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Possible unconventional superconductivity in substituted BaFe2As2 revealed by magnetic pair-breaking studies.

Rosa PF, Adriano C, Garitezi TM, Piva MM, Mydeen K, Grant T, Fisk Z, Nicklas M, Urbano RR, Fernandes RM, Pagliuso PG - Sci Rep (2014)

Bottom Line: An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively.From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function.Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Instituto de Física "Gleb Wataghin", UNICAMP, Campinas-SP, 13083-859, Brazil [2] University of California, Irvine, California 92697-4574, USA.

ABSTRACT
The possible existence of a sign-changing gap symmetry in BaFe2As2-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe1.9M0.1As2 (M = Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

No MeSH data available.


Related in: MedlinePlus

X-Band ESR lines at T = 150 K for powdered crystals of BaFe1.9M0.1As2 (M = Cu, Mn) and BaFe1.895Mn0.005Co0.1As2.The spectra were normalized by the concentration of paramagnetic probes in order to clearly compare their intensities. The solid lines are Lorentzian fits to the spectra (sample grain size smaller than the skin depth48). It is worth mentioning that, in order to obtain the ESR signal, the sample surface must be completely clean and free of In-flux. The ESR signals for both samples were calibrated at 300 K using a strong pitch standard sample with 4.55 × 1015 spins/cm.
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f3: X-Band ESR lines at T = 150 K for powdered crystals of BaFe1.9M0.1As2 (M = Cu, Mn) and BaFe1.895Mn0.005Co0.1As2.The spectra were normalized by the concentration of paramagnetic probes in order to clearly compare their intensities. The solid lines are Lorentzian fits to the spectra (sample grain size smaller than the skin depth48). It is worth mentioning that, in order to obtain the ESR signal, the sample surface must be completely clean and free of In-flux. The ESR signals for both samples were calibrated at 300 K using a strong pitch standard sample with 4.55 × 1015 spins/cm.

Mentions: To investigate such magnetic IPB scenario, we performed ESR – a powerful spin probe technique sensitive to the presence of local moments and their coupling to the conduction electrons48. In agreement with the expectation that Cu and Mn ions have local moments, our ESR data reveal an intense resonance line for M = Cu and Mn, but not for M = Co and Ni. Fig. 3 shows the X-Band ESR lines normalized by the concentration of paramagnetic ions at T = 150 K for fine powders of gently crushed single crystals. The Lorentzian fitting of the spectra reveals a linewidth of ΔH = 600(60) G and a g-value of g = 2.08(3) for M = Cu. For M = Mn, g = 2.04(3) and the linewidth is slightly larger, ΔH = 750(80) G, indicating stronger Mn-Mn interactions. Finally, for M = Mn and Co, g = 2.05(3) and ΔH = 670(70) G. For all samples, the calibrated number of resonating spins at room-T is in good agreement with the concentrations obtained from Energy Dispersive Spectroscopy (EDS). As expected, the ESR intensity, which is proportional to S(S + 1), was found to be roughly twelve times larger for M = Mn samples, as compared to the M = Cu sample. These results also indicate that the oxidation states of Cu and Mn are indeed Cu2+ (S = 1/2) and Mn2+ (S = 5/2). In the former case, Cu+ (3d10 state) would not display an ESR resonance line since it is not a paramagnetic ion. In the case of copper, Cu+ (3d10 state) would not display an ESR resonance line since it is not a paramagnetic probe with unpaired electrons. In the case of manganese, for Mn3+ (S = 2) and Mn4+ (S = 3/2) ions, one would expect a distinct ESR response (i.e., different g-value and calibrated signal intensity). Consequently, one can infer that there is no effective charge doping into the system, as suggested previously both experimentally and theoretically68. Furthermore, our ESR results agree with other indirect probes that also suggest localized Cu2+ and Mn2+ moments in chemically-substituted Ba1225051525349. We note that the detailed analysis of the ESR data confronted with Eu-substituted BaFe2−xMxAs2 (M = Co, Ni, Cu, Mn, and Ru) requires further technical discussion. Therefore, it will be the focus of a separated report1315.


Possible unconventional superconductivity in substituted BaFe2As2 revealed by magnetic pair-breaking studies.

Rosa PF, Adriano C, Garitezi TM, Piva MM, Mydeen K, Grant T, Fisk Z, Nicklas M, Urbano RR, Fernandes RM, Pagliuso PG - Sci Rep (2014)

X-Band ESR lines at T = 150 K for powdered crystals of BaFe1.9M0.1As2 (M = Cu, Mn) and BaFe1.895Mn0.005Co0.1As2.The spectra were normalized by the concentration of paramagnetic probes in order to clearly compare their intensities. The solid lines are Lorentzian fits to the spectra (sample grain size smaller than the skin depth48). It is worth mentioning that, in order to obtain the ESR signal, the sample surface must be completely clean and free of In-flux. The ESR signals for both samples were calibrated at 300 K using a strong pitch standard sample with 4.55 × 1015 spins/cm.
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Related In: Results  -  Collection

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Show All Figures
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f3: X-Band ESR lines at T = 150 K for powdered crystals of BaFe1.9M0.1As2 (M = Cu, Mn) and BaFe1.895Mn0.005Co0.1As2.The spectra were normalized by the concentration of paramagnetic probes in order to clearly compare their intensities. The solid lines are Lorentzian fits to the spectra (sample grain size smaller than the skin depth48). It is worth mentioning that, in order to obtain the ESR signal, the sample surface must be completely clean and free of In-flux. The ESR signals for both samples were calibrated at 300 K using a strong pitch standard sample with 4.55 × 1015 spins/cm.
Mentions: To investigate such magnetic IPB scenario, we performed ESR – a powerful spin probe technique sensitive to the presence of local moments and their coupling to the conduction electrons48. In agreement with the expectation that Cu and Mn ions have local moments, our ESR data reveal an intense resonance line for M = Cu and Mn, but not for M = Co and Ni. Fig. 3 shows the X-Band ESR lines normalized by the concentration of paramagnetic ions at T = 150 K for fine powders of gently crushed single crystals. The Lorentzian fitting of the spectra reveals a linewidth of ΔH = 600(60) G and a g-value of g = 2.08(3) for M = Cu. For M = Mn, g = 2.04(3) and the linewidth is slightly larger, ΔH = 750(80) G, indicating stronger Mn-Mn interactions. Finally, for M = Mn and Co, g = 2.05(3) and ΔH = 670(70) G. For all samples, the calibrated number of resonating spins at room-T is in good agreement with the concentrations obtained from Energy Dispersive Spectroscopy (EDS). As expected, the ESR intensity, which is proportional to S(S + 1), was found to be roughly twelve times larger for M = Mn samples, as compared to the M = Cu sample. These results also indicate that the oxidation states of Cu and Mn are indeed Cu2+ (S = 1/2) and Mn2+ (S = 5/2). In the former case, Cu+ (3d10 state) would not display an ESR resonance line since it is not a paramagnetic ion. In the case of copper, Cu+ (3d10 state) would not display an ESR resonance line since it is not a paramagnetic probe with unpaired electrons. In the case of manganese, for Mn3+ (S = 2) and Mn4+ (S = 3/2) ions, one would expect a distinct ESR response (i.e., different g-value and calibrated signal intensity). Consequently, one can infer that there is no effective charge doping into the system, as suggested previously both experimentally and theoretically68. Furthermore, our ESR results agree with other indirect probes that also suggest localized Cu2+ and Mn2+ moments in chemically-substituted Ba1225051525349. We note that the detailed analysis of the ESR data confronted with Eu-substituted BaFe2−xMxAs2 (M = Co, Ni, Cu, Mn, and Ru) requires further technical discussion. Therefore, it will be the focus of a separated report1315.

Bottom Line: An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively.From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function.Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Instituto de Física "Gleb Wataghin", UNICAMP, Campinas-SP, 13083-859, Brazil [2] University of California, Irvine, California 92697-4574, USA.

ABSTRACT
The possible existence of a sign-changing gap symmetry in BaFe2As2-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe1.9M0.1As2 (M = Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

No MeSH data available.


Related in: MedlinePlus