Well-defined Spin Research Articles

Towards atom-scale spin-selective electron emitters based on strong-field Freeman resonant ionization

To produce electrons with well-defined spin via photoionization, realizing orbital-resolved ionization is the prerequisite due to spin-orbital coupling. In this paper, we numerically and experimentally demonstrate that the electrons emitted from two original energy degenerated orbitals ($j=3/2, m=1$ and $j=3/2, m=\ensuremath{-}1$) of Xe atoms can be well separated in the energy domain through selectively achieving Freeman resonance during strong-field ionization using a circularly polarized laser field. This orbital resolved Freeman resonant ionization leads to strong spin sensitivity without the need of fine laser frequency and intensity tuning. This result enables the production of high-degree spin-polarized electrons by energy gating, which provides a unique degree of freedom for advancing many promising applications.

Artificial Double-Helix for Geometrical Control of Magnetic Chirality.

Chirality plays a major role in nature, from particle physics to DNA, and its control is much sought-after due to the scientific and technological opportunities it unlocks. For magnetic materials, chiral interactions between spins promote the formation of sophisticated swirling magnetic states such as skyrmions, with rich topological properties and great potential for future technologies. Currently, chiral magnetism requires either a restricted group of natural materials or synthetic thin-film systems that exploit interfacial effects. Here, using state-of-the-art nanofabrication and magnetic X-ray microscopy, we demonstrate the imprinting of complex chiral spin states via three-dimensional geometric effects at the nanoscale. By balancing dipolar and exchange interactions in an artificial ferromagnetic double-helix nanostructure, we create magnetic domains and domain walls with a well-defined spin chirality, determined solely by the chiral geometry. We further demonstrate the ability to create confined 3D spin textures and topological defects by locally interfacing geometries of opposite chirality. The ability to create chiral spin textures via 3D nanopatterning alone enables exquisite control over the properties and location of complex topological magnetic states, of great importance for the development of future metamaterials and devices in which chirality provides enhanced functionality.

Supramolecular Porous Organic Nanocomposites for Heterogeneous Photocatalysis of a Sulfur Mustard Simulant.

Efficient heterogeneous photosensitizing materials require both large accessible surface areas and excitons of suitable energies and with well-defined spin structures. Confinement of the tetracationic cyclophane (ExBox4+ ) within a nonporous anionic polystyrene sulfonate (PSS) matrix leads to a surface area increase of up to 225 m2 g-1 in ExBox•PSS. Efficient intersystem crossing is achieved by combining the spin-orbit coupling associated to Br heavy atoms in 1,3,5,8-tetrabromopyrene (TBP), and the photoinduced electron transfer in a TBP⊂ExBox4+ supramolecular dyad. The TBP⊂ExBox4+ complex displays a charge transfer band at 450 nm and an exciplex emission at 520 nm, indicating the formation of new mixed-electronic states. The lowest triplet state (T1 , 1.89 eV) is localized on the TBP and is close in energy with the charge separated state (CT, 2.14 eV). The homogeneous and heterogeneous photocatalytic activities of the TBP⊂ExBox4+ , for the elimination of a sulfur mustard simulant, has proved to be significantly more efficient than TBP and ExBox+4 , confirming the importance of the newly formed excited-state manifold in TBP⊂ExBox4+ for the population of the low-lying T1 state. The high stability, facile preparation, and high performance of the TBP⊂ExBox•PSS nanocomposites augur well for the future development of new supramolecular heterogeneous photosensitizers using host-guest chemistry.

Separation of the valley exciton-polariton in two-dimensional semiconductors with an anisotropic photonic crystal

Excitons in two-dimensional (2D) transition metal dichalcogenides (TMDC) are stable at room temperature because of high exciton binding energies. They can be selectively addressed based on the unique optical selection rules from angular momentum conservation for the $K/{K}^{\ensuremath{'}}$ valley state and the helicity of circularly polarized light. When coupled with the optical modes in optical cavities, excitons can form exciton-polaritons, exploiting which in 2D TMDC may lead to optoelectronic devices for room temperature operation. The valley degree of freedom of the excitons, however, is mostly lost when forming exciton-polaritons because the cavity mode usually does not have a well-defined spin angular momentum. Here, we theoretically demonstrate that the valley information of exciton-polaritons can be preserved and resolved in a photonic cavity made of birefringent materials. Because of the optical anisotropy, the guided resonance modes have a net transverse spin angular momentum and selectively couple to exciton-polaritons with the corresponding valley state. In the strong-coupling regime, the exciton-polariton behaves in a way like the Rashba effect in the solid. The dispersion of the $K/{K}^{\ensuremath{'}}$ exciton-polariton splits in momentum space based on its valley state, similar to electron spins in Rashba systems. Realizing valley-dependent exciton-polaritons affords a possibility to explore valley exciton dynamics in a strongly coupled system and will contribute to the study of excitonic, polaritonic devices, Bose-Einstein condensation, and superfluidity in semiconductors.

Three-Electrode Device for Applying Two-Dimensional Vector Electric Fields to Single InAs Quantum Dots

InAs/GaAs quantum dots (QDs) and quantum dot molecules (QDMs) are self-assembled semiconductor nanostructures that can trap a single electron or hole with well-defined spin projections. QDs and QDMs have excellent optical properties and have long been of interest for incorporation into quantum optoelectronic devices ranging from single photon sources to multi-bit quantum computers. The properties of single QDs, or carriers confined within those QDs, can be tuned by external electric fields, which provides an important tool for the development of scalable and tunable devices. Deterministic charging of a QD with a single electron or hole is an important tool for quantum devices and is well-established under the application of growth-direction electric fields in a diode structure. Here, we report a new charging mechanism for a single QD in a 3-electrode device that does not contain a vertical diode and can be used to control electric field profiles in two dimensions. We fabricate the device with E-beam lithography and characterize photoluminescence from single QDs under different bias configurations. Using a combination of experimental data and COMSOL band structure calculations, we explain how the charging originates in the electrical-injection of holes induced by lateral electric fields. We discuss the applications for this device and the potential for full 2-D electric field control of a single QD and QDM.

Black holes: The next generation—repeated mergers in dense star clusters and their gravitational-wave properties

When two black holes merge in a dense star cluster, they form a new black hole with a well-defined mass and spin. If that "second-generation" black hole remains in the cluster, it will continue to participate in dynamical encounters, form binaries, and potentially merge again. Using a grid of 96 dynamical models of dense star clusters and a cosmological model of cluster formation, we explore the production of binary black hole mergers where at least one component of the binary was forged in a previous merger. We create four hypothetical universes where every black hole born in the collapse of a massive star has a dimensionless Kerr spin parameter, $\chi_{\rm birth}$, of 0.0, 0.1, 0.2, or 0.5. We show that if all stellar-born black holes are non-spinning ($\chi_{\rm birth} = 0.0$), then more than 10% of merging binary black holes from clusters have components formed from previous mergers, accounting for more than 20% of the mergers from globular clusters detectable by LIGO/Virgo. Furthermore, nearly 7% of detectable mergers would have a component with a mass $\gtrsim 55M_{\odot}$, placing them clearly in the mass "gap" region where black holes cannot form from isolated collapsing stars due to the pulsational-pair instability mechanism. On the other hand, if black holes are born spinning, then the contribution from these second-generation mergers decreases, making up as little as 1% of all detections from globular clusters when $\chi_{\rm birth} = 0.5$. We make quantitative predictions for the detected masses, mass ratios, and spin properties of first- and second-generation mergers from dense star clusters, and show how these distributions are highly sensitive to the birth spins of black holes.

Inner and outer ring states of MoS2 quantum rings: Energy spectrum, charge and spin currents

We investigate the energy levels and persistent currents of MoS2 quantum rings having different shapes and edge types in the presence of a perpendicular magnetic field by means of the tight-binding approach. We find states localized at the inner and outer boundaries of the ring. These energy levels exhibit different magnetic field dependences for the inner and outer ring states due to their different localization properties. They both exhibit the usual Aharanov–Bohm oscillations but with different oscillation periods. In the presence of spin–orbit coupling, we show distinct spin and charge persistent currents for inner and outer ring states. We find well-defined spin currents with negligibly small charge currents. This is because the local currents of spin-up and -down states flow in opposite directions.

Transverse optical pumping of spin states

Optical pumping is an efficient method for initializing and maintaining atomic spin ensembles in a well-defined quantum spin state. Standard optical pumping methods orient the spins by transferring photonic angular momentum to spin polarization. Generally the spins are oriented along the propagation direction of the light due to selection rules of the dipole interaction. Here we present and experimentally demonstrate that by modulating the light polarization, angular momentum perpendicular to the optical axis can be transferred efficiently to cesium vapor. The transverse pumping scheme employs transversely oriented dark states, allowing for control of the trajectory of the spins on the Bloch sphere. This new mechanism is suitable and potentially beneficial for diverse applications, particularly in quantum metrology.

Multiplet-to-net CIDEP conversion by MW-pulses with adiabatically ramped amplitude

ABSTRACTA method is proposed to manipulate electron spin order of spin-correlated radical pairs. As radical pairs are often born in a well-defined spin state, e.g. in the singlet state, they acquire Chemically Induced Dynamic Electron Polarisation (CIDEP). In the case of singlet-state preparation CIDEP is of the multiplet (or anti-phase) type resulting in reduction of EPR (electron paramagnetic resonance) signals due to overlap of absorptive and emissive lines in the spectrum. Here we propose to convert the singlet spin order into net magnetisation of the radical pair by applying a microwave field, with its amplitude slowly (adiabatically) reduced to zero. We demonstrate that by properly choosing the microwave frequency one can completely convert the singlet order into net polarisation of the radical pair with significant enhancement of the signal as compared to multiplet CIDEP. Calculations show that the technique is operative for both weakly coupled and strongly coupled spin pairs. Potential applications of the method are discussed.

Effective nonlinear model for electron transport in deformable helical molecules

The helical conformation of electric dipoles in some chiral molecules, such as DNA and bacteriorhodopsin, induces a spin-orbit coupling that results in a sizable spin selectivity of electrons. The local deformation of the molecule about the moving electron may affect the spin dynamics due to the appearance of bright solitons with well-defined spin projection onto the molecule axis. In this work, we introduce an effective model for electron transport in a deformable helical molecular lattice that resembles the nonlinear Kronig-Penney model in the adiabatic approximation. In addition, the continuum limit of our model is achieved when the dipole-dipole distance is smaller than the spatial extent of the bright soliton, as discussed by E. D\'{\i}az et al. [N. J. Phys. 20, 043055 (2018)]. In this limit, our model reduces to an extended Davydov model. Finally, we also focus on perturbations to the bright soliton that arise naturally in the context of real helical molecules. We conclude that the continuum approximation provides excellent results in more complex scenarios.

Extended exchange interactions stabilize long-period magnetic structures in Cr1∕3NbS2

The topologically protected, chiral soliton lattice is a unique state of matter offering intriguing functionality, and it may serve as a robust platform for storing and transporting information in future spintronic devices. While the monoaxial chiral magnet Cr1∕3NbS2 is known to host this exotic state in an applied magnetic field, its detailed microscopic origin has remained a matter of debate. Here, we work towards addressing this open question by measuring the spin wave spectrum of Cr1∕3NbS2 over the entire Brillouin zone with inelastic neutron scattering. The well-defined spin wave modes allow us to determine the values of several microscopic interactions for this system. The experimental data are well-explained by a Heisenberg Hamiltonian with exchange constants up to the third nearest neighbor and an easy plane magnetocrystalline anisotropy term. Our work shows that both the second and third nearest neighbor exchange interactions contribute to the formation of the helimagnetic and chiral soliton lattice states in this robust three-dimensional magnet.

Doping evolution of spin fluctuations and their peculiar suppression at low temperatures in Ca(Fe1−xCox)2As2

Results of inelastic neutron scattering measurements are reported for two annealed compositions of ${\mathrm{Ca}(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2},x=0.026$ and 0.030, which possess stripe-type antiferromagnetically ordered and superconducting ground states, respectively. In the AFM ground state, well-defined and gapped spin waves are observed for $x=0.026$, similar to the parent ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ compound. We conclude that the well-defined spin waves are likely to be present for all $x$ corresponding to the AFM state. This behavior is in contrast to the smooth evolution to overdamped spin dynamics observed in ${\mathrm{Ba}(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$, wherein the crossover corresponds to microscopically coexisting AFM order and SC at low temperature. The smooth evolution is likely absent in ${\mathrm{Ca}(\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$ due to the mutual exclusion of AFM ordered and SC states. Overdamped spin dynamics characterize paramagnetism of the $x=0.030$ sample and high-temperature $x=0.026$ sample. A sizable loss of magnetic intensity is observed over a wide energy range upon cooling the $x=0.030$ sample, at temperatures just above and within the superconducting phase. This phenomenon is unique amongst the iron-based superconductors and is consistent with a temperature-dependent reduction in the fluctuating moment. One possible scenario ascribes this loss of moment to a sensitivity to the $c$-axis lattice parameter in proximity to the nonmagnetic collapsed tetragonal phase and another scenario ascribes the loss to a formation of a pseudogap.

Electrical Initialization of Electron and Nuclear Spins in a Single Quantum Dot at Zero Magnetic Field.

The emission of circularly polarized light from a single quantum dot relies on the injection of carriers with well-defined spin polarization. Here we demonstrate single dot electroluminescence (EL) with a circular polarization degree up to 35% at zero applied magnetic field. The injection of spin-polarized electrons is achieved by combining ultrathin CoFeB electrodes on top of a spin-LED device with p-type InGaAs quantum dots in the active region. We measure an Overhauser shift of several microelectronvolts at zero magnetic field for the positively charged exciton (trion X+) EL emission, which changes sign as we reverse the injected electron spin orientation. This is a signature of dynamic polarization of the nuclear spins in the quantum dot induced by the hyperfine interaction with the electrically injected electron spin. This study paves the way for electrical control of nuclear spin polarization in a single quantum dot without any external magnetic field.

Modification of Transition-Metal Redox by Interstitial Water in Hexacyanometalate Electrodes for Sodium-Ion Batteries.

A sodium-ion battery (SIB) solution is attractive for grid-scale electrical energy storage. Low-cost hexacyanometalate is a promising electrode material for SIBs because of its easy synthesis and open framework. Most hexacyanometalate-based SIBs work with aqueous electrolyte, and interstitial water in the material has been found to strongly affect the electrochemical profile, but the mechanism remains elusive. Here we provide a comparative study of the transition-metal redox in hexacyanometalate electrodes with and without interstitial water based on soft X-ray absorption spectroscopy and theoretical calculations. We found distinct transition-metal redox sequences in hydrated and anhydrated NaxMnFe(CN)6·zH2O. The Fe and Mn redox in hydrated electrodes are separated and are at different potentials, leading to two voltage plateaus. On the contrary, mixed Fe and Mn redox in the same potential range is found in the anhydrated system. This work reveals for the first time how transition-metal redox in batteries is strongly affected by interstitial molecules that are seemingly spectators. The results suggest a fundamental mechanism based on three competing factors that determine the transition-metal redox potentials. Because most hexacyanometalate electrodes contain water, this work directly reveals the mechanism of how interstitial molecules could define the electrochemical profile, especially for electrodes based on transition-metal redox with well-defined spin states.

Zero Quantum Coherence in a Series of Covalent Spin-Correlated Radical Pairs

Photoinitiated subnanosecond electron transfer within covalently linked electron donor-acceptor molecules can result in the formation of a spin-correlated radical pair (SCRP) with a well-defined initial singlet spin configuration. Subsequent coherent mixing between the SCRP singlet and triplet ms = 0 spin states, the so-called zero quantum coherence (ZQC), is of potential interest in quantum information processing applications because the ZQC can be probed using pulse electron paramagnetic resonance (pulse-EPR) techniques. Here, pulse-EPR spectroscopy is utilized to examine the ZQC oscillation frequencies and ZQC dephasing in three structurally well-defined D-A systems. While transitions between the singlet and triplet ms = 0 spin states are formally forbidden (Δms = 0), they can be addressed using specific microwave pulse turning angles to map information from the ZQC onto observable single quantum coherences. In addition, by using structural variations to tune the singlet-triplet energy gap, the ZQC frequencies determined for this series of molecules indicate a stronger dependence on the electronic g-factor than on electron-nuclear hyperfine interactions.

Higher-Spin Theory of the Magnetorotons.

Fractional quantum Hall liquids exhibit a rich set of excitations, the lowest energy of which are the magnetorotons with dispersion minima at a finite momentum. We propose a theory of the magnetorotons on the quantum Hall plateaux near half filling, namely, at filling fractions ν=N/(2N+1) at large N. The theory involves an infinite number of bosonic fields arising from bosonizing the fluctuations of the shape of the composite Fermi surface. At zero momentum there are O(N) neutral excitations, each carrying a well-defined spin that runs integer values 2,3,…. The mixing of modes at nonzero momentum q leads to the characteristic bending down of the lowest excitation and the appearance of the magnetoroton minima. A purely algebraic argument shows that the magnetoroton minima are located at qℓ_{B}=z_{i}/(2N+1), where ℓ_{B} is the magnetic length and z_{i} are the zeros of the Bessel function J_{1}, independent of the microscopic details. We argue that these minima are universal features of any two-dimensional Fermi surface coupled to a gauge field in a small background magnetic field.

High-Fidelity and Ultrafast Initialization of a Hole Spin Bound to a Te Isoelectronic Center in ZnSe.

We demonstrate the optical initialization of a hole-spin qubit bound to an isoelectronic center (IC) formed by a pair of Te impurities in ZnSe, an impurity-host system providing high optical homogeneity, large electric dipole moments, and potentially advantageous coherence times. The initialization scheme is based on the spin-preserving tunneling of a resonantly excited donor-bound exciton to a positively charged Te IC, thus forming a positive trion. The radiative decay of the trion within less than 50ps leaves a heavy hole in a well-defined polarization-controlled spin state. The initialization fidelity exceeds 98.5% for an initialization time of less than 150ps.

Direct observation of the skyrmion Hall effect

The well-known Hall effect describes the transverse deflection of charged particles (electrons or holes) in an electric-current carrying conductor under the influence of perpendicular magnetic fields, as a result of the Lorentz force. Similarly, it is intriguing to examine if quasi-particles without an electric charge, but with a topological charge, show related transverse motion. Chiral magnetic skyrmions with a well-defined spin topology resulting in a unit topological charge serve as good candidates to test this hypothesis. In spite of the recent progress made on investigating magnetic skyrmions, direct observation of the skyrmion Hall effect in real space has, remained elusive. Here, by using a current-induced spin Hall spin torque, we experimentally observe the skyrmion Hall effect by driving skyrmions from creep motion into the steady flow motion regime. We observe a Hall angle for the magnetic skyrmion motion as large as 15 degree for current densities smaller than 10 MA/cm^(2) at room temperature. The experimental observation of transverse transport of skyrmions due to topological charge may potentially create many exciting opportunities for the emerging field of skyrmionics, including novel applications such as topological selection.

Irreversible magnetic-field dependence of ferromagnetic resonance and inverse spin Hall effect voltage in CoFeB/Pt bilayer

Magnetic field (H) sweeping direction dependences of the mixed voltage Vmix induced by the inverse-spin Hall effect(ISHE) and spin-rectified effect (SRE) in a CoFeB (5nm)/Pt (10nm) bilayer structure are investigated using the ferromagnetic resonance in the TE mode cavities and coplanar waveguide methods. Conventionally, the magnitude of ISHE voltage VISH (symmetric) excluding the SRE (antisymmetric component) was unavoidably separated from the fitting curve of Vmix (a sum of a symmetric and an antisymmetric part) for one direction of H-source. By studying the ratio of the two voltage parts with the bi-directional H sweeping, the optimized VISH (no SRE condition) value which also include a well-defined spin Hall angle can be obtained via the linear response relation of ISHE and SRE components.

2'-Alkynylnucleotides: A Sequence- and Spin Label-Flexible Strategy for EPR Spectroscopy in DNA.

Electron paramagnetic resonance (EPR) spectroscopy is a powerful method to elucidate molecular structure through the measurement of distances between conformationally well-defined spin labels. Here we report a sequence-flexible approach to the synthesis of double spin-labeled DNA duplexes, where 2'-alkynylnucleosides are incorporated at terminal and internal positions on complementary strands. Post-DNA synthesis copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions with a variety of spin labels enable the use of double electron-electron resonance experiments to measure a number of distances on the duplex, affording a high level of detailed structural information.