Spin stiffness

Mathematically it can be defined by the following equation:

${\displaystyle \rho _{s}={\cfrac {\partial ^{2}}{\partial \theta ^{2}}}{\cfrac {E_{0}(\theta )}{N}}|_{\theta =0}}$

where ${\displaystyle E_{0}}$ is the ground state energy, ${\displaystyle \theta }$ is the twisting angle, and N is the number of lattice sites.

Start off with the simple Heisenberg spin Hamiltonian:

${\displaystyle H_{\mathrm {Heisenberg} }=-J\sum _{<i,j>}\left[S_{i}^{z}S_{j}^{z}+{\cfrac {1}{2}}(S_{i}^{+}S_{j}^{-}+S_{i}^{-}S_{j}^{+})\right]}$

Now we introduce a rotation in the system at site i by an angle θ_i around the z-axis:

${\displaystyle S_{i}^{+}\longrightarrow S_{i}^{+}e^{i\theta _{i}}}$

${\displaystyle S_{i}^{-}\longrightarrow S_{i}^{-}e^{-i\theta _{i}}}$

Plugging these back into the Heisenberg Hamiltonian:

${\displaystyle H(\theta _{ij})=-J\sum _{<i,j>}\left[S_{i}^{z}S_{j}^{z}+{\cfrac {1}{2}}(S_{i}^{+}e^{i\theta _{i}}S_{j}^{-}e^{-i\theta _{j}}+S_{i}^{-}e^{-i\theta _{i}}S_{j}^{+}e^{i\theta _{j}})\right]}$

now let θ_ij = θ_i - θ_j and expand around θ_ij = 0 via a MacLaurin expansion only keeping terms up to second order in θ_ij

${\displaystyle H\approx H_{\mathrm {Heisenberg} }-J\sum _{<ij>}\left[\theta _{ij}J_{ij}^{(s)}-{\cfrac {1}{2}}\theta _{ij}^{2}T_{ij}^{(s)}\right]}$

where the first term is independent of θ and the second term is a perturbation for small θ.

${\displaystyle J_{ij}^{s}={\cfrac {i}{2}}(S_{i}^{+}S_{j}^{-}-S_{i}^{-}S_{j}^{+})}$ is the z-component of the spin current operator

${\displaystyle T_{ij}={\cfrac {1}{2}}(S_{i}^{+}S_{j}^{-}+S_{i}^{-}S_{j}^{+})}$ is the "spin kinetic energy"

Consider now the case of identical twists, θ_x only that exist along nearest neighbor bonds along the x-axis Then since the spin stiffness is related to the difference in the ground state energy by

${\displaystyle E(\theta )-E(0)=N\rho _{s}\theta _{x}^{2}}$

then for small θ_x and with the help of second order perturbation theory we get:

${\displaystyle \rho _{s}={\cfrac {1}{N}}\left[{\cfrac {1}{2}}\langle T_{x}\rangle +\sum _{\nu \neq 0}{\cfrac {|\langle 0|j_{x}^{(s)}|\nu \rangle |^{2}}{E_{\nu }-E_{0}}}\right]}$

• Spin wave

• S.E. Krüger; R. Darradi; J. Richter; D.J.J. Farnell (2006). "Direct calculation of the spin stiffness of the spin-(1/2) Heisenberg antiferromagnet on square, triangular, and cubic lattices using the coupled-cluster method". Physical Review B. 73 (9): 094404. arXiv:cond-mat/0601691. Bibcode:2006PhRvB..73i4404K. doi:10.1103/PhysRevB.73.094404.
• J. Bonča; J.P. Rodriguez; J. Ferrer; K.S. Bedell (1994). "Direct calculation of spin stiffness for spin-1/2 Heisenberg models". Physical Review B. 50 (5): 3415–3418. arXiv:cond-mat/9405069. Bibcode:1994PhRvB..50.3415B. doi:10.1103/PhysRevB.50.3415. PMID 9976600. S2CID 32495059.
• T. Einarsson; H.J. Schulz (1994). "Direct Calculation of the Spin Stiffness in the J₁−J₂ Heisenberg Antiferromagnet". Physical Review B. 51 (9): 6151–6154. arXiv:cond-mat/9410090v1. Bibcode:1995PhRvB..51.6151E. doi:10.1103/PhysRevB.51.6151. PMID 9979543. S2CID 22218061.
• B.S. Shastry; B. Sutherland (1990). "Twisted boundary conditions and effective mass in Heisenberg–Ising and Hubbard rings". Physical Review Letters. 65 (2): 243–246. Bibcode:1990PhRvL..65..243S. doi:10.1103/PhysRevLett.65.243. PMID 10042589.
• R.R.P. Singh; D.A. Huse (1989). "Microscopic calculation of the spin-stiffness constant for the spin-(1/2) square-lattice Heisenberg antiferromagnet". Physical Review B. 40 (10): 7247–7251. Bibcode:1989PhRvB..40.7247S. doi:10.1103/PhysRevB.40.7247. PMID 9991112.
• R. G. Melko, A. W. Sandvik, and D. J. Scalapino1 (2004). "Two-dimensional quantum XY model with ring exchange and external field". Physical Review B. 69 (10): 100408–100412. arXiv:cond-mat/0311080. Bibcode:2004PhRvB..69j0408M. doi:10.1103/PhysRevB.69.100408. S2CID 119491422.CS1 maint: multiple names: authors list (link)