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A new unconventional (possibly d-wave) superconductor: LaAg 1-x Mn x

S. N. Kaul School of Physics University of Hyderabad INDIA. A new unconventional (possibly d-wave) superconductor: LaAg 1-x Mn x. Electron 2007. Organization of the Talk. Introduction Sample preparation and Characterization Normal - to - Superconducting Transition Anomalous behaviour

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A new unconventional (possibly d-wave) superconductor: LaAg 1-x Mn x

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  1. S. N. Kaul School of Physics University of Hyderabad INDIA A new unconventional (possibly d-wave) superconductor: LaAg1-xMnx Electron 2007

  2. Organization of the Talk • Introduction • Sample preparation and Characterization • Normal - to - Superconducting Transition • Anomalous behaviour • Absence of Long - range magnetic order • Normal state: Presence of antiferromagnetic spin fluctuations • Conclusions

  3. Magnetically – mediated superconductivity

  4. Polycrystalline samples of LaAg1-cMnc (c = 0, 0.025, 0.05, 0.1, 0.2, 0.3) alloys prepared by arc and RF induction melting techniques. Sample characterization by XRD, SEM and EDAX. Synthesis and characterization To explore the possibility of observing superconductivity at elevated temperaturesinduced by antiferromagnetic spin fluctuations in a 3 - dimensional nearly antiferromagnetic metal.

  5. 10 m 10m SEM Micrographs Induction Melted LaAg0.9Mn0.1 Arc Melted

  6. Energy dispersive absorption of x-rays 1. Global average composition matches with nominal composition. 2. Compositional fluctuations are more in arc-melted samples.

  7. X-ray diffraction patterns CsCl (bcc) structure retained even in the highest Mnconcentration Minority phase less than 1 volume % of the majority phase.

  8. Neutron diffraction patterns Neutron wavelength  = 1.31 Å

  9. Neutron diffraction patterns Neutron wavelength  = 1.31 Å

  10. Variation of Lattice parameter ‘a’ with Mn concentration and its thermal evolutionfor a given c

  11. Transition from normal to superconducting state Elecrical resistivity and its temperature derivative ac susceptibility The peak in d / dT and ``(T) at H = 0 Oe is identified as TC.

  12. ac susceptibility X = 0.1 X = 0.08

  13. Upper Critical field HC 2 At any given temperature Critical field HC2 is equal for c = 0.1(x = 0.05) and c = 0.05  = [0 / 2 Hc2(0)]1/2  = 185 Å – 215 Å  is 1.5 times smaller than the quasi particle mean free path deduced from the normal state residual resistivity.

  14. Superconducting – Normal Phase Diagram TC is consistently higher in the arc melted samples than the induction-melted counterparts indicates the sensitivity of TC to local fluctuations in the Mn concentration. TC abruptly increases from 0.37 K to 5 K at Mn concentration x = 0.05 indicating thereby that there exists a threshold Mn concentration.

  15. Zero - field - cooled (ZFC) and field - cooled (FC) magnetization Zero-field-cooled magnetization at H = 200 Oe [in agreement with ’ (H =0)] corresponds to screening of H over 90 – 95 % sample volume. Field-cooled magnetization at the same measuring field indicates that the Meissner (flux expulsion) fraction is as small as 10 - 15 %.

  16. Magnetic Hysteresis loops X = 0.05 X = 0.08 X = 0.1

  17. Suppression of the jump at Tc by Mn substitution Cn = Cen + Cln = T + A (T / D)3 Cn / T =  + (A / D3) T2 N (EF) = ( 3 / 2 2 kB2)  C(TC) = 1.43  TC

  18. Existence of Superconducting Gap Cm = Ces - Cen Ces /  Tc = a exp(-  Tc/ T )

  19. Mn concentration dependence of Superconducting Gap Cm /  Tc = a exp(-  Tc/ T )

  20. Effect of annealing

  21. Effect of annealing

  22. Antiferromagnetic spin fluctuations and their suppression by magnetic field X = 0.05 X = 0.05 Cm (H) = Cm(0) [1 – B(H)*T3/2 ]

  23. Antiferromagnetic Spin fluctuation contribution to specific heat C(TC) = 1.43  TC C(TC) = 230 mJ / mole K for c = 0.2 C(TC) = 350 mJ / mole K for c = 0.3 Cm(TC)  3 mJ / mole K Cm(T) decreases in accordance with T3/2 power law around and above TC. This observation is consistent with the theoretical prediction that antiferromagnetic spin fluctuationsin a nearly antiferromagnetic metal (near the magnetic instability) give rise to T3/2 variation for the excess contribution to specific heat.

  24. Antiferromagnetic spin fluctuation contribution to resistivity (T,0) and (T,H) vary as T3/2 in a certain temperature range around TC. X = 0.05 Theory also predicts a T3/2 variation for (T,0) and (T,H) in a nearly antiferromagnetic metal arising from antiferromagnetic spin fluctuations. X = 0.1 Negative magnetoresistivity reflects the suppression of antiferromagnetic spin fluctuations by magnetic field.

  25. Suppression of spin fluctuations with magnetic field. X = 0.05 Suppression is faster for x = 0.05 than x = 0.1, because spin fluctuations are more stiff in x = 0.1. X = 0.1  (T, H) =  (0, H) [1 – A(H)*T3/2 ]

  26. Curie-Weiss behaviour of susceptibility in the normal state (T) = [C / (T + )] + 0 Characteristic temperature, , is negative indicating the existence of antiferromagnetic spin correlations. Large magnetic susceptibility reflects the exchange-enhanced antiferromagnetic spin fluctuation contribution. Magnetic moment per Mn atom is 4.0 B

  27. Magnetization measurements in the normal state Indicates the presence of both ferromagnetic and antiferromagnetic interactions but the latter are predominant.

  28. Absence of long – range magnetic order

  29. Absence of long – range magnetic order

  30. Suppression of antiferromagnetic spin fluctuation contribution to Magnetoresistance by magnetic field

  31. A brief summary of Results CsCl structure, which is unfavorable for conventional superconductivity, is retained even for highest Mn concentration.  Abrupt increase in the superconducting transition temperature TC, from 0.37 K to 5 K, suggests existence of a threshold Mn concentration  0.05. Since the magnetic Mn atomsare responsible forthe antiferromagnetic spin fluctuations, the magnetic instability is reached from the paramagnetic side at a certain threshold value of Mn concentration when antiferromagnetic spin fluctuations are correlated over long distances and hence are more effective for electron pairing.  Cp(T) neither goes through a peak (associated with long range anti- ferromagnetic order; thereby signalling the absence of long-range antiferromagnetic order) nor exhibits a jump at TC.

  32.  Debye temperature D exhibits a very weak (about 4%) variation with Mn concentration and therefore the Mn substitution has hardly any effect on the phonon spectrum of LaAg and yet TC is sensitive to Mn concentration. The BCS relation, TC = 1.14 D exp[-1 / V N(EF)] predicts TCvalues approximately one order of magnitude higher than the observed ones Above observations thus completely rule out the electron - phonon mechanism for superconductivity in the present system

  33. Conclusions • Curie-Weiss behaviour of susceptibility in the normal state reflects the existence of exchange-enhanced antiferromagnetic spin correlations. • Neutron diffraction confirms theabsence of long-range antiferromagnetic order and of the structural phase transformation at or near TC. • According to the spin fluctuation theories, the observed T3/2 variation of Cm(T), (T,0) and (T,H) arises from antiferromagnetic spin fluctuations near the magnetic instability. • Negative magnetoresistivity reflects the suppression of spin fluctuations by the magnetic field. • The observation that the T3/2 variation of Cm(T) extends to temperatures < TC implies that antiferromagnetic spin fluctuations persist in the superconducting state. Antiferromagnetic spin fluctuation mediated electron pairing is the most likely mechanism for superconductivity

  34. Collaborators Sanjeev Kumar, University of Hyderabad Dr. L. Fernández Barquín, Dr. J. Rodríguez Fernández Condensed Matter Group, University of Cantabria, Spain. Unconventional Superconductivity in LaAg1-xMnx: relevance of Spin Fluctuation Mediated pairing S.N. Kaul, S. Kumar, J. Rodríguez Fernández, and L. Fernández Barquín Europhys. Lett., 74, 138-144 (2006) Exchange-enhanced spin fluctuations in the normal state of a new unconventional superconductor S. Kumar, S.N. Kaul ,J. Rodríguez Fernández, and L. Fernández Barquín J. Magn. Magn. Mater, at press.

  35. Thank you

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