Flux flow resistivity of superconducting thin film

The flux flow resistivity _f has been investigated in the range of reduced temperature 0.5–0.99 for indium thin films having the properties of dirty, type II superconductors. In this temperature range _f was found to show a linear dependence on magnetic field strength for low values of the field. The temperature-dependent viscosity coefficient of the flux flow was obtained and compared with the microscopic theories of Gor'kov and Kopnin (GK) and others. Reasonable agreement with the GK theory was obtained except in the vicinity ofT _c. NearT _c, the viscosity coefficient [in units of ₀H_c(t)_n ^–1c^–2] showed a remarkable decrease beyond a maximum, contrary to the theoretical prediction. Although this behavior is not yet well understood, it seemed to appear asd/ became small. The nonlinearity of the current(I)-voltage(V) curves is also discussed. It was found that a linearI–V curve is found only in the region of temperature and magnetic field where the conditionI _p<1/10I_c is satisfied, whereI _p is the depinning current andI _c is the effective critical current. In the measurements the temperature rise of the film specimens was recorded and the effect due to thermal heating of the specimens was estimated.On leave from Department of Electronic Engineering, Faculty of Engineering, Kyushu Industrial University, Fukuoka, Japan.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7?x

The effects of addition to YBa₂Cu₃O_7–x of lithium halides (YBa₂Cu₃O_7–x (LiF) _y , and YBa₂Cu₃O_7–x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7–x (LiF) _y and 91.30 K in YBa₂Cu₃O_7–x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _cB ^–, with a lithium-concentration-dependent . The voltage–current characteristics follow a law typical for granular superconductors, V(I–I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Magnetic behavior of weak links in bulk superconducting YBa2Cu3O7?x: Ag composite

The magnetization of a polycrystalline composite YBa₂Cu₃O_7–x :Ag_0.1 in the superconducting state is measured in the presence of a small a.c. magnetic field. With increase in field amplitude, the magnetizationM(t) changes its shape from nearly sinusoidal to square-wave like and then to a two-peak structure. A similar behavior of the magnetizationM(t), calculated from the critical-state model with intergranular critical currentJ _c =c[(H ₀ +¦H¦)]^–1 is obtained. A very good fit is found between the theoretical and experimental results, and the parameters of the model are estimated.     

Josephson Plasma in Various High-T c Cuprates

Josephson plasma in various high-T _c cuprates with and without magnetic field is studied by using the sphere resonance method. For Bi ₂ Sr ₂ CaCu ₂ O ₈₊, the plasma in a zero magnetic field exists at 5 cm ^–1 for a slightly overdoped sample (T _c = 85 K) and shifts to 11 cm ^–1 as the doping increases (T _c = 71 K). For SmLa _1–x Sr _x CuO _3.95 (T* phase), two peaks appear at 11 and 30 cm ^–1 in a zero magnetic field, and both peaks shift to lower frequencies as the magnetic field increases. These peaks are identified as the Josephson plasma of the intrinsic Josephson junction at the fluorite-type Sm ₂ O ₂ block layer and the rocksalt-type (La,Sr) ₂ O _2– block layer, respectively. This indicates that the T* phase can be regarded as the ···S/I/S/I/S/I/S/I/S··· (···superconductor/insulator1/superconductor/insulator2/superconductor···) -type Josephson junction array.     

Magnetic field dependence of relaxation times in nonequilibrium superconductors

We have investigated the effects of a magnetic field H on nonequilibrium quasiparticle relaxation processes in superconductors by analyzing the I–V characteristics of long tin microbridges in a parallel field near T _c (H). Following Skocpol, Beasley, and Tinkham, we take the differential resistance of a given phase-slip center (PSC) to be approximately equal to the normal resistance R _n of a nonequilibrium region of the bridge with length 2(D_R)^1/2, where D is the quasiparticle diffusion constant and _R is the transverse mode (branch imbalance) relaxation time. The magnitude and the temperature and field dependence of _R as inferred from our data agree well with those of the transverse mode relaxation time in the presence of pair-breaking, derived by Schmid and Schön. The longitudinal mode disequilibrium is interpreted in terms of local heating proportional to IV, and the resulting distortions in the I–V characteristics are corrected. Most of our samples contain a deliberate weak spot with a depressed critical current I _c, which isolates a single PSC and lowers the heat dissipation(~I _c ²R_n).Research supported in part by NSF and ONR.     

Isochoric specific heat of sulfur hexafluoride at the critical point: Laboratory results and outline of a spacelab experiment for the d1-mission in 1985

The specific heat at constant volume c_v shows a weak singularity at the critical point. Renormalization group techniques have been applied, predicting a universal critical behavior which has to be experimentally confirmed for different systems. In this paper an experiment is presented to measure the specific heat of SF₆ along the critical isochore (_c=0.737 g·cm^–3), applying a continuous heating method. The results cover a temperature span of –1.5×10^–2< <1.70×10^–2 [=(T–T _c)/T _c] and were strongly affected by gravity effects that emerge in the sample of 1-mm hydrostatic height near the critical point. Using regression analysis, data were fitted with functions of the form c _v/R=A × ¦¦^– + B for the one-phase state and c _v/R=A × ¦¦^– + B for the twophase state. Within their error bounds the critical values (==0.098, A/A=1.83) represent the measurements for the temperature span 3.5×10^{–5< ¦¦<2.0×10 –3}, in good agreement with theoretical predictions. In order to exclude density profiles in the specimen, which are unavoidable in terrestrial experiments due to the high compressibility of fluids at the critical point and the gravity force, a space-qualified scanning ratio calorimeter has been constructed, which will permit long-term c_v measurements under microgravity (-g) conditions. The experiment will be part of the German Spacelab mission in October 1985. The significant features of the apparatus are briefly sketched.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Voltage-current characteristics of polycrystalline (Bi,Pb)2Sr2Ca2Cu3O10 superconductor at different magnetic fields and temperatures

The voltage-current characteristics of polycrystalline (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ superconductor were investigated at temperatures from 80 to 105 K under magnetic field up to 0.4 T. In magnetic field considerable broadening in the transition width occurs which depends on the processing conditions and microstructure of the sample. I–V data below T_c were fitted to a power law expression V I to obtain . We have found that decreases rather rapidly with increase of magnetic field and temperature, in low magnetic fields and at temperatures quite below T_c, respectively. Similar behavior was observed for the variation of the critical current as a function of temperature and magnetic field. The values are found to change linearly as a function of critical current at relatively low values of I_c.     

Linear response and instability of vortex lattice in type-II superconductors

The wave vector dependent linear response of the vortex lattice in type-II superconductors is studied by calculating the superfluid density and is shown to have a singular behavior with respect to the long wave length perturbation of vector potential. The calculations are carried out in terms of the nonlinear elastic theory of the vortex lattice, which is, in principle, a low temperature expansion, and the terms up to the first order in temperature are taken into account, which is beyond the usual Gaussian elastic theory. The superfluid density is found to behave as ~ k_BT ¦k¦² log ¦k¦,for the small wave vector,k,perpendicular to the external magnetic field, where Tand k_B are the temperature and the Boltzmann constant, respectively. This behavior causes the divergence of the magnetic susceptibility at a critical wave vector, k_c.We show that this behavior is associated with an instability of vortex lattice to the externally applied perturbation.     

Electrical resistivity in ferromagnetic NiCu alloys near the Curie point

Direct measurements of the temperature derivative of the electrical resistivity d/dT have been made on several ferromagnetic NiCu alloys with high Cu content (20, 30, 40, 50.4 at %). For Ni₈₀Cu₂₀ and Ni₇₀Cu₃₀ the results closely resemble the general pattern found in pure ferromagnets: d/dT - b – a ln ¦¦, for 10^–3 ¦¦ 10^–1, with a and b constants and = (T/T _c )– 1. In Ni₆₀Cu₄₀ the rounding off of the magnetic transition is already so pronounced that a similar analysis is hardly significant. In the Ni_49.6Cu_50.4 sample negative values of d/dT were confirmed above ~165 K. The possible theoretical implications of the results are briefly discussed.Work supported by Instituto de Alta Cultura, Portugal (Project PF1).     

Magnetic field properties of 1 at.% transition metal substituted Nb3Ge

Measurements of the critical magnetic field slope near the superconducting transition temperature T _care reported for samples of Nb_1–x Ge _x , (Nb_0.99Ti_0.01)_1–x Ge _x , and (Nb_0.99Zr_0.01)_1–x Ge _x for 0.15x0.25. The samples investigated were produced by trisputtering onto heated polycrystalline alumina substrates. The critical magnetic field slope near T _cfor substituted samples is comparable to that of Nb₃Ge alone. The highest T _cvalues, the highest resistance ratios, and the narrowest transitions, however, occur for samples with excess transition metal component.Supported by PSC-BHE Research Award Program.     

Effect of magnetic field and temperature on the voltage-current characteristics of YBCO superconductor

Voltage-current characteristics of YBCO superconductor was studied under magnetic field up to 0.4 T at different temperatures below T_c. The critical temperature decreases and the transition width broadens under magnetic field. V-I data below T_c were fitted to a power law expression V I^(T,B) in which (T,B) is found to decrease with increase of magnetic field and temperature, gradually approaching unity as T approaches T_c, being independent of magnetic field. Similarly, (T,B) approaches unity as magnetic field increases being independent of temperature.     

Change in the nature of hydrodynamic cavitation in nonuniform magnetic fields

It is known that in nonuniform magnetic fields the precavitation properties of aqueous media change, leading to an increase in the irreversible physicochemical changes.Notation l length of zone II - D and d diameters of tubes I, III, and II - p_I, p_II, p_III pressures in regions I, II, and III - p_cr critical pressure at which cavitation occurs - p_cr and p _cr ⁰ critical pressures in the magnetic field and when there is no magnetic field - [V_I, V_II, V_III] velocities of the liquid in regions I, II, and III - V_II, lim velocity of the liquid at which breakdown of the hydrated layer occurs for a certain value of the induction - V_cr and V _cr ⁰ critical velocities at which cavitation occurs in the magnetic field and when there is no magnetic field - p_a atmospheric pressure - p_sv saturation-vapor pressure at the given temperature - density of the liquid - kinematic viscosity - Re Reynolds number - Re_cr critical Reynolds number - c_gf and c_gd concentrations of free and dissolved gases in the magnetic field and when there is no magnetic field - c_gf and c_gd, and c _gf ⁰ and c _gd ⁰ concentrations of free and dissolved gases in the magnetic field and when there is no magnetic field - _sc space-charge density - electrical conductivity in the volume of the liquid - _b electrical conductivity in the boundary layer - _l , _g, _d dielectric constants of the liquid in the volume, of the gas in the bubbles, and of the diffusion layer - j, j_b, j_i, and j_T current density of the general, boundary layer, induced and current flow - f_MHD and f_EHD volume forces of magnetohydrodynamic and electrodynamic nature (per unit volume) - p_MHD pressure in the liquid due to the action of the magnetohydrodynamic forces - ₀ limiting shear stress in the liquid - B magnetic induction - E electric field strength in the volume of the liquid Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 35, No. 5, pp. 842–850, November, 1978.     

Angular dependence of transport current near critical at fields to 4 T in metalorganic thin films

Values of the transport current have been obtained as a function of angle of the magnetic field to the plane of the thin film YBa₂Cu₃O_7–d (YBCO) prepared by metalorganic deposition. Current flowed in thea, b planes at various angles to the applied magnetic fields to 4.2 T. Films with thicknesses near 350 nm were studied. For these thin films andHc a secondary maximum is observed whenJ _c> 1 MA/cm₂. NearHa, b a large increase inI _c is observed, and the sharpness, field dependence, and field directional dependence is discussed. The response to currents aboveI _c is determined and the fit to a power lawVI _n shows a strong correlation betweenI _c andn.     

Observation of tunneling and magneto-tunneling out of a 2D Wigner crystal

We measured the escape rates of surface state electrons from an electron layer confined at the liquid helium-vacuum interface in the temperature range of 30–450mK, and for densities 0.02–2.2×10⁸ cm ^–2. Below 200mK the escape rates were temperature independent and converged to the expected single particle tunneling rates in the zero density limit, where correlations are negligible. The single particle rates were enhanced exponentially as the density was increased up to a critical densityn _c. Atn _c the escape process becomes extremely nonlinear. As the barrier is raised so that the escape rates decrease below 5.0×10^–4sec^–1, a new very weakly density and external field dependent mechanism seems to dominate the escape. Thermally activated escape was observed above 250mK and the activation energies were in good agreement with the expected values. Upon application of a magnetic field in the plane of the electron layer, the rates become strongly temperature and field dependent even at the lowest temperaturesT40mK. At these temperatures and low densities rates are suppressed four orders of magnitude in a few kGauss magnetic fields.     

Low Temperature Specific Heat of CeRu2Si2 at the Field Induced Metamagnetic Instability

We report specific heat measurements in a broad temperature range between 0.06 and 20 K on single crystalline CeRu₂Si₂ in magnetic fields applied parallel to the c-direction around the critical field B _M , where the metamagnetic transition from the itinerant to the localized state takes place. In the critical field B _M c=7.8 T a distinct deviation from the usual Fermi- liquid behavior is found down to our lowest temperature. Below 1.8 K C/T varies as (1–aT), while for 1.8 K<T<20 K a power law T ^–1 with =0.66 is found. Small deviations from the critical magnetic field leads to the recovery of a Fermi-liquid ground state. The observed behavior of the specific heat at the field induced magnetic instability is discussed in the context of a non-Fermi-liquid state and compared with those for the field-induced nFl CeCu_6–x Ag _x and the recent observation of nFl behavior at B _M in UPt₃.     

Magnetic and nonmagnetic impurities in some A-15 structure systems. I. Superconducting properties

We present measurements of the critical temperature T _c and the critical field H _c2 of the A-15 structure systems Nb _y ,Pt_100–y–x M _x where M = Fe and Ru (y = 73–78). T _c is influenced by two effects: one due to the magnetic Fe atoms, which causes a strong decrease, and the other due to the nonmagnetic effect of the Fe atoms, which causes a smaller but non-negligible effect. For the study of the nonmagnetic effect we introduced Ru impurities instead of Fe. Anomalous high critical field values are found as T 0, and the curves H _c2 versus T do not follow the WHH theory generalized to include magnetic impurities. The introduction of an empirical parameter permits us to fit our experimental results. This abnormal behavior of H _c2 is discussed in terms of strong-coupling and anisotropy effects.     

Decay rate of critical fluctuations in steam and in dilute steam-NaCl mixtures

The decay rate of critical fluctuations in steam and in a steam-NaCl mixture has been investigated experimentally with the aid of photon correlation spectroscopy. For pure steam, the measurements have been performed along seven isochores [(¦– _c¦)/_c<0.09] as a function of the temperatureT for (T–T_t)<1 K. The results have been compared with the values predicted by the renormalization-group theory written as a modification of the classical mode coupling theory. The agreement between experiment and theory is satisfactory along the critical isochore, but larger deviations are noted for _c when approching the transition temperatureT _t. The decay rate of a 0.1% (molar) dilute mixture of NaCl in H₂O has been measured along some near-critical isochores as a function of temperature. Its behavior, which is very different from that observed for pure steam, is dicussed.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Hall-Effect study of YNi2B2C superconductor

Hall-effect and critical magnetic fields H_c2(T) have been studied for YNi₂B₂C (prepared using high pressure technique) at H130 kOe and 1.6T300K. The normal state Hall coefficient R_H was found to be negative (R_H=–1.43.10^–11 cm/Oe at T=4.2 K and H=130 kOe) and slightly growing in absolute value with increase of temperature. The dependence of the Hall voltage on magnetic field in the mixed superconducting state shows no sign reversal as was observed earlier for some high-T_c systems. The values of ¦H_c2/ T¦and H_c2(0) are found to be 4.8 kOe/K and 47 kOe respectively.