21 research outputs found
Enhancing quantum exchanges between two oscillators
We explore the extent to which two quantum oscillators can exchange their
quantum states efficiently through a three-level system which can be spin
levels of colored centers in solids. High transition probabilities are obtained
using Hamiltonian engineering and quantum control techniques. Starting from a
weak coupling approximation, we derive conditions on the spin-oscillator
interaction Hamiltonian that enable a high fidelity exhange of quanta. We find
that these conditions cannot be fulfilled for arbitrary spin-oscillator
coupling. To overcome this limitation, we illustrate how a time-dependent
control field applied to the three-level system can lead to an effective
dynamic that performs the desired exchange of excitation. In the strong
coupling regime, an important loss of fidelity is induced by the dispersion of
the excitation onto many Fock states of the oscillators. We show that this
detrimental effect can be substantially reduced by suitable control fields,
which are computed with optimal control numerical algorithms.Comment: 11 figure
Singular relaxation of a random walk in a box with a Metropolis Monte Carlo dynamics
We study analytically the relaxation eigenmodes of a simple Monte Carlo
algorithm, corresponding to a particle in a box which moves by uniform random
jumps. Moves outside of the box are rejected. At long times, the system
approaches the equilibrium probability density, which is uniform inside the
box. We show that the relaxation towards this equilibrium is unusual: for a
jump length comparable to the size of the box, the number of relaxation
eigenmodes can be surprisingly small, one or two. We provide a complete
analytic description of the transition between these two regimes. When only a
single relaxation eigenmode is present, a suitable choice of the symmetry of
the initial conditions gives a localizing decay to equilibrium. In this case,
the deviation from equilibrium concentrates at the edges of the box where the
rejection probability is maximal. Finally, in addition to the relaxation
analysis of the master equation, we also describe the full eigen-spectrum of
the master equation including its sub-leading eigen-modes
Spin-dependent recombination probed through the dielectric polarizability.
Despite residing in an energetically and structurally disordered landscape, the spin degree of freedom remains a robust quantity in organic semiconductor materials due to the weak coupling of spin and orbital states. This enforces spin-selectivity in recombination processes which plays a crucial role in optoelectronic devices, for example, in the spin-dependent recombination of weakly bound electron-hole pairs, or charge-transfer states, which form in a photovoltaic blend. Here, we implement a detection scheme to probe the spin-selective recombination of these states through changes in their dielectric polarizability under magnetic resonance. Using this technique, we access a regime in which the usual mixing of spin-singlet and spin-triplet states due to hyperfine fields is suppressed by microwave driving. We present a quantitative model for this behaviour which allows us to estimate the spin-dependent recombination rate, and draw parallels with the Majorana-Brossel resonances observed in atomic physics experiments.This work was supported by the Engineering and Physical Sciences Research Council [Grants
No. EP/G060738/1]. A. D. C. acknowledges support from the E. Oppenheimer Foundation and St Catharine's
College, Cambridge. S. L. B. is grateful for support from the EPSRC Supergen SuperSolar Project, the Armourers
and Brasiers Gauntlet Trust and Magdalene College, Cambridge.This is the final published version of the article. It was originally published in Nature Communications (Bayliss et. al, Nature Communications 2015, 6, 8534, doi:10.1038/ncomms9534). The final version is available at http://dx.doi.org/10.1038/ncomms953
Spin-dependent recombination probed through the dielectric polarizability.
Despite residing in an energetically and structurally disordered landscape, the spin degree of freedom remains a robust quantity in organic semiconductor materials due to the weak coupling of spin and orbital states. This enforces spin-selectivity in recombination processes which plays a crucial role in optoelectronic devices, for example, in the spin-dependent recombination of weakly bound electron-hole pairs, or charge-transfer states, which form in a photovoltaic blend. Here, we implement a detection scheme to probe the spin-selective recombination of these states through changes in their dielectric polarizability under magnetic resonance. Using this technique, we access a regime in which the usual mixing of spin-singlet and spin-triplet states due to hyperfine fields is suppressed by microwave driving. We present a quantitative model for this behaviour which allows us to estimate the spin-dependent recombination rate, and draw parallels with the Majorana-Brossel resonances observed in atomic physics experiments.This work was supported by the Engineering and Physical Sciences Research Council [Grants
No. EP/G060738/1]. A. D. C. acknowledges support from the E. Oppenheimer Foundation and St Catharine's
College, Cambridge. S. L. B. is grateful for support from the EPSRC Supergen SuperSolar Project, the Armourers
and Brasiers Gauntlet Trust and Magdalene College, Cambridge.This is the final published version of the article. It was originally published in Nature Communications (Bayliss et. al, Nature Communications 2015, 6, 8534, doi:10.1038/ncomms9534). The final version is available at http://dx.doi.org/10.1038/ncomms953
Spin-dependent recombination probed through the dielectric polarizability
Despite residing in an energetically and structurally disordered landscape, the spin degree of freedom remains a robust quantity in organic semiconductor materials due to the weak coupling of spin and orbital states. This enforces spin-selectivity in recombination processes which plays a crucial role in optoelectronic devices, for example, in the spin-dependent recombination of weakly bound electron-hole pairs, or charge-transfer states, which form in a photovoltaic blend. Here, we implement a detection scheme to probe the spin-selective recombination of these states through changes in their dielectric polarizability under magnetic resonance. Using this technique, we access a regime in which the usual mixing of spin-singlet and spin-triplet states due to hyperfine fields is suppressed by microwave driving. We present a quantitative model for this behaviour which allows us to estimate the spin-dependent recombination rate, and draw parallels with the Majorana–Brossel resonances observed in atomic physics experiments
Long range electronic transport in DNA molecules deposited across a disconnected array of metallic nanoparticles
We report in detail our experiments on the conduction of DNA
molecules over a wide range of temperature deposited across slits in a few
nanometers thick platinum film. These insulating slits were fabricated using
focused ion beam etching and characterized extensively using near field and
electron microscopy. This characterization revealed the presence of metallic Ga
nanoparticles inside the slits, as a result of the ion etching. After
deposition of DNA molecules, using a protocol that we describe in
detail, some of the slits became conducting and exhibited superconducting
fluctuations at low temperatures. We argue that the observed conduction was due
to transport along DNA molecules, that interacted with the Ga nanoparticles
present in the slit. At low temperatures when Ga becomes superconducting,
induced superconductivity could therefore be observed. These results indicate
that minute metallic particles can easily transfer charge carriers to attached
DNA molecules and provide a possible reconciliation between apparently
contradictory previous experimental results concerning the length over which
DNA molecules can conduct electricity
Geminate and nongeminate recombination of triplet excitons formed by singlet fission.
We report the simultaneous observation of geminate and nongeminate triplet-triplet annihilation in a solution-processable small molecule TIPS-tetracene undergoing singlet exciton fission. Using optically detected magnetic resonance, we identify recombination of triplet pairs directly following singlet fission, as well as recombination of triplet excitons undergoing bimolecular triplet-triplet annihilation. We show that the two processes give rise to distinct magnetic resonance spectra, and estimate the interaction between geminate triplet excitons to be 60 neV.EPSRC [grant no. EP/J017361/1 and EP/G060738/1]. E. Oppenheimer Foundation and St. Catherine's College, Cambridge. NSF [CMMI- 1255494].This is the author accepted manuscript. The final version is available at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.238701
Site-selective measurement of coupled spin pairs in an organic semiconductor
From organic electronics to biological systems, understanding the role of
intermolecular interactions between spin pairs is a key challenge. Here we show
how such pairs can be selectively addressed with combined spin and optical
sensitivity. We demonstrate this for bound pairs of spin-triplet excitations
formed by singlet fission, with direct applicability across a wide range of
synthetic and biological systems. We show that the site-sensitivity of exchange
coupling allows distinct triplet pairs to be resonantly addressed at different
magnetic fields, tuning them between optically bright singlet (S=0) and dark
triplet, quintet (S=1,2) configurations: this induces narrow holes in a broad
optical emission spectrum, uncovering exchange-specific luminescence. Using
fields up to 60 T, we identify three distinct triplet-pair sites, with exchange
couplings varying over an order of magnitude (0.3-5 meV), each with its own
luminescence spectrum, coexisting in a single material. Our results reveal how
site-selectivity can be achieved for organic spin pairs in a broad range of
systems.Comment: 8 pages, article, 7 pages, supporting informatio
Enhancing quantum exchanges between two oscillators
We explore the extent to which two quantum oscillators can exchange their quantum states efficiently through a three-level system which can be spin levels of colored centers in solids. High transition probabilities are obtained using Hamiltonian engineering and quantum control techniques. Starting from a weak coupling approximation, we derive conditions on the spin-oscillator interaction Hamiltonian that enable a high fidelity exhange of quanta. We find that these conditions cannot be fulfilled for arbitrary spin-oscillator coupling. To overcome this limitation, we illustrate how a time-dependent control field applied to the three-level system can lead to an effective dynamic that performs the desired exchange of excitation. In the strong coupling regime, an important loss of fidelity is induced by the dispersion of the excitation onto many Fock states of the oscillators. We show that this detrimental effect can be substantially reduced by suitable control fields, which are computed with optimal control numerical algorithms