arXiv subjects

S. Sergeenkov

Publications and source records attributed to S. Sergeenkov.

At least 37 records · Page 2Linked to original sources

arXiv2008

Magnetic and transport properties of electron-doped superconducting thin films: Pairing symmetry, pinning and spin fluctuations

In this Chapter we review our latest results on magnetic (AC susceptibility) and transport (resistivity) properties of Pr1.85Ce0.15CuO4 (PCCO) and Sm1.85Ce0.15CuO4 (SCCO) thin films grown by pulsed laser deposition technique. Three main topics of our studies will be covered. We start with a thorough discussion of the pairing symmetry mechanisms in optimally-doped SCCO thin films based on the extracted with high accuracy temperature profiles of penetration depth. In particular, we found that above and below a crossover temperature T*=0.22T_C, our films are best-fitted by a linear and quadratic dependencies, respectively, with physically reasonable values of d-wave node gap parameter and paramagnetic impurity scattering rate. Our next topic is related to the flux distribution in our films. More precisely, we present a comparative study on their pinning ability at low magnetic fields extracted from their AC susceptibilities. Depending on the level of homogeneity of our films, two different types of the irreversibility line have been found. The obtained results are described via the critical-state model taking into account the low-field grain-boundary pinning. And finally, we demonstrate our recent results on the normal state temperature behavior of resistivity R(T) for the high-quality optimally-doped SCCO thin films. In addition to the expected contributions from the electron-phonon and electron-electron scattering processes, we also observed an unusual step-like behavior of R(T) around T=87K which we attributed the to the manifestation of thermal excitations due to spin fluctuations induced by Sm3+ moments through Cu2+-Sm3+ interaction.

cond-mat.supr-con↗

arXiv2007

Influence of Zeeman splitting and thermally excited polaron states on magneto-electrical and magneto-thermal properties of magnetoresistive polycrystalline manganite La_{0.8}Sr_{0.2}MnO_3

Some possible connection between spin and charge degrees of freedom in magneto-resistive manganites is investigated through a thorough experimental study of the magnetic (AC susceptibility and DC magnetization) and transport (resistivity and thermal conductivity) properties. Measurements are reported in the case of well characterized polycrystalline La_{0.8}Sr_{0.2}MnO_3 samples. The experimental results suggest rather strong field-induced polarization effects in our material, clearly indicating the presence of ordered FM regions inside the semiconducting phase. Using an analytical expression which fits the spontaneous DC magnetization, the temperature and magnetic field dependences of both electrical resistivity and thermal conductivity data are found to be well reproduced through a universal scenario based on two mechanisms: (i) a magnetization dependent spin polaron hopping influenced by a Zeeman splitting effect, and (ii) properly defined thermally excited polaron states which have to be taken into account in order to correctly describe the behavior of the less conducting region. Using the experimentally found values of the magnetic and electron localization temperatures, we obtain L=0.5nm and m_p=3.2m_e for estimates of the localization length (size of the spin polaron) and effective polaron mass, respectively.

cond-mat.mtrl-sci↗

arXiv2007

Possible manifestation of spin fluctuations in the temperature behavior of resistivity in Sm_{1.85}Ce_{0.15}CuO_4 thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity $ρ(T)$ is observed in the optimally-doped $Sm_{1.85}Ce_{0.15}CuO_4$ thin films around $T_{sf}=87K$ and attributed to manifestation of strong spin fluctuations induced by $Sm^{3+}$ moments with the energy $\hbar ω_{sf}=k_BT_{sf}\simeq 7meV$. In addition to fluctuation induced contribution $ρ_{sf}(T)$ due to thermal broadening effects (of the width $ω_{sf}$), the experimental data are found to be well fitted accounting for residual (zero-temperature) $ρ_{res}$, electron-phonon $ρ_{e-ph}(T)=AT$ and electron-electron $ρ_{e-e}(T)=BT^2$ contributions. The best fits produced $ω_p=2.1meV$, $τ_0^{-1}=9.5\times 10^{-14}s^{-1}$, $λ=1.2$, and $E_F=0.2eV$ for estimates of the plasmon frequency, the impurity scattering rate, electron-phonon coupling constant, and the Fermi energy, respectively.

cond-mat.supr-con↗

arXiv2007

Irreversibility line and low-field grain-boundary pinning in electron-doped superconducting thin films

AC magnetic susceptibilities of electron-doped Pr_{1.85}Ce_{0.15}CuO_4 (PCCO) and Sm_{1.85}Ce_{0.15}CuO_4 (SCCO) granular thin films have been measured as a function of temperature and magnetic-field strength. Depending on the level of homogeneity of our films, two different types of the irreversibility line (IL) defined as the intergrain-loss peak temperature in the imaginary part of susceptibility have been found. The obtained results are described via the critical-state model taking into account the low-field grain-boundary pinning. The extracted pinning-force densities in more granular SCCO films turn out to be four times larger than their counterparts in less granular PCCO films.

cond-mat.supr-con↗

arXiv2007

Influence of structural disorder on low-temperature behavior of penetration depth in electron-doped high-T_C thin films

To probe the influence of structural disorder on low-temperature behavior of magnetic penetration depth in electron-doped high-T_C superconductors, a comparative study of high-quality Pr_{1.85}Ce_{0.15}CuO_4 (PCCO) and Sm_{1.85}Ce_{0.15}CuO_4 (SCCO) thin films is presented. The obtained results confirm a d-wave pairing mechanism in both samples, substantially modified by impurity scattering (which is more noticeable in less homogeneous SCCO films) at the lowest temperatures. The value of the extracted impurity scattering rate correlates with the quality of our samples and is found to be much higher in less homogeneous films with lower T_C.

cond-mat.supr-con↗

arXiv2007

Probing pairing symmetry of Sm_{1.85}Ce_{0.15}CuO_4 via highly-sensitive voltage measurements: Evidence for strong impurity scattering

Using a highly-sensitive home-made mutual-inductance technique, temperature profiles of the magnetic penetration depth $λ(T)$ in the optimally-doped $Sm_{1.85}Ce_{0.15}CuO_4$ thin films have been extracted. The low-temperature behavior of $λ(T)$ is found to be best-fitted by linear $Δλ(T)/λ(0)= \ln(2)k_BT/Δ_0$ and quadratic $Δλ(T)/λ(0)=Γ^{-1/2}Δ_0^{-3/2}T^2$ laws above and below $T=0.22T_C$, respectively, which clearly indicates the presence of d-wave pairing mechanism dominated by strong paramagnetic scattering at the lowest temperatures. The best fits produce $Δ_0/k_BT_C=2.07$ and $Γ/T_C=0.25(T_C/Δ_0)^3$ for the estimates of the nodal gap parameter and impurity scattering rate.

cond-mat.supr-con↗

arXiv2007

On the origin of unusual transport properties observed in densely packed polycrystalline CaAl_{2}

A possible origin of unusual temperature behavior of transport coefficients observed in densely packed polycrystalline CaAl_{2} compound [M. Ausloos et al., J. Appl. Phys. 96, 7338 (2004)] is discussed, including a power-like dependence of resistivity with $ρ\propto T^{-3/4}$ and N-like form of the thermopower. All these features are found to be in good agreement with the Shklovskii-Efros localization scenario assuming polaron-mediated hopping processes controlled by the Debye energy.

cond-mat.mtrl-sci↗

arXiv2007

Thermal expansion of Josephson junctions as an elastic response to an effective stress field

By introducing a concept of thermal expansion (TE) of a Josephson junction as an elastic response to an effective stress field, we study (both analytically and numerically) the temperature and magnetic field dependence of TE coefficient $α$ in a single small junction and in a square array. In particular, we found that in addition to {\it field} oscillations due to Fraunhofer-like dependence of the critical current, $α$ of a small single junction also exhibits strong flux driven {\it temperature} oscillations near $T_C$. We also numerically simulated stress induced response of a closed loop with finite self-inductance (a prototype of an array) and found that $α$ of a $5\times 5$ array may still exhibit temperature oscillations provided the applied magnetic field is strong enough to compensate for the screening induced effects.

cond-mat.supr-con↗

arXiv2005

Magnetic properties of artificially prepared ordered two-dimensional shunted and unshunted Nb-AlO_x-Nb Josephson Junction Arrays

In this Chapter, we present our latest experimental results (along with their theoretical interpretation) related to some unusual magnetic properties of artificially prepared ordered two-dimensional Josephson junction arrays (of both shunted and unshunted Nb-AlO_x-Nb tunnel junctions). Namely, we address the following topics: (i) influence of non-uniform critical current density profile on magnetic field behavior of AC susceptibility, (ii) origin of the so-called dynamic reentrance phenomenon, and (iii) manifestation of novel geometric quantization effects in temperature behavior of AC magnetic response.

cond-mat.supr-con↗

arXiv2005

Elastic properties of polycrystalline YBa_2Cu_3O_7: Evidence for granularity induced martensitic behavior

In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa_2Cu_3O_7 prepared by the sol-gel method. The quality of all samples was checked by x-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse-echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval (T < 200K). The obtained results show a strong hysteretic behavior in the ultrasonic attenuation (in addition to usually observed hysteretic behavior for the velocity) which strongly supports the existence of a martensitic-like phase above the superconducting critical temperature T_C. We argue that this peculiar behavior can be attributed to the granularity present in the samples.

cond-mat.supr-con↗

arXiv2005

On the origin of reentrance in 2D Josephson Junction Arrays

A comparative study of the magnetic properties of shunted and unshunted two-dimensional Josephson junction arrays (2D-JJA) is presented. Using a single-plaquette approximation of the 2D-JJA model, we were able to successfully fit all our experimental data (for the temperature, AC and DC field dependencies of susceptibility) and demonstrate that the dynamic reentrance of AC susceptibility is directly linked to the value of the Stewart-McCumber parameter β_C. Based on extensive numerical simulations, a phase diagram β_C vs β_L is plotted which demarcates the border between the reentrant and non-reentrant behavior in the arrays.

cond-mat.supr-con↗

arXiv2005

Influence of nonuniform critical current density profile on magnetic field behavior of AC susceptibility in 2D Josephson Junction Arrays

Employing mutual-inductance measurements we study the magnetic field dependence of complex AC susceptibility of artificially prepared highly ordered (periodic) two-dimensional Josephson junction arrays of unshunted Nb-AlO_x-Nb junctions. The observed behavior can be explained assuming single-plaquette approximation of the overdamped model with an inhomogeneous critical current distribution within a single junction.

cond-mat.supr-con↗

arXiv2005

Higher harmonics of ac voltage response in narrow strips of YBa2Cu3O7 thin films: Evidence for strong thermal fluctuations

We report on measurements of higher harmonics of the ac voltage response in strips of YBa2Cu3O7 thin films as a function of temperature, frequency and ac current amplitude. The third (fifth) harmonic of the local voltage is found to exhibit a negative (positive) peak at the superconducting transition temperature and their amplitudes are closely related to the slope (derivative) of the first (Ohmic) harmonic. The peaks practically do not depend on frequency and no even (second or fourth) harmonics are detected. The observed data can be interpreted in terms of ac current induced thermal modulation of the sample temperature added to strong thermally activated fluctuations in the transition region.

cond-mat.supr-con↗

arXiv2005

Manifestation of geometric quantization effects in temperature behavior of AC magnetic response of Josephson Junction Arrays

By improving resolution of home-made mutual-inductance measurements technique, a pronounced step-like structure (with the number of steps n=4 for all AC fields) has been observed in the temperature dependence of AC susceptibility in artificially prepared two-dimensional Josephson Junction Arrays (2D-JJA) of unshunted Nb-AlO_x-Nb junctions with β_L(4.2K)=30. Using a single-plaquette approximation of the overdamped 2D-JJA model, we were able to successfully fit our data assuming that steps are related to the geometric properties of the plaquette. The number of steps n corresponds to the number of flux quanta that can be screened by the maximum critical current of the junctions. The steps are predicted to manifest themselves in arrays with the inductance related parameter β_L(T) matching a "quantization" condition β_L(0)=2π(n+1).

cond-mat.supr-con↗

arXiv2001

Universal Temperature Behavior of Remanent Magnetization Observed in Low-Tc and High-Tc Josephson Junction Arrays

A comparative study of the magnetic remanence exhibited by tridimensional Josephson junction arrays in response to an excitation with an AC magnetic field is presented. The observed temperature behavior of the remanence curves for disordered arrays fabricated from three different materials (Nb, YBa_2Cu_3O_7 and La_{1.85}Sr_{0.15}CuO_4) is found to follow the same universal law (based on the explicit temperature expressions for the activation energy and the inductance-dominated contribution to the magnetization of the array within the framework of the phase-slip model) regardless of the origin of the superconducting electrodes of the junctions which form the array.

cond-mat.supr-con↗

arXiv1999

Anomalous temperature behavior of resistivity in lightly doped manganites around a metal-insulator phase transition

An unusual temperature and concentration behavior of resistivity in $La_{0.7}Ca_{0.3}Mn_{1-x}Cu_xO_3$ has been observed at slight $Cu$ doping ($0\leq x \leq 0.05$). Namely, introduction of copper results in a splitting of the resistivity maximum around a metal-insulator transition temperature $T_0(x)$ into two differently evolving peaks. Unlike the original $Cu$-free maximum which steadily increases with doping, the second (satellite) peak remains virtually unchanged for $x<x_c$, increases for $x\ge x_c$ and finally disappears at $x_m\simeq 2x_c$ with $x_c\simeq 0.03$. The observed phenomenon is thought to arise from competition between substitution induced strengthening of potential barriers (which hamper the charge hopping between neighboring $Mn$ sites) and weakening of carrier's kinetic energy. The data are well fitted assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites.

cond-mat.str-el↗

arXiv1999

Probing the field-induced variation of the chemical potential in Bi(2)Sr(2)CaCu(2)O(y) via the magneto-thermopower measurements

Approximating the shape of the measured in $Bi_2Sr_2CaCu_2O_y$ magneto-thermopower (TEP) $ΔS(T,H)$ by asymmetric linear triangle of the form $ΔS(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T_c-T)$ with positive $B ^{-}(H)$ and $B ^{+}(H)$ defined below and above $T_c$, we observe that $B ^{+}(H)\simeq 2B ^{-}(H)$. In order to account for this asymmetry, we explicitly introduce the field-dependent chemical potential of holes $μ(H)$ into the Ginzburg-Landau theory and calculate both an average $ΔS_{av}(T,H)$ and fluctuation $ΔS_{fl}(T,H)$ contributions to the total magneto-TEP $ΔS(T,H)$. As a result, we find a rather simple relationship between the field-induced variation of the chemical potential in this material and the above-mentioned magneto-TEP data around $T_c$, viz. $Δμ(H)\propto S_p(H)$.

cond-mat.supr-con↗

arXiv1999

Influence of Cu on spin-polaron tunneling in the ferromagnetic state of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) from the resistivity data

Nearly a 50% decrease of resistivity ρ(T,x) due to just 4% Cu doping on the Mn site of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) is observed. Attributing the observed phenomenon to the substitution induced decrease of the spin polaron energy E_s(x) below the Curie point T_C(x)=T_C(0)(1-x), all data are found to be well fitted by the nonthermal coherent tunneling expression ρ(T,x) = ρ_0*exp(-γM^2(T,x)) assuming M(T,x)=M_R(x)+M_0(x)*tanh{\sqrt{[T_C(x)/T]^2-1}} for the magnetization in the ferromagnetic state. The best fits through all the data points yield M_0(x)= \sqrt(1-x)M_0(0), M_R(x)=\sqrt(x)M_0(0), and E_s(x)=E_s(0)(1-x)^4 for the Cu induced modifications of the Mn spins dominated zero-temperature spontaneous magnetization, the residual paramagnetic contribution, and spin-polaron energy, respectively, with E_s(0)=0.12 eV.

cond-mat.str-el↗