The mesoscopic X-ray edge problem: Boundary effects enhance photoabsorption in quantum dots.

     Nano- or mesoscopic systems, such as nanoparticles or quantum dots, may differ significantly in their properties from that of macroscopic samples, such as metallic probes: In the featured article we focus on the absorption properties for X-rays, i.e., the photoabsorption cross section at the so-called K-edge. We find it to characteristically differ in quantum dots,  where the signal is peaked, from the rounded photoabsorption signal at the Fermi edge that has been  long known for bulk metals. The origin of this difference lies in the existence of a system  boundary – that is, if you will, exactly the property that makes a sample to be in the nano-/mesoscopic rather than in a macroscopic regime: Other than metallic Bloch waves, the electronic wave function drops to zero at mesoscopic system boundaries (see figure below). The wave function derivative has to take, as a direct consequence, relatively large values at the same time. These enter the photoabsorption cross section via the dipole matrix element, dominate the overall photoabsorption signal, and eventually explain the large absorption values of mesoscopic samples in contrast to metallic probes. Moreover, we had previously seen characteristic boundary effects in the photoabsorption signal for graphene (Röder et al., Europhys.Lett. 2011). These examples illustrate the application potential as well at the importance of edge and boundary effects in the continuing miniaturization of devices.