Spin Degeneracy Research Articles

Topological-edge-state spin transport in asymmetric three-terminal silicenelike nanodevice

We theoretically investigate the topological-edge-state spin transport in asymmetric three-terminal silicene-like nanodevice. Since silicene-like materials are honeycomb structures with considerable spin-orbit interaction (SOI), they possess both Dirac electron and topology insulator behaviors. In the three-terminal silicene-like nanodevice, the SOI realizes helical edge state and brings fully spin polarization selectively without external field. Firstly, we find that the spin degeneracy breaking gives rise to spin-polarized transport, i.e., up-spin electron and down-spin electron propagating to different leads from the top lead. The distribution of edge-state spin-dependent current in the real space indicates that an up-/down-spin channel to the left/right lead is opened at the interface of the present nanodevice. Secondly, the spin-polarized transport behavior has a competition with the effect of asymmetric transport, which prefers propagating the up- and down-electrons from top lead to the same (right) lead. Interestingly, as the geometric size variation is considered, the results show that the width increase of the horizontal armchair (top vertical zigzag) lead reinforces the spin-polarized (asymmetric) transport. However, when both the armchair and zigzag leads increase simultaneously, the spin-polarized transport becomes the dominant effect. Therefore, this edge-state spin-polarized transport behavior is topologically protected and very robust as the whole geometric size of the nanodevice increases. These properties of the topological-edge-state spin transport enable the asymmetric three-terminal silicene-like nanodevice a spin filter or a spin valve, and might contribute to the silicene-like nanocircuit engineering and spintronics application.

Triplet correlations in Cooper pair splitters realized in a two-dimensional electron gas

Cooper pairs occupy the ground state of superconductors and are typically composed of maximally entangled electrons with opposite spin. In order to study the spin and entanglement properties of these electrons, one must separate them spatially via a process known as Cooper pair splitting (CPS). Here we provide the first demonstration of CPS in a semiconductor two-dimensional electron gas (2DEG). By coupling two quantum dots to a superconductor-semiconductor hybrid region we achieve efficient Cooper pair splitting, and clearly distinguish it from other local and non-local processes. When the spin degeneracy of the dots is lifted, they can be operated as spin-filters to obtain information about the spin of the electrons forming the Cooper pair. Not only do we observe a near perfect splitting of Cooper pairs into opposite-spin electrons (i.e. conventional singlet pairing), but also into equal-spin electrons, thus achieving triplet correlations between the quantum dots. Importantly, the exceptionally large spin-orbit interaction in our 2DEGs results in a strong triplet component, comparable in amplitude to the singlet pairing. The demonstration of CPS in a scalable and flexible platform provides a credible route to study on-chip entanglement and topological superconductivity in the form of artificial Kitaev chains.

Spin filtration in generalized Sierpinski triangles in presence of Rashba spin–orbit interaction

In this work, we theoretically investigate the phenomenon of spin polarization in generalized Sierpinski gasket (SPG) triangles (, , , ) in the presence of Rashba spin–orbit interaction (RSOI) within a tight-binding framework. The geometrical construction is addressed for all four types of generalized SPG triangles using an iterative function system that begins with a fixed triangle of any arbitrary side length. The geometry of various types of fractal networks has a major impact on the degeneracy of the energy spectrum. It is shown that only the two-terminal setup in the presence of RSOI is incapable of breaking the spin degeneracy, which necessitates the use of a three-terminal setup to break the time-reversal symmetry. The effect of flip map induced anisotropic hopping integrals on three-terminal spin-resolved transmission probabilities is studied. In several cases, we find almost spin polarization in our considered lattices. We also analyze how spin polarization varies with RSOI strength and fractal generations. Our results indicate that fractal lattices might be suitable functional elements for designing future spin-based electronic devices.

The classical Heisenberg model on the centred pyrochlore lattice

The centred pyrochlore lattice is a novel geometrically frustrated lattice, realized in the metal-organic framework Mn(ta)_22 [Phys. Rev. Research 5, L022018 (2023)] where the basic unit of spins is a five site centred tetrahedron. Here, we present an in-depth theoretical study of the J_1-J_2J1−J2 classical Heisenberg model on this lattice, using a combination of mean-field analytical methods and Monte Carlo simulations. We find a rich phase diagram with low temperature states exhibiting ferrimagnetic order, partial ordering, and a highly degenerate spin liquid with distinct regimes of low temperature correlations. We discuss in detail how the regime displaying broadened pinch points in its spin structure factor is consistent with an effective description in terms of a fluid of interacting charges. We also show how this picture holds in two dimensions on the analogous centred kagome lattice and elucidate the connection to the physics of thin films in (d+1d+1) dimensions. Furthermore, we show that a Coulomb phase can be stabilized on the centred pyrochlore lattice by the addition of further neighbour couplings. This demonstrates the centred pyrochlore lattice is an experimentally relevant geometry which naturally hosts emergent gauge fields in the presence of charges at low energies.

Anomalous conductance quantization of a one-dimensional channel in monolayer WSe2

Among quantum devices based on 2D materials, gate-defined quantum confined 1D channels are much less explored, especially in the high-mobility regime where many-body interactions play an important role. We present the results of measurements and theory of conductance quantization in a gate-defined one-dimensional channel in a single layer of transition metal dichalcogenide material WSe2. In the quasi-ballistic regime of our high-mobility sample, we report conductance quantization steps in units of e2/h for a wide range of carrier concentrations. Magnetic field measurements show that as the field is raised, higher conductance plateaus move to accurate quantized values and then shift to lower conductance values while the e2/h plateau remains locked. Based on microscopic atomistic tight-binding theory, we show that in this material, valley and spin degeneracies result in 2 e2/h conductance steps for noninteracting holes, suggesting that symmetry-breaking mechanisms such as valley polarization dominate the transport properties of such quantum structures.

Valley and spin splittings in a valley layer locked monolayer via atomic adsorption

To date, a number of valley materials have been discovered with spin-valley or valley-layer couplings. It is highly desirable to realize the interplay of more electronic degrees of freedom in a valley material. Based on the first-principles calculations, we demonstrate the valley and spin degeneracy liftings in the band structure of a TiSiCO monolayer, with the help of the selective expression of the layer pseudospin via atomic adsorption. The introduction of the transition-metal adatoms provides an effective electric field and magnetic proximity effect, giving rise to considerable valley and spin splittings. The larger valley splitting and the largest spin splitting for the band-edge states have magnitudes of up to 190 meV and 148 meV, respectively. The valley and spin splittings can be further tuned by applying an external electric field. According to the modified band structure, various interlayer excitons with different combinations of spins and electric dipoles are selectively created under the optical field of appropriate frequencies. The tunable spin and valley splittings in atom-adsorbed TiSiCO monolayer offer opportunities for exploring the interactions between spin, valley, and layer pseudospins and designing advanced optoelectronic devices.

Quadriexciton Binding Energy in Electron–Hole Bilayers

Excitonic condensation and superfluidity have recently received a renewed attention, due to the fabrication of bilayer systems in which electrons and holes are spatially separated and form stable pairs known as indirect excitons. Dichalcogenides- and graphene-based bilayers are nowadays built and investigated, giving access to systems with (i) only spin degeneracy and (ii) spin and valley degeneracy. Simulation studies performed in the last decades at T=0 for simple, model electron–hole bilayers, as function of the interlayer distance and in-layer carrier density, have revealed in case (i) the formation of biexcitons in a tiny region of the parameter space and in case (ii) the formation of stable compounds made of four electrons and four holes (quadriexcitons) in a sizable region of the parameter space. Of some interest is the relation of the properties of isolated biexcitons (quadriexcitons) and those of their finite-density counterpart. In fact, the isolated biexciton has been repeatedly studied in the last years with simulations and other techniques. No simulations, instead, are available to our knowledge for the isolated quadriexciton, for which we present here results of the first quantum Monte Carlo (QMC) study. Stability with respect to the dissociation into biexcitons and the pair correlations while varying the interlayer distance d are discussed.

Controllable and enormous spin splitting in antiferromagnetic MnPSe3 through interfacial coupling with Janus MoSSe

The manipulation of spin and valley degree of freedom in two-dimensional materials is desirable for emerging applications in next-generation electronics. Here, we report a regulation of the spin splitting in monolayer antiferromagnetic MnPSe3 through stacking van der Waals (vdW) heterostructures with transition-metal dichalcogenides (TMDs). Different TMDs including Janus MoSSe, MoSe2, and MoS2 are engaged and the spin and valley dependences on stacking configurations, interfacial atoms, and interlayer spacings are comprehensively investigated based on first-principles calculations. The results show that spin degeneracy of MnPSe3 can be lifted through the interfacial interaction in the vdW heterostructures. Compared with the MnPSe3 interfacing with traditional TMDs (MoSe2 and MoS2), those with Janus TMDs (MoSSe or MoSeS) have larger spin splitting. A maximal spin splitting of 50.8 meV at valence band maximum is achieved in MoSSe/MnPSe3 heterostructure, and such splitting is further enhanced to 243.0 meV when the interlayer spacing is reduced to 2.9 Å. It is found that the different hybridizations between the d-orbitals of the two inequivalent Mn atoms in MnPSe3 and the above TMDs lead to the occurrence of net magnetic moments, and thus induce spin splitting. This work paves a novel way to modify the spin-valley properties of two-dimensional semiconductors.

Spin-orbit-coupling-induced band splitting in two-dimensional hybrid organic-inorganic perovskites: Importance of organic cations

We investigate how the spin-orbit-coupling-induced lifting of the spin degeneracy depends indirectly on constituting organic motifs in two-dimensional hybrid organic-inorganic perovskites. We examine experimentally synthesized structures using first-principles electronic structure theory. In particular, the effect of the organic spacer cation packing and of the methyl ammonium orientation within the inorganic layer are elucidated. Both the organic spacer packing and subtle changes in methyl ammonium orientations can cause enough distortions of lead iodide octahedral cages to induce the spin splitting. Interestingly, the momentum-independent persistent spin texture splitting rather than the traditional Rashba/Dresselhaus splitting is observed for a structure in which organic spacer molecules are modified with fluorine substitution. Our calculation on these experimentally reported structures highlights the important role of the organic constituents in these hybrid organic inorganic perovskites for the spin splitting.

A type-II MnPSe3/GeC heterostructure with tunable spin and valley splitting

The ternary chalcogenide MnPSe3 monolayer recently gets a lot of attention because of its two nonequivalent energy valleys. The existence of spin–orbit coupling (SOC) effect can cause the valley splitting of around 24 meV. However, the spin degeneracy is still kept in MnPSe3 monolayer owing to the antiferromagnetic coupling between Mn2+ ions. The interlayer coupling effect in the van der Waals (vdW) heterostructures can break the symmetry of MnPSe3 and induce the spin splitting. Here, we simulate the MnPSe3/GeC vdW heterostructure and investigate the effects of the stacking and interlayer coupling effect on both spin and valley splitting. The excitation mode of valley excitons is greatly related to the stacking of heterostructures. The interlayer excitons can only be formed in one of the configurations with C3 symmetry according to optical selection transition rule. The interlayer coupling effect is modulated by changing interlayer distance. With vertical compressive strain (−20%), the spin and valley splitting higher than 50 meV and 30 meV can be obtained in MnPSe3/GeC vdW heterostructures, respectively. This study suggests that the stacking and vertical strain are both efficient approaches to modulate the spin and valley splitting through the interlayer coupling effect in the vdW heterostructures.

Modulation of antichiral edge states in zigzag honeycomb nanoribbons by side potentials

The antichiral edge states induced by the modified Haldane model have been predicted in the previous studies. In this study, other types of antichiral edge states are proposed by applying the side potentials composed of a potential field, staggered electric field, and exchange field along the boundaries of zigzag honeycomb nanoribbons (zHNRs). Their corresponding transport properties are investigated. The results show that the side potentials can lift the spin degeneracy of the edge modes, inducing five types of antichiral edge states. By calculating the spin-dependent energies in K’ and K valleys of the edge modes, an interpretation for generating antichiral edge states is proposed. In addition, the spin/charge switcher in the three-terminal device consisting of zHNRs is developed based on the induced edge states. We believe that these results can be used in the design of future spintronic devices.

Gapless Spin Liquid and Nonlocal Corner Excitation in the Spin-1/2 Heisenberg Antiferromagnet on Fractal

Motivated by the mathematical beauty and the recent experimental realizations of fractal systems, we study the spin-1/2 antiferromagnetic Heisenberg model on a Sierpiński gasket. The fractal porous feature generates new kinds of frustration to exhibit exotic quantum states. Using advanced tensor network techniques, we identify a quantum gapless-spin-liquid ground state in fractional spatial dimension. This fractal spin system also demonstrates nontrivial nonlocal properties. While the extremely short-range correlation causes a highly degenerate spin form factor, the entanglement in this fractal system suggests a long-range scaling behavior. We also study the dynamic structure factor and clearly identify the gapless excitation with a stable corner excitation emerged from the ground-state entanglement. Our results unambiguously point out multiple essential properties of this fractal spin system, and open a new route to explore spin liquid and frustrated magnetism.

Global phase diagram of charge-neutral graphene in the quantum Hall regime for generic interactions

Monolayer graphene at charge neutrality in a quantizing magnetic field is a quantum Hall ferromagnet. Due to the spin and valley (near) degeneracies, there is a plethora of possible ground states. Previous theoretical work, based on a stringent ultra short-range assumption on the symmetry-allowed interactions, predicts a phase diagram with distinct regions of spin-polarized, canted antiferromagnetic, inter-valley coherent, and charge density wave order. While early experiments suggested that the system was in the canted antiferromagnetic phase at a perpendicular field, recent scanning tunneling studies universally find Kekul\'e bond order, and sometimes also charge density wave order. Recently, it was found that if one relaxes the stringent assumption mentioned above, a phase with coexisting canted antiferromagnetic and Kekul\'e order exists in the region of the phase diagram believed to correspond to real samples. In this work, starting from the continuum limit appropriate for experiments, we present the complete phase diagram of $\nu=0$ graphene in the Hartree-Fock approximation, using generic symmetry-allowed interactions, assuming translation invariant ground states up to an intervalley coherence. Allowing for a sublattice potential (valley Zeeman coupling), we find numerous phases with different types of coexisting order. We conclude with a discussion of the physical signatures of the various states.

Photonic Dirac cavities with spatially varying mass term.

In recent years, photonics has proven itself as an excellent platform for emulation of relativistic phenomena. Here, we show an example of relativistic-like trapping in photonic system that realizes Dirac-like dispersion with spatially inhomogeneous mass term. The modes trapped by such cavities, their energy levels, and corresponding orbitals are then characterized through optical imaging in real and momentum space. The fabricated cavities host a hierarchy of photonic modes with distinct radiation profiles directly analogous to various atomic orbitals endowed with unique characteristics, such as pseudo-particle-hall symmetry and spin degeneracy, and they carry topological charge which gives rise to radiative profiles with angular momentum. We demonstrate that these modes can be directionally excited by pseudo-spin-polarized boundary states. In addition to the fundamental interest in the structure of these pseudo-relativistic orbitals, the proposed system offers a route for designing new types of nanophotonic devices, spin-full resonators and topological light sources compatible with integrated photonics platforms.

Field-induced magnetic phases in a qubit Penrose quasicrystal.

Unveiling the fundamental dynamics of naturally or artificially formed magnetic quasicrystals in the presence of an external magnetic field remains a difficult problem that may have implications for the design of information processing devices. By embedding a qubit magnetic Penrose quasicrystal into a quantum annealer, we were able to reproduce the formation of magnetic phases driven by specific physical parameter selections, allowing us to distinguish a wide range of frustrated magnetic configurations at the single-spin scale. In our experiments, we observe some spins dynamically activate, while others remain static, all within an average magnetization space defined by competing structural and magnetic degrees of freedom. Static spin structure factors reveal ferromagnetic and ferrimagnetic modulations that are compatible with a variety of spin textures. This research demonstrates that introducing structural aperiodicity in magnetic devices that exploit spin degeneracy in a single, richly intraconnected finite object can enable the engineering of quantum states in both the effective low-temperature and thermally excited regimes.

Twisted bilayer zigzag-graphene nanoribbon junctions with tunable edge states

Stacking two-dimensional layered materials such as graphene and transitional metal dichalcogenides with nonzero interlayer twist angles has recently become attractive because of the emergence of novel physical properties. Stacking of one-dimensional nanomaterials offers the lateral stacking offset as an additional parameter for modulating the resulting material properties. Here, we report that the edge states of twisted bilayer zigzag graphene nanoribbons (TBZGNRs) can be tuned with both the twist angle and the stacking offset. Strong edge state variations in the stacking region are first revealed by density functional theory (DFT) calculations. We construct and characterize twisted bilayer zigzag graphene nanoribbon (TBZGNR) systems on a Au(111) surface using scanning tunneling microscopy. A detailed analysis of three prototypical orthogonal TBZGNR junctions exhibiting different stacking offsets by means of scanning tunneling spectroscopy reveals emergent near-zero-energy states. From a comparison with DFT calculations, we conclude that the emergent edge states originate from the formation of flat bands whose energy and spin degeneracy are highly tunable with the stacking offset. Our work highlights fundamental differences between 2D and 1D twistronics and spurs further investigation of twisted one-dimensional systems.

Spontaneous spin momentum locking and anomalous Hall effect in BiFeO3

The studies for the discovery of new nontrivial degrees of freedom of Bloch electrons are at the forefront of condensed matter physics. The focus remains on the spin-orbit coupling (SOC)-induced pseudospin valley state and its interplay with time inversion asymmetry. Here we propose a distinct magnetic order-protected, momentum-dependent spin-splitting valley (SSV) state in polar bulk ${\mathrm{BiFeO}}_{3}$. The octahedral distortion-induced spatial asymmetry of the superexchange interaction path breaks spin degeneracy and antiferromagnetic time-reversal symmetry. The SSV polarization is caused by crystal symmetry-related intrinsic SOC and leads to nonzero anomalous Hall conductivity (AHC) and magneto-optical effects. The anomalous Hall conductivity and magneto-optical effects are expected to be most prominent if magnetic and electric dipoles are orthogonal. Due to the ferroelectric nature of ${\mathrm{BiFeO}}_{3}$, such unconventional magneto-optical effect and AHC in fully compensated magnetic order offers straightforward tunability under an electric field.

Spin-split collinear antiferromagnets: A large-scale ab-initio study

It was recently discovered that, depending on their symmetries, collinear antiferromagnets can actually break the spin degeneracy in momentum space, even in the absence of spin-orbit coupling. Such systems, recently dubbed altermagnets, are signalled by the emergence of a spin-momentum texture set mainly by the crystal and magnetic structure, relativistic effects playing a secondary role. Here we consider all collinear q = 0 antiferromagnetic compounds in the MAGNDATA database allowing for spin-split bands. Based on density-functional calculations for the experimentally reported crystal and magnetic structures, we study more than sixty compounds and introduce numerical measures for the average momentum-space spin splitting. We highlight some compounds that are of particular interest, either due to a relatively large spin splitting, such as CoF2 and FeSO4F, or because of their low-energy electronic structure. The latter include LiFe2F6, which hosts nearly flat spin-split bands next to the Fermi energy, as well as RuO2, CrNb4S8, and CrSb, which are spin-split antiferromagnetic metals.

Tunable Valley and Spin Splittings in VSi2N4 Bilayers

The control and manipulation of the valley and spin degrees of freedom have received great interest in fundamental studies and advanced information technologies. Compared with magnetic means, it is highly desirable to realize more energy-efficient electric control of valley and spin. Using the first-principles calculations, we demonstrate tunable valley and spin degeneracy splittings in VSi2N4 bilayers, with the aid of the layered structure and associated electric control. Depending on different interlayer magnetic couplings and stacking orders, the VSi2N4 bilayers exhibit a variety of combinations of valley and spin degeneracies. Under the action of a vertical electric field, the degeneracy splittings become highly tunable for both the sign and the magnitude. As a result, a series of anomalous Hall currents can be selectively realized with varied indices of valley and spin. These intriguing features offer a practical way for designing energy-efficient devices based on the couplings between multiple electronic degrees of freedom.

A quantum-classical Liouville formalism in a preconditioned basis and its connection with phase-space surface hopping.

We revisit a recent proposal to model nonadiabatic problems with a complex-valued Hamiltonian through a phase-space surface hopping (PSSH) algorithm employing a pseudo-diabatic basis. Here, we show that such a pseudo-diabatic PSSH (PD-PSSH) ansatz is consistent with a quantum-classical Liouville equation (QCLE) that can be derived following a preconditioning process, and we demonstrate that a proper PD-PSSH algorithm is able to capture some geometric magnetic effects (whereas the standard fewest switches surface hopping approach cannot capture such effects). We also find that a preconditioned QCLE can outperform the standard QCLE in certain cases, highlighting the fact that there is no unique QCLE. Finally, we also point out that one can construct a mean-field Ehrenfest algorithm using a phase-space representation similar to what is done for PSSH. These findings would appear extremely helpful as far as understanding and simulating nonadiabatic dynamics with complex-valued Hamiltonians and/or spin degeneracy.