Spin Detection Research Articles

Spin‐Spacetime Censorship

AbstractQuantum entanglement and relativistic causality are key concepts in theoretical works seeking to unify quantum mechanics and gravity. In this article, a gedanken experiment that couples the spin to spacetime is proposed, and is then analyzed in the context of quantum information by using different approaches to quantum gravity. Both classical gravity theory and certain quantum theories predict that around a spin‐half particle, the spherical symmetry of spacetime is broken by its magnetic field or merely by its intrinsic angular momentum. It is asserted that any spin‐related deviation from spherical symmetry, upon appropriate measurement, can violate relativistic causality and quantum no‐cloning. To avoid these violations, the measurable spacetime around the particle's rest frame shall typically remain spherically symmetric, potentially as a back‐action by the act of a covariant measurement, or due to a quantized spin‐dependence of the magnetic field. This way, this gedanken experiment suggests a censorship mechanism preventing the possibility of spacetime‐based spin detection, which can shed light on the interface between quantum mechanics and gravity. Since this proposed gedanken experiment is independent of any specific theory, it is suitable for testing the coupling of quantum matter and spacetime in present and future candidate theories of quantum gravity.

Large Tunneling Magnetoresistance in van der Waals Ferromagnet/Semiconductor Heterojunctions.

2D layered chalcogenide semiconductors have been proposed as a promising class of materials for low-dimensional electronic, optoelectronic, and spintronic devices. Here, all-2D van der Waals vertical spin-valve devices, that combine the 2D layered semiconductor InSe as a spacer with the 2D layered ferromagnetic metal Fe3 GeTe2 as spin injection and detection electrodes, are reported. Two distinct transport behaviors are observed: tunneling and metallic, which are assigned to the formation of a pinhole-free tunnel barrier at the Fe3 GeTe2 /InSe interface and pinholes in the InSe spacer layer, respectively. For the tunneling device, a large magnetoresistance (MR) of 41% is obtained under an applied bias current of 0.1 µA at 10 K, which is about three times larger than that of the metallic device. Moreover, the tunneling device exhibits a lower operating bias current but a more sensitive bias current dependence than the metallic device. The MR and spin polarization of both the metallic and tunneling devices decrease with increasing temperature, which can be fitted well by Bloch's law. These findings reveal the critical role of pinholes in the MR of all-2D van der Waals ferromagnet/semiconductor heterojunction devices.

Detection of single 13C spins coupled to NV center via dynamical decoupling design

The adjusted Carr–Purcell–Meiboom–Gill (CPMG) sequence is mathematically simulated to obtain the 13C nuclear spin signals coupled to a negatively-charged nitrogen-vacancy (NV−) center in diamond. The pulse interval ratio and scanning frequency of the sequence were precisely adjusted to achieve the high-resolution 13C nuclear spin detection. The frequency resolution of the sequence was 5.8 times improved than that of the original CPMG sequence, and the full width at half maximum of the coherence dips were less than 1.7 kHz. The adjusted method can be extended to other dynamical decoupling sequence, and offers a promising approach to improve identification of the nuclear spins in coupled electron-nuclear spin systems.

Control of spin–charge conversion in van der Waals heterostructures

The interconversion between spin and charge degrees of freedom offers incredible potential for spintronic devices, opening routes for spin injection, detection, and manipulation alternative to the use of ferromagnets. The understanding and control of such interconversion mechanisms, which rely on spin–orbit coupling, is therefore an exciting prospect. The emergence of van der Waals materials possessing large spin–orbit coupling (such as transition metal dichalcogenides or topological insulators) and/or recently discovered van der Waals layered ferromagnets further extends the possibility of spin-to-charge interconversion to ultrathin spintronic devices. Additionally, they offer abundant room for progress in discovering and analyzing novel spin–charge interconversion phenomena. Modifying the properties of van der Waals materials through proximity effects is an added degree of tunability also under exploration. This Perspective discusses the recent advances toward spin-to-charge interconversion in van der Waals materials. It highlights scientific developments which include techniques for large-scale growth, device physics, and theoretical aspects.

Spin Pumping of an Easy-Plane Antiferromagnet Enhanced by Dzyaloshinskii-Moriya Interaction.

Recently, antiferromagnets have received revived interest due to their significant potential for developing next-generation ultrafast magnetic storage. Here, we report dc spin pumping by the acoustic resonant mode in a canted easy-plane antiferromagnet α-Fe_{2}O_{3} enabled by the Dzyaloshinskii-Moriya interaction. Systematic angle and frequency-dependent measurements demonstrate that the observed spin-pumping signals arise from resonance-induced spin injection and inverse spin Hall effect in α-Fe_{2}O_{3}-metal heterostructures, mimicking the behavior of spin pumping in conventional ferromagnet-nonmagnet systems. The pure spin current nature is further corroborated by reversal of the polarity of spin-pumping signals when the spin detector is switched from platinum to tungsten which has an opposite sign of the spin Hall angle. Our results reveal the intriguing physics underlying the low-frequency spin dynamics and transport in canted easy-plane antiferromagnet-based heterostructures.

Phase-sensitive quantum spectroscopy with high-frequency resolution

Classical sensors for spectrum analysis are widely used but lack micro- or nanoscale spatial resolution. On the other hand, quantum sensors, capable of working with nanoscale precision, do not provide precise frequency resolution over a wide range of frequencies. Using a single spin in diamond, we present a measurement protocol for quantum probes which enables full signal reconstruction on a nanoscale spatial resolution up to potentially 100\,GHz. We achieve $58\,\mathrm{nT/\sqrt{Hz}}$ amplitude and $0.095\,\mathrm{rad/\sqrt{Hz}}$ phase sensitivity and a relative frequency uncertainty of $10^{-12}$ for a $1.51\,\mathrm{GHz}$ signal within $10\,\mathrm{s}$ of integration. This technique opens the way to quantum spectrum analysis methods with potential applications in electron spin detection and nanocircuitry in quantum technologies.

Robust and fast post-processing of single-shot spin qubit detection events with a neural network

Establishing low-error and fast detection methods for qubit readout is crucial for efficient quantum error correction. Here, we test neural networks to classify a collection of single-shot spin detection events, which are the readout signal of our qubit measurements. This readout signal contains a stochastic peak, for which a Bayesian inference filter including Gaussian noise is theoretically optimal. Hence, we benchmark our neural networks trained by various strategies versus this latter algorithm. Training of the network with 106 experimentally recorded single-shot readout traces does not improve the post-processing performance. A network trained by synthetically generated measurement traces performs similar in terms of the detection error and the post-processing speed compared to the Bayesian inference filter. This neural network turns out to be more robust to fluctuations in the signal offset, length and delay as well as in the signal-to-noise ratio. Notably, we find an increase of 7% in the visibility of the Rabi oscillation when we employ a network trained by synthetic readout traces combined with measured signal noise of our setup. Our contribution thus represents an example of the beneficial role which software and hardware implementation of neural networks may play in scalable spin qubit processor architectures.

Magnetically modified in-situ N-doped Enteromorpha prolifera derived biochar for peroxydisulfate activation: Electron transfer induced singlet oxygen non-radical pathway

Herein, in-situ N-doped Enteromorpha prolifera derived magnetic biochar (MBC) was prepared by loading Fe3O4. It can effectively activate peroxodisulfate (PDS) to degrade tetracycline (TC) and easy recycling. The removal rate of TC reached 87.2%, and its possible degradation pathway was revealed through a liquid chromatography-mass spectrometer. This work first proposes the mechanism of in-situ N-doping and Fe synergistic effect on PDS activation. Unlike the well-reported role of N doping in activating PDS, except for the edge pyridine N plays a significant role in the activation of PDS. After the load of Fe, the synergistic effect of Fe and graphite N induces a non-radical path dominated by singlet oxygen (1O2) due to the excellent electron transfer function. Through chemical quenching experiment, electron spin detection, and electrochemical analysis, the mechanism of PDS activation by MBC was thoroughly investigate. This research will deepen the understanding of the mechanism of transition metals and carbon materials in synergistically driving PDS activation, and guide biochar-mediated PDS activation in environmental remediation.

Proposed rapid detection of nuclear spins with entanglement-enhanced sensors

Recently, there have been significant developments to detect nuclear spins with nitrogen vacancy (NV) centers in diamond. However, due to the nature of the short range dipole–dipole interaction, it takes a long time to detect distant nuclear spins with the NV centers. Here, we propose a rapid detection of nuclear spins with an entanglement between the NV centers. We show that the necessary time to detect the nuclear spins with the entanglement is several orders of magnitude shorter than that with separable NV centers. Our results pave the way for new applications in nanoscale nuclear magnetic resonance spectroscopy.

Spatial spin flipping and spin switching phenomena on a Y-shaped graphene nanoribbon ferromagnetic junction with Rashba spin orbit coupling and strain

We theoretically study a controllable spatial spin converter and spin valve by using a Y-shaped even-chain zigzag graphene nanoribbon (ZGR) junction with Rashba spin orbit coupling (SOC) and strain. By calculating the spin-dependent conductance spectra of different tunneling terminals through the multi-terminal Landauer formula, we find that the spin up electron can flip its spin orientation, and convert to spin down one on the left electrode, the same spin converting phenomena can be inhibited on the right terminal. The above spin flipping and its valve phenomena are derived from the interactions of the Rashba SOC, strain and magnetization layout. When the strain’s orientation is along (perpendicular) to the zigzag chain’s direction, the valley valve effect of the even-chain ZGR is remained, the Rashba SOC takes little effect on the spatial spin switching, one can get 100% polarized spin up (down) electrons at different terminals of the Y-shaped ZGR due to the valley valve effect. When the strain is changed to other direction, the valley valve effect is partly destroyed, Rashba SOC can enhance the spin flipping conductance and break the ON/OFF transport states of spin valve. Our investigations might be useful in designing a multi-parameter controllable spin valve and spin detector based on a multi-terminal graphene nanoribbon junction.

Theory for electrical detection of the magnon Hall effect induced by dipolar interactions

We derive the anomalous Hall contributions arising from dipolar interactions to diffusive spin transport in magnetic insulators. Magnons, the carriers of angular momentum in these systems, are shown to have a non-zero Berry curvature, resulting in a measurable Hall effect. For yttrium iron garnet (YIG) thin films we calculate both the anomalous and magnon spin conductivities. We show that for a magnetic field perpendicular to the film the anomalous Hall conductivity is finite. This results in a non-zero Hall signal, which can be measured experimentally using Permalloy strips arranged like a Hall bar on top of the YIG thin film. We show that electrical detection and injection of spin is possible, by solving the resulting diffusion-relaxation equation for a Hall bar. We predict the experimentally measurable Hall coefficient for a range of temperatures and magnetic field strengths. Most strikingly, we show that there is a sign change of the Hall coefficient associated with increasing the thickness of the film.

Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs

Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids.

Engineering the spin conversion in graphene monolayer epitaxial structures

Spin Hall and Rashba–Edelstein effects, which are spin-to-charge conversion phenomena due to spin–orbit coupling (SOC), are attracting increasing interest as pathways to manage rapidly and at low consumption cost the storage and processing of a large amount of data in spintronic devices as well as more efficient energy harvesting by spin-caloritronics devices. Materials with large SOC, such as heavy metals (HMs), are traditionally employed to get large spin-to-charge conversion. More recently, the use of graphene (gr) in proximity with large SOC layers has been proposed as an efficient and tunable spin transport channel. Here, we explore the role of a graphene monolayer between Co and a HM and its interfacial spin transport properties by means of thermo-spin measurements. The gr/HM (Pt and Ta) stacks have been prepared on epitaxial Ir(111)/Co(111) structures grown on sapphire crystals, in which the spin detector (i.e., top HM) and the spin injector (i.e., Co) are all grown in situ under controlled conditions and present clean and sharp interfaces. We find that a gr monolayer retains the spin current injected into the HM from the bottom Co layer. This has been observed by detecting a net reduction in the sum of the spin Seebeck and interfacial contributions due to the presence of gr and independent from the spin Hall angle sign of the HM used.

Theory of the electric field controlled antiferromagnetic spin Hall oscillator and detector

A theory of an electrically controlled terahertz-frequency auto-oscillator is developed here within the framework of the antiferromagnetic sigma model. The structure is based on a trilayer heterostructure: a piezoelectric ceramic, NiO-based antiferromagnet (AFM), and a heavy metal. The authors demonstrate that the frequency of the AFM resonance of the layered structure in a passive subcritical regime can be controlled by the external electric field applied to the piezoelectric, by choosing a proper piezoelectric material, and by the magnitude of the electric current flowing in the heavy metal.

Room-temperature two-terminal magnetoresistance ratio reaching 0.1% in semiconductor-based lateral devices with L21-ordered Co2MnSi

We report on the highest two-terminal magnetoresistance (MR) ratio at room temperature in semiconductor-based lateral spin-valve devices. From first-principles calculations, we predict energetically stable ferromagnet–semiconductor heterointerfaces consisting of Co2MnSi (CMS) and Ge(111) upon insertion of Fe atomic layers. Using low-temperature molecular beam epitaxy, we demonstrate L21-ordered CMS epilayers at 80 °C on Ge(111), where the CMS layer can be utilized as a spin injector and detector. Two-terminal MR ratios as high as 0.1% are achieved in n-Ge-based lateral spin-valve devices with CMS/Fe/Ge Schottky tunnel contacts annealed at 200 °C. This study will open a path for semiconductor-based spintronic devices with a large MR ratio at room temperature.

Probing hole spin transport of disorder quantum dots via Pauli spin-blockade in standard silicon transistors

Single hole transport and spin detection is achievable in standard p-type silicon transistors owing to the strong orbital quantization of disorder based quantum dots. Through the use of the well acting as a pseudo-gate, we discover the formation of a double-quantum dot system exhibiting Pauli spin-blockade and investigate the magnetic field dependence of the leakage current. This enables attributes that are key to hole spin state control to be determined, where we calculate a tunnel coupling t c of 57 μeV and a short spin−orbit length l SO of 250 nm. The demonstrated strong spin−orbit interaction at the interface when using disorder based quantum dots supports electric-field mediated control. These results provide further motivation that a readily scalable platform such as industry standard silicon technology can be used to investigate interactions which are useful for quantum information processing.

A new imaging concept in spin polarimetry based on the spin-filter effect.

The concept of an imaging-type 3D spin detector, based on the combination of spin-exchange interactions in the ferromagnetic (FM) film and spin selectivity of the electron-photon conversion effect in a semiconductor heterostructure, is proposed and demonstrated on a model system. This novel multichannel concept is based on the idea of direct transfer of a 2D spin-polarized electron distribution to image cathodoluminescence (CL). The detector is a hybrid structure consisting of a thin magnetic layer deposited on a semiconductor structure allowing measurement of the spatial and polarization-dependent CL intensity from injected spin-polarized free electrons. The idea is to use spin-dependent electron transmission through in-plane magnetized FM film for in-plane spin detection by measuring the CL intensity from recombined electrons transmitted in the semiconductor. For the incoming electrons with out-of-plane spin polarization, the intensity of circularly polarized CL light can be detected from recombined polarized electrons with holes in the semiconductor. In order to demonstrate the ability of the solid-state spin detector in the image-type mode operation, a spin detector prototype was developed, which consists of a compact proximity focused vacuum tube with a spin-polarized electron source [p-GaAs(Cs,O)], a negative electron affinity (NEA) photocathode and the target [semiconductor heterostructure with quantum wells also with NEA]. The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range 0.5-3.0 eV and up to 1.3 keV was studied in image-type mode. The figure of merit as a function of electron kinetic energy and the target temperature is determined. The spin asymmetry of the CL intensity in a ferromagnetic/semiconductor (FM-SC) junction provides a compact optical method for measuring spin polarization of free-electron beams in image-type mode. The FM-SC detector has the potential for realizing multichannel 3D vectorial reconstruction of spin polarization in momentum microscope and angle-resolved photoelectron spectroscopy systems.

Phase-Adaptive Dynamical Decoupling Methods for Robust Spin-Spin Dynamics in Trapped Ions

Quantum platforms based on trapped ions are main candidates to build a quantum hardware with computational capacities that largely surpass those of classical devices. Among the available control techniques in these setups, pulsed dynamical decoupling (pulsed DD) revealed as a useful method to process the information encoded in ion registers, whilst minimising the environmental noise over them. In this work, we incorporate a pulsed DD technique that uses random pulse phases, or correlated pulse phases, to significantly enhance the robustness of entangling spin-spin dynamics in trapped ions. This procedure was originally conceived in the context of nuclear magnetic resonance for nuclear spin detection purposes, and here we demonstrate that the same principles apply for robust quantum information processing in trapped-ion settings.

Polarization amplification by spin-doping in nanomagnetic/graphene hybrid systems

The generation of nonequilibrium electron spin polarization, spin transport, and spin detection are fundamental in many quantum devices. We demonstrate that a lattice of magnetic nanodots enhances the electron spin polarization in monolayer graphene via carrier exchange. We probed the spin polarization through a resistively detected variant of electron spin resonance and observed resonance amplification mediated by the presence of the nanodots. Each nanodot locally injects a surplus of spin-polarized carriers into the graphene, and the ensemble of all spin hot spots generates a nonequilibrium electron spin polarization in the graphene layer at macroscopic lengths. This occurs whenever the interdot distance is comparable or smaller than the spin diffusion length.

Deep learning enhanced individual nuclear-spin detection

The detection of nuclear spins using individual electron spins has enabled diverse opportunities in quantum sensing and quantum information processing. Proof-of-principle experiments have demonstrated atomic-scale imaging of nuclear-spin samples and controlled multi-qubit registers. However, to image more complex samples and to realize larger-scale quantum processors, computerized methods that efficiently and automatically characterize spin systems are required. Here, we realize a deep learning model for automatic identification of nuclear spins using the electron spin of single nitrogen-vacancy (NV) centers in diamond as a sensor. Based on neural network algorithms, we develop noise recovery procedures and training sequences for highly non-linear spectra. We apply these methods to experimentally demonstrate the fast identification of 31 nuclear spins around a single NV center and accurately determine the hyperfine parameters. Our methods can be extended to larger spin systems and are applicable to a wide range of electron-nuclear interaction strengths. These results pave the way towards efficient imaging of complex spin samples and automatic characterization of large spin-qubit registers.