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Although ferromagnetism in metals has been known for thousands of years, its mechanism re-mains a hard-core problem of condensed matter physics. Its origin is deeply quantum mechanical: Pauli’s exclusion principle drives spin-polarization even though Coulomb interaction itself is spin-independent. However, the well-known Stoner criterion neglects correlations and cannot explain why electrons remain paramagnetic even under very strong interactions. The Curie-Weiss metal state above the Curie temperature is another challenge: Its spin-susceptibility is local-moment-like while its charge-channel remains metallic. In spite of these difficulties, we have proved theorems rigorously setting up ferromagnetic ground states over a large region of electron fillings. The Curie-Weiss metal phase and critical scalings are studied via the sign-problem free quantum Monte-Carlo simulations in a numerically “exact” way. We further generalize ferromagnetism to the types of unconventional symmetries (e.g. p-wave). Consequentially, spin-orbit coupling is generated as an order parameter via non-relativistic many-body physics of Pomeranchuk instabilities. This is a conceptually new mechanism since spin-orbit coupling is conventionally viewed as a rel-ativistic and single-particle effect. Possible experimental realizations in transition metal oxides and rare earth compounds and detections by neutron scatterings are discussed.
References
1) Zi Cai, Congjun Wu, “Symplectic ferromagnetism and phase transitions in multi-component fermionic systems”, arXiv:2003.08566
2) Yi Li, E. H. Lieb, Congjun Wu, “Exact Results on Itinerant Ferromagnetism in Multi-orbital Systems on Square and Cubic Lattices”, Phys. Rev. Lett. 112, 217201(2014).
3) Shenglong Xu, Yi Li, Congjun Wu, “Sign-Problem-Free Quantum Monte Carlo Study on Thermodynamic Properties and Magnetic Phase Transitions in Orbital-Active Itinerant Ferromag-nets”, Phys. Rev. X 5, 021032, (2015).
4) Congjun Wu, and Shou-Cheng Zhang, “Dynamic generation of spin-orbit coupling”, Phys. Rev. Lett. 93, 36403(2004).
5) Congjun Wu, Kai Sun, Eduardo Fradkin, and Shou-Cheng Zhang, “Fermi liquid instabilities in the spin channel”, Phys. Rev. B 75, 115103 (2007).
Congjun Wu received his Ph.D. in physics from Stanford University in 2005, and did his postdoctoral research at the Kavli Institute for Theoretical Physics, University of California, Santa Barbara, from 2005 to 2007. He became an Assistant Professor in the Department of Physics at the University of California, San Diego (UCSD) in 2007, an Associate Professor of Physics at UCSD in 2011, and a Professor of Physics at UCSD in 2017. He was elected to be Fellow of American Physical Society in 2018. His research interests are exploring new states of matter and reveling their organizing principles, including quantum magnetism, superconductivity, topological states, mathematical physics, and the numerical method of quantum Monte Carlo simulations.
Venue: TDLI Meeting Room 300
Here is the video link of this talk:
https://vshare.sjtu.edu.cn/open/eae7cba1b18b66c2fff15dc591379dec