Spin fluctuations in superconductors near magnetic instabilities

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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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Collin
Broholm

Johns Hopkins Institute for Quantum Matter


Superconductivity & spin fluctuations


CeCoIn5



Spin resonance


ZEEMAN effect



Condensation energy



FeSe
x
Te
1
-
x



Spin resonance



Competition for momentum space


Conclusions



CeCoIn5 (PRL
100
,

087001

(
2008
)
)


C. Stock,


ISIS


Y. Zhao,


IQM
-
JHU


H. J.
Khang
,


NIST


C.
Petrovic
,


BNL


FeSe
0.4
Te
0.6

(PRL
103
, 067008 (2009))


Y.
Qiu
,


NIST


W.
Bao
,


Remnin

Univ.


Y. Zhao,


IQM
-
JHU


V.
Stanev
,


IQM
-
JHU


Y. C.
Gasparovic
,

NIST


S. Chang,


NIST


J.
Hu
,


Tulane U.


B.
Qian
,


Tulane U.


M. Fang,


Tulane U.


Z. Mao,


Tulane U.


FeSe
0.4
Te
0.6

(Unpublished)


W.
Bao
,


Remnin

U.


V.
Thampy


IQM
-
JHU


J.
Wen


IQM
-
JHU


J. Rodriguez


NIST



Funding:

Neutrons:

Ce
Co
In
5

:

HF superconductor
T
c
=2.3 K

Ce
Ir
In
5

:

HF superconductor
T
c
=0.38 K

Ce
Rh
In
5

:

HF
Antiferromagnet

T
N
=3.8 K

Fisk, Thompson,
Petrovic
,…

Nicklas

et al. (2007)

Kenzelmann

et al (2008)


~300
×
10 mm
2

x
-
tals


Fixed by
Fomblin

H
-
free
pump oil


Edge aligned


3
o

FWHM mosaic


CaFe2As2

CeCoIn
5

CeCoIn
5



7 1
c


Ǻ



10 1
ab


Ǻ



1 1 1
2 2 2
c

Q

Normal State: relaxation




Superconductor: Resonance


0.30 0.15meV
 
0
0.60 0.03 meV
0.1meV

 
 
9

4/14/2008

Resonance energy less
T
-
dependent
than expected for
D
(T)

T (Kelvin)

Increased Damping is largest effect

of heating through
T
c

T (Kelvin)

Bulut & Scalapino (1993)





D   D
c
p Q p


2 2
x y
p p
D  
p
Resonance in CeCoIn
5



1 1 1
2 2 2
Zero
-
moment sum
-
rule (
Scalapino

& White,
Demler

& Zhang ):

First
-
moment sum
-
rule (
Hohenberg

& Brinkman):







3
2
0
3 d
d cos
4
B
g
  





 
  

 
q
R R
q
q
S S q q R R


2
1
2
0
d,,
  
  


 
 

 
 

q q
q S S
S H
To use it we must know the form of the spin
Hamiltonian

4f CF
K RKKY
  
H H H H


K
K
 

R
R
s R S
H
Kondo:

1
2
,,
K K
 

 
 
 
 
 
q q
S S
H H
Crystal field

1 1
2 2
,,
z z
CF CF

 
 
 
 
 
q q
S S
H H
m m
CF n n
nm
B


H O
RKKY Exchange

RKKY
J
 

 

RR R R
RR
S S
H




1
2
,,2 1 cos
RKKY
J
 
 


 

 
     
 
 

q q RR R R
RR
S S S S q R R
H






2
RKKY 0
2
0
Tr,d
4
c
B
g

   
 


D  D

q
H






0 0
d d
ref
T
T
E T C T T C T T
D  
 
Changes in near neighbor

RKKY exchange dominate


through T
C


Compare:

To net condensation energy




q
S






10
0 0
d d
K
T
E T C T T C T T
   
D  
 






2
RKKY
2
0
Tr d
4
c
B
g

   
 




q
H

Appearance of
q
-
dependent resonance
indicates energy
reduction through RKKY



The correlation length
however does not
change substantially

Hsu F et al. PNAS 2008;105:14262
-
14264

Z. Mao et al. (2009)

~20 mm

Qiu

et al. PRL (2009)

FeSe
0.4
Te
0.6


Superconducting CeCoIn5 and
FeSeTe

both have spin resonance
excitations at similar reduced energies


Heavy
Fermion

CeCoIn5:


First moment sum
-
rule indicates d
-
wave superconductivity is
stabilized by a reduction of RKKY exchange energy


ZEEMAN splitting indicates a spin
-
doublet resonance


Iron superconductor FeSe
0.4
Te
0.6
:


Complex Wave vector dependence in indicates competing nesting
instabilities


Frozen short range spin correlations coexist in the optimally doped
sample but are unaffected by superconductivity.


Detailed characterization of the field and wave vector
dependence of the spin resonance in a range of magnetic
superconductors is needed to elucidate its significance