(j)Transport anomalies and suppressionof superconductivity in La2_x_yNdyBaxCu04

Xiuzeng Baa

Department of Physics) Faculty of Science) Hiroshima University)
Higashi-Hiroshima 739) Japan(January, 1997)

Contents
Abstract . . . . .

3

1. Introduction

4

2. Experimental........ .

7

2.1 Sample preparation ..

7

2.2 Measurement methods

7

2.3 experimental setup for thermopower measurement

8

3. Experimental Results............... . ..........

11

3.1 Results for La2-x-y Ndy BaxCu04 (x=1/8, 0~y~0.60).

11

3.2 Results for La2-x-y Ndy BaxCu04 (y=0.20, 0~x~0.25) . .

13

3.3 Comparison between LNBCO and LBCO . . . . .

15

4. Discussion.

...............................

16

4.1 Structural phase transition and transport properties

16

4.2 Structural phase transition and the suppression of superconductivity

16

4.3 Transport properties and the suppression of superconductivity . .

17

5. Conclusion

24

Acknowledgments

25

References

26

Figures.

30

Abstract
It is well kno\vn that a striking suppression ofsuperconductivityof

La 2_xB~Cu04 occurs in a narrow range of x around 1/8, accompanied by a structuralphase ' transition from a mid-temperature orthorhombic (OMT) phase to a lowtemperature tetragonal (TLT) phase below about 60 K. It seems that the suppression ofsuperconductivity requires both a Ba concentration of x=1/8 and a distortion of thelattice from the OMT phase to the TLT phase. For the first condition, partial
replacement of La 3+ by Th 4+ (La 2- x - y ThyB,\Cu0 4) clearly reveals that the holeconcentration of p=x-y=1/8 per Cu is of essential importance to the suppression ofsuperconductivity, instead of Ba concentration of x=1/8. For the second condition,
however, it is not clear to date what specific role the structural phase transition playsin the severe suppression of Te around p=1/8. The purpose of this study is to help clarify
what role the structural phase transition plays and what relationship there exists among
the anomalies of transport properties, the structural phase transitions and the suppressionof superconductivity around X= 1/8.
The interplay among transport properties, structural phase transitions andsuperconductivity in La2_x_yN~B,\Cu04 has been studied. We found that thetemperature at which the thermopower SeT) begins to show unusual decrease on coolingcoincides with that of the minimum in the resistivity for the samples with x around 1/8.
Moreo\ er, the appearance of negative thermopower accompanies the suppression ofsuperconductivity. However, we did not find any anomaly in the thermopower and in
the resistivity at the structural phase transition temperature Td2 for any of the samples
in this study. The results show that in this series of compounds, the suppression ofsuperconductivity around x=1/8 is not directly associated with the structural phasetransition but with the profound change of the transport properties .

3

1. Introduction

It is well kno\vn that the curious disappearance of bulk superconductivity of

La 2_xBax Cu0 4 (LBCO) occurs in a narrow range of x around 1/8 [1] accompanied bya structural phase transition from a mid-temperature orthorhombic (OMT) phase [2Jwith space group Bmab to a low-temperature tetragonal (TLT) phase with space group
P4i ncm

below about 60 K [3-6].

It was believed that the suppressionofsuperconductivity requires both a Ba concentration of x=1/8 and a distortion of thelattice from the OMT phase to the TLT phase. The origin of the suppression ofsuperconductivity in the narrow range of x around 1/8, which is called the 1/8 problem,
has attracted considerable interest. It is hoped that the clarification of this anomaly will
give a· clue to the mechanism of superconducti\'ity in high- Tc cuprates.
In order to investigate the origin of the anon1alies and the relationship between the
structural phase transition and the striking suppression of superconductivity, we havepreviously studied ffi'o series of samples of La 2 - x - y ThyBax Cu0 4 (LTBCO): the one has
a fixed value of y (y=0.020) and varied values of x (0.007$x::;0.170) [7,8] and the otherhas a fixed value of x (x=0.145) and varied values of y (0::;y::;0.04)[9]. For the firstseries of san1ples the center of sharp depression of Tc' as well as of the structural phasetransition, shifts from x=1/8 for LBCO to x=0.145 for LTBCO with y=0.020 [shown in
Fig.1(a)]. For the second series of samples the TLT phase is most stable at y=0.020 andthe striking suppression of superconductivity is also centered at y=0.020 [shown in
Fig.1(b)]. The partial replacement of trivalent ions La 3+

byquadrivalent ionsTh4+ (La 2- x - y ThyBax Cu0 4) clearly reveals that the suppression of superconductivity iscentered at x=1/8+y rather than at x=1/8. The same conclusion has been drawn by studyof substitution with other rare earth element [10]. On the other hand, with partial
replacement of La 3+ by trivalent ions R3+ (La2_x_y~BaxCu04' R=Gd, Oy) [11], theminimum of Tc is still at x=1/8. These substitute studiies of different valent ion for La 3 +indicate that a particular value for the concentration of holes per Cu, p=1/8, is ofessential importance to the suppression of superconductivity, instead of Ba concentration4

of x=1/8.
For the second condition, many efforts [12-16] have been made which focus on
the effects of the structural phase transitions at low temperatures on the suppression ofsuperconductivity. However, it is not clear to date what specific role the structural phasetransition plays in the severe suppression of Te around p=1/8. The previous studies on
the element substitution effects for Ba in LBCO [6,16] show a strong correlation
between the lattice instability and electronic properties. The disappearance of the TLT
phase is correlated with the recovery of Te in the Ba-site substituted systems(La 2_xBax_yMyCU04. M=Sr and Ca). Nevertheless, in high pressure studies of LBCO
around x=1/8 [17,18] , Te increases until the TLT phase is completely suppressed bypressure. However even at the pressure of 20 kbar at \vhich the TLT phase is suppressed[18] , the reduction in Te is still evident atx=1/8 [17]. It therefor seems that the structuralphase transition is not the direct origin of the suppression of Te near p=1/8. The purposeof this study is to help clarify what role the structural phase transition plays and what
relationship there exists among the anomalies of transport properties, the structural phasetransitions and the suppression of superconductivity around x=1/8.
Because Nd is stable only as trivalent ions in oxides and has a smaller ionic radiusthan La, the substitution for La 3+ by Nd 3+ will not change the density of holes but willchange the crystal structure even for the samples with the same Ba concentration.
Therefore we can readily control the hole density p and the structural phase transi60n
temperatures by changing Ba concentration x and Nd concentration y, respectively. In
the present study

we have studied

two series of polycrystalline samples of

La 2_x_y N~BaxCu04 (LNBCO). For the first series of samples we have chosen the fixedvalue x (Ba)=1/8 to investigate the effects of Nd doping in LBCO on the structural phasetransition, transport properties and superconductivity at x=1/8. For the second series ofsamples we have chosen the fixed value y(Nd)=0.20 because this value changes Td2
substantially without destructing superconductivity severely. The interplay among thetransport properties, superconductivity and structural phase transitions was investigatedby electrical resistivity,

thermoelectric power, diamagnetic susceptibility,

temperature powder X -ray diffraction, and ultrasonic velocity measurements.

5

low-

Our previous study [19] indicated that the structural phase transition temperature
Td2 (OMT - TLT) is enhanced by SOK by partial substitution of Nd withy=0.20 in LBCO

with x=1/8. A subsequent study of transport properties indicated no anomalous behavior
at Td2 [20]. The present results show further that the unusual decrease of thermopower
on cooling occurs just below the temperature (we call Tmin) at which the resistivity takes
the minimum. The suppression of superconductivity is connected with the appearanceof negative thermopower. There is no evidence that a structural phase transition occurs
at about the temperature Tmin by X-ray diffraction as well as by ultrasonic velocity
measurement. Conversely we do not find any anomaly of the transport properties atabout Td2 . These mean that the suppression of superconductivity for LNBCO with x
around 1/8 is not directly related with the structural phase transition but is caused by the
instability of the electronic state.

In Section 2 of this thesis we present the sample preparation and the measurement
methods. We also give a brief description about the homemade experimental setup forthe thermopower measurement. In Section 3 we show the experimental results of various
measurements. In Section 4 we discuss the experimental results and propose a simple
phenomenological density-of-states model to describe the origin of the negative
thermopower. Finally in Section S we give our conclusion.

6

2. Experimental procedure

2.1 Sample preparation
In this study two series of polycrystalline samples of La2_x_yN~BaxCu04(LNBCO) were prepared: the one has a fixed value of x (x=1 /8) and varied values ofy (0:::;:y:::;:0.60 : y=O, O.OS, 0.10, 0.20, 0040, 0.60) and the other has a fixed value of y(y=0.20) and varied values ofx (0:::;:x:::;:0.2S0 : x=O, 0.02,0.04,0.06, 0.07S, 0.100, 0.110,

0.120, 0.12S, 0.130, 0.140, 0.lS0, 0.160, 0.170, 0.17S, 0.190, 0.200, 0.210, 0.22S,
0.2S0). In order to make a comparison between LNBCO and LBCO, we also preparedtwopolycrystalline

samplesof La 2 _x Bax Cu0 4 with x=0.14 and x=O.16. The

polycrystalline samples were prepared by solid-state reaction from appropriate mixturesof dried powders of La 2 0 3 , BaC0 3 , CuO and Nd 20 3 in stoichiometric ratio. Each ofthese po\vders has a purity of 99.99%. The powders were mixed, pelletized, and reacted
at 1000 °C in air for 24 hours. Then the pellets were reground, pelletized, and reacted
at 10S0 °C for 24 hours. This procedure was repeated again but reacted at a slightly
higher temperature of 1100 °C. Finally, the pellets were annealed in flowing oxygen at
SOO °C for 96 hours to minimize oxygen deficiencies. The samples were confinned to
be of single phase by powder X -ray diffraction.
Each of the annealed samples has a weight of about 109 with a diameter of about
20mm and a thickness of about 8mm. The samples were then cut with a diamond saw.
The dimensions

of

the

are approximately l.Ox2.0x12.0 mm 3 ,cut samples

l.Oxl.Sx10.0 mm 3 and 2.0x3.0x4.S mm 3 for resistivity, thermopower and ultra onic
velocity measurements

respectively. For low-ternperature X-ray diffraction anddiamagnetic susceptibility measurements, the samples were ground into powder withweights of about 300mg and 400mg, respectively.

2.2 Measurement methods
To investigate the structural phase transitions, we perfonned powder X-ray
diffraction in a wide temperature range from room temperature down to SK using a
7

flow-type 4He cryostat. To determine the superconducting transition temperature, wemeasured diamagnetic susceptibility by a SQUID m.agnetometer on cooling in a fieldof 3.0 De. Electrical resistivity was measured by an ordinary four-probe method witha constant current of 10mA. Thermopower was measured by a conventional method witha homemade apparatus. We have controlled the temperature gradient at 1.4K/8mm in theentire temperature range from room temperature down to 5K for every thermopower
measurement. The linearity of the measured thermal electromotive force with respect
to the temperature gradient was carefully checked.
For a sensitive detection of a possible structural phase change at Tmin' We havealso measured the ultrasonic velocity for La1.68Ndo.20Bao.12Cu04 at a frequency of 17MHz.

2.3 Experimental set up for the thermopower measurement
As is well known, a temperature gradient over a metal sample will give a voltage
difference from the cold end to the hot end of the metal sample. Thermopower isdefined as the thermo electromotive force per unit increment in temperature. As isshown in Fig.2, we can determine the thermopower of the sample Ss by measuring thevoltages V 1 and V 2 . V 1 can be expressed as(1 )where SA is the thermopower of the copper lead at a given temperature and Ss is thatof the sample. Then,(2)

8

This gives to a good accuracy(3)

in which SA and Ss are the values at T = To + ~ToI2. In the same way,(4)

where SB is the thermopower of the alloy constantan lead. Combine Eq.3 with Eq.4,

SAB

(5)

where ' SAB=SA -SB' From Eq.3 the thermopower of the sample can be expressed as(6)

With Eg.S, we express Eq.6 as(7)

Eq.7 is the fundamental equation which we used to calculated the thermopower of thesamples in this study. We have used the values of the international industrial standards

9

for the relative thermopower SAB [21]. The absolute thermopower of the copper lead SAv,ras given by reference 22. Because at lower temperatures the absolute thermopower ofcopper is very sensitive to impurities and defects of specific material used, we havecalibrated SA below SOK by using a superconducting sample of YBCO. As SAB and SA
are known the thermopower of the sample Ss is obtained by measuring VI and V 2 ,The experimental

setup for the measurement of thermopower is shown

schematically in Fig.3 and the sample holder was depicted in FigA. As shown in FigA,
the sample is attached betv.'een tv.'o copper blocks: the cold block and the hot block. The
tv.'o blocks are linked up by a brass bolt with the thermal conductance low enough toestablish the temperature gradient but high enough compared with that of the sample.
Thus the heat will flow mainly through the brass bolt and the thermal resistance between
the tv.'o copper blocks v,rill not be affected by the size or the shape of the samples. Twopairs of thermocouples were connected to the sample. We have used thermocouple pairsof type T (copper and constantan). This thermocouple can be used in the temperature
range from 3K to 673K. Two heaters are attached to the two copper blocks. The mainheater on the cold block is used to establish the systelm temperature (T). The sub heateron the hot block is used to establish a temperature gradient in the sample by heating the
hot block to a higher temperature (T+ 11. T). Two resistance thermometers in the cold
block are used to measure the system temperature. One is the platinum resistancethermometer (PRT) whkh is used at higher temperatures (T>SOK); the other is thegermanium resistance thermometer (GRT) whkh is used at lower temperatures (T<S5K).
The sample holder is mounted in a vacuum can. After the sample is set up, thevacuum is pumped to a typical pressure of 4x10- 6 Torr. This high vacuum is needed tohold a stable temperature inside the vacuum can. Then the vacuum can i put into adouble glass dewar. In the outside dewar we fill liquid nitrogen and in the inner dewar
liquid helium. Bet\veen the two liquids there exists a vacuum space which is u ed topre\'ent the helium to boil off too fast. By the DC current supplies (as shown in Fig.3),
through the Temperature Controller we can control the system temperature at any given
temperature (from SK to room temperature) with an appropriate temperature gradient (inour experiment we let 11. T/ I1./ = l.4K/Smm). After the system temperature becomes stable,10we measure VI and V 2 by the DMM. A micro-computer is used to evaluate Ii T, SA' SAB
and Ss' and the data for T, VI' V 2, liT, SA' SAB and Ss are stored into a disc and printed
out by a printer at the same time.

113. Experimental Results

3.1 Results for La2_x_y N~Ba.xCu04 (x=1 /8, 0~y~0.60)
Powder X-ray diffraction study at low temperatures revealed that with any Ndconcentration investigated in this study La1.875-yN~Bao.125Cu04 (0~y~0.60) undergoes
the same sequence of structural phase transitions

as La1.875Bao.125Cu04. Figure.5(a)shows a part of the powder X -ray diffraction spectra at some selected temperatures for

y=0.20 and x=1/8. As temperature decreases the structure first transforms from a hightemperature tetragonal (THT) phase to the OMT phase at the temperature Tdl and thentransforms to the TLT phase at some lower temperature T d2 . In this study we define Td2
as the temperature at which the ratio of the integrated intensity of the TLT peaks to thetotal peak intensity becomes 50 %. The former transition (THT-OMT) is of the second
order, whereas the latter transition (OMT - TLT) is of the first order. As shown in
Fig.5(a), at 300K there exists only one peak which is (220) diffraction peak of the THT
phase. With decreasing temperature it transforms to two peaks which are (040) and(400) peaks of the OMT phase. At 150K these two peaks are clearly visible. Below
130K a new peak, the (400) peak of the TLT phase, appears between these two peaks.
It grows with the expense of the intensities of the OMT peaks as temperature decreases.

The OMT peaks in our samples did not disappear entirely even at 5K, but their intensity
was less than 16 % of the total peak intensity.
Nd concentration dependence of Tdl and Td2 is plotted in Fig.6. As shown, both
Tdl and Td2 increase substantially with increasing Nd concentration y. With increasing

Nd concentration, Td2 increases from 65K (V=O) up to 135K 0)=0.60) for the same holeconcentration of p= 1/8, although it tends to saturate at higher values of y.
Diamagnetic susceptibility of the powdered samples of La2_x_y N~BaxCu04(x=1 /8) on cooling in a field of 3.0 Oe is shown in Fig.7. Only a weak diamagnetism
is perceived for small Nd concentrations. With increasing y, the diamagnetism becomes
unnoticeable and Curie-like behavior due to the localized moment of Nd 3+ becomesappreciable. After subtracting the latter contribution using measured paramagnetic
12

susceptibility at 10 kOe, we estimate the diamagnetic contribution to be at most about
1 % of the volume fraction for any y.
Fig.8 sho~rs the temperature dependence of the resistivity, showing a distinctive
upturn of the resistivity below Tmin=60-70K. With decreasing temperature a two-stepsuperconducting transition was observed. The higher one, a small negative step at about
30 K, is most likely due to the filamentary part of the specimen. The lower one, agradual decrease toward zero below 10 K, may indicate the intrinsic superconducting
transit.ion. However it is weak superconductivity as probed by the flux expulsion
measurements (at most 1 % volume fraction of perfect diamagnetism). Above T min theresistivity shows the metal properties. We did not found any anomaly in the resisti ity
at Td2 for all the samples as shown in Fig.8.
The variation of the thermopower S(n is shown in Fig.9. The overall temperaturedependence is not much affected by the change in Nd concentration y. It shows only asmall decrease in the magnitude with increasing y. At higher temperatures thethermopower first increases slowly with decreasing temperature and thendecreases

slowly ~rith further decreasing temperature below about lS0-200K. Below T=60-70K(which we call T* in this study), S begins to fall down sharply, crosses zero rapidly,
attains the largest negative value at about 2SK and then returns toward zero as thetemperature approaches zero.
We summarized the Nd concentration dependence of T d2 , Tmin ~nd T* in Fig.10.
As sho~rn, Td2 increase substantially \vith increasing Nd concentrationy . With increasing
Nd concentration, Td2 increases from 6SK (y=O) up to 13SK (y=0.60) for the same holeconcentration of p=1/8. In contrast, T min and T* show a much weaker y dependence thanT d2 · At lower Nd concentration (near y=O), the values of Tmin' T* and Td2 are nearly thesame. With increasing Nd concentration, Tmin and T* agree with each other but Td2
becomes larger than Tmin and T* by about GOK for the sample with y=0.40. It should be
noted that Td2 coincides with Tmin and T* in LBCO without Nd (y=O), suggesting thatthe interplay bemreen structural phase transjtjon and charge instability becomes
important only in the immediate vicinity of that composition.

13

3.2 Results for La2_x_yN~B~CU04 (y=0.20, 0:Sx:s0.2S)
Low-temperature power X-ray diffraction study revealed that the samples withhigher Ba concentration (x~0.10 , y=0.20) undergoes the same sequence of structuralphase transition as La1.87SBao.12SCu04 as described in Section 3.1. For the samples withlower 'Ba concentration (x<0.10 , y =0.20) another first order structural phase transition,from the OMT phase to the low-temperature orthorhombic (OLD phase consistent withspace group Pccn, was observed. This transition is shown in Fig.S(b) with a part of thepowder X-ray diffraction spectra at selected temperatures for y=0.20 and x=0.07S. At
300K the existing two peaks are (040) and (400) diffraction peaks of the OMT phase.
Below lOOK 1\\'0 new peaks of the OLT phase appear between the (040) and (400)peaks of the OMT phase. These four peaks coexist down to SK.
Ba-concentration dependence of Tdl and Td2 for La2_x _yN~BaxCu04 (y=0.20,
O::;x:sO.2S) is plotted in Fig.11. Compared with Nd--free LBCO, for which structuralphase transition (OMT - TLD occurs only in a narrow range of x around 1/8, the Ndsubstituted system has higher values of Tdl and Td2 for the same Ba concentration andexhibits the structuralphase transition(OMT -TLD

in a wider range of x

(0.100:Sx:s0 .200).
Temperature dependence of the magnetic susceptibilities for these samples areshown in Fig.12. The Ba concentration dependence of Te and the magnetic susceptibility
at SK are summarized in Fig.13. Here we define Te as the temperature at which thesample exhibits 1 % volume fraction of perfect diamagnetism. As shown in Figs.12 and13, the suppression of superconductivity is not restricted in the immediate vicinity of

x=1/8 but spreads over somewhat wider range of x (O.120:Sx:S0.140).
The temperature dependence of the thermopower is shown in Fig.14. At higher
temperatures the thermopower S(1) decreases systematically with increasing Ba
concentration x, and increases slowly with decreasing temperature for gi en x. At lower
temperatures S(1) approaches zero in three kinds of 'ways depending on x as sho\\Tn in
Fig.1S. For the overdoped samples , such as the one \.vith x=0.2S , S(1) decreases slowly
with decreasing temperature and monotonically approaches zero at T=O. For the14

superconductors, for example \vith x=O.190, S (T) begins to fall dO\\Tn sharply at sometemperature slightly above Tc and drops down to zero in the superconducting state. For
the samples with x around 1/8 (O.120:$x~O.140) , for which the superconductivity issuppressed, SeT) begins to fall down sharply at the ternperature T * , crosses zero rapidly,
attains the largest negative value and then return toward zero as the temperatureapproaches zero.
Fig.16 shows the relationship bet\\reen the anomaly of resistivity and that ofthermopower. Ba concentration dependence of T e, T d2 , T min and T* \\'as summarized in
Fig.17 . For the same Ba concentration, the values of Tmin and T* are nearly the samebut the value of T d2 is about 50K higher than those of Tmin and T*; the thermopower
begins to drop at the temperature of minimum in resistivity. It is also evident from
Fig.16 that there is no anomaly at Td2 either in the resistivity or in the thermoelectric
power at least for the polycrystalline samples.
Moreover, as we already noted above, the suppression of superconductivity not
only occurs at x=1/8 but spreads 0\ er somewhat wider range of x (O.120~x~O . 140). The
disappearance of superconductivity at x=O .140 is caused by the introduction of Nd which
induces the structural phase transition (OMT - TLD, indicating that the TLT phase is notfavorable to superconductivity. Nevertheless the TLT phase itself is not the direct originof the severe suppression of superconductivity around x=1/8. As shown in Figs.13 and17, bulk superconductivity exists under the TLT phase for the samples with x far awayfrom 1/8. Among them, at x=O.100 and x=O.170, the volume fractions of perfectdiamagnetism are higher than 16% at 5K even on powdered samples.
As a sensitive probe of a possible structural phase change at Tmin' we have alsomeasured the ultrasonic velocity for La1.68Ndo.20Bao.12Cu04. As shown in Fig.18
obvious hardening is observed at about 125K, which is the response to the structuralphase transition (OMT-TLT). This temperature is consistent with Td2 (onset) determinedby powder X-ray diffraction. At T min and T* , however, we did not find any anomaly
in the' T -dependence of the ultrasonic velocity. Therefore there is no evidence for
additional structural phase transition at the temperatures of T min and T*.

15

3.3 Comparison between LNBCO and LBCO
To compare with LBCO let us now focus on samples with x=O.140 and x=O.160.
The results are shown in Fig.19. For the same Ba concentration of x, compared with
LBCO (y=O), a visible difference SCT) appears only at lower temperatures connectedwith the difference in superconducting properties. At x=O.140 for the sample of LNBCO
CTc<4~)SeT) becomes negative below 30K associated with the suppression ofsuperconductivity. But for the sample of LBCO CTc=25K) such negative S does notappear. At x=O.160 SCT) of both samples are similar, reflecting that the samples with y=O
and y=O.20 are both superconductors with Tc=28K a.nd 18K, respectively. As we haveshown in Section 3.1 negative SCT) also appears for the samples of La2_x_yN~BaxCu04(.x=1/8,

O~y~O.60) for which superconductivity is suppressed. Similar phenomenon ofnegative thermopower
CLa2_x_y~SrxCu04)

was

also

observed

in

the

rare

earth

doped

LSCO

associated with the suppression of superconductivity [23]. It is

important to note in Fig.19 that although Td2 is very different between LBCO andLNBCO for the same value of x, we did not find any anomaly in the thermopower at
Td2 for these four polycrystalline samples. Rather,

it is clear that the suppression ofsuperconductivity has some relation with the appearance of the negative values ofthermopower.

16

4. Discussion

4.1 Structural phase transition and transport properties
In the samples of La2_x_yN~BaxCu04 (x=1 /8, 0~y~0.60) the structural phasetransition (OMT-TLT) temperature Td2 increases substantially with increasing Nd
concentration y. As we have shown, with increasing Nd concentration, Td2 increases
from 6SK (y=O) up to 13SK (y=0.60) for the same hole concentration of p=1/8. In
contrast, Tmin and T * show a much weaker y dependence than Td2 . At lower Ndconcentration (near y=O), the values of Tmin' T* and Td2 are nearly the same. With
increasing Nd concentration, Tmin and T* agree 'Nith each other but Td2 becomes larger
than T min and T* by about 60K for the sample \vith y=0.40 (shown in Fig. 10). For the
samplesof

La2_x_yN~BaxCu04(y=0.20,0.120$x~O.140)

with

a

given

Ba

concentration, the values of Tmin and T* are nearly the same (60-70K) but the value of
Td2 is about SOK higher than those of Tmin and T* (shown in Fig.17). It is clear that theanomalies of the transport properties and the structural phase transition occur at different
temperatures and that there is no anomaly at Td2 neither in the resistivity nor in thethem10power for the samples with higher Nd concentration. This seems that there is no
direct relation between the anomalies of the transport properties and the structural phasetransition. It should be noted that Td2 coincides with T* in LBCO without Nd (y=0),
suggesting that the interpla) between structural phase transition and charge instability
becomes important only in the immediate vicinity of that composition. One the other
hand it is noticed that the anomalies of transport properties, such as the minimum of
resistivity and the negative thermopower, are observed only in the TLT phase for the
samples in our study. Therefore the TLT phase seerns to be a necessary condition for
the anomalies of the transport properties to appear.

4.2 Structural phase transition and superconductivity
As wehave already noted above, compared with Nd-free LBCO, for which

structural phase transition (OMT - TLT) and suppression of superconductivity occur only17

in a narrow range of x around 1/8, the Nd-substituted system exhibits the structuralphase transition eOMT - TLT) in a wider range of x

eO.100$.x~0.200)

and thesuppression of superconductivity not only occurs at x=1/8 but spreads over somewhat
wider range of x eO.120~x~0.140). The disappearance of superconductivity at x=0.140
is caused by the introduction of Nd which induces the structural phase transition (OMTTLT), indicating that the TLT phase is not favorable to superconductivity. Neverthelessthe TLT phase itself is not the direct origin of the severe suppression ofsuperconductivity around x=l/S. As sho\vn in Figs.13 and 17, bulk superconductivity
exists even under the TLT phase for the samples with x far away from 1/8. Moreover,as we have mentioned in Section 1, in high pressure studies of LBCO around X= 1/8[17,18], Tc increases until the TLT phase is completely suppressed by pressure. Howevereven at the pressure of 20 kbar at which the TLT phase is suppressed [18], the reduction
in Tc is still evident at x=1/8 [17].

4.3 Transport properties and the suppression of superconductivity
We ha\ e demonstrated that there is no direct relation between the anomalies of thetransport properties and the structural phase transition. In contrast, there exists a definite
relation betv.'een the anomaly of the resistivity and that of the thermopower. The sharpchange in SeT) coincides clearly \vith the minimum of the resistivity for the samples
with x around 1/8: when the resistivity takes the minimum, the thermopower begins todrop and become negative at lower temperatures.
We argue that the anomalies of the transport properties are caused by charge
localization. Charge localization is clearly recognized in the transport properties: at Tmin
the sample changes from metallic to nonmetallic behavior; the thermopower begins todrop sharply below T* and becomes negative at lower temperatures for the samples withx around l/S. This bchavior of the thermopower is attributed to the development of a
gap-like structure in the density of states . We will discuss below that the sharp decreasein the thermopower starting at T* is consistent with the charge localization.
As shown in Fig.14, thermopower decreases systematically with increasing Ba
concentration at higher temperatures but changes drastically at lower temperatures. We
18plotted the Ba concentration dependence of the thermopower in Fig.20 at several
selected temperatures. At higher temperatures all of the samples have positive values of

SCT). At lower temperatures negative SCT) appears for the samples with x around 1/8 assho\\Tn in Fig.20 for 30K and 10K.
For a sufficiently pure metal the thermopower SCI) is approximately linear intemperature and shows a narrow phonon drag peak at lower temperatures [22,24]. This
phonon drag thermopower is often diminished by impurities. But for high- Tc cuprates
the thermopower is more complicated than that of a usual metal. In general, thethermopower of high- Tc cuprates decreases with increasing hole concentratjon p; S(P)is positive for lightly doped materials and negative for highly over-doped materials. The
details of T dependence are quite different among different high- Tc cuprates. For LBCO
system, as mentioned above, SCT) shows a strong and broad peak at high temperatures.
This behavior of the thermopower has also been reported for other materials based on
La 2 Cu0 4 [20,23,25-34]. For YBa 2 Cu 30 7 system [29,32,35-47], SCT) shows a weakertemperature-linear dependence and takes negative values for the samples with high holeconcentration. For Bi 2Sr 2 CaCu 2 0 s system [43,45,46], SCT) contains a term which varies
linearly with temperature with a large negative slope. Before discussing the negative
thermopower of LNBCO let us first briefly review how the thermopower of high-Tc
cuprates has been interpreted.
Because the basic physics of high- Tc cuprates is described by the Hubbard model,
it is natural to interpret the experimental results using the predictions of the Hubbard
model for the thermopower which considers correlated hopping [26,28]. In the hightemperature limit, if kBT is greater than the bandwidth W but much less than the on-site
Coulomb repulsion U , the thermopower is expressed [48,49] as:sk~ ( ln2 + In-.l~ )
lei
I-p

(8)

where p is the number of hole per Cu site. The ln2 term comes from the spin degree of19freedom and is absent if Cu 2+ ions order magnetically such as in pure La 2 Cu04 .
According to Eq.8, S decreases as p increases but remains positive as long as p<1/3.
This is at least consistent with the experimental result that the thennopower decreasesv.,rith the increase of hole doping, although the condition of kBT greater than W is notsatisfied for the cuprates. According to Eq.(8) the thennopower has a contribution fromspin entropy which should be quenched by a large magnetic field. Yu et a1. [29] reported
that in studying the effect of the large magnetic field on the thennopower for YBCO
their experimental results, howe\'er, show a field independent thennopov.,rer up to 30T.
There seems to be some problems to apply this theory to high- Tc cuprates.
Kang et a1. [42] proposed the magnon-drag effect on the thennopower. Theysuggested that even in superconducting samples the presence of antiferomagnetic
fluctuations gives rise to an extra contribution to the thennopower as a re ult ofinteraction with electrons. However, the experimental confirmation needs to be made.
The magnon-drag thennopower has a similar expression to that of the phonon-dragthennopo'Ner except that the phonon specific heat is replaced by magnetic specific heat.
Trodahl [50] has tried to interpret the complex T dependence of the thennopowerof high- Tc cuprates by a model in which the total thennopower (Snet) consists of twocontributions: one is the diffusion thermopower (Sd) which is proportional to temperatureand has a negative value, and the other is the phonon drag thennopower (S g) which
depends only weakly on temperature and has a positive value. He argued that because
the phonon-phonon scattering remains weaker than phonon-electron scattering up to
300K,· the phonon drag thennopower is strong even at room temperature. The phonondrag thennopower Sg consists of positive and negative contdbution, depending on therelative
direction of the velocity change of the electron with respect to the phonon velocity. Thebalance between the nvo contributions is controlled by the hole doping. As holeconcentration is raised negative contribution increases and the total Sg decreases. Thismodel seems to reproduce reasonably the experimental results of thennopov.,rer of high-Tc cuprates.
If such a tenn is assumed for our experimental results shown in Fig.14, Snet is20mainly decided by Sg for all of the samples in this study at high temperatures (T>T*)
and also for the samples with x not around 1/8 at low temperatures because Snet ispositive or zero (in superconducting state). But for the samples with x around 1/8 atlower temperatures (T<T*) Snet takes negative values, showing that Snet is decided
mainly by Sd' The diffusion thennopower can be expressed by the Mott fonnula [22,24]:(9)

where kB is Boltzman's constant, IeI is the elerrlentary charge, and 0(£) is the
hypothetical conductivity for the Fermi level at £. Using a well known expression of0( £F) for the isotropic three dimensional case

(10)\\Fe express Eq.(9) as(11 )Here vF is the Fermi velocity, r is quasiparticle life time and }l( £F) is the density ofstates at the Fermi level. In this expression, we argue that the first term plays a key rolein the appearance of negative SeT) at lower temperatures for the samples with x around

1/8. In order to describe the origin of the negative thermopower, we consider a simple
phenomenological density-of-states model as sketched in Fig.21, in which it is assumed
that the Fermi le\"el £F shifts by doping. Such a model is in fact supported by the results
21

of ultrasonic measurements on single crystals of LSCO [51]. At higher temperatures(T>T*) as sho\\Tn in Fig.21(a), the density of states at the Fermi level decreases withincreasing cF' According to Eg.(11), a negative ene.rgy-derivative of the density ofstates will give a positive contribution to the diffusion thermopower. The first term will
give a positive contribution and this contribution will decrease with increasing hole
doping; but the total diffusion thermopower is negative. At lower temperatures (T<T*)as shown in Fig. 21(b), for the samples around x= 1/8 the density of states splits into two
peaks, reflecting the formation of a gap due to the charge localization. The formationof the gap-like structure may lead to a positive energy derivative of the density of statesat the Fermi level, to support the observed large negative contribution to thermopower
according to Eg.(11). We believe that this is the main reason that thermopower takenegative values for the samples with x around 1/8. The energy dependence of rand v F
may also give substantial effects on the appearance of the negative thermopower andfurther theoretical work is needed.
We have shown that the suppression of superconductivity around x=1/8 is directly
associated with the sharp change of the transport properties. This change is likely causedby the localization of the electrons near the Fermi level. It is important to note that thischange occurs below about 70K, which is about 50I{ lower than the structural phasetransition temperature T d2 . This means that the suppression of superconductivity iscaused by the instability of the electronic states near the Fermi level. This result is
consistent with the formation of charge order as suggested by recent electron diffraction[52] as well as neutron diffraction [53] studies.
In the electron diffraction experiment for the :~ample LNBCO with y=0.20 and

x=1/8, Ito et al. [52] observed the [ 1/2 1/2 0 ] type superlattice below 70K showing theformation of charge order, whereas they observed that the transition temperature Td2 isabout 110K for that sample. These values well correspond to T* and T d2 , respectively
in the present study.
Similar superlattice was observed in La2_x_yN~ySrxCu04 with x=0.12 and y=0.40
by neutron diffraction [53]. Tranguada et al. reported that they observed two kinds ofsuperlattice peaks below 70K: spin-related and charge-related peaks. That experiment22

gives a suggestion that there exists a static ordered stripe of charge and spin at low
temperatures in this compound. The ordered charge stripe has a period of 4a, four times
larger than that of the lattice parameter a. This causes the appearance of an energy gap
at J[ /4a in the reciprocal lattice, leading to the suppression of the superconductivity.
Although the atomic displacements in the TLT phase seem to be favorable to theobserved superlattice associated with the charge ordering [53], we still do not fully
under~tand why the charge localization occurs only in the TLT phase.

23

5. conclusion

By our experiment, we conclude that the structural phase transition (OMT- TLT)
itself is not the direct origin of the suppression of superconductivity in LNBCO around

p=1/8, although the TLT phase is not favorable to superconductivity. In contrast, thesuppression of superconductivity in LNBCO around p==1/8 is directly associated with the
sharp change of the normal-state transport properties. The anomalies of the transport
properties are attributed to the charge localization. This means the suppression ofsuperconductivity is caused by the instability of the electronic states near the Fermi levelat temperatures lower than Td2 for the structural phase transition (OMT - TLT).
Let us summarize a scenario for the suppression of superconductivity for x around
1/8. As the temperature decreases below Tmin' the electrons begin to localize and thecharge order is formed. The density of states split into two peaks and the gap-likestruct~reis produced. The sharp change of the transport properties reflects such an

instability of the electronic state. The charge ordering reduces the density of states at the
Fermi level and suppresses the superconductivity. The connection between theappearance of negative thermopower and the suppression of superconductivity can beinterpreted naturally by this picture.

24

Acknowledgements

I would like to express my sincere appreciations to Professor Toshizo Fujita andAssociate Professor Y oshiteru Maeno for their useful guidance, strong encouragement,
valuable advice and critical reading of this thesis. I wish to thank Dr.Takashi Suzuki and
Fumihiko Nakamura for fruitful discussion and useful advice. I am particularly gratefulto Dr.Koji Yoshida, Mr.Yasuki Tanaka and J0rgen Nyhus for their help in experiments
during a long time. I am also indebted to all of the students of our group for theirfriendly sentiments. I would like to thank all members of the Department of Physics fortheir hospitalities during the years I study in Hiroshi:ma University.

25References[1]

A.R. Moodenbaugh, Y. Xu, M. Suenaga, TJ. Folkerts and R. N. Shelton,
Phys. Rev. B 38 (1988) 4596.

[2]

Both abbreviations THT and HIT for the tetragonal phase at hightemperatures have been widely used at least since mid 1970's in the studyof organic K2NiF 4-type compounds. In light of the orthorhombic phaseat lowtemperatures foundlater in Nd-substituted La 2_x Srx Cu0 4, wemuch prefer to place 0 (for orthorhombic, or T for tetragonal) in front anddesignate Bmab and Pccn phases as OMT (mid,- temperature orthorhombic)
and OLT (low-temperature orthorhombic), respectively. Some authors use
LTO (low temperature orthorhombic) and LTLO (low temperature lessorthorhombic) for these phases.
[3]

J.D. Axe, D. E. Cox, K. Mohanty, H. Moudden, A. R. Moodenbaugh, Y. Ku
and Y . R. Thurston, IBM J. Res Develop. 33 (1989) 382.

[4]

T. Suzuki and T. Fujita, J. Phys. Soc. Jpn. S8 (1989) 1883; Physica C 1S9(1989) 11l.

[5]

J.D . Axe, A.H. Moudden, D. Hohlwein, D.E. Cox, K .M. Mohanty, A.R.
Moodenbaugh, and Y.W. Xu, Phys. Rev. Lett. 62 (1989) 275l.

[6]K . Yoshida, F. Nakamura, Y. Tanaka, Y. Maeno and T. Fujita, Physica C
203 (1994) 37l.

[7]

Y. Maeno, N . Kakehi, M. Kato and T. Fujita, Phys. Rev. B 44 (1991) 7753.

[8]

Y. Maeno, F. Nakamura, T. Suzuki and T. Fujita, JJAP series 7'.(Mechanisms of Superconductivity" (1992) 9l.

[9]X.Z. Bao, Y. Maeno, Y. Tanaka, K. Yoshida and T. Fujita, Advances in the
Superconductivity VI (1994) 375.

[10] Y. Koike, T. Kawaguchi, N . Watanabe, T. Noji, and Y. Saito, Solid State
Commun. 79 (1991) 155.
[11] X.Z. Bao, Y. Maeno, F.Nakamura, S. Nishizaki and T. Fujita, Advances in26

Superconductivity vn (1995) 24l.
[12] M.K. Crawford, R.L. Harlow, E.M. McCarron, ·W.E. Fameth, J.D. Axe, H.
Chou and Q.Huang, Phys. Rev. B 44 (1991) 7749.
[13] B. Buchner, M. Cramm, M. Braden, W. Braunisch, O. Hoffels, W. Schnelle,
R. Muller, A. Freimuth, W. Schlabitz, G. Heger, D.l. Khomskii and D.
Wohlleben, Europhys. Lett. 21 (1993) 953.
[14] S.J.L. Billinge, G.H. Kwei, A.C. Lawson, J.D. Thompson and H. Takagi,
Phys. Rev. Lett. 71 (1993) 1903.
[15] B. Buchner, M. Breuer and A. Freimuth, Phys. Rev. Lett. 73(1994)184l.
[16] F. Nakamura, K. Yoshida, Y. Tanaka, X.Z. Bao, Y. Maeno and T. Fujita, J.
Supercon. 7 (1994) 33.
[17] N. Yamada and M. Ido, Physica C 203 (1992) 240.
[18] S. Katano, S. Funahashi, N. Mori, Y. Ueda and J.A. Femandez-Baca, Phys.
F-ev. B 48 (1993) 6549.
[19] Y . Tanaka, Y . Maeno, F. Nakamura and T. Fujita, Physica B 194-196(1994) 2053.
[20] J. Yamada, M. Sera, M. Sato, T. Takayama, M. Takata and M. Sakata, J.
Phys . Soc. Jpn. 63 (1994) 2314.
[21 J R.P. Reed and A .F. Clark, Materials at Lo~1 Temperatures (American
Society of Metals, Ohio, 1993).
[22] D.K.C. MacDonald, Thermoelectricity (John \Viley & Sons, New York,
1962).
[23J T. Suzuki, M. Sera, T . .Hanaguri and T. Fukase, Phys. Rev. B 49 (1994)
12392.
[24J R.D. Barnard, Thermoelectricity in metals and alloys (Taylor & Francis,
London, 1972).
[25J J.T . Chen, C.J. McEwan, L.E. Wenger and E.M. Logothetis: Phys. Rev. B 35
(1987) 7124.
[26J J.R. Cooper, B. Alavi, L-W. Zhou, W.P . Beyermann and G. Gruner, Phys.
Rev . B 35 (1987) 8794.
27

[27] M.F. Hundley, A. Zettl, A. Slacy and Marvin L. Cohen, Phys. Rev. B 35 (1987)
8800.
[28] C. Uher, A.B. Kaiser, E. Gmelin and L. Walz, Phys. Rev. B 36 (1987) 5676.
[29] R.C. Yu, MJ. Naughton, X. Yan, P.M. Chaikin, F. Holtzberg, R.L. Greene,
J. Stuart and P. Davies, Phys. Rev. B 37 (1988) 7963.
[30] M. Sera, Y. Ando, S. Kondoh, K. Fukuda, M. Sato, I. Watanabe, S. Nakashima
and K. Kumagai, Solid State Commun. 69 (1989) 85I.
[31] Y. Nakamura and S. Uchida, Phys. rev. B 46 (1992) 5841
[32] M. Sera, T. Nishikawa and M. Sato, J. Phys. Soc. Jpn. 62 (1993) 28I.
[33] J. Takeda, T. Nishikawa and M. Sato, Physica C 231 (1994) 293.
[34] T. Suzuki, M. Sera and T. Fukase, Physica C 235-240 (1994) 1315.
[35] N. Mitra, J. Trefny, M. Young and B. Yarar, Phys. Rev. B 36 (1987) 558I.
[36] C. Uher, A.B. Kaiser, E. Gmelin and L. Walz, Phys. Rev. B 36 (1987) 5676.
[37] U. Gottwick, R. Held, G. Sparn, F. Steglish, H. Rietschel, D. Ewert, B. Renker,
W. Bauhofer, S. von Molnar, M. Wilhelm and H.E. Hoenig, Europhys. Lett.,4(10), (1987)1183.
[38] H.J. Trosahl and A. Mawdsley, Phys. Rev. B 36 (1987) 888I.
[39] S.C. Lee, J.H. Lee, B.J. Suh,S.H. Moon, CJ. LilTI and Z.G. Khim, Phys. Rev. B
37 (1988) 2285.
[40] A.P. Goncalves, I.C. Santus, E.B. Lopes, R.T. Henriques, M. Almeida and M.O.
Figueiredo, Phys. Rev. B 37 (1988) 7476.
[41] J. Clayhold, S. Hagen, Z.Z. Wang, N.P. Ong, J.M. Tarascon and P. Barboux, Phys.
Rev. B 39 (1989) 777.
[42] W.N. Kang, K.C. Cho, Y.M. Kim and M.Y. Choi, Phys. Rev. B 39 (1989)
2763.
[43] A.B. Kaiser and G. Mountjoy, Phys. Rev. B 43 (1991) 6266.
[44] J.L. Cohn, E.F. Skelton, S.A. Wolf and J.Z. Liu, Phys. Rev. B 45 (1992) 13140.
[45] S.D. Oberlelli, J.R. Cooper and J.L. Tallon, Phys. Rev. B 46 (1992) 14928.
[46] J.R. Cooper and A. Carrington, Advances in Superconductivity V (1993) 95.
[47] J.W. Cochrane, GJ. Russell and D.N. Matthews, Physica C 232 (1994) 89.
28

[48] P.M. Chaikin and G. Beni, Phys. Rev. B 13 (1976) 647.
[49] J.F. Kwak and G. Beni, Phys. Rev. B 13 (1976) 652.
[50] H.J. Trodahl, Phys. Rev. B 51 (1995) 6175.
[51] S. Sakita, T. Suzuki, F. Nakamura, M. Nohant Y. Maeno and T.Fujita,
Physica B 219 & 210 (1996) 216.
[52] K. Ito, Y. Koyama, Y. Wakabayashi, and Y. Inoue, Advances inSuperconductivity vm (1996) 239.
[53] J.M. Tranquada, BJ. Sternlieb, J.D. Axe, Y. Nakamura and S. Uchida,
Nature 375 (1995) 561.

29

p=x-y0.145
60

La 2 _ y-xTh)3a xCuO <4

0.125
0.105
La 2 _x_yTh yEa xCu0.c
x =0.145

130(a)jon,.,(b)vI;

Td2 (y=0 .02)

OM

40

Q'

g

90%

,

f-

o

-.,?

/.

( 'TlT)

f-...

,. Tc

.x (y=0 .02)

0 -0

40

Td2
,

/
"-

20

\ ;\/ "'-0,
\

0

9

I

onset

"-

20

TLT

(. o.tT)'I~' ",
,
,
I

I

Tc

(y=0.00)

30%

".
.,,'
"

;

/

\

\
\

\

Tc

0

0

0

0.20
X (8a)

0.000

0.020

0.040

y (Th)

Fig. 1. Superconductivity transition temperature Tc and the low-temperature structuralphase transition temperature Td2 of Th-substituted La 2_x Bax Cu0 4 . P = x - y = 0.125,
is the essential condition which leads to the low-temperature anomalies in
La 2- x - ,ThyBax Cu0 4 as functions of both x and y. Tc is defined at the temperature at
witch the sample is of 1 % volume fraction of perfect diamagnetism. Td2 is defined as
the temperature at which the integrated intensity ratio of the TLT phase becomes 90 %in Fig.l(a) and 30% in Fig.l(b) .

+ <;> V1 (;> -

v

-(

Tl

)

2~

f--"-BTo

To + AT- I--

S:..-----A/

fig. 2 . Experiment principle for thermopower. S: sample, A: copper, B: constantan . The
thennopower of the sample at given temperature can be attained by measuring VI and

V2 ,

30

Temperature
ContJroller

Thermoeters

DC Current
SupplyMicnr
Computer

Thcrmocou lesPrinter
DMM:VtV 2(nV)

f==I~-~--------+ DC Current

Sub Heater

SupplyFig. 3. General lay out of the setup for the measurement of thermopower.

~------ Main

Heater

Thermome t ers

Cold Block
Thermocouples

Sub Heater

Fjg. 4. Sample holder for the measurement of thermopower.

31

La2-x_yNdyBaxCU04
(a)

Jt%%

I
(b)

y=0.200
x=0.125y=0.200
x=0.075~' \ 300K

300K

~

+-'
(f)

C
Q)
+-'

C

69.5 ·

70.0

70.5

71.0 69.5

70.0

70.5

71.0

2 e (degree)

Fig. 5. Powder X-ray diffraction spectra of La2_x_yNdyBaxCu04 with (a) x=1/8, y=0.20
and (b) x=O.075, y=0.20 at selected temperatures. Circles represent the data points andsolid CUf\ es indicate the results of multiple-peak fittings.
32

La2-x_yNdyBaxCuC)4
I

/

300

>-

I

I

I

x-O.125
I

I

I

THT//

-

/

•

I
SZ

200

.1 Td1
OMT

f-

-

~

jr

d2

0---

0----

0-----

100 -

/

TLT
(+OMT)

o/'o

-

) /

I

o

I

I

I

0.20

0.40

I

I

0.60

I

0.80

y(Nd)

Fig. 6. Nd concentration dependence of Tdl Ce ) and Td2 CO). Td2 is defined as thetemperature at which the integrated intensity ratio of the TLT phase becomes 50%.

33

o

0.4

--

o y=0.60

o

OJ

---E

0.40
0.20
0.10
0.05
0.00

=:J
Q)

0)

I

o

0.2

(s

o
x

0

0

+

-r-

o

'----"

~
~

o

@

0.0

-0.2

o

10

20

30

40

so

T(K)

fig. 7. Magllctiz(1tioll of powdcrcd s(1l11plcs of L12_x_yNdyBtlx Cu0 4 ~r=1/8) mCasurcd
by tl SQUID magnctometer on cooling in (1 field of 3.0 De.
34

3.0

2.0

......--.....

1.0

E

u

G
OJ

0.0

I

0

-~

Q

a

~~~~I--~~I--_~~I--~~I
50
100
150
200

o

T(K)
Fig. 8. Variation of the resistivity with temperature, showing a distinctive upturn below
Tmin =60-70K. There is no anomaly in the resistivity at Td2 .
35

60~--~----~--~----~----~--~

y 0
40

20

I

Cf)

200

100

300

T(K)

Fig. 9 . Temperature dependence of the thermopower seT). A sharp change of thethermopower is observed at T* =60-70K, below which S(T) decreases rapidly towards
negative values. Tl1ere is 110 anomaly in the thermopower at Td2 ,

36

y

/

THT//

300

/

•

I

.1 Td1
OMT

200

f
~
Td2

----

-TLT-

100

-./'" - 0 - a

0

rn
o

o

o

0.20

0.40
y(Nd)

7~

min

t
0.60

0.80

Fig. ] o. Nd concentration dependence of Tdl ce), Td2 (0), Tmill (0) and T* (D). Td2 isdefined as the temperature at which the integrated intensity ratio of the TLT phasebecomes 50%. Top ends of the vertical bars represent the onset of the structural phasetransition, and bottom ends the temperature at which the intensity of the TLT peaks
begins to saturate. For a given Nd concentration, the values of Tmin and T * arc nearly
the same but the value of Td2 is higher than those of Tmin and T* for the samples withhigher Nd concentration.

37

OMT

THT

200

\

5Z
~

Td2

\

0----

100

Td1

0

o--o ~

\
\

0

0 /1

\
\

\

f

\

-OLT
---

(+OMT)

00.00

TLT
i (+OMT)
II

I

\

\
\
0

\
\

0.10

0.20

0.30

x(8a)

Fjg. 11. Ba concentration dependence of Tdl and Td2 for La2_x_yNdyBaxCu04 (y=O.20).
38

o

--

~~ 0.140-~~-.-.

O. 1 2 0 ~;--+___ -r/

--

0)

0.210

:J

E

t

/ ;

(f)

I

o

~

/
0.200;

Q)

s

/

-1

~

Fe H=3.0 Oe<:;t:X}P0

powdered samples

~

-2

o

10

20

30

T(K)

Fig. 12. Magnetization of pO'vvdered samples of La2_x_rN~, BaxCu04 O!=O.20) measured
by a SQUID magnetometer on cooling in a field of 3".0 Ue.
39

La2-x-yNdyBaxCU04

y 0.2030 r-----,-----r--.-----r---,-,----.------,---,---------,---,
powdered samples
Fe f-f=3.0 Oe

20

10__ O. 0 r--~--------'ro...lU=::(~-----~

--

OJ
::J

E
Q) -1.00)Ioi- 2 .0~

T~5K- 3 .0 '----L----'-----L---...1..-----1_----L-_---L-_~--L-----L_.J
0.00
0.05 0.10
0.15
0.20
0.25

x(8a)

Fig. 13. B(l concentration dependence of Te and magnetic susceptibility at SK. Te is
defined as the temperature at which a sample exhibits 1% volume fraction of perfectdiamagnetism.

40

40

-->

".---.....

~

20

::J..

Cf)

0.250
o~~~~~--------------------------~-20~~~~--~~~~-~--~~~~~

o

100
T(K)

200

Fig. 14. Temperature dependence of the thermopower of La2_x_yN~BaxCu04 (y=O.20)
41

La2_x_yNdyBaxCU04

y-0.20x 0.125
40

sz

-->
:::t

20

(f)

o

-20~~~~~~~~-~~~~

o

100

200

T(K)

Pig. 15. At lowcr tcmpcraturcs S(7) approachcs zcro jn thrce kinds of ways dcpcnding
on x for thc samplcs of U12_x_yNdyBaxCu04 (y=O.20).
42

3......---.,.

E

u

2

OJ
I

o
S1

0.160

Q

~~......~......,..

-i'" -i'"~,.~.+..r: t: +

t

0.250

. x=0.120

40

0.130

......---.,.

-->
~

~20

Cf)

0.250
.--..~+t++tf-H+f 111111111111 f 1111111"_11

1HIIf!H+l!HH " " " I " III I I -++tH+H+ ~-20~~~~~~~~~~-~~-L-L~

o

50
T(K)

100

150

Pig. 10. RclC1lionship bctwccn thc anomaly of rcsistivity and that of thcrmopowcr. forthc SZlI11C Ba conccntration thc valucs of TJIlill and T* (irc ncarly thc S;Ul1C with x aroundl /S. Tmill is dcfincd as thc tcmpcraturc at which thc rcsistivity of thc samplc takcs amininlUm. T* is dcfincd ,IS thc tcmpcraturc at which the thcrmopowcr begins to fall
down sharply.

43

120

o'0~Td2/0/1 TLT
1

80

OMT

\

~O!

10:f3
I 8

OLT
40

I
I

I
I
I

o
\

I

-~\
.

'T,

C

o
0.00

0.20

0.10

0.30

x(8a)

Fig. 17. Ba concentration dependence of Tc (.), !d2 (0), Tmin (0) and T* (D). For thesame Ba concentration, the values of Tmin and T are nearly the same but the value of
Td2 is about 50K higher than those of Tmin and T * .

44

La2-x_yNdyBaxCU04

y'-0.20x 0.120

1.06~.

I Tmin~.

SZ 1.04

17 MHz

0
LO

.,.-

II

r----:::::J

TLT~ 1.02

OMT:::::J

Td2

1.00

o

50

100

150

T(K)
Fig. 18. Temperature dependence of the ultrasonic velocity of LNBCO with x=0.120
and y=0.20. At Tmin and r* there is no anomaly in this curve.
45

La2_X_yNd yBaxC u () 460
x=0.140
Td2

40
Td2

. ---~
Y 0...--...
~--->:::::i. 20----CJ)

y=0.20

Td2

I

0

/y 0

20

"'y 0.20
x=O.160_20L---~----_LI----~--_L----~--~

o

200

100

300

T(K)

Fig. 19. Comparison of thermopower between LBCO (y==O) and LNBCO (y=O.20).

46

La2_x_yNdyBaxCu04

y=0.20

50
0

40
30

-->

720

T 90 K~10-50K:::~30K

Cf)

0

~10K

-10

-28 .10

0.20

0.30

x(8a)

Fig. 20 . Ba concentration dependence of thermopower at selected temperatures.

47

La2_x_yNdyBaxCU04

y 0.20t(a)(b)
High temperatures I Low temperatures

x<O.120 #,~~x>0.140

CF

CF

Fig. 21.
A simple model of the density of states for x<1/8, x=1/8 and x> 1/8 . (a) at
high temperatures (b) at low temperatures.

48

[1]

Relation between transport, structural and superconductive properties in
La2_x_yN~B~CU04

X. Z. Bao, J. Nyhus, Y. Maeno, Y. Tanaka, S. Sakita and T. Fujita,

Physica C 271 (1996) 256-264.

[2]

Changes in the transport properties and the suppression of superconductivity

in La2_x_yN~B~CU04 at x=1/8

X.Z. Bao, Y. Maeno, Y. Tanaka, J. Nyhus and T. Fujita,

Advances in Superconductivity VITI (1996) 235-238.

[1]Superconductivity and Structural Phase Transition in lA2_xBaxCu04 withLa Partially Replaced by Th

x.z. Baa, Y. Maeno, Y. Tanaka, K. Yoshida and T. Fujita,
Advances in Superconductivity VI (1994) 375-378.

[2]

Effect of Tb Substitution on the Superconductivity of La 2 _x Bax Cu0 4

X.Z. Baa, Y. Maeno, F. Nakamura, S. Nishizaki and T. Fujita,

Advances in Superconductivity vn (1995) 241-244.

[3]

Stability of the low-temperature tetragonal phase in La 2_x Bax Cu0 4 andrelated compounds

F. Nakamura, K. Yoshida, Y. Tanaka, X.Z. Baa, Y. Maeno and T. Fujita

J. Supercon. 7 (1994) 33-35.