arXiv subjects

A. Calloni

Publications and source records attributed to A. Calloni.

3 recordsLinked to original sources

arXiv2026

Unoccupied spin texture of ordered Bi films on vicinal Ge substrates

Vicinal surfaces offer a compelling testing ground in surface science: breaking the high symmetry of a singular surface introduces periodic step arrays that can modify surface states on the terraces and influence epitaxial ordering; the interplay between these effects governs morphology and electronic structure, especially in heavy, spin--orbit-coupled materials. We combine low-energy electron diffraction (LEED) and spin-polarized inverse photoemission (SP-IPES) with ab-initio calculations for the Bi$(1\,1\,1)$ reference, for the first time on a true high-index vicinal bismuth surface. On the ordered Bi$(9\,9\,11)$ vicinal film selected by Ge$(2\,2\,3)$, the measured unoccupied-state spin-sign sequence matches the calculated initial-state spectral function of Bi$(1\,1\,1)$ terraces. Conversely, the pseudo-cubic Bi domains retained on Ge$(4\,4\,3)$ and Ge$(3\,3\,1)$ exhibit a characteristic spin-polarized surface-state response referenced to the macroscopic vicinal surface. These results show that the substrate cut selects not only the morphology of the Bi film, but also the crystallographic reference frame of its unoccupied spin texture.

cond-mat.mtrl-sci↗

arXiv2018

Tuning spin-charge interconversion with quantum confinement in ultrathin Bi/Ge(111) films

Spin-charge interconversion (SCI) phenomena have attracted a growing interest in the field of spintronics as means to detect spin currents or manipulate the magnetization of ferromagnets. The key ingredients to exploit these assets are a large conversion efficiency, the scalability down to the nanometer scale and the integrability with opto-electronic and spintronic devices. Here we show that, when an ultrathin Bi film is epitaxially grown on top of a Ge(111) substrate, quantum size effects arising in nanometric Bi islands drastically boost the SCI efficiency, even at room temperature. Using x-ray diffraction (XRD), scanning tunneling microscopy (STM) and spin- and angle-resolved photoemission (S-ARPES) we obtain a clear picture of the film morphology, crystallography and electronic structure. We then exploit the Rashba-Edelstein effect (REE) and inverse Rashba-Edelstein effect (IREE) to directly quantify the SCI efficiency using optical and electrical spin injection.

cond-mat.mtrl-sci↗

arXiv2014

Condensation of holes into antiferromagnetic droplets in the organic semiconductor (DOEO)$_4$[HgBr$_4$]TCE

Changes of the electronic structure accompanied by charge localization and a transition to an antiferromagnetic ground state were observed in the (DOEO)$_4$[HgBr$_4$]TCE organic semiconductor. Localization starts in the region of about 150 K and the antiferromagnetic state occurs below 60 K. The magnetic moment of the crystal contains contributions of antiferromagnetic inclusions (droplets), individual paramagnetic centers formed by localized holes and free charge carriers at 2 K. Two types of inclusions of 100-400 nm and 2-5 nm sizes were revealed by transmission electron microscopy. Studying the symmetry of the antiferromagnetic droplets (100-400 nm inclusions) and individual localized holes by electron spin resonance (ESR) revealed fingerprints of the antiferromagnetic resonance spectra of the spin correlated droplets as well as paramagnetic resonance spectra of the individual localized charge carriers. Photoelectron spectroscopy in the VUV, soft and hard X-ray range shows temperature-dependent effects upon crossing the critical temperature. The substantially different probing depths of soft and hard X-ray photoelectron spectroscopy yield information on the surface termination. The combined investigation using soft and hard X-ray photons to study the same sample results in details of electronic structure including structural aspects at the surface.

cond-mat.str-el↗