Charge Configuration Memory Devices: Energy Efficiency and Switching Speed

Current trends in data processing have given impetus for an intense search of new concepts of memory devices with emphasis on efficiency, speed, and scalability. A promising new approach to memory storage is based on resistance switching between charge-ordered domain states in the layered dichalcogenide 1T-TaS2. Here we investigate the energy efficiency scaling of such charge configuration memory (CCM) devices as a function of device size and data write time τW as well as other parameters that have bearing on efficient device operation. We find that switching energy efficiency scales approximately linearly with both quantities over multiple decades, departing from linearity only when τW approaches the ∼0.5 ps intrinsic switching limit. Compared to current state of the art memory devices, CCM devices are found to be much faster and significantly more energy efficient, demonstrated here with two-terminal switching using 2.2 fJ, 16 ps electrical pulses.

Nonequilibrium sub-10 nm spin-wave soliton formation in FePt nanoparticles

Magnetic nanoparticles such as FePt in the L10 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub-10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.

Ultrafast manipulation of the NiO antiferromagnetic order via sub-gap optical excitation

Wide-band-gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes...

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

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.

Tracking the evolution of non-equilibrium phases requires measurements over a wide range of timescales. Here, using a combination of femtosecond spectroscopy and scanning tunneling microscopy, the authors map out a temporal phase diagram of metastable states in a charge-ordered material 1T-TaS₂.

Quantum jamming transition to a correlated electron glass in 1T-TaS2

Distinct many-body states may be created under non-equilibrium conditions through different ordering paths, even when their constituents are subjected to the same fundamental interactions. The phase-transition mechanism to such states remains poorly understood. Here, we show that controlled optical or electromagnetic perturbations can lead to an amorphous metastable state of strongly correlated electrons in a quasi-two-dimensional dichalcogenide. Scanning tunnelling microscopy reveals a hyperuniform pattern of localized charges, whereas multitip surface nanoscale conductivity measurements and tunnelling spectroscopy show an electronically gapless conducting state that is different from conventional Coulomb glasses and many-body localized systems. The state is stable up to room temperature and shows no signs of either local charge order or phase separation. The mechanism for its formation is attributed to a dynamical localization of electrons through mutual interactions. Theoretical calculations confirm the correlations between localized charges to be crucial for the state's unusual stability.

Strain-Induced Metastable Topological Networks in Laser-Fabricated TaS2 Polytype Heterostructures for Nanoscale Devices

The stacking of layered materials into heterostructures offers diverse possibilities for generating deformed moiré states arising from their mutual interaction. Here we report self-assembled two-dimensional nanoscale strain networks formed within a single prismatic (H) polytype monolayer of TaS₂ created in situ on the surface of an orthorhombic 1T-TaS₂ single crystal by a low-temperature laser-induced polytype transformation. The networks revealed by scanning tunneling microscopy (STM) take on diverse configurations at different temperatures, including extensive double stripes and a twisted 3-gonal mesh of connected 6-pronged vertices. The resulting phase diagram can be understood to be a consequence of thermally driven minimization of discommensurations between the H and 1T layers. Nontrivial dislocation defects of embedded 2- and 4-gonal structures are shown to be associated with local inhomogeneous strains. The creation of metastable heterostructures by laser quench at cryogenic temperatures in combination with STM manipulation of local strain demonstrates nanoscale control of topological defects in transition metal dichalcogenide heterostructures may be utilized in the fabrication of nanoscale electronic devices and neural networks.

Controlling the metal-to-insulator relaxation of the metastable hidden quantum state in 1T-TaS2

Controllable switching between metastable macroscopic quantum states under nonequilibrium conditions induced either by light or with an external electric field is rapidly becoming of great fundamental interest. We investigate the relaxation properties of a "hidden" (H) charge density wave (CDW) state in thin single crystals of the layered dichalcogenide 1T-TaS2, which can be reached by either a single 35-fs optical laser pulse or an ~30-ps electrical pulse. From measurements of the temperature dependence of the resistivity under different excitation conditions, we find that the metallic H state relaxes to the insulating Mott ground state through a sequence of intermediate metastable states via discrete jumps over a "Devil's staircase." In between the discrete steps, an underlying glassy relaxation process is observed, which arises because of reciprocal-space commensurability frustration between the CDW and the underlying lattice. We show that the metastable state relaxation rate may be externally stabilized by substrate strain, thus opening the way to the design of nonvolatile ultrafast high-temperature memory devices based on switching between CDW states with large intrinsic differences in electrical resistance.

Multichannel photodiode detector for ultrafast optical spectroscopy

Construction and characterization of a multichannel photodiode detector based on commercially available components with high signal to noise of ∼10(6) and a rapid frame rate, suitable for time resolved femtosecond spectroscopy with high repetition femtosecond sources, is presented.

Ultrafast switching to a stable hidden quantum state in an electronic crystal

Hidden states of matter may be created if a system out of equilibrium follows a trajectory to a state that is inaccessible or does not exist under normal equilibrium conditions. We found such a hidden (H) electronic state in a layered dichalcogenide crystal of 1T-TaS2 (the trigonal phase of tantalum disulfide) reached as a result of a quench caused by a single 35-femtosecond laser pulse. In comparison to other states of the system, the H state exhibits a large drop of electrical resistance, strongly modified single-particle and collective-mode spectra, and a marked change of optical reflectivity. The H state is stable until a laser pulse, electrical current, or thermal erase procedure is applied, causing it to revert to the thermodynamic ground state.