arXiv subjects

Mario Lodari

Publications and source records attributed to Mario Lodari.

9 recordsLinked to original sources

arXiv2025

Using Landau quantization to probe disorder in semiconductor heterostructures

Understanding scattering mechanisms in semiconductor heterostructures is crucial to reducing sources of disorder and ensuring high yield and uniformity in large spin qubit arrays. Disorder of the parent two-dimensional electron or hole gas is commonly estimated by the critical, percolation-driven density associated with the metal-insulator transition. However, a reliable estimation of the critical density within percolation theory is hindered by the need to measure conductivity with high precision at low carrier densities, where experiments are most difficult. Here, we connect experimentally percolation density and quantum Hall plateau width, in line with an earlier heuristic intuition, and offer an alternative method for characterizing semiconductor heterostructure disorder.

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arXiv2025

Optical spin pumping in silicon

The generation of an out-of-equilibrium population of spin-polarized carriers is a keystone process for quantum technologies and spintronics alike. It can be achieved through the so-called optical spin orientation by exciting the material with circularly polarized light. Although this is an established technique for studying direct band-gap semiconductors, it has been proven limited in materials like Si that possess weak oscillator strengths for the optical transitions. In this study, we address the problem by presenting an all-optical analog of the spin pumping method. This involves the optical creation of a non-equilibrium spin population within an absorber, which subsequently transfers spin-polarized carriers to a nearby indirect gap semiconductor, resulting in polarized emission from the latter. By applying this concept to a Ge-on-Si heterostructure we observe luminescence from Si with an unrivaled polarization degree as high as 9%. The progressive etching of the absorbing layer, assisted by magneto-optic experiments, allows us to ascertain that the polarized emission is determined by effective spin injection aided by the carrier lifetime shortening due to extended defects. These findings can facilitate the use of highly promising spin-dependent phenomena of Si, whose optical exploitation has thus far been hampered by fundamental limitations associated with its peculiar electronic structure.

cond-mat.mes-hall↗

arXiv2025

Buried unstrained germanium channels: a lattice-matched platform for quantum technology

Strained Ge ($\epsilon$-Ge) and strained Si ($\epsilon$-Si) buried quantum wells have enabled advanced spin-qubit quantum processors. However, in the absence of suitable lattice-matched substrates, $\epsilon$-Ge and $\epsilon$-Si are deposited on defective, metamorphic SiGe substrates, which may impact device performance and scaling. Here an alternative platform is introduced, based on the heterojunction between unstrained Ge and a lattice-matched strained SiGe ($\epsilon$-SiGe) barrier, eliminating the need for metamorphic buffers altogether. In a structure with a 52-nm-thick $\epsilon$-SiGe barrier, a low-disorder two-dimensional hole gas is demonstrated with a high-mobility of 1.33$\times$10$^5$ cm$^2$/Vs and a low percolation density of 1.4(1)$\times$10$^1$$^0$ cm$^-$$^2$. Quantum transport shows that holes confined in the buried unstrained Ge channel have a strong density-dependent in-plane effective mass and out-of-plane $g$-factor, pointing to a significant heavy-hole$-$light-hole mixing in agreement with theory. Measurements of Zeeman spin-split levels in quantum point contacts further highlight this character, showing a two-fold larger in-plane $g$-factor in Ge than in $\epsilon$-Ge. The prospect of strong spin-orbit interaction, isotopic purification, and of hosting superconducting pairing correlations make this platform appealing for fast quantum hardware and hybrid quantum systems.

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arXiv2022

Wafer-scale low-disorder 2DEG in $^{28}$Si/SiGe without an epitaxial Si cap

We grow $^{28}$Si/SiGe heterostructures by reduced-pressure chemical vapor deposition and terminate the stack without an epitaxial Si cap but with an amorphous Si-rich layer obtained by exposing the SiGe barrier to dichlorosilane at 500 {\deg}C. As a result, $^{28}$Si/SiGe heterostructure field-effect transistors feature a sharp semiconductor/dielectric interface and support a two-dimensional electron gas with enhanced and more uniform transport properties across a 100 mm wafer. At T = 1.7 K we measure a high mean mobility of (1.8$\pm$0.5)$\times$10$^5$ cm$^2$/Vs and a low mean percolation density of (9$\pm$1)$\times$10$^{10}$ cm$^{-2}$. From the analysis of Shubnikov-de Haas oscillations at T = 190 mK we obtain a long mean single particle relaxation time of (8.1$\pm$0.5) ps, corresponding to a mean quantum mobility and quantum level broadening of (7.5$\pm$0.6)$\times$10$^4$ cm$^{2}$/Vs and (40$\pm$3) $\mu$eV, respectively, and a small mean Dingle ratio of (2.3$\pm$0.2), indicating reduced scattering from long range impurities and a low-disorder environment for hosting high-performance spin-qubits.

cond-mat.mes-hall↗

arXiv2021

Atomic fluctuations lifting the energy degeneracy in Si/SiGe quantum dots

Electron spins in Si/SiGe quantum wells suffer from nearly degenerate conduction band valleys, which compete with the spin degree of freedom in the formation of qubits. Despite attempts to enhance the valley energy splitting deterministically, by engineering a sharp interface, valley splitting fluctuations remain a serious problem for qubit uniformity, needed to scale up to large quantum processors. Here, we elucidate and statistically predict the valley splitting by the holistic integration of 3D atomic-level properties, theory and transport. We find that the concentration fluctuations of Si and Ge atoms within the 3D landscape of Si/SiGe interfaces can explain the observed large spread of valley splitting from measurements on many quantum dot devices. Against the prevailing belief, we propose to boost these random alloy composition fluctuations by incorporating Ge atoms in the Si quantum well to statistically enhance valley splitting.

cond-mat.mes-hall↗

arXiv2021

InSbAs two-dimensional electron gases as a platform for topological superconductivity

Topological superconductivity can be engineered in semiconductors with strong spin-orbit interaction coupled to a superconductor. Experimental advances in this field have often been triggered by the development of new hybrid material systems. Among these, two-dimensional electron gases (2DEGs) are of particular interest due to their inherent design flexibility and scalability. Here we discuss results on a 2D platform based on a ternary 2DEG (InSbAs) coupled to in-situ grown Aluminum. The spin-orbit coupling in these 2DEGs can be tuned with the As concentration, reaching values up to 400 meV$\unicode{xC5}$, thus exceeding typical values measured in its binary constituents. In addition to a large Land\'e g-factor $\sim$ 55 (comparable to InSb), we show that the clean superconductor-semiconductor interface leads to a hard induced superconducting gap. Using this new platform we demonstrate the basic operation of phase-controllable Josephson junctions, superconducting islands and quasi-1D systems, prototypical device geometries used to study Majorana zero modes.

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arXiv2020

Vanishing Zeeman energy in a two-dimensional hole gas

A clear signature of Zeeman split states crossing is observed in Landau fan diagram of strained germanium two-dimensional hole gas. The underlying mechanisms are discussed based on a perturbative model yielding a closed formula for the critical magnetic fields. These fields depend strongly on the energy difference between the top-most and the neighboring valence bands and are sensitive to the quantum well thickness, strain, and spin-orbit-interaction. The latter is a necessary feature for the crossing to occur. This framework enables a straightforward quantification of the hole-state parameters from simple measurements, thus paving the way for its use in design and modelling of hole-based quantum devices.

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arXiv2020

Effect of quantum Hall edge strips on valley splitting in silicon quantum wells

We determine the energy splitting of the conduction-band valleys in two-dimensional electrons confined to low-disorder Si quantum wells. We probe the valley splitting dependence on both perpendicular magnetic field $B$ and Hall density by performing activation energy measurements in the quantum Hall regime over a large range of filling factors. The mobility gap of the valley-split levels increases linearly with $B$ and is strikingly independent of Hall density. The data are consistent with a transport model in which valley splitting depends on the incremental changes in density $eB/h$ across quantum Hall edge strips, rather than the bulk density. Based on these results, we estimate that the valley splitting increases with density at a rate of 116 $μ$eV/10$^{11}$cm$^{-2}$, consistent with theoretical predictions for near-perfect quantum well top interfaces.

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arXiv2019

Ballistic superconductivity and tunable $π$-junctions in InSb quantum wells

Two-dimensional electron gases (2DEGs) coupled to superconductors offer the opportunity to explore a variety of quantum phenomena. These include the study of novel Josephson effects, superconducting correlations in quantum (spin) Hall systems, hybrid superconducting qubits and emergent topological states in semiconductors with spin-orbit interaction (SOI). InSb is a well-known example of such a strong SOI semiconductor, however hybrid superconducting devices in InSb quantum wells remain unexplored. Here, we interface InSb 2DEGs with a superconductor (NbTiN) to create Josephson junctions (JJs), thus providing the first evidence of induced superconductivity in high quality InSb quantum wells. The JJs support supercurrent transport over several microns and display clear signatures of ballistic superconductivity. Furthermore, we exploit the large Landé g-factor and gate tunability of the junctions to control the current-phase relation, and drive transitions between the $0$ and $π$-states. This control over the free energy landscape allows us to construct a phase diagram identifying these $0$ and $π$-regions, in agreement with theory. Our results establish InSb quantum wells as a promising new material platform to study the interplay between superconductivity, SOI and magnetism.

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