Showing papers in "Journal of Low Temperature Physics in 1984"

Electric and magnetic-properties of the kondo lattice compound cecu2si2

Abstract: Electric, magnetic, and thermoelectric properties of Ce x La1−x Cu2Si2 (0⩽x⩽1) compounds have been studied over a wide temperature interval 0.04 ⩽T⩽300 K in magnetic fieldsH⩽40 kOe. The paramagnetic-magnetic ordering transition temperatureT M is found to rise from ∼0.32 K for cerium concentrationx=0.2 to 1.6 K forx=0.6. A further increase inx from ∼0.8 to 1.0 leads to a decrease inT M . Simultaneously, the susceptibility kink is smeared out and atx≈1.0 it is transformed into temperature-independent enhanced Pauli paramagnetism. The magnetic phase diagram has been found to be similar to that proposed by Doniach for the one-dimensional Kondo-necklace model. The Kondo-lattice compound CeCu2Si2 exhibits a superconducting transition atT c ⋍0.5 K. The variation of the magnetic properties of Ce x La1−x Cu2Si2 from magnetic ordering at 0.2≲x≲0.8 to the nonmagnetic superconducting state atx → 1.0 is caused by the crossover from the magnetic regimeT RKKY≫T K (in which the RKKY temperatureT RKKY exceeds the Kondo temperatureT K) to the nonmagnetic singlet ground state corresponding to the situation whenT K≫T RKKY. This crossover is accompanied by a sharp increase in the low-temperature Hall coefficientR H(T) in Ce x La1−x Cu2Si2 compounds atx → 1. At the same time, a minimum of the negative Seebeck coefficient with a high amplitude appears at 10p KL, a continuous transition from the Kondo lattice to the intermediate valence state is observed, which is accompanied by a complete smearing out of the resonance near the Fermi level. Therefore the Kondo lattices represent a new class of solids, which can be characterized as the link between stable magnetism of metals with a deep 4f level and unstable magnetism associated with fluctuating valence. This novel state can be described by a set of anomalous low-temperature properties related to the giant Abrikosov-Suhl resonance near the Fermi level.

Magnetization of a two-dimensional electron gas

Abstract: The oscillations of magnetization of a two-dimensional free electron gas with field are derived first in the ideal situationT=0 and sharp Landau levels without electron spin and then for finiteT and for broadened levels. Explicit results are obtained in the limiting approximations that the broadening (or kT) is small or large compared with the separation of Landau levels, and the special situations of a field-independent Fermi energy and a field-independent electron density are discussed. The modifications introduced by taking account of electron spin are then considered, and finally the steady magnetic susceptibility superimposed on the oscillations is discussed.

New methods for nuclear cooling into the microkelvin regime

Abstract: We describe the philosophy and practice of a new method of nuclear cooling in which the copper refrigerant is immersed directly in the3He sample to be cooled using a guard cell configuration. The method has been used to cool liquid3He to ∼120 µK. We also describe a variant of the method intended for cooling metallic samples, by which a platinum NMR thermometer has been cooled to ∼13 µK. Finally, in an appendix we suggest a very simple nuclear cooling method utilizing the copper flakes used in the manufacture of paint, which will cool liquid3He to around 1 mK with a minimum of cryogenic effort.

Relaxation processes in a condensed Bose gas

Abstract: We develop, on the basis of the self-consistent mean-field approximation, the kinetic theory for a dilute Bose gas below the Bose-Einstein transition temperature. The collision operator in the Boltzmann equation is calculated by golden rule arguments, and the momentum and temperature dependences of the scattering rates are determined. As an application, we consider the relaxation of a nonequilibrium distribution of the quasiparticles. We discuss the relevance of our calculation for the (hypothetical) condensed state of spinpolarized hydrogen.

Transition temperatures and critical fields of Nb/Cu superlattices

Abstract: We present experimental results on the superconducting properties of Nb/Cu superlattices. The transition temperature of the superlattice implies a decrease in the transition temperature of single Nb films as a function of layer thickness. This is interpreted as due to the mean free path-induced decrease in the density of states at the Fermi surface, in agreement with experimentally measured magnetic susceptibilities. The temperature dependence of anisotropic critical fields is in qualitative agreement with predictions based on effective mass theories. However, the behavior of the angular dependence is considerably more complicated. In addition to the anisotropy due to the layering, there is an anisotropy due to surface superconductivity. These results are discussed in light of theories of anisotropic critical fields.

Simulation and optimization of a dc SQUID with finite capacitance

Abstract: This paper deals with the calculations of the noise and the optimization of the energy resolution of a dc SQUID with finite junction capacitance. Up to now noise calculations of dc SQUIDs were performed using a model without parasitic capacitances across the Josephson junctions. As the capacitances limit the performance of the SQUID, for a good optimization one must take them into account. The model consists of two coupled nonlinear second-order differential equations. The equations are very suitable for simulation with an analog circuit. We implemented the model on a hybrid computer. The noise spectrum from the model is calculated with a fast Fourier transform. A calculation of the energy resolution for one set of parameters takes about 6 min of computer time. Detailed results of the optimization are given for products of inductance and temperature ofLT=1.2 and 5 nH K. Within a range of β and β c between 1 and 2, which is optimum, the energy resolution is nearly independent of these variables. In this region the energy resolution is near the value calculated without parasitic capacitances. Results of the optimized energy resolution are given as a function ofLT between 1.2 and 10 mH K.

The transverse acoustic impedance of He II

Abstract: The complex shear acoustic impedance of liquid He II has been measured at frequenciesf(=ω/2π) of 205, 341, and 478 MHz from 30 mK to the λ-point Tλ (2176 K) The impedanceZ was found from the temperature dependence of the quality factor and the resonant frequency of a thickness shear mode quartz crystal resonator immersed in the liquid The relationship for a hydrodynamic viscous liquidZ(T)=(1−i)(πfηρ n )1/2 was used to measure the temperature dependence of the viscosity η(T) using tabulated values of the normal fluid density ρ n (T) Deviations from hydrodynamic behavior occurred when the viscous penetration depth was less than the superfluid healing length, the phonon mean free path, and the roton mean free path Near the λ-point,Z(T)/Z(Tλ) was frequency dependent and a value for the superfluid healing lengtha=(010±001)e−2/3 nm was found, where e=(Tλ−T)/Tλ The effects of van der Waals forces near the crystal surface were also observed and a layer model was used to interpret the measurements Below 18 K only rotons contribute significantly toZ and we determined the roton relaxation time as τ r =85×10−14 T −1/3 exp (865/T) sec Below 12 K, ωτ r >1 and we investigated the breakdown of hydrodynamics in this region ForT<06 K the resonant frequency of the crystals decreased by Δf/f=2×10−7, but the origin of this effect is not yet known

Intermediate valence behavior of ternary cerium and uranium transition metal borides

Abstract: Low-temperature specific heat, electrical resistivity, ac magnetic susceptibility, and dc magnetization measurements were made on ternary cerium and uranium transition metal borides with the general formula CeT3B2 (T=Co, Ru, Rh, and Ir) and UT3B2 (T=Co, Ru, and Ir). The compound CeRu3B2 was found to exhibit superconductivity below 0.68 K. The values of the electronic specific heat coefficient range from 9.7 mJ/mole-K2 for CeCo3B2 to 64 mJ/mole-K2 for UIr3B2. The electrical resistivity versus temperature curves of all of the compounds exhibit negative curvature and are reminiscent of valence fluctuation behavior. In the case of CeIr3B2, the electrical resistivity attains a maximum value at 180 K, while the dc magnetic susceptibility has a temperature dependence that is typical of intermediate valence Ce compounds, approaching a finite value at zero temperature. The electrical resistivity of the ferromagnetic compound CeRh3B2 reveals a rapid decrease in spin disorder resistivity below 120 K. The dc magnetic susceptibility of this material can be described as the sum of a constant term and a Curie-Weiss contribution with an effective magnetic moment of 1.01 µB per formula unit and a Curie-Weiss temperature of 119 K. Magnetization measurements on CeRh3B2 yield a saturation magnetic moment of 0.37 µB per formula unit and a Curie temperature of 113 K.

Structure and properties of the dissipative phase-slip state in narrow superconducting filaments with and without inhomogeneities

Abstract: Using the generalized time-dependent Ginzburg-Landau equations for dirty superconductors nearT c we study the oscillatory phase-slip solutions in quasione-dimensional superconductors at arbitrary pair-breaking parameter and calculate their properties. For small pair breaking (“low temperature”), a static approximation can be used, which allows analytic evaluation of most quantities and connects the theory to simple models. The influence of two types of short weak regions that pin the phase slips is considered. They tend to decrease the differential resistance and influence its temperature dependence.

Elementary pinning potential in type II superconductors nearH c2

Abstract: The elementary pinning potential of a small defect in a type II superconductor is calculated near the upper critical magnetic field. The calculation applies at all temperatures and all background impurity contents of the metal. The pinning potential is found to decay according to the inverse impurity parameter when other impurities are added. A very simple formula for the pinning potential is obtained in the Ginzburg-Landau region. The boundary condition to be imposed on a Ginzburg-Landau theory at small defects is derived.

Two-stage nuclear demagnetization refrigerator reaching 27 µK

Abstract: A two-stage nuclear demagnetization refrigerator with large cooling capacity has been constructed and successfully operated. The first stage consists of 11 moles of the Van Vleck paramagnet PrNi5 in an initial field of 6 T, and the second stage consists effectively of 19 moles of copper in an initial field of 7.6 T. The minimum temperature as measured with pulsed platinum NMR is 27 µK, the lowest ever obtained in a low-field experimental region.

Upper critical fields of the superconducting layered compounds Nb 1- x Ta x Se 2

Abstract: The upper critical fieldsH c2 (T) of the superconducting layered compounds Nb1−x Ta x Se2 (0≤x≤0.20) have been measured with the magnetic field oriented either perpendicular or parallel to the layer plane. Just belowT c , positive (upward) curvature is seen inH c2 (T) for both field directions and for all compositions. The amount of positive curvature is not a function of crystal quality. At low temperatures,H c2 (T) displays enhanced linearity as compared to the prediction of the isotropic theory, and shows no evidence of Pauli paramagnetic limiting for either field orientation. The critical field slopesdH c2 /dT nearT c change only slightly with composition.H c2 (T) has been calculated for two Fermi surface models for NbSe2 using the band structure calculated by Wexler and Woolley. TheseH c2 (T) calculations indicate that Fermi surface anisotropy can explain the positive curvature and enhanced linearity seen in the experimental data. The calculations also show that theH c2 (T) data are better explained with a Fermi surface that consists of open, undulating cylinders with an additional smaller closed piece, than with a Fermi surface model that consists only of the open, undulating cylinders of the original Wexler and Woolley model.

Electron-electron interaction and weak localization

Abstract: A consistent description of the effects of inelastic electron-electron scattering processes on weak localization in thin metal films is presented. Different techniques are used to treat strongly inelastic processes and quasielastic collisions, with special attention to the treatment of the latter. The calculations give an upper bound for the decay rate of the particle-particle propagator. This rate is proportional to the temperature and depends rather strongly on the conductance of the film. The calculation of the magnetoconductance connects this rate with measurable quantities and also reveals a dependence on the magnetic field. Finally, a description that avoids the splitting into two energy regions is presented.

Spin-independent transport parameters for superfluid3He-B

Abstract: The scalar kinetic equation for Bogoliubov quasiparticles in the B phase of superfluid3He is discussed and the collision integral is represented in a compact form. For the cases of shear and second viscosity and diffusive thermal conductivity the problem is reduced to solving one-dimensional integral equations. The quasiparticle interaction enters via weighted angular averages of the normal state scattering amplitude. The effect of strong coupling renormalization of the gap function is accounted for. The transport coefficients are exactly related to relaxation parameters that describe how the system tends toward local equilibrium. For low temperatures the transport parameters are evaluated exactly, including corrections of orderT/T c. The results are compared with those of a previous paper in which an approximate form of the collision operator was used, as well as with results of a variational approach and with recent experimental data.

The Josephson pendulum as a nonlinear capacitor

Abstract: In the quantum electrodynamic circuit theory of weak link constrictions, the singly connected weak link exhibits energy bands in charge space. Here, it is shown that charge space energy bands arise also in the quantum mechanical treatment of a Josephson pendulum.

Magnetic coupling of 3 He with a fluorocarbon substrate

Abstract: We observe a nuclear interaction between3He and the19F spin species of a fluorocarbon substrate. The magnetization of a3He film is strongly linked to that of the19F substrate spins. We use this interaction as a probe of the3He to identify several features important in surface relaxation.

Superconductivity of the linear chain compound Tl2Mo6Se6

Abstract: We report on transport properties of chain-like compounds M2Mo6X6 with M=In or Tl and X=Se or Te. Only Tl2Mo6Se6 is found to be superconducting, with a critical temperature between 5.5 and 6.6 K. The superconducting properties are extremely dependent on the sample preparation. We have determined large azimuthal and polar anisotropies by magnetoresistance measurements. Properties of rare-earth-substituted samples, such as Eu2−x Tl x Mo 6 Se 6 , are also reported.

Unusual behavior of superconductors near the tricritical Lifshitz point

Abstract: The properties of superconductors near the tricritical Lifshitz (TL) point are studied. Unusual behavior of H/sub c//sub 2/(T) near the TL point in three-dimensional (3D) systems is found, as well as an oscillatory character of H/sub c//sub 2/(T) in quasi-2D systems. The oscillatory character of the critical current J/sub c/ of the S-S'-S junction as a function of thickness and temperature is also studied. Predictions are compared to experimental data in such quasi-2D systems as T/sub a/S/sub 2/(pyridine).

Heat transfer between liquid3He and sintered metal heat exchangers

Abstract: A new model to explain the unexpectedly large heat transfer between liquid3He and sintered metal heat exchangers is described and evaluated. The heat transfer results from a direct coupling of3He quasiparticles in the pores to vibrational modes of the sintered metal powder. It is proposed that for a range of temperatures below 20 mK the dominant vibrational modes of the sinter are localized oscillations involving a few powder particles with frequencies distributed over a constant density of states. The3He is then treated as a Fermi gas in a set of boxes corresponding to the pores in the sinter. The vibrating or shaking boxes transfer energy to the3He quasiparticles inside the box. The calculated heat transfer between liquid3He and the vibrational modes of sintered metal heat exchangers isQ/(VΔT)∼4×10−15T/d3 W m−3 K−1, where Q is the heat flow for a temperature differenceΔT, V is the volume of the sinter (metal and helium), andd is the powder particle diameter in meters; this has the observed linear temperature dependence, and in magnitude is larger than or comparable to published results from several laboratories. The heat transfer between the vibrational modes and the electron gas in the metal sinter that is needed to complete the heat path can also be described by the same model with the result that the electron-“phonon” coupling is significantly larger than the3He-“phonon” coupling. When the model is applied to heat transfer between liquid3He-4He mixtures and sinter the calculated results are again comparable to or larger than those measured. The postulated localized oscillator modes give a specific heat, linear in temperature, that is in reasonable agreement with measurements for pressed powder by Pohl and Tait.

NMR Studies of Single Crystals of H2. VII. Orientational Ordering in the HCP Phase

Abstract: We report an analysis of NMR lineshape measurements in hcp single crystals of H2 at ortho concentrationsX between 0.15 and 0.55 over the temperature range where the orientational ordering becomes substantial. The normalized rms order parameter\(\bar \sigma \) is obtained from the second momentM2 and is plotted against the reduced orientational entropySred derived from calorimetric measurements. For comparison, we present calculations for a single-particle model, for isolated pairs and triangles ofo-H2 molecules. They confirm that single-particle models as used by previous authors cannot satisfactorily account for the orientational ordering in the hcp phase. Rather, cluster configurations where the lowest excited states have a quite different ordering arrangement from that of the ground state, yet only a small energy change, need to be invoked. In a\(\bar \sigma \) versus 1−Sred plot, the hcp phase ordering is contrasted with that in the ordered cubic phase. In the Appendix, the experimental sample cell design at Duke University and the observed NMR line shapes are compared with those of the groups at Saclay and Harvard University under similar conditions ofT andX and the differences in results are discussed.

Hall effect in bismuth in quantizing magnetic fields

Abstract: The Hall effect has been investigated in the very low-temperature limit in bismuth, when the lattice scattering is dominant in solids. The energy band structure of bismuth carriers is assumed to follow the modified nonellipsoidal, nonparabolic (MNENP) model. Results show that the Hall coefficient and Hall angle oscillate with the dc magnetic field, and the period and amplitude of the oscillations increase with the dc magnetic field. We also compare our numerical results with those found using other types of energy bands.

Concentration dependence ofD, T 1, andT 2 for dilute solutions of3He in solid4He

Abstract: Measurements of the spin diffusion coefficientD and NMR relaxation timesT1 andT2 are reported for dilute solutions of3He in solid4He at two molar volumes, 20.95 and 20.7 cm3. The weakly interacting impuriton model, for whichD−1 is proportional to impurity concentration, is observed only at fractional impurity concentrationx3 below 3 × 10−4. Forx3 around 10−3,T1 andT2 are controlled by the formation and breakup of3He2 molecules.

Point-contact noise spectroscopy of phonons in metals

Abstract: The spectral density of low-frequency electrical fluctuations (noise) in pure metal point contacts is found to depend nonmonotonically on the potential difference at low temperatures. For contacts satisfying the conditions for the ballistic regime of electron current flow the noise is made up of a monotonic part proportional to the square of the dc voltage (1/f noise) and a “spectral part” with extrema at biases corresponding to the characteristic phonon energies. Comparison of the noise spectrum part for sodium with the calculated contribution introduced by Umklapp processes in the point-contact electron-phonon interaction function shows that these characteristics are similar. Analysis of noise spectra and dispersion curves of phonons in the main crystallographic directions for three metals (sodium, copper, and tin) makes it possible to establish that the positions of maxima in the noise spectra correspond to the energies at which Umklapp processes are allowed. At the same time the positions of minima correlate well with the energies of phonons possessing maximum wave vectors and minimum group velocities. The latter regularity is attributed to stimulated emission of coherent phonons due to normal processes. This emission reduces the noise due to the correlation between separate electron-phonon scattering events. Thus, the spectral density of low-frequency fluctuations as a function of dc potential difference (the “noise spectrum”) is found to contain more detailed information about phonons, enabling one to determine their energies at a number of characteristic points on the dispersion curves.

Theory of helium under heat flow near the λ point. II. Dynamics of phase change

Abstract: The He I-He II interface is a crucial aspect in the transformation processes between the superfluid and normal fluid phases. Its motion is investigated when temperatures and heat flows at boundaries deviate from those of a stationary coexistence state. As a unique feature, the heat flow to the interface from the He I side can be mostly transmitted to the He II side by thermal counterflow, and the latent heat generation (or absorption) at the interface becomes negligibly small. In any case the interfacial motion is so slow that the temperature on the He II sideT∞ is still given by the stationary relationTλ−T∞ ∝Q3/4, whereTλ is the critical temperature and Q is the heat flow. The temperature profile and the interfacial position are calculated in some nonstationary cases. To this end a simple approximation scheme is developed. First, the interface can propagate with a constant velocity and the superfluid region can expand as a shock wave. Second, if the heat flow at the warmer boundaryQw and that in the He II regionQ− are fixed at different values, the length of the He I region yi changes in time as (d/dt)yi1+p = const ∝Qw − Q−, wherep=1/(1−xλ), and xλ is the critical exponent of the thermal conductivity. In particular, ifyi=0 att=0 andQw>Q−, the normal fluid region emerges asyi∝t1/(1+p) at the warmer boundary. Third, ifQ− and the temperature at the warmer boundary are fixed, the interfacial position approaches an equilibrium position exponentially in time. The uniqueness of the problem arises from the superfluidity on the He II side and the strong critical singularity of the thermal conductivity on the He I side.

Electronic structure and low-temperature properties of V x Nb1−x N alloys

Abstract: The vanadium and niobium nitrdes V x Nb1−x N are studied theoretically and experimentally for varying compositionsx. Self-consistent linear muffin-tin orbital band calculations are performed for various ordered supercells corresponding tox=0.0, 0.25, 0.50, 0.75, and 1.0. The band results are used to calculate electron-phonon coupling and spin susceptibility enhancement factors in order to study the variations of specific heat and superconducting transition temperatures with composition. A relative localization of, in particular, the V 3d states is found for the mixed compositions. The superconducting temperatures are found to be more reduced for intermediate compositions than what is expected from a simple interpolation between pure VN and NbN. Effects of spin fluctuations are important for the alloys containing vanadium, but are probably larger than what has been found in our theoretical study, and also containingq≠0 contributions. A small peak in the density of states near the Fermi energy is associated with a flat band near theW point, and causes a shoulder in density of state-dependent properties forx near 0.9.

Heat Capacity Analysis of a Large Number of Chevrel-Type Superconductors

Abstract: Low-temperature specific heat measurements wee performed on a series of binary, pseudobinary, ternary, and pseudoternary molybdenum chalcogenides M/sub x/Mo/sub 6/X/sub 8/. The measurements covered a sufficiently wide temperature range to yield the relevant moments of the phonon spectra in addition to the electronic term. From the data in the superconduting state strong coupling parameters were also obtained, particularly the gap ratios 2..delta..(0)/kT/sub c/. The superconducting transition temperature of the Chevrel-type phases essentially depends on the bare electronic density of states as the Fermi level and, moreover, on the atomic order pertaining to the Mo/sub 6/ and certain chalcogen sites. For the ordered compounds, the relation lambdaapprox. =an/sub b/s(E/sub F/) holds on the average. The factor aapprox. =1.25 eV Mo atom spin is practically equal to that found for Nb-based A15-type compounds.

The decay rate of critical fluctuations in3He-4He mixtures near the gas-liquid critical point

Abstract: We have measured the critical light scattering intensity and the Rayleigh line shape for3He and for3He-4He mixtures with compositionX(3He)=0.95, 0.79, and 0.63 along their respective critical isochores near the plait point. The experimental linewidth of3He is compared with the calculated one from heat conductivity and equation of state measurements, and satisfactory agreement is obtained. For mixtures, gravity effects in our cell of finite height prevent us from reaching the critical point along a path at strictly constant composition and density. HenceTc cannot be determined directly. Using the prediction that the scattered light intensity in the mixtures has the same diverging behavior as for the pure fluid, we determine the reduced temperaturet≡[T − Tc(X)]/Tc from the intensity. The measured Rayleigh line shape can be expressed by a single decay rate Γ as a function oft for a given scattering angle of the light beam. Our experiments show that Γ in the mixtures is only weakly dependent on composition. Our analysis leads to the determination of the mass diffusion coefficientD, which is found to be nearly independent of composition and nearly equal to the thermal diffusivityDT measured for3He. The results are discussed in the light of the predictions from mode coupling theory.

Transport properties in3He-4He mixtures near the superfluid transition

Abstract: We report measurements of the temperature, density, and concentration gradients in /sup 3/He-/sup 4/He mixtures, induced by a vertical heat flux. The flat horizontal cell included two superposed capacitors and the density was determined by means of the dielectric constant method. The experiments were carried out on mixtures with mole fractions X/sub 3/ = 0.37, 0.15, and 0.05 at saturated vapor pressure, with special emphasis on the region near the superfluid transition. Our measurements under steady-state conditions give conductivity kappa, the thermal diffusion ratio k/sub T/, and the coefficient of thermal expansion. We describe the singular behavior of these quantities in the neighborhood of T/sub lambda/(X). In the superfluid phase, we test with fair success a relation by Khalatnikov between grad X/grad T and several static properties. From the relaxation times needed to attain steady-state conditions, and in combination with measured static and transport properties, we obtain in the normal phase the mass diffusion D, which diverges strongly as T/sub lambda/ is approached. In the superfluid phase, we test successfully a scaled relation that results from the solution of Khalatnikov's hydrodynamic equations. From our data the dispersion relations for scattered light are calculated: GAMMA/sub 0//q/sup 2/ in both the normalmore » and the superfluid phases and GAMMA/sub 2//q/sup 2/ in the normal phase.« less

Specific heat, pressure, and the Grüneisen relation in solid hydrogen

Abstract: We report systematic measurements ofC v and (∂P/∂T) v in solid hydrogen below 1 K and for orthohydrogen molar concentrations 0.58≥X ortho≥0.20. This is the temperature and concentration range associated with a proposed quadrupolar glass phase. The results show no overtly glassy properties such as remanence or hysteresis upon warming and cooling, and there is no evidence of anomalous thermal relaxations. We have calculated the quadrupolar Gruneisen constant from our data and find that γ is only a slowly varying function of temperature and orthohydrogen concentration. TheC v results are compared with theoretical models and the behavior of a corresponding classical solid, N2-argon. There are several similarities between the N2-argon and H2 behaviors, but the semiempirical single-particle model does not correspond well with the experimental results.

Upper critical field and related properties of superconducting amorphous alloys Zr-Si

Abstract: The upper critical fieldH c2 has been measured for the superconducting amorphous alloys Zr100−x Si x withx=13, 15, and 17 over a wide temperature range down to 50 mK. Measurements for related properties, such as the depinning current densityJ p , pinning force densityF p , and flux flow resistivity ρ f , reveal that these systems are well characterized as typically soft and dirty type II superconductors with an extremely lowJ p of 1 A/cm2 andF p of 1.5×104 N/m3 ath (≡H/H c2)=0.5 andt (≡T/T c )=0.5. The temperature dependence ofH c2 is well fitted by a curve of the Werthamer-Helfand-Hohenberg theory with Maki parameter α=1.7−1.5 and spin-orbit interaction parameter λso=1−2.4. These results indicate that the Pauli spin-paramagnetic effect plays an important role in the low-temperature behavior ofH c2 in these low-T c and high-H c2 amorphous superconductors. Corrections of the electron-phonon coupling (λep=0.6) to the Pauli spin-paramagnetic limiting field lead to a reduced importance of the spin-orbit interaction effect, with λso less than 1.0. A discussion is given of the structural inhomogeneities in an amorphous phase, which are sensitively reflected in the strength of the fluxoid line pinning interaction (J p ,F p ) and are intimately related to the nonintrinsic anomaly, observed in many amorphous alloys, thatH c2 depends linearly on temperature even at low temperatures, exceeding the theoretically allowedH c2 values.