Posted in | Spectrometers

Angle-Resolved Photoemission Spectroscopy Cryostat for Analyzing Photoelectrons

The electronic structure of the surface of solids is typically studied using angle-resolved photoemission spectroscopy (ARPES).

The ARPES method uses a laser or synchrotron beam to interact with a sample, leading to the emission of photoelectrons. The material’s Fermi surface can be mapped by inspecting the momentum and energy of the photoelectrons. Although ARPES experiments are usually carried out in the 2–10 K range, the latest research trends generally need temperatures that are below 1 K (sub-K).

In 2008, Janis Research launched the world’s first continuous operating high cooling power “wet” He-3 ARPES cryostat. The company successfully achieved a base temperature of 0.9 K. Since then, analogous systems have been set up at top research centers worldwide. However, it became costly to perform an uninterrupted operation with these bulky sub-K ARPES systems because of the recent shortage of helium in the world.

Hence, to address researchers’ demand, Janis Research has effectively come up with another world-first cryostat system for the ARPES technique. The dry He-3 system is a continuously operating, high cooling power cryogen-free sub-K cryostat. It avoids the need (and cost) for liquid helium, reduces the size of the cryostat considerably, and also simplifies the design.

photoelectron emission

Cryostat Design

The dry sub-K ARPES cryostat from Janis Research has been developed and fabricated to enable real UHV operation. It can be baked to 90 °C. The cryostat includes a two-stage pulse tube refrigerator (PTR) that cools two internal platforms as well as thermal shields, to 35 K (first stage) and 3 K (second stage).

This PTR is initially mounted to a z-motion linear translation stage through a metal-sealed CF flange, and then raised and removed from the cryostat at the time of vacuum baking. It is finally lowered into position for standard operation. Two separate closed loops of recirculating He-3 and He-4 gases are precooled by heat exchangers placed on the PTR and platforms.

Using two independent He-3 condensers, the He-3 gas can be converted into liquid; the selection of condenser relies on experimental needs and is made by adjusting the valve. The condensed liquid is collected by using a He-3 pot, and the sample is mounted by using tapped holes in the He-3 pot. Within the thermal shields, accurately aligned slots enable the laser or synchrotron radiation to reach the sample.

The thermal shields are also mounted with movable shutters, which are mechanically connected together. Both movable shutters can be opened at the same time to mount and remove the sample using a wobble stick, and can be subsequently closed to enhance thermal performance. The system is precooled by using a multifunction mechanical heat switch, and is operated in variable temperature (He-4) mode. The He-3 gas is circulated with the help of a large-capacity, gas-sealed root pump.

Sliding sample access shutter

Sliding sample access shutter

Operating Principle

The system can be used in three separate modes.

He-4 Mode: Only He-4 gas is circulated in this mode; circulation of He-3 gas is not needed. After He-4 gas is precooled via the heat exchangers, it is condensed through impedance and enters a 1 K pot. A base temperature of ~1.3 K is achieved by pumping He-4 gas. The mechanical heat switch is configured to allow a direct thermal connection between the He-3 pot and the 1 K pot. This results in variable temperature operation ranging from ~4 K to 320 K. In particular, this operating mode can be used easily, and provides rapid thermal cycling of samples across a wide range of temperatures.

1 K Operation Mode: The mechanical heat switch is opened in this mode. External valves are configured to guide the circulating He-3 gas via a heat exchanger within the 1 K pot. After the He-3 gas is precooled, it is condensed by expansion through impedance. It then accumulates within the He-3 pot and cools it to 500 mK.

JT Operation Mode: The mechanical heat switch is opened in this mode, and the external valves are designed to guide the circulating He-3 gas to the JTCU. The He-3 exhaust stream precools the incoming gas. This gas is condensed by expansion through impedance and collected in the He-3 pot, cooling it to 550 mK.

Summary

Janis Research’s dry sub-K ARPES cryostatis a simple multi-functional tool that is well suited for an extensive range of research applications. The cryostat system provides scientists the means to realize temperatures down to 300 mK with beam slits closed, sub-K temperatures in ARPES configuration, and also high cooling power together with infinite experimental duration.

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