Anomalous Peak Effect Research Articles

Peak effects induced by particle irradiations in 2H-NbSe2

Various peak effects in 2H-NbSe2 single crystals induced by particle irradiations were studied. 3 MeV proton irradiation magnified the peak effect induced by order–disorder (O-DO) transition of vortices, where the peak field shifts from high fields to low fields with increasing irradiation dose. For the peak effect in NbSe with splayed columnar defects (CDs), as the splayed angle increases, peak field gradually shifts from high fields to low fields. Numerical calculations have been conducted to investigate the mechanism of the peak effect. The calculated results exhibit excellent agreement with experimental observations. Analyses of field dependence of J reveal the formation of non-uniform J flow with increasing splayed angle, which plays a crucial role in inducing the self-field peak effect in superconductors with splayed CDs. In samples with symmetric splayed CDs with respect to the c-axis generated by 800 MeV Xe and 320 MeV Au ions, coexistence of O-DO transition-induced peak effect and self-field peak effect was observed. In the case of 320 MeV Au irradiated samples, when the splay angle is small, the two peak effects transform into a broad peak, which has similarity to the anomalous peak effect observed in iron-based superconductors. Interestingly, the broad anomalous peak effect is strongly suppressed when the external magnetic field is applied parallel to one of splayed CDs.

Anisotropic Critical Current Densities in Ba0.6K0.4Fe2As2 with Splayed Columnar Defects

Enhancement and non-monotonic field dependence of critical current density (Jc ), called anomalous peak effect, were observed in Ba1−x K x Fe2As2 with splayed columnar defects. In principle, anisotropy of in-plane Jc should be present in this superconductor since the introduced splayed columnar defects introduce anisotropy in the plane. This study reports in-plane anisotropy of Jc observed in 2.6 GeV U irradiated Ba0.6K0.4Fe2As2 with splay angles of ±15° and ±20°. Results show that Jc component parallel to the splay plane is larger than the perpendicular component, and the parallel component contribute to the anomalous peak effect at around B Φ/3 more.

Effects of Asymmetric Splayed Columnar Defects on the Anomalous Peak Effect in Ba0.6K0.4Fe2As2

In order to enhance the critical current density ($J_c$) of superconductors, introduction of columnar defects (CDs) through swift-particle irradiation is effective. By dispersing the direction of CDs (splayed CDs), not only further enhancement of $J_c$ has been confirmed but also an anomalous peak effect (APE) in $J_c$ at a certain magnetic field determined by the irradiation dose was observed. It has been proposed that the APE arises from the suppression of kink motion and/or the effects of vortex entanglement in systems with splayed CDs. In this study, we measure $J_c$ properties of optimally K-doped Ba-122-type iron-based superconductor Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ single crystals with splayed CDs that are introduced asymmetrically with respect to the $c$-axis by irradiating 2.6 GeV U ions. We discuss the significance of the average direction of splayed CDs to the APE. We also discuss the relationship between the APE and the entanglement of vortices.

Effects of Splayed Columnar Defects on Critical Current Density in CaKFe4As4

Introduction of columnar defects to superconductors through particle irradiation enhances their critical current density (J c) . Further enhancement of J c by dispersing the direction of columnar defects has been confirmed. Moreover, in such systems with splayed columnar defects, an anomalous peak effect in J c at a certain magnetic field determined by the irradiation dose accompanied by the in-plane anisotropy of J c between those parallel and perpendicular to the splay direction were observed. We introduce splayed columnar defects to CaKFe4As4 single crystals, which was recently found as a new type of iron-based superconductors (1144-type IBS), by irradiating 2.6 GeV U and 320 MeV Au ions and measure their J c properties.

Anomalous peak effect in iron-based superconductors Ba1−xKxFe2As2 ( x ≈ 0.69 and 0.76) for magnetic-field directions close to the ab plane and its possible relation to the spin paramagnetic effect

We report magnetic torque measurements on iron-pnictide superconductors Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($x \approx$ 0.69 and 0.76) up to an applied field of $B_a$ = 45 T. The peak effect is observed in torque-vs-field curves below the irreversibility field. It is enhanced and becomes asymmetric as the field is tilted from the $c$ axis. For field directions close to the $ab$ plane, increasing- and decreasing-field curves peak at markedly different fields, and exhibit a sharp jump, suggestive of a first-order phase transition, on the high- and the low-field side of the peak, respectively. Complicated history dependence of the torque is observed in the peak-effect region. We construct and discuss the temperature ($T$)--applied-magnetic-field ($B_a$) phase diagram. Since the upper critical field for the $ab$-plane direction is comparable to the Pauli limit, we also consider possible influence of the spin paramagnetic effect on the anomalous peak effect.

Field-driven transition in Ba1−xKxFe2As2 with splayed columnar defects

Columnar defects in type-II superconductors serve as artificial pinning centers, which lead to enhancement of critical current density. However, little is known about the details of vortex motion in iron-based superconductors with columnar defects. In this study, we performed magnetization measurements in Ba0.6K0.4Fe2As2 single crystals with splayed columnar defects for several directions of magnetic field in order to investigate an anomalous peak effect in the system. We report results of detailed magnetization measurements, and discuss possible origins of the field-driven transition.

From an Anomalous Peak Effect to a Second Magnetization Peak in Nb-rich Nb-Ti Alloys

We investigated the evolution of the DC magnetic hysteresis curves of Nb 0.89Ti 0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 ∘C in vacuum and cold rolling (∼ 50 % thickness reduction). Starting with a rectangular piece (∼ 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak (where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

Giant increase of critical current density and vortex pinning in Mn doped KxFe2−ySe2 single crystals

We report a comparative study of the critical current density (Jc) and vortex pinning among pure and Mn doped KxFe2−ySe2 single crystals. It is found that the Jc values can be greatly improved by Mn doping and post-quenching treatment when comparing to pristine pure sample. In contrast to pure samples, an anomalous second magnetization peak (SMP) effect is observed in both 1% and 2% Mn doped samples at T = 3 K for H∥ab but not for H∥c. Referring to Dew-Hughes and Kramer's model, we performed scaling analyses of the vortex pinning force density vs magnetic field in 1% Mn doped and quenched pristine crystals. The results show that the normal point defects are the dominant pinning sources, which probably originate from the variations of intercalated K atoms. We propose that the large nonsuperconducting K-Mn-Se inclusions may contribute to the partial normal surface pinning and give rise to the anomalous SMP effect for H∥ab in Mn doped crystals. These results may facilitate further understanding of the superconductivity and vortex pinning in intercalated iron-selenides superconductors.

The influence of the buffer layer architecture on transport properties for BaFe1.8Co0.2As2 films on technical substrates

A low and almost temperature independent resistance in the normal state and an anomalous peak effect within the normal-superconducting transition have been observed in BaFe1.8Co0.2As2/Fe bilayers, prepared on ion beam assisted deposition-MgO/Y2O3 buffered technical substrates. A resistor network array sufficiently reproduces this effect, assuming an increase of the electrical conductance between tape and film with decreasing buffer layer thickness. Based on this model, we evaluated the influence of this effect on the critical current density and successfully reconstructed the superconducting transition of the bilayer.

Complex mixed state of the Pauli-limited superconductor CeCoIn5

Magnetization measurements were performed on ${\mathrm{CeCoIn}}_{5}$ at temperatures down to 20 mK and magnetic fields up to 17 T applied along different crystallographic orientations. For field configurations nearly parallel to the $ab$ plane ($\ensuremath{\theta}\ensuremath{\lesssim}{40}^{\ensuremath{\circ}}$ and $T\ensuremath{\le}50\phantom{\rule{0.16em}{0ex}}\mathrm{mK}$), we have found an intriguing vortex dynamics regime revealed by a hysteretic and metastable anomalous peak effect (APE), which gives evidence of surface barrier effects enhanced by antiferromagnetic fluctuations in the mixed state of ${\mathrm{CeCoIn}}_{5}$. Furthermore, we have observed crossover features in the torque and magnetization traces at fields below ${H}_{c2}$, which are consistent with vortices lattice phase transitions and with the anomalies speculated to be the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state in ${\mathrm{CeCoIn}}_{5}$. All of the above features were found to be dramatically perturbed in ${\mathrm{Ce}}_{0.98}{\mathrm{Gd}}_{0.02}{\mathrm{CoIn}}_{5}$.

Magnetic characterization of polycrystalline NdFeAsO 0.88F 0.12 oxypnictide superconductor

The superconducting properties of polycrystalline NdFeAsO 0.88F 0.12 oxypnictide were studied by both DC and AC magnetization measurements. The zero-field cooled (ZFC) magnetic susceptibility, χ( T), measured under the magnetic field of 0.5 T shows a dramatic decrease at about 11 K. The imaginary component of the first harmonics of the AC magnetic susceptibility, χ″( T), increases with the increasing DC field H dc below 11 K. These results indicate the onset of robust intergranular superconductivity at low temperatures. The magnetic hysteresis loops show an anomalous double central peak effect at low temperatures, with one peak at positive fields in decreasing positive external fields. This phenomenon suggests the granular character of this system. The paramagnetism of Nd 3+ ions tilts the magnetic hysteresis loops and broadens the hysteresis width Δ M. After correction for the paramagnetism, the temperature and field dependence of the average magnetization critical current density J cm was obtained. The related mechanism was discussed.

Flux Dynamic Changes by Neutron Irradiation in BISCCO: High Harmonics AC Susceptibility Analysis

High harmonics ac susceptibility measurements joint with neutron irradiations on quasi-bi-dimensional high temperature superconductor (quasi-2D-HTSC) are good tools to study the flux dynamics and its interaction with pinning processes in these superconductors. Flux neutron intensity of shows a deep change in the flux pinning dynamics in Bi-Sr-Ca-Cu-O (BSCCO) system. Third harmonic susceptibility signal increases in amplitude after the neutron irradiation, followed by a rise of the pinning and critical current. Moreover, after the irradiation, the measurements underline the demise of the anomalous peak effect (PE) associated with a three-dimensional/two-dimensional (3D/2D) flux lattice transition.

Anomalous peak-effect in type-II superconductors: A competition between bulk pinning and a surface barrier

In this work, the joint influence of the surface barrier and bulk pinning on the magnetic-field dependence of the critical current Ic for bulk type-II superconductors is investigated. It is shown that in the weak magnetic field H0, there is a section in the dependence Ic(H0) in which Ic increases with the growth of H0; this increase results in a pronounced peak in the dependence Ic(H0) – the anomalous peak-effect. This effect is explained by the combined influence of the surface barrier and bulk pinning. Two well-known models – the Bean model and the Kim–Anderson model – of the critical state were analysed in order to describe the anomalous peak-effect quantitatively.

Experimental evidence for a dimensional crossover of the vortex ensemble in BSCCO (Bi 1.8Pb 0.4Sr 1.8Ba 0.2Ca 2Cu 3O x) by multi-harmonic ac susceptibility measurements

We have studied a portion of H– T phase diagram of the anisotropic Bi2223 bulk system using multi-harmonic ac susceptibility measurements ( χ n ). Based on the behavior of the third harmonic modulus, ∣ χ 3∣, the irreversibility line (IL) was obtained from the onset of ∣ χ 3∣ and an anomalous peak effect (PE) was observed. It has been shown that the anomalous peak is due to the crossover between a three-dimensional (3D) and a quasi-two dimensional (quasi-2D) peculiarity of vortex dynamics, the crossover magnetic field being H cr ≈ 0.1 T. The obtained portion of B irr( T) is well described by the melting line of the flux lattice. The 3D flux fluctuation part (low field and high temperature) is described by the nearly parabolic temperature dependence B = B 0( T c/ T − 1) n with n = 1.58 (sample 1) and n = 1.48 (sample 2). The other region (high field and low temperature) is well described by the temperature dependence of the quasi-2D flux fluctuations, T m ( H ) = T m 2 D [ 1 + b / ( ln B / B cr ) 1 / ν ] corresponding to the weak interaction between pancake vortices from adjacent planes. The 2D limit temperature was determined as T m 2 D = 36.7 K (sample 1) and T m 2 D = 27.3 K (sample 2).

Evidence for unconventional superconductivity inUPt3from magnetic torque studies

We studied the anisotropic magnetic response of the internal superconducting phases of ${\mathrm{UPt}}_{3}$ and its anisotropic magnetic susceptibility with a capacitive torque meter which is very sensitive in high fields. Experiments were performed at temperatures down to 20 mK and at various angles between the c axis (hexagonal structure) and $\stackrel{\ensuremath{\rightarrow}}{B},$ ranging from 16\ifmmode^\circ\else\textdegree\fi{} to 82\ifmmode^\circ\else\textdegree\fi{}. The samples were four single crystals grown with different methods and subjected to different annealing procedures. The normal state susceptibility has a maximum around 20 K for $\stackrel{\ensuremath{\rightarrow}}{B}$ in the $a\ensuremath{-}b$ plane which we followed up to 14 T. It may arise from hybridized uranium ion states split by the hexagonal crystal field. The magnetization curves in the superconducting (SC) regime show strong irreversibilities which are highly sample dependent. They are not correlated with the internal SC phase lines but continue up to a line of fields that lies parallel to the ${B}_{c2}$ curve and even follows its kink at the tetracritical point ${(T}^{\ensuremath{\star}}{,B}^{\ensuremath{\star}}).$ In the cleanest sample this line is shifted to fields well below the $B\ensuremath{-}C$ internal phase line which then manifests itself in a pronounced kink of the magnetization curve indicating an enhanced Ginzburg-Landau parameter $\ensuremath{\kappa}.$ In another sample the $B\ensuremath{-}C$ phase line between two of the three internal SC states could be detected even in the hysteretic region. The enhanced Ginzburg-Landau parameter $\ensuremath{\kappa}$ means a larger penetration depth and/or a shorter coherence length, clear evidence for the unconventional character of the $B\ensuremath{-}C$ phase transition. With our cleanest sample we also observe an anomalous peak effect, a region of enhanced flux pinning near ${B}_{c2},$ which is probably related to the Fulde-Ferrell-Larkin-Ovchinnikov state. In yet another sample we find a crossing of the up-down magnetization curves, also near ${B}_{c2},$ but with reversed orientation of the magnetization loops. We interpret this in terms of different flux pinning in the two main crystal directions, possibly in relation to the peak effect which is, however, masked in this sample by strong irreversibilities.

Magnetization measurements on anomalous peak effect in detwinned YBCO single crystals

Magnetization measurements for detwinned YBCO single crystal were accomplished. Characteristic peak effect can be seen. The second peaks can be expressed with an empirical formula J c =J 0 ln(H/H ∗) . Characteristic fields H ∗ divide the two different vortex states.

Anomalous magnetization in the superconducting mixed state of CeRu 2: a study with vibrating sample magnetometer

Magnetic response of the superconducting mixed state of CeRu 2 is studied using a vibrating sample magnetometer (VSM) and a comparison is made with the results obtained earlier with the SQUID magnetometer. While confirming various results associated with the anomalous peak-effect in CeRu 2, the present VSM study also reveals some interesting new features which were not observed in the SQUID-magnetometer studies.

Anomalous peak-effect in CeRu2: effect of disorder

The irreversible magnetisation in C-15 Laves phase superconductor CeRu2 is being studied vis-a-vis the disorder in the vortex phase induced by the material defects present in different CeRu2 samples. The study clearly reveals that the peak-effect in CeRu2 cannot be explained on the basis of collective pinning model. The results on 5at.% Nd-doped CeRu2 sample appear to be indicative of a possible correlation between the peak-effect and the normal state paramagnetism of CeRu2.

Equilibrium and driven vortex phases in the anomalous peak effect

We report experimental tests of the present models of the peak effect in weakly disordered type-II superconductors. Our results favour the scenario in which the peak effect arises from a crossover between the Larkin pinning length and a rapidly falling elastic length in a vortex phase populated with thermally excited topological defects. A thickness dependence study of the onset of the peak effect at varying driving currents suggests that both screw and edge dislocations are involved in the vortex lattice disordering. The driven dynamics in threedimensional samples are drastically different from those in two-dimensional samples. We suggest that this may be a consequence of the absence of a Peierls potential for screw dislocations in a three-dimensional vortex line lattice.

Microstructure and Micromechanical Properties of SiC-W Multilayer Films

SiC-W multilayer thin films with various modulation wavelengths were prepared by a magnetron sputtering system. Using low-angle x-ray diffraction (LXD), their interface microstructure and the modulation period were studied. The mechanical properties of these films were investigated using an ultramicrohardness (UMH) tester with loads smaller than 20 mN. It was shown that the UMH values of multilayer films varies with applied load L and modulation wavelength. When L was 5 mN, was 10 nm, the UMH reached a maximum, 19.869 GPa, which was 1.02 times higher than that of the homogeneously mixed film. The anomalous peak effect of UMH varying with the modulation wavelength is discussed.