PHYSICAL REVIEW B 83, 245117 (2011) Thermoelectric evidence for high-field anomalies in the hidden order phase of URu 2 Si 2 Liam Malone, 1 Tatsuma D. Matusda, 2,3 Arlei Antunes, 4 Georg Knebel, 2 Valentin Taufour, 2 Dai Aoki, 2 Kamran Behnia, 5 Cyril Proust, 1 and Jacques Flouquet 2 1 Laboratoire National des Champs Magn´ etiques Intenses, UPR 3228 (CNRS-INSA-UJF-UPS), Toulouse F-31400, France 2 INAC, SPSMS, CEA Grenoble, 17 Rue des Martyrs, F-38054 Grenoble, France 3 ASRC, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, JAPAN 4 LNCMI, CNRS, 25 Rue des Martyrs, F-38042 Grenoble, France 5 Laboratoire Photons Et Matirere (UPMC-CNRS), ESPCI, F-75005 Paris, France (Received 20 December 2010; revised manuscript received 9 May 2011; published 22 June 2011) Measurements of the thermoelectric coefficients of URu 2 Si 2 in a high magnetic field could imply topological Fermi surface changes even deep inside the hidden order state. A change is observed in the thermopower as a function of the field applied parallel to the easy axis of magnetization, which could signify a change in the Fermi surface characteristics. The maximum of the thermopower coincides with previously measured anomalies in resistivity at H ∗ = 23 T. We analyze our results in terms of a Lifshitz transition in URu 2 Si 2 originating from the hidden order Pauli depairing on a given subband. DOI: 10.1103/PhysRevB.83.245117 PACS number(s): 71.27.+a, 74.70.Tx I. INTRODUCTION At T 0 ∼ 17 K, URu 2 Si 2 undergoes a second order phase transition into a hidden order (HO) state. 1 This transition demonstrates a large drop in entropy, but to date no order parameter has been conclusively observed. There are various theoretical suggestions 2–8 for the hidden order parameter. Transport and thermodynamic probes point to a significant Fermi surface reconstruction at T 0 (Refs. 9–11), which leads to a large drop in carrier density. This decrease in carrier density corresponds to a decreased Fermi temperature and furthermore this weakness is reinforced by the large effective mass of the quasiparticles. Thus a moderate magnetic field (H ) field can have a large effect on the Fermi surface characteristics. In addition, a magnetic field, applied parallel to the c axis, suppresses the hidden order and induces a cascade of transitions above 35 T (Ref. 12) ending with a paramagnetic metal above 40 T. Recent resistivity and Hall measurements 13 on high-quality single crystals have revealed anomalies and additional quantum oscillation frequencies at H ∗ ∼ 23 T implying additional structure in the HO phase or even a possible phase transition. Thermodynamic measurements 14,15 show no evidence of anomalies in this field range. This may point to a more subtle topological change in the Fermi surface or “Lifshitz transition.” If this is the case then the interplay between the Fermi surface, the HO order parameter and magnetic field could lead to fascinating insights into this compound and a better understanding of Fermi surface changes in the presence of strong correlations. Measurements of the thermoelectric coefficients at low temperature can provide valuable information of the Fermi surface characteristics. Measurements of thermoelectric power on compounds such as CeColn 5 (Ref. 16) and YbRh 2 Si 2 (Ref. 17) have helped elucidate the nature of the electronic state. In the case of URu 2 Si 2 , previous measurements have concentrated on a large increase in the thermoelectric power and Nernst signal on entering the hidden order state, 10,18 which is a result of the Fermi surface reconstruction and on the non-Fermi liquid-like behavior observed for currents applied along the a axis. 19 However, no previous study has combined low temperature and high field to study H ∗ . Since Lifshitz 20 first proposed the “2.5” type transition, there have been several theoretical 21,22 and experimental studies 23 of thermopower around a topological transition which show the thermopower is very sensitive to Fermi surface changes. An early proof of this has been given in a study of CeRu 2 Si 2 through its pseudometamagnetic transition. 24 Recent measurements of thermopower 25 on the high temperature cuprates are another example of this and have provided evidence for previously unexpected changes in the Fermi surface. Figure 1 shows a schematic temperature-field phase di- agram for URu 2 Si 2 when the field is applied along the c axis. 15,26–28 The hidden order state can be suppressed with a field of 35 T. At this point several unknown phases (possibly due to the quantum critical point created when suppressing the HO state to T = 0) are observed and then at higher fields a paramagnetic metal is recovered. Recent resistivity and Hall measurements at low temperatures (down to 250 mK) show anomalies at H ∗ ∼ 23 T (Ref. 13). Several transport studies of the Fermi surface of URu 2 Si 2 have been performed. 26,29–31 However, despite recent progress 31 they cannot account for the correlated Sommerfeld coefficient observed by heat capacity, implying there are still some unobserved Fermi surface branches. The angle depen- dence of the frequencies implies a roughly isotropic Fermi tem- perature. Angle-resolved photoemission experiments 32 report a significant Fermi surface change through T 0 due to the heavy electron bands crossing the Fermi surface, in good agreement with a symmetry change inside the tetragonal class. 2,3,8,31 In this paper we report measurements of the thermopower of URu 2 Si 2 in magnetic fields up to 28 T to observe H ∗ .A change in thermopower is observed approaching H ∗ when the field is applied along the c axis of the tetragonal crystal. These results could be explained by a field-induced Lifshitz transition at H ∗ originating from a specific sheet of the Fermi surface. Complimentary measurements with the field applied along the a axis show little change as a function of field. This raises 245117-1 1098-0121/2011/83(24)/245117(5) ©2011 American Physical Society