  A quantum spin liquid (QSL) is a state of matter where unpaired electrons'
spins in a solid are quantum entangled, but do not show magnetic order in the
zero-temperature limit. Because such a state may be important to the
microscopic origin of high-transition temperature superconductivity and useful
for quantum computation, the experimental realization of QSL is a long-sought
goal in condensed matter physics. Although neutron scattering experiments on
the two-dimensional (2D) spin-1/2 kagome-lattice ZnCu3(OD)6Cl2 and effective
spin-1/2 triangular lattice YbMgGaO4 have found evidence for a continuum of
magnetic excitations, the hallmark of a QSL carrying 'fractionalized quantum
excitations', at very low temperature, magnetic and nonmagnetic site chemical
disorder complicates the interpretation of the data. Recently, the
three-dimensional (3D) Ce3+ pyrochlore lattice Ce2Sn2O7 has been suggested as a
clean, effective spin-1/2 QSL candidate, but there is no evidence of a spin
excitation continuum. Here we use thermodynamic, muon spin relaxation ({\mu}
SR), and neutron scattering experiments on single crystals of Ce2Zr2O7, a
compound isostructural to Ce2Sn2O7, to demonstrate the absence of magnetic
ordering and the presence of a spin excitation continuum at 35 mK, consistent
with the expectation of a QSL. Since our neutron diffraction and diffuse
scattering measurements on Ce2Zr2O7 reveal no evidence of oxygen deficiency and
magnetic/nonmagnetic ion disorder as seen in other pyrochlores, Ce2Zr2O7 may be
the first example of a 3D QSL material with minimum magnetic and nonmagnetic
chemical disorder.
