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B. C. Sales

Publications and source records attributed to B. C. Sales.

At least 91 records · Page 5Linked to original sources
arXiv2008

Renormalized behavior and proximity to a magnetic quantum critical point in BaCo$_2$As$_2$

We report synthesis and single crystal measurements of magnetic, transport and thermal properties of single crystalline BaCo$_2$As$_2$ as well as first principles calculations of the electronic structure and magnetic behavior. These results show that BaCo$_2$As$_2$ is a highly renormalized paramagnet in proximity to a quantum critical point, presumably of ferromagnetic character and that BaFeNiAs$_2$ behaves similarly. These results are discussed in relation to the properties of Ba(Fe,Co)$_2$As$_2$ and Ba(Fe,Ni)$_2$As$_2$, which are superconducting for low Co and Ni concentrations.

cond-mat.supr-con↗
arXiv2008

Spin susceptibility, phase diagram, and quantum criticality in the electron-doped high Tc Superconductor Ba[Fe(1-x)Co(x)]2As2

We report a systematic investigation of Ba[Fe(1-x)Co(x)]2As2 based on transport and 75-As NMR measurements, and establish the electronic phase diagram. We demonstrate that doping progressively suppresses the uniform spin susceptibility and low frequency spin fluctuations. The optimum superconducting phase emerges at x_c~0.08 when the tendency toward spin ordering completely diminishes. Our findings point toward the presence of a quantum critical point near x_c between the SDW (spin density wave) and superconducting phases.

cond-mat.supr-con↗
arXiv2008

Evidence for Strong Itinerant Spin Fluctuations in the Normal State of CeFeAsO(0.89)F(0.11) Iron-Oxypnictides

The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.

cond-mat.supr-con↗
arXiv2008

C-axis Phonons in Fe-As Based Superconductors Investigated by Inelastic X-ray Scattering

In Fe-As based superconductors magnetism and superconductivity show strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. We investigated c-axis polarized phonons in doped and undoped BaFe2As2 by inelastic X-ray scattering. Phonon peak positions and linewidths did not vary significantly as a function of doping either by Co or by K. The linewidth of the Fe vibration shows unusual wavevector-dependence, which may be due to hybridization with in-plane modes. Comparison with the density functional theory shows significant difference for the energy of the As-As vibrations (Raman active at the zone center) but not for the Fe-As vibrations (infrared-active at the zone center). This behavior cannot be explained by a 10% softening of Fe-As interaction strength as proposed previously for in-plane polarized vibrations.

cond-mat.supr-con↗
arXiv2008

Charge order, dynamics, and magneto-structural transition in multiferroic LuFe$_2$O$_4$

We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and Mössbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field.

cond-mat.str-el↗
arXiv2008

Charge order in LuFe2O4: antiferroelectric ground state and coupling to magnetism

X-ray scattering by multiferroic LuFe2O4 is reported. Below 320 K, superstructure reflections indicate an incommensurate charge order with propagation close to (1/3,1/3,3/2). The corresponding charge configuration, also found by electronic structure calculations as most stable, contains polar Fe/O double-layers with antiferroelectric stacking. Diffuse scattering at 360 K, with (1/3,1/3,0) propagation, indicates ferroelectric short-range correlations between neighboring double-layers. The temperature dependence of the incommensuration indicates that charge order and magnetism are coupled.

cond-mat.str-el↗
arXiv2008

Pressure effects on the electron-doped high Tc superconductor BaFe(2-x)Co(x)As(2)

Application of pressures or electron-doping through Co substitution into Fe sites transforms the itinerant antiferromagnet BaFe(2)As(2) into a superconductor with the Tc exceeding 20K. We carried out systematic transport measurements of BaFe(2-x)Co(x)As(2) superconductors in pressures up to 2.5GPa, and elucidate the interplay between the effects of electron-doping and pressures. For the underdoped sample with nominal composition x = 0.08, application of pressure strongly suppresses a magnetic instability while enhancing Tc by nearly a factor of two from 11K to 21K. In contrast, the optimally doped x=0.20 sample shows very little enhancement of Tc=22K under applied pressure. Our results strongly suggest that the proximity to a magnetic instability is the key to the mechanism of superconductivity in iron-pnictides.

cond-mat.supr-con↗
arXiv2008

NMR measurements of intrinsic spin susceptibility in LaFeAsO(0.9)F(0.1)

We will probe the intrinsic behavior of spin susceptibility chi_(spin) in the LaFeAsO(1-x)F(x) superconductor (x ~ 0.1, Tc ~ 27K) using 19-F and 75-As NMR techniques. Our new results firmly establish the pseudo-gap behavior with Delta_(PG)/kB ~ 140K. The estimated magnitude of chi_(spin) at 290K, ~1.8x10^(-4) [emu/mol-Fe], is approximately twice larger than that in high Tc cuprates. We also show that chi_(spin) levels off below ~50K down to Tc.

cond-mat.supr-con↗
arXiv2008

59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)

We report an NMR investigation of the superconductivity in BaFe(2)As(2) induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron-pnictide superconductors is quite different from the widely accepted physical picture of high Tc cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap Delta 560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc decreases for both along the c-axis and ab-plane, and is consistent with the singlet pairing scenario.

cond-mat.supr-con↗
arXiv2008

Temperature and Spatial Dependence of the Superconducting and Pseudogap of NdFeAsO0.86F0.14

The discovery of superconductivity with a critical temperature exceeding 55 K in the iron-oxypnictides and related compounds has quite suddenly given the community a new set of materials - breaking the tyranny of copper. This new class of materials raises fundamental questions related to the origin of the electron pairing in the superconducting state and to the similarity to superconductivity in the cuprates. Here, we report spatially resolved measurements using scanning tunneling microscopy/spectroscopy (STM/STS) of the newly discovered iron-based layered superconductor NdFeAsO0.86F0.14 (Tc = 48 K) as a function of temperature. The tunneling spectra at 17 K show a suppression of spectral intensity within +/- 10 meV, indicative of the opening of the superconducting gap (SG). Below Tc, the sample exhibits two characteristic gaps - a large one (18 meV) and a small one (9 meV) - existing in different spatial locations. Both gaps are closed above Tc at the bulk Tc, but only the small gap can be fitted with a superconducting gap function. This gap displays a BCS - like order parameter. Above Tc, at the same location where the small gap was observed, a pseudogap (PG) opens abruptly at a temperature just above Tc and closes at 120 K. In contrast to the cuprates, the SG and PG have competing order parameters.

cond-mat.supr-con↗
arXiv2008

Effect of pressure on the superconducting critical temperature of La[O_{0.89}F_{0.11}]FeAs and Ce[O_{0.88}F_{0.12}]FeAs

We have performed several high-pressure resistivity experiments on the recently discovered superconductors La[O_{0.89}F_{0.11}]FeAs and Ce[O_{0.88}F_{0.12}]FeAs. At ambient pressure, these materials have superconducting onset temperatures T_c of 28 K and 44 K, respectively. While the T_c of La[O_{0.89}F_{0.11}]FeAs goes through a maximum between 10-68 kbar, in qualitative agreement with a recent report by Takahashi et al., the T_c of Ce[O_{0.88}F_{0.12}]FeAs decreases monotonically over the measured pressure range. At 265 kbar, the T_c of the cerium-based compound has been suppressed below 1.1 K.

cond-mat.supr-con↗
arXiv2008

Phonon Density of States of LaFeAsO1-xFx

We have studied the phonon density of states (PDOS) in LaFeAsO1-xFx with inelastic neutron scattering methods. The PDOS of the parent compound(x=0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x~0.1). Good agreement is found between the experimental PDOS and first-principle calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity.

cond-mat.supr-con↗
arXiv2008

Comparative High Field Magneto-Transport of Rare Earth Oxypnictides with Maximum Transition Temperatures

The recent discovery of a new class of superconducting oxypnictides with high transition temperatures may have profound implications for understanding unconventional high-temperature superconductivity. Like the cuprates, the oxypnictides seem to manifest an interleaving of charge donor and superconducting layers emerging upon doping of an antiferromagnetic parent semi-metal. Here we report magneto-transport measurements of three rare earth (Re = La, Nd, Sm) oxypnicide compounds with the transition temperatures near the maximum reported to date, in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Our resistivity, Hall coefficient and critical magnetic fields data suggest that these oxypnictide superconductors bridge the gap between MgB$_2$ and YBaCu$_3$O$_{7-x}$ as far as electromagnetic and vortex properties are concerned.

cond-mat.supr-con↗
arXiv2008

Evidence for the Spin Density Wave in LaFeAsO

This paper has been withdrawn by the authors due to errors in the X-ray diffraction data. Other measured data are not affected; however, the errors significantly change the interpretation and conclusions, and thus warrant withdrawal and later resubmission.

cond-mat.supr-con↗
arXiv2008

Evidence for electromagnetic granularity in the polycrystalline iron-based superconductor LaO0.89F0.11FeAs

The new rare-earth arsenate superconductors are layered, low carrier density compounds with many similarities to the high-Tc cuprates. An important question is whether they also exhibit weak-coupling across randomly oriented grain-boundaries. In this work we show considerable evidence for such weak-coupling by study of the dependence of magnetization in bulk and powdered samples. Bulk sample magnetization curves show very little hysteresis while remanent magnetization shows almost no sample size dependence, even after powdering. We conclude that these samples exhibit substantial electromagnetic granularity on a scale approximating the grain size, though we cannot yet determine whether this is intrinsic or extrinsic.

cond-mat.supr-con↗
arXiv2008

Very High Field Two-Band Superconductivity in LaFeAsO_0.89F_0.11

The ongoing search for new superconductors has recently yielded a new family of oxypnictides composed of alternating La_2O_{2-x}F_x and Fe_2As_2 layers [1-4] with transition temperatures T_c of 25-28 K, which can be raised to 40-43 K by replacing La with Ce [5] or Sm [6] or to 52 K by replacing La with Nd and Pr [7, 8]. Recent experiments and band structure calculations have suggested an unconventional multiband superconductivity in the layers of paramagnetic Fe ions, which would normally destroy superconductivity in the traditional mechanism of the s-wave Cooper pairing. Here we report very high-field resistance measurements up to 45T, which show a remarkable enhancement of the upper critical fields B_c2 at low temperatures, as compared to those expected from the already high slopes of dB_c2/dT ~ 2T/K near T_c . The deduced B_c2(0) ~ 63-65 T exceeds the paramagnetic limit, consistent with strong coupling and important two-band effects in LaFeAsO_0.89F_0.11. We argue that oxypnictides are emerging as a new class of high-field superconductors surpassing the B_c2 of Nb_3Sn, MgB_2, and the Chevrel phases and perhaps approaching the 100T field benchmark of the high-T_c cuprates.

cond-mat.supr-con↗
arXiv2008

Three Dimensional Magnetic Correlations in Multiferroic LuFe2O4

We present single-crystal neutron diffraction measurements on multiferroic LuFe2O4 showing phase transitions at 240 and 175 K. Magnetic reflections are observed below each transition indicating that the magnetic interactions in LuFe2O4 are 3-dimensional (3D) in character. The magnetic structure is refined as a ferrimagnetic spin configuration below the 240 K transition. While 3D magnetic correlations persists below 175 K, a significant broadening of the magnetic peaks is observed along with the build up of a diffuse component to the magnetic scattering.

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