Potential for spin-based information processing in a thin-film molecular semiconductor

Abstract

Organic semiconductors are studied intensively for applications in electronics and optics1, and even spin-based information technology, or spintronics2. Fundamental quantities in spintronics are the population relaxation time (T1) and the phase memory time (T2): T1 measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and T2 measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine3, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, T1 and T2 are respectively 59 ms and 2.6 μs, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 μs and 1 μs, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range4. T2 is more than two orders of magnitude greater than the duration of the spin manipulation pulses, which suggests that copper phthalocyanine holds promise for quantum information processing, and the long T1 indicates possibilities for medium-term storage of classical bits in all-organic devices on plastic substrates.

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Figure 1: Copper phthalocyanine films.
Figure 2: Concentration dependence of decoherence and relaxation times.
Figure 3: Temperature dependence of decoherence and relaxation times.
Figure 4: Controlling a single qubit.

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Acknowledgements

S.H. and Z.W. thank EPSRC (EP/F039948/1) for the award of a First Grant. S.H. and S.D. thank Kurt J. Lesker and EPSRC for a CASE award. Work at UCL and Imperial College was supported by the EPSRC Basic Technologies grant Molecular Spintronics (EP/F041349/1 and EP/F04139X/1). G.W.M. is supported by the Royal Society. I.S.T. thanks IARPA, NSERC (grant CNXP 22R81695) and PITP for support.

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Contributions

M.W. conducted the electron spin resonance measurements with input and supervision from G.A. and C.W.M.K. S.D., J.A.G. and Z.W. made and characterized the samples with input and supervision from S.H.. M.W., G.W.M., A.M.S., A.J.F., C.W.M.K. and G.A. analysed data, I.S.T. performed theoretical work, and M.W. wrote the manuscript.

Corresponding authors

Correspondence to Marc Warner or Gabriel Aeppli.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Stretched-exponential T1 fits.

Inversion recovery echoes for varying CuPc concentrations fitted with stretched exponentials. The T1 decay of each echo magnitude can also be fitted to a stretched exponential, Aexp(−x/k)β, which is a form characteristic of the random environment that the CuPc molecules experience. In particular, the more isolated molecules will show slower relaxation34. However, because the stretched exponential is a phenomenological fit, it must be interpreted with care, particularly in cases where the underlying distribution of relaxation times is highly non-trivial. This is the case in this work, where relaxation times depend strongly on long-range dipolar interactions and, therefore, the finite size of the crystallites35.

Extended Data Figure 2 Decay times of stretched-exponential fits.

Decay times extracted from the fits in Extended Data Fig. 1 and plotted against CuPc concentration. The concentration dependence of T1 is not greatly affected by the change in fit. This allows the interpretation of the data based on the simpler mono-exponential fits (main text).

Extended Data Figure 3 Power-law exponents of stretched-exponential fits.

Magnitudes of the power-law exponent, β, in the fits in Extended Data Fig. 2 plotted against CuPc concentration. In a uniform environment, β = 1 for the population of spins. The greater is the deviation from this value, the larger is the proportion of long-lived isolated spins relative to the average.

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Warner, M., Din, S., Tupitsyn, I. et al. Potential for spin-based information processing in a thin-film molecular semiconductor. Nature 503, 504–508 (2013). https://doi.org/10.1038/nature12597

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