Resistivity, optical, and angle-resolved photoemission experiments reveal
unusual one-dimensional electronic properties of highly anisotropic
SrNbO$_{3.41}$. Along the conducting chain direction we find an extremely small
energy gap of only a few meV at the Fermi level. A discussion in terms of
typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to
provide a satisfactory explanation for the unique properties of SrNbO$_{3.41}$.Comment: 4 pages, 3 figure

A. Herrnberger

B. Gorshunov

C. A. Kuntscher

F. Lichtenberg

F. Mayr

H. J. Schulz

J. Mannhart

M. Dressel

M. Grioni

P. Haas

R. Peierls

S. Nanamatsu

S. Schuppler

English

arXiv

Crossref

Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound
                    
                      
                        
                          S

Resistivity, optical, and angle-resolved photoemission experiments reveal unusual one-dimensional electronic properties of highly anisotropic SrNbO3.41. Along the conducting chain direction, we find an extremely small energy gap of only a few meV at the Fermi level. A discussion in terms of typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to provide a satisfactory explanation for the unique properties of SrNbO3.41.LS

Schuppler, S.

Haas, P.

Gorshunov, B.

Dressel, M.

Grioni, M.

Lichtenberg, F.

Herrnberger, A.

Mayr, F.

Mannhart, J.

Kuntscher, C. A.

Infoscience - École polytechnique fédérale de Lausanne

Extremely small energy gap in the quasi-one-dimensional conducting chain compound SrNbO3.41

Kuntscher, Christine A.

Gorshunov, B. P.

Dressel, Martin

Lichtenberg, Frank

Herrnberger, Alexander

Mayr, Franz

Mannhart, Jochen

Online-Publikationserver Augsburg

OPUS Augsburg

VOLUME 89, NUMBER 23 P H Y S I C A L R E V I E W L E T T E R S 2 DECEMBER 2002Extremely Small Energy Gap in the Quasi-One-DimensionalConducting Chain Compound SrNbO3:41C. A. Kuntscher,1,2 S. Schuppler,1 P. Haas,2 B. Gorshunov,2,* M. Dressel,2 M. Grioni,3 F. Lichtenberg,4 A. Herrnberger,4F. Mayr,4 and J. Mannhart41Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsruhe, Germany21. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany3Institut de Physique Appliquée, Ecole Polytechnique Fédérale, CH-1015 Lausanne, Switzerland4Institut für Physik, EKM, Universität Augsburg, Universitätsstrasse 1, D-86135 Augsburg, Germany(Received 23 March 2001; published 18 November 2002)236403-1Resistivity, optical, and angle-resolved photoemission experiments reveal unusual one-dimensionalelectronic properties of highly anisotropic SrNbO3:41. Along the conducting chain direction, we find anextremely small energy gap of only a few meV at the Fermi level. A discussion in terms of typical 1Dinstabilities (Peierls, Mott-Hubbard) shows that neither seems to provide a satisfactory explanation forthe unique properties of SrNbO3:41.DOI: 10.1103/PhysRevLett.89.236403 PACS numbers: 71.10.Pm, 71.20.–b, 78.20.–e, 79.60.–iEa=1.5meVEa=1.8meVacSrNbO3.41(a) (b)0 50 100 150 200 250 30010-410-310-210-1100101102103||a||b||cabcUII UResistivityρ(Ωcm)Temperature (K)Ea=3.3meVFIG. 1. (a) Projection along the b axis of the SrNbO3:4 crystalstructure; Nb atoms are hidden within the NbO6 octahedra(light grey). White (grey) circles: Sr (O) atoms. (b) dc resis-tivity  versus temperature of SrNbO3:41 along the three crystalaxes, with the respective measurement geometry. In the rangeperature (RT) x-ray powder diffraction pattern indicatessingle-phase composition with lattice constants a 20–40 K (grey bar), one finds an activated behavior  /expfEa=kBTg with Ea as indicated.A one-dimensional (1D) interacting electron system nolonger exhibits the quasiparticles known from Fermiliquid theory and is instead a Tomonaga-Luttinger liquid(TLL) whose properties also include spin-charge separa-tion and a power-law behavior of correlation functions [1].This 1D theory does not, however, directly apply to real1D materials since some coupling between the 1D entities(such as chains or similar) is always present, renderingthe systems quasi-1D and facilitating 2D and 3D orderingeffects. Real 1D materials are thus conducive to variousinstabilities masking the predicted TLL characteristics[2]. Electron-phonon coupling, for instance, can lead to agapped charge-density wave (CDW) ground state (Peierlstransition), and electron-electron interaction may causethe opening of a Mott-Hubbard (MH) gap at commensu-rate band filling.Recently, a new quasi-1D material was synthesized [3]which belongs to the series SrNbO3:5x of perovskite-related transition-metal oxides, with characteristicsspanning a wide range: Depending on the oxygen stoi-chiometry, these niobium oxides exhibit quasi-1D metal-lic character (x  0:1) [4] or ferroelectricity (x  0) witha very high transition temperature [5]. In this Letter, wefocus on the unusual electronic properties of the quasi-1Dmetallic compound SrNbO3:41 which were determined bydc resistivity, optical spectroscopy, and angle-resolvedphotoemission (ARPES). Along the conducting chaindirection, an extremely small energy gap of only a fewmeV, much smaller than for other quasi-1D compounds, isobserved at the Fermi level. The gap is discussed in termsof a Peierls and a MH scenario.SrNbO3:41 single crystals of typical size 3 20:2 mm3 were grown [3,6] by the floating zone meltingtechnique. The precise oxygen content was determinedthermogravimetrically. The analysis of the room tem-0031-9007=02=89(23)=236403(4)$20.003:99 A, b  5:67 A, and c  32:46 A, in good agreementwith earlier work [7]. The basic structural building blocksare NbO6 octahedra [see Fig. 1(a)] grouped into slabswhich extend parallel to the a; b plane and forSrNbO3:4 are five octahedra wide along c. Only alongthe a axis the octahedra are connected continuously viatheir apical oxygen atoms, forming 1D chains.The dc resistivity of SrNbO3:41 along the three axes wasmeasured using a four-point configuration [6]. Polarizedreflection measurements along the a and b axes were 2002 The American Physical Society 236403-102000400060000.990.9050 K(eV) 100 K200 K300 K5 KE||aConductivity σ1 ( Ω-1cm-1) 101 102 103 1045 K300 KRν (cm-1)101 102 103 10402004006008000.990.905 K300 KE||b Frequency (cm-1)101 102 103 104 5 K300 Kν (cm-1) R10-3 10-2 10-1 100SrNbO3.41 FIG. 2. Frequency dependent conductivity of SrNbO3:41 fordifferent temperatures and the polarization E set parallel to thea and b axes (insets: reflectivity R used for Kramers-Kroniganalysis). For Ejja, the low-temperature peak around 40 cm1indicates excitations across a gap 2  5 meV.VOLUME 89, NUMBER 23 P H Y S I C A L R E V I E W L E T T E R S 2 DECEMBER 2002performed for frequencies 6–34 000 cm1 by utilizingvarious spectrometers; for details, see Ref. [8]. To accom-plish the Kramers-Kronig analysis, we used a Hagen-Rubens extrapolation for ! ! 0 and a power law (!4)at high frequencies. High-resolution (E  15 meV)ARPES data were recorded with a Scienta ESCA-300analyzer using the He I line of a discharge lamp. Thesamples were cleaved at RT and a base pressure of 11010 mbar to expose an (001) surface, and cooled downto 25 K. By Laue diffraction, the crystals were orientedto 1. The angular resolution was set to 0:5, andthe Fermi level EF was determined to 1 meV accuracyfrom the Fermi cutoff of a freshly evaporated Au filmrecorded immediately afterwards at the same experimen-tal conditions.Figure 1(b) shows the dc resistivity  versus tempera-ture T of SrNbO3:41 along the three axes. It is highlyanisotropic with very low, metal-like values along the adirection [a300 K  4:6 104cm] and a RT an-isotropy of a:b:c  1:102:104. The variation of with temperature is smaller within the a; b plane thanalong c. In all directions, a broad maximum or shoulder isseen between 200 and 50 K, and below 50 K we find anincrease of , which in the range 20–40 K can bedescribed by an activated behavior  / expfEa=kBTgwith activation energies Ea  2–3 meV. In the range60–130 K, a (along the chains) has a metallic T depen-dence, whereas above 130 K it slightly decreases withincreasing temperature (thermally activated hopping be-tween metallic strands may explain this). A similar tem-perature dependence is observed for b; however, not asclearly developed. There are no signs of metallic behavioralong c.The anisotropy in the electrical properties of SrNbO3:41is also clearly seen in the optical response (Fig. 2). Thereflectivity R displayed in the insets of Fig. 2 shows asharp plasma edge for the polarization Eka with highvalues (R  1) at low frequencies, indicating metallicconductivity. In contrast, for Ekb the material is insulat-ing, and we mainly see phonon contributions between 40and 1000 cm1 showing significant changes with tem-perature which will be discussed elsewhere [8]. For bothpolarization directions, the low frequency (! ! 0) con-ductivity 1 agrees, in general, well with the measured dcdata (Fig. 1), reproducing its rather complicated tempera-ture behavior. At RT, the Eka conductivity contains aDrude-like contribution at low frequencies followed byseveral phonon lines between 100 and 1000 cm1 and abroad midinfrared band around 1500 cm1. Upon reduc-ing the temperature, 1 exhibits substantial changes atfrequencies lower than 100 cm1, leading to the appear-ance of a peak around 40 cm1 which is already quitestrong at 50 K. At 5 K, this peak has shifted slightly tohigher frequencies and has grown even stronger. We as-cribe this feature to single-particle excitations across anenergy gap in the electronic density of states with25 K  5 meV.236403-2The momentum resolved electronic properties probedby ARPES reveal a quasi-1D band structure forSr0:9La0:1NbO3:39 [4] as well as for SrNbO3:41 [8]: A dis-persing band is found only along one high-symmetry line,- X , parallel to the chain direction a, and seems to crossEF; a second band near EF does not show discernibledispersion. Neither band disperses along the transversedirection - Y . The Fermi surface (FS) is 1D [4], withnesting vector 2kF of about 1=3 of the Brillouin zone.Further insight is gained by band structure calculationsin the local density approximation [4,9] which demon-strate that among the three inequivalent Nb sites per slaba predominant [9] contribution to the density of occupiedstates at EF comes from the central Nb site; the contri-butions of the other two Nb sites rapidly fall off withdistance away from the center. Distortion is small only forthe NbO6 octahedra based on the central Nb sites, andone can thus clearly relate the quasi-1D character of thedispersing band to these chains of almost ‘‘perfect’’ oc-tahedra in the middle of the slabs. The importance of thecentral chains is also demonstrated by comparing ARPESresults for SrNbO3:41 with those for Sr0:8La0:2NbO3:50 [8],where the central chains are missing since there the slabsare only four octahedra wide: For the latter, the 1Dcharacter of the band structure has almost vanished.236403-2VOLUME 89, NUMBER 23 P H Y S I C A L R E V I E W L E T T E R S 2 DECEMBER 2002The quasi-1D FS with almost complete nesting makesSrNbO3:41 prone to instabilities opening a gap at EF, andwe thus concentrate on the region around kF in order tovery sensitively detect such signatures. Figure 3(a) showsthe corresponding high-resolution ARPES spectra along- X near EF. At   1:5, the peak associated withthe strongly dispersing band is located at  160 meVbinding energy and is rather broad, but sharpens up asit approaches EF and appears to cross it. We define kF(  0) as the emission direction for which the leadingedge midpoint of the spectrum is closest to EF. Becauseof a 2 1 superstructure [4], the peak reappears alreadyat kF  5:5 and disperses towards higher binding ener-gies for increasing . Figure 3(b) depicts the ARPESspectra around kF for a narrow angular step size of 0:25together with the Fermi cutoff of a Au film. Starting from0:5 and going to higher , the leading edge midpointapproaches EF but does not reach it for any of the spectra.At kF, the spectral weight at the Fermi level is signifi-cantly suppressed (by  40%) compared to the leadingedge midpoint, and we find a gap between the leadingedge midpoint and EF of 25 K  4 meV. Althoughthis ‘‘leading edge’’ method is known to underestimate0.3 0.2 0.1 0.0 -0.1 -0.20°-0.25°-0.5°-1°-1.5°0.25°0.5°1°2°3°4°5°5.5°6°6.25°6.5°7°kFΓ∆θ = X80 40 0 -40∆θ =  -0.5˚ -0.25˚ 0˚ 0.25˚ 0.5˚Photoemission IntensityBinding Energy (eV)Binding Energy (meV)(a)(b)SrNbO3.41(Au)Binding Energy (eV)kF(c)kF*kF  *0.3  0.2   0.1     0   FIG. 3. ARPES spectra of SrNbO3:41 along - X near kFat T  25 K. The angle  is the emission angle with respectto kF (  0). (a) Spectra between the two ‘‘crossing points’’(kF, kF) of the strongly dispersing band; peak positions aremarked by dots. (b) Spectra around kF in 0:25 steps. Agap   5 meV is seen when compared to the Fermi cutoffof a gold film (thin solid line). (c) Intensity plot of spectra,taken between kF and kF (step size 0:5) and normalized toconstant total intensity; a weak, dispersing shadow band can beidentified.236403-3the gap size somewhat for a peaklike structure, a fit (notshown) of the kF spectrum, similar to Ref. [10] andaccounting for the peak structure, gives only a slightlylarger result, 25 K  5 2 meV, corroborating theleading edge result. Possible extrinsic effects such ascharging or too coarse an angular step size can also beexcluded as a source of the gap [11]. The gap is thusclearly established and with a size of about 5 meV is, infact, the smallest one found in ARPES for any quasi-1Dcompound.The intensity plot of high-resolution ARPES spectra in0:5 steps and normalized for clarity to constant totalintensity is shown in Fig. 3(c). It indicates additionalstates between kF and kF, where the strongly dispersingband is unoccupied: Beyond kF a ‘‘shadow band’’ is vis-ible which disperses away from the gap edge and exhibitsa symmetry consistent with the 2 1 superstructure. Itssmall intensity is compatible with the small gap size [12].As shadow bands are rather weak in general a clearidentification in experiment has been possible only forvery few quasi-1D materials [12,13].The different experiments on SrNbO3:41 all yield clearevidence for an energy gap at EF along the conductingchains: a rise in the dc resistivity, a strong peak in theoptical conductivity, as well as a significant suppressionof spectral weight in ARPES. All results lead to charac-teristic energies of a few meV— agreeing well enough todemonstrate that the gap does exist and suggesting acommon origin. Some spread is present, as expected,due to the different nature of the excitation for the varioustechniques (the optical gap, e.g., is somewhat smaller,possibly due to excitonic effects).Most surprising is the gap’s extreme smallness, makingit unique among quasi-1D materials; the unusual proper-ties of the gap, however, turn out to defy easy explana-tion: (i) A lattice superstructure with sufficiently smalldisplacement amplitudes would, of course, be able toreproduce the gap and its small size. It cannot, however,plausibly explain why the gap should be situated right atEF, considering the large width (* 2 eV [4,9]) of thequasi-1D band. The instabilities of quasi-1D systems, onthe other hand, by their very nature cause a gap located atEF and thus are more likely candidates for an explanationof the gap properties observed for SrNbO3:41 — suchas (ii) a MH-type insulating state with a gap at EF dueto electron-electron interaction; for this, the filling levelof the quasi-1D band would have to be commensurate,1=3. This 1=3 filling, however, would require long-rangeinteraction and a TLL parameter K < 1=3 for a MH gapto open [14], and the very strong correlation implied bythe latter appears implausible for SrNbO3:41: Correlationeffects are generally small for 4d systems such asniobium oxides. The possibility of a MH gap is thusunlikely yet cannot be fully excluded based on ourdata as theoretical results for 1=3 filling are still scarce.(iii) A Peierls instability caused by electron-phononcoupling appears possible, too, leading to a CDW236403-3VOLUME 89, NUMBER 23 P H Y S I C A L R E V I E W L E T T E R S 2 DECEMBER 2002state for sufficiently low temperatures with a gap at EF[15,16]. The gap size is related to the mean-field (MF)transition temperature TMFP , and in the weak-couplinglimit 20  3:52kBTMFP ; from the ARPES gap225 K  10 meV, one can then estimate TMFP 40 K [17]. Only below this temperature the gap (orits precursor, the fluctuation-induced pseudogap) canexist; in this sense TMFP strictly limits the temperaturerange where any gap-related features can possibly beobserved. Yet, for SrNbO3:41 one clearly sees the up-turn in aT (Fig. 1) to start at  55 K, and the low-energy peak for Eka in optics (Fig. 2) is almost fullydeveloped already at 50 K, suggesting an on-set well above that temperature. A simple Peierlspicture therefore is also insufficient to provide an expla-nation for the observed small gap [18]. Inconsisten-cies between the experimental results and theoreticalmodels have been found for other quasi-1D systems aswell, such as the paradigmatic NbSe43I, TaSe42I,and K0:3MoO3. The latter two are CDW systems yetshow clear discrepancies to the Peierls picture (e.g.,ARPES line shape and Fermi cutoff above TP), whichwere attributed to correlation effects [2,19]. In contrast,quasi-1D SrNbO3:41 has no obvious Peierls transition,little tendency to correlation, and the relevant energyscale is drastically smaller. It seems to be a system withcharacteristics distinctly its own, thus expanding theparameter space where real 1D systems remain to be fullyunderstood.In summary, we consistently observe in ARPES, dcresistivity, and optics that SrNbO3:41 is a compoundwith 1D electronic characteristics around EF; as expectedfor such a system, instabilities lead to an energy gap at EF.Most surprising is its small gap size: only a few meV byall techniques and thus much smaller than for otherquasi-1D compounds. Further analysis shows the experi-mental findings to appear inconsistent with validityranges for both the Peierls and Mott-Hubbard picture.These aspects promise more general implications forthe understanding of real 1D systems.We thank B. Götz, G. Hammerl, A. Loidl, L. Perfetti,C. Rojas, C. Schneider, R. Schulz, I. Vobornik, J. Voit, andH. Winter for valuable help and fruitful discussions. Thiswork was supported by the DAAD, the BMBF (ProjectNo. 13N6918/1), and the DFG.2364*Permanent address: General Physics Institute, Academyof Sciences, Vavilov 38, Moscow 117942, Russia.[1] J. Voit, Rep. Prog. Phys. 58, 977 (1995).03-4[2] M. Grioni and J. Voit, in Electron Spectroscopies Appliedto Low-Dimensional Materials, edited by H. P. Hughesand H. I. Starnberg (Kluwer, Dordrecht, 2000).[3] F. Lichtenberg et al., Z. Phys. B 84, 369 (1991);T. Williams et al., J. Solid State Chem. 103, 375(1993).[4] C. A. Kuntscher et al., Phys. Rev. B 61, 1876 (2000).[5] S. Nanamatsu et al., J. Phys. Soc. Jpn. 30, 300 (1971);S. Nanamatsu, M. Kimura, and T. Kawamura, J. Phys.Soc. Jpn. 38, 817 (1975).[6] F. Lichtenberg et al., Prog. Solid State Chem. 29, 1(2001).[7] S. C. Abrahams et al., Acta Crystallogr. Sect. B 54, 399(1998).[8] C. A. Kuntscher et al. (to be published).[9] H. Winter, S. Schuppler, and C. A. Kuntscher, J. Phys.Condens. Matter 12, 1735 (2000).[10] N. P. Armitage et al., Phys. Rev. Lett. 86, 1126 (2001).[11] Charging: With a;b;c (25 K) from Fig. 1, estimates ofpossible charging effects induced by the total photo-emission current turn out to be 3 orders of magnitudesmaller than the gap size observed. Angular step size:Simulated ARPES spectra within the Fermi liquid modelfor a band with the experimental dispersion near kF andaccounting for the experimental angular and energybroadening show that (i) the gap indeed closes at kF,and that (ii) a step size of 0:25 (implying that the actualkF can be ’’missed‘‘ by 0:125 at most) is clearly suffi-cient to rule out that a gap with  > 1 meV could be dueto this effect.[12] J. Voit et al., Science 290, 501 (2000).[13] V. Vescoli et al., Phys. Rev. Lett. 84, 1272 (2000);J. Schäfer et al., ibid. 87, 196403 (2001).[14] H. J. Schulz, in Strongly Correlated Electronic Materials,edited by K. S. Bedell et al. (Addison-Wesley, Reading,MA, 1994).[15] R. Peierls, Quantum Theory of Solids (Oxford UniversityPress, London, 1955).[16] The fact that no signatures for a transition are obvious inresistivity (Fig. 1) or detectable in thermal expansion[P. Nagel, V. Pasler, and C. Meingast (unpublished)]may be due to the low number of electrons involved inthe 1D character of SrNbO3:41; it does not by itself meanthat a Peierls transition does not occur.[17] A BCS-type approximation for T was used for theestimate.[18] The structural affinity of SrNbO3:41 to ferroelectricSrNbO3:50 may be of importance: The central chainsresponsible for the 1D electronic character are coupledto a network of polarizable NbO6 octahedra structurallyequivalent to those giving rise to the high dielectricconstant in the ferroelectric, and a certain influence onthe electronic properties of the chains is conceivable.[19] L. Perfetti et al., Phys. Rev. Lett. 87, 216404 (2001).236403-4

OPUS - Augsburg University Publication Server

https://opus.bibliothek.uni-augsburg.de/opus4/files/42564/PhysRevLett.89.236403.pdf

Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain
  Compound SrNbO$_{3.41}$

Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound SrNbO $_{3.41}$

Abstract

Similar works

Full text

Available Versions

Crossref

Infoscience - École polytechnique fédérale de Lausanne

Online-Publikationserver Augsburg

OPUS Augsburg

OPUS - Augsburg University Publication Server

Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound SrNbO3.41_{3.41}3.41​

Abstract

Similar works

Full text

Available Versions

Crossref

Infoscience - École polytechnique fédérale de Lausanne

Online-Publikationserver Augsburg

OPUS Augsburg

OPUS - Augsburg University Publication Server

Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound SrNbO $_{3.41}$