Open Access

Quantifying domain population in multiferroics is required to understand domain nucleation/switching processes and achieve on-demand domain control. We report an approach based on nuclear magnetic resonance spectroscopy for the accurate measurement of volume fractions of multiferroic domains in bulk crystals. We demonstrate on a benchmarking system, GaV4Se8, that the electric quadrupole interaction of the 71Ga and the hyperfine field at the 51V nuclei are proper microscopic probes of the ferroelectric polarization and the ferromagnetic moment, respectively. We use the anisotropy of these local quantities to determine the multiferroic domain population, controlled here by both electric and magnetic fields. The sensitivity of this local-probe technique to site symmetries facilitates domain quantification in a wide range of anisotropic magnets, ferroelectrics, and multiferroics.

The current study generates profound atomistic insights into doping-induced changes of the optical and electronic properties of the prototypical PTCDA/Ag(111) interface. For doping K atoms are used, as KxPTCDA/Ag(111) has the distinct advantage of forming well-defined stoichiometric phases. To arrive at a conclusive, unambiguous, and fully atomistic understanding of the interface properties, we combine state-of-the-art density-functional theory calculations with optical differential reflectance data, photoelectron spectra, and X-ray standing wave measurements. In combination with the full structural characterization of the KxPTCDA/Ag(111) interface by low-energy electron diffraction and scanning tunneling microscopy experiments (ACS Nano2016, 10, 2365–2374), the present comprehensive study provides access to a fully characterized reference system for a well-defined metal–organic interface in the presence of dopant atoms, which can serve as an ideal benchmark for future research and applications. The combination of the employed complementary techniques allows us to understand the peculiarities of the optical spectra of K2PTCDA/Ag(111) and their counterintuitive similarity to those of neutral PTCDA layers. They also clearly describe the transition from a metallic character of the (pristine) adsorbed PTCDA layer on Ag(111) to a semiconducting state upon doping, which is the opposite of the effect (degenerate) doping usually has on semiconducting materials. All experimental and theoretical efforts also unanimously reveal a reduced electronic coupling between the adsorbate and the substrate, which goes hand in hand with an increasing adsorption distance of the PTCDA molecules caused by a bending of their carboxylic oxygens away from the substrate and toward the potassium atoms.