Donald Anthony Gajewski

Localized vibrational modes in metallic solids

Filled skutterudite antimonides, are cubic compounds with the formula RM4Sb12, where R is a rare-earth element (such as La or Ce), and M is a transition metal (for example, Fe or Co). The rare-earth ion is weakly bound in an oversized atomic cage formed by the other atoms. Its presence has been shown to cause a dramatic reduction in the lattice component of the thermal conductivity, while having little effect on the electronic properties of the compound. This combination of properties makes filled skutterudites of interest as thermoelectric materials. It has been suggested that localized, incoherent vibrations of the rare-earth ion are responsible for the reduction in thermal conductivity, but no direct evidence for these local vibrational modes exists. Here we report the observation of local modes in La-filled skutterudites, using heat capacity, elastic constant and inelastic neutron scattering measurements. The La atoms show unusual thermodynamic behaviour, characterized by the presence of two low-energy localized modes. Our results suggest that consideration of local modes will play an important role in the design of the next generation of thermoelectric materials.

Heavy fermion behaviour of the cerium-filled skutterudites and

The low temperature properties of polycrystalline samples of the filled skutterudites and , as well as the unfilled skutterudites and , have been investigated by means of electrical resistivity, specific heat and magnetic susceptibility measurements. The resistivity of exhibits a rather abrupt drop-off with decreasing temperature near 100 K; this drop-off temperature increases with increasing applied hydrostatic pressure, which is reminiscent of the onset of coherence in so-called Kondo lattice materials. The compounds and exhibit values of the electronic specific heat coefficient and Pauli susceptibility at low temperature which are enhanced over those of the lanthanum-filled and unfilled skutterudites. These quantities yield a Wilson ratio of order unity, which indicates that they both correspond to the properties of itinerant electrons. These transport, magnetic and thermodynamic properties suggest a moderately heavy fermion ground state in and .

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.

Extraordinary behaviour of the Y1−xPrxBa2Cu3O7−δ system

Investigations of Y1−xMxBa2Cu3O7−δ (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofTc withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y1−xPrxBa2Cu3O7−δ single crystals reveal aT2 dependence of the cotangent of the Hall angle in the normal state and a “negative Hall anomaly” belowTc in the superconducting state, in agreement with recent reports. Our research shows that the depth, Δ, of the negative Hall signal scales withT/Tc and that the maximum value of Δ decreases linearly withx and vanishes atx≈0.24. Magnetoresistance measurements on Y1−xPrxBa2Cu3O7−δ single crystals indicate that the irreversibility lineH(T*) obeys a universal scaling relation characterized by anm=3/2 power law nearTc, with a crossover to a more rapid temperature dependence of belowT/Tc≈0.6, similar to that observed for polycrystalline specimens.

U 5f spectral weight variation in UPd3-xPtx

Abstract We report a photoemission study of the 5f spectral weight variation in UPd 3− x Pt x . Relative to a previous study the results show both agreement and very significant differences. New spectral detail is resolved.

Fermi- and non-Fermi-liquid ground states in M1−xUxPd3 (M Sc, Y, La, Pr, Zr, Th) systems

Abstract A growing number of chemically substituted intermetallic compounds of Ce and U exhibit non-Fermi-liquid (NFL) behavior in their low temperature physical properties and apparently constitute a new class of strongly correlated f-electron materials. In this paper, we update the experimental situation for the archetypal NFL f-electron system Y1−xUxPd3 and briefly describe recent experiments on the related systems M1−xUxPd3 (M  Sc, La, Pr, Zr, Th).

A very simple and inexpensive apparatus for detecting superconducting transitions via magnetic screening

We report a very simple and inexpensive technique for detecting the screening of the B field by high temperature superconductors via the change in the inductance of a coil containing the sample. While related to previously reported techniques for measurement magnetic susceptibility via changes in inductance, the apparatus we report does not require lock-in amplification and is suitable for general physics labs.

Dynamics of flux creep in underdoped single crystals of Y{sub 1-x}Pr{sub x}Ba{sub 2}Cu{sub 3}O{sub 7-{delta}}

Transport as well as magnetic relaxation properties of the mixed state were studied on strongly underdoped Y_1-xPr_xBa_2Cu_3O_7-d crystals. We observed two correlated phenomena - a coupling transition and a transition to quantum creep. The distribution of transport current below the coupling transition is highly nonuniform, which facilitates quantum creep. We speculate that in the mixed state below the coupling transition, where dissipation is nonohmic, the current distribution may be unstable with respect to self-channeling resulting in the formation of very thin current-carrying layers.

Flux jump avalanches in torque studies of single crystal YBa2Cu3O7−δ

Abstract A series of avalanche-like jumps are observed in the mixed state of single crystal YBa 2 Cu 3 O 7− δ (YBCO) superconductors. Emerging as a saw-tooth pattern in torque vs. sample orientation in magnetic field, these jumps are discontinuous on our most resolute angular scale. While reminiscent of the classical flux jump instability, the present jumps are instead proposed to be associated with the layered nature of the material and twin boundary (TB) pinning, the combination of which promotes a crossover from a tilted to a kinked vortex structure.