Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
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22 R. Shankar<br />
broken<br />
critical fan<br />
u∗<br />
symmetric<br />
Fig. 6. The generic phase diagram for a second-order quantum phase transition.<br />
The horizontal axis represents the coupling constant which can be tun<strong>ed</strong> <strong>to</strong> take one<br />
across the transition. The vertical axis is usually the temperature in bulk quantum<br />
systems, but is 1/g here, since in our system one of the roles play<strong>ed</strong> by g is that of<br />
the inverse temperature<br />
without actually traversing it. As we move further <strong>to</strong> the left, we reach the<br />
strongly-coupl<strong>ed</strong> symmetry-broken phase, with a non-zero order parameter.<br />
It can be shown that in our problem, 1/g plays the role of T . One way <strong>to</strong> see<br />
is this that in any large N theory N stands in front of the action when written<br />
in terms of the collective variable. That is t<strong>ru</strong>e in this case well for g. (Hereg<br />
also enters at a subdominant level inside the action, which makes it hard <strong>to</strong><br />
pr<strong>ed</strong>ict the exact shape of the critical fan. The bot<strong>to</strong>m line is that we can see<br />
the critical point at finite 1/g. In addition one can also raise temperature or<br />
bias voltage <strong>to</strong> see the critical fan.<br />
Subsequent work has shown, in more familiar examples that Landau interactions,<br />
that the general picture depict<strong>ed</strong> here is t<strong>ru</strong>e in the large g limit:<br />
upon adding sufficiently strong interactions the Universal Hamil<strong>to</strong>nian gives<br />
way <strong>to</strong> other descriptions with broken symmetry [33, 34]. 3<br />
Acknowl<strong>ed</strong>gement<br />
It is a pleasure <strong>to</strong> thank the organizers of this school especially Professors<br />
Dieter <strong>Heiss</strong>, Nithaya Chetty and Hendrik Geyer for their stupendous hospitality<br />
that made all of decide <strong>to</strong> revisit South Africa as soon as possible.<br />
3 The only result that is not exact in the large g limit is the critical value u ∗ m<br />
since the input value of um at ET is relat<strong>ed</strong> in a non-universal way <strong>to</strong> the Landau<br />
parameter. In other words, the Landau coupling um settles down <strong>to</strong> its fix<strong>ed</strong> point<br />
value at an energy scale that is much larger than ET .W<strong>ed</strong>onotknowhowit<br />
flows as we reach energies inside ET wherein our RMT assumptions are valid.<br />
1/g<br />
u