arXiv subjects

R. S. Gonnelli

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

At least 55 records · Page 3Linked to original sources

arXiv2004

A Point-Contact Study of the Superconducting Gaps in Al-Substituted and C-Substituted MgB_2 Single Crystals

We present the results of directional point-contact spectroscopy in state-of-the-art Mg_{1-x}Al_xB_2 and Mg(B_{1-y}C_{y})_2 single crystals produced at ETH, Zurich. Fitting the conductance curves of our point contacts, that always feature Andreev reflection structures, we obtained the doping dependence of the gap amplitudes. The results are discussed in comparison with other experimental findings and relevant theoretical predictions. We conclude that the physics of Al-substituted crystals at x > 0.09 might be governed by phase segregation, while C-substituted crystals unexpectedly show a doping-induced transition to single-gap superconductivity at y=0.132.

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arXiv2004

Critical Field of Al-Doped MgB2 Samples: Correlation with the Suppression of Sigma-Band Gap

In this Letter, the study of the effect of Al substitution on the upper critical field, Bc2, in AlxMg1-xB2 samples is presented. We find a straightforward correlation between Bc2 and the sigma-band gap, Delta_sigma, evaluated by point-contact measurements. Up to x=0.2 Bc2 can be well described within a clean limit model and its decrease with x is directly related to the suppression of Delta_sigma. For larger doping we observed the crossover to the dirty regime driven mostly by the strong decrease of Delta_sigma rather than by the increase of the sigma-band scattering rate

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arXiv2003

Point-contact study of gap amplitude and symmetry in RuSr_2GdCu_2O_8

We present the results of point-contact spectroscopy measurements in polycrystalline RuSr_2GdCu_2O_8 samples previously characterized from both the structural and the electrical point of view. AC susceptibility and resistivity measurements, as well as SEM and optical microscopy, indicate that the polycrystalline material is made up of small, weakly connected grains. Possibly because of this morphology, point-contact measurements turned out to be rather difficult. Best reproducibility of the conductance curves was achieved in point contacts obtained by pressing Au or Pt-Ir tips on the surface of the samples. The low-temperature conductance curves reproducibly show a clear zero-bias cusp that might be interpreted as due to Andreev reflection at the normal metal-superconductor interface in the case where the superconducting order parameter has a pure d-wave symmetry - even though alternative explanations cannot be ruled out. In this paper we show that these curves can indeed be fitted, in the framework of the generalised BTK model, by using pure d-wave symmetry and a gap of the order of 6 meV. Finally, we present the temperature dependence of the conductance curves from 4.2 K up to the critical temperature of the junctions.

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arXiv2003

Point-Contact Spectroscopy in MgB_2: from Fundamental Physics to Thin-Film Characterization

In this paper we highlight the advantages of using point-contact spectroscopy (PCS) in multigap superconductors like MgB_2, both as a fundamental research tool and as a non-destructive diagnostic technique for the optimization of thin-film characteristics. We first present some results of crucial fundamental interest obtained by directional PCS in MgB_2 single crystals, for example the temperature dependence of the gaps and of the critical fields and the effect of a magnetic field on the gap amplitudes. Then, we show how PCS can provide useful information about the surface properties of MgB_2 thin films (e.g. Tc, gap amplitude(s), clean or dirty-limit conditions) in view of their optimization for the fabrication of tunnel and Josephson junctions for applications in superconducting electronics.

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arXiv2003

The Magnetic-field Dependence of the Gaps in a Two-band Superconductor: A Point-contact Study of MgB_2 Single Crystals

We present the results of directional point-contact measurements in MgB_2 single crystals, in magnetic fields up to 9 T parallel to the c axis. By fitting the conductance curves of our point contacts -- showing clear Andreev-reflection features -- with a generalized Blonder-Tinkham-Klapwijk model, we were able to extract the values of the two gaps Delta_sigma and Delta_pi. The comparison of the resulting Delta_sigma(B) and Delta_pi(B) curves to the theoretical predictions of the two-band model in dirty limit, recently appeared in literature, allows the first direct test of this model and gives a clear and quantitative proof that the pi band, even in the best single crystals, is in the moderate dirty limit.

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arXiv2003

The determination of the electron-phonon interaction from tunneling data in the two-band superconductor MgB2

We calculate the tunneling density of states (DOS) of MgB2 for different tunneling directions, by directly solving the real-axis, two-band Eliashberg equations (EE). Then we show that the numeric inversion of the standard single-band EE, if applied to the DOS of the two-band superconductor MgB2, may lead to wrong estimates of the strength of certain phonon branches (e.g. the E_2g) in the extracted electron-phonon spectral function alpha^(2)F(omega). The fine structures produced by the two-band interaction turn out to be clearly observable only for tunneling along the ab planes in high-quality single crystals. The results are compared to recent experimental data.

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arXiv2003

Directional point-contact spectroscopy of MgB2 single crystals in magnetic fields: two-band superconductivity and critical fields

The results of the first directional point-contact measurements in MgB2 single crystals, in the presence of magnetic fields up to 9 T either parallel or perpendicular to the ab planes, are presented. By applying suitable magnetic fields, we separated the partial contributions of the sigma and pi bands to the total Andreev-reflection conductance. Their fit with the BTK model allowed a very accurate determination of the temperature dependency of the gaps (Delta_sigma and Delta_pi), that resulted in close agreement with the predictions of the two-band models for MgB2. We also obtained, for the first time with point-contact spectroscopy, the temperature dependence of the (anisotropic) upper critical field of the sigma band and of the (isotropic) upper critical field of the pi band.

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arXiv2003

Can one extract the electron-phonon-interaction from tunneling data in case of the multigap superconductor MgB$_2$?

In the present work we calculate the tunneling density of states (DOS) of MgB% $_{2}$ for different tunneling directions by directly solving the two-band Eliashberg equations (EE) in the real-axis formulation. This procedure reveals the fine structures of the DOS due to the optical phonons. Then we show that the numeric inversion of the standard \emph{single-band} EE (the only available method), when applied to the \emph{two-band} DOS of MgB$_{2}$, may lead to wrong estimates of the strength of certain phonon branches (e.g. the $E_{2g}$) in the extracted electron-phonon spectral function $α^{2}F(ω)$. The fine structures produced by the two-band interaction at energies between 20 and 100 meV turn out to be clearly observable only for tunneling along the $ab$ planes, when the extracted $α^{2}F(ω)$ contains the combination $α^{2}F_{σσ}(ω)$\textbf{+}$α^{2}F_{σπ}(ω)$, together with a minor $α^{2}F_{ππ}(ω)$\textbf{+}$α^{2}F_{πσ} (ω)$ component. Only in this case it is possible to extract information on the $σ$-band contribution to the spectral functions. For any other tunneling direction, the $π$-band contribution (which does not determine the superconducting properties of MgB$_{2}$) is dominant and almost coincides with the whole $α^2F(ω)$ for tunneling along the c axis. Our results are compared with recent experimental tunneling and point-contact data.

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arXiv2002

Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals

Directional point-contact spectroscopy measurements were performed for the first time in state-of-the-art MgB_2 single crystals. The selective suppression of the superconductivity in the "pi" band by means of a suitable magnetic field allowed separating the partial contribution of each band to the total point-contact conductance. By fitting the partial conductance curves sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental uncertainty. Their temperature dependence was found to agree well with the predictions of the two-band models for MgB_2.

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arXiv2002

Direct evidence for two-band superconductivity in MgB_2 single crystals from directional point-contact spectroscopy in magnetic fields

We present the results of the first directional point-contact spectroscopy experiments in high-quality MgB_2 single crystals. Due to the directionality of the current injection into the samples, the application of a magnetic field allowed us to separate the contributions of the sigma and pi bands to the total conductance of our point contacts. By using this technique, we were able to obtain the temperature dependency of each gap independent of the other. The consequent, strong reduction of the error on the value of the gap amplitude as function of temperature allows a stricter test of the predictions of the two-band model for MgB_2.

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arXiv2002

Point-contact spectroscopy in MgB2 single crystals in magnetic field

We present the results of a spectroscopic study of state-of-the-art MgB2 single crystals, carried out by using a modified point-contact technique. The use of single crystals allowed us to obtain point contacts with current injection either parallel or perpendicular to the ab planes. The effect of magnetic fields up to 9 T on the conductance spectra of these contacts is here thoroughly studied, for both B parallel and perpendicular to the ab planes. The complete thermal evolution of the upper critical field of the "pi" band is determined for the first time, and quantitative information about the upper critical field of the "sigma" band and its anisotropy is obtained. Finally, by exploiting the different effect of a magnetic field applied parallel to the ab planes on the two band systems, the partial contributions of the "sigma" and "pi" bands to the total conductance are obtained separately. Fitting each of them with the standard BTK model yields a great reduction of the uncertainty on Delta_sigma and Delta_pi, whose complete temperature dependence is obtained with the greatest accuracy.

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arXiv2002

Possible d+id scenario in La_{2-x}Sr_{x}CuO_4 by point-contact measurements

We analyze the results of point-contact measurements in La_{2-x}Sr_{x}CuO_{4} (LSCO) previously reported as a clear evidence of the separation between gap and pseudogap in this copper oxide. Here we show that, in addition to this, the conductance curves of our point-contact junctions -- showing clear Andreev reflection features -- can be interpreted as supporting a nodeless d_{x^2-y^2}+id_{xy}-wave symmetry of the gap in LSCO. The results of our analysis, in particular the doping dependence of the subdominant d_{xy} gap component, are discussed and compared to the predictions of different theoretical models.

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arXiv2002

The possible explanation of electric-field-doped C60 phenomenology in the framework of Eliashberg theory

In a recent paper (J.H. Schon, Ch. Kloc, R.C. Haddon and B. Batlogg, Nature 408 (2000) 549) a large increase in the superconducting critical temperature was observed in C60 doped with holes by application of a high electric field. We demonstrate that the measured Tc versus doping curves can be explained by solving the (four) s-wave Eliashberg equations in the case of a finite, non-half-filled energy band. In order to reproduce the experimental data, we assume a Coulomb pseudopotential depending on the filling in a very simple and plausible way. Reasonable values of the physical parameters involved are obtained. The application of the same approach to new experimental data (J.H. Schon, Ch. Kloc and B. Batlogg, Science 293 (2001) 2432) on electric field-doped, lattice-expanded C60 single crystals (Tc=117 K in the hole-doped case) gives equally good results and sets a theoretical limit to the linear increase of Tc at the increase of the lattice spacing.

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arXiv2001

Andreev reflection in Au/La_{2-x}Sr_{x}CuO_{4} point-contact junctions: separation between pseudogap and phase-coherence gap

We made point-contact measurements with Au tips on La_{2-x}Sr_{x}CuO_{4} samples with 0.08 < x < 0.20 to investigate the relationship between superconducting gap and pseudogap. We obtained junctions whose conductance curves presented typical Andreev reflection features at all temperatures from 4.2 K up to T_c^A close to the bulk T_c. Their fit with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The doping dependence of the low temperature dominant isotropic gap component Delta_{s} follows very well the T_{c} vs. x curve. These results support the separation between the superconducting (Andreev) gap and the pseudogap measured by angle-resolved photoemission spectroscopy (ARPES) and tunneling.

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arXiv2001

Temperature and junction-type dependency of Andreev reflection in MgB2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of high-quality single-crystal-like MgB2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In out-of-plane, high-pressure point contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Delta = 2.6 +/- 0.2 (clean limit). These results support the two-gap model of superconductivity, the presence of a modified layer at the surface of the crystals and an important and non-conventional role of the impurities in MgB2.

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arXiv2001

Josephson effect in MgB_2 break junctions

We present the first observation of the DC and AC Josephson effect in MgB_2 break junctions. The junctions, obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson effect. By irradiating the junctions with microwaves we observe clear Shapiro steps spaced by the ideal $ΔV$ value. The temperature dependence of the DC Josephson current and the dependence of the height of the steps on the microwave power are obtained. These results are a direct prove for the existence of pairs with charge 2e in this new metallic superconductor and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.

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arXiv2001

Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/La_{2-x}Sr_{x}CuO_4 point-contact junctions

We present new Au/La_{2-x}Sr_{x}CuO_{4} (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 <= x <= 0.2. Andreev reflection features disappear at about the bulk Tc, giving no evidence of gap for T > Tc. The fit of the normalized conductance at any T < Tc supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the Tc(x) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x <= 0.2.

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arXiv2001

Tunneling conductance of SIN junctions with different gap symmetries and non-magnetic impurities by direct solution of real-axis Eliashberg equations

We theoretically investigate the effect of various symmetries of the superconducting order parameter Delta(omega) on the normalized tunneling conductance of SIN junctions by directly solving the real-axis Eliashberg equations (EEs) for a half-filled infinite band, with the simplifying assumption mu*=0. We analyze six different symmetries of the order parameter: s, d, s+id, s+d, extended s and anisotropic s, by assuming that the spectral function alpha^{2}F(Omega) contains an isotropic part alpha^{2}F(Omega)_{is} and an anisotropic one, alpha^{2}F(Omega)_{an}, such that alpha^{2}F(Omega)_{an} = g alpha^{2}F(Omega)_{is}, where g is a constant. We compare the resulting conductance curves at T=2 K to those obtained by analytical continuation of the imaginary-axis solution of the EEs, and we show that the agreement is not equally good for all symmetries. Then, we discuss the effect of non-magnetic impurities on the theoretical tunneling conductance curves at T=4 K for all the symmetries considered. Finally, as an example, we apply our calculations to the case of optimally-doped high-T_{c} superconductors (HTSC). Surprisingly, although the possibility of explaining the very complex phenomenology of HTSC is probably beyond the limits of the Eliashberg theory, the comparison of the theoretical curves calculated at T=4 K with the experimental ones obtained in various optimally-doped copper-oxides gives fairly good results.

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