P. Schweiss

Lack of coupling between superconductivity and orthorhombic distortion in stoichiometric single-crystalline FeSe

The coupling between superconductivity and orthorhombic distortion is studied in vapor-grown FeSe single crystals using high-resolution thermal-expansion measurements. In contrast to the Ba122-based (Ba122) superconductors, we find that superconductivity does not reduce the orthorhombicity below

Nematic susceptibility of hole-doped and electron-doped BaFe2As2 iron-based superconductors from shear modulus measurements.

{T}_{c}

Evidence of Strong Correlations and Coherence-Incoherence Crossover in the Iron Pnictide Superconductor KFe2As2

. Instead we find that superconductivity couples strongly to the in-plane area, which explains the large hydrostatic pressure effects. We discuss our results in light of the spin-nematic scenario and argue that FeSe has many features that are quite different from typical Fe-based superconductors.

Calorimetric evidence of multiband superconductivity in Ba ( Fe 0.925 Co 0.075 ) 2 As 2 single crystals

The nematic susceptibility, χφ, of hole-doped Ba(1-x)K(x)Fe2As2 and electron-doped Ba(Fe(1-x)Co(x))2As2 iron-based superconductors is obtained from measurements of the elastic shear modulus using a three-point bending setup in a capacitance dilatometer. Nematic fluctuations, although weakened by doping, extend over the whole superconducting dome in both systems, suggesting their close tie to superconductivity. Evidence for quantum critical behavior of χφ is, surprisingly, only found for Ba(Fe(1-x)Co(x))2As2 and not for Ba(1-x)K(x)Fe2As2--the system with the higher maximal Tc value.

3D-XY CRITICAL FLUCTUATIONS OF THE THERMAL EXPANSIVITY IN DETWINNED YBA2CU3O7-DELTA SINGLE CRYSTALS NEAR OPTIMAL DOPING

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe2As2. Both the Sommerfeld coefficient (γ≈103 mJ mol(-1) K(-2)) and the Pauli susceptibility (χ≈4×10(-4)) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of KFe2As2 to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hunds coupling between orbitals.

Pressure versus Concentration Tuning of the Superconductivity in Ba(Fe1-xCox)2As2

We report on the determination of the electronic heat capacity of a slightly overdoped (x = 0.075) Ba(Fe1-xCox)2As2 single crystal with a Tc of 21.4 K. Our analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band s-wave order parameter with gap amplitudes 2D1(0)/kBTc=1.9 and 2D2(0)/kBTc=4.4. Our result is consistent with the recently predicted s+- order parameter [I. I. Mazin et al., Phys. Rev. Lett. 101, 057003 (2008)].

The effect of chemical doping and hydrostatic pressure on Tc of Y1−yCayBa2Cu3Ox single crystals

The strong coupling of superconductivity to the orthorhombic distortion in YBa2Cu3O7-d makes possible an analysis of the superconducting fluctuations without the necessity of subtracting any background. The present high-resolution capacitance dilatometry data unambiguously demonstrate the existence of critical, instead of Gaussian, fluctuations over a wide temperature region (+/- 10 K) around Tc. The values of the amplitude ratio A+/A-=0.9-1.1 and the leading scaling exponent |alpha|<0.018, determined via a least-squares fit of the data, are consistent with the 3D-XY universality class. Small deviations from pure 3D-XY behavior are discussed.

Multiband Superconductivity in KFe2As2 : Evidence for one Isotropic and several Liliputian Energy Gaps

In the iron arsenide compound BaFe 2 As 2 , superconductivity can be induced either by a variation of its chemical composition, e.g., by replacing Fe with Co, or by a reduction of the unit-cell volume through the application of hydrostatic pressure p . In contrast to chemical substitutions, pressure is expected to introduce no additional disorder into the lattice. We compare the two routes to superconductivity by measuring the p dependence of the superconducting transition temperature T c of Ba(Fe 1- x Co x ) 2 As 2 single crystals with different Co content x . We find that T c ( p ) of underdoped and overdoped samples increases and decreases, respectively, tracking quantitatively the T c ( x ) dependence. To clarify to which extent the superconductivity relies on distinct structural features we analyze the crystal structure as a function of x and compare the results with that of BaFe 2 As 2 under pressure.

Transverse and longitudinal magnetic-field responses in the Ising ferromagnets URhGe, UCoGe, and UGe2

We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals under high He pressure. For each Ca content various samples with different oxygen contents have been prepared to probe the influence of Ca on Tc(x), dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated nh values from Tc and Tc,max for each sample. It is shown that in the overdoped region dTc/dp can be described by a pressure induced charge transfer with dnh/dp ≈ 3.7⋅10 -3 GPa -1 and a dTc,max/dp value of 0.8 K/GPa, irrespective of the Ca content. In the underdoped region additional pressure effects lead to a peak in dTc/dp at ≈0.11 holes/CuO2 plane. However, with increasing Ca content this peak is strongly depressed. This is explained in terms of an increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp at very low nh values can be assigned to the ortho II ordering in the CuO chain system.

Crystal and electronic structure of solid C76

We report a detailed low-temperature thermodynamic investigation (heat capacity and magnetization) of the superconducting state of KFe 2 As 2 for H || c axis. Our measurements reveal that the properties of KFe 2 As 2 are dominated by a relatively large nodeless energy gap (∆ 0 = 1.9 k B T c) which excludes d x 2 −y 2 symmetry. We prove the existence of several additional extremely small gaps (∆ 0 < 1.0 k B T c) that have a profound impact on the low-temperature and low-field behavior, similar to MgB 2 , CeCoIn 5 and PrOs 4 Sb 12. The zero-field heat capacity is analyzed in a realistic self-consistent 4-band BCS model which qualitatively reproduces the recent laser ARPES results of Okazaki et al. (Science 337 (2012) 1314). Our results show that extremely low-temperature measurements, i.e. T < 0.1 K,will be required in order to resolve the question of the existence of line nodes in this compound.