We have investigated the structural, magnetic, and electric properties of ferromagnetic ${\mathrm{BiMnO}}_3$ with a highly distorted perovskite structure. At $T_E=750\mathrm{}–770\mathrm{K},$ a centrosymmetric–to–non-centrosymmetric structural transition takes place, which describes of the ferroelectricity in the system. The changes in the dielectric constant were induced by the magnetic ordering ${(T}_M\approx 100\mathrm{K})$ as well as by the application of magnetic fields near $T_M.$ These features are attributed to the inherent coupling between the ferroelectric and ferromagnetic orders in the multiferroic system.

Removing electrons from the CuO ₂ plane of cuprates alters the electronic correlations sufficiently to produce high-temperature superconductivity. Associated with these changes are spectral-weight transfers from the high-energy states of the insulator to low energies. In theory, these should be detectable as an imbalance between the tunneling rate for electron injection and extraction—a tunneling asymmetry. We introduce atomic-resolution tunneling-asymmetry imaging, finding virtually identical phenomena in two lightly hole-doped cuprates: Ca _1.88 Na _0.12 CuO ₂ Cl ₂ and Bi ₂ Sr ₂ Dy _0.2 Ca _0.8 Cu ₂ O _8+δ . Intense spatial variations in tunneling asymmetry occur primarily at the planar oxygen sites; their spatial arrangement forms a Cu-O-Cu bond-centered electronic pattern without long-range order but with 4 a ₀ -wide unidirectional electronic domains dispersed throughout ( a ₀ : the Cu-O-Cu distance). The emerging picture is then of a partial hole localization within an intrinsic electronic glass evolving, at higher hole densities, into complete delocalization and highest-temperature superconductivity.

The unusual property of negative thermal expansion is of fundamental interest and may be used to fabricate composites with zero or other controlled thermal expansion values. Here we report that colossal negative thermal expansion (defined as linear expansion <−10⁻⁴ K⁻¹ over a temperature range ~100 K) is accessible in perovskite oxides showing charge-transfer transitions. BiNiO₃ shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer that is shifted to ambient pressure through lanthanum substitution for Bi. Changing proportions of coexisting low- and high-temperature phases leads to smooth volume shrinkage on heating. The crystallographic linear expansion coefficient for Bi_0.95La_0.05NiO₃ is −137×10⁻⁶ K⁻¹ and a value of −82×10⁻⁶ K⁻¹ is observed between 320 and 380 K from a dilatometric measurement on a ceramic pellet. Colossal negative thermal expansion materials operating at ambient conditions may also be accessible through metal-insulator transitions driven by other phenomena such as ferroelectric orders.

Understanding the role of competing states in the cuprates is essential for developing a theory for high-temperature superconductivity. We report angle-resolved photoemission spectroscopy experiments which probe the 4 a ₀ × 4 a ₀ charge-ordered state discovered by scanning tunneling microscopy in the lightly doped cuprate superconductor Ca _2–x Na _x CuO ₂ Cl ₂ . Our measurements reveal a marked dichotomy between the real- and momentum-space probes, for which charge ordering is emphasized in the tunneling measurements and photoemission is most sensitive to excitations near the node of the d -wave superconducting gap. These results emphasize the importance of momentum anisotropy in determining the complex electronic properties of the cuprates and places strong constraints on theoretical models of the charge-ordered state.