arXiv subjects

Brian C. Sales

Publications and source records attributed to Brian C. Sales.

At least 55 records · Page 3Linked to original sources

arXiv2011

Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling

The effects of various transition metal dopants on the electrical and thermal transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal conductivity data among Os, Ir and Co doped alloys indicates strong electron-phonon coupling in this compound. Because of this interaction, the common approximation of dividing the total thermal conductivity into independent electronic and lattice components (κTotal = κelectronic + κlattice) fails for these alloys. The effects of grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also reported. The thermal conductivity can be lowered by about 50% with little or no effect on the electrical resistivity or Seebeck coefficient. This results in ZTmax = 0.125 at 100 K, still about a factor of five too low for solid-state refrigeration applications.

cond-mat.str-el↗

arXiv2011

Unusual Phase Transitions and Magnetoelastic Coupling in TlFe1.6Se2 Single Crystals

Structural, magnetic, electrical transport, and heat capacity data are reported for single crystals of TlFe1.6Se2. This compound crystallizes in a tetragonal structure similar to the ThCr2Si2 structure, but with vacancies in the Fe layer. The vacancies can be ordered or disordered depending on temperature and thermal history. If the vacancies are ordered, the basal plane lattice constant increases from a to \sqrt{5}a. Antiferromagnetic order with the Fe spins along the c-axis occurs below T_N ~ 430K as shown by single crystal neutron diffraction and the magnetic structure is reported. In addition, for the vacancy ordered crystal, two other phase transitions are found at T_1 ~ 140K, and T_2 ~ 100K. The phase transitions at T_1 and T_2 are evident in heat capacity, magnetic susceptibility, resistivity data, a and c lattice parameters, and in the unusual temperature dependence of the magnetic order parameter determined from neutron scattering. The phase transitions at T_1 and T_2 result in significant changes in the magnetic moment per iron, with 1.72(6)μ_B observed at 300K, 2.07(9)μ_B at 140\,K, 1.90(9)\,μ_B at 115\,K, and 1.31(8)μ_B for 5\,K if the same "block checkerboard" magnetic structure is used at all temperatures. The phase transitions appear to be driven by small changes in the c lattice constant, large magnetoelastic coupling, and the localization of carriers with decreasing temperature.

cond-mat.supr-con↗

arXiv2011

Structural and Magnetic Phase Transitions in NdCoAsO under High Pressures

We have investigated structural and magnetic phase transitions under high pressures in a quaternary rare earth transition metal arsenide oxide NdCoAsO compound that is isostructural to high temperature superconductor NdFeAsO. Four-probe electrical resistance measurements carried out in a designer diamond anvil cell show that the ferromagnetic Curie temperature and anti-ferromagnetic Neel temperature increase with an increase in pressure. High pressure x-ray diffraction studies using a synchrotron source show a structural phase transition from a tetragonal phase to a new crystallographic phase at a pressure of 23 GPa at 300 K. The NdCoAsO sample remained anti-ferromagnetic and non-superconducting to temperatures down to 10 K and to the highest pressure achieved in this experiment of 53 GPa. A P-T phase diagram for NdCoAsO is presented to a pressure of 53 GPa and low temperatures of 10 K.

cond-mat.supr-con↗

arXiv2011

Formation of Collapsed Tetragonal Phase in EuCo2As2 under High Pressure

The structural properties of EuCo2As2 have been studied up to 35 GPa, through the use of x-ray diffraction in a diamond anvil cell at a synchrotron source. At ambient conditions, EuCo2As2 (I4/mmm) has a tetragonal lattice structure with a bulk modulus of 48 +/-4 GPa. With the application of pressure, the a-axis exhibits negative compressibility with a concurrent sharp decrease in c-axis length. The anomalous compressibility of the a-axis continues until 4.7 GPa, at which point the structure undergoes a second-order phase transition to a collapsed tetragonal (CT) state with a bulk modulus of 111 +/- 2 GPa. We found a strong correlation between the ambient pressure volume of 122 parents of superconductors and the corresponding tetragonal to collapsed tetragonal phase transition pressures

cond-mat.mtrl-sci↗

arXiv2011

Search for pressure induced superconductivity in CeFeAsO and CeFePO iron pnictides

The CeFeAsO and CeFePO iron pnictide compounds were studied via electrical transport measurements under high-pressure. In CeFeAsO polycrystals, the magnetic phases involving the Fe and Ce ions coexist up to 15 GPa, with no signs of pressure-induced superconductivity up to 50 GPa. For the CeFePO single crystals, pressure stabilizes the Kondo screening of the Ce 4f-electron magnetic moments.

cond-mat.str-el↗

arXiv2011

Anomalous Compressibility Effects and Superconductivity in EuFe2As2 under High Pressures

The crystal structure and electrical resistance of the structurally-layered EuFe2As2 have been studied up to 70 GPa and down to temperature of 10 K, using a synchrotron x-ray source and the designer diamond anvils. The room-temperature compression of the tetragonal phase of EuFe2As2 (I4/mmm) results in an increase in the a-axis and a rapid decrease in c-axis with increasing pressure. This anomalous compression reaches a maximum at 8 GPa and the tetragonal lattice behaves normal above 10 GPa with a nearly constant c/a axial ratio. The rapid rise in superconducting transition temperature (Tc) to 41 K with increasing pressure is correlated to this anomalous compression and a decrease in Tc is observed above 10 GPa. We present P-V data or equation of state of EuFe2As2 in both the ambient tetragonal phase and the high pressure collapsed tetragonal phase to 70 GPa.

cond-mat.supr-con↗

arXiv2010

Control of Correlations in Sr4V2O6Fe2As2 by Chemical Stoichiometry

We show using a combination of powder X-ray and neutron diffraction, first principles calculations, temperature- and field-dependent magnetization, heat capacity and resistivity data that the superconducting behavior of `Sr$_4$V$_2$O$_6$Fe$_2$As$_2$' is dependent on synthesis conditions, particularly, heating profiles result in unintentional chemical doping. This compound can be tuned from a state in which the vanadium electrons are itinerant with a high electronic density of states, to a state where the vanadium-oxide layers are insulating and presumably antiferromagnetic.

cond-mat.supr-con↗

arXiv2010

Iron substitution in NdCoAsO: crystal structure and magnetic phase diagram

The effects of replacing small amounts of Co with Fe in NdCoAsO are reported. Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10, 0.15, and 0.20) are studied and the results compared to previous reports for NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe replaces Co on the transition metal (T) site, the T-As distance increases, and the As tetrahedra surrounding the T-site become more regular. Electrical resistivity and magnetization measurements indicate that the three magnetic phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these transitions are not observed above 1.8 K for x = 0.20. Based on these results, the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system is constructed.

cond-mat.str-el↗

arXiv2010

Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

cond-mat.supr-con↗

arXiv2010

Magnetic Phase Transitions in NdCoAsO

Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase transitions below room temperature. The crystal and magnetic structures of NdCoAsO have been determined by powder neutron diffraction, and the effects of the phase transitions on physical properties are reported. Near 69 K a ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and corresponds to the antiferromagnetic ordering, with the same propagation vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In addition, transport properties and heat capacity results are presented, and show anomalies at all three phase transitions.

cond-mat.supr-con↗

arXiv2009

Unusual Relationship between Magnetism and Superconductivity in FeTe$_{0.5}$Se$_{0.5}$

We use neutron scattering, to study magnetic excitations in crystals near the ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.

cond-mat.supr-con↗

arXiv2009

Suppression of spin density wave by isoelectronic substitution in PrFe(1-x)Ru(x)AsO

We have studied the effects of the isoelectronic substitution of Ru for Fe in polycrystalline samples of the spin density wave (SDW) material PrFeAsO. Crystal structures from powder x-ray diffraction at room temperature and transport properties from 2-300 K are reported. The SDW is completely suppressed upon Ru substitution. The distortion of the tetrahedral coordination environment of the transition metal site increases as the Ru concentration increases, which may be related to the absence of superconductivity above 2 K. Band structure calculations show that the larger size of Ru 4$d$ orbitals is primarily responsible for the suppression of magnetism as Ru is substituted into the Fe layer. The experimental results indicate that long-ranged magnetic order of Pr moments is suppressed at Ru concentrations as low as 10%.

cond-mat.supr-con↗

arXiv2009

Aligned crystallite powder of NdFeAsO$_{0.86}$F$_{0.14}$: magnetic hysteresis and penetration depth

We report the basal-plane critical current and superfluid density of magnetically aligned NdFeAsO$_{0.86}$F$_{0.14}$ powder. This sample has individual crystallite grains permanently oriented with their c axis along the external field. Magnetic irreversibilities at high field suggest strong flux pinning of basal-plane critical currents, with monotonic field dependence and no evidence of the "fishtail" effect. The small particles provide a sensitive indicator of \textit{dc} flux penetration, and allow analysis of the temperature dependence of $ab-$plane London penetration depth $λ_{ab,\mathrm{L}}$, which is quadratic at low $T$. This feature may not necessarily be due to the nodes in the gap, but may be rather a sign of a strong pair-breaking. A quantitative determination of the absolute magnitude of $λ_{ab,\mathrm{L}}$ is hindered by the need for accurate knowledge of the particle size distribution.

cond-mat.supr-con↗

arXiv2009

Neutron scattering patterns show Superconductivity in FeTe0.5Se0.5 likely results from itinerant electron fluctuations

The discovery of the Fe pnictide superconductors generated great interest as the structure consists of planes of a magnetic material quite similar to the cuprate superconductors. Fe(Te0.5Se0.5) is a particularly simple system whose planes are isostructural to the FeAs layers found in the originally discovered superconductors of this type. We report here neutron scattering measurements on this material that provide an understanding of the superconductivity. Since the information about the Fermi surface is available both from photoemission and band structure calculations for FeTe, FeSe and other related materials the neutron spectra can be used to see if the itinerant electron picture is valid. The results are consistent with a picture where there are both electron and hole Fermi surfaces that make exact (pi,pi) transitions possible. This would normally favor either a spin or charge density wave state. However, our measurements show the extent of the region where (pi,pi) transitions take place and demonstrate that there are a much larger number of transitions near pi,pi). The near (pi,pi) transitions are observed both above and below Tc and are expected to be strongly pairing. The superconductivity can be attributed to these excitations while the exactly (pi,pi) transitions produce the narrow resonance excitation that appears below Tc.

cond-mat.supr-con↗

arXiv2009

Hole-doped BaFe2-xCrxAs2 Crystals: A Case of Non-superconductivity

We investigate the physical properties and electronic structure upon Cr-doping in the iron arsenide layers of BaFe2As2. This form of hole-doping leads to suppression of the magnetic/structural phase transition in BaFe2-xCrxAs2 for x > 0, but does not lead to superconductivity. For various x values, temperature dependence of the resistivity, specific heat, magnetic susceptibility, Hall coefficient, and single crystal x-ray diffraction data are presented. The materials show signatures of approaching a ferromagnetic state with x, including a metamagnetic transition for x as little as 0.36, an enhanced magnetic susceptibility, and a large Sommerfeld coefficient. Such results reflect renormalization due to spin fluctuations and they are supported by density functional calculations at x = 1. Calculations show a strong interplay between magnetic ordering and chemical ordering of Fe and Cr, with a ferromagnetic ground state. This ferromagnetic ground state is explained in terms of the electronic structure. The resulting phase diagram is suggestive that superconductivity does not derive simply from the suppression of the structural/magnetic transitions.

cond-mat.supr-con↗

arXiv2009

BaT2As2 Single Crystals (T = Fe, Co, Ni) and Superconductivity upon Co-doping

The crystal structure and physical properties of BaFe2As2, BaCo2As2, and BaNi2As2 single crystals are surveyed. BaFe2As2 gives a magnetic and structural transition at TN = 132(1) K, BaCo2As2 is a paramagnetic metal, while BaNi2As2 has a structural phase transition at T0 = 131 K, followed by superconductivity below Tc = 0.69 K. The bulk superconductivity in Co-doped BaFe2As2 below Tc = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe1.84Co0.16As2 indicate the inter-band scattering due to Co is weak.

cond-mat.supr-con↗

arXiv2009

Influence of the rare-earth element on the effects of the structural and magnetic phase transitions in CeFeAsO, PrFeAsO, and NdFeAsO

We present results of transport and magnetic properties and heat capacity measurements on polycrystalline CeFeAsO, PrFeAsO, and NdFeAsO. These materials undergo structural phase transitions, spin density wave-like magnetic ordering of small moments on iron, and antiferromagnetic ordering of rare earth moments. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient, and magnetoresistance are reported. The magnetic behavior of the materials have been investigated using Mossbauer spectroscopy and magnetization measurements. Transport and magnetic properties are affected strongly by the structural and magnetic transitions, suggesting significant changes in the band structure and/or carrier mobilities occur, and phonon-phonon scattering is reduced upon transformation to the low temperature structure. Results are compared to recent reports for LaFeAsO, and systematic variations in properties as the identity of Ln is changed are observed and discussed. As Ln progresses across the rare-earth series from La to Nd, an increase in the hole contributions to Seebeck coefficient, and increases in magnetoresistance and the Hall coefficient are observed in the low temperature phase. Analysis of hyperfine fields at the iron nuclei determined from Mossbauer spectra indicates that the moment on Fe in the orthorhombic phase is nearly independent of the identity of Ln, in apparent contrast to reports of powder neutron diffraction refinements.

cond-mat.supr-con↗

arXiv2009

Structure and Anisotropic Properties of BaFe2-xNixAs2 (x = 0, 1, 2) Single Crystals

The crystal structure, anisotropic electrical resistivity and magnetic susceptibility, as well as specific heat results from single crystals of BaFe2As2, BaNi2As2, and BaFeNiAs2 are surveyed. BaFe2As2 properties demonstrate the equivalence of C(T), Fisher's d(XT)/dT, and dp/dT results in determining the antiferromagnetic transition at TN = 132(1) K. BaNi2As2 shows a structural phase transition from a high-temperature tetragonal phase to a low-temperature triclinic (P-1) phase at T0 = 131 K.The superconducting critical temperature for BaNi2As2 is well below T0 and at Tc = 0.69 K. BaFeNiAs2 does not show any sign of superconductivity to 0.4 K and exhibits properties similar to BaCo2As2, a renormalized paramagnetic metal.

cond-mat.supr-con↗