Thermal Transport and Strong Mass Renormalization 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>

Several quantum paramagnets exhibit magnetic-field-induced quantum phase transitions to an antiferromagnetic state that exists for H c1 ≤ H ≤ H c2. For some of these compounds, there is a significant asymmetry between the low- and high-field transitions. We present specific heat and thermal conductivity measurements in NiCl2-4SC(NH2)2, together with calculations which show that the asymmetry is caused by a strong mass renormalization due to quantum fluctuations for H ≤ H c1 that are absent for H ≥ H c2. We argue that the enigmatic lack of asymmetry in thermal conductivity is due to a concomitant renormalization of the impurity scattering.