arXiv subjects

M. A. Green

Publications and source records attributed to M. A. Green.

At least 37 records · Page 2Linked to original sources

arXiv2012

Spin fluctuations and superconductivity in powders of Fe_1+xTe_0.7Se_0.3 as a function of interstitial iron concentration

Using neutron inelastic scattering, we investigate the role of interstitial iron on the low-energy spin fluctuations in powder samples of Fe_{1+x}Te_{0.7}Se_{0.3}. We demonstrate how combining the principle of detailed balance along with measurements at several temperatures allows us to subtract both temperature-independent and phonon backgrounds from S(Q,ω) to obtain purely magnetic scattering. For small values of interstitial iron (x=0.009(3)), the sample is superconducting (T_{c}=14 K) and displays a spin gap of 7 meV peaked in momentum at wave vector q_{0}=(π,π) consistent with single crystal results. On populating the interstitial iron sites, the superconducting volume fraction decreases and we observe a filling in of the low-energy magnetic fluctuations and a decrease of the characteristic wave vector of the magnetic fluctuations. For large concentrations of interstitial iron (x=0.048(2)) where the superconducting volume fraction is minimal, we observe the presence of gapless spin fluctuations at a wave vector of q_{0}=(π,0). We estimate the absolute total moment for the various samples and find that the amount of interstitial iron does not change the total magnetic spectral weight significantly, but rather has the effect of shifting the spectral weight in Q and energy. These results show that the superconducting and magnetic properties can be tuned by doping small amounts of iron and are suggestive that interstitial iron concentration is also a controlling dopant in the Fe_{1+x}Te_{1-y}Se_{y} phase diagram in addition to the Te/Se ratio.

cond-mat.supr-con↗

arXiv2011

Non-collinear spin-density wave antiferromagnetism in FeAs

We present the magnetic structure of the itinerant monoarsenide, FeAs, with the B31 structure. Powder neutron diffraction confirms incommensurate modulated magnetism with wavevector $\mathbf{q} = (0.395\pm0.001)\mathbf{c}^*$ at 4 K, but cannot distinguish between a simple spiral and a collinear spin-density wave structure. Polarized single crystal diffraction confirms that the structure is best described as a non-collinear spin-density wave arising from a combination of itinerant and localized behavior with spin amplitude along the b-axis direction being (15 $\pm$ 5)% larger than in the a-direction. Furthermore, the propagation vector is temperature dependence, and the magnetization near the critical point indicates a two-dimensional Heisenberg system. The nature of the magnetism in the simplest iron arsenide is of fundamental importance in understanding the interplay between localized and itinerant magnetism and superconductivity.

cond-mat.mtrl-sci↗

arXiv2011

Interstitial iron tuning of the spin fluctuations in Fe1+xTe

Using neutron inelastic scattering, we investigate the low-energy spin fluctuations in Fe1+xTe as a function of both temperature and interstitial iron concentration. For Fe1.057(7)Te the magnetic structure is defined by a commensurate wavevector of (1/2,0,1/2). The spin fluctuations are gapped with a sharp onset at 7 meV and are three dimensional in momentum transfer, becoming two dimensional at higher energy transfers. On doping with interstitial iron, we find in Fe1.141(5)Te the ordering wavevector is located at the (0.38, 0, 1/2) position and the fluctuations are gapless with the intensity peaked at an energy transfer of 4 meV. These results show that the spin fluctuations in the Fe1+xTe system a can be tuned not only through selenium doping, but also with interstitial iron. We also compare these results with superconducting concentrations and in particular the resonance mode in the Fe_1+xTe_1-ySe_y system.

cond-mat.supr-con↗

arXiv2011

On the Structure of Vacancy Ordered Superconducting Alkali Metal Iron Selenide

With single crystal X-ray diffraction studies, we compare the structures of three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2, K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a ({\sqrt}5 x {\sqrt}5 x 1) super cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250K of a = b = 8.729(2) Å and c = 14.120(3) Å, a = b = 8.7186(12) Å and c = 14.0853(19) Å and at 295 K, a = b = 8.8617(16) Å and c = 15.304(3) Å for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty, whilst the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2) % to 85.3(3) %. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se - F - Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors distinguishes themselves from the structure of the non-superconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site as well as lower alkali metal occupancy that ensures an exact Fe2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity.

cond-mat.supr-con↗

arXiv2011

A Novel Large Moment Antiferromagnetic Order in K0.8Fe1.6Se2 Superconductor

The discovery of cuprate high Tc superconductors has inspired searching for unconventional su- perconductors in magnetic materials. A successful recipe has been to suppress long-range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point, which is replicated in recent discovery of iron-based high TC superconductors. The long-range magnetic order coexisting with superconductivity has either a small magnetic moment or low ordering temperature in all previously established examples. Here we report an exception to this rule in the recently discovered potassium iron selenide. The superconducting composition is identified as the iron vacancy ordered K0.8Fe1.6Se2 with Tc above 30 K. A novel large moment 3.31 μB/Fe antiferromagnetic order which conforms to the tetragonal crystal symmetry has the unprecedentedly high an ordering temperature TN = 559 K for a bulk superconductor. Staggeredly polarized electronic density of states thus is suspected, which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstance.

cond-mat.supr-con↗

arXiv2011

Electrical Properties of Conductive Ge Nanocrystal Thin Films Fabricated by Low Temperature In-situ Growth

Thin films composed of Ge nanocrystals embedded in amorphous SiO2 matrix (Ge-NCs TFs) were prepared using a low temperature in-situ growth method. Unexpected high p-type conductivity was observed in the intrinsic Ge-NCs TFs. Unintentional doping from shallow dopants was excluded as a candidate mechanism of hole generation. Instead, the p-type characteristic was attributed to surface state induced hole accumulation in NCs, and the hole conduction was found to be a thermally activated process involving charge hopping from one NC to its nearest neighbor. Theoretical analysis has shown that the density of surface states in Ge-NCs is sufficient to induce adequate holes for measured conductivity. The film conductivity can be improved significantly by post-growth rapid thermal annealing and this effect is explained by a simple thermodynamic model. The impact of impurities on the conduction properties was also studied. Neither compensation nor enhancement in conduction was observed in the Sb and Ga doped Ge-NCs TFs, respectively. This could be attributed to the fact that these impurities are no longer shallow dopants in NCs and are much less likely to be effectively activated. Finally, the photovoltaic effect of heterojunction diodes employing such Ge-NCs TFs was characterized in order to demonstrate its functionality in device implementation.

cond-mat.mtrl-sci↗

arXiv2010

Interstitial Iron Controlled Superconductivity in Fe1+xTe0.7Se0.3

The superconducting series, Fe(Te,Se), has a complex structural and magnetic phase diagram that is dependent on composition and occupancy of a secondary interstitial Fe site. In this letter, we show that superconductivity in Fe1+xTe0.7Se0.3 can be enhanced by topotactic deintercalation of the interstitial iron, demonstrating the competing roles of the two iron sites. Neutron diffraction reveals a flattening of the Fe(Te,Se)4 tetrahedron on Fe removal of iron and an increase in negative thermal expansion within the ab plane that correlates with increased lattice strain. Inelastic neutron scattering shows that a gapped excitation at 6 meV, evolves into gapless paramagnetic scattering with increasing iron; similar to the fluctuations observed for non-superconducting Fe1+xTe itself.

cond-mat.supr-con↗

arXiv2010

Antiferromagnetic ordering in the absence of a structural distortion in Ba(Fe{1-x}Mn{x})2As2

Neutron and x-ray diffraction studies of Ba(Fe{1-x}Mn{x})2As2 for low doping concentrations (x <= 0.176) reveal that at a critical concentration, 0.102 < x < 0.118, the tetragonal-to-orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of (1/2 1/2 1) persists. Among all of the iron arsenides this observation is unique to Mn-doping, and unexpected because all models for "stripe-like" antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of Ba(Fe{1-x}TM{x})2As2 compounds (TM = Transition Metal), and models for magnetic ordering in the iron arsenide compounds.

cond-mat.supr-con↗

arXiv2009

Nature of the Magnetic Order in BaMn2As2

Neutron diffraction measurements have been performed on a powder sample of BaMn2As2 over the temperature T range from 10 K to 675 K. These measurements demonstrate that this compound exhibits collinear antiferromagnetic ordering below the Neel temperature T_N = 625(1) K. The ordered moment mu = 3.88(4) mu_B/Mn at T = 10 K is oriented along the c axis and the magnetic structure is G-type, with all nearest-neighbor Mn moments antiferromagnetically aligned. The value of the ordered moment indicates that the oxidation state of Mn is Mn^{2+} with a high spin S = 5/2. The T dependence of mu suggests that the magnetic transition is second-order in nature. In contrast to the closely related AFe2As2 (A = Ca, Sr, Ba, Eu) compounds, no structural distortion is observed in the magnetically ordered state of BaMn2As2.

cond-mat.supr-con↗

arXiv2009

Glassy Spin Freezing and Gapless Spin Dynamics in a Spatially Anisotropic Triangular Antiferromagnet Ag2MnO2

Using elastic and inelastic neutron scattering techniques, we show that upon cooling a spatially anisotropic triangular antiferromagnet Ag$_2$MnO$_2$ freezes below $T_f ~\sim 50$ K into short range collinear state. The static spin correlations are extremely two-dimensional, and the spin fluctuations are gapless with two characteristic relaxation rates that behave linearly with temperature.

cond-mat.str-el↗

arXiv2009

Lattice distortion and magnetic quantum phase transition in CeFeAs(1-x)P(x)O

We use neutron scattering to show that replacing the larger arsenic with smaller phosphorus in CeFeAs(1-x)P(x)O simultaneously suppresses the AF order and orthorhombic distortion near x = 0.4, providing evidence for a magnetic quantum critical point. Furthermore, we find that the pnictogen height in iron arsenide is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity.

cond-mat.supr-con↗

arXiv2009

One-dimensional magnetic fluctuations in the spin-2 triangular lattice α-NaMnO2

The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T ~ 45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Delta ~ 7.5 meV) has been analysed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k_B ~ 73 K), along the monoclinic b-axis and a sizeable easy-axis magnetic anisotropy (|D|/k_B ~ 3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.

cond-mat.str-el↗

arXiv2009

Superconductivity at 2.2K in La3Ni4P4O2

We report the observation of superconductivity in La3Ni4P4O2 at 2.2 K. The layer stacking in this compound results in an asymmetric distribution of charge reservoir layers around the Ni2P2 planes. The estimated Wilson ratio, Rw ~ 5, indicates the presence of a strongly enhanced normal state susceptibility, but many of the basic superconducting characteristics are conventional. The estimated electronic contribution to the specific heat, gamma ~ 6.2 mJ mol-Ni^-1K^-2, is about 2/3 of that found in layered nickel borocarbide superconductors.

cond-mat.supr-con↗

arXiv2008

Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering

Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.

cond-mat.supr-con↗

arXiv2008

Incommensurate magnetic order in the alpha-Fe(Te,Se) superconductor systems

Magnetic spin fluctuations is one candidate to produce the bosonic modes that mediate the superconductivity in the ferrous superconductors. Up until now, all of the LaOFeAs and BaFe2As2 structure types have simple commensurate magnetic ground states, as result of nesting Fermi surfaces. This type of spin-density-wave (SDW) magnetic order is known to be vulnerable to shifts in the Fermi surface when electronic densities are altered at the superconducting compositions. Superconductivity has more recently been discovered in alpha-Fe(Te,Se), whose electronically active antifluorite planes are isostructural to the FeAs layers found in the previous ferrous superconductors and share with them the same quasi-two-dimensional electronic structure. Here we report neutron scattering studies that reveal a unique complex incommensurate antiferromagnetic order in the parent compound alpha-FeTe. When the long-range magnetic order is suppressed by the isovalent substitution of Te with Se, short-range correlations survive in the superconducting phase.

cond-mat.supr-con↗

arXiv2008

Magnetic order in BaFe2As2, the parent compound of the FeAs based superconductors in a new structural family

In addition to higher Tc compared with the ubiquitous cuprates for a material composed of a single electronically active layer, the newly discovered LnFeAsO superconductors offer additional compositional variation. In a similar fashion to the CuO2 layers in cuprates, the FeAs layers now dominate the electronic states that produce superconductivity. Cuprate superconductors distinguish themselves structurally by adopting different stacking of the Cu-O and electronically inactive "spacer" layers. Using the same structural philosophy, materials with the formula (A,K)Fe2As2,A=Ba or Sr have been reported and possess a Tc~38 K. Here, we report the neutron diffraction studies of BaFe2As2 that shows, in contrast to previous studies on the LnFeAsO materials, an antiferromagnetic transition which concurs with first-order structural transition. Although the magnetic and structural transitions occur differently in the AFe2As2 and LnFeAsO-type materials, this work clearly demonstrates that the complete evolution to a low symmetry structure is a pre-requirement for the magnetic order.

cond-mat.supr-con↗

arXiv2008

Structural and magnetic phase diagram of CeFeAsO1-xFx and its relationship to high-temperature superconductivity

We use neutron scattering to study the structural and magnetic phase transitions in the iron pnictides CeFeAsO1-xFx as the system is tuned from a semimetal to a high-transition-temperature (high-Tc) superconductor through Fluorine (F) doping x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a stripe like commensurate antiferromagnetic order with decreasing temperature. With increasing Fluorine doping, the structural phase transition decreases gradually while the antiferromagnetic order is suppressed before the appearance of superconductivity, resulting an electronic phase diagram remarkably similar to that of the high-Tc copper oxides. Comparison of the structural evolution of CeFeAsO1-xFx with other Fe-based superconductors reveals that the effective electronic band width decreases systematically for materials with higher Tc. The results suggest that electron correlation effects are important for the mechanism of high-Tc superconductivity in these Fe pnictides.

cond-mat.supr-con↗

arXiv2008

Intrinsic Properties of Stoichiometric LaOFeP

DC and ac magnetization, resistivity, specific heat, and neutron diffraction data reveal that stoichiometric LaOFeP is metallic and non-superconducting above T = 0.35 K, with gamma = 12.5 mJ/mol*K. Neutron diffraction data at room temperature and T = 10 K are well described by the stoichiometric, tetragonal ZrCuSiAs structure and show no signs of structural distortions or long range magnetic ordering, to an estimated detectability limit of 0.07 uB/Fe. We propose a model, based on the shape of the iron-pnictide tetrahedron, that explains the differences between LaOFeP and LaOFeAs, the parent compound of the recently discovered high-Tc oxyarsenides, which, in contrast, shows both structural and spin density wave (SDW) transitions.

cond-mat.supr-con↗