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The ultrasonic attenuation of the heavy-fermion superconductor UPt3 is investigated as a function of temperature, magnetic field strength, and frequency. It is shown that in the presence of a dc magnetic field at least one metastable and two stable phases exist in the superconducting state.PACS numbers: 64.70. Kb, 62.80.+f, 74.30.Gn Until recently He was known as the only existing superfluid Fermi liquid in nature where the formation of the superfluid pairs is via triplet pairing. Since the discovery of superconductivity in some of the so-called "heavy-fermion" systems, x however, whose low-temperature behavior may also be well described 2 by Landau's Fermi-liquid approach, there is an increasing number of experimental and theoretical findings which support speculations that some of these compounds-and in particular 3,4 UPt3-are promising candidates for anisotropic odd-parity-pairing superconductivity. One of the characteristic features of an odd-parity-pairing superfluid (or superconducting) state is the larger number of possible spin configurations than in the singlet case which give rise to the various phases in superfluid 3 He and to a phase diagram which is profoundly influenced by a magnetic field. In 3 He ultrasonic experiments 5 have substantially contributed to the identification of the different superfluid phases and to the understanding of the physical nature of the pairing mechanism. Therefore, the idea suggests itself that ultrasonic experiments should also provide valuable information about phase transitions and the pairing mechanism in superconducting heavy-fermion compounds.It is the main purpose of this paper to present ultrasonic attenuation experiments in the superconducting state of UPt3 and to demonstrate for the first time that-as in superfluid 3 He-the phase diagram is strongly affected by a dc magnetic field so that, in the presence of a dc magnetic field, at least one metastable and two stable superconducting phases exist in UPt3.The measurements where performed on a single crystal of UPt3 of about 7 mm length and 7 mm diameter whose c axis forms an angle of about 21° with the cylinder axis. Longitudinal sound was propagated along the c axis and generated by means of a 30-MHz overtone A'-cut quartz transducer. A conventional ultrasonic pulse spectrometer was used to measure the ultrasonic attenuation coefficient a, and the ac susceptibility (at 30 Hz) of the sample was monitored simultaneously. In all experiments the dc magnetic field Bo was oriented along the cylinder axis of the specimen, thereby giving rise to some difficulties in the calculation of the true local field because, in the sample investigated here, no common coordinate system exists in which both the magnetic susceptibility and the "demagnetization factor" are diagonal tensors. On the other hand, this orientation guarantees that Bo has a component perpendicular to the ultrasonic wave vector q. The latter is a necessary condition to make sure that zero sound may couple to the three collective modes 6 (normal flapping, cl...

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