Quantum magnetism in molecular spin ladders probed with muon-spin spectroscopy

Abstract

We present the results of muon-spin spectroscopy (${\mu }^{+}$SR) measurements on the molecular spin ladder system (Hpip)${}_2$CuBr${}_{4(1-x)}$Cl${}_{4x}$, [Hpip=(C${}_5$H${}_{12}$N)]. Using transverse field ${\mu }^{+}$SR we are able to identify characteristic behaviour in each of the regions of the phase diagram of the $x=0$ strong-rung spin ladder system (Hpip)${}_2$CuBr${}_4$. Comparison of our results to those of the dimer-based molecular magnet Cu(pyz)(gly)(ClO${}_4$) shows several common features. We locate the crossovers in partially disordered (Hpip)${}_2$CuBr${}_{4(1-x)}$Cl${}_{4x}$ ($x=0.05$), where a region of behaviour intermediate between quantum disordered and Luttinger liquid-like is identified. Our interpretation of the results incorporates an analysis of the probable muon stopping states in (Hpip)${}_2$CuBr${}_4$ based on density functional calculations and suggests how the muon plus its local distortion can lead to a local probe unit with good sensitivity to the magnetic state. Using longitudinal field ${\mu }^{+}$SR we compare the dynamic response of the $x=1$ strong-rung material (Hpip)${}_2$CuCl${}_4$ to that of the strong-leg material (C${}_7$H${}_{10}$N)${}_2$CuBr${}_4$ (known as DIMPY) and demonstrate that our results are in agreement with predictions based on interacting fermionic quasiparticle excitations in these materials.

arXiv comment16 pages, 13 figures

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