Infrastructure

Dilution Refrigerator Facilities

To perform advanced transport and optical measurements down to ultralow temperatures, the group operates multiple He³/He⁴ dilution refrigerators. Temperatures down to 10 mK can be reached and the samples can be exposed to magnetic fields as high as 21.5 T. One of these systems is equipped with a 9T-3T-1T vector magnet. Sample rods with various capabilities are available. These include: sample rotation, microwave excitation using a waveguide and/or coaxial line, optical excitation for photoluminescence and Raman spectroscopy using optical fibers. Surface acoustic wave excitation and detection are also available.

Dry Cryostat Facility

Transport studies at temperatures down to 1.5 K proceed in our dry cryostat facility. Three systems are available. They are each equipped with a variable temperature insert as well as an axial superconducting magnet. The sample temperature can be varied continuously from 1.5 K up to 300 or 400 K and the maximum available perpendicular field in these systems is 12 or 14 T, depending on the system. For one of these systems a sample rod with a rotatable sample stage is available, so that it is possible to change the magnetic field orientation with respect to the sample surface in-situ from out-of-plane to in-plane.

Glovebox Facility

The fabrication of van der Waals heterostructures through stacking and twisting of layers of 2D materials is carried out in our glovebox facility offering an inert argon gas atmosphere, so that it is also possible to handle materials that are sensitive to ambient air. The gloveboxes host, among others, equipment for the transfer and stacking of 2D materials, an atomic force microscope and a microscope geared for the accurate placement of electrolyte drops for on-chip single device electrochemistry driven intercalation experiments.

Cleanroom Facility

The group operates a small clean room facility that hosts all necessary equipment for the exfoliation of 2D materials, identification of the layer thicknesses as well as the preparation of stamps used for the transfer and stacking of 2D materials to fabricate van der Waals heterostructures. Advanced electron beam lithography, etching and deposition of metals proceeds in the large central clean room facility of the institute.

Microwave/Terahertz Optical Facility

Together with the Department Solid State Spectroscopy, we operate a microwave and terahertz optical facility that allows photoconductivity, transmission, absorption as well as interference experiments in the microwave and terahertz frequency range (up to about 800 GHz). Despite the long wavelengths, the electromagnetic radiation is treated as an optical beam offering unmatched control over the polarization properties (linear, circular or even elliptical). The sample can be mounted in an optical cryostat both in the Faraday as well as the Voigt geometry. The cryostat is equipped with a radial supercondicting magnet offering fields up to 11 T. The microwave and terahertz radiation are generated with continuously tunable backward wave oscillators. The accessible frequency range matches well with the energy scale of important excitations in low dimensional systems such as spin resonance, cyclotron resonance, plasmons as well as moiré bandgaps.

Atomic Force Microscopy

Our atomic force microscope (AFM) is located in an argon filled glovebox. It is an important tool during sample fabrication. In addition to recording the topography and identifying the thickness of 2D material sheets, it is possible to map the conductance, study magnetic interactions and do force spectroscopy. AFM “ironing” by applying a constant force with the tip on fully processed van der Waals heterostructures can significantly enhance the transport quality of such heterostructures. The AFM is equipped with a magnetic stage that allows to apply an in-plane field up to 0.5 T or an out-of-plane field up to 0.2 T.

Optics Facility

Magneto-optical luminescence and Raman spectroscopy studies can be performed with laser diodes, or a tunable continuous wave titanium:sapphire laser as the excitation source and a double or triple grating spectrometer equipped with a liquid nitrogen cooled CCD camera as detector. The optics facility is connected via glass fibers to any of the available magnetic field systems with variable temperature or dilution refrigerator inserts. For time-resolved transport and optical studies femtosecond laser sources are available. They offer pulse widths down to 100 fs at wavelengths of 1550 nm (glass fiber laser) and between 700 and 900 nm (titanium:saphire laser), respectively. A second harmonic generation stage allows the use of fiber laser systems also at 780 nm. With an optical parametric oscillator the wavelength range of the titanium:sapphire laser can be extended from 1100 to 1700 nm. The laser pulses can be guided into any cryostat system using a single mode glass fiber. The dispersion of the glass fiber is compensated with a grating dispersion compensator. Auto- correlators and spectrum analyzers are available for pulse and beam characterization.