Quantum phase transition discovered in a quasi-2D system consisting of pure spins

Publicado em 01 julho 2021

Figure 1: Water phase diagram. It shows the start of the primary transition and the coexistence of the liquid and gaseous states along the black line. The transition ends at the critical point marked with an asterisk. Credits: Julio Larrea, Nature

Pure quantum systems can undergo a phase transition similar to the classical phase transition between the liquid and gaseous states of water. However, at the quantum level, particles that spin in the state resulting from the phase transition exhibit collective intertwined behavior. This unexpected observation opens a new path for the manufacture of materials with topological properties that are useful for spintronics applications and quantum computing.

This discovery was made through international cooperation led by Julio Larrea, a professor at the Institute of Physics (IF-USP) at the University of Sao Paulo, Brazil. Larrea is the first author of an article on research published in. Nature..

“We have the first experimental evidence of a first-order quantum phase transition in a quasi-two-dimensional system consisting entirely of spins, a breakthrough study from both experimental development and theoretical interpretation perspectives,” Larea said. It was.

To understand the importance of this discovery, it is helpful to examine the classical phase transitions illustrated by changes in water conditions and their quantum analogs illustrated by the Mott metal-insulator transition.

“The change in water state that occurs at 100 ° C under standard atmospheric pressure is the so-called first-order transition, which is characterized by discontinuous jumps in molecular density, that is, the number of water molecules. Per unit volume varies significantly from state to state, “says Larea. “This first-order discontinuous transition changes with pressure and temperature until it is completely suppressed by so-called pressure. Critical point Water generated at 374 ° C and 221 bar. At the critical point, the transition is quadratic, or continuous. “

Near the critical point, density fluctuations are infinitely correlated on the atomic length scale, resulting in unusual behavior of water properties. As a result, the material exhibits a unique state that is different from both gas and liquid (see Figure 1).


Figure 2: SrCu2(BO3).2 Spin system phase diagram showing the start of the first-order transition at absolute zero. The primary transition ends at a critical point, similar to the water diagram. However, unlike what happens in water, spin systems present a new state of order that is purely quantum and strongly correlated. It is an antiferromagnetic state. Credits: Julio Larrea, Nature

“In quantum materials, the metal-insulator transition of Mott is a rare example of a first-order transition. Unlike ordinary metals and insulators with non-interacting free electrons, the Mott state involves strong interactions between electron charges. , The actions that make up the group, “explained Larea. “The energy scale of these interactions is so low that first-order quantum phase transitions between metals and insulators can occur at absolute zero, which is the lowest possible temperature. Interactions between charges. Changes with temperature and pressure. It is suppressed at the critical point. As the critical point approaches, the volume charge density, which is the amount of charge per unit volume, changes rapidly, which may cause new states such as supercondensation. there is.”

In the above two examples, the phenomenon involves giant particles such as water molecules and electrons. The question raised by the researchers is whether the concept of phase transitions can be extended to massless quantum systems, such as spin-only systems (understood as quantum signs of matter related to magnetic states). How was it? This kind of situation has never been observed before.

“The material we used was the frustrating quantum antiferromagnet SrCu.2(BO3).2“Mr. Larea said. specific heat Analysis of small samples under extreme temperature conditions at the same time [to 0.1 kelvin],pressure [to 27 kilobar] And magnetic field [to 9 tesla].. Specific heat is a physical property that gives a measure of the internal energy of a system, from which we can infer various types of regular or disordered quantum states and possible electronic or intertwined spin states. “

According to Larrea, using samples exposed to cryogenic, high-pressure, and strong magnetic fields to obtain these measurements with the accuracy required to reveal correlated quantum states is a daunting experimental task. did. The experiment was conducted in Lausanne, Switzerland, at the Quantum Magnetic Laboratories of the Swiss Federal Institute of Technology Lausanne (LQM-EPFL), led by Henrik Lausanne. The accuracy of the measurements motivated theoretical collaborators, led by Frederick Mira (EPFL) and Philippe Corboz (University of Amsterdam), to develop state-of-the-art computational methods for interpreting the various observed anomalies. ..

“Our results showed unexpected signs of quantum Phase transition In a pure spin system, “First we observed the quantum phase transition between two different types of intertwined spin states, the dimer state. [spins correlated at two atomic sites] And the condition of the placket [spins correlated at four atomic sites].. This first-order transition ends at a critical point at a temperature of 3.3 Kelvin and a pressure of 20 kilobars.The critical point of water and SrCu2(BO3).2 The spin system has similar characteristics, and the states that appear near the critical point of the spin system conform to different descriptions of Ising-type physics. The term Ising refers to a model of statistical mechanics named after the German physicist Ernst Ising (1900). -98).

“It has also been observed that there is a discontinuity in magnetic particle density at this critical point, where triplets or states correlate with various configurations of spin orientation, leading to the emergence of pure quantum antiferromagnetic states. “It was,” said Larrea (see Figure 2).

Larrea’s next step is to learn more about the critical and intertwined spin states that appear near the critical point, the nature of discontinuous and continuous quantum phase transitions, and the energy scales that represent electron-electron interactions and correlations. Spins and charges that lead to quantum states such as superconductivity. “For this purpose, we plan to conduct research at pressures around critical points and at higher pressures,” he said. To this end, a new facility, the Institute for Quantum Materials in the Extreme State (LQMEC), has been established in collaboration with Valentina Martelli, a professor of experimental physics at IF-USP.

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