Spin-lattice and spin-electronic interactions in the van der Waals semiconductor $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$

We report the results of a temperature-dependent reflection spectroscopy study, across a wide energy range ($\sim 0.01$ to 3 eV), of unsubstituted and 40% Zn substituted for Co, for the transition-metal phosphorus trichalcogenide, $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. We observe a transition from a paramagnetic to antiferromagnetic state with a Néel temperature at $T_{\mathrm{N}}\sim 120$ K that is completely suppressed in Zn-substituted samples. At 300 K we identify four narrow ($\sim 1$ meV) infrared active phonon modes while at 70 K (below $T_{\mathrm{N}}$) we observe that the low-energy phonons, dominated by Co motion, resolve into two modes. These low-energy modes are asymmetric, indicating coupling to a broad electronic continuum. We also report a broad ($\sim 30$ meV) low-temperature infrared absorption band that appears near $T_{\mathrm{N}}$ that we suggest is determined by a multiphonon-assisted 2-magnon absorption process. At 300 K in higher-energy spectra, we observe considerable absorption starting from $0.20\pm 0.02$ eV which we associate with inter-$\mathrm{C}{\mathrm{o}}^{2+}$ ion $3d$ transitions. At temperatures below $T_{\mathrm{N}}$ the number of electronic absorption bands increases from 4 to 6, indicating a lowering of the symmetry around the $\mathrm{C}{\mathrm{o}}^{2+}$ ions. On substituting 40% Zn for Co, the antiferromagnetic transition is suppressed along with temperature-dependent changes in the phonon and electronic spectra. The temperature-dependent spectral changes indicate strongly correlated behavior between the infrared active lattice vibrations, the electronic excitations, and magnetism in unsubstituted $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$.

Figure 1

The crystal structure of $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ viewed along the $a$ axis (top), illustrating the 2D layers separated by a van der Waals gap, and along the $c$ axis (bottom) illustrating the Co honeycomb lattice. Each Co atom is surrounded by six S atoms with the P dimers filling every third Co position and thereby sitting at the center of a Co honeycomb lattice. Note that each P atom of the dimer is connected to only three S atoms, thus with the dimer forming a ${\mathrm{P}}_2{\mathrm{S}}_6$ unit. The rectangles indicate a unit cell. The structure is based on published atomic positions [49] and was generated using the mercury software package from the Cambridge Crystallographic Data Centre (CCDC).

Figure 2

(a) Out-of-plane θ-2θ x-ray diffractograms for a crystal of $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ at 300 K. (b) Temperature dependence of the $c$-axis lattice parameter $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. XRD data collected with Cu Kα radiation (${\lambda }_{\mathrm{K}\alpha 1}=0.154056$ nm).

Figure 3

(a) The temperature-dependent magnetic susceptibility of $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ (blue) and 40% Zn substituted for Co (red), when the field is orientated perpendicular to the $ab$ plane. $T_{\mathrm{N}}$ for $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ is indicated at 120 K by the vertical gray dashed line illustrating the lack of a transition in the 40% Zn-substituted crystal. The small anomaly near 45 K is due to oxygen contamination. (b) The temperature dependence of the heat capacity for $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. The dotted line is an estimate of the lattice contribution highlighting the antiferromagnetic contribution at the transition temperature of $\sim 120$ K as indicated by the vertical gray dashed line.

Figure 4

The reflectivity of $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystals (black dots) for a temperature (a) above $T_{\mathrm{N}}$ at 230 K, and (b) a temperature below $T_{\mathrm{N}}$ at 70 K. The best-fit calculated reflectivity, based on the dielectric function described in the text, is also illustrated (red line).

Figure 5

(a) Comparing the energy-dependent conductivity, ${\sigma }_1\left(\omega \right)$, for $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ at 230 K (red) and 70 K (blue). There is an approximate 40% loss in oscillator strength in the $569\text{−}\mathrm{c}{\mathrm{m}}^{-1}$ mode. The inset highlights the low-energy region to emphasize the splitting and energy shifts of the low-energy modes below the transition temperature, $T_{\mathrm{N}}$. (b) The temperature dependence of the relative spectral weight of the $569\text{−}\mathrm{c}{\mathrm{m}}^{-1}$ (blue) and the broad mid-IR band (red). The uncertainty in the spectral weight of the $569\text{−}\mathrm{c}{\mathrm{m}}^{-1}$ mode is $\sim 20\%$. We have also included the relative temperature dependence of the (0,1,0) neutron-scattering Bragg peak (black) where the authors note that the error bars are smaller than the symbols [49].

Figure 6

(a) The 70 K reflectivity of 40% Zn substituted for Co in $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ (black) along with a model (red) discussed in the text. The weak oscillation in $R$ between about 300 and $500\mathrm{c}{\mathrm{m}}^{-1}$ we assign to interference fringes in the top layers of the exfoliated crystals. (b) The 70 K energy-dependent conductivity, ${\sigma }_1\left(\omega \right)$, of 40% Zn-substituted $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$.

Figure 7

(a) The temperature dependence of the vibrational modes in unsubstituted $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. The Néel temperature is indicated by the vertical dotted-dashed line at 120 K and the red points emphasize the impact of the magnetic transition on the phonons. (b) The temperature dependence of the phonons in 40% Zn-substituted $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ illustrating the lack of any change at the magnetic transition.

Figure 8

(a) The high-energy energy-dependent conductivity at 300 K (red) and 72 K (blue) for $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ illustrating the change in its electronic behavior in passing through $T_{\mathrm{N}}$. (b) The energy-dependent conductivity for $\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ with 40% Co substitute by Zn at 300 and 76 K.