Anomalous Local Transport in Microfabricated Sr 2 RuO 4 -Ru Eutectic Junction

1. 2009. 9. 4 NDSN2009@Nagoya Univ. Anomalous Local Transport in Microfabricated Sr2RuO4-Ru Eutectic Junction AISTHiroshi Kambara Satoshi Kashiwaya Tokyo Univ of Sci. Hiroshi Yaguchi Hokkaido Univ. Yasuhiro Asano Nagoya Univ. Yukio Tanaka Kyoto Univ. Yoshiteru Maeno

2. Introduction Sr2RuO4 (Tc=1.5 K) Chiral p-wave superconductor (spin-triplet pairing) How is the local transport property? If chiral domain exists, how is the transport property affected? spin orbital by Deguchi and Maeno Measurement of local transport characteristics of Sr2RuO4-Ru eutectic junction made by microfabrication. Observation of anomalous hysteresis in V-I L S

3. Sr2RuO4-Ru eutectic system~3-K phase superconductivity~ Sr2RuO4 – Ru eutectics Ru inclusion T < 1.5 K (S-S’-S) 3 > T > 1.5 K (S-N-S) T ~ 3 K Ru Sr2RuO4 10 mm weak link (N) 3-K phase (S) 1.5-K phase (S’) Maeno et al., PRL 81, 3765 (1998). Pure Sr2RuO4(1.5-K phase) Sr2RuO4-Ru(3-K phase) p-wave superconducting junction is naturally formed.

4. Extract a channel of Sr2RuO4-Ru junction Focusing on the local superconducting channel by controlling the number of Ru-inclusions FIB milled Ru initial initial If microbridge is thin and narrow enough. R (normalized) R (normalized) c-axis FIB-1 50 mm FIB Sr2RuO4-Ru Kink structures appear after FIB milling because only a few channels are left. FIB-2 I- I // ab V- < t10 20 V+ I+ 20 unit:[mm] microbridge

5. dV/dI-I characteristics dV/dI V T < 1.5 K (S-S’-S) 3 > T > 1.5 K (S-N-S) Ic1 Ic0 I 0 weak link (N) 1.5-K phase (S’) Ic0 Ic1 Model of Josephson junctions in series N or 1.5-K N or 1.5-K 3-K 3-K 3-K Ru Ru Ru S-N-S S-S’-S Ic Ic2 Each channel Ic1 Ic0 T 0 Tc0

6. T < Tc0 Parity of the 3-K phase 3-K phase 1.5-K phase 3-K phase T > Tc0 s s s s s 0 0 normal 3-K 3-K s p 0 p s p - + 0 p p p p p p p s - - - - + + + + - - - + + + - + + + + + + + 3-K phase is also p-wave. (∵ 1.5-K phase is p-wave.) Monotonic increase of Ic

7. Sample configuration (I // ab and I // c) V- I- V+ I+ Ru 10 mm microbridge I // ab 3 mm Ru Sr2RuO4-Ru 5 mm c ab sample 1 I I I // c 0.7mm Sr2RuO4-Ru slit c sample 2 glue ab 10 mm

8. V-I & dV/dI-I characteristics (Anomalous hysteresis) sample 1 sample 2 I // ab I // c Anomalous hysteresis is observed for both I//ab and I//c direction.

9. How are V-I characteristics anomalous? Isw2 Isw1 V Sr2RuO4-Ru Usual switching Intrinsic JJs in Bi2212 3rd 2nd 1st 0 Ic0 I’c0 I’’c0 I • Anomalous features • Voltage decreases at Isw. • It switches to a lower Rn (normal resistance) branch with larger Ic. • (3) Opposite hysteresis loop compared to typical Josephson junction (JJ) s. V current 0 Isw1 I Isw2 NOT usual JJs! (Ic)

10. Magnetic field effect FC sample 1 ② H // c ① cf) Hc1(0)  70 G (1.5-K phase) Deguchi, Mao, Maeno, JPSJ(2004). No change in the hysteresis between FCand ZFC. ZFC NO influence of vortices !

11. Possible origin of the hysteresis • What is the degree of freedom of Ic ? • To explain unusual phenomena, • Assuming chiral domain, • anomalous hysteresis is explained (but not perfectly) by • Inhomogeneous channel model • 3-K phase(kx or ky)+1.5-K phase(kx±iky) coexistence model • or other mechanisms?

12. Possible origin of the anomalous hysteresis (a) Inhomogeneous channel model (pinned at a defect) chiral domain wall Ru initial state Ru Small SDW ∵different order parameter overlap px-ipy px+ipy Ic SDW Sfilament transfer of Cooper pair under DC current Larger SDW → Icincrease Ic’ DC current Anomalous hysteresis in dV/dI-I and V-I domain wallmotion→ variation of cross section ⇒ variation of Ic Ic∝Sfilament (parallel domain) … NO hysteresis Ic∝SDW (antiparallel domain) … Hysteresis!

13. Possible origin of the anomalous hysteresis (b) 3-K phase+1.5-Kphase coexistence model (kx or ky) (kx±iky) (1) Initial state (3) I = Isw→Ic larger DW DW + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - Free energy (2) I > 0 Isw Assuming 3-K phase symmetry (kx) along the edge because of broken translational symmetry. V 0 Ic0 I’c0 I

14. Questions for the chiral domain model (a) Inhomogeneous channel model ・Reproducibility of the hysteresis (appearance and shape) is quite high. → Antiparallel domain always forms in the same position?Pinning at lattice defects may be possible. ・Hysteresis survives after magnetic field cooling. kx state (Time reversal conserving state) at edge is mixed? → It may weaken the magnetic moment due to the chiral state. combined with model (b) (b) 3-K phase+1.5-Kphase coexistence model (kx or ky) (kx±iky) ・Hysteresis exists along I//c. 　→ Spatial variation in the order parameter along c-axis should be small. ・Isw1 and Isw2 is separated largely. 　→ Phase change of kx states for both edges should occur at I=Isw1≒Isw2? Each edge may not be the same.

15. Summary of the anomalous hysteresis No-magnetic field effect Hysteresis Multiple switch total Model I // ab I // c Inhomogeneous channel Chiraldomain 3-K + 1.5-K (kx) (kx+iky) ?( ) ?( ) ?( ) ?( ) ? × × Vortex × ? …question …contrary …OK! …probably OK Vortex origin is ruled out.

16. Local transport measurementshave been done (I//ab, I//c) for Sr2RuO4-Rueutectic samples made by FIB process. • p-wave Josephson junctions are formed (3-K / 1.5-K / 3-K). • Anomalous hysteresis was observed for both current directions. • (voltage drop, opposite hysteresis loop) • Chiral domain is a possible origin with • (a) Inhomogeneous channel model or • (b) 3-K phase+1.5-K phasemodel • or others? Summary