3 research outputs found
Two-step stabilization of orbital order and the dynamical frustration of spin in the model charge-transfer insulator KCuF3
We report a combined experimental and theoretical study of KCuF3, which
offers - because of this material's relatively simple lattice structure and
valence configuration (d9, i.e., one hole in the d-shell) - a particularly
clear view of the essential role of the orbital degree of freedom in governing
the dynamical coupling between the spin and lattice degrees of freedom. We
present Raman and x-ray scattering evidence that the phase behaviour of KCuF3
is dominated above the Neel temperature (T_N = 40 K) by coupled orbital/lattice
fluctuations that are likely associated with rotations of the CuF6 octahedra,
and we show that these orbital fluctuations are interrupted by a static
structural distortion that occurs just above T_N. A detailed model of the
orbital and magnetic phases of KCuF3 reveals that these orbital fluctuations -
and the related frustration of in-plane spin-order-are associated with the
presence of nearly degenerate low-energy spin-orbital states that are highly
susceptible to thermal fluctuations over a wide range of temperatures. A
striking implication of these results is that the ground state of KCuF3 at
ambient pressure lies near a quantum critical point associated with an
orbital/spin liquid phase that is obscured by emergent Neel ordering of the
spins; this exotic liquid phase might be accessible via pressure studies.Comment: 13 pages, 3 figure
Theory of the composition dependence of the band offset and sheet carrier density in the GaN/AlxGa1-xN heterostructure
We present a systematic study of the sheet carrier density and valence-band offset in the GaN/AlxGa1-xN(0001) heterostructure as a function of x from ab initio density-functional methods. We find that the calculated sheet carrier density increases rapidly with x for xless than or equal to0.3 in good agreement with experiments, but beyond this concentration, it quickly saturates to a value of about 2x10(13) cm(-2). The band offset shows a small asymmetry between the Ga-face and N-face interfaces and changes more or less linearly with x, although a small bowing is found. The layer-projected densities of states indicate the formation of the two-dimensional electron gas at the Ga-face interface and confirm the absence of interface states in the gap. (C) 2004 American Institute of Physics