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Sign Problem Free Monte Carlo Simulation of
Itinerant Ferromagnetism
Shenglong Xu (UCSD)Yi Li (Princeton)
Congjun Wu (UCSD)
2014 CCCQS Workshop Évora
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Mean Field Argument
Stoner Criterion:
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Lieb-Mattis (1962): No 1D FM
Nagaoka FM(1966): Single hole+Infinite U
Mielke 1991: Flat-band FM Divergent DOS
Non Perturbative result
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Open question on Itinerant FM
Non-perturbative treatment on finite temperature is lacking
Stoner mean field + RPA calculation does not lead to Curie- Weiss behavior of spin susceptibility
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Y. Li, E. Lieb, C. Wu Phys. Rev. Lett. 112, 217201 2014
Exact Results for Itinerant Ferromagnetism in Multi-orbital Systems on Square and Cubic Lattices
Fully polarized ground state at strong coupling limit
Generic filling (0<n<2)
Itinerant system
No-node ground state
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Model: P orbital Hubbard model
Directional Hopping
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Sign Problem Free Monte Carlo
The off diagonal elements of the Hamiltonian is negative/zero in a suitable basis1 2 3 4
1
2
3
4
Hopping:
Hund’s Coupling:
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Stochastic Series Expansion + Directed loop
OF Syljuåsen, AW Sandvik Physical Review E, 2002
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Result: V=0, t=1, J=2
Compressibility satures to a finite value when T approaches 0, indicating metallic phases
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The ground state is fully polarized state
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Result: Large V (V=8)
Gapped excitation at quarter filling
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FM energy scale: T0 (V=0)
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Antiferromagnetic orbital order at quarter filling
FM energy scale: T0 (V=8)
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The effective moment
Spin ½ local moment
Spin 1 local moment
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Low temperature
D. P. Arovas and A. Auerbach, Phys. Rev. B 38, 316(1988)
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Summary
Hamiltonian with fully polarized ground state
Local moment like spin susceptibility
Metallic compressibility (V=0)Antiferromagnetic orbital mott insulator (quarter filling and strong V)
Spin susceptibility crosses over to exponential growth at low temperature