Single crystals of the Kitaev spin-liquid candidate $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ have been studied to determine the low-temperature bulk properties, the structure, and the magnetic ground state. Refinements of x-ray diffraction data show that the low-temperature crystal structure is described by space group $C2/m$ with a nearly perfect honeycomb lattice exhibiting less than 0.2% in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at ${T}_{N}=7$ K. The magnetic order below this temperature exhibits a propagation vector of $k=(0,1,1/3)$, which coincides with a three-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14 K can be introduced by deformations of the crystal that result in regions in the crystal with a two-layer stacking sequence. The best-fit symmetry-allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$ plane, with a zigzag configuration in each honeycomb layer. The three-layer repeat out-of-plane structure can be refined as a ${120}^{\ensuremath{\circ}}$ spiral order or a collinear structure with a spin direction of ${35}^{\ensuremath{\circ}}$ away from the $a$ axis. The collinear spin configuration yields a slightly better fit and also is physically preferred. The average ordered moment in either structure is less than 0.45(5) ${\ensuremath{\mu}}_{B}$ per ${\mathrm{Ru}}^{3+}$ ion.

Low-temperature crystal and magnetic structure of α -RuCl 3

In iron-based superconductors the interactions driving the nematic order (that breaks four-fold rotational symmetry in the iron plane) may also mediate the Cooper pairing. The experimental determination of these interactions, which are believed to depend on the orbital or the spin degrees of freedom, is challenging because nematic order occurs at, or slightly above, the ordering temperature of a stripe magnetic phase. Here, we study FeSe (ref. )-which exhibits a nematic (orthorhombic) phase transition at Ts = 90 K without antiferromagnetic ordering-by neutron scattering, finding substantial stripe spin fluctuations coupled with the nematicity that are enhanced abruptly on cooling through Ts. A sharp spin resonance develops in the superconducting state, whose energy (∼4 meV) is consistent with an electron-boson coupling mode revealed by scanning tunnelling spectroscopy. The magnetic spectral weight in FeSe is found to be comparable to that of the iron arsenides. Our results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.

/pdf/strong-interplay-between-stripe-spin-fluctuations-nematicity-1ek5ey8cy7.pdf

Strong interplay between stripe spin fluctuations, nematicity and superconductivity in FeSe

A quantum spin liquid (QSL) is a state of matter where unpaired electrons' spins in a solid are quantum entangled, but do not show magnetic order in the zero-temperature limit. Because such a state may be important to the microscopic origin of high-transition temperature superconductivity and useful for quantum computation, the experimental realization of QSL is a long-sought goal in condensed matter physics. Although neutron scattering experiments on the two-dimensional (2D) spin-1/2 kagome-lattice ZnCu3(OD)6Cl2 and effective spin-1/2 triangular lattice YbMgGaO4 have found evidence for a continuum of magnetic excitations, the hallmark of a QSL carrying 'fractionalized quantum excitations', at very low temperature, magnetic and nonmagnetic site chemical disorder complicates the interpretation of the data. Recently, the three-dimensional (3D) Ce3+ pyrochlore lattice Ce2Sn2O7 has been suggested as a clean, effective spin-1/2 QSL candidate, but there is no evidence of a spin excitation continuum. Here we use thermodynamic, muon spin relaxation ({\mu} SR), and neutron scattering experiments on single crystals of Ce2Zr2O7, a compound isostructural to Ce2Sn2O7, to demonstrate the absence of magnetic ordering and the presence of a spin excitation continuum at 35 mK, consistent with the expectation of a QSL. Since our neutron diffraction and diffuse scattering measurements on Ce2Zr2O7 reveal no evidence of oxygen deficiency and magnetic/nonmagnetic ion disorder as seen in other pyrochlores, Ce2Zr2O7 may be the first example of a 3D QSL material with minimum magnetic and nonmagnetic chemical disorder.

Experimental signatures of a three-dimensional quantum spin liquid in effective spin-1/2 Ce2Zr2O7 pyrochlore

Bulk rutile RuO_{2} has long been considered a Pauli paramagnet. Here we report that RuO_{2} exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05μ_{B} as evidenced by polarized neutron diffraction. Density functional theory plus U (DFT+U) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard U of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high Neel temperature and small itinerant moments make RuO_{2} unique among ruthenate compounds and among oxide materials in general.

Itinerant Antiferromagnetism in RuO_{2}.

Weyl (WSMs) evolve from Dirac semimetals in the presence of broken time-reversal symmetry (TRS) or space-inversion symmetry. The WSM phases in TaAs-class materials and photonic crystals are due to the loss of space-inversion symmetry. For TRS-breaking WSMs, despite numerous theoretical and experimental efforts, few examples have been reported. In this Article, we report a new type of magnetic semimetal Sr1-yMn1-zSb2 (y,z<0.1) with nearly massless relativistic fermion behaviour (m*=0.04-0.05m0, where m0 is the free electron mass). This material exhibits a ferromagnetic order for 304K < T < 565K, but a canted antiferromagnetic order with a ferromagnetic component for T < 304K. The combination of relativistic fermion behaviour and ferromagnetism in Sr1-yMn1-zSb2 offers a rare opportunity to investigate the interplay between relativistic fermions and spontaneous TRS breaking.

A magnetic topological semimetal Sr1-yMn1-zSb2 (y, z< 0.1)

A four-circle neutron diffractometer with a new multi-wafer 331 Si monochromator has been installed and commissioned on a thermal beamline at the High Flux Isotope Reactor at Oak Ridge National Laboratory. The instrument is well suited to studies of nuclear and magnetic structures as a function of composition and temperature, resolving symmetry changes (lattice distortions and local structural changes), mapping the evolution of complex magnetic phases, determining hydrogen bonding, analyzing nuclear and spin densities, mapping diffuse scattering, and exploring fiber diffraction. Three incident wavelengths are available, 1.000, 1.536 and 2.540 A, with intensities of 2.5 × 106, 2.2 × 107 and 8.0 × 106 neutrons cm−2 s−1, respectively. Either high-resolution or high-intensity modes are possible by horizontal bending of the monochromator. With increased bending of the monochromator, the incident flux on the sample passes through a maximum, increasing by ×2.0 for 1.000 A, by ×3.5 for 1.536 A and by ×3.5 for 2.540 A, as compared to the flat condition. The flux increases because the lattice strain in the silicon crystals increases. The ω-scan peak width increases with monochromator curvature and this width versus scattering angle flattens. Given these effects, the monochromator bending can be adjusted to deliver high intensity primarily for crystal structure refinements or high resolution for resolving symmetry changes. In addition to the traditional step-scanning mode, a more efficient continuous-scanning mode was developed, and both these are implemented through a LabView-based control program, i.e. a modified version of the SPICE software package. A 4 K closed-cycle helium refrigerator is permanently mounted on the χ-circle of the goniometer to provide temperature control between 4 and 450 K.

Four-circle single-crystal neutron diffractometer at the High Flux Isotope Reactor

Over the past year, new 1 m×1 m neutron detectors have been installed at both the General Purpose SANS (GP-SANS) and the Bio-SANS instruments at HFIR, each intended as an upgrade to provide improved high rate capability. This paper presents the results of characterization studies performed in the detector test laboratory, including position resolution, linearity and background, as well as a preliminary look at high count rate performance.

Characterization of the neutron detector upgrade to the GP-SANS and Bio-SANS instruments at HFIR

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Four-circle single-crystal neutron diffractometer at the High Flux Isotope Reactor

Characterization of the neutron detector upgrade to the GP-SANS and Bio-SANS instruments at HFIR