Terahertz control of surface topology probed with subatomic resolution

Abstract

Light-induced phase transitions offer a method to dynamically modulate topological states in bulk complex materials. Yet, next-generation devices demand nanoscale architectures with contact resistances near the quantum limit and precise control over local electronic properties. The layered material WTe$_2$ has gained attention as a likely Weyl semimetal, with topologically protected linear electronic band crossings hosting massless chiral fermions. Here, we demonstrate a topological phase transition facilitated by light-induced shear motion of a single atomic layer at the surface of bulk WTe$_2$, thereby opening the door to nanoscale device concepts. Ultrafast terahertz fields enhanced at the apex of an atomically sharp tip resonantly couple to the key interlayer shear mode of WTe$_2$ via a ferroelectric dipole at the interface, inducing a structural phase transition at the surface to a metastable state. Subatomically resolved differential imaging, combined with hybrid-level density functional theory, reveals a shift of 7 $\pm$ 3 picometres in the top atomic plane. Tunnelling spectroscopy links electronic changes across the phase transition with the electron and hole pockets in the band structure, suggesting a reversible, light-induced annihilation of the topologically-protected Fermi arc surface states in the top atomic layer.

Figures (17)

• Figure 1: Figure 1 $\mid$ Shear motion in WTe2 driven by tip-enhanced terahertz fields.a, Unit cells for the orthorhombic Td phase (blue and black spheres) and monoclinic 1T' phase (red and white spheres). The solid black line highlights the $\approx$4 tilt of the unit cell for 1T' with respect to Td (dashed black line). b, The terahertz-pulse-train incident on the scanning tunnelling microscopy (STM) tip is modulated at $f_\mathrm{THz}$ and the terahertz-pulse-induced contribution to the total tunnel current ($I_\mathrm{STM}$) is isolated with a lock-in amplifier, producing both in-phase ($I_\mathrm{X}$) and out-of-phase ($I_\mathrm{Y}$) components. A steady-state bias ($V_\mathrm{d.c.}$) continuously tunnels electrons at the tip apex, while a terahertz pulse with peak amplitude $E_\mathrm{THz,pk}$ incident on the tip (red curve), drives a phase transition at the tip apex and/or tunnels electrons between the tip and sample. c, Terahertz time-domain spectroscopy (THz-TDS) of the tunnel junctionJelic_atomicTHzTDS_2024 shows resonances at 0.26THz, 0.60THz, 1.46THz and 2.24THz (dashed black lines) in both the spectral amplitude (top, purple points) and phase (orange points). The THz-TDS shown here utilizes a reference waveform acquired on a gold surfaceJelic_atomicTHzTDS_2024 (Extended Data Fig. \ref{['fig:ext-validation']}), $E_\mathrm{Au}(A_\mathrm{Au},\varphi_\mathrm{Au}) = \mathfrak{F}\{ E_\mathrm{Au}(t) \}$, with the same tip apex and incident terahertz alignment as the experiments on WTe2. See Methods for parameters during waveform acquisition. The data is shown as mean values $\pm$ standard deviation of five individual scans. Bottom: schematic of the shear mode associated with a phase transition in the topmost layer of WTe2, occurring at a frequency of 0.26THz (arrow lengths are exaggerated for clarity). d, STM topography scans acquired at $V_\mathrm{d.c.} = \qty{10}{mV}$ and $I_\mathrm{d.c.} = \qty{100}{pA}$ with incident $E_\mathrm{THz,pk}$ at 13V/cm (top), 18V/cm (top middle), 23V/cm (middle), 32V/cm (bottom middle) and 36V/cm (bottom). Colourmap range [5,35] pm; image size $\qty{10}{nm} \times \qty{2}{nm}$; scan speed 4.3nm/s. e, Horizontal cross-sections of the respective topography scans in d (dashed green line). The cross-sections in e are vertically offset for clarity.
• Figure 1: Extended Data Figure 1 $\mid$ Phonon dispersion of Td-WTe2 calculated using DFT with the hybrid PBE0 functional. Calculated phonon band structure of a two-layer slab along the $\Gamma-$X$-$S$-$Y$-\Gamma$ path. See Extended Data Fig. \ref{['fig:ext-banddiagrams']}a for a schematic of the Brillouin zone.
• Figure 1: Supplementary Figure 1 $\mid$ Picometre resolution tunnelling microscopy of a terahertz-driven phase transition in WTe2.a, Out-of-phase THz-STM image, $I_\mathrm{Y}(x,y)$, acquired at $V\textsubscript{d.c.}$ = 10 mV, $I\textsubscript{d.c.}$ = 100 pA and $V\textsubscript{THz,pk}$ = $-$1.7 V. Scan size 6 nm $\times$ 8 nm; scan speed 4.3 nm/s. b, Perspective view of $I_\mathrm{Y}(x,y)$ from Fig. \ref{['fig:fig3']}b overlaid onto a 3D texture generated by the simultaneously acquired $I\textsubscript{d.c.}(x,\,y)$, where $I\textsubscript{d.c.}$ spans 100 pA to 220 pA. c, Picometre-scale image from Fig. \ref{['fig:fig3']}d. The colourmap in c spans [$-1,\,+1$] pA. The white box in a shows where b was acquired, while the grey box in b shows where c was acquired.
• Figure 2: Figure 2 $\mid$ Differential atomic imaging of a terahertz-field-driven phase transition.a, Schematic showing the different interaction regimes of WTe2 within a THz-STM tunnel junction while an optical chopper modulates the incident terahertz-pulse-train (top). Experimental oscilloscope traces are shown for the three interaction regimes. Regime I: $E_\mathrm{THz,pk}$ does not drive any measurable effects as the tip-enhanced terahertz electric field strength lies below the phase transition onset ($E_\mathrm{PT}$). Regime II: $E_\mathrm{THz,pk}$ minimally drives tunnelling but is sufficient to induce a phase transition between Td-WTe2 and 1T'-WTe2, which can be detected via the 90 out-of-phase component of the modulated tunnel current ($I_\mathrm{Y} \neq 0$) when a small d.c. bias is also present. Regime III: terahertz-driven tunnelling becomes dominant, while the differential current, $I_\mathrm{Y}$, between Td-WTe2 and 1T'-WTe2 subsides, since a metastable state emerges as the primary phase at the surface near the tip apex. b, $I_\mathrm{THz}$-$V_\mathrm{THz,pk}$ curve on the WTe2 surface acquired at $V_\mathrm{d.c.} = \qty{-10}{mV}$ and $z = z_0$ with the tip height, $z_0$, set by $V_0 = \qty{10}{mV}$, $I_0 = \qty{100}{pA}$. The solid grey line shows the in-phase component ($I_\mathrm{X}$), while the solid black line shows the out-of-phase component ($I_\mathrm{Y}$). c, THz-STM images of a $\qty{5}{nm} \times \qty{5}{nm}$ area showing both $I_\mathrm{X}(x,y)$ and $I_\mathrm{Y}(x,y)$ for a WTe2 surface defect acquired at $V_\mathrm{d.c.} = \qty{-5.5}{mV}$ (left), $V_\mathrm{d.c.} = \qty{0}{V}$ (middle) and $V_\mathrm{d.c.} = \qty{+5.5}{mV}$ (right). The left and right image pairs were acquired at $I_\mathrm{d.c.} = \qty{100}{pA}$ and $V_\mathrm{THz,pk} = \qty{-3.3}{V}$, while the middle image pair was acquired at $V_\mathrm{THz,pk} = \qty{-2.8}{V}$ and the tip was approached 150 pm to enhance lightwave-driven tunnelling ($z = z_0 - \qty{150}{pm}$). d, Conventional THz-STM rectified current map, $I_\mathrm{X}(x,y)$, of a $\qty{42}{nm} \times \qty{28}{nm}$ area acquired at constant height with $V_\mathrm{d.c.} = \qty{0}{V}$, $V_\mathrm{THz,pk} = \qty{-1.7}{V}$ and $z = z_0 - \qty{50}{pm}$. The tip height $z_0$ was set by $V_\mathrm{d.c.} = \qty{10}{mV}$, $I_\mathrm{d.c.} = \qty{100}{pA}$. e, Out-of-phase THz-STM image, $I_\mathrm{Y}(x,y)$, of the same area acquired at $V_\mathrm{d.c.} = \qty{10}{mV}$, $V_\mathrm{THz,pk} = \qty{-2.2}{V}$ and $I_\mathrm{d.c.} = \qty{100}{pA}$. Insets: Symmetrized 2D Fourier transforms with dimensions $\qty{3}{nm^{-1}} \times \qty{4}{nm^{-1}}$ (non-symmetrized are shown in Supplementary Fig. \ref{['sifig:THzSTMqpi']}). Scan speed $\qty{4.3}{nm/s}$ for c-e. A 100pm full-width-half-maximum (FWHM) 2D Gaussian was convolved with the images in c-e for noise reduction.
• Figure 2: Extended Data Figure 2 $\mid$ Fourier analysis of WTe2 STM topography under terahertz pulse illumination.a, STM topography acquired at $V_\mathrm{d.c.} = \qty{10}{mV}$ and $I_\mathrm{d.c.} = \qty{100}{pA}$ with $V_\mathrm{THz,pk}$ at 0.7V (left), 1.0V (middle left), 1.3V (middle), 1.7V (middle right) and 2.0V (right). Chopper frequency 477 Hz; image size $\qty{10}{nm} \times \qty{10}{nm}$; scan speed 4.3nm/s. b-d, Symmetrized 2D Fourier transforms ($\qty{8}{nm^{-1}} \times \qty{8}{nm^{-1}}$) of the images in a with light blue (c) and white (d) arrows denoting Fourier peaks where horizontal cross-sections are shown in c (light blue arrow) and d (white arrow). e, Difference image between the right and left 2D Fourier transforms in b.