S. A. Crooker’s research while affiliated with Los Alamos National Laboratory and other places

GaN has emerged to be a major semiconductor akin to silicon due to its revolutionary impacts in solid state lighting, critically enabled by p-type doping, and high-performance radio-frequency and power electronics. Suffering from inefficient hole doping and low hole mobility, quantum oscillations in p-type GaN have not been observed, hindering fundamental studies of valence bands and hole transport in GaN. Here, we present the first observation of quantum oscillations of holes in GaN. Shubnikov-de Haas (SdH) oscillations in hole resistivity are observed in a quantum-confined two-dimensional hole gas at a GaN/AlN interface, where polarization-induced doping overcomes thermal freeze-out, and a sharp and clean interface boosts the hole mobility enough to unmask the quantum oscillations. These holes degenerately occupy the light and heavy hole bands of GaN and have record-high mobilities of ~1900 cm2/Vs and ~400 cm2/Vs at 3K, respectively. We use magnetic fields up to 72 T to resolve SdH oscillations of holes from both valence bands to extract their respective sheet densities, quantum scattering times, and the effective masses of light holes (0.5-0.7 m0) and heavy holes (1.9 m0). SdH oscillations of heavy and light holes in GaN constitute a direct metrology of valence bands and open new venues for quantum engineering in this technologically important semiconductor. Like strained silicon transistors, strain-engineering of the valence bands of GaN is predicted to dramatically improve hole mobilities by reducing the hole effective mass, a proposal that can now be explored experimentally, particularly in a fully fabricated transistor, using quantum oscillations. Furthermore, the findings of this work suggest a blueprint to create 2D hole gases and observe quantum oscillations of holes in related wide bandgap semiconductors such as SiC and ZnO in which such techniques are not yet possible.

Covalent 2D magnets such as Cr2Te3, which feature self‐intercalated magnetic cations located between monolayers of transition‐metal dichalcogenide material, offer a unique platform for controlling magnetic order and spin texture, enabling new potential applications for spintronic devices. Here, it is demonstrated that the unconventional anomalous Hall effect (AHE) in Cr2Te3, characterized by additional humps and dips near the coercive field in AHE hysteresis, originates from an intrinsic mechanism dictated by the self‐intercalation. This mechanism is distinctly different from previously proposed mechanisms such as topological Hall effect, or two‐channel AHE arising from spatial inhomogeneities. Crucially, multiple Weyl‐like nodes emerge in the electronic band structure due to strong spin‐orbit coupling, whose positions relative to the Fermi level is sensitively modulated by the canting angles of the self‐intercalated Cr cations. These nodes contribute strongly to the Berry curvature and AHE conductivity. This component competes with the contribution from bands that are less affected by the self‐intercalation, resulting in a sign change in AHE with temperature and the emergence of additional humps and dips. The findings provide compelling evidence for the intrinsic origin of the unconventional AHE in Cr2Te3 and further establish self‐intercalation as a control knob for engineering AHE in complex magnets.

Tuning the density of resident electrons or holes in semiconductors provides crucial insight into the composition of excitonic complexes that are observed as absorption or photoluminescence resonances in optical studies. Moreover, we can change the way these resonances shift and broaden in energy by controlling the quantum numbers of the resident carriers with magnetic fields and doping levels, and by selecting the quantum numbers of the photoexcited or recombining electron-hole (e-h) pair through optical polarization. We discuss the roles of distinguishability and optimality of excitonic complexes, showing them to be key ingredients that determine the energy shifts and broadening of optical resonances in charge-tunable semiconductors. A distinguishable e-h pair means that the electron and hole undergoing photoexcitation or recombination have quantum numbers that are not shared by any of the resident carriers. An optimal excitonic complex refers to a complex whose particles come with all available quantum numbers of the resident carriers. All optical resonances may be classified as either distinct or indistinct depending on the distinguishability of the e-h pair, and the underlying excitonic complex can be classified as either optimal or suboptimal. The universality of these classifications, inherited from the fundamental Pauli exclusion principle, allows us to understand how optical resonances shift in energy and whether they should broaden as doping is increased. This understanding is supported by conclusive evidence that the decay of optical resonances cannot be simply attributed to enhanced screening when resident carriers are added to a semiconductor. Finally, applying the classification scheme in either monolayer or moire heterobilayer systems, we relate the energy shift and amplitude of the neutral exciton resonance to the compressibility of the resident carrier gas.

We use magnetoresistance measurements at high magnetic field (μ0H≤65 T) and low temperature (T≥500 mK) to gain fresh insights into the behavior of the upper critical field Hc2 in superconducting ultrathin FeSe films of varying degrees of disorder, grown by molecular beam epitaxy on SrTiO3. Measurements of Hc2 across samples with a widely varying superconducting critical temperature (1.2 K ≤Tc≤21 K) generically show similar qualitative temperature dependence. We analyze the temperature dependence of Hc2 in the context of Werthamer-Helfand-Hohenberg (WHH) theory. The analysis yields parameters that indicate a strong Pauli paramagnetic pair-breaking mechanism which is also reflected by pseudoisotropic superconductivity in the limit of zero temperature. In the lower Tc samples, we observe a spin-orbit scattering-driven enhancement of Hc2 above the strongly-coupled Pauli paramagnetic limit. We also observe clear deviations from WHH theory at low temperature, regardless of Tc. We attribute this to the multiband superconductivity of FeSe and possibly to the emergence of a low-temperature, high-field superconducting phase.

When doped with a high density of mobile charge carriers, monolayer transition-metal dichalcogenide (TMD) semiconductors can host new types of composite many-particle exciton states that do not exist in conventional semiconductors. Such multiparticle bound states arise when a photoexcited electron-hole pair couples not to just a single Fermi sea that is quantum-mechanically distinguishable (as in the case of conventional charged excitons or trions), but rather couples simultaneously to multiple Fermi seas, each having distinct spin and valley quantum numbers. Composite six-particle “hexciton” states were recently identified in electron-doped WSe2 monolayers, but under suitable conditions they should also form in all other members of the monolayer TMD family. Here we present spectroscopic evidence demonstrating the emergence of many-body hexcitons in charge-tunable WS2 monolayers (at the A-exciton) and MoSe2 monolayers (at the B-exciton). The roles of distinguishability and carrier screening on the stability of hexcitons are discussed.

The optical properties of lead halide perovskite semiconductors in vicinity of the bandgap are controlled by excitons, so that investigation of their fundamental properties is of critical importance. The exciton Landé or g‐factor gX is the key parameter, determining the exciton Zeeman spin splitting in magnetic fields. The exciton, electron, and hole carrier g‐factors provide information on the band structure, including its anisotropy, and the parameters contributing to the electron and hole effective masses. Here, gX is measured by reflectivity in magnetic fields up to 60 T for lead halide perovskite crystals. The materials band gap energies at a liquid helium temperature vary widely across the visible spectral range from 1.520 up to 3.213 eV in hybrid organic–inorganic and fully inorganic perovskites with different cations and halogens: FA0.9Cs0.1PbI2.8Br0.2, MAPbI3, FAPbBr3, CsPbBr3, and MAPb(Br0.05Cl0.95)3. The exciton g‐factors are found to be nearly constant, ranging from +2.3 to +2.7. Thus, the strong dependences of the electron and hole g‐factors on the bandgap roughly compensate each other when combining to the exciton g‐factor. The same is true for the anisotropies of the carrier g‐factors, resulting in a nearly isotropic exciton g‐factor. The experimental data are compared favorably with model calculation results.

Cesium lead bromide (CsPbBr 3 ) is a representative material of the emerging class of lead halide perovskite semiconductors that possess remarkable optoelectronic properties. Its optical properties in the vicinity of the band gap energy are greatly contributed by excitons, which form exciton‐polaritons due to strong light‐matter interactions. We examine exciton‐polaritons in solution‐grown CsPbBr ³ crystals by means of circularly‐polarized reflection spectroscopy measured in high magnetic fields up to 60 T. The excited 2P exciton state is measured by two‐photon absorption. Comprehensive modelling and analysis provides detailed quantitative information about the exciton‐polariton parameters: exciton binding energy of 32.5 meV, oscillator strength characterized by longitudinal‐transverse splitting of 5.3 meV, damping of 6.7 meV, reduced exciton mass of 0.18 m 0 , exciton diamagnetic shift of 1.6 μ eV/T ² , and exciton Landé factor g X =+2.35. We show that the exciton states can be well described within a hydrogen‐like model with an effective dielectric constant of 8.7. From the measured exciton longitudinal‐transverse splitting we evaluate the Kane energy of E p =15 eV, which is in reasonable agreement with values of 11.8−12.5 eV derived from the carrier effective masses. This article is protected by copyright. All rights reserved.

Direct detection of spontaneous spin fluctuations, or “magnetization noise,” is emerging as a powerful means of revealing and studying magnetic excitations in both natural and artificial frustrated magnets. Depending on the lattice and nature of the frustration, these excitations can often be described as fractionalized quasiparticles possessing an effective magnetic charge. Here, by combining ultrasensitive optical detection of thermodynamic magnetization noise with Monte Carlo simulations, we reveal emergent regimes of magnetic excitations in artificial “tetris ice.” A marked increase of the intrinsic noise at certain applied magnetic fields heralds the spontaneous proliferation of fractionalized excitations, which can diffuse independently, without cost in energy, along specific quasi-1D spin chains in the tetris ice lattice.

Quantum light emitters capable of generating single photons with circular polarization and non-classical statistics could enable non-reciprocal single-photon devices and deterministic spin–photon interfaces for quantum networks. To date, the emission of such chiral quantum light relies on the application of intense external magnetic fields, electrical/optical injection of spin-polarized carriers/excitons or coupling with complex photonic metastructures. Here we report the creation of free-space chiral quantum light emitters via the nanoindentation of monolayer WSe2/NiPS3 heterostructures at zero external magnetic field. These quantum light emitters emit with a high degree of circular polarization (0.89) and single-photon purity (95%), independent of pump laser polarization. Scanning diamond nitrogen-vacancy microscopy and temperature-dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in the antiferromagnetic order of NiPS3, both of which are co-localized by strain fields associated with the nanoscale indentations.