Interaction between spin, lattice, and charge in non-centro\-symmetric correlated metals

• Applicant: Professor Dr. Rudolf Hackl
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 269710404

• Final Report Year: 2022

Final Report Abstract

Ordering phenomena and new phases play an important role in complex materials where quantum mechanical effects dominate the physical properties. Arguably, the best known phenomena are magnetism and superconductivity. The superconducting ground state is characterized by perfect diamagnetism and vanishing resistivity. Magnetic ground states may be considerably more complicated. In all modern superconducting materials such as heavy fermion systems, copper-oxygen compounds (cuprates) or ironbased pnictides or chalcogenides magnetic and superconducting phases are in close proximity and are believed to be interrelated. Therefore, the study of complex magnetism beyond ferro- or antiferromagnetism has usually aspects with broader vistas. Yet, phenomena like skyrmion textures, frustration, stripes and fluctuation continue to attract attention on their own since they have an impact on transport properties which may facilitate new functionalities and applications. However, the link between magnetism and superconductivity remains one of the most vexing problems in condensed matter physics. Although this project focused originally on the study of materials without inversion center, displaying complex magnetic textures and superconductivity, the work went into the direction of superconductivity in materials having other types of magnetic and electronic instabilities. The essential reasons for the change of direction were technical difficulties and funding problems. In spite of that approximately 15, partially high profile publications resulted from the research. The central results are (1) the observation and theoretical description of sub-leading Cooper pairing channels in iron pnictides and chalcogenides, (2) the study and explanation of a phonon anomaly in the iron-based materials and (3) the discovery of frustrated anti-ferromagnetism in FeSe. (1) We consider the analysis of the pairing states in iron-based materials the most remarkable result of the project. It demonstrates how the pairing attraction between two electrons of a Cooper pair varies as a function of momentum. This momentum dependence is a crucial information for understanding any type of superconductor. In the pnictides and chalcogenides, the pairing interaction is determined by the topology of the Fermi surface. Therefore, pairing induced by magnetic interactions (spin fluctuations) appears to be more likely than by lattice instabilities. We hope that this is also helpful for improving these superconductors towards applications. (2) The anomalous intensity of the As A1g phonon below the structural transition preceding magnetic ordering indicates low- or high-energy anisotropies. The dependence of the anomaly on laser energy shows that high-energy states are involved and modified by magnetic ordering. (3) FeSe is an exceptional member of the iron-based materials since it does not order magnetically at low temperature. However, since bulk FeSe is superconducting below 9 K one may argue to be in the wrong position of the phase diagram. For applied pressure, magnetic order and superconductivity are observed simultaneously. Thus, FeSe is distinctly different from the pnictides. It was suggested that FeSe has itinerant and nearly local magnetic moments in parallel. Our Raman scattering results along with theoretical considerations indeed indicate that short range magnetic ordering of local moments exists. However, due to a ratio J1/J2 of the nearest and next-nearest neighbor magnetic interaction is close to two, magnetism is predicted and observed to be frustrated meaning that the system is not in the ground state.

Publications

• Critical spin fluctuations and the origin of nematic order in in Ba(Fe1-xCox)2As2. Nature Phys. 12, 560 (2016)
  F. Kretzschmar, T. Böhm, U. Karahasanović, B. Muschler, A. Baum, D. Jost, J. Schmalian, S. Caprara, M. Grilli, C. Di Castro, J. G. Analytis, J.-H. Chu, I. R. Fisher, R. Hackl
  
• Indication of subdominant d-wave interaction in superconducting CaKFe4As4. Phys. Rev. B 98, 020504(R) (2018)
  D. Jost, J.-R. Scholz, U. Zweck, W. R. Meier, A. E. Böhmer, P. C. Canfield, N. Lazarević and R. Hackl
  
• Interplay of lattice, electronic and spin degrees of freedom in detwinned BaFe2As2: A Raman scattering study. Phys. Rev. B 98, 075113 (2018)
  A. Baum, Ying Li, M. Tomić, N. Lazarević, D. Jost, F. Löffler, B. Muschler, T. Böhm, J.-H. Chu, I. R. Fisher, R. Valentí, I. I. Mazin, R. Hackl
  
• Magnetic excitations and amplitude fluctuations in insulating cuprates. Phys. Rev. B 97, 024407 (2018)
  N. Chelwani, A. Baum, T. Böhm, M. Opel, F. Venturini, A. Erb, H. Berger, L. Forró, and R. Hackl
  
• Microscopic origin of Cooper pairing in the iron-based superconductor Ba1-xKxFe2As2. npj Quantum Materials 3:48 (2018)
  T. Böhm, F. Kretzschmar, A. Baum, M. Rehm, D. Jost, R. Hosseinian Ahangharnejhad, R. Thomale, C. Platt, T. A. Maier, W. Hanke, B. Moritz, T. P. Devereaux, D. J. Scalapino, S. Maiti, P. J. Hirschfeld, P. Adelmann, T. Wolf, Hai-Hu Wen, and R. Hackl
  
• Frustrated spin order and stripe fluctuations in FeSe. Commun. Phys. 2:14 (2019)
  A. Baum, H. N. Ruiz, N. Lazarević, Yao Wang, T. Böhm, R. Hosseinian Ahangharnejhad, P. Adelmann, T. Wolf, Z. V. Popović, B. Moritz, T. P. Devereaux, and R. Hackl
  
• Fluctuations and pairing in Fe-based superconductors: Light scattering experiments. Topical review: J. Phys.: Condens. Matter 32, 413001 (2020)
  N. Lazarević and R. Hackl Topical review: J. Phys.: Condens
  
• Raman Study of Cooper Pairing Instabilities in (Li1-xFex)OHFeSe. Phys. Rev. Lett. 125, 217002 (2020)
  G. He, D. Li, D. Jost, A. Baum, P.P. Shen, X.L. Dong, Z.X. Zhao, R. Hackl
  
• Calculation of an Enhanced A1g Symmetry Mode Induced by Higgs Oscillations in the Raman Spectrum of High-Temperature Cuprate Superconductors. Phys. Rev. Lett. 127, 197001 (2021)
  M. Puviani, A. Baum, S. Ono., Y. Ando, R. Hackl, and D. Manske
  
• Probing charge density wave phases and the Mott transition in 1T-TaS2 by inelastic light scattering. Phys. Rev. B 103, 245133 (2021)
  S. Djurdjić Mijin, A. Baum, J. Bekaert, A. Solajić,1 J. Pešić, Y. Liu, Ge He, M. V. Milošević, C. Petrovic, Z. V. Popović, R. Hackl, and N. Lazarević