Field-induced non-Fermi-liquid resistivity of stoichiometric YbAgGe single crystals

We have investigated hexagonal YbAgGe down to 70 mK by measuring the magnetic-field and temperature dependence of the resistivity rho of single crystals in fields up to 14 T. Our results extend the H-T phase diagram to the lowest temperatures for H applied in the basal plane and along the c axis. In particular, critical fields for the suppression of several magnetic phases are determined. The temperature dependence of rho(T) is unusual: whereas at low H, rho(T) reveals a temperature exponent n >= 2, we find 1 <= n < 1.5 and strong enhancement of the temperature dependence of rho(T) close to and beyond the highest critical field for each field direction. For H applied in the basal plane, at high fields a conventional T-2 dependence of rho(T) is reached above 10 T accompanied by an approach to saturation of a strong drop in the residual resistivity. YbAgGe appears to be one of few Yb-based stoichiometric systems, where quantum-critical behavior may be induced by a magnetic field.