A direct measurement of the London penetration depth in the high-Tc superconductor YBa2Cu3O6.92 has been made using low-energy μSR with an effective background suppression method. The average magnetic field versus mean depth was measured in the Meissner state of high-purity detwinned crystals of YBa2Cu3O6.92. The resulting magnetic field profiles along the a and b axes are consistent with a local London model beyond 10 nm but there are deviations close to the surface. The absolute values of λa and λb extrapolated to zero temperature are 126.1±1.2±3 and 105.5±1.0±3 nm, respectively. These results are compared with other less direct methods and extend the use of low-energy μSR to small crystals.
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