Pulse measurement of the I–V characteristics in Bi-2212 intrinsic junctions

Current-voltage (I–V) characteristics are studied in the intrinsic Josephson junctions of Bi₂Sr₂CaCu₂O_y single crystals. In order to examine the influence of self-heating, a current pulse (∼0.2 μsec) is applied to the mesas of 40 μmϕx0.15 μm patterned on the crystal. As a consequence, in contrast to small characteristic voltageV _c in the continuous-current measurement, theV _c data is found comparable to the BCS value. Moreover, theI–V curve is nearly ohmic forl>l _c , implying that the nonlinearity under the continuous current is due to heating. The quasiparticle resistance forT<T _c is also presented by an estimate from the characteristic voltage.     

Magnetic, electrical, and crystallographic properties of thin La1−x SrxMnO3 films

A study is reported of the magnetic, electrical, and crystallographic properties of La_1−x Sr_xMnO₃ (0.15⩽x⩽0.23) epitaxial films grown on single-crystal substrates of (001)ZrO₂(Y₂O₃) having the fluorite structure and (001)LaAlO₃ having the perovskite structure. It was found that films with close compositions for x=0.15 and 0.16, grown on different substrates, have different properties, namely, the film on a fluorite substrate is semiconducting and has a coercive strength 30 times that of the film on a perovskite substrate; the temperature dependence of electrical resistance of the latter film has a maximum around the Curie point T _C and follows metallic behavior for T<T _C. These differences are explained as due to different structures of the films. The x=0.23 film on the perovskite substrate has been found to exhibit a combination of giant magnetoresistance at room temperature with a resistance of ≈300 Ω which is useful for applications. The maxima in resistance and absolute value of negative magnetoresistance are accounted for by the existence of two-phase magnetic states in these films. Fiz. Tverd. Tela (St. Petersburg) 40, 290–294 (February 1998)     

Inherent diode isolation in programmable metallization cell resistive memory elements

The feasibility of a storage element with inherent rectifying or isolation properties for use in passive memory arrays has been demonstrated using a programmable metallization cell structure with a doped (n-type) silicon electrode. The Cu/Cu–SiO₂/n-Si cell used in this study switches via the formation of a nanoscale Cu filament in the Cu–SiO₂ film which results in the creation of a Cu/n-Si Schottky contact with soft reverse breakdown characteristics. The reverse bias leakage current in the on-state diode is dependent on the programming current employed as this influences the area of the electrodeposit and hence the area of the Cu/n-Si junction. The programming current also controls the on-state resistance of the device, allowing multi-level cell (MLC) operation, in which discrete resistance levels are used to represent multiple logical bits in each physical cell. The Cu/Cu–SiO₂/n-Si elements with heavily doped silicon electrodes were readily erasable at voltage less than −5 V which allows them to be re-programmed. Lightly doped silicon electrode devices were not able to be erased due to their very high reverse breakdown voltage but exhibited extremely low leakage current levels potentially allowing them to be used in low energy one-time programmable arrays.     

Contribution of Andreev reflection to the increase in the resistance of the normal metal in a bimetallic N-S structure

We study the resistive properties of 3D normal-metal-superconductor systems in the pure mean-free-path limit l _N,S ≫ξ(T) (l _N,S are the mean free paths in the metals, and ξ is the coherence length) at liquid helium temperatures. In contrast to the situation where l≪ξ, which is common in experiments involving either sandwiches or mesoscopic samples, here the N-S system exhibits unusual temperature behavior that cannot be described by existing theories of boundary resistance. What is most remarkable is a rise in normal resistance in regions that do not incorporate the N-S boundary as the temperature decreases, with asymptotic behavior resembling that of the temperature curve of the gap of a superconductor in contact with a normal metal. We show that this effect, not observed earlier in 3D systems, is due to the nonequivalence of the cross sections of scattering by normal-metal impurities of electron and hole excitations in conditions of Andreev reflection. We also show that in standard measurements of the contribution of the N-S boundary lying between the test contacts, this effect is masked by accompanying effects, the proximity effect and the boundary resistance, whose estimate requires taking into account the presence on the N-S boundary of an electrostatic barrier of the Schottky type, a barrier that redistributes the probabilities of ordinary and Andreev reflections of quasiparticles in the nonequilibrium conditions due to current flow. Zh. éksp. Teor. Fiz. 113, 1064–1070 (March 1998)     

Anomalous hall effect in HoB12

Within a wide temperature range of 1.8–300 K, the Hall effect in holmium dodecaboride is investigated. The measurements of the angular dependences of the Hall resistance ρ_H(φ, T, H) for HoB₁₂ performed in a high magnetic field up to 80 kOe at helium and intermediate temperatures made it possible to separate the normal and anomalous contributions to the Hall effect. Analysis of the anomalous component behavior in the paramagnetic and Néel phases suggests the appearance of the 5d-state magnetic polarization (the spin polaron effect) and makes it possible to compare the revealed features of the Hall coefficient R _H(T, H) with the H-T magnetic phase diagram of HoB₁₂. Original Russian Text ? N.E. Sluchanko, D.N. Sluchanko, V.V. Glushkov, S.V. Demishev, N.A. Samarin, N.Yu. Shitsevalova, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 9, pp. 691–694.     

Zero differential resistance in a double quantum well at high filling factors

The differential resistance r _xx in a GaAs double quantum well with two occupied size-quantization subbands have been studied at temperatures T = 1.6–4.2 K in magnetic fields B < 0.5 T. It has been found that differential resistance r _xx vanishes at the maxima of magneto-intersubband oscillations with an increase in the direct current I _dc. It has been shown that the discovered r _xx ≈ 0 state appears under the condition 2R _c E _H/ħω_c < 1/2, where R _c is the cyclotron radius of electrons at the Fermi level, E _H is the Hall electric field induced by the current I _dc, and ω_c is the cyclotron frequency.     

Effect of illumination on the space charge limited current in organic bulk heterojunction diodes

Current–voltage (J–V) characteristics of organic bulk heterojunction diodes based on an interpenetrating network of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl esters (PCBM) have been studied in the dark and under halogen lamp illumination. The diodes contained 1:1 and 1:0.6 weight ratios of P3HT and PCBM. For both diodes the currents measured in dark (J _d , commonly known as the dark current) in forward bias are found to agree with the space charge limited current (SCLC). The illuminated current consists of a current due to applied voltage (J _da ) and the light generated current (J _L ). J _da  extracted from the illuminated current agrees well with Shockley’s diffusion and recombination current. This observation shows that illumination changes the SCLC into Shockley’s diffusion and recombination current. The forward current under illumination has been observed to be greater than the dark current, which is contrary to the photo–voltaic (PV) theory. This result is well explained by the change of SCLC into Shockley’s diffusion and recombination current. Former address of S.C. Jain: IMEC, Kapeldreef 75, 3001 Leuven, Belgium.     

Superconducting current in hybrid structures with an antiferromagnetic interlayer

It is shown experimentally that the superconducting current density in Nb/Au/Ca_{1 − x} Sr _x CuO₂/YBa₂Cu₃O_{7 − δ} hybrid superconducting heterostructures with a Ca_{1 − x} Sr _x CuO₂ anti-ferromagnetic (AF) cuprate interlayer is anomalously high for interlayer thicknesses d _M = 10–50 nm and the characteristic damping length for superconducting correlations is on the order of 10 nm. The experimental results are explained on the basis of theoretical analysis of a junction of two superconductors (S′ and S) connected by a magnetic multilayer with the AF ordering of magnetization in the layers. It is shown that with such a magnetization ordering, anomalous proximity effect determined by the singlet component of the condensate wavefunction may take place. As a result, the critical currents in S′/I/AF/S and S′/I/N/S structures (I denotes an insulator, and N, the normal metal) may coincide in order of magnitude even when the thickness of the AF interlayer considerably exceeds the decay length of the condensate wavefunction in ferromagnetic layers.     

Influence of electrical and optical losses on fundamental solar-cell parameters

Summary We present here an analysis of the dependence of the light-generated current on the current at maximum power in lossy solar cells. that is, cells with both series and shunt resistances present simultaneously — that is, a real cell. It is essentially seen that this dependence is highly sensitive to series resistance. However, for cells with series resistance of the order of 3 Ω, ϖI _g/ϖI _mp=1. We also look at the influence of TiO_x antireflection coatings on the open-circuit voltage and short-circuit current in silicon solar cells.

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Riassunto Si presenta un'analisi della dipendenza della fotocorrente dalla corrente di massima potenza nelle celle solari quando sono contemporaneamente presenti le resistenze serie e di shunt. Su tale dipendenza influisce molto la resistenza serie. Tuttavia, se questa è dell'ordine di 3 Ω, è ϖI _g/ϖI _mp=1. Si studia anche l'influenza di rivestimenti antiriflettenti di TiO_x sulla tensione di circuito aperto, sulla corrente di circuito aperto e sulla corrente di corto circuito nelle celle di silicio.

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Extraordinary magnetoresistance: sensing the future

Simulations utilising the finite element method (FEM) have been produced in order to investigate aspects of circular extraordinary magnetoresistance (EMR) devices. The effect of three specific features on the resultant magnetoresistance were investigated: the ratio of the metallic to semiconducting conductivities (σ _M /σ _S ); the semiconductor mobility; and the introduction of an intermediate region at the semiconductormetal interface in order to simulate a contact resistance. In order to obtain a large EMR effect the conductivity ratio (σ _M /σ _S ) is required to be larger than two orders of magnitude; below this critical value the resultant magnetoresistance effect is dramatically reduced. Large mobility semiconductors exhibit larger EMR values for a given field (below saturation) and reduce the magnetic field required to produce saturation of the magnetoresistance. This is due to a larger Hall angle produced at a given magnetic field and is consistent with the mechanism of the EMR effect. Since practical magnetic field sensors are required to operate at low magnetic fields, high mobility semiconductors are required in the production of more sensitive EMR sensors. The formation of a Schottky barrier at the semiconductor-metal interface has been modelled with the introduction of a contact resistance at the semiconductor-metal interface. Increasing values of contact resistance are found to reduce the EMR effect with it disappearing altogether for large values. This has been shown explicitly by looking at the current flow in the system and is consistent with the mechanism of the EMR effect. The interface resistance was used to fit the simulated model to existing experimental data. The best fit occurred with an interface with resistivity of 1.55×10⁻⁴ m (overestimate). The EMR effect holds great potential with regard to its future application to magnetic field sensors. The design of any such devices should incorporate high mobility materials (such as graphene) along with the specific features presented in this paper in order to produce effective magnetic field sensors.     

Characteristic features of the formation of the resistive state in a Bi2Sr2CaCu2Oy crystal

The effect of a magnetic field H⊥(ab) on the transverse current-voltage characteristics (IVCs) of the mixed state of a single crystal of the layered superconductor Bi₂Sr₂CaCu₂O_y (BSCCO) is investigated. It is established that in a wide range of temperatures and fields above the irreversibility line the initial part of the IVC is described by the law V∝I ^γ with γ≃1. As the current increases further, this law is replaced by a section where V∝exp(I). It is established that the multivalued, multibranch characteristics, interpreted as a manifestation of an internal Josephson effect, do not change appreciably when the crystal passes into a state with nonzero linear resistance. The character of the dependence of the characteristic switching current on the first resistive branch, I _J (H,T), is determined. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 543–548 (25 October 1999)     

Weak-link bi-epitaxial Josephson junctions in a YBa2Cu3O7−δ film on BaZrO3/CeO2/SrTiO3

A bi-epitaxial (001)YBa₂Cu₃O_7−δ /(110)BaZrO₃/(001)CeO₂ three-layer heterostructure has been grown on (100)SrTiO₃ by laser ablation. The epitaxial relations between the layers making up the heterostructure were derived from x-ray diffraction data. The I _cR_n product for the bi-epitaxial Josephson junctions thus obtained was within 1–1.5 mV for 4.2 K, and 30–60 μV for 77 K. The normal resistance R _n=(2–5 Ω) was practically independent of temperature. The magnetic field dependences of I _c had typically a clearly pronounced main maximum, followed by distorted subsequent peaks. Interaction of the Josephson ac current with self-induced electromagnetic waves at the 45° grain boundary and with external microwave radiation (f=11 GHz) produced current steps in the I-V characteristics of the bi-epitaxial junctions at the corresponding voltages. Fiz. Tverd. Tela (St. Petersburg) 39, 1732–1738 (October 1997)     

Exchange Interactions and Curie Temperatures in Dilute Magnetic Semiconductors

We discuss the origin of ferromagnetism in dilute magnetic semiconductors based on ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb. We use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. Curie temperatures (T _C) are calculated from first-principles by using a mapping on a Heisenberg model in a mean field approximation. It is found that if impurity bands are formed in the gap, as it is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic √c dependence of T _C as a function of the Mn concentration c. On the other hand, if the d-states are localized, as in (Ga, Mn)Sb, Zener's p–d exchange prevails resulting in a linear c-dependence of T _C. In order to have more precise estimations of T _C, effective exchange coupling constants J _ij 's are calculated by using the formula of Liechtenstein et al. It is found that the range of the exchange interaction in (Ga, Mn)N is very short. On the other hand, in (Ga, Mn)As the interaction is weaker but long ranged. Monte Carlo simulations show that the T _C values of (Ga, Mn)N are very low since percolation is difficult to achieve for small concentrations and the mean field approximation strongly overestimates T _C. Even in (Ga, Mn)As the percolation effect is still important.     

Effect of a pulsed magnetic field on the transverse resistance of a layered superconductor

The field dependences of the transverse resistance of a single crystal of the layered superconductor Bi₂Sr₂CaCu₂O_y (BSCCO-2212) with T _c 0≃92 K are studied in magnetic fields up to 50 T in the perpendicular orientation H⊥(ab). It is established that in the resistive region the resistance is a power-law function of the field, and the temperature dependence of the barrier height for flux creep is obtained. It is found that in a wide temperature range, 50–125 K, the transverse magnetoresistance of the crystal in the normal state and under conditions of superconductivity suppression by a strong magnetic field is negative and can be approximated by a linear law with a temperature-dependent slope. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 420–425 (10 September 1998)     

Effect of helical edge states on the tunneling conductance in ferromagnet/noncentrosymmetric superconductor junction

Helical edge states exist in the mixed spin-singlet and spin-triplet phase of a noncentrosymmetric (NCS) superconductor [Y. Tanaka, T. Yokoyama, A.V. Balatsky, N. Nagaosa, Phys. Rev. B 79, 060505(R) (2009)]. In this article we have considered a planar ferromagnetic metal/NCS superconductor tunnel junction and have studied the effect of these helical edge states which manifests itself through the charge and spin tunneling conductance across the junction. We have shown the behavior of conductance for the entire range of variation of γ = Δ _-/Δ ₊ where Δ _± are the order parameters in the positive and negative helicity bands of the NCS superconductor. There exists a competition between the Rashba parameter α and the exchange energy E _ex which is crucial for determining the variation of the conductance with the applied bias voltage across the junction. We have found a nonzero spin current across the junction which appears due to the exchange energy in the Ferromagnet and modulates with the bias voltage. It also changes its profile when the strength of the exchange energy is varied.     

Anomalous Hall effect in granular alloys

A theoretical study is performed of the anomalous Hall effect in granular alloys with giant magnetoresistance. The calculation is carried out within the Kubo formalism and the Green’s function method. The mechanism of asymmetric scattering of the spin-polarized current carriers is considered with allowance for a size effect associated with scattering not only by one grain, but also with more complicated processes of transport among two and three grains. It is shown that scattering of conduction electrons by the interfaces of the grains and the matrix has a substantial effect on the magnitude of the anomalous Hall effect and determines its sign. In general, correlation between the quantities ρ _H and ρ ² is absent, where ρ _H is the Hall resistivity and ρ is the total resistivity of the granular alloy. However, numerical calculation shows that for certain values of the model parameters ρ _H∼ρ ^3.8 and for these same parameter values the amplitude of the giant magnetoresistance reaches 40%, which is found to be in quantitative agreement with the experimental data for Co₂₀Ag₈₀ alloys [P. Xiong, G. Xiao, J. Q. Wang et al., Phys. Rev. Lett. 69, 3220 (1992)]. It is also shown that increasing the resistivity of the matrix leads to a significant growth in the anomalous Hall effect, and more substantial growth for alloys with small grain size, which is in good agreement with experiment [A. B. Pakhomov, X. Yan, and Y. Xu, J. Appl. Phys. 79, 6140 (1996); [X. N. Jing, N. Wang, and A. B. Pakhomov, Phys. Rev. B 53, 14032 (1996)]. Zh. éksp. Teor. Fiz. 112, 2198–2209 (December 1997)     

Critical current of an inhomogeneous Josephson junction at an intergrain boundary with a random distribution of dislocations

The critical current J _c(θ) of an intergrain boundary is calculated as a function of the contact misorientation angle θ of the granules. It is assumed that the ordering parameter is suppressed in regions near boundaries with an enhanced mechanical stress induced by randomly distributed surface dislocations. The stress distribution function is determined using a probabilistic approach. Assuming that the weak coupling at the boundary is Josephson coupling, an analytic expression is found for the angular dependence J _c(θ) (for tilt and twist boundaries). The magnitude of the residual critical current of a boundary in a strong magnetic field is estimated. Fiz. Tverd. Tela (St. Petersburg) 40, 393–402 (March 1998)     

Current-voltage characteristics of SIS structures with localized states in the material of the barrier layer

Elastic resonant tunneling through a single localized state in an insulating layer (I-layer) situated in the constriction zone between two thick superconducting electrodes is investigated theoretically, and the current-voltage characteristic (IVC) of the structure is calculated. The accompanying analysis leads to the prediction that an appreciable current can flow through the structure, not at |eV|=2Δ (Δ is the modulus of the order parameter of the superconducting electrodes) as in the case of an ordinary SIS junction, but at |eV|⩾Δ, and also that the IVC can acquire segments of negative differential resistance in the case of tunneling through a single localized state. Averaging of the IVC over an ensemble of localized states distributed uniformly throughout the volume of the I-layer and with respect to the energy near the chemical potential min the limit Γ₀/Δ≫1 (Γ₀ is the half-width of the resonance line of the localized state) produces a smaller excess current than in a junction of the SNS type. It is shown that the IVC’s exhibit a transition from an excess current to a deficit current as Γ₀ decreases in the high-voltage range. Zh. éksp. Teor. Fiz. 114, 687–699 (August 1998)     

Hyperfine Interactions of N Isotopes in TiO2 and Quadrupole Moment of 16N

Hyperfine interactions of ^12,14N in BN(hexagonal) crystal were studied by detecting β-NQR(¹²N) and FT-NMR(¹⁴N). A β-NMR of ¹⁶N (I ^π=2⁻,T _1/2=7.13 s) in MgO crystal was detected to determine the magnetic moment of ¹⁶N to be |μ(¹⁶N:2⁻)|=(1.986±0.001)μ_N. Also, the β-NQR of ^12,16N in TiO₂ crystal were detected to determine |Q(¹⁶N:2⁻)|=(17.9±1.7) mb. An abnormally small effective charge for neutrons is required to account for |Q(¹⁶N:2⁻)|. This revised version was published online in September 2006 with corrections to the Cover Date.     

Electron—Phonon Interaction,Phonon and Electronic Structures of Layered Electride Ca2N

The phonon and electronic properties, the Eliashberg function and the temperature dependence of resistance of electride Ca₂N are investigated by the DFT-LDA (density functional theory in local density approximation) plane-wave method. The phonon dispersion, the partial phonon density of states and the atomic eigenvectors of zero-center phonons are studied. The electronic band dispersion and partial density of states conclude that Ca₂N is a metal and the Ca 3p, 4s and N 2p orbitals are hybridized. For the analysis of an electron-phonon interaction and its contribution of the Eliashberg function to resistance was calculated and a temperature dependence of resistance due to electron-phonon interaction was found.