Nonzero Spin Research Articles

Universal intrinsic higher-rank spin tensor Hall effect

The spin Hall effect with nonzero transverse spin current but vanishing charge current has important applications in spintronics. Owing to the half-spin nature of electrons, the spin transport has hitherto been restricted to the rank-1 spin vector, whereas higher-rank spin tensors exist and play important roles in larger-spin ($\ensuremath{\ge}1$) systems. For instance, there are five linearly independent rank-2 spin quadrupole tensors, characterizing the spin nematics, in a spin-1 system. While these internal spin-tensor degrees of freedom substantially enrich the quantum phases and dynamics of large-spin ultracold gases, the quantum transport of spin tensors remains largely unexplored yet. Here we investigate the higher-rank spin tensor current and introduce the concept of the spin tensor Hall effect (STHE) in a large-spin system. We find that a net transverse spin tensor current can be driven by an external field in the longitudinal direction, while no lower-rank spin or charge current exists. Significantly, we identify a universal rank-2 spin tensor Hall conductivity $q/8\ensuremath{\pi}$ (with the carrier charge $q$) in a spin-1 model with intrinsic spin-orbit coupling, which is independent of the detailed model parameters. The STHE requires the violation of time-reversal symmetry (TRS) but can be protected by a unique pseudo-TRS associated with the rank-2 spin tensor. An experimental scheme is proposed to realize and detect the STHE with pseudospin-1 ultracold fermionic atoms and through the spin tensor accumulation. A general route towards the generalization of the STHE for larger spins is provided, using the SU(2) subalgebra and generalized Gell-Mann matrix representation of the $\mathrm{SU}(N$) group. Our work reveals an intrinsic spin tensor transport phenomenon and introduce a new member to the Hall effect families, which may pave the way to spin-tensor-tronics.

Topological Hall effect driven by short-range magnetic order in EuZn2As2

Short-range (SR) magnetic orders such as magnetic glass orders or fluctuations in a quantum system usually host exotic states or critical behaviors. As the long-range (LR) magnetic orders, SR magnetic orders can also break time-reversal symmetry and drive the non-zero Berry curvature leading to novel transport properties. In this work, we report that in EuZn$_2$As$_2$ compound, besides the LR A-type antiferromagnetic (AF) order, the SR magnetic order is observed in a wide temperature region. The magnetization measurements and electron spin resonance (ESR) measurements reveal the ferromagnetic (FM) correlations for this SR magnetic order which results in an obvious anomalous Hall effect above the AF transition. Moreover the ESR results reveal that this FM SR order coexists with LR AF order exhibiting anisotropic magnetic correlations below the AF transition. The interactions of LR and SR magnetism evolving with temperature and field can host non-zero spin charility and berry curvature leading the additional topological Hall contribution even in a centrosymmetric simple AF system. Our results indicate that EuZn$_2$As$_2$ is a fertile platform to investigate exotic magnetic and electronic states.

Hexagonal warping effect in the Janus group-VIA binary monolayers with large Rashba spin splitting and piezoelectricity.

In this paper, the electronic band structure, Rashba effect, hexagonal warping, and piezoelectricity of Janus group-VIA binary monolayers STe2, SeTe2, and Se2Te are investigated based on density functional theory (DFT). Due to the inversion asymmetry and spin-orbit coupling (SOC), the STe2, SeTe2 and Se2Te monolayers exhibit large intrinsic Rashba spin splitting (RSS) at the Γ point with the Rashba parameters 0.19 eV Å, 0.39 eV Å, and 0.34 eV Å, respectively. Interestingly, based on the k·p model via symmetry analysis, the hexagonal warping effect and a nonzero spin projection component Sz arise at a larger constant energy surface due to nonlinear k3 terms. Then, the warping strength λ was obtained by fitting the calculated energy band data. Additionally, in-plane biaxial strain can significantly modulate the band structure and RSS. Furthermore, all these systems exhibit large in-plane and out-of-plane piezoelectricity due to inversion and mirror asymmetry. The calculated piezoelectric coefficients d11 and d31 are about 15-40 pm V-1 and 0.2-0.4 pm V-1, respectively, which are superior to those of most reported Janus monolayers. Because of the large RSS and piezoelectricity, the studied materials have great potential for spintronic and piezoelectric applications.

Multiple surface resonance electronic spin states in the strong topological metal Zr2Te2P

Nonzero spin polarization occurs in nonmagnetic materials induced by spin-orbital coupling when inversion symmetry is broken. Understanding the origin of spin polarization is essential for finding effective methods of manipulating the electron spin. Recently, nontrivial Dirac node arcs with a novel spin texture have been reported in a tetradymite family ${M}_{2}{\text{Te}}_{2}X$ (with $M=\text{Ti}$, Zr, or Hf and $X=\text{P}$ or As) and have potential applications in spintronics. Here, combining spin- and angle-resolved photoemission spectroscopy and first-principles calculations, we have unambiguously distinguished the spin-polarized topological surface states from the bulk continuum states in ${\text{Zr}}_{2}{\text{Te}}_{2}\text{P}$, and report that surface resonance Rashba-type spin splittings and spin-momentum-layer locked bulk states exist along with the Dirac node arcs in reciprocal space in adjacent energy ranges. Our work shows the importance of resonance and hybridization between surface and bulk states to their electronic spin textures and provides a platform for developing spintronics devices.

Probing details of spin-orbit coupling through Pauli spin blockade

Spin-orbit interaction (SOI) plays a fundamental role in many low-dimensional semiconductor and hybrid quantum devices. In the rapidly evolving field of semiconductor spin qubits, SOI is an essential ingredient that can allow for ultrafast qubit control. The exact manifestation of SOI in a given device is, however, often both hard to predict theoretically and probe experimentally. Here, we develop a detailed theoretical connection between the leakage current through a double quantum dot in Pauli spin blockade and the underlying SOI in the system. We present a general analytic expression for the leakage current, which allows to connect experimentally observable features to both the magnitude and orientation of an effective spin-orbit field acting on the moving carriers. Motivated by the large recent interest in hole-based quantum devices, we further zoom in on the case of Pauli blockade of hole spins, assuming a strong transverse confinement potential. In this limit we also find an analytic expression for the current at low external magnetic field, that includes the effect of hyperfine coupling of the hole spins to randomly fluctuating nuclear spin baths. This result can be used to extract information about both hyperfine and spin-orbit coupling parameters for hole spins in devices with a significant fraction of nonzero nuclear spins.

Nickel substitution induced reentrant spin glass behavior in EuGa4

Typically, atomic scale disorder is essential to show glassy behavior and amorphous materials can be the optimum candidates in this regard. Generating local disorder in an ordered material by substitution of another metal is a challenging task. In this paper, we successfully substituted Ni in ${\mathrm{EuGa}}_{4}$ (${\mathrm{EuNi}}_{0.37}{\mathrm{Ga}}_{3.63}$) to induce spin glass (SG) behavior. The compound was characterized by single-crystal x-ray diffraction, dc magnetization, frequency-dependent ac susceptibility, isothermal magnetization at various temperatures, spin relaxation, specific heat, and electric resistivity. Two distinct anomalies were observed in dc susceptibility $\ensuremath{\sim}16.5$ and 5.2 K corresponding to long-range antiferromagnetic (AFM) ordering and SG transition, respectively. Their characteristic peaks in specific heat capacity further support the AFM and SG behavior. The irreversibility temperature is fitted with the Almeida-Thouless equation, confirming the Ising SG. The SG behavior is further confirmed by shift of maxima to a high temperature in ac susceptibility with increasing frequency. Nonzero spin relaxation after 7200 s, fitted with stretched exponential model, provides the value of $\ensuremath{\beta}=0.41$. This material can be identified as a possible reentrant SG as AFM and SG behavior coexist in the low-temperature range. Additionally, a spin-flip transition is observed in the AFM region ($5.2\phantom{\rule{0.28em}{0ex}}\mathrm{K}<T<16.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). Our first-principles calculations show Ni substitution induces the distortion in electronic band structure as compared with parent ${\mathrm{EuGa}}_{4}$. A minimal difference between the different magnetic configurations for Ni-substituted ${\mathrm{EuGa}}_{4}$ suggests the frustration in the lattice, pointing toward SG behavior.

Spin-flux attachment by dimensional reduction of vortices

The description of a system of vortices in terms of dual fields provides a window to new phases of the system. It was found recently that dualizing a 3+1-d boson-fermion system leads to a system of fermions and vortices interacting via a 2-form field through a non-local term. Here we explore some consequences of such an interaction when the degrees of freedom of the system are confined to a 2+1-d space-time. In particular, we show that the vortices in the 2+1-d system are attached to the fermions via their non-zero spin magnetic moment in a way similar to the phenomenon of flux attachment in Chern-Simons gauge theory coupled to matter. We also show that such flux attached particles exhibit fractional statistical behaviour like anyons. Thus our model provides a realization of anyons without Chern-Simons theory.

Photonic topological phases in Tellegen metamaterials.

We investigate the photonic topological phases in Tellegen metamaterials characterized by the antisymmetric magnetoelectric tensors with real-valued quantities. The underlying medium is considered a photonic analogue of the topological semimetal featured with a displaced Weyl cone in the frequency-wave vector space. As the 'spin'-degenerate condition is satisfied, the photonic system consists of two hybrid modes that are completely decoupled. By introducing the pseudospin states as the basis for the hybrid modes, the photonic system is described by two subsystems in terms of the spin-orbit Hamiltonians with spin 1, which result in nonzero spin Chern numbers that determine the topological properties. Surface modes at the interface between two Tellegen metamaterials with opposite sign of the magnetoelectric parameter exist at their common gap in the wave vector space, which are analytically formulated by algebraic equations. In particular, two types of surface modes are tangent to or wrapping around the Weyl cones, which form a pair of bended and a pair of twisted surface sheets. At the Weyl frequency, the surface modes contain a typical and two open Fermi arc-like states that concatenate to yield an infinite straight line. Topological features of the Tellegen metamaterials are further illustrated with the robust transport of surface modes at an irregular boundary.

Cooperative dynamics in two-component out-of-equilibrium systems: molecular ‘spinning tops’

We study the two-dimensional Langevin dynamics of a mixture of two types of particles that live respectively at two different temperatures. Dynamics is constrained by an optical trap and the dissimilar species interact via a quadratic potential. We realize that the system evolves toward a peculiar non-equilibrium steady-state with a non-zero probability current possessing a non-zero curl. This implies that if the particles were to have a finite-size and therefore a rotational degree of freedom, they would experience a torque generated by the non-zero local curl and spin around their geometric centers, like ‘spinning top’ toys. Our analysis shows that the spinning motion is correlated and also reveals an emerging cooperative behavior of the spatial components of the probability currents of dissimilar species.

Deep absorption in SDSS J110511.15+530806.5

Aims. We study the origin of the anomalous deep absorption in a spectrum of the SDSS J110511.15+530806.5 distant quasar (z = 1.929) obtained by the Sloan Digital Sky Survey in Data Release 14 of the optical catalog. We aim to estimate the velocity of absorbing material, and we show that this material considerably affects our measurements of the black hole (BH) mass in massive quasars with the use of common virial mass estimators. Methods. The spectral shape of the quasar was modeled assuming that the accretion disk emission is influenced by a hot corona, warm skin, and absorbing material located close to the nucleus. The whole analysis was undertaken with XSPEC models and tools. The overall spectral shape was represented with the AGNSED model, while the deep absorption is well described by two Gaussians. Results. The observed spectrum and the fitting procedure allowed us to estimate the BH mass in the quasar as 3.52 ± 0.01 × 109 M⊙, the nonzero BH spin is a* = 0.32 ± 0.04, and the accretion rate is ṁ = 0.274 ± 0.001. The velocities of the detected absorbers lie in the range of 6330–108 135 km s−1. When we consider that absorption is caused by the C IV ion, one absorber is folding toward the nucleus with a velocity of 73 887 km s−1. We derived a BI index of about 20 300 km s−1 and a mass outflow rate up to 38.5% of the source accretion rate. Conclusions. The high absorption observed in SDSS J110511.15+530806.5 is evidence of fast winds that place the source in the group of objects on the border with UFO (ultra-fast outflows), strong broad absorption line, and fast failed radiatively accelerated dusty outflow (FRADO). This absorption affects the BH mass measurement by two orders of magnitude as compared to virial mass estimation.

ANALYSIS OF DPPH AND MnCl₂ USING ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY

EPR spectroscopy, in a constant-frequency field-swept experiment, was used to determine the resonant magnetic field, g-factor, line width, and hyperfine constant for diphenyl picrylhydrazyl at room temperature and manganese (II) chloride at cryogenic temperatures (using liquid nitrogen). Diphenyl picrylhydrazyl was determined to be a free radical, indicated by a g-factor of 2.022, with a resonant magnetic field of 3.23 kG at a microwave frequency of 9.14 GHz. The manganese (II) chloride was found to have six signal peaks, indicating hyperfine interaction of a nucleus with non-zero spin with unpaired electrons (spin was determined to be 5/2).

Elucidating Λ-doublet splittings and rotational quantum number-dependent collisional broadenings in 2∏1/2 and 2∏3/2 spin-split sub-bands of NO at 5.2 µm

High-resolution ro-vibrational spectroscopic features of nitric oxide (NO) are particularly interesting because of the unpaired electron and non-zero nuclear spin of the nitrogen atom. Here, ultra-sensitive cavity ring-down spectroscopy (CRDS) coupled with an external-cavity quantum cascade laser (EC-QCL) radiation source was employed to measure the rotationally-resolved fine structure Λ-doublet splittings between the parity doublet e and f components in the (2∏1/2 - 2∏1/2) and (2∏3/2 - 2∏3/2) allowed sub-bands of the v = 1 ← 0 fundamental vibrational band of NO molecule near 5.2 µm. Subsequently, we determined several principal spectroscopic parameters such as the Λ-doubling constants, vibrational transition dipole moments and the Herman-Wallis coefficients for both e and f Λ-doublet components by probing various R-branch rotational lines (J = 0.5 to 23.5) of the studied spin-split sub-bands of NO associated with spin–orbit interaction. In addition, we performed the pressure broadening effect on the Λ-doublet splittings in collision with three vital perturbing gases at room temperature (296 K) and accurately determined the pressure broadening coefficients, γi (e,f) in cm-1atm−1 [(i = He, Ar and Zero air (mainly N2 + O2)] along with their dependence on the rotational quantum number (J). We observed the pronounced collision-induced rotational quantum effect and the result of rotationally inelastic collision in the system for each collision partner. All these measured high-resolution new spectroscopic parameters over 33 ro-vibrational transitions via an EC-QCL based CRDS method will help significantly in interpreting fundamental molecular properties of this diatomic NO molecule.

Toward the determination of the gluon helicity distribution in the nucleon from lattice quantum chromodynamics

We present the first exploratory lattice quantum chromodynamics (QCD) calculation of the polarized gluon Ioffe-time pseudo-distribution in the nucleon. The Ioffe-time pseudo-distribution provides a frame-independent and gauge-invariant framework to determine the gluon helicity in the nucleon from first principles. We employ a high-statistics computation using a $32^3\times 64$ lattice ensemble characterized by a $358$ MeV pion mass and a $0.094$ fm lattice spacing. We establish the pseudo-distribution approach as a feasible method to address the proton spin puzzle with successive improvements in statistical and systematic uncertainties anticipated in the future. Within the statistical precision of our data, we find a good comparison between the lattice determined polarized gluon Ioffe-time distribution and the corresponding expectations from the state-of-the-art global analyses. We find a hint for a nonzero gluon spin contribution to the proton spin from the model-independent extraction of the gluon helicity pseudo-distribution over a range of Ioffe-time, $\nu\lesssim 9$.

Magnetic field-induced nontrivial spin chirality and large topological Hall effect in kagome magnet ScMn6Sn6

RMn6Sn6 (R = rare earth element) kagome magnets have attracted much attention owing to their potential for realizing the emerging topological properties in both reciprocal and real spaces. One of the RMn6Sn6 members, ScMn6Sn6, is predicted to possess room temperature-stabilized chiral spin textures arising from frustrated exchange interactions, but further experimental evidence has not been well established yet. In this work, we fabricate high-quality ScMn6Sn6 single crystals and systematically study their magnetoelectric transport properties. A large topological Hall effect is observed within the temperature range from 100 to 320 K with the magnetic field applied along the parallel direction of the kagome plane. This observation suggests that the spin textures in ScMn6Sn6 have a nonzero scalar spin chirality over a wide temperature range. Our results identify ScMn6Sn6 as a promising member of the rare earth element kagome magnets that hosts chiral spin textures at room temperature.

Intrinsic anomalous spin Hall effect

Charge-spin interconversion in magnetic materials is investigated by using the first-principles calculations. In addition to the conventional spin Hall effect (SHE) that requires mutual orthogonality of the charge current, spin-flow direction, and spin polarization, the recently proposed anomalous SHE (ASHE) is confirmed in Mn2Au and WTe2. The interaction of the order parameter with conduction electrons leads to sizeable non-zero spin Berry curvatures that give rise to anomalous spin Hall conductivity (ASHC). Our calculations show that the ASHE is intrinsic and originates from the order-parameter-controlled spin-orbit interaction, which generates an extra anomalous effective field. A useful relationship among the order parameter, the spin Berry curvature, and the ASHC is revealed. Our findings provide a new avenue for generating and detecting arbitrary types of spin currents.

Photonic Weyl semimetals in pseudochiral metamaterials

We investigate the photonic topological phases in pseudochiral metamaterials characterized by the magnetoelectric tensors with symmetric off-diagonal chirality components. The underlying medium is considered a photonic analogue of the type-II Weyl semimetal featured with two pairs of tilted Weyl cones in the frequency-wave vector space. As the ’spin’-degenerate condition is satisfied, the photonic system consists of two hybrid modes that are completely decoupled. By introducing the pseudospin states as the basis for the hybrid modes, the photonic system is described by two subsystems in terms of the spin-orbit Hamiltonians with spin 1, which result in nonzero spin Chern numbers that determine the topological properties. Surface modes at the interface between vacuum and the pseudochiral metamaterial exist in their common gap in the wave vector space, which are analytically formulated by algebraic equations. In particular, the surface modes are tangent to both the vacuum light cone and the Weyl cones, which form two pairs of crossing surface sheets that are symmetric about the transverse axes. At the Weyl frequency, the surface modes that connect the Weyl points form four Fermi arc-like states as line segments. Topological features of the pseudochiral metamaterials are further illustrated with the robust transport of surface modes at an irregular boundary.

Chirality-Induced Spin Selectivity in Heterochiral Short-Peptide-Carbon-Nanotube Hybrid Networks: Role of Supramolecular Chirality.

Supramolecular short-peptide assemblies have been widely used for the development of biomaterials with potential biomedical applications. These peptides can self-assemble in a multitude of chiral hierarchical structures triggered by the application of different stimuli, such as changes in temperature, pH, solvent, etc. The self-assembly process is sensitive to the chemical composition of the peptides, being affected by specific amino acid sequence, type, and chirality. The resulting supramolecular chirality of these materials has been explored to modulate protein and cell interactions. Recently, significant attention has been focused on the development of chiral materials with potential spintronic applications, as it has been shown that transport of charge carriers through a chiral environment polarizes the carrier spins. This effect, named chirality-induced spin selectivity or CISS, has been studied in different chiral organic molecules and materials, as well as carbon nanotubes functionalized with chiral molecules. Nevertheless, this effect has been primarily explored in homochiral systems in which the chirality of the medium, and hence the resulting spin polarization, is defined by the chirality of the molecule, with limited options for tunability. Herein, we have developed heterochiral carbon-nanotube-short-peptide materials made by the combination of two different chiral sources: that is, homochiral peptides (l/d) + glucono-δ-lactone. We show that the presence of a small amount of glucono-δ-lactone with fixed chirality can alter the supramolecular chirality of the medium, thereby modulating the sign of the spin signal from "up" to "down" and vice versa. In addition, small amounts of glucono-δ-lactone can even induce nonzero spin polarization in an otherwise achiral and spin-inactive peptide-nanotube composite. Such "chiral doping" strategies could allow the development of complementary CISS-based spintronic devices and circuits on a single material platform.

Room-Temperature Quantitative Quantum Sensing of Lithium Ions with a Radical-Embedded Metal-Organic Framework.

Recent advancements in quantum sensing have sparked transformative detection technologies with high sensitivity, precision, and spatial resolution. Owing to their atomic-level tunability, molecular qubits and ensembles thereof are promising candidates for sensing chemical analytes. Here, we show quantum sensing of lithium ions in solution at room temperature with an ensemble of organic radicals integrated in a microporous metal-organic framework (MOF). The organic radicals exhibit electron spin coherence and microwave addressability at room temperature, thus behaving as qubits. The high surface area of the MOF promotes accessibility of the guest analytes to the organic qubits, enabling unambiguous identification of lithium ions and quantitative measurement of their concentration through relaxometric and hyperfine spectroscopic methods based on electron paramagnetic resonance (EPR) spectroscopy. The sensing principle presented in this work is applicable to other metal ions with nonzero nuclear spin.

Spin-dependent polaron transport in helical molecules

We study thermal effects on spin transport along a deformable helical molecule in the presence of chiral-induced spin–orbit coupling. The carrier–lattice interaction is modeled by the well-established Peyrard–Bishop–Holstein model within the Langevin approach to include temperature as a stochastic noise. The carrier–lattice interaction causes the occurrence of polaron states in the molecule. We demonstrate the existence of two well-differentiated spin-dependent polaron transport regimes as a function of temperature. In the low-temperature regime, the spatial separation of the two spin-dependent polaron wave-packets results in a nonzero spin current. On the contrary, the spin current becomes negligible if the temperature of the system is high enough. Finally, we characterize this transition and estimate the critical temperature at which it takes place.

Altermagnetism and magnetic groups with pseudoscalar electron spin

We revise existing group-theoretical approaches for a treatment of nonrelativistic collinear magnetic systems with perfect translation invariance. We show that full symmetry groups of these systems, which contain elements with independent rotations in the spin and configuration spaces (spin groups), can be replaced by magnetic groups consisting of elements with rotations acting only on position vectors. This reduction follows from modified transformation properties of electron spin, which in the considered systems becomes effectively a pseudoscalar quantity remaining unchanged upon spatial operations but changing its sign due to an operation of antisymmetry. We introduce a unitary representation of the relevant magnetic point groups and use it for a classification of collinear magnets from the viewpoint of antiferromagnetism-induced spin splitting of electron bands near the center of Brillouin zone. We prove that the recently revealed different altermagnetic classes correspond in a unique way to all nontrivial magnetic Laue classes, i.e., to the Laue groups containing the operation of antisymmetry only in combination with a spatial rotation. Four of these Laue classes are found compatible with a nonzero spin conductivity. Subsequent inspection of a simple model allows us to address briefly the physical mechanisms responsible for the spin splitting in real systems.