Ferromagnetic quantum criticality in the new heavy-fermion system YbNi{sub 4}P{sub 2}

We present measurements of the ac-susceptibility, specific heat (C) and thermal expansion ({alpha}) on single crystals of the new quasi-one-dimensional heavy-fermion system YbNi{sub 4}P{sub 2}. YbNi{sub 4}P{sub 2} is a Kondo-lattice system that orders ferromagnetically at a very low temperature T{sub c}=0.17 K. Ferromagnetism can be suppressed by arsenic substitution x on phosphorus site and a ferromagnetic (FM) quantum critical point (QCP) is approached at x=0.08. We provide evidence that such a FM-QCP exists in YbNi{sub 4}(P{sub 0.92}As{sub 0.08}){sub 2} by showing that (i) the magnetic transition remains ferromagnetic between x=0 and x=0.08, (ii) no phase transition nor Fermi-liquid ground state down to 0.02 K can be observed at x=0.08, and finally (iii) the Grueneisen ratio {Gamma}={alpha}/C strongly diverges with decreasing temperature as {Gamma}{proportional_to}T{sup -0.3}.