Evidence for electron-hole crystals in a Mott insulator.

The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron-hole crystals in a doped Mott insulator, namely, α-RuCl3, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru-Ru bonds, respectively. Moreover, a gate-induced transition of electron-hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron-hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials.

High-Mobility Compensated Semimetals, Orbital Magnetization, and Umklapp Scattering in Bilayer Graphene Moiré Superlattices.

Twist-controlled moiré superlattices (MSs) have emerged as a versatile platform for realizing artificial systems with complex electronic spectra. The combination of Bernal-stacked bilayer graphene (BLG) and hexagonal boron nitride (hBN) can give rise to an interesting MS, which has recently featured a set of unexpected behaviors, such as unconventional ferroelectricity and the electronic ratchet effect. Yet, the understanding of the electronic properties of BLG/hBN MS has, at present, remained fairly limited. Here, we combine magneto-transport and low-energy sub-THz excitation to gain insights into the properties of this MS. We demonstrate that the alignment between BLG and hBN crystal lattices results in the emergence of compensated semimetals at some integer fillings of the moiré bands, separated by van Hove singularities where the Lifshitz transition occurs. A particularly pronounced semimetal develops when eight holes reside in the moiré unit cell, where coexisting high-mobility electron and hole systems feature strong magnetoresistance reaching 2350% already at B = 0.25 T. Next, by measuring the THz-driven Nernst effect in remote bands, we observe valley splitting, indicating an orbital magnetization characterized by a strongly enhanced effective gv-factor of 340. Finally, using THz photoresistance measurements, we show that the high-temperature conductivity of the BLG/hBN MS is limited by electron-electron umklapp processes. Our multifaceted analysis introduces THz-driven magnetotransport as a convenient tool to probe the band structure and interaction effects in van der Waals materials and provides a comprehensive understanding of the BLG/hBN MS.

Fundamental Limits of Few-Layer NbSe2 Microbolometers at Terahertz Frequencies.

The rapid development of infrared spectroscopy, observational astronomy, and scanning near-field microscopy has been enabled by the emergence of sensitive mid- and far-infrared photodetectors. Superconducting hot-electron bolometers (HEBs), known for their exceptional signal-to-noise ratio and fast photoresponse, play a crucial role in these applications. While superconducting HEBs are traditionally crafted from sputtered thin films such as NbN, the potential of layered van der Waals (vdW) superconductors is untapped at THz frequencies. Here, we introduce superconducting HEBs made from few-layer NbSe2 microwires. By improving the interface between NbSe2 and metal leads, we overcome impedance mismatch with RF readout, enabling large responsivity THz detection (0.13 to 2.5 THz) with a minimal noise equivalent power of 7 pW/ Hz and nanosecond-range response time. Our work highlights NbSe2 as a promising platform for HEB technology and presents a reliable vdW assembly protocol for custom bolometer production.

Zero-Bias Photodetection in 2D Materials via Geometric Design of Contacts.

Structural or crystal asymmetry is a necessary condition for the emergence of zero-bias photocurrent in light detectors. Structural asymmetry has been typically achieved via p-n doping, which is a technologically complex process. Here, we propose an alternative approach to achieve zero-bias photocurrent in two-dimensional (2D) material flakes exploiting the geometrical nonequivalence of source and drain contacts. As a prototypical example, we equip a square-shaped flake of PdSe2 with mutually orthogonal metal leads. Upon uniform illumination with linearly polarized light, the device demonstrates nonzero photocurrent which flips its sign upon 90° polarization rotation. The origin of zero-bias photocurrent lies in a polarization-dependent lightning-rod effect. It enhances the electromagnetic field at one contact from the orthogonal pair and selectively activates the internal photoeffect at the respective metal-PdSe2 Schottky junction. The proposed technology of contact engineering is independent of a particular light-detection mechanism and can be extended to arbitrary 2D materials.

Ultralow-noise Terahertz Detection by p-n Junctions in Gapped Bilayer Graphene.

Graphene shows strong promise for the detection of terahertz (THz) radiation due to its high carrier mobility, compatibility with on-chip waveguides and transistors, and small heat capacitance. At the same time, weak reaction of graphene's physical properties on the detected radiation can be traced down to the absence of a band gap. Here, we study the effect of electrically induced band gap on THz detection in graphene bilayer with split-gate p-n junction. We show that gap induction leads to a simultaneous increase in current and voltage responsivities. At operating temperatures of ∼25 K, the responsivity at a 20 meV band gap is from 3 to 20 times larger than that in the gapless state. The maximum voltage responsivity of our devices at 0.13 THz illumination exceeds 50 kV/W, while the noise equivalent power falls down to 36 fW/Hz1/2.

Terahertz Photoconductivity in Bilayer Graphene Transistors: Evidence for Tunneling at Gate-Induced Junctions.

Photoconductivity of novel materials is the key property of interest for design of photodetectors, optical modulators, and switches. Despite the photoconductivity of most novel 2d materials having been studied both theoretically and experimentally, the same is not true for 2d p-n junctions that are necessary blocks of most electronic devices. Here, we study the sub-terahertz photocoductivity of gapped bilayer graphene with electrically induced p-n junctions. We find a strong positive contribution from junctions to resistance, temperature resistance coefficient, and photoresistivity at cryogenic temperatures T ∼ 20 K. The contribution to these quantities from junctions exceeds strongly the bulk values at uniform channel doping even at small band gaps of ∼10 meV. We further show that positive junction photoresistance is a hallmark of interband tunneling, and not of intraband thermionic conduction. Our results point to the possibility of creating various interband tunneling devices based on bilayer graphene, including steep-switching transistors and selective sensors.

Visualizing atomic structure and magnetism of 2D magnetic insulators via tunneling through graphene.

The discovery of two-dimensional (2D) magnetism combined with van der Waals (vdW) heterostructure engineering offers unprecedented opportunities for creating artificial magnetic structures with non-trivial magnetic textures. Further progress hinges on deep understanding of electronic and magnetic properties of 2D magnets at the atomic scale. Although local electronic properties can be probed by scanning tunneling microscopy/spectroscopy (STM/STS), its application to investigate 2D magnetic insulators remains elusive due to absence of a conducting path and their extreme air sensitivity. Here we demonstrate that few-layer CrI3 (FL-CrI3) covered by graphene can be characterized electronically and magnetically via STM by exploiting the transparency of graphene to tunneling electrons. STS reveals electronic structures of FL-CrI3 including flat bands responsible for its magnetic state. AFM-to-FM transition of FL-CrI3 can be visualized through the magnetic field dependent moiré contrast in the dI/dV maps due to a change of the electronic hybridization between graphene and spin-polarised CrI3 bands with different interlayer magnetic coupling. Our findings provide a general route to probe atomic-scale electronic and magnetic properties of 2D magnetic insulators for future spintronics and quantum technology applications.