Evidence of a magnetic Bose glass in ${\left({\text{CH}}_3\right)}_2{\text{CHNH}}_3\text{Cu}{\left({\text{Cl}}_{\left\{0.95\right\}}{\text{Br}}_{\left\{0.05\right\}}\right)}_3$ from neutron diffraction

We report the single-crystal study of the bulk magnetization and neutron-scattering measurements on a quantum $S=1/2$ spin ladders system $\text{IPA-Cu}{\left({\text{Cl}}_{0.95}{\text{Br}}_{0.05}\right)}_3$ with quenched disorder. In zero field, the disordered spin liquid phase is preserved as in pure ${\text{IPA-CuCl}}_3$. Due to the bond randomness, a different Bose glass phase was directly observed in $H_c<H<H^{\prime }$, which separates the spin liquid phase from the unconventional Bose-Einstein condensation phase. The observed finite value of boson compressibility $\left(dM/dH\right)$ and lack of field-induced three-dimensional long-range order are consistent with the theoretical prediction.

Figure 1

(Color online) The excitation spectrum (false color plot) measured in $\text{IPA-Cu}{\left({\text{Cl}}_{0.95}{\text{Br}}_{0.05}\right)}_3$ at $T=1.5\text{K}$ in zero applied field reveals sharp dispersive quasiparticles with an energy gap $\Delta =1.24\left(1\right)\text{meV}$. A typical energy scan (left panel, symbols) shows a well-defined peak with an energy width entirely due to experimental resolution (solid line, calculation).

Figure 2

(Color online) Signatures of various quantum phases in $\text{IPA-Cu}{\left({\text{Cl}}_{0.95}{\text{Br}}_{0.05}\right)}_3$. The uniform magnetization (a, circles, from Ref. 10) represents the density of bosons in the ground state. The rate of increase in uniform magnetization is the boson compressibility (a, squares). The thin solid lines correspond to the disorder-free parent compound ${\text{IPA-CuCl}}_3$. (b) The magnetic Bragg intensity measured at the maximum represents the effective BEC order parameter. The measured temperature dependence of $H^{\prime }$ is shown in the inset. The solid curve is a guide for the eyes.

Figure 3

(Color online) Neutron scans across the $\left(-0.5,-1.5,0\right)$ point measured in ZFC and FC $\text{IPA-Cu}{\left({\text{Cl}}_{0.95}{\text{Br}}_{0.05}\right)}_3$ samples at $T=30\text{mK}$ in $H=13\text{T}$ applied field (symbols), in the BEC phase. The magnetic peaks are broader than experimental resolution (dashed lines), indicating that only short-range correlations of the BEC order parameter are present. The solid lines are Voigt function fits to the data.