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Bulk Superconductivity in Bismuth oxy-sulfide Bi

Shiva Kumar Singh

, Anuj Kumar

, Bhasker Gahtori

, Shruti Kirtan

, Gyaneshwar Sharma

, Satyabrata

Patnaik

and Veer P.S. Awana

#,*

Quantum Phenomena and Applications Division, National Physical Laboratory (CSIR)

Dr. K. S. Krishnan Road, New Delhi-110012, India

School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110067, India

ABSTRACT: Very recent report

on observation of supercon-

ductivity in Bi

could potentially reignite the search for su-

perconductivity in a broad range of layered sulfides. We report

here synthesis of Bi

at 500

C by vacuum encapsulation

technique and its basic characterizations. Bi

is contaminated

by small amounts of Bi

and Bi impurities. The majority phase

is tetragonal I4/mmm space group with lattice parameters a =

3.9697(2)Å, c = 41.3520(1)Å. Both AC and DC magnetization

measurements confirmed that Bi

is a bulk superconductor

with superconducting transition temperature (T

) of 4.4K. Iso-

thermal magnetization (MH) measurements indicated closed loops

with clear signatures of flux pinning and irreversible behavior.

The lower critical field (H

) at 2K, of the new superconductor is

found to be ~15 Oe. The magneto-transport R(T, H) measure-

ments showed a resistive broadening and decrease in T

(ρ=0) to

lower temperatures with increasing magnetic field. The extrapo-

lated upper critical field H

(0) is ~ 31kOe with a corresponding

Ginzburg-Landau coherence length of ~100Å . In the normal state

the ρ ~ T

is not indicated. Hall resistivity data show non-linear

magnetic field dependence. Our magnetization and electrical

transport measurements substantiate the appearance of bulk su-

perconductivity in as synthesized Bi

. On the other hand

same temperature heat treated Bi is not superconducting, thus

excluding possibility of impurity driven superconductivity in the

newly discovered Bi

superconductor.

The discovery of superconductivity at 26K in LaO

1−x

FeAs

has ignited a gold rush in the search of new superconductors.

Besides popular Fe based pnictides

2,3

and chalcogenides

, some

new interesting systems have also appeared. To name some, they

are CeNi

0.8

, BiOCuS

and doped LaCo

. The supercon-

ducting transition temperatures of these systems are around 4K.

These compounds are layered with relatively large unit cells and

mimic the superconducting characteristics of CuO

based HTSc

cuprates and FeAs based pnictides. A comprehensive theoretical

understanding of the mechanisms of CuO

and FeAs based high

temperature superconductivity is still awaited. The hybridization

of Cu-O and Fe-As in these strongly correlated systems along

with their multiband character has been of prime interest to the

scientific community

8,9

. After the recent observations of super-

conductivity in BiOCuS

and doped LaCo

, it is a pertinent

question to ask if CuS and CoB could also play the same role as

of CuO

and FeAs. In this regards, it is worth mentioning that

although superconductivity of BiOCuS could not be reproduced

the CeNi

0.8

and doped LaCo

still lack independent confir-

mation. For example the volume fraction of superconductivity in

CeNi

0.8

is very small

. In this sense, the observation of super-

conductivity at around 4K in Bi

has once again started the

debate; whether this newest series of superconductivity is intrinsic

or not. It is suggested that superconductivity of Bi

is BiS

based and doping mechanism is similar to that of cuprates and

pnictides

12,13

. The central question is whether the observed super-

conductivity in Bi

is intrinsic or it is being triggered by Bi

impurity in the matrix.

Bismuth has been a part of various superconducting com-

pounds, such as Bi based High Temperature cuprates (BSSCO)

15,16

and CeNi

0.8

compounds. On the other hand, pure

Bismuth is found in several phases, out of which ordinary

rhombohedral Bi phase is non-superconducting

17,18

, while some

other phases are found to be superconducting

19-23

. Various crystal-

lographic phases of pure Bi, which are superconducting in the

bulk phase, are Bi II, III and V (high-pressure phases of Bi) with

= 3.9K, 7.2K, and 8.5K

19-21

respectively. The fcc Bi phase

superconducts with T

~ 4K

; and amorphous Bi with T

= 6K

In the current communication, we report on the extensive char-

acterization of the newly discovered

superconductor.

The synthesized Bi

is crystallized in tetragonal structure

with space group I4/mmm. The main phase of the sample is con-

taminated with small impurities of Bi and Bi

. Bi

com-

pound is found to be bulk superconducting at around 4.4K, as

confirmed from magnetization and transport measurements. Inter-

estingly same route heat treated pure Bi is non-superconducting.

Bi is in rhombohedral phase and hence is non-superconducting.

Our results conclude that superconductivity of Bi

is intrinsic

and not driven by Bi impurity phase.

was synthesized by solid state reaction route via vacu-

um encapsulation. High purity Bi, Bi

and S were weighed in

right stoichiometric ratio and ground thoroughly in the glove box

under high purity argon atmosphere. The powders were subse-

quently pelletized and vacuum-sealed (10

-4

Torr) in separate

quartz tubes. Sealed quartz ampoules were placed in box furnace

and heat treated at 500

C for 18h followed by cooling to room

temperature naturally. The process was repeated twice. The X-ray

diffraction (XRD) pattern of the compounds was recorded on

Rigaku diffractometer. Rietveld refinement of XRD pattern was

carried out using FullProf software. The magnetization and

transport measurements were carried out using 14 Tesla Cryogen-

ic PPMS (Physical Property Measurement System).

The synthesized Bi

sample is gray in color. On the other

hand, Bi sample is of shiny silver color. The room temperature

XRD pattern for synthesized Bi and Bi

samples are shown in

Figure l (a).

Figure 1 (a). Rietveld refined room temperature X-ray diffraction

(XRD) patterns of Bi

and same temperature heat treated Bi.

Figure 1 (b). The schematic unit cell of the Bi

compound.

Color code: Voilet-Bismuth, Yellow-Sulfur and Green-Oxygen.

The Bi

sample is crystallized in tetragonal structure with

space group I4/mmm. Rietveld refinement of XRD patterns are

carried out using reported

Wyckoff positions. The positions and

lattice parameters are further refined. The lattice parameters thus

obtained are a = 3.9697 (2)Å and c = 41.3520 (1)Ǻ. The Wyckoff

positions of the Bi

compound are given in Table 1. The XRD

pattern of same temperature heat treated Bi, is also depicted in

Figure 1 (a), which is crystallized in clean rhombohedral phase. It

can be concluded from XRD results that the synthesized Bi

sample is nearly single phase with some impurities of

rhombohedral Bi and Bi

. Rhombohederal Bi is reported to be

non-superconducting

17,18

The structure of Bi

is still under debate within the space

group crystallization of I4/mmm or I -42m

. The representative

unit cell of the compound in I4/mmm space group crystallization

is shown in Figure 1 (b). The layered structure includes Bi

(rock-salt type)

(fluorite type) and SO

layers. Supercon-

ductivity is induced in BiS

layer due to Bi-6p and S-3p orbitals

hybridization. The theoretical calculations

show that, bands are

derived from Bi-6p

and in-plane S-3p

orbitals. These are dominat-

ing bands for electron conduction and superconductivity.

Atom

site

Fractional

occupancy

Bi1

0.0000

0.0583 (4)

Bi2

0.0000

0.2074 (2)

Bi3

0.0000

0.3821 (2)

0.0000

0.1383 (1)

0.0000

0.2890(1)

0.0000

0.5000

1/2

0.0000

0.5000

0.0884(1)

0.0000

0.3053(1)

0.4793(2)

16n

1/4

Table 1. Rietveld refined Wyckoff positions and fractional

occupancies of the atoms in Bi

Various atoms with their respective positions are labeled in

Figure 1(b) and their coordinates are provided in Table 1. Bismuth

(Bi1, Bi2 and Bi3) and Sulfur (S1 and S2) atoms occupy the 4e (0,

0, z) site. S3 atom is at 2b (0, 0, ½) site. O1 is situated at 8g (0,

½, z) and the O2 atom is positioned at 16n (0, y, z) site. The struc-

tural refinement indicates that the molecular composition is

2.25

. It is the normalization of Bi

composition by ¾.

The superconducting phase (i.e. Bi

or Bi

2.25

) is 25%

deficient composition of the parent Bi

2.5

(Bi

) com-

pound

Figure 2. Temperature variation of DC Magnetization in ZFC and

FC mode for Bi

compound at 20Oe, onset T

is identified at

4.4K. Inset shows the expanded part of the same plot indicating

irreversible behavior.

The DC magnetic susceptibility of Bi

sample is shown in

Figure 2. The magnetization is done in both, FC (Field cooled)

and ZFC (Zero-field-cooled) protocols under applied magnetic

field of 20Oe.The compound shows sharp superconducting onset

at 4.4K, which is clear from the zoomed inset of Figure 2. There

is evidence for substantial flux trapping too. The bifurcation of

FC and ZFC below T

marks the irreversible region. The shielding

fraction as evidenced from ZFC diamagnetic susceptibility is

~95%. Both FC and ZFC magnetization data confirm the appear-

ance of bulk superconductivity in Bi

. In order to exclude the

role of Bi impurity in superconductivity of Bi

, we measured

the magnetization of the same temperature (500

C) heat treated Bi

and found the same to be non-superconducting (plot not shown).

It can be seen in Figure 1, that the 500

C heat treated Bi is crystal-

lized in non-superconducting

17,18

rhombohedral phase. This ex-

cludes the possibility of un-reacted Bi driven superconductivity in

. In fact the sufficient superconducting volume fraction

and large shielding (~95%) of our studied sample, discards the

possibility of the minor impurity phase driven superconductivity.

The AC susceptibility versus temperature χ(T) behavior of the

sample is exhibited in Figure 3. AC susceptibility is done

at 1kHz and 10Oe AC drive field. DC applied field is kept zero to

check the superconducting transition temperature and is increased

to 5kOe and 10kOe to further check the AC loses in the mixed

state. Both the real (χ

) and imaginary (χ

) part of AC susceptibil-

ity were measured. Real part (χ

) susceptibility shows sharp transi-

tion to diamagnetism at around 4.4K, confirming bulk supercon-

ductivity. The imaginary part on the other hand exhibits a single

sharp peak in positive susceptibility at around the same tempera-

ture. Presence of single sharp peak in χ

is reminiscent of better

superconducting grains coupling in studied Bi

superconduc-

tor. Under applied DC field of 5kOe the χ

diamagnetic transition

is shifted to lower temperature of 2.6K and the corresponding χ

peak is broadened and shifted to same lower temperature. This is

usual for a type-II superconductor. At 10kOe DC field neither χ

nor χ

show any transitions, indicative of rapid suppression of

superconductivity.

Figure 3. AC susceptibility χ(T) behavior of the Bi

sample at

frequency 1 kHz and AC drive amplitude 10Oe under various (0,

5, 10kOe) DC applied fields.

Figure 4 shows the isothermal MH curve of the sample at 2K,

up to applied field of 3kOe. Upper inset of the figure shows the

same up to 1kOe. The MH curve (lower inset) shows that the ini-

tial flux penetration and the deviation from linearity marks lower

critical field (H

) of this compound ~15 Oe (at 2K). Wide open

MH loop of the studied Bi

compound demonstrates bulk

superconductivity.

Figure 4. Isothermal magnetization with field (MH) at 2K in an

applied field up to 3kOe. Insets of the figure show the same in

smaller field ranges. H

(2K) is estimated to be 15Oe.

Figure 5 depicts the resistivity versus temperature (ρ-T) meas-

urement with and without applied magnetic field. The resistivity

of the sample decreases with temperature and confirms supercon-

ductivity with onset T

~4.4 K. The normal state conduction (inset

Figure 5) is of metallic type and a T

fitting is found to be inap-

propriate implying non-Fermi liquid behavior. With applied field

of 1, 2 and 5kOe, the T

(ρ = 0) decreases to 3.2, 2.7 and 2K.With

further higher fields of 10 and 20kOe, the T

(ρ = 0) state is not

observed and only T

(onset) is seen. As sketched in Figure 5, we

have estimated upper critical field H

(T) by using the conserva-

tive procedure of intersection point between linear slope lines of

normal state resistivity and superconducting transition line. While

the applicability of WHH (Werthamer-Helfand-Hohenberg) ap-

proximation can be debated in this new superconductor, a simplis-

tic single band extrapolation leads to H

(0) (= - 0.69 T

/dT|

) value of 31kOe. From this the Ginzburg-Landau co-

herence length



x



H

)

1/2

is estimated to be ~100

Å

Figure 5. Resistivity vs. temperature (ρ-T) behavior of Bi

various applied fields of 0, 1, 2, 5, 10 and 20kOe in superconduct-

ing region; inset shows the zero field ρ-T in extended temperature

range of 2-300K.

A strong magneto-resistance in the normal state is also seen

that can possibly be ascribed to reasons similar to extra Mg impu-

rity in MgB

i.e. due to Bi impurity in the matrix. It is predicted

by the theoretical calculations that superconductivity in BiS

lay-

ers is of multiband type

13,24

. In Figure 6 we plot the Hall resistivi-

ty as a function of magnetic field at 10K. The dominance of elec-

tronic charge carrier in normal state conduction mechanism is

confirmed. Strong non-linearity is observed with increasing mag-

netic field that is suggestive of the deviations from single band

analysis.

The Hall coefficient [Figure 6 Inset (a)] is field dependent and

the carrier concentration at low field is estimated to be ~1.53×10

per cm

at 10K that increases to ~2.4×10

per cm

at 300K.

Figure 6. Hall resistivity is plotted as a function of magnetic field

at T = 10K. Inset (a) shows variation of Hall coefficient as a func-

tion of field. Inset (b) shows normalized magneto-resistance at

5K, implying non- H

dependence.

In the inset (b) of Figure 6, we show 











versus H

5K for fields up to 20kOe. One of the established features of

multiband superconductivity is the dependence 







 H

in

low field rangeEvidently, in this regime, this dependence is not

seen. Taken in totality, we can conclude that while our Hall resis-

tivity data may demand incorporation of more rigorous analysis,

the magneto-resistance aspects in Bi

could be well due to Bi

impurity.

In conclusion we have synthesized the new layered sulfide

superconductor and established its bulk superconductivi-

ty by magnetization and transport measurements. Detailed

Reitveld analysis determines the molecular composition as

2.25

. The coherence length is estimated to be ~100Å. A

departure from strong electron-electron correlation in the normal

state is indicated. The Hall resistivity yields non-linear magnetic

field dependence.

AUTHOR INFORMATION

Corresponding Author

Dr. V. P. S. Awana, Senior Scientist

E-mail: [email protected]

Ph. +91-11-45609357, Fax-+91-11-45609310

Author Contributions

All authors contributed equally.

ACKNOWLEDGMENT

This work is supported by DAE-SRC outstanding investigator

award scheme on search for new superconductors. Authors from

NPL would like to thank their Director Prof. R.C. Budhani for his

keen interest in the present work. Shiva Kumar and Anuj Kumar

are thankful to CSIR-India for providing the financial support

during their research. Shruti and G. Sharma acknowledge UGC

for research fellowships. S. Patnaik thanks AIRF, JNU for the

PPMS facility.

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