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Abstract

We have studied the temperature-pressure phase diagram of CeCoSi by electrical-resistivity experiments under pressure. Our measurements revealed a very unusual phase diagram. While at low pressures no dramatic changes and only a slight shift of the Neel temperature T-N (approximate to 10 K) are observed, at about 1.45 GPa a sharp and large anomaly, indicative of the opening of a spin-density wave gap, appears at a comparatively high temperature T-S approximate to 38 K. With further increasing pressure, T-S shifts rapidly to low temperatures and disappears at about 2.15 GPa, likely continuously in a quantum critical point, but without evidence for superconductivity. Even more surprisingly, we observed a clear shift of T-S to higher temperatures upon applying a magnetic field. We discuss two possible origins for T-S : magnetic ordering of Co and a metaorbital type of transition of Ce.