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John F. Klem

Publications and source records attributed to John F. Klem.

7 recordsLinked to original sources
arXiv2026

Flat-Band Generation in InAs/GaSb Quantum Wells through Vertically Engineered Heterostructures

Quantum materials constitute a novel category of substances wherein quantum effects and electron-electron (e-e) interactions give rise to unforeseen phenomena on a macroscopic scale. Of particular interest within the realm of quantum materials are flat bands, which promote heavy conduction electrons and enhance e-e correlation effects. While the engineering of such flat bands has been demonstrated in graphene and two-dimensional transition metal dichalcogenides moiré superlattices and in lithography defined semiconductor moiré superlattices, conventional tear-and-stack fabrication methods face challenges due to inevitable twist-angle disorder, strain, and relaxation effects, leading to issues with reproducibility and scalability. Here, we explore the creation and modification of flat bands through vertically engineered III-V semiconductor heterostructures, without the need for twisting. These artificial quantum materials offer a reproducible and scalable means for producing high-quality flat-band materials via molecular beam epitaxy growth. Our investigation includes magnetotransport and infrared magneto-spectroscopy studies of quad-layer InAs/GaSb quantum wells, accompanied by k*p band structure calculations, which illustrate the flattening of bands in vertically designed heterostructures.

cond-mat.mes-hall↗
arXiv2026

Buried Dirac points in quantum spin Hall insulators: Implications for Majorana Kramers pair-based quantum computing

For heterostructures formed by a quantum spin Hall insulator (QSHI) placed in proximity to a superconductor (SC), no external magnetic field is necessary to drive the system into a phase supporting topological superconductivity with Majorana zero energy states, making them very attractive for the realization of non-Abelian states and fault-tolerant qubits. Despite considerable work investigating QSHI edge states, there is still an open question about their resilience to large magnetic fields and the implication of such resilience for the formation of a quasi-1D topological superconducting state. In this work, we investigate the transport properties of helical edge states in a QSHI-SC junction formed by a InAs/GaSb (15nm/5nm) double quantum well and a superconducting tantalum (Ta) constriction. We observe a robust conductance plateau up to 2 T, signaling resilient edge state transport. Such resilience is consistent with the Dirac point for the edge states being buried in the bulk valence band. Using a modified Landauer-Buttiker analysis, we find that the conductance is consistent with 98% Andreev reflection probability owing to the high transparency of the InAs/GaSb-Ta interface. We further theoretically show that a buried Dirac point does not affect the robustness of the quasi-1D topological superconducting phase, and favors the hybridization of Majorana Kramer pairs and fermionic modes in the QSHI resulting in extended MKP states, highlighting the subtle role of buried Dirac points in probing MKPs.

cond-mat.supr-con↗
arXiv2025

Optical characterization of deep level defects in n-type Al$_x$In$_y$Ga$_{1-x-y}$P for development of solid-state photomultiplier analogs

Characterizing intrinsic defects is an important step in evaluating materials for new optoelectronic device applications. For photomultipliers, suppressing dark currents is critical, but there exists a tradeoff between maximizing the band gap while remaining sensitive to the wavelength of interest, and minimizing the incorporation of new defects by growing not-yet-optimized alloys. We present a series of capacitance-based measurements, including deep level optical spectroscopy, steady-state photocapacitance and illuminated capacitance-voltage, on photodiodes with lightly \textit{n}-type Al$_x$In$_y$Ga$_{1-x-y}$P absorber regions. Several deep levels are identified, including one near midgap. While the inclusion of aluminum increases each trap density by approximately 10x, the hole capture cross section also appears to decrease, suggesting that Shockley-Read-Hall dark currents may be suppressed. These materials may be good candidates for development into silicon photomultiplier analogs with wider bandgap for scintillator applications.

physics.app-ph↗
arXiv2025

Lithography defined semiconductor moires with anomalous in-gap quantum Hall states

Quantum materials and phenomena have attracted great interest for their potential applications in next-generation microelectronics and quantum-information technologies. In one especially interesting class of quantum materials, moire superlattices (MSL) formed by twisted bilayers of 2D materials, a wide range of novel phenomena are observed. However, there exist daunting challenges such as reproducibility and scalability of utilizing 2D MSLs for microelectronics and quantum technologies due to their exfoliate-tear-stack method. Here, we propose lithography defined semiconductor moires superlattices, in which three fundamental parameters, electron-electron interaction, spin-orbit coupling, and band topology, are designable. We experimentally investigate quantum transport properties in a moire specimen made in an InAs quantum well. Strong anomalous in-gap states are observed within the same integer quantum Hall state. Our work opens up new horizons for studying 2D quantum-materials phenomena in semiconductors featuring superior industry-level quality and state-of-the-art technologies, and they may potentially enable new quantum information and microelectronics technologies.

cond-mat.mes-hall↗
arXiv2014

Electrodynamic modeling of strong coupling between a metasurface and intersubband transitions in quantum wells

Strong light-matter coupling has recently been demonstrated in sub-wavelength volumes by coupling engineered optical transitions in semiconductor heterostructures (e.g., quantum wells) to metasurface resonances via near fields. It has also been shown that different resonator shapes may lead to different Rabi splittings, though this has not yet been well explained. In this paper, our aim is to understand the correlation between resonator shape and Rabi splitting, and in particular determine and quantify the physical parameters that affect strong coupling by developing an equivalent circuit network model whose elements describe energy and dissipation. Because of the subwavelength dimension of each metasurface element, we resort to the quasi-static (electrostatic) description of the near-field and hence define an equivalent capacitance associated to each dipolar element of a flat metasurface, and we show that this is also able to accurately model the phenomenology involved in strong coupling between the metasurface and the intersubband transitions in quantum wells. We show that the spectral properties and stored energy of a metasurface/quantum-well system obtained using our model are in good agreement with both full-wave simulation and experimental results. We then analyze metasurfaces made of three different resonator geometries and observe that the magnitude of the Rabi splitting increases with the resonator capacitance in agreement with our theory, providing a phenomenological explanation for the resonator shape dependence of the strong coupling process.

physics.optics↗
arXiv2014

Superconducting proximity effect in inverted InAs/GaSb quantum well structures with Ta electrodes

We present our recent electronic transport results in top-gated InAs/GaSb quantum well hybrid structures with superconducting Ta electrodes. We show that the transport across the InAs-Ta junction depends largely on the interfacial transparency, exhibiting distinct zero-bias behavior. For a relatively resistive interface a broad conductance peak is observed at zero bias. When a transparent InAs-Ta interface is achieved, a zero-bias conductance dip appears with two coherent-peak-like features forming at bias voltages corresponding to the superconducting gap of Ta. The conductance spectra of the transparent InAs-Ta junction at different gate voltages can be fit well using the standard Blonder-Tinkham-Klapwijk theory.

cond-mat.mes-hall↗