Optical conductivity of URu₂Si_2-xP_x from the hidden-order to the antiferromagnetic phase

We report the optical spectroscopy of the URu₂Si_2-xP_x heavy fermion chemical substitution series. For x=0, this material is the now well-studied URu₂Si₂ compound with a Kondo temperature of 370K. In this material, the hybridization between heavy f electrons with conduction electrons creates a crossover to a Kondo liquid state having coherent transport properties below 70K. At 17.5K, a second-order mean-field transition creates an electronically ordered state whose origin remains unknown, commonly called the hidden order (HO) phase. Finally below 1.5K an unconventional superconducting phase emerges. The HO phase disappears for x≥0.035 and, at much higher doping x>0.25, an antiferromagnetic phase (AFM) emerges at low temperature (T_N~40K for x=0.28).
Here we investigated the passing between the HO and AFM phase by measuring the optical conductivity from 300K to 5K for x=0.02, 0.23 and 0.28 samples. Distinct optical signatures are observed. For x=0.02 (HO), a 6.5meV gap opens at low temperature in accordance with values observed in undoped samples. For the x=0.28 (AFM) a distinct higher energy gap is seen for T<T_N. At intermediate doping x=0.23, no particular spectral feature has been found. The spectral weight transfer of these various gap openings will be presented and discussed.