The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

Intermetallic compounds containing $f$-electron elements display a wealth of superconducting phases, which are prime candidates for unconventional pairing with complex order parameter symmetries. For instance, superconductivity has been found at the border of magnetic order as well as deep within ferromagnetically and antiferromagnetically ordered states, suggesting that magnetism may promote rather than destroy superconductivity. Superconducting phases near valence transitions or in the vicinity of magnetopolar order are candidates for new superconductive pairing interactions such as fluctuations of the conduction electron density or the crystal electric field, respectively. The experimental status of the study of the superconducting phases of $f$-electron compounds is reviewed.

Superconducting phases of f -electron compounds

In this review, we summarize the present status of theories of the spin fluctuation (SF) mechanism in explaining anomalous or non-Fermi liquid behaviours and unconventional superconductivity (SC) in strongly correlated electron systems around the antiferromagnetic (AF) quantum critical point. We refer to the high-Tc cuprates, two-dimensional organic compounds and heavy electron systems. We argue that the SF theories are successful in explaining at least the essential part of the problems, indicating that AF SF is the common origin of SC in these systems. This review is concluded by several comments on remaining problems which may require some additions to the SF theory, like the pseudo-gap phenomena observed in the hole-underdoped cuprates.

https://iopscience.iop.org/article/10.1088/0034-4885/66/8/202/pdf

Antiferromagnetic spin fluctuation and superconductivity

Most materials expand upon heating. Although rare, some materials expand on cooling, and are said to exhibit negative thermal expansion (NTE); but the property is exhibited in only one crystallographic direction. Such materials include silicon and germanium1 at very low temperature (<100 K) and, at room temperature, glasses in the titania–silica family2, Kevlar, carbon fibres, anisotropic Invar Fe-Ni alloys3, ZrW2O3 (ref. 4) and certain molecular networks5. NTE materials can be combined with materials demonstrating a positive thermal expansion coefficient to fabricate composites exhibiting an overall zero thermal expansion (ZTE). ZTE materials are useful because they do not undergo thermal shock on rapid heating or cooling. The need for such composites could be avoided if ZTE materials were available in a pure form. Here we show that an electrically conductive intermetallic compound, YbGaGe, can exhibit nearly ZTE—that is, negligible volume change between 100 and 400 K. We suggest that this response is due to a temperature-induced valence transition in the Yb atoms. ZTE materials are desirable to prevent or reduce resulting strain or internal stresses in systems subject to large temperature fluctuations, such as in space applications and thermomechanical actuators.

/pdf/zero-thermal-expansion-in-ybgage-due-to-an-electronic-y8s2zh7od8.pdf

Zero thermal expansion in YbGaGe due to an electronic valence transition

The solidification pressure of Daphne 7373, which is widely used as a pressure medium in high pressure studies, was examined at room temperature. Using a new generation clamp-type pressure cell, we found that Daphne 7373 solidifies at 2.2 GPa at room temperature. This is exactly on the natural extrapolation of the melting curve obtained at lower pressures and temperatures in our previous report. The solidification pressure of Daphne 7373 is twice as high as that of another well-known medium Fluorinert 77/70 (0.9 GPa). This allows us to hold hydrostatic pressure even in the newly developed BeCu–NiCrAl clamp-type pressure cell, which exceeds the limit of 1.5 GPa generated by a conventional BeCu cell.

https://iopscience.iop.org/article/10.1143/JJAP.46.3636/pdf

Solidification of High-Pressure Medium Daphne 7373

Pressure-induced valence instability of the heavy-fermion compound CeInCu2

A high-pressure apparatus which we can use at low temperature and high magnetic field has been designed for making measurements of thermal and transport properties of condensed matter. The details of the apparatus and some examples of the measurements are described and briefly discussed. It is proved that the apparatus is suitable for use up to hydrostatic pressure of ~ 3.5 GPa, down to ~ 2.0 K and up to ~ 11 T.

https://iopscience.iop.org/article/10.1088/0953-8984/14/44/506/pdf

High-pressure apparatus for the measurement of thermal and transport properties at multi-extreme conditions

High pressure apparatus has been designed to measure the physical properties of condensed matter at high magnetic field H(T) and at low temperature T(K). It is shown that the apparatus can be used in the ranges of P, T and H, 0<P≤25 kbar, 1.3≤T≤350 K and 0≤H≤5 T, respectively. Some examples of the measurements of electrical resistivity, thermal expansion coefficients, magnetoresistance and magnetostriction are reported briefly. The results are discussed on the basis of the pressure change of the Kondo temperature TK. Some difficulties in carrying out the high pressure experiment are pointed out.

Design of High Pressure Apparatus to Use at Low Temperature and High magnetic Field

A high-pressure apparatus has been constructed to measure the physical properties of condensed matters at low temperatures and high magnetic fields. The apparatus has a versatile usage in the research of solid state physics. We can measure the thermal and transport properties of condensed matters at “multi-extreme” conditions, i.e., high pressure, low temperature and high magnetic field. Several examples of the measurements using this apparatus are reported briefly.

Tomoko Kagayama

Papers

Pressure-induced valence instability of the heavy-fermion compound CeInCu2

High-pressure apparatus for the measurement of thermal and transport properties at multi-extreme conditions

Design of High Pressure Apparatus to Use at Low Temperature and High magnetic Field

Versatile high-pressure apparatus for use at low temperatures and high magnetic fields

Effect of pressure on the metal-insulator transition temperature in thiospinel CuIr2S4