Chemistry Reference
In-Depth Information
superposition, entanglement, identical particles with fermi or Bose charac-
ters, anyons in 2 dimensions, topological order and so on make even simple
looking quantum systems much more
unpredictable
and complex than one
can comprehend.
Let us discuss a well known case first - the case of cuprate superconduc-
tors. Soon after the discovery of superconductivity in La
2
−x
Ba
x
CuO
4
by
Bednorz and Muller new family members of the cuprate family were synthe-
sised and superconducting Tc went up to 163 K (under pressure) in one of
the family members. This was a surprise. An old idea of resonating valence
bond (RVB) state was revived and found to be relevant for cuprates. RVB
theory exposed Mott insulators as seat of high Tc superconductivity. A new
electronic mechanism appeared. The superconducting order parameter was
found to be unconventional, a d-wave order with nodal quasi particles, with
rich consequences. The idea of quantum spin liquid got sharpened and no-
tions like spinon, holon, pseudo fermi surface, dimer models, topological
order, topological term in non linear sigma models, dynamically gener-
ated RVB gauge fields, Chern-Simons theory etc., were born. Whatever
happened during 1986-88 were were exciting and unexpected developments
with rich consequences. In cuprates quantum mechanics was at its best in
producing a variety of novel phenomena in a Mott insulator template.
3
He exemplifies quantum complexity well. Volovik's topic,
The Universe
in a Helium Droplet
[
6
]
is an elaboration of quantum complexity. A simple
monoatomic
3
He system, because of its fermionic property and light mass
has all the rich consequences that we have discovered in the normal and
superfluid phases.
It is amusing that we also encounter complexity in the quantum world,
when we go down the length and time scales, within the experimentally
accessible regime. Nuclear physics is rich and complex. Elementary par-
ticle physics and the standard model description of electromagnetic, weak
and strong interaction is complex. It has nearly 19 parameters and a rich
variety of unexpected particles and phenomena. Going down further, into
frontiers beyond possibilities of experimentation at the present time, we en-
counter supersymmetry, quantum gravity and superstring theory. Physical
realizations apart, they have connected our physics friends to a rich and
profoundly complex quantum world and world of mathematics.
The complexity we are talking about are with respect to equilibrium
properties or equilibrium states. The world of non equilibrium quantum
phenomena is also rich and varied. Thanks to experiments such as femto
second spectroscopy, new experiments in meso and nanoscopic systems,
