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Deals with anomalies in the low-temperature specific heat and thermopower of cerium-based Kondo lattice systems. Information on cerium-based intermetallic compounds with lattice parameters; Details of normal kondo lattices; Measurements of a superconducting kondo lattice.

Historically heavy-fermion superconductors have been the first discovered unconventional superconductors and can to some extent be regarded as model systems for high- T c superconductors. A hallmark of the unconventional superconductivity in high- T c cuprates has been the observation of a spin resonance in the magnetic excitation spectrum probed by inelastic neutron scattering. We review the situation in heavy-fermion superconductors, the possible existence of a spin resonance in the superconducting state and its implications. • Spin resonances in inelastic neutron scattering are a characteristic of unconventional superconductivity. • Only very few heavy-fermion superconductors are known which show a spin resonance in superconducting state. • Presentation of heavy-fermion superconductors with spin resonance and possible dispersion of resonance mode (with figures). • Some heavy-fermion superconductors without spin resonance are mentioned and possible reasons are given. [ABSTRACT FROM AUTHOR]

Abstract: A variety of unconventional pairing mechanisms seem to be operating in heavy-fermion superconductors. We address briefly Cooper pairing mediated by magnetic excitons in UPd2Al3 and by valence fluctuations in pressurized CeCu2Si2. Moreover, evidence from inelastic neutron scattering for HF superconductivity being mediated by quantum critical spin-density-wave fluctuations in CeCu2Si2 is discussed. [Copyright &y& Elsevier]

Abstract: CeCu2Si2 plays a crucial role in studying heavy fermion superconductivity and related phenomena. Both the superconducting and normal state properties support the existence of two quantum phase transitions in pressurized CeCu2(Si1−x Ge x )2. Superconductivity in the low-pressure region occurs around an antiferromagnetic quantum critical point, indicating magnetically mediated superconductivity. A novel superconducting state, likely paired via valence fluctuations, develops around a first-order isostructural volume-collapse transition at high pressures while crossing from the heavy fermion state to the intermediate valence state. In this paper, we briefly review the recent status of CeCu2Si2, in particular its unique properties under pressure. [Copyright &y& Elsevier]

We discuss physical properties of two metallic clathrate compounds with low carrier concentration: Eu8Ga16Ge30, the first clathrate known to date, for which the ‘guest’ sites are fully occupied by ‘rattling’ lanthanide ions, and Ba6Ge25. Eu8Ga16Ge30 exists in two modifications which both contain divalent Eu ions and order ferromagnetically at low temperatures (TC=10.5 K and 36 K, respectively). In Ba6Ge25 some of the ‘rattling’ Ba atoms lock, perhaps randomly, into split positions near 200 K. A BCS-like superconducting phase transition takes place at Tc=0.14 K. Tc increases almost 20-fold, as the structural distortion is suppressed under hydrostatic pressure. [Copyright &y& Elsevier]

We report on a joint theoretical and experimental study of the vibrational dynamics of the type-I clathrates A8Sn44□2 (A = K, Rb, Cs, and □ stands for a vacancy) by high-resolution inelastic neutron scattering experiments combined with low-temperature specific heat measurements (2–300 K). Ab initio lattice dynamics calculations were performed on hypothetical vacancy-free A8Sn46 clathrates in order to determine the phonon dispersions and vibrational density of states Z (ω). The temperature dependence of the generalized vibrational density of states (GVDOS) was traced from 420 K down to 50 K, paying particular attention to the low-energy region of the GVDOS spectra. In the Cs and Rb compounds, the inelastic signal at low energies is dominated by several peaks mainly associated with the dynamics of the alkali metal atoms A in the polyhedral cages of the clathrate structure. In contrast, the low-energy spectrum of the K compound features a more pronounced contribution of the weighed modes of the framework Sn atoms. Upon cooling, the A-weighted modes soften regardless of the nature of the alkali element. The shift observed is similar for the three compounds and of small amplitude, suggestive of a dominant quasi-harmonic behavior above 50 K. The distinct dynamics of the K atoms in comparison to Cs and Rb is further demonstrated by the analyses of the low-temperature specific heat data, indicating that the low-energy Sn-weighted modes cannot be described by a Debye model with Einstein-like contributions. [ABSTRACT FROM AUTHOR]

We present investigations on the magnetic susceptibility χ( T), the specific heat coefficient γ( T) = C( T)/ T, and the spin-lattice relaxation (SLR) rate on the two isostructural iron aluminides YFe2Al10 and YbFe2Al10. Below 10 K, both systems display a monotonous field-dependent increase with decreasing temperature in χ( T), γ( T) and the SLR coefficient 27 R( T) = 27(1/ T1 T). These divergences are associated with spin fluctuations near a quantum critical point (QCP). Most probably, the QCP is of ferromagnetic (FM) nature, but so far no magnetic order is observed in either system down to 0.35 K. The application of moderate fields of a few Tesla suppresses the low- T upturn in the properties. The comparison between zero-field and high field measurements allow for the determination of enhancement factors in χ( T), the γ( T), and the spin-lattice relaxation coefficient (SLRC) 27 R( T). Surprisingly, the relative enhancement in γ( T) and the T-dependence [∝ ln(1/ T)] are very similar for both systems. In the SLRC, the enhancement is larger for the Yb-system but both show ln(1/ T) behavior for T → 0. The divergences are very similar because the Yb ions in YbFe2Al10 are in an intermediate valence state at low temperatures and contribute only very little towards the observed spin fluctuations. This indicates strongly the same underlying physics and the same energy scales for both systems. We propose the presence of dominant q = 0 low-energy FM spin fluctuations from weak itinerant Fe as the origin for the observed logarithmic divergences. [ABSTRACT FROM AUTHOR]

High-resolution measurements of the coefficient of thermal expansion, α(T), on the superconductors κ-(BEDT-TTF)2X with X=Cu[N(CN)2]Br and Cu(NCS)2 reveal distinct anomalies at T* around 40 K—the same temperature where the opening of a pseudo-gap has been inferred from anomalies in various thermal and magnetic properties. Judging from the shape of the α anomalies, we tend to assign these features to broadened second-order phase transitions. Employing a thermodynamic analysis, the signs of the uniaxial-pressure coefficients of T* can be determined and are found to be strictly anticorrelated to those of Tc. We propose that T* marks a density-wave (DW) instability involving only the small, quasi-1D portions of the Fermi surface (FS) as opposed to a pseudo-gap on the whole FS. Our scenario implies that the DW and superconductivity form on disjunct parts of the FS and compete for stability. [Copyright &y& Elsevier]

Abstract: We present the first data of physical properties of the cubic compound . An effective magnetic moment well below the full free-ion value is found. An abrupt rise in the electrical resistivity of marks the entrance into a charge ordered phase below 60K. The specific heat of has a very large peak at low temperatures. [Copyright &y& Elsevier]

The thermoelectric properties of the type-I clathrate Ba8AuxSi46-x (4.10 ≤ x ≤ 6.10) were characterized from 300 to 700 K. Increasing the Au concentration leads to a transition from an n-type (x < 5.43) to a p-type (x ≥ 5.43) electrical conduction. The experimental data are well described by a single-parabolic-band model assuming a single scattering mechanism of the charge carriers in this temperature range. The lattice thermal conductivity, inferred from degeneracy-adjusted Lorenz numbers, is low regardless of the composition. However, the measured values are significantly lower in the p-type samples possibly due to a combination of a higher degree of disorder in the crystal structure at high Au contents and an enhanced phonon-charge carrier coupling. Even though high thermopower values are achieved, the high electrical resistivity remains the main obstacle to push the dimensionless figure of merit ZT (∼0.2 at around 600 K for x = 5.59) beyond the level of the best Si-based clathrate compounds. [ABSTRACT FROM AUTHOR]

We report on the synthesis, crystallographic and transport properties of the Zintl phase BaGe5, which crystallizes in a new clathrate-type structure. This compound was synthesized by the decomposition of the type-I clathrate Ba8Ge43[larger_open_square]3 subjected to annealing treatment at 623, 673 and 793 K. Electrical resistivity, thermopower and thermal conductivity measurements were performed in the temperature range 2 - 773 K and complemented by magnetization, specific heat and Hall experiments below room temperature. Additional information on the chemical bonding and electronic band structure in BaGe5 was obtained through the electron localizability indicator (ELI) and the total density of states, all calculated within the all-electron full-potential local orbital method (FLPO). In agreement with the chemical bonding and electronic band structure calculations, electrical resistivity and specific heat data show that BaGe5 is a semiconductor. The complex crystal structure of BaGe5 contributes to the low thermal conductivity which displays a conventional crystalline-like behavior. Further measurements were carried out on samples annealed at 623, 673 and 793 K for four up to 30 days to probe possible variations of the crystal structure and electronic properties as a function of the annealing temperature and time. Even though the annealing temperature does not alter the semiconducting nature of this material, differences in the absolute values of the transport properties were unveiled in samples annealed for short-time periods. These differences are significantly reduced in samples which underwent long-time annealing treatment even though the measured curves do not merge completely. [ABSTRACT FROM AUTHOR]

Highly textured FeSb2 films were produced on quartz wafers by a sputtering method. Their resistivity and Seebeck coefficient (S) were measured and a maximum absolute value of S∼160 μV K-1 at 50 K was obtained. Hall measurements were employed to study the charge carrier concentrations and Hall mobilities of the FeSb2 films. By comparing with the transport properties of FeSb2 single crystals and an extrinsically doped FeSb1.98Te0.02 single crystal, the thermoelectric properties of the FeSb2 films are demonstrated to be dominated by the intrinsic properties of FeSb2 at a high charge carrier concentration. [ABSTRACT FROM AUTHOR]

We present thermopower measurements on Yb( Rh 1 - x Co x ) 2 Si 2 . Upon cobalt substitution, the Kondo temperature is decreasing and the single large thermopower minimum observed for YbRh 2 Si 2 splits into two minima. Simultaneously, the absolute thermopower values are strongly reduced due to a weaker exchange coupling between the 4f and the conduction electron states with increasing x. Pure YbCo 2 Si 2 is considered a stable trivalent system. Nevertheless, we still observe two minima in the thermopower indicative of weak residual Kondo scattering. This is in line with results from photoemission spectroscopy revealing a tiny contribution from Yb 2 + . The value at the high-T minimum in S(T) is found to be proportional to the Sommerfeld coefficient for the whole series. This unexpected finding is discussed in relation to recent measurements of the valence and Fermi surface evolution with temperature. [ABSTRACT FROM AUTHOR]

Presents a study which analyzed the leading non-linear term of the susceptibility of the singlet-ground-state heavy-fermion superconductors UPd[sub2]Al[sub3]. Calculation of the crystal-electric-field (CEF) approach for singlet-ground-state systems; Correlations of magneto-elastic couplings; Measurement of the CEF level scheme.

Abstract: We present low-temperature thermal expansion measurements on the Sn-substituted heavy fermion antiferromagnet CeRhIn5−x Sn x for 0⩽ x ⩽0.36 in which T N(x) is linearly suppressed from 3.8 K at x =0 to zero at x c ≈0.4. The application of the Ehrenfest relation allows to calculate the initial uniaxial and hydrostatic pressure dependences dT N/dP at various x. The observed non-linear variation with x is interpreted in terms of the Doniach diagram by an increase of the 4f-conduction electron hybridization induced by Sn-doping. As no traces of superconductivity are observed close to x c, this system is ideally suited for the study of the magnetic quantum critical point. [Copyright &y& Elsevier]

Abstract: The results of measurements of the thermopower , thermal conductivity , and Hall coefficient of CeRhIn are presented in order to correlate the behaviour in these properties with the previously reported intermediate-valent-related phenomena. Extrema are found in both and near the susceptibility-derived excitation temperature of . The Hall coefficient is found to be small and positive, suggesting a metallic-like carrier concentration. [Copyright &y& Elsevier]

SmB, a so-called Kondo insulator, is recently discussed as a candidate material for a strong topological insulator. We present detailed atomically resolved topographic information on the (0 0 1) surface from more than a dozen SmBsamples. Atomically flat,in situcleaved surfaces often exhibit B- and Sm-terminated surfaces as well as reconstructed and non-reconstructed areascoexistingon different length scales. The terminations are unambiguously identified. In addition, electronic inhomogeneities are observed which likely result from the polar nature of the (0 0 1) surface and may indicate an inhomogeneous Sm valence at the surface of SmB. In addition, atomically resolved topographies on a (1 1 0) surface are discussed. [ABSTRACT FROM AUTHOR]

Heavy fermion metals provide a prototype setting to study quantum criticality. Experimentally, quantum critical points have been identified and studied in a growing list of heavy fermion compounds. Theoretically, Kondo destruction has provided a means to characterize a class of unconventional quantum critical points that goes beyond the Landau framework of order-parameter fluctuations. Among the prominent evidence for such local quantum criticality have been measurements in YbRh 2 Si 2 . A rapid crossover is observed at finite temperatures in the isothermal field dependence of the Hall coefficient and other transport and thermodynamic quantities, which specifies a T * B line in the temperature ( T )–magnetic field ( B ) phase diagram. Here, we discuss what happens when temperature is raised, by analyzing the ratio of the crossover width to the crossover position. With this ratio approaching unity at T ≳ 0.5 K , YbRh 2 Si 2 at zero magnetic field belongs to the quantum-critical fluctuation regime, where the single-particle spectral function has significant weight at both the small and large Fermi surfaces. This implies that, in this temperature range, any measurements sensitive to the Fermi surface will also see a significant spectral weight at the large Fermi surface. The angle-resolved photoemission spectroscopy experiments recently reported for YbRh 2 Si 2 at T > 1 K are consistent with this expectation, and therefore support the association of the T * B line with the physics of Kondo destruction. [ABSTRACT FROM AUTHOR]

The superconducting pairing state of LaPt4Ge12 is studied by measuring the magnetic penetration depth λ(T,B) and superfluid density ρs(T) using a tunnel-diode-oscillator (TDO)-based method and transverse-field muon-spin rotation (TF-μSR) spectroscopy. The penetration depth follows an exponential-type temperature dependence at T«Tc, but increases linearly with magnetic field at T = 1.5 K. A detailed analysis demonstrates that both λL(T) and ρsTD0(T), measured in the Meissner state using the TDO method, are well described by a two-gap γ model with gap sizes of Δ1(0) = 1.31 kBTc and Δ2(0) = 1.80kBTc, and weak interband coupling. In contrast, ρsμSR(T), derived from the pSR data, can be fitted by a single-gap BCS model with a gap close to Δ2(0). We conclude that LaPt4Ge12 is a marginal two-gap superconductor and the small gap Δ1 seems to be suppressed by a small magnetic field applied in the μSR experiments. In comparison, the 4 f electrons in PrPt4Ge12 may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity. [ABSTRACT FROM AUTHOR]

We present measurements of the thermopower S(T) on CePd1−xRhx between 2 K and 300 K. For low Rh content, the system behaves as a ferromagnetic Kondo system with a Curie temperature TC of about 6 K, a Kondo scale smaller than TC, and an overall crystal electric field splitting of 210 K. As the Rh content increases TC 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. [ABSTRACT FROM AUTHOR]

We present the evolution of the low-temperature thermodynamic, galvanomagnetic and thermoelectric properties of the type-I clathrate Ba8NixGe46−x−y□y with the Ni concentration studied on polycrystalline samples with 0.0 ≤x≤ 6.0 by means of specific heat, Hall effect, electrical resistivity, thermopower and thermal conductivity measurements in the 2–350 K temperature range and supported by first-principles calculations. The experimental results evidence a 2a× 2a× 2a supercell described in the space group Ia3̅d for x≤ 1.0 and a primitive unit cell a×a×a (space group Pm3̅n) above this Ni content. This concentration also marks the limit between a regime where both electrons and holes contribute to the electrical conduction (x≤ 1.0) and a conventional, single-carrier regime (x ＞ 1.0). This evolution is traced by the variations in the thermopower and Hall effect with x. In agreement with band structure calculations, increasing the Ni content drives the system from a nearly-compensated semimetallic state (x = 0.0) towards a narrow-band-gap semiconducting state (x = 4.0). A crossover from an n-type to a p-type conduction occurs when crossing the x = 4.0 concentration i.e. for x = 4.1. The solid solution Ba8NixGe46−x−y□y therefore provides an excellent experimental platform to probe the evolution of the peculiar properties of the parent type-I clathrate Ba8Ge43□3 upon Ge/Ni substitution and filling up of the vacancies, which might be universal among the ternary systems at low substitution levels. [ABSTRACT FROM AUTHOR]

We present magnetization, specific heat, and 27 A1NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T ) divergence of the low -T specific heat coefficient in zerofield, which indicates a ground state of correlated electrons. From our hard-x-ray photoemission spectroscopy study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of about 3.1μB. The enhanced value of the Sommerfeld-Wilson ratio and the dynamic scaling of the spin-lattice relaxation rate divided by T[27(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. 27(1/T1T) simultaneously tracks the valence fluctuations from the 4f Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature; the latter evolve out of an Yb 4f admixed conduction band. [ABSTRACT FROM AUTHOR]

Electron-like carriers in bismuth are described by the Dirac Hamiltonian, with a band mass becoming a thousandth of the bare electron mass along one crystalline axis1. The existence of three anisotropic valleys offers electrons an additional degree of freedom, a subject of recent attention2. Here, we map the Landau spectrum by angle-resolved magnetostriction, and quantify the carrier number in each valley: while the electron valleys keep identical spectra, they substantially differ in their density of states at the Fermi level. Thus, the electron fluid does not keep the rotational symmetry of the lattice at low temperature and high magnetic field, even in the absence of internal strain. This effect, reminiscent of the Coulomb pseudogap in localized electronic states, affects only electrons in the immediate vicinity of the Fermi level. It presents the most striking departure from the non-interacting picture of electrons in bulk bismuth. [ABSTRACT FROM AUTHOR]

We report low-temperature specific heat, C(T), measurements on (Yb1-xLux)4As3, with x = 0.01 and x = 0.03, where nonmagnetic Lu atoms are randomly distributed on antiferromagnetic S = 1/2 Heisenberg chains with J / kB = 28 K. The observed reduction of C below 15 K with increasing x is accurately described by quantum transfer matrix simulations without any adjustable parameter, implying that the system is an excellent experimental realization of segmented quantum spin chains. Finite-size effects consistent with conformal-field theory predictions are leading to the formation of an effective low-energy gap. The size of the gap increases with Lu content and accounts for the impurity-driven reduction of the specific heat. For both concentrations our results verify experimentally the low-temperature scaling behavior established theoretically and also confirm the value of J determined from pure Yb4As3 [ABSTRACT FROM AUTHOR]

We present research on the superconducting properties of NbxRe1–x (x=0.13–0.38) obtained by measuring the electrical resistivity ρ(T), magnetic susceptibility χ(T), specific heat CP(T), and London penetration depth Δλ(T). It is found that the superconducting transition temperature Tc decreases monotonically with an increase of x. The upper critical field Bc2(T) for various x can be nicely scaled by its corresponding Tc. The electronic specific heat Ce(T)/T, penetration depth Δλ(T), and superfluid density ρs(T) demonstrate exponential behavior at low temperatures and can be well fitted by a one-gap BCS model. The residual Sommerfeld coefficient γ0(B) in the superconducting state follows a linear field dependence. All these properties suggest an s-wave BCS-type of superconductivity with a very large Bc2(0) for NbxRe1–x (0.13≤x≤0.38). [ABSTRACT FROM AUTHOR]

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity, and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands. [ABSTRACT FROM AUTHOR]

YbRh2Si2 is a prototypical system for studying unconventional antiferromagnetic quantum criticality. However, ferromagnetic correlations are present which can be enhanced via isoelectronic cobalt substitution for rhodium in Yb(Rh1-xCox)2Si2. So far, the magnetic order with increasing x was believed to remain antiferromagnetic. Here, we present the discovery of ferromagnetism for x = 0.27 below TC = 1.30 K in single crystalline samples. Unexpectedly, ordering occurs along the c axis, the hard crystalline electric field direction, where the g factor is an order of magnitude smaller than in the basal plane. Although the spontaneous magnetization is only 0.1 μB/Yb it corresponds to the full expected saturation moment along c taking into account partial Kondo screening. [ABSTRACT FROM AUTHOR]

We study the superconducting properties of the noncentrosymmetric compound LaNiC2 by measuring the London penetration depth Δλ(T ), the specific heat C(T, B) and the electrical resistivity ρ(T, B). Both Δλ(T ) and the electronic specific heat Ce(T ) exhibit behavior at low temperatures that can be described in terms of a phenomenological two-gap Bardeen-Cooper-Schrieffer (BCS) model. The residual Sommerfeld coefficient in the superconducting state, γ0(B), shows a rapid increase at low fields and then an eventual saturation with increasing magnetic field. A pronounced upturn curvature is observed in the upper critical field Bc2(T ) near Tc. All these experimental observations support the existence of two-gap superconductivity in LaNiC2. [ABSTRACT FROM AUTHOR]

Kondo insulators and in particular their noncubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu4Sn6 employing optical reflectivity measurements in broad frequency and temperature ranges. The low-energy optical conductivity shows semiconductor-like features within the a-a plane but a Drude-like response along the c axis, signaling weak metallicity. Local density approximation plus dynamical mean-field theory calculations reproduce these observations qualitatively and help to identify their origin. For large parts of the Brillouin zone, the strongly correlated band structure presents a narrow direct gap within the Kondo resonance. Only for the c direction, due to anisotropy in the hybridization, there is a pronounced band crossing at the Fermi level. [ABSTRACT FROM AUTHOR]

Ferromagnetic correlations are discussed on the basis of shift K4 f and spin-lattice-relaxation (SLR) rate R4 f( T) = (1/ T1 T)4 f investigations for YbNi4P2 and YbRh2Si2. For YbRh2Si2 the coexistence of antiferromagnetic (afm) and ferromagnetic (fm) fluctuations at the edge of magnetic order is strongly related to the non-Fermi-liquid behaviour (NFL). The analysis of the NMR results for YbNi4P2 in the modified Korringa framework for correlated local moment metals yields the enhancement factor $\cal K$( α) which is a measure of the q -space dependence of the complex dynamic susceptibility. Surprisingly the $\cal K$( α) behavior of YbNi4P2is very similar to YbRh2Si2and evidences strongly the evolution of an afm ( q ≠ 0) component in the q = 0 dominated complex dynamic susceptibility χ'' ( q,ωn,T) at low temperatures and in small fields. This suggests the presence of ferro- and antiferromagnetic fluctuations at the QCP and the freezing out of the afm ones by applying magnetic fields and tuning towards FL state. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

YFe2Al10 forms in a well-ordered crystal structure, with a unique site for the magnetic atom Fe. At elevated temperatures YFe2Al10 behaves electronically as a good metal. At low temperatures on the other hand, signatures of correlated behavior develop and eventually non-Fermi-liquid scaling dominates all of the physical properties: the electrical resistivity develops a negative temperature coefficient below 20 K, and the specific heat Cp( T)/ T and χ( T) both assume a power-law increase upon lowering the temperature below ≃3 K. Lowest temperature studies have failed to find magnetic ordering in YFe2Al10 in spite of compelling evidence that there is impending order in this compound and that it is indeed of ferromagnetic character. Here we report on measurements of thermal and electronic transport as well as heat capacity studies on the series of compounds YFe2 + δAl10 with δ ≤ |0.1|, with the purpose of testing the stability of the low-temperature divergences in physical properties against off-stoichiometric Fe. [ABSTRACT FROM AUTHOR]

We report measurements of the London penetration depth Δλ(T) and the electronic specific heat Ce(T) on high-quality single crystals of the filled skutterudite superconductor PrPt4Ge12 (Tc~ 8 K). Both quantities show a weak temperature dependence at T ≪ Tc, following Δ λ ~ Tn (n = 3.2 ± 0.1) and Ce/T ~ T28. Such temperature dependences deviate from both conventional s-wave-type and nodal superconductivity. A detailed analysis indicates that the superfluid density ps(T), derived from the penetration depth, as well as the electronic specific heat can be consistently described in terms of a two-gap model, providing strong evidence of multiband superconductivity for PrPt4Ge12. [ABSTRACT FROM AUTHOR]

We have investigated the optical conductivity of the prominent valence-fluctuating compounds EuIr2Si2 and EuNi2P2 in the infrared energy range to get new insights into the electronic properties of valence-fluctuating systems. For both compounds, we observe upon cooling the formation of a renormalized Drude response, a partial suppression of the optical conductivity below 100 meV, and the appearance of a midinfrared peak at 0.15 e V for EuIr2Si2 and 0.13 eV for EuNi2P2. Most remarkably, our results show a strong similarity with the optical spectra reported for many Ce- or Yb-based heavy-fermion metals and intermediate valence systems, although the phase diagrams and the temperature dependence of the valence differ strongly between Eu systems and Ce- or Yb-based systems. This suggests that the hybridization between 4f and conduction electrons, which is responsible for the properties of Ce and Yb systems, plays an important role in valence-fluctuating Eu systems. [ABSTRACT FROM AUTHOR]

We report magnetization, specific heat, and NIMR investigations on YFe2AI10 over a wide range of temperature and magnetic field and zero field (NQR) measurements. Magnetic susceptibility, specific heat, and spin-lattice relaxation rate divided by T(1/ T1 T) follow a weak power law (~[T-0.4) temperature dependence, which is a signature of the critical fluctuations of Fe moments. The value of the Sominerfeld-Wilson ratio and the linear relation between 1/ T1 T and suggest the existence of ferromagnetic correlations in this system. No magnetic ordering down to 50 mK in Cp(T)/ T and the unusual T and H scaling of the bulk and NMR data are associated with a magnetic instability which drives the system to quantum criticality. The magnetic properties of the system are tuned by field wherein ferromagnetic fluctuations are suppressed and a crossover from quantum critical to Fermi-liquid behavior is observed with increasing magnetic field. [ABSTRACT FROM AUTHOR]

The filled skutterudite compound CePt4Ge12 is situated close to the border between the intermediate valence of Ce and heavy-fermion behavior. Substitution of Ge by Sb drives the system into a strongly correlated and, ultimately, upon further increasing the Sb concentration, an antiferromagnetically ordered state. Our experiments evidence a delicate interplay of emerging Kondo physics and the formation of a local 4f moment. An extended non-Fermi-liquid region, which can be understood in the framework of a Kondo-disorder model, is observed. Band-structure calculations support the conclusion that the physical properties are governed by the interplay of electron supply via Sb substitution and the concomitant volume effects. [ABSTRACT FROM AUTHOR]

Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac susceptibility, specific heat, and muon-spin relaxation (μSR). Short-range static magnetism occurs below the freezing temperature Tg ≈ 0.7 K, which prevents the system from accessing a putative ferromagnetic quantum critical point. In the μSR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time- field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms. [ABSTRACT FROM AUTHOR]

The heavy fermion systems present a unique platform in which strong electronic correlations give rise to a host of novel, and often competing, electronic and magnetic ground states. Amongst a number of potential experimental tools at our disposal, measurements of the Hall effect have emerged as a particularly important one in discerning the nature and evolution of the Fermi surfaces of these enigmatic metals. In this article, we present a comprehensive review of Hall effect measurements in the heavy fermion materials, and examine the success it has had in contributing to our current understanding of strongly correlated matter. Particular emphasis is placed on its utility in the investigation of quantum critical phenomena which are thought to drive many of the exotic electronic ground states in these systems. This is achieved by the description of measurements of the Hall effect across the putative zero-temperature instability in the archetypal heavy fermion metal YbRh2Si2. Using the CeMIn5 (with M=Co, Ir) family of systems as a paradigm, the influence of (antiferro-)magnetic fluctuations on the Hall effect is also illustrated. This is compared to prior Hall effect measurements in the cuprates and other strongly correlated systems to emphasize on the generality of the unusual magnetotransport in materials with non-Fermi liquid behavior. [ABSTRACT FROM PUBLISHER]

We have successfully determined the hitherto unknown sign of the B44 Stevens crystal-field parameter of the tetragonal heavy-fermion compound CeCu2Si2 using vector &qcirc;-dependent nonresonant inelastic x-ray scattering experiments at the cerium N4,5 edge. The observed difference between the two different directions, &qcirc; || [100] and &qcirc; || [110], is due to the anisotropy of the crystal-field ground state in the (001) plane and is observable only because of the utilization of higher than dipole transitions possible in nonresonant inelastic x-ray scattering. This approach allows us to go beyond the specific limitations of dc magnetic susceptibility, inelastic neutron scattering, and soft x-ray spectroscopy, and provides us with a reliable information about the orbital state of the 4/ electrons relevant for the quantitative modeling of the quasiparticles and their interactions in heavy-fermion systems. [ABSTRACT FROM AUTHOR]

We studied the heavy-fermion system YbNi4P2, which presents strong ferromagnetic correlations, using the local 31P NMR probe over a wide field (0.2-8.6 T) and temperature (1.8-200 K) range. The 31P NMR Knight shift provides the static spin susceptibility which tracks the bulk susceptibility whereas the spin-lattice relaxation rate31 ( 1/T1) provides information about the fluctuations of the Yb 4f moment. The Korringa law is valid over a wide range of temperature and field. The Korringa product 31(1/T1TK2S0) ≪ 1 gives evidence for the presence of strong ferromagnetic correlations. A31(l/T1T) ∼ T-3/4 behavior was found over two decades in temperature. [ABSTRACT FROM AUTHOR]

A detailed study of the spin dynamics of the S-state ions Gd3+ and Eu2+ in the filled skutterudites Lai-jAjPtGe (R = Gd, Eu) is reported. The spin dynamics is investigated directly by means of Gd3+ and Eu2+electron spin resonance, performed at X-band (&ap;9.4 GHz) and g-band (&ap;34 GHz) frequencies in the temperature intervals 8 < T < 300 K and 1.5 < T < 300 K, respectively. The ESR parameters provide direct evidence for the vibrational behavior of the Gd3+ ions but not for the Eu2+ ions. These results are interpreted in light of recent discussions about the spin relaxation in cage systems. In particular, the Gd3+ spin relaxation in La0.9Gd0.1Pt4Ge12 provides evidence for the existence of an extra phonon mode with an Einstein temperature θE &ap; 24 K in this system. The work suggests that the so-called "rattling" modes have a general and important role that should be taken into account in the study of spin dynamics in cage systems. [ABSTRACT FROM AUTHOR]

We report on the observation of large, steplike anomalies in the magnetization (&DELTA;M = 0.74 /&MU;B/Ce), in the magnetostriction (&DELTA;l/l&RING; = 2.0 x 10 -3), and in the magnetoresi stance in polycrystals of the paramagnetic heavy-fermion system CeTiGe at a critical magnetic field ß0HC&THKAP; 12 Tat low temperatures. The size of these anomalies is much larger than those reported for the prototypical heavy-fermion metamagnet CeRu2Si2. Furthermore, hysteresis between increasing and decreasing field data indicate a real thermodynamic, first-order type of phase transition, in contrast to the crossover reported for CeRe2Si2. Analysis of the resistivity data shows a pronounced decrease of the electronic quasiparticle mass across HC. These results establish CeTiGe as a rare metamagnetic Kondo-lattiee system, with an exceptionally large, metamagnetic transition of first-order type at a moderate field. [ABSTRACT FROM AUTHOR]

We have determined the ground-state wave functions and crystal-field-level schemes of CeRh2Si2 and CeRu2Si2 using linear polarized soft x-ray-absorption spectroscopy (XAS) and inelastic neutron scattering, We find large crystal-field splittings and ground-state wave functions which are made of mainly Jz = | ±5/2) with some amount of |∓3/2) in both the compounds. The 4f° contribution to the ground state of several members of the CeM2Si2 family with M = (Cu, Ru. Rh. Pd, and Au) has been determined with XAS, and the comparison reveals a trend concerning the derealization of the f electrons. Absolute numbers are extracted from scaling to results from hard x-ray photoelectron spectroscopy on CeRu2Si2 by Yano et al. [Phys. Rev. B 77. 0351IX (2008)]. [ABSTRACT FROM AUTHOR]

The evolution of spin dynamics from the quantum critical system YbRh2Si2 to the stable bivalent Yb system YbRh2Si2 was investigated by electron spin resonance (ESR) spectroscopy. While the Kondo temperature changes by one order of magnitude, all compositions of the single-crystalline series Yb(Rh1-xCox )2Si2 show well-defined ESR spectra with a clear Yb3+ character for temperatures below &ap;20 K. With increasing Co content, the ESR g factor along the c direction strongly increases, indicating a continuous change of the ground-state wave function and, thus, a continuous change of the crystal electric field. The linewidth presents a complex dependence on the Co content and is discussed in terms of the Co-doping dependence of the Kondo interaction, the magnetic anisotropy, and the influence of ferromagnetic correlations between the 4f states. The results provide evidence that, for low Co doping, the Kondo interaction allows narrow ESR spectra despite the presence of a large magnetic anisotropy, whereas at high Co concentrations, the linewidth is controlled by ferromagnetic correlations. A pronounced broadening due to critical correlations at low temperatures is only observed at the highest Co content. This might be related to the presence of incommensurate magnetic fluctuations. [ABSTRACT FROM AUTHOR]

Hall effect and magnetoresistance measurements were carried out on a single-phase polycrystalline sample of the type-I clathrate Ba8Ge43⇔3 in the 2-350 K temperature range. These experiments were complemented by electronic band structure and Fermi surface calculations performed by an all-electron, full potential first-principles method. These calculations have revealed the presence of three pieces of the Fermi surface originating from both holelike and electronlike bands crossing the Fermi level suggesting that both types of carriers play a role in the electrical conduction of this material. These results were experimentally confirmed via a strongly temperature-dependent and dominant holelike signal in the Hall coefficient and a breakdown of Kohler's rule in the magnetoresistance data. We analyze the Hall coefficient and magnetoresistance data within the framework of a two-band model with temperature-dependent carrier concentrations and mobilities and discuss its limits in capturing the essential features governing the magnetotransport. Combined with the temperature dependences of the thermopower and the electrical resistivity, all these results suggest that Ba8Ge43⇔3 can be classified as a nearly compensated semimetal. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of the clathrate Ba8AuxSi46-x were synthesized for 0.2 ≤ x ≤ 10. The homogeneity range of the type-I clathrate phase was determined to be 3.63 ≤ x ≤ 6.10 after annealing at 900 °C, while a lower Au concentration (x ≈ 2.2) was obtained by steel-quenching. Quasisingle phase materials were obtained for 4.10 ≤ x ≤ 6.10. In this composition range, thermoelectric properties, including electrical resistivity, thermopower, and thermal conductivity, were investigated between 2 and 350 K. These experiments were complemented by low-temperature specific heat and Hall-effect measurements (2-300 K). First-principles calculations were carried out to determine the evolution of the electronic structure as a function of x. Both theoretical and experimental results evidence a progressive evolution, with the Au content, from a metallic-like behavior towards a highly doped semiconducting state which develops around x = 5.43. At this concentration, a crossover from n- to p-type conduction occurs, suggesting that the present system satisfies the Zintl-Klemm concept, which predicts a transition at x = 5.33. This crossover is traced by Hall-effect data indicating a dominant electronlike response for x ≤ 5.43, which turns into a holelike signal at higher x values. Analysis of the data based on a single-parabolic-band model under the assumption of a single scattering mechanism of the charge carriers proved to adequately describe the transport properties in the compositional range investigated. Interestingly, the temperature dependence of the lattice thermal conductivity is strongly influenced by the Au concentration: the typical behavior of crystalline insulators in the n-type compounds evolves into a glasslike dependence in the p-type samples. The series Ba8AuxSi46-x thus provides an excellent testing ground for the interplay between crystal structure, electronic properties, and lattice thermal conductivity in type-I clathrates. [ABSTRACT FROM AUTHOR]

We present a new Kondo-lattice system, YbNi4P2, which is a clean heavy-fermion metal with a severely reduced ferromagnetic (FM) ordering temperature at TC = 0.17 K, evidenced by distinct anomalies in susceptibility, specific heat and resistivity measurements. The FM nature of the transition, with only a small ordered moment of ~0.05μB, is established by a diverging susceptibility at TC with huge absolute values in the ferromagnetically ordered state, severely reduced by small magnetic fields. Furthermore, YbNi4P2 is a stoichiometric system with a quasi-one-dimensional crystal and electronic structure and strong correlation effects which dominate the low-temperature properties. This is reflected by a stronger-than-logarithmically diverging Sommerfeld coefficient and a linear-in-T resistivity above TC, which cannot be explained by current theoretical predictions. These exciting characteristics are unique among all correlated electron systems and make this an interesting material for further in-depth investigations. [ABSTRACT FROM AUTHOR]

We report on a comprehensive study of the magnetic properties of YbCo2Si2 investigated using a single crystal. All data confirm a stable trivalent Yb state carrying a well-localized 4f moment, as well as a non-magnetic Co state. We observed a sizeable anisotropy in the magnetic susceptibility χ(T ), with the moment in the basal plane twice as large as that along the c-direction. Combining this result with published neutron-scattering data puts strong constraints on the values of the crystalline electrical field (CEF) parameters. Our calculation yields one possible solution with a Γ7 CEF ground state. A peak in the easy-plane susceptibility at low temperatures demonstrates that the previously reported magnetic ordering at TN ≈ 1.65K is of antiferromagnetic (AFM) nature. In addition, we observed sharp anomalies in all measured properties, evidencing a further phase transition at TL ≈ 0.9 K, found to be first order, which corresponds to a change in the AFM structure. Both transitions are suppressed by a magnetic field B = 2 T. Below TL the specific heat, C4f, shows a well-defined T 3 dependence, as expected for AFM spin-wave excitations. Surprisingly, the resistivity, ρ(T ), also shows a T 3 power law in the same temperature range, instead of the expected T 5. This discrepancy might be related to more complex magnetic interactions, as also evidenced by the multiple transitions. By extending the linear fit of C4f/T versus T ² to T = 0, the Sommerfeld coefficient γ0 = 0.13 JK-2 mol-1 is extracted. Although magnetism in YbCo2Si2 is dominated by the RKKY exchange interaction, the enhanced 0 and a weak Kondo-like increase in ρ(T ) indicate the presence of some interactions between the conduction electrons and the 4f moments. [ABSTRACT FROM AUTHOR]

The entanglement of quantum states is both a central concept in fundamental physics and a potential tool for realizing advanced materials and applications. The quantum superpositions underlying entanglement are at the heart of the intricate interplay of localized spin states and itinerant electronic states that gives rise to the Kondo effect in certain dilute magnetic alloys. In systems where the density of localized spin states is sufficiently high, they can no longer be treated as non-interacting; if they form a dense periodic array, a Kondo lattice may be established. Such a Kondo lattice gives rise to the emergence of charge carriers with enhanced effective masses, but the precise nature of the coherent Kondo state responsible for the generation of these heavy fermions remains highly debated. Here we use atomic-resolution tunnelling spectroscopy to investigate the low-energy excitations of a generic Kondo lattice system, YbRh2Si2. We find that the hybridization of the conduction electrons with the localized 4f electrons results in a decrease in the tunnelling conductance at the Fermi energy. In addition, we observe unambiguously the crystal-field excitations of the Yb3+ ions. A strongly temperature-dependent peak in the tunnelling conductance is attributed to the Fano resonance resulting from tunnelling into the coherent heavy-fermion states that emerge at low temperature. Taken together, these features reveal how quantum coherence develops in heavy 4f-electron Kondo lattices. Our results demonstrate the efficiency of real-space electronic structure imaging for the investigation of strong electronic correlations, specifically with respect to coherence phenomena, phase coexistence and quantum criticality. [ABSTRACT FROM AUTHOR]

Abstract: The detailed growth of FeSb2 films formed on quartz (0001) substrates by magnetron sputtering is reported. FeSb2 films with different orientations and compositions can be produced by adjusting the Ar working gas pressure and the substrate temperature. By employing FeSb2 thin layers produced at different substrate temperatures as templates, <101>-, <120>- and <002>-textured FeSb2 films were produced under identical growth conditions. The thermoelectric properties of film samples grown at different temperatures were measured and the effects of Sb and FeSb impurities were investigated. [Copyright &y& Elsevier]