arXiv subjects

Michael E. Flatté

Publications and source records attributed to Michael E. Flatté.

At least 37 records · Page 2Linked to original sources

arXiv2025

Singly occupied 4$f$ antiferromagnetic insulators: CePO$_4$ and CeVO$_4$

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 CePO$_4$ and tetragonal CeVO$_4$ exhibiting localized occupied and unoccupied Ce $4f$ states with $4f-4f$ transition characteristics. Interestingly, in CePO$_4$, O $2p$ and P $3p$ states hybridize negligibly with Ce $4f$ states, while in CeVO$_4$, 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 CePO$_4$ and CeVO$_4$ 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 $C_1$ site symmetry of Ce, lacking inversion symmetry in CePO$_4$, is relevant for quantum memories and $D_{2d}$ site symmetry of Ce exhibiting inversion symmetry in CeVO$_4$ is relevant for quantum transduction.

cond-mat.str-el↗

arXiv2025

Stability, electronic quantum states, and magnetic interactions of Er$^{3+}$ ions in Ga$_2$O$_3$

The chemical, structural, mechanical, and dynamical stabilities of the $α$- and $β$-Ga$_2$O$_3$ are confirmed from respective negative formation energies, negative cohesive energies, favorable elastic constants, and positive phonon frequencies. The phonon dispersions indicate that the Ga-O bonds are uniform in the $α$-phase, while they vary in the $β$-phase due to the anisotropic polyhedral movement. The defect formation energy analysis confirms that both Er-doped phases prefer Er$^{3+}$ state. The underestimated band gaps of the pristine phases from $ab~initio$ calculations are corrected by employing the hybrid functional calculations. The site preference energy analysis indicates partial occupation of Er in the octahedral site of Ga. Anisotropic nature of hyperfine tensor coefficients of Er are similar in both phases. Calculated magnetic exchange interaction between two Er dopants is negative for $α$ and positive for $β$, indicating antiferromagnetic ground state in the former and the ferromagnetic ground state in the latter. A large values of Dzyaloshinskii-Moriya interactions (DMIs) are obtained along the $x$ direction in the $α$ and along the $y$ direction in the $β$. The analysis of dielectric constants and refractive indices of both pristine and Er doped phases shows a good agreement with available experimental values. The calculated optical anisotropy is slightly higher in $β$ than those in $α$, which is due to the involvement of lower symmetry in $β$. The crystal field coefficients (CFCs) calculated from DFT are used to analyze 4$f$ multiplets and 4$f$ - 4$f$ transitions. Thus calculated lowest energy level of the first excited state to the lowest energy level of the ground state is about 1.53~$μ$m, which is in a good agreement with available experiment and it falls within the quantum telecommunication wavelength range.

cond-mat.mtrl-sci↗

arXiv2025

Refining spectroscopic calculations for trivalent lanthanide ions: a revised parametric Hamiltonian and open-source solution

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 orthogonal operators to provide an alternative parametric description. Based on experimental data available in the literature, a new set of parameter values for the ions in LaF$_3$ and LiYF$_4$ is presented. Additionally, we provide calculations of spontaneous emission rates and oscillator strengths for magnetic dipole transitions in the LaF$_3$ crystal host. To ensure the replicability of our findings, we make available the open-source code $\mathbf{\texttt{qlanth}}$, accompanied by a comprehensive set of electronic files to serve as an updated reference for future calculations.

physics.atom-ph↗

arXiv2025

Ab initio calculations of erbium crystal field splittings in oxide hosts

We present an effective ab initio method to calculate the crystal field coefficients of an erbium (Er3+) ion experiencing different local site symmetries in several wide-band-gap oxides, and then evaluate crystal field splittings of these Er3+ ions for their ground and excited states. The optical transitions between the ground state (Z) and excited state (Y) manifolds of the environmentally shielded 4f states of these Er3+ ions have wavelengths ~1.5 microns and thus have potential applications to quantum communications and quantum memories. These results are in excellent agreement with recent low-temperature measurements, provided the inadequate calculation of the 4f shell screening is adjusted by reducing the radial extent of the 4f wavefunctions by approximately a factor of 2.

cond-mat.mtrl-sci↗

arXiv2025

Ab initio calculations of erbium crystal field splittings in oxide hosts: role of the 4f radial wave function

We expand here our description of a newly developed simple and effective ab initio method of calculating crystal field coefficients (CFCs) of rare-earth atoms in insulating hosts, focusing on Er in wide band gap oxide hosts MgO, ZnO, TiO$_2$, CaWO$_4$, and PbWO$_4$, which exemplify different local site symmetries. These hybrid functional calculations, which incorporate a portion of exact exchange from Hartree-Fock theory, reproduce the experimentally identified insulating band gaps of these oxides. The negative values of cohesive and formation energies confirm the structural and chemical stability of these oxides, whereas the defect formation energies indicate that Er doped ZnO, CaWO$_4$, and PbWO$_4$ are easier to form compared to Er doped MgO and TiO$_2$. Er doped in these oxide hosts exhibits a spin magnetic moment of $\sim$ 3 $μ_B$ confirming the $3+$ valence state. The CFCs of these hosted Er are determined from charge densities and potentials obtained from non-spin-polarized calculations, involving a 4\textit{f} core approximation. These CFCs are subsequently used to solve an effective Hamiltonian and generate the crystal field splitting of Er 4f. These calculated Er 4f energy levels are in good agreement with available experiments.

cond-mat.mtrl-sci↗

arXiv2024

Robust Edge States from Band Topology in a Damped One-dimensional Magnonic Crystal

The presence or absence of topologically-produced edge states of a crystal are robust to disorder; their stability in the presence of decay is less clear. For topologically nontrivial bosonic systems with finite particle lifetimes, such as photonic, phononic, or magnonic structures, a natural hypothesis suggests that if the linewidth from particle decay exceeds the gap between neighboring bands, then topological features such as Berry phases or edge states will lose their protection. Here we show that topological properties are significantly more robust than this, by assessing the properties of a one-dimensional magnonic crystal as the damping is increased. Even when the damping greatly exceeds the gap between neighboring bands the Zak phase of those bands is nearly unchanged, and the edge states remain clearly visible in micromagnetic simulations of microwave transmission. These results clarify the understanding of robust topological properties and bulk-boundary correspondence.

cond-mat.mes-hall↗

arXiv2024

Enhancement of Microwave to Optical Spin-Based Quantum Transduction via a Magnon Mode

We propose a new 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 $1$ GHz 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.

quant-ph↗

arXiv2024

Implications of the electron-phonon coupling in CuPb$_9$(PO$_4$)$_6$O for superconductivity: an \textit{ab initio} study

We report $ab~initio$ calculations of the electronic and vibrational properties in CuPb$_9$(PO$_4$)$_6$O, including the electron-phonon coupling strength via strong-coupling Migdal-Eliashberg theory. We verify the presence of appealing flat electronic bands near the Fermi level, a strong hybridization between the Cu $3d$ and O $2p$ states, and soft low-energy phonons, which can suggest high-temperature superconducting behavior. However, the electron-phonon coupling strength appears insufficient to overcome the Coulomb repulsion between an electron pair and thus does does not support high-temperature superconductivity in CuPb$_9$(PO$_4$)$_6$O via the conventional electron-phonon Migdal-Eliashberg mechanism. Even neglecting Coulomb repulsion of the electron pair we find this electron-phonon coupling suggests a superconducting transition temperature less than 2~K.

cond-mat.supr-con↗

arXiv2023

Distinguishing erbium dopants in Y$_2$O$_3$ by site symmetry: \textit{ ab initio} theory of two spin-photon interfaces

We present a first-principles study of defect formation and electronic structure of erbium (Er)-doped yttria (Y$_2$O$_3$). This is an emerging material for spin-photon interfaces in quantum information science due to the narrow linewidth optical emission from Er dopants at standard telecommunication wavelengths and their potential for quantum memories and transducers. We calculate formation energies of neutral, negatively, and positively charged Er dopants and find the charge neutral configuration to be the most stable, consistent with experiment. Of the two substitutional sites of Er for Y, the $C_2$ (more relevant for quantum memories) and $C_{3i}$ (more relevant for quantum transduction), we identify the former as possessing the lowest formation energy. The electronic properties are calculated using the Perdew-Burke-Ernzerhof (PBE) functional along with the Hubbard $U$ parameter and spin-orbit coupling (SOC), which yields a $\sim$ 6 $μ_B$ orbital and a $\sim$ 3 $μ_B$ spin magnetic moment, and 11 electrons in the Er $4f$ shell, confirming the formation of charge-neutral Er$^{3+}$. This standard density functional theory (DFT) approach underestimates the band gap of the host and lacks a first-principles justification for $U$. To overcome these issues, we performed screened hybrid functional (HSE) calculations, including a negative $U$ for the $4f$ orbitals, with mixing ($α$) and screening ($w$) parameters. These produced robust electronic features with slight modifications in the band gap and the $4f$ splittings depending on the choice of tuning parameters. We also computed the many-particle electronic excitation energies and compared them with experimental values from photoluminescence.

cond-mat.mtrl-sci↗

arXiv2023

Spin-Dependent Capture Mechanism for Magnetic Field Effects on Interface Recombination Current in Semiconductor Devices

Electrically detected magnetic resonance (EDMR) and near-zero field magnetoresistance (NZFMR) are techniques that probe defect states at dielectric interfaces critical for metal-oxide-semiconductor (MOS) electronic devices such as the Si/SiO$_2$ MOS field effect transistor (MOSFET). A comprehensive theory, adapted from the trap-assisted recombination theory of Shockley, Read, and Hall, is introduced to include the spin-dependent recombination effects that provide the mechanism for magnetic field sensitivity.

cond-mat.mtrl-sci↗

arXiv2023

Spin Hall conductivity in Bi$_{1-x}$Sb$_x$ as an experimental test of bulk-boundary correspondence

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.

cond-mat.mes-hall↗

arXiv2023

Room Temperature Dynamics of an Optically Addressable Single Spin in Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) hosts pure single-photon emitters that have shown evidence of optically detected electronic spin dynamics. However, the electrical and chemical structure of these optically addressable spins is unknown, and the nature of their spin-optical interactions remains mysterious. Here, we use time-domain optical and microwave experiments to characterize a single emitter in h-BN exhibiting room temperature optically detected magnetic resonance. Using dynamical simulations, we constrain and quantify transition rates in the model, and we design optical control protocols that optimize the signal-to-noise ratio for spin readout. This constitutes a necessary step towards quantum control of spin states in h-BN.

cond-mat.mes-hall↗

arXiv2023

Fine structure splitting cancellation in highly asymmetric InAs/InP droplet epitaxy quantum dots

We find the single exciton's fine structure splitting (FSS), which splits its degenerate ground state manifold into singlets, nearly vanishes in highly asymmetric quantum dots due to the cancellation of splitting effects with markedly different origin. The dots simulated are those that emerge on top of etch pits through the droplet epitaxy growth process; these etch pit dots break square ($C_{4v}$) spatial symmetry, which has been previously associated with small FSS. Configuration interaction calculations predict a vanishing FSS at a specific finite etch pit displacement from the center of the dot, for a structure far from square symmetry. We thus predict that highly asymmetric quantum dots may still display negligible fine structure splitting, providing new avenues for high-fidelity generation of indistinguishable, polarization entangled photon pairs on demand.

cond-mat.mes-hall↗

arXiv2023

Magnon-mediated qubit coupling determined via dissipation measurements

Controlled interaction between localized and delocalized solid-state spin systems offers a compelling platform for on-chip quantum information processing with quantum spintronics. Hybrid quantum systems (HQSs) of localized nitrogen-vacancy (NV) centers in diamond and delocalized magnon modes in ferrimagnets-systems with naturally commensurate energies-have recently attracted significant attention, especially for interconnecting isolated spin qubits at length-scales far beyond those set by the dipolar coupling. However, despite extensive theoretical efforts, there is a lack of experimental characterization of the magnon-mediated interaction between NV centers, which is necessary to develop such hybrid quantum architectures. Here, we experimentally determine the magnon-mediated NV-NV coupling from the magnon-induced self-energy of NV centers. Our results are quantitatively consistent with a model in which the NV center is coupled to magnons by dipolar interactions. This work provides a versatile tool to characterize HQSs in the absence of strong coupling, informing future efforts to engineer entangled solid-state systems.

quant-ph↗

arXiv2023

Magnon Diffusion Length and Longitudinal Spin Seebeck Effect in Vanadium Tetracyanoethylene (V[TCNE]$_x$, $x \sim 2$)

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 ($λ_m>1$ $μ$ m) and magnon lifetime for V[TCNE]$_x$ ($τ_{th}\approx 1-10$ $μ$ s) exceeding YIG ($τ_{th}\sim 10$ 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.

cond-mat.mtrl-sci↗

arXiv2023

$\textit{In situ}$ electric-field control of ferromagnetic resonance in the low-loss organic-based ferrimagnet V[TCNE]$_{x\sim 2}$

We demonstrate indirect electric-field control of ferromagnetic resonance (FMR) in devices that integrate the low-loss, molecule-based, room-temperature ferrimagnet vanadium tetracyanoethylene (V[TCNE]$_{x \sim 2}$) mechanically coupled to PMN-PT piezoelectric transducers. Upon straining the V[TCNE]$_x$ films, the FMR frequency is tuned by more than 6 times the resonant linewidth with no change in Gilbert damping for samples with $α= 6.5 \times 10^{-5}$. We show this tuning effect is due to a strain-dependent magnetic anisotropy in the films and find the magnetoelastic coefficient $|λ_S| \sim (1 - 4.4)$ ppm, backed by theoretical predictions from DFT calculations and magnetoelastic theory. Noting the rapidly expanding application space for strain-tuned FMR, we define a new metric for magnetostrictive materials, $\textit{magnetostrictive agility}$, given by the ratio of the magnetoelastic coefficient to the FMR linewidth. This agility allows for a direct comparison between magnetostrictive materials in terms of their comparative efficacy for magnetoelectric applications requiring ultra-low loss magnetic resonance modulated by strain. With this metric, we show V[TCNE]$_x$ is competitive with other magnetostrictive materials including YIG and Terfenol-D. This combination of ultra-narrow linewidth and magnetostriction in a system that can be directly integrated into functional devices without requiring heterogeneous integration in a thin-film geometry promises unprecedented functionality for electric-field tuned microwave devices ranging from low-power, compact filters and circulators to emerging applications in quantum information science and technology.

cond-mat.mtrl-sci↗

arXiv2023

Engineering Spin Coherence in Core-Shell Diamond Nanocrystals

Diamond nanocrystals can harbor spin qubit sensors capable of probing the physical properties of biological systems with nanoscale spatial resolution. These diamond nanosensors can readily be delivered into intact cells and even living organisms. However, applications beyond current proof-of-principle experiments require a substantial increase in sensitivity, which is generally limited by surface-noise-induced spin dephasing and relaxation. In this work, we significantly reduce magnetic surface noise by engineering core-shell structures, which in combination with dynamical decoupling result in qubit coherence times (T2) ranging from 52us to 87us - a drastic improvement over the 1.1us to 35us seen in bare particles. This improvement in spin coherence, combined with an overall increase in particle fluorescence, corresponds to a two-order-of-magnitude reduction in integration time. Probing qubit dynamics at a single particle level, furthermore, reveals that the noise characteristics fundamentally change from a bath with spins that rearrange their spatial configuration during the course of an experiment to a more dilute static bath. The observed results shed light on the underlying mechanisms governing spin dephasing in diamond nanocrystals and offer an effective noise mitigation strategy based on engineered core-shell structures.

quant-ph↗

arXiv2023

Interplay between charge and spin noise in the near-surface theory of decoherence and relaxation of $C_{3v}$ symmetry qutrit spin-1 centers

Decoherence and relaxation of solid-state defect qutrits near a crystal surface, where they are commonly used as quantum sensors, originates from charge and magnetic field noise. A complete theory requires a formalism for decoherence and relaxation that includes all Hamiltonian terms allowed by the defect's point-group symmetry. This formalism, presented here for the $C_{3v}$ symmetry of a spin-1 defect in a diamond, silicon cardide, or similar host, relies on a Lindblad dynamical equation and clarifies the relative contributions of charge and spin noise to relaxation and decoherence, along with their dependence on the defect spin's depth and resonant frequencies. The calculations agree with the experimental measurements of Sangtawesin $\textit{et al.}$, Phys. Rev. X $\textbf{9}$, 031052 (2019) and point to an unexpected importance of charge noise.

quant-ph↗