The theory of phase transitions driven by thermal fluctuations is one of the centerpieces of condensed matter physics. Even at zero temperature however, quantum fluctuation can destroy order and drive a phase transition that is tuned by some nonthermal parameter (doping, magnetic field, pressure, strain …). Experiments cannot access absolute zero temperature, but the presence of the quantum phase transition creates a region of parameter space where quantum and thermal fluctuations compete. An appropriate theoretical description has not yet been established for this region, but various experiments have found strong deviations from the standard (and highly successful) theory of metals, Landau’s theory of the Fermi liquid.
Most interestingly, superconductivity emerges close to the supposed QCP in the phase diagram of a large number of unconventional superconductors, including the iron-pnicitdes, cuprates and heavy fermion. The problem is however much broader, and includes non-superconducting materials like Sr3Ru2O7, YbAlB4 and YbRh2Si2.
|Iridates in high magnetic field
We recently discovered a three-dimensional honeycomb structure that is close connected to the Kitaev spin liquid, which was originally theorized only in two-dimensions.
Quasi-one dimensional systems are thought to show quantum critical behavior near a Luttinger liquid quantum critical point.