This work combines theory and experiment to examine the mechanisms underlying the harmonic generation of magnons. We develop a nonlinear spin-wave framework that is directly analogous to harmonic generation in nonlinear optics, and combine it with scanning nitrogen-vacancy (NV) center magnetometry to image and quantify magnonic harmonic generation in a Ni_81Fe_19/Pt microstripe. Within this framework, the harmonic response arises from nonlinear magnetization dynamics localized at strongly inhomogeneous textures, such as the sample edges and domain walls, that act as anharmonic confining potentials. Scanning probe imaging confirms that the harmonic response is correspondingly nonuniform and concentrated near the sample edges. We measure an expected nonlinear power-law scaling, a systematic shift toward larger wavevector excitations at higher harmonic order, and a spin-selective response indicative of an increasingly chiral harmonic stray field. These results provide a microscopic understanding of magnonic harmonic generation and highlight its potential for engineering nonlinear functionality in magnonic systems.

Spin defects in diamond are promising platforms for quantum sensing. The longest electron spin relaxation times (T_1) at room temperature for solid-state defects are observed in nitrogen vacancy centers in diamond, which can reach 6.67 ms, and substitutional nitrogen ("P1 centers") in diamond, which exhibit a T_1 of 2 ms. No other solid-state defect has exhibited millisecond-scale spin relaxation times at room temperature thus far. Here, we characterize the spin properties of the WAR5 defect in diamond with pulsed electron spin resonance. The observed T_1 is one of the longest for solid-state spin defects: 0.97(27) ms at room temperature and 14.38(19) min at 4 K. The observed coherence time (T_2) is 246(7) μs, which can be extended to 6.49(34) ms at 4 K with dynamical decoupling. Furthermore, we demonstrate optical spin polarization with a range of wavelengths from 405 nm to 500 nm and propose potential zero-phonon line candidates.

The historical calculation of spectroscopic properties for trivalent lanthanide ions is a complex multistep process that has been prone to inaccuracies. In this work, we revise the parametric semi-empirical Hamiltonian and address long-standing discrepancies in the literature. We also resurface the distinctions between orthogonal and non-orthogonal operators, and use orthogonalized operators to provide an alternative parametric description. Based on experimental data available in the literature, an updated set of parameter values for the canonical case of lanthanide ions in LaF3 is presented. Additionally, we provide calculations of spontaneous emission rates and oscillator strengths for magnetic dipole transitions in the LaF3 crystal host. To ensure the replicability of our findings, we make available the open-source code qlanth, accompanied by a comprehensive set of electronic files to serve as an updated reference for future calculations.

We report a combined scanning tunneling microscopy and atomistic theoretical study of Fe adatoms on the Bi_2Te_3(111) surface. Topographic imaging at 4.5 K shows Fe adatoms in fcc and hcp hollow sites exhibit a threefold-symmetric contrast, consistent with the C_3v symmetry of the adsorption site. However, simultaneously acquired differential conductance (dI/dV) maps reveal a pronounced reduction in symmetry, evidenced by differential contrast observed at nearest-neighbor Te sites. Density functional theory calculations show that the Fe/Bi_2Te_3 system undergoes a static Jahn–Teller distortion, reducing the adsorption symmetry from C_3v to C_1v, with the distorted configuration favored by 72.5 meV. Orbital-projected density of states calculations show that the occupied states near the Fermi level are dominated by d_xz and d_yz orbitals, whereas the unoccupied states are primarily of d_z^2, d_x^2-y^2 and d_xy character. The local density of states from these orbitals is in good qualitative agreement with experimental dI/dV spectra. Furthermore, simulated local-density-of-states maps using a tight-binding Green's function approach are in good agreement with experimental dI/dV maps, confirming that the reduced symmetry originates from the C_1v structural distortion.

Nitrogen-vacancy (NV) based quantum sensors hold great potential for real-time single-cell sensing with far-reaching applications in fundamental biology and medical diagnostics. Although highly sensitive, the mapping of quantum measurements onto cellular physiological states has remained an exceptional challenge. Here we introduce a novel quantum sensing modality capable of detecting changes in cellular activity. Our approach is based on the detection of environment-induced charge depletion within an individual particle that, owing to a previously unaccounted transverse dipole term, induces systematic shifts in the zero-field splitting (ZFS). Importantly, these charge-induced shifts serve as a reliable indicator for lipopolysaccharide (LPS)-mediated inflammatory response in macrophages. Furthermore, we demonstrate that surface modification of our diamond nanoprobes effectively suppresses these environment-induced ZFS shifts, providing an important tool for differentiating electrostatic shifts caused by the environment from other unrelated effects, such as temperature variations. Notably, this surface modification also leads to significant reductions in particle-induced toxicity and inflammation. Our findings shed light on systematic drifts and sensitivity limits of NV spectroscopy in a biological environment with ramification on the critical discussion surrounding single-cell thermogenesis. Notably, this work establishes the foundation for a novel sensing modality capable of probing complex cellular processes through straightforward physical measurements.

As electronic devices approach the atomic limit, the charge dynamics of individual dopant atoms increasingly constrain performance, stability, and coherence. In scanning tunnelling microscopy (STM), donor ionization is typically interpreted as a static threshold process arising from tip-induced band bending. Here we show that the ionization of individual sulfur donors in InAs is intrinsically dynamic and governed by the local electric field. Using MHz-frequency STM noise spectroscopy with atomic-scale spatial mapping, we resolve pronounced random telegraph noise that is invisible in time-averaged tunnelling spectra. A bias-dependent model quantitatively links the noise spectra to microscopic ionization and neutralization processes of the donor states, enabling direct extraction of nanosecond charge-state lifetimes. The switching rate is strongly bias dependent, demonstrating that the electric field continuously drives charge-state transitions. Unexpectedly, we show that the degenerately doped bulk leads to a sharp bias-dependent onset of donor ionization as the donor level crosses the Fermi level, giving rise to a characteristic shoulder in the noise power spectrum that is captured by our model. These results establish donor ionization as a non-equilibrium dynamical process with nontrivial contribution by the bulk electrons, and identify impurity switching as a universal nanoscale charge-noise mechanism relevant to quantum devices.

Nitrogen-vacancy (NV) centers in diamond are utilized extensively as quantum sensors for imaging fields at the nanoscale. The ultrahigh sensitivity of NV magnetometers has enabled the detection and spectroscopy of individual electron spins, with potentially far-reaching applications in condensed matter physics, spintronics, and molecular biology. However, the surfaces of these diamond sensors naturally contain electron spins, which create a background signal that can be hard to differentiate from the signal of the target spins. In this study, we develop a surface modification approach that eliminates the unwanted signal of these so-called dark electron spins. Our surface passivation technique, based on coating diamond surfaces with a thin titanium oxide (TiO2) layer, reduces the dark spin density. The observed reduction in dark spin density aligns with our findings on the electronic structure of the diamond-TiO2 interface. The reduction, from a typical value of 2000 & micro;m-2 to a value below that set by the detection limit of our NV sensors (200 & micro;m-2), results in a twofold increase in Hahn-echo coherence time of near surface NV centers. Furthermore, we derive a comprehensive spin model that connects dark spin relaxation with NV coherence, providing additional insights into the mechanisms behind the observed spin dynamics. Our findings are directly transferable to other quantum platforms, including nanoscale solid-state qubits and superconducting qubits.

We present a quantitative theory for simulating the electrically detected magnetic resonance (EDMR) of silicon vacancy-related spin pairs in silicon carbide using steady-state Lindblad master equations. In our theory, we consider V1a and V2a deep level silicon vacancies near the (0/-) charge state transition level in proximity to a previously identified nitrogen-related complex, the incomplete K-center, due to the hyperfine, spin structure, and Landé g factor of the shallow state. Our theory describes recent room temperature measurements attributed to V1a silicon vacancies, with reasonable extracted parameters for defect spin coherence times and electrical transport rates. At lower temperatures we predict that the shallow level hyperfine structure may be spectrally resolvable. Finally, we predict the EDMR spectrum of V2a silicon vacancy-related spin pairs and predict that two-photon, double quantum transitions of the silicon vacancy's negative charge state can be electrically read-out for enhanced magnetic field sensing.

We use multiband real space Green's functions computed using open-boundary conditions for clean GaN to exactly solve the potential-scattering Dyson equation to obtain the electronic structure of single nitrogen and gallium vacancies. From these vacancy solutions, we compute the local density of states as well as the Fermi contact and anisotropic contributions to the hyperfine field in the vicinity of the defect. These quantities directly affect electrically-detected magnetic resonance signals, which can be used to identify these defects when present in GaN devices.

Spintronic, spin caloritronic, and magnonic phenomena arise from complex interactions between charge, spin, and structural degrees of freedom that are challenging to model and even more difficult to predict. This situation is compounded by the relative scarcity of magnetically-ordered materials with relevant functionality, leaving the field strongly constrained to work with a handful of well-studied systems that do not encompass the full phase space of phenomenology predicted by fundamental theory. Here we present an important advance in this coupled theory-experiment challenge, wherein we extend existing theories of the spin Seebeck effect (SSE) to explicitly include the temperature-dependence of magnon non-conserving processes. This expanded theory quantitatively describes the low-temperature behavior of SSE signals previously measured in the mainstay material yttrium iron garnet (YIG) and predicts a new regime for magnonic and spintronic materials that have low saturation magnetization, $M_S$, and ultra-low damping. Finally, we validate this prediction by directly observing the spin Seebeck resistance (SSR) in the molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene (V[TCNE]$_x$, $x \sim 2$). These results validate the expanded theory, yielding SSR signals comparable in magnitude to YIG and extracted magnon diffusion length ($\lambda_m>1$ $\mu$ m) and magnon lifetime for V[TCNE]$_x$ ($\tau_{th}\approx 1-10$ $\mu$ s) exceeding YIG ($\tau_{th}\sim 10$ ns). Surprisingly, these properties persist to room temperature despite relatively low spin wave stiffness (exchange). This identification of a new regime for highly efficient SSE-active materials opens the door to a new class of magnetic materials for spintronic and magnonic applications.

Rydberg states of atoms in vacuum are now well recognized as a resource for quantum technologies. Donors in semiconductors also display analogous states, which have been proposed for similar applications. While they benefit from permanent locations in their host crystals, electron-lattice coupling leads to much shorter excited-state lifetimes than for neutral atoms in vacuum. Here we provide a quantitative description of donor-phonon kinetics, creating a basis for engineering donor systems in realistic material stacks for quantum devices. Our theory incorporates both form factors for the Rydberg states, which given their large extents in real space provide strong selectivity in momentum space, and tabulated deformation potentials for all six phonon branches throughout the Brillouin zone. By confronting this framework with carefully controlled time-resolved free electron laser measurements, we show that the widely quoted position of the silicon conduction band minimum, k_0, is inconsistent with observed donor relaxation rates and that quantitative agreement is obtained for a value further from the X-point than commonly assumed. This stringent experiment-theory comparison establishes donor relaxation as a precision metrology for conduction band parameters and scattering processes in silicon, with consequences spanning from quantum devices to classical electronics.

Shallow nitrogen-vacancy (NV) centers in diamond are promising nanoscale quantum sensors, yet their coherence is strongly limited by surface-induced noise. Surface adsorbates are widely believed to be a major source of decoherence. Here, we test this assumption by characterizing shallow single NV centers under ultrahigh vacuum (UHV) conditions, where the diamond surface is kept free of adsorbates, and comparing their behavior to ambient conditions. Surprisingly, we observe a 4x reduction in the Hahn echo coherence time T2 in UHV. By combining Hahn echo measurements in the single-quantum (SQ) and double-quantum (DQ) bases, we separate contributions from different noise sources and find that both electric and magnetic noise are enhanced in UHV. In contrast, T1 measurements reveal an increased DQ T1 in UHV, indicating suppressed electric field noise in the 100 MHz frequency regime. These results point to a modification of the surface noise spectrum upon adsorbate removal, with different frequency regimes arising from distinct microscopic mechanisms. Specifically, we find that the low frequency noise is consistent with increased surface charge in UHV that can be compensated by surface adsorbates in ambient conditions. Our findings highlight a complex and previously underappreciated role of surface adsorbates in shaping the noise environment of shallow NV centers, with important implications for nanoscale quantum sensing.

In this work, three InAs/ superlattices with different periods were investigated using photoluminescence and photoreflectance measurements, and their band structure was simulated using a 14 bulk-band k p model. The structures were studied by analysing the evolution of the spectral features in temperature and excitation power to determine the origin of optical transitions. After identifying which of these are related to the superlattice mini-bands, a rich collection of observed higher-order optical transitions was compared with refractive-index calculations. This procedure was used to adjust the parameters of the theoretical model, namely the bowing parameters of the InAsSb valence band offset and bandgap. It was also shown that the spectroscopy of the higher-order states combined with numerical modelling of the refractive index is a powerful tool for improvement of the material parameters, presenting a new approach to material studies of advanced semiconductor heterostructures.

We report an ab initio investigation of functionalized and 3d-electron-doped Cr2C MXenes. Upon functionalization, the Cr2C becomes chemically, dynamically, and mechanically stable, and it exhibits magnetic semiconducting behavior. Cr2CF2 stands out as a wide band gap semiconductor, possessing superexchange interaction mediated by F atoms within the layer; however, the applied strain transforms it from an indirect to a direct band gap semiconductor. Strong spin-phonon coupling found in Cr2CH2 is supported by the distorted Cr spin density due to the hydrogen environment. Two magnon branches, associated with two sublattice spins, are found in the ferromagnetic Cr2CO2 and antiferromagnetic Cr2CF2. Depending on the types of 3d-electron dopants and functionalization, Cr2C MXenes (except for Cr2CO2) change from an indirect band gap magnetic semiconductor to different states of electronic and magnetic matter, including an exotic direct band gap magnetic bipolar semiconductor. In addition, we reveal a band inversion between the two highest valence bands in the Fe-doped Cr2CCl2.

We propose a method for converting single microwave photons to single optical sideband photons based on spinful impurities in magnetic materials. This hybrid system is advantageous over previous proposals because (i) the implementation allows much higher transduction rates (10^{3} times faster at the same optical pump Rabi frequency) than state-of-the-art devices, (ii) high-efficiency transduction is found to happen in a significantly larger space of device parameters (in particular, over 1GHz microwave detuning), and (iii) it does not require mode volume matching between optical and microwave resonators. We identify the needed magnetic interactions as well as potential materials systems to enable this speed up using erbium dopants for telecom compatibility. This is an important step towards realizing high-fidelity entangling operations between remote qubits and will provide additional control of the transduction through perturbation of the magnet.

A Mn dopant in a III-V semiconductor produces a highly-entangled, coherent triplet ground state not fully captured by single-determinant theories of electron structure. We directly construct an analytic form for its ground-state wavefunction, finding surprising spin-charge correlations not revealed by semiclassical calculations. Spin-correlated circulating currents associated with the dopant yield remarkably large magnetic fringe fields of ∼1μT at distances of ∼ 10 nm from Mn in GaAs, potentially detectable by NV-diamond magnetometry while the dopant spin coherently precesses.

We present an ab initio investigation of the long-range charge density wave (CDW) order and superconducting properties of the pristine and lithiated NbSe_2 monolayer. Stable CDW structures are obtained through atomic reconstruction driven by soft-mode distortions and lithiation, respectively, lead to significant electronic modifications that suppress the CDW order. This suppression is attributed to anisotropic atomic distortions, along with a reduction in the electronic density of states at the Fermi level. As a result, the electron–phonon coupling strength is suppressed, particularly in the lithiated structure, due to reduced contributions from low-frequency phonons, primarily associated with in-plane Nb vibrations. Finally, we observe a sizable anisotropy in the superconducting gap on the Fermi surface, with a superconducting transition temperature of approximately 8 K in the distorted, and 4 K in the lithiated, CDW NbSe_2 monolayer.

Bulk-boundary correspondence is a foundational principle underlying the electronic band structure and physical behavior of topological quantum materials. Although it has been rigorously tested in topological systems where the physical properties involve charge currents, it remains unclear whether bulk-boundary correspondence should also hold for non-conserved spin currents. We study charge-to-spin conversion in a canonical topological insulator, Bi$_{1-x}$Sb$_x$, to address this fundamentally unresolved question. We use spin-torque ferromagnetic resonance measurements to accurately probe the charge-to-spin conversion efficiency in epitaxial Bi$_{1-x}$Sb$_x$~thin films of high structural quality spanning the entire range of composition, including both trivial and topological band structures, as verified using {\it in vacuo} angle-resolved photoemission spectroscopy. From these measurements, we deduce the effective spin Hall conductivity (SHC) and find excellent agreement with the values predicted by tight-binding calculations for the intrinsic SHC of the bulk bands. These results provide strong evidence that the strong spin-orbit entanglement of bulk states well below the Fermi energy connects directly to the SHC in epitaxial Bi$_{1-x}$Sb$_x$~films interfaced with a metallic ferromagnet. The excellent agreement between theory and experiment points to the generic value of analyses focused entirely on bulk properties, even for topological systems involving non-conserved spin currents.

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional platform for quantum sensing due to its optically addressable spin defects, such as the negatively charged boron vacancy (VB-). Spectral overlap of spin transitions due to large hyperfine interactions has limited its magnetic sensitivity. Here, we demonstrate spin-selective excitation of VB- spin defects in hBN driven by a circularly polarized microwave. Using a cross-shaped microwave resonance waveguide, we superimpose two orthogonally linearly polarized microwaves shifted in phase from an FPGA to generate circularly polarized microwaves. This enables selective spin |0⟩ → |-1⟩ or |0⟩ → |1⟩ excitation of VB- defects, as confirmed by optically detected magnetic resonance experimentally and supported computationally. We also investigate the influence of the magnetic field on spin-state selectivity. Our technique enhances the hBN platform for quantum sensing through better spin state control and magnetic sensitivity at low and zero fields.

Rare-earth containing wide band gap oxides, which provide spin-photon interface and narrow linewidth optical emission, are getting significant attention as the most promising candidate materials in advancing quantum transduction and memories. Here, from ab initio calculations, we identify antiferromagnetic ground states in structurally preferred monoclinic CePO4 and tetragonal CeVO4 exhibiting localized occupied and unoccupied Ce 4f states with 4f-4f transition characteristics. Interestingly, in CePO4, O 2p and P 3p states hybridize negligibly with Ce 4f states, while in CeVO4, V 3d and O 2p states hybridize and appear as extended states in between the occupied and unoccupied Ce 4f states. Here, phonon calculations and analysis identify and differentiate Raman active phonon modes along with the spin phonon coupling of Ce in both CePO4 and CeVO4 that ultimately lead to different 4f ground-state crystal-field multiplets, which are critical to accurately describe electronic transitions for foundational quantum transduction and memories. Further, the identified C1 site symmetry of Ce, lacking inversion symmetry in CePO4, is relevant for quantum memories and D2d site symmetry of Ce exhibiting inversion symmetry in CeVO4 is relevant for quantum transduction.