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 / moleK)
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 / moleK
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 / moleK)
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
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