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PHYSICAL REVIEW LETTERS VOLUME 55, NUMBER 1S 7 OCTOBER 1985 Low-Temperature State of UCu5'. Formation of Heavy Electrons in a Magnetically Ordered Material H. R. Ott, H. Rudigier, and E. Felder Laboratorium fur Festkorperphysik, Eidgenossiche Technische Hochschule Ho-nggerberg, CH 8093 -Zurich, Switzerland Z. Fisk Materials Science and Technology Division, Los Alamos National Laboratory, Los A/amos, New Mexico 87545 B. Batlogg A TckT Bell Laboratories, Murray Hill, New Jersey 07974 (Received 8 July 1985) The formation of a heavy-electron state in a magnetically ordered material is established by mea- surements of the low-temperature specific heat of UCu~ and UAgCu4. In UCus this state under- goes a continuous but hysteretic phase transition which removes parts of the Fermi surface with a high density of electronic states and leads to a resistivity increase of almost an order of magnitude. PACS numbers: 65.40.Em, 72. 15.Eb, 75.50.Ee In two previous publications, UCuq was identified at 15 K. . This con- as ordering antiferromagnetically clusion was based on data from measurements of the and from neutron-diffraction magnetic susceptibility Later measurements of the specific experiments. heat and the electrical resistivity confirmed the phase transition but also led to the conclusion, in the course of a more general investigation of UNi5 „Cu„com- pounds, that UCu5 is, in fact, an intermediate-valent compound. 3 It was argued that the drastic change in the properties of the compounds in this series implies that for 4 & x & 5 the uranium ions adopt an electron- ic configuration that is intermediate between U + and U +, fluctuating in time. Among other indications, the abrupt increase of the low-temperature electronic specific heat for x exceeding 4 was a major argument for this conclusion. The electronic specific-heat pa- rameter y was obtained from data in the temperature range between 1. 5 and 30 K by extrapolation of a c~/T vs T2 plot to T = 0 K. An anomalous increase of c~/T with decreasing temperature below 4 K was ascribed to the onset of a Schottky-type contribution to the specif- ic heat due to the spontaneous splitting of nuclear lev- els in the magnetically ordered matrix. In this Letter we demonstrate that this increase in c~/T is due to many-body effects that are now a fami- liar feature of heavy-electron materials and that UCu5 undergoes another phase transition around 1 K which is, so far, of unknown origin but, more important, in- volves the heavy-mass quasiparticles that lead to the low-temperature enhanced specific heat. To our knowledge, this is the first example of such a distinct enhancement effect that occurs in a magnetically or- dered material. Our reasoning is based on data obtained from mea- surements of the specific heat and the electrical resis- tivity that were made on well-annealed polycrystalline samples of UCuq and UAgCu4. The specific heat was measured between 0. 15 and 21 K. Specimens that were cut from the same respective buttons were used for measurements of the electrical resistivity below room temperature, extending to 0. 4 K in the case of UCu5 and to 1. 2 K for UAgCu4. In UCu5 the distance between adjacent U atoms of 4.96 A is quite large. In principle, one would there- fore expect an integral occupancy of the Sf-electron shell of the U ions. As mentioned above this was first put in question by van Daal et al. who concluded that although U in UNi5, with a U-U distance of 4. 80 A, adopts the Sf' configuration, no integral valence can be assumed for U in UCu5. In the work of Schneider et al. , 5 however, photoemission data indicate that the 5 f-electron-state occupation barely changes between UNi5 and UCu5 and these authors concluded that in both cases, hybridization effects between U 5 and Ni or Cu 3d electrons were important. There was also no evidence for two different final-state Sf' multiplets in the valence-band spectrum of UCu~ that would indi- cate valence fluctuations between U + and U + states. The experimental results that we present below also rather indicate itinerant-Sf-electron behavior that is most likely due to hybridization with 3d electrons. In Fig. 1 we show the results of our measurements of the specific heat cp of UCu5 and UAgCu4 between 0. 15 and 21 K. For temperatures above 1. 5 K we plot c~/T vs T2 and in the insets we display c~ vs T for tem- peratures below 0. 6 K. For UCu5 we confirm the data of van Daal et al. obtained for T 1. 5 K with the main features of an anomaly induced by magnetic or- dering around 15 K and the upturn of c~/T with de- creasing temperature below 4 K. Replacement of Cu by Ag obviously results in quite different changes of 1985 The American Physical Society f

physical review letters

Low-Temperature State of U<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>5</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>: Formation of Heavy Electrons in a Magnetically Ordered Material

Low-Temperature State of UCu5: Formation of Heavy Electrons in a Magnetically Ordered Material