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The concept of a charge density wave (CDW) permeates much of condensed matter physics and chemistry. CDWs have their origin rooted in the instability of a one-dimensional system described by Peierls. The extension of this concept to reduced dimensional systems has led to the concept of Fermi surface nesting (FSN), which dictates the wave vector (qCDW) of the CDW and the corresponding lattice distortion. The idea is that segments of the Fermi contours are connected by qCDW, resulting in the effective screening of phonons inducing Kohn anomalies in their dispersion at qCDW, driving a lattice restructuring at low temperatures. There is growing theoretical and experimental evidence that this picture fails in many real systems and in fact it is the momentum dependence of the electron-phonon coupling (EPC) matrix element that determines the characteristic of the CDW phase. Based on the published results for the prototypical CDW system 2H-NbSe2, we show how well the q-dependent EPC matrix element, but not the FSN, can describe the origin of the CDW. We further demonstrate a procedure of combing electronic band and phonon measurements to extract the EPC matrix element, allowing the electronic states involved in the EPC to be identified. Thus, we show that a large EPC does not necessarily induce the CDW phase, with Bi2Sr2CaCu2O8+δ as the example, and the charge-ordered phenomena observed in various cuprates are not driven by FSN or EPC. To experimentally resolve the microscopic picture of EPC will lead to a fundamental change in the way we think about, write about, and classify charge density waves. [ABSTRACT FROM AUTHOR]

Ferroelectric random access memory is still challenging in the feature of combination of room temperature stability, non-destructive readout and high intensity storage. As a non-contact and non-destructive information readout method, surface potential has never been paid enough attention because of the unavoidable decay of the surface potential contrast between oppositely polarized domains. That is mainly due to the recombination of the surface movable charges around the domain walls. Here, by introducing a laser beam into the combination of piezoresponse force microscopy and Kelvin probe force microscopy, we demonstrate that the surface potential contrast of BiFeO3 films can be recovered under light illumination. The recovering mechanism is understood based on the redistribution of the photo-induced charges driven by the internal electric field. Furthermore, we have created a 12-cell memory pattern based on BiFeO3 films to show the feasibility of such photo-assisted non-volatile and non-destructive readout of the ferroelectric memory. [ABSTRACT FROM AUTHOR]

Chemical doping in materials is known to give rise to emergent phenomena. These phenomena are extremely difficult to predict a priori, because electron-electron interactions are entangled with local environment of assembled atoms. Scanning tunneling microscopy and low energy electron diffraction are combined to investigate how the local electronic structure is correlated with lattice distortion on the surface of Sr3(Ru1-xMnx)2O7, which has double-layer building blocks formed by (Ru/Mn)O6 octahedra with rotational distortion. The presence of doping-dependent tilt distortion of (Ru/Mn)O6 octahedra at the surface results in a C2v broken symmetry in contrast with the bulk C4v counterpart. It also enables us to observe two Mn sites associated with the octahedral rotation in the bulk through the "chirality" of local electronic density of states surrounding Mn, which is randomly distributed. These results serve as fingerprint of chemical doping on the atomic scale. [ABSTRACT FROM AUTHOR]

In complex materials, how correlation between charge, spin, and lattice affects the emergent phenomena remains unclear. The newly discovered iron-based high-temperature superconductors and related compounds present to the community a prototype family of materials, where interplay between charge, spin, and lattice degrees of freedom can be explored. With the occurrence of structural, magnetic, and superconducting transitions in the bulk of these materials, creating a surface will change the delicate balance between these phases, resulting in new behavior. A surface lattice dynamics study on (001) Ba(Fe1-xCox)2As2, through electron energy loss spectroscopy measurements, reveals unusual temperature dependence of both the phonon frequency and line width in the low-temperature orthorhombic phase. The rate of change of phonon frequency with temperature is gigantic, two orders of magnitude larger than in the bulk. This behavior cannot be explained using conventional models of anharmonicity or electron-phonon coupling; instead, it requires that a large surface-spin-charge-lattice coupling be included. Furthermore, the higher surface-phase-transition temperature driven by surface stabilization of the low-temperature orthorhombic phase seems to turn the first-order transition (bulk) into the second-order type, equivalent to what is observed in the bulk by applying a uniaxial pressure. Such equivalence indicates that the surface mirrors the bulk under extreme conditions. [ABSTRACT FROM AUTHOR]

The signature of correlated electron materials (CEMs) is the coupling between spin, charge, orbital and lattice resulting in exotic functionality. This complexity is directly responsible for their tunability. We demonstrate here that the broken symmetry, through cubic to orthorhombic distortion in the lattice structure in a prototype manganite single crystal, La0.69Ca0.31MnO3, leads to an anisotropic magneto-elastic response to an external field, and consequently to remarkable magneto-transport behavior. An anomalous anisotropic magnetoresistance (AMR) effect occurs close to the metal-insulator transition (MIT) in the system, showing a direct correlation with the anisotropic field-tuned MIT in the system and can be understood by means of a simple phenomenological model. A small crystalline anisotropy stimulates a "colossal" AMR near the MIT phase boundary of the system, thus revealing the intimate interplay between magneto- and electronic-crystalline couplings. [ABSTRACT FROM AUTHOR]

Mott transitions, which are metal-insulator transitions (MITs) driven by electron-electron interactions, are usually accompanied in bulk by structural phase transitions. In the layered perovskite Ca1.9Sr0.1RuO4, such a first-order Mott MIT occurs in the bulk at a temperature of 154 kelvin on cooling. In contrast, at the surface, an unusual inherent Mott MIT is observed at 130 kelvin, also on cooling but without a simultaneous lattice distortion. The broken translational symmetry at the surface causes a compressional stress that results in a 1500/o increase in the buckling of the Ca/Sr-O surface plane as compared to the bulk. The Ca/Sr ions are pulled toward the bulk, which stabilizes a phase more amenable to a Mott insulator ground state than does the bulk structure and also energetically prohibits the structural transition that accompanies the bulk MIT. [ABSTRACT FROM AUTHOR]

Reports that insulin inhibits transcription of IRS-2 gene in rat liver through an insulin response element (IRE) that resemble IREs of other insulin-repressed genes. Insulin-mediated inhibition of IRS-2 gene transcription in rat liver; Putative IRE sequence in promoter region of mouse and human IRS-2 genes as compared with rat PEPCK gene.

Demonstrates that the amount of [3H]-membrane lipid release is a regulated aspect of apoptosis in lymphoid cells. Study of apoptosis in cells exposed to vincristine or etoposide; Correlation of formation and release of apoptotic bodies and cell to membrane lipid release; Arrest in the maturation and release of apoptotic bodies with inhibition of caspases.

The electrical modulation of the magnetoresistance (MR) from -70% to 80% under a small magnetic field of 200 Oe near room temperature is found in multi-p-n heterostructures of SrTiO3-δ/La0.9Sr0.1MnO3/SrTiO3-δ/La0.9Sr0.1MnO3/Si we fabricated. The mechanism causing the modulation of MR by applied bias is proposed as the interface competition effect in the multilayer heterojunctions. Our results of the present structure are expected to meet the high desire for the application of large electronic modulation of MR near room temperature. [ABSTRACT FROM AUTHOR]

The ordered (100) surface of layered In4Se3 single crystals is characterized by semiconducting quasi-one-dimensional indium (In) chains. A band with significant dispersion in the plane of the surface is observed near the valence band maximum. The band exhibits an anisotropic dispersion with ∼1 eV band width along the In chain direction. The dispersion of this band is largely due to the hybridization of In-s and Se-p orbitals, but the hybridization between In-s and Se-p and In-p and Se-p orbitals is also critical in establishing the band gap. [ABSTRACT FROM AUTHOR]

The magnetic field-dependent transport of Pr0.6Ca0.4MnO3 has been investigated to expose the field-induced melting of orbital ordering. The critical field and critical temperature for the melting of the orbital ordered state depend on the orientation of the applied magnetic field, seemingly independent of the nature of the magnetic phase. The crystalline c axis is the hard axis of the field-induced melting, but in-plane anisotropy also exists, correlating with the anisotropic charge-exchange-(CE) type orbital-ordered structure. These findings demonstrate that the anisotropic melting coincides with the orbital ordering anisotropy, thus indicating a strong spin-orbital coupling. [ABSTRACT FROM AUTHOR]

High-resolution scanning tunneling microscopy measurements on the (001) surface of BaFe²As², a parent compound of Fe-based high-T( superconductors, reveal a complex surface with coexisting structural phases. Large areas display a squarelike (√2 × √2)R45° structure with surface-pinned antiphase domains. The domain walls exhibit C2η symmetry, in contrast with the bulk C2„ geometric structure. We argue that the strong spin-lattice coupling at the surface results in the coexistence of structure and spin antiphase domain boundaries with C² symmetry. [ABSTRACT FROM AUTHOR]

Atomically resolved structural and electronic properties of FeTe1-xSex (x = 0 and 0.45) have been studied with scanning tunneling microscopy/spectroscopy (STM/STS). In contrast to the extreme flatness of the Te-terminated FeTe surface, nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe0.55Se0.45. A statistical counting of the two kinds of atoms has the same ratio as that in the bulk. Remarkably, there is no electronic phase separation seen in the tunneling spectroscopy. This indicates that the optimally doped superconductor is chemically inhomogeneous but electronically homogeneous, in contrast to many correlated electron materials. [ABSTRACT FROM AUTHOR]

Ferroelectric (FE) distortions in a metallic material were believed to be experimentally inaccessible because itinerant electrons would screen the long-range Coulomb interactions that favor a polar structure. It has been suggested by Anderson and Blount [P. W. Anderson, E. I. Blount, Phys. Rev. Lett. 14, 217-219 (1965)] that a transition from paraelectric phase to FE phase is possible for a metal if, in the paraelectric phase, the electrons at the Fermi level are decoupled from the soft transverse optical phonons, which lead to ferroelectricity. Here, using Raman spectroscopy combined with magnetotransport measurements on a recently discovered FE metal LiOsO3, we demonstrate active interplay of itinerant electrons and the FE order: Itinerant electrons cause strong renormalization of the FE order parameter, leading to a more gradual transition in LiOsO3 than typical insulating FEs. In return, the FE order enhances the anisotropy of charge transport between parallel and perpendicular to the polarization direction. The temperature-dependent evolution of Raman active in-plane 3Eg phonon, which strongly couples to the polar-active out-ofthe- plane A2u phonon mode in the high-temperature paraelectric state, exhibits a deviation in Raman shift from the expectation of the pseudospin-phonon model that is widely used to model many insulating FEs. The Curie-Weiss temperature (θ ≈ 97 K) obtained from the optical susceptibility is substantially lower than Ts, suggesting a strong suppression of FE fluctuations. Both line width and Fano line shape of 3Eg Raman mode exhibit a strong electron- phonon coupling in the high-temperature paraelectric phase, which disappears in the FE phase, challenging Anderson/Blount's proposal for the formation of FE metals. [ABSTRACT FROM AUTHOR]

Many ultrathin films of transition metal oxides exhibit nonmetallic behavior, in contrast to their metallic bulk counterpart, thus displaying a metal-insulator transition (MIT) as the film thickness is reduced. The nature of this MIT has been a long-standing issue in the epitaxial oxide research community. Here, we report the processing dependence of the critical thickness (tc) of MIT and the origin of the insulating phase in La2/3Sr1/3MnO3 (LSMO) films. A tc of 3 unit cells (u.c.) is achieved by minimizing oxygen vacancies under optimal growth conditions, diminishing the epitaxial strain with a tunable buffer layer and suppressing surface strain by film capping. The electrical transport measurements demonstrate that the nonmetallic behavior in LSMO thin films is an unavoidable result of localization initiated by inherent disorder but amplified by the reduction in dimensionality. [ABSTRACT FROM AUTHOR]

Low energy electron diffraction, scanning tunneling microscopy and spectroscopy, and first-principles spin-dependent density functional theory are utilized to investigate the geometric, electronic, and magnetic structures of the stripe-ordered (1 x 2) surface of Ca(Fei-xCox)2As2 (x = 0, 0.075). The surface is terminated with a 50% Ca layer. Compared to the bulk, the surface Ca layer has a large inward relaxation (∼0.5 A), and the underneath As-Fe2-As layer displays a significant buckling. First-principles calculations show that the (1 x 2) phase is stabilized by the bulk antiferromagnetic spin ordering through the spin-charge-lattice coupling. Strikingly, a superconducting gap (∼7 meV at 7.4 K) is observed to spatially coexist with the (1 x 2) phase (x = 0.075 compound). This implies the coexistence of both superconductivity and AFM ordering at the surface. [ABSTRACT FROM AUTHOR]

Chemical doping in materials is known to give rise to emergent phenomena. These phenomena are extremely difficult to predict a priori, because electron-electron interactions are entangled with local environment of assembled atoms. Scanning tunneling microscopy and low energy electron diffraction are combined to investigate how the local electronic structure is correlated with lattice distortion on the surface of Sr3(Ru1-xMnx)2O7, which has double-layer building blocks formed by (Ru/Mn)O6 octahedra with rotational distortion. The presence of doping-dependent tilt distortion of (Ru/Mn)O6 octahedra at the surface results in a C2v broken symmetry in contrast with the bulk C4v counterpart. It also enables us to observe two Mn sites associated with the octahedral rotation in the bulk through the "chirality" of local electronic density of states surrounding Mn, which is randomly distributed. These results serve as fingerprint of chemical doping on the atomic scale. [ABSTRACT FROM AUTHOR]

We report on a combined scanning tunneling microscopy and density functional theory calculation study of the SrTiO3(110)-(4×1) surface. It is found that antiphase domains are formed along the [11̅ 0]-oriented stripes on the surface. The domain boundaries are decorated by defect pairs consisting of Ti2O3 vacancies and Sr adatoms, which relieve the residual stress. The formation energy of and interactions between vacancies result in a defect superstructure. It is suggested that the density and distributions of defects can be tuned by strain engineering, providing a flexible platform for the designed growth of complex oxide materials. [ABSTRACT FROM AUTHOR]

In complex materials, how correlation between charge, spin, and lattice affects the emergent phenomena remains unclear. The newly discovered iron-based high-temperature superconductors and related compounds present to the community a prototype family of materials, where interplay between charge, spin, and lattice degrees of freedom can be explored. With the occurrence of structural, magnetic, and superconducting transitions in the bulk of these materials, creating a surface will change the delicate balance between these phases, resulting in new behavior. A surface lattice dynamics study on (001) Ba(Fe1-xCox)2As2, through electron energy loss spectroscopy measurements, reveals unusual temperature dependence of both the phonon frequency and line width in the low-temperature orthorhombic phase. The rate of change of phonon frequency with temperature is gigantic, two orders of magnitude larger than in the bulk. This behavior cannot be explained using conventional models of anharmonicity or electron-phonon coupling; instead, it requires that a large surface-spin-charge-lattice coupling be included. Furthermore, the higher surface-phase-transition temperature driven by surface stabilization of the low-temperature orthorhombic phase seems to turn the first-order transition (bulk) into the second-order type, equivalent to what is observed in the bulk by applying a uniaxial pressure. Such equivalence indicates that the surface mirrors the bulk under extreme conditions. [ABSTRACT FROM AUTHOR]

Neutron scattering studies reveal an unusual E-type antiferromagnetic (AFM) structure in Mn-substituted single crystals of Sr3Ru2O7 (x = 0.16). The material behaves as quasi-two-dimensional (2D) antiferromagnet with in-plane (ab) long-range ordering and exhibits only single-bilayer (5-6Å) ferromagnetic correlations along the c direction below TN = 78 K. However, the critical behavior of the staggered magnetization, the AFM order parameter, does not reflect the expected behavior of a 2D magnetic phase transition. Such an unusual magnetic structure deviates from what would be expected from the conventional spin-lattice coupling scenario in this class of materials. [ABSTRACT FROM AUTHOR]

The observation of substantially enhanced superconductivity of single-layer FeSe films on SrTiO3 has stimulated intensive research interest. At present, conclusive experimental data on the corresponding electron-boson interaction is still missing. Here we use inelastic electron scattering spectroscopy and angle resolved photoemission spectroscopy to show that the electrons in these systems are dressed by the strongly polarized lattice distortions of the SrTiO3, and the indispensable nonadiabatic nature of such a coupling leads to the formation of dynamic interfacial polarons. Furthermore, the collective motion of the polarons results in a polaronic plasmon mode, which is unambiguously correlated with the surface phonons of SrTiO3 in the presence of the FeSe films. A microscopic model is developed showing that the interfacial polaron-polaron interaction leads to the superconductivity enhancement. [ABSTRACT FROM AUTHOR]

Charge transfer and electron-phonon coupling (EPC) are proposed to be two important constituents associated with enhanced superconductivity in the single unit cell FeSe films on oxide surfaces. Using high-resolution electron energy loss spectroscopy combined with first-principles calculations, we have explored the lattice dynamics of ultrathin FeSe films grown on SrT103. We show that, despite the significant effect from the substrate on the electronic structure and superconductivity of the system, the FeSe phonons in the films are unaffected. The energy dispersion and linewidth associated with the Fe- and Se-derived vibrational modes are thickness and temperature independent. Theoretical calculations indicate the crucial role of antiferromagnetic correlation in FeSe to reproduce the experimental phonon dispersion. Importantly, the only detectable change due to the growth of FeSe films is the broadening of the Fuchs-Kliewer (F-K) phonons associated with the lattice vibrations of SrTiO3(001) substrate. If EPC plays any role in the enhancement of film superconductivity, it must be the interfacial coupling between the electrons in FeSe film and the F-K phonons from substrate rather than the phonons of FeSe. [ABSTRACT FROM AUTHOR]

Bilayered Sr3Ru2O7 is an unusual metamagnetic metal with inherently antiferromagnetic (AFM) and ferromagnetic (FM) fluctuations. Partial substitution of Ru by Mn results in the establishment of a metal-insulator transition (MIT) at TMIT and AFM ordering at TM in Sr3(Ru1-xMnx)2O7. Using elastic neutron scattering, we investigated the effect of Mn doping on the magnetic structure, in-plane magnetic correlation lengths and their correlation to the MIT in Sr3(Ru1-xMnx)2O7 (x=0.06 and 0.12). With the increase of Mn doping (x) from 0.06 to 0.12 or the decrease of temperatures for x=0.12, an evolution from an in-plane short-range to long-range antiferromagnetic (AFM) ground state occurs. For both compounds, the magnetic ordering has a double-stripe configuration, and the onset of magnetic correlation with an anisotropic behavior coincides with the sharp rise in electrical resistivity and specific heat. Since it does not induce a measurable lattice distortion, the double-stripe antiferromagnetic order with anisotropic spin texture breaks symmetry from a C4v crystal lattice to a C2v magnetic sublattice. These observations shed light on an age-old question regarding the Slater versus Mott-type MIT. [ABSTRACT FROM AUTHOR]

Heteroepitaxial growth of transition-metal oxide films on the open (111) surface of SrTi O 3 results in significant restructuring due to the polar mismatch. Monitoring the structure and composition on an atomic scale of LaNiO3/SrTiO 3 (111) interface as a function of processing conditions has enabled the avoidance of the expected polar catastrophe. Using atomically resolved transmission electron microscopy and spectroscopy as well as low-energy electron diffraction, the structure of the thin film, from interface to the surface, has been studied. In this paper, we show that the proper processing can lead to a structure that is ordered, coherent with the substrate without intermediate structural phase. Angle-resolved x-ray photoemission spectroscopy shows that the oxygen content of thin films increases with the film thickness, indicating that the polar mismatch is avoided by the presence of oxygen vacancies. [ABSTRACT FROM AUTHOR]

The coupling between the electrical transport properties of La2/3Sr1/3MnO3 (LSMO) thin films and structural phase transitions of SrTiO3 (STO) substrates at Ts=105K has been investigated. We found that the electrical resistivity of LSMO films exhibit a "cusp" at Ts, which is greatly amplified by tuning films to the verge of metallic and insulating phases, i.e., to the boundary of two delicate competing electronic states. Our results demonstrate that small amounts of strain can tip the subtle balance of competing interactions and tune the electronic properties in correlated electron materials. [ABSTRACT FROM AUTHOR]

Using atomically resolved imaging and spectroscopy, we reveal a remarkably deep polarization in nonferroelectric SrTiO3 near its interface with an ultrathin nonmetallic film of La2/3Sr1/3MnO3. Electron holography shows an electric field near the interface in SrTiO3, yielding a surprising spontaneous polarization density of ~ 21μC/cm². Combining the experimental results with first-principles calculations, we propose that the observed deep polarization is induced by the electric field originating from oxygen vacancies that extend beyond a dozen unit cells from the interface, thus providing important evidence of the role of defects in the emergent interface properties of transition metal oxides. [ABSTRACT FROM AUTHOR]

Double-layered Sr3Ru2O7 has received phenomenal consideration because it exhibits a plethora of exotic phases when perturbed. New phases emerge with the application of pressure, magnetic field, or doping. Here, we show that creating a surface is an alternative and effective way to reveal hidden phases that are different from those seen in the bulk by investigating the surface properties of Sr3(Ru1-xMnx)2O7. Driven by the tilt distortion of RuO6 octahedra, the surface of Sr3Ru2O7 is less metallic than the bulk. In contrast, because of the vanishing of tilt and enhanced rotation with Mn doping, the surface of Sr3(Ru0.84Mn0.16)2O7 is metallic, while the bulk is insulating. Our result demonstrates that the electronic and structural properties at the surface are intimately coupled and consistent with quasitwo-dimensional character. [ABSTRACT FROM AUTHOR]

The significant role of interfacial coupling in the enhancement of superconductivity in FeSe films on SrTiO3 has been widely recognized, but the explicit origin of this coupling is yet to be identified. Here, by surface phonon measurements using high-resolution electron energy loss spectroscopy, we found the electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with the electrons therein. The mode-specific electron-phonon coupling constant for the ~92 meV F-K phonon is ~0.25 in single-layer FeSe on SrTiO3. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. It is unambiguously shown that the SrTiO3 F-K phonon penetrating into FeSe is essential for interfacial superconductivity enhancement. [ABSTRACT FROM AUTHOR]

We have systematically investigated the annealing effect on the structure and composition of the polar surface of SrTiO3 (111) starting with an ex situ chemical etch. The relative surface concentration between Ti and Sr strongly depends on both the annealing temperature and the oxygen processing. There is a critical annealing temperature at which the maximum concentration ratio of Ti to Sr is achieved while still maintaining a (1?1) surface structure. We demonstrate that with proper processing it is possible to avoid surface reconstruction over a broad temperature range. Our results provide an optimal temperature window for epitaxial film growth. [ABSTRACT FROM AUTHOR]