slide1
Download
Skip this Video
Download Presentation
A Critical Look at Criticality

Loading in 2 Seconds...

play fullscreen
1 / 38

A Critical Look at Criticality - PowerPoint PPT Presentation


  • 87 Views
  • Uploaded on

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

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' A Critical Look at Criticality' - maeko


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

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

slide2

Co-workers/Supervision:

Prof. Aad Pruisken

ITF, UvA

Leonid Ponomarenko

Dr. Anne de Visser

WZI, UvA

slide3

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

slide4

Quantum Hall Effect: Basic Ingredients

2D Electron Gas (disorder!)

Low Temperatures (0.1-10 K)

High Magnetic Fields (20-30 T)

slide5

The making of a 2DEG

MBE/MOCVD/CBE/LPE:

InGaAs

Spacer (InP)

Si-doped InP

Substrate (InP)

EF(Fermi Energy)

slide6

The making of a 2DEG - II

InGaAs

Spacer (InP)

Si-doped InP

Substrate (InP)

EF(Fermi Energy)

slide7

The making of a 2DEG - III

4-point resistance measurement:

I

I

Vxy

Vxx

Hall bar geometry: Etching & Contacts

slide8

The Hall Effect: Classical

Magnetotransport:

(Ohm’s law)

Drude (classical):

slide9

The Hall Effect: Quantum (Integer)

Magnetotransport:

i =1

rxy=h/ie2

i =2

i =4

slide10

2D Density of States (DOS)

B=0:

2D DOS is constant

B>0:

DOS becomes series of d-functions:

Landau Levels

energy separation:

slide11

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:

slide12

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

slide14

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?

slide15

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?

slide16

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

slide18

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)

slide19

Our own ‘benchmark’ experiment on PI

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

slide20

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

slide21

Our own ‘benchmark’ experiment on PP

Something is not quite right…

K=0.48

K=0.35

slide22

Leonid’s density gradient explanation…

L. Ponomarenko, AIO colloq. December 4, 2002

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

slide23

Leonid’s density gradient explanation…

L. Ponomarenko, AIO colloq. December 4, 2002

slide24

Leonid’s density gradient explanation…

L. Ponomarenko, AIO colloq. December 4, 2002

slide25

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

slide26

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

slide27

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

slide28

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…

slide29

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)

slide38

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.

ad