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J Ma,
O Delaire,
A F May,
C E Carlton,
M A McGuire,
L H Vanbebber,
D L Abernathy,
G Ehlers,
Tao Hong,
A Huq,
Wei Tian,
V M Keppens,
Y Shao-Horn, B C Sales
[show abstract]
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ABSTRACT: Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.
Nature Nanotechnology 06/2013; · 27.27 Impact Factor
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A. D. Christianson,
M. D. Lumsden,
K. Marty,
C. H. Wang,
S. Calder,
D. L. Abernathy,
M. B. Stone,
H. A. Mook,
M. A. McGuire,
A. S. Sefat, B. C. Sales,
D. Mandrus,
E. A. Goremychkin
[show abstract]
[hide abstract]
ABSTRACT: The Fe1+yTe1-xSex series of materials is one of the prototype families of
Fe-based superconductors. To provide further insight into these materials we
present systematic inelastic neutron scattering measurements of the low energy
spin excitations for x=0.27, 0.36, 0.40, 0.49. These measurements show an
evolution of incommensurate spin excitations towards the (1/2 1/2 0) wave
vector with doping. Concentrations (x=0.40 and 0.49) which exhibit the most
robust superconducting properties have spin excitations closest to (1/2 1/2 0)
and also exhibit a strong spin resonance in the spin excitation spectrum below
Tc. The resonance signal appears to be closer to (1/2 1/2 0) than the
underlying spin excitations. We discuss the possible relationship between
superconductivity and spin excitations at the (1/2 1/2 0) wave vector and the
role that interstitial Fe may play.
05/2013;
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[show abstract]
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ABSTRACT: Millimeter sized single crystals of Mo3Sb7 are grown using the self-flux technique and a thorough characterization of their structural, magnetic, thermal and transport properties is reported. The structure parameters for the high-temperature cubic phase and the low-temperature tetragonal phase were, for the first time, determined with neutron single crystal diffraction. Both X-ray powder diffraction and neutron single crystal diffraction at room temperature confirmed that Mo3Sb7 crystallizes in Ir3Ge7-type cubic structure with space group Im[`3]m. The cubic-tetragonal structure transition at 53 K is verified by the peak splitting of (4 0 0) reflection observed by X-ray single crystal diffraction and the dramatic intensity change of (12 0 0) peak observed by neutron single crystal diffraction. The structural transition is accompanied by a sharp drop in magnetic susceptibility, electrical resistivity, and thermopower while cooling. A weak lambda anomaly was also observed around 53 K in the temperature dependence of specific heat and the entropy change across the transition is estimated to be 1.80 J/molMo K. The temperature dependence of magnetic susceptibility was measured up to 750 K and it follows a Curie-Weiss behavior above room temperature. Analysis of the low-temperature magnetic susceptibility suggests a spin gap of 110 K around 53 K. A typical phonon thermal conductivity was observed in the low temperature tetragonal phase. A glassy phonon thermal conductivity above 53 K suggests a structural instability in a wide temperature range. Superconductivity was observed at 2.35 K in the as-grown crystals and the dimensionless specific heat jump \triangleC(T)/gnTc was determined to be 1.49, which is slightly larger than the BCS value of 1.43 for the weak-coupling limit.
Physical Review B 02/2013; · 3.69 Impact Factor
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[show abstract]
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ABSTRACT: The chiral helimagnet Cr1/3NbS2 has been investigated by magnetic, transport
and thermal properties measurements on single crystals and by first principles
electronic structure calculations. From the measured field and temperature
dependence of the magnetization for fields applied perpendicular to the c axis,
the magnetic phase diagram has been constructed in the vicinity of the phase
transitions. A transition from a paramagnetic to a magnetically ordered phase
occurs near 120 K. With increasing magnetic field and at temperatures below 120
K, this material undergoes transitions from a helimagnetic to a soliton-lattice
phase near 900 Oe, and then to a ferromagnetic phase near 1300 Oe. The
transitions are found to strongly affect the electrical transport. The
resistivity decreases sharply upon cooling near 120 K, and the spin
reorientation from the helimagnetic ground state to the commensurate
ferromagnetic state is evident in the magnetoresistance. At high fields a large
magnetoresistance (55 % at 140 kOe) is observed near the magnetic transition
temperature. Heat capacity and electronic structure calculations show the
density of states at the Fermi level is low in the magnetically ordered state.
Effects of spin fluctuations are likely important in understanding the behavior
of Cr1/3NbS2 near and above the magnetic ordering transitions.
09/2012;
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ABSTRACT: The layered 5d transition metal oxide Sr2IrO4 has been shown to host a novel
Jeff=1/2 Mott spin orbit insulating state with antiferromagnetic ordering,
leading to comparisons with the layered cuprates. Here we study the effect of
substituting Mn for Ir in single crystals of Sr2Ir0.9Mn0.1O4 through an
investigation involving bulk measurements and resonant x-ray and neutron
scattering. We observe a new long range magnetic structure emerge upon doping
through a reordering of the spins from the basal plane to the c-axis with a
reduced ordering temperature compared to Sr2IrO4. The strong enhancement of the
magnetic x-ray scattering intensity at the L3 edge relative to the L2 edge
indicates that the Jeff=1/2 state is robust and capable of hosting a variety of
ground states.
Physical Review B 08/2012; 86:220403. · 3.69 Impact Factor
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P. Vilmercati,
C. Parks Cheney,
F. Bondino,
E. Magnano,
M. Malvestuto,
M. A. McGuire,
A. S. Sefat, B. C. Sales,
D. Mandrus,
D. J. Singh,
M. D. Johannes,
N. Mannella
[show abstract]
[hide abstract]
ABSTRACT: We use core-valence-valence (CVV) Auger spectra to probe the Coulomb
repulsion between holes in the valence band of Fe pnictide superconductors. By
comparing the two-hole final state spectra to density functional theory
calculations of the single particle density of states, we extract a measure of
the electron correlations that exist in these systems. Our results show that
the Coulomb repulsion is highly screened and can definitively be considered as
weak. We also find that there are differences between the 1111 and 122 families
and even a small variation as a function of the doping, x, in Ba(Fe1 xCox)2As2.
We discuss how the values of the hole-hole Coulomb repulsion obtained from our
study relate to the onsite Coulomb parameter "U" used in model and first
principles calculations based on dynamical mean field theory, and establish an
upper bound for its effective value. Our results impose stringent constraints
on model based phase diagrams
06/2012;
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ABSTRACT: The noncentrosymmetric ferromagnet Cr11Ge19 has been investigated by electrical transport, AC and DC magnetization, heat capacity, x-ray diffraction, resonant ultrasound spectroscopy, and first principles electronic structure calculations. Complex itinerant ferromagnetism in this material is indicated by nonlinearity in conventional Arrott plots, unusual behavior of AC susceptibility, and a weak heat capacity anomaly near the Curie temperature (88 K). The inclusion of spin wave excitations was found to be important in modeling the low temperature heat capacity. The temperature dependence of the elastic moduli and lattice constants, including negative thermal expansion along the c axis at low temperatures, indicates strong magneto-elastic coupling in this system. Calculations show strong evidence for itinerant ferromagnetism and suggest a noncollinear ground state may be expected.
Phys. Rev. B. 06/2012; 85(22).
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[show abstract]
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ABSTRACT: Lattice dynamics and magnetic excitations are investigated to elucidate the
origin of magneto-dielectric effects in the S=1/2 two-dimensional quantum spin
compound K$_2$V$_3$O$_8$. We find evidence for lattice instabilities at 110 K
and 60 K as optical phonon anomalies and a soft mode at 26 cm$^{-1}$ in A$_1$
symmetry. Two-magnon excitations in B$_1$ symmetry show an unconventional
double-peak structure and temperature dependence. This suggests the existence
of a split mode near the zone boundary caused by a mixing of spin and lattice
modes.
04/2012;
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[show abstract]
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ABSTRACT: We present inelastic neutron scattering measurements and first principles calculations examining the intermetallic marcasite CrSb(2). The observed spin-wave dispersion implies that the magnetic interactions are strongly one-dimensional with antiferromagnetic chains parallel to the crystalline c axis. Such low-dimensional excitations are unexpected in a semiconducting intermetallic system. Moreover, we observe a clear anisotropic thermal conductivity indicating that the magnetic anisotropy enhances thermoelectric properties along particular crystallographic directions.
Physical Review Letters 04/2012; 108(16):167202. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The noncentrosymmetric ferromagnet Cr11Ge19 has been investigated by
electrical transport, AC and DC magnetization, heat capacity, x-ray
diffraction, resonant ultrasound spectroscopy, and first principles electronic
structure calculations. Complex itinerant ferromagnetism in this material is
indicated by nonlinearity in conventional Arrott plots, unusual behavior of AC
susceptibility, and a weak heat capacity anomaly near the Curie temperature (88
K). The inclusion of spin wave excitations was found to be important in
modeling the low temperature heat capacity. The temperature dependence of the
elastic moduli and lattice constants, including negative thermal expansion
along the c axis at low temperatures, indicate strong magneto-elastic coupling
in this system. Calculations show strong evidence for itinerant ferromagnetism
and suggest a noncollinear ground state may be expected.
04/2012;
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P. Vilmercati,
A. Fedorov,
F. Bondino,
F. Offi,
G. Panaccione,
P. Lacovig,
L. Simonelli,
M. A. McGuire,
A. S. M. Sefat,
D. Mandrus, B. C. Sales,
T. Egami,
W. Ku,
N. Mannella
[show abstract]
[hide abstract]
ABSTRACT: A direct and element-specific measurement of the local Fe spin moment has
been provided by analyzing the Fe 3s core level photoemission spectra in the
parent and optimally doped CeFeAsO1-xFx (x = 0, 0.11) and Sr(Fe1 xCox)2As2 (x =
0, 0.10) pnictides. The rapid time scales of the photoemission process allowed
the detection of large local spin moments fluctuating on a 10-15 s time scale
in the paramagnetic, anti-ferromagnetic and superconducting phases, indicative
of the occurrence of ubiquitous strong Hund's magnetic correlations. The
magnitude of the spin moment is found to vary significantly among different
families, 1.3 \muB in CeFeAsO and 2.1 \muB in SrFe2As2. Surprisingly, the spin
moment is found to decrease considerably in the optimally doped samples, 0.9
\muB in CeFeAsO0.89F0.11 and 1.3 \muB in Sr(Fe0.9Co0.1)2As2. The strong
variation of the spin moment against doping and material type indicates that
the spin moments and the motion of itinerant electrons are influenced
reciprocally in a self-consistent fashion, reflecting the strong competition
between the antiferromagnetic super-exchange interaction among the spin moments
and the kinetic energy gain of the itinerant electrons in the presence of a
strong Hund's coupling. By describing the evolution of the magnetic
correlations concomitant with the appearance of superconductivity, these
results constitute a fundamental step toward attaining a correct description of
the microscopic mechanisms shaping the electronic properties in the pnictides,
including magnetism and high temperature superconductivity.
03/2012;
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A. Möchel,
I. Sergueev,
H.-C. Wille,
J. Voigt,
M. Prager,
M. B. Stone, B. C. Sales,
Z. Guguchia,
A. Shengelaya,
V. Keppens,
R. P. Hermann
[show abstract]
[hide abstract]
ABSTRACT: The lattice dynamics of the filled skutterudite YbFe4Sb12 was studied by resonant ultrasound spectroscopy and an anomalous softening in the temperature dependence of the elastic constants at ∼50 K was observed. This anomaly can not be explained by the dynamics of the filler, in contrast to other filled skutterudites. We have further investigated the origin of this anomaly using macroscopic and microscopic measurements. A rearrangement of the spectral weight of the Yb phonon states was observed in the temperature dependence of the density of phonon states, obtained by inelastic neutron scattering. We suggest that the anomaly is due to a change of the Yb valence state and that the anomaly and the phonon spectral weight rearrangement have the same origin.
Physical Review B 01/2012; 84(18). · 3.69 Impact Factor
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[show abstract]
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ABSTRACT: We report the temperature dependence of the resistivity and thermoelectric power (TEP) under hydrostatic pressure of the itinerant antiferromagnet BaFe2As2 and the electron-doped superconductor Ba(Fe0.9Co0.1)2As2. We observe a hole-like contribution to the thermopower below the structural/magnetic (S-M) transition in the parent compound that is suppressed in magnitude and temperature with pressure. Pressure increases the contribution of electrons to transport in both the doped and undoped compound. In the 10% Co-doped sample we used a two-band model for thermopower to estimate the carrier concentrations and determine the effect of pressure on the band structure.
Phys. Rev. B. 08/2011; 84(7).
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O Delaire,
J Ma,
K Marty,
A F May,
M A McGuire,
M-H Du,
D J Singh,
A Podlesnyak,
G Ehlers,
M D Lumsden, B C Sales
[show abstract]
[hide abstract]
ABSTRACT: Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic-transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.
Nature Material 06/2011; 10(8):614-9. · 32.84 Impact Factor
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E A Goremychkin,
R Osborn,
C H Wang,
M. D. Lumsden,
M. A. McGuire,
A. S. Sefat, B. C. Sales,
D. Mandrus,
H M Rønnow,
Y Su,
A. D. Christianson
[show abstract]
[hide abstract]
ABSTRACT: We report inelastic neutron scattering measurements of crystal field
transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with
fluorine produces additional crystal field excitations, providing evidence that
there are two distinct charge environments around the rare earth ions, with
probabilities that are consistent with a random distribution of dopants on the
oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have non-magnetic and
magnetic ground states, respectively, indicating that the enhancement of Tc
compared to LaFeAsO1-xFx is not due to rare earth magnetism.
Physical Review B 05/2011; · 3.69 Impact Factor
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J E Sonier,
W Huang,
C V Kaiser,
C Cochrane,
V Pacradouni,
S A Sabok-Sayr,
M D Lumsden, B C Sales,
M A McGuire,
A S Sefat,
D Mandrus
[show abstract]
[hide abstract]
ABSTRACT: It is shown that attempts to accurately deduce the magnetic penetration depth λ of overdoped BaFe(1.82)Co(0.18)As₂ single crystals by transverse-field muon spin rotation (TF μSR) are thwarted by field-induced magnetic order and strong vortex-lattice disorder. We explain how substantial deviations from the magnetic field distribution of a nearly perfect vortex lattice by one or both of these factors is also significant for other iron-arsenic superconductors, and this introduces considerable uncertainty in the values of λ obtained by TF μSR.
Physical Review Letters 03/2011; 106(12):127002. · 7.37 Impact Factor
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D. A. Zocco,
R. E. Baumbach,
J. J. Hamlin,
M. Janoschek,
I. K. Lum,
M. A. McGuire,
A. S. Sefat, B. C. Sales,
R. Jin,
D. Mandrus,
J. R. Jeffries,
S. T. Weir,
Y. K. Vohra,
M. B. Maple
[show abstract]
[hide abstract]
ABSTRACT: 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 for hydrostatically applied pressures up to 15 GPa, and with no signs of pressure-induced superconductivity up to 50 GPa for the less hydrostatic pressure techniques. For the CeFePO single crystals, pressure further stabilizes the Kondo screening of the Ce 4f-electron magnetic moments.
Phys. Rev. B. 03/2011; 83(9).
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[show abstract]
[hide abstract]
ABSTRACT: We report the temperature dependence of the resistivity and thermoelectric
power under hydrostatic pressure of the itinerant antiferromagnet BaFe2As2 and
the electron-doped superconductor Ba(Fe0.9Co0.1)2As2. We observe a hole-like
contribution to the thermopower below the structural-magnetic transition in the
parent compound that is suppressed in magnitude and temperature with pressure.
Pressure increases the contribution of electrons to transport in both the doped
and undoped compound. In the 10% Co-doped sample, we used a two-band model for
thermopower to estimate the carrier concentrations and determine the effect of
pressure on the band structure.
03/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe0.92Co0.08)2As2 superconductors. We have measured two samples extracted from the same batch: first sample has been measured just after preparation with no additional heat treatment. The sample shows Tc=20 K, residual specific heat γ0=3.6 mJ/mol K2 and a Schottky-like contribution at low temperatures. A second sample has been annealed at 800 °C for two weeks and shows Tc=25 K and γ0=1.4 mJ/mol K2. By subtracting the lattice specific heat, from pure BaFe2As2, the temperature dependence of the electronic specific heat has been obtained and studied. For both samples the temperature dependence of Cel(T) clearly indicate the presence of low-energy excitations in the system. Their specific heat data cannot be described by single clean s- or d-wave models and the data requires an anisotropic gap scenario which may or may not have nodes.
Journal of Physics Conference Series 02/2011; 273(1):012094.
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E A Goremychkin,
R Osborn,
C H Wang,
M D Lumsden,
M A McGuire,
A S Sefat, B C Sales,
D Mandrus,
H M Ronnow,
Y Su,
A D Christianson
[show abstract]
[hide abstract]
ABSTRACT: We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of T-c compared to LaFeAsO1-xFx is not due to rare-earth magnetism.
Physical Review B 01/2011; 83(21). · 3.69 Impact Factor
