A joint research group of Osaka University, Osaka City University, and the University of Tokyo created artificial atoms composed of carbon nanotubes and generated two different types of Kondo states.We have elucidated the relationship between the types of Kondo states and quantum fluctuations using the world's highest level current noise measurement technology.
The electrical resistance of a metal decreases with a decrease in temperature, but starts to increase at a certain temperature.The phenomenon that indicates an increase in resistance at low temperatures is the Kondo effect, which occurs due to the interaction between the spins of impurities contained in metals and the spins of conduction electrons. It was first elucidated by Jun Kondo in 1964.The Kondo effect is caused by fluctuations in spin degrees of freedom (quantum fluctuations), but when electrons also have degrees of freedom such as the direction of motion, different quantum fluctuations occur, resulting in different types of Kondo effects.The SU (2) Kondo effect is realized when the degree of freedom is only spin, and the SU (4) Kondo effect is realized when there are other degrees of freedom.
Therefore, the research group created artificial atoms using carbon nanotubes that correspond to impurities in metals. Two electrons were generated in the artificial atom and the electrons flowed.It was confirmed that the electrons in the artificial atom are in the SU (2) Kondo state, which has a degree of freedom in the direction of motion along the tube in addition to the degree of freedom in spin.It was discovered that by further applying a magnetic field, the SU (4) Kondo state changes to the SU (4) Kondo state due to the interaction between the spin and the magnetic field.This change was reproduced by theoretical calculation, and the Wilson ratio, which is an index of quantum fluctuation, was also calculated.In addition, the current noise contained in the current is also investigated to detect the effective charge.It was demonstrated that the effective charge and quantum fluctuations change continuously as the type of Kondo state changes.
This result will lead to understanding of quantum multi-body phenomena such as superconductivity and control of quantum fluctuations, and is expected to develop material science such as the development of new functions of materials.
Paper Information: [Physical Review Letters] Quantum Fluctuations along Symmetry Crossover in Kondo-correlated Quantum Dot
