A new phase diagram of $Pr_{1-x}Ca_xMnO_3$ for x $\le$ 0.3 is derived that suggests a necessary revaluation of the phase diagram of other manganites in that doping region. Rather than an orbital ordered phase reaching up to high temperatures 850 K for x=0.1 and 950 K for x=0, we propose a loss of spontaneous order already near room temperature. Above this temperature, the phase is characterized by a finite orbital polarization and octahedral tilt pattern. The tilt pattern couples to the Jahn-Teller distortion and thus induces a remaining orbital order, which persists up to high temperatures, where the tilt order is lost as well. This explains the experimental observation of orbital order up to high temperatures. Anomalies at a temperature 220-260 K have been observed in epitaxial thin films of doping x=0.1 for photovoltaic effect, electric transport, magnetisation, optical and ultrafast transient pump probe studies. The onset of the polaron photovoltaic effect and the increase of the hot polaron relaxation time below $T_{OO}$ suggest a change in the orbital order. Finite-temperature simulations based on a tight-binding model with carefully adjusted parameters from first-principles calculations exhibit an orbital-order phase transition at $T_{OO} \approx$ 300 K for x=0.1. This is consistent with the experimental observation of a change in temperature dependent lattice parameter for bulk samples of the same doping at 300 K for x=0.1 and 350 K for x=0, typical for a second order phase transition.

The phase diagram of Pr_1-xCa_xMnO_3 is modified x ≤ 0.3, which suggests a reevaluation of the phase diagram of other manganites in that doping region. Rather than an orbital ordered phase reaching up to high temperatures of approximately 800-1100 K, we propose a loss of spontaneous orbital order already near room temperature. Above this temperature, the phase is characterized by a finite orbital polarization and octahedral tilt pattern. The tilt pattern couples to the Jahn-Teller distortion and thus induces a remaining orbital order, which persists up to high temperatures, where the tilt order is lost as well. This explains the experimental observation of orbital order up to high temperatures. The reevaluation of the orbital order transition is based on observed anomalies of various physical properties at a temperatures of 220-260 K in epitaxial thin films of Pr_1-xCa_xMnO_3 x=0.1, i.e.in the photovoltaic effect, electric transport, magnetization, optical and ultrafast transient pump probe studies. Finite-temperature simulations based on a tight-binding model with carefully adjusted parameters from first-principles calculations exhibit an orbital order phase transition at T_OO≈ 300 K for x=0.1. This is consistent with the experimental observation of a temperature dependent change in lattice parameter for bulk samples of the same doping at 300 K for x=0.1 and 350 K for x=0, typical for a second order phase transition. Since our reassignment of the orbital order phase transition towards lower temperatures challenges a well-established and long-accepted picture, we provide results of multiple complementary measurements as well as a detailed discussion.

The rich phase diagram of bulk Pr$_{1-x}$Ca$_{x}$MnO$_3$ resulting in a high tunability of physical properties gave rise to various studies related to fundamental research as well as prospective applications of the material. Amongst others, the structural orthorhombic to pseudo-cubic transition that occurs at high temperatures is of major interest. In more detail, due to static Jahn-Teller distortions and tilting of the octahedra spanned by manganese and its surrounding oxygen atoms, a pronounced lattice constant anisotropy is present in the low temperature phase reacting very sensitively to small compositional changes. In this paper, we show that a structural transition exists for $x=0.1$ in ion-beam sputtered thin films as well and point out differences to the bulk by employing in-situ heating nano-beam electron diffraction to follow the temperature dependence of lattice constants. In addition, we demonstrate that controlling the environment during heating, i.e. preventing oxygen loss, is crucial in order to avoid irreversible structural changes.

Harvesting of solar energy by hot carriers from optically induced intraband transitions offers new perspectives for photovoltaic energy conversion. Clearly, mechanisms slowing down hot-carrier thermalization constitute a fundamental core of such pathways of third-generation photovoltaics. The intriguing concept of hot polarons stabilized by long-range phonon correlations in charge-ordered strongly correlated three-dimensional metal-oxide perovskite films has emerged and been demonstrated for ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}\mathrm{O}}_{3}$ at low temperature. In this work, a tailored approach to extending such processes to room temperature is presented. It consists of a specially designed epitaxial growth of two-dimensional Ruddlesden-Popper ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{Mn}\mathrm{O}}_{4}$ films on $\mathrm{Nb}$:${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ with a charge-ordering transition at ${T}_{\mathrm{CO}}$ \ensuremath{\sim} 320 K. This opens the route to a different phonon-bottleneck strategy of slowing down carrier relaxation by strong coupling of electrons to cooperative lattice modes.

Received 18 October 2016DOI:https://doi.org/10.1103/PhysRevB.94.159902©2016 American Physical Society

Small polaron optical properties are studied comprehensively in thin film samples of the narrow bandwidth manganite Pr1-xCaxMnO3 by optical absorption spectroscopy as a function of doping and temperature. A broad near infrared double-peak absorption band in the optical conductivity spectras is observed and interpreted in the framework of photon-assisted small polaron intersite hopping and on-site Jahn-Teller excitation. Application of quasiclassical small polaron theory to both transitions allows an approximate determination of polaron specific parameters like the polaron binding energy, the characteristic phonon energy, as well as the Jahn-Teller splitting energy as a function of temperature and doping. Based on electronic structure calculations, we consider the impact of the hybridization of O 2p and Mn 3d electronic states on the Jahn-Teller splitting and the polaron properties. The interplay between hopping and Jahn-Teller excitations is discussed in the alternative pictures of mixed valence Mn3+/Mn4+ sites (Jahn-Teller polaron) and equivalent Mn(3+x)+ sites (Zener polaron). We give a careful evaluation of the estimated polaron parameters and discuss the limitations of small polaron quasiclassical theory for application to narrow bandwidth manganites.

We report here on the presence of two different nonvolatile resistive switching mechanisms in Pt-Pr0.67Ca0.33MnO3-Pt sandwich structures based on pulsed electrical transport measurements. As a function of pulse length, amplitude and temperature, the devices show two different switching regimes. The first is positive switching (PS) where a high resistance state (HRS) evolves at positive bias at the top electrode in the voltage range of U approximate to 0.5-1.2 V and pulse lengths t(p) approximate to 10(-7) s. In addition, we observe a cross over to negative switching (NS) for U > 1 V and t(p) approximate to 10(-3) s. Here, the HRS evolves at negative bias applied at the top electrode. We present strong evidence that both switching mechanisms take place at the interface between Pr0.67Ca0.33MnO3 and the top electrode. Based on finite element simulations of the temperature evolution during the electrical pulses, we show that the onset of Joule heating is characteristic of the PS regime, whereas drastic temperature increases of several hundred Kelvin evolve during NS. Based on the observed different timescales, pulse amplitudes and temperature dependences of PS and NS, respectively, we suggest that two different switching mechanisms are involved: a fast, short range exchange of oxygen at the interface with the metallic electrode for PS and a slower, long range redistribution of oxygen in the entire PCMO film for the NS.

The colossal magnetoresistance effect (CMR), the drop of the electric resistance by orders of magnitude in a strong magnetic field, is a fascinating property of strongly correlated electrons in doped manganites. Here, we present a detailed analysis of the magnetotransport properties of small polarons in thin films of the low bandwidth manganite Pr0.68Ca0.32MnO3 with different degrees of preparation- induced octahedral disorder. The crystal and defect structure is investigated by means of high-resolution transmission electron microscopy. We apply the small polaron theory developed by Firsov and Lang in order to study the hopping mobility in the paramagnetic phase and its changes due to the formation of the antiferromagnetic charge ordered (CO) and the ferromagnetic metallic phases. Although it represents a single particle theory, reasonable estimates of small polaron properties such as formation energy, activation energy and transfer integral are possible, if the effects of interactions and disorder are taken into account. Beyond the well-known effect of the magnetic double exchange on the transfer integral, we show that the emergence of band transport of small polarons in the CMR transition sensibly depends on the degree of octahedral disorder, the polaron-polaron interactions and the resulting long range order leading to a structural phase transition in the CO phase.

The identification of the cross-plane electric transport mechanisms in different resistance states of metal–oxide sandwich structures is essential for gaining insights into the mechanisms of resistive switching (RS). Here, we present a systematic study of cross-plane electric transport properties of Pr0.67Ca0.33MnO3 (PCMO) thin films sandwiched by precious Pt metal electrodes. We observe three different transport regimes: ohmic, nonlinear and RS. The nonlinear regime is associated with colossal magneto-resistance (CMR) and colossal electro-resistance (CER) effects. In contrast to RS, the CMR and CER are volatile resistance effects which persist only during application of strong magnetic or electric fields and they are restricted to low temperatures. At low current densities, the device resistance is dominated by small polaron hopping transport of the PCMO film. At higher electric current densities near the switching threshold, the interface resistance starts to dominate and remarkably also exhibits thermally activated transport properties. Our studies also shed light onto the interplay of colossal resistance effects and RS: at low temperatures, RS can be only induced by reduction of the PCMO resistivity through CMR and CER. This clearly demonstrates the key role of the current density for controlling the amplitude of non-volatile resistive changes. Conversely, the CMR can be used as a probe for the switching induced changes in disorder and correlations. At small switching amplitudes, we observe slight changes in polaron activation energy which can be attributed to changes at the interface. If the switching amplitude exceeds 1000% and more, the CMR effect in the device can be reversibly changed. This indicates persistent changes in electronic or lattice structure of large regions within the PCMO film.

Separating out effects of point defects and lattice strain on thermal conductivity is essential for improvement of thermoelectric properties of SrTiO3. We study relations between defects generated during deposition, induced lattice strain, and their impact on thermal conductivity κ in homoepitaxial SrTiO3 films prepared by ion-beam sputtering. Lowering the deposition temperature gives rise to lattice expansion by enhancement of point defect density which increases the hardness of the films. Due to a fully coherent substrate-film interface, the lattice misfit induces a large biaxial strain. However, we can show that the temperature dependence of κ is mainly sensitive on the defect concentration.

No AccessSynthetic Polymer-Polymer CompositesAug 2012Polylactide Based Bio-Resorbable Bone Nails: Improvements of Strength and Stiffness by Microfibrillar ReinforcementK. Friedrich, J. Hoffmann, A. A. Almajid, M. EvstatievK. FriedrichSearch for more papers by this author, J. HoffmannSearch for more papers by this author, A. A. AlmajidSearch for more papers by this author, M. EvstatievSearch for more papers by this authorhttps://doi.org/10.3139/9781569905258.018SectionsAboutPDF ToolsAdd to FavoritesDownload CitationTrack CitationsCopy LTI LinkPDF key 'share (en)' returned an object instead of string.FacebookTwitterEmailLinkedIn previous chapternext chapter FiguresReferencesRelatedDetails 2012Pages: 627-640Print ISBN: 978-1-56990-510-4eISBN: 978-1-56990-525-8 Copyright & Permissions© 2012 Carl Hanser Verlag GmbH & Co. KGPDF DownloadLoading ...

The electrically induced persistent resistance change in perovskite Pr0.7Ca0.3MnO3 films sandwiched by metallic electrodes is analyzed with respect to noble electrode materials (Pt, Au, and Ag) and geometric arrangement by electrical transport measurements. Comparing switching behavior in symmetric and asymmetric electrode interfaces gives evidence for identifying the active, single interface in the switching process. The interaction of two opposing interfaces can lead to an observed switching polarity inversion in different current density regimes in the otherwise well defined bipolar behavior. The different noble metals exhibit a quite similar switching behavior, but a lower interfacial resistance seems to favor switching.

Combining pulse-probe measurements as well as local transport measurements in an electron microscope system by a simultaneous monitoring of the structural changes, we show that the nonvolatile electric pulse induced resistance change in Ca-doped praseodymium manganite is related to a polaron order-disorder transition, modified by electronic band bending in the vicinity of an interface to a metallic electrode. A pronounced resistance change requires a critical distance between the two electrode and/or oxide interfaces to form an insulating incommensurate polaron-ordered phase during the initialization of the device. Based on these observations, a qualitative model for the electronic structure of the metal-oxide interface is developed.

The temperature-dependent resistivity and the colossal resistance effect induced by strong electric fields of the small-band Pr(1-x)Ca(x)MnO(3) (PCMO) manganites are analysed with respect to the influence of the Ca doping, post-annealing, the prehistory of the electric stimulation, and the physical dimensions of the sample. Despite the phase separation between charge and orbital ordered and disordered phases, PCMO reveals the properties of a homogeneous medium with a conductivity governed by the hopping of small polarons if the electric field is not too strong. In contrast, high electric fields induce a structural transition which gives rise to a glassy behaviour in the transient regime. In the low resistance state the small activation energy of charge carrier hopping implies a transition to large polaron hopping.

Our previous work showed that Pr 1− x Ca x MnO 3 ( x = 0.2–0.5) thin films reveal an electronic phase separation, in which the low-temperature phase is orbital- and charge-ordered of the Zener-polaron type [Ch. Jooss, T. Beetz, L. Wu, M. Beleggia, R. Klie, M. Schofield, Y. Zhu, S. Schramm, J. Hoffmann, submitted for publication]. Applying a magnetic field leads to a ferromagnetic and conducting phase, which gives rise to the colossal magneto resistance effect (CMR). In this article we show that the orbital- and charge-ordered state could be significantly modified by different kind of lattice defects. Quenched disorder, present after the film deposition, can suppress long-range ordering. In addition, extended defects like twin boundaries act as a nucleation center for ordered domains, i.e. the hysteretic properties depend on the density of these defects. Well-ordered artificial anti-phase boundaries (APB) can be introduced by an epitaxial growth on vicinal substrates, leading to anisotropic transport properties with respect to the orientation of the APB.

Polarons, the combined motion of electrons in a cloth of their lattice distortions, are a key transport feature in doped manganites. To develop a profound understanding of the colossal resistance effects induced by external fields, the study of polaron correlations and the resulting collective polaron behavior, i.e., polaron ordering and transition from polaronic transport to metallic transport is essential. We show that static long-range ordering of Jahn–Teller polarons forms a polaron solid which represents a new type of charge and orbital ordered state. The related noncentrosymmetric lattice distortions establish a connection between colossal resistance effects and multiferroic properties, i.e., the coexistence of ferroelectric and antiferromagnetic ordering. Colossal resistance effects due to an electrically induced polaron solid–liquid transition are directly observed in a transmission electron microscope with local electric stimulus applied in situ using a piezo-controlled tip. Our results shed light onto the colossal resistance effects in magnetic field and have a strong impact on the development of correlated electron-device applications such as resistive random access memory (RRAM).

The insulator-metal (IM)-transition in Pr1-xCaxMnO3(PCMO) is of particular interest because it can be induced by avariety of external forces, such as magnetic and electric fields,photon exposure and hydrostatic pressure. In this paper, we presenta comparative study of the IM-transition in magnetic and electricfields for epitaxial thin films prepared by pulsed laser deposition.The transport data as a function of applied field or temperaturegive strong evidence for the presence of electronic phaseseparation. However, the observed different IM-transitions in magnetic and electric fields indicate that two different areas of spatially inhomogeneous electronic ground states in the phase diagram of PCMO are involved.

For the production of high-current-carrying, long-length superconducting wires or tapes, it is necessary to use biaxially textured metallic substrates or buffer layers. Though being highly textured, the deposited superconducting film exhibits a complex defect structure which (locally) suppresses the critical current and alternates characteristically the magnetic flux distribution seen in magneto-optical imaging. In this paper, we report on pulsed laser deposited YBaCuO films on biaxially textured yttrium-stabilized ZrO2 (YSZ) and Gd2Zr2O7 (GZO) buffers which were grown by ion beam assisted deposition (IBAD) on polycrystalline substrates. The current-limiting defect structure turns out to resemble closely a combination of a dense distribution of pinhole-like induced growth distortions and a fine grain boundary network. The current suppression is caused on the one hand by the dense packing of pinhole-like defects. On the other hand, we observe a substantial current anisotropy being related to the surface morphology of the buffer layers and the direction of the IBAD-beam.

In contrast to the classical composites, microfibrillar reinforced composites are not prepared via melt blending of the matrix and the reinforcing material. In fact, the reinforcing elements of this composite, the microfibrils, are created during processing. This advantage allows the manufacturing of a full biodegradable composite material with improved mechanical properties. Basing on a blend of two common biodegradable polymers, polylactide and polyglycolide, this new composite shows promising values under ambient conditions.