arXiv subjects

Robert J. McQueeney

Publications and source records attributed to Robert J. McQueeney.

25 records · Page 2Linked to original sources

arXiv2017

Pressure induced half-collapsed-tetragonal phase in CaKFe$_4$As$_4$

We report the temperature-pressure phase diagram of CaKFe$_4$As$_4$ established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe$_4$As$_4$ is suppressed and then disappears at $p$ $\gtrsim$ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe$_4$As$_4$ under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for $p$ $\geq$ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe$_4$As$_4$: a half-collapsed-tetragonal phase.

cond-mat.supr-con↗

arXiv2015

Neutron-scattering measurements of the spin excitations in LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$: Evidence for a sharp enhancement of spin fluctuations by nematic order

Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{\rm S}$, sets in well above the stripe antiferromagnetic ordering at $T_{\rm N}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{\rm S}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.

cond-mat.str-el↗

arXiv2013

Magnetic structures and the Ce-Fe coupling induced Fe spin reorientation in CeFeAsO single crystal

Neutron and synchrotron resonant X-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in single crystal CeFeAsO. The RXMS of magnetic reflections at the Ce $L_{\rm II}$-edge shows a magnetic transition that is specific to the Ce antiferromagnetic long-range ordering at $T_\texttt{Ce}\approx$ 4 K with short-range Ce ordering above $T_\texttt{Ce}$, whereas neutron diffraction measurements of a few magnetic reflections indicate a transition at $T^{*}\approx$ 12 K with unusual order parameter. Detailed order parameter measurements on several magnetic reflections by neutrons show a weak anomaly at 4 K which we associate with the Ce ordering. The successive transitions at $T_\texttt{Ce}$ and $T^{*}$ can also be clearly identified by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at $T^{*}\approx$ 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the \textit{ab} plane. The Fe spin reorientation transition and short-range Ce ordering above $T_\texttt{Ce}$ reflect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through $T^{*}$ and $T_\texttt{Ce}$ is proposed.

cond-mat.str-el↗

arXiv2013

Magnetoelastic coupling and charge correlation lengths in a twin domain of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0.047$): A high-resolution X-ray diffraction study

The interplay between structure, magnetism and superconductivity in single crystal Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.047) has been studied using high-resolution X-ray diffraction by monitoring charge Bragg reflections in each twin domain separately. The emergence of the superconducting state is correlated with the suppression of the orthorhombic distortion around \emph{T}$_\texttt{C}$, exhibiting competition between orthorhombicity and superconductivity. Above \emph{T}$_\texttt{S}$, the in-plane charge correlation length increases with the decrease of temperature, possibly induced by nematic fluctuations in the paramagnetic tetragonal phase. Upon cooling, anomalies in the in-plane charge correlation lengths along $a$ ($ξ_{a}$) and $b$ axes ($ξ_{b}$) are observed at \emph{T}$_\texttt{S}$ and also at \emph{T}$_\texttt{N}$ indicative of strong magnetoelastic coupling. The in-plane charge correlation lengths are found to exhibit anisotropic behavior along and perpendicular to the in-plane component of stripe-type AFM wave vector (101)$_{\rm O}$ below around \emph{T}$_\texttt{N}$. The temperature dependence of the out-of-plane charge correlation length shows a single anomaly at \emph{T}$_\texttt{N}$, reflecting the connection between Fe-As distance and Fe local moment. The origin of the anisotropic in-plane charge correlation lengths $ξ_{a}$ and $ξ_{b}$ is discussed on the basis of the antiphase magnetic domains and their dynamic fluctuations.

cond-mat.str-el↗

arXiv2009

Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2

The discovery of a new family of high Tc materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent, for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked, however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal to orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behavior under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

cond-mat.supr-con↗

arXiv2009

Magnetic and lattice coupling in single-crystal SrFe$_2$As$_2$: A neutron scattering study

A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe$_2$As$_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_\texttt{O}$ = $T_\texttt{N}$ = (201.5 $\pm$ 0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1-2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than \emph{T}$_\texttt{O}$, we suggest that the T-O transition may be an order-disorder transition.

cond-mat.str-el↗

arXiv2008

Fermi-surface induced modulation in an optimally doped YBCO superconductor

We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO superconductor with a wavevector along CuO chains, {\it i.e.} ${\bf q}_1$=(0,$δ$,0). The value of $δ\sim0.21$ is twice the Fermi wavevector ($2{\bf k}_F$) along {\bf b*} connecting nearly nested FS `ridges'. The ${\bf q}_1$ modulation exists only within O-vacancy-ordered islands (characterized by ${\bf q}_0$=$(\frac14,0,0))$ and persists well above and below $T_c$. Our results are consistent with the presence of a FS-induced charge-density wave.

cond-mat.supr-con↗