Critical Properties at the Field-Induced Bose-Einstein Condensation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>NiCl</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mtext mathvariant="normal">−</mml:mtext><mml:mn>4</mml:mn><mml:mi>SC</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:msub><mml:mi>NH</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mn>2</mml:mn></mml:msub></mml:math>

We report new magnetization measurements on the spin-gap compound NiCl(2)-4SC(NH2)2 at the low-field boundary of the magnetic field-induced ordering. The critical density of the magnetization is analyzed in terms of a Bose-Einstein condensation of bosonic quasiparticles. The analysis of the magnetization at the transition leads to the conclusion for the preservation of the U(1) symmetry, as required for Bose-Einstein condensation. The experimental data are well described by quantum Monte Carlo simulations.

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