Magnetic Field Penetration Depth Research Articles

Positive muon Knight shift and spin relaxation in heavy fermion superconductors UPt 3 and UBe 13

We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductors UBe 13 and UPt 3. In both materials we find that the muon Knight shift is unchanged in the superconducting state, consistent with odd-parity pairing (such as p-wave). The magnetic field penetration depths in UPt 3 and UBe 13 are extremely long, greater than 10000 Å. We find no evidence of a magnetic transition in UBe 13 below 10 K.

Basic similarities among cuprate, bismuthate, organic, Chevrel-phase, and heavy-fermion superconductors shown by penetration-depth measurements.

Muon-spin-relaxation and bulk measurements of the magnetic-field penetration depth suggest that the cuprate high-${\mathit{T}}_{\mathit{c}}$, bismuthate, organic Chevrel-phase, and heavy-fermion systems possibly belong to a unique group of superconductors characterized by high transition temperatures ${\mathit{T}}_{\mathit{c}}$ relative to the values of ${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$ (carrier density/effective mass). This feature distinguishes these exotic superconductors from ordinary BCS superconductors.

Unconventional superconductivity in the organic superconductor, κ-(BEDT-TTF)2Cu(NCS)2

Unconventional natures of the superconductivity in the organic superconductor, κ-(BEDT-TTF)2Cu(NCS)2, were revealed by complex susceptibility and 1H-NMR relaxation measurements. The temperature dependence of magnetic-field penetration depth, λ, was found to follow a power law at low temperatures, instead of an exponential behavior expected in the BCS theory. This result strongly suggests anisotropic superconductivity of gapless nature. The existence of new relaxation mechanism related to vortex dynamics in the superconducting state was confirmed.

Superconductivity of UPt3 studied by μ+SR and neutron scattering (abstract)

The magnetic field penetration depth λ in single-crystalline UPt3 has been measured by muon spin relaxation ( μ+SR). The temperature dependence of λ−2 is nonexponential and anisotropic decreasing from its T=0 value proportional to T when measured perpendicular to the c axis but with a higher-power ∥c. Our data can be accounted for in detail by a superconducting gap with a line of nodes in the basal plane and axial point nodes. By neutron scattering we furthermore show that the antiferromagnetic order parameter of UPt3 is perturbed by the development of superconductivity. We discuss the implications of these results on the symmetry of the superconducting states of UPt3.

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe13, URu2Si2 and U6Fe. The magnetic field penetration depth in the superconducting state of UBe13 is greater than 10000 A, as no increase in the transverse field relaxation rate is observed belowTc. A sharp increase in the precession frequency is seen, starting atTc. This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U6Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atTc=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.

The comparative study of irreversibility effects in Nb foil and high-temperature superconducting ceramics by μSR

We present the results of investigations of superconducting niobium and high temperature ceramical superconductor La1.9Sr0.1CuO4 by the μSR technique. The experiments with the niobium sample confirm high reliability of the μSR-technique in determining such characteristics of type II superconductors asTc,Hc1,Hc2, the magnetic field penetration depth λ, and the critical current densityJc. The analysis of the field dependencies of the distribution width and mean value of the magnetic fields on the muon when the samples are magnetized is carried out. A qualitative difference in the behaviour of the magnetic field distribution width in Nb and LaSrCuO is revealed. Whilst the niobium data are well described in the frame of the critical state model, application of a similar approach to the high-Tc superconductor does not give a satisfactory description of our experimental results.

Anisotropy inc-axis oriented YBa2Cu3O7−δ

Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa2Cu3O7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration depth anisotropy are observed forB‖c andBc. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux pinning characteristics are suggested as the fundamental cause of the additional effects.

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd2−xCexCuO4−y and a large single crystal of Nd2CuO4−y. We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La2−xSrxCuO4−y, although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd2CuO4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.

Muon spin relaxation studies on high- Tc, organic, heavy-fermion, and Chevrel phase superconductors

We report recent progress of positive muon spin relaxation (μSR) studies on high- T c oxide, organic, heavy-fermion, and Chevrel phase superconductors. μSR measurements of the magnetic field penetration depth λ in more than 30 different hole-doped cuprate compounds have demonstrated strong correlations between T c and 1/λ 2∝ n s/ m∗ (carrier density / effective mass). These correlations can be extended to the organic (BEDT-TTF) 2Cu(NCS) 2 and possibly to Chevrel phase systems. In particular, we found that there are only ∼5–10 superconding pairs in a coherence area ξ 2 on the two-dimensional conducting planes in both the cuprate and organic systems. The penetration depths in UBe 13 and UPt 3 are found to be so long that the μSR measurements can only provide a lower limit on λ ⩾ 10 000 Å. The muon Knight shift is essentially independent of temperature below T c in these heavy fermion systems, indicating a constant spin susceptibility and odd-parity (most likely p-wave) pairing.

Bipolaron theory of the low magnetic field properties of high-temperature superconductors

The low magnetic field properties of the layered high-temperature superconductors are studied in the frame of the two-dimensional heavy charged Bose-gas model. The temperature dependencies of the magnetic field penetration depth λ H ( T) and of the lower critical field H c1( T) are calculated in the London limit. The discrepancy between the temperature dependence of the penetration depth from the μSR measurements and that from the ac susceptibility and magnetization measurements is discussed.

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenT c and the relaxation rate σ αn s/m * (carrier density/effective mass) of hole-doped high-T c cuprate superconductors (La2, Sr3)CuO4, YBa2Cu3O7 (Y1−xPrx)Ba2Cu3O7, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)2Cu(NCS)2.

Effect of density, magnetic field modulation and magnetic field penetration depth on the microwave absorption properties of YBa2Cu3O7 –δsuperconductors

Magnetic field sweep hysteresis of the low-field microwave absorption (LFMA) in YBa2Cu3O7 –δ is stronger for low magnetic field modulation, low temperatures and low microwave power. The field-sweep hysteresis is related to the pinning of magnetic fluxons. Under conditions favouring substantial fluxon pinning, noise-like structures are superimposed on the LFMA signal. The LFMA signal amplitude depends on magnetic flux pinning, the relative contributions of intergranular and intragranular currents, the distribution of Josephson junctions and the variation in the magnetic field penetration depth. The hysteresis behaviour is similar for samples with different densities at low and high modulation amplitudes. However, the LFMA amplitude decreases with increasing density. The effective magnetic field penetration depth is measured by electron paramagnetic resonance line broadening of spin probes adsorbed on the sample surface. The results are discussed in terms of several current models for the LFMA effect.

Temperature dependence of the magnetic field penetration depth in YBa2Cu3O7−δ measured on ultra fine powder

Abstract The temperature dependence of the magnetic field penetration depth λ( T ) in superconducting YBa 2 Cu 3 O 7−δ has been determined by measuring the AC susceptibility of fine powder at the frequency 10 5 Hz. The reason for noticeable changes in λ at low temperatures has been found out: strong scattering of carriers by low-frequency excitations. The coupling constant which controls the superconductivity has been estimated. At low temperatures a paramagnetic signal superimposed to the superconductivity one has been observed. It is probably due to alteration in the interaction of the magnetic moments near the surface of the particles.

Magnetic field penetration depth of an organic superconductor: Evidence for anisotropic superconductivity of gapless nature.

The temperature dependence of the magnetic-field penetration depth \ensuremath{\lambda} of the organic superconductor \ensuremath{\kappa}-[bis(ethylenedithio)-tetrathiafulvalene${]}_{2}$Cu(NCS${)}_{2}$ has been determined by means of complex susceptibility measurements for single crystals. \ensuremath{\lambda} was found to follow a power law, \ensuremath{\lambda}(T)/\ensuremath{\lambda}(0)-1\ensuremath{\propto}(T/${\mathit{T}}_{\mathit{c}}$${)}^{2}$, at low temperatures instead of the exponential behavior expected from BCS theory. This result is considered as the first evidence for anisotropic superconductivity with nodes of the gap parameter on the Fermi surface in this material.

The effective microwave surface impedance of high T c thin films

The dependence of the effective surface impedance Zeff=Reff+iXeff of superconducting thin films on the film thickness d, on the magnetic field penetration depth λ, and on the dielectric properties of the substrate material is investigated theoretically by means of impedance transformations. It was found that the effective surface resistance Reff can be expressed by RSf(d/λ)+Rtrans where RS is the intrinsic surface resistance of the superconductor. The function f(d/λ) describes the altered current density distribution in the film. Rtrans arises from power transmission through the film. It depends on d and λ as well as on the dielectric properties of the substrate material and is significantly altered in the case of a resonant background. The effective surface reactance Xeff of a superconducting thin film can be expressed by XS cosh(d/λ) where XS=ωμ0λ is the intrinsic surface reactance. Measurements of Zeff at 87 GHz have been performed for YBa2Cu3O7−δ thin films grown epitaxially by laser ablation on SrTiO3, MgO, and LaAlO3. With the best films, Reff (77 K) values of 21 mΩ and RS (77 K) values of 8 mΩ were achieved. The temperature dependence of λ was found to be in good agreement to both weak-coupling BCS theory in the clean limit and the empirical two-fluid model relation with λ (0 K) values ranging from 140 to 170 nm and 205 to 250 nm, respectively.

Anomalous temperature dependences of London penetration depth in layered superconductors

It is predicted that under appropriate conditions anomalous temperature dependences of the magnetic field penetration depth and of the lower critical field can appear in layered superconductors. The possibility of such dependences in BiSrCaCuO and TlBaCaCuO superconductors is discussed.

Complex susceptibility and penetration depth of the organic superconductor, (BEDT-TTF) 2 Cu(NCS) 2

Abstract We have measured the complex susceptibility for single crystals of the organic superconductor, (BEDT-TTF) 2 Cu(NCS) 2 and derived the magnetic field penetration depth λ . The λ parallel to the layer at lowest temperatures is of the order of 1 mm, which is exceptionally large among all the superconductors to date. The low temperature variation of λ is not consistent with the conventional BCS or Josephson coupling model, but is compatible with anisotropic pairing of gapless nature.

Experimental determination of the absolute value of the London penetration depth in the heavy fermion superconductors UBe 13 und UPT 3

We report on measurements of the absolute value of the magnetic field penetration depth in single crystal specimens of UBe13 and UPt3 using a new technique. The experiments yield an absolute value at T=0, λ(0) = (1100 +/−200) nm in UBe13andλ(0) = (1900+/t-200) nm in UPt3. The results support our previous claim that the Fermiliquidparameter Fs1 should be of order 1 or less and give additional hints to the possible pair states.

Polaron theory of high-Tc superconductors

It is shown that the ordinary electron-phonon interaction can produce a high Tc as a result of the polaron narrowing of the electron band, which is not considered by the traditional theory of strong-coupling superconductors, based on the Migdal-Eliashberg equations. The main properties of high-Tc superconductors are explained by New (polaron) theory of strong-coupling superconductors /3–5/ namely the value of Tc, of the magnetic field penetration depth λL and its temperature dependence, of the normal electron heat capacity γM′ of the thermoelectric power S, the upward curvatury of the upper critical field Hc2, the small value of the coherence length, the temperature dependence and the value of the heat capacity jump ΔC, the anomalous pressure dependence of Tc, the tunneling spectrum, the temperature dependence of sound velocity and some other properties.

Magnetic field penetration depth of polycrystalline (Y,Gd)Ba2Cu3O7, grain-aligned YBa2Cu3O7, and single-crystal Bi2Sr2CaCu2O8.

The magnetic field penetration depths \ensuremath{\lambda}(T) of the title compounds are determined from high-field (H=2.5 to 5 T) magnetization M versus applied magnetic field H isotherms, by use of a new method of data analysis in the region ${H}_{c1}$\ensuremath{\ll}H\ensuremath{\ll}${H}_{c2}$ where M is reversible and linear in the logarithm of H. All of the \ensuremath{\lambda}(T) results are consistent with behavior expected from BCS theory, indicating a nodeless superconducting order parameter. The magnitudes of \ensuremath{\lambda}(0) are similar to reported values obtained using independent methods.