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The interface between the insulators LaAlO$_3$ and SrTiO$_3$ accommodates a two-dimensional electron liquid (2DEL) -- a high mobility electron system exhibiting superconductivity as well as indications of magnetism and correlations. While this flagship oxide heterostructure shows promise for electronics applications, the origin and microscopic properties of the 2DEL remain unclear. The uncertainty remains in part because the electronic structures of such nanoscale buried interfaces are difficult to probe, and is compounded by the variable presence of oxygen vacancies and coexistence of both localized and delocalized charges. These various complications have precluded decisive tests of intrinsic electronic and orbital reconstruction at this interface. Here we overcome prior difficulties by developing an interface analysis based on the inherently interface-sensitive resonant x-ray reflectometry. We discover a high charge density of 0.5 electrons per interfacial unit cell for samples above the critical LaAlO$_3$ thickness, and extract the depth dependence of both the orbital and electronic reconstructions near the buried interface. We find that the majority of the reconstruction phenomena are confined to within 2 unit cells of the interface, and we quantify how oxygen vacancies significantly affect the electronic system. Our results provide strong support for the existence of polarity induced electronic reconstruction, clearly separating its effects from those of oxygen vacancies., Accepted in Physical Review Materials

We present a comprehensive study of single crystals of ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$, a putative Kitaev honeycomb magnet, focusing on its low-temperature phase behaviors. A new thermal phase transition is identified at 31.0 K, below which the system develops a two-dimensional (2D) long-range magnetic order. This order precedes the well-known three-dimensional (3D) order below 26.7 K, and is likely driven by strongly anisotropic interactions. Surprisingly, excitations from the 3D order do not support the order's commonly accepted ``zigzag'' nature, and are instead consistent with a ``triple-$\mathbf{q}$'' description, which has remained hitherto unexplored in the study of Kitaev honeycomb magnets. The 3D order exerts a fundamental feedback on high-energy excitations that likely involve orbital degrees of freedom, and it features strongly scattered spin waves until at much lower temperatures, a sign for ground state near degeneracy and frustration. These findings constrain microscopic models and render ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$ a spin-orbit entangled frustrated magnet that hosts very rich physics.

The authors measure the effects of uniaxial tensile-stress on a prototypical stripe-ordered cuprate, showing that a modest amount of stress creates large changes in the phase diagram.

In strongly correlated systems the strength of Coulomb interactions between electrons, relative to their kinetic energy, plays a central role in determining their emergent quantum mechanical phases. We perform resonant x-ray scattering on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, a prototypical cuprate superconductor, to probe electronic correlations within the CuO$_2$ plane. We discover a dynamic quasi-circular pattern in the $x$-$y$ scattering plane with a radius that matches the wave vector magnitude of the well-known static charge order. Along with doping- and temperature-dependent measurements, our experiments reveal a picture of charge order competing with superconductivity where short-range domains along $x$ and $y$ can dynamically rotate into any other in-plane direction. This quasi-circular spectrum, a hallmark of Brazovskii-type fluctuations, has immediate consequences to our understanding of rotational and translational symmetry breaking in the cuprates. We discuss how the combination of short- and long-range Coulomb interactions results in an effective non-monotonic potential that may determine the quasi-circular pattern., Comment: This is a post-peer-review, pre-copyedit version of an article published in Nature Communications. The final authenticated version is available online at: https://doi.org/10.1038/s41467-020-20824-7. Supplementary materials are available through the published version in Nature Communications

Charge density wave (CDW) order has been shown to compete and coexist with superconductivity in underdoped cuprates. Theoretical proposals for the CDW order include an unconventional $d$-symmetry form factor CDW, evidence for which has emerged from measurements, including resonant soft x-ray scattering (RSXS) in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO). Here, we revisit RSXS measurements of the CDW symmetry in YBCO, using a variation in the measurement geometry to provide enhanced sensitivity to orbital symmetry. We show that the $(0\ 0.31\ L)$ CDW peak measured at the Cu $L$ edge is dominated by an $s$ form factor rather than a $d$ form factor as was reported previously. In addition, by measuring both $(0.31\ 0\ L)$ and $(0\ 0.31\ L)$ peaks, we identify a pronounced difference in the orbital symmetry of the CDW order along the $a$ and $b$ axes, with the CDW along the $a$ axis exhibiting orbital order in addition to charge order., 17 pages, 4 figures + supplementary information

Epitaxial TiO(001) film grown by MBE is found to be the only metal and superconductor among 3d transition-metal monoxides., Titanium monoxide (TiO), an important member of the rock salt 3d transition-metal monoxides, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti2+. We adapt a closely lattice-matched MgO(001) substrate and report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO thin films, showing that TiO is metal but in proximity to Mott insulating state. We observe a transition to the superconducting phase below 0.5 K close to that of Ti metal. Density functional theory (DFT) and a DFT-based tight-binding model demonstrate the extreme importance of direct Ti–Ti bonding in TiO, suggesting that similar superconductivity exists in TiO and Ti metal. Our work introduces the new concept that TiO behaves more similar to its metal counterpart, distinguishing it from other 3d transition-metal monoxides.

Using resonant x-ray diffraction and Raman spectroscopy, we study charge correlations and lattice dynamics in two model cuprates, ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ and ${\mathrm{HgBa}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{6+\ensuremath{\delta}}$. We observe a maximum of the characteristic charge order temperature around the same hole concentration ($p\ensuremath{\approx}0.09$) in both compounds, and concomitant pronounced anomalies in the lattice dynamics that involve the motion of atoms in and/or adjacent to the ${\mathrm{CuO}}_{2}$ layers. These anomalies are already present at room temperature, and therefore precede the formation of the static charge correlations, and we attribute them to an instability of the ${\mathrm{CuO}}_{2}$ layers. Our finding implies that the charge order in the cuprates is an emergent phenomenon, driven by a fundamental variation in both lattice and electronic properties as a function of doping.

CrAs is a well-known helimagnet with the double-helix structure originating from the competition between the Dzyaloshinskii-Moriya interaction (DMI) and antiferromagnetic exchange interaction $J$. By resonant soft-x-ray scattering, we observe the magnetic peak (0 0 ${q}_{m}$) that emerges at the helical transition with ${T}_{S}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}267.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Intriguingly, the helimagnetic domains significantly shrink on cooling below $\ensuremath{\sim}255\phantom{\rule{4pt}{0ex}}\mathrm{K}$, opposite to the conventional thermal effect. The weakening of DMI on cooling is found to play a critical role here. It causes the helical wave vector to vary, ordered spins to rotate, and extra helimagnetic domain boundaries to form at local defects, thus leading to the anomalous shrinkage of helimagnetic domains. Our results indicate that the size of magnetic helical domains can be controlled by tuning DMI in certain helimagnets.

Hole-doped perovskite bismuthates such as ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ and ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate ${\mathrm{SrBiO}}_{3}$ on SrO-terminated ${\mathrm{SrTiO}}_{3}$ (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature codeposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. ${\mathrm{SrBiO}}_{3}$ has a pseudocubic lattice constant $\ensuremath{\approx}4.25$ \AA{} and an $\ensuremath{\approx}8.8%$ lattice mismatch on ${\mathrm{SrTiO}}_{3}$ substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature codeposition, the recrystallization method can produce higher quality 1- to 6-unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlated-electron perovskite rare-earth nickelates, RNiO(3) (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott–Hubbard electron–electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO(3) can be controlled via an electric field, leading to analog resistance switching behavior. This study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence.

Magnetic ordering phenomena have a profound influence on the macroscopic properties of correlated-electron materials, but their realistic prediction remains a formidable challenge. An archetypical example is the ternary nickel oxide system RNiO3 (R = rare earth), where the period-four magnetic order with proposals of collinear and non-collinear structures and the amplitude of magnetic moments on different Ni sublattices have been subjects of debate for decades. Here we introduce an elementary model system - NdNiO3 slabs embedded in a non-magnetic NdGaO3 matrix - and use polarized resonant x-ray scattering (RXS) to show that both collinear and non-collinear magnetic structures can be realized, depending on the slab thickness. The crossover between both spin structures is correctly predicted by density functional theory and can be qualitatively understood in a low-energy spin model. We further demonstrate that the amplitude ratio of magnetic moments in neighboring NiO6 octahedra can be accurately determined by RXS in combination with a correlated double cluster model. Targeted synthesis of model systems with controlled thickness and synergistic application of polarized RXS and ab-initio theory thus provide new perspectives for research on complex magnetism, in analogy to two-dimensional materials created by exfoliation., This is the accepted version of the manuscript. The revised manuscript according to peer reviews was published online in Nature Physics on July 23, 2018

In this Letter, we report a resonant x-ray scattering measurement of stripelike charge order in the 1/8th doped component of electronically phase-separated, orthorhombic ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+y}$. This observation is coupled to the absence of any resonant (001) peak, which at different resonant energies has been identified with the presence of low-temperature-tetragonal-like structural tilt patterns or nematicity in the CuO planes. Thus, we provide evidence that structural pinning is not necessary for the formation of static charge stripes and that the relationship between charge nematicity and stripes may not be simple.

The electronic-structural modulations of ${\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}{\mathrm{Te}}_{2}$ ($0\ensuremath{\leqq}x\ensuremath{\leqq}0.12$) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructures with wave vectors of $\mathbf{Q}=(1/5\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/5)$, $(1/8\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/8)$, and $(1/6\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/6)$ are observed on the same sample of ${\mathrm{IrTe}}_{2}$ at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around $\mathbf{Q}=(1/5\phantom{\rule{4pt}{0ex}}0\phantom{\rule{4pt}{0ex}}\ensuremath{-}1/5)$ are observed for doped samples ($0.02\ensuremath{\leqq}x\ensuremath{\leqq}0.05$). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge-density wave (CDW) can coexist with superconductivity. The analysis of the energy-dependent REXS and RIXS line shape reveals the importance of the Te $5p$ state rather than the Ir $5d$ states in the formation of the spatial modulation of these systems. The phase diagram reexamined in this work suggests that the CDW incommensurability may correlate with the emergence of superconducting states such as ${\mathrm{Cu}}_{x}{\mathrm{TiSe}}_{2}$ and ${\mathrm{Li}}_{x}{\mathrm{TaS}}_{2}$.

Charge-ordered ground states permeate the phenomenology of 3d-based transition metal oxides, and more generally represent a distinctive hallmark of strongly-correlated states of matter. The recent discovery of charge order in various cuprate families fueled new interest into the role played by this incipient broken symmetry within the complex phase diagram of high-Tc superconductors. Here we use resonant X-ray scattering to resolve the main characteristics of the charge-modulated state in two cuprate families: Bi2201 and YBCO. We detect no signatures of spatial modulations along the nodal direction in Bi2201, thus clarifying the inter-unit-cell momentum-structure of charge order. We also resolve the intra-unit-cell symmetry of the charge ordered state, which is revealed to be best represented by a bond-order with modulated charges on the O-2p orbitals and a prominent d-wave character. These results provide insights on the microscopic description of charge order in cuprates, and on its origin and interplay with superconductivity., Comment: A high-resolution version with supplementary material can be found at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/CDW_symmetry.pdf

Picking out the elusive stripes Copper-oxide superconductors have periodic modulations of charge density. Typically, the modulation is not the same for the whole crystal, but breaks up into small nanosized domains. Bulk experiments show that the density is modulated along both axes in the copper-oxide plane, but it is not clear whether this is true only on the scale of the whole crystal or also locally, for each domain. Comin et al. analyzed the charge order in the compound YBa 2 Cu 3 O 6+y , using resonant x-ray scattering, and found that it was consistent with a local unidirectional, so-called stripy, ordering. Science , this issue p. 1335

Polar discontinuities and redox reactions provide alternative paths to create two-dimensional electron liquids (2DELs) at oxide interfaces. Herein, we report high mobility 2DELs at interfaces involving SrTiO3 (STO) achieved using polar La7/8Sr1/8MnO3 (LSMO) buffer layers to manipulate both polarities and redox reactions from disordered overlayers grown at room temperature. Using resonant x-ray reflectometry experiments, we quantify redox reactions from oxide overlayers on STO as well as polarity induced electronic reconstruction at epitaxial LSMO/STO interfaces. The analysis reveals how these effects can be combined in a STO/LSMO/disordered film trilayer system to yield high mobility modulation doped 2DELs, where the buffer layer undergoes a partial transformation from perovskite to brownmillerite structure. This uncovered interplay between polar discontinuities and redox reactions via buffer layers provides a new approach for the design of functional oxide interfaces., Nano Letters, 2017

The electronic structure of ${\mathrm{BaFe}}_{2}{X}_{3}$ ($X$ = S and Se) and ${\mathrm{CsFe}}_{2}{\mathrm{Se}}_{3}$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe $L$ edges for ${\mathrm{BaFe}}_{2}{X}_{3}$ exhibit two components, indicating that itinerant and localized Fe $3d$ sites coexist. Substantial x-ray linear dichroism is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe ladder even above the N\'eel temperature ${T}_{\mathrm{N}}$. Direct exchange interaction along the legs of the Fe ladder stabilizes the orbital and antiferromagnetic orders in ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$, while the ferromagnetic molecular orbitals are realized between the rungs in ${\mathrm{CsFe}}_{2}{\mathrm{Se}}_{3}$.

Designing materials to function in harsh environments, such as conductive aqueous media, is a problem of broad interest to a range of technologies, including energy, ocean monitoring and biological applications. The main challenge is to retain the stability and morphology of the material as it interacts dynamically with the surrounding environment. Materials that respond to mild stimuli through collective phase transitions and amplify signals could open up new avenues for sensing. Here we present the discovery of an electric-field-driven, water-mediated reversible phase change in a perovskite-structured nickelate, SmNiO3. This prototypical strongly correlated quantum material is stable in salt water, does not corrode, and allows exchange of protons with the surrounding water at ambient temperature, with the concurrent modification in electrical resistance and optical properties being capable of multi-modal readout. Besides operating both as thermistors and pH sensors, devices made of this material can detect sub-volt electric potentials in salt water. We postulate that such devices could be used in oceanic environments for monitoring electrical signals from various maritime vessels and sea creatures.

Physical review / B 96(13), 134510 (2017). doi:10.1103/PhysRevB.96.134510, We present a detailed synchrotron x-ray scattering study of the charge-density-wave (CDW) order in simple tetragonal $HgBa_{2}CuO_{4+δ}$ (Hg1201). Resonant soft x-ray scattering measurements reveal that short-range order appears at a temperature that is distinctly lower than the pseudogap temperature and in excellent agreement with a prior transient reflectivity result. Despite considerable structural differences between Hg1201 and $YBa_2Cu_3O_{6+δ}$, the CDW correlations exhibit similar doping dependencies, and we demonstrate a universal relationship between the CDW wave vector and the size of the reconstructed Fermi pocket observed in quantum oscillation experiments. The CDW correlations in Hg1201 vanish already below optimal doping, once the correlation length is comparable to the CDW modulation period, and they appear to be limited by the disorder potential from unit cells hosting two interstitial oxygen atoms. A complementary hard x-ray diffraction measurement, performed on an underdoped Hg1201 sample in magnetic fields along the crystallographic c axis of up to 16 T, provides information on the form factor of the CDW order. As expected from the single-$CuO_2$-layer structure of Hg1201, the CDW correlations vanish at half-integer values of L and appear to be peaked at integer $L$. We conclude that the atomic displacements associated with the short-range CDW order are mainly planar, within the $CuO_2$ layers., Published by APS, Woodbury, NY

Emergent phenomena in transition-metal-oxide heterostructures such as interface superconductivity and magnetism have been attributed to electronic reconstruction, which, however, is difficult to detect and characterise. Here we overcome the associated difficulties to simultaneously address the electronic degrees of freedom and distinguish interface from bulk effects by implementing a novel approach to resonant X-ray reflectivity (RXR). Our RXR study of the chemical and valance profiles along the polar (001) direction of a LaCoO3 film on NdGaO3 reveals a pronounced valence-state reconstruction from Co3+ in the bulk to Co2+ at the surface, with an areal density close to 0.5 Co2+ ions per unit cell. An identical film capped with polar (001) LaAlO3 maintains the Co3+ valence over its entire thickness. We interpret this as evidence for electronic reconstruction in the uncapped film, involving the transfer of 0.5e− per unit cell to the subsurface CoO2 layer at its LaO-terminated polar surface. A new experimental design affords insight into electronic reconstruction at the interface between oxides. A number of unexpected phenomena have been seen when one oxide meets another, with a two-dimensional electron gas in the SrTiO3/LaAlO3 system being a well-known example. The underpinning mechanism remains up for debate however: the transfer of electronic charge between interfaces has been suggested – termed ‘electronic reconstruction ’ – but an unambiguous demonstration has been hampered by experimental difficulties. An international team lead by Vladimir Hinkov from University of Wurzburg now applies elementspecific resonant x-ray reflectivity to study the LaCoO3/vacuum interface, demonstrating a change in Co valence state in LaCoO3 from 3+ in the bulk to 2+ at this interface. This data provides compelling experimental evidence for electronic reconstruction at oxide interfaces.

Understanding the interplay between charge order (CO) and other phenomena (e.g. pseudogap, antiferromagnetism, and superconductivity) is one of the central questions in the cuprate high-temperature superconductors. The discovery that similar forms of CO exist in both hole- and electron-doped cuprates opened a path to determine what subset of the CO phenomenology is universal to all the cuprates. Here, we use resonant x-ray scattering to measure the charge order correlations in electron-doped cuprates (La2-xCexCuO4 and Nd2-xCexCuO4) and their relationship to antiferromagnetism, pseudogap, and superconductivity. Detailed measurements of Nd2-xCexCuO4 show that CO is present in the x = 0.059 to 0.166 range, and that its doping dependent wavevector is consistent with the separation between straight segments of the Fermi surface. The CO onset temperature is highest between x = 0.106 and 0.166, but decreases at lower doping levels, indicating that it is not tied to the appearance of antiferromagnetic correlations or the pseudogap. Near optimal doping, where the CO wavevector is also consistent with a previously observed phonon anomaly, measurements of the CO below and above the superconducting transition temperature, or in a magnetic field, show that the CO is insensitive to superconductivity. Overall these findings indicate that, while verified in the electron-doped cuprates, material-dependent details determine whether the CO correlations acquire sufficient strength to compete for the ground state of the cuprates., Supplementary information available upon request

Resonant X-ray scattering (RXS) has recently become an increasingly important tool for the study of ordering phenomena in correlated electron systems. Yet, the interpretation of the RXS experiments remains theoretically challenging due to the complexity of the RXS cross-section. Central to this debate is the recent proposal that impurity-induced Friedel oscillations, akin to quasiparticle interference signals observed with the scanning tunneling microscope (STM), can lead to scattering peaks in the RXS experiments. The possibility that quasiparticle properties can be probed in RXS measurements opens up a new avenue to study the bulk band structure of materials with the orbital and element selectivity provided by RXS. Here, we test these ideas by combining RXS and STM measurements of the heavy fermion compound CeMIn$_5$ (M = Co, Rh). Temperature and doping dependent RXS measurements at the Ce-M$_4$ edge show a broad scattering enhancement that correlates with the appearance of heavy f-electron bands in these compounds. The scattering enhancement is consistent with the measured quasiparticle interference signal in the STM measurements, indicating that quasiparticle interference can be probed through the momentum distribution of RXS signals. Overall, our experiments demonstrate new opportunities for studies of correlated electronic systems using the RXS technique., Comment: accepted to Science Advances

Fine questions our interpretation of unidirectional-stripes over bidirectional-checkerboard, and illustrates his criticism by simulating a momentum space structure consistent with our data and corresponding to a checkerboard-looking real space density. Here we use a local rotational-symmetry analysis to demonstrate that the simulated image is in actuality composed of locally unidirectional modulations of the charge density, consistent with our original conclusions., Response to original comment: B.V. Fine, Science 351, 235 (2016) (arxiv version at http://arxiv.org/abs/1602.00888)

In underdoped cuprate superconductors, a rich competition occurs between superconductivity and charge density wave (CDW) order. Whether rotational symmetry breaking (nematicity) occurs intrinsically and generically or as a consequence of other orders is under debate. Here we employ resonant x-ray scattering in stripe-ordered (La,M)_2CuO_4 to probe the relationship between electronic nematicity of the Cu 3d orbitals, structure of the (La,M)_2O_2 layers, and CDW order. We find distinct temperature dependences for the structure of the (La,M)_2O_2 layers and the electronic nematicity of the CuO_2 planes, with only the latter being enhanced by the onset of CDW order. These results identify electronic nematicity as an order parameter that is distinct from a purely structural order parameter in underdoped striped cuprates., Comment: 23 pages including supplementary information, 3 main figures, 4 supplementary figures

Vanadium dioxide $({\mathrm{VO}}_{2})$ undergoes an unusual insulator-metal transition (IMT), and after decades of study, the origin of the IMT remains hotly debated. Here, by analyzing spectral-weight transfers (SWTs) of x-ray absorption spectroscopy at the $\mathrm{V}\phantom{\rule{0.28em}{0ex}}{L}_{3,2}$ and O K edges on specially designed ${\mathrm{VO}}_{2}$ films, we observe ${d}_{||}\phantom{\rule{0.28em}{0ex}}({d}_{{x}^{2}\ensuremath{-}{y}^{2}})$ band splitting at the $\mathrm{V}\phantom{\rule{0.28em}{0ex}}{L}_{3,2}$ edges across the IMT, accompanied by anomalous SWTs as high as $\ensuremath{\sim}12\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ at the $\mathrm{O}\phantom{\rule{0.28em}{0ex}}K$ edge, indicating strong electronic correlations. Surprisingly, a few oxygen vacancies induce dramatic SWTs at the $\mathrm{O}\phantom{\rule{0.28em}{0ex}}K$ edge, but the sample remains conducting. Supported by theoretical calculations, we find that in the metallic state, direct $\mathrm{V}(3{d}_{\ensuremath{\parallel}})\text{-V}(3{d}_{\ensuremath{\parallel}})$ and $\text{O}(2p)\text{-V}(3{d}_{\ensuremath{\parallel}})$ hybridized orbital correlations are screened by $\text{O}(2p)\text{-V}(3{d}_{\ensuremath{\pi}})$ hybridized orbitals, while in the insulating state they are strongly correlated due to changes in the oxygen orbital occupancy. Our result shows the importance of screenings and electronic correlations for IMTs in ${\mathrm{VO}}_{2}$.

The understanding of the origin of superconductivity in cuprates has been hindered by the apparent diversity of intertwining electronic orders in these materials. We combined resonant x-ray scattering (REXS), scanning-tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to observe a charge order that appears consistently in surface and bulk, and in momentum and real space within one cuprate family, Bi 2 Sr 2 − x La x CuO 6 + δ . The observed wave vectors rule out simple antinodal nesting in the single-particle limit but match well with a phenomenological model of a many-body instability of the Fermi arcs. Combined with earlier observations of electronic order in other cuprate families, these findings suggest the existence of a generic charge-ordered state in underdoped cuprates and uncover its intimate connection to the pseudogap regime.

Recent theories of charge density wave (CDW) order in high temperature superconductors have predicted a primarily d CDW orbital symmetry. Here, we report on the orbital symmetry of CDW order in the canonical cuprate superconductors La1.875Ba0.125CuO4 (LBCO) and YBa2Cu3O6.67 (YBCO), using resonant soft x-ray scattering and a model mapped to the CDW orbital symmetry. From measurements sensitive to the O sublattice, we conclude that LBCO has predominantly s' CDW orbital symmetry, in contrast to the d orbital symmetry recently reported in other cuprates. Additionally, we show for YBCO that the CDW orbital symmetry differs along the a and b crystal axes and that these both differ from LBCO. This work highlights CDW orbital symmetry as an additional key property that distinguishes the different cuprate families. We discuss how the CDW symmetry may be related to the "1/8--anomaly" and to static spin ordering., 7 pages, 4 figures + supplementary (16 pages, 14 figures)

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate on a microscopic scale into hole-rich and hole-poor regions. We report resonant elastic x-ray scattering measurements of stripe-ordered La(1.475)Nd(0.4)Sr(0.125)CuO(4) at the Cu L and O K absorption edges that identify an additional feature of stripe order. Analysis of the energy dependence of the scattering intensity reveals that the dominant signature of the stripe order is a spatial modulation in the energies of Cu 3d and O 2p states rather than the large modulation of the charge density (valence) envisioned in the common stripe paradigm. These energy shifts are interpreted as a spatial modulation of the electronic structure and may point to a valence-bond-solid interpretation of the stripe phase.

We have studied the cubic to tetragonal phase transition in epitaxial SrTiO$_3$ films under various biaxial strain conditions using synchrotron X-ray diffraction. Measuring the superlattice peak associated with TiO$_6$ octahedra rotation in the low temperature tetragonal phase indicates the presence of a phase transition whose critical temperature is a strong function of strain, with T$_C$ as much as 50K above the corresponding bulk temperature. Surprisingly, the lattice constants evolve smoothly through the transition with no indication of a phase change. This signals an important change in the nature of the phase transition due to the epitaxy strain and substrate clamping effect. The internal degrees of freedom (TiO$_6$ rotations) have become uncoupled from the overall lattice shape., 4 pages, 3 figures, REVTeX 4

Recently, charge density wave (CDW) order in the ${\mathrm{CuO}}_{2}$ planes of underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}$ was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned sample of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.75}$ with ortho-III oxygen ordering in the chain layer. We show that the ortho-III CDW order in the chains is distinct from the CDW order in the planes. The ortho-III structure gives rise to a commensurate superlattice reflection at $Q=[0.33\text{ }0\text{ }L]$ whose energy and polarization dependence agrees with expectations for oxygen ordering and a spatial modulation of the Cu valence in the chains. Incommensurate peaks at [0.30 0 $L$] and [0 0.30 $L$] from the CDW order in the planes are shown to be distinct in $Q$ as well as their temperature, energy, and polarization dependence, and are thus unrelated to the structure of the chain layer. Moreover, the energy dependence of the CDW order in the planes is shown to result from a spatial modulation of energies of the Cu $2p$ to $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ transition, similar to stripe-ordered 214 cuprates.

There are increasing indications that superconductivity competes with other orders in cuprate superconductors, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of $\bf \sim 3.2$ lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$ with hole concentrations $0.09 \leq p \leq 0.13$ per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature, $T_c$; further cooling below $T_c$ abruptly reverses the divergence of the charge correlations. In combination with prior observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge-density-wave instability that competes with superconductivity., Comment: to appear in Science

We describe the design, construction and performance of a 4-circle in-vacuum diffractometer for resonant elastic soft x-ray scattering and reflectivity. The diffractometer, installed on the REIXS beamline at the Canadian Light Source, includes 9 in-vacuum motions driven by in-vacuum stepper motors and operates in ultra-high vacuum at base pressure of 2 x10^-10 Torr. Cooling to a base temperature of 18 K is provided with a closed-cycle cryostat. The diffractometer includes a choice of 3 photon detectors: a photodiode, a channeltron and a 2D sensitive channelplate detector. Along with variable slit and filter options, these detectors are suitable for studying a wide range of phenomena having both weak and strong diffraction signals. Example measurements of diffraction and reflectivity in Nd-doped (La,Sr)_2CuO_4 and thin film (Ga,Mn)As are shown., Comment: 9 pages, 8 figures

We have successfully grown epitaxial ${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ films with a small crystalline mosaic using pulsed laser deposition. With synchrotron radiation, the x-ray-diffraction peaks associated with charge stripes have been successfully observed for relatively thick films. Anomalies due to the charge-ordering transition have been examined using four-point probe resistivity measurements. X-ray scattering provides direct evidence for suppression of the stripe phase in thinner samples; the phase disappears for film thicknesses $\ensuremath{\leqslant}2600\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The suppression appears to be a result of shrinking the stripe phase domains. This may reflect the stripe phase progressing from nematic to isotropic.

SrTiO$_3$ thin films were used as a model system to study the effects of strain and epitaxial constraint on structural phase transitions of perovskite films. The basic phenomena revealed will apply to a variety of important structural transitions including the ferroelectric transition. Highly strained SrTiO$_3$ films were grown on different substrates, providing both compressive and tensile strain. The measured strain-temperature phase diagram is qualitatively consistent with theory, however the increase in the phase transition temperature is much larger than predicted. Due to the epitaxial strain and substrate clamping, the SrTiO$_3$ lattice is tetragonal at all temperatures. The phase transitions involve only changes in internal symmetry. The low temperature phase under tensile strain has a unique structure with orthorhombic $Cmcm$ space group but a tetragonal lattice, an interesting consequence of epitaxial constraint., 4 pages, 3 figures

Three different film systems have been systematically investigated to understand the effects of strain and substrate constraint on the phase transitions of perovskite films. In SrTiO$_3$ films, the phase transition temperature T$_C$ was determined by monitoring the superlattice peaks associated with rotations of TiO$_6$ octahedra. It is found that T$_C$ depends on both SrTiO$_3$ film thickness and SrRuO$_3$ buffer layer thickness. However, lattice parameter measurements showed no sign of the phase transitions, indicating that the tetragonality of the SrTiO$_3$ unit cells was no longer a good order parameter. This signals a change in the nature of this phase transition, the internal degree of freedom is decoupled from the external degree of freedom. The phase transitions occur even without lattice relaxation through domain formation. In NdNiO$_3$ thin films, it is found that the in-plane lattice parameters were clamped by the substrate, while out-of-plane lattice constant varied to accommodate the volume change across the phase transition. This shows that substrate constraint is an important parameter for epitaxial film systems, and is responsible for the suppression of external structural change in SrTiO$_3$ and NdNiO$_3$ films. However, in SrRuO$_3$ films we observed domain formation at elevated temperature through x-ray reciprocal space mapping. This indicated that internal strain energy within films also played an important role, and may dominate in some film systems. The final strain states within epitaxial films were the result of competition between multiple mechanisms and may not be described by a single parameter., REVTeX4, 14 figures

We have investigated the out-of-plane lattice relaxation related to the ferroelectric transitions in epitaxial BaTiO$_3$ (BTO) films using synchrotron X-ray diffraction. Under either compressive strain or tensile strain, there is evidence for two structural phase transitions as a function of temperature. The transition temperature $T_C$ is a strong function of strain, which can be as much as 100 K above the corresponding $T_C$ in bulk. Under compressive strain, the tetragonality of BTO unit cell implies that the polarization of the first ferroelectric phase is out-of-plane, while under tensile strain, the polarization is in-plane. The transitions at lower temperature may correspond to the $aa\to r$ or $c\to r$ transitions, following the notations by Pertsev \textit{et al}. The orientations of the domains are consistent with theoretical predictions.