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The recent capability to perform angle-resolved photoemission spectroscopy (ARPES) with improved energy and angle resolution over a wide photon energy range has provided new information about f-electron systems. Results for two rather different systems, 5f-electron heavy fermion metal URu2Si2 and 4f-electron mixed-valent SmB6 are compared and contrasted.

The Kondo properties of the impurity Anderson model provide a theoretical framework for relating thermodynamic and angle integrated electron spectroscopy data in many heavy fermion materials. We describe the success and the challenges of this approach, summarize a detailed analysis of CeSi2, and give a perspective on the relation to the lattice Anderson model.

We have investigated the electrodynamic response of the heavy-electron compounds U2Zn1 7, UPd2Al3, UCu5and URu2Si2. Particular emphasis has been devoted to the optical evidence of the antiferromagnetic phase transitions at TN= 9.7 K, 14 K, 15 K and 17 K for U2Zn1 7, UPd2Al3, UCu5and URu2Si2, respectively. In the excitation spectrum of UCu5and URu2Si2, we found an absorption in the far-infrared, which develops below TNand is ascribed to the excitation across a spin-density-wave type gap, suggesting that the antiferromagnetic phase transition might be itinerant in nature, and invokes a Fermi surface instability. Since this gap-like feature is absent in U2Zn1 7and UPd2Al3, we argue that these latter compounds belong to a characteristically different class of antiferromagnets, representative of the heavy-electron compounds with an ordering of essentially localized magnetic moments. The antiferromagnetic ordering then leads to a suppression of the spin-flip mechanism below TN. At low temperatures, we observe for all compounds the formation of a narrow Drude-like resonance in the optical conductivity, which is ascribed to the electrodynamic response of the heavy-quasiparticles, and is indicative of the progressive development of the many-body coherent Kondo state, coexisting with both types of magnetic ordering. In this review, we also present the evolution of the optical properties due to Ni- and Redoping in UCu5and URu2Si2, respectively. The optical evidence of the itinerant antiferromagnetic ordering is suppressed in both compounds upon doping and particularly for the URu2Si2compound this is consistent with a crossover to a ferromagnetic ground state upon Re-doping. © Springer-Verlag 1997.

The flux dynamics at low magnetic fields in UPt3 shows a clear distinction between the relaxation of bulk vortices and those close to the surface. In addition, in the high-temperature A-phase, vortices trapped in the bulk of the specimen after cycling it in a magnetic field creep out as expected, while bulk vortices in the B-phase remain strongly pinned, indicating that an intrinsic, novel pinning mechanism exists in the low-temperature superconducting phase of UPt3.

The basic properties of heavy fermion materials are briefly summarized. Several recent issues in the physics of heavy fermion materials are discussed, including the unconventional superconductivity of UPt 3 , non Fermi liquid behavior in strongly correlated f-electron systems, and the possible observation of the Fulde-Ferrel-Larkin-Ovchinnikov spatially nonuniform superconducting state.

Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for three f-electron systems: M1−xUxPd3 (M = Sc, Y), U1−xThxPd2Al3, and UCu5−xPdx. The emerging systematics of NFL behavior in f-electron systems, based on the present sample of nearly ten f-electron systems, is updated. Many of the f-electron systems exhibit the following temperature dependences of the electrical resistivity p, specific heat C, and magnetic susceptibility χ for T ≪ T0, where To is a characteristic temperature: P(T) ∼ 1 –aT/T0, where a 0, C(T)/T ∼ (-1/To) In (T/bT0), and χ(T) ∼ 1 −c(T/To)1/2. In several of the f-electron systems, the characteristic temperature To can be identified with the Kondo temperature Tk.

We have performed muon spin relaxation (\ensuremath{\mu}SR) measurements of polycrystalline specimens of ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{U}}_{\mathit{x}}$${\mathrm{Pd}}_{3}$ with x=0.1, 0.2, 0.4, and ${\mathrm{UPd}}_{4}$. We demonstrate that ${\mathrm{UPd}}_{4}$ and ${\mathrm{Y}}_{0.6}$${\mathrm{U}}_{0.4}$${\mathrm{Pd}}_{3}$ exhibit spin glass order, while the predominant volume fraction remains nonmagnetic at T\ensuremath{\rightarrow}0 in ${\mathrm{Y}}_{0.8}$${\mathrm{U}}_{0.2}$${\mathrm{Pd}}_{3}$ and the muon spin relaxation rate becomes even smaller in ${\mathrm{Y}}_{0.9}$${\mathrm{U}}_{0.1}$${\mathrm{Pd}}_{3}$, indicating the disappearance of magnetic order with decreasing U concentration x. These results are discussed in terms of the competition between the crystalline electric field splitting ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CEF}}$ and the exchange coupling J.

Evidence for non Fermi liquid (NFL) behavior in y1−x UxPd3 and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Low temperature electrical resistivity, specific heat, and magnetic susceptibility measurements on the Th1−xUxPd2Al3 system reveal unconventional Kondo behavior with NFL low temperature characteristics. Magnetic susceptibility measurements on UCu3.5Pd1.5, which has previously been shown to exhibit NFL behavior, are presented. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity ϱ ∼ 1−aT with either positive or negative coefficient a, a logarithmically diverging specific heat C/T ∼−lnT, and T1/2 asymptotic behavior of the magnetic susceptibility χ ∼ 1 − T1/2.

ac-magnetic-susceptibility [chi][sub ac]([ital T]) measurements as a function of frequency [ital f] and excitation field [ital h][sub ac] have been performed on a single crystal of the heavy-fermion superconductor URu[sub 2]Si[sub 2] in magnetic fields [ital H] up to 30 kOe applied along the [ital c] axis. The apparent superconducting transition temperature [ital T][sub [ital c]] shows an unexpected and approximately logarithmic increase (decrease) with [ital f] ([ital h][sub ac]) that is enhanced with [ital H], which suggests the existence of an irreversibility line below the upper critical field [ital H][sub [ital c]2]([ital T]) in URu[sub 2]Si[sub 2]. Isothermal [chi][sub ac] vs [ital H] sweeps were used to define [ital H][sub [ital c]2]([ital T]) which was independent of [ital f] and [ital h][sub ac].

Single crystals Ln2-xCexCuO4-y (Ln = Nd, Pr, Sm; 0⩽x⩽0.2) were grown in CuO flux and characterized by means of X-ray diffraction and microprobe analysis, electrical resistivity, and magnetic susceptibility. There is a large variation of the Ce content from crystal to crystal within a given batch and the distribution width decreases with increasing Ce starting composition. The temperature dependence of the normal-state electrical resistivity determined superconducting critical temperature Tc are very sensitive to the heat treatment. The magnetic susceptibility of Ln2-xCexCuO4-y single crystals, measured f or two orientations of the crystals with respect to the applied field H, reveals a high anisotropy and indicates the presence of crystalline electric-field (CEF) effects.

Magnetoresistance measurements on superconducting Nd[sub 2[minus][ital x]]Ce[sub [ital x]]CuO[sub 4[minus][ital y]] single crystals with [ital T][sub [ital c]] varying from 7 to 19 K were made with magnetic fields [ital H] applied parallel ([ital H][parallel][ital c]) and perpendicular ([ital H][perpendicular][ital c]) to the tetragonal c axis. For [ital H][parallel][ital c], the resistive superconductive transition curves gradually develop structure at low temperatures and high magnetic fields as [ital T][sub [ital c]] decreases. For the crystals with [ital T][sub [ital c]][approx lt]11 K, this structure evolves into a double resistive transition with a resistivity maximum near [ital T][approx]1.1 K, followed by an extremely sharp transition into the superconducting state. In this region, the resistivity exhibits non-Ohmic behavior for low current densities and Ohmic behavior for larger measuring currents. The resistivity for [ital H][perpendicular][ital c] does not exhibit any anomalous behavior or a maximum. The irreversibility line inferred from these measurements is sample dependent.

Angle-resolved photoemission measurements for bulk single crystals of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ with x=0.15 show that the spectral weight which builds in the gap of the x=0 insulator displays a Luttinger Fermi surface, the first such observation for a material which is electron doped and which must be driven metallic by alloying. This finding greatly strengthens our previous ocnclusion that the metal's chemical potential lies in gap-filling spectral weight, and sharpens the distinction between theories in which this does or does not occur.

This paper presents a review of the electronic structure of Y1−xUxPd3, along with new data on Fermi level tuning in alloy and in Th1−xUxPd3.

The resisitive upper critical field H c2 as a function of temperature T has been determined for polycrystalline specimens of the isostructural antiferromagnetic heavy-fermion superconductors UT 2 Al 3 (T=Ni, Pd). The zero-temperature superconducting coherence lengths ξ 0 =190 A (77 A) for T=Ni (Pd) have been estimated from the initial slope of H c2 (T), and analyses of previously reported specific-heat data yield and effective mass enhancement m * /m e ∼40 (49) for T=Ni (Pd), where m e is the free-electron mass

The effect of thorium impurities on superconductivity and antiferromagnetism, which coexist below the superconducting critical temperature T c , has been investigated in the heavy electron compound URu 2 Si 2 . The substitution of up to 5 at% Th for U was found to (1) enhance the low temperature normal state magnetic susceptibility and electronic specific heat coefficient γ, (2) broaden the superconducting and antiferromagnetic transitions, (3) suppress the specific heat jump associated with the superconductivity at T c and antiferromagnetism at the Neel temperature T N , and (4) induce a Kondo-like minimum in the electrical resistivity at low temperatures. The increase in γ and decrease in T N and the antiferromagnetic energy gap Δ with negative “chemical pressure” associated with the substitution of Th for U correlates with the decrease in γ and increase in T N and Δ upon application of an external pressure.

An overview is given of some recent issues and results concerning the single-particle electronic structure of strongly correlated electron systems. Specific topics include (i) the α−γ transition in cerium metal, (ii) the Kondo resonance in Y 1−x U x Pd 3 and (iii) the insulator-to-metal transition in superconducting cuprates.

For x≤0.3 in the alloy system Y 1-x U x Pd 3 , transport data display signatures of Kondo behavior with the Kondo temperature increasing as x decreases. For the same range of x, the inverse photoemission spectrum shows a growth of U 5f spectral weight near the Fermi energy. We interpret this Fermi-energy weight as the first identification of the Kondo resonance in a uranium material

We have performed a neutron-scattering investigation of the heavy-fermion compound ${\mathrm{Uru}}_{1.2}$${\mathrm{Re}}_{0.8}$${\mathrm{Si}}_{2}$, in order to verify the existence of long-range ferromagnetic order and characterize the properties of the ordered phase. This study consisted of measurements of elastic scattering using polarized and unpolarized neutrons, measurements of low-angle critical scattering, and the determination of the temperature (T) dependence of the order parameter. The critical scattering measurements at wave vector Q=0.08 A\r{} $^{\mathrm{\ensuremath{-}}1}$ revealed a sharp peak at T\ensuremath{\approxeq}30 K, which was taken as the Curie temperature ${\mathit{T}}_{\mathit{C}}$. The value of the ordered moment estimated from the elastic-scattering data is \ensuremath{\approxeq}0.49${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ at 10 K, which is in good agreement with the value of 0.46${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ obtained previously from magnetization measurements. A polarized-neutron beam elastically scattered off the {112} planes was only partially depolarized upon traveling through the sample. The partial depolarization of the beam suggests that ${\mathrm{URu}}_{1.2}$${\mathrm{Re}}_{0.8}$${\mathrm{Si}}_{2}$ is either a weak ferromagnet, or that the domains are extremely small. Extensive short-range correlations precede the onset of long-range order. Preliminary results of inelastic scattering are discussed.

We report the observation of anomalies in the irreversibility line H ( T ∗ ) and resistive upper critical field H c2 ( T ) for two different samples of the electron-doped superconductor Sm 1.85 Ce 0.15 CuO 4− y , a specimen consisting of magnetically aligned single-crystal grains and a single crystal. The irreversibility line conforms to a power a law dependence in the H-T plane with an exponent that changes at T ∗ T c ≅0.5 and H ≅1 kOe m ≅1.8 in low fields to m ≅4.2 in high fields, for applied magnetic fields parallel to the c -axis. These observations may indicate two different temperature and/or field dependences for the pinning potential. A single value of m =1.6 is observed for the exponent of the irreversibility line for H ⊥ c in the field and temperature range studied. The resistively determined H c2 ( T ) data taken with H ‖ c show the same power law behavior, with a change in the value of the exponent at T T c ≅0.5 and H ≅10 kOe from m =1.6 in low fields to m =2.4 in high fields. For H ⊥ c , the H c2 ( T ) data are characterized by a single exponent m =1.4 in the field and temperature range studied.

Although previous resistive measurements of the superconducting transition under applied magnetic field on a Sm1.85Ce0.15CuO4-y single crystal did not show much dissipation effects, magnetic relaxation data on another superconducting single crystal yield a small flux pinning energy, of the order of 20 meV at T = 6 K when H⌈c, similar to hole-doped superconductors. In the electron-doped compounds, the normal-state resistivity, which exhibits substantial anisotropy, does not follow a linear temperature dependence, but rather a T2 law up to about 200 K, consistent with Fermi-liquid theory for electron-electron scattering. In superconducting Nd2-xCexCuO4-y single crystals, both in-plane and out-of- plane resistivities exhibit metallic behavior with T2 dependence.

Specific heat C(T) and upper critical field Hc2(T) measurements have been performed on two single crystal specimens (denoted A and B) of the antiferromagnetic heavy-electron superconductor URu2Si2. Specific heat measurements on both single crystals reveal two distinct jumps, indicative of two superconducting phases. This is reminiscent of the antiferromagnetic heavy-electron compound UPt3 in which multiple superconducting transitions have been observed and attributed to coupled antiferromagnetic and multicomponent superconducting order parameters, although two superconducting phases associated with two different states of the crystal cannot be ruled out. The relative magnitudes of the two specific heat jumps observed in crystal B suggest that the two superconducting phases occupy nearly equal volume fractions of the crystal. At temperatures below the lower jump, the specific heat of both crystals can be described by C3(T) = γs(0)T+AT3 with γ3(0) ≈ 0.5γn(0), where γn(0) is the value of the normal-state electronic specific heat coefficient γn(T), extrapolated to T = 0. The two critical temperatures inferred from the specific heat jumps in crystal B, measured in magnetic fields H between 0 and 15kOe applied parallel to the c-axis, have a similar dependence on H. Resistive measurements of Hc2(T) on specimens from crystal A between 0 and 60 kOe reveal a kink near 2 kOe for H⌈c and strong positive curvature in Hc2(T) below ∼3 kOe for H⌈a.

We report on the static and dynamic magnetic behavior of a Sm 1.85 Ce 0.15 CuO 4− y single crystal for fields ranging from 20 Oe to 6 kOe. The irreversibility line, extracted from static magnetization versus temperature measurements in different magnetic fields H , shows power law behavior with an exponent that changes at ∼ 1 koe from 1.7 for fields H ⩽1 kOe to 3.5 for H ⩾ 1 kOe. The magnetic relaxation study shows that for values of the applied field greater than 600 Oe, an extrapolation of the normalized relaxation rate S n to zero temperature gives a non-zero value. Thismay be indicative of nonthermally activated relaxation processes. For 1 kOe, S n versus temperature exhibits two peaks, a possible indication of the presence of two pinning mechanisms. A distribution of activation energies for the pinning barriers was inferred to explain the experimental observations. The distribution functions were calculated from magnetic relaxation data taken in different applied magnetic fields.

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe13, URu2Si2 and U6Fe. The magnetic field penetration depth in the superconducting state of UBe13 is greater than 10000 A, as no increase in the transverse field relaxation rate is observed belowTc. A sharp increase in the precession frequency is seen, starting atTc. This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U6Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atTc=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.

dc flux-relaxation experiments were performed on single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ at low magnetic fields. A sharp, time-dependent crossover in the logarithmic magnetization decay rate was observed in two different crystals. Consideration of the size dependence of the relaxation rate in one of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals suggests that the crossover in the relaxation rate is due to a transition from surface-controlled to bulk-controlled relaxation behavior. Our observations seem to indicate that, over a certain range of temperatures and magnetic fields, surface pinning may play a significant role in the flux-relaxation behavior of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

The temperature T dependence of the upper critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ of the heavy-electron superconductors ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ (x=0 and 2.9%) doped with various concentrations of Gd has been determined from low-frequency ac magnetic susceptibility measurements in magnetic fields up to 60 kOe. The ${\mathit{H}}_{\mathit{c}2}$(T) curves for ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Gd}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ samples deviate from the ${\mathit{H}}_{\mathit{c}2}$(T) curves of ${\mathrm{UBe}}_{13}$ near ${\mathit{T}}_{\mathit{c}}$, which is consistant with the depairing of superconducting electrons via the Zeeman interaction between the spins of the superconducting electrons and the exchange field associated with the Gd spins. This suggests that ${\mathrm{UBe}}_{13}$ exhibits singlet superconductivity. In contrast, the ${\mathit{H}}_{\mathit{c}2}$(T) curves for (${\mathrm{U}}_{0.97}$${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$ doped with Gd scale with ${\mathit{H}}_{\mathit{c}2}$(T) of pure (${\mathrm{U}}_{0.97}$${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$ and do not reflect superconducting depairing by the Gd ions. These results are consistent with either strong spin-orbit scattering due to the presence of Th in ${\mathrm{UBe}}_{13}$, or to a qualitatively different type of superconductivity involving triplet spin pairing in (${\mathrm{U}}_{0.97}$${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$. Measurements of the temperature dependence of ${\mathit{H}}_{\mathit{c}2}$ for ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{R}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ compounds where R=La, Lu, and Ce for various compositions x as well as ${\mathrm{U}}_{0.985}$${\mathit{R}}_{0.015}$${\mathrm{Be}}_{13}$ compounds for R=Tb, Dy, Ho, and Er are also presented and compared with the Gd-doped ${\mathrm{UBe}}_{13}$ system. The results of low-temperature specific-heat measurements of ${\mathrm{UBe}}_{13}$ doped with various concentrations of Gd are also discussed.

The superconducting transition temperature Tc and the temperature dependence of the upper critical field Hc2 were measured resistively on polycrystalline samples in the (Gd1−xPrx)Ba2Cu3O7−δ system. The experimental results reveal that Tc decreases monotonically with increasing x and vanishes at x≈0.49. Whereas Hc2 of a sample with x=0.10 has an almost linear temperature dependence and the superconducting transition broadens with increasing magnetic field, a sample with x=0.45 shows strong positive curvature in the Hc2 versus temperature curve and narrowing of the superconducting transition width with increasing magnetic field. Both phenomena observed in the sample with x=0.45 can be explained qualitatively in terms of a negative exchange interaction between the superconducting charge carriers and the Pr 4f electrons.

We have performed a neutron scattering study of the heavy fermion compound URu 1.2 Re 0.8 Si 2 , in order to verify the occurrence of long range ferromagnetic order. This study consisted of measurements of elastic, inelastic, and polarized neutron scattering, measurements of low angle critical scattering, and the determination of the temperature dependence of the order parameter. We found (1) a sharp peak in the critical scattering at ≈30 K for the wave vector Q = 0.08 A −1 , and (2) an enhancement of the intensity at the position of the {1 0 1} nuclear Bragg reflection below the Curie temperature. These measurements suggest the occurrence of long range magnetic order. The value of the ordered magnetic moment is estimated to be ≈(0.49±0.05)μ B at 10 K, in good agreement with the 0.46μ B value obtained from magnetization measurements.

We present calculations of resonance photoemission spectra for Nd{sub 2}CuO{sub 4}. The calculations use a model Hamiltonian for which all parameters for the valence electrons are obtained from {ital ab initio} calculations or atomic data. Most features of the calculated sp agree well with experiment where comparison is possible. A substantial dependence on the polarization of the light is predicted for the occurrence of resonance behavior, and for the off-resonance intensity of the local singlet and one of the satellites.

Ce compose presente deux transitions de phase electroniques aux basses temperatures. On etudie l'effet d'une substitution chimique de metaux de transition au moyen de l'analyse des variations de la resistivite electrique, de la susceptibilite magnetique et de la chaleur specifique dans URu 2−x M x Si 2 pour x≤0,2

The temperature dependence of the anisotropic upper critical magnetic field, {ital H}{sub {ital c}2}(T), has been determined resistively in antiferromagnetic Sm{sub 1.85}Ce{sub 0.15}CuO{sub 4{minus}{ital y}} single crystals. We observe the first evidence in high-{ital T}{sub {ital c}} copper oxides for the interaction between superconductivity and antiferromagnetic ordering of rare-earth ions; estimates of the exchange coupling {ital J}{approx}0.1 eV and the Ginzburg-Landau coherence lengths {xi}{sub {ital ab}}=79 A and {xi}{sub {ital c}}=14.7 A are obtained. In both Nd{sub 1.84}Ce{sub 0.16}CuO{sub 4{minus}{ital y}} and Sm{sub 1.85}Ce{sub 0.15}CuO{sub 4{minus}{ital y}}, the {ital H}{sub {ital c}2} data above {ital T}{sub {ital N}} can be described by {ital H}{sub {ital c}2}({ital T})/{ital H}{sub {ital c}2}{sup +}(0)=(1{minus}{ital T}/{ital T}{sub {ital c}}){sup 1.6}.

The electronic structures of single-crystal and ceramic samples of the electron-doped superconductor ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ with x=0 and 0.15 have been studied using resonant photoemission spectro- scopy. Comparing these spectra with previous results for ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ shows that for both electron and hole doping the Fermi level lies at nearly the same energy, in states that fill in the x=0 insulator gap. We point out that this would occur if the states at the Fermi level obey a Luttinger-type sum rule, i.e., according to standard theory, if the metallic state is a normal Fermi liquid.

We have conducted optical investigations on the URu 2− x Re x Si 2 (0 x x is varied. In the antiferromagnetic phase ( x x > 0.4).

Magnetoresistance measurements on a superconducting Nd2 - xCexCuO4 - y single crystal with Tc ≈ 7K were made with magnetic fields H applied parallel (H‖c) and perpendicular (H⊥c) to the tetragonal c-axis. For H‖c, the resistive superconductive transition curves exhibit a double transition at low temperatures and high magnetic fields. This double resistive transition has a maximum near T ≈ 1.1 K, followed by an extremely sharp transition into the superconducting state. For applied fields of 1 kOe, the resistivity first goes to zero, increases again to a finite value and then goes through a second transition. In this region, the resistivity exhibits non-ohmic behavior for low current densities and ohmic behavior for larger measuring currents. The resistivity for H⊥c does not exhibit any anomalous behavior or a maximum for 0 ≤ H ≤ 60 kOe and 4K ≤ T ≤ 7 K.

Muon spin relaxation (μSR) measurements have been performed on polycrystalline specimens of Y 1− x U x Pd 3 with x =0.1, 0.2, 0.4 and on UPd 4 . Spin-glass-like magnetic order was observed in UPd 4 at T g ≅13 K and in Y 0.6 U 0.4 P d 3 at T g ≅12 K. The average static random internal field H int at the muon site is found to decrease very rapidly with U concentration x , indicating that the ordered static U moment rapidly disappears with decreasing U concentration around x ≅0.2. Our results are consistent with the conjecture that Kondo screening eliminates the magnetic moment around this concentration.

The upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) for the magnetic field applied parallel to the c axis of a ${\mathrm{Sm}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ single crystal has been determined from an analysis of magnetoresistance data in terms of the Ginzburg-Landau fluctuation theory for layered superconductors. The fluctuations are predominantly two dimensions (2D) away from ${\mathit{T}}_{\mathit{c}}$(H) with a crossover to 3D as ${\mathit{T}}_{\mathit{c}}$(H) is approached. The ${\mathit{H}}_{\mathit{c}2}$(T) curve shows negative curvature and is in excellent agreement with the Werthamer, Helfand, Hohenberg, and Maki theory for conventional superconductors without paramagnetic limiting effects. The values of the initial slope (${\mathit{dH}}_{\mathit{c}2}$/dT${)}_{\mathit{T}\mathit{c}}$=-7.38 kOe/K, coherence length \ensuremath{\xi}(0)=48.4 \AA{}, and orbital critical field ${\mathit{H}}_{\mathit{c}2}$(0)=97.6 kOe, derived from this analysis, are in good agreement with the values we previously obtained from fits of the Hao-Clem variational model to reversible magnetization data below ${\mathit{T}}_{\mathit{c}}$.

The correlations that enhance the thermodynamic quantities are particularly strong in CeAl 3 . Renormalized parameters are also observed in transport, and we will report measurements of the conductivity and dielectric constant at millimeter wave frequencies demonstrating these effects. A narrow zero-frequency mode develops at low temperatures whose oscillator strength is consistent with a dynamical mass m ∗ of order 10 3 m e . As expected, the relaxation time τ ∗ is also v ery large.

Substitution of rare earths ($R$), Th, and Zr for U in U${\mathrm{Pt}}_{3}$ depresses the superconducting transition temperature ${T}_{c}$ at an initial rate ($\frac{\ensuremath{\Delta}{T}_{c}}{x}$), where $x$ is the impurity concentration, that increases linearly with increasing residual resistivity ${\ensuremath{\rho}}_{0}$, indicating that the primary pair breaking mechanism is impurity potential scattering. The scaling of ($\frac{\ensuremath{\Delta}{T}_{c}}{x}$) with ${\ensuremath{\rho}}_{0}$ is strong evidence for anisotropic superconductivity in U${\mathrm{Pt}}_{3}$. The absence of a correlation of ($\frac{\ensuremath{\Delta}{T}_{c}}{x}$) with the de Gennes factor of the $R$ ions suggests that the superconducting order parameter in the $A$ phase of U${\mathrm{Pt}}_{3}$ has odd parity.

Measurements of the irreversibility line H(${\mathit{T}}^{\mathrm{*}}$) of ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.97}$, a system in which the superconducting transition ${\mathit{T}}_{\mathit{c}}$ can be varied from 0 to 92 K, show that H(${\mathit{T}}^{\mathrm{*}}$) obeys a universal scaling relation characterized by an m=3/2 power law near ${\mathit{T}}_{\mathit{c}}$, with a crossover to a more rapid temperature dependence below T/${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}0.6. The applicability of the scaling formula to other high ${\mathit{T}}_{\mathit{c}}$ superconductors is tested, with the results suggesting that both the crossover phenomenon and the power law temperature dependence are ubiquitous, scale-invariant properties of high temperature superconductors.

We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductor UPt3 in external fields Hext∥c. We find that the muon Knight shift is unchanged in the superconducting state, consistent with odd‐parity pairing (such as p wave). The transverse field relaxation is observed to be strongly field dependent, decreasing with increasing field. Below Tc the increase is barely detectable in an applied field of 4 kG∥c. On the basis of the high field measurements, we estimate the low temperature penetration depth to be λ(T→0)≥ (R18)11 000 A.

History of the muon spin relaxation (μSR) measurements dates back to the late 1950’s and 60’s following the discovery of parity violation. The application of μSR to condensed matter physics has steadily developed1 in the 70′s and early 80′s; mainly in the study of magnetic properties of ferro- or antiferromagnets and spin glasses. Extensive μSR studies on high-Tc systems2–4 have increased the recognition of this technique as one of the very powerful experimental methods in the study of magnetism and superconductivity.

We have investigated flux creep in single crystals of UPt 3 (Hc) and UBe 13 (Hc 4 ) in the temperature 5 mK c and for magnetic fields H

Measurements of the temperature and concentration dependence of the magnetic susceptibility and electrical resistivity for U1 − xThxPd2Al3 are presented. The magnetic susceptibility data scale with the concentration of U ions are large regions of x and T. The electrical resistivity shows a smooth evolution from the Kondo lattice regime to the single-ion Kondo behavior as Th concentration x is increased. A lack of saturation in resistivity in the samples with x = 0.8 and 0.6, and a logarithmic divergence in χ(T) at low temperatures makes the U1 − xThxPd2Al3 system a good candidate for a two-channel Kondo effect.

We have conducted optical and point-contact spectroscopy (PCS) investigations on the URu2 - xRexSi2 (0 0.4). We discuss our experimental findings in relation to the itinerant nature of the magnetic phase transitions.

Low-teperature specific-heat C measurements have been made on the high-T/sub c/ superconductors RBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ (R = Nd, Sm, Gd, Dy, and Er; deltaapprox. =0.1) for 0.5approx.

Low-temperature specific-heat measurements have been carried out between 0.5 and 30--50 K on the high-${T}_{c}$ copper oxide superconductors ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (R=Y, Eu, Ho, Tm, and Yb). The specific heat of the ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compounds below ${T}_{c}$ can be resolved into a contribution of the form ${C}_{e}$(T)=\ensuremath{\gamma}'T with a finite \ensuremath{\gamma}' and a lattice contribution that consists of Debye and Einstein terms. Specific-heat data for the R${\mathrm{Ba}}_{3}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compounds with R=Ho, Tm, and Yb exhibit no features due to magnetic order above 0.5 K, but reveal electronic Schottky anomalies associated with crystalline electric field (CEF) splitting of the Hund's-rules ground-state multiplet of the ${R}^{3+}$ ions. The Schottky anomalies can be described by calculations based on tentative energy-level schemes for the ${\mathrm{Ho}}^{3+}$, ${\mathrm{Tm}}^{3+}$, and ${\mathrm{Yb}}^{3+}$ ions in the CEF which are presented. Specific-heat data for the ${\mathrm{EuBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compound, which can be separated into a \ensuremath{\gamma}'T term, a Debye contribution, and a low-temperature upturn, are consistent with a nonmagnetic J=0 ground state for ${\mathrm{Eu}}^{3+}$, as expected from Hund's rules.

The low-temperature properties of M${\mathrm{Fe}}_{4}$${\mathrm{P}}_{12}$ (M=La, Pr, Nd, and Th) single crystals have been studied by means of electrical-resistivity, magnetization, specific-heat, and magnetoresistivity measurements. Superconductivity among these compounds is known to occur only in ${\mathrm{LaFe}}_{4}$${\mathrm{P}}_{12}$, which has a superconducting transition temperature ${T}_{c}$ of \ensuremath{\sim}4 K. The compounds ${\mathrm{PrFe}}_{4}$${\mathrm{P}}_{12}$ and ${\mathrm{NdFe}}_{4}$${\mathrm{P}}_{12}$ display features that suggest the occurrence of antiferromagnetic ordering below \ensuremath{\sim}6.2 K and ferromagnetic ordering below \ensuremath{\sim}2 K, respectively. Isothermal magnetization curves for ${\mathrm{PrFe}}_{4}$${\mathrm{P}}_{12}$ below 6 K reveal a spin-flop or metamagnetic transition.

Transport, thermal, and magnetic data for the heavy electron system ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ indicate that a charge- or spin-density-wave transition opens an energy gap of \ensuremath{\sim}11 meV over a portion of the Fermi surface below ${\mathrm{T}}_{0}$\ensuremath{\approxeq}17.5 K and demonstrate that bulk superconductivity occurs below ${\mathrm{T}}_{\mathrm{c}}$\ensuremath{\approxeq}1.5 K. The pressure dependences of ${\mathrm{T}}_{0}$ and ${\mathrm{T}}_{\mathrm{c}}$ support this interpretation. The unusually large initial slope of the upper critical magnetic field (9.2 T/K) is consistent with the high values of the electronic-specific-heat coefficient and the electrical resistivity.

We have measured electron paramagnetic resonance (EPR), electrical resistivity, and dc magnetic susceptibility from 2 K – 300 K for the high T c oxide superconductor EuBa 2 Cu 3 O 9−x , either undoped or doped with 3d ions (Cr, Mn, Fe, Ni, Co, or Zn), which presumably substitute at the Cu sites. We have observed an EPR line at low temperatures (T ≤ 40 K), which exhibits an increase in intensity and decrease in field for resonance as the temperature is lowered. The EPR linewidth is also temperature dependent and exhibits a minimum at about 15 K. In some of the samples another EPR signal is observed over the entire temperature range studied, with properties that depend on sample preparation conditions. This signal is likely to be due to small amounts of an additional phase. The behavior and origin of these EPR signals are discussed. The variation of T c with 3d ion concentration over the range (1 – 8%) is also presented.

Frequency-dependent conductivity measurements are reported in the dc to 150-GHz range in the moderately heavy-fermion compound CePd/sub 3/. A Drude expression is found with a renormalized relaxation time tau/sup */, and a renormalized plasma frequency ..omega../sub p//sup */ at low temperatures. This experiment represents the first direct measurement of these quantities, and the enhanced mass obtained from the frequency-dependent conductivity compares favorably with that determined from thermodynamic measurements.

The series of rare-earth (R)-barium copper oxides with nominal composition (RBa2)Cu3O9−σ and the prototype compound (YBa2)Cu3O9−σ have been prepared from the R (except Pm), Y, and Cu oxides and Ba carbonate by sintering, followed by arc-melting and annealing in pure oxygen. X-ray diffraction measurements show that, except for R = Ce, Tb, and Lu, the compounds have the perovskite-type structure reported by Cava et al. for high superconducting transition temperature Tc (YBa2)Cu3O9−σ. Electrical resistivity and magnetic susceptibility vs temperature measurements reveal super-conductive onsets above 90 K for all R elements, except La, Ce, Pr, and Tb, for which no superconductivity was observed down to 4.2 K. Light (heavy) rare-earth compounds tend to have relatively little (large) orthorhombic splitting in their X-ray patterns, relatively high (low) normal-state resistivities with negative (positive) temperature coefficients, and broad (sharp) superconductive transitions. The negligible destructive effect of the R magnetic moments on superconductivity indicates that the superconducting electrons are primarily associated with the Cu-O polyhedral clusters and interact only weakly with the R ions, suggesting that the (RBa2)Cu3O9−σ compounds may constitute an interesting new class of “high Tc” magnetic superconductors.