ I ₇ transition corresponds one band at 1.9 μm for both polarizations E ∥ c and E ⊥ c. As the temperature increases from 80 to 293 K, a low-energy band with a peak at 2.04 μm for E ⊥ c and a peak at 2.07 μm for E ∥ c arises associated with transitions from an excited Stark level of the ground ⁵ I ₈ multiplet to the Stark levels of the ⁵ I ₇ multiplet and with an increase in the population of the initial Stark level, the energy of which is ～100 K.