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The heavy-fermion superconductor CeCoIn5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state with a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa

The heavy-fermion superconductor CeCoIn 5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state with a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn 5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa ≤ P ≤1.5 GPa, 0 T ≤µ 0 H ≤14 T, and 100 mK ≤ T ≤4 K). Our results prove the stability of the the FFLO phase under pressure. It even expands, while the Pauli-limiting becomes weaker and the AFM-SF are suppressed. This shows the intriguing influence of the AFM-SF on the FFLO state.

For the prototypical heavy-fermion (HF) metal CeCu2Si2 superconductivity extends to surprisingly high pressures (PO10 GPa), quite different from other Ce-based HF superconductors, like CePd2Si2, where superconductivity is confined to a narrow P-range centered around an antiferromagnetic quantum critical point (QCP). By deliberately reducing the HF mean free path, i.e. by moderate Ge-substitution for Si, superconductivity could be weakened and two distinct superconducting regimes could be established at low and high P, respectively. The low-P regime is centered at a magnetic QCP. The nature of the magnetic phase could recently be unraveled with the aid of neutron diffractometry and appears to be an incommensurate low-moment spin-density-wave phase. q 2005 Published by Elsevier Ltd.

CePt 3 Si is a novel ternary compound exhibiting antiferromagnetic order at T N ≈ 2.2 K and superconductivity (SC) at T c ≈ 0.75 K . Large values of H c 2 ′ ≈ - 8.5 T / K and H c 2 ( 0 ) ≈ 5 T indicate Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interaction with a characteristic temperature T K ≈ 8 K . NMR and μ SR measurements evidence coexistence of SC and long-range magnetic order on a microscopic scale. Moreover, CePt 3 Si is the first heavy fermion SC without an inversion symmetry. This gives rise to a novel type of NMR relaxation rate 1 / T 1 which is very unique and never reported before for other heavy fermion superconductors. Studies of Si/Ge substitution allow us to establish a phase diagram.

Single crystals of EuCu2Ge2 were grown and characterized using electrical resistivity, magnetization, specific heat and magnetoresistance measurements. The crystals exhibit antiferromagnetic transitions at TN1 = 9 K and TN2 = 5 K. The TN of the flux-grown single crystals reported here are lower than that reported for the polycrystalline sample (TN = 13 K) in the literature (Felner and Nowik 1978 J. Phys. Chem. Solids 39 763). The magnetoresistance is positive in the ordered state and negative in the paramagnetic state. The magnetic order could not be suppressed up to a pressure of 25 kbar.

We discuss a series of thermodynamic, magnetic, and electrical transport experiments on the two heavy fermion compounds CeNi2Ge2 and YbRh2Si2 in which magnetic fields, B, are used to tune the systems from a non-Fermi liquid (NFL) into a field-induced FL state. Upon approaching the quantum-critical points from the FL side by reducing B we analyze the heavy quasiparticle (QP) mass and QP-QP scattering cross sections. For CeNi2Ge2 the observed behavior agrees well with the predictions of the spin-density wave (SDW) scenario for three-dimensional (3D) critical spin-fluctuations. By contrast, the observed singularity in YbRh2Si2 cannot be explained by the itinerant SDW theory for neither 3D nor 2D critical spinfluctuations. Furthermore, we investigate the magnetization M(B) at high magnetic fields. For CeNi2Ge2 a metamagnetic transition is observed at 43 T, whereas for YbRh2Si2 a kink-like anomaly occurs at 10 T in M vs B (applied along the easy basal plane) above which the heavy fermion state is completely suppressed.

The effect of pressure on the low-temperature states of the newly discovered clathrate Ba6Ge25 is investigated by means of measurements of the electrical resistivity. At ambient pressure, Ba6Ge25 undergoes a two-step structural phase transition between 230 and 180 K from metallic behaviour to a highresistivity state characterized by a mean free path of about 3 A. Interestingly, a Bardeen–Cooper–Schrieffer-like (BCS-like) superconducting transition occurs at TC ≈ 0.24 K fro mt he resulting ‘bad metal’. With increasing pressure, the structural phase transition is depressed but TC increases drastically. TC reaches am aximum value of 3.85 K at the critical pressure pC ≈ 2. 8G Pa, where the structural distortion is completely suppressed and the system exhibits metallic behaviour. Higher pressures lead to a slight decrease of TC.

We present a detailed pressure study of the electrical resistivity ρ(T) and the specific heat C(T) of the non-Fermi-liquid (NFL) compound YbRh2Si2 and of ρ(T) for a single crystal in which 5 at.% of Si is replaced by isoelectronic Ge. The magnetic phase diagram is deduced up to p ~ = 2.5 GPa. A comparison of the effects of the volume change introduced by doping and/or by hydrostatic pressure will be given. We show that the NFL behaviour observed in ρ(T) as well as the magnetic phase diagram are not influenced by the disorder introduced by alloying.

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.

We investigate the low temperature properties of the recently discovered clathrates Ba 6 Ge 25 and Na 2 Ba 4 Ge 25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba 6 Ge 25 undergoes a two-step structural phase transition between 230 K and 180 K from metallic behavior to a high-resistivity state. A superconducting transition occurs at T_{C}\approx 0.24\,\hbox{K} out of the resulting bad metal ( \rho_{0}\approx 1.5\,\hbox{m}\Omega\;\hbox{cm} ). With increasing pressure, the structural phase transition is shifted to lower temperature but T C increases drastically. T C reaches a maximum value of 3.85 K at the critical pressure p_{C}\approx 2.8\,\hbox{GPa} , where the structural distortion is completely suppressed and the system exhibits metallic behavior. On replacing 1/3 of the Ba atoms with Na (Na 2 Ba 4 Ge 25 ), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher (T_{C}(p = 0) \approx 1.05\,\hbox{K}) than in th...

We report the first observation of non-Fermi-liquid (NFL) effects in a clean Yb compound at ambient pressure and zero magnetic field. The electrical resistivity and the specific-heat coefficient of high-quality single crystals of YbRh(2)Si(2) present a linear and a logarithmic temperature dependence, respectively, in more than a decade in temperature. We ascribe this NFL behavior to the presence of (presumably) quasi-2D antiferromagnetic spin fluctuations related to a very weak magnetic phase transition at T(N) approximately 65 mK. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the stabilization of magnetic order.

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.

The crystallographic and low-temperature properties of the equiatomic Yb-based compounds YbPtIn, YbRhSn (hexagonal ZrNiAl structure), and YbNiGa (orthorhombic $\ensuremath{\epsilon}$-TiNiSi structure) are studied by means of x-ray diffraction, magnetic susceptibility, magnetization, electrical resistivity $(\ensuremath{\rho}),$ magnetoresistance, thermoelectric power, and specific-heat ${(C}_{P})$ measurements. The magnetic susceptibility follows a Curie-Weiss law with effective magnetic moments close to the value of ${\mathrm{Yb}}^{3+}$ and shows strong magnetocrystalline anisotropy. At low temperature the three compounds exhibit the typical properties of antiferromagnetic (AFM) Kondo lattices, namely a logarithmic increase of $\ensuremath{\rho}(T),$ enhanced electronic ${C}_{P}$ coefficients ${\ensuremath{\gamma}}_{o},$ and a reduced entropy gain below the ordering temperature. The values of ${\ensuremath{\gamma}}_{o}g350 {\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ are among the largest of known Yb-based magnetic Kondo lattices. Two AFM transitions in each compound evidence a competition between single-ion anisotropy and frustration of the magnetic interactions due to the topology of the underlying crystal structure. The ground-state properties are discussed and compared with those of other f-electron compounds showing similar characteristics concerning magnetic frustration.

The low-temperature properties of a family of ytterbium-based compounds with the composition ${\mathrm{Yb}}_{2}{T}_{3}{X}_{9}$ $(T=\mathrm{Rh}, \mathrm{Ir}; X=\mathrm{Al}, \mathrm{Ga})$ are studied by means of magnetic susceptibility, electrical resistivity, thermoelectric power, and specific-heat ${C}_{P}$ measurements. ${\mathrm{Yb}}_{2}{\mathrm{Rh}}_{3}{\mathrm{Al}}_{9}$ and ${\mathrm{Yb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Al}}_{9}$ show the typical properties of heavy-fermion antiferromagnets, namely they order at ${T}_{N}=3.5 \mathrm{K}$ and ${T}_{N}=5.5 \mathrm{K},$ respectively, their ${C}_{P}/T$ data extrapolate to ${\ensuremath{\gamma}}_{o}=230 \mathrm{mJ}/\mathrm{mol} \mathrm{Yb} {\mathrm{K}}^{2}$ below 0.5 K, and their entropy gain at ${T}_{N}$ reaches $0.6R\mathrm{ln}2$ only. The transport data show evidence of low-lying crystal-electric-field levels separated by ${\ensuremath{\Delta}}_{\mathrm{CEF}}\ensuremath{\simeq}30 \mathrm{K}$ from the ground-state doublet. On the contrary, ${\mathrm{Yb}}_{2}{\mathrm{Rh}}_{3}{\mathrm{Ga}}_{9}$ and ${\mathrm{Yb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$ show mixed-valent behavior, as observed in most Yb-based intermetallics. The effect of the ligands on the ground-state properties is discussed and compared with other lanthanide- and actinide-based isostructural ${R}_{2}{T}_{3}{X}_{9}$ homologues.

Pronounced non-Fermi-liquid (NFL) effects in thermodynamic $(C,\ensuremath{\alpha})$ and transport $(\ensuremath{\rho})$ properties at ambient pressure $(p)$ and zero field are reported for high-purity samples of the undoped heavy-fermion compound ${\mathrm{CeNi}}_{2}{\mathrm{Ge}}_{2}$. The origin of the NFL effects is not yet clear. Novel phase transitions are observed at both finite and ambient pressure. Under $p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.7\mathrm{GPa}$, a novel, perhaps spin-density-wave-type, phase transition occurs at ${T}_{m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.9\mathrm{K}$. At $p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$, incipient superconductivity below $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}100\mathrm{mK}$ is found in the two samples with lowest residual resistivities, ${\ensuremath{\rho}}_{0}l0.45\ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm}$.

We report muon spin relaxation (μ+SR) studies on the magnetic phase diagram of Ce(Cu1-xNix)2Ge2 polycrystals for 0.5≤ x ≤ 0.8. A sharp magnetic transition, evidenced by the appearance of a fast Gaussian relaxation component σ, has been observed in the x = 0.5 alloy at 4.0 K in zero applied field. The average local field at the stopping sites of the muons, extracted from σ, exhibits a linear temperature dependence. We associate these features with an incommensurate spin density wave (SDW) ordering. Magnetic ordering, either long range or short range, and signatures of non-Fermi liquid behaviour have not been observed down to 2.0 K at x = 0.8.

We report resistivity, $\ensuremath{\rho}(T)$, and specific-heat, $C(T)$, results on near stoichiometric $\mathrm{CeCu}{}_{2}{\mathrm{Si}}_{2}$ samples, in the vicinity of a quantum critical point (QCP). The latter is defined by ${T}_{A}\ensuremath{\rightarrow}0$, where ${T}_{A}\ensuremath{\lesssim}0.8\mathrm{K}$ marks the transition into a spin-density--wave-type ``phase $A$'' which competes with heavy-fermion superconductivity below ${T}_{c}\ensuremath{\approx}0.65\mathrm{K}$. Upon approaching the QCP, $\ensuremath{\rho}(T)$ and $C(T)$ behave very disparately, suggesting a breakup of the heavy quasiparticles. Very surprising observations are being made for samples with ${T}_{A}g0$ also.

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.

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 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.

We studied the evolution of superconductivity (sc) and antiferromagnetism (afm) in the heavy fermion compound CePt 3 Si with hydrostatic pressure. We present a pressure–temperature phase diagram established by electrical transport measurements. Pressure shifts the superconducting transition temperature, T c , to lower temperatures. Antiferromagnetism is suppressed at a critical pressure P c ≈ 0.5 GPa .

In BaxC60 superconductivity (sc) at Tc=(6.8±0.2) K is associated with the x=4 phase. Here we present pressure studies of Tc of a BaxC60 sample (dTc/dp= −1.9 K/GPa) which exhibits a large volume fraction of the x=4 phase. In the alkali-fullerides K60C60 and Rb3C60 Tc is governed by the electronic density of states N(EF) (Tc ∝exp{−1/N(EF)V}). The dependence of N(EF) on volume can be approximated by N(EF)=N0 d eff 3, where deff is the surface to surface distance of neighboured C60-molecules. This relationship is corroborated by (TC,d)-data of various compounds of the form A3−xBxC60 (A: K, Rb; B: K, Rb, Cs; x≤2). Strong deviation from this behavior is found for the alkaline-fulleride superconductor Ba4C60. This deviation is attributed to the hybridization of alkaline-metal s-, d-states and C60 π-states.

We present results of low-temperature calorimetric and resistive measurements on the isostructural heavy-fermion compounds and . `Non-Fermi-liquid' effects are established which suggest the nearness of an antiferromagnetic quantum critical point (QCP) in both systems. The observed deviations from the properties of a Landau Fermi liquid (FL) may be related to anomalous energy dependences of both the quasiparticle mass and the quasiparticle - quasiparticle scattering cross section. For , a moderately heavy FL can be recovered by application of moderate values of either magnetic field or hydrostatic pressure. For p = 1.7 GPa a novel, non-superconducting, phase transition has been discovered at .

The thermal conductivity {kappa} of untwinned single crystals of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (T{sub C}=91 K) has been measured along the a- ({kappa}{sub a}) and b-axis ({kappa}{sub b}) for 0.1K30 K {kappa}{sub b} is found to be larger than {kappa}{sub a}, while for 0.2 K< T

We present the results of specific-heat and resistivity measurements as a function of temperature, magnetic field and hydrostatic pressure on the Kondo lattice CeNi2Ge2, the heavy-fermion superconductors CeCu2Si2 and UBe13 as well as the low-carrier-density system Yb4As3. “Non-Fermi-liquid” effects in the low-temperature normalstate properties of the three former systems are consistent with the existence of a “nearby” quantum critical point, presumably of antiferromagnetic type. Yb4As3, though showing the outward appearance of a Landau-type heavy-fermion metal, behaves very differently, i.e. as an extreme two-fluid system.

A series of BaxC60 samples were prepared with stoichiometries ranging from x = 3 to x = 6. The lattice structure was analysed by using X-ray diffraction (XRD) investigations and the superconducting properties were determined by applying acsusceptibility and dc-magnetization (SQUID) techniques in external magnetic dc fields up to 2 T. Beyond x=3, XRD-results reveal the presence of more than one phase in the samples and, in contrast to the Ba3C60 sample, superconductivity around Tc = (6.8±0.2) K occurs. Three intrinsic BaxC60 phases (x = 3, 4, 6) are identified, and superconductivity is associated to the x = 4 composition which exhibits the largest superconducting volume fraction (∼ 22 %) in this series. Furthermore first estimates of the upper critical field of Ba4C60 are presented. The slope of the upper critical field ( ∂ B c2 ∂ T ) near Tc is one order of magnitude smaller than in alkali-metal doped C60 compounds. The whole Bc2(T)-curve is compared with the Werthamer-Helfand-Hohenberg (WHH) theory in the dirty and clean limit. We find zero temperature values for the upper critical field Bc2(0) = (2.4±0.3) T and for the corresponding Ginzburg-Landau coherence length ξ GL (0) = (116±7) A .

Measurements of specific heat (0.3 K < T < 15 K, P < 1.2 GPa) and electrical resistance (1.2 K < T < 300 K, P < 2 GPa) under hydrostatic pressure on the heavy-fermion system Ce(Cu 1−x Ni x ) 2 Ge 2 (x ≤ 0.1) were performed to discriminate between effects of chemical substitution and purely steric effects on the 4f-ligand hybridization in Kondo-lattice systems. The latter is effective in forming heavy-mass quasiparticles or generating heavy-fermion band magnetism (HFBM). We find the Kondo temperature T K to be governed by volume, while magnetic-ordering transitions at T NIH and T NIL for 0 < X ≤ 0.1 CANNOT BE REPRODUCED IN CECU 2 Ge 2 by a corresponding change in volume. Entropy calculations lead us to conclude that the lower transition (T NIL ) reflects a reorientation of the magnetic moments within the antiferromagnetically ordered phase and may be a precursor of a HFBM state. Its absolute value critically depends on ligand periodicity and/or density-of-states effects

In order to examine the nature of the superconducting order parameter in UBe13, we measured the specific heat, C, as a function of temperature, T, under uniaxial stress along 〈1 0 0〉 axis, for both “L”- (T c ≈0.75 K ) and “H”-type (0.85⩽T c ⩽0.95 K ) crystals. With increasing pressure, a linear decrease of the superconducting transition temperature, Tc, was observed ( ∂ T c / ∂ p≃8.5 mK/GPa for “L”-type and ∂ T c / ∂ p≃50 mK/GPa for “H”-type) but no split of the transition could be resolved unambiguously for either of the two samples within our measurement resolution.

Among heavy-fermion (HF) superconductors, CeCoIn 5 exhibits a record high value of T c =2.3 K at ambient pressure [1]. CeCoIn 5 belongs to a new class of HF-superconductors that crystallize in the tetragonal HoCoGa 5 -structure. This structure can be regarded as alternating layers of CeIn 3 and CoIn 2 . Bulk CeIn 3 undergoes a transition from an antiferromagnetic (AFM) state at ambient pressure ( T N =10.2 K) to a superconducting state with very low T C =0.15 K at a critical pressure p c =2.8 GPa [2] at which long range magnetic order vanishes. It is, therefore, regarded as a possible candidate for magnetically mediated superconductivity (SC). We report on measurements of the heat capacity of CeCoIn 5 at hydrostatic pressures p h 1.5 GPa. While T c increases with increasing pressure, the effective mass of the quasi-particles m eff decreases, as indicated by the ratio C / T | T c . As a working hypothesis based on theories of a nearly antiferromagnetic Fermi-liquid (NAFFL), this may be interpreted as the s...

In the quest for new superconductor compounds which adopt the superconducting state at increasingly higher transition temperatures T c , a non-phonon mediated coupling between the charge carriers seems to play a key role. In order to enhance our understanding of such unconventional coupling mechanisms, we studied a new family of heavy fermion (HF) superconductors CeTIn 5 (T: transition metal) whose properties point toward the realization of unconventional superconductivity (SC): the specific heat, thermal conductivity and nuclear spin-lattice relaxation rate of CeIrIn 5 and CeCoIn 5 decrease as a power law of temperature instead of exponentially for T < T c . We report on measurements of the heat capacity of CeIrIn 5 and CeCoIn 5 at hydrostatic pressures p h 1.6 GPa. In both compounds, T c increases with increasing pressure, while the mass of the quasi-particles m eff decreases, as indicated by the ratio C / T | T c . As a working hypothesis based on theories of a nearly antiferromagnetic Fermi-liquid (NA...

We report measurements of the resistivity and the specific heat, performed as a function of temperature, magnetic field, and hydrostatic pressure on the antiferromagnetic (T N =14 K) heavy-fermion superconductor (T c ≃2 K) UPd 2 A 3 . Our results suggest a coexistence of two subsystems with more localized 5f states responsible for the magnetic properties and less localized states responsible for the superconducting properties. The latter are compatible with an order parameter of octagonal d-wave symmetry

Hydrostatic pressure experiments on single crystals of the magnetically ordered heavy fermion compound CeCu 2 Ge 2 for P > 0.1 GPa revealed a pressure dependence of the magnetic ordering temperature ( dT N / dP = −0.1 K/GPa ), which can be brought into quantitative agreement with theoretical calculations. The results are suitable to explain a difference observed between the magnetic phase diagram of Ce ( Cu 1− x Ni x )2 Ge 2 and the calculated dependence of T N on the local exchange coupling constant g.

Heavy-fermion compounds are ideally suited to study cooperative phenomena in highly correlated electron systems. We discuss local-moment magnetism and heavy-fermion band magnetism in the exemplary systems CeCu 2 Ge 2 and Ni-rich Ce(Cu 1− x Ni x )2 Ge 2, respectively. In addition, the coexistence of long-range antiferromagnetic order and heavy-fermion superconductivity in UM 2 Al 3 (M: Ni, Pd) will be addressed.

We present thermal-conductivity $\ensuremath{\kappa}$ and thermopower S measurements on high-quality single crystalline CeNiSn along the three crystallographic axes $a, b,$ and c in the temperature range between 100 mK and 7 K and in magnetic fields up to 8 T applied along the a axis. Both $\ensuremath{\kappa}$ and S are highly anisotropic. However, characteristic features that may be attributed to the opening of a pseudogap in the electronic density of states (DOS) at the Fermi energy $\ensuremath{\eta}$ below 10 K are seen for all three crystallographic directions. These features are strongly suppressed by a magnetic field of 8 T applied along the a axis. At the lowest temperatures we have evidence for the presence of a residual metallic DOS at $\ensuremath{\eta},$ again for all three directions. The saturation of the reduced Lorenz number ${L/L}_{0}$ to a value distinctly higher than one is an interesting feature which deserves further investigation.

In the low dimensional spin System α′-NaV 2 O 5 , a charge ordering phase transition and the formation of a spin-singlet state are almost degenerate at T 0 ≅34 K . To foster our understanding of the interplay between charge and magnetic degrees of freedom, we have investigated the influence of chemical substitution on the phase transition by measuring the specific heat of doped single crystals. Significant changes have been observed already at low substitution levels. While isoelectrical substitution (Li, K) leads to a uniform decrease of T 0 and of the size of the anomaly Δ c p at T 0 , non-isoelectrical substitution (Ca) leads to a strong reduction of Δ c p but only to a weak decrease of T 0 . Once the phase transition has disappeared, a linear term and an enhanced cubic term β d appears in c p between 2 and 19 K, c p = γT + β d T 3 , which we attribute to magnetic excitation of the effective S =1/2 spin chain and to valence fluctuation, respectively.

The results of elastic, quasielastic, and inelastic neutron-scattering studies on polycrystalline CeM 2 Ge 2 (M=Ag, Au, and Ru) are presented. All compounds reveal long-range magnetic order at low temperatures. Ferromagnetic (M=Ru), antiferromagnetic (M=Au), and incommensurate (M=Ag) structures were detected. Using time-of-flight (TOF) techniques, the crystalline electric-field splittings were determined. With high-resolution TOF experiments the temperature and wave-vector dependence of the magnetic relaxation rate was studied

We report measurements of the low-temperature thermal conductivity of YBa2Cu3O7−δ (δ≃0.1) single crystals (Tc=84 K) both parallel (κa, b) and perpendicular (κc) to the CuO2 planes. Whereasκc (T) is found to be identical, within experimental resolution, with the phonon contributionκph (T),κa, b (T) contains an additional term linear in temperature,AT. We ascribeAT to the contribution of unpaired electronic carriers residing in the “chain layers”. Measurements performed in external magnetic fieldsB≦8 T support this interpretation. Our observations can be explained by an “internal multilayer” (IML) model in which it is assumed that strong superconductivity is generated within the CuO2 layers and weak superconductivity is induced in the “chain layers” by the proximity effect. The fit of the experimental results to the IML model reveals that approximately 15% of the electronic carriers remain unpaired in YBa2Cu3O7 belowT=1 K.

We have determined the pressure response of magnetism in C60TDAE, where TDAE is tetrakis (dimethylamino) ethylene, and its temperature dependent susceptibility above and below the magnetic ordering temperature TC=16.1K. The magnetism is depressed very rapidly with applied pressure. We consider several possible interpretations for the magnetic transition and its pressure dependence and conclude that an itinerant-ferromagnet description is most likely.

Direct measurements of the pressure dependence of the superconducting transition temperature {ital T}{sub {ital c}} for single-phase Rb{sub 3}C{sub 60} provide for the first time substantial evidence that {ital T}{sub {ital c}} in {ital A}{sub 3}C{sub 60} compounds may be a universal function of the lattice constant (inter-C{sub 60} spacing). Determination of the lower ({ital H}{sub {ital c}1}) and upper ({ital H}{sub {ital c}2}) critical fields, and consequently the superconducting penetration depth and coherence length, is also reported. Compared to K{sub 3}C{sub 60}, the measured quantities are consistent with a 15% larger electronic-state density in Rb{sub 3}C{sub 60} arising from a greater inter-C{sub 60} spacing in this compound.

Low-temperature (T) heat-capacity measurements under hydrostatic pressure of up to p=2.1 GPa have been performed on single-crystalline CeCu2Si2. A broad superconducting (SC) region exists in the T-p phase diagram. In the low-pressure region antiferromagnetic spin fluctuations and in the high-pressure region valence fluctuations had previously been proposed to mediate Cooper pairing. We could identify these two distinct SC regions. We found different thermodynamic properties of the SC phase in both regions, supporting the proposal that different mechanisms might be implied in the formation of superconductivity.

A monotonous increase of the Kondo temperature in Ce(Cu1−xNix)2Ge2 from 7 (x = 0) to 30 K (x = 1) is accompanied by drastic changes of ground state properties: for x⩽0.2, a modulated magnetic structure (q01 = (0.28, 0.28, 0.54)) involving Kondo-reduced local Ce moments ( μ s = 0.74μ B Ce for x = 0) forms below TN1(x). TN1 = 4. 1 K for CeCu2Ge2 is strongly depressed upon increasing x. At x ≲ 0.2, a different modulation develops below TN2(x) which becomes maximum (≃4 K) for x = 0.5. Since this is characterized by a very small value of q02 (=(0, 0, 0.13) at x = 0.5) and a gradually decreasing ordered moment (reaching μs ≲ 0.2μB/Ce for x ⩾0.65), we ascribe it to “heavy fermion band magnetism”. For 0.02⩽x⩽0.3, the two modulations seem to be superposed at sufficiently low temperatures. A non-magnetic heavy fermion ground state exists for x > 0.75.

In the archetypal strongly correlated electron superconductor CeCu2Si2 and its Ge-substituted alloys CeCu2(Si1-xGex)2 two quantum phase transitions--one magnetic and one of so far unknown origin-can be crossed as a function of pressure. We examine the associated anomalous normal state by detailed measurements of the low temperature resistivity (rho) power-law exponent alpha. At the lower critical point (at pcl, 1

We report low-temperature ( T ⩾ 40 mK ) magnetization measurements on YbRh2Si2 and YbIr2Si2 at magnetic fields up to 11.5 T and under hydrostatic pressure P ⩽ 1.35 GPa . The magnetization slope in YbRh2Si2 changes drastically under pressure, while that of YbIr2Si2 is less sensitive to pressure. Isofield M ( T ) curves of YbIr2Si2 under pressure up to 1.35 GPa saturate at low temperatures without signature of a magnetic transition. Thus, YbIr2Si2 has Landau Fermi liquid ground state up to 1.35 GPa and larger pressure is needed to reach the quantum critical point and to induce magnetic order.

CeCoIn5 exhibits the highest superconducting transition temperature (Tc = 2.3 K) among the Ce‐based heavy‐fermion compounds. Power‐law dependencies in specific heat (C), thermal conductivity (κ), and nuclear spin‐lattice relaxation rate (1/T1) for T ≪ Tc point to an unconventional superconducting state. Within the superconducting state specific‐heat shows an anomaly in the vicinity of the upper critical field Hc2 = 11.6 T for magnetic field parallel to the c‐axis. This phase‐transition is discussed as possible realization of a Fulde‐Ferrell‐Larkin‐Ovchinnikov (FFLO) phase. On the other hand, CeCoIn5 is located close to an antiferromagnetic (AFM) quantum critical point (QCP) at ambient pressure and underlying magnetic fluctuations may be responsible for the specific‐heat feature. We present first results of a specific‐heat study under pressure and in high magnetic fields to clarify the origin of this phase.

We review the current state of experimental and theoretical investigations of heavy fermion superconductors. We discuss most of the Ce-based compounds like Ce122, Ce115, Ce218 and Ce131 classes and U-based superconductors like UBe_13 and UPd_2Al_3. In the former the emphasis is on the connection to quantum critical phenomena and non-Fermi liquid behaviour. Recent neutron scattering and hydrostatic pressure results on SDW/SC competition in the Ce122 system are included. For the U-compounds we discuss the significance of dual models with both localised and itinerant 5f electrons for mass enhancement and superconducting pair formation. Itinerant spin fluctuation theories for unconventional superconductivity are also reviewed.

We investigated the evolution of the magnetism in the alloy $\mathrm{Ce}{\mathrm{Cu}}_{2}{({\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x})}_{2}$ by means of low-temperature thermal expansion measurements on large single crystals with $0.01\ensuremath{\leqslant}x\ensuremath{\leqslant}0.45$. The results evidence a new magnetic phase diagram more complex than that obtained in previous studies on polycrystals. The two main features are a second order transition from a paramagnetic to an antiferromagnetic state with a transition temperature ${T}_{N}(x)$ continuously increasing with Ge content and a first order transition corresponding to some change in the magnetically ordered structure at ${T}_{1}(x)\ensuremath{\leqslant}{T}_{N}(x)$. ${T}_{1}(x)$ and ${T}_{N}(x)$ seem to merge at $x\ensuremath{\approx}0.25$ leading to a tetracritical point at this concentration. An analysis of the Gr\"uneisen parameter suggests that at this critical concentration a transition from rather localized f electrons for $xg0.25$ to composite heavy fermions for $xl0.25$ occurs. This strongly supports the itinerant scenario for the quantum-critical point observed in pure $\mathrm{Ce}{\mathrm{Cu}}_{2}{\mathrm{Si}}_{2}$.