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Thanks to high resolution and polarization analysis, resonant inelastic x-ray scattering (RIXS) magnetic spectra of La2CuO4, Sr2CuO2Cl2 and CaCuO2 reveal a rich set of properties of the spin 1/2 antiferromagnetic square lattice of cuprates. The leading single-magnon peak energy dispersion is in excellent agreement with the corresponding inelastic neutron scattering measurements. However, the RIXS data unveil an asymmetric lineshape possibly due to odd higher order terms. Moreover, a sharp bimagnon feature emerges from the continuum at (1/2,0), coincident in energy with the bimagnon peak detected in optical spectroscopy. These findings show that the inherently complex spin spectra of cuprates, an exquisite manifestation of quantum magnetism, can be effectively explored by exploiting the richness of RIXS cross sections., 7 pages, 3 figures

The coupling between lattice vibration quanta and valence electrons can induce charge-density modulations and decisively influence the transport properties of materials, e.g., leading to conventional superconductivity. In high-critical-temperature superconductors, where electronic correlation is the main actor, the actual role of electron-phonon coupling (EPC) is being intensely debated theoretically and investigated experimentally. We present an in-depth study of how the EPC strength can be obtained directly from resonant inelastic x-ray scattering (RIXS) data through the theoretical approach derived by Ament et al. [Europhys. Lett. 95, 27008 (2011)]. The role of the model parameters (e.g., phonon energy ω0, intermediate state lifetime 1/Γ, EPC matrix element M, and detuning energy Ω) is thoroughly analyzed, providing general relations among them that can be used to make quantitative estimates of the dimensionless EPC g=(M/ω0)2 without detailed microscopic modeling. We then apply these methods to very high-resolution Cu L3-edge RIXS spectra of three Nd1+xBa2−xCu3O7−δ films. For the insulating antiferromagnetic parent compound, the value of M as a function of the in-plane momentum transfer is obtained for Cu-O bond-stretching (breathing) and bond-bending (buckling) phonon branches. For the underdoped and the nearly optimally doped samples, the effects of Coulomb screening and of charge-density-wave correlations on M are assessed. In light of the anticipated further improvements of the RIXS experimental resolution, this work provides a solid framework for an exhaustive investigation of the EPC in cuprates and other quantum materials.

We investigate the CuL3 edge resonant inelastic x-ray scattering (RIXS) spectra of a quasi-1D antiferromagnet Ca2CuO3. In addition to the magnetic excitations, which are well-described by the two-spinon continuum, we observe two dispersive orbital excitations, the 3d(xy) and the 3d(yz) orbitons. We carry out a quantitative comparison of the RIXS spectra, obtained with two distinct incident polarizations, with a theoretical model. We show that any realistic spin-orbital model needs to include a finite, but realistic, Hund's exchange J(H) approximate to 0.5 eV. Its main effect is an increase in orbiton velocities, so that their theoretically calculated values match those observed experimentally. Even though Hund's exchange also mediates some interaction between spinon and orbiton, the picture of spin-orbit separation remains intact and describes orbiton motion in this compound.

A study of the interface structure of superconducting and non-superconducting CaCuO2/SrTiO3 heterostructures grown on NdGaO3(110) substrates is reported. Using the combination of high resolution x-ray reflectivity and surface diffraction, the crystallographic structure of superconducting and non-superconducting samples has been investigated. The analysis has demonstrated the excellent sharpness of the CaCuO2/SrTiO3 interface (roughness smaller than one perovskite unit cell). Furthermore, we were able to discriminate between the superconducting and the non-superconducting phase. In the former case, we found an increase of the spacing between the topmost Ca plane of CaCuO2 block and the first TiO2 plane of the overlaying STO block, relative to the non-superconducting case. These results are in agreement with the model that foresees a strong oxygen incorporation in the interface Ca plane in the superconducting heterostructures.

Fundamental understanding of interface mechanisms that generate unexpected physical properties in the cuprate/manganite heterostructures is essential for possible applications as spintronic devices. Here, CaCuO2/La1-xSrxMnO3(CCO/LSMO) superlattices are investigated, where the infinite-layer cuprate CCO does not have apical oxygen for Cu in the Ca-plane so that the Mn-O-Cu superexchange coupling can be only present at the MnO2-(La,Sr)O-CuO(2)interface. Two different doping states for the manganites are studied, namelyx= 0.1 andx= 0.3, corresponding to the ferromagnetic insulating and ferromagnetic metallic states, respectively, in the manganite phase diagram. Linear and circular dichroism in X-ray absorption by synchrotron radiation at Cu and Mn L-edges clearly demonstrate that in the absence of apical oxygen in the cuprate block, the magnetic coupling between the LSMO and CCO is weakly ferromagnetic when LSMO is metallic, while it is antiferromagnetic when LSMO is insulating, along with an increased Cu 3d(3z(2)-r(2)) and Mn 3d(3z(2)-r(2)) orbital occupation. It is proposed that the Mn 3d valence band upward shift driven by the enhancement of Mn(3+)content in the underdoped sample enhances the orbital hybridization. The stronger hybridization at the interface MnO2-(La,Sr)O-CuO(2)gives rise to the antiferromagnetic coupling between cuprate and underdoped manganite.

Physical review materials 3(9), 094406 (2019). doi:10.1103/PhysRevMaterials.3.094406, The electronic properties of CaCuO$_2$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) superlattices are determined by the electronic structure of the structural units and in particular their interfaces. The electronic structure of LSMO is governed by a metal-insulator transition, which is controlled by the thickness of the units and the sample temperature, resulting in a systematic downward band shift for metallic samples (i.e., thick LSMO units, low temperature). We present a systematic study of the changes in the valence-band structure and screening features in Mn 2p and Cu 2p core-level spectra. The results show that hybridization of Cu 3d orbitals with out-of-plane O 2p orbitals can be systematically tuned by controlling the band alignment at the interface via the metal-to-insulator transition of the LSMO units. This opens a new route to rational design of functional interfaces and control of orbital reconstructions., Published by APS, College Park, MD

We demonstrate that combining standing-wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La$_{1.85}$Sr$_{0.15}$CuO$_{4}$(LSCO) and a half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the RIXS excitations. The magnetic excitations are found to arise from the LSCO/LSMO interfaces, and there is also a suggestion that one of the dd excitations comes from the interfaces. SW-RIXS measurements of correlated-oxide and other multilayer heterostructures should provide unique layer-resolved insights concerning their orbital and magnetic excitations, as well as a challenge for RIXS theory to specifically deal with interface effects., Comment: 18 pages, 3 figures, plus supplemental materials (14 pages)

We report a study of the structural phase transitions induced by pressure in bulk black phosphorus by using both synchrotron x-ray diffraction for pressures up to 12.2 GPa and Raman spectroscopy up to 18.2 GPa. Very recently black phosphorus attracted large attention because of the unique properties of few-layers samples (phosphorene), but some basic questions are still open in the case of the bulk system. As concerning the presence of a Raman spectrum above 10 GPa, which should not be observed in an elemental simple cubic system, we propose a new explanation by attributing a key role to the non-hydrostatic conditions occurring in Raman experiments. Finally, a combined analysis of Raman and XRD data allowed us to obtain quantitative information on presence and extent of coexistences between different structural phases from similar to 5 up to similar to 15 GPa. This information can have an important role in theoretical studies on pressure-induced structural and electronic phase transitions in black phosphorus.

The asymmetry between electron and hole doping in high critical-temperature superconducting (HTS) cuprates is key information for the understanding of Cooper pair formation mechanisms. Despite intensive studies on different cuprates, a comprehensive description of related magnetic and charge excitations is still fragmentary. In the present work, artificial cuprates were used to cover the entire phase diagram within the same HTS family. In particular, Cu ${L}_{3}$-edge resonant inelastic x-ray scattering (RIXS) measurements were performed on artificial $n$- and $p$-type infinite layer (IL) epitaxial films. Beside several similarities, RIXS spectra show noticeable differences in the evolution, with doping level, of magnetic and charge intensity and damping. Compatible trends can be found in spectra measured on bulk cuprates, as well as in theoretical calculations of the spin dynamical structure factor $S(\mathbf{q},\ensuremath{\omega})$. The findings give a deeper insight into the evolution of collective excitations across the cuprate phase diagram, and on underlying general features, only connected to the doping type. Moreover, they pave the way to the exploration of general properties of HTS physics over a broad range of conditions, by means of artificial compounds not constrained by the thermodynamic limitations governing the chemical stability of bulk materials.

A systematic study by reversible and hysteretic electrochemical strain microscopy (ESM) in samples of cerium oxide with different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in electrical conduction mechanism and related surface activity, such as water adsorption and dissociation with subsequent proton liberation. We have measured the behavior of the reversible hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first-order reversal curve method. The measurements have been performed in much smaller temperature ranges with respect to alternative measuring techniques. Complementing our study with hard X-ray photoemission spectroscopy and irreversible scanning probe measurements, we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity clearly emerges from all of our experimental results. We find that at lower Sm concentration, proton conduction is prevalent, featured by lower activation energy and higher electrical conductivity. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.

We perform infrared conductivity measurements on a series of CaCuO$_2$/SrTiO$_3$ heterostructures made by the insulating cuprate CaCuO$_2$ (CCO) and the insulating perovkite SrTiO$_3$ (STO). We estimate the carrier density of various heterostructures, with different level of hole doping from the integral of the optical conductivity and we measure the corresponding degree of correlation by estimating the ratio between the Drude weight and the integral of the infrared spectrum. The analysis demonstrates a large degree of correlation, which increases as the doping is reduced. The experimental results can be reproduced by Dynamical Mean-Field Theory calculations which strongly support the role of correlations in the CCO/STO heterostructures and their similarities with the most common cuprate superconductors. Our results suggest that cuprate superconductors can be looked at as natural superlattices, where the properties of the CuO$_2$ conducting planes and charge reservoir blocks can be completely disentangled., Comment: 5 pages, 5 figures

In high Tc superconductors the magnetic and electronic properties are determined by the probability that valence electrons virtually jump from site to site in the CuO2 planes, a mechanism opposed by on-site Coulomb repulsion and favored by hopping integrals. The spatial extent of the latter is related to transport properties, including superconductivity, and to the dispersion relation of spin excitations (magnons). Here, for three antiferromagnetic parent compounds (single-layer Bi2Sr0.99La1.1CuO6+delta, double-layer Nd1.2Ba1.8Cu3O6 and infinite-layer CaCuO2) differing by the number of apical atoms, we compare the magnetic spectra measured by resonant inelastic x-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy. We observe that the absence of apical oxygens increases the in-plane hopping range and, in CaCuO2, it leads to a genuine 3D exchange-bond network. These results establish a corresponding relation between the exchange interactions and the crystal structure, and provide fresh insight into the materials dependence of the superconducting transition temperature., 9 pages, 4 figures, 1 Table, 42 references

We measured high resolution Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS) of the undoped cuprates La$_2$CuO$_4$, Sr$_2$CuO$_2$Cl$_2$, CaCuO$_2$ and NdBa$_2$Cu$_3$O$_6$. The dominant spectral features were assigned to $dd$ excitations and we extensively studied their polarization and scattering geometry dependence. In a pure ionic picture, we calculated the theoretical cross sections for those excitations and used them to fit the experimental data with excellent agreement. By doing so, we were able to determine the energy and symmetry of Cu-3$d$ states for the four systems with unprecedented accuracy and confidence. The values of the effective parameters could be obtained for the single ion crystal field model but not for a simple two-dimensional cluster model. The firm experimental assessment of $dd$ excitation energies carries important consequences for the physics of high $T_c$ superconductors. On one hand, having found that the minimum energy of orbital excitation is always $\geq 1.4$ eV, i.e., well above the mid-infrared spectral range, leaves to magnetic excitations (up to 300 meV) a major role in Cooper pairing in cuprates. On the other hand, it has become possible to study quantitatively the effective influence of $dd$ excitations on the superconducting gap in cuprates., 22 pages, 11 figures, 1 table

Low dimensional superconducting systems have been the subject of numerous studies in the recent past, with the aim of achieving a higher and higher critical temperature (T c ). The recent improvement in film deposition techniques has allowed the realization of artificial heterostructures, with atomically flat surfaces and interfaces, where novel properties appear that are not present in the single constituent. For instance, quasi-2D superconductivity was found at the interface between different oxides. In this review we analyze, in particular, the quasi-2D superconductivity occurring at the interface between two non-superconducting oxides, mostly cuprates. Throughout a comparison of the superconducting properties of different oxide heterostructures and superlattices, we propose a phenomenological explanation of the behavior of the T c as a function of the number of conducting CuO2 planes. This is achieved by introducing two different interactions between the superconducting 2D sheets. This interpretation is finally extended also to standard high T c cuprates, contributing to the solution of the long-standing question of the dependence of T c on the number of CuO2 planes in these systems.

The pressure dependencies at room temperature of Raman and infrared (IR) data on VO 2 are presented up to 19 and 15 GPa, respectively, together with preliminary X-ray diffraction (XRD) data collected up to 42 GPa. Both Raman and XRD indicate the persistence of a monoclinic lattice up the highest pressures. Observed modifications in the pressure dependence of Raman phonon frequencies, IR spectral weight, and unit cell volume at ∼10 GPa allow us to identify two regimes at low and high pressure and to conjecture a transition to a new unknown monoclinic phase where applying pressure activates a metallization mechanism.

The pressure effects on the Jahn–Teller (JT) distortion of three representative compounds belonging to the LaMn1−xGaxO3 (x = 0.2, 0.3, 0.4) family were widely investigated by means of X-ray diffraction and Raman spectroscopy. A compound with a fully coherent JT distorted structure (x = 0.2), one with regular octahedra (x = 0.6) and one in an intermediate configuration (x = 0.3) were selected. A pressure induced transition from the orthorhombic Pbnm phase towards structures with higher symmetry was observed in all the samples. Both Raman and X-ray data confirm that the most important structural effect of pressure is that of reducing the octahedral distortion. The appearance of a feature in the lattice parameter behavior connected to a structural instability was also detected, pointing out the key role of the JT distortion in stabilizing the manganite structures. On the other hand, the complete suppression of the JT distortion in the high-pressure phases cannot be claimed. The Raman spectra collected on the more distorted compounds (x = 0.2, 0.3) reveal clearly the coexistence of domains of distorted and more regular octahedra in a certain pressure range. The first sketch of the pressure vs. Ga-content phase diagram was drawn.

At interfaces between complex oxides it is possible to generate electronic systems with unusual electronic properties, which are not present in the isolated oxides. One important example is the appearance of superconductivity at the interface between insulating oxides, although, until now, with very low Tc. We report the occurrence of high Tc superconductivity in the bilayer CaCuO2/SrTiO3, where both the constituent oxides are insulating. In order to obtain a superconducting state, the CaCuO2/SrTiO3 interface must be realized between the Ca plane of CaCuO2 and the TiO2 plane of SrTiO3. Only in this case can oxygen ions be incorporated in the interface Ca plane, acting as apical oxygen for Cu and providing holes to the CuO2 planes. In addition, a detailed hole doping spatial profile can be obtained by scanning transmission electron microscopy and electron-energy-loss spectroscopy at the O K edge, clearly showing that the (super)conductivity is confined to about 1–2 CaCuO2 unit cells close to the interface with SrTiO3. The results obtained for the CaCuO2/SrTiO3 interface can be extended to multilayered high Tc cuprates, contributing to explaining the dependence of Tc on the number of CuO2 planes in these systems.

Scanning probe bias techniques have been used as a method to locally dope thin epitaxial films of La(2)CuO(4) (LCO) fabricated by pulsed laser deposition. The local electrochemical oxidation of LCO very efficiently introduces interstitial oxygen defects in the thin film. Details on the influence of the tip voltage bias and environmental conditions on the surface morphology have been investigated. The results show that a local uptake of oxygen occurs in the oxidized films.

Yttrium-doped barium zirconate (BZY) thin films recently showed surprising electric transport properties. Experimental investigations conducted mainly by electrochemical impedance spectroscopy suggested that a consistent part of this BZY conductivity is of protonic nature. These results have stimulated further investigations by local unconventional techniques. Here, we use electrochemical strain microscopy (ESM) to detect electrochemical activity in BZY films with nanoscale resolution. ESM in a novel cross-sectional measuring setup allows the direct visualization of the interfacial activity. The local electrochemical investigation is compared with the structural studies performed by state of art scanning transmission electron microscopy (STEM). The ESM and STEM results show a clear correlation between the conductivity and the interface structural defects. We propose a physical model based on a misfit dislocation network that introduces a novel 2D transport phenomenon, whose fingerprint is the low activation energy measured.

At interfaces between complex oxides it is possible to generate electronic systems with unusual electronic properties, which are not present in the isolated oxides. One important example is the appearance of superconductivity at the interface between insulating oxides, although, until now, with very low T(c). We report the occurrence of high T(c) superconductivity in the bilayer CaCuO(2)/SrTiO(3), where both the constituent oxides are insulating. In order to obtain a superconducting state, the CaCuO(2)/SrTiO(3) interface must be realized between the Ca plane of CaCuO(2) and the TiO(2) plane of SrTiO(3). Only in this case can oxygen ions be incorporated in the interface Ca plane, acting as apical oxygen for Cu and providing holes to the CuO(2) planes. A detailed hole doping spatial profile can be obtained by scanning transmission electron microscopy and electron-energy-loss spectroscopy at the O K edge, clearly showing that the (super)conductivity is confined to about 1-2 CaCuO(2) unit cells close to the interface with SrTiO(3). The results obtained for the CaCuO(2)/SrTiO(3) interface can be extended to multilayered high T(c) cuprates, contributing to explaining the dependence of T(c) on the number of CuO(2) planes in these systems.

Given the layered structure of cuprate high-transition-temperature superconductors (HTS), essentially constituted by infinite layer (IL) blocks, containing the CuO2 planes, separated by charge reservoir (CR) blocks, the use of heterostructures and superlattices, made by stacking HTS with different magnetic and conductivity properties (metallic, superconducting, insulating), such as YBa2Cu3O7−x, PrBa2Cu3O7−x, and Bi2Sr2CunCan−1Ox, have been extensively exploited to investigate many fundamental issues in HTS physics. Successively, superconducting heterostructures made by a metallic (but non-superconducting) cuprate and an insulating cuprate have been successfully synthesized, such as CaCuO2/BaCuO2+x and La2CuO4/La2−xSrxCuO4, with the idea to mimic the intrinsic IL/CR structure of HTS, revealing the circumstance that interface effects are important for the occurrence of high T c superconductivity. The discovery of 2D electron gas at the interface between two insulating perovskites induced us to develop a new kind of heterostructures based on an insulating cuprate with IL structure, CaCuO2 (CCO), and an insulating perovskite, SrTiO3 (STO), with the idea that the 2D electron gas eventually formed at the interface could dope the CuO2 planes in CaCuO2 block, giving rise to high T c superconductivity. Indeed, CCO/STO superlattices are superconducting at T c = 50 K. The doping of CCO occurs thanks to extra oxygen ions, which are incorporated at the CCO/STO interface during the growth in strongly oxidizing conditions. The CR role is thus here played by the interface layer, and the superconductivity is confined at the interface within few CCO unit cells. Structural features of these heterostructures can be engineered in a wide range and, consequently, their superconducting properties are studied. The highlights of these investigations are reviewed here.

We have studied effect of X-ray illumination on charge ordering superstructures in oxygen doped La 2 CuO 4+δ superconducting system. Formation of long range crystalline phase of polarons is favored in this system by mobile counter-ions (interstitial oxygen ions in the La 2 O 2 layer) in the temperature range 200–350 K. We find a clear signature of disorder to order like X-ray induced phase transition. This transition appears to show up beyond a threshold dose of about 1016 photons/cm2, the value which increases with decreasing temperature.

The increase of Tc with x in Al 1 − x Mg x B 2 is controlled by the Fermi level tuning at a "shape resonance" (i.e., the 2D-3D cross-over, at x = 0.66, of the topology of the Fermi surface of σ holes in the superlattice of boron mono-layers intercalated by Al 1 − x Mg x ions) and by the tensile "micro-strain" in the boron sub-lattice (due to the lattice misfit between the boron and the intercalated layers). The softening of the E2g phonon frequency with increasing boron tensile micro-strain e in the range 3% < e < 6% shows the increasing electron-lattice interaction. The linear scaling, for 0.66 < x < 1, of Tc vs. the Fermi temperature TF of the σ holes shows a constant coupling strength with kFξ0 = 90 (where kF is the Fermi wave vector and ξ0 is the Pippard coherence length) that points toward a vibronic pairing mechanism.

Here we report the evidence of a disorder-to-order photo-induced phase transition (PIPT) under X-ray (h x = 12.4 keV) illumination in oxygen doped La 2 CuO 4 + i . The commensurate striped polaron crystalline phase is induced by the irradiation of X-rays in a cuprate perovskite at hole doping i ¨ 1/8 and high Cu-O micro-strain, k >7%. The nucleation of long range crystalline phases of holes on the oxygen 2 p x,y orbitals in the CuO 2 plane associated with pseudo Jahn Teller local lattice distortions (LLD) is favored by mobile counter-ions (interstitial oxygen ions in the La 2 O 2 layer) in the temperature range 200-350 K. The PIPT appears beyond a threshold dose of 10 17 photons/cm 2 at 220 K and a power law of the polaron crystal growth is observed as a function of the X-ray illumination dose.

Epitaxial heterostructures consisting of an yttrium doped barium cerate, BaCe0.8Y0.2O3,(BCY) layer sandwiched between two yttrium doped barium zirconate, BaZr0.8Y0.2O3,(BZY) thin layers have been deposited on insulating (001) MgO substrates by pulsed laser deposition. The first BZY layer was aimed at improving the lattice match with the MgO substrate, the second at protecting the BCY layer. Ionic conductivity has been studied in the 300 °C–600 °C temperature range as a function of the BCY thickness in dry air and wet 5% H2 in Ar atmosphere. In both atmospheres, heterostructures showed a conductivity slightly larger than that of BCY pellets sintered under optimized conditions. Such a result has been attributed to the absence of blocking grain boundaries in the epitaxial heterostructures due to their good crystallographic quality. The BCY sandwich heterostructures showed an improved chemical stability relative to standard BCY pellets.

International audience; Here we report a structural and electronic investigation of the recently discovered CaCuO2/SrTiO3 superconducting artificial superlattices based on two insulating oxides. We show that Tc = 50 K is reached in the superlattices grown on NdGaO3 substrate under optimized growth conditions. If the superlattices are grown on LaAlO3 substrate the Tc is drastically reduced due to the much higher lattice mismatch between CaCuO2 and LaAlO3, which has lattice parameters substantially smaller than NdGaO3. The consequent in-plane lattice contraction also makes the incorporation of extra oxygen atoms more difficult, leading to a decrease of hole doping and Tc. Finally, x-ray absorption spectroscopy study as a function of the thickness of the CaCuO2 block in CaCuO2/SrTiO3 superconducting superlattices has revealed the important role played by the interfaces as the location of hole doping and superconductivity in these systems.

We have carried out a comparative study of the low-temperature dependence of the magnetic penetration depth λ in (1) superconducting NbN films grown on R -plane sapphire and MgO substrates and (2) Bi2212 single crystals with different doping levels ranging from under- to over-doping. The NbN films display textured or epitaxial microstructure depending on growth conditions. The thermally activated dependence of λ ( T ) characteristic of conventional s-wave superconductors was found only in the epitaxial or thin textured NbN films. All thick textured films exhibit below 4 K a linear dependence Δλ ∼ T characteristic of a gapless superconducting state. This is quantitatively explained by a model of granularity-induced thermal phase fluctuations of the order parameter. As for Bi2212, we have found in all crystals a linear low-temperature dependence Δλ ab ∼ T in the ab -plane. The slope Δλ ab / ΔT displays a pronounced minimum ∼8 A/K at optimum doping. This value is in good agreement with the d-wave model proposed for cuprates. However, this model fails to account for the large slope values >40 A/K observed at both under- and over-doping. These values are quantitatively explained by the same model of phase fluctuations adopted for the textured NbN films, in spite of the difference between these two superconducting systems.

We have investigated the charge ordering in an oxygen doped La 2 CuO 4 crystal by diffuse high-resolution x-ray diffraction using synchrotron radiation. The micro-strain ε in the CuO 2 plane is close to the micro-strain ε0 for the formation of static charge ordered phases. The crystal shows the co-existence of a first phase with an insulating crystal of charge strings and a second superconducting phase of "supestripes". The first insulating phase is due to charges at the critical doping 1/8 self trapped into a crystal of ordered strings of finite length (~60 Å) and a commensurate period of 4 lattice units. The superconducting phase shows diffuse diffraction peaks with short coherence length due to fluctuations of stripes with an incommensurate period of about 4.8 unit cells and length longer than 500 Å.

We report experimental evidence fur the phase diagram of doped cuprate superconductors as a function of the micro-strain epsilon of the planar Cu-O bund length, measured by Cu K-edge EXAFS, and hole doping delta. The local lattice distortions are measured by EXAFS and the charge ordering is measured by synchrotron radiation diffuse X-ray diffraction. This phase diagram shows a QCP at P(epsilon (c)delta (c)) where for epsilon > epsilon (c) charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. Thr function T(c) (epsilon, delta) of two variables shows its maximum at the strain QCP. The critical fluctuations near tills strain QCP give the self-organization of a metallic superlattice of quantum wires "superstripes" that favors the amplification of the critical temperature.

We discuss the transformation of a network of strings consisting of charges self-trapped by linear cooperative local lattice distortions into an inhomogeneous phase of stripes and itinerant carriers in cuprate superconductors. This scenario is observed by X-ray diffraction in oxygen doped La2CuO4 where the doped charges at the critical doping 1/8 are self trapped into a crystal of ordered strings of finite length. Above this critical density in the superconducting phase the stripes co-exist with itinerant carriers.

We have measured high-resolution Ru K-edge x-ray absorption spectra for the intermetallic La1-xCexRu2 system. Multiple-scattering calculations are made to explore the origins of different features observed in the experimental x-ray absorption near-edge structure (XANES) and their variation with Ce doping. The experiments and theoretical analysis demonstrate that the Ce doping in La1-xCexRu2 has a direct influence on the states at the Fermi level and hence the hybridization between the f and d states induced by doping.

Penetration depth λ ( T ) has been derived from the measurements of temperature-dependent complex conductivity on Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) superconducting system using a single-coil inductance method. Temperature dependence of λ ( T ) in the vicinity of the superconducting transition temperature has been used to explore the fluctuations of the superconducting order parameter. Doping dependence of these fluctuations reveals that the fluctuation region in the underdoped system is far greater than in the optimally doped system. We find a linear doping dependence of the fluctuation region below T c , suggesting that superfluid density might be the only parameter to control these fluctuations.

The superconducting properties of (CaCuO2)n/(SrTiO3)m artificial superlattices have been investigated via transport measurements under the application of external magnetic fields. The coherence lengths in the plane of the substrate and in the direction perpendicular to it (ξab and ξc, respectively) have been measured while varying m, the thickness of the SrTiO3 block. The results show that with increasing m, i.e. with increasing structural anisotropy, the superconducting anisotropy γ = ξab/ξc decreases. This apparent anomalous relation between the structural and the superconducting anisotropies suggests that γ is more affected by local doping at the interface rather than by the separation between the superconducting blocks. This interpretation of the experimental results has been confirmed by both the irreversibility lines and the magnetic field dependence of the activation energy for fluxon motion.

Here we report on interface reconstruction in the recently discovered superconducting artificial superlattices based on insulating ${\mathrm{CaCuO}}_{2}$ and ${\mathrm{SrTiO}}_{3}$ blocks. Hard x-ray photoelectron spectroscopy shows that the valence-band alignment prevents any electronic reconstruction by direct charge transfer between the two blocks. We demonstrate that the electrostatic built-in potential is suppressed by oxygen redistribution in the alkaline-earth-metal interface planes. By using highly oxidizing growth conditions, the oxygen coordination in the reconstructed interfaces may be increased, resulting in the hole doping of the cuprate block and thus in the appearance of superconductivity.

The energy of magnetic and orbital excitations of copper-oxide systems have been assessed in bulk materials, but little is known about how they change in thin films or heterostructures. Although in these samples the epitaxial strain is known to alter important physical properties, such as Tc, the actual relation to fundamental excitations remains to be established. Here we determine how the magnetic interactions and orbital splitting scale with lattice constant under strain in the prototypical insulating cuprate CaCuO2. We extract the scaling power laws both experimentally, using Cu L3 resonant inelastic x-ray scattering, and theoretically, with ab initio quantum chemical calculations. This combined approach quantifies the large impact of small lattice variations on the magnetic and orbital energy scales and opens the way to the production of strain-engineered samples. © 2013 American Physical Society.

Raman spectra of CaCuO2/SrTiO3 superlattices show clear spectroscopic marker of two structures formed in CaCuO2 at the interface with SrTiO3. For non-superconducting superlattices, grown in low oxidizing atmosphere, the 425 cm(-1) frequency of oxygen vibration in CuO2 planes is the same as for CCO films with infinite layer structure (planar Cu-O coordination). For superconducting superlattices grown in highly oxidizing atmosphere, a 60 cm(-1) frequency shift to lower energy occurs. This is ascribed to a change from planar to pyramidal Cu-O coordination because of oxygen incorporation at the interface. Raman spectroscopy proves to be a powerful tool for interface structure investigation. (c) 2013 AIP Publishing LLC.

We report the occurrence of superconductivity, with maximum T${}_{c}=40$ K, in superlattices (SLs) based on two insulating oxides, namely CaCuO${}_{2}$ and SrTiO${}_{3}$. In these (CaCuO${}_{2}$)${}_{n}$/(SrTiO${}_{3}$)${}_{m}$ SLs, the CuO${}_{2}$ planes belong only to CaCuO${}_{2}$ block, which is an antiferromagnetic insulator. Superconductivity, confined within a few unit cells at the CaCuO${}_{2}$/SrTiO${}_{3}$ interface, shows up only when the SLs are grown in a highly oxidizing atmosphere, because of extra oxygen ions entering at the interfaces. The hole doping is obtained by charge transfer from the interface layers, which thus act as charge reservoir.

Using high-resolution resonant inelastic x-ray scattering (RIXS) excited at the Cu ${L}_{3}$ edge we have investigated the ligand field ($dd$) and magnetic excitations in (CaCuO${}_{2}$)${}_{n}$/(SrTiO${}_{3}$)${}_{n}$ superlattices ($n=2,3$) and compared them to those of a 14-nm-thick CaCuO${}_{2}$ film. The $dd$ excitation spectrum reveals a pyramidal coordination of Cu ions at the CaCuO${}_{2}$/SrTiO${}_{3}$ interfaces. In the three samples, spin excitations are in the form of dispersing magnons, with similar spectral intensity but reduced dynamics in SLs with respect to pure CaCuO${}_{2}$. By fitting the dispersions within linear spin wave theory we have obtained the leading term of the in-plane superexchange parameters: $J=127$ meV, 138 meV, and 157 meV for $n=2$, 3 SLs, and CaCuO${}_{2}$, respectively. These results demonstrate that the antiferromagnetic order is preserved in these SLs down to very small cuprate layer thickness and despite the chemical and structural alterations at the interface. This finding opens the way to the production of artificial Cu-based high-temperature superconductors where the charge reservoir layer is constituted by the interface itself.

The effect of chemical and hydrostatic pressure has been studied systematically in a selected system belonging to the 1111 family of iron pnictide high-temperature superconductors. The results show a surprising similarity between the trend of critical temperature vs hydrostatic pressure for isoelectronic samples with di ff erent rare earths (RE) on the RE site and samples of the SmFeAsO(1-x)F(x) series with di ff erent doping levels. These results open new questions about the underlying mechanism for superconductivity in iron pnictides.

We performed in situ angle-resolved photoemission spectroscopy measurements on ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_{3}$ thin and ultrathin films deposited onto ${\text{SrTiO}}_{3}$ substrates in order to determine their electronic band structure. We directly identified, in ultrathin films, an enhancement of the occupation of the out-of-plane $\text{Mn}\text{ }3d\text{ }{e}_{g}$ $(3{z}^{2}\ensuremath{-}{r}^{2})$ band at the expense of the in-plane $3d\text{ }{e}_{g}$ $({x}^{2}\ensuremath{-}{y}^{2})$ band. Such a disproportionate change in orbital occupation is interpreted as the origin of the strong depression of the magnetotransport properties at the interface between manganite films and substrates.

Recently it was predicted theoretically and confirmed experimentally that in cuprates single-magnon dispersions can be mapped out with resonant inelastic x-ray scattering (RIXS) at the copper ${L}_{3}$ edge. To further establish RIXS as a viable technique we investigate the momentum and incident photon polarization dependence of the single-magnon spectral weight in a variety of layered undoped antiferromagnetic compounds. The agreement of experimental and theoretical results bolsters the assignment of RIXS spectral features to single magnons. This detailed analysis allows to disentangle single-magnon scattering from other spectral contributions. Moreover, it is a necessary premise for future research aimed at investigating processes that modulate spectral weights beyond the predictions of linear spin-wave theory.

The effect of crystallographic interface orientation on the electric transport properties of fully strained La2/3Sr1/3MnO3 films grown onto LaAlO3 substrates has been investigated. It is found that, relative to the (001) orientation, the (110) orientation strongly enhances the transport properties for film thickness in the range between 3 and 12 nm. Such an effect was ascribed to reduced [relative to the (001) substrates] tetragonal distortion induced by epitaxy onto (110)-oriented substrates. The reduced tetragonal distortion quenches the occupational imbalance between the Mn eg orbitals thus, ultimately, reinforcing the ferromagnetic double exchange transport mechanism.

Multigap superconductivity has been discussed since the 1950’s. Nevertheless, the topic remained essentially speculative until 2001, when for the first time multigap superconductivity has been demonstrated to take place in a real material, as MgB2. In the last few years evidence of multigap superconductivity has been claimed for several different classes of superconductors. Thanks to the high brightness of THz synchrotron radiation we addressed multiband superconductivity on two model systems: The prototype multiband superconductor MgB2 and the A‐15 superconductor V3Si.

We report on THz reflectivity results on multigap superconductors: The prototype multiband superconductor MgB 2 , the A-15 compound V 3 Si, and Co-doped BaFe 2 As 2 . Our results highlight the applicability of a parallel resistor model to describe the THz properties of multigap superconductors.

High-pressure (P) Raman $(0\ensuremath{\le}\text{P}\ensuremath{\le}12\text{ }\text{GPa})$ and Infrared $(0\ensuremath{\le}\text{P}\ensuremath{\le}24\text{ }\text{GPa})$ spectra are collected on two samples of the ${\text{LaMn}}_{1\ensuremath{-}x}{\text{Ga}}_{x}{\text{O}}_{3}$ series: $x=0.2$ and 0.6 with cooperative Jahn-Teller distorted and regular ${\text{MnO}}_{6}$ octahedra, respectively. Raman spectra are also collected at $\text{P}=0$ on varying Ga-content $(0\ensuremath{\le}x\ensuremath{\le}0.8)$. A remarkable octahedral symmetrization is observed on increasing P in the $x=0.2$ sample, as well as $x$ at $\text{P}=0$. The $x$-driven symmetrization process is homogeneous whereas, compressing the lattice, a phase-separated regime (regular/distorted ${\text{MnO}}_{6}$) is observed at intermediate P. The comparison between the spectra of the two samples in the high-pressure regime strongly suggests the presence of small residual distortion in the $x=0.2$ even at the highest P. Infrared data show that P is effective only in the $x=0.2$ sample (P-driven band-gap filling) suggesting the relevance of orbital and magnetic order on the metallization process. Our results provide a unique experimental base to clarify the origin of the peculiar pressure behavior of the parent ${\text{LaMnO}}_{3}$.

The various phases observed in all cuprate superconductors [superconducting (SC), spin-glass (SG), and antiferromagnetic (AFM)] were investigated with respect to oxygen-isotope (16O/18O) effects, using here as a prototype system of cuprates Y1-xPrxBa2Cu3O7-delta. All phases exhibit an isotope effect which is strongest where the respective phase terminates. In addition, the isotope effects on the magnetic phases (SG and AFM) are sign reversed as compared to the one on the superconducting phase. In the coexistence regime of the SG and SC phase a two-component behavior is observed where the isotope induced decrease of the superfluid density leads to a corresponding enhancement in the SG related density.

Raman and infrared transmission and reflectivity measurements were carried out at room temperature and high pressure (0--15 GPa) on ${\text{V}}_{1\ensuremath{-}x}{\text{Cr}}_{x}{\text{O}}_{2}$ compounds. Raman spectra were collected at ambient conditions on the $x=0.007$ and 0.025 materials, which are characterized by different insulating monoclinic phases ($M3$ and $M2$, respectively), while infrared spectra were collected on the $x=0.025$ sample only. The present data were compared with companion results on undoped ${\text{VO}}_{2}$ [E. Arcangeletti et al., Phys. Rev. Lett. 98, 196406 (2007)], which is found at ambient conditions in a different, third insulating monoclinic phase, named $M1$. This comparison allowed us to investigate the effects of different extents of structural distortions (Peierls distortion) on the lattice dynamics and the electronic properties of this family of compounds. The pressure dependence of the Raman spectrum of ${\text{VO}}_{2}$ and Cr-doped samples shows that all the systems retain the monoclinic structure up to the highest explored pressure, regardless the specific monoclinic structure ($M1$, $M2$, and $M3$) at ambient condition. Moreover, the Raman spectra of the two Cr-doped samples, which exhibit an anomalous behavior over the low-pressure range $(Pl8\text{ }\text{GPa})$, merge into that of ${\text{VO}}_{2}$ in the high-pressure regime and are all found into a common monoclinic phase (a possible fourth kind phase). Combining Raman and infrared results on both the ${\text{VO}}_{2}$ and the present data, we found that a common metallic monoclinic phase appears accessible in the high-pressure regime at room temperature for both undoped and Cr-doped samples independently of the different extents of Peierls distortion at ambient conditions. This finding differs from the behavior observed at ambient pressure, where the metallic phase is found only in conjunction with the rutile structure. The whole of these results suggests a major role of the electron correlations, rather than of the Peierls distortion, in driving the metal-insulator transition in vanadium dioxide systems, thus opening to new experimental and theoretical queries.