1. Anomalous Local Transport in MicrofabricatedSr 2 RuO 4 -Ru Eutectic Junction 2009. 9. 4 NDSN2009@Nagoya Univ. AISTHiroshi Kambara Satoshi Kashiwaya Tokyo Univ of Sci. Hiroshi Yaguchi Hokkaido Univ.Yasuhiro Asano Nagoya Univ.Yukio Tanaka Kyoto Univ.Yoshiteru Maeno

2. Introduction How is the local transport property? If chiral domain exists, how is the transport property affected? Sr 2 RuO 4 (T c =1.5 K) Chiral p-wave superconductor (spin-triplet pairing) L S spinorbital by Deguchi and Maeno Measurement of local transport characteristics of Sr 2 RuO 4 - Ru eutectic junction made by microfabrication. Observation of anomalous hysteresis in V-I

3. Sr 2 RuO 4 -Ru eutectic system ~3-K phase superconductivity~ Sr 2 RuO 4 -Ru (3-K phase) Pure Sr 2 RuO 4 (1.5-K phase) Sr 2 RuO 4 – Ru eutectics 10  m Ru inclusion Maeno et al., PRL 81, 3765 (1998). T < 1.5 K (S-S’-S) 1.5-K phase (S’) Ru 3-K phase (S) Sr 2 RuO 4 T ~ 3 K weak link (N) 3 > T > 1.5 K (S-N-S) p-wave superconducting junction is naturally formed.

4. Extract a channel of Sr 2 RuO 4 -Ru junction Focusing on the local superconducting channel by controlling the number of Ru-inclusions 20 c-axis < t 10 Sr 2 RuO 4 -Ru I+I+ I- V+ V- unit:[  m] Ru 50  m FIB milled microbridge I // ab R (normalized) initial FIB-2 FIB FIB-1 Kink structures appear after FIB milling because only a few channels are left. If microbridge is thin and narrow enough.

5. dV/dI-I characteristics I I c0 I c1 I c0 I c1 V 0 dV/dI T < 1.5 K (S-S’-S) 1.5-K phase (S’) weak link (N) 3 > T > 1.5 K (S-N-S) Ru 3-K N or 1.5-K N or 1.5-K Model of Josephson junctions in series T T c0 0 IcIc Each channel S-S’-S S-N-S I c0 I c1 I c2

6. Parity of the 3-K phase s + s - p +-  3-K phase 1.5-K phase 3-K phase s + s + s + s + p +- p + - p +- p +- p +- 0  0 3-K phase is also p-wave. ( ∵ 1.5-K phase is p-wave.) 3-K normal 3-K s + s + 0 p +- p +- 0 Monotonic increase of I c T > T c0 T < T c0

7. Sample configuration (I // ab and I // c) I-I- c ab V+V+ I+I+ V-V- microbridge Sr 2 RuO 4 -Ru sample 1 I // ab I // c I slit sample 2 Ru 10  m 5  m 3  m Sr 2 RuO 4 -Ru 0.7  m ab I 10  m c glue

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

9. How are V-I characteristics anomalous? Anomalous features (1)Voltage decreases at I sw. (2)It switches to a lower R n (normal resistance) branch with larger I c. (3) Opposite hysteresis loop compared to typical Josephson junction (JJ) s. Sr 2 RuO 4 -Ru I c0 I’ c0 I’’ c0 I sw1 I sw2 1st 2nd 3rd I V 0 Usual switching Intrinsic JJs in Bi2212 I V 0 I sw1 I sw2 (I c ) current

10. Magnetic field effect cf ) H c1 (0)  70 G (1.5-K phase) Deguchi, Mao, Maeno, JPSJ(2004). sample 1 ① ② NO influence of vortices ! FC ZFC No change in the hysteresis between FC and ZFC. H // c

11. Possible origin of the hysteresis What is the degree of freedom of I c ? To explain unusual phenomena, Assuming chiral domain, anomalous hysteresis is explained (but not perfectly) by (a) Inhomogeneous channel model (b) 3-K phase(k x or k y )+1.5-K phase(k x ±ik y ) coexistence model or other mechanisms?

12. Possible origin of the anomalous hysteresis domain wall motion→ variation of cross section ⇒ variation of I c Ru p x +ip y p x -ip y Ru initial state IcIc S filament S DW I c ∝ S filament (parallel domain) … NO hysteresis I c ∝ S DW (antiparallel domain) … Hysteresis! chiral domain wall (pinned at a defect) DC current under DC current Ic’Ic’ transfer of Cooper pair (a) Inhomogeneous channel model Small S DW ∵ different order parameter overlap Larger S DW → I c increase Anomalous hysteresis in dV/dI-I and V-I

13. Possible origin of the anomalous hysteresis (b) 3-K phase+1.5-K phase coexistence model (k x or k y ) (k x ±ik y ) (3) I = I sw → I c larger + - + - + - + - + - + - + - + - (1) Initial state DW + - + - + - + - + - + - (2) I > 0 + - + - + - + - + - + - Free energy Assuming 3-K phase symmetry (k x ) along the edge because of broken translational symmetry. I c0 I’ c0 I sw I V 0

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. k x 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-K phase coexistence model (k x or k y ) (k x ±ik y ) ・ Hysteresis exists along I//c. → Spatial variation in the order parameter along c-axis should be small. ・ I sw1 and I sw2 is separated largely. → Phase change of k x states for both edges should occur at I=I sw1 ≒ I sw2 ? Each edge may not be the same.

15. Summary of the anomalous hysteresis Hysteresis Inhomogeneous channel Model 3-K + 1.5-K (k x ) (k x +ik y ) Vortex I // abI // c ? No- magnetic field effect × …OK!…probably OK ? …question × …contrary × total Multiple switch Chiral domain ?( ) Vortex origin is ruled out. ?( )

16. Local transport measurements have been done (I//ab, I//c) for Sr 2 RuO 4 -Ru eutectic 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 phase model or others? Summary