arXiv subjects

I. K. Yanson

Publications and source records attributed to I. K. Yanson.

83 records · Page 5Linked to original sources

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↗

arXiv2002

Phonon structure in I-V characteristic of MgB$_{2}$ point-contacts

The search of the phonon structure at the above-gap energies was carried out for $d^{2}V/dI^{2}(V)$ spectra of MgB$_{2}$ point contacts with a normal metal. The two-band model is assumed not only for the gap structure in $dV/dI(V)$-characteristics, but also for phonons in $d^{2}V/dI^{2}(V)$ point-contact spectra, with up to the maximum lattice vibration energy. Since the current is carried mostly by charges of 3D-band, whereas the strong electron-phonon interaction occurs in 2D-band, we observe the phonon peculiarities due to ''proximity'' effect in {\it k}-space, which depends on the variation of interband coupling through the elastic scattering.

cond-mat.supr-con↗

arXiv2002

Superconducting energy gap distribution in c-axis oriented MgB$_2$ thin film from point contact study

We have analyzed about 100 voltage-dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis oriented MgB$_2$ 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 a single-gap features result in 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 $Δ_{S}=2.45\pm 0.15$ meV and $Δ_{L}=7.0\pm 0.45$ meV in MgB$_2$ with gap ratio $2.85\pm 0.15$. Our observations prove further a widely discussed multi-gap scenario for MgB$_2$, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.

cond-mat.supr-con↗

arXiv2001

Superconducting gap and electron-phonon interaction in MgB$_2$ thin film studied by point contacts

The superconducting order parameter and electron-phonon interaction (EPI) in a MgB$_{2}$ c-axis oriented thin film are investigated by point contacts (PCs). Two superconducting gaps $Δ_{Small}\approx 2.6$ meV and $Δ_{Large}\approx 7.4$ meV have been established, which fairly well correspond to the theoretical gap ratio 1:3. The reconstructed PC EPI function along c-axis has above 30 meV similarities with the phonon DOS, while a pronounced maximum around 20 meV is resolved pointing out that the low frequency phonon mode can be of importance for pairing. We have estimated the Fermi velocity in MgB$_2$ about 5$\times 10^7$ cm/s.

cond-mat.supr-con↗

arXiv2001

Superconducting gap and pair breaking in CeRu2 studied by point contacts

The superconducting gap in a CeRu$_{2}$ single crystal is investigated by point contacts. BCS-like behavior of the gap $Δ$ in the temperature range below T$_{c}^{*}<$T$_{c}$, where T$_{c}$ is the critical temperature, is established, indicating the presence of a gapless superconductivity region (between T$_{c}^{*}$ and T$_{c}$). The pair-breaking effect of paramagnetic impurities, supposedly Ce ions, is taken into consideration using the Scalski-Betbeder-Matibet-Weiss approach based on Abrikosov-Gorkov theory. It allows us to recalculate the superconducting order parameter $Δ^α $ (in the presence of paramagnetic impurities) and the gap $Δ^{P}$ (in the pure case) for the single crystal and for the previously studied polycrystalline CeRu$_{2}$. The value 2$Δ^{P}$(0)$ \approx $2 meV, with 2$Δ^{P}$(0)$/$k$_{B}$T$_{c} \approx $3.75, is found in both cases, indicating that CeRu$_{2}$ is a ``moderate'' strong-coupling superconductor.

cond-mat.supr-con↗

arXiv2001

Crossover from Electronic to Atomic Shell Structure in Alkali Metal Nanowires

After making a cold weld by pressing two clean metal surfaces together, upon gradually separating the two pieces a metallic nanowire is formed, which progressively thins down to a single atom before contact is lost. In previous experiments [1,2] we have observed that the stability of such nanowires is influenced by electronic shell filling effects, in analogy to shell effects in metal clusters [3]. For sodium and potassium at larger diameters there is a crossover to crystalline wires with shell-closings corresponding to the completion of additional atomic layers. This observation completes the analogy between shell effects observed for clusters and nanowires.

cond-mat.mes-hall↗

arXiv2000

Point-Contact Spectroscopy of Superconductors

The point-contact spectroscopy, in contrast to the tunneling spectrocopy, considers small electrical contacts with direct conductivity. In the normal state, it enables one to measure the spectral function of electron-boson interaction. In the superconducting state, new features appear connected with the energy dependence of excess current. The non-linearities of the current-voltage characteristic are due to: i) The inelastic scattering of electron quasiparticles in the contact region; ii) The energy dependence of the superconducting energy gap, and iii) The non-equilibrium superconducting effects. These effects are discussed from the experimental point of view

cond-mat.supr-con↗

arXiv2000

Enhancement of Kondo Temperature in Nanometer-Size Point Contacts

Point-contact spectroscopy is applied to study the energy dependence of paramagnetic impurities in noble metals. The samples are in the form of the so-called "mechanically controllable break-junctions" where the investigated piece of alloy makes a nanowire connecting two bulk electrodes.The lateral dimensions of the bridge are of the order of a few nm and can be continuously changed. Three qualitative dependences manifest the size effect while decreasing the contact diameter: 1) Intensity of Kondo peak due to Kondo scattering in point-contact spectra decreases slower than the nonlinearities due to phonon scatterings, 2) The width of the Kondo peak becomes broder, and 3) Due to the Zeeman energy, the splitting of Kondo peak in external and internal (for a spin glass) fields, is suppressed. Explanation of the phenomena is given in terms of the theory of Zarand and Udvardi where the local density of states (LDOS) of conduction electron fluctuates strongly inside a nanowire due to interference of electronic states. Since Kondo effect is a local probe of LDOS, the impurities located close to the maxima of LDOS show the increase of the Kondo temperature and give the primary contribution to the contact resistance.

cond-mat.mes-hall↗

arXiv2000

Andreev-reflection and Point-Contact Spectroscopy of Superconducting Rare Earth Transition Metal Borocarbides

The superconducting energy gap and quasiparticle density of states (DOS) is measured by means of Andreev-reflection spectroscopy in normal metal - superconductor point contacts. It is found that both in paramagnetic and in antiferromagnetically ordered states (Tm- and Dy-compounds) the DOS has the BCS-like form while in Er- the dip in the temperature dependence of the gap is observed at the Neel temperature. In Ho-compound, the non-BCS-like form of DOS is observed in the temperature region where the incommensurate spin density waves exist along the axes a and c . Inelastic point-contact spectroscopy performed on these compounds shows that the low-frequency phonons are important for the superconducting state which are strongly mixed with the magnetic excitation branches in magnetic superconductors.

cond-mat.supr-con↗

arXiv2000

Observation of Supershell Structure in Alkali Metal Nanowires

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross-section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles, such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure.

cond-mat.mes-hall↗

arXiv1999

Observation of shell structure in sodium nanowires

The quantum states of a system of particles in a finite spatial domain in general consist of a set of discrete energy eigenvalues; these are usually grouped into bunches of degenerate or close-lying levels, called shells. In fermionic systems, this gives rise to a local minimum in the total energy when all the states of a given shell are occupied. In particular, the closed-shell electronic configuration of the noble gases produces their exceptional stability. Shell effects have previously been observed for protons and neutrons in nuclei and for clusters of metal atoms. Here we report the observation of shell effects in an open system - a sodium metal nanowire connecting two bulk sodium metal electrodes, which are progressively pulled apart. We measure oscillations in the statistical distribution of conductance values, for contact cross-sections containing up to a hundred atoms or more. The period follows the law expected from shell-closure effects, similar to the abundance peaks at `magic numbers' of atoms in metal clusters.

cond-mat.mes-hall↗