Infrastructure

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.

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.

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.

We operate cryostats optimized for different temperature ranges. Cooling with liquid Helium-4 inside a variable temperature insert allows experiments from room temperature down to 1.5 K. Helium-3 systems cover the 1.5 K to 0.3 K temperature range. With a mixture of Helium-3 and Helium-4 temperatures from 700 mK all the way down to 10 mK can be reached in dilution refrigerators. Each of these systems is equipped with a superconducting magnet. The highest available field is 20 T. In order to serve a broad range of experimental needs, a wide set of sample holders has been developed. They allow sample rotation, microwave irradiation, optical luminescence, Raman spectroscopy, microwave photoconductivity and surface acoustic wave studies.

Because in condensed matter the energy of excitations such as for instance plasmons, the electron spin and cyclotron resonance mode frequently falls into the microwave part of the electromagnetic spectrum, microwave photoconductivity or absorption measurements represent powerful probes. Solid-state microwave sources and spectrum analyzers are available up to 50 GHz. Backward wave oscillators extend the accessible frequency range up to 700 GHz. Sample holders with semi-rigid coaxial lines or waveguides are available even for dilution refrigerator systems, so that at low microwave excitation levels temperatures down to 40 mK can be reached. A quasi-optical setup offers full control over the polarization state of the micro- wave radiation and allows polarization dependent studies down to 1.5 K.

The confocal Raman microscope offers simultaneously optical access and atomic force microscopy (AFM) with an AFM tip mounted on a large aperture microscope objective with a magnification of 100. The confocal optical part allows focusing laser light down to the diffraction limit. Two wavelengths are available: 488 nm and 633 nm. By scanning either the optical beam or the sample stage, spatial maps of the elastically (Rayleigh) and inelastically (Raman) scattered light can be obtained. Studies with circularly polarized light are possible. The AFM part gives access to the topography but also to electric, magnetic and thermodynamic properties. Combining the two technologies enables modes like tip-enhanced Raman and fluorescence microscopy (TERS, TEFS) or scanning near-field optical microscopy (SNOM).

Our ambient atomic force microscope with crosstalk elimination offers a lateral resolution down to 5 nm. It is an important tool that provides feedback during sample fabrication. It possesses a height resolution of better than 0.1 nm. Single atomic layers of for instance graphene, boron-nitride and molybdenum-disulfide can be distinguished. In addition to recording the topography, it is possible to map the conductance, study magnetic interactions and do force spectroscopy. Due to an acoustic enclosure and an active piezo table for decoupling purposes, scan acquisition times of hours are possible for acquiring high resolution images up to 4096 x 4096 pixels within a scan range of 50 x 50 µm. Such scans aid precision alignment in electron beam lithography used to pattern leads, gates or etching masks.

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. An optical cryostat with four large windows and a variable temperature insert allows direct optical access in both the Voigt and Faraday geometry.  Magnetic fields upto 11 T can be applied and the lowest accessible temperature in this optical cryostat is approximately 1.5 K.

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 the compact, turnkey glass fiber laser system 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 a cryostat system equipped with a 17 T superconducting magnet 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.