Ni2X2 (X = pnictide,chalcogenide, or B) based superconductors

We review the properties of Ni-based superconductors which contain Ni₂X₂ (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi₂As₂. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in T_c in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.     

Competing energy scales in high‐temperature superconductors: Ultrafast pump–probe studies

We present a review of photoexcited quasiparticle dynamics of cuprate and pnictide high‐temperature superconductors in regimes (temperature, doping) where different phases such as superconductivity, spin‐density‐wave (SDW) and pseudogap phases coexist or compete with one another. We start with the overdoped cuprate superconductor Y_1–xCa_x Ba₂Cu₃O_7–δ, where the superconducting gap and pseudogap coexist in the superconducting state. In another cuprate Tl₂Ba₂Ca₂Cu₃O_y, we ob‐ serve a competition between SDW and superconducting orders deep in the superconducting state. Finally, in the underdoped iron pnictide superconductor (Ba,K)Fe₂As₂, SDW order forms at 85 K, followed by superconductivity at 28 K. We also find the emergence of a normal‐state order that suppresses SDW at a temperature T * ～ 60 K and argue that this normal‐state order is a precursor to superconductivity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Local moment spin resonance in a superconductor

The ESR of Gd in CeRu₂ and LaRu₂ has been observed in the normal and superconducting state. The temperature dependence of the linewidth in the superconducting state follows a trend expected from NMR measurements in superconductors.     

Spin Hall magnetoresistance in Nb/Y3Fe5O12 hybrids

We investigate the spin Hall magnetoresistance (SMR) in niobium (Nb) attached to Y₃Fe₅O₁₂ near the superconducting critical temperature (T_c) of Nb. The SMR vanishes after cooling the sample below T_c, and recovers if the temperature is raised. When a magnetic field larger than the critical field of Nb is applied, the SMR re‐emerges with an enhanced magnitude even if the temperature is below T_c. The experimental results demonstrate that the SMR could be completely suppressed by the coupling between superconducting condensation and spin–orbit interaction in superconductors. In addition to the fundamental physics on the charge–spin interactions in superconductors, our work adds a different dimension to superconducting spintronics. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Comparison of the local structures of Ca0.82La0.18FeAs2 and Ba0.64K0.36Fe2As2 pnictide superconductors using atomic pair distribution function analysis

A comparative local structure study of pnictide superconductors Ca_0.82La_0.18FeAs₂ (112-type, T_c∼ 40 K) and Ba_0.64K_0.36Fe₂As₂ (122-type, T_c∼ 37 K), using room temperature x-ray total scattering measurements is reported. The Fe–As superconducting active layer is found to be globally similar in both the systems consisting of edge-sharing FeAs_4/4 tetrahedra as in all the iron-pnictide superconductors discovered so far. Although optimally superconducting, the active layer in these compounds is found to sustain a large local inhomogeneity. These results thus imply that a nanoscopic manipulation of the Fe–As active layer, rather than its isotropic structural tuning, is the key parameter to control the superconducting properties of the iron-based systems.     

Nonlinear effects of the thermal stabilization theory of applied superconducting materials

The stability of dissipative states of a Bi₂Sr₂CaCu₂O₈ applied superconducting composite cooled by liquid helium or hydrogen with continuously increasing current-voltage characteristics described by a power equation has been studied. It has been shown that, under intensive cooling conditions, special conditions of thermal stabilization can exist for applied superconducting materials (technical superconductors) if the nonlinear temperature dependences of critical current density and specific resistance of the stabilizing matrix are taken into account. First, the minimum currents of existence and propagation of a normal zone can be absent. Second, the intensive cooling of a technical superconductor substantially increases the range of stable currents in the diapason of supercritical currents. Third, during the irreversible propagation of the thermal instability, the temperature of a technical superconductor can increase under conditions close to adiabatic, despite cooling by liquid coolant. These effects should be taken into account upon determining the conditions for overheating a technical superconductor. The numerical experiments have been compared with the results that follow from the thermal stabilization theory of combined superconductors, which assumes a linear temperature dependence of critical current density of a superconductor and a step-wise transition from the superconducting to normal state.     

Anomalous ultrasonic attenuation and velocity change in LaAl2

The attenuation and velocity of longitudinal waves has been measured in superconducting and normal LaAl₂ in the frequency range from 15 to 225 MHz. Both in the superconducting and in the normal state an unusually large damping has been found superimposed on the relatively small attenuation due to the well known electron-phonon interaction. On cooling into the superconducting state the attenuation starts to drop only at temperatures well below T_c and — as in insulating glasses — it rises again below 0.8 K. In our view this indicates that two-level systems similar to the ones found in amorphous superconductors may be present in our crystalline LaAl₂ sample and are responsible for the observed acoustic anomalies.     

Workshop on fast structural changes

Copper-oxide (cuprate) high-temperature superconductors are doped Mott insulators. The undoped parent compounds are antiferromagnetic insulators, and superconductivity occurs only when an appropriate number of charge carriers (electrons or holes) are introduced by doping. All cuprate materials contain CuO₂ planes (Figure 1a) in their crystal structure; the doped carriers are believed to go into these CuO₂ planes, which are responsible for high-temperature superconductivity. High-temperature superconductors are characterized by their unusual physical properties, both in the superconducting state (below the superconducting transition temperature T_c) and in the normal state (above T_c). Since the discovery of high-temperature superconductivity in 1986 [1], these unusual physical properties and the mechanism of superconductivity have been prominent issues in condensed matter physics [2].     

The Cu valence in the highT c superconductors and in monovalent,divalent and trivalent copper oxides determined from XPS core level spectroscopy

A number of theoretical models have been developed to explain the unexpected high temperature superconductivity in La-Me-Cu-oxides (Me=Ba, Sr) and Y-Ba-Cu-oxides. Some of these models invoke charge fluctuations on the copper ions in the superconductors between a 2⁺ and a 3⁺ state. In order to test these possibilities we have measured the Cu–2p _3/2-core level spectra of NaCuO₂ in which the copper ion is in a 3⁺ state and compared it with the core line position in pure CuO and to superconducting oxides. The data strongly suggest, that there is, if any very little Cu³⁺ in the superconducting compounds present. However, we notice, that in comparison to trivalent and monovalent copper oxides the Cu–2p _3/2 line in CuO and the superconducting oxides is unexpectedly broad. The cause of this large linewidths remains so far unexplained.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Phase transitions in a MgB<Subscript>2</Subscript> granular BCS superconductor in weak magnetic fields

The character of temperature dependences of the electric conductivity of MgB₂ granular BCS superconductors at temperatures of ~35–45 K in external magnetic fields H _ext of up to ~2 kOe is studied. An increase in the superconducting transition width ΔT _c with an increase in Hext is found. The presence of a system of weak links in MgB₂-based granular superconductors is established. On the basis of experimental data, MgB₂ granular superconductor is assigned to two-level superconducting systems and the H–T phase diagram is constructed.     

Influence of lattice instability on the superconducting properties of “La3S4”

The normal state properties (the electronic specific heat constant, Debye temperature and electrical resistivity) and superconducting state properties [the superconducting transition temperature, T_c, and the upper critical field at 0 K, H_c²(0)] have been studied in the La₃S₄-La₂S₃ system. The superconducting properties and the electronic specific heat constant exhibit the maximum values in the alloy with the lowest sulfur content that does not undergo a low temperature crystallographic transformation. At lower sulfur contents the alloys exhibit a cubic to tetragonal transformation at ～80 K with a serious degradation in their superconducting properties, especially H_c² (0). These alloys clearly illustrate that materials which are almost but not quite unstable are good superconductors, relative to the more stable compositions.     

A brief review on <Emphasis Type="Italic">μ</Emphasis>SR studies of unconventional Fe- and Cr-based superconductors

Muon spin relaxation/rotation (μSR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remain a puzzle. Very recently double layered Fe-based super- conductors having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D structure have drawn considerable attention. Here we present a brief review of the characteristics of a few selected Fe- and Cr-based superconducting materials and highlight some of the major outstanding problems, with an emphasis on the superconducting pairing symmetries of these materials. We focus on μSR studies of the newly discovered superconductors ACa₂Fe₄As₄F₂ (A = K, Rb, and Cs), ThFeAsN, and A₂Cr₃As₃ (A = K, Cs), which were used to determine the superconducting gap structures, the presence of spin fluctuations, and to search for time reversal symmetry breaking in the superconducting states. We also briefly discuss the results of μSR investigations of the superconductivity in hole and electron doped BaFe₂As₂.     

HIGH-PRESSURE SYNTHESIS OF MgB2 SUPER-CONDUCTOR WITH Tc ABOVE 39 K

We report on the high-pressure synthesis and superconductivity of MgB₂ intermetallic compounds. The compounds have been obtained through high-pressure sintering of the mixtures of magnesium and boron fine powders under 5.0 GPa and at ～1000℃ for 30 min. Magnetic measurements using a SQUID magnetometer show the sharp bulk superconducting transition above 39 K; the four-probe dc resistivity measurements indicate the highly-conductive normal state and sharp superconducting transition. The results highlight that high-pressure synthesis would be a promising way to promote the studies of this new kind of intermetallic superconductors.     

The vortex-like excitations detected by Nernst signals in high-T c superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

On the effect of boundary conditions on I–V characteristics of HTSC tunnel junctions

The pairing potential distribution over the thickness of superconducting CuO₂ layers in cuprate HTSCs is determined within the Ginzburg–Landau (GL) theory using the microscopic justification of this theory by Gor’kov. It is found that the pairing potential in them is significantly suppressed due to the effect of non-superconducting interlayers, which results in a decrease in the critical temperature of these superconductors. The temperature dependences of the effective energy gap and current–voltage (I–V) characteristic of tunnel junctions of the “break junction” type made of these superconductors are calculated.     

The vortex-like excitations detected by Nernst signals in high-Tc superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

Development of the method for low-temperature investigations of HTSC systems on an X-ray electron magnetic spectrometer

Peculiarities of the chemical structure of bulk polycrystalline samples of the high-temperature superconductors Bi₂Sr₂CaCu₂O₈, BiSrCaCu₂O_5.5, BiSrCaCu₃O₈, and YBa₂Cu₃O_{7 ? δ} have been investigated in detail at room and superconducting temperatures on an X-ray electron magnetic spectrometer equipped with an attachment for low-temperature studies. It is shown that covalent bonding is formed at a superconducting temperature between copper and oxygen due to Cu²⁺ ions. Due to the enhancement of the d(Cu)–p(O) hybridization of copper and oxygen electrons in the superconducting state, the d-electron density increases near E _F. The occurrence of additional peaks in the O1s and Sr3d (Ba3d) spectra after transition of the system to the superconducting state indicates changes in the nearest environment of O and Sr (Ba) atoms, in particular, the transition of Sr atoms to a higher oxidation state.     

Can the Bogoliubov–de Gennes equation be interpreted as a ‘one-particle’ wave equation?

The solutions of the Bogoliubov–de Gennes (BdG) equation are usually interpreted as the excitations from the superconducting ground state. This viewpoint is not easily applied to a strongly coupled heterojunction since the ground state changes across the interface and it is not clear how the ground state should be connected across the heterointerface. In this paper, we present a different viewpoint that does not suffer from this conceptual drawback. We show that the BdG equation can be viewed as a ‘one-particle’ wave equation whose eigenstates (including the negative energy states) can be filled up systematically to describe the superconducting state, in much the same way that we fill the eigenstates of the Schrödinger equation to describe normal conductors. The only difference is that we need to start from a special vacuum | V〉, consisting of a full band of down-spin electrons, instead of the usual vacuum devoid of all particles. Any quantity of interest, A (such as the charge density or the current density), can be interpreted as the sum of a ‘vacuum contribution’A_{V AC}due to the vacuum | V〉 and a one-particle contribution A_BdGdue to the filled eigenstates of the BdG equation. This picture is easily applied even to strongly coupled heterojunctions since the vacuum | V〉 is the same on both sides of a heterointerface. As such, we believe it puts the scattering theory of transport for superconductors on a firmer conceptual basis.