Unusual magnetic properties of the low-dimensional quantum magnet Na2V3O7

TL;DR

This study investigates the low-temperature magnetic properties of Na2V3O7, revealing complex behavior near a quantum critical point, including field-dependent specific heat maxima, a low-temperature susceptibility cusp, and evidence of dimerized V moments.

Contribution

The paper provides the first detailed low-temperature measurements of specific heat, susceptibility, and NMR on Na2V3O7, highlighting its proximity to a quantum critical point and the role of dimerization in its magnetic behavior.

Findings

Field-dependent maxima in specific heat shift with magnetic field.

Susceptibility exhibits a cusp indicating a phase transition at very low temperatures.

Magnetic behavior is dominated by a small fraction of V moments at the lowest temperatures.

Abstract

We report the results of low-temperature measurements of the specific heat Cp(T), ac susceptibility chi(T) and 23Na nuclear magnetic resonance NMR of Na2V3O7. At liquid He temperatures Cp(T)/T exhibits broad field-dependent maxima, which shift to higher temperatures upon increasing the applied magnetic field H. Below 1.5 K the ac magnetic susceptibility chi(T) follows a Curie-Weiss law and exhibits a cusp at 0.086 mK which indicates a phase transition at very low temperatures. These results support the previous conjecture that Na2V3O7 is close to a quantum critical point (QCP) at mu_{0}H = 0 T. The entire data set, including results of measurements of the NMR spin-lattice relaxation 1/T1(T), reveals a complex magnetic behavior at low temperatures. We argue that it is due to a distribution of singlet-triplet energy gaps of dimerized V moments. The dimerization process evolves over a rather broad temperature range around and below 100 K. At the lowest temperatures the magnetic properties are dominated by the response of only a minor fraction of the V moments.