mK Temperatures Research Articles

Application of response surface methodology to the temperature fluctuation suppression structure in the cryostats.

The cryocooler-based cryostat typically suffers from an intrinsic 200 mK temperature oscillation originating from the periodic variation of the cryocooler's working fluid. In this paper, a numerical study is performed to investigate the impact of a thermal link (a temperature fluctuation suppression structure)'s geometric parameters on a cryostat's performance. An optimization model is developed to minimize a cryostat flange's temperature fluctuation and deviation. We considered three factors during the analysis: the number of the thermal link wires n, the length of the thermal link l, and the radius of the thermal link wires r. Two primary evaluation measures are the temperature fluctuation Tp-p and the temperature deviation of the flange Ri. The results are analyzed using Response Surface Methodology. The final predicted optimum combination of geometrical parameters for the thermal link is n = 8, l = 36mm, and r = 0.8mm. The optimization results are in good agreement with our model, and the deviations in predicted values are less than 1.6%. The model and results presented here to optimize the thermal link provide helpful guidance for cryostat optimization in experiments.

Electron-phonon coupling of epigraphene at millikelvin temperatures measured by quantum transport thermometry

We investigate the basic charge and heat transport properties of charge neutral epigraphene at sub-kelvin temperatures, demonstrating a nearly logarithmic dependence of electrical conductivity over more than two decades in temperature. Using graphene's sheet conductance as an in situ thermometer, we present a measurement of electron-phonon heat transport at mK temperatures and show that it obeys the T4 dependence characteristic for a clean two-dimensional conductor. Based on our measurement, we predict the noise-equivalent power of ∼10−22 W/Hz of the epigraphene bolometer at the low end of achievable temperatures.

A scanning tunneling microscope capable of electron spin resonance and pump-probe spectroscopy at mK temperature and in vector magnetic field.

In the last decade, detecting spin dynamics at the atomic scale has been enabled by combining techniques such as electron spin resonance (ESR) or pump-probe spectroscopy with scanning tunneling microscopy (STM). Here, we demonstrate an ultra-high vacuum STM operational at milliKelvin (mK) temperatures and in a vector magnetic field capable of both ESR and pump-probe spectroscopy. By implementing GHz compatible cabling, we achieve appreciable RF amplitudes at the junction while maintaining the mK base temperature and high energy resolution. We demonstrate the successful operation of our setup by utilizing two experimental ESR modes (frequency sweep and magnetic field sweep) on an individual TiH molecule on MgO/Ag(100) and extract the effective g-factor. We trace the ESR transitions down to MHz into an unprecedented low frequency band enabled by the mK base temperature. We also implement an all-electrical pump-probe scheme based on waveform sequencing suited for studying dynamics down to the nanoseconds range. We benchmark our system by detecting the spin relaxation time T1 of individual Fe atoms on MgO/Ag(100) and note a field strength and orientation dependent relaxation time.

Gravitational Forces Between Nonclassical Mechanical Oscillators

Interfacing quantum mechanics and gravity is one of the great open questions in natural science. Micromechanical oscillators have been suggested as a plausible platform to carry out these experiments. We present an experimental design aiming at these goals, inspired by Schm\"ole et al., Class. Quantum Grav. 33, 125031 (2016). Gold spheres weighing on the order a milligram will be positioned on large silicon nitride membranes, which are spaced at submillimeter distances from each other. These mass-loaded membranes are mechanical oscillators that vibrate at $\sim 2$ kHz frequencies in a drum mode. They are operated and measured by coupling to microwave cavities. First, we show that it is possible to measure the gravitational force between the oscillators at deep cryogenic temperatures, where thermal mechanical noise is strongly suppressed. We investigate the measurement of gravity when the positions of the gravitating masses exhibit significant quantum fluctuations, including preparation of the massive oscillators in the ground state, or in a squeezed state. We also present a plausible scheme to realize an experiment where the two oscillators are prepared in a two-mode squeezed motional quantum state that exhibits nonlocal quantum correlations and gravity the same time. Although the gravity is classical, the experiment will pave the way for testing true quantum gravity in related experimental arrangements. In a proof-of-principle experiment, we operate a 1.7 mm diameter Si$_3$N$_4$ membrane loaded by a 1.3 mg gold sphere. At 10 mK temperature, we observe the drum mode with a quality factor above half a million at 1.7 kHz, showing strong promise for the experiments. Following implementation of vibration isolation, cryogenic positioning, and phase noise filtering, we foresee that realizing the experiments is in reach by combining known pieces of current technology.

A CUPID Li2100MoO4 scintillating bolometer tested in the CROSS underground facility

A scintillating bolometer based on a large cubic Li2100MoO4 crystal (45 mm side) and a Ge wafer (scintillation detector) has been operated in the CROSS cryogenic facility at the Canfranc underground laboratory in Spain. The dual-readout detector is a prototype of the technology that will be used in the next-generation 0ν2β experiment CUPID . The measurements were performed at 18 and 12 mK temperature in a pulse tube dilution refrigerator. This setup utilizes the same technology as the CUORE cryostat that will host CUPID and so represents an accurate estimation of the expected performance. The Li2100MoO4 bolometer shows a high energy resolution of 6 keV FWHM at the 2615 keV γ line. The detection of scintillation light for each event triggered by the Li2100MoO4 bolometer allowed for a full separation (∼8σ) between γ(β) and α events above 2 MeV . The Li2100MoO4 crystal also shows a high internal radiopurity with 228Th and 226Ra activities of less than 3 and 8 μBq/kg, respectively. Taking also into account the advantage of a more compact and massive detector array, which can be made of cubic-shaped crystals (compared to the cylindrical ones), this test demonstrates the great potential of cubic Li2100MoO4 scintillating bolometers for high-sensitivity searches for the 100Mo 0ν2β decay in CROSS and CUPID projects.

High Noise Margin, Digital Logic Design Using Josephson Junction Field-Effect Transistors for Cryogenic Computing

As compute demands and their large energy costs continue to rise in the datacenter, it is imperative to identify low energy compute solutions. One approach is to investigate device schemes based on collective mode phenomena arising at low temperatures where power dissipation and switching voltage can be extremely low. Additionally, such devices could also have applications in quantum computing where such a low power logic/memory scheme could possibly be integrated onto the qubit plane, which is typically at ~10 mK temperature. Josephson Junction Field Effect Transistor (JJFET) is a promising candidate for low power operation with zero voltage drop across its drain-source terminals while operating in the superconducting regime. JJFET logic gate is typically realized as a n-type JJFET connected in a common-source configuration with a current source load. This results in high noise margin for logic-1 input but not for both logic-1 and 0. In this paper, we propose an overdamped region, n-type JJFET based digital logic using a cascaded common-source configuration-based design yielding high noise margin for both the logic inputs. DC noise margin sensitivity analysis is performed for the bias current and threshold voltage modulation. The noise margin can be further improved (approaches ideal inverter case) by cascading multiple common source stages yielding higher inverter gain.

Ultra-low temperature adiabatic demagnetization refrigerator for sub-Kelvin region

<sec>With the development of space observation, quantum technology and other frontier scientific research fields, the demand for ultra-low temperature refrigeration in sub-Kelvin region is increasing. Compared with dilution refrigeration and sorption refrigeration, adiabatic demagnetization refrigeration (ADR) has the outstanding advantages of high efficiency, compact, gravity independence and accessibility of working materials, which make ADR a promising technology for sub-Kelvin cooling.</sec><sec>A single-stage ultro-low temperature adiabatic demagnetization refrigerator is designed and developed. The thermodynamic principle and quantitative analysis are presented, from the macroscopic and microcosmic view, and operating results show good agreement with the theoretical value.</sec><sec>This refrigerator is precooled to 3 K by a GM-type refrigerator, with 252 g gadolinium gallium garnet (monocrystalline) used as a working medium. The maximum magnetic field of 4 T is provided by a superconducting coil. Flexible heat connection is used between the pre-cooler and ADR, so heat generated by vibration decreases. From (3 K, 4 T), the lowest temperature can reach 0.47 K by adiabatic demagnetization, which is consistent with the result drawn from the entropy data. In a constant-temperature-control mode, the demagnetization rate is controlled by a feedback loop, so the temperature can be held in the presence of a load. A cooling capacity of 2.7 J is provided at 1 K, with temperature fluctuation being lower than 0.5 mK, and the second thermodynamic efficiency of adiabatic demagnetization refrigeration is 57%. at 0.8 K, the cooling capacity is 1.2 J.</sec><sec>Future work on improving the performance includes the improving of the on-off ratio of the heat switch, so, the irreversible loss caused by the heat transfer temperature difference in conduction state can be reduced. Improving the heat transfer performance of the salt pill, the heat can be ejected in a shorter period.</sec><sec>This refrigerating machine is the first Chinese adiabatic demagnetization refrigeration system that can be operated in circulation, which is expected to be the 1<sup>st</sup> stage of a three-stage adiabatic demagnetization refrigeration system in a 50 mK temperature zone. This study lays a foundation for further developing continuous multistage adiabatic demagnetization refrigeration at ultra-low temperature.</sec>

Experimental study of PVT and phase-transition properties of binary water+n-hexane mixture near the upper critical endpoint

The PρT and phase transition (LLV and LV) properties (PS,TS,ρS) of binary water+n-hexane mixture with fixed concentration of 0.5 mol fraction of n-hexane (equimolar mixture) were measured. Measurements were focused in the immediate vicinity of the upper critical endpoint (UCEP) and supercritical regions in order to closely study the features of the mixture critical behavior near the UCEP. Measurements were made along 15 liquid and vapor isochores between (63.16 and 554.41) kg·m−3 and over a temperature range from (373.15 to 673.15) K at pressures up to 53.3 MPa. Phase transition phenomena on isochoric heating of the binary mixture was detailed study. For each isochore most measurements were made in the immediate vicinity of the LLV and LV phase transition temperatures (in the single-, two-, and three-phase regions) where the break of the P-T isochore's slopes are observing. Temperatures and pressures (PS,TS) at the LLV and LV phase transition points for each fixed density (isochore, ρ) were measured using the isochoric P-T break-point technique in the immediate vicinity of the UCEP of the mixture. The UCEP property data TUCEP = 469.15 K, PUCEP = 5.229 MPa, and ρUCEP = 263.95 kg·m−3 for the binary H2O + n-hexane mixture (x = 0.5 mol fraction of n-hexane) were extracted from the detailed PρT and three-phase equilibrium data (PLLV, TLLV, ρLLV) measurements near the UCEP. The combined expanded uncertainty of the density, ρ, pressure, P, temperature, T, and concentration, x, measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.2% (density in the liquid phase), 0.3% (density in the gas phase), 0.5% (density in the critical and supercritical regions); 0.05% (pressure); 15 mK (temperature); and 0.01 mol% (concentration). The excess molar volumes (VmE) of the binary mixture were calculated using the measured molar volumes Vm of the mixture and the reference molar volumes of the pure components (water and n-hexane) for various experimental temperatures and pressures in the supercritical region. Also, the measured PρT data at low densities (vapor phase) and high temperatures were used to calculated the second, third, and cross-term virial coefficients of the mixture.

Ultra-sensitive hybrid diamond nanothermometer.

Nitrogen-vacancy (NV) centers in diamond are promising quantum sensors because of their long spin coherence time under ambient conditions. However, their spin resonances are relatively insensitive to non-magnetic parameters such as temperature. A magnetic-nanoparticle-nanodiamond hybrid thermometer, where the temperature change is converted to the magnetic field variation near the Curie temperature, were demonstrated to have enhanced temperature sensitivity (n}{}11{rm{,,mK,,H}}{{rm{z}}^{ - 1/2}}) (Wang N, Liu G-Q and Leong W-H et al. Phys Rev X 2018; 8: 011042), but the sensitivity was limited by the large spectral broadening of ensemble spins in nanodiamonds. To overcome this limitation, here we show an improved design of a hybrid nanothermometer using a single NV center in a diamond nanopillar coupled with a single magnetic nanoparticle of copper-nickel alloy, and demonstrate a temperature sensitivity of n}{}76{rm{,,mu K,,H}}{{rm{z}}^{ - 1/2}}. This hybrid design enables detection of 2 mK temperature changes with temporal resolution of 5 ms. The ultra-sensitive nanothermometer offers a new tool to investigate thermal processes in nanoscale systems.

Experimental study of the critical and supercritical phenomena in ternary mixture of water + 1-propanol + n-hexane

The liquid-gas phase transition (PS, TS, ρS) and PρT properties of the ternary water + 1-propanol + n-hexane mixture with fixed concentration of 0.3333 mole fraction of each component (equimolar mixture) were measured. Measurements were focused in the immediate vicinity of the critical and supercritical regions in order to closely observe the features of the mixture critical behavior. Measurements were made along 29 liquid and vapor isochores between (25.03 and 646.59) kg·m−3 and over the temperature range from (373.15 to 673.15) K at pressures up to 60 MPa. For each isochore most measurements were made in the immediate vicinity of the liquid-gas phase transition temperature (in the single- and two-phase regions) where the break of the P-T isochore's slopes is observing. Temperatures and pressures (TS, PS) at the liquid-gas phase transition point for each fixed density (isochore, ρ) were measured using the isochoric P–T break point technique in the immediate vicinity of the critical point of the mixture. The critical property data TC = 510.85 K, PC = 6.09 MPa, and ρC = 255.22 kg·m−3 for the ternary mixture were extracted from the detailed PρT and (PS, TS, ρS) measurements near the critical point. The combined expanded uncertainty of the density, ρ, pressure, P, temperature, T, and concentration, x, measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.2% (density in the liquid phase), 0.3% (density in the gas phase); 0.5% (density in the critical and supercritical regions); 0.05% (pressure), 15 mK (temperature), and 0.01 mol% (concentration). The excess molar volumes (VmE) of the ternary mixture were calculated using the measured molar volumes Vm of the mixture and the reference molar volumes of the pure components for various experimental temperatures and pressures in the supercritical region. Also, the measured PρT data at low densities (vapor phase) were used to calculated the second and third virial coefficients of the mixture at high temperatures.

Experimental results of ADR cooling tuned for operation at 50 mK or higher temperature

Several astrophysics missions are currently planned, with significant European participations. To reach their targeted performances, cooling down to temperature of 50 or 100 mK, depending on the instrument design is required. Multi stage Adiabatic Demagnetization Refrigerator (ADR) cooling can provide such performances and is well adapted for space. At the early stage of the mission definition various cryogenics design are considered implying different cooling requirements. A 3-stage ADR cooler demonstration model for space has been used to demonstrate and validate several operation modes. Results will be presented for a 100 mK cooling including a continuous 400 mK stage as well as for 300 mK continuous cooler. A specific focus has also been put on 50 mK temperature measurements demonstrating the tight thermal stability, well below 1 μk that can be reached with this technology.

Lasing on a narrow transition in a cold thermal strontium ensemble

Highly stable laser sources based on narrow atomic transitions provide a promising platform for direct generation of stable and accurate optical frequencies. Here we investigate a simple system operating in the high-temperature regime of cold atoms. The interaction between a thermal ensemble of $^{88}$Sr at mK temperatures and a medium-finesse cavity produces strong collective coupling and facilitates high atomic coherence which causes lasing on the dipole forbidden $^1$S$_0 \leftrightarrow ^3$P$_1$ transition. We experimentally and theoretically characterize the lasing threshold and evolution of such a system, and investigate decoherence effects in an unconfined ensemble. We model the system using a Tavis-Cummings model, and characterize velocity-dependent dynamics of the atoms as well as the dependency on the cavity-detuning.

Controlling the thermal conductance of silicon nitride membranes at 100 mK temperatures with patterned metal features

Freestanding micromachined membranes are often used for thermal isolation in electronic devices such as photon sensors. The degree of thermal isolation plays an important role in determining device performance, and so the ability to suppress the thermal conductance of a membrane without increasing its size or decreasing its mechanical strength is of practical importance. We present a simple method that controllably reduces the thermal conductance of silicon nitride membranes by as much as 56% at temperatures near 100 mK. The thermal conductance suppression is achieved by depositing one additional metal layer patterned into islands or rings onto the membrane surface. Complex impedance and noise measurements of superconducting transition-edge sensors fabricated using this technique show that their noise performance is not degraded.

Dispersion of Lifetimes of Excited States of Single Molecules in Organic Matrices at Ultralow Temperatures

Abstract: Fluorescence excitation spectra of single terrylene molecules in transparent naphthalene and polyethylene matrices at ultralow (30–100 mK) temperatures have been studied under the conditions where the widths of zero-phonon spectral lines are determined only by the lifetime T of the excited electronic state. The experimentally observed dispersion of T values for identical molecules is attributed to local field effects, which are responsible for the dependence of T on the effective value of refractive index n of the matrix, which is characteristic of the localization region of each molecule. It has been shown that the dependence T(n) for ensembles of point emitters in organic matrices is satisfactorily explained within the model of a “virtual cavity” around the emitter inside a continuous medium, as well as within the developed quantum kinetic approach including various contributions of the local environment to the lifetime T. The recalculation of average T values to the corresponding n values with the use of the expressions obtained for T(n) has shown that the difference of the calculated refractive indices of naphthalene and polyethylene from the well-known tabulated n values is less than 1%.

Microscale direct measurement of localized photothermal heating in tissue-mimetic hydrogels

Photothermal hyperthermia is proven to be an effective diagnostic tool for cancer therapy. The efficacy of this method directly relies on understanding the localization of the photothermal effect in the targeted region. Realizing the safe and effective concentration of nano-particles and the irradiation intensity and time requires spatiotemporal temperature monitoring during and after laser irradiation. Due to uniformities of the nanoparticle distribution and the complexities of the microenvironment, a direct temperature measurement in micro-scale is crucial for achieving precise thermal dose control. In this study, a 50 nm thin film nickel resistive temperature sensor was fabricated on a 300 nm SiN membrane to directly measure the local temperature variations of a hydrogel-GNR mixture under laser exposure with 2 mK temperature resolution. The chip-scale approach developed here is an effective tool to investigate localization of photothermal heating for hyperthermia applications for in-vitro and ex-vivo models. Considering the connection between thermal properties, porosity and the matrix stiffness in hydrogels, we present our results using the interplay between matrix stiffness of the hydrogel and its thermal properties: the stiffer the hydrogel, the higher the thermal conductivity resulting in lower photothermal heating. We measured 8.1, 7.4, and 5.6 °C temperature changes (from the room temperature, 20 °C) in hydrogel models with stiffness levels corresponding to adipose (4 kPa), muscle (13 kPa) and osteoid (30 kPa) tissues respectively by exposing them to 2 W/cm2 laser (808 nm) intensity for 150 seconds.

PρTx properties of binary R134a/R245fa system

The gaseous PVTx properties of 1,1,1,2-tetrafluoroethane (R134a) + 1,1,1,3,3-pentafluoropropane (R245fa) mixtures were measured at pressures from 124 to 650 kPa, temperatures from 300.15 to 335.15 K and the compositions of R134a from 0.30 to 0.55 mass fraction using the Burnett isothermal expansion method. The uncertainty of temperature measurement was estimated to be within ±10 mK temperature, the pressure was within ±4 kPa, the mass fraction was within ±0.3%, and the compression factor was within ±0.06%. Based on the experimental PVTx property data, the densities and gas virial equation of the mixed refrigerants at the different temperatures and pressures were fitted. It provided detailed data for the further research on the performance of the Organic Rankine Cycle adopting the new alternative refrigerant.

Mapping Hot Spots at Heterogeneities of Few-Layer Ti3C2 MXene Sheets.

Structural defects and heterogeneities play an enormous role in the formation of localized hot spots in 2D materials used in a wide range of applications from electronics to energy systems. In this report, we employ scanning thermal microscopy (SThM) to spatially map the temperature rise across various defects and heterogeneities of titanium carbide (Ti3C2T x; T stands for surface terminations) MXene nanostructures under high electrical bias with sub-50 mK temperature resolution and sub-100 nm spatial resolution. We investigated several Ti3C2T x flakes having different thicknesses as well as heterogeneous MXene structures incorporating line defects or vertical heterojunctions. High-resolution temperature rise maps allow us to identify localized hot spots and to quantify the nonuniformity of the temperature fields across various morphological features. The results show that the local heating is most severe in vertical junctions of MXene flakes and is highly affected by nonuniform conduction due to the presence of line defects. These results provide a direct insight into the power dissipation of MXene-based devices and the roles of various heterogeneities that are inherent to the material synthesis process. This study provides a guideline for how a better understanding of the structure-property-processing correlations and further optimization of the synthesis routes could improve the lifetime, safety, and operation limits of the MXene-based devices.

Study of the Nonlinear Dynamics of Micro-resonators Based on a Sn-Whisker in Vacuum and at mK Temperatures

The dynamics of micro-resonators (or any mechanical resonators) can be studied by two complementary methods allowing the measurements in two different domains: (i) in the frequency domain - by the frequency sweeps using cw-excitation, and (ii) in the time domain - by the pulse techniques, when the free-decay oscillations are investigated. To broaden the knowledge about the intrinsic mechanical properties of micro-resonators based on a Sn-whisker we used both methods. We show that the dynamics of the Sn-whisker can be described by a phenomenological theory of the Duffing oscillator. Furthermore, we present the results of theoretical analysis based on the Duffing's model provided in the time and frequency domains, and we show that these results are qualitatively the same with those measured experimentally.

Design and analysis of optical micro-two-ring resonator temperature sensor with graphene

Micrometer-sized optical temperature sensors with graphene have been designed and analyzed in order to reach the highest temperature sensitivity. The main idea is to control the temperature of any system by surface graphene material in a micro-two-ring resonator path. By rising the temperature of the system, graphene with 5000 W / mK temperature conductivity senses the increase and applies it in the sensor resonance peaks. The prominent features of these sensors are small dimensions, high finesse and sensitivity, safe from electromagnetic interference, high bandwidth, low weight, and low cost. Electronic sensors suffer from electrical disturbances and electromagnetic interference, and sometimes do not function properly. Therefore, this micrometer optical temperature sensor with 44 × 40 μm dimensions and high sensitivity and finesse in all industrial and medical fields is necessary. The materials used in this sensor are graphene, Si, and SiO2, which are natural elements and have very high temperature stability. Free-spectral range reaches to 820 GHz and full width at half maximum to 21 GHz, and this quantity increases the finesse and sensitivity of the system, the time delay of sensor is 1.21 ps, which shows the low light dispersion and fast performance time.

Collapse of the Kondo state and ferromagnetic quantum phase transition in YbFe2Zn20

We present the electrical resistivity data under application of pressures up to $\sim$ 26 GPa and down to 50 mK temperatures on YbFe$_2$Zn$_{20}$. We find a pressure induced magnetic phase transition with an onset at $p_c$=18.2$\pm$0.8 GPa. At ambient pressure, YbFe$_2$Zn$_{20}$ manifests a heavy fermion, nonmagnetic ground state and the Fermi liquid behavior at low temperatures. As pressure is increased, the power law exponent in resistivity, $n$, deviates significantly from Fermi liquid behavior and tends to saturate with $n$ = 1 near $p_c$. A pronounced resistivity maximum, $T_\text{max}$, which scales with Kondo temperature is observed. $T_\text{max}$ decreases with increasing pressure and flattened out near $p_c$ indicating the suppression of Kondo exchange interaction. For $p>p_c$, $T_\text{max}$ shows a sudden upward shift, most likely becoming associated with crystal electric field scattering. Application of magnetic field for $p>p_c$ broadens the transition and shifts it toward the higher temperature, which is a typical behavior of the ferromagnetic transition. The magnetic transition appears to abruptly develop above $p_c$, suggesting probable first-order (with changing pressure) nature of the transition; once stabilized, the ordering temperature does not depend on pressure up to $\sim$ 26 GPa. Taken as a whole, these data suggest that YbFe$_2$Zn$_{20}$ has a quantum phase transition at $p_c$ = 18.2 GPa associated with the avoided quantum criticality in metallic ferromagnets.