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Kicked rotor in Wigner phase space - The University of Texas at Austin

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Fortschr. Phys. 51, No. 4–5 (2003) 485<br />

C Resonance and anti-resonance<br />

For the<strong>phase</strong><strong>space</strong>analysis <strong>of</strong> theresonanceweneed to evalu<strong>at</strong>ethesum<br />

≡<br />

∞<br />

Sl−r(κ; x)Sr(κ ′ ; x). (30)<br />

W (+)<br />

l<br />

r=−∞<br />

<strong>The</strong>anti-resonance<strong>in</strong>volves thesum<br />

≡<br />

∞<br />

Sl−r(κ; x)Sr(κ; x + rπ). (31)<br />

W (−)<br />

l<br />

r=−∞<br />

Westart our discussion with thesum W (−)<br />

l and first show th<strong>at</strong> it is closely rel<strong>at</strong>ed to the sum W (+)<br />

l .For<br />

this purposewe<strong>in</strong>troducethenew <strong>in</strong>tegr<strong>at</strong>ion variable¯y ≡−y <strong>in</strong> the expansion coefficient<br />

which yields<br />

Sr(κ; x) ≡ 1<br />

π<br />

2π<br />

−π<br />

Sr(κ; x + rπ) = 1<br />

π<br />

2π<br />

dy e iry −iκ[V (x+y)−V (x−y)]<br />

e<br />

−π<br />

d¯y e i(−r)¯y e −iκ[V (x+rπ−¯y)−V (x+rπ+¯y)] .<br />

Due to the periodicity properties V (x +2mπ) =V (x) and V (x +(2m +1)π) =−V (x) <strong>of</strong> thepotential<br />

wef<strong>in</strong>d thesymmetry rel<strong>at</strong>ion<br />

Sr(κ; x + rπ) =S−r(κ; x)<br />

which br<strong>in</strong>gs thesum W (−)<br />

l<br />

W (−)<br />

l<br />

=<br />

∞<br />

r=−∞<br />

<strong>in</strong>to theform<br />

Sl−r(κ; x)S−r(κ; x).<br />

It is therefore convenient to evalu<strong>at</strong>e the sums<br />

≡<br />

∞<br />

Sl−r(κ; x)S±r(κ ′ ; x).<br />

C (±)<br />

l<br />

r=−∞<br />

For this purposewesubstitutethedef<strong>in</strong>ition <strong>of</strong> Sr, Eq. (32), <strong>in</strong>to thesum and <strong>in</strong>terchangethesumm<strong>at</strong>ion<br />

and <strong>in</strong>tegr<strong>at</strong>ion which leads to<br />

C (±)<br />

l<br />

1<br />

=<br />

4π2 π<br />

−π<br />

dy<br />

π<br />

−π<br />

dy ′<br />

<br />

r<br />

e −ir(y∓y′ )<br />

<br />

e ily e −i[κV(x,y)+κ′ V(x,y ′ )] .<br />

<strong>The</strong> sum <strong>in</strong> the curly brackets represents a comb <strong>of</strong> delta functions, Eq. (29), which allows us to perform<br />

the<strong>in</strong>tegral over y ′ , th<strong>at</strong> is<br />

C (±)<br />

l<br />

1<br />

=<br />

2π<br />

π<br />

−π<br />

(32)<br />

dy e ily e −i(κ±κ′ )V(x,y) = Sl(κ ± κ ′ ; x). (33)

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