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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.

The point-contact spectra of the Andreev reflection dV / dI curves of the superconducting rare-earth nickel borocarbide ErNi₂B₂C (Tc≈11K) have been analyzed in the «one-gap» and «two-gap» approximations using the generalized Blonder–Tinkham–Klapwijk model and the Beloborod'ko model allowing for the pair-breaking effect of magnetic impurities. Experimental and calculated curves have been compared not only in shape, but in magnitude as well, which provide more reliable data for determining the temperature dependence of the energy gap (or superconducting order parameter) Δ(T). The anisotropic effect of antiferromagnetic ordering at TN≈6K on the superconducting gap/order parameter has been determined: as the temperature is lowered, Δ decreases by∼25% in the c-direction and only by∼4% in the ab-plane. It is found that the pair-breaking parameter increases in the vicinity of the magnetic transitions, the increase being more pronounced in the c-direction. The efficiency of the models was tested for providing Δ(T) data for ErNi₂B₂C from Andreev reflection spectra.

Contains fulltext : 72630.pdf (Author’s version preprint ) (Open Access) 4 p.

It is shown that the two-gap approximation is applicable for describing the dV/dI(V) spectra of LuNi₂B₂C–Ag point contacts in a wide interval of temperatures. The values and the temperature dependences of the large and the small gaps in the ab plane and in the c direction were estimated using the generalized BTK model [A. Plecenik, M. Grajacar, S. Benacka P. Seidel, A. Pfuch, Phys. Rev. B49, 10016 (1994)]] and the equations of [S.I. Beloborodko, Fiz. Nizk. Temp. 29, 868 (2003) [Low Temp. Phys. 29, 650 (2003)]. In the BCS extrapolation the critical temperature of the small gap is 10 K in the ab plane and 14.5 K in the c direction. The absolute values of the gaps are ∆₀ab = 2.16 meV and ∆₀с = 1.94 meV. For the large gaps the critical temperature Tc coincides with the bulk, Tc = 16.8 K, and their absolute values are very close, being about 3 meV in both orientations. In the c direction the contributions to the conductivity from the small and the large gaps remain practically identical up to 10–11 K. In the ab plane the contribution from the small gap is much smaller and decreases rapidly as a temperature rises.

In strong-coupling superconductors with a short electron mean free path the self-energy effects in the superconducting order parameter play a major role in the phonon manifestation of the point-contact spectra at the above-gap energies. We derive asymptotic expressions for the nonlinear conductivity of tunnel, ballistic, and diffusive point contacts in the case eV>>Delta, and show that these expressions not only qualitatively, but also semiquantitatively correspond to the measurements of the phonon structure in the point-contact spectra for the pi band of MgB2 thin films.

Analysis of the point-contact spectroscopy (PCS) data on the new dramatic 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 correlation between the gap value and intensity of the high-Tc driving force — the E₂g boron vibration mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison.

The superconducting gap in a CeRu₂ single crystal is investigated by point contacts. BCS-like behavior of the gap Δ in the temperature range below Tc* (Tc*

High-frequency point-contact (PC) spectroscopy is used to investigate the relaxation processes kinetics in the intermetallic rare-earth compound PrNi₅. A difference is observed in the spectral response between the conventional low-frequency PC data and the response signal to microwave and farinfrared radiation of PrNi₅-Cu point contacts. This difference is connected with the f-shell electronic levels (CEF levels) and phonon temporal dynamics. The phonon reabsorption contribution to the spectra above the Debye energy decreases for microwave and far-infrared frequencies. However, the crystal-field contribution to the spectra at 4.2 meV is enhanced for high frequencies reflecting the relaxation processes specific for these Fermi-statistics electron excitations. The characteristic frequency for CEF-level relaxation is evaluated as ~200 GHz.

The measured dV/dI(V) curves of point contacts between the intermetallic compound CeRu2 in the superconducting state and the normal metal Cu correspond fairly well to the Blonder-Tinkham -Klapwijk model. It is used to obtain temperature and magnetic field dependence of the superconducting gap D. The D(T) dependence corresponds well to the BCS theory with 2D(0)/kbTc*=3.1±0.1. Additionally, a region of gapless superconductivity between Tc*»4,4-5,4K and Tcbulk»6.2K is found. The gap decreases approximately linearly in a magnetic field and vanishes in a field Bc*»3.5T, which is well below the upper critical field Bc2»6T and close to the irreversibility field.

The point-contact (PC) d²V/dI²-spectra of HoNi₂B₂C and YNi₂B₂C reveal structure at applied voltages corresponding to the phonon frequencies. At about 4 meV a maximum is observed in the phonon density of states by analogy to the soft-phonon structure in neutron scattering experiments for LuNi₂B₂C [ P. Dervenagas et al., Phys. Rev. B52, R9839 (1995)] and YNi₂B₂C [H. Kawano et al., Cech. J. Phys. 46, S2-825 (1996) Phys. Rev Lett. 77, 4628 (1996)]. In the Ho compound the low-energy phonon peak is suppressed by an applied magnetic field in an anisotropic way, pointing to an interaction between the phonons and the magnetic systems Surprisingly, in the nonmagnetic Y compound the 4-meV peak is also suppressed by a magnetic field. In the Ho-compound contacts which show the «quasi-thermal» behavior, the detailed magnetic-field and temperature dependences of the PC spectra suggest that the magnetic order is destroyed due to the coupled phonon-magnon subsystem which is driven out of equilibrium by electrons that pass through the contact, by analogy with the nonequilibrium phonon-induced destruction of the superconducting state in point contacts [I. K. Yanson et al., JETP Lett 45, 543 (1987)]. The PC electron-phonon interaction (EPI) spectral functions are reconstructed and the estimates for the λ-parameter yield values of the order of 0.1. Comparison with PC EPI spectra of nonsuperconducting and nonmagnetic LaNi₂B₂C [I K. Yanson et al, Phys Rev. Lett. 78, 935 (1997)], as well as the comparative study of PC EPI and Andreev-retlection spectra tor various contacts with superconducting Ho and Y compounds suggest that the low-energy part ot the electron-quasi-particle interaction spectral function is responsible for the Cooper pairing in these materials.

The low temperature dVldl(V) characteristics are investigated for point contacts of various sizes (d ≈ 2-35 nm) using a mechanically controlled breakjunction of a Kondo alloy CuFe (0.1 at. %). A size effect, being manifested in an increase in the effective Kondo temperature with decreasing contact size, is found both on voltage and magnetic field dV/dl dependences. In accordance with the prediction of the Zarand-Udvardi theory the enhancement of the Kondo temperature in the CuFe alloy appears to be quantitatively much smaller than in CuMn alloy of the same concentration. This enhancement is due to the strong fluctuations of the local electron density of states in the ultrasmall metallic contacts. The Kondo temperature of a magnetic impurity provides a local probe of these fluctuations.

We have measured the dV/dI vs. V characteristics of point contacts between the heavy-fermion superconductor URu₂Si₂ and the conventional superconductors Zn and NbTi. Contacts between URu₂Si₂ and Zn behave like a superconductor—normal metal—superconductor junction with a thick normal layer in the heavy-fermion part of the contact and show Andreev-reflection type structures related to both the heavy-fermion and the conventional superconductors. In contrast, contacts between URu₂Si₂ and NbTi become superconducting at low bias currents. The data indicate that proximity induced superconductivity is important for the coupling of the two types of superconductivity. With such contacts we also fabricated a closed-loop setup with two NbTi contacts on a URu₂Si₂ sample, which showed SQUID oscillation in a small magnetic field.