Magnetoconductance Research Articles

Electrical tuning of the band alignment and magnetoconductance in an n-type ferromagnetic semiconductor (In,Fe)As-based spin-Esaki diode

We report a strong bias dependence of the magnetoconductance (MC) of a spin-Esaki diode composed of n+-type ferromagnetic semiconductor (FMS) (In,Fe)As and p+-type Be doped InAs grown on a p+-InAs (001) substrate by molecular beam epitaxy. When the bias voltage V is increased above 450 mV in the forward bias, we found that the MC, measured at 3.5 K under a magnetic field H of 1 T in the in-plane [110] direction, changes its sign from positive to negative and its magnitude rises rapidly from 0.5% at V < 450 mV to −7.4% at V = 650 mV. Furthermore, the MC magnitude decreases as cos2(θ) when rotating H from the in-plane [110] direction to the perpendicular [001] direction, where θ is the angle between H and the [110] axis. Using a two-fluid model, we explain both the magnitude and the anisotropy of the MC based on the evolution of the spin-Esaki diode's band profile with V. This analysis provides insights into the density of states and spin-polarization of the conduction band and the Fe-related impurity band in n-type FMS (In,Fe)As.

Magnetic field dependence of the current flowing in the spin-coated chlorophyll thin films

The magnetic dependence of the current flowing in the spin coated chlorophyll films on a patterned Cu PCB substrate has been presented. Chlorophyll was isolated from Spirulina sp and deposited by spin coated methods. The reducing of current by the change of magnetic field (magneto conductance effect) was performed by inducing the magnetic field parallel to the inplane of film at room temp. The magnetoconductance ratio decreases as the increase of voltage. It was indicated that the origin of carrier charge in chlorophyll films should be different with the carrier charge injection (electron).

High-field magnetoconductance in La-Sr manganites of FM and AFM ground states

Large-grain La1–xSrxMnO3 ceramic samples of compositions x = 0.45 and 0.55, representing the ferromagnetic (FM) and A-type antiferromagnetic (AFM) ground states, were produced via classical sintering at 1500 °C of cold-pressed sol-gel prepared single-phase nanoparticles. Using the same precursors, nanogranular forms of both manganite ceramics were prepared by fast spark plasma sintering at low temperature of 900 °C, which limits the growth of crystal grains. The magnetotransport of both the bulk and nanogranular forms was investigated in a broad range of magnetic fields up to 130 kOe and analyzed on the basis of detailed magnetic measurements. Both the large-grain and nanogranular systems with x = 0.45, possessing a pure FM state with similar Curie tempereature TC ≈ 345 K), show nearly the same conductivity enhancement in external fields when expressed relatively to the zero-field values. This positive magnetoconductance (MC) can be separated into two terms: (i) the hysteretic low-field MC that reflects the field-induced orientation of magnetic moments of individual grains, and (ii) the high-field MC that depends linearly on external field. In the case of large-grain ceramics with x = 0.55, a partially ordered FM state formed below TC = 264 K is replaced by pure A-type AFM ground state below 204 K. This A-type AFM state is characterized by positive magnetoconductance that is essentially of quadratic dependence on external field in the investigated range up to 130 kOe. On contrary, the nanogranular product with x = 0.55 exhibits a mixed FM/AFM state at low temperatures, and, as a consequence, its magnetotransport combines the features of FM and A-type AFM systems, in which the quadratic term is much enhanced and clearly dominates at high fields. For interpretation of observed behaviors, the theory of grain-boundary tunneling is revisited.

Modulating the line shape of magnetoconductance by varying the charge injection in polymer light-emitting diodes

We fabricate the phenyl-substituted poly(p-phenylene vinylene) copolymer (super yellow, SY-PPV)-based polymer light-emitting diodes (PLEDs) with different device architectures to modulate the injection of opposite charge carriers and investigate the corresponding magnetoconductance (MC) responses. At the first glance, we find that all PLEDs exhibit the positive MC responses. By applying the mathematical analysis to fit the curves with two empirical equations of a non-Lorentzian and a Lorentzian function, we are able to extract the hidden negative MC component from the positive MC curve. We attribute the growth of the negative MC component to the reduced interaction of the triplet excitons with charges to generate the free charge carriers as modulated by the applied magnetic field, known as the triplet exciton-charge reaction, by analyzing MC responses for PLEDs of the charge-unbalanced and hole-blocking device configurations. The negative MC component causes the broadening of the line shape in MC curves.

Tuning the polarity of organic magnetic field effects in polymer light-emitting diodes by incorporating a colloidal quantum dots thin layer

Organic magnetic field effects (MFEs) have attracted a lot of interests in foundation research and practical applications. In this work, a thin layer of colloidal quantum dots QDs was incorporated into the planar heterojunction-based polymer light-emitting diodes (PLEDs) as a sensitizer to tune the organic MFEs. As a result, the magneto-electroluminescence (MEL) and magneto-conductance (MC) in the poly [2-(4-(3′,7′-dimethyloctyloxyphenyl)-1,4-phenylene vinylene)] (P-PPV)-based PLEDs can be tuned synchronously from positive to negative. The current and temperature dependences of the negative MEL and MC were studied elaborately, showing normal and anomalous dependences compared to those of typical positive MEL and MC, respectively. The underlying mechanism of negative MEL and MC was examined. It is revealed that efficient Förster energy transfer between P-PPV and QDs is responsible for the effective tuning of MEL and MC via promoting the reverse intersystem crossing between electron-hole pairs.

Mesoscopic 2D Charge Transport in Commonplace PEDOT:PSS Films

AbstractThe correlation between the transport properties and structural degrees of freedom of conducting polymers is a central concern in both practical applications and scientific research. In this study, the existence of mesoscopic 2D coherent charge transport in poly(3,4‐ethylenedioxythiophene):poly‐(styrenesulfonate) (PEDOT:PSS) film is demonstrated by performing structural investigations and high‐field magnetoconductance (MC) measurements in magnetic fields of up to 15 T. The researchers have succeeded in observing marked positive MCs reflecting 2D electronic states in a conventional drop‐cast film. This low‐dimensional feature is surprising, since PEDOT:PSS—a mixture of two different polymers—seems to be significantly different from crystalline 2D materials from the viewpoint of structural inhomogeneity, especially in popular drop‐cast thick films. The results of the structural experiments suggest that such 2D transport originates from the nanometer‐scale self‐assembled laminated structure, which is composed of PEDOT nanocrystals wrapped by insulating sheets consisting of amorphous PSSs. These results indicate that charge transport in the PEDOT:PSS film can be divided into two regimes: mesoscopic 2D coherent tunneling and macroscopic 3D hopping among 2D states. These findings elucidate the hieratical nature of charge transport in the PEDOT:PSS film, which can provide new insight into a recent engineering concern, i.e., the anisotropic conductance.

Unraveling the Characteristic Shape for Magnetic Field Effects in Polymer-Fullerene Solar Cells.

Spin-dependent effects in organic solar cells (OSCs) are responsible for tuning the electric current when an external magnetic field is applied. Here, we report the magnetic field effect (MFE) on wide-bandgap (WBG) solar cells based on the polymers PBDT(O)-T1 and PBDT(Se)-T1 blended with PC70BM. Furthermore, we propose an experimental method based on the electrical transport (i–V) measurements to unveil the negative magneto conductance (MC) at small bias. The observed curves in a double-logarithmic scale display a particular S-like shape, independent of the OSC power conversion efficiency (PCE) or MC amplitudes. Additionally, from the slope of the S-like shape curve, it is possible to identify the fullerene concentrations that would result in the minimum MC and the maximum PCE. Our work opens up a door to find more patterns to describe MFE and PCE in polymer–fullerene solar cells, without the application of external magnetic or luminous sources.

Weak antilocalization induced by Rashba spin-orbit interaction in layered III-VI compound semiconductor GaSe thin films

Magnetoconductance (MC) at low temperature was measured to investigate spin-related transport affected by spin-orbit interaction (SOI) in III-VI compound $n$-type GaSe thin films. Results reveal that MC shows weak antilocalization (WAL). Its temperature and gate voltage dependences reveal that the dominant spin relaxation is governed by the D'yakonov-Perel' mechanism associated with the Rashba SOI. The estimated Rashba SOI strength in GaSe is much stronger than that of III-V compound GaAs quantum wells, although the energy gap and spin split-off band in GaSe closely resemble those in GaAs. The angle dependence of WAL amplitude in the in-plane magnetic field direction is almost isotropic. This isotropy indicates that the strength of the Dresselhaus SOI is negligible compared with the Rashba SOI strength. The SOI effect in $n$-GaSe thin films differs greatly from those of III-V compound semiconductors and transition-metal dichalcogenides.

Weak localization and electron–electron interaction in GaN nanowalls

We investigate the magneto-transport properties of GaN nanowalls grown by molecular-beam epitaxy on a sapphire substrate. The temperature dependent resistivity shows an upturn at T < 200 K due to localization behaviour at low temperatures. Magneto-conductance (MC) measurements at various temperatures from 3 K to 200 K were done in a magnetic field up to ±15 T applied parallel and perpendicular to the substrate. MC curves clearly show a sharp cusp like behaviour due to the weak-localization phenomenon and are fitted with Hikami–Larkin–Nagaoka equation (HLN). The phase coherence length deduced from the fit shows a decreasing behaviour with increasing temperature. The plot of phase coherence time (τφ) with T−1 shows a linear behaviour from 10 K down to 3 K denoting that the electron–electron interaction is the prime de-phase mechanism in the sample. The carrier density and mobility deduced from Hall measurements did not vary with temperatures which further substantiate that the interaction of the electrons in GaN nanowall plays a major role in the quantum correction.

Triplet induced competition between scattering and dissociation process in exciton-charge reaction

Magneto-conductance ( MC ) refers to the relative change rate of conduction current with and without external magnetic field B and is used as the sensitive detection tools to study the carrier transport characteristics in organic semiconductor devices. This provides a non-contact approach to tune conduction current properties because the carrier transport shows strong dependence on magnetic field. In general, the magnetic phenomenon where the values of conduction current of device are growing at constant bias upon the application of external magnetic field is named positive MC effect. Inversely, when the B -induced current value is decreased it is named negative MC effect. Recently, researchers had found that the scattering and dissociation processes of triplet-charge interaction (TQI) respectively caused the positive and negative MC in organic polymer materials, the charge transfer states (CTS) leaded to dissociation process of TQI, and the exciton states (ES) leaded to scattering processes, and the rivalry between the two states could tune the MC between positive and negative. However, it remains further study whether the micro mechanism of positive and negative MC in small molecule materials is similar to that in polymer materials. In this work, organic light-emitting diodes (OLEDs) that based on small molecule materials had been fabricated through doping the 4-(Dicyanomethylene)-2-methyl-6-(4- dimethylaminostyryl)-4H-pyran (DCM) into 4,40-N,N0-dicarbazole-biphenyl (CBP) as an active layer. Here, we demonstrate that the MC of the device could be changed between positive and negative which were caused by the scattering and dissociation processes of TQI by fitting the MC curves with the combination of two non-Lorentzian empirical equations. The electroluminescence spectrum of CBP: x %DCM shows that small molecule materials were difficult to form CTS, although it had similar theory with polymer materials. As a result, both the scattering process and dissociation process in TQI had been caused by ES in CBP: x %DCM, and it was necessary to study the transformation mechanism between these two processes in ES. Therefore, it had been found that changing the injection currents, the temperature and the doped concentration could alter the concentration of triplet excitons, consequently leading to the positive and negative MC . This means that the scattering process and dissociation process in TQI have opposite dependences on the concentration of triplet excitons in CBP: x %DCM. The scattering process plays a major role when the concentrations of triplet excitons are large enough, yielding positive MC . On the other hand, the dissociation process plays an important role when the concentrations of triplet excitons are low enough, producing negative MC . This finding provides a new pathway to efficiently tune the organic magneto-conductance, and further to understand the microscopic mechanism of the interaction between charges and excite states in organic optoelectronic devices.

Electrical modulation of weak-antilocalization and spin–orbit interaction in dual gated Ge/Si core/shell nanowires

Magnetic transport of holes in Ge/Si core/shell nanowires (NWs) is investigated under the control of dual electrical gating. The strength of the spin–orbit interaction (SOI) is analyzed from the weak-antilocalization (WAL) of the magnetoconductance (MC) as a function of a perpendicular magnetic field. By superimposing a small alternating signal on the voltage offset of both gates the universal conductance fluctuations are largely removed from the averaged MC traces, enabling a good fitting to WAL theory models. The tuning of both spin lifetime and the SOI strength is observed in the NWs with dual gating while the carrier density is kept constant. We observe an enhancement of spin lifetime with the mean free path due to the effect of geometrical confinement. The measured SOI energy of 1–6 meV may arise from the dipole coupled Rashba SOI, which is predicted to be one order of magnitude larger than the conventional Rashba coefficient in the Ge/Si core/shell NW system. A clear electrostatic modulation of SOI strength by a factor of up to three implies that Ge/Si NWs are a promising platform for the study of helical states, Majorana fermions and spin–orbit qubits.

Identification of both bipolaron and electron-hole pair contributions to organic magnetoresistance in a regioregular polythiophene device

We study organic diodes of regioregular polythiophene (RR-P3HT) fabricated with various anode and cathode materials to determine the importance of bipolarons and electron-hole pairs for magnetoconductance (MC). RR-P3HT is selected because it can display both negative or positive MC depending on the electrodes used to fabricate the devices. Near-unipolar, electron-only, devices show negative MC at all voltages. We attribute this to the bipolaron mechanism. Both a negative (low voltage) and positive (high voltage) MC are measured in bipolar devices. The positive and negative MC are attributed to the electron-hole pair mechanism and the bipolaron mechanism, respectively. This conclusion is supported by an analysis of the electroluminescence efficiency versus voltage and the magnetic field effect on the electroluminescence quantum efficiency.

Thickness dependence of organic magnetoconductance of polymeric semiconductor device based on an ambipolar polymer AnE-PVstat

We explore the magnetoconductance (MC) of polymer thin-film device, based on an anthracene containing poly (p-phenylene-ethynylene)-alt-poly (p-phenylene-venylene) (PPE-PPV) denoted AnE-PVstat, for different thicknesses to clarify the role of electron–hole (e–h) pairs on the MC effect. Here, by changing the thickness of the active layer, we observed a positive MC which reaches up 1.5% at a magnetic field of 425mT at room temperature. We found that the increase of the thickness is accompanied by the decrease of the MC effect. To analyze the experimental data, we used the electron–hole (e–h) pair model based on the Stochastic Liouville Equation with spin mixing by hyperfine fields and different singlet and triplet recombination and dissociation rates kS, kT, qS and qT. The theoretical calculations are well consistent with the experimental results.

Simultaneous Sign Change of Magneto-Electroluminescence and Magneto-Conductance in Polymer/Colloidal Quantum Dot Nanocomposites

Nanocomposites of conjugated polymer and colloidal quantum dots (QDs) have attracted considerable attention for optoelectronic and photovoltaic applications. Here the effects of magnetic field on the electroluminescence and current in poly[2-(4-(3′,7′-dimethyloctyloxyphenyl)-1,4-phenylenevinylene] (P-PPV)/CdSe-CdS-ZnS QDs composites are studied. Our results show that by increasing the concentration of CdSe-CdS-ZnS QDs in the hybrid nanocomposites from 0 to 25 wt %, the polarities of magneto-electroluminescence (MEL) and magneto-conductance (MC) are simultaneously changed from positive to negative. In addition, the amplitudes of negative MEL and MC are enhanced as the temperature decreases, showing an abnormal temperature dependence. We attribute these magnetic field effects to CdSe-CdS-ZnS QDs-induced direction reverse in spin-mixing of loosely bound polaron pairs in P-PPV matrix prior to energy transfer to QDs. With this study we show that incorporating QDs in polymer matrix can strongly influence spin-s...

Correlated disordered electrons and organic magnetoconductance

The purpose of this paper is to explore the magnetoconductance (MC) effect in an organic semiconductor. By taking into account the external magnetic field and the hyperfine interaction, we determine the magnetoconductance (MC) from a numerical solution by using a steady-state rate equation for correlated electrons in Gaussian disordered systems. The ingredient of this model is that at thermal equilibrium, the spins of the polarons relax in the same direction as the local magnetic fields. We have investigated the MC dependence of the external magnetic field, the hyperfine interaction, the carrier densities, the intra-site coulomb energy, and the applied electrical field. We show that the interplay between the Hubbard energy and the charge-carrier density leads to positive and negative magnetoconductance. The theoretical calculations are in qualitative agreement with the experimental results.

Study of magnetoconductance effect in silicon nanowires formed by chemical etching in HF/AgNO3 solution: Effect of etching time

The magneto-transport proprieties of silicon nanowires (SiNWs) for various etching time at room temperature has been studied. SiNWs were formed by metal-assisted chemical etching of crystalline silicon in AgNO3-based chemical solutions. Scanning electron microscopy (SEM) shows that the SiNWs may have different structures and their lengths depending on the etching time. We found that the electrical conductance and magnetoconductance (MC) effect are extremely depended on the etching time. MC measurements at room temperature revealed a positive MC and this effect is important and it reaches up to 9% at a magnetic field of 0.5T. The latter effect can be discussed in terms of quasi-one-dimensional (quasi-1D) weak localization (WL) theory.

Photoconductivity and magnetoconductance effects on vacuum vapor deposition films of weak charge-transfer complexes.

Thin films of weak charge-transfer (CT) complexes (pyrene/dimethylpyromellitdiimide or pyrene/pyromellitic dianhydride) were prepared on an interdigitated platinum electrode by vacuum vapor deposition. Their photoconductivity and magnetoconductance (MC) effects were investigated, and mobile triplet excitons (probably CT excitons) were detected by time-resolved ESR (TRESR) at room temperature. The MC effect on the photocurrent was observed and analyzed by quantum-mechanical simulation assuming two types of collision mechanisms between the electron and hole carriers and between the trapped triplet excitons and mobile carriers. A successful simulation was achieved when the parameters (g, D, E, and polarization) determined by TRESR and the effective hyperfine splitting estimated from an ab initio molecular-orbital calculation were used.

Traps as interaction sites for hyperfine mixing: The origin of magnetoconductance in organic light-emitting diodes

The origin of magnetoconductance (MC) in organic light-emitting diodes under bipolar injection conditions was investigated using devices containing pristine Super-Yellow poly(phenylene vinylene) (SY-PPV) or SY-PPV:phenyl-C61-butyric acid methyl ester (PCBM) (x wt%) blends as the active layers. In pristine SY-PPV device, it was found that the low-field component of MC was always larger than the high-field component. Additionally, the low-field component increased and then saturated with increasing the electrical stressing time, whereas the high-field component remained unchanged. These behaviors were analyzed using empirical formula (containing a Lorentzian and a non-Lorentzian function), which suggested that the dominant mechanism in the MC response was hyperfine mixing between single and triplet polaron pairs that occurred on trap sites. The specific role of these traps, providing interaction sites for hyperfine mixing, was confirmed by controlling the lifetime of the trapped polaron-pairs states by doping the active layer with PCBM.

Identify triplet-charge interaction in rubrene-based diodes using magneto-conductance: Coexistence of dissociation and scattering channels

Although it is known that triplet excitons can be quenched via triplet-charge interaction (TQI), it is still unclear how this process occurs in rubrene-based devices. We found that magneto-conductance (MC) can be used to probe the detailed mechanism of TQI in rubrene-based organic light-emitting diodes and observed the coexistence of negative and positive MC responses in the high-field region when holes and electrons were the dominant charged species at different interfaces adjoining the rubrene layer, respectively. Further analysis suggests that the negative MC response was originated from the dissociation of triplet excitons by holes, while the positive MC response was due to electron scattering by triplet excitons. The MC responses of the devices were examined under different injection currents and temperatures to confirm our hypothesis. This work gives significant insight into mechanisms of TQI in organic semiconductors, which will allow for the design of new and improved devices.

Analysis of the mechanism for magneto-conductance induced by triplet-charge interaction by energy resonance of excitons in rubrene

To analyze the mechanism for magneto-conductance (MC) induced by triplet-charge interaction (TCI), the rubrene-based organic emitting devices with Al, LiF/Al and Ca acting as metallic cathodes were fabricated, respectively. The MC in Al-cathode device decreases monotonously with increasing external magnetic field, which is obviously different from the one in LiF/Al- or Ca-cathode device. Using the energy resonance of singlet (S) and triplet (T) excitons in rubrene, the singlet fission and triplet fusion was changed to alter the ratio of T. The charge (C) was also changed by metallic cathode modulated electronic injection barrier. Consequently, the TCI was substantially modulated. The results suggested that the negative MC curves were due to the quenching of T exciton by trapped charges via detrapped channel (T+Ct→S0+C), rather than by dissociation or scattering of excessive charges. It was also suggested that the MC of LiF/Al- and Ca-cathode devices with balanced carriers injection is an order of magnitude smaller than the one of Al-cathode device. Additionally, the MC decreases in low-field region ( B field. The weak MC in balanced devices is not due to the weakness of TCI, but to the assumption that the existence of detrapping and trapping channels has a weak impact of on electronic current. This is because the traps in active layer of rubrene are fully occupied by minority electrons which are easier injected from cathode. Hence, the carrier traps in quenching way paly a primary role in the MC response of TCI. Our work suggests that producing plenty enough un-occupied carrier traps in organic functional layer is a promising approach to utilize the MC of TCI.