Discovery of the Quantum Critical Point
Common phase transitions are those that occur as a function of temperature changes. For example, ice starts to transform into liquid water at 0℃, and liquid water transforms into water vapor at 100℃. Similarly, magnetic materials become non-magnetic at the critical temperature, but there are phase transitions that do not depend on temperature. They occur near absolute zero (-273.15℃) and are related to quantum fluctuations. A study involved experiments under extreme conditions, especially ultra-low temperature and strong magnetic fields, and accompanied by a theoretical explanation of the experimental results, explored this type of situation, and studied the phenomenon of quantum critical points in extremely unusual transitions .
Italian researcher Valentina Martelli and Peruvian professor Julio Larrera are both professors at the Institute of Physics of the University of São Paulo, Brazil. Both participated in this research. The experimental part was led by Professor Silke Paschen. Performed in the laboratory of the Technical University of Vienna, Austria. This theoretical work found and explained the evidence for two consecutive quantum critical points related to the double breakdown of the Kondo effect.
Experiments of Quantum Fluctuations
The experiment was carried out with heavy fermion Ce3Pd2OSi6, which is a compound of cerium, palladium and silicon. With the support of the Sao Paulo Research Foundation, Larrea will continue his research through the "Topology and Singular Quantum State Research under Extreme Conditions" project. The starting point of these transitions is the strong correlation between electrons and certain materials, which allows us to understand this state change. Various collective interactions can affect electrons, and one possible state is the so-called "strange metal". In heavy fermions, electron transport is similar to that of ordinary metals, but electrons are strongly correlated, and the collective behavior is as if they form a single quasi-particle to transport charge.
This does not happen in a quantum phase transition, so this state is called "strange". What has been observed in the experiment is that the performance of physical properties such as electrical resistance is very different from the classical electron transport in metals. This phenomenon occurs at extremely low temperatures, very close to absolute zero. When the temperature drops to such a low temperature, the thermodynamic fluctuations have almost disappeared, and quantum fluctuations have been observed, forming the "medium for interactions between electrons." ". Prior to the publication of the study, most of these experiments focused on electronic correlations leading to so-called simultaneous touring and localized electronic magnetic materials.
Magnetic Moment During the Test
"Heavy" because they are related to quasiparticles with large effective mass. These materials also have magnetic moments, so in addition to charged quasiparticles, they are also related to quasiparticles shielded by conductive electrons or magnetic moments. Each magnetic moment shield can be coupled to adjacent magnetic moments in the lattice, creating a magnetic sequence throughout the material. In the case of Ce3Pd2OSi6, this sequence is antiferromagnetic, which means that the magnetic moments in the lattice are coupled in an anti-parallel manner. At the quantum critical point, this magnetic order can be suppressed by applying a magnetic field without being affected by thermodynamic control parameters. The Kondo singlet decomposes, and the electrons coupled to this magnetic order are simply separated.
This is not inconsistent with the basic principles of quantum mechanics, but it is very different from what is described in basic physics textbooks. Because the magnetic moment is defined relative to spin, the suppression of the magnetic order creates a situation where electrons seem to lack spin. This quantum critical point based on magnetic order has previously been published in other studies. The difference in this study is that in addition to the dipole magnetic sequence, the material also exhibits a quadrupole magnetic sequence generated by the electron orbit. The phase diagram is almost a graphical summary of the study, so it shows two quantum critical points: one is the breakdown of the dipole sequence, and the other is the breakdown of the quadrupole sequence.