Magneto-optical properties and charge-spin coupling in the molecular ${\left(2,3\text{-dmpyH}\right)}_2{\text{CuBr}}_4$ spin-ladder material

We investigated the magnetic and optical properties of ${\left(2,3\text{-dmpyH}\right)}_2{\text{CuBr}}_4$, an unusual antiferromagnetic quantum spin ladder with strong rail interactions that can be saturated in a 30 T field. This transition drives a field-induced color change, a dramatic example of charge-spin coupling in a molecular material. Spin-density calculations reveal that electronic structure is sensitive to the magnetic state because magnetic orbital character depends on the spin arrangement between adjacent ${\text{CuBr}}_4^{2-}$ chromophores. This finding suggests that molecular architecture and the magnetic arrangement between molecular units might be used to control magnetochromism.

Figure 1

(Color online) Magnetization of ${\left(2,3\text{-dmpyH}\right)}_2{\text{CuBr}}_4$ at 4.0 K, 1.6 K, and 480 mK for $H\parallel b$. The 480 mK data show $H_{c1}$ and $H_{c2}$ most clearly. Upper inset: closeup view of $H_{c1}$. Lower inset: 300 K structure of ${\left(2,3\text{-dmpyH}\right)}_2{\text{CuBr}}_4$ (Ref. 6). The organic counterions are omitted for clarity.

Figure 2

(Color online) (a) Absorption of ${\left(2,3\text{-dmpyH}\right)}_2{\text{CuBr}}_4$ in single-crystal and isotropic pellet forms. Inset: closeup view of single-crystal data showing the polarization dependence. The black dashed arrow indicates the range of our magneto-optical investigation. (b) Schematic view of the $d$ manifold excitations of ${\text{CuBr}}_4^{2-}$ from Ref. 17. (c) Low-temperature magneto-optical response at 0, 20, 23, 27, 35, and 53 T. Inset: closeup view of the absolute value of the absorption difference spectra at the same fields (20, 23, 27, 35, and 53 T).

Figure 3

(Color online) (a) Low-temperature magnetization of ${\left(2,3\text{-dmpyH}\right)}_2{\text{CuBr}}_4$ compared with the magneto-optical contrast. (b) Spin-density distribution difference $\Delta \rho ={\rho }_{\text{AFM}}-{\rho }_{\text{FM}}$, where ${\rho }_{\text{AFM}}$ and ${\rho }_{\text{FM}}$ are the calculated spin-density distributions of the AFM and FM states of ${\left(2,3\text{-dmpyH}\right)}_2{\text{CuBr}}_4$. The region of blue (darker) [red (lighter)] color indicates that the FM state has a higher (lower) density than the AFM state.