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-based materials; A View on valence-fluctuating EuIr
SiUsachov, D. Yu*; Tarasov, A. V.*; Schulz, S.*; Bokai, K. A.*; Tupitsyn, I. I.*; Poelchen, G.*; Seiro, S.*; Caroca-Canales, N.*; Kliemt, K.*; Mende, M.*; et al.
Physical Review B, 102(20), p.205102_1 - 205102_11, 2020/11
被引用回数：9 パーセンタイル：53.43(Materials Science, Multidisciplinary)Using photoelectron diffraction (PED) and magnetic dichroism measurements, we explore the electronic and magnetic properties of the near-surface region of the valence-fluctuating material EuIrSi. We present and discuss the methodology for modeling of the 4 photoemission spectra, PED patterns and magnetic dichroism for rare-earth-based materials. For the Eu-terminated surface we found that the topmost Eu layer is divalent and exhibits a ferromagnetic order below 10 K. The valency of the next Eu layer, that is the 5th atomic layer, is about 2.8 at low temperature that is close to the valency in the bulk. The properties of the Si-terminated surface are drastically different. The first subsurface Eu layer (4th atomic layer below the surface) behaves divalently and orders ferromagnetically at much higher temperature. Experimental data indicate, however, that there is an admixture of trivalent Eu in this layer, resulting in its valency of about 2.1. The next deeper lying Eu layer (8th atomic layer below the surface) behaves mixed-valently, but the estimated valency of 2.4 is notably lower than the value in the bulk. The obtained results create a background for further studies of exotic surface properties of 4-based materials, and allow to derive information related to valency and magnetism of individual rare-earth layers in a rather extended area near the surface.
Si as a reference for the Luttinger theorem and antiferromagnetism in trivalent heavy-fermion YbRhSiG
ttler, M.*; Generalov, A.*; 藤森 伸一; Kummer, K.*; Chikina, A.*; Seiro, S.*; Danzenb
cher, S.*; Koroteev, Yu. M.*; Chulkov, E. V.*; Radovic, M.*; et al.
Nature Communications (Internet), 10, p.796_1 - 796_7, 2019/02
被引用回数：9 パーセンタイル：59.53(Multidisciplinary Sciences)Application of the Luttinger Theorem (LT) to the canonical heavy-fermion Kondo Lattice (KL) material YbRhSi suggests that its large 4f-derived Fermi surface (FS) in the paramagnetic (PM) regime should be similar in shape and volume to that of the divalent local moment anti-ferromagnet (AFM) EuRhSi in its PM regime. This leads to the tempting opportunity to explore a new experimental realization of the LT in general and how the large FS may change upon the AFM transition below 70 mK in YbRhSi in particular. A detailed knowledge of the FS reconstruction might be essential to disclose the properties of this phase, which is a precursor of quantum criticality and superconductivity. Using angle-resolved photoemission spectroscopy (ARPES), we observe a large FS for PM EuRhSi essentially the same as the one seen in YbRhSi in the KL state at a temperature of 1 K. Across the EuRhSi AFM transition we found an extensive fragmentation of the FS due to Brillouin zone folding, intersection and resulting hybridization of the Doughnut and Jungle gym Fermi-surface sheets. Our results on EuRhSi indicate that the formation of the AFM state in YbRhSi is very likely also connected with large changes in the FS, which have to be taken into account in the controversial analysis and discussion of anomalies observed at the quantum critical point in this system.
SiMufti, N.*; 金子 耕士; Hoser, A.*; Gutmann, M.*; Geibel, C.*; Krellner, C.*; Stockert, O.*
Physical Review B, 94(4), p.045116_1 - 045116_8, 2016/07
被引用回数：2 パーセンタイル：10.64(Materials Science, Multidisciplinary)We report on the results of powder and single crystal neutron diffraction to investigate the magnetic order in YbCoSi below the N
el temperature 
,K in detail. Two different magnetically ordered phases can clearly be distinguished. At lowest temperatures a commensurate magnetic structure with a propagation vector 
is found, while the intermediate phase (
,K) is characterized by an incommensurate magnetic structure with 
. The magnetic structure in YbCoSi is in marked contrast to all other known 
CoSi compounds ( = rare earth element) likely due to some itineracy of the Yb 4f states being responsible for the magnetism.
Si金子 耕士; Stockert, O.*; Mufti, N.*; Kiefer, K.*; Klingner, C.*; Krellner, C.*; Geibel, C.*; Steglich, F.*
Journal of Physics; Conference Series, 200, p.032031_1 - 032031_4, 2010/02
被引用回数：5 パーセンタイル：84.49(Materials Science, Multidisciplinary)YbCoSi with the tetragonal ThCrSi-type structure is an isoelectronic compound to YbRhSi. At zero magnetic field YbCoSi undergoes two magnetic transitions at 
=0.9,K and 
=1.7,K,and exhibits a complex magnetic phase diagram for small applied fields, lower than 
=2,T. Therefore, YbCoSi is not only a good reference to YbRhSi but also an interesting system in its own to study its magnetic properties. In order to reveal the magnetic structure of YbCoSi, a neutron powder diffraction experiment was carried out. At zero field, several superlattice reflections were clearly observed at 0.5,K. By heating up to 1.1,K, to the intermediate phase, a substantial change in both, the peak position and the intensity ratio of the magnetic reflection was found. We will propose a possible magnetic structure for both phases.
Siにおける磁気転移金子 耕士; Stockert, O.*; Mufti, N.*; Kiefer, K.*; Klingner, C.*; Krellner, C.*; Geibel, C.*; Steglich, F.*
no journal, ,
YbCoSiは、
=0.9K, 
=1.7Kの2段の磁気転移を示すことが明らかとなった。また2T以下の低磁場で多段の磁気転移を示す、複雑な磁気相図が報告された。YbCoSiの磁気構造の決定とともに、この系の磁気相互作用を明らかにする目的で、中性子回折実験を行った。実験は、ベルリンのBER-II原子炉に設置されたE2において行った。粉末中性子回折パターンから、常磁性状態(2.0K)の結果を差し引いたものにおいて、明瞭な磁気反射の観測に成功した。0.48Kで観測された磁気反射は、1.1Kへの昇温に伴い、低角側にシフトするとともに、その強度比が大きく変化することを見いだした。今回観測された磁気反射は、磁気秩序を示す他のCoSi(=希土類)において観測される100, 001とは異なっており、YbCoSiの磁気秩序相は、単純な整合反強磁性構造ではないことを示している。当日は単結晶の結果と合わせて、その磁気構造について報告する。
Si金子 耕士; Stockert, O.*; 田端 千紘; 桑原 慶太郎*; 鬼柳 亮嗣; 中尾 朗子*; Krellner, C.*; Geibel, C.*
no journal, ,
Novel physical phenomena, such as superconductivity and non-Fermi liquid behavior, emerge in the vicinity of a quantum critical point (QCP). YbRhSi has been extensively studied owing to its proximity to an antiferromagnetic (AFM) QCP at ambient pressure. The AFM order at 
= 70 mK is suppressed by a weak external magnetic field. An inelastic neutron scattering study revealed a competition between ferromagnetic and incommensurate spin fluctuations at q' close to the zone center in YbRhSi, and evolution of resonance-like response at q=0 under magnetic fields. These features suggest an importance of ferromagnetic fluctuations in this system, where as a static AFM order was not confirmed at q'. This fact indicates that the AFM order in YbRhSi may have a different characteristic wave vector. As the latest study discovers emergence of superconductivity at very low temperature, recurring interests on nature of the AFM order burst. A search for an AFM Bragg reflection in YbRhSi has extreme difficulties arising from extremely low , and correspondingly a tiny ordered moment. To overcome these difficulties, we focus on isoelectronic substitution, where substitution of Rh with Co leads to a stabilization of the magnetic order. The huge enhancement of through the Co doping provides us with an opportunity to study the magnetic order closely. Our latest single crystal neutron scattering experiment was carried out on a lower Co concentration of 0.18. A new form of the AFM ordering vector was revealed in the ground state. This provides new insights into magnetic properties of YbRhSi.
