arXiv subjects

I. K. Yanson

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

At least 73 records · Page 4Linked to original sources

arXiv2005

Atomic size oscillations in conductance histograms for gold nanowires and the influence of work hardening

Nanowires of different nature have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for gold wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials and we discover a new set of periodic structures, which we interpret as due to the atomic discreteness of the contact size for the three principal crystal orientations.

cond-mat.mes-hall↗

arXiv2005

Spectroscopy of phonons and spin torques in magnetic point contacts

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts. Our results show that impurity scattering at the N/F interface is the origin of the new single-interface spin torque effect.

cond-mat.mtrl-sci↗

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↗

arXiv2005

Distribution of the superconducting gap in an YNi2B2C film studied by point contact spectroscopy

The differential resistances $R_d=dV/dI(V)$ of point contacts between a normal metal and a c axis oriented YNi2B2C film ($T_c$ = 15.2K) in the superconducting (SC) state have been investigated. $R_d(V)$ contains clear "gap" features connected with processes of Andreev reflection at the boundary between normal metal and superconductor that allow the determination of the SC gap $Δ$ and its temperature and magnetic field dependence. A distribution of $Δ$ from $Δ_min\approx$ 1.5 meV to $Δ_max\approx$ 2.4 meV is revealed; however the critical temperature $T_c$ in all cases corresponded to that of the film. The value 2$Δ_max/k_BT_c\approx$3.66 is close to the BCS value of 3.52, and the temperature dependence $Δ(T)$ is BCS-like, irrespective of the actual $Δ$ value. It is supposed that the distribution of $Δ$ can be attributed to a gap anisotropy or to a multiband nature of the SC state in YNi2B2C, rather than to the presence of nodes in the gap.

cond-mat.supr-con↗

arXiv2004

Point Contact Spectroscopy of Superconducting Gap Anisotropy in Nickel Borocarbide Compound LuNi2B2C

Point contacts are used to investigate the anisotropy of the superconducting energy gap in LuNi2B2C in the ab plane and along the c axis. It is shown that the experimental curves should be described assuming that the superconducting gap is non-uniformly distributed over the Fermi surface. The largest and the smallest gaps have been estimated by two-gap fitting models. It is found that the largest contribution to the point-contact conductivity in the c direction is made by a smaller gap and, in the ab plane by a larger gap. The deviation from the one-gap BCS model is pronounced in the temperature dependence of the gap in both directions. The temperature range, where the deviation occurs, is for the c direction approximately 1.5 times more than in the ab plane. The Γparameter, allowing quantitatively estimate the gap anisotropy by one-gap fitting, in c direction is also about 1.5 times greater than in the ab plane. Since it is impossible to describe satisfactorily such gap distribution either by the one- or two-gap models, a continuous, dual-maxima model of gap distribution over the Fermi surface should be used to describe superconductivity in this material.

cond-mat.supr-con↗

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

Realization of an N-shaped IVC of nanoscale metallic junctions using the antiferromagnetic transition

We have observed at low temperatures (<8K) hysteretic I(V) characteristics for sub-mkm (~200nm) metallic break-junctions based on the heavy-fermion compound UPd2Al3. Degrading the quality of the contacts by in situ increasing the local residual resistivity or temperature rise reduces the hysteresis. We demonstrate that those hysteretic I(V) curves can be reproduced theoretically by assuming the constriction to be in the thermal regime. Our calculations show that such anomalous I(V) curves are due to the sharp increase of ρ(T) of UPd2Al3 near the Neel temperature T_N ~ 14K. From this point of view each metal with similar ρ(T) should produce similar hysteretic I(V) curves. As example we show calculations for the rare-earth manganite La{0.75}Sr{0.25}MnO3, a system with colossal magnetoresistance. In this way we demonstrate that nano-sized point contacts can be non-linear devices with N-shaped I(V) characteristics, i. e. with negative differential resistance, that could serve like Esaki tunnel diodes or Gunn diodes as amplifiers, generators, and switching units. Their characteristic response time is estimated to be less than 1ns for the investigated contacts.

cond-mat.mes-hall↗

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↗

arXiv2004

Electronic shell effects and the stability of alkali nanowires

Experimental conductance histograms for Na nanowires are analyzed in detail and compared to recent theoretical results on the stability of cylindrical and elliptical nanowires, using the free-electron model. We find a one-to-one correspondence between the peaks in the histograms and the most stable nanowire geometries, indicating that several of the commonly observed nanowires have elliptical cross sections.

cond-mat.mes-hall↗

arXiv2004

Phonon structure in point-contact spectra of MgB$_2$

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 above-gap energies. We compare the expressions for the nonlinear conductivity of tunnel, ballistic, and diffusive point-contacts and show that these expression are similar and correspond to the measurements of the phonon structure in the point-contact spectra for the $π$-band of MgB$_{2}$.

cond-mat.supr-con↗

arXiv2004

Shell-effects in heavy alkali-metal nanowires

We supplement our previous observations of the shell effect in alkali-metal nanowires (Li, Na, K) with data extended to the heavy alkalis Rb and Cs. Our observations include: i) a non-monotonous dependence of conductance-histogram peak heights on atomic weight, ii) a rapid transition to an atomic shell structure at elevated temperatures, and iii) a ''reverse'' atomic-electronic shell transition, caused by the closeness to the liquid state.

cond-mat.mes-hall↗

arXiv2004

The antiferromagnetic transition of UPd2Al3 break-junctions: A new realization of N-shaped current-voltage characteristics

We have investigated metallic break junctions of the heavy-fermion compound UPd2Al3 at low temperatures between 0.1K and 9K and in magnetic fields up to 8T. Both the current-voltage I(V) characteristics and the dV/dI (V) spectra clearly showed the superconducting ($T_{\rm c}\simeq$ 1.8K) as well as the antiferromagnetic ($T_{\rm N}\simeq$14K) transition at low temperatures when the bias voltage is raised. The junctions with lateral size of order 200nm had huge critical current densities around $5\times 10^{10} A/m^2 at the antiferromagnetic transition and hysteretic I(V) characteristics. Degrading the quality of the contacts by in situ increasing the local residual resistivity reduced the hysteresis. We show that those hysteretic I(V) curves can be reproduced theoretically by assuming the constriction to be in the thermal regime. It turns out that these point contacts represent non-linear devices with N-shaped I(V) characteristics that have a negative differential resistance like an Esaki tunnel diode.

cond-mat.mes-hall↗

arXiv2004

Advances in point-contact spectroscopy: two-band superconductor MgB2 (A review)

Analysis of the point-contact spectroscopy (PCS) data on the new dramatic high-T$_c$ superconductor MgB$_2$ 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-T$_c$ driving force -- the $E_{2g}$ boron vibration mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison.

cond-mat.supr-con↗

arXiv2003

Point-Contact Spectroscopy

The history is described of how one of the most commonly used electric circuit components, an ordinary electric contact, has become a powerful tool for the physicists to study various mechanisms of electron scattering in metals. The physical principles of spectroscopy of quasi-particle excitations in metals by means of point contacts (PCs) whose dimensions range from only a few to tens of nanometers are presented in a popular form.

physics.pop-ph↗

arXiv2003

Self-energy Effect of Superconducting Energy Gap in Point-Contact Spectra of MgB$_2$

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 of the phonon structure in MgB$%_{2}$ in the case $eV\ggΔ$ for tunnel, ballistic, and diffusive point-contacts and show that these expressions not only qualitatively, but also semi-quantitatively correspond to the measurements of the phonon structure in the point-contact spectra for the $π$-band of MgB$_{2}$ c-axis oriented thin films.

cond-mat.supr-con↗

arXiv2003

Point-contact spectroscopy of two-band superconductor MgB2

The progress in investigation of two-band superconductor MgB$_2$ by the point-contact spectroscopy (PCS) is given. Results of study of superconducting gap temperature and magnetic field dependence for two-dimensional $σ$ and three-dimensional $π$ band and electron-phonon-interaction spectral function are presented. Correlation between the gap value and the intensity of the high T$_c$ driving force -- $E_{2g}$ boron vibration mode, is provided. PCS data on some nonsuperconducting transition metal diborides are surveyed for comparison.

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

Search for E_2g phonon modes in MgB_2 single crystals by point-contact spectroscopy

The electron-phonon interaction in magnesium diboride MgB_2 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 MgB_2 and they better match the harmonic phonons model.

cond-mat.supr-con↗