arXiv subjects

W. Ratcliff

Publications and source records attributed to W. Ratcliff.

8 recordsLinked to original sources

arXiv2020

Electronic and magnetic properties of stoichiometric CeAuBi$_{2}$

We report the electronic and magnetic properties of stoichiometric CeAuBi$_{2}$ single crystals. At ambient pressure, CeAuBi$_{2}$ orders antiferromagnetically below a Néel temperature ($T_{N}$) of 19 K. Neutron diffraction experiments revealed an antiferromagnetic propagation vector $\hatτ = [0, 0, 1/2]$, which doubles the paramagnetic unit cell along the $c$-axis. At low temperatures several metamagnetic transitions are induced by the application of fields parallel to the $c$-axis, suggesting that the magnetic structure of CeAuBi$_{2}$ changes as a function of field. At low temperatures, a linear positive magnetoresistance may indicate the presence of band crossings near the Fermi level. Finally, the application of external pressure favors the antiferromagnetic state, indicating that the 4$f$ electrons become more localized.

cond-mat.str-el↗

arXiv2011

Antiferromagnetic order and superlattice structure in nonsuperconducting and superconducting RbyFe(1.6+x)Se2

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb$_y$Fe$_{1.6+x}$Se$_2$. For the insulating Rb$_{y}$Fe$_{1.6+x}$Se$_2$, neutron polarization analysis and single crystal neutron diffraction unambiguously confirm the earlier proposed $\sqrt{5}\times\sqrt{5}$ block antiferromagnetic structure. For superconducting samples ($T_c=30$ K), we find that in addition to the tetragonal $\sqrt{5}\times\sqrt{5}$ superlattice structure transition at 513 K, the material develops a separate $\sqrt{2}\times \sqrt{2}$ superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb$_{y}$Fe$_{1.6+x}$Se$_2$ is phase separated with coexisting $\sqrt{2}\times \sqrt{2}$ and $\sqrt{5}\times\sqrt{5}$ superlattice structures.

cond-mat.supr-con↗

arXiv2009

Cd-doping effects in Ce2MIn8 (M = Rh and Ir) heavy fermion compounds

Low temperature magnetic properties of Cd-doped Ce2MIn8 (M = Rh and Ir) single crystals are investigated. Experiments of temperature dependent magnetic susceptibility, heat capacity and electrical resistivity measurements revealed that Cd-doping enhances the antiferromagnetic (AFM) ordering temperature from TN = 2.8 K (x = 0) to TN = 4.8 K (x = 0.21) for Ce2RhIn8-xCdx and induces long range AFM ordering with TN = 3.8 K (x = 0.21) for Ce2IrIn8-xCdx. Additionally, X-ray and neutron magnetic scattering studies showed that Cd-doped samples present below TN a commensurate antiferromagnetic structure with a propagation vector (1/2,1/2,0). The resolved magnetic structures for both compounds indicate that the Cd-doping tends to rotate the direction of the ordered magnetic moments toward the ab-plane. This result suggests that the Cd-doping affects the Ce3+ ground state single ion anisotropy modifying the crystalline electrical field (CEF) parameters at the Ce3+ site. Indications of CEF evolution induced by Cd-doping were also found in the electrical resistivity measurements. Comparisons between our results and the general effects of Cd-doping on the related compounds CeMIn5 (M = Co, Rh and Ir) confirms the claims that the Cd-doping induced electronic tuning is the main effect favoring AFM ordering in these compounds.

cond-mat.str-el↗

arXiv2009

Short-range incommensurate magnetic order near the superconducting phase boundary in Fe(1+d)Te(1-x)Se(x)

We performed elastic neutron scattering and magnetization measurements on Fe(1.07)Te(0.75)Se(0.25) and FeTe(0.7)Se(0.3). Short-range incommensurate magnetic order is observed in both samples. In the former sample with higher Fe content, a broad magnetic peak appears around (0.46,0,0.5) at low temperature, while in FeTe(0.7)Se(0.3) the broad magnetic peak is found to be closer to the antiferromagnetic (AFM) wave-vector (0.5,0,0.5). The incommensurate peaks are only observed on one side of the AFM wave-vector for both samples, which can be modeled in terms of an imbalance of ferromagnetic/antiferromagnetic correlations between nearest-neighbor spins. We also find that with higher Se (and lower Fe) concentration, the magnetic order becomes weaker while the superconducting temperature and volume increase.

cond-mat.supr-con↗

arXiv2009

Spin-lattice order in frustrated ZnCr2O4

Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice order in highly frustrated ZnCr$_2$O$_4$ where magnetic chromium ions occupy the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of anti-aligning spins surrounding the triangular faces of tetrahedra is resolved by establishing weak interactions on each triangle through an intricate lattice distortion. The resulting spin order is however, not simply a Néel state on strong bonds. A complex co-planar spin structure indicates that antisymmetric and/or further neighbor exchange interactions also play a role as ZnCr$_2$O$_4$ resolves conflicting magnetic interactions.

cond-mat.str-el↗

arXiv2008

Observation of a continuous phase transition in a shape-memory alloy

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic transition temperatures (T_M=64K and 45K, respectively). In each composition, elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's T_M. Although the new Bragg peaks appear in a discontinuous manner in the Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near the transition temperature are in favorable accord with a mean-field approximation. A Landau-theory-based fit to the pressure dependence of the transition temperature suggests the presence of a critical endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a quantum saturation temperature θ_s=48.3 +/- 3.7K.

cond-mat.mtrl-sci↗

arXiv2006

Structural Anomalies at the Magnetic and Ferroelectric Transitions in $RMn_2O_5$ (R=Tb, Dy, Ho)

Strong anomalies of the thermal expansion coefficients at the magnetic and ferroelectric transitions have been detected in multiferroic $RMn_2O_5$. Their correlation with anomalies of the specific heat and the dielectric constant is discussed. The results provide evidence for the magnetic origin of the ferroelectricity mediated by strong spin-lattice coupling in the compounds. Neutron scattering data for $HoMn_2O_5$ indicate a spin reorientation at the two low-temperature phase transitions.

cond-mat.str-el↗

arXiv2002

Emergent excitations in a geometrically frustrated magnet

Frustrated systems are ubiquitous and interesting because their behavior is difficult to predict. Magnetism offers extreme examples in the form of spin lattices where all interactions between spins cannot be simultaneously satisfied. Such geometrical frustration leads to macroscopic degeneracies, and offers the possibility of qualitatively new states of matter whose nature has yet to be fully understood. Here we have discovered how novel composite spin degrees of freedom can emerge from frustrated interactions in the cubic spinel ZnCr2O4. Upon cooling, groups of six spins self-organize into weakly interacting antiferromagnetic loops whose directors, defined as the unique direction along which the spins are aligned parallel or antiparallel, govern all low temperature dynamics. The experimental evidence comes from a measurement of the magnetic form factor by inelastic neutron scattering. While the data bears no resemblance to the atomic form factor for chromium, they are perfectly consistent with the form factor for hexagonal spin loop directors. The hexagon directors are to a first approximation decoupled from each other and hence their reorientations embody the long-sought local zero energy modes for the pyrochlore lattice.

cond-mat.str-el↗