Jan Nyéki

Topological superconductors, characterized by spin-triplet Cooper pairing, are important for exploring unconventional pairing mechanisms and protected quantum states. Yet experimentally established odd-parity, spin-triplet superconductors remain scarce. Here we demonstrate that the heavy-fermion compound YbRh ₂ Si ₂ hosts distinct magnetic-field-tuned superconducting states, both Pauli limited and beyond this limit, revealed by high-resolution measurements of the complex electrical impedance. We also find that superconductivity is abruptly suppressed at the critical field associated with the primary antiferromagnetic transition of this compound. The onset of electro-nuclear spin density wave order enhances the superconductivity. We propose that this behaviour can be explained by the formation of a pair density wave that boosts a selected spin-triplet superconducting order parameter. Together, our findings indicate odd-parity superconductivity in YbRh ₂ Si ₂ and point to one of the superconducting states being the topological helical phase.

We previously reported the discovery of a two-dimensional He 4 supersolid; a state with intertwined density wave and superfluid order, observed in the second layer of He 4 adsorbed on graphite. In this Letter we provide direct evidence for the density wave order, obtained by doping the layer with a small concentration of He 3 atoms (impuritons). The heat capacity, magnetization and NMR relaxation times of the He 3 were measured over a wide temperature range from 200 μ K to 500 mK. They provide evidence for changes in the ground state in the second layer film as the amount of He 4 is increased at various fixed He 3 doses. Clear evidence is obtained for a solid second layer film, matching the regime of superfluid response previously observed in pure He 4 films.

Improved accessibility to the microkelvin temperature regime is important for future research in quantum materials; for quantum information science; and for applications of quantum sensors. Here we report the design and performance of a microkelvin platform based on a nuclear demagnetization stage, engineered and well optimized for operation on a standard cryogen-free dilution refrigerator. PrNi5 is used as the dominant refrigerant. The platform provides a large area for mounting experiments in an ultralow temperature, low electromagnetic noise environment. The performance is characterized using current sensing noise thermometry. Temperatures as low as 395 $\mu$K have been reached, and a protocol has been established in which it is possible to operate experiments below 1 mK for 95% of the time, providing an efficient cryogen-free microkelvin environment for a wide range of science applications

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.

We describe here the design and performance of a current sensing noise thermometer using a low T_c DC SQUID as the front end amplifier. The DC SQUID is used to measure the thermal noise current in a resistor and the temperature is then obtained from the Nyquist formula. The thermometer is fast, absolute and precise and is usable over a wide temperature range below 4.2 K, in principle down to well below 1 mK. The excellent energy sensitivity of the DC SQUID, operated at fixed temperature, enables the use of a relatively large noise resistor, in the mΩ range. This requires relatively short averaging times when measuring the spectrum of noise fluctuations. We have shown that it is possible to determine absolute temperature with a precision of 1% in a measuring time of 10 seconds with an amplifier noise temperature, T_N, of the order of 30 µK, and to an accuracy better than 0.3%. The percentage precision is independent of temperature for temperatures much greater than T_N. Our method of heat sinking the noise resistor ensures proper cooling of the electrons. We incorporate a fixed point device for checking the gain calibration. We have cooled the thermometer successfully to below 1 mK, achieving a minimum electron temperature of 300 µK. We present the results of a preliminary comparison with a ³He melting curve thermometer (MCT) above 4.5 mK, and with a platinum NMR thermometer down to the lowest temperatures.