2010. 3. 12 PSM2010@Yokohama STM/STS study of surface electronic STM/STS study of surface electronic density of states of Sr 2 RuO 4 & density of states of Sr 2 RuO & 4 Unconventional local transport Unconventional local transport characteristics in microfabricated characteristics in microfabricated Sr 2 RuO 4 -Ru eutectic crystals Ru eutectic crystals Sr 2 RuO 4 - AIST Hiroshi Kambara AIST Hiroshi Kambara AIST Satoshi Kashiwaya Tokyo Univ. of Sci. Hiroshi Yaguchi Hokkaido Univ. Yasuhiro Asano Nagoya Univ. Yukio Tanaka Kyoto Univ. Yoshiteru Maeno Univ. Tokyo Hiroshi Fukuyama
Introduction Time reversal Sr 2 RuO 4 (T c =1.5 K) symmetry breaking L ˆ d ( k ) z ( k ik ) 0 x y spin orbital ( k ik ), ( k ik ) x y x y Existence of chiral domain S SR Luke et al. (1998) by Deguchi and Maeno Kerr effect Xia et al. (2006) Chiral p-wave superconductor (spin-triplet pairing) Rich internal degrees of freedom in the Cooper pair ! Novel phenomena are predicted theoretically: half quantum vortex, anomalous proximity effect, etc. challenging subjects
Outline What are the local electronic states and properties? 1. STM/STS study of local density of states Surface sensitive A cleaved surface (SrO-layer) does not show superconductivity. STM image (5 nm × 5 nm) Sr 2 RuO 4 2. Local transport characteristics of microfabricated crystals Local caracteristics p x -ip y Surface insensitive p x +ip y extracted from bulk Anomalous hysteretic p x -ip y feature in V-I p x +ip y characteristics suggests the existence of chiral domain
1. 1. STM/STS study of surface STM/STS study of surface electronic density of states electronic density of states
STM and Tunnel spectra on a cleaved surface Cleaved topmost surface is usually a SrO-layer. 5 nm × 5 nm Sr-atom 2 The most cleavable dI/dV [arb. unit] plane SrO 19 K Sr RuO 2 1 7.3 K O T = 100 mK c Ru b 0 -100 0 100 a V [mV] Sr 2 RuO 4 Non-superconducting gap: ～ 5 meV (～ 50 K)
STM and Tunnel spectra on a cleaved surface Cleaved topmost surface is usually a SrO-layer. 5 nm × 5 nm SrO layer Sr-atom 2 dI/dV [arb. unit] 19 K RuO 2 layer? (Rare) C. Lupien et al ., cond-mat/0503317. 1 7.3 K T = 100 mK 0 -100 0 100 V [mV] C. Lupien et al ., cond-mat/0503317. Non-superconducting gap: ～ 5 meV (～ 50 K) Electronic structure on a cleaved SrO-surface is different from that of superconductivity.
Cleaving-temperature dependence of Sr 2 RuO 4 90 nm × 90 nm 300 K T cleave = 7 K Cleaving 100 K temperature 90 nm × 90 nm 5 nm × 5 nm 5 nm × 5 nm 5 nm × 5 nm Flat surface with Disordered surface atomic resolution (no atomic image) T = 45 mK T = 42 mK 3 3 All samples were cleaved dI/dV [nA/V] dI/dV [nA/V] at ultrahigh vacuum. 2 2 STM images were obtained at T ~ 40 mK. V 0.47 1 1 Recently, similar 0 0 -50 0 50 -50 0 50 experiments were Anderson V [mV] V [mV] Non- reported by Pennec localization E superconducting E et al ., PRL (2008). in 3D F gap Altshuler and Aronov , Solid State Commun. 30 , 115 (1979).
Non-superconducting surface of Sr 2 RuO 4 junction Surface-sensitive measurement is not SRO/SRO straightforward to study the superconductivity of Ru Sr 2 RuO 4 . SRO/Ru/SRO Mao et al ., PRL 87 , 37003 (2001). Junction resistance between SRO/SRO increases at T < 25 K → Non-superconducting surface layer
2. 2. Unconventional local transport Unconventional local transport characteristics in microfabricated characteristics in microfabricated Sr 2 RuO 4 -Ru eutectic crystals Ru eutectic crystals Sr 2 RuO 4 -
Sr 2 RuO 4 -Ru eutectic system ~ 3-K phase superconductivity ~ Sr 2 RuO 4 – Ru eutectics Pure Sr 2 RuO 4 (1.5-K phase) Ru Sr 2 RuO 4 -Ru inclusion (3-K phase) 10 m Maeno et al ., PRL 81 , 3765 (1998). T < 1.5 K 3 > T > 1.5 K T ~ 3 K (S-S’-S) (S-N-S) Ru Sr 2 RuO 4 weak link (N) 3-K phase (S) 1.5-K phase (S’) p-wave superconducting junctions are naturally formed.
Transport characteristics in microfabricated Sr 2 RuO 4 -Ru junction V Sr 2 RuO 4 - I c0 I c1 Ru I // ab Ru FIB 1) Kink ⇒ I c I- milled V- < t 10 20 I 0 I c0 I c1 V+ I+ 2) Anomalous hysteresis 0.8 microbridge 20 2.7 K c-axis unit:[ m] → 50 m dV/dI (normalized at 4.2 K) ← → The surface state does not influence 0.6 2.4 K this 4-probe configuration. 2.2 K 1.0 c199-4 0.4 1.9 K R (normalized) Extraction of initial R / R 4.2 K superconducting FIB-1 0.5 linkage channels 0.2 1.7 K FIB-2 without averaging 1.5 K over bulk sample 1.4 K 0 1.3 K 0 -4 -2 0 2 4 1 2 3 4 I (mA) T (K)
Sample configurations (I // ab and I // c) V - I - V + 10 m I + Ru microbridge 3 m I // ab Ru Sr 2 RuO 4 -Ru 5 m c ab sample 1 V - I - V + I + I 0.7 Sr 2 RuO 4 m I -Ru c I // c glue ab slit 10 m sample 2 slit Ru SIM image obtained after the top and I side surfaces were slightly milled. a region milled c FIB milling was done after transport from side measurements.
V-I & dV/dI-I characteristics ( Anomalous hysteresis) I sw2 20 I sw2 I sw1 0.2 I sw1 T = 0.80 K T = 1.3 K sample 2 10 sample 1 0.1 V (mV) V ( V) 0 0 -0.1 → → -10 ← ← I sw1 → → I sw1 -0.2 I sw2 -20 I sw2 dV/dI (normalized) dV/dI (normalized) I // ab I // c 4 0.4 2 0.2 0 0 -4 -2 0 2 4 -4 -2 0 2 4 I (mA) I (mA) Negative dV/dI is not observed → switching phenomena Anomalous hystereses are observed for both I//ab and I//c directions.
How are V-I characteristics anomalous? Usual switching V Sr 2 RuO 4 -Ru I sw2 Intrinsic JJs I sw1 V 3rd in Bi2212 2nd 1st 0 I c0 I’ c0 I’’ c0 I 0 I sw1 I sw2 (I c ) I Anomalous features (1) Voltage decreases at I sw . (2) It switches to a lower R n current (normal resistance) branch with larger I c . (3) Opposite hysteresis loop compared to typical Josephson junction (JJ) s.
Magnetic field effect No change ! dV/dI (normalized) sample 1 sample 1 4.2 K FC (130 G // c) T = 1.3 K (dV/dI) / (dV/dI) ZFC T = 1.3 K 0.5 H = 0 (dV/dI) / (dV/dI) 4.2 K 1 dV/dI (normalized) 0 0 130 G -4 -2 0 2 4 2 I (mA) 350 G 3 No change ! -1 0 G Anomalous hysteresis is Anomalous hysteresis is 4 NOT due to a magnetic NOT due to a magnetic -4 -2 0 2 4 vortex! vortex! I (mA) The curves are offset by -0.5 unit for clarity. cf ) H c1 (0) 70 G (1.5-K phase) Deguchi, Mao, Maeno, JPSJ(2004).
Anomalous J c enhancement In usual case, L R bridge variation in 0.15 S thickness c199-4 c199-20 L c199-8 width L / R bridge (m/ ) c199-9 W 0.1 c359-2 S = Wt I // ab c359-7 t: thickness 0.05 usual Critical current density: J c T = 4.2 K I 0 J const . c S c c199-4 (1.4 K) 10 4 S-independent c199-20 (1.3 K) c199-8 (1.2 K) c199-9 (1.3 K) J c (A/cm 2 ) c359-2 (1.4 K) Unusual ! J c increases for small S. c359-7 (1.3 K) 10 3 high J c along edge k x J c (0) = 500 A/cm 2 - + low J c inside for bulk pure Sr 2 RuO 4 + k x ±ik y - + 10 2 (by Deguchi and Maeno) - k x - + 0 1 2 Edge channels seem [ 10 3 ] S ( m 2 ) to be formed. ※ J c is not J c (0) at T = 0.
Possible origin of the anomalous hysteresis Chiral domain wall motion through the 3-K phase (k x ) and 1.5-K phase (k x ±ik y ) coexistence region Frustration (1) Initial state DW A - + - + + + + + + - - + + - - + - - - - - + - + B (2) I > 0 (3) I = I sw1, I sw2 A - + + Domain - k x -phase flips + + wall moves + + + - - + + - - + under DC - - - - - + No frustration + current. - B Frustration I sw1 I sw2 V No frustration pair potential for x 1.5K 3K A B 1.5K node 3K A B 0 I c0 I’ c0 I’’ c0 I
Summary Summary STM/STS at Sr 2 RuO 4 surface STM/STS at Sr 2 RuO 4 surface Low temperature (T<100 K) cleaved surface (SrO-layer) shows non-superconducting gap. Room temperature cleaved surface shows disordered electronic states. The surface electronic states are different from those of bulk superconductivity. Local transport measurement for microfabrication sample Local transport measurement for microfabrication sample Microfabrication technique with FIB was applied to Sr 2 RuO 4 -Ru eutectic crystals. Local superconducting channels were successfully extracted. Anomalous hysteresis of V-I characteristics was observed for both I//ab and I//c directions. It suggests that internal degrees of freedom of the chiral p-wave state. Chiral domain wall motion by DC current is a possible origin of the anomalous hysteresis.
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