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Tuning the Kondo Effect in Yb (Fe1−X Cox)2 Zn20

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Tuning the Kondo Effect in Yb (Fe1−X Cox)2 Zn20 Physics and Astronomy Publications Physics and Astronomy 4-15-2017 Tuning the Kondo effect in Yb (Fe1−x Cox)2 Zn20 Tai Kong Iowa State University and Ames Laboratory Valentin Taufour Iowa State University and Ames Laboratory Sergey L. Bud’ko Iowa State University and Ames Laboratory, [email protected] Paul C. Canfield Iowa State University and Ames Laboratory, [email protected] Follow this and additional works at: https://lib.dr.iastate.edu/physastro_pubs Part of the Condensed Matter Physics Commons The complete bibliographic information for this item can be found at https://lib.dr.iastate.edu/physastro_pubs/523. For information on how to cite this item, please visit http://lib.dr.iastate.edu/howtocite.html. This Article is brought to you for free and open access by the Physics and Astronomy at Iowa State University Digital Repository. It has been accepted for inclusion in Physics and Astronomy Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Tuning the Kondo effect in Yb (Fe1−x Cox)2 Zn20 Abstract We study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe1−xCox)2Zn20 (0≤x≤1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe2Zn20 can be well described by a Kondo model with degeneracy N = 8 and a TK∼30 K. The ground state of YbCo2Zn20 is close to a Kondo state with degeneracy N = 2 and a much lower TK∼ 2 K, even though the total crystalline electric field (CEF) splittings are similar for YbFe2Zn20 and YbCo2Zn20. Upon Co substitution, the coherence temperature of YbFe2Zn20 is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4≲x≲ 0.9, the ground state remains roughly the same, which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence in resistivity data down to 500 mK within this substitution range. The coherence reappears at around x≳ 0.9 and the coherence temperature increases with higher Co concentration levels. Disciplines Condensed Matter Physics Comments This article is published as Kong, Tai, Valentin Taufour, Sergey L. Bud'ko, and Paul C. Canfield. "Tuning the Kondo effect in Yb (Fe 1− x Co x) 2 Zn 20." Physical Review B 95, no. 15 (2017): 155103. DOI: 10.1103/ PhysRevB.95.155103. Posted with permission. This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/physastro_pubs/523 PHYSICAL REVIEW B 95, 155103 (2017) Tuning the Kondo effect in Yb(Fe1−xCox)2Zn20 Tai Kong, Valentin Taufour,* Sergey L. Bud’ko, and Paul C. Canfield Ames Laboratory, U.S. Department of Energy, and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA (Received 20 September 2016; revised manuscript received 7 February 2017; published 3 April 2017) We study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe1−x Cox )2Zn20 (0 x 1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe2Zn20 can be well described by a Kondo model with degeneracy N = 8andaTK ∼ 30 K. The ground state of YbCo2Zn20 is close to a Kondo state with degeneracy N = 2 and a much lower TK ∼ 2 K, even though the total crystalline electric field (CEF) splittings are similar for YbFe2Zn20 and YbCo2Zn20. Upon Co substitution, the coherence temperature of YbFe2Zn20 is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4 x 0.9, the ground state remains roughly the same, which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence in resistivity data down to 500 mK within this substitution range. The coherence reappears at around x 0.9 and the coherence temperature increases with higher Co concentration levels. DOI: 10.1103/PhysRevB.95.155103 I. INTRODUCTION much higher critical pressure, around 10 GPa, was proposed in order to reach a QCP [12]. With an effective negative The RT M Zn (R = rare-earth elements; TM= transition 2 20 pressure induced by Cd substitution, the hybridization between metals) series of compounds was discovered two decades + Yb3 4f electrons in YbFe Zn and the conduction electrons ago, in 1997 [1]. The compounds crystallize in a cubic 2 20 becomes weaker [15]. Apart from pressure induced ordering, CeCr Al -type structure (space group Fd3¯m) where R ions 2 20 metamagnetic transitions were reported for YbCo Zn at occupy a single crystallographic site. The nearest and next- 2 20 high magnetic fields [16–19]. Crystalline electric field (CEF) nearest neighbors of R ions are all Zn and thus varying schemes for YbCo Zn have been proposed based on specific the transition metal does not significantly alter the local 2 20 heat and anisotropic magnetization measurements. The first environment of the R3+. Even though more than 85% of and the second excited CEF levels are around 10 and 25 K the atomic constituents are zinc, these compounds exhibit above the ground-state doublet [20,21]. Inelastic neutron- myriad physical properties depending on the rare earth and scattering measurements show some excitations that might be transition metal that are involved [2–6]. When the rare-earth related to these proposed CEF schemes [22]. Experimentally, element is Yb, there are six closely related YbTM Zn heavy 2 20 band structure had only been reported for YbCo Zn via fermion compounds for TM = Fe, Ru, Os, Co, Rh, Ir [6,7]. 2 20 quantum oscillations [23], with a heavy ground state being Among these six compounds, YbCo Zn has the largest 2 20 strongly suppressed by increasing field. In a zero-field limit, Sommerfeld coefficient, γ ∼ 7900 mJ/mol K2. This value is the mass of the quasiparticles was extrapolated to be 100–500 comparable to the record holding YbBiPt [8], and is more than times the free-electron mass. an order of magnitude larger than the other members in this 2 Given that (i) the first- and second-nearest neighbors of family, which have γ values ranging from 520 mJ/mol K + Yb3 in YbTM Zn do not have TMand (ii) as TMchanges for YbFe Zn to 740 mJ/mol K2 for YbRh Zn [6]. The 2 20 2 20 2 20 from Fe to Co the values of T and γ change by an order of reason behind the dramatic difference between YbCo Zn K 2 20 magnitude, it is of interest to see how the strongly correlated and the other five YbTM2Zn20 compounds is still not clear, electron state evolves in Yb(Fe − Co ) Zn for 0 x 1. In although band-structure calculations reveal that the 4f level 1 x x 2 20 this paper, we report the temperature-dependent resistivity and is closer to the Fermi level in YbCo2Zn20 than in YbFe2Zn20. specific heat data on 19 members of the Yb(Fe − Co ) Zn Upon Co substitution, the d band is gradually filled, which is 1 x x 2 20 series and track the effects of band filling and disorder on accompanied by a drop in d-band energy [9]. the coherence and Kondo temperatures as well as amounts Having the largest γ value among all the YbTM Zn 2 20 of entropy removed by thermal depopulation of CEF levels compounds, YbCo Zn has been studied intensively ever 2 20 versus Kondo state. since its discovery [6]. Upon application of pressure, there is an indication that the heavy Fermi-liquid regime can be sup- II. EXPERIMENTAL METHODS pressed, followed by the appearance of an antiferromagnetic ordering for P>1GPa[10]. It has, therefore, been argued that Single crystals were grown using a high-temperature solu- YbCo2Zn20 is close to a quantum critical point (QCP) [10,11]. tion growth technique [2,6]. The starting molar stoichiometry Similar suppressions of the Fermi-liquid regime under pressure was Yb:TM:Zn = 2:4:94. Bulk elemental material [Yb from were also observed for TM= Fe/Rh/Ir [12–14]. In the case of Ames Laboratory Material Preparation Center (99.9% abso- 2 YbFe2Zn20, γ = 520 mJ/mol K and is thus likely to be further lute purity); Fe (99.98%), Co (99.9+%), and Zn (99.999%) away from a QCP in terms of the Doniach diagram; indeed, a from Alfa Aesar] was packed in a frit-disc crucible set [24] and sealed in a silica tube under ∼0.25 bar of Ar atmosphere. The ampoule assembly was then heated up to 900◦C over ◦ ◦ *Current address: Department of Physics, University of California 3 h; dwelt at 900 C for 10 h, and then cooled to 600 C Davis, Davis, CA 95616, USA. over 100 h. At 600◦C, the remaining Zn rich solution was 2469-9950/2017/95(15)/155103(8) 155103-1 ©2017 American Physical Society KONG, TAUFOUR, BUD’KO, AND CANFIELD PHYSICAL REVIEW B 95, 155103 (2017) 1.0 value of the WDS determined concentration changes close to linearly with the nominal concentration, in the middle of the substitution range, the variation of the substitution level 0.8 is large. The variation is based on WDS results measured at different spots on the sample as well as different samples 0.6 in the same batch. Near the two ends of the series, the variation in substitution level is significantly smaller. The Co 0.4 concentrations that are used to denote each batch of sample x (WDS) in the following text are the averaged values of measured concentrations.
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