O. E. Kvitnitskaya

Search for E(2g) phonon modes in MgB2 single crystals by point-contact spectroscopy.

The electron-phonon interaction in magnesium diboride MgB2 single crystals is investigated by point-contact (PC) spectroscopy. For the first time the electron coupling with E(2g) phonon modes is resolved in the PC spectra. The correlation between intensity of the extremely broad E(2g) modes in the PC spectra and value of the superconducting gap is established. Our observations favor current theoretical models for electron-phonon mediated superconductivity in MgB2, and they better match the harmonic phonon model.

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.

Exploring point-contact spectra of Ba1−xNaxFe2As2in the normal and superconducting states

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 2\Delta/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.

Josephson effect and Andreev reflection in Ba1–xNaxFe2As2 (x = 0.25 and 0.35) point contacts

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–xNaxFe2As2 (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 =  ℏω. 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–xNaxFe2As2. 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.

Study of the Electron-Phonon Interaction in Metal Diborides MeB2 (Me = Zr, Nb, Ta, Mg) by Point-Contact Spectroscopy

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB2 (Me = Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter λ ≲ 0.1 were estimated. The data are consistent with the measured surface phonon dispersion curves. The low λ value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB2 films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB2 a two-band model is exploited and the proximity effect in the k-space is suggested.

Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state

Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI—the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and 3.5 meV were identified.Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI - the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Delta = 2.5 and 3.5 meV were identified.

Single 20 meV boson mode inKFe2As2detected by point-contact spectroscopy

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.

Reconstruction of the electron-phonon interaction function in Ta, 2H-NbSe2 and MgB2 from spectra of S–c–N microcontacts

Using Ta, 2H-NbSe2 and MgB2 as an example it is shown that it is possible to reconstruct qualitatively a function of the electron-phonon interaction from point-contact spectra in a superconducting state. The limits and the restrictions of this method are also shown. The results obtained are compared with available literature data.

Point-contact investigations of challenging superconductors: two-band MgB2, antiferromagnetic HoNi2B2C, heavy-fermion UPd2Al3, paramagnetic MgCNi3

Abstract An overview on recent efforts in point-contact (PC) spectroscopy of title superconductors is given. Distinct phonon features and crystalline-electric field effects are observed in PC spectra of HoNi2B2C. Results of study of superconducting (SC) gap and excess current versus temperature and magnetic field reflecting specific multi-band electronic structure in MgB2 are presented. The nature of the extremely nonlinear I ( V ) curves in the antiferromagnetic (AF) and SC state are elucidated for UPd2Al3 break junctions and MgCNi3 point contacts.

Superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy

FeSe single crystals have been studied by soft point-contact Andreev-reflection spectroscopy. Superconducting gap features in the differential resistance dV/dI(V) of point contacts such as a characteristic Andreev-reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing dV/dI within the extended two-gap Blonder-Tinkham-Klapwijk model allows to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field indicating a weak decrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25 point contacts results in the averaged gap values = 1.8+/-0.4meV and =1.0+/-0.2 meV and reduced values 2 /kTc=4.2+/-0.9 and 2 /kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent decreasing with both temperature and magnetic field. No signatures in the dV/dI spectra were observed testifying a gapless superconductivity or presence of even smaller gaps.