The Overhauser Instability

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1 The Overhauser Instability Zoltán Radnai and Richard Needs TCM Group ESDG Talk 14th February 2007 Typeset by FoilTEX

2 Introduction Hartree-Fock theory and Homogeneous Electron Gas Noncollinear spins and Spiral Spin Density Waves The Overhauser Instability Results Interpretation

3 Noncollinear spins Collinear: definite spin (up or down) with respect to global quantization axis. Particles are distinguishable, simplifies calculation. Noncollinear: spin directions that are not parallel to the global quantization axis and the spin direction can vary with position. Wavefunction depends on position and spin coordinates.

4 Spin density or Magnetization density: net magnetic moment due to spin. Vector function of position. Noncollinear eyxample: Spiral Spin Density Wave (magnetization wavevector q)

5 Generalized Hartree-Fock theory HF theory of noncollinear spins. Orbitals have full spin dependence. Can write orbitals as function of space-spin coordinates ψ(x). Can write orbitals as two-component spinors: ψ(r) = ( ψ1 (r) ψ 2 (r) ). Determinant of spinors: Ψ = 1 N! det ψ i (r j ). Total energy is E = Ψ Ĥ Ψ. Evaluate variation with respect to orbitals, subject to orthonormality constraint.

6 Gives single-particle HF equation (depending on form of Ĥ) such as: ( ˆK + Û + ˆV Ĵ)ψ k (r) = ɛ kψ k (r) (1) This is a 2x2 matrix equation. ˆK (kinetic energy), Û (external potential), ˆV (direct term) and Ĵ (exchange term) are 2x2 matrices of spatial operators. For spin independent to noncollinearity. Ĥ, Ĵ can still have off-diagonal components, giving rise Collinear case is special case with ψ(r) = φ(r)χ, where χ is a spin eigenstate. Reduces problem to Unrestricted HF theory. HF equation needs to be solved self-consistently.

7 Overhauser Instability What is ground state of Homogeneous Electron Gas in Hartree-Fock theory? High-density (low r s ) limit: paramagnet. Low-density (high r s ): ferromagnet. Collinear states. Overhauser: paramagnet is never ground state, but instead a Spiral Spin Density Wave is. Proof and analytical solution for 1D with repulsive δ-function interaction. (A. W. Overhauser, Giant spin density waves, Phys. Rev. Lett. 4, p.462 (1960)) Proof of existence for 3D with Coulomb interaction. (A. W. Overhauser, Spin density waves in an electron gas, Phys. Rev. 128, p.1437 (1962))

8 Hartree-Fock theory of Homogeneous Electron Gas Hamiltonian: Ĥ = i i + i U(r i) V could be Coulomb or Ewald interaction. i,j i V (r i, r j ) + λ Single-particle HF equation is self-consistently solved by SSDW orbitals: ( ) ψ k (r) = e ik r cos( 1 2 θ k) e i1 2 q r sin( 1 2 θ k) e +i1 2 q r k is plane-wave vector, q is magnetization wave vector (constant), and the orbital has its spin pointing in (θ, q r) direction (spiral in space). Mixing up spin at k 1 2 q with down spin at k q.

9 Paramagnet and ferromagnet are special cases. They correspond to particular choices of occupation and θ k. Example: paramagnet is two overlapping spheres in k-space:

10 Total energy: Analytical part E = k 1 2 {k } q2 k q cos θ k 1 1 2Ω (ξ is Ewald self-image term) k,k k 4π k k 2 cos 21 2 (θ k θ k )+ 1 2 Nξ (2) Single-particle HF equation: [( ) Kk1 0 0 K k2 ( )] ( Jk1 J ko cos( 1 2 θ k) J ko J k2 sin( 1 2 θ k) ) = ɛ k ( cos( 1 2 θ k) sin( 1 2 θ k) ) (3)

11 Numerical part Aim Find occupation of k-space and form of θ k self-consistently. Find value of q that gives lowest energy. Start by choosing initial occupation of orbitals and θ k. Two schemes: best guess at solution (Overhauser) and paramagnetic occupation with randomized angles. Iterate to self-consistency. Consistency of result: total energy vs. sum of eigenvalues. Numerically tricky, false local minima.

12 Results: Energy vs. System size SDW HF r_s Energy per particle (a.u.) Paramagnet Overhauser Random angles Particle number Energy from best value of q

13 Results: Energy vs. System size SDW HF Extrapolation r_s Energy per particle (a.u.) Ferromagnet Paramagnet Overhauser Random angles /(N^(2/3)) Extrapolating to infinite system size

14 Results: Energy vs. q SDW HF r_s 1, N Energy difference from paramagnet per particle (a.u.) -6e-07-8e-07-1e e e e e q (a.u.) Lowest energy is achieved when q is near 2k F (3.84 a.u.).

15 Results: Energy vs. r s SDW HF N 35114, paramagnet random angles 0 Energy diff from paramagnet (a.u.) -5e r_s Effect gets smaller at higher density, tricky at r s = 1.0.

16 Nature of the instability single double single Occupation in k-space at start Occupation in k-space at end

17 4 Nature of the instability SDW HF Angles N10082 rs 5 q18 ky=0 kz=0 Start Converged 3 Theta kx

18 Nature of the instability Instability is associated with formation of a gap. The spiral spin density is a short-wavelength periodic structure. SDW HF Eigenvalues N10082 rs5 q18 ky=0 kz=0 0 Energy eigenvalue (a.u.) Occupied Unoccupied kx

19 Conclusions Demonstrated the existence of the instability for a range of densities. nature is qualitatively as predicted by Overhauser. Its There is always a Spiral SDW solution that has lower energy than paramagnet. Work in progress: other types of instabilities, densities below the para/ferromagnetic crossover. Work in progress: Quantum Monte Carlo (VMC) calculations using the HF orbitals.

20 Acknowledgments Financial support was provided by the Gates Cambridge Trust and Trinity College. Computational resources were provided by TCM Group.

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