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

Abstract

We present the results of muon-spin spectroscopy ( μ ^+ SR) measurements on the molecular spin ladder system (Hpip) _2 CuBr _4(1− _x _) Cl _4 _x , [Hpip = (C _5 H _12 N)]. Using transverse field μ ^+ 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 _4 _x ( 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 μ ^+ 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.

Keywords

• spin ladders
• muon spectroscopy
• quantum phase transitions
• muon site determination
• Luttinger liquid parameters
• Luttinger liquids