Solid-state physics

A historic experiment redesigned

Large quasiparticles known as Rydberg excitons have been detected in a natural crystal of copper oxide. The result may find use in applications such as single-photon logic devices. See Letter p.343

An exciton is a quasiparticle in a solid-state system comprising an electron and a hole (the absence of an electron). It has an energy spectrum akin to that of a hydrogen atom, and so may be considered as an artificial hydrogen atom in a solid-state environment, with the hole playing the part of the hydrogen's proton. The concept of excitons was first formulated in the early 1930s by Yakov Frenkel1, who predicted their existence in molecular crystals. A few years later, Gregory Wannier2 and Nevill Mott3 described these electron–hole bound states for inorganic semiconductors. In 1952, Evgeniy Gross and Nury Karryjew4 discovered these Wannier–Mott excitons experimentally in a copper oxide (Cu2O) semiconductor. Now, on page 343 of this issue, Kazimierczuk et al.5 report how they have redesigned this historic experiment to find excitons in a natural crystal of copper oxide. The excitons extend across some tens of billions of lattice sites of the crystal.

Gross and Karryjew's discovery marked the beginning of 'excitonics' — an area of solid-state physics that holds promise for applications in optoelectronics and in information and communication technologies6. For their studies, Gross and Karryjew selected crystals of copper oxide, and, using a spectrograph, identified eight dark lines in the material's transmission spectrum. Such absorption dips indicated the energies of optically induced transitions from the crystal's ground state to excited states with principal quantum numbers n = 2, 3, ... 9 (Fig. 1). The transition energies scaled with n in a similar way to those of a hydrogen atom. This result proved that hydrogen-like quasiparticles, excitons, can be generated in these semiconductor crystals by photoabsorption.

Figure 1: The spectral signatures of excitons.
figure1

Ruben Seisyan/Ref. 4

The diagram shows the spectral absorption dips (black) associated with the hydrogen-like transitions of Wannier–Mott excitons in a copper oxide crystal. In their historic 1952 experiment4, Gross and Karryjew identified the eight dips that correspond to transition energies En, with n = 2, 3, ... 9. The transition with energy E1 is forbidden by optical-selection rules and cannot be observed. Kazimierczuk et al.5 have observed excitons in a natural crystal of copper oxide with n as large as 25. The energies are given in electronvolts.

In their study, Kazimierczuk et al. performed high-resolution transmission spectroscopy of an extremely high-quality natural crystal of copper oxide found at the Tsumeb mine in Namibia using laser light of tunable frequency and ultralow spectral linewidth (corresponding to roughly 1.2 megahertz). Taking advantage of the narrow linewidth of the laser and the high purity of the crystal, the authors have measured transmission spectra of the material with a spectral resolution of 5 nanoelectronvolts — an extremely high value for optical spectroscopy experiments. Analysis of the spectra revealed absorption lines associated with excitons with principal quantum numbers as large as n = 25. The size of an exciton increases as n2, with n = 25 corresponding to a huge quasiparticle filling a sphere of diameter more than 2 μm — about ten times the wavelength of the light needed to create this exciton (see Fig. 1 of the paper5). Moreover, the authors found that excitons that formed at different locations in the crystal had, within the experimental accuracy of the measurements, identical spectral lines. This observation confirmed the extraordinary quality of the sample.

Such enormous quasiparticles, called Rydberg excitons, exhibit unusual quantum phenomena. One of them is giant diamagnetism, which is related to the excitons' ability to counteract an applied magnetic field. This results in a blueward shift of the excitons' spectral lines. Giant diamagnetism in copper oxide was first observed by Gross and colleagues7, and the present experiments confirm this. Another phenomenon observed by Kazimierczuk and co-workers is Rydberg blockade, which manifests as a reduction in excitonic absorption with increasing laser power for lines that correspond to large n. Rydberg blockade means that only a limited number of large Rydberg excitons is permitted within a given volume of the crystal. The effect could be used to make nonlocal all-optical switches and single-photon logic devices.

Kazimierczuk and colleagues' discovery of giant Rydberg excitons opens up new avenues for the field of excitonics. Furthermore, the standard descriptions of light–matter interaction, in a regime in which the size of the exciton resulting from the interaction exceeds the wavelength of light used to create it, need to be revised. The long-standing assumption that extra boundary conditions for the exciton wavefunction, such as Pekar boundary conditions8, must be included in the classical calculation of optical excitations in crystals acquires new importance in view of the nonlocal optical properties introduced in the crystals by giant Rydberg excitons: the light-induced creation of such an exciton at point A in a crystal may strongly modify the crystal's optical properties at point B if B is separated from A by more than 1 μm. Also, the fine structure of the absorption lines associated with excitons of large n may be complex and unusual owing to multiple emission and absorption of virtual photons by the excitons9 and interactions between the spins and orbital momenta of the excitons' electrons.

Finally, with n = 25 having now been achieved, the question arises of whether researchers might be able to observe excitons with principal quantum numbers as large as n = 50, for which the exciton would have a diameter of about 1 mm — and so would, in principle, be visible to the naked eye. Answering this question would require Kazimierczuk and colleagues' experiment to be performed at millikelvin temperatures (the present experiments were done at 1.2 K), with a spectral resolution of about 1 neV. This might seem challenging, but is perhaps not impossible.

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Correspondence to Sven Höfling or Alexey Kavokin.

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Höfling, S., Kavokin, A. A historic experiment redesigned. Nature 514, 313–314 (2014). https://doi.org/10.1038/514313a

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