Ultrafast Memory materials
The search for metastable state switched by ultrafast laser pulses or electrical pulses has led to several patent applications and international projects and collaborations.
An important new area of research is related to the search for new metastable states which have both fundamental and practical importance. The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T–TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states.
Dual vortex charge order in a metastable state created by an ultrafast topological transition in 1T-TaS2.
Many body systems in complex materials undergoing non-equilibrium phase tran- sitions may self-organise into ordered metastable emergent states with new and un- expected functionalities. Recent observation of long-living metastable states induced by a single ultrafast optical pulse gave access to previously unexplored area of non-stroboscopic and real space measurements for systems out of equilibrium. Here, using large-area scanning tunneling microscopy we reveal an intricate chiral vortex structure and complex tiling of charged electron domains in the metastable metallic state in 1T-TaS2 created by a non-equilibrium topological transition initiated by a single femtosecond optical pulse. A Moir ́e analysis shows that the interference of non-equilibrium nested Fermi surface electrons leads to the creation of charge vortices D⃗ on a length scale of ∼70 nm. On a much smaller scale of ∼ 5 nm, domain configurational patterns appear, which show bound vortex-antivortex pairs, discommensurations, domain wall crossings and kinks, consistent with a rapidly quenched Berezinskii-Kosterlitz-Thouless transition. Long-range ordered state emerges finally through domain phase locking in space, thus supporting the fundamental concept that macroscopic states can be created out of equilibrium. Revealing the detailed mechanism for the transition opens the way to design of long-range charge-ordered metastable states with intricate emergent properties under controlled non-equilibrium conditions. The paper is currently under review, and the latest preprint can be found at arXiv: 1704.08149v2.
Ultrafast Switching to a Stable Hidden Quantum State in an Electronic Crystal
Science 11 April 2014, Vol. 344, Issue 6180, pp. 177-180
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