Walter Schnelle

A guest-free germanium clathrate

The challenges associated with synthesizing expanded semiconductor frameworks with cage-like crystal structures continue to be of interest. Filled low-density germanium and silicon framework structures have distinct properties that address important issues in thermoelectric phonon glass–electron crystals, superconductivity and the possibility of Kondo insulators. Interest in empty framework structures of silicon and germanium is motivated by their predicted wide optical bandgaps of the same magnitude as quantum dots and porous silicon, making them and their alloys promising materials for silicon-based optoelectronic devices. Although almost-empty Na1-xSi136 has already been reported, the synthesis of guest-free germanium clathrate has so far been unsuccessful. Here we report the high-yield synthesis and characteristics of germanium with the empty clathrate-II structure through the oxidation of Zintl anions in ionic liquids under ambient conditions. The approach demonstrates the potential of ionic liquids as media for the reactions of polar intermetallic phases.

Superconductivity in Weyl semimetal candidate MoTe2

Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.

Atomic Interactions in the p-Type Clathrate I Ba8Au5.3Ge40.7

Single crystals of Ba(8)Au(5.3)Ge(40.7) [space group Pm(3)n (No. 223), a = 10.79891(8) Å] were prepared by a Bridgman technique. The crystal structure refinement based on single-crystal X-ray diffraction data does not reveal any vacancies in the Au/Ge framework or in the cages. In addition to the ionic bonding between Ba and the anionic framework, a direct interaction between Ba and Au atoms was identified in Ba(8)Au(5.3)Ge(40.7) by applying the electron localizability indicator. As expected by the chemical-bonding picture, Ba(8)Au(5.3)Ge(40.7) is a diamagnet and shows p-type electrical conductivity with a hole carrier concentration of 7.14 × 10(19) cm(-3) at 300 K and very low lattice thermal conductivity of ≈0.6 W m(-1) K(-1) at 500 K. The thermoelectric figure of merit ZT of single crystals of Ba(8)Au(5.3)Ge(40.7) attains 0.3 at 511 K and reaches 0.9 at 680 K in a polycrystalline sample of closely similar composition. This opens up an opportunity for tuning of the thermoelectric properties of materials in the Ba-Au-Ge clathrate system by changing the chemical composition.

Discovery of a Superhard Iron Tetraboride Superconductor

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

Superconducting state in SrFe2-xCoxAs2 by internal doping of the iron arsenide layers.

In the strontium iron-cobalt arsenides SrFe2-xCoxAs2 (0.2 < or = x < or = 0.4) superconductivity with T_{c} up to 20 K is observed in magnetic susceptibility, electrical resistivity, and specific heat data. This first observation of bulk superconductivity induced by electron doping in this family of compounds-despite strong disorder in the Fe-As layer-favors an itinerant electronic theory in contrast to the strongly correlated cuprates and renders a p- or d-wave pairing unlikely. The magnetic ordering present in SrFe2As2 is rapidly suppressed by substitution of Fe by Co. Density functional theory calculations show that this is due to a rigid downshift of the Fe-3d_{x;{2}-y;{2}}-related band edge in the density of states.

Exchange bias up to room temperature in antiferromagnetic hexagonal Mn3Ge

Mn3.04Ge0.96 has a hexagonal crystal structure, which can be stabilized by high-temperature annealing, and shows antiferromagnetic order with a small ferromagnetic component of less than 0.1μB and a coercivity of 0.45 T. In the ordered phase, magnetization curves M(H) exhibit an exchange bias of 62 mT at T = 2 K after field cooling, which is observable up to room temperature. The exchange anisotropy is suggested to originate from the exchange interaction between the host of triangular-antiferromagnetic Mn3Ge units and embedded ferrimagnetic-like clusters. Such clusters develop when excess Mn atoms occupy empty Ge sites in the original triangular-antiferromagnetic structure of Mn3Ge.

Large anomalous Hall effect driven by a nonvanishing Berry curvature in the noncolinear antiferromagnet Mn3Ge

A large anomalous Hall effect is observed in the triangular antiferromagnet Mn3Ge arising from an intrinsic electronic Berry phase. It is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that the noncolinear triangular antiferromagnet Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals; the magnitude of the anomalous conductivity is ~500 (ohm·cm)−1 at 2 K and ~50 (ohm·cm)−1 at room temperature. The angular dependence of the anomalous Hall effect measurements confirms that the small residual in-plane magnetic moment has no role in the observed effect except to control the chirality of the spin triangular structure. Our theoretical calculations demonstrate that the large anomalous Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect of 1100 (ħ/e) (ohm·cm)−1, comparable to that of platinum. The present results pave the way toward the realization of room temperature antiferromagnetic spintronics and spin Hall effect–based data storage devices.

The heat capacity of MgCr2O4, FeCr2O4, and Cr2O3 at low temperatures and derived thermodynamic properties

Abstract The heat capacity of synthetic eskolaite, Cr2O3, and of the synthetic spinels magnesiochromite, MgCr2O4, and chromite, FeCr2O4 were measured from 1.5 K to 340 K. For MgCr2O4, a substantial magnetic contribution to the entropy is revealed by a sharp peak in the heat capacity curve at 12.55 ± 0.05 K, which indicates the transition to antiferromagnetic long-range order. Integration of the heat capacity curve yields a value of 118.3 ± 1.2 J/(mol·K) for the standard entropy at 298.15 K, which is in excellent agreement with that calculated from phase equilibria studies on the reaction MgCr2O4+ SiO2 = Cr2O3 + MgSiO3. The new calorimetric results for Cr2O3 indicate a standard entropy at 298.15 K of 82.8 ± 0.8 J/(mol·K). The measurements for FeCr2O4 show three distinct heat capacity anomalies, one of which (peaking at 36.5 ± 0.2 K) was missed by previous low temperature heat capacity measurements, which only extend down to 53 K. Integration of the heat capacity curve yields a value for the standard entropy at 298.15 K of 152.2 ± 3.0 J/(mol·K) for FeCrO4, some 6 J/ (mol·K) greater than the previous calorimetric value. These low-temperature heat capacity data were combined with high-temperature heat content measurements from the literature to derive heat capacity equations for all three phases to 1800 K. The resulting heat capacity equations were then used to extract revised recommended values of the standard enthalpies of formation and entropies of MgCr2O4 and Cr2O3 from phase equilibrium data. For FeCr2O4, the phase equilibrium data are of dubious accuracy, the enthalpy of formation is only approximate

Perovskite-like Mn2O3: a path to new manganites.

Among complex oxides, perovskite-based manganites play a special role in science and technology. They demonstrate colossal magnetoresistance, and can be employed as memory and resistive switching elements or multiferroics. The perovskite structure ABO3 has two different cation sites: B-sites that are octahedrally coordinated by oxygen, and cuboctahedrally-coordinated (often heavily distorted) Asites. The magnetic and transport properties of perovskite manganites are largely determined by the Mn O Mn interactions in the perovskite framework of corner-sharing MnO6 octahedra. Although the A cations do not directly participate in these interactions, they control the Mn valence and the geometry of the Mn O Mn bonds. Complex phenomena, such as charge and orbital ordering, often accompany chemical substitutions on the A-site. Requirements on formal charge and ionic radius are usually different for cations adopting theA or B positions and prevent A/B mixing. Small and often highly charged transition-metal B-cations are unfavorable for the large 12coordinated A-site. Partial filling of the A-position with transition metals is, nevertheless, possible in a unique class of A-site ordered perovskites AA’3B4O12 (where A= alkali, alkali-earth, rare-earth, Pb, or Bi cations, A’=Cu or Mn, and B= transition metals, Ga, Ge, Sb, or Sn). A key ingredient of such compounds is the A’ cation that should be prone to a first-order Jahn–Teller effect (Cu or Mn). An oxygen environment suitable for such transition-metal cations at the A’ position is created by the aaa octahedral tilt system (in Glazer s notation) with a notably large magnitude of the tilt (for example, in CaCu3Ti4O12 the Ti O Ti bond angle is only 140.78). The tilt creates a square-planar anion coordination, favorable for Jahn–Teller-active A’ cations. The ap= ffiffiffi

AFe2As2 (A = Ca, Sr, Ba, Eu) and SrFe2-xTMxAs2 (TM = Mn, Co, Ni): crystal structure, charge doping, magnetism and superconductivity

The electronic structure and physical properties of the pnictide compound families REOFeAs (RE = La, Ce, Pr, Nd, Sm), AFe2As2 (A = Ca, Sr, Ba, Eu), LiFeAs and FeSe are quite similar. Here, we focus on the members of the AFe2As2 family whose sample composition, quality and single-crystal growth are more controllable compared with the other systems. Using first-principles band structure calculations, we focus on understanding the relationship between the crystal structure, charge doping and magnetism in AFe2As2 systems. We will elaborate on the tetragonal to orthorhombic structural distortion along with the associated magnetic order and anisotropy, the influence of doping on the A site and the Fe site and the changes in the electronic structure as a function of pressure. Experimentally, we investigate the substitution of Fe in SrFe2 xTMxAs2 by other 3d transition metals, TM = Mn, Co or Ni. In contrast to a partial substitution of Fe by Co or Ni (electron doping), a corresponding Mn partial substitution does not lead to the suppression of the antiferromagnetic order or the appearance of superconductivity. Most of the calculated properties agree well with the measured properties, but several of them are sensitive to the As z position. For a microscopic understanding of the electronic structure of this new family of superconductors, this structural feature related to the Fe-As interaction is crucial, but its correct ab initio treatment still remains an open question.