Observation of multiple metastable states induced by electric pulses in the hysteresis temperature range of 1T−TaS2

Abstract

The electric pulses induced responses of 1T-TaS 2 in the commensurate charge-density-wave phase in hysteresis temperature region (160∼210 K) have been investigated. We observed an abrupt jump of the resistance excited by pulse, followed by a slow relaxation process in a time scale of ∼100 s.At a fixed temperature, the various electric pulses can drive the system to multi meta-stable states.We propose that the spontaneous evolution or slow relaxation of the system by pulse excitations corresponds to the rearrangements of the textures of CCDW domains. 1The strongly correlated electronic materials are the interesting systems for condensed matter scientists, as the carriers couple together and thus novel physical properties appear 1 .The layered transition-metal dichalcogenides drive much attention in recent years, esp 1T-TaS 2 : the observed ultra-fast resistance switching 2,3 , the supercooled nearly commensurate charge-density-wave (CDW) phase at lower temperatures 4 , photosensitivity from visible to terahertz at room temperature 5 , and electrically driven reversible insulator-metal phase transition 4,6 . Though the microscopic mechanisms have not been disclosed until now, it is believed that the associated transport properties are attributed to the novel electron structures.1T-TaS 2 is one of the classical two-dimensional (2D) CDW systems. At room temperature, it reveals the nearly commensurate (NC) CDW. With decreasing temperature, a commensurate (C) CDW phase appears below 180 K whereas for heating process the CCDW state maintains up to 220 K, above which the NC phase establishes 7-9 . Scanning tunnelling microscopy has revealed that the NCCDW phase consists of trigonally packed CCDW domains separated by metallic regions that are not fully distorted 9 . As expected, the CCDW ground state is the starting point to understand the charge dynamics of the system 4,10-13 . Besides the observations of hidden states and supercooled states at lower temperatures, the region 160 ∼ 210 K is also interesting due to the remarkable hysteresis in dc transport properties, also known as the temperature region of metastability 14 . Photoemission spectroscopy, infrared conductivity and thermoelectric potential also exhibit hysteresis features [15][16][17] . Naively, understanding the properties in the hysteresis region would be inevitably related the CCDW and NCCDW phases.Because of the close proximity of the various competing charge ordered phases in energy, several external perturbations or excitations can effectively modulated the CCDW phase 2,4,18,19 , raising interesting questions about the dynamics near the phase transitions.Based on the accumulated results, the charge transport properties of 1T-TaS 2 are determined by sample dimensions, thermal history, photo excitation, electric field and pressure. These characteristics are believed to be supported by the intrinsic electrons and lattice structures, which range from microscopic processes of single carriers, through mesoscopic ordering of polarons, up to macroscopic frustra...

Observation and manipulation of a phase separated state in a charge density wave material

“…22 In the FFT of the C-like region only the fundamental, three-fold symmetric CDW modes are observed. It is important to note that the observed state is not simply an NC state with asymmetrically arranged domain walls nor is it an NC state with a domain wall period different from the equilibrium NC value as has been previously suggested, 14,17 but rather the state is specifically composed of defined, irregularly shaped NC-like and C-like domains.…”

“…Previous results in ultrathin flakes of 1T-TaS 2 have indicated that within the hysteresis region of the NC-C transition, the resistivity of the material can be bidirectionally switched, driven reversibly through intermediate states whose resistivities lie between the extrema values associated with the NC and C states. [14][15][16][17] While this property suggests the use of this material as a memory element in future technologies, the nanoscopic nature of these non-equilibrium intermediate phases remains an open question. Efforts to characterize the CDW phases in 1T-TaS 2 have typically focussed on bulk samples, within regions of the phase diagram where the CDW states are well-defined.…”

Multiple Nonvolatile Resistance States Tuned by Electric Pulses in the Hysteresis Temperature Range of 1T‐TaS2

“…In 1 T ‐TaS 2 , the changes of transport properties are often determined by thermal history and electrical or laser pulses. [ 28,38–41 ] The typical temperature dependent resistivity of a bulk crystal measured covering the NC→ C phase transition is shown in Figure 2 a. The hysteresis loop between cooling and warming defines the temperature region of metastability—an activation barrier separates the energy minima.…”

Photocurrent Imaging of Multi-Memristive Charge Density Wave Switching in Two-Dimensional 1T-TaS2

“…Phase-change materials, which display different intrinsic resistivities across a first-order phase transition, have attracted particular attention as candidates for memristors due to their fast switching speeds and robust behavior 2 . Recently, several groups have demonstrated memristive switching in devices incorporating the layered, charge density wave (CDW) material, 1T-TaS2 [3][4][5][6][7][8][9] . Upon warming from the insulating, commensurate (C) CDW phase, the resistivity of bulk(-like) 1T-TaS2 abruptly decreases by an order of magnitude at ~220K upon entering the nearly commensurate (NC) CDW phase, which consists of ~10nm-sized C-phase regions separated by a network of more conductive discommensuration channels [10][11][12][13][14] .…”

“…Phase-change materials, which display different intrinsic resistivities across a first-order phase transition, have attracted particular attention as candidates for memristors due to their fast switching speeds and robust behavior . Recently, several groups have demonstrated memristive switching in devices incorporating the layered, charge density wave (CDW) material, 1T-TaS 2 . − Upon warming from the insulating, commensurate (C) CDW phase, the resistivity of bulk­(like) 1T-TaS 2 abruptly decreases by an order of magnitude at ∼220 K upon entering the nearly commensurate (NC) CDW phase, which consists of ∼10 nm-sized C-phase regions separated by a network of more conductive discommensuration channels. − At constant temperature, both volatile and nonvolatile transitions between these two states have been induced electrically by DC and pulsed voltages/currents. , In ultrathin flakes, metastable states with intermediate resistance levels have further been observed under equilibrium conditions, , although the switching behavior demonstrated so far has been unidirectional at a given temperature and/or semipermanent, requiring temperature cycling in order to reset the device.…”