Electronic and vibrational properties of low-dimensional perovskites Sr$_{1-y}$La$_y$NbO$_{3.5-x}$

0.97 MB·English

by  C. A. Kuntscher

Electronic and vibrational properties of low-dimensional perovskites Sr La NbO
1−y y 3.5−x
C. A. Kuntscher
Forschungszentrum Karlsruhe, Institut fu¨r Festk¨orperphysik, D-76021 Karlsruhe, Germany
and 1. Physikalisches Institut, Universit¨at Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany
5 S. Schuppler
0 Forschungszentrum Karlsruhe, Institut fu¨r Festk¨orperphysik, D-76021 Karlsruhe, Germany
0
2 P. Haas, B. Gorshunov,∗ and M. Dressel
n 1. Physikalisches Institut, Universit¨at Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany
a
J M. Grioni

4 Institut de Physique Appliqu´ee, Ecole Polytechnique F´ed´erale, CH-1015 Lausanne, Switzerland
1
F. Lichtenberg
]
Experimentalphysik VI, Institut fu¨r Physik, EKM, Universit¨at Augsburg, Universit¨atsstr. 1, D-86135 Augsburg, Germany
i
c (February 2, 2008)
s
-
By angle-resolved photoemission spectroscopy and polarization-dependent infrared reflectiv-
l
r ity measurements the electronic and vibrational properties of low-dimensional perovskites
t
m Sr1−yLayNbO3.5−x werestudiedalongdifferentcrystaldirections. Theelectronicbehaviorstrongly
depends on the oxygen and La content, including quasi-one-dimensional metallic and ferroelectric
.
at insulating behavior. An extremely small energy gap at the Fermi level is found for SrNbO3.41 and
m SrNbO3.45 along the conducting direction at low temperature, suggestive for a Peierls-type insta-
bility. Despite the similar nominal carrier density, for Sr0.8La0.2NbO3.50 the quasi-one-dimensional
-
d metallic character is suppressed and no gap opening is observed, which can be explained by differ-
n encesinthecrystalstructure. Electron-phononinteractionappearstoplayanimportantroleinthis
o series of compounds.
c
[

2 I. INTRODUCTION TABLEI. StudiedniobatesofthetypeSrnNbnO3n+2. For
v each composition the stacking sequence of the (n=5)- and
8 (n=4)-subunits per unit cell and the nominal number z of
8 The observation of quasi-one-dimensional (quasi-1D) Nb4d conduction electrons per formula unit are given.
5 metallic behavior of some members of the perovskite-
5 related series Sr1−yLayNbO3.5−x, with 0 x 0.1, raised stacking z
0 interest recently.1,2 It comprises compoun≤ds≤with a wide sequence
4 range of electronic properties,3 including a ferroelec- SrNbO3.41 -5-5-5-5- 0.18
0 tric insulator,semiconducting compounds, and quasi-1D SrNbO3.45 -4-5-4-5- 0.1
/
t metals. They have an orthorhombic, perovskite-related SrNbO3.50 -4-4-4-4- 0
a
m crystalstructurewith lattice parametersa 3.9˚A,b 5.5 Sr0.8La0.2NbO3.50 -4-4-4-4- 0.2
˚A, and c depending on the oxygen conten≈t,3–5 and≈be- Sr0.7Ba0.1La0.2NbO3.50 -4-4-4-4- 0.2
d- long to the homologousseries of the type SrnNbnO3n+2. Sr0.6Ca0.2La0.2NbO3.50 -4-4-4-4- 0.2
n The main building blocks of the structure are NbO6 oc-
o tahedra, which are grouped into slabs extending along
c
the (a,b) plane. The width of the slabs along c is de- depending on the position within the slab. Along the
:
v termined by the oxygen content and directly given by a axis the octahedra are connected continuously via the
Xi the parametern: While for quasi-1Dmetallic SrNbO3.40 apical oxygen sites forming 1D chains, as is illustrated
(n=5)the slabsarefiveoctahedrawide, ferroelectric,in- in Ref. 2. The oxygen content not only determines the
r
a sulatingSrNbO3.50(n=4)consistsoffouroctahedrawide crystal structure but also the number of Nb4d conduc-
slabs. For defined oxygen stoichiometries in the range tionelectrons. A seconddoping channelwhich, however,
0 x 0.1 the structure exhibits a well-ordered stacking does not affect the width and stacking sequence of the
≤ ≤
sequenceofthe (n=5)-and(n=4)-subunits,listedinTa- slabsis the substitution of divalentSr by a higher-valent
ble I for the investigated compounds.3,5,6 A schematic element like La. The nominal numbers of Nb4d conduc-
drawing of the idealized crystal structure of three rep- tion electrons per formula unit for the present niobates,
resentative SrNbO3.50−x compounds is shown in Fig. 1. derived within an ionic picture, are listed in Table I.
The real crystal structure exhibits a considerable distor- According to dc resistivity measurements, the elec-
tion of the NbO6 octahedra, with different magnitude tronicpropertiesofSrNbO3.41 andSr0.8La0.2NbO3.50 are
1
b top view of NbO octahedra a 1D antiferromagnetic spin-1/2 Heisenberg spin chain
6
were proposed.10
c =5 The conduction mechanism of the compounds
n
=4 SrNbO3.41, SrNbO3.45, and Sr0.8La0.2NbO3.50 was stud-
n ied by dielectric measurements in the radio-frequency
=4 range along the c direction.11 They suggest a hopping
n
5 transport of charge carriers with polaronic character
=5 n= along c. The results from EPR studies on SrNbO3.4110
n with the magnetic field applied parallel to the b and c
4
=
n axis are in agreement with a variable range hopping in
1D. Furthermore, 93Nb NMR measurements indicate a
4 =5 fluctuating quadrupole field and seem to resemble some
= n
4 n of the characteristics of the prototypical charge-density-
=
n wave (CDW) compound NbSe3.10
The most puzzling finding for the series
5 5 Sr1−yLayNbO3.50−x is an extremely small energy gap
4 = =
n= n n at the Fermi energy for SrNbO3.41 along the chain
direction.2 In this manuscript we will show that signa-
tures of a very small energy gap are also present for the
SrNbO SrNbO SrNbO
3.50 3.44 3.40 compound SrNbO3.45. In order to understand the ob-
FIG. 1. Schematic drawing of the idealized, i.e., servedquasi-1Dmetalliccharacterandtheunusualprop-
non-distorted, crystal structures of three compounds of the ertiesatlowtemperature,wegiveanoverviewoftheelec-
series SrnNbnO3n+2. The NbO6 units in the middle of the tronic and vibrational properties of various compounds
slabs, which presumably are the conducting channels of the of the series Sr1−yLayNbO3.5−x, as obtained by optical
lattice, are marked by thick lines.
spectroscopy in the infrared frequency range and angle-
resolved photoemission spectroscopy (ARPES). We will
discuss our findings for the various compounds with re-
highly anisotropic,withlowest,metal-likeresistivityval-
ues along the a direction.3 The temperature dependence spect to differences in the crystal structure. Based on
the comparisonofourresultsfromopticsandARPESto
of the resistivity along the a axis, ρ , consists of several
a
the dc resistivity data, we propose a possible picture for
regimes; taking SrNbO3.41 as an example, one observes
thetransportmechanismanddiscusstheroleofelectron-
the followingbehavior: Startingfromthe lowesttemper-
phonon coupling in these niobate compounds.
ature, the resistivity decreases strongly, showing a min-
The manuscript is organized as follows: The experi-
imum around 50 K and increases between 50 and 130
≈ mental details are given in Sec. II, followed by the pre-
K. Above 130 K ρ decreases slightly with increasing
a
≈ sentation of the experimental results in Sec. III. We dis-
temperature. The resistivity behavior below 50 K can
cusstheelectronicandvibrationalpropertiesoftheseries
be describedby anactivatedbehaviorρ exp[E /(k T)]
a B
withactivationenergiesE =2-3meV.2,3∝Anexplanation Sr1−yLayNbO3.5−x in Sec. IV and conclude with a sum-
a
mary of the findings in Sec. V.
for the temperature behavior of ρ above 50 K is so far
a
missing. A similar temperature dependence is observed
for Sr0.8La0.2NbO3.50.
II. EXPERIMENT
On the other hand, SrNbO3.50 (n=4) is a band insu-
lator. It is ferroelectric below T =1613K with a sponta-
c
neouspolarizationalongthebaxis.7,8Thecompoundun- The investigated SrNbO3.41, SrNbO3.45, SrNbO3.50,
dergoes two further phase transitions, namely a second- and(Sr,Ba,Ca)0.8La0.2NbO3.50singlecrystalswithatyp-
order phase transition to an incommensurate phase at ical size of 3 2 0.2 mm3 were grown by a floating zone
× ×
488 K and another ferroelectric transition at 117 K with melting process, and their oxygen content was deter-
anadditionalcomponentofthespontaneouspolarization minedbythermogravimetricanalysis.3ARPESdatawere
appearing along the c axis.9 recorded with a Scienta ESCA-300 analyzer. The He Iα
The magnetic properties of the investigated niobium (21.22 eV) line of a standard gas discharge lamp was
oxide compounds were described in Ref. 3. Except used. With a fixed analyzer setup the sample was ro-
SrNbO3.50,allcompoundsshowasimilarbehavior: With tated around one axis; the rotation angle θ determines
increasing temperature, between 4 and 50 K the mag- the emission direction of the photoelectrons relative to
netic susceptibility decreases steeply due to paramag- thesurfacenormalandthusthecomponentk ofthepho-
netic impurities, followed by an increase between 50 and toelectronmomentumalongthestudieddirectionaccord-
400 K. This increase is not yet understood; explanations ingtok=√2mEkinsinθ/¯h. Thecrystals,whichhadbeen
◦
in terms of thermally activated charge carriers excited oriented to 1 by Laue diffraction before introducing
±
across an energy gap or the Bonner-Fisher model for them into the UHV system, were cleavedat a base pres-
2
sure of 1×10−10 mbar and cooled down. ARPES spec- q q = q q =
tra with energy resolution ∆E=30 meV were recorded X
attemperatureT=75K.The samplesurfacequalitywas 20° 16°Y
checked periodically by comparison of valence band fea-
tures. The angular resolution of the ARPES spectra 16°
amounts to ∆θ= 0.5◦ corresponding to ∆k=0.04 ˚A−1. 12°
± 12°
TheFermienergyE wasdeterminedto1meVaccuracy
F 8°
from the Fermi cutoff of a freshly evaporated gold film
y 8°
recordedimmediatelyafterwardsatthesameexperimen- sit
tal conditions. nten 4° 4°
For the reflectivity measurements the samples were n I
carefullypolishedonapolishingdiscusingdiamondpaste sio 0°G G 0G°G
with a grain size of 1 µm to obtain flat and shiny sur- mis 1.2 0.8 0.4 0.0 -0.4
faces. Near-normal incidence reflection measurements otoe -4° BBinidnidnign Eg( nbEe)nrgeyr g(my e(eVV) )
with the electric field E of the incident light parallel to Ph -8°
theaandbaxeswereperformedforfrequencies20-10000
cm−1usingaFourier-transformspectrometerBrukerIFS -12°
113v. Theenergyresolutionwassetto1cm−1 inthefar-
infraredrangeandto2-4cm−1inthemid-infrared(MIR) -16°
and near-infraredrange. For SrNbO3.41, SrNbO3.45, and -20°
Sr0.8La0.2NbO3.50 the low frequency reflectivity (6-30 X
cm−1) was studied with a quasi-optical submillimeter 1.2 0.8 0.4 0.0 -0.4
(submm)spectrometer12usingbackward-waveoscillators Binding Energy (eV)
(a)
which produce monochromatic, polarized radiation con-
FIG. 2. ARPES spectra of SrNbO3.41, normalized to the
tinuously tunable in frequency. For SrNbO3.41 the re- sametotalintensity,along(a)Γ¯-X¯ and(b)Γ¯-Y¯ at75Kwith
flectivityspectrawereextendedto34000cm−1 byutiliz- ∆E=30 meV.
ing a Bruker IFS 66v/S spectrometer. The use of opti-
cal cryostates enabled temperature-dependent measure-
mentsbetween5and300K.Toobtainabsolutereflectiv- III. RESULTS
ities the spectra were divided by the reflectivity spectra
of an Al mirror. A. SrNbO3.41: ARPES, optics
To obtain the optical conductivity σ1(ν) by Kramers-
KroniganalysisthereflectivityspectraR(ν)ofSrNbO3.41 The k-dependent near-EF electronic structure, as
and SrNbO3.45 along the highly conducting direction studiedbyARPES,reflectsthestronglyanisotropicchar-
were extrapolated to zero frequency according to the acter of SrNbO3.41 (see Fig. 2): One observes two bands
Hagen-Rubens relation.13 A constant extrapolation to near E , with 600 and 200 meV binding energy
F
ν=0 was chosen for the perpendicular direction. For in- (BE) at the Γ¯ p≈oint. Along≈Γ¯-X¯ (Fig. 2a), i.e., along
sulating SrNbO3.50 the value R(0) for the constant low- the chains, the band at lower BE shows no dispersion,
frequency extrapolation was calculated from the dielec- whereas the secondband disperses stronglytowards E ,
F
tricconstantǫ1at1kHz(ǫ1,a=75,ǫ1,b=43)8accordingto gains intensity at around θ =8◦, and loses intensity be-
R(0)=[(1 √ǫ1)/(1+√ǫ1)]2. FortheSr0.2La0.8NbO3.50 tween θ =8◦ and θ =10◦|s|uggestive of a Fermi surface
spectra eith−er a Hagen-Rubens or a constant extrapola- (FS) cr|o|ssing. At|θ| 14◦ the peak reappears and dis-
tion was used, depending on the low-frequency behav- perses away from E| |±for higher θ . The reappearance
F
iorofthemeasuredreflectivityatdifferenttemperatures. of the peak can be explained in |te|rms of a 2 1 super-
Forthehigh-frequencyextrapolationofallspectraanν−4 structure which was also observed by low en×ergy elec-
decay was assumed. tron diffraction1,14 and neutron scattering,15 and seems
In the case of SrNbO3.45 the reflectivity data were to be a characteristic property of all members of the
squupepnlceymreanntgeed1b0y-2t0racnms−m1isosniona sminegalseucrreymsetanltsofinthtihckenferses- Sthre1−byaLnadyNarboOun3.d5−Γ¯xasnedriXe¯s.givTehseanpeaffraebctoilviceedleiscpterorsniomnaossf
≈20 µm using a Mach-Zehnder interferometer arrange- of m∗/m0=0.71. Along the perpendicular direction Γ¯-Y¯
ment. Fromthe independent determinationofthe trans- (Fig. 2b) both bands hardly show any dispersion.
mission coefficient and the phase shift we were able to The anisotropiccharacteris confirmedby the strongly
directly calculate the complex dielectric function ǫ(ν) by polarization-dependentreflectivityR(ν)showninFig.3.
utilizing the Fresnel equations.12 ItexhibitsaplasmaedgeforE aandhighvalues( 1)at
k ≈
lowfrequenciesindicatingmetallicbehavior. Incontrast,
for E b the considerably lower reflectivity demonstrates
k
theinsulatingcharacterperpendiculartothechains. The
3
10-3 10-2 10-1 100(eV) 10-3 10-2 10-1 100(eV)
1.0
SrNbO
0.8 1.000 5K 3.41 6000 30100K0K 550KK SrNbO
0.6 R 5K100K 300K 4000 3.41
0.4 0.975 -1m)
0.950 200K E||a -1c 2000 200K
ctivity R 000...028 030100K1 n (c30m0K-1) 102 E||b sWuctivity (1 6000 ((ab)) EE||||ab
e d
efl on
R 0.6 5K C 400

0.4
200
0.2 300K 5K
0
0.0 101 102 103 104
101 102 103 104 Frequency (cm-1)
Frequency (cm-1) FIG. 4. Optical conductivity σ1 of SrNbO3.41 at different
FIG. 3. Reflectivity R of SrNbO3.41 at different tempera- temperatures for Eka and Ekb, respectively. For Eka the
tures for Eka and Ekb, respectively. Inset: Low-frequency low-temperature peak around 40 cm−1 indicates excitations
reflectivityfor E||a; open (closed) circles: submm reflectivity across agap 2∆≈5meV.Alsoshown arethedcconductivity
data at 300 K (5 K). values for the a axis at 300 K (filled triangle), 200 K (open
square), 100 K (filled square), 50 K (open circle), and 5 K
(filled circle).
corresponding optical conductivity σ1 (Fig. 4) for E a
k
consists of a Drude-like component followed by phonon
modesandarelativelybroadMIRbandcenteredaround These findings confirmthe high-resolutionARPESdata,
1500 cm−1. A rich phonon spectrum is observed along which show a gap between the leading edge midpoint of
both directions; for E b 76 lines can be resolved at RT, the spectrum and EF of ∆(25 K) 4 meV for Γ¯-X¯.2 The
withthe strongestmodkesat144,296,304,561,572,577, gapopeningasobservedbyoptics≈andARPESalongthe
583, 595, 603, 612, and 680 cm−1. The strong double- chain direction is in agreement with the dc resistivity
peakstructureintheE bspectrumcenteredaround5000 data along the a direction, although there the upturn
cm−1 can be related tokinterband transitions. occurs at a lower temperature ( 50 K) than the devel-
≈
During coolingdownconsiderablechangescanbe seen opment of the low-frequency peak in the optical conduc-
in the electronic and vibrational part of the spectra: tivity (below 100 K), as was already pointed out in Ref.
For E b at least six new modes appear with decreasing 2. InFig.4wealsoincludedthedcconductivityresults;3
tempekrature. Along the perpendicular direction, E a, above50Kthedcvaluesareobviouslylowerthantheop-
a strong peak appears around 40 cm−1 in the optikcal tical conductivity values at 5 cm−1. This suggests that
conductivityspectrum(see Fig.4),whichstartsbuilding theopticalconductivityσ1 cannotbedescribedbyasim-
up below 100 K. With decreasing temperature the peak ple Drude-type behavior. Fitting the σ1(ω)-spectra with
shiftsslightlytohigherfrequenciesandbecomesstronger. the Drude-Lorentz model indeed shows that a fit of rea-
As discussed in Ref. 2 this feature can be ascribed to sonable quality (not shown) requires the introduction of
single-particle excitations across a gap in the electronic two Drude peaks, e.g., with σ1,1(ω 0)=2006 Ω−1cm−1
densityofstates(DOS)with2∆(5K) 5meV.Thedevel- and σ1,2(ω 0)=2356 Ω−1cm−1 at→300 K, which is in
opmentofthepeakisalreadyindicate≈dinthereflectivity good agreem→ent with the corresponding dc conductivity
at5Kbytheloweringandflatteningofthelow-frequency value.
partofthe spectrum(see insetofFig.3). The gapopen-
ing at low temperature strongly influences the dielectric
constantǫ1(ν 0): Assumingasimplepictureofasemi- B. SrNbO3.50: Optics
→
conductor with an energy gap h¯ω between the narrow
g
valenceandconductionband,therelationǫ1(ν =0)≈ωg−2 The reflectivity spectra of SrNbO3.50 (insets of Fig. 6)
is expected to hold.13 This explains the measured spec- are significantly different from those of SrNbO3.41. The
trum ǫ1(ν) of SrNbO3.41 for E a (Fig. 5), where at 5 overalllow reflectivity for both polarization directions is
K high positive values are foundk for ν 0. In contrast, in accordance with the insulating nature of the mate-
at 300 K ǫ1(ν 0) is negative as expe→cted for a metal. rial. The non-conducting character is also illustrated in
→
4
10-3 10-2 10-1 100(eV) 10-2 10-1 100(eV)
1500 1.0
10000 E||a E||a R 5 K
0.5 300 K
5000 SrNbO3.41 1000
1 0 SrNbO3.45 -1m) 5 K 0 102n (cm10-13) 104
ent -1c 500 SrNbO
a 300 K 3.50
ctric const -5010000 E||b Ws (uctivity 1 6000 E||b 1R .05 K
e d 0.5
el n
Di 50 Co 400 300 K
0.0
5 K 102 103 104
0 n (cm-1)
200
300 K
-50
-100 0
101 Freq10u2ency (cm10-13) 104 10F2 requency 1(0c3m-1) 104
atF3I0G0.K5.(tDhiieclkecltirniecs)coannsdta5ntKǫ1(tohfiSnrlNinbeOs)3.f4o1raEndkaSraNndbOE3k.4b5, KFfoIrGE.6k.aOanpdticEaklbc,ornedsupcetcitviivteylyσ.1IonfseStrsN: bROefl3.e5c0taivtit3y00Raunsded5
for Kramers-Kronig analysis.
respectively;open(closed)circles: datapointsat300K(5K)
directly determined by transmission measurements on a thin
platelet. properties between the two compounds: The E a reflec-
k
tivity R at RT is high and almost 1 at low frequencies
and shows a pseudo plasma edge, whereas for E b the
the optical conductivity spectra (Fig. 6) which do not
k
overall reflectivity is low and no plasma edge is found.
contain electronic excitations in the low-frequency range
(the onset of interband transitions is around 2000 cm−1 Correspondingly, for E a the optical conductivity con-
k
tains a Drude-like peak due to itinerant electrons which
for both polarizations) but only vibrational excitations.
Perpendicular to the chains, i.e., for E b, a rich phonon is superimposed by vibrational modes between 50 and
k 800 cm−1, followed by a pronounced MIR band cen-
spectrum is found. At RT one can resolve 24 infrared
activemodes forE b,withthe strongestaround54,100, tered around 2000 cm−1 with a vibrational fine struc-
147,176,275,295,k338,567,and601cm−1,and12modes ture. In contrast, for E b no Drude peak is observed in
k
for E a, the strongest being around 112, 173, 295, and σ1(ν)butalargenumberofvibrationalexcitationsupto
561cmk−1. Aqualitativeassignmentofthephononmodes 900cm−1 followedbyinterbandtransitionsabove 2000
is giveninthe discussion. The mode around54cm−1 for cm−1. Thus, also SrNbO3.45 shows a quasi-1D m≈etal-
E bis closeto the softmode foundby Ramanscattering lic characterat RT, with a metal-like behavior along the
exkperiments16 which is related to the ferroelectric tran- chains and an insulating behavior along the perpendicu-
lar direction.
sitionat1615K.Itisexpectedtobe infraredactive,and
The temperature dependence of the low-frequency op-
indeed was already observed in earlier reflectivity and
transmission studies.17 During cooling down additional tical properties along the chains agrees qualitatively
with the dc resistivity,19 being almost temperature-
modes appear in agreement with the results of Ref. 17,
independent between 300 K and 100 K and increases
andat5Ktherearealtogether24and51modesresolved
≈
for E a and E b, respectively. The number of observed below 100 K with decreasing temperature. Substantial
k k changes occur in the low-frequency ( 50 cm−1) optical
modes at RT and 5 K is lower than expected from the
≤
factor group analysis;17 the missing modes might be ei- response of SrNbO3.45 at low temperatures (<100 K):
One observes a lowering and flattening of the reflectiv-
ther masked by the other contributions or too weak to
ity (see inset of Fig. 7) which signals the development
be resolved.
of a new excitation. According to the optical conductiv-
ity (Fig. 7) between 0 and 400 cm−1 a redistribution of
spectral weight from low to high frequencies occurs for
C. SrNbO3.45: Optics
temperatures below 150 K, resulting in the development
of a relatively broad and asymmetric peak, which has
Previous ARPES measurements1,18 on SrNbO3.45 re- a maximum around 60 cm−1 ( 7 meV) and is superim-
vealedaquasi-1DbandstructurenearEF similartothat posedbyalargenumberofphon≈onexcitations. Thepeak
of SrNbO3.41. Also the optical response shown in Fig. 7 intensityincreaseswithdecreasingtemperature. Onecan
clearly demonstrates strong similarities in the electronic
5
10-3 10-2 10-1 100(eV) q q = q q =
2000 SErN||abO3.45 01R..50 300 K5 K 23°X 16°Y
1500
150 K 0101 102 103 104 19° 12°
-1m) 1000 n (cm-1) 15°
c
-1 500 300 K 8°
sWuctivity (1 8000 100 K 5 2K00 K 01R..50 E||b n Intensity 1731°°° 4°
ond 60 0 300K5 K ssio 0G°G 0G°G
C 400 101 n1 (0c2m -11)03 1004.0 otoemi -3° 1.2Bin0d.8ing E(0bn.4)ergy0 .(0eV)-0.4
h
P -7°
200
5 K
300 K -11°
0
101 102 103 104
-15°
Frequency (cm-1)
FIG. 7. Optical conductivity σ1 of SrNbO3.45 at differ- -19°X
ent temperatures for Eka and Ekb, respectively; closed cir-
1.2 0.8 0.4 0.0 -0.4
cles: data points directly determined by transmission mea- Binding Energy (eV)
surements on a thin platelet. For E||a the low-temperature (a)
ptaetaikonisnaσc1rowsisthaigtaspm2a∆xi≈m7ummeaVro.uInndse6t0s:cRme−fl1ecintidviictayteRseuxsceid- Γ¯-XF¯IGan.d8.(bA)RΓ¯-PY¯ESats7p5ecKtrwaitohf ∆SrE0.=8L3a00m.2NeVb.O3.50 along (a)
for Kramers-Kronig analysis; open (closed) circles: submm
reflectivity data at 300 K (5 K). θ 7◦ is even less pronounced.14 The results for the
| |≤
(Sr,Ba,Ca)0.8La0.2NbO3.50 suggest that there are two
ascribethedevelopmentofthelow-frequencypeaktothe near-EF bands with similar BEs: One band is almost
opening of an energy gap with decreasing temperature. dispersionlessalongbothΓ¯-X¯ andΓ¯-Y¯;the secondband
The presence of a small gap at low temperature is also hasaparabolic-likedispersion,althoughveryweak,along
indicated by the high values of the corresponding dielec- Γ¯-X¯ with an intensity drop near EF suggestive of a FS
tricconstantǫ1at5Kforν 0(Fig.5),aswasdiscussed crossing,andisdispersionlessalongΓ¯-Y¯. Tosomeextent
for SrNbO3.41 in section III→A. In summary, SrNbO3.45 the band dispersions of Sr0.8La0.2NbO3.50 thus resemble
rinegsembobtlhesathqeuealseic-1trDonmiceptarollpicercthieasraocftSerrNabnOd3t.4h1eboypsehnoinwg- SthroNsbeOo3f.4S1rtNhbeOd3i.s4p1er(ssieoenFailgo.n2g)Γ¯.-HX¯oiwsemveurc,hcwomeapkaerre,di.et.o,
the quasi-1D character of the band structure is less de-
of an energy gap of only a few meV size below 150 K.
veloped.
Further insight was gained by temperature-dependent
D. Sr0.8La0.2NbO3.50: ARPES, optics preofllaecritzivaittiyonmEeasbutrheme eonvtesrafollrloEwkar,ebfle(sceteivFitiyg.a9n)d. Fthoer tahbe-
k
TheelectronicpropertiesofSr0.8La0.2NbO3.50nearEF sence of a Drude component in σ1(ν) demonstrate the
insulating characterof the materialperpendicular to the
were studied by ARPES along the two high-symmetry
lines Γ¯-X¯ andΓ¯-Y¯; the results aredepictedin Fig.8. At chains. For Eka the reflectivity at RT is metal-like with
theΓ¯ pointweobserveabroadpeakaround350meVBE. relatively high values at low frequencies, decreases for
AlongΓ¯-X¯ thepeakslowlydispersestowardsE accom- higher frequencies, and finally drops rapidly above 6000
F ◦ cm−1. The corresponding optical conductivity shows
panied by an intensity increase and between θ =7 and
◦ | | a very weak Drude-like contribution and a pronounced
θ =9 the intensity of the peak slightly drops; at higher
| | MIR absorption peak around 3000 cm−1. Between 300
emission angles a broad peak is still visible with almost
constantBE( 350meV).AlongΓ¯-Y¯ nodispersionofthe K and 50 K the low-frequency optical conductivity is
≈ in reasonable agreement with the dc conductivity,3 but
broadpeakisobserved. Alongbothdirectionsandforall
at 5 K it is lower than the dc value. The overall op-
emission angles the overall photoelectron intensity near
E is quite low. We also studied the ARPES spectra tical response of Sr0.8La0.2NbO3.50 is similar to that of
F
for Sr0.7La0.2Ba0.1NbO3.50 and Sr0.6La0.2Ca0.2NbO3.50 SrNbO3.41 (see Fig. 4) and SrNbO3.45 (see Fig. 7), but
there are important differences: The overall reflectiv-
(not shown), having the same nominal carrier density
as Sr0.8La0.2NbO3.50 (see Table I): For these two com- ity of Sr0.8La0.2NbO3.50 along the chains is lower in the
pounds the dispersion of the broad peak along Γ¯-X¯ for far-infrared range and the plasma edge is hardly devel-
6
1 0-2 10-1 100(eV) 10-2 10-1 (eV)
1.0 4000
400 300 K R E||a SrNbO
5 K 0.5 E||a SrNbO3.41
300 3000 300 K SrNbO3.45
0.0 3.5
-1-1cm) 200 120100n K1(c0m2-31)0100 3K 104 -1m) 2000
W (1 100 -1c 1000
s Conductivity 2300 000 150 K1 11000320 K 05 K1 K03 100014R...050 SEr0|.|8bLa0.2NbO3.5 sWConductivity (1 4000 3E0||0b K* * * *
n (cm -1) 200
100 300 K 5 K
0 0
102 103 104 100 1000
Frequency (cm-1) Frequency (cm-1)
FIG. 9. Optical conductivity σ1 of Sr0.8La0.2NbO3.50 at FIG.10. Opticalconductivityσ1 ofSrNbO3.50,SrNbO3.45,
different temperatures for Eka and Ekb, respectively. Also and SrNbO3.41 at RT for Eka and Ekb, respectively. The
shown are the dc conductivity values for the a axis at 300 K asterisks mark the phonon lines of SrNbO3.50 for Eka which
(filled triangle), 200 K (open square), 100 K (filled square), survive in the conducting compounds. The arrow indicates
50K(opencircle),and5K(filledcircle). Insets: Reflectivity theferroelectric soft mode of SrNbO3.50.
R used for Kramers-Kronig analysis; open (closed) circles:
submm reflectivity data at 300 K (5 K).
caldensity approximation(LDA) for SrNbO3.41:1,20 The
predominant contribution to the DOS near E is at-
F
oped; both findings indicate only weak metallic charac- tributed to the Nb site in the middle of the slab, while
ter. Thus, the optical conductivity of Sr0.8La0.2NbO3.50 theothertwo21 Nbsitescontributeconsiderablyless(see
for E a contains a much weaker Drude-like term. Im- Fig.6(a)inRef.20). ThecorrespondingNbO6octahedra
k
portantly, with decreasing temperature the development in the middle of the slabs (marked by thick lines in Fig.
of a low-frequency peak is not observed, in contrast to 1)aretheleastdistortedones,withtheNbatomslocated
what is found for SrNbO3.41 and SrNbO3.45 (see Figs. 4 right in the center of the octahedra; for all other octa-
and 7). hedra the Nb atoms are considerably shifted away from
the central octahedral position. The quasi-1D metallic
character is thus related to the central chains of least
IV. DISCUSSION distorted octahedra in the middle of the slabs. The in-
fluence of the centraloctahedralchains onthe electronic
According to the experimental results, the electronic characteristics is illustrated by the experimental results
properties of the SrNbO3.50−x compounds strongly de- of Sr0.8La0.2NbO3.50 compared to those of SrNbO3.41:
pend on the oxygen content, which not only determines Both compounds nominally contain the same density of
the crystal structure, but also the number of conduction Nb4d conduction electrons (see Table I). However, in
electronsinthematerial(seeTableI). Toillustratethese Sr0.8La0.2NbO3.50theslabsareonlyfouroctahedrawide,
differences we plot in Fig. 10 the optical conductivity i.e., the central chains are missing. According to the
at RT for the three compositions SrNbO3.41, SrNbO3.45, resistivity3 and optical results presented here, this com-
and SrNbO3.50 for E a,b. A contribution of itinerant poundshowsamuchweakermetallic behaviorcompared
electrons appears in tkhe E a spectra: A Drude peak is to SrNbO3.41, and the quasi-1D character of its band
observed for SrNbO3.45, whkich is even more pronounced structure, as observed by ARPES, has almost vanished.
for SrNbO3.41 in full agreement with the higher density This is in agreement with the absence of a gap opening
of conduction electrons for the latter compound, while forSr0.8La0.2NbO3.50 alongthechains(seeFig.11): The
itthseinasbusleantcinegofchaaDrarcutdere.peak for SrNbO3.50 accounts for ionptteincasiltycoonfduthcteivwiteyakforDErudaehpaesakdeocbresaesrevdedatin5tKhe, bRuTt
k
Despite their layeredcrystal structure, SrNbO3.45 and a low-frequency peak has not developed, in contrast to
SrNbO3.41 show a pronounced quasi-1D metallic charac- whatisfoundforSrNbO3.41andSrNbO3.45. Alltheseex-
terwhichcanberelatedtothe chain-likearrangementof perimentalfindingsshowthatthequasi-1Dmetallicchar-
NbO6 octahedra along the a axis. A more detailed pic- acterandtherelatedgapopeningobservedforSrNbO3.41
ture is providedby bandstructure calculationsin the lo- andSrNbO3.45areindeedlinkedtothecentraloctahedral
7
10-2 10-1 100(eV) 10-2 10-1 (eV)
500
6000 E||a SrNbO
-1m) E||a, T S=r5N KbO 400 300 K Sr0.8La03..25NbO3.5
c 3.41
-1 4000 SrNbO3.45 300
Wy ( Sr0.8La0.2NbO3.50
ductivit 2000 -1m) 200 * *
Con -1c 100
0 102 103 104 Wvity ( 0 E||b
Frequency (cm-1) ucti 300 K
anFdISGr.0.181L.a0O.2pNtibcOal3c.5o0nadtuc5tiKvitfyorσE1koaf.SrNbO3.41,SrNbO3.45, Cond 200

chains.
ThecompoundsSrNbO3.50,SrNbO3.45,andSrNbO3.41
have a complex crystal structure and thus exhibit rich 0
100 1000
phonon spectra for both polarizationdirections (see Fig. Frequency (cm-1)
10). ForEkathemaingroupsofphononmodesobserved FIG. 12. Optical conductivity σ1 of SrNbO3.50 and
faonrdS5r4N0bcOm3−.510,awndhimcharakreedcbenytaesrteedriasrkosuinndF1ig1.01,01,7s0u,r3v0iv0e, Sasrt0e.8rLisak0s.2mNabrOk3t.5h0eapthRonTonforlinEeksaoafnSdrNEbkOb,3.r5e0spfoerctEivkealyw. hTihche
in the conducting members, with only small frequency surviveintheconductingcompound. Thearrowindicatesthe
shifts and a masking due to the contribution of itinerant ferroelectric soft mode of SrNbO3.50.
electrons. The similarities in the vibrational properties
are more obvious for the polarization direction perpen-
dicular to the chains, E b, since no Drude contribution mode was assigned to the soft mode of the ferroelectric
is present: The main grkoups of phonon lines, centered phase transition at 1615 K, based on Raman scattering
around 53, 100, 150, 290, and 600 cm−1, are present in experiments.16Itwasalsoobservedinrecentopticalmea-
all three compounds. A qualitative assignment to the surements and related to NbO6 octahedra tiltings.17 In
lattice dynamics can be achieved by a comparison with agreement with Ref. 17, this soft mode is temperature
other niobium oxides with a similar network of corner- independent between 300 and100 K and shifts to higher
sharing NbO6 octahedra: The high-frequency infrared- frequenciesbelow100K.Theprecisetemperaturedepen-
active modes of KNbO3 (NaNbO3) were assigned to the dence is, however,difficult to determine due to the over-
internalmodesofNbO6 octahedra,namelythe660(675) lap with another mode which appears around 50 cm−1
and 550 (510) cm−1 modes to the stretching modes and at 100 K during cooling down.
≈
the 375 (375) cm−1 mode to the bending mode.22 This Finally, we want to discuss the puzzling electronic
is in agreement with later Raman and infrared mea- propertiesofthe conductingmembersofthe presentnio-
surements on LiNbO3 and Ba2NaNb5O15.23 The lower- bate series at low temperature, in particular the open-
frequency lines (ν<200 cm−1) were assigned to transla- ing of an energy gap at EF found for SrNbO3.41 and
tional modes of the cations.23 The influence of La dop- SrNbO3.45. Several aspects make an explanation of the
ing on the optical properties is illustrated in Fig. 12 gap opening in terms of a Peierls-type scenario most
whereweplotthe conductivityspectraofSrNbO3.50 and likely: (i) The FS nesting1 could drive the lattice to-
Sr0.8La0.2NbO3.50. ForE aanelectronicbackgroundre- wardsaPeierlsdistortion;(ii)theshadowbandobserved
lated to itinerant carrierskin the Sr0.8La0.2NbO3.50 spec- for SrNbO3.41 in ARPES2 would be consistent with a
trumisvisible. Twoofthemaingroupsofphononmodes superstructure with wave vector q=2kF;24 (iii) the ap-
of SrNbO3.50 (marked by asterisks) are also present in pearance of phonon lines with decreasing temperature
the (weakly) conducting material, but they are broader suggests a structural phase transition such as a Peierls
andstronglydamped. Interestingly,therelativelystrong instability. However, there remain some inconsistencies
modes observed in SrNbO3.50 around 110 and 170 cm−1 withinaPeierlsscenario: Incontrasttowhatisexpected
appeartobecompletelysuppressedinSr0.8La0.2NbO3.50, for CDW formation,no sharpanomalyis found indc re-
indicating a strong damping of the cation translational sistivity; neither do thermal expansion measurements25
modes by partial substitution of Sr by La. Along the b show indications of a structural phase transition. Fur-
directionthesimilaritiesinthevibrationalpropertiesare thermore, an estimate of the mean-field transition tem-
alsoobvious. Inparticular,themodearound54cm−1 for peraturebasedontheobservedgapsizeisnotcompatible
E b, indicated by an arrow in Fig. 10 and 12 is present with the temperature development of dc resistivity and
inkallfourcompoundsdiscussedhere. ForSrNbO3.50 this optical conductivity.2
8
Further open issues are the transport mechanism in V. SUMMARY
SrNbO3.41 and SrNbO3.45 along the chain direction and
the closely related temperature dependence of the dc re- By angle-resolved photoemission spectroscopy and
sistivity ρa, showing different regimes. The strong up- polarization-dependent reflectivity measurements in the
turn of ρa below 50 K with cooling down can be ex- infraredwestudiedtheelectronicandvibrationalproper-
plained by the opening of a gap, in agreement with ties of severalmembers of the series Sr1−yLayNbO3.50−x
ARPES and optics. The temperature dependence above (0 x 0.1) along and perpendicular to the chains of
50 K may be due to contributions of quasiparticles with Nb≤O6≤octahedra. SrNbO3.50 is a ferroelectric insulator
different characteristics. While the metallic tempera- with a rich phonon spectrum for both polarization di-
ture dependence between 50 K and 130 K suggests a rections E a,b. At room temperature the compounds
band-like motion of electrons, above 130 K the hop- SrNbO3.41kand SrNbO3.45 exhibit a metallic behavior
ping of more localized charge carriers appears to domi- with a strongly dispersing band along the chains (E a)
nate. Apossiblemechanismforthelocalization/trapping andaninsulating,nondispersivecharacterintheperpken-
of charge carriers could be strong electron-phonon cou- dicular direction (E b); they are thus quasi-1D metals.
pling, which might also explain the observationof a pro- Despite the similarckarrierdensity,for Sr0.8La0.2NbO3.50
nouncedMIRabsorptionbandintheopticalconductivity the quasi-1D metallic character is much less developed.
alongthe chaindirection(see Figs.4 and7). Similar ab- Based on a comparison between the experimental re-
sorption bands in related quasi-1D metallic compounds sults of SrNbO3.41, SrNbO3.45, and Sr0.8La0.2NbO3.50
were recently interpreted in terms of excitations of pola- the quasi-1D metallic character observed in this series
ronic quasiparticles.26,27 Further spectroscopic informa- canbe relatedto the centralchains ofleastdistorted oc-
tion,e.g.,fromtemperature-dependentARPESmeasure- tahedra located in the middle of the slabs. This is in
ments, is needed to clarify a possible polaron formation fullagreementwiththeresultsfromLDAbandstructure
in the studied niobates. calculations. We propose a possible transport mecha-
In conclusion, we determined the electronic and vi- nism for the compounds SrNbO3.41 and SrNbO3.45, with
brationalpropertiesoftheperovskite-relatedcompounds a band-like contribution of mobile carriers and a con-
Sr1−yLayNbO3.5−xandrelatedtheresultstotheircrystal tribution due to the hopping motion of more localized
structure. Apossible transportmechanismof SrNbO3.41 charges dominating above 130 K. At low temperature
and SrNbO3.45 was discussed and the importance of the opening of an extremely small energy gap at EF is
electron-phonon coupling in these systems suggested, observed for SrNbO3.41 and SrNbO3.45 along the chain
possibly leading to polaron formation. To explain the direction, which might be related to a Peierls-type in-
low-temperature behavior a Peierls picture appears to stability, with electron-phonon coupling as the driving
be the most appropriate, but is insufficient to describe mechanism. However, not all findings can be explained
all the findings. An important aspect might be the withinaPeierlspicture,andwespeculatethatthehighly
closestructuralaffinityofthequasi-1DmetalsSrNbO3.41 polarizable structural units surrounding the conducting
andSrNbO3.45 totheferroelectricSrNbO3.50: SrNbO3.41 channels might have an appreciable influence.
(and the (n=5)-slabs in SrNbO3.45) contains the same
structural building blocks as SrNbO3.50, but with addi-
tionalchainsofbasicallyundistortedNbO6 octahedrain- Acknowledgements
sertedinthe center ofthe slabs. FerroelectricSrNbO3.50
shows relatively high values of the static dielectric con-
We are grateful to L. Perfetti, C. Rojas, and I.
stantǫ1atRT(ǫ1,a=75,ǫ1b=43,andǫ1,c=46)8indicating Vobornik for valuable technical help. We thank H. Win-
highly polarizable structural units. A further important
ter for fruitful discussions. Financial support by the
resultis the observationofthe phononmode at54 cm−1
DAAD, the BMBF (project No. 13N6918/1), and the
in the E b conductivity spectra of all present niobate
k Deutsche Forschungsgemeinschaft is gratefully acknowl-
compounds (see Fig. 10 and 12), which for SrNbO3.50 edged.
can be assigned to the ferroelectric soft mode. Since
the soft modes in ferroelectrics in general lead to elec-
trical polarization, the presence of the 54 cm−1 mode
in SrNbO3.41 and SrNbO3.45 indicates a polarizability of
their lattices. The highly conducting channels of octa-
hedra in the middle of the slabs, which are responsible
for the 1Dmetallic transport,canthus be viewedas em- ∗
Present address: General Physics Institute, Russian
bedded in a highly polarizable medium, and a certain Academy of Sciences, Vavilov 38, Moscow 117942, Russia.
influence of this medium on the electronic properties of 1C. A. Kuntscher, S. Gerhold, N. Nu¨cker, T. R. Cummins,
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10