Intercalating Architecture for the Design of Charge Density Wave in Metallic MA2Z4 Materials.

We present a novel approach to induce charge density waves (CDWs) in metallic MA2Z4 materials, resembling the behavior observed in transition metal dichalcogenides (TMDCs). This method leverages the intercalating architecture to maintain the same crystal field and Fermi surface topologies. Our investigation reveals that CDW instability in these materials arises from electron-phonon coupling (EPC) between the d band and longitudinal acoustic (LA) phonons, mirroring TMDC's behavior. By combining α-MA2Z4 with 1H-MX2 materials in a predictive CDW phase diagram using critical EPC constants, we demonstrate the feasibility of extending CDW across material families with comparable crystal fields and reveal the crucial role in CDW instability of the competition between ionic charge transfer and electron correlation. We further uncover a strain-induced Mott transition in ß2-NbGe2N4 monolayer featuring star-of-David patterns. This work highlights the potential of intercalating architecture to engineer CDW materials, expanding our understanding of CDW instability and correlation physics.

Picosecond Femtojoule Resistive Switching in Nanoscale VO2 Memristors.

Beyond-Moore computing technologies are expected to provide a sustainable alternative to the von Neumann approach not only due to their down-scaling potential but also via exploiting device-level functional complexity at the lowest possible energy consumption. The dynamics of the Mott transition in correlated electron oxides, such as vanadium dioxide, has been identified as a rich and reliable source of such functional complexity. However, its full potential in high-speed and low-power operation has been largely unexplored. We fabricated nanoscale VO2 devices embedded in a broadband test circuit to study the speed and energy limitations of their resistive switching operation. Our picosecond time-resolution, real-time resistive switching experiments and numerical simulations demonstrate that tunable low-resistance states can be set by the application of 20 ps long, <1.7 V amplitude voltage pulses at 15 ps incubation times and switching energies starting from a few femtojoule. Moreover, we demonstrate that at nanometer-scale device sizes not only the electric field induced insulator-to-metal transition but also the thermal conduction limited metal-to-insulator transition can take place at time scales of 100s of picoseconds. These orders of magnitude breakthroughs can be utilized to design high-speed and low-power dynamical circuits for a plethora of neuromorphic computing applications from pattern recognition to numerical optimization.

Halide Perovskite Artificial Solids as a New Platform to Simulate Collective Phenomena in Doped Mott Insulators.

The development of quantum simulators, artificial platforms where the predictions of many-body theories of correlated quantum materials can be tested in a controllable and tunable way, is one of the main challenges of condensed matter physics. Here we introduce artificial lattices made of lead halide perovskite nanocubes as a new platform to simulate and investigate the physics of correlated quantum materials. We demonstrate that optical injection of quantum confined excitons in this system realizes the two main features that ubiquitously pervade the phase diagram of many quantum materials: collective phenomena, in which long-range orders emerge from incoherent fluctuations, and the excitonic Mott transition, which has one-to-one correspondence with the insulator-to-metal transition described by the repulsive Hubbard model in a magnetic field. Our results demonstrate that time-resolved experiments provide a quantum simulator that is able to span a parameter range relevant for a broad class of phenomena, such as superconductivity and charge-density waves.

Phase Diagram of High-Temperature Electron-Hole Quantum Droplet in Two-Dimensional Semiconductors.

Quantum liquids, systems exhibiting effects of quantum mechanics and quantum statistics at macroscopic levels, represent one of the most exciting research frontiers of modern physical science and engineering. Notable examples include Bose-Einstein condensation (BEC), superconductivity, quantum entanglement, and a quantum liquid. However, quantum liquids are usually only stable at cryogenic temperatures, significantly limiting fundamental studies and device development. Here we demonstrate the formation of stable electron-hole liquid (EHL) with the quantum statistic nature at temperatures as high as 700 K in monolayer MoS2 and elucidate that the high-temperature EHL exists as droplets in sizes of around 100-160 nm. We also develop a thermodynamic model of high-temperature EHL and, based on the model, compile an exciton phase diagram, revealing that the ionized photocarrier drives the gas-liquid transition, which is subsequently validated with experimental results. The high-temperature EHL provides a model system to enable opportunities for studies in the pursuit of other high-temperature quantum liquids. The results can also allow for the development of quantum liquid devices with practical applications in quantum information processing, optoelectronics, and optical interconnections.

Observation of Mott instability at the valence transition of f-electron system.

Mott physics plays a critical role in materials with strong electronic correlations. Mott insulator-to-metal transition can be driven by chemical doping, external pressure, temperature and gate voltage, which is often seen in transition metal oxides with 3d electrons near the Fermi energy (e.g. cuprate superconductor). In 4f-electron systems, however, the insulator-to-metal transition is mostly driven by Kondo hybridization and the Mott physics has rarely been explored in experiments. Here, by combining the angle-resolved photoemission spectroscopy and strongly correlated band structure calculations, we show that an unusual Mott instability exists in YbInCu4 accompanying its mysterious first-order valence transition. This contrasts with the prevalent Kondo picture and demonstrates that YbInCu4 is a unique platform to explore the Mott physics in Kondo lattice systems. Our work provides important insight for the understanding and manipulation of correlated quantum phenomena in the f-electron system.

Exotic Magnetic Anisotropy Near Digitized Dimensional Mott Boundary.

The magnetic anisotropy of low-dimensional Mott systems exhibits unexpected magnetotransport behavior useful for spin-based quantum electronics. Yet, the anisotropy of natural materials is inherently determined by the crystal structure, highly limiting its engineering. The magnetic anisotropy modulation near a digitized dimensional Mott boundary in artificial superlattices composed of a correlated magnetic monolayer SrRuO3 and nonmagnetic SrTiO3 , is demonstrated. The magnetic anisotropy is initially engineered by modulating the interlayer coupling strength between the magnetic monolayers. Interestingly, when the interlayer coupling strength is maximized, a nearly degenerate state is realized, in which the anisotropic magnetotransport is strongly influenced by both the thermal and magnetic energy scales. The results offer a new digitized control for magnetic anisotropy in low-dimensional Mott systems, inspiring promising integration of Mottronics and spintronics.

Giant nonlinear Hall effect in twisted bilayer WSe2.

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism for generating second-harmonic electrical Hall signals under time-reversal-symmetric conditions. Here, we introduce a new approach to engineering an NHE by using twisted moiré structures. We found that the twisted WSe2 bilayer exhibited an NHE when the Fermi level was tuned to the moiré flat bands. When the first moiré band was half-filled, the nonlinear Hall signal exhibited a sharp peak with a generation efficiency that was at least two orders of magnitude greater than those obtained in previous experiments. We discuss the possible origins of the diverging generation efficiency in twisted WSe2 based on resistivity measurements, such as moiré-interface-induced correlation effects and mass-diverging-type continuous Mott transition. This study demonstrates not only how interaction effects can combine with Berry curvature dipoles to produce novel quantum phenomena, but also the potential of NHE measurements as a new tool for studying quantum criticality.

Picoampere Dark Current and Electro-Opto-Coupled Sub-to-Super-linear Response from Mott-Transition Enabled Infrared Photodetector for Near-Sensor Vision Processing.

Light-intensity selective superlinear photodetectors with ultralow dark current can provide an essential breakthrough for the development of high-performing near-sensor vision processing. However, the development of near-sensor vision processing is not only conceptually important for device operation (given that sensors naturally exhibit linear/sublinear responses), but also essential to get rid of the massive amount of data generated during object sensing and classification with noisy inputs. Therefore, achieving the giant superlinear photoresponse while maintaining the picoampere leakage current, irrespective of the measurement bias, is one of the most challenging tasks. Here, Mott material (vanadium dioxide) and silicon-based integrated infrared photodetectors are developed that show giant superlinear photoresponse (exponent >18) and ultralow dark current of 4.46 pA. Specifically, the device demonstrates an electro-opto-coupled insulator-to-metal transition, which leads to outstanding photocurrent on/off ratio (>106 ), a high responsivity (>1 mA W-1 ), and excellent detectivity (>1012  Jones), while maintaining response speed (τr  = 6 µs and τf  = 10 µs). Further, intensity-selective near-sensor processing is demonstrated and night vision pattern reorganization even with noisy inputs is exhibited. This research will pave the way for the creation of high-performance photodetectors with potential uses, such as in night vision, pattern recognition, and neuromorphic processing.

Harnessing the Metal-Insulator Transition of VO2 in Neuromorphic Computing.

Future-generation neuromorphic computing seeks to overcome the limitations of von Neumann architectures by colocating logic and memory functions, thereby emulating the function of neurons and synapses in the human brain. Despite remarkable demonstrations of high-fidelity neuronal emulation, the predictive design of neuromorphic circuits starting from knowledge of material transformations remains challenging. VO2 is an attractive candidate since it manifests a near-room-temperature, discontinuous, and hysteretic metal-insulator transition. The transition provides a nonlinear dynamical response to input signals, as needed to construct neuronal circuit elements. Strategies for tuning the transformation characteristics of VO2 based on modification of material properties, interfacial structure, and field couplings, are discussed. Dynamical modulation of transformation characteristics through in situ processing is discussed as a means of imbuing synaptic function. Mechanistic understanding of site-selective modification; external, epitaxial, and chemical strain; defect dynamics; and interfacial field coupling in modifying local atomistic structure, the implications therein for electronic structure, and ultimately, the tuning of transformation characteristics, is emphasized. Opportunities are highlighted for inverse design and for using design principles related to thermodynamics and kinetics of electronic transitions learned from VO2 to inform the design of new Mott materials, as well as to go beyond energy-efficient computation to manifest intelligence.

A Pair of 2D Quantum Liquids: Investigating the Phase Behavior of Indirect Excitons.

Long-lived indirect excitons (IXs) exhibit a rich phase diagram, including a Bose-Einstein condensate (BEC), a Wigner crystal, and other exotic phases. Recent experiments have hinted at a "classical" liquid of IXs above the BEC transition. To uncover the nature of this phase, we use a broad range of theoretical tools and find no evidence of a driving force toward classical condensation. Instead, we attribute the condensed phase to a quantum electron-hole liquid (EHL), first proposed by Keldysh for direct excitons. Taking into account the association of free carriers into bound excitons, we study the phase equilibrium between a gas of excitons, a gas of free carriers, and an EHL for a wide range of electron-hole separations, temperatures, densities, and mass ratios. Our results agree reasonably well with recent measurements of GaAs/AlGaAs coupled quantum wells.

Disentangling Structural and Electronic Properties in V2O3 Thin Films: A Genuine Nonsymmetry Breaking Mott Transition.

Phase transitions are key in determining and controlling the quantum properties of correlated materials. Here, by using the combination of material synthesis and photoelectron spectroscopy, we demonstrate a genuine Mott transition undressed of any symmetry breaking side effects in the thin films of V2O3. In particular and in contrast with the bulk V2O3, we unveil the purely electronic dynamics approaching the metal-insulator transition, disentangled from the structural transformation that is prevented by the residual substrate-induced strain. On approaching the transition, the spectral signal evolves slowly over a wide temperature range, the Fermi wave-vector does not change, and the critical temperature is lower than the one reported for the bulk. Our findings are fundamental in demonstrating the universal benchmarks of a genuine nonsymmetry breaking Mott transition, extendable to a large array of correlated quantum systems, and hold promise of exploiting the metal-insulator transition by implementing V2O3 thin films in devices.

Reversible Insulator-Metal Transition by Chemical Doping and Dedoping of a Mott Insulator.

The chemical carrier doping of molecular Mott insulators has been poorly investigated to date due to its difficulty. In this study, iodine doping of a molecular Mott insulator, lithium phthalocyanine crystallized in the x-form (x-LiPc), was performed to obtain metallic x-LiPcI. Crystal structure analysis revealed that iodine atoms penetrated channels of x-LiPc and formed one-dimensional chains. The Raman spectroscopy of x-LiPcI indicated the existence of linear I5 - , demonstrating a transition from a half-filled band of the Mott insulating state to a 2/5-filled band of the metallic state. Electrical resistivity measurements confirmed the metallic nature of x-LiPcI, whereas a thermally activated behavior was observed for pristine x-LiPc. Furthermore, the x-LiPc Mott insulator was reproduced by dedoping iodine from x-LiPcI, suggesting that the electronic state can be reversibly tuned between the Mott insulating and metallic states by chemical doping and dedoping.

Sub-Band Filling, Mott-like Transitions, and Ion Size Effects in C60 Single Crystal Electric Double Layer Transistors.

Electric double layer transistors (EDLTs) based on C60 single crystals and ionic liquid gates display pronounced peaks in sheet conductance versus gate-induced charge. Sheet conductance is maximized at electron densities near 0.5 e/C60 and is suppressed near 1 e/C60. The conductance suppression depends markedly on the choice of ionic liquid cation, with small cations favoring activated transport and essentially a complete shutdown of conductance at ∼1 e/C60 and larger cations favoring band-like transport, higher overall conductances at all charge densities up to 1.7 e/C60, and weaker suppression at 1 e/C60. Displacement current measurements on C60 EDLTs with small cations show clear evidence of sub-band filling at 1 e/C60, which correlates very well with the minimum in the C60 sheet conductance. Overall, the data suggest a significant Mott-Hubbard-like energy gap opens up in the surface density of states for C60 crystals gated with small cations. The causes of this energy gap may include both electron-electron repulsion and electron-cation attraction at the crystal/ionic liquid interface. The energy gap suppresses the insulator-to-metal transition in C60 EDLTs, but it can be manipulated by choice of electrolyte.

Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2.

The density-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron-hole pairs within a WSe2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron-hole plasma above 7 × 1012 cm-2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.

An Artificial Mechano-Nociceptor with Mott Transition.

Intelligent touch sensing is now becoming an essential part of various human-machine interactions and communication, including in touchpads, autonomous vehicles, and smart robotics. Usually, sensing of physical objects is enabled by applied force/pressure sensors; however, reported conventional tactile devices are not able to differentiate sharp and blunt objects, although sharp objects can cause unavoidable damage. Therefore, it is central issue to implement electronic devices that can classify sense of touch and simultaneously generate pain signals to avoid further potential damage from sharp objects. Here, concept of force-enabled nociceptive behavior is proposed and demonstrated using vanadium oxide-based artificial receptors. Specifically, versatile criteria of bio-nociceptor like threshold, relaxation, no adaptation, allodynia, and hyperalgesia behaviors are triggered by pointed force, but the device does not mimic any of these by the force applied by blunt objects; thus, the proposed device classifies the intent of touch. Further, supported by finite element simulation, the nanoscale dynamic is unambiguously revealed by conductive atomic force microscopy and results are attributed to the point force-triggered Mott transition, as also confirmed by temperature-dependent measurements. The reported features open a new avenue for developing mechano-nociceptors, which enable a high-level of artificial intelligence within the device to classify physical touch.

Mott transition, magnetic and orbital orders in the ground state of the two-band Hubbard model using variational slave-spin mean field formalism.

We study the ground state phase diagram of the degenerate two-band Hubbard model at integer fillings as a function of onsite Hubbard interactionUand Hund's exchange couplingJ. We use a variational slave-spin mean field method which allows symmetry broken states to be studied within the computationally less intensive slave-spin mean field formalism. The results show that at half-filling, the ground state at smallerUis a Slater antiferromagnet with substantial local charge fluctuations. AsUis increased, the antiferromagnetic (AF) state develops a Heisenberg behavior, finally undergoing a first-order transition to a Mott insulating AF state at a critical interactionUcwhich is of the order of the bandwidth. Introducing the Hund's couplingJcorrelates the system more and reducesUcdrastically. At quarter-filling with one electron per site, the ground state at smallerUis paramagnetic metallic. At finiteJ, as interaction is increased beyond a lower critical valueUc1, it goes to a fully spin polarized ferromagnetic state coexisting with an antiferro-orbital order. Further increase inUbeyond a higher critical valueUc2results in the Mott insulating state where local charge fluctuation vanishes.

Ground state of a three-band Hubbard model with Hund's coupling: Janus-faced behavior in presence of magnetic order.

We study the ground state of the three-band degenerate Hubbard model on a square lattice at integer fillings using the variational slave-spin mean field method. At half-filling, the method reproduces the well known result that the ground state is antiferromagnetic (AF) insulating at smaller values of Hubbard onsite repulsionU, while it becomes Mott insulating with Néel AF order at higherU. Away from half-filling, for two particles per site, we show that the model supports a ferromagnetic (FM) metallic state with fully polarized spins at sufficiently largeU. The FM state occurs irrespective of the value of Hund's couplingJ. The ferromagnetism atJ= 0 can be explained by the Stoner mechanism while that forJ> 0 is shown to arise from the superexchange process. At this band filling, the Hund's couplingJis known to have the Janus-faced effect on electronic correlations where it enhances correlations at smallerUwhile reducing it at higherU. We show that these two effects are separated by the paramagnetic (PM) to FM transition point. The former effect is obtained at the PM state while the latter occurs in the FM state. The FM phase also occurs for one particle per site but here Hund's couplingJreduces the effect of electronic correlations at allU.

New insights into the structural properties of κ-(BEDT-TTF)2Ag2(CN)3 spin liquid.

Here, the first accurate study is presented of the room-temperature and 100 K structures of one of the first organic spin liquids, κ-(BEDT-TTF)2Ag2(CN)3. It is shown that the monoclinic structure determined previously is only the average one. It is shown that the exact structure presents triclinic symmetry with two non-equivalent dimers in the unit cell. But surprisingly this does not lead to a sizeable charge disproportionation between dimers. The difference from the analogue compound κ-(BEDT-TTF)2Cu2(CN)3 which also presents a spin liquid phase is discussed in detail. The data provided here show the importance of the anionic layer and in particular the transition metal position in the process of symmetry breaking. The possible impact of the symmetry breaking, albeit weak, on the spin-liquid mechanism and the influence of various disorders on the physical properties of this system is also discussed.

Giant and Tunable Optical Nonlinearity in Single-Crystalline 2D Perovskites due to Excitonic and Plasma Effects.

Materials with large optical nonlinearity, especially in the visible spectral region, are in great demand for applications in all-optical information processing and quantum optics. 2D hybrid Ruddlesden-Popper-type halide perovskites (RPPs) with tunable ultraviolet-to-visible direct bandgaps exhibit large nonlinear optical responses due to the strong excitonic effects present in their multiple quantum wells. Using a microscopic Z-scan setup with femtosecond laser pulses tunable across the visible spectrum, it is demonstrated that single-crystalline lead halide RPP nanosheets possess unprecedentedly large nonlinear refraction and absorption coefficients near excitonic resonances. A room-temperature insulator (exciton)-metal (plasma) Mott transition is found to occur near the exciton resonance of the thinnest qunatum-well RPPs, boosting the nonlinear response. Owing to the rapidly changing refractive index near resonance, a single RPP crystal can exhibit different nonlinear functionalities across the excitation spectrum. The results suggest that RPPs are efficient nonlinear materials in the visible waveband, indicating their potential use in integrated nonlinear photonic applications such as optical modulation and switching.

Vacancy-Driven Robust Metallicity of Structurally Pinned Monoclinic Epitaxial VO2 Thin Films.

Vanadium dioxide (VO2) is a strongly correlated material with 3d electrons, which exhibits temperature-driven insulator-to-metal transition with a concurrent change in the crystal symmetry. Interestingly, even modest changes in stoichiometry-induced orbital occupancy dramatically affect the electrical conductivity of the system. Here, we report a successful transformation of epitaxial monoclinic VO2 thin films from a conventionally insulating to permanently metallic behavior by manipulating the electron correlations. These ultrathin (∼10 nm) epitaxial VO2 films were grown on NiO(111)/Al2O3(0001) pseudomorphically, where the large misfit between NiO and Al2O3 were fully relaxed by domain-matching epitaxy. Complete conversion from an insulator to permanent metallic phase is achieved through injecting oxygen vacancies ( x ∼ 0.20 ± 0.02) into the VO2- x system via annealing under high vacuum (∼5 × 10-7 Torr) and increased temperature (450 °C). Systematic introduction of oxygen vacancies partially converts V4+ to V3+ and generates unpaired electron charges which result in the emergence of donor states near the Fermi level. Through the detailed study of the vibrational modes by Raman spectroscopy, hardening of the V-V vibrational modes and stabilization of V-V dimers are observed in vacuum-annealed VO2 films, providing conclusive evidence for stabilization of a monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO2- x thin films, resulting in metallic characteristics at room temperature. With these results, we propose a defect engineering pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO2.