Experimental Evidence for Crossed Andreev Reflection

We report on electronic transport properties of mesoscopic superconductor-ferromagnet spin-valve structures. Two ferromagnetic iron leads form planar tunnel contacts to a superconducting aluminum wire, where the distance of the two contacts is of the order of the coherence length of the aluminum. We observe a negative non-local resistance which can be explained by crossed Andreev reflection, a process where an electron incident from one of the leads gets reflected as a hole into the other, thereby creating a pair of spatially separated, entangled particles.Comment: LT24 conference proceeding, 2 pages, 2 figure

Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures

We have measured the non-local resistance of aluminum-iron spin-valve structures fabricated by e-beam lithography and shadow evaporation. The sample geometry consists of an aluminum bar with two or more ferromagnetic wires forming point contacts to the aluminum at varying distances from each other. In the normal state of aluminum, we observe a spin-valve signal which allows us to control the relative orientation of the magnetizations of the ferromagnetic contacts. In the superconducting state, at low temperatures and excitation voltages well below the gap, we observe a spin-dependent non-local resistance which decays on a smaller length scale than the normal-state spin-valve signal. The sign, magnitude and decay length of this signal is consistent with predictions made for crossed Andreev reflection (CAR).Comment: RevTeX, 4 page

A mechanism for the non-Fermi-liquid behavior in CeCu_{6-x}Au_x

We propose an explanation for the recently observed non-Fermi-liquid behavior of metallic alloys CeCu_{6-x}Au_x: near x=0.1, the specific heat c is proportional to T \ln (T_0/T) and the resistivity increases linearly with temperature T over a wide range of T. These features follow from a model in which three-dimensional conduction electrons are coupled to two-dimensional critical ferromagnetic fluctuations near the quantum critical point, x_{c}=0.1. This picture is motivated by the neutron scattering data in the ordered phase (x=0.2) and is consistent with the observed phase diagram.Comment: 4 pages, LaTeX, 3 figure

Multidimensional entropy landscape of quantum criticality

The Third Law of Thermodynamics states that the entropy of any system in equilibrium has to vanish at absolute zero temperature. At nonzero temperatures, on the other hand, matter is expected to accumulate entropy near a quantum critical point (QCP), where it undergoes a continuous transition from one ground state to another. Here, we determine, based on general thermodynamic principles, the spatial-dimensional profile of the entropy S near a QCP and its steepest descent in the corresponding multidimensional stress space. We demonstrate this approach for the canonical quantum critical compound CeCu6-xAux near its onset of antiferromagnetic order. We are able to link the directional stress dependence of S to the previously determined geometry of quantum critical fluctuations. Our demonstration of the multidimensional entropy landscape provides the foundation to understand how quantum criticality nucleates novel phases such as high-temperature superconductivity.Comment: 14 pages, 4 figure

Electronic structure of single-crystalline Mgx_xAl1−x_{1-x}B2_2 probed by x-ray diffraction multipole refinements and polarization-dependent x-ray absorption spectroscopy

X-ray diffraction multipole refinements of single-crystalline Mg$_x$Al$_{1-x}$B$_2$ and polarization-dependent near-edge x-ray absorption fine structure at the B 1$s$ edge reveal a strongly anisotropic electronic structure. Comparing the data for superconducting compounds ($x= 0.8$, 1.0) with those for the non-superconductor ($x=0$) gives direct evidence for a rearrangement of the hybridizations of the boron $p_z$ bonds and underline the importance of holes in the $\sigma$-bonded covalent $sp^2$ states for the superconducting properties of the diborides. The data indicate that Mg is approximately divalent in MgB$_2$ and suggest predominantly ionic bonds between the Mg ions and the two-dimensional B rings. For AlB$_2$ ($x=0$), on the other hand, about 1.5 electrons per Al atom are transferred to the B sheets while the residual 1.5 electrons remain at the Al site which suggests significant covalent bonding between the Al ions and the B sheets. This finding together with the static electron deformation density points to almost equivalent electron counts on B sheets of MgB$_2$ and AlB$_2$\@, yet with a completely different electron/hole distribution between the $\sigma$ and $\pi$ bonds