Quantum spin liquids can arise from Kitaev magnetic interactions, and exhibit fractionalized excitations with the potential for a topological form of quantum computation. This review surveys recent experimental and theoretical progress on the pursuit of phenomena related to Kitaev magnetism in layered and exfoliatable materials, which offer numerous opportunities to apply powerful techniques from the field of atomically thin materials. We primarily focus on the antiferromagnetic Mott insulatorα-RuCl3, which exhibits Kitaev couplings and is readily exfoliated to single- or few-layer sheets, and thus serves as a test bed for developing probes of Kitaev phenomena in atomically thin materials and devices. We introduce the Kitaev model and how it is realized inα-RuCl3and other material candidates; and coverα-RuCl3synthesis and fabrication into van der Waals heterostructure devices. A key discovery is a work-function-mediated charge transfer that heavily dopes both theα-RuCl3and proximate materials, and can enhance Kitaev interactions by up to 50%. We further discuss a wide range of recent results in electronic transport and optical and tunneling spectroscopies ofα-RuCl3devices. The experimental techniques and theoretical insights developed forα-RuCl3establish a framework for discovering and engineering superior two-dimensional Kitaev materials that may ultimately realize elusive quantum spin liquid phases.

Characterizing the low-energy dynamics of quantum materials is crucial to our understanding of strongly correlated electronic states. Yet, it remains experimentally challenging to investigate such dynamics with high spectroscopic resolution in both frequency and momentum space, particularly in two-dimensional correlated systems. Here, we leverage Nitrogen-Vacancy (NV) centers in diamond as a powerful and non-invasive tool to study thin-film Bi_2Sr_2CaCu_2O_8+δ (BSCCO), revealing several distinct dynamical phenomena across the superconducting phase diagram. At zero magnetic field and low temperatures, NV depolarization (T_1) noise spectroscopy captures the low-frequency (GHz-scale) magnetic noise generated by nodal superconducting quasiparticle excitations, in agreement with Bardeen-Cooper-Schrieffer (BCS) mean-field theory. Near the critical temperature T_c ≈ 90 K, supercurrent-fluctuation-induced noise leads to a sharp reduction of the NV T_1. By carefully analyzing the temperature-scaling of T_1, we observe clear deviations from the BCS prediction, reflecting the importance of order parameter fluctuations and enabling the determination of both static and dynamical critical exponents. When a small field is applied, we detect a broad and asymmetric reduction of NV T_1 near T_c; the field-induced smearing of the transition unveils the presence of a vortex liquid phase. Finally, NV decoherence (T_2) noise spectroscopy allows us to characterize magnetic noise at even lower MHz-scale frequencies and obtain evidence for complex vortex-solid fluctuations well below T_c. Our results establish quantum noise spectroscopy as a versatile platform for probing dynamical phenomena in superconductors, with frequency and length scales complementary to existing techniques.

The negatively charged boron-vacancy center (V-B) in hexagonal boron nitride (hBN) has recently emerged as a highly promising quantum sensor. Compared to the nitrogen-vacancy (NV) center in diamond, the change with temperature of the spin transition energy of V-B is more than an order of magnitude larger, making it a potential nanoscale thermometer with superior sensitivity. However, the underlying mechanism of the observed large temperature dependence remains an open question. In this work, using isotopically purified h10B15N, we systematically characterize the zero-field splitting, hyperfine interaction, and spin-relaxation time of V-B from 10 to 350 K. We carry out first-principles calculations of the V-B spin-phonon interaction and show that a secondorder effect from finite-temperature phonon excitations is responsible for the observed changes in experiments. By fitting our experimental results to a physically motivated model, we extract the dominant phonon mode which agrees well with our simulations. Finally, we investigate the dynamic nuclear spin-polarization process at cryogenic temperatures. Our results provide key insights in V-B centers and their utilization as nanoscale thermometers and phonon sensors.

Charge transfer at material interfaces governs a wide range of physical properties, from electronic band structures to emergent collective excitations. In two-dimensional (2D) material heterostructures, charge transfer phenomena play important roles in enabling novel quantum phases, proximity effects, and tunable plasmonic responses. One representative charge transfer interface is formed between alpha-RuCl3, a van der Waals material with high electron affinity, and graphene. Significant charge transfer across this interface induces the formation of charge-transfer plasmon polaritons (CPPs), hybrid excitations between light and charge oscillations. However, previous studies found that as the charge transfer process takes place, alpha-RuCl3 becomes lossy, which limits the quality factor of CPPs. Here, we investigate CPPs down to 10 K using a home-built scattering-type scanning near-field optical microscope (s-SNOM) optimized for low-temperature measurements. Our study reveals a dramatic suppression of plasmon loss channels below 40 K, contributing to a significant enhancement in the plasmonic quality factor. This reduction in loss is likely attributed to the blue shift of the correlation-induced Mott gap in alpha-RuCl3 with decreasing temperature, along with the reduction of phonon scattering at low temperature. Our results highlight the potential of using s-SNOM and CPPs to study complex 2D interfaces and reveal correlated electron dynamics in the underlying material.

Moiré superlattices in transition metal dichalcogenide heterostructures provide a platform to engineer many-body interactions. Here, we realize a bichromatic moiré superlattice in an asymmetric WSe2/WS2/WSe2 heterotrilayer by combining R- and H-stacked bilayers with mismatched moiré wavelengths. This structure hosts fermionic quadrupolar moiré trions-interlayer excitons bound to an opposite-layer hole-with vanishing dipole moments. These trions arise from hybridized moiré potentials enabling multiple excitonic orbitals with tunable interlayer coupling, allowing control of excitonic and electronic ground states. We show that an out-of-plane electric field could effectively reshape moiré excitons and interlayer-intralayer electron correlations, driving a transition from interlayer to intralayer Mott states with enhanced Coulomb repulsion. The asymmetric stacking further enriches excitonic selection rules, broadening opportunities for spin-photon engineering. Our results demonstrate bichromatic moiré superlattices as a reconfigurable platform for emergent quantum states, where quadrupolar moiré trion emission may enable coherent and entangled quantum light manipulation.

We offer theoretical predictions for the frequency of the resonant frequency of transport for the hydrodynamic description of bilayer graphene, as well as provide quantification for the relative strength of this signal throughout phase space. Our calculations are based on classical fluid dynamics equations derived from the Boltzmann equation for bilayer graphene in arXiv:1901.07039, and suggest that while this resonance is accessible to current experimental techniques, the same mechanism which causes the hydrodynamic resonance to differ from the Fermi liquid value is responsible for a significant broadening of the peak.

Single photon detectors (SPDs) are essential technology in quantum science, quantum network, biology, and advanced imaging. To detect the small quantum of energy carried in a photon, conventional SPDs rely on energy excitation across either a semiconductor bandgap or superconducting gap. While the energy gap suppresses the false-positive error, it also sets an energy scale that can limit the detection efficiency of lower energy photons and spectral bandwidth of the SPD. Here, we demonstrate an orthogonal approach to detect single near-infrared photons using graphene calorimeters. By exploiting the extremely low heat capacity of the pseudo-relativistic electrons in graphene near its charge neutrality point, we observe an electron temperature rise up to 2 K using a hybrid Josephson junction. In this proof-of-principle experiment, we achieve an intrinsic quantum efficiency of 87 second (per hour) at operation temperatures as high as 1.2 K. Our results highlight the potential of electron calorimetric SPDs for detecting lower-energy photons from the mid-IR to microwave regimes, opening pathways to study space science in far-infrared regime, to search for dark matter axions, and to advance quantum technologies across a broader electromagnetic spectrum.

We perform infrared magnetospectroscopy of Landau level (LL) transitions in dual-gated bilayer graphene. At ν=4 when the zeroth LL (octet) is filled, two resonances are observed indicating the opening of a gap. At ν=0 when the octet is half-filled, multiple resonances disperse nonmonotonically with increasing displacement field, D, perpendicular to the sheet, showing a phase transition at modest displacement fields from a canted antiferromagnet (CAFM) to the layer-polarized state, with a gap that opens linearly in D. When D=0 and ν is varied, resonances at ±ν show an electron-hole asymmetry with multiple line splittings as the octet is progressively filled. The ν=4 data show good agreement with predictions from a mean-field Hartree-Fock calculation when accounting for multiple tight-binding terms in a four-band model of bilayer graphene. However, even by incorporating a valley interaction anisotropy tuned to the CAFM ground state, only partial agreement is found at ν=0. Our results suggest additional physics is required to understand bilayer graphene at half-filling.

Probing electrical and magnetic properties in aqueous environments remains a frontier challenge in nanoscale sensing. Our inability to do so with quantitative accuracy imposes severe limitations, for example, on our understanding of the ionic environments in a diverse array of systems, ranging from novel materials to the living cell. The Nitrogen-Vacancy (NV) center in fluorescent nanodiamonds (FNDs) has emerged as a good candidate to sense temperature, pH, and the concentration of paramagnetic species at the nanoscale, but comes with several hurdles such as particle-to-particle variation which render calibrated measurements difficult, and the challenge to tightly confine and precisely position sensors in aqueous environment. To address this, we demonstrate relaxometry with NV centers within optically-trapped FNDs. In a proof of principle experiment, we show that optically-trapped FNDs enable highly reproducible nanomolar sensitivity to the paramagnetic ion, (\mathrm{Gd}^{3+}). We capture the three distinct phases of our experimental data by devising a model analogous to nanoscale Langmuir adsorption combined with spin coherence dynamics. Our work provides a basis for routes to sense free paramagnetic ions and molecules in biologically relevant conditions.

In high-sensitivity bolometers and calorimeters, the photon absorption often occurs at a finite distance from the temperature sensor to accommodate antennas or avoid the degradation of superconducting circuitry exposed to radiation. As a result, thermal propagation from the input to the temperature readout can critically affect detector performance. In this report we model the performance of a graphene bolometer, accounting for electronic thermal diffusion and dissipation via electron-phonon coupling at low temperatures in three regimes: clean, supercollision, and resonant scattering. Our results affirm the feasibility of a superconducting readout without Cooper-pair breaking by mid- and near-infrared photons, and provide a recipe for designing graphene absorbers for calorimetric single-photon detectors. We investigate the tradeoff between the input-readout distance and detector efficiency, and predict an intrinsic timing jitter of ~2.7 ps. Based on our result, we propose a spatial-mode-resolving photon detector to increase communication bandwidth.

Spin defects in van der Waals materials offer a promising platform for advancing quantum technologies. Here, we propose and demonstrate a powerful technique based on isotope engineering of host materials to significantly enhance the coherence properties of embedded spin defects. Focusing on the recently-discovered negatively charged boron vacancy center ( V_B^- ) in hexagonal boron nitride (hBN), we grow isotopically purified h 10 B 15 N crystals. Compared to V_B^- in hBN with the natural distribution of isotopes, we observe substantially narrower and less crowded V_B^- spin transitions as well as extended coherence time T 2 and relaxation time T 1 . For quantum sensing, V_B^- centers in our h 10 B 15 N samples exhibit a factor of 4 (2) enhancement in DC (AC) magnetic field sensitivity. For additional quantum resources, the individual addressability of the V_B^- hyperfine levels enables the dynamical polarization and coherent control of the three nearest-neighbor 15 N nuclear spins. Our results demonstrate the power of isotope engineering for enhancing the properties of quantum spin defects in hBN, and can be readily extended to improving spin qubits in a broad family of van der Waals materials.

Defects are crucial in determining a variety of material properties especially in low dimensions. In this work, we study point defects in monolayer alpha-phase Ruthenium (III) chloride (alpha-RuCl3), a promising candidate to realize quantum spin liquid with nearly degenerate magnetic states. Our first-principles simulations reveal that Cl vacancies, Ru vacancies, and oxygen substitutional defects are the most energetically stable point defects. Besides, these point defects break the magnetic degeneracy: Cl vacancies and oxygen substitutional defects energetically favor the zigzag-antiferromagnetic configuration while Ru vacancies favor the ferromagnetic configuration, shedding light on understanding the observed magnetic structures and further defect engineering of magnetism in monolayer {\alpha}-RuCl3. We further calculated their electronic structures and optical absorption spectra. The polarization symmetry of optical responses provides a convenient signature to identify the point defect types and long-range magnetic orders.

Optically active spin defects in van der Waals materials are promising platforms for modern quantum technologies. Here we investigate the coherent dynamics of strongly interacting ensembles of negatively charged boron-vacancy ( V_B^- ) centers in hexagonal boron nitride (hBN) with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different dephasing sources, we observe more than 5-fold improvement in the measured coherence times across all hBN samples. Crucially, we identify that the many-body interaction within the V_B^- ensemble plays a substantial role in the coherent dynamics, which is then used to directly estimate the concentration of V_B^- . We find that at high ion implantation dosage, only a small portion of the created boron vacancy defects are in the desired negatively charged state. Finally, we investigate the spin response of V_B^- to the local charged defects induced electric field signals, and estimate its ground state transverse electric field susceptibility. Our results provide new insights on the spin and charge properties of V_B^- , which are important for future use of defects in hBN as quantum sensors and simulators.

The observation of magnetic diode behavior with ultra-low forward voltage renders new venue for energetically efficient spintronic device research in the unconventional system of two-dimensional permalloy honeycomb lattice. However, detailed understanding of temperature and magnetic field tuning of diode behaviors are imperative to any practical application. In this report, our study unveils many important properties of magnetic diode that not only pave the way for practical applications, but also underlines the role of emergent phenomena of magnetic charge correlation on honeycomb vertices. We find that magnetic diode behavior persists across a broad temperature range. In a surprising observation, magnetic field application tends to induce a peculiar reentrant characteristic where diode behavior is suppressed in remnant field but reappears after warming to room temperature. Analysis of $I-V$ data suggests a modest energy gap, $∖sim$ 0.03 - 0.1 eV, which is comparable to magnetic Coulomb’s interaction energy between emergent magnetic charges on honeycomb vertices in the reverse biased state. It affirms the role of magnetic charge correlation in unidirectional conduction in 2D honeycomb lattice. The experimental results are expected to spur the utilization of magnetic diode in next generation spintronic device applications.

We investigate the electronic properties of a graphene and α-ruthenium trichloride (α-RuCl3) heterostructure using a combination of experimental techniques. α-RuCl3 is a Mott insulator and a Kitaev material. Its combination with graphene has gained increasing attention due to its potential applicability in novel optoelectronic devices. By using a combination of spatially resolved photoemission spectroscopy and low-energy electron microscopy, we are able to provide a direct visualization of the massive charge transfer from graphene to α-RuCl3, which can modify the electronic properties of both materials, leading to novel electronic phenomena at their interface. A measurement of the spatially resolved work function allows for a direct estimate of the interface dipole between graphene and α-RuCl3. Their strong coupling could lead to new ways of manipulating electronic properties of a two-dimensional heterojunction. Understanding the electronic properties of this structure is pivotal for designing next generation low-power optoelectronics devices.

We investigate the electronic properties of a graphene and α-ruthenium trichloride (hereafter RuCl_3) heterostructure, using a combination of experimental and theoretical techniques. RuCl_3 is a Mott insulator and a Kitaev material, and its combination with graphene has gained increasing attention due to its potential applicability in novel electronic and optoelectronic devices. By using a combination of spatially resolved photoemission spectroscopy, low energy electron microscopy, and density functional theory (DFT) calculations we are able to provide a first direct visualization of the massive charge transfer from graphene to RuCl_3, which can modify the electronic properties of both materials, leading to novel electronic phenomena at their interface. The electronic band structure is compared to DFT calculations that confirm the occurrence of a Mott transition for RuCl_3. Finally, a measurement of spatially resolved work function allows for a direct estimate of the interface dipole between graphene and RuCl_3. The strong coupling between graphene and RuCl_3 could lead to new ways of manipulating electronic properties of two-dimensional lateral heterojunction. Understanding the electronic properties of this structure is pivotal for designing next generation low-power opto-electronics devices.

We present a non-magnetic, fully-integrated, superconducting non-reciprocal bandpass filter using spatio-temporal inductance modulation in coupled resonator networks, thus combining the functionalities of a conventional bandpass filter with an isolator. The bandpass filter is based on a capacitively-coupled shunt resonator architecture where the resonators are implemented as a parallel combination of finger capacitors and a chain of 10 SQUIDs which behave as flux-controlled nonlinear inductors. The operation principle of the device has been studied and optimized for non-reciprocal performance using Floquet spectral analysis where the time-modulated SQUIDs are modeled using frequency translational admittance matrices. The bandpass filter is implemented in a single-layer superconducting process. Operating at a base temperature of 20 mK, we measured a low insertion loss of 1.3 dB in the forward direction. In the reverse direction, the measured isolation is as high as +25 dB at the center frequency and >+15 dB across a bandwidth of 250 MHz.

Recent experiments on the coupling of the in-plane motional state of electrons floating on the surface of liquid helium to a microwave resonator have revealed the importance of helium surface fluctuations to the coherence of this motion. Here we investigate these surface fluctuations by studying the resonance properties of a superconducting coplanar waveguide (CPW) resonator filled with superfluid helium, where a significant fraction of the resonator's electromagnetic mode volume is coupled to the surface dynamics of the liquid. We present preliminary results on real-time CPW resonator frequency shifts driven by helium fluctuations, which are quantified via their power spectral density and compared with measurements using a commercial accelerometer. We find that a considerable contribution to the CPW resonator noise originates from the mechanical vibrations of the helium surface generated by the pulse tube (PT) cryocooler on the cryostat on which the experiments were performed.

Background: Emerging multi-pixel X-ray source technology enables new designs for X-ray imaging systems. The power of multi-pixel X-ray sources with a fixed anode is limited by focal spot power density. Purpose: The purpose of this study is to optimize the W-diamond target and predict its performance in multi-pixel X-ray sources. Methods: X-ray intensity and energy deposition in the W-diamond target with different thicknesses of tungsten film and incident electron energies was calculated with the Geant4 Monte Carlo toolkit. COMSOL Multiphysics software was used to analyze the transient and stationary heat transfer in the thin-film W-diamond target. The maximum tube power and X-ray output intensity were predicted for both transmission and reflection target configurations. Results: The maximum focal spot power density was limited by either the graphitization of the diamond substrate or the melting point of the W target. With optimal W-target thickness, the maximum transmission X-ray intensities are about 40%-50% higher than the maximum reflection intensities. Thin-film W-diamond targets allow four to five times more maximum power input and produce six to seven times higher transmission X-ray intensity in continuous mode compared with conventional reflection W thick targets. Depending on the focal spot size, reducing the X-ray pulse duration can further enhance the tube power. Conclusions: Multi-pixel X-ray sources using this W-diamond target design can produce significantly higher X-ray output than traditional thick tungsten targets without major modification of the tube design.

We demonstrate ultrasharp (≲10 nm) lateral p-n junctions in graphene using electronic transport, scanning tunneling microscopy, and first-principles calculations. The p-n junction lies at the boundary between differentially doped regions of a graphene sheet, where one side is intrinsic and the other is charge-doped by proximity to a flake of α-RuCl3 across a thin insulating barrier. We extract the p-n junction contribution to the device resistance to place bounds on the junction width. We achieve an ultrasharp junction when the boundary between the intrinsic and doped regions is defined by a cleaved crystalline edge of α-RuCl3 located 2 nm from the graphene. Scanning tunneling spectroscopy in heterostructures of graphene, hexagonal boron nitride, and α-RuCl3 shows potential variations on a sub 10 nm length scale. First-principles calculations reveal that the charge-doping of graphene decays sharply over just nanometers from the edge of the α-RuCl3 flake.