arXiv subjects

O. E. Kvitnitskaya

Publications and source records attributed to O. E. Kvitnitskaya.

30 records · Page 2Linked to original sources

arXiv2011

Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films

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 2Δ\simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV. Temperature dependence of the gap Δ(T) can be fitted well by BCS curve giving reduced gap ratio 2Δ/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.

cond-mat.supr-con↗

arXiv2010

Point-contact study of the LuNi2B2C borocarbide superconducting film

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 approach, 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.

cond-mat.supr-con↗

arXiv2008

The Superconducting Gap Behavior in the Antiferromagnetic Nickel-Borocarbide Compounds RNi2B2C (R=Dy, Ho, Er, Tm) Studied by Point-Contacts Spectroscopy

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-superconductor 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=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.

cond-mat.supr-con↗

arXiv2007

Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF) (T_N=5.2K) HoNi2B2C single crystals in the normal and two different superconducting (SC) states (T_c=8.5K and $T_c^*=5.6K) are reported. The PC study of the electron-boson(phonon) interaction (EB(P)I) spectral function reveals pronounced phonon maxima at 16, 22 and 34meV. For the first time the high energy maxima at about 50meV and 100meV are resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitations with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the 10-meV peak in PC EPI constant λ_PC is evaluated as 20-30%, while contribution of the high energy modes at 50 and 100meV amounts about 10% for each maxima, so the superconductivity might be affected by CEF excitations. The SC gap in HoNi2B2C exhibits a standard single-band BCS-like dependence, but vanishes at $T_c^*=5.6K<T_c, with 2Δ/kT_c^*=3.9. The strong coupling Eliashberg analysis of the low-temperature SC phase with T_c^*=5.6K =T_N, coexisting with the commensurate AF structure, suggests a sizable value of the EPI constant λ_s=0.93. We also provide strong support for the recently proposed by us ''Fermi surface (FS) separation'' scenario for the coexistence of magnetism and superconductivity in magnetic borocarbides, namely, that the superconductivity in the commensurate AF phase survives at a special (nearly isotropic) FS sheet without an admixture of Ho 5d states. Above T_c^* the SC features in the PC characteristics are strongly suppressed pointing to a specific weakened SC state between T_c* and T_c.

cond-mat.supr-con↗

arXiv2006

Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C

The point-contact (PC) spectroscopy study of the electron-phonon(boson) interaction (EP(B)I) spectral function in HoNi2B2C reveals phonon maxima at 16 and 22meV and 34meV. For the first time the pronounced high energy maxima at about 50meV and 100meV were resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitation with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the CEF peak in EP(B)I constant λ_{PC} is evaluated as 20-30%, while contribution of the high frequency modes at 50 and 100meV amounts about 10% for each maxima. PC Andreev reflection measurements below the critical temperature T_c=8.5K reveals two different superconducting (SC) states separated by T_c^*=5.6K which is close to the Neel temperature T_N=5.3K. Below T_c^* the gap Δin HoNi2B2C exhibits a standard single-band BCS-like behavior with 2Δ/kT_c^*=3.9. Above T_c^* the gap features in PC spectra are strongly suppressed pointing to a peculiar SC state between T_c^* and T_c, where incommensurate magnetic order develops.

cond-mat.supr-con↗

arXiv2006

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 RNi2B2C 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^*= 5.6K< T_c=8.5K, where a specifc magnetic ordering starts to play a role. For R=Tm (T_c=10.5 K) a decrease of the SC gap is observed below 5 K.

cond-mat.supr-con↗

arXiv2005

Point-contact spectroscopy of the normal state excitations in PrOs4Sb12

Point-contact (PC) investigations of the heavy-fermion (HF) superconductor PrOs4Sb12 in the temperature range between 1.5 and 4.2K and in a magnetic field up to 8T are presented. The main feature in PC spectra, the second derivative of the I-V curves, in the normal state is a peak at about 1-3mV. This low energy peak is smeared by the temperature rise and suppressed (or splitted) by a magnetic field. The origin of the peak is likely excitation of the Pr^3+ ion from the ground $Γ_1$ state to the upper $Γ_5$ state, considering the generally accepted crystalline electric field (CEF) schema in PrOs4Sb12. Absence of the visible features of other CEF transitions in the PC spectra testifies to the dominant coupling of the conducting electrons with this low-lying (about 1meV in energy) excitation. Thus, this interaction is important for ascertainment of the nature of HF and superconducting state in PrOs4Sb12.

cond-mat.str-el↗

arXiv2004

Excess current in point contacts on two-band superconductor MgB$_2$ in magnetic field

Series of $I(V)$ characteristic and bias-dependent differential resistance $dV/dI(V)$ for point-contacts on the single crystals of two-band superconductor MgB$_2$ were measured in magnetic field up to 9 T. We have obtained magnetic field dependences of the excess current on the $I(V)$ curves and analyzed them using Koshelev and Golubov (Phys. Rev. Lett. {\bf 90}, 177002 (2003)) theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state our data can be qualitatively described utilizing theoretical magnetic filed behavior of the $σ$ and $π$-band superconducting order parameters and the corresponding averaged electronic density of states in MgB$_2$.

cond-mat.supr-con↗

arXiv2004

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

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.

cond-mat.supr-con↗

arXiv2002

Study of the electron-phonon interaction in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact spectroscopy

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter $λ\lesssim 0.1$ 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 MgB_2 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 MgB$_2$ a two-band model is exploited and the proximity effect in the {\bf k}-space is suggested.

cond-mat.supr-con↗

arXiv2002

Electron-phonon interaction in transition metal diborides TB_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 lambda<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 the superconductivity in MgB_2.

cond-mat.supr-con↗

arXiv2001

Magnetic state in URu2Si2, UPd2Al3 and UNi2Al3 probed by point contacts

The antiferromagnetic (AFM) state has been investigated in the three heavy-fermion compounds URu2Si2, UPd2Al3, and UNi2Al3 by measuring dV/dI(V) curves of point contacts at different temperatures (1.5-20 K) and magnetic fields (0-28 T). The zero-bias maximum in dV/dI(V) for URu2Si2 points to a partially gapped Fermi-surface related to the itinerant nature of the AFM state contrary to UPd2Al3 where analogous features have not been found. The AFM state in UNi2Al3 has more similarities with URu2Si2. For URu2Si2, the same critical field of about 40 T along the easy c axis is found for all features in dV/dI(V) corresponding to the Neel temperature, the gap in the electronic density of states, and presumably the ordered moments.

cond-mat.str-el↗