arXiv subjects

Alexander Pawlis

Publications and source records attributed to Alexander Pawlis.

13 recordsLinked to original sources

arXiv2026

Understanding and Designing Phase Change Materials: Insights from Atom Probe Tomography

Phase Change Materials (PCMs) can be rapidly and reversibly switched between their amorphous and crystalline state; a transition which is accompanied by a pronounced change of optoelectronic properties. Here progress is reviewed to explain these property changes, focusing on advances by atom probe tomography (APT). This technique classifies bonding by providing two crucial bonding descriptors. Most important is the Probability of Multiple Events (PME), which is related to the likelihood that more than one ion is dislodged per successful laser pulse in laser assisted field evaporation. Crystalline PCMs are characterized by a PME above 55%, not found for metals or iono-covalent solids. This confirms that crystalline PCMs employ a unique bonding mechanism coined metavalent bonding (MVB). While crystalline PCMs employ MVB, amorphous PCMs behave as covalent solids characterized by a much lower PME. PCMs thus change their bonding upon crystallization, consistent with quantum-chemical calculations of bonding. Crystalline solids with a high PME lie in a narrow conductivity range between metals and iono-covalent solids, indicative for a competition between electron localization and delocalization. A map quantifying chemical bonding locates metavalent solids in a region where approximately one electron is shared between adjacent atoms and bonding is not too ionic. This quantum chemical bonding map is now used to find and explain property trends relevant for PCMs in various application domains.

cond-mat.mtrl-sci↗

arXiv2026

Optically Active Single Hole Spin in ZnSe

Semiconductor hole spins offer a pathway to extended coherence times by decoupling from nuclear magnetic noise, while their spin-orbit coupling enables fast all-electrical control. In ZnSe, however, realizing this potential has been limited by p-doping challenges. Here, we circumvent this limit by optically activating acceptors within the ZnSe quantum well. We isolate a single-hole spin bound to a shallow acceptor, confirmed by antibunching and accessed via the fast (244 ps) radiative recombination of a bound exciton. Magnetic and Raman spectroscopy of the ground state reveal an effective hole g-factor of 0.7 and an optical resonance linewidth of 26.7 GHz. Complementary first-principles simulations, together with the experimental results, provide evidence that points toward nitrogen as the most likely acceptor impurity. These results introduce a promising new platform for optically active spin qubits and single-photon sources in ZnSe.

quant-ph↗

arXiv2026

Interplay between Aharonov-Bohm and Altshuler-Aronov-Spivak oscillations in phase-pure GaAs/InAs core/shell nanowires of different lengths

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 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 the $h/2e$-periodic Altshuler--Aronov--Spivak oscillations and its $h/4e$-periodic higher harmonics are enhanced. The additional phase rigidity seen in the $h/3e$-periodic oscillations is attributed to phase rigidity propagating from the neighbouring 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 quasi-ballistic transport with persistent Aharonov--Bohm-, and phase-rigid Altshuler--Aronov--Spivak-type oscillations despite few scattering centers.

cond-mat.mes-hall↗

arXiv2025

Electrical Control of Optically Active Single Spin Qubits in ZnSe

Electrons bound to shallow donors in ZnSe quantum wells are promising candidates for optically addressable spin qubits and single-photon sources. However, their optical coherence and indistinguishability are often limited by spectral broadening arising from charge fluctuations in the local environment. Here, we report electrical control of single donor qubits in ZnSe quantum wells. The applied field induces a DC Stark shift that tunes the emission energy over a range exceeding 30 times the inhomogeneous linewidth, effectively compensating for emitter-to-emitter variations. Concurrently, the field stabilizes trap occupancy, yielding a twofold reduction in optical linewidth and the suppression of spectral wandering. A statistical model based on trap dynamics qualitatively reproduces these observations and elucidates the mechanism of field-assisted charge noise suppression. Our results identify electrical control as a versatile pathway to significantly improve optical and spin addressability.

quant-ph↗

arXiv2025

Aharonov-Bohm and Altshuler-Aronov-Spivak oscillations in the quasi-ballistic regime in phase-pure GaAs/InAs core/shell nanowires

The realization of various qubit systems based on high-quality hybrid superconducting quantum devices, is often achieved using semiconductor nanowires. For such hybrid devices, a good coupling between the superconductor and the conducting states in the semiconductor wire is crucial. GaAs/InAs core/shell nanowires with an insulating core, and a conductive InAs shell fulfill this requirement, since the electronic states are strongly confined near the surface. However, maintaining a good crystal quality in the conducting shell is a challenge for this type of nanowire. In this work, we present phase-pure zincblende GaAs/InAs core/shell nanowires and analyze their low-temperature magnetotransport properties. We observe pronounced magnetic flux quantum periodic oscillations, which can be attributed to a combination of Aharonov-Bohm and Altshuler-Aronov-Spivak oscillations. From the gate and temperature dependence of the conductance oscillations, as well as from supporting theoretical transport calculations, we conclude that the conducting states in the shell are in the quasi-ballistic transport regime, with few scattering centers, but nevertheless leading to an Altshuler-Aronov-Spivak correction that dominates at small magnetic field strengths. Our results demonstrate that phase-pure zincblende GaAs/InAs core/shell nanowires represent a very promising alternative semiconductor nanowire-based platform for hybrid quantum devices.

cond-mat.mes-hall↗

arXiv2024

Generation of Coherent Quantum Light from a Single Impurity-Bound Exciton

Impurity-bound excitons in II-VI direct-bandgap semiconductors are promising optically active solid-state spin qubits that combine exceptional optical quantum efficiency with an ultra-low spin noise environment. Previous studies on single impurities relied on incoherent optical excitation to generate photons. However, many quantum applications require resonant driving of quantum emitters to precisely control optical transitions and maintain coherence of the emission. Here, we demonstrate coherent optical emission of quantum light from a resonantly driven single impurity-bound exciton in ZnSe. The resonantly driven emitter exhibits bright quantum light emission that preserves the phase of the resonant drive, validated through polarization interferometry. Resonant excitation enables us to directly measure the Debye-Waller factor, determined to be 0.94, which indicates high efficiency emission to the zero-phonon line. Time-resolved resonance fluorescence measurements reveal a fast optically-driven ionization process that we attribute to Auger recombination, along with a slower spontaneous ionization process having a lifetime of 21 {\mu}s due to charge tunneling from the impurity. We show that incoherent, low-power laser pumping efficiently stabilizes the charge of the impurity-bound exciton on the timescale of 9.3 ns, recovering the resonance fluorescence emission from the bound exciton. These results pave the way for coherent optical and spin control of the single impurity states through resonant excitation of impurity-bound excitons in II-VI semiconductors.

quant-ph↗

arXiv2024

Fabrication and characterization of InAs nanowire-based quantum dot structures utilizing buried bottom gates

Semiconductor nanowires can be utilized to create quantum dot qubits. The formation of quantum dots is typically achieved by means of bottom gates created by a lift-off process. As an alternative, we fabricated flat buried bottom gate structures by filling etched trenches in a Si substrate with sputtered TiN, followed by mechanical polishing. This method achieved gate line pitches as small as 60 nm. The gate fingers have low gate leakage. As a proof of principle, we fabricated quantum dot devices using InAs nanowires placed on the gate fingers. These devices exhibit single electron tunneling and Coulomb blockade.

cond-mat.mes-hall↗

arXiv2024

Two-dimensional photonic crystal cavities in ZnSe quantum well structures

ZnSe and related materials like ZnMgSe and ZnCdSe are promising II-VI host materials for optically mediated quantum information technology such as single photon sources or spin qubits. Integrating these heterostructures into photonic crystal (PC) cavities enables further improvements, for example realizing Purcell-enhanced single photon sources with increased quantum efficiency. Here we report on the successful implementation of two-dimensional (2D) PC cavities in strained ZnSe quantum wells (QW) on top of a novel AlAs supporting layer. This approach overcomes typical obstacles associated with PC membrane fabrication in strained materials, such as cracks and strain relaxation in the corresponding devices. We demonstrate the attainment of the required mechanical stability in our PC devices, complete strain retainment and effective vertical optical confinement. Structural analysis of our PC cavities reveals excellent etching anisotropy. Additionally, elemental mapping in a scanning transmission electron microscope confirms the transformation of AlAs into AlOx by post-growth wet oxidation and reveals partial oxidation of ZnMgSe at the etched sidewalls in the PC. This knowledge is utilized to tailor FDTD simulations and to extract the ZnMgSe dispersion relation with small oxygen content. Optical characterization of the PC cavities with cross-polarized resonance scattering spectroscopy verifies the presence of cavity modes. The excellent agreement between simulation and measured cavity mode energies demonstrates wide tunability of the PC cavity and proves the pertinence of our model. This implementation of 2D PC cavities in the ZnSe material system establishes a solid foundation for future developments of ZnSe quantum devices.

cond-mat.mtrl-sci↗

arXiv2024

Flux-periodic supercurrent oscillations in an Aharonov-Bohm-type nanowire Josephson junction

Phase winding effects in hollow semiconductor nanowires with superconducting shells have been proposed as a route to engineer topological superconducting states. We investigate GaAs/InAs core/shell nanowires with half-shells of epitaxial aluminium as a potential platform for such devices, where the thin InAs shell confines the electron wave function around the GaAs core. With normal contacts we observed pronounced $h/e$ flux periodic oscillations in the magnetoconductance, indicating the presence of a tubular conductive channel in the InAs shell. Conversely, the switching current in Josephson junctions oscillates with approximately half that period, i.e. $h/2e$, indicating transport via Andreev transport processes in the junction enclosing threading magnetic flux. On these structures, we systematically studied the gate-, field-, and temperature-dependent evolution of the supercurrent. Results indicate that Andreev transport processes can occur about the wire circumference indicating full proximitization of the InAs shell from the half-shell superconducting contacts.

cond-mat.mes-hall↗

arXiv2023

Cavity-enhanced single photon emission from a single impurity-bound exciton

Impurity-bound excitons in ZnSe quantum wells are bright single photon emitters--a crucial element in photonics-based quantum technology. But to achieve the efficiencies required for practical applications, these emitters must be integrated into optical cavities that enhance their radiative properties and far-field emission pattern. In this work, we demonstrate cavity-enhanced emission from a single impurity-bound exciton in a ZnSe quantum well. We utilize a bullseye cavity structure optimized to feature a small mode volume and a nearly Gaussian far-field transverse mode that can efficiently couple to an optical fiber. The fabricated device displays emission that is more than an order of magnitude brighter than bulk impurity-bound exciton emitters in the ZnSe quantum well, as-well-as clear anti-bunching, which verifies the single photon emission from the source. Time-resolved photoluminescence spectroscopy reveals a Purcell-enhanced radiative decay process with a Purcell factor of 1.43. This work paves the way towards high efficiency spin-photon interfaces using an impurity-doped II-VI semiconductor coupled to nanophotonics.

quant-ph↗

arXiv2022

Correlations between cascaded photons from spatially localized biexcitons in ZnSe

Radiative cascades emit correlated photon pairs, providing a pathway for the generation of entangled photons. The realization of a radiative cascade with impurity atoms in semiconductors, a leading platform for the generation of quantum light, would therefore provide a new avenue for the development of entangled photon pair sources. Here we demonstrate a radiative cascade from the decay of a biexciton at an impurity-atom complex in a ZnSe quantum well. The emitted photons show clear temporal correlations revealing the time--ordering of the cascade. Our result establishes impurity atoms in ZnSe as a potential platform for photonic quantum technologies using radiative cascades.

quant-ph↗

arXiv2022

Single quantum emitters with spin ground states based on Cl bound excitons in ZnSe

Defects in wide-bandgap semiconductors are promising qubit candidates for quantum communication and computation. Epitaxially grown II-VI semiconductors are particularly promising host materials due to their direct bandgap and potential for isotopic purification to a spin-zero nuclear background. Here, we show a new type of single photon emitter with potential electron spin qubit based on Cl impurities in ZnSe. We utilize a quantum well to increase the binding energies of donor emission and confirm single photon emission with short radiative lifetimes of 192 ps. Furthermore, we verify that the ground state of the Cl donor complex contains a single electron by observing two-electron satellite emission, leaving the electron in higher orbital states. We also characterize the Zeeman splitting of the exciton transition by performing polarization-resolved magnetic spectroscopy on single emitters. Our results suggest single Cl impurities are suitable as single photon source with potential photonic interface.

quant-ph↗

arXiv2009

Indistinguishable photons from independent semiconductor single-photon devices

We demonstrate quantum interference between photons generated by the radiative decay processes of excitons that are bound to isolated fluorine donor impurities in ZnSe/ZnMgSe quantum-well nanostructures. The ability to generate single photons from these devices is confirmed by auto-correlation experiments, and indistinguishability of single photons from two independent devices is confirmed via a Hong-Ou-Mandel dip. These results indicate that donor impurities in appropriately engineered semiconductor structures can portray atom-like homogeneity and coherence properties, potentially enabling scalable technologies for future large-scale optical quantum computers and quantum communication networks.

quant-ph↗