The study of phonon modes of Cu 1− x Tl x Ba 2Ca 3Cu 4O 12− y superconductor thin films by FTIR...

Physica C 407 (2004) 103–114

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The study of phonon modes of Cu1�xTlxBa2Ca3Cu4O12�y

superconductor thin films by FTIR absorption spectroscopy

Nawazish A. Khan a,*, M. Mumtaz a, K. Sabeeh a,M.I.A. Khan b, Mushtaq Ahmed b

a Materials Science Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistanb National Physical and Standard Laboratories, 16 H-9 Islamabad, Pakistan

Received 26 January 2004; accepted 23 April 2004

Available online 9 June 2004

Abstract

The superconducting properties of Cu1�xTlxBa2Ca3Cu4O12�y thin films prepared by amorphous phase epitaxial

(APE) method have been studied, by resistivity measurements, critical current density measurements, infrared spec-

troscopy and X-ray diffraction. The resistivity measurements of all the samples have shown metallic variation down to

onset of superconductivity and Tc in the range of 95–112 K. The critical current densities of samples were also measuredand it was observed that the samples with higher thallium contents in the unit cell have low critical current density, Jc(�103 A/cm2) while lower thallium contents in the material give, higher critical current density Jc (�106 A/cm2). The

XRD measurements showed the material to be single phase and oriented along c-axis. The /-scan of (1 0 3) reflection ofCu1�xTlx-1234 material showed the crystal to be a-axis oriented. The main emphasis of this research work is on thestudy of phonon modes of vibration of different atoms in the unit cell of Cu1�xTlxBa2Ca3Cu4O12�y superconductor. The

effects of post-annealing, of the samples in air atmosphere, on the phonon modes absorption is also studied. Three

major phonon modes around 450–475, 900 and 1200 cm�1 in Cu1�xTlx-1234 have been observed. These modes are

assigned to the vibrations of apical oxygen, O3 and C–O respectively. The apical oxygen mode at 464 cm�1 is hardened

to 474 cm�1 when the preparation temperature is increased from 905 to 920 �C while these modes substantially reduce inintensity after the post-annealing in air at 650 �C. The 945 cm�1 O3 mode is softened to 880 �C when the synthesis

temperature is above 910 �C. The softening of O3 mode increases the critical temperature of the material, which also

showed that the presence of O3 atoms provide maximum stability to T1 atoms of the unit cell. The post-annealing of the

materials in air atmosphere at 650 �C substantially reduces the intensity of 465–475 and 850 cm�1 mode. The reduced

intensity of these modes shifts the material towards highly resistive regime and superconductivity is destroyed.

� 2004 Elsevier B.V. All rights reserved.

PACS: 74.76.)w; 74.76.Bz; 74.72.)h; 74.72.)JtKeywords: Thin films; Cu1�xTlxBa2Ca3Cu4O12�y superconductors; Low anisotropy; Long coherence length; High Jc; Infrared activephonon modes; Epical oxygen; Charge reservoir layers

* Corresponding author. Tel.: +92-51-2875906; fax: +92-51-9210256.

E-mail address: [email protected] (N.A. Khan).

0921-4534/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2004.04.028

mail to: [email protected]

104 N.A. Khan et al. / Physica C 407 (2004) 103–114

1. Introduction

Since the discovery of high temperature super-

conductivity in cuprates, many systems based on

Y–Ba–Cu–O, Bi–Sr–Ca–Cu–O, Tl–Ba–Ca–Cu–Oand Hg–Ba–Ca–Cu–O [1–4] have been developed.

These compounds have a critical temperature of

90, 110, 120 and 136 K respectively. Of these

compounds Tl–Ba–Ca–Cu–O and Hg–Ba–Ca–

Cu–O have higher critical temperatures as com-

pared to the rest of the compounds and look

promising from the application viewpoint, but the

presence of toxic thallium and mercury in thesetwo compounds makes them not very attractive

for their use in the device fabrication. On the other

hand Bi–Sr–Ca–Cu–O does not have any toxic

element and also have Tc ¼ 110 K but the high

superconducting anisotropy (c ¼ 29) of this com-

pound makes it not much attractive for applica-

tion in the devices.

Another high temperature superconductor Cu-1223 (CuBa2Ca2Cu3O10) is promising in the cup-

rate family [5] due to its low superconductor

anisotropy as compared to the previous discussed

systems. One of the compounds in the cuprate

family (Cu-1234) has a superconductor anisotropy

(c ¼ 1:6) even lower than Cu-1223 and long

coherence length (n ¼ 10� 10�8 cm) which makesthis compound attractive from application pointof view [6]. The normal pressure synthesis of Cu-

1234 has not yet become possible but a close

derivative of this compound in the form of thin

films of Cu1�xTlxBa2Ca3Cu4O12�y(Cu1�xTlx-1234)

has been prepared [7]. The presence of thallium in

this compound acts as a structure stabilizer, which

makes it possible to synthesize this compound at

lower pressure. Moreover, this synthesis route hasreduce thallium to a tolerable level and even we

can remove the volatile thallium from the final

compound. This will open a new way to the

preparation of best compounds with low super-

conductor anisotropy in the cuprate family for the

application purpose. The study of their phonon

modes hitherto not done and the results of such

studies will be presented in the present paper.The infrared absorption measurements of the

Cu1�xTlx-1234 thin films synthesized at various

synthesis temperatures were done and their

annealing experiments were carried out in air

atmosphere. The effects of removal of O3 atom

from charge reservoir layer and the deleterious

effects of this removal on apical oxygen mode have

been discussed.

2. Experimental

In the preparation of thin films of

Cu1�xTlxBa2Ca3Cu4O12�y, the amorphous phase

epitaxy (APE) method is used, which is thallium

treatment of amorphous phase at elevated tem-

peratures. The amorphous phase was depositedon an SrTiO3 substrate by rf-sputtering from a

stoichiometric target with a composition of

CuBa2Ca3Cu4Oy. During the deposition of films

on SrTiO3 substrate, the sputtering power of 100

Watt was used and the chamber pressure was

maintained to 25 mTorr (20 mTorr Ar and 5m

Torr O2). The 6 h sputtering under afore men-

tioned sputtering parameters produced films with1 lm thickness. Through this technique we can

transform amorphous phase of Cu0:5Ba2Ca3Cu4Oy

films to a crystalline superconducting phase of

Cu0:5Tl0:5Ba2Ca3Cu4Oy by treating it with precur-

sor pellet containing thallium. Thallium acts as a

structure stabilizer, reaction rate accelerator and it

has chemical effects for the development of layered

cuprate superconductors. Thallium treatment ofthe amorphous phase was carried out in a gold

capsule at 890–925 �C for 45 min. The mechanismof the growth kinetics had shown that [7] the for-

mation of Cu1�xTlx-1234 thin films was accom-

plished from Cu1�xTlx-1212 and Cu1�xTlx-1223 by

successive introduction of CuO2 planes in these

phases. The best synthesis temperature for

Cu1�xTlx-1234 films was found to be 910 �C butthis phase could also be synthesized as a single

phase at lower temperatures (�890 �C). Howeverthe low temperature synthesis results in higher

thallium contents in the final compound. From the

X-ray diffraction measurements the c-axis lengthwas found to increase with the increase of the

thallium contents. The partial substitution of Cu

for Tl in the Cu1�xTlx-1234 phase was achieved bydeveloping equilibrium vapor pressure of thallium

in the gold capsule which transforms amorphous

N.A. Khan et al. / Physica C 407 (2004) 103–114 105

Cu0:5TlxBa2Ca3Cu4O12�y to a crystalline Cu0:5-

Tl0:5Ba2Ca3Cu4O12�y phase.

Four-probe method was used for the resistivity

measurements of the superconducting samples.

The contacts to the surface of the superconducting

thin films were made by the deposition of gold(Au) by electron beam evaporation of gold (Au) at

the surface of the thin film. The samples after Au

deposition were annealed at 400 �C for 30 min [8].The contacts between the conducting wires and

sputtered gold dots to the surface of the films were

developed by the silver paste. Four probe method

was also used for Jc measurements using the litho-graphic patterned samples, the size of the patternwas 50–100 lm. The criterion of 1 lV/cm was

followed during Jc, measurements.For the FTIR spectrum of superconductor thin

films, the background spectrum was taken by

placing SrTiO3; substrate. The substrate was then

removed from the spectrometer and thin film

sample of Cu1�xTlx-1234 was placed inside the

spectrometer chamber for the sample spectrummeasurements. The numbers of scans for the

background and sample spectrum were 15 and 50

respectively. The spectral resolution of the spec-

trometer during FTIR measurements was 1 cm�1.

The CuKa radiations were used for determining

the crystal structure of the material.

3. Results and discussion

The superconducting compound Cu1�xTlxBa2-

Ca3Cu4O12�y of cuprate family has P4/mmm space

group and simple tetragonal structure. The unit

cell has a Cu1�xT1xBa2O4�d charge reservoir layer

and four CuO2 planes, as shown in Fig. 1. Three

Ca atoms separate these CuO2 planes and two Baatoms separate charged reservoir layers with CuO2

planes. The CuO2 planes separated by Ca atoms

are called central planes or S-planes. However, the

planes separated by Ca atoms on one side and with

Cu1�xTlxBa2O4�d charged reservoir on the other

side, are called P-planes. The P-planes are over

doped with the carriers while S-planes are opti-

mally doped with the carriers. The P-planes alsoserve as a bridge for the supply of the carriers from

charge reservoir layer to S-planes. The copper

atom in the S-planes is named as Cu(2) and the

oxygen atom in this plane as O(4). The copper

atom in the P-planes is Cu(1) and the oxygen atom

in this plane is called O(1). The oxygen atom

bridging the Cu1�xTlxBa2O4�d charge reservoir

layer and P-plane is apical oxygen and is named asO(2) atom. This atom practically controls the

supply of carriers to the P-planes and their doping

from the charge reservoir layer. The oxygen atom

at the center of Cu1�xTlx plane is named as O3

atom or Od of the charge reservoir layer.

3.1. XRD of Cu1�xTlxBa2O4�d (Cu1�xTlx-1234)

thin film

The representative XRD of Cu1�xTlx-1234

superconductor thin films is shown in Fig. 2. All

the (0 0 ‘) lines are manifesting the crystal orien-tation along c-axis. Typical c-axis length of

Cu1�xTlx-1234 material is 18.70 �A [7]. The c-axislength in Tl-1234 material, observed in previous

studies is 19.1 �A [9]. The smaller c-axis length inour Cu1�xTlx-1234 than Tl-1234 is another mani-

festation of our material to be Cu1�xTlx-1234. It is

observed in previous studies and by our work on

Cu1�xTlx-based superconductors that c-axis length[7] increases by the enhanced inclusion of Tl in the

charge reservoir layers of Cu1�xTlx-based super-

conductor. The /-scan of (1 0 3) reflection showedthe films to be oriented along a-axis. Theseobservations showed that the films were bi-axially

oriented. The c-axis of the films is normal to thesurface of the substrate. The c-axis of the films inthe present studies is in the direction of incident

laser light used for the FTIR absorption mea-

surements.

3.2. Resistivity measurements

The resistivity measurements of Cu1�xTlx-1234

samples are shown in Fig. 3. A metallic variation

of resistivity from room temperature down to

onset of superconductivity is witnessed in this fig-

ure. It could be seen from this figure that degree of

room temperature resistivity (associated with the

residual resistivity of the samples) is higher in thesamples prepared at lower temperature. However,

the zero resistivity critical temperature and

Fig. 1. Unit cell of Cu1�xTlxBa2Ca3Cu4O12�y superconductor.

Fig. 2. Typical XRD of single phase Cu1�xTlx-1234 thin film.

Fig. 3. R vs. T at different thallium treatment temperatures: (a)

920 �C, (b) 905 �C, (c) 895 �C.



Tc(onset) in the samples are almost identical. Theonset of critical temperature is 110 K and zero

resistivity critical temperature is 100 K in all the

samples. The possible reason for the increased

residual resistivity in the low temperature prepared

Cu1�xTlx-1234 samples is higher amount of thal-lium in Cu1�xTlxBa2O4�d charge reservoir layer.

Higher amount of thallium in the charge reservoir

layer makes the material more insulating in nature,

which possibly results in increased residual resis-

tivity. The high temperature synthesis (�910–920�C) results in a relatively smaller amount of thal-lium in the charge reservoir layer, which make the

charge reservoir layer more metallic in character.Thallium is a volatile material and its degree of

volatility increases with increased preparation

temperature. In the materials synthesized at higher

Fig. 4. (a–f) Critical current density meas

temperatures, the charged reservoir layers are de-

pleted of thallium and become more metallic in

character and room temperature resistivity is de-

creased. The slopes of the resistivity curves are

same in all the samples prepared at different syn-

thesis temperatures, showing identical types ofmaterial under different preparation temperatures.

3.3. Critical current density measurements of

Cu1�xTlx-1234

The critical current density measurements of

Cu1�xTlx-1234 thin film samples are shown in Fig.

4(a)–(f). The current density measurements weredone using lithographic patterned samples and

following the criterion of 1 lV/cm. The samplesprepared at 920 and 910 �C have critical current

urements of Cu1�xTlx-1234 samples.

Fig. 4 (continued)

Fig. 5. R vs. T of superconductor sample of Cu1�xTlx-1234

prepared below 900 �C.


densities 1.06� 106 and 1� 106 A/cm2 respectively

and their IV characteristics are shown in Fig. 4(a)

and (b). The samples prepared at 900 �C have

critical current densities from 0.28� 103 to3.3� 105 A/cm2 and their IV characteristics are

shown in Fig. 4(c)–(f). The higher critical current

density in 920 and 910 �C prepared sample is

possibly due to smaller amount of thallium in the

charge reservoir layer. Smaller amount of thallium

in Cu1�xTlxBa2O4�d makes it more conducting

than insulating in character and increases the

overall Fermi velocity of the carriers. The in-creased Fermi velocity increases the coherence

length along c-axis and decreases the supercon-ductor anisotropy. These Jc results are consistentwith resistivity measurements. The increased

resistivity of low temperature synthesized material

is due to presence of larger amount of thallium in

the charge reservoir layer.

3.4. Infrared absorption measurements of Cu1�xTlx-

1234 samples prepared below 900 �C

The resistivity measurement of Cu1�xTlx-1234

sample prepared at 895 �C is shown in Fig. 5.

Metallic variation of resistivity from room tem-

perature down to onset of superconductivity is

observed in this sample. The onset of supercon-ductivity is at 120 K and zero resistivity at 110 K.

The infrared absorption measurements of one of

the representative Cu1�xTlx-1234 samples prepared

at 895 �C is shown between the wave numbers

range 400–4000 cm�1 (Fig. 6(a) and (b)). Three

absorption bands around 450–475, 800–1000 and

1300 cm�1 could be seen in this spectrum. Coupled

with this spectrum is an absorption band ofmoderate free carrier absorption from 1500 to

4000 cm�1. The moderate free carrier absorption is

possibly due to larger amount of thallium in the

Cu1�xTlx Ba2O4�d charge reservoir layer. The more

insulating charge reservoir layer results in lower

carrier density in material and less intensity of the

Fig. 6. (a) FTIR absorption spectra of Cu1�xTlx-1234 samples

prepared below 900 �C before annealing. (b) Extended FTIR

absorption spectra of Cu1�xTlx-1234 samples prepared below

900 �C before annealing.


band of free carrier absorption band between 1400and 4000 cm�1. If the free carrier density is higher

in the material, the intensity of absorption band

between 1400 and 4000 cm�1 would be higher.

In layered cuprate superconductors the infrared

absorption between the wave number range 50–

350 cm�1 are suggested to be due to the vibrations

of Cu, Ba, Ca and Tl atoms while the absorption

band above 400 cm�1 are associated with thevibrations of lighter oxygen atoms i.e. of chain-

oxygen, planer oxygen or apical oxygen [10–14].

The two major infrared absorption bands around

450 and 800 cm�1 are expected in our Cu1�xTlx-

1234 superconductors between 400 and 1000 cm�1.

These types of modes are theoretically predicted in

Tl-1223 and Tl-1234 at 450 and 650 cm�1 for

apical oxygen and O3 atom of the charge reservoir

layer. These infrared absorption modes have not

been observed and reported hitherto in literature

to the best of our knowledge in Cu1�xTlx-1234

superconductor. The infrared absorption mode

observed around 454 cm�1 in our Cu1�xTlx-1234 isassigned to apical oxygen and a band of absorp-

tion around 914 cm�1 to O3 atom in the

Cu1�xTlxBa2O4�d charge reservoir layer. The

changed position of O3 mode appearing at 914

cm�1 instead of 650 cm�1 is due to changed nature

of charge reservoir layer. The position of this

mode to appear at 650 cm�1 was predicted for

Tl-1223 and Tl-1234, the changed nature ofcharge reservoir layer from TlBa2O4�d to

Cu1�xTlxBa2O4�d may possibly forces this mode to

vibrate at higher wave numbers. An absorption

band around 1300 cm�1 is assigned to the C–O

single band vibration. Carbon in the material is

inadvertently included during the synthesis of

Cu1�xTlx-1234 superconductor. This inclusion of

the carbon in the unit cell is brought about duringthe synthesis of Cu1�xTlx-1234 from the target,

which is prepared from CuO, BaCO3 and CaCO3.

The data available on carbon oxygen compounds

suggested that carbon oxygen single bond, double

bond and triple bond are observed between 1100

and 1600 cm�1. The carbon oxygen single bonds

are observed at lower wave number side, while

carbon oxygen double bonds at relatively higherwave number side. The mode at 1300 cm�1 is

possibly due to carbon oxygen double bond

(C¼O), while the mode at 1240 cm�1 is due to

carbon and oxygen single bond (C–O). This car-

bon appearing in the infrared absorption mea-

surements was present in the amorphous phase

which also survives in Cu1�xTlxBa2Ca3Cu4O12�d

superconductor. In the previous studies on (Cu,C)Ba2Ca2Cu3O10�d system with C¼ 0.02–0.19, it hasbeen observed that carbon cannot be detected by

XRD, which had become possible by our FTIR

absorption measurements.

The 452 cm�1 apical oxygen mode and 914 cm�1

O3 oxygen mode seem to be coupled together, the

intensity of the later mode, however, depends on

the concentration of O3 in the unit cell. However,the appearance of 452 cm�1 mode remains visible,

as long as the unit cell is intact and not broken.


The position of 914 cm�1 mode however can vary,

depending upon the concentration of O3 in the

unit cell and its coupling with the Tl3þ and Cu2þ

atoms of the charge reservoir layer. Although the

intensity of a mode depends upon the concentra-

tion of the material, but its position depends uponthe bonding with the attached atoms. Interestingly

in our Cu1�xTlx-1234 superconductor, the position

of O3 mode is sensitive to the concentration of O3

in the Cu1�xTlx Ba2O4�d charge reservoir layer, as

the relative ratio of Cu:Tl will determine the ulti-

mate bond strength.

3.5. Infrared absorption measurements of Cu1�xTlx-

1234 samples prepared above 900 �C

In Fig. 7 the resistivity measurements of the

samples used in FTIR absorption measurements

are shown. All the samples have metallic behavior

from room temperature down to onset of super-

conductivity as for as their resistivity variation

with temperature is concerned. The onset ofsuperconductivity in these samples is from 110 to

120 K with zero resistivity critical temperature

from 95 to 112 K. These samples were prepared at

Fig. 7. R vs. T of superconductor sample of Cu1�xTlx-1234

prepared above 900 �C.

905, 910, 915 and 920 �C. The high temperaturesynthesis of these materials results in small amount

of thallium in the charge reservoir layer, which was

confirmed by X-ray diffraction measurements. The

more metallic charge reservoir layer results in in-

creased carrier concentration and increased Fermi-velocity of the carrier and longer coherence

lengths. The synthesis of Cu1�xTlx-1234 above 900

�C, results in material with partial O3 deficient

sites in the Cu1�xTlxBa2 O4�d charge reservoir

layer. This is reflected in the FTIR absorption

measurements of these films (Fig. 8(a) and (b)).

The 464–474 cm�1 band of apical oxygen is present

with enhanced relative intensity. The peak posi-tions of the apical oxygen modes are at higher

wave numbers side, as compared to one observed

in the samples prepared below 900 �C, whichshows that lower amount of thallium is present in

charge reservoir layer and the c-axis length is de-creased. The decreased c-axis length also decreasesthe bond length of apical oxygen mode and pro-

motes this mode to vibrate at higher wave numberside. The relative position of apical oxygen mode

as a function of synthesis temperature is shown in

Fig. 9(a). It could be seen that increased synthesis

temperature decreases the amount of thallium in

the charge reservoir layer, which, results in a de-

crease in c-axis length and makes the apical oxygenmode to vibrate at higher wave number side (Fig.

9(a)). The O3 mode vibrations in our Cu1�xTlx-1234 samples are observed at 800–940 cm�1. If the

position of center of bond of O3 mode is plotted

versus synthesis temperature of Cu1�xTlx-1234

thin films material, it is observed that in 905 and

910 �C synthesized materials, this mode is posi-

tioned around 945 cm�1. However, this mode is

substantially softened to 880 cm�1 in 915 and 920

�C synthesized material. The most possible reasonfor this softening is reduced amount of thallium in

the charge reservoir layer, and O3 atoms attached

to it. The charge reservoir layer Tl3þ have about

50% higher affinity of oxygen than Cu2þ have (as

the thallium with oxygen is in Tl2O3 form and

copper CuO form). The decreased amount of

thallium relaxes the a-axis of the unit cell and O3

mode is softened and appears at 880 cm�1. Thelarger sized atom of thallium in the charge reser-

voir layer possibly increases the size of a-axis. If we

Fig. 8. (a) FTIR absorption spectra of Cu1�xTlx-1234 samples prepared above 900 �C before annealing. (b) Extended FTIR

absorption spectra of Cu1�xTlx-1234 samples prepared above 900 �C before annealing.


ignore 905 �C synthesized sample by taking it at

the boundary of low and high temperature syn-

thesis, and plot Tc(onset) and Tc(R ¼ 0) of 910, 915and 920 �C synthesized versus the hardening of

apical oxygen mode, it can be seen from Fig. 9(b)

that hardening of apical oxygen mode increases

both critical onset and Tc(R ¼ 0) temperature is of

the material. This also leads to a conjecture that by

improving inter-planer coupling, the critical tem-

perature of the materials can be enhanced Fig.

9(b).

3.6. Post-annealing at 650 �C in air of samples

synthesized below 900 �C

In order to see the effects of loss of O3 atom

from the charge reservoir layer on the phonon

characteristics of Cu1�xTlx-1234 superconductor

samples, annealing experiments were done at

650 �C. Cu1�xTlx-1234 samples post-annealed at

650 �C in air remain unchanged in their phonon-characteristics. In the EDX measurements these

types of samples have shown higher thallium

contents in the charge reservoir layer Cu1�xTlx-

Ba2O4�d (x ¼ 1). Therefore, 890 �C prepared

sample have almost TlBa2O4�d type of charge

reservoir layer, as the end product in these mate-

rials is TlBa2Ca3Cu4O12�d. The dominant presence

of Tl3þ in the charge reservoir layer gives maxi-mum stability to the O3 atom and it is not lost by

annealing the samples at 650 �C in air. Therefore,the phonon modes remain unaffected with the

post-annealing, which shows high stability of O3

atoms in the charge reservoir layer. The increased

Tl contents in the charge reservoir layer makes it

more insulating and the free carriers concentration

in the conducting layers is decreased, which is also


prepared below 900 �C after annealing at 650 �C in air. (b)

Extended FTIR absorption spectra of Cu1�xTlx-1234 samples

prepared below 900 �C after annealing at 650 �C in air.

Fig. 9. (a) Synthesis temperatures versus wave number of

Cu1�xTlx-1234 superconductor samples. (b) TcðR ¼ 0Þ versushardening of apical oxygen modes of Cu1�xTlx-1234 samples.


manifested in Fig. 10(a) and (b). It also shows that

symmetric charge reservoir layer gives maximum

stability to O3 atoms in the charge reservoir layer,

however, the doping mechanism of CuO2 planes is

substantially quenched.

3.7. Post-annealing at 650 �C (in air) of samples

synthesized above 900 �C

Cu1�xTlx-1234 samples prepared at tempera-

tures higher than 900 �C after the post-annealing

at 650 �C in air become highly resistive (few kilo

ohms) and their resistivity was substantially en-

hanced at low temperatures (few hundred kilo

ohms). The infrared absorption measurements ofthese samples are given in Fig. 11(a) and (b). The

473 cm�1 mode of apical oxygen is substantially


prepared above 900 �C after annealing at 650 �C in air. (b)

Extended FTIR absorption spectra of Cu1�xTlx-1234 samples

prepared above 900 �C after annealing at 650 �C in air.


reduced in intensity, and is softened to 450 cm�1.The O3 mode is also around 800–829 cm

�1 is also

substantially reduced in intensity. The free carrier

absorption band starting from 1500 to 4000 cm�1,

however, does not change substantially in inten-

sity. This shows that although the free carriers are

available for conductivity, the charge transfer

mechanism, which ultimately brings about the

optimum carriers in CuO2 planes is substantiallyquenched, (as the apical oxygen mode and O3

modes responsible for this mechanism are signifi-

cantly reduced in their intensity after the post-

annealing in air) and superconductivity is not

achieved at lower temperatures (100 K). The in-

creased resistance of the samples after post-

annealing in air may be attributed to the reductionin intensity of O3 vibrational mode, which supplies

carriers to the CuO2 planes, which ultimately

determine conductivity. The optimum concentra-

tion of carriers in CuO2 planes attribute desired

Fermi velocity VF to carriers which fixes the lengthof Fermi vector KF required for optimum super-

conductivity. So these annealing experiments

manifested the importance of apical oxygen modeand O3 mode for superconductivity. These obser-

vations have also shown the importance of apical

oxygen mode and the role it plays for the charge

transfer mechanism at low temperatures, which

ultimately determines superconductivity in these

materials. These observations manifested that even

if large supply of the carriers is available, the

interaction between the carriers (which bringsabout superconductivity) is possibly brought

about by the wave functions of oxygen modes. The

detailed study of mechanism of doping in this

material is underway.

4. Conclusions

In conclusions we have studied the original re-

sults of characterization of Cu1�xTlx-1234 thin

films by resistivity, critical current density, XRD

and FTIR absorption measurements. The

emphasis of these studies was on FTIR absorption

measurements and we have tried to develop a

correlation between phonon modes and mecha-

nism of superconductivity. The XRD measure-ments have shown the material to be single phase

and oriented along c-axis. The /-scan of (1 0 3)reflection of Cu1�xTlx-1234 material have shown

the crystal to be a-axis oriented. The material is b-axially oriented. The resistivity measurements of

the samples have shown metallic variation of

resistivity with temperature. The zero resistivity

critical temperature is observed from 95 to 112 K.The Jc measurements have shown that higher

thallium contents in the final compound results in


low critical current density, Jc (�103 A/cm2) while

lower thallium contents in the material give, higher

critical current density Jc (�106 A/cm2). These

observations also lead to a conjecture that we can

prepare Cu-1234 material from Cu1�xTlx-1234 by

removing volatile thallium from the charge reser-voir layer. The infrared absorption measurements

have also shown that there are three major phonon

modes around 450–475, 880–900 and 1200 cm�1 in

Cu1�xTlx-1234. The first two modes between 450–

475 and 880–900 cm�1 are due to apical oxygen

and O3 atom in the charge reservoir layer of

Cu1�xTlx-1234 respectively. These two modes are

present in our pre-annealed samples, while thesemodes substantially reduce in intensity after the

post-annealing in air at 650 �C for 4 h.The 464 cm�1 mode is hardened to 474 cm�1 if

the preparation temperature of the material is in-

creased from 905 to 920 �C. The increased syn-thesis temperature reduces the amount of thallium

in the charge layer while with lower synthesis

temperature higher thallium contents are retainedin the final compound. Above 910 �C synthesis ofmaterial, there is a substantial removal of thallium

from the charge reservoir layer and 945 cm�1 O3

mode is softened to 880 �C. Both the hardening of464 cm�1 mode to 474 cm�1 and softening of O3

mode from 945 to 880 cm�1 increase the critical

temperature of the material. The increase of criti-

cal temperature is possibly linked with improvedinter-planer coupling in Cu1�xTlx-1234 material.

The improved inter-planer coupling increases car-

rier’s Fermi velocity and coherence length along c-axis and zero resistivity temperature is increased

from 100 to 112 K. Based on this improved inter-

planer coupling we can enhance the critical tem-

peratures of these materials to higher values and

their Jc can be enhanced.

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The study of phonon modes of Cu 1− x Tl x Ba 2Ca 3Cu 4O 12− y superconductor thin films by FTIR...

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