1

S. Haddad LPMC, Département de Physique, Faculté des

Sciences de Tunis, Tunisia

S. Charfi-KaddourLPMC, Faculté des Sciences de Tunis, Tunisia

M. HéritierLPS, Orsay, (unité mixte de Recherche) CNRS-Paris

XI, FranceR. Bennaceur

LPMC, Faculté des Sciences de Tunis, Tunisia

Interplay between SDW and superconductivity in the quasi-one organic

superconductor (TMTSF)2ClO4

2

Acknowledgments

Theory ExperimentsC. Bourbonnais (Sherbrooke) D. Jérome (Orsay)A. G. Lebed (Arizona) Y. Maeno (Kyoto)

N. Joo (Orsay)

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Why quasi-one organic superconductors?

(TMTSF)2ClO4: an exotic organic conductor

Effect of disorder on the interplay between Superconductivity and SDW

Effect of a magnetic field: new field induced SDW (FISDW) phases

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Why superconductors ?

Uses for superconductors:

Also as … Electric generator (99% more efficient than ordinary one)SQUID (Superconducting Quantum Interference Device): sensing weak magnetic field Military: antenna and in detecting mines (US NAVY) …

IRM Accelerator LHC (CERN)

Maglev in Japan: fast, safe and economic

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The future of Superconductors

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Why organic superconductors ?

Dream: towards room temperature superconductors !!!

Problems: difficulties to synthesize such materials until…

Little’s Proposition (1964): look for organic conductors with one dimensional character to get high Tc !!!

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Why organic superconductors ?

But: there are still open questions !!!

1979: discovery of organic superconductivity in a quasi-one dimensional salt (TMTSF)2PF6 (D. Jérome’s group) …then in Bechgaard salts denoted by (TMTSF)2X (X=PF6

-, ClO4-

…)

Interplay between SC/ SDW: coexistence or competition?

SDW

SC

BUT : TC=1.2 K (so low !!!)

a complete laboratory for physicists intensive study

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Needle like

Crystal structure of (TMTSF)2X

TMTSFX

TMTSF=tétraméthyl-tétraséléna-fulvalèneX= anion: Br-, PF6

-; ClO4-…

c

ba

9

ab

c

Organic chains of TMTSF molecules

Conducting planestbta

Key parameters of (TMTSF)2X

tc

t’b

tc « tb « ta c «b« a quasi-1D conductors

10

0 10 20 30 40 50 60 70 80

1

10

100

1000

T*

T ()B

r

ClO

4

PF6X = B

F 4

PF6

Tem

pera

ture

(K)

(TMTTF)2 X

M-H

Pressure (kbar)

SC

METAL

AF SDWSP

(TMTSF)2 X

1D LL

2D FL

Phase diagram of Bechgaard salts

AF SC T. Vuletic, et al. Eur. Phys. J. B 25, 319 (2002)

(TMTSF)2ClO4 is superconducting at ambient pressure (Tc= 1.2 K)

ClO

4

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(TMTSF)2ClO4 : What makes it so special ?

TMTSFClO4

ab

c

ClO4 anions are noncentrosymmetric

2 possible orientations or

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(TMTSF)2ClO4 : anion ordering

Anion ordering in (TMTSF)2ClO4

ClO4 TMTSFb

c

t b

Slowly cooled sample (relaxed):

ClO4 anions order along b direction at TAO = 24 K

Rapidly cooled sample (quenched):

ClO4 anions disordered ! V = 0

Periodic potential:

V (y) =V cos( /b y)

V: anion potential

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2VFermi surface of (TMTSF)2ClO4

k ┴

k

Fermi surface of (TMTSF)2X without anion ordering

Dispersion relation of relaxed (TMTSF)2ClO4

bktVbktkkvk bbFF 2cos2 cos4 222

Two-band energy spectrum

BBA A

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Joo et al., Euro. Phys. Lett. 72, 645 (2005)

Effect of cooling rate

In the quenched samples:

pure magnetic state (SDW)

Puzzles !!!

SC

SDW

relaxed samples (slowly cooled) :

Superconducting (SC)

For the intermediate cooling rate:

both SC and magnetism.

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TSCSC/

SDW

TSDW

Cooling rate

T

effect of cooling rate: generic phase diagram

metal

Pure SC

Pure SDW

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Model :Interplay of superconductivity and magnetic

phases

Anion ordering two band energy spectrum

(k)

k

A B

kFA

kFBEg

Bands separated by gap Eg

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Method: perturbative renormalization group theory (Bourbonnais et al.)

Eg

tb

MV

system set of coupled chains

singlet superconductivity (SSc)

MV CDW SDW

triplet superconductivity(tSc)

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m

Scattering processes:(g-ology model)

g(1) processes g(2) processes

m m

m

m

m m

m m

m m

m

m m

m

(1)0g (1)

bg

(1)tg(1)

fg

m

m m

m

m

m m

m m

m

m

m

m

m

m

m

m

(2)0g (2)

bg

(2)fg(2)

tg

Most divergent the most dominant fluctuation.

MV

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Eg = 5 K

Rapid cooling

Eg = 8 K

Intermediate cooling

Eg = 15 K

Slow cooling

Scaling flows of the most divergent MV

tb = 300 K, EF = 2000 K, Tcross= 170 K, = 0.6 (i)g

Singlet SC Coexisting singlet SC/SDW

Pure SDW

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Phase diagram

pure singlet SC (SCs) phase(SCs + SDW) pure SDW

Limits RG calculations:

Is there coexistence or

segregation

between SC and SDW ?

Cooling rateS. Haddad et al. to appear in J. Low Temp. Phys.

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Experiments

Phase segregation !

Ordered ClO4 region

SC

SC

SCSDW

Disordered ClO4 region

decrease cooling rate(decrease disorder)

Next step: compare free energies (pure SC, pure SDW, SC+SDW)

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Effect of a transverse magnetic fieldH

a

b

c

Organic chains

Cascade of field-induced SDW (FISDW) phases

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Other puzzle: Effect of a high magnetic field

Generic Temperature field phase diagram in the absence of anion

ordering:

2

4

6

8

10

Tem

pera

ture

(K)

5 10 15 20 25

N=0

N=123

metal

Cascade of FISDW phases :already explained within the Quantized nesting model (Lebed, Gor’kov, Maki, Héritier, Montambaux, Lederer).

Magnetic field (T)

SDW phase inside an original SDW state !!!

Temperature field phase diagram in the (TMTSF)2ClO4 (Chung et al. 2000)

SDW I

SDW III

SDW IISDW IV

?

??

Ok-Hee Chung et al., PRB 61 (2000)

25

2

4

6

8

10

Tem

pera

ture

(K)

5 10 15 20 25

N=0

N=123

metal

Magnetic field (T)

High field phases correspond lowest N values

Focus on N=0 and N=1 phases

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Gvt

VJF

b40 G=eHb/hc, b interchain distance

In the presence of magnetic field: effective anion gap

27Bq0

Aq0 q1

Intraband nesting: N=0 phase

Osada et al. (Phys. Rev. Lett. 1992)

Interband nesting: N=1 phase

(k)

k

A B

N=0Tow nesting vectors

k

A B

N=1one nesting vector

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Instability criteria

Tvg

gTQ

g F

mm

b ,4 21 ,

21 1

)2(

)2(

00

)2(00

N=0 FISDW phase: Generalized Stoner Criterion

Term describing the overlap of the tow SDW components appearing

on the two bands

Intraband term

N=1 FISDW phase: standard Stoner Criterion

, 1 21 1

11)2(

)2(

)2( TQgg

gff

t

MFT + RG

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Thermodynamics:Competition between the N=0 and the N=1 phase

F

T

F1

F0

T1 T0

T*1

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Temperature-field phase diagram

Experiments (Chung et al. 2000)

?

?

?

Our model

S. Haddad et al. Phys. Rev. Lett., 89, 087001 (2002)S. Haddad et al. Phys. Rev B, 72, 085104 (2005)

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Effect of a parallel magnetic field

H

a

b

c

Organic chainsa

Confinement inthe (a,b) plane

b

Free of bird flu !

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Experiment (Danner et al. 1997)

Our model

submitted to Eur. Phys. J. B

Joo et al. 2006

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Quantum mechanical calculation

Layer index

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Other models in competition with our !!!

Index layer

Probability in transverse direction

A. G. Lebed, Phys. Rev. Lett. (2005)

But, does not explain the resistance

behavior

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What should be next ?

symmetry of the gap in (TMTSF)2ClO4? triplet or singlet ?

Interplay between Superconductivity and SDW : coexistence or competition ?

SDW

Sc