Dilution refrigerator | EPFL Graph Search

A 3He/4He dilution refrigerator is a cryogenic device that provides continuous cooling to temperatures as low as 2 mK, with no moving parts in the low-temperature region. The cooling power is provided by the heat of mixing of the helium-3 and helium-4 isotopes. The dilution refrigerator was first proposed by Heinz London in the early 1950s, and was experimentally realized in 1964 in the Kamerlingh Onnes Laboratorium at Leiden University. The field of dilution refrigeration is reviewed by Zu et al. Theory of operation The refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4. When cooled below approximately 870 millikelvins, the mixture undergoes spontaneous phase separation to form a 3He-rich phase (the concentrated phase) and a 3He-poor phase (the dilute phase). As shown in the phase diagram, at very low temperatures the concentrated phase is essentially pure 3He, while the dilute phase contains about 6.6% 3He and 93.4% 4He

Order and dynamics in Kagome like compound Cu2OSO4 and other quantum magnets

Virgile Yves Favre

This thesis presents results of studies of novel compounds modeling complex fundamental physics phenomena. Cu2OSO4 is a copper based magnetic Mott Insulator system, where spin half magnetic moments form a new type of lattice. These intrinsically quantum pins are exhibiting atypical magnetic order and spin dynamics. The recent success in the growth of large single crystals of Cu2OSO4 enabled to perform measurements probing its static and fluctuating properties. The peculiarity of this sample is that its atoms are forming layers, with a geometry close to the intensively studied Kagomé lattice, but with a third of its spins replaced by dimers. This quantum magnetism system has been probed in its bulk, by the means of heat capacity and DC-susceptibility measurements, revealing a transition to a magnetically long range ordered state upon cooling, the details of which are revealed by neutron scattering. Single crystal inelastic neutron scattering shed light on the spin-dynamics in the system, with clear spin waves appearing as fluctuations around the peculiar ground state of the system: a 120 degrees spin configuration where the magnetic moment of the spin-dimer causes the sample to be globally ferrimagnetic. The presented results indicate that Cu2OSO4 represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored. The magnetic excitations of the compound can be modeled by a yet-to-be-understood internal effective mean-field that no simple magnetic coupling seems to reproduce. K2Ni2(SO4)3 is another compound that allows for the existence of non-trivial topological phases. This thesis presents results of the study of the unusual magnetic behavior of K2Ni2(SO4)3. No clear sign of well-established magnetic long range order has been observed down to dilution temperatures. Neutron scattering reveals the details of the competition between frustration and fluctuations that prevent order from settling in. Low temperature spin excitations take the form of a continuum at 500 mK, but also of broad, energy independent continua at higher temperatures. Bulk and neutron scattering measurements are put in perspective and linked together with a view to building up a better understanding of how quantum spin liquids can be stabilized in general, and in particular in this model compound. Finally, the last contribution of this thesis to the field of condensed matter physics regards the establishment of a state-of-the-art technique to fit heat capacity and unit cell volume of samples to try and make the extraction of magnetic information from specific heat measurements more robust. This newly-developed technique consists in modeling lattice contributions with better accuracy by using data from multiple experimentally accessible quantities to consolidate the fitting scheme. This method has been cautiously applied to several compounds at the forefront of research in experimental physics.

EPFL2021

Development of a Millikelvin Scanning Tunneling Microscope for Applications in Ultra High Vacuum and High Magnetic Fields

Maximilian Assig

In this thesis we discuss the realization of a scanning tunneling microscope (STM) operating at temperatures below 20 mK. This is accomplished by attaching the STM to a dilution refrigerator. We can apply high magnetic fields up to 14 T perpendicular and 0.5 T parallel to the sample surface. A full ultra high vacuum (UHV) preparation chamber is attached to the cryostat allowing in situ preparation of the samples to investigate. This setup allows us to investigate low temperature phase transitions in solids and offers the possibility for ultimate energy resolution tunneling spectroscopy measurements on the atomic scale. The concept of realizing the mK-STM is given. A main point of concern was the design of the STM unit. The choice of materials should be well adapted for the use at very low temperatures and in high magnetic fields. A special concern are thermal expansion coefficients and thermal conductivity. In addition, we analyzed the variation of the tip sample distance for a given external excitation for our STM design and two variations of the Besoke-STM by using a one dimensional mass spring model. This gives us the possibility to extract an optimization criterion for our design to limit the response of the tip sample distance to vibrations that are transmitted to the STM. The external damping mechanism, which ensures a low mutual vibration level of the STM as well as the grounding concept for the electronics is explained. To characterize the STM at operating temperatures of 1.5 K, 800 mK and 15 mK we measured topography images of Au(111) and Cu(111) surfaces. This allows us to extract the stability between tip and sample. A statistical analysis of the noise in the tunneling current without activated feedback loop yields a stability of 2.7±0.1 pm in a frequency range f ≈ 0.03 – 25 Hz at 800 mK. To verify if tip and sample are cooled efficiently, we measured the temperature at their positions with additional thermometers. We achieved 17 mK at the tip and 20 mK at the sample position, respectively. The energy resolution of the STM setup was verified by taking tunneling spectra between a superconductor and a normal conducting metal. These spectra could be fitted using the BCS theory of superconductivity. From those ts we can extract the upper limit of the effective temperature of the electrons which is Teff = 87 ± 3 mK for our best measurement. This corresponds to an energy resolution of 3.5 kbTeff = 26 ± 1 µeV. Further, the tunneling conductance between two superconductors was investigated. A sub-gap structure, which can be related to Andreev reflections in asymmetric tunneling junction was observed. In addition, at zero bias voltage a peak due to the Josephson effect can be measured. Side peaks with intensities that scale with this zero bias conductance peak appear most probably due to the interaction of Cooper pairs with the electromagnetical environment in which the junction is placed. Investigating the dependence of the conductance between a superconducting tip and a normal conducting sample on externally applied magnetic fields leads to the appearance of shoulders in the coherence peaks. These shoulders shift linearly with the external magnetic field. Such an effect has been measured for planar tunnel junctions and can be explained with the lifting of the spin degeneracy of the quasi-particle density of states in a superconductor. It was observed for the first time by Meservey, Tedrow and Fulde. The fact, that we are now able to prepare STM tips showing the Meservey-Tedrow-Fulde effect enables us to measure the absolute spin polarization at EF with atomic resolution.

EPFL2011

Ultra Low Temperature Susceptometer and Magnetometer

Julian Piatek

The first part of this thesis discusses technical details relating to measurements of magnetic properties at ultra low temperatures. The implementation of AC susceptibility at temperatures down to 30 mK is introduced and used as a platform to showcase selected quantum magnets measured during the thesis. Each presented system illustrates a particular strength of AC susceptibility. This is followed by in-depth analysis of the design and implementation of a new solution for a SQUID magnetometer capable of running below 100 mK. The system employs a piezomotor to move the sample inside a dilution fridge, rather than the existing designs, which involve moving the entire dilution fridge. Furthermore, the system is completely modular, allowing for rapid removal from the fridge, and opening the possibility to use it on virtually any commercial dilution refrigerator. The latter part of the thesis presents a comprehensive study of a new family of model magnets, LiHox Er1−x F4, which combines the Ising spins of ferromagnetic LiHoF4 with the XY ones of antiferromagnetic LiErF4. The temperature-doping (T − x) phase diagram has been studied using AC susceptibility, and three key regions investigated in detail using additional neutron scattering experiments and mean-field calculations. The first region, x ≳ 0.6, corresponds to an Ising ferromagnet, where Tc (x) decreases linearly and faster than what mean-field predicts. At T < TC a so-called embedded spin-glass state is observed. The second region, 0.6 ≳ x ≳ 0.3, undergoes a spin-glass transition, where needle-like spin clusters form along the Ising axis below Tg (x) ∼ 0.4 − 0.5 K. Applying a field along the Ising axis at T < 200 mK produces a thermal runaway in the x = 0.50 sample, when the field reaches a value of H = 0.029 ± 0.002 T. The final region, x ≲ 0.3, corresponds to an antiferromagnetically coupled spin-glass, which shows archetypal spin-glass behaviour.

EPFL2012