Search

The universality of the strange metal phase in many quantum materials is often attributed to the presence of a quantum critical point (QCP), a zero-temperature phase transition ruled by quantum fluctuations. In cuprates, where superconductivity hinders direct QCP observation, indirect evidence comes from the identification of fluctuations compatible with the strange metal phase. Here we show that the recently discovered charge density fluctuations (CDF) possess the right properties to be associated to a quantum phase transition. Using resonant x-ray scattering, we studied the CDF in two families of cuprate superconductors across a wide doping range (up to $p$=0.22). At $p^*\approx$0.19, the putative QCP, the CDF intensity peaks, and the characteristic energy $\Delta$ is minimum, marking a wedge-shaped region in the phase diagram indicative of a quantum critical behavior, albeit with anomalies. These findings strengthen the role of charge order in explaining strange metal phenomenology and provide insights into high-temperature superconductivity., Comment: 36 pages, 4 figures, 9 supplementary figures

Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface Bi$_2$Se$_3$/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface Bi$_2$Se$_3$/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that superconducting quantum interference device (SQUID) magnetometry of Bi$_2$Se$_3$/EuSe heterostructures reveals precisely the magnetic phase diagram known from EuSe, including the ferrimagnetic phase below 5 K, without any apparent changes from the bulk behavior. Choosing a temperature of 10 K to search for magnetic enhancement, we determine an upper limit for a possible magnetic coercive field of 3 mT. Using interface sensitive x-ray absorption spectroscopy we verify the magnetic divalent configuration of the Eu at the interface without contamination by Eu3+, and by x-ray magnetic circular dichroism (XMCD) we confirm at the interface the magnetic hysteresis obtained by SQUID. XMCD data obtained at 10 K in a magnetic field of 6 T indicate a magnetic spin moment of mz,spin = 7 $\mu$B/Eu$^{2+}$, in good agreement with the SQUID data and the expected theoretical moment of Eu2+. Subsequent XMCD measurements in zero field show, however, that sizable remanent magnetization is absent at the interface for temperatures down to about 10 K., Comment: 11 figures, 35 references

The recent observation of superconductivity in infinite-layer Nd$_{1-x}$Sr$_x$NiO$_2$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of LaNiO$_3$ embedded in insulating LaGaO$_3$, and used layer-selective topotactic reactions to reduce the nickelate layers to LaNiO$_{2}$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction and found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition metal oxides., Comment: 12 pages, 10 figures

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

The La-based '214' cuprates host several symmetry breaking phases including superconductivity, charge and spin order in the form of stripes, and a structural othorhombic-to-tetragonal phase transition. Therefore, these materials are an ideal system to study the effects of uniaxial stress onto the various correlations that pervade the cuprate phase diagram. We report resonant x-ray scattering experiments on $\textrm{La}_{1.475}\textrm{Nd}_{0.4}\textrm{Sr}_{0.125}\textrm{Cu}\textrm{O}_{4}$ (LNSCO-125) that reveal a significant response of charge stripes to uniaxial tensile-stress of $\sim$ 0.1 GPa. These effects include a reduction of the onset temperature of stripes by $\sim$ 50 K, a 29 K reduction of the low-temperature orthorhombic-to-tetragonal transition, competition between charge order and superconductivity, and a preference for stripes to form along the direction of applied stress. Altogether, we observe a dramatic response of the electronic properties of LNSCO-125 to a modest amount of uniaxial stress., Comment: 6 pages, 3 figures, Supplemental material available upon request

We report a ground state with strongly coupled magnetic and charge density wave orders mediated via orbital ordering in the layered compound \tbt. In addition to the commensurate antiferromagnetic (AFM) and charge density wave (CDW) orders, new magnetic peaks are observed whose propagation vector equals the sum of the AFM and CDW propagation vectors, revealing an intricate and highly entwined relationship. This is especially interesting given that the magnetic and charge orders lie in different layers of the crystal structure where the highly localized magnetic moments of the Tb$^{3+}$ ions are netted in the Tb-Te stacks, while the charge order is formed by the conduction electrons of the adjacent Te-Te layers. Our results, based on neutron diffraction and resonant x-ray scattering reveal that the charge and magnetic subsystems mutually influence each other via the orbital ordering of Tb$^{3+}$ ions., Comment: 5 pages, 4 figures

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect that might be used for high precision metrology and edge channel spintronics. In conjunction with superconductors, they could host chiral Majorana zero modes which are among the contenders for the realization of topological qubits. Recently, it was discovered that the stable 2+ state of Mn enables the formation of intrinsic magnetic topological insulators with A1B2C4 stoichiometry. However, the first representative, MnBi2Te4, is antiferromagnetic with 25 K N\'eel temperature and strongly n-doped. Here, we show that p-type MnSb2Te4, previously considered topologically trivial, is a ferromagnetic topological insulator in the case of a few percent of Mn excess. It shows (i) a ferromagnetic hysteresis with record high Curie temperature of 45-50 K, (ii) out-of-plane magnetic anisotropy and (iii) a two-dimensional Dirac cone with the Dirac point close to the Fermi level which features (iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy and (v) a magnetically induced band gap that closes at the Curie temperature as demonstrated by scanning tunneling spectroscopy. Moreover, it displays (vi) a critical exponent of magnetization beta~1, indicating the vicinity of a quantum critical point. Ab initio band structure calculations reveal that the slight excess of Mn that substitutionally replaces Sb atoms provides the ferromagnetic interlayer coupling. Remaining deviations from the ferromagnetic order, likely related to this substitution, open the inverted bulk band gap and render MnSb2Te4 a robust topological insulator and new benchmark for magnetic topological insulators.

V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD) we directly probe the magnetism at the V site and in zerofield. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a time scale of minutes which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point., Comment: 12 pages, 3 figures

We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of Co$_{0.975}$Ge$_{0.025}$Cr$_2$O$_4$ (Ge-CCO) and CoCr$_2$O$_4$ (CCO), under low- and high-magnetic fields, from 0.2 T up to 6.5 T. A characterization of Ge-CCO at a low magnetic field is performed and the results are compared to those of pure CCO. The ferrimagnetic phase transition temperature $T_C \approx 95$ K and the multiferroic transition temperature $T_S \approx 27$ K in Ge-CCO are comparable to those observed in CCO. In Ge-CCO, the ordering wave vector $\textit{(qq0)}$ observed below $T_S$ is slightly larger compared to that of CCO, and, unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity and therefore the electric polarization was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals., Comment: 20 pages, 12 figures

We study the magnetic structure of the "stuffed" (Tb-rich) pyrochlore iridate Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$, using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir $L_3$ and Tb $M_5$ edges, which provide selective sensitivity to Ir $5d$ and Tb $4f$ magnetic moments, respectively. At the Ir $L_3$ edge, we found the onset of long-range ${\bf k}={\bf 0}$ magnetic order below $T_{N}^\text{Ir}\sim$ 71 K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of $\mu_{5d}^{\text{Ir}} \approx 0.34(3)\,\mu_B$ at 5 K. At the Tb $M_5$ edge, long-range ${\bf k}={\bf 0}$ magnetic order appeared below $\sim40$ K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the $M_5$ edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb$_{2}$Ir$_{2}$O$_{7}$, for which $T_{N}^{\text{Ir}} =130$ K and $\mu_{5d}^{\text{Ir}}=0.56\,\mu_B$, indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates., Comment: 9 pages, 6 figures

In order to explain the emergence of the anomalous pseudogap state and high-temperature superconductivity in the cuprates, intense research activity over three decades has focused on unravelling the connection between the various instabilities of the underdoped regime. In the high-temperature superconductor (Ca$_x$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_3$O$_y$ (CLBLCO) isovalent chemical substitution produces smooth changes to the CuO$_2$ plane buckling and the Cu(II)-to-apical-oxygen distance, allowing us to study the interdependence of charge-density-wave (CDW) order, superconductivity and the pseudogap at constant hole doping in two adiabatically connected representations of the 123 cuprate structure. In this study, resonant soft x-ray scattering measurements reveal the first observation of incommensurate CDW correlations in CLBLCO and demonstrate a lack of correlation between $T_{\text{CDW}}$ and the pseudogap crossover temperature ($T^{\ast}$). This result disfavours a scenario in which the opening of the pseudogap at $T^{\ast}$ results from fluctuating CDW correlations.

We used high-resolution resonant inelastic x-ray scattering (RIXS) at the Ni $L_3$ edge to simultaneously investigate high-energy interband transitions characteristic of Ni-O bond ordering and low-energy collective excitations of the Ni spins in the rare-earth nickelates $R$NiO$_3$ ($R$ = Nd, Pr, La) with pseudocubic perovskite structure. With the support of calculations based on a double-cluster model we quantify bond order (BO) amplitudes for different thin films and heterostructures and discriminate short-range BO fluctuations from long-range static order. Moreover we investigate magnetic order and exchange interactions in spatially confined $R$NiO$_3$ slabs by probing dispersive magnon excitations. While our study of superlattices (SLs) grown in the (001) direction of the perovskite structure reveals a robust non-collinear spin spiral magnetic order with dispersive magnon excitations that are essentially unperturbed by BO modulations and spatial confinement, we find magnons with flat dispersions and strongly reduced energies in SLs grown in the $(111)_{\text{pc}}$ direction that exhibit collinear magnetic order. These results give insight into the interplay of different collective ordering phenomena in a prototypical 3$d$ transition metal oxide and establish RIXS as a powerful tool to quantitatively study several order parameters and the corresponding collective excitations within one experiment., Comment: 13 pages, 8 figures, accepted in Physical Review B

We report the direct observation of a magnetic-feld induced long-wavelength spin spiral modulation in the chiral compound Ba3TaFe3Si2O14. This new spin texture emerges out of a chiral helical ground state, and is hallmarked by the onset of a unique contribution to the bulk electric polarization, the sign of which depends on the crystal chirality. The periodicity of the feld induced modulation, several hundreds of nm depending on the field value, is comparable to the length scales of mesoscopic topological defects such as skyrmions, merons and solitons. The phase transition and observed threshold behavior are consistent with a phenomenology based on the allowed Lifshitz invariants for the chiral symmetry of langasite, which intriguingly contain all the ingredients for the possible realization of topologically stable antiferromagnetic skyrmions.

Spin-orbit coupling in magnetic systems lacking inversion symmetry can give rise to non trivial spin textures. Magnetic thin films and heterostructures are potential candidates for the formation of skyrmions and other non-collinear spin configurations as inversion symmetry is inherently lost at their surfaces and interfaces. However, manganites, in spite of their extraordinarily rich magnetic phase diagram, have not yet been considered of interest within this context as their spin-orbit coupling is assumed to be negligible. We demonstrate here, by means of angular dependent X-ray linear dichroism experiments and theoretical calculations, the existence of a noncollinear antiferromagnetic ordering at the surface of ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_3$ thin films whose properties can only be explained by an unexpectedly large enhancement of the spin-orbit interaction. Our results reveal that spin-orbit coupling, usually assumed to be very small on manganites, can be significantly enhanced at surfaces and interfaces adding a new twist to the possible magnetic orders that can arise in electronically reconstructed systems., Comment: 8 pages, 5 figures

Incommensurate charge order (CO) has been identified as the leading competitor of high-temperature superconductivity in all major families of layered copper oxides, but the perplexing variety of CO states in different cuprates has confounded investigations of its impact on the transport and thermodynamic properties. The three-dimensional (3D) CO observed in YBa$_2$Cu$_3$O$_{6+x}$ in high magnetic fields is of particular interest, because quantum transport measurements have revealed detailed information about the corresponding Fermi surface. Here we use resonant X-ray scattering to demonstrate 3D-CO in underdoped YBa$_2$Cu$_3$O$_{6+x}$ films grown epitaxially on SrTiO$_3$ in the absence of magnetic fields. The resonance profiles indicate that Cu sites in the charge-reservoir layers participate in the CO state, and thus efficiently transmit CO correlations between adjacent CuO$_2$ bilayer units. The results offer fresh perspectives for experiments elucidating the influence of 3D-CO on the electronic properties of cuprates without the need to apply high magnetic fields.

Resonant elastic X-ray scattering (REXS) at the Eu $M_5$ edge reveals an antiferromagnetic structure in layered EuCd$_2$Sb$_2$ at temperatures below $T_\textrm{N}$ = 7.4 K with a magnetic propagation vector of $(0,0,1/2)$ and spins in the basal plane. Magneto-transport and REXS measurements with an in-plane magnetic field show that features in the magnetoresistance are correlated with changes in the magnetic structure induced by the field. Ab initio electronic structure calculations predict that the observed spin structure gives rise to a gapped Dirac point close to the Fermi level with a gap of $\Delta E \sim$0.01 eV. The results of this study indicate that the Eu spins are coupled to conduction electron states near the Dirac point.

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3 and SrTiO3. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from correlations between transition metal and oxygen ions. Strong correlations thus offer a rich playground to engineer functional interfaces but their compatibility with the classical band alignment picture remains an open question. Here we show that beyond differences in electron affinities and polar effects, a key parameter determining charge transfer at correlated oxide interfaces is the energy required to alter the covalence of the metaloxygen bond. Using the perovskite nickelate (RNiO3) family as a template, we probe charge reconstruction at interfaces with gadolinium titanate GdTiO3. X-ray absorption spectroscopy shows that the charge transfer is thwarted by hybridization effects tuned by the rare-earth (R) size. Charge transfer results in an induced ferromagnetic-like state in the nickelate, exemplifying the potential of correlated interfaces to design novel phases. Further, our work clarifies strategies to engineer two-dimensional systems through the control of both doping and covalence., Comment: Work supported by ERC Consolidator grant MINT (Contract No. 615759)

Layer-by-layer oxide molecular beam epitaxy has been used to synthesize cuprate-nickelate multilayer structures of composition (La$_2$CuO$_4$)$_m$/LaO/(LaNiO$_3$)$_n$. In a combined experimental and theoretical study, we show that these structures allow a clean separation of dopant and doped layers. Specifically, the LaO layer separating cuprate and nickelate blocks provides an additional charge that, according to density functional theory calculations, is predominantly accommodated in the interfacial nickelate layers. This is reflected in an elongation of bond distances and changes in valence state, as observed by scanning transmission electron microscopy and x-ray absorption spectroscopy. Moreover, the predicted charge disproportionation in the nickelate interface layers leads to a thickness-dependent metal-to-insulator transition for $n=2$, as observed in electrical transport measurements. The results exemplify the perspectives of charge transfer in metal-oxide multilayers to induce doping without introducing chemical and structural disorder.

Resonant x-ray scattering at the Dy $M_5$ and Ni $L_3$ absorption edges was used to probe the temperature and magnetic field dependence of magnetic order in epitaxial LaNiO$_3$-DyScO$_3$ superlattices. For superlattices with 2 unit cell thick LaNiO$_3$ layers, a commensurate spiral state develops in the Ni spin system below 100 K. Upon cooling below $T_{ind} = 18$ K, Dy-Ni exchange interactions across the LaNiO$_3$-DyScO$_3$ interfaces induce collinear magnetic order of interfacial Dy moments as well as a reorientation of the Ni spins to a direction dictated by the strong magneto-crystalline anisotropy of Dy. This transition is reversible by an external magnetic field of 3 T. Tailored exchange interactions between rare-earth and transition-metal ions thus open up new perspectives for the manipulation of spin structures in metal-oxide heterostructures and devices., Comment: PRL, in press

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., Comment: Supplementary information available upon request

The interplay between charge density waves (CDWs) and high-temperature superconductivity is currently under intense investigation. Experimental research on this issue is difficult because CDW formation in bulk copper-oxides is strongly influenced by random disorder, and a long-range-ordered CDW state in high magnetic fields is difficult to access with spectroscopic and diffraction probes. Here we use resonant x-ray scattering in zero magnetic field to show that interfaces with the metallic ferromagnet La$_{2/3}$Ca$_{1/3}$MnO$_3$ greatly enhance CDW formation in the optimally doped high-temperature superconductor YBa$_2$Cu$_3$O$_{6+\delta}$ ($\bf \delta \sim 1$), and that this effect persists over several tens of nm. The wavevector of the incommensurate CDW serves as an internal calibration standard of the charge carrier concentration, which allows us to rule out any significant influence of oxygen non-stoichiometry, and to attribute the observed phenomenon to a genuine electronic proximity effect. Long-range proximity effects induced by heterointerfaces thus offer a powerful method to stabilize the charge density wave state in the cuprates, and more generally, to manipulate the interplay between different collective phenomena in metal oxides., Comment: modified version published in Nature Materials

Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi$_{1-x}$Mn$_{x}$)$_{2}$Se$_{3}$ is a prototypical magnetic topological insulator with a pronounced surface band gap of $\sim100$ meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a novel mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection., Comment: 26 pages, 7 figures

Using resonant soft x-ray scattering as a function of both temperature and magnetic field, we reveal a large number of almost degenerate magnetic orders in SrCo6O11. The Ising-like spins in this frustrated material in fact exhibit a so-called magnetic devil's staircase. It is demonstrated how a magnetic field induces transitions between different microscopic spin configurations, which is responsible for the magnetoresistance of SrCo6O11. This material therefore constitutes a unique combination of a magnetic devil's staircase and spin valve effects, yielding a novel type of magnetoresistance system., Comment: 5 pages, 5 figures

We have used polarized Raman scattering to probe lattice vibrations and charge ordering in 12 nm thick, epitaxially strained PrNiO$_3$ films, and in superlattices of PrNiO$_3$ with the band-insulator PrAlO$_3$. A carefully adjusted confocal geometry was used to eliminate the substrate contribution to the Raman spectra. In films and superlattices under tensile strain, which undergo a metal-insulator transition upon cooling, the Raman spectra reveal phonon modes characteristic of charge ordering. These anomalous phonons do not appear in compressively strained films, which remain metallic at all temperatures. For superlattices under compressive strain, the Raman spectra show no evidence of anomalous phonons indicative of charge ordering, while complementary resonant x-ray scattering experiments reveal antiferromagnetic order associated with a modest increase in resistivity upon cooling. This confirms theoretical predictions of a spin density wave phase driven by spatial confinement of the conduction electrons., Comment: PRL, in press

We report the results a comprehensive study of charge density wave (CDW) correlations in untwinned YBCO6+x single crystals with 0.4 0.09) an enhancement of the signal when an external magnetic field up to 6 T is applied in the superconducting state. For samples with p~0.08, where prior work has revealed a field-enhancement of incommensurate magnetic order, the RXS signal is field-independent. This supports a previously suggested scenario in which incommensurate charge and spin orders compete against each other, in addition to individually competing against. We discuss the relationship of these results to stripe order 214, the pseudogap phenomenon, superconducting fluctuations, and quantum oscillations., Comment: revised version. To appear in Phys. Rev. B

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, Bi2201. 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., Comment: A high resolution version can be found at http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/Bi2201_CDW_REXS_STM.pdf"

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and quantum confinement of the valence electrons. The data are discussed in the light of recent theoretical predictions., Comment: 5 pages, 3 figures

We have used resonant x-ray scattering to determine the momentum dependent charge correlations in YBa$_2$Cu$_3$O$_{6.55}$ samples with highly ordered chain arrays of oxygen acceptors (ortho-II structure). The results reveal nearly critical, biaxial charge density wave (CDW) correlations at in-plane wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length are enhanced as superconductivity is weakened by an external magnetic field. Analogous experiments were carried out on a YBa$_2$Cu$_3$O$_{6.6}$ crystal with a dilute concentration of spinless (Zn) impurities, which had earlier been shown to nucleate incommensurate magnetic order. Compared to pristine crystals with the same doping level, the CDW amplitude and correlation length were found to be strongly reduced. These results indicate a three-phase competition between spin-modulated, charge-modulated, and superconducting states in underdoped YBa$_2$Cu$_3$O$_{6+\delta}$., Comment: 6 pages, 3 figures revised version, to appear in Phys. Rev. Lett

Resonant (elastic) soft x-ray scattering (RSXS) offers a unique element, site, and valence specific probe to study spatial modulations of charge, spin, and orbital degrees of freedom in solids on the nanoscopic length scale. It cannot only be used to investigate single crystalline materials. This method also enables to examine electronic ordering phenomena in thin films and to zoom into electronic properties emerging at buried interfaces in artificial heterostructures. During the last 20 years, this technique, which combines x-ray scattering with x-ray absorption spectroscopy, has developed into a powerful probe to study electronic ordering phenomena in complex materials and furthermore delivers important information on the electronic structure of condensed matter. This review provides an introduction to the technique, covers the progress in experimental equipment, and gives a survey on recent RSXS studies of ordering in correlated electron systems and at interfaces.

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

Soft resonant x-ray Bragg diffraction (SRXD) at the Ho M$_{4,5}$ edges has been used to study Ho $4f$ multipoles in the combined magnetic and orbitally ordered phase of HoB$_2$C$_2$. A full description of the energy dependence for both $\sigma$ and $\pi$ incident x-rays at two different azimuthal angles, as well as the ratio $I_\sigma/I_\pi$ as a function of azimuthal angle for a selection of energies, allows a determination of the higher order multipole moments of rank 1 (dipole) to 6 (hexacontatetrapole). The Ho 4f multipole moments have been estimated, indicating a dominant hexadecapole (rank 4) order with an almost negligible influence from either the dipole or the octupole magnetic terms. The analysis incorporates both the intra-atomic magnetic and quadrupolar interactions between the 3d core and 4f valence shells as well as the interference of contributions to the scattering that behave differently under time reversal. Comparison of SRXD, neutron diffraction and non resonant x-ray diffraction shows that the magnetic and quadrupolar order parameter are distinct. The $(0 0 1/2)$ component of the magnetic order exhibits a Brillouin type increase below the orbital ordering temperature T$_Q$, while the quadrupolar order increases more sharply. We conclude the quadrupolar interaction is strong, but quadrupolar order only occurs when the magnetic order gives rise to a quasi doublet ground state, which results in a lock-in of the orbitals at T$_Q$., Comment: 16 pages, 11 figures

We report the study of magnetic and orbital order in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films grown on (LaAlO$_3$)$_{0.3}$-(SrAl$_{0.5}$Ta$_{0.5}$O$_3$)$_{0.7}$ (LSAT) (011)$_c$. In a new experimental approach, the polarization and energy dependence of resonant soft x-ray scattering are used to reveal significant modifications of the magnetic order in the film as compared to the bulk, namely (i) a different magnetic ordering wave vector, (ii) a different magnetic easy axis and (iii) an additional magnetic reordering transition at low temperatures. These observations indicate a strong impact of the epitaxial strain on the spin order, which is mediated by the orbital degrees of freedom and which provides a promising route to tune the magnetic properties of manganite films. Our results further demonstrate that resonant soft x-ray scattering is a very suitable technique to study the magnetism in thin films, to which neutron scattering cannot easily be applied due to the small sample volume., Comment: 5 pages, 3 figures

We report combined soft and hard x-ray scattering studies of the electronic and lattice modulations associated with stripe order in La$_{1.875}$Ba$_{0.125}$CuO$_4$ and La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$. We find that the amplitude of both the electronic modulation of the hole density and the strain modulation of the lattice is significantly larger in La$_{1.875}$Ba$_{0.125}$CuO$_4$ than in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ and is also better correlated. The in-plane correlation lengths are isotropic in each case; for La$_{1.875}$Ba$_{0.125}$CuO$_4$, $\xi^{hole}=255\pm 5$ \AA\ whereas for La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$F, $\xi^{hole}=111\pm 7$ \AA. We find that the modulations are temperature independent in La$_{1.875}$Ba$_{0.125}$CuO$_4$ in the low temperature tetragonal phase. In contrast, in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$, the amplitude grows smoothly from zero, beginning 13 K below the LTT phase transition. We speculate that the reduced average tilt angle in La$_{1.875}$Ba$_{0.125}$CuO$_4$ results in reduced charge localization and incoherent pinning, leading to the longer correlation length and enhanced periodic modulation amplitude., Comment: 6 pages, 4 figures

We report the observation of a magnetic polarization of the O\,$2p$-states in YMn$_2$O$_5$ through the use of soft X-ray resonant scattering at the oxygen $K$-edge. Remarkably, we find that the temperature dependence of the integrated intensity of this signal closely follows the macroscopic electric polarization, and hence is proportional to the ferroelectric order parameter. This is in contrast to the temperature dependence observed at the Mn\,$L_3$-edge, which reflects the Mn magnetic order parameter. First principle calculations provide a microscopic understanding of these results and show that a spin-dependent hybridization of O\,$2p$- and Mn\, 3d-states results in a purely electronic contribution to the ferroelectric polarization, which can exist in the absence of lattice distortions.

Resonant-magnetic x-ray scattering (RMXS) near the vanadium $L_{2,3}$-absorption edges has been used to investigate the low temperature magnetic structure of high quality \bvs\ single crystals. Below $T_N$ = 31 K, the strong resonance revealed a triple-incommensurate magnetic ordering at wave vector (0.226 0.226 $\xi$) in the hexagonal notation, with $\xi$ = 0.033. The simulations of the experimental RMXS spectra with a time-dependent density functional theory indicate an antiferromagnetic order with the spins polarized along $a$ in the monoclinic structure., Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett

We studied the (001/2) diffraction peak in the low-temperature phase of magnetite (Fe3O4) using resonant soft x-ray diffraction (RSXD) at the Fe-L2,3 and O-K resonance. We studied both molecular-beam-epitaxy (MBE) grown thin films and in-situ cleaved single crystals. From the comparison we have been able to determine quantitatively the contribution of intrinsic absorption effects, thereby arriving at a consistent result for the (001/2) diffraction peak spectrum. Our data also allow for the identification of extrinsic effects, e.g. for a detailed modeling of the spectra in case a "dead" surface layer is present that is only absorbing photons but does not contribute to the scattering signal., Comment: to appear in Phys. Rev. B

We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias., Comment: 4 pages, 3 figures

We report an experimental study of \nco, a Mott insulator containing chains of edge-sharing CuO$_4$ plaquettes, by polarized x-ray absorption spectroscopy (XAS), resonant magnetic x-ray scattering (RMXS), magnetic susceptibility, and pyroelectric current measurements. The XAS data show that the valence holes reside exclusively on the Cu$^{2+}$ sites within the copper-oxide spin chains and populate a $d$-orbital polarized within the CuO$_4$ plaquettes. The RMXS measurements confirm the presence of incommensurate magnetic order below a N\'eel temperature of $T_N = 11.5$ K, which was previously inferred from neutron powder diffraction and nuclear magnetic resonance data. In conjunction with the magnetic susceptibility and XAS data, they also demonstrate a new "orbital" selection rule for RMXS that is of general relevance for magnetic structure determinations by this technique. Dielectric property measurements reveal the absence of significant ferroelectric polarization below $T_N$, which is in striking contrast to corresponding observations on the isostructural compound \lco. The results are discussed in the context of current theories of multiferroicity., Comment: 7 pages, 7 figures

Sizeable b and c components of the 4f moments and pronounced circular dichroism are observed in the ferroelectric phase of DyMnO_3 by soft X-ray diffraction at the Dy-M_5 resonance. This points to cycloidal order of the 4f moments and indicates that inversion-symmetry breaking in this material is not characteristic of the Mn spins alone. The circular dichroism allows to image chiral domains that are imprinted on the surface of a DyMnO$_3$ single crystal, exploiting the local charging by the X-ray beam via the photoelectric effect. These findings suggest a novel approach to control and image domains and domain walls in multiferroic materials.

We studied the stripe phase of La1.8Sr0.2NiO4 using neutron diffraction, resonant soft x-ray diffraction (RSXD) at the Ni L2,3 edges, and resonant x-ray diffraction (RXD) at the Ni K threshold. Differences in the q-space resolution of the different techniques have to be taken into account for a proper evaluation of diffraction intensities associated with the spin and charge order superstructures. We find that in the RSXD experiment the spin and charge order peaks show the same temperature dependence. In the neutron experiment by contrast, the spin and charge signals follow quite different temperature behaviors. We infer that fluctuating magnetic order contributes considerably to the magnetic RSXD signal and we suggest that this result may open an interesting experimental approach to search for fluctuating order in other systems by comparing RSXD and neutron diffraction data.

X-ray diffraction with photon energies near the Ru L$_2$-absorption edge was used to detect resonant reflections characteristic of a G-type superstructure in RuSr$_2$GdCu$_2$O$_8$ single crystals. A polarization analysis confirms that these reflections are due to magnetic order of Ru moments, and the azimuthal-angle dependence of the scattering amplitude reveals that the moments lie along a low-symmetry axis with substantial components parallel and perpendicular to the RuO$_2$ layers. Complemented by susceptibility data and a symmetry analysis of the magnetic structure, these results reconcile many of the apparently contradictory findings reported in the literature., Comment: to appear in PRL

We investigated the electronic structure of the SrTiO$_3$/LaAlO$_3$ superlattice (SL) by resonant soft x-ray scattering. The (003) peak, which is forbidden for our "ideal" SL structure, was observed at all photon energies, indicating reconstruction at the interface. From the peak position analyses taking into account the effects of refraction, we obtained evidence for electronic reconstruction of Ti 3d and O $2p$ states at the interface. From reflectivity analyses, we concluded that the AlO$_2$/LaO/TiO$_2$/SrO and the TiO$_2$/SrO/AlO$_2$/LaO interfaces are quite different, leading to highly asymmetric properties., Comment: 5 pages, 4 figures

We have employed Soft and Hard X-ray Resonant Magnetic Scattering and Polarised Neutron Diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial Ni$_{81}$Fe$_{19}$(111)/CoO(111) exchange biased bilayer. The combination of these scattering tools provides unprecedented detailed insights into the still incomplete understanding of some key manifestations of the exchange bias effect. We show that the several orders of magnitude difference between the expected and measured value of exchange bias field is caused by an almost anisotropic in-plane orientation of antiferromagnetic domains. Irreversible changes of their configuration lead to a training effect. This is directly seen as a change in the magnetic half order Bragg peaks after magnetization reversal. A 30 nm size of antiferromagnetic domains is extracted from the width the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and x-ray scattering. A reduced blocking temperature as compared to the measured antiferromagnetic ordering temperature clearly corresponds to the blocking of antiferromagnetic domains. Moreover, an excellent correlation between the size of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio is found, providing a comprehensive understanding of the origin of exchange bias in epitaxial systems., Comment: 8 pages, 5 figures, submitted

Resonant soft X-ray scattering with photon energies near the O K and the Cu L3 edges was used to study charge ordering in the system La_{1.8-x}Eu_{0.2}Sr_xCuO_4 as a function of temperature for x = 0.125 and 0.15. From the superstructure diffraction intensities a charge ordering with a doping dependent wave vector is derived which is in this system well below the transition temperature of the low-temperature tetragonal phase but well above the onset of spin ordering. This indicates that charge ordering is the primary driving force for the formation of stripe-like phases in two-dimensional doped cuprates. Analysis of the lineshape of the scattered intensity as a function of photon energy yields evidence for a high hole concentration in the stripes., Comment: 4 pages and 2 figures