Spin Degeneracy Research Articles

Electrically controlled spin polarization in suspended GaAs quantum point contacts

We report on the observation of the lateral electric spin polarization effect in a suspended GaAs-based quantum point contact (QPC) separated from the substrate. The effect manifests itself in the experiment as the appearance of an additional half-integer plateaus at 0.5×2e2/h when the asymmetric voltage is applied to the side gates in zero magnetic field. The appearance of the plateaus has been attributed to the spin degeneracy lifting caused by the spin-orbit coupling associated with the lateral electric field in the asymmetrically biased QPC. We have experimentally demonstrated that, despite the relatively small g-factor in GaAs, the observation of the spin polarization in the GaAs-based QPCs becomes possible after the suspension due to the enhancement of the electron-electron interaction and the effect of the electric field guiding. These features are caused by a partial confinement of the electric field lines within a suspended semiconductor layer with a high dielectric constant.

Dephasing of InAs quantum dot p-shell excitons studied using two-dimensional coherent spectroscopy.

The dephasing mechanisms of p-shell and s-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of s-shell excitons is lifetime limited, whereas p-shell excitons exhibit significant pure dephasing due to scattering between degenerate spin states. At elevated temperatures, quadratic exciton-phonon coupling plays an important role in both s-shell and p-shell exciton dephasing. We show that multiple p-shell states are also responsible for stronger phonon dephasing for these transitions.

Transition between Electron Localization and Antilocalization and Manifestation of the Berry Phase in Graphene on a SiC Surface

It is shown that the transport properties of graphitized silicon carbide are controlled by a surface graphene layer heavily doped with electrons. In weak magnetic fields and at low temperatures, a negative magnetoresistance is observed due to weak localization. A crossover in the magnetoresistance from weak localization to weak antilocalization (the latter is the manifestation of the isospin in graphene) is observed for the first time in samples of this kind at elevated temperatures. A pronounced pattern of Shubnikov–de Haas oscillations is observed in strong magnetic fields (up to 30 T). This pattern demonstrated fourfold carrier spectrum degeneracy due to the double spin and double valley degeneracies. Also, the manifestation of the Berry phase is observed. The effective electron mass is estimated to be m* = 0.08m0, which is characteristic of graphene with a high carrier concentration.

Spin–orbit interaction effects in ZnO/ZnS core–shell and ZnS/ZnO inverted core–shell quantum dots

Rashba and Dresselhaus spin–orbit interactions (SOIs), external fields and dielectric environment effects on hydrogenic impurity states in core–shell (ZnO/ZnS) and inverted core–shell (ZnS/ZnO) quantum dots were studied. We employed the finite difference method to achieve energy eigenvalues of the system. Our results demonstrate that SOIs, external fields and dielectric environment change drastically the impurity states. Besides, the spin degeneracy is removed when both the SOIs and the Zeeman effect are simultaneously considered.

Magnonic Floquet Hofstadter butterfly

We introduce the magnonic Floquet Hofstadter butterfly in the two-dimensional (2D) insulating honeycomb ferromagnet using a combination of spin wave theory and quantum field theory. We show that hopping magnetic dipole moment (i.e. magnon quasiparticles) in the 2D insulating honeycomb ferromagnet accumulates the Aharonov–Casher phase when irradiated by an oscillating space- and time-dependent electric field. We study two different cases. In the first case, we consider the effects of only space-dependent electric field. In this case, we realize the magnonic Hofstadter spectrum with similar fractal structure as graphene subject to a perpendicular magnetic field, but with no spin degeneracy due to broken time-reversal symmetry by the ferromagnetic order. In addition, the magnonic Dirac points and Landau levels occur at finite energy as expected in a bosonic system. Remarkably, this discrepancy does not affect the topological invariant of the system. Consequently, the magnonic Chern number assumes odd values and the magnon Hall conductance gets quantized by odd integers. In the second case, we study the effects of both space- and time-dependent electric field. In this case, the theoretical framework is studied by the Floquet formalism. We show that the magnonic Floquet Hofstadter spectrum emerges entirely from the oscillating space- and time-dependent electric field, which is in stark contrast to electronic Floquet Hofstadter spectrum, where irradiation by circularly polarized light and a perpendicular magnetic field are applied independently. We study the deformation of the fractal structure at different laser frequencies and amplitudes, and analyze the topological phase transitions associated with gap openings in the magnonic Floquet Hofstadter butterfly.

Suppressed and enhanced spin polarization in the 1ML-Pb/Ge(1 1 1)-1 [formula omitted] 1 system

It is shown, based on the density functional theory (DFT), that the spin splitting of surface electronic states induced by a Pb monolayer on the Ge(111)-1 × 1 surface depends on the bonding structure of the Pb/Ge interface and may be either significant, ∼0.8 eV, (H3/T4) or completely suppressed (T1). It is also shown that for the former the bonding configuration facilitates an emergence of unquenched orbital angular momentum L along Γ-K-M direction, which then lifts the spin degeneracy of the electronic states through the spin-orbit coupling (SOC) of electrons in Pb. Along with the DFT data, we present a complementary analysis conducted withing the orbital Rashba framework, which provides an additional insight into the SOC-induced splitting.

Electronic structure calculation of vanadium‐and scandium‐based endohedral fullerenes VSc2N@C2n (2n = 70, 76, 78, 80)

AbstractWe present candidate structures for the most stable isomers for the VSc2N@C70, VSc2N@C76, VSc2N@C78, and VSc2N@C80 using a systematic procedure that involves all possible isomers of the host fullerene cages. Subsequently, a detailed investigation of structural and electronic properties of the lowest energy isomers is performed using density functional theory in combination with large polarized Gaussian basis sets. The search correctly identifies the experimentally observed VSc2N@C80 isomer as the most stable structure. The structural analysis shows that only VSc2N@C70 has a non‐IPR cage among the four endohedral fullerenes. Respectively, VSc2N@C70 and VSc2N@C76 have nearly degenerate spin states with total spin S = 0 and S = 1. All the lowest energy cages are energetically stable and show significant electron accepting capacity comparable to C60.

Scalar charge of black holes in Einstein-Maxwell-dilaton theory

We show that the monopole scalar charge of black holes in Einstein--Maxwell--dilaton theory is proportional to the electric potential at the event horizon, with a proportionality factor given by (minus) the scalar coupling constant. We also show that the scalar charge, in the weak electric charge limit, does not depend on the black hole spin. This result can be very useful to circumvent spin degeneracy issues when testing the theory against gravitational waves observations.

Electric Control of the Edge Magnetization in Zigzag Stanene Nanoribbons from First Principles

There has been tremendous interest in manipulating electron and hole-spin states in low-dimensional structures for electronic and spintronic applications. We study the edge magnetic coupling and anisotropy in zigzag stanene nanoribbons, by first-principles calculations. Taking into account considerable spin-orbit coupling and ferromagnetism at each edge, zigzag stanene nanoribbon is insulating and its band gap depends on the inter-edge magnetic coupling and the magnetization direction. Especially for nanoribbon edges with out-of-plane antiferromagnetic coupling, two non-degenerate valleys of edge states emerge and the spin degeneracy is tunable by a transverse electric field, which give full play to spin and valley degrees of freedom. More importantly, both the magnetic order and anisotropy can be selectively controlled by electron and hole doping, demonstrating a readily accessible gate-induced modulation of magnetism. These intriguing features offer a practical avenue for designing energy-efficient devices based on multiple degrees of freedom of electron and magneto-electric couplings.

Attractive and driven interactions in quantum dots: Mechanisms for geometric pumping

We analyze time-dependent transport through a quantum dot with electron-electron interaction that is statically tunable to both repulsive and attractive regimes, or even dynamically driven. Motivated by the recent experimental realization [Hamo et. al, Nature 535, 395 (2016)] of such a system in a static double quantum dot we compute the geometric pumping of charge in the limit of weak tunneling, high temperature and slow driving. We analyze the pumping responses for all pairs of driving parameters (gate voltage, bias voltage, tunnel coupling, electron-electron interaction). We show that the responses are analytically related when these different driving protocols are governed by the same pumping mechanism, which is characterized by effective driving parameters that differ from the experimental ones. For static attractive interaction we find a characteristic pumping resonance despite the 'attractive Coulomb blockade' that hinders stationary transport. Moreover, we identify a pumping mechanism that is unique to driving of the interaction. Finally, although a single-dot model with orbital pseudo-spin describes most of the physics of the mentioned experimental setup, it is crucial to account for the additional (real-) spin degeneracies of the double dot and the associated electron-hole symmetry breaking. This is necessary because the pumping response is more sensitive than DC transport measurements and detects this difference through pronounced qualitative effects.

Magnetization on nitrogen in extended honeycomb carbon layers from first principles: Case studies of CxN (x = 2, 6, 12)

Identification of unusual onset of spin polarization of N(p) states is reported. Full saturation up to 3μB (Bohr Magnetons) is demonstrated within extended two-dimensional carbon networks of CxN (x = 2, 6, 12) hexagonal structures refined based on density functional theory calculations. The cohesive energy is found to increase along the extension of the carbon network which opens perspective to experimental investigations through reactive deposition of thin films. From establishing the energy-volume equations of states of the two carbon rich nitrides done with assuming spin degenerate (non spin polarized) and spin-polarized configurations, the ground state is identified as ferromagnetic in both C6N and C12N. The electronic density of states plots show vanishing intensities at the Fermi level, whence the integer magnetizations. The variation of magnetization with volume points to strongly ferromagnetic behavior.

Revised model for thermopower and site inversion in Co3O4 spinel

In recent years, the fundamental understanding of thermopower in strongly correlated systems as it relates to spin and orbital degeneracy has advanced, but the consequences for determining cation distribution have not been considered. In this work we compare measurements of the electrical conductivity and thermopower in ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ spinel with different models for the thermopower based on different assumptions of the cation distributions. Using thermopower measurements we calculate the in situ cation distribution according to three different models: case (i) the original Heikes equation, case (ii) Koshibae and co-workers' modified Heikes equation, and case (iii) a new model, using the modified Heikes equation but with contributions from both octahedral and tetrahedral sites. We find that only the modified Heikes equation that includes contributions from both octahedral and tetrahedral sites satisfies the constraints of stoichiometry in the spinel structure. The findings suggest either complete $(\ensuremath{\sim}100%)$ inversion of the spinel structure if no change in spin state, or a combination of a minimum 40% inversion with a change in spin state of the octahedral ${\mathrm{Co}}^{3+}$ cation.

Half-metallicity in zigzag phosphorene nanoribbons with magnetic edges

Phosphorene nanoribbons with magnetic edges are potentially attractive for nanoelectronic and spintronic applications. Here, the attention is focused on electronic and magnetic properties of these materials. Using a self-consistent computational method it is shown that, in general, two types of configuration with spontaneous edge magnetization can be responsible for lifting the spin degeneracy, and consequently for the appearance of the half-metallic behavior. Namely, in addition to the well-known configuration with edge states ferromagnetically coupled along each edge and aligned in parallel between the two edges, there is another one (much less explored) with edge states ferromagnetically aligned on only one edge. The ground state magnetic configurations strongly depend on the chemical potential position (electron filling factor), and can be controlled by means of the external perpendicular gate voltage, and by changing the nanoribbon width. It is also shown that the magnetic edge configurations responsible for the half-metallicity can be stabilized with ferromagnetic substrates, due to the proximity effect.

Spin Splitting in Quasi‐One Dimensional Systems

Spin Splitting in Quasi‐One Dimensional Systems

Magnetotransport and lateral confinement in an InSe van der Waals Heterostructure

In the last six years, indium selenide (InSe) has appeared as a new van der Waals heterostructure platform which has been extensively studied due to its unique electronic and optical properties. Such as transition metal dichalcogenides (TMDCs), the considerable bandgap and high electron mobility can provide a potential optoelectronic application. Here we present low-temperature transport measurements on a few-layer InSe van der Waals heterostructure with graphene-gated contacts. For high magnetic fields, we observe magnetoresistance minima at even filling factors related to two-fold spin degeneracy. By electrostatic gating with negatively biased split gates, a one-dimensional channel is realized. Close to pinch-off, transport through the constriction is dominated by localized states with charging energies ranging from 2 to 5 meV. This work opens new possibility to explore the low-dimensional physics including quantum point contact and quantum dot.

Molecular Engineering of High Energy Barrier in Single-Molecule Magnets Based on [MoIII(CN)7]4− and V(II) Complexes

Molecular engineering of high energy barrier Ueff in single-molecule magnets (SMMs) of general composition MoIIIkVIIm based on orbitally-degenerate pentagonal-bipyramidal [MoIII(CN)7]4− complexes with unquenched orbital momentum and high-spin V(II) complexes is discussed. In these SMMs, the barrier originates exclusively from anisotropic Ising-type exchange interactions −Jxy(SixSjx + SiySjy) − JzSizSjz in the apical cyano-bridged pairs MoIII–CN–VII, which produce a double-well energy profile with a doubly degenerate ground spin state ±MS. It is shown that the spin-reversal barrier Ueff is controlled by anisotropic exchange parameters Jz, Jxy, and the number n of apical MoIII–CN–VII groups in a SMM cluster, Ueff ~ 0.5|Jz − Jxy|n; it can reach a value of many hundreds of wavenumbers (up to 741 cm−1). This finding provides a very efficient straightforward strategy for further scaling Ueff to high values (>1000 cm−1) by means of enhancing exchange parameters Jz, Jxy, and increasing the number of [MoIII(CN)7]4− complexes in a SMM molecule.

Dynamic nuclear polarization at high Landau levels in a quantum point contact

We demonstrate a way to polarize and detect nuclear spin in a gate-defined quantum point contact operating at high Landau levels. Resistively-detected Nuclear Magnetic Resonance (RDNMR) can be achieved up to the $5$th Landau level and at a magnetic field lower than $1$ T. We are able to retain the RDNMR signals in a condition where the spin degeneracy of the first 1D subband is still preserved. Furthermore, the effects of orbital motion on the first 1D subband can be made smaller than those due to electrostatic confinement. This developed RDNMR technique is a promising means to study electronic states in a quantum point contact near zero magnetic field.

Optical properties of uniaxially strained graphene on transition metal dichalcogenide substrate

The uniaxially strained graphene monolayer on transition metal dichalcogenide (GrTMD) substrate, constituting a van der Waals heterostructure (vdWH), is found to possess unusual intra-band plasmon dispersion ([Formula: see text]) with stronger incarceration compared to that of a standalone, doped graphene for finite doping in the long wavelength limit. The intra-band absorbance of GrTMD is found to be an increasing (decreasing) function of the strain field (frequency) at a given frequency (strain field). It is also observed that whereas the strain field is responsible for the valley polarization, a Rashba coupling-dependent pseudo Zeeman term arising due to the interplay of substrate-induced interactions is found to bring about the spin degeneracy lifting and the gate voltage tunable spin polarization. The latter turns out to be inversely proportional to the square root of the carrier concentration.

Double line groups: structure, irreducible representations and spin splitting of the bands

Double line groups are derived, structurally examined and classified within 13 infinite families. Their irreducible representations, found and tabulated, single out the complete set of conserved quantum numbers in fermionic quasi-one-dimensional systems possessing either translational periodicity or incommensurate helical symmetry. Spin–orbit interaction is analyzed: the induced orbital band splitting and the consequent removal of the spin degeneracy are completely explained. Being incompatible with vertical mirror symmetry, as well as with simultaneous invariance under time-reversal and horizontal (roto)reflections, spin splitting and spin polarized currents may occur only in the systems with the first and the fifth family double line group symmetry. The effects are illustrated on carbon nanotubes.

Effects of Zeeman splitting on spin transportation in a three-terminal Rashba ring under a weak magnetic field

Spin-dependent transport in one-dimensional (1D) three-terminal Rashba rings is investigated under a weak magnetic field, and we focus on the Zeeman splitting (ZS) effect. For this purpose, the interaction between the electron spin and the weak magnetic field has been treated by perturbation theory. ZS removes the spin degeneracy, and breaks both the time reversal symmetry and the spin reversal symmetry of the ring system. Consequently, all conductance zeros are lifted and turned into conductance dips. Aharonov-Bohm (AB) oscillations can be found in both branch conductances and the total conductance as a function of the magnetic field. In a relatively high magnetic field, the decoherence caused by ZS decreases the amplitude of the branch conductance and increases that of the total conductance. The results have been compared with those reported in the published literature, and a reasonable agreement is obtained. The conductance as a function of the Rashba spin-orbit coupling (RSOC) strength has also been investigated. As the RSOC strength increases, the role of ZS becomes weaker and weaker; ZS can even be neglected when B ≤ 0.1 T.