Low-temperature state of UCu5: Formation of heavy electrons in a magnetically ordered material.
Low-temperature state of UCu5: Formation of heavy electrons in a magnetically ordered material.
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DOI：
10.1103/physrevlett.55.1595
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发表时间：
1985-10
影响因子：
8.6
通讯作者：
Ott;Rudigier;Felder;Fisk;Batlogg
中科院分区：
文献类型：
--
作者：
Ott;Rudigier;Felder;Fisk;Batlogg
物理评论信件卷55，编号1 S 1985年10月7日UCu 5 '的低温状态。磁有序材料H中重电子的形成。R.奥特，H。Rudigier和E. Felder Festkorperphysik，Eidgenossiche Technische Hochschule Ho-nggerberg，CH 8093 -苏黎世，瑞士Z.菲斯克材料科学与技术部，洛斯阿拉莫斯国家实验室，洛斯A/阿莫斯，新墨西哥州87545 B。Batlogg A TckT贝尔实验室，Murray Hill，新泽西07974（1985年7月8日接收）磁有序材料中重电子态的形成是通过测量UCu~ 4和UAgCu ~ 4的低温比热来确定的。在UCus中，这种状态经历连续但滞后的相变，其去除了具有高电子态密度的费米表面的部分，并导致电阻率增加几乎一个数量级。PACS编号：65.40.Em，72。15.Eb，75.50.Ee在之前的两篇文章中，UCuq被确定为15 K。.这一结论是基于中子衍射磁化率测量和后来的具体实验测量数据得出的。热和电阻率证实了相变，但也导致了结论，在对UNi 5·Cu·化合物的更一般的研究过程中，UCu 5实际上是中间价化合物。[3]有人认为，这一系列化合物性质的急剧变化意味着，对于4 & x & 5，铀离子采取了一种电子构型，它介于U +和U +之间，随时间波动。除其他迹象外，低温电子比热的突然增加x超过4是这一结论的主要论据。电子比热参数y是从1.通过将c~/T与T ~ 2的关系曲线外推到T = 0 K，得到了温度为5 K和30 K时的C~/T曲线。c~/T在4K以下随温度降低而异常增加，这是由于磁有序基体中原子核能级的自发分裂对比热的Schottky型贡献的开始。在这封信中，我们证明了c~/T的增加是由于多体效应，这是现在重电子材料的一个常见特征，UCu 5在1 K附近经历了另一个相变，到目前为止，这是未知的原因，但更重要的是，涉及导致低温增强比热的重质量准粒子。据我们所知，这是在磁性有序材料中发生的这种明显增强效应的第一个例子。我们的推理是基于从测量的比热和电阻率的UCuq和UAgCu 4的良好退火的多晶样品上获得的数据。比热在0. 15和21 K。从相同的相应按钮切割的样品用于测量低于室温的电阻率，延伸至0。4 K的情况下的UCu 5和1。UAgCu 4为2K。在UCu 5中，相邻U原子之间的距离为4.96 A，这是相当大的。原则上，人们会因此期望U离子的Sf电子壳层的积分占据。如上所述，这首先是由货车Daal等人提出的问题，他们得出结论，虽然U在UNi 5中，U-U距离为4。80 A，采用Sf'构型，UCu_5中的U不能假定为整数价。然而，在Schneider等人的工作中，5光电发射数据表明，5 f电子态占据在UNi 5和UCu 5之间几乎没有变化，这些作者得出结论，在这两种情况下，U 5和Ni或Cu 3d电子之间的杂化效应是重要的。也没有证据表明UCu~+的价带谱中存在两个不同的末态Sf'多重态，这将指示U ~+和U ~+态之间的价态涨落。我们下面给出的实验结果也表明了巡游Sf电子的行为，这很可能是由于与3d电子的杂化。在图1中，我们显示了我们对UCu 5和UAgCu 4在0. 15和21 K。对于温度高于1.我们绘制了c~/T与T2的关系图，在插图中，我们显示了温度低于0.六千对于UCu 5，我们证实了货车Daal等人获得的T1的数据。其主要特征是在15 K附近出现磁感应异常，在4 K以下c~/T随温度下降而上升。用Ag代替Cu显然导致了1985年美国物理学会f
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