Detlev Grützmacher

Control of magnetic order through optical manipulation of orbital states can be realized in a wide range of materials, including insulators. In the latter case, electrons are excited into different electronic states, whose character may be a complex result of spin-orbit-lattice coupling. Here, we use a combination of polarization-resolved photoluminescence spectroscopy and density functional theory calculations to study excited Cr ³⁺ ions in bulk, insulating, antiferromagnetic CrCl ₃ . Our results demonstrate a temperature-dependent preferential polarization direction below ~17 K, which reorients in the presence of an applied in-plane magnetic field. Density functional theory calculations reveal a local distortion of the ligand field and the generation of out-of-plane spin and orbital moments, following the excitation of Cr ³⁺ ion. These changes link the photoluminescence polarization to the magnetic state of CrCl ₃ . Our findings provide insight into the electronic structure of excited-state Cr ³⁺ ions and suggest that CrCl ₃ is a promising candidate for opto-orbitronic applications.

Conventional Si Complementary Metal Oxide Semiconductor (CMOS) technologies face increasing challenges in improving energy efficiency while maintaining device performance. Here, SiGeSn/GeSn heterostructures are employed as a unified Group‐IV platform for complementary vertical nanowire gate‐all‐around (GAA) MOSFETs and tunnel field‐effect transistors (TFETs) fabricated on Si wafers. The wider‐bandgap SiGeSn source/drain regions introduce band offsets to the GeSn channel, enabling heterostructure band engineering for leakage suppression and tunneling‐pathway control. Compared with all‐GeSn reference devices, the SiGeSn source/drain architecture suppresses drain‐induced barrier lowering and off‐state leakage without modifying the GeSn channel. The same heterostructure platform also enables complementary TFET operation, where the n‐TFET exhibits a subthreshold swing of 60 mV dec ^{− 1} and a two‐order‐of‐magnitude enhancement in on/off current ratio. The performance difference between p‐ and n‐TFETs provides experimental evidence of the different efficiencies of indirect Γ‐to‐L and direct Γ‐to‐Γ tunneling processes. These results demonstrate that SiGeSn/GeSn heterostructures can be used to suppress leakage currents and engineer tunneling pathways, establishing SiGeSn/GeSn heterostructures as a unified Group‐IV platform for future energy‐efficient CMOS technologies beyond conventional Si.

In GaAs/InAs core/shell nanowires, comprising a tubular conducting shell, interference phenomena observed under an axial field and originating from closed-loop states encircling the insulating core provide an ideal platform for superconducting quantum devices that utilize effects such as Aharonov-Bohm–type or Altshuler-Aronov-Spivak–type conductance oscillations. Both effects are different in nature with respect to phase rigidity because of interference of non-time-reversed or time-reversed paths, respectively. Since their occurrence is largely governed by averaging effects, which depend on sample dimensions and the transport regime, we present a systematic study of flux-periodic oscillations of phase-pure zinc-blende GaAs/InAs core/shell nanowires as a function of gate voltage for samples with different contact separation lengths. Our analysis shows that with increasing contact separation length, averaging effects result in gradual reduction of h / e -periodic Aharonov-Bohm–type oscillations, while relative to that the h / 2 e -periodic Altshuler-Aronov-Spivak oscillations and its h / 4 e -periodic higher harmonics are enhanced. The additional phase rigidity seen in the h / 3 e -periodic oscillations is attributed to phase rigidity propagating from the neighboring lower harmonics. Our tight-binding transport simulations on nanowires of different lengths which contain only a few scattering centers confirm the experimental observations regarding the different harmonics and their phase rigidity. Together, our experimental and simulation findings indicate quasiballistic transport with persistent Aharonov-Bohm–type and phase-rigid Altshuler-Aronov-Spivak–type oscillations despite few scattering centers.

Improvements in circuit design and, more recently, in materials and surface cleaning have contributed to a rapid development of coherent superconducting qubits. However, organic resists commonly used for shadow evaporation of Josephson junctions (JJs) pose limitations due to residual contamination, poor thermal stability, and compatibility under typical surface-cleaning conditions. To provide an alternative, we developed an inorganic SiO2/Si3N4 on-chip stencil lithography mask for JJ fabrication. The stencil mask is resilient to aggressive cleaning agents and it withstands high temperatures up to 1200 °C, thereby opening new avenues for JJ material exploration and interface optimization. To validate the concept, we performed shadow evaporation of Al-based transmon qubits followed by stencil mask liftoff using vapor hydrofluoric acid, which selectively etches SiO2. We demonstrate an average T1≈75±11 μs over a 200 MHz frequency range across multiple cooldowns for one device, and T1≈44±8 μs for a second device. These results confirm the compatibility of stencil lithography with state-of-the-art superconducting quantum devices and motivate further investigations into materials engineering, film deposition, and surface cleaning techniques.

Stencil mask lithography is an advanced technique for fully in situ fabricating Josephson junctions, which is increasingly being used for multiterminal Josephson junctions. This study provides information on the optimal mask design and mask reliability. For this, 270 mask designs were systematically fabricated and investigated under a scanning electron microscope. Reliable statements are made about mask yield, minimal dimensions, and systematic dependencies on the number of superconducting terminals. We find that stencil mask lithography can be used reliably for fabricating multiterminal Josephson junctions, enabling lateral mask dimensions down to 40 nm on average.

The Josephson diode effect, where the switching current magnitude depends on its direction, arises when both time-reversal and inversion symmetries are broken, often achieved by a combination of spin-orbit interaction and applied magnetic fields. Taking advantage of the strong spin-orbit coupling inherent in three-dimensional topological insulators, we study this phenomenon in Nb / Bi 0.8 Sb 1.2 Te 3 / Nb Josephson weak-link junctions. Under an in-plane magnetic field perpendicular to the current direction, we observe a pronounced Josephson diode effect with efficiencies up to 7%. A crucial component of this behavior is the nonsinusoidal current-phase relationship and an anomalous phase shift, which we attribute to the presence of a ballistic supercurrent component due to the surface states. These findings open up new avenues for harnessing and controlling the Josephson diode effect in topological material systems.

Phase-change memory (PCM) is a promising candidate for in-memory computation and neuromorphic computing due to its high endurance, low cycle-to-cycle variability, and low read noise. However, among other factors, its performance strongly depends on the post-lithography fabrication steps. This study examines the impact of reactive ion etching (RIE) on PCM device performance by evaluating different etching gas mixtures, CHF₃:O₂, H₂:Ar, and Ar, and determining their impact on key device characteristics, particularly initial resistance and cycling stability. The present study demonstrates that a two-step etching approach in which the capping layer is first removed using H₂:Ar and the underlying GST layer is subsequently etched using physical Ar sputtering ensures stable and reliable PCM operation. In contrast, chemically reactive gases negatively impact the initial resistance, cycling stability, and device lifetime, likely due to alterations in the material composition. For the cycling stability evaluation, an advanced measurement algorithm utilizing the aixMATRIX setup by aixACCT Systems is employed. This algorithm enables automated testing, dynamically adjusting biasing parameters based on cell responses, ensuring a stable ON/OFF ratio and high-throughput characterization.

We study the frustration pattern of a square lattice with fabricated Nb-Pt-Nb four-terminal Josephson junctions. The four-terminal geometry gives rise to a checkerboard pattern of alternating fluxes f , f ′ piercing the plaquettes, which stabilizes the Berezinskii-Kosterlitz-Thouless transition even at irrational flux quanta per plaquette, due to an unequal repartition of integer flux sum f + f ′ into alternating plaquettes. This type of frustrated frustration manifests as a beating pattern of the dc resistance, with state configurations at the resistance dips gradually changing between the conventional zero- and half-flux states. Hence, the four-terminal Josephson junction array offers a promising platform to study previously unexplored flux and vortex configurations and provides an estimate on the spatial expansion of the four-terminal Josephson junction central weak link area.

Heterostructures comprised of a magnetic topological insulator (MTI) placed in the proximity of an s -wave superconductor have emerged as a platform for the practical realization of Majorana bound states (MBSs). More specifically, it has been theoretically predicted that MBS can appear in proximitized MTI nanoribbons (PNRs) in the quantum anomalous Hall regime. As with all MBS platforms, disorder and device imperfections can be detrimental to the formation of robust and well-separated MBSs that are suitable for fusion and braiding experiments. Here, we identify the optimal conditions for obtaining a topological superconducting gap that is against disorder, with spatially separated MBSs in PNRs, and introduce a figure of merit that encompasses these conditions. Particular attention is given to the thin-film limit of magnetic topological insulators (MTIs), where the hybridization of the surface states cannot be neglected, and to the role of electron-hole asymmetry in the low-energy physics of the system. Based on our numerical results, we find that (1) MTI thin films that are normal (rather than quantum spin Hall) insulators for zero magnetization are favorable, (2) strong electron-hole asymmetry causes the stability and robustness of MBS to be very different for chemical potentials above or below the Dirac point, and (3) the magnetization strength should preferably be comparable to the hybridization or confinement energy of the surface states, whichever is largest.