Systematic Evolution of the Magnetotransport Properties of Bi_{2}Sr_{2-x}La_{x}CuO_{6} with

a r X i v :c o n d -m a t /9906268v 1 [c o n d -m a t .s u p r -c o n ] 17 J u n 1999

Systematic Evolution of the Magnetotransport

Properties of Bi 2Sr 2?x La x CuO 6with

Carrier Concentration

Yoichi Ando,1,2T.Murayama,1,2and S.Ono 1

1Central Research Institute of Electric Power Industry,Tokyo 201-8511,Japan 2

Department of Physics,Science University of Tokyo,Tokyo 162-8601,Japan

We report that it is possible to obtain a series of high-quality crystals of Bi 2Sr 2?x La x CuO 6,of which the transport properties have been believed to be “dirtier”than those of other cuprates.In our crystals,the normal-state transport properties display behaviors which are in good accord with other cuprates;for example,in the underdoped region the in-plane resistivity ρab shows the pseudogap feature and in the overdoped region the T dependence of ρab changes to T n with n >1.The characteristic temperatures of the pseudo-gap deduced from the resistivity and the Hall coe?cient data are presented.

PACS numbers:74.25.Fy,74.62.Dh,74.72.Hs A useful way to elucidate the origin of the peculiar normal-state prop-erties of high-T c cuprates is to study their systematic evolution upon chang-ing the carrier concentration.Bi 2Sr 2CuO 6(Bi-2201)system is an attrac-tive candidate for such studies,because the carrier concentration can be widely changed by partially replacing Sr with La (to underdope)or Bi with Pb (to overdope).1Moreover,this system allows us to study the normal-state in a wider temperature range,because the optimum T c (achieved in Bi 2Sr 2?x La x CuO 6with x ?0.41,2)is about 30K,which is lower than the opti-mum T c of La 2?x Sr x CuO 4.However,a number of problems have been known so far for Bi-2201crystals:(i)the transport properties of Bi-2201are quite non-reproducible even among crystals of nominally the same composition;3,4(ii)the residual resistivity of ρab is usually large (the smallest value reported to date is 70μ?cm 4,5),as opposed to other systems where the residual resis-tivity in high-quality crystals is negligibly small;and (iii)the temperature

Y.Ando,T.Murayama,and S.

Ono

20406080ρ

a b

[ μ ? c m ]

Temperature [K]

R H

[ 10 -3

c m 3

C

-1 ]

Temperature [K]

Fig. 1.T dependence of (a)ρab and (b)R H of the BSLCO crystals with various x .Note that the extrapolated residual resistivity of x =0.44sample is 25μ?cm,which is the smallest value to date for Bi-2201or BSLCO.dependence of the Hall coe?cient R H is weak and thus the cotangent of the Hall angle θH does not obey the T 2law,3while cot θH ～T 2has been almost universally observed in other cuprates.

Here we report the transport properties of a series of high-quality Bi-2201crystals,in which the normal-state transport properties display behav-iors that are in good accord with other cuprates.We show data on ρab (T )and R H (T )for a wide range of carrier concentrations,from which we extract the characteristic temperatures for the pseudogap.

The single crystals of Bi 2Sr 2?x La x CuO 6(BSLCO)are grown using a ?oating-zone technique in 1atm of ?owing oxygen.Note that pure Bi-2201is an overdoped system,1and increasing La doping brings the system from overdoped region to underdoped region.The actual La concentrations in the crystals are determined with the ICP analysis.Here we report crystals

Magnetotransport Properties of Bi 2Sr 2?x La x CuO 6

c o t θH

a t 10 T [ 10 2

]

T

2.05

[104

K

2.05

]

T

1.70

[104

K 1.70

]

T

1.95

[104

K

1.95

]Fig.2.Plots of cot θH vs T αfor (a)x =0.74,(b)x =0.66,and (c)x =0.44.with x =0.24,0.30,0.44,0.57,0.66,and 0.74,for which the zero-resistance T c is 24,30,33.3,29.2,21.4K,and 17.3K,respectively.The optimum doping is achieved with x ?0.4,which is consistent with previous reports on BSLCO.1,2The optimum zero-resistance T c of 33K (which is in agreement with the Meissner-onset T c )is,to our knowledge,the highest value ever reported for Bi-2201or BSLCO system.

Figure 1(a)shows the T dependence of ρab for the six x values in zero ?eld.Clearly,both the magnitude of ρab and its slope show a systematic decrease with increasing carrier concentration (decreasing x ).One can see that it is only at the optimum doping that ρab shows a good T -linear be-havior:In the underdoped region,ρab (T )shows a downward deviation from the T -linear behavior,which has been discussed to mark the pseudogap.6In the overdoped region,ρab (T )shows an upward curvature in the whole temperature range;the T dependence of ρab in the overdoped region can be well described by ρab =ρ0+AT n (with n =1.14and 1.27for x =0.30and 0.24,respectively),which is a behavior known to be peculiar for the overdoped cuprates.7,8Shown in Fig.1(b)is the T dependence of R H for the six sam-ples.Here again,a clear evolution of R H with x is observed;the change in the magnitude of R H at 300K suggests that the carrier concentration is actually reduced roughly by a factor of 4upon increasing x from 0.24to 0.74.Note that the T dependence of R H is stronger than those previously reported.3,9In our data,a pronounced peak in R H (T )is clearly observed for all carrier concentrations and the position of the peak shifts systematically to higher temperatures as the carrier concentration is reduced.

Y.Ando,T.Murayama,and S.Ono

[ ρa b

(T ) - ρ0

] / a T

Temperature [K]

T [K ]

1-x

Fig. 3.(a)Plots of (ρab (T )?ρ0)/aT vs T for the optimally-doped and underdoped samples.(b)Phase diagram to show the two characteristic tem-peratures for the pseudogap,T ?and T 0,and the superconducting transition temperature T c .T 0is de?ned by the peak in R H (T ).

We observed that the cotangent of the Hall angle,cot θH ,obeys a power-law temperature dependence,T α,where αis nearly 2in underdoped samples (x =0.74and 0.66)but shows a systematic decrease with increasing carrier concentration.10Figure 2shows the plots of cot θH vs T αfor the two under-doped samples (x =0.74and 0.66)and the optimally-doped sample (x =0.44).We note that the T 2law of cot θH is con?rmed for the ?rst time for Bi-2201in our crystals.A particularly intriguing fact here is that cot θH of the optimally-doped sample changes as T 1.7,not as T 2,while ρab shows a good T -linear behavior.This suggests that the Fermi-liquid-like behavior of the

Hall scattering rate,τ?1

H

～T 2,may not be a generic feature of the optimally-doped cuprates.The upward deviation from the T 2behavior evident in Fig.2(a)for the most underdoped sample (x =0.74)is likely to be related to the opening of the pseudogap.11,12

Due to the limited space,we will concentrate below on the implication of our data to the pseudogap in Bi-2201.As we noted above,the downward deviation from the T -linear behavior in ρab (T )has been associated with the pseudogap and the onset of the deviation at T ?gives a characteristic temper-ature for the pseudogap.6Figure 3(a)shows the plot which emphasizes the deviation from the T -linear behavior to determine T ?.We should mention that this type of plot is subject to some arbitrariness and thus the errors in T ?are inevitably large.(Interestingly,in Fig.3(a),even the optimally-

Magnetotransport Properties of Bi2Sr2?x La x CuO6

doped sample shows a T?which is well above T c.)It has recently been recognized that there are two di?erent characteristic temperatures for the pseudogap,13and T?corresponds to the higher characteristic temperature for the pseudogap.Also,it was proposed very recently that the peak in the T dependence of R H may mark the lower characteristic temperature T0for the pseudogap,11,12as does the NMR relaxation rate,ARPES,or the tunnelling spectroscopy.14In our samples,the peak in R H(T)moves to higher temper-atures as the carrier concentration is reduced,which is consistent with the behavior of the pseudogap.We note,however,that it is not clear whether the peak in R H(T)observed in the overdoped samples really corresponds to the pseudogap.For a more detailed discussion on the relation between the peak in R H(T)and the pseudogap,please refer to Ref.[12].

Figure3(b)shows the phase diagram,T vs1?x,for our BSLCO.Note that the horizontal axis is taken to be1?x for convenience;this way,the left hand side of the plot corresponds to underdoping.One can see in Fig.3(b) that a signi?cant portion of the phase diagram has been covered and all the plotted temperatures,T?,T0,and T c,show good systematics with the carrier concentration.It is intriguing that the magnitudes of the characteristic temperatures for the pseudogap is quite similar to other cuprates,14while the T c of the present system is the lowest among the major cuprates.

In summary,we present the data of the in-plane resistivity,Hall coe?-cient,and the Hall angle of a series of high-quality La-doped Bi-2201crystals in a wide range of carrier concentrations.The normal-state transport prop-erties of our Bi-2201crystals show systematics that can be considered to be “canonical”to cuprates.The characteristic temperatures for the pseudogap were deduced from the data to construct a phase diagram.

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