With the long-term support from the National Science and Technology Council (NSTC), a theoretical physics research team led by Distinguished Professor Chung-Hou Chung at the Department of Electrophysics, National Yang-Ming Chiao-Tung University has collaborated with researchers at Brookhaven National Laboratory (BNL) to reveal the mystery of quantum critical entangled strange metal state in rare-earth superconductors. Their research was published in Nature Communications in February 2023.  This study paves the way for understanding the mechanism for high-temperature superconductors, which has been a puzzle for the condensed matter physics community for the past 35 years. The breakthrough shows significance in both basic science and technological applications. This milestone collaboration between Taiwan and the United States demonstrates the leadership and innovation of Taiwan's fundamental science research team in international scientific collaborations.

The Mystery and Mechanism of Strange Metal States in Quantum Critical Superconductors 
It is well-known that the resistance of normal metals decreases in a temperature-square fashion on cooling at low temperatures before they superconduct, so-called Fermi liquid. However, over the last three decades, un-conventional metallic behaviors (or called “strange metal”), distinct from the normal metals, have been discovered in various new quantum materials, such as high-temperature cuprate superconductors, rare-earth superconductors, organic superconductors and two-dimensional twisted bi-layer graphene. These strange metals show linear-in-temperature resistance and logarithimic-in-temperature divergent specific heat coefficient. More recently, an even more exotic strange metal state called "Planckian metal" state, showing linear-in-temperature scattering rate with coefficient inversely proportional to the Planck constant, were discovered in rare-earth and cuprate high Tc superconductors near the unstable "quantum critical point". 

However, the underlying mechanisms for these strange metal states are largely unknown, and therefore constitute an outstanding open problem in condensed matter physics. The research team is dedicated to understanding and revealing the mystery of these strange quatum states of matter. 

Due to competing quantum ground states, quantum mechanics leads to unstable quantum critical states near quantum phase transition point, or the quantum critical point. These quantum crtical states are new states of matter with maximal quantum entanglement between electrons. The strange metal state is an example of quantum critical states. They can be regarded as a new type of matter, and involve in the quantum entanglement of a large number of electrons, called 'quantum entangled many-body state”, caused by the enhanced quantum critical fluctuations at low temperatures. In other words, in addition to ordinary metals, insulators, superconductors and semiconductors that are widely known, there is a new state of matter “quantum critical state “ in nature, carrying significance in both fundamental science and applications.

In this study, the mystery of the Planckian metal in strongly-correlated rare-earth superconductors is revealed for the first time by a theoretical-experimental collaboration. The experimental results confirm that the Planckian metal state is indeed a quantum critical state near the quantum critical point. Moreover, a microscopic theoretical mechanism based on quantum critical charge fluctuations and "quantum many-body entanglement" between electron’s charge and spin degrees of freedom is proposed. This theory, for the first time, successfully explains the Planckian metal and its relation to the quantum critical phenomenon in strongly correlated unconventional superconductors.  Due to very similar quantum critical charge fluctuations as the origin of the Planckian strange metal state, this study paves the way for revealing the mystery of the Planckian strange metal, the precursor of superconducting state, in high-temperature cuprate superconductors. Therefore, it marks an important step towards revealing the mechanism for high-temperature superconductivity in cuprates, which has been an outstanding puzzle for the condensed matter physics community for the past 35 years.

Meanwhile, understanding these strange metallic behaviors is important for technological applications as it is a necessary step to design, predict and eventually enhance the critical temperature of high-temperature superconductors, which shows great technological advances in energy conservation and developing new technology, such as: transmitting electrical power without energy lost, developing quantum computing and quantum information technology.

A New Milestone in Taiwan-U.S. Science Collaboration
This Taiwan-US collaboration was conceived by Prof. Chung in 2018, and he has led the whole research project. Since the fall of 2019, Dr. Cedomir Petrovic from the BNL experimental team in the US has joined the collaboration and provided the experimental data for a class of rare-earth quantum critical superconductors. Prof. Chung and Dr. Yung-Yeh Chang from the Taiwan team analyzed the experimental data and developed the microscopic theory for the experimental results. After three and a half years of collaborative efforts, the proposed theoretical mechanism successfully captures the experimental phenomena of the Planckian strange metal and was finally published in the Nature Communications in February 2023. The success of this collaboration demonstrates the leadership and dominating role of Taiwan's fundamental science research team in international collaborations, and marks a new milestone in Taiwan-US basic science cooperation.

Link to original posting:

https://www.nature.com/articles/s41467-023-36194-9

Figure

Top Figure: Finite temperature phase diagrams of the rare-earth heavy-fermion superconductor Ce1-xNdxCoIn5 as functions of magnetic fields and Nd doping (Top Left) and as a function of Nd doping at a fixed magnetic field (Top Right).Bottom Figure: The Planckian strange metal state showing quantum critical behaviors in electrical resistivity (Bottom Left), electron scattering rate (Bottom Middle), and specific heat coefficient (Bottom Right).

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