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Strong electron correlations can give rise to extraordinary properties of metals with renormalized quasiparticles which are at the basis of Landau's Fermi liquid theory. Near a quantum critical point, these quasiparticles can be destroyed and non-Fermi liquid behavior ensues. YbRh$_2$Si$_2$ is a prototypical correlated metal as it exhibits quasiparticles formation, formation of Kondo lattice coherence and quasiparticle destruction at a field-induced quantum critical point. Here we show how, upon lowering the temperature, the Kondo lattice coherence develops and finally gives way to non-Fermi liquid electronic excitations. By measuring the single-particle excitations through scanning tunneling spectroscopy down to 0.3 K, we find the Kondo lattice peak emerging below the Kondo temperature $T_{\rm K} \sim$ 25 K, yet this peak displays a non-trivial temperature dependence with a strong increase around 3.3 K. At the lowest temperature and as a function of an external magnetic field, the width of this peak is minimized in the quantum critical regime. Our results provide a striking demonstration of the non-Fermi liquid electronic excitations in quantum critical metals, thereby elucidating the strange-metal phenomena that have been ubiquitously observed in strongly correlated electron materials., 7 pages, 5 figures

We investigate the influence of small amounts of Ir and La substitution on the low-temperature thermal conductivity κ and thermopower S of YbRh2Si2. While the thermal conductivity is reduced in both cases due to the introduction of additional impurities, the effect on the thermopower is non-uniform: Ir substitution leads to a reduction of the absolute value of the thermopower. This is attributed largely to an enhanced disorder scattering. On the other hand, La substitution increases the absolute thermopower values. We discuss the possibility that this is due to band structure modifications upon doping into the Yb sublattice.

We present measurements of the thermopower $S(T)$ on ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}$ between 2 K and 300 K. For low Rh content, the system behaves as a ferromagnetic Kondo system with a Curie temperature ${T}_{\mathrm{C}}$ of about 6 K, a Kondo scale smaller than ${T}_{\mathrm{C}}$, and an overall crystal electric field splitting of 210 K. As the Rh content increases ${T}_{\mathrm{C}}$ is suppressed, while the average Kondo scale gets larger. Simultaneously, the presence of different Ce environments leads to a broad distribution of local Kondo scales ranging from very small values to above 50 K. As a consequence, large thermopower values are observed over an extended temperature range. Close to the critical concentration we find power-law dependencies of $S/T$ vs $T$ down to 2 K. For a Rh content of $x=0.95$ we may show explicitly that the thermopower contains contributions from Ce sites with different local energy scales.

Low-temperature specific heat measurements on single crystalline YbRh 2 Si 2 in small magnetic fields are reported. The phase diagram around its quantum critical point is deduced which confirms the antiferromagnetic and the Fermi-liquid region as deduced from transport measurements. Furthermore, the specific heat reveals that the additional crossover line at T * which was recently observed in field-dependent transport and thermodynamic measurements is independent of the Fermi liquid regime.

Thermodynamic and transport properties of the La-diluted Kondo lattice ${\mathrm{CeNi}}_{2}{\mathrm{Ge}}_{2}$ were studied in a wide temperature range. The Ce-rich alloys ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Ni}}_{2}{\mathrm{Ge}}_{2}$ were found to exhibit distinct features of the coherent heavy Fermi liquid. At intermediate compositions ($0.7\ensuremath{\le}x\ensuremath{\le}0.9$), non-Fermi liquid properties have been observed, followed by the local Fermi liquid behavior in the dilute limit. The $4f$-electron contribution to the specific heat was found to follow the predictions of the Kondo-impurity model in both the local as well as the coherent regimes, with the characteristic Kondo temperature decreasing rapidly from about 30 K for the parent compound ${\mathrm{CeNi}}_{2}{\mathrm{Ge}}_{2}$ to about 1 K in the most dilute samples. The specific heat does not show any evidence for the emergence of a new characteristic energy scale related to the formation of the coherent Kondo lattice.

A quantum critical point (QCP) arises at a continuous transition between competing phases at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau's Fermi liquid description, the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations which possess the quantum numbers of the bare electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh$_2$Si$_2$. We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann-Franz (WF) law above the critical field, $B_c$. This is also expected at $B < B_c$, where weak antiferromagnetic order and a Fermi liquid phase form below 0.07 K ($B = 0$). However, at the critical field the low-temperature electrical conductivity suggests a non-Fermi-liquid ground state and exceeds the thermal conductivity by about 10%. This apparent violation of the WF law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles breaks down. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations. Our finding brings new insights into understanding deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials., 16 pages, 4 figures, and supplemental information

We present low-temperature thermopower results, S(T), on the heavy-fermion compound YbRh 2 Si 2 in the vicinity of its field-induced quantum critical point (QCP). At B = 0, a logarithmic increase of ―S(T)/T between 1 and 0.1 K reveals strong non-Fermi-liquid behavior. A pronounced downturn of ―S(T)/T below T max = 0.1 K and a sign change from negative to positive S(T) values at T 0 ≈ 30 mK are observed on the low-field side of the Kondo breakdown crossover line T * (B). In the field-induced, heavy Landau-Fermi-liquid regime, S(T)/T assumes constant, negative values below T LFL . A pronounced crossover in the ―S(B)/T isotherms at T * (B) sharpens with decreasing T and seems to evolve toward a steplike function for T → 0. This is attributed to an abrupt change of the Fermi volume upon crossing the unconventional QCP of YbRh 2 Si 2 .

We report on precise low-temperature specific-heat measurements, C(T), of YbRh2Si2 in the vicinity of the antiferromagnetic phase transition on a single crystal of superior quality (residual resistivity ratio of approximately 150). We observe a very sharp peak at T_{N}=72 mK with absolute values as high as C/T=8 J/mol K2. A detailed analysis of the critical exponent alpha around T_{N} reveals alpha=0.38 which differs significantly from those of the conventional universality classes in the Ginzburg-Landau theory, where alpha< or =0.11. Thermal-expansion measurements corroborate this large positive critical exponent. These results provide insight into the nature of the critical magnetic fluctuations at a temperature-driven phase transition close to a quantum critical point.

In the search for effective thermoelectric materials the class of strongly correlated electron systems has become one the main research topics for lowtemperature Peltier cooling applications. In this paper we present results for a number of different compounds which exhibit strong correlations. The described heavy fermion metals have a particularly large thermopower S(T ) below 30 K. The enhancement of the thermoelectric power is related to the large degeneracy of the ground-state f multiplet. The maximum in S(T ) can be shifted by volume changes which cause a change of the hybridization. Some correlated semiconductors which have a huge thermoelectric power at low temperatures are also described. As an example, we consider FeSb2 which has a thermoelectric power exceeding −30,000μV/K at 10 K. Due to its semiconducting behavior, this material exhibits a record power factor PF = S2σ , where σ is the electrical conductivity.

The heavy-fermion compound YbRh2Si2 exhibits an antiferromagnetic (AFM) phase transition at an extremely low temperature of TN = 70 mK. Upon applying a tiny magnetic field of Bc = 60 mT the AFM ordering is suppressed and the system is driven toward a field-induced quantum critical point (QCP). Here, we present low-temperature thermopower S(T) measurements of high-quality YbRh2Si2 single crystals down to 30 mK. S(T) is found negative with comparably large values in the paramagnetic state. In zero field no Landau-Fermi-liquid (LFL) like behavior is observed within the magnetically ordered phase. However, a sign change from negative to positive appears at lowest temperatures on the magnetic side of the QCP. For higher fields B > Bc a linear extrapolation of S to zero clearly evidences the recovery of LFL regime. The crossover temperature is sharply determined and coincides perfectly with the one derived from resistivity and specific heat investigations., LT25 conference proceedings in Journal of Physics: Conference Series

We report on thermal transport properties of the heavy-fermion compound YbRh 2 Si 2 . Thermal conductivity κ and thermopower S measurements on single-crystalline samples of YbRh 2 ( Si 1 - x Ge x ) 2 with x = 0 and 0.05 in the temperature range between 2 and 300 K are presented. In this region, the transition from incoherent to coherent Kondo scattering and crystal electric field excitations play a major role in the transport and scattering processes of the system.

We present results of high-resolution, low-temperature measurements of the Hall coefficient, thermopower, and specific heat on stoichiometric YbRh2Si2. They support earlier conclusions of an electronic (Kondo-breakdown) quantum critical point concurring with a field induced antiferromagnetic one. We also discuss the detachment of the two instabilities under chemical pressure. Volume compression/expansion (via substituting Rh by Co/Ir) results in a stabilization/weakening of magnetic order. Moderate Ir substitution leads to a non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor screened by the Kondo effect. The so-derived zero-temperature global phase diagram promises future studies to explore the nature of the Kondo breakdown quantum critical point without any interfering magnetism., minor changes, accepted for publication in JPSJ

Large thermopower values at low temperatures are commonly observed for heavy fermion systems. Their thermopower is similarly enhanced compared to ordinary metals as the Sommerfeld coefficient of the electronic specific heat. Here, we present low-T resistivity ?(T) and thermopower S(T) results of CeTiGe and Ce1?xLaxTiGe which show enhanced absolute thermopower values of 60-70 ?V/K around 15 K. CeTiGe has been identified to exhibit the full Ce J = 5/2 multiplet ground state. The strongly enhanced thermopowers at low temperatures is, thus, attributed to the highly degenerate ground state. Upon substituting La on the Ce site, we study the effect of negative chemical pressure on the Kondo scale of CeTiGe. Combined with the metallic resistivity ?, the power factor PF = S2/?, which is of interest for thermoelectric applications, is in the same order as common thermoelectric materials.

Investigations of the susceptibility, electrical resistivity, specific heat and thermopower of CeTiGe at low temperatures show that this compound is a Kondo lattice system with an enhanced Sommerfeld coefficient γ≈0.3 J K(-2) mol(-1) and where the whole J = 5/2 multiplet is involved in the formation of the ground state. No magnetic order was observed down to 0.4 K. In the temperature range below 10 K we observed Fermi-liquid behavior as indicated by a ρ(T)∼T(2) dependence in the electrical resistivity and a linear specific heat and thermopower. Because of these results we classify CeTiGe as a moderate heavy-fermion system with a non-magnetic ground state.