Van-der-Waals magnets: Frustrated magnetism, magnetotransport, and optically driven excitations

• Applicant: Professor Dr. Urban Seifert
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449890867

• Final Report Year: 2024

Final Report Abstract

In this project, we were motivated by recent advances in 2D (van der Waals) heterostructure engineering to investigate questions pertaining to quantum magnets and their interaction with additional degrees of freedom. We have considered three main research lines: First, moire het-´ erostructures of transition metal dichalcogenides can exhibit Mott insulating behaviour, also at certain fractional fillings, and therefore provide a new platform for quantum magnetism. We have formulated a general theory for interaction-induced charge-ordered Mott insulators and possible ´ spin liquid states that they may host. To study weakly doped dilute Mott insulators in moire heterostructures, we have investigated the interplay of itinerant charge carries and local moments in an effective Kondo lattice model, finding that the formation of spin polarons stabilizes ferromagnetism at weak doping densities, in striking agreement with experimental results. In a second research line, we were motivated by the weak intrinsic elastic rigidity of two-dimensional van der Waals layers. We have studied the impact of magnetoelastic couplings on gapless spin liquids and deconfined quantum critical points, finding that they exhibit a rare (2 + 1)-dim. example of the spin- Peierls instability that occurs in S = 1/2 spin chains. Our theory makes predictions for the critical temperatures and phonon energy scales that bound the stability of these states, with a power-law form of the phase boundary as a function of the monopole scaling dimension ∆. Third, we have collaborated with experimental groups to explore and theoretically model the magnetic field control of the direction of Neel antiferromagnetic order in MnPSe3 and pump-probe spectroscopy of magnons in NiPS3.

Publications

• Mott insulators in moiré transition metal dichalcogenides at fractional fillings: Slave-rotor mean-field theory. Physical Review B, 108(15).
  Song, Zhenhao; Seifert, Urban F. P.; Luo, Zhu-Xi & Balents, Leon
  
• Magnetic field control of continuous Neel vector rotation and Neel temperature in a van der Waals antiferromagnet
  Z. Ni, U. Seifert, A. V. Haglund, N. Huang, D. G. Mandrus, L. Balents & L. Wu
  
• Spin Polarons and Ferromagnetism in Doped Dilute Moiré-Mott Insulators. Physical Review Letters, 132(4).
  Seifert, Urban F. P. & Balents, Leon
  
• Spin-Peierls instability of deconfined quantum critical points. Physical Review B, 110(12).
  Hofmeier, David; Willsher, Josef; Seifert, Urban F. P. & Knolle, Johannes
  
• Spin-Peierls instability of the U(1) Dirac spin liquid. Nature Communications, 15(1).
  Seifert, Urban F. P.; Willsher, Josef; Drescher, Markus; Pollmann, Frank & Knolle, Johannes
  
• Topological and magnetic phase transitions in the bilayer Kitaev-Ising model. Physical Review B, 109(2).
  Vijayvargia, Aayush; Seifert, Urban F. P. & Erten, Onur
  
• Wegner's Ising gauge spins versus Kitaev's Majorana partons: Mapping and application to anisotropic confinement in spin-orbital liquids. SciPost Physics, 16(6).
  Seifert, Urban F. P. & Moroz, Sergej
  
• Distinct Optical Excitation Mechanisms of a Coherent Magnon in a van der Waals Antiferromagnet. Physical Review Letters, 134(6).
  Allington, Clifford J.; Belvin, Carina A.; Seifert, Urban F. P.; Ye, Mengxing; Tai, Tommy; Baldini, Edoardo; Son, Suhan; Kim, Junghyun; Park, Jaena; Park, Je-Geun; Balents, Leon & Gedik, Nuh
  
• Stability of algebraic spin liquids coupled to quantum phonons. Physical Review Research, 7(4).
  Ferrari, Francesco; Willsher, Josef; Seifert, Urban F. P.; Valentí, Roser & Knolle, Johannes