Frederick Casper

Electronic and magnetic phase diagram of β -Fe 1.01 Se with superconductivity at 36.7 K under pressure

In this letter, we report that the superconductivity transition temperature in beta-Fe1.01Se increases from 8.5 to 36.7 K under applied pressure of 8.9 GPa. It then decreases at higher pressure. A dramatic change in volume is observed at the same time Tc rises, due to a collapse of the separation between the Fe2Se2 layers. A clear transition to a linear resistivity normal state is seen on cooling at all pressures. No static magnetic ordering is observed for the whole p-T phase diagram. We also report that at higher pressure (starting around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal NiAs-type structure and displays non-magnetic, insulating behavior. The inclusion of electron correlation in band structure caculations is necessary to describe this behavior, signifying that such correlations are important in this chemical system. Our results strongly support unconventional superconductivity in beta-Fe1.01Se.

Tetragonal-to-Orthorhombic Structural Phase Transition at 90 K in the Superconductor Fe1.01Se

In this letter we show that superconducting Fe1.01Se undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that non-superconducting Fe1.03Se does not. Further, high resolution electron microscopy study at low temperatures reveals an unexpected additional modulation of the crystal structure of the superconducting phase involving displacements of the Fe atoms, and that the non-superconducting material shows a distinct, complex nanometer-scale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.

Half-metallic ferromagnetism with high magnetic moment and high Curie temperature in Co2FeSi

Co2FeSi crystallizes in the ordered L21 structure as proven by x-ray diffraction and Moβbauer spectroscopy. The magnetic moment of Co2FeSi was measured to be about 6μB at 5 K. Magnetic circular dichroism spectra excited by soft x-rays were taken to determine the element-specific magnetic moments of Co and Fe. The Curie temperature was measured with different methods to be (1100±20)K. Co2FeSi was found to be the Heusler compound as well as the half-metallic ferromagnet with the highest magnetic moment and Curie temperature.

Half-Heusler compounds: novel materials for energy and spintronic applications

Half-Heusler compounds are an impressive class of materials with a huge potential for different applications such as future energy applications and for spintronics. The semiconducting Heusler compounds can be identified by the number of valence electrons. The band gap can be tuned between 0 and 4 eV by the electronegativity difference of the constituents. Magnetism can be introduced in these compounds by using rare-earth elements, manganese or ‘electron’ doping. Thus, there is a great interest in the fields of thermoelectrics, solar cells and diluted magnetic semiconductors. The combination of different properties such as superconductivity and topological edge states leads to new multifunctional materials, which have the potential to revolutionize technological applications. Here, we review the structure, the origin of the band gap and the functionalities of semiconducting half-Heusler compounds.

Design scheme of new tetragonal Heusler compounds for spin-transfer torque applications and its experimental realization.

Band Jahn-Teller type structural instabilities of cubic Mn(2)YZ Heusler compounds causing tetragonal distortions can be predicted by ab initio band-structure calculations. This allows for identification of new Heusler materials with tunable magnetic and structural properties that can satisfy the demands for spintronic applications, such as in spin-transfer torque-based devices.

Systematical, experimental investigations on LiMgZ (Z = P, As, Sb) wide band gap semiconductors

This work reports on the experimental investigation of wide band gap compounds LiMgZ (Z = P, As, Sb), which are promising candidates for opto-electronics and anode materials for lithium batteries. The compounds crystallize in the cubic (C1b) MgAgAs structure (space group ). The polycrystalline samples are synthesized by solid-state reaction methods. X-ray and neutron diffraction measurements show homogeneous, single-phased samples. The electronic properties are studied using the direct current method. Additionally, UV–Vis diffuse reflectance spectra are recorded in order to investigate the band gap nature. The measurements show that all compounds exhibit semiconducting behaviour with direct band gaps of 1.0–2.3 eV depending on the Z element. A decrease in the peak widths in the static 7Li nuclear magnetic resonance spectra with increasing temperature is observed, which can be directly related to an increase in Li ion mobility.

Efficient spin injector scheme based on Heusler materials.

We present a rational design scheme intended to provide stable high spin polarization at the interfaces of the magnetoresistive junctions by fulfilling the criteria of structural and chemical compatibilities at the interface. This can be realized by joining the semiconducting and half-metallic Heusler materials with similar structures. The present first-principles calculations verify that the interface remains half-metallic if the nearest interface layers effectively form a stable Heusler material with the properties intermediately between the surrounding bulk parts. This leads to a simple rule for selecting the proper combinations.

Topological insulators and thermoelectric materials

Topological insulators (TIs) are a new quantum state of matter which have gapless surface states inside the bulk energy gap [1–4]. Starting with the discovery of two-dimensional TIs, the HgTe-based quantum wells [5, 6], many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families [7], the HgTe family and the Bi2Se3 family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials [8–10]. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and thermoelectrics, and give examples of compound classes where both good thermoelectric properties and topological insulators can be found. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure

In this letter, we report that the superconductivity transition temperature in beta-Fe1.01Se increases from 8.5 to 36.7 K under applied pressure of 8.9 GPa. It then decreases at higher pressure. A dramatic change in volume is observed at the same time Tc rises, due to a collapse of the separation between the Fe2Se2 layers. A clear transition to a linear resistivity normal state is seen on cooling at all pressures. No static magnetic ordering is observed for the whole p-T phase diagram. We also report that at higher pressure (starting around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal NiAs-type structure and displays non-magnetic, insulating behavior. The inclusion of electron correlation in band structure caculations is necessary to describe this behavior, signifying that such correlations are important in this chemical system. Our results strongly support unconventional superconductivity in beta-Fe1.01Se.

Topological Insulators from a Chemist’s Perspective

Heavy stuff: Topological insulators are formed of heavy atoms and host special surface or edge states. The electronic structure is characterized by a Dirac cone within a bulk band gap (see picture) that is generated by strong spin-orbit coupling. A chemists perspective in terms of bonds, bands, symmetry, and nuclear charge is provided.