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Atomically thin ferromagnetic and conducting electron systems are highly desired for spintronics, because they can be controlled with both magnetic and electric fields. We present (SrRuO3)1-(SrTiO3)5 superlattices and single-unit-cell-thick SrRuO3 samples that are capped with SrTiO3. We achieve samples of exceptional quality. In these samples, the electron systems comprise only a single RuO2 plane. We observe conductivity down to 50 mK, a ferromagnetic state with a Curie temperature of 25 K, and signals of magnetism persisting up to approximately 100 K., The version published at Phys. Rev. X (open access) contains a large amount of additional material compared to the version published here

Homologous series are layered phases that can have a range of stoichiometries depending on an index n. Examples of perovskite-related homologous series include (ABO3)nAO Ruddlesden–Popper phases and (Bi2O2) (An−1BnO3n+1) Aurivillius phases. It is challenging to precisely control n because other members of the homologous series have similar stoichiometry and a phase with the desired n is degenerate in energy with syntactic intergrowths among similar n values; this challenge is amplified as n increases. To improve the ability to synthesize a targeted phase with precise control of the atomic layering, we apply the x-ray diffraction (XRD) approach developed for superlattices of III–V semiconductors to measure minute deviations from the ideal structure so that they can be quantitatively eradicated in subsequent films. We demonstrate the precision of this approach by improving the growth of known Ruddlesden–Popper phases and ultimately, by synthesizing an unprecedented n = 20 Ruddlesden–Popper phase, (ATiO3)20AO where the A-site occupancy is Ba0.6Sr0.4. We demonstrate the generality of this method by applying it to Aurivillius phases and the Bi2Sr2Can–1CunO2n+4 series of high-temperature superconducting phases.

The engineering of phononic resonances in ferroelectric structures appears as a new knob in the designand realization of novel multifunctional devices. In this work we experimentally study phononicresonators based on insulating (BaTiO 3 , SrTiO 3 ) and metallic (SrRuO 3 ) oxides. We experimentallydemonstrate the confinement of acoustic waves in the 100 GHz frequency range in a phonon nanocavity,the time and spatial beatings resulting from the coupling of two different hybrid nanocavities forming anacoustic molecule, and the direct measurement of Bloch-like oscillations of acoustic phonons in a systemformed by 10 coupled resonators. By means of coherent phonon generation techniques we study thephonon dynamics directly in the time-domain. The metallic SrRuO 3 introduces a local phonon generatorand transducer that allows for the spatial, spectral and time-domain monitoring of the complex generatedwaves. Our results introduce ferroelectric cavity systems as a new tool for the study of complex wavelocalization phenomena at the nanoscale. Fil: Bruchhausen, Axel Emerico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. University of Konstanz; Alemania Fil: Lanzillotti Kimura, Norberto Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Université Paris - Saclay; Francia Fil: Jusserand, B.. Universite Pierre et Marie Curie; Francia Fil: Soukiassian, A.. Cornell University; Estados Unidos Fil: Schlom, D. G.. Cornell University; Estados Unidos Fil: Dekorsy, T.. University of Konstanz; Alemania Fil: Fainstein, Alejandro. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina

Infrared reflectance spectra of the polar optic phonons in a [(BaTiO3)(8)/(SrTiO3)(4)](50) superlattice over the temperature range 8 to 650 K are reported. The spectra exhibit lattice vibration features typical of the perovskite constituents, BaTiO3 and SrTiO3. Using the effective medium approximation for multilayer systems and a fitting procedure, we were able to successfully simulate the infrared reflectivity of the superlattice structures and extract the parameters and dielectric functions of the phonons. When applied at varying temperatures, this approach provides the parameters of the zone-center polar phonons and their temperature dependence. Comparing these results with x-ray diffraction we identified some anomalies in phonon behavior related to the phase transitions that the superlattice undergoes.

Polarized infrared reflectance of ferroelectric [(BaTiO)/(SrTiO)] superlattices grown on DyScO, TbScO, EuScO and SmScOsubstrates by reactive molecular beam epitaxy was measured. Infrared spectra (30–3000 cm) were taken independently from the (110) cut of bare substrate and the substrate covered with superlattice in two polarizations: along the -axis and perpendicular to it. The phonon parameters and dielectric function of the superlattice were extracted using a fitting procedure to study the influence of the strain induced by different substrates. The phonon frequency dependence on the substrate-induced strain was varying from 75 to 95 cm. The superlattice static permittivity was estimated from that data, which varies from 150 to 270 depending on substrate and polarization. The highest permittivity was obtained for the EuScO substrate.

Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

A short review of the study of thermal properties of the nanostructured ferroelectrics is given. An application of the modern calorimetric technique to a comparative study of phase transitions in the ferroelectric nanostructured materials (polycrystalline and epitaxial thin films, superstructures, ferroelectric heterostructures, substrate-induced ferroelectricity) is considered.

Oxide heterostructure superlattices constitute a new family of materials with tunable ferroelectric properties. While theoretical models predict the presence of nanosized ferroelectric domains in these films, they had not been observed as the magnitude of the response functions challenges the limits of experimental detection. Here, a new protocol in a precise variant of piezoforce microscopy is used to image domains in BaTiO(3)/SrTiO(3) superlattices. Comparison of experimentally determined polarization to predictions of phase-field calculations is in quantitative agreement. Additionally, a combination of theory and experiment is used to determine the magnitude of internal electric field within the thin film, in a procedure that can be generalized to all ferroelectric films.

It is shown that the information about thermophysical parameters of the nanostructured ferroelectrics is a useful supplement to the traditional x-ray and dielectric data. The results of the study of the temperature dependence of heat capacity of BaTiO 3 /MgO epitaxial thin films by dynamical 3ω method for the thickness range 50–450 nm are presented. The blurred specific heat anomalies corresponding to the phase transitions were observed, and the temperature of the transitions increases with decreasing of film thickness compared with T c in a bulk sample. The causes of the severe smearing of specific heat anomalies as well as pronounced nonlinearity of the dependence of the transition temperature on the film thickness are discussed. Specific heat data for ferroelectric superlattices (BaTiO 3 ) n /(Sr TiO 3 ) m /Sr TiO 3 were obtained for the first time for the films with the average thicknesses near 100 nm.

Commensurate BaTiO3/SrTiO3 superlattices were grown by reactive molecular-beam epitaxy on four different substrates: TiO2-terminated (001) SrTiO3, (101) DyScO3, (101) GdScO3, and (101) SmScO3. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create commensurate BaTiO3/SrTiO3 superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions. The rocking curve full width half-maximum of the superlattices was as narrow as 7 arc s (0.002°). High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. Temperature-dependent ultraviolet Raman and XRD were used to reveal the paraelectric-to-ferroelectric transition temperature (TC). Our results demonstrate the importance of finite size and strain effects on the TC of BaTiO3/SrTiO3 superlattices. In addition to probing finite size and strain effects, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control.

We report properties of polycrystalline MgB2 films deposited on thin flexible yttium-stabilized zirconia (YSZ) substrates by hybrid physical-chemical vapor deposition. The MgB2 films show a transition temperature of 38.9 K with a narrow superconducting transition (0.1 K). The self-field critical current density in the films reaches 107 A/cm2 at low temperatures. These properties do not change after repeated bending over a radius of 10 mm. Low microwave surface resistance comparable to those obtained in epitaxial MgB2 films on single-crystalline sapphire substrate was observed. The result shows promise of polycrystalline MgB2 films for applications in superconducting digital circuits as well as in coated conductor wires.

Hybrid physical–chemical vapor deposition (HPCVD) has been the most effective technique for depositing MgB2 thin films. It generates high magnesium vapor pressures and provides a clean environment for the growth of high purity MgB2 films. The epitaxial pure MgB2 films grown by HPCVD show higher-than-bulk Tc due to tensile strain in the films. The HPCVD films are the cleanest MgB2 materials reported, allowing basic research, such as on magnetoresistance, that reveals the two-band nature of MgB2. The carbon-alloyed HPCVD films demonstrate record-high Hc2 values promising for high magnetic field applications. The HPCVD films and multilayers have enabled the fabrication of high quality MgB2 Josephson junctions.

A two-dimensional ferromagnet is an interesting system both for basic understanding of magnetism and for spintronic applications. However, making such a system is not easy as ferromagnetism becomes more and more unstable with reducing the thickness of a ferromagnet. For example, in oxide systems such as SrRuO 3 , La 1−x Sr x MnO 3 , etc, ferromagnetism is lost around the thickness of a few unit cells [1, 2]. Recent theoretical studies have shown that a single SrRuO 3 layer can exhibit half-metallicity when it is sandwiched between SrTiO 3 lattices [3, 4]. The conduction electrons are predicted to exist only in RuO 2 layers, which means that the system is a two-dimensional half-metal. In this work, in order to demonstrate the theoretical prediction, we have fabricated high-quality SrRuO 3 /SrTiO 3 superlattices using molecular beam epitaxy. Magnetic and transport measurements show a clear signature of ferromagnetism and conductivity.

Carbon-doped MgB/sub 2/ thin films have been produced with hybrid physical-chemical vapor deposition (HPCVD) by adding a carbon-containing metalorganic magnesium precursor, bis(methylcyclopentadienyl)magnesium, to the carrier gas. The amount of the carbon added, thus the carbon content in the films, was controlled by the flow rate of a secondary hydrogen gas flow through the precursor bubbler. X-ray diffraction and electron microscopy showed that the carbon-doped MgB/sub 2/ films are textured with c-axis oriented columnar nano-grains and highly resistive amorphous areas at the grain boundaries. When the amount of carbon in the films increases, the resistivity increases dramatically while T/sub c/ decreases much more slowly as the current-carrying cross section is reduced by the grain boundaries. The temperature-dependent part of the resistivity, /spl Delta//spl rho//spl equiv//spl rho/(300 K)-/spl rho/(50 K), increases only modestly until the highly resistive grain boundaries completely cut off the conducting path. The impact of the reduced cross section on critical current density J/sub c/ is discussed.

We present a Raman spectroscopy study of the lattice-dynamical properties of the BaxSr1−xTiO3 thin films with x=0.05, 0.1, 0.2, 0.35, and 0.5 in the temperature range of 5–300K. Soft phonon modes were observed, and the temperature dependence of their frequencies and Raman intensities were investigated. The Raman spectra of the BaxSr1−xTiO3 films are compared with the single crystals of the same compositions. The essential differences in the lattice-dynamical properties of the thin films and single crystals are observed, such as the forbidden first-order Raman scattering in the films well above the phase-transition temperature in bulk, hardening of the soft phonon modes in films compared to crystals, and significantly larger range of the soft-mode overdamping in the films. On the other hand, the lattice-dynamics behavior in the BaxSr1−xTiO3 films is found similar to that of the relaxor ferroelectrics. Analogous to the relaxors, the presence of the polar nanoregions existing in the films at the temperatures...

We report the transient optical pump-probe reflectivity measurements on single and double layer cuprate single crystals and thin films of ten different stoichiometries. We find that with sufficiently low fluence the relaxation time (τR) of all samples exhibits a power law divergence with temperature (T ): τR ∝ T �3±0.5 . Further, the divergence has an onset temperature above the superconducting transition temperature for all superconducting samples. Possible causes of this divergence are discussed.

It has long been known that oxygen doping, cation doping and strain all strongly influence the superconducting transition temperature, T/sub c/, of high temperature superconductors (HTS). These factors become especially important when thin films are prepared for electronic devices where sample uniformity and reproducibility are crucial. To date, there has been no detailed understanding of the comparative roles that each parameter plays in determining T/sub c/ and thus predicting what T/sub c/ will result from a particular process is problematic. We have shown that the very different strain and doping dependencies in the prototypical HTS material, LSCO, can be reduced to a common dependence upon the conductivity. The shape of the common phase boundary is related to the metal-insulator transition in accordance with that recently discovered for many other superconducting systems. The implication for this and other high T/sub c/ systems is that only one, easily measured parameter, conductivity, need be determined or controlled.

The recently discovered superconductor MgB2 with Tc at 39 K has great potential in superconducting electronics. In this paper, we review the deposition techniques used for MgB2 thin films in the light of a thermodynamic study of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique. This thermodynamic study identifies a growth window in the pressure–temperature phase diagram, in which the magnesium pressure is very high for likely in situ growth temperatures. A Hybrid Physical–Chemical Vapor Deposition (HPCVD) technique that successfully achieves such a high Mg pressure is shown to produce in situ epitaxial MgB2 thin films with bulk superconducting properties.

Artificially layered ferroelectric superlattices have enormous appeal from both a technological and a fundamental standpoint. However, it is difficult to measure and monitor their properties on nan...

The recently discovered superconductor MgB2 with Tc at 39 K has great potential in superconducting microelectronics. Thermodynamics studies with the calculation of phase diagrams (CALPHAD) modelling technique show that due to the high volatility of Mg, MgB2 is only thermodynamically stable under fairly high Mg overpressures for likely in situ growth temperatures. This provides a helpful insight into the appropriate processing conditions for MgB2 thin films, including the identification of the pressure–temperature region for adsorption-controlled growth. The initial MgB2 thin films were made by pulsed laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature of 34 K and a zero-field critical current density of 1.3 × 106 A cm−2 were obtained. The qualities of these films are limited by the thermodynamic stability conditions, which favour deposition techniques that can maintain a high flux of Mg.

E.H.S. and D.G.S. acknowledge support by the National Science Foundation (NSF) MRSEC program (DMR-1420620). In numerous symmetric θ-2θ scans of phase-pure epitaxial complex oxide thin films grown on single-crystal substrates, we observe x-ray diffraction peaks that correspond to neither the film nor the substrate crystal structure. These peaks are the result of multiple, sequential diffraction events that occur from both the film and the substrate. The occurrence of so-called "hybrid" reflections, while described in the literature, is not widely reported within the complex oxide thin-film community. We describe a simple method to predict and identify peaks resulting from hybrid reflections and show examples from epitaxial complex oxide films belonging to three distinct structural types. Publisher PDF

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature ( T c ) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO 3 layers in BaTiO 3 /SrTiO 3 superlattices are not only ferroelectric (with T c as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO 3 layers adjacent to them. T c was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO 3 and SrTiO 3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Atomic-resolution elemental maps of materials obtained using energy-loss spectroscopy in the scanning transmission electron microscope (STEM) can contain artifacts associated with strong elastic scattering of the STEM probe. We demonstrate how recent advances in instrumentation enable a simple and robust approach to reduce such artifacts and produce atomic-resolution elemental maps amenable to direct visual interpretation. The concept is demonstrated experimentally for a (BaTiO3)8/(SrTiO3)4 heterostructure, and simulations are used for quantitative analysis. We also demonstrate that the approach can be used to eliminate the atomic-resolution elastic contrast in maps obtained from lower-energy excitations, such as plasmon excitations.

It is demonstrated that soft modes in 240 nm think ${\text{BaTiO}}_{3}/{\text{SrTiO}}_{3}$ ferroelectric superlattices can be probed by infrared reflectivity measurements, which allows us to estimate the intrinsic in-plane dielectric constants of such thin superlattice slabs. The infrared reflectance spectrum of the ${({\text{BaTiO}}_{3})}_{8}/{({\text{SrTiO}}_{3})}_{4}$ superlattice epitaxially grown a ${\text{DyScO}}_{3}$ single-crystal substrate reveals a strong polar mode with a transverse optic frequency of about $70\text{ }{\text{cm}}^{\ensuremath{-}1}$ and moderate damping (about $30\text{ }{\text{cm}}^{\ensuremath{-}1}$). Our analysis of the experimental results is combined with the first-principles based theoretical evaluation of optical polar modes in a model system representing the 60-atom periodic superlattice unit cell. This hybrid approach suggests that the observed soft mode is associated with a Slater-type polar mode confined mostly in the ${\text{SrTiO}}_{3}$ component layer.

We report pump-probe time resolved reflectivity experiments in a hybrid air--Ni metal--${\mathrm{BaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ oxide mirror phonon cavity. We demonstrate that the generated coherent acoustic phonon spectra of the impulsively excited metallic film can be inhibited or enhanced in the phonon cavity with respect to a Ni film directly grown on a ${\mathrm{SrTiO}}_{3}$ substrate. The experiments are compared with simulations that highlight the role of the phonon density of states in the coherent acoustic emission, extending concepts at the base of the optical Purcell effect to the field of phononics.

We demonstrate the dramatic effect of film thickness on the ferroelectric phase transition temperature Tc in strained BaTiO3 films grown on SrTiO3 substrates. Using variable-temperature ultraviolet Raman spectroscopy enables measuring Tc in films as thin as 1.6 nm, and a film thickness variation from 1.6 to 10 nm leads to Tc tuning from 70 to about 925 K. Raman data are consistent with synchrotron x-ray scattering results, which indicate the presence of 180 degrees domains below Tc, and thermodynamic phase-field model calculations of Tc as a function of thickness.

We report a UV-Raman study of folded acoustic vibrations in epitaxial ferroelectric ${\mathrm{BaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ superlattices. The folded acoustic doublets show an anomalous temperature dependence disappearing above the ferroelectric transition, which is tuned by varying the thickness of the ${\mathrm{BaTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ layers. A mechanism involving the acoustic phonon modulation of the spatially periodic ferroelectric polarization explains the observed temperature dependence. These results demonstrate the strong coupling between sound, charge, and light in these multifunctional nanoscale ferroelectrics.

We report the magnetic and transport properties of [(SrMnO3)(n)/(LaMnO3)(2n)](m) superlattices grown by molecular-beam epitaxy on (100) SrTiO3 with periodicities n=1, 2, 3, 4, 5, 6, 8, and 16. Although the superlattice constituents, LaMnO3 and SrMnO3, are both antiferromagnetic insulators, for small n (n

We report Raman study of oxygen reduced single crystal strontium titanate. Oxygen reduction leads to the appearance of the forbidden first-order Raman peaks, as well as spectral features attributed to the local vibrational modes associated with oxygen vacancies. This assignment is supported by ab initio calculations of phonon modes in $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ with introduced oxygen vacancies. Raman studies of re-oxidized samples show the same spectra as the initial single crystals. Comparison of Raman spectra of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ thin films and reduced $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ single crystals demonstrates the importance of other factors such as polar grain boundaries in the lattice dynamical behavior of thin films.

We report structural, electrical, and thermopower properties of epitaxial and topotaxial NaxCoO2 thin films on (0001) sapphire substrate. Topotaxial NaxCoO2 films were prepared by converting an epitaxial Co3O4 film to NaxCoO2 by annealing in Na vapor and epitaxial NaxCoO2 films were obtained by pulsed laser deposition. All the films are c-axis oriented. For topotaxial films, annealing in different Na vapor pressures resulted in films with different Na concentrations, which showed distinct transport properties. For directly deposited epitaxial films by pulsed laser deposition, deposition parameters are found to control the Na concentration and hence the film properties. The largest thermoelectric power of the samples made by different methods is found to be similar in the range of 70-100 μV/K at room temperature

The lattice dynamical properties of ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$ single crystals with $x=0.05$, 0.1, 0.2, 0.35, and 0.5 have been studied by Raman spectroscopy in the temperature range $5--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}.$ We present the data on the composition and temperature dependence of the phonon modes in ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$, in particular, the soft modes. At low temperatures, the well-defined $E$ and ${A}_{1}$ soft modes have been observed in the crystals of all compositions studied. For low $\mathrm{Ba}$ content $(xl0.2)$ the soft modes extrapolate to zero and remain underdamped in the entire temperature range, indicating the displacive type of the ferroelectric phase transition. For $x\ensuremath{\gtrsim}0.2$, i.e., in the composition range of the ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$ phase diagram where three ferroelectric phases exist, the soft modes exhibit less softening and become heavily damped in the temperature range corresponding to the orthorhombic and tetragonal phases. This demonstrates that the order-disorder-type behavior becomes stronger with increasing $\mathrm{Ba}$ content. However, no first-order Raman scattering in the cubic phase was observed in ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$ of compositions studied, in contrast to pure ${\mathrm{BaTiO}}_{3}$. The temperature behavior of the ${A}_{1}$ soft mode indicates that the ferroelectric phase transition is of the first order in ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$ with $x\ensuremath{\gtrsim}0.2$, and turns into the second order for smaller $\mathrm{Ba}$ concentrations.

We report a systematic increase of the superconducting transition temperature T(c) with a biaxial tensile strain in MgB2 films to well beyond the bulk value. The tensile strain increases with the MgB2 film thickness, caused primarily by the coalescence of initially nucleated discrete islands (the Volmer-Weber growth mode.) The T(c) increase was observed in epitaxial films on SiC and sapphire substrates, although the T(c) values were different for the two substrates due to different lattice parameters and thermal expansion coefficients. We identified, by first-principles calculations, the underlying mechanism for the T(c) increase to be the softening of the bond-stretching E(2g) phonon mode, and we confirmed this conclusion by Raman scattering measurements. The result suggests that the E(2g) phonon softening is a possible avenue to achieve even higher T(c) in MgB2-related material systems.

We have studied phase transitions in epitaxial ${\mathrm{BaTiO}}_{3}$ thin films by Raman spectroscopy. The films are found to remain in a single ferroelectric phase over the temperature range from 5 to 325 K. The low-temperature phase transitions characteristic of bulk ${\mathrm{BaTiO}}_{3}$ (tetragonal-orthorhombic-rhombohedral) are absent in the films. X-ray diffraction shows that the ${\mathrm{BaTiO}}_{3}$ films are under tensile strain due to the thermal expansion mismatch with the buffer layer. A phase-field calculation of the phase diagram and domain structures in ${\mathrm{BaTiO}}_{3}$ thin films predicts, without any priori assumption, that an orthorhombic phase with in-plane polarization is the thermodynamically stable phase for such values of tensile strain and temperature, consistent with the experimental Raman results.

Femtosecond optical reflectivity measurements of La 2-x Sr x CuO 4 , La 2 CuO 4+y , Bi 2 Sr 2 CuO 6+z and Bi 2 Sr 2 CaCu 2 O 8+ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55 15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies- no longer perturbative- the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec. 1. INTRODUCTION There have been several reports of studying cuprates and simple metals using the technique of femtosecond optical reflectivity.[1-22] The technique provides direct information on carrier dynamics in these systems.[2] In metals, the classic model is called the ‘two-temperature’ model.[1] Developed in the U.S.S.R. in 1956, this model assumes that after light is absorbed by electrons, these photoelectrons (PE’s) thermalize immediately with the other electrons (TE’s) and establish an effective electron temperature. The TE’s then relax by dissipating energy to the lattice on a relaxation time ( ) that is typically the time scale of lattice vibrations (~ 1ps). The system then, much more slowly, removes the energy from the illuminated region by thermal conduction (~ 1-10 ns). Several reports on the cuprates have shown that the relaxation time changes across the superconducting phase transition temperature (T

Epitaxialstraine ectsonsup erconductingandtransp ortprop ertiesofLa2xSrCuO4+ X.Xia,Weidong Si,X.H.Zeng,A. SoukiassianC.L. JiabandK. UrbanaDepartmentofPhysics,ThePennsylvaniaStateUniversity,ark,A16802, USAbInstitutfurFestkorp erforschung,orscungszentrumJulichGmbH,D-52425 J h, GermanyABSTRACTEpitaxial strain in La2xSrCuO4+ thin lms (0 x0:30) is controlled by using SrLaAlO4bu er layers ofdi erentthicknesses on SrTiO3substrates.We found that compressive epitaxial strain results in higherTcforall the Sr concentrations.Better oxygenation by co oling the lms in ozone/molecular oxygen mixture also leadsto higherTc.In undop ed and lightly-dop ed ultrathin lms, the samples are insulating under tensile strain, butsup erconductingwhenthestrainissu cientlycompressive.Wesuggestthatepitaxiala ectsinsertionofinterstitialoxygen,whichisresp onsiblefortheobservede ects.Hallmeasurements con rmchangeincarrier density lmsofdi erentstrain.TheHallanglealsohanges withepitaxialepitaxial strain dep endence of the slop e in theT2dep endence of the cotangent of the Hall angle is di erent forunderdop ed and optimally-dop ed samples.Keywords:Epitaxial strain, high temp erature sup erconductor, thin lms1. INTRODUCTIONAmong various high temp erature sup erconductors (HTS), the dop ed La2CuO4comp ound (the \214" comp ound)has b een the desirable prototyp e cuprate system for the study of HTS mechanisms.1The two-dimensional CuO2layers are far separated and weakly coupled in the \214" comp ound, and there are no charge reservoir laers.The hole doping is achieved by either inserting interstitial O2(La2CuO4+ or LCO) or substituting La3+bySr2+(La2xCuO4or LSCO).2Epitaxial strain haslong b eenknown toa ecttheprop ertiesof\214" thin lms.3{5Usinga\blo ck-by-blo ck"molecularbeamepitaxytechnique,Lo cquetetal.describ edalargestraine ectin\214"(x=0:1)thin lmsgrown onSrTiO3(STO)andSrLaAlO4(SLAO)substrates.6The\214" lmintensilestrainonSTOhasareducedTcof10K,whilethe lmincompressive strainonSLAOanenhanced49K.This enhancementofTcis much larger than in anyhydrostatic or uniaxial pressure exp eriments.7, 8Suchenhancementhasb eenattributedtothemo dulationofCuO2plane-chargereservoir distance,9andsuppression of the low-temp erature tetragonal phase,10which has b een suggested to pin charge strip es.11Thestrip es, mostly dynamic and uctuating in space and time, refers to charges concentrating along sp ontaneouslygenerated domain walls b etween antiferromagnetic insulating regions.Static strip es have b een observed in Nd-dop ed \214" crystals near thex= 1/8 comp osition,11{13where the suppression ofTcis also rep orted.14{16Ifa compressive strain suppresses the static strip es, it may enhancesTc.10Here we present results thatshow thatb esides strain,oxygenation isalso imp ortant forthe prop erties of\214" lms.17Further, the compressive epitaxial strain and insertion of interstitial oxygen are coupled to eachother:in b oth cases thea-axis of the lattice shrinks and thec-axis expands, which relieves the internal strain dueto the b ond length mismatchbetween the CuO2and LaO layers. Using epitaxial strain, insulator-sup erconductor(I-S)transitioncanbeinducedinundop edandlightly-dop ed\214" lmsthroughtheenhancedinsertionofinterstitial oxygen.18Our Hall e ect measurementshows higher carrier densities in more compressively strained lms, however the Hall angle also changes with epitaxial strain.Further author information:Send e-mail to X.X.X.:xxx4@psu.edu

Thin films of BaxSr1−xTiO3 with Ba contents x = 0.05, 0.1, 0.2 and 0.5, grown by pulsed laser deposition on SrTiO3 and LaAlO3 substrates with SrRuO3 buffer layers, were studied by Raman spectroscopy. Soft phonon modes were observed, and the temperature behavior of their frequencies and Raman intensities in the range from 5 to 300 K was investigated. The temperature dependence of the soft phonon frequencies and the splitting of the triply degenerated soft mode of the cubic phase into two components of A and E symmetries indicate the ferroelectric phase transition. The E soft mode line is clearly seen in Raman spectra at temperatures away from the ferroelectric phase transition, but it becomes overdamped over a broad range of temperatures around the phase transition. The relative Raman intensity of hard phonon modes as well as the A soft mode decreases gradually with increasing the temperature over a broad range of temperatures. This indicates that the ferroelectric phase transition in the thin films is the...

Polarized infrared reflectance spectra of a BaTiO3/SrTiO3 superlattice deposited on the (110) cut of SmScO3 substrate and bare substrate have been studied in the broad temperature range from 10 to 650 K. Strong infrared anisotropy of the SmScO3 substrate was observed and explained as a projection of B1u and B2u + B3u phonons into the (110) plane. In the polarization parallel to the c-axis below 200 cm−1, an anomaly in the temperature dependence of the spectra was observed. Fitting the superlattice and substrate spectra, the parameters of optical phonons and their temperature dependence were determined. The superlattice phonon frequencies show only classical behavior, it means frequency hardening on cooling. Anisotropy was also found in the superlattice phonon frequencies and their contributions to static permittivity.

The vibrational properties of barium strontium titanate thin films were studied by Raman spectroscopy in the temperature range from 5 to 300 K. The films were grown by pulsed laser deposition on SrTiO3 and LaAlO3 substrates with SrRuO3 buffer layers. Soft phonons are observed in Raman spectra of BaxSr1−xTiO3 films with Ba contents x = 0.05, 0.1, 0.2 and 0.5. The temperature dependence of the soft phonon frequencies and the splitting of the triply degenerated soft mode into two components of A and E symmetries indicate the ferroelectric phase transition. The E soft mode line is overdamped over a broad range of temperatures near the ferroelectric phase transition, while it is clearly seen in the spectra at temperatures away from the phase transition. The relative Raman intensity of the A soft mode and hard modes decreases gradually over a broad range of temperatures. This indicates a broad ferroelectric phase transition in the thin films. Comparison of Raman spectra for films grown on SrTiO3 and LaAlO3 substrates shows the influence of strain on the temperature of the ferroelectric phase transition.

A variant of piezo force microscopy was used to characterize the effect of strain on polarization in [(BaTiO3)n/(SrTiO3)m]p superlattices. The measurements were compared to theoretical predictions based on phase-field calculations. When polarization is constrained to be perpendicular to the substrate, the measured polarization and domain morphology agree quantitatively with the predictions. This case allows the presence of an internal electric field in the thin film to be identified. The measured trend in piezoelectric response with strain state was in qualitative agreement with predictions, and the differences were consistent with the presence of internal electrical fields. Clear differences in domain morphology with strain were observed; and in some cases, the lateral anisotropic strain appeared to influence the domain morphology. The differences in magnitude and morphology were attributed to the internal electric fields and anisotropic strains.

We have grown Bi2Sr2Co2Oy thin films on LaAlO3 (100) and fused silica substrates by pulsed laser deposition. The films on LaAlO3 are c-axis oriented and partially in-plane aligned with multiple domains, while the films on fused silica are preferred c-axis oriented without in-plane alignment. The Seebeck coefficient and resistivity of films on both substrates are comparable to those of single crystals. An oxide p-n heterojunction was formed by depositing Bi2Sr2Co2Oy film on Nb-doped SrTiO3 single crystal, which showed a rectifying behavior. These thin films and heterostructures may be used for future thermoelectric applications.

A hybrid physical–chemical vapor deposition process using a pocket heater was developed for the growth of high quality epitaxial large-area MgB2 thin films and c-axis textured MgB2 thick films. This technique is able to independently control the substrate and Mg source temperatures and maintain sufficient Mg overpressure to ensure phase stability. The two-inch large-area MgB2 thin films showed uniform superconducting properties with the superconducting transition temperature Tc of about 40 K, residual resistivity ratio (RRR) of about 10, and critical current density Jc of about 107 A cm−2 (0 T, 5 K). The thick films (~10 µm) on sapphire substrates showed a maximum Tc of 40 K and RRR of 15, and a Jc of 1.6 × 106 A cm−2 at low applied magnetic fields even at 20 K. High quality thick films also have been obtained on metal substrates.

The complete elastic tensor of single crystal GdScO3 was determined using resonant ultrasound spectroscopy (RUS) in combination with ab initio calculations. The experimental determination of all nine elastic constants also provides a method for probing the dynamic lattice properties for this recently developed orthorhombic material. The experimentally determined elastic constants differed from theoretical values on average by 10%, and all but three of the nine elastic constants varied by less than 10%. These results indicate that ab initio calculations are now sufficiently accurate for the precise determination of the elastic tensor using RUS as the sole experimental source.

Low residual resistivity in superconducting thin films is required for their applications in radio frequency (RF) cavities. Here we report on clean epitaxial MgB2 films fabricated by the ex situ annealing of B films, grown by chemical vapour deposition, in Mg vapour. The films show sharp superconducting transitions at about 40 K and a high residual resistivity ratio of about 10. The result indicates that a clean precursor B film and a contamination-free annealing procedure are important for a viable MgB2 film fabrication process for RF cavity applications.

We studied the phase transitions, domain morphologies, and polarizations in BaTiO3∕SrTiO3 superlattices grown on SrTiO3 substrates. Using the phase field approach, we discovered the remarkable influence of film/substrate interfacial coherency on the ferroelectricity of the SrTiO3 layers within a superlattice: it is an orthorhombic ferroelectric for an incoherent interface while it exhibits only induced polarization by the adjacent BaTiO3 layers for a coherent interface. We presented the domain morphologies within individual BaTiO3 and SrTiO3 layers which have different ferroelectric symmetries. The results are compared to ultraviolet Raman spectroscopy and variable temperature x-ray diffraction measurements.

We study high quality molecular-beam epitaxy grown BaTiO3/SrTiO3 superlat-tices using ultraviolet Raman spectroscopy. In the low energy spectral region, acoustic phonon doublets are observed. These are due to the artificial superlattice periodicity and consequent folding of the acoustic phonon dispersion. From the study of samples with different BaTiO3/SrTiO3 layer thicknesses the effective sound velocities within each of the layers are obtained.

The phase transitions of superlattices into single- and multidomain states were studied using a mesoscale phase-field model incorporating structural inhomogeneity, micromechanics, and electrostatics. While the predictions of transition temperatures of BaTiO3∕SrTiO3 superlattices into multidomains show remarkably good, quantitative agreement with ultraviolet Raman spectroscopic and variable-temperature x-ray diffraction measurements, the single-domain assumption breaks down for superlattices in which the nonferroelectric layer thickness exceeds the characteristic domain size in the ferroelectric layers.