Nicolas Ubrig

Spontaneous symmetry breaking, driven by competing interactions and quantum fluctuations, is fundamental to understanding ordered electronic phases. Although electrically neutral, optical excitations like excitons can interact through their dipole moment, raising the possibility of optically active ordered phases. The effects of spontaneous ordering on optical properties remains underexplored. The excitonic Mott insulating state recently observed in semiconducting moiré crystals may help clarify this question. Here we present evidence for an in-plane ferroelectric phase of dipolar moiré excitons driven by strong exciton–exciton interactions. We reveal a speed-up of photon emission at late times and low densities in excitonic decay. This counterintuitive behaviour is attributed to collective radiance, linked to the transition between disordered and symmetry-broken ferroelectric phases of moiré excitons. Our findings provide evidence for strong dipolar intersite interactions in moiré lattices, demonstrate collective photon emission as a probe for moiré quantum materials and a path for exploring cooperative optical phenomena in strongly correlated systems.

Two‐dimensional (2D) van der Waals magnets show strong interconnection between their electrical, magnetic, and structural properties. Here, the emergence of a luminescent transition is revealed upon crossing the Néel transition temperature of CrPS₄, a layered antiferromagnetic semiconductor. This luminescent transition occurs above the lowest absorption level. The optical transitions are attributed to excited states of the t_2g orbitals of the Cr³⁺ ions, which are influenced by the distortion of the octahedral crystal field. Specifically, the vicinity of the Néel temperature, the distortion switches from an anti‐polar to a polar arrangement, thereby not only promoting an additional luminescent pathway but also significantly strengthening the static dipole moment detected by a marked enhancement in the intensity of the second harmonic generation. These results strongly encourage further investigation into the multiferroic properties and potential coupling mechanisms in CrPS₄.

In all van der Waals layered antiferromagnetic semiconductors investigated so far, a negative magnetoresistance has been observed in vertical transport measurements, with characteristic trends that do not depend on applied bias. Here, we report vertical transport measurements on a layered antiferromagnetic semiconductor CrPS 4 that exhibit a drastically different behavior, namely, a strongly bias-dependent, positive magnetoresistance that is accompanied by pronounced oscillations for devices whose thickness is smaller than 10 nm. We establish that this unexpected behavior originates from transport being space-charge limited and not injection limited as for the layered antiferromagnetic semiconductors studied earlier. Our analysis indicates that the positive magnetoresistance and the oscillations only occur when electrons are injected into in-gap defect states, whereas when electrons are injected into the conduction band, the magnetoresistance vanishes. We propose a microscopic explanation for the observed phenomena that combines concepts typical of transport through disordered semiconductors with known properties of the CrPS 4 magnetic state, thus capturing all basic experimental observations. Our results illustrate the need to understand, in detail, the nature of transport through vdW magnets in order to extract information about the nature of the order magnetic states and its microscopic properties. Published by the American Physical Society 2025

Transistors realized on the 2D antiferromagnetic semiconductor CrPS₄ exhibit large magnetoconductance due to magnetic-field-induced changes in the magnetic state. The microscopic mechanism coupling the conductance and magnetic state is not understood. We identify it by analyzing the evolution of the parameters determining the transistor behavior─carrier mobility and threshold voltage─with temperature and magnetic field. For temperatures T near the Néel temperature T_N, the magnetoconductance originates from a mobility increase due to the applied magnetic field that reduces spin fluctuation induced disorder. For T ≪ T_N, instead, what changes is the threshold voltage, so that increasing the field at fixed gate voltage increases the density of accumulated electrons. The phenomenon is explained by a conduction band-edge shift correctly predicted by the ab initio calculations. Our results demonstrate that the band structure of CrPS₄ depends on its magnetic state and reveal a mechanism for magnetoconductance that had not been identified earlier.

In twisted two-dimensional (2D) magnets, the stacking dependence of the magnetic exchange interaction can lead to regions of ferromagnetic and antiferromagnetic interlayer order, separated by non-collinear, skyrmion-like spin textures. Recent experimental searches for these textures have focused on CrI₃, known to exhibit either ferromagnetic or antiferromagnetic interlayer order, depending on layer stacking. However, the very strong uniaxial anisotropy of CrI₃ disfavors smooth non-collinear phases in twisted bilayers. Here, we report the experimental observation of three distinct magnetic phases—one ferromagnetic and two antiferromagnetic—in exfoliated CrBr₃ multilayers, and reveal that the uniaxial anisotropy is significantly smaller than in CrI₃. These results are obtained by magnetoconductance measurements on CrBr₃ tunnel barriers and Raman spectroscopy, in conjunction with density functional theory calculations, which enable us to identify the stackings responsible for the different interlayer magnetic couplings. The detection of all locally stable magnetic states predicted to exist in CrBr₃ and the excellent agreement found between theory and experiments, provide complete information on the stacking-dependent interlayer exchange energy and establish twisted bilayer CrBr₃ as an ideal system to deterministically create non-collinear magnetic phases.

Using field‐effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2D magnetic semiconductors cause carrier localization, preventing transistor operation. Here, it is shown that exfoliated layers of CrPS₄—a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1 eV—allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, conductance measurements as a function of temperature and magnetic field are performed to determine the full magnetic phase diagram, which includes a spin‐flop and a spin‐flip phase. The magnetoconductance, which depends strongly on gate voltage, is determined. reaching values as high as 5000% near the threshold for electron conduction. The gate voltage also allows the magnetic states to be tuned, despite the relatively large thickness of the CrPS₄ multilayers employed in the study. The results show the need to employ 2D magnetic semiconductors with sufficiently large bandwidth to realize properly functioning transistors, and identify a candidate material to realize a fully gate‐tunable half‐metallic conductor.

Using field-effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2D magnetic semiconductors cause carrier localization, preventing transistor operation. Here, we show that exfoliated layers of CrPS$_4$ -- a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1 eV -- allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, we perform conductance measurements as a function of temperature and magnetic field, to determine the full magnetic phase diagram, which includes a spin-flop and a spin-flip phase. We find that the magnetoconductance depends strongly on gate voltage, reaching values as high as 5000 % near the threshold for electron conduction. The gate voltage also allows the magnetic states to be tuned, despite the relatively large thickness of the CrPS$_4$ multilayers employed in our study. Our results show the need to employ 2D magnetic semiconductors with sufficiently large bandwidth to realize properly functioning transistors, and identify a candidate material to realize a fully gate-tunable half-metallic conductor.