STM/STS study of surface electronic STM/STS study of surface - - PowerPoint PPT Presentation

STM/STS study of surface electronic STM/STS study of surface electronic density of states of Sr density of states of Sr2

2RuO

RuO4

4

& & Unconventional local transport Unconventional local transport characteristics in microfabricated characteristics in microfabricated Sr Sr2

2RuO

RuO4

4-

• Ru eutectic crystals

Ru eutectic crystals

• 2010. 3. 12 PSM2010@Yokohama

AIST AIST Hiroshi Kambara 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

Sr2RuO4 (Tc=1.5 K)

) ( ˆ ) (

y x

ik k z k d     

spin

• rbital

L S

by Deguchi and Maeno

Time reversal symmetry breaking

) ( ), (

y x y x

ik k ik k  

Chiral p-wave superconductor (spin-triplet pairing)

Existence of chiral domain SR Luke et al. (1998) Kerr effect Xia et al. (2006)

Rich internal degrees of freedom in the Cooper pair ! Novel phenomena are predicted theoretically: half quantum vortex, anomalous proximity effect, etc. challenging subjects

Anomalous hysteretic feature in V-I characteristics suggests the existence of chiral domain

Outline What are the local electronic states and properties?

• 1. STM/STS study of local density of states
• 2. Local transport characteristics of microfabricated

crystals

px+ipy px-ipy

px+ipy px-ipy Local caracteristics extracted from bulk

A cleaved surface (SrO-layer) does not show superconductivity.

STM image (5 nm×5 nm)

Sr2RuO4 Surface sensitive Surface insensitive

1. 1. STM/STS study of surface STM/STS study of surface electronic density of states electronic density of states

• 100

100 1 2 V [mV] dI/dV [arb. unit]

T = 100 mK 7.3 K 19 K

STM and Tunnel spectra on a cleaved surface

Non-superconducting gap:  ～ 5 meV (～ 50 K)

5 nm×5 nm

Cleaved topmost surface is usually a SrO-layer. Sr-atom a b c

Sr2RuO4

Ru O Sr The most cleavable plane

SrO RuO2

• 100

100 1 2 V [mV] dI/dV [arb. unit]

T = 100 mK 7.3 K 19 K

STM and Tunnel spectra on a cleaved surface

SrO layer RuO2 layer? (Rare)

Electronic structure on a cleaved SrO-surface is different from that of superconductivity.

5 nm×5 nm

Cleaved topmost surface is usually a SrO-layer. Sr-atom

• C. Lupien et al., cond-mat/0503317.

Non-superconducting gap:  ～ 5 meV (～ 50 K)

• C. Lupien et al., cond-mat/0503317.

90 nm × 90 nm 5 nm × 5 nm

Disordered surface (no atomic image)

Cleaving-temperature dependence of Sr2RuO4

90 nm × 90 nm 5 nm × 5 nm 5 nm × 5 nm

All samples were cleaved at ultrahigh vacuum. STM images were

• btained at T~40 mK.

Recently, similar experiments were reported by Pennec et al., PRL (2008).

• 50

50 1 2 3 V [mV] dI/dV [nA/V] T = 42 mK

• 50

50 1 2 3 V [mV] dI/dV [nA/V] T = 45 mK

V0.47

Flat surface with atomic resolution

F

E E   

Altshuler and Aronov, Solid State Commun. 30, 115 (1979).

Anderson localization in 3D

Tcleave = 7 K 300 K 100 K

Cleaving temperature Non- superconducting gap

Non-superconducting surface of Sr2RuO4

Junction resistance between SRO/SRO increases at T < 25 K → Non-superconducting surface layer

junction

Ru SRO/SRO SRO/Ru/SRO

Surface-sensitive measurement is not straightforward to study the superconductivity of Sr2RuO4.

Mao et al., PRL 87, 37003 (2001).

2. 2. Unconventional local transport Unconventional local transport characteristics in microfabricated characteristics in microfabricated Sr Sr2

2RuO

RuO4

4-

• Ru eutectic crystals

Ru eutectic crystals

Sr2RuO4-Ru eutectic system ~3-K phase superconductivity~

Sr2RuO4-Ru (3-K phase) Pure Sr2RuO4 (1.5-K phase)

Sr2RuO4 – 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) Sr2RuO4 T ~ 3 K weak link (N) 3 > T > 1.5 K (S-N-S)

p-wave superconducting junctions are naturally formed.

• 4
• 2

2 4 0.2 0.4 0.6 0.8 I (mA)

2.7 K 2.4 K 2.2 K 1.9 K 1.7 K 1.5 K 1.4 K 1.3 K

→ ← →

dV/dI (normalized at 4.2 K)

Transport characteristics in microfabricated Sr2RuO4-Ru junction

20 20 c-axis < t10 Sr2RuO4- Ru I+ I- V+ V- unit:[m]

Ru

50 m

FIB milled microbridge

I // ab

1 2 3 4 0.5 1.0 T (K) R / R4.2 K

c199-4

R (normalized) initial FIB-2 FIB-1

2) Anomalous hysteresis I Ic0 Ic1 Ic0 Ic1 V 1) Kink ⇒ Ic

Extraction of superconducting linkage channels without averaging

• ver bulk sample

The surface state does not influence this 4-probe configuration.

Sample configurations (I // ab and I // c)

I- c ab V+ I+ V- microbridge Sr2RuO4-Ru

sample 1

I // ab

Ru Ru 10 m 5 m 3 m Sr2RuO4

• Ru

0.7 m

ab I

10 m

c

glue V+ I+

I // c

I slit

sample 2

I- V-

I

Ru

c

a region milled from side

slit

SIM image obtained after the top and side surfaces were slightly milled. FIB milling was done after transport measurements.

Anomalous hystereses are observed for both I//ab and I//c directions.

V-I & dV/dI-I characteristics (Anomalous hysteresis)

• 20
• 10

10 20 V (V)

T = 1.3 K

• 4
• 2

2 4 0.2 0.4 dV/dI (normalized) I (mA)

→ ← →

sample 1

I // ab

• 4
• 2

2 4 2 4 dV/dI (normalized)

T = 0.80 K

• 0.2
• 0.1

0.1 0.2 I (mA) V (mV)

→ ← →

I // c

sample 2

Isw1 Isw2 Isw1 Isw2 Isw2 Isw2 Isw1 Isw1 Negative dV/dI is not observed → switching phenomena

How are V-I characteristics anomalous?

Anomalous features (1) Voltage decreases at Isw. (2) It switches to a lower Rn (normal resistance) branch with larger Ic. (3) Opposite hysteresis loop compared to typical Josephson junction (JJ) s. Sr2RuO4-Ru

Ic0 I’c0 I’’c0

Isw1

Isw2 1st 2nd 3rd I V

Usual switching Intrinsic JJs in Bi2212 I V

Isw1 Isw2 (Ic)

current

• 4
• 2

2 4

• 1

I (mA) (dV/dI) / (dV/dI) 4.2 K T = 1.3 K

1 2 3 4

350 G H = 0 130 G 0 G

The curves are offset by -0.5 unit for clarity.

• 4
• 2

2 4 0.5

I (mA)

(dV/dI) / (dV/dI)

4.2 K

T = 1.3 K

Magnetic field effect

cf) Hc1(0)  70 G (1.5-K phase) Deguchi, Mao, Maeno, JPSJ(2004). sample 1

FC (130 G // c) ZFC

No change !

Anomalous hysteresis is NOT due to a magnetic vortex! Anomalous hysteresis is NOT due to a magnetic vortex!

dV/dI (normalized) dV/dI (normalized)

sample 1

No change !

1 2 [103] 102 103 104 Jc (A/cm2) S (m2)

c199-4 (1.4 K) c199-20 (1.3 K) c199-8 (1.2 K) c199-9 (1.3 K) c359-2 (1.4 K) c359-7 (1.3 K)

※ Jc is not Jc(0) at T = 0.

Unusual !

Anomalous Jc enhancement

0.05 0.1 0.15 L / Rbridge (m/)

c199-4 c199-20 c199-8 c199-9 c359-2 c359-7

usual

thickness width

T = 4.2 K Jc (0) = 500 A/cm2

for bulk pure Sr2RuO4 (by Deguchi and Maeno)

L W t: thickness I // ab S = Wt

S L Rbridge   . const S I J

c c

 

In usual case,

Critical current density: Jc

S-independent variation in

Jc increases for small S.

Edge channels seem to be formed. high Jc along edge low Jc inside +

• +
• +
• +
• kx

kx±iky kx

Possible origin of the anomalous hysteresis

Chiral domain wall motion through the 3-K phase (kx) and 1.5-K phase (kx±iky) coexistence region

(1) Initial state +

• +
• +
• +
• +
• +
• DW

Ic0 I’c0

Isw1

I V

Isw2

I’’c0 (2) I > 0 +

• +
• +
• +
• +
• +
• Domain

wall moves under DC current. 1.5K 3K B A

pair potential for x

Frustration

node

1.5K 3K B A

No frustration

+

• +
• +
• +
• +
• +
• A

B +

• +
• +
• +
• +
• +
• A

B (3) I = Isw1, Isw2 kx-phase flips

Frustration No frustration

STM/STS at Sr STM/STS at Sr2

2RuO

RuO4

4 surface

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 Sr2RuO4-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

• f freedom of the chiral p-wave state. Chiral domain wall motion

by DC current is a possible origin of the anomalous hysteresis.

Summary Summary