Magnetic Spin Ladder(C5H12N)2CuBr4: High-Field Magnetization and Scaling near Quantum Criticality

The magnetization, $M(H\le 30\mathrm{T},0.7\mathrm{}\le T\le 300\mathrm{K})$, of $({\mathrm{C}}_5{\mathrm{H}}_{12}\mathrm{N}{)}_2{\mathrm{CuBr}}_4$ has been used to identify this system as an $S=1/2\mathrm{}$ Heisenberg two-leg ladder in the strong-coupling limit, $J_{\perp }=13.3\mathrm{}\mathrm{K}$ and $J_{\parallel }=3.8\mathrm{}\mathrm{K}$, with $H_{c1\mathrm{}}=6.6\mathrm{}\mathrm{T}$ and $H_{c2\mathrm{}}=14.6\mathrm{}\mathrm{T}$. An inflection point in $M(H,T=0.7\mathrm{}\mathrm{K})$ at half saturation, $M_s/2\mathrm{}$, is described by an effective $\mathrm{XXZ}$ chain. The data exhibit universal scaling behavior in the vicinity of $H_{c1\mathrm{}}$ and $H_{c2\mathrm{}}$, indicating that the system is near a quantum critical point.