A new unconventional (possibly

winkwellmadeΠολεοδομικά Έργα

15 Νοε 2013 (πριν από 4 χρόνια και 1 μήνα)

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Electron 2007

S. N. Kaul

School of Physics

University of Hyderabad

INDIA

A new unconventional (possibly
d
-
wave) superconductor:

LaAg
1
-
x
Mn
x


Electron 2007

Electron 2007

Organization of the Talk


Introduction



Sample preparation and Characterization



Normal
-

to
-

Superconducting Transition



Anomalous behaviour



Absence of Long
-

range magnetic order



Normal state: Presence of antiferromagnetic spin fluctuations



Conclusions

Electron 2007

Magnetically


mediated superconductivity

Electron 2007

Synthesis and characterization



Polycrystalline samples of
LaAg
1
-
c
Mn
c



(c = 0, 0.025, 0.05, 0.1, 0.2, 0.3) alloys prepared by arc and
RF induction melting techniques.


Sample characterization by XRD, SEM and EDAX.

To explore the possibility of observing

superconductivity


at elevated temperatures

induced by
antiferromagnetic spin
fluctuations

in a 3
-

dimensional nearly antiferromagnetic metal.

Electron 2007

10

m

10


m

Induction Melted

Arc Melted

LaAg
0.9
Mn
0.1

SEM Micrographs

Electron 2007

Nominal Composition


LaAg
1
-
C
Mn
C

Actual Composition

Minority Phase

(Preparation Method)

Majority Phase

LaAg
1
-
x
Mn
x

Minority Phase

La
1
-
x’
Ag
1
-
x’
Mn
x’

La

Ag

Mn

La

Ag

Mn

La

Ag

Mn

1.00

1.00

0.00

1.00(1)

1.00

0.00

-

-

-

Absent
(Arc Melted)

1.00

0.975

0.025

1.000(5)

0.975(5)

0.025(1)

-

-

-

Absent
(Arc Melted)

1.00

0.95

0.05

0.950(1)

1.00(1)

0.05(2)

-

-

-

Absent
(Arc Melted)

1.00

0.95

0.05

1.00(1)

0.950(1)

0.05(2)

-

-

-

Absent
(Arc Melted)

1.00

0.90

0.10


0.95(1)

1.00(1)

0.05(2)

0.4

0.4

0.2

Present
(Arc Melted)

1.00

0.90

0.10

0.950(5)

1.000(5)

0.05(1)

0.4

0.4

0.2

Present
(Induction Melted)

1.00

0.80

0.20

0.92(1)

1.00(1)

0.08(2)

0.35

0.35

0.3

Present
(Arc Melted)

1.00

0.80

0.20

0.920(1)

1.00(5)

0.08(1)

0.35

0.35

0.3

Present
(Induction Melted)

1.00

0.70

0.30

0.900(5)

1.000(5)

0.10(1)

0.3

0.3

0.4

Present
(Induction Melted)

Energy dispersive absorption of x
-
rays

1. Global average composition matches with nominal composition.

2. Compositional fluctuations are more in arc
-
melted samples.

Electron 2007

20
40
60
80
100
0
5
10
0
5
10
0
5
10
0
5
c = 0.0



c = 0.05
c = 0.1


c = 0.2
2

(degrees)


(100)
c = 0.3
(321)
(310)
(200)
(211)
(220)
(110)


Intensity (10
2
cps)
CsCl (bcc) structure retained even

in the highest Mn

concentration

X
-
ray diffraction patterns

Minority phase less than 1 volume % of the
majority phase.

-400
-200
0
200
400
600
800
20
30
40
50
60
70
80
90
100
Y obs
Y cal
Y obs - Y cal
Bragg position
Intensity (Arbitrary units)
2

(
0
)
c = 0.1
(AM)
100
110
200
211
220
310
222
321
Electron 2007

Neutron

diffraction

patterns

-5000
0
5000
1 10
4
1.5 10
4
2 10
4
20
40
60
80
100
120
140
2 K
50 K
Difference Plot
Intensity (Arbitrary units)
2

(
o
)
c = 0.05
(100)
(110)
(111)
(200)
(210)
(220)
(310)
(222)
(321)
(330)
(420)
(332)
(422)
(521)
(431)
Neutron

wavelength



= 1.31
Å

Electron 2007

-5000
0
5000
1 10
4
1.5 10
4
2 10
4
20
40
60
80
100
120
140
c = 0.2
Y obs
Y cal
Y obs - Y cal
Braag position
Intensity ( Arbitrary units)
2


(
0
)
(100)
(110)
(111)
(200)
(210)
(211)
(220)
(310)
(222)
(321)
(330)
(420)
(332)
(422)
(432)
(521)
Neutron

diffraction

patterns

Neutron

wavelength



= 1.31
Å

Electron 2007

Variation of Lattice parameter ‘a’ with Mn concentration

and its thermal evolution

for a given c

Electron 2007

Transition from normal to superconducting state

The peak in d


/ dT and

``(T)

at H = 0 Oe is identified as T
C
.

Elecrical resistivity and its

temperature derivative

ac susceptibility


Electron 2007

0
1
2
3
4
1
2
3
4
5
6
7
La
0.92
Ag
1.00
Mn
0.08
T (K)

´´
ac
(10
-3
emu/g)
-12
-8
-4
0
H = 0 Oe
H = 500 Oe
H = 1000 Oe
H = 1500 Oe
H = 2000 Oe
H = 2500 Oe
H = 3000 Oe
H = 3500 Oe
H = 4000 Oe
H = 5000 Oe

´
ac
(10
-3
emu/g)
X = 0.1


ac susceptibility


X = 0.08


0
1
2
3
4
1
2
3
4
5
6
7
T (K)

" (10
-3
emu/gOe)
La
0.90
Ag
1.00
Mn
0.10
-16
-12
-8
-4
0
H = 0 Oe
H = 500 Oe
H = 1000 Oe
H = 1500 Oe
H = 2000 Oe
H = 2500 Oe
H = 3500 Oe
H = 4000 Oe
H = 4250 Oe

' (10
- 3

emu/gOe)
Electron 2007

At any given

temperature Critical

field H
C2

is equal for

c = 0.1(x = 0.05)

and c = 0.05

0
1
2
3
4
2
3
4
5
x = 0.05 (c =0.1)

''
x = 0.05 = d

/dT
x = c = 0.05 =

''
H
C2
(kOe)
T (K)
0
2
4
6
2
3
4
5
x = 0.05
x = 0.08
H
C 2
(T) (kOe)
T (K)
d

/dT
0
2
4
6
1
2
3
4
5
6
x = 0.05
x = 0.08
T (K)
H
C2
(T)


´´
0
1
2
3
2
3
4
5
d

/dT

´´
H
C2
(kOe)
LaAg
1 - x
Mn
x
(c = 0.1)
T (K)
x = 0.05
Upper Critical field H
C 2




= [

0
/ 2

H
c2
(0)]
1/2



is 1.5 times smaller than the quasi particle mean free

path deduced from the normal state residual resistivity.




㴠=㠵
Å



215 Å

Electron 2007

Superconducting


Normal Phase Diagram

T
C
is consistently higher in the

arc melted samples than the

induction
-
melted counterparts

indicates the sensitivity of T
C

to

local fluctuations in the Mn

concentration.

T
C

abruptly increases from 0.37 K
to 5 K at Mn concentration x = 0.05
indicating thereby that there exists
a threshold Mn concentration.

Electron 2007

Zero
-

field
-

cooled (ZFC) and field
-

cooled (FC) magnetization

Zero
-
field
-
cooled magnetization at

H = 200 Oe [in agreement with


’ (H =0)] corresponds to screening

of H over 90


95 % sample volume.

Field
-
cooled magnetization at the
same measuring field indicates
that the Meissner (flux expulsion)
fraction is as small as 10
-

15 %.


-0.6
-0.4
-0.2
0
La
0.95
Ag
1.00
Mn
0.05
T
C
= 5.0 K
FC
ZFC
-2
-1.5
-1
-0.5
0
M (emu/g)
La
0.92
Ag
1.00
Mn
0.08
T
C
= 5.4 K
-2
-1
0
1
2
3
4
5
6
7
T (K)
La
0.90
Ag
1.00
Mn
0.10
T
C
= 4.6 K
Electron 2007

Magnetic Hysteresis loops

-0.2
-0.1
0
0.1
0.2
1.9 K
2.9K
4.15 K
-2
-1
0
1
M (emu/g)
LaAg
0.9
Mn
0.1
H (kOe)
-6
-4
-2
0
2
4
6
2K
3K
4.2 K
M (emu/g)
LaAg
0.8
Mn
0.2
-6
-4
-2
0
2
4
-10
-5
0
5
10
1.93 K
2.93 K
4.15 K
M (emu/g)
H (kOe)
LaAg
0.70
Mn
0.30
X = 0.05

X = 0.08

X = 0.1

Electron 2007

C
n

= C
en

+ C
ln
=



T + A (T /

D
)
3


C
n
/ T =


+⡁⼠

D
3
) T
2


N (E
F
) = ( 3 / 2

2

k
B
2
)



C(T
C
) = 1.43



C

Suppression of the jump at T
c
by Mn substitution

Electron 2007

C
es
/



c

= a exp(
-




c

/ T )

Existence of Superconducting Gap

C
m
= C
es

-

C
en

Electron 2007

Mn concentration dependence of Superconducting Gap

C
m
/



c

= a exp(
-




c

/ T )

Electron 2007

-3
-2
-1
0
1
2
3
4
5
6
7
LaAg
0.95
Mn
0.05
0 Oe
50 Oe
100 Oe
200 Oe
300 Oe
500 Oe
1000 Oe
1500 Oe
T (K)

' (10
- 5
emu/gOe)
0
2
4
6
8
0
5
10
15
20
c = 0.05 annealed
c = 0.025

'
(10
-5
emu/gOe)
T (K)
0
0.1
0.2
0.3
0.4
0
100
200
300
x = Mn0.025
x = Mn0.05 (Annealed)
x = Mn0.05

(10
- 4
emu/g Oe)
T (K)
13
14
15
16
0
2
4
6
8
10
12
14
c = 0.05 annealed
c = 0.025

''
(10
- 5
emu / gOe)
T (K)
Effect of annealing

-3
-2
-1
0
1
2
3
4
5
6
7
LaAg
0.95
Mn
0.05
0 Oe
50 Oe
100 Oe
200 Oe
300 Oe
500 Oe
1000 Oe
1500 Oe
T (K)

' (10
- 5
emu/gOe)
Electron 2007

Effect of annealing

Electron 2007

0
10
20
0
10
20
30
C
m
(m J / mole-K)
x = 0.05 AM


T
3/2

(
K
3/2
)
H = 9T
H = 7T
H = 5T
H = 3T
H = 2T
H = 1T
0
20
40
60
80
100
-2.5
-2.0
-1.5
-1.0
-0.5


B
(
mJ / mole-K
5/2

)

H (kOe)
X = 0.05

Antiferromagnetic spin fluctuations and their suppression by magnetic field

0
10
20
0
10
20
30
x = 0.05
T
c
= 4.9 K
T
C
= 4.43 K
= AM
= IM


C
m
(mJ / mole-K)
T
3/2
K
3/2
X = 0.05

C
m

(H) = C
m
(0) [1


B(H)*T
3/2
]

Electron 2007


C(T
C
) = 1.43



C



C(T
C
) = 230 mJ / mole K


for c = 0.2


C(T
C
) = 350 mJ / mole K


for c = 0.3


C
m
(T
C
)


㌠m䨠潬攠K

C
m
(T) decreases in accordance with T
3/2

power law around and above T
C.

This observation is consistent with the theoretical prediction that
antiferromagnetic

spin fluctuations

in a nearly antiferromagnetic metal (
near the magnetic instability)

give rise to

T
3/2

variation

for the excess contribution to specific heat.

Antiferromagnetic Spin fluctuation contribution to specific heat

0.0
0.8
1.6
2.4
0
20
40
(T / T
C
)
3/2
= 0.05
= 0.1
= 0.2
= 0.3

(X 1/2)


c = 0.3
c = 0.2
c = 0.1
c = 0.05
C
m
(m J / mole K)

Electron 2007

Antiferromagnetic spin fluctuation contribution to resistivity


(T,0) and

(T,H) vary as T
3/2

in a
certain temperature range around T
C
.

Negative magnetoresistivity reflects

the suppression of antiferromagnetic
spin fluctuations by magnetic field.

Theory also predicts a T
3/2

variation for

(T,0) and

(T,H) in a nearly
antiferromagnetic metal arising from
antiferromagnetic spin fluctuations.

8.5
9.0
0.5
1.0
1.5
2.0
2.5
10.0
10.5
11.0

0 T

4 T

6 T

8 T




(T, H) (

cm)
c = 0.3
c = 0.1
(T / T
C
)
3/2


X = 0.1

X = 0.05

Electron 2007

Suppression of spin fluctuations


with magnetic field.

Suppression is faster for


x = 0.05 than x = 0.1,

because spin fluctuations
are more stiff in x = 0.1.

0
1
2
3
12
16
20
0
1
2
12
14
16
H
1/3

(
Oe
1/3
)

A
(
n

cm K
-3/2

)
H
1/2

(
10
2
Oe
1/2
)
c = 0.3
c = 0.1





(T, H) =


⠰Ⱐ䠩⁛ㄠ


A(H)*T
3/2
]

X = 0.1

X = 0.05

Electron 2007


Characteristic temperature,

,

is
negative

indicating the existence


of antiferromagnetic spin correlations
.


(T) = [C / (T +

)]+

0

Large magnetic susceptibility reflects the exchange
-
enhanced

antiferromagnetic spin fluctuation contribution.

Curie
-
Weiss behaviour of susceptibility in the normal state

Magnetic moment per Mn atom is 4.0

B


Electron 2007

0
1
2
4.9K
5.9K
6.9K
7.8K
8.8K
14.8K
19.6K
0
20
40
60
80
100
LaAg
0.90
Mn
0.10
M (emu/g)
H (kOe)
0
1
2
3
4.2 K
10 K
20 K
30 K
M (emu/g)
LaAg
0.8
Mn
0.2
0
1
2
3
0
20
40
60
80
100
5.2K
5.9K
6.9K
7.8K

8.8K

9.8K

11.8K

14.8

19.7K

29.6K

39.5K
LaAg
0.70
Mn
0.30
M (emu/g)
H (kOe)
Magnetization measurements in the normal state

Indicates the presence of both

ferromagnetic and antiferromagnetic

interactions but the latter are predominant.

Electron 2007

0
50
100
150
200
250
4.9K
5.9K
6.9K
7.8K
8.8K
14.8K
19.6K
2
4
6
8
La
0.95
Ag
1.00
Mn
0.05
M
2
(G
2
)
H / M (10
3
)
0
4
8
12
16
0
1000
2000
3000
4000
5000
La
0.95
Ag
1.00
Mn
0.05
4.9 K
5.9 K
6.9 K
7.8 K
8.8 K
14.8 K
19.6 K
M (G)
H / (T +

) ( Oe K
- 1
)

Absence of long


range magnetic order

Electron 2007

Absence of long


range magnetic order

0
100
200
300
400
2
3
4
5
6
7
M
2
(G
2
)
H / M
(10
3
)
x = 0.1
0
5
10
15
20
0
800
1600
2400
3200
La
0.90
Ag
1.00
Mn
0.10
5.2K
6.6K
7.8K
9.8K
11.8K
14.8K
19.7K
29.6K
39.5K
M (G)
H / (T +


= 25 K
(Oe K
-1
)
Electron 2007

Suppression of antiferromagnetic spin fluctuation

contribution to Magnetoresistance by magnetic field

0
50
100
150
200
250
300
-0.08
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
0.01
T
dip
~ 25 K


La
0.90
Ag
1.00
Mn
0.10

II
/

(0) = [

II
(H) -

(0)] /

(0)
T (K)
H = 0.6 T
H = 5 T
H = 1 T
H = 6 T
H = 2 T
H = 7 T
H = 3 T
H = 8 T
H = 4 T
0
2
4
6
8
-0.08
-0.06
-0.04
-0.02
0.00
6
5
4
3
2
1



II
/

(0)

H
(
10
4
Oe
)
1 = 7K
2 = 10K
3 = 20K
4 = 40K
5 = 50K
6 = 60K
Electron 2007



CsCl structure, which is
unfavorable

for conventional superconductivity, is
retained even for highest Mn concentration.



Abrupt increase in t
he superconducting transition temperature T
C
, from 0.37 K
to 5 K, suggests existence of a
threshold

Mn concentration


0.05. Since the
magnetic Mn atoms

are responsible for

the antiferromagnetic spin fluctuations,
the magnetic instability is reached from the paramagnetic side at a certain
threshold value of Mn concentration when antiferromagnetic spin fluctuations
are correlated over long distances and hence are more effective for electron
pairing.




C
p
(T) neither goes through a peak (associated with long range anti
-


ferromagnetic order; thereby signalling the absence of

long
-
range


antiferromagnetic

order) nor exhibits a jump at T
C
.



A brief summary of Results

Electron 2007








Debye temperature

D

exhibits a very weak (about 4%) variation with
Mn concentration and therefore the Mn substitution has hardly any
effect on the phonon spectrum of LaAg and yet T
C

is sensitive to Mn
concentration.






The BCS relation,
T
C

= 1.14

D

exp[
-
1 / V N(E
F
)]


predicts T
C

values approximately
one order of magnitude higher
than the
observed ones


Above observations thus completely rule out the electron
-

phonon
mechanism for superconductivity in the present system

Electron 2007

Conclusions



Curie
-
Weiss

behaviour of susceptibility in the normal state reflects the
existence
of exchange
-
enhanced antiferromagnetic

spin correlations.



Neutron diffraction

confirms the

absence of long
-
range

antiferromagnetic order

and of the

structural phase

transformation at or near T
C
.



According to the spin fluctuation theories, the observed T
3/2
variation of C
m
(T),

⡔,〩0慮a

⡔,䠩H慲a獥猠晲潭o
antiferromagnetic

spin fluctuations

near the
magnetic instability.



Negative magnetoresistivity reflects the
suppression of spin fluctuations by the
magnetic field.



The observation that the T
3/2

variation of C
m
(T) extends to temperatures < T
C

implies that
antiferromagnetic

spin fluctuations
persist

in the superconducting
state.

Antiferromagnetic spin fluctuation mediated electron
pairing is the most likely mechanism for superconductivity

Electron 2007

Unconventional Superconductivity in LaAg
1
-
x
Mn
x
:

relevance of Spin Fluctuation Mediated pairing

S.N. Kaul, S. Kumar, J. Rodríguez Fernández, and L. Fernández Barquín

Europhys. Lett., 74, 138
-
144 (2006)


Exchange
-
enhanced spin fluctuations in the normal state of a

new unconventional superconductor


S. Kumar, S.N. Kaul

,
J. Rodríguez Fernández, and L. Fernández Barquín

J. Magn. Magn. Mater, at press.



Collaborators


Sanjeev Kumar, University of Hyderabad


Dr. L. Fern
á
ndez Barqu
í
n, Dr. J. Rodr
í
guez Fern
á
ndez


Condensed Matter Group, University of Cantabria, Spain
.

Electron 2007

Thank you