An angle-resolved photoemission study is reported on Ca 2 CuO 2 Cl 2 , a parent compound of high- T c superconductors. Analysis of the electron occupation probability, n( k ), from the spectra shows a steep drop in spectral intensity across a contour that is close to the Fermi surface predicted by the band calculation. This analysis reveals a Fermi surface remnant, even though Ca 2 CuO 2 Cl 2 is a Mott insulator. The lowest energy peak exhibits a dispersion with approximately the |cos k x a – cos k y a | form along this remnant Fermi surface. Together with the data from Dy-doped Bi 2 Sr 2 CaCu 2 O 8+δ , these results suggest that this d -wave–like dispersion of the insulator is the underlying reason for the pseudo gap in the underdoped regime.

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than 50 years of research. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. High-resolution resonant inelastic x-ray scattering revealed dispersive charge excitations across the Mott gap in a high-critical temperature parent cuprate (Ca(2)CuO(2)Cl(2)), shedding light on the anisotropy of the Mott gap. These charge excitations across the Mott gap can be described within the framework of the Hubbard model.

The resistivity p,b( T) and anisotropic magnetization M ( T) of ErNi28&C crystals were measured as a function of applied magnetic field (H) from 2 to 300 K to study the magnetic and superconducting properties with H parallel and perpendicular to the c axis (H~~c and Hlc). Low-temperature specific-heat measurements for H=O show a lambda-shaped anomaly associated with antiferromagnetic (AF) ordering at TN=(5. 85+0. 15) K. The p, b(T) and M(T) data show a superconducting transition at T, =10.5 K, long-range AF ordering at T& =6.0 K, and coexistence of superconductivity and antiferromagnetism below T&. From the p, b(T) and M( T) data, the Tz was found to be independent of H up to H =20 kG for H~~c, whereas for Hjc T& decreased as -H from 6.0 K at H=O to Tn=3. 2 K at H=18 kG. The M(T} data show a change in the easy axis direction from H~~c above 150 to Hlc below 150 K. This change in anisotropy is associated with the anomalously small magnitude of the crystalline electric field B2 term. The superconducting upper critical magnetic fields H, z(T) for H~~c and Hlc, determined from M(T) and p,b(H, T) data, show anomalies for both field orientations near T&. However, the local minimum of H, z(T) near T~, seen previously for both Hlc and H~~c in a HoNi2B2C crystal, was found only for H~~c in ErNi, B,C. This anisotropy in H, 2( T) is likely a result of the anisotropy of the Er sublattice magnetization, specifically the anisotropy of T~(H). The depth of the local minimum in H, z for H~~c for ErNi2B&C near T& is comparable to the one for HoNi2B2C at 5 K.