The noncentrosymmetric superconductor Re6Zr has attracted much interest for its possible unconventional superconducting state with broken time-reversal symmetry. Here we report Re-185/187 nuclear quadrupole resonance measurements on Re6Zr (T-c = 6.72 K) and the isostructural compounds Re27Zr5 (T-c = 6.53 K) and Re24Zr5 (T-c = 5.00 K). The nuclear spin-lattice relaxation rate 1/T-1 shows a coherence peak below T-c and decreases exponentially at low temperatures in all three samples. The superconducting gap Delta derived from the 1/T-1 data is 2 Delta = 3.58k(B)T(c), 3.55k(B)T(c), and 3.51k(B)T(c) for Re6Zr, Re27Zr5, and Re24Zr5, respectively, which is close to the value of 3.53k(B)T(c) expected for weak-coupling superconductivity. These data suggest conventional s-wave superconductivity with a fully opened gap in this series of compounds.
