July 05, 2011

Picosecond switch for superconductors

A high-temperature superconductor can now be switched on and off within a trillionth of a second. A team including physicists from the University of Oxford and the Max Planck Research Group for Structural Dynamics at the University of Hamburg has realised an ultrafast superconducting switch by using intense terahertz pulses. This experiment opens up the possibility to discover more about the still unsettled cause of this type of superconductivity, and also hints at possible applications for ultrafast electronics in the future.

Nature Photonics - Bi-directional ultrafast electric-field gating of interlayer charge transport in a cuprate superconductor

The high-temperature superconductor used by the Hamburg scientists has been known for a long time. It is a crystal based on lanthanum cuprate (La2CuO4) to which a specific quantity of strontium has been added (La1,84Sr0,16CuO4). Its transition temperature is minus 233 degrees Celsius. Although it is not yet completely clear how the superconductivity arises here, essential elements are known: “The crystal is formed by copper-oxygen planes which lie on top of each other like the pages of a book,” explains Cavalleri. The electrons can only move within these planes; the current transport therefore only occurs in two dimensions.

If the material is cooled below 40 Kelvin, a link is suddenly created between these two planes. Physicists explain this using the wave model, according to which the electrons are pictured not as particles, but as waves. Below the transition temperature the electrons from neighbouring planes overlap, and this allows the electric charge carriers to change from one plane to the other. Current is suddenly transported in all three spatial dimensions: the superconducting state has been created.

Cavalleri and his colleagues then wanted to know whether this transport between the layers can be deliberately interrupted and switched on again. In theory this is possible if a very strong electric field is applied at right angles to the layers. However, applying such a field is impractical. “This causes the crystal to heat upand the superconductivity collapses,” explains Cavalleri. The solution was to send in an ultrashort pulse of light to manipulate the superconductor.

This so-called terahertz pulse is an electromagnetic wave, similar to light, but with a much longer wavelength. It has an electric field that briefly destroys the coupling of the electron waves between the planes when it penetrates into the crystal. This is only successful if the electric field strength of the pulse is very high, in the order of several ten thousand volts per centimetre. And it must be short enough that it does not heat up the crystal.

The experiment, which Andreas Dienst designed and carried out in Oxford, succeeded as anticipated: for the short time of less than one picosecond (10-12 seconds) as the pulse interacts with the superconductor, the coupling between the planes, and thus the superconductivity, was interrupted before subsequently returning. The superconductor does not suffer in this process and can be switched as often as one likes.

In cuprate superconductors, tunnelling between planes makes three-dimensional superconductive transport possible. However, the interlayer tunnelling amplitude is reduced when an order-parameter-phase gradient between planes is established. As such, interlayer superconductivity along the c-axis can be weakened if a strong electric field is applied along the c-axis. In this Letter, we use high-field single-cycle terahertz pulses to gate interlayer coupling in La1.84Sr0.16CuO4. We induce ultrafast oscillations between superconducting and resistive states and switch the plasmon response on and off, without reducing the density of Cooper pairs. In-plane superconductivity remains unperturbed, revealing a non-equilibrium state in which the dimensionality of the superconductivity is time-dependent. The gating frequency is determined by the electric field strength. Non-dissipative, bi-directional gating of superconductivity is of interest for device applications in ultrafast nanoelectronics and represents an example of how nonlinear terahertz physics can benefit nanoplasmonics and active metamaterials.

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