Liquid Helium Temperature Research Articles

An experimental and analytical investigation to determine thermal conductivity of epoxy-filler composites for space applications

Thermal conductivity of epoxy-filler composite based Thermal Interface Materials (TIMs) is of utmost importance for thermal management systems used in space applications. Previous studies have shown that depending on the filler particle mixed into the epoxy, thermal conductivity of the composite can either be increased or decreased. Towards that, we have selected four different filler particles (micron sized) – aluminium, copper, zinc and silver to enhance the thermal conductivity of the base epoxy. Thermal conductivity of these 4 epoxy-filler composites has been determined experimentally using an indigenous developed setup at the temperatures ranging from 4.2 K to 323 K. The values have also been reported at various volume fractions (up to 15 %). In addition, after each preparation as the filler particles are mixed mechanically into the epoxy, they are randomly distributed, and this can affect the composite’s thermal conductivity (for the same volume fraction). The experimental determination of thermal conductivity for all the possible distributions is a time consuming, and cumbersome task. In the literature, thermal conductivity values are usually reported for few possible distributions. Therefore, we have developed a novel analytical model to predict all the possible values of thermal conductivity for a specific volume fraction. The predicted values compare well with our experimental data reported in this work at temperatures ranging between 338 K (above room temperature) to 4.2 K (liquid helium temperature). The predicted values are also in good agreement with the experimental values of different fillers such as red mud, pine wood dust and glass fiber etc. from the literature. Additionally, in comparison to other theoretical models like Lewis-Nielsen, Rule of mixture, and Poisson’s distribution, the present model predicts the thermal conductivity values more precisely. This work will be significant in the designing of components where heat transfer plays an important role towards the safety of the component.

Dynamic Nuclear Polarization with Conductive Polymers

AbstractThe low sensitivity of liquid‐state nuclear magnetic resonance (NMR) can be overcome by hyperpolarizing nuclear spins by dissolution dynamic nuclear polarization (dDNP). It consists of transferring the near‐unity polarization of unpaired electron spins of stable radicals to the nuclear spins of interest at liquid helium temperatures, below 2 K, before melting the sample in view of hyperpolarized liquid‐state magnetic resonance experiments. Reaching such a temperature is challenging and requires complex instrumentation, which impedes the deployment of dDNP. Here, we propose organic conductive polymers such as polyaniline (PANI) as a new class of polarizing matrices and report 1H polarizations of up to 5 %. We also show that 13C spins of a host solution impregnated in porous conductive polymers can be hyperpolarized by relayed DNP. Such conductive polymers can be synthesized as chiral and display current induced spin selectivity leading to electron spin hyperpolarization close to unity without the need for low temperatures nor high magnetic fields. Our results show the feasibility of solid‐state DNP in conductive polymers that are known to exhibit chirality‐induced spin selectivity.

Intracage vibrations and Zeeman effect in Y3N@C80 single-molecule transistors

Clusterfullerenes, which contain a cluster rather than single-atom inclusions, exhibit more complex internal structures and greater degrees of freedom for motion. Trimetallic nitride clusterfullerenes have attracted significant attention due to their diversity and potential applications, among which Y3N@C80 stands out for its charge-transfer characteristics in electronic excitations, owing to the unique distribution of molecular orbitals near the Fermi level. Here, we have fabricated single-molecule transistor devices using Y3N@C80. Transport measurements at liquid helium temperature revealed a series of excited state energy levels, which were matched to corresponding vibrational modes through comparison with Raman spectra and density functional theory calculations. Additionally, we measured charge-state-dependent magnetic responses, revealing the electron and spin-filling patterns of the molecular orbitals in Y3N@C80. These results enhance our understanding of the dynamics and molecular spin–orbit characteristics of clusterfullerenes, indicating their potential for multifunctional applications.

Integrated thermo-optic phase shifters for laser-written photonic circuits operating at cryogenic temperatures

Abstract Integrated photonics offers compact and stable manipulation of optical signals in miniaturized chips, with the possibility of changing dynamically their functionality by means of integrated phase shifters. Cryogenic operation of these devices is becoming essential for advancing photonic quantum technologies, accommodating components like quantum light sources, single photon detectors and quantum memories operating at liquid helium temperatures. In this work, we report on a programmable glass photonic integrated circuit (PIC) fabricated through femtosecond laser waveguide writing (FLW) and controlled by thermo-optic phase shifters both in a room-temperature and in a cryogenic setting. By taking advantage of a femtosecond laser microstructuring process, we achieved reliable PIC operation with minimal power consumption and confined temperature gradients in both conditions. This advancement marks the first cryogenically-compatible programmable FLW PIC, paving the way for fully integrated quantum architectures realized on a laser-written photonic chip.

Large reversible magnetocaloric effect in rare-earth molybdate RE2(MoO4)3 (RE: Gd and Tb) compounds

We report on the synthesis, structural, DC magnetic susceptibility studies on Gd2(MoO4)3 and Tb2(MoO4)3 polycrystals, prepared by conventional solid-state reaction method. Temperature and magnetic field (H) dependent magnetization and heat capacity measurements have been carried out to estimate the isothermal magnetic-entropy change (−ΔSm) and adiabatic temperature change (ΔTadi) in both rare-earth molybdates. No obvious long-range magnetic ordering can be found down to 2 K in both studied compounds. The estimated maximum value of –ΔSm at 3.5 K for 70 kOe is 31.1 J kg−1 K−1 and 16.7 J kg−1 K−1 in Gd2(MoO4)3 and Tb2(MoO4)3, respectively. Further, the calculated total-adiabatic temperature change for Gd2(MoO4)3 was 12.8 K, and Tb2(MoO4)3 has shown 9.8 K. The difference in the observed magnetocaloric and adiabatic temperature responses of Tb2(MoO4)3 compared to Gd2(MoO4)3 could be attributed to the presence of orbital angular moment of Tb3+ ions and relatively low-value of magnetization. Both the systems show a large magnetocaloric effect (MCE) (−ΔSm ∼ 12 J kg−1 K−1) even in low applied H (<20 kOe) attainable by a permanent magnet. Large magnetocaloric behaviour of Gd2(MoO4)3 and Tb2(MoO4)3 systems at a moderate field change along with very low electrical conductivity and the absence of thermal and magnetic field hysteresis in magnetization make them conducive to their use as promising magnetic refrigerants in the vicinity of liquid-helium temperatures.

Understanding vortex dynamics in CaK(Fe,Ni)4As4 and Ba(Fe,Co)2As2 single crystals under the influence of random point disorder

Abstract We report on the influence of doping on vortex dynamics in 3 MeV proton-irradiated single crystals of CaK(Fe1−x Ni x )4As4 (1144, x = 0.015, 0.025, and 0.03) and Ba(Fe1−x Co x )2As2 (x = 0.04, 0.062, 0.066 and 0.074). Non-irradiated crystals of the 1144 system display superconducting critical temperatures ranging from 31 K for x = 0.015–20.5 K, as doping increases to 0.03. On the other hand, pristine crystals of the 122 system show Tc values between 14.6 and 23.6 K, with the maximum Tc occurring at intermediate doping levels. The fluence was set at 3 × 1016 p cm−2, resulting in a decrease in the Tc by around 1.5 K for all samples and significantly affecting the vortex dynamics by reducing the flux creep relaxation compared to previously reported values for unirradiated crystals. Parameters such as vortex pinning energy U 0 and the glassy exponent μ dependencies on doping and magnetic field strength are identified. For the 1144 system, U 0 reaches values approaching 500 K for small fields in samples with Tc = 29.3 K (x = 0.015), systematically decreasing to around 200 K as Tc falls below 20 K. Furthermore, U 0 decreases as the field increases to 3 T for the same sample, varying from approximately 250 K to 100 K as Tc decreases. These changes are typically accompanied by modifications in μ, gradually increasing from values around 1 towards 1.5, corresponding to small bundle relaxation in the collective creep theory. Despite differences in the substitutional disorder and magnetic phase diagram with respect to the 1144 system, the results for 122 single crystals follow a similar tendency in which U 0 usually reduces and μ increase rise as the applied magnetic field is increased. Due to moderate U 0 in these systems (few hundreds of kelvins), the resulting decay of persistent current at liquid helium temperatures is primarily determined by a balance between U 0 and bundle size contribution. These findings provide valuable insights for potential applications of these systems, particularly in the context of intrinsic superconducting parameters and the resulting pinning landscape.

Silane functionalization of graphene nanoplatelets

The lack of a facile route for the functionalization of highly sp2 hybridized, low oxygen content (<10 atm%, C:O > 9:1) graphene nanoplatelets (GNP) has greatly hindered full exploitation of these materials. Moreover, the highly conductive and paramagnetic properties of GNP can exclude the use of solid-state nuclear magnetic resonance spectroscopy (SS-NMR) and electron paramagnetic resonance spectroscopy (EPR), even at liquid helium temperatures, to prove binding of silane to the GNP surface is successful. Here, three model silanes, 3-Mercaptopropyltrimethoxysilane, 3-Methacryloxypropyltrimethoxysilane and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane were successfully bound to the surface of GNP, each following either a condensation silanization reaction or a pathway dependent on the R-group of each silane. Several techniques were employed, but critically a combination of X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis-mass spectrometry (TGA-MS) in argon and air confirmed successful binding of the silane to the GNP. The approach adopted makes available the pendant R-group on the silane for interaction or reaction with polymers and a route to significantly modifying the properties of polymers and surfaces.

Essential improvement of the JT cryocooler working at liquid helium temperature for space: Efficient and lightweight

Due to long lifetime, low level vibration and negligible electromagnetic interference, the Joule-Thomson (JT) cryocooler working at liquid helium temperature has been used in space. However, its cooling capacity and thermodynamic efficiency still need to be further improved under a certain mass limit, which is an essential improvement for space-efficient application of the JT cryocooler. Therefore, in this study, optimization design is carried out for a JT cryocooler working at liquid helium temperature. Based on the modification of Stirling cryocooler, pulse tube cryocooler and JT compressor, the developed JT cryocooler can provide a cooling capacity of 0.36 W at 4.18 K while the total input power and the total mass (without cryostat) are 1157 W and 26.8 kg, respectively. Compared with the literature research, it can be found that the developed JT cryocooler is suitable for space applications.

A Study on the Liquid Helium Temperature Tensile Property of Fe-21Cr-15Ni-5Mn-2Mo Austenitic Stainless Steel after Solution Treatment.

A novel non-magnetic Fe-21Cr-15Ni-5Mn-2Mo austenitic stainless steel with high strength and plasticity has been developed. The microstructure and liquid helium temperature (4.2 K) tensile properties of the top and bottom samples of large-size forged flat steel after solution treatment at 1090 °C were investigated. The results showed that the average grain size of the bottom sample (48.0 ± 6.7 μm) was smaller than that of the top sample (58.8 ± 15.3 μm), and the MX precipitates and Z phases were distributed in the matrix of the samples. The 4.2 K strengths of the samples at the top and bottom were high, and large amounts of annealing twin boundaries played a certain role in strengthening. After cryogenic tensile testing, large amounts of deformation twins, stacking faults, and dislocations were generated inside the austenite grains of both samples, which helped the material to obtain higher plasticity and strength. The top and bottom samples possessed excellent synergies of strength and plasticity at 4.2 K, and the 4.2 K tensile properties of the top sample were as follows: ultimate tensile strength (UTS) of 1850 MPa, yield strength (YS) of 1363 MPa, and elongation (EL) of 26%. The tested steel is thus believed to meet the requirements of combined excellent strength and plasticity within a deep cryogenic environment, and it would be a promising material candidate for manufacturing superconducting coil cases to serve in new generation fusion engineering.

High-precision temperature control of helium Joule–Thomson cryocooler: Long-term temperature stability

To meet the refrigeration requirements of scientific instruments for mK-level temperature stability over a long-time range in the liquid helium temperature, the temperature characteristics and high-precision control strategies of the helium Joule-Thomson cryocooler (JTC) have been experimentally investigated for the first time. Based on the Ideal Gas Law, the influencing mechanism of pre-cooling temperature, gas reservoir volume, and liquid phase volume fraction on refrigeration temperature were qualitatively analyzed and experimentally verified. An active control studies of pre-cooling temperature, low-pressure and heat load were carried out to verify the feasibility of active temperature control of helium JTC. Finally, a long-term temperature stability of ±1 mK/h was successfully achieved when a multi-stage active control strategy was adopted, which is the optimal long-term temperature stability of helium JTC that have been reported to date.

Dynamic Nuclear Polarization with Conductive Polymers.

The low sensitivity of liquid-state nuclear magnetic resonance (NMR) can be overcome by hyperpolarizing nuclear spins by dissolution dynamic nuclear polarization (dDNP). It consists of transferring the near-unity polarization of unpaired electron spins of stable radicals to the nuclear spins of interest at liquid helium temperatures, below 2 K, before melting the sample in view of hyperpolarized liquid-state magnetic resonance experiments. Reaching such a temperature is challenging and requires complex instrumentation, which impedes the deployment of dDNP. Here, we propose organic conductive polymers such as polyaniline (PANI) as a new class of polarizing matrices and report1H polarizations of up to 5%. We also show that13C spins of a host solution impregnated in porous conductive polymers can be hyperpolarized by relayed DNP. Such conductive polymers can be synthesized as chiral and display current induced spin selectivity leading to electron spins hyperpolarization close to unity without the need for low temperatures nor high magnetic fields. Our results show the feasibility of solid-state DNP in conductive polymers that are known to exhibit chirality-induced spin selectivity.

Two-level system loss: Significant not only at millikelvin

Tow-level system (TLS) loss in amorphous dielectric materials has been intensively studied at millikelvin temperatures due to its impact on superconducting qubit devices and incoherent detectors. However, the significance of TLS loss in superconducting transmission lines at liquid helium temperatures remains unclear. This study investigates TLS loss in amorphous SiO2 at liquid helium temperatures (about 4 K) within a frequency range of 130–170 GHz, using niobium microstrip and coplanar waveguide resonators. Our results demonstrate notable power and temperature dependence of dielectric loss, with the dielectric loss and quasiparticle loss exchanging dominance at around 4 K. These findings are consistent with TLS models and provide crucial insight for the design of superconducting devices operating at liquid helium temperatures.

Atomic resolution scanning transmission electron microscopy at liquid helium temperatures for quantum materials

Fundamental quantum phenomena in condensed matter, ranging from correlated electron systems to quantum information processors, manifest their emergent characteristics and behaviors predominantly at low temperatures. This necessitates the use of liquid helium (LHe) cooling for experimental observation. Atomic resolution scanning transmission electron microscopy combined with LHe cooling (cryo-STEM) provides a powerful characterization technique to probe local atomic structural modulations and their coupling with charge, spin and orbital degrees-of-freedom in quantum materials. However, achieving atomic resolution in cryo-STEM is exceptionally challenging, primarily due to sample drifts arising from temperature changes and noises associated with LHe bubbling, turbulent gas flow, etc. In this work, we demonstrate atomic resolution cryo-STEM imaging at LHe temperatures using a commercial side-entry LHe cooling holder. Firstly, we examine STEM imaging performance as a function of He gas flow rate, identifying two primary noise sources: He-gas pulsing and He-gas bubbling. Secondly, we propose two strategies to achieve low noise conditions for atomic resolution STEM imaging: either by temporarily suppressing He gas flow rate using the needle valve or by acquiring images during the natural warming process. Lastly, we show the applications of image acquisition methods and image processing techniques in investigating structural phase transitions in Cr2Ge2Te6, CuIr2S4, and CrCl3. Our findings represent an advance in the field of atomic resolution electron microscopy imaging for quantum materials and devices at LHe temperatures, which can be applied to other commercial side-entry LHe cooling TEM holders.

Magnetic properties and large low-field magnetocaloric effect of RFe2Si2 (R = Ho, Tm) compounds

Nowadays, magnetic refrigerant technology has drawn much attention for its environmental friendliness. Those magnetic materials possessing giant low-field magnetocaloric effect (MCE) performance are of great importance for practical applications, especially at low temperatures. Herein, we present a detailed study on polycrystalline magnetocaloric compounds HoFe2Si2 and TmFe2Si2. HoFe2Si2 exhibits a transition from antiferromagnetic to paramagnetic phase at 2.2 K, while no long-range magnetic ordering is observed in TmFe2Si2 even temperature down to 2 K. For both compounds, they showed large low-field magnetic entropy change (−ΔSM) with the peak values of 10.6 and 7.9 J kg−1 K−1 under field change of 0–2 T for HoFe2Si2 and TmFe2Si2, respectively, which is comparable or even larger than some low-temperature magnetocaloric materials. In addition, the characteristic of second-order magnetic transitions of both compounds were confirmed on basis of Arrott plots and mean-field theory criterion. The low magnetic ordering temperatures, large low-field MCE performance along with the feature of second order phase transition for HoFe2Si2 and TmFe2Si2 indicate that both compounds are promising candidate for magnetic refrigerant materials at liquid helium temperature.

Evidence of the inverse proximity effect in tunnel magnetic josephson junctions

Magnetic Josephson junctions (MJJs) are a special class of hybrid systems where antagonistic correlations coexist, thus providing a key for advances in weak superconductivity, superconducting spintronics, and quantum computation. So far, the memory properties of MJJs have been mostly investigated in view of digital electronics and for spintronic devices at liquid-helium temperature. At the operating temperature of quantum circuits, a magnetic order can rise in a superconductor (S) at the S/ferromagnet (F) interface, i.e., the inverse proximity effect (IPE), thus leading to a significant modification of the magnetic field patterns in MJJs. In this work, we have carried out a comparative investigation of the magnetic behavior of tunnel MJJs with a strong ferromagnetic layer inserted in the layout of both Nb and Al JJs, respectively. The comparative analysis validates the crucial role of the temperature, the fundamental scaling energies of S/F coupling systems, and the transparency of the S/F interface. This investigation points out that the IPE is a key aspect to consider when designing tunnel MJJs operating well below 4 K and thus in the perspective of hybrid superconducting quantum architectures.

Ohmic contacts on SnO2 produced by hydrogen plasma treatment

This study introduces a method for creating Ohmic contacts to tin oxide (SnO2) by subjecting the sample surface to hydrogen plasma treatment at moderate temperatures of about 300 °C. This process generates a surface layer of metallic tin droplets, forming suitable electrical contacts. The contacts exhibit remarkable durability and demonstrate Ohmic behavior down to liquid helium temperatures.

High-Resolution Cryogenic Spectroscopy of Single Molecules in Nanoprinted Crystals.

We perform laser spectroscopy at liquid helium temperatures (T = 2 K) to investigate single dibenzoterrylene (DBT) molecules doped in anthracene crystals of nanoscopic height fabricated by electrohydrodynamic dripping. Using high-resolution fluorescence excitation spectroscopy, we show that zero-phonon lines of single molecules in printed nanocrystals are nearly as narrow as the Fourier-limited transitions observed for the same guest-host system in the bulk. Moreover, the spectral instabilities are comparable to or less than one line width. By recording super-resolution images of DBT molecules and varying the polarization of the excitation beam, we determine the dimensions of the printed crystals and the orientation of the crystals' axes. Electrohydrodynamic printing of organic nano- and microcrystals is of interest for a series of applications, where controlled positioning of quantum emitters with narrow optical transitions is desirable.

Synthesis, Structure, and Magnetic Properties of Cyanurates RE5(C3N3O3)(OH)12 (RE = Gd-Lu): Cryogenic Magnetocaloric Candidate Gd5(C3N3O3)(OH)12.

Rare-earth (RE)-based frustrated magnets are fertile playgrounds for discovering exotic quantum phenomena and exploring adiabatic demagnetization refrigeration applications. Here, we report the synthesis, structure, and magnetic properties of a family of rare-earth cyanurates RE5(C3N3O3)(OH)12 (RE = Gd-Lu) with an acentric space group P6̅2m. Magnetic susceptibility χ(T) and isothermal magnetization M(H) measurements manifest that RE5(C3N3O3)(OH)12 (RE = Gd, Dy-Yb) compounds exhibit no magnetic ordering down to 2 K, while Tb5(C3N3O3)(OH)12 shows long-range magnetic ordering around 3.6 K. Among them, magnetically frustrated spin-7/2 Gd5(C3N3O3)(OH)12 shows long-range magnetic ordering around 1.25 K and a large magnetocaloric effect with a maximum magnetic entropy change ΔSm of up to 58.1 J kg-1 K-1 at ΔH = 7 T at liquid-helium temperature regimes.

Influence of swift heavy ions irradiation on optical and luminescence properties of Y3Al5O12 single crystals

Optical and luminescence properties of Y3Al5O12 (YAG) single crystals preliminary irradiated by swift heavy ions were studied. Swift heavy Xe ions with fluences ranging from 6·1010 to 2·1012 ions/cm2 were utilized for the irradiation of nominally undoped YAG single crystals. A stable strong induced absorption observed in the 200–600 nm spectral range correlates with the irradiation fluence. It is suggested that several centers are responsible for this induced absorption in YAG single crystals and their possible origin (F-type centers) is proposed and discussed. The swift heavy ions irradiation strongly modifies the luminescence properties of YAG, namely, the excitonic emission at liquid helium temperature is drastically suppressed in heavily irradiated crystals.

A Brilliant Magnetic Refrigerant Operating Near Liquid Helium Temperature: Enhanced Magnetocaloric Effect in Ferromagnetic EuTi0.75Al0.125Zr0.125O3

AbstractRare earth‐based perovskites have become an attractive research interest in the field of cryogenic magnetic refrigerants due to their unique advantages in practical applications. The remarkable magnetocaloric effect (MCE) renders EuTiO3 a potential magnetic refrigerant in the liquid helium temperature range. More impressively, the tunability between antiferromagnetism (AFM) and ferromagnetism (FM) provides the feasibility of tailoring the magnetism and enhancing the magnetocaloric performance. In this study, the magnetism of EuTi0.75Al0.125Zr0.125O3 is investigated in depth through first‐principles calculations and experimental methods. Both theoretical calculations and experimental results reveal that it exhibits significant ferromagnetism due to the AFM‐FM magnetic transition promoted by the co‐substitution of Al and Zr. Lattice expansion and altered electronic interactions are responsible for the FM behavior, which leads to a significant enhancement of the MCE. With the field change of 0−1 T, the peak values of magnetic entropy change (−ΔSM), refrigerating capacity (RC), and adiabatic temperature change (ΔTad) reach 18.9 J kg−1 K−1, 77.7 J kg−1, and 7.4 K, respectively. More surprisingly, the values of maximum magnetic entropy change () and maximum adiabatic temperature change for EuTi0.75Al0.125Zr0.125O3 reach 11.4 J kg−1 K−1 and 3.7 K under the field change of 0−0.5 T, respectively. The remarkable magnetocaloric performance proves it to be a brilliant magnetic refrigerant operating near liquid helium temperature.