We report the discovery of a large negative thermal expansion (NTE) up to α=−25×10−6K−1 (α: coefficient of linear thermal expansion) in Ge-doped anti-perovskite manganese nitrides Mn3AN (A=Cu,Zn,Ga). This gigantic NTE is several to ten times higher than that of commercially available NTE materials. The discontinuous lattice expansion seen in pure Mn3AN is broadened by Ge substitution over a wide temperature window, at widest ΔT∼100K, around room temperature. Such a large, isotropic and nonhysteretic NTE is desirable for practical applications.

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The absence of resistivity saturation in many strongly correlated metals, including the hightemperature superconductors, is critically examined from the viewpoint of optical conductivity measurements. Coherent quasiparticle conductivity, in the form of a Drude peak centred at zero frequency, is found to disappear as the mean free path (at ω = 0) becomes comparable to the interatomic spacing. This basic loss of coherence at the so-called Mott-Ioffe-Regel (MIR) limit suggests that the universality of the MIR criterion is preserved even in the presence of strong electron correlations. We argue that the shedding of spectral weight at low frequencies, induced by strong correlation effects, is the primary origin of the extended positive slope of the resistivity to high temperatures observed in all so-called 'bad metals'.Moreover, in common with those metals which exhibit resistivity saturation at high temperatures, the scattering rate itself, as extracted from optical spectra, saturates at a value consistent with the MIR limit. We consider possible implications that this ceiling in the scattering rate may have for our understanding of transport within a wide variety of bad metals and suggest a better method for analysing their optical response.

The experimental results are presented on the in-plane resistivity for Zn-substituted single crystals of YBa 2 Cu 3 O 72y and La 22x Sr x CuO 4 with various hole densities. The primary effect of Zn is to produce a large residual resistivity (r 0 ) as a potential scatterer in the unitarity limit. In the underdoped regime, due also to low carrier density in the CuO 2 plane, only a few percent Zn is sufficient for r 0 to reach the critical value near the universal two-dimensional resistance h͞4e 2 and to induce a superconductor-insulator transition. By contrast, the universal behavior is not seen in the highly doped regime, suggestive of a radical change in the electronic state. PACS numbers: 74.25.Fy, 74.62.Dh, Zn substitution in copper oxide superconductors has a strong influence on the critical temperature T c and thus offers an opportunity to characterize the high-T c superconducting state. Many experimental efforts have been conducted to explain the T c depression in relation with a "normal" impurity in the d-wave pairing state [1,2]. Since a small concentration of Zn impurities introduced into the CuO 2 plane produces a significant change in the low-energy spin fluctuations as evidenced by the NMR [1,3] and neutron scattering [4,5] experiments, it is argued that the Zn substitution provides a "smoking gun" for the spin-fluctuation-mediated pairing mechanism in high-T c cuprates [6].Zn in the CuO 2 plane is itself a nonmagnetic impurity with a closed d shell and is expected to be a strong potential scatterer for charge carriers. Recently, Chien, Wang, and Ong [7] have investigated the Zn-substitution effect on the normal-state charge transport in YBa 2 Cu 3 O 7 and estimated a large scattering cross section of the Zn impurity. As the scattering cross section of Zn impurity is related with the parameters characterizing the electronic state of the high-T c cuprates, Zn is expected to be an effective probe for detailed study of the evolution of the electronic state with doping.We present the results of resistivity measurements on the single crystals of Zn-substituted YBa 2 Cu 3 O 72y (Y123) and La 22x Sr x CuO 4 (La214) over a wide range of Zn content and doped hole density. We demonstrate how the Zn-induced residual resistivity varies with hole density and show that the superconductor-insulator transition in two dimensions is readily induced due to very large residual resistivity in the underdoped regime. It is highlighted that Zn probes a remarkable difference in the electronic state between underdoped and highly doped superconducting regime.Single crystals of La 22x Sr x Cu 12z Zn z O 4 were grown using the traveling-solvent-floating-zone (TSFZ) method [8]. In the present work the crystals with x 0.10 (T c 27 K), 0.15 (T c 37 K), 0.20 (T c 32 K), andx 0.30 (T c 0 K) were prepared with Zn content ranging up to 0.04. Zn-substituted single crystals of YBa 2 ͑Cu 12z Zn z ͒ 3 O 72y were grown by the CuO-BaO self-flux method up to z 0.04. The oxygen content was adjusted to have a value between 6.63 and 6.93 by anneal...

Most materials expand upon heating. However, although rare, some materials contract upon heating. Such negative thermal expansion (NTE) materials have enormous industrial merit because they can control the thermal expansion of materials. Recent progress in materials research enables us to obtain materials exhibiting negative coefficients of linear thermal expansion over −30 ppm K −1 . Such giant NTE is opening a new phase of control of thermal expansion in composites. Specifically examining practical aspects, this review briefly summarizes materials and mechanisms of NTE as well as composites containing NTE materials, based mainly on activities of the last decade.