Quantum Criticality

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.

Figure3Dec13Iridates 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.

PhaseDiag_colorplotP-substituted BaFe2As2

P substituted iron-pnictide materials are perhaps the most striking example of the pnictide superconductors of quantum critical materials. To date, the exact mechanism of high-temperature superconductivity has not been completely understood. The fact that superconductivity emerges in the close vicinity of QCP for different unconventional SCs raises the question if there is a more fundamental relation between quantum critical fluctuations and this ordering phenomenon.

Li0.9Mo6O17

Quasi-one dimensional systems are thought to show quantum critical behavior near a Luttinger liquid quantum critical point.