The Experts below are selected from a list of 5613 Experts worldwide ranked by ideXlab platform
Renato Gonnelli - One of the best experts on this subject based on the ideXlab platform.
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point contact Andreev Reflection spectroscopy in fe te se films multiband superconductivity and electron boson coupling
arXiv: Superconductivity, 2014Co-Authors: Dario Daghero, Giovanni Ummarino, Paola Pecchio, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Ichiro Tsukada, Seiki Komiya, Renato GonnelliAbstract:We report on a study of the superconducting order parameter in Fe(Te$_{1-x}$Se$_{x}$) thin films (with different Se contents: x=0.3, 0.4, 0.5) by means of point-contact Andreev-Reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude $\Delta_E \simeq 2.75 k_B T_c$ and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude $\Delta_H \simeq 1.75 k_B T_c$. The existence of this gap and its amplitude are confirmed by PCARS measurements in Fe(Te$_{1-x}$Se$_{x}$) single crystals. The values of the two gaps $\Delta_E$ and $\Delta_H$, once plotted as a function of the local critical temperature $T_c^A$, turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.
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point contact Andreev Reflection spectroscopy in fe te se films multiband superconductivity and electron boson coupling
Superconductor Science and Technology, 2014Co-Authors: Dario Daghero, Giovanni Ummarino, Paola Pecchio, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Ichiro Tsukada, Seiki Komiya, Renato GonnelliAbstract:We report on a study of the superconducting order parameter in thin films (with different Se contents: ) by means of point-contact Andreev-Reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude . The existence of this gap and its amplitude are confirmed by PCARS measurements in single crystals. The values of the two gaps and , once plotted as a function of the local critical temperature , turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.
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the order parameter symmetry and fermi surface topology of 122 fe based superconductors a point contact Andreev Reflection study
arXiv: Superconductivity, 2013Co-Authors: Renato Gonnelli, Mauro Tortello, Dario Daghero, Paola Pecchio, Sara Galasso, V A Stepanov, Z Bukowski, N D Zhigadlo, J Karpinski, K IidaAbstract:We report on the results of directional point-contact Andreev-Reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev Reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds.
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point contact Andreev Reflection spectroscopy in fe based superconductors multigap superconductivity and strong electron boson interaction
Journal of Superconductivity and Novel Magnetism, 2012Co-Authors: Mauro Tortello, Dario Daghero, Giovanni Ummarino, V A Stepanov, N D Zhigadlo, J Karpinski, J Jiang, Renato GonnelliAbstract:We performed point-contact Andreev-Reflection measurements in Ba(Fe1−xCox)2As2 single crystals (x=0.1, Tc=24.5 K) and SmFeAsO1−xFx polycrystals (x=0.2, Tc=52 K). The spectra indicate the presence of two superconducting gaps with no line nodes on the Fermi surface, but also feature additional structures related to the electron–boson interaction (EBI). From the spectra, it is possible to extract the characteristic energy Ω0 of the mediating boson. In Co-doped Ba-122, we obtain Ω0=12 meV that coincides with the spin-resonance energy observed in neutron-scattering experiments. In Sm-1111, Ω0=20 meV fulfils the relation Ω0=4.65kBTc inferred from neutron-scattering results on other Fe-based superconductors. The strong electron–boson coupling may also explain some anomalies in the PCAR conductance curves (e.g., the excess conductance at high energy) which sometimes prevent a good fit of the curves with models based on constant, BCS-like order parameters.
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directional point contact Andreev Reflection spectroscopy of fe based superconductors fermi surface topology gap symmetry and electron boson interaction
Reports on Progress in Physics, 2011Co-Authors: Dario Daghero, Mauro Tortello, Giovanni Ummarino, Renato GonnelliAbstract: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.
Pratap Raychaudhuri - One of the best experts on this subject based on the ideXlab platform.
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inter landau level Andreev Reflection at the dirac point in a graphene quantum hall state coupled to a nbse 2 superconductor
Physical Review Letters, 2018Co-Authors: Manas Ranjan Sahu, Pratap Raychaudhuri, Xin Liu, A K Paul, Sourin Das, J K Jain, Anindya DasAbstract:Superconductivity and the quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of the quantum Hall effect with a superconductor can provide fertile ground for realizing exotic topological excitations such as non-Abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we report observation of Andreev Reflection at the junction of a quantum Hall edge state in a single layer graphene and a quasi-two-dimensional niobium diselenide (${\mathrm{NbSe}}_{2}$) superconductor. Our principal finding is the observation of an anomalous finite-temperature conductance peak located precisely at the Dirac point, providing a definitive evidence for inter-Landau-level Andreev Reflection in a quantum Hall system. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in Andreev physics. This study paves the way for investigating analogous Andreev Reflection in a fractional quantum Hall system coupled to a superconductor to realize exotic quasiparticles.
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Andreev Reflection near the dirac point at the graphene nbse 2 junction
Physical Review B, 2016Co-Authors: Manas Ranjan Sahu, Pratap Raychaudhuri, Anindya DasAbstract:Despite extensive search for about a decade, specular Andreev Reflection (SAR) has only recently been realized in the bilayer graphene-superconductor interface. The experimental observation of retro to specular Andreev Reflection is not only fundamentally important, but also has potential application in quantum computing, etc. Here we have carried out the transport measurements at the van der Waals interface of single-layer graphene and ${\mathrm{NbSe}}_{2}$ superconductor. We investigate the Andreev Reflection near the Dirac point by measuring the differential conductance as a function of Fermi energy and bias energy. We find that the normalized conductance (${G}_{Tl{T}_{c}}/{G}_{TgTc}$) becomes suppressed as we pass through the Dirac cone, which manifests the transition from retro to nonretro-type Andreev Reflection. The suppression indicates the blockage of Andreev Reflection beyond a critical angle (${\ensuremath{\theta}}_{c}$) of the incident electron with respect to the normal between the single-layer graphene and the superconductor junction. However, the observation of SAR was restricted due to the finite Fermi-energy broadening. The results are compared with a theoretical model of the corresponding setup.
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temperature dependence of transport spin polarization in nd ni 5 from point contact Andreev Reflection
Physical Review B, 2007Co-Authors: Sourin Mukhopadhyay, Pratap Raychaudhuri, Devang A Joshi, C V TomyAbstract:We report a study in which Point contact Andreev Reflection (PCAR) spectroscopy using superconducting Nb tip has been carried out on NdNi5, a ferromagnet with a Curie temperature of TC~7.7K. The measurements were carried out over a temperature range of 2-9K which spans across the ferromagnetic transition temperature. From an analysis of the spectra, we show that (i) the temperature dependence of the extracted value of transport spin polarization closely follows the temperature dependence of the spontaneous magnetization; (ii) the superconducting quasiparticle lifetime shows a large decrease close to the Curie temperature of the ferromagnet. We attribute the latter to the presence of strong ferromagnetic spin fluctuations in the ferromagnet close to the ferromagnetic transition temperature.
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transport spin polarization in srruo3 measured through point contact Andreev Reflection
Physical Review B, 2003Co-Authors: Pratap Raychaudhuri, A P Mackenzie, J W Reiner, M R BeasleyAbstract:We report a study in which Andreev Reflection using a Nb point contact is used to measure the transport spin polarization of the 4d itinerant ferromagnet SrRuO 3 in the ballistic limit of transport. The degree of transport spin polarization is comparable to that of the hole-doped rare-earth manganites. We conclude that the large transport spin polarization results mainly from a difference in the Fermi velocities between the majority- and minority-spin channels in this material.
Dario Daghero - One of the best experts on this subject based on the ideXlab platform.
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point contact Andreev Reflection spectroscopy in fe te se films multiband superconductivity and electron boson coupling
arXiv: Superconductivity, 2014Co-Authors: Dario Daghero, Giovanni Ummarino, Paola Pecchio, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Ichiro Tsukada, Seiki Komiya, Renato GonnelliAbstract:We report on a study of the superconducting order parameter in Fe(Te$_{1-x}$Se$_{x}$) thin films (with different Se contents: x=0.3, 0.4, 0.5) by means of point-contact Andreev-Reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude $\Delta_E \simeq 2.75 k_B T_c$ and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude $\Delta_H \simeq 1.75 k_B T_c$. The existence of this gap and its amplitude are confirmed by PCARS measurements in Fe(Te$_{1-x}$Se$_{x}$) single crystals. The values of the two gaps $\Delta_E$ and $\Delta_H$, once plotted as a function of the local critical temperature $T_c^A$, turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.
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point contact Andreev Reflection spectroscopy in fe te se films multiband superconductivity and electron boson coupling
Superconductor Science and Technology, 2014Co-Authors: Dario Daghero, Giovanni Ummarino, Paola Pecchio, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Ichiro Tsukada, Seiki Komiya, Renato GonnelliAbstract:We report on a study of the superconducting order parameter in thin films (with different Se contents: ) by means of point-contact Andreev-Reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude . The existence of this gap and its amplitude are confirmed by PCARS measurements in single crystals. The values of the two gaps and , once plotted as a function of the local critical temperature , turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.
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the order parameter symmetry and fermi surface topology of 122 fe based superconductors a point contact Andreev Reflection study
arXiv: Superconductivity, 2013Co-Authors: Renato Gonnelli, Mauro Tortello, Dario Daghero, Paola Pecchio, Sara Galasso, V A Stepanov, Z Bukowski, N D Zhigadlo, J Karpinski, K IidaAbstract:We report on the results of directional point-contact Andreev-Reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev Reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds.
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point contact Andreev Reflection spectroscopy in fe based superconductors multigap superconductivity and strong electron boson interaction
Journal of Superconductivity and Novel Magnetism, 2012Co-Authors: Mauro Tortello, Dario Daghero, Giovanni Ummarino, V A Stepanov, N D Zhigadlo, J Karpinski, J Jiang, Renato GonnelliAbstract:We performed point-contact Andreev-Reflection measurements in Ba(Fe1−xCox)2As2 single crystals (x=0.1, Tc=24.5 K) and SmFeAsO1−xFx polycrystals (x=0.2, Tc=52 K). The spectra indicate the presence of two superconducting gaps with no line nodes on the Fermi surface, but also feature additional structures related to the electron–boson interaction (EBI). From the spectra, it is possible to extract the characteristic energy Ω0 of the mediating boson. In Co-doped Ba-122, we obtain Ω0=12 meV that coincides with the spin-resonance energy observed in neutron-scattering experiments. In Sm-1111, Ω0=20 meV fulfils the relation Ω0=4.65kBTc inferred from neutron-scattering results on other Fe-based superconductors. The strong electron–boson coupling may also explain some anomalies in the PCAR conductance curves (e.g., the excess conductance at high energy) which sometimes prevent a good fit of the curves with models based on constant, BCS-like order parameters.
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directional point contact Andreev Reflection spectroscopy of fe based superconductors fermi surface topology gap symmetry and electron boson interaction
Reports on Progress in Physics, 2011Co-Authors: Dario Daghero, Mauro Tortello, Giovanni Ummarino, Renato GonnelliAbstract: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.
Anindya Das - One of the best experts on this subject based on the ideXlab platform.
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inter landau level Andreev Reflection at the dirac point in a graphene quantum hall state coupled to a nbse 2 superconductor
Physical Review Letters, 2018Co-Authors: Manas Ranjan Sahu, Pratap Raychaudhuri, Xin Liu, A K Paul, Sourin Das, J K Jain, Anindya DasAbstract:Superconductivity and the quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of the quantum Hall effect with a superconductor can provide fertile ground for realizing exotic topological excitations such as non-Abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we report observation of Andreev Reflection at the junction of a quantum Hall edge state in a single layer graphene and a quasi-two-dimensional niobium diselenide (${\mathrm{NbSe}}_{2}$) superconductor. Our principal finding is the observation of an anomalous finite-temperature conductance peak located precisely at the Dirac point, providing a definitive evidence for inter-Landau-level Andreev Reflection in a quantum Hall system. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in Andreev physics. This study paves the way for investigating analogous Andreev Reflection in a fractional quantum Hall system coupled to a superconductor to realize exotic quasiparticles.
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Andreev Reflection near the dirac point at the graphene nbse 2 junction
Physical Review B, 2016Co-Authors: Manas Ranjan Sahu, Pratap Raychaudhuri, Anindya DasAbstract:Despite extensive search for about a decade, specular Andreev Reflection (SAR) has only recently been realized in the bilayer graphene-superconductor interface. The experimental observation of retro to specular Andreev Reflection is not only fundamentally important, but also has potential application in quantum computing, etc. Here we have carried out the transport measurements at the van der Waals interface of single-layer graphene and ${\mathrm{NbSe}}_{2}$ superconductor. We investigate the Andreev Reflection near the Dirac point by measuring the differential conductance as a function of Fermi energy and bias energy. We find that the normalized conductance (${G}_{Tl{T}_{c}}/{G}_{TgTc}$) becomes suppressed as we pass through the Dirac cone, which manifests the transition from retro to nonretro-type Andreev Reflection. The suppression indicates the blockage of Andreev Reflection beyond a critical angle (${\ensuremath{\theta}}_{c}$) of the incident electron with respect to the normal between the single-layer graphene and the superconductor junction. However, the observation of SAR was restricted due to the finite Fermi-energy broadening. The results are compared with a theoretical model of the corresponding setup.
L F Cohen - One of the best experts on this subject based on the ideXlab platform.
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Andreev bound states in superconductor ferromagnet point contact Andreev Reflection spectra
Physical Review B, 2017Co-Authors: K A Yates, L Olde A B Olthof, M E Vickers, Dharmalingam Prabhakaran, M Egilmez, J W A Robinson, L F CohenAbstract:As charge carriers traverse a single superconductor ferromagnet interface, they experience an additional spin-dependent phase angle that results in spin mixing and the formation of a bound state called the Andreev bound state. Here we explore whether point contact Andreev Reflection can be used to detect the Andreev bound state and, within the limits of our experiment, we extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7-Pb junctions, together with a compilation of literature data on highly spin polarized systems, we suggest that the existence of the Andreev bound state would resolve a number of long standing controversies in the literature of Andreev Reflection, as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly, we find that for high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have cho sen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.
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investigation of superconducting gap structure in tbfeaso0 9f0 1 using point contact Andreev Reflection
New Journal of Physics, 2009Co-Authors: K A Yates, K Morrison, Jennifer A Rodgers, George B S Penny, Jw G Bos, J P Attfield, L F CohenAbstract:Bulk samples of TbFeAsO0.9 F0.1 (Tcon = 50 K) were measured by point contact Andreev Reflection spectroscopy. The spectra show unambiguous evidence for multiple gap-like features plus the presence of high-bias shoulders. By measuring the spectra as a function of temperature with both gold and superconducting niobium tips, we establish that the gap-like features are associated with the superconducting order parameter in this material. We discuss whether the well-defined zero-bias conductance peak that we observe infrequently is associated with a nodal superconducting order parameter.
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investigation of superconducting gap structure in tbfeaso _ 0 9 f _ 0 1 using point contact Andreev Reflection
arXiv: Superconductivity, 2008Co-Authors: K A Yates, K Morrison, Jennifer A Rodgers, George B S Penny, Jw G Bos, Paul J Attfield, L F CohenAbstract:Bulk samples of TbFeAsO$_{0.9}$F$_{0.1}$ (T$_{c}$(on) = 50K) were measured by point contact Andreev Reflection spectroscopy. The spectra show unambiguous evidence for multiple gap-like features plus the presence of high bias shoulders. By measuring the spectra as a function of temperature with both gold and superconducting niobium tips, we establish that the gap-like features are associated with superconducting order parameter in this material. We discuss whether the well defined zero bias conductance peak that we observe infrequently is associated with a nodal superconducting order parameter.
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Andreev Reflection spectroscopy of niobium point contacts in a magnetic field
Physical Review B, 2005Co-Authors: Y Miyoshi, Y Bugoslavsky, L F CohenAbstract:The Blonder-Tinkham-Klapwijk model of the point-contact Andreev Reflection (PCAR) spectroscopy does not provide an adequate description of experiments in the presence of a magnetic field. We demonstrate this using a junction between a niobium tip and a copper film. We modify the theory to explicitly take into account the contribution to conductance that stems from the normal vortex cores in the superconductor. These results have important implications for the interpretation of transport spin polarization measurements using the PCAR technique. We demonstrate that stray magnetic fields can be responsible for the experimentally observed dependence of the spin polarization on the strength of the interface barrier and potential misassignment of the inferred spin polarization.