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

Abstract

Metastable self-organized electronic states in quantum materials are of fundamental importance, displaying emergent dynamical properties that may be used in new generations of sensors and memory devices. Such states are typically formed through phase transitions under non-equilibrium conditions and the final state is reached through processes that span a large range of timescales. Conventionally, phase diagrams of materials are thought of as static, without temporal evolution. However, many functional properties of materials arise as a result of complex temporal changes in the material occurring on different timescales. Hitherto, such properties were not considered within the context of a temporally-evolving phase diagram, even though, under non-equilibrium conditions, different phases typically evolve on different timescales. Here, by using time-resolved optical techniques and femtosecond-pulse-excited scanning tunneling microscopy (STM), we track the evolution of the metastable states in a material that has been of wide recent interest, the quasi-two-dimensional dichalcogenide 1T-TaS2. We map out its temporal phase diagram using the photon density and temperature as control parameters on timescales ranging from 10−12 to 103 s. The introduction of a time-domain axis in the phase diagram enables us to follow the evolution of metastable emergent states created by different phase transition mechanisms on different timescales, thus enabling comparison with theoretical predictions of the phase diagram, and opening the way to understanding of the complex ordering processes in metastable materials.

Snapshots of a light-induced metastable hidden phase driven by the collapse of charge order

“…As in the 2.5 mJ/cm 2 fluence case, the steady-state reflectivity value reached after the first each shot differs by only a modest amount from the value at 8 ps (or 1 ps) delay following excitation, which indicates that switching at this fluence also occurs on a picosecond time scale. Note that the switching observed here is entirely reversible upon thermal cycling (see note S2), which rules out any formation of the amorphous (or A) state known to persist up to room temperature (26,33). We also note that at all fluences, including the first shots at the two high fluences, the relaxation kinetics show a subpicosecond decay component followed by a slower multipicosecond decay (the values are indicated in table S1).…”

“…In the fast-cooling limit (electron-phonon scattering time shorter than the order parameter growth time t c ), the time needed to form the metastable H state and the domain size are ( 46 , 53 )tc~τ4normalαln1ζ, normalξ~ξ02/normalαln1ζwhere ζ is the Ginzburg parameter (or “Ginzburg number”) that determines the accuracy of the mean field picture. From the STM images of the H states induced by optical and electrical pulses ( 26 , 44 , 54 ), the bare coherence length (roughly the width of the domain walls at a temperature much lower than T c ) is about ξ 0 = 3 nm, while the domain size is about ξ = 10 nm. Together with the new observation that the evolution time is t c = 3.7 ps, we obtain the Ginzburg parameter and the intrinsic relaxation time of the in-plane CDWnormalζ∼4×10−3, normalτ∼2.7ps.…”

Quantum billiards with correlated electrons confined in triangular transition metal dichalcogenide monolayer nanostructures

“…The ET structures (Fig. 1f ) are created by a controlled exposure of a freshly exfoliated 1T-TaS 2 single crystal to laser pulses in ultrahigh vacuum at 80 K 22 , where the majority of the top surface is transformed to the 1H polytype, but ET structures of 1T polytype remain structurally unchanged 23 , 24 . The domains have atomically defined sides parallel to the crystal axes of the 1T layer, matching the lattice structure of the surrounding 1H layer, forming a perfect ET shape with edges at 60° to each other (Fig.…”

Charge Configuration Memory (CCM) Device – A Novel Approach to Memory

“…The phase diagram of 1T-TaS 2 is very rich, exhibiting different charge-density-wave (CDW) states, Mott transition, polaronic ordering [10], metastable state [8] and even superconductivity at higher pressures [11]. At temperatures above 540 K, the material behaves as a simple metal, but when cooled down below that temperature it undergoes a phase transition into an incommensurate (IC) CDW due to a combination of Coulomb repulsion [12] and Fermi surface nesting [13].…”