M. Brian Maple

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 .

Interplay between superconductivity and magnetism

Abstract The f-electrons in rare earth and actinide intermetallic compounds are responsible for a rich variety of novel superconducting and magnetic states that have been discovered during the past two decades. In this paper, we briefly review these remarkable superconducting and magnetic states, how they are believed to originate, and the f-electron materials in which they are found. Examples of phenomena that are produced by superconducting-magnetic interactions in certain ternary and quaternary rare earth compounds include the coexistence of superconductivity and antiferromagnetism, the destruction of superconductivity by the onset of ferromagnetism at a second critical temperature Tc2 below the superconducting critical temperature Tc1, accompanied by a new sinusoidally modulated magnetic state that coexists with superconductivity in a narrow temperature interval above Tc2, and the induction of superconductivity by the application of high magnetic fields. Multiple superconducting phases have been discovered in several U “heavy fermion” compounds, f-electron materials in which the electrons have enormous effective masses as high as several hundred times the mass of the free electron. The multiple superconducting phases are apparently the result of the interaction of an unconventional superconducting state with a coexisting weakly antiferromagnetic state. This unconventional superconducting state is presumed to involve electrons that are paired in states with angular momentum greater than zero via antiferromagnetic spin fluctuations, and is similar to the superconducting state that is believed by many (but not all!) researchers to be responsible for high-Tc superconductivity in the cuprates.

Electronic Pairing in Exotic Superconductors

Investigations of rare earth, Aactinide, organic and oxide compounds have yielded several new classes of exotic superconductors. These include magnetically ordered superconductors, A15 superconductors, buckyball superconductors, heavy‐electron superconductors, organic superconductors and high‐Tc oxide superconductors. These materials have properties significantly different from those of conventional superconductors such as Al and Zn, which are described well by the Bardeen‐Cooper‐Schrieffer model of superconductivity. We carefully distinguish between the BCS model and the more general BCS theory. In the BCS theory superconductivity arises, loosely speaking, from electron pairs that behave essentially as bosons and undergo macroscopic condensation to the lowest energy state at the critical temperature Tc The BCS model, presented in 1957, further specifies that the pairing is mediated by exchange of quantized lattice vibrations (phonons) between the electrons, yielding pairs with zero spin S (spin singlet) ...

Limits on superconductivity-related magnetization in Sr2RuO4 and PrOs4Sb12 from scanning SQUID microscopy

We present scanning superconducting quantum interference device microscopy data on the superconductors Sr2RuO4 Tc=1.5 K and PrOs4Sb12 Tc=1.8 K. In both of these materials, superconductivity-related timereversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However, in neither Sr2RuO4 nor PrOs4Sb12 do we observe spontaneous superconductivity-related magnetization. In Sr2RuO4, many experimental results have been interpreted on the basis of a pxipy superconducting order parameter. This order parameter is expected to give spontaneous magnetic induction at sample edges and order parameter domain walls. Supposing large domains, our data restrict domain wall and edge fields to no more than 0.1% and 0.2% of the expected magnitude, respectively. Alternatively, if the magnetization is of the expected order, the typical domain size is limited to 30 nm for random domains or 500 nm for periodic domains.

On the existence of heavy fermion ytterbium compounds

Author(s): Fisk, Z; Maple, MB | Abstract: Evidence for heavy fermion behavior in ytterbium compounds is examined.

Ferromagnetic ordering and simultaneous fast magnetization tunneling in a Ni4 single-molecule magnet.

Low-temperature heat capacity and oriented single-crystal field-cooled and zero-field-cooled magnetization data for the single-molecule magnet [Ni(hmp)(dmb)Cl](4) are presented that indicate the presence of ferromagnetic ordering at approximately 300 mK, which has little effect on the magnetization relaxation rates.

Strongly Correlated Electron Phenomena in f-Electron Materials

A series of experiments on strongly correlated electron phenomena in f-electron materials, performed during the past three and one half decades, are briefly reviewed. The phenomena considered include the Kondo effect in superconductors, demagnetization of rare earth ions in metals, valence fluctuations of rare earth ions in metals, heavy fermion superconductors, and non-Fermi liquid behavior in f-electron systems. Recent experiments on heavy fermion behavior and unconventional superconductivity in the filled skutterudite compound PrOs 4 Sb 12 are also discussed. The fundamental interactions responsible for the Kondo effect play a prominent role in the physics of strongly correlated electron phenomena in f-electron materials.

Transport and magnetic properties of (M = Pd, Pt)

The substitution of transition metals for Cu in the heavy-fermion compound leads to unusual low-temperature properties. For example, the substitution of Pd suppresses long-range magnetic order and results in non-Fermi-liquid behaviour. In this report, we compare properties and phase diagrams for and based upon electrical resistivity and dc magnetic susceptibility measurements. In the Pt system, we find a rapid suppression of magnetic ordering similar to that seen in the Pd system. The electrical resistivity and dc magnetic susceptibility of exhibit low-temperature behaviour for suggestive of the non-Fermi-liquid behaviour found in for . The electrical resistivity of for reaches a concentration-dependent minimum before increasing at lower temperatures. At higher Pt concentrations , the electrical resistivity develops a broad peak at low temperatures, as in , suggesting short-range magnetic order although irreversibility was not seen in the magnetic susceptibility. The similarities of and differences between the two systems are discussed. The possibility of non-Fermi-liquid behaviour in is considered.

Ultrasonic Investigation of Off-Center Rattling in Filled Skutterudite Compound NdOs4Sb12

The off-center rattling of Nd rare-earth ions in the filled skutterudite compound NdOs 4 Sb 12 has been investigated by ultrasonic measurement. The longitudinal C 11 mode for frequencies between 34 and 253 MHz shows a marked frequency dependence in elastic constant and ultrasonic attenuations at two different temperatures centered at around 45 and 15 K. The relaxational frequency dependence of ultrasonic dispersion reveals a thermally activated Γ 23 -type off-center motion of Nd-ions, involving local charge fluctuations with Γ 23 symmetry in the (OsSb 3 ) 4 cage. An attempt time τ 0,(1) =7.5×10 -12 s and an activation energy E 1 =337 K were obtained from fits to the dispersion at around 45 K. The presence of another dispersion at lower temperatures of around 15 K implies that the rattling in NdOs 4 Sb 12 has an additional low-energy excitation characterized by a lower activation energy E 2 ∼67 K with an attempt time τ 0,(2) =5.1×10 -11 s.

Novel Types of Superconductivity in F‐Electron Systems

During the last one and one‐half decades, experiments on compounds and alloys of rare earth and actinide elements with partially filled f‐electron shells have revealed novel superconducting phenomena. Among these are the reappearance of resistance at a second critical temperature due to the interaction of magnetic moments with conduction electrons, the coexistence of superconductivity and antiferromagnetism, superconductivity induced by magnetic fields and superconductivity due to electrons with large effective masses.