Magnetic and phononic dynamics in the two-ladder quantum magnet (C5H9NH3)2CuBr4

TL;DR

This study investigates the interplay between magnetic and phononic excitations in the soft quantum magnet (C5H9NH3)2CuBr4, revealing how magnetic correlations influence lattice vibrations and identifying key low-energy magnetic gaps.

Contribution

It provides the first detailed analysis of coupled spin and lattice dynamics in a soft, low-dimensional quantum magnet using high-resolution neutron spectroscopy.

Findings

Identification of two magnetic gaps at 0.41 meV and 0.55 meV confirming a two-ladder spin structure

Observation of a localized phonon mode at around 2 meV affected by magnetic correlations

Detection of a 5% decrease in phonon frequency with decreasing temperature

Abstract

In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Can such an interplay occur in a structurally soft system with comparable elastic and magnetic energy scales? Here we study the metal-organic material (C5H9NH3)2CuBr4 (Cu-CPA), in which an explanation of the low-lying excitations depends crucially on a full understanding of both the spin and lattice subsystems. We report high-resolution neutron spectroscopy enabled by large, deuterated single-crystals that reveal how both sectors are affected by the recently discovered structural phase transition. By measuring over several Brillouin zones, we disentangle the vibrational contribution to the spectrum in order to obtain an accurate estimate of the quasi-one-dimensional magnetic signal. The low-energy magnetic excitations are dominated by two gaps, $\Delta$ b = 0.41 meV and $\Delta$ a = 0.55 meV, which contribute with equal intensity ratios, confirming that Cu-CPA realizes a two-ladder spin Hamiltonian, and we deduce the magnetic interaction parameters of both ladders. The phonon spectrum contains a highly localized mode at an anomalously low-energy around 2 meV. This characteristic frequency drops by approximately 5 percent as magnetic correlations become established with decreasing temperature, and we connect this behavior with the location and structure of the cyclopentylammonium rings.