Understanding the color properties of ${\left({\mathrm{C}}_5{\mathrm{H}}_9\mathrm{N}{\mathrm{H}}_3\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ in high magnetic fields

We report the optical and magneto-optical properties of ${\left({\mathrm{C}}_5{\mathrm{H}}_9\mathrm{N}{\mathrm{H}}_3\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$, an $S=1∕2$ low-dimensional molecular antiferromagnet. Complementary electronic structure calculations demonstrate that the color properties of this material are dominated by excitations of the $\mathrm{Cu}{\mathrm{Br}}_4^{2-}$ chromophore. In an applied magnetic field, ${\left({\mathrm{C}}_5{\mathrm{H}}_9\mathrm{N}{\mathrm{H}}_3\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ displays a rich magnetochromic response that is particularly strong in the direction of the bromide-bromide bridge pairs between the chains. This effect is attributed to a field-induced reorientation of the $\mathrm{Cu}{\mathrm{Br}}_4^{2-}$ anions. These color changes are correlated with critical fields in the magnetization.

Figure 1

The ladderlike molecular structure of ${\left(\mathrm{CPA}\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ in the ab plane at $300\mathrm{K}$ (a) and $200\mathrm{K}$ (b). For clarity, the organic cations are not shown.

Figure 2

Variable temperature reflectance (a) and optical conductivity (b) of ${\left(\mathrm{CPA}\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ along the $a$ direction, corresponding to the response mainly along the double bromide-bromide bridge pairs. Data were collected at $80\mathrm{K}$ (solid line), $250\mathrm{K}$ (dashed line), $270\mathrm{K}$ (dotted line), and $300\mathrm{K}$ (dotted-dashed line). The insets in both figures focus on the phonon modes of ${\left(\mathrm{CPA}\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ at $300\mathrm{K}$.

Figure 3

Predicted optical excitations from filled $p$- and $d$-block energy levels to the singly occupied $6A_1$ level of the $\mathrm{Cu}{\mathrm{Br}}_4^{2-}$ chromophore along the (a) $b$ axis, (b) $a$ axis, and (c) $c$ axis, corresponding to the directions of $x$, $y$, and $z$, respectively. Predicted excitations energies (in eV) are shown. Relevant orbitals in the $\mathrm{Cu}{\mathrm{Br}}_4$ chromophore are also diagrammed.

Figure 4

The normalized magneto-optical response $R\left(H\right)∕R\left(0\mathrm{T}\right)$, (a) of ${\left(\mathrm{CPA}\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ along the $a$ direction (bromide-bromide bridge pairs) at $4.2\mathrm{K}$. The field-dependent integrated area (b) of the features between $2.00–2.6\mathrm{eV}$ (square) and that centered at $2.75\mathrm{eV}$ (circle). Changes in slope and inflection points in the data are denoted by vertical dashed lines.

Figure 5

The normalized magneto-optical response $R\left(H\right)∕R\left(0\mathrm{T}\right)$, (a) of ${\left(\mathrm{CPA}\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ along the single bromide-bromide chains ($b$ direction) at $4.2\mathrm{K}$. For viewing purposes, the energy axis is smaller than that in Fig. 4. The field-dependent integrated area (b) of the features centered at $1.25\mathrm{eV}$ (square), $1.60\mathrm{eV}$ (circle), $1.80\mathrm{eV}$ (triangle), and $2.00\mathrm{eV}$ (diamond). Changes in slope and inflection points in the data are denoted by vertical dashed lines.

Figure 6

The field-dependent molar magnetization $M$ of ${\left(\mathrm{CPA}\right)}_2\mathrm{Cu}{\mathrm{Br}}_4$ (solid line) and derivative of the magnetization with respect to the field $dM∕dH$ (dashed line) at $0.74\mathrm{K}$ (Ref. 63). Magnetization shows a clear change at $20\mathrm{T}$ and an estimated change near $2\mathrm{T}$ (due to thermal occupation of the excited states that smear the magnetization) that are assigned as upper and lower critical fields, respectively. The magnetization saturates above $H_{c2}$ (Ref. 63). Nacent features are also found at intermediate fields. The latter appear as inflection points in the $dM∕dH$ data.