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First 90 T magnet in operation at LNCMI-Toulouse

In parallel with the workhorses 60 T and 70 T magnets for users, we pursue our efforts in order to increase the available magnetic field at LNCMI in Toulouse. We have designed and built a new insert for our dual coil system successfully tested up 90.8 T that is the new LNCMI record of non-destructive pulsed magnetic field. This magnet complete our catalogue of high field magnet with the 80 T insert (see EMFL News n°4/2013 p.3) and the 80 T monolithic magnet (see EMFL News n°1/2014 p.3). This inner coil is made of 6.5 mm² copper/niobium composite conductor reinforced with Toyobo Zylon fibres. The winding diameter has been reduced from 14 mm in the 80 T version to 9.5 mm. While the unchanged outer magnet is energized with the 14 MJ capacitor bank, the new inner magnet is energized with the 1.15 MJ mobile bank and could also be used with one 3 MJ module of our new 6 MJ mobile capacitor bank. Liquid nitrogen temperature is available under helium gas atmosphere in 7 mm diameter. First experimental results in these conditions are shown below and demonstrate the usability of this magnet. Temperatures down to 1.5 K are also available in 4 mm diameter. Pulse duration is approximately the same as the 80 T version, i.e. 9.5 ms from 30 T to 90 T as shown on Figure 1. Thanks to a rapid cooling technique the wait time between two 90 T pulses is less than 90 minutes. At the time of writing this article we have made 20 shots above 80 T, and there is no sign of defect or fatigue. We will continue our efforts in order to offer higher magnetic field, longer pulse durations and higher repetition rates. Figure 1 : Time dependence of the magnetic field during the 90.8 T pulse as measured by an in-situ pick-up coil. Inset: field profile around the maximum field showing the inner magnet pulse.

We present below the first results obtained with our magnet in very preliminary studies of simple perovskite films. Solid-state perovskite-based solar cells have made a dramatic impact on emerging PV research (Refs 1-5). Evolving from dye sensitized solar cells, perovskite based cells have demonstrated remarkably high power conversion efficiency of over 15% (4, 5), broad light absorption, and open-circuit voltages (Voc) of over 1.1 V, by employing an organometallic halide perovskite, CH3NH3PbI3-xClx, as the absorber. This progress has occurred very rapidly but to date, the basic electronic properties of the perovskites are hardly known, with considerable uncertainties in the effective masses of the electrons and holes and the values of the exciton binding energies. We have measured the transmission of the white light through the sample as a function of magnetic field. Typical spectra are presented in Figure 2 showing the magnetic field induced exciton energy shift. Figure 2: Transmission spectra through perovskite thin film measured in temperature 77 K.

Références : 1. Lee, MM et al., Science 338, (2012), 643.

2. Ball, JM et al., Energy & Environmental Science, 6, (2013), 1739.

3. Liu, M; Johnston, M. B.; Snaith, H. J., Nature, 501 (2013) 395.

4. Stranks, SD et al., Science 342 (2013) 341.

5. Wang J T-W., et al., Nano Lett. In press (2013).