Conventional type-II Superconductors Research Articles

Crystal orientation-dependent superconductivity in titanium nitride films

High-quality titanium nitride (TiN) films with different crystal orientations (001, 110, and 111) obtained under the same growth conditions are systematically investigated by both ultra-low temperature scanning tunneling microscope/spectroscopy and transport experiments. Our results reveal that all of them are conventional type-II superconductors, which exhibit spatially homogeneous superconducting properties. The superconductivity is uniform between surface and bulk. Intriguingly, the TiN (111) film has the highest transition temperature (Tc) but the lowest upper critical field (Hc2). This crystal orientation-dependent superconductivity could be explained by the fact that TiN (001) and TiN (110) films are dirtier than TiN (111). Our results suggest that (111)-oriented TiN is superior to design certain superconducting devices, including Josephson junction devices. The crystallographic orientation could offer an effective controlling parameter for designing TiN-based superconducting devices.

A Quenched Disorder in the Quantum-Critical Superconductor CeCoIn5.

Emergent inhomogeneous electronic phases in metallic quantum systems are crucial for understanding high-Tc superconductivity and other novel quantum states. In particular, spin droplets introduced by nonmagnetic dopants in quantum-critical superconductors (QCSs) can lead to a novel magnetic state in superconducting phases. However, the role of disorders caused by nonmagnetic dopants in quantum-critical regimes and their precise relation with superconductivity remain unclear. Here, the systematic evolution of a strong correlation between superconductive intertwined electronic phases and antiferromagnetism in Cd-doped CeCoIn5 is presented by measuring current-voltage characteristics under an external pressure. In the low-pressure coexisting regime where antiferromagnetic (AFM) and superconducting (SC) orders coexist, the critical current (Ic ) is gradually suppressed by the increasing magnetic field, as in conventional type-II superconductors. At pressures higher than the critical pressure where the AFM order disappears, Ic remarkably shows a sudden spike near the irreversible magnetic field. In addition, at high pressures far from the critical pressure point, the peak effect is not suppressed, but remains robust over the whole superconducting region. These results indicate that magnetic islands are protected around dopant sites despite being suppressed by the increasingly correlated effects under pressure, providing a new perspective on the role of quenched disorders in QCSs.

Unusual superconductivity in the topological nodal-line semimetal candidate Sn x NbSe2-δ

We report the superconductivity of the topological nodal-line semimetal candidate Sn x NbSe2-δ with a noncentrosymmetric crystal structure. The superconducting transition temperature T c of Sn x NbSe2-δ drastically varies with the Sn concentration x and the Se deficiency δ, and reaches 12 K, relatively higher than those of known topological superconductors. The upper critical field of this compound shows unusual temperature dependence, inconsistent with the WHH theory for conventional type-II superconductors. In a low-Tc sample, the zero-temperature limit of the upper critical field parallel to the ab plane exceeds the Pauli paramagnetic limit estimated from the simple BCS weak coupling model by a factor of ∼ 2, suggestive of unusual superconductivity stabilized in Sn x NbSe2-δ . Together with the robust superconductivity against disorder, these observations indicate that Sn x NbSe2-δ is a promising candidate to explore topological superconductivity.

Universality and unconventional enhancement of flux-flow resistivity in Ba(Fe1−xCox)2As2

Measurements of the current-voltage characteristics were performed on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with doping level $0.044 \leq x \leq 0.1$. An unconventional increase in the flux-flow resistivity $\rho_{\rm ff}$ with decreasing magnetic field was observed across this doping range. Such an abnormal field dependence of flux-flow resistivity is in contrast with the linear field dependence of $\rho_{\rm ff}$ in conventional type-II superconductors, but is similar to the behavior recently observed in the heavy-fermion superconductor CeCoIn$_5$. A significantly enhanced $\rho_{\rm ff}$ was found for the x=0.06 single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized $\rho_{\rm ff}$ scales with normalized field and temperature. The doping level dependence of these parameters strongly suggests that the abnormal upturn flux-flow resisitivity is likely related to the enhancement of spin fluctuations around the vortex cores of the optimally doped samples.

Pressure Effect on the Einstein-Like Phonon Mode in Superconducting $$\hbox {YB}_{6}$$ YB 6

$$\hbox {YB}_{6}$$ is known as a conventional type-II BCS superconductor in which the strong coupling superconductivity with $$2\Delta /k_{B}T_{c}\approx 4.1$$ is mediated by the Einstein-like phonon mode of Y atoms located at $$\hbar \omega _{E}\approx 8\,\hbox {meV}$$ . We have investigated the pressure effect on $$\omega _{E}$$ by Raman scattering up to 14 GPa. The received linear pressure increase in this mode $$d\omega _{E}/dp\approx $$ 0.21 meV/GPa yields an isothermal Gruneisen coefficient $$\gamma _{T}= -\partial \hbox {ln}\omega _{E}/\partial \hbox {ln} V = 3.85$$ . Moreover, the pressure effect on the electron–phonon interaction $$\lambda $$ calculated from the McMillan–Allen–Dynes expression for the superconducting transition temperature was determined to be $$\partial \hbox {ln}{\lambda }/\partial \hbox {ln}V \cong 7.2$$ .

Giant paramagnetic Meissner effect in multiband superconductors.

Superconductors, ideally diamagnetic when in the Meissner state, can also exhibit paramagnetic behavior due to trapped magnetic flux. In the absence of pinning such paramagnetic response is weak, and ceases with increasing sample thickness. Here we show that in multiband superconductors paramagnetic response can be observed even in slab geometries, and can be far larger than any previous estimate - even multiply larger than the diamagnetic Meissner response for the same applied magnetic field. We link the appearance of this giant paramagnetic response to the broad crossover between conventional Type-I and Type-II superconductors, where Abrikosov vortices interact non-monotonically and multibody effects become important, causing unique flux configurations and their locking in the presence of surfaces.

How the macroscopic current correlates with the microscopic flux-line distribution in a type-II superconductor: an experimental study

We present a study of the real-space flux-line lattice (FLL) of pristine and neutron irradiated conventional type-II superconductors using scanning tunnelling microscopy. Our work is focused on the magnetic field range, where the critical current density shows a second peak as a result of neutron irradiation. Scanning tunnelling microscopy images, including more than 2000 flux lines, are used to evaluate various microscopic parameters describing the disorder of the FLL, such as the defect density, the nearest neighbour distances and correlation functions. These parameters are compared with the macroscopic critical current density of the samples. The results show a direct correlation of the micro- and macroscopic properties. We observe a clear transition from an ordered to a disordered lattice at the onset of the second peak. Moreover, we discuss the defects of the FLL and their accumulation to large clusters in the second peak region.

Temperature dependence of the upper critical field of FeSe single crystals

We present a careful study of the resistive superconducting transition in FeSe single crystals down to $T=40$ mK in continuous magnetic fields up to 30 T applied perpendicular and parallel to the $ab$ plane. In the $\mathbf{H}\ensuremath{\parallel}\mathbf{c}$ geometry the temperature dependence of the resistive upper critical field ${H}_{c2}^{*}$, determined as the field at which the in-plane resistivity in the transition region is 90$%$ of the normal state resistivity is down to temperatures $T/{T}_{c}<0.006$, is in close agreement with the Werthamer-Helfand-Hohenberg (WHH) theoretical curve which describes the behavior of the upper critical field in conventional type-II superconductors. In contrast, for the $\mathbf{H}\ensuremath{\parallel}\mathbf{ab}$ geometry, the data depart from the WHH model with increasing applied magnetic field according to the paramagnetic limitation of superconductivity. An anisotropy parameter $\ensuremath{\gamma}$ in our FeSe crystals decreases with decreasing temperature and FeSe becomes nearly isotropic when the temperature $T\ensuremath{\rightarrow}0$.

New germanide superconductors with the type-I clathrate type structure

We discovered that the new germanide superconductors of A8(Al,Ge)46 (A=Rb, Cs). A8(Al,Ge)46 have the type-I clathrate type structure (cubic, space group: Pm3¯n (No. 223)). The superconducting transition temperature is confirmed at around 3.6K for Rb8(Al0.4Ge0.6)46 and 4K for Cs8(Al0.4Ge0.6)46, respectively. The lattice constants of these compounds are estimated to be a=1.082(6)nm for Rb8(Al0.4Ge0.6)46 and a=1.087(8)nm for Cs8(Al0.4Ge0.6)46. The M–H curve at 1.8K shows that the Rb8(Al0.4Ge0.6)46 is categorized to be a conventional type-II superconductor. The superconducting parameters of Rb8(Al0.4Ge0.6)46 are determined using the thermodynamic data and Ginzburg–Landau relations.

Anomalous metallic state above the upper critical field of the conventional three-dimensional superconductor AgSnSe2with strong intrinsic disorder

We report superconducting properties of AgSnSe2 which is a conventional type-II superconductor in the very dirty limit due to intrinsically strong electron scatterings. While this material is an isotropic three-dimensional (3D) superconductor with a not-so-short coherence length where strong vortex fluctuations are NOT expected, we found that the magnetic-field-induced resistive transition at fixed temperatures becomes increasingly broader toward zero temperature and, surprisingly, that this broadened transition is taking place largely ABOVE the upper critical field determined thermodynamically from the specific heat. This result points to the existence of an anomalous metallic state possibly caused by quantum phase fluctuations in a strongly-disordered 3D superconductor.

Superconductivity in Ba(TM,Si)2(TM = Pd, Pt, Cu, Ag, and Au) with the AlB2-Type Structure

We succeeded in synthesizing BaTM x Si 2- x (TM = Pd, Pt, Cu, Ag, and Au) superconductors with the AlB 2 -type structure and performed detailed thermodynamic measurements of the superconducting state in BaTM x Si 2- x to determine the superconducting parameters [ H c1 (0), H c2 (0), λ(0), ξ(0), and κ GL ] using Ginzburg–Landau relations. The observed parameters indicated that BaTM x Si 2- x can be categorized into a conventional type-II superconductor. The AlB 2 -type structure appears at x between 0.2 and 0.8, and the lattice parameters systematically changed with x . The superconductivity was observed at a TM concentration x = 0.2–0.6. T c was almost constant between x = 0.2 and 0.3 and observed to decrease with increasing TM concentration (0.3 ≤ x ≤0.6). The specific heat data indicated that the superconductivity in BaTM x Si 2- x exists in a weak coupling regime in the BCS theory. Moreover, we discussed the role of the TM element in BaTM x Si 2- x using the observed data.

Superconducting state in (W, Ta)5SiB2

We characterize the superconducting state in a boro-silicide (W, Ta)5SiB2, with T c of 6.5 K by means of magnetization, electrical resistivity, and specific heat measurements. As x increased, the transition temperature T c abruptly enhances from 5.8 to 6.5 K. The magnetization versus magnetic field ( M - H ) curve indicated that (W, Ta)5SiB2 was a conventional type-II superconductor. The estimated lower critical field H c1(0) and upper critical field H c2( T ) are about 121 Oe and 14.7 kOe, respectively. The penetration depth λ(0) and coherence length ξ(0) are calculated to be approximately 369 and 14.9 nm, respectively, using Ginzburg-Landau (GL) equations. Specific heat data shows the superconductivity in W4.5Ta0.5SiB2 belongs to a week-coupling BCS superconductor. Finally, we discuss the increasing of T c in of (W, Ta)5SiB2 system.

Spectroscopic Evidence for Competing Order-Induced Pseudogap Phenomena and Unconventional Low-Energy Excitations in High-T c Cuprate Superconductors

The low-energy excitations of cuprate superconductors exhibit various characteristics that differ from those of simple Bogoliubov quasiparticles for pure d_{x^2-y^2}-wave superconductors. Here we report experimental studies of spatially resolved quasiparticle tunnelling spectra of hole- and electron-type cuprate superconductors that manifest direct evidences for the presence of competing orders (COs) in the cuprates. In contrast to conventional type-II superconductors that exhibit enhanced local density of states (LDOS) peaking at zero energy near the centre of field-induced vortices, the vortex-state LDOS of YBa_2Cu_3O_{7-\delta} (Y-123) and La_{0.1}Sr_{0.9}CuO_2 (La-112) remains suppressed inside the vortex core, with pseudogap (PG)-like features at an energy larger (smaller) than the superconducting (SC) gap \Delta_{SC} in Y-123 (La-112). Energy histograms of the SC and PG features reveal steady spectral shifts from SC to PG with increasing magnetic field H. These findings may be explained by coexisting COs and SC: For hole-type cuprates with PG above T_c, the primary CO gap (V_{CO}) is larger than \Delta_{SC} and the corresponding COs are charge/pair-density waves with wave-vectors parallel to (\pi,0)/(0,\pi). For electron-type cuprates without PG above T_c, V_{CO} is smaller than \Delta_{SC} and the CO wave-vector is along (\pi,\pi). This CO scenario may be extended to the ARPES data to consistently account for the presence (absence) of Fermi arcs in hole- (electron)-type cuprates. Fourier transformation of the vortex-state LDOS in Y-123 further reveals multiple sets of energy-independent wave-vectors due to field-enhanced pair- and spin-density waves. These results imply important interplay of SC with low-energy collective excitations.

Nernst effect of a new iron-based superconductor LaO1−xFxFeAs

We report the first Nernst effect measurement on a new iron-based superconductor LaO1−xFxFeAs (x=0.1). In the normal state, the Nernst signal is negative and very small. Below Tc, a large positive peak caused by vortex motion is observed. The flux flowing regime is quite large compared to the conventional type-II superconductors. The sharp decrease in the Nernst signal at the depairing magnetic field Hc2 is not obvious even for temperatures close to Tc. Furthermore, a clear deviation of the Nernst signal from normal state background and an anomalous suppression of off-diagonal thermoelectric current in the normal state between Tc and 50 K are observed. We argue that this anomaly in the normal state Nernst effect could correlate with the spin-density wave (SDW) fluctuations although the contribution of superconducting fluctuations cannot be excluded.

Ni substitution effect on structure and superconductivity properties in Li 2(Pd 1−xNi x) 3B boride system

The crystal structure and basic superconductivity parameters in Li 2(Pd 1− x Ni x ) 3B are systematically studied by means of X-ray diffraction method, transport properties and magnetization measurement. The limiting solubility of Ni substitution at Pd site was less than 0.2. The lattice parameter a and superconducting transition temperature T c decrease gradually with increasing Ni content x. Low critical field H c1 and upper critical field H c2 were estimated from the isothermal magnetization ( M– H) measurements, and the results showed a decreased tendency with increase in x. From H c1(0) and H c2(0), the coherence length ξ(0), penetration depth λ(0), Ginzburg–Landau parameter κ(0) and thermodynamic critical field H c(0) were estimated and the results are compared with the related systems. The present results supported that Li 2(Pd 1− x Ni x ) 3B system is a conventional type-II superconductor.

Magnetic susceptibility in a superconducting coaxial cylindrical multi-shell structure

Although an intergranular superconducting system is definitely different from a conventionaltype-II superconductor in several aspects such as flux shape, flux size, and flux behaviors,its magnetic properties have been described by the critical state model in an equivalentmanner to those of a type-II superconductor’s. Instead of the critical state model, wesuggest another primitive model which is able to describe the magnetic properties of anintergranular system. We have made a model calculating the magnetic susceptibility of asuperconducting coaxial cylindrical multi-shell structure zero-field-cooled under an externalfield and have applied the model to susceptibility versus temperature data of aYBa2Cu3Ox polycrystal. Calculation results by this model concurred well with the experimental data. Aphysical view point of this model is that an intergranular superconducting systemzero-field-cooled under an external field can be treated to be a hypothetical coaxialmulti-shell structure.

Superconductivity in ternary silicide NaAlSi with layered diamond-like structure

We have succeeded in synthesizing a new member of ternary silicide NaAlSi with a superconducting transition temperature T c−onset = 7 K by high pressure technique. The crystal structure of NaAlSi is the layered diamond-like structure with a space group of P4/ nmm (no. 129, Z = 3) and without inversion symmetry. The superconducting properties in NaAlSi have been investigated by means of DC and AC magnetic susceptibility measurements. These magnetic responses indicate that NaAlSi belongs to a conventional type-II superconductor with a Ginzbrug–Landau parameter κ = 13.8. From the band structure calculations, we found that the hole band of Si p- and electron Al s-orbitals play an important role for the superconductivity in this system.

Scanning magnetic imaging ofSr2RuO4

We magnetically imaged the $ab$-plane surface of single crystals of the unconventional superconductor ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$, including one sample with an array of microholes, using scanning superconducting quantum interference device and Hall probe microscopy in a dilution refrigerator at low applied magnetic fields. The images show dilute trapped vortices, as would be expected in conventional type-II superconductors, and no other magnetic features. We found no direct signs of the spontaneous magnetization that would be expected in a time-reversal symmetry-breaking (TRSB) superconductor. These measurements set upper limits on the presence of TRSB signatures in this material.

Shrinking Magnetic Vortices inV3Sidue to Delocalized Quasiparticle Core States: Confirmation of the Microscopic Theory for Interacting Vortices

We report muon spin rotation measurements on the conventional type-II superconductor V3Si that provide clear evidence for changes to the inner structure of a vortex due to the delocalization of bound quasiparticle core states. The experimental findings described here confirm a key prediction of recent microscopic theories describing interacting vortices. The effects of vortex-vortex interactions on the magnetic and electronic structure of the vortex state are of crucial importance to the interpretation of experiments on both conventional and exotic superconductors in an applied magnetic field.

Ullah-Dorsey Scaling of Fluctuation Conductivity Near the Superconducting Transition in (LaSe)1.14(NbSe2)

Magnetotransport measurements with the quasi two-dimensional misfit-layer compound (LaSe)1.14(NbSe2) with critical temperature Tc = 1.23 K are presented. The temperature dependence of the upper critical magnetic field Hc2 has been determined from an analysis of magnetoresistance data through the scaling procedure obtained theroretically by Ullah and Dorsey for the fluctuation conductivity. We show that in contrast to a direct determination of Hc2(T) with a positive curvature the temperature dependence of Hc2 obtained via the scaling procedure reveals a negative curvature in agreement with the Werthammer-Helfand-Hohenberg theory for conventional type-II superconductors.