Evidence for Dimer Crystal Melting in the Frustrated Spin-Ladder BiCu2PO6

TL;DR

This study uses inelastic light scattering to explore spin excitations in BiCu$_2$PO$_6$, revealing a complex spectrum of bound states and evidence for a quantum phase transition from a dimer crystal to a resonating valence bond state.

Contribution

It provides the first detailed characterization of spin excitations and bound states in BiCu$_2$PO$_6$, highlighting the role of frustration and interladder interactions in dimer melting.

Findings

Observation of low-energy singlets and a broad triplon continuum

Detection of interladder and intraladder bound states with specific polarization dependence

Evidence for a Z$_2$ quantum phase transition from a dimer to RVB state

Abstract

In the spin ladder compound BiCu$_2$PO$_6$ there exists a decisive dynamics of spin excitations that we classify and characterize using inelastic light scattering. We observe low-energy singlets and a broad triplon continuum extending from 36 cm$^{-1}$ to 700 cm$^{-1}$ in ($aa$), ($bb$), and ($cc$) light scattering polarizations. Though isolated spin ladder physics can roughly account for the observed excitations at high energies, frustration and interladder interactions need to be considered to fully describe the spectral distribution and scattering selection rules at low and intermediate energies. More significantly, an interladder singlet bound mode at 24 cm$^{-1}$, lying below the continuum, shows its largest scattering intensity in interladder ($ab$) polarization. In contrast, two intraladder bound states at 62 cm$^{-1}$ and 108 cm$^{-1}$ with energies comparable to the continuum are observed with light polarization along the leg ($bb$) and the rung ($cc$). We attribute the rich spectrum of singlet bound modes to a melting of a dimer crystal. Our study provides evidence for a Z$_2$ quantum phase transition from a dimer to a resonating valence bond state driven by singlet fluctuations.