Nearly Room-Temperature Ferromagnetism in a Pressure-Induced Correlated Metallic State of the van der Waals Insulator ${\mathrm{CrGeTe}}_3$

A complex interplay of different energy scales involving Coulomb repulsion, spin-orbit coupling, and Hund’s coupling energy in 2D van der Waals (vdW) material produces a novel emerging physical state. For instance, ferromagnetism in vdW charge transfer insulator ${\mathrm{CrGeTe}}_3$ provides a promising platform to simultaneously manipulate the magnetic and electrical properties for potential device implementation using few nanometers thick materials. Here, we show a continuous tuning of magnetic and electrical properties of a ${\mathrm{CrGeTe}}_3$ single crystal using pressure. With application of pressure, ${\mathrm{CrGeTe}}_3$ transforms from a ferromagnetic insulator with Curie temperature $T_C\sim 66\mathrm{K}$ at ambient condition to a correlated 2D Fermi metal with $T_C$ exceeding $\sim 250\mathrm{K}$. Notably, absence of an accompanying structural distortion across the insulator-metal transition (IMT) suggests that the pressure induced modification of electronic ground states is driven by electronic correlation furnishing a rare example of bandwidth-controlled IMT in a vdW material.

Figure 1

(a) A single layer of ${\mathrm{CrGeTe}}_3$. In $\mathrm{Cr}{X}_3$, the place of Ge-Ge dimers remains vacant. (b) Crystalline electric field splitting of Cr $3d$ orbitals into $t_{2g}$ manifolds and $e_g$ manifolds. $\mathrm{\Delta }$ is the energy difference between $t_{2g}$ levels and $e_g$ levels. $J_H^{\mathrm{Te}}$ is the Hund’s coupling energy at the Te site. (c) Schematic picture of FM superexchange interaction between Cr $e_g$ orbitals via two Te $p$ orbitals. $t_{pd}(t_{p^{\prime }{d}^{\prime }})$ is the virtual hopping between $e_g(e_g^{\prime })$ and $p(p^{\prime })$ orbitals. $\theta$ is the geometrical $\mathrm{Cr}─\mathrm{Te}─\mathrm{Cr}$ bond angle. (d) In plane resistivity, ${\rho }_{ab}$. (e) Zero field-cooled (ZFC) magnetic susceptibility $\chi$ at $H=0.1\mathrm{T}$ applied along $c$ axis. Inset: temperature derivative of $d\chi /dT$.

Figure 2

(a) Top: ZFC dc susceptibility of ${\mathrm{CrGeTe}}_3$ crystal $S1$ up to 1.73 GPa. Bottom: temperature dependence of $d\chi /dT$. Field-cooled (FC) susceptibility for crystal (b) $S2$ and (c) $S3$. Field dependence of magnetization, $M(H)$, for (d) $S2$ and (e) $S3$ at $T=1.8\mathrm{K}$ under different pressure. Red arrows represent the direction of increasing pressure. (f) Curie-Weiss plot, $1/\chi$ vs $T$, of crystal $S3$ at some selected pressure.

Figure 3

(a) In plane resistivity ${\rho }_{ab}$ of ${\mathrm{CrGeTe}}_3$ at several pressures ranging from 0 to 11.0 GPa. The magenta dotted line represents an estimate of the Mott-Ioffe-Regel limit of resistivity, ${\rho }_M^{2\mathrm{D}}$. (b) ${\rho }_{ab}$ in logarithmic temperature scale. Red solid lines are $-\log (T)$ fit to ${\rho }_{ab}$. (c) $\mathrm{\Delta }{\rho }_{ab}=({\rho }_{ab}-{\rho }_0)$ vs $T^2$ for $P\ge 7.5\mathrm{GPa}$, where ${\rho }_0$ is the residual resistivity. The black line is a linear fit to $\mathrm{\Delta }{\rho }_{ab}$ at 7.5 GPa confirming a Fermi liquid state below $T_{\mathrm{FL}}$. (d) Out of plane resistivity ${\rho }_c$ and (e) resistivity anisotropy ${\rho }_c/{\rho }_{ab}$ at some selected pressure.

Figure 4

(a) Pressure-temperature phase diagram of ${\mathrm{CrGeTe}}_3$. Color scale represents the magnitude of ${\rho }_{ab}$. ${\theta }_{\mathrm{CW}}$ and $T_C^{\chi }$ are estimated from magnetic susceptibility. $T_C^{\rho }$ and $T_{\mathrm{FL}}$ are determined from ${\rho }_{ab}$. (b) Pressure dependence of effective moment, ${\mu }_{\mathrm{eff}}$, and saturation magnetic moment $M_S$ (at $T=1.8\mathrm{K}$). (c) and (d) are pressure dependence of ${\rho }_c$ and ${\rho }_{ab}$ at $T=10\mathrm{K}$, respectively.