A superconducting transition edge sensor array for synchrotron soft x-ray emission spectroscopies of low-dimensional...

We present a new instrument for soft x-ray absorption and emission spectroscopy based on the latest generation of transition edge sensor array optimized for soft x-ray detection, built on microwave superconducting quantum interference device multiplexing readout, and cooled with a dilution refrigerator. Its extreme collecting efficiency enables spectroscopy measurements on dilute systems. It also enables shorter acquisition time and large energy-range spectra. We describe the design and operation of the spectrometer and characterize its performance in terms of energy resolution, photon collecting efficiency, and stability. A resolution of 0.7-1.8 eV in the energy range 260-900 eV with a high detection rate of up to 10 000 photons per second across the array is achieved with minimal performance degradation. In addition, the use of the dilution refrigerator to cool the detector results in a robust and stable energy calibration over time. The spectrometer is attached to a dedicated ultra-high vacuum sample chamber equipped with a fully motorized sample cryostat for experiments at temperatures between 10 and 300 K, multi-sample mounting, and electric field-gated devices. The spectrometer-sample chamber is installed at a beamline providing full polarization control. We demonstrate the capabilities of the setup using two representative examples of XES on extremely low-concentration systems, namely monolayer hexagonal boron nitride and the K3[Fe(CN)6] molecular system at sub-millimolar concentration.