Effect of electron doping the metamagnet Sr_3-yLa_yRu₂O₇

We study the effects of adding electrons to the conduction bands of the itinerant metamagnet Sr3Ru2O7 using substitution of La3+ onto the Sr2+ site. Small changes to the chemical potential have a large effect: adding only 0.03 electrons per Ru reduces the electronic specific-heat coefficient by 30%, but makes no corresponding change to the temperature (similar to 8 K) at which the electronic specific-heat coefficient shows its maximum. The observations are incompatible with a simple rigid-band shift, raising the possibility that a many-body resonance plays a key role in the physics of Sr3Ru2O7.