In-plane optical spectra of Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$: Overdoping and disorder effect on residual conductivity

0.39 MB·English

by  E. Uykur

In-plane optical spectra of Y Ca Ba Cu O : Overdoping and disorder effect on
1−x x 2 3 7−δ
residual conductivity
E. Uykur, K. Tanaka, T. Masui, S. Miyasaka, and S. Tajima
Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, Japan
We measured the temperature dependence of the in-plane polarized reflectivity spectra of twin-
free Y1−xCaxBa2Cu3O7−δ single crystals with different Ca-concentrations (x = 0,0.11 and 0.16)
fromoptimallydopedtoheavilyoverdopedregion. Lowenergyopticalconductivityspectrashowed
2 a Drude-like residual conductivity at temperatures far below the superconducting transition tem-
1 perature, which indicates the presence of unpaired-normal carriers in the superconducting state.
0 Comparing the spectra at a fixed Ca-content or at a fixed doping level, we have revealed that the
2 carrier overdoping increases unpaired carriers in addition to those induced by the Ca-disorder. We
alsofoundthesuperconductingbehavioroftheone-dimensionalCuOchainsfortheCa-freesamples.
n
a
PACSnumbers: 74.25.Gz, 74.72.-h
J
Keywords: (Y,Ca)Ba2Cu3O7−δ,far-infrared,in-planeoptical spectra,chainconductivity
9

]
n INTRODUCTION in order to discuss the origin of residual conductivity.
o So far, there have been few studies for the in-
c The overdopedregime of cuprate superconductors has plane optical spectra of overdoped cuprates except
-
r not been intensively investigated, compared to the well for Tl2Ba2CuO6+δ (Tl2201) and Bi2Sr2CaCu2O8+δ
p studied pseudogap phase in the underdoped regime [1], (Bi2212)[7, 8]. In the present work, we prepared de-
u
s although many unresolved problems remain in the for- twinnedsinglecrystalsofY1−xCaxBa2Cu3O7−δ forvari-
. mer. One of the reasons for this situation is that it ous Ca- and oxygen-contents. In order to separate the
t
a is believed that in the overdoped regime, the system is effects of overdoping and Ca-disorder, we studied the
m
approaching a conventional Fermi liquid state and thus spectral change with Ca-content by fixing the doping
- nothing peculiar happens. The other is a technical rea- level, as well as the change with doping by fixing the
d
n sonthatonlylimitedmaterialssuchasTl-basedcuprates Ca-content. These comparisons have revealed that over-
o can achieve the overdoped state. doping definitely increases the unpaired carriers. While
c Among some anomalies in the overdoped regime, we Ca-substitution also enhances residual conductivity, its
[ focus here onthe lowenergyopticalresponse. When the effect is weaker than the Zn-substitution effect.
1 system goes into the superconducting state with d-wave Moreover,for highly oxygenatedY-123,we observeda
v symmetry, the optical conductivity is suppressed below clear conductivity suppression due to the superconduct-
9 the gap energy (2∆) and gradually decreases towards inggapopeningintheCu-Ochainspectra. Itseemsthat
7
zero at ω = 0. However, in many cuprates, a substan- thedecreaseofoxygendeficiencyleadstohighconductiv-
6
1 tial amount of conductivity remains finite at ~ω ≪ 2∆, ityinthechainsandthustoasuperconductingresponse.
. forming a Drude-like spectrum even at T ≈ 0. This
1
residual conductivity suggests the presence of unpaired
0
EXPERIMENTS AND SAMPLE
2 normal carriers in the superconducting state. Impurity
CHARACTERIZATION
1 pair-breaking could be one of the sources for this resid-
v: ual conductivity, since Zn-doped cuprates exhibit a sim-
i ilar low frequency response [2]. The problem is that this Y1−xCaxBa2Cu3O7−δ single crystals (for x = 0, 0.11
X and 0.16) were grown by using a pulling technique ex-
anomaly can be observed even in impurity-free samples
r plained elsewhere [9]. As-grown crystals were cut into
[3]. Its origin is unknown.
a
pieces with ab-plane surfaces that were not smaller than
The increase of the residual conductivity with carrier
4×4 mm2. Then the pieces were detwinned under uni-
overdoping was first reported in the c-axis spectra of
YBa2Cu3O7−δ (Y-123) [4, 5] and Y1−xCaxBa2Cu3O7−δ axial pressure (cid:16)∼8 kg/mm2(cid:17). For reflectivity measure-
(Y/Ca-123) [6], followed by the study of the in-plane ments, the sample surfaces were mechanically polished
spectra of Tl2Ba2CuO6+δ [7]. It seems to be related to by using Al2O3 powder gradually as fine as 0.3 µm. To
overdoping,but not conclusive because disorder-induced obtain optimally doped crystals, samples were annealed
◦
pair-breaking cannot be ignored in most cases. For ex- at 500 and 575 C for x = 0 and x = 0.11, respectively,
ample, Ca-substitution for Y in YBa2Cu3O7−δ lowers in an oxygen atmosphere for about 1 month. For the
themaximumT valuepresumablybecauseitintroduces overdoped region, all samples were annealed at 350 ◦C
c
disorders into the CuO2-planes, while it also introduces under oxygen flow for about 2 months. The supercon-
carrier-holes into the system. Therefore, we need to dis- ducting transition temperatures were determined by the
tinguishtheeffectsofcarrier-overdopingandCa-disorder dc susceptibility measurements as follows: 93.5 and 87
2
1.0 7
Tc 93.5 K Tc 87 K Room Temp. p Tc x
x = 0 x = 0.11 6 E//a 0.16, 93.5 K, 0
vity 300 K 300 K -1 m) 5 00..128, , 9705 KK,, 00.11
ti 10 K 10 K 1 c 4 0.22, 70 K, 0.16
c0.5 -
le 3 0 3
ef E//b E//b (1
R 1 2
E//a E//a
1
(a) (b)
0.0 0
0 7500 15000 0 7500 15000 0 500 1000 1500 2000 2500 3000
-1
-1
Frequency (cm ) Frequency (cm )
FIG. 1. The polarized in-plane reflectivity spectra in the op- FIG. 2. Room temperature a-axis optical conductivity from
timally doped samples with (a) x=0 and (b) x=0.11. p=0.16 to p=0.22. Solid symbols indicate the dc value for
each sample.
K for x = 0 and x = 0.11, respectively, in the optimally
doped regime, while in the overdoped region, T values chains, because in order to keep a constant doping level,
c
decreaseddownto90Kforx=0,75Kforx=0.11,and we reduced the oxygen concentration in the chains for
70Kforx=0.16. Thedopinglevels(p)weredetermined the Ca-substituted samples [11]. The estimated δ-values
from T , assuming the empirical formula between p and were 0.12 for x=0 and 0.257 for x=0.11, respectively.
c
T [10]. DC resistivity wasmeasuredby a standardfour
c
probe method.
RESULTS
The temperature dependent reflectivity measurements
were performed with a Bruker 80v Fourier transform in-
frared(FTIR) spectrometer from ∼70 to ∼20000cm−1 Overdoping effect on a-axis residual conductivity
with E//a and E//b polarizations at various tempera-
turesfrom∼10to∼300K.Thesampleandthereference Inthe overdopedregion,the samples withx=0,0.11,
mirror (Au for far-infrared and middle infrared regions, and 0.16 have been studied. The doping levels (p) were
Ag for near infrared and visible regions) were placed determined as 0.18, 0.2, and 0.22 for these samples, re-
into a He-flow cryostat. Their spectra were compared spectively. Figure 2 shows the comparison of the a-axis
successively at each measured temperature by checking σ1(ω) spectra at room temperature, together with the
their positions with a He-Ne laser. The optical con- optimal doping spectrum for x = 0. The increase of the
ductivity spectra were obtained from the measured re- opticalconductivitydemonstratesasuccessfulincreasein
flectivity spectra by using the Kramers-Kronig (K-K) carrier concentration. For the Ca-substituted samples,
transformation with the Hagen-Rubens extrapolation in a peak-like structure evolves in the low-energy region
the low-energy region. For the analysis, we used room- (∼ 300 cm−1) in the normal state, which becomes more
temperature reflectivity spectra in the higher-energy re- pronounced with increasing Ca-concentration. With de-
gion up to 40 eV, which were measured with the use of creasingtemperature,thispeakstructuremergesintothe
synchrotronradiation at UV-SOR, Institute for Molecu- Drude-like conductivity. This feature is not only depen-
lar Science (Okazaki). We also used ω−4 extrapolation dent of the Ca-content but also of the oxygen content.
above 40 eV. The origin of this Ca-induced peak is unknown at the
Figures 1(a) and (b) show the in-plane polarized re- moment.
flectivity spectra of the optimally doped crystals with With increasing carrier-doping, the characteristics of
x = 0 and 0.11 in a wide frequency range. The 10 K the optical spectra change dramatically. Figure 3 shows
spectra are also given for E//a to demonstrate that the the reflectivity [(a)-(d)] and conductivity [(e)-(h)] spec-
appreciable temperature dependence is observed only in tra of the optimally doped sample with x= 0 and three
the low energy region. A clear a-b anisotropy can be overdoped samples with x = 0, 0.11, and 0.16, respec-
seenbothsamples,whichguaranteesthatourdetwinning tively. The conductivity spectra were calculated from
treatment was successful. The b-axis reflectivity edge the reflectivity spectra through the Kramers-Kronig(K-
for the Ca-substituted sample is located at lower energy K) transformation. The low frequency conductivity can
compared to the Ca-free sample. This shift can be ex- besmoothlyconnectedtothedcvaluesdeterminedbydc
plainedbythedecreaseofthecarrierconcentrationinthe resistivitymeasurement,whichconfirmsthereliabilityof
3
some absorption within the superconducting gap.
Intheconductivityspectra,fortheCa-freesample,the

1.00 σ1-suppressionatlowfrequenciesassociatedwiththe su-
300 K x = 0 p = 0.16 5 perconducting condensation can be seen while the sup-
0.95 100 K 300 K 4 pression weakens in the overdoped sample [Fig. 3(f)].
60 K 100 K
0.90 10 K 60 K 3 With further increasing p (p = 0.2 and 0.22), a clear
superconducting gap could not be observed, except for
10 K 2
a small suppression of the conductivity, which indicates
0.85 1
Tc 93.5 K possible gapless superconductivity in this state. In this
0.80 (a) E//a (e) 0 overdoped region, σ1(ω) shows a distinct Drude-like in-
crease toward ω = 0. The Drude-like peak for p = 0.22
T c 90 K x = 0 4
0.95 is broader than that for p = 0.2, which indicates the in-
E//a p = 0.18
3 crease of the residual conductivity with doping, namely,
0.90 the increase of the unpaired normal carriers. The cal-
ctivity 0.85 121 (10 3 rceusliadtuedal sppaercttraatl1w0eiKghatsre(S4.W31×=10R61600Ω00−σ1(cωm)d−ω2), 8o.f01th×e
fle 0.80 (b) (f) 0c -1 106 Ω−1cm−2, 19.92× 106 Ω−1cm−2, and 30.15× 106
Re m Ω−1cm−2 for samples in Figs. 3(e), (f), (g), and (h),
T c 75 K x = 0.11 ) -1
0.95 respectively.
E//a p = 0.2 4
The strong increase of residual conductivity in Fig. 3
0.90
seems to be the effect of carrier-overdoping. However, it
2 isnotobviouswhetherthisisonlyduetooverdopingorif
0.85
the disorder introduced by Ca-substitution contributes.

(c) (g) To extract the overdoping effect solely, we compare the
0.80 0
spectra of optimally doped and overdoped samples with
T c 70 K x = 0.16 8
0.95 E//a p = 0.22 the same Ca-content. Figures 4(a) and (b) show the
6 normal state (100 K) and superconducting state (10 K)
0.90 σ1(ω)intheoptimallydopedandintheoverdopedregion
4
for x = 0 and 0.11, respectively. For both Ca-contents,
0.85
2 the missing area is reduced with carrier overdoping. For
(d) (h) the Ca-free sample, the residualconductivity at the low-
0.80 0
0 500 1000 15000 500 1000 1500 est temperature (10 K) is enhanced in the overdoped
-1
Frequency (cm ) sample,comparedtothatoftheoptimallydopedsample.
For x=0.11, this increase is more pronounced. As a re-
sult,thesuppressionduetosuperconductingtransitionis
FIG. 3. Temperature dependent a-axis reflectivity spectra
almost erased by the Drude-like increase of the conduc-
(left panels) of optimally doped sample with (a) x = 0 and
threeoverdopedsampleswith(b)x=0,(c)x=0.11,and(d) tivity. Since we can observe a similar increase of resid-
x = 0.16. Right panels show the corresponding optical con- ual conductivity for two sets of samples with different
ductivity spectra. (e) for (a), (f) for (b), (g) for (c), and (h) Ca-content, it is concluded that the carrier-overdoping
for (d), respectively. Solid symbols show the dc conductivity intrinsically enhances the residual conductivity.
values at 300 K.
Ca-substitution effect at the optimum doping
our K-K transformation. The spectra for the optimally
dopedsamplewithx=0arealmostidenticaltothedata Since we work with the Ca-substituted samples, it is
published so far [2,12, 13]. The major changewith tem- necessary to examine whether the Ca-disorder effect ex-
perature occurs in the low energy region, as can be seen ists. Figure 5 is a comparison of the spectra of Ca-free
in Fig. 1. In the normal state, the reflectivity increases and Ca-substituted samples with the same doping level
withloweringfrequency,asexpectedinametallicsystem. (p=0.16). InFigs. 5(a)and(b) wefoundthatthe step-
In the superconducting state, a well known step-like be- like feature in the reflectivity spectrumis getting weaker
havior [14–16] is seen below 1000 cm−1. withCa-substitution. Thereflectivitybelow∼400cm−1
This behavior is clearly seen in the optimally doped is slightly lowerforthe Ca-substituted sample,whichin-
sample while it weakens with overdoping and is almost dicates some absorption due to normal carriers.
suppressed for the heavily overdoped sample. Moreover, Figures5(c)and(d)arethecorrespondingopticalcon-
the reflectivity below 200 cm−1 at 10 K becomes lower ductivity spectra. Since the doping levels are the same
with increasing p and x, which implies an increase of (p = 0.16) in both samples, the differences can be at-
4

5 1.00 5
300 K x = 0 p = 0.16
4 x = 0 E//a 100 K, p = 0.16 0.95 100 K 300 K 4
10 K, p = 0.16 60 K 100 K
100 K, p = 0.18 10 K 60 K 3
3 10 K, p = 0.18 0.90 10 K
3-1-1 (10cm)1012 (a) Reflectivity000...988505 (aT) cE //a93.5 KTc E/8/a7 K xp == 00..1116 (c) 01241 (10cm) 3-1-1
x = 0.11 E//a 100 K, p = 0.16 3
6 10 K, p = 0.16 0.90
100 K, p = 0.2 2
10 K, p = 0.2 0.85
4 1

(b) (d)
0.80 0
2 0 500 1000 15000 500 1000 1500
-1
Frequency (cm )
(b)
0 FIG. 5. Temperature dependent a-axis reflectivity spectra
0 500 1-1000 1500 (left panels) in the optimally doped region for (a) x=0 and
Frequency (cm ) (b) x = 0.11. Right panels show the corresponding optical
conductivityspectra. Solidsymbolsshowthedcconductivity
FIG. 4. Comparisons of a-axis optical conductivity in the valuesfor 300 K.
normal state (100 K) and the superconducting state (10 K)
for YBa2Cu3O7−δ with (a) x= 0 and (b) x=0.11. In both
panels, thespectra of theoptimally doped sample (p=0.16) state and the suppression of the conductivity with the
andtheoverdopedsample(p=0.18orp=0.2)arecompared. superconductingtransitioncanbeseenasexpected. The
mid-infraredabsorptiondueto disorderedchains[17,18]
between ∼ 1000 cm−1 and ∼ 3000 cm−1 is shifting to-
tributed solely to the Ca-substitution effects. A notice- wardslowerfrequencieswithoverdoping. Theb-axiscon-
able change with Ca-substitution is seen in the super- ductivity values are higher than the a-axis values (Figs.
conducting state, while there is no substantial differ- 6(c) and (f)), which indicates the chain contribution to
ence between the normal state spectra for x = 0 and the optical conductivity.
0.11. The suppression of the conductivity due to the
The optical conductivity for the chain (σ ) is
CHAIN
superconducting condensation becomes weaker and the
estimated by subtracting the a-axis conductivity (σ1a)
missing area decreases with Ca-substitution. Therefore, from the b-axis one (σ1b) as σCHAIN = σ1b −σ1a. Fig-
we conclude that the increase of residual conductivity
ures 7(a) and (b) show the chain conductivity in the op-
and the decrease in the missing area are also caused by
timally doped and overdoped sample, respectively. The
Ca-substitution, not only overdoping. Here it should
conductivity in the disorderedchains will occur with the
be noted that the normal state spectra for the opti-
hopping mechanism [19]. On the other hand, for the
mallydopedsamplesdonotappreciablychangewithCa-
highly ordered chain structure, a metallic behavior can
substitution,whichisconsistentwiththedcconductivity
beexpected. Assupportforthisidea,wecanseetheshift
behavior.
of the mid-infrared absorption to lower energies and the
development of a Drude-like spectrum in the chain re-
sponse of the highly oxygenated sample. Moreover, at
b-axis spectra and the superconducting chain 10K,adistinctsuppressiondue tosuperconductingcon-
behavior densation can be observed. So far, although the higher
conductivity in the b-axis spectra has been considered
Sofar,wehavefocusedonthea-axisspectrawhichgive to indicate the contribution of the chains to the elec-
information about the two-dimensional CuO2 planes. tronic conductivity, most of the reported chain spectra
Next, we discuss the b-axis polarized spectra. b-axis showed neither a Drude-like profile nor a superconduct-
measurements are also important for the Y-123 system ing response. Figure 7 demonstrates that with decreas-
to extract the chain contribution. Figures 6(a) and (d) ing oxygen deficiency, the chains contribute to the su-
showthe low-frequencyb-axisreflectivity spectrafor the perfluid density, as well. The superfluid response of the
optimally doped and overdoped YBa2Cu3O7−δ, respec- intrinsically metallic CuO chains would be the result of
tively,whileFigs. 6(b)and(e)arethecorrespondingop- the proximity effect between chains and superconduct-
tical conductivity. The metallic behavior in the normal ing CuO2 planes,which is theoreticallypredicted in Ref.
5

4
1.00 1.00
E//b x = 0 (a) (d) Tc 93.5 K 300 K
y 300 K
eflectivit 00..9905 Tc 9 3316.000500 KK KK 00..99 05 1610000 KK K -1 m) 23 xp == 00.16 1610000 KK K
R 0.85 10 K 0.85 E//bx = 0 c
Tc 90 K -1 1
0.80 0.80 3 0
-1 cm-1) 68 TEc// b 9x3 =.5 0K (b) 68 ET/c/b 9x0 = K 0 (e) (1CHAIN 30 Tc 90 K (a)
3 0 4 4 x = 0
1
( p = 0.18
1 2 2 2

0 0
1) (c) (f) 1
m- 8 300 K 8 300 K
c E//b E//b (b)
-1 6 E//a 6 E//a 0
3 0 500 1000 1500 2000 2500 3000
0 4 4
1 -1
( Frequency (cm )
1 2 2
0 0 FIG.7. Temperaturedependentchainconductivityforx=0
0 1000 2000 3000 0 1000 2000 3000
in the (a) optimally doped region and (b) overdoped region.
-1

Frequency (cm )
FIG.6. Temperaturedependentb-axisreflectivityspectrafor
DISCUSSIONS
x = 0 in the (a) optimally doped region and (d) overdoped
region. (b) and (e) show the corresponding optical conduc-
tivity spectra for (a) and (d), respectively. (c) and (f) show
It is possible to calculate the superfluid density with
the a-axis and b-axis optical conductivity in the room tem-
two different methods. The first method is through the
perature. Solid symbols show the dc conductivity values at
missingareaofthea-axisopticalconductivity. Wecalcu-
300 K.
cla2t/e8dλt2he=m4iπssnineg2/amre∗a,,Ausi=ngR1o100n00l0y0[rσe1lnia(bωl)e−daσt1as(dωo)w]dnωt=o
L s
100cm−1 forallthe samples. Here,the spectraat100K
and 10 K were used for σ1n(ω) and σ1s(ω), respectively.
20. Although the missing area in the chain conductivity Theobtainedvaluesforλ−2(∼ ω2 )areplottedinFig. 8
L ps
spectrum was reported for the oxygendeficient Y-123 in asafunctionofthedopinglevel(p). Thesecondmethod
Ref. 21, we observed such a superconducting response is estimation from the imaginary part of the conductiv-
only in the highly oxygenated Y-123. ity, σ2(ω), which is equal to ωp2s/4πω at ω ≪ 2∆. The
values obtained by the two methods are in good agree-
Another possible source of the anisotropy of σ1b−σ1a mentwitheachotherwithintheerrorbarsinFig. 8. The
may be the stripe effect within the CuO2 plane. A λL value for the optimally dopedCa-free samplewasde-
clear in-plane anisotropy was observed in the spectra of termined as 1520 ˚A. This value is consistent with the
lightly doped La2−xSr2CuO4 [22] and YBa2Cu3Oy [23], reported data [2, 24]. In Fig. 8, w∗e see a systematic de-
whichwasattributedtothenematicnatureofthecharge creaseinsuperfluiddensity(ns/m )withincreasingpas
stripes. In Y-123, the CuO chain is supposed to play a wellaswithincreasingCa-concentration. Althoughthere
roleinalignmentofthestripes,whileithasalmostnodi- is an effect from Ca-substitution, as we showed in previ-
rect contribution to the conductivity. Comparedto this ous section,it is demonstratedthat carrieroverdopingis
datafortheunderdopedY-123[23],themagnitudeofthe also an effective factor. Distinguishing the Ca-disorder
difference σ1b −σ1a is very large in the present result, effect from the overdoping effect, we can conclude that
which indicates a huge contribution of the CuO chain to unpairedcarriersareintrinsicallyinducedbyoverdoping.
the b-axis conductivity. Therefore, we believe that the The decrease of λ−L2 with overdoping was also observed
anisotropy observed in this study is mainly caused by in muon spin rotation (µSR) measurements for Tl2201
the CuO chainconductivity andthe effect of the stripes, [25,26]andCa-dopedY-123[27],whichsuggestsanelec-
even if it exists, is much weaker than the chain conduc- tronic inhomogeneity in these systems.
tivity. Scanning tunnelling microscopy (STM) studies re-
6
rate is completely suppressed. This superconductivity-
60 induced feature is getting weaker with Ca-substitution,
Y1-xCaxBa2Cu3O7- as is expected from Fig. 5.
50 x = 0
93.5 K When the doping level is further increased, several
90 K x = 0.11
40 x = 0.16 changes are observed. First, the bump around ∼ 1000
-2 m) 87 K YBa2(Cu1-xZnx)3O7- cm−1 vanishesgraduallywiththeincreasingdopinglevel
30 x = 0.004 a nd the T-independent high frequency part becomes T-
(
-2 L20 dependent(Figs. 9(c)and(d)). Thedisappearanceofthe
75 K bumpfeatureis notonlydue to Ca-substitutionbutalso
85 K 70 K due to overdoping (comparison of Figs. 9(b) and (c)).
10
The second noticeable change is the decrease in scatter-
0 ing rate in spite of the increase of Ca-disorder. It is con-
0.14 0.16 0.18 0.20 0.22 0.24 sideredthatthecarrierdopingeffectovercomestheeffect
of disorder. Thirdly, the ω-dependence changes from ω-
doping level (p) linear towards ω2- behavior. The latter is an indication
FIG. 8. The doping dependence of the superfluid density ofthe Fermiliquid nature ofthe system. Forthe heavily
(∼ λ−L2) of Y1−xCaxBa2Cu3O7−δ single crystals for x = overdoped region a small upturn is also observed below
0, 0.11 and 0.16. The ◮ shows the superfluid density of 500 cm−1 in Fig. 9(d), which becomes stronger with de-
YBa2(Cu1−xZnx)3O7−δ for comparison (Ref. 2). Temper- creasingtemperature. Asimilarbehaviorobservedinthe
atures are Tc valuesfor each sample.
heavilyoverdopedregionisdiscussedindetailforBi2212
[33] and Tl2201 [7] systems. The upturn of the low en-
ergyscatteringrateusuallyexplainedbythechargelocal-
vealednanoscalespatialvariationsintheelectronicstate
ization induced by impurity in disorder introduced sys-
oftheoverdopedBi2Sr2Cu2Oy [28,29],contrarytomany tems or by local disordered octahedral distortion for the
models of high temperature superconductors that con-
Tl2201 system.
sider electronically homogeneous systems. The increase
of normal carriers in the superconducting state in the The other importantfinding in Fig. 9 is that the scat-
overdoped regime with increasing carrier density is also tering rate is insensitive to Ca-concentration. It is seen
suggested by other methods. The loss of the specific when we compare the spectra for the same doping level
heat anomaly and the increase in the zero-temperature (See Figs. 9(a) and (b)). The Ca-insensitive behavior
specific heat coefficient [γ(0)] for (La,Sr)2CuO4 and wasalso observedin the conductivity spectra andthe dc
TlSr2CaCu2O7−δindicatethestrongincreaseofunpaired conductivityvaluesinFig. 5. Thesefactsimply thatthe
carriers in the overdoped region [30]. In NMR measure- Ca atoms in the Y-layer do not act as strong scattering
ments,theT-lineardependenceofthenuclearspin-lattice centersfor the carriersin the CuO2 planes. The effectof
relaxationrate(1/T1)wasobservedatlowtemperatures disordersin the blocking layerswasintensively discussed
in overdoped TlSr2CaCu2O7−δ, which was interpreted in terms of a weak scattering center in ref. 34. The Ca-
to reflect the existence of the residual density of states substitutioninthe presentsystemisconsideredtogivea
at the Fermi level, suggesting gapless superconductivity similar weak scattering effect.
[31]. Moreover,recentlythedecreaseoftheMeissnervol- This weakscattering is insharpcontrastto the strong
ume fraction was observed on field cooling of the mag- scatteringdue to the in-plane disordersuch asin Zn. Zn
netic susceptibility measurements [32]. This indicates a is a well-known impurity which causes pair-breaking in
decrease in superconducting carrier density due to the a d-wave superconductor. The strong Zn-effect can be
phase separation in the overdoped (La,Sr)2CuO4. seeninthe radicalreductionofsuperfluid densityinFig.
Finally, we discuss the Ca-substitution effects on the 8, where the λ−2 estimated from the optical spectrum
L
scatteringrate. Thea-axisscatteringratewascalculated of Zn-substituted Y-123 (Y-123(Zn)) is also plotted [2].
by using the equation, 1/τ(ω) = ω2/4π·Re(1/σ(ω)), as Although T values are almost the same (87K and 85K)
p c
shown in Fig. 9. The ω values are determined with the in optimally doped Y/Ca-123and Y-123(Zn),the super-
p
Ferrell-Glover-Tinkham (FGT) sum rule by integration fluiddensityismuchsmallerinY-123(Zn)thaninY/Ca-
uptoω =10000cm−1 as∼1.66×104cm−1,∼1.69×104 123. Therefore, in the case of Ca-substitution, the T -
c
cm−1, ∼ 1.92×104 cm−1, and ∼ 2.29×104 cm−1 for suppressionaccompaniedwiththereductioninsuperfluid
x=0,x=0.11(optimallydoped),x=0.11(overdoped), density and with the increase in unpaired carriers must
andx=0.16,respectively. Fortheoptimallydopedsam- bedistinguishedfromtheusualimpuritypair-breakingin
ple with x = 0 (Fig. 9(a)), the normal state scattering a d-wave superconductor. We need another mechanism
ratesshowanalmostlinearincreasewithω upto∼3000 toexplaintheT suppressionwithoutincreasingresidual
c
cm−1. At the superconducting transition, a clear bump resistivity. It should give the answer for the problem of
is formed around ∼1000 cm−1 and the low ω scattering the blocking layer effect on T .
c
7
ductivity, a clear superconducting gap feature could not
be observed because of the huge residual conductivity.
The comparison of the spectra between optimally
doped and heavily overdoped samples with the same
3 (a) E//a x = 0
Ca-contents showed that the residual conductivity
Tc 93.5 K p = 0.16
increases with carrier-overdoping. Since we successfully
2 removed the Ca-disorder effect in the experiment, we
conclude that the increase of residual conductivity is
300 K
an intrinsic property of the high temperature cuprate
100 K
1 60 K superconductors.
10 K WealsocomparedtheCa-substitutedandCa-freesam-
ples at the fixed doping level (p = 0.16) to observe the
0
Ca-substitution effect on the residual conductivity. As
3 (b) E//a x = 0.11
a result, we found that the disorder effect due to Ca-
Tc 87 K p = 0.16
substitution also causes the increase of the residual con-
2 ductivity.
Most of the b-axis spectra are characterized by the
) mid-infrared absorption due to hopping conduction in
1
- m 1 the chain in addition to the plane response. Only for
c
3 the heavily oxygenatedCa-free Y-123 sample, where the
0
1 0 one-dimensional chain structure shows a highly ordered
(
a 3 (c) E//a x = 0.11 structure,thechainitselfshowsasuppressionofconduc-
1/ Tc 75 K p = 0.2 tivitybelow Tc. Apossiblereasonforthe superconduct-
ing chain behavior would be the proximity coupling ef-
2 fect of the chains with two-dimensional superconducting
CuO2 planes.
1

ACKNOWLEDGEMENTS
0
3 (d) E//a x = 0.16 This work was supported by the New Energy and In-
Tc 70 K p = 0.22 dustrialTechnologyDevelopmentOrganization(NEDO),
and the Grant-in-Aid for Scientific Research from the
2
Ministry of Education, Culture, Sports, Science and
Technology of Japan.
1

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