Large Transport Properties of Bi$_2$Sr$_{2-x}$La$_x$CaCu$_2$O$_{8+δ}$ Single Crystals Grown by a Floating-Zone Method

4 Pages·0.45 MB·English

by  Takenori Fujii

Large Transport Properties of Bi Sr La CaCu O Single Crystals Grown by a
2 2−x x 2 8+δ
Floating-Zone Method
Takenori Fujii1,2∗ and Ichiro Terasaki1,2.
1Department of Applied Physics,
Waseda University, Tokyo 169-8555, Japan.
2PREST, Japan Science and Technology Corporation,
Kawaguchi 332-0012, JAPAN.
3
0
0 (Dated: February 2, 2008)
2
A parentinsulator of theBi-based high-Tc superconductorBi2Sr2−xLaxCaCu2O8+δ was success-
n fully grown byatravelingsolvent floatingzonemethod. Thein-planeresistivity ρab shows metallic
a behavior (dρab/dT > 0) near room temperature, although the magnitude of the resistivity is far
J
abovetheMottlimit for metallicconduction. TheHallmobility oftheparentinsulatordoesnotso

1 muchdifferfrom thatofanoptimally dopedsample. Theseresultssuggest thatthesystemisnota
2 simpleinsulatorwithawell-definedgap,andtheholeintheparentinsulatorisessentiallyitinerant,
as soon as it is doped.

]
l
e PACSnumbers: 74. 25. Fy,74. 25. Dw,74. 72. Hs
-
r
t
s INTRODUCTION EXPERIMENTAL
.
t
a
m
- Twocrystalsof40%LadopedBi2Sr2CaCu2O8+δ were
d It is well known that the high T superconductivity usedinthisstudy. OneisacrystalinwhichallLaatoms
c
n occurs with doping carriers into the antiferromagnetic were substituted for Sr site (Bi2(Sr,La)2CaCu2O8+δ),
o
and the other is a crystal in which La atoms were sub-
(AF) insulator. The AF insulator is quite different from
c
[ a conventionalinsulator, and shows various peculiar fea- stituted for Sr and Ca sites (Bi2(Sr,Ca,La)3Cu2O8+δ).
They were grown by TSFZ method with a growth
tures. One of the most peculiar features is that in the
1
v lightly doped La2−xSrxCuO4 (LSCO) metallic behavior velocity of 0.5mm/h in a mixed gas flow of O2
(20%) and Ar (80%). The starting composition of
0 of the in-plane resistivity ρab (dρab/dT > 0) near room
7 temperature was observed, even though the magnitude the feed rods were Bi2.1Sr1.1La0.8Ca1.0Cu2.0O8+δ
3 of the in-plane resistivity is far above the Mott limit [1]. for Bi2(Sr,La)2CaCu2O8+δ and
1 This metallic behavior is considered to be responsible Bi2.1Sr1.5La0.8Ca1.0Cu2.0O8+δ for
0
for the appearance of band dispersion within the Mott- Bi2(Sr,Ca,La)3Cu2O8+δ. Very large single crys-
3 tals with a size of 2 × 4 × 0.1 mm3 were suc-
0 Hubbard gap seen in the angle-resolved photoemission
/ spectroscopy (ARPES) measurement [2]. The electric cessfully grown. By energy dispersive x-ray anal-
t
a states of the doped parent insulator, on the other hand, ysis (EDX), the actual compositions were deter-
m seem to be quite different between LSCO and the Bi- mined to be Bi2.01Sr1.11La0.84Ca1.00Cu2.00O8+δ
- based cuprates. The charge-stripe order is stabilized for Bi2(Sr,La)2CaCu2O8+δ, and
d in LSCO, whereas it is much weaker in the Bi-based Bi2.04Sr1.31La0.86Ca0.76Cu2.00O8+δ for
n
o cuprates. LSCO has been most frequently used for sys- Bi2(Sr,Ca,La)3Cu2O8+δ. In the case of
c tematic studies of the heavily underdoped region of the Bi2(Sr,Ca,La)3Cu2O8+δ, excess La replaces the Ca
: site. Figure 1 shows the XRD pattern for the as-grown
v cuprates, because a heavily underdoped sample is easy
i to synthesize. By contrast, there are only a few reports Bi2(Sr,La)2CaCu2O8+δ singlecrystal. Allthepeakswere
X indexed as (0 0 l) peaks of the Bi-2212 phase with no
onthelightlydopedBi2Sr2CaCu2O8+δ [3,4],becauseit
r trace of impurities. The c-axis length was evaluated to
is difficult to grow high-quality single crystals of lightly
a
doped region. We have previously grown the parent in- be 30.18˚A for Bi2(Sr,La)2CaCu2O8+δ, and30.38˚A for
sulator of Bi2Sr2CaCu2O8+δ by flux method, where the Bi2(Sr,Ca,La)3Cu2O8+δ, which is smaller than that of
trivalent lanthanide, such as Dy and Er, is substituted Bi2Sr2CaCu2O8+δ (30.86 ˚A). This is due to the larger
ion radius of the Sr atom (1.13 ˚A) than that of the La
for divalent Ca [3]. Here, we havesuccessfully grownthe
(1.05˚A)andCa(1.00˚A)atoms. Thein-planeresistivity
parent insulator of Bi2Sr2−xLaxCaCu2O8+δ by a travel-
ing solvent floating zone (TSFZ) method. of Bi2(Sr,La)2CaCu2O8+δ was measured using standard
four-probe method, whereas the in-plane resistivity of
the highly insulating sample Bi2(Sr,Ca,La)3Cu2O8+δ
was measured using a two-probe method with constant
∗E-mailaddress:[email protected] voltage(wherethesampleresistanceismuchhigherthan
2
105 105
10
Intensity (cps) 111111110000000023451234 (002)(002)((ab))(004)(004) (006)(006) (008)(008) (00 10)(00 10) (00 12)(00 12)(00 14)(00 14) (00 16)(00 16)BBii2(00 18)(00 18)2((SSrr,C,(00 20)(00 20)Laa,)L2aC(00 22)(00 22))a(cid:21)CCC(00 24)(00 24)uuu22OO K(00 26)(00 26)88++add WResistivity (cm) 1111111100000000---01234321 WResistivity (mcm)B02468Bi02i(2S(rS,5rC0,LaBTB,aLie)1i2m22a0((CS0)Sp3raCer,CL,r1Cua5au2t0a)u2O2,OrLC8e28+a aa0+(d)Cs0Kd(cid:2)A3- uCaga)r2ssr2 uOo-a-52gg0wn8Orr+nnood8e3w+wa0d0ln(cid:2)n
0 50 100 150 200 250 300
101 Temperature (K)
0 10 20 30 40 50 60 70 80
2q (degree)
FIG. 2: Temperature dependence of the in-plane resis-
FIG. 1: X-ray diffraction pattern for (a) as-grown tivities ρab for Bi2(Sr,La)2CaCu2O8+δ (closed circles) and
Bi2(Sr,La)2CaCu2O8+δ(b)as-grownBi2(Sr,Ca,La)3Cu2O8+δ Bi2(Sr,Ca,La)3Cu2O8+δ (opencircles)withvariousannealing
single crystals. conditions. Inset: ρab of Bi2(Sr,La)2CaCu2O8+δ isplottedin
linear scale to emphasize metallic behavior.
the contact resistance). The voltage for both samples
is quite different from a conventional insulator (usually
was confirmed to vary linearly with the current in the
called as a “bad metal” [6]).
measured range from 4.2 to 300 K. The thermopower
Figure 3 shows the temperature dependence of the
was measuredusing a steady-state technique from 4.2 to
300 K. The Hall coefficient measurement was done by in-planethermopowerforBi2(Sr,La)2CaCu2O8+δ (closed
sweeping the magnetic field which was applied along the circles) and Bi2(Sr,Ca,La)3Cu2O8+δ (open circles). For
both samples, the thermopower decreases with decreas-
caxis(c//H)from-7to7Tatfixedtemperatures. The
ing temperature, which is consistent with our previ-
Hall resistance was confirmed to be linear in magnetic
field. The crystals were annealed in the O2 or Ar ous results of Bi2Sr2RCu2O8+δ (R=Dy, Er) [7]. The
◦ magnitude of the thermopower increases by Ar anneal-
atmosphere at 600 C for 10 h to change the oxygen
contents as is similar to Bi2Sr2CaCu2O8+δ [5]. ing and decreases by O2 annealing, which means that
the thermopower decreases with increasing oxygen con-
tents. Since the room temperature thermopower is con-
sidered to be a universal measure of the doping level [8],
RESULTS AND DISCUSSION
this result indicates that the carrier doping is caused
by the excess oxygen. Note that the room-temperature
Figure 2 shows the temperature dependence of thermopower and the magnitude of the in-plane resis-
the in-plane resistivities for Bi2(Sr,La)2CaCu2O8+δ tivity of Bi2(Sr,La)2CaCu2O8+δ are smaller than those
(closed circles) and Bi2(Sr,Ca,La)3Cu2O8+δ (open cir- of Bi2(Sr,Ca,La)3Cu2O8+δ. Although these two com-
cles). Bi2(Sr,Ca,La)3Cu2O8+δ showsinsulating behavior poundscontainapproximatelythesameamountofLa(x
(dρ /dT < 0) in the measured temperature range. The ∼ 0.8), the carrier concentration is different. For exam-
ab
magnitude of the in-plane resistivity at room tempera- ple, the carrier concentration per Cu p are estimated to
ture is about 30 mΩcm for an as-grown sample. It in- be 0.02 for as-grown Bi2(Sr,Ca,La)3Cu2O8+δ, and 0.04
creasesto 120 mΩcm by annealingin Ar, which suggests for as-grown Bi2(Sr,La)2CaCu2O8+δ. One explanation
that the carrier concentration decreases with decreas- forthis isthatthe substitutionofLaforCasite causesa
ing the oxygen content. Bi2(Sr,La)2CaCu2O8+δ shows disorderintothe CuO2 plane to make the dopedcarriers
metallic behavior (dρ /dT > 0) above 150 K, and the localized. Anotherexplanationisthatthe amountofthe
ab
magnitude of the in-plane resistivity is about 5 mΩcm excess oxygen is different.
at room temperature. In the inset of Fig. 2, the in- Figure 4 shows the temperature dependence of the
plane resistivity of Bi2(Sr,La)2CaCu2O8+δ is plotted in Hall coefficient RH for as-grownBi2(Sr,La)2CaCu2O8+δ
linearscaletoseethemetallicbehaviorclearly. Although (closed circles) and as-grown Bi2(Sr,Ca,La)3Cu2O8+δ
observed metallic behavior seems to suggest the Boltz- (open circles). The Hall coefficients monotonically in-
manntransport,themagnitudeofthein-planeresistivity crease with decreasing temperature for both samples.
isfarabovetheMottlimitformetallicconduction,which Theroomtemperaturecarrierconcentrationisestimated
3
300
than that of Bi2Sr2CaCu2O8+δ. In our previous work of
the parent insulator Bi2Sr2RCu2O8+δ (R = Dy, Er), the
250 mobility was too small to measure the Hall coefficient
K) Bi2(Sr,Ca,La)3Cu2O8+d(cid:2) precisely [10]. A similar large mobility at 300 K was re-
V/ 200 Ar anneal portedinthelightlydopedLSCO,whereonly1%ofhole
as-grown
m( doping causes the metallic behavior [1].
r
we 150 In the case of a conventional semiconductor, the mo-
po bility does not decrease with decreasing carrier concen-
o
m 100 tration. It is rather possible that the mobility becomes
r
e largewithdecreasingcarrierconcentrationbecauseofthe
h
T as-grown suppressionoftheelectron-electronscattering. While, in
50 O anneal
2 the case of the doped antiferromagnetic insulator, such
Bi2(Sr,La)2CaCu2O8+d as high-Tc superconductor, the hole is replaced by the
0
0 50 100 150 200 250 300 spininthenextsitewheneveritmoves. Then,ifthehole
moves independently, there is the energy loss of the or-
Temperature (K)
der of the antiferromagnetic correlation energy and the
mean free pass becomes as small as the lattice constant.
FIG. 3: Temperature dependence of the ther-
The fact that the mobility of the parent insulator does
mopower for Bi2(Sr,La)2CaCu2O8+δ (closed circles) and
Bi2(Sr,Ca,La)3Cu2O8+δ (opencircles)withvariousannealing not so muchdiffer fromthat of an optimally doped sam-
conditions. ple does not agree with above simple consideration. It
strongly suggests that the doped holes form a chargeor-
0.25 der, suchas chargestripe or phase separation,and move
2.4
2.2 Bi2(Sr,La)2CaCu2O8+d in a group. Thus, the doped hole in the parent insulator
as-grown
2 is essentially itinerant and shows metallic behavior.
C) 0.2 Vs) 1.8
3m/ 2m/ 1.6
c
nt (c 0.15 m ( 11..24 CONCLUSION
cie 1 Bi2(Sr,Ca,La)3Cu2O8+d
ffi 0.1 0.80 50 100 150 200 2as5-0grow3n00 In conclusion, we have successfully grown large sin-
coe Temperature (K) gle crystals of the parent insulator of Bi2Sr2CaCu2O8+δ
Hall 0.05 Bi2(Sr,Ca,La)3Cu2O8+d as-grown bwye hsuavbestmitueatisnugreLdathfeorinS-pr.lanWe ritehsisutisvinitgyt,htehseermcroypsotwalesr,
Bi(Sr,La)CaCuO as-grown
2 2 2 8+d and Hall coefficient along the CuO2 plane. the in-plane
0 resistivity of the most conducting sample shows metallic
0 50 100 150 200 250 300 behaviorabove150K.Althoughthemagnitudeofthein-
Temperature (K) planeresistivityoftheparentinsulatorisabout100times
larger than that of optimum doped Bi2Sr2CaCu2O8+δ,
FIG. 4: Temperature dependence of the Hall coefficient the mobility is not so much different from that of the
for as-grown Bi2(Sr,La)2CaCu2O8+δ (closed circles) and as- superconducting compounds. These results suggest that
grownBi2(Sr,Ca,La)3Cu2O8+δ(opencircles). Inset: Hallmo- the system is not a conventional insulator with a well-
bility µ≡RH/ρab plotted as a function of temperature.
defined gap, and the hole in the parent insulator is es-
sentially itinerant, as soon as it is doped. Our single
crystal shows relatively large mobility compared to Ca-
from the relation n = (eR )−1 to be 4 × 1020 cm−3
H site-substitutedsingle crystals,whichindicates a smaller
(cpm−∼3(0p.0∼4)0.f0o1r)fBoir2B(Si2r,(LSar,)C2Ca,aLCau)32COu82+Oδ,8+aδn,dwh1ic×ha1g0re20e disorder in CuO2 plane.
well with the carrier concentration estimated from the
room temperature thermopower. The Hall mobility cal-
culated using the Hall coefficient and the resistivity as
µ≡R /ρ is shownin the inset of Fig. 4. It should be [1] Y. Ando, A. N. Lavrov, S. Komiya, K. Segawa, X. F.
H ab
emphasizedthatthemobilityisrelativelylarge;themag- Sun;Phys. Rev.Lett. 87 (2001) 017001.
nitude of which (2.2 cm2/Vs for Bi2(Sr,La)2CaCu2O8+δ [2] T. Yoshida, X. J. Zhou, T. Sasagawa, W. L. Yang, P.
and 1.8 cm2/Vs for Bi2(Sr,Ca,La)3Cu2O8+δ) is only VFu.jBimoogrdia,nHo.vE,iAsa.kLi,aZn.z-aXra.,SZh.enH,uTs.saKinak,eTsh.iMtai,zSo.kaUwcah,idAa;.
5 times smaller than that of Bi2Sr2CaCu2O8+δ (10 cond-mat/0206469.
cm2/Vs) [9], although the magnitude of the in-plane re- [3] T. Kitajima, T. Takayanagi, T. Takemura, I. Terasaki;
sistivity of Bi2(Sr,La,Ca)3Cu2O8+δ is 100 times larger J. Phys.Condens. Matter 11 (1999) 3169.
4
[4] C. Kendziora, L. Follo, D. Mandrus, J. Hartge, P. [8] S. D. Obertelli, J. R. Cooper, J. L. Tallon; Phys. Rev.
Stephens,L. Mihaly; Phys. Rev. B45 (1992) 13025. B46 (1992) 14928.
[5] T.Watanabe,T.Fujii,A.Matsuda;Phys.Rev.Lett.79, [9] A.Maeda,M.Hase,I,Tsukada,K.Noda,S.Takebayashi,
2113 (1997). K. Uchinokura;Phys. Rev. B41 (1990) 6418.
[6] V. J. Emery, S. A. Kivelson; Phys. Rev. Lett. 74, 3253 [10] T. Takemura, Master thesis 2001, Waseda Univ. (in
(1995). Japanese).
[7] T. Takemura, T. Kitajima, T. Sugaya, I. Terasaki; J.
Phys.Condens. Matter 12 (2000) 6199.