Search

The clathrate I Ba8Ge43□3 [space group $$ Ia\bar{3}d $$ , no. 230, a = 21.307(1) A] has been synthesized as a single phase and characterized by x-ray powder diffraction and metallographic analysis. Electrical and thermal transport measurements have been performed in the temperature range of 5 K to 673 K. Ba8Ge43□3 displays the electrical resistivity of a poor metal at low temperatures, with semiconducting-like behavior appearing above 300 K.

We report low-temperature measurements of the electrical resistivity of Yb ( Rh 1 - x Co x ) 2 Si 2 single crystals with 0 ≤ x ≤ 0.12 . The isoelectronic substitution of Co on the Rh site leads to a decrease of the unit cell volume which stabilizes the antiferromagnetism. Consequently, the antiferromagnetic transition temperature increases upon Co substitution. For x = 0.07 Co content a subsequent low-temperature transition is observed in agreement with previous susceptibility measurements and results on YbRh 2 Si 2 under hydrostatic pressure. Above the Neel transition the resistivity follows a linear temperature dependence of a non-Fermi liquid similar to that of YbRh 2 Si 2 .

The new example of a ferromagnetic Kondo lattice, CeRuPO, is a good candidate which offers the opportunity to investigate the physical properties near a ferromagnetic instability. Macroscopic experiments evidenced a Kondo temperature T K = 10 K and Curie temperature of T C = 14 K . We have investigated the pressure–temperature phase diagram of CeRuPO by means of electrical resistivity measurements on high quality single crystals in the temperature interval from 1.8 to 300 K. The pressure dependence of the ordering temperature follows the anticipated behavior of a Ce-based Kondo lattice system, where pressure is expected to suppress the magnetic order. The critical pressure for the suppression of the transition temperature to zero is estimated to be p c ≈ 3.2 GPa .

A variety of unconventional pairing mechanisms seem to be operating in heavy-fermion superconductors. We address briefly Cooper pairing mediated by magnetic excitons in UPd 2 Al 3 and by valence fluctuations in pressurized CeCu 2 Si 2 . Moreover, evidence from inelastic neutron scattering for HF superconductivity being mediated by quantum critical spin-density-wave fluctuations in CeCu 2 Si 2 is discussed.

We report on inelastic neutron scattering experiments on the heavy-fermion superconductor CeCu2Si2 to study the magnetic excitation spectrum in single crystals. Elastic short-range magnetic correlations have been observed in the superconducting state. While the low-energy magnetic response seems to be quasielastic in the normal state above T c ≈ 600 mK , a truly inelastic response is detected in the superconducting state well below T c . This inelastic response is discussed in connection to the properties of the superconducting state.

CeCu 2 Si 2 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 CeCu 2 (Si 1− 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 CeCu 2 Si 2 , in particular its unique properties under pressure.

In this paper, a brief survey is given on a number of research activities devoted to exploring the relationship between superconductivity and magnetism in f-electron systems. The starting point for these activities has been the pioneering work of 1958 by Matthias and coworkers illustrating the antagonistic nature of the two phenomena. Subsequent efforts concerned the investigation of Kondo superconductors and Kondo-lattice systems (in the 1970s), heavy-fermion metals (in the 1980s and 90s) and quantum critical materials (in the last decade). The latter systems are especially interesting as they promise a deeper insight into the mutual interplay between unconventional superconductivity and magnetism.

In our search for clathrates containing rare-earth elements we have discovered the three new clathrate-I phases K 6 Eu 2 Ga 10 Ge 36 , K 6 Eu 2 Zn 5 Ge 41 , and K 6 Eu 2 Cd 5 Ge 41 . The Eu atoms occupy only the smaller cages (pentagondodecahedra) leading to a fully ordered arrangement of the guest atoms. X-ray absorption spectroscopy and magnetic susceptibility measurements reveal that at room temperature and above Eu is close to divalent in all compounds. Magnetization measurements at low temperatures, however, may indicate slightly reduced magnetic moments for K 6 Eu 2 Ga 10 Ge 36 and K 6 Eu 2 Zn 5 Ge 41 . The ferromagnetic ordering temperatures of 3.8, 4.0, and 9.3 K for K 6 Eu 2 Ga 10 Ge 36 , K 6 Eu 2 Zn 5 Ge 41 , and K 6 Eu 2 Cd 5 Ge 41 , respectively, are considerably reduced with respect to the 36 K observed for the type-I clathrate β - Eu 8 Ga 16 Ge 30 where all cages are occupied by Eu.

Recently it has been shown that at any quantum critical point (QCP) thermal expansion is more singular than specific heat and that the resulting Gruneisen ratio divergence characterizes the nature of the critical fluctuations. Here, we report a comparative study on heavy fermion and d-metal systems. In Ce 0.98 Ni 2 + x Ge 2 - x polycrystals ( 0 ⩽ x ⩽ 0.015 ) an extreme sensitivity of the ground-state behavior on small changes of composition is observed. For YbRh 2 ( Si 1 - x Ge x ) 2 the Gruneisen ratio changes sign between the antiferromagnetic and paramagnetic state and diverges with a fractional exponent in the quantum critical regime. In the alloy Ni x Pd 1 - x the absence of a Gruneisen ratio divergence is incompatible with a ferromagnetic QCP.

Early results on the dilute Kondo alloy (La, Ce)Al2 as well as on the Kondo-lattice system CeAl2 are discussed, before previous and more recent investigations on CeCu2Si2, the first heavy-fermion superconductor, are addressed. After a brief review of the highly unusual properties of the non-Fermi-liquid metal YbRh2Si2, a short remark concerning the interplay between quantum criticality and heavy-fermion superconductivity concludes the paper.

Zero-field μ SR measurements have been performed on CeCu2Si2 single crystals. Antiferromagnetic order shows in an initial rapid damping of the μ + polarization. In a single crystal exhibiting antiferromagnetic order and superconductivity, there exists clear evidence of phase separation of both phenomena with a first order transition from antiferromagnetism at higher temperature to superconductivity at low temperature.

We discuss first the physical properties of the prototypical heavy-fermion superconductor CeCu 2 Si 2 and, second, the relationship of its superconductivity with both a spin-density-wave-type quantum phase transition and a weak valence transition of Ce that occurs at high pressure. We, then, briefly address the isostructural heavy-fermion compound YbRh 2 Si 2 , displaying a local type of quantum critical behavior which appears to be detrimental to superconductivity.

We report on extensive neutron diffraction on single crystalline CeCu 2 ( Si 1 - x Ge x ) 2 to determine the antiferromagnetic order in the regime with low Ge content, initially called A phase due to the unknown origin of the magnetic order. Starting from the x = 0.45 alloy we followed the magnetic order to lower Ge concentrations and to pure CeCu 2 Si 2 . For all Ge concentrations x ⩽ 1 an incommensurate magnetic order with a propagation vector close to τ = ( 0.25 0.25 0.5 ) is observed. The magnetic order is determined by the nesting properties of the Fermi surface.

The first 1 H, 2 D, 63 Cu NMR experiments in pulsed high field magnets are reported. The resonances were observed near the field maximum. Examples at field strengths up to 46 T are shown. The experimental set-up and strategies for calibrating the magnets will be discussed. The obtainable signal-to-noise and resolution shows that NMR in pulsed high field magnets is able to become a valuable research tool.

We discuss recent results obtained for the heavy-fermion metals UPd2Al3 and YbRh2Si2. UPd2Al3 is the first among all superconductors for which tunneling and inelastic neutron-scattering data highlight a non-phononic, i.e., magnetic-exciton mediated, pair state. YbRh2Si2 represents a model system exhibiting pronounced non-Fermi liquid effects above a weak antiferromagnetic phase transition at T N =70 mK . Upon approaching the quantum critical point (TN→0), by low doping with Ge, one observes for T K disparate behavior in the temperature dependences of both the electrical resistivity and the electronic specific heat as well as a Curie–Weiss law in the uniform magnetic susceptibility, implying uncompensated large 4f moments. These observations indicate a break up of the composite quasiparticles into their local f-spin and itinerant conduction-electron parts.

The tetragonal compound UAs 1-x Se 1+x shows two-level-system (TLS) Kondo effect in ferromagnetic state (Curie temperature T c ∼ 100-120 K). We have examined its ab-plane electrical resistivity dependence on temperature (down to 30 mK), on magnetic field (up to 14 T) and on external pressure (up to 19.3 kbar). Observed saturation of the TLS-Kondo resistivity ρ K (T) ∼ 1 - AT n below the Kondo temperature, T K can be characterised by gradual decrease of n from the Fermi liquid value n = 2 just below T K to n = 1 in the range ∼ 3.5-7 K and to n = 1/2 for over one decade of temperature below ∼3.5 K. The coefficient A from the n = 1 range is scaled with T K increasing from 33 to 49 K, when T c (x) varies from 101.9 (0.048) to 108.5 K (0.029). Either longitudinal or transverse magnetoresistance is positive and low (

We discuss to which extent the concepts of Zintl phases and of ‘phonon glasses and electron single crystals’ apply to type-I clathrates. In (β-) Eu 8 Ga 16 Ge 30 the presence of residual charge carriers appears to be related to a slight off-stoichiometry of the samples pointing to the validity of the Zintl concept in stoichiometric samples. The low and almost stoichiometry independent mobilities of (β-) Eu 8 Ga 16 Ge 30 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Ge 30 seriously question the validity of the ‘electron single crystal’ concept for type-I clathrates. The temperature dependence of the thermal conductivity of a Ba 8 Ga 16 Ge 30 single crystal indicates that tunneling states play a central role in producing ‘phonon glass’-like thermal conductivities.

The hybridization between a periodic lattice of almost localized (core like) electrons (of lanthanides or actinides) and band electrons has challenged researchers for over 30 years to unravel its microscopic origin and the many puzzling physical phenomena related to it. Among such phenomena are very unusual normal state properties, which differ strongly from Landau–Fermi liquid (FL) behavior or exciting unconventional forms of superconductivity. Here, we report on experimental studies of a recently discovered new class of these heavy fermion superconductors: CeTIn5 (T: transition metal). Our studies point towards the realization of unconventional superconductivity in these compounds. In both CeIrIn5 and in CeCoIn5 the specific heat C(T), thermal conductivity κ(T) and nuclear spin-relaxation rate decrease as a power law of temperature instead of exponentially for T

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.

The semimetallic quasi-one-dimensional S= 1 2 antiferromagnet Yb4As3 has been studied by performing low-temperature (T) and high magnetic-field (B) measurements of the specific heat, C(T,B), magnetization, M(T,B), AC-susceptibility, χAC(T,B), and electrical resistivity, ρ(T,B). At finite transverse magnetic fields, a gap Δ(B) is induced in the low-energy magnetic excitation spectrum. Our C(T,B) measurements reveal a Δ(B)∼B2/3 dependence for B⩽9 T, in accordance with predictions of the quantum sine-Gordon model. At higher fields the Δ(B) curve levels-off gradually. In the isothermal magnetization taken at 0.6 K no saturation occurs up to 60 T. We also present new results on spin-glass behavior below 0.15 K caused by a weak ferromagnetic interchain coupling and disorder. Finally, we concentrate on the electrical transport properties. Shubnikov-de Haas oscillations, arising from a low-density system of mobile As-4p holes, are recorded in magnetic fields up to 60 T. We estimate the effective mass and the mean-free path of these carriers and discuss spin-splitting effects.

We investigate a new route to quasi-one-dimensional spin-chain systems which originates from the charge-ordering transition out of a homogeneous mixed-valence state. We present evidence that Yb4As3 and P, Sb doped mixed crystals are well described by this mechanism. Many thermodynamic and low-temperature transport heavy-fermion-like properties can be explained by the existence of low-lying quasi-one-dimensional spin excitations in the Yb3+-chains. The observation of soliton excitations in a transverse external field gives further support to the existence of spin chains in Yb4As3. We also present recent results on spin-glass behavior at very low temperature caused by the interchain coupling and disorder. In addition, the yet unexplained magnetotransport effects are discussed.

We discuss strong deviations from the signatures of a heavy Landau–Fermi liquid already at p=0 that are observed for normal (n)-state CeCu2Si2, for CeNi2Ge2 and YbRh2Si2. No quantum critical point could yet be established in the case of CeNi2Ge2. For this compound, preliminary investigations of the chemical phase diagram have revealed certain similarities to CeCu2Si2. The non-Fermi-liquid (NFL) effects observed for YbRh2Si2 suggest a quasi-2D system of magnetic fluctuations. We also discuss briefly disparities between resistivity and specific-heat results in all three compounds as well as the relationship between a NFL n-state and the occurrence of heavy-fermion superconductivity.

We report on low-temperature thermal-expansion and specific-heat measurements on single crystals of the heavy-fermion superconductor UBe13. A new “line of anomalies”, B*(T), in the superconducting state is discovered. The evolution of this feature is studied as a function of Th doping on several U1−xThxBe13 polycrystals (x⩽0.03), giving rise to a new “line of anomalies”, TL(x), in the T–x phase diagram. TL(x) hits the multi-critical point at xcr≈0.019 and merges smoothly into the Tc2 line that marks the lower of the two phase transitions at x>xcr. Our data strongly suggest that the anomaly at TL(x) represents the precursor of the transition at Tc2(x) for 0.019

We report on Hall effect measurements on YbRh2Si2 single crystals with different residual resistivity and on LuRh2Si2 single crystal. The temperature dependence of the linear-response Hall coefficient of YbRh2Si2 is described by the anomalous Hall effect and the normal contribution incorporating multi-band effects. Sample dependencies at low T are found and explained by slight changes in the charge-carrier concentrations. © 2007 Elsevier B.V. All rights reserved.

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.

We report low-temperature measurements of the magnetic susceptibility of Yb ( Rh 1 - x M x ) 2 Si 2 ( M = Ir , Co ) single crystals for 0 ⩽ x Ir ⩽ 0.17 and 0 ⩽ x Co ⩽ 0.12 . Whereas Ir-doping increases the unit-cell volume and weakens the antiferromagnetic ordering, Co-doping induces chemical pressure and stabilizes antiferromagnetism. Our results prove that Ir-substitution tunes the ground state from the magnetically ordered into the paramagnetic regime. For Co-substitution with x Co ⩾ 0.07 two subsequent antiferromagnetic transitions are observed.

The well known intermetallic compound YbRh 2 Si 2 (tetragonal ThCr 2 Si 2 structure) is a heavy fermion system which orders antiferromagnetically at T N = 70 mK . Application of a magnetic field suppresses the Neel temperature continuously to zero and thus accesses a magnetic-field induced quantum critical point (QCP). The Hall coefficient shows a broadened step-like decrease as a function of applied field which extrapolates, at T = 0 , to a sharp step at the QCP, indicating that this QCP is not of spin density wave type. The iso-structural compound YbIr 2 Si 2 (I-type) does not order magnetically down to the lowest temperatures but does, nevertheless, show non-Fermi liquid behaviour in wide regions of the temperature-field phase diagram. Here we use field-dependent Hall-effect measurements as a probe of a putative nearby QCP in YbIr 2 Si 2 .

We report far-infrared optical properties of the Heavy Fermion compounds YbRh2Si2 and YbIr2Si2 for photon energies down to 2 meV and temperatures 0.4–300 K. Upon heavy quasiparticle formation the optical conductivities of both compounds develop a narrow Drude-like response above which a pseudogap structure appears. Compared to YbRh2Si2 the pseudogap of YbIr2Si2 manifests different structures reflecting differences in Kondo scale and low-energy electronic structure.

We report Hall Effect measurements under pressure in the normal state of CeCoIn5, from 60 to 355 mK and for fields exceeding the superconducting upper critical field H c 2 , with the field oriented parallel to [0 0 1]. At low pressures, the field dependence of the Hall coefficient exhibits a scaling consistent with the one reported in the normal state at higher temperatures, but at odds with the Δ H / T scaling expected near a field tuned quantum critical point. The breakdown of this scaling at higher pressures, concomitant with the suppression of spin fluctuations, suggests that it is a hallmark of the spin fluctuations.

We present the first data of physical properties of the cubic compound Yb 3 Ir 4 Ge 13 . An effective magnetic moment well below the full free-ion Yb + 3 value is found. An abrupt rise in the electrical resistivity of Yb 3 Ir 4 Ge 13 marks the entrance into a charge ordered phase below 60 K. The specific heat of Yb 3 Ir 4 Ge 13 has a very large peak at low temperatures.

We present low-temperature thermal expansion measurements on the nominally 10% Cd doped CeCoIn5. While the superconducting transition temperature is monotonically suppressed, an antiferromagnetic phase evolves in CeCoIn5 by Cd-doping. For the uniaxial pressure dependence of the Neel temperature along c, we find ( ∂ T N / ∂ p ) ∥ c = 0.206 K / GPa . The magnetic field dependence (for B ∥ c ) of T N is stronger compared to CeRhIn5. As no traces of a superconducting transition are resolved in thermal expansion along the c-axis, we estimate a lower limit of the in-plane pressure dependence to ( ∂ T c / ∂ p ) ⊥ c = 0.38 K / GPa .

By means of Raman scattering, we investigated single-crystalline samples of the new inorganic spin-Peierls compound α′-NaV2O5 = 34 with TSP = 34 K. Phonons with anomalous broad line shape indicating a strong spin-phonon coupling above TSP were detected. Below TSP several additional modes could be observed. Different temperature dependencies of these modes are interpreted as a first hint that low-energy modes are related to the opening of the spin-Peierls gap observed in neutron scattering experiments while other modes are related to the lowering of the lattice symmetry due to the spin-Peierls transition.

We have examined an influence of the As–Se chemical composition on the low- T upturn of resistivity ρ ( T ) (below ∼50 K) and the peak contribution (at ∼30 K) to thermoelectric power S ( T ), both due to the orbital Kondo (OK) effect, in UAs 1− x Se 1+ x ferromagnet ( T C between 85 and 117 K). The ρ ( T ) upturn decreases monotonously with x decreasing below 0.074 and vanishes leaving behind ferromagnetic metal-like ρ ( T ) behaviour for crystals with the lowest x examined. At the same time the peak contribution to S ( T ) vanishes, too. Both the ρ ( T ) upturn and the peak contribution to S ( T ) are reduced for x >0.074. The role of Se excess in a formation of two-level systems in uranium arsenoselenide is discussed.

63,65Cu field sweep NMR investigations on a series of EuCu2(Ge1−x Six)2 powder samples ( x = 0.3 , 0.7 , 1 ) are presented. EuCu2(Ge, Si )2 is unique among Eu-intermetallics because, upon tuning from a divalent antiferromagnetically ordered state for x = 0 to a valence fluctuating state for x = 1 , Heavy Fermion behaviour was found around x≈0.7. The Knight shift 63,65K is negative for all stoichiometries. Hyperfine fields are largest for the antiferromagnetically ordered Ge-rich compound ( x = 0.3 ) and smallest for the valence fluctuating pure silicon compound ( x = 1 ). For the x = 0.3 compound, above TN≈19 K, 63K(T) exhibits a Curie–Weiss like behaviour (≈ −10% at 20 K) whereas for EuCu2Si2 a temperature-independent shift of 63K(T)≈−1.5% is found. Our results are discussed in terms of formation of heavy quasiparticles and/or presence of valence fluctuations of Eu.

The results of low-temperature normal-state resistivity measurements of La 2−x Sr x CuO 4 (LSCO) and Bi 2 Sr 2−x La x CuO y (Bi-2201) using 61-T pulsed magnetic fields are summarized. Measurements of LSCO with various Sr doping reveal (1) an insulator-to-metal crossover in the normal state that takes place near optimum doping and (2) unusual log(1/ T ) divergence of both ϱ ab and ϱ c of insulating underdoped samples. In particularly clean Bi-2201 crystals, metallic in-plane resistivity and insulating c -axis resistivitiy are found to coexist down to the lowest experimental temperature. Recent measurements of the low-temperature normal-state Hall effect on Bi-2201 find an essentially temperature-independent Hall coefficient below 20 K, which suggest that the unusual insulating behavior does not come from conventional disorder-enhanced electron interactions.

We present a comparison of the effect of different dopants on both spin-Peierls and the Neel transition in CuGeO3. The composition dependence of the Neel-transition temperature TN can be scaled to a universal dependence for both in- and off-chain dopants taking into consideration the changes of the ionic radii. This suggests local distortions to play an important role for the effect of the doping. All in-chain dopants lead to a similar linear decrease of TSP; the slope differing only slightly within a factor of 1.5 for different dopands. The much weaker effect on TSP of the two off chain dopants Si and Ti scales to a common behaviour if the chemical pressure is taken into account. Furthermore, we found for in-chain dopants a clear correlation between the transition temperature TSP and the spin-Peierls gap.

The anomalous magnetic and superconducting properties in a series of non-stoichiometric Ce1 + xCu2 + ySi2 samples have been investigated by the Cu NQR. In a CeCu2.05Si2 sample regarded as the best quality to date, the superconducting state is within the H-T phase diagram embedded in the unusual A-phase dominated by critical magnetic fluctuations, whereas this A-phase is markedly suppressed in the off-stoichiometric compounds, Ce1.025Cu2Si2 and CeCu2.2Si2 with lower Tc. No difference in the relaxation behavior in the superconducting state was observed between both type of samples.

In Yb pnictides with anti-Th 3 p 4 structure, the valence of Yb decreases from about 2.25 in Yb 4 As 3 to about 2 in Yb 4 Bi 3 with increasing weight of pnictogen. In Yb 4 As 3 , charge ordering around 300 K is observed and in the charge ordered phase heavy fermion behavior is seen in spite of extremely low carrier concentration. On the other hand, Yb 4 Sb 3 is a valence fluctuating system. So, to make systematic studies from Yb 4 As 3 to Yb 4 Sb 3 we grew single crystals of Yb 4 (As 1− x Sb x ) 3 . With increasing Sb content, the absolute value of the resistivity decreases due to the increase of the number of carriers and the charge ordering temperature decreases reflecting the increase of the screening effect, and at 29% charge ordering seems to disappear. The γ-value, however, is still large even in Sb 29% sample. The large γ-value of Yb 4 (As 1− x Sb x ) 3 is considered to be independent ofthe carrier concentration.

Using Raman scattering we studied the effect of substitutions on 1D spin fluctuations in CuGeO_3 observed as a spinon continuum in frustration induced exchange scattering. For temperatures below the spin-Peierls transition (T_{SP}=14K) the intensity of this continuum at 120-500 cm^{-1} is exponentially suppressed and transferred into a 3D two-magnon density of states. Besides a spin-Peierls gap-induced mode at 30 cm^{-1} and additional modes at 105 and 370 cm^{-1} are observed. Substitution of Zn on the Cu-site and Si on the Ge-site of CuGeO_3 quenches easily the spin-Peierls state. Consequently a suppression of the spin-Peierls gap observable below T_{SP}=14K as well as a change of the temperature dependence of the spinon continuum are observed. These effects are discussed in the context of a dimensional crossover of this compound below T_{SP} and strong spin-lattice interaction., 9 pages, 2 eps figures included

We present transport and thermodynamic measurements on two high-quality single crystals of CeCu2Si2 with different ground-state properties: crystal #1 (“A/S”-type) shows a competition of superconductivity and a presumably magnetically “A-phase”. The latter develops out of a low-temperature state where the resistivity varies as ϱ − ϱ0 = aT2 indicative of a coherent heavy Fermi liquid. By contrast, non-Fermi-liquid behavior is found for crystal #2 (“S”-type) which lacks the A-phase for B

NQR and NMR measurements have been performed for two types of heavy fermion compounds, UNi 2 Al 3 ( T N = 4.6 K, T c = 1 K) and UPd 2 Ga 3 ( T N = 13 K). In UNi 2 Al 3 , an appreciably broadened 27 Al-NQR spectrum was observed below T N , indicating that the magnetic structure is not planar type such as UPd 2 Al 3 , but an incommensurate spin density wave or helical type one. In the superconducting state, 1/ T 1 decreases with no coherence peak just below T c , similarly to the other heavy fermion superconductors. In UPd 2 Ga 3 , two well-separated sets of Ga-NQR spectra were observed in the paramagnetic state, implying that Ga atoms occupy two different sites.

We present a reinvestigation of the complex magnetic phase diagram of the heavy-fermion system Ce(Cu1-xNix)2Ge2 by employing measurements of the specific heat, C(T), and thermal expansion, α(T), on high-quality single crystals with x = 0, 0.02, 0.05, 0.5 and 0.8 in magnetic fields B ⩽ 8T. In addition to what was observed in polycrystalline samples multiple phase-transition anomalies could be resolved in these single crystals for x < 0.5 and 1.5 K

We investigated the non-monotonous evolution of superconductivity with pressure ( P ) in a single crystal of CeCu 2 ( Si 0.9 Ge 0.1 ) 2 ( P = 0 : antiferromagnetic order at T N ≈ 1.2 K , superconductivity at T c ≈ 0.35 K ). Electrical resistivity measurements were performed at low temperatures T ⩾ 50 mK under nearly hydrostatic pressure conditions ( 0 P 4.5 GPa ) . The existence of two distinct superconducting domes is observed in the T – P phase diagram, one at 0 P 2.2 GPa and the other at 2.2 P 4.5 GPa . Surprisingly, our preliminary data suggest that the T – P phase diagram is more rich in structure than reported previously.

Measurements of specific heat, electrical resistivity, thermo-electric power, thermal conductivity, and the Hall coefficient on the semiconducting compound CeRu 4 Sn 6 are presented. In C / T , the electronic specific heat, a - ln T divergent temperature dependence develops below 2 K, reaching 396 mJ / mole K 2 at 0.36 K. The zero-field resistivity increases down to 0.36 K after a weak minimum at 3 K. The interpretation of these characteristics in terms of a low carrier density are further borne out by data of the thermal transport.

A 23Na ( I = 3 2 ) NMR study is presented on the itinerant ferromagnets NaFe4Sb12 and the isostructural Ca-substituted compound Na0.5Ca0.5Fe4Sb12 performed in 7.05 and 11.74 T in the temperature range from 4 to 300 K. Both compounds show a bulk ferromagnetism below 85 K. Static (Knight-shift) and dynamic (spin-lattice relaxation) measurements are described in the framework of the SCR theory for itinerant d band metals.

U 2 Ru 2 Sn has been classified as the first tetragonal U-based Kondo insulator. Here, we present measurements of the thermal conductivity κ and thermopower S of high-quality single-crystalline U 2 Ru 2 Sn along and perpendicular to the tetragonal c-axis, in the temperature range between 100 mK and 1 K, in zero field and in a magnetic field of 6 T. Below 400 mK, the phonon contribution to κ ( T ) shows a T 2 behaviour for both directions that can be attributed to phonons scattered by electrons. S ( T ) presents a linear behaviour in the whole temperature range. S is positive along the c-axis and negative perpendicular to the c-axis. Using a one-band model the effective mass m * is estimated to be 2 m 0 along and 16 m 0 perpendicular to the c-axis, where m 0 is the free-electron mass. This indicates that U 2 Ru 2 Sn has a highly anisotropic residual density of states within the pseudogap.

We use high-resolution dilatometry and electrical resistivity down to 20 mK and millitesla magnetic field steps to search for a possible quantum critical (end-) point (QC(E)P) in CeRu 2 (Si 1 - x Ge x ) 2 with x = 0.00 and 0.02 at the metamagnetic transition (MMT), B m = 7.8 and 6.8 T, respectively. We do not find any evidence for QCEP since (i) the peak height and FWHM of the magnetostrictive anomaly saturate below 0.2 K and (ii) the thermal expansion and the electrical resistivity indicate the formation of a Landau–Fermi liquid (LFL) state below 0.3 K even at B = B m . We speculate that the metamagnetic crossover represents a Fermi surface reconstruction that is fully completed below 0.2 K.

We report DC-magnetization, M ( B ) , measurements on YbRh 2 Si 2 under hydrostatic pressure of 0.64 GPa at magnetic fields up to 11.5 T and at temperatures down to 0.04 K. The evolution of low- T and low- B anomalies indicate that AF ordering is stabilized with pressure. The magnetic field at which the Yb-4f-electrons are completely localized is drastically reduced from 9.5 T at ambient pressure to 6.1 T at 0.64 GPa.

We present low-temperature thermal expansion measurements on the heavy fermion system CeCu 5.8 Ag 0.2 , which is located at an antiferromagnetic (AF) quantum critical point (QCP). At zero magnetic field, the volume expansion coefficient divided by temperature shows a logarithmic divergence upon cooling below 1 K. This temperature dependence is incompatible with the predictions of the itinerant spin-density wave theory for an AF QCP. The application of magnetic fields leads to a cross-over to Landau Fermi liquid behavior as expected for a zero-field QCP.

We report on ρ ( T ) measurements of YbRh 2 Si 2 for 0.1 K T 300 K up to 15 GPa. The ( T , p ) phase diagram can be divided in three pressure ranges: (i) For p 4.1 GPa the ordering temperature T N increases strongly, followed by (ii) a quasi-pressure-independent regime of T N in the range 4.1 GPa p 8 GPa . After a sudden increase of T N , the system eventually (iii) exhibits a weak pressure dependence above ≈ 10 GPa with T N ≈ 7 K at 15 GPa. Moreover, the reversible pressure dependencies of several maxima in ρ ( T ) are analyzed according to these pressure ranges.