The response of the surface states at then-type Si-SiO₂ interface to the different a.c. signal frequencies has been studied. The response values have been computed from both the measured capacitance voltage (C-V) and conductance-voltage (G-V) techniques. The results presented show that the frequency response of the effective density of states to different a.c. signal frequencies is proportional to the log of the applied frequencies.

Hill-Coleman’s single frequency conductance technique for the determination of surface state density has been extended upto 2 kHz. A.c. conductance (G_m) and capacitance (C_m)versus gate bias (V_G) curves were obtained at various signal frequencies. Shift of the observed peaks in theG_m versus V_G curves for different signal frequencies was utilized for the determination of surface state density at different surface potentials (φ_s). Determination of surface state density for differentφ_s values was also done by Nicollian-Goetzberger method and the results compared. Results obtained by Hill-Coleman technique compare reasonably well with those obtained by the other method.

High-T_c superconductors are EPR silent but on a little deoxygenation of the high-T_c materials and their constituents, they yield rich but complex spectra. Spectra of (1) CuO, (2) BaCuO₂, (3) CaCuO₂, (4) Y₂Cu₂O₅, (5) La₂CuO₄, (6) La_2−xM_xCuO₄ (M=Sr, Ba), (7) Y based-123, (8) Bi based-2201, 2212, 2223, (9) Tl based-2223 and (10) Hg based-1212, 1223 have been studied. One thing common to all these materials is the CuO₂ plane which gets fragmented on deoxygenation and the inherent antiferromagnetic coupling is partially destroyed which results in the appearance of the spectra.

The spectra recorded have been identified to be due to (1) Cu-monomer, (2) Cu-dimer, (3) Cu-tetramer, (4) Cu-octamer and (5) one signal at very low field which could not be identified because there was no structure in it and may be due to fragments higher than octamers. Very big fragments do not give any spectra because the original AF order probably remains intact in them. It is expected that when the fragments become magnetically isolated from the bulk, they produce EPR spectra. Most of the spectra have been analyzed and their spin-Hamiltonian parameters determined. The spectra of these species vary a little in terms of g-value and fine-structure splitting constant from sample to sample or even in the same sample and this may be attributed to some extra oxygen attachments retained with these species. Most frequently occurring species is the Cu-tetramer, (CuO)₄. As (CuO)₄ represents the unit cell of the all important two-dimensional CuO₂ plane of the high-T_c materials, its spectra have been argued to provide some clue to the mechanism of high-T_c superconductivity. The tetramer (CuO)₄ is a four one-half spin system and is essentially 16-fold degenerate by Heisenberg isotropic exchange, it is split into 6 components: one pentet, three triplets and two singlets. In superconductors the pentet appears to be the ground state and in the non-superconducting constituents the singlets seem to form the ground state as revealed by the temperature variation studies. In the case of La_1.854Sr_0.146CuO₄ we have found the signature of quantum stripe formation. The high-T_c superconductivity theories involving spin bag, antiferromagnetic spin fluctuations and magnons can be explained on the basis of Cu-tetramers.