arXiv subjects

R. S. Gonnelli

Publications and source records attributed to R. S. Gonnelli.

66 records · Page 4Linked to original sources

arXiv2000

Electron-Phonon Coupling Origin of the resistivity in YNi_{2}B_{2}C Single Crystals

Resistivity measurements from 4.2 K up to 300 K were made on YNi_{2}B_{2}C single crystals with Tc=15.5 K. The resulting rho(T) curve shows a perfect Bloch-Grueneisen (BG) behavior, with a very small residual resistivity which indicates the low impurity content and the high cristallographic quality of the samples. The value lambda_{tr}=0.53 for the transport electron-phonon coupling constant was obtained by using the high-temperature constant value of d(rho)/dT and the plasma frequency reported in literature. The BG expression for the phononic part of the resistivity rho_{ph}(T) was then used to fit the data in the whole temperature range, by approximating alpha^{2}_{tr}F(Omega) with the experimental phonon spectral density G(Omega) multiplied by a two-step weighting function to be determined by the fit. The resulting fitting curve perfectly agrees with the experimental points. We also solved the real-axis Eliashberg equations in both s- and d-wave symmetries under the approximation alpha^{2}F(Omega)= alpha^{2}_{tr}F(Omega). We found that the value of lambda_{tr} here determined in single-band approximation is quite compatible with Tc and the gap Delta experimentally observed. Finally, we calculated the normalized tunneling conductance, whose comparison with break-junction tunnel data gives indication of the possible s-wave symmetry for the order parameter in YNi_{2}B_{2}C.

cond-mat.supr-con↗

arXiv2000

Anomalies in the ab-plane resistivity of strongly underdoped La_{2-x}Sr_{x}CuO_{4} single crystals: possible charge stripe ordering?

We present and discuss the results of measurements of the ab-plane resistivity of La_{2-x}Sr_{x}CuO_{4} single crystals with small Sr contents (x=0.052-0.075). The resistivity was obtained in the temperature range between 4.2 K and 300 K by using an AC Van der Pauw technique. When the temperature is lowered, the rho_{ab}(T) curves show a transition from a linear behaviour to a semiconducting-like one. The deviation from the linearity occurs at a doping-dependent temperature T_{ch}. This effect can be interpreted as due to a progressive pinning of preexistent charge stripes. In the same framework, the appearance of anomalous sharp peaks in the rho_{ab}(T) curves of superconducting samples just above T_{c} may be explained by a transition from a nematic to a more ordered smectic electronic stripe phase.

cond-mat.supr-con↗

arXiv1999

Solution of real-axis Eliashberg equations with different pair symmetries and tunneling density of states

The real-axis direct solution of the Eliashberg equations for the retarded electron-boson interaction in the half-filling case and in the presence of impurities is obtained for six different symmetries of the order parameter: $s$, $s+\mathrm{i}d$, $s+d$, $d$, anisotropic-$s$ and extended-$s$. The spectral function is assumed to contain an isotropic part $α_{is}^{2}F(Ω) $ and an anisotropic one $α_{an}^{2}F(Ω)$ such that $α_{is}^{2}F(Ω)=g\cdotα_{an}^{2}F(Ω)$, where $g$ is a constant, and the Coulomb pseudopotential $μ^{\ast}$ is set to zero for simplicity. The density of states is calculated for each symmetry at $T= 2, 4, 40$ and 80 K. The resulting curves are compared to those obtained by analytical continuation of the imaginary-axis solution of the Eliashberg equations and to the experimental tunneling curves of optimally-doped Bi 2212 crystals.

cond-mat.supr-con↗

arXiv1999

s- and d-wave solution of Eliashberg equations with finite bandwidth

In this work, we discuss the results of the direct solution of the Eliashberg equations with finite bandwidth, in the cases of s- and d-wave symmetry for the pair wave function and in the presence of scattering from impurities. We show that the reduction of the critical temperature $T_{\mathrm{c}}$ due to the finite bandwidth depends on the value of the bandwidth itself, but is almost independent of the symmetry of the order parameter. The same happens for the shape of $Z(ω)$ and $Δ(ω)$. Moreover, we discuss the effect of the finite bandwidth on the shape of the quasiparticle density of states. The results clearly indicate that the infinite bandwidth approximation leads to an underestimation of the electron-boson coupling constant.

cond-mat.supr-con↗

arXiv1999

3D-melting features of the irreversibility line in overdoped Bi$_2$Sr$_2$CuO$_6$ at ultra-low temperature and high magnetic field

We have measured the irreversible magnetization of an overdoped Bi$_2$Sr$_2$CuO$_6$ single crystal up to B=28 T and down to T=60 mK, and extracted the irreversibility line $B_{\rm irr}(T)$: the data can be interpreted in the whole temperature range as a 3D-anisotropic vortex lattice melting line with Lindemann number $c_{\rm L}=0.13$. We also briefly discuss the applicability of alternative models such as 2D- and quantum melting, and the connection with magnetoresistance experiments.

cond-mat.supr-con↗

arXiv1999

ab-plane resistivity and possible charge stripe ordering in strongly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals

We have measured the ab-plane resistivity of La$_{2-x}$Sr$_x$CuO$_4$ single crystals with small Sr content (x=0.052 $÷$ 0.075) between 4.2 and 300 K by using the AC Van der Pauw technique. As recently suggested by Ichikawa et al., the deviation from the linearity of the $ρ_{\mathrm{ab}}(T)$ curve starting at a temperature T$_{\mathrm{ch}}$ can be interpreted as due to a progressive slowing down of the fluctuations of pre-formed charge stripes. An electronic transition of the stripes to a more ordered phase could instead be responsible for some very sharp anomalies present in the $ρ_{\mathrm{ab}}(T)$ of superconducting samples just above $T_{\mathrm{c}}$.

cond-mat.supr-con↗

arXiv1999

Resistivity and electron-phonon coupling in YNi$_2$B$_2$C single crystals

In this work, we present the results of precise measurements of the resistivity of YNi$_2$B$_2$C single crystals with $T_{\rm c}$ = 15.5 K as a function of the temperature, and analyze the experimental data in the framework of the Bloch-Grüneisen theory and electron-phonon coupling. The transport electron-phonon spectral function that best fits the resistivity data is then inserted in the real-axis Eliashberg equations, which are directly solved to determine the normalized tunneling conductance both in s- and d-wave symmetry.

cond-mat.supr-con↗

arXiv1999

The irreversibility line of overdoped Bi_{2+x}Sr_{2-(x+y)}Cu_{1+y}O_{6 +- delta} at ultra-low temperatures and high magnetic fields

The irreversible magnetization of the layered high-T_{c} superconductor Bi_{2+x}Sr_{2-(x+y)}Cu_{1+y}O_{6 +- delta} (Bi-2201) has been measured by means of a capacitive torquemeter up to B=28 T and down to T=60 mK. No magnetization jumps, peak effects or crossovers between different pinning mechanisms appear to be present. The deduced irreversibility field B_{irr} can not be described by the law B_{irr}(T)=B_{irr}(0)(1-T/T_{c})^n based on flux creep, but an excellent agreement is found with the analytical form of the melting line of the flux lattice as calculated from the Lindemann criterion. The behavior of B_{irr}(T) obtained here is very similar to the resistive critical field of a Bi-2201 thin film, suggesting that magnetoresistive experiments are likely to be strongly influenced by flux lattice melting.

cond-mat.supr-con↗

arXiv1998

Real-axis direct solution of the d-wave Eliashberg equations and the tunneling density of states in optimally doped Bi2Sr2CaCu2O{8+x}

In this work we calculate the direct solution of the equations for the retarded electron-boson interaction in the case of d-wave symmetry for the pair wave function and in the real axis formulation. We use a spectral function containing an isotropic part and an anisotropic one: alpha^2(Omega,phi,phi')F(Omega)=[alpha^2]_s*F(Omega)+[alpha^2]_d*F(Omega)*cos(2 phi)*cos(2phi') and make the simple assumption: [alpha^2]_d*F(Omega)=g_d*[alpha^2]_s*F(Omega) where g_d is a constant. For appropriate values of the isotropic electron-boson coupling constant lambda_s and the anisotropic one lambda_d, solutions are obtained with only d-wave symmetry for the order parameter and only s-wave one for the renormalization function. We have employed the real axis formulation in order to compare the theoretical curves to the tunneling density of states of the optimally-doped high-Tc superconductor Bi2Sr2CaCu2O{8+x}. The results of our numerical simulations are able to fit very well the value of the gap, the critical temperature and the shape of the density of states in the whole energy range, as recently determined in our break-junction tunneling experiments. At T > Tc the theoretical conductance still shows a broaden peak that disappears at T ~ 140 K.

cond-mat.supr-con↗

arXiv1997

Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions

An intrinsic gap depression at the Superconductor-Insulator interface due to the very short value of the coherence length in High-Tc Superconductors [HTSs] is considered, in the framework of a mixed (s+id)-wave pair symmetry for the order parameter ranging from pure s to pure d-wave. This gap depression acts as the main physical agent causing the relevant reduction of IcRn(T) values with respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with various experimental data is obtained with both pure s-wave and pure d-wave symmetries of the order parameter, but with amounts of gap depression depending on the pair symmetry adopted. Regardless of the pair symmetry considered, these results prove the importance of the surface order-parameter depression in the correct interpretation of the Ic(T)Rn(T) data in HTS SIS junctions. In a case of planar YBCO-based junction the use of the de Gennes condition allowed us to tentatively obtain an upper limit for the amount of d-wave present in the order parameter of YBCO.

cond-mat.supr-con↗

arXiv1997

Breakdown of Migdal's theorem and intensity of electron-phonon coupling in high-Tc superconductors

In this article we quantify the possible effects of the breakdown of Migdal's theorem on the electron-phonon (e-p) coupling constant, on the critical temperature and on the superconducting gap by examining different kinds of superconducting materials either with low and high critical temperature. We use the theoretical approach developed by Grimaldi, Pietronero and Strassler [PRB 52, 10516 & 10530 (1995)] on experimental data taken both from literature and from our recent break-junction tunneling experiments in Bi_2Sr_2CaCu_2O_{8+x} (BSCCO) [Physica C 275, 162 (1997) and cond-mat/9612220]. The results show that a large violation of the Migdal's theorem (as in BSCCO) yields to a large increase of the observed e-p coupling constant and, in a first approximation and by direct solution of the Eliashberg equations, to large but different increases of Tc and gap. The same theory gives no modifications when applied to low-Tc conventional superconductors. In BSCCO, the renormalized values of the e-p coupling constant and of the ratio 2Delta/kTc (1.85 and 4.98, respectively) are compatible with a strong-coupling electron-phonon origin of superconductivity.

cond-mat.supr-con↗

arXiv1996

Determination of the tunneling electron-phonon spectral function in high-Tc superconductors with energy dependence of the normal density of states

We discuss the limits and the correct utilization of the standard program for the inversion of Eliashberg equations and the determination of the electron-phonon spectral function and the coulomb pseudopotential from tunneling measurements in high-Tc superconductors. In order to compare the calculated density of states with the experimental one, we introduce the results of the inversion procedure, applied to our recent tunneling data in Bi2Sr2CaCu2O{8+x} (BSCCO) single-crystal break junctions with Tc=93 K, in a direct program for the solution of Eliashberg equations. Most of the observed differences between theoretical and experimental curves at energy greater than gap can be explained by a smooth energy dependence of the normal density of states that we introduced in the direct solution of the Eliashberg equations. The electron-phonon coupling constant determined in a self-consistent way is equal to 2.69, giving evidence for a strong electron-phonon interaction in BSCCO. Similar results have been obtained in 7 break-junction tunneling curves measured on the same BSCCO single crystals.

cond-mat.supr-con↗