The single-layer perovskite cobaltates have attracted enormous attention due
to the recent observation of hour-glass shaped magnetic excitation spectra
which resemble the ones of the famous high-temperature superconducting
cuprates. Here, we present an overview of our most recent studies of the spin
and charge correlations in floating-zone grown cobaltate single crystals. We
find that frustration and a novel kind of electronic and magnetic nano phase
separation are intimately connected to the appearance of the hour-glass shaped
spin excitation spectra. We also point out the difference between nano phase
separation and conventional phase separation.Comment: * plenary talk SUPERSTRIPES conference 201

Castellan, J. P.

Drees, Y.

Gutowski, O.

Komarek, A. C.

Lamago, D.

Li, Z. W.

Piovano, A.

Ricci, A.

Rotter, M.

Rütt, U.

Schmidt, W.

Sobolev, O.

Sprung, M.

Tjeng, L. H.

English

arXiv

The single-layer perovskite cobaltates have attracted enormous attention due to the recent observation of hour-glass shaped magnetic excitation spectra which resemble the ones of the famous high-temperature superconducting cuprates. Here, we present an overview of our most recent studies of the spin and charge correlations in floating-zone grown cobaltate single crystals. We find that frustration and a novel kind of electronic and magnetic nano phase separation are intimately connected to the appearance of the hour-glass shaped spin excitation spectra. We also point out the difference between nano phase separation and conventional phase separation

DESY

Electronic and Magnetic Nano Phase Separation in Cobaltates $La_{2−x}Sr_{x}CoO_{4}$

Z. W. Li

Y. Drees

A. Ricci

D. Lamago

A. Piovano

M. Rotter

W. Schmidt

O. Sobolev

U. Rütt

O. Gutowski

M. Sprung

J. P. Castellan

L. H. Tjeng

A. C. Komarek

Springer - Publisher Connector

Electronic and Magnetic Nano Phase Separation in Cobaltates La2−x Sr x CoO4

Li, Z.

Ruett, U.

Castellan, J.

Tjeng, L.

Komarek, A.

MPG.PuRe

Electronic and Magnetic Nano Phase Separation in Cobaltates La2-xSrxCoO4

DESY Publication Database

KITopen

J Supercond Nov Magn (2016) 29:727–731DOI 10.1007/s10948-015-3302-4ORIGINAL PAPERElectronic and Magnetic Nano Phase Separationin Cobaltates La2−xSrxCoO4Z. W. Li1 ·Y. Drees1 ·A. Ricci2 ·D. Lamago3,4 ·A. Piovano5 ·M. Rotter1 ·W. Schmidt6 ·O. Sobolev7,8 ·U. Rütt2 ·O. Gutowski2 ·M. Sprung2 ·J. P. Castellan3,4 ·L. H. Tjeng1 ·A. C. Komarek1Received: 10 October 2015 / Accepted: 19 November 2015 / Published online: 23 December 2015© The Author(s) 2015. This article is published with open access at Springerlink.comAbstract The single-layer perovskite cobaltates haveattracted enormous attention due to the recent observa-tion of hour-glass shaped magnetic excitation spectra whichresemble the ones of the famous high-temperature super-conducting cuprates. Here, we present an overview of ourmost recent studies of the spin and charge correlations infloating-zone grown cobaltate single crystals. We find thatfrustration and a novel kind of electronic and magnetic nano A. C. KomarekAlexander.Komarek@cpfs.mpg.de1 Max-Planck-Institute for Chemical Physics of Solids,Nöthnitzer Str. 40, 01187 Dresden, Germany2 Deutsches Elektronen-Synchrotron DESY, Notkestr. 85,22603 Hamburg, Germany3 Laboratoire Léon Brillouin, CEA/CNRS, 91191 Gif-surYvette Cedex, UMR12 CEA-CNRS, Bât 563 CEA Saclay,France4 Institute of Solid State Physics, Karlsruhe Instituteof Technology, 76021 Karlsruhe, Germany5 Institut Laue-Langevin (ILL), 6 Rue Jules Horowitz, 38043Grenoble, France6 Jülich Centre for Neutron Science JCNS, ForschungszentrumJülich GmbH, Outstation at ILL, CS 20156, 71 avenuede Martyrs, 38042 Grenoble, France7 Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM-II),TU München, Lichtenbergstr. 1, 85747 Garching, Germany8 Georg-August-Universitát Göttingen, Institutfür Physikalische Chemie, Tammannstrasse 6,37077 Göttingen, Germanyphase separation are intimately connected to the appearanceof the hour-glass shaped spin excitation spectra. We alsopoint out the difference between nano phase separation andconventional phase separation.Keywords Magnetic excitation spectra · Hour-glass ·Cobaltate · Cuprate · Superconductor · Neutron ·Synchrotron1 IntroductionThe hour-glass magnetic excitation spectrum has fascinatedphysicists over years. This spectrum has been observedin high-temperature superconducting cuprates, and it iswidely believed that fluctuating charge stripes are involvedin the physics of these intriguing materials [1, 2]. Besides,charge stripes also Fermi surface effects have been pro-posed for the emergence of these hour-glass spectra incuprates [3]. Recently, Boothroyd et al. observed an hour-glass magnetic spectrum in the copper-free Co oxide mate-rial La2−xSrxCoO4 implying that Fermi surface effects arenot needed for the emergence of hour-glass spectra sincethis material is insulating [4]. The undoped parent materialLa2CoO4 is an antiferromagnetic insulator with a Néel-temperature of roughly 275 K [6]. In contrast to the high-temperature superconducting cuprates La2−xSrxCuO4, thecobaltates remain insulating up to high hole-doping levelsof x ∼ 1 [7]. Also, the antiferromagnetic correlations withLa2CoO4-like character which are centered at the planarantiferromagnetic wavevector persist to very high hole-doping ranges [8], higher than in the cuprates where incom-mensurate magnetic satellites have been observed alreadyabove ∼2 % Sr-doping [9]. A clearly incommensurate anti-ferromagnetic regime with well-separated incommensurate728 J Supercond Nov Magn (2016) 29:727–731antiferromagnetic peaks can be observed only above ∼33 %of hole-doping in the cobaltates [8]. At half-doping, a robustcheckerboard charge order (CBCO) has been reported inthese cobaltates (La1.5Sr0.5CoO4) with a charge orderingtemperature of the order of ∼800 K [10, 11]. It was shownthat the Co2+ and Co3+ ions in La1.5Sr0.5CoO4 are in thehigh spin and in the nonmagnetic low-spin state, respec-tively, [11, 12].2 Results and DiscussionAt one-third doping, i.e., in La5/3Sr1/3CoO4, broad incom-mensurate magnetic peaks have been observed around third-integer positions in reciprocal space. Also, in the isostruc-tural nickelates La2−xSrxNiO4 [5], magnetic peaks can beobserved at about the same positions in reciprocal spacearound 1/3-hole-doping, see Fig. 1a. These nickelates areknown to exhibit a robust diagonal charge stripe phase[5]. Hence, it is natural to expect the existence of chargestripe ordered phases also in La5/3Sr1/3CoO4, with the notethat the correlation length of these diagonal charge stripeswas thought to be very low in the cobaltate correspond-ing to the broadness of the magnetic peaks. However, using100 keV hard X-rays at the synchrotron, we were able toshow that in La5/3Sr1/3CoO4 the charge correlations essen-tially occur with CBCO character only, see Fig. 1b [13, 14].Also, in La1.6Sr0.4CoO4, the charge ordering peaks were notobserved at the incommensurate positions that correspondto the magnetic peak positions, but, instead at half-integerpeak positions typical for CBCO [8, 14]. The CBCO char-acter of the charge correlations in La5/3Sr1/3CoO4 andLa1.6Sr0.4CoO4 is also reflected in the topmost 1 Co-Obond-stretching phonon dispersions that resemble the onein the half-doped CBCO ordered cobaltate La1.5Sr0.5CoO4where an anomalous phonon softening can be observedat half-integer propagation vectors, see magenta arrow inFig. 2. This is unlike to the behaviour in the undopedparent material La2CoO4 and also unlike to the behaviourin charge stripe ordered nickelates [13].Interestingly, nickelates around half-doping also exhibitCBCO at high temperatures (∼480 K) and become chargestripe ordered on cooling to low temperatures (∼180 K)[15, 16]. This re-arrangement of charges into stripes at lowtemperatures seems to be hampered in the cobaltates whichalso have a much more robust CBCO with distinctly higherCBCO ordering temperatures than in the nickelates. Anapparent difference between cobaltates and nickelates is thehigher insulating properties of cobaltates according to thespin-blockade mechanism [12] as well as the stability ofboth Co2+ and Co3+ oxidation states (charges are local-ized at Co-sites and not at the oxygen ions) together witha very large difference in their ionic sizes (the ratio of theCo2+ and Co3+ ionic radii amounts to ∼1.367 [17]). Allthese effects together hamper the hopping of electrons fromCo2+ to Co3+ sites and, thus, probably the re-arrangementof charges into stripes at low temperatures.The absence of charge stripes and Fermi surfaces inLa5/3Sr1/3CoO4 and La1.6Sr0.4CoO4 forced us to find analternative mechanism for the emergence of the hour-glassspectrum [8, 13]. We proposed that frustration effects indisordered CBCO phases produce the incommensurabil-ities in La2−xSrxCoO4 [8]. Even more intriguing is themicroscopic origin behind this mechanism. For cobaltatesbelow half-doping additional electrons have to be intro-duced into the Co-oxygen planes, i.e., Co2+ ions have tobe inserted at nonmagnetic Co3+ sites. This electron dopingof CBCO phases will create undoped La2CoO4-like islandson the nanometer scale. Within these undoped islands, largenearest-neighbour exchange interactions J occur, whereasthe surrounding regions with CBCO character only containmuch weaker exchange interactions J ′ across the holes,Fig. 1 a Magnetic neutronscattering intensities at lowtemperatures for one-third hole-doped cobaltate and nickelate,respectively, [13]. b The chargecorrelations observed at 285 Kin La5/3Sr1/3CoO4 [13]J Supercond Nov Magn (2016) 29:727–731 729Fig. 2 a Polarization patterns for two high-frequency 1 Co-O bond-stretching phonon dispersions with propagation vectors of (1/3 1/3 0)and (1/2 1/2 0), respectively, are shown. As can be seen, one wouldexpect a coupling of a phonon mode to charge correlations that havethe same propagation vector as the phonon mode. b The Co-O bond-stretching phonon dispersions of La2−xSrxCoO4 with x = 0 (yellow),1/3 (blue), 0.4 (red), and 1/2 (green) [8]. For a better comparison,the dispersion of La2CoO4 has been shifted to higher energies. Theblue/red/green arrow marks the position of the propagation vector ofa 1/3-doped and of a 40 % hole-doped charge stripe phase and of aCBCO phase. As can bee seen, La5/3Sr1/3CoO4 exhibits absolutely noanomalies in the phonon dispersion at propagation vectors correspond-ing to a 1/3-hole-doped charge stripe phase (blue arrow). Instead, aphonon softening occurs at the zone boundary which is indicativefor CBCO. Also, La1.6Sr0.4CoO4 exhibits a similar phonon soften-ing (magenta arrow) like the half-doped CBCO ordered cobaltateLa1.5Sr0.5CoO4see Fig. 3. This gives rise to a new situation—whereasnanometer-sized undoped islands can be excited to highenergies the surrounding CBCO ordered regions are not ableto follow these high-frequency oscillations. Hence, besideselectronic (i.e. charge) also magnetic nano phase separa-tion occurs in these cobaltates. The high energy excitationswithin the hour-glass shaped magnetic spectra appear to bebasically hosted in nano phase separated undoped islandsFig. 3 The Co2+ ions (green spheres with arrows symbolizing thespin) and the nonmagnetic Co3+ ions (black spheres) within theab-plane are schematically drawn for our nano phase separation sce-nario. By doping of electrons at Co3+-sites in a checkerboard chargeordered matrix (blue) undoped islands (red) will be created. Withinthe undoped islands the nearest-neighbour exchange interactions J arequite large. However, in the surrounding CBCO regions, only weakexchange iteractions J ′ across the holes exist [14]whereas the remaining hole-rich regions can be excited onlyat lower energies such that the so-called hour-glass spectrumis formed from the coupling of these two very different typeof excitations.The magnetic part of our nano phase separation scenariowas corroborated by our study of the hole-doping depen-dence and the temperature dependence of the hour-glassspectra [13]. The charge part of our nano phase separationscenario was also verified by microdiffraction measure-ments on La5/3Sr1/3CoO4 at the synchrotron, where wescanned a CBCO peak reflection intensity with a micro-focused beam across the surface. The domain size distri-bution exhibits islands with larger and islands with smallerdomain sizes (correlation lengths). Using the statisticalmethod developed in Ref. [18], we analyzed the probabil-ity density function (PDF) and observed an almost perfectpower-law behaviour that reflects a scale-invariant (fractal-like) domain size distribution which nicely confirms ournano phase separation scenario in cobaltates [13]. Usingthese new tools, it has been possible to discover a common(structural) nano scale phase separation scenario character-ized by the coexistence of competing granular networks ofdifferent striped orders [18–33]. However, so far, a rigor-ous study of the dynamical interplay among these multipleorders is still missing but a perspective might come fromthe use of the coherent x-ray beams available at the newgeneration synchrotron facilities and free electron lasers[34].Here, we would like to emphasize that nano phase sep-aration appears to be different from conventional phaseseparation as is schematically shown in Fig. 4.Whereas con-ventional phase separation will basically lead to a trivial730 J Supercond Nov Magn (2016) 29:727–731Fig. 4 The first two columns schematically show the magnetic excita-tion spectra of the undoped parent material La2CoO4 (phase A) and ofthe checkerboard charge ordered half-doped material La1.5Sr0.5CoO4(phase B). The reported [11] extremely weak optical mode in thespectrum of La1.5Sr0.5CoO4 has been omitted since it could not bedetected with a triple axis-spectrometer in Ref. [8] in the accessibleQ-space and can be neglected for these reasons). The third columnschematically shows the expected observations for conventional phaseseparation with any domain sizes down to the micro-meter scale veryroughly. Finally, the last column schematically shows the situation fornano phase separation. When domain sizes shrink to the nano scale,the magnetic spectra are no longer a trivial superposition of the spectraof phases A and B. Instead, a novel spectrum with the characteris-tics of an hour-glass spectrum appears. Thus, nano phase separationis distinct from conventional phase separation and might induce newphysical propertiessuperposition of magnetic excitation spectra of the con-tributing phases, electronic and magnetic nano phase sep-aration is able to create new physical properties. As isshown schematically in Fig. 4, the energy scale and widthof the magnetic excitations are altered and even the entirespectrum looks different from a simple superposition ofthe spectra of the contributing phases since the outwards-dispersing branches are suppressed in the low energy partof the spectrum. Whereas the bulk properties of a materialare usually governed by the volume properties of the con-tributing phases if conventional phase separation occurs, itis this volume of the contributing phases which has almostvanished as soon as nano phase separation occurs. This is,then, responsible for new physical properties as is shown forthe example of the hour-glass shaped magnetic excitationspectra in cobaltates.Acknowledgments Open access funding provided by Max PlanckSociety. A. C. K. thanks J. Zaanen and G. Seibold for valuablediscussions.Open Access This article is distributed under the terms of theCreative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricteduse, distribution, and reproduction in any medium, provided you giveappropriate credit to the original author(s) and the source, provide alink to the Creative Commons license, and indicate if changes weremade.References1. Tranquada, J.M., Woo, H., Perring, T.G., Goka, H., Gu, G.D.,Xu, G., Fujita, M., Yamada, K.: Nature 429, 534 (2004)2. Vojta, M., Vojta, T., Kaul, R.K.: Phys. Rev. Lett. 97, 097001(2006)3. Eremin, I., Morr, D.K., Chubukov, A.V., Bennemann, K.H.,Norman, M.R.: Phys. Rev. Lett. 94, 147001 (2005)4. Boothroyd, A.T., Babkevich, P., Prabhakaran, D., Freeman, P.G.:Nature 471, 341–344 (2011)5. Tranquada, J.M., Buttrey, D.J., Sachan, V., Lorenzo, J.E.: Phys.Rev. Lett. 73, 1003 (1994)6. Babkevich, P., Prabhakaran, D., Frost, C.D., Boothroyd, A.T.:Phys. Rev. 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Electronic and Magnetic Nano Phase Separation in Cobaltates La₂₋ₓSrₓCoO₄

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Electronic and magnetic nano phase separation in cobaltates
  La$_{2-x}$Sr$_{x}$CoO$_4$

Electronic and magnetic nano phase separation in cobaltates La $_{2-x}$ Sr $_{x}$ CoO $_4$

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Electronic and magnetic nano phase separation in cobaltates La2−x_{2-x}2−x​Srx_{x}x​CoO4_44​

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Electronic and magnetic nano phase separation in cobaltates La $_{2-x}$ Sr $_{x}$ CoO $_4$