Mauro Tortello | Politecnico di Torino (original) (raw)

Papers by Mauro Tortello

Journal of Membrane Science, 2010

Central European Journal of Physics, 2009

Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superco... more Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superconductor LaFeAsO0.9F0.1 with resistive T c on ∼ 27 K. The observation of two pairs of peaks in the low-temperature Andreev-reflection spectra clearly indicates the presence of two order parameters. The behavior of the two gaps as a function of temperature, obtained by fitting the conductance curves by means of the generalized two-band Blonder-Tinkham-Klapwijk model, shows some anomalies. A theoretical analysis performed within the two-band Eliashberg theory with a generic electron-boson coupling can reproduce the low-temperature value of the two gaps but generally fails in giving a satisfactory fit of their overall temperature dependence, indicating the rich and complex physics of these newly discovered superconductors.

Superconductor Science & Technology, 2009

We present a systematic characterization of Mg1-xLixB2 polycrystalline samples with nominal Li co... more We present a systematic characterization of Mg1-xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li

Physical Review B, 2009

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides R... more The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides REFeAsO1-xFx ( RE=rare earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on LaFeAsO1-xFx gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, Δ1=2.8-4.6meV , which scales with the local Tc of the contact; a larger unconventional OP that gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases on increasing the Tc of the contact; a pseudogaplike feature (i.e., a depression in the conductance around zero bias) that survives in the normal state up to T∗˜140K (close to the Néel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in LaFeAsO1-xFx .

Superconductor Science & Technology, 2012

Journal of Physics-condensed Matter, 2008

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg... more We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg_{1-x}Al_{x}B_{2} single crystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-band Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes Delta_{sigma} and Delta_{pi}. The gap values agree rather well with other PCAR results in Al-doped crystals and polycrystals up to x=0.2 reported in literature, and extend them to higher Al contents. In the low-doping regime, however, we observed an increase in the small gap Delta_{pi} on increasing x (or decreasing the local critical temperature of the junctions, T_{c}^{A}) which is not as clearly found in other samples. On further decreasing T_{c}^{A} below 30 K, both the gaps decrease and, up to the highest doping level x=0.32 and down to T_{c}^{A}= 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashberg theory shows that this gap trend can be explained as being mainly due to the band filling and to an increase in the interband scattering which is necessary to account for the increase in Delta_{pi} at low Al contents (x < 0.1). We suggest to interpret the following decrease of Delta_{pi} for T_{c}^{A} < 30 K as being governed by the onset of inhomogeneity and disorder in the Al distribution that partly mask the intrinsic effects of doping and is not taken into account in standard theoretical approaches.

Superconductor Science & Technology, 2009

We studied the effects of co-doping with Li and Al on the energy gaps of MgB2 by performing point... more We studied the effects of co-doping with Li and Al on the energy gaps of MgB2 by performing point-contact Andreev-reflection spectroscopy (PCAR) in polycrystalline Mg1-x(AlαLi1-α)xB2 samples with x<=0.4. Even though the lattice parameters and the critical temperature of the compound simply scale with the effective Al content αx, irrespective of the Li concentration, the energy gaps do not. In particular, for a given effective Al content, the comparison with Mg1-y(Al)yB2 with y = αx shows that the σ bandgap is practically the same while the π bandgap is higher. A clear gap merging is observed in the most doped sample (x = 0.4) when Tc<20 K. The results are discussed within the two-band Eliashberg theory and compared to the outcomes of first-principles calculations of the effects of Li and Al co-doping on the electronic structure of magnesium diboride.

Physica C-superconductivity and Its Applications, 2009

Journal of Superconductivity and Novel Magnetism, 2009

Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-b... more Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-based superconductor LaFeAsO1−x Fx in order to investigate the energy gaps. The Andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Their values, as determined by fitting the conductance curves measured in junctions with local T c =27.3–28.6 K within the two-band BTK model, are Δ 1=2.75–3.8 meV for the small gap and Δ 2=7.9–10.2 meV for the larger one, respectively. The absence of zero-bias conductance peaks and considerations regarding the nonperfectly directional current injection in PCAR experiments and the polycrystalline nature of the samples rule out the possibility of nodal gaps in the superconductor. Moreover, the conductance curves show pseudogap-like features coexisting, at low temperature, with superconductivity and disappearing close to the Néel temperature of the parent compound, T N≈140 K.

Physical Review Letters, 2010

Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1-xCox)2As2 single cryst... more Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1-xCox)2As2 single crystals (Tc=24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The PCAR spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Omega_b(T) is obtained, very similar to the spin-resonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s+- Eliashberg model by using an electron-boson spectral function peaked at Omega_0 = 12 meV ~ Omega_b(0).

Journal of Superconductivity and Novel Magnetism, 2007

We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Alx... more We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Alx B2 crystals and polycrystals with x up to 0.32, recently grown at ETH (Zurich) and University of Genova, respectively. The differential conductance curves of our “soft” point contacts were measured as a function of temperature from 4.2 K up to the Andreev critical temperature T c A , at which the normal-state conductance is recovered. The gap amplitudes, Δσ and Δπ , were obtained through a two-band BTK fit of the normalized conductance curves. The results extend previous data to the range of heavy Al doping and show that contrary to expectations, no gap merging is observed up to x=0.32 (i.e., down to T c A =9 K). The analysis of the gap trends within the two-band Eliashberg theory shows that band filling is the main effect of Al doping, even though a small increase in interband scattering might be necessary to account for the experimental data in single crystals.

Journal of Superconductivity and Novel Magnetism, 2009

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in Sm... more We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO0.8F0.2 state-of-the-art polycrystals, with critical temperature T c≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB2 and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to T c, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ1=6.15±0.45 meV and Δ2=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same T c. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.

Physical Review B, 2009

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides R... more The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides REFeAsO_(1-x)F_x (RE = rare earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on LaFeAsO_(1-x)F_x gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, Delta1 = 2.8-4.6 meV, which scales with the local Tc of the contact; a larger unconventional OP that gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases on increasing the Tc of the contact; a pseudogaplike feature (i.e. a depression in the conductance around zero bias), that survives in the normal state up to T* ~ 140 K (close to the Neel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations (AF SF) coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in LaFeAsO_(1-x)F_x.

Journal of Superconductivity, 2005

We report the results of directional point-contact measurements in Mg(B 1−x_{1-x}1−x C x)2_{x})_{2}x)2 si... more We report the results of directional point-contact measurements in Mg(B 1−x_{1-x}1−x C x)2_{x})_{2}x)2 single crystals. The amplitudes of the gaps, Deltapi\Delta_{\pi}Deltapi and Deltasigma\Delta_{\sigma}Deltasigma , were determined for each C content by fitting the experimental low-temperature normalized conductance curves of our “soft” point contacts with the BTK model generalized to the two-band case. We found that, on increasing the carbon content, Deltasigma\Delta_{\sigma}Deltasigma decreases almost linearly with TcT_{c}Tc and Deltapi\Delta_{\pi}Deltapi slightly increases until, at x=0.132x=0.132x=0.132 (where Tc=19T_{c}=19Tc=19 K), they assume the same value Delta=3.2pm0.9\Delta =3.2 \pm 0.9Delta=3.2pm0.9 meV. This result is confirmed by the temperature and magnetic-field dependence of the conductance curves at this C content, which do not show any evidence of two distinct gap values. In particular, the Δ versus T curve follows very well a standard BCS curve, with a gap ratio 2Delta/kBTc=3.92\Delta /k_{B} T_{c}=3.92Delta/kBTc=3.9 . These experimental findings are compared to the theoretical predictions of the two-band model in the Eliashberg formulation.

Superconductor Science & Technology, 2012

Central European Journal of Physics, 2009

Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superco... more Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superconductor LaFeAsO0.9F0.1 with resistive T {/c on } ∼ 27 K. The observation of two pairs of peaks in the low-temperature Andreev-reflection spectra clearly indicates the presence of two order parameters. The behavior of the two gaps as a function of temperature, obtained by fitting the conductance curves by means of the generalized two-band Blonder-Tinkham-Klapwijk model, shows some anomalies. A theoretical analysis performed within the two-band Eliashberg theory with a generic electron-boson coupling can reproduce the low-temperature value of the two gaps but generally fails in giving a satisfactory fit of their overall temperature dependence, indicating the rich and complex physics of these newly discovered superconductors.

Physical Review Letters, 2008

We present the first results of directional point-contact spectroscopy in high quality CaC6 sampl... more We present the first results of directional point-contact spectroscopy in high quality CaC6 samples both along the ab plane and in the c-axis direction. The superconducting order parameter \Delta(0), obtained by fitting the Andreev-reflection (AR) conductance curves at temperatures down to 400 mK with the single-band 3D Blonder-Tinkham-Klapwijk model, presents two different distributions in the two directions of the main current injection, peaked at 1.35 and 1.71 meV, respectively. By ab-initio calculations of the AR conductance spectra, we show that the experimental results are in good agreement with the recent predictions of gap anisotropy in CaC6.

Reports on Progress in Physics, 2011

Point-contact Andreev-reflection spectroscopy (PCAR) has proven to be one of the most powerful to... more Point-contact Andreev-reflection spectroscopy (PCAR) has proven to be one of the most powerful tools in the investigation of superconductors, where it provides information on the order parameter (OP), a fundamental property of the superconducting state. In the past 20 years, successive improvements of the models used to analyze the spectra have continuously extended its capabilities, making it suited to study new superconductors with 'exotic' properties such as anisotropic, nodal and multiple OPs. In Fe-based superconductors, the complex compound- and doping-dependent Fermi surface (FS) and the predicted sensitivity of the OP to fine structural details present unprecedent challenges for this technique. Nevertheless, we show here that PCAR measurements in Fe-based superconductors carried out so far have already greatly contributed to our understanding of these materials, despite some apparent inconsistencies that can be overcome if a homogeneous treatment of the data is used. We also demonstrate that, if properly extended theoretical models for Andreev reflection are used, directional PCAR spectroscopy can provide detailed information not only on the amplitude and symmetry of the OPs, but also on the nature of the pairing boson, and even give some hints about the shape of the FS.

Physical Review Letters, 2006

We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 singl... more We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3 K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model. Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principle band structure calculations.

Journal of Membrane Science, 2010

Central European Journal of Physics, 2009

Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superco... more Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superconductor LaFeAsO0.9F0.1 with resistive T c on ∼ 27 K. The observation of two pairs of peaks in the low-temperature Andreev-reflection spectra clearly indicates the presence of two order parameters. The behavior of the two gaps as a function of temperature, obtained by fitting the conductance curves by means of the generalized two-band Blonder-Tinkham-Klapwijk model, shows some anomalies. A theoretical analysis performed within the two-band Eliashberg theory with a generic electron-boson coupling can reproduce the low-temperature value of the two gaps but generally fails in giving a satisfactory fit of their overall temperature dependence, indicating the rich and complex physics of these newly discovered superconductors.

Superconductor Science & Technology, 2009

We present a systematic characterization of Mg1-xLixB2 polycrystalline samples with nominal Li co... more We present a systematic characterization of Mg1-xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li

Physical Review B, 2009

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides R... more The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides REFeAsO1-xFx ( RE=rare earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on LaFeAsO1-xFx gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, Δ1=2.8-4.6meV , which scales with the local Tc of the contact; a larger unconventional OP that gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases on increasing the Tc of the contact; a pseudogaplike feature (i.e., a depression in the conductance around zero bias) that survives in the normal state up to T∗˜140K (close to the Néel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in LaFeAsO1-xFx .

Superconductor Science & Technology, 2012

Journal of Physics-condensed Matter, 2008

We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg... more We present new results of point-contact Andreev-reflection (PCAR) spectroscopy in single-phase Mg_{1-x}Al_{x}B_{2} single crystals with x up to 0.32. Fitting the conductance curves of our point contacts with the two-band Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes Delta_{sigma} and Delta_{pi}. The gap values agree rather well with other PCAR results in Al-doped crystals and polycrystals up to x=0.2 reported in literature, and extend them to higher Al contents. In the low-doping regime, however, we observed an increase in the small gap Delta_{pi} on increasing x (or decreasing the local critical temperature of the junctions, T_{c}^{A}) which is not as clearly found in other samples. On further decreasing T_{c}^{A} below 30 K, both the gaps decrease and, up to the highest doping level x=0.32 and down to T_{c}^{A}= 12 K, no gap merging is observed. A detailed analysis of the data within the two-band Eliashberg theory shows that this gap trend can be explained as being mainly due to the band filling and to an increase in the interband scattering which is necessary to account for the increase in Delta_{pi} at low Al contents (x < 0.1). We suggest to interpret the following decrease of Delta_{pi} for T_{c}^{A} < 30 K as being governed by the onset of inhomogeneity and disorder in the Al distribution that partly mask the intrinsic effects of doping and is not taken into account in standard theoretical approaches.

Superconductor Science & Technology, 2009

We studied the effects of co-doping with Li and Al on the energy gaps of MgB2 by performing point... more We studied the effects of co-doping with Li and Al on the energy gaps of MgB2 by performing point-contact Andreev-reflection spectroscopy (PCAR) in polycrystalline Mg1-x(AlαLi1-α)xB2 samples with x<=0.4. Even though the lattice parameters and the critical temperature of the compound simply scale with the effective Al content αx, irrespective of the Li concentration, the energy gaps do not. In particular, for a given effective Al content, the comparison with Mg1-y(Al)yB2 with y = αx shows that the σ bandgap is practically the same while the π bandgap is higher. A clear gap merging is observed in the most doped sample (x = 0.4) when Tc<20 K. The results are discussed within the two-band Eliashberg theory and compared to the outcomes of first-principles calculations of the effects of Li and Al co-doping on the electronic structure of magnesium diboride.

Physica C-superconductivity and Its Applications, 2009

Journal of Superconductivity and Novel Magnetism, 2009

Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-b... more Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-based superconductor LaFeAsO1−x Fx in order to investigate the energy gaps. The Andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Their values, as determined by fitting the conductance curves measured in junctions with local T c =27.3–28.6 K within the two-band BTK model, are Δ 1=2.75–3.8 meV for the small gap and Δ 2=7.9–10.2 meV for the larger one, respectively. The absence of zero-bias conductance peaks and considerations regarding the nonperfectly directional current injection in PCAR experiments and the polycrystalline nature of the samples rule out the possibility of nodal gaps in the superconductor. Moreover, the conductance curves show pseudogap-like features coexisting, at low temperature, with superconductivity and disappearing close to the Néel temperature of the parent compound, T N≈140 K.

Physical Review Letters, 2010

Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1-xCox)2As2 single cryst... more Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1-xCox)2As2 single crystals (Tc=24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The PCAR spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Omega_b(T) is obtained, very similar to the spin-resonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s+- Eliashberg model by using an electron-boson spectral function peaked at Omega_0 = 12 meV ~ Omega_b(0).

Journal of Superconductivity and Novel Magnetism, 2007

We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Alx... more We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Alx B2 crystals and polycrystals with x up to 0.32, recently grown at ETH (Zurich) and University of Genova, respectively. The differential conductance curves of our “soft” point contacts were measured as a function of temperature from 4.2 K up to the Andreev critical temperature T c A , at which the normal-state conductance is recovered. The gap amplitudes, Δσ and Δπ , were obtained through a two-band BTK fit of the normalized conductance curves. The results extend previous data to the range of heavy Al doping and show that contrary to expectations, no gap merging is observed up to x=0.32 (i.e., down to T c A =9 K). The analysis of the gap trends within the two-band Eliashberg theory shows that band filling is the main effect of Al doping, even though a small increase in interband scattering might be necessary to account for the experimental data in single crystals.

Journal of Superconductivity and Novel Magnetism, 2009

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in Sm... more We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO0.8F0.2 state-of-the-art polycrystals, with critical temperature T c≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB2 and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to T c, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ1=6.15±0.45 meV and Δ2=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same T c. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.

Physical Review B, 2009

The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides R... more The nature and value of the order parameters (OPs) in the superconducting Fe-based oxypnictides REFeAsO_(1-x)F_x (RE = rare earth) are a matter of intense debate, also connected to the pairing mechanism which is probably unconventional. Point-contact Andreev-reflection experiments on LaFeAsO_(1-x)F_x gave us direct evidence of three energy scales in the superconducting state: a nodeless superconducting OP, Delta1 = 2.8-4.6 meV, which scales with the local Tc of the contact; a larger unconventional OP that gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases on increasing the Tc of the contact; a pseudogaplike feature (i.e. a depression in the conductance around zero bias), that survives in the normal state up to T* ~ 140 K (close to the Neel temperature of the undoped compound), which we associate to antiferromagnetic spin fluctuations (AF SF) coexisting with superconductivity. These findings point toward a complex, unconventional nature of superconductivity in LaFeAsO_(1-x)F_x.

Journal of Superconductivity, 2005

We report the results of directional point-contact measurements in Mg(B 1−x_{1-x}1−x C x)2_{x})_{2}x)2 si... more We report the results of directional point-contact measurements in Mg(B 1−x_{1-x}1−x C x)2_{x})_{2}x)2 single crystals. The amplitudes of the gaps, Deltapi\Delta_{\pi}Deltapi and Deltasigma\Delta_{\sigma}Deltasigma , were determined for each C content by fitting the experimental low-temperature normalized conductance curves of our “soft” point contacts with the BTK model generalized to the two-band case. We found that, on increasing the carbon content, Deltasigma\Delta_{\sigma}Deltasigma decreases almost linearly with TcT_{c}Tc and Deltapi\Delta_{\pi}Deltapi slightly increases until, at x=0.132x=0.132x=0.132 (where Tc=19T_{c}=19Tc=19 K), they assume the same value Delta=3.2pm0.9\Delta =3.2 \pm 0.9Delta=3.2pm0.9 meV. This result is confirmed by the temperature and magnetic-field dependence of the conductance curves at this C content, which do not show any evidence of two distinct gap values. In particular, the Δ versus T curve follows very well a standard BCS curve, with a gap ratio 2Delta/kBTc=3.92\Delta /k_{B} T_{c}=3.92Delta/kBTc=3.9 . These experimental findings are compared to the theoretical predictions of the two-band model in the Eliashberg formulation.

Superconductor Science & Technology, 2012

Central European Journal of Physics, 2009

Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superco... more Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superconductor LaFeAsO0.9F0.1 with resistive T {/c on } ∼ 27 K. The observation of two pairs of peaks in the low-temperature Andreev-reflection spectra clearly indicates the presence of two order parameters. The behavior of the two gaps as a function of temperature, obtained by fitting the conductance curves by means of the generalized two-band Blonder-Tinkham-Klapwijk model, shows some anomalies. A theoretical analysis performed within the two-band Eliashberg theory with a generic electron-boson coupling can reproduce the low-temperature value of the two gaps but generally fails in giving a satisfactory fit of their overall temperature dependence, indicating the rich and complex physics of these newly discovered superconductors.

Physical Review Letters, 2008

We present the first results of directional point-contact spectroscopy in high quality CaC6 sampl... more We present the first results of directional point-contact spectroscopy in high quality CaC6 samples both along the ab plane and in the c-axis direction. The superconducting order parameter \Delta(0), obtained by fitting the Andreev-reflection (AR) conductance curves at temperatures down to 400 mK with the single-band 3D Blonder-Tinkham-Klapwijk model, presents two different distributions in the two directions of the main current injection, peaked at 1.35 and 1.71 meV, respectively. By ab-initio calculations of the AR conductance spectra, we show that the experimental results are in good agreement with the recent predictions of gap anisotropy in CaC6.

Reports on Progress in Physics, 2011

Point-contact Andreev-reflection spectroscopy (PCAR) has proven to be one of the most powerful to... more Point-contact Andreev-reflection spectroscopy (PCAR) has proven to be one of the most powerful tools in the investigation of superconductors, where it provides information on the order parameter (OP), a fundamental property of the superconducting state. In the past 20 years, successive improvements of the models used to analyze the spectra have continuously extended its capabilities, making it suited to study new superconductors with 'exotic' properties such as anisotropic, nodal and multiple OPs. In Fe-based superconductors, the complex compound- and doping-dependent Fermi surface (FS) and the predicted sensitivity of the OP to fine structural details present unprecedent challenges for this technique. Nevertheless, we show here that PCAR measurements in Fe-based superconductors carried out so far have already greatly contributed to our understanding of these materials, despite some apparent inconsistencies that can be overcome if a homogeneous treatment of the data is used. We also demonstrate that, if properly extended theoretical models for Andreev reflection are used, directional PCAR spectroscopy can provide detailed information not only on the amplitude and symmetry of the OPs, but also on the nature of the pairing boson, and even give some hints about the shape of the FS.

Physical Review Letters, 2006

We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 singl... more We present the first results of directional point-contact measurements in Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3 K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model. Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principle band structure calculations.