Reinhard K. Kremer

Band Filling and Interband Scattering Effects in MgB2: Carbon versus Aluminum Doping

We argue, based on band structure calculations and the Eliashberg theory, that the observed decrease of T(c) of Al and C doped MgB2 samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant lambda by the variation of the density of states as a function of electron doping is sufficient to capture the experimentally observed behavior. Further, we also explain the long standing open question of the experimental observation of a nearly constant pi gap as a function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant pi gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only.

Complicated magnetic behavior in one-dimensional nickel(III) chain complex [1-(4′-cyanobenzyl)pyridinium][Ni(mnt)2](mnt2−=maleonitriledithiolate)

Abstract A new ion-pair complex [1-(4′-cyanobenzyl)pyridinium] [Ni(mnt)2] (1), in which mnt2−=maleonitriledithiolate, have been fabricated and its X-ray single crystal structural analyses at 293, 180 and 140 K shown that the [Ni(mnt)2]− anions and [CNBzPy]+ cations form a well-separated stacking column along c-axis direction, within which [Ni(mnt)2]− anions are uniformly spaced to give a one-dimensional (1-D) chain structure. Bulk magnetic properties of this complex have been investigated in the temperature range of 2–400 K and shown there exists spin kink at ∼190 K. In low temperature region, weak ferromagnetic behavior occurs in 1.

Nonlinear Hall effect and multichannel conduction in LaTiO3/SrTiO3 superlattices

We report magnetotransport properties of heterointerfaces between the Mott insulator LaTiO{sub 3} and the band insulator SrTiO{sub 3} in a delta-doping geometry. At low temperatures, we have found a strong nonlinearity in the magnetic field dependence of the Hall resistivity, which can be effectively controlled by varying the temperature and the electric field. We attribute this effect to multichannel conduction of interfacial charges generated by an electronic reconstruction. In particular, the formation of a highly mobile conduction channel revealed by our data is explained by the greatly increased dielectric permeability of SrTiO{sub 3} at low temperatures and its electric field dependence reflects the spatial distribution of the quasi-two-dimensional electron gas.

CRYSTAL AND ELECTRONIC STRUCTURES OF SCAUGE, CEAUGE, AND LUAUGE: A TRANSITION FROM TWO- TO THREE-DIMENSIONAL AUGE POLYANIONS

Abstract New germanides ScAuGe, TmAuGe, and LuAuGe were prepared by melting mixtures of the elements in an arc furnace and subsequent annealing at 1070 K. The structures of ScAuGe and LuAuGe were refined from X-ray single-crystal diffractometer data: P6 3 mc (No. 186), Z = 2, a = 430.82(5) pm, c = 684.58(10) pm, V = 0.1100(1) nm 3 , wR 2 = 0.0688, 275 F o 2 values, ten variables for ScAuGe; P6 3 mc (No. 186), Z = 2, a = 437.75(4) pm, c = 711.38(6) pm, V = 0.1181(1) nm 3 , wR 2 = 0.0340, 355 F o 2 values, 11 variables, and a batch scale factor of 0.47(3) for LuAuGe. The lattice constants for TmAuGe are a = 439.08(4) pm, c = 716.59(7) pm, and V = 0.1196(1) nm 3 . The crystal structures of these germanides are derived from the CaIn 2 -type structure by an ordered arrangement of Au and Ge atoms at the indium position. The crystal chemistry of ScAuGe and LuAuGe is compared with that of the recently reported cerium compound. Although the AuGe intralayer distances at 259.6 pm in CeAuGe, 260.5 pm in LuAuGe, and 257.6 pm in ScAuGe are similar, the AuGe interlayer distances at 364.2 pm in CeAuGe, 292.7 pm in LuAuGe, and 275.2 pm in ScAuGe differ significantly. Thus, the [AuGe] polyanions are changed from two-dimensional layers in CeAuGe to a three-dimensional network of distorted tetrahedra in ScAuGe. Chemical bonding within the structures was investigated by TB-LMTO-ASA band structure calculations. The energy bands, the densities of states and the valence charge densities are discussed. Bonding is characterized by intralayer bonds between Au and Ge within the puckered AuGe hexagons. In addition, in the case of ScAuGe strong bonds are directed from the Au atoms of one layer to the Ge atoms of the neighbouring layer. Weaker interlayer bonding is observed in LuAuGe and no interlayer interaction in CeAuGe, as already indicated by the pronounced increase of the interlayer distances.

Weak ferromagnetism and magnetically modulated microwave absorption at low magnetic fields in 1,3,5-triphenyl-6-oxoverdazyl

Abstract The magnetic susceptibility and microwave properties of powder samples of 1,3,5-triphenyl-6-oxoverdazyl were investigated between room temperature and 1.8 K. The high-temperature susceptibility follows a Curie-Weiss law with a Curie constant of 0.38(1) emu K mol −1 and a Weiss constant of −12(2) K. At low temperature we find 1D magnetic behaviour evidenced by a broad maximum in the susceptibility. Below 4.9 K weak ferromagnetism is detected which is attributed to a canting of the antiferromagnetic sublattices of about 0.13°. Strong nonresonant microwave absorption appears below 4 K which exhibits a hysteresis when measured in a small scanned external field. Possible origins of the weak ferromagnetism and the nonresonant microwave absorption are discussed.

Ferromagnetic ordering in CeAuGe

Abstract CeAuGe was prepared by reaction of the elemental components in an arc-melting furnace and subsequent annealing at 800°C. Its hexagonal crystal structure (NdPtSb-type) was refined from single-crystal data of an inversion twin: P6 3 mc, Z = 2, a = 446.03(7) pm, c = 793.60(12) pm, V = 0.1367(1) nm 3 , wR 2 = 0.060, 213 F 2 values, 11 variables and BASF= 0.52(3). In the paramagnetic state, the susceptibility of CeAuGe shows Curie-Weiss behaviour above 50 K with μ exp = 2.55(5)μ B and Θ P = −5(1) K. CeAuGe orders ferromagnetically at 10.0(2) K. Its saturation magnetic moment at 5 K is 1.09(2)μ B /Ce. Electrical resistivity measurements show metallic behaviour.

Thermodynamics of Two-Band Superconductors: The Case of MgB2

Thermodynamic properties of the multiband superconductor MgB2 have often been described using a simple sum of the standard BCS expressions corresponding to sigma- and pi bands. However, it is a priori not clear if this approach is working always adequately, particularly in cases of strong interband scattering. Here we compare the often used approach of a sum of two independent bands using BCS-like alpha model expressions for the specific heat, entropy, and free energy to the solution of the full Eliashberg equations. The superconducting energy gaps, the free energy, the entropy, and the heat capacity for varying interband scattering rates are calculated within the framework of two-band Eliashberg theory. We obtain good agreement between the phenomenological two-band alpha model with the Eliashberg results, which delivers the theoretical verification to use the alpha model as a useful tool for a reliable analysis of heat capacity data. For the thermodynamic potential and the entropy we demonstrate that only the sum over the contributions of the two bands has physical meaning.

CuBr2--a new multiferroic material with high critical temperature.

A new multiferroic material, CuBr(2) , is reported for the first time. CuBr(2) has not only a high transition temperature (close to liquid nitrogen temperature) but also low dielectric loss and strong magnetoelectric coupling. These findings reveal the importance of anion effects, in the search for the high temperature multiferroics materials among these low-dimensional spin systems.

X-ray structural investigation of an untwinned single crystal of orthorhombic YBa2Cu3O6.93

Abstract Crystals of YBa 2 Cu 3 O 6.93 were grown in a SnO 2 crucible. Parts of these crystals were untwinned and isolated for X-ray structure investigations at seven temperatures between 103 and 303 K. The positions of the chain atoms Cu(1) and O(4) are occupied at 100 and 93%, respectively. There is no evidence for oxygen atoms between the chains (site O(5) in ( 1 2 0 0) ). The temperature-dependent changes of the lattice constants and the positional shifts of O(1), Cu(2), and Ba are discussed.

Quantum Behavior of Water Molecules Confined to Nanocavities in Gemstones.

When water is confined to nanocavities, its quantum mechanical behavior can be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores of a beryl crystal lattice and observe a rich and highly anisotropic set of absorption lines in the terahertz spectral range. Two bands can be identified, which originate from translational and librational motions of the water molecule isolated within the cage; they correspond to the analogous broad bands in liquid water and ice. In the present case of well-defined and highly symmetric nanocavities, the observed fine structure can be explained by macroscopic tunneling of the H2O molecules within a six-fold potential caused by the interaction of the molecule with the cavity walls.