arXiv subjects

Michael A. McGuire

Publications and source records attributed to Michael A. McGuire.

At least 91 records · Page 5Linked to original sources

arXiv2014

Orbital occupancy and charge doping in iron-based superconductors

Iron-based superconductors (FBS) comprise several families of compounds having the same atomic building blocks for superconductivity, but large discrepancies among their physical properties. A longstanding goal in the field has been to decipher the key underlying factors controlling TC and the various doping mechanisms. In FBS materials this is complicated immensely by the different crystal and magnetic structures exhibited by the different families. In this paper, using aberration-corrected scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), we observe a universal behavior in the hole concentration and magnetic moment across different families. All the parent materials have the same total number of electrons in the Fe 3d bands; however, the local Fe magnetic moment varies due to different orbital occupancy. Although the common understanding has been that both long-range and local magnetic moments decrease with doping, we find that, near the onset of superconductivity, the local magnetic moment increases and shows a dome-like maximum near optimal doping, where no ordered magnetic moment is present. In addition, we address a longstanding debate concerning how Co substitutions induces superconductivity in the 122 arsenide family, showing that the 3d band filling increases a function of doping. These new microscopic insights into the properties of FBS demonstrate the importance of spin fluctuations for the superconducting state, reveal changes in orbital occupancy among different families of FBS, and confirm charge doping as one of the mechanisms responsible for superconductivity in 122 arsenides.

cond-mat.supr-con↗

arXiv2014

Effect of Eu magnetism on the electronic properties of the candidate Dirac material EuMnBi2

The crystal structure and physical properties of the layered material EuMnBi2 have been characterized by measurements on single crystals. EuMnBi2 is isostructural with the Dirac material SrMnBi2 based on single crystal x-ray diffraction, crystallizing in the I4/mmm space group (No. 139). Magnetic susceptibility measurements suggest antiferromagnetic (AFM) ordering of moments on divalent Eu ions near T_N=22K. For low fields, the ordered Eu moments are aligned along the c-axis, and a spin-flop is observed near 5.4T at 5K. The moment is not saturated in an applied field of 13T at 5K, which is uncommon for compounds containing Eu^{2+}. The magnetic behavior suggests an anisotropy enhancement via interaction between Eu and the Mn moments that appear to be order antiferromagnetically below approximately 310K. A large increase in the magnetoresistance is observed across the spin-flop, with absolute magnetoresistance reaching approximately 650% at 5K and 12T. Hall effect measurements reveal a decrease in the carrier density below T_N, which implies a manipulation of the Fermi surface by magnetism on the sites surrounding the Bi square nets that lead to Dirac cones in this family of materials.

cond-mat.str-el↗

arXiv2014

Tuning the magnetic and structural phase transitions of PrFeAsO via Fe/Ru spin dilution

Neutron diffraction and muon spin relaxation measurements are used to obtain a detailed phase diagram of Pr(Fe,Ru)AsO. The isoelectronic substitution of Ru for Fe acts effectively as spin dilution, suppressing both the structural and magnetic phase transitions. The temperature of the tetragonal-orthorhombic structural phase transition decreases gradually as a function of x. Slightly below the transition temperature coherent precessions of the muon spin are observed corresponding to static magnetism, possibly reflecting a significant magneto-elastic coupling in the FeAs layers. Short range order in both the Fe and Pr moments persists for higher levels of x. The static magnetic moments disappear at a concentration coincident with that expected for percolation of the J1-J2 square lattice model.

cond-mat.supr-con↗

arXiv2014

Competing magnetic phases and field-induced dynamics in DyRuAsO

Analysis of neutron diffraction, dc magnetization, ac magnetic susceptibility, heat capacity, and electrical resistivity for DyRuAsO in an applied magnetic field are presented at temperatures near and below those at which the structural distortion (T_S = 25 K) and subsequent magnetic ordering (T_N = 10.5 K) take place. Powder neutron diffraction is used to determine the antiferromagnetic order of Dy moments of magnitude 7.6(1) mu_B in the absence of a magnetic field, and demonstrate the reorientation of the moments into a ferromagnetic configuration upon application of a magnetic field. Dy magnetism is identified as the driving force for the structural distortion. The magnetic structure of analogous TbRuAsO is also reported. Competition between the two magnetically ordered states in DyRuAsO is found to produce unusual physical properties in applied magnetic fields at low temperature. An additional phase transition near T* = 3 K is observed in heat capacity and other properties in fields greater than about 3 T. Magnetic fields of this magnitude also induce spin-glass-like behavior including thermal and magnetic hysteresis, divergence of zero-field-cooled and field-cooled magnetization, frequency dependent anomalies in ac magnetic susceptibility, and slow relaxation of the magnetization. This is remarkable since DyRuAsO is a stoichiometric material with no disorder detected by neutron diffraction, and suggests analogies with spin-ice compounds and related materials with strong geometric frustration.

cond-mat.str-el↗

arXiv2013

The influence of spin fluctuations on the thermal conductivity in superconducting Ba(Fe_{1-x}Co_x)_2As_2

The thermal conductivity of electron-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals is investigated below 200K, with an emphasis on the behavior near the magnetic and superconducting (T_c) transition temperatures. An enhancement of the in-plane thermal conductivity $κ_{ab}$ is observed below T_c for all samples, with the greatest enhancement observed near optimal doping. The observed trends are consistent with the scattering of heat carriers by low-energy magnetic excitations. Upon entering the superconducting state, the formation of a spin-gap leads to reduced scattering and an enhancement in $κ(T)$. Similarly, an enhancement of $κ$ is observed for polycrystalline BaFe2As2 below the magnetic transition, and qualitative differences in $κ(T)$ between single crystalline and polycrystalline BaFe2As2 are utilized to discuss anisotropic scattering. This study highlights how measuring $κ$ near $T_c$ in novel superconductors can be useful as a means to probe the potential role of spin fluctuations.

cond-mat.supr-con↗

arXiv2013

Origin of the phase transition in IrTe2: structural modulation and local bonding instability

We used X-ray/neutron diffraction to determine the low temperature (LT) structure of IrTe2. A structural modulation was observed with a wavevector of k =(1/5, 0, 1/5) below Ts?285 K, accompanied by a structural transition from a trigonal to a triclinic lattice. We also performed the first principles calculations for high temperature (HT) and LT structures, which elucidate the nature of the phase transition and the LT structure. A local bonding instability associated with the Te 5p states is likely the origin of the structural phase transition in IrTe2.

cond-mat.str-el↗

arXiv2013

Thermoelectric Properties of Polycrystalline NiSi3P4

The Hall and Seebeck coefficients, electrical resistivity and thermal conductivity of polycrystalline NiSi3P4 were characterized from 2 to 775K. Undoped NiSi3P4 behaves like a narrow gap semiconductor, with activated electrical resistivity ρbelow room temperature and a large Seebeck coefficient of ~400uV/K at 300K. Attempts to substitute boron for silicon resulted in the production of extrinsic holes, yielding moderately-doped semiconductor behavior with ρincreasing with increasing temperature above ~150\,K. Hall carrier densities are limited to approximately 5x10^{19}/cm^3 at 200K, which would suggest the solubility limit of boron is reached if boron is indeed incorporated into the lattice. These extrinsic samples have a Hall mobility of ~12cm^2/V/s at 300K, and a parabolic band equivalent effective mass of ~3.5 times the free electron mass. At 700,K, the thermoelectric figure of merit zT reaches ~0.1. Further improvements in thermoelectric performance would require reaching higher carrier densities, as well as a mechanism to further reduce the lattice thermal conductivity, which is ~5W/m/K at 700K. Alloying in Ge results in a slight reduction of the thermal conductivity at low temperatures, with little influence observed at higher temperatures.

cond-mat.mtrl-sci↗

arXiv2013

Transport, Thermal, and Magnetic Properties of the Narrow-Gap Semiconductor CrSb2

Resistivity, Hall effect, Seebeck coefficient, thermal conductivity, heat capacity, and magnetic susceptibility data are reported for CrSb2 single crystals. In spite of some unusual features in electrical transport and Hall measurements below 100 K, only one phase transition is found in the temperature range from 2 to 750 K corresponding to long-range antiferromagnetic order below T_N ~ 273 K. Many of the low temperature properties can be explained by the thermal depopulation of carriers from the conduction band into a low mobility band located approximately 16 meV below the conduction band edge, as deduced from the Hall effect data. In analogy with what occurs in Ge, the low mobility band is likely an impurity band. The Seebeck coefficient, S, is large and negative for temperatures from 2 to 300 K ranging from ~ -70\muV/K at 300 K to -4500\muV/K at 18 K. A large maximum in |S| at 18 K is likely due to phonon drag with the abrupt drop in |S| below 18 K due to the thermal depopulation of the high mobility conduction band. The large thermal conductivity between 10 and 20 K (~350 W/m/K) is consistent with this interpretation, as are detailed calculations of the Seebeck coefficient made using the complete calculated electronic structure. These data are compared to data reported for FeSb2, which crystallizes in the same marcasite structure, and FeSi, another unusual narrow-gap semiconductor.

cond-mat.str-el↗

arXiv2012

Spin reorientation in TlFe1.6Se2 with complete vacancy ordering

The relationship between vacancy ordering and magnetism in TlFe1.6Se2 has been investigated via single crystal neutron diffraction, nuclear forward scattering, and transmission electron microscopy. The examination of chemically and structurally homogenous crystals allows the true ground state to be revealed, which is characterized by Fe moments lying in the ab-plane below 100K. This is in sharp contrast to crystals containing regions of order and disorder, where a competition between c-axis and ab-plane orientations of the moments is observed. The properties of partially-disordered TlFe1.6Se2 are therefore not associated with solely the ordered or disordered regions. This contrasts the viewpoint that phase separation results in independent physical properties in intercalated iron selenides, suggesting a coupling between ordered and disordered regions may play an important role in the superconducting analogues.

cond-mat.supr-con↗

arXiv2012

Structural and physical properties of layered oxy-arsenides LnRuAsO (Ln = La, Nd, Sm, Gd)

Polycrystalline samples of LaRuAsO, NdRuAsO, SmRuAsO, and GdRuAsO have been synthesized and studied using powder x-ray diffraction, electrical transport, magnetization, and heat capacity measurements. Variations in structural properties across the series reveal a trend toward more ideal tetrahedral coordination around Ru as the size of the rare earth element is reduced. The lattice parameters of these Ru compounds show a more anisotropic response to variation in Ln than their Fe analogues, and significant anisotropy in thermal expansion is also observed. Transport measurements show metallic behavior, and carrier concentrations near 10^21 - 10^22 electrons per cm^3 are inferred from simple analysis of Hall effect measurements. Anomalies in resistivity, magnetization, and heat capacity indicate antiferromagnetic ordering of rare earth moments at 5 K for GdRuAsO, 4.5 K for SmRuAsO, and <2 K for NdRuAsO. Magnetization measurements on LaRuAsO show no evidence of a magnetic moment on Ru. Observed behaviors are compared to those reported for similar Fe and Ru compounds.

cond-mat.str-el↗

arXiv2012

Physical properties of Ce3-xTe4 below room temperature

The physical properties of polycrystalline Ce3-xTe4 were investigated by measurements of the thermoelectric properties, Hall coefficient, heat capacity, and magnetization. The fully-filled, metallic x=0 compound displays a soft ferromagnetic transition near 4K, and analysis of the corresponding heat capacity anomaly suggests a doublet ground state for Ce^{3+}. The transition is suppressed to below 2K in the insulating x=0.33 composition, revealing that magnetic order in Ce3-xTe4 is driven by an RKKY-type interaction. The thermoelectric properties trend with composition as expected from simple electron counting, and the transport properties in Ce3Te4 are observed to be similar to those in La3Te4. Trends in the low temperature thermal conductivity data reveal that the phonons are efficiently scattered by electrons, while all compositions examined have a lattice thermal conductivity near 1.2W/m/K at 200K.

cond-mat.mtrl-sci↗

arXiv2012

Neutron Diffraction Study of anomalous negative thermal expansion in non-superconducting PrFe1-xRuxAsO

Neutron powder diffraction has been used to investigate the structural and magnetic behavior of the isoelectronically doped Fe pnictide material PrFe1-xRuxAsO. Substitution of Ru for Fe suppresses the structural and magnetic phase transitions that occur in the undoped compound PrFeAsO. Contrary to the behavior usually observed in 1111 pnictide materials, the suppression of both the structural and magnetic transitions does not result in the emergence of superconductivity or any other new ground state. Interestingly, PrFeAsO itself shows an unusual negative thermal expansion (NTE) along the c-axis, from 60K down to at least 4K; this does not occur in superconducting samples such as those formed by doping with fluorine on the oxygen site. We find that NTE is present for all concentrations of PrFe1-xRuxAsO with x ranging from 0.05 to 0.75. These results suggest that the absence of superconductivity in these materials could be related to the presence of NTE.

cond-mat.supr-con↗

arXiv2012

Evolution of the nuclear and magnetic structures of TlFe1.6Se2 with temperature

The evolution of the nuclear and magnetic structures of TlFe$_{1.6}$Se$_2$ was determined in the temperature range of 5-450\,K using single crystal neutron diffraction. The Fe layers in these materials develop a corrugation in the magnetically ordered state. A canting away from the block checkerboard magnetic structure is observed in the narrow temperature range between approximately 100 and 150\,K. In this same temperature range, an increase in the corrugation of the Fe layers is observed. At lower temperatures, the block checkerboard magnetic structure is recovered with a suppressed magnetic moment and abrupt changes in the lattice parameters. Microstructure analysis at 300\,K using atomic-resolution Z-contrast scanning transmission electron microscopy reveals regions with ordered and disordered Fe vacancies, and the iron content is found to be uniform across the crystal. These findings highlight the differences between the alkali-metal and thallium materials, and indicate competition between magnetic ground states and a strong coupling of magnetism to the lattice in TlFe$_{1.6}$Se$_2$.

cond-mat.supr-con↗

arXiv2012

Properties of single crystalline AZn2Sb2 (A=Ca,Eu,Yb)

Single crystals of CaZn2Sb2, EuZn2Sb2, and YbZn2Sb2 were grown from melts of nominal composition AZn5Sb5 (A=Ca,Eu,Yb) with the excess melt being removed at 1073\,K. The electrical transport properties are consistent with those previously reported for polycrystalline samples. This confirms that the $p$-type carrier concentrations ranging from 2x10^{19}cm^{-3} to ~1x10^{20}cm^{-3} are intrinsic to these materials. Also consistent with transport in polycrystalline materials, the carrier mobility is found to be lowest in CaZn2Sb2, suggesting the trends in mobility and thermoelectric efficiency within these compounds are inherent to the material systems and not due to inhomogeneity or impurities in polycrystalline samples. These results suggest CaZn2Sb2 has the strongest coupling between the doping/defects and the electronic framework. Magnetization measurements reveal an antiferromagnetic transition near 13\,K in EuZn2Sb2, and the observed magnetic anisotropy indicates the spins align parallel and anti-parallel to $c$ in the trigonal lattice. Powder neutron diffraction on polycrystalline samples of CaZn2Sb2 and YbZn2Sb2 reveals smooth lattice expansion to 1000\,K, with $c$ expanding faster than $a$. The Debye temperatures calculated from specific heat capacity data and the isotropic displacement parameters are found to correlate with the carrier mobility, with the CaZn2Sb2 displaying the largest Debye temperature and smallest mobility.

cond-mat.mtrl-sci↗

arXiv2011

Effect of Molybdenum 4d Hole Substitution in BaFe2As2

We investigate the thermodynamic and transport properties of molybdenum-doped BaFe2As2 (122) crystals, the first report of hole doping using a 4d element. The chemical substitution of Mo in place of Fe is possible up to ~ 7%. For Ba(Fe1-xMox)2As2, the suppression rate of the magnetic transition temperature with x is the same as in 3d Cr-doped 122 and is independent of the unit cell changes. This illustrates that temperature-composition phase diagram for hole-doped 122 can be simply parameterized by x, similar to the electron-doped 122 systems found in literature. Compared to 122 with a coupled antiferromagnetic order (TN) and orthorhombic structural transition (To) at ~ 132 K, 1.3% Mo-doped 122 (x = 0.013) gives TN = To = 125(1) K according to neutron diffraction results and features in specific heat, magnetic susceptibility and electrical resistivity. The cell volume expands by ~ 1% with maximum Mo-doping and TN is reduced to ~ 90 K. There is a new T* feature that is identified for lightly Cr- or Mo-doped (< 3%) 122 crystals, which is x dependent. This low-temperature transition may be a trace of superconductivity or it may have another electronic or magnetic origin.

cond-mat.supr-con↗

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↗