F. Rullier-Albenque 1, H. Alloul 2 1

Transcription

1 Distinct Ranges of Superconducting Fluctuations and Pseudogap in Cuprates Glassy29-2/7/29 F. Rullier-Albenque 1, H. Alloul 2 1 Service de Physique de l Etat Condensé, CEA, Saclay, France 2 Physique des Solides, Université Paris-Sud, Orsay, France Pulsed magnetic field experiments C. Proust, B. Vignolle Laboratoire National des Champs Magnétiques Pulsés, Toulouse, France Single crystals D. Colson, A. Forget, SPEC Electron irradiation Laboratoire des Solides Irradiés (Ecole Polytechnique)

2 Distinct Ranges of Superconducting Fluctuations and Pseudogap in Cuprates Glassy29-2/7/29 Introduction Detection of Superconducting fluctuations, Nernst effect High magnetic field measurements in pure YBCO (underdoped, optimally doped and slightly overdoped) Pseudogap and fluctuation ranges Influence of disorder Conclusion

3 Relation between pseudogap and superconducting states? PG and SC states are in competition PG = precursor to SC T * T * T * Two energy scales in the underdoped region No clear experimental evidence of a T* line below T c Superconducting fluctuations? Glassy29-2/7/29

4 How to separate normal state from precursor superconducting effects Using a physical property which is different in the N and in the SC states Decrease of resistivity Diamagnetism above T c In underdoped cuprates : presence of pseudogap Nernst effect E B J y = y hot T x cold Transverse Electric field E y in response to a temperature gradient x T in presence of a perpendicular magnetic field B In normal state : Nernst coefficient very small In the mixed state of superconductors : mobile vortices Nernst signal In cuprates: anomalous Nernst signal above T c signature of phase fluctuations Glassy29-2/7/29

5 Anomalous Nernst effect in the normal state of cuprates La 2-x Sr x Cu O 4 T* vortex Nernst signal onset T = 12 K, ~ 9K above T c Significant Nernst signal at T>Tc Effect more pronounced in underdoped samples Y.Wang et al, PRB (26) Signature of superconducting fluctuations in the normal state T c = loss of long range phase coherence Possible implications for the physics of the pseudogap regime: preformed pairs? Y. Wang et al, PRB 64 (21) Glassy29-2/7/29

6 Importance of disorder in high-t c cuprates Glassy29-2/7/29 Influence of disorder on the Metal-insulator transition Comparison with low-t c cuprates F. Rullier-Albenque et al., Europhys. Lett. (28) SEE POSTER SC fluctuations in pure YBCO Influence of disorder

7 e y ( V/K) Glassy29-2/7/29 (nv/kt) K 1 62K 64K 68K 11K 7K 15K 2K 85K B(T) Nernst effect in pure YBCO K 45K 55K T c = 57K Rapid drop of the Nernst signal at T c In pure YBCO 6.6, the Nernst signal extends up to ~85K E H T x B B s n YBCO 6.6 T c =57K S tan xy / B /B H T onset : 85K S tan y xy 1 xy xy F. Rullier-Albenque et al., PRL(26) xy

8 Temperature extension of the Nernst signal in pure YBCO.cm) (nv/kt) Glassy29-2/7/29.cm) (nv/kt) Optimally doped YBCO Underdoped YBCO T onset T onset T(K) The temperature range of the Nernst signal increases with decreasing doping BUT T onset is higher in optimally doped than in underdoped YBCO T onset follows T c and not T *

9 H dependence Y. Wang et al., PRB (26) Nernst signal and flux flow resistivity nodal quasiparticles In a two fluid model with n s s << n ( / ) Superconducting contribution 2 a trans H e N With increasing magnetic field The Nernst signal remains quite large The resistivity approaches saturation to the «normal state»value very quickly Glassy29-2/7/29 Harris et al. PRL (1995)

10 Transverse magnetoresistivity in YBCO 6.6 High field measurements H//c 2 4 H (T) 6K 63K 7K 8K 9K 1K 12K 15K For T < 12K ~ H 2 once H> H c Weak field limit w c << H c H 2 (T 2 ) 1K 12K 15K F. Rullier-Albenque et al., PRL 27 Glassy29-2/7/29

11 Glassy29-2/7/29 a trans (T -2 ) Transverse magnetoresistivity in optimally doped YBCO 6.95 High field measurements K YBCO 6.95 : T c = 93K 1K H c 17K Good agreement between low field and high field measurements K 12K 1-5 High field measurements.2 15K H 2 (T 2 ) 13K Impossible to completely suppress the superconductivity 15K above T c / ( / ) 2 a transh 1-6 Low field measurements Modified Kohler s rule: a trans 1/ T 4

12 Glassy29-2/7/29 H (T) High field measurements in YBCO 6.6 / ( / ) H c H 2 (T 2 ) w 1 T much lower than T * ~3K c 2 c w 2 c 8K 9K 1K 11K 117K a trans Irreversibility line Phase diagram H(T) YBCO 6.6 vortex liquid + superconducting fluctuations YBCO 6.6 our results from high magnetic field measurements low field results Harris et al. vortex solid T onset from Nernst measurements normal state T c

13 .cm) Normal state resistivity in YBCO K K.1 1K 11K.5 117K H 2 (T 2 ) Extrapolations at low field Intercept at H=T 1 5 LSCO 55T bt 2 Underdoped YBCO 6.6 is metallic down to low T Normal state resistivity Glassy29-2/7/29 G.S. Boebinger etal., PRL (1996)

14 .cm) Superconducting contribution to the conductivity excess conductivity (1 3 (.m) -1 ) YBCO 6.8 AL -2D 1 AL -3D Gaussian fluctuations Aslamazov-Larkin 2D ln( T / T ) ( T T )/ T c 2 e () 16 d 1 1/ 2 3D () 32 () e 2 c c 1 YBCO ~7 YBCO 6.95 YBCO 6.8 YBCO ln(t/t ) c For =.1, 2D-AL gives () with d=11.7a (. m) Might explain results found from O 6.8 to O ~7 for <. Something else needed for O Glassy29-2/7/29

15 fl (1 3 (.m) -1 ) Effects of fluctuations on magnetoconductivity excess fluctuations (1 3 (.m) -1 ) YBCO T=13K K 13K K 11K H 2 (T 2 ) (H) Far from the transition : H K 12K 125K K 13K H 2 (T 2 ) K 11K 115K 123K H 2 (T 2 ) Glassy29-2/7/29 fl exp ( H / H ) 2 H ~26T, whatever the hole doping

16 .cm).cm) Pseudogap and onset of fluctuations Deviations (.cm) 3 25 T * YBCO 6.8 : T c = 85K 2 15 Fluctuations T c T * Pseudogap Zoom Glassy29-2/7/ Pseudogap ~21K Fluctuations ~13K

17 H c (T) Pseudogap and onset of fluctuations.cm) In optimally dopedybco 6.95 T c ~14K Linear extrapolation T * ~12K 5 4 Range of SC fluctuations Onset of fluctuations ~14K Pseudogap temperature ~12K Glassy29-2/7/29

18 .cm) Pseudogap and onset of fluctuations 9 Slightly overdoped YBCO ~7 14K 8 YBCO 6.95 T c =93K K YBCO ~7 T c =92.5K In YBCO ~7, no signature of a pseudogap down to 15K Glassy29-2/7/29

19 Phase diagram of pure YBCO 3 YBCO Hole doping Crossing of the Pseudogap and fluctuation lines Onset of fluctuations follows T c and not T * Very different from LSCO Role of defects? Glassy29-2/7/29

20 H (T).cm).cm) Influence of disorder on T c and T * - optimally doped YBCO 6.95 Electron irradiation to decrease T c T c ~ 95K T c = 7K T c = 93K T(K) T * ~12K 4 3 Normal state SC fluctuations T c decreases with T c T * is not affected by disorder Glassy29-2/7/29

21 H (T) Glassy29-2/7/29 T c, T (K) Disorder and superconducting fluctuations in YBCO 6.6 H c( T) Hc() 1 ( T / Tc ) 2 T always larger than Nernst T c 8 1 T c 6 75 T T c (K) T c T c Introduction of disorder expands the domain of superconducting fluctuations above T c () H c With decreasing T c and are depressed T c BUT remain quite large The pseudogap temperature is not affected

22 (nv/kt) M (arb.units) Comparison between La-Bi221 and irradiated YBCO 6.6 «Pure» underdoped Bi 2 Sr 2-y La y Cu O 6 y=.5 - T c = 28.9K H c2 is nearly unchanged from low T to above T c T c = 28K 1 Wang et al, PRB 64 (21) T onset ~ 75K Stan H /B xy / B Electron irradiated YBCO 6.6 T c = 24.6K (onset of magnetization) -2 Irradiated YBCO 6.6 T c = 25K Wang et al, PRB (26)

23 Superconducting fluctuations, pseudogap and disorder T Pure T? n h disordered n h Glassy29-2/7/29

24 Conclusion Magnetoresistance under high magnetic field = powerful probe for superconducting fluctuations Fluctuating conductivity only suppressed above a threshold field H c which is found to vanish at T c >> T c T c always higher than T onset for Nernst signal In pure optimally doped YBCO, superconducting fluctuations do appear above the pseudogap temperature T * is not related to the pairing energy scale In the overdoped part of the phase diagram, some might have named pseudogap something which is rather linked to superconducting fluctuations (ARPES, STM) Thorough treatment of superconducting fluctuations is still required Glassy29-2/7/29

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