Volume 40, Issue 10, October 2014
Index of content:
40(2014); http://dx.doi.org/10.1063/1.4900695View Description Hide Description
This review is devoted to the studies of the resistive state in wide superconducting films induced by dc and ac currents in the absence of an external magnetic field. It is found experimentally that the current-voltage characteristics and their parameters are well described by the Aslamazov–Lempitskii theory of the resistive vortex state, which has long been lacking an adequate evidence to support it. There is experimental evidence that the vortex mechanism of resistivity is absent for currents exceeding the maximum current at which the resistive vortex state exists in wide films. The non-vortex nature of phase slip lines (PSL) in wide films is proved experimentally. It is shown that the phase-slip lines arise in a wide superconducting film subjected to an external microwave irradiation with the power exceeding some critical value. The resistance of these phase-slip lines depends on irradiation frequency. Results obtained in studies of PSL in wide films are similar to those obtained for the phase-slip centers in narrow channels. This suggests the identical nature of the phase-slip processes in these two phenomena. It is established that the current-carrying resistive state of a wide film arises due to two different mechanisms that occur one after the other: the penetration into the film and transverse motion of the Pearl vortices induced by the magnetic field of the transport current and the emergence of the phase-slip lines of the superconducting order parameter.
40(2014); http://dx.doi.org/10.1063/1.4898791View Description Hide Description
40(2014); http://dx.doi.org/10.1063/1.4897410View Description Hide Description
Unconventional high-temperature superconductivity in MgB2:La0:65Sr0:35MnO3 (MgB:LSMO) nanocomposite has been found recently [V. N. Krivoruchko and V. Yu. Tarenkov, Phys. Rev. B 86, 10502 (2012)]. In this report, the symmetry of the nanocomposite superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that of the ordinary s- wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm, we calculated the contribution of MgB2 in the total composite's conductivity and estimated the magnitude of the electron–phonon effects originated from MgB2 in I–V characteristics of the composite at above-gap energies. It was found that distinctive features observed in the PC spectra of the MgB:LSMO samples and conventionally attributed to the electron–phonon interaction cannot be related to the MgB2 phonons. It is argued that the detected singularities may be a manifestation of the electron-spectrum renormalizations due to strong magnetoelastic (magnon–phonon) interaction in LSMO.
40(2014); http://dx.doi.org/10.1063/1.4897411View Description Hide Description
Andreev-reflection spectroscopy of elemental superconductors in contact with nonmagnetic normal metals reveals that the strength of normal-reflection varies only slightly. This observation imposes strong constrictions on the three possible normal-reflection mechanisms: tunneling through a dielectric barrier, reflection due to the different electronic properties of the two electrodes, and diffusive transport caused by elastic scattering in the contact region. We discuss in detail the role played by Fermi-surface mismatch, represented by the different Fermi velocities on both sides of the contact interface. We find that it is at least not the dominant mechanism and possibly completely absent in the Andreev-reflection process.
40(2014); http://dx.doi.org/10.1063/1.4897412View Description Hide Description
We show that self sustained mechanical vibrations in a model magnetic shuttle device can be driven by both the charge and the spin accumulated on the movable central island of the device. Different scenarios for how spin- and charge-induced shuttle instabilities may develop are discussed and shown to depend on whether there is a Coulomb blockade of tunneling or not. The crucial role of electronic spin flips in a magnetically driven shuttle is established and shown to cause giant magnetoresistance and dynamic magnetostriction effects.
40(2014); http://dx.doi.org/10.1063/1.4897413View Description Hide Description
Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N) and films of amorphous ferromagnet (F) Co40Fe40B20 of different thickness, exchange-biased by antiferromagnetic Mn80Ir20 are studied. A surface spin valve effect in the conductance of such F–N contacts is observed. The effect of exchange bias is found to be inversely proportional to the Co40Fe40B20 film thickness. This behavior as well as other magneto-transport effects we observe on single exchange-pinned ferromagnetic films are similar in nature to those found in conventional three-layer spin-valves.
40(2014); http://dx.doi.org/10.1063/1.4897414View Description Hide Description
I(V) characteristics and their first derivatives of ScS and ScN-type (S is superconductor, c is constriction, N is normal metal) point contacts (PCs) based on Ba1– x Na x Fe2As2 (x = 0.25 and 0.35) were studied. ScS-type PCs with S = Nb,Ta, and Pb show Josephson-like resistively shunted I(V) curves with microwave induced Shapiro steps which satisfy relation 2 eV = . The IcRN product (Ic is critical current, RN is normal state PC resistance) in these PCs is found to be up to 1.2 mV. All this data with the observed dependence of the Ic on the microwave power of ScS PCs with Pb counter electrode indicates the presence of the singlet s-wave type pairing in Ba1– x Na x Fe2As2. From the dV/dI(V) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap Δ ratio 2Δ/kBTc = 3.6 ± 1 for the compound with x = 0.35 was evaluated. Analysis of these dV/dI(V) at high biases V, that is well above Δ, testifies transition to the thermal regime in PCs with a voltage increase.
40(2014); http://dx.doi.org/10.1063/1.4897415View Description Hide Description
The electronic gap structure of Nb 3Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2Δ = 4–5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible hump-like structures at the biases of about ±20 and ±50 mV. Such a coexistence of gap and hump structures resembles the situation found in the high-Tc copper-oxide superconductors. Above the superconducting critical temperature Tc ∼ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in connection to the structural phase transition occurring in Nb 3Sn.
40(2014); http://dx.doi.org/10.1063/1.4898793View Description Hide Description
The response of copper-permalloy (Ni80Fe20) point microcontacts to microwave irradiation (8–12 GHz) is investigated systematically. The effects of the external magnetic field strength, transport current flowing through the contact, and microwave intensity and frequency on the measured signal are studied. The contributions to the contact electrical resistance owing to giant and anisotropic magnetoresistance are analyzed. The experimentally determined positions of the resonance features are in good agreement with Kittel's formula for FMR in films in parallel magnetic fields. The resonance signal is observed against the background of an additional contribution owing to rectification of the rf field on the nonlinear current-voltage characteristic of the contact. Two mechanisms for the resonance response are discovered: synchronous self-detection from mixing of the rf current with temporal variations in the resistance and a bolometric response associated with the steady-state variation in the contact resistance when magnetization precession is excited. The amplitude of the resonance signal is linearly proportional to the transport current through the contact and to the intensity of the microwave irradiation.
40(2014); http://dx.doi.org/10.1063/1.4898792View Description Hide Description
The method of recording the current-voltage characteristic was used to study the processes occurring at the surface of a point contact conductance channel. The transport characteristics of the point contact in a liquid medium are studied for the first time. The current states of the conductance channel corresponding to the reversible, transient, and irreversible regimes of charge transport in a point contact are studied and identified. In the irreversible range of bias voltage, the famous cyclical effect of electromechanical switching is observed on the contact, which governs the growth and dissolution of dendritic point contacts of the test sample. The electric resistance of the point-contact structure changes over time, going through stages of increase, decrease, and stabilization. Subsequently, the stages of this process are repeated multiple times, reflecting the cyclical nature of the changes in the physiochemical properties of the test object. The current-voltage characteristic of the point contact has a step structure, due to shell effects. Copper point contact conductance histograms, formed spontaneously in the electric field under the influence of the shell effect, are built using the obtained curves. There is a demonstrated presence of preferred current states for the conductance channel, serving as evidence of the quantum nature of conductance changes in the process of dendritic point contact formation.
Frustration phenomena in Josephson point contacts between single-band and three-band superconductors40(2014); http://dx.doi.org/10.1063/1.4897416View Description Hide Description
Within the formalizm of Usadel equations the Josephson effect in dirty point contacts between single-band and three-band superconductors is investigated. The general expression for the Josephson current, which is valid for arbitrary temperatures, is obtained. We calculate current-phase relations for very low temperature and in the vicinity of the critical temperature. For three-band superconductors with broken time-reversal symmetry (BTRS) point contacts undergo frustration phenomena with different current-phase relations, corresponding to φ-contacts. For three-band superconductors without BTRS we have close to sinusoidal current-phase relations and absence of the frustration, excepting the case of very low temperature, where under certain conditions two ground states of the point contact are realized. Our results can be used as the potential probe for the detection of the possible BTRS state in three-band superconducting systems.