Coherent Spin Transport Research Articles

Synthesis of α-Bi/SrTiO3 heterostructure through the Eu-induced reduction of Bi2O2Se/SrTiO3

Thin Bi films, especially noncentrosymmetric α-phase Bi films (α-Bi), have attracted considerable attention in recent years due to their intriguing physical properties such as ferroelectricity, nonlinear optical response, and coherent spin transport. However, the current experimental preparation of α-Bi films still presents substantial challenges, resulting in only isolated α-Bi islands being achieved. In this study, α-Bi/SrTiO3 (α-Bi/STO) was synthesized from a Bi2O2Se/STO (BOS/STO) heterostructure by depositing a Eu layer and reducing a BOS film. The so-formed α-Bi/STO interface features metallic conductivity with electron mobility of 7.7 × 104 cm2/V s and ultralow resistivity of 1 nΩ cm at 2 K, as well as high residual resistance ratios R300 K/R2 K up to 2353. Furthermore, we observed a complex weak antilocalization–weak localization–weak antilocalization crossover with increasing magnetic field at temperatures below 10 K. Our work presents the synthesis of α-Bi films, which could potentially serve as a valuable platform for experimental validation of diverse theoretical predictions associated with α-Bi heterostructures. Furthermore, this special synthesis method provides valuable insights into the preparation of other metastable films.

Coherent spin transport in a copper protein.

The coherent electron/spin transport in azurin, a species of copper protein, was calculated based on the Landauer model. The research is motivated by the fast electron transport and spin selectivity/polarization in azurin, which have been reported in relation to the chiral-induced spin selectivity of the peptide structure. The calculated spin polarization of copper proteins was large. This phenomenon was strongly influenced by the spin density of the atoms in the ligand group, whereas the contribution of copper was negligible. The results suggest that spin polarization in copper proteins is enhanced by that of the ligand groups. The predicted spin polarization aligns primarily with the scanning tunneling microscope-based break-junction technique to study the electronic properties of single-molecule junctions. Computational techniques employed in this study are nonequilibrium Green's functions (NEGF) and density functional theory (DFT) based on the Landauer model, implemented using the QuantumATK software (Synopsys Inc.). The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was adopted for spin-polarized generalized gradient approximation (SGGA). The valence atomic orbitals were constructed using the wavefunctions of the SIESTA package, which was based on the norm-conserving Troullier-Martins relativistic pseudopotentials for describing core electrons. The mesh used for real-space integration was 150 Ha.

Coherent spin transport in an Fe-S protein

Spin transport in a molecule was recently examined to discuss the chiral-induced spin selectivity (CISS) effect. This effect has been particularly reported in metalloproteins because the presence of transition metals improves spin polarization in a helical peptide structure. Among metalloproteins, iron-sulfur proteins have ubiquitous redox properties in photosynthesis and respiration. Iron can enhance spin polarization. In this study, the coherent spin transport in an Fe-S protein was investigated using first-principles calculations based on the Landauer formula. Proteins containing the [4Fe-4S] cluster were used for the calculations. The results of the spin transmission pathway revealed spin flow via Fe-S clusters.

Identifying and Constructing Complex Magnon Band Topology.

Magnetically ordered materials tend to support bands of coherent propagating spin wave, or magnon, excitations. Topologically protected surface states of magnons offer a new path toward coherent spin transport for spintronics applications. In this work we explore the variety of topological magnon band structures and provide insight into how to efficiently identify topological magnon bands in materials. We do this by adapting the topological quantum chemistry approach that has used constraints imposed by time reversal and crystalline symmetries to enumerate a large class of topological electronic bands. We show how to identify physically relevant models of gapped magnon band topology by using so-called decomposable elementary band representations, and in turn discuss how to use symmetry data to infer the presence of exotic symmetry enforced nodal topology.

Control of dephasing in spin qubits during coherent transport in silicon

One of the key pathways toward scalability of spin-based quantum computing systems lies in achieving long-range interactions between electrons and increasing their interconnectivity. Coherent spin transport is one of the most promising strategies to achieve this architectural advantage. Experimental results have previously demonstrated high-fidelity transportation of spin qubits between two quantum dots in silicon and identified possible sources of error. In this theoretical study, we investigate these errors and analyze the impact of tunnel coupling, magnetic field, and spin-orbit effects on the spin transfer process. The interplay between these effects gives rise to double dot configurations that include regimes of enhanced decoherence that should be avoided for quantum information processing. These conclusions permit us to extrapolate previous experimental conclusions and rationalize the future design of large-scale quantum processors.

First principles study of coherent electron/spin transport across metallothionein: A cadmium-binding protein

Metallothioneins (MTs) play an important role in controlling the concentration of heavy metals in the body by forming metal-thiolate clusters. In addition, the proteins can scavenge free radicals, to exhibit an antioxidant effect. To clarify the precise physiological function of these proteins, it is necessary to examine the non-equilibrium state of electron/spin transport in the molecule. In this study, coherent spin transport in MTs connected to metal electrodes has been calculated to examine the behavior of the molecule toward spin flow (radicals) in the body. The estimated values of conductance were found to be very low, while the spin polarization appears to derive considerable spin filtering. Furthermore, spin polarization did not localize on cadmium but on the sulfur atoms of cadmium-thiolate clusters. These results confirmed that cysteine plays an important role in the scavenging of free radicals even when forming the cadmium-thiolate cluster.Significance Statement.The purpose of this study is to better understand how spin polarization (induced radical) flows in metallothionein, even if it forms cadmium-thiolate clusters. This is important because it can be known where the radical is stored and processed in the body. Our results provide a guide to examine non-equilibrium electron transient in the metallothionein by observing the single-molecule electric conduction with scanning tunneling microscope.

Coherent spin transport in a lanthanide-binding protein

Coherent spin transport in a lanthanide-binding protein

Coherent spin transport in a multi-heme protein molecule

• Chiral-induced spin selectivity (CISS) • Multi-heme protein. • Coherent spin transport. Recently, the chiral-induced spin selectivity (CISS) characteristics of protein molecules with helical peptide structures have been reported. Furthermore, the interaction between CISS-caused spin and paramagnetic ions has been studied to enhance the magnetoresistance of memory and magnetic sensing devices. In this study, a first-principles approach was used to examine spin transport in a multi-heme protein using the Landauer model. Although the molecular size was so large that the conductance values were very low, spin polarization was observed in the molecule. The presence of heme enhanced spin polarization. However, in some cases, the presence of multiple hemes did not increase the spin polarization as expected because the spin transmission route between the electrodes deviated from the hemes. Therefore, the presence of multiple hemes does not necessarily effectively amplify spin polarization.

Enhanced Berry Curvature Dipole and Persistent Spin Texture in the Bi(110) Monolayer.

Nonvanishing Berry curvature dipole (BCD) and persistent spin texture (PST) are intriguing physical manifestations of electronic states in noncentrosymmetric 2D materials. The former induces a nonlinear Hall conductivity while the latter offers a coherent spin current. Based on density-functional-theory (DFT) calculations, we demonstrate the coexistence of both phenomena in a Bi(110) monolayer with a distorted phosphorene structure. Both effects are concurrently enhanced due to the strong spin-orbit coupling of Bi while the structural distortion creates internal in-plane ferroelectricity with inversion asymmetry. We further succeed in fabricating a Bi(110) monolayer in the desired phosphorene structure on the NbSe2 substrate. Detailed atomic and electronic structures of the Bi(110)/NbSe2 heterostructure are characterized by scanning tunneling microscopy/spectroscopy and angle-resolved-photoemission spectroscopy. These results are consistent with DFT calculations which indicate the large BCD and PST are retained. Our results suggest the Bi(110)/NbSe2 heterostructure as a promising platform to exploit nonlinear Hall and coherent spin transport properties together.

Dynamically stabilized spin superfluidity in frustrated magnets

We study the onset of spin superfluidity, namely coherent spin transport mediated by a topological spin texture, in frustrated exchange-dominated magnetic systems, engendered by an external magnetic field. We show that for typical device geometries used in nonlocal magnetotransport experiments, the magnetic field stabilizes a spin superflow against fluctuations, up to a critical current. For a given current, the critical field depends on the precessional frequency of the texture, which can be separately controlled. We contrast such dynamic stabilization of a spin superfluid to the conventional approaches based on topological stabilization.

Tunnel magnetoresistance of a natural protein molecule

The coherent spin transport in a natural protein molecule sandwiched between ferromagnetic electrodes was analyzed based on first-principle calculations in the presence of spin–orbit coupling. The helical peptides exhibited high absolute values of the tunnel magnetoresistance ratio above 0.50. The energy level of the highest-occupied molecular orbital (HOMO) was very close to the Fermi level and sensitively responded to a change in the magnetic configuration of the electrodes. In addition, the HOMO had wave functions consisting of atomic orbitals at specific amide groups that were derived from the close-packed helical peptide structure.

Self-stabilizing exchange-mediated spin transport

Long-range spin transport in magnetic systems can be achieved by means of exchange-mediated spin textures with robust topological winding -- a phenomenon referred to as spin superfluidity. Its experimental signatures have been discussed in antiferromagnets which are nearly free of dipolar interaction. However, in ferromagnets, which possess non-negligible dipole fields, realization of such spin transport has remained a challenge. Using micromagnetic simulations, we investigate coherent exchange-mediated spin transport in extended thin ferromagnetic films. We uncover a two-fluid state, in which the long-range spin transport by spin textures co-exists with spin waves, as well as a soliton-screened spin transport regime at high spin injection biases. Both states are associated with distinct spin texture reconstructions near the spin injection region and sustain spin transport over large distances.

Room-Temperature Spin Transport in Cd3As2.

As the need for ever greater transistor density increases, the commensurate decrease in device size approaches the atomic limit, leading to increased energy loss and leakage currents, reducing energy efficiencies. Alternative state variables, such as electronic spin rather than electronic charge, have the potential to enable more energy-efficient and higher performance devices. These spintronic devices require materials capable of efficiently harnessing the electron spin. Here we show robust spin transport in Cd3As2 films up to room temperature. We demonstrate a nonlocal spin valve switch from this material, as well as inverse spin Hall effect measurements yielding spin Hall angles as high as θSH = 1.5 and spin diffusion lengths of 10-40 μm. Long spin-coherence lengths with efficient charge-to-spin conversion rates and coherent spin transport up to room temperature, as we show here in Cd3As2, are enabling steps toward realizing actual spintronic devices.

Coherent spin transport in a natural helical protein molecule

• Spin transport calculation based on Landauer–Büttiker formula. • NEGF-DFT in the presence of spin–orbit coupling. • Spin polarization in a helical protein molecule. Coherent spin transport in a protein molecule connected to the gold electrodes was calculated using the Landauer–Büttiker formula with NEGF-DFT. A natural helical peptide and a non-helical oligoglycine were used as the model protein molecules. The NEGF-DFT calculations were performed in the presence of spin–orbit coupling. The helical peptide provided a certain spin polarization; however, oligoglycine suppressed the spin polarization. It was found that an amide group located on a specific region overlapping with the frontier orbital exhibited a relatively high spin polarization. It further suggested that the helical peptide structure derived from the closely-packed protein structure was related to spin polarization in the presence of spin–orbit coupling.

Coherent spin transport through helical edge states of topological insulator

We study coherent spin transport through helical edge states of topological insulator tunnel-coupled to metallic leads. We demonstrate that unpolarized incoming electron beam acquires finite polarization after transmission through such a setup provided that edges contain at least one magnetic impurity. The finite polarization appears even in the fully classical regime and is therefore robust to dephasing. There is also a quantum magnetic field-tunable contribution to the polarization, which shows sharp identical Aharonov-Bohm resonances as a function of magnetic flux—with the period hc/2e—and survives at relatively high temperature. We demonstrate that this tunneling interferometer can be described in terms of ensemble of flux-tunable qubits giving equal contributions to conductance and spin polarization. The number of active qubits participating in the charge and spin transport is given by the ratio of the temperature and the level spacing. The interferometer can effectively operate at high temperature and can be used for quantum calculations. In particular, the ensemble of qubits can be described by a single Hadamard operator. The obtained results open wide avenue for applications in the area of quantum computing.

Simulated coherent electron shuttling in silicon quantum dots

Shuttling of single electrons in gate-defined silicon quantum dots is numerically simulated. A minimal gate geometry without explicit tunnel barrier gates is introduced, and used to define a chain of accumulation mode quantum dots, each controlled by a single gate voltage. One-dimensional potentials are derived from a three-dimensional electrostatic model, and used to construct an effective Hamiltonian for efficient simulation. Control pulse sequences are designed by maintaining a fixed adiabaticity, so that different shuttling conditions can be systematically compared. We first use these tools to optimize the device geometry for maximum transport velocity, considering only orbital states and neglecting valley and spin degrees of freedom. Taking realistic geometrical constraints into account, charge shuttling speeds up to $\sim$300 m/s preserve adiabaticity. Coherent spin transport is simulated by including spin-orbit and valley terms in an effective Hamiltonian, shuttling one member of a singlet pair and tracking the entanglement fidelity. With realistic device and material parameters, shuttle speeds in the range 10-100 m/s with high spin entanglement fidelities are obtained when the tunneling energy exceeds the Zeeman energy. High fidelity also requires the inter-dot valley phase difference to be below a threshold determined by the ratio of tunneling and Zeeman energies, so that spin-valley-orbit mixing is weak. In this regime, we find that the primary source of infidelity is a coherent spin rotation that is correctable, in principle. The results pertain to proposals for large-scale spin qubit processors in isotopically purified silicon that rely on coherent shuttling of spins to rapidly distribute quantum information between computational nodes.

Structure-dependent spin transport in a DNA molecule

A DNA molecule formed by a cytosine-guanine sequence ((CG)n) exhibits a conformational transition from the B-form to the Z-form by varying the ionic environment. This study reports coherent spin transport in a molecule of double strand 5′-(CG)n-3′ (n = 2, 3, 4) under a magnetic proximity effect from ferromagnetic nickel electrodes. The calculation was performed by using a Landauer-Büttiker formula based on a non-equilibrium Green’s function with density functional theory. The conformational transition led to about a 10 times change in conductance, which is similar to the experimental results. In addition, the absolute value of the tunnel magnetoresistance ratio had a tendency to decrease with the conformational transition. The calculation result suggests that not only a change in conductance but also a change in the magnetoresistance ratio can be used to monitor the conformation of a DNA molecule sensitively.

Condensation of indirect excitons

Abstract

Electrical manipulation of spin pumping signal through nonlocal thermal magnon transport

We study the magnon transport in the nonlocal configuration composed of two Pt strips on top of a yttrium iron garnet, with and without the presence of RF microwaves generated by an on-chip antenna. We find that the spin-Hall induced thermal magnon heating/cooling, the Oersted field, and the Joule heating generated by the alternating current in the Pt injector can significantly influence the spin-pumping signal measured by the Pt detector in the presence of RF microwaves, forcing the spin-pumping voltage to show up in the first and second harmonic signals in the nonlocal magnon transport measurement. These results indicate that the nonlocal magnon transport configuration can serve as a structure to electrically detect and manipulate the spin-pumping signal. Furthermore, certain caution is needed when studying the interplay between incoherent magnon and coherent magnon spin transport in the nonlocal transport configuration, since the change in microwave-induced spin-pumping voltage can overwhelm the incoherent magnon transport signals.

Coherent spin transport through a six-coordinate FeN6 spin-crossover complex with two different spin configurations

Due to the magnetic bistability, single-molecule spin-crossover (SCO) complexes have been considered to be the most promising building blocks for molecular spintronic devices. Here, we explore the SCO behavior and coherent spin transport properties of a six-coordinate FeN $_6$ complex with the low-spin (LS) and high-spin (HS) states by performing extensive first-principles calculations combined with non-equilibrium Green's function technique. Theoretical results show that the LS $\leftrightarrow$ HS spin transition via changing the metal-ligand bond lengths can be realized by external stimuli, such as under light radiation in experiments. According to the calculated zero-bias transmission coefficients and density of states as well as the $I$ - $V$ curves under small bias voltages of FeN $_6$ SCO complex with the LS and HS states sandwiched between two Au electrodes, we find that the examined molecular junction can act as a molecular switch, tuning from the OFF (LS) state to the ON (HS) state. Moreover, the spin-down electrons govern the current of the HS molecular junction, and this observed perfect spin-filtering effect is not sensitive to the detailed anchoring structure. These theoretical findings highlight this examined six-coordinate FeN $_6$ SCO complex for potential applications in molecular spintronics.