Session E36: Spin in Organic Semiconductors and 2D Systems

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Organic semiconductors have recently been found to have a comparably large spin diffusion time and length [1]. This makes them ideal candidates for spintronic devices. However, spin injection and transport properties in organic semiconductors have yet to be fully understood.
The efficiency of spin injection at ferromagnetic resonance (FMR) from a ferromagnetic material into an adjacent non-magnetic material is given by the spin mixing conductance g_eff. It can be quantified by measuring the linewidth broadening of the FMR absorption of the ferromagnet due to an increase in Gilbert damping caused by spin injection into the adjacent non-magnetic material. Here, we use this technique to systematically study spin injection from a metallic ferromagnet permalloy, Ni₈₀Fe_20, into dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), one of the best performing small molecule organic semiconductors to date. The unique tunability of organic materials by molecular design allows us to study the impact of interfacial properties on the spin injection efficiency systematically. We show that both, spin injection efficiency at the interface as well as the spin diffusion length can be tuned sensitively by the interfacial molecular structure and side chain substitution of the molecule [2].
[1] S. Watanabe*, K. Ando* et al., Nature Physics, 10, 308−313 (2014)
[2] A. Wittmann et al., Physical Review Letters, 124, 027204 (2020)   

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Hybrid organic-inorganic perovskites (HOIPs) are a new class of solution-processed semiconductors with remarkable performance in photovoltaics. Beyond the broad optoelectronic applications, they are great candidates for spintronics because of the long spin lifetime and predicted large spin-orbit coupling (SOC) effects for potential spin manipulation. Our previous study has revealed long spin lifetime (T₂^*) exceeding 1 ns at 4 K on MAPbI₃ polycrystalline films, which is unusual given the large SOC of Pb and I. A critical question is whether the long spin lifetime is an intrinsic property of MAPbI₃ or related to extrinsic factors, such as high defect concentrations or grain boundaries. Recently we have made significant progress in single crystal growth and surface treatment, resulting in a reduction in defect densities of several orders of magnitude. We measured time-resolved Kerr rotation (TRKR) on these high quality MAPbI3 single crystals, and showed that the spin lifetime improves by an order of magnitude, which suggests the intrinsic origin of this property. Combined with results from complementary optical measurements, we will also discuss the interplay between carrier dynamics and spin dynamics upon optical excitation of spin-polarized carriers.   

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Vanadium tetracyanoethylene, V(TCNE)₂, is a room temperature ferrimagnetic semiconductor with a T_c ~ 600 K, which has very low loss ferromagnetic resonance and spin-wave propagation. Previous first principles calculations of the electronic structure have indicated an indirect band gap at 0.8 eV, substantially larger than the 0.5 eV from experimental inference from the conductivity activation energy. [1] The study of Ref. 1 used a local-orbital calculation with B3LYP hybrid functional. Here we explore the electronic structure using a plane-wave code VASP and XC functional Heyd-Scuseria-Ernzerhof (HSE06). We confirm that the structure of VTCNE has a triclinic unit cell with each V atom surrounded by 6 organic ligands, as found in Ref. 1. However, in contrast to the previous study we find a direct band gap of 0.4 eV, instead of an indirect band gap, located at a different k-point. This band gap better agrees with the experiment. We also studied magnetic anisotropy, the optical properties, magnetic dynamics, and magnetoelastic properties.

[1] G. C. De Fusco, L. Pisani, B. Montanari, and N. M. Harrison, Phys. Rev. B 79, 085201 (2009).   

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We report measurements of the spin relaxation times T₁ and T₂ of charge carrier spin states in the π-conjugated polymer SY-PPV at room temperature under low static magnetic field conditions (1mT ≤ B₀ ≤ 4mT), in near absence of spin polarization, using a pulsed electrically detected magnetic resonance (pEDMR) scheme, where spin-permutation symmetry dependent charge carrier recombination currents are detected after short, coherent (pulsed) Hahn-echo and inversion recovery pulse trains [1] are applied to organic light emitting diodes (OLED) devices. As we performed these measurements at B₀ ∼ the oscillating magnetic resonant driving field B₁ [2, 3], these experiments required arbitrary waveform generation (AWG) for the direct synthesis of the coherent RF-pulse sequences. Since B₀ was also similar to hydrogen induced random local hyperfine fields in our experiments, we could observe changes of T₁ with small changes of B₀, causing a magnetic field dependence of the spin-dependent electric device current. [1] W. J. Baker et al., Phys. Rev. Lett. 108, 267601 (2012). [2] D. P. Waters et al., Nature Physics 11, 910 (2015). [3] L. Dreher et al., Phys. Rev. B 91,075314 (2015).   

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We study nonlinear magnetic resonance phenomena such as the Bloch-Siegert shift and multiphoton magnetic dipole transitions [1] in the high-drive regime using electrically detected magnetic resonance (EDMR) spectroscopy of organic light emitting diodes with conductive polymers as active layers [2]. EDMR allows for the detection of magnetic resonance at very low magnetic fields, where spin-polarization is nearly non-existent. However, the measured spin-dependent electrical currents can be obscured by the superposition of randomly-occurring artifact signals induced by the spin-resonant radiation. Here, we demonstrate the use of an amplitude-modulated lock-in detection scheme that allows us to isolate these two electric current signatures, taking advantage of their different dynamic time scales. We show a significant improvement in signal-to-noise over unmodulated, direct detection of spin-dependent currents and observe the presence of two-photon magnetic dipole transitions [1]. [1] S. Jamali, V. V. Mkhitaryan, et al., arXiv:2010.02170; [2] S. Jamali et al., Nano Lett., 17, 4648 (2017).   

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Using the nonequilibrium Green's function combined with density-functional theory, we study the transport properties of the zigzag graphene nanoribbon (ZGNR). When an external transverse electric field is applied, a ZGNR can exhibit half-metallic characteristics with its conductive channel localized at the edge positions. Accordingly, an in-plane dual-gated spin-valve device is proposed. Thanks to its unique design, the proposed device overcomes the bottleneck of current leakage and avoids contact issues. Remarkably, a 100% spin injection efficiency and a giant tunnel magnetoresistance of up to 10⁷ are realized, which represent much better performance than that of traditional magnetic tunnel junctions. In addition, we also explore the effect of architecture reconstruction and impurity doping on spin-polarized transport. It is found that, in general, the tunneling process is hindered if disorder occurs at the conductive edge. By contrast, if we put the deformation or impurities away from the conductive edge, the transport properties may be barely degraded or even improved, depending on the type of disorder. It should be possible to apply these ubiquitous underlying principles to other materials, which could inspire the design of spintronic devices with high performance.   

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We compute charge carrier mobility in graphene as a function of temperature due to phonon scattering and scattering from adsorbed magnetic molecules from the complex band structure, using the Quantum Espresso suite. Carriers with finite lifetimes due to scattering may be represented by generalized Bloch states with complex energies. Our method determines the constant complex potential that must be added to a perfect crystal to induce the scattering effects seen in a crystal with defects, which we deduce from a series of complex band calculations. The mean scattering lifetime is computed from the imaginary part of this complex potential, and the carrier mobility is obtained from the scattering lifetime using the Boltzmann transport theory. Mobility is calculated for different magnetic molecule configurations to demonstrate that mobility measurements may be used to detect and distinguish different magnetic molecules.   

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Low-frequency charge noise is ubiquitous in low dimensional devices. A high level of charge noise prevents stable operation of devices and can be a severe problem for many applications. We observe that GaAs/AlGaAs QPC devices with an Al2O3 dielectric between the metal gates and semiconductor exhibit significantly lower charge noise compared to devices with only Schottky gates and no dielectric. Additionally, the devices with Schottky gates exhibit drift over time towards lower conductance, while the devices with the dielectric drift towards higher conductance. Temperature dependence measurements suggest that in devices with Schottky gates noise is dominated by tunneling from the gates to trap sites in the semiconductor, and when this mechanism is suppressed by inclusion of a dielectric, thermally activated hopping between trap sites becomes the dominant source of noise.   

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Controlling the spin dynamics and spin lifetimes is one of the main challenges in spintronics. To this end, the study of the spin diffusion in two-dimensional electron gases (2DEGs) shows that when the Rashba and Dresselhaus spin-orbit couplings (SOC) are balanced, a persistent spin helix regime arises. There, a striped spin pattern shows a long lifetime, limited only by the cubic Dresselhaus SOC, and its dynamics can be controlled by in-plane drift fields. Here, we derive a spin diffusion equation for non-degenerate two-subbands 2DEGs. We show that the intersubband scattering rate enters as a new knob to control the spin dynamics, which is defined by the overlap of the square of the wavefunctions, and can be controlled by electric fields. We find that for large intersubband couplings the dynamics follows an effective diffusion matrix given by approximately half of the subband-averaged matrices. This extra 1/2 factor arises from Matthiessen's rule summing over the intra- and intersubband scattering rates and leads to a reduced diffusion constant and larger spin lifetimes. We illustrate our findings with numerical solutions of the diffusion equation with parameters extracted from realistic Schrödinger-Poisson calculations.   

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Mxenes are the most competent in 2D-material field and most promising candidate as in technical field, medical fields, and for sustainable environment as water filtration, radiation hazard reduction and so on. Employing the pseudo-potential based density functional theory (DFT) with general gradient approximation (GGA), and GGA with coulomb exchange interaction correction (GGA + U), electro-magnetic and mechanical properties are calculated. The structural, and the chemical bonding stability of the Ti₃C₂ and its derivatives were investigated through the calculation of formation and cohesive energies. We investigate pristine Ti₃C₂ (mxene) is anti ferro-magnetic (AFM) with spingapless semiconductor. Further, the mxene with N-layer substitution on one of the site of C-layer as Ti₃-C-N shows metallic ferromagnetic behavior where as double Hf-layer substitution on the site of Ti-layer in mxene as Hf-Ti-Hf-C-N shows AFM with metallic nature, respectively. The elasticity and
plasticity were investigated by bi-axial straining (x-y plane) up to 25% of relaxed parameters.   

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The collective dynamics of electrons with Dirac-like dispersions, such as in doped graphene, is not well described by the usual semilocal approximations of (TD)DFT, which are based on the traditional electron gas. Instead, we use orbital-based approximations, most notably the Singwi-Tosi-Land-Sjolander (STLS) approach, generalized to systems with noncollinear spin. We calculate spin-wave dispersions in magnetized two-dimensional systems of itinerant electrons.   

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Magnons, low energy spin excitations in magnetic systems, have been recently observed in the integer quantum Hall effect in graphene, where magnons were generated and detected by non-equilibrium populations of integer edge channels. In this work, we study spin relaxation at a filling factor 2/3, where the transition between polarized (p) and unpolarized (u) states can be controlled by electrostatic gates. When current is injected from p to u region or vice versa, angular momentum conservation is satisfied by polarizing a nuclear bath via hyperfine interaction. Nuclear bath polarization can be measured independently in u and p regions via an Overhauser shift of the ferromagnetic transition. We found that for spin-lowering injection nuclei in both u and p regions are polarized, indicating omnidirectional spin diffusion, while for the spin-raising injection only nuclei in the p region are polarized. We attributed this asymmetry to the preferred generation of magnons in the p ferromagnetic region which efficiently carries angular momentum away from the u-p boundary.   

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Vanadium tetracyanoethylene (V[TCNE]_x≈2) is an organic-based ferrimagnet, with an ultra-low magnetic damping rate, comparable to yttrium iron garnet (YIG). Its mild growth conditions allow integration with a wide variety of processes. V[TCNE]_x≈2 is air sensitive, but its lifetime in ambient conditions can be extended to several weeks by encapsulating it with epoxy while preserving its low magnetic damping. However, its ageing mechanism is not well studied. Here we use confocal microscopy, microfocused Raman spectroscopy, and ferromagnetic resonance to study V[TCNE]_x≈2 and its ageing process. We identify Raman peaks in agreement with density functional theory calculations. These calculations, using the VASP code, help distinguish the frequency ranges for contributions from CN, CC and VN bonds. By comparing optical features with ferromagnetic resonance measurements, we monitor the V[TCNE]_x≈2 film as it ages. We find a correlation between a reduction in the effective magnetization 4πM_s-H_k and changes in Raman intensity as well as increases in the photoluminescence of the material. These results enable optical measurements that locally probe V[TCNE]_x≈2 film quality.