A Critical Look at Criticality
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A Critical Look at Criticality. The influence of macroscopic inhomogeneities on the critical behavior of quantum Hall transitions. Dennis de Lang. AIO Colloquium, June 18, 2003 Van der Waals-Zeeman Institute. Co-workers/Supervision :. Prof. Aad Pruisken ITF, UvA. Leonid Ponomarenko

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A critical look at criticality

A Critical Look at Criticality

The influence of macroscopic inhomogeneities on the

critical behavior of quantum Hall transitions

Dennis de Lang

AIO Colloquium, June 18, 2003

Van der Waals-Zeeman Institute


A critical look at criticality

Co-workers/Supervision:

Prof. Aad Pruisken

ITF, UvA

Leonid Ponomarenko

Dr. Anne de Visser

WZI, UvA


A critical look at criticality

Outline:

Quantum Hall Effect:

essentials

quantum phase transitions (critical behavior)

motivation

Experiments and remaining puzzles

PI vs. PP transitions

Modelling macroscopic inhomogeneities

Conclusions and Outlook


A critical look at criticality

Quantum Hall Effect: Basic Ingredients

2D Electron Gas (disorder!)

Low Temperatures (0.1-10 K)

High Magnetic Fields (20-30 T)


A critical look at criticality

The making of a 2DEG

MBE/MOCVD/CBE/LPE:

InGaAs

Spacer (InP)

Si-doped InP

Substrate (InP)

EF(Fermi Energy)


A critical look at criticality

The making of a 2DEG - II

InGaAs

Spacer (InP)

Si-doped InP

Substrate (InP)

EF(Fermi Energy)


A critical look at criticality

The making of a 2DEG - III

4-point resistance measurement:

I

I

Vxy

Vxx

Hall bar geometry: Etching & Contacts


A critical look at criticality

The Hall Effect: Classical

Magnetotransport:

(Ohm’s law)

Drude (classical):


A critical look at criticality

The Hall Effect: Quantum (Integer)

Magnetotransport:

i =1

rxy=h/ie2

i =2

i =4


A critical look at criticality

2D Density of States (DOS)

B=0:

2D DOS is constant

B>0:

DOS becomes series of d-functions:

Landau Levels

energy separation:


A critical look at criticality

2D states (B=0,T=0) are localized, but

extended states in center of

Landau Levels

2D Density of States (DOS)

B=0:

2D DOS is constant

broadening due to disorder

B>0:

DOS becomes series of d-functions:

Landau Levels

energy separation:


A critical look at criticality

Localized to extended states transition

Scaling theory : (Pruisken, 1984)

Localization length: x~| B-Bc| -c

Phase coherence length: Lf ~ T-p/2

(effective sample size)

rij ~ gij(T -k(B-Bc)) ; k = p/2cp relates L (sample size) and Tc relates localization length x and B


A critical look at criticality

Integer quantum Hall effect


A critical look at criticality

Plateaus: “Quantum Hall states”: bulk is localized. Current travels on the edges (edge states)

Transitions: “Extended states”current travels through the bulk

Integer quantum Hall effect

T 0 behavior?

Universality?


A critical look at criticality

Motivation…

T 0 behavior?

QHE transitions are second order (quantum) phase transitions…

… there should be an associated critical exponent

Universality?

… since all LLs are in principle identical, the critical exponent of each transition should be in the same universality class.

How does macro-disorder

result in chaos?


A critical look at criticality

Outline:

Quantum Hall Effect:

essentials

quantum phase transitions (critical behavior)

motivation

Experiments and remaining puzzles

PI vs. PP transitions

Modelling macroscopic inhomogeneities

Conclusions and Outlook


A critical look at criticality

Measuring T –dependence in PP transitions


A critical look at criticality

Historical ‘benchmark’ experiments on PP

  • InGaAs/InP

    • H.P.Wei et al. (PRL,1988): PP=0.42 (left)

  • AlGaAs/GaAs

    • S.Koch et al. (PRB, 1991):

    •  ranges from 0.36 to 0.81

    • H.P.Wei et al. (PRB, 1992): ’scaling’ (PP=0.42 ) only below 0.2 K

n=1.5

n=2.5

n=2.5

n=1.5

n=3.5

(Wei et al., 1988)


A critical look at criticality

Our own ‘benchmark’ experiment on PI

de Lang et al., Physica E 12 (2002); to be submitted to PRB


A critical look at criticality

Our own ‘benchmark’ experiment on PI

Hall resistance is quantized (T 0)

k=0.57 (non-Fermi Liquid value !!)

Inhomogeneities can be recognized, explained and disentangled

[h/e2]

[h/e2]

Contact misalignment

Macroscopic carrier density variations

Pruisken et al., cond-mat/0109043


A critical look at criticality

Our own ‘benchmark’ experiment on PP

Something is not quite right…

K=0.48

K=0.35


A critical look at criticality

Leonid’s density gradient explanation…

L. Ponomarenko, AIO colloq. December 4, 2002

Ponomarenko et al., cond-mat/0306063, submitted to PRB


A critical look at criticality

Leonid’s density gradient explanation…

L. Ponomarenko, AIO colloq. December 4, 2002


A critical look at criticality

Leonid’s density gradient explanation…

L. Ponomarenko, AIO colloq. December 4, 2002


A critical look at criticality

Outline:

Quantum Hall Effect:

essentials

quantum phase transitions (critical behavior)

motivation

Experiments and remaining puzzles

PI vs. PP transitions

Modelling macroscopic inhomogeneities

Conclusions and Outlook


A critical look at criticality

Modelling preliminaries:

Transport results can be explained by means of density gradients.

n2Dn2D(x,y)

Resistivity components:

rij rij (x,y)

Electrostatic boundary value problem


A critical look at criticality

Scheme – I

Calculate the ‘homogeneous’ r0, rH through Landau Level addition/substraction

r0PI = exp(-X) ; rHPI =1 X=Dn/n0(T)

r0P

sPI= (rPI)-1 e.g.s0PI =

(r0PI)2+(rHPI)2

s0PP(k) = s0PI(k)sHPP(k) = sHPI(k) + k

rPP(k)= (sPP(k))-1

k=0

k=1

k=2


A critical look at criticality

Scheme – II

Expansion of ji, r0 , rH to 2nd order in x,y…

r0(x,y)= r0(1+axx+ayy+axxx2+ayyy2+axyxy)

rH(x,y)= rH(1+bxx+byy+bxxx2+byyy2+bxyxy)

jx(x,y)= jx (1+axx+ayy+axxx2+ayyy2+axyxy)

jy(x,y)= jy (1+bxx+byy+bxxx2+byyy2+bxyxy)

22 parameters…


A critical look at criticality

Scheme – III

Appropriate boundary conditions & limitations:

W/2

?

- W/2

- L/2

L/2

jy(y=W/2) = 0 (b.c.)

j = 0 (conservation of current)

E = 0 (electrostatic condition)


Result only in terms of a ij b ij r xx r xx r 0 r h a ij b ij r xy r xy r 0 r h a ij b ij

Result ONLY in terms of aij, bij :rxx= rxx(r0, rH, aij, bij ) rxy =rxy (r0, rH, aij, bij )

Scheme – IV

  • jx, jy using b.c.

  • Ei =  rijjj

  • Vx,y= dx,y Ex,y

  • Ix=dy jx

  • R =V / I

… use Taylor expansion in x,y to obtain aij, bij as function of nx andny :n(x,y) =n0 (1+nx/n0 x + ny/n0 y)


A critical look at criticality

Results: 1.5 % gradient along x


A critical look at criticality

Results: 1.5 % gradient along x


A critical look at criticality

Results: 1.5 % gradient along x


A critical look at criticality

Results: 3.0 % gradient along y


A critical look at criticality

Results: 3.0 % gradient along y


A critical look at criticality

Results: 3.0 % gradient along y


A critical look at criticality

Results: ‘realistic’ gradient along x,y

nx< ny < 5%


A critical look at criticality

Conclusions …

Realistic QH samples show different critical exponents for different transitions within the same sample.

Inhomogeneity effects on the critical exponent can only be disentangled at the PI transition.

Density gradients of a few percent (<5%) can vary the value of the critical exponents of PP transitions by about 10-15%.

Experimentally obtained values of the maximum of sxx often show a noticable T-dependence. This can be explained by a carrier density ‘gradient’ along the width of the Hall bar. It is also an indication that the obtained critical exponent is underestimated.

Reported ‘universal’ values of PP transition exponents should be viewed with great care and scrutiny.


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