ABSTRACT.  

Hans Boschker and Jochen Mannhart

Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany

Combining the power and possibilities of heterostructure engineering with the collective and emergent properties of quantum materials, quantum-matter heterostructures [1] open a new arena of solid-state physics. Here we provide a review of interfaces and heterostructures made of quantum matter. As we will show, unique electronic states can be engineered in these structures, giving rise to unforeseeable opportunities for scientific discovery and potential applications.

[1] “Quantum-Matter Heterostructures”, H. Boschker and J. Mannhart, Annual Reviews of Condensed Matter Physics, 8, April 2017.

ABSTRACT.  

Morgan Trassin

ETH Zürich, Switzerland

morgan.trassin@mat.ethz.ch

In ferroelectric thin films, the polarization state, i. e. orientation and domain architecture, defines the macroscopic ferroelectric properties such as the switching dynamics. Ferroelectric domain engineering is in permanent evolution from the epitaxial strain tuning to the chemical control on interface atomic termination. Technology promising complex polar flux closure or vortices architecture have been recently demonstrated in ferroelectric heterostructures. However the mechanism involved in the formation of these complex polar states remains unexplored. The optical second harmonic generation process is an efficient and non-invasive tool for thin films ferroic properties probing [1,2]. Here, we investigate the emergence of the ferroelectric polarization in ultra-thin ferroelectric and multiferroic films and monitor in situ the optical non-linear response of the film during the growth. We find that, the ferroelectric critical thickness and domain state can be measured in situ during the film deposition. Using a combination of epitaxial strain engineering and surface termination control in (BiFeO₃/SrRuO₃)n multilayers, we determine the BiFeO₃ net polarization orientation in each layer and in real-time, exempt from substrate contribution. Our work provides direct observation of ferroelectric states during the growth as well as new insights towards further control of ferroelectric based heterostructure.

[1] M. Trassin et al., Adv. Mater 27, 4871 (2015)

[2] De Luca et al., Adv. Mater 1605145 (2016)

ABSTRACT.  

J.M. Dekkers

Drienerlolaan 5 (building 46), 7522 NB, Enschede, The Netherlands

matthijn.dekkers@solmates.nl

It is well known that Pulsed Laser Deposition (PLD) is a very flexible and versatile technique allowing fast optimization of new and complex material thin films. The unique features of PLD allow for the integration of “Beyond Moore” materials in CMOS and new devices. However, mainly because of the sample size, the developed materials and processes in PLD research tools only just make it into demonstrator devices. In order to make it into commercial applications, next generation PLD equipment is needed to bridge the gap between demonstrator and the prototype – pilot – production stages.   

Since 2006 Solmates developed PLD systems for large substrate dimensions and stable processing. The current Solmates PLD platform is the next step beyond fundamental PLD research. The reliable hardware is flexible for fast process optimization and allows uniform thin film deposition up to 200 mm wafers or 200 mm² glass panels with high reproducibility. The automated software ensures easy operation and stable performance. These characteristics enable the integration of PLD thin films in applications for (pilot) production and commercialization.

In this contribution the Solmates core technology will be presented. As a first example wafer-level integration of epitaxial PZT and PMN-PT thin films on silicon is demonstrated. The results of this work are the first milestone in the development of a piezoelectric memory.

In another example, the PZT is integrated in silicon photonics for strain-optical modulators. These devices are a key component for phased-array antennas that will enable 5G data communication. Pilot production of these devices that rely on PLD-deposited piezo materials is scheduled for 2019.

ABSTRACT.  

F. Hensling

Forschungszentrum Jülich, Germany

f.hensling@fz-juelich.de

The reduction of oxides during annealing and growth in low pressure processes is a widely known problem. We hence investigate the influence of mere annealing and of growth in vacuum systems to shed light on the reasons behind the reduction of perovskites. When comparing the existing literature regarding the reduction of the perovskite model material SrTiO₃ it is conspicuous that one finds different oxygen pressures required to achieve reduction for vacuum annealing and for chemically controlled reducing atmospheres. The unraveling of this discrepancy is of high interest for low pressure physical vapor depositions of thin films heterostructures to gain further understanding of the reduction of the SrTiO₃. For thermal annealing, our results prove the attached measurement devices (mass spectrometer/ cold cathode gauge) to be primarily responsible for the reduction of SrTiO₃ in the deposition chamber by shifting the thermodynamic equilibrium to a more reducing atmosphere. We investigated the impact of our findings on the pulsed laser deposition growth at low pressure for LaAlO₃/SrTiO₃. During deposition the reduction triggered by the presence of the laser plume dominates and the impact of the measurement devices plays a minor role. During post annealing a complete reoxidization of samples is inhibited by an insufficient supply of oxygen. [F.Hensling et al. doi:10.1038/srep39953]

ABSTRACT.  

C.A.J. Damen¹, R.Groenen¹, G. Koster², G. Rijnders²

¹Twente Solid State Technology, P.O. Box 256 7500 AG Enschede, The Netherlands
²MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

groenen@tsst.nl

Pulsed Laser Deposition (PLD) has proven over the last decades to be a versatile thin film deposition technique for stoichiometric synthesis of materials. These include complex transition metal oxides, which offer a variety of exploitable properties. Despite this rich potential for use in electronics, actual industrial applications are relatively few as they rely on growth of films on substrate materials with sizes suitable for industrial applications, where silicon wafers define the standard of CMOS technology. This demands upscaling of the PLD process to grow high quality films on silicon wafers.

TSST B.V. has developed a PLD system with which the growth of oxide heterostructures on 4” silicon wafers is studied and optimised. For thin film growth on silicon wafers using PLD, two important challenges have been the focus of research in recent years. Firstly, epitaxial growth of oxides on as-received silicon wafers is intrinsically prohibited by a native silicon dioxide layer. Growth of YsZ buffer layers has proven to be an effective method to reduce and remove the native silicon dioxide layer, thus forming an epitaxial basis for oxide growth. Secondly, in PLD the dimensions of the plasma plume are smaller than commonly used industrial wafer sizes, thus covering only part of the wafer with material. This has been solved by scanning the plasma plume over the full wafer area.

We present the results on growth of SrRuO₃//Pb(Zr_0.48, Ti_0.52)O₃ (PZT) and La_0.67Sr_0.33MnO₃ (LSMO) on optimized high quality YSZ//CeO₂ buffer layers on 4'' silicon wafers. Film morphology and structural characteristics have been investigated with Atomic Force Microscopy and X-Ray Diffraction. We show that over the full wafer, films are highly crystalline with atomically sharp interfaces. Subsequently, investigation of the magnetic and ferroelectric properties of LSMO and PZT shows behaviour comparable to films deposited with small scale systems. Futhermore, we present the results of the effect of an additional SrRuO3 buffer layer on the growth temperature dependent structural and magnetic properties of LSMO films, where LSMO films show ferromagnetic behaviour for growth temperatures as low as 250°C. We speculate that the occurance of this LSMO high quality crystal growth at these remarkable low growth temperatures could be understood by an improved surface diffusion induced by the SRO buffer layer.

ABSTRACT.  

Thorsten Schmitt

Paul Scherrer Institut, Swiss Light Source, CH-5232 Villigen PSI, Switzerland

thorsten.schmitt@psi.ch

Resonant inelastic X-ray scattering (RIXS) is a powerful bulk-sensitive photon-in / photon-out spectroscopic probe of the electronic structure of condensed matter with atomic and orbital sensitivity. It is a unique tool for studying excitations from the electronic ground state in correlated transition-metal oxides, being directly sensitive to lattice-, charge-, orbital- and spin-degrees of freedom. In this talk, we report RIXS investigations of the LaTiO₃ layers in (LaTiO₃)_n/(LaAlO₃)₅ superlattices undergoing a transition from Ti³⁺ to Ti⁴⁺ oxidation state upon reducing n and thickness as well as temperature-driven metal-insulator transitions in thin films of CaVO₃.

(LaTiO₃)_n/(LaAlO₃)₅  superlattices (SL) composed of a band-insulator (LaAlO₃) and a Mott-insulator (LaTiO₃) present an enhanced insulating character when n is reduced. We prepared a set of SLs (n=10, 5 and 2 unit cells) and investigated these with X-ray absorption spectroscopy (XAS) and RIXS. XAS shows a clear change in the Ti valence going progressively from the nominal Ti³⁺ (3d¹, n=10 u.c.) for bulk LaTiO₃ to an almost pure Ti⁴⁺ (3d⁰, n=2 u.c.). RIXS reveals two spectral developments when reducing the LaTiO₃ thickness n: 1) reduction of intra-t_2g / intra-e_g splitting and increase of t_2g to e_g separation and 2) increase of the charge transfer excitation spectral weight. The changes in the energy of the orbital levels observed as a function of n reveal a clear change of the local TiO₆ distortion. We suggest that an inverse Jahn-Teller effect, inducing the octahedra to assume higher symmetry, is responsible for the observed orbital energy shifts. This peculiar effect is partially caused by strain, triggering a 3d¹ → 3d⁰ electron transition at the interfacial Ti sites.

Bulk CaVO₃ is a correlated paramagnetic metal. Thin films of CaVO₃ undergo a metal-insulator transition (MIT) when the thickness is reduced below ca. 20 u.c.. Our XAS and RIXS measurements at the V L-edge across this dimensionality driven MIT in CaVO₃ reveal a large transfer of spectral weight from fluorescent to Raman modes upon entering the insulating state. We observe a large reduction in the charge excitation bandwidth and V-O covalence across the thickness and temperature-driven MIT. Further analysis of the charge modes suggests a bandwidth-controlled MIT, assisted by the presence of strong correlations.

ABSTRACT.  

Naor Vardi¹, Elihu Anouchi¹, Tony Yamin¹, Srimanta Middey², Michael Kareev², Jak Chakhalian², Yonatan Dubi³ and Amos Sharoni¹

¹ Department of Physics, Bar Ilan University, Ramat-Gan, IL-5290002, Israel
² Department of Physics, University of Arkansas, Fayetteville, Arkansas, 72701, USA
³ Department of Chemistry, Ben Gurion University, Be'er Sheva, IL-841050, Israel

amos.sharoni@biu.ac.il

Transition metal oxides (TMOs) are complex electronic systems which exhibit a multitude of collective phenomena. Two archetypal examples are VO₂ and NdNiO₃, which undergo a metal-insulator phase-transition (MIT), the origin of which is still under debate. We have discovered a new kind of memory effect in both systems, manifest through an increase of resistance at a specific temperature, which is set by reversing the temperature-ramp from heating to cooling during the MIT, thus we call it ‘Ramp Reversal Memory’. The characteristics of this memory effect do not coincide with any previously reported history or memory effects in similar systems. From a broad range of experimental features, supported by theoretical modelling, we claim that the main ingredients for the effect to arise are the spatial phase-separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local critical temperature of the phase transition. We predict that similar ramp-reversal effects exist also in other systems.

ABSTRACT.  

J. Sławińska,¹ P.K. Das,² I. Vobornik,² J. Fujii,² J.M. Kahk,³ A. Regoutz,³ D.O. Scanlon,^4,5 B.J. Morgan,⁶ E. Plekhanov,¹ D. Di Sante,¹ Y.S. Huang,⁷ W.R. Branford,⁸ S. Picozzi,¹ G. Panaccione² and D.J. Payne³

¹Consiglio Nazionale delle Ricerche, CNR-SPIN, UOS L’Aquila, Sede Temporanea Chieti, 66100 Chieti (Italy)
²Istituto Officina dei Materiali (IOM)-CNR, Laboratorio TASC, Trieste, Italy
³Department of Materials, Imperial College London, United Kingdom.
⁴University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, London, United Kingdom.
⁵Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, United Kingdom.
⁶Department of Chemistry, University of Bath, Claverton Down Road, Bath, UK.
⁷Department of Electric Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan
⁸Blackett Laboratory, Department of Physics, Imperial College, London, United Kingdom.

jagoda.slawinska@spin.cnr.it

5d transition metal oxides are known to reveal a wide range of exotic states and phenomena, including spin-orbit-assisted Mott insulating state, spin Hall effect, spin liquid behavior, topological Weyl semimetallic state, all mostly driven by spin-orbit coupling (SOC). Here, we investigate the electronic structure of the rutile binary iridate, IrO₂. By combining ARPES measurements with DFT-based calculations (with and without SOC), we unveil the relevant features of the band structure, including spin-orbit induced avoided crossing, likely to play a key role in the spin Hall effect. The good agreement between theory and experiments shows that the single-particle DFT description of IrO₂ band structure is adequate, without the need of invoking any treatment of correlation effects beyond DFT. The results show features strongly dependent on the k vector normal to the (110) surface probed in experiment (pointing to the 3D nature of IrO₂ electronic structure), coexisting with undispersed bands. The discussion of the orbital character of the relevant bands crossing the Fermi levels sheds more light on SOC-driven phenomena in this material.

ABSTRACT.  

Jagoda Sławińska, Silvia Picozzi

Consiglio Nazionale delle Ricerche,  Istituto SPIN, UOS L'Aquila, Sede di lavoro CNR-SPIN di Chieti c/o Univ. Chieti-Pescara "G. D'Annunzio", 66100 Chieti, Italy

jagoda.slawinska@spin.cnr.it

During the STSM project we have studied theoretically, employing DFT and Green’s functions calculations, several spin-orbit based phenomena in non-magnetic oxides and other materials. In particular, we have analyzed the spin-orbit avoided crossing features in IrO₂ which play an essential role in the strong spin Hall effect observed in this material; importantly we have also identified them in experimental ARPES spectra. Among non-oxide materials we have investigated the effect of hidden spin polarization in BaNiS₂ and spin textures of  bulk and surface bands in ferroelectric GeTe (also covered with Fe(111) layers). Currently, we are studying the presence (and related properties) of 2DEG on the reconstructed (001) surface of anatase TiO₂ and comparing the results with corresponding experimental ARPES spectra.

ABSTRACT.  

Giovanna Canu¹, Maria Teresa Buscaglia¹, Giorgia Confalonieri², Monica Dapiaggi², Lavinia Curecheriu³, Oana Condurache³, Marco Holzer⁴, Marco Deluca^4,5, Liliana Mitoseriu³, Vincenzo Buscaglia¹

¹ ICMATE-CNR, Via De Marini 6, 16149 Genoa, Italy
² Department of Earth Sciences, University of Milan, via Botticelli 23, 20133 Milan, Italy
³ Alexandru Ioan Cuza University, Faculty of Physics, 11 Blvd. Carol I, 700506 Iasi, Romania
⁴ Materials Center Leoben Forschung GmbH, Roseggerstraße 12, 8700 Leoben, Austria
⁵Institut für Struktur- und Funktionskeramik, Montanuniversität Leoben, Peter Tunner Straße 5, 8700 Leoben, Austria

vincenzo.buscaglia@ge.icmate.cnr.it

Dense (97-99% relative density) ceramic samples with composition BaCe_xTi_1-xO₃ (x = 0.02, 0.05, 0.06, 0.10, 0.12, 0.15, 0.175, 0.20 and 0.30) were prepared by the conventional solid-state route starting from fine precursor powders and sintered at 1450-1500 °C. The average crystal structure at different temperatures in the range 100-400 K has been determined from the Rietveld refinement of high-energy X-ray diffraction data collected at ESRF. The pair distribution function analysis has been used to investigate the local structure and the level of disorder. The phase transitions and the evolution of polar order with composition have been studied using dielectric permittivity measurements at 100 Hz-1 MHz between -150 and 150 °C, variable temperature Raman spectroscopy (from -195 to 165 °C) and differential scanning calorimetry (-100 – 150 °C). The data provide a detailed picture of the BaCe_xTi_1-xO₃ system and a tentative composition-temperature phase diagram will be proposed. The results indicate that the three transitions (rhombohedral/orthorhombic, orthorhombic/tetragonal and tetragonal/cubic) typical of BaTiO₃ (x = 0) meet at a critical point located at x » 0.10 and T = 115 °C. Evolution of the polar order from conventional ferroelectric to diffuse phase transition behavior and to relaxor state is observed with increasing x. Relaxor behavior is found at x ≥ 0.20. In contrast to the homologous systems BaSn_xTi_1-xO₃ and BaZr_xTi_1-xO₃, a diffuse ferro/para transition is already observed at x = 0.05 as a consequence of the high level of lattice disorder originated from the much larger ionic radius of Ce⁴⁺ (0.87 Å) in comparison to Ti⁴⁺ (0.605 Å).

ABSTRACT.  

Kumara Cordero-Edwards^1*, Patrycja Paruch², Gustau Catalán^1,3

¹ICN2 - Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, 08193 Bellaterra Spain
²DPMC-MaNEP, Université de Genève, Quai Ernest-Ansermet 24, 1211 Geneva, Switzerland
³Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, E-08010 Barcelona, Spain

kumara.cordero@icn2.cat

One of the most studied multiferroics is bismuth ferrite (BiFeO₃), which is ferroelectric, ferroelastic, and antiferromagnetic at room temperature. In this material the magnetization measurement seems to have a different value from what would be expected, based on his symmetry. On the other hand, LaFeO₃ is an interesting material to study because it has the same lattice parameter of BiFeO₃, but is not ferroelectric. This means that the domain walls of this material are pure ferroelastic and not a combination of ferroelectric and ferroelastic, as in BiFeO₃. By comparing the internal structure, magnetic ordering, and oxygen content of each material, the magnetic properties could be separated from the magnetoelectric properties, enabling us to know which part of the properties are because ferroelectricity, and which are because of ferroelasticity. During the STSM thin films of BiFeO₃, LaFeO₃, and LaNiO₃ on DyScO₃ substrates were grown by radio frequency off-axis magnetron sputtering. In order to characterize these films X-ray diffraction, AFM, and PFM measurements were also performed. At the present, we are trying to visualize the magnetization in domain walls of bismuth ferrite (BiFeO₃) and lanthanum ferrite (LaFeO₃), to study and compare the internal structure, magnetic ordering, and oxygen content of the ferroelastic walls.

ABSTRACT.  

J.C. Gonzalez-Rosillo¹, B.Arndt², R.Dittmann², R.Ortega-Hernandez^1,3, J. Suñe³, M.Coll¹, X.Obradors¹, A. Palau¹, T. Puig¹

¹Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Barcelona, Spain
² Institute of Solid State Research, Forschungszentrum Juelich, 52425 Juelich, Germany
³ Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Barcelona, Spain

jgonzalez@icmab.es

In recent years, huge efforts have been made in the research community to overcome the limitations in Flash and RAM memories, and consequently, new emerging technologies are competing in the race towards a new paradigm in information storage. In particular, nonvolatile memories based on the Resistive Switching (RS) effect [1], has emerged with excellent performance and it is thought to be the future substitute of present memories.

This phenomenon has been observed in many oxide systems, in particular in perovskite complex oxides, which are materials showing outstanding properties giving rise to exotic physical phenomena due to the strong electronic correlation, such as metal-insulator transitions (MIT). This is the case of the metallic perovskite La_1-xSr_xMnO₃ (LSMO) family compounds, which are able to display Volume RS effects induced by the MIT and therefore, small changes in carrier concentration can induce huge resistance changes, thus enabling tuning of robust novel ReRAM based electronics devices [2,3]. The mechanism underlying this phenomenon is still unclear although oxygen vacancies mobility plays an important role in the mechanism underneath this phenomenon.

In this regard, we have confirmed the strong influence of different atmospheres on the resistive switching properties on bare LSMO thin films. The resistance ratio is strongly diminished when changing from ambient conditions (R_off/R_on ~103-104) to vacuum (R_off/R_on ~2). This decrease does not take place when the LSMO layer is capped with a CeO₂ thin layer, which acts as an oxygen reservoir, In addition, the switching dynamics, was investigated in Ag/CeO₂/LSMO/CeO₂/Ag devices by means of pulse measurements. An extreme non-linear behavior in the device response is found. The device can switch seven orders of magnitude faster, from the second to the nanosecond regime, by increasing the amplitude of the pulse from 3V to 4.25V. We believe that these experiments have contributed to a better understanding of the physical mechanism behind the RS effect in these metallic complex oxides films showing MIT.

[1] R. Waser, R. Dittmann et al., Adv. Mater., 2009, 21, 2632 -2663 

[2] C. Moreno et al. Nano Lett. 2010 10, 3828-3835 

[3] R. Ortega-Hernandez et al. Mic. Eng, 2015, 137, 37-40

ABSTRACT. T. Schenk¹, M. H. Park¹, M. Pešić¹, M. Hoffmann¹, C. Richter¹, S. Mueller², H. Mulaosmanovic¹, F. P. G. Fengler¹, S. Slesazeck¹, T. Mikolajick^1,3, U. Schroeder¹

¹ NaMLab gGmbH, Noethnitzer Str. 64, D-01187 Dresden, Germany
² Ferroelectric Memory GmbH, c/o NaMLab gGmbH Noethnitzer Str. 64, D-01187 Dresden
³ Chair of Nanoelectronic Materials, TU Dresden, D-01062 Dresden, Germany

Tony.Schenk@NaMLab.com

Ten years have passed since ferroelectricity in Si:HfO₂ has been discovered at the memory company Qimonda in Dresden. In the frame of their research on capacitor dielectrics for dynamic random access memories (DRAMs), they looked for methods to increase the permittivity of hafnia. At that time, the admixture of e.g. SiO₂[1] was a common approach to stabilize cubic or tetragonal polymorphs, which exhibit a higher permittivity than the monoclinic bulk phase. Suspicious humps in the small-signal capacitance characteristics (a standard check for non-linear dielectric behavior) motivated polarization hysteresis and interferometric displacement measurements, which further pointed toward the presence of ferroelectricity. Since the first publication by Böscke and co-workers[2] in 2011, the number of research groups with both fundamental and application-oriented backgrounds has been tremendously growing. Fluorite-type ferroelectrics have been suggested for or even successfully demonstrated in a wealth of applications including ferroelectric memories, electrostatic supercapacitors, energy harvesting, electrocaloric cooling and steep-slope transistors [3,4].
Starting with a brief synopsis of the history, the focus of this talk will be on the outstanding properties of this material class and the experimental evidence of the originally suggested orthorhombic Pca2₁ phase. It will be explained why hafnia/zirconia-based thinfilms revived the research on ferroelectric memories. Recent progress in the field of ferroelectric capacitors and field effect transistors will be discussed. Finally, an outline of open challenges is intended to stimulate the discussion and motivate future studies.

[1] A. Toriumi et al. ECS Trans., 1, 5, 185-197 (2006).
[2] T. S. Böscke et al., Appl. Phys. Lett., 99, 10, 102903 (2011).
[3] M. H. Park et al., 27, 11, 1811-1831(2015).
[4] M. Hoffmann et al., Adv. Funct. Mater., 47, 8643-8649 (2016)

ABSTRACT.  

M. Asa¹, C. Rinaldi¹, S. Varotto¹, M. Cantoni¹ and R. Bertacco¹

¹Department of Physics, Politecnico di Milano, via G. Colombo 81, 20133 Milano

marco.asa@polimi.it

Thanks to the tunnelling electro resistance (TER) effect, i.e. the variation of the tunnelling resistance observed upon reversal of the ferroelectric polarization, Tunnelling junctions with ferroelectric barriers (FTJs) are currently under investigation for different applications. In fact, besides the potential use of FTJs as memory cells with the remanent polarization representing the digital information [1], the viscous dynamics of ferroelectrics makes possible to use these devices also as memristors with potential application as synapses in neuromorphic computing networks [2]. In this regard, we studied the switching behaviour of BaTiO₃ (BTO) ferroelectric thin films on La1/3Sr2/3MnO3 (LSMO) bottom electrodes both grown by Pulsed Laser Deposition (PLD) on SrTiO₃ (001) substrates. By means of Piezo Force Microscopy (PFM) we verified a viscous switching well described by the Merz law [3]. We then realized Pt/BTO/LSMO FTJs through a three-step optical lithography process and ion milling. In the fully-fabricated devices, the application of proper pulse trains allows for the continuous tuning of the junction resistance within a maximum ON/OFF ratio of about 10. We show that the modulation of resistance is determined both by the amplitude and the duration of the voltage pulses. The latter makes possible to observe Spike Timing Dependent Plasticity (STDP) which is the fundamental learning process in adaptive neural networks [4]. With appropriate stimulation signals, we demonstrate STDP in our ferroelectric memristors on a time scale comparable with the one of biological synapses (1-100 ms), envisaging potential development of actual neuromorphic networks based on this kind of devices.

[1] V. Garcia and M. Bibes, “Ferroelectric Tunnel Junctions for Information Storage and Processing.,” Nat. Commun., vol. 5, p. 4289, Jan. 2014.

[2] A. Chanthbouala, V. Garcia, R. O. Cherifi, K. Bouzehouane, S. Fusil, X. Moya, S. Xavier, H. Yamada, C. Deranlot, N. D. Mathur, M. Bibes, A. Barthélémy, and J. Grollier, “A Ferroelectric Memristor.,” Nat. Mater., vol. 11, no. 10, pp. 860–4, Oct. 2012.

[3] W. J. Merz, “Domain formation and domain wall motions in ferroelectric BaTiO3 single crystals,” Phys. Rev., vol. 95, no. 3, pp. 690–698, 1954. [4] G. Bi, M. Poo, “Synaptic Modification by Correlated Activity: Hebb’s Postulate Revisited”, Annu. Rev. Neurosci., vol, 24, pp. 139–166, 2001.

ABSTRACT.  

Hong Jian Zhao¹, Alessio Filippetti², Pietro Delugas³, Enric Canadell⁴, Laurent Bellaiche⁵, Vincenzo Fiorentini⁶, and Jorge Íñiguez¹

¹Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 5 avenue des Hauts-Fourneaux, L-4362 Esch/Alzette, Luxembourg
²CNR-IOM SLACS Cagliari, Istituto Officina dei Materiali, Cittadella Universitaria, Monserrato (CA) 09042-I, Italy
³Scuola Internazionale Superiore di Studi Avanzati, Via Bonomea 265, 34136 Trieste, Italy
⁴Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain
⁵Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA
⁶Dipertimento di Fisica, Università di Cagliari, Cittadella Universitaria, Monserrato (CA) 09042-I, Italy

hongjian.zhao@list.lu

Recently, a lot of work focuses on the discovery of the so-called “metallic ferroelectrics” [1-4], which exhibit both metallicity and ferroelectricity (or at least, an inversion-symmetry breaking distortion). In particular, doping normal ferroelectrics with charge carriers seem to be a promising strategy to create such metallic ferroelectrics from insulating/semiconducting ferroelectrics. However, it is well known that the free carriers will screen the electrostatic (dipole-dipole) interactions, which favor the polar order. As a result, the free carriers are generally expected to preclude the ferroelectric phase. One typical example is BaTiO₃, whose polar distortion disappears upon electronic doping [5, 6]. On the other hand, the polar distortion of PbTiO₃ still coexists with the metallicity [7]. This lead us to think about (1) the classification of ferroelectrics based on the existence or disappearance of polar distortion upon doping; and (2) the physical mechanisms for the differentiated behavior. Here, by first-principles simulations, we show that the polar distortion of many ferroelectrics can coexist with metallicity, and even be enhanced by the electron or hole doping. Our results allow us to classify ferroelectrics in two families according to their response to carrier doping. As a by-product of this work, we obtain evidence that moderate levels of doping can effectively act as an applied hydrostatic pressure in many compounds, making it possible to induce a variety of structural transitions.

[1] Y. Shi et al, Nat. Mats. 12, 1024-1027 (2013).

[2] A. Filippetti et al, Nat. Comm. 7, 11211 (2016).

[3] D. Puggioni et al, Nat. Comm. 5, 3432 (2014).

[4] T.H. Kim et al, Nature 533, 68–72 (2016).

[5] Y. Iwazaki et al, Phys. Rev. B 86, 214103 (2012).

[6] Y. Wang et al, Phys. Rev. Lett. 109, 247601 (2012).

[7] X. He et al, Phys. Rev. B 94, 224107 (2016)..

ABSTRACT.  

R. Wördenweber¹, J. Schwarzkopf², Yang Dai¹, Biya Cai¹, J. Schubert¹

¹Peter Grünberg Institute, Forschungszentrum Jülich, D-52425 Jülich, Germany
²Leibniz Institute for Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, Germany

r.woerdenweber@fz-juelich.de

Due to their tendency to form ionic states, transition metal oxides are not only highly interesting for various applications, their properties can also easily be engineered by relatively simple means. Well known examples are high-temperature superconductors, superisolators, or high-k materials. For example, ferroelectric oxides can show extremely high permittivity (> 20.000) and piezoelectricity, however they do this only close to the ferroelectric phase transition Tc which is typically far from room temperature. Therefore it is of interest to shift Tc towards room temperature without losing too much of the extraordinary properties of these ferroelectric oxide. In this contribution, we present a way to engineer the ferroelectric transition temperature, and thus permittivity, piezoelectricity, and conductivity, of oxide films via strain and stoichiometry modifications. Anisotropic biaxial strain (tensile or compressive) is generated in (K,Na)NbO₃ and (Ba,Sr)TiO₃ thin films (20-100 nm) via epitaxial growth on single-crystalline oxide substrates with different lattice mismatch with respect to the film. The resulting shift of the transition temperature (increase or decrease of Tc by several 100K are obtained for tensile or compressive strain, respectively) depends on the mismatch (up to 2%) and are explained via the Landau thermodynamic theory. The shift of T_c is accompanied by shifts of the peaks in the permittivity, piezoelectricity and conductivity. Depending on the type of ferroelectric these shifts can be advantageous for different application. For instance the hard ferroelectric (K,Na)NbO₃ shows excellent piezoelectric properties just below Tc that can be utilized for SAW-type applications, whereas the soft ferroelectric (Ba,Sr)TiO₃ shows a tunable conductivity around Tc that might be used in tunable memristor or artificial synapses approaches. We will show examples for both possible applications.

ABSTRACT.  

J. Schwarzkopf¹, L. von Helden¹, D. Braun¹, Y. Dai², R. Wördenweber², M. Hanke³ and M. Schmidbauer²

¹Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany
²Peter Grünberg Institute, Forschungszentrum Jülich, D-52425 Jülich, Germany
³Paul-Drude Institute for Solid State Electronics, Hausvogteiplatz 5-7, 10117 Berlin, Germany

jutta.schwarzkopf@ikz-berlin.de

Formation of ferroelectric domains has a large impact on the macroscopic ferro- and piezoelectric properties of functional oxide layers. Of particular interest are periodic domain structures with monoclinic symmetry due to their enhanced piezoelectric coefficients. These monoclinic phases have been predicted for K_xNa_1-xNbO₃ thin films grown under anisotropic lattice strain, which can be systematically varied by the application of different oxide substrates. Thus, understanding and engineering of ferroelectric domain pattern at the nanoscale are provided which is essential for a transfer from fundamental research to technological applications like surface acoustic wave devices or piezoelectric sensors. In this study exemplarily, two different domain configurations in K_xNa_1-xNbO₃ thin films grown by metal-organic chemical vapor deposition will be discussed. The appropriate substrate-film composition has been chosen on the basis of linear elastic theory calculations. Experimentally, the ferroelectric domain structure was investigated by means of x-ray diffraction (XRD) and piezoresponse force microscopy (PFM). First, K_0.7Na_0.3NbO₃ thin films were grown under anisotropic, almost uniaxial lattice strain on (110) TbScO₃ substrates. The resulting domain pattern exhibits periodically arranged stripe domains, which are aligned along the psuedocubic [-110]_pc as well as [110]_pc direction. Monoclinic M_C domains, in which the pseudocubic unit cells are sheared alternatingly in ±[100]_pc and ±[010]_pc direction, have been verified by PFM and XRD. Moreover, for films with a thickness of 45 nm, the propagation of surface acoustic waves – first and third harmonic – were observed occurring only along the [100]_pc and [010]_pc directions coinciding with the monoclinic shearing directions. In a second example, the domain structure of K_0.9Na_0.1NbO₃ thin films on (110) NdScO₃ substrates will be discussed. Since in this configuration the (001)_pc and (100)_pc oriented phases have almost equal elastic strain energy density, the resulting domain structure is a periodic coexistence of (001)_pc-oriented M_C domains and (100)_pc-oriented a₁a₂ domains. Thus, a herringbone pattern evolves that is characterized by an alternating arrangement of domains with inclined vertical and pure lateral electrical polarization, respectively. Here, the monoclinic symmetry in combination with polarization discontinuities at the a₁a₂/M_C domains walls is expected to result in enormous piezoelectric coefficients.

ABSTRACT.  

Monica Ciomaga Hatnean¹ , Martin R. Lees¹ , Oleg A. Petrenko¹ , Claudia Decorse² , Elsa Lhotel³ , Sylvain Petit⁴ , Romain Sibille⁵ , Michel Kenzelmann⁵ , and Geetha Balakrishnan¹

¹ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
² SP2M-ICMMO, UMR 8182, Universite Paris-Sud 11, F-91405 Orsay, France
³ Institut Neel, CNRS and Univ. Grenoble Alpes, F-38042 Grenoble, France
⁴ Laboratoire Leon Brillouin, CEA, CNRS, Universite Paris-Saclay, CEA-Saclay, F-91191 Gif-sur-Yvette, France
⁵ Laboratory for Scientific Developments and Novel Materials, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

G.Balakrishnan@warwick.ac.uk

The availability of large, high quality single crystals of rare earth titanate pyrochlore oxides has enabled significant progress to be made in the study of geometrically frustrated magnets. These materials have been studied in great detail and yet their fascinating magnetic properties (such as spin ice, spin glass, spin liquid or long-range magnetic ordered states) continue to be puzzling. One of the most exciting avenues of future research in the field is into materials which exhibit novel magnetic ground states, such as quantum spin liquid and quantum spin ice. In the search for new frustrated magnets that display quantum effects, we have turned our attention to less studied pyrochlore systems, such as rare earth zirconates and hafnates R2M2O7 (R = Rare Earth, M = Zr or Hf). We will show that with the advances we have made in the production of crystals of the rare earth zirconates and hafnates, it is now possible to investigate these novel classes of pyrochlores in greater depth. This talk will address the recent developments in the study of these new frustrated pyrochlore magnets and discuss the magnetic properties [1-5] of these systems.

[1] Single crystal growth, structure and magnetic properties of Pr2Hf2O7 pyrochlore M. Ciomaga Hatnean et al J. Phys-Cond. Matter 29 075902 (2017)

[2] Antiferroquadrupolar correlations in the quantum spin ice candidate of Pr2Hf2O7 S. Petit et al Phys. Rev. B 94, 165153 (2016)

[3] Observation of magnetic fragmentation in spin ice S.Petit et al Nature Physics 12 746-750 (2016)

[4] Candidate quantum spin ice in the pyrochlore of Pr2Hf2O7 R. Sibille et al Phys. Rev. B 94 024436 (2016)

[5] Zirconate pyrochlore frustrated magnets: Crystal growth by the floating zone technique M. Ciomaga Hatnean et al Crystals 6 79 (2016)

ABSTRACT.  

P. Jiménez-Cavero^1,2, I. Lucas^1,2,3, A. Anadón^1,2, R. Ramos^4,5, T. Niizeki^4,5, M. H. Aguirre^1,2,6,3, P. A. Algarabel^2,7, K. Uchida^8,9,10, M. R. Ibarra^1,2,3,6, E. Saitoh^4,5,10,11, and L. Morellón^1,2,3

¹Instituto de Nanociencia de Aragón, Universidad de Zaragoza, 50018 Zaragoza, Spain
²Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain
³Fundación INA, 50018 Zaragoza, Spain
⁴WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
⁵Spin Quantum Rectification Project, ERATO, Japan Science and Technology Agency, Sendai 980-8577, Japan
⁶Laboratorio de Microscopías Avanzadas, Universidad de Zaragoza, 50018 Zaragoza, Spain
⁷Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza and Consejo Superior de Investigaciones Científicas, 50009 Zaragoza, Spain
⁸National Institute for Materials Science, Tsukuba 305-0047, Japan
⁹PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan
¹⁰Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
¹¹Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan

pjcavero@unizar.es

The generation of spin currents in absence of free carriers due to magnon excitations is a major topic in thermal spintronics. Magnetic insulators are ideal systems due to the lack of Joule losses limiting the design of new devices. In this work, epitaxial thin films of maghemite (γ−Fe₂O₃), a classic ferrimagnetic insulating iron oxide, have been fabricated using Pulsed Laser Deposition technique (PLD) from a magnetite (Fe₃O₄) target and further in-situ annealing. Once the structural, electrical and magnetic properties have been optimized, we have prepared γ−Fe₂O₃/Pt bilayers and excited a spin current by a thermal gradient in the system (spin Seebeck effect, SSE), detected on the Pt layer by means of inverse spin Hall effect (ISHE). Given the insulating behavior of maghemite, the origin of the thermospin effect lies on the only contribution of magnon spin current. We have obtained a SSE coefficient of 0.5(1) μV/K at room temperature. We have also performed SSE experiments as a function of temperature, finding the magnon diffusion length in maghemite to be within the range of tens of nanometers, in agreement with previous reports [1] for conducting iron oxide magnetite. Our results thus support the relevance of spin currents of magnonic origin in magnetic iron oxides.

[1] A. Anadón et al., Appl. Phys. Lett., 109, 012404 (2016)

ABSTRACT.  

Blai Casals¹, Rafael Cichelero¹, Pablo García-Fernández², Javier Junquera², David Pesquera¹, Mariano Campoy-Quiles¹, Ingrid C. Infante¹, Florencio Sánchez¹, Josep Fontcuberta¹, Gervasi Herranz¹

¹Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, Bellaterra 08193, Catalonia, Spain
²Departamento CITIMAC, Universidad de Cantabria, Avda. de los Castros s/n, E-39005 Santander (Cantabria), Spain

gherranz@icmab.cat

In transition metal oxides, a subtle balance between different competing electronic and structural energy scales leads often to a complex phase diagram. One paradigmatic case is that of colossal magnetoresistance (CMR) oxides, in which the magnetic structure and electrical conductivity can be largely modified by magnetic fields, temperature or strain. Here we focus on the magneto-optical response of CMR manganites, which arises when polarized light interacts with magnetic materials. The magneto-optical activity is caused by changes in polarization state of light, so that a rotation or ellipticity is induced proportional to the magnetization.

These changes in polarization are nowadays exploited commercially to control the flux of light along optical fibers, in devices such as optical circulators and rotators. However, these are generally bulky, and inappropriate for integrated electronics. This is the reason why there is large interest to increase the magneto-optical response, e.g., by exploiting photonic and plasmonic effects [1,2]. Here we uncover a novel physical mechanism by which the magneto-optic activity is enhanced in a dramatic way in optimally doped ferromagnetic manganites [3]. The phenomenon is related to the specific magneto-optical response of polarons, which gives an extra contribution to that of the Drude-like response of delocalized electrons. The emergence of polarons alters locally the electronic structure, so that the magneto-optical activity is increased by more than one order of magnitude around the ferromagnetic transition. This phenomenon is only observed for a relatively narrow range of wavelengths and temperatures. We present a theoretical model in which the massive amplification of the gyrotropic response is explained in terms of spin-reversing polaron jumping induced by photons of high enough energy. In contrast, lower energy photons that do not change spin do not induce the gyrotropic enhancement. Interestingly, the showcased material is La_2/3Ca_1/3MnO₃, for which the extraordinary gyrotropic response is observed near room temperature. Summing up, the observed optical phenomenon gives an added functionality –unseen previously in any manganite or other magnetic oxides– and puts a new perspective on applications.

[1] J. M. Caicedo et al., ACS Nano 5 2957 (2011).

[2] M. Rubio-Roy et al., Langmuir, 28, 9010 (2012).

[3] B. Casals et al., Physical Review Letters 117, 026401 (2016).

ABSTRACT.  

D. Sutter¹, C. G. Fatuzzo², S. Moser³ , M. Kim^4,5, R. Fittipaldi^6,7, A. Vecchione^{6, 7}, V. Granata^6,7, Y. Sassa⁸, F. Cossalter¹, G. Gatti², M. Grioni², H. M. Ronnow² , N. C. Plumb⁹, C.E. Matt⁹, M. Shi⁹, M. Hoesch¹⁰, T. K. Kim¹⁰, T.-R. Chang¹¹, H.-T. Jeng^11,12, C. Jozwiak³, A. Bostwick³, E. Rotenberg³, A. Georges^4,5,13, T. Neupert,¹ and J. Chang¹

¹Physik-Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
²Institute of Physics, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland
³Advanced Light Source (ALS), Berkeley, California 94720, USA
⁴College de France, 75231 Paris Cedex 05, France
⁵Centre de Physique Theorique, Ecole Polytechnique, CNRS, Univ Paris-Saclay, 91128 Palaiseau, France
⁶CNR-SPIN, I-84084 Fisciano, Salerno, Italy
⁷Dipartimento di Fisica "E.R. Caianiello", Universita di Salerno, I-84084, Salerno, Italy
⁸Department of Physics and Astronomy, Uppsala University, S-75121 Uppsala, Sweden
⁹Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
¹⁰Diamond Light Source, Harwell Campus, Didcot, OX11 0DE, United Kingdom
¹¹Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
¹²Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
¹³Department of Quantum Matter Physics, University of Geneva, 1211 Geneva 4, Switzerland

dsutter@physik.uzh.ch

Ca₂RuO₄ is an archetypal example for multi-band Mott physics including spin-orbit and Hund's coupling. For decades, the mechanism underlying its Mott insulating state has remained elusive. This talk will present the complete low-energy ruthenium band structure as observed by ARPES in the paramagnetic insulating state of Ca₂RuO₄. These results suggest that Ca₂RuO₄ is a unique example of an orbital differentiated conventional band and Mott insulator. The talk we make a strong effort to explain how this conclusion is reached independently from both DMFT calculations and a purely phenomenological DFT model.

ABSTRACT.  

Esin Eren, Ceyda Alver, Gözde Yurdabak Karaca, Aysegul Uygun Oksuz

Department of Chemistry, Suleyman Demirel University, Faculty of Arts and Science, 32260 Isparta, Turkey

eso_eren@yahoo.com.tr, ayseguluygun@sdu.edu.tr

Vanadium oxide (V₂O₅) is a considerably used inorganic electrochromic material because of its interesting electrochemical performance [1]. V₂O₅ displays both anodic and cathodic electrochromism behaviors. However, deficiencies of V₂O₅ thin film-based electrochromic device (ECD) are poor reversibility, low electrical conductivity. Hybrids of V₂O₅ were proposed to solve these problems and enhance the electrochromic feature [2].

The one of aims of this study is to utilize poly[3,4-ethylenedioxythiophene]:polystyrene sulfonate (PEDOT:PSS) coated electrospun polyethylene terephthalate (PET) mat as conductive electrode to replace indium tin oxide (ITO)-based conductive electrodes for electrochromic textile applications. The other aim is to carry out plasma nanocoating of PEDOT onto the surface of V₂O₅ powders using rf rotating plasma modification method. Thin films of hybrid powder and V₂O₅ powders were prepared via the electron beam evaporation onto conductive electrospun PET mats. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used for characterization of materials. Chronoamperometry (CA) and Chronocoulometry (CP) were adopted to characterize the electrochemical activities of both V₂O₅-PEDOT and V₂O₅ coated conductive PET nanofiber.

[1] E. Eren, G. Y. Karaca, C. Alver, A. U. Oksuz, European Polymer Journal 84 (2016) 345–354

[2] Y. Liu, C. Jia, Z. Wan, X. Weng, J. Xie, L. Deng, Sol. Energy Mater. Sol. Cells 132 (2015) 467–475.

Authors gratefully acknowledge the TÜBİTAK /COST (Project No: 114M877) for financial support to this study.