An accurate equation for the V−T characteristic of GaAs diode thermometers in the 4–300 K range

GaAs diode thermometers have been demonstrated to be reliable and reproducible transducers in both the liquid nitrogen and helium range, but so far no equation has been available for interpolating the voltage-temperature characteristic, at constant current, with an accuracy better than 1 K.An equation is developed: V = A + Blog_e (T/T₁ + 1) + C(log_e(T/T₁ + 1)² + …] which determines the V?T characteristic with a voltage accuracy of 0.01%, corresponding to 0.1 K on the whole 4–300 K range.     

Cryogenic low noise 2.2–3 GHz amplifier

A simple design of one stage, low power cryogenic amplifier at 2.2–3 GHz range is presented. The design was numerically simulated by freely available microwave library SuperMix. The amplifier constructed according to the numerically optimized design was measured in cryogen-free refrigerator. The measured noise temperature T_N ≈ 6 K and gain G ≈ 15.5 dB are in good agreement with numerical simulations.     

Reflection spectra of lithium hydride (Deuteride) crystals in the 4–35 eV energy range

The reflection spectra of LiH and LiD single crystals cleaved in ultrahigh vacuum (2.8 × 10^?10 Torr) at a temperature of 8 K have been studied using synchrotron radiation in the 4–35 eV photon energy range. The electron structure of lithium hydride in the vacuum ultraviolet range is interpreted. A method of rapid determination of the isotope composition of mixed LiH _x -LiD_{(1 ? x)} crystals is proposed that is based on the measurement of reflection spectra in the exciton range.     

Realisation, Dissemination, and Comparison of the ITS-90 and the PLTS-2000 Below 1 K at PTB

The International Temperature Scale of 1990 (ITS-901 and the Provisional Low Temperature Scale (PLTS-2000)² are the internationally agreed temperature scales in the low temperature range. At temperatures between 0.65 K and 5 K, the ITS-90 is defined by polynomials relating the vapour pressure p _v of ⁴He and ³He to the temperature T ₉₀, whereas the PLTS-2000 is defined by a polynomial relating the ³He megting pressure p _m to the temperature T ₂₀₀₀ in the range from 0.9 mK to 1 K. Here, we describe the methods and the cryogenic facilities used for the realisation and dissemination of the ITS-90 and the PLTS-2000 as well as the level of accuracy, which is achieved at PTB. We focus on how the pressures are measured traceably to the national pressure standard. Using this experimental basis, we plan to compare directly the vapour and megting pressure of ³He in the overlapping temperature range from 0.65 K to 1 K.     

Prototype of a microwave generator with a high-temperature superconductor disk resonator in a feedback circuit with a low phase-noise level

Phase noise of a prototype of microwave generator with a disk resonator based on high-temperature superconductor (HTSC) Y₁Ba₂Cu₃O_{7 ? δ} films in the feedback circuit has been measured for the first time. The disk resonator was cooled to a temperature of 77 K, while the other generator components remained at room temperature. The generator carrier frequency was 7.5 GHz. Phase noises have been measured for different offsets from the carrier frequency. For example, the minimum measured phase-noise level was found to be 136.2 dB/Hz for a 10-kHz offset from the carrier frequency. It is shown that the results obtained correspond to the classical Leeson formula with flicker noise neglected. Thus, a disk resonator based on Y₁Ba₂Cu₃O_{7 ? δ} films does not introduce additional noise (in comparison with the amplifier noise) at small offsets and, correspondingly, is promising for microwave generators with extremely low phase noise. The possibility of further decrease in the phase-noise level by increasing the Q factor of the HTSC disk resonator and optimizing its housing design is discussed.     

Cryogenic Integrated Offset Compensation for Time Domain SQUID Multiplexing

Superconducting QUantum Interference Device (SQUID) multiplexing is a common technique in the use of large arrays of Transition Edge Sensors (TES). A?Time Domain Multiplexer (TDM) combines input TES signals into one output signal using several SQUIDs. Different TES, SQUID and amplifier characteristics induce unavoidable different offsets on the multiplexed signal. Additionally, given the periodicity of the SQUID characteristic, the Flux Locked Loop (FLL) operating point is only defined modulo?Φ ₀. This can lead to a large output offset. In multiplexed mode, the difference between offsets associated with different pixels can induce a parasitic signal which is often larger than that of the TES. These offset signals drastically constrain the readout dynamic range and thus the maximum gain allowed. They also limit the signal-to-noise ratio, the FLL stability and the multiplexing frequency. Offsets in SQUID readout are discussed and offset compensation for TDM is presented. The dynamic calibration and compensation on a simplified 4:1?TDM are demonstrated in simulation. Dynamic offset compensation is being implemented on a cryogenic SiGe integrated circuit operated at 4?K for 128:1?TDM.     

Temperature dependence of In1−xGaxSb reflectivity in the far infrared

Far infrared reflectivity spectra of polycrystalline In_1?xGa_xSb were measured and numerically analyzed using the classical dispersion formula and also a fitting procedure based on the modified plasmon-phonon interaction model in the temperature range from 10 K to 300 K. Optical parameters were calculated and discussed. A local mode belonging to the GaSb rich end and two-mode behavior were observed at low temperatures.     

Magnetocaloric Effect in Nanopowders of Pr0.67Ca0.33Fe x Mn1−x O3

Magnetocaloric effect in nanopowders of Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ (x=0,0.1,0.3,0.4), in the vicinity of magnetic phase transitions, was investigated. It is shown that Pr_0.67Ca_0.33MnO₃ exhibits the largest magnetic entropy change (ΔS _M ) of 0.61 J/kg/K at 208 K upon 0.5 T magnetic field variation. Furthermore, the ΔS _M distribution of the Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ is much more uniform than that of gadolinium. Because of these results, nanopowders of Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ have some potential applications for magnetic refrigerants in an extended high-temperature range. Moreover, it can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.     

Modeling of low-temperature specific heat data for Ge27As13S60 and As40S60 glasses by means of the phenomenologically modified soft potential model

Experimental results for the low-temperature specific heat C_p of Ge₂₇As₁₃S₆₀ and As₄₀S₆₀ glasses, measured from 70 K down to 0.54 K, are presented. A maximum in the temperature dependence of the scaled specific heat C_p/T³, appearing in the Boson peak range 2-10 K, and a minimum of this quantity in the tunneling-state range below 1 K, typical for all the glassy materials, were obtained. The presented C_p data are modeled in the frames of a phenomenologically modified soft potential model, applicable in wide low-temperature range. The character of the low-energy excitations in the investigated Ge-As chalkogenides is clarified.     

Preparation and X-ray diffraction, dielectric, and Mössbauer characterization of Co1 − x Ni x Cr2O4 solid solutions

Co_{1 ? x} Ni _x Cr₂O (0 ≤ x ≤ 1) ceramic samples have been characterized by X-ray diffraction and relative dielectric permittivity ?(T), loss tangent tan δ(T) (0.1–200 kHz), and thermally stimulated depolarization current (TSDC) measurements in the range 100–350 K. The samples were shown to consist of spinel solid solutions with a cubic (0 < x ≤ 0.98) or tetragonal (0.99 ≤ x < 1) structure. Increasing the Ni content of the samples from 0 to 100 at % increases their tanδ and 1/ρ by three to four orders of magnitude. The ?(T), tan δ(T), and TSDC of the samples with 0.2 ≤ x ≤ 0.6 have anomalies at T ₁ ～ 220 K and T ₂ ～ 240 K, which point to a transition to a polar state below T ₂. Samples containing 1–4 at % ⁵⁷Fe as an impurity were characterized by Mössbauer spectroscopy in the range 54–330 K and were shown to be magnetically ordered at 78 K. We conclude that the solid solutions have ferroelectric properties with a Curie temperature T ₂ and magnetoelectric multiferroic properties.     

Construction and use of a pulsed copper nuclear magnetic resonance thermometer

A pulsed nuclear magnetic resonance thermometer, constructed for temperature measurements below 0.5 K, is described. The nuclear free precession signal of copper nuclei is recorded with a gated low noise amplifier and with a phase sensitive detector gated to integrate the signal over an adjustable number of periods. The physical significance of the signal is discussed. A comparison of the resonance thermometer against a slurry type cerium magnesium nitrate (CMN) thermometer for the temperature region 10 mK to 100 mK is presented. The deviation Δ = T_NMR − T_CMN, representing the departure of the CMN thermometer from a Curie law behaviour, was measured as (0.5 ± 0.2) mK. In experiments with a nuclear refrigeration cryostat the resonance thermometer was calibrated against a nuclear orientation thermometer which provided an independent absolute temperature standard. At the low temperature end, from 1 mK to 10 mK, the linearity of the thermometer in T⁻¹ was confirmed by measurements of the nuclear spin-lattice relaxation time.     

Large-Area YBa2Cu3O7 − x Films on Sapphire with Excellent Microwave Power Handling Capability

Crack-free thick YBa₂Cu₃O_{7 ? x} films are prepared on CeO₂ buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces (“peak-to-valley” roughness <10 nm), high critical currents (J _C > 2 MA/cm² at 77 K and 0 T), and low microwave surface resistances (R _s(77K) ?.4mω and R _s(4.2K) ?.110μω at 19.15GHz) comparable to the best values reported in the literature for YBCO films on structurally better matched substrates. These thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B _HF) up to 54, 37, and 17.4 mT at 4.2, 50, and 77 K, respectively, which for the lower temperatures were limited by the available power of the 25-W HF amplifier. The high-power performance, which to our knowledge belongs to the best reported so far for unpatterned YBa₂Cu₃O_{7 ? x} films, was achieved without any degradation of the samples despite frequent thermal cycling.     

Two-point calibration of standard capsule platinum resistance thermometers for the range 13.81 K to 273.15 K

A method of calibrating a standard capsule platinum resistance thermometer (prt) over the range 13.81 K to 273.15 K is described. Measurements of the resistance of the prt are needed at only two fixed points, the boiling point of He⁴ (4.2 K) and the ice-point (273.15 K), both of which are easy to realize. For a prt with a residual resistance ratio (R_{4.2 K}/R_{273.15 K}) of less than 4 × 10^?4, the method provides a calibration on ipts 68 with an uncertainty of 20 mK over the entire range 13.81 K to 273.15 K. For prts with residual resistance ratios between 4 × 10^?4 and 7 × 10^?4, the calibration uncertainty is 75 mK from 13.81 K to 40 K and 20 mK from 40 K to 273.15 K.     

Phase Diagram of Hydrogen in Palladium

Hydrogen in palladium, Pd-H(D), is an interesting system because of the highly mobile hydrogen and the presence of a phase boundary below 100 K. Experimentally, however, the nature of this transition has not been established. Historically this transition around 55 to 100 K has been thought to be an order-disorder transition. Such a transition would produce a phase boundary with anomalies at specific hydrogen concentrations corresponding to the specific ordered structures. In order to check this phase boundary we have performed a detailed study of the hydrogen concentration dependence of the specific heat of PdH _x over the temperature range from below 0.5 K to above 100 K using PdH _x specimens with x up to 0.8753. The measured heat capacity has been analyzed as the sum of contributions due to the lattice specific heat of Pd, the electronic specific heat of PdH _x , and the excess contribution caused by hydrogenation of the specimen. The excess specific heat result shows a sharp peak which indicates a phase boundary with transition temperature T ₁=55 K to 85 K depending linearly on the hydrogen concentration from x=0.6572 to 0.8753. We do not observe anomalies at specific x values as would be expected for the specific ordered structures.     

Signal and noise characteristics of a terahertz frequency-selective YBa2Cu3O7−x Josephson detector

The performance characteristics of a frequency-selective Josephson detector based on a symmetrical [001]-tilt YBa₂Cu₃O_7?x bicrystal junction have been studied in the terahertz frequency range. At an external radiation frequency of 0.692 THz and a detector temperature of 55 K, the volt-watt responsivity reaches (7 ± 2) × 10⁴ V/W, which corresponds to theoretical estimates calculated with allowance for thermal fluctuations in the junction. The noise equivalent power (2.9 ± 0.9) × 10^–13 W/Hz^1/2 and the power dynamic range D = 47 ± 3 dB of the detector are determined by the excess 1/f type noise in the junction. It is shown that NEP values up to 5 × 10^?15 W/Hz^1/2 and D above 60 dB can be reached with high-frequency modulators or pulsed sources of terahertz radiation.     

Control of substrate surface temperature in millisecond annealing technique using thermal plasma jet

Temporal variations of substrate surface temperature in scanning Ar thermal plasma jet has been investigated based on an analysis of transient changes in optical reflectivity. The accuracy of the temperature measurement has been evaluated to be 30 K at temperature around 1760 K. The maximum surface temperature (T_max) is controlled in the range from ∼ 960 to ∼ 1780 K with keeping the annealing duration (t_a) around ∼ 3 ms by changing the Ar gas flow rate (f) and distance between the plasma jet and the substrate (d) under a constant scanning speed (ν) of 500 mm/s.     

Preparation of New Composite Magnetocaloric Compounds by Modifying the Annealing Temperature of La0.8Ca0.2?x □ x MnO3 Perovskite

In this paper, a new method to broaden the range of the magnetic refrigeration temperature by changing the annealing temperature was proposed. Series of La_0.8Ca_0.2?x ?? _x MnO₃(0.00??x??0.20) compounds were prepared by solid-state reaction and annealed firstly at a temperature of 1473 K (S1) and then at 1073 K (S2). Morphologic and structural studies have revealed that the decrease of the annealing temperature modifies the grain size and the structure of these compounds. The magnetic measurements have shown that the annealing at low temperature (1073 K) increases the magnetization and enhances the one-electron bandwidth, which induces an increase of the Curie temperature for 183 K (S1) to 241 K (S2) for the x=0.00 sample. The magnetocaloric investigation has exposed that the decrease of the annealing temperature induces a change from a second-order magnetic phase transition to first-order one for S1 and S2 compounds, respectively. Also, we have found that the Relative Cooling Power (RCP) factor remains almost constant as a function of calcium-deficiency concentration (x) and the annealing temperature. Finally, we have deduced that we can use composite magnetocaloric compounds, exploiting a mixing of the same compounds annealed at two different temperatures (1473 K; S1) and (1073?K; S2), for refrigeration over the temperature range 175?C264?K.     

Elastic and Thermal Properties of Double Doped Orthovanadate: Eu1−x (La0.254Y0.746) x VO3

We present the effect of A-site double doping (La_0.254Y_0.746) on elastic and thermal properties of EuVO₃ in a wide doping range (0≤x≤1), using a modified rigid ion model (MRIM). Various lattice distortions and their relation to bulk modulus has been investigated systematically. The effect of the lattice distortions on thermodynamic properties of Eu_1?x (La_0.25Y_0.74) _x VO₃ has been explored by an atomistic approach. The computed results emphasize that the Debye temperature decreases while the specific heat increases with increase in doping concentration (x). The computed temperature dependent (1 K≤T≤300 K) specific heat trends are in accordance with the corresponding experimental data at various compositions. Future scope of MRIM has also been discussed.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇-δ superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The ～0.2μm thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇-δ powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 mΩ, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.     

Equilibrium vacancy parameters and limit temperature of nonequilibrium melting in osmium

For osmium, based on experimental data on enthalpy and the averaged heat capacity obtained by the method of mixing within the temperature range of 1150?C2960 K, for the first time the parameters of the equilibrium vacancies for this metal have been determined: the vacancy formation energy E = 1.8 eV, the vacancy concentration at melting c = 3.3%, and the vacancy formation entropy S = 25.6 J/(mol K). The limit temperature of the onset of nonequilibrium melting of osmium T _m = 4256 K and its relative value T _lim/T _m = 1.30 has been calculated.