CHAPTER 1 INTRODUCTION TO SUPERCONDUCTIVITYshodhganga.inflibnet.ac.in/bitstream/10603/22633/8/08_chapter_1.pdf · A superconductor is characterized by zero electrical resistance.

CHAPTER 1

INTRODUCTION TO SUPERCONDUCTIVITY

1.1 SUPERCONDUCTIVIY-BASIC PROPERTIES

1.2 THEORITICALS MODELS

1.3 APPLICATIONS

1.4 SUPERCONDUCTING MATERIALS

1.5 BISCCO SUPERCONDUCTORS-A REVIEW

1.6 AIM OF THE PRESENT WORK

1.7 REFERENCES

Chapter-1

1

1.1. SUPERCONDUCTIVITY – BASIC PROPERTIES

The Phenomenon of Superconductivity.

Superconductivity was discovered by the Dutch physicist Heike Kamerlingh Onnes

in 1911 [1]. This was in fact a consequence of the liquefaction of Helium gas by him in

1908 which opened up a new temperature regime down to 1K (-272oC) hitherto unknown.

Investigating the electrical conduction of the frozen metal at these low temperatures,

Kamerlingh Onnes stumbled upon the discovery that was to have far reaching

technological significance. The electrical resistance of mercury completely vanished below

the temperature of 4.2K. Superconductivity was thus born.

A superconductor is characterized by zero electrical resistance. This test is often the

quickest way to establish superconductivity. The temperature below which the resistance

of the material vanishes is called the transition temperature,Tc.

The most important characteristic of a superconductor is the Messier effect,

whereby a superconductor expels magnetic flux from within and becomes a perfect

diamagnetic, provided the external applied magnetic field is small. The metal to

superconductor transition is purely electronic in origin and involves a second order phase

transition showing an anomaly in heat capacity at Tc. In addition, superconductors exhibit

the Josephson effect whereby a DC super current flows between two superconductors

separated by a thin insulating barrier without any voltage drop below Tc.

Type I and Type II Superconductors.

As mentioned earlier, a superconductor is any material that undergoes a transition

from the normal state to the resistance-less state below Tc. However, depending on their

response to an externally applied magnetic field, superconductors can be classified into

two types (Type I and Type II). If H is the applied magnetic field and M is the

magnetization, the magnetic induction B is given by :

B = H + 4ΠM. … (1)

Since a superconductor expels magnetic flux from within, B=0. Equation (1) now

can be written as

Chapter-1

2

(M/H) = χ = - Π41 … (2)

where χ is the magnetic susceptibility. Thus, in the super conducting state, χ is negative.

When H>Hc, the critical magnetic field, superconductivity will be destroyed, though T<Tc.

Superconductors which exhibit such a behaviour are termed as Type I, with Hc varying

from 100-500Oe depending on the material. On the other hand, a Type II superconductor

passes from the perfect diamagnetic state at low magnetic fields to a mixed state (vortex

state) and finally to a sheathed state before attaining the normal state of the metal. The

magnetic field values correspondingly are called Hcl and Hc2. The super conductive state

below Hc is perfectly diamagnetic and identical to the Type I behaviour. In the region

between Hc and Hcl, the material is in the mixed state in which the superconducting regions

are enclosed by normal regions in the form of vortices. High Hcl values of the order of 10-

300kOe are usually encountered for Type II superconductors. Between Hcl and Hc2, the

superconductor has a sheath of current carrying super conductive material at the body

surface and above Hc2, the normal metallic state exists throughout the material.

Many pure elements, alloys and some conventional superconductors exhibit Type I

behaviour. However, the majority of super conducting alloys and compounds including

ceramic oxide superconductors exhibit the Type II behaviour. For practical applications

Type II superconductors are the useful materials since they have large Hc2 values and still

retain superconductivity.

1.2 THEORETICAL MODELS

Superconductivity has always been a very fascinating but difficult problem for

theoreticians. Fritz and Hens London derived two equations describing zero resistance and

the Messer effect [2].

j

= ne2E/m = E)πλ4/(c 2L

2

where Lλ = 2/12 )λne4/(mc 2 … (3)

B)π4(c/jxΔ 2L

= … (4)

Chapter-1

3

Lλ has the units of length and is known as the London penetration depth. Lλ is the

fundamental length that characterizes a superconductor and is a measure of the exponential

penetration over which the shielding super current flows. Another independent length of

equal importance is the coherence length ξ and it is the length over which the super

conducting order is smoothened. The intrinsic coherence length 0ξ is described by the

equation.

0ξ = /λhv2 F Eg … (5)

where Eg = energy gap, vF = electron velocity at the Fermi surface. Type I and Type II

superconductors were characterized according to where λ < ξ or λ > ξ . Later Ginzburg

and Landau [3] described superconductivity in terms of quantum mechanics. Ginzburg-

Landau parameter k is given by

k = c2

c /hλeH22 … (6)

k < 2/1 for Type I and k > 2/1 for Type II superconductors.

These were phenomenological theories which describe the observed phenomena

without explaining the theory at microscopic level. The theory proposed by Bardeen,

Cooper and Schrieffer [4] is the first major successful microscopic theory to explain

superconductivity.

The BCS Theory

The BCS theory essentially explains how an electron-phonon interaction can lead

to an electron-electron attraction to form a Cooper pair and hence superconductivity.

According to the theory, an electron moving through the lattice momentarily distorts the

lattice and gets scattered. This will create a concentration of positive charge around the

region. A second electron is attracted to this positively charged region. Thus the two

electrons can form a bound pair (Cooper pair) with an opposite spin (and also momentum)

with a binding energy characteristic of the material. The existence of Cooper pairs in the

super conducting state can successfully explain many typical properties of superconductors

like zero resistance, Meissner effect, anomaly in the heat capacity (Cp) and vanishing of

See beck coefficient (S). The prediction of the energy gap (2∆ ) and explanation of the

Chapter-1

4

isotope effect (TcαM0.5), where M is the mass of the isotope of the metal) are some of the

other consequences of the BCS pairing mechanism. The transition temperature, according

to BCS theory is

Tc = 1.14 θD exp [-1/UD )( Fε ] … (7)

where θD = Debye temperature, U = electron – lattice interaction and D )( Fε = electron

density of orbitals at the Fermi level. The energy gap ∆ was explained in terms of Tc as

∆ = 3.2 kBTc [1-T/Tc]1/2 … (8)

This theory has successfully explained the occurrence of superconductivity in the

conventional superconductors, but was found to be inadequate to explain the high

temperature superconductivity. There are two reasons for this. The first one is that

superconductivity is due to phonon induced pairing of electrons. The second and perhaps

the most important one is that superconductivity probably arises not from Cooper pair

condensation but from condensation of new quasi particle of charge +e which are called

holons.

The RVB Theory

One of the leading theories for the high temperature superconductors is the

Resonance Valence Band (RVB) model proposed by Anderson [5]. According to this

theory, superconductivity is due to the formation of a condensate consisting of holon pairs.

A holon is essentially an empty site with the rest of the electrons singlet bonded and

resonating among various valence bond configuration in a coherent way. When such an

empty site is filled with one electron, it forms a spinon which is a neutral fermion; a spinon

is an unpaired spin in a sea of resonating singlet pairs. Thus the holons and spinons are

quasi particles of an RVB superconductor.

Other Theories

Since the explanation of HTSC by RVB theory, there have been many more

theories on the same subject. Among them are: Spin-bag mechanism [6] which explains

the pairing as due to the effective interaction between two holes which overcome the short

range coulomb repulsion, and other mechanisms involving plasmons and excitons [7, 8].

Chapter-1

5

Ginzburg [9] has proposed a model where a possible electron-phonon interaction might be

responsible for high Tc superconductivity and Mott [10] has proposed a model similar to

bipolaron theory.

However, these theories could not explain the anomalous state properties of

cuprate. With more and more investigations on physical properties of these high-Tc cuprate

superconductors, there is still a ray of hope for a complete theory in near future.

1.3 APPLICATIONS

The basic properties of superconductors (viz., zero electrical resistance, Meissner

effect, Josephson effect) have been successfully exploited for their practical applications.

Fig.1.1 summarizes the practical applications of superconducting materials.

Fig.1.1 Application tree of high Tc superconductors.

The three fundamental parameters that determine the economic feasibility for the

applications are : (i) Critical temperature (Tc), (ii) Critical current (Jc) and (iii) Critical

magnetic field (Hc2). The higher the above three values for a superconductor, the better it

Chapter-1

6

will be for practical applications. Due to the limitations of refrigeration systems to

maintain materials below their Tc and also due to the high cost of liquid He, the progress

made in this direction has been rather slow, but with the advent of high Tc

superconductors, an economic feasibility now exists for the realization of many of these

practical applications.

1.4. SUPERCONDUCTING MATERIALS

A large number and wide variety of solid metallic conductors have been found to

exhibit the phenomenon of superconductivity ever since its discovery in 1911.

Superconductivity is thus not an uncommon phenomenon and in fact, more than 1000

different compounds are presently known. However, most of them possess Tc less than

10K and a handful of binary and ternary compounds or alloys possess Tc in the range

10-23K. Since the discovery of high Tc copper oxide superconductors in 1987by Bednorz

and Müller there has been a dramatic increase in Tc’s over a short period and this is shown

in Fig.1.2; the survey of various kinds of superconductors is given in Table 1.

Fig.1.2 Discovery of high temperature of superconductivity

Chapter-1

7

Table – 1 History of Superconductivity

Year

1911 Kemerlingh Onnes Discovery of superconductivity (Berlin Physikalisch Technology (Hg) Rcichsanstalt) The Nobel Prize (1913) 1933 Meissner and Ochsenfeld Discovery of Perfect diamagnetism (Univ. of IIIinois) (Meissner effect) 1957 Bardeen, Cooper and Schrieffer Announcement of BCS theory (Univ. of Cambridge) The Nobel Prize (1972) 1961 Josephson Discovery of Josephson effect (Univ. of Cambridge) The Nobel Prize (1973) 1986 Bednorz and Muller Suggestion of possible high Tc (IBM Research Division oxide superconductors. Zurich Laboratory) The Nobel Prize (1987) 1987 Shoji Tanaka et al Discovery of high Tc (Univ. of Tokyo) superconductors. (Tc = 30K, La-Ba-Cu-O) 1987 Paul Chu Tc increased to 94 K. (Univ. of Houston) (Y-Ba-Cu-O) 1988 H. Maeda et al Tc increased to 110 K. (STA, National Research (Bi-Sr-Ca-Cu-O) Laboratory of Metals) 1988 Hermann et al Tc increased to 125 K. (Arkansus Univ.) Perkin et al (Tl-Ba-Cu-Ca-O) (IBM, Almaden Lab.) 1993 A. Schilling et al & Tc increased to 134 K. S.N. Putilin et al (Hg-Ba-Ca-Cu-O) Low Tc Materials

Low Tc superconducting materials can be classified as (i) Elements, alloys,

intermetallics etc. and (ii) Ternary systems which include oxides, chalcogenides etc. The

list of low Tc compounds is shown in Table. 2.

Chapter-1

8

Table – 2 Various classes of superconducting materials

SUPERCONDUCTING MATERIALS ELEMENTS AND ALLOYS TERNARY SYSTEMS Hg, Sn, Pb, V, Nb, 1. Oxides Mo, Nb-Ti SrTiO3-x. AxWO3 (A = Rb, Cs); BaPb1-xBixO3 (0.35<x<0.15); Li1+xT2-xO`4, INTERMETALLICS AND Ba0.6K0.4BiO3, LixNbO2, La2-xMxCuO4 COMPOUNDS (M = Ca, Sr, Ba; x = 0.2) 1. NbN, NbC, TaC (fcc, NaCl struc) 2. Nb3Sn, V3Si, Mo3Os (β -w struc) 2. Chalcogenides 3. ZrV2, CeRu2 (Cubic Leaves phase) a. SnNbS3, PbTaS3, Ag3Pd2S 4. La2C3, (Y-Th)2C3 (Pu2C3 struc) b.

AxMX2 (M = Nb, Ta, Mo, W; 5. PdHx, NbHx X = S, Se, A=metal)

6. MoN (Hex) Layer intercalation Compounds

7. Upt3, Ube13 c. CuRh2X4 (X = S, Se, Spinels)

d. Chevrel Phases : AxMo6X8 (A = Metal; X = S, Se), Mo6S6I2

BINARY COMPOUNDS NbO, NbX2, TaX2 3. Misc. Ternaries (X = S, Se) a. Borides, LnRh4B4 (Ln =

Rare earth or Y); La3S4, Zr3S4, Graphite LuRuB2, LaRh3B2 Interc, Compds (KxC) b. Carbides :

Mo3Al2C; Mo2BC c. Quaternary Borocarbides : Y-Ni-B-C d. Silicides : CeCu2Si2; Ln2Fe3Si5;

ORGANICS AND MISC. e. Germanides : Ln5M4Ge10; Ln3M4Ge13; (TMTSeF)2X (X=PF6, C104), LnM2Ge2 (M

= Pd, Pt, Ru, Os) (BEDT-TTF)2I3, (SN)x. f.

Stannides : AMxSny (A = Metal or Ln, M = Rh, Os; x ≈1.0; y = 3.0-4.0) g. Pnictides FULLERENE COMPOUNDS ZrRuX (X=P, As), NbPS; LnM4X12 K3C60, Rb3C60, Rb2.7Tl2.2C60 (M=Fe, Ru)

Chapter-1

9

High Tc materials.

Researchers have shown that at present, there are about five different classes of

high Tc (>77K) superconductors. The various known high Tc superconductors based on

copper oxides and their Tc’s are given in Table 3.

Table – 3 Classification of copper oxide high- Tc superconducting materiasl

Number Composition Space Group Abbreviation Tc

la La2CuO4 14/mm 214-T 22K lb P42/ncm Bmab Fmmm 2 Nd2CuO4 14/mmm 214-T 25 K 3 (Nd, Ce, Sr)2 CuO4 P4/mmm 214-T* 4 (La,Sr)2CaCu2O6 2126 60 K 5a YBa2Cu3O6 P4/mmm 123-T 5b YBa2Cu3O7 Pmmm 123-O 92 K 6 YBa2Cu4O8 Ammm 124 80 K 7 Y2Ba4Cu7O15 Ammm 247 40 K 8 (Ba,Nd)2(Nd,Ce)2Cu3O8 14/mmm 223 40 K 9a Pb2YSr2Cu3O8 P4/mmm 2123 70 K 9b Cmmm 10a Bi2Sr2CuO6 Amma Bi-2201 20 K 10b A2/a 10c C2 11a Bi2Sr2Ca1Cu2O8 Fmmm Bi-2212 85 K 11b Amaa 12 Bi2Sr2Ca2Cu3O10 14/mmm Bi-2223 110 K 13a Tl2Ba2CuO6 14/mmm T1-2201 11 K 13b Fmmm 14 Tl2Ba2CaCu2O8 14/mmm T1-2212 110 K 15 Tl2Ba2Ca2Cu2O10 14/mmm T1-2223 125 K 16 Tl2Ba2Ca3Cu4O12 14/mmm T1-2234 17 TlBa2CuO5 P4/mmm Tl-1201 18 TlBa2CaCu2O7 P4/mmm Tl-1212 90 K 19 TlBa2Ca2Cu3O9 P4/mmm Tl-1223 110 K 20 TlBa2Ca3Cu4O11 P4/mmm Tl-1234 122 K 21 HgBa2Cu1O4 Hg-1201 94 K 22 HgBa2Ca1Cu2O6 Hg-1212 128 K 23 HgBa2Ca2Cu3O8 Hg-1223 134 K

Chapter-1

10

The first ever superconductor with a transition temperature above the boiling point

of liquid N2 (77K) has been reported by Chu and co-workers [11] for a multiphasic starting

composition of Y1.2Ba0.8CuO4 with a Tc of 90K. Almost immediately many researchers

throughout the world established that the actual single phase material is YBa2Cu3O7 (123

phase) [12, 13]. This discovery of superconductivity at an unusually high transition

temperature has created worldwide interest and excitement. The extensive research work

done on copper-based mixed oxides has resulted in the discovery of more high Tc super

conducting systems. They are based on : Bi-Sr-Ca-Cu-O system with Tc ‘s in the range 75-

110K; Tl-Ba-Ca-Cu-O system with Tc >100K and Hg-Ba-Ca-Cu-O system with Tc >130K.

Superconductivity near 70 K has also been discovered in Pb-Sr-Ca(Ln)-Cu-O system.

Structural investigations on YBa2Cu3O7 have shown that this system is an

orthorhombically distorted perovskite with two Cu-O sheets in the ab plane and Cu-O

chains along the b-axis. The lattice parameters and the Tc of Y-123 have indicated that Y

can be replaced by other magnetic rare earths (except Pr), retaining the super conducting

characteristics. Two more super conducting phases in the Y-Ba-Cu-O system have been

identified. They are YBa2Cu3O8 and Y2Ba4Cu7O14 with Tc ~80K and ~40K respectively.

These two phases also possess an orthorhombic symmetry like YBa2Cu3O7.

High temperature superconductivity in the Bismuth and Thallium cuprates of

Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O families have attracted considerable attention in the

last few years. Historically, Michel et.al [14] reported superconductivity in Bi2Sr2Cu1Oy

but with a very low Tc. Maeda et.al [15] reported onset of superconductivity around 110K

in Bi-Sr-Ca-Cu-O composition. Several workers have since investigated this system

[16-20]. Both the Bismuth and Thallium cuprates confirm the general formula

A2B’n+1-xBx”CunO2n+4 (A=Tl, Bi, and B’, B” = Ca, Sr, Ba) [21-23]. T’cs have been found to

depend on the number of copper layers. The general problem with the Bi-Ca-Sr-Cu-O

system appears to be the difficulty in getting pure phases, especially with regard to n=1

and n=3 members. The crystal structures of n=1, 2 and 3 phases is shown in Fig.1.3 and the

structural data is summarized in Table 4.

Chapter-1

11

Fig.1.3 Crystal structures of Bi-based high Tc Superconductors.

Chapter-1

12

Table : 4. Crystal structure data of Bi-based high Tc superconductors.

S.L. Tc Space Group

Crystal Structure

Hc1 Hc2 ξ (0) λ (0) Lattice Param., A

Layer Sequence

No.

1.

2.

3.

4.

Superconductors Bi2Sr2CuO6

Bi2Sr2Ca1Cu2O8

Bi2Sr2Ca2Cu2O10

Bi2-

xPbxSr2Ca2Cu3O10

K

4-20

83-85

105-106

107-115

--

Amaa A2/a C2

Fmmm Amaa

14/mm

m

--

Orthorhom

Orthorhom

Tetra

Tetra

Tetra

mT

-

40

6.5-4.2

11-90

T -

1 400 1190

110

61-416

nm

-

14 110.2

0.7-1.7

2.3

nm

-

195

40-150

88-260

a b c 5.362 5.362 24.622 5.408 5.413 30.871 3.812 - 30.66 5.41 - 37.10 5.41 - 37.10

-BiO-SrO-Cuo2-SrO-BiO -BiO-SrO-CuO2-Ca-CuO2 -SrO-BiO -BiO-SrO-CuO2-Ca-CuO2 –Ca-CuO2-SrO-BiO- -PbO/BiO-SrO-CuO2-Ca -CuO2-Ca-CuO2 -SrO-PbO/BiO-

1 : Vertical to C axis 11 : Parallel to C axis

Chapter-1

13

The Pb2Sr2(Ca,Ln)Cu3O8 superconductors were reported by Cava et al [24] and

Subramanian et al [25] which shows a Tc of 70K. The structural features of this compound

are also similar to other high Tc superconducting materials.

The recent discovery of superconductivity in HgBa2CuO4 below 94K suggested the

possibility of considerably enhanced critical temperature Tc in similar compounds with

more than one Cu-O plane per unit cell of the crystal lattice [26]. Variations of the critical

temperature by changing external parameters have been established [27, 28] in these Hg-

compounds. Replacing Hg by Pb and/or oxygen annealing brought about changes in Tc by

more than 10%. Results obtained for Pb-substituted Hg-1223 compounds suggest that this

series is the first example of cuprates containing three Cu-O layers in which the doping

level may continuously be altered from an under doped to an over doped situation [27].

The maximum value for Tc that the researchers [27] obtained by varying the chemical

composition is ~134.3K. Application of external pressure enhances Tc [28]. Saturation of

this tendency occurs at pressures as high as 250kbar, where Tc values of the order of 150K

have been recorded.

Despite the progress made in the understanding and performance of high

temperature superconductors, the dream of achieving a room temperature superconductor

still remains elusive. Also, there is no firmly established theory to explain the properties of

cuprate superconductors. Superconductivity thus has interesting and challenging future

from the view point of both theorists and experimentalists.

1.5. BISCCO SUPERCONDUCTORS – A REVIEW

Following the discovery of high temperature superconductivity in the bismuth

cuprates of Bi-Sr-Ca-Cu-O family [14-17], the system has attracted considerable attention

in the last few years because of their many advantages like:(i) higher transition temperature

Tc, (ii) relative inertness to the oxygen content, (iii) chemical stability and (iv) possibility

of incorporation of additional copper oxide layers. Numerous studies have been made to

synthesize and stabilize these phases by varying the processing parameters like cation

nonstoichiometry [29, 30], heat treatment conditions [31, 32], cooling rate [33, 34],

pressure [35, 36] etc.

Chapter-1

14

Various substitutional studies in BISSCO system have been done to explore the

possible increase in Tc and to study their physical properties. Kanai et al [37] have studied

the effect of 34 different dopants in the BiSrCaCu2Oy ceramics. They classified the

dopants into four groups depending on their solubility and substitutional properties in

BISCCO structure. Dopants in the first group i.e. Fe, Co, Ni and Zn dissolve and substitute

the Cu and significantly reduce the Tc ‘s of the high Tc and low Tc phases. The second

group of elements, i.e. rare earth elements substitute Ca resulting in suppression of Tc and

finally become insulators. The dopants in the third group, i.e. alkaline metals and

phosphorus have a strong tendency to decompose the superconducting phase. The

remainder of dopants (B, Be, Si, Ge, V, Mo, W, Ga, Cr, Mg, Nb, Sn, Sb, Hf, Zr, Mn, Sc,

Cd and Ba) were classified as the fourth group. Superconductivity of the high and low Tc

phases of the fourth group remain almost unaffected.

The lead and antimony doping to the Bi-system has been extensively studied by

several uthors [38-41]. It was found that the substitution of Bi with Pb and Sb increases the

Tc substantially and stabilized the high Tc (2223) phase. The detailed substitutional

effects of Sb, Sn and In on the physical properties of BISCCO were reported by Nkum and

others [42, 43]. Sb and Sn addition was also found to increase the Jc of the system [44].

Doping of noble metals (Au, Ag and Pt) has also been reported [45, 46]. The studies

revealed that Au and Pt doping suppresses superconductivity whereas the Ag addition in

small amounts was found to uneffect the superconductivity and decreases Tc ‘s at higher

Ag content. Silver addition was however found to improve the current density and also

improved the mechanical properties of the system [46, 47].

The substitutional effects of other metals such as Ba, Mg, Mo etc. at Bi site were

also investigated [48, 49]. At higher concentrations of these metals, the compounds were

found to be insulators. The other dopant studies included the doping of Ga and alkali

metals like Na, K and Li at Ca site in Bi-2212 system [50-52]. These dopants increased

oxygen content thereby decreasing the Tc. The effect of various 3d metal doping for Cu has

been investigated [53, 56]. The Tc ‘s were found to decrease with increasing dopant

concentration. The depression in Tc was found to be more with Fe doping than Zn or Mn

doping, and it was also found that n=2 structure forms with all substitutions but the n=3

structure forms with Ti, V, Mn and Zn only.

Chapter-1

15

The oxygen site doping effects were also studied in the BISCCO system. The

fluorine doping was found to increase the transition temperature and also the flux pinning

[57].

Besides these substitutions, the effect of addition of compounds like Ca2CuO3,

Ca2PbO4 and YBCO were also done [58-64]. Dou et al [58] and others [59, 60] studied the

influence of excess Ca2CuO3 and found that this phase acts as the flux pinning center and

when added in optimum concentration, it will improve the super conducting properties. A

clear distinction between under doped and over doped regions in the Tc suppression was

reported by Klug et al [61] in the substituted high Tc cuprates. Similar results were

observed with Ca2PbO4 and (Ag+Ca2CuO3) additions [62, 63]. It was also found that the

addition of these impurity phases improves the current densities. Imao et al [64] have

studied the effect of YBCO addition to BPSCCO system. They found the presence of two

phases and that Tc ‘s have decreased with increasing YBCO concentration.

Effect of Trivalent ion doping on BISCCO-2212 system.

In all the high temperature Cu-oxide based superconductors the phase diagram is

such that these materials behave like Mott-Hubbard insulators in the low-carrier

concentration region and superconductors in the intermediate range, while in the heavily

doped region they are normal metal-like. In general, the three types of behaviour viz.

metallic, superconducting and semiconducting can be brought about easily by different

aliovalent substitutions or changes in the oxygen stoichiometry which can effectively alter

the carrier concentration in the system. In the case of Bi-based superconductors, the RE

substitution for Ca has been found to be more feasible as the crystal structure of the parent

system remains invariant with these substitutions even though a significant change in the

carrier concentration is affected due to doping of 3+ cation for 2+ cation. Hence to bring

about the metal to insulator transition in Bi-based superconductors, RE substitution seems

to the right choice. It can also be noted from the previous studies that the plane containing

Ca does not have any oxygen and Ca occupies an identical crystallographic site as Y in the

YBa2Cu3O7 compound [65]. Hence it is possible that Y and other rare earths can be

substituted at the Ca site in these compounds.

Chapter-1

16

There have been many reports on the Ln3+ ion (Ln=Y, Nd, Eu, Gd, Lu etc.)

substitution at the Ca site in Bi-2212 compounds. The structural analysis of these

compounds showed that all the samples exhibit a single phase nature with small

orthorhombicity [66-69]. Tarascon et al [70] have studied the superconducting properties

of the substituted 85K material and found that they are independent of the dopant whether

it is magnetic or nonmagnetic. Complete solid solution exists for compounds of the form

Bi2Sr2(CaRE)1Cu2Oy (RE= rare earth, Y) except for La, Pr, Yb and Lu. Single phase nature

of the compounds for the whole range of substitution was also observed by Wakata et al

[71] and others [72, 73] whereas Fukushima et al [74] and Zhengnan et al [75] have

observed change in phase at higher dopant concentration. The variation of lattice

parameters with RE concentration depicted that the c-parameter decreases monotonically

with a simultaneous increase in a-parameter. Such a decrease in c-parameter may be a

consequence of either the increase in the oxygen content due to higher valent cation

substitution or due to lower ionic size of the substituent ion than Ca, existing in the same

eightfoldcoordination in the system. Decrease in the c-parameter emananting from the

increase in the oxygen content as a result of either treating the compound in the reducing

atmosphere or by RE ion substitution at the Ca site has been reported by several workers

[76, 77] .

Xue et al [78] have observed in the Gd doped Bi-2212 system the CuO2-Gd-CuO2

separation is larger than that of CuO2-Ca-CuO2 although the ionic size of Gd3+ is less than

Ca2+. The excess positive charge on Gd3+ causes repulsion between the CuO2 layers

thereby increasing the CuO2-CuO2 plane separation. However, with an increase in RE

concentration, the oxygen content increases [65, 70, 79] and this excess oxygen is

incorporated in between the BiO double layers. Consequently, the net positive charge and

hence the repulsion between the Bi2O2 layers decreases causing the slab sequence, SrO-

BiO-SrO to shrink [78, 80]. This decrease offsets the increase in the CuO2-RE-CuO2

separation and hence the decrease in the c-lattice parameter is due to the incorporation of

excess oxygen within the Bi2O2 layers. The simultaneous increase in a-lattice parameter is

expected, since the extra electrons introduced by the dopant ions reduce the effective

valence on Cu resulting in an increase in the Cu-O bond length, which is observed as an

increase in the a-parameter. This result was also evident from the relation between oxygen

Chapter-1

17

nonstoichiometry and structural changes observed by Munakata et al [81] in Y substituted

Bi-2212 system.

There are few studies on structural modulation in the literature [82-86]. The

decrease in the modulation periodicity with Y content was attributed to the valence

changes of Y3+ for Ca2+. When Y3+ was substituted for Ca2+, the coulomb repulsion

between Cu2+ and Y3+ would become large, and it would increase the length of Cu-O

layers along the b-axis. Thus the lattice mismatch along the b-axis, between Cu-O sheets

and Bi2O2 layers, would be enhanced. The enhanced lattice mismatch seems to be relaxed

by the short modulation periodicity. The length of the a, b-axes become long with increase

in the Y content, which would also be attributed to the coulomb repulsion between Cu2+

and Y3+.

Another change that occurred with RE substitution is the suppression in

superconducting properties. It was observed from electrical and magnetization

measurements that Tc decreases monotonically with increase in dopant concentration [87-

93]. This is because the RE ions substituted at the Ca site in Bi-2212 reduce the number of

holes in Cu-O planes. Tc can be related to the density of states at the Fermi level through

the hole number. Fukushima et al [74] have concluded that the change in electrical

properties by RE substitution is caused by the change of the Cu average valence, namely

the hole concentration in Cu-O levels. A relation between Tc and the density of holes per

CuO2 plane was presented by Groen et al [94] and others [95-98] for Bi-2212. The hole

concentration was varied by replacing Ca partly by Y and by changing the oxygen content

of the samples. The phase diagram of Bi-2212 showed an unambiguous relation between

Tc and the density of holes per CuO2 plane. Superconductivity does not occur at doping

levels upto about 0.07 holes/Cu ion. With increasing hole content Tc showed a broad

maximum at 0.20 holes/cu ion and at higher doping levels Tc started decreasing. Nowik et

al [99] have observed similar result and suggested that there is competition between

superconductivity in the CuO2 planes and antiferromagnetism in the same planes.

Substitution of Ca by Y decreases the number of hole charge carriers, destroys

superconductivity and allows magnetic order to be formed. Thus at higher dopant

concentration (~0.50), a transition from metallic to semi conducting (anti ferromagnetic

insulators) was observed. In the insulating regime, the resistivity behaviour was found to

Chapter-1

18

turn from two dimensional (2D) to three dimensional (3D) variable range hopping (VRH)

[87, 96, 100-103]. Jayaram et al [87] have explained this as due to the localization of super

conducting grains and the clear VRH behaviour was seen only when the localization length

is smaller than the coherence length. According to Pham et al [104] the metal to insulator

transition can be explained within the charge transfer model which arises with the increase

in oxygen by Ca/Y substitution. These excess oxygen, intercalated in the Bi-O layers,

oxidize bismuth, decreasing the charge transfer between the Cu-O bands and the Bi-O

bands.

The changes in oxygen content and Cu valence is another important observation

made by Ca/RE substitution. Kawano et al [105] have determined valences of Bi and Cu

ions in Y substituted Bi-2212 by a coulometric titration technique and observed a decrease

in Cu valency from 2.16 to 2.04 with increasing Y content from 0 to 1. Superconductivity

was found to decrease from 3.12 to 2.99 as the Y content increased. This change in the Bi

valency was correlated with the change in the structural modulation period and with its

transformation from an incommensurate to a commensurate state. The results of Karppinen

and co workers [106, 107] and Kambe et al [108] have also indicated the incorporation of

excess oxygen and reduction in both Bi and Cu valences with the substitution of Y for Ca.

It was found that both of these phenomena occurred simultaneously.

The charge states of Cu in Y substituted Bi-2212 compounds with different Y

contents have been investigated by several authors by means of X-ray photoelectron

spectroscopy [109-114]. It was shown that doping with Y leads to the diminution of hole

numbers in the CuO2 planes.

The temperature dependence of the Thermo Electric Power of Y substituted Bi-

2212 samples was reported in the literature by researchers in the temperature range of 77-

300K [115-118]. The advantage in studying such a system is that the carrier concentration

can be varied from as low as 0.003/Cu ion to 0.44/Cu ion by changing RE from 1.0 to 0.0.

But the reported information on TEP has always been contradicting. Mandal et al [115]

have obtained positive and large TEP values for large x (Y content) and negative small

TEP values for x=0.00 sample. They have discussed the results following strong

correlation and the Nagaosa and Lee model. Munakata et al [116] have investigated the

Chapter-1

19

effects of oxygen nonstoichiometry and substitution of composition elements on the TEP

of Bi-2212 system substituted with Y. Their experiments studying the Seebeck coefficients

for the Bi-2212 sample sintered in air yielded negative values for S in the temperature

range from 310K down to Tc. However, the S for the sample where y-control-is 0.20 was

negative around room temperature, then became positive around 270K, and remained

positive down to Tc. From these results, it was considered that the characteristic behaviour

of S was controlled by the Cu valence. To explain these results, they have considered that

the Femi surface geometry for the mid gap states was two dimensional. The conflicting

reports in the literature regarding the values of Seebeck coefficient S and its sign is due to

different heat treatments [117, 118]. It is therefore very important to perform the S

measurements at high temperatures. Ponnambalam et al [119] have measured thermo

power of Y substituted Bi-2212 high Tc oxides at elevated temperatures and suggested a

mixed type conduction for these compounds.

There are very few reports available in the literature on simultaneous substitution

of the Bi-based 2212 superconductor with moderate amounts of RE3+ and Pb2+. The partial

substitution of Pb was reported to have stabilized the crystal structure [80, 94, 120].

Similar result was suggested by other workers from the structure properties of Pb and Y

substituted BISCCO-2212 system [121-123]. The double substitution was found to

improve the super conducting properties by strengthening the flux pinning forces. A

correlation between the distribution of different elements in these samples and the

influence of this correlation on the crystal lattice parameters was obtained by Lei et al

[124]. They have attributed such a correlation to the correlation of the combined charge 5+

of the corresponding ions: Ca3+Bi3+ and Y3+Pb2+.

There are also few reports on the effects of trivalent ion (Y, Sm, Nd, Gd etc.)

substitutions for Ca in the Bi-2223 system [125-130]. All these studies have reported a

degradation of the high-Tc phase. This was explained on the basis of decrease in hole

carrier concentration. Simon et al [126] studied the effect of Gd substitution for Ca and

found that the magnetic Gd3+ ion depresses the superconducting transition temperature.

Furukawa et al [127] and Iwai et al [128] studied the effect of Y substitution and found that

the solubility of Y is limited. The depression in Tc and high Tc 2223 phase was also

observed by Nanda Kishore et al [129, 130] in Sm and Gd substituted Bi-2223 system.

Chapter-1

20

This was attributed to the change in the hole concentration. Conversion of the 2223 phase

into the 2212 phase and the degradation of the superconducting properties appear to be

predominant in Gd doped samples as compared to the Sm doped ones, and this was

explained in terms of its higher magnetic moment.

The above survey indicates that the fundamental issues in high temperature

superconductivity concern not only the super conducting state properties but also the

normal state. A detailed study of transition between the normal metal to insulator state may

provide rich new physics. For these studies solid solutions such as Bi2

Ca1-xRExSr2Cu2OY (RE=rare earth) serve as useful materials.

It can also be seen from the above survey that most of the work on Bi-2212 materials was focused on samples with dopant concentration near that required for an optimal super conducting transition temperature with comparatively little work being done on samples at the cross over to the non-super conducting regime. Such studies are of fundamental interest to enhance the understanding of the super conducting state and to provide basic information of normal state properties. It is worth noticing that most of the studies in these systems are reported in Y substituted Bi-2212 compounds whereas less amount of work has been done in other RE doped systems. Substitution of Ca2+ by magnetic rare earths ions will allow the study of the interplay between magnetism and super conductivity. One of the objectives of this proposal will be to study other possible substitutions in order to induce eventually a magnetic state and their implications with respect to superconductivity.

With regards to TEP, the behavior of TEP in the normal state of high temperature superconductors is not yet completely understood. Even the sign of the TEP is still undetermined. Moreover, transport properties of RE substituted BISCCO compounds have not been studied extensively at elevated temperatures. These studies are essential in understanding the insulating properties of end compounds which in turn will give fruitful information regarding transport mechanisms in the normal state. Also, there are very few reports on properties of Bi-2212 system where both Pb and RE are simultaneously substituted for bismuth and calcium respectively. With this in view, in the present investigation, a detailed study on Pb and Gd substituted Bi-2212 system has been undertakine.

Chapter-1

21

1.6 AIM OF THE PRESENT WORK

In order to understand Tc suppression and the mechanism of metal insulator

transition due to the substitution of rare earth ions at calcium site in Bi-2212

superconductors, systematic studies have been undertaken in the present investigation.

(i) Preparation of Bi1.7Pb0.3Sr2Ca1-xGdxCu2Oy (x=0.0-1.0) compounds by solid state

ceramic technique.

Role of impurity addition in different concentrations of Gd on the super conducting

properties in general has been investigated in the BPSCCO-2212 system. These materials

have been characterized by using the following

(ii) X-ray diffraction (XRD)

Powder X-ray diffraction technique has been used for the determination of phase

purity of the samples and also for the calculation of lattice parameters.

(iii) Iodometric titration :

The oxygen content in the materials has been estimated from iodmetric titration.

(iv) DC electrical resistance:

The super conducting state, onset of super conducting transition Tc (onset) and the

transition temperature Tc (0) have been determined by DC electrical resistance technique.

Conductivity mechanism in the insulating regime has been arrived at.

(v) Thermoelectric power (TEP) :

Room temperature Thermoelectric power of the samples has been determined using

differential technique. The nature and the concentration of charge carriers has been

determined.

(vi) AC magnetic susceptibility:

Using AC complex susceptibility technique, onset temperature for diamagnetism,

phase purity and the quality of the sample have been estimated.

(vii) DC magnetization:

DC magnetization measurements have been used to determine the super conducting

volume of the samples.

Chapter-1

22

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CHAPTER 1 INTRODUCTION TO SUPERCONDUCTIVITYshodhganga.inflibnet.ac.in/bitstream/10603/22633/8/08_chapter_1.pdf · A superconductor is characterized by zero electrical resistance.

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