Cryostat

Low-temperature STM measurements were performed using a cryostat (Oxford Instruments) equipped with an UHV insert hosting a Tribus STM head (Sigma Surface Science) operated at a temperature T = 1.9 K. All measurements have been performed using electrochemically etched tungsten tips. Before measurements, the tips were conditioned on a Ag(111) single crystal. Spectroscopic data have been obtained using the lock-in technique and a bias voltage modulation in between 1 and 20 meV at a frequency of 793 Hz, with the amplitude progressively increasing with the scanning bias. dI/dU maps have been acquired simultaneously to topographic images.

CrBr₃ multilayers were mechanically exfoliated from the crystals discussed in the section of crystal growth. Tunnel junctions of multilayer graphene/CrBr₃/multilayer graphene were assembled using a pick-and-lift technique with stamps of PDMS/PC. To avoid degradation of thin CrBr₃ multilayers, the exfoliation of CrBr₃ and the heterostructure stacking process were done in a glove box filled with Nitrogen gas, and the whole tunneling junction was encapsulated with hBN before being taken out. Conventional electron beam lithography, reactive-ion etching, electron-beam evaporation (10 nm/50 nm Cr/Ar) and lift-off process were used to make edge contacts to the multilayer graphene. The thickness of the layers was determined by atomic force microscope measurements performed outside the glove box, on the encapsulated devices. Transport measurements were performed in a cryostat from Oxford Instruments, using home-made low-noise electronics.

The powdered sample without further preparation was placed in a standard 5 mm glass sample tube, which was subsequently placed inside the probe of the NQR spectrometer. ³⁵ Cl NQR spectra were recorded using NMR/NQR Tecmaq Redstone spectrometer by the single pulse method in the frequency range 35–37 MHz. The pulse duration was 3 µs, followed by 1 ms acquisition and subsequent 400 ms delay time. The number of scans was ca. 3000. The temperature was controlled using an Oxford Instrument cryostat and stabilized with the precision better than ±1 K.

The sample was ground to give uniform particle distribution and placed in a 40 × 10 × 2 mm quartz cuvette to ensure optical thickness. A cuvette sample holder with a white polytetra­fluoro­ethyl­ene (PTFE) block spacer was used to load the sample into an Oxford Instruments Cryostat. The sample was irradiated with an Ocean Optics halogen light source and an Avantes AvaSpec-2048-2 CCD detector (placed at an acute angle to minimize detection of specular reflectance) collected the reflectance spectra, which were recorded using AvaSoft basic software. Cryostat temperature control was performed using an Oxford Intelligent Temperature Controller and each temperature was stabilized for 10 min or until ±0.1 K before recording the spectrum. A white PTFE block was used to record a reference spectrum before each data set collection. Irradiation was carried out using a 405 nm laser po­inter or UV LEDs after an initial ground-state spectrum was collected. The diffuse reflectance spectra are illustrated as percent reflectance versus wavelength and Kubelka–Munk function, F(R), versus wavelength. If S is independent of λ, then F(R) versus λ is equivalent to the absorption spectrum for a diffuse reflector. To allow basic trends to be easily observed, moving averages were applied to data during analysis.