The Semiconductor Physics Department consists of the following **Research Groups**, according to the different types of fabrication and characterization of semiconductor nanostructures:

**Nanoscale Semiconductors (Rastelli); Nanophotonics (Trotta); ****SiGe Nanostructures (Schäffler, Fromherz, Brehm, Rauter)****; Topological Insulators (Springholz); ****X-Ray Scattering (Stangl)**

## OUR RESEARCH HIGHLIGHTS

**Xueyong Yuan, **Fritz Weyhausen-Brinkmann, Javier Martín-Sánchez, Giovanni Piredda, Vlastimil Křápek, Yongheng Huo**, Huiying Huang, Christian Schimpf, **Oliver G. Schmidt, Johannes Edlinger, Gabriel Bester**, Rinaldo Trotta, Armando Rastelli**

*Uniaxial stress flips the natural quantization axis of a quantum dot for integrated quantum photonics*

**Nature Communications 9, 3058 (2018)**

*********************************************************************************************************************************************************************************************************************************************************************************************

**Daniel Huber, Marcus Reindl, Saimon Filipe Covre da Silva, Christian Schimpf, Javier Martín-Sánchez, Huiying Huang**, Giovanni Piredda, Johannes Edlinger, **Armando Rastelli, and Rinaldo Trotta***:*

*Strain-Tunable GaAs Quantum Dot: A Nearly Dephasing-Free Source of Entangled Photon Pairs on Demand*

**Phys. Rev. Lett. 121, 033902 (2018).**

*********************************************************************************************************************************************************************************************************************************************************************************************

Entangled photon pairs are a key resource for quantum communication. We have now achieved the highest degree of polarization entanglement for photon pairs emitted by quantum dots by combining: 1) High-quality GaAs quantum dots grown by molecular beam epitaxy at our institute and embedded in planar cavities for enhanced light extraction, 2) resonant two-photon excitation for on demand operation, and 3) micro-structured piezoelectric actuators for removing the effect of residual anisotropy in the quantum dot potential. This work shows that quantum dots have the potential to fulfill the criteria of a perfect entangled-photon source.

*********************************************************************************************************************************************************************************************************************************************************************************************

J. Krempaský, S. Muff, J. Minár, N. Pilet, M. Fanciulli, A. P. Weber, E. B. Guedes, M. Caputo, E. Müller, **V. V. Volobuev**, M. Gmitra, C. A. F. Vaz, V. Scagnoli, **G. Springholz**, and J. H. Dil*:*

*Operando Imaging of All-Electric Spin Texture Manipulation in Ferroelectric and Multiferroic Rashba Semiconductors*

**Phys. Rev. X 8, 021067 (2018)**

The control of the electron spin by external means is a key issue for spintronic devices. Using spin- and angle-resolved photoemission spectroscopy (SARPES) with three-dimensional spin detection, we demonstrate operando electrostatic spin manipulation in ferroelectric α−GeTe and multiferroic Ge1−xMnxTe. We demonstrate for the first time electrostatic spin manipulation in Rashba semiconductors due to ferroelectric polarization reversal. In multiferroic Ge1−xMnxTe operando SARPES reveals switching of the perpendicular spin component due to electric-field-induced magnetization reversal. This provides firm evidence of magnetoelectric coupling which opens up functionality with a multitude of spin-switching paths in which the magnetic and electric order parameters are coupled through ferroelastic relaxation paths. This work thus provides a new type of magnetoelectric switching intertwined with Rashba-Zeeman splitting in a multiferroic system.

A. B. Henriques, X. Gratens, P. A. Usachev, V. A. Chitta, and **G. Springholz**:

*Ultrafast Light Switching of Ferromagnetism in EuSe*

**Phys. Rev. Lett. 120, 217203 (2018)**

We demonstrate that light resonant with the band gap forces the antiferromagnetic semiconductor EuSe to enter ferromagnetic alignment in the picosecond timescale. A photon generates an electron-hole pair, whose electron forms a supergiant spin polaron of magnetic moment of nearly 6000 Bohr magnetons. By increasing the light intensity, the whole of the illuminated region can be fully magnetized. The key to the novel large photoinduced magnetization mechanism is the huge enhancement of the magnetic susceptibility when both antiferromagnetic and ferromagnetic interactions are present in the material and are of nearly equal magnitude, as is the case in EuSe.

B. Mandl, M. Keplinger, M. E. Messing, D. Kriegner, R. Wallenberg, L. Samuelson, **G. Bauer, J. Stangl**, V. Holy, K. Deppert:

*Self-Seeded Axio-Radial InAs–InAs*_{1–x}P_{x }Nanowire Heterostructures beyond “Common” VLS Growth

**Nano Lett. 18, 144-151 (2018)**

_{1–x}P

_{x }Nanowire Heterostructures beyond “Common” VLS Growth

Semiconductors are essential for modern electronic and optoelectronic devices. To further advance the functionality of such devices, the ability to fabricate increasingly complex semiconductor nanostructures is of utmost importance. Nanowires offer excellent opportunities for new device concepts; heterostructures have been grown in either the radial or axial direction of the core nanowire but never along both directions at the same time. This is a consequence of the common use of a foreign metal seed particle with fixed size for nanowire heterostructure growth. In this work, we present for the first time a growth method to control heterostructure growth in both the axial and the radial directions simultaneously while maintaining an untapered self-seeded growth. This is demonstrated for the InAs/InAs_{1–x}P_{x} material system. We show how the dimensions and composition of such axio-radial nanowire heterostructures can be designed including the formation of a “pseudo-superlattice” consisting of five separate InAs_{1–x}P_{x} segments with varying length. The growth of axio-radial nanowire heterostructures offers an exciting platform for novel nanowire structures applicable for fundamental studies as well as nanowire devices. The growth concept for axio-radial nanowire heterostructures is expected to be fully compatible with Si substrates.

L. Schweickert, K. D. Jöns, K. D. Zeuner, **S. F. Covre da Silva, H. Huang, T. Lettner, M. Reindl**, J. Zichi,** R. Trotta, A. Rastelli**, V. Zwiller:

*On-demand generation of background-free single photons from a solid-state source*

**Appl. Phys. Lett. 112, 093106 (2018)**

Our collaborators at KTH Stockholm demonstrated an unprecedented single photon purity in the solid-state which exceeds the performance of any available single photon source to date. The used quantum dot source was grown in Linz.

**V. Volobuiev**, P. S. Mandal, M. Galicka, O. Caha, J. Sanchez-Barriga, D. Di Sante, A. Varykhalov, A. Khiar, S. Picozzi, **G. Bauer**, P. Kacman, R. Buczko, O. Rader, **G. Springholz**:

*Giant Rashba Splitting in Pb*_{1– x}Sn_{x}Te (111) Topological Crystalline Insulator Films Controlled by Bi Doping in the Bulk

_{1– x}Sn

_{x}Te (111) Topological Crystalline Insulator Films Controlled by Bi Doping in the Bulk

**Adv. Mater. 29, 1604185 (2017**)

**The topological properties of lead‐tin chalcogenide topological crystalline insulators** can be widely tuned by temperature and composition. It is shown that bulk Bi doping of epitaxial Pb_{1‐}* _{x}*Sn

*Te (111) films induces a giant Rashba splitting at the surface that can be tuned by the doping level. Tight binding calculations identify their origin as Fermi level pinning by trap states at the surface.*

_{x}**Highlight of Semiconductor Science and Technology 2017**

**Semicond. Sci. Technol. 32 (2017) 02LT01**

The article* "Photoluminescence enhancement through vertical stacking of defect-engineered Ge on Si quantum dots*", by H. Groiss, L. Spindlberger, P. Oberhumer, F. Schäffler, T. Fromherz, M. Grydlik and M. Brehm,

has been chosen for the 2017 Semiconductor Science and Technology (SST) highlight collection.

**Highlight in Nanotechnology 2017**

**Nanotechnology 28, 392001 (2017)**

The topical review** "Site-controlled and advanced epitaxial Ge/Si quantum dots: fabrication, properties and applications" **by

**Moritz Brehm and Martyna Grydlik**/ was included in the Highlights of Nanotechnology 2017.

This collection includes outstanding articles and Topical Reviews published in the journal during 2017. These articles were selected on the basis of a range of criteria including referee endorsements, presentation of outstanding research and popularity with our online readership. The articles will be free to read until the end of December 2018. Our article is open access anyways, thanks to Funding from the FWF.

P. S. Mandal, **G. Springholz, V. V. Volobuev**, O. Caha, A. Varykhalov, E. Golias,** G. Bauer**, O. Rader, J. Sánchez-Barriga:

*Topological quantum phase transition from mirror to time reversal symmetry protected topological insulator*

**Nature Communications 8, 968 (2017)**

Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z_{2} class, which possess an odd number of metallic surface states with dispersion of a Dirac cone. Topological crystalline insulators are merely protected by individual crystal symmetries and exist for an even number of Dirac cones. Here, we demonstrate that Bi-doping of Pb_{1−x}Sn_{x}Se (111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator.

B. A. Assaf, T. Phuphachong, E. Kampert, **V. Volobuiev**, P. S. Mandal, J. Sanchez-Barriga, O. Rader, **G. Bauer, G. Springholz**, L. A. de Vaulchier, Y. Guldner:

*Negative Longitudinal Magnetoresistance from the Anomalous N=0 Landau Level in Topological Materials*

*Negative Longitudinal Magnetoresistance from the Anomalous N=0 Landau Level in Topological Materials*

**Phys. Rev. Lett. 119, 106602 (2017)**

Negative longitudinal magnetoresistance (NLMR) is shown to occur in topological materials in the extreme quantum limit, when a magnetic field is applied parallel to the excitation current. We perform pulsed and dc field measurements on Pb1−xSnxSe epilayers where the topological state can be chemically tuned. The NLMR is observed in the topological state, but is suppressed and becomes positive when the system becomes trivial. In a topological material, the lowest N=0 conduction Landau level disperses down in energy as a function of increasing magnetic field, while the N=0 valence Landau level disperses upwards. This anomalous behavior is shown to be responsible for the observed NLMR. Our work provides an explanation of the outstanding question of NLMR in topological insulators and establishes this effect as a possible hallmark of bulk conduction in topological matter.

**H. Groiß, M. Glaser, M. Schatzl, M. Brehm**, D. Gerthsen, D. Roth, P. Bauer, **F. Schäffler**

*Free-running Sn precipitates: an efficient phase separation mechanism for metastable Ge*_{1−x}Sn_{x}

_{1−x}Sn

### Scientific Reports 7, 16114 (2017)

**M. Schatzl, F. Hackl, M. Glaser, P. Rauter, M. Brehm, L. Spindlberger,** A. Simbula, M. Galli, **T. Fromherz, F. Schäffler**