kpenvelope: self-consistent six-band k.p envelope-function solver for wurtzite heterostructures

Abstract

Experimental-conditions release: the two knobs every measurement actually has -- a temperature and lab units -- plus an honest convergence report. Added Finite-temperature subband filling. fill_subbands_thermal implements the closed form n_i = dos_i kT ln(1 + exp((E_i - E_F)/kT)) for 2D parabolic hole subbands, and temperature_K is now a parameter of solve_self_consistent, solve_self_consistent_hetero and fill_subbands_kgrid (Fermi-Dirac occupation factor on the k-grid). The default 0 reproduces the historical cold filling exactly -- the same code path, asserted bit for bit in the tests. The Boltzmann constant in eV/K (KB_EV_PER_K) is computed from the two exact SI defining constants (k_B and e, both exact since the 2019 redefinition), not typed by hand. Lab units in and out. sheet_density_from_cm2 / sheet_density_to_cm2: exact powers of ten between the cm^-2 a lab quotes and the nm^-2 the solver speaks -- the single most common way to be wrong by orders of magnitude when driving the solver from measured numbers. SelfConsistentResult.converged. An iteration-starved run now reports its failure instead of hiding it in a residual the caller must remember to inspect. Changed The k-grid filler's undersized-window refusal extends to finite temperature (a Fermi tail reaching the grid edge is refused, as the T = 0 Fermi disc was). README rewritten: organized by what the package does rather than by release history, in plainer language, same facts and citations. Anchors (asserted in tests/test_thermal.py, not stated) KB_EV_PER_K from the exact SI definitions agrees with the CODATA eV/K value to 1e-14. The thermal closed form matches direct numerical quadrature of the Fermi-Dirac occupation over the constant 2D DOS (two independent code paths) to 1e-10 at several Fermi levels. temperature_K = 0 routes through the identical T = 0 filler (array_equal), the T -> 0 limit converges to it, and neutrality is exact (1e-15) at 4.2, 77 and 300 K. Warming moves holes from the top subband into lower ones -- the physics the temperature knob controls. On the exactly parabolic demo model the finite-T k-grid filler agrees with the closed-form thermal filler; an undersized k-window is refused. The self-consistent loop at 300 K converges with exact neutrality; the T = 0 default reproduces the historical run bit for bit; a max_iter-starved run reports converged = False. 4.6e13 cm^-2 = 0.46 nm^-2 exactly; conversions round-trip exactly. Full test suite: 54 passed (Python 3.9-3.13). See CHANGELOG.md for details.