  Recent predictions and experimental observations of high Tc superconductivity
in hydrogen-rich materials at very high pressures are driving the search for
superconductivity in the vicinity of room temperature. We have developed a
novel preparation technique that is optimally suited for megabar pressure
syntheses of superhydrides using pulsed laser heating while maintaining the
integrity of sample-probe contacts for electrical transport measurements to 200
GPa. We detail the synthesis and characterization, including four-probe
electrical transport measurements, of lanthanum superhydride samples that
display a significant drop in resistivity on cooling beginning around 260 K and
pressures of 190 GPa. Additional measurements on two additional samples
synthesized the same way show resistance drops beginning as high as 280 K at
these pressures. The loss of resistance at these high temperatures is not
observed in control experiments on pure La as well as in partially transformed
samples at these pressures, and x-ray diffraction as a function of temperature
on the superhydride reveal no structural changes on cooling. We infer that the
resistance drop is a signature of the predicted room-temperature
superconductivity in LaH10, in good agreement with density functional structure
search and BCS theory calculations.
