Yu. G. Naidyuk

Point-contact spectroscopy of heavy-fermion systems

A review of heavy-fermion compounds investigated in their normal and superconducting states by point-contact spectroscopy is presented. The Joule heating appears to have a considerable influence on the conductivity in metallic point contacts in both the normal and superconducting states. Nonetheless, some spectroscopic information can be obtained from the point-contact measurements, and Josephson effects are observed for certain heavy-fermion superconductors. We review in detail very recent development in this field, including experiments using mechanically controllable break junctions covering the regime from metallic conductivity to truly vacuum tunnelling.

Advances in point-contact spectroscopy: two-band superconductor MgB2 (Review)

Analysis of the point-contact spectroscopy (PCS) data on the dramatic new high-Tc superconductor magnesium diboride MgB2 reveals quite different behavior of two disconnected σ and π electronic bands, deriving from their anisotropy, different dimensionality, and electron–phonon interaction. PCS allows direct registration of both the superconducting gaps and electron–phonon interaction spectral function of the two-dimensional σ and three-dimensional π band, establishing the correlation between the gap value and the intensity of the high-Tc driving force—the E2g boron vibrational mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison.

Electron-phonon interaction in transition-metal diborides T B 2 ( T = Zr , Nb , Ta ) studied by point-contact spectroscopy

The electron-phonon interaction (EPI) in transition-metal diborides TB 2 (T=Zr,Nb,Ta) is investigated by point-contact (PC) spectroscopy. The PC EPI functions were recovered and the EPI parameters λ≤0.1 were estimated for all three compounds. Common and distinctive features between the EPI functions for those diborides are discussed also in connection with superconductivity in MgB 2 .

Superconducting energy gap distribution in c-axis oriented MgB2 thin film from point contact study

We have analyzed about a hundred voltage-dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis-oriented MgB2 thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with single-gap features result in a more broad maximum at 3.5 meV. The double-gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps: ΔS=2.45±0.15 meV and ΔL=7.0±0.45 meV in MgB2 with the gap ratio ΔL/ΔS=2.85±0.15. Our observations further prove a widely discussed multigap scenario for MgB2, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.

Surface spin-valve effect.

We report an observation of spin-valve-like hysteresis within a few atomic layers at a ferromagnetic interface. We use phonon spectroscopy of nanometer-sized point contacts as an in situ probe to study the mechanism of the effect. Distinctive energy phonon peaks for contacts with dissimilar nonmagnetic outer electrodes allow localizing the observed spin switching to the top or bottom interfaces for nanometer thin ferromagnetic layers. The mechanism consistent with our data is energetically distinct atomically thin surface spin layers that can form current- or field-driven surface spin-valves within a single ferromagnetic film.

Hot electrons in magnetic point contacts as a photon source

We propose to use a point contact between a ferromagnetic and a normal metal in the presence of a magnetic field for creating a large inverted spin population of hot electrons in the contact core. The key point of the proposal is that when these hot electrons relax by flipping their spin, microwave photons are emitted, with a frequency tunable by the applied magnetic field. While point contacts are an established technology, their use as a photon source is a new and potentially very useful application. We show that this photon emission process can be detected by means of transport spectroscopy and demonstrate stimulated emission of radiation in the 10-100GHz range for a model point contact system using a minority-spin ferromagnetic injector. These results can potentially lead to new types of lasers based on spin injection in metals.

Spectroscopy of phonons and spin torques in magnetic point contacts.

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts. Our results show that impurity scattering at the N/F interface is the origin of the new single-interface spin torque effect.

Stimulated emission and absorption of photons in magnetic point contacts

Point contacts between high anisotropy ferromagnetic SmCo5 and normal metal Cu are used to achieve a strong spin-population inversion in the contact core. Subjected to microwave irradiation in resonance with the Zeeman splitting in Cu, the inverted spin population relaxes through stimulated spin-flip photon emission, detected as peaks in the point-contact resistance. Resonant spin-flip photon absorption is detected as resistance minima, corresponding to sourcing the photon field energy into the electrical circuit. These results demonstrate fundamental mechanisms that are potentially useful in designing metallic spin-based lasers.

Competition of multiband superconducting and magnetic order in ErNi2B2C observed by Andreev reflection

Point contacts (PC) Andreev reflection dV/dI spectra for the antiferromagnetic (TN6 K) superconductor (Tc11 K) ErNi2B2C have been measured for the two main crystallographic directions. The observed retention of the Andreev reflection minima in dV/dI up to Tc directly points to an unusual superconducting order parameter (OP) vanishing at Tc. The temperature dependence of the OP was obtained from dV/dI using the recent theory of Andreev reflection including the pair-breaking effect. For the first time the existence of two superconducting OPs in ErNi2B2C is shown. A distinct decrease of both OPs as temperature is lowered below TN is observed.

Point-contact spectroscopy of the nickel borocarbide superconductors RNi2B2C (R = Y, Dy, Ho, Er, Tm, Lu)

An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi 2 B 2 C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron-boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R = Dy, Ho, Er and Tm, while for R = Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R = Ho and near 6 meV in R = Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R = Y, Lu. At low temperature the SC gap in R = Ho exhibits a standard single-band BCS-like dependence, which vanishes above T*c ≃ 5.6 K < T c ≃ 8.5 K, where a specific magnetic ordering starts to play a role. For R = Tm (Tc ∼ 10.5 K) a decrease of the SC gap is observed below 5 K.