Collapse of layer dimerization in the photo-induced hidden state of 1T-TaS2

Abstract

Photo-induced switching between collective quantum states of matter is a fascinating rising field with exciting opportunities for novel technologies. Presently very intensively studied examples in this regard are nanometer-thick single crystals of the layered material 1T-TaS2, where picosecond laser pulses can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is believed to be connected to the switching between metastable collective quantum states, but the microscopic nature of this so-called hidden quantum state remained largely elusive up to now. Here we determine the latter by means of state-of-the-art x-ray diffraction and show that the laser-driven IMT involves a marked rearrangement of the charge and orbital order in the direction perpendicular to the TaS2layers. More specifically, we identify the collapse of inter-layer molecular orbital dimers, which are a characteristic feature of the insulating phase, as a key mechanism for the non-thermal IMT in 1T-TaS2, which indeed involves a collective transition between two truly long-range ordered electronic crystals.The layered transition metal dichalcogenides (TMDs) form a vast class of materials hosting diverse non-trivial quantum phenomena such as spin-valley polarization [1], Ising-superconductivity [2] or intertwined electronic orders [3,4]. All these intriguing electronic effects along with the natural suitability of TMDs for the preparation of quasi two-dimensional (2D) nano-sheets render them highly appealing for next-generation technologies [5][6][7][8].1T-TaS 2 is a particularly interesting and extensively studied TMD in which external tuning parameters like temperature, pressure or chemical substitution span a particularly complex electronic phase diagram. Apart from several charge density waves (CDWs) this phase diagram also features pressure-induced superconductivity and a so-called Mott-phase, which stands out due to its semiconducting electronic transport properties [3,9].Remarkably, besides the aforementioned states that can be reached in thermal equilibrium, femto to picosecond optical or electrical pulses can launch a nonequilibrium IMT into a previously hidden and persistent metallic CDW-state [7,10,11]. The discovery of this so-called hidden CDW (HCDW) has sparked wide excitement as it might provide a new platform for memory device applications. Accordingly, in recent years, a significant number of experimental and theoretical studies aimed at pinning down the microscopic mechanism of this non-equilibrium IMT that is believed to be connected to a reorganization of the CDW-order. However, despite significant efforts to determine the microscopic processes underlying this novel IMT have been made [12][13][14][15][16][17], a clear picture remains elusive.In this article we address this open issue directly by means of high-resolution synchrotron x-ray diffraction (XRD) in combination with laser pumping. Our experiments enable examination of the laser-driven transition and in-particular the HCDW-order in 1T-TaS 2 nanosheets wi...

A time-domain phase diagram of metastable states in a charge ordered quantum material

“…Also, the suppression of the CDW diffraction peaks is in agreement with the decrease of the AM peak intensity in the transient optical spectroscopy data reported here 27,32 . Finally, we note that the H state boundaries on long timescales are also consistent with the recent XRD measurements 37 . Comparing the appearance of the H state on short and long timescales we see that the H state immediately appears at low uences, but it takes much longer to stabilize at higher uences.…”

“…TrED measurements 32 revealed a number of transient phases on the picosecond timescale dependent on photon density, with a semi-continuous rotation of the CDW ordering wavevector with respect to the crystal axes as a function of laser uence. The ordering wavevector angles observed in TrED are consistent with the angles obtained from the Fourier transformed STM images 19 and with recent static Xray measurements of the H state 37 . X-ray diffraction showed electronic domain fusion dynamics associated with the formation of the IC phase, described in terms of coherent domain formation on the picosecond timescale 35 followed by diffusive processes on a time-scale of ps 31 .…”

A time-domain phase diagram of metastable states in a charge ordered quantum material

“…TrED measurements 35 revealed a number of transient phases on the picosecond timescale dependent on photon density, with a semicontinuous rotation of the CDW ordering wavevector with respect to the crystal axes as a function of laser fluence. The ordering wavevector angles observed in TrED are consistent with the angles obtained from the Fourier transformed STM images 16 and with recent static X-ray measurements of the H state 40 . X-ray diffraction showed electronic domain fusion dynamics associated with the formation of the IC phase, described in terms of coherent domain formation on the picosecond timescale 38 followed by diffusive processes on a time-scale of~100 ps 34 .…”

Mottness versus unit-cell doubling as the driver of the insulating state in 1T-TaS2

“…that the bulk stacking structure likely alternates between T A and a vector drawn randomly from three versions of T C related by rotations of 120 ∘ , in a partially disordered patternsee Supplementary Note 1. The dimerisation of the stacking structure into bilayers, and the disorder, have also been discussed in the interpretation of recent experimental works 21,22 ). Put simply, if the electronic unit-cell contains two SDs, the total number of electrons per cell is even, leaving the highest occupied band filled and allowing an insulator without invoking Mott.…”