Magnetoconductance Research Articles

Magnetoluminescence of light-emitting field-effect transistors based on alpha sexithiophene

We demonstrated the effect of a magnetic field on the luminous intensity and electric current of light-emitting field-effect transistors (LEFETs) based on alpha sexithiophene (α-6T). Sublimate-grade α-6T was thermally deposited on an n+-Si/300 nm-SiO2 substrate with patterned asymmetric gold-aluminum electrodes to fabricate a bottom-contact LEFET. We observed an increase in luminous intensity of approximately 1.3% under a magnetic field of 100 mT. A possible explanation for this is that the magnetic field increased the probability of singlet formation at the α-6T/Al interface. While the magneto-electroluminescence (MEL) was reported to be derived from the magneto-conductance (MC) in ordinary light emitting diodes, the MEL in LEFET was independent with MC. This indicates that the luminous efficiency can be improved by optimizing the magnetic field effect.

Theoretical, numerical, and experimental study of a flying qubit electronic interferometer

We discuss an electronic interferometer recently measured by Yamamoto et al. This "flying quantum bit" experiment showed quantum oscillations between electronic trajectories of two tunnel-coupled wires connected via an Aharanov-Bohm ring. We present a simple scattering model as well as a numerical microscopic model to describe this experiment. In addition, we present new experimental data to which we confront our numerical results. While our analytical model provides basic concepts for designing the flying qubit device, we find that our numerical simulations allow to reproduce detailed features of the transport measurements such as in-phase and anti-phase oscillations of the two output currents as well as a smooth phase shift when sweeping a side gate. Furthermore, we find remarkable resemblance for the magneto conductance oscillations in both conductance and visibility between simulations and experiments within a specific parameter range.

Magnetoconductance responses of triplet polaron pair charge reaction in hyperfine coupling regime

This study investigates a distinct low-field negative magnetoconductance (MC) response under illumination in pentacene:fullerene heterojunction devices. The low-field negative MC response is attributed to a triplet polaron pair ((PP)3) charge reaction associated with the hyperfine interaction. Applying a magnetic field can reduce the intensity of the reaction causing the negative MC response. Additionally, the low-field negative MC response changes with the blending ratios of fullerene in pentacene active layer, the applied bias voltages, and applying device structure with bulk heterojunction or planar heterojunction configuration, because the modulations of the internal electric field. Consequently, the results indicate that the (PP)3 charge reaction induces the low-field negative MC, which is correlated with the local hyperfine field, external magnetic field, and electric field.

Tunable magneto-conductance and magneto-electroluminescence in polymer light-emitting electrochemical planar devices

We report studies of magneto-conductance (MC) and magneto-electroluminescence (MEL) in polymer light-emitting electrochemical planar devices using “super-yellow” poly-(phenylene vinylene). We observed consistent negative MC while MEL becomes positive when electroluminescence quantum efficiency (ELQE) increases. At an optimal ELQE, the MC has a much narrower width than the MEL, indicating that the MC and MEL do not share a common origin. However, MC reverses and has the same width as MEL when exposed to a threshold laser power. We show that the e-h pair model can explain the positive MEL and MC while the negative MC can be explained by the bipolaron model.

Enhancement of magneto-conductance in n-Si/n-PS/NPB structures at room temperature

Abstract Hybrid organic–inorganic semiconductor heterojunction with a sandwich structure have been prepared and studied. The inorganic semiconductor is n-type Porous Silicon (n-PS) elaborated on n-type crystalline silicon, the used conjugated polymer is the N,N′-diphenyl-N,N′-bis(1-naphthyl-pheny1)-(1,1′-biphenyl)-4,4′-diamine (NPB). Current–voltage ( I – V ) at transverse static magnetic field effect was used to study the electrical properties of the devices at room temperature. The electrical parameters such as the ideality factor ‘ n ’, the barrier height and the series resistance are determined from the I – V curve. We report the observed magneto-conductance (MC) in a weak magnetic field. The observed positive MC was enhanced when we partially filled pores with the NPB. This effect reaches up to 4.7% at a magnetic field of 0.8 T.

Diffusing magnetic Tb impurities and magnetotransport in strongly spin-polarized Bi films

As a contribution to electronic transport within strongly spin-polarized surface states and its modification by adsorption of magnetic impurities we studied the adsorption of Tb (atomic magnetic moment 10 ${\ensuremath{\mu}}_{B}$) on epitaxial Bi(111) films by means of surface sensitive (magneto)conductance and low-energy electron diffraction. Surface diffusion turned out to be non-negligible even at substrate temperatures of 10 K. The Tb adatoms finally nucleate at intrinsic defects of the Bi(111) surface, where the Tb impurities act as dopants but not as scatterers. Nevertheless, time-dependent measurements allowed to determine also single-particle Tb scattering properties, as also supported by simulations of adsorption kinetics and time-dependent conductance. The magnetoconductance properties are characterized by small charge transfer (0.05 $e$/atom) and strong spin-orbit scattering, which in this case results only in strong reduction of the weak antilocalization effect but not a reversal to weak localization as for Fe and Co [see L\"ukermann, Sologub, Pfn\"ur, Klein, Horn-von Hoegen, and Tegenkamp, Phys. Rev. B 86, 195432 (2012)]. Although Tb has a magnetic moment, which is by far higher than for adsorbed Fe and Co, it turns out that the $f$ electrons of Tb play essentially no role in scattering of the conduction electrons, yielding an even smaller scattering cross section than that for Fe and Co. The adatom coordination (interstitial or on the surface) may also play an important role.

Large magnetic field effects in electrochemically doped organic light-emitting diodes

Large negative magnetoconductance (MC) of \ensuremath{\sim}12$%$ is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages (${V}_{\mathrm{bias}}$). Simultaneously, a positive magnetoefficiency (M\ensuremath{\eta}) of 9$%$ is observed at ${V}_{\mathrm{bias}}$ $=$ 2 V. At higher bias voltages, both the MC and M\ensuremath{\eta} diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive M\ensuremath{\eta} effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet:triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.

Modulating the competition between dissociation and spin mixing in electron–hole pairs: An investigation of ultra-small field induced magnetoconductance responses in blended devices

The ultra-small field induced magnetoconductance (MC) responses in Supper Yellow Poly(phenylenevinylene) (SY-PPV):Phenyl-C61-butyric acid methyl ester (PCBM) blends have been investigated to clarify the role of competition ratio by changing the dissociation rate. It is found that the widths of the ultra-small field induced MC curves broaden from 0.4mT (0wt.%) to 1.9mT (4wt.%). The characteristics of electroluminescence-voltage suggest that the width broaden is assigned to an increase in the competition ratio induced by an increase in the dissociation rate when PCBM is blended. This conclusion is further verified by an empirical formula fitting.

Quantum Corrections Crossover and Ferromagnetism in Magnetic Topological Insulators

Revelation of emerging exotic states of topological insulators (TIs) for future quantum computing applications relies on breaking time-reversal symmetry and opening a surface energy gap. Here, we report on the transport response of Bi2Te3 TI thin films in the presence of varying Cr dopants. By tracking the magnetoconductance (MC) in a low doping regime we observed a progressive crossover from weak antilocalization (WAL) to weak localization (WL) as the Cr concentration increases. In a high doping regime, however, increasing Cr concentration yields a monotonically enhanced anomalous Hall effect (AHE) accompanied by an increasing carrier density. Our results demonstrate a possibility of manipulating bulk ferromagnetism and quantum transport in magnetic TI, thus providing an alternative way for experimentally realizing exotic quantum states required by spintronic applications.

Magneto-Conductance in Tri-(8-Hydroxyquinoline) Aluminum-Based Organic Light Emitting Diodes

The origin of magneto-conductance (MC) in tri-(8-hydroxyquinoline)-aluminum (Alq3)-based organic light emitting diodes is investigated. Our results clearly show that the generated MC is related to the singlet polaron pair dissociation. Further studies on the MC in an electron blocking layer N,N′-bis(lnaphthyl)-N,N′-diphenyl-l,l′-biphentl-4,4′-diamine (NPB) and a hole blocking layer 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi)-based devices indicate that the holes reaching the cathode from the dissociation of singlet polaron pairs on Alq3 are the main cause of the MC generation. It is found that the MC can be significantly reduced by doping a red fluorescence dye DCJTB as a hole trapper in Alq3.

Magnetoconductance response due to the triplet exciton–charge interaction in organic light-emitting diodes

The magnetoconductance (MC) effects in three organic light-emitting diodes have been measured over a range of operating current and temperatures. The peculiar roles played by the hyperfine field and triplet excitons are outlined by fitting the MC traces using the sum of two non-Lorentzian functions. The MC response evident at large magnetic fields is confirmed to be resulted from the triplet exciton–charge interaction by investigating its variation with the triplet population. The remarkable agreement between the experimental data and fit lines demonstrates that the method used in this work could help to identify the correct models for understanding the fundamental mechanism behind the magnetic field effects universally observed in organic devices.

Traps and trions as origin of magnetoresistance in organic semiconductors

The large effect of a small magnetic field on the current, magnetoconductance (MC), in organic semiconductors---so-called organic magnetoresistance---has puzzled the field of organic spintronics during the last decade. Although the microscopic mechanisms regarding spin mixing are well understood by now, it is still unknown which pairs of spin carrying particles are influencing the current in such a drastic manner. Here, a model for the MC is presented based on the spin selective formation of metastable trions from triplet exciton-polaron pairs. Additionally, the magnetic-field and voltage dependence of the MC are experimentally investigated in materials showing large effects. Using a combination of analytical and numerical calculations, it is shown that the MC is perfectly described by a process in which trions are created at polaron trap sites.

Theoretical investigation on magnetic field effect in organic devices with asymmetrical molecules

A mechanism of organic magnetic field effect (OMFE) based on Lorentz effect in organic light-emitting devices with asymmetrical molecules is suggested and the magnetoconductance (MC) value is calculated. By considering the collision of a positive and a negative charged polaron and exciton formation in the organic layer through a non-adiabatic dynamic process, it is found that the exciton yield can be changed by applying a magnetic field due to the Lorentz effect on the moving polarons’ phase factors. By calculating the current through the device and the MC, we obtain that the calculated MC is well consistent to some experimental observations. It is also found that the MC value is sensitive to the asymmetrical structure and the electron–phonon (e–ph) coupling of the organic material, which explains why magnetic effect in an organic semiconductor is much more apparent than its inorganic counterpart.

Investigating the magnetic field effect on electron-hole pair in organic semiconductor devices

By constructing dynamic equations including electrons, holes and their pair densities, we calculate the magnetoconductance (MC) and the magnetoelectroluminescence (MEL) separately. It is indicated that MC and MEL may result from different response on the applied magnetic field. MC is from the scattering of polarons by magnetic field related triplet excitons, while MEL is mainly from magnetic field related conversion between singlet and triplet electron-hole pairs. Furthermore, we discuss the relation between MC and MEL. The theoretical calculations are well consistent with the experimental results.

Magnetic field effect spectroscopy of C60-based films and devices

We performed spectroscopy of the magnetic field effect (MFE) including magneto-photoinduced absorption (MPA) and magneto-photoluminescence (MPL) at steady state conditions in annealed and pristine fullerene C60 thin films, as well as magneto-conductance (MC) in organic diodes based on C60 interlayer. The hyperfine interaction has been shown to be the primary spin mixing mechanism for the MFE in the organics. In this respect, C60 is a unique material because 98.9% of the carbon atoms are 12C isotope, having spinless nucleus and thus lack hyperfine interaction. In spite of this, we obtained substantial MPA (up to ∼15%) and significant MC and MPL in C60 films and devices, and thus mechanisms other than the hyperfine interaction are responsible for the MFE in this material. Specifically, we found that the MFE(B) response is composed of narrow (∼10 mT) and broad (>100 mT) components. The narrow MFE(B) component is due to spin-dependent triplet exciton recombination in C60, which dominates the MPA(B) response at low pump intensities in films, or the MC response at small current densities in devices. In contrast, the broad MFE(B) component dominates the MPA(B) response at high pump intensities (or large current densities for MC(B)) and is attributed to spin mixing in the polaron pairs spin manifold due to g-factor mismatch between the electron- and hole-polarons in C60. Our results show that the organic MFE has a much broader scope than believed before.

Antagonistic responses between magnetoconductance and magnetoelectroluminescence in polymer light-emitting diodes

Antagonistic responses between magnetoconductance (MC) and magnetoelectroluminescence (MEL) in the polymer light-emitting diodes with an interfacial layer between Al cathode and active layer are simultaneously measured. As the interfacial layer (tetraoctylammonium bromide) is used, the significant increase in the number of injected negative polarons and the blocking of positive polarons promote the triplets-(free polaron) reaction and provide a good explanation for the reason that electroluminescence (EL) efficiency is maximal in the trap free space charge limited current regime at high bias. By fitting of MC and MEL curves using Lorentzian and non-Lorentzian empirical equations, three magnetic field dependent mechanisms, which are the intersystem crossing between singlet/triplet polaron pairs, the triplets-(free polaron) reaction, and the triplets-(trapped polaron) reaction are elucidated. The distribution of the three components is tunable by varying the applied electric field, which primarily modulates the triplets-(free polaron) reaction rate. The results pave a new route toward understanding the mechanism of organic spintronics for developing of multifunctional devices.

Experimental investigation on the origin of magneto-conductance and magneto-electroluminescence in organic light emitting devices

We investigate the underlying mechanisms of the magnetic field effects (MFEs), magneto-conductance (MC) and magneto-electroluminescence (MEL), in organic semiconductors by testing three kinds of devices, the electron-only devices, the hole-only devices and the ambipolar devices. There is no or very small MCs in the unipolar devices and much larger MC in the ambipolar devices. The results suggest that the ambipolar injection or exciton formation is a dominant factor to obtain large MCs, and the MEL is in principle a primary effect. Our results are consistent with the electron–hole pair model.

The influence of the excition recombination zone on the organic magnetic-field effect

In this work we explore the influence of the exciton recombination zone (RZ) on magnetic-field effect in tris-(8-hydroxyquinolinato) aluminum (Alq3) based organic light-emitting diodes by changing the thickness of Alq3. The magneto-electroluminescence and magneto-conductance (MC) in these devices are investigated at various temperatures and bias voltages. It is found that the sign of MC changes from positive to negative, and then back to positive with the reduction of the thickness of Alq3 at 50 K. The phenomenon observed is ascribed to the change of the exciton density in the exciton RZ. Based on the mechanisms including the hyperfine mixing, the triplet-charge interaction and interfacial dissociation or quenching of excitons, the observed results are explained qualitatively.

Probing spin-charge relation by magnetoconductance in one-dimensional polymer nanofibers

Polymer nanofibers are one-dimensional organic hydrocarbon systems containing conducting polymers where the non-linear local excitations such as solitons, polarons and bipolarons formed by the electron-phonon interaction were predicted. Magnetoconductance (MC) can simultaneously probe both the spin and charge of these mobile species and identify the effects of electron-electron interactions on these nonlinear excitations. Here we report our observations of a qualitatively different MC in polyacetylene (PA) and in polyaniline (PANI) and polythiophene (PT) nanofibers. In PA the MC is essentially zero, but it is present in PANI and PT. The universal scaling behavior and the zero (finite) MC in PA (PANI and PT) nanofibers provide evidence of Coulomb interactions between spinless charged solitons (interacting polarons which carry both spin and charge).

Porosity dependence of positive magnetoconductance in n‐type porous silicon

AbstractPositive magnetoconductance (MC) on n‐type porous silicon (PS) based devices was observed at room temperature for low static magnetic field (under 6000 G). We found that the measured MC decreases when the film porosity is increased. Obtained results were analyzed by means of the quasi‐1D weak localization (WL) theory. From the dependence of the MC vs. applied magnetic field, we determine the phase coherence length Lϕ. Good agreement between theoretical and experimental results was found (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)