In-Operando Spatiotemporal Imaging of Coupled Film-Substrate Elastodynamics During an Insulator-to-Metal Transition.

The drive toward non-von Neumann device architectures has led to an intense focus on insulator-to-metal (IMT) and the converse metal-to-insulator (MIT) transitions. Studies of electric field-driven IMT in the prototypical VO2 thin-film channel devices are largely focused on the electrical and elastic responses of the films, but the response of the corresponding TiO2 substrate is often overlooked, since it is nominally expected to be electrically passive and elastically rigid. Here, in-operando spatiotemporal imaging of the coupled elastodynamics using X-ray diffraction microscopy of a VO2 film channel device on TiO2 substrate reveals two new surprises. First, the film channel bulges during the IMT, the opposite of the expected shrinking in the film undergoing IMT. Second, a microns thick proximal layer in the substrate also coherently bulges accompanying the IMT in the film, which is completely unexpected. Phase-field simulations of coupled IMT, oxygen vacancy electronic dynamics, and electronic carrier diffusion incorporating thermal and strain effects suggest that the observed elastodynamics can be explained by the known naturally occurring oxygen vacancies that rapidly ionize (and deionize) in concert with the IMT (MIT). Fast electrical-triggering of the IMT via ionizing defects and an active "IMT-like" substrate layer are critical aspects to consider in device applications.

Bipolaronic Nature of the Pseudogap in Quasi-One-Dimensional (TaSe4)2I Revealed via Weak Photoexcitation.

The origin of the pseudogap in many strongly correlated materials has been a longstanding puzzle. Here, we present experimental evidence that many-body interactions among small Holstein polarons, i.e., the formation of bipolarons, are primarily responsible for the pseudogap in (TaSe4)2I. After weak photoexcitation of the material, we observe the appearance of both dispersive (single-particle bare band) and flat bands (single-polaron sub-bands) in the gap by using time- and angle-resolved photoemission spectroscopy. Based on Monte Carlo simulations of the Holstein model, we propose that the melting of pseudogap and emergence of new bands originate from a bipolaron to single-polaron crossover. We also observe dramatically different relaxation times for the excited in-gap states in (TaSe4)2I (∼600 fs) compared with another 1D material Rb0.3MoO3 (∼60 fs), which provides a new method for distinguishing between pseudogaps induced by polaronic or Luttinger-liquid many-body interactions.

Strong electron-phonon coupling driven pseudogap modulation and density-wave fluctuations in a correlated polar metal.

There is tremendous interest in employing collective excitations of the lattice, spin, charge, and orbitals to tune strongly correlated electronic phenomena. We report such an effect in a ruthenate, Ca3Ru2O7, where two phonons with strong electron-phonon coupling modulate the electronic pseudogap as well as mediate charge and spin density wave fluctuations. Combining temperature-dependent Raman spectroscopy with density functional theory reveals two phonons, B2P and B2M, that are strongly coupled to electrons and whose scattering intensities respectively dominate in the pseudogap versus the metallic phases. The B2P squeezes the octahedra along the out of plane c-axis, while the B2M elongates it, thus modulating the Ru 4d orbital splitting and the bandwidth of the in-plane electron hopping; Thus, B2P opens the pseudogap, while B2M closes it. Moreover, the B2 phonons mediate incoherent charge and spin density wave fluctuations, as evidenced by changes in the background electronic Raman scattering that exhibit unique symmetry signatures. The polar order breaks inversion symmetry, enabling infrared activity of these phonons, paving the way for coherent light-driven control of electronic transport.

Layered Semiconductor Cr0.32Ga0.68Te2.33 with Concurrent Broken Inversion Symmetry and Ferromagnetism: A Bulk Ferrovalley Material Candidate.

The valleytronic state found in group-VI transition-metal dichalcogenides such as MoS2 has attracted immense interest since its valley degree of freedom could be used as an information carrier. However, valleytronic applications require spontaneous valley polarization. Such an electronic state is predicted to be accessible in a new ferroic family of materials, i.e., ferrovalley materials, which features the coexistence of spontaneous spin and valley polarization. Although many atomic monolayer materials with hexagonal lattices have been predicted to be ferrovalley materials, no bulk ferrovalley material candidates have been reported or proposed. Here, we show that a new non-centrosymmetric van der Waals (vdW) semiconductor Cr0.32Ga0.68Te2.33, with intrinsic ferromagnetism, is a possible candidate for bulk ferrovalley material. This material exhibits several remarkable characteristics: (i) it forms a natural heterostructure between vdW gaps, a quasi-two-dimensional (2D) semiconducting Te layer with a honeycomb lattice stacked on the 2D ferromagnetic slab comprised of the (Cr, Ga)-Te layers, and (ii) the 2D Te honeycomb lattice yields a valley-like electronic structure near the Fermi level, which, in combination with inversion symmetry breaking, ferromagnetism, and strong spin-orbit coupling contributed by heavy Te element, creates a possible bulk spin-valley locked electronic state with valley polarization as suggested by our DFT calculations. Further, this material can also be easily exfoliated to 2D atomically thin layers. Therefore, this material offers a unique platform to explore the physics of valleytronic states with spontaneous spin and valley polarization in both bulk and 2D atomic crystals.

Strong room-temperature bulk nonlinear Hall effect in a spin-valley locked Dirac material.

Nonlinear Hall effect (NLHE) is a new type of Hall effect with wide application prospects. Practical device applications require strong NLHE at room temperature (RT). However, previously reported NLHEs are all low-temperature phenomena except for the surface NLHE of TaIrTe4. Bulk RT NLHE is highly desired due to its ability to generate large photocurrent. Here, we show the spin-valley locked Dirac state in BaMnSb2 can generate a strong bulk NLHE at RT. In the microscale devices, we observe the typical signature of an intrinsic NLHE, i.e. the transverse Hall voltage quadratically scales with the longitudinal current as the current is applied to the Berry curvature dipole direction. Furthermore, we also demonstrate our nonlinear Hall device's functionality in wireless microwave detection and frequency doubling. These findings broaden the coupled spin and valley physics from 2D systems into a 3D system and lay a foundation for exploring bulk NLHE's applications.

Large Exchange Coupling Between Localized Spins and Topological Bands in MnBi2 Te4.

Magnetism in topological materials creates phases exhibiting quantized transport phenomena with potential technological applications. The emergence of such phases relies on strong interaction between localized spins and the topological bands, and the consequent formation of an exchange gap. However, this remains experimentally unquantified in intrinsic magnetic topological materials. Here, this interaction is quantified in MnBi2 Te4 , a topological insulator with intrinsic antiferromagnetism. This is achieved by optically exciting Bi-Te p states comprising the bulk topological bands and interrogating the consequent Mn 3d spin dynamics, using a multimodal ultrafast approach. Ultrafast electron scattering and magneto-optic measurements show that the p states demagnetize via electron-phonon scattering at picosecond timescales. Despite being energetically decoupled from the optical excitation, the Mn 3d spins, probed by resonant X-ray scattering, are observed to disorder concurrently with the p spins. Together with atomistic simulations, this reveals that the exchange coupling between localized spins and the topological bands is at least 100 times larger than the superexchange interaction, implying an optimal exchange gap of at least 25 meV in the surface states. By quantifying this exchange coupling, this study validates the materials-by-design strategy of utilizing localized magnetic order to manipulate topological phases, spanning static to ultrafast timescales.

Heteroanionic Control of Exemplary Second-Harmonic Generation and Phase Matchability in 1D LiAsS2-xSex.

The isostructural heteroanionic compounds ß-LiAsS2-xSex (x = 0, 0.25, 1, 1.75, 2) show a positive correlation between selenium content and second-harmonic response and greatly outperform the industry standard AgGaSe2. These materials crystallize in the noncentrosymmetric space group Cc as one-dimensional 1/∞ [AsQ2]- (Q = S, Se, S/Se) chains consisting of corner-sharing AsQ3 trigonal pyramids with charge-balancing Li+ atoms interspersed between the chains. LiAsS2-xSex melts congruently for 0 ≤ x ≤ 1.75, but when the Se content exceeds x = 1.75, crystallization is complicated by a phase transition. This behavior is attributed to the ß- to α-phase transition present in LiAsSe2, which is observed in the Se-rich compositions. The band gap decreases with increasing Se content, starting at 1.63 eV (LiAsS2) and reaching 1.06 eV (ß-LiAsSe2). Second-harmonic generation measurements as a function of wavelength on powder samples of ß-LiAsS2-xSex show that these materials exhibit significantly higher nonlinearity than AgGaSe2 (d36 = 33 pm/V), reaching a maximum of 61.2 pm/V for LiAsS2. In comparison, single-crystal measurements for LiAsSSe yielded a deff = 410 pm/V. LiAsSSe, LiAsS0.25Se1.75, and ß-LiAsSe2 show phase-matching behavior for incident wavelengths exceeding 3 µm. The laser-induced damage thresholds from two-photon absorption processes are on the same order of magnitude as AgGaSe2, with S-rich materials slightly outperforming AgGaSe2 and Se-rich materials slightly underperforming AgGaSe2.

Interlayer magnetophononic coupling in MnBi2Te4.

The emergence of magnetism in quantum materials creates a platform to realize spin-based applications in spintronics, magnetic memory, and quantum information science. A key to unlocking new functionalities in these materials is the discovery of tunable coupling between spins and other microscopic degrees of freedom. We present evidence for interlayer magnetophononic coupling in the layered magnetic topological insulator MnBi2Te4. Employing magneto-Raman spectroscopy, we observe anomalies in phonon scattering intensities across magnetic field-driven phase transitions, despite the absence of discernible static structural changes. This behavior is a consequence of a magnetophononic wave-mixing process that allows for the excitation of zone-boundary phonons that are otherwise 'forbidden' by momentum conservation. Our microscopic model based on density functional theory calculations reveals that this phenomenon can be attributed to phonons modulating the interlayer exchange coupling. Moreover, signatures of magnetophononic coupling are also observed in the time domain through the ultrafast excitation and detection of coherent phonons across magnetic transitions. In light of the intimate connection between magnetism and topology in MnBi2Te4, the magnetophononic coupling represents an important step towards coherent on-demand manipulation of magnetic topological phases.

Ultrasensitive electrode-free and co-catalyst-free detection of nanomoles per hour hydrogen evolution for the discovery of new photocatalysts.

High throughput theoretical methods are increasingly used to identify promising photocatalytic materials for hydrogen generation from water as a clean source of energy. While most promising water splitting candidates require co-catalyst loading and electrical biasing, computational costs to predict them a priori become large. It is, therefore, important to identify bare, bias-free semiconductor photocatalysts with small initial hydrogen production rates, often in the range of tens of nanomoles per hour, as these can become highly efficient with further co-catalyst loading and biasing. Here, we report a sensitive hydrogen detection system suitable for screening new photocatalysts. The hydrogen evolution rate of the prototypical rutile TiO2 loaded with 0.3 wt. % Pt is detected to be 78.0 ± 0.8 µmol/h/0.04 g, comparable with the rates reported in the literature. In contrast, sensitivity to an ultralow evolution rate of 11.4 ± 0.3 nmol/h/0.04 g is demonstrated for bare polycrystalline TiO2 without electrical bias. Two candidate photocatalysts, ZnFe2O4 (18.1 ± 0.2 nmol/h/0.04 g) and Ca2PbO4 (35.6 ± 0.5 nmol/h/0.04 g) without electrical bias or co-catalyst loading, are demonstrated to be potentially superior to bare TiO2. This work expands the techniques available for sensitive detection of photocatalytic processes toward much faster screening of new candidate photocatalytic materials in their bare state.

Tunable Nanoscale Evolution and Topological Phase Transitions of a Polar Vortex Supercrystal.

Understanding the phase transitions and domain evolutions of mesoscale topological structures in ferroic materials is critical to realizing their potential applications in next-generation high-performance storage devices. Here, the behaviors of a mesoscale supercrystal are studied with 3D nanoscale periodicity and rotational topology phases in a PbTiO3 /SrTiO3 (PTO/STO) superlattice under thermal and electrical stimuli using a combination of phase-field simulations and X-ray diffraction experiments. A phase diagram of temperature versus polar state is constructed, showing the formation of the supercrystal from a mixed vortex and a-twin state and a temperature-dependent erasing process of a supercrystal returning to a classical a-twin structure. Under an in-plane electric field bias at room temperature, the vortex topology of the supercrystal irreversibly transforms to a new type of stripe-like supercrystal. Under an out-of-plane electric field, the vortices inside the supercrystal undergo a topological phase transition to polar skyrmions. These results demonstrate the potential for the on-demand manipulation of polar topology and transformations in supercrystals using electric fields. The findings provide a theoretical understanding that may be utilized to guide the design and control of mesoscale polar structures and to explore novel polar structures in other systems and their topological nature.

Extreme Ultraviolet Second Harmonic Generation Spectroscopy in a Polar Metal.

The coexistence of ferroelectricity and metallicity seems paradoxical, since the itinerant electrons in metals should screen the long-range dipole interactions necessary for dipole ordering. The recent discovery of the polar metal LiOsO3 was therefore surprising [as discussed earlier in Y. Shi et al., Nat. Mater. 2013, 12, 1024]. It is thought that the coordination preferences of the Li play a key role in stabilizing the LiOsO3 polar metal phase, but an investigation from the combined viewpoints of core-state specificity and symmetry has yet to be done. Here, we apply the novel technique of extreme ultraviolet second harmonic generation (XUV-SHG) and find a sensitivity to the broken inversion symmetry in the polar metal phase of LiOsO3 with an enhanced feature above the Li K-edge that reflects the degree of Li atom displacement as corroborated by density functional theory calculations. These results pave the way for time-resolved probing of symmetry-breaking structural phase transitions on femtosecond time scales with element specificity.

Relativistic spacetime crystals.

Periodic space crystals are well established and widely used in physical sciences. Time crystals have been increasingly explored more recently, where time is disconnected from space. Periodic relativistic spacetime crystals on the other hand need to account for the mixing of space and time in special relativity through Lorentz transformation, and have been listed only in 2D. This work shows that there exists a transformation between the conventional Minkowski spacetime (MS) and what is referred to here as renormalized blended spacetime (RBS); they are shown to be equivalent descriptions of relativistic physics in flat spacetime. There are two elements to this reformulation of MS, namely, blending and renormalization. When observers in two inertial frames adopt each other's clocks as their own, while retaining their original space coordinates, the observers become blended. This process reformulates the Lorentz boosts into Euclidean rotations while retaining the original spacetime hyperbola describing worldlines of constant spacetime length from the origin. By renormalizing the blended coordinates with an appropriate factor that is a function of the relative velocities between the various frames, the hyperbola is transformed into a Euclidean circle. With these two steps, one obtains the RBS coordinates complete with new light lines, but now with a Euclidean construction. One can now enumerate the RBS point and space groups in various dimensions with their mapping to the well known space crystal groups. The RBS point group for flat isotropic RBS spacetime is identified to be that of cylinders in various dimensions: mm2 which is that of a rectangle in 2D, (∞/m)m which is that of a cylinder in 3D, and that of a hypercylinder in 4D. An antisymmetry operation is introduced that can swap between space-like and time-like directions, leading to color spacetime groups. The formalism reveals RBS symmetries that are not readily apparent in the conventional MS formulation. Mathematica script is provided for plotting the MS and RBS geometries discussed in the work.

Electric Field-Induced Polarization Responses of Noncentrosymmetric Crystalline Biopolymers in Different Frequency Regimes - A Case Study on Unidirectionally Aligned ß-Chitin Crystals.

A dielectric medium containing noncentrosymmetric domains can exhibit piezoelectric and second-harmonic generation (SHG) responses when an electric field is applied. Since many crystalline biopolymers have noncentrosymmetric structures, there has been a great deal of interest in exploiting their piezoelectric and SHG responses for electromechanical and electro-optic devices, especially owing to their advantages such as biocompatibility and low density. However, exact mechanisms or origins of such polarization responses of crystalline biopolymers remain elusive due to the convolution of responses from multiple domains with varying degrees of structural disorder or difficulty of ensuring the unidirectional alignment of noncentrosymmetric domains. In this study, we investigate the polarization responses of a noncentrosymmetric crystalline biopolymer, namely, unidirectionally aligned ß-chitin crystals interspersed in the amorphous protein matrix, which can be obtained naturally from tubeworm Lamellibrachia satsuma (LS) tube. The mechanisms governing polarization responses in different dynamic regimes covering optical (>1013 Hz), acoustic/ultrasonic (103-105 Hz), and low (10-2-102 Hz) frequencies are explained. Relationships between the polarization responses dominant in different frequencies are addressed. Also, electromechanical coupling responses, including piezoelectricity of the LS tube, are quantitatively discussed. The findings of this study can be applicable to other noncentrosymmetric crystalline biopolymers, elucidating their polarization responses.

Subterahertz collective dynamics of polar vortices.

The collective dynamics of topological structures1-6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices3. A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.

Shear-induced unidirectional deposition of bacterial cellulose microfibrils using rising bubble stream cultivation.

In crystalline cellulose I, all glucan chains are ordered from reducing ends to non-reducing ends. Thus, the polarity of individual chains is added forming a large dipole within the crystal. If one can engineer unidirectional alignment (parallel packing) of cellulose crystals, then it might be possible to utilize the material properties originating from polar crystalline structures. However, most post-synthesis manipulation methods reported so far can only achieve the uniaxial alignment with bi-directionality (antiparallel packing). Here, we report a method to induce the parallel packing of bacterial cellulose microfibrils by applying unidirectional shear stress during the synthesis and deposition through the rising bubble stream in a culture medium. Driving force for the alignment is explained with mathematical estimation of the shear stress. Evidences of the parallel alignment of crystalline cellulose Iα domains were obtained using nonlinear optical spectroscopy techniques.

Cocrystalline Polymer Films Exhibiting Second-Order Nonlinear Optical Properties.

Tailored polymer materials exhibiting high-glass transition temperatures, cross-linked matrices, and/or strong intermolecular interactions containing electric-field poled nonlinear optical (NLO) chromophores are promising materials for applications in optical telecommunication, high-performance computing, and data transmission. Although the current design parameters have led to significant advances in NLO materials, we introduce an alternative, yet highly effective, approach in which a NLO chromophore is cocrystallized with a polymer, forming a noncentrosymmetric hybrid host-guest complex. Specifically, poly(ethylene oxide) (PEO) and 2-chloro-4-nitroaniline (CNA) will cocrystallize and exhibit second harmonic generation (SHG) activity due to the formation of a noncentrosymmetric cocrystalline unit cell where the chromophore exhibits acentric alignment. Furthermore, the hybrid PEO/CNA films exhibit interesting SHG activity at elevated temperature in which SHG intensity decreases to zero when the cocrystal orientation randomizes due to sample melting. Aligning and maintaining a cocrystalline domain orientation via the formation of hybrid host-guest complexes, while imparting SHG properties, is an innovative approach for creating materials exhibiting SHG properties.