Doping Effects on the Charge-Density-Wave Dynamics in Blue Bronze

0.34 MB·English

by  S. Yue

EPJ manuscript No.
(will be inserted by the editor)
5
0
0
Doping Effects on the Charge-Density-Wave Dynamics
2

n in Blue Bronze
a
J
S. Yue1, C. A. Kuntscher1a, M. Dressel1, S. van Smaalen2, F. Ritter3, and W. Assmus3
4
1 1 1. Physikalisches Institut,Universit¨at Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart,Germany
2 Lehrstuhlfu¨r Kristallographie, Universit¨at Bayreuth, 95440 Bayreuth,Germany
]
i 3 Physikalisches Institut,Universit¨at Frankfurt,60054 Frankfurt,Germany
c
s
- Received: February 2, 2008
l
r
t
m Abstract. The temperature dependences of the dc resistivity and the nonlinear transport properties in
. pure, Rb-doped, and W-doped blue bronze K0.3MoO3 single crystals are presented. In comparison with
t the Rb doping, the W doping has larger effects on the electrical transport properties and the Peierls
a
transition. Inparticular,themaximum inthetemperaturedependenceof thethresholdfieldfor nonlinear
m
transport,observedinpureandRb-dopedsamplesaround100K,isabsentinW-dopedK0.3MoO3.These
- resultsarediscussedwithrespecttotheproposedincommensurate-commensuratetransitionofthecharge-
d
density-waveand its interaction with impurities and normal carriers.
n
o
c PACS. 71.45.Lr Charge-density-wavesystems – 71.30.+h Metal-insulator transition and otherelectronic
[ transitions

1
v 1 Introduction CDW is depinned, it begins to slide through the crystal
2
andcontributesadditionallytotheconductivity,resulting
3
3 Charge-density-waves (CDW) in low-dimensional solids in a nonlinear current-voltage (I −V) response [8]. Be-
1 attract interest since R. Peierls introduced this broken- low approximately T = 40 K for K0.3MoO3 a more com-
0 plicated I −V response is observed, including a second
symmetry ground state half a century ago [1]. Their dy-
5 threshold field E∗ above which a sharp increase of the
namics has been subject to numerous investigations as T
0 current by many orders of magnitude occurs [9,10,11].
soonas the firstexperimentalrealizationbecame possible
/
t inthe1970s.Thepresentstateofknowledgeiscompiledin The temperature dependence of the first threshold field
a
E is commonly explained by a sort of two-fluid model
m severalreviews[2,3,4,5,6,7].BelowthePeierlstransitions, T
containing the electrons condensed in the CDW and the
anenergygapopensattheFermienergyleadingtoanin-
- remaining normal electrons [12]; the approach describes
d sulating ground state. In the CDW phase the electronic
the Coulomb interaction between the CDW and uncon-
n transporthascontributionsfromthe normalchargecarri-
densedchargecarriersthermallyexcitedabovethePeierls
o ersexcitedacrossthegap(likeasemiconductor)andfrom
c gap. Within this model, with rising temperature the un-
the collectiveresponseofthe CDW condensate.Although
: condensedcarrierdensityincreases,leadingto anincreas-
v there exist a general idea about the main phenomena, a
ing damping of the CDW by dissipative normal currents
i largenumberofobservationsarestillpuzzlingandthede-
X andthustoanincreaseofE .However,whenthetemper-
tails are not understood in depth. One of the key issues T
r oftheCDWdynamicsistheinteractionofthecondensate aturerisesaboveT =100KadecreaseofET isobserved,
a resulting in a maximum of threshold field E (T) around
with the underlying lattice and in particular with lattice T
this temperature [8]. Similar observations were reported
imperfections.
for the CDW compound TaS3 [13]. The change in the
Molybdenum bronze A0.3MoO3, where A = K, Rb temperaturedependence ofE (T)mightbe attributedto
or Tl, is among the most-extensively studied quasi-one- T
an incommensurate-commensurate (I-C) transition of the
dimensional compounds, which develops a CDW along
CDW which happens around T = 100 K for blue bronze.
the b axis below approximately 180 K [5]. In contrast to
However,smalldeviations fromcomplete commensurabil-
ideal systems the CDW condensate cannot move freely,
ity were found down to 4 K [14,15,16,17] which somehow
but is pinned to the lattice by impurities and defects.
disturbs this picture. Interestingly, in comparison with
This is easily seen, for instance, if an electric field is ap-
plied along the chain direction exceeding a critical value, K0.3MoO3 and TaS3, for NbSe3 with two CDWs, which
are always incommensurate with the underlying lattice,
theso-calledfirstthresholdfieldE ;asaconsequencethe
T a very different temperature dependence of E (T) was
T
a email: [email protected] found[18]:ThethresholdfielddivergesatthePeierlstran-
2 S. Yueet al.: Doping Effects on the Charge-Density-WaveDynamics in Blue Bronze

sition temperature TP and exhibits a minimum slightly 102
5
below T . This behavior could be well described by the
P
Fukuyama-Lee-RicemodelforimpuritypinningoftheCDW
K ) 4
[19,20] taking into account thermal fluctuations of the 0
0
tChDeWmapxhiamsuem[21i]n.FEoTr(KT0).3nMeaorOT3 d=iffe1r0e0ntKg,robuuptsiotsbsoerrivgeind 00 K ) rrrrT ) / ( 3 23
remTaihnes paepcuuzlizalrei.ties of the CDW motion may be clar- ( 3 101 rrrr ( 1
ified by studying the doping dependence of ET(T). In rrrrT ) / 140 160 180 200 220
bluebronzetwotypesofsubstitutionarepossible,namely ( T ( K )
replacing K by isoelectronic Rb and the substitution of rrrr
Mo by isoelectronic W. The two doping channels have a
verydifferenteffectonthetransportproperties[22,23].In 100
blue bronze the oxygen octahedrons with the transition
metal in the center form conducting chains along which 100 150 200 250 300
the CDW develops. Obviously W substitution strongly T ( K )
disturbstheone-dimensionalconductionandthedevelop-
ment of the CDW, hence already a very small W concen- Fig. 1. Dc resistivity along the b axis, normalized to
trationcausesalargeshiftofthePeierlstransitiontolower the room temperature value, for Rb0.3MoO3 (black
temperature and a considerable broadening of the phase short dot), (K0.5Rb0.5)0.3MoO3 (blue short dash),
transition. For this so-called strong pinning the phase of (K0.833Rb0.167)0.3MoO3 (green dash dot dot), K0.3MoO3
the CDW is adjusted at each impurity site [19]. In con- (red dash dot), K0.3Mo0.995W0.005O3 (cyan dot) and
trast, the Rb dopants occupying the K sublattice only K0.3Mo0.99W0.01O3 (purple dash). The inset shows the
enlargement of the results near the Peierls transition temper-
causesomedisorderinthe crystalpotentialandthus only
atureTp.
play a minor role in the pinning mechanism; it is called
weak pinning [19]. Measurements of ET(T) of K0.3MoO3
with 50% Rb doping for temperatures below 80 K show
Fig. 1 shows the temperature dependence of the dc
a similar behavior as in the pure compound [24]. How-
resistivity along the chain direction b normalized to the
ever, the temperature dependence of the first threshold
room temperature value for a series of pure and doped
field for other Rb doping levels, and in particular for W-
blue bronze crystals. The pure samples K0.3MoO3 and
doped blue bronze compounds has not been measured up
Rb0.3MoO3 show a sharp metal-semiconductor transition
to now. Here we presenta comprehensive study of E (T)
T atthe PeierlstransitiontemperatureT . The mixedcom-
p
foraseriesofRb-aswellasofW-dopedbluebronzecom-
pounds with 16.7%and 50%substitution of K by Rb still
pounds and discuss the origin of the maximum threshold
exhibit a clear transition T which is slightly shifted to
p
observed around T =100 K.
lower temperatures. In contrast, for samples with small
Wdoping(0.5%,1%,and2%,whoseresultsareverysim-
ilar to those of 1% doping and therefore not shown), the
2 Experiments and Results phase transition is significantly broadened. These results
are consistent with previous reports [22].
To determine the Peierls transition temperature more
Pure,Rb-doped,andW-dopedbluebronzeK0.3MoO3sin- precisely, we plot the derivative D(T) = −∂lnσ/∂T−1
gle crystals (see Table 1) were prepared by the temper-
versustemperatureinFig.2.Exceptforthe sampleswith
ature gradient flux method [25] and electrolytic reduc-
1%and2%(notshown)Wdoping,allcurvesclearlyshow
tion of the molten salts of A2CO3-MoO3 for pure blue
a peak around T ; the width ∆T is a measure of the
bronzeA0.3MoO3(A=KandRb),K2CO3-Rb2CO3-MoO3 sharpness of the tPransition [27]. BoPth the Rb and the W
for Rb-doped K0.3MoO3, and K2CO3-MoO3-WO3 for W-
doped K0.3MoO3. Different concentrationsof the dopants
in the melt determine the doping levels in the obtained
Table 1. List of the studied blue bronze samples with the
crystals. For Rb-doped samples, the doping ratio is al-
most the same as that in the melt, but for the W-doped Peierls transition temperature Tp and thewidth of thetransi-
compounds, the doping ratio is nearly 2 to 3 times bigger tion∆TP [fullwidthathalfmaximumofthederivativeD(T)].
tahtaynpitchaaltsiiznetohfe4m×e4l×t [02.35,m26m].3T.hIneooubrtadinceedlescatmricpallestrhaanvse- Sample Tp (K) ∆TP (K)
port measurements a four-probe configuration was used. Rb0.3MoO3 180.5 7.3
Thingoldwireswereanchoredbysilverpaint.Inthe con- (K0.5Rb0.5)0.3MoO3 170.7 36.3
tact regions, indium films were covered by gold films for (K0.833Rb0.167)0.3MoO3 175.1 18.5
stable contacting at low temperature. In this way, a typ- K0.3MoO3 179.2 11.2
ical contact resistance of around 1 Ω was achieved. For K0.3Mo0.995W0.005O3 160.7 > 50
samples with the same doping level but grown by differ- K0.3Mo0.99W0.01O3 ≈140 —
ent preparationmethods we obtained similar results. K0.3Mo0.98W0.02O3 ≈140 —
S. Yueet al.: Doping Effects on the Charge-Density-WaveDynamics in Blue Bronze 3

0.30
180 K
0.25 100 160 1K40 K (a)
) 120 K
0.20 -1m 100 K
D(T) ( eV ) 00..1105 -1WWWW ( c10-1 90CLcl 89CKck88CJcj87CIc8i6CH8ch5CG8c4gC8F3cCf8E2cC8eD1cC8d0C7Cc9cB7Cc8bA7Bc77ZaB6b7YzB5b7BXy47bWB3x7bVB2w7bUB17vbB6T0uB6b9SBb6t8R6sB7Qb6B6brP6B5bqO6B4bNp63BbM6o2Bb6nL1B5mK0b5BbB9J5lB85kbIH7B5b6jGb5B55BiFbh45BEb35gDBb52BCfb41AeBb40AdA4b9AA4cZ8b4Y7baA46XaWAa45z4aA4aV4Ay3UA4xa23AwTa13SA03vRAa9a3uQA83AaP73taOsA63aNA53raM43qAa33apLA22AKoA21nJ20maAa29AH2IA8aG2A7lkaa2F6A2a5EA2jD42ahiC3a12g1aBZ11AYaf01WXea91da81V71cU16bzT51aS1y4R3xQw29P1Ov80u7NM6t5s4LK3rq2Jp1HoIGmnFEDlkCBjAhigefdcba 80 K
ssss 10-2 70 K
K MoO
0.05 0.3 3 66 K
58 K
0.00
10-3 10-2 10-1 100 101

120 140 160 180 200 220 E (V/cm )
T ( K )
170 K
Fig. 2. The derivative D = −∂lnσ/∂T−1 ver- 100 150 K (b)
130 K
sus temperature for Rb0.3MoO3 (black full circles),
110 K
K(fu0.l3lMogOre3en (ospqeunaresr)e,d (Kci0r.c5lResb),0.5)0(.K3M0.8o3O3R3b0.(1o6p7)e0n.3MbolOu3e 1 ) 10-1 90 K
squares), K0.3Mo0.995W0.005O3 (full cyan triangles), and -m 70 K
K0.3Mo0.99W0.01O3 (open purple triangles). 1 c10-2 50 K
-
WWWW ( 10-3
doping lead to a shift of the transition to lower temper- K Mo W O
ssss
0.3 0.995 0.005 3
ature and a broadening of the transition (see Table 1);
30 K
however, these effects are considerably bigger in the case 10-4
of W substitution. 10-4 10-3 10-2 10-1 100
Wealsostudiedthedcconductivityofpureanddoped
E (V/cm )
bluebronzesinglecrystalsalongthechaindirectionbasa
function of the applied electrical field. Two examples are
Fig. 3. Dc conductivity along the b axis as a function
displayed in Fig. 3. For temperatures below T the con-
p of applied electric field for several temperatures below the
ductivity increases rapidly when the applied electric field
Peierls transition temperature Tp for (a) K0.3MoO3 and (b)
exceedsthethresholdfieldE duetothedepinningofthe
T K0.3Mo0.995W0.005O3. The dashed lines are guides to the eye
CDW.ThevaluesofE weredeterminedgraphicallyfrom
T for thevariation of thethreshold field with temperature.
the I −V characteristics. Fig. 4 summarizes the temper-
ature dependences of the threshold field for blue bronze
samples with different substitutions as indicated. the threshold field changed completely: E (T) increases
T
monotonously with decreasing temperature, and no max-
imum is observed within our experimental temperature
3 Discussion range.
The non-monotonous temperature behavior of E (T)
T
The main results of our experiments can be summarized observed for the CDW phase in blue bronze is puzzling.
asfollows:(i)StartingfromthePeierlstransitionatT = ThemostcommonexplanationistheI-Ctransitionofthe
P
180K,forpureK0.3MoO3andRb0.3MoO3compoundsET CDW around T = 100 K. However, several observations
firstincreasesuponcoolingandthenstartstodecrease,re- contradictthishypothesis:IftheCDWbecomescommen-
sulting in a maximum in the behavior of E (T) around surate with the underlying lattice for temperatures below
T
T =100K;thisagreeswithpreviousdata[8].(ii)Thesub- T =100 K, it should be strongly pinned and thus E (T)
T
stitution of K by Rb and of Mo by W does not influence is expected to increase below T = 100 K, in contrast to
theabsolutevalueofE significantly;infact,fortempera- the experimental findings. Furthermore, substituting Mo
T
turesabove≈50KtheE valuesfortheW-dopedsamples byWaddspinningcenters,butdoesnotchangetheCDW
T
are even slightly smaller than those for the pure samples, nesting vector and its temperature dependence [14]; nev-
as found previously [24]. It is known that the absolute erthelessforW-dopedcompoundsthemaximuminE (T)
T
valueofE dependsonthepropertiesofthesample,such is absent.
T
as impurity concentration, cross section, and current dis- Certainly, there is no abrupt transition from an in-
tribution. (iii) For all the Rb-doped samples the thresh- commensurate to a commensurate CDW, but a contin-
old field E (T) goes through a maximum value around uous adjustment of the CDW to the underlying lattice;
T
T = 100 K when the temperature is lowered, like for the maybe this processsaturates aroundT =100K [14]. It is
pure compound. However, even for very small W substi- known that down to T = 4 K small deviations from per-
tution on the Mo sites, the temperature dependence of fectcommensurationremain[15,16,17].Resultsfromelec-
4 S. Yueet al.: Doping Effects on the Charge-Density-WaveDynamics in Blue Bronze

0.5
(a)
Weak pinning
0.4
)
m
0.3
c T
V/ E I-C transition of the CDW
( T 0.2
E
0.1

0.0 CDW damping by norm al currents

(b)
1.2
Strong pinning
)
m
c Screening of impurity potential
V/ 0.8
E ( T ET
I-C transition of the CDW
0.4
0.0
20 40 60 80 100 120 140 160 180
T ( K ) CDW damping by normal currents
30 60 90 120 150 180
Fig. 4. Temperature dependence of the threshold field
ET along the b axis for (a) Rb0.3MoO3 (full cir- T ( K )
cles), K0.3MoO3 (open circles), (K0.833Rb0.167)0.3MoO3 (full
squares), and (K0.5Rb0.5)0.3MoO3 (open squares), and for (b) Fig. 5. Scheme of the various mechanisms relevant for the
K0.3Mo0.995W0.005O3 (full triangles) and K0.3Mo0.99W0.01O3 temperaturedependenceofthethresholdfield,ET(T),forweak
(open triangles). pinning(top) and strong pinning(bottom).
tron spin resonance experiments indicate the spatial co-
existence of commensurate and incommensurate regions; of the CDW [19]. For strong pinning the CDW distorts
when lowering the temperature from T = 53 K to 25 K its phase and amplitude locally at each impurity site; the
the commensurate fraction increases from 5% to a value CDWcoherencelengthisthereforeonlyontheorderofthe
above20%[17].Itwasproposed[28]thateventhe incom- impurity spacing. In the case of weak pinning, the CDW
mensurate CDW consists of regions which are essentially adjusts its overall phase distribution within its coherence
commensurate with the lattice separated by discommen- length, which is much larger than the impurity spacing.
surations;theselatterarenarrowregionswherethe phase While the substitution of K by Rb pins the CDW only
or displacement changes rapidly. At low temperature the weakly,Wdopingcausesstrongpinning ofthe CDW[22].
CDW will then not move as a rigid object but propagate The last two mechanisms will be strongly influenced
bydepinneddiscommensurations,likethemotionofgrain by uncondensed electrons. According to Sneddon’s model
boundaries and dislocations in the case of plastic defor- [12]thepresenceofuncondensedchargecarrierswillcause
mation. a damping of the CDW mode, since dissipative normal
TherearedifferenteffectsrelevantfortheCDWtrans- currents are induced by the dynamic deformations of the
port, which might also influence the temperature depen- CDW. This is in agreement with previous investigations
dence of the threshold field: (i) The CDW can interact ofthenonlinearcharacteristicsofpurebluebronze,where
with the underlying lattice in very different ways. Since a strong influence of the single-particle conductivity on
fora commensurateCDW the interactionwith the lattice the collectiveCDW responsehadbeenfound[29]. Within
is strong, mainly incommensurate domains of the CDW this picture E (T) increases with rising temperature, as
T
contribute to the collective transport. (ii) The CDW is the fraction of uncondensed carriers increases. Also the
not rigid,but subject to internal deformations,leading to strength of the interaction of the CDW with lattice im-
a non-zero threshold field for nonlinear transport. These perfections, like impurities or disorder, is influenced by
dynamic deformations of the CDW and dissipative nor- the presence of normal charge carriers. This is in partic-
mal currents need to be taken into account in order to ular important in the case of strong pinning, since un-
explainE (T).(iii)Theinteractionofthe CDWwithim- condensedcarrierssmoothentheCDWphasedeformation
T
purities ordisordereither leadsto strongorweakpinning induced by the strong pinning centers.
S. Yueet al.: Doping Effects on the Charge-Density-WaveDynamics in Blue Bronze 5
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T
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leading to a decrease of E (T). On the other hand, the
T
11. S. Martin, R. M. Fleming, and L. F. Schneemeyer, Phys.
screening of the impurity potential by uncondensed car-
Rev.B 38, 5733 (1988).
riers will also depend on their density and thus on tem-
12. L. Sneddon,Phys.Rev. B 29, 719 (1984).
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Phys. Rev.B 31, 899 (1985).
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T 15. M.Sato,H.Fujishita,andS.Hoshino,J.Phys.C16,L877
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20. H.FukuyamaandP.A.Lee,Phys.Rev.B17,535(1978).
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21. K. Maki and A. Virosztek, Phys. Rev.B 39, 9640 (1989).
pinningonthecharacteristicsofthePeierlstransitionand
22. L.F. Schneemeyer,F.J. DiSalvo,S.E. Spengler,and J.V.
on the first threshold field of non-linear conductivity, we
Waszczak, Phys.Rev.B 30, 4297 (1984).
studied the electrical transport properties of a series of
23. L. G. Schneemeyer, R. M. Fleming, and S. E. Spengler,
pure,Rb-doped,andW-dopedbluebronzeK0.3MoO3sin- Solid State Commun. 53, 505 (1985).
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wood, and E. A.Rietman, Phys. Rev.B 32, 4088 (1985).
the threshold field E (T) exhibits a maximum around
T 25. K. V. Ramanujachary,M. Greenblatt, and W. H. McCar-
T = 100 K which is correlated with the incommensu- roll, J. of Crystal Growth 70, 476, 1984.
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28. W. L. McMillan, Phys.Rev. B 14, 1496 (1976).
5 Acknowledgements 29. G. Mihaly, P. Beauchene, J. Marcus, J. Dumas, and C.
Schlenker,Phys. Rev.B 37, 1047 (1988)
The authorswouldlike to thank KazumiMaki,Attila Vi-
rosztek, and Silvia Tomic for helpful discussion and G.
Untereinerfortechnicalsupport.Financialsupportbythe
DeutscheForschungsgemeinschaft(SPP1073)isacknowl-
edged.
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