K. Grube

Ni-based superconductor : Heusler compound ZrNi2Ga

This work reports on the novel Heusler superconductor ZrNi2Ga. Compared to other nickel-based superconductors with Heusler structure, ZrNi2Ga exhibits a relatively high superconducting transition temperature of Tc=2.9 K and an upper critical field of 1.5 T. Electronic structure calculations show that this relatively high transition temperature is caused by a van Hove singularity, which leads to an enhanced density of states at the Fermi energy. The van Hove singularity originates from a higher order valence instability at the L-point in the electronic structure. The enhanced density of states at the Fermi level was confirmed by specific heat and susceptibility measurements. Although many Heusler compounds are ferromagnetic, our measurements of ZrNi2Ga indicate a paramagnetic state above Tc and could not reveal any traces of magnetic order down to temperatures of at least 0.35 K. We investigated in detail the superconducting state with specific heat, magnetization, and resistivity measurements. The resulting data show the typical behavior of a conventional, weakly coupled BCS (s-wave) superconductor.

Giant pressure effect in oxygen deficient YBa2Cu3Ox

Abstract The pressure effect on T c of polycrystalline and single crystalline YBa 2 Cu 3 O x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x > 6.93 the single crystals show a negative d T c /d p , as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find d T c /d p = 0.4 K/GPa which points to pressure effects on T c aside from charge transfer. In the underdoped region x T c /d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, d T c /d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the d T c /d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa 2 Cu 3 O x structure and can be explained by considering the anisotropic structure of YBa 2 Cu 3 O x . The decrease of the c -axis lattice parameter results in a charge transfer to the CuO 2 -planes mainly [1], whereas the compression of the a - and b -axis lattice parameter is known to produce different pressure effects which are responsible for the peak in d T c /d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T c to the equilibrium value T c ( p ) at this pressure with a time constant depending on the oxygen content x . A decrease x results in a peak effect in d T c /d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, d T c /d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x T c increase (d T c /d p ) O due to pressure induced oxygen ordering via oxygen motion between unit cells.

The effect of high hydrostatic pressure on Tc of YBa2Cu3Ox as a function of the oxygen content

Abstract The AC-susceptibility χ′, χ′ of YBa 2 Cu 3 O x polycrystals has been determined under hydrostatic He-pressures up to 0.8 GPa. The experimental data allow a separation to be made of the pressure effect on the superconducting transition temperature T c into two parts, the pressure effect from the weak link structure of the polycrystalline material and the true pressure effect of the bulk material. These investigations have been performed on a series of polycrystalline samples of YBa 2 Cu 3 O x with various oxygen content. Each sample has been cut from the same pellet followed by a careful adjustment of the oxygen content by a heat treatment in air. The true pressure effect of the bulk material d T c /d p shows an increase from ≈ 0.4 K/GPa to ≈ 4 K/GPa for an oxygen content decreasing from x ≈ 6.93 to x ≈ 6.77. If the oxygen content is reduced further, d T c /d p is almost constant, except for a narrow maximum of ≈ 7 K/GPa at x ≈ 6.72. This maximum can be related to the oxygen content, where the transition temperature of YBa 2 Cu 3 O x changes from the 90 K to the 60 K plateau.

Pauli-limited multiband superconductivity in KFe2As2.

The upper critical field H(c2)(T) of the multiband superconductor KFe2As2 has been studied via low-temperature thermal expansion and magnetostriction measurements. We present compelling evidence for Pauli-limiting effects dominating H(c2)(T) for H || a, as revealed by a crossover from second- to first-order phase transitions to the superconducting state in the magnetostriction measurements down to 50 mK. Corresponding features were absent for H || c. To our knowledge, this crossover constitutes the first confirmation of Pauli limiting of the H(c2)(T) of a multiband superconductor. The results are supported by modeling Pauli limits for single-band and multiband cases.

Towards the Identification of a Quantum Critical Line in the ( p , B ) Phase Diagram of CeCoIn 5 with Thermal-Expansion Measurements

The low-temperature thermal expansion of CeCoIn(5) single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient β. Considerable deviations of β(T) from Fermi-liquid behavior occur already within the superconducting region of the (B, T) phase diagram and become maximal at the upper critical field B(c2)(0). However, β(T) and the Grüneisen parameter Γ are incompatible with a quantum critical point at B(c2)(0), but allow for a quantum criticality shielded by superconductivity and extending to negative pressures for B<B(c2)(0). We construct a tentative (p, B, T) phase diagram of CeCoIn(5) suggesting a quantum critical line in the (p, B) plane.

Separation of the intrinsic pressure effect on Tc of YBa2Cu3O6.7 from a Tc enhancement caused by pressure-induced oxygen ordering

Abstract The superconducting transition temperature Tc of polycrystalline YBa2Cu3O6.7 has been investigated under purely hydrostatic He-gas pressure. In a series of experiments, it has been shown that the degree of oxygen ordering depends on the applied pressure and the storage temperature TS of the sample. At temperatures below ≈240 K, there is no indication for oxygen ordering. When pressure is applied at such low temperatures, the Tc values show no relaxation effects, and (dTc/dp)i=7.4 K/GPa is observed which we believe is intrinsic. At temperatures above 240 K, the oxygen sublattice reacts reversibly to the applied temperature and pressure. Changes of the storage temperature or pressure cause Tc relaxations with time constants that are temperature dependent. When the relaxations were completed the effects of pressure and storage temperature on Tc are independently determined to be: (1) a pressure-independent effect of the storage temperature Tc with dTc/dTS=−0.02, (2) a temperature-independent effect of pressure-induced oxygen ordering with (dTc/dp)o=4.1 K/GPa. For temperatures above 240 K, the intrinsic and the oxygen ordering effect add to dTc/dp=11.5 K/GPa.

C60 under pressure-bulk modulus and equation of state

C60 has been investigated under pressure up to 13 GPa using angular dispersive X-ray scattering and a diamond anvil cell. The resolution of the experimental setup allows to examine the volume decrease dV/dp under pressure even for pressures of a tenth of a GPa. The obtained data of numerous experimental runs result in a bulk modulus of 13.4 GPa, which is much smaller than the value reported by Duclos et al. [1]. At 170 K and 70 K a bulk modulus of 14.2 GPa and 14.7 GPa was obtained, respectively. The pressure induced fcc-sc transition at 300 K was clearly visible at approx. 0.3 GPa with a jump in the lattice parameter of 0.05 Å. With increasing pressure we found an extreme change in dV/dp, which disables the usage of common equations of state (EOS), like the Murnaghan [2] or Birch [3] equation. Considering the small compressibility of the fullerence molecules we suggest a modified EOS to describe the experimental data.

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

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.

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

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.

Entropy Evolution in the Magnetic Phases of Partially Frustrated CePdAl

In the heavy-fermion metal CePdAl, long-range antiferromagnetic order coexists with geometric frustration of one-third of the Ce moments. At low temperatures, the Kondo effect tends to screen the frustrated moments. We use magnetic fields B to suppress the Kondo screening and study the magnetic phase diagram and the evolution of the entropy with B employing thermodynamic probes. We estimate the frustration by introducing a definition of the frustration parameter based on the enhanced entropy, a fundamental feature of frustrated systems. In the field range where the Kondo screening is suppressed, the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration. Based on our experiments, this field range may be a promising candidate to search for a quantum spin liquid.