Quantum phase transition in the heavy-fermion compoundYbIr2Si2

Abstract

We investigate the pressure-temperature phase diagram of YbIr2Si2 by measuring the electrical resistivity ρ(T ). In contrast to the widely investigated YbRh2Si2, YbIr2Si2 is a paramagnetic metal below pc ≃ 8 GPa. Interestingly, a first-order, presumably ferromagnetic, transition develops at pc. Similar magnetic properties were also observed in YbRh2Si2 and YbCu2Si2 at sufficiently high pressures, suggesting a uniform picture for these Yb compounds. The ground state of YbIr2Si2 under pressure can be described by Landau Fermi-liquid (LFL) theory, in agreement with the nearly ferromagnetic Fermi-liquid (NFFL) model. Moreover, evidence of a weak valence transition, characterized by a jump of the Kadowaki-Woods (KW) ratio as well as an enhancement of the residual resistivity ρ0 and of the quasiparticle-quasiparticle scattering cross section, is observed around 6 GPa. The study of quantum critical phenomena has attracted considerable attention because of the fascinating physical properties caused by quantum fluctuations. In the Ce-based heavy fermion (HF) systems, unconventional superconductivity, most likely paired via antiferromagnetic (AFM) spin fluctuations, has been widely observed around a spin-density-wave (SDW) type quantum critical point (QCP) [1]. On the other hand, YbRh 2 Si 2 has been established as a model system to study quantum physics at a "local" QCP [2], around which no superconductivity has yet been observed at T > 10 mK. Recent efforts have been largely concentrated on weakly first-order quantum phase transitions (QPT), e.g., the ferromagnetic (FM) transition in MnSi [3,4], the metamagnetic transition in Sr 3 Ru 2 O 7 [5], and the valence transition inThe HF compound YbRh 2 Si 2 undergoes an AFM transition at T N = 70 mK [7]. A small magnetic field or a slight expansion of the unit cell by substituting Si with Ge can eventually suppress the AFM order at a QCP [2,8], at which the conventional LFL theory breaks down (see the inset of Fig. 1). As tuning away from the QCP, LFL behavior immediately recovers at the lowest temperature. On the other hand, the weak AFM transition in YbRh 2 Si 2 is stabilized by applying pressure [7,9,10]. In particular, the magnetic phase undergoes a first-order transition from a small-moment state (AFM-type) to a large-moment state around 10 GPa [9]. Furthermore, it was argued that in YbRh 2 Si 2 FM quantum critical fluctuations dominate over a wide range in the phase diagram except for the close vicinity of the AFM QCP [11,12]. In order to better understand the nature of the "local" QCP, YbIr 2 Si 2 , a sister compound of YbRh 2 Si 2 , was recently synthesized by Hossain et al. [13]. YbIr 2 Si 2 (I-type) is a moderate HF compound with a paramagnetic ground state at zero pressure and, therefore, was expected to cross a magnetic QCP by applying a small pressure of . A large-moment order, presumably FM-type, develops at a critical pressure pc. The ground-state properties in both the high-p phase and the paramagnetic state can be described by LFL theory. Uniquely, in YbRh2Si2...

Pressure-induced Magnetic Transition in a Single Crystal of YbCo2Zn20

“…As a result, we conclude that the magnetic ordering transition occurs at T M above P c . It is worth noting that P c ($ 1 GPa) is considerably lower than that of other Yb compounds, [3][4][5][6][7] which is an advantage in investigating quantum critical phenomena. We believe that the relatively smaller P c in YbCo 2 Zn 20 is simply due to its proximity to the magnetic QCP at ambient pressure, as inferred from the small T K .…”

“…[3][4][5][6][7] The key point here is that Yb ions fluctuate between the nonmagnetic Yb 2þ (J ¼ 0) and the magnetic Yb 3þ (J ¼ 7=2) states. Since the ionic volume of the magnetic Yb 3þ state is smaller than that of the nonmagnetic Yb 2þ one, applying pressure stabilizes the magnetic Yb 3þ configuration and induces the appearance of a magnetically ordered state in contrast to the Ce case.…”

Metallic quantum ferromagnets

“…The former is magnetically (presumably AFM) ordered below 0.7 K, whereas the latter is a paramagnet at ambient pressure. Yuan et al (2006) found that by applying pressure the system in its P -type structure undergoes a first-order QPT to an ordered phase at a critical pressure p c ≈ 8 GPa. The nature of the ordered phase is suspected to be FM, but additional investigations are needed.…”

Ferromagnetism in YbCu2Si2at high pressure

“…4 In other systems evidence is more indirect: FM fluctuations have been shown in YbRh 2 Si 2 5 from transport measurements it was also suggested that the order that appears at high pressure (8 GPa) in YbRh 2 Si 2 and YbIr 2 Si 2 might be FM. 6 Magnetoresistance at high pressures also point to possible FM correlations and order in YbNi 2 Ge 2 .…”