M. C. de Andrade

Non fermi liquid ground states in strongly correlated f-electron materials

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 or > 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.

Non fermi liquid behavior in strongly correlated f-electron materials

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.

Evidence for a Common Physical Description of Non-Fermi-Liquid Behavior in Chemically Substituted f -Electron Systems

The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat C(T) and magnetic susceptibility {chi}(T) of many chemically substituted f -electron systems is analyzed within the context of a recently developed theory based on Griffiths{close_quote} singularities. Measurements of C(T) and {chi}(T) in the systems Th{sub 1{minus}x}U {sub x}Pd{sub 2}Al{sub 3} , Y{sub 1{minus}x}U {sub x}Pd{sub 3} , and UCu{sub 5{minus}x }M{sub x} (M=Pd,thinspPt ) are found to be consistent with C(T)/T{proportional_to}{chi}(T){proportional_to}T{sup {minus}1+{lambda}} predicted by this model with {lambda}{lt}1 in the NFL regime. These results suggest that the NFL properties observed in a wide variety of disordered f -electron systems can be described within the context of a common physical picture. {copyright} {ital 1998} {ital The American Physical Society}

Single-ion scaling of the low-temperature properties of f-electron materials with non-Fermi-liquid groundstates

Certain chemically substituted Ce and U compounds have low-temperature physical properties that exhibit non-Fermi-liquid (NFL) characteristics and apparently constitute a new class of strongly correlated f-electron materials. The NFL behaviour takes the form of weak power law or logarithmic divergences in the temperature dependence of the physical properties that scale with a characteristic temperature T 0 , which, in some systems, can be identified with the Kondo temperature T K . These systems have complex temperature T-chemical substituent composition x phase diagrams, which contain regions displaying the Kondo effect, NFL behaviour, spin glass freezing, magnetic order, quadrupolar order, and, sometimes, even superconductivity. Possible origins of the NFL behaviour include a multichannel Kondo effect and fluctuations of an order parameter in the vicinity of a second-order phase transition at T = 0 K. Recent experiments on the systems Y 1-x U x Pd 3 and U 1-x M x Pd 2 Al 3 (M = Th, Y) are reviewed. In the Y 1-x U x Pd 3 and U 1-x Th x Pd 2 Al 3 systems, the low-temperature physical properties in the NFL regime scale with the U concentration and T K , suggesting that single-ion effects are responsible for the NFL behaviour.

Superconductivity to 21 K in intermetallic thorium-based boride carbides

Abstract We report the observation of superconductivity in intermetallic thorium-based boride carbides. For ThM2B2C, superconducting Tcs of 6 K (M=Ni), 6.5 K (M=Pt), and 14.5 K (M=Pd) have been measured. These compounds are isostructural with the recently reported LnNi2B2C superconductors. We have also observed bulk superconductivity in an as-yet-undetermined thorium-palladium-boron-carbon quaternary phase at 21.5 K. Critical-field measurements on the two palladium-based superconductors, as well as the pressure dependence of Tc for the 21.5 K superconductor, are reported.

DC and RF Measurements of Serial Bi-SQUID Arrays

SQUID arrays are promising candidates for low-profile antennas and low-noise amplifier applications. We present the integrated circuit designs and results of dc and radio frequency measurements of wideband serial arrays based on the integration of linear bi-SQUID cells forming a superconducting quantum interference filter (bi-SQUID SQIF). Various configurations of serial array designs are described. The measured linearity, power gain, and noise temperature are analyzed and compared. The experimental results are matched to results of mathematical modeling. A serial bi-SQUID SQIF arrays are integrated into a coplanar waveguide, and symmetrically grounded to corresponding sides of the coplanar waveguide. The radio frequency output comes out from the central common line, which is also used for dc biasing, and forms a symmetrical balanced output. The signal and dc flux biasing line is designed as coplanar lines passed in parallel over each bi-SQUID cell in a bidirectional fashion concentrating magnetic flux inside of each cell. Serial bi-SQUID SQIF arrays are fabricated on 5 mm × 5 mm chips using a standard HYPRES niobium 4.5 kA/cm2 fabrication process.

Development of 2-D Bi-SQUID Arrays With High Linearity

We develop a two-dimensional (2-D) superconducting quantum interference filter (SQIF) array based on the recently introduced high-linearity tri-junction bi-SQUIDs (superconducting quantum interference device). Our bi-SQUID SQIF array design is based on a tight integration of individual bi-SQUID cells sharing inductances with adjacent cells. We provide extensive computer simulations, analysis, and experimental measurements, in which we explore the phase dynamics and linearity of the array voltage response. The nonuniformity in inductances of the bi-SQUIDs produces a pronounced zero-field single antipeak in the voltage response. The antipeak linearity and size can be optimized by varying the critical current of the additional junction of each bi-SQUID. The layout implementation of the tight 2-D array integration leads to a distinct geometrical diamond shape formed by the merged dual bi-SQUID cells. Different-sized 2-D arrays are fabricated using the standard HYPRES niobium 4.5 kA/cm2 fabrication process. The measured linearity, power gain, and noise properties will be analyzed for different array sizes and the results will be compared with circuit simulations. We will discuss a design approach for the electrically small magnetic field antenna and low-noise amplifiers with high bandwidth based on these 2-D bi-SQUID SQIF arrays. The results from this work will be used to design chips densely and completely covered in bi-SQUIDs that have optimized parameters such as linearity and power gain.

Anomalies in the irreversibility line and upper critical field of the electron-doped superconductor Sm1.85Ce0.15CuO4−y

Abstract 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.

Structure and composition analysis of the phases in the system Th-Pd-B-C containing superconductors with Tc = 14.5 K and Tc = 21 K

Abstract Several specimens in the system Th-Pd-B-C with nominal compositions ThPd 3 B 2 C and ThPd 3 B 3 C (as melted and annealed), containing superconducting phases exhibiting T c s of 14.5 K and 21 K, have been studied with EPMA, electron diffraction, EDX element analysis and HREM. A number of phases (approximate compositions are given) have been observed: (1) ThPd 2 B 2 C, I-centered tetragonal, a = 0.375, c = 1.07 nm, which can be related to the T c of 14.5 K; (2) ThPd 0.65 B 4.7 , P-type cubic, a = 0.42 nm, with short-range ordered superstructure resulting in diffuse streaks in the electron-diffraction pattern; this phase is suggested to be the 21 K superconducting phase; (3) ThPd 3 P-type hexagonal, a = 0.58, c = 0.98 nm; (4) ThPd 8 B 3 , I-centred orthorhombic a = 0.84, b = 0.90, c = 1.67 nm; (5) ThPd 3 B 2 C, with unknown structure; (6) ThB 4 , tetragonal a = 0.72, c = 0.41 nm; (7) graphitic C. Annealing of the specimen ThPd 3 B 2 C at 1050°C results in the disappearance of ThPd 2 B 2 C and the loss of the T c at 14.5 K but a strong increase of the 21 K superconducting fraction with a corresponding strong increase of the phases ThPd 0.65 B 4.7 and ThPd 3 . HREM indicates that ThPd 0.65 B 4.7 adopts a modified CaB 6 structure in which the cube face, 1 2 , 1 2 , 0, which is vacant in CaB 6 , is partially occupied by Pd with the removal of some B atoms. The superstructure indicates a short-range ordering (clustering) of the Pd atoms.

HREM on Tc=14.5 K superconducting ThPd2B2−xC

Abstract The compound ThPd 2 B 2 C has been studied with high-resolution electron microscopy and electron diffraction. ThPd 2 B 2− x C adopts the LuNi 2 B 2 C structure with a layer sequence (B y Ni 2 B z LuC) n in which the B composition and the exact position of the C are still uncertain. No intergrowths or planar defects are observed. The c -axis in the thinner parts of the crystals (