VO2 features concomitant structural and metal-insulator transitions. This poses a challenge for understanding the underlying mechanism: Is the transition triggered by a structural or by an electronic instability? The two scenarios are expected to produce very different pretransitional fluctuations above TC. By combining magnetic susceptibility, IR reflectivity, and x-ray diffuse scattering measurements, we observe that metallic VO2 features strong electronic and structural fluctuations toward the insulating monoclinic phase. By measuring resonant diffuse x-ray scattering across the vanadium K edge, we search for a potential decoupling between electronic and structural ordering in these fluctuations, finding no evidence of it. While our results do not completely rule out pure electronic fluctuations, they constrain them, favoring the interpretation that the VO2 metal-insulator transition is triggered by a structural instability. Our work offers a unique approach to solve similar problems in other strongly correlated systems.

Achieving both volatile and nonvolatile resistive switching in the same material platform is a key milestone towards the development of dense hardware-based neuromorphic computing. We show that, by suitable choice of electrode metals, a single oxide film can feature both types of behavior. We demonstrate this for two oxides featuring an insulator-to-metal transition: SmNiO3 and NdNiO3. When a current is applied across two-terminal Pt microdevices, the metal/oxide interface is ohmic and most of the voltage drops horizontally along the oxide film, triggering the transition and inducing volatile resistive switching. However, when Ti electrodes are used, a TiOx layer forms at the metal/oxide interface, across which a strong electric field develops upon application of a current, inducing nonvolatile resistive switching due to vacancy migration. These two electrical behaviors can be observed at the same temperature and coexist at the microscale, leading to different possibilities for implementing neuristors and memristors in the same microcircuit.

Many correlated oxides feature an insulator-to-metal transition as the temperature is increased. In thin films, this transition can be electrically induced by localized Joule heating, resulting in volatile resistive switching. Considering the importance of thermal effects, the thermal conductivity of the underlying substrate is expected to play a key role. Despite this, its influence has not been experimentally explored. Here, we compare the resistive switching of VO2 films grown on two substrates with very different thermal conductivities k: sapphire [k<<^>> 50 W/(m K)] and mica [k<<^>> 0.5 W/(m K)]. While the overall features of the electrical switching are similar, the VO2-mica devices need around one order of magnitude less power to be switched, and their switching time is shorter. This can be understood by the improved thermal insulation offered by the substrate, which keeps the heat within the film. Our work shows that thermal insulators, such as mica, are a promising platform for energy-efficient volatile resistive switching.

The epitaxial growth of complex oxides enables the production of high-quality films, yet substrate choice is restricted to certain symmetry and lattice parameters, thereby limiting the technological applications of epitaxial oxides. In comparison, the development of free-standing oxide membranes gives opportunities to create novel heterostructures by nonepitaxial stacking of membranes, opening new possibilities for materials design. Here, we introduce a method for writing, with atomic precision, ionically bonded crystalline materials across the gap between an oxide membrane and a carrier substrate. The process involves a thermal pretreatment, followed by localized exposure to the raster scan of a scanning transmission electron microscopy (STEM) beam. STEM imaging and electron energy-loss spectroscopy show that we achieve atomically sharp interface reconstructions between a 30-nm-thick SrTiO3 membrane and a niobium-doped SrTiO3(001)-oriented carrier substrate. These findings indicate new strategies for fabricating synthetic heterostructures with novel structural and electronic properties.

Recent theoretical studies have suggested that transition metal perovskite oxide membranes can enable surface phonon polaritons in the infrared range with low loss and much stronger subwavelength confinement than bulk crystals. Such modes, however, have not been experimentally observed so far. Here, using a combination of far-field Fourier-transform infrared (FTIR) spectroscopy and near-field synchrotron infrared nanospectroscopy (SINS) imaging, we study the phonon-polaritons in a 100 nm thick freestanding crystalline membrane of SrTiO3 transferred on metallic and dielectric substrates. We observe a symmetric-antisymmetric mode splitting giving rise to epsilon-near-zero and Berreman modes as well as highly confined (by a factor of 10) propagating phonon polaritons, both of which result from the deep-subwavelength thickness of the membranes. Theoretical modeling based on the analytical finite-dipole model and numerical finite-difference methods fully corroborate the experimental results. Our work reveals the potential of oxide membranes as a promising platform for infrared photonics and polaritonics.

We investigate phonon lifetimes in VO2 single crystals. We do so in the metallic state above the metal-insulator transition (MIT), where strong structural fluctuations are known to take place. By combining inelastic X-ray scattering and Raman spectroscopy, we track the temperature dependence of several acoustic and optical phonon modes up to 1000 K. Contrary to what is commonly observed, we find that phonon lifetimes decrease with decreasing temperature. Our results show that pre-transitional fluctuations in the metallic state give rise to strong electron-phonon scattering that onsets hundreds of degrees above the transition and increases as the MIT is approached. Notably, this effect is not limited to specific points of reciprocal space that could be associated with the structural transition.

We investigate the oxygen isotope effect on the VO2 metal-insulator transition. Using an alternative method, we synthesize V16 O2 and V18 O2 crystals, finding a 1%-3% phonon softening and a 1.3 K increase in the metal-insulator transition temperature for the latter. A simple calculation, further confirmed by density functional theory, shows that this shift can be attributed to changes in the lattice internal energy. Our results show that lattice dynamics plays a key role in setting the electronic transition temperature, indicating that electronic and structural degrees of freedom remain strongly coupled at the transition.

Materials tuned to a quantum critical point display universal scaling properties as a function of temperature $T$ and frequency $\omega$. A long-standing puzzle regarding cuprate superconductors has been the observed power-law dependence of optical conductivity with an exponent smaller than one, in contrast to $T$-linear dependence of the resistivity and $\omega$-linear dependence of the optical scattering rate. Here, we present and analyze resistivity and optical conductivity of La$_{2-x}$Sr$_x$CuO$_4$ with $x=0.24$. We demonstrate $\hbar\omega/k_{\mathrm{B}} T$ scaling of the optical data over a wide range of frequency and temperature, $T$-linear resistivity, and optical effective mass proportional to $\sim \ln T$ corroborating previous specific heat experiments. We show that a $T,\omega$-linear scaling Ansatz for the inelastic scattering rate leads to a unified theoretical description of the experimental data, including the power-law of the optical conductivity. This theoretical framework provides new opportunities for describing the unique properties of quantum critical matter.

Phonon polaritons are promising for infrared applications due to a strong light-matter coupling and subwavelength energy confinement they offer. Yet, the spectral narrowness of the phonon bands and difficulty to tune the phonon polariton properties hinder further progress in this field. SrTiO3 - a prototype perovskite oxide - has recently attracted attention due to two prominent far-infrared phonon polaritons bands, albeit without any tuning reported so far. Here we show, using cryogenic infrared near-field microscopy, that long-propagating surface phonon polaritons are present both in bare SrTiO3 and in LaAlO3/SrTiO3 heterostructures hosting a two-dimensional electron gas. The presence of the two-dimensional electron gas increases dramatically the thermal variation of the upper limit of the surface phonon polariton band due to temperature dependent polaronic screening of the surface charge carriers. Furthermore, we demonstrate a tunability of the upper surface phonon polariton frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that oxide interfaces are a new platform bridging unconventional electronics and long-wavelength nanophotonics.

Some correlated materials display an insulator-to-metal transition as the temperature is increased. In most cases this transition can also be induced electrically, resulting in volatile resistive switching due to the formation of a conducting filament. While this phenomenon has attracted much attention due to potential applications, many fundamental questions remain unaddressed. One of them is its characteristic lengths: what sets the size of these filaments, and how does this impact resistive switching properties. Here we use a combination of wide-field and scattering-type scanning near-field optical microscopies to characterize filament formation in NdNiO3 and SmNiO3 thin films. We find a clear trend: smaller filaments increase the current density, yielding sharper switching and a larger resistive drop. With the aid of numerical simulations, we discuss the parameters controlling the filament width and, hence, the switching properties.

The frustrated magnet Nd_2Ir_2O_7, where strong correlations together with spin-orbit coupling play a crucial role, is predicted to be a Weyl semimetal and to host topological pairs of bulk Dirac-like valleys. Here we use an external magnetic field to manipulate the localized rare earth 4f moments coupled to the 5d electronic bands. Low energy optical spectroscopy reveals that a field of only a few teslas suffices to create charge compensating pockets of holes and electrons in different regions of momentum space, thus introducing a valley population shift that can be tuned with the field.

Doped strontium titanate SrTiO3 (STO) is one of the most dilute superconductors known today. The fact that superconductivity occurs at very low carrier concentrations is one of the two reasons that the pairing mechanism is not yet understood, the other is the role played by the proximity to a ferroelectric instability. In undoped STO, ferroelectric order can in fact be stabilized by substituting 16O with its heavier isotope 18O. Here we explore the superconducting properties of doped and isotope-substituted SrTi(18O16 y O1−y)3−δ for 0 ≤ y ≤ 0.81 and carrier concentrations between 6×1017 and 2 × 1020 cm−3 (δ < 0.02). We show that the superconducting Tc increases when the 18O concentration is increased. For carrier concentrations around 5×1019 cm−3 this Tc increase amounts to almost a factor 3, with Tc as high as 580 mK for y = 0.74. When approaching SrTiO3 the maximum Tc occurs at a much smaller carrier densities than for pure SrTiO3. Our observations agree qualitatively with a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft mode.

A liquid of superconducting vortices generates a transverse thermoelectric response. This Nernst signal has a tail deep in the normal state due to superconducting fluctuations. Here, we present a study of the Nernst effect in two-dimensional heterostructures of Nb-doped strontium titanate (STO) and in amorphous MoGe. The Nernst signal generated by ephemeral Cooper pairs above the critical temperature has the magnitude expected by theory in STO. On the other hand, the peak amplitude of the vortex Nernst signal below T_{c} is comparable in both and in numerous other superconductors despite the large distribution of the critical temperature and the critical magnetic fields. In four superconductors belonging to different families, the maximum Nernst signal corresponds to an entropy per vortex per layer of ≈k_{B}ln2.

Clément Collignon, 2, ∗ Yudai Awashima, Ravi, Xiao Lin, † Carl Willem Rischau, ‡ Anissa Acheche, Baptiste Vignolle, 5 Cyril Proust, Yuki Fuseya, 6 Kamran Behnia, and Benoit Fauqué § JEIP, USR 3573 CNRS, Collège de France, PSL Research University, 11, place Marcelin Berthelot, 75231 Paris Cedex 05, France Laboratoire de Physique et d’Étude des Matériaux (ESPCI Paris CNRS Sorbonne Université), PSL Research University, 75005 Paris, France Department of Engineering Science, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan Laboratoire National des Champs Magnétiques Intenses (LNCMI-EMFL), CNRS ,UGA, UPS, INSA, Grenoble/Toulouse, France Institut de Chimie de la Matière Condensée, Bordeaux, France Institute for Advanced Science, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan (Dated: January 25, 2021)

Nd2Ir2O7 is a correlated semimetal with the pyrochlore structure, in which competing spin–orbit coupling and electron–electron interactions are believed to induce a time-reversal symmetry-broken Weyl semimetal phase characterized by pairs of topologically protected Dirac points at the Fermi energy1–4. However, the emergent properties in these materials are far from clear, and exotic new states of matter have been conjectured5–7. Here, we demonstrate optically that, at low temperatures, the free carrier spectral weight is proportional to T2, where T is the temperature, as expected for massless Dirac electrons. However, we do not observe the corresponding T3 term in the specific heat. That the system is not in a Fermi liquid state is further corroborated by the charge carrier scattering rate approaching critical damping and the progressive opening of a correlation-induced gap at low temperatures. These observations cannot be reconciled within the framework of band theory of electron-like quasiparticles and point towards the effective decoupling of the charge transport from the single particle sector. Transport and optical conductivity measurements reveal the non-Fermi liquid behaviour in correlated semimetal Nd2Ir2O7. The result implies the emergent collective charge transport in this compound, not reconcilable with conventional band theory.

Lightly doped III-V semiconductor InAs is a dilute metal, which can be pushed beyond its extreme quantum limit upon the application of a modest magnetic field. In this regime, a Mott-Anderson metal-insulator transition, triggered by the magnetic field, leads to a depletion of carrier concentration by more than one order of magnitude. Here, we show that this transition is accompanied by a two-hundred-fold enhancement of the Seebeck coefficient which becomes as large as 11.3mV.K$^{-1}\approx 130\frac{k_B}{e}$ at T=8K and B=29T. We find that the magnitude of this signal depends on sample dimensions and conclude that it is caused by phonon drag, resulting from a large difference between the scattering time of phonons (which are almost ballistic) and electrons (which are almost localized in the insulating state). Our results reveal a path to distinguish between possible sources of large thermoelectric response in other low density systems pushed beyond the quantum limit.

SrTiO3 is an insulating material which, using chemical doping, pressure, strain or isotope substitution, can be turned into a ferroelectric material or into a superconductor. The material itself, and the two aforementioned phenomena, have been subjects of intensive research of Karl Alex Müller and have been a source of inspiration, among other things, for his Nobel prize-winning research on high temperature superconductivity. An intriguing outstanding question is whether the occurrence of ferroelectricity and superconductivity in the same material is just a coincidence, or whether a deeper connection exists. In addition there is the empirical question of how these two phenomena interact with each other. Here we show that it is possible to induce superconductivity in a two-dimensional layer at the interface of SrTiO3 and LaAlO3 when we make the SrTiO3 ferroelectric by means of 18O substitution. Our experiments indicate that the ferroelectricity is perfectly compatible with having a superconducting two-dimensional electron system at the interface. This provides a promising avenue for manipulating superconductivity in a non centrosymmetric environment.

The soft ferroelectric phonon in SrTiO3 observed with optical spectroscopy has an extraordinarily strong spectral weight which is much stronger than expected in the limit of a perfectly ionic compound. This "charged phonon" effect in SrTiO3 is caused by the close-to-covalent character of the Ti-O ionic bond and implies a strong coupling between the soft ferroelectric phonon and the interband transitions across the 3-eV gap of SrTiO3. We demonstrate that this coupling leads, in addition to the charged phonon effect, to a pairing interaction involving the exchange of two transverse optical phonons. This process owes its relevance to the strong electron-phonon coupling and to the fact that the interaction mediated by a single transverse optical phonon vanishes at low electron density. We use the experimental soft phonon spectral weight to calculate the strength of the biphonon mediated pairing interaction in the electron-doped material and show that it is of the correct magnitude when compared to the experimental value of the superconducting critical temperature. Biphonon exchange is therefore an important pairing mechanism at low doping, and may be the key to understanding the occurrence of superconductivity in doped SrTiO3 and other low electron density materials.

The fate of electric dipoles inside a Fermi sea is an old issue, yet poorly explored. Sr _1-x Ca _x TiO _3 hosts a robust but dilute ferroelectricity in a narrow ( 0.0018 < x < 0.02 ) window of substitution. This insulator becomes metallic by removal of a tiny fraction of its oxygen atoms. Here, we present a detailed study of low-temperature charge transport in Sr _1-x Ca _x TiO _3-δ , documenting the evolution of resistivity with increasing carrier concentration ( n ). Below a threshold carrier concentration, n^* (x) , the polar structural-phase transition has a clear signature in resistivity and Ca substitution significantly reduces the 2 K mobility at a given carrier density. For three different Ca concentrations, we find that the phase transition fades away when one mobile electron is introduced for about 7.9± 0.6 dipoles. This threshold corresponds to the expected peak in anti-ferroelectric coupling mediated by a diplolar counterpart of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. Our results imply that the transition is driven by dipole–dipole interaction, even in presence of a dilute Fermi sea. Charge transport for n < n^* (x) shows a non-monotonic temperature dependence, most probably caused by scattering off the transverse optical phonon mode. A quantitative explanation of charge transport in this polar metal remains a challenge to theory. For n≥n^* (x) , resistivity follows a T-square behavior together with slight upturns (in both Ca-free and Ca-substituted samples). The latter are reminiscent of Kondo effect and most probably due to oxygen vacancies.