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Polar metals are commonly defined as metals with polar structural distortions. Strict symmetry restrictions make them an extremely rare breed as the structural constraints favor insulating over metallic phase. Moreover, no polar metals are known to be magnetic. Here we report on the realization of a magnetic polar metal phase in a BaTiO3/SrRuO3/BaTiO3 heterostructure. Electron microscopy reveals polar lattice distortions in three-unit-cells thick SrRuO3 between BaTiO3 layers. Electrical transport and magnetization measurements reveal that this heterostructure possesses a metallic phase with high conductivity and ferromagnetic ordering with high saturation moment. The high conductivity in the SrRuO3 layer can be attributed to the effect of electrostatic carrier accumulation induced by the BaTiO3 layers. Density-functional-theory calculations provide insights into the origin of the observed properties of the thin SrRuO3 film. The present results pave a way to design materials with desired functionalities at oxide interfaces., Polar metals—metals with polar structural distortions—are known not to be magnetic. Here, the authors demonstrate a magnetic polar metal phase in a BaTiO3/SrRuO3/BaTiO3 heterostructure displaying high conductivity and ferromagnetic ordering with high saturation moment.

Transition metal oxide thin films and heterostructures have attracted remarkable and continuous attention due to the incredible variety of electronic and magnetic properties. A significant challeng...

Significance Electric polarization due to polar structural distortion is a central characteristic of an ferroelectric (FE) material, which is normally unattainable in a metallic system because itinerant electrons screen the internal electric field. A possible exception for a polar metal is if the itinerant electrons are decoupled with FE distortion, as proposed by Anderson and Blount. Here, we reveal an unusual FE phase transition in recently discovered polar metal LiOsO3, different from the typical transition in insulating FEs, with dramatic temperature-dependent electron−phonon coupling. Our results provide a look at models for FE transitions with the interplay between FE dipoles and itinerant electrons, suggesting that an improvement of the Anderson and Blount model is needed., 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-of-the-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.

The surface of SrTiO3 (001) is considered to be weakly polar, and in this work, we study the validity of this notion. It exhibits a surface structural distortion, quantified here using low energy electron diffraction at room temperature. Structural analysis shows the presence of strong surface rumpling in the TiO2 terminated surface with the oxygen atoms moving outward and Ti atoms moving inward. Density functional calculations confirm the measured rumpling, and experimental data show the distortion is localized at the surface. Angle-dependent core-level X-ray photoemission spectroscopy (XPS) shows that the surface rumpling strongly impacts the electronic structure of the surface. This observation is reinforced by density functional theory, which demonstrates that the valence state of Ti at the surface is reduced while O is enhanced, where we found the Ti–O bonds are more covalent near the surface. Our results show that surface rumpling is accompanied by a change in the bond hybridization of Ti–O at the s...

Charge density wave (CDW) is an important concept in condensed matter physics, germane to a number of physical phenomena. But the origin of CDW is still under debate, partly because the origin and properties of CDW are highly material-dependent. The concept of a CDW has been applied to many materials without a clear definition of the fundamental nature of CDW. As a result, misconceptions about CDW can be seen in the literature. In this review, we will try to describe and explain the possible existing misconceptions associated with the origin of CDWs.

An unusual Fe-H bonding rather than conventional OH bonding is identified at Fe3O4 (001) surface. This abnormal behavior is associated with the oxygen vacancies which exist on the surface region but also penetrate deep into the bulk Fe3O4. In contrast, OH bonding becomes preferential as generally expected on an ozone processed surface, which has appreciably less oxygen vacancies. Such bonding site selective behavior, depending on oxygen vacancy concentrations, is further confirmed with DFT calculations. The results demonstrate an opportunity for tuning the chemical properties of oxide surfaces or oxide clusters.

Complex oxide interfaces have been one of the central focuses in condensed matter physics and material science. Over the past decade, aberration corrected scanning transmission electron microscopy and spectroscopy has proven to be invaluable to visualize and understand the emerging quantum phenomena at an interface. In this paper, we briefly review some recent progress in the utilization of electron microscopy to probe interfaces. Specifically, we discuss several important challenges for electron microscopy to advance our understanding on interface phenomena, from the perspective of variable temperature, magnetism, electron energy loss spectroscopy analysis, electronic symmetry, and defects probing., Comment: 45 pages, 7 figures

Metallic oxide ${\mathrm{SrVO}}_{3}$ represents a prototype system for the study of the mechanism behind thickness-induced metal-to-insulator transition (MIT) or crossover in thin films due to its simple cubic symmetry with one electron in the $3d$ state in the bulk. Here we report a deviation of chemical composition and distortion of lattice structure existing in the initial 3 unit cells of ${\mathrm{SrVO}}_{3}$ films grown on ${\mathrm{SrTiO}}_{3}$ (001) from its bulk form, which shows a direct correlation to the thickness-dependent MIT. In situ photoemission and scanning tunneling spectroscopy indicate a MIT at the critical thickness of \ensuremath{\sim}3 unit cells (u.c.), which coincides with the formation of a $(\ensuremath{\surd}2\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}2)\phantom{\rule{0.28em}{0ex}}R{45}^{\ensuremath{\circ}}$ surface reconstruction. However, atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy show depletion of Sr, and a change of V valence, thus implying the existence of a significant amount of oxygen vacancies in the 3 u.c. of ${\mathrm{SrVO}}_{3}$ near the interface. Transport and magnetotransport measurements further reveal that disorder, rather than electron correlations, is likely to be the main cause for the MIT in the ${\mathrm{SrVO}}_{3}$ ultrathin films.

Interfaces between transition metal oxides are known to exhibit emerging electronic and magnetic properties. Here we report intriguing magnetic phenomena for La(2/3)Sr(1/3)MnO(3) films on an SrTiO(3) (001) substrate (LSMO/STO), where the interface governs the macroscopic properties of the entire monolithic thin film. The interface is characterized on the atomic level utilizing scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS), and density functional theory (DFT) is employed to elucidate the physics. STEM-EELS reveals mixed interfacial stoichiometry, subtle lattice distortions, and oxidation-state changes. Magnetic measurements combined with DFT calculations demonstrate that a unique form of antiferromagnetic exchange coupling appears at the interface, generating a novel exchange spring-type interaction that results in a remarkable spontaneous magnetic reversal of the entire ferromagnetic film, and an inverted magnetic hysteresis, persisting above room temperature. Formal oxidation states derived from electron spectroscopy data expose the fact that interfacial oxidation states are not consistent with nominal charge counting. The present work demonstrates the necessity of atomically resolved electron microscopy and spectroscopy for interface studies. Theory demonstrates that interfacial nonstoichiometry is an essential ingredient, responsible for the observed physical properties. The DFT-calculated electrostatic potential is flat in both the LSMO and STO sides (no internal electric field) for both Sr-rich and stoichiometric interfaces, while the DFT-calculated charge density reveals no charge transfer/accumulation at the interface, indicating that oxidation-state changes do not necessarily reflect charge transfer and that the concept of polar mismatch is not applicable in metal−insulator polar−nonpolar interfaces.

Understanding and manipulating properties emerging at a surface or an interface require a thorough knowledge of structure-property relationships. We report a study of a prototype oxide system, $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ grown on $\mathrm{SrTi}{\mathrm{O}}_{3}(001)$, by combining in situ angle-resolved x-ray photoelectron spectroscopy, ex situ x-ray diffraction, and scanning transmission electron microscopy/spectroscopy with electric transport measurements. We find that $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ films thicker than 20 unit cells (u.c.) exhibit a universal behavior with no more than one u.c. intermixing at the interface but at least 3 u.c. of Sr segregation near the surface, which is (La/Sr)O terminated. The conductivity vs film thickness shows the existence of nonmetallic layers with thickness $\ensuremath{\sim}6.5\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.16em}{0ex}}\mathrm{u}.\mathrm{c}.$, which is independent of film thickness but mainly relates to the deviation of Sr concentration near the surface region. Below 20 u.c., the surface of the films appears mixed, (La/Sr)O with ${\mathrm{MnO}}_{2}$ termination. Decreasing film thickness to less than 10 u.c. leads to the enhanced deviation of chemical composition in the films and eventually drives the film insulating. Our observation offers a natural explanation for the thickness-driven metal-nonmetal transition in thin films based on the variation of film stoichiometry.

The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) ${\mathrm{Ba}}_{2}{\mathrm{CoO}}_{4}$. In spite of isolated ${\mathrm{CoO}}_{4}$ tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to ${T}_{\mathrm{N}}=26$ K. More unexpectedly, spin dynamics displays quasi-two-dimensional spin-wave dispersion with an unusually large spin gap, and one-dimensional magnetoelastic coupling. Our results indicate that ${\mathrm{Ba}}_{2}{\mathrm{CoO}}_{4}$ is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and spin states in oxides.

To circumvent the limitations associated with sandwich ELISA for tissue biomarker quantitation, Quantitative Dot Blot method (QDB) was proposed using antibodies clinically validated for immunohistochemistry (IHC), as this method requires only one primary antibody in the analysis. The protein levels of four breast cancer tissue biomarkers, including Estrogen Receptor (ER), Progesterone Receptor (PR), Ki67 and Her2, were absolutely quantitated successfully in 190 frozen breast tissue biopsies, and the results were further verified with provided IHC results. We propose QDB method as an alternative platform to Sandwich ELISA for absolute quantitation of tissue biomarkers with significantly reduced developing effort and time.

Thickness-driven phase transitions have been widely observed in many correlated transition metal oxides materials. One of the important topics is the thickness-driven metal to insulator transition in half-metal La2/3Sr1/3MnO3 (LSMO) thin films, which has attracted great attention in the past few decades. In this article, we review research on the nature of the metal-to-insulator (MIT) transition in LSMO ultrathin films. We discuss in detail the proposed mechanisms, the progress made up to date, and the key issues existing in understanding the related MIT. We also discuss MIT in other correlated oxide materials as a comparison that also has some implications for understanding the origin of MIT.

Dislocations often occur in thin films with large misfit strain as a result of strain energy accumulation and can drastically change the film properties. Here the structure and dislocations in oxide heterostructures with large misfit strain are investigated on atomic scale. When grown on SrTiO3 (001), the dislocations in both the monolithic BaTiO3 thin film and its superlattices with SrIrO3 appear above a critical thickness around 6 nm. The edge component of the dislocations is seen in both cases with the Burgers vector of a ⟨100⟩. However, compared to monolithic BaTiO3, the dislocation density is slightly lower in BaTiO3/SrIrO3 superlattices. In the superlattice, when considering the SrTiO3 lattice constant as the reference, BaTiO3 has a larger misfit strain comparing with SrIrO3. It is found that in both cases, the formation of dislocation is only affected by the critical thickness of the film with larger lattice misfit (BaTiO3), regardless of the existence of a strong octahedral tilt/rotation mismatch at BaTiO3/SrIrO3 interface. Our findings suggest that it is possible to control the position of dislocations, an important step toward defect engineering.

Transition metal oxides (TMOs) exhibit exotic magnetic properties in both naturally formed and artificially structured materials, often difficult to understand in conventional wisdom. Magnetic insulator Ba2CoO4 has mystified the community, because the CoO4 tetrahedron appears to be completely isolated with the nearest Co atoms far apart (~5 A), making it difficult to account for long-range magnetic ordering seen experimentally using only Co. By theoretically investigating magnetism and relating our findings to experimental observations in bulk Ba2CoO4, we illustrate for the first time that the magnetic moment on oxygen atoms are the origin of the unexpected long-range magnetic ordering and low magnetic dimensionality. We find that the magnetic moment is not only localized on Co atoms, as assumed in all conventional data analysis, but also distributed on its tetrahedrally-coordinated O atoms. The total magnetic moment of the CoO4 building block is 4.63μB, with the magnetic moment on Co being only 3.08μB. Therefore, the magnetic building block is CoO4, not Co. Our first principles calculations are capable of explaining the origin of the unique magnetic response, including the presence of long-range magnetic ordering with two-dimensional character and a one-dimensional magnetoelastic behavior. Having oxygen contribute to the magnetic moment may be identified as a universal property of magnetic TMOs, which would require a reconsideration of conventional models.

The ferromagnetic metal La2/3Sr1/3MnO3 (LSMO) is an important correlated oxide material for oxide spintronics. However, the existence of a dead layer not only degrades device performance but also hinders the ultimate miniaturization of devices. Intense research has been made to eliminate the dead layer and understand its origin. Here, we have investigated the dead layer of (110)-oriented LSMO films on SrTiO3 (STO) substrates in which there is no polar discontinuity across the interface, in contrast to the (001)-oriented LSMO/STO interface. Our results reveal an 8 unit cell (u.c.) dead layer for (110) LSMO. Angle resolved x-ray photoemission demonstrating Sr segregation at the surface. Additionally, the broken symmetry at LSMO/vacuum interfaces also degrades the transport property. By introducing a capping layer, the film conductivity is enhanced. Finally, an ultrathin limit dead layer of 5 u.c. (=1.36 nm) has been obtained by introducing both the LaMnO3 capping layer and the buffer layer to compensate the excess holes at both LSMO/STO and Vacuum/LSMO interfaces and also to reduce the structural distortion at the top surface of LSMO. Our work paves the way toward eliminating the LSMO dead layer for spintronics application and provides a route to engineer the intriguing physical properties of oxide heterointerfaces.

Due to their unique structure that hosts geometric frustration, spinel oxides with magnetic ions in both the tetrahedral and octahedral environments undergo multiple spin rearrangement transitions. From the magnetization and thermal properties measurements of MnFe2O4, we have found three magnetic states occurring at TFI-1 = 575 K, TFI-2 = 50 K, and Tx = 15 K. Detailed analyses of the specific heat and thermal conductivity reveal a significant contribution from the low-temperature magnetic excitations. Especially, there is a large negative magneto-thermal conductivity with a maximum magnitude near TFI-2, which becomes positive below Tx. The possible origin of each magnetic state is discussed.

Understanding oxide interface-induced effects requires controlled epitaxial growth of films on well-defined substrate surfaces. While conventional film growth on ex situ prepared substrates has proven to be a successful route, the choices of appropriate substrates with atomically defined surfaces are limited. Here, by depositing $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ on $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ (001), we present an alternative method of growing oxide thin films on in situ cleaved surfaces of layered-structured substrates. Cleaving $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ at low temperature in ultrahigh vacuum exposes an atomically flat, solely SrO-terminated surface with up to micrometer-scale terraces. The deposition of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ spontaneously diminishes the surface $\mathrm{Ru}{\mathrm{O}}_{6}$ in-plane rotational distortion of the substrate and results in a cubic-like perovskite film structure with (La/Sr)-O layer termination. The interface is atomically sharp without obvious deviation of lattice spacing and chemical valence, except in the first unit cell where Ru-Mn intermixing is observed. These results demonstrate that film growth on a cleaved substrate can be an alternative route to obtain well-defined interfaces and in addition increase the availability of substrates for future oxide films.

Extended defects are known to have critical influences in achieving desired material performance. However, the nature of extended defect generation is highly elusive due to the presence of multiple nucleation mechanisms with close energetics. A strategy to design extended defects in a simple and clean way is thus highly desirable to advance the understanding of their role, improve material quality, and serve as a unique playground to discover new phenomena. In this work, we report an approach to create planar extended defects-antiphase boundaries (APB) -with well-defined origins via the combination of advanced growth, atomic-resolved electron microscopy, first-principals calculations, and defect theory. In La2/3Sr1/3MnO3 thin film grown on Sr2RuO4 substrate, APBs in the film naturally nucleate at the step on the substrate/film interface. For a single step, the generated APBs tend to be nearly perpendicular to the interface and propragate toward the film surface. Interestingly, when two steps are close to each other, two corresponding APBs communicate and merge together, forming a unique triangle-shaped defect domain boundary. Such behavior has been ascribed, in general, to the minimization of the surface energy of the APB. Atomic-resolved electron microscopy shows that these APBs have an intriguing antipolar structure phase, thus having the potential as a general recipe to achieve ferroelectric-like domain walls for high-density nonvolatile memory.

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 SrTiO3. 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 SrTiO$_3$(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., 13 gages, 8 figures

We have studied $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films grown on (3\ifmmode\times\else\texttimes\fi{}1)-reconstructed $\mathrm{SrTi}{\mathrm{O}}_{3}$ (110) substrates. Films with thicknesses less than the critical thickness of ${\ensuremath{\theta}}_{\mathrm{c}}\ensuremath{\cong}8$ unit cells are insulating in the measured temperature ($T$) range (2--400 K). However, films with thicknesses slightly over ${\ensuremath{\theta}}_{\mathrm{c}}$ exhibit reentrant nonmetallic behavior at low temperatures in addition to the normally observed metal-insulator transition at higher temperatures. In contrast, the magnetization does not show signs of low-$T$ transitions. Such reentrance of a low-$T$ nonmetallic phase is affected by the film thickness as well as the density of oxygen vacancies. The electrical resistivity analysis reveals that localization effects are responsible for the reentrant nonmetallic behavior, which is enhanced with reduced film thickness.

Bilayered $\mathrm{S}{\mathrm{r}}_{3}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$ 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 ${T}_{\mathrm{MIT}}$ and AFM ordering at ${T}_{\mathrm{M}}$ in $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{R}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x})}_{2}{\mathrm{O}}_{7}$. 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 $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{R}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x})}_{2}{\mathrm{O}}_{7}$ ($x=0.06$ 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 ${C}_{4v}$ crystal lattice to a ${C}_{2v}$ magnetic sublattice. These observations shed light on an age-old question regarding the Slater versus Mott-type MIT.

Plasmons, the collective excitations of electrons in the bulk or at the surface, play an important role in the properties of materials, and have generated the field of "plasmonics." We report the observation of a highly unusual acoustic plasmon mode on the surface of a three-dimensional topological insulator (TI) Bi_{2}Se_{3}, using momentum resolved inelastic electron scattering. In sharp contrast to ordinary plasmon modes, this mode exhibits almost linear dispersion into the second Brillouin zone and remains prominent with remarkably weak damping not seen in any other systems. This behavior must be associated with the inherent robustness of the electrons in the TI surface state, so that not only the surface Dirac states but also their collective excitations are topologically protected. On the other hand, this mode has much smaller energy dispersion than expected from a continuous media excitation picture, which can be attributed to the strong coupling with surface phonons.

Heteroepitaxial growth of transition-metal oxide films on the open (111) surface of $\mathrm{SrTi}{\mathrm{O}}_{3}$ results in significant restructuring due to the polar mismatch. Monitoring the structure and composition on an atomic scale of $\mathrm{LaNi}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (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.

Interfaces with subtle difference in atomic and electronic structures in perovskite ABO 3 heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. In this article, we report an integrated study of unusual transport, magnetic, and structural properties of Pr 0.67 Sr 0.33 MnO 3 (PSMO) films and La 0.67 Sr 0.33 MnO 3 (LSMO) films of various thicknesses on SrTiO 3 (STO) substrate. In particular, using atomically resolved imaging and electron energy-loss spectroscopy (EELS), we measured interface related local lattice distortion, BO 6 octahedral rotation and cation-anion displacement induced polarization. In the very thin PSMO film, an unexpected interface-induced ferromagnetic polaronic insulator phase was observed during the cubic-to-tetragonal phase transition of the substrate STO, due to the enhanced electron-phonon interaction and atomic disorder in the film. On the other hand, for the very thin LSMO films we observed a remarkably deep polarization in non-ferroelectric STO substrate near the interface. Combining the experimental results with first principles calculations, we propose that the observed deep polarization is induced by an 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.

Using atomically resolved imaging and spectroscopy, we reveal a remarkably deep polarization in nonferroelectric $\mathrm{SrTi}{\mathrm{O}}_{3}$ near its interface with an ultrathin nonmetallic film of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$. Electron holography shows an electric field near the interface in $\mathrm{SrTi}{\mathrm{O}}_{3}$, yielding a surprising spontaneous polarization density of $\ensuremath{\sim}21\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$. 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.

Double-layered $\mathrm{S}{\mathrm{r}}_{3}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$ 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 $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{R}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x})}_{2}{\mathrm{O}}_{7}$. Driven by the tilt distortion of $\mathrm{Ru}{\mathrm{O}}_{6}$ octahedra, the surface of $\mathrm{S}{\mathrm{r}}_{3}\mathrm{R}{\mathrm{u}}_{2}{\mathrm{O}}_{7}$ is less metallic than the bulk. In contrast, because of the vanishing of tilt and enhanced rotation with Mn doping, the surface of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{R}{\mathrm{u}}_{0.84}\mathrm{M}{\mathrm{n}}_{0.16})}_{2}{\mathrm{O}}_{7}$ 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.

The significant role of interfacial coupling on the superconductivity enhancement in FeSe films on SrTiO3 has been widely recognized. But the explicit origination of this coupling is yet to be identified. Here by surface phonon measurements using high resolution electron energy loss spectroscopy, we found electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with electrons therein. The mode-specific electron-phonon coupling (EPC) constant for the ~92 meV F-K phonon is ~0.25 in the 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 in the interfacial superconductivity enhancement., Main text: 6 pages, 3 figures; Supplementary Material: 8 pages, 6 figures. An error in the EPC constant calculation was corrected in this version and several new references are added. The authors would like to thank Dr. Yan Wang (University of Tennessee) for pointing out the error

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 x 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., Accepted to Phys. Rev. B

Significance Raman scattering is a powerful technique to probe optical phonons in solids. It usually involves an electron-mediated three-step process, involving photon–electron, electron–phonon, and electron–photon interactions. In principle, manipulating electrons, for instance by applying a magnetic field, should affect Raman phonon intensity, yet there is no direct experimental measurement to demonstrate this. In this work we report the first realization to our knowledge of the idea in a prototype material, MoS 2 . From monolayer and bilayer to bulk MoS 2 we observe a dramatic modification of Raman phonon intensity induced by magnetic field. Such a giant magneto-optical effect appearing at a monoatomic layer level and its technological implications for magnetic-optical devices should inspire a new branch of inelastic light scattering.

Ever since the discovery of high-Tc superconductivity in layered cuprates, the roles that individual layers play have been debated, due to difficulty in layer-by-layer characterization. While there is similar challenge in many Fe-based layered superconductors, the newly-discovered Ca10(Pt4As8)(Fe2As2)5 provides opportunities to explore superconductivity layer by layer, because it contains both superconducting building blocks (Fe2As2 layers) and intermediate Pt4As8 layers. Cleaving a single crystal under ultra-high vacuum results in multiple terminations: an ordered Pt4As8 layer, two reconstructed Ca layers on the top of a Pt4As8 layer, and disordered Ca layer on the top of Fe2As2 layer. The electronic properties of individual layers are studied using scanning tunneling microscopy/spectroscopy (STM/S), which reveals different spectra for each surface. Remarkably superconducting coherence peaks are seen only on the ordered Ca/Pt4As8 layer. Our results indicate that an ordered structure with proper charge balance is required in order to preserve superconductivity.

Homoepitaxial SrTiO3(110) film is grown by molecular beam epitaxy in ultra-high vacuum with oxygen diffusing from substrate as the only oxidant. The resulted oxygen vacancies (VOs) are found to be spatially confined within few subsurface layers only, forming a quasi-two-dimensional doped region with a tunable high concentration. Such a δ-function distribution of VOs is essentially determined by the thermodynamics associated with the surface reconstruction, and facilitated by the relatively high growth temperature. Our results demonstrate that it is feasible to tune VOs distribution at the atomic scale by controlling the lattice structure of oxide surfaces.

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.

High-resolution scanning tunneling microscopy (STM) measurements on BaFe2As2-one of the parent compounds of the iron-based superconductors-reveals a (1x1) As-terminated unit cell on the (001) surface. However, there are significant differences of the surface unit cell compared to the bulk: only one of the two As atoms in the unit cell is imaged and domain walls between different (1x1) regions display a C2 symmetry at the surface. It should have been C2v if the STM image reflected the geometric structure of the surface or the orthorhombic bulk. The inequivalent As atoms and the bias dependence of the domain walls indicate that the origin of the STM image is primarily electronic not geometric. We argue that the surface electronic topography mirrors the bulk spin structure of BaFe2As2, via strong orbital-spin coupling.

Core-level spectra of the Mn-doped Sr3Ru2O7 and Srn+1RunO3n+1 (n = 1, 2 and 3) crystals are investigated with X-ray photoelectron spectroscopy. Doping of Mn to Sr3Ru2O7 considerably affects the distribution of core-level spectral weight. The satellite of Ru 3d core levels exhibits a substantial change with doping, indicating an enhanced electron localization across the doping- induced metal-insulator transition. However, the Ru 3p core levels remain identical with Mn-doping, thus showing no sign of doping-induced multiple Ru valences. In the Srn+1RunO3n+1 (n = 1, 2 and 3), the Ru 3d core-level spectra are similar, indicating that the chemical bonding environment around Ru ions remains the same for different layered compounds. Meanwhile the Sr 3d shallow core levels shift to higher binding energy with increasing n, suggesting their participation in Sr-O bonding with structural evolution., Comment: 6 pages with 6 figures, to be published in PRB

${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ exhibits properties that are characteristic of the parent compounds of the newly discovered iron (Fe)-based high-${T}_{C}$ superconductors. By combining real-space imaging of scanning tunneling microscopy and spectroscopy ($\mathrm{STM}+\mathrm{STS}$) with momentum-space quantitative low-energy electron diffraction (LEED), we have identified the surface plane of cleaved ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ crystals as the As terminated Fe-As layer---the plane where superconductivity occurs. LEED and $\mathrm{STM}+\mathrm{STS}$ data on the ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}(001)$ surface indicate an ordered arsenic (As) terminated metallic surface without reconstruction or lattice distortion. It is surprising that STM images the different Fe-As orbitals associated with the orthorhombic structure, but not the As atoms in the surface plane.

Neutron scattering has been used to investigate the evolution of long- and short-range charge-ordered (CO), ferromagnetic, and antiferromagnetic (AF) correlations in single crystals of Pr(1-x)Ca(x)MnO(3). The existence and population of spin clusters as reflected by short-range correlations are found to drastically depend on the doping and temperature. Concentrated spin clusters coexist with long-range canted AF order in a wide temperature range in the x=0.3 while clusters do not appear in the x=0.4 crystal. In contrast, both CO and AF order parameters in the x=0.35 crystal show a precipitous decrease below similar to 35 K where spin clusters form. These results provide direct evidence of magnetic phase separation and demonstrate a critical doping x(c) (close to x=0.35) that divides the inhomogeneous from homogeneous CO ground state.

The surface structural phases of Ca2-xSrxRuO4 are investigated using quantitative low energy electron diffraction. The broken symmetry at the surface enhances the structural instability against the ...

Neutron scattering results are presented for spin-wave excitations of three ferromagnetic metallic $A_{1-x}A^{\prime}_{x}$MnO$_3$ manganites (where $A$ and $A^\prime$ are rare- and alkaline-earth ions), which when combined with previous work elucidate systematics of the interactions as a function of carrier concentration $x$, on-site disorder, and strength of the lattice distortion. The long wavelength spin dynamics show only a very weak dependence across the series. The ratio of fourth to first neighbor exchange ($J_4/J_1$) that controls the zone boundary magnon softening changes systematically with $x$, but does not depend on the other parameters. None of the prevailing models can account for these behaviors., Comment: Submitted to Phys. Rev. Lett