Working Group New Materials, Transport, Thermodynamics and Mesoscopics

Fe-based Superconductors

Superconductivity in the Fe-based materials occurs in the vicinity of a magnetic/structural critical point, suggesting that magnetic/structural fluctuations may be important for the pairing mechanism. The nematic susceptibility, i.e., the susceptibility of the electronic order driving the structural transition, has turned out to be a good measure of such fluctuations. In order to gain access to the nematic susceptibility of hole-doped Ba_1-xK_xFe₂As₂ and electron-doped Ba(Fe_1-xCo_x)₂As₂ superconductors, a new technique was developed by which the shear modulus is measured using a three-point bending setup in a capacitance dilatometer. Nematic fluctuations, although weakened by doping, were found to extend over the whole superconducting dome in both systems, suggesting their close link to superconductivity. Evidence for quantum critical behavior of the nematic susceptibility was, surprisingly, only found for Ba(Fe_1-xCo_x)₂As₂, the system with the lower maximal T_c value. Further, a scaling relation between nuclear magnetic resonance (NMR) and shear modulus data in the tetragonal phase of electron-doped Ba(Fe_1-xCo_x)₂As₂ compounds was found, which provides strong evidence for a magnetically-driven structural transition.

Nematic susceptibility (color-coded) of hole- and electron-doped BaFe2As2 superconductors as a function of dopant concentration and temperature.

Heavy-Fermion Compounds

Continuous magnetic phase transitions driven to zero temperature by a non-thermal control parameter yielding a quantum critical point (QCP). The excitations close to the QCP as measured, e.g., by the specific heat, do not follow the predictions of Fermi-liquid theory. Furthermore, often novel phases emerge in the proximity of a QCP. The heavy-fermion system CeCoIn₅ is a well-established member of the growing family of compounds exhibiting unconventional superconductivity close to antiferromagnetic order. Locating the the exact position of a QCP is however difficult, because it is masked by superconductivity. Our thermal-expansion and magnetostriction measurements on CeCoIn₅ single crystals indicate that the pronounced deviations from Fermi-liquid behavior arise from a quantum critical line, i.e., an antiferromagnetic phase boundary in the field and pressure plane at zero temperature that is shielded by superconductivity. Together with pressure and doping data, a three-dimensional phase diagram showing this phase boundary could be constructed.

(p,B,T) phase diagram of CeCoIn5 (AF: antiferromagnetic phase, SC: superconducting phase, QC: quantum critical line in the (B,p) plane at T=0, not observable for small magnetic fields due to superconductivity).

Electronic Transport in Normalconducting and Superconducting Nanostructures
(R. Schäfer)

We fabricate and investigate single-charge devices of normal metals and superconductors. Superconducting tunnel contacts ('Josephson contacts') may carry a supercurrent if the phase of the superconducting order parameter on both sides of the contact is suitably fixed. However, fixing the phase phase of the superconducting condensate excludes determination of the involved charge. This leads to a peculiar transport behavior of mesoscopic Josephson-junction devices which is the focus of our present research. In particular, we investigate chains where supercurrents can be switched on and off by magnetic fields.

Scanning electron micrograph of a Josephson junction chain formed by bone shaped aluminum islands. The actual Josephson junctions result from overlaps of the islands seen at the top and bottom of the right inset.

Current through such a chain of 256 mesoscopic Josephson junction at 0.01 K as function of voltage and magnetic field.

Crystal Growth (A.-A. Haghighirad)

Our laboratory is equipped for the preparation of samples in single-crystalline and polycrystalline form. Single crystals are grown using mainly growth from the melt or flux, as well as vapor-growth techniques. The growth parameters are optimized according to a detailed compositional and structural characterization of the crystals in order to continuously improve the degree of perfection of the crystals. If required crystals with a particular impurity doping concentration can be grown.

For sample preparation, chamber furnaces, top-loading furnaces and tubular furnaces up to working temperatures of 1800 °C, Czochralski furnace, 3-zone furnace, induction furnace, arc-melt furnace, mirror furnace, autoclave furnace are available.

Superconducting and related materials:
TiN, ZrN, HfN, TiC, YBa₂Cu₃O_x-family (La-Lu), BaCuO₂, Ba₂CuO₃, CuO, (Nd_1-xCe_x)₂CuO₄, (Gd_1-xNd_x)₂CuO₄, BaFe₂As₂, SrFe₂As₂, CaFe₂As₂, KFe₂As₂, RbFe₂As₂, CsFe₂As₂, K_xFe₂Se₂, FeAs, FeSe, FeTe, FeTe₂, FeI₂, MgB₂, AlB₂, (Ca/Sr)CuO₂, Ca(Al_0.5Si_0.5)₂, NbSe₂.
Quantum critical materials:
YbNiSi₃, YbNiGe₃, YbPdSn, LaCu₆, CeCu_6-xAu_x,YbB₂, YbB₄, YbAlB₄, CrB₂, Cr₂B₃, CrAlB₄.
Magnetic and other materials:
BaNi₂V₂O₈, BaNi_2-xMg_xV₂O₈, Ni₃V₂O₈, Co₃V₂O₈, MnSi (Al, Co, Fe), Ba₂V₃O₉, Ba₂V₂O₇, Ba₃V₄O₁₃, Cs₂CuCl₄, Ti₄O₇, Zn₄Sb₃, YBaCo₄O₇, La₂Mo₂O₉, Co₃TeO₆.

Various single crystals and Ta2NiS5 dendrites

Left: Hybrid inorganic-organic perovskite crystals MAPbX3 (X = Cl,Br,I) Right: Cu2OSeO3 crystal