Girsh Blumberg (Rutgers): CaMn2Sb2 (CMS) with buckled hexagonal Mn plane structure: removal of Hund's spin blockade

Blogged by Eoin O'Farrell

The tittle compound is a new material, on which the erstwhile blogger found just 2 papers in the literature; in the spirit of new materials the cover slide showed several single crystals a few mm in dimension. The structure is the 122 structure (made famous by the FeAs superconductors) and the physics is somewhat related to that presented earlier in the week by Christian Haule.

The Mn ion is in the 2+ state and the 3d orbitals are half filled so Hund's rule gives a large spin 5/2 state. The interesting physics is in the MnSb bilayer which can be viewed as a hexagonal bilayer wherein the Mn ion coordinates with 3 Mn ions in the adjacent layer. Dr. Blumberg explained that the reason to consider this material as strongly correlated is the difference between the bare bandwidth and that measured from Raman is about 2 orders of magnitude.

The behavior of CMS is as follows: at 300K the material is a paramagnetic metal, at 200K CMS becomes weakly ferromagnetic and the resistivity increases following a standard activated behavior and then at ~100K orders AFM. Finally at low temperature the resistivity has increased by about 6 orders of magnitude. The main experimental exhibit is Raman spectroscopy and the explanation of this by hopping in the AFM state, this was used to explain the difference between the optical gap (~1eV) and the transport gap (40meV).

Hybridization between the Mn-Sb produces a finite (~10%) occupancy of 3d6 states on the Mn site, so that adjacent Mn sites are like so:

This gives rise to a flat band with bandwidth 10meV, but  Dr. Blumberg stuck to a localized, hopping picture in his explanation.

The key to resolving the discrepancy between the optical gap and the transport gap was the binding of pairs of polarons by the Heisenberg interaction. This gives rise to the possibility of certain photon assisted or activated hopping mechanisms which change the component Sz=5/2 -> 2 and reduce the transport gap below the optical gap.

This also explained why DMFT calculations did not capture the full renormalization of the bandwidth.

Questions:

1.

Andrey Chubukov: Where in the Raman spectrum is the 2 magnon peak.

Answer: We think all the features of the Raman spectrum can be explained, but not due to a 2 magnon peak

2.

Unidentified interrogator: How large is J in the Heisenberg interaction?

Answer: It's about 7meV

3.

Piers Coleman: Has the Fe version of this material been made?

Answer: For the pnictides there is a similar [unspecified - ed]  material, but it is insulating, The Fe compounds are expected to be less strongly interacting

4.

Hide Takagi: Please say more about the FM phase?

Answer: In the FM phase nearest neighbor interactions are still AFM but there is no phase rigidity. [Note this should be consistent with the resistivity which showed no significant change between at the AFM/FM phase transition.]

5.

Natalia Perkins (chair)

Why is the renormalization so large?

Answer: DMFT finds some renormalization from the bare LDA, the remaining renormalization comes from strong correlations.