Posted in | Spectrometers

Cryogen-Free Sub-Kelvin Angle-Resolved Photoemission Spectroscopy System

Angle-Resolved Photoemission Spectroscopy (ARPES) is a technique used to examine the electronic structure of the surface of solids. The ARPES method involves using a synchrotron or laser beam to interact with a sample, causing the emission of photoelectrons.

The Fermi surface of the material can be mapped by examining the momentum and energy of the photoelectrons. ARPES experiments are mostly carried out in the 2–10 K range, but new research developments generally require temperatures below 1 K (sub-K).

In 2008, Janis Research developed the world’s first continuous operating high cooling power “wet” He-3 ARPES cryostat, realizing a base temperature of 0.9 K. Analogous systems have since been fitted at top research facilities globally, but the latest worldwide helium shortage has made continuous operation of these large sub-K ARPES systems expensive.

In response to the demands of researchers, Janis Research has successfully created another world-first, continuously operating high cooling power cryogen-free (“dry”) sub-K cryostat for ARPES. This dry He-3 system eliminates the necessity (and cost) to use liquid helium, while also streamlining the design and greatly decreasing the cryostat size.

Cryostat Design

The Janis Research dry sub-K ARPES cryostat is designed and engineered for true UHV operation, and can be baked to 90 °C. A two-stage pulse tube refrigerator (PTR) is provided for cooling two internal platforms and thermal shields to 35 K (first stage) and 3 K (second stage). The PTR is fixed by metal-sealed CF flange to a z-motion linear translation stage.

The PTR is raised and detached from the cryostat during vacuum baking, and then lowered into position for regular operation. Heat exchangers placed on the PTR and platforms precool two separate closed loops of recirculating He-4 and He-3 gas. Two independent He-3 condensers can be used to change the He-3 gas into liquid; the choice of condenser is based upon experimental needs and is made by valve adjustment.

A He-3 pot is used to collect the condensed liquid, and tapped holes in the He-3 pot are used for sample mounting. Exactly aligned slots in the thermal shields enable the synchrotron radiation or laser to reach the sample. Movable shutters are also fixed on the thermal shields, and are mechanically connected together.

The two shutters can be opened simultaneously for sample mounting and removal using a wobble stick, and then shut to enhance thermal performance.

A multifunctional mechanical heat switch is utilized to precool the system, and to run the system in variable temperature (He-4) mode. A large-capacity, gas-sealed root pump is employed to circulate the He-3 gas.

Sliding sample access shutter

Sliding sample access shutter

Operating Principal

The system can be run in three diverse modes.

He-4 Mode: In this mode, only He-4 gas is circulated; He-3 circulation is not necessary. After being precooled via the heat exchangers, He-4 gas is condensed through impedance and enters a 1 K pot. When He-4 gas is pumped, it results in a base temperature of ~1.3 K.

The mechanical heat switch is configured to offer a direct thermal connection between the 1 K pot and the He-3 pot, resulting in variable temperature operation in the range of ~4 K to 320 K. This operating mode is particularly user-friendly, and enables faster thermal cycling of samples across a broad temperature range.

1 K Operation Mode: The mechanical heat switch is opened in this mode. External valves are configured such that the circulating He-3 gas is directed via a heat exchanger inside the 1 K pot. He-3 gas is precooled and then condensed after expansion through impedance, collected within the He-3 pot, which is cooled to 500 mK.

JT Operation Mode: The mechanical heat switch is opened in this mode, and the external valves are configured to guide the circulating He-3 gas to the JTCU. The incoming gas is precooled to ~2 K by the He-3 exhaust stream, and then condensed by expansion through impedance, thus cooling the He-3 pot to 550 mK.

Summary

The Janis Research dry sub-K ARPES cryostat is an easy-to-use, multi-functional tool, ideal for a broad range of research applications. The system provides scientists the means to realize temperatures as low as 300 mK with closed beam slits, sub-K temperatures in ARPES configuration, as well as high cooling power and unrestricted experimental duration.

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