Static Nuclear Magnetisation of Aluminium Measured by Its Influence on the Superconducting Critical Field

We have measured the temperature dependence of the superconducting critical field of Al in the temperature range 17K T 600mK. The external magnetic field necessary to suppress superconductivity in Al decreases significantly with decreasing T below about 1 mK. This effect is due to the paramagnetic contribution of the nuclear magnetic moments of Al to the total magnetic flux. Our method enables us to determine the static nuclear magnetisation of the Al nuclei. The results are in agreement with the theory for a Langevin nuclear paramagnet. In addition, we have measured the spin lattice relaxation time ₁ . For the Korringa constant we get ₁ ·T = (2.0±0.2)K sec in good agreement with previous NMR results.     

The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

The Impact of Nuclear Magnetism on Superconductivity in a Metal with Nuclear Electric Quadrupole Splitting: Indium

We report the first investigation of the impact of nuclear magnetism on superconductivity in the tetragonal metal indium. We have measured the superconducting critical field B_c(T) and in its vicinity the nuclear magnetic heat capacity at ultralow temperatures, 170 KT200 mK. We compare the measured quantities with calculations which consider the nuclear magnetic Zeeman and the dominating nuclear electric quadrupole interaction in indium. The heat capacity data support the occurence of a positive sign of the electrical field gradient at nuclear sites and in consequence the existence of a nuclear low spin ground state. Surprisingly, at lowest investigated temperatures, 170 KT1 mK, the reduction of the critical field B_c(T) clearly exceeds the size of the calculated magnetization ₀ M(B_c, T) which is limited by the nuclear low spin ground state. In all other materials the interplay of nuclear magnetism and superconductivity has been studied so far (Al, AuAl ₂ , AuIn ₂ , Rh, and Sn), the bare nuclear magnetization appeared as an upper limit of the reduction of the critical field.     

Electrical Properties of Background-Limited Membrane-Isolation Transition-Edge Sensing Bolometers for Far-IR/Submillimeter Direct-Detection Spectroscopy

We built and measured the electrical properties of membrane-isolation transition-edge sensing bolometers (TESs) suitable for background-limited far-IR/submillimeter direct-detection spectroscopy. Each TES consists of a Mo/Au bilayer patterned onto a suspended, thermally isolated absorber that is connected to the substrate through four Si _x N _y beams deposited by low pressure chemical vapor deposition (LPCVD). We fabricated TESs with straight and meander support beams. The dimensions of the meander (straight) support beams are 700 μm (700 μm) long by 0.25 μm (0.5 μm) thick by 0.35 μm (0.5 μm) wide. We measured I–V characteristics for these TESs and determined that the thermal conductance G equals 72 fW/K (straight) and 19 fW/K (meander) for our best devices. The thermal conductance exhibits a T ^1/2 dependence with temperature which is evidence of effective elastic scattering of the acoustic phonon modes. The transition temperatures T _c for the same TESs are T _c =137 mK (straight) and T _c =71 mK (meander). If we assume the TESs are temperature-fluctuation noise limited, then the derived noise equivalent power (NEP) equals 1.9×10⁻¹⁹ W/Hz^1/2 (straight) and 6.1×10⁻²⁰ W/Hz^1/2 (meander), using our measured values for G and T _c . The meander-beam TES has a derived NEP that is close to two orders of magnitude lower than the state-of-the-art. Finally, we measured an effective time constant τ of about 300 ms (straight) and 400 ms (meander) using electrical and optical pulses. These values for the NEP and τ for the meander-beam TES meet the requirements for the Background-Limited far-IR/Submillimeter Spectrograph (BLISS), a proposed NASA instrument.      

Heat capacity of titanium hydride at millikelvin temperatures

The heat capacity C of a TiH_1.98 bulk sample in zero and 72 mT magnetic fields has been measured at 8 mKT 600 mK. In zero magnetic field the sample shows metallic behavior. In a magnetic field the protons give an additional nuclear heat capacity contribution, whose value is in good agreement with the calculated one. From our data we also obtain an upper limit for the nuclear spin-lattice relaxation time ₁ and for the molecular hydrogen impurity concentration in the sample. ForT 10 mK, no superconducting or any other phase transition has been observed.     

Nuclear specific heat of thallium

We have measured the nuclear specific heat of a 0.79 mole thallium sample of 5 N+ purity in the temperature range 70 µKT20 mK and in magnetic fields from 20 mTB228 mT. The experimental results agree with the theoretical expectation for the specific heat of a nuclear paramagnet with the properties of Tl. Our results for the nuclear specific heat imply that the static properties of bulk Tl seem to differ from the dynamic, surface-sensitive properties of Tl samples investigated in former nuclear magnetic resonance experiments.     

Simultaneous Measurements of Thermal,Electrical, and Acoustic Properties of BaTiO<Subscript>3</Subscript> – New Feature of 403 K Phase Transition

In order to obtain useful information on the transient process of phase transitions in ferroelectrics by various methods including calorimetry, the “mK-stabilized cell” of a small size has been developed. It is based on the heat flux differential scanning calorimeter (DSC) and has a temperature stability better than 1 mK. The “cell” can be used to change the temperature under nearly quasi-static conditions by an infinitesimally small rate not only on heating but also on cooling while passing through the transition points. It enables simultaneous measurements of endothermic heat and exothermic heat along with dielectric constants, displacement currents, etc. with a high degree of temperature resolution. X-ray diffraction measurements for sensing thermal anomalies are also possible by a minor modification of the “cell.” Precise and simultaneous measurements of thermal, electrical, and acoustic properties were carried out at the 403 K phase transition in BaTiO₃ single crystal grown by the top-seeded solution growth method. It has been clarified that the exothermic heat at the transition on cooling has more useful information than the endothermic heat on heating; in the cooling process two thermal anomalies occur separately at T₁ and T₂ although the transition is in a narrow temperature range. It is recognized from other methods that the nature of the transition on cooling is not of a single but of multiple steps. Resonant ultrasonic measurements using the “cell” were carried out, after developing a new excitation method. The sample does not have a simple softening approaching the transition point on cooling but has different elastic moduli for the two thermal anomalies at T₁ and T₂. The dielectric constant also has an intermediate constant value between T₁ and T₂. The crystal structure in the room temperature phase below the transition point has been determined by X-ray diffraction. In this region, tetragonal and monoclinic structures coordinating with each other exist. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Very Low Temperature Electronic Transport in Al-Pd-Re Quasicrystalline Alloys

Electrical resistivities of two icosahedral (I) Al-Pd-Re alloys have been measured between room temperature and mK temperatures. One quasicrystalline (QC) polygrain Al-Pd-Re sample exhibited insulating behavior in its resistivities, increasing by a factor of r=R(4 K)/R(300 K)=7.76; its room temperature resistivity was 9,890 μΩ cm. A “phenomenological” expression fitted the conductivity data well between 300 K to 0.5 K. Below 0.4 K a crossover to an activated variable-range hopping law was observed. Low temperature magnetoresistance ratio data and fits using the wave function shrinkage theory are presented. A second QC Al-Pd-Re sample had a small resistance temperature ratio r=2.12. The room temperature resistivity was extremely large, ρ(300 K)≈40,980 μΩ cm. Its conductivity could be described well using a simple temperature power law between 300 K to 20 K. Below 20 K there was a crossover to a new behavior. Below 1 K, the conductivity could be fitted using a very weakly insulating power law where σ(T)≈11.37T ^0.032 in (Ω cm)⁻¹, suggesting that this sample is located just below the metal-insulator transition. The magnetoconductivity data could not be fitted successfully using the 3D weak localization (WL) theory and inserting into it physical and realistic fitting magnitudes for the inelastic magnetic field B _in.      

Electronic Transport Behaviors of Insulating Icosahedral Al–Pd–Re Bulk Samples

Electronic transport measurements are summarised on two insulating icosahedral quasicrystalline (IQC) Al_69.0 Pd_22.8 Re_8.2 samples. Data were taken between 0.020 K ≤ T ≤ 292 K and in magnetic fields Bs up to 18 T. For a weakly insulating IQC, the electronic conductivity followed the expression σ(T) = σ₀T^zexp(T/ T₀) over four decades of temperature. For a strongly insulating sample, the conductivity above 0.3 K followed simple temperature power laws, σ (T) = σ₀ T^z. Below 0.2 K, the conductivity displayed an activated variable-range hopping (VRH) law. These fits included a conductivity contribution arising from the presence of a second metallic phase, which caused “weak saturation” of the measured resistances below 1 K.     

Van vleck paramagnetism of the diluted intermetallic compounds Pr1−x Y x Ni5 at fields up to 15 T

We have measured the 16 Hz susceptibility of the diluted Van Vleck paramagnets Pr_1−x Y _x Ni₅ withx=0.00, 0.05, 0.10. 0.20 at temperatures 1.5 K≤T≤50 K and magnetic fields up to 15T. Their Van Vleck type magnetic susceptibility χ(T, B, x) strongly depends on the field atB≥6 T. The temperatureT _max(B, x) of the maximum of χ(T, B, x) decreases at increasing the field from zero to 15 T by approximately one order of magnitude for all Pr_1−x Y _x Ni₅ compounds. Changing the dilutionx from zero to 0.20, the field whereT _max(B, x) strongly drops is increased from 9 T to 11 T. Our data agree qualitatively with the predictions of a point charge model which considers the Zeeman term in addition to the electronic exchange and the dominating crystal field contributions.     

Heat transport in dilute mixtures of3He in superfluid4He

We report new measurements of the effective thermal conductivity K_eff and relaxation time τ in dilute mixtures of³He in superfluid⁴He, with molar concentrationsX≤10⁻³. The temperature range extended fromT≈1.4 K toT _λ. Both K_cff and τ are found to agree with theoretical predictions, in contrast to previous experiments where significant differences were observed. A new thermal conductivity cell design was used which almost completely eliminates extraneous volumes and surfaces, and the earlier results are explained in relation to these design changes.     

Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C,B, and rare earth elements

MnAl alloys are attractive candidates to potentially replace rare earth hard magnets because of their superior mechanical strength, reasonable magnetic properties, and low cost. In this study, the phase transitions and magnetic properties of melt spun Mn₅₅Al₄₅ based alloys doped with C, B, and rare earth (RE) elements were investigated. As-spun Mn–Al, Mn–Al–C, and Mn–Al–C–RE ribbons possessed a hexagonal ε crystal structure. Phase transformations between the ε and the L1₀ (τ) phase are of interest. The ε → τ transformation occurred at ~500 °C and the reversed τ → ε transformation was observed at ~800 °C. Moderate carbon addition promoted the formation of the desired hard magnetic L1₀ τ-phase and improved the hard magnetic properties. The Curie temperature T _C of the τ phase is very sensitive to the C concentration. Dy or Pr doping in MnAlC alloy had no significant effect on T _C. Pr addition can slightly improve the magnetic properties of MnAlC alloy, especially J_S. Doping B could not enhance the magnetic properties of MnAl alloy since B is not able to stabilize either the ε phase or the L1₀ hard magnetic τ phase.     

Anisotropy of Superconducting Single Crystal SmFeAsO0.8F0.2 Studied by Torque Magnetometry

Single crystals of the oxypnictide superconductor SmFeAsO_0.8F_0.2 with T _c≃45(1) K were investigated by torque magnetometry. The crystals of mass ≤0.1 μg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ≃8 at T≲T _c to γ≃23 at T≃0.4T _c. This unusual behavior of γ signals unconventional superconductivity.      

Specific heat studies in Ho-Ba-CuO superconductors: Fermionic and bosonic contributions

The specific heats of superconducting HoBa₂Cu₃O_7-δ (T _c≅ 92 K) have been theoretically investigated in the temperature domain 70 ≤T ≤110 K. The bosonic (phonons) contribution to the specific heat is estimated from Debye model in the harmonic approximation for high temperature expansion (T > θ_D/2π) using the moments of the phonon density of states. The fermionic constituent as the electronic specific heat is deduced using a suitable trial function above and belowT _c. As a next step the contribution of specific heat by charge oscillations (plasmons) are obtained. The theoretical results from bosonic and fermionic terms are then compared with the experimental results. We find that the specific heats from electronic as well as plasmon term are only a fraction of lattice specific heat and in particular, plasmons do not influence the thermal conduction significantly. The implications of the above analysis are discussed.     

Simulation of a glass transition in a hot-wire experiment using time-dependent heat capacity

The transient hot-wire method is used for simultaneous measurements of the thermal conductivity λ and the heat capacity per unit volumepc _p and yields a peak in λ and a dip inpc _p near a glass transition. Through simulations, it is shown that these anomalous results arise due to a time dependence inc _p , which is described by a fractional exponential function:c _p (t)=c _p (liquid)+[c _p (glass)-c _p (liquid)c ^-(t,τ)β], where τ is the heat capacity relaxation time and β is a sample dependent parameter (0<β≤1). By a comparison with experimental data for cyclohexanol and glycerol, it is demonstrated that this model can be used to reproduce the peak and the dip as well as the temperature at which these occur. In addition, it is shown that the maximum in λ occurs at τ-0.3 s, whereas τ of the minimum inpc _p is dependent on β and moves from 0.4 to 1 s for a change in β from 1 to 0.5. The difference in τ between the peak and the dip is in agreement with the experimental results. It is concluded that the anomalies reveal glass forming characteristics such as a rough classification in terms of strong and fragile glass formers.     

Effect of oxygen deficiency (δ) on normal state resistivity of YBa2Cu3O7-δ superconductors

We have investigated theoretically the effect of oxygen deficiency (δ) on normal state resistivity (ρ) as well as its temperature dependence in YBa₂Cu₃O_7-δ superconductors. This has been based on a potential which incorporates the structure factors and various interactions for double two-dimensional (2-D) conducting CuO₂ plane. Using the Coulomb and electron-phonon terms of the interaction potential, we have then worked out the coupling strength (γ) for neighbouring electrons linked via 2-D acoustic phonons (hω_). Furthermore, the scattering time (τ _e-ph) due to electron-phonon interaction is deduced. The variations inτ _e-ph andρ _e-ph are studied with oxygen deficiency (δ) which is in the range of 0·0≤δ≤1·0, and the results thus obtained are found to be consistent with the earlier reported data. The residual resistivityρ ₀ obtained by extrapolation from experimental data together withρ _e-ph will predict the nearly-linear behaviour of normal state resistivity at temperatures (T) [90≤T≤300 K] in YBa₂Cu₃O_7-δ superconductors.     

Epitaxial Si Sensors at Low Temperatures: Non-Linear Effects

Cryogenic bolometric sensors made from epitaxially grown Si:As have been tested down to 40 mK. The sensors were grown by chemical vapour deposition with a doped layer 8.4 μm thick. The dopant concentration was measured using SIMS and was constant, ±1%, with an excellent box profile. Arsenic concentrations up to 7.5×10¹⁸ cm⁻³ were achieved. Above 100 mK the low power resistanceR(T) followed the variable range hopping law, or Efros-Shklovskii law for a Coulomb gap,R(T)=R ₀ exp(T ₀/T)^1/2 withT ₀∼25 K, typically. A double sensor arrangement was used to measure the electronphonon coupling in the sensors and the phonon coupling to the heat sink. As the dc current bias through a sensor was increased, spontaneous voltage oscillations were observed across the sensor below 100 mK, which limited the sensitivity of the sensors in this region. These are circuit-limited oscillations between high and low resistance states. A phase diagram was established for the spatio-temporal coexistence of the two states, with a critical temperatureT _c=115 mK. We show that this is an intrinsic phase transition within a thermal model of the electron-phonon coupling. For a resistance-temperature characteristic given by the Efros-Shklovskii law we findT _c=0.00512T ₀, independent ofR ₀ and the coupling strength. This predictsT _c=115±4 mK in this case. The model gives excellent agreement for the critical voltage and current, by assuming that the breakdown occurred via the formation of a filamentary region of high current density and high electron temperature. At higher currents, the response was temperature independent and given byI(E)=I(0) exp{−(E ₀/E)^1/2} whereE is the average applied electric field andE ₀∼380 V/cm, in agreement with a thermal model which includes the phonon-phonon coupling to the heat sink.     

Thermal and magnetic properties of copper potassium tutton salt below 0.15 K

The specific heat and the ac susceptibility of copper potassium tutton salt have been measured between 0.01 and 0.15 K. The magnetic phase transition from the paramagnetic to the canted ferromagnetic state was observed at 29.5 mK in zero field. From the obtained electronic entropy curve this salt is considered to be a Heisenberg-type ferromagnet. The copper nuclear specific heat of the hyperfine splitting is estimated to beC _N =1.1×10^–5 R/ (T ²/[K²]), which is one order smaller than the value calculated from previous results of the paramagnetic resonance.     

3He melting pressure thermometry

High-precision measurements of the³He melting pressure versus temperature have been made from 500 K to 25 mK using a⁶⁰Co nuclear orientation primary thermometer and a Pt NMR susceptibility secondary thermometer. Temperatures for the fixed points on the melting curve are: the superfluid A transition T_A = 2.505 mK, the A-B transition T_AB = 1.948 mK, and the solid ordering temperature t_n = 0.934 mK. These fixed points and a functional form for P(T) constitute a convenient temperature scale, based on a primary thermometer, usable to well below 1 mK.     

Electron Magnetic Resonance,Neutron Diffraction and ac Susceptibility Study of CaMn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ru{<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> (x ≤ 0.40) Manganite System

Electron magnetic resonance (EMR), neutron powder diffraction (NPD) and ac susceptibility techniques were employed for studying the crystallographic structure and magnetic ordering in CaMn_1−xRu_xO₃ (x ≤ 0.40) manganite system. EMR measurements were done on polycrystalline samples at 120 ≤ T ≤ 500 K. High temperature EMR spectra of pristine antiferromagnetic (AFM) CaMnO₃ show a singlet Lorentzian-like line, whose intensity diminishes, zeroing at Neel temperature T _N=120 K. Strong broadening of paramagnetic (PM) lines with increase of Ru-content (Δ H_pp ∼ 1 T for x=0.10) was found. Upon cooling low-doped (x ≤ 0.06) samples remain AFM, whereas higher doped ones (0.10 ≤ x ≤ 0.40) clearly show progressive appearance of ferromagnetic (FM) phases. Thus, EMR evidences that Ru-doping modifies both PM and AFM states and creates an inhomogeneous phase separated FM and AFM ground states at x0.06. Complementary measurements of NPD and ac susceptibility corroborate the complex character of magnetic ordering, revealed by EMR. The changes of the magnetic ordering in CaMn_1−xRu_xO₃ supposed to be solely determined by doping of Mn-sites with Ru.