Imaging of electrically controlled van der Waals layer stacking in 1T-TaS2

TL;DR

This work uses in situ microbeam X-ray diffraction and fluorescence to image electrically induced, non-thermal hidden CDW switching in 1T-TaS2 at cryogenic temperatures. The authors reconstruct 3D reciprocal-space maps to reveal a long-range, non-filamentary switching channel that extends beneath the electrodes, driven by a combination of charge injection and lattice strain. They show that the electrically switched HCDW is structurally equivalent to the optically induced counterpart, underscoring a common non-thermal pathway to the same local energy minimum. Finite element simulations connect the observed switching geometry to device design and current density, providing a framework for engineering bulk, low-power vdW memory devices with 3D imaging capabilities.

Abstract

Van der Waals (vdW) materials exhibit a variety of states that can be switched with low power at low temperatures, offering a viable cryogenic "flash memory" required for the classical control electronics for solid-state quantum information processing. In 1T-TaS2, a non-volatile metallic 'hidden' state can be induced from an insulating equilibrium charge-density wave ground state using either optical or electrical pulses. Given that conventional memristors form localized, filamentary channels which support the current, a key question for design concerns the geometry of the conduction region in highly energy-efficient 1T-TaS2 devices. Here, we report in operando micro-beam X-ray diffraction, fluorescence, and concurrent transport measurements, allowing us to spatially image the non-thermal hidden state induced by electrical switching of 1T-TaS2. Our results reveal a long-range ordered, non-filamentary switched state that extends well below the electrodes, implying that the self-organized, collective growth of the hidden phase is driven by a combination of charge flow and lattice strain. Our unique combination of techniques showcases the potential of non-destructive, three-dimensional X-ray imaging to study bulk switching properties in microscopic detail, namely electrical control of the vdW layer stacking.

Figures (14)

• Figure 1: In operando macro- and microscopic measurement of phase switching. a Schematic of the synchrotron beamline including the undulator (1), a toroidal mirror (2), the monochromator (3) and Kirkpatrick-Baez focusing mirrors (4). The 1.5 $\times$ 2.5µm sized X-ray beam is directed at the 1$T$-TaS$_2$ device (5) in a $^4$He cryostat that can be moved using translation stages. The device is electrically contacted, allowing for resistance measurements and application of current pulses. X-ray fluorescence (6) and diffraction (7) is recorded simultaneously on respective detectors. b Optical image of the 1$T$-TaS$_2$ device. c Au fluorescence map highlighting the electrodes. d Diffraction patterns measured at 6K at selected X-ray energies with the lattice (013) and (014) Bragg reflections, as well as one CCDW peak. e Conversion of the 2D detector images taken at various X-ray energies to 3D (h, k, l) reciprocal space. fIn situ resistance as a function of temperature and time with A the unswitched CCDW state, B and C the partially- and fully-switched HCDW states, respectively.
• Figure 2: Electrically-induced non-thermal phase switching. a-c Spatially-resolved intensity at the reciprocal space positions of the (013) lattice reflection, as well as a nearby CCDW and HCDW peak in the unswitched state A. The lattice and CCDW peaks are observed across the whole flake, whereas no HCDW signal is found. d-f and g-i show the respective peak intensities in the partially- B and fully-switched state C. The CCDW signal is suppressed in the lower left corner of the flake where the HCDW signal emerges. Color scales show a minimal intensity of 0 counts and maxima of $2.5 \cdot 10^3$ (lattice), $1.3 \cdot 10^3$ (CCDW) and $8.0 \cdot 10^2$ (HCDW) for an integration time of 100ms. Dashed lines indicate the position of the electrodes.
• Figure 3: Momentum- and real-space structure of the CDW states. a 2D (k, l) projection in reciprocal lattice units (r.l.u.) of the CCDW peak in the unswitched state A (dark blue), obtained by integrating the intensity along the in-plane h direction and normalizing per pixels. b Measurement of the fully-switched state C in the light blue region (inset of e), showing a CCDW and only a very faint HCDW signal. c,d Respective projection measured in the light and dark purple regions (inset of e), close to and in the vicinity of the electrode gap, respectively. Fewer pixels in those regions result in poorer statistics of the projections. e Out-of-plane projection of the data shown in a-d. Dashed lines on the spatial map in the inset indicate the location of the electrodes.
• Figure 4: Bulk electrical switching. 3D tomographic representation of the device in a the partially- B and b fully-switched state C. The vertical dimension represents the normalized switching depth, defined by the ratio of the HCDW signal and the total (CCDW + HCDW) intensity. 180-nm-thick Pd/Au electrodes (yellow) are on top of the 500-nm thick 1$T$-TaS$_2$ flake, where the CCDW region and the HCDW switching channel are depicted in blue and orange, respectively. Width and depth are not to scale. The arrow indicates the electron (e$^-$) flow upon application of the current pulse. Cuts through the flake parallel (green) and perpendicular (pink) to the electrode gap are shown on the bottom left and right, respectively. Going from B to C, the in-gap HCDW order extends both laterally towards the $-$ electrode and in volume.
• Figure 5: Elements in the 1T-TaS$_2$ device detected by X-ray fluorescence. a Total fluorescence intensity measured using an incoming X-ray energy of 9.52keV, below the Ta L-edge Thompson2001. Peak fitting traces elements such as Ti, Fe, Ni and Cu on the device and sample holder (via the respective K-edges), as well as elastic and Compton scattering. b Respective fluorescence signal measured at 12keV, above the Ta and Au L-edge Thompson2001, where in addition to the elements observed in a, also Ta and Au are detected. c Optical image of the device. d Ni signal mapped spatially across the device showing traces along the electrodes and the flake. e,f The Ta and Au signals outline the flake and electrodes, respectively. The minimal intensity in d-f is 0 counts, and the maximal intensity is $6.7$ counts (Ni), $9.6$ counts (Ta), and $67$ counts (Au), measured using an integration time of 100ms.