Low temperature specific heat of 12442-type KCa_2Fe_4As_4F_2 single crystals

Low-temperature specific heat(SH) is measured for the 12442-type KCa_2Fe_4As_4F_2 single crystal under different magnetic fields.A clear SH jump with the height of ?C/T|_(T_c)= 130 mJ mol~(-1) K~(-2) is observed at the superconducting transition temperature T_c.It is found that the electronic SH coefficient ?γ(H) quickly increases when the field is in the low-field region below 3T and then considerably slows down the increase with a further increase in the field, which indicates a rather strong anisotropy or multi-gap feature with a small minimum in the superconducting gap(s). The temperature-dependent SH data indicate the presence of the T~2 term, which supplies further information and supports the picture with a line-nodal gap structure. Moreover, the onset point of the SH transition remains almost unchanged under the field as high as 9 T, which is similar to that observed in cuprates, and places this system in the middle between the BCS limit and the Bose-Einstein condensation.     

NMR and NQR studies on transition-metal arsenide superconductors LaRu_2As_2,KCa_2Fe_4As_4F_2,and A_2Cr_3As_3

We report ~(75)As-nuclear magnetic resonance(NMR) and nuclear quadrupole resonance(NQR) measurements on transition-metal arsenides LaRu_2As_2, KCa_2Fe_4As_4F_2, and A_2Cr_3As_3. In the superconducting state of La Ru_2As_2, a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T1 just below Tc, which indicates that LaRu_2As_2 is a full-gap superperconducor. For KCa_2Fe_4As_4F_2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate RAF= Tc1/Tab1 is small and temperature independent,implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa_2Fe_4As_4F_2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A_2Cr_3As_3(A = Na, K, Rb, Cs), we calculate the electric field gradient by first-principle method and assign the ~(75)As-NQR peaks to two crystallographically different As sites, paving the way for further NMR investigation.     

Superconducting vortex pinning with artificial magnetic nanostructures

This review is dedicated to summarizing the recent research on vortex dynamics and pinning effects in superconducting films with artificial magnetic structures. The fabrication of hybrid superconducting/magnetic systems is presented together with the wide variety of properties that arise from the interaction between the superconducting vortex lattice and the artificial magnetic nanostructures. Specifically, we review the role that the most important parameters in the vortex dynamics of films with regular array of dots play. In particular, we discuss the phenomena that appear when the symmetry of a regular dot array is distorted from regularity towards complete disorder including rectangular, asymmetric, and aperiodic arrays. The interesting phenomena that appear include vortex-lattice reconfigurations, anisotropic dynamics, channeling, and guided motion as well as ratchet effects. The different regimes are summarized in a phase diagram indicating the transitions that take place as the characteristic distances of the array are modified respect to the superconducting coherence length. Future directions are sketched out indicating the vast open area of research in this field.     

Coexistence of Polaronic States and Superconductivity in Iron-Pnictide Compound Ba_2Ti_2Fe_2As_4O

The electronic structure of iron-pnictide compound superconductor Ba_2Ti_2Fe_2As_4O, which has metallic intermediate Ti_2O layers, is studied using angle-resolved photoemission spectroscopy. The Ti-related bands show a‘peak-dip-hump' line shape with two branches of dispersion associated with the polaronic states at temperatures below around 120 K. This change in the spectra occurs along with the resistivity anomaly that was not clearly understood in a previous study. Moreover, an energy gap induced by the superconducting proximity effect opens in the polaronic bands at temperatures below T_c(~21 K). Our study provides the spectroscopic evidence that superconductivity coexists with polarons in the same bands near the Fermi level, which provides a suitable platform to study interactions between charge, lattice and spin freedoms in a correlated system.     

Hysteretic vortex pinning in superconductor-ferromagnet nanocomposites

This Letter reports the observation of hysteresis in the vortex pinning in a superconductor-ferromagnetic epitaxial nanocomposite consisting of fcc Gd particles incorporated in a Nb matrix. We show that this hysteretic pinning is associated with magnetic reversal losses in the Gd particles and is fundamentally different in origin to pinning interactions previously observed for ferromagnetic particles or other microstructural features.     

Superconducting density of states and vortex cores of 2H-NbS2

Scanning tunneling microscopy and spectroscopy measurements in the superconducting dichalcogenide 2H-NbS2 show a peculiar superconducting density of states with two well-defined features at 0.97 and 0.53 meV, located, respectively, above and below the value for the superconducting gap expected from the single band s-wave BCS model (Delta=1.76k_(B)T_(c)=0.9 meV). Both features have a continuous temperature evolution and disappear at T_(c)=5.7 K. Moreover, we observe the hexagonal vortex lattice with radially symmetric vortices and a well-developed localized state at the vortex cores. The sixfold star shape characteristic of the vortex lattice of the compound 2H-NbSe2 is, together with the charge density wave order, absent in 2H-NbS2.     

Superconducting pairs with extreme uniaxial anisotropy in URu2Si2

We report magnetic field orientation-dependent measurements of the superconducting upper critical field in high quality single crystals of URu(2)Si(2) and find the effective g factor estimated from the Pauli limit to agree remarkably well with that found in quantum oscillation experiments, both quantitatively and in the extreme anisotropy (≈10(3)) of the spin susceptibility. Rather than a strictly itinerant or purely local f-electron picture being applicable, the latter suggests the quasiparticles subject to pairing in URu(2)Si(2) to be "composite heavy fermions" formed from bound states between conduction electrons and local moments with a protected Ising behavior. Non-Kramers doublet local magnetic degrees of freedom suggested by the extreme anisotropy favor a local pairing mechanism.     

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB₂ tapes have been measured at temperatures T≥28 K in magnetic fields B≤0.9 T. The irreversibility line B_irr(T) for doped tape shows a stepwise variation with a kink around 0.3 T. Such B_irr(T) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B_?) and is very different from a smooth B_irr(T) variation in undoped MgB₂ samples. The microstructure studies of nanoparticle doped MgB₂ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.     

Geometrical surface vortex pinning in superconducting films

A model is proposed for vortex pinning in a superconducting film with a rough surface. The model relates the critical current to the steepness of the surface relief and, at a high vortex concentration, to the distance between neighboring steepness maxima on the paths of vortex motion. The dependence of the critical current density on the thickness of a high-T_c superconducting film is measured in a weak magnetic field. Its behavior can be explained by the pinning at the stepped surface relief.     

Observation of vortex pinning in Bose-Einstein condensates

We report the observation of vortex pinning in rotating gaseous Bose-Einstein condensates. Vortices are pinned to columnar pinning sites created by a corotating optical lattice superimposed on the rotating Bose-Einstein condensates. We study the effects of two types of optical lattice: triangular and square. In both geometries we see an orientation locking between the vortex and the optical lattices. At sufficient intensity the square optical lattice induces a structural crossover in the vortex lattice.     

Superconducting vortex line in Josephson junction

The superconducting ring closed with the half infinite plane Josephson junction is considered. The external magnetic flux is introduced in the ring with the external source supplied solenoid.The conditions of stability are found for the superconducting vortex in the plane Josephson junction. The function φ(φ_x) is derived. Here φ is the total magnetic flux in the hole. This function differentiates from the same function in the case of the ring closed. with the point contact.     

Controlled vortex motion in multiple interpenetrating pinning arrays

We study the effect of multiple interpenetrating pinning arrays on the vortex motion in the presence of an ac driving force, f _d (t), by using extensive molecular dynamics (MD) simulations. Firstly, the response to a square ac wave f _d (t) has been explored for the vortices interacting with a periodic square pinning array which has different pinning strengths and sizes. The effect of the type of an ac drive and its amplitude on the oscillatory dynamics of vortices have been investigated in detail. For very low displacements of the vortices, we have found that the single-particle model can produce results analytically similar to the ones obtained by the MD simulations. It is shown that the collective motion of vortices can be controlled easily by varying the number of multiple interpenetrating square pinning lattices (N _SPSL). A regular sequence of peaks has been observed for N _SPSL = 3 in the time evolution of the average velocity of the vortices (i.e., V? _{x - t} curves). The number of peaks (N _peak) strongly depends on the magnitude of f _d (t), and increases with increasing the magnitude of f _d . The close relation between N _peak and f _d is considered as an indication of controlling vortex motion in a multiple periodic pinning structure. Finally, the variation of the power spectrum of noise S(ν) with N _SPSL has been investigated. For N _SPSL = 3, it has been found that the plastic motion of the row of vortices evolves at low frequencies, i.e., 1/ν behavior, whereas, at high frequencies, S(ν) shows a typical behavior of Gaussian white noise.     

Flux vortex pinning in a simple model system

Results of numerical calculations of flux vortex pinning by random one and two dimensional arrays of identical pinning centres are reported. Three different methods for determining the critical state flux gradient are described and the critical state concept is shown to break down for samples smaller than a certain size.     

Quenched Fe moment in the collapsed tetragonal phase of Ca_(1-x)Pr_x Fe_2As_2

We report 75As NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1-xPrxFe2As2 . In both cases of x = 0.075, 0.15, a large increase of νq upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structure transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/75T1 is enhanced and forms a peak at T ≈ 25 K, which may be caused by the magnetic ordering of the Pr3+ moments or spin dynamics of mobile domain walls.     

Metamagnetic transitions and anomalous magnetoresistance in EuAg_4As_2 crystals

In this work,we systematically studied the magnetic and transport properties of EuAg4As2 single crystals.It was found that the two antiferromagnetic transitions(TN1=10 K and TN2=15 K)were driven to lower temperatures by an applied magnetic field.Below TN1,two successive metamagnetic transitions were observed when a magnetic field was applied in the ab plane(H//abplane).For both H//ab and H//c,EuAg4As2 showed a positive,unexpectedly large magnetoresistance(up to 202%)in lower magnetic fields below TN1,and a large negative magnetoresistance(up to-78%)at high fields/intermediate temperatures,thus presenting potential applications in magnetic sensors.Finally,the magnetic phase diagrams of EuAg4As2 were constructed for both H//ab and H//c using the resistivity and magnetisation data.     

Multiple magnetic transitions in single crystals of Ce_(12)Fe_(57.5)As_(41) and La_(12)Fe_(57.5)As_(41)

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce_(12)Fe_(57.5)As_(41)and La_(12)Fe_(57.5)As_(41).The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds.Both compounds exhibited multiple magnetic orders within 2-300 K and metamagnetic transitions at various fields.Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce_(12)Fe_(57.5)As_(41)and La_(12)Fe_(57.5)As_(41),respectively,followed by antiferromagnetic type spin reorientations near Curie temperatures.The magnetic properties underwent complex evolution in the magnetic field for both compounds.An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce_(12)Fe_(57.5)As_(41).The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure.A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce_(12)Fe_(57.5)As_(41).A temperature-field phase diagram was present for these two rare earth systems.In addition,a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150-300 K,which is rarely found in 3D-based compounds.It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.     

Nonequilibrium dynamics in type-II superconductors with inhomogeneous vortex pinning

We study numerically the dynamics relating to negative vortex motion in inhomogeneous pinning systems. We show that this dynamical phenomenon results from the internal field effect produced by the growing local barriers with decreasing temperature. We find that the negative motion is characterized by a peak of negative voltage or resistance in resistance–temperature transport measurements. We also demonstrate that the time window to observe the negative motion is determined by the magnitude of driving force in addition to the temperature scanning rate.     

Mode locking in ac-driven vortex lattices with random pinning

We find mode-locking steps in simulated current-voltage characteristics of ac-driven vortex lattices with random pinning. For low frequencies there is mode locking above a finite ac force amplitude, while for large frequencies there is mode locking for any small ac force. This is correlated with the nature of temporal order in the different regimes in the absence of ac drive. The mode-locked state is a frozen solid pinned in the moving reference of frame, and the depinning from the step shows plastic flow and hysteresis.     

Effective temperature in driven vortex lattices with random pinning

We study numerically correlation and response functions in nonequilibrium driven vortex lattices with random pinning. From a generalized fluctuation-dissipation relation, we calculate an effective transverse temperature in the fluid moving phase. We find that the effective temperature decreases with increasing driving force and becomes equal to the equilibrium melting temperature when the dynamic transverse freezing occurs. We also discuss how the effective temperature can be measured experimentally from a generalized Kubo formula.