Irreversibility Temperature Research Articles

11B NMR study of vortex motion in magnesium-diboride

We have performed 11B nuclear magnetic resonance (NMR) measurements to investigate vortex dynamics in the polycrystalline MgB 2 superconductor. 11B NMR spectrum, shift and transverse relaxation rate 1 / T 2 were measured down to 4.3 K at ∼ 1.8 T . The spectrum below T c exhibits a typical local field distribution for a vortex lattice under magnetic field. The peak point of the spectrum shifts toward low magnetic field due to incomplete field penetration. Below the irreversibility temperature T irr , 1 / T 2 data, probing the slow motion of vortices, shows a single peak with a small change of the rate, contrary to the results of nickel borocarbides. Also, the relaxation profile changes from Lorentzian decay below T c to Gaussian decay at lower temperature. It strongly suggests that thermal fluctuation of vortices is reduced at low temperature and the vortex motion is much weaker compared with nickel borocarbides.

Clean-limit pair breaking and two-dimensional vortex dynamics in ferromagnet-superconductor heterostructures

Superconductivity in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ films of thickness ${d}_{\mathrm{Y}}<500\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ sandwiched between ferromagnetic and nonmagnetic epitaxial layers has been probed. The irreversibility temperature ${T}_{\mathrm{irr}(\mathrm{H})}$ in the vortex state maps onto a clean-limit pair-breaking temperature ${T}_{\mathrm{c}(\mathrm{H})}$, and a universal relation exists between the pair-breaking field ${H}_{\mathrm{pb}}$ and an effective thickness ${d}_{\mathrm{eff}}(<{d}_{\mathrm{Y}})$ of the superconductor. The vortex response in these two-dimensional (2D) films indicates a crossover temperature $T*$, which separates two regimes of flux motion controlled by formation of dislocations $(T>T*)$, and migration of preexisting dislocations and dislocation pairs $(T<T*)$ in the 2D flux line lattice.

Magnetic study of Fe 65Ni 20Nb 6B 9 nanocomposite alloys

The alloy Fe 65Ni 20Nb 6B 9 was obtained from the elemental constituents in a high-energy planetary ball mill and subsequently thermally treated at 873 and 900 K in order to obtain the equilibrium phases. The as-prepared nanocrystalline alloy consists primarily of metastable BCC α-Fe(Ni) nanocrystals while the treated ones consist of a mixture of BCC (ferromagnetic) and FCC (paramagnetic at room temperature) phases. Hysteresis loops at 5 and 300 K present low remanence and coercivity. As-prepared sample exhibits the best soft magnetic properties. In all samples, the susceptibility curves suggest magnetic collective (long-range order) behavior with a maximum between 70 and 90 K. This feature is caused by nanometric-sized magnetic particles. The huge difference between blocking and irreversibility temperatures in the field cooling and zero-field cooling scans indicates a wide grain-size distribution.

Spin dynamics in the mixed phase of La 2− xSr xCuO 4 ( x=0.10, x=0.17)

We present an inelastic neutron scattering study of the spin dynamics in the mixed phase of the high- T c superconductor La 2− x Sr x CuO 4. In the overdoped regime ( x=0.17, T c=37 K) the application of a magnetic field induces a spectral weight redistribution centered around the zero-field spin gap ( Δ SG≈6.5 meV). Moreover the opening of the spin gap seems to track the irreversibility temperature (related to the melting of the vortex lattice) rather than T c, indicating an unusual interplay between the mesoscopic and microscopic properties in La 2− x Sr x CuO 4. In the underdoped regime ( x=0.10, T c=29 K) our zero-field data indicate a strong decrease (or even vanishing) of the spin gap in the superconducting state and no dependence upon the application of a magnetic field.

Effects of magnetic field on the cuprate high‐Tc superconductor La2–xSrxCuO4

AbstractThis article discusses neutron scattering measurements on the cuprate, high transition temperature superconductor La2–xSrxCuO4 (LSCO) in an applied magnetic field. LSCO is a type‐II superconductor and magnetic flux can penetrate the material via the formation of vorticies. Phase coherent superconductivity characterized by zero resistance is suppressed to the lower field‐dependent irreversibility temperature (Tirr(H)) and occurs when the vortices freeze into a lattice. Because superconductivity is destroyed within the vortex cores, an investigation of the vortex state provides information about the ground state that would have appeared had superconductivity not intervened. Our measurements reveal that both optimally doped LSCO (x = 0.16, Tc = 38.5 K) and underdoped LSCO (x = 0.10, Tc = 29 K) have an enhanced antiferromagnetic response in a field. Measurements of the optimally doped system for H = 7.5 T show that inelastic sub‐gap spin fluctuations first disappear with the loss of finite resistivity at Tirr, but then reappear at a lower temperature with increased lifetime and correlation length compared to the normal state. In the underdoped system elastic antiferromagnetism develops below Tc in zero field, and is significantly enhanced by application of a magnetic field; phase coherent superconductivity is then established within the antiferromagnetic phase at Tirr. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Bose-glass transition in Ag/BSCCO-2223 irradiated tapes

In this work we show how strongly the interaction between pancakes in the ab-planes and Josephson vortices (JVs) across them is affected by columnar defect (CD) implantation, due to heavy-ion irradiation. By means of the irreversibility temperatures, obtained by the onset of the third harmonic of the a.c. susceptibility, the Bose-glass transition is put in evidence. This transition is limited in phase space size by the JV density, modulated by increasing the in-plane component of the applied field, in Ag/BSCCO-2223 tapes irradiated with 0.25 GeV Au-ions producing CDs reaching the 15% of the depth of the tape.

Performance analysis for an irreversible variable temperature heat reservoir closed intercooled regenerated Brayton cycle

In this paper, the theory of finite time thermodynamics is used in the performance analysis of an irreversible closed intercooled regenerated Brayton cycle coupled to variable temperature heat reservoirs. The analytical formulae for dimensionless power and efficiency, as functions of the total pressure ratio, the intercooling pressure ratio, the component (regenerator, intercooler, hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficients, the heat reservoir inlet temperature ratio, and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio, are derived. The intercooling pressure ratio is optimized for optimal power and optimal efficiency, respectively. The effects of component (regenerator, intercooler and hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies, the pressure recovery coefficients, the heat reservoir inlet temperature ratio and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio on optimal power and its corresponding intercooling pressure ratio, as well as optimal efficiency and its corresponding intercooling pressure ratio are analyzed by detailed numerical examples. When the heat transfers between the working fluid and the heat reservoirs are executed ideally, the pressure drop losses are small enough to be neglected and the thermal capacity rates of the heat reservoirs are infinite, the results of this paper replicate those obtained in recent literature.

Temperature modulation of the visible and near infrared absorption and scattering coefficients of human skin

We determine temperature effect on the absorption and reduced scattering coefficients (mu(a) and mu(s)(')) of human forearm skin. Optical and thermal simulation data suggest that mu( a) and mu(s)(') are determined within a temperature-controlled depth of approximately 2 mm. Cutaneous mu(s)(') change linearly with temperature. Change in mu(a) was complex and irreversible above body normal temperatures. Light penetration depth (delta) in skin increased on cooling, with considerable person-to-person variations. We attribute the effect of temperature on mu(s)(') to change in refractive index mismatch, and its effect on mu(a) to perfusion changes. The reversible temperature effect on mu (s)(' ) was maintained during more than 90 min. contact between skin and the measuring probe, where temperature was modulated between 38 and 22 degrees C for multiple cycles While temperature modulated mu(s)(' ) instantaneously and reversibly, mu(a) exhibited slower response time and consistent drift. There was a statistically significant upward drift in mu(a) and a mostly downward drift in mu( s)(') over the contact period. The drift in temperature-induced fractional change in mu(s)(') was less statistically significant than the drift in mu(s)('). Deltamu( s)(') values determined under temperature modulation conditions may have less nonspecific drift than mu(s)(') which may have significance for noninvasive determination of analytes in human tissue.

Structure, magnetic, magnetocaloric and magnetoresistance properties of La 1− xPb xMnO 3 perovskite

La 1− x Pb x MnO 3 ( x=0.1, 0.2, 0.3, 0.4, and 0.5) perovskites were prepared by a solid-state reaction. Except for x=0.5 (cubic) and x=0.4 (rhombohedral), the structure of the other compositions was pseudo-rhombohedral with P1 symmetry. The particle size of the grains is depending on the Pb content of the samples. The Curie temperature T c increases from 235 K for x=0.1–310 K for x=0.2 and is almost constant (about 360 K) for x⩾0.3. The field-cooled and zero-field-cooled thermomagnetic curves measured at low field show a split below a so-called irreversibility temperature T r, which is somewhat smaller than T c except for x=0.1, where it is 270 K. From a series of magnetic isotherms the magnetic entropy changes Δ S( T) were determined for a field step of 500 Oe. The maximum value of Δ S max increases with increasing x till x=0.3 and then decreases with further increasing x. The conductivity of perovskites is metallic at low temperatures and semiconducting at high temperatures. Magnetoresistance measurements have been performed.

TEM analysis of Gd 5(Si xGe l−x)4, where x=1/2

The material Gd 5(Si x Ge l− x )4, with x=1/2 was investigated using transmission electron microscopy. The material possesses a complex crystallographic structure and undergoes a unique, reversible, transformation at low temperatures and an irreversible high temperature transformation. The high, room and low temperature microstructures of arc-melted and heat treated samples are reported. Direct observation of the transformation with a discussion on the mechanism of the reversible, low temperature, phase transformation is detailed.

First-order vortex phase transition in κ-type BEDT-TTF organic superconductors

We measured the local magnetization using a micro-Hall probe in a magnetic field perpendicular to the conduction layer to investigate the mixed state of the quasi-two-dimensional organic superconductors κ-(BEDT-TTF) 2Cu(NCS) 2. A sharp magnetization jump was observed in the vicinity of the irreversible temperature ( T irr) below ∼80 Oe, demonstrating the first-order phase transition. Meanwhile, much broader step structure above higher magnetic field than 80 Oe appears below T irr. The magnetic field regime of 80–100 Oe is likely to be connected with the second peak line in the magnetic field–temperature phase diagram.

Dissipative resistivity and activation energy U( T, H) in the textured superconducting MgB 2 bulk

The temperature dependence of dissipative resistivity has been measured by the flux transform configuration in applied magnetic field up to 7.5 T perpendicular to the surface of a textured MgB 2 bulk. The temperature dependence of the resistivity follows the Bloch–Gruneisen formula, ρ(T)=ρ 0+A T Θ m∫ 0 Θ/T x m dx (1− e −x)( e x−1) , showing an electron–phonon scattering mechanism. The resistive transition becomes broader with increasing applied field H. The field dependence of the characteristic temperature T th, peak temperature T p and irreversibility temperature T irr shows a good power law, H= H 0(1− T/ T i (0)) n , with n=1.18±0.02 and H 0=37±1 T for H∥ c. The dissipative resistivity for H>1 T can be scaled by the thermally activated flux creep model, ρ= ρ 0exp(− U e/ T) with U e=(1− T/ T p) 2.3exp(14−0.776 H). Moreover, the dynamic of vortices exhibits a strong 3D behavior.

Transport and magnetic properties of superconducting MgB 2

We have investigated the transport and magnetic properties of the recently discovered superconducting materials MgB 2. We obtained irreversibility temperature T r( H) and irreversibility field H r( T) from magnetic measurements. The irreversibility temperature T r( H) was determined from the merging temperature of the magnetic moment observed in the zero field cooling process in to the one in field cooling process. We also defined the transport irreversibility temperature B c2 from resistivity measurements at zero-resistivity in the presence of magnetic field. Those results of the irreversibility lines are plotted in H– T plane. The magnetic irreversibility lines are consistent with each other, while the transport one locates at the higher temperature side.

VORTEX MAGNETISM IN THE HIGH-TEMPERATURES SUPERCONDUCTOR La2-xSrxCuO4

There is strong evidence that magnetic interactions play a crucial role in the mechanism driving high-temperature superconductivity in cuprate superconductors. To investigate this further we have done a series of neutron scattering measurements on La 2-x Sr x CuO 4 (LSCO) in an applied magnetic field. Below Tc the field penetrates the superconductor via an array of normal state metallic inclusions or vortices. Phase coherent superconductivity characterized by zero resistance sets in at the lower field-dependent irreversibility temperature (Tirr). We have measured optimally doped LSCO (x = 0.16, Tc = 38.5 K ) and underdoped LSCO (x = 0.10, Tc = 29 K ); both have an enhanced antiferromagnetic response in a field. Measurements of the optimally doped system at H = 7.5 T show that sub-gap spin fluctuations first disappear with the loss of finite resistivity at Tirr, but then reappear at a lower temperature with increased lifetime and correlation length compared to the normal state. In the underdoped system elastic antiferromagnetism develops below Tc in zero field, and is significantly enhanced by application of a magnetic field. Phase coherent superconductivity is then established within the antiferromagnetic phase at Tirr; thus, the situation in underdoped LSCO is the reverse of that for the optimally doped LSCO where the zero-resistance state develops first before the onset of antiferromagnetism.

FIELD-INDUCED ANTIFERROMAGNETISM IN THE HIGH-TEMPERATURE SUPERCONDUCTOR La2-xSrxCuO4

There is strong evidence that magnetic interactions play a crucial role in the mechanism driving high-temperature superconductivity in cuprate superconductors. To investigate this we have done a series of neutron scattering measurements on La 2-x Sr x CuO 4 (LSCO) in an applied magnetic field. Below Tc the field penetrates the superconductor via an array of normal state metallic inclusions or vortices. Phase coherent superconductivity characterized by zero resistance sets in at the lower field-dependent irreversibility temperature (Tirr). We have measured optimally doped LSCO (x = 0.16, Tc = 38.5 K ) and under-doped LSCO ( x = 0.10, Tc = 29 K ); both have an enhanced antiferromagnetic response in a field. Measurements of the optimally doped system at H = 7.5 T show that sub-gap spin fluctuations first disappear with the loss of finite resistivity at Tirr, but then reappear at a lower temperature with increased lifetime and correlation length compared to the normal state. In the under-doped system elastic antiferromagnetism develops below Tc in zero field, and is significantly enchanced by application of a magnetic field. Phase coherent superconductivity is then established within the antiferromagnetic phase at Tirr; thus, the situation in under-doped LSCO is the reverse of that for the optimally doped LSCO where the zero-resistance state develops first before the onset of antiferromagnetism.

The second magnetization step in Bi2Sr2CaCu2O8+δ single crystals

The temperature dependence of magnetization in different magnetic fields for the optimally-doped and overdoped Bi2Sr2CaCu2O8+δ single crystals was systematically investigated. It was found that the second transition occurs in the reversible region of the magnetization, and the transition temperature TIIC(H) decreases with increasing magnetic field and deviates from the irreversibility temperature Tirr(H) in the H–T phase diagram. The second step, existing in the vortex liquid state, can be regarded as due to the second-order transition from a low-temperature vortex liquid with line tension to a high-temperature vortex liquid without line tension, which is different from the vortex glass to vortex liquid transition.

Magnetic properties and microstructure of carbon encapsulated Ni nanoparticles and pure Ni nanoparticles coated with NiO layer

Two kinds of nickel nanoparticles—carbon encapsulated Ni nanoparticles Ni(C) and pure Ni nanoparticles coated with NiO layers Ni(O) are successfully prepared. Structural characterizations (HR-TEM, SAED, and XRD) reveal their distinct morphological properties. Magnetization measurements for the assemblies of two kinds of Ni nanoparticles show a larger coercivity and remanence by a deviation between the zero-field-cooled and the field-cooled magnetization below the irreversibility temperature, T irr, for the assemblies of Ni(O) particles. This deviation may be explained as a typical nanocluster–glass behavior (collective behavior) due to ferromagnetic dipole–dipole interaction effects among the assemblies of Ni(O) particles. However, Ni(C) particles exhibit modified superparamagnetic properties above the average blocking temperature of T B, which is determined to be around 115 K at 1000 Oe. Moreover, a gradual decrease in saturation magnetization is observed, which is attributed to the nanocrystalline nature of the encapsulated particles, coupled with possible carbon solution in Ni nanocrystals.

The effect of Er-substitution on the superconducting properties of MTG-YBaCuO crystals

The high quality Er-doping Y0.8Er0.2Ba2Cu3O7−δ (YErBCO) crystal is grown by the melted-textured-growth (MTG) method. The microstructure photograph shows that the MTG-YErBCO crystal has an obvious layered structure. Its magnetization loops show that the irreversibility field is decreased by Er-doping for H//c and that there is an anomalous behaviour, a second peak, in the magnetization curve for H//ab. The critical current density along the ab-plane (Jcab(77 K, 0 T) = 4.75 × 104 A cm−2) is higher than that along the c-axis (Jcc(77 K, 0 T) = 470 A cm−2). The field dependence of the irreversibility temperature Tirr follows a power law, H = H0(1 − Tirr(H)/Tirr(0))1.3, with H0 = 550 T for H//ab and 51 T for H//c, respectively. The field dependence of the activation energy Ue shows a different behaviour for H//ab and for H//c, namely a linear relation, U = 49131–3006H, for H//ab and a power law, U ∝ H−0.68, for H//c, respectively. The anisotropy factor γ = 15 was obtained from the angular dependences of the activation energy and the irreversibility temperature. A possible reason for the effect of Er addition on the flux-pinning behaviour has been discussed.

MgB<SUB>2</SUB>の磁化緩和

We have investigated the transport and magnetic properties of the recently discovered superconducting MgB2. We obtained irreversibility temperature Tr(H) and irreversibility field Hr(T) from magnetic measurement and irreversibility field Bc2 from resistivity measurement. The irreversibility temperature Tr(H), was determined from the merging temperature of the magnetic moment observed in the zero field cooling (ZFC) process in to the one in field cooling (FC) process. We define the transport irreversibility line from the temperature at zero-resistivity in the presence of magnetic field. We also measured the magnetic relaxation of the remanent. The sample was cooled in zero field and the field above H*, where the flux front penetrated at the center of the sample. The time decay of the remanent moment was measured for about 105 s. The relaxation of the remanent moment showed the linear ln (t) dependence in the whole observed temperature range. Those results of the irreversibility lines are plotted in H-T plane. The magnetic irreversibility lines are consistent with each other, while the transport one locates at the higher temperature side.

( Ta 1−x Nb x ) 2 O 5 films produced by atomic layer deposition: Temperature dependent dielectric spectroscopy and room-temperature I–V characteristics

Temperature dependent ac dielectric spectroscopy and room-temperature I–V characterization were performed on atomic layer deposited (Ta1−xNbx)2O5 films. The high frequency permittivity, as well as the dc conductivity of the films, were found to increase with increasing Nb content. The conduction mechanism in the mixed Ta–Nb oxide films was of the Poole–Frenkel type, while the high field conduction in pure Ta2O5 was space-charge limited. The activation energy for dc conduction was higher in mixed Ta–Nb oxides compared to pure Ta2O5 and Nb2O5 films. Irreversible changes in the conduction mechanism took place upon heat treatment above a certain irreversibility temperature. This temperature was higher for the mixed oxides than for the binary ones.