We study light absorption by nearly monodisperse PbS nanocrystals grown in a glass matrix. The absorption spectra demonstrate a well-resolved structure at energies above the first absorption peak. The absorption cross sections are calculated with an empirical tight-binding model accounting for the quantum confinement, the band anisotropy, and valley splittings. Absorption spectra measured at 4 K quantitatively agree with the atomistic calculations. This allows us to unambiguously ascribe the much-debated second absorption peak to the p–p optical transition and directly measure the splitting of excited p states.

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The physics of interacting nuclear spins in solids is well interpreted within the nuclear spin temperature concept. A common approach to cooling the nuclear spin system is adiabatic demagnetization of the initial, optically created, nuclear spin polarization. Here, the selective cooling of 75As spins by optical pumping followed by adiabatic demagnetization in the rotating frame is realized in a nominally undoped GaAs/(Al,Ga)As quantum well. The lowest nuclear spin temperature achieved is 0.54 μK. The rotation of 6 kG strong Overhauser field at the 75As Larmor frequency of 5.5 MHz is evidenced by the dynamic Hanle effect. Despite the presence of the quadrupole induced nuclear spin splitting, it is shown that the rotating 75As magnetization is uniquely determined by the spin temperature of coupled spin-spin and quadrupole reservoirs. The dependence of heat capacity of these reservoirs on the external magnetic field direction with respect to crystal and structure axes is investigated.

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