STM and STS investigations of the topological insulator...

Fabian Craes University of Cologne Capri Spring School 2011

STM and STS investigations of the topological insulator Bi2Te3

F. Craes, E. Sela, T. Michely, A. Rosch, C. Busse


STM and STS

•  Metallic tip scanning the surface in 1nm distance. 5 main parameters determine scanning tunneling microscopy (STM): I, U, x, y, z

•  STM mode constant current: fix I and U, vary z(x,y) à topography (constant density of states)

•  Scanning tunneling spectroscopy (STS): measure variable density of states, e.g. I(V)


STS


STS

•  measure dI/dU directly by Lock-In detection: best signal-to-noise ratio

•  Ubias + AC modulation f •  detect current harmonic with modulation frequency f •  amplitude directly proportional to dI/dU, thus density

of states à real space picture of LDOS


Clean surfaces

stick method

view into the UHV chamber

Generating a clean surface by in-situ crystal cleaving in UHV


Topological insulator Bi2Te3

(2.5x2.5)nm², U=-0.5V

(20x20)nm², I=0.7nA, U= -0.6V


Bi2Te3 – scattering of topological surface states

real space reciprocal space


•  How to measure LDOS with an STM? •  solution: reflection of electronic statesà formation of standing waves à modulation of density of states:

Measure standing wave patterns of LDOS at point defects and steps (energy resolved Friedel oscillations)

•  method: measure dI/dU by Lock-in detection

Surface states and STM


•  How to measure LDOS with an STM? •  solution: reflection of electronic statesà formation of standing waves à modulation of density of states:

Measure standing wave patterns of LDOS at point defects and steps (energy resolved Friedel oscillations)

•  method: measure dI/dU by Lock-in detection

Surface states and STM

kSW kin

kout


Bi2Te3 – scattering of topological surface states

• direct backscattering (q1) forbidden by TRI • scattering solely enabled by nesting vectors

Zhang, T. et al., PRL 103, 2009

Fu et al., PRL 103, 2009


Bi2Te3 scattering at a step

-20mV

50mV 80mV

100mV

124mV

141mV 181mV

dI/dU map of standing waves on Bi2Te3(111)

U x=26nm, I=0.1nA,T=7.5K

surface step


Bi2Te3 scattering at a step

-20mV

50mV 80mV

100mV

124mV

141mV 181mV

U x=26nm, I=0.1nA,T=7.5K

surface step


Bi2Te3 scattering at a step determine E(k) by fitting y-averaged linescans of dI/dU for each energy with sinc-function

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surface step E=100meV


Bi2Te3 scattering at a step determine E(k) by fitting y-averaged linescans of dI/dU for each energy with sinc-function

ED

Linear dispersion verified!


Bi2Te3 scattering at magnetic defects (40x40)nm², U=0.7V, I=0.1nA, T=7.5K

•  idea: breaking TRI à use magnetic defects

•  evaporate 0.02ML Eu on Bi2Te3(111) surface at T=20K

•  Eu: huge magnetic moment


(10x10)nm², U=33mV, I=0.24nA

Bi2Te3 - scattering at magnetic defects

K M

FT


Bi2Te3 - scattering at magnetic defects (40x29)nm², U=700mV, I=0.1nA, T=7.5K

dI/dU map

(40x40)1/nm², U=700mV, I= 0.1nA

FFT of dI/dU map

K M

FT


Bi2Te3 - scattering at magnetic defects (40x29)nm², U=700mV, I=0.1nA, T=7.5K

dI/dU map

(40x40)1/nm², U=700mV, I= 0.1nA

FFT of dI/dU map

à direct backscattering?

K M


Bi2Te3 - breaking TRI? calculated intensity from magnetic point scattering

intensity from magnetic scattering vanishes, only potential scattering left

see also Guo and Franz, PRB 81, 2010


summary

•  scattering at step: linear dispersion relation of Dirac fermions could be observed on samples

•  scattering at magnetic point defects does not show direct backscattering (consistent with our theory: presence of time-reversed process)


outlook •  measure scattering at strong local magnetic moments

(magnetic clusters) in order to suppress temporal fluctuations

•  use magnetic tip in order to suppress time reversed processes

•  Consistency with measurements of Okada et al.: new scattering channels on Bi2-xFexTe3+d? Scattering into other states?

Okada et al., arXiv:1011.4913, 2010


S1: theory


S2: STS

•  STS difficulties: –  tip not featureless – really a free electron gas as

assumed in many calculations? –  tunneling transmission probability: impossible to

measure states that do not overlap with states of tip

–  lacks chemical sensitivity


S3: LTSTM system AG Michely

•  UHV 10-13mbar – 1bar àsample clean for several

weeks •  temperature range 6K – 300K •  2 chambers: STM + preparation •  sample preparation at 20K – 1500K


S4: Bi2Te3 - scattering of topological surface states literature

Alpichshev et al. ARPES Zhang et al. STS Alpichshev et al. STS AG Michely STS

ED


S5: Clean surfaces

•  STM essentially needs clean surfaces •  Possible ways to obtain:

–  Ion bombardment and high temperuture treatment (mainly one component systems)

–  in-situ growth (e.g. molecular beam epitaxy (MBE), not too many components)

–  in-situ cleaving: method of choice for complex materials, especially layered ones

STM and STS investigations of the topological insulator...

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