First principles studies of GeTe based dilute magnetic semiconductors

Thermoelectric GeTe with Diverse Degrees of Freedom Having Secured Superhigh Performance

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh-performance GeTe-based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density-of-state resonant distortion are employed to further enhance the thermoelectric performance of GeTe-based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon-phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe-based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe-based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.

Entanglement and manipulation of the magnetic and spin-orbit order in multiferroic Rashba semiconductors

Entanglement of the spin–orbit and magnetic order in multiferroic materials bears a strong potential for engineering novel electronic and spintronic devices. Here, we explore the electron and spin structure of ferroelectric α-GeTe thin films doped with ferromagnetic Mn impurities to achieve its multiferroic functionality. We use bulk-sensitive soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to follow hybridization of the GeTe valence band with the Mn dopants. We observe a gradual opening of the Zeeman gap in the bulk Rashba bands around the Dirac point with increase of the Mn concentration, indicative of the ferromagnetic order, at persistent Rashba splitting. Furthermore, subtle details regarding the spin–orbit and magnetic order entanglement are deduced from spin-resolved ARPES measurements. We identify antiparallel orientation of the ferroelectric and ferromagnetic polarization, and altering of the Rashba-type spin helicity by magnetic switching. Our experimental results are supported by first-principles calculations of the electron and spin structure.

In α-GeTe, ferroelectric polarization acts to break inversion symmetry of the lattice and induce a strong Rashba-type spin splitting of the electronic band structure. Here, the authors study how this effect competes with Zeeman splitting due to ferromagnetic exchange coupling in Mn-doped GeTe.

Ferroelectric phase transition in multiferroic Ge1-x Mn x Te driven by local lattice distortions

The evolution of local ferroelectric lattice distortions in multiferroic Ge_1-x Mn _x Te is studied by x-ray diffraction, x-ray absorption spectroscopy and density functional theory. We show that the anion/cation displacements smoothly decrease with increasing Mn content, thereby reducing the ferroelectric transition from 700 to 100 K at x = 0.5, where the ferromagnetic Curie temperature reaches its maximum. First principles calculations explain this quenching by different local bond contributions of the Mn 3d shell compared to the Ge 4s shell in excellent quantitative agreement with the experiments.