Allenspach, StephanPuphal, PascalLink, JoosepHeinmaa, IvoPomjakushina, EkaterinaKrellner, CorneliusLass, JakobTucker, Gregory S.Niedermayer, ChristofImajo, ShusakuKohaema, YoshimitsuKindo, KoichiKramer, SteffenHorvatic, MladenJaime, MarceloMadsen, AlexanderMira, AntoniettaLaflorencie, NicolasMila, FredericNormand, BruceRuegg, ChristianStern, RaivoWeickert, Franziska2021-07-032021-07-032021-07-032021-06-0410.1103/PhysRevResearch.3.023177https://infoscience.epfl.ch/handle/20.500.14299/179755WOS:000662063500006Classical and quantum phase transitions (QPTs), with their accompanying concepts of criticality and universality, are a cornerstone of statistical thermodynamics. An excellent example of a controlled QPT is the field-induced ordering of a gapped quantum magnet. Although numerous "quasi-one-dimensional" coupled spin-chain and -ladder materials are known whose ordering transition is three-dimensional (3D), quasi-two-dimensional (2D) systems are special for multiple reasons. Motivated by the ancient pigment Han purple (BaCuSi2O6), a quasi-2D material displaying anomalous critical properties, we present a complete analysis of Ba0.9Sr0.1CuSi2O6. We measure the zero-field magnetic excitations by neutron spectroscopy and deduce the spin Hamiltonian. We probe the field-induced transition by combining magnetization, specific-heat, torque, and magnetocalorimetric measurements with nuclear magnetic resonance studies near the QPT. With a Bayesian statistical analysis and large-scale Quantum Monte Carlo simulations, we demonstrate unambiguously that observable 3D quantum critical scaling is restored by the structural simplification arising from light Sr substitution in Han purple.Physics, MultidisciplinaryPhysicsbose-einstein condensationground-statemagnetization plateausphase-transitionsfieldtemperaturemodelantiferromagnettext::journal::journal article::research article