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Pumping

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Pumping

Uniform conductor: no bias, no current

length along wire

some charge shifted to left and right

length along wire

length along wire

length along wire

back to phase 1

Two parameter pumping in 1d wire

Example taken from P.W.Brouwer Phys. Rev.B 1998

Berry phase associated to two-parameter pumping

Bouwer formulation for Two parameter pumping assuming linear response to parameters X1, X2

Circuit in parameter space

There is a clear connection with the Berry phase (see e.g. Di Xiao,Ming-Che-Chang, Qian Niu cond-mat 12 Jul 2009).

A circuit is not enough: one needs singularities inside. The magnetic charge that produces the Berry magnetic field is made of quantized Dirac monopoles arising from degeneracy. The pumping is quantized (charge per cycle= integer).

But this is not the only kind of pumping discovered so far.

time-periodic gate voltage

Mono-parametric quantum charge pumping ( Luis E.F. Foa Torres PRB 2005)

quantum charge pumping in an open ring with a dot embedded in one of its arms.

The cyclic driving of the dot levels by a single parameter leads to a pumped current when a static magnetic flux is simultaneously applied to the ring.

The direction of the pumped current can be reversed by changing the applied magnetic field (imagine going to the other side of blackboard).

The response to the time-periodic gate voltage is nonlinear.

The pumping is not adiabatic.No pumping at zero frequency.

The pumping is not quantized.

See also: Cini-Perfetto-Stefanucci,PHYSICAL REVIEW B 81, 165202 (2010)

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Another view of same quantum effect described above

It must be possible to make all in reverse!

Interaction with Bcurrent vortex->magnetic moment of ring current in wires Bias

Bias U current in wires vortex magnetic moment of ring

Interaction with magnetic field proportional to U^3

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This is Magnetic pumping

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Model: laterally connected ring, same phase drop on all red bonds

Different distribustions of the phase drop among the bonds are equivalent in the static case, but not here. This choice is simplest.

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this is emf first clockwise then counterclockwise

the ring remains excited

the ring remains charged

similar charge is sent to right wire

charge is sent to left wire

Half flux in and then out.

Charging of ring with no net pumping

We may avoid leaving the ring excited by letting it swallow integer fluxons

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Pumping by an hexagonal ring – insertion of 6 fluxons (Bchirality)

ring returns to ground state

emf always same way

pumping is achieved

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Pumping by an hexagonal ring – insertion of 6 fluxons (Bchirality)

effect of 6 fluxons in 200 time units

Rebound due to finite leads

effect of 6 fluxons in 300 time units

effect of 6 fluxons in 100 time units

If the switching time grows the charge decreases. It is not adiabatic and not quantized!

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What happened? We got 1-parameter pumping (only flux varies)

Charge not quantized- no adiabatic result

Linearity assumption fails and one may have nonadiabatic 1 parameter pumping

We got a strikingly simple and general case where linearity assumption that holds in the classical case fails due to quantum effects. In the present time-dependent problem the roles of cause and effect are interchanged.

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Memory storage

Insertion of 3.5 flux quanta into a ring with 17 sides connected to a junction (left wire atoms have energy level 2 in units of the hopping integral th, right wire atoms have energy level 0). The figure shows the phase pulse and the geometry. Time is in units of the inverse of the hopping integral.

Right: expectation value of the ring Hamiltonian. The ring remains excited long after the pulse. It remembers.

Charge on the ring . The ring remains charged after the pulse. It remembers.

Fine! But memory devices must be erasable.

How can we erase the memory?

Same calculation as before performed in the 17-sided ring, but now with the A–B bond cut between times t = 30and t = 70.

The ring energy and occupation tend to return to the

values they had at the beginning, and the memory of the flux is

thereby erased.

Graphene

Unit cell

a

Lower resistivity than silver-Ideal for spintronics (no nuclear moment, little spin-orbit) and breaking strength = 200 times greater than steel.

a=1.42 Angstrom

Corriere della sera 15 febbraio 2012

the lattice is bipartite

a

b

= basis

Primitive vectors

Reciprocal lattice vectors

To obtain the BZ draw the smallest G vectors and the straightlinesthrough the centres of all the G vectors: the interior of the hexagonis the BZ.

K’

K

K

M

G

K’

K’

K

BZ and important points.

K

K

M

G

K’

K’

K

BZ and important points.

M

K

K

M

M

G

K’

K’

K

BZ and important points.

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Tight-binding model for the p bands: denoting by a and b the two kinds of sites the main hoppings are:

b

a

Jean Baptiste Joseph Fourier

Why 2 component? Itis the amplitude of being in sublattice a or b.

(upper band)

- Band Structure of graphene

Note the cones at K and K’ points

no gap

Expansion of band structure around K and K’ points

Expansion of band structure around K and K’ points

But the 2 components are for the 2 sublattices

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