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Mandrus, D.

Publications and source records attributed to Mandrus, D..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Spectroscopic Evidence for Strong Quantum Spin Fluctuations with Itinerant Character in YFe 2 Ge 2

We report x-ray absorption and photoemission spectroscopy of the electronic structure in the normal state of metallic YFe 2 Ge 2 . The data reveal evidence for large fluctuating spin moments on the Fe sites, as indicated by exchange multiplets appearing in the Fe 3s core-level photoemission spectra, even though the compound does not show magnetic order. The magnitude of the multiplet splitting is comparable to that observed in the normal state of the Fe-pnictide superconductors. This shows a connection between YFe 2 Ge 2 and the Fe-based superconductors even though it contains neither pnictogens nor chalcogens. Finally, the implication is that the chemical range of compounds showing at least one of the characteristic magnetic signatures of the Fe-based superconductors is broader than previously thought.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetotransport of proton-irradiated BaFe 2 As 2 and BaFe 1.985 Co 0.015 As 2 single crystals

In this paper, we study the magnetotransport properties of the ferropnictide crystals BaFe 2 As 2 and BaFe 1.985 Co 0.015 As 2 . These materials exhibit a high field linear magnetoresistance that has been attributed to the quantum linear magnetoresistance model. In this model, the linear magnetoresistance is dependent on the concentration of scattering centers in the material. By using proton-beam irradiation to change the defect scattering density, we find that the dependence of the magnitude of the linear magnetoresistance on scattering quite clearly contravenes this prediction. Finally, a number of other scaling trends in the magnetoresistance and high field Hall data are observed and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magnetic ordering in the frustrated J 1 - J 2 Ising chain candidate BaNd 2 O 4

The AR 2 O 4 family (R = rare earth) has recently been attracting interest as a new series of frustrated magnets, with the magnetic R atoms forming zigzag chains running along the c axis. In this paper, we have investigated polycrystalline BaNd 2 O 4 with a combination of magnetization, heat-capacity, and neutron powder diffraction measurements. Magnetic Bragg peaks are observed below T N = 1.7 K, and they can be indexed with a propagation vector of k = (0,1/2,1/2). The signal from magnetic diffraction is well described by long-range ordering of only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations (double Néel state). Furthermore, low-temperature magnetization and heat-capacity measurements reveal two magnetic-field-induced spin transitions at 2.75 and 4 T for T = 0.46 K. The high-field phase is paramagnetic, while the intermediate-field state may arise from a spin transition of the long-range ordered Nd chains. Finally, one possible candidate for the field-induced ordered state corresponds to an up-up-down intrachain spin configuration, as predicted for a classical J 1 -J 2 Ising chain with a double Néel ground state in zero field.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Infrared pseudogap in cuprate and pnictide high-temperature superconductors

We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors, YBa2Cu3Oy and BaFe2As2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500cm-1 followed by a featureless continuum between 500 and 1500cm-1 in the conductivity data and a significant suppression in the scattering rate below 700–900 cm-1. The latter result allows us to identify the energy scale associated with the pseudogap ΔPG. We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100–200 cm-1, which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model, which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below ΔPG is solely due to the pseudogap formation, whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism

36 MATERIALS SCIENCE↗
DOE OSTI2014

Polymorphism, phase transitions, and thermal expansion of K3Lu(PO4)2

Alkali rare-earth double phosphates have been studied for use as long-wavelength scintillators for -ray detection using Si photodiodes. Single-crystal and powder x-ray diffraction (XRD) and powder neutron diffraction have been used to study the structure as a function of temperature. K3Lu(PO4)2 crystallizes with a hexagonal unit cell at room temperature, space group P 3. The Lu ion is six-coordinated to the oxygen atoms of the phosphate groups. Two lower-temperature phases were characterized using single-crystal XRD and powder neutron diffraction. The first transition occurs at 230 K with a transformation to a monoclinic P21/m space group symmetry, and the Lu retains six coordination. The second phase transition occurs at 130 K, with a large change in the cell volume, keeping the same P21/m space group symmetry; however, one of the phosphate groups rotates to increase the coordination of the Lu ion to seven. This is an unusual example of an isosymmetric phase transition with a coordination change, driven by temperature. High-temperature powder neutron diffraction and high-temperature powder XRD have been used to study the thermal expansion of K3Lu(PO4)2 and indicate a large thermal expansion anisotropy. The crystallographic axes with largest changes account for the structural collapse, which rotates the phosphate group to increase the Lu coordination. The lowest temperature form of K3Lu(PO4)2 is the same as the room temperature form for all the lighter RE compounds of the same type, which is not surprising, given the lighter (larger) RE ions would prefer a higher coordination number.

Farmer, James Matthew↗
DOE OSTI2014

Jeff=1/2 Mott spin-orbit insulating state close to the cubic limit in Ca4IrO6

The J_eff=1/2 state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and strong evidence for this state in Sr2IrO4, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical J_eff=1/2 state, impacting the development of a robust model of this novel insulating and magnetic state. We present experimental and theoretical results that not only show Ca4IrO6 hosts the state, but furthermore uniquely resides in the limit required for a canonical unmixed J_eff=1/2 state.

Calder, Stuart A↗