The thermoelectric coefficients have been measured down to a very low temperature for the Yb-based heavy-fermion compounds β-YbAlB4 and YbRh2Si2, often considered as model systems for the local quantum criticality case. We observe a striking difference in the behavior of the Seebeck coefficient S in the vicinity of their respective quantum critical point (QCP). Approaching the critical field, S/T is enhanced in β-YbAlB4, but drastically reduced in YbRh2Si2. The ratio of thermopower to specific heat remains constant for β-YbAlB4, but it is significantly reduced near the QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The interplay between valence and magnetic quantum criticality and the additional possibility of a Lifshitz transition crossing the critical field under magnetic field are discussed as the origin of the different behaviors of these compounds.
The thermal conductivity measurements have been performed on the heavy-fermion compound YbRh2Si2 down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at Bc = 0.66 T c-axis. In the limit as T → 0, we find that the Wiedemann-Franz law is satisfied within experimental error at the QCP despite the destruction of the standard signature of Fermi liquid. Our results place strong constraints on models that attempt to describe the nature of unconventional quantum criticality of YbRh2Si2.PACS numbers: 71.27.+a, 71.10.Hf, 74.70.Tx A quantum critical point (QCP) defines a second-order transition at zero temperature that is driven by external parameters, such as pressure, magnetic field, and chemical substitution [1][2][3]. Near the QCP, the Fermi liquid (FL) behavior is destroyed by diverging quantum fluctuations, and in consequence anomalous properties quoted as non-Fermi liquid (NFL) behaviors show up. Given that a number of intriguing phenomena, e.g., unconventional superconductivity [4], exotic electronic states [5], and nontrivial spin states [6], are found in the vicinity of the QCP, it is essential to understand the nature of the quantum criticality.A key element in the debate about the quantum criticality in heavy fermion metals is concerning whether the quantum critical physics can be understood by the conventional spin-fluctuation theories [7][8][9], or whether a new framework, invoking the critical breakdown of the Kondo effect at the QCP is required [10,11]. A major difference of the two scenarios relies on quite different fates for the FL state. In the conventional scenario [7][8][9], fluctuations are concentrated at hot spots on the Fermi surface (FS), so that the FL state is retained on part of the FS at the QCP. In the Kondo breakdown scenario [10,11], by contrast, fluctuations are thought to cover the entire FS, leading to a strong breakdown of the quasiparticle picture [10,11]. What is desperately needed for differentiating between the two scenarios is therefore to conclusively determine whether the heavy quasiparticles survive at the QCP.A crucial test of this issue is a verification of the Wiedemann-Franz law at the QCP, which states that the ratio of the thermal conductivity κ to the electrical conductivity σ is a universal constant in the T → 0 limit:WΩ/K 2 is the Sommerfeld value. A violation of this law would imply a profound breakdown of the FL theory [12][13][14][15]. Experimentally, however, the Wiedemann-Franz law appears to be universal as T → 0 and no material has been reported to violate this law up to date. (We note that a deviation from the Wiedemann-Franz law has been reported in CeCoIn 5 [16] as we will discuss later).In the Letter, we have chosen to study the WiedemannFranz law in YbRh 2 Si 2 , a tetragonal heavy fermion compound [17]. YbRh 2 Si 2 has provided a rare opportunity to probe the electronic properties near the QCP by using the magnetic field B as a tuning parameter. The very weak antiferromagnetic (AF) order (T N ∼ 0.07 K) is suppressed by a small...
Thermal transport measurements have been performed on single-crystalline Co-doped BaFe 2 As 2 down to 0.1 K and under magnetic fields up to 7 T. Significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity below T c as an indication of the enhancement of the quasiparticle mean-free path. Moreover, we find a sizable residual T -linear term in thermal conductivity, possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair breaking. Our findings support a pairing symmetry compatible with the theoretically predicted sign-reversing s-wave state.KEYWORDS: iron pnictide superconductor, Co-doped BaFe 2 As 2 , thermal transport, signreversing s-wave stateThe symmetry of the order parameter is essential for identifying the superconducting pairing mechanism. In conventional superconductors (e.g., Al and Pb), the effective electron interaction is mediated by phonons, which gives rise to the isotropic s-wave pairing symmetry.On the other hand, electron pairs glued by magnetic interactions form unconventional pairing states: p-, d-wave states and so on. So far, such unconventional superconducting states have been found in a number of materials on the boarder between magnetism and superconductivity. 1 These findings suggest that a system close to magnetic instability provides a fertile field for unconventional superconductivity. A new family of superconductors containing layers of iron pnictides bear resemblance to unconventional superconductors such as high-T c cuprates with a two-dimensional electronic structure and a magnetic order proximity to the superconducting phase. 2, 3 Therefore, an exotic superconducting pairing state can be naively expected in this system. In fact, an intriguing pairing state of sign-reversing s-wave symmetry has been theoretically proposed. 4,5 Here, we report the first thermal transport evidence of a novel pairing state in Co-doped BaFe 2 As 2 . In particular, we find a sizable residual T -linear term of the thermal conductivity, possibly due to the impurity-induced in-gap state. In addition, significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity originating from
We present the result of the thermal conductivity κ and the thermopower S measurements of heavy fermion compound YbRh 2 Si 2 down to 40 mK and under magnetic fields up to 5 T. In zero field, κ(T )/T shows an upturn at the antiferromagnetic (AF) transition temperature T N = 70 mK. The lorentz ratio L/L 0 shows about 0.8 down to T N and approaches to 1 as T → 0 K, indicating the dominant contribution of the electrons to the thermal conductivity. The upturn of κ(T )/T , therefore, may account for the enhancement of the mean free path of the electrons due to the AF ordering. The suppression of L/L 0 above T N suggests the presence of the inelastic scattering. On the other hand, the absolute value of S (T )/T steeply decreases at low temperatures near the quantum critical point (QCP), whereas S (T )/T becomes almost temperature-independent down to lowest temperature in the Fermi-liquid (FL) region. Moreover, the ratio of the Seebeck coefficient to the specific heat is suppressed strongly in the vicinity of the QCP while the ratio shows a constant value of the order of unity for FL region. Similar suppression of the ratio is also observed near the AF QCP in CeCoIn 5 .
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