Cooling low-dimensional electron systems into the microkelvin regime

Abstract

Two-dimensional electron gases (2DEGs) with high mobility, engineered in semiconductor heterostructures host a variety of ordered phases arising from strong correlations, which emerge at sufficiently low temperatures. The 2DEG can be further controlled by surface gates to create quasi-one dimensional systems, with potential spintronic applications. Here we address the long-standing challenge of cooling such electrons to below 1 mK, potentially important for identification of topological phases and spin correlated states. The 2DEG device was immersed in liquid 3He, cooled by the nuclear adiabatic demagnetization of copper. The temperature of the 2D electrons was inferred from the electronic noise in a gold wire, connected to the 2DEG by a metallic ohmic contact. With effective screening and filtering, we demonstrate a temperature of 0.9 ± 0.1 mK, with scope for significant further improvement. This platform is a key technological step, paving the way to observing new quantum phenomena, and developing new generations of nanoelectronic devices exploiting correlated electron states.

Superconductivity in AuNiGe Ohmic contacts to a GaAs-based high mobility two-dimensional electron gas

“…We have achieved this using 4mm×4mm samples, see schematic in Fig. 1(a) inset, and elsewhere we have used one of these samples to demonstrate 5 an electron temperature of T e = 1 mK, with a a) Electronic mail: james.nicholls@rhul.ac.uk heat leak to the 2DEG in the fW range. In this Letter we report the unexpected discovery that the AuNiGe contacts become superconducting below 1 K. The superconductivity in the ohmic contact appears as a variable series resistance to the 2DEG sample, and will hinder how the electrons in the 2DEG are cooled to ultra-low temperatures.…”

“…Although the R c of the contacts are lower below T c , the overriding effect will be their reduced ability to cool the 2DEG because superconductors have low thermal conductivities. 23 Using noise thermometry on a 4mm×4mm similar to sample A, where the electrons are cooled in a 3 He immersion cell, Levitin et al 5 report that at 1-3 mK the thermal conductance through the contacts is about 10% of that expected from applying the Wiedemann-Franz law to their normal state electrical resistances R C ≈ 1 Ω. Given the widespread use of AuNiGe contacts annealed at 400-450 • C for both GaAs-and InGaAs-based 2DEGs, it is possible superconducting contacts were used in previous studies and the superconductivity in low magnetic fields (< 0.15 T) could be the factor why it has been historically difficult to cool GaAs-based 2DEGs below 50 mK until recently.…”

Quantum bath suppression in a superconducting circuit by immersion cooling

“…Derived from recent advances in ultra-low temperature technology and the cooling of electronic systems to sub-mK temperatures 28 , 29 we construct an immersion cell suitable for a superconducting quantum circuit. Cooling is achieved by placing the circuit, in our case an NbN superconducting resonator 30 on a sapphire substrate, inside the immersion cell, as shown in Fig.…”

Microstructuring YbRh2Si2 for resistance and noise measurements down to ultra-low temperatures

“…Using the observed T s = 1.60 mK at T 0 = 1.44 mK we estimate Q0 ≈ 0.1 pW from equation (8). Thorough shielding of the sample and filtering of the measurement lines [73] are expected to reduce Q0 by 1-2 orders of magnitude, opening temperatures well below 1 mK to microstructured devices and enabling driven resistance measurements, that can clearly distinguish between the sample and contact resistances.…”