Spin-polarized Ground State Research Articles

First principles study of magnetism induced by topological frustration of bowtie-shaped graphene nanoflake

Fine-tuning magnetic states by understanding topological frustration inducing magnetic mechanism should allow greater flexibility for the design of graphene-based spintronics. Based on first-principles calculations, it is predicted that bowtie-shaped graphene nanoflake (GNF) is of spin-polarized ground state exhibiting antiferromagnetic (AFM) ordering between two individual triangular GNFs. It is demonstrated that strength of antiferromagnetic coupling of both symmetric and asymmetric bowtie-shaped GNF displays strong zero-energy-state-orientated behavior due to non-trivial nature of topological frustration, with implications for designing graphene nanostructures with predefined magnetic states. It also proposes a specific example of structures that can serve as nanoscale molecular logic gates composed by asymmetric bowtie-shaped GNFs units, which augment the special antiferromagnetic function through structural configuration of multi-bowtie-shaped GNFs.

Ferromagnetism of undoped anatase TiO2 based on the first-principles calculations

Compared with conventional semiconductors, the diluted magnetic semiconductors, in which the cations are substituted by transition metal ions, have attracted a great deal of attention due to their promising applications in spintronics. Recently, the unexpected room temperature ferromagnetism has been found in many undoped oxides. These findings challenge our understanding of magnetism in these systems, because neither cations nor anions have unpaired d or f electrons. Generally, the candidate defects responsible for the unexpected ferromagnetism must fulfill two conditions at the same time: (i) the defects should prefer a spin-polarized ground state with a nonzero local magnetic moments; (ii) the exchange interactions between local magnetic moments induced by defects should be ferromagnetic energetically. Among these oxides, TiO2 has recently attracted much attention because of its unique properties and potential applications in spintronics, laser diodes and biomaterials. In order to explore the origin of ferromagnetism in such an undoped TiO2 system, the electronic structures and magnetic properties of oxygen vacancy (VO) and Ti vacancy (VTi) in anatase TiO2 have been studied systematically by the first-principles calculation based on the density functional theory with the LDA+U method (UTi-3d = 5.8 eV). It is found that two electrons introduced by VO are captured by two neighbor Ti4+ ions, and thereby the Ti4+ ions are restored to Ti3+ ions with opposite spin orientation. Therefore, the single VO cannot induce local magnetic moment. The defect energy level locates near the Fermi level for VTi. Six oxygen atoms neighboring VTi constitute an octahedron, and the defect energy level is split into a single state A, a double state E and a triple state T in the octahedral crystal field. The occupation of four unpaired electrons introduced by six oxygen atoms is a+1t+3t-0e0 (subscripts + and - mean up-spin and down-spin, respectively), and the VTi can induce 4 B local moments. Furthermore, the magnetic coupling interaction between local magnetic moments induced by two VTi is ferromagnetic, and the magnetic coupling constant (JO) is 88.7 meV. It means the ferromagnetism can continue up to room-temperature. The VO cannot induce local magnetic moment, but it can enhance the coupling strength between two VTi, which can explain the origin of ferromagnetism observed experimentally in undoped anatase TiO2, i.e., the VTi induces local magnetic moment, while VO enhances the long range ferromagnetic coupling interaction between VTi. Especially, for the ferromagnetic coupling between local magnetic moments, we have proposed the second type direct exchange interaction model, which has been recommended in detail.

Investigations on Magnetic Properties of Cr-Doped LiZnAs by First-Principle Calculations

First-principle calculations were performed to investigate the electronic structures and magnetic properties of Cr-doped LiZnAs. The Cr-doped LiZnAs system with Cr concentration of 6.25 % should be nonmagnetic, while the system favors spin-polarized ground states with Cr-doped concentration up to 12.5 %. The magnetic coupling results show that Cr-doped LiZnAs prefer the ferromagnetic stable state. The ferromagnetic interactions in the Cr–Cr pair can be attributed to the p-d exchange interactions as Cr↑–As↓–Cr↑. The calculations on the formation energies of vacancies reveal that VLi requires a much lower energy than VZn and VAs. VLi, VZn, and VAs can induce ferromagnetism in Cr-doped LiZnAs.

Spontaneous appearance of a low-dimensional magnetic electron system on semiconductor nanostructures

We report on the large-scale density-functional calculations that show the emergence of the spin-polarized ground states in nanofacets self-organized on SiC (0001) surfaces. We first reveal that the nanofacet formed by bunching of single bilayer steps induces peculiar electron states localized at but extended along step edges, showing the flat-band characteristics. The electron states are of C-dangling-bond characters mixed with the back-bond character of neighboring edge Si atoms. We find that the resulting flat bands lead to the spin polarization near the step edges by H passivation of the terrace Si atoms. Interestingly, either ferromagnetic or antiferromagnetic chains appear along the step edges on the SiC nanofacet and the location of such magnetic chains can be controlled by manipulating the H passivation of atoms near the step edges. These findings open a possibility of the appearance of new magnetic functions on the covalent semiconductor surfaces without magnetic elements.

Colossal magnetoresistance in amino-functionalized graphene quantum dots at room temperature: manifestation of weak anti-localization and doorway to spintronics.

In this work, we have demonstrated the signatures of localized surface distortions and disorders in functionalized graphene quantum dots (fGQD) and consequences in magneto-transport under weak field regime (∼1 Tesla) at room temperature. Observed positive colossal magnetoresistance (MR) and its suppression is primarily explained by weak anti-localization phenomenon where competitive valley (inter and intra) dependent scattering takes place at room temperature under low magnetic field; analogous to low mobility disordered graphene samples. Furthermore, using ab-initio analysis we show that sub-lattice sensitive spin-polarized ground state exists in the GQD as a result of pz orbital asymmetry in GQD carbon atoms with amino functional groups. This spin polarized ground state is believed to help the weak anti-localization dependent magneto transport by generating more disorder and strain in a GQD lattice under applied magnetic field and lays the premise for future graphene quantum dot based spintronic applications.

Field-induced conductance switching by charge-state alternation in organometallic single-molecule junctions.

Charge transport through single molecules can be influenced by the charge and spin states of redox-active metal centres placed in the transport pathway. These intrinsic properties are usually manipulated by varying the molecule's electrochemical and magnetic environment, a procedure that requires complex setups with multiple terminals. Here we show that oxidation and reduction of organometallic compounds containing either Fe, Ru or Mo centres can solely be triggered by the electric field applied to a two-terminal molecular junction. Whereas all compounds exhibit bias-dependent hysteresis, the Mo-containing compound additionally shows an abrupt voltage-induced conductance switching, yielding high-to-low current ratios exceeding 1,000 at bias voltages of less than 1.0 V. Density functional theory calculations identify a localized, redox-active molecular orbital that is weakly coupled to the electrodes and closely aligned with the Fermi energy of the leads because of the spin-polarized ground state unique to the Mo centre. This situation provides an additional slow and incoherent hopping channel for transport, triggering a transient charging effect in the entire molecule with a strong hysteresis and large high-to-low current ratios.

Slippage in stacking of graphene nanofragments induced by spin polarization.

Spin polarization and stacking are interesting effects in complex molecular systems and are both presented in graphene-based materials. Their possible combination may provide a new perspective in understanding the intermolecular force. The nanoscale graphene structures with zigzag edges could possess spin-polarized ground states. However, the mechanical effect of spin polarization in stacking of graphene nanofragments is not clear. Here we demonstrate the displacement between two stacked rhombic graphene nanofragments induced by spin polarization, using first-principles density-functional methods. We found that, in stacking of two rhombic graphene nanofragments, a spin-polarized stacked conformation with zero total spin is energetically more favorable than the closed-shell stacking. The spin-polarized conformation gives a further horizontal interlayer displacement within 1 angstrom compared with the closed-shell structure. This result highlights that, besides the well-known phenomenologically interpreted van der Waals forces, a specific mechanism dependent on the monomeric spin polarization may lead to obvious mechanical effects in some intermolecular interactions.

Tuning magnetic splitting of zigzag graphene nanoribbons by edge functionalization with hydroxyl groups

The electronic properties and relative stability of zigzag graphene nanoribbons are studied by varying the percentage of hydroxyl radicals for edge saturation using first principle calculations. The passivated structures of zigzag graphene nanoribbon have spin-polarized ground state with antiferromagnetic exchange coupling across the edge and ferromagnetic coupling along the edges. When the edges are specially passivated by hydroxyl, the potentials of spin exchange interaction across the two edges shift accordingly, resulting into a spin-semiconductor. Varying the concentration of hydroxyl groups can alter the maximum magnetization splitting. When the percentage of asymmetrically adsorbed hydroxyl reaches 50%, the magnetization splitting can reach a value as high as 275 meV due to the asymmetrical potential across the nanoribbon edges. These results would favor spintronic device applications based on zigzag graphene nanoribbons.

Exploring d0 magnetism in doped SnO2–a first principles DFT study

In search of d0 magnetism, the magnetic behavior of SnO2 with cation substitution from group1A (Li, Na, K) and group 2A (Be, Mg, Ca) elements has been systematically studied using Density Functional Theory (DFT). While an impurity from group 1A elements switches on ferromagnetism at a lower concentration itself, sufficient hole density is required for a group 2A impurity to create a spontaneous spin polarized ground state, a finding that has not been reported in earlier investigations. Our DFT results predict for the first time that impurities from group 2A (Mg, Ca) in SnO2 can promote room temperature ferromagnetism. Further, the emergence of ferromagnetism due to doping from group 1A elements, which injects three holes per defect, has been mapped successfully onto a modified Hubbard model from the literature. Doping of a single Na atom in the supercell (concentration 6.25 at%) makes the system ferromagnetic, with a magnetic moment close to 3.0μB per defect, and a Curie temperature of 815K, obtained in the mean field approximation. This agrees closely with a model prediction of 750K. Finally, the triggering of ferromagnetism by an impurity atom from group 2A, which adds two holes per defect in the system, implies that the prescription of three holes per defect given in the literature is not a necessary criterion for hole induced ferromagnetism. Rather, the analysis of the density of states and ferromagnetic coupling indicate that the system needs a critical hole concentration to activate ferromagnetism, by pushing the Fermi level inside the valence band in order to satisfy the Stoner criterion.

Energetics, Electron States, and Magnetization in Nearly Zigzag-Edged Graphene Nano-Ribbons

Energetics, spin polarization, and electron states of graphene nano-ribbons (GNRs) are investigated by using the density functional theory. To consider realistic shapes of GNRs in nature, we focus on the nearly zigzag-edged GNRs (NZE-GNRs) which have finite-length zigzag edges connected by another short zigzag edge (kink). Through the systematic calculations, we have clarified roles of nano-shapes of NZE-GNRs in its stability, the spin polarization, and the electron states. Edge states localized near the zigzag edge, the flat bands, and the consequent spin-polarized ground states exist when NZE-GNRs have long enough zigzag edges or wide enough width. In contrast, we have found that the large kink size hinders their appearance. We have also calculated the edge formation energies of various NZE-GNRs. The relative stability of the edge is enhanced when the zigzag edge is short or the width is narrow.

Neutral collective modes in spin-polarized fractional quantum Hall states at filling factors13,25,37, and49

We determine the lowest and higher order collective modes in both spin-conserving and spin-reversed sectors by calculating energy differences of the appropriate linear combinations of different levels of composite-fermion-excitons and the fully spin-polarized ground states at filling factors $\nu=1/3$, 2/5, 3/7, and 4/9. Apart from providing the detailed study of previously reported modes that have also been observed in the experiments, we predict additional higher energy modes at different filling factors. The lowest and the next higher spin-conserving modes have equal number of "magneto-rotons" and the number is the same as the number of filled effective Landau-like levels of composite fermions. The higher energy modes at $\nu =1/3$ merge with the lowest mode at long-wavelength. The spin-conserving modes do not merge at other filling factors. Apart from showing zero-energy spin-wave mode at zero momentum, thanks to Larmor's theorem, the lowest spin-reversed modes at the ferromagnetic ground states of $\nu=2/5$, 3/7, and 4/9 display one or more "spin-rotons" at negative energies signalling the unstable fully polarized ground states at sufficiently small Zeeman energies. The high energy spin-reversed modes also have spin-rotons but at positive energies. The energies of these excitations depend on the finite width of the quantum well as the Coulomb interaction gets screened. We determine finite thickness correction to the Coulomb interaction by the standard method of local density approximation and use them to calculate the critical energies such as rotons, long-wavelength, and short-wavelength modes which are detectable in inelastic light scattering experiments.

Effects of Edge Oxidation on the Stability and Half‐Metallicity of Graphene Quantum Dots

A comprehensive first-principles theoretical study of the electronic properties and half-metallic nature of zigzag edge-oxidized graphene quantum dots (GQDs) is carried out by using density functional theory (DFT) with the screened exchange hybrid functional of Heyd, Scuseria and Ernzerhof (HSE06). The oxidation schemes include -OH, -COOH and -COO groups. We identify oxidized GQDs whose opposite spins are localized at the two zigzag edges in an antiferromagnetic-type configuration, showing a spin-polarized ground state. Oxidized GQDs are more stable than the corresponding fully hydrogenated GQDs. The partially hydroxylated and carboxylated GQDs with the same size exhibit half-metallic state under almost the same electric-field intensity whereas fully oxidized GQDs behave as spin-selective semiconductors. The electric-field intensity inducing the half metal increases with the length of the partially oxidized GQDs, ranging from M=4 to 7.

Tunable Spin-orbit Coupling and Quantum Phase Transition in a Trapped Bose-Einstein Condensate

Spin-orbit coupling (SOC), the intrinsic interaction between a particle spin and its motion, is responsible for various important phenomena, ranging from atomic fine structure to topological condensed matter physics. The recent experimental breakthrough on the realization of SOC for ultra-cold atoms provides a completely new platform for exploring spin-orbit coupled superfluid physics. However, the SOC strength in the experiment is not tunable. In this report, we propose a scheme for tuning the SOC strength through a fast and coherent modulation of the laser intensities. We show that the many-body interaction between atoms, together with the tunable SOC, can drive a quantum phase transition (QPT) from spin-balanced to spin-polarized ground states in a harmonic trapped Bose-Einstein condensate (BEC), which resembles the long-sought Dicke QPT. We characterize the QPT using the periods of collective oscillations of the BEC, which show pronounced peaks and damping around the quantum critical point.

Fluorine-induced local magnetic moment in graphene: A hybrid DFT study

Recent experimental evidence that fluorinated graphene creates local magnetic moments around F adatoms has not been supported by semilocal density-functional theory (DFT) calculations where the adsorption of an isolated F adatom induces no magnetic moment in graphene. Here, we show that such an incorrect prediction of the nonmagnetic ground state is due to the self-interaction error inherent in semilocal exchange-correlation functionals. The present hybrid DFT calculation for an isolated F adatom on graphene predicts not only a spin-polarized ground state with a spin moment of 1 bohr magneton, but also a long-range spin polarization caused by the bipartite nature of the graphene lattice as well as the induced spin polarization of the graphene states. The results provide support for the experimental observations of local magnetic moments in fluorinated graphene.

First-principles analysis of ferromagnetism in hexagonal and rhombohedral BN

In view of experimental reports of ferromagnetic behaviors in the as-prepared defective BN, the effect of B and N vacancies on the magnetic properties and electronic structure of h-BN and r-BN are investigated using first-principles calculations within the generalized gradient approximation (GGA) and GGA+U scheme. The B and N vacancy induces a set of spin-polarized defect states above the valence band and below the conduction band, respectively, resulting in the formation of local magnetic moment. It is noticed that the U correction for 2p electrons plays important role in obtaining spin-polarized ground states of the h-BN and r-BN systems with B vacancy. Furthermore, the magnetic coupling between the moments induced by two B vacancies is long-range ferromagnetic and the coupling can be attributed to the p–p hybridization interaction involving holes, whereas the coupling between the moments induced by two N vacancies is anti-ferromagnetic. The calculated results suggest that the observed d0 ferromagnetism in the as-prepared defective BN should be attributed to the B vacancies.

Spin-polarized ground states and ferromagnetic order induced by low-coordinated surface atoms and defects in nanoscale magnesium oxide

We investigate the effect of surface defects and the related low-coordinated surface atoms on the defect-induced magnetism in MgO nanocrystallites using hybrid density functional theory calculations. It has been demonstrated that when Mg vacancies are introduced at the surface or near surface of cubelike MgO clusters, a magnetic state ($S$ $\ensuremath{\ge}$ 1) becomes lower in total energy than the nonmagnetic singlet state ($S$ = 0) by several electron volts, resulting in the robust spin-polarized ground state. The total spin $S$ of the clusters in their ground state is equal to the number of the surface Mg vacancies introduced. The resulting spin density is not only located at the surrounding O atoms neighbor to the Mg vacancy site but is also extended to the low-coordinated surface O atoms along the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction, forming ferromagneticlike domains. This directional spin delocalization allows a remote (\ensuremath{\sim}1 nm or longer) vacancy-vacancy interaction, eventually leading to a long-range ferromagnetic interaction.

Structural, electronic and dynamical stability of heavy metal iron pernitride: a spin polarized first-principles study

First principles calculations using ab initio density functional theory using the ultrasoft pseudopotential method and generalized gradient approximations (GGA) have been carried out for heavy metal iron-pernitride (FeN2). We have performed both non-spin and spin polarized calculations and found that the spin polarized ground state is energetically more favourable with a magnetic moment of 1.86 μ B. The calculated electronic band structure, density of states and contour plot suggest that natural iron-pernitride is metallic. The high bulk modulus of FeN2 confirms that the FeN2 has lower compressibility and high hardness. The present values of the bulk modulus, N-N bond length, magnetic moment and optimized structure parameters agree with the previous studies. A detailed analysis of the phonon dispersion curves allows us to conclude that the spin polarized phonon dispersion curves contain positive frequencies throughout the Brillouin zone and hence confirms the dynamical stability. The eigen displacements of Raman and infrared active phonon modes at the zone centre for FeN2 are discussed in detail. The total and partial phonon density of states are also reported along with lattice specific heat and Debye temperature.

Electronic structure and spin-polarization of boron-nitride nanoflake

Boron-nitride graphene-like monolayer possesses a similar atomic arrangement to that of the famous graphene. However, due to the large difference in electronegetivity between boron and nitrogen atoms, the electronic properties of the two nanomaterials are different significantly. Here, we report on our theoretical investigation of the electronic structure and spin-polarization of zigzag-edged boron-nitride triangular nanoflake using a Hubbard model and the first-principles calculations within density-functional theory. Our numerical results indicate that in contrast to graphene nanoflake with spin-polarized ground state, the boron-nitride nanoflak has the zero-energy state that is either empty or fully occupied, and its ground state is thus spin-unpolarized which breaks the Lieb's law. However, the electron occupation and spin-polarization of the zero-energy state of boron-nitride nanoflake can be tuned by doping it with electrons or holes. These results are expected to offer the theoretical basis for the applications of boron-nitride nanomaterials in spintronics.

Magnetic instability and pair binding in aromatic hydrocarbon superconductors.

Understanding magnetism and electron correlation in many unconventional superconductors is essential to explore mechanism of superconductivity. In this work, we perform a systematic numerical study of the magnetic and pair binding properties in recently discovered polycyclic aromatic hydrocarbon (PAH) superconductors including alkali-metal-doped picene, coronene, phenanthrene, and dibenzopentacene. The π-electrons on the carbon atoms of a single molecule are modelled by the one-orbital Hubbard model, and the energy difference between carbon atoms with and without hydrogen bonds is taking into account. We demonstrate that the spin polarized ground state is realized for charged molecules in the physical parameter regions, which provides a reasonable explanation of local spins observed in PAHs. In alkali-metal-doped dibenzopentacene, our results show that electron correlation may produce an effective attraction between electrons for the charged molecule with one or three added electrons.

Spectroscopic evidence for spin-polarized edge states in graphitic Si nanowires

The step edges on the Si(553)–Au surface undergo a 1 × 3 reconstruction at low temperature which has recently been interpreted theoretically as the ×3 ordering of spin-polarized silicon atoms at the edges of the graphitic Si nanowires on this vicinal surface. This predicted magnetic ground state has a clear spectroscopic signature—a silicon step-edge state at 0.5 eV above the Fermi level—that arises from strong exchange splitting and hence would not occur without spin polarization. Here we report spatially resolved scanning tunneling spectroscopy data for these nanowires. At low temperature we find an unoccupied state at 0.5 eV above every third step-edge silicon atom, in excellent agreement with the spin-polarized ground state predicted theoretically. This spin-polarized state survives up to room temperature where the position of the spins rapidly fluctuates among all Si step-edge sites.