Magnetic field sensors based on organic thin-film materials have attracted considerable interest in recent years as they can be manufactured at very low cost and on flexible substrates. However, the technological relevance of such magnetoresistive sensors is limited owing to their narrow magnetic field ranges (∼30 mT) and the continuous calibration required to compensate temperature fluctuations and material degradation. Conversely, magnetic resonance (MR)-based sensors, which utilize fundamental physical relationships for extremely precise measurements of fields, are usually large and expensive. Here we demonstrate an organic magnetic resonance-based magnetometer, employing spin-dependent electronic transitions in an organic diode, which combines the low-cost thin-film fabrication and integration properties of organic electronics with the precision of a MR-based sensor. We show that the device never requires calibration, operates over large temperature and magnetic field ranges, is robust against materials degradation and allows for absolute sensitivities of <50 nT Hz−1/2.

The effects of external magnetic fields on the electrical conductivity of organic semiconductors have been attributed to hyperfine coupling of the spins of the charge carriers and hydrogen nuclei. We studied this coupling directly by implementation of pulsed electrically detected nuclear magnetic resonance spectroscopy in organic light-emitting diodes (OLEDs). The data revealed a fingerprint of the isotope (protium or deuterium) involved in the coherent spin precession observed in spin-echo envelope modulation. Furthermore, resonant control of the electric current by nuclear spin orientation was achieved with radiofrequency pulses in a double-resonance scheme, implying current control on energy scales one-millionth the magnitude of the thermal energy.

Pairs of charge-carrier spins in organic semiconductors constitute four-level systems that can be driven electromagnetically 1 . Given appropriate conditions for ultrastrong coupling 2 -weak local hyperfine fields B hyp , large magnetic resonant driving fields B 1 and low static fields B 0 that define Zeeman splittingthe spin-Dicke e ect, a collective transition of spin states, has been predicted 3 . This parameter range is challenging to probe by electron paramagnetic resonance spectroscopy because thermal magnetic polarization is negligible. It is accessed through spin-dependent conductivity that is controlled by electron-hole pairs of singlet and triplet spin-permutation symmetry without the need of thermal spin polarization 4 . Signatures of collective behaviour of carrier spins are revealed in the steady-state magnetoresistance of organic light-emitting diodes (OLEDs), rather than through radiative transitions. For intermediate B 1 , the a.c.-Zeeman e ect appears. For large B 1 , a collective spin-ensemble state arises, inverting the current change under resonance and removing power broadening, thereby o ering a unique window to ambient macroscopic quantum coherence.Macroscopic phase coherence is a hallmark of many exotic states of matter such as superconductivity, ferromagnetism or BoseEinstein condensation. Such coherence may also emerge between two-level systems, where it is mediated by electromagnetic fields, as described by the Dicke effect in collisional narrowing 5 and superradiance 6 . Collective behaviour may already arise within a pair of interacting two-level systems 7 , an observation that can potentially be extended to the prototypical two-level system of an electron spin. For pairs of charge-carrier spins in organic semiconductors, with driving fields B 1 exceeding the hydrogen-induced random local hyperfine field 1 B hyp and approaching the magnitude of the static magnetic field B 0 , a collective macroscopic spin phase has been predicted to emerge 3 . Under these conditions, when the spin-Rabi splitting becomes comparable to the Zeeman splitting, the electromagnetic field links individually resonant spin pairs together, forming a spin-Dicke state analogous to that in the dipolar Dicke effect [5][6][7] . These macroscopic effects are observable through measurements of electronic recombination rates, which depend on spin-permutation symmetry of the pair 8 .We monitor the electron-hole recombination current in an OLED, where positive and negative charges are injected into a thin film of an organic semiconductor from opposite electrodes. As the charges drift through the material, they can capture each other on intermolecular length scales owing to weak dielectric screening. These weakly coupled intermolecular electron-hole pairs 9 can ultimately recombine on individual molecules to form a molecular excited state, or exciton, which gives rise to electroluminescence. The subsequent discussion focuses on carrier pairs and not on excitons, which have spin S = 0 or 1. Because the carriers possess spi...

We report room-temperature pulsed electrically detected magnetic resonance measurements of the dark conductivity of films of the fullerene derivative [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) under bipolar (electron-hole) and unipolar (electron-rich) injection conditions. Directly after material deposition, no detectable spin-dependent processes are observed, yet after storage under ambient conditions for more than a day, two distinct spin-dependent mechanisms are found under bipolar injection, suggesting the involvement of degradation-induced electronic states. Spin-Rabi beat oscillation measurements show that at least one of these processes is due to weakly spin-coupled pairs with s = 1/2. The absence of these signals when hole injection is impeded by a barrier suggests that they are due to spin-dependent recombination. The presence of recombination confirms that fullerenes are both electron and hole acceptors, with important consequences for the design, operation, and understanding of plastic solar cells. Electron-hole recombination can occur within homogeneous domains of either the donor or the acceptor of the bulk heterojunction structure, constituting an important dissipative channel in addition to the established interfacial bimolecular recombination loss.

Conceptual inventory surveys are routinely used in education research to identify student learning needs and assess instructional practices. Students might not fully engage with these instruments because of the low stakes attached to them. This paper explores tests that can be used to estimate the percentage of students in a population who might not have taken such surveys seriously. These three seriousness tests are the pattern recognition test, the easy questions test, and the uncommon answers test. These three tests are applied to sets of students who were assessed either by the Force Concept Inventory, the Conceptual Survey of Electricity and Magnetism, or the Brief Electricity and Magnetism Assessment. The results of our investigation are compared to computer simulated populations of random answers.