Interlayer Coupling of a Two-Dimensional Kondo Lattice with a Ferromagnetic Surface in the Antiferromagnet CeCo2P2.

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

Spin Orientation of Two-Dimensional Electrons Driven by Temperature-Tunable Competition of Spin-Orbit and Exchange-Magnetic Interactions.

Finding ways to create and control the spin-dependent properties of two-dimensional electron states (2DESs) is a major challenge for the elaboration of novel spin-based devices. Spin-orbit and exchange-magnetic interactions (SOI and EMI) are two fundamental mechanisms that enable access to the tunability of spin-dependent properties of carriers. The silicon surface of HoRh2Si2 appears to be a unique model system, where concurrent SOI and EMI can be visualized and controlled by varying the temperature. The beauty and simplicity of this system lie in the 4f moments, which act as a multiple tuning instrument on the 2DESs, as the 4f projections parallel and perpendicular to the surface order at essentially different temperatures. Here we show that the SOI locks the spins of the 2DESs exclusively in the surface plane when the 4f moments are disordered: the Rashba-Bychkov effect. When the temperature is gradually lowered and the system experiences magnetic order, the rising EMI progressively competes with the SOI leading to a fundamental change in the spin-dependent properties of the 2DESs. The spins rotate and reorient toward the out-of-plane Ho 4f moments. Our findings show that the direction of the spins and the spin-splitting of the two-dimensional electrons at the surface can be manipulated in a controlled way by using only one parameter: the temperature.

X-ray absorption microscopy of bacterial surface protein layers: X-ray damage.

The electronic structure of individual sheets of the bacterial surface protein layer (S layer) of Bacillus sphaericus NCTC 9602 was studied using a photoemission electron microscope (PEEM) operating in near-edge X-ray absorption fine structure spectroscopy mode. The laterally resolved measurements performed at the C 1s, N 1s, and O 1s thresholds on fresh samples revealed characteristic differences compared to the laterally integrated data, where substrate contributions were taken along with the protein signals. During the PEEM experiments an irradiation-induced increase of the C-C bond density at the cost of the densities of the C-O and C-N bonds related to a rearrangement of the contributing atoms of the S layer deposited onto a Si substrate was observed. The critical irradiation doses for the C-O and C-N bond breaking and formation of the new C-C bonds were derived.

Photoemission and near-edge X-ray absorption fine structure studies of the bacterial surface protein layer of Bacillus sphaericus NCTC 9602.

The electronic structure of the regular, two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602 has been examined by photoemission (PE) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both the O 1s and the N 1s core-level PE spectra show a single structure, whereas the C 1s core-level spectrum appears manifold, suggesting similar chemical states for each oxygen atom and also for each nitrogen atom, while carbon atoms exhibit a range of chemical environments in the different functional groups of the amino acids. This result is supported by the element-specific NEXAFS spectra of the unoccupied valence electronic states, which exhibit a series of characteristic NEXAFS peaks that can be assigned to particular molecular orbitals of the amino acids by applying a phenomenological building-block model. The relative contributions of the C-O, C-N, and C-C bond originating signals into the C 1s PE spectrum are in good agreement with the number ratios of the corresponding bonds calculated from the known primary structure of the bacterial surface protein. First interpretation of the PE spectrum of the occupied valence states is achieved on the basis of electronic density-of-states calculations performed for small peptides. It was found that mainly the pi clouds of the aromatic rings contribute to both the lowest unoccupied and the highest occupied molecular orbitals.

Electronic structure of regular bacterial surface layers.

We report photoemission and near-edge x-ray absorption fine structure measurements of the occupied and unoccupied valence electronic states of the regular surface layer of Bacillus sphaericus, which is widely used as the protein template for the fabrication of metallic nanostructures. The two-dimensional protein crystal shows a semiconductorlike behavior with a gap value of approximately 3.0 eV and the Fermi energy close to the bottom of the lowest unoccupied molecular orbital. We anticipate that these results will open up new possibilities for the electric addressability of biotemplated low-dimensional hybrid structures.