UPd<sub>2</sub>Al<sub>3</sub> is a heavy-fermion superconductor with a hexagonal crystal structure and critical temperature T<sub>c</sub>=2.0K that was discovered in 1991. Furthermore, UPd<sub>2</sub>Al<sub>3</sub> orders antiferromagnetically at T<sub>N</sub>=14K, and UPd<sub>2</sub>Al<sub>3</sub> thus features the unusual behavior that this material, at temperatures below 2K, is simultaneously superconducting and magnetically ordered. Later experiments demonstrated that superconductivity in UPd<sub>2</sub>Al<sub>3</sub> is magnetically mediated, and UPd<sub>2</sub>Al<sub>3</sub> therefore serves as a prime example for non-phonon-mediated superconductors.
Heavy-fermion superconductivity was discovered already in the late 1970s (with CeCu<sub>2</sub>Si<sub>2</sub> being the first example), but the number of heavy-fermion compounds known to superconduct was still very small in the early 1990s, when Christoph Geibel in the group of Frank Steglich found two closely related heavy-fermion superconductors, UNi<sub>2</sub>Al<sub>3</sub> (T<sub>c</sub>=1K) and UPd<sub>2</sub>Al<sub>3</sub> (T<sub>c</sub>=2K), which were published in 1991. At that point, the T<sub>c</sub>=2.0K of UPd<sub>2</sub>Al<sub>3</sub> was the highest critical temperature amongst all known heavy-fermion superconductors, and this record would stand for 10 years until CeCoIn<sub>5</sub> was discovered in 2001.
The overall metallic behavior of UPd<sub>2</sub>Al<sub>3</sub>, e.g. as deduced from the dc resistivity, is typical for a heavy-fermion material and can be explained as follows: incoherent Kondo scattering above approximately 80 K and coherent heavy-fermion state (in a Kondo lattice) at lower temperatures. Upon cooling below 14 K, UPd<sub>2</sub>Al<sub>3</sub> orders antiferromagnetically in a commensurate fashion (ordering wave vector (0,0,1/2)) and with a sizable ordered magnetic moment of approximately 0.85 ü<sub>B</sub> per uranium atom, as determined from neutron scattering.
The metallic heavy-fermion state is characterized by a strongly enhanced effective mass, which is connected to a reduced Fermi velocity, which in turn brings about a strongly suppressed transport scattering rate. Indeed, for UPd<sub>2</sub>Al<sub>3</sub> optical Drude behavior with an extremely low scattering rate was observed at microwave frequencies. This is the 'slowest Drude relaxation' observed for any three-dimensional metallic system so far.
Superconductivity in UPd<sub>2</sub>Al<sub>3</sub> has a critical temperature of 2.0K and a critical field around 3T. The critical field does not show anisotropy despite the hexagonal crystal structure. For heavy-fermion superconductors it is generally believed that the coupling mechanism cannot be phononic in nature. In contrast to many other unconventional superconductors, for UPd<sub>2</sub>Al<sub>3</sub> there actually exists strong experimental evidence (namely from neutron scattering and tunneling spectroscopy ) that superconductivity is magnetically mediated.
In the first years after the discovery of UPd<sub>2</sub>Al<sub>3</sub> it was actively discussed whether its superconducting state can support a FuldeâÂÂFerrellâÂÂLarkinâÂÂOvchinnikov (FFLO) phase, but this suggestion was later refuted.