Adiabatic Nuclear Demagnetization Research Articles

Magnetic Cooling and Vibration Isolation of a Sub-kHz Mechanical Resonator

We report recent progress toward the realization of a sub-mK, low-vibration environment at the bottom stage of a dry dilution refrigerator for use in mechanical tests of quantum mechanics. Using adiabatic nuclear demagnetization, we have cooled a silicon cantilever force sensor to Tapprox 1 mK. The temperature of the tip holder of the cantilever chip was determined via a primary magnetic flux noise thermometer. The quality factor of the cantilever continues to increase with decreasing temperature, reaching Qapprox 4cdot 10^4 at 2 mK. To demonstrate that the vibration isolation is not compromised, we report the detection of the thermal motion of the cantilever down to T approx 20 mK, only limited by the coupling to the SQUID readout circuit. We discuss feasible improvements that will allow us to probe unexplored regions of the parameter space of continuous spontaneous localization models.

Cooling low-dimensional electron systems into the microkelvin regime

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.

On-chip Thermometry for Microwave Optomechanics Implemented in a Nuclear Demagnetization Cryostat

We report on microwave optomechanics measurements performed on a nuclear adiabatic demagnetization cryostat, whose temperature is determined by accurate thermometry from below 500$~\mu$K to about 1$~$Kelvin. We describe a method for accessing the on-chip temperature, building on the blue-detuned parametric instability and a standard microwave setup. The capabilities and sensitivity of both the experimental arrangement and the developed technique are demonstrated with a very weakly coupled silicon-nitride doubly-clamped beam mode of about 4$~$MHz and a niobium on-chip cavity resonating around 6$~$GHz. We report on an unstable intrinsic driving force in the coupled microwave-mechanical system acting on the mechanics that appears below typically 100$~$mK. The origin of this phenomenon remains unknown, and deserves theoretical input. It prevents us from performing reliable experiments below typically 10-30$~$mK; however no evidence of thermal decoupling is observed, and we propose that the same features should be present in all devices sharing the microwave technology, at different levels of strengths. We further demonstrate empirically how most of the unstable feature can be annihilated, and speculate how the mechanism could be linked to atomic-scale two level systems. The described microwave/microkelvin facility is part of the EMP platform, and shall be used for further experiments within and below the millikelvin range.

A microkelvin cryogen-free experimental platform with integrated noise thermometry

We report experimental demonstration of the feasibility of reaching temperatures below 1 mK using cryogen-free technology. Our prototype system comprises an adiabatic nuclear demagnetization stage, based on hyperfine-enhanced nuclear magnetic cooling, integrated with a commercial cryogen-free dilution refrigerator and 8 T superconducting magnet. Thermometry was provided by a current-sensing noise thermometer. The minimum temperature achieved at the experimental platform was 600 μK. The platform remained below 1 mK for over 24 h, indicating a total residual heat-leak into the experimental stage of 5 nW. We discuss straightforward improvements to the design of the current prototype that are expected to lead to enhanced performance. This opens the way to widening the accessibility of temperatures in the microkelvin regime, of potential importance in the application of strongly correlated electron states in nanodevices to quantum computing.

Method for cooling nanostructures to microkelvin temperatures

We propose a new scheme aimed at cooling nanostructures to microkelvin temperature based on the well established technique of adiabatic nuclear demagnetization: we attach each device measurement lead to an individual nuclear refrigerator, allowing efficient thermal contact to a microkelvin bath. On a prototype consisting of a parallel network of nuclear refrigerators, temperatures of ∼1 mK simultaneously on ten measurement leads have been reached upon demagnetization, thus completing the first steps toward ultracold nanostructures.

Electrical resistivity of intermetallic compound PrInAg 2 at ultra low temperatures

Resistivity of the Heusler-type paramagnetic intermetallic compound PrInAg 2 is measured by the SQUID technique, down to 0.18 mK. The sample is cooled by an adiabatic Cu nuclear demagnetization refrigerator. No evidence of any phase transition is observed at any temperature. However, a ‘+’ T 1/2 dependence of resistivity is observed below 100 mK.

Nuclear cooling and spin properties of rhodium down to picokelvin temperatures

The lowest temperatures ever measured have been produced by cascade adiabatic nuclear demagnetization of copper and rhodium. Ultimately, spontaneous antiferromagnetic ordering of rhodium nuclei is anticipated. High-sensitivity SQUID-NMR measurements at high nuclear polarization in low magnetic fields have been used to study the mutual interactions between the nuclear spins of rhodium in the metal.

Digitally controlled current source for demagnetization refrigerators

The construction and electrical characteristics of a programmable current source for nuclear adiabatic demagnetization refrigerators are described. The influence of the digitally controlled current source on the demagnetization process efficiency is also discussed.

Nuclear refrigeration and thermometry at microkelvin temperatures

The present status and problems of refrigeration and thermometry at micro. kelvin temperatures will be discussed. It will be shown that a better understanding of internal, time dependent heat leaks and of thermal boundary resistances as well as further progress in thermometry are necessary to reduce the present minimum temperature of about IO I~K to which matter has been refrigerated. 1. PRESENT STATUS OF NUCLEAR REFRIGERATION The ambition of physicists to investigate m~ttter under extreme conditions and to look for new phenomena under these conditions has led to considerable progress in the techniques of obtaining ultralow temperatures. In the millikelvin temperature range refrigeration by the continuous 3I-Ie-4He dilution process and thermometry by various techniques are well advanced and thoroughly understood. 1-3 For the microkelvin temperature range the only known method for refrigeration is adiabatic demagnetization of nuclear magnetic moments and the only thermometric technique applied at microkelvin temperatures is nuclear magnetic resonance on 195pt.l-s Even though both nuclear adiabatic demagnetization refrigeration as well as nuclear magnetic resonance thermometry have been substantially advanced during the last two decades, 3-1~ they have not been developed to their full potential and the experimentalist still encounters a number of difficulties in applying them. The schematic of the low temperature part of a typical nuclear refrigerator is shown in Fig. 1. The nuclear refrigeration stage, usually 10+ 100 moles of Cu are exposed to a magnetic field of typically 8 T and precooled by a 3He-4He dilution refrigerator to a starting temperature of about 10ink for the adiabatic demagnetization. The thermal contact between the dilution refrigerator and the Cu nuclear stage is provided by a superconducting heat switch. The starting conditions for the demagnetization are determined by the cooling power of the SHe-4He dilution

Observation of nuclear ferromagnetic ordering in silver at negative nanokelvin temperatures.

The ferromagnetically ordered state in the nuclear spin system of silver has been reached at negative absolute temperatures by adiabatic nuclear demagnetization at entropies below 0.82 ln2. The ordering, caused by the antiferromagnetic Ruderman-Kittel interaction, was observed below -1.9 nK as a saturation of susceptibility close to -1 and as an increase of the NMR frequencies. Comparison with recent mean-field calculations by Vierti\o and Oja suggests a domain configuration. The phase diagram of silver nuclei at T0 is outlined in the magnetic field versus entropy plane.

Spontaneous nuclear magnetic ordering in copper and silver at nano- and picokelvin temperatures

Owing to the weak mutual interactions, spontaneous nuclear magnetic ordering in metallic copper and silver occurs at 60 nK and 560 pK, respectively. These extremely low spin temperatures can be reached by two-stage adiabatic nuclear demagnetization. Spin ordering has been investigated by employing magnetic susceptibility measurements on copper and silver and by using neutron diffraction techniques on copper. Three antiferromagnetic phases in the field-entropy plane have been discovered in copper, caused by competition between the dipolar and Ruderman-Kittel exchange interactions; only one ordered state has been found in silver. Negative spin temperatures have been produced in silver as well, and a clear ferromagnetic tendency was observed when T < 0. The theoretically calculated spin-spin interactions, ordering temperatures, magnetic phase diagrams and ordered spin structures are in good overall agreement with experimental data for these two metals.

Optimization of nuclear demagnetization process

Abstract On the basis of a relatively simple thermal model of an adiabatic nuclear demagnetization apparatus a system of differential equations has been obtained. A numerical solution of this system describes the demagnetization process and a subsequent warming-up process. These results are confirmed by comparison with experimental data. The model suggested allows an optimal law of magnetic field decrease in demagnetization to be obtained.

The behaviour of nuclear spin systems at spin temperatures below 1μK

At present two techniques are used to study nuclear spin systems at submicro-Kelvin temperatures. The first technique uses brute force nuclear polarization and adiabatic nuclear demagnetization. The second one uses dynamic nuclear polarization and adiabatic demagnetization in the rotating frame. A third method is envisaged using microwave optical nuclear polarization. In Leiden the second method is used to study the proton spin system in Ca(OH) 2. A ferromagnetic ordering with a domain structure and a transverse helical ordering were produced and studied.

Orientation dependent static magnetic susceptibility of copper nuclei below 1 μK

The static nuclear susceptibility of copper has been measured perpendicular and parallel to thin wires in a two-stage adiabatic nuclear demagnetization refrigerator. The results show that χ  ∥(0) < χ  ⊤(0) below 500 nK.

Neutron diffraction study of nuclear magnetic ordering

We describe the results obtained by neutron diffraction in the detection and study of several nuclear ordered phases-antiferromagnetic and ferromagnetic - in lithium hydride. This is preceded by a survey of the method of production of ordering, by cooling the nuclear spins down to the microKelvin range in a two-step process: dynamic nuclear polarization followed by adiabatic nuclear demagnetization in high field.

Metallic thermal connectors for use in nuclear refrigeration

Measurements of the electrical resistance at 4.2 K are used to infer the thermal conductance at ultralow temperatures of (1) copper magnet wires with Formvar and Allex insulations, (2) inert gas welded and silver-soldered joints, and (3) various demountable metallic connectors. The purpose of this study is to determine a wire suitable for use as the refrigerant in a nuclear adiabatic demagnetization refrigerator and an appropriate method of joining the wire to a heat exchanger in a 3He cell. It was found that ordinary magnet wire insulated with Allex can easily be annealed to have a very low resistance at low temperatures. Further, annealed copper-to-copper welds are indistinguishable from bulk OFHC copper, while alloying makes the resistance of silver-soldered joints somewhat larger than would be calculated from the bulk resistivity of the silver solder. An improved version of the ’’squeeze’’ connector and a simple pressure contact using a stainless steel screw are described. These have too high a resistance for nuclear refrigeration but are good enough for most other low-temperature applications. Finally, a large copper tapered screw joint was found to make relatively poor thermal contact.

Principles of nuclear magnetic ordering

We describe in this article the principles of production and observation of magnetic ordering in systems of nuclear spins subjected to dipole-dipole interactions. The cooling necessary for producing ordering, which concerns only the nuclei, is obtained by a two-step process: dynamic polarization in a high field followed by adiabatic nuclear demagnetization. The study is mostly limited to adiabatic demagnetization in the rotating frame, for which the effective nuclear spin-spin interactions are truncated dipolar interactions. We list briefly a number of measurements that can be made, mostly relevant to magnetic resonance techniques, together with the information they yield on the ordering. The prediction of the nature of ordered structures, both at positive and negative temperatures, is made through the use of the local Weiss-field approximation. In the case of simple cubic systems of spins 1/2, one predicts the occurrence of three different antiferromagnetic structures. The Weiss-field approximation, occasionally supplemented with high-temperature approximation to spin temperature theory, is finally used for predicting as a function of entropy various properties of the antiferromagnetic states: sublattice magnetizations, transition entropy, transverse and longitudinal susceptibilities, transition field, and shape of the fast-passage dispersion signal.

Nuclear cooling in PrCu6

Nuclear adiabatic demagnetization experiments have been performed in the Van Vleck paramagnet PrCu6 in order to determine the very-low-temperature nuclear magnetic properties of this material and to test its efficiency as a very low-temperature refrigerator. It is found that a temperature of 2.7 mK can be reached from an initial temperature of 40 mK and an initial field of 20 kOe. Susceptibility and specific heat measurements above 2.7 mK indicate the presence of both nuclear quadrupole and exchange interactions. The transition to an antiferromagnetically ordered state among the Pr nuclei is expected to occur below 2.7mK.

Hyperfine Enhanced Nuclear Magnetic Cooling in Van Vleck Paramagnetic Intermetallic Compounds

In strongly Van Vleck paramagnetic rare-earth alloys or compounds, large hyperfine fields can be induced at the rare-earth nucleus with moderate external fields. These fields are useful to generate low temperatures by the method of nuclear adiabatic demagnetization. Experiments in the Van Vleck paramagnets PrPt5 and PrTl3 are described in which end temperatures of 3.5 and 1.5 mK are obtained starting from about 60 mK and 20 kOe. The possibility of nuclear ferromagnetism in PrTl3 is discussed and compared with observations.

Low‐temperature physics runs both hot and cold

The domain of the low‐temperature physicist now extends down as low as microdegrees Kelvin and, on the high side, up to the rather startling temperature of 108 K. Valid claims to these two extreme regions of temperature were staked out by two papers presented at the recent 12th International Conference on Low Temperature Physics at Kyoto. In the first, P. M. Berglund, G. J. Ehnholm, R. G. Gylling, O. V. Lounasmaa and R. P. So/vik (Helsinki University of Technology) reported the successful operation of a large cryostat that has reached temperatures as low as 500 microdeg K. The system, which combines dilution refrigeration with the relatively new technique of nuclear adiabatic demagnetization, promises to open the way for physicists to study the properties of matter at temperatures an order of magnitude lower than has previously been possible.