Search

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 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]

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]

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]

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]

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]

Galvanomagnetic and thermoelectric properties including Hall effect, electrical resistivity, thermopower, and thermal conductivity of polycrystalline type-I clathrates Ba8NixSi46-x (2.6 ⩽ × ⩽ 3.8) have been studied in the 2- to 350-K temperature range. Further characterization of the electronic properties of these compounds has been performed via low-temperature specific heat and magnetic susceptibility measurements (2-300 K). The electronic band structure, density of states, dispersion curves, and Fermi surface were calculated within the full-potential local-orbital method (FPLO). These calculations have not only revealed that the electronic band structure evolves in a nonrigid manner but have also shown that the density of states at the Fermi level strongly varies with x. The variations in the band structure have been experimentally confirmed by transport properties and specific heat measurements. Regardless of the Ni content, the Fermi surface shows disconnected electron and hole sections that appear consistent with thermopower data indicating that both types of carriers contribute to the electrical conduction in these materials. Magnetic susceptibility measurements have indicated that the Ni atoms do not carry any magnetic moment in these compounds. All the investigated samples exhibit metallic-like behavior resulting in moderate thermopower values and thus, in low dimensionless thermoelectric figures of merit ZT. [ABSTRACT FROM AUTHOR]

We present a study of the evolution of magnetism from the quantum critical system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2. Single crystals of Yb(Rh1-xCox)2Si2 were grown for 0 ⩽ x ⩽ 1 and studied by means of magnetic susceptibility, electrical resistivity, and specific heat measurements, as well as photoemission spectroscopy. The results evidence a complex magnetic phase diagram, with a nonmonotonic evolution of TN and two successive transitions for some compositions resulting in two tricritical points. The strong similarity with the phase diagram of YbRh2Si2 under pressure indicates that Co substitution basically corresponds to the application of positive chemical pressure. Analysis of the data proves a strong reduction of the Kondo temperature TK with increasing Co content, TK becoming smaller than TN for x≈0.5, implying a strong localization of the 4f electrons. Furthermore, low-temperature susceptibility data confirm a competition between ferromagnetic and antiferromagnetic exchange. The series Yb(Rh1-xCox)2Si2 provides an excellent experimental opportunity to gain a deeper understanding of the magnetism at the quantum critical point in the vicinity of YbRh2Si2 where the antiferromagnetic phase disappears (TN → 0). [ABSTRACT FROM AUTHOR]

Yb(Rh1-xCox)2Si2 is a model system to address two challenging problems in the field of strongly correlated electron systems. The first is the intriguing competition between ferromagnetic (FM) and antiferromagnetic (AFM) order when approaching a magnetic quantum critical point (QCP). The second is the occurrence of magnetic order along a very hard crystalline electric field (CEF) direction, i.e., along the one with the smallest available magnetic moment. Here, we present a detailed study of the evolution of the magnetic order in this system from a FM state with moments along the very hard c direction at x=0.27 towards the yet unknown magnetic state at x=0. We first observe a transition towards an AFM canted state with decreasing x and then to a pure AFM state. This confirms that the QCP in YbRh2Si2 is AFM, but the phase diagram is very similar to those observed in some inherently FM systems like NbFe2 and CeRuPO, which suggests that the basic underlying instability might be FM. Despite the huge CEF anisotropy the ordered moment retains a component along the c axis also in the AFM state. The huge CEF anisotropy in Yb(Rh1-xCox)2Si2 excludes that this hard-axis ordering originates from a competing exchange anisotropy as often proposed for other heavy-fermion systems. Instead, it points to an order-by-disorder based mechanism. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance measurements were performed on CeCu2Si2 in the presence of a magnetic field close to the upper critical field μ0Hc2 in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature 1/T1T abruptly decreased below the SC transition temperature Tc (H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T1T was enhanced just below Tc (H) and exhibited a broad maximum when magnetic fields close to μ0Hc2 (0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below Tc (H). This enhancement of 1/T1T, which was recently observed in the organic superconductor κ - (BEDT - TTF)2 Cu (NCS)2, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase. [ABSTRACT FROM AUTHOR]

The authors discuss the article "Zeeman-Driven Lifshitz Transition: A Model for the Experimentally Observed Fermi-Surface Reconstruction in YbRh2Si2." They say that the observed Fermi-surface reconstruction and multiple energy scale in continuous suppression of antiferromagnetic (AFM) order in YbRh2Si2 showed a novel class of quantum critical point (QCP). They explore the letter by Hackl and Vojta (HV) describing the Zeeman-induced Lifshitz transition.

The site-diluted compound (Yb1-xLux)4As3 is a scarce realization of the linear Heisenberg antiferromagnet partitioned into finite-size segments and is an ideal model compound for studying field-dependent effects of quenched disorder in the one-dimensional antiferromagnets. It differentiates from the systems studied so far in two aspects--the type of randomness and the nature of the energy gap in the pure sample. We have measured the specific heat of single-crystal (Yb1-xLux)4As3 in magnetic fields up to 19.5 T. The contribution C⊔ arising from the magnetic subsystem in an applied magnetic field perpendicular to the chains is determined. Compared to pure Yb4As3, for which C⊔ indicates a gap opening, for diluted systems a nonexponential decay is found at low temperatures which is consistent with the thermodynamic scaling of the specific heat established for a Bose-glass phase. [ABSTRACT FROM AUTHOR]

We address the origin of the magnetic-field-independent -|A|T1/2 term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs1.58Se0.39, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the nonmagnetic version of the two-channel Kondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a C4 symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the low superconducting transition temperature Tc≈0.14 K of ZrAs1.58Se0.39 compared to the free-of-vacancies homologue ZrP1.54S0.46 (Tc≈3.7 K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers. [ABSTRACT FROM AUTHOR]

We investigated the superconducting order parameter of the filled skutterudite LaPt4Ge12, with a transition temperature of Tc=8.3 K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting s-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series Pr1-xLaxPt4Ge12. They suggest compatible order parameters for the two end compounds LaPt4Ge12 and PrPt4Ge12. This is not consistent with a single s-wave gap in LaPt4Ge12 considering previous reports of unconventional and/or multiband superconductivity in PrPt4Ge12. [ABSTRACT FROM AUTHOR]

71Ga NQR, magnetization, and specific-heat measurements have been performed on polycrystalline Ge-doped FeGa3 samples. A crossover from an insulator to a correlated local moment metal in the low-doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, xC=0.15, 71(1/T1T) exhibits a pronounced T-4/3 power law over two orders of magnitude in temperature, which indicates three-dimensional quantum critical ferromagnetic fluctuations. Furthermore, for the ordered x=0.2 sample (TC≈6 K), (1/T1T)71 could be fitted well in the frame of Moriya's self-consistent renormalization theory for weakly ferromagnetic systems with 1/T1T∼χ. In contrast to this, the low-doping regime nicely displays local moment behavior where 1/T1T∼χ² is valid. For T→0, the Sommerfeld ratio γ=(C/T) is enhanced (70mJ/moleK² for x=0.1), which indicates the formation of heavy 3d electrons. [ABSTRACT FROM AUTHOR]

A correction to the article "Field-tuned critical fluctuations in YFe2Al10: Evidence from magnetization, 27Al NMR, and NQR investigations" in the 2012 issue is presented.

The superconducting order parameter of the first heavy-fermion superconductor CeCu&#95;{2}Si&#95;{2} is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here, we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy-fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.

Heavy fermion compounds exhibiting a ferromagnetic quantum critical point have attracted considerable interest. Common to two known cases, i.e., CeRh&#95;{6}Ge&#95;{4} and YbNi&#95;{4}P&#95;{2}, is that the 4f moments reside along chains with a large interchain distance, exhibiting strong magnetic anisotropy that was proposed to be vital for the ferromagnetic quantum criticality. Here, we report an angle-resolved photoemission study on CeRh&#95;{6}Ge&#95;{4} in which we observe sharp momentum-dependent 4f bands and clear bending of the conduction bands near the Fermi level, indicating considerable hybridization between conduction and 4f electrons. The extracted hybridization strength is anisotropic in momentum space and is obviously stronger along the Ce chain direction.The hybridized 4f bands persist up to high temperatures, and the evolution of their intensity shows clear band dependence. Our results provide spectroscopic evidence for anisotropic hybridization between conduction and 4f electrons in CeRh&#95;{6}Ge&#95;{4}, which could be important for understanding the electronic origin of the ferromagnetic quantum criticality.

The "failed Kondo insulator" CeNiSn has long been suspected to be a nodal metal, with a node in the hybridization matrix elements. Here we carry out a series of Nernst effect experiments to delineate whether the severely anisotropic magnetotransport coefficients do indeed derive from a nodal metal or can simply be explained by a highly anisotropic Fermi surface. Our experiments reveal that despite an almost twentyfold anisotropy in the Hall conductivity, the large Nernst signal is isotropic. Taken in conjunction with the magnetotransport anisotropy, these results provide strong support for an isotropic Fermi surface with a large anisotropy in quasiparticle mass derived from a nodal hybridization.

Superconductivity in the heavy-fermion compound CeCu2Si2 is a prototypical example of Cooper pairs formed by strongly correlated electrons. For more than 30 years, it has been believed to arise from nodal d-wave pairing mediated by a magnetic glue. Here, we report a detailed study of the specific heat and magnetization at low temperatures for a high-quality single crystal. Unexpectedly, the specific-heat measurements exhibit exponential decay with a two-gap feature in its temperature dependence, along with a linear dependence as a function of magnetic field and the absence of oscillations in the field angle, reminiscent of multiband full-gap superconductivity. In addition, we find anomalous behavior at high fields, attributed to a strong Pauli paramagnetic effect. A low quasiparticle density of states at low energies with a multiband Fermi-surface topology would open a new door into electron pairing in CeCu2Si2.

A distinctly temperature-dependent Nernst coefficient, ν, which is strongly enhanced over that of LaCu(2)Si(2), is observed between T=2 and 300 K for CeCu(2)Si(2) and Ce(0.8)La(0.2)Cu(2)Si(2). The enhanced ν(T) is determined by the asymmetry of the on-site Kondo (conduction electron -4f electron) scattering rate. Taking into account the measured Hall mobility, μ(H), the highly unusual thermopower, S, of these systems can be semiquantitatively described by S(T)=-ν(T)/μ(H)(T), which explicitly demonstrates that the thermopower originates from the local Kondo scattering process over a wide temperature range from far above to well below the coherence temperature (≈20 K for CeCu(2)Si(2)). Our results suggest that the Nernst effect can act as a proper probe of local charge-carrier scattering. This promises to impact on exploring the unconventional enhancement of the thermopower in correlated materials suited for potential applications.

Thermodynamic and transport properties of the La-diluted Kondo lattice CeNi(2)Ge(2) were studied in a wide temperature range. The Ce-rich alloys Ce(1-x)La(x)Ni(2)Ge(2) were found to exhibit distinct features of the coherent heavy Fermi liquid. At intermediate compositions (0.7≤x≤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 CeNi(2)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.

We carried out specific-heat and ac-susceptibility experiments under hydrostatic pressure to investigate the interplay of spin-density-wave antiferromagnetism (A) and superconductivity (S) in single-crystalline AS-type CeCu(2)Si(2). We find evidence for a line of magnetic-field- and pressure-tuned quantum critical points in the normal state in the zero-temperature magnetic field-pressure plane. Our analysis suggests an extension of this line into the superconducting state and corroborates the close connection of the underlying mechanisms leading to the formation of the antiferromagnetic and the superconducting states in AS-type CeCu(2)Si(2)., (© 2011 American Physical Society)

We report on the magnetic excitation spectrum in the normal state of the heavy-fermion superconductor CeCu92)Si(2) on approaching the quantum critical point (QCP). The magnetic response in the superconducting state is characterized by a transfer of spectral weight to energies above a spin excitation gap. In the normal state, a slowing-down of the quasielastic magnetic response is observed, which conforms to the scaling expected for a QCP of spin-density-wave type. This interpretation is substantiated by an analysis of specific heat data and the momentum dependence of the magnetic excitation spectrum. Our study represents the first direct observation of an almost critical slowing-down of the normal state magnetic response at a QCP when suppressing superconductivity. The results strongly imply that the coupling of Cooper pairs in CeCu(2)Si(2) is mediated by overdamped spin fluctuations.

The tetragonal heavy-fermion compound CeCu₂Si₂ exhibits a superconducting ground state (S type, T(c) = 0.67 K) close to a magnetic instability. Here, we present angle-resolved resistivity measurements of the upper critical field H(c2). In-plane rotation of S-type CeCu₂Si₂ single crystals reveals a fourfold oscillation of H(c2). An extended weak-coupling BCS model for a d-wave symmetry including strong Pauli-limiting effects confirms the aforementioned angular dependence and points towards d(xy) symmetry of the order parameter.

The canonical view of heavy fermion quantum criticality assumes a single quantum critical point separating the paramagnet from the antiferromagnet. However, recent experiments on Yb-based heavy fermion compounds suggest the presence of non-Fermi liquid behavior over a finite zero-temperature region. Using detailed susceptibility and transport measurements we show that the classic quantum critical system, Ge-substituted YbRh(2)Si(2), also displays such behavior. We advance arguments that this is not due to a disorder-smeared quantum critical point, but represents a new class of metallic phase.

We present low-temperature thermopower results, S(T), on the heavy-fermion compound YbRh2Si2 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} approximately 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 YbRh2Si2.

We report on a comprehensive study of CePd(1-x)Rh(x) (0.6 0, providing evidence for the absence of a quantum critical point. Instead, a peculiar "Kondo-cluster-glass" state is found for x >or= 0.65, and the non-Fermi-liquid effects in the specific heat, ac susceptibility, and magnetization are compatible with the quantum Griffiths phase scenario.

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&#95;{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&#95;{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.

An electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to approximately 8 T reveals a strongly anisotropic signal in the Kondo state below approximately 25 K. A similarity between the T dependence of the ESR parameters and that of the specific heat and the 29Si nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions. Tuning the Kondo effect on the 4f states with magnetic fields approximately 2-8 T and temperature 2-25 K yields a gradual change of the ESR g factor and linewidth which reflects the evolution of the Kondo state in this Kondo lattice system.

The heavy-fermion metal YbRh2Si2 is studied by low-temperature magnetization M(T) and specific-heat C(T) measurements at magnetic fields close to the quantum critical point (H&#95;{c}=0.06 T, H perpendicularc). Upon approaching the instability, dM/dT is more singular than C(T), leading to a divergence of the magnetic Grüneisen ratio Gamma&#95;{mag}=-(dM/dT)/C. Within the Fermi-liquid regime, Gamma&#95;{mag}=-G&#95;{r}(H-H&#95;{c};{fit}) with G&#95;{r}=-0.30+/-0.01 and H&#95;{c};{fit}=(0.065+/-0.005) T which is consistent with scaling behavior of the specific-heat coefficient in YbRh2(Si0.95Ge0.05)&#95;{2}. The field dependence of dM/dT indicates an inflection point of the entropy as a function of magnetic field upon passing the line T;{ small star, filled}(H) previously observed in Hall and thermodynamic measurements.

The ground state properties of CeFePO, a homologue of the new high temperature superconductors RFePnO1-xFx, were studied by means of susceptibility, specific heat, resistivity, and NMR measurements on polycrystals. All the results demonstrate that this compound is a magnetically nonordered heavy fermion metal with a Kondo temperature TK approximately 10 K, a Sommerfeld coefficient gamma=700 mJ/mol K2, and a mass enhancement factor of the order of 50. Analysis of the susceptibility data and of the spin relaxation time indicates that the strong electronic correlation effects originate from the Ce-4f electrons rather than from Fe-3d electrons. An enhanced Sommerfeld-Wilson ratio R=5.5 as well as a Korringa product S0/T1TK2 approximately 0.065 well below 1 indicate the presence of ferromagnetic correlations. Therefore, CeFePO appears to be on the nonmagnetic side of a ferromagnetic instability.