Cryogenic Superconducting Magnet Systems

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.

Microwave Sources

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.

Combined Confocal Raman and Atomic Force Setup

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).

Atomic Force Microscope

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.

Chemical Vapor Deposition System

With the chemical vapor deposition system it is possible to grow large-area graphene layers on metallic foils of for instance copper or platinum. The system is based on a three zone furnace reaching a maximum temperature of 1200°C. It is equipped with precision mass flow controllers covering different ranges of flow rates: 0.8-40, 10-500 and 40-2000 sccm. Both liquid as well as gaseous precursors can be used. The attached pumping system enables both ambient as well as low pressure growth conditions. The system can also be operated for etching in the presence of oxygen. Intrinsic defects or prepatterned holes in graphene enlarge and can be converted at high temperatures into hexagonal shaped holes with edges having well-defined zig-zag chirality.

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. 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.

Femtosecond Lasers

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.
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