DIP meeting report – Technion , March 2013

Yoav Kalcheim (HU)

Gad Koren, Tal Kirzhner (Technion)

Jason Robinson, Mehmet Eglimez (Cambridge)

Oded MilloRacah Institute of Physics, The Hebrew University of Jerusalem

cos

F

exx v

EP

d

Clean limit: Order parameter ~ sin(d/ξF)/(d/ξF) ; ξF = ħvF/2Eex

Dirty limit: Order parameter ~ exp(-d/ξF)cos(d/ξF) ; ζF = (ħD/2Eex)1/2

ξF ~ nm (typically)

Oscillations and decay of SC properties observed in many cases

Long-ranged PE in S-F bilayers: 1. Crossed Andreev reflections at DWs?2. Proximity induced triplet pairing.

PE in SC-F junctions: the FFLO mechanism

-Px Px0

SCSC FM

-Px+ΔPx Px+ΔPx

FM

0

Proximity induced triplet-pairing at F-S interfaces

Spin active interface + interface magnetization different than in the bulk.M. Eschrig et al., Phys. Rev. Lett. 90, 137003 (2003)

Superconductor

Ferromagnet

sizewalldomain

Long Range PE

Magnetic inhomogeneitymay cause triplet pairing (e.g., DWs).A. F. Volkov and K. B. Efetov, Phys. Rev. Lett. 102 (2009)

Triplet pairing:

The symmetry of the induced triplet order parameter

Superconductor

Ferromagnet

sizewalldomain

Long Range PENote:

Anisotropic order parameters are sensitive to disorder.

Odd-frequency triplet: s- or d-wave

Even-frequency triplet: p-wave

La2/3Ca1/3MnO3 (LCMO)/(100)YBCO bilayers

LCMO: Domain walls of ~20nm at 4.2K

(>>ξS~2nm in YBCO). CARE suppressed.

Good lattice matching between LCMO and YBCO high interface transparency.

Our samples: Epitaxially grown(15-50nm) LCMO layers on 135nm (100)YBCO.

Y. Kalcheim, et al., Phys. Rev. B 83, 064510 (2011)

• Penetration depth much larger thanthe expected

Overview of spectral features

[a-d] - 17nm LCMO, [e] – 30nm LCMO

Non superconducting: a) metallic-like spectra. b) insulator-like wide gaps.

S. Chen et al., APL. 82, 1242 (2003).

Induced superconductivity: c) proximity induced

superconducting gap. d) and e) – zero bias

conductance peaks -hallmark of a sign changing order parameter.

ζF = (ħD/2Eex)1/2

Localized gaps – domain walls?

On few occasions gaps appeared along well defined lines of a width on the order of the LCMO domain walls.

CARE cannot account for gaps in regions 20 times larger than YBCO’s coherence length.

Triplet SC via Efetov & Volkov?

15 nm LCMO

Non-localized SC-like spectral features

Not confined to DWs – evidence for Eschrig’s model. ZBCPs – evidence for p- or d-wave order parameter.

17 nm LCMO

Zero bias conductance peaks

ZBCPs were commonly found embedded within gaps.

ZBCP splitting was also observed.

No correlation with topography.

Y. Kalcheim, et al., Phys. Rev. B 83, 064510 (2011)

Tunneling cone

- +

Possible symmetry of the order parameter

(odd frequency) (even frequency)d-wave p-wave

ZBCP splitting

dx2-y2+idxy or d+isPx + iPy

N(ε

)/N0

G. Annunziata, M. Cuoco, C. Noce, A. Sudbo, and J. Linder, arXiv:1009.3014 (2010).

Sharoni et al., Phys. Rev. B, 65, 134526 (2002)

Do the ZBCP features reflect the underlying d-wave symmetry of the YBCO or do they reflect the genuine symmetry of the induced order parameter?

Is the induced order parameter intrinsically anisotropic (d-wave or p-wave)?

Study bilayers with the e-doped Pr1.85Ce0.15CuO4 (PCCO) HTSC

(d-wave SC that does not show ZBCPs)

-20 -15 -10 -5 0 5 10 15 20

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

dI

/dV

(arb

. u.)

Sample Bias (mV)

4.2 K

200 nm

Spectrum measured on the bare PCCO film

• Only gaps, Δ ~ 3 meV.

• No ZBCPs observed ; not on top of crystallites nor on facets

-20 -15 -10 -5 0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

sample bias (mV)

dIdV

(arb

. uni

ts)

=3meV=0.39meV Z=1.5

datafit

Fits to tunneling model into a d-wave SC

4.2 K

1 μm

• Only gaps, Δ ~ 3 meV.

• No ZBCPs observed ; not on top of crystallites nor on facets

50 n

m

Correlation between spectra and surface morphology (c-axis)

1

-40 -20 0 20 400.00.20.40.60.81.01.21.41.6

Δd=16 meVdI/d

V (a

rb. u

.)

1

Sample Bias (mV)

[001]

2

-60 -40 -20 0 20 40 600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

2

Δd=16 meV

dI/d

V (a

rb. u

.)

Sample Bias (mV)

-30 -20 -10 0 10 20 300.5

1.0

1.5

2.0

2.5

3.0

3.5

Sample Bias (mV)

[110]

A. Sharoni et. al., Europhys. Lett., 54, 675 (2001).

YBCO

(b)

-5 0 5 100

0.2

0.4

0.6

0.8

1

Sample Bias (mV)

dI/d

V (a

rb. u

nits

)

gapped region

40nm

(a) (b)

STS on 15 nm LCMO on PCCO bilayer

• Most typically: gapped regions within metallic regions.

• Gaps found up to LCMO thicknesses of 25 nm.

Y. Kalcheim, J. Robinson, M. Eglimez, O. Millo, PRB 85, 104504 (2012)

-10 -5 0 5 100.7

0.8

0.9

1.0

1.1

dI/d

V (a

rb. u

.)sample bias (mV)

Observation of zero bias conductance peaks

• Appeared less abundantly compared to the gaps

• ZBCP in an intrinsic property of the induced OP.

The induced OP can be intrinsically anisotropic

-10 -5 0 5 10 150.9

1

1.1

1.2

1.3

1.4

1.5

1.6

sample bias (mV)

dI/d

V (a

rb. u

nits

)

10nm25nm 25 nm LCMO

Y. Kalcheim, J. Robinson, M. Eglimez, O. Millo, PRB 85, 104504 (2012)

-10 -5 0 5 100.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

Sample Bias (mV)dI

/dV

(arb

. uni

ts)

So far:• Localized long-range proximity effects in LCMO/YBCO and LCMO/PCCO bilayres.

• Results on the LCMO/YBCO and LCMO/PCCO bilayres may be explained by the induction of triplet pairing in the LCMO.

• ZBCPs imply that the proximity induced order-parameter has anisotropic orbital symmetry, p-wave or d-wave, and even a complex OP (p + ip).

Q?: What is the mechanism leading to triplet-pairing?- Look at the effect of magnetic-field application: Volkov-Efetov: Proximity effect (triplet-pairing) along DWs – suppressedEschrig: Proximity effect (triplet-pairing) – enhanced.

Y. Kalcheim, J. Robinson, M. Eglimez, O. Millo, PRB 85, 104504 (2012)

Y. Kalcheim, T. Kirzhner, G. Koren, O. Millo, Phys. Rev. B 83, 064510 (2011)

-30 -20 -10 0 10 20 30

0.6

0.8

1.0

1.2

1.4

dI

/dV

(arb

. u.)

Sample Bias (mV)Gray colors – before magnetic field application“warm” colors – after B = 60 mTBlue-green colors – after B = 150 mT > Bsat

• Application of magnetic field enhances initially (B=0) small gaps, but does not affect much initially large gaps.• Some ZBCPs appeared, but in this case it seems like that their magnitude and abundance was not affected by the field.

B = 150 mTB = 0

-30 -20 -10 0 10 20 30

0.7

0.8

0.9

1.0

1.1

dI/d

V (a

rb. u

.)

Sample Bias (mV)

14nm LCMO(100)YBCO

STO

STM tip

H

14 LCMO on YBCO

Non-monotonic dependence of the (somewhat shifted) ZBPC on magnetic field. Enhancement up to about the saturation filed and then suppression.

17nm LCMO(100)YBCO

STO

STM tip

H

17 LCMO on YBCO

-30 -20 -10 0 10 20 300.7

0.8

0.9

1

1.1

1.2

1.3

1.4

Sample Bias (mV)

dI/d

V (a

rb. u

nits

)

-30 -20 -10 0 10 20 300.75

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

Sample Bias (mV)

dI/d

V (a

rb. u

nits

)

“anomalous” tunneling spectra varying with time under B

H=175mT

H=0

-30 -20 -10 0 10 20 30

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

Sample Bias (mV)

dI/d

V (a

rb. u

nits

)

12345

H=70mT 14nm LCMO(100)YBCO

STO

STM tip

H

Taken over the course of a few minutes. Each curve represents an average over a set of similar spectra. Delay between each measurement set is tens of seconds/

-30 -20 -10 0 10 20 300.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

Sample Bias (mV)

dI/d

V (a

rb. u

nits

)

H=116mT

stable

Smaller ZBCPs evolved into the larger ones (green-yellow) within ~ 1 min.

start - end Variations may reflect changes in local magnetization.

t

-15 -10 -5 0 5 10 15

1.0

1.1

1.2

dI/d

V (a

rb. u

.)

Sample Bias (mV)-15 -10 -5 0 5 10 15

0.6

0.7

0.8

0.9

1.0

1.1

1.2

dI

/dV

(arb

. u.)

Sample Bias (mV)

‘cold’ colors – before application of magnetic field‘warm’ colors – after application of B = 180 mT

It appears that magnetic field (even after ramping down back to zero) enhances the PE-induced spectral features, both gaps and ZBCPs.

T = 4.2 K T = 4.2 K

15 nm LCMO and PCCO15nm LCMO(100)PCCO

NGO

STM tip

H

12 nm LCMO and PCCO

-15 -10 -5 0 5 10 15

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

dI/d

V (a

rb. u

.)

Sample Bias (mV)

before applying field

after B = 180 mT250 nm

12nm LCMO(100)PCCO

NGO

STM tip

H

Again, magnetic field enhances gaps and in this case maybe also ZBCPs

-10 -5 0 5 10

0.8

1.0

1.2

dI/d

V (a

rb. u

.)

Sample Bias (mV)

15 nm LCMO and PCCO

In regions where the gaps are well developed before B applications, turning on the magnetic field does not change much.

Black-blue curve – before B application“warm colored” curves – after B = 150 mT

15nm LCMO(100)PCCO

NGO

STM tip

H

Effect of magnetic field application:• In general, well developed (large and deep) gaps are not affected much.

• Small gaps usually become more pronounced by the application of magnetic field.

- this effect increases up to the saturation field, after which the gaps may somewhat reduce.

- this was observed also for ZBCPs, but we have less data.

• Variations of spectra with time (non-monotonic) under magnetic field. Changes in local magnetization structure?

Outlook

PCCO / YBCO

LCMO

NGO

STM tip

• Investigate induced triplet pairing in the SC.- PCCO may be better: ZBCPs will provide more direct(less ambiguous) evidence for induced triplet pairing.

Increased TC of Nb films upon chemical linking Au dots

1Yossi Paltiel, 1Eran Kazir, 2Yoav Kalcheim, 2Oded Millo1Department of Applied Physics,2Racah Institute of Physics, and the Center for Nanoscience and Nanotechnology,The Hebrew University of Jerusalem

ISF - Center of Excellence programISF - BIKURADIP , DARPA

Nb

Si

Si O

O

Au

O

O

O

CH2

CH2

CH2C

Si

Si O

Au

O

O

CH2

CH2

CH2C

Si

Si O

Au

O

O

CH2

CH2

CH2C

Au nanoparticles attached to Au by di-Silane molecules

7.2 7.4 7.6 7.8 8.0 8.20.0

0.2

0.4

0.6

0.8

1.0

Tc=7.9KTc=7.65KH=0 OeI=2mA

R

[]

T [K]

Nb+D-Silane+10nM Nb Before Attachment

Small but reproducible enhancement of Tc

150 nm Nb film

Small but reproducible enhancement of Tc

• Relative enhancement increases as the Nb film thickness reduces.

0.95 1.00 1.05 1.10 1.15 1.200

2

4diSilaneand 10 nm

dR

/dT

[

]

Normalized Temperature

H=0 TI=2mA

BareNb

MPS and10 nm

0.85 0.90 0.95 1.00 1.05 1.100

2

4

6

MPS and 5nm

diSilaneand 10nm

diSilane and 5nm

H=0 TI=2 mA

dR/d

T [

]

Normalized Temperature

Bare sample

7.5 8.0 8.5 9.00.00

0.05

0.10

0.15

R[

]

T[K]

H=0 TI=2 mA

BareNb

OTS and10 nm

di-Silanelinker, ~3 nm

MPSlinker, ~0.5 nm

OTSnon-linker, ~3 nm

• Chemical linking ability for charge transfer is essential• Not a Coulomb screening effect

150 nm Nb 50 nm Nb 150 nm Nb

7.2 7.6 8.0 8.40.0

0.2

0.4

0.6

0.8

1.0

R[

]

T [K]

I=2 mAH=0 Oe

6.2 6.4 6.6 6.8 7.00.0

0.2

0.4

0.6

0.8

1.0H=0.9 TI=2mA

Tc=6.65KTc=6.6K

R[

]T [K]

Effect of Au nanoparticles removal

• Tc enhancement effect reduces as NP density reduces. • Effect much smaller under application of matching magnetic (inset).- effect of flux-creep eliminated after linking (inset, red curve).

Au-NPs

Scanning tunneling spectroscopy - linked dots

• Enhanced zero-bias conductance on Au dots when TC increases.

• Far away from Au dots, a reduced Nb SC gap observed.

di-Silane

T = 4.2 K

30 20 10 0 10 20 300

1

2

3

4

5

6

7

Sample Bias (mV)-4 -2 2 40

MPS

Scanning tunneling spectroscopy - linked dots

• Proximity-gaps appeared on Au dots when TC reduced (MPS only).

Andreev reflections through the linker molecules.

0.5

Sample bias (mV)-2 0 2

dI/d

V(n

orm

.)

-10 -5 0 5 10100.94

0.96

0.98

1

1.02

1.04

1.06

1.08

Sample Bias [mV]

Den

sity

of s

tate

s [a

.u.]

-2 20Sample bias (mV)

dI/d

V(n

orm

.) 1.1

0.9

1.0

MPSlinker, ~0.5 nm

Scanning tunneling spectroscopy – unlinked dots

• Coulomb-blocked observed.

Here the tunnel-barrier between the Au-dot an Au-substrate is high.

-200 -100 0 100 200 300

-1

-0.5

0

0.5

1

Sample Bias (mV)

Cur

rent

(nA

)

Au dot on OTS

4.2 K

• Plasmon excitation effect on TC?

Effect of illumination wavelength on TC

400 500 600 7007.2

7.4

7.6

7.8

T C

[K]

W avelength [nm ]

di-Silanelinker, ~3 nm

Co nanoparticles linked to Nb film.

Linking process:

* Cleaning the surface for 10 min in plasma Argon* di-Silane molecule adsorption.* Co adsorption from Toluene solution: two dips process, each dip for 2 hours, washing with Toluene between the 2 dips.

Co

7.50 7.52 7.54 7.56 7.58 7.60

0.0

0.2

0.4

0.6

0.8

Tc=7.565 KTc=7.545 K

R[O

hm]

T[K]

For poor quality of Nb

Before adsorption

After adsorption

Co

Co nanoparticles linked to Nb film.

High quality Nb film

9.05 9.10 9.15 9.20 9.25 9.30-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Tc=9.2KR

esis

t [O

hm]

Temperature [T]

REF REF DS+Co NP DS+Co NP

Tc=9.14K

After adsorptionBefore adsorption

Co

Co nanoparticles linked to Nb film.

Summary

• TC enhancement of Nb films upon chemically linking Au NPs.

• Not a Coulomb screening effect – chemical linking essential.

• Origin not yet understood.

• DOS on the Au-NPs is modified. Either a ZBCP developed, when TC is enhanced, or induced mini-gap when TC reduces.

• Small enhancement also upon attaching ferromagnetic Co NPs.