Session V14: Focus Session: Spins in Carbon - Spins and Magnetism in Carbon

Inducing Magnetization by Flexing Graphene Nanoribbon

Zigzag graphene nanoribbons (ZGNRs) are antiferromagnetic in the ground state with zero net magnetization due to the compensation of contributions from opposite edges. The uniform deformations (both shear and axial) do not produce magnetization due to the symmetry restrictions. However, we report the results of first-principles calculations that predict that the induction of net magnetization in the graphene nanoribbon upon non-uniform strain applied to the nanoribbon. Using density functional theory (DFT) method implemented in SIESTA code, we show that the bending or twisting of nanoribbon produces magnetization because in the presence of strain gradient the induced magnetization on opposite edges are not compensating each other. We estimate an average magnetization of $\sim $ 3.3$\mu _{B}$ that produced from the bending of nanoribbon with the sinusoidal profile $\delta $x=Asin(2$\pi $z/L) with A= 3{\AA} and L=87.4 {\AA} (z=0..L/2, i.e. the half of the period). Our study suggests that the induced magnetization can be used for the control of magnetic structure in graphene including the trap of the domain walls.   

Numerical studies of the magnetism in graphene nanoribbons and graphene dot

Within determinant quantum Monte Carlo simulation, the magnetic properties of graphene nanoribbons and graphene dot are studied. It is predicted that the armchair graphene nanoribbons show carrier mediated ferromagnetism as electron filling is lower than 0.8. The uniform magnetic susceptibility increase as the width of nanoribbons decrease, and it increases greatly as the next-nearest-neighbor hopping energy increases. It is also shown that strain may induce magnetism in doped square graphene dot having zigzag edge. The resultant ferromagnetism in graphene-based system may facilitate the development of spintronics.   

g-Tensor control in a bent carbon nanotube quantum dot

We report low-temperature transport measurements of a carbon nanotube quantum dot containing a bend along its length. The bend occurred naturally in CVD growth, with a bend angle of 140 degrees and a radius of curvature of $\sim $1um. The device was contacted across the bend, with a global back gate and a top gate proximal to one arm of the bend. We measured the magnetic field angle dependence of conductance, tunneling rates, and bandgap in a 3-axis vector magnetic field. We characterize key signatures of carbon nanotube bends in the Kondo regime, comparing these dependencies in perpendicular and in-plane magnetic fields. We also demonstrate control of the electron spin g-tensor using gate voltages. Support from IBM, NSF-MWN, NSF-NRI through the INDEX Center, and Harvard University is acknowledged.   

Magnetic impurities and Kondo conductance anomalies in nanotubes: the importance of being ``in''

Transition metal impurities should yield zero bias anomalies in the conductance of well contacted metallic carbon nanotubes, but relevant temperatures and lineshapes cannot be anticipated without accurate ab initio calculations. Applying the density functional plus numerical renormalization group approach of Lucignano et al [1] to Co and Fe impurities in (4,4) and (8,8) nanotubes, we discover a huge difference of behaviour between outside versus inside impurity adsorption. The predicted Kondo temperatures and zero bias anomalies, tiny when the impurity is outside the nanotube [2] turn large and strongly radius dependent inside, owing to a change of symmetry of the magnetic orbital. These results foreshadow interesting field and temperature dependent nanotube electrical transport, to be addressed in future experiments.\\[4pt] [1] P. Lucignano, et al., Nature Materials 8, 563 (2009).\\[0pt] [2] P. Baruselli, et al., Physica E (2011) doi:10.1016/j.physe.2011.05.005   

Spin-orbit coupling and electronic transport in carbon nanotubes in external fields

We have investigated theoretically the role of spin-orbit coupling (SOC) on the conductance response of carbon nanotubes (CNT) in the presence of external electric and magnetic fields. We use an equilibrium Green's function formalism to calculate the spin resolved conductance by using a four-orbital orthogonal tight-binding representation in real space, taking into account curvature effects that induce orbital hybridization and are responsible for sizable SOC enhancements [1]. Different directions of external magnetic and electric fields (transverse and longitudinal to the CNTs), as well as length and chirality of the CNT, are shown to strongly affect the transport behavior of the systems. In particular, this results in stronger SOC effects for tubes with smaller radii. The interplay of electric and magnetic fields on the possible spin polarization of conductance will be discussed, as the sizable SOC effects result in effective control of the spin selective transport in these versatile nanoeletronic devices.\\ $[1]$ J. Klinovaja et al. PRL 106, 156809 (2011).   

Spin-orbit induced strong coupling of a single spin to a nanomechanical resonator

We theoretically investigate the coupling of an electron spin to vibrational motion due to curvature-induced spin-orbit coupling in suspended carbon nanotube quantum dots. Our estimates indicate that, with current capabilities, a quantum dot with an odd number of electrons can serve as a realization of the Jaynes-Cummings model of quantum electrodynamics in the strong-coupling regime. A quantized flexural mode of the suspended tube plays the role of the optical mode and we identify two distinct two-level subspaces, at small and large magnetic field, which can be used as qubits in this setup. The strong intrinsic spin-mechanical coupling allows for detection, as well as manipulation of the spin qubit, and may yield enhanced performance of nanotubes in sensing applications.   

Distinctive Magneto Conductance and Universal Scaling in One Dimensional Polymer Nanofibers

The conducting polymers are one dimensional organic hydrocarbon systems where the non-linear local excitations such as solitons, polarons and bipolarons were predicted based on the electron-phonon interactions. The local excitations have different spin-charge relations in different kinds of polymers. In this work, the magneto conductance (MC) of polymer nanofibers is investigated in high magnetic field at low temperature to understand both spin (magneto) and charge (conductance) of the charge carriers simultaneously. We discovered the distinctive zero MC in polyacetylene nanofibers while the finite MC in polyaniline and polythiophene nanofibers. On the other hand, the charge transports of polymer nanofibers as a function of temperature and bias are observed to be scaled onto the universal curve. We conclude that the universal scaling and the zero MC (the finite MC) in polyacetylene (polyaniline and polythiophene) nanofibers are from the interacting spinless charged solitons (interacting polarons which have both spin and charge).   

Magneto-Photoinduced Absorption in Organic Polymer Films

In order to elucidate the underlying mechanism of magneto-conductivity (MC) in OLEDs we studied magneto-photoinduced absorption (MPA) response in polymer films. The films were based on the MEH-PPV polymer in three different forms, namely: pristine film; film exposed to prolonged UV illumination; and MEH-PPV/PCBM blend having weight ratio 1:1. In pristine film we show that the MPA at low excitation intensity is due to sublevel spin mixing of triplet excitons; whereas at high excitation intensity the MPA is dominated by the triplet-triplet annihilation process. In UV illuminated MEH-PPV films that support photogenerated polarons we show that the MPA is due to sublevel spin-mixing of polaron-pairs via the hyperfine interaction with the closest hydrogen atoms to the chain. This mechanism also explains the MC response of OLED based on MEH-PPV, since its response is similar to that of MPA. Finally we found that the MPA in MEH-PPV/PCBM blend films is dominated by spin mixing of polaron-pair on the polymer and fullerene molecules, via the $\Delta $g mechanism. Supported by the NSF DMR-1104495, the NSF MRSEC at the UoU, and the BSF program.   

Tunnel magnetoresistance in Self-Assembled Monolayers Based Tunnel Junctions

Organic/molecular spintronics is a rising research field at the frontier between spintronics and organic chemistry. Organic molecule and semiconductors were first seen as promising for spintronics devices due to their expected long spin lifetime. But an exciting challenge has also been to find opportunities arising from chemistry to develop new spintronics functionalities. It was shown that the molecular structure and the ferromagnetic metal/molecule hybridization can strongly influence interfacial spin properties going from spin polarization enhancement to its sign control in spintronics devices. In this scenario, while scarcely studied, self-assembled monolayers (SAMs) are expected to become perfect toy barriers to further test these tailoring properties in molecular magnetic tunnel junctions (MTJs). Due to its very high spin polarization and air stability LSMO has positioned itself as the electrode of choice in most of the organic spintronics devices. We will present a missing building block for molecular spintronics tailoring: the grafting and film characterization of organic monofunctionalized long alkane chains over LSMO. We have obtained 35\% of magnetoresistance in LSMO/SAMs/Co MTJs. We will discuss the unusual behaviour of the bias voltage dependence of the TMR.   

Electrical spin injection from an organic-based ferrimagnet in a hybrid organic/inorganic heterostructure

The development of organic-based magnets with room temperature magnetic ordering and semiconducting functionality promises to broaden the field of semiconductor spintronics, providing a route to all-organic spintronic devices and hybrid organic/inorganic structures capable of exploiting the multifunctionality and ease of production in organic systems as well as the well established spintronic functionality of inorganic materials. Our work demonstrates the successful extraction of spin polarized current from the organic-based room temperature ferrimagnetic semiconductor V[TCNE]$_{x}$ ($x\sim $2, TCNE: tetracyanoethylene; $T_{C} \quad >$ 400 K, $E_{G}$~$\sim $~0.5~eV, \textit{$\sigma $}$\sim $ 10$^{-2}$ S/cm) and its subsequent injection into a GaAs/AlGaAs light-emitting diode (LED) [1]. The spin current is detected by monitoring the polarization state of the photons emitted from the LED structure and tracks the magnetization of V[TCNE]$_{x\sim 2}$, is weakly temperature dependent, and exhibits heavy hole / light hole asymmetry. This result opens the door to a new class of active, hybrid spintronic devices with multifunctional behavior defined by the optical, electronic and chemical sensitivity of the organic layer. In addition, spin transport in these hybrid structures provides the opportunity to leverage well-characterized inorganic materials as a probe of spin physics in organic and molecular systems and to explore the impact of the hybrid interface on spin injection efficiency. Initial studies exploring the impact of surface passivation of the inorganic layer with self-assembled monolayers of various chemistries will be presented, and additional experimental probes of the interfacial exchange interaction will be discussed. \\[4pt] [1] ``Electrical Spin Injection from an Organic-Based Ferrimagnet in a Hybrid Organic-Inorganic Heterostructure,'' Lei Fang, K. Deniz Bozdag, Chia-Yi Chen, P.A. Truitt, A.J. Epstein and E. Johnston-Halperin, \textit{Phys. Rev. Lett.} \textbf{106}, 156602 (2011).   

The effect on the properties of V(TCNE) grown on bare substrates and self assembled monolayers

V(TCNE) is a room temperature Tc larger than 400K organic based magnetic semiconductor. It has been shown that this fully-spin polarized material can be used as a spin injector in hybrid organic inorganic spin-light emitting diode (spin-LED) as well as hybrid and all organic spin valves [1,2]. Attempts to improve the spin signal from a spin LED device are focusing on surface passivation of the III-V heterostructure surface by including a self--assembled monolayer (SAM). We present a comparison of the properties of V(TCNE)grown by chemical vapor deposition (CVD) on various SAM's and bare substrates. We find that the successful growth of uniform, thin films of CVD V(TCNE) requires careful purification of the SAM precursors. We observe bulk magnetic properties of V(TCNE) grown on SAM's to be consistent with or better than films grown on bare substrates. Therefore the presence of the SAM does not appear to adversely affect the desired properties of the V(TCNE) for use as a spin injector, dramatically expanding the range of potential hybrid device geometries. \\[4pt] [1] L. Fang, Phys Rev Lett, 106 (15), (2011). \\[0pt] [2] J. W. Yoo, Nat Mater, 9 (2010).   

Spin multiplicity and symmetry breaking in vanadium-benzene complexes

Despite use of vanadium-benzene complexes in spintronics applications [1], reliable theoretical and experimental knowledge of energetics, dissociation energy, spin multiplicity, etc. of these systems is missing. Fixed-node DMC calculations have been done with the quest to elucidate electronic and atomic structure of vanadium-benzene half-sandwiches. At variance with DFT results which favor either low- or high-spin state, depending on the functional used [2], DMC predicts degenerate energies for spin multiplicities 2, 4, and 6, irrespective of DFT functional used to fix the nodal hypersurfaces. Ultimately, we predict high-spin ground-state, based on comparison of experimental/theory ionization energy [3]. Based again on DMC, we predict vastly different gaps for spin-up/-down electrons. The DMC results indicate that both DFT [2] as well as experimental results [4] may be biased. [1] V. V. Maslyuk \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 097201 (2006) [2] R. Pandey, B. K. Rao, P. Jena, M. A. Blanco, J. Am. Chem. Soc. \textbf{123}, 3799 (2001) [3] T. Kurikawa, \textit{et al.}, Organomentallics \textbf{18}, 1430 (1999). [4] R. L. Hettich, T. C. Jackson, E. M. Stanko, B. S. Freiser, J. Am. Chem. Soc. \textbf{108}, 5086 (1986).   

Total Scattering Study of Vanadium Tetracyanoethylene

Vanadium tetracyanoethylene powder prepared in deuterated dichloromethane (henceforth V-TCNE) was studied using neutron and X-ray diffraction. V-TCNE is a molecule-based magnet that has been shown to display magnetic order above room temperature\footnote{J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, and J.S. Miller, Science 252, 1415-1417 (1991).} as well as photocontrol of magnetism at cryogenic temperatures.\footnote{J.-W. Yoo, R.S. Edelstein, D.M. Lincoln, N.P. Raju, and A.J. Epstein, Phys. Rev. Lett. 99 157205 (2007).} To date, all reported synthesis preparations of V-TCNE yield amorphous compounds that lack Bragg peaks in diffractograms. In the absence of long range structural order, diffraction experiments may still elucidate short range structural order. The experimental results, which display short range correlations, will be presented and compared to Monte Carlo simulations.