Nanoscale Electronic Phase Separation Driven by Fe-site Ordering in Fe\textsubscript{5-x}GeTe\textsubscript{2}

Abstract

Understanding how local structural order governs electronic correlations is essential for revealing the microscopic mechanism underlying emergent behavior in two-dimensional magnets. In the layered van der Waals ferromagnet Fe\textsubscript{5-x}GeTe\textsubscript{2}, intrinsic Fe-site disorder provides a natural platform to probe this interplay. Here, we establish a direct atomic scale correlation between Fe-site ordering and local electronic structure by combining high-resolution scanning tunneling microscopy with density functional theory calculations. Scanning tunneling microscopy resolves two coexisting surface phases, a $\sqrt{3} \times \sqrt{3}$ superstructure associated with ordered Fe(1) configurations and an undistorted $1 \times 1$ hexagonal Te lattice in Fe(1)-deficient regions. Spatially resolved spectroscopy shows that the $\sqrt{3}$-ordered domains exhibit metallic behavior, whereas Fe(1) vacant areas display a suppressed density of states(DOS) near the Fermi level, indicative of pseudogapped electronic states. The nanoscale coexistence of these distinct electronic responses provides direct evidence of electronic phase separation driven by Fe-site ordering. First-principles calculations reveal that symmetry allowed hybridization between Fe 3d and Te 5p orbitals reconstructs the low-energy electronic structure, giving rise to the contrasting tunneling signatures of ordered and disordered phases. Bias-dependent local DOS simulations reproduce the experimentally observed contrast evolution and reveal that hybridization induced out of plane orbital character governs the spatial modulation of tunneling conductance. These results provide a microscopic framework linking atomic-scale structural order to nanoscale electronic inhomogeneity in van der Waals magnets.

Figures (8)

• Figure 1: (a) Side view of the crystal structure of Fe5–xGeTe2. The rectangle outlines the unit cell, and the positions of Fe, Ge, and Te atoms at their respective sites are indicated. (b) Representative x-ray diffraction pattern collected from a crystal facet with the c-axis oriented normal to the surface, (inset) an optical image of the corresponding single crystal. (c–e) Magnetic characterization of bulk Fe5–xGeTe2 single crystals. (c) Temperature dependence of magnetization under zero-field cooling for in-plane ($H \parallel ab$, Mab) and out-of-plane ($H \parallel c$, Mc) applied fields of 500 Oe. The blue arrows mark transition temperatures 90K, 180K and 250K. (d) M–H curves measured with $H \parallel ab$ at five temperatures from 40K to room temperature, including the transition temperatures. (e) M–H curves measured with $H \parallel c$ at five temperatures from 4K to 300K.
• Figure 2: (a) STM topograph (U = -200 mV, I = 190 pA) of the Te-terminated surface of Fe5–xGeTe2, highlighting two regions exhibiting distinct structural orderings. (b) Crystal structure of Fe5–xGeTe2 portraying possible split-site occupancies of Fe(1) and Ge atoms. (c) STM topograph (10 nm $\times$ 10 nm) (U = -200 mV, I = 190 pA) acquired from region I (marked by blue square) in panel (a). (d) Corresponding fast Fourier transform (FFT) image, the peaks marked by blue and red circles represent the 1 × 1 hexagonal lattice and the $\sqrt{3} \times \sqrt{3}$ superstructure, respectively. (e) High-resolution (U = -200 mV, I = 190 pA) STM scan (5 nm $\times$ 5 nm) acquired from region II (marked by green square) in panel (a) and (f) its corresponding FFT image showing only the hexagonal lattice.
• Figure 3: Schematic representation and STM topographs of the two $\sqrt{3} \times \sqrt{3}$ superstructure phases. (a) Side view and (b) top view atomic model of the UDD configuration. The red rhombus marks the unit cell, while the blue triangle highlights Te trimers. (c) STM topograph (5 nm × 5 nm) of the UDD phase (U = -200 mV, I = 250 pA), (inset) atoms within the hexagonal lattice superimposed for clarity. (d) Side view and (e) top view atomic model of the DUU configuration. (f) STM topograph (5 nm × 5 nm) of the DUU phase (U = -200 mV, I = 250 pA). Half-red, full-red, blue, and half-grey spheres denote Fe(1), Fe, Te, and Ge atoms, respectively.
• Figure 4: Simulated STM images obtained from density functional theory calculations, plotted at a fixed isosurface value and overlaid with the corresponding ball model to indicate the atomic positions. (a) Relaxed UDD structure and (b) relaxed undistorted hexagonal structure, shown as top views along the $z$ direction. Bright regions correspond to enhanced local density of states (LDOS) at the selected bias, while darker regions indicate reduced LDOS. Blue, red, and green spheres denote Te, Fe, and Ge atoms, respectively. The crystallographic axes are indicated in panel (b).
• Figure 5: (a) STM topograph (10 nm × 10 nm) of a region with coexisting $\sqrt{3}a \times \sqrt{3}a$ ordering and the undistorted $1a \times 1a$ phase (U = -200 mV, I = 190 pA). (b) Representative differential conductance spectra acquired on the $\sqrt{3}a \times \sqrt{3}a$ (red) and $1a \times 1a$ (black) regions; the spectroscopic measurement points are indicated in Figure \ref{['Figure5']}a by red and black triangles, respectively.