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Scanning Tunneling Microscopy

Local probes at high magnetic fields ?

Hermann Suderow

Laboratorio de Bajas Temperaturas

Departamento de Física de la Materia Condensada

Instituto de Ciencia de Materiales Nicolás Cabrera

Universidad Autónoma de Madrid (UAM)

Microscopyoptical

Always fascinating to obtain real space images

C. Zeiss, Jena

Antoni van Leeuwenhoek

(1632-1723) Delft

Franz Griendel von Ach

MicroscopyOptical, SEM, SPM

SPM at cryogenic temperatures

Cryogenic operation of a scanning probe microscope eliminates Brownian motion altogether, provides for resolution in energy and enables macroscopic

quantum behavior

Cryogenic scanning probe microscopy eliminates Brownian motion altogether and

enables macroscopic quantum behavior4

D. Eigler, IBM

Atomic scale manipulation with cryogenic scanning probe microscopy

T=4.2 K

Status of cryogenic scanning probe microscopy

What ?

Tunneling spectroscopy

Tunneling microscopy

Spectroscopy at high magnetic fields

pA

Piezoelectrics

V

Ayx 20, ≈∆

STM:Binnig and Rohrer (1982)

1Å ~ 250 mV

z

F eEVNzVIΦ−≈ 025.1)(),(

∆ I = 1 nA ~ ∆ z = 1Å

Tunneling current between a metallic tip and a sample:

Overlap between tip and sample’s wavefunctions. Bardeen’s formalism

( ) ( )( ) ( )

( )

( )∫

∑

∇−∇=

+

=

−+−=

−

**2

2

2

;

;

1

1)(

12

µµψψψψ

ψ

ψ

δπ

ννµν

νν

µµ

νµµννµν

µ

dSm

M

E

E

e

Ef

EEMeVEfEfe

I

Tk

EE

B

F

h

h

STM:

The tunneling current

Ideal tip, low bias voltage :

( ) ( ) ( )FF EEEI ,2

00dd ρδψ ν

νν =−∝∑

d0

STM: Contour map of the local density of states

STM:

The tunneling current

-4 -3 -2 -1 0 1 2 3 40

1

2

3

4∆=1mV

T=0.3 K

T=1 K

Bias voltage (eV/∆∆∆∆0)

Norm

aliz

ed tun

nelli

ng c

ondu

cta

nce

dEV

eVEfEN

dV

dI

∂

−∂∝ ∫

)()(

Low temperature spectroscopy

87 µeV = 1 K

22)(

∆−=

E

EEN

ϕψψ ie=

2

,

2

,,

)(kEkk

kk

rvv

rr

βα

βααβ

ε

ψψ

∆+=

∝∆−

),;( yxEN

Tunneling spectroscopy with STM:

vacuum Giaever tunneling

-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.20.0

0.5

1.0

1.5

2.0

2.5T = 150 mK

200 G

∆ = 0.28 meV

Nor

mal

ized c

onduc

tanc

eBias voltage (mV)

The superconducting gap through local tunneling spectroscopy at very low

temperatures

Geneva, Cornell, Paris, Tokyo, …High Tc superconductors

-1 0 10.0

0.5

1.0

1.5

MgB2

norm

alize

d con

ductance

Normalized bias voltage

Two band superconductors

-4 -3 -2 -1 0 1 2 3 40.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

T = 0.15 K

Bias voltage (mV)

ErNi2B

2C

Tc=11K

Normalize

d conductance

-5 -4 -3 -2 -1 0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

TmNi2B

2C

Tc = 10.5 K

Bias voltage (mV)

T = 0.8 K

Nor

mal

ized c

onduc

tanc

e

Magnetic superconductors

-2 - 1 0 1 20 .0

0 .5

1 .0

1 .5

2 .0

Tc= 1 .8 5 K

P rO s4S b

12

Normalized con

ductanc

e

B ia s vo lta ge / ∆0 ( = 1 .7 3 k

BT

c)

Sr2RuO4

BULK superconductingdensity of states

Averaged over part of the Fermi surface

- 3 - 2 - 1 0 1 2 30 .0

0 .5

1 .0

1 .5

2 .0

2 .5

Nor

mal

ized

con

duc

tanc

e

B ia s v o lt a g e (m V )-3 -2 -1 0 1 2 3

0.0

0.5

1.0

1.5

No

rmal

ized

co

nd

uct

ance

Bias voltage (mV)

2H-NbSe2

1.7nm

2H-NbS2

NbSe2 and NbS2 : two two-gap superconductors

1/λCDW

1/a

Charge order in 2H-NbSe2 at 100 mK

2H-NbSe2

2H-NbS2

0.2

9 n

m

1.2

5 n

m

0.35 nm

Se

Nba

b

c

( )%13 ±= aCDWλ1.7nm

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

No

rmal

ized

den

sity

of

Stat

es

Energy (meV)

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.50

1

2

3

4

5

No

rmal

ized

co

nd

uct

ance

Bias voltage (mV)

2H-NbS2 Electronic density of states vs. temperature

dEV

eVEfEN

dV

dIS

∂

−∂∝ ∫

)()( ( )ENS

Two gap superconductor NbS2

0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

1.2 ∆

0=1.73 k

BT

c

∆ (

meV

)

Temperature (K)

2H-NbS2 Maxima of the gap distribution (two gap) vs. temperature

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

No

rmal

ized

den

sity

of

stat

es

Energy (meV)

Two gap superconductor NbS2

URu2Si2

Kondo systems

Strong coupling features

Andreev features

0 1 2 3 4 5 6 7 8 9 10 11 120.00

0.25

0.50

0.75

1.00

1.25

1.50

Q = 0.58

Q = 0.59

Q=0.57

Q = 0.55 - 0.56

H (

Te

sla

)

Temperatura (K)

Phase diagrams

Tunneling spectroscopy

What ?

Tunneling spectroscopy

Tunneling microscopy

Spectroscopy at high magnetic fields

pA

Piezoelectrics

V

Ayx 20, ≈∆

STM:Binnig and Rohrer (1982)

1Å ~ 250 mV

z

F eEVNzVIΦ−≈ 025.1)(),(

∆ I = 1 nA ~ ∆ z = 1Å

H

TTc

Hc1

Hc2

Hc

Meissner state

Mixed state

Normal phase

H

Meissner state

Mixed state

H

d

H(T)/50d(nm) ≈

r

s

2

nΨ ====

H

λξJ

J

Microscopy of the superconducting gap in type II materials

2

02

2πξ

φ=cH

ϕψψ ie=

βααβ ψψ,, kkrr

−∝∆ 22

)(∆−

=E

EEN

The vortex lattice through STM

Measure far below Tc (here 7 K)

0.6 nm

0 mV

0.4 µS

dI/dV

0.85 µS

21nm

0 µS

dI/dV

2.23 µS

-2 -1 0 1 20.0

0.4

0.8

1.2

1.6

2.0

2.4

Se atom

Hole

Co

nd

uct

ance

(µS

)

Bias voltage (mV)

0 mV

Atomic size tunneling spectroscopy inside vortices in 2H-NbSe2

T=100 mK

Local tunneling spectroscopy in the mixed state of superconductors

MgB2

Geneva, Paris, Argonne, Madrid

V3Si and Chevrel phasesMaryland, GenevaHigh Tc materials:

Geneva , Paris, Tokyo, Cornell , Princeton, …

Nickel borocarbidesTokyo, Argonne, Madrid

FeAsBeijing, Harvard, Tokyo …

NbSe2 and related compoundsBell labs, Paris, Madrid, Leiden

Temperature between (0.1K-2.1K)144 images

8 min each one

Several images at each T

Direct observation of thermally induced vortex depinning

Physics Today, see youtube channel physics updatehttp://www.youtube.com/watch?v=7fgNpqgZWKYhttp://www.youtube.com/user/citecnomik1

Spectroscopy at high magnetic fields

pA

How ?

Principles of design of STMs : From first large devices …

First cryogenic STM in Madrid

1988

100 mK + 10 T“Superinsulator”Vortex physicsNanostructures

10 mK + 9 TCurrent Drive STS

Heavy Fermions (Kondo lattice)

100 mK, 3D VECTOR MAGNETLayered materials

300 mK, 13 T -> 17 T Superconducting tips

100 mK

Facilities in Madrid

1. Spectroscopy

2. Macroscopic movement over the sample

3. Tip and sample preparation methods

STM/S in a dilution refrigerator• Compact design to guarantee mechanical stability (piezoelectric drive)• Home made electronics, cryogenics and mechanics

2 µm

V

nmyx 20, ≈∆

Principle of operation of a scanning tunneling microscope

Scanning window in the µm range with sub atomic

resolution

Fine positioning piezoelectrics +

XY

xyz

Z

Coarse motor

Position without

heating with nm resolution

Stiffness

fmicroscope

fvibration isolation

effm

kf

π2

10 =

Designing a cryogenic scanning tunneling microscope

Soft and heavy

Stiff and light

Tip-sample motion shall be in phase

Mezclas de 3He y 4HeSTM holder for dilution refrigeration

� STM head is posed on fiber glass loom� Specific precision motion system using a series of ropes with a room temperature actuator without measurable heating at 100 mK

Mezclas de 3He y 4HeSTM electronics at LBTUAM

IndustrialPC

DACADC

Powersupply

+/- 15 V

+/- 140 V

OP276xPA243

Bias voltageRF filters

I-V converter

Tunnel current

RF

filters

Motor Z’Switchable

filter’

RF

filters

6 PIEZOSCANNING

DRIVES

… Power supply

+/- 15 V

Power supplyRF

filters

PA243

Resolution in spectroscopy of <15 µµµµeV (<150 mK)

RF filters

XY

xyz

Z

Spectroscopy at high magnetic fields

pA

X

Z

Compact system

Madrid: 17 Tsuperconducting magnet

Compact system

just for tunneling

maximally simplified

Microscope for high field superconducting solenoid

Web page of LBTUAMwww.uam.es/lbtuam

Workshop on high magnetic field science and technologyMiraflores de la Sierra, 6-9 November 2012

www.uam.es/inc

Organizers : H. Suderow and I. Guillamón

A. Maldonado, J.A. Galvis Echeverry, P. Kulkarni,A. Buendía,

I. Guillamón, J.G. Rodrigo, S. VieiraLaboratorio de Bajas Temperaturas

Dpto. Física de la Materia CondensadaInstituto de Ciencia de Materiales Nicolás Cabrera

Universidad Autónoma de Madrid (UAM)

J. Sesé, R. Córdoba, A. Fernández Pacheco, J.M. de Teresa, R. IbarraICMA, Unizar, Instituto de Nanociencia de Aragón

F. GuineaInstituto de Ciencia de Materiales de Madrid,

Consejo Superior de Investigaciones Científicas, Madrid

S. Bud’ko, P.C. CanfieldAmes Laboratory, Ames – USA

S. BannerjeeIIT Kanpur – India

T. Baturina, Novosibirsk, Russia, V. Vinokur, Argonne, USA

L. Cario, Nantes; P. Rodiere, P. Lejay, J.P. Brison, Dai Aoki, J. FlouquetInstitut Néel and SPSMS/DRFMC

E. Navarro Moratalla, C. Martí Gastaldo, E. Coronado, ICMol Valencia

V. Tissen, Chernogolovka, Russia

Scanning probe microscopy

The SEGAINVEX team (headed by M. Pazos)

www.lbtuam.es

Increasing the field in stepsNo time variation of flux distribution

Pinning produces spatial variation of BCritical state

FL=-FP

Vortex bundles with weak pinning at 100 mK

33 STS images0.04 T steps

0 1 2 3 40

1

2

3

4

5

6

Cam

po

Crí

tico

Hc2

(T

)

Temperatura (K)