Anisotropic X-Ray Scattering of Transiently Oriented Water

We study the structural dynamics of liquid water by time-resolved anisotropic x-ray scattering under the optical Kerr effect condition. In this way, we can separate the anisotropic scattering decay of 160 fs from the delayed temperature increase of ∼0.1  K occurring at 1 ps and quantify transient changes in the O-O pair distribution function. Polarizable molecular dynamics simulations reproduce well the experiment, indicating transient alignment of molecules along the electric field, which shortens the nearest-neighbor distances. In addition, analysis of the simulated water local structure provides evidence that two hypothesized fluctuating water configurations exhibit different polarizability.

A Superconducting Praseodymium Nickelate with Infinite Layer Structure

A variety of nickel oxide compounds have long been studied for their manifestation of various correlated electron phenomena. Recently, superconductivity was observed in nanoscale infinite layer nickelate thin films of Nd_0.8Sr_0.2NiO₂, epitaxially stabilized on SrTiO₃ substrates via topotactic reduction from the perovskite precursor phase. Here, we present the synthesis and properties of PrNiO₂ thin films on SrTiO₃. Upon doping in Pr_0.8Sr_0.2NiO₂, we observe superconductivity with a transition temperature of 7-12 K and robust critical current density at 2 K of 334 kA/cm². These findings indicate that superconductivity in the infinite layer nickelates is relatively insensitive to the details of the rare earth 4f configuration. Furthermore, they motivate the exploration of a broader family of compounds based on two-dimensional NiO₂ planes, which will enable systematic investigation of the superconducting and normal state properties and their underlying mechanisms.

Superconducting Dome in Nd_{1-x}Sr_{x}NiO_{2} Infinite Layer Films

We report the phase diagram of Nd_{1-x}Sr_{x}NiO_{2} infinite layer thin films grown on SrTiO_{3}. A superconducting dome spanning 0.125<x<0.25 is found, remarkably similar to cuprates, albeit over a narrower doping window. However, while cuprate superconductivity is bounded by an insulator for underdoping and a metal for overdoping, here we observe weakly insulating behavior on either side of the dome. Furthermore, the normal state Hall coefficient is always small and proximate to a continuous zero crossing in doping and in temperature, in contrast to the ∼1/x dependence observed for cuprates. This suggests the presence of both electronlike and holelike bands, consistent with band structure calculations.

Electronic structure of the parent compound of superconducting infinite-layer nickelates

The search continues for nickel oxide-based materials with electronic properties similar to cuprate high-temperature superconductors^1-10. The recent discovery of superconductivity in the doped infinite-layer nickelate NdNiO₂ (refs. ^11,12) has strengthened these efforts. Here, we use X-ray spectroscopy and density functional theory to show that the electronic structure of LaNiO₂ and NdNiO₂, while similar to the cuprates, includes significant distinctions. Unlike cuprates, the rare-earth spacer layer in the infinite-layer nickelate supports a weakly interacting three-dimensional 5d metallic state, which hybridizes with a quasi-two-dimensional, strongly correlated state with [Formula: see text] symmetry in the NiO₂ layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare-earth intermetallics^13-15, which are well known for heavy fermion behaviour, where the NiO₂ correlated layers play an analogous role to the 4f states in rare-earth heavy fermion compounds. This Kondo- or Anderson-lattice-like 'oxide-intermetallic' replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.

Electrochemical generation of liquid and solid sulfur on two-dimensional layered materials with distinct areal capacities

It has recently been shown that sulfur, a solid material in its elementary form S₈, can stay in a supercooled state as liquid sulfur in an electrochemical cell. We establish that this newly discovered state could have implications for lithium-sulfur batteries. Here, through in situ studies of electrochemical sulfur generation, we show that liquid (supercooled) and solid elementary sulfur possess very different areal capacities over the same charging period. To control the physical state of sulfur, we studied its growth on two-dimensional layered materials. We found that on the basal plane, only liquid sulfur accumulates; by contrast, at the edge sites, liquid sulfur accumulates if the thickness of the two-dimensional material is small, whereas solid sulfur nucleates if the thickness is large (tens of nanometres). Correlating the sulfur states with their respective areal capacities, as well as controlling the growth of sulfur on two-dimensional materials, could provide insights for the design of future lithium-sulfur batteries.

Electrotunable liquid sulfur microdroplets

Manipulating liquids with tunable shape and optical functionalities in real time is important for electroactive flow devices and optoelectronic devices, but remains a great challenge. Here, we demonstrate electrotunable liquid sulfur microdroplets in an electrochemical cell. We observe electrowetting and merging of sulfur droplets under different potentiostatic conditions, and successfully control these processes via selective design of sulfiphilic/sulfiphobic substrates. Moreover, we employ the electrowetting phenomena to create a microlens based on the liquid sulfur microdroplets and tune its characteristics in real time through changing the shape of the liquid microdroplets in a fast, repeatable, and controlled manner. These studies demonstrate a powerful in situ optical battery platform for unraveling the complex reaction mechanism of sulfur chemistries and for exploring the rich material properties of the liquid sulfur, which shed light on the applications of liquid sulfur droplets in devices such as microlenses, and potentially other electrotunable and optoelectronic devices.

Heteroepitaxial vertical perovskite hot-electron transistors down to the monolayer limit

Two-dimensional heterostructures combined with vertical geometries are candidates to probe and utilize the physical properties of atomically-thin materials. The vertical configuration enables a unique form of hot-carrier spectroscopy as well as atomic-scale devices. Here, we present the room-temperature evolution of heteroepitaxial perovskite hot-electron transistors using a SrRuO₃ base down to the monolayer limit (∼4 Å). As a fundamental electronic probe, we observe an abrupt transition in the hot-electron mean free path as a function of base thickness, coinciding with the thickness-dependent resistive transition. As a path towards devices, we demonstrate the integrated synthesis of perovskite one-dimensional electrical edge contacts using water-soluble and growth-compatible Sr₃Al₂O₆ hard masks. Edge-contacted monolayer-base transistors exhibit on/off ratios reaching ∼10⁸, complete electrostatic screening by the base manifesting pure hot-electron injection, and excellent scaling of the output current density with device dimensions. These results open new avenues for incorporating emergent phenomena at oxide interfaces and in heterostructures.

Two-dimensional heterostructures combined with vertical geometries trigger superior functionalities in fundamental studies and applications. Here, the authors report vertical perovskite hot-electron transistors integrated with perovskite one-dimensional edge contacts down to the monolayer limit.

Terahertz-Driven Stark Spectroscopy of CdSe and CdSe-CdS Core-Shell Quantum Dots

The effects of large external fields on semiconductor nanostructures could reveal much about field-induced shifting of electronic states and their dynamical responses and could enable electro-optic device applications that require large and rapid changes in optical properties. Studies of quasi-dc electric field modulation of quantum dot (QD) properties have been limited by electrostatic breakdown processes observed under high externally applied field levels. To circumvent this, here we apply ultrafast terahertz (THz) electric fields with switching times on the order of 1 ps. We show that a pulsed THz electric field, enhanced by a microslit field enhancement structure (FES), can strongly manipulate the optical absorption properties of a thin film of CdSe and CdSe-CdS core-shell QDs on the subpicosecond time scale with spectral shifts that span the visible to near-IR range. Numerical simulations using a semiempirical tight binding model show that the band gap of the QD film can be shifted by as much a 79 meV during these time scales. The results allow a basic understanding of the field-induced shifting of electronic levels and suggest electro-optic device applications.

A Two-Dimensional MoS2 Catalysis Transistor by Solid-State Ion Gating Manipulation and Adjustment (SIGMA)

A variety of methods including tuning chemical compositions, structures, crystallinity, defects and strain, and electrochemical intercalation have been demonstrated to enhance the catalytic activity. However, none of these tuning methods provide direct dynamical control during catalytic reactions. Here we propose a new method to tune the activity of catalysts through solid-state ion gating manipulation and adjustment (SIGMA) using a catalysis transistor. SIGMA can electrostatically dope the surface of catalysts with a high electron concentration over 5 × 10¹³ cm^-2 and thus modulate both the chemical potential of the reaction intermediates and their electrical conductivity. The hydrogen evolution reaction (HER) on both pristine and defective MoS₂ were investigated as model reactions. Our theoretical and experimental results show that the overpotential at 10 mA/cm² and Tafel slope can be in situ, continuously, dynamically, and reversibly tuned over 100 mV and around 100 mV/dec, respectively.

Freestanding Oxide Ferroelectric Tunnel Junction Memories Transferred onto Silicon

Crystalline oxide ferroelectric tunnel junctions enable persistent encoding of information in electric polarization, featuring nondestructive readout and scalability that can exceed current commercial high-speed, nonvolatile ferroelectric memories. However, the well-established fabrication of epitaxial devices on oxide substrates is difficult to adapt to silicon substrates for integration into complementary metal-oxide-semiconductor electronics. In this work, we report ferroelectric tunnel junctions based on 2.8 nm-thick BaTiO₃ films grown epitaxially on SrTiO₃ growth substrates, released, and relaminated onto silicon. The performance of the transferred devices is comparable to devices characterized on the oxide substrate, suggesting a viable route toward next-generation nonvolatile memories broadly integrable with different materials platforms.

Oxygen Evolution Reaction Activity in IrO x/SrIrO3 Catalysts: Correlations between Structural Parameters and the Catalytic Activity

Understanding how structural properties affect the oxygen evolution reaction (OER) of a catalyst can reveal important information not only on the catalytic mechanism but also on the general design strategy of OER catalysts. We report a variation of ∼0.15 V in the overpotential of the recently discovered IrO _x/SrIrO₃ OER catalysts, which directly correlates with the structural parameters of the as-synthesized SrIrO₃ epitaxial films. This variation is caused by both extrinsic area enhancement and intrinsic electronic structure modification driven by defect formation. These correlations not only indicate that microscopic film defects play an important role in the activity of the IrO _x/SrIrO₃ catalyst but also provide readily accessible parameters predictive of the activity post-transformation to IrO _x/SrIrO₃. Establishing strong associations between the catalytic activity and key structural and electronic parameters, rather than synthetic variables, provides important guidance to control and study these complex catalysts independent of the synthetic technique.

Superconducting Tunneling Spectroscopy of Spin-Orbit Coupling and Orbital Depairing in Nb:SrTiO_{3}

We have examined the intrinsic spin-orbit coupling and orbital depairing in thin films of Nb-doped SrTiO_{3} by superconducting tunneling spectroscopy. The orbital depairing is geometrically suppressed in the two-dimensional limit, enabling a quantitative evaluation of the Fermi level spin-orbit scattering using Maki's theory. The response of the superconducting gap under in-plane magnetic fields demonstrates short spin-orbit scattering times τ_{so}≤1.1  ps. Analysis of the orbital depairing indicates that the heavy electron band contributes significantly to pairing. These results suggest that the intrinsic spin-orbit scattering time in SrTiO_{3} is comparable to those associated with Rashba effects in SrTiO_{3} interfacial conducting layers and can be considered significant in all forms of superconductivity in SrTiO_{3}.

Measurement of elastoresistivity at finite frequency by amplitude demodulation

Elastoresistivity, the relation between resistivity and strain, can elucidate the subtle properties of the electronic structure of a material and is an increasingly important tool for the study of strongly correlated materials. To date, elastoresistivity measurements have predominantly been performed with quasi-static (DC) strain. In this work, we demonstrate a method using AC strain in elastoresistivity measurements. A sample experiencing AC strain has a time-dependent resistivity, which modulates the voltage produced by an AC current; this effect produces time-dependent variations in resistivity that are directly proportional to the elastoresistivity, and which can be measured more quickly, with less strain on the sample, and with less stringent requirements for temperature stability than the previous DC technique. Example measurements between 10 Hz and 3 kHz are performed on a material with a large, well-characterized and temperature dependent elastoresistivity: the representative iron-based superconductor Ba(Fe_0.975Co_0.025)₂As₂. These measurements yield a frequency independent elastoresistivity and reproduce results from previous DC elastoresistivity methods to within experimental accuracy. We emphasize that the dynamic (AC) elastoresistivity is a distinct material-specific property that has not previously been considered.

Carrier density and disorder tuned superconductor-metal transition in a two-dimensional electron system

Quantum ground states that arise at atomically controlled oxide interfaces provide an opportunity to address key questions in condensed matter physics, including the nature of two-dimensional metallic behaviour often observed adjacent to superconductivity. At the superconducting LaAlO3/SrTiO3 interface, a metallic ground state emerges upon the collapse of superconductivity with field-effect gating and is accompanied with a pseudogap. Here we utilize independent control of carrier density and disorder of the interfacial superconductor using dual electrostatic gates, which enables the comprehensive examination of the electronic phase diagram approaching zero temperature. We find that the pseudogap corresponds to precursor pairing, and the onset of long-range phase coherence forms a two-dimensional superconducting dome as a function of the dual-gate voltages. The gate-tuned superconductor-metal transitions are driven by macroscopic phase fluctuations of Josephson coupled superconducting puddles.

Ultralow Damping in Nanometer-Thick Epitaxial Spinel Ferrite Thin Films

Pure spin currents, unaccompanied by dissipative charge flow, are essential for realizing energy-efficient nanomagnetic information and communications devices. Thin-film magnetic insulators have been identified as promising materials for spin-current technology because they are thought to exhibit lower damping compared with their metallic counterparts. However, insulating behavior is not a sufficient requirement for low damping, as evidenced by the very limited options for low-damping insulators. Here, we demonstrate a new class of nanometer-thick ultralow-damping insulating thin films based on design criteria that minimize orbital angular momentum and structural disorder. Specifically, we show ultralow damping in <20 nm thick spinel-structure magnesium aluminum ferrite (MAFO), in which magnetization arises from Fe³⁺ ions with zero orbital angular momentum. These epitaxial MAFO thin films exhibit a Gilbert damping parameter of ∼0.0015 and negligible inhomogeneous linewidth broadening, resulting in narrow half width at half-maximum linewidths of ∼0.6 mT around 10 GHz. Our findings offer an attractive thin-film platform for enabling integrated insulating spintronics.

Gate-Induced Metal-Insulator Transition in MoS2 by Solid Superionic Conductor LaF3

Electric-double-layer (EDL) gating with liquid electrolyte has been a powerful tool widely used to explore emerging interfacial electronic phenomena. Due to the large EDL capacitance, a high carrier density up to 10¹⁴ cm^-2 can be induced, directly leading to the realization of field-induced insulator to metal (or superconductor) transition. However, the liquid nature of the electrolyte has created technical issues including possible side electrochemical reactions or intercalation, and the potential for huge strain at the interface during cooling. In addition, the liquid coverage of active devices also makes many surface characterizations and in situ measurements challenging. Here, we demonstrate an all solid-state EDL device based on a solid superionic conductor LaF₃, which can be used as both a substrate and a fluorine ionic gate dielectric to achieve a wide tunability of carrier density without the issues of strain or electrochemical reactions and can expose the active device surface for external access. Based on LaF₃ EDL transistors (EDLTs), we observe the metal-insulator transition in MoS₂. Interestingly, the well-defined crystal lattice provides a more uniform potential distribution in the substrate, resulting in less interface electron scattering and therefore a higher mobility in MoS₂ transistors. This result shows the powerful gating capability of LaF₃ solid electrolyte for new possibilities of novel interfacial electronic phenomena.

Gate-Induced Interfacial Superconductivity in 1T-SnSe2

Layered metal chalcogenide materials provide a versatile platform to investigate emergent phenomena and two-dimensional (2D) superconductivity at/near the atomically thin limit. In particular, gate-induced interfacial superconductivity realized by the use of an electric-double-layer transistor (EDLT) has greatly extended the capability to electrically induce superconductivity in oxides, nitrides, and transition metal chalcogenides and enable one to explore new physics, such as the Ising pairing mechanism. Exploiting gate-induced superconductivity in various materials can provide us with additional platforms to understand emergent interfacial superconductivity. Here, we report the discovery of gate-induced 2D superconductivity in layered 1T-SnSe₂, a typical member of the main-group metal dichalcogenide (MDC) family, using an EDLT gating geometry. A superconducting transition temperature T_c ≈ 3.9 K was demonstrated at the EDL interface. The 2D nature of the superconductivity therein was further confirmed based on (1) a 2D Tinkham description of the angle-dependent upper critical field B_c2, (2) the existence of a quantum creep state as well as a large ratio of the coherence length to the thickness of superconductivity. Interestingly, the in-plane B_c2 approaching zero temperature was found to be 2-3 times higher than the Pauli limit, which might be related to an electric field-modulated spin-orbit interaction. Such results provide a new perspective to expand the material matrix available for gate-induced 2D superconductivity and the fundamental understanding of interfacial superconductivity.

Imaging and tuning polarity at SrTiO3 domain walls

Electrostatic fields tune the ground state of interfaces between complex oxide materials. Electronic properties, such as conductivity and superconductivity, can be tuned and then used to create and control circuit elements and gate-defined devices. Here we show that naturally occurring twin boundaries, with properties that are different from their surrounding bulk, can tune the LaAlO₃/SrTiO₃ interface 2DEG at the nanoscale. In particular, SrTiO₃ domain boundaries have the unusual distinction of remaining highly mobile down to low temperatures, and were recently suggested to be polar. Here we apply localized pressure to an individual SrTiO₃ twin boundary and detect a change in LaAlO₃/SrTiO₃ interface current distribution. Our data directly confirm the existence of polarity at the twin boundaries, and demonstrate that they can serve as effective tunable gates. As the location of SrTiO₃ domain walls can be controlled using external field stimuli, our findings suggest a novel approach to manipulate SrTiO₃-based devices on the nanoscale.

Two-dimensional limit of crystalline order in perovskite membrane films

Long-range order and phase transitions in two-dimensional (2D) systems-such as magnetism, superconductivity, and crystallinity-have been important research topics for decades. The issue of 2D crystalline order has reemerged recently, with the development of exfoliated atomic crystals. Understanding the dimensional limit of crystalline phases, with different types of bonding and synthetic techniques, is at the foundation of low-dimensional materials design. We study ultrathin membranes of SrTiO₃, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released after synthesis by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO₃ membrane lattice collapses below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition. The transition is likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.

Terahertz-Driven Luminescence and Colossal Stark Effect in CdSe-CdS Colloidal Quantum Dots

Optical properties of colloidal semiconductor quantum dots (QDs), arising from quantum mechanical confinement of charge, present a versatile testbed for the study of how high electric fields affect the electronic structure of nanostructured solids. Studies of quasi-DC electric field modulation of QD properties have been limited by electrostatic breakdown processes under high externally applied electric fields, which have restricted the range of modulation of QD properties. In contrast, here we drive CdSe-CdS core-shell QD films with high-field THz-frequency electromagnetic pulses whose duration is only a few picoseconds. Surprisingly, in response to the THz excitation, we observe QD luminescence even in the absence of an external charge source. Our experiments show that QD luminescence is associated with a remarkably high and rapid modulation of the QD bandgap, which changes by more than 0.5 eV (corresponding to 25% of the unperturbed bandgap energy). We show that these colossal energy shifts can be explained by the quantum confined Stark effect even though we are far outside the regime of small field-induced shifts in electronic energy levels. Our results demonstrate a route to extreme modulation of material properties and to a compact, high-bandwidth THz detector that operates at room temperature.

Electrical tuning of a quantum plasmonic resonance

Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light-matter interaction. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wavelength λ_F of the electrons. Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles, systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements, as well as imperfect control over size, shape, faceting, surface reconstructions, contamination, charging effects and surface roughness in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal. An ionic liquid (IL) is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.

High electron mobility and quantum oscillations in non-encapsulated ultrathin semiconducting Bi2O2Se

High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research. However, air-stable ultrathin semiconducting materials with superior performances remain elusive at present. Here, we report ultrathin films of non-encapsulated layered Bi₂O₂Se, grown by chemical vapour deposition, which demonstrate excellent air stability and high-mobility semiconducting behaviour. We observe bandgap values of ∼0.8 eV, which are strongly dependent on the film thickness due to quantum-confinement effects. An ultrahigh Hall mobility value of >20,000 cm² V^-1 s^-1 is measured in as-grown Bi₂O₂Se nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition and at the LaAlO₃-SrTiO₃ interface, making the detection of Shubnikov-de Haas quantum oscillations possible. Top-gated field-effect transistors based on Bi₂O₂Se crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm² V^-1 s^-1), large current on/off ratios (>10⁶) and near-ideal subthreshold swing values (∼65 mV dec^-1) at room temperature. Our results make Bi₂O₂Se a promising candidate for future high-speed and low-power electronic applications.