Pure quantum systems can undergo phase transitions analogous to the classical phase transition between the liquid and gaseous states of water. At the quantum level, however, the particle rotates in states that arise from phase transitions, showing collectively entangled behavior. This unexpected observation offers a new way to produce materials with topological properties that are useful in spintronics and quantum computing applications.
The discovery was made by an international collaborator led by Julio Larea, a professor at the Institute of Physics at the University of Sao Paulo (IF-USP) in Brazil. Larea is the first author of an article on the study published in Nature.
“We received the first experimental evidence of a first-order quantum phase transition in a quasi-two-dimensional system consisting entirely of spins. This was an innovative study in terms of both experimental development and theoretical interpretation,” Larea said.
To understand the significance of this discovery, it will help to study the classical phase transition, which can be illustrated by the change in water state, and its quantum analogue, illustrated by the Mott metal-insulator transition.
“The change in the state of water that occurs at 100 ° C at standard atmospheric pressure is what we call a first-order transition. It is characterized by a continuous jump in molecular density. In other words, the number of water molecules per unit volume varies. drastically between the two states, “Larea said. “This interrupted first-order transition develops according to pressure and temperature until it is completely suppressed at the so-called critical point of water, which occurs at 374 ° C and 221 bar. At the critical point, the transition is of the second order, t. .f. continuously. “
Near the critical point, the properties of water behave abnormally, as density fluctuations are infinitely correlated on the atomic length scale. As a result, the material exhibits a unique state that differs from both gas and liquid.
“In quantum matter, the Mott-insulator transition is a rare example of a first-order transition. Unlike ordinary metals and insulators, which have free electrons that do not interact, the Mott state involves strong interaction between electronic charges, configuring collective behavior.” , Larea explained. “The energy scales of these interactions are very low, so the first-order quantum phase transition between metal and insulator can occur at absolute zero, which is the lowest possible temperature. The interaction between charges varies with temperature and pressure, while suppressed at the critical point. As the critical point approaches, the bulk charge density, which is the amount of charge per unit volume, changes so dramatically that it can cause new states of matter such as superconductivity. “
In the two examples mentioned, the phenomena include massive particles such as water molecules and electrons. The question posed by the researchers is whether the concept of phase transition can be extended to massless quantum systems, as a system built solely of spins (of course as a quantum manifestation of matter associated with magnetic states). Such a situation has never been observed before.
“The material we used was a frustrated quantum antiferromagnet SrCu 2 (BO 3) 2 “, Said Larea.” We measured the specific heat of small samples at simultaneously extreme temperature conditions [to 0.1 kelvin], pressure [to 27 kilobar] and magnetic field [to 9 tesla]. Specific heat is a physical property that gives us a measure of the internal energy in the system, and from this we can draw conclusions about different types of ordered or disordered quantum states and possible electronic states or tangled spin states. “
Obtaining these measurements with the precision needed to detect correlated quantum states using samples subjected to extremely low temperatures, high pressure, and strong magnetic fields was a great experimental challenge, according to Larrea. The experiments were conducted in Lausanne, Switzerland, at the Quantum Magnetism Laboratory of the Federal Polytechnic School of Lausanne (LQM-EPFL), headed by Henrik Ronnow. The accuracy of the measurements motivated the theoretical collaborators led by Frederick Mila (EPFL) and Philippe Corbos (University of Amsterdam) to develop modern computational methods to interpret the various anomalies observed.
“Our results showed unexpected manifestations of quantum phase transitions in pure spin systems,” Larea said. “First, we observed a quantum phase transition between two different types of entangled spin states, the dimeric state. [spins correlated at two atomic sites] and the condition of the plaque [spins correlated at four atomic sites]. This first-order transition ends at the critical point, at a temperature of 3.3 kelvins and a pressure of 20 kilobar. Although the critical points of water and SrCu 2 (BO 3) 2 the spin system has similar characteristics, the states that appear near the critical point of the spin system correspond to a different description of physics of the Ising type. “The term Ising refers to a model of statistical mechanics named after the German physicist Ernst Ising (1900) -98).
“We also noticed that this critical point has a break in the magnetic density of the particles, with triplets or states correlated in different spin orientation configurations, leading to the appearance of a pure quantum antiferromagnetic state,” Larea said.
The next step for Larrea is to learn more about the criticality and entangled spin states that occur near the critical point, the nature of the discontinuous and continuous quantum phase transitions, and the energy scales that represent the interactions and correlations between electron spins and charges leading to quantum states such as superconductivity. “To this end, we plan to conduct a study with pressure around the critical point and higher pressures,” he said. For this purpose, a new facility is created – Laboratory for Quantum Matter in Extreme Conditions (LQMEC) in collaboration with Valentina Marteli, Professor in the Department of Experimental Physics at IF-USP.
Water and quantum magnets share critical physics
J. Larrea Jiménez et al, Quantum magnetic analogue of the critical point of water, Nature (2021). DOI: 10.1038 / s41586-021-03411-8