Organic semiconductors offer a unique environment to probe the hyperfine coupling of electronic spins to a nuclear spin bath. We explore the interaction of spins in electron-hole pairs in the presence of inhomogeneous hyperfine fields by monitoring the modulation of the current through an organic light emitting diode under coherent spin-resonant excitation. At weak driving fields, only one of the two spins in the pair precesses. As the driving field exceeds the difference in local hyperfine field experienced by electron and hole, both spins precess, leading to pronounced spin beating in the transient Rabi flopping of the current. We use this effect to measure the magnitude and spatial variation in hyperfine field on the scale of single carrier pairs, as required for evaluating models of organic magnetoresistance, improving organic spintronics devices, and illuminating spin decoherence mechanisms.

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.

Polaron pairs are intermediate electronic states that are integral to the optoelectronic conversion process in organic semiconductors. Here, we report on electrically detected spin echoes arising from direct quantum control of polaron pair spins in an organic light-emitting diode at room temperature. This approach reveals phase coherence on a microsecond time scale, and offers a direct way to probe charge recombination and dissociation processes in organic devices, revealing temperature-independent intermolecular carrier hopping on slow time scales. In addition, the long spin phase coherence time at room temperature is of potential interest for developing quantum-enhanced sensors and information processing systems which operate at room temperature.

Pulsed electrically detected magnetic resonance offers a unique avenue to distinguish between polaronpair (PP) and triplet-exciton polaron (TEP) spin-dependent recombination, which control the conductivity and magnetoresistivity of organic semiconductors. Which of these two fundamental processes dominates depends on carrier balance: by injecting surplus electrons we show that both processes simultaneously impact the device conductivity. The two mechanisms are distinguished by the presence of a half-field resonance, indicative of TEP interactions, and transient spin beating, the signature of PPs. Coherent spin Rabi flopping in the half-field (triplet) channel is observed, demonstrating that the triplet exciton has an ensemble phase coherence time of at least 60 ns, offering insight into the effect of carrier correlations on spin dephasing.

A study of donor electron spins and spin-dependent electronic transitions involving phosphorous ( 31 P) atoms in proximity of the (111) oriented crystalline silicon (c-Si) to silicon dioxide (SiO 2 ) interface is presented for [ 31 P] = 10 15 cm −3 and [ 31 P] = 10 16 cm −3 at about liquid 4 He temperatures (T = 5 K − 15 K). Using pulsed electrically detected magnetic resonance (pEDMR), spin-dependent transitions between the 31 P donor state and two distinguishable interface states are observed, namely (i) P b centers which can be identified by their characteristic anisotropy and (ii) a more isotropic center which is attributed to E ′ defects of the SiO 2 bulk close to the interface. Correlation measurements of the dynamics of spin-dependent recombination confirm that previously proposed transitions between 31 P and the interface defects take place. The influence of these electronic near-interface transitions on the 31 P donor spin coherence time T 2 as well as the donor spin-lattice relaxation time T 1 is then investigated by comparison of spin Hahn-echo decay measurements obtained from conventional bulk sensitive pulsed electron paramagnetic resonance and surface sensitive pEDMR, as well as surface sensitive electrically detected inversion recovery experiments. The measurements reveal that both T 2 and T 1 of 31 P donor electrons spins in proximity of energetically lower interface states at T ≤ 13 K are reduced by several orders of magnitude.

Substitutional donor atoms in silicon are promising qubits for quantum computation with extremely long relaxation and dephasing times demonstrated. One of the critical challenges of scaling these systems is determining inter-donor distances to achieve controllable wavefunction overlap while at the same time performing high fidelity spin readout on each qubit. Here we achieve such a device by means of scanning tunnelling microscopy lithography. We measure anti-correlated spin states between two donor-based spin qubits in silicon separated by 16 ± 1 nm. By utilising an asymmetric system with two phosphorus donors at one qubit site and one on the other (2P−1P), we demonstrate that the exchange interaction can be turned on and off via electrical control of two in-plane phosphorus doped detuning gates. We determine the tunnel coupling between the 2P−1P system to be 200 MHz and provide a roadmap for the observation of two-electron coherent exchange oscillations.

In this work we perform direct single-shot readout of the singlet-triplet states in exchange coupled electrons confined to precision-placed donor atoms in silicon. Our method takes advantage of the large energy splitting given by the Pauli-spin blockaded (2,0) triplet states, from which we can achieve a singleshot readout fidelity of 98.4 AE 0.2%. We measure the triplet-minus relaxation time to be of the order 3 s at 2.5 T and observe its predicted decrease as a function of magnetic field, reaching 0.5 s at 1 T. DOI: 10.1103/PhysRevLett.119.046802 An increased ability to control and manipulate quantum systems is driving the field of quantum computation forward [1][2][3][4]. The spin of a single electron in the solid state has long been utilized in this context [5][6][7][8][9][10][11], providing a superbly clean quantum system with two orthogonal quantum states that can be measured with over 99% fidelity [12]. As a natural next step, the coupling of two electrons at separate sites has been studied in gate-defined quantum dots [5,13,14], as well as in donor systems [15][16][17]. In addition to being the eigenstates for two coupled spins, the singlet-triplet (ST) states of two electrons can form a qubit subspace, and have previously been utilized for quantum information processing [6,[18][19][20][21][22]. Unlike in gate-defined quantum dots, donor systems do not require electrodes to confine electrons. The resulting decrease in physical complexity makes donor nanodevices very appealing for scaling up to many electron sites [15].In the (1,1) charge configuration the ST states are eigenstates if the exchange coupling is greater than any difference in Zeeman energy between the two spins. The singlet and three triplet states are split only by the Zeeman energy in the cases of jT þ i ¼ j↑↑i and jT − i ¼ j↓↓i, and an exchange energy, J, for the singlet jSi ¼ ðj↑↓i − j↓↑iÞ= ffiffi ffi 2 p and jT 0 i ¼ ðj↑↓i þ j↓↑iÞ= ffiffi ffi 2 p states. However, in the (2,0) configuration all triplet states split from the singlet jSð2; 0Þi by a larger exchange interaction, Δ ST , measured in previous works to be > 5 meV for donors [23]. The triplet states are therefore blocked from tunneling from the ð1; 1Þ → ð2; 0Þ charge configuration, known as Pauli spin blockade.Typically, direct ST readout is performed by charge discrimination between the (1,1) and (2,0) states below the ST energy splitting Δ ST . However, this relies on the charge sensor having a large enough differential capacitive coupling to each dot to discriminate between the two charge states. This is not possible in some architectures due to symmetry constraints, in particular, for donors it is advantageous for multiple donor sites to be coupled equally to a charge sensor for independent readout and/or loading. The tightly confined electron wave function at each donor site therefore necessitates that they are equidistant from the charge sensor. As a consequence ð1; 1Þ ↔ ð2; 0Þ charge transfer signals are often too small to detect directly in this architecture.Until now s...

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