Quasi-one-dimensional quantum spin liquid in the $\rm {Cu(C_4H_4N_2)(NO_3)_2}$ insulator

TL;DR

This paper investigates the quantum spin liquid behavior in the quasi-one-dimensional insulator Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$, demonstrating how magnetic fields tune it towards a quantum critical point characterized by fermion condensation, with implications for thermodynamic scaling.

Contribution

It introduces the concept of fermion condensation quantum phase transition (FCQPT) to explain thermodynamic properties of CuPzN and constructs its temperature-magnetic field phase diagram.

Findings

Thermodynamic properties are governed by a quantum spin liquid with spinons.

Magnetic field tunes CuPzN towards a FCQPT where spinon effective mass diverges.

Constructed a $T-H$ phase diagram resembling heavy-fermion systems.

Abstract

We analyze measurements of the magnetization, differential susceptibility and specific heat of quasi-one dimensional insulator Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$ (CuPzN) subjected to magnetic fields. We show that the thermodynamic properties are defined by quantum spin liquid formed with spinons, with the magnetic field tuning the insulator CuPzN towards quantum critical point related to fermion condensation quantum phase transition (FCQPT) at which the spinon effective mass diverges kinematically. We show that the FCQPT concept permits to reveal and explain the scaling behavior of thermodynamic characteristics. For the first time, we construct the schematic $T-H$ (temperature---magnetic field) phase diagram of CuPzN, that contains Landau-Fermi-liquid, crossover and non-Fermi liquid parts, thus resembling that of heavy-fermion compounds.