Helical Spin Research Articles

Scattering Amplitudes and BCFW in $\mathcal{N}=2^*$ Theory

We use massive spinor helicity formalism to study scattering amplitudes in \mathcal{N}=2^*𝒩=2* super-Yang-Mills theory in four dimensions. We compute the amplitudes at an arbitrary point in the Coulomb branch of this theory. We compute amplitudes using projection from \mathcal{N}=4𝒩=4 theory and write three point amplitudes in a convenient form using special kinematics. We then compute four point amplitudes by carrying out massive BCFW shift of the amplitudes. We find some of the shifted amplitudes have a pole at z=\inftyz=∞. Taking the residue at z=\inftyz=∞ into account ensures little group covariance of the final result.

Long-lived phantom helix states in Heisenberg quantum magnets

Exact solutions for quantum many-body systems are rare but provide valuable insights for the description of universal phenomena such as the non-equilibrium dynamics of strongly interacting systems and the characterization of new forms of quantum matter. Recently, specific solutions of the Bethe ansatz equations for integrable spin models were found. They are dubbed phantom Bethe states and can carry macroscopic momentum yet no energy. Here, we show experimentally that there exist special helical spin patterns in anisotropic Heisenberg chains which are long-lived, relaxing only very slowly in dynamics, as a consequence of such states. We use these phantom spin-helix states to directly measure the interaction anisotropy, which has a major contribution from short-range off-site interactions. We also generalize the theoretical description to higher dimensions and other non-integrable systems and find analogous stable spin helices, which should show non-thermalizing dynamics associated with so-called quantum many-body scars. These results have implications for the quantum simulation of spin physics, as well as many-body dynamics.

First-principles study of the crystal and magnetic structures of multiferroic Cu2OCl2

Recently, the discovery of multiferroicity in pyrochlore-like compound Cu2OCl2 has generated significant interest, and several studies have been performed in this area. This transition metal oxychloride is unique because the divalent copper atoms create an correlated insulator and the pyrochlore lattice tends to frustrate spins. From neutron powder diffraction measurements, an incommensurate magnetic order of the ordering vector emerges below the Néel temperature of 70 K. At this temperature or slightly above, ferroelectricity (FE) or antiferroelectricity, accompanying a lattice distortion, has been observed. Experimentally, some discrepancies remain. In this paper, we report our first-principles simulation results by evaluating the possible lattice and spin spiral states. We found that the Fddd structure is not more stable than , which is supported by our reexamination of the x-ray diffraction data. In addition, we find that after we include magnetism in the calculation, it predicts that the lattice with a helical (proper screw) spin structure is energetically more stable than other spin configurations. Our results indicate charge-order-driven FE that subsequently induces magnetism.

Quantum Spin Hall Phases in Graphene and the Atomically Thin Transition Metal Dichalcogenide Family

AbstractThe quantum‐spin‐Hall (QSH) phase with helical edge spin states, in which opposite‐moment electron spins counter‐propagate within a topologically preserved time reversal symmetry, is attracting considerable attention. The helical edge spin currents allow zero‐emission energy spintronic‐devices with small error rates by modulating with external bias voltages. Recently, QSH phases and possible 2D topological insulating states have been reported in various atomically thin layers (such as graphene and the transition metal dichalcogenide family). Particularly, (twisted) bi‐layer graphene on heavy substrates is becoming a novel candidate for robust QSH phases (including Chern insulating phases), nevertheless, graphene itself lacks spin‐orbit coupling owing to the light carbon atoms. Here, research of QSH phases focusing on graphene is reviewed, particularly for using heavy adatom decoration and heavy substrates. They will open the door to next‐generation spintronic architectures.

Breakdown of Spin-to-Helicity Locking at the Nanoscale in Topological Photonic Crystal Edge States.

We measure the local near-field spin in topological edge state waveguides that emulate the quantum spin Hall effect. We reveal a highly structured spin density distribution that is not linked to a unique pseudospin value. From experimental near-field real-space maps and numerical calculations, we confirm that this local structure is essential in understanding the properties of optical edge states and light-matter interactions. The global spin is reduced by a factor of 30 in the near field and, for certain frequencies, flipped compared to the pseudospin measured in the far field. We experimentally reveal the influence of higher-order Bloch harmonics in spin inhomogeneity, leading to a breakdown in the coupling between local helicity and global spin.

Switching of the information backflow between a helical spin system and non-Markovian bath

The dissipative dynamics of the spin chain coupled to the non-Markovian magnonic reservoir was studied. The chirality of the chain is formed due to the magnetoelectric coupling. We explored the sign of the trace distance derivative and found the alternating positive/negative periods in system’s time evolution. The negative sign is associated with the flow of information from the system to the bath and decrease in states distinguishability, while the positive sign is related to the flow of the information in the opposite direction and increase in distinguishability. We found the distinct effect of the applied electric and magnetic fields. While the Dzyaloshinskii–Moriya interaction and external electric field lead to reshuffling of the periods, the applied magnetic field leads to the swift positive–negative transitions. Thus, in the helical quantum rings coupled to the non-Markovian magnonic baths, it is possible to control the directions of information flow through the external fields.

Magnetic moments and spin structure in single-phase B20 Co1+xSi1−x (x = 0.043)

Neutron powder diffraction (NPD) and x-ray magnetic circular dichroism (XMCD) spectroscopy are employed to investigate the magnetism and spin structure in single-phase B20 Co1.043Si0.957. The magnetic contributions to the NPD data measured in zero fields are consistent with the helical order among the allowed spin structures derived from group theory. The magnitude of the magnetic moment is (0.3 ± 0.1)μB/Co according to NPD, while the surface magnetization probed by XMCD at 3 kOe is (0.18–0.31)μB/Co. Both values are substantially larger than the bulk magnetization of 0.11μB/Co determined from magnetometry at 70 kOe and 2 K. These experimental data indicate the formation of a helical spin phase and the associated conical states in high magnetic fields.

Fermi surface chirality induced in a TaSe2 monosheet formed by a Ta/Bi2Se3 interface reaction.

Spin-momentum locking in topological insulators and materials with Rashba-type interactions is an extremely attractive feature for novel spintronic devices and is therefore under intense investigation. Significant efforts are underway to identify new material systems with spin-momentum locking, but also to create heterostructures with new spintronic functionalities. In the present study we address both subjects and investigate a van der Waals-type heterostructure consisting of the topological insulator Bi2Se3 and a single Se-Ta-Se triple-layer (TL) of H-type TaSe2 grown by a method which exploits an interface reaction between the adsorbed metal and selenium. We then show, using surface x-ray diffraction, that the symmetry of the TaSe2-like TL is reduced from D3h to C3v resulting from a vertical atomic shift of the tantalum atom. Spin- and momentum-resolved photoemission indicates that, owing to the symmetry lowering, the states at the Fermi surface acquire an in-plane spin component forming a surface contour with a helical Rashba-like spin texture, which is coupled to the Dirac cone of the substrate. Our approach provides a route to realize chiral two-dimensional electron systems via interface engineering in van der Waals epitaxy that do not exist in the corresponding bulk materials.

Real-space observation of non-collinear spin structure in centrosymmetric TbGa rare-earth magnet

The exploration of non-collinear spin texture in the f-electron-based rare-earth magnets has attracted fascinating interest for its fundamental physics and potential spintronic applications. Here, the real-space observation of nanometric helical spin order with the period down to 20 nm in centrosymmetric TbGa magnets was presented by Lorentz transmission electron microscopy. It was found that the helical period and the initial temperature for the appearance of the periodic helix can be tuned by the thickness of single-crystal TbGa plates (71–169 nm). Furthermore, the domain evolution under the perpendicular magnetic fields and with the shape constriction reveals the high stability of the stripe domains.

The hypersimplex canonical forms and the momentum amplituhedron-like logarithmic forms

In this paper we provide a formula for the canonical differential form of the hypersimplex Δ k,n for all n and k. We also study the generalization of the momentum amplituhedron to m = 2, which has been conjectured to share many properties with the hypersimplex, and we provide counterexamples for these conjectures. Nevertheless, we find interesting momentum amplituhedron-like logarithmic differential forms in the m = 2 version of the spinor helicity space, that have the same singularity structure as the hypersimplex canonical forms.

Spatiotemporal spin dynamics of two-dimensional electron gas with ballistic motion in persistent spin helix state

The spatiotemporal spin dynamics of two-dimensional electron spins with ballistic motion in a GaAs quantum well is investigated using time- and spatially resolved magneto-optical Kerr rotation measurements. The ballistic movement of photoexcited spins, which maintains the width of the excitation spot size, results in a wave-packet-like motion of the spin distribution. The time evolution of the spin distribution measured at different excitation intensities reveals that the spin diffusion slows down and the wave-packet-like motion disappears at higher photoexcited spin densities, at which the spin distribution converges to the helical spin mode observed in the diffusive regime. The variation in the spatiotemporal spin distribution can be controlled by spin diffusion, while the spin precession length is determined by the strength of the spin-orbit interaction.

Identification of a state of persistent spin helix in a parallel magnetic field, and exploration of its transport properties

The tensors of conductivity and spin susceptibility of a two-dimensional electron gas (2DEG) with equal Rashba and Dresselhaus spin-orbit interaction constants [the persistent spin helix (PSH) state] in a parallel magnetic field are calculated. Applying a parallel magnetic field to the PSH state leads to the appearance of a saddle point in the spectrum and, accordingly, to a Van Hove singularity (VHS), whose amplitude increases indefinitely as the magnetic field strength $(\mathbf{b})$ approaches to some critical value ${b}_{cr}$. The presence of a VHS in the density of states is an important factor determining the conductivity and spin susceptibility tensors. When only the lower spin subband is filled, the off-diagonal elements of the conductivity tensor are nonzero; that is, a Hall voltage arises due to the anisotropy of the Fermi surface and scattering. In the region where two spin subbands are filled, the diagonal and off-diagonal components of the spin susceptibility tensor are equal, and the off-diagonal terms of the conductivity tensor vanish. A sharp decrease in the conductivity and spin susceptibility at the beginning of the filling of the second spin subband, the ratio of the diagonal component of the spin susceptibility to the off-diagonal one, and also the number of critical points in the spectrum make it possible to establish the PSH 2DEG state.

Linear dichroism and polarization controllable persistent spin helix in two-dimensional ferroelectric ZrOI2 monolayer

Linear dichroism and polarization controllable persistent spin helix in two-dimensional ferroelectric ZrOI2 monolayer

Photoinduced transient states of antiferromagnetic orderings in La1/3Sr2/3FeO3 and SrFeO3−δ thin films observed through time-resolved resonant soft x-ray scattering

The relationship between the magnetic interaction and photoinduced dynamics in antiferromagnetic perovskites is investigated in this study. In La1/3Sr2/3FeO3 thin films, commensurate spin ordering is accompanied by charge disproportionation, whereas SrFeO3−δ thin films show incommensurate helical antiferromagnetic spin ordering due to increased ferromagnetic coupling compared to La1/3Sr2/3FeO3. To understand the photoinduced spin dynamics in these materials, we investigate the spin ordering through time-resolved resonant soft x-ray scattering. In La1/3Sr2/3FeO3, ultrafast quenching of the magnetic ordering within 130 fs through a nonthermal process is observed, triggered by charge transfer between the Fe atoms. We compare this to the photoinduced dynamics of the helical magnetic ordering of SrFeO3−δ . We find that the change in the magnetic coupling through optically induced charge transfer can offer an even more efficient channel for spin-order manipulation.

Field-induced topological Hall effect in antiferromagnetic axion insulator candidate EuIn2As2

Magnetic topological materials have attracted significant attention due to their potential realization of a variety of novel quantum phenomena. ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ has recently been theoretically recognized as a long-awaited intrinsic antiferromagnetic bulk axion insulator. However, experimental studies of the transport properties arising from the topological states in this material are scarce. In this paper, we perform detailed magnetoresistance (MR) and Hall measurements to study the magnetotransport properties of this material. We find that the transport is strongly influenced by the spin configuration of the Eu moments from the concomitant change in the field dependence of the MR and that of the magnetization below the N\'eel temperature. Most importantly, an anomalous Hall effect (AHE) and a large topological Hall effect (THE) are observed. We suggest that the AHE is originated from a nonvanishing net Berry curvature due to the helical spin structure and that the THE is attributed to the formation of a noncoplanar spin texture with a finite scalar spin chirality induced by the external magnetic field in ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$. Our studies provide a platform to understand the influence of the interplay between the topology of electronic bands and the field-induced magnetic structure on magnetoelectric transport properties. In addition, our observations give a hint to realize axion insulator states and higher-order topological insulator states through manipulating the magnetic state of ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$.

Evidence for a single-layer van der Waals multiferroic.

Multiferroic materials have attracted wide interest because of their exceptional static1-3 and dynamical4-6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices1,9,10. Here we report the discovery of type-II multiferroic order in a single atomic layer of the transition-metal-based van der Waals material NiI2. The multiferroic state of NiI2 is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Combining birefringence and second-harmonic-generation measurementswith theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.

Commensurate helicoidal order in the triangular layered magnet Na2MnTeO6

The trigonal layered quaternary tellurate ${\mathrm{Na}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ has been studied by means of various techniques to clarify its magnetic properties. The crystal structure of this compound is based on the triangular arrangement of all cations in the parallel layers with the space group $P\overline{3}1c$. By using symmetry analysis of the magnetic neutron scattering data, we have found that the solution for the magnetic structure corresponds to the magnetic Shubnikov group $R{\overline{3}}^{\ensuremath{'}}{c}^{\ensuremath{'}}$ (No. 167.4.1337). ${\mathrm{Mn}}^{4+}$ ions in an octahedral environment form a triangular network where all spins are directed from the center of each triangle. Overall magnetic structure in ${\mathrm{Na}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ is commensurate $120{}^{\ensuremath{\circ}}$ spin helix with propagation vector $\mathbit{k}=(1/3,1/3,1/3)$ in variance with planar spin structure in structurally equivalent ${\mathrm{Li}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ with magnetic propagation vector $\mathbf{k}=(1/3,1/3,0)$. The magnetization measurements show that ${\mathrm{Na}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ experiences an antiferromagnetic order at ${T}_{\mathrm{N}}=5.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. NMR, electron spin resonance, and thermodynamics experiments demonstrate the extended temperature region of 2D short-range correlations well above the ordering temperature.

Observation of Short‐Period Helical Spin Order and Magnetic Transition in a Nonchiral Centrosymmetric Helimagnet

AbstractThe search for materials exhibiting nanoscale spiral order continues to be fuelled by the promise of emergent inductors. Although such spin textures have been reported in many materials, most of them exhibit long periods or are limited to operate far below room temperature. Here, the real‐space observation of an ordered helical spin order with a period of 3.2 nm in a nonchiral centrosymmetric helimagnet MnCoSi at room temperature via multiangle and multiazimuth approach of Lorentz transmission electron microscopy (TEM) is presented. A magnetic transition from the ordered helical spin order to a cycloidal spin order below 228 K is clearly revealed by in situ neutron powder diffraction and Lorentz TEM, which is closely correlated with temperature‐induced variation in magneto‐crystalline anisotropy. These results reveal the origin of spiral ordered spin textures in nonchiral centrosymmetric helimagnet, which can serve as a new strategy for searching materials with nanoscale spin order with potential applications in emergent electromagnetism.

Magnetization and spin resonances in helical spin systems

In this work, magnetization and magnetization dynamics in helical-ordered spin systems are considered. By employing the Nelder–Mead simplex numerical algorithm, the dependence of spin orientations on magnetic field is calculated for spin systems with arbitrary helicity. As demonstrated, in a certain case, the dependence of magnetization on magnetic field shows a clear dependence on helicity. Next, magnetization dynamics in helical-ordered spin systems with arbitrary helicity is studied by employing the Suhl–Smit–Beljers approach. Two types of resonance responses are examined: the molecular field response and collective spin resonance modes. Both types of responses also demonstrate clear dependence on helicity. Thus, it is demonstrated that the overall combination of magnetization measurements and ferromagnetic resonance studies defines the helical angle.

Realizing Persistent-Spin-Helix Lasing in the Regime of Rashba-Dresselhaus Spin-Orbit Coupling in a Dye-Filled Liquid-Crystal Optical Microcavity

In the presence of Rashba-Dresselhaus coupling, strong spin-orbit interactions in liquid-crystal optical cavities result in a distinctive spin-split entangled dispersion. Spin coherence between such modes gives rise to an optically persistent spin helix. In this paper, we introduce optical gain in such a system, by dispersing a molecular dye in a liquid-crystal microcavity, and demonstrate an optically persistent spin-helix lasing in the Rashba-Dresselhaus regime.