Nodal-line semimetal HMTSF-TCNQ: Anomalous orbital diamagnetism and charge density wave

TL;DR

This paper explores the electronic states of the organic charge-transfer complex HMTSF-TCNQ, revealing topological semimetal behavior with nodal lines, and studies the effects of charge-density-wave transition on its magnetic and electronic properties.

Contribution

The study combines first-principles calculations with a tight-binding model to analyze the topological semimetal state and CDW phase, and links theoretical predictions with experimental measurements of orbital diamagnetism.

Findings

Normal state is a topological semimetal with open nodal lines.

CDW phase deforms nodal lines into closed shapes sensitive to the order parameter.

An anomalous low-temperature plateau in orbital diamagnetism is observed and explained.

Abstract

This study investigates the electronic states and physical quantities of an organic charge-transfer complex HMTSF-TCNQ, which undergoes a charge-density-wave (CDW) phase transition at temperature $T_c\simeq 30$ K. A first-principles calculation is utilized to determine that the normal state is a topological semimetal with open nodal lines. Besed on the first-principles calculation, we develop a tight-binding model to investigate the electronic state in detail. Below $T_c$, the CDW phase is examined in the tight-binding scheme using the mean-field approximation. It is shown that the open nodal lines are deformed into closed ones, and their shapes are sensitive to the order parameter. Using this tight-binding model, we theoretically evaluate the temperature dependencies of two physical quantities: the spin-lattice relaxation time $T_1$ and the orbital magnetic susceptibility. In particular, an anomalous plateau is obtained at low temperatures in the orbital diamagnetism. We presume that this anomalous plateau originates owing to the conflict between the interband diamagnetism, impurity scattering, and the nodal line deformation. We also conduct an experiment to investigate the orbital magnetism, and the results are in excellent quantitative agreement with the theory.