Tunneling of the Dislocations in bcc ³He Crystals

The attenuation and the velocity of the longitudinal sound wave in low density bcc 3 He crystals containing various amounts of 4 He impurities from <10 ppm to 1600 ppm were studied at various temperatures, sound frequencies and stress amplitudes. It was found that a large number of dislocations were multiplied by the isotopic phase separation. Most of them are considered to be of screw type, and their irreversible motion causes the amplitude dependent attenuation and velocity change. The critical shear stress amplitude above which the attenuation increases does not depend on the 4 He concentration nor on the temperature. Its magnitude is estimated to be τ c =5×10 -5 µ, where µ is the shear modulus. Because τ c is only 1/6 of the Peierls stress and independent of T , it is concluded that the screw dislocations penetrate the Peierls potential barrier by tunneling under the applied stress τ>τ c .

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