Influence of Ball-Milling Treatment of B Original Powder on the Phase Formation and Critical Current Density of Graphite Doped MgB_{2}

In present work, the sintering process and superconducting properties of graphite doped MgB \(_{2}\) prepared with milled B original powder were investigated. It is found that ball milling treatment of B original powder obviously suppresses the solid–solid reaction between Mg and B, whereas it enhances their liquid–solid reaction during the subsequent sintering process of these graphite doped MgB \(_{2}\) bulks. Ball milling treatment of B original powder can also promote C substitution for B sites in MgB \(_{2}\) crystal lattice in the graphite-doped samples, and thus obviously increase their values of \(J_{c}\) at high fields. Moreover, ball milling also refines MgB \(_{2}\) grains, enhancing grain boundary pinning and \(J_{c}\) at high fields.     

Effects of Initial Cooling Conditions and Measurement Heights on the Levitation Performance of Bulk MgB_{2} Superconductor at Different Measurement Temperatures

The levitation properties of MgB \(_{2}\) prepared by hot press at 200  \(^{\circ }\) C and then pellet/closed tube method has been investigated. The vertical and lateral levitation forces ( \(F_{z}\) and \(F_{x})\) on a cylindrical NdFeB permanent magnet (PM) below a disk-shaped bulk MgB \(_{2}\) were measured during the vertical and lateral traverses of the PM in different cooling heights (CHs) and measurement heights (MHs) at temperatures of 20, 25 and 30 K to investigate the effect of the initial CH and MH on the levitation performance of MgB \(_{2}\) . For temperatures below 30 K, it was observed that \(F_{z}\) increases with increasing CH. However, a minute variation in \(F_{z}\) and a big hysteresis loop are observed at 30 K. From the lateral traverses, it was obtained that the \(F_{z}\) with attractive character increases with decreasing MH and the hysteresis effect increases for small MHs due to the increment of the magnetic field intensity which the sample feels with decreasing MH. In addition, it was seen that the character of \(F_{x}\) varies depending on both MH and measurement temperature. The higher hysteresis obtained in \(F_{x}\) than in \(F_{z}\) during lateral traverses implies that the motion of the flux lines in MgB \(_{2}\) is especially in lateral rather than vertical direction. Finally in this study, it was shown that the levitation performance of MgB \(_{2}\) depends not only on the measurement temperature but also on the CH and MH conditions. These results can be useful for optimizing the levitation performance of MgB \(_{2}\) superconductors for potential applications.     

Fabrication of Nb/Al Superconducting Tunnel Junction Using Ozone Gas

An ozone (O \(_{3})\) oxidation process was introduced for Nb/Al-based superconducting tunnel junctions (STJs) in order to form defect-free tunnel barriers at high critical current and to improve the energy resolution ( \(\Delta E\) ) for X-rays. The dependence of critical current ( \(J_\mathrm{C})\) and leak current ( \(I_\mathrm{leak})\) on the O \(_{3}\) exposure was measured to optimize the oxidation condition. The 50-square- \(\upmu \) m STJs produced by the O \(_{3}\) oxidation process exhibited an extremely small \(I_\mathrm{leak}\) of less than 50 pA. As expected, the lower or shorter the O \(_{3}\) exposure, the higher \(J_\mathrm{C}\) and the smaller the normal resistance ( \(R_\mathrm{N})\) . However, the maximum \(J_\mathrm{C}\) was 8 A/cm \(^{2}\) at an O \(_{3}\) exposure of 0.72 Pa min, which is much smaller than those of STJs with the conventional O \(_{2}\) oxidation process. It is expected that the high \(J_\mathrm{C}\) of 1,000 A/cm \(^{2}\) , at which a 9-eV-energy resolution for 277 eV photons is predicted, can be reached by an O \(_{3}\) exposure of 3.5 \(\times \) 10 \(^{-4}\) Pa min.     

Effective Thermal-Conductivity Measurement on Germanate Glass–Ceramics Employing the 3\omega Method at High Temperature

It can be noted that the germanate glass–ceramic is a functional material with excellent thermal stability which can be used in optical devices. The temperature-dependent effective thermal conductivities of CaO–BaO–CoO–Al \(_{2}\) O \(_{3}\) –SiO \(_{2}\) –GeO \(_{2}\) glass–ceramics from 295.5 K to 780 K are determined using a \(3\omega \) method. One of the main advantages for the \(3\omega \) method is to diminish radiation errors effectively when the temperature is as high as 1000 K. Thermal conductivities of CaO–BaO–CoO–Al \(_{2}\) O \(_{3}\) –SiO \(_{2}\) –GeO \(_{2}\) increase with a rise in temperature. Effective thermal conductivities of a sample increase from \(1.55~\hbox {W}\cdot \hbox {m}^{-1}\cdot \hbox {K}^{-1}\) at 295.5 K to \(7.64~\hbox {W}\cdot \,\hbox {m}^{-1}\cdot \hbox {K}^{-1}\) at 698.1 K. The effective thermal conductivity of CaO–BaO–CoO–Al \(_{2}\) O \(_{3}\) –SiO \(_{2}\) –GeO \(_{2}\) glass–ceramic increases with a rise of temperature. This investigation can be used as a basis for the measurement of thermal properties of ceramic materials at higher temperature.     

Primitive elements with prescribed trace

Let \(q\) be a power of a prime number \(p\) . Let \(n\) be a positive integer. Let \(\mathbb {F}_{q^n}\) denote a finite field with \(q^n\) elements. In this paper, we consider the existence of the some specific elements in the finite field \(\mathbb {F}_{q^n}\) . We get that when \(n\ge 29\) , there are elements \(\xi \in \mathbb {F}_{q^n}\) such that \(\xi +\xi ^{-1}\) is a primitive element of \(\mathbb {F}_{q^n}\) , and \(\mathrm{Tr}(\xi ) = a, \mathrm{Tr}(\xi ^{-1}) = b\) for any pair of prescribed \(a, b \in \mathbb {F}_q^*\) .     

An extension of the (strong) primitive normal basis theorem

An extension of the primitive normal basis theorem and its strong version is proved. Namely, we show that for nearly all \(A = {\small \left( \begin{array}{cc} a&{}b \\ c&{}d \end{array} \right) } \in \mathrm{GL}_2(\mathbb {F}_{q})\) , there exists some \(x\in \mathbb {F}_{q^m}\) such that both \(x\) and \((-dx+b)/(cx-a)\) are simultaneously primitive elements of \(\mathbb {F}_{q^m}\) and produce a normal basis of \(\mathbb {F}_{q^m}\) over \(\mathbb {F}_q\) , granted that \(q\) and \(m\) are large enough.     

Thermal Stability of \beta -PtO_2 Investigated by Simultaneous Thermal Analysis and Its Influence on Platinum Resistance Thermometry

We present thermogravimetric and differential scanning calorimetric studies of PtO \(_2\) powders measured in different atmospheres. In synthetic air a mass loss of 11.4 % is found at the decomposition temperature \(T_\mathrm {D}\)  = 595  \(^{\circ }\hbox {C}\) which can be attributed to the reduction of PtO \(_2\) . In a helium atmosphere the mass loss is 12.0 % and is found at 490  \(^{\circ }\hbox {C}\) . Subsequent heating in air leads to another oxidation process above \(T_\mathrm {D}\) and a reduction at 800  \(^{\circ }\hbox {C}\) . The second oxidation and reduction process is strongly suppressed when the powder is heated in He. The remaining mass above \(T_\mathrm {D}\) does not comply with a reduction path PtO \(_2 \rightarrow \) PtO \(\rightarrow \) Pt. Differential scanning calorimetry shows an endothermic reaction at \(T_\mathrm {D}\) in synthetic air as well as in helium which corresponds with the mass loss. These measurements imply that the powder can be assigned to be \(\beta \) -PtO \(_2\) . Furthermore, catalytic activity of the PtO \(_2\) powder is evidenced by mass spectrometry to be present below 460  \(^{\circ }\hbox {C}\) . Finally, the impact of these findings on the stability of platinum resistance thermometers is discussed.     

Containerless Measurements of Density and Viscosity for a Cu_{48}Zr_{52} Liquid

The densities of solid and liquid Cu \(_{48}\) Zr \(_{52}\) and the viscosity of the liquid were measured in a containerless electrostatic levitation system using optical techniques. The measured density of the liquid at the liquidus temperature (1223 K) is (7.02 \(\pm \) 0.01) g \(\cdot \) cm \(^{-3}\) and the density of the solid extrapolated to that temperature is (7.15 \(\pm \) 0.01) g \(\cdot \) cm \(^{-3}\) . The thermal expansion coefficients measured at 1223 K are (6.4 \(\pm \) 0.1) \(\,\times \,10^{-5}\) K \(^{-1}\) in the liquid phase and (3.5 \(\pm \) 0.3) \(\,\times \,10^{-5}\) K \(^{-1}\) in the solid phase. The viscosity of the liquid, measured with the oscillating drop technique, is of the form \(A\exp \left[ \left( {{E}_{0}}+{{E}_{1}}\left( 1/T-1/{{T}_{0}} \right) \right) \times \left( 1/T-1/{{T}_{0}} \right) \right] \) , where \({{T}_{0}}=1223\) K, \(A= (0.0254 \pm 0.0004)\) Pa \(\cdot \) s, \({{E}_{0}}\) =  (8.43 \(\pm \) 0.26) \(\,\times \,10^3\) K and \({{E}_{1}}\) =  (1.7 \(\pm \) 0.2) \(\,\times 10^7\) K \(^{2}\) .     

Effect of Grain Size of BaTiO_{3}Ceramics on Thermal Expansion and Electrical Properties

The thermal expansion behavior and electrical resistivity of BaTiO \(_{3}\) ceramics with different grain sizes were investigated. When they were heated and subsequently cooled in the range from 25  \(^{\circ }\) C to 200  \(^{\circ }\) C, the expansion and contraction curves of BaTiO \(_{3}\) ceramics with grain sizes of 600 nm and 1500 nm were not matched well to each other, and abnormal contraction and expansion behaviors were observed. For 30 nm and 150 nm BaTiO \(_{3}\) ceramics, the expansion and contraction curves basically are straight lines during heating. The linear thermal expansion coefficients ( \(\alpha _\mathrm{L}\) ) and the electrical resistivity of BaTiO \(_{3}\) ceramics were also measured. Experimental results showed that the value of \(\alpha _\mathrm{L}\) increases and the electrical resistivity decreases gradually with reducing grain size. This phenomenon can be attributed to the combination effect of the grain boundary and oxygen vacancies.     

Theoretical Study on the Upper Critical Field of a Layered Superconductor NbSe_2

Based on two-band Ginzburg-Landau theory, we study the temperature and angular dependences of the upper critical field for superconducting NbSe \(_2\) crystals. We introduce two variational parameters in our approach, and provide the explicit equations to determine the upper critical field in arbitrary direction. Our analyses strongly support previous scanning tunneling microscopy results and specific heat data, pointing to the existence of two energy gaps in this compound. Our results thus suggest that besides MgB \(_{2}\) , NbSe \({_2}\) is another example of two-gap superconductors.     

Fully Quantum Cross Second Virial Coefficients for the Three-Dimensional He–H_{2} Pair

A recent high-accuracy three-dimensional potential is used to compute the cross second virial coefficient \(B_{12}(T)\) between helium and molecular hydrogen. These calculations fully account for both quantum effects (with the path-integral Monte Carlo method) and the flexibility of the hydrogen molecule. The effect of flexibility is relatively small (only slightly larger than the expanded uncertainty of our results), but the full quantum mechanical approach is essential to obtain correct results at cryogenic temperatures. Values are calculated from 8 K to 2000 K; the uncertainty of the potential is propagated into uncertainties of \(B_{12}\) . Similar calculations are performed for He with the isotopologues D \(_{2}\) , T \(_{2}\) , HD, HT, and DT. Comparison is made with the experimental data for the He/H \(_{2}\) binary, and with the limited data available for He/D \(_{2}\) and He/HD. The calculated \(B_{12}(T)\) ’s are generally consistent with the experimental results, but have lower uncertainties.     

Four-Channel Current-Biased Kinetic Inductance Detectors Using MgB_2 Nanowires for Sensing Pulsed Laser Irradiation

We recently proposed the idea of a novel sort of superconducting detector, i.e., a current-biased kinetic inductance detector (CB-KID). This detector is different from a current-biased transition edge detector studied previously, and is able to sense a change in kinetic inductance \(L_k\) given by \(L_{k} = \Lambda _{k}l/S = m_{s}l/n_{s}{q_{s}}^{2}S\) ( \(\Lambda _{k}\) ; kinetic inductivity, \(m_s\) ; mass of Cooper pair, \(n_s\) ; density of Cooper pairs, \(q_s\) ; charge of Cooper pair, \(l\) ; length of device, \(S\) ; cross sectional area) under a constant dc bias current \(I_b\) . In the present work, we first extend this idea to construct a multi-channel CB-KIDs array made of 200-nm-thick MgB \(_2\) thin-film meanderline with 3- \(\upmu \) m thin wire. We succeeded in observing clear signals for imaging from the four-channel CB-KIDs at 4 K by irradiating focused pulsed laser. A scanning laser spot can be achieved by an XYZ piezo-driven stage and an optical fiber with an aspheric focused lens. We can see typical signals from all 4 channels at 4 K, and obtain the positional dependence of the signal as the contour in XY plane. Our CB-KIDs can be used as neutron detectors by utilizing energy released from a nuclear reaction between \(^{10}\) B and cold neutron.     

Thermodynamics of the Spin-1/2 Two-Leg Ladder Compound (C_5H_{12}N)_2CuBr_4

The thermodynamic behavior of the spin \(S=1/2\) antiferromagnetic two-leg ladder compound (C \(_5\) H \(_{12}\) N) \(_{2}\) CuBr \(_{4}\) in a uniform magnetic field is studied using numerical and analytical approaches. The entropy \(S(H,T)\) and specific heat \(C(H,T)\) are calculated. The specific heat shows various behaviors in different regions of the magnetic field. The field dependence of the specific heat is almost symmetric about the average of quantum critical fields in complete agreement with experimental results. In addition, it is found that during an adiabatic demagnetization process, temperature drops in the vicinity of the field induced zero-temperature quantum phase transitions.     

Fusion Diagrams in the \mathrm{BiBO}_{3}–\mathrm{YbBO}_{3} and \mathrm{Bi}_{4}\mathrm{B}_{2}\mathrm{O}_{9}–\mathrm{YbBO}_{3} Systems

A calculation model of the Gibbs energy of ternary oxide compounds from the binary components was used. Thermodynamic properties of \(\mathrm{Yb}_{2} \mathrm{O}_{3}\) – \(\mathrm{Bi}_{2}\mathrm{O}_{3}\) – \(\mathrm{B}_{2}\mathrm{O}_{3}\) ternary systems in the condensed state were calculated. Thermodynamic data of binary and ternary compounds were used to determine the stable sections. The probability of reactions between the corresponding components in the \(\mathrm{Yb}_{2} \mathrm{O}_{3}\) – \(\mathrm{Bi}_{2} \mathrm{O}_{3}\) – \(\mathrm{B}_{2} \mathrm{O}_{3}\) system was estimated. Fusibility diagrams of systems \(\mathrm{BiBO}_{3}\) – \(\mathrm{YbBO}_{3}\) and \(\mathrm{Bi}_{4} \mathrm{B}_{2} \mathrm{O}_{9}\) – \(\mathrm{YbBO}_{3}\) were studied by physical–chemical analysis. The isothermal section of the phase diagram of \(\mathrm{Yb}_{2} \mathrm{O}_{3}\) – \(\mathrm{Bi}_{2} \mathrm{O}_{3}\) – \(\mathrm{B}_{2} \mathrm{O}_{3}\) at 298 K is built, as well as the projection of the liquid surface of \(\mathrm{BiBO}_{3}\) – \(\mathrm{B}_{2} \mathrm{O}_{3}\) – \(\mathrm{YbBO}_{3}\) .     

Limit laws for the maxima of stationary chi-processes under random index

Let \(\{\chi _{k}(t), t\ge 0\}\) be a stationary \(\chi \) -process with \(k\) degrees of freedom. In this paper, we consider the maxima \(M_{k}(T)= \max \{\chi _{k}(t), \forall t\in [0,T]\}\) with random index \(\mathcal {T}_{T}\) , where \(\mathcal {T}_{T}/T\) converges to a non-degenerate distribution or to a positive random variable in probability, and show that the limit distribution of \(M_{k}(\mathcal {T}_{T})\) exists under some additional conditions.     

First- and Second-Order Phase Transition in Parallel Triple Quantum Dots: The Roles of Symmetric and Asymmetric Hopping

By means of the numerical renormalization group method, I study the quantum phase transition (QPT) and the electronic transport in parallel triple quantum dot system with symmetric and/or asymmetric hopping. For symmetric hopping \(t_{1} = t_{2}\) and zero magnetic field \(B = 0\) , I find a first order transition between spin quadruplet and doublet as \(t_{1}\) ( \(t_{2}\) ) increases. With increasing \(B\) , a second order QPT between \(S_{z} = 1/2\) of the doublet and \(S_{z} = 3/2\) of the quadruplet is observed. For asymmetric hopping \(t_{1} \ne t_{2}\) , the QPT depends closely on the other hopping. For fixed \(t_{1} < \varGamma \) , where \(\varGamma \) is the hybridization function between the dots and the leads, a first order transition is observed as \(t_{2}\) increases, while for \(t_{1} \ge \varGamma \) , a crossover occurs. In the presence of \(B\) , the transition between \(S_{z} = 1/2\) and \(S_{z} = 3/2\) is a first order QPT for \(t_{1} < \varGamma \) , while a second order for \(t_{1} \ge \varGamma \) .     

({p, \rho,T, x}) Properties of \text{ CO}_{2}/Propane Binary Mixtures at 280 K to 440 K and 3 MPa to 200 MPa

The (p, \(\rho \) , T, x) properties of binary mixtures of CO \(_{2}\) (volume fraction purity 0.99999) and propane (mole fraction purity 0.9999) ( \(x_{1}\) CO \(_{2}+x_{2}\) propane; \(x_{1} = 0.1744\) , 0.3863, 0.5837, and 0.7732) were measured in the compressed liquid phase using a metal-bellows variable volumometer. Measurements were conducted from 280 K to 440 K and 3 MPa to 200 MPa. The expanded uncertainties ( \(k = 2\) ) were estimated to be temperature, \(<\) 3 mK; pressure, 1.5 kPa ( \(p\le 7\)  MPa), 0.06 % (7 MPa \(< p\le 50\)  MPa), 0.1 % (50 MPa \(< p\le 150\)  MPa), 0.2 % ( \(p> 150\)  MPa); density, 0.10 %; and composition, \(4.4\times 10^{-4}\) . At \(p >100\)  MPa and 280 K or 440 K, the uncertainties in density measurements increase to 0.14 % and 0.22 %, respectively. The data were compared with available equations of state. The excess molar volumes, \(v_\mathrm{m}^\mathrm{E}\) , of the mixtures were calculated and plotted as a function of temperature and pressure.     

Quasiparticle Freeze-Out in Superconducting Tunnel Junction X-ray Detectors with Killed Base Electrode

The current–voltage characteristics of superconducting tunnel junction (STJ) X-ray detectors were measured in the temperature range from 4.2 to 0.1 K. The freeze-out of the thermal tunneling current was compared between an STJ detector with a traditional Nb/Al/Al \(_{2}\) O \(_{3}\) /Al/Nb layer structure and a Ti/Nb/Al/Al \(_{2}\) O \(_{3}\) /Al/Nb/NbN detector whose low-gap Ti film kills the X-ray response of the base electrode. The current decrease and the linear low-temperature I(V) characteristics for the detector with the killed electrode can be qualitatively explained by tunneling current contributions from the subgap states of the Ti film. The data are analyzed on the basis of the proximity theory in the dirty limit.     

Toward a Detector/Readout Architecture for the Background-Limited Far-IR/Submm Spectrograph (BLISS)

We have built and tested 32-element linear arrays of absorber-coupled transition-edge sensors (TESs) read out with a time-division SQUID multiplexer. This detector/readout architecture is designed for the background-limited far-IR/submm spectrograph (BLISS) which is a broadband (35–433  \(\upmu \) m), grating spectrometer consisting of six wavebands each with a modest resolution of R \(\sim \) 700. Since BLISS requires the effective noise equivalent power (NEP) of the TESs to equal 1  \(\times \)  10 \(^{-19}\)  W/Hz \(^{1/2}\) , our detectors consist of very long (1–2 mm), narrow (0.4 \(\upmu \) m), and thin (0.25 \(\upmu \) m) Si \(_{x}\) N \(_{y}\) support beams that reduce the thermal conductance G between the substrate and the optical absorber. The thermistors of our lowest noise TESs consist of iridium with \(T_{c}=130\) mK. We have measured the electrical properties of arrays of these Ir TESs with various meander and straight support beams and absorber shapes and found that G is \(\sim \) 30 fW/K (meander) and \(\sim \) 110 fW/K (straight), the electrical NEP is 2–3  \(\times \)  10 \(^{-19}\) W/Hz \(^{1/2}\) (meander and straight), and the response time \(\tau \) is 10–30 ms (meander) and 2–5 ms (straight). To reduce spurious or “dark” power from heating the arrays, we mounted the arrays into light-tight niobium boxes and added custom L/R and L/C low-pass chip filters into these boxes to intercept dark power from the bias and readout circuit. We found the average dark power equals 1.3 and 4.6 fW for the boxes with L/R and L/C chip filters, respectively. We have built arrays with \(T_{c}= 70\)  mK using molybdenum/copper bilayers and are working to lower the dark power by an order of magnitude so we can demonstrate NEP \(~=~1~\times \)  10 \(^{-19}\)  W/Hz \(^{1/2}\) with these arrays. PACS numbers: 85.25.Pb; 95.85.Gn; 95.85.Fm; 63.22. \(+\) m     

Application of Non-Isothermal Thermogravimetric Method to Interpret the Decomposition Kinetics of \hbox {NaNO}_{3}, \hbox {KNO}_{3}, and \hbox {KClO}_{4}

The non-isothermal thermogravimetric method was used to study the thermal decomposition of \(\hbox {KClO}_{4}, \hbox {KNO}_{3}\) , and \(\hbox {NaNO}_{3}\) at heating rates of (5, 10, 15, and 20)  \(\hbox {K}\cdot \hbox {min}^{-1}\) . The activation energy of thermal decomposition reactions was computed by isoconversional methods of Ozawa–Flynn–Wall, Kissinger–Akahiro–Sunose, and Friedman equations. Also, the kinetic triplet of the thermal decomposition of salts was determined by the model-fitting method of the modified Coats–Redfern equation. The activation energies of \(\hbox {KClO}_{4}, \hbox {KNO}_{3}\) , and \(\hbox {NaNO}_{3}\) of (293 to 307, 160 to 209, and 192 to 245)  \(\hbox {kJ}\cdot \hbox {mol}^{-1}\) , respectively, are obtained by non–isothermal isoconversional methods. The modified Coats and Redfern method showed that the most probable mechanism functions \(g(\alpha )\) of \([-\hbox {ln}(1 - \alpha )]^{1/3}\) (model A3: Arami–Erofeev equation) and \((1 - \alpha )^{-1}- 1\) (model F2: second order) can be used to predict the decomposition mechanisms of \(\hbox {KClO}_{4}\) , \(\hbox {KNO}_{3}\) , and \(\hbox {NaNO}_{3}\) , respectively.