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Charging and noise as probes of non-abelian quantum Hall states Ady Stern (Weizmann)

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Charging and noise as probes of non-abelian quantum Hall states

Ady Stern (Weizmann)

with: D.E. Feldman, Eytan Grosfeld, Y. Gefen,

B.I. Halperin, Roni Ilan, A. Kitaev, K.T. Law, K. Schoutens

- Non-abelian quantum Hall states – what they are and where they are on the map of the fractional quantum Hall effect
- Bulk and edge in non-abelian quantum Hall states
- Experimental consequences of non-abelian quantum Hall states
- Interferometers
- Coulomb blockade

More precise, and relaxed, presentations of the subject:

1. Anyons in the QHE – a pedagogical introduction

AS, Annals of physics, 2008

2. Review paper “Non Abelian Anyons and Topological Quantum Computation”

by Nayak, Simon, Stern, Freedman and Das Sarma

on the arxiv/soon on RMP

3. Next week, in a talk I will give here at the GGI

I

B

+++++++++

- Defining properties:
- A quantum Hall state – vanishing longitudinal resistivity, quantized Hall resistivity. Gapped bulk.
- Current flows with no dissipation, along the gapless edge.
- 2. In the presence of localized quasi-particles, the ground state is degenerate, and the degeneracy is exponential in the number of quasi-particles
- 3. Local perturbations (phonons, photons, etc.) do not couple ground states. The (almost) only way to shift the system from one ground state to another is by having quasi-particles braid one another.

1

2

2

3

1

3

position of

quasi-particles

…..

Permutations between quasi-particles positions

unitary transformations in the ground state subspace

determined by the topology of the trajectories

Non-abelian quantum Hall states – location on the map

- 1. The fractional quantum Hall effect is a state of
- Dissipationless flow of current
- Quantized Hall resistivity
- 2. Understanding it by mapping onto another system where current flows with no dissipation.
- Two possibilities:
- The integer quantum Hall effect – composite fermion theory
- Bose Einstein condensate (Bosons at zero magnetic field, or nearly zero) – Moore-Read-Rezayi non-abelian states
- Both are based on flux attachment

H y(z1..zN)= E y(z1..zN)

Interacting electrons at a partially filled Landau level, with 1/n flux quanta per electron.

Define a new wave function

y(z1..zN) =Pi<j (zi-zj) a F (z1..zN)

The new wave function describes interacting particles subjectedto flux quanta per electron

The statistics of the new particles is determined by the value ofa.

Even a: fermionic Odd a: bosonic

Fractional a: anyonic

Mapping the fractional onto the integer - Even a :

Reducing the magnetic field and increasing the filling factor from a fraction of a Landau level to an integer number of Landau levels, keeping the statistics fermionic

ne=p/(ap+1)

ncf= p

ne=p/(ap+1)

p filled Landau levels

Abelian excitations

Non-abelian quantum Hall states:

choosing such that the composite particles feel no magnetic field, with the goal of Bose condensing these particles.

Good news: the composite particles are at zero field

Bad news: they are not necessarily bosons.

Examples:

n=1/3 – attaching three flux quanta to each electron turns it into a boson and cancels the magnetic field – good

But:

n=1/2 – attaching two flux quanta to each electron cancels the magnetic field, but turns the electron into a fermion.

Moreover:

n=2/3 – attaching 1.5 flux quanta to each electron cancels the magnetic field, but turns the electron into an anyon.

How does one Bose-condense particles which are not bosons?

Answer (Bardeen, Cooper, Schrieffer):

pairs of fermions may condense like bosons

similarly, clusters of k-anyons with statistics of p/k may condense like bosons

Examples:

n=1/2 (the Moore-Read state of the n=5/2 state) – Bose condensate of pairs of composite fermions

n=2/5 (the “Fibonacci anyon” state) a condensate of clusters of three anyons

a bosonic phase is accumulated upon encircling

Other ways of looking at these states exist (Cappelli, Georgiev, Todorov)

Properties of such condensates

- Bulk excitations:
- A Bose condensate has topological excitations – vortices.
- If the boson is a cluster of k particles, the vortex carries a flux of 1/k, and a charge of e/(k+2).
- On a compact geometry,
- # of vortices minus # of anti-vortices
- must be a multiple of k
- 4. Clusters may disintegrate and populate inner core states.
- If these inner core states are modes of zero energy
- The ground state becomes degenerate in the presence of (anti)vortices, and a non-abelian quantum Hall state is formed.

Properties of such condensates

- Bulk excitations:
- A Bose condensate has topological excitations – vortices.
- If the boson is a cluster of k particles, the vortex carries a flux of 1/k, and a charge of e/(k+2).
- 3. Clusters may disintegrate and populate inner core states.
- If these inner core states are modes of zero energy
- The ground state becomes degenerate in the presence of (anti)vortices, and a non-abelian quantum Hall state is formed. Also, a vortex and an anti-vortex have two ways to annihilate one another.

- The mere existence of the quantum Hall effect forces the edge to have a charged chiral gapless mode – a Luttinger liquid (Wen)
- In non-abelian states, the edge has another gapless mode, which is neutral.
- Both the quasi-particle operator and the electron operator affect the state of the two edge modes – the charged and the neutral.

term

Number of

q.p.’s in the interference loop,

even

odd

even

Brattelli diagram (for k=2)

Interferometers:

The interference term depends on the number and quantum state of the bulk quasi-particles.

S

M-Z

D2

F-P

D2

D1

S1

Interferometers:

Main difference: the interior edge is/is not part of interference loop

For the M-Z geometry every tunnelling quasi-particle advances the system along the Brattelli diagram

(Feldman, Gefen, Law PRB2006)

G2

G3/2

G1/2

G1

G4/2

G2/2

G3

Interference

term

Number of

q.p.’s in the interference loop

- The system propagates along the diagram, with transition rates assigned to each bond.
- The rates have an interference term that
- depends on the flux
- depends on the bond (with periodicity of 4)

1-p

a well-designed coin

p

I2

- The probability p, always <<1, varies according to the outcome of the tossing. It depends on flux and on the number of quasi-particles that have already tunneled.
- Consider two extremes (two different values of the flux):
- If all rates are equal, there is just one value of p, and the usual binomial story applies – Fano factor of 1/4.
- But:

The other extreme – some of the bonds are “broken”

Charge flows in “bursts” of many quasi-particles. The maximum expectation value is around 12 quasi-particles per burst – Fano factor of about three.

Effective charge span the range from 1/4 to about three. The dependence of the effective charge on flux is a consequence of unconventional statistics.

Charge larger than one is due to the Brattelli diagram having more than one “floor”, which is due to the non-abelian statistics

In summary, flux dependence of the effective charge in a Mach-Zehnder interferometer may demonstrate non-abelian statistics at n=5/2

Two pinched-off point contacts define a quantum dot

Coulomb blockade !

current

n=5/2

area S, B

A Coulomb blockade peak appears in the conductance through the dot whenever the energy cost for adding an electron is zero:

For a fixed magnetic field B, what is the area separation between consecutive peaks?

Chiral Luttinger liquid mode –

charging energy

leads invariably to an equal area separation between consecutive peaks

The energy of the parafermion edge mode needs to be added.

The second (“parafermionic”) edge mode

- accommodates the un-clustered Nmod(k) electrons (for an energy cost) energy is periodic with k
- the energy cost is determined in a Bohr-Sommerfeld manner, but in a way that depends both on Nmod(k) and on the number of quasi-particles localized in the bulk. This number varies with the magnetic field.

When the energy of the parafermionic mode is added, the peaks move and bunch. The bunching depends on the number of localized quasi-particles.

E

1

3/4

N

N+1

Bunching of the Coulomb peaks to groups of n and k-n –

A signature of the Zk states

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