Discontinuous quantum phase transitions

Image taken from fig 4. of Ulrich Schneider, Realizing discontinuous quantum phase transitions in a strongly correlated driven optical lattice, Nature Physics (2022). DOI: 10.1038/s41567-021-01476-w. www.nature.com/articles/s41567-021-01476-w

Phase transitions are an everyday occurrence. Water turns to steam when it is boiled or water vapor fogs up a cold window pane. However their fundamental dynamics are not well understood. A recent paper from the University of Cambridge aims to shed some light on how quantum fluctuations play a part in first order quantum phase transitions.

A first order phase transition is one where there is a latent energy. The first derivative of free energy is discontinuous. Practically, this means water being heated increases in temperature until it starts evaporating, at which point the temperature stays constant until all the water is converted into steam. A second order phase transition is where the second derivative of free energy is discontinuous. From the perspective of temperature, it goes up continuously and there isn’t a region of constant temperature. Such is the case of magnetization in ferrous metals.

Researchers at Cambridge University were able to control the continuity of this transition. Up until now, this has been demonstrated in systems with weak interactions. The Cambridge researchers achieved this in a strongly correlated many-body system. Their experimental subjects were ultracold atoms arranged in a one-dimensional crystal lattice. Shaking the lattice at its resonant frequency was able to couple the first two bands of the lattice. The atoms from the lowest energy band were able to jump to the first excited band, forming a superfluid.

For weak amplitudes of this shaking (still at the resonant frequency), the transition is discontinuous and the system experiences a sort of hysteresis, where it remains suspended in a metastable state. However for strong amplitudes, the transition is continuous and smoothly becomes a superfluid.

This is strong evidence that phase transitions close to absolute zero are caused by quantum fluctuations, rather than the traditional thinking of thermal fluctuations. This new finding serves as a stepping stone to potentially unraveling the key role of quantum fluctuations in discontinuous transitions, such as those that occurred in the early Universe.

The research paper can be downloaded for free through your UofT account:

https://www.nature.com/articles/s41567-021-01476-w