arXiv subjects

R. J. Birgeneau

Publications and source records attributed to R. J. Birgeneau.

At least 55 records · Page 3Linked to original sources

arXiv2013

Magnetic neutron diffraction study of Ba(Fe1-xCox)2As2 critical exponents through the tricritical doping

We present temperature dependent magnetic neutron diffraction measurements on Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for $x$ = 0.039, 0.022, and 0.021 as-grown single crystals. Our investigations probe the behavior near the magnetic tricritical point in the ($x$,$T$) plane, $x_{tr} \approx $0.022, as well as systematically exploring the character of the magnetic phase transition across a range of doping values. All samples show long range antiferromagnetic order that may be described near the transition by simple power laws, with $β$ =~0.306$\pm$0.060 for $x$ =~0.039, $β$ =~0.208$\pm$0.005 for $x$ =~0.022, and $β$ =~0.198$\pm$0.009 for $x$ =~0.021. For the $x$ =~0.039 sample, the data are reasonably well described by the order parameter exponent $β$ =~0.326 expected for a 3D Ising model while the $x$ =~0.022 and $x$ =~0.021 samples are near the $β$ =~0.25 value for a tricritical system in the mean-field approximation. These results are discussed in the context of existing experimental work and theoretical predictions.

cond-mat.supr-con↗

arXiv2013

Lifetimes of antiferromagnetic magnons in two and three dimensions: experiment, theory, and numerics

A high-resolution neutron spectroscopic technique is used to measure momentum-resolved magnon lifetimes in the prototypical two- and three-dimensional antiferromagnets Rb2MnF4 and MnF2, over the full Brillouin zone and a wide range of temperatures. We rederived theories of the lifetime resulting from magnon-magnon scattering, thereby broadening their applicability beyond asymptotically small regions of wavevector and temperature. Corresponding computations, combined with a small contribution reflecting collisions with domain boundaries, yield excellent quantitative agreement with the data.

cond-mat.str-el↗

arXiv2012

Thermal evolution of the full three-dimensional magnetic excitations in the multiferroic BiFeO3

The idea of embedding and transmitting information within the fluctuations of the magnetic moments of spins (spin waves) has been recently proposed and experimentally tested. The coherence of spin waves, which describes how well defined these excitations are, is of course vital to this process, and themost significant factor that affects the spin-wave coherence is temperature. Here we present neutron inelastic scattering measurements of the full threedimensional spin-wave dispersion in BiFeO3, which is one of the most promising functional multiferroic material, for temperatures from 5K to 700K. Despite the presence of strong electromagnetic coupling, the magnetic excitations behave like conventional magnons over all parts of the Brillouin zone. At low temperature the spin-waves are well-defined coherent modes, described by a classical model for a G-type antiferromagnet. A spin-wave velocity softening is already present at room temperature, and more pronounced damping occurs as the magnetic ordering temperature TN \sim 640K is approached. In addition, a strong hybridization of the Fe 3d and O 2p states is found to modify the distribution of the spin-wave spectral weight significantly, which implies that the spins are not restricted to the Fe atomic sites as previously believed.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic order tuned by Cu substitution in Fe1.1-zCuzTe

We study the effects of Cu substitution in Fe1.1Te, the non-superconducting parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at \sim 60 K without Cu, are monotonically depressed with increasing Cu content. By 10% Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of (0.5-d, 0, 0.5) with d being the incommensurability of 0.02, and correlation length of 12 angstrom along the a axis and 9 angstrom along the c axis. With 4% Cu, both transition temperatures are at 41 K, though short-range incommensurate order at (0.42, 0, 0.5) is present at 60 K. With further cooling, the incommensurability decreases linearly with temperature down to 37 K, below which there is a first order transition to a long-range almost-commensurate antiferromagnetic structure. A spin anisotropy gap of 4.5 meV is also observed in this compound. Our results show that the weakly magnetic Cu has large effects on the magnetic correlations; it is suggested that this is caused by the frustration of the exchange interactions between the coupled Fe spins.

cond-mat.supr-con↗

arXiv2012

Effect of uniaxial strain on the structural and magnetic phase transitions in BaFe$_2$As$_2$

We report neutron scattering experiments probing the influence of uniaxial strain on both the magnetic and structural order parameters in the parent iron pnictide compound, BaFe$_2$As$_2$. Our data show that modest strain fields along the in-plane orthorhombic b-axis can affect significant changes in phase behavior simultaneous to the removal of structural twinning effects. As a result, we demonstrate in BaFe$_2$As$_2$ samples detwinned via uniaxial strain that the in-plane C$_4$ symmetry is broken by \textit{both} the structural lattice distortion \textit{and} long-range spin ordering at temperatures far above the nominal (strain-free), phase transition temperatures. Surprising changes in the magnetic order parameter of this system under relatively small strain fields also suggest the inherent presence of magnetic domains fluctuating above the strain-free ordering temperature in this material.

cond-mat.supr-con↗

arXiv2012

A neutron scattering study of the under-doped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, x=0.09 and 0.17 self-flux grown single crystals and the universality of the tricritical point

We present a combination of elastic neutron scattering measurements in zero and 14.5 T and magnetization measurements in zero and 14 T on under-doped superconducting Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ x=0.17, and the same measurements in zero field on a non-superconducting crystal with x=0.09. The data suggest that the under-doped materials may not be electronic phase separated but rather have slightly inhomogeneous potassium doping. The temperature dependence of the magnetic order parameter (OP) below the transition of the sample with x=0.09 is more gradual than that for the case of the un-doped BaFe$_{2}$As$_{2}$, suggesting that this doping may be in the vicinity of a tricritical point. We advance therefore the hypothesis that the tricritical point is a common feature of all superconducting 122s. For the x=0.17 sample, while T$_{c}$ is suppressed from $\approx$17 K to $\approx$8 K by a magnetic field of 14 T, the intensity of the magnetic Bragg peaks (1 0 3) at 1.2 K is enhanced by 10$%$ showing competition of superconductivity (SC) and antiferromagnetism (AFM). The intensity of the magnetic Bragg peaks (1 0 3) in the (T$_{c}$, T$_{N}$) temperature interval remain practically unchanged in 14.5 T within a 10$%$ statistical error. The present results are discussed in the context of the existing literature.

cond-mat.supr-con↗

arXiv2012

Temperature-dependent transformation of the magnetic excitation spectrum on approaching superconductivity in Fe1-x (Ni/Cu)x Te0.5 Se0.5

Spin excitations are one of the top candidates for mediating electron pairing in unconventional superconductors. Their coupling to superconductivity is evident in a large number of systems, by the observation of an abrupt redistribution of magnetic spectral weight at the superconducting transition temperature, Tc, for energies comparable to the superconducting gap. Here we report inelastic neutron scattering measurements on Fe-based superconductors, Fe1-x (Ni/Cu)x Te0.5 Se0.5, that emphasize an additional signature. The overall shape of the low energy magnetic dispersion changes from two incommensurate vertical columns at T >> Tc to a distinctly different U-shaped dispersion at low temperature. Importantly, this spectral reconstruction is apparent for temperature up to ~3Tc. If the magnetic excitations are involved in the pairing mechanism, their surprising modification on the approach to Tc demonstrates that strong interactions are involved.

cond-mat.supr-con↗

arXiv2012

Zn-induced spin dynamics in overdoped La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$

Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_y$O$_4$ ($x=0.25$, $y\approx0.01$) are measured via inelastic neutron scattering techniques. As Zn$^{2+}$ is substituted onto the Cu$^{2+}$-sites, an anomalous enhancement of the local spin susceptibility $χ^{\prime\prime}(ω)$ appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector \textbf{Q}$=(π, π, 0)$ that coexists with the known incommensurate SDW excitations at \textbf{Q}$_{HK}=(π\pmδ,π), (π,π\pmδ)$. Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu$^{2+}$-spins, and the simultaneous suppression of T$_c$ as AF fluctuations are slowed in proximity to Zn-impurities suggests the continued importance of high energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.

cond-mat.supr-con↗

arXiv2011

Widespread spin polarization effects in photoemission from topological insulators

High resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi$_2$Se$_3$ using a recently developed high-efficiency spectrometer. The topological surface state's helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.

cond-mat.mtrl-sci↗

arXiv2011

Interplay between magnetism and superconductivity in iron-chalcogenide superconductors: crystal growth and characterizations

In this review, we present a summary of the results on single crystal growth of two types of iron-chalcogenide superconductors, Fe(1+y)Te(1-x)Se(x) (11), and A(x)Fe(2-y)Se(2) (A= K, Rb, Cs, Tl, Tl/K, Tl/Rb), using Bridgman, zone-melting, vapor self-transport, and flux techniques. The superconducting and magnetic properties (the latter gained mainly from neutron scattering measurements) of these materials are reviewed to demonstrate the connection between magnetism and superconductivity. It will be shown that for the 11 system, while static magnetic order around the reciprocal lattice position (0.5, 0) competes with superconductivity, spin excitations centered around (0.5, 0.5) are closely coupled to the materials' superconductivity; this is made evident by the strong correlation between the spectral weight around (0.5, 0.5) and the superconducting volume fraction. The observation of a spin resonance below the superconducting temperature, Tc, and the magnetic-field dependence of the resonance, emphasize the important role spin excitations play in the superconductivity. Generally, these results illustrate the similarities between the iron-based and cuprate superconductors. In A(x)Fe(2-y)Se(2), superconductivity with Tc ~ 30 K borders an antiferromagnetic insulating phase; this is closer to the behavior observed in the cuprates but differs from that in other iron-based superconductors.

cond-mat.supr-con↗

arXiv2011

First and second order magnetic and structural transitions in BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$

We present here high resolution magnetization measurements on high-quality BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$, 0$\leq$x$\leq$0.046 as-grown single crystals. The results confirm the existence of a magnetic tricritical point in the ($x$,$T$) plane at x$^{m}_{tr}$$\approx$0.022 and reveal the emergence of the heat capacity anomaly associated with the onset of the structural transition at x$^{s}$$\approx$0.0064. We show that the samples with doping near x$^{m}_{tr}$ do not show superconductivity, but rather superconductivity emerges at a slightly higher cobalt doping, x$\approx$0.0315

cond-mat.supr-con↗

arXiv2011

Smectic-A and Smectic-C Phases and Phase Transitions in Liquid Crystal-Aerosil Gels

High-resolution X-ray scattering studies of the nonpolar thermotropic liquid crystal 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (\bar{8}S5) in aerosil gel nano-networks reveal that the aerosil-induced disorder significantly alters both the nematic to Smectic-A and Smectic-A to Smectic-C phase transitions. The limiting Smectic-A correlation length follows a power-law dependence on the aerosil density in quantitative agreement with the limiting lengths measured previously in other Smectic-A liquid crystal gels. The Smectic-A to Smectic-C liquid crystalline phase transition is altered fundamentally by the presence of the aerosil gel. The onset of the Smectic-C phase remains relatively sharp but there is an extended coexistence region where Smectic-A and Smectic-C domains can exist.

cond-mat.soft↗

arXiv2010

Heat capacity study of BaFe$_{2}$As$_{2}$: effects of annealing

Heat-capacity, X-ray diffraction, and resistivity measurements on a high-quality BaFe$_{2}$As$_{2}$ sample show an evolution of the magneto-structural transition with successive annealing periods. After a 30-day anneal the resistivity in the (ab) plane decreases by more than an order of magnitude, to 12 $μΩ$cm, with a residual resistance ratio $\sim$36; the heat-capacity anomaly at the transition sharpens, to an overall width of less than K, and shifts from 135.4 to 140.2 K. The heat-capacity anomaly in both the as-grown sample and after the 30-day anneal shows a hysteresis of $\sim$0.15 K, and is unchanged in a magnetic field $μ_{0}$H = 14 T. The X-ray and heat-capacity data combined suggest that there is a first order jump in the structural order parameter. The entropy of the transition is reported.

cond-mat.supr-con↗

arXiv2010

Antiferromagnetic Critical Fluctuations in BaFe$_2$As$_2$

Magnetic correlations near the magneto-structural phase transition in the bilayer iron pnictide parent compound, BaFe$_2$As$_2$, are measured. In close proximity to the antiferromagnetic phase transition in BaFe$_2$As$_2$, a crossover to three dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about T$_N$. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of BaFe$_2$As$_2$ and their evolution near the anticipated crossover to three dimensional critical behavior and long-range order are discussed.

cond-mat.supr-con↗

arXiv2010

Magnetic nature of the 500 meV peak in $\rm La_{2-x}Sr_{x}CuO_4$ observed with resonant inelastic x-ray scattering at the Cu $K$-edge

We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at ${\bf q}=(π,0)$ in resonant inelastic x-ray scattering (RIXS) experiments on $\rm La_2CuO_4$. The intensity of this peak persists above the Néel temperature (T$_{N}$=320 K), but decreases gradually with increasing temperature, reaching zero at around T=500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon $\rm B_{1g}$ Raman peak in $\rm La_2CuO_4$, and with suitable rescaling, agrees with the Raman peak shifts in $\rm EuBa_2Cu_3O_6$ and $\rm K_2NiF_4$. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.

cond-mat.str-el↗

arXiv2009

Muon Spin Rotation Measurements of Heterogeneous Field Response in Overdoped La2-xSrxCuO4

Transverse-field muon spin rotation measurements of overdoped La2-xSrxCuO4 reveal a large broadening of the local magnetic field distribution in response to applied field, persisting to high temperatures. The field-response is approximately Curie-Weiss like in temperature and is largest for the highest doping investigated. Such behaviour is contrary to the canonical Fermi-liquid picture commonly associated with the overdoped cuprates and implies extensive heterogeneity in this region of the phase diagram. A possible explanation for the result lies in regions of staggered magnetization about dopant cations, analogous to what is argued to exist in underdoped systems.

cond-mat.str-el↗

arXiv2009

Universal magnetic and structural behaviors in the iron arsenides

Commonalities among the order parameters of the ubiquitous antiferromagnetism present in the parent compounds of the iron arsenide high temperature superconductors are explored. Additionally, comparison is made between the well established two-dimensional Heisenberg-Ising magnet, K$_2$NiF$_4$ and iron arsenide systems residing at a critical point whose structural and magnetic phase transitions coincide. In particular, analysis is presented regarding two distinct classes of phase transition behavior reflected in the development of antiferromagnetic and structural order in the three main classes of iron arsenide superconductors. Two distinct universality classes are mirrored in their magnetic phase transitions which empirically are determined by the proximity of the coupled structural and magnetic phase transitions in these materials.

cond-mat.supr-con↗

arXiv2009

Phase diagram of the PrFeAsO$_{1-x}$F$_{x}$ superconductor

The electronic phase diagram of PrFeAsO$_{1-x}$F$_{x}$ (0$\leq$x$\leq$0.225) has been determined using synchrotron X-ray powder diffraction, magnetization and resistivity measurements. The structural transition temperature is suppressed from 154 K to $\approx$120 K and the magnetic phase transitions of both iron and praseodymium ions are completely suppressed by x$\approx$0.08 fluorine doping, coinciding with the emergence of superconductivity. The optimal doping is x$\approx$0.15 when T$_{C}$=47 K, while the maximum solubility of fluorine in PrFeAsO$_{1-x}$F$_{x}$ is reached around x=0.22. The structural, magnetic and superconducting phase diagram is presented.

cond-mat.supr-con↗