Superconductor Ba0 Research Articles

Two-Dimensional Superconductivity and Anomalous Vortex Dissipation in Newly Discovered Transition Metal Dichalcogenide-Based Superlattices.

Properties of layered superconductors can vary drastically when thinned down from bulk to monolayer owing to the reduced dimensionality and weakened interlayer coupling. In transition metal dichalcogenides (TMDs), the inherent symmetry breaking effect in atomically thin crystals prompts novel states of matter such as Ising superconductivity with an extraordinary in-plane upper critical field. Here, we demonstrate that two-dimensional (2D) superconductivity resembling those in atomic layers but with more fascinating behaviors can be realized in the bulk crystals of two new TMD-based superconductors Ba0.75ClTaS2 and Ba0.75ClTaSe2 with superconducting transition temperatures 2.75 and 1.75 K, respectively. They comprise an alternating stack of H-type TMD layers and Ba-Cl layers. In both materials, intrinsic 2D superconductivity develops below a Berezinskii-Kosterlitz-Thouless transition. The upper critical field along the ab plane () exceeds the Pauli limit (μ0Hp); in particular, Ba0.75ClTaSe2 exhibits an extremely high ≈ 14 μ0Hp and a colossal superconducting anisotropy (/) of ∼150. Moreover, the temperature-field phase diagram of Ba0.75ClTaSe2 under an in-plane magnetic field contains a large phase regime of vortex dissipation, which can be ascribed to the Josephson vortex motion, signifying an unprecedentedly strong fluctuation effect in TMD-based superconductors. Our results provide a new path toward the establishment of 2D superconductivity and novel exotic quantum phases in bulk crystals of TMD-based superconductors.

Superconducting state in Ba1−x Sr x Ni2As2 near the quantum critical point

In the phase diagram of the nickel-based superconductor Ba1−x Sr x Ni2As2, T c has been found to be enhanced sixfold near the quantum critical point (QCP) x = 0.71 compared with the parent compound. However, the mechanism is still under debate. Here, we report a detailed investigation of the superconducting properties near the QCP (x ≈ 0.7) by utilizing scanning tunneling microscopy and spectroscopy. The temperature-dependent superconducting gap and magnetic vortex state were obtained and analyzed in the framework of the Bardeen–Cooper–Schrieffer model. The ideal isotropic s-wave superconducting gap excludes the long-speculated nematic fluctuations while preferring strong electron–phonon coupling as the mechanism for T c enhancement near the QCP. The lower than expected gap ratio of Δ/(k B T c) is rooted in the fact that Ba1−x Sr x Ni2As2 falls into the dirty limit with a serious pair breaking effect similar to the parent compound.

Theoretical Investigation of the Interplay of Superconductivity and Magnetism in Ba1−xKxFe2As2 Superconductor in a Two-Band Model by Using the Bogoliubov Transformation Formalism

The main focus of this article is to investigate the theoretical interplay of magnetism and superconductivity in a two-band model for the iron-based superconductor Ba1−xKxFe2As2. On the basis of experimental results, the two-band model Hamiltonian was considered. We obtained mathematical statements for the superconductor Ba1−xKxFe2As2 superconducting (SC) transition temperature, spin-density-wave (SDW), transition temperature, superconductivity order parameter, and SDW order parameter from the Bogoliubov transformation formalism and the model Hamiltonian. Furthermore, an expression for the dependence of the SDW transition temperature on the SDW order parameter and the dependence of the SC transition temperature on the SDW order parameter was obtained for Ba1−xKxFe2As2. By substituting the experimental and theoretical values of the parameters in the derived statements, phase diagrams of the SC transition temperature versus the SDW order parameter and the SDW transition temperature versus the SDW order parameters have been plotted to demonstrate the dependence of the SDW order parameter on transition temperatures. By combining the two-phase diagrams, we depicted the possible coexistence of superconductivity and magnetism for the Ba1−xKxFe2As2 superconductor. The phase diagrams of temperature versus SC order parameter and temperature versus SDW order parameter were also plotted to show the dependence of order parameters on temperature for the Ba1−xKxFe2As2 superconductor.

Theoretical Study of Some Properties of High‐Temperature Two‐Band Model Iron‐Based Superconductor Ba1−xRbxFe2As2

This work focuses on the theoretical investigations of some properties of high‐temperature two‐band model iron‐based superconductor Ba1−xRbxFe2As2. The mathematical expressions of superconducting parameters are treated by the temperature‐dependent Green's function formalism. The superconducting order parameters of the material for electron, hole bands and for the interband are plotted as a function of temperature. The parameters decrease with increasing temperature and vanish at the superconducting transition temperature (Tc) of Ba1−xRbxFe2As2. The values of the superconducting order parameters and coupling strength in electron intraband, hole intraband, and the electron–hole interband are computed quantitatively. It is found that the pairing potential of Ba1−xRbxFe2As2 increases as a function of temperature. The phase diagrams of temperature‐dependent electron and hole intrabands and density of states versus excitation energy are plotted and it is observed that both decrease as the excitation energy increases. The impacts of temperature, pairing potential, and transitional temperature on condensation energy are also investigated and the results show that condensation energy decreases with the increase of temperature and pairing potential and vanishes at Tc of Ba1−xRbxFe2As2. Furthermore, the density of states is plotted for electron and hole‐intrabands at different temperatures versus excitation energy, and it is perceived that the density of states decreased gradually as the excitation energy increases and vanished at low‐temperature values. The current results are compatible with previous findings.

Investigation of the Interplay Between Spin Density Wave and Superconductivity in High‐Temperature Iron‐Based Superconductor Ba1−xRbxFe2As2

The current research work deals with the theoretical investigation of the interplay between spin density wave (SDW) and superconductivity in a two‐band model high‐temperature iron‐based superconductor by using the double‐time temperature‐dependent Green's function formalism. The superconductivity transition temperature () versus the spin density wave order parameter () for the electron and hole intrabands as well as the interband for the iron‐based superconductor has been plotted. With an increase in of the material, the superconductivity transition temperature decreases. In both intra‐ and interband pairings, the temperature at which the spin density wave transition () rises with increasing . The interplay between superconductivity and spin density wave of the electron and hole intraband interactions for . Furthermore, the phase diagram of the spin density wave parameter () versus temperature has been plotted, and the spin density wave parameter () is observed to decrease as the temperature rises and vanishes at the spin density wave transition temperature ().

Theoretical Investigation of the Superconducting and Thermodynamic Properties of Two-Band Model High-Temperature Iron-Based Superconductor Ba1–xNaxFe2As2

This work presents the theoretical investigation of the superconducting and thermodynamic properties of the two-band model high-temperature iron-based superconductor Ba1-xNaxFe2As2. By developing a model Hamiltonian and by employing the well-knownvdouble-time temperature-dependent Green’s function formalism, we have computed the superconducting orderbparameters for the electron and hole intra- and inter-band transitions, superconducting transition temperature, densitiesbof states, and condensation energies. Furthermore, the electronic specific heat and the entropy for electron and hole intra-band transitions have been determined. By using appropriate experimental data and some credible approximations of the parameters in the computed expressions, we have found the phase diagrams of superconducting order parameters versus the temperature, superconducting critical temperature versus the inter-band interaction potential, temperature dependences of the electronic specific heat and entropy for electron and hole intra-band transitions, and densities of states for the electron and hole intra-band transitions as functions of the excitation energy at different values of the temperature. Finally, the phase diagrams of the condensation energy versus the temperature, inter-band pairing potential at T = 0 K versus the condensation energy, and condensation energy versus the superconducting transition temperature (TC) have been drawn. In some of the phase diagrams, the comparison between theoretical and experimental values has been made. The results are in a good agreement with previous findings.

Tracking the Meissner Hole and Relaxation of Magnetization in (Ba,Rb)Fe2As2

We investigated the unstable interface between vortices and anti-vortices in single crystals of iron-based superconductors (Ba0.67Rb0.33)Fe2As2 (Tc ∼ 37.6 K). These interfaces, also known as the Meissner hole, were captured with magneto-optical imaging in both pristine crystal and one with artificial point defects. Local excess current were observed flowing along the interface. In addition, we confirmed that the creep mode of such an interface follows the prediction of the critical state model, accompanied with a finite relaxation rate of magnetization.

Terahertz pulse-driven collective mode in the nematic superconducting state of Ba1\u2212xKxFe2As2

We investigate the collective mode response of the iron-based superconductor Ba1−xKxFe2As2 using intense terahertz (THz) light. In the superconducting state a THz Kerr signal is observed and assigned to nonlinear THz coupling to superconducting degrees of freedom. The polarization dependence of the THz Kerr signal is remarkably sensitive to the coexistence of a nematic order. In the absence of nematic order the C4 symmetric polarization dependence of the THz Kerr signal is consistent with a coupling to the Higgs amplitude mode of the superconducting condensate. In the coexisting nematic and superconducting state the signal becomes purely nematic with a vanishing C4 symmetric component, signaling the emergence of a superconducting collective mode activated by nematicity.

Dispersion of neutron spin resonance mode in Ba0.67K0.33Fe2As2 * *Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0704200, 2018YFA0305602, 2017YFA0303100, and 2017YFA0302900), the National Natural Science Foundation of China (Grant Nos. 11822411 and 11961160699), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (CAS) (Grant Nos. XDB25000000 and XDB07020300), K.C.Wong Education

We report an inelastic neutron scattering investigation on the spin resonance mode in the optimally hole-doped iron-based superconductor Ba0.67K0.33Fe2As2 with Tc= 38.2 K. Although the resonance is nearly two-dimensional with peak energy E R ≈ 14 meV, it splits into two incommensurate peaks along the longitudinal direction ([H,0,0]) and shows an upward dispersion persisting to 26 meV. Such dispersion breaks through the limit of total superconducting gaps Δ tot = |Δk | + |Δ k+Q | (about 11–17 meV) on nested Fermi surfaces measured by high resolution angle resolved photoemission spectroscopy (ARPES). These results cannot be fully understood by the magnetic exciton scenario under s±-pairing symmetry of superconductivity, and suggest that the spin resonance may not be restricted by the superconducting gaps in the multi-band systems.

Coexistence of magnetism and superconductivity in thin films of the Fe-based superconductor Ba1−xLaxFe2As2

Magnetization measurements have been performed to understand the role of the magnetic structure on the superconducting properties of epitaxial thin films of Ba1−xLaxFe2As2 (x = 0.08, 0.13, and 0.18) deposited on single crystal (001)-oriented MgO substrates by pulsed laser deposition. All samples exhibit a reentrant-spinglass like behavior at normal state. At lower temperatures, we observe the same magnetic state coexisting with superconductivity and it is also observed a prominent non-linear giant diamagnetism in an intermediate temperature range just above the superconducting phase transition temperature. Furthermore, no significant change in the magnetic domain structure was detected by the onset of superconductivity. Based on their magnetic states, we claim that each domain (as a disconnected superconducting island) has its own bulk superconducting properties. Finally, we discussed the dual character played by the La atoms in the superconducting properties. That duality character has been also confirmed by analyzing resistivity data.

Orbital selectivity of layer-resolved tunneling in the iron-based superconductor Ba0.6K0.4Fe2As2

We use scanning tunneling microscopy/spectroscopy (STM/S) to elucidate the Cooper pairing of the iron pnictide superconductor Ba0.6K0.4Fe2As2. By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer-identity. By cross-referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct Fe-3d orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a new and general concept that, for complex multi-orbital material, tunneling on different terminating layers can feature orbital selectivity.

31P NMR studies of an iron-based superconductor Ba0.5Sr0.5Fe2(As1−x P x )2 with T c = 29 K

We report 31P NMR studies of an oriented polycrystalline superconductor of Ba0.5Sr0.5Fe2(As1−x P x )2 with x ∼ 0.4 (T c = 29 K) at T = 14−325 K and B 0 = 1 and 5 T. The 31P Knight shift Kab at B 0 ⊥ c shows a nearly T-independent uniform spin susceptibility above T c and a spin singlet decrease below T c. The 31P nuclear spin-lattice relaxation rate 1/T 1 shows an asymptotic behavior of a + bT (a and b are constants) at T > 100 K and the minimum at 40 K with an upturn toward T c. The a term in 1/T 1 indicates the presence of two-dimensional antiferromagnetic spin fluctuations. The negative θ = −15 K of the Curie-Weiss-type antiferromagnetic spin susceptibility in the analysis of 1/T 1 T suggests antiferromagnetic instability in the superconducting state. Discussions are made from the self-consistent renormalization (SCR) theory for the spin fluctuations with interlayer correlation.

Synthesis of Iron-Based Superconductor Ba0.6K0.4Fe2As2 by Mechanical Alloying

A polycrystalline sample of superconductor Ba0.6K0.4Fe2As2 was synthesized by mechanical alloying, which was confirmed by the X-ray diffraction analysis. The sample heat treatment time is reduced more than twice in comparison with the heat treatment time in the solid-phase synthesis method. The superconducting transition temperature of the synthesized sample is 37 K, which is close to the critical temperature for the Ba0.6K0.4Fe2As2 single crystal.

Comparative study of vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals

We investigate the vortex dynamics in two typical hole doped iron based superconductors CaKFe4As4 (CaK1144) and Ba0.6K0.4Fe2As2 (BaK122) with similar superconducting transition temperatures. It is found that the magnetization hysteresis loop exhibits a clear second peak effect in BaK122 in wide temperature region while it is absent in CaK1144. However, a second peak effect of critical current density versus temperature is observed in CaK1144, which is however absent in BaK122. The different behaviors of second peak effect in BaK122 and CaK1144 may suggest distinct origins of vortex pinning in different systems. Magnetization and its relaxation have also been measured by using dynamical and conventional relaxation methods for both systems. Analysis and comparison of the two distinct systems show that the vortex pinning is stronger and the critical current density is higher in BaK122 system. It is found that the Maley’s method can be used and thus the activation energy can be determined in BaK122 by using the time dependent magnetization in wide temperature region, but this is not applicable in CaK1144 system. Finally we present the different regimes with distinct vortex dynamics in the field-temperature diagram for the two systems.

Microscopic origin of Cooper pairing in the iron-based superconductor Ba1\u2212xKxFe2As2

Resolving the microscopic pairing mechanism and its experimental identification in unconventional superconductors is among the most vexing problems of contemporary condensed matter physics. We show that Raman spectroscopy provides an avenue towards this aim by probing the structure of the pairing interaction at play in an unconventional superconductor. As we study the spectra of the prototypical Fe-based superconductor Ba1−xKxFe2As2 for 0.22 ≤ x ≤ 0.70 in all symmetry channels, Raman spectroscopy allows us to distill the leading s-wave state. In addition, the spectra collected in the B1g symmetry channel reveal the existence of two collective modes which are indicative of the presence of two competing, yet sub-dominant, pairing tendencies of d_{x^2 - y^2} symmetry type. A comprehensive functional Renormalization Group and random-phase approximation study on this compound confirms the presence of the two sub-leading channels, and consistently matches the experimental doping dependence of the related modes. The consistency between the experimental observations and the theoretical modeling suggests that spin fluctuations play a significant role in superconducting pairing.

Structural phase transitions, phase separation and physical properties for the 122-system iron-based superconductors

The discovery of iron-based superconductors stimulated the second research upsurge for high-temperature superconductors, which have been considered as one of the significant research field from both academic and technological points of view. It is noted that the studies of iron-based superconductors have brought a great development of a large number of experimental technologies and theoretical researches in superconductivity physics. The remarkable correlations between structural transitions and multiple ordered states have been observed in the typical iron-based superconductors. According to the microstructure analyses and in-situ transmission electron microscopy (TEM) investigations, AFe2As2 (A=Ba, Sr and Ca) iron-based superconducting materials often show up a structural transition from tetragonal phase to orthorhombic phase from room temperature down to 20 K, resulting in visible twinning domains in the orthorhombic phase. The tetragonal SrFe2As2 samples, consistent with X-ray and neutron-diffraction data, undergo the tetragonal- orthorhombic phase transition at about 205 K and show clear twin domains in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudoperiodic structural modulation with a periodicity of around 40 nm at room temperature. This quasi-periodicity structural modulation is likely related to the local structural distortions within the Ca layers. In situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase. Phase separation and structural inhomogeneity as critical structural issues have been extensively investigated in a variety of strongly correlated systems. The phase separations associated with structural domains result visibly structural alterations in K y Fe2− x Se2 system. Structural investigations by means of TEM on K0.8Fe x Se2 and KFe x Se2, with 1.5≤ x ≤1.8, have revealed a rich variety of microstructure phenomena. Materials with 1.5≤ x ≤1.6 often show a superstructure modulation along the [310] zone-axis direction, and this modulation can be well interpreted by the Fe-vacancy order, which likely yields a superstructure phase of K2Fe4Se5. The superconducting K0.8Fe x Se2 and KFe x Se2 (1.7≤ x ≤1.8) materials contain clear phase separation, in particular, along the c-axis direction, recognizable as visible parallel lamellae in the crystals; this fact suggests that the superconducting phase could have the Fe-vacancy disordered state. The main changes of physical properties in the AFe2As2 materials have also been discussed. The substitution of A-element in AFe2As2 have a significant effect on the structural properties and spin density wave (SDW), then leads to the appearance and change of superconductivities. Polycrystalline samples of Ba1− x Sr x Fe2As2 (0≤ x ≤1) and Ba1− x Sr x Fe1.8Co0.2As2 (0≤ x ≤1) were synthesized by a solid state reaction method. Structural analysis by means of X-ray diffraction shows that the lattice parameters and unit cell volume decrease monotonically with the increase of x for Ba1– x Sr x Fe2As2. The measurements of transport properties demonstrate that the average size of the Ba(Sr)-site cations could evidently influence the SDW behavior in Ba1− x Sr x Fe2As2 and superconductivity in Ba1− x Sr x Fe1.8Co0.2As2 as well. The critical temperature for SDW ( T SDW) increases with the Sr substitution for Ba in Ba1− x Sr x Fe2As2 and, on the other hand, the superconducting T c decreases with the increase of Sr content in Ba1− x Sr x Fe1.8Co0.2As2. The inhomogeneous distributions of Ba/Sr ions and structural distortions in Ba0.5Sr0.5Fe2As2 have been investigated by TEM observations.

Possible coexistence of double-Q magnetic order and chequerboard charge order in the re-entrant tetragonal phase of Ba0.76K0.24Fe2As2

We investigate the re-entrant tetragonal phase in the iron-based superconductor Ba0 .76K0.24Fe2As2 by DC magnetization and thermoelectrical measurements. The reversible magnetization confirms by a thermodynamic method that the spin alignment in the re-entrant C4 phase is out-of-plane, in agreement with an itinerant double-Q magnetic order [Allred et al., Nat. Phys. 12, 493 (2016)]. The Nernst coefficient shows the typical unusually large negative value in the stripe-type spin density wave (SDW) state owing to the Fermi surface reconstruction associated with SDW and nematic order. At the transition into the re-entrant C4 tetragonal phase it hardly changes, which could indicate that instead of a complete vanishing of the associated charge order, the spin reorientation could trigger a redistribution of the charges to form a secondary charge order, e.g. in form of a chequerboard-like pattern that no longer breaks the rotational C4 symmetry.

Ablation laser fluence as an effective parameter to control superconductivity in Ba1−xKxBiO3 films

Potassium doping in insulating BaBiO3 induces superconductivity, with high superconducting transition temperatures, Tc, of up to 31 K in bulk. We investigated growth control of superconducting properties of BKBO films, by varying laser fluence using pulsed laser deposition technique. As cation stoichiometry, especially potassium concentration in BKBO films, was sensitively changed with laser fluence, we were able to precisely control Tc of BKBO films. Following the trend of the bulk phase diagram, Tc showed the highest value of 24.5 ± 0.5 K at the optimal stoichiometry. This result can provide optimal guidance for the synthesis of high-quality BKBO films, and demonstrates the effectiveness of laser fluence to study emerging superconducting phenomena in PLD-grown complex oxide thin films.

Synthesis, Structure, and Properties of the Superconductor Ba1−x K z Bi1−y Pb y O 3

The series of BaBi 1−y Pb y O 3(0 ≤y≤1) has been synthesized by high-temperature solid-state reaction. X-ray and selected area electron diffraction data indicate that BaBi 1−y Pb y O 3(0 ≤y≤ 0.88) crystallizes in space group P1 and BaBi 1−y Pb y O 3 (0.93 ≤y≤1) crystallizes in space group Imma. The samples with the nominal formula BaBi 1−y Pb y O 3(y = 0.70, 0.80, 0.90) shows superconductivity with transition temperature (T c) around 11 K. When BaBi 1−y Pb y O 3 (0 ≤ y≤0.80) was treated with KOH and KF at 450 ∘C, the corresponding samples with the nominal formula Ba 1−x K z Bi 1−y Pb y O 3 (0 ≤y≤ 0.80) were obtained, which shows superconductivity with T c changing from 30 to 11 K.

Electric field gradient wave (EFGW) in iron-based superconductor Ba0.6K0.4Fe2As2 studied by Mössbauer spectroscopy

The optimally doped ‘122’ iron-based superconductor Ba0.6K0.4Fe2As2 has been studied by 57Fe Mössbauer spectroscopy versus temperature ranging from 4.2K till 300K with particular attention paid to the superconducting transition around 38K. The spectra do not contain magnetic components and they exhibit quasi-continuous distribution of quadrupole split doublets. A distribution follows the electric field gradient (EFG) spatial modulation (wave) – EFGW. The EFGW is accompanied by some charge density wave (CDW) having about an order of magnitude lesser influence on the spectrum. The EFGW could be modeled as widely separated narrow sheets with the EFG increasing from small till maximum value almost linearly and subsequently dropping back to the original value in a similar fashion – across the sheet. One encounters very small and almost constant EFG between sheets. The EFGW shape and amplitude as well as the amplitude of CDW are strongly affected by a superconducting transition. All modulations are damped significantly at transition (38K) and recover at a temperature being about 14K lower. The maximum quadrupole splitting at 4.2K amounts to about 2.1mm/s, while the dispersion of CDW seen on the iron nuclei could be estimated far away from the superconducting gap opening and at low temperature as 0.5el./a.u.3. It drops to about 0.3el./a.u.3 just below transition to the superconducting state.