We report an experimental and theoretical investigation of the electron-boson interaction in KFe2As2 by point-contact (PC) spectroscopy, model, and ab-initio LDA-based calculations for the standard electron-phonon Eliashberg function. The PC spectrum
viz. the second derivative of the I - V characteristic of representative PC exhibits a pronounced maximum at about 20meV and surprisingly a featureless behavior at lower and higher energies. We discuss phonon and non-phonon (excitonic) mechanisms for the origin of this peak. Analysis of the underlying source of this peak may be important for the understanding of serious puzzles of superconductivity in this type of compounds.
We present study of derivatives of current-voltage I(V) characteristics of point-contacts (PCs) based on Ba{1-x}Na{x}Fe2As2 (x=0.25) in the normal and superconducting state. The detailed analysis of dV/dI(V) data (also given in Appendix A) shows that
the thermal regime, when temperature increases with a voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at least at high biases V above the superconducting (SC) gap Delta. In this case, specific resistivity rho (T) in PC core is responsible for a peculiar dV/dI(V) behavior, while a pronounced asymmetry of dV/dI(V) is caused by large value of thermopower in this material. A reproducible zero-bias minima detected on dV/dI(V) at low biases in the range pm(6--9)mV well below the SC critical temperature T_c could be connected with the manifestation of the SC gap Delta. Evaluation of these Andreev-reflection-like structures on dV/dI(V) points out to the preferred value of 2Delta/kT_c approx 6. The expected second gap features on dV/dI(V) are hard to resolve unambiguously, likely due to impurity scattering, spatial inhomogeneity and transition to the mentioned thermal regime as the bias further increases. Suggestions are made how to separate spectroscopic features in dV/dI(V) from those caused by the thermal regime.
An asymmetry as a function of the direction of current flow is observed in the current-voltage characteristic (CVC) and its first and second derivatives for point heterocontacts between pure metals {Cu, Ni, Fe) as well as between these metals and dil
ute alloys (CuFe, CuMn). It is shown that the observed asymmetry is caused by thermoelectrical phenomena (Seebeck, Peltier, and Thompson effects), observed when the temperature inside the contact differs from the temperature of the bath. In the low energy range (less than or of the order of the Debye energy) the asymmetry of the CVC is affected mainly by the Seebeck effect, while at high energies and for larger contacts (lower resistance) the contributions of all noted effects are of the same order of magnitude. A technique is proposed for determining the temperature of a heterocontact by measuring the CVC in two polarities. It is established that in the intermediate (between the diffusion and ballistic) state of flight of the electrons through the constriction the temperature of the heterocontact increases linearly with the voltage on it, and all the more rapidly the larger the contact and the more impurity in it.
We present a study of the influence of an external magnetic field H and an electric current I on the spin-valve (SV) effect between a ferromagnetic thin film (F) and a sharp tip of a nonmagnetic metal (N). To explain our observations, we propose a mo
del of a local surface SV which is formed in such a N/F contact. In this model, a ferromagnetic cluster at the N/F interface plays the role of the free layer in this SV. This cluster exhibits a larger coercive field than the bulk of the ferromagnetic film, presumably due to its nanoscale nature. Finally, we construct a magnetic state diagram of the surface SV as a function of I and H.
Point-contact (PC) investigations on the title compound in the normal and superconducting (SC) state (Tc=10,6 K) are presented. The temperature dependence of the SC gap of TmNi2B2C determined from Andreev-reflection (AR) spectra using the standard si
ngle-gap approximation (SGA) deviates from the BCS behavior in displaying a maximum at about Tc/2. A refined analysis within the two-gap approximation provides evidence for the presence of a second gap twice as large as the main gap (the first one), while the latter is close to that within the SGA. This way, TmNi2B2C expands the number of nickel borocarbide superconductors which exhibit a clear multiband character. Additionally, for the first time reentrant features were found in the AR spectra for some PCs measured in a magnetic field. The PC spectroscopy of the electron-boson interaction in TmNi2B2C in the normal state reveals a pronounced phonon maximum at 9.5meV and a more smeared one around 15 meV, while at higher energies the PC spectra are almost featureless. Additionally, the most intense peak slightly above 3meV observed in the PC spectra of TmNi2B2C is presumably caused by crystalline-electric-field (CEF) excitations. The peak near 1meV detected for some PC spectra is connected with a modification of the CEF probably due to boron or carbon vacancies, allowing to probe the local stoichiometry by PC spectroscopy.
Point-contact (PC) Andreev-reflection (AR) measurements of the superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have been carried out. The value of the gap is distributed in the range 2Delta simeq 5-10 meV (for Re=Sm) with a
maximum in the distribution around 6 meV. Temperature dependence of the gap Delta(T) can be fitted well by BCS curve giving reduced gap ratio 2Delta /kT_c^*simeq 3.5 (here T_c^* is the critical temperature from the BCS fit). At the same time, an expected second larger gap feature was difficult to resolve distinctly on the AR spectra making determination reliability of the second gap detection questionable. Possible reasons for this and the origin of other features like clear-cut asymmetry in the AR spectra and current regime in PCs are discussed.
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 s
pin-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 for designing metallic spin-based lasers.
We present point-contact (PC) Andreev-reflection measurements of a superconducting epitaxial c-axis oriented nickel borocarbide film LuNi2B2C (Tc=15.9 K). The averaged value of the superconducting gap is found to be 2.6 +/-0.2 meV in the one-gap appr
oach, whereas the two-gap approach results in 2.14+/-0.36 meV and 3.0+/-0.27 meV. The better fit of the Andreev-reflection spectra for the LuNi2B2C - Cu PC obtained by the two-gap approach provides evidence for multiband superconductivity in LuNi2B2C. For the first time, PC electron-phonon interaction (EPI) spectra have been measured for this compound. They demonstrate pronounced phonon maximum at 8.5+/-0.4meV and a second shallow one at 15.8+/-0.6 meV. The electron-phonon coupling constant estimated from the PC EPI spectra turned out to be small (~ 0.1), like in other superconducting rare-earth nickel borocarbides. Possible reasons for this are discussed.
The point-contact (PC) spectra of the Andreev reflection dV/dI curves of the superconducting rare-earth nickel borocarbide ErNi2B2C (Tc=11 K) have been analyzed in the one-gap and two-gap approximations using the generalized Blonder-Tinkham-Klapwijk
(GBTK) model and the Beloborodko (BB) 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) Delta(T). The anisotropic effect of antiferromagnetic ordering at T_N =6 K on the superconducting gap/order parameter has been determined: as the temperature is lowered, Delta(T) 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 Delta(T) data for ErNi2B2C from Andreev reflection spectra.
An general survey of the superconducting (SC) gap study in the title compounds by point-contact (PC) spectroscopy is presented. The SC gap was determined from dV/dI of PCs employing the well-known theory of conductivity for normal metal-superconducto
r PCs accounting Andreev reflection. The theory was modified by including pair-breaking effects considering the presence of magnetic rare-earth ions. A possible multiband structure of these compounds was also taken into account. The PC study of the gap in the Er-compound (TN=6K<Tc=11K) gives =evidence for the presence of two SC gaps. Additionally, a distinct decrease of both gaps is revealed for R = Er in the antiferromagnetic (AF) state. For R = Tm (TN=1.5K<Tc=10.5K) a decrease of the SC gap is observed below 4-5K, while for R = Dy (TN=10.5K>Tc=6.5K) the SC gap has a BCS-like dependence in the AF state. The SC gap for R = Ho (TN=5.2K<Tc=8.5K) exhibits below T*=5.6K a single-band BCS-like dependence vanishing above T*, where a specific magnetic order occurs. The difference in the SC gap behavior in the title compounds is attributed to different AF ordering.
