Low energy spin fluctuations in a ferromagnetic spin system at a quantum critical point

• Applicant: Dr. Rajib Sarkar
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2012 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 217573298

• Final Report Year: 2015

Final Report Abstract

In summary, in this DFG funding period I have performed µSR experiments on polycrystalline YbNi4 (P1−x Asx )2 with x = 0, 0.04, 0.08, and 0.13 down to T = 20 mK and NMR studies on grain aligned YbNi4 P2 , respectively. I show that in stoichiometric YbNi4 P2 the ordered Yb3+ moment below TC = 0.15 K is strongly reduced to 0.046 µB and with increasing As substitution it decreases continuously to values smaller than 0.005 µB at x ≈ xc . In the paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate 1/T1 T ∝ T −n with an exponent value that decreases across the FM QCP from 1.13 for x = 0 to 1.01 for x = 0.13. This behavior can not be explained by any existing theory of FM quantum criticality. Finally, from the measured spin autocorrelation time, we find that the spin fluctuations are very slow and becomes even slower for x → xc . The present µSR experiments show that by introducing a small amount of As at the P site in the system YbNi4 (P1−x Asx )2 the magnetic order remains homogeneous and the static ordered Yb3+ moment is suppressed continuously from about 0.04 µB to values lower than 0.005 µB for x → xc = 0.1. There is no evidence for disorder effect in the µSR samples. Notably, in the paramagnetic region of the phase diagram and across the FM QCP the dynamic muon-spin relaxation rate 1/T1 T ∝ T −n with n ≈ 1. The critical fluctuations are very slow in the pure compound and become even slower when approaching the FM QCP. All these findings support the presence of a clean FM QCP in this system. However, the power laws observed in the T -dependence of thermodynamic quantities and in 1/T1 T remain to be understood. On the other hand, in the NMR experiments 31 K vs χ plots confirm grains to be aligned along the c-direction, as expected from χ anisotropy at high T . For in-plane measurements, we observe two peaks as expected from the local in-plane anisotropy. The analysis of the in-plane Knight shifts allows for a first estimation of the anisotropic in-plane local susceptibilities. Present NMR data evidence a large basal plane local anisotropy in YbNi4 P2 which was so far not accessible by other experiments. So far these NMR and µSR results are not communicated for the archive journal publications. However, the present status of the results clearly suggest that these deserve publication. These aforementioned work is directly related to the project with the title ”Low energy spin fluctuations in a ferromagnetic spin system at a quantum critical point”. In addition to that I was actively involved in many projects in our group as reflected in the list of my publications.

Publications

• Static and dynamic susceptibility in the putative ferromagnetic quantum critical system YbNi4 P2 probed by 31 P-NMR. Physics Status solidi B 250, 519-521 (2013)
  R. Sarkar, P. Khuntia, J. Spehling, C. Krellner, C. Geibel, H-H Klauss, and M. Baenitz
  
• Unconventional magnetism in multivalent charge-ordered YbPtGe2 probed by 195 Pt- and 171 Yb-NMR. Phys. Rev. B 88, 201101(R)(2013)
  R. Sarkar, R. Gümeniuk, A. Leithe-Jasper, W. Schnelle, Y. Grin, C. Geibel, and M. Baenitz
  
• CDW order and unconventional s-wave superconductivity in Ba1−x N ax T i2 Sb2 O. Journal of Physics: Conference Series 551, 012026 (2014)
  S. Kamusela, P. Materne, H. Maeter, R. Sarkar, T. Goltz, L. Harnagea, S. Wurmehl, B. Büchner, H. Luetkens, and Hans-Henning Klauss
  
• Interplay of magnetism and superconductivity in EuFe2 (As1−x Px )2 single crystals probed by muon spin-rotation and 57Fe Mösbauer. Journal of Physics: Conference Series 551, 012025 (2014)
  T. Goltz, S Kamusella, P. Materne, J. Spehling, H. S. Jeevan, P. Gegenwart, R. Sarkar, and H-H Klauss
  
• Magnetic order and spin dynamics in La2 OFe2 O2 Se2 probed by 57 Fe Mssbauer, 139 La NMR, and muon spin relaxation spectroscopy. Phys. Rev. B 90, 184408 (2014)
  M. Günter, S. Kamusella, R. Sarkar, T. Goltz, H.-H. Klauss, H. Luetkens, G. Pascua, K.-Y. Choi, and H. Zhou
  
• Multigap superconductivity in locally non-centrosymmetric SrPtAs: An 75 As nuclear quadrupole resonance investigations. Phys. Rev. B 90, 220503(R)(2014)
  F. Brückner, R. Sarkar, M. Günter, H. Kühne, H. Luetkens, T. Neupert, A. P. Reyes, P. L. Kuhns, P. K. Biswas, T. Sturzer, D. Johrend, and H.-H. Klauss
  
• Microscopic coexistence of superconductivity and magnetism in Ca1−x Nax Fe2 As2. Phy. Rev 92, 134511 (2015)
  P. Materne, S. Kamusela, R. Sarkar, T. Goltz, J. Spehling, H. Maeter, L. Harnagea, S. Wurmehl, B. Büchner, H. Luetkens, C. Timm, and Hans-Henning Klauss
  
• Sn-NMR investigations on superconducting Ca3 Ir4 Sn13 : Evidence for multigap BCS superconductivity. Physica B 479, 51-53 (2015)
  R. Sarkar F. Brückner, M. Günter, C. Petrovic, K. Wang, H. Luetkens, P. K. Biswas, E. Morenzoni, A. Amato, H-H. Klauss
  
• Superconducting properties and pseudogap from preformed Cooper pairs in the triclinic (CaFe1−x Ptx As)1 0Pt3 As8. Phys. Rev. B 91, 104515 (2015)
  M. A. Surmach, F. Brückner, S. Kamusella, R. Sarkar, P. Y. Portnichenko, J. T. Park, G. Ghambashidze, H. Luetkens, P. K. Biswas, W. J. Choi, Y. I. Seo, Y. S. Kwon, H.-H. Klauss, and D. S. Inosov