Small Spin Research Articles

A room-temperature four-terminal spin field effect transistor

In analogy to the central role of transistors in conventional electronics, a key device in spintronics, spin field effect transistor (spin FET), has been predicted theoretically. Several approaches have been explored, but their applications remain a considerable challenge due to the small spin signals and low working temperature. In this work, we report a four-terminal spin FET using an individual SWNT with two partially open segments and the working mechanisms is closely related to the magnetic moments at these segments. The spin-related signals ( R spin ) can be as large as several hundred Ohm and the spin FET works at room temperature under atmospheric conditions. R spin can be modulated with gate voltages and external magnetic fields. The realization of spin FET may lead to the development of integrated circuit of spintronics, which is fundamentally different from charge-based conventional electronics. • Room-temperature ferromagnetic semiconductor: semiconducting SWNT with open segments. • Pure spin current induced by a conventional charge current in a fully separated path. • Spin FET working at room temperature under atmospheric conditions. • Ohmic contact between SWNT and Mo is an efficient spin injector/detector. • R spin can be modulated with gate voltages and external magnetic fields.

Frustrated quantum spins at finite temperature: Pseudo-Majorana functional renormalization group approach

The pseudofermion functional renormalization group (PFFRG) method has proven to be a powerful numerical approach to treat frustrated quantum spin systems. In its usual implementation, however, the complex fermionic representation of spin operators introduces unphysical Hilbert-space sectors which render an application at finite temperatures inaccurate. In this work we formulate a general functional renormalization group approach based on Majorana fermions to overcome these difficulties. We, particularly, implement spin operators via an $\text{SO}(3)$ symmetric Majorana representation which does not introduce any unphysical states and, hence, remains applicable to quantum spin models at finite temperatures. We apply this scheme, dubbed pseudo-Majorana functional renormalization group (PMFRG) method, to frustrated Heisenberg models on small spin clusters as well as square and triangular lattices. Computing the finite-temperature behavior of spin correlations and thermodynamic quantities such as free energy and heat capacity, we find good agreement with exact diagonalization and the high-temperature series expansion down to moderate temperatures. We observe a significantly enhanced accuracy of the PMFRG compared to the PFFRG at finite temperatures. More generally, we conclude that the development of functional renormalization group approaches with Majorana fermions considerably extends the scope of applicability of such methods.

Temperature dependence of spin—orbit torque-driven magnetization switching in in situ grown Bi2Te3/MnTe heterostructures

We report the temperature dependence of the spin–orbit torque (SOT) in the in situ grown Bi2Te3/MnTe heterostructures by molecular beam epitaxy. By appropriately designing the film stack, robust ferromagnetic order with high Curie temperature and strong perpendicular magnetic anisotropy is established in the MnTe layer. Meanwhile, the sharp hetero-interface warrants highly efficient spin current injection from the conductive topological insulator (TI) channel. Accordingly, SOT-driven magnetization switching is observed up to 90 K with the critical current density within the 106 A⋅cm−2 range. More importantly, the temperature-dependent harmonic measurement data can be divided into two categories, namely, the spin Hall effect of the TI bulk states gives rise to a relatively small spin Hall angle in the high-temperature region, whereas the spin-momentum locking nature of the interfacial Dirac fermions leads to the enhancement of the SOT strength once the topological surface states become the dominant conduction channel at deep cryogenic temperatures. Our results offer direct evidence of the underlying mechanism that determines the SOT efficiency and may set up a suitable platform to realize TI-based spin–orbit applications toward room temperature.

The Fascinating World of Low-Dimensional Quantum Spin Systems: Ab Initio Modeling.

In recent times, ab initio density functional theory has emerged as a powerful tool for making the connection between models and materials. Insulating transition metal oxides with a small spin forms a fascinating class of strongly correlated systems that exhibit spin-gap states, spin–charge separation, quantum criticality, superconductivity, etc. The coupling between spin, charge, and orbital degrees of freedom makes the chemical insights equally important to the strong correlation effects. In this review, we establish the usefulness of ab initio tools within the framework of the N-th order muffin orbital (NMTO)-downfolding technique in the identification of a spin model of insulating oxides with small spins. The applicability of the method has been demonstrated by drawing on examples from a large number of cases from the cuprate, vanadate, and nickelate families. The method was found to be efficient in terms of the characterization of underlying spin models that account for the measured magnetic data and provide predictions for future experiments.

Supersymmetric spin\u2013phonon coupling prevents odd integer spins from quantum tunneling

Quantum tunneling of the magnetization is a phenomenon that impedes the use of small anisotropic spin systems for storage purposes even at the lowest temperatures. Phonons, usually considered for temperature dependent relaxation of magnetization over the anisotropy barrier, also contribute to magnetization tunneling for integer spin quantum numbers. Here we demonstrate that certain spin–phonon Hamiltonians are unexpectedly robust against the opening of a tunneling gap, even for strong spin–phonon coupling. The key to understanding this phenomenon is provided by an underlying supersymmetry that involves both spin and phonon degrees of freedom.

Conditional Action and Imperfect Erasure of Qubits.

We consider state changes in quantum theory due to “conditional action” and relate these to the discussion of entropy decrease due to interventions of “intelligent beings” and the principles of Szilard and Landauer/Bennett. The mathematical theory of conditional actions is a special case of the theory of “instruments”, which describes changes of state due to general measurements and will therefore be briefly outlined in the present paper. As a detailed example, we consider the imperfect erasure of a qubit that can also be viewed as a conditional action and will be realized by the coupling of a spin to another small spin system in its ground state.

Magnetic ground state and electron-doping tuning of Curie temperature in Fe3GeTe2 : First-principles studies

Intrinsic magnetic van der Waals (vdW) materials have attracted much attention, especially Fe$_{3}$GeTe$_{2}$ (FGT), which exhibits highly tunable properties. However, despite vast efforts, there are still several challenging issues to be resolved. Here using a first-principles linear-response approach, we carry out comprehensive investigation of both bulk and monolayer FGT. We find that although the magnetic exchange interactions in FGT are frustrated, our Monte Carlo simulations agree with the total energy calculations, confirming that the ground state of bulk FGT is indeed ferromagnetic (FM). A tiny electron doping reduces the magnetic frustration, resulting in significant increasing of the Curie temperature. We also calculate the spin-wave dispersion, and find a small spin gap as well as a nearly flat band in the magnon spectra. These features can be used to compare with the future neutron scattering measurement and finally clarify the microscopic magnetic mechanism in this two-dimensional family materials.

Thermodynamic properties of spin-imbalance harmonically trapped one-dimensional attractive 6Li atomic gas

The thermodynamic properties of 6Li atomic gas system, with imbalanced spin populations trapped in one-dimension, were systematically investigated using the Static Fluctuation Approximation. The two-body interaction used is an attractive contact potential. The effects of gas parameter [Formula: see text] and spin polarization [Formula: see text], on the thermodynamic properties and effective magnetic field were investigated. We observed a decrease in [Formula: see text] and an enhancement in [Formula: see text] and [Formula: see text] with increasing [Formula: see text]. At strong interaction and at [Formula: see text], the behavior of entropy with [Formula: see text] indicated two different phases. At small spin polarization [Formula: see text], the system could be in Fulde–Ferrell–Larkin Ovchinnikov (FFLO) state, while above [Formula: see text], the system might be in normal state. In addition, we found a clear decrease in both [Formula: see text] and [Formula: see text] and an enhancement in [Formula: see text] with the increase of the interaction strength. Our results are consistent with the reported results obtained by the mean-field Bogoliubov–de Gennes method, the Bardeen–Cooper–Schrieffer (BCS) approximation and Nozieres–Schmitt–Rink (NSR) theory.

Terahertz Spin-to-Charge Conversion by Interfacial Skew Scattering in Metallic Bilayers.

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin-based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin-to-charge-current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni81 Fe19 , Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin-orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.

On the Mutual Relationships between Molecular Probe Mobility and Free Volume and Polymer Dynamics in Organic Glass Formers: cis-1,4-poly(isoprene).

We report on the reorientation dynamics of small spin probe 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) in cis-1,4-poly(isoprene) (cis-1,4-PIP10k) from electron spin resonance (ESR) and the free volume of cis-1,4-PIP10k from positron annihilation lifetime spectroscopy (PALS) in relation to the high-frequency relaxations of cis-1,4-PIP10k using light scattering (LS) as well as to the slow and fast processes from broadband dielectric spectroscopy (BDS) and neutron scattering (NS). The hyperfine coupling constant, 2Azz′(T), and the correlation times, τc(T), of cis-1,4-PIP10k/TEMPO system as a function of temperature exhibit several regions of the distinct spin probe TEMPO dynamics over a wide temperature range from 100 K up to 350 K. The characteristic ESR temperatures of changes in the spin probe dynamics in cis-1,4-PIP10k/TEMPO system are closely related to the characteristic PALS ones reflecting changes in the free volume expansion from PALS measurement. Finally, the time scales of the slow and fast dynamics of TEMPO in cis-1,4-PIP10k are compared with all of the six known slow and fast relaxation modes from BDS, LS and NS techniques with the aim to discuss the controlling factors of the spin probe reorientation mobility in polymer, oligomer and small molecular organic glass-formers.

Observable shape of black hole photon rings

Motivated by the prospect of measuring a black hole photon ring, in previous work we explored the interferometric signature produced by a bright, narrow curve in the sky. Interferometric observations of such a curve measure its "projected position function" $\mathbf{r}\cdot\hat{\mathbf{n}}$, where $\mathbf{r}$ parameterizes the curve and $\hat{\mathbf{n}}$ denotes its unit normal vector. In this paper, we show by explicit construction that a curve can be fully reconstructed from its projected position, completing the argument that space interferometry can in principle determine the detailed photon ring shape. In practice, near-term observations may be limited to the visibility amplitude alone, which contains incomplete shape information: for convex curves, the amplitude only encodes the set of projected diameters (or "widths") of the shape. We explore the freedom in reconstructing a convex curve from its widths, giving insight into the shape information probed by technically plausible future astronomical measurements. Finally, we consider the Kerr "critical curve" in this framework and present some new results on its shape. We analytically show that the critical curve is an ellipse at small spin or inclination, while at extremal spin it becomes the convex hull of a Cartesian oval. We find a simple oval shape, the "phoval", which reproduces the critical curve with high fidelity over the whole parameter range.

Evolution of the Huygens Probe Spin During Parachute Descent

The anomalous spin of the Huygens probe on Titan is simulated with a simple model, which is also applied to the (also anomalous) spin history from a terrestrial parachute drop test. The model considers two parameters: a “setting angle,” which defines an equilibrium spin rate for a given descent speed, and a coefficient, which defines how quickly the probe spin changes to reach that equilibrium. The parameters required to reproduce the descent histories are compared with values derived from wind-tunnel tests and estimated by construction. Not only did both descents see spin in the opposite direction from that intended to be driven by small canted spin vanes, but both the Titan descent and the drop test require nonconstant torque parameters, requiring configuration changes and/or external torques. In both cases, unexplained short-term spin evolution occurs while under a small “stabilizer” parachute. In the Titan case, additional spin evolution anomalies appear consistent with the off-nominal deployment of an instrument boom.

Exploring the Organometallic Route to Molecular Spin Qubits: The [CpTi(cot)] Case

AbstractThe coherence time of the 17‐electron, mixed sandwich complex [CpTi(cot)], (η8‐cyclooctatetraene)(η5‐cyclopentadienyl)titanium, reaches 34 μs at 4.5 K in a frozen deuterated toluene solution. This is a remarkable coherence time for a highly protonated molecule. The intramolecular distances between the Ti and H atoms provide a good compromise between instantaneous and spin diffusion sources of decoherence. Ab initio calculations at the molecular and crystal packing levels reveal that the characteristic low‐energy ring rotations of the sandwich framework do not yield a too detrimental spin‐lattice relaxation because of their small spin–phonon coupling. The volatility of [CpTi(cot)] and the accessibility of the semi‐occupied, non‐bonding d orbital make this neutral compound an ideal candidate for single‐qubit addressing on surface and quantum sensing in combination with scanning probe microscopy.

Giant perpendicular magnetic anisotropy induced by double degeneracy of d-orbitals in W/MgO/Ag heterojunction

Large perpendicular magnetic anisotropy energy (MAE) is crucial for designing high-density information storage devices. Here, employing first-principles calculations, it has shown the possibility to boost the magnetic anisotropy using a single W atom functionalized by radical of hydroxy (OH) on the MgO(001)/Ag(001) heterojunction. The OH-functionalized system exhibited the giant perpendicular MAE of 214.76 meV, which outperformed all the reported heterojunctions. The underlying mechanism for the enormous MAE is attributed to the doubly degenerated d-orbitals around the Fermi level caused by a special OH-ligand field which alters the spin-orbit coupling Hamiltonian and enhances the magnetic anisotropy. Meanwhile, the small spin magnetic moment of 1.829 μB is obtained due to the strong OH-ligand field. This work provides a new approach to generate high perpendicular MAE that is very promising for nanoscale magnetic devices.

Predicting oxidation and spin states by high-dimensional neural networks: Applications to lithium manganese oxide spinels.

Lithium ion batteries often contain transition metal oxides such as LixMn2O4 (0 ≤ x ≤ 2). Depending on the Li content, different ratios of MnIII to MnIV ions are present. In combination with electron hopping, the Jahn-Teller distortions of the MnIIIO6 octahedra can give rise to complex phenomena such as structural transitions and conductance. While for small model systems oxidation and spin states can be determined using density functional theory (DFT), the investigation of dynamical phenomena by DFT is too demanding. Previously, we have shown that a high-dimensional neural network potential can extend molecular dynamics (MD) simulations of LixMn2O4 to nanosecond time scales, but these simulations did not provide information about the electronic structure. Here, we extend the use of neural networks to the prediction of atomic oxidation and spin states. The resulting high-dimensional neural network is able to predict the spins of the Mn ions with an error of only 0.03 ℏ. We find that the Mn eg electrons are correctly conserved and that the number of Jahn-Teller distorted MnIIIO6 octahedra is predicted precisely for different Li loadings. A charge ordering transition is observed between 280 K and 300 K, which matches resistivity measurements. Moreover, the activation energy of the electron hopping conduction above the phase transition is predicted to be 0.18 eV, deviating only 0.02 eV from experiment. This work demonstrates that machine learning is able to provide an accurate representation of both the geometric and the electronic structure dynamics of LixMn2O4 on time and length scales that are not accessible by ab initio MD.

Insight into the Topological Nodal Line Metal YB2 with Large Linear Energy Range: A First-Principles Study.

The presence of one-dimensional (1D) nodal lines, which are formed by band crossing points along a line in the momentum space of materials, is accompanied by several interesting features. However, in order to facilitate experimental detection of the band crossing point signatures, the materials must possess a large linear energy range around the band crossing points. In this work, we focused on a topological metal, YB2, with phase stability and a P6/mmm space group, and studied the phonon dispersion, electronic structure, and topological nodal line signatures via first principles. The computed results show that YB2 is a metallic material with one pair of closed nodal lines in the kz = 0 plane. Importantly, around the band crossing points, a large linear energy range in excess of 2 eV was observed, which was rarely reported in previous reports that focus on linear-crossing materials. Furthermore, YB2 has the following advantages: (1) An absence of a virtual frequency for phonon dispersion, (2) an obvious nontrivial surface state around the band crossing point, and (3) small spin–orbit coupling-induced gaps for the band crossing points.

Estimating black hole masses: Accretion disk fitting versus reverberation mapping and single epoch

We selected a sample of 28 Type 1 active galactic nuclei for which a black hole mass has been inferred using the reverberation mapping technique and single epoch scaling relations. All 28 sources show clear evidence of the “Big Blue Bump” in the optical-UV band whose emission is produced by an accretion disk (AD) around a supermassive black hole. We fitted the spectrum of these sources with the relativistic thin AD model KERRBB in order to infer the black hole masses and compared them with those from Reverberation mapping and Single epoch methods, discussing the possible uncertainties linked to such a model by quantifying their weight on our results. We find that for the majority of the sources, KERRBB is a good description of the AD emission for a wide wavelength range. The overall uncertainty on the black hole mass estimated through the disk fitting procedure is ∼0.45 dex (which includes the uncertainty on fitting parameters such as e.g., spin and viewing angle), comparable to the systematic uncertainty of reverberation mapping and single epoch methods; however, such an uncertainty can be ≲0.3 dex if one of the parameters of the fit is well constrained. Although all of the estimates are affected by large uncertainties, the masses inferred using the three methods are compatible if the dimensionless scale factorf(linked to the unknown kinematics and geometry of the Broad Line Region) is assumed to be larger than one. For the majority of the sources, the comparison between the results coming from the three methods favors small spin values. To check the goodness of the KERRBB results, we compared them with those inferred with other models, such as AGNSED, a model that also accounts for the emission originating from an X-ray corona: using two sources with a good data coverage in theXband, we find that the masses estimated with the two models differ at most by a factor of ∼0.2 dex.

Spin Detection via Parametric Frequency Conversion in a Membrane Resonator

Recent demonstrations of ultracoherent nanomechanical resonators introduce the prospect of new protocols for solid state sensing applications. Here, we propose to use two coupled ultracoherent resonator modes on a Si$_3$N$_4$ membrane for the detection of small nuclear spin ensembles. To this end, we employ parametric frequency conversion between nondegenerate modes. The nondegenerate modes result from coupled degenerate resonators, and the parametric conversion is mediated by periodic inversions of the nuclear spins in the presence of a magnetic scanning tip. We analyze potential noise sources and derive the achievable signal-to-noise ratio with typical experimental parameter values. Our proposal reconciles the geometric constraints of optomechanical systems with the requirements of scanning force microscopy and brings forth a new platform for spin-phonon interaction and spin imaging.

Roles of easy-plane and easy-axis XXZ anisotropy and bond alternation in a frustrated ferromagnetic spin- 12 chain

The spin-$1/2$ Heisenberg chain with a ferromagnetic first-neighbor exchange coupling $J_1$ and an antiferromagnetic second-neighbor $J_2$ has a Haldane dimer ground state with an extremely small spin gap. Thus, the ground state is readily altered by perturbations. Here, we investigate the effects of XXZ exchange magnetic anisotropy of both the easy-axis and easy-plane types and an alternation in $J_1$ on the ground state, the spin gap, and magnetic properties of the frustrated ferromagnetic spin-$1/2$ chain. It is found that there are two distinct dimerized spin-gap phases, in one of which the spin gap and the magnetic susceptibility are extremely small around the SU(2) symmetric case and in the other they are moderately large far away from the SU(2) symmetric case. A small alternation in the amplitude of $J_1$ rapidly shortens the pitch of spin correlations towards the four-spin periodicity, as in the limit of $J_1/J_2\to0$. These effects are not sufficient to quantitatively explain overall experimentally observed magnetic properties in the quasi-one-dimensional spin-gapped magnetoelectric cuprate Rb$_2$Cu$_2$Mo$_3$O$_{12}$ that exhibits ferroelectricity stabilized by a magnetic field. Our results are also relevant to Cs$_2$Cu$_2$Mo$_3$O$_{12}$, where the ferromagnetic intrachain and antiferromagnetic interchain order has recently been found, in a single chain level. We also reveal the nature of symmetry-protected topological phase transitions in the model by mapping onto effective spin-1 chain models.

Composition dependence of spin Seebeck voltage in YIG/Pt100−XRuX, Pt100−XCuX, and Pt100−X(Cu0.5Ru0.5)X

We investigated the composition effect on spin Seebeck coefficient in YIG/Pt100−XRuX, YIG/Pt100−XCuX, and YIG/Pt100−X(Cu0.5Ru0.5)X systems. In all systems the composition of around 50 (at%) showed the maximum spin Seebeck coefficient, which is considered as the result from the extrinsic spin Hall effect improvement since the resistivity was also increased with doping Ru and Cu into Pt host. In terms of the spin Seebeck coefficient dependence on doped element, YIG/Pt100−XCuX was the largest, followed in order by YIG/Pt100−X(Cu0.5Ru0.5)X, and YIG/Pt100−XRuX, although both of Cu and Ru have equally small spin Hall angle in the single state. The X-ray diffraction analysis showed that Cu doping most widely decreased the unit cell size of alloy, and consequently increased the resistivity. It is considered as a reason of improvement of the extrinsic spin Hall angle and the spin Seebeck voltage.