Charge Spin Density Research Articles

Graphene Nanocomposites Templated from Cage-Containing Metal-Organic Frameworks for Oxygen Reduction in Li-O2 Batteries

Recently, the physical and chemical properties of carbon, such as crystal structure, morphology, and defect chemistry, have been demonstrated to significantly impact the ORR activity of carbon-based cathode in Li-O2 batteries.(1, 2) Compared to other carbon catalysts studied, transition metal based nitrogen-doped nanocarbon materials (e.g., graphene, tube, onion-like carbon) have been known as the most promising class of non-precious metal ORR catalysts in various electrolytes for electrochemical energy conversion and storage devices,(3-7) including nonaqueous Li-O2 batteries.(8) When nitrogen atoms are doped into carbon lattices, electronic and geometric structures of carbon will be significantly modified. This doping process leads to non-uniform distribution of the spin and atomic charge density, which is very important to oxygen adsorption and activity enhancement.(9) Meanwhile, the resulting defects and disorders can become active sites to adsorb O2 and Li+, increasing the density of reaction sites and thus enhancing the discharge voltage as well as capacity of Li-O2 batteries.(10) Recently, i n situ formed nitrogen-doped graphene nanostructures were derived from heteroatom polymers (polyaniline, a nitrogen/carbon precursor) via a high-temperature approach and are found to be crucial for the ORR activity enhancement in Li+-O2 nonaqueous electrolyte.(8) These highly graphitized nanocarbons (e.g., nanotube, fiber, onion-like carbon, and graphene) seem to provide a robust matrix for hosting the active sites.(11) The catalyst activity and durability have been directly linked to the carbon morphology and structures in situ formed during the catalyst synthesis via high-temperature treatments. Here, in an effort to develop high-performance ORR catalysts, we have been interested in investigating the templating effect of cage-containing metal-organic frameworks (MOFs) on the formation of novel nanocarbon composites and on the resulting ORR activity in Li-O2 batteries. Due to the accessibility of metal-cation centers, the variety of building blocks, and the high micropore surface area, MOF is able to offer unique opportunities in synthesizing efficient ORR catalysts.(12) However, to date, the studied MOFs as precursors for catalyst synthesis are limited to commercially available microporous ones. The use of specially designed MOFs with unique structures for preparing high-performance ORR catalysts remains unexplored. In this work, we report the first demonstration of a newly-developed Co containing MOF material with giant polyhedral cages (cage size ~1.8 nm) as a template in preparation of transition metal-nitrogen-carbon ORR catalysts, especially for nonaqueous Li-O2 battery cathodes. As a result, a Fe-based catalyst dominant by nitrogen-doped graphene/graphene tube (tubular graphene) nanocomposites was prepared through heat-treating a novel nitrogen/carbon precursor dicyandiamide (DCDA) in the presence of iron species. The in situ formation of nitrogen-doped graphene/graphene tube templated by the MOF may provide a new route for preparation of carbon nanocomposites for electrochemical energy applications. 1. R. R. Mitchell, B. M. Gallant, C. V. Thompson and Y. Shao-Horn, Energ Environ Sci, 4, 2952 (2011). 2. J. Xiao, D. H. Mei, X. L. Li, W. Xu, D. Y. Wang, G. L. Graff, W. D. Bennett, Z. M. Nie, L. V. Saraf, I. A. Aksay, J. Liu and J. G. Zhang, Nano Lett., 11, 5071 (2011). 3. G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 332, 443 (2011). 4. Z. H. Wen, S. Q. Ci, F. Zhang, X. L. Feng, S. M. Cui, S. Mao, S. L. Luo, Z. He and J. H. Chen, Adv. Mater., 24, 1399 (2012). 5. Y. Y. Shao, F. Ding, J. Xiao, J. Zhang, W. Xu, S. Park, J. G. Zhang, Y. Wang and J. Liu, Adv. Func. Mater., 23, 987 (2013). 6. M. Lefevre, E. Proietti, F. Jaouen and J. P. Dodelet, Science, 324, 71 (2009). 7. G. Wu, M. Nelson, S. Ma, H. Meng, G. Cui and P. K. Shen, Carbon, 49, 3972 (2011). 8. G. Wu, N. H. Mack, W. Gao, S. G. Ma, R. Q. Zhong, J. T. Han, J. K. Baldwin and P. Zelenay, ACS Nano, 6, 9764 (2012). 9. M. D. Esrafili, Comp. Theor. Chem., 1015, 1 (2013). 10. Y. L. Li, J. J. Wang, X. F. Li, J. Liu, D. S. Geng, J. L. Yang, R. Y. Li and X. L. Sun, Electrochem. Commun., 13, 668 (2011). 11. G. Wu and P. Zelenay, Acc. Chem. Res., 46, 1878 (2013). 12. A. Morozan and F. Jaouen, Energ Environ Sci, 5, 9269 (2012). Figure 1

First principles study of structural, elastic, electronic and magnetic properties of Mn-doped AlY (Y=N, P, As) compounds

We investigate zinc-blende phase Al0.75Mn0.25Y (Y=N, P, As) compounds using full-potential linear-augmented-plane wave plus local-orbital method. For computing structural and elastic properties the Generalized Gradient Approximation (GGA) has been used; whereas the electronic and magnetic properties are examined at the optimized GGA lattice parameters by employing modified Becke and Johnson local density approximation. All these compounds are found to be stable in ferromagnetic ordering in the zinc-blende structure which is supported by the computed elastic constants. The nature of electronic band structure are calculated and the nature of band gaps in the doped system is analyzed. The results are examined to identify exchange mechanism which is the main source of introducing ferromagnetism in the compounds under investigation. Spin charge density contour plots in the (110) plane and the evaluation of s–p and p–d exchange constants (N0α and N0β) are evaluated for understanding bonding and exchange splitting process, respectively.

Photoinduced hole injection into a self-assembled π-extended G-quadruplex.

We have prepared a G-quadruplex (GQ-1) that incorporates an 8-(4'-aminophenylethynyl)guanine (GEAn) electron donor covalently attached to a 4-aminonaphthalene-1,8-imide (ANI) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) electron acceptor (GEAn-ANI-NDI, 1). In the presence of KPF6 in tetrahydrofuran (THF), 1 self-assembles into a monodisperse, C4-symmetric GQ-1 with small spatial intraquadruplex overlap between the ANI-NDI units. Photoexcitation of monomeric 1 induces the two-step charge transfer GEAn-(1)*ANI-NDI → GEAn(+•)-ANI(-•)-NDI → GEAn(+•)-ANI-NDI(-•) that occurs in τ(CS1) = 5 ps and τ(CS2) = 330 ps, respectively, while charge recombination in ca. 300 ns. Sharpening of the GEAn(+•) transient absorption and a shift of the ethynyl vibrational frequency in 1 were observed, concomitant with the stepwise electron transfer from ANI(-•) to NDI. Formation of GQ-1 from 1 in THF increases the secondary charge-shifting rate (τ(CS2) = 110 ps) and results in no change in ethynyl vibrational frequency. Charge recombination in GQ-1 is slowed by enhanced radical-pair intersystem crossing driven by the greater number of hyperfine couplings in the assembly. Moreover, time-resolved EPR spectroscopy shows that the spin-spin-exchange interaction (J) between the radicals of GEAn(+•)-ANI-NDI(-•) within GQ-1 is smaller than that of 1, suggesting that the spin (charge) density in GEAn(+•) is more dispersed in GQ-1. The spectroscopic results are consistent with hole sharing among the guanines within the G-quadruplex that is kinetically competitive with the formation of GEAn(+•). This suggests that G-quadruplexes can serve as effective hole conduits in ordered donor-acceptor assemblies.

Tailoring the structural and magnetic properties of Cu-doped ZnO by ±bc-axis pressure**Project supported by the National Natural Science Foundation of China (Grant Nos. 51031004 and 51272078), the Natural Science Foundation of Guangdong, China (Grant No. S2012010008124), the National Basic Research Program of China (Grant No. 2015CB921202), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014),

The structural and magnetic properties of the Cu-doped ZnO (ZnO:Cu) under c-axis pressure were studied using first-principle calculations. It was found that the ZnO:Cu undergoes a structural transition from Wurtzite to Graphite-like structure at a c-axis pressure of 7–8 GPa. This is accompanied by an apparent loss of ferromagnetic stability, indicating a magnetic transformation from a ferromagnetic state to a paramagnetic-like state. Further studies revealed that the magnetic instability is closely related to the variation in crystalline field originated from the structural transition, which is in association with the overlapping of spin–charge density between the Cu2+ and adjacent O2-.

Mapping of the DFT Spin Configuration Energies of Cr8Cd Molecular Ring onto the Energy Structure of Falicov-Kimball Model

A comprehensive study of electronic and magnetic properties of a recently synthesized nonametalic chromiumbased heteronuclear molecule Cr8CdF9(O2C − C(CH3)3)18 is presented, using DFT and Falicov Kimball (FK) model approach. The magnetic moments are calculated and the spin charge density map is discussed. The exchange coupling parameters between transition metals ions are extracted, taking into account all the nonequivalent spin con gurations. It is demonstrated that the energies of the spin con gurations can be reproduced by the FK model with a set of parameters consistent with that for the parent Cr8 molecule. For molecular ring considered, the ground state corresponds to the antiferromagnetic con guration and the ferromagnetic con guration yields the highest energy.

Mechanical control of magnetism in oxygen deficient perovskite SrTiO3.

Mechanical control of magnetism in perovskite oxides is an important and promising approach in spintronics. Based on the first-principles calculations, we demonstrate that a negative pressure leads to a great enhancement of magnetic moment in deficient SrTiO3 with oxygen vacancies, whereas a positive pressure results in the gradual disappearance of magnetism. Spin charge density, Bader charge analysis and electronic density of states successfully elucidate the origin and underlying physics of the enhancement and disappearance of magnetism. It is found that the split electronic states of dz(2), dyz and dzx in the 3d orbitals of Ti atoms remarkably contribute to the occupancy of majority spin states under negative pressure, which induces a large magnetic moment. Under positive pressure, however, the equal occupancy of both majority and minority t2g and eg states leads to the disappearance of magnetization. In addition, both negative and positive pressures can largely lower the vacancy formation enthalpy, suggesting that the oxygen vacancy is preferable with pressure. Our findings may provide a mechanism to achieve the pressure control of magnetization in nonmagnetic perovskite oxides.

Anomalous Influence of an External Magnetic Field on the Electron Energy Spectrum of Ferromagnetic Semiconductors with Strong p,d-hybridization

We investigate the influence of magnetic field on the optical conductivity and electron spectra of lanthanum manganites, which are used as magneto-optical switches. It has been shown that a narrow band of heavy fermions appears in the gap. The chemical potential is located in this band due to hole doping. For this reason, heavy fermions are responsible for the magnetism, optical properties and charge transfer. The scattering of heavy fermions by spin fluctuations and charge density fluctuations induced by them, lead to the Anderson localization. This effect determines the anomalous relationship between the magnetic and electron subsystems. The formulated model explains the observation and magnetorefractive magnetoresistive effects for identical values of temperatures and magnetic fields.

Spin imbalance in hybrid superconducting structures with spin-active interfaces

We consider a heterostructure consisting of a normal metal and a superconductor separated by a spin-active interface. At finite-bias voltages, spin-filtering and spin-mixing effects at the interface allow for an induced magnetization (spin imbalance) on the superconducting side of the junction, which relaxes to zero in the bulk. Such interfaces are also known to host a pair of in-gap Andreev bound states which were recently observed experimentally. We show that these states are responsible for the dominant contribution to the induced spin imbalance close to the interface. Motivated by recent experiments on spin-charge density separation in superconducting aluminum wires, we propose an alternative way to observe spin imbalance without applying an external magnetic field. We also suggest that the peculiar dependence of the spin imbalance on the applied bias voltage permits an indirect bound-state spectroscopy.

The Study on Zero-Temperature Gap of Superconductor Having the Coexistence of SDW and CDW

In the field of superconductivity the most interesting area is the studying of the mechanism of the high Tc superconductor (HTS). Understanding of this mechanism, we can raise the critical temperature (Tc). There are many proposals theoretical descriptions for HTS. We focus on the spin density wave (SDW) and charge density wave (CDW), the static wave of spin density and charge density arising from the nesting property of Fermi surface that were observed in the superconductor. The model Hamiltonian of superconductor having SDW and CDW coexistence was studied by Greens function method. The analytic expression of zero-temperature gap of superconductor with the coexistence of the SDW and CDW were derived. By varying the effective potential, the phase diagram of the SDW and CDW was constructed then the band structure and density of states (DOS) of coexistent phase were also studied.

Charge- and spin-density waves observed through their spatial fluctuations by coherent and simultaneous x-ray diffraction

Spatial uctuations of spin density wave (SDW) and charge density wave (CDW) in chromium have been compared by combining coherent and simultaneous X-ray diffraction experiments. Despite their close relationship, spatial fluctuations of the spin and of the charge density waves display a very different behavior: the satellite reflection associated to the charge density displays speckles while the spin one displays an impressive long-range order. This observation is hardly compatible with the commonly accepted magneto-elastic origin of CDW in chromium and is more consistent with a purely electronic scenario where CDW is the second harmonic of SDW. A BCS model taking into account a second order nesting predicts correctly the existence of a CDW and explains why the CDW is more sensitive to punctual defects.

Local magnetic properties of $$L\times L$$ L × L square lattice with an Extended Hubbard Model

Within an analytical diagonalization, it is possible to exactly solve a large class of models useful for the study of strongly correlated electrons systems. Here, we perform an exact diagonalization of the two-dimensional Extended Hubbard Model with first nearest neighbor repulsive $$V$$ and spin-spin interaction $$J$$ at one-eight filling. The analysis of the ground state energy and spin gap behaviors as a function of $$V$$ and $$J$$ terms allows us to study some local magnetic properties of $$2D$$ ferromagnetic correlated electrons system distributed in finite square lattice. The obtained results show the existence of competition between Spin Density Wave and Charge Density Wave with the possibility to have a phase transition between these two waves at a critical value $$V_{c}$$

Topological defects of Néel order and Kondo singlet formation for the Kondo-Heisenberg model on a honeycomb lattice

Heavy fermion systems represent a prototypical setting to study magnetic quantum phase transitions. A particular focus has been on the physics of Kondo destruction, which captures quantum criticality beyond the Landau framework of order-parameter fluctuations. In this context, we study the spin one-half Kondo-Heisenberg model on a honeycomb lattice at half filling. The problem is approached from the Kondo destroyed, antiferromagnetically ordered insulating phase. We describe the local moments in terms of a coarse grained quantum non-linear sigma model, and show that the skyrmion defects of the antiferromagnetic order parameter host a number of competing order parameters. In addition to the spin Peierls, charge and current density wave order parameters, we identify for the first time Kondo singlets as the competing orders of the antiferromagnetism. We show that the antiferromagnetism and various competing singlet orders can be related to each other via generalized chiral transformations of the underlying fermions. We also show that the conduction electrons acquire a Berry phase through their coupling to the hedgehog configurations of the N\'eel order, which cancels the Berry phase of the local moments. Our results demonstrate the competition between the Kondo-singlet formation and spin-Peierls order when the antiferromagnetic order is suppressed, thereby shedding new light on the global phase diagram of heavy fermion systems at zero temperature.

Spin polarized electrons in a metallic quantum wire

We obtain explicit expressions for the spin-resolved density response functions of a spin polarized metallic quantum wire by including the first-order exchange and self-energy contributions to the random-phase approximation. These are used to calculate the spin-resolved static structure factors, pair-correlation functions, and correlation energy of the system for various values of the spin-polarization parameter ζ. The inclusion of first-order correction is found to bring the present approximate theory in overall very good agreement with the recent quantum Monte Carlo simulations by Shulenburger et al. [L. Shulenburger, M. Casula, G. Senatore, R.M. Martin, J. Phys. A 42, 214021 (2009)]. Particularly, we find excellently close results for the fully spin-polarized (i.e., ζ = 1) electrons for even larger coupling (r s ~ 4), implying a fairly good description of exchange correlations. Interestingly, there is no crossover found from para- to ferro-magnetic ground state. The static charge and spin susceptibilities, and spin sound are obtained analytically and found to be in good agreement with the available quantum Monte Carlo simulations. Furthermore, we present results for the wavevector dependent static charge density, spin density, and charge-spin density susceptibilities for some wire parameters. The dispersion of spin sound has also been obtained and found to be strongly dependent on wire width and density parameter.

A double-QM/MM method for investigating donor-acceptor electron-transfer reactions in solution.

We developed a double-quantum mechanical/molecular mechanical (d-QM/MM) method for investigation of full outer-sphere electron transfer (ET) processes between a donor and an acceptor (DA) in condensed matter. In the d-QM/MM method, which employs the novel concept of multiple QM regions, one can easily specify the number of electrons, spin states and appropriate exchange-correlation treatment in each QM region, which is especially important in the cases of ET involving transition metal sites. We investigated Fe(2+/3+) self-exchange and Fe(3+) + Ru(2+) → Fe(2+) + Ru(3+) in aqueous solution as model reactions, and demonstrated that the d-QM/MM method gives reasonable accuracy for the redox potential, reorganization free energy and electronic coupling. In particular, the DA distance dependencies of those quantities are clearly shown at the density functional theory hybrid functional level. The present d-QM/MM method allows us to explore the intermediate DA distance region, important for long-range ET phenomena observed in electrochemistry (on the solid-liquid interfaces) and biochemistry, which cannot be dealt by the half-reaction scheme with the conventional QM/MM.

Spin-charge separation in bipolar spin transport in (111) GaAs quantum wells

We present a microscopic theory for transport of the spin polarized charge density wave with both electrons and holes in the $(111)$ GaAs quantum wells. We analytically show that, contradicting to the commonly accepted belief, the spin and charge motions are bound together only in the fully polarized system but can be separated in the case of low spin polarization or short spin lifetime even when the spatial profiles of spin density wave and charge density wave overlap with each other. We further show that, the Coulomb drag between electrons and holes can markedly enhance the hole spin diffusion if the hole spin motion can be separated from the charge motion. In the high spin polarized system, the Coulomb drag can boost the hole spin diffusion coefficient by more than one order of magnitude.

Spin chaos manifestation in a driven quantum billiard with spin-orbit coupling

The coupling of orbital and spin degrees of freedom is the source of many interesting phenomena. Here, we study the electron dynamics in a quantum billiard --a mesoscopic rectangular quantum dot-- with spin-orbit coupling driven by a periodic electric field. We find that both the spatial and temporal profiles of the observables demonstrate the transition to chaotic dynamics with qualitative modifications of the power spectra and patterns of probability and spin density. The time dependence of the wavefunctions and spin density distributions indicates spin-charge separation {seen in the decay of the spin-charge density correlators}. This new spin chaos effect can be experimentally verified leading to a better understanding of the interplay between spin and spatial degrees of freedom, relevant to fundamental and applied quantum physics.

Vortex pattern and differential conductance in hole underdoped high-temperature superconductors with competing d-wave and antiferromagnetic orders

Vortex charges in copper oxide high-temperature superconductors with hole underdoping are investigated by Bogoliubov–de Gennes equations based on a model Hamiltonian with competing antiferromagnetic order and d-wave superconducting order (DSC) with variation of temperature. For the DSC, spin density wave (SDW) and charge density wave (CDW), the transition from stripe to checkerboard pattern and to 2D structure may occur with variation of temperature. Moreover, both spatial profile of the electron density distribution and intensity of the differential conductance are strongly affected by the SDW order around the vortex cores. The transitions from one-dimensional to isotropic two-dimensional structures of the vortex charge and spin-density wave orders may occur by tuning the temperature.

Altered magnetism and new electronic length scales in magneto-electric La2/3Sr1/3MnO3–BiFeO3 heterointerface

We map out the charge-spin density profile of magneto-electric La2/3Sr1/3MnO3 (LSMO)–BiFeO3 (BFO) heterostructure using soft x-ray resonant magnetic reflectivity. We show that the spatial extent of interface orbitals can extend over a few lattice periods even for an atomically sharp interface. While LSMO magnetization is depleted at the interface, BFO does develop a weak magnetic moment mostly near the interface, probably due to a proximity-induced charge-transfer process. Our study reveals that simultaneous control of electronic and magnetic interfaces is essential in realizing the potential of oxide devices.

Simulation of a hump structure in the optical scattering rate within a generalized Allen formalism and its application to copper oxide systems

We propose a possible way to simulate a hump structure in the optical scattering rate. The optical scattering rate of correlated charge carriers can be defined within an extended Drude model formalism. When some electron- and hole-doped copper oxide systems are in spin density or charge density wave phases they show hump structures in their optical scattering rates. The hump structures have not yet been simulated or clearly understood. We are able to simulate the hump structure by using a peak followed by a dip feature in the normalized density of states within a generalized Allen formalism. We observe that reversing the order of the dip and peak gives completely different features in the optical scattering rate; a peak–dip (dip–peak) results in a hump (a valley) in the scattering rate. We also obtain the real parts of the optical conductivity and reflectance spectra from the simulated optical scattering rate and compare them with published experimental spectra. From these comparisons we conclude that the peak–dip order can give the hump structure that is observed experimentally in copper oxide systems. Finally we fit two published optical spectra with our new model and discuss our results and the possible origin of the dip or peak features in the normalized density of states.

First-principles investigations of structural, elastic, electronic and magnetic properties of Ga1−x Mn x P and In1−x Mn x P

We employ the full-potential linearized augmented plane wave plus local orbital (FP L/APW + lo) method based on the density functional theory (DFT) in order to investigate the structural, elastic, electronic, and magnetic properties of ordered dilute ferromagnetic semiconductors Ga1−x Mn x P and In1−x Mn x P at (x = 0.25) in the zinc blende phase, using generalized gradient approximation, GGA (PBE). To our knowledge the elastic constants of these compounds have not yet been measured or calculated, hence our results serve as a first quantitative theoretical prediction for future study. Results of calculated electronic structures and magnetic properties reveal that both Ga0.75Mn0.25P and In0.75Mn0.25P have stable ferromagnetic ground state, and they are ideal half-metallic (HM) ferromagnetic at their equilibrium lattice constants. Also we show the nature of the bonding from the charge spin-densities calculations. The calculated total magnetic moments are 4.0 μB per unit cell for both Ga0.75Mn0.25P and In0.75Mn0.25P, which agree with the Slater–Pauling rule quite well, and we observe that p–d hybridization reduces the local magnetic moment of Mn from its free space charge value and produces smaller local magnetic moments on the nonmagnetic Ga, In and P sites. The values of N 0α and N 0β exchange constants confirm the magnetic nature of these compounds. From the robust half-metallicity of Ga0.75Mn0.25P and In0.75Mn0.25P as a function of lattice constant is also investigated.