arXiv subjects

Michael E. Flatte

Publications and source records attributed to Michael E. Flatte.

At least 19 recordsLinked to original sources

arXiv2026

Divergence between long- and short-wavelength magnon damping in spinel ferrites

The realization of practical, high-speed magnonic devices requires engineering magnetic materials with low dissipation over wide frequency ranges. While optical and microwave probes are used to infer the damping of low energy/long wavelength modes, the degree to which these $q\sim0$ properties translate into higher-energy, finite-momentum modes remains an important open question. Here, we utilize a combination of ferromagnetic resonance (FMR) and resonant inelastic x-ray scattering on spinel ferrites Li$_{0.5}$Al$_x$Fe$_{2.5-x}$O$_4$ to probe magnons in both the short- and long-wavelength limits. We observe that aluminum substitution both markedly reduces the magnon bandwidth and drastically shortens the high-$q$ magnon lifetimes, in sharp contrast to the ultralow magnon damping inferred from FMR. These findings demonstrate a disparity between how non-magnetic substituents impact magnon damping in the long- and short-wavelength limits, providing a new perspective for assessing candidate materials for magnonic devices.

cond-mat.mtrl-sci↗

arXiv2026

Broken site symmetry of Fe adatoms on Bi$_2$Te$_3$

We report a combined scanning tunneling microscopy and atomistic theoretical study of Fe adatoms on the Bi$_2$Te$_3$(111) surface. Topographic imaging at $4.5$~K shows that 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$_2$Te$_3$ 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 consistent with experimental $dI/dV$ maps, confirming that the reduced symmetry originates from the $C_{1v}$ structural distortion.

cond-mat.mes-hall↗

arXiv2026

Surface adsorbates suppress low-frequency noise for shallow nitrogen-vacancy centers

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.

quant-ph↗

arXiv2026

A defect in diamond with millisecond-scale spin relaxation time at room temperature

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) $μ$s, 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.

cond-mat.mtrl-sci↗

arXiv2020

Extraction of Isotropic Electron-Nuclear Hyperfine Coupling Constants of Paramagnetic Point Defects from Near-Zero Field Magnetoresistance Spectra via Least Squares Fitting to Models Developed from the Stochastic Quantum Liouville Equation

We report on a method by which we can systematically extract spectroscopic information such as isotropic electron-nuclear hyperfine coupling constants from near-zero field magnetoresistance spectra. The method utilizes a least squares fitting of models developed from the stochastic quantum Liouville equation. We applied our fitting algorithm to two distinct material systems: Si/SiO2 MOSFETs, and a-Si:H MIS capacitors. Our fitted results and hyperfine parameters are in reasonable agreement with existing knowledge of the defects present in the systems. Our work indicates that the NZFMR response and fitting of the NZFMR spectrum via models developed from the stochastic quantum Liouville equation could be a relatively simple yet powerful addition to the family of spin-based techniques used to explore the chemical and structural nature of point defects in semiconductor devices and insulators.

cond-mat.mtrl-sci↗

arXiv2020

Tunable tunnel barriers in a semiconductor via ionization of individual atoms

We report scanning tunneling microscopy studies of individual adatoms deposited on an InSb(110) surface. The adatoms can be reproducibly dropped off from the STM tip by voltage pulses, and impact tunneling into the surface by up to ~100x. The spatial extent and magnitude of the tunneling effect are widely tunable by imaging conditions such as bias voltage, set current and photoillumination. We attribute the effect to occupation of a (+/0) charge transition level, and switching of the associated adatom-induced band bending. The effect in STM topographic images is well reproduced by transport modeling of filling and emptying rates as a function of the tip position. STM atomic contrast and tunneling spectra are in good agreement with density functional theory calculations for In adatoms. The adatom ionization effect can extend to distances greater than 50 nm away, which we attribute to the low concentration and low binding energy of the residual donors in the undoped InSb crystal. These studies demonstrate how individual atoms can be used to sensitively control current flow in nanoscale devices.

cond-mat.mes-hall↗

arXiv2019

Fermi Level Dependent Spin Pumping from a Magnetic Insulator into a Topological Insulator

Topological spintronics aims to exploit the spin-momentum locking in the helical surface states of topological insulators for spin-orbit torque devices. We address a fundamental question that still remains unresolved in this context: does the topological surface state alone produce the largest values of spin-charge conversion efficiency or can the strongly spin-orbit coupled bulk states also contribute significantly? By studying the Fermi level dependence of spin pumping in topological insulator/ferrimagnetic insulator bilayers, we show that the spin Hall conductivity is constant when the Fermi level is tuned across the bulk band gap, consistent with a full bulk band calculation. The results suggest a new perspective, wherein "bulk-surface correspondence" allows spin-charge conversion to be simultaneously viewed either as coming from the full bulk band, or from spin-momentum locking of the surface state.

cond-mat.mtrl-sci↗

arXiv2014

Long-lived spin plasmons in a spin-polarized two-dimensional electron gas

Collective charge-density modes (plasmons) of the clean two-dimensional unpolarized electron gas are stable, for momentum conservation prevents them from decaying into single-particle excitations. Collective spin-density modes (spin plasmons) possess no similar protection and rapidly decay by production of electron-hole pairs. Nevertheless, if the electron gas has a sufficiently high degree of spin polarization ($P>1/7$, where $P$ is the ratio of the equilibrium spin density and the total electron density, for a parabolic single-particle spectrum) we find that a long-lived spin-plasmon---a collective mode in which the densities of up and down spins oscillate with opposite phases---can exist within a "pseudo gap" of the single-particle excitation spectrum. The ensuing collectivization of the spin excitation spectrum is quite remarkable and should be directly visible in Raman scattering experiments. The predicted mode could dramatically improve the efficiency of coupling between spin-wave-generating devices, such as spin-torque oscillators.

cond-mat.mes-hall↗

arXiv2014

Synthesis of Luminescent Eu defects in diamond

Lanthanides are vital components in lighting, imaging technologies and future quantum memory applications due to their narrow optical transitions and long spin coherence times. Recently, diamond has become a preeminent platform for realization of many experiments in quantum information science. In this work, we demonstrate a promising approach to incorporate Eu ions into single crystal diamond and nanodiamonds, providing a means to harness the exceptional characteristics of both lanthanides and diamond in a single material. Polyelectrolytes are used to electrostatically assemble Eu(III) chelate molecules on diamond and subsequently chemical vapor deposition is employed for the growth of a high quality diamond crystal. Photoluminescence, cathodoluminescence and time resolved fluorescence measurements show that the Eu atoms retain the characteristic optical signature of Eu(III) upon incorporation into the diamond lattice. Computational modelling supports the experimental findings, corroborating that Eu3+ in diamond is a stable configuration within the diamond bandgap. The versatility of the synthetic technique is further illustrated through the creation of the well-studied Cr defect center. Together these defect centers demonstrate the outstanding chemical control over the incorporation of impurities into diamond enabled by the electrostatic assembly together with chemical vapour deposition growth.

cond-mat.mtrl-sci↗

arXiv2008

Magnetic circular dichroism from the impurity band in III-V diluted magnetic semiconductors

The magnetic circular dichroism of III-V diluted magnetic semiconductors, calculated within a theoretical framework suitable for highly disordered materials, is shown to be dominated by optical transitions between the bulk bands and an impurity band formed from magnetic dopant states. The theoretical framework incorporates real-space Green's functions to properly incorporate spatial correlations in the disordered conduction band and valence band electronic structure, and includes extended and localized electronic states on an equal basis. Our findings reconcile unusual trends in the experimental magnetic circular dichroism in III-V DMSs with the antiferromagnetic p-d exchange interaction between a magnetic dopant spin and its host.

cond-mat.mtrl-sci↗

arXiv2006

All-electrical control of single ion spins in a semiconductor

We propose a method for all-electrical initialization, control and readout of the spin of single ions substituted into a semiconductor. Mn ions in GaAs form a natural example. In the ion's ground state the Mn core spin magnetic moment locks antiparallel to the spin and orbital magnetic moment of a bound valence hole from the GaAs host. Direct electrical manipulation of the ion spin is possible because electric fields manipulate the orbital wave function of the hole, and through the spin-orbit coupling the spin is reoriented as well. Coupling two or more ion spins can be achieved using electrical gates to control the size of the valence hole wave function near the semiconductor surface. This proposal for coherent manipulation of individual ionic spins and controlled coupling of ionic spins via electrical gates alone may find applications in extremely high density information storage and in scalable coherent or quantum information processing.

quant-ph↗

arXiv2006

Atom-by-Atom Substitution of Mn in GaAs and Visualization of their Hole-Mediated Interactions

The discovery of ferromagnetism in Mn doped GaAs [1] has ignited interest in the development of semiconductor technologies based on electron spin and has led to several proof-of-concept spintronic devices [2-4]. A major hurdle for realistic applications of (Ga,Mn)As, or other dilute magnetic semiconductors, remains their below room-temperature ferromagnetic transition temperature. Enhancing ferromagnetism in semiconductors requires understanding the mechanisms for interaction between magnetic dopants, such as Mn, and identifying the circumstances in which ferromagnetic interactions are maximized [5]. Here we report the use of a novel atom-by-atom substitution technique with the scanning tunnelling microscope (STM) to perform the first controlled atomic scale study of the interactions between isolated Mn acceptors mediated by the electronic states of GaAs. High-resolution STM measurements are used to visualize the GaAs electronic states that participate in the Mn-Mn interaction and to quantify the interaction strengths as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga1-xMnxAs structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples. Our experimental methods also provide a realistic approach to create precise arrangements of single spins as coupled quantum bits for memory or information processing purposes.

cond-mat.mtrl-sci↗

arXiv2004

Dynamical nuclear polarization and nuclear magnetic fields in semiconductor nanostructures

We investigate the dynamic nuclear polarization from the hyperfine interaction between nonequilibrium electronic spins and nuclear spins coupled to them in semiconductor nanostructures. We derive the time and position dependence of the induced nuclear spin polarization and dipolar magnetic fields. In GaAs/AlGaAs parabolic quantum wells the nuclear spin polarization can be as high as 80% and the induced nuclear magnetic fields can approach a few gauss with an associated nuclear resonance shift of the order of kHz when the electronic system is 100% spin polarized. These fields and shifts can be tuned using small electric fields. We discuss the implications of such control for optical nuclear magnetic resonance experiments in low-dimensional semiconductor nanostructures.

cond-mat.mtrl-sci↗

arXiv2004

Teleportation of electronic many-qubit states via single photons

We propose a teleportation scheme that relies only on single-photon measurements and Faraday rotation, for teleportation of many-qubit entangled states stored in the electron spins of a quantum dot system. The interaction between a photon and the two electron spins, via Faraday rotation in microcavities, establishes Greenberger-Horne-Zeilinger entanglement in the spin-photon-spin system. The appropriate single-qubit measurements, and the communication of two classical bits, produce teleportation. This scheme provides the essential link between spintronic and photonic quantum information devices by permitting quantum information to be exchanged between them.

cond-mat.mes-hall↗

arXiv2004

Nuclear Spin Dynamics in Parabolic Quantum Wells

We present a detailed analytical and numerical analysis of the nuclear spin dynamics in parabolic quantum wells. The shallow potential of parabolic quantum wells permits substantial modification of the electronic wave function in small electric fields. The nuclear spin relaxation via the hyperfine interaction depends on the electronic local density of states, therefore the local nuclear relaxation time depends sensitively on the electric field. For an inhomogeneous nuclear magnetization, such as generated by dynamic nuclear polarization, the total nuclear magnetization dynamics can similarly be altered. We examine this effect quantitatively and the effect of temperature, field, well thickness, and nuclear spin diffusion.

cond-mat.mtrl-sci↗

arXiv2003

Measuring the entanglement of coupled spins by multiphoton interference

We propose an experimental method to measure the entanglement of coupled spins, each in a separate quantum dot, by means of multiphoton interference patterns generated through the scattering of two laser beams off the quantum dots. We calculate the N-photon quantum correlations measured by N detectors on an image plane. Using two perpendicular laser beams, either many correlation measurements on a time ensemble or a single correlation measurement on a spatial ensemble of the many-qubit state is sufficient to retrieve all the possible amplitudes of a many-qubit state.

cond-mat.mes-hall↗

arXiv2003

Non-magnetic semiconductor spin transistor

We propose a spin transistor using only non-magnetic materials that exploits the characteristics of bulk inversion asymmetry (BIA) in (110) symmetric quantum wells. We show that extremely large spin splittings due to BIA are possible in (110) InAs/GaSb/AlSb heterostructures, which together with the enhanced spin decay times in (110) quantum wells demonstrates the potential for exploitation of BIA effects in semiconductor spintronics devices. Spin injection and detection is achieved using spin-dependent resonant interband tunneling and spin transistor action is realized through control of the electron spin lifetime in an InAs lateral transport channel using an applied electric field (Rashba effect). This device may also be used as a spin valve, or a magnetic field sensor. The electronic structure and spin relaxation times for the spin transistor proposed here are calculated using a nonperturbative 14-band k.p nanostructure model.

cond-mat.mtrl-sci↗

arXiv2003

Electric field tunability of nuclear and electronic spin dynamics due to the hyperfine interaction in semiconductor nanostructures

We present formulas for the nuclear and electronic spin relaxation times due to the hyperfine interaction for nanostructed systems and show that the times depend on the square of the local density of electronic states at the nuclear position. A drastic sensitivity (orders of magnitude) of the electronic and nuclear spin coherence times to small electric fields is predicted for both uniformly distributed nuclear spins and for $\d$-doped layers of specific nuclei. This sensitivity is robust to nuclear spin diffusion.

cond-mat↗