139La NMR evidence for phase solitons in the ground state of overdoped manganites
D. Koumoulis, N. Panopoulos, A. Reyes, M. Fardis, M. Pissas, A. Douvalis, T. Bakas, D. Argyriou, G. Papavassiliou
aa r X i v : . [ c ond - m a t . s t r- e l ] O c t La NMR evidence for phase solitons in the ground state of overdoped manganites
D. Koumoulis , N. Panopoulos , A. Reyes , M. Fardis , M. Pissas ,A. Douvalis , T. Bakas , D. Argyriou & G. Papavassiliou ∗ Institute of Materials Science, NCSR, Demokritos, 153 10 Aghia Paraskevi, Athens, Greece National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA Physics Department, University of Ioannina, P.O. Box 1186, GR-451 10, Ioannina, Greece and Helmholtz-Zentrum Berlin fuer Materialien und Energie (HZB), Glienicker Strasse 100, D-14109 (Dated: November 24, 2018)
Hole doped transition metal oxides are famousdue to their extraordinary charge transport prop-erties, such as high temperature superconduc-tivity (cuprates) and colossal magnetoresistance(manganites) . Astonishing, the mother systemof these compounds is a Mott insulator , whereasimportant role in the establishment of the metal-lic or superconducting state is played by the waythat holes are self-organized with doping . Ex-periments have shown that by adding holes theinsulating phase breaks into antiferromagnetic(AFM) regions, which are separated by hole richclumps (stripes) with a rapid change of the phaseof the background spins and orbitals . However,recent experiments in overdoped manganites ofthe La − x Ca x MnO (LCMO) family have shownthat instead of charge stripes , charge in thesesystems is organized in a uniform charge den-sity wave (CDW) . Besides, recent theoret-ical works predicted that the ground state is in-homogeneously modulated by orbital and chargesolitons , i.e. narrow regions carrying charge ± e/ , where the orbital arrangement varies veryrapidly. So far, this has been only a theoreticalprediction. Here, by using La Nuclear Mag-netic Resonance (NMR) we provide direct evi-dence that the ground state of overdoped LCMOis indeed solitonic. By lowering temperature thenarrow NMR spectra observed in the AFM phaseare shown to wipe out, while for
T < K a verybroad spectrum reappears, characteristic of anincommensurate (IC) charge and spin modula-tion. Remarkably, by further decreasing temper-ature, a relatively narrow feature emerges fromthe broad IC NMR signal, manifesting the ap-pearance of a solitonic modulation as T → . The presence of an IC spin-density modulationand the formation of charge stripes in AFM tran-sition metal oxides was first observed in supercon-ducting La − x Sr x CuO and their insulating nickelatecounterparts . Inelastic neutron scattering experi-ments have shown that holes in these materials tendto localize into periodically arranged antiphase domainwalls, separating AFM regions, which propagate diago-nally through the CuO (respectively NiO ) layers . Inaddition, a link between stripe ordering and the wipe-out effect (disappearance) of the NMR/NQR signal was observed , which indicates that most probably thestripe phase in these systems is a slowly fluctuating,strongly correlated fluid over an extended temperaturerange.In case of the LCMO family for x ≥ .
5, electrondiffraction experiments unveiled an IC charge modula-tion with wave vector q k a ⋆ , which is associated withorbital and AFM spin ordering. This charge modulationhas been considered as signature of charge stripes arisingfrom the ordered arrangement of alternating Mn +3 andMn +4 ions. However, a number of recent experimentsput into question this kind of Mn +3 and Mn +4 chargealternation ; instead they suggest the formation of acharge modulation wave with a uniform periodicity forall x ≥ . . A collective sliding of the charge system ina moderate electric field was also reported at half dop-ing, x = 0 . . These experimental results have beenexplained in the framework of a CDW model withnonzero and possibly high itineracy. Recently, it has beenalso proposed that the nanoscale spin and orbital textureobserved in overdoped manganites is possibly explainedin the framework of an IC solitonic ground state , thatis produced by orbital solitons carrying charge equal to ± e/ . However, until now there is no experimentalevidence about the existence of orbital or spin solitons;only a uniform IC modulation has been reported to ex-ist in overdoped LCMO for T ≥ . Here, by us-ing La NMR in magnetic field 4 . T < . Diffractionof a neutron or electron beam by long-period spin andcharge density modulations yields sattelite Bragg peaks,which provide information about the spatial period andorientation of the corresponding modulation, whereas theintensity provides a measure of the modulation ampli-tude. A complementary and equally powerful techniquein the study of modulated structures is NMR. In the past,NMR has been succesfully applied in the study of struc-tural IC phases , as well as CDW and SDW systems .The ability of NMR to provide information about theappearance and evolution of an IC spin modulation isclearly demonstrated in Figure 1. ( x ) x IC (solitonic)
IC (plane wave)
Frequency [MHz]
Commensurate
FIG. 1: The spatial variation of the phase ϕ ( x ) of a SDW andthe corresponding NMR lineshapes for three different cases:(i) A spin modulation with constant phase ϕ ( x ) (lower panel),corresponding to a commensurate spin configuration with thecrystal-lattice periodicity. (ii) An IC plane wave phase mod-ulation (middle panel), and (iii) an IC modulation with phasesolitons (upper panel). Assuming an 1 − D spin density wave (SDW), themodulation wave varies in space according to formula ρ = A cos( ϕ ( x )) . In magnetic systems the NMRfrequency is proportional to the local hyperfine field B hf = (1 /γ ¯ h ) C< S > , (C is the hyperfine coupling con-stant and < S > the average electron spin probed by theresonating nuclei ); hence, the NMR frequency will beaccordingly modulated, ν = ν + ν cos( ϕ ( x )). This givesrise to a characteristic NMR frequency distribution de-picted by formula, f ( ν ) ∝ ν | sin( ϕ ) dϕdx | . In case of anIC modulation, the phase of the modulation wave ϕ ( x )varies in space according to the Sine-Gordon (SG) equa-tion, d ϕdx = w sin( mϕ ( x )) , where m is a commen-surability factor defining the crystal symmetry , and w a constant depending on the electronic properties of thesystem and the amplitude of the modulation wave .Figure 1 presents the spatial variation of ϕ ( x ) by solvingthe SG equation for two cases: (i) A nearly plane waveIC modulation with w << w ≤ m = 1, in accordance with theoretical studies for 1 − D CDWs . This gives a phase shift accross solitons equalto ∆ ϕ = 2 π . Other works predict ∆ ϕ = ± π/ ± e/ , while in systems with coexist-ing charge and spin order a change in phase equal to ± π has been predicted, even in the presence of higher order commensurability (e.g. m = 3 or 4) . For reasons ofcomparison a commensurate modulation with constantphase ϕ ( x ) is also presented in the left lower panel ofFigure 1, which corresponds to spin configuration withthe crystal-lattice periodicity. Simulated NMR spectrafor the three cases (with initial phase ϕ = π/ → IC (plane wave) → IC(solitonic)phase transitions, the NMR signal is expected to trans-form from a narrow symmetric lineshape in the commen-surate phase, to a broad frequency distribution in the ICplane wave limit, and finally to the composite signal ofthe right upper panel.
20 24 28 32 36 20 24 28 32 36 x=0.50 20 K10 K 5.5 K3 Kx=0.55 40K wipe out wipe out
220 K80K50 K
FIG. 2:
La NMR lineshapes of the AFM signal as a func-tion of temperature for x = 0 .
55 and 0 .
63. The blue line inthe left panel is the spectrum for x = 0 .
50 at 4K, and theblue (red) lines in the left panel are the spectra for x = 0 . .
69) at 3K.
Figure 2 shows
La NMR lineshapes for LCMO with x = 0 .
55 and 0 .
63 in the temperature range 3K to 200K.A striking similarity is observed between the simulationsof Figure 1 and the temperature evolution of the spectrain Figure 2. For 80K ≤ T ≤ ≈
29 MHz,which remains almost unshifted by entering the AFMphase ( T N ≈ T < T N the sig-nal intensity decreases rapidly by cooling and disappearsat ≈ T c cuprates and nickelates ,and will be discussed in more detail below. Unexpect-edly, a very broad spectrum reappears for T ≤
30K ,which is characteristic of an IC modulation. By furtherlowering temperature a relatively narrow peak emergesfrom the broad IC NMR signal, which closely resem-bles the spectrum in the upper right panel of Figure1. This is strongly suggesting that by lowering tem-perature, the CDW/SDW breaks into alternating com-mensurate and discommensurate regions giving rise tothe particular soliton-modulated NMR signal. A similartemperature evolution of the NMR spectra was observedin LCMO x = 0 .
50, 0 .
66, and 0 .
69. However, the ICmodulation appears to reduce at higher doping values,as indicated by the narrowing of the IC NMR lineshapeswith increasing doping in Figure 2. This is in confor-mity with electron diffraction experiments, which exhibita decrease of the modulation wavevector according to re-lation, q s = (1 − x ) a ∗ . La Ca x MnO T [ s e c ] Temperature [K]Wipeout effect region
FIG. 3:
La NMR spin-spin relaxation time ( T ) as a func-tion of temperature of LCMO x = 0 . , .
63 and 0 .
69. Thetemperature range without experimental points (30 − T values (wipeout). Another distinguishing feature in Figure 2 is the strongwipeout effect of the NMR signal, which takes place inthe temperature range 30K to 60K. The reason for thecomplete disappearance of the NMR signal is the ex-treme shortening of the spin-spin relaxation time T , asshown in Figure 3. This makes part of the La nucleito relax so fast that the NMR signal decays before itcan be measured. In general, the T shortening by cool-ing is ascribed to slowing down of CDW/SDW fluctua-tions in accordance with relevant measurements in othersystems . The presence of slow collective fluctu-ations reconciles with resistivity measurements, whereapplication of a moderate external electric field invokessliding of the CDW .Of considerable interest is the x = 0 . T − x magnetic phasediagram, where coexistence of a relatively strong FMphase component with the AFM phase is observed. Bylowering temperature this system undergoes a Paramag-netic (PM) to Ferromagnetic (FM) phase transition at T c ≈
20 25 30 35 40 45 50 55 60 65
La 4 K La Frequency [MHz]
20 K40 K wipe out
140 K160 K70 K115 K190 K240 K270 K298 K10 K
130 K La Ca MnO Temperature [K] coolingwarming T c T N /T co Temperature [K] T [ s e c ] FM AFM FM AFM T c T N /T co F r equen cy [ M H z ] FIG. 4:
La NMR spectra for x = 0 .
50 at various temper-atures. For
T <
70K the AFM signal component wipes out,while for
T <
30K the broad IC NMR signal marks the ap-pearance of an IC modulated AFM phase. The upper rightpanel shows the spin-spin relaxation time T as a function oftemperature for both the FM and AFM signal components.The lower right panel shows the corresponding signal frequen-cies. tion at T N ≈ T N ≈ . Electron and neutron diffraction exper-iments revealed the formation of a commensurate chargemodulation for T < T N , which becomes IC at highertemperatures. Figure 4 shows the evolution of the LaNMR signal from room temperature down to 3K. Atroom temperature the PM NMR signal is located at ≈ ≈ . T N the AFM signal component starts to grow at ≈
29 MHz,which exhibits the same behaviour as in all other stud-ied samples; the narrow AFM NMR signal wipes out at ≈ . The upper right panel of Figure4 shows T vs. T . As expected, a strong hysterestic be-haviour between T c and T N is shown by the T of the FMsignal component. On the other hand, the T of the AFMsignal component exhibits the same behaviour as in allother systems presented in Figure 3. We also notice thatthe high temperature AFM signal is sufficiently narrowerthan the PM signal, which supports the idea that in thistemperature regime the NMR signal is motionally nar-rowed, due to strong fast fluctuations of the modulationwave. Another important remark is that the onset of thewipeout effect is accompanied with a strong frequencyshift and significant broadening of the FM NMR signal.This indicates that the FM phase component does notconsist of isolated FM islands embedded into an AFMmatrix, but is rather strongly interacting with the AFMphase component.In summary, our La NMR measurements provideevidence that the ground state in overdoped LCMO man-ganites comprises an IC soliton-modulated charge andspin density wave. At higher temperatures the modula-tion wave transforms to a uniform IC plane wave, whichis subjected to strong slow fluctuations, as implied bythe complete wipeout effect of the NMR signal. This isa completely new result, which urges to reconsider ouropinion about the low temperature electronic properties of overdoped manganites. Even more, the fundamen-tal mechanism governing the establishment and evolutionof the stripe phase appears to be common in overdopedmanganites with cuprates and nickelates. In all these sys-tems charge order is established at a higher temperaturesthan AFM order, while the growth of the IC modulationat low temperatures is accompanied with a strong wipe-out of the NMR signal. Further experiments are neededin order to examine whether the solitonic ground state isa generic property of striped AFM transition metal oxidecompounds. Dagotto E., Open questions in CMR manganites, relevanceof clustered states and analogies with other compoundsincluding the cuprates.
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